knobil and neill's physiology of reproduction · the steroid receptors 1101 4. female...

Knobil and Neill’s Physiology of Reproduction, Fourth Editionhttp://dx.doi.org/10.1016/B978-0-12-397175-3.00025-9 © 2015 Elsevier Inc. All rights reserved.

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C H A P T E R

25Steroid Receptors in the Uterus and Ovary

April K. Binder, Wipawee Winuthayanon, Sylvia C. HewittLaboratory of Reproductive and Developmental Toxicology, National Institute of Environmental Health Sciences,

NC, USA

John F. CouseTaconic Farms, Albany Operations, Rensselaer, NY, USA

Kenneth S. KorachLaboratory of Reproductive and Developmental Toxicology, National Institute of Environmental Health Sciences,

NC, USA

INTRODUCTION

The ovarian-derived sex steroid hormones dictate the uterine estrous or menstrual cycle in mammals and are therefore essential to the establishment and maintenance of pregnancy. The uterine response to the preovulatory rise in circulating estradiol (E2) is required to prepare the tissue for the forthcoming rise in progesterone (P) that accompanies ovulation and is critical to embryo implantation. This physiological coordination between the ovary and uterus is common to the large majority of mammals studied and the spectrum of actions and effects of the sex steroids in uterine tissue are mediated by their cognate NRs. Estrogen was first described almost 100 years ago as a substance that induced estrus. We now know from many studies over the past 50 plus years that its physiological activity is broader than simply induc-ing a secretory response in the reproductive tract—estro-gen also affects multiple organs that previously were not considered estrogen responsive. The mechanistic theory for explaining the biological actions of steroid hormones came in 1960 when Elwood Jensen and colleagues first described the uptake and retention of labeled E2 in certain tissues that, because of this retention, became referred to as estrogen target tissues. The ovary and uterus were amongst the tissues showing this property. The hormone tissue retention concept was then shown to be reflective

of other steroid hormones, exemplified by progesterone, glucocorticoid, and thyroid hormones. The Jensen and Gorski laboratories were then the first to show that the tissue retention of estrogen was due to the presence of specific proteins contained within those tissues, which became referred to as estrogen receptors (ER). Numer-ous research groups followed with studies showing that the receptor proteins were hormone specific and their presence was linked to some biological activity in cer-tain tissues. Subsequent development some 20 years later of the first antibodies to the ER allowed for more direct detection and qualification of the protein levels in tissues under different physiological states or disease conditions. Although initially the dramatic physiologi-cal effects of estrogens on the mammalian uterus were appreciated and well described, later investigations began to elucidate the precise mechanisms and signaling pathways involved. Our understanding of steroid hormone action in the uterus has been greatly advanced by utilization of new technologies with the generation of knockout and knock-in models, including receptor or co-activator null, tissue and cell selective null, and mutant knock-in models in mice. These models are also used in combination with microarray, RNA-seq, and ChIP-seq methods that allow for comprehensive mapping of inter-action of nuclear receptors (NRs) with chromatin and modulation of genomic response to steroids in uterine

25. STEROID RECEPTORS IN THE UTERUS AND OVARY1100

4. FEMALE REPRODUCTIVE SYSTEM

tissues. Together, these models and techniques have led to better understanding of the molecular details of steroid receptor roles in biological processes.

Receptor-mediated actions of progestins, androgens, and estrogens are central to reproduction. Perhaps unique among the various categories of signaling pathways, the sex steroid hormones and their cognate receptors are surely mentioned, if not discussed in detail, in every chapter of this volume. The sex steroid hormones are integrated into every aspect of mamma-lian reproductive physiology in both sexes, including sexual maturation and development, gametogenesis, hypothalamic–pituitary control of gonadal function, sexual and maternal behavior, pregnancy, and lactation. Disruption of the signaling pathways for any one of the three gonadal steroids leads to reduced fecundity, if not infertility, due to aberrations in multiple organ systems. Therefore, a thorough understanding of the sex steroid receptors in terms of their expression in reproductive and endocrine tissues, their mechanism of action, their role in reproductive processes, and their interaction with nonsteroidal signaling pathways is instrumental to our ability to manage infertility and reproductive disease.

This chapter principally covers the signaling pathways for progestins, androgens, glucocorticoids, and estro-gens, and the role of each in ovarian and uterine func-tion in mammals. Because of the importance of the sex steroids and the likelihood that each is discussed in several other chapters of this volume, it is appropriate to first discuss the structure, mechanism of action, and signaling pathways for each of the sex steroid recep-tors. Sections then follow on the role of sex steroids in ovarian and uterine function. We chose to divide each of these sections according to the sex steroid receptors: the estrogen receptors (ERs), progesterone receptors (PRs), androgen receptors (ARs), and glucocorticoid receptors (GRs). The section Sex Steroid Receptors and Ovarian Function discusses what is currently known concerning each particular sex steroid signaling pathway in terms of its expression pattern and distribution within the ovary, and its intraovarian role in granulosa cell proliferation and differentiation, thecal cell function, and ovulation. Because a separate chapter (Chapter 23) is dedicated to the corpus luteum, the role of the sex steroid receptors in luteal function is not covered in detail. The section Sex Steroid Receptors in Uterine Function discusses what is currently known concerning each sex steroid signaling pathway in terms of its expression pattern and distribu-tion within the uterus and its role in uterine cell prolifera-tion and differentiation. New since the previous editions of this volume is the development of tissue selective null or “knock-in” mouse models for each of the sex ste-roids. The study of each of these models has already and will continue to make enormous contributions to

our understanding of the reproductive role of each receptor signaling pathway in toto. Therefore, we include a detailed description of the ovarian and uter-ine phenotypes for each of these models in their respec-tive sections.

THE STEROID RECEPTORS

The steroid receptors are part of the family of nuclear receptors (NRs) that is ubiquitous throughout the animal kingdom. The members of the NR fam-ily fulfill a plethora of functions and are integral to the development and maintenance of multiple physi-ological systems. Continued discovery of new NRs has led to a considerable expansion of the NR family and demanded a system of categorization. NRs have been assigned to three classes.1–3 Class I includes the steroid receptors (ER, PR, AR, GR), which interact with high specificity to their steroid ligand as well as to DNA motifs comprised of inverted-repeat hexanucleotides separated by a 3-base pair (bp) spacer. The Nuclear Receptors Nomenclature Committee has developed an NR nomenclature scheme that is based largely on sequence homology and evolutionary comparisons and divides the over 60 NRs into seven subfamilies and a varied number of groups within each subfamily.4 The ERs (ERα and ERβ) are the sole members of the NR3A subgroup, ERα and ERβ being NR3A1 and NR3A2, respectively.4 The PR and AR are the third and fourth members of the NR3C subgroup that also includes the GR and the mineralocorticoid receptor.4 The official gene names for these steroid receptors are ESR1 and ESR2 for ERα and ERβ, respectively, PGR for proges-terone receptor, AR for androgen receptor, and GCR for glucocorticoid receptor. The general properties of the sex steroid receptor family and those mechanisms of receptor action that are most relevant to ovarian and reproductive tract function are outlined following.

Genes, mRNA, and Regulation

The genes encoding the sex steroid receptors exhibit a highly conserved structural organization. The human PR (PGR or NR3C3), AR (AR or NR3C4), ERα (ESR1 or NR3A1), ERβ (ESR2 or NR3A2), and GR (GCR or NR3C1) genes are each composed of eight coding exons, and vary in length from 40 kb (ERβ) to >140 kb (ERα).5–8 In each case, the N-terminal domain (NTD) of the receptor is usually encoded by a single exon, the two zinc fingers of the DNA binding domain, each encoded by separate exons (2 and 3), and the ligand binding domain (LBD), encoded by exons 4–8 (Figure 25.1).

THE STEROID RECEPTORS 1101


Estrogen ReceptorThe human ESR1 (ERα) cDNA was first cloned in

19859–11 and has since been isolated from numerous addi-tional species.12,13 A second ER gene, termed ESR2 (ERβ), was discovered in 1996 in rat14 and human15 tissues and has since been cloned from many species.13 Unlike the A and B forms of PR and AR, ERα and ERβ are not isoforms but rather distinct receptors encoded by two genes on different chromosomes. The ERα proteins are 595 and 599 amino acids in length in human and mice, respectively, with an approximate molecular weight of 66 kDa (Figure 25.1).8,16–18 Multiple promoter and regu-latory regions in the 5′-untranslated sequences of the human and rat ESR1 gene have been described, yet all possess the same single open reading frame.8 Numerous naturally occurring variants of the ESR1 mRNA in nor-mal and neoplastic tissues of several species have been described, but the existence of corresponding proteins remains controversial.18

The promoter region of the ESR1 gene has been relatively well characterized and indicates a complex regulation of expression.8,19–21 Cicatiello et al.22

demonstrated that the proximal 0.4 kb of the mouse Esr1 promoter is sufficient to provide for widespread but specific expression of a reporter construct in vivo. Regulatory elements that may provide for AP-1, Sp1, and ER autoregulation of the human ESR1 promoter have been described.23,24 Decreased ESR1 expression is linked to receptor-mediated actions of vitamin D25 and increased intracellular cAMP or mitogen-activated protein kinase activity.23 Increased methylation of the ESR1 promoter is implicated in reducing ER levels, especially in tumorigenic tissues.23

The ESR2 genes of multiple species yield numerous transcripts that range from 1 to >9 kb in length, in contrast to the single predominant transcript of ∼7 kb transcribed from the ESR1 gene.18 Initial descriptions of human and rodent ERβ projected a protein of 485 amino acids. However, it is now apparent that translation of the ESR2 mRNA initiates upstream of these original open reading frames and yields a receptor of 549 amino acids in rodents and 530 amino acids in humans, each with an approxi-mate molecular weight of 60–63 kDa (Figure 25.1).8 Therefore, ERβ is slightly smaller than ERα, and most of this difference lies within the N-terminus. A number of

FIGURE 25.1 The steroid family of receptors and endogenous ligands. (A) Schematic illustration of the structural organization of the sex steroid nuclear receptors. The more highly conserved C and E domains are depicted as open boxes and the less well-conserved A/B, D, and F domains as filled bars. The F domain is unique to the estrogen receptors (ER). The functions of the modular domains are indicated as are the two known transcriptional activation function domains, AF-1 and AF-2 (as well as AF-3 found only in PRB). The AF-1 domain is harbored in the A/B region and exhibits constitutive activity in vitro, whereas the AF-2 domain lies with the ligand binding (E) domain and is critical to ligand-induced receptor activation. Both domains synergize during ligand-activated receptor actions. NLS, nuclear localization signal. (B) Comparison of the human steroid receptors, the androgen receptor (AR), estrogen receptors α (ERα) and β (ERβ), progesterone receptor B (PRB) and A (PRA) and the glucocorticoid receptor (GR). The amino acids that compose each domain of the human forms are indicated. For PRA, the numbers refer to the amino acid residues in reference to PRB.



variant transcripts of the ESR2 gene have been described; however, unlike ERα, there is growing evidence that some of these variants may coexist with the wild-type (WT) receptor form in certain tissues (Figure 25.2). Portions of the mouse Esr2 gene have been characterized and pos-sess potential binding sites for numerous transcription factors8,26 and the initiation of transcription from at least two distinct untranslated exons.27

Progesterone ReceptorTwo distinct promoters in the PGR gene provide for

the generation of two major PR isoforms, PRB (114 kDa) and PRA (94 kDa), in most species except rabbit, which possess PRB only. The two PR forms are identical in all regions except the NTD, which is truncated by 128–164 amino acids in PRA, depending on the species (Figure 25.1). Both PR isoforms are expressed at relatively equal levels in tissues, although differences in the ratio have been noted in certain tissues. A third isoform, termed PR-C, an N-terminally truncated form that lacks the

full A/B domain and first zinc finger of the C domain, has been described, but its expression as a protein is controversial.29,30 The level of PR expression is modu-lated by ER-mediated effects of E2 in certain but not all tissues, and this is especially true in the female reproductive tract.31–34 Several naturally existing PGR variant transcripts harboring exon deletions or distinct 5′-untranslated sequences have been described but are not well characterized in terms of protein expression or functionality.27,30,35

Androgen ReceptorThe AR gene is located on the X chromosome, and

therefore genetic males possess only a single copy.7,36,37 Two distinct start sites in the AR gene are used to produce two isoforms, AR-B (110 kDa) and AR-A (87 kDa), which differ only in the N-terminus ( Figure 25.1).12,38 However, in contrast to the PR, the two known AR isoforms exhibit only subtle functional dif-ferences. A unique feature of the AR gene relative to

FIGURE 25.2 Human (h) and mouse (m) estrogen receptor variant transcripts. ERs are shown, with the general modular structure of ste-roid hormone receptors illustrating the N-terminal (A/B) domain, DNA binding (C) domain, hinge (D) region, ligand binding (E) domain, and C-terminal (F) domain (see Figure 25.1 for details). Top: hERα full-length and truncated variants (hERα-46 and hERα-36), lacking N-terminal AF-1. Bottom: Full-length hERβ (or ERβ1) with % sequences similarity in comparison to ERα indicated above it. Reported hERβ isoforms are shown beneath. hERβ2 is also called ERβCx, and encodes an alternate C-terminal region, depicted by the striped fill, also seen in hERβ4 and 5. hERβcx is unable to bind E2 but acts as a dominant negative receptor when heterodimerized with ERα in vitro.28 hERβΔexon5 is produced by deletion of exon 5, and produced a truncated protein due to a frame shift leading to translational termination, and has been found in other mammals.18,28 mERβ2 contains a 54-bp insert (INS) that codes for an additional 18 amino acids in the ligand binding (E) domain and has only been found in rodents. mERβ2 exhibits an approximate 35-fold decreased affinity for E2 relative to WT mERβ and a significantly reduced transactivational activ-ity in vitro.18 mERβΔexon5, mERβΔexon6, and mERβΔexon5&6 lack parts of the LBD. Source: Redrawn with permission based on a figure from Ref. 18.



its sex steroid receptor counterparts is the presence of polymorphic repeats of glutamine and glycine in the NTD, the former of which has been linked to certain chronic diseases and cancer in humans.39 There are binding sites for a wide array of transcription factors in the AR promoter, including Sp1, cAMP-response ele-ment binding (CREB) protein, and c-myc, suggesting complex tissue-specific regulation. Direct androgen-mediated autoregulation of AR expression has also been shown.39

Glucocorticoid ReceptorAlthough not historically recognized for its roles in

uterus and ovary, GR is ubiquitously expressed, and more recent studies have demonstrated important inter-actions between GR and sex steroid receptor mediated signaling.40,41 GR isoforms, are produced by alternate splicing leading to different c-terminal amino acids. GRα is the primary isoform, whereas GRβ is unable to bind GR ligands and can behave as a dominant negative inhibitor of GRα activity.42

Receptor Structure

Common to all members of the NR family is a mod-ular structure of domains, each of which harbors an autonomous function that is critical to total receptor action.1,2,12,43–47 The sex steroid receptors are composed of five functional modules, an N-terminal domain (NTD) or A/B domain, the DNA binding (C) domain, a hinge (D) region, and an LBD (E) (Figure 25.1). The ERs also possess a unique C-terminal F domain of unknown func-tion. Our understanding of the NR functional domains and their importance to overall receptor activity is largely derived from the in vitro study of artificially gen-erated mutant receptors and more recently from X-ray crystallography studies.

NTD or A/B DomainThe NTD or A/B domains of the NR family members

greatly vary in length and share little homology among the steroid receptors, although some structural features are conserved (Figure 25.1). Crystallography studies of the steroid receptor NTD have been largely unsuc-cessful because this portion of the receptor fluctuates in aqueous solutions. However, evidence suggests that intramolecular interactions between the A/B and other receptor domains are likely to induce a more structured NTD.1,37,45 Current models of NR signaling incorporate the flexibility of intrinsically disordered (ID) regions of the receptor, including the NTD, into a mechanism of allosteric interaction and coordination of ligands, DNA motifs, and NR domain functions.1,45,48 The NTD of each of the sex steroid receptors harbors the transcriptional activation function-1 (AF-1) domain and provides for cell and promoter-specific activity of the receptor as well as a site for interaction with co-receptor proteins (Table 25.1). The AF-1 domain alone can confer constitutive transcrip-tional (i.e., ligand-independent) activity when linked to a heterologous NR DNA binding domain in vitro, but this function is largely overcome in the context of the whole receptor.12 Posttranslational modifications to the A/B domain can dramatically affect the overall behavior of the receptor and are thought to be an important mechanism for the modulation of AF-1 functions.17 Phosphoryla-tion of the A/B domain is the most well-characterized posttranslational modification and occurs in all three sex steroid receptors via the actions of multiple intracel-lular signaling pathways, including mitogen-activated protein kinase pathways,49 the cAMP/protein kinase A (PKA) pathway, and cyclin-dependent kinases.12,37,39,43,50

The extended N-terminal sequences that are unique to PRB (Figure 25.1) provide a third transcriptional AF-3 domain in this isoform that is lacking in PRA. This may allow for PRB-specific expression of certain P regulated

TABLE 25.1 Steroid Receptor Co-Regulator Complexes

Complex Functions Comments References

SRC1, SRC2, SRC3 Interact with Helix12 of agonist-bound NR, interact with SWI/SNF, histone modifiers

51,52

Mediator “Bridges” NR and transcriptional “machinery” (RNA Pol II) to control transcription

Made up of >20 subunits, MED 1-31, arranged in 3 modules (head, middle, tail)

53,54

SWI/SNF Regulate access to enhancer sequences via chromatin remodeling, ATPase activity

Made up of 9+ subunits, examples include BRG1, BRM, BAF subunits

55

Histone Modifiers Modify histones to increase or decrease transcription

Acetyltransferase (HAT; e.g., p300/CBP), deacetylase (HDAC; e.g., NCoR), Methyl transferase (e.g., PMRT/ CARM), de-methylase

56,57

26S Proteasome “Clears” transcriptional modulatory proteins to facilitate subsequent transcription, transcriptional termination

Structure made up of 20S catalytic core particles (CP), 19S regulatory particles (RP)

58,59



FIGURE 25.3 The nuclear receptor response elements and primary structure of the DNA binding domain. Top: Shown is the canonical core hormone response element (HRE) for the estrogen receptors (ERE) and glucocorticoid receptors (GRE). These sequences are located in the regulatory regions of sex steroid target genes and provide a site for receptor binding and transactivational activity. A full HRE consists of two core palindromic sequences arranged as an inverted repeat, always separated by three bp (denoted by XXX). Note that a change from GT to AA at positions two and three of the core sequence modifies the ERE to a GRE. Bottom (A) The DNA binding domain (C) of the sex steroid receptors is highly conserved and composed of two zinc fingers and a C-terminal extension (CTE). The ERα DBD is shown. Each zinc finger is composed of four conserved cysteine residues that coordinate to chelate a single zinc ion. Helix 1 of the first zinc finger contains the P-box residues involved in the discrimination of the HRE. Residues in the second zinc finger (helix 2) form the D box that provides a dimerization interface. The CTE is critical



genes.29,50,60 In general, PRB is a stronger activator of transcription versus PRA when acting on a hormone response element (HRE)–driven gene construct in vitro.60 Furthermore, antagonist-bound PRB acts as a strong transcriptional activator in certain cell and promoter contexts, whereas antagonist-bound PRA is inactive.29

The NTD of the AR is especially autologous in vitro and is likely more important to overall AR transactivation than the C-terminal AF-2 domain (described later).1,37–39 Furthermore, the first 30 residues of the AR NTD are highly conserved and important for interactions with the LBD that provide for agonist-induced stabilization of the receptor.37 Unique to the AR among the sex steroid receptors are a series of amino acid repeat sequences in the NTD, most notably the polymorphic tracts of gluta-mine and glycine, of which certain lengths are thought to be linked to various human diseases or cancer.37,38

The greatest structural disparities between ERα and ERβ lie within the A/B domain, which is approxi-mately 30 amino acids shorter in ERβ and exhibits only >20% homology.8,61,62 This divergence likely accounts for the many functional differences that have been revealed from comparative studies of the two ER forms.8,17,18,62 In general, ERβ tends to be a less effec-tive transcriptional activator compared with ERα when acting in a classic estrogen response element (ERE)–driven mechanism in vitro.18 An ERα/ERβ chimera in which the A/B domain of ERβ has been replaced with that of ERα is able to activate transcription much better than the native ERβ.61 Furthermore, certain antagonists (e.g., tamoxifen) that exhibit some agonist-like proper-ties through AF-1 when bound to ERα exhibit no such activity with ERβ.61,63

DNA Binding or C DomainThe C domain of the NR family members is that

portion of the receptor that specifically functions to rec-ognize and bind to the cis-acting enhancer sequences, or HREs, that are located within the regulatory regions of target genes (Figure 25.3). It is the most highly conserved (55–80%) region among the NR family members.2,46,64 The C domains of ERα and ERβ are practically identi-cal (>95% homology) in most species and are therefore expected to exhibit a similar affinity for the same HREs.62 The functionality of the C domain is provided by a motif of two zinc fingers, each composed of four cysteine residues that chelate a single Zn2 ion, and are always encoded by separate exons within the gene (Figure 25.3).

Crystallography studies indicate a highly conserved structure consisting of dual α-helices positioned per-pendicular to each other1,45,46 (Figure 25.3). Amino acids in the C-terminal “knuckle” of the first zinc finger form the “P box” (proximal box) of the DNA binding domain and confer sequence specificity to the receptor; hence, the proximal zinc finger is often referred as forming the “recognition helix” (Figure 25.3).64 Amino acids at the N-terminal “knuckle” of the second zinc finger form the “D-box” (distal box) and are more specifically involved in differentiating the “spacer” sequence within the HRE as well as providing an interface for receptor dimeriza-tion (Figure 25.3).

The consensus HREs that sex steroid receptors bind are composed of two 6-bp palindromic sequences arranged as an inverted repeat and always separated by a 3-bp spacer (Figure 25.3). Because the P-box residues are identical among the AR, PR, and GR, these receptors bind a common consensus HRE consisting of two 6-bp palindromic half-sites (5′-PuG[G/A]ACA) arranged as an inverted repeat and always separated by a 3-bp spacer (Figure 25.3). The consensus ERE bears the same arrangement but is composed of a unique palindromic half-site (5′-PuGGTCA) (Figure 25.3). The inverted-repeat arrangement of the palindromic sequences is unique to sex steroid receptor HREs and dictates that the receptors homodimerize in a “head-to-head” position when bound to DNA. Evidence indicates that the AR can uniquely dimerize in a “head-to-tail” fashion on an HRE of the monomer 5′-GGTTCT. More recent structural analysis has revealed the importance of the 10-30 amino acid carboxy terminal extension (CTE) of the DBD in DNA interaction.1,45,46,48 Although this region is variable between steroid receptors, it is crucial for DNA binding, particularly for sequence selectivity of DNA binding, by extending the interaction surfaces between the receptor and the DNA.

Hinge Region or D DomainHistorically, the D domain was thought to primarily

serve to connect the more highly conserved C and E domains of the receptor.12 The previously described CTE extends into the hinge region, which also harbors a nuclear localization signal, and influences cellular compartmentalization of receptors, as well as sites of posttranslation modifications.48 Current mechanisms suggest this nonconserved and intrinsically disordered (ID) domain is important for intramolecular allosteric

for monomeric DNA binding. Diagram includes amino acids found in ERα. (B) Amino acid sequences in the vicinity of the P box and CTE regions of the steroid receptors are compared. The underlined residues in the P-box sequences are critical for HRE selectivity. Amino acid substitutions made within the P box to create the NERKI and the EAAE DNA binding–deficient ERα mouse lines are also shown. The EAAE mutations also includes an amino acid change in the “tip” of the first finger that isn’t in the P box. (C) Shown is a ribbon diagram of the DNA binding (C) domain illustrating the putative arrangement of helix 1 and helix 2 crossing at right angles to form the core of the DNA binding domain that recognizes a hemi-site of the response element. Source: (A) reproduced with permission from Ref. 43 and (C) reproduced with permission from Ref. 65.



interactions involving the N-terminal and ligand binding domain (LBD)48 (described earlier in the section NTD or A/B Domain). This type of flexible structural interaction works to allow rapid response to diverse modulators governing changes in biological environments.48

LBD or E DomainThe LBD or E domain of the steroid receptors is a

highly structured multifunctional region that primar-ily serves to specifically bind ligand and provide for hormone-dependent transcriptional activity.12 An AF-2 domain located in the C-terminus of the E domain mediates this latter function. The AF-2 domain is subject to posttranslational modifications12 and is an especially strong activator of transcription in the ER and PR but is markedly weaker in the AR, where it is more involved in interactions with residues in the NTD.37,38 Also harbored within the E domain is a strong receptor dimerization interface, sites for interaction with heat shock proteins, and nuclear localization signals.12,48 Although there is minimal homology in the primary sequence of the LBD for the sex steroid receptors, comparative studies of the crystal structures of liganded and unliganded LBDs indi-cate a highly conserved structural arrangement. These structural studies indicate that the LBD is composed of 11 α-helices (H1, and H3–H12) arranged in a three-layer α-helical sandwich to create a hydrophobic ligand bind-ing pocket near the C-terminus of the receptor.48 The resulting shape and volume of the ligand binding pocket is larger than necessary to accommodate the correspond-ing ligand, suggesting that key interactions between the ligand and specific receptor residues are more critical to conferring ligand specificity.61 Receptor binding to an agonist ligand leads to rearrangement of the LBD such that H11 is repositioned and H12 swings back toward the core of domain to form a “lid” over the binding pocket. This agonist-induced repositioning of H12 leads to the formation of a hydrophobic cleft, or “NR box”, by helices 3, 4, and 5 on the receptor surface, constituting the AF-2, which serves to recruit co-activators (Table 25.1) to the receptor complex. In contrast, receptor antagonists are unable to induce a similar repositioning of H12, leading to a receptor formation that is incompatible with co-activator recruitment and is therefore less likely to activate transcription. The LBDs of PRB and PRA are identical and provide for high affinity binding to P.12,50 The LBD of the AR in humans, rats, and mice is identical and provides for high affinity binding of two endogenous androgens, T and 5α-hydroxyT (DHT), the latter of which binds with much greater affinity.38,66 Numerous high affinity synthetic ligands for PR and AR have been developed, including the agonists R5020 and R1181, respectively (Table 25.2).12 The LBDs of ERα and ERβ exhibit less than 60% homology but bind the endogenous ligand, E2, with similar affinity (ERα, 0.1 nM; ERβ, 0.4 nM).8,12,17 Both ER

forms also bind the synthetic estrogen diethylstilbestrol (DES) with relatively equal affinity.67 However, given the divergence in homology, it is not surprising that ERα and ERβ exhibit measurable differences in their affinity for other endogenous steroids and xenoestrogens.8,17 Natural and synthetic steroidal and nonsteroidal ER agonists and antagonists have been described, some of which show specificity for one or the other ER subtype (Table 25.2), illustrating differences between the LBDs of ERα and ERβ, and provide for powerful pharmacological tools to discern the overall function of each ER (Table 25.2). The most widely used ER subtype selective ligands currently in use are propylpyrazole (PPT), an ERα selective ago-nist, and diarylpropionitrate (DPN), an agonist showing preference, but not exclusive selectivity, towards ERβ.8

F DomainAmong the sex steroid receptors, only ERs possess

a well-defined F domain (Figure 25.1). This region is relatively unstructured and harbors little known func-tion, although some data indicate a role in co-activator recruitment, dimerization, and receptor stability.61,62,68–70

Co-Regulatory ComplexesAll steroid receptors interact with co-regulatory mol-

ecules, co-activators, and co-repressors.51,71 The primary co-activator interaction for steroid receptors (SR) is with p160/SRC (steroid receptor co-activator) 1, 2, and 3.52,72,73 SRC1 (NCOA1), SRC2 (GRIP1, TIF2), and SRC3 (pCIP, RAC3, ACTR, TRAM, A1B1) interact with helix 12 of SRs via “LXXLL” motifs in their nuclear receptor interacting domain, which are leucine rich regions with “X” desig-nating any amino acid.52 SRCs also contain activation domains that recruit secondary molecules such as p300, and a bHLH-PAS motif within the N-terminal region, which can interact with other transcription factors.52 The elegant complexity of co-activator composition and func-tion has been increasingly revealed. Steroid receptors and SRCs function as a nexus interacting with massive multimeric complexes, such as the SWI/SNF chroma-tin remodeler, mediator complex, or proteasomes (Table 25.1).73 These interactions facilitate coordination of the specific functions necessary to allow appropriate gene and cell selective access to chromatin, via modifications of histones or members of co-regulatory complexes.74 In this way, co-activators dynamically mediate and coor-dinate processes necessary to accomplish transcription, including initiation, elongation, termination, and clear-ing or turnover of the transcriptional modulators.

Receptor Mechanisms of Action

Our understanding of the mechanisms by which steroid hormones and their cognate intracellular recep-tors influence cell function and behavior has expanded



profoundly since 1988 when Clark and Markaverich reviewed the field for the inaugural edition of this vol-ume. Much of that earlier chapter remains contempo-rary in reference to general receptor biochemistry and ligand-dependent activation, which is now referred to as the “classical” or ligand-dependent direct-DNA binding model of receptor function (Figure 25.4). In the years since, numerous discoveries have been made that illuminate the complexity of sex steroid receptor signaling in cells and tissues, such as the discovery of additional receptor forms and variants (e.g., ERβ) and high-resolution crystallography of receptor domains. The entrée into the “omics” era has facilitated massive expansion of study of transcriptional regulation and chromatin remodeling. In addition, several alterna-tive receptor signaling mechanisms that diverge from the classic model have become apparent, including “tethering” of the sex steroid receptor to heterologous DNA-bound transcription factors to provide for steroid regulation of genes that lack HRE sequences within

their promoter (Figure 25.4); plasma membrane ste-roid signaling, often referred to as “nongenomic” ste-roid actions; and ligand-independent “cross talk” with intracellular and second messenger systems that pro-vide for sex steroid receptor activation in the absence of the cognate steroid ligand (Figure 25.4). The existence of multiple receptor-mediated signaling pathways likely accounts for the refined control and plasticity of tissue responses to sex steroids. These modes of sex steroid receptor action as currently understood are dis-cussed following.

Ligand-Dependent Actions: Direct or ClassicalThe classic model of steroid receptor action states

that the receptor resides in the nucleus or cytoplasm but is sequestered in a multiprotein inhibitory com-plex in the absence of hormone (Figure 25.4). The lipo-philic steroid ligands are able to freely diffuse across the plasma and nuclear membranes and bind their cognate receptor. The binding of ligand results in a

TABLE 25.2 Structures of Endogenous and Synthetic Agonists and Antagonists of Steroid Receptors



conformational change in the receptor, transforming it to an “activated” state that is now available for homodi-merization, increased phosphorylation, and binding to an HRE within target gene promoters, and interaction with chromatin and transcriptional mediators. NRs seem to be preferentially recruited to open regions of chromatin.75 Studies using MCF7 breast cancer cells indicate that FoxA1 acts as a pioneering factor, pro-viding accessible regions in the chromatin that recruit ERα.76–79 The ligand-HRE-bound receptor complex then engages co-activator molecules as described before,52 leading to modulation of transcription rates of responding genes. This classic steroid receptor mechanism is dependent on the functions of both AF-1 and AF-2 domains of the receptor, which synergize via the recruitment of co-activator proteins, most notably the p160 family members.52 Depending on the cell and target gene promoter context, the DNA-bound receptor complex may positively or negatively affect expression of the downstream target gene. Initially, study of NR-mediated gene regulation was carried out on a gene-by-gene basis using a handful of known hormone regulated transcripts, such as the ER target genes PS2, lactotransferrin, and PR or the AR target gene prostate specific antigen. Now, after numerous comprehensive analyses of hormonally regulated transcriptional profiles, using microarray and more recently RNA-seq, thousands of NR targets have been found in various cell lines and tissues.

In general, PRB can better or uniquely activate transcription versus PRA when acting on progesterone response element–regulated genes in various cell types in vitro due to the presence of a third AF-3 in the NTD of PRB.50,60 Furthermore, PRA can repress the transcriptional activities of PRB in certain cell and promoter contexts,50 suggesting that PRA may modulate P responsiveness in certain tissues. Interestingly, PRA can also repress the actions of other heterologous NRs, including the AR, ER, and GR.12,50,60 The variant, PR-C, is primarily cytosolic and lacks the A/B domain and the 5′ portion of the C domain, and has been shown to bind P and enhance the actions of PRB and PRA,80 but the pres-ence of encoded protein and a potential biological role remains controversial.30,81 Some evidence for a role in P signal dampening during labor has been described for PR-C.82 When acting via a classic ERE-driven mechanism in vitro, ERα homodimers and ERα/ERβ heterodimers tend to be stronger activators of transcription com-pared with ERβ homodimers.18,61 Corroborating in vivo evidence of differential regulation and heterodimer formation by the ER subtypes has been difficult to gen-erate. However, a microarray study by Lindberg et al.83 using bone tissue from ovariectomized mice found that ERβ generally inhibits ERα-mediated gene expression, but in the absence of ERα, ERβ can partially provide some E2-stimulated gene expression. No evidence for ERα or ERβ preferential regulation of specific transcripts or promoters has been forthcoming; rather, it seems the

FIGURE 25.4 Ligand-dependent and ligand-independent nuclear receptor mechanisms. The direct “classic” model of sex steroid receptor (SR) action involves direct interaction between SR bound to hormone (triangles) and HRE; the tethered pathway utilizes indirect “tethering” of SR to genes via interactions with other transcription factors (TF). “Nongenomic” signaling is initiated by membrane-localized receptors modulating extranuclear second messenger (SM) signaling pathways. Ligand-independent responses occur as a result of transduction of membrane receptor signaling, such as growth factors, to nuclear SR. Source: Adapted with permission from Ref. 18.



important factor is which receptors are expressed in a responding tissue.

Indirect/Tethered Actions (HRE Independent)Ligand-activated steroid receptors can stimulate

the expression of genes that lack a conspicuous HRE within their promoter. This has been especially demon-strated for ERα84–86 but is also known to occur for PR.87 This mechanism of HRE-independent steroid receptor activation is postulated to involve a “tethering” of the ligand-activated receptor to transcription factors that are directly bound to DNA via their respective response elements (Figure 25.4), This mechanism involves a mediator component (e.g., p160) between ER and AP-1 versus direct interaction.85 Interestingly, ERβ is unable to enhance the actions of a DNA-bound AP-1 complex when bound to E2 but can do so in the presence of certain selective estrogen receptor modulators (SERMs) such as tamoxifen, raloxifene, and ICI 164,384.18 A simi-lar HRE-independent mechanism of sex steroid receptor regulation has been documented for genes that possess a GC-rich region or Sp1 binding site within the promoter, upon which the actions of a bound Sp1 complex can be enhanced by ERα.84 These mechanisms have primarily been demonstrated using in vitro cell culture models.

Nongenomic ActionsAll models of sex steroid receptor action described thus

far influence cellular phenotypes by acting in the nucleus and modulating target gene expression. The required gene transcription and mRNA translation is a relatively slow process; microarray studies demonstrate that initial transcriptional responses occur beginning about 60 min after administering hormones. Therefore, the numer-ous examples described to date in which steroids affect cellular dynamics within seconds to minutes of exposure have remained difficult to explain within the context of the previous models. Rapid cellular changes such as increased intracellular calcium or cAMP or activation of kinase signaling cascades have been attributed to sex steroid exposure of varied cell types and tissues.88,89 Because these steroid effects do not involve direct steroid receptor activation of gene transcription, they are often collectively referred to as representing “nongenomic” pathways of steroid action. Indeed, several of the docu-mented effects lack a conspicuous nuclear component or have even been demonstrated to occur in enucleated cells, yet others can ultimately affect gene expression. Therefore, rather than nongenomic, some have proposed these types of steroid actions to be loosely categorized as plasma membrane–associated steroid signaling events. Although there are numerous examples of putative membrane-associated steroid signaling effects in repro-ductive tissues and processes, a thorough description of these mechanisms is beyond the scope of this chapter.

An obstacle to our better understanding of membrane-associated effects of steroids is the lack of data on the nature of the cell-surface steroid “receptors”. Questions remain concerning whether the membrane-associated receptors are identical or variant forms of the sex steroid NR or instead distinct receptors altogether. P is known to rapidly induce maturation in Xenopus oocytes that are arrested in the G2 phase of meiosis I,90 without requiring protein synthesis. Most evidence indicates this process begins at plasma membrane progesterone bind-ing sites that are unique from the classic nuclear forms of PR, called progesterone receptor membrane component 1 (PGRMC1),91 although cDNA homologs of the PR were cloned from Xenopus and found to be associated with P-induced oocyte maturation.89 In other studies, P is able to inhibit apoptosis of rat granulosa cells in vitro via a membrane-bound receptor that is immunoreactive to antisera directed against the LBD of the classic PR.89

Rapid effects of E2 have been described in a vast array of tissues, including a rapid activation of endo-thelial nitric oxide synthase in endothelial cells, poten-tiation of currents induced by the ion channel agonist kainite in hippocampal neurons, and influx of calcium in uterine endometrial cells.88,90 The evidence of an asso-ciation between ERα and plasma membrane–bound components has increased, supporting the view that this mechanism may indeed account for certain rapid effects of E2.92,93 In this sense, the various nongenomic actions of E2 are often categorized according to their sus-ceptibility to inhibition by classical ER antagonists (e.g., ICI type compounds).92 E2-induced activation of mem-brane ion channels, endothelial nitric oxide synthase, and mitogen-activated protein kinase kinase are all inhibited by ER antagonists and therefore likely involve membrane-associated ERα or ERβ.92 In contrast, exam-ples of E2-induced PKA and protein kinase C (PKC) activation are not inhibited by ICI compounds and are therefore not likely to involve the nuclear ER form.92 A recent study utilized a mutated form of the mouse ERα engineered to remain sequestered outside the nucleus (ERαH2NES), regardless of estrogen ligand. This form of ER did not mediate transcriptional responses but maintained estrogen-induced MAPK phosphorylation.94 Targeting steroid receptors to the membrane involves palmitoylation, which is facilitated by HSP27.88 The palmitoylation promotes interaction with caveolin-1, which then results in localization of the receptor in membrane caveolin rafts. Another potential mediator of rapid membrane localized hormone response is the G protein-coupled receptor GPER (originally referred to as GPR30), which is activated by E2.95 GPER-null mice lack reproductive phenotypes,96 although effects on the degrees of uterine responses elicited by E2 have been observed with G15, a GPER selective antagonist, suggesting a potential role for GPER in modulating



ERα-mediated outcomes.97 A multiprotein complex of extracellular signal–related kinases, the inner plasma membrane component of tyrosine kinase receptors (c-Src), and the nuclear sex steroid receptors92,98–101 has also been described as a mediator of rapid response to E. A different mechanism speculates that androgens or E2 may act as extracellular signaling hormones via binding to sex hormone binding globulin that is already bound to the cell membrane via a G protein–coupled-like sex hormone binding globulin cell-surface receptor.102

Ligand-Independent Actions: Membrane Receptor Cross Talk

We now have ample evidence that the sex steroid receptors can be activated via intracellular second messenger and signaling pathways, allowing for the induction of sex steroid target genes in the absence of steroid ligand, or the enhancement of steroid hormone signaling (Figure 25.4).29,103–105 Polypeptide growth fac-tors are able to activate ERα-mediated gene expression via mitogen-activated protein kinase activation of ERα in the absence of E2 (Figure 25.4). Similarly, interleu-kin-6 stimulation of cells leads to increased AR-medi-ated gene expression,38,106 and cyclin-dependent kinases have been shown to activate PR-regulated gene expres-sion.29 The intracellular signaling molecule cAMP, a common second messenger of G protein–coupled recep-tors and an activator of the PKA pathway, can stimu-late increased PR-, AR-, and ER-mediated transcription of target genes in the absence of steroid ligand.29,103 A well-characterized example of ligand-independent cAMP activation of PR occurs after dopamine stimula-tion of dopaminergic neurons107 and is the mechanism by which sexual behavior is induced in female rodents. Likewise, growth factors are able to mimic the effects of E2 in the rodent uterus via E2 independent activation of ERα.109,110 Most interesting are recent studies indicat-ing that the MAP kinase protein ERK is co-recruited to chromatin with ERα.111 Ligand-independent activation of sex steroid receptors is believed to rely largely on cel-lular kinase pathways that alter the phosphorylation state of the receptor and/or its associated proteins (e.g., co-activators, heat shock proteins) (Figure 25.4). As in the classic ligand-dependent mechanism described earlier, specific receptor domains are critical to ligand-indepen-dent activation as well. ER activation by peptide growth factor–signaling pathways appears to be more depen-dent on AF-1 functions, whereas the effects of increased intracellular cAMP are postulated to depend on the AF-2 domain and do not require a functional AF-1.103

Receptor AntagonistsStructural analyses of the sex steroid receptors

provide insight into the agonist/antagonist actions of endogenous or synthetic ligands (Table 25.2). Certain

ligands possess “mixed” agonist/antagonist activity that is dependent on the receptor, cell, and promoter context, leading to the more descriptive terms of SERMs, selective PR modulator, and selective AR modulator. As described in the subsection Receptor Mechanisms of Action of the section Steroid Receptors, sex steroid receptor agonists interact with the LBD to generate an ordered array of 11 α-helices that is most conducive to receptor interaction with co-activator proteins and/or promotes disassociation of co-repressor proteins. In turn, antagonist ligands generally fail to generate the necessary changes in receptor structure that induce full transcriptional activity.48,112–114

Antiprogestins have a number of applications in repro-ductive medicine, including use as contraceptives due to their ability to prevent implantation or ovulation.115 The currently available PR antagonists are all competi-tive inhibitors of P binding to the PR LBD.115 Type I anti-progestins, such as ZK 98299 (onapristone), bind PR but fail to induce receptor phosphorylation and induce weak receptor binding to HREs.115,116 In contrast, type II anti-progestins, such as RU 486 (mifepristone, Table 25.2), bind PR and promote receptor phosphorylation, homodimer-ization, and binding of the receptor complex to an HRE, but are unable to activate transcription.50 Crystallogra-phy data indicate that RU 486 binding to PR induces an arrangement of helix 12 that is not conducive to binding of steroid receptor co-activators and may even promote recruitment of co-repressors such as NCoR.50 Type II anti-progestins do possess some PR agonist activity when act-ing in the context of the cAMP/PKA signaling pathway in certain cell types.50

The field of antiestrogens and SERMS has been an area of intense research because the potential clinical applications include treatments for fertility, cardiovas-cular disease, osteoporosis, cognitive function, post-menopausal symptoms, and breast and gynecological cancers.18 The best-characterized antiestrogens are com-petitive inhibitors of E2 binding to ER and have been classified into two major groups. The type I class of ER antagonists includes the triphenylethlylene compounds, tamoxifen (Table 25.2), hydroxy-tamoxifen, and raloxi-fene and are characterized by mixed agonist/antagonist activity depending on the receptor, cell, and promoter context.18 Upon binding the ER, these compounds selec-tively inhibit AF-2 function but leave AF-1–mediated receptor functions intact, thus explaining the selective agonist activity. In contrast, the type II compounds are considered pure antagonists, the most well char-acterized being ICI 182,780 (Table 25.2), which is a 7α-substituted derivative of E2. This compound binds the ER but possesses extended side chains that prevent co-activator association with the NR box of helix 12 and may even promote interaction with co-repressors as well as increased receptor degradation,18 resulting in

SEX STEROID RECEPTORS IN UTERINE FUNCTION 1111


decreased ER protein. Compounds of this type have now been designated as selective estrogen receptor degraders (SERDs). ER-selective nonsteroidal ligands exploit dif-ferences between the LBDs of ERα and ERβ and provide powerful pharmacological tools to discern the overall function of each ER (Table 25.2).

Clinical use of antiandrogens has focused largely on the treatment of prostate cancer.38,117 The best- characterized AR antagonists include the steroidal compound cyprot-erone acetate (Table 25.2) and nonsteroidal compounds hydroxyflutamide (Table 25.2), casodex, and nilu-tamide. Hydroxyflutamide is considered a pure AR antagonist that competitively binds AR and reduces AR transcriptional activity by preventing interaction with co-activators.117,118

SEX STEROID RECEPTORS IN UTERINE FUNCTION

Estrogen Receptor Signaling in Uterine Function

The ovarian-derived sex steroid hormones dictate the uterine estrous or menstrual cycle in mammals and are therefore essential to the establishment and maintenance of pregnancy. The uterine response to the preovulatory rise in circulating E2 is required to pre-pare the tissue for the forthcoming rise in P that accom-panies ovulation and is critical to embryo implantation. This physiological coordination between the ovary and uterus is common to the large majority of mammals studied, and the spectrum of actions and effects of the sex steroids in uterine tissue are mediated by their cog-nate NRs. Estrogen was first described almost 100 years ago as a substance that induced estrus.119 Although initially the dramatic physiological effects of estrogens on the mammalian uterus have been appreciated and well described, later investigations begun to eluci-date the precise mechanisms and signaling pathways involved in estrogen responses. Our understanding of steroid hormone action in the uterus has been greatly advanced by utilization of new technologies with the generation of knockout and knock-in models, includ-ing receptor or co-activator null, tissue and cell selec-tive null, and mutant knock-in models in mice. These models are also used in combination with microarray, RNA-seq, and ChIP-seq methods that allow for com-prehensive mapping of interaction of NRs with chroma-tin and modulation of genomic response to steroids in uterine tissues. Together, these models and techniques have led to better understanding of the molecular details of steroid receptor roles in biological processes. The following sections discuss what is currently known and hypothesized about sex steroid receptor actions in the mammalian uterus.

ER Expression in the UterusRODENTS

ERα is present in the female reproductive tract of rodents throughout late fetal and neonatal development, puberty, and adulthood.120,121 In mice, ERα transcript has been detected in the blastocyst, but immunoreac-tivity first appears in the mesenchymal cells of the fetal uterus as early as gestational day 15,120,121 whereas epi-thelial expression is delayed until the late fetal period but increases substantially on postnatal day 4 and peaks on day 16.122,123 The level of ERα transcripts in the murine uterus during neonatal development closely mirrors the patterns indicated by earlier immunohisto-chemical studies.124 There is reportedly no ERβ immu-noreactivity in the neonatal mouse uterus, although transcripts are detected at a modest level.124 In adult mice, ERα transcripts and immunoreactivity are quite high in uterine epithelial cells before ovulation125 but decrease during pregnancy, whereas expression in the stroma continues.126 In contrast, ERβ expression is rela-tively low in uteri of virgin and pregnant mice.124,127,128 Adult rats exhibit a similar uterine ER expression pat-tern such that ERα transcripts and immunoreactivity are easily detectable in the epithelium, stoma, and myome-trium, and levels peak during the proestrous period of increased circulating E2 levels.129 The evidence of ERβ expression in the rat uterus is conflicting; whereas ERβ expression has been reported to be in approximately 40% of rat uterine epithelial and stromal cells regardless of the estrous cycle stage,129 others report a total lack of detectable ERβ.130 These observations are plagued by the inconsistent quality of antibodies against ERβ. In ovari-ectomized mice, ERα and ERβ proteins were detected in the epithelial, stromal, and myometrial cells,131–133 indi-cating that removal of ovarian hormones alters the ER expression pattern. Although ERβ protein is present in the mouse uterus, there is overwhelming functional evi-dence that ERα is responsible for mediating the many effects of estrogens in the uterus. As described follow-ing, only ERα-null female mice exhibit a severely attenu-ated response to acute and chronic estrogen (e.g., E2 or DES) treatment in terms of uterine growth and altered gene expression, as well as exhibit impaired embryo implantation.110,134,135 Furthermore, categorical studies have shown that the toxic effects of neonatal estrogen exposure in the neonatal female mouse uterus are clearly mediated by ERα.136,137

DOMESTIC ANIMALS

In the neonatal ovine uterus, ERα mRNA and protein levels are highest in the developing glandular structures; detectable but lower in the luminal epithelia, stroma, and vascular endothelial cells; and absent in the myome-trium.138,139 In the adult ewe, ERα expression is detected



in all uterine cell types and is highest on day 1 of the 16- to 18-day cycle, concurrent with peak plasma E2 levels.138,140 During days 1–6, when P levels rise and E2 declines, ERα expression decreases accordingly and does not begin to rise again until the end of the cycle nears on days 11–15.138,140 A similar pattern of uterine ERα expression is reported during the 21-day bovine cycle.141 In situ hybridization and immunohistochemical analy-ses indicate maximum ERα levels on days 1–3, especially in the glandular and stromal cells; these levels generally decrease by day 6, although cells comprising the deep glands maintain ERα expression.142 The observed cycli-cal changes in ERα expression in ovine and bovine uteri are consistent with steroid regulation of receptor lev-els. Indeed, E2 treatment of ovariectomized cows leads to increased ERα expression in the glands, stroma, and luminal epithelia, whereas P alone or in combination with E2 elicits a decrease in ERα levels.142

PRIMATES

Early investigations in human uterine tissue found that levels of ER immunoreactivity or high-affinity E2 binding vary during the menstrual cycle such that peak expression occurs in the mid- to late proliferative phase and then decreases at ovulation and commencement of the secretory phase.143–145 This peak in uterine ER levels at the time of rising plasma E2 levels is similar to several other species and suggests that expression is autoregu-lated. However, maintenance of uterine ER expression in postmenopausal women suggests that nonsteroidal regulatory factors are also involved.145 The ovulatory decline in ERα levels among all uterine cell types is con-current with the rise in circulating P levels and suggests that PR-mediated P actions may act to downregulate ER expression. Such PR-mediated decrease in ERα has been reported in experimental systems as well.146 A distinct gradient of ER levels among the functional components of the human uterus was also reported such that levels are highest in the fundus and progressively decrease in those tissues closer to the cervix.147,148

The discovery of ERβ forced a reevaluation of uter-ine ER expression because the early studies discussed previously did not use methods that adequately distin-guished between the two ER forms. More recent studies definitively show that ERα is the predominant form in the human uterus and accounts for the increased recep-tor levels that occur during the proliferative phase.149 ERα transcripts and protein are especially high in glan-dular epithelial and stromal cells during the prolif-erative phase and exhibit a dramatic decrease among uterine tissues upon entering the secretory phase of the cycle.149 The levels of ERα transcripts and immunoreac-tivity in the uterine myometrium are much lower and exhibit little change during the menstrual cycle.149 Still, comparatively higher ERα levels are described in the

subendometrial myometrium during the proliferative phase and are postulated to be involved in uterine peri-staltic activity.149

ERβ expression in the human uterus is much lower relative to ERα but is generally present in the same cell types and exhibits comparative changes during the menstrual cycle.149,150 There is one report of increased ERβ immunoreactivity in the uterine stroma during the secretory phase.150 ERβ immunoreactivity is espe-cially detectable in vascular smooth muscle cells and increases therein during the secretory phase, suggesting a role for this receptor form in vascularization.150 The human-specific ERβ isoform, ERβcx, also called ERβ2 (Figure 25.2), is also detected in the uterus and is present along with full-length ERβ (ERβ1) in the functional and basal layer endometrial glands.151 Interestingly, ERβcx immunoreactivity decreases in the basal layer glandu-lar cells during the mid-secretory phase, whereas little change occurs in the level of full-length ERβ,151 suggest-ing that differential expression of ERβ isoforms over the course of the menstrual cycle may modify the cellular responses to E2.

Nonhuman primates exhibit uterine ER expression patterns similar to those described in humans. ERα immunoreactivity in baboon uteri is highest in endome-trial glandular, stroma, and myometrial smooth muscle cells during the proliferative phase and decreases in all cell types upon commencement of the secretory phase.152 A similar ERα expression pattern in epithelial, stromal, and myometrial cells is reported in the cynomolgus mon-key uterus.153 ERα immunoreactivity is also reported in the vasculature, including the spinal arteries, of the baboon uterus.152 Ovariectomy followed by exogenous E2 or E2 plus P treatments in baboons leads to changes in uterine ER expression that mirror the menstrual cycle, supporting the direct role of ovarian steroids in the regu-lation of uterine ER levels.152 Considerable levels of ERβ transcripts are also detected in the epithelial, stromal, and myometrial cells of cynomolgus monkey uteri.154 In contrast, ERβ immunoreactivity is reportedly low in normal uterine tissue from baboons, although higher levels are observed in diseased glands associated with endometriosis.155

Uterine Phenotypes in Mouse Models of Disrupted Estrogen Signaling

The murine models of disrupted estrogen signaling have proven invaluable to experimental investigation of estrogen actions in the uterus and the contribution of each ER form to these functions (Table 25.3). In addition to the ER-null models are two independently derived lines of mice that lack the capacity to synthesize E2 due to disruption of the Cyp19 gene.131,156 Following, we will describe how these different mouse models have helped to delineate the role of ER in the uterus.


TABLE 25.3 Uterine Phenotypes in Mice Null or Mutated for Sex Steroid Receptors and Signaling

Mutated or Null for Sex Steroid Receptors and Signaling Uterine Phenotypes References

Esr1−/− (homozygous null alleles for ERα: αERKO and Ex3αERKO)

Normal uterine development but exhibits hypoplastic uteriInsensitive to the proliferative and differentiating effects of endogenous growth factors and exogenous E2Implantation defect*Exhibit decidualizationInfertile

135,157–161

NERKI+/− (one mutated allele of two-point mutation in ERα DBD and one WT allele)

Normal uterine development but exhibits hyperplastic uteriHypersensitive to estrogenInfertile

132

KIKO (ERAA/−) (one mutated allele of two-point mutation in DNA binding domain of ERα and one ERαKO allele)

Normal uterine developmentInsensitive to the proliferative effects of exogenous E2 treatmentInfertile

162,163

ERαEAAE/EAAE (homozygous animal of 4-point mutation of DBD ERα)

Normal uterine development but exhibits hypoplastic uteriLoss of E2-induced uterine transcriptsInfertile

164

ERαAF-10 (deletion of amino acids 2–128 on ERα) Normal uterine development and architectureBlunted E2 responseInfertile

165,166

ERαAF-20 (deletion of amino acids 543–549 on ERα) Normal uterine development but exhibits hypoplastic uteriInsensitive to E2 treatmentInfertile

167

ENERKI (ERαG525L) (homozygous animal of 1-point mutation in LBD of ERα)

Normal uterine development but exhibits hypoplastic uteriInsensitive to E2 treatmentIGF1 induced slight uterine epithelial proliferation compared to control littermates (nonhomogenous pattern)Infertile

168

AF2ERKI/KI (homozygous knock-in of 2-point mutation in LBD of ERα)

Normal uterine development but exhibits hypoplastic uteriInsensitive to E2 treatmentER antagonists and partial agonist (ICI 182,780 and TAM) induced uterine epithelial proliferationGrowth factor did not induce the uterine epithelial cell proliferationInfertile

169

Wnt7aCre+;Esr1f/f (uterine epithelial cell–specific deletion of ERα)

Normal uterine developmentSensitive to E2- and growth factor–induced epithelial cell proliferationSelective loss of E2-target gene responseImplantation defectInfertile

170

Esr2−/− (homozygous null alleles for ERβ: βERKO, Ex3βERKO, and **ERβST

L−/L−)Exhibit grossly normal uterine development and functionSensitive to E2 treatmentSome Esr2−/− lines reported elevated uterine epithelial proliferation after E treatment compared to WTSome are completely sterile (due to ovarian phenotype)

158,171–173

αβERKO (homozygous null for both ERα and ERβ) Normal uterine development but exhibit hypoplastic uteri, similar to those of Esr1−/−

Insensitive to E2Infertile

158,174

Cyp19a1−/− (homozygous null aromatase: ArKO) Normal uterine development but exhibits hypoplastic uteriSensitive to E2-induced epithelial cell proliferationInfertile

131,156,175

Pgr−/− (homozygous null alleles for PRA and PRB: PRKO)

Exhibit grossly normal uterine developmentImplantation and decidualization defectsExhibit epithelial hyperplasia in response to E2Loss of P-induced proliferative switch in uterine epithelial and stromal cellsInfertile

176–178

(Continued)



Mutated or Null for Sex Steroid Receptors and Signaling Uterine Phenotypes References

Wnt7aCre+;Pgrf/−(uterine epithelial cell–specific deletion of PR)

Normal uterine developmentLoss of P-induced uterine epithelial P target gene transcriptionAbnormal proliferative switch of uterine epithelial and stromal cellsImplantation and decidualization defectInfertile

179

PRKO and PRd/d (homozygous null alleles for PR) Normal uterine developmentAbnormal epithelial proliferation in response to PImplantation and decidualization defectInfertile

176

PRA−/− (homozygous null alleles for PRA: PRAKO) Normal uterine developmentResemble PRKO phenotypesInfertile

180

PRB−/− (homozygous null alleles for PRB: PRBKO) Normal uterine developmentNormal implantation and decidualization responsesDo not exhibit epithelial hyperplasia in response to E2Normal fertility

181,182

AR−/− (homozygous null alleles for AR: ARKO) Normal uterine development, function, and maintenance of pregnancySlightly longer uterine horns compared to controlsSlightly reduced uterine circumferenceReduced uterine hypertrophic response to gonadotropinsIncrease placenta size (placentamegaly)Subfertile

183,184

AR−/− in-frame deletion of exon 3, loss of DNA binding activity

No overt uterine phenotypeSubfertile

185

SPARKI (homozygous knock-in of mutated DBD of AR)

Normal female fertility, no uterine phenotype reported 186

* Ex3αERKO females have a similar uterine phenotype to the original αERKO except for exhibiting a decidualization defect, which may be due to the splice variants in the original αERKO that retains ER activities.** ERβST

L−/L− females are the only line of ERβ knockout animals that are reported to be completely sterile.

TABLE 25.3 Uterine Phenotypes in Mice Null or Mutated for Sex Steroid Receptors and Signaling—cont’d

MICE LACKING ERαUnlike the AR, which is mutated in the naturally

occurring Tfm rat and mouse, no known mutations of the ERα have been reported, thus the deletion of ERα was thought to be lethal. Similarly, only one male patient and one female patient with ERα mutation have been discov-ered.187,188 The male patient’s mutation results in severe truncation of the ERα protein due to a stop codon in the A/B domain. The female patient has a point mutation in her ERα LBD that decreases activity by reducing E binding more than 200-fold. ERα-null mice were the first experimentally generated steroid-receptor-null animal, reported in 1993, and preceded the discovery of ERβ in 1996. There are currently numerous reported lines of ERα-null mice and additional lines of mice with mutations in functional domains of ERα. Three separate lines of ERα-null mice were generated: the αERKO, first described by Lubahn et al. in 1993,157 the ERαKO (or Ex3αERKO), described by Dupont et al. in 2000158 and by Hewitt in 2010,159 and ERα−/−, described by Antonson et al. in 2012.160 Homologous recombination was employed to either disrupt (αERKO) or completely excise (ERαKO

and ERα−/−) exon 3158,160 of the murine Esr1 (ERα) gene. The uterine estrogenic response in αERKO females dif-fers from the latter two lines, as αERKO animals have a low level of truncated ERα protein produced from a splice variant, which preserves some residual biological functions,189 but all ERα-null female mice are infertile. Recently, an ERα-null rat has been derived using zinc finger nuclease (ZFN) genome editing. All phenotypes examined thus far were previously seen in the ERα-null mice, including infertility due to hypoplastic uteri, poly-cystic ovaries, and ovulation defects.190 Interestingly, the recently described female patient with homozygous ERα mutation also has cystic ovaries and a small uterus despite elevated circulating E.188

MICE LACKING ERβThe first description of ERβ-null mice followed the

discovery of ERβ by only two years. Prior to generation of the ERβ-null model, very little was known as to the role of ERβ in biological processes, making it difficult to predict possible phenotypes. Therefore, the ERβ-null mice have served a tremendous role in providing insight



into the importance of the newly discovered ER form to female fertility, and studies to date indicate ERβ plays a particularly important role in ovarian function. Four dif-ferent lines of ERβ-null mice have been described. The βERKO was first described by Krege et al. in 1998,171 and the ERβKO, or Ex3βERKO, was described by Dupont et al. in 2000.158 Homologous recombination was employed in both lines to disrupt exon 3158 of the murine Esr2 (ERβ) gene. As described to date, the reproductive, endocrine, and ovarian phenotypes of both lines are indistinguishable, with both exhibiting female subfertil-ity. Then, in 2002, Shughrue et al. reported the third line of ERβKO animals, however, no uterine or ovarian phenotypes were reported.191 Recently, ERβKOST

L−/

L− animals, which contain LoxP sites flanking exon 3 of Esr2, were generated using the Cre/loxP recombination system.172 Interestingly, female mice from this recently described ERβKOST

L−/L− colony were reported to be ster-ile due to an ovarian defect.

MICE LACKING ER α AND βMuch insight into the physiological roles of E2 has

been gained from the study of mice lacking one or the other known ER forms. Still, conclusions drawn from these models are confounded by possible compensatory mechanisms provided by the opposite ER in each respec-tive ERKO model. Furthermore, mice lacking only one ER form will lose possible ERα/ERβ heterodimer-mediated actions.192,193 Therefore, compound ER-null mice (i.e., αβERKO) represent an opportunity to elucidate potential compensatory and cooperative actions of ERα and ERβ. The two reported lines of compound ER-null mice are the αβERKO, described by Couse et al. in 1999,174 and the ERαβKO, described by Dupont et al. in 2000.158 Both were generated by crossbreeding animals heterozygous for the respective individual ER-null mice and, as described to date, exhibit comparable reproductive, endocrine, and ovarian phenotypes as discussed in the subsection Mice Lacking ER α and β within the section Ovarian Pheno-types in Mouse Models of Disrupted Estrogen Signaling of this chapter. Shughrue et al. generated an additional ERαβKO line in 2002, however, their reproductive func-tion was not described, as the study focused on ER-medi-ated estrogen responses in the brain.191

MICE LACKING CYP19

Estrogens are produced by aromatase cytochrome P450, the product of Cyp19 gene. Female mice with disruption of circulating estrogen production exhibit altered female reproduction.131,156,175 There are three ani-mal models of Cyp19-null (called ArKO). Fisher et al. reported the first mouse line in 1998, which disrupted exon 9 of Cyp19 gene, as the region is a highly conserved region among all aromatase.156 Later, in 1998, Honda et al. reported a mouse line with targeted disruption of

exons 1 and 2 of the Cyp19 gene.175 Toda et al. generated the most recent mouse line of Cyp19-null in 2001 with a targeted disruption of exon 9 of the Cyp19 gene.131 These ArKO female phenotypes are indistinguishable.131,156,175 Briefly, these mice are infertile due to ovarian dysfunc-tion marked by cystic follicles and a failure to respond to exogenous gonadotropins. This phenotype is described in detail in the subsection Mice lacking Cyp19 within the section Ovarian Phenotypes in Mouse Models of Disrupted Estrogen Signaling of this chapter.

UTERINE PHENOTYPES IN GLOBAL ER- OR CYP19-NULL MICE

Females within each respective model exhibit a simi-lar phenotypic syndrome. Female mice lacking ERα or aromatase are infertile due to dysfunction of numer-ous physiological systems, including the ovary (see the section Estrogen Receptor Signaling in Ovarian Func-tion) and uterus, whereas ERβ-null females exhibit reduced or lost fecundity that is largely attributable to ovarian dysfunction. A level of caution is warranted when making phenotypic comparisons between the ER-null and Cyp19-null models because sensitivity to maternally derived estrogens may provide a more normal developmental environment during gestation in Cyp19-null mice, and sensitivity to dietary estrogens during adulthood is able to abate several phenotypes in Cyp19-null mice.194 The reported uterine phenotypes are summarized in Table 25.3.

All lines of ER-null females exhibit uteri that possess the expected tissue compartments, myometrium, endo-metrial stroma, and epithelium134,159,174 (Figure 25.5). However, in females lacking functional ERα or Cyp19, uteri are overtly hypoplastic and exhibit severely reduced weights relative to WT littermates,131,134,156,195 whereas ERβ-null females possess uteri that are grossly normal and responsive to ovarian-derived steroids134 (Figure 25.5). The uterine endometrium of ERα-null females is severely hypotrophic, poorly organized, and possesses a paucity of glandular structures (Figure 25.5(B)).159,196 The luminal and glandular epithelial cells in ERα-null uteri are severely immature with fewer glands and con-sistently exhibit a cuboidal morphology, versus the tall columnar morphology and basal location of the nucleus of an “estrogenized” epithelium in WT uteri (Figure 25.5(B)). Therefore, fetal, neonatal, and perinatal devel-opment of the female reproductive tract in mice is largely independent of ERα- and ERβ-mediated actions, but estrogen responsiveness and sexual maturation of the adult uterus are ablated after the loss of functional ERα. The totality of the ERα-null phenotype and lack of any overt uterine abnormalities in ERβ-null females suggest that ERβ has little function in mediating estrogen actions in the uterus. Moreover, ERαβ-null also demonstrated a similar uterine phenotype as ERα-null (Figure 25.5).197



Weihua et al. reported that ERβ-null females exhibited a slightly aberrant uterine growth response after estro-gen replacement; however, the uterine bioassay was conducted in immature intact, not ovariectomized adult, animals.133 In addition, Wada-Hiraike et al. showed that in immature females, loss of ERβ leads to increased uter-ine epithelial proliferation induced by E2 compared to WT uteri.173 Although ERβ-null females are subfertile, when pregnancies are established they are sustained to term,171 indicating uterine competence. More recent findings suggest that loss of ERβ leads to complete ste-rility due to a defect in ovarian function,158,172 described in more detail in the subsection Mice Lacking ERβ of the section Ovarian Phenotypes in Mouse Models of Disrupted Estrogen Signaling of this chapter.

MICE WITH UTERINE-SPECIFIC DELETION OF ERαCurrently, only a single mouse model with

tissue-specific deletion of ERα in the uterus has been published. Our laboratory has described selective

deletion of ERα, using the Cre/loxP recombination sys-tem, by crossing Wnt7aCre+198 with Esr1f/f animals159 to generate mice with deletion of ERα in uterine epithe-lial cells (Wnt7aCre+;Esr1f/f). The expression of ERα in the uterine luminal and glandular epithelium of these animals was ablated, while the ERα expression in the stromal cells and other uterine cells remains intact.170 The epithelial ERα was ablated not only in the uterus in this mouse line170 but also in the oviduct (unpublished data). As expected, based on findings in the global ERα knockouts, loss of uterine epithelial ERα has no effect on female reproductive tract development. Uterine his-tological analysis showed a similar uterine morphology as wild-type control.170 The Wnt7aCre+;Esr1f/f uteri are sensitive to 24 h treatment of E2, as the uterine epithe-lial proliferation is preserved (Figure 25.6). However, Wnt7aCre+;Esr1f/f uteri lack a complete uterine response to E2, apparent following a 3-day uterine bioassay, which demonstrated a blunted growth response and increased apoptotic activity in Wnt7aCre+;Esr1f/f compared to the

FIGURE 25.5 Gross uterine morphology of adult female estrogen receptor knockout mouse models. (A) Female reproductive phenotype of wild-type (WT) control, ERα-null, ERβ-null, and ERαβ-null. (B) Histology of uterine and vaginal tissue of WT, ERα-null and ERβ-null females. Top: Uterine cross-sections from representative adult WT, ERα-null and ERβ-null adult females illustrating the presence of a normal uterine architecture in all. The WT uterine section illustrates a normal myometrium (Myo), endometrial stroma (End), and columnar luminal epithe-lium (arrowhead). The ERα-null uterine section illustrates the characteristic hypoplasia of each anatomical compartment, a slightly disorganized endometrial stroma, and cuboidal luminal (arrowhead) and glandular epithelium. The ERβ-null uterine section is indistinguishable from that of the WT. Bottom: Vaginal cross-sections from tissue adult WT, ERα-null, and ERβ-null female mice. The WT and ERβ-null vaginal tissues illustrate a normal stroma (Str) and hypertrophied epithelium (Epi) showing estrogen-induced stratification and cornification (arrow). In contrast, the ERα-null vaginal tissue indicates complete estrogen insensitivity, as illustrated by a thin epithelium (Epi) and total lack of cornification. Source: Reproduced with permission from Refs 197 and 134.



control uteri. Additionally, a lack of ERα expression in the uterine epithelial cells contributes to complete infertility, partly due to an implantation defect.170 This suggests that uterine epithelial ERα is dispensable for early uterine proliferative responses but crucial for com-plete biological responses induced by E2, as well as for embryo implantation.

MICE WITH MUTATED DNA BINDING DOMAINS OF ERα

The generation of mouse lines with mutation in the specific domains of ERα allows us to dissect the biologi-cal functionality of individual domains of the receptor in vivo. To date, there are two mouse lines with mutations that are designed to disrupt the DNA binding function of the ERα that have been “knocked-in” (KI) at the ERα gene locus. The first line was generated by replacing critical P-box amino acids E207 and G208 with alanines (Figure 25.3(B)). This line was named “Nongenomic ER knock-in” (NERKI), as these mutations were intended to restrict ERα signaling to the nongenomic and tethered mecha-nisms (Figure 25.4). Female NERKI+/− animals that have one mutated allele and one WT allele132 were infertile, exhibiting a highly novel hyperplastic uterine phenotype, so NERKI+/− males were crossed with ERα-null hetero-zygous (WT/KO) females to produce mice with one NERKI mutated allele and one deleted Esr1 allele, called ERα KIKO or ERαAA/− as described by O’Brien et al. in 2006.162 The second line of DNA binding domain knock-in animals possessed mutation of four amino acids in the first zinc finger of the Esr1 gene, substituting Y at position

201 with E, and in the critical P box (Figure 25.3(B)), K at position 210 with A, K at position 214 with A, and R at position 215 with E (Figure 25.3(B)) as described by Ahlbory-Dieker et al. in 2009 (called ERαEAAE/EAAE).164

The NERKI+/− females have normal uterine development but exhibit hyperplastic uteri and are hyper-sensitive to estrogen.132 This suggests that disruption of the ERα binding to ERE while maintaining the presence of a normal WT allele leads to an aberrant overstimu-lated uterine response to estrogen. Interestingly, these NERKI+/− are infertile and exhibit a uterine abnormality of enlarged hyperplastic endometrial glands despite possessing normal levels of circulating sex steroids.132

ERαAA/− females have normal uterine development. Initially, O’Brien et al. reported that ERαAA/− females, with mutation of the DNA binding domain, maintained proliferative responses—induced by E2.162 However, in subsequent studies, no uterine proliferation was observed.163,199 Ahlbory-Dieker et al. showed that, unlike the NERKI+/−, females heterozygous for the ERαEAAE mutation are fertile. The homozygous ERαEAAE/EAAE females have normal reproductive tract development, but uteri are severely hypoplastic (Figure 25.7(A)), simi-lar to global ERα-null uteri. Additionally, ERαEAAE/EAAE uteri do not respond to E treatment, as the estrogen-regulated genes (such as Pgr, Cdkn1a, and Igf1) failed to be upregulated in ERαEAAE/EAAE compared to control uteri. The females from these two mouse lines with point mutations in the DNA binding domain of ERα are infer-tile. Thus the physiological function of the DNA binding domain of ERα is crucial for female reproduction.

FIGURE 25.6 Estrogen-induced epithelial DNA synthesis in the uterus of adult ovariectomized mice is ERα dependent but occurs indepen-dent of epithelial ERα. Shown are cross-sections of uterine tissues from ovariectomized WT control, ERα-null, or uterine epithelial ERαKO mice treated for 24 h with vehicle or E2. DNA synthesis as indicated by nuclear EdU incorporation is detected in the luminal epithelia (arrowheads) of vehicle-treated WT uteri, and this is dramatically increased after E2 treatment of WT or epithelial ERαKO uteri. In contrast, ERα-null uteri exhibit a basal level of DNA synthesis that is unchanged after E2 treatment, indicating that E2-induced proliferation of the uterine epithelium is dependent on functional ERα (Scale bar = 100 μM). Hoeschst was used as a counterstain to visualize the tissue. Source: Adapted with permission from Ref. 170



MICE WITH MUTATED AF-1 OR AF-2 DOMAINS OF ERαAs discussed in the section Receptor Structure, AF-1

and AF-2 are important for ER transcriptional activ-ity (Figure 25.1). A single mouse line with amino acids 2–128 deleted from exon 1 of Esr1, which removes the AF domain, called ERαAF-10, was described by Billon-Gales et al. in 2009.165 There are three reported mouse lines with mutation in AF-2 domain of ERα. In the first line, the ani-mals have a single point mutation in ERα of G at position 525 to L in the ligand binding domain (LBD), and this model was described by Sinkevicius et al. in 2008 (called “Estrogen-nonresponsive ERα Knock-in or ENERKI” or ERαG525L).168 Billon-Gales et al. generated a second line of LBD mutation animals in 2011, with amino acids 543–549 deleted from the LBD of ERα, removing helix 12 and thus AF-2 functionality (called ERαAF-20).167 One month after the second line was published, Arao et al.

reported the third line of animals with mutations within LDB of ERα169 with two point mutations of L at position 543 and 544 to A (called AF2ERKI/KI animals). All females from the ERαAF-10, ERαG525L, ERαAF-20, and AF2ERKI/KI mouse lines are sterile.165,167–169

ERαAF-10 females exhibited minimal uterine wet weight gain compared to ER+/+ uteri after treatment with E2 pellets for two consecutive weeks ( Figure 25.7(B)), while ERαAF-20 females did not respond ( Figure 25.7(C)).165–167 This indicates that the ERαAF-2 functional domain contributes to minimal uterine weight increase induced by E2 in the absence of AF-1. Both lines of AF-2-mutated animals (ERαG525L and AF2ERKI/KI) display severely hypoplastic uteri (Figure 25.7(D,E)) and lack uterine growth response to E2 treatment.167–169 Inter-estingly, uterine wet weight can be increased by using the synthetic ERα agonist PPT (Table 25.2) in ERαG525L

FIGURE 25.7 Gross uterine morphology and histological analysis of mice with knock-in of mutated ERα. (A) Female homozygous ERαEAAE/EAAE mice develop hypoplastic uteri (indicated by arrow) compared to heterozygous control (ERαEAAE/+). (B) Uterine weight of ERα+/+, ERα−/−, and ERαAF-10 or (C) ERαAF-2+/+ and ERαAF-20 mice treated with placebo control or E2 (80 μg/kg per day for 2 weeks). (D) Adult reproduc-tive tract histology from WT control and ERαG525L mice. H&E staining demonstrated that ERαG525L animals exhibit hypoplastic uteri compared to WT control uteri. Insets depict gross repro-ductive tract morphology. Scale bar = 100 μm. (E) H&E staining of uterine tissues from WT control (+/+) and AF2ERKI/KI mice. Insets depict gross reproductive tract morphology. Scale bar: 100 μm. Source: Reproduced and modified with permission from Refs. 164,165,167–169.



or by using the ER antagonists ICI 182,780 or tamoxifen (Table 25.2) in AF2ERKI/KI females.168,169 The ability of the antagonists to mediate responses seems to be due to a unique conformation of the LBD of the AF2ER that leads to AF-1-dependent transcriptional activity.169,200 Arao et al. also demonstrated that the uterine response to ICI or tamoxifen includes increased DNA synthesis in the uterine epithelial cells of AF2ERKI/KI. The growth factor IGF1 (insulin-like growth factor 1) induced mini-mal uterine epithelial proliferation in ERαG525L and was ineffective in AF2ERKI/KI uteri.168,169 Together, these findings indicated that both AF-1 and AF-2 activation domains of ERα contribute to a normal regulation of uterine growth and reproductive functions. As the AF domains mediate ER-co-regulator interaction (Table 25.1), this emphasizes the importance of effective ERα co-activator protein recruitment for successful uterine E2 response. Similarly, mice lacking sufficient SRC-1 co-activator (Src1−/−) exhibit measurably diminished uterine response to E2.201

Uterine Phenotypes in Mouse Models with Mutations in Transcriptional Proteins that Affect ER FunctionMICE WITH UTERINE-SPECIFIC DELETION OF COUP-TFII

Chicken ovalbumin upstream promoter transcription factor II (COUP-TFII), encoded from the Nr2f2 gene, is a transcription factor, belonging to nuclear receptor super-family. COUP-TFII is expressed in uterine stromal cells and is crucial for female reproduction.202–204 The tissue-specific deletion of COUP-TFII (COUP-TFIId/d) in the uterus using PgrCre+ crossed with COUP-TFIIf/f animals demonstrated that COUP-TFIId/d females exhibited normal uterine devel-opment but were completely infertile, in part, due to lack of implantation.202 Kurihara et al. also showed an enhanced estrogenic response in the epithelial cells of COUP-TFIId/d uteri, as E-responsive genes lactotransferrin (Ltf) and mucin 1 (Muc1) were elevated during the receptive win-dow, which suggested that COUP-TFII modulates uterine epithelial ERα activity during pregnancy.202 Inhibition of ERα activity using ER antagonist (ICI 182,789) partially res-cued implantation in COUP-TFIId/d females and corrected the expression level of Ltf and Muc1.203 These results sug-gest COUP-TFII plays an essential role in regulating uter-ine ERα activity during early pregnancy in rodents.

MICE WITH UTERINE-SPECIFIC DELETION OF REA

Repressor of estrogen receptor activity (REA) is an ER co-regulator that represses ER activity both in vitro and in vivo.205–209 In Chinese hamster ovarian (CHO) cells, REA is shown to be a direct co-regulator of ERα and ERβ, but not PR or RAR.205 In 2005, Park et al. gener-ated a mouse line with a targeted deletion of REA exon sequence encoding amino acids 12–201 (heterozygous

REA or REA+/−).208 Gross uterine morphology of REA+/− appeared normal compared to control littermates. How-ever, the REA+/− females exhibited hypersensitivity to E2 treatment, as uterine weight and luminal epithelial cell height and proliferation were increased compared to E2-treated control uteri. In addition, E2-induced uterine genes (such as C3 and Ltf) were also elevated in E2-treated REA+/− compared to E2-treated control uteri.208

Due to the embryonic lethality of REA−/−,208 uterine deletion of REA was generated using PgrCre+ crossed with REAf/f, producing REAf/d (heterozygous deletion) and REAd/d (homozygous deletion) animal models. Uterine epithelial cell hyperproliferation after E2 treatment was seen in REAf/d, similar to the E2 response of REA+/− uteri.207 The enhanced uterine weight increase by E2 in REAf/d females is in part due to increased levels of the aquaporin water transport gene (Aqp4). The levels of Aqp3 and Aqp5 induced by E2 treatment were similar between control and REAf/d uteri, but higher levels of Aqp4 were reached in REAf/d than control uteri after E treatment.207 REAf/d females were subfertile, while REAd/d females were com-pletely infertile. No ovulation defect was found in REAd/d females, rather REAd/d females showed altered uterine growth and maturation, as well as implantation, decidual-ization, and placentation defects.207 Aberrant apoptosis is observed in REAd/d uteri, as reflected by drastic increases in both TUNEL and active caspase-3 levels in REAd/d uteri compared to wild-type uteri.207 These findings suggest that REA regulates proper ER responsiveness in vivo and is cru-cial for ER-mediated female uterine function.

MICE WITH UTERINE-SPECIFIC DELETION OF MSX1 AND MSX2

Mammalian homeobox genes, Msx1 and Msx2, are crucial for organogenesis and embryo development. Nallasamy et al. reported that the selective ablation of either Msx1 (PgrCre+;Msx1f/f, called Msx1d/d) or Msx2 (PgrCre+;Msx2f/f, called Msx2d/d) in the uterus lead to sub-fertility in females, however, deletion of both Msx1 and Msx2 in the uterus (Msx1d/dMsx2d/d), resulted in com-plete infertility.210 This indicates that Msx1 and Msx2 func-tions can partially compensate for each other. The female infertility in Msx1d/dMsx2d/d females is in part due to an implantation defect.210 Deletion of Msx1 and Msx2 in the uterus causes uterine hypersensitivity to estrogen due to an increase in uterine ERα activity. Increased ER phos-phorylation in uterine epithelial and subepithelial stromal cells, resulting from high fibroblast growth factor (FGF) expression, were observed.210 Female mice with uterine Msx1 and Msx2 deletion exhibited not only an altered ERα activity but also showed aberrant expression of Wnt/β-catenin signaling molecules.210 This indicates that homeo-box genes Msx1 and Msx2 in the uterus are mediators of physiological ERα activity, which subsequently play piv-otal roles in establishing successful pregnancy.



MICE WITH UTERINE-SPECIFIC DELETION OF NCOA6

Nuclear receptor co-activator-6 (NCOA-6) is an ERα co-activator. Kawagoe et al. reported that NCOA-6 regulates uterine ER activity by using selective deletion of Ncoa6 in the uterus (PgrCre+;Ncoa6f/f, called Ncoa6d/d).211 Ovulation, fertilization rates, and blastocyst development are all nor-mal in Ncoa6d/d females; however, embryo implantation fails. The implantation defect is in part due to an increased level of the ERα co-activator, SRC3, in the uterine epithe-lial cells of Ncoa6d/d, which causes elevated uterine epithe-lial ERα activity compared to wild-type uteri. Moreover, a diminished PR activity, particularly during the implan-tation period, was observed. Implantation was partially rescued and the level of MUC1 was corrected in uterine luminal epithelia by treating Ncoa6d/d females with the ER antagonist ICI 182 780.211 These findings suggest that loss of uterine NCOA-6 causes implantation failure due to an aberrant ER-estrogen hypersensitivity.

MICE WITH UTERINE-SPECIFIC DELETION OF MIG6

Mitogen-inducible gene 6 (Mig-6, encoded from the Errfi1 gene) is a downstream target of P-PR and SRC1 action in the uterus.108 However, uterine-specific dele-tion of Mig-6 (Mig-6d/d or PgrCre+;Mig-6f/f) demonstrated altered ERα signaling resulting in uterine hyperplasia.108 Mig-6d/d uteri are enlarged, with weights approximately 3-fold more than wild-type (Mig-6f/f) uteri. Immunohis-tochemical analysis demonstrated that Mig-6d/d uteri exhibit hyperproliferation of the endometrium, with increased phospho-H3 staining, as a result of increased ERα expression and phosphorylation.108 The ERα targeted

genes, lactoferrin (Ltf), chloride channel calcium–activated 3 (Clca3), and complement component 3 (C3) were also elevated in Mig-6d/d uteri in the presence of both E and P, however, the PR-targeted genes amphiregulin (Areg) and follistatin (Fst) remained comparable to wild-type uteri. Ovariectomized Mig-6d/d females that were treated with E for 3 months exhibited endometrial cancer. How-ever, co-treatment with E and P attenuated the pathology in the uterus of E treated Mig-6d/d, although the uterine hyperplasia was still observed in Mig-6d/d females. Addi-tionally, the expression level of MIG-6 in women with endometrioid carcinoma was elevated compared to nor-mal endometrium.108 These findings together suggest that Mig-6, as a tumor suppressor in both rodents and human, plays a crucial role in uterine growth in response to E in part by mediating the protective action of P.108

MICE WITH UTERINE-SPECIFIC DELETION OF HAND2

Heart and neural crest derivatives expressed 2 (Hand2) is regulated by P in the uterus, as protein expression of HAND2 is induced in the uterine stromal cells after P treat-ment.212 PRKO uteri showed loss of Hand2 expression, indicating the direct regulation of Hand2 by PR.212 Selec-tive ablation of Hand2 in the uterus (Hand2d/d) using the PgrCre+;Hand2f/f animal model demonstrated an elevated uterine ER activity.212 Loss of uterine Hand2 expression in the stromal cells leads to aberrant fibroblast growth factor receptor (FGFR) signaling, which subsequently induced ERK signaling and ERα activity in the uterine epithelium (Figure 25.8), resulting in complete infertility. The infer-tility in Hand2d/d females is in part due to an implanta-tion defect as a result of enhanced uterine epithelial ERα

FIGURE 25.8 Schematic representation of paracrine and autocrine mechanisms of uterine proliferation in response to hormones. E2 stimulation of proliferation of the uterine epithelium requires the presence of functional ERα in the under-lying stroma independent from epithelium, indicating that E2/ERα actions in the stroma induce the secretion of paracrine factors that then act on the epithelium to stimulate prolifera-tion. Progesterone (P) acts through epithelial and stromal PRs to inhibit the proliferative response of the epithelium to E, while inducing proliferation of the underlying stroma. BM, basement membrane. (For detail see text.)



activity during the implantation period.212 This suggests that Hand2 expression in the uterus, orchestrated by an action of PR, mediates a proper physiological ERα activity (Figure 25.8) in response to ovarian hormones, leading to successful pregnancy establishment.

Vaginal and Cervical Phenotypes in Mouse Models of Disrupted Estrogen Signaling

The vaginal mucosa is composed of an epithelium and underlying stroma, possesses significant levels of ER,213 and is highly sensitive to estrogens.214 Endogenous or exogenous estrogens induce stromal differentiation, epi-thelial proliferation, and epithelial keratin synthesis to produce the stratified layer of cornified cells that lines the vaginal lumen.214–216 These changes in the vaginal mucosa are used experimentally to estimate the level of circulat-ing gonadal steroids and approximate the estrous cycle stage. Despite exposure to elevated endogenous E2 levels, the vaginal tissue of ERα-null females lacks any indica-tions of estrogenization (Figure 25.5(B)).134 Exogenous administration of E2 or DES also elicits no discernible vaginal response in ERα-null females.196 In contrast, the vaginal mucosa of ERβ-null females undergoes the nor-mal cyclic changes as dictated by ovarian sex steroids (Figure 25.5(B)). An effect on the vaginal mucosa simi-lar to that observed in ERα-null females is produced in rodents after prolonged ovariectomy or exposure to ER antagonists.217–219 Therefore, estrogenization of the vagi-nal mucosa in rodents, which is critical to mating, is ERα dependent. Using a human papilloma virus (HPV) trans-genic model of cervical cancer, Chung et al. demonstrated that estrogen promotes development of HPV-induced cervical cancer through ERα, as αERKO HPV–transgenic animals were protected from developing disease.220,221

Progesterone Receptor Expression in the Uterus

In the murine uterus, PR transcript and protein levels are low on days 1–2 of pregnancy with the highest expres-sion occurring in the epithelia and subepithelial stroma.126 PR levels rise in these same tissues on days 3 and 4 of preg-nancy, leading up to implantation.126,222 Receptor-medi-ated E2 actions are a primary regulator of PR expression in the rodent uterus, but these actions are complex and spe-cific to the different uterine compartments. Ovariectomy leads to increased PR levels in the uterine luminal epithe-lium that can be reduced upon exogenous E2 treatments, indicating E2 acts to repress PR expression in this uterine compartment.128,223 Recombination experiments of stro-mal and epithelial tissues similar to those described earlier, as well as studies using epithelial selective ERα deletion, indicate this to be a paracrine-mediated effect that requires functional ERα in the stroma.125,170 Simultaneous with causing decreased PR expression in the epithelium, E2 treatments lead to increased PR levels in the uterine stroma

of ovariectomized mice.128,223 This is an ERα dependent regulation, as PR is found only in epithelial cells, regard-less of E2 treatment in αERKO. Additionally, epithelial ERα is dispensable for this response as increased PR expression in the stromal cells is preserved in the absence of epithelial ERα.170 These observations have been largely confirmed in studies using a PR-lacZ reporter mouse that represents Pgr gene promoter activity as β-galactosidase activity in situ.224 In these studies, β-galactosidase activity representative of PR expression is observed in luminal and glandular epi-thelium after ovariectomy and decreased after estrogen treatment.224 P treatments also lead to decreased PR-lacZ expression in all cell types.224

In the adult ovine and bovine uterus, PR expression is highest during the early proliferative phase and pres-ent in all uterine cell types but is most abundant in the stroma and myometrium.138,140–142 PR levels decline as the cycle progresses to the secretory phase, initially in the stroma and myometrial cells.138,140–142

In the human uterus, PR immunoreactivity is increased in all cell types during the proliferative phase but mea-surably lower than ER levels.143,145 During the secretory phase, PR levels are maintained in the stroma and myo-metrium but decrease in the glandular epithelium such that levels are eventually undetectable.143,145 Studies using PR isoform selective antibodies have shown that the decrease in PR levels that occurs in the glandular epi-thelium during the mid-secretory phase is largely repre-sentative of PRA, leading to an increased PRB/PRA ratio in these cells.225 In stroma, where fewer cells expressed PR relative to glandular epithelium, PRA is more abun-dant.225 During the late secretory phase, PRA levels in the stroma decrease but remain detectable, whereas stromal PRB expression decreases occur earlier and are signifi-cantly diminished by the late secretory phase.225 The pre-dominance of PRA in the glandular epithelium during the early secretory phase may serve to inhibit further estro-gen-mediated proliferation, as the uteri in PRA-null mice exhibit an exaggerated proliferative response to estro-gens.181 As P levels rise during the mid-secretory phase, PRB is concentrated in nuclear foci, suggesting active transcriptional activity.226 A similar finding of focal-con-centrated PRB is reported in endometrial cancer cells in postmenopausal women, suggesting that inappropriate PRB activity may be associated with uterine cancer.226 The pattern of PR expression in uteri of nonhuman primates is similar to that described in humans. In female baboons, PR levels are increased during the proliferative phase and then decrease accordingly during the secretory phase.152

Uterine Phenotypes in Mouse Models with Disrupted Progesterone SignalingMICE LACKING PR

There are several reported lines of PR-null mice. Mice lacking both nuclear PR isoforms (PRKO) were first



described by Lydon et al. in 1995 and were generated via targeted disruption of exon 1 of the murine Pgr gene.176 In 2006, Hashimoto-Partyka et al. generated the PR (Pgrf/f)-targeted mutation in exon 1 of PR using EIIa-Cre promoter, which expressed Cre recombinase in the pre-implantation embryo in order to create a deletion of exon 1 and loss of PR protein.177 In 2010, Fernandez-Valdivia et al. described a mouse line that had a deletion of PR exon 2 and loss of PR protein using Pgrf/f and ZP3-Cre promoter expressing mice (called PRd/d).178

Mice lacking only PRA (PRAKO)180 or PRB (PRBKO)182,227 were generated via a Cre/loxP-based gene targeting strategy that mutated the respective start codons for each isoform while preserving the transla-tional reading frame of the other. Comparative studies in all models have greatly furthered our understand-ing of the divergent functions for the two PR isoforms. Recently, Franco et al. generated an epithelial cell spe-cific deletion of both PRA and PRB in the uterus (called Wnt7a-Cre+PRf/−) using Wnt7a-Cre expressing promoter crossed with Pgrf/− animals.179

PR-null mice exhibit normally developed uteri that pos-sess the expected uterine architecture (Table 25.3).181,182 During the secretory phase of the murine estrous cycle, decreasing circulating levels of E2 concurrent with rising P cause a shift in uterine proliferation from the epithelial to stroma cells.228 Estrogen, through ERα, is known to stim-ulate uterine epithelial cell proliferation in adult mouse uteri,125,159,229 and P exhibits the antiproliferative effect on the action of E2 (Figure 25.8).230,231 Therefore, PR-mediated P actions negatively modulate E2-induced proliferation of the uterine epithelium.228 In addition, ovariectomized PR-null females treated with daily injections of E2 and P for 3 weeks exhibit abnormally large fluid-filled uteri that are characterized by a thickened uterine wall due to extensive extracellular edema, hyperproliferation of the glandular epithelia resulting in a disordered cellular arrangement, and acute inflammation of the endometrium.176 The hyperestrogenic response in PR-null uteri is similar to that observed in ovariectomized mice after prolonged exposure to unopposed estrogens229 and provides strong support of the modulatory actions of PR in the uterus.176 Similar experiments in isoform-specific PR-null mice indicate this phenotype is reproduced in PRA-null mice only, whereas PRB-null mice exhibit a normal uterine response, indicat-ing that PRA is primarily responsible for negatively modu-lating the uterine response to E2.180–182

Uterine Phenotypes in Mouse Model with Mutations in Transcriptional Proteins That Mediate PR FunctionMICE WITH UTERINE-SPECIFIC DELETION OF P160 (SRCs)

The SRCs mediate P-dependent action in the uterus.232 Immunohistochemical analysis in the uteri of pregnant

mice demonstrated the spatiotemporal expression pat-tern of SRC1 and SRC2 in the luminal epithelial, stromal as well as secondary decidual zone. The uterine expres-sion pattern of SRC1 and SRC2 was similar to the pat-tern of PR expression during pregnancy.232 In uteri from ovariectomized animals, the majority of SRC1 and SRC2 was expressed in the luminal and glandular epithelium, much like PR expression.232 Both E and P, either alone or administered together, increased the expression of SRC1 and SRC2 in the uterine stromal cells.232

There are several reports describing SRC-null mice and resulting effects on female reproductive functions.201,232–234 Global disruption of the Src1 gene demonstrated that both males and females were fertile, however, females exhibited a blunted response to ovarian hormones.201 Treatment of Src1−/− females with E2 for 3 consecutive days resulted in less uterine weight increase than in WT, similarly, arti-ficial induction of decidualization resulted in less stromal differentiation of Src1−/− uteri.201 Due to a lethality of Src2 global knockout,235 uterine-specific SRC2 deletion (Src2d/d) was generated using PgrCre+ crossed with Src2f/f animals.233 Src2d/d females were infertile, in part, due to an implanta-tion and decidualization defect.232,233,236 E2-induced uter-ine epithelial cell proliferation was preserved in Src2d/d uteri.233 The decidualization markers such as Bmp2, Ptgs2 (Cox-2), and Fst were significantly decreased in Src2d/d when compared to wild-type uteri.233 A complete absence of decidual response was observed in Src2d/d uteri in a Src1−/− background.233 However, Src3−/− females had no overt uterine phenotype.232 These findings indicate that of the three SRCs, SRC2 is most crucial for P-dependent uter-ine function.

MICE WITH UTERINE-SPECIFIC DELETION OF COUP-TFII

COUP-TFII regulates uterine ER function during early pregnancy as mentioned earlier. Additionally, the COUP-TFIId/d animal model also demonstrated that COUP-TFII is a downstream effector for PR-mediated decidualiza-tion.202 Loss of uterine COUP-TFII resulted in a defect in decidualization, as reflected by lack of expression of decidual cell markers (WNT4 and BMP2).203 Treatment of COUP-TFIId/d females with ICI 182,780 restores the expression of WNT4 and BMP2 in the decidual cells.203 Together, the findings from uterine-specific deletion of COUP-TFII suggest that COUP-TFII regulates P signaling during implantation and decidualization by controlling uterine ER activity.

MICE WITH UTERINE-SPECIFIC DELETION OF REA

As discussed earlier, REA is crucial for uterine responses to ovarian hormones and establishment of pregnancy. Using REAd/d animals also demonstrated that loss of uterine REA led to aberrant PR function, which resulted in a decidualization defect.207 The decidual cell



markers, Wnt4 and Bmp2, were not increased in REAd/d uteri after artificial decidualization. The defect in decidu-alization is due to loss of PR expression after uterine REA ablation.207 These findings indicate that uterine REA expression is not only crucial for ER activity but also for PR function to establish and maintain pregnancy.

MICE WITH UTERINE-SPECIFIC DELETION OF FKBPs

FK506 Binding Proteins 4 (FKBP4; encoded by Fkbp52 gene) and 5 (FKBP5; encoded by Fkbp51 gene) are immu-nophilin family co-chaperones that interact with PR in the absence of the ligands.237 Fkbp4 is expressed in the luminal and glandular epithelial cells during implanta-tion, whereas a minimal level was detected in the uter-ine stromal cells, similar to PR expression.237 During the decidual response, Fkbp4 was highly expressed in both primary and secondary decidual zones. Although the expression of Fkbp5 also showed a similar pattern, the level of expression was much lower than that of Fkbp4.237 The global knockout of FKBP5 was fertile and showed no impairment of uterine functions.237,238 However, the global knockout of FKBP4 caused complete female infertility,237,238 due to implantation and decidualization defects,237,238 suggesting that FKBP4 not only binds to PR but is crucial for PR action during pregnancy.

Uterine Response to Estradiol and Progesterone

Changes in PhysiologyEstrogens stimulate a complex process of epithelial

proliferation and differentiation in the sexually mature uterus that leads to the formation of a multilayered secre-tory endometrium. This effect is critical to provide a suit-able intrauterine environment for the establishment and maintenance of pregnancy. Early studies in neonatal and prepubertal rodents found that both the uterine stroma and epithelium proliferate in response to estrogens (Figure 25.9);239 however, estrogen-induced mitogenesis in uteri of sexually mature rodents is limited to the epithe-lium.229 Therefore, sexual maturation of the rodent uterus is not simply marked by the presence of ER or an estrogen response but is rather the acquired capacity to undergo synchronized phases of proliferation and differentiation as dictated by the ovarian-derived sex steroids. The lack of estrogen-induced uterine epithelial proliferation in ERα-null uteri indicates the essential role of ERα in this process (Figure 25.6). Although ERα is present in both epithelial and stromal compartments, tissue recombination stud-ies and study of the uterine epithelial-specific ERα null indicate the proliferative epithelial response to estrogens is indirect and dependent on stromal ERα actions (Figure 25.6 and a working model in Figure 25.8).170,240

Treatment of ovariectomized mice with estrogens (e.g., E2 or DES) has long served as an experimental

model to mimic the uterine events that occur during the estrous phase of the rodent cycle or immediately after the preovulatory E2 surge. Morphological and biochemical changes occur in the rodent uterus after estrogen stimu-lation following an established biphasic temporal pat-tern241,242 (Table 25.4). Estrogen-stimulated changes in the rodent uterus that occur early, within the first 6 h after treatment, include increases in nuclear ER occupancy, water imbibition, vascular permeability and hyperemia, prostaglandin release, glucose metabolism, eosinophil infiltration, gene expression (e.g., c-fos), and lipid and protein synthesis (Table 25.4). Recent ERα ChIP-Seq pro-files from uterine tissues showed that the receptor preoc-cupies chromatin sites and that E2 treatment increases ERα recruitment.243 These processes are then accompa-nied by a delayed response that peaks after 24–72 h and includes dramatic increases in RNA and DNA synthe-sis, epithelial proliferation, and differentiation toward a more columnar secretory phenotype, dramatic increases in uterine weight, and continued gene expression (e.g., lactotransferrin) (Table 25.4). Ovariectomized mice exhibit a three- to four-fold increase in uterine weight after three daily treatments with E2 or DES, whereas no such response is observed in the uteri of ERα-null females.157,159,244 The early phase effects of water imbibi-tion and hyperemia as well as the late-phase effects of increased DNA synthesis and epithelial proliferation are absent in ERα-null uteri159,189,196 (Figure 25.6). Interest-ingly, females that are heterozygous for the Esr1 gene

FIGURE 25.9 Estrogenic response in immature uteri in mice (21-day-old). DNA synthesis as indicated by nuclear EdU incorpora-tion is detected in vehicle-treated uterine epithelial cells (arrowhead). However, DNA synthesis is dramatically increased in the luminal and glandular epithelia as well as stromal cells in the uteri 16 h after E2 treatment. Hoescht was used as a counterstain to visualize the tissue. Scale bar = 100 μm. Arrowhead indicates EdU positive epithelial cell.



disruption possess approximately one-half the normal level of ERα in the uterus, but their response to estro-gen treatment is comparable with wild-type females. The total lack of response to estrogens in ERα-null uteri as well as a lack of late biological response in epithelial ERα knockout uteri provide strong evidence that ERα is required to mediate the full biochemical and biological uterine response to estrogens.159,170

Molecular Mechanisms of Estrogen-Induced Uterine Proliferation

Numerous studies have elucidated the molecular mechanisms of E2-induced uterine epithelial cell pro-liferative responses in animal models. For example, the transcription factor CCAAT enhancer binding protein beta (C/EBPβ) is involved in hormone-induced uter-ine proliferation.245 Maximum uterine expression of C/EBPβ is induced 1 h after E2 treatment in both epithelial

and stromal cells.245,246 ICI 182,786 (ER antagonist) strongly inhibited E2-induced Cebpb transcript in the uterus, suggesting an ER-dependent expression of C/EBPβ.247 In addition, loss of epithelial ERα in the uterus did not alter E2-induced Cebpb expression, indicating that Cebpb expression is independent of epithelial ER.170 This points to the action of estrogen through ERα as the major mediator of C/EBPβ expression in the uterus. Indeed, the deletion of C/EBPβ (C/EBPβ−/−) leads to a lack of the E-induced uterine proliferative response245 as reflected by the absence of mitotic activity, S-phase activity, and an increase in apoptotic activity in the uterine epithelial cells.246 In addition to a blunted uterine growth response to hormones, the C/EBPβ−/− females also exhibit complete infertility247 due to implantation and decidualization defects.245

Pan et al. demonstrated that the uterine expression of minichromosome maintenance proteins (MCMs), a complex required for DNA synthesis initiation, is induced after E2 treatment, specifically MCM2 and MCM3248 (Figure 25.8). MCM2 activity is crucial and required for DNA synthesis in the uterine epithelial cells.249 The DNA replication of uterine epithelial cells induced by E2 is attenuated by P action, which will be discussed further in a later section.

Estrogen–Growth Factor Cross Talk in the UterusThe autocrine and paracrine actions of polypeptide

growth factors are an integral component of the uter-ine response to estrogens. The uterine response to E2 is modulated by stromal factors, such as IGF1, that are induced by E2 and then impact epithelial responses.250 Igf1 transcript is increased with concomitant decrease of Igfbp3242 and activation of the IGF1 receptor and down-stream effectors following E2 treatment.251 Igf1 transcript is increased in both stromal and epithelial compartments of the uterus by E2, with greater signal apparent in the stroma (Figure 25.10(A)).252 Igf1 has been demonstrated to play an essential role in the uterine growth response, as Igf1 null mice lack a full uterine proliferative response, and more specifically, lack G2/M progression of the epi-thelial cells following E2 stimulation.253 Additionally, transgenic mice overexpressing Igfbp1, which sequesters, and therefore decreases, the amount of available IGF1, have an attenuated uterine response to E2.254 Uterine response is restored by transplanting Igf1KO uterine tis-sue into a WT host,255 which demonstrated the paracrine effect of the host Igf1. Further, E2 treatment results in the activation of downstream mediators of Igf1 signaling, including the Igf1 receptor, IRS1,251 AKT, and inhibition of GSK3β,252 leading to nuclear translocation of CCND1 and epithelial proliferation. Additionally, picropodo-phyllin (PPP), an IGF1R inhibitor, blunted the prolifer-ative effect of E2. This effect of PPP on E2- stimulated uterine proliferation could be reversed, however, by

TABLE 25.4 Biphasic Response of the Rodent Uterus to Estrogens

EARLY UTEROTROPIC RESPONSES (WITHIN 6 H)

Nuclear localization of estrogen receptor/recruitment to chromatin

Activation of receptor-tyrosine kinase pathways

Changes in gene expression (induction/repression of “early” genes)

Increased vascular permeability

Increased protein synthesis

Water imbibition

Hyperemia

Eosinophil infiltration

Albumin accumulation

Increased electrolytes

Lysozyme labilization

Increased cyclic nucleotides, prostaglandins, and associated enzyme activity

Increased glucose metabolism and associated enzyme activity

Calcium influx

Increased lipid synthesis

LATE UTEROTROPIC RESPONSES (WITHIN 24 H)

Second peak of nuclear localization of estrogen receptor

Changes in gene expression (induction/repression of “late” genes)

Increased protein synthesis

Increased DNA synthesis

Epithelial proliferation in “waves”

Cellular hypertrophy

Overall increase in uterine dry weight



co-treatment with PPP and the GSK3β inhibitor, SB415286 (Figure 25.10(B)).252 These findings elegantly illustrate a direct role for IGF1R-initiated signaling through AKT and GSK3β on uterine response to E2.

In addition, IGF1 or EGF treatment of ovariecto-mized wild-type mice elicits a pattern of global gene expression similar to that induced by E2, although some estrogen-specific genes are revealed.256 IGF1 treatment was shown to increase the expression of an ERE-driven luciferase reporter gene in transgenic mice, providing the first in vivo evidence of E2-independent ER activation by growth factor.257 ER-null mice provide an excellent in vivo model for the study of ER–growth factor “cross talk” mechanisms in the uterus. ERα-null uteri express wild-type levels of functional EGFR but are unrespon-sive to the mitogenic actions of EGF, confirming the interaction of these two signaling systems.258 However, not all EGF responses are lacking in the uteri of αERKO females, as upregulation of the c-fos gene by this growth factor remains intact. Similar studies have demonstrated that the uterine response to IGF1 is compromised in

ERα-null females.259 Cunha and colleagues125,250,255 used the ERα-null mice in a series of tissue recombination experiments to further demonstrate interaction between ER and growth factor signaling in the murine reproduc-tive tract. In these studies, uterine stoma and epithelium are enzymatically disassociated from prepubertal mice and recombined with corresponding tissue from animals of different treatments or genotypes. The resulting chi-meric stromal-epithelial unit is then implanted under the kidney capsule of an ovariectomized nude mouse that is then treated with various hormonal combina-tions. A caveat to these studies is that neonatal tissues are used and may not accurately reflect the uterine phys-iology of sexually mature females. Nonetheless, these methods have been effectively used to demonstrate that estrogen-induced proliferation of uterine and vaginal epithelium requires functional ERα in the underlying stroma only,250 whereas estrogen-induced increases in secretory products (e.g., LTF, complement C3, keratins) in the uterine or vaginal epithelium requires functional ERα in both uterine compartments. These findings have now been confirmed in sexually mature intact uterine tissue using uterine epithelial cell–selective ERα-null mice.170 In addition, treatment of uterine epi-thelial ERα-null females with IGF1 or EGF mimics the uterine epithelial cell DNA synthesis stimulated by E2 (Figure 25.11). These studies strongly support a paracrine mechanism of estrogen-mediated epithelial proliferation that requires ERα in the stroma (working model in Figures 25.8 and 25.11). Interestingly, microarray stud-ies indicate that the growth factor–induced genomic response is intact in ERα-null uteri, although epithelial proliferation is absent,256 suggesting a greater complex-ity to the estrogen–growth factor signaling “cross talk” than was originally perceived. GF signaling is thought to mainly utilize AF-1 via MAPK phosphorylation sites in the N-terminus. Thus is was expected that IGF1 or EGF treatment of AF2ERKI/KI mice would lead to uterine growth, however, no response was seen,169 indicating functional AF-2 is also involved in this mechanism.

Antiproliferative Functions of PR in the Endometrium

Tong and Pollard demonstrated that P inhibits E-induced uterine proliferation via the inhibition of DNA synthesis and PCNA expression (a component of DNA polymerase gamma, which is required for S-phase entry).228 Additionally, Ray and Pollard demonstrated that Krupple-like transcription factor 4 (KLF4) expres-sion is induced whereas KLF15 is suppressed by E2 treatment in the uterine epithelial cells.249 Conversely, co-treatment with P and E decreased KLF4 expression and increased KLF15 in the uterus.249 In addition, over-expression of uterine KLF15 using an adenovirus leads to a lack of E2-induced uterine epithelial proliferative

FIGURE 25.10 E2 treatment stimulates uterine epithelial prolif-eration through IGF1 signaling. (A) In situ hybridization of transverse sections of uteri of control (1 and 4) and E2-treated (2, 3, 5, and 6) mice probed by using antisense (1, 2, 4, and 5) or sense (3 and 6) probes. Dark precipitate represents the hybridization signal and indicates an upregulation of Igf1 after E2 treatment principally in the stroma but also in the luminal and glandular epithelium. (Magnification: 1–3, 10×; 4–6, 40×.) (B) E2 signals through IGF1 and GSK3β to induce DNA syn-thesis in the uterine epithelium. BrdU incorporation in uterine section from ovariectomized mice 15 h after the following treatments: (1) con-trol, (2) E2 (subcutaneous, s.c.), (3) E2 (s.c.) + PPP (picropodophyllin; IGF1R inhibitor) given intraluminally, and (4) E2 (s.c.) + PPP + SB415286 (GSK3β inhibitor) given intraluminally. Source: Reproduced and modified with permission from Ref. 252. Copyright (2007) National Academy of Sci-ences, USA.



response due to the loss of MCM2 expression,249 dem-onstrating that KLF15, through the action of P, inhibits E2-induced uterine epithelial DNA synthesis by inhibi-tion of MCM2 expression. Moreover, P also diminished E2 induction of Cyclin D1 nuclear translocalization and expression of Cyclin A, which consequently decreased the phosphorylation of pRb and p107.228,260,261 Together, these findings described the mechanism by which P inhibits cell cycle activity induced by E2 in the uterine cells (see Figure 25.8).

P not only inhibits the uterine epithelial pro-liferation but also contributes to an induction of stromal cell proliferation in the presence of E2. Recom-bination experiments using uterine tissues from PR-null mice demonstrate that the antiproliferative functions of P in the uterine epithelium are para-crine mediated and require PRA in the stroma125,262 (Figure 25.12). However, selective deletion of PR in the uterine epithelial cells (Wnt7a-Cre+;PRf/− animal model) demonstrated that epithelial PR expression is dispensable for E2-induced uterine weight increase and induction of

stromal cell proliferation but required for the P-medi-ated inhibition of epithelial cell proliferative response induced by E2.179,228 Wnt7a-Cre+PRf/− animals exhibit an aberrant uterine expression of MCM3 and Cyclin D1,179 which are important mediators of E2-induced uterine epithelial proliferative responses.

Moreover, P treatment also inhibits the pro-inflam-matory activity induced by estrogen.263 The inflamma-tory response observed in PR-null uteri after prolonged estrogen/P treatment is extensive and consists of “marked” infiltration of polymorphonuclear leukocytes into the endometrial stroma, mucosal epithelium, and uterine luminal fluid.176 In vitro studies have shown that P inhibits the expression of chemotactic cytokines for neutrophil and lymphocyte infiltration264,265 and may reduce prostaglandin E levels in uterine decidual and chorionic tissue during early pregnancy.266 Therefore, PR-mediated P actions may be critical to suppressing the uterine inflammatory response that may accompany embryo implantation.176,267 The uterine levels of macro-phages, neutrophils, and Ltf are increased in both WT and PRKO uteri after unopposed E treatment. The pro-inflammatory activity is decreased by co-treatment of E with P in WT, but not in PRKO uteri,223,263 thus PR func-tion is crucial to control uterine inflammatory response during the ovarian cycle.

Roles of PR in the Rodent Uterus during PregnancyPR-mediated P actions in the uterus are critical to pre-

paring the uterine endometrium for pregnancy. Embryo implantation is a highly complex process that requires synchronized cooperation between the blastocyst and uterine endometrium (see Chapter 38). Circulating E2 levels peak at ovulation and elicit a cascade of prolif-eration and differentiation in the luminal and glandu-lar epithelium of the uterus, including induction of PR expression in the endometrial stroma and myome-trium.223 Postovulatory increases in circulating P then

FIGURE 25.11 Growth factors are able to mimic the proliferative effects of E2 in the mouse uterus. Shown are uterine cross-sections from ovariectomized WT, ERα-null, and uterine epithe-lial ERα-knockout (uterine epithelial ERαKO) mice treated for 24 h with vehicle, E2, epidermal growth factor (EGF), or insulin-like growth factor 1 (IGF1). Uterine tissue was then subjected to immunohis-tochemistry for the antigen Ki67, a marker of cell proliferation. As expected, E2 treatment induces a dramatic increase in Ki67 immunoreactivity in uter-ine luminal and glandular epithelium, and EGF and IGF1 mimic this effect in the absence of endogenous E2. Source: Reproduced and modified with permission from Refs 159,170.

FIGURE 25.12 Progesterone inhibits E2-induced uterine epithe-lial cell proliferation and stimulates stromal cell proliferation. BrdU incorporation in the uterine tissues from ovariectomized mice treated with control (no treatment), E2 (15 h), P (daily for 4 days), and PE2 (P for 4 days and E2 on the fourth day for 15 h). Source: Reproduced and modified with permission from Ref. 228.



cause decidualization, a complex process that involves massive proliferation and differentiation of the endome-trial stroma along with localized increases in vascular permeability and edema.34,268,269 This process involves the synthesis and interaction of numerous hormones and signaling pathways, including prolactin, cytokines, pros-taglandins, and extracellular matrix components.268,269 The result is a remarkable swelling of the uterine stroma that is thought to be necessary for implantation by forc-ing the uterus to close down on the blastocyst.268,269 The final stage of apposition is a grasping of the blastocyst and ultimate attachment to the uterine wall, a process thought to be dependent on secondary rises in ovarian-derived E2.268,269 Therefore, preparation of the uterus for blastocyst implantation is dependent on the multi-functional and sometimes opposing effects of E2 and P. In addition, PR-null models suggest that PRA is crucial not only for implantation but also the decidualization process as female mice lacking only PRA fail to exhibit a uterine decidual response, illustrating the importance of PRA for establishing successful pregnancy.176,178,180 Recently, Franco et al. demonstrated that the epithelial PR is responsible for establishing successful pregnancy as the deletion of uterine epithelial PR contributes to an impaired embryo attachment, implantation, and decid-ual response.179 Details of implantation and decidualiza-tion mechanisms are discussed in Chapter 38.

Maintenance of Progesterone Action in the Absence of ER

The Pgr gene is a well-described target of estrogen-induced expression via the classic model of ER action, especially in the uterus.32,33 The lack of E2-induced increases in Pgr expression in ERα-null uteri confirms the regulatory dependence on ERα action.189 Therefore, it was hypothesized that disruption of the ERα gene may subsequently result in abnormally low levels of PR in ERα-null uteri and thereby render this tissue refrac-tory to P as well. However, assays for Pgr expression and P binding in ERα-null uteri indicate PR levels are only reduced by approximately half.110,161,189 Furthermore, a greater proportion of PR is nuclear localized in ERα-null uteri (≈25%) relative to wild-type (≈5%).161 Western blots indicate no difference in the relative levels of PRA and PRB between genotypes, with PRA consistently pres-ent in greater amounts in both.161 Therefore, a loss of ERα action in the uterus has not led to a complete lack of PR or to altered and preferential transcription from one of the two Pgr gene promoters. Furthermore, sev-eral P-dependent actions are preserved in ERα-null uteri, including P-induced expression of amphiregulin and calcitonin and the uterine decidual response,161 but not embryo attachment or implanation.135

The postovulatory nadir in circulating E2 levels has long been known to be critical to uterine decidualization

in mice.270 Studies have shown that the ER antagonist ICI 182,780 prevents artificially induced uterine decid-ualization in wild-type mice, indicating involvement of ER signaling.161 Therefore, it is surprising that ERα-null mice exhibit a uterine response when exposed to an artificial model of hormonal and mechanical induc-tion of uterine decidualization.161,271 However, this may partly be due to the residual ERα protein produced by the splice variant found in the uterus of the αERKO line made by targeted disruption.189 Nevertheless, these findings suggest that an altered “organization” of the uterine tissue or compensatory pathways might provide for hormonally driven uterine decidualization in ERα-null mice. Alternatively, some genes detected at the time of implantation, including the gap junction pro-tein connexin 26272 and the cytokine leukemia inhibitory factor,135 are regulated by dual pathways. One involves estrogen- stimulated ERα, and the ability to induce via this mechanism is lost in the αERKO. The second pathway is initiated by decidualization-associated signals and is ERα independent and retained in the αERKO. This indicates a redundancy of regulatory mechanisms that allows reten-tion of gene regulation in the αERKO and accounts for the ERα-independent uterine decidualization response observed in ERα-null mice. Recent studies demonstrated that de novo E2 synthesis occurs locally during decidual-ization, especially on day 6 and 7 of pregnancy in mice,273 indicating an important role for local E2 synthesis for establishment of successful pregnancy.

Androgen Receptor Signaling in Uterine Function

AR Expression in the UterusAR is present in uterine tissues of multiple spe-

cies,130,274–278 although the function of androgen sig-naling in the uterus remains unclear. In rodents, ARs are present in all uterine cell types but most highly expressed in the myometrium, where expression may be positively regulated by estrogens.276,279 In humans, ARs are also detected in the myometrial and endometrial uterine tissues, and levels increase during the prolifera-tive phase.280

MICE LACKING AR

There are several reported lines of AR-null as well as mice with knock-in of mutated forms of AR (reviewed in Refs 281,282). Tfm mice were first described in 1970 and are a naturally existing androgen-resistant mutant283 with an inactivating mutation of the Ar gene.284,285 Comparable inactivation mutations of the AR gene and resulting phenotypes are well described in rats and humans.286 Because the Ar gene is located on the X chro-mosome and Tfm males are infertile, it is impossible to breed for XX female mice that are homozygous for the



mutation. Lyon and Glenister overcame this challenge more than 30 years ago by utilizing embryo aggrega-tion to generate a limited number of Tfm chimeric males that were fertile and carried germ cells harboring the Ar mutation.287 More recently, AR-null female mice were generated via a Cre/loxP targeting scheme that allows for tissue and temporal specific disruption of the Ar gene and therefore the generation of fertile male carriers of the targeted Ar allele.185,288–290 Exon 2 of the murine Ar gene was targeted for deletion in nearly all lines that have been described in the literature.183,184,289,290 Addi-tionally, one line of mice with in-frame deletion of exon 3 of the Ar gene has been generated, however, the uter-ine phenotype of this line was has not been described.185 The exon 2 AR-null females have normal uterine devel-opment, although the uterine circumference is smaller than that of wild-type uteri (Table 25.3).183,184 Hu et al. demonstrated that the AR-null uteri had increased uter-ine horn length, but smaller uterine diameter, as a result of decreased endometrial and myometrial areas dur-ing diestrus, when compared to wild-type uteri.183,184 These AR-null females were subfertile, due to impaired folliculogenesis (discussed later in the section Ovarian Phenotypes in Mouse Models of Disrupted Androgen Signaling). Still, studies to date indicate that AR-null females exhibit reproductive phenotypes that are quite similar to those originally described in Tfm/Tfm females.

Recently, a mouse line was developed in which muta-tion of the AR DNA binding domain alters the ability of AR to bind to selective androgen response elements. This mouse line was generated by Schauwaers et al. with a targeted mutation of 12 amino acids in the second zinc-finger of the DNA binding domain at the exon 3 of AR (called SPARKI).186 The heterozygous or homozygous SPARKI females do not have an overt phenotype and have normal fertility,186 indicating selective AR DNA binding is not critical for female reproductive organ development and function.

Treatment of hypophysectomized rats with DHT is known to cause increased uterine weight in rodents, indicating that ligand-dependent AR actions can affect uterine responses.291 Several studies also showed that DHT induced uterine weight increase in αERKO ani-mals.292,293 Upregulation of AR expression is observed in αERKO uteri after DHT treatment, suggesting that DHT exerts its action in the uterus through AR in the absence of ERα.292 In addition, AR agonists increased myometrial thickness in the uteri of ovariectomized rats but inhib-ited estrogen-induced epithelial proliferation.264 Similar observations were made in female mice, which exhib-ited not only uterine epithelial cell proliferation but also myometrial smooth muscle cell proliferation after DHT treatment.294 AR-null female mice exhibit relatively nor-mal uteri that are somewhat hypoplastic, although this phenotype may be more representative of decreased

E2 synthesis in the ovaries rather than a role for AR in maintaining uterine weight (Table 25.3).183 Furthermore, AR-null females are able to establish and maintain preg-nancies to term.183 Microarray studies have indicated that DHT leads to a pattern of uterine gene regulation that is remarkably similar to that elicited by estrogens but less robust,292,293 with 86% of the DHT response consisting of a subset of estrogen responses. The fold response of the overlapping genes was in general more robust after estrogen treatment versus DHT. Thus, despite differences in biological outcomes, the global genomic patterns elicited by estrogens and nonaroma-tizable androgens largely overlap. Genes noted included those involved in metabolism, tissue growth and remod-eling, transcription, protein synthesis and processing, and signal transduction.293 Suppression of AR expres-sion in human endometrial stromal cells (HESCs) using siRNA led to a decrease in decidual cell proliferation and differentiation.295 These findings suggest that AR plays a minimal role for uterine growth and differentiation but is not essential for uterine development.

Glucocorticoid Receptor Signaling in Uterine Function

GR Expression in the UterusThe uterus is not considered a classic target tissue of

glucocorticoid action, although GRs are present through-out the cell types of the rodent uterus, including uterine natural killer cells (uNK cells).40,296

MICE LACKING GR

Homozygous GR-null animals generated via homolo-gous recombination in embryonic stem cells die at birth due to respiratory failure.297 Therefore, the generation of tissue-specific GR deletion in female reproductive tis-sues is crucial for studying the role of GR in reproductive functions.

Uterine Functions of GRThe limited experimental data available indicate that

GR-mediated glucocorticoid actions are present in the uterus.40 Treatment with the GR agonist, dexametha-sone, did not increase the weight of immature rat uterus compared to E.40,298 However, dexamethasone inhib-its the uterine weight increase induced by estrogen in the rodent uteri.40,298,299 Surprisingly, stimulation of the GR signaling pathway by dexamethasone treatment in immature rats elicits a pattern of gene expression that is remarkably similar to, but not as robust as, estro-gen.40 However, the molecular mechanism by which dexamethasone inhibits uterotropic action of estrogen remains unclear. A recent study reported that E-induced human uterine leiomyoma cell proliferation is sup-pressed by GR activation.41 Gilz (glucocorticoid-induced



leucine zipper) is crucial for immune-related function of glucocorticoid activity.300 Whirledge and Cidlowski also demonstrated that E suppressed Gilz gene expression induced by dexamethasone in human uterine epithelial (ECC1) cells.300 This suggests that estrogen and gluco-corticoid exert interplay between immune-responsive functions in the uterus.

GR activity not only exhibits inhibitory effects on estrogenic action in the uterus, but may also play important roles in the decidual response, as it is known that GR is expressed in uterine natural killer (uNK) cells,296 and that the uNK cells play important roles for establishment and maintenance of successful preg-nancy (reviewed in Ref. 301). However, the role of GR in uNK cell activity in the uterus remains unclear.

MicroRNAs in Uterine Reproductive Functions

MicroRNA (miRNA) are very small RNA molecules (18–24 nt) that are transcribed as larger primary RNAs (priRNA), either within introns of encoding transcripts or from miRNA encoding promoters. priRNA is pro-cessed to a shorter stem and loop pre-miRNA form by the activity of the DGCR8/DROSHA microprocessor complex. The pre-miRNA is then exported from the nucleus by exportin 5, to the cytoplasm where DICER cleaves the loop, leaving miRNA, which then functions to interact with mRNA, targeting it for degradation or repressing translation.302,303 miRNA is particularly criti-cal to female reproductive tract function, as mouse mod-els with deletion of DICER in these tissues lose fertility. Estrogen has been demonstrated to regulate levels of miRNA in uterine tissue.304,305

The biological roles of Dicer are crucial for a nor-mal development of limbs,306 muscle,307 and lung.308 In addition, recent findings demonstrated that expression of Dicer is not only critical for female germ line biologi-cal functions309 but also for embryo implantation.310 Although miRNAs are expressed in the female reproduc-tive tract, their precise physiological function remains unclear. To study the role of dicer and miRNAs dur-ing female reproduction, several studies used Dicer1fl/

fl crossed with Amhr2Cre/+ animals to generate a specific deletion of dicer in the mesenchymal layer of female reproductive tract (called Dicer1fl/fl;Amhr2Cre/+).311–313 Female mice with a lack of Dicer1 expression in the mes-enchyme exhibit a complete infertility.311–313 Loss of Dicer1 has no effect on uterine development as the uterine epi-thelial cells, glands, stroma, and myometrium are pres-ent in Dicer1fl/fl;Amhr2Cre/+ females.311,313 Uterine horns of Dicer1fl/fl;Amhr2Cre/+ females are shorter and hypo-plastic when compared to control (Dicer1fl/fl) uteri.311,313 Additionally, the Dicer1fl/fl;Amhr2Cre/+ uteri respond nor-mally to an artificial decidual stimulation.312 An ovarian defect was observed as increased apoptosis within the

granulosa cells of Dicer1fl/fl;Amhr2Cre/+ compared to wild-type ovaries.312 Moreover, Dicer1fl/fl;Amhr2Cre/+ females exhibit an aberrant oviductal morphology with enlarged and fluid-filled cystic oviducts.311–313 No embryos were found in Dicer1fl/fl;Amhr2Cre/+ uteri, but were discovered to be retained within the oviduct.312,313 The oviducts from Dicer1fl/fl;Amhr2Cre/+ females have an augmented inflammatory response seen as an increase in recruitment of lymphocytes and macrophages into the oviductal tissues.313 Moreover, Dicer1fl/fl;Amhr2Cre/+ females are also defective in embryo transport as trans-ferred blue beads were mostly retained in the Dicer1fl/fl; Amhr2Cre/+ oviduct, whereas in WT females the beads moved through the oviduct and were found to be in the uterine lumen.313 A lack of a proper embryo transport in Dicer1fl/fl;Amhr2Cre/+ appears to be due to a uterotubal junction defect, resulting in retrograde uterine flow into the oviduct.313 Additionally, the expression of Wnt/β-catenin signaling molecules was altered in the Dicer1fl/fl; Amhr2Cre/+ oviduct.311,313 A recent finding from a uter-ine-specific Dicer deletion using PgrCre/+ and Dicerfl/fl animals demonstrated that the females are infertile.314 Loss of uterine dicer leads to uterine developmental defects, including absence of glandular epithelium and increased apoptosis in the stromal cells.314 Stromal cell proliferation could not be induced in Dicer1fl/fl;PgrCre/+ uteri treated with estrogen and P.314 A decidualization defect was also found in Dicer1fl/fl;PgrCre/+, indicat-ing loss of uterine response to P.314 Additionally, loss of uterine dicer expression also leads to a dysregulation in Wnt/β-catenin signaling molecules, similar to the find-ings reported in Dicer1fl/fl;Amhr2Cre/+ females. These findings together suggest that miRNAs, produced by Dicer processing, play pivotal roles in female repro-ductive functions needed for establishing successful pregnancy.

Changes in Uterine Gene Expression

The dramatic physiological changes that occur in the uterus in response to steroid hormones are presumably the ultimate effects of equally dramatic changes in gene expression among the uterine cells. It is unlikely that the E2–ER complex is directly involved in mediating the whole genomic response in the uterus but more plau-sibly serves to stimulate a cascade of downstream sig-naling pathways that act to amplify the estrogen action. However, early investigations of the genomic response to estrogens in the rodent uterus discovered a handful of genes that are directly regulated via the classic ER mode of action, including Pgr and Ltf. The uteri of ERα-null females fail to exhibit estrogen-induced increases in Pgr and Ltf expression,174 indicating the importance of ERα to this response. Furthermore, estrogen- stimulated increases in PR in the rodent uterus are localized to



the stromal and myometrial compartments, whereas increased Ltf is limited to luminal and glandular epi-thelium,223 indicating that ERα functions are critical to induced gene expression in multiple uterine cell types. Further complexity of estrogen action in the uterus is illustrated by the simultaneous induction of PR expres-sion in the myometrium and stroma while eliciting a decrease in PR levels in the luminal epithelium.223 Simi-larly, the c-fos gene is rapidly induced by estrogen in the uterus, and this is ERα dependent.159,242

Microarray analysis of gene expression has signifi-cantly advanced understanding of genomic response of the rodent uterus to E2. Numerous studies have used microarray techniques to map the global gene expression patterns after estrogen exposure in the uterus and largely demonstrate that the biphasic uterine response to estro-gens, so well characterized by physiological indicators discussed earlier (Table 25.4), is mirrored by the global changes in gene expression (Figure 25.13).242,256,315–320 The clearly defined patterns of early and late response genes found in mouse uterine tissues are completely lacking in ERα-null uteri.159,242 The identified genes fall into functional groupings, including signal transduc-tion, gene transcription, metabolism, protein synthesis and processing, immune function, and cell cycle. Sur-prisingly, the expression levels of a striking number of genes are repressed by estrogen in the mouse uterus, and these effects were either absent in ERα-null uteri or relieved by co-treatment with ER antagonists, indicating that ERα is also actively involved in this process.159,242 Comparative gene array analyses have also been con-ducted on human endometrial tissues during the prolif-erative and secretory phases of the menstrual cycle and have revealed gene expression patterns that are similar to those described in rodents.321–325

Microarray analyses have also been used to map the uterine response to P and have identified numerous tar-get genes, including those involved in immune function, metabolism, growth factor regulation, signal transduc-tion, and extra- and intracellular structure.326–328 These observations have identified numerous signals and mechanisms important for normal uterine function and involved in uterine cancer (Table 25.5).34

Microarray approaches have been utilized to inves-tigate the molecular responses to xenoestrogens, such as the industrial chemical bisphenol A (BPA) and the pesticide metabolite 2,2-bis(p-Hydroxyphenyl)-1,1,1-trichloroethane (HPTE). Microarray transcript profiles reveal that both xenoestrogens induce responses that are highly correlated to E2 response early (2 h), but less cor-related later (24 h), similar to a pattern seen with a weak estrogen such as estriol.336 Accordingly, BPA and HPTE, like estriol, are unable to induce uterine growth to the same degree as E2.336

FIGURE 25.13 Top: Schematic representation of uterine biphasic responses that occur following a single injection of E into an ovariectomized mouse (see also Table 25.4). Bottom: Transcriptional profile of uterine RNA, fold changes of transcripts in comparison of vehicle-treated animals at indicated times after E injec-tion. White and black boxes highlight transcripts that typify early and late phases, respectively.

TABLE 25.5 Steroid Receptor Uterine Target Genes from Microarray Studies

Steroid Targets References

P Ihh, Hoxa10, FoxO1, Bmp2, FKBP52, Cebpb, Wnt pathway, Calbindin 9k, Pparg, Lox 12/15,Mig-6, Ihh, Klk5, Klk6, Bcl2, Fst

232,245,326,328–331

E cell cycle regulators CcnB1+2, Cdc2a, p21, Mad2l2, Cdc25c, Gadd45g, DNA replication licensing MCM2, Aqp, thioredoxin pathway, Mig-6, Wnt signaling, Cyr61, RAMP3, Aqp5, Bop1/Scx, Muc1, Inhbb,

199,242,248,332–334

T ATM/Gadd45g pathway 335

Dex Ad1, Mgla, Ykt6, Tis11, Id3, Gilz 40,300

SEX STEROID RECEPTORS AND OVARIAN FUNCTION 1131


Whole transcriptome analyses are now routinely incor-porated into studies of disruptions in signaling pathways underlying uterine phenotypes of mouse models such as those described in Table 25.3. Thus, microarray compari-sons have now become just one of many tools employed for investigation of uterine functions. The data are rou-tinely deposited in the publically accessible site GEO (Gene Expression Omnibus; http://www.ncbi.nlm.nih. gov/geo/). Table 25.5 lists examples of steroid-regulated uterine transcripts discovered in microarray studies.

Chip-seq

More recently, technologies to facilitate evaluation of sites of transcription factor interaction with chromatin (by enriching a DNA binding protein, such as ERα, that has been cross-linked in situ to chromatin, with immu-noprecipitation [chromatin immunoprecipitation or ChIP], followed by hybridizing the associated DNA to a chip tiled with promoter region sequences [ChIP-Chip] or by “next generation” massively parallel sequencing [ChIP-seq]) have been developed and widely utilized to study sites of nuclear receptor interaction (Figure 25.14).75,337–341 Initial studies focused on ERα binding in MCF7 breast cancer cells, and several similar studies fol-lowed, which are summarized and compared in several review articles,97,342–345 and reported that most sites were distal from transcriptional start sites (TSS) or were in intronic regions, rather than adjacent to TSS, as models of NR regulation of target transcripts had hypothesized. These comprehensive maps of cis-acting transcriptional regulators have been dubbed “cistromes”. The initial ERα cistrome–associated sequences were evaluated for enrichment of transcription factor motifs, and confirmed binding to the experimentally defined “ERE” sequence. In the case of the MCF7 cells, enrichment of motifs for forkhead binding factors (Fox) was apparent. Owing to the abundant expression of the FoxA1 member of the Fox family, a potential role for FoxA1 in estrogen response was pursued with an arsenal of bioinformatic, Next Gen sequencing, and biological studies that demonstrated the ability of FoxA1 to “pioneer”, and thus make acces-sible, regions of the chromatin that were subsequently targeted by ERα.76

Most ERα cistromes reported have utilized in vitro cultured cell models, although ERα profiles from mouse liver tissue have been obtained.346 A ChIP-seq study by Hewitt et al. examining ERα binding sites in mouse uterine tissue indicated that, much like the MCF7 breast cancer study, most ERα sites were not proximal to TSS.243 Subsequent comprehensive ChIP-Chip or ChIP-seq studies have been published for the PR in mouse uterus,347 and T47D breast cancer and leio-myoma cells348 and for the AR in mouse epididymis349 skeletal muscle myoblasts,350 ZR-75-1 breast cancer

cells351 and LnCAP prostate cancer cells.352 GR binding has been evaluated in mouse liver tissue,353 mouse epi-thelial cells,354 and A549 lung cancer cells.355 We have learned that NRs bind to thousands of sites within the cellular chromatin, and that not all potential HREs in every cell demonstrate cognate NR binding. Rather, it is apparent that chromatin exhibits “pre-opened” regions destined to recruit NR.356 For ER in MCF7, FoxA1 establishes ER and AR accessible regions; for other cells or other NRs this function might be mediated by other “pioneers”, such as AP1 for GR.354 The accessible chro-matin regions are co-localized within nuclear “hubs” that seem to optimize frequency of interaction with NR.356 ChIP-seq is also used to locate other molecules involved in chromatin remodeling and transcriptional regulation, and to examine activating or repressive his-tone modifications or “marks”. These maps of relative locations and dynamics of NR and chromatin compo-nents greatly enhance our understanding of hormone response mechanisms.339–342,345

SEX STEROID RECEPTORS AND OVARIAN FUNCTION

The functions of the sex steroids and their cognate receptors in ovarian function are especially complex and multifaceted. P, T, and E2 are all synthesized and secreted by the ovary during folliculogenesis and act via both extraovarian (i.e., endocrine) and intraovarian (i.e., para-/autocrine) pathways to pro-foundly influence all aspects of ovarian function. The endocrine actions of the sex steroids in the hypotha-lamic–pituitary axis are critical to the regulation of gonadotropin secretion and the ovarian cycle and are described in more detail in other chapters of this book. In turn, our appreciation of the extent and importance of the para-/autocrine actions of sex steroids within the ovary has increased substantially over the past years. Herein, we specifically review the literature concerning the expression of the sex steroid receptors (Table 25.6) in the mammalian ovary and employ the described null and mutated mouse models that have been developed as a platform to focus on recent rev-elations concerning the intraovarian roles of steroid signaling in ovarian function. A caveat to experimen-tal testing of steroid action and steroid receptor stud-ies in a tissue such as the ovary is that the organ is also producing the steroid being studied. Androgen, estrogen, and progesterone are all synthesized by the ovary; therefore, removal and replacement stud-ies to analyze response are impossible. Thus use of mutant or null mice allows evaluation of the pheno-type reflecting loss of functional interactions between steroids and their receptors.



FIGURE 25.14 (A) Schematic (Source: Modified with permission from Ref. 337), shows method to evaluate the “cistrome” of a steroid recep-tor or other chromatin interacting factors using ChIP–Chip or ChIP-seq. Cells or tissues are treated with a chemical, often formaldehyde, to cross-link DNA binding proteins to DNA. Then, chromatin (DNA and attached proteins) is isolated and fragmented with sonication or another method to break the chromatin into small pieces (approx. 500 bp). The protein of interest is immunoprecipitated with an antibody to enrich DNA fragments bound to the protein. Cross-linking is reversed to allow the enriched DNA to be purified. For ChIP–chip, DNA is amplified and labeled and hybridized to a chip spotted with promoter regions. For ChIP-seq, the DNA is sequenced using massively parallel (Deep) sequencing. The sequence “reads” are mapped onto known genomic regions, resulting in peaks indicating regions of binding of the immuno-precipitated DNA binding protein. Both methods are normalized in comparison to “input” chromatin that has not been immunoprecipitated. (B) A screenshot showing an example from ERα and RNA polymerase II cistromes in the vicinity of the Fos transcript from a mouse uterine dataset243 is shown.



Estrogen Receptor Signaling in Ovarian Function

Estrogen Receptor Expression in the OvaryThere is a plethora of evidence for ER expression in

somatic cell types of the mammalian ovary, and numer-ous reviews describe the importance of ERα and ERβ in ovarian function. As early as 1969, Stumpf demonstrated the specific uptake of [3H]-E2 in rat ovaries using dry-mount autoradiography.357 Richards later demonstrated that the predominance of high-affinity E2 binding sites in the rat ovary is found in the granulosa cell compo-nent and that these levels are regulated by estrogen and gonadotropins.358,359 Successful cloning of the individual ER genes from multiple species and contin-ued development of better ER-specific immunoglobu-lin allow for more detailed characterization of ERα and ERβ expression and regulation in the mammalian ovary. It should be pointed out that immunodetection and expression patterns of steroid receptors are depen-dent on the quality of the antibodies used. Such anti-bodies often vary between studies of ovarian tissue as described following and may explain both species and tissue differences.

ER expression patterns are well conserved among mammalian species, although distinct differences are apparent in late stage follicles. A high level of ERβ expression in granulosa cells of growing follicles is conserved among rodents, large animals, and primates. ERα expression is exclusive to thecal/interstitial cells in the stroma of the ovary in rats and mice (Table 25.6, mice) but this compartmental expression pattern does not hold true in hamsters, domestic animals, monkeys, or humans (Figure 25.15). In fact, current data indi-cate that granulosa cells of preovulatory follicles in large animals and primates express comparable levels of both ERα and ERβ, and, in humans, ERα may pre-dominate.360 Similarly, thecal cells in large animal and human ovaries possess both ER forms. These data force

us to consider a greater potential role for ERα/ERβ het-erodimers in the ovaries of large animals and primates that may not occur in rodents. The more restricted expression pattern of ERα and ERβ in rodent ovaries does not exclude possible cooperative action between the two receptors, as suggested by the unique ovar-ian phenotypes in compound ER-null mice compared to each single knockout ovarian phenotype (Table 25.7).158,174 Although much has been learned concern-ing ER localization, it remains difficult to delineate which of the incongruent findings among species are truly representative of divergent expression patterns or are more attributable to disparities in techniques and immunoreagents. This is further confounded by evi-dence that levels of ER mRNA do not always correspond with the levels of immunoreactive protein within the ovarian compartments.360 The following section reviews the literature concerning the expression patterns of ERα and ERβ in the ovaries of rodents, domestic animals, and primates, and the changes in expression patterns that occur during folliculogenesis. Altered expression has been implicated in diseases affecting ovarian function (including polycystic ovarian syndrome and ovarian cancer); however, this will not be addressed.

Estrogen Receptor αRODENTS

ERα is localized to the ovarian interstitial/stroma, thecal cells of growing follicles, and surface epithe-lium in rat129,130,274,366–372 and mouse124,373 ovaries. This expression pattern is not evident in hamsters374 where both receptors are detectable to varied degrees through-out the different somatic cell types of the ovary. In the fetal rat, ERα expression is detectable shortly after the first indications of gonadal differentiation.375 Postnatal rat and mouse ovaries exhibit ERα mRNA shortly after birth, but levels remain relatively constant; whereas ERβ levels rise substantially during this period.124,376 Immunoreactivity for ERα in neonatal rat ovaries indi-cates ERα expression is limited to thecal/interstitial cells and the ovarian surface epithelia and absent in granulosa cells and oocytes.366,375 A similar pattern of ERα expression occurs in fetal hamster ovaries, which exhibit immunoreactivity as early as gestational day 14 and significant increases thereafter during the neonatal period.377 Yang et al. report that a noticeable increase in ERα immunoreactivity occurs in the granulosa cells and oocytes of primordial follicles during neonatal days 8–15.377

In adult rats and mice, ERα expression continues to be exclusively localized to thecal cells of growing fol-licles, interstitial/stromal cells, and the ovarian surface epithelium129,130,367,369,370,372,378,379 (Figure 25.15). Electro-phoresis mobility shift assays (EMSA) of nuclear extracts from rat granulosa cells indicate that virtually all of the

TABLE 25.6 Localization of Steroid Receptor Expression in the Mouse Ovary

Steroid Receptor Granulosa Theca Oocyte Stroma

ERα + +++ – ++

ERβ +++ – – –

PRA +++ + – ++

PRB + + – ++

AR +++ ++ + +/−

GR +++ ++ – ND

Aromatase +++ – – –

ND: Not determined.–: Not expressed.+/−, +, ++, +++: Intensity of signal indicated by number of + symbols.



TABLE 25.7 Ovarian Phenotypes in E Signaling Mutant and Null Mouse Models

Mutated or Null for Sex Steroid Signaling Ovarian Phenotypes Hormone Levels References

Esr1−/− (homozygous null alleles for ERα animal)

Anovulatory and infertileHemorrhagic and cystic pathologyNo CLs present in ovarian sectionsIncreased expression of steroidogenic enzymesReduced response or failure to respond to exogenous gonadotropins

Elevated T, E2 and LHNormal FSH & P

157–160

NERKI+/− (one mutated allele with 2-point mutation in DNA binding zinc finger of ERα and one WT allele)

Anovulatory and infertileNo plugs observed in NERKI females after superovulation and natural matingSuperovulation was able to partially restore ovulation in NERKI mice, however also increased hemorrhagic follicles in the ovaries

Normal LH, FSH and E2Reduced P

132

KIKO (ERAA/−) (one mutated allele of 2-point mutation in DNA binding domain of ERα and one ERαKO allele)

Anovulatory and infertileNo CLs present in ovarian sections

Normal E2 and P 162

ENERKI (ERαG525L) (homozygous animal of 1-point mutation in LBD of ERα)

AnovulatoryHemorrhagic and cystic ovarian pathologyIncreased number of atretic antral follicles and no CLsHyperplastic theca cells in response to LH (data not shown)

Elevated serum E2, T and LHNormal FSH

168

ERαEAAE/EAAE (homozygous animal of 4-point mutation of DBD ERα)

InfertileHemorrhagic ovaries

Not reported 164

AF2ERKI/KI (homozygous animal of 2-point mutation in LBD of ERα)

Anovulatory and infertileHemorrhagic and cystic ovarian pathologyNo CLs present

Elevated serum LH and E2 169

Cyp17cre;ERaflox/flox

(Theca cell specific ERα knockout)Fertility normal in young mice, but 6 month old animals have reduced fertility and longer estrous cycle

Elevated T at both 2 & 6 monthsNormal FSHDecreased LH at 2 months with further decrease at 6 months

361,362

Esr2−/− (homozygous null alleles for ERβ: βERKO, Ex3βERKO, and **ERβST

L−/L−)

Subfertile although some lines are infertileReduced (or lack of) response to exogenous gonadotropinsApproximately 1/5 the number of large preovulatory follicles after superovulation compared to WT miceLack of COC expansion

Normal LH and FSH 158,171–173,363

ERαAF-10 Ovarian phenotype not reported Unknown 165

ERαAF-20 Ovarian phenotype not reported Unknown 167

αβERKO Anovulatory and infertileNo CLs and few large folliclesOvarian transdifferentiation to Sertoli-like cells that express Sox9Altered expression of steroidogenic enzymes

Elevated LH and TNormal FSH and P

158,174

Cyp19a1−/− Anovulatory and infertileHemorrhagic and cystic pathologyNo CLs present in ovarian sectionsPoor response to exogenous gonadotropins, although treatment with E2 in addition to gonadotropins improves ovulatory responseOvarian transdifferentiation to Sertoli-like cells that express Sox9

No E2Elevated LH, FSH and T

131,156,175,194,364,365

**ERβSTL−/L− females are the only line of ERβ knockout animals that are sterile.



specific ERE-bound complexes are supershifted by anti-ERβ but not anti-ERα immunoglobulin, providing fur-ther support that rat granulosa cells possess very little ERα.367,371 Western blot analyses of whole cell or nuclear extracts from isolated rat granulosa cells do indicate a low level of ERα protein of 61 kDa,371 and use of laser capture microdissection (LCM) shows some expres-sion of ERα mRNA in granulosa cells (Figure 25.15).363 Unlike ERβ, ERα expression remains relatively constant throughout the rat estrous cycle.380 Furthermore, no change in ERα levels is detected in granulosa cells cul-tured in FSH/T over a period of 72 h; however, a 24-h treatment with forskolin induces a marked increase in ERα immunoreactivity in the nuclei.371

ERα localization in the adult hamster ovary is some-what divergent from that in rats and mice. Yang et al. report moderate clear ERα immunoreactivity in the-cal/interstitial cells, but also appreciable immunoreac-tive granulosa cells of small preantral follicles.374 In the granulosa cells of antral follicles, ERα immunoreactivity

is strongest among those in proximity to the forming antrum.374 FSH treatment (twice a day for 2 consecutive days) or a single injection of E2 elicits a significant induc-tion of ERα expression in both thecal and granulosa cells in hypophysectomized hamsters.374 Still, Western blot analyses of whole ovarian homogenates from hamsters indicate an ERβ:ERα ratio of 14:1 during the follicular phase, indicating that ERβ predominates. However, the rapid decline in ERβ and concurrent increase in ERα that occurs just prior to and shortly after the gonadotropin surge adjusts this ratio to 2:1, suggesting that ERβ plays a more predominant role in follicular growth and ERα is more important during luteinization374 in the hamster ovary.

DOMESTIC ANIMALS

In fetal bovine ovaries, ERα was shown to be local-ized in all cell types involved with follicle development, including granulosa cells, oocytes, and epithelial cells throughout development, with expression becoming

FIGURE 25.15 (A) Immunohistochemistry for ERα and ERβ in adult mouse ovary. Immunohistochemistry for ERα (top) indicates specific nuclear immunoreactivity in thecal/interstitial cells and thecal cells (TC) in and around a preantral follicle. Granulosa cells lack any measurable immunoreactivity for ERα. The staining around the outer surface of the oocyte is nonspecific and not representative of ERα immunoreactivity. Immunohistochemistry for ERβ (bottom) indicates specific nuclear immunoreactivity in the granulosa cells (GC) throughout a preantral follicle. Thecal cells (TC) lack any measurable immunoreactivity for ERβ, but the surrounding thecal/interstitial cells exhibit some cytoplasmic staining. Immunohistochemistry was done by Dr. Madhabananda Sar (see Ref. 366 for protocol). (B) Pure populations of granulosa and theca cells were isolated from large preovulatory follicles using laser capture microdissection (LCM), and RNA was reverse transcribed and real time PCR was performed using primers specific for ERα and ERβ.



more localized to thecal and some stromal cells during the later stages of development.381 In adult bovine ova-ries, ERα immunoreactivity is relatively high in thecal cells of secondary and tertiary follicles, substantially weaker in granulosa cells of tertiary follicles, and totally absent in primordial, primary, and secondary follicles.382 In agreement, Barisha et al. found ERα transcripts are detectable in both granulosa and thecal cell fractions, but levels are substantially higher in the latter cell type.383 Furthermore, thecal ERα expression appears to be dif-ferentially regulated as it is almost four-fold higher in follicles of 20–180 mm versus those <0.5 mm, whereas ERβ expression in thecal cells remains constant among follicles of different sizes.383 ERα expression in granu-losa cells, although much lower relative to theca, also increased with advanced follicle size.383 In porcine ova-ries, ERα immunoreactivity is detected in both theca interna and granulosa cells, but only in large follicles; and even then ERα immunoreactivity is considerably moderate relative to ERβ.384 In ovine ovaries, ERα mRNA and immunoreactivity is detected in both cell types but predominates in granulosa cells and is particularly high in cumulus oophorus cells of small antral follicles.385

PRIMATES

The first reported study of ERα immunoreactivity rhe-sus or cynomolgus monkey ovaries found that ERα was undetectable in all ovarian cell types except the surface epithelia, regardless of menstrual stage.386 However, a later study in baboon ovaries using the same antibody found nuclear ERα immunoreactivity in 30–40% of the granulosa cells of healthy antral follicles, and detectable but less intense staining in granulosa cells of preantral follicles.387 This same study found the stroma, inter-stitial, and thecal cells to be largely unlabeled.387 Pau et al. produced similar findings of ERα expression in the granulosa cells of rhesus monkey ovaries by in situ hybridization.388

High-affinity E2 binding sites in normal human ovar-ian tissue were first described several decades ago,389–391 but these techniques were not able to differentiate between the two ER forms. Iwai et al. used anti-ERα antisera similar to that in the nonhuman primate stud-ies to demonstrate immunoreactivity in the granulosa cells of antral and preovulatory follicles in human ovary but a total absence in primordial, preantral follicles, atretic follicles, and thecal/interstitial cells.392 Pelletier and El-Alfy393 used a different antihuman ERα antisera and reported results that are in contrast to those of Iwai et al.,392 i.e., a total lack of ERα immunoreactivity in granulosa cells but clear nuclear staining of theca interna cells, interstitial gland cells, and ovarian surface epithe-lia. A possible explanation for this discrepancy may be a lack of large antral follicles in samples evaluated by Pel-leteir and El-Alfy since both studies agree that ERα is not

detectable in the granulosa cells of less mature follicles. In support of this explanation is a report by Taylor and Al-Azzawi in which ERα immunoreactivity is illustrated in the granulosa cells of what is clearly a large antral follicle when using antisera raised against the bovine ERα.394 Indeed, Saunders et al. described ERα immuno-reactivity that is limited to granulosa cells of large antral follicles and undetectable in smaller follicles in the human ovary.395 Also, in agreement with Pelletier and El-Alfy,393 Saunders et al. found ERα immunoreactivity in thecal cells of preantral and antral follicles, and ovar-ian surface epithelia.395

ERα transcripts are detectable in human nonluteinized granulosa cells396 and granulosa-luteal cells collected at the time of oocyte retrieval during in vitro fertilization (IVF) procedures.396,397 In fact, Jakimiuk et al. found ERα mRNA and protein levels are two- to four-fold higher in granulosa versus thecal cells of small antral follicles in human ovaries.360 Furthermore, ERα protein levels remain elevated in granulosa cells but decrease in the-cal cells of dominant follicles.360 In luteinized human granulosa cells, Chiang et al. found ERα levels to remain relatively constant over a period of 10 days in culture but always much lower than ERα mRNA levels.398 Recent reports in human ovarian samples collected during elec-tive hysterectomy demonstrate that ERα is expressed in granulosa cells of antral follicles, with weak to no posi-tive immunohistochemical staining in luteinized granu-losa cells399 supporting previous data.

Estrogen Receptor βIn the nonpregnant ovaries of most mammalian

species, ERβ is clearly the predominant ER form and is most often exclusively localized to granulosa cells of follicles from the primary to preovulatory stage. This expression pattern is documented in the ovaries of rats,14,130,366,367,370–372,380,400–402 mice,124,373,379,402 ham-sters,374 cows,383,403,404 sheep,405,406 pigs,402,407 nonhu-man primates,274,388 and humans.393,394,402,408,409

RODENTS

In developing rat ovaries, ERβ expression is detectable shortly after the first indications of gonadal differentia-tion on gestational day 14, and levels increase thereafter during prenatal development.375 A second, more robust increase in ERβ expression occurs during days 10–15 of neonatal development in rat ovaries,376 and immunore-activity is largely localized to granulosa cells of growing follicles and notably absent in primordial follicles and oocytes.366,375 Developing mouse ovaries exhibit a com-parable pattern ERβ expression.124 In contrast, ERα is limited to thecal and interstitial cells366,375 and exhibits little change in levels with increased age124,376 in devel-oping rat and mouse ovaries. The hamster ovary exhib-its a fairly similar ontogeny as ERβ is detectable as early



as gestational day 13 and increases thereafter to peak on postnatal day 10, throughout which ERβ immunoreac-tivity is predominantly localized to granulosa cells but detectable in some interstitial cells and oocytes.377

In the ovaries of prepubertal and adult rats and mice, ERβ immunoreactivity is nearly exclusive to the nuclei of granulosa cells of healthy follicles at all advanced stages of folliculogenesis124,129,130,366–370,374,402 and is nota-bly decreased or absent in atretic follicles129,366 (Figure 25.15). Primordial follicles, thecal cells, oocytes, ovarian surface epithelium, and luteal cells lack ERβ immunore-activity in adult rat and mouse ovaries,129,130,366–369,374,402 although some cytoplasmic staining in the latter cell type is reported when using certain antisera.129,366 Gran-ulosa cell specific expression of ERβ in the rat ovary was reproduced using three separate anti-rat ERβ antisera, two raised against residues 467–485 and a third raised against residues 54–71.369 Furthermore, independent studies using in situ hybridization produced results con-gruent with the immunohistochemical findings of strict localization of ERβ mRNA to granulosa cells of healthy growing follicles,14,129,375,401 although Bao et al. reported scattered but specific hybridization for ERβ mRNA in thecal cells of healthy medium and large follicles in the rat ovary.400

A study by Saunders et al.370 stands in contrast to those just discussed. In this study, strong nuclear immunoreac-tivity is described throughout the different somatic cell types of the rat ovary, including thecal, interstitial, and luteal cells, as well granulosa cells of maturing follicles when using an anti-rat ERβ antisera raised against resi-dues 196–213. Yang et al. reported similar findings of ERβ immunoreactivity in thecal/interstitial cell prepa-rations from hamster ovary using different ERβ-specific antisera.374 Still, the results of Saunders et al. are prob-lematic because they include reports of substantial ERβ immunoreactivity in certain reproductive tissues of the rat, such as the oviduct and uterus,370 that are otherwise thought to possess relatively low or null levels of ERβ mRNA and immunoreactivity. Two obvious differences between the immunohistochemical study of Saunders et al.370 and others are the aforementioned use of differ-ent antisera as well as different tissue fixative.

The previous discrepant findings notwithstanding, ERβ is clearly the predominant ER form present in gran-ulosa cells of healthy growing follicle in the rodent ova-ries. Recent isolation of pure granulosa and theca cells from mouse ovaries using laser capture microdissection demonstrates that ERβ mRNA expression is solely in granulosa cells (Figure 25.15). Any specific distribution of ERβ throughout the subpopulations of granulosa cells of growing follicles has been difficult to ascertain. In general, ERβ immunoreactivity is continuous and strong among the mural cells and relatively uniform among those layers closer to the antrum and oocyte.366,367,369

However, not all granulosa cells of healthy growing fol-licles in the rodent ovary express ERβ, as negative cells are randomly distributed throughout the follicle.366,367

Data from Western blots of nuclear extracts from whole ovaries or isolated granulosa cells support the immunohistochemical data that ERβ is the predominant receptor form in rodent granulosa cells.367,371 Electropho-resis mobility shift assays in which whole cell extracts from ovaries or isolated granulosa cells of rats provide further evidence of the predominance of ERβ.367,371,410 Sharma et al. found that antisera raised against resi-dues 54–71 of rat ERβ produces the optimum results on Western blot and detects four immunoreactive bands of 58/52 to 46/44 kDa in size, the 58 kDa protein presum-ably being full-length ERβ (ERβ1 in Figure 25.2).371 Anti-sera raised against the AF-2 (residues 182–485) domain of rat ERβ detects a single specific protein of 60 kDa in whole-cell extracts from both rat and mouse ovaries that comigrates with the putative full-length ERβ.367 Choi et al.402 and Hiroi et al.368 produced comparable findings of immunoreactive proteins at 60 and 55 kDa in rat and mouse ovarian extracts when using either a monoclonal antibody mapped to residues 272–285 of human ERβ or antisera raised against C-terminal residues 467–485 of rat ERβ, respectively. In the hamster, a single ERβ-immunoreactive protein of 54 kDa is detected.374 To date, ERβ antibodies are not as specific as those manufactured for ERα, and produce bands in the ERβ-null tissues simi-lar to those observed in WT tissues (unpublished data from Korach laboratory) such that further characteriza-tion of ERβ localization is challenging.

Several different ERβ isoforms are present in rodent ovaries, notably ERβ2 (Figure 25.2), ERβ-Δ3, and ERβ2-Δ3, the latter being a compound form of the two former vari-ants. ERβ-Δ3 is an exon 3 deletion that results in a recep-tor lacking the C-terminal zinc-finger of the DNA binding domain and therefore unable to bind an ERE.193 ERβ2 is an especially interesting variant because it possesses an insert of 18 amino acids in the N-terminal region of the LBD (Figure 25.2) that causes a 35-fold reduction in affin-ity for E2 and a 1000-fold decrease in E2-induced trans-activational activity in vitro.193 This isoform is not found in human ovary (Figure 25.2). Pettersson et al.193 specu-late that ERβ2 may be especially attuned to mediating E2 actions within healthy growing follicles where intrafol-licular E2 levels greatly exceed that required to activate wild-type ER forms. Differential RT-PCR indicates a 1:1 ratio of ERβ1:ERβ2 mRNA in the ovary of adult371,410,411 and neonatal rats,323 while transcripts encoding ERβ1-Δ3 or ERβ2-Δ3 variants are detectable but at substantially lower levels.410,411 Pettersen et al. demonstrated that FLAG-tagged clones of ERβ1 and ERβ2 expressed in 293T cells co-migrate at approximately 60 kDa,411 sug-gesting that resolution of the two isoforms by Western blot may be difficult. However, several studies illustrate



two distinct ERβ-specific bands appear on Western blots from rat367,368,371,410 and mouse402 ovarian extracts. Fur-thermore, O’Brien et al. fractionated rat granulosa cell preparations in a 7.5% SDS-PAGE from which immu-noreactive protein bands presumed to be ERβ1 (60 kDa) and ERβ2 (62 kDa) were resolved; they further remarked that an equal ratio of ERβ1:ERβ2 transcripts does not correlate to actual protein amount by Western blot, as much greater amounts of ERβ1 protein are present.410 In retrospect, previous [3H]-E2 binding data also supports a predominance of ERβ1 versus ERβ2 in rat granulosa cells. Because recombinant ERβ1 and ERβ2 differentially bind E2 with affinities of 0.14 and 5.1 nM, respectively,411 one would expect differentiation of the two forms when assessed by Scatchard plot analysis. Therefore, the ear-lier studies using [3H]-E2412 and more recent studies using [125I]-17α-iodovinyl-11β-methoxyE2367 that report a single, saturable, high-affinity binding factor with a Kd = 0.4 nM in rat ovarian or granulosa cell extracts are congruent with ERβ1 as the predominant form present. Similar data of a single E2 binding component with a Kd = 1–1.4 nM in hamster ovaries is also reported.413,414

The majority of studies on the regulation of β expres-sion during folliculogenesis focus on the rat ovary. While some report high but relatively constant levels of ERβ mRNA leading up to proestrus,380 more detailed studies indicate a substantial increase in ERβ expression that peaks

in medium sized (275–450 mm) follicles possessing clearly defined antrum.400 Most studies agree on the precipitous decline in ERβ levels that occur shortly after the ovulatory gonadotropin surge and the low levels that remain during estrus in rats380 and hamsters374 (Figure 25.16). In retro-spect, it is evident that the decreasing effect of the gonado-tropin surge on ERβ levels was first indicated by Richards in 1975, in which a 75% decrease in high-affinity E2 bind-ing sites in granulosa cells of hypophysectomized, FSH/E2-primed rats was found to occur following a single LH treatment.358 This effect of LH on ERβ expression has been reproduced in vivo in gonadotropin-primed rats367,380 and mice415 in which a single hCG injection led to a rapid decrease in ERβ mRNA and protein levels of more than 50% within 6–9 h (Figure 25.16). Similarly, granulosa cells isolated from pregnant mares serum gonadotropin (PMSG)-stimulated rats and exposed to hCG in vitro exhibit a comparable loss of ERβ protein when assessed by western blot367 or EMSA.371

Therefore, induction of the LH-signaling pathway in differentiated granulosa cells is the primary stimulus for rapidly decreased ERβ expression. Evidence of a direct role of LH comes from findings that only preovulatory follicles that co-express LH receptor exhibit a decline in ERβ expression; while smaller, LH-receptor negative follicles maintain ERβ expression as late as 24 h post-hCG exposure.367,380 The inhibitory effect of LH on ERβ

FIGURE 25.16 Regulation of ERβ expression in adult rat ovaries during the estrous cycle or after exogenous gonadotropin treatments. (A) Quantification of gene expression from Northern blot analysis (not shown) for ERα and ERβ in adult rat ovaries during the estrous cycle. Band intensities were measured on a phosphorimager and normalized to an S16 internal control for each time point. mRNA levels are shown relative to the level at E1100 (set to 1.0). Sera from animals were used to determine LH concentrations; the onset of the LH surge was observed at 1600 h of proestrous, and the peak was observed at 1800 h of proestrous. Annotation at bottom indicates the estrous cycle stage and hour of tissue collec-tion: E, estrous; M, metestrous; D, diestrous; P, proestrous. (Source: Reproduced with permission from Ref. 380.) (B) Reverse transcriptase polymerase chain reaction was used to quantify ERβ mRNA levels in the ovaries of immature rats after treatment with PMSG alone or PMSG for 48 h followed by hCG. The ratio of ERβ/S16 of control rats with no hormonal treatment was set to 1.0. Shown are the mean ± SE (n = 4). (Source: Reproduced with permission from Ref. 380) (C) ERβ immunoreactivity in rat ovaries treated with vehicle (Veh), PMSG for 48 h, or PMSG for 48 h followed by hCG. ERβ immunoreactivity is detected in granulosa cells of small and large antral follicles in Veh PMSG animals. Stars indicate the location of antra in antral follicles. PMSG-treated rats were injected with an ovulatory dose of hCG and ovaries isolated after 3, 9, 12, or 24 h. The expression of ERβ protein in granulosa cells 9 h after hCG is reduced in large antral follicles (left), greatly reduced in preovulatory follicles (center), but does not change in small antral follicles (right). Similar expression is observed 12 h after hCG administration. One day (24 h) after hCG treatment, ERβ expres-sion is not detected in corpora lutea (small arrowheads) but is highly expressed in preantral (right) and small antral follicles. Magnification 250×. Source: Reproduced with permission from Ref. 367.



expression can be reproduced by exposing differentiated granulosa cells to either forskolin or TPA,380,410 activa-tors of the protein kinase-A and protein kinase-C path-ways, respectively, and thought to mimic the effects of LH stimulation.416 Furthermore, forskolin or TPA treat-ment reproduce the same temporal pattern of decreased ERβ expression that follows LH or hCG exposure,417 sug-gesting a common mechanism. Guo et al. demonstrated both activators and mimics of the LH-signaling pathway elicit the rapid decline in granulosa cell ERβ levels by decreasing the stability of ERβ transcripts rather than via a repression of gene expression.417

In contrast to the peri-ovulatory decrease in ERβ expression that occurs, there is little known about the positive regulation of ERβ expression in rat granulosa cells. Induction of folliculogenesis by PMSG or FSH in hypophysectomized rats leads to a significant rise in E2 binding among granulosa cells, presumably repre-sentative of ERβ,358,418 but this is likely due more to an increased number of granulosa cell population rather than direct gonadotropin-stimulated ERβ expression. The presence of ERβ in the granulosa cells of primary follicles, considered to be insensitive to direct gonado-tropin stimulation, supports a minor role for FSH in ERβ expression.366,375 Furthermore, ERβ levels exhibit little change in immature rat or mouse ovaries 48 h after a single injection with PMSG,367,380,415 and no reports exist of altered ERβ expression in the ovaries of mice null for FSH signaling.419–421 Sharma et al. found that ERβ pro-tein levels in gonadotropin-primed rat granulosa cells evaluated are highest if assessed after isolation and decrease steadily to undetectable levels within 72 h in culture, even in the presence of FSH and T.371 However, this pattern was not totally reproduced at the level of ERβ mRNA, which drops by only 55% when cultured for the first 24 h in the absence of hormones but stabilizes upon the addition of FSH and T,371 suggesting that FSH may be more important to the maintenance rather than stimulation of granulosa cell ERβ expression. In contrast to the previous findings, preovulatory rat granulosa cells allowed to lose ERβ expression following long-term (6 day) culture exhibit a significant rise in levels 48 h after exposure to forskolin, a direct PKA activator and thought to mimic FSH signaling.371 Interestingly, FSH may have a greater positive influence on ERβ expres-sion in the hamster ovary where a substantial increase in expression in preantral and antral follicles is observed in hypophysectomized females following 1–2 days of treat-ment with ovine FSH;374 and ovaries of neonatal ham-sters in which prenatal FSH action is inhibited exhibit significant decreases in ERβ expression.377

The existing evidence of E2 regulation of ERβ expres-sion is conflicting. Treatment of hypophysectomized rats with E2 for 3–4 days results in steady and dramatic increases in E2 binding sites in granulosa cells,358,422 but

once again this is more likely due to an increased gran-ulosa cell population. However, Tonetta et al. demon-strated that FSH-induced increases in E2 binding sites in granulosa cells of hypophysectomized rats are blocked by co-administration of an ER antagonist, suggesting an autoregulatory element for ER expression.418 In contrast, Kim and Greenwald found little change in the number of E2 binding sites in hamster ovaries following E2 or DES treatment, although animals were exposed for only 1–2 days, and therefore significant gains in granulosa cell number may not have been achieved.413 Drummond et al. reported that 1–4 days of treatment with DES has no discernible effect on the levels of ERβ1 or ERβ2 tran-scripts in immature rat ovaries.376 Yang et al. found that hypophysectomized hamsters exhibit an almost six-fold increase in ERβ expression as detected by immu-nohistochemistry 24 h after a single injection of 0.1 mg E2- valerate.374 Furthermore, rat granulosa cells isolated from E2-primed versus untreated animals exhibit a four- to six-fold higher basal estrogenic activity on an ERE-luciferase reporter construct, suggesting that individual cellular levels of ERβ are increased by E2 treatment.371 Also, isolated rat granulosa cells exhibit an increased nuclear intensity for ERβ immunoreactivity 1.5 h after E2 exposure that is maintained for 24 h but totally lost by 48 h.371 Still, the failure of granulosa cells to totally sustain ERβ expression when maintained in FSH and T, and therefore capable of synthesizing endogenous E2, is puzzling. It is plausible that E2 regulation of ERβ expres-sion in granulosa cells may require the actions of ERα in thecal cells, which cannot be mimicked when granulosa cells are isolated in culture. Preservation of ERβ expres-sion in preantral follicles of ERα-null ovaries argues against this hypothesis.127,373 Recent work in KGN cells suggest that GIOT-4, a cofactor regulated by FSH, may act to increase ERβ expression as well as work in com-bination with ERβ to regulate target genes in follicles.423 Although this work was done primarily in KGN ovar-ian cells, it was also reported that GIOT-4 and ERβ are co-localized in growing follicles providing support for the co-regulation. The expression of GIOT-4 has not been examined in in vitro–cultured granulosa cells. Therefore, although the signaling factors that may be most impor-tant to inducing ERβ expression remain unknown, it is clear that the level of ERβ is highly dependent upon the state of granulosa cell differentiation as cells from pri-mary, growing, and preovulatory follicles exhibit diver-gent expression patterns and mechanisms of regulation.

DOMESTIC ANIMALS

Descriptions of ERβ expression in the ovaries of domestic animals are limited but generally indicate pat-terns that are congruent with rodent ovaries. In the fetal bovine ovary, ERβ is localized in the cell types associated with follicle growth, predominantly in pregranulosa



and granulosa cells. In early development, epithelial cells show expression; however, this becomes punctate as follicles begin to develop.381 In adult bovine ovaries, Rosenfeld et al. found that ERβ mRNA and immunore-activity is restricted to granulosa cells of small and large antral follicles with no detectable levels in thecal cells.403 A follow-up study further demonstrated substantial ERβ immunoreactivity in cumulus oophorus and antral gran-ulosa cells relative to mural granulosa, as well as men-tioned significant ERβ expression in oocytes,404 the latter finding being common only to hamsters.374,377 In con-trast to the immunohistochemical data, Walther et al.424 and Berisha et al.383 found ERβ transcripts are detectable in both granulosa and thecal cells. In ovine ovaries, ERβ is highly expressed in granulosa cells of growing folli-cles,405,406 and levels exhibit little change over the course of the estrous cycle.406 However, Jansen et al. report via in situ hybridization that ERβ mRNA is greatest in fol-licles ≤3 mm and declines in larger follicles during the early follicular phase.405 Both reports describe low but detectable ERβ expression in thecal cells of growing folli-cles in the sheep ovary405,406 as well as in ovarian surface epithelia.406 In porcine ovaries, ERβ is almost equally expressed among the granulosa and theca interna of medium and large follicles, as well appreciable detection in the oocyte and ovarian surface epithelia.384 Like the rodent, atretic follicles in porcine ovaries exhibit reduced ERβ expression.384 LaVoie et al. found that ERβ mRNA levels in whole ovarian lysates exhibit little change dur-ing the porcine estrous cycle and are relatively equal when comparing follicles of 1–5 mm in size.407

An interesting commonality among bovine, ovine, and porcine ovaries is the presence of a truncated ERβ iso-form lacking most of the ligand binding domain, termed ERβΔLBD in the cow424 and ERβ-Δ5 in sheep and pig406,407 (Figure 25.2). In all three species, ERβ-Δ5 is a deletion of exon 5 and encodes a receptor isoform that is truncated 15 residues into the LBD and terminates with four to nine heterologous residues depending on the species.406,407,424 LaVoie et al.407 and Choi et al.402 produced Western blot evidence that ERβ-Δ5 may indeed be translated as a 35 kDa protein in porcine ovaries. Given that ERβ-Δ5 lacks the LBD, it predictably exhibits little estrogen-induced transactivational activity when overexpressed in vitro but does possess measurable ligand-independent transactivational activity that is three-fold lower than full-length ERβ, suggesting preservation of AF-1 func-tions.407 ERβ-Δ5 is also able to reduce the in vitro activity of full-length ERβ by approximately 40% when present in excess.407,424 Thus ERβ-Δ5 might modulate ERβ activity in an environment of high hormone concentrations, such as is found with estrogens in granulosa cells.

PRIMATES

In ovaries of known fertile cynomolgus monkeys, Pelletier et al. localized ERβ expression via in situ

hybridization to granulosa cells of follicles at all differ-ent stages of development, as well as substantial labeling in theca interna and ovarian surface epithelia.154 Similar findings are described by Pau et al.388 in granulosa cells of large follicles in rhesus monkey ovary. In the baboon ovary, Pepe et al. found ERβ immunoreactivity is abun-dant in granulosa cells of follicles at all stages, including large antral follicles but notably lacking in thecal cells.425 Western blots of baboon ovarian homogenates indicate two specific ERβ protein bands at 55 and 63 kDa.425 An extensive immunohistochemical study in marmoset ovaries using antisera raised against human ERβ found extensive immunoreactivity in granulosa cells of pri-mary through to mid- and late follicular stage follicles, including those with a large antrum, but a clear lack of staining in atretic follicles.395 No specific pattern of ERβ immunoreactivity was apparent among subpopulations of granulosa cells within any one follicle.395 In contrast to baboon ovaries but in agreement with cynomolgus monkey ovaries, thecal cells of large growing follicles as well as the ovarian surface epithelia in marmoset ovaries possess appreciable levels of ERβ immunoreactivity.395

High-affinity E2 binding sites in normal human ovarian tissue were described several decades ago.389–391 Al-Timimi et al. found 45 of 89 normal ovaries from premenopausal women to possess detectable E2 binding sites; whereas all of the (n = 10) postmenopausal ovaries assayed were devoid of detectable binding.390 Vierikko et al. reported high- affinity E2 binding in a similar percentage of pre-menopausal ovaries but did find 67% of postmeno-pausal samples were also positive, although the levels of E2 binding in both are notably lower than that for P.391 The advent of better reagents has allowed for differentiation of the two ER forms within human ovaries. ERβ expres-sion was first detected in human ovary by Northern blot analysis.15,408,426,427 Follow-up studies using RNase-protec-tion assays or RT-PCR indicate a relatively equal ratio of ERα:ERβ transcripts in mixed-cell homogenates from ova-ries of pre-and postmenopausal women but exclusively ERβ transcripts in luteinized granulosa cells isolated from women undergoing IVF.408,427 Hillier et al. report equal levels of ERα and ERβ mRNA in both nonluteinized and luteinized granulosa cells from normal human ovaries, although levels of both ER forms are reduced in the latter.396 Another study comparing ERβ mRNA and protein levels within isolated thecal and granulosa cells of small antral follicles found mRNA levels are two-fold higher in thecal cells but protein levels are similar in both cell types.360

Three reports describing immunohistochemical local-ization of ERβ in human ovaries produced comparable findings of specific immunoreactivity among granulosa cells of follicles from primary to late antral stage, and specific, but considerably lower, levels in thecal cells of preantral and antral follicles.393–395 Furthermore, all three demonstrate substantial ERβ immunoreactivity in human ovarian surface epithelia.393–395 Hillier et al. also found



ERβ transcripts are detectable in primary cultures of human ovarian surface epithelial cells;396 however, simi-lar evaluations in cell lines derived from human ovarian surface epithelium indicate no detectable ERβ mRNA.427

There is currently little known about ERβ regulation in human ovaries. Jakimiuk et al. found ERβ expression is significantly decreased in thecal and granulosa cells of dominant follicles relative to those at earlier stages, yet ERβ protein levels did not mirror this difference.360 In primary cultures of human granulosa–luteal cells, ERβ is more highly expressed relative to ERα and levels of the former increase 1.6-fold over a 10-day period dur-ing which spontaneous luteinization is believed to occur, while ERα levels remain constant throughout.398 Treat-ment of human granulosa-luteal cells maintained in cul-ture for 7 days with hCG reduces ERβ and ERα expression by almost 50% within 24 h, and this effect is mimicked by activators of protein kinase A (forskolin) or protein kinase C (TPA).398 Recent examination of human ovaries found that ERβ1 and ERβ2 are expressed in granulosa-luteal cells as well as endothelial cells at all stages of the luteal phase. This study also used in vitro luteinized granulosa cells, treated with hCG or E2. Treatment of the lutein-ized granulosa cells with hCG reduced expression of ERα, ERβ1, and ERβ2 by approximately 50%, while treat-ment with E2 reduced expression of ERα and ERβ1 with no significant change in ERβ2,399 suggesting that human granulosa cells may respond to LH by reducing ERβ in a manner similar to that found in rodent ovaries.

Studies to date indicate that human tissues express a unique C-terminal variant of ERβ termed ERβcx

426 or human ERβ2409,428 (Figure 25.2). ERβcx is a 495–amino acid–long receptor form in which the 61 C-terminal resi-dues are replaced by a heterologous string of 26 amino acids encoded by an alternative downstream exon.426,428 Transcripts encoding ERβcx are detectable in multi-ple human tissues but most especially in ovary, testis, thymus, and spleen and are often at levels equal to or greater than wild-type ERβ.426,428 Immunohistochem-istry employing ERβcx-specific antisera indicate the variant is nuclear localized in granulosa cells of early follicles.409 The physiological function of ERβcx remains unclear. It is unable to bind E2, but in vitro studies indi-cate the isoform may preferentially heterodimerize with ERα and act as a dominant negative modulator of ERα action. Rodents also possess an ERβ variant (ERβ2) that poorly binds E2, but this receptor form does not inhibit wild-type ERα or ERβ activity.193 Furthermore, categori-cal studies have shown that the rodent ERβ2 variant is not detected in human tissues.409,429

Ovarian Phenotypes in Mouse Models of Disrupted Estrogen SignalingMICE LACKING ERα

Several lines of ERα-null mice have been characterized as listed in Table 25.7. Neonatal and prepubertal ERα-null

mice exhibit relatively normal ovaries except for signs of premature folliculogenesis as indicated by the sporadic presence of large antral follicles.158,373 Adult ERα-null females are anovulatory and hence infertile, and exhibit ovaries that possess normal pre- and small antral-stage follicles, multiple hemorrhagic cysts, absence of corpora lutea, reduced number of interstitial glandular cells,

FIGURE 25.17 Ovarian phenotypes in ER-null mice. (A–C) Shown are cross-sections from representative adult ovaries of wild-type (A), ERβ-null (B), and ERα-null (C) female mice. Wild-type and ERβ-null ovaries each exhibit all stages of folliculogenesis except corpora lutea, although large antral follicles are sparse in the ERβ-null ovary. In contrast, ERα-null ovaries are characterized by several large, hemorrhagic, and cystic follicles; a sparse number of follicles at the early stages of proliferation; and a lack of corpora lutea. (D) Cross-section of a representative ERα-null ovary after prolonged treatment with a GnRH antagonist. Circulating LH levels were reduced in ERβ-null females by treatment with a GnRH antagonist (60 μg Antide) every 48 h from the age of 28–53 days. Ovaries were collected within 24 h of the final treatment. The characteristic ovar-ian phenotypes of ERα-null mice (shown in C) are prevented by reduc-ing circulating LH levels, indicating these more dramatic phenotypes are secondary to the loss of ERα actions in the hypothalamic pituitary axis that are necessary to maintain proper LH levels and not due to the loss of ERα within the ovary. (Source: Reproduced from Ref. 415.) (E) Ribonucle-ase protection assays for the steroidogenic enzymes in adult wild-type (WT) and ERα-null ovaries treated either with vehicle (V) or a GnRH antagonist (A) every 48 h for 12 days (as described above). ERα-null ova-ries exhibit increased expression of Cyp17 and Cyp19, both of which are reduced to normal after reduction of gonadotropin levels. ERα-null ova-ries also uniquely express Hsd17b3, a Leydig cell–specific enzyme, and this expression is dependent on gonadotropin stimulation as it is ablated after the reduction of circulating LH levels with GnRH-antagonist treat-ments. All samples are normalized to β-actin (Actb) mRNA levels. Source: Reproduced with permission from Ref. 431.



and mast cell infiltration in the interstitium134,158,373,430 (Figure 25.17) (Table 25.7). The cystic and hemorrhagic follicles likely originate from antral follicles that fail to ovulate due to acyclicity (i.e., lack of an LH surge) and therefore become atretic and accumulate within the ovary. They are characterized by a mural granulosa cell layer of one to several cells thick surrounding an enor-mous fluid-filled antrum containing blood and immune cells; a degenerating ovum if visible at all; elevated FSH-receptor and LH-receptor expression in the granulosa cell layer, and a hypertrophied theca (Figure 25.17) that also exhibits elevated LH-receptor levels.373,415 There-fore, ERα is not required for the recruitment and early growth of follicles or the induction of gonadotropin receptors in thecal and granulosa cells, but is vital to the later stages of folliculogenesis in the mouse ovary.158,373

ERα-null (αERKO) females exhibit a severely dis-rupted reproductive hormonal milieu, which in toto represents the cause and effect of the overt ovarian phe-notypes415,431 (Table 25.7). Plasma LH level in αERKO females is elevated three to eight-fold relative to their wild-type littermates, while FSH levels remain nor-mal.431 This is interesting in light of the fact that ovariec-tomy of normal WT mice leads to elevation of circulating FSH and suggests that the αERKO differs from ovari-ectomized WT females. Therefore the ovarian pheno-types of thecal hypertrophy,373 elevated steroidogenic enzyme expression, and increased sex steroid synthesis in αERKO females are congruent with hypergonado-tropic-hypergonadism.431 A synopsis of evidence that supports acyclicity and chronically elevated LH as a primary cause of the other ovarian phenotypes in ERα-null females includes: (1) the cystic and hemorrhagic fol-licles appear after the onset of puberty (approximately 40 days of age),373 (2) the cystic follicles and elevated ste-roidogenesis (Figure 25.17) are prevented when plasma LH levels are reduced to normal via treatments with a GnRH antagonist,415,432 (3) transgenic mice possessing elevated LH but functional ERα exhibit a comparable ovarian phenotype433–435 that is rescued by periodic ovulatory doses of exogenous hCG to induce luteiniza-tion,436,437 and (4) immature αERKO females successfully ovulate, albeit with a reduced number of oocytes com-pared to wild-type mice, and form some corpora lutea when treated with exogenous gonadotropins prior to the onset of the cystic phenotype.415,430,432 Others pro-pose that aberrantly high intraovarian histamine levels due to infiltration of mast cells in the interstitium may also contribute to cyst formation in ERα-null ovaries.158 Therefore, the previous findings indicate that a primary role of ERα in murine ovarian function may be extra-ovarian, i.e., as an essential mediator of the endocrine actions of E2 in the hypothalamic–pituitary axis that are critical to gonadotropin regulation. Indeed, prolonged treatment of female rodents with antiestrogens that

cross the blood–brain barrier (e.g., ZM-189,154, EM-800) and produce an αERKO-like gonadotropin profile lead to a similar ovarian phenotype; whereas treatments with tamoxifen, a receptor antagonist that does not alter gonadotropin secretion generates no such effects in the ovary.217–219

A prominent phenotype in αERKO ovaries that may be indicative of an intraovarian role for ERα is their elevated capacity to synthesize androgens.431 Relative to their wild-type littermates, αERKO females possess plasma levels of androstenedione and T that are increased 3 and 40-fold, respectively.431 Indeed, LH is the primary stim-ulus of thecal cell androgen synthesis and plasma LH and thecal cell LH-receptor levels are both significantly increased in αERKO females (as discussed previously). Therefore, it is not totally unexpected that αERKO ova-ries exhibit increased expression of the enzymes neces-sary for androgen synthesis, most notably a remarkable increase of Cyp17 expression,431 the enzyme necessary for the final step of androstenedione synthesis (Figure 25.17). However, the elevated plasma androgens found in αERKO females is likely not due solely to LH-mediated hyperstimulation of the theca. E2 from granulosa cells is proposed to mediate an intraovarian short feedback loop upon thecal cells to negatively modulate androgen synthesis during the later stages of folliculogenesis by primarily targeting CYP17 activity.438 Given that ERα is the dominant ER form expressed in rodent thecal cells, the elevated Cyp17 expression found in αERKO ovaries suggests ERα mediates this effect of E2 on androgen syn-thesis. Additional support for a specific ERα-mediated effect on thecal cell steroidogenesis comes from our find-ings that chronically elevated LH in wild-type and ERβ-null females leads to a much more moderate increase in Cyp17 expression and androgen synthesis relative to αERKO females.439 Furthermore, individually cultured ERα-null follicles in vitro secrete substantially more androgens relative to similarly propagated wild-type follicles even though the level of gonadotropin stimula-tion is held constant.440 Therefore, it may be concluded that ERα is paramount to maintaining proper androgen synthesis in rodent females via: (1) endocrine actions in the hypothalamic–pituitary axis that negatively modu-late LH secretion, and (2) intraovarian actions on thecal cells to negatively modulate Cyp17 expression.

The granulosa cell–specific enzymes necessary for E2 synthesis, Cyp19 and Hsd17b1, are also expressed at elevated levels in αERKO ovaries.431 As a result, plasma E2 levels are typically increased almost 10-fold relative to wild-type littermates.431 It is likely that the plasma androstenedione levels discussed earlier would be even greater in αERKO females if ovarian aromatase activity was not also equally elevated and providing for effi-cient conversion.431 Although these findings in αERKO females suggest that ERα may also negatively modulate



E2 synthesis in granulosa cells, there is little precedent for this hypothesis. The chronically elevated plasma LH in αERKO females is also not likely to positively influence granulosa cell E2 synthesis as most evidence indicates that LH stimulation of granulosa cells leads to decreased aromatase activity.441–443 Furthermore, indi-vidually cultured ERα-null follicles in vitro continue to exhibit heightened E2 synthesis relative to similarly propagated wild-type follicles in an environment of con-trolled FSH and LH stimulation.440 Instead, increased aromatase activity in ERα-null ovaries is likely due to the positive actions of estrogens on FSH-induced granu-losa cell steroidogenesis442,444–447 which are presumably mediated by ERβ and therefore remain intact in ERα-null ovaries. Androgens have also been shown to augment FSH induction of E2 synthesis442,448,449 and therefore the elevated androstenedione and T levels characteristic of αERKO females may also contribute to increased E2 syn-thesis in granulosa cells.

The several attempts to induce ovulation of ERα-null females collectively indicate an age-dependent effect of the loss of ERα. Schomberg et al. report that 4-month-old αERKO females do not successfully ovu-late following exogenous treatments with PMSG and hCG, although this conclusion was based solely on the absence of corpora lutea rather than actual oocyte num-bers.373 Two later studies focused on younger αERKO females (3–5 weeks) and produced comparable results of successful ovulation (oocytes in the oviduct) and for-mation of functional corpora lutea, although the oocyte yield was reduced compared to age-matched wild-type females.415,430 Oocytes harvested from ERα-null females successfully undergo in vitro fertilization, indicating that ERα may not be important to oocyte function.415 In contrast, Dupont et al. report that the other line of ERα-null females (ERαKO) fail to ovulate or exhibit corpora lutea following exogenous gonadotropin treatments even at 21–25 days of age.158 Studies in immature αERKO females of pure C57BL6 background (versus the mixed background of earlier studies) found that these mice can elicit a response to gonadotropin-induced ovulation.432 This discrepancy in in vivo ovulatory success between the αERKO and ERαKO mice may be due to differences in genetic strain, the considerable disparity in the doses of gonadotropin used, or the small sample size in the Dupont et al. (n = 3). In further support of a minor role for ERα in ovulation, Emmen et al. demonstrated that individual ERα-null follicles propagated and induced to ovulate in vitro behave no differently than similarly cul-tured wild-type follicles.440

The discrepancies in the ovulatory response described herein may also be due to the splice variant of ERα expressed in the αERKO that was able to respond to exogenous gonadotropins (albeit reduced number [∼15 oocytes] compared to wild-type [∼41 oocytes]).415 These

mice were found to have expressed a splice variant in some tissues, and to circumvent this, a new ERα-null mouse model was made by Cre/loxP-mediated recombi-nation (Ex3αERKO).159 The ovarian phenotype and hor-monal milieu of these mice is similar to that observed in the αERKO mice,134,157 however the ability of these animals to respond to exogenous gonadotropins has not been fully studied to date. Preliminary data suggest reduced response to exogenous gonadotropins, as less than 50% ovulate and those that do respond ovulate a significantly lower number of oocytes (preliminary data Korach laboratory); however, further work is necessary to confirm this finding and fully characterize the ovula-tory response of the Ex3αERKO mice.

Recently, a woman with homozygous mutation of ERα leading to severe E resistance was reported.188 It is interesting to note that, similar to the ERα null mouse lines, she had cystic ovaries and elevated E, but unlike the mouse models, her LH and T levels were normal or nominally elevated. This is perplexing in light of the mechanisms discussed previously proposing the key role of LH elevation in causing the hemorrhagic cystic ovar-ian phenotype, and suggests notable interspecies differ-ences in the mechanisms underlying these processes.

MICE WITH OVARIAN-SPECIFIC DELETION OF ERαIn rodent species, ERα is expressed predominately

in theca cells, while ERβ is expressed in granulosa cells. Global loss of ERα in mice leads to a severe ovarian phe-notype with large hemorrhagic and cystic phenotype due increased serum LH from loss of negative feedback in the hypothalamic–pituitary axis. While examination of ovarian function in these global ERα-null mice has provided important data on the necessity of ERα for ovarian function, the models make it difficult to delin-eate the importance of ERα specifically in the theca cells of the ovary. To circumvent this, Bridges et al. generated a mouse model with Cre-recombinase under control of the Cyp17 promoter so that it is expressed specifically in the-cal cells in the female mouse, and when crossed with a ERαf/f mouse deletes expression of ERα in thecal cells in the ovary.361 Further characterization of this mouse and ovarian function in the absence of ERα demonstrated that these mice prematurely lose fertility.362

Theca cell–specific ERα knockout (thEsr1KO) mice did not have an altered estrous cycle, fertility as mea-sured by the number of offspring born, or ovarian response to exogenous gonadotropins (superovulation) in animals aged 2 months; however, these measures were significantly reduced in thEsr1KO females at 6 months of age (Table 25.7).362 In response to exogenous gonado-tropins the thEsr1KO animals release similar numbers of oocytes to WT at 2 months of age (27 in WT versus 22 in thEsr1KO); however, by 6 months there are signifi-cant reductions in oocytes released (22 in WT versus 6 in



thEsr1KO) and the ovaries show an increase in the num-ber of cystic/hemorrhagic follicles.362 Additionally, the oocytes collected from the thEsr1KO ampulla appeared to be more degenerated than WT oocytes, however, further studies are required to examine the viability of oocytes collected from thEsr1KO mice. The age-related reduction in fertility coincides with an increase in the length of estrous cycle these animals have at 6 months of age,362 demonstrating that ERα is important for mainte-nance of fertility, but its expression in thecal cells is dis-pensable for normal ovarian response in younger mice.

Global ERα-null mice have increased serum LH, which contributes to the cystic and hemorrhagic phe-notype presumably due to loss of negative feedback in the hypothalamic–pituitary axis. Loss of ERα specifically in the theca cell layer leads to a reduction of serum LH in mice at 2 months of age, and a further reduction at 6 months.362 This reduction in LH in the thEsr1KO ani-mals was not due to reduced gonadotropes in the ante-rior pituitary, as there was actually an increase in the number of gonadotrope cells. Moreover, no difference is seen in the amount of LH stored in these cells as exam-ined by immunofluorescence.362 This confirms that the excess LH in the global ERα-null models is due to loss of ER in the hypothalamic–pituitary axis, and suggests that ERα expression in the ovary may be important for positive feedback and/or normal LH secretory patterns, although this needs to be tested experimentally.

Previous work has suggested that androgen synthe-sis is negatively regulated by estrogen through para-crine signaling actions, which can be examined in the thEsr1KO mouse model. Examination of serum hormone levels at both 2 and 6 months shows that T is increased,362 confirming that ERα is necessary for maintenance of nor-mal T levels. Examination of the expression of several steroidogenic enzymes found that Cyp17 expression was increased at both 2 and 6 months following PMSG and hCG stimulation, while Cyp19 expression was not signif-icantly different between WT and thEsr1KO animals.362 While expression of Cyp17 is increased, the protein con-centration of CYP17 was also increased in thEsr1KO ovaries in both aged animals and younger mice. CYP17 levels were also examined via immunohistochemistry during diestrus, when CYP17 activity and expression should be minimal, and increased CYP17 was observed in interstitial cells in the thEsr1KO ovary compared to wild-type ovaries.362 The collective data support the notion that ERα signaling is necessary to support proper androgen synthesis in the ovary.

MICE WITH MUTATED DNA BINDING DOMAINS OF ERα

As discussed earlier in this chapter, the ERs may act as transcription factors on certain genes in the absence of a classical ERE within the regulatory sequences. This

alternative mechanism of ER action, often referred to as “tethering”, is thought to occur via interaction with tran-scription factors that are in fact bound directly to DNA (Figure 25.4). To develop an in vivo model for the study of ERα signaling via ERE-independent pathways, Jakacka et al. generated mice that possess a mutant form of the zinc-finger of the ERα DBD.132 Because this mutation was incorporated into the endogenous Esr1 gene (i.e., a “knock-in”), transcriptional expression of the mutant ERα is presumably no different than that of an undis-rupted Esr1 gene. Female animals that are heterozygous for the ERα mutation, termed NERKI+/− or ERαAA/+ mice, are infertile and exhibit distinct ovarian pheno-types characterized by follicles of all stages of growth but no corpora lutea and lipid-filled cells throughout the stroma (Table 25.7).132 The latter phenotype is similar to that of female mice lacking functional steroid acute regulatory protein (STAR) and correlates with reduced StAR expression in NERKI+/− ovaries.132 Upon stimula-tion with exogenous gonadotropins, NERKI+/− ovaries exhibit multiple large hemorrhagic follicles similar to those found in ERα-null ovaries.132 NERKI+/− females exhibit normal plasma gonadotropin and E2 levels and proper expression of steroidogenic enzymes in the ovary.132 Still, induced ovulation in NERKI+/− females is only partially successful in causing some oocytes to be released and the formation of corpora lutea, as sev-eral preovulatory follicles fail to rupture and become hemorrhagic.132

Without further data it is difficult to determine the exact cause of the NERKI+/− ovarian phenotypes. Jakacka et al. postulate that the mutant ERα may act as a dominant-negative form of ER, however, the mutant ERα does not exhibit this property in vitro.132 Further-more, prolactin (Prl) is a known ERE-regulated gene and exhibits no change in expression in NERKI+/− females.132 An imbalance between ERE-dependent and -indepen-dent pathways or tissue-specific inhibitory effects of the mutant ERα may also explain some of the resulting phenotypes.132

To further explore the role of the DNA binding domain in ERα function in the ovary, both the ERAA/− (also called “KIKO”) and EREAAE mouse models (described in the section Mice with Mutated DNA Binding Domains of ERα) were developed,162,168 both expressing only ERα that is unable to bind directly to ERE DNA motifs. KIKO were found to be anovulatory and infertile (Table 25.7).162 Further characterization of the ovaries of these mice found follicles at most stages of development, how-ever, there was a lack of corpora lutea in the ovary sec-tions. The lack of corpora lutea and an estrous cycle in these mice suggests that they do not ovulate, however, the ability of these mice to respond to exogenous gonad-otropins was not examined. These animals have some of the ovarian phenotypes identified in the NERKI+/−,



demonstrating that the DNA binding activity of ERα is important for normal ovarian function. ERαEAAE/EAAE females are infertile and have a hemorrhagic and cys-tic ovarian pathology (Table 25.7)164 similar to the ovar-ian phenotypes observed in the other models that carry a mutation in the DNA binding domain. While further characterization of the ovarian phenotype in these ani-mals is necessary to determine the exact cause of the ovarian phenotypes, the data to date suggest that DNA binding is an essential activity of ERα in normal ovarian function.

The KIKO animals have normal levels of E2 and P (Table 25.7),162 and hormone levels were not examined in the ERαEAAE/EAAE mouse model although ovarian phenotypes were similar between the two mouse mod-els,164 suggesting that regulation of serum steroid levels are not dependent on ERα DNA binding activities. Fur-thermore, the role of the DNA binding domain in ERα in tissues other than the ovary (such as the pituitary) could contribute to some of the observed phenotypes. Tissue-specific knock-in mutations to examine the role of direct DNA binding of ERα in ovarian function experiments could provide more mechanistic properties of the role of ERα, however, these experiments have not been done to date.

MICE WITH MUTATED AF-1 OR AF-2 DOMAINS OF ERαWhile the models presented to date discuss the role of

the DNA binding domain of ERα, there are other impor-tant domains of the nuclear receptor that may also con-tribute to the role of ERα in the ovary. To examine this, two different mouse models were developed with muta-tions in the ligand binding domain of ERα. In the first model, a single point mutation was made in the LBD (Figure 25.1) where a leucine was substituted in place of residue 525 where normally a glycine resides. This mutation alters the binding pocket, preventing normal ligand binding due to the increased side chain present in leucine compared to the single methyl group present in glycine. The knock-in mouse model ERαG525L, hereaf-ter referred to as ENERKI, is anovulatory with a hem-orrhagic and cystic ovarian pathology (Table 25.7).168 The animals were found to have an increased number of atretic antral follicles, which presumably became the hemorrhagic/cystic follicles observed due to lack of rupture. Furthermore, these animals did not have any corpora lutea present, similar to the ERα suggesting that they do not ovulate,168 demonstrating the importance of ligand binding ERα for normal ovarian function.

The AF-1 and AF-2 domains of ERα are impor-tant for ER function as demonstrated in vitro reporter assays. Mouse models were developed with each AF domain deleted, and uterine function was determined as described earlier in this chapter; however, the ovar-ian phenotype or hormone levels of these mice were not

reported (Table 25.7).165,167 To examine the importance of the AF-2 domain, a knock-in mutation in the AF-2 domain of ERα was made such that E2 is no longer able to transactivate E-dependent transcription.169 This two-point mutation in the AF-2 domain was introduced into a knock-in mouse referred to as AF2ERKI/KI. The mice are anovulatory and infertile (Table 25.7),169 similar to the global αERKO knockout mouse models developed to date.134,157,159 The ovaries of these mice also resemble the ovaries in ERα-null mouse models, in that they are hem-orrhagic and cystic and lack presence of corpora lutea.169 Furthermore, the endocrine milieu of these mice is simi-lar to the global ERα-null animals; the AF2ERKI/KI mice have elevated serum LH and E2 (Table 25.7)169 presum-ably due to lack of negative feedback in the hypotha-lamic–pituitary axis. The ovarian functions of other mice with AF-1 or AF-2 disruptions (ERαAF-10 and ERαAF-20) were not reported (Table 25.7). These models suggest that the ligand binding domain of ERα is required for normal ovarian function; however, the results differ slightly from the tissue-specific ERα knockout mouse described earlier. Therefore, further work using a tissue-specific knock-in mouse model would be useful in deter-mining the mechanistic role of the different domains of ERα in the ovary.

MICE LACKING ERβSeveral facets of ovarian physiology thought to be

dependent on the paracrine actions of E2 are clearly maintained in ERα-null and ERα-mutated ovaries, including granulosa cell proliferation, LH-receptor and aromatase expression, antrum formation, and the attenuation of atresia. The preservation of these estro-gen actions in ERα-null ovaries strongly suggests their dependence on ERβ, which continues to be expressed in αERKO granulosa cells.373,431 Neonatal ovaries from ERβ-null mice exhibit no gross abnormali-ties,134,158,171,450 however, there are some differences in the expression of extracellular matrix proteins at this time of development.451 Adult ERβ-null ovaries pos-sess all stages of folliculogenesis,158,171,440,450 a slight but perceptible increase in atretic follicles,171,440,450 and a paucity of corpora lutea171,440 (Table 25.7; Fig-ure 25.17). Cheng et al. found that among 5-month-old ERβ-null females, less than 30% exhibit corpora lutea compared to 100% incidence in wild-type litter-mates.450 Furthermore, when corpora lutea are pres-ent in ERβ-null ovaries, there are rarely more than two versus upwards of seven in wild-type females.450 Con-tinuous mating studies with ERβ-null females indicate a severe impairment in fecundity, but there is discern-ible variability in this phenotype among age-matched animals.158,171 Consistent among βERKO pregnancies is reduced litter size of approximately one-third rela-tive to wild-type.158,171



However, whereas some βERKO females exhibit the expected number of pregnancies over a 4 month period, others become pregnant only once while still others exhibit total infertility.171,172 This variability in fecun-dity among ERβ-null females remains puzzling but is reported in both independently generated lines.158,171 Given that the ERβ-null lines represent distinct target-ing schemes for the Esr2 gene on different genetic back-grounds, the observed variability in fertility is likely related to the loss of ERβ functions. A similar variabil-ity in oocyte yield is found when immature ERβ-null females are induced to ovulate by exogenous gonado-tropin treatments.158,171,172,432 A third ERβ-null mouse model was recently developed in which all mice were found to be infertile and unable to respond to exogenous gonadotropins.172 These animals also had a significant reduction in the number of corpora lutea present in the ovaries of the ERβ-null mice.172 While subfertile ver-sus infertile phenotypes are observed in the different ERβ-null mouse lines, it should be noted that although some mice did produce pups and respond to exogenous gonadotropins, the number was significantly lower than that observed in WT mice, demonstrating the impor-tance of ERβ in normal ovarian function. Therefore, the observed sub/infertility in ERβ-null females appears to originate from disrupted ovarian function that is best characterized to date as infrequent and inefficient spon-taneous ovulation. A lack of evidence indicating embryo resorption during gestation in mated βERKO females171 rules out an extraovarian contribution, although such phenotypes have not been categorically studied.

Histological evaluation of ERβ-null ovaries follow-ing induced ovulation reveals multiple preovulatory follicles possessing underdeveloped antra and minimal cumulus expansion.158,171,172,432 Krege et al.171 report obvious corpora lutea in immature βERKO ovaries 20 h after hCG treatment, but Dupont et al. remark that no luteal cells were observed in similarly treated ERβKO ovaries that did not ovulate.158 However, immature βERKO females do indeed exhibit functional corpora lutea following induced ovulation, as indicated by histo-logical evaluation of the ovaries and a substantial rise in plasma P levels following induced ovulation.432 Lutein-ization and increased P synthesis is also observed when individually propagated ERβ-null follicles are exposed to an hCG bolus in vitro.440 Furthermore, luteinization occurs in unruptured preovulatory follicles both in vivo and in vitro,134,171,440 resulting in “trapped” oocytes simi-lar to those observed in other null mouse models.176,452

ERβ-null females exhibit a relatively normal repro-ductive endocrine milieu.431 Basal gonadotropin lev-els are within the range of control values,431 although one report remarks that plasma LH levels are slightly increased in βERKO females.453 This notwithstand-ing, ERβ-null females clearly do not exhibit the ovarian

phenotypes that are associated with chronically elevated LH.415,435 Therefore, ERα is clearly the principal receptor involved in mediating the negative feedback effects of E2 on gonadotropin secretion from the hypothalamic–pituitary axis. Sex steroid hormone levels in ERβ-null females are also comparable to wild-type littermates.431 Additionally, the overtly estrogenized uterine and vaginal tissues observed in ERβ-null females indicate sufficient ovarian estrogen synthesis in the absence of functional ERβ.134 This preservation of normal E2 levels in βERKO females is surprising given the evidence that estrogens are necessary to maximize FSH stimulation of aromatase activity in granulosa cells.442,444–447

Recent studies into the role of ERβ in normal ovula-tory function have found that ERβ is necessary in gran-ulosa cells for normal response to both gonadotropins FSH and LH. Granulosa cells isolated from ERβ-null ovaries and grown in culture had reduced levels of cAMP in response to FSH stimulation compared to gran-ulosa cells isolated from WT ovaries.454 The reduced sig-naling response to FSH was also observed in granulosa cells from in vitro–stimulated ovaries, and whole fol-licles grown in vitro.454,455 The reduced cAMP accumu-lation after FSH stimulation coincided with a reduction of LH-receptor (officially Lhcgr) mRNA accumulation.455 This suggests that granulosa cells in ERβ-null ovaries are unable to stimulate increased LH-receptor expression, which could contribute to the inability of these cells to respond to LH intracellular signaling components363,455 and ultimately ovulate. Still, βERKO females clearly do not exhibit a normal ovarian cycle, as evidenced by vagi-nal smears indicating animals in persistent estrous and the paucity of corpora lutea in the ovary. This suggests the βERKO female mounts a preovulatory rise in E2 that is insufficient to induce the gonadotropin surge from the hypothalamic–pituitary axis, and/or a diminished abil-ity of the βERKO ovary to respond to the LH surge. As mentioned before, androgens also augment FSH-induc-tion of E2 synthesis442,448,449 and provide for compensa-tory actions in ERβ-null granulosa cells.

MICE LACKING ER α AND βCongruent with the loss of ERα, αβERKO females are

infertile and do not spontaneously ovulate.158,174 Adult αβERKO ovaries possess structures appropriate to the normal ovary, including primordial and growing fol-licles, although the latter possess an underdeveloped antrum, reduced granulosa number and a thin poorly structured theca (Table 25.7).158,174 Cystic follicles that are somewhat characteristic of those found in ERα-null ovaries are also present but are not as large or as hemor-rhagic in αβERKO ovaries and are more apt to exhibit only a thin layer of granulosa cells.158,174 We have used mice with chronically elevated expression of LH from a transgene (α-LHβCTP mice) to show that the formation



of hemorrhagic and cystic follicles in the mouse ovary are due to ovarian acyclicity compounded by chroni-cally elevated plasma LH in the presence of normal FSH levels.435,437 The occurrence of hemorrhagic follicles in α-LHβCTP transgenic ERα-null mice and the absence of hemorrhagic follicles in α-LHβCTP transgenic ERβ-null mice439 demonstrates a role of ERβ in development of the cysts. Therefore, the absence of hemorrhagic and cystic follicles in αβERKO ovaries indicates an impor-tant intraovarian role for ERβ in this pathology. A recent compound αβERKO mouse model that does not express the splice variant for ERα159 has elevated plasma LH and hemorrhagic and cystic ovarian phenotype observed in the ERα-null mice.456 The discrepancy observed in these mice suggests that the presence of the splice vari-ant, genetic background, or mode of knockout in the mouse models may contribute to the ovarian pathology observed. Further studies are necessary to determine the role of ERα or ERβ in development of hemorrhagic fol-licles within the ovary.

A most remarkable feature of adult αβERKO ova-ries is the presence of seminiferous tubule-like struc-tures that often occupy large portions of the gonad but are conspicuously absent in ERα- or ERβ-null ova-ries158,174 (Figure 25.18). Morphological observations

indicate these structures are the “ghosts” of atretic follicles as they possess an intact basal lamina, par-tial layers of granulosa cells, and an invariably degen-erating oocyte174 (Figure 25.18). Furthermore, these structures are postpubertal as Couse et al. report no such structures in αβERKO ovaries at 10 days of age174 and Dupont et al. concur in another line of ERαβKO ovary as late as 23 days of age.158 The defining characteristic of the “ghost” follicles that led to their ini-tial description as testis-like seminiferous tubules is the overt presence of Sertoli-like cells in the lumen (Table 25.7 and Figure 25.18).158,174 Several morphological fea-tures of these cells in both αβERKO and ERαβKO ovaries are strongly indicative of a Sertoli cell phenotype (Figure 25.18), including: (1) alignment with the basal lamina of the follicle wall, (2) a tripartite nucleolus, (3) numer-ous veil-like cytoplasmic processes extending inward toward the lumen, (4) ectoplasmic specializations that are unique to Sertoli cells in prepubertal testis, and (5) immunoreactivity for Müllerian-inhibiting substance and sulfated glycoprotein-2.

Granulosa and Sertoli cells fulfill analogous game-togenic roles as “nurse” cells in the ovary and testis, respectively, and are postulated to derive from a com-mon embryological precursor cell during early gonadal

FIGURE 25.18 Ovarian phenotype in adult mice lacking both ERα and ERβ. (A, B) Low-power magnification of repre-sentative adult wild-type (A) and ERαβ–null (B) ovaries. The wild-type ovary exhibits all stages of folliculogenesis, including a preovulatory follicle (right) and corpus luteum (right, bottom). The ERαβ-null ovary exhibits a few relatively healthy maturing follicles but is overwhelmed by multiple “sex-reversed” follicles. (C–F) High-power magnification of representative follicles from adult ERαβ-null ovaries (C, 66×; D, 330×). A healthy follicle shows a single oocyte (O), several layers of granulosa cells (GC), an intact basal lamina, and thecal cells (TC) (E, 66×; F, 330×). A representative “sex-reversed” follicle in which there is no lon-ger evidence of an oocyte and the somatic cells have undergone redifferentiation to a Sertoli-like cell (SC) phenotype. Preserva-tion of the basement membrane (BM) of the follicles provides for the “tubular” appearance that assimilates testicular cords. The Sertoli-like cells (SC) possess the characteristic tripartite nucleolus and veil-like cytoplasmic extensions (F). Scale bars: E ×100 μm; D, F ×10 μm. Source: Reproduced with permission from Ref. 174.



differentiation.457 Thus, the origin of the Sertoli-like cells in αβERKO ovaries remains a perplexing question. Do they originate from a bi-potent precursor popula-tion present in the αβERKO ovary at birth, or are they the result of granulosa cell transdifferentiation fol-lowing oocyte death and follicle atresia? A report of extra- follicular Sertoli cells in the ERαβKO ovary158,458 supports the existence of an embryological precursor cell population, yet similar interstitial cell populations are not found in ovaries from the other αβERKO.174 In turn, the following findings that are common to both lines support a pathway of granulosa cell transdifferentiation: (1) the Sertoli-like cells are not apparent prior to 30 days of age, (2) the spherical shape of the “tubules” suggested by two-dimensional histology suggests they originate from a once-healthy follicle in which only the basement membrane remains, and (3) the appearance of Sertoli cells is strongly correlated with oocyte death and fol-licle atresia.158,174 Furthermore, Sox9 is highly expressed in both αβERKO and ERαβKO ovaries174 and localized to the Sertoli-like cells.458 SOX9 is an Sry-related tran-scription factor that is critical to normal Sertoli cell dif-ferentiation during testis development in rodents and humans.459,460 Transgenic overexpression of Sox9 leads to phenotypic male gonadal development in XX mice and therefore acts “downstream” of the Y-linked male-determining gene, Sry.461,462 In strong favor of a pathway of granulosa cell transdifferentiation, Sox9 expression in ERαβKO ovaries is restricted to the granulosa cells of atretic follicles and precedes the overt appearance of Sertoli-like cells but is not detectable in the granulosa cells of primordial follicles.458 Furthermore, those cells expressing Sox9 are initially present with the confines of the follicle “ghosts” and appear in the intrafollicular spaces of the ovary only after the phenotype has pro-gressed to an advanced stage,458 supporting a follicular (i.e., granulosa) origin.

Similar phenotypes of “sex-reversal” are reported in fetal rodent ovaries following: (1) transplantation of the fetal ovary to an adult host,463,464 (2) in vitro exposure to purified Müllerian inhibiting substance (MIS),465 (3) transgenic MIS overexpression in vivo,453 (4) transgenic Sox9 overexpression in vivo,461,462 and (5) following tar-geted disruption of the Wnt4 gene in mice.466 Although the αβERKO ovarian phenotype shares a number of morphological similarities with the previous findings, including aberrant expression of the MIS, SGP-2 and Sox9 genes, a remarkable distinction in the αβERKO ovary is the postnatal onset of the phenotype. The previ-ous descriptions of ovarian sex reversal are reported to occur or even require fetal ovarian tissue.465,467 Therefore, the observed “sex-reversal” adult αβERKO ovaries is the first to be described in the postnatal mouse gonad, indi-cating that the potential of female ovarian somatic cells to redifferentiate into Sertoli-like cells may be present

throughout life in the mouse. Recently, postnatal repro-gramming of ovaries to testis with cells that are positive for SOX9 expression and cells resembling Sertoli cells was reported in adult mice lacking FOXL2.468 This pro-vided a second postnatal model for this ovarian transdif-ferentiation, and suggested that FOXL2 is an important transcription factor necessary for granulosa cell differenti-ation and ovarian function. Interestingly, the Ex3αβERKO mouse model shows loss of Foxl2/FOXL2 expression fol-lowed by increased Sox9/SOX9 expression and appear-ance of Sertoli-like cells in the ovary of adult mice.456,469

A causal link between the loss of all ER function and postnatal morphological ovarian sex reversal is unclear. Dupont et al. report that female mice possessing only one functional ERα allele but lacking functional ERβ also exhibit the αβERKO ovarian phenotype, suggesting a gene-dosage effect for ERα.158 The majority of docu-mented cases of similar phenotypes in the mammalian ovary are preceded by massive germ cell loss.470 The remarkable oocyte attrition that occurs in the αβERKO ovaries suggests a similar mechanism occurs in this model; yet it remains unclear if the progressive loss of germ cells in αβERKO ovaries illustrates the loss of critical intraovarian estrogen signaling that is perhaps dependent upon ERα/ERβ cooperation. A role of direct estrogen/ER actions in gonadal differentiation is sup-ported by reports of sex reversal in turtles and whiptail lizards following developmental exposure to aromatase inhibitors.471,472 The mouse Sox9 promoter lacks an obvi-ous ERE but does possess two consensus binding sites for GATA-1,460 a transcription factor expressed in Sertoli cells during differentiation of mouse testis but repressed by germ cells in adult testis.473 Furthermore, ERα can repress the transcriptional activities of GATA-1,474 sug-gesting that aberrant Sox9 expression leading to Sertoli-cell differentiation in αβERKO ovaries may be due to increased GATA-1 activity following the loss of ERα and other potential oocyte-derived inhibitory factors. As a similar phenotype is observed in FOXL2 knockout ova-ries, it has been hypothesized that Foxl2/FOXL2 is regu-lated by ERE-dependent transcription,468 and loss of this regulation contributes to the Sertoli-cell phenotype in the ovaries of knockout mice. This does not account for the lack of transdifferentiation during ovarian develop-ment or in neonatal animals,158,174,468 which suggests that this “sex-reversal” or transdifferentiation phenotype is a complex trait that requires aberrant regulation/expres-sion of multiple genes. Current studies are underway with the Ex3αβERKO mouse model to look at ovarian gene expression and identify possible candidate genes that may contribute to the granulosa cell transdifferen-tiation observed.456,469

Female αβERKO animals exhibit a gonadotropin pro-file that is similar to αERKO females, although plasma LH levels are noticeably higher. Therefore, the hormonal



milieu in αβERKO females may impact the ovarian “sex reversal” observed.174,431 Still, transgenic mice possess-ing chronically elevated plasma LH also display a pro-gressive loss of oocytes but lack any evidence of ovarian “sex reversal”.435,439,475 Interestingly, αβERKO ovaries do not exhibit αERKO-like increases in ovarian aromatase or E2 synthesis.431 This finding provides further support that elevated LH and precursor substrates, combined with the positive actions of ERβ, lead to enhanced E2 synthesis in ERα-null ovaries—a scenario that would be disrupted in αβERKO ovaries. The dramatic loss of germ cells in the αβERKO ovary may also impact gonadotropin responsiveness of granulosa cells as oocyte-derived fac-tors are known to positively influence steroido genesis;476 further studies are necessary to determine how germ cell loss contributes to the observed phenotype.

MICE LACKING CYP19

In the ovary, CYP19, or aromatase, is expressed exclu-sively in the granulosa cells (Table 25.6). Initial reports of Cyp19-null ovaries indicated no phenotypic difference from WT, suggesting estrogen-independent signaling mechanisms were involved in eliminating the pheno-types of the ERKO mice.477 It was not until Cyp19-null mice were fed a soy-free diet that phenotypes appeared. These observations were some of the first to directly demonstrate dietary estrogen contamination in feed. Therefore, to study the phenotypes associated with loss of estrogens, Cyp19 null are kept on soy-free diets. Under these conditions, Cyp19-null mice have ovaries with all stages of folliculogenesis, including multiple large antral follicles but no corpora lutea at 10–14 weeks of age (Table 25.7).131,156,364 At 21 weeks of age, Cyp19-null ovaries continue to exhibit all stages of folliculo-genesis, but larger follicles possess a reduced number of granulosa cells and become enlarged, cystic, and hem-orrhagic, similar to ERα-null follicles.364 By 1 year of age, Cyp19-null ovaries are characterized by a paucity of normal follicles, an increased number of cystic and hemorrhagic follicles, significant collagen deposition, and massive macrophage infiltration into the intersti-tium (Table 25.7).364 Several findings indicate infertility among Cyp19-null females due to an inability to spon-taneously ovulate, including: (1) absence of corpora lutea at all ages,131,156,364 (2) lack of pregnancies during con-tinuous mating,131 and (3) total failure to ovulate follow-ing exogenous gonadotropin treatments.131 Recently, it has been demonstrated that co-treatment of Cyp19-null mice with E2 and PMSG followed by hCG can rescue the anovulatory phenotype,365 demonstrating the necessity of E2 signaling in addition to the gonadotropins for nor-mal ovulatory function.

Surprisingly, initial descriptions of the ovarian phe-notypes in Cyp19-null females did not include remarks of Sertoli-like cells similar to those found in αβERKO

females. However, Britt et al. later demonstrated that mutant females maintained on a diet free of phytoes-trogens exhibit a clearly exacerbated ovarian phenotype by 6 weeks of age that included hemorrhagic and cystic follicles, seminiferous tubule–like structures possessing Sertoli-like cells, and an almost 40-fold increase in Sox9 expression.194 By 16 weeks of age, the tubule-like struc-tures and Sertoli cells occupy up to 80% of the ovary and are strikingly similar to those found in αβERKO ovaries in both morphology and gene expression.194 Additional supporting evidence of Sertoli-like cells in Cyp19-null ovaries is documentation of Sertoli cell-specific ectoplas-mic specializations and immunoreactivity for Espin, an important component of Sertoli cell junctions in testis.194 Recent microarray analysis has identified 450 genes that are differentially expressed in Cyp19-null ovaries com-pared to wild-type ovaries, with 291 showing increased expression and 159 showing reduced expression.478 Several of these genes have been implicated in testis development, however, further studies and confirma-tion are necessary to determine potential candidates that may contribute to ovarian transdifferentiation. Interest-ingly, a similar phenotype of “sex reversal” has not been described in the ovaries of a second line of Cyp19-null females.131

As expected, the reproductive hormonal milieu in female Cyp19-null mice is severely disrupted. Plasma E2 levels131 and ovarian aromatase activity156 are below the level of detection, supporting the existence of a single aromatase-encoding gene in mice. In turn, plasma T lev-els in Cyp19-null females are 10-fold that found in wild-type littermates.131,156 Increased plasma androgens in Cyp19-null females are likely the combined effect of an accumulation of androgen precursors that occurs in the absence of aromatization to E2, and increased synthesis due to hyperstimulation of the ovarian theca by five-fold increased plasma LH.156,364 Thus, increased plasma LH and androgens are characteristic of mice lacking ERα-mediated estrogen actions following the loss of recep-tor (ERα-null animals) or ligand (Cyp19-null animals). However, one striking difference between Cyp19-null and ERα-null females is the four- to six-fold increase in plasma FSH that is unique to the former.364 Interest-ingly, Cyp19-null females exhibit the above abnormal hormone milieu regardless of phytoestrogen content in the diet,194 suggesting that dietary phytoestrogens may attenuate some but not all of the effects that follow the loss of endogenous E2 synthesis.

Studies have indeed shown that Cyp19-null mice still possess functional ER signaling.479 Tonic E2 replacement therapy in Cyp19-null females over a period of 21 days leads to restored gonadotropin homeostasis, improved follicular development in the ovary, a reduced presence of Sertoli-like cells and Sox9 expression, and restora-tion of ovulation and corpora lutea formation.479,480 In



a similar study, Toda et al. report that E2 administration to Cyp19-null females every fourth day for 4 weeks also ameliorates the ovarian phenotypes but did not restore ovulation.131 Ovulation was partially restored when the mice were treated with E2 in addition to exogenous gonadotropins,365 although oocyte numbers were still significantly lower than those observed in WT mice. Furthermore, treatment of the Cyp19-null mice with a GnRH antagonist and E2 replacement was able to restore normal gonadotropin levels, while treatment with an antiandrogen had no effect. This demonstrates that loss of E2 in these animals contributes to the ovarian pheno-type observed and also indicates that an indirect effect from excess androgens does not directly contribute to the excess gonadotropins, hemorrhagic follicles, or Ser-toli-like cells present in the Cyp19-null mice.481

Intraovarian Roles of Estradiol in Ovarian Function

The importance of the extraovarian or endocrine roles of estrogen signaling in ovarian function has long been recognized and is the precedent for steroid-based oral contraceptives.482,483 In contrast, the significance of estrogen signaling within the ovary is not well under-stood due to the inherent difficulties associated with the study of a particular hormone’s action within the very tissue from which it is synthesized and secreted. Past in vivo investigations employing ER antagonists217–219,484 or aromatase inhibitors484–486 to inhibit hormone recep-tor actions within the ovary have provided informative but often equivocal findings217–219 due to the following limitations: (1) the enormous levels of endogenous E2 in the ovary are difficult to overcome by pharmacological administration of a receptor antagonist, (2) the enormous levels of aromatase activity in the ovary are difficult to overcome by pharmacological administration of an enzyme inhibitor, (3) the effects of a pharmacological ER antagonist due to actions directly in the ovary versus actions in the hypothalamus or pituitary are difficult to discern, (4) the well-characterized agonists (e.g., E2 and DES) comparably activate both ERα and ERβ, (5) early antagonists were not selective for the two known ERs, and (6) the two ERs may respond differently to agonist or antagonist ligands. The generation of ER and Cyp19-null mice, and development of improved ER-selective ligands or SERMs61,487,488 have contributed to better understanding over the years.

The abundance of data on the localization of ERα and ERβ within rodent follicles allows for a simplified model of estrogen actions within the ovary. For example, given that ERα predominates in thecal cells, does it mediate the inhibitory effects of E2 on LH-stimulated androgen synthesis? In turn, as ERβ clearly is the predominant receptor in granulosa cells, is it responsible for mediat-ing the known estrogen augmentation of FSH actions?

This section employs the contributions from studies of the pertinent null- or mutated-mouse models as a plat-form to discuss the current view on intraovarian estro-gen/ER signaling.

GRANULOSA CELL PROLIFERATION

A number of studies in immature hypophysectomized rodents have demonstrated that DES or E2 treatments over 2–4 days leads to a marked increase in ovarian weight489 a significant rise in the number of medium-sized preantral follicles and increased DNA synthesis in both thecal and granulosa cells,422,490–492 indicating a direct and gonadotropin-independent effect of E2 on the ovary. Similar data have been reported follow-ing DES treatment of intact immature rats.493 However, there is equally convincing data that sequential treat-ment of immature hypophysectomized rodents with DES followed by FSH results in a synergistic response in terms of increased ovarian weight and DNA syn-thesis, and elicits several major indices of granulosa cell differentiation, i.e., antrum formation and acquisi-tion of LH-receptor expression, that are not observed with either hormone alone.359,490,491,494 Furthermore, co-administration of an antiestrogen (cis-clomiphene) inhibits FSH-induced increases in ovarian weight and follicular growth in hypophysectomized rats, suggest-ing that ER-mediated effects are necessary for a full response to gonadotropin.495 The synergistic actions of E2 on FSH-induced granulosa cell proliferation have been replicated in vitro,496 although others report that FSH-mediated growth of isolated murine follicles is not affected by anti-E2 antisera or ER antagonist (ICI 182,780).497 The capacity of E2 to enhance the granulosa cell response to FSH is not believed to rely on estrogen-induced increases in FSH-receptor levels, although some evidence indicates such an effect may play a greater role in mice versus rats.359,492,494 These differences will be easier to study with the recently described ERα-null rat,190 which can be used to compare the roles of ERα in rat versus mouse ovarian functions. Still, pretreatment of hypophysectomized rats with antiestrogen (CI628) inhibits FSH-induced increases in granulosa cell FSH receptor levels.418

It is generally agreed that the synergistic response of granulosa cells to estrogens and discovery of ERβ and its substantial expression in the granulosa cells of growing follicles and absence in atretic or lutein-ized follicles strongly suggests that the trophic actions of estrogens are direct and ERβ mediated. Support-ing data of a greater importance of ERβ versus ERα in estrogen-induced granulosa cell proliferation comes from a report of Hegele-Hartung et al. in which the ERβ-selective agonist 8-vinylestra-1,3,5-(10)-triene-3,17β-diol induces increased ovarian weight and granulosa cell proliferation to levels comparable to that elicited by



17β-E2 in hypophysectomized rats; whereas the ERα-selective agonist 3,17-dihydroxy-19-nor-17α-pregna-1,3,5 (10)-triene-21,16α-lactone has little effect.488 However, although these data indicate that E2/ERβ actions may indeed enhance FSH-induced granulosa cell prolifera-tion, several findings question the overall importance of this intraovarian role. First, ERβ-null ovaries exhibit a relatively normal number of growing preantral fol-licles with no obvious reduction in the granulosa cell population.158,171,498 Furthermore, immature ERβ-null females treated with PMSG exhibit the expected increase in the number of small and large preantral follicles.158,440 In addition, granulosa cell tumors, due to the loss of inhibin-α (Inha) in mice continue to proliferate regard-less of the presence or absence of functional ERβ.499 It is possible that ERα may provide some compensatory actions as ERβ-null follicles show increased expression of ERα mRNA.455 Furthermore, Cyp19-null mice are pre-sumably devoid of E2-dependent ER action but exhibit ovaries possessing all stages of follicle growth, each with a normal complement of granulosa cells.364 Therefore, the data suggest that although estrogen-induced granu-losa cell proliferation is likely mediated by ERβ, it is not obligatory to gonadotropin-induced follicle growth in rodent ovaries.

ERβ may facilitate gonadotropin-induced granulosa cell proliferation through the induction of cyclin-D2 (Ccnd2), which enhances progression of the cell cycle from the G0 to S phase. Targeted disruption of the Ccnd2 in mice indicates this cyclin protein is obligatory to gran-ulosa cell proliferation as Ccnd2-null ovaries lack fol-licles beyond the small preantral stage.500 Furthermore, the arrest in folliculogenesis observed in Ccnd2-null mice is not rescued by exogenous FSH treatment in vivo or in vitro, indicating the critical downstream role of cyclin-D2 in gonadotropin-mediated granulosa cell pro-liferation.500 In rats, Ccnd2 expression is localized to the granulosa cells of early growing follicles and is signifi-cantly induced by PMSG or FSH treatments.501 How-ever, E2 also induces Ccnd2 expression in rat granulosa cells, and although the response is less rapid relative to that elicited by FSH, the overall induction by E2 is higher and better sustained over 24 h.501 Furthermore, E2 elicits a concomitant decrease in the expression of Cdkn1b (p27), a cyclin-dependent kinase inhibitor.501 E2 induction of Ccnd2 in undifferentiated rat granulosa cells in vitro is inhibited by the ER antagonist ICI 164,384, indicating this to be an ER-mediated event, presumably ERβ.501 Therefore, parallel pathways of FSH and E2 converge to induce and maintain Ccnd2 expression in granulosa cells, thereby forcing cell cycle progression and subsequent proliferation.501 Interestingly, adult ERβ-null ovaries possess normal granulosa cell expression of Ccnd2450 as well as exhibit the expected induction following PMSG exposure,432 further supporting the hypothesis that FSH

and E2 work in parallel. Therefore, these observations and the absence of any gross deficiency in granulosa cells in ERβ-null ovaries suggests that FSH may play a greater role in inducing Ccnd2 expression and granulosa cell proliferation. Interestingly, E2 specifically induces Ccnd2 expression in keratinocytes via the cAMP/PKA signaling pathway that leads to activation of CREB,502 the same intracellular pathway employed by FSH sig-naling in granulosa cells. These findings suggest that E2 may also function within the FSH signaling pathway to induce Ccnd2 in granulosa cells.

A second mechanism by which E2 may amplify FSH-induced granulosa cell proliferation is via interactions with the ovarian IGF1 axis. Like Ccnd2-null mice, Igf1-null mice exhibit a total failure of follicles to progress beyond the small preantral stage.503,504 This arrest in folliculogenesis in Igf1-null ovaries is primarily due to a >50% reduction in FSH receptor levels in the granu-losa cells, severely hampering their response to FSH.504 However, Kadakia et al. demonstrated that Igf1-null granulosa cells are also refractory to E2-induced prolif-eration and exhibit a 50% lower rate of DNA synthesis and decreased Ccnd2 induction following E2 treatment in vivo.505 The poor response of Igf1-null granulosa cells to E2 may be due to a likewise reduction in ERβ levels as IGF1 is reportedly necessary for the maintenance of ERβ expression in granulosa cells in vitro.506 There are cur-rently no reports describing the levels of ERβ expression in the ovaries of Igf1–null mice. Interestingly, Richards et al. demonstrated that E2 significantly upregulates the expression of several components of the IGF1 signal-ing pathway in rat granulosa cells, including the IGF1 receptor-β subunit, the glucose transporter, Glut-1, and the forkhead family member, FKHR (Foxo1).506 These findings suggest that E2 may enhance IGF1 actions in granulosa cells by regulating targets important for cel-lular energy flow, glucose metabolism, and cell sur-vival.506 In summary, IGF1 actions are clearly necessary to provide for sufficient FSH signaling in granulosa cells, which in turn, is necessary for the induction of aroma-tase activity and E2 synthesis, thereby providing ligand for ERβ to enhance the effects of IGF1, hence forming an autocrine regulatory pathway to promote granulosa cell proliferation.506 The extent to which these findings may translate to the human ovary remains to be determined since IGFII, rather than IGF1, is the predominant and gonadotropin-regulated IGF form in the human ovary.507

The previous discussion provides a compelling argu-ment that the mitogenic actions of E2 on granulosa cells are direct and ERβ mediated. However, thecal cell ER actions may also be involved. For example, ERα in the thecal cells may respond to granulosa cell–derived E2 by inducing the synthesis and secretion of a paracrine-acting growth factor(s) that then becomes the principal stimulus for granulosa cell proliferation. Indeed, Dorrington and



colleagues have proposed that thecal-derived transform-ing growth factor-β (TGFβ) is estrogen induced and stim-ulates granulosa cell proliferation.508,509 Similar examples of growth factor–mediated cooperative actions between theca and granulosa cells are postulated to regulate fol-licle steroid production.510,511 Still, ERα-null follicles do not exhibit any gross deficits in granulosa cell number and possess a normal number of preovulatory follicles following gonadotropin stimulation.415 ERα-null follicles in vitro show normal growth, however, they do not reach the same size as wild-type follicles.440 It is plausible that a dramatic effect on granulosa cell proliferation may only become apparent following the loss of both putative estrogen pathways, i.e., the direct (ERβ-mediated) and indirect (ERα-mediated) actions—hence explaining the unique somatic cell phenotypes observed in the ovaries of compound ER-null158,174 and older Cyp19-null mice364 on a soy-free diet in absence of estrogen ligand.

GRANULOSA CELL DIFFERENTIATION

The process of granulosa cell differentiation that occurs during progression from a preantral to preovula-tory follicle has been an area of intense research. Fully differentiated preovulatory follicles in mammalian ovaries are distinctly characterized by: (1) a large fluid-filled antrum, (2) acquisition of LH responsiveness, (3) significant increases in aromatase (CYP19) activity, and (4) increased inhibin synthesis. Although FSH is the pri-mary stimulus for both granulosa cell proliferation and differentiation, these two processes appear to be inde-pendent as the undersized follicles in Ccnd2-null ova-ries still develop an antrum, possess sufficient aromatase activity, and are responsive to exogenous LH stimula-tion.502 Therefore, granulosa cell proliferation and dif-ferentiation are separate gonadotropin-induced events during folliculogenesis.

The absolute requirement of FSH signaling in the process of granulosa cell differentiation is illustrated by the lack of antral follicles, aromatase and E2 synthe-sis, and granulosa cell LH receptor in mice null for FSH action.420,512,513 Igf1-null follicles exhibit a similar failure to differentiate; this is postulated to be due to a lack of sufficient FSH-receptor expression.503 However, numer-ous studies in rat ovaries have demonstrated that FSH induction of antrum formation,490 aromatase expres-sion and activity,442,444–447 and LH-receptor expres-sion484,514,515 require E2 for maximum effect. FSH acts via a classic heterotrimeric G protein–coupled receptor pathway that may activate multiple intracellular second messenger systems, the most well characterized being activation of adenylyl cyclase, which leads to intracel-lular accumulation of adenosine 3′,5′-monophosphate (cAMP) and activation of protein kinase A (PKA).416,494 The long-recognized synergism between E2 and FSH on granulosa cell differentiation is believed to be due

to the capacity of E2 to both amplify the level of FSH-stimulated intracellular cAMP359,441,494,515,516 and aug-ment the downstream actions of cAMP itself.494,515 The mechanisms involved remain poorly understood and are unlikely to involve estrogen-induced increases of FSH receptor levels but appear more likely to rely on E2’s capacity to positively modulate the granulosa cell adenylyl cyclase system.441,494

ER-mediated estrogen activation of the adenylyl cyclase system and subsequent cAMP-regulated gene expression has been demonstrated in uterine and breast cancer cells.517 Furthermore, in vitro and in vivo stud-ies indicate that ERα or ERβ can activate ERE-dependent gene transcription via the cAMP/PKA signaling path-way in the absence of E2.93,103,518 This pathway involves the CREB protein,519 which is also known to be critical to FSH induction of gene expression.442,520,521 There-fore, given the existing evidence that E2 is required to maximize the FSH response, combined with the known cooperative activity between the ER and PKA signaling pathways, it may be reasonable to expect that a loss of ER action in the ovary would cause severe deficits in granu-losa cell differentiation. Indeed, the ovarian phenotypes in ERβ-null females in terms of antrum formation, aro-matase expression, and LH responsiveness collectively represent an attenuated response to FSH-induced granu-losa cell differentiation in the absence of ERβ. Addition-ally, ERβ-null granulosa cells and follicles have a reduced response to FSH marked by reduced cAMP accumula-tion in vivo and in vitro.454,455 The reduction in cAMP can be overcome when cells are stimulated with forskolin to activate cAMP accumulation,455 demonstrating that altered cAMP/PKA signaling contributes to the reduced granulosa cell differentiation observed in ERβ-null ova-ries and suggests that maximal activation requires cross talk between FSH and ER-mediated signaling pathways. Possible downstream mediators were recently identified in a microarray study,363 providing key targets to further study in the future.

ANTRUM FORMATION

Goldenberg et al. first demonstrated that both FSH and estrogen are required for full antrum formation in preovulatory follicles of hypophysectomized rat ova-ries.490 Wang and Greenwald produced similar data in hypophysectomized mice, reporting that FSH plus E2 leads to an average of >70 large antral follicles per ovary versus an average of six or none when treated with FSH or E2 alone, respectively.492 The requirement of FSH plus E2 for antrum formation holds true when rat preantral follicles are grown in vitro.522 Others have shown that exposure to aromatase inhibitors during gonadotropin-induced folliculogenesis also reduces the number of healthy antral follicles in rats,485,486 although contradictory findings are reported when follicles are



similarly exposed in vitro.523 Independent evaluations of ovaries from three separate lines of ERβ-null mice indicate a paucity of follicles with fully developed antra, even following standard gonadotropin stimula-tion.158,172,440 These data strongly suggest that the vital role of E2 in FSH-induced antrum formation is mediated by ERβ. A role for ERα in antrum formation cannot be excluded but is unlikely given the hallmark phenotype in ERα-null ovaries is the invariable presence of severely oversized antral follicles, lending to their description as “cystic”.157–159,373,415 Furthermore, a role of aberrant LH action cannot be discounted, as mice overexpressing a mutant LH receptor have large cystic and hemorrhagic follicles similar to those observed in ERα-null mice.524 While it is possible to hypothesize that ERβ may con-tribute to the formation of exaggerated antrum due to hyperstimulation of the receptor, the data supporting this idea are not clear. For example, mice that lack both ER forms (αβERKO),158,171 or transgenic βERKO females that have elevated LH,439 do not have large cystic fol-licles, although a new line of compound αβERKO mice (Ex3αβERKO) do have large cystic follicles456 as do the Cyp19-null mice.480 This contradictory data suggests that the mechanism behind antrum formation may not be due solely to overstimulation of ERβ, and multiple mechanisms may exist. Furthermore, mice lacking ERα specifically in the theca cells develop cystic follicles as they age in response to excess gonadotropins, which is not observed in younger mice,362 suggesting a more complex mechanism contributes to antrum formation.

As mentioned, FSH alone will induce the formation of small antra in growing follicles whereas E2 has no such effect in vivo and in vitro, but both hormones are required for full antrum development.490,492,522 Com-pared to the total failure of antrum formation among follicles in the murine models of disrupted FSH signal-ing,420,503,512,513,525 similarly sized follicles in ERβ-null ovaries exhibit an initiation of antrum formation but fail to develop to a full preovulatory stage.158 Quantitative analyses of immature ERβ-null ovaries 48 h after PMSG stimulation indicate an increased number of small antral follicles but fewer large (preovulatory) follicles relative to wild-type females, suggesting an arrest in antrum for-mation.440 Likewise, a large percentage of ERβ-null fol-licles fail to reach maximum size when grown in culture with FSH and E2.440 These follicles also fail to accumu-late maximal cAMP levels after FSH stimulation,455 sug-gesting that multiple signaling pathways may contribute to antrum formation. These data support the role of FSH actions in the initiation of antrum formation, but in the absence of ERβ-mediated E2 augmentation, maximum antrum development is prohibited.

Because so little is known about the physiology of antrum formation, the role of ERβ-mediated estrogen actions is speculative. It has long been proposed that

antrum development follows gonadotropin-stimulated increases in granulosa cell proteoglycan secretion, which act to increase osmotic pressure within the fol-licle and cause the influx of water from the ovarian vasculature.526,527 McConnell et al. report that water movement into mouse follicles in vitro occurs via a transcellular pathway and is likely mediated by aqua-porins-7, -8, or -9 on the surface of granulosa cells.528 Although there is currently no evidence of estrogen regulation of aquaporin expression in the ovary, E2 is known to influence fluid transport in the uterus via regulation of aquaporins -1,332 -2, and -3,529 and 5 and 8.242,530 Furthermore, ERα is principally involved in the regulation of Na+ and water transport across the effer-ent ductules in the rodent testis,531 suggesting ERα may play a similar role in follicles. Antrum formation may not rely totally on hydromechanical mechanisms but is likely to require cell–cell interactions among the granulosa cell layer. Gore-Langton and Daniel dem-onstrated in vitro that rat preantral follicles lacking an intact basement membrane and therefore unable to provide a sealed environment exhibit a reorganization of granulosa cells that is unmistakably antrum-like in response to FSH plus E2, supporting a role for cell–cell interaction during antrum formation.522 A lack of antral follicles in the ovaries of mice lacking the gap junction component connexin-37 (Cx37) further supports a need for cell–cell interactions among granulosa cells.532 E2 is known to dramatically increase the number of gap junctions between granulosa cells533 as well as regulate ovarian expression of certain component proteins.534–536 Additionally, the extracellular matrix may be important to antrum formation. Mice lacking ERβ exhibit altered expression of several extracellular matrix proteins, including increased expression of Collagen 11a1 and Nidogen 2 at both the mRNA and protein level in both prepubertal and adult ovaries,451,500 which may alter the follicle composition and contribute to loss of antrum in these mice. Future studies of a potential regulatory role for E2 in the expression of the aquaporins, connexin, and extracellular matrix proteins may reveal a precise role for ERβ in antrum formation.

AROMATASE AND ESTRADIOL SYNTHESIS

Acquisition of aromatase activity and E2 synthesis is a hallmark of healthy preovulatory follicles in mamma-lian ovaries. In monotocous species, including women, 90% of the circulating E2 is estimated to originate from the dominant follicle in the ovary. Aromatase (CYP19) expression in rat, mouse, bovine, marmoset, and human ovaries, via activation of the gonad-specific promoter II,537 is exclusive to and highest in the granulosa cells of fully differentiated healthy follicles (Table 25.6).400,538–

540 Likewise, E2 levels are consistently highest in the follicular fluid of healthy preovulatory, and substantially



lower in overtly atretic, follicles. The endocrine actions of ovarian-derived E2 are critical to preparing the female reproductive tract for a forthcoming pregnancy and prim-ing the hypothalamic–pituitary axis for generation of the gonadotropin surge. However, there is equally abundant evidence that E2 acts in a paracrine or autocrine man-ner to augment FSH stimulation of aromatase activity in

granulosa cells and thereby positively influences its own rate of synthesis (Figure 25.19). This pattern of CYP19 expression described herein strongly resembles the pro-file of ERβ during folliculogenesis, suggesting that the E2-induced enhancement of aromatase activity in granu-losa cells is ERβ mediated.440

FIGURE 25.19 Synergistic action of sex steroids and FSH in the induction of Cyp19 expression and aromatase activity in rodent granulosa cells. (A) Shown is the effect of in vitro treatment with the synthetic estrogen DES on FSH-induced aromatase activity in granu-losa cells isolated from immature hypophysectomized rats. Granu-losa cells were cultured for 3 days in the presence or absence of FSH (10 ng/ml) with or without increasing concentrations of DES. Three days later, the cells were washed with medium and reincubated for a 5-h test interval in medium supplemented with androstenedione. The accumulation of estrogen (as measured by radioimmunoassay) during this test period is taken as a measure of the level of aromatase activ-ity. C, controls. (Source: Reproduced with permission from Ref. 445.) (B, C) Shown is the effect of FSH and sex steroids on Cyp19 expression and aromatase activity in cultured granulosa cells. Granulosa cells were isolated from 26-day-old rats that had been untreated (unprimed) or treated with E2 (1.5 mg/day) for 3 days (E-primed). Cells were incu-bated in serum-free medium with 5α-dihydroT (DHT) at 20 nM, FSH (50 ng/ml), or FSH in combination with DHT (20 nM), T (20 nM), or E2 (20 nM). Cells were also incubated in medium containing 5% fetal bovine serum (FBS), with FSH alone (50 ng/ml), or in combination with T (20 nM). After 48 h in culture, cells were collected for RNA extraction and evaluation of Cyp19 expression by Northern Blot analy-sis (B) or for measurement of aromatase activity (C). Source: Reproduced with permission from Ref. 442.



FSH is clearly the principal stimulus for the acquisi-tion of aromatase activity in granulosa cells. Once again, female mice lacking FSH (Fshb-null mice)419,541 or FSH receptor (Fshr-null mice)421,513 exhibit phenotypes that are consistent with a severe reduction in circulating estrogens, including uterine hypoplasia and elevated plasma levels of LH. Furthermore, the ovaries of Fshb-null mice exhibit a six-fold reduction in Cyp19 transcripts,419 although these data are not corroborated in Fshr-null mice.421 FSH regulation of Cyp19 expression in granulosa cells is believed to require the actions of multiple transcrip-tions factors, including steroidogenic factor-1 (SF-1) and β-catenin,542 CREB protein, GATA-4 and CBP, the former three of which possess cognate response elements within the rodent and human CYP19 promoter II.441,520,521,543

The mechanism by which E2 augments FSH stimula-tion of aromatase activity or Cyp19 expression in granulosa cells is unclear but shown to occur in isolated granulosa cell cultures,442,444–447 ruling out the influence of a the-cal cell–derived factor and supporting a direct role for ERβ. Coadministration of FSH and an ER antagonist (tamoxifen) to granulosa cells either in vivo or in vitro inhibits the expected induction of aromatase activity;544 although others report conflicting data using a different antagonist (ICI 182,780) in whole follicle culture experi-ments.497 More recently, Emmen et al.440 and Rodriguez et al. found that ERβ-null follicles grown in vitro do indeed secrete significantly less E2 relative to wild-type follicles.455 The absence of a consensus ERE within the rat or human Cyp19 promoter II makes it unlikely that the effect of E2 is via a direct ERβ mechanism (Figure 25.4). However, ERβ may interact with the cAMP/PKA path-way, SF-1, CREB, GATA-4, CBP, or other unknown tran-scription factors that act to promote Cyp19 expression. Indeed, ERβ has been shown to associate with both CREB519 and CBP545,546 in heterologous in vitro systems. The ERs are also known to influence GATA-directed gene expression via direct interactions with multiple members of the GATA family of transcription factors.474,547 There-fore, there is ample evidence to support ER interaction with the multitude of transcription factors known to influence Cyp19 expression, but the exact nature by which ERβ is involved requires further study.

ACQUISITION OF LUTEINIZING HORMONE RECEPTOR

In contrast to the constitutive expression of LH receptor in thecal cells, granulosa cells express LH receptor only in follicles in the late preovulatory stage. This limited expression of LH receptor to the granulosa cells of healthy preovulatory follicles provides for an intraovarian mechanism that ensures only those follicles that are suitable for ovulation acquire the capacity to respond to the LH surge. As with the other markers of follicle differentiation discussed previously, FSH is the

primary stimulus of LH-receptor (Lhcgr) expression in preovulatory granulosa cells. However, FSH exposure alone leads to a minimal increase of LH-receptor lev-els in the ovaries of hypophysectomized rats, and then only after 4 days of treatment; whereas pretreatment of animals with E2 for 4 days prior to FSH exposure leads to an enormous induction of LH-receptor levels that are limited to the granulosa cells of preovulatory follicles (Figure 25.20).359 This requirement for FSH plus E2 and the temporal pattern of LH-receptor expression has been reproduced in rat granulosa cells in vitro, with maxi-mum levels reached after 48–72 h.515,548,549 Lhcgr mRNA levels mirror the LH-receptor protein levels and exhibit a comparable FSH/E2 regulation in rat granulosa cells.550 Further confirmation that estrogens are required to aug-ment FSH-induced LH-receptor expression in granulosa cells comes from studies that have collectively shown the following: (1) only estrogens (e.g., E2, diethylstilbestrol) or substrates for aromatization (e.g., androstenedione, T, estrone) are effective,515,548,549 (2) nonaromatizable androgens (e.g., DHT) and P have little effect (Figure 25.20),515,549 (3) co-treatment with aromatase inhibitors, ER antagonists, or 17β-E2-specific antisera block FSH induction of LH receptor484,514,548 and hCG-induced ovu-lation,485 and (4) Lhcgr levels are decreased five-fold in the ovaries of Cyp19-null mice.479 The poor ovulatory response of ERβ-null ovaries to an ovulatory dose of the LH analog, hCG,171,432 strongly suggests that ERβ medi-ates the synergistic actions of E2 during FSH induction of the Lhcgr gene in granulosa cells.134,171 In contrast, the granulosa cells of large antral follicles in ERα-null ova-ries exhibit abnormally high levels of LH receptor, even after becoming enlarged and cystic.134,373

The complexity of the Lhcgr promoter is illustrated by the differential regulation between thecal and preovula-tory granulosa cells. Not surprisingly, FSH induction of LH-receptor expression in granulosa cells occurs via the cAMP/PKA pathway.551–554 However, the proximal pro-moter of the Lhcgr gene in both humans and rats lacks a consensus cAMP response element (CRE) similar to those that confer FSH regulation of the Cyp19 promoter II.555,556 Studies indicate that a GC-rich region possess-ing a cluster of Sp1 binding sites and an ERE half site contributes to FSH and 8-bromo-cAMP induction of the rat Lhcgr promoter in vitro.557–559 Furthermore, multiple nonconsensus cAMP-response elements within this GC-rich region of the rat Lhcgr promoter provide a plat-form for assembling an undefined complex of nuclear proteins from granulosa cell extracts that confer cAMP induction.556,560 With such limited data concerning Lhcgr regulation, it is difficult to speculate where the actions of ERβ may impact LH responsiveness in granulosa cells. ERβ-null granulosa cells have reduced cAMP accumu-lation after FSH signaling and also show reduced Lhcgr accumulation compared to wild-type granulosa cells.454



The reduced expression suggests it may be downstream of cAMP, and in vitro follicle culture studies found increased Lhcgr expression when follicles were treated with forskolin to induce cAMP accumulation.455

As discussed before, E2 enhances the level and action of FSH-stimulated increases in intracellular cAMP to maximize Cyp19 expression, and it is likely that a similar mechanism is employed for induction of LH-receptor levels. However, there are notable differences in the E2/FSH regulation of Cyp19 versus Lhcgr expres-sion. FSH induction of Lhcgr expression in preovulatory granulosa cells is augmented by estrogens only,515,548,549 whereas nonaromatizable androgens or estrogens act through their respective receptor pathways to enhance FSH induction of Cyp19 expression.442 This divergence in regulation is critical since androgen augmentation of FSH on Cyp19 expression allows for FSH/AR-mediated initiation of E2 synthesis in preantral follicles shortly after thecal cells begin synthesizing aromatizable pre-cursors; whereas the specificity for ER/E2 actions in the FSH induction of the Lhcgr gene provides that LH receptors are acquired only by those follicles that have sufficient E2 synthesis, and are hence healthy and suit-able for ovulation. Secondly, Farookhi and Desjardins549 demonstrated that FSH induction of Lhcgr but not Cyp19 expression is lost when estrogen-pretreated granulosa cells are dispersed in culture by EGTA-disruption of cell–cell contacts.549 A requirement for intact cell–cell contacts for E2/FSH-mediated Lhcgr expression may ensure that atretic follicles, which often exhibit a fragmented and less compact granulosa cell population, do not acquire the capacity to respond to an LH surge and ovulate an unhealthy oocyte.

ESTRADIOL MEDIATED NEGATIVE FEEDBACK ON THECAL CELL FUNCTION

Like granulosa cells, follicular thecal cells also undergo a process of differentiation during progression from a preantral to preovulatory follicle.561 Fully differenti-ated thecal cells are best characterized by their unique capacity to convert cholesterol to C19 steroids, primarily androstenedione.561 A principal element of the two-cell, two-gonadotropin paradigm of steroidogenesis in maturing follicles (Figure 25.21) is that thecal cells exclusively pos-sess the 17β-hydroxylase:C17,20-lyase activity necessary for synthesizing androgens, forcing the granulosa cell layer to be dependent upon the theca as a source of aromatizable precursors.562 Thecal cell synthesis of androstenedione is considered the rate-limiting step in E2 production by preovulatory follicles and involves the sequential enzy-matic actions of P450scc (CYP11), P45017α/17,20-lyase (CYP17) and Δ5-3β-hydroxysteroid dehydrogenase (3β-HSD 1).562 It is generally accepted that LH regulates thecal cell ste-roidogenesis via the induction of CYP17 and CYP11 activ-ity,561 while 3β-HSD 1 is constitutive.562 The requirement of LH stimulation to induce CYP17 activity in thecal cells

FIGURE 25.20 Synergistic action of E2 and FSH in the induction of LH receptor in rodent granulosa cells. (A) Induction of Lhcgr (LH/CG-R) mRNA in granulosa cells of hypophysectomized rats after treat-ment with FSH and/or E2. Animals were untreated (H) or injected with 1.5 mg E2/day for 3 days (HE), 1 mg FSH twice daily for 2 days (HF), or a sequential combination of E then FSH (HEF). Animals were euthanized after treatment and granulosa cells isolated from the ovaries for total RNA extraction. Top: Northern blot of 20 μg RNA/lane probed for Lhcgr (LH/CG-R) mRNA. (PO GC: pro-ovulatory granulosa cell RNA) (Source: Reproduced with permission from Ref. 550.) (B) Effect of antiestrogens on the induction of LH-receptor levels by FSH, forskolin, or 8-bromo-cAMP. Granulosa cells were isolated from immature rats implanted with DES pellets for 4 days. 2 × 105 cells were cultured for 48 h with FSH (100 ng), forskolin (10 μM), or 8-bromo-cAMP (5 nM), with or without E2, 10−8 M in the absence or presence of antiestrogens, keoxifene (K, 1 μM), or tamoxifen (T, 1 μM). Media were then removed and cells were analyzed for LH-receptor content by radiolabeled binding assay. The first bar on the left side of the figure is FSH (A), forskolin (B), or 8-bromo-cAMP (C) alone. Control cells bound less than 10 pg hCG/2 × 105 cells, and the data are not shown. Source: Reproduced with permission from Ref. 514.



is effectively illustrated by the 12-fold reduction in Cyp17 expression in ovaries of LH-receptor (Lhcgr)-null mice,563 as well as the five-fold increase in expression in the ova-ries of transgenic mice that possess chronically elevated LH levels.439 Likewise, chronically elevated plasma LH leads to a comparable increase in Cyp17 expression and androstenedione synthesis in both ERα-null and ERβ-null ovaries and cultured follicles, indicating that neither ER is obligatory to the positive regulation of thecal cell steroidogenesis.431,439,564

Therefore, androgens are necessary to serve as both a stimulus of and substrate for aromatase activity in granulosa cells. However, a proper androgen:estrogen ratio is critical as an intrafollicular milieu of increased androgens is strongly associated with atresia.565 It is gen-erally believed that an intraovarian mechanism exists to negatively modulate thecal cell androgen synthesis so that levels do not surpass the aromatase capacity of the granulosa cell layer. Indeed, defects in this intraovarian mechanism are postulated to be an underlying cause of the excess thecal cell androgen synthesis that is a hall-mark of polycystic ovarian syndrome (PCOS) in women.

There is ample evidence that granulosa cell–derived E2 may mediate a feedback loop on thecal cells to decrease further androgen synthesis in thecal cells,566 however, the mechanism remains unclear. Magoffin posed the question several years ago as to whether E2 inhibition of androgen synthesis occurs at the level of steroidogenic enzyme expression or via direct inhibition of enzymatic activity.561 Indeed, estrogens have been shown to inhibit CYP17 activity in both thecal and Leydig cell prepara-tions while having no effect on activities of the upstream steroidogenic enzymes.566–571 Early studies found that E2 can compete with androstenedione for CYP17 enzymatic activity;568 however, this effect could not be reproduced in ovarian lysates in vitro.572 Additional studies in dis-persed rat ovarian cells found that E2 has no effect on the number of LH receptors or level of hCG-stimulated cAMP synthesis, further supporting a direct E2 inhibition of CYP17 activity as a mechanism for regulating andro-gen synthesis.570 However, other evidence supports a role for ER-mediated regulation of Cyp17 expression at the transcriptional level, including demonstrations that an antiestrogen blocks the expected drop in CYP17

FIGURE 25.21 Model of sex steroid receptor actions in the ovary during follicular differentiation. The process of follicle differentiation from preantral to preovulatory follicle is marked in granulosa cells by the induction of aromatase (CYP19) activity for the synthesis of E2 and LH-receptor (LH-R) expression. This process is principally dependent on stimulation of thecal cells by LH and granulosa cells by FSH. However, there is substantial evidence of a modulatory role for the sex steroid receptors in this process. The above model illustrates the putative expression pat-tern and actions of the sex steroid receptors during follicle differentiation. Some aspects of the above model are supported by experimental data, whereas others are more speculation at this time (see text for details). (I) Preantral Follicle: In undifferentiated preantral follicles, LH stimulation of thecal cells (which constitutively express LH-R) leads to de novo synthesis of androstenedione (A4) and T. A4 and T diffuse across the base-ment membrane of the follicle and into the granulosa cells. A4 may be converted to T in granulosa cells. Thecal-cell derived or locally synthesized T activates the androgen receptor (AR), and this action synergizes with FSH to induce aromatase (CYP19) expression in the granulosa cells. The induction of E2 synthesis commences the process of follicle differentiation. (II) Preovulatory Follicle: Increasing E2 synthesis in the granulosa cells leads to activation of ERβ and induces differentiation by synergizing with FSH to further increase CYP19 expression and E2 synthesis and induce LH-R expression in the granulosa cells. Ligand-dependent ERβ actions may also lead to decreased AR expression, allowing the role of A4 and T to shift from that of a ligand for AR-mediated actions to that as substrates for E2 synthesis. Increasing E2 levels eventually activate ERα in thecal cells to reduce further androgen synthesis by decreasing CYP17 activity, forming a negative feedback loop to ensure the proper estrogen-to-androgen ratio in the follicle. LH Surge: Climaxing E2 levels in the circulation facilitate the release of the LH surge from the pituitary, which stimulates ovulation and terminal differentiation of the granulosa cells. Only those follicles that possess properly differentiated granulosa cells (i.e., have acquired sufficient LH-R) are able to respond to the LH surge, which causes a dramatic rise in PR expression that occurs in the preovula-tory granulosa cells within hours of the LH surge, whereas ERβ and CYP19 levels are both dramatically decreased.



activity that occurs just prior to ovulation in rats,573 and that E2 significantly reduces Cyp17 expression in the tes-tes of rats574 and fish.575 Our findings that ERα-null ova-ries exhibit increased Cyp17 expression and enzymatic activity despite a milieu of significantly elevated E2 also supports a mechanism of ERα-mediated suppression of Cyp17 transcription in thecal cells versus direct inhibi-tion of CYP17 activity.431 Furthermore, Taniguchi dem-onstrated that in vitro–cultured ERα-null follicles have increased Cyp17 expression similar to that observed in WT follicles cultured in the presence of an aromatase inhibitor.564 Treatment of ERα-null follicles with E2 or an ERα-specific agonist as well as an aromatase inhibitor reduced Cyp17 expression to that observed in wild-type vehicle treated follicles.564 Therefore, the excess andro-gen synthesis observed in ERα-null ovaries is due to the loss of ERα-mediated suppression of Cyp17 expression, indicating the existence of intraovarian feedback actions of E2 that are mediated by ERα and critical to maintain-ing homeostasis of androgen levels in the female mouse.

OVULATION

E2 clearly facilitates the acquisition of several gonado-tropin-dependent effects during folliculogenesis, includ-ing: (1) the proper cellular organization of the follicle, e.g., antrum and cumulus oocyte complex; (2) the necessary enzymatic activity, e.g., E2 synthesis; and (3) the essen-tial receptor signaling pathways; e.g., LH receptor. All of these characteristics define a healthy differentiated follicle and are necessary for a proper response to the gonadotro-pin surge and expulsion of a competent oocyte. However, a need for estrogen signaling in the ovulating follicle or following the postgonadotropin surge remains unclear. Selvaraj et al.485 found that coadministration of immature rats with PMSG and an aromatase inhibitor (Fadrazole) leads to a reduced number of healthy antral follicles that culminates with a severely reduced ovulatory response to a bolus of hCG. These data support the intrafollicular role of E2 in preparing competent preovulatory follicles for ovulation. However, in an additional experiment, Selvaraj et al. once again treated immature rats with PMSG but delayed administration of the aromatase inhibitor by 40 h such that exposure occurred just 6 h prior to hCG induced ovulation.485 These animals exhibit an equally poor response in terms of follicular rupture and recoverable oocytes from the oviduct; and this effect is abated by coad-ministration of E2 just prior to induced ovulation. These data in turn suggest that E2 does facilitate the ovulatory process. However, inhibition of E2 synthesis is known to cause severe intraovarian485 and intrafollicular523 accu-mulations of P and androgens that could also lead to a poor ovulatory response. In contrast to the above in vivo findings, Hu et al. reports that isolated mouse follicles grown exposed to an aromatase inhibitor in vitro exhibit a normal ovulatory response to hCG, including mucifi-cation of the cumulus–oocyte complex.523 Furthermore,

detrimental effects of aromatase inhibition such as those reported in the rat ovary are not observed in similarly treated hamsters, rabbits, or monkeys.486

Multiple investigations of the ovulatory capacity of ER-null and CYP19-null mice have been conducted. As described before, ERα-null females are anovulatory throughout life and gonadotropin-induced ovulation is unsuccessful in ERα-null females at 4 months of age when the ovarian cystic phenotype is advanced.373 However, immature (≤28 days of age) ERα-null (αERKO) females do respond to induced ovulation and yield recoverable oocytes in the oviduct at an average yield of 14.5 oocytes/ERα-null female versus 40.6 oocytes/wild-type female.415 Similar data has been reported in ERα-null females as old as 5 weeks of age.415,430 Interestingly, Dupont et al. report that immature females from their ERα-null (ERαKO) line exhibit a total failure to ovulate in response to exog-enous gonadotropin treatments, although this study involved only three females.158 Preliminary studies sug-gest Ex3αERKO mice159 have a significantly reduced ovu-lation response, where less than 50% of mice ovulate in response to exogenous gonadotropins, with few oocytes produced from those that do ovulate (Korach laboratory, unpublished data). These observations suggest that the response observed previously415 was possibly due to the presence of residual ERα splice variant expressed in the αERKO mice,159,189 thus further studies are underway to confirm these preliminary studies.

The cause of the reduced ovulatory response to exog-enous gonadotropins in ERα-null females is unclear and suggests a facilitatory role for ERα in follicular rupture. However, the expected induction of several genes that are critical to follicle rupture, such as P receptor, prosta-glandin-synthase-2,576 occurs in gonadotropin-primed ERα-null ovaries shortly after hCG treatment.415,432 Fur-thermore, ERα-null ovaries form functional corpora lutea following induced ovulation, indicating that granulosa cells of ERα-null preovulatory follicles possess the capac-ity to respond to LH. Therefore, the reduced ovulatory response in ERα-null ovaries is more likely attributable to premature increases in endogenous plasma LH415 and subsequent ovarian androgen synthesis.415,430 Although immature ERα-null ovaries do not yet manifest the overt morphological effects of LH-hyperstimulation, e.g., cys-tic follicles, they do exhibit elevated LH-receptor lev-els.415 Cumulative damage in the ovary from chronic LH hyperstimulation likely causes the age-related reduction in gonadotropin-induced ovulatory capacity in ERα-null females discussed previously. Indeed, Armstrong and col-leagues demonstrated in rats that treatment with PMSG preparations possessing increased LH activity lead to a reduced ovulatory response to subsequent hCG stimu-lation577,578 and that this effect is likely due to inappro-priately high stimulation of androgen synthesis during follicle maturation.578 Studies showing ERα-null follicles allowed to differentiate in vitro under controlled hormonal



stimulation exhibit a rate of hCG-induced rupture that is comparable to similarly cultured wild-type follicles fur-ther indicating that any role for ERα in the ovulatory pro-cess is minimal.440 The ERα was found to be necessary for ovulatory response in aging animals, as thEsr1KO mice had normal ovulatory response at 2 months that was severely dampened by the time the mice were 6 months old.362 The premature loss of fertility in these mice sug-gests that ERα is important for normal ovulatory function; however, the precise mechanism has yet to be elucidated.

The loss of functional ERβ clearly leads to ovarian defects that are inhibitory to ovulation. Both lines of ERβ-null females exhibit severely reduced fertility due to ovar-ian defects; however, unlike ERα-null females, ERβ-null animals do spontaneously ovulate.158,171 Krege et al. found that immature ERβ-null (βERKO) females exhibit a severely reduced oocyte yield of 6 oocytes/female versus 34 oocytes/female among age-matched wild-types following gonadotropin-induced ovulation.171 Dupont et al. report a similar average yield in immature ERβKO females (17.6 versus 37 oocytes/female), even when excluding those females that do not ovulate at all.158 A third line of ERβ-null mice was found to be unresponsive to exogenous gonado-tropins, and no animals were able to ovulate (n = 19).172 Collectively, even though some animals are able to respond with some oocytes ovulated, the models demonstrate the necessity of ERβ for optimal ovulatory response.

Immature ERβ-null ovaries collected 20 h after hCG-induced ovulation reveal multiple preovulatory follicles that retain their oocyte and fail to undergo expansion of the cumulus–oocyte complex.158,171,432 Furthermore, hCG induction of prostaglandin-syn-thase 2 and PR, two events that are critical to follicular rupture, are significantly compromised in immature βERKO ovaries.432 ERβ-null follicles induced to ovu-late in vitro exhibit an equally poor rate of rupture and gene induction following hCG exposure.440 The sum of in vivo and in vitro data from ERβ-null mice strongly indicates an inability to fully respond to an endogenous LH surge or an exogenous bolus of hCG. This is likely due to insufficient acquisition of LH-receptor expres-sion among granulosa cells of ERβ-null preovulatory follicles.432,440,455 Indeed, Clemens et al. found that the ER-antagonist ICI 164,384 inhibits FSH-induced differ-entiation of rat granulosa cells in vitro, as illustrated by a severely compromised induction of PR expres-sion following stimulation of the cAMP/PKA signaling pathway.579 However, if exposure to the ER antagonist is delayed until the time of ovulatory stimulation, lit-tle effect is observed,579 further supporting a role for ERβ in granulosa cell differentiation and LH-receptor expression rather than during ovulation. ERβ-null fol-licles cultured in vitro also have a reduced ovulatory response and reduced cAMP accumulations after FSH stimulation.455 Interestingly, treatment of these fol-licles with forskolin to increase cAMP levels was able

to increase the ovulation of these follicles in vitro,455 suggesting that ERβ is necessary for maximal ovulatory response downstream of gonadotropins, although ERβ is not directly contributing to follicle rupture.

Evidence of any cooperative action between ERα and ERβ in ovulation may be derived from studies of the compound ER-null and CYP19-null mice. The single report of induced ovulation in immature ERα/ERβ-null (ERαβKO) females describes a total failure of ovulation.158 Not surprisingly, ovaries from these females exhibited underdeveloped preovulatory follicles that failed to exhibit cumulus–oocyte complex expansion or luteiniza-tion,158 indicating an attenuated response to hCG similar to that in ERβ-null ovaries. Investigations in both lines of Cyp19-null females produced comparable results of a total failure to ovulate and luteinize.131,580 Interestingly, Huynh et al. remarked that cumulus–oocyte complexes lacking oocytes were recovered from the oviducts of 4-week-old Cyp19-null females following induced ovu-lation, indicating that some elements of follicle rupture may occur after degradation of the oocyte.580 The more severe ovulatory phenotypes in compound ER-null and Cyp19-null females suggest that the intraovarian func-tions of both ERα and ERβ are necessary for ovulation but not necessarily for the process of follicular rupture. Recent studies in Cyp19-null mice suggest that treat-ment with E2 as well as exogenous gonadotropins could stimulate some ovulation,365 suggesting the importance of the steroid and gonadotropin hormones in regulating ovulation.

ERα MEDIATED REPRESSION OF LEYDIG CELL DEVELOPMENT IN THE OVARY

As discussed in earlier sections, female mice lacking ERα action due either to the loss of receptor (αERKO, thEsr1KO, and αβERKO) or ligand (Cyp19-null) exhibit abnormally high levels of plasma T. For example, the average plasma T level in adult αERKO and αβERKO females is 5.5 ng/ml and 2.1 ng/ml, respectively, versus 0.14 ng/ml in wild-type females.431 In females of the two existing Cyp19-null lines, Fisher et al. report average plasma T levels in mutant females to be 2.3 ng/ml ver-sus 0.24 ng/ml in wild-type;156 while Toda et al. report 1.4 ng/ml in the mutant females versus 0.13 ng/ml in wild-type.131 These levels of plasma T in ERα-null and Cyp19-null females surpass the lower limits character-istic of normal male mice (2–10 ng/ml)131,156,581 and are obviously very uncharacteristic of normal females. The mouse ovary possesses the enzymatic machinery nec-essary for T synthesis, although it is not a major prod-uct. The two-cell, two-gonadotropin paradigm of ovarian steroidogenesis (Figure 25.21) postulates that thecal cell–derived androstenedione serves as a substrate for two sequential reactions in granulosa cells, CYP19-mediated aromatization to estrone followed by a reduc-tion to E2 by 17β-hydroxysteroid dehydrogenase type I



(HSD17β1).561 The murine form of 17β-HSD 1 is reported to efficiently reduce androstenedione to T as well as estrone to E2; whereas the human form may not possess this activity.582 Therefore, the accumulation of andro-stenedione that occurs in ERα-null and Cyp19-null ova-ries due to LH hyperstimulation of the theca in both, and the absence of aromatization in the latter,156,431 provides for sufficient substrate for 17β-HSD 1 conversion to T. However, further investigations have discovered that ERα-null431 and Cyp19-null479 ovaries uniquely possess substantial levels of 17β-hydroxysteroid dehydrogenase type III (Hsd17b3), a Leydig cell–specific enzyme in tes-tes that specifically functions to reduce androstenedione to T.583–587 Remarkably, the level of Hsd17b3 expression in αERKO, αβERKO,431 and Cyp19-null479 ovaries is comparable to that found in testes of wild-type males. Therefore, the abnormally high capacity of ERα-null and Cyp19-null ovaries to synthesize T may be attributed to the accumulation of androstenedione and its conversion to T by 17β-HSD 3 rather than 17β-HSD 1.

Hsd17b3 expression in Leydig cells is primarily LH regulated.585 The ectopic expression in ERα-null ovaries is also dependent on LH hyperstimulation as expression is eradicated following prolonged treatment of animals with a GnRH antagonist.431 In ERα-null females, Hsd17b3 expression is limited to the ovarian interstitium and possi-bly the theca.588 However, it is unclear whether these cells are Leydig-like and suggestive of an “organizational” defect in ERα-null ovaries or are thecal/interstitial type cells within which the loss of ERα leads to loss of Hsd17b3 repression, i.e., an “activational” defect. Evidence to sup-port the latter hypothesis includes: (1) testes from ERα-null males also exhibit elevated Hsd17b3 expression and T synthesis,589 (2) female mice possessing chronically elevated LH but intact ERα signaling do not exhibit ovar-ian Hsd17b3 expression,439 and (3) prolonged E2 therapy in Cyp19-null females eradicates Hsd17b3 expression.479 In turn, there exists supporting evidence for the former hypothesis that a defect during ovarian differentiation may occur in the absence of ERα. Thecal and Leydig cells are believed to derive from a common bi-potent embryo-logical precursor cell during gonadal differentiation. In Cyp19-null ovaries, Britt et al. have described the pres-ence of cells resembling the mature Leydig cells of tes-tes, in that they possess a tubulovesicular arrangement of mitochondrial cristae, an annular nucleolus, and an abundance of smooth endoplasmic reticulum in whorl-like formations.194 These Leydig cell–like ultrastructural features were also observed in ERα-null ovaries.588 These Leydig-like cells in Cyp19-null ovaries are located in the interstitial regions and often proximal to the basement membrane of atretic follicles that exhibit Sertoli-like cells in the follicle lumen.194 Indeed, E2 is a potent inhibitor of Leydig cell proliferation in vitro and in vivo,590 and E2 replacement in Cyp19-null females reduces the presence

of Leydig-like cells in the ovaries.479 Therefore, ERα may be critical to promoting thecal cell differentiation and inhibiting the Leydig cell phenotype during ovarian dif-ferentiation. A similar phenotype of Leydig cell differen-tiation, Hsd17b3 expression, and “masculinization” of the ovary and reproductive tract is reported in female mice lacking Wnt4 function.466

DISRUPTED ESTROGEN SIGNALING IN HUMANS

There is one case of a human female that is autosomal recessive for an inactivating mutation of the ESR1 (ERα) but no patients with ESR2 (ERβ) mutation reported. There are cases of human females that are autosomal recessive for inactivating mutations of the CYP19 gene and therefore lack the ability to synthesize E2.591–597 The CYP19 mutated females present with androgen-induced pseudohermaphrodism at birth,591–593 and amenarche, polycystic ovaries, and an absence of female second-ary sex characteristics at puberty.591,593,594 Three reports describe elevated plasma FSH and LH levels, leading to hypergonadism that manifests as increased plasma androgens and virulization.591,593,594 The female with Esr1 mutation has polycystic ovaries and elevated E2. The ovarian pathology exhibited is somewhat similar to that observed in ERα-null females and compatible with a diagnosis of PCOS.593 To date there are no indications of “sex reversal” in terms of male cell types or gene expres-sion in the ovaries of CYP19-null human females. Estro-gen and P replacement effectively alleviates all of the above phenotypes.591,593,594

Polymorphisms of the human ESR1 gene have been linked to breast cancer, cardiovascular disease, osteo-porosis cognitive function, and multiple reproductive anomalies.598–603 A polymorphic (TA)n repeat within the ESR1 promoter region has been linked to premature ovarian failure604 but has no effect on plasma levels of E2, sex-hormone binding globulin (SHBG), T, or FSH in premenopausal women during the follicular phase.605 Two PvuII single nucleotide polymorphisms (SNP) in the ERS1 gene have been described to date. The first of these is a glutamine-to-glycine transformation at amino acid 400606 and has received little investigative attention. The second is an anonymous SNP located in intron 1, 400 bp upstream of exon 2,607 and is found in approximately 30% of women.608–612 Although the intronic ESR1 PvuII SNP does not affect ERα receptor levels,607 it may be associated with: (1) poor performance in women undergoing in vitro fertilization,608,612 (2) increased plasma E2 and andro-stenedione levels,610,612 and (3) late onset of menarche and/or menopause.609,611,613,614 Two SNP of the ESR2 gene have been described to date, an RsaI SNP leads to a silent nucleotide change in exon 5, and an AluI SNP occurs in the 3′-untranslated region of exon 8.615 Sundarrajan et al. found that Chinese women that are homozygous for either the RsaI or AluI SNP of the ESR2 gene are more



prone to exhibit ovulatory defects, menstrual disorders, and elevated LH; while women that are homozygous for both exhibit more severe idiopathic ovulatory defects.616

Progesterone Receptor Signaling in Ovarian Function

PR Expression in the OvaryThe first report of saturable, high-affinity P binding

sites in the rat ovary is that of Schreiber and colleagues in 1979617,618 and has since been followed by similar descriptions in human389–391,619 and cow619,620 ovaries. Later immunohistochemical studies primarily localized ovarian PR expression to the theca of large preovulatory follicles, the surface epithelia, and stromal/interstitium, and found this expression pattern is well conserved among species, including mice,621,622 pig,623 mon-keys,386,624 and humans.392,625,626 Numerous studies in species ranging from rodents to large domestic animals and primates agree that granulosa cells possess mini-mal PR expression throughout folliculogenesis except for the period 2–4 h after the preovulatory gonadotropin surge, whereupon an enormous induction of PR expres-sion occurs in preovulatory granulosa cells and peaks just prior to follicle rupture386,392,415,621–624,627–630 (Figure 25.22). This dramatic increase in PR expression in the granulosa cells of ovulatory follicles is absolutely essen-tial to follicle rupture.182,631 This induction of PR expres-sion in ovulating follicles is quite transient and lasts less than 12 h in rodent ovaries415,621,628 but is maintained throughout luteinization and corpus luteum formation in porcine623 and primate386,392,624 ovaries.

The regulation of ovarian PR expression prior to the preovulatory surge is poorly characterized. PMSG treat-ment of immature rats leads to a four-fold increase in cyto-solic P binding sites632 and a modest two-fold increase in Pgr mRNA levels628 after 48 h, indicating that FSH or subsequent E2 signaling may stimulate these basal levels of PR expression. In contrast, the mechanisms underly-ing the dramatic induction of PR expression in preovu-latory granulosa following the gonadotropin surge are

relatively well characterized. Over the course of the estrous cycle in rats, Pgr mRNA levels are increased >30-fold during a 4 h period (1800–2200 h) on the evening of proestrus and then return to baseline shortly after (2400 h).628 The increased expression is almost exclusive to the mural granulosa cells of healthy preovulatory fol-licles in rats,628,633 indicating that only fully differenti-ated granulosa cells possess the capacity for such rapid induction of the Pgr gene (Figure 25.22). Although PR expression climaxes within hours after plasma E2 and LH levels climax, the following evidence indicates LH is most directly involved: (1) expression peaks just 2 h after plasma LH levels climax versus >8 h after peak plasma E2 levels,628 (2) hCG treatment 48 h after PMSG exposure leads to a comparable increase in PR expression, whereas PMSG and concomitant increases in E2 have a minimal effect,415,621,628 (3) treatment of hypophysectomized or immature rats, or differentiated rat granulosa cells with E2 has no marked effect on ovarian PR expression,422,634 (4) inhibition of the LH surge by pentobarbital abates the increase in PR expression despite having no effect on the rise in plasma E2 levels,628 (5) LH but not E2 elicits a dra-matic PR induction in differentiated granulosa cells from rat634,635 or porcine629 ovaries in vitro, (6) an ER antago-nist (ICI 164,384) does not inhibit forskolin-induced PR expression in differentiated rat granulosa cells,579 (7) the putative ERE-like region of the rat Pgr promoter does not bind ERα or ERβ in ovarian extracts,579 and (8) ER (mainly ERβ) levels are rapidly decreasing at the time of rising PR expression in preovulatory granulosa cells.367,371,380,415

The above data provide a convincing argument that the rapid and transient induction of PR expression in preovulatory granulosa cells is directly mediated by the LH surge. Clearly only granulosa cells of preovulatory follicles in vivo or fully differentiated granulosa cells in vitro possess the capacity to respond to the LH surge and increase PR expression, presumably due to their unique acquisition of LH receptor.579,634 Indeed, granu-losa cells isolated from estrogen-primed immature rats require a 48-h exposure to physiological concentrations of FSH and T to acquire LH induction of PR expression.579 LH-mediated induction of the intracellular cAMP/pro-tein kinase-A pathway is likely the principal stimulus for increased PR expression in preovulatory granulosa cells as large doses of FSH, forskolin, or (Bu)2cAMP also elicit a comparable response in rat ovaries in vivo634 and dif-ferentiated rat634,635 or porcine629 granulosa cells in vitro. However, GnRH, PMA, or EGF exposure of differenti-ated rat granulosa cells in vitro elicits a comparable induction and temporal pattern of PR expression, sug-gesting that stimulation of protein kinase C or tyrosine kinase signaling cascades may also be involved.635 In fact, Sriraman et al. report that forskolin and PMA synergize to induce increased PR expression in undifferentiated

FIGURE 25.22 Regulation of PR expression in mouse ovaries and granulosa cells during gonadotropin-induced ovulation. (A, B) Immunohistochemistry for PR in the ovary of a PMSG-primed mouse 4 h after hCG treatment, illustrating the dramatic induction of PR immunoreactivity in the granulosa cells of preovulatory follicles (POF) but not surrounding preantral follicles. (B, higher magnification of A.)



primary rat granulosa cells.636 However, studies in fully differentiated rat granulosa cells demonstrate that for-skolin-induction of PR expression is fully blocked by co-treatment with a PKA inhibitor (H89) but only mini-mally affected by a PKC inhibitor (Cal-C),579 indicating a greater importance for the former signaling pathway.

The mechanisms by which LH activation of the PKA pathway leads to increased PR expression in preovula-tory granulosa cells remain unclear. The Pgr promoter is complex, consisting of distal and proximal regions and able to give rise to multiple transcripts.31,33 Initial stud-ies concluded that an estrogen-response element-like region (ERE3) within the proximal rat Pgr promoter is important to LH induction,579 but later studies that better modeled the structure of the rat Pgr promoter indicated these sequences are dispensable.636 Sriraman et al.636 demonstrated that two Sp1/Sp3 binding sites within the proximal promoter of the mouse Pgr gene bind Sp1/Sp3 and are essential to forskolin/PMA-induced expression. Furthermore, LH induction of Pgr also involves MAPK activation as mice lacking ERK1/2 in granulosa cells do not induce expression of Pgr in response to an hCG ovula-tory signal.637 While these mice show reduced expression, the direct targets of ERK1/2 remain unknown at this time. Recent evidence in porcine granulosa suggests that andro-gens may regulate Pgr expression, where treatment with either T or an AR antagonist alone reduces Pgr expression, while co-treatment leads to increased Pgr expression.638 Further characterization into the molecular mechanisms involved in this regulation is necessary. Therefore, unlike the uterus where Pgr is estrogen regulated, estrogen does not have the same effect on ovarian Pgr.

Differential expression of the two PR isoforms, PRA and PRB, among thecal and granulosa cells further illustrates the complexity of PR expression in the ovary. Western blot analyses following LH induction indicate a PRA:PRB ratio of 3:1 in differentiated granulosa cells from rats635 and 2:1 ratio in whole ovaries from mice.621 In rat ovaries, PRA

immunoreactivity is restricted to the thecal cells of prean-tral and antral follicles prior to the gonadotropin surge; whereas PRB immunoreactivity is detected in both thecal and mural granulosa cells throughout the stages of follicu-logenesis and shows less variation over the course of the estrous cycle.622 The enormous induction of PR expression that occurs in preovulatory granulosa cells on the evening of proestrus is primarily representative of PRA, resulting in a 2:1 ratio of PRA:PRB in these cells.622 In turn, the more moderate LH-induced increases in PR expression in the-cal cells is predominantly PRB.622 During the metestrous stage and rising P synthesis that follow ovulation, PRB is the sole isoform in thecal cells of preantral and antral fol-licles.622 The differential expression of PRA and PRB sug-gests that specific roles exist for each isoform.

To examine this possibility, Sriraman et al. recently examined genes that were induced by PRA or PRB in granulosa cells in vitro. In the absence of P ligand, PRA regulated 41 genes, while PRB only showed differential expression of seven genes compared to control cells.639 When the granulosa cells were treated with a PR ligand in vitro, PRA-transduced cells showed an increase in 42 genes, while PRB-transduced cells only showed 27 tran-scripts that were differentially expressed, while only three transcripts were shared between the two PR iso-forms.639 The differential expression of genes in granu-losa cells transduced with either PR isoform supports the hypothesis that specific roles exist for each isoform and provide target genes for future study.

Ovarian Phenotypes in Mouse Models of Disrupted Progesterone Signaling

Several mouse lines have been developed that are null for both PR forms and exhibit grossly normal ovaries but are anovulatory and therefore infertile176–178 (Table 25.8). Furthermore, gonadotropin-induced ovulation of PR-null females is unable to rescue the anovulatory phenotype as no oocytes could be recovered following treatment.176,178

TABLE 25.8 Ovarian Phenotypes in PR-Null Mouse Models


Pgr−/− (homozygous null alleles for PRA and PRB: PRKO)

Anovulatory and infertileFailure to respond to superovulation— large follicles present in ovary with trapped oocytesNo follicle ruptureNo CL

Elevated LHNormal FSH

176–178

PRA−/− (homozygous null alleles for PRA: PRAKO)

Anovulatory and infertileReduced to no response to superovulationLarge follicles present with trapped oocytes similar to PRKONo follicle rupture

Not reported 180

PRB−/− (homozygous null alleles for PRB: PRBKO)

FertileOvaries appear normalRespond to superovulation similar to wild-type

Not reported 182,227



Instead, PR-null ovaries following induced ovulation exhibit multiple large, preovulatory follicles, each pos-sessing a healthy, competent oocyte but a total failure to rupture176,178 (Figure 25.23). The follicular cells of the unruptured preovulatory follicles undergo luteinization and form functional corpora lutea within the expected time frame, suggesting that some elements of an LH response are intact177,178,182,633 (Figure 25.23). Similar experiments in isoform-specific PR-null females indicate a more severe phenotype following the loss of PRA relative to PRB, as

females lacking PRA ovulate an average of 9 oocytes/female versus 32 oocytes/female among wild-type litter-mates; whereas PRB females exhibit no deficits in oocyte yield.180,182,227 This finding that the PRA isoform is more critical to ovulation180 is congruent with the above studies of Gava et al. that demonstrate this isoform is much more dramatically induced by the LH surge in preovulatory granulosa cells. This also supports the finding that PRA in the presence of ligand regulates more genes in granu-losa cells in vitro compared to cells expressing PRB.639 Still, the absolute failure of ovulation even after gonadotropin induction in females lacking both PR forms indicates a level of cooperation between PRA and PRB that is vital to follicular rupture.180 Recently, a mouse model was devel-oped to use for tissue-specific deletion of PR,177 however to date only global knockout models have been published that show a similar phenotype as previously reported.176,178

Intraovarian Role of Progesterone in Ovarian FunctionFOLLICLE GROWTH AND DIFFERENTIATION

Although immunohistochemical studies indicate that both PR isoforms are expressed at basal levels through-out thecal and granulosa cells,622 there is little evidence that P action profoundly influences follicle growth and differentiation in the rodent ovary. Administration of P or synthetic progestins to immature or hypophysecto-mized rats has no obvious effect in the ovary in terms of estrogen receptor levels358,422 or gonadotropin/estrogen-induced follicle growth and maturation.358,640 In addi-tion, P has no measurable influence on FSH-induced granulosa cell proliferation496 or growth and differentia-tion of whole follicles497 in vitro. Perhaps most indicative of a minor role for P signaling during folliculogenesis prior to ovulation is the lack of any overt phenotype in follicle growth, maturation, or steroidogenesis in the ovaries of PR-null mice (Table 25.8).176–178,641 Indeed, Robker et al. demonstrated that the expected FSH- or PMSG-induced increases in Cyp19 and Lhcgr expression in mature follicles occurs in PR-null ovaries.633

OVULATION

PR expression in the ovary is relatively low through-out folliculogenesis except during the 4–6 h period imme-diately after the ovulatory gonadotropin surge, during which an enormous induction of PR expression occurs in granulosa cells of ovulating follicles628,634,635 and is synchronized with an equally acute increase in ovar-ian P synthesis. These phenomena alone are indicative of an important role for PR-mediated P signaling dur-ing ovulation. Experimental evidence of a critical role for PR in ovulation comes from studies in which ovula-tion is blocked when peri-ovulatory ovaries or follicles are exposed to: (1) anti-P antisera,642 (2) inhibitors of P synthesis,643–645 or (3) PR antagonists.621,646,647 Finally, the

FIGURE 25.23 Ovarian response to gonadotropin-induced ovula-tion in PR-null mice. (A) Transverse section of a typical ovary isolated from a 6-week-old wild-type mouse (+/+) treated with an intraperito-neal injection of 5 IU of PMSG, followed 48 h later with 5 IU of hCG, and killed 24 h later. Note the presence of numerous corpora lutea (CL). Scale bar = 100 μm. (B) A representative cross-section of an ovary from an age-matched PR-null (−/−) female that was hormonally treated exactly as the wild-type described above. Note the unusual presence of several unruptured follicles (UF) as indicated. Scale bar = 100 μm. (C) High magnification of a corpus luteum in the wild-type ovary, exhibit-ing the characteristic hypertrophied luteal cells (LC). Scale bar = 50 μm. (D) High magnification of an unruptured follicle present in a PR-null ovary, exhibiting an intact oocyte (O) with a zona pellucida and granu-losa cells (GC) that have undergone cumulus expansion. Note the lack of luteinization among the GCs in the PR-null follicle compared to those in the wild-type (C). Scale bar = 50 μm. (Source: (A–D) reproduced with permission from Ref. 176.) (E) Average number of oocytes (±SEM) released per mouse after gonadotropin-induced ovulation in wild-type (WT), PRA-null (PRAKO−/−), PRB-null (PRBKO−/−), and PR-null (PRKO−/−) mice; n = 8 per test group. Source: Reproduced with permis-sion from Ref. 182.



anovulatory phenotype of PR-null mice discussed before provides definitive evidence that PR actions are required in the mammalian ovary during the period just prior to ovulation.176–178,180 PR-null females exhibit multiple large, preovulatory but unruptured follicles following induced ovulation.176 Therefore, follicle growth and dif-ferentiation are unaffected by the loss of PR functions as PR-null follicles exhibit fully formed antra and increased Cyp19 and Lhcgr expression following FSH or PMSG treatment.176,633 Furthermore, although follicle rupture is impaired, PR-null ovaries exhibit several indications that other LH- or hCG-induced responses are intact, including: (1) rapidly decreased Cyp19 expression,633 (2) dramatic induction of Ptgs2 expression,633 (3) expan-sion of the cumulus–oocyte complex,176 and (4) lutein-ization and formation of functional corpora lutea.182,633 These findings in PR-null females are congruent with earlier studies using P synthesis inhibitors645,648 or PR antagonists,646 which also report that ovulatory levels of prostaglandins and steroids are largely unaffected by inhibition of P signaling.

The previous data clearly indicate that PR actions are highly specific and critical for follicle rupture. Stud-ies have discovered that PR-null ovaries fail to exhibit LH-induced expression of several proteases, includ-ing ADAMTS-1, ADAM8, and cathepsin-L, which are postulated to be involved in degradation of the follicle wall and extracellular matrix that is necessary for oocyte extrusion (Figure 25.24). ADAMTS-1 (Adamts1) and ADAM8 are members of the A disintegrin and metal-loproteinase family of proteases and are dramatically increased in granulosa cells of preovulatory follicles 12 h after hCG treatment, after peak PR expression and

coinciding with a period of follicular rupture.644,649 A similar pattern of peri-ovulatory ADAMTS-1 expression is documented in preovulatory follicles of porcine,650 equine,630 and primate651 ovaries. Cathepsin-L (Ctsl) is also significantly increased following hCG exposure in granulosa cells of preovulatory follicles and also peaks 12 h after treatment.633,652 PR-null females,633 as well as wild-type female rats exposed to a P synthesis inhibitor near the time of hCG treatment644,653 fail to exhibit hCG-stimulated increases in Adamts-1 expression, indicating this effect of LH is primarily dependent on PR-mediated P action. ADAMTS-1-null female mice exhibit a severely compromised ovulation and a phenotype of large, pre-ovulatory but unruptured, follicles following gonado-tropin-induced ovulation.654,655

Interestingly, PRA-null mice exhibit a severe ovula-tory phenotype similar to total PR-null females but pos-sess normal LH-stimulated induction of Adamts-1 and Ctsl during induced ovulation,182 suggesting possible compensatory actions by PRB but also questioning the importance of ADAMTS-1 and cathepsin-L to follicle rupture. ADAM8 was recently shown to be regulated specifically by PRA in reporter assays, while PRB was unable to regulate the promoter of ADAM8 in vitro.649 This could explain why PRA-null mice show LH-stim-ulated induction of Adamts-1 and Ctsl yet are unable to ovulate, although further studies are needed to confirm this.

Several other genes have been identified downstream of PR and suggested to be important in follicle rupture and ultimately ovulation. Since ovulation is discussed more fully in another chapter (Chapter 22) of this vol-ume, we will not provide in-depth description, although

FIGURE 25.24 Ovarian expression of ADAMTS-1 in PR-null ovaries during gonadotropin-induced ovula-tion. Immature heterozygous (PR+/−) and homozygous PR-null (PRKO) mice were left untreated or injected with PMSG and euthanized 46 h later, or were treated with PMSG for 46 h followed by hCG and euthanized 7, 12, and 24 h later. Reverse transcriptase polymerase chain reac-tion analysis of total RNA from whole ovaries indicates that ADAMTS-1 expression is dramatically increased by hCG in PMSG-primed mice, but this induction is lacking in PR-null ovaries, indicating dependence on PR function. ADAMTS-1 expression was normalized to expression of ribosomal protein L19. Source: Reproduced with permission from Ref. 633.



we will briefly describe what is known to date. These include SNAP25, synaptosomal-associated protein 25, an LH-regulated gene that has decreased expression in PR-null mice but not Ptgs2 mice.656 Furthermore, Snap25 was PR regulated in in vitro reporter assays, and the PR antagonist RU486 was able to inhibit this regulation. SNAP25 acts to alter cytokine and chemokine secretion in granulosa cells, which may act to facilitate follicle rup-ture.656 Endothelin 2 (Edn2) was identified to be an LH-regulated gene with undetectable levels in PR-null mice that plays a role in follicle rupture.657,658 Recent studies have shown that Edn2 is regulated by PRA,639 and stud-ies in rats have shown that it acts to stimulate smooth muscle contraction and follicle constriction.659 EDN2 provides the mechanical actions that help to facilitate fol-licle rupture and successful ovulation. This is supported by its regulation specifically by PRA, which is essential for ovulation, whereas loss of PRB doesn’t reduce ovula-tory response as described previously.

Numerous transcription factors have also been iden-tified as PR targets, and knockout mouse models have demonstrated that these factors are necessary for ovula-tion in mice. Hypoxia inducible factor 1 (Hif1) is induced by FSH in rat granulosa cells cultured in vitro,660,661 and have also been found to be regulated by PR.662 Further-more, the expression of several HIF family members, including Hif1a, Hif2a, and Hif1b, are reduced in PR-null ovaries662,663 suggesting an important role in follicle rup-ture. To further explore this possibility, pharmacological inhibitor of HIF activity was shown to block ovulation and inhibit follicle rupture,662 demonstrating the impor-tance of HIF activity in regulating ovulation. Another nuclear receptor, PPARG, is also necessary for follicle rupture as demonstrated by targeted deletion in granu-losa cells in mice that had no response to exogenous gonadotropins.664 The action of PPARG was found to be mediated by several PR target genes, including Il6 and Edn2,664 demonstrating that several targets are regulated downstream of PR and LH signaling to coordinate follicle rupture and ovulation.

Androgen Receptor Signaling in Ovarian Function

AR Expression in the OvaryAR expression has been documented in the ovaries

of multiple species, including mouse,373,450 rat,130,665–668 pig,669,670 sheep,671 cow,672 monkey,274,395,673–675 and human.395,625,676–679 The ovarian pattern of AR expres-sion is well conserved among species, with levels being detectable throughout the stages of folliculogenesis except primordial follicles, with the highest expression in the granulosa cells of small preantral follicles, detect-able expression in thecal/interstitial cells, and little to no expression in luteal cells (Table 25.6).

In the ovaries of multiple species, Ar/AR expression among growing follicles is inversely correlated with the extent of granulosa cell differentiation (Figure 25.25). A mechanism to decrease AR levels during follicle matura-tion is consistent with the need to reduce sensitivity to intrafollicular androgens, which rise to levels sufficient for aromatization to E2, as activating AR ligands are det-rimental to the health of preovulatory follicles.680 In the ovaries of gonadotropin-stimulated rats, Testsuka and Hillier found that large antral follicles (>400 μm) exhibit a 2.75-fold higher level of Cyp19 expression relative to small follicles (<200 μm) but a 51% decrease in Ar expres-sion.665 In rat ovaries, Ar expression is first apparent in early postnatal development in follicles of the preantral stage.681 In adult ovaries, decreasing AR immunoreac-tivity occurs during follicle differentiation; a decrease is first apparent in mural granulosa cells and then pro-gresses in those cells closest to the antrum.667 Interest-ingly, cells composing the cumulus–oocyte complex of preovulatory follicles are believed to be the last to dif-ferentiate682 and maintain Ar expression throughout.667 This gradient of AR expression suggests that AR signal-ing correlates with the differentiation state of granulosa cells and follicle development.

In primate and human ovaries, AR expression is also highest in the granulosa cells of small growing follicles, yet evidence of an inverse correlation with follicle dif-ferentiation is conflicting.625,674,675,678,684 Hillier et al. found AR immunoreactivity in the marmoset ovary is most abundant in granulosa cells of healthy pre- to small antral follicles and low or absent in preovulatory folli-cles of late follicular stage.674 In contrast, minimal differ-ences in granulosa cell AR expression between preantral and large antral follicles are reported in rhesus monkey ovaries.675 Furthermore, healthy follicles at late stages of maturation in human ovaries are described to possess significant AR immunoreactivity.625,678,679 A recent report has suggested that AR expression is highest in small antral follicles and decreases as the follicle matures,685 suggesting that message levels may not correspond with AR protein levels in human follicles.

The regulatory factors for AR expression in the ovary are poorly understood. Small preantral follicles in rats maintain high AR levels after hypophysectomy indicat-ing that gonadotropins are not required to induce AR expression.668 In fact, most evidence indicates that FSH or PMSG-induced differentiation of granulosa cells is the primary cause of reduced AR expression; Campo et al. demonstrated that PMSG treatment of rats leads to the replacement of high-affinity, low-capacity androgen binding sites by nonsaturable, low-affinity binding sites in the ovary.686 Similarly, immature rats treated with recombinant FSH over 48 h exhibit a 65% reduction in ovarian Ar mRNA levels and a further decrease when LH is included.668 However, neither FSH nor (BR)-cAMP



FIGURE 25.25 Relationship between expression of androgen receptor and aromatase (CYP19) during folliculogenesis. (A) Expres-sion of androgen receptor (Ar) and aromatase (Cyp19) expression in gran-ulosa cells of small (∼200 μm), medium (200–400 μm), and large (>400 μm) follicles from immature rats after treatment with PMSG. Expression was quantified by RNase-protection assay and normalized to 18S rRNA (not shown); data are expressed as percentage of control values (SEM of three separate trials, each consisting of 8–10 animals). a, b: P <0.05; a, c: P <0.01 (by ANOVA with the Newman–Keuls test). (Source: Reproduced with permission from Ref. 491.) (B) Hypothetical model of androgen uti-lization during folliculogenesis. As follicular development progresses, thecal androgen production gradually increases. During the early stages of follicular differentiation, androgens act via androgen receptor (AR) to enhance FSH-induced differentiation, including the stimulation of Cyp19 expression. During the final stages of follicular development, androgens primarily serve as a substrate for CYP19-mediated E2 syn-thesis under stimulation by FSH and LH. This differential regulation of AR and CYP19 may be important in shifting androgen utilization from action to metabolism, thereby ensuring a healthy transition of a follicle from the early maturation to full maturation stage. Source: Reproduced with permission from Ref. 683.

affect Ar mRNA levels in cultured rat granulosa cells in vitro.665 This suggests that a paracrine interaction with theca and/or stromal cells is necessary for reduced Ar expression observed in granulosa cells. Interestingly, DHT also elicits a 20% reduction in Ar expression in rat granulosa cells in vitro that is prevented by co-treatment with FSH.665 In contrast, T reportedly has little effect on AR expression in rhesus monkey ovaries.675 There is

increasing evidence that E2 may play a role in decreas-ing granulosa cell AR levels during follicle differentia-tion. Like DHT, E2 exposure of rat granulosa cells also leads to a 20% reduction in Ar transcripts, but this effect is unabated by FSH.665 Furthermore, granulosa cells retrieved from untreated hypophysectomized rats respond well to DHT plus FSH and exhibit increased Cyp19 expression accordingly; but the effect of DHT is



lost in granulosa cells isolated from E2-primed hypoph-ysectomized rats, suggesting that estrogen pretreatment decreases AR expression.442 Indeed, mature follicles in ERβ-null ovaries are reported to possess aberrantly high AR expression relative to similarly staged follicles in wild-type ovaries,450 supporting the idea that ER may act to decrease Ar expression. While the hormonal regu-lation of Ar expression is unclear, recent reports suggest that a member of the orphan nuclear receptor family, Nur77, is required for maximal Ar expression in murine ovaries. Nur77 regulates several steroidogenic enzymes in the ovary, including those involved in androgen syn-thesis,687 and recent reports demonstrate 20% reduction in Ar expression in the granulosa cells from Nur77-null mice.688 Dia and colleagues were able to show that NUR77 bound directly to the promoter regulatory region of the Ar gene in mouse granulosa cells, provid-ing insight into one of the transcriptional factors neces-sary for maximal AR expression in the ovary.688

Ovarian Phenotypes in Mouse Models of Disrupted Androgen SignalingMICE LACKING AR

Tfm mice were first described in 1970 and are a natu-rally existing androgen-resistant mutant283 due to an inactivating mutation of the Ar gene.284,285 With the advent of gene targeting techniques, comparable inac-tivation mutations of the AR gene and resulting phe-notypes have been described in rats and humans.286 Tfm/Tfm female mice are fertile but exhibit a noticeably shortened reproductive life span that becomes appar-ent after approximately four litters compared to control (Tfm/X or X/X) females; however, individual litter sizes were not different.287 Histological evaluation of ovaries from the Tfm/Tfm breeding females indicated a sparse number of healthy follicles and a hypertrophied inter-stitium, both illustrative of premature ovarian failure (POF).287 The difficulties in generating Tfm/Tfm female mice limited more extensive studies.

Several global AR-null mouse models have been gen-erated via a Cre/loxP targeting scheme that allows for tis-sue and temporal specific disruption of the Ar gene and therefore the generation of fertile male carriers of the targeted Ar allele183,185,288,290 (Table 25.9). In one study, global AR-null females made through deletion of exon 1 exhibit normal fecundity only during the first 12 weeks of continuous breeding, after which they produce a reduced number of litters per female (2.3 versus 3.3 in wild-type) and less offspring per litter (4.5 versus 9.8 in wild-type), while others become totally infertile.183 The POF observed in AR-null females is remarkably similar to that described before in Tfm/Tfm females.287 Adult AR-null females possess ovaries exhibiting a normal number of growing and antral follicles but notably fewer corpora lutea183 and reduced circulating P (Table 25.9),689

suggesting that folliculogenesis is preserved but ovulation and luteinization are disrupted. Gonadotropin-induced ovulation of immature AR-null females leads to a severely reduced ovulatory yield and oocytes that often exhibit a less condensed cumulus–oocyte complex.183 This latter finding is especially interesting in light of the preserved Ar expression in the cumulus oophorus cells of preovulatory follicles. Additional evidence that AR-null ovaries fail to fully respond to an ovulatory dose of hCG is an inadequate induction of Pgr, hyaluro-nyl synthetase-2 (Has2), and tumor necrosis factor-α-stimulated gene 6 (Tnfaip6),183 all factors that are critical to expansion of the cumulus–oocyte complex and fol-licle rupture. Furthermore, granulosa cells of ovulatory follicles in AR-null ovaries fail to terminally differenti-ate and luteinize following hCG-induced ovulation, as indicated by decreased expression of cyclin-dependent kinase inhibitor 1A (Cdkn1a) and cytochrome P450scc (Cyp11).183 These animals were generated by targeted deletion of exon 2 and demonstrate that AR signaling is important for normal ovarian function and to prevent POF in rodents.

A second global knockout mouse model was devel-oped by Shiina et al., where AR was deleted through Cre/loxP mediated removal of exon 1.690 This animal model is similar to that described before in that the female mice show an age-dependent decrease in fecundity, however, these mice also have a decrease in number of offspring per litter in animals aged 8 weeks old (4.5 versus 8.3 in wild-type). At 8 weeks of age, these mice also have decreased number of CL and an increased number of unhealthy and atretic follicles.690 Knockout of AR in exon 1 also causes a POF phenotype where the number of off-spring decreases over time concomitant with a decrease in the number of healthy follicles, suggesting defects in folliculogenesis. By 32 weeks of age, 40% of knockout females are infertile, similar to that observed in Tfm/Tfm females and the exon 2–deleted AR-null mice,183,283 and at 40 weeks of age all of the mice are infertile.690 No dif-ference was observed in genes involved in steroidogen-esis within the ovary, and serum hormone levels for E2, P, and T are within normal rages similar to WT controls, providing evidence that AR is not necessary for normal steroidogenesis in the ovary (Table 25.9)690 in contrast to reduced P observed in the exon 2–deleted AR-null female.689 Microarray analysis was done on ovaries from these animals to find potentially altered transcriptional networks that might contribute to the POF phenotype. Several genes implicated in oocyte-granulosa cell com-munication were found to be increased,690 including kit ligand (Kitl). Kitl is a downstream target of AR as demonstrated by induction of Kitl expression following DHT treatment, while co-treatment with DHT and the antiandrogen flutamide attenuated the increased Kitl expression.690 This report suggests that AR signaling has



an important role in the oocyte-granulosa cell regula-tory loop,690 although future work is needed to identify potential mechanisms of action.

These knockout mouse models demonstrated an important role for AR in normal ovarian function and suggest that loss of AR contributes to POF (Table 25.9). Both AR-null models lack all AR protein due to a trun-cated message and premature stop codon. A third AR-null mouse model was developed by Walters et al. where an in-frame deletion of exon 3 was made that led to produc-tion of an AR protein lacking the second zinc finger and is therefore truncated and unable to bind DNA.185 Muta-tions in this region of the AR gene have been identified

in human patients with androgen insensitivity syn-drome (AIS), demonstrating the importance of binding DNA to AR activities in normal physiological state.693–695 This mouse model allows for analysis of the direct effects of classical genomic AR signaling in the ovary compared to the AR-null models that lack all AR protein. Dele-tion of the DNA binding domain in female mice led to a subfertile phenotype where the females homozygous for the deletion had less offspring per litter compared to both heterozygous and WT females.185 Interestingly, the AR(exon 3)-null females had reduced numbers of CL similar to other AR-null mice,290,690 demonstrating the importance for intact AR signaling for this process.

TABLE 25.9 Ovarian Phenotypes in AR-Mutant or Null Mouse Models


Tfm/Tfm mice Fertile but have a reduced number of pups/litter as they ageShorted reproductive life span

Not reported 283

AR−/− (exon 2) Reduced fertility and a shortened reproductive life spanAdults exhibit grossly normal ovaries with reduced number of CLAbnormal estrous cycleImmature females have reduced response to gonadotropin-induced ovulation

Reduced POther hormones not reported

183,290

AR−/− (exon 1) Reduced fertility and POF phenotype where animals have a shortened reproductive life spanIncreased number of atretic follicles and reduced number of CLAltered ovarian gene expression in immature and adult animals

Normal serum E2, P, T, LH, and FSH

690

AR−/− (inframe deletion of exon 3, loss of DNA binding activity)

Delayed production of first litterDecreased pups/litter and reduced number of CLAR+/− animals had age-dependent reduction in pups/litterIncreased number of unhealthy antral folliclesNormal response to exogenous gonadotropins, but a reduced number of oocytes ovulated during natural mating

Normal serum E2, T, LH and FSHIncreased intraovarian T in 10–12 week old mice

185

AMHCre+Arf/+;Arf/f (exon 3 floxed, loss of DNA binding activity in large preantral to antral follicles)

Subfertile due to reduced number of litters and age dependent decrease in total number of pups bornDecreased fertility over time with a cumulative decrease in pups born per dam3-month-old animals have increased large preantral and antral follicle numbers6-month-old animals have increased length of estrous cycleReduced cumulus expansion and oocyte/embryo viability

Normal serum LH and FSH 691

Amhr2Cre+;Arf/f (exon 2 floxed) GCARcKO

Reduced fertilityAltered follicle progression and development8–9 week mice: normal estrous cycle, reduced pups/litter, reduced number of oocytes from natural mating, however, no differences observed when treated with exogenous gonadotropins24 weeks: increased length of estrous cycle, reduced number of pups born per dam, reduced number of oocytes from natural mating and treatment with endogenous gonadotropins

Not reported 692

Gdf9Cre+;Arf/f (exon 2 floxed, oocyte specific loss of AR)

Normal female phenotype except androgens were unable to promote oocyte maturation in vitro

Not reported 692

SPARKI (homozygous KI of mutated DBD of AR)

Normal female phenotype Not reported 186



Furthermore, these mice also had an age-dependent reduction in fecundity similar to the other AR-null mod-els,183,690 and females heterozygous for the AR(exon3) deletion also showed an age-dependent reduction in offspring born at 6 months of age, suggesting a dosage effect.185 An increase was observed in the number of unhealthy antral follicles, although AR(exon3)-null mice did not have significant changes in overall follicle devel-opment even in animals aged 10 months185 in contradic-tion to reports in AR-null ovaries with reduced follicle numbers.183 While these models have indicated some differences in the role of AR in normal folliculogenesis, all demonstrate an increase in the number of atretic fol-licles, demonstrating the importance of AR signaling in maintaining ovarian health over time. AR-null mice have reduced response to gonadotropin-induced ovula-tion,183 and examination of the AR(exon3)-null animals found that they respond to exogenous gonadotropin stimulation with similar numbers of oocytes ovulated; however, a decrease was observed in oocytes ovulated when examined after natural mating.185 This suggests that AR is necessary for normal ovulatory response; however, excess gonadotropins are able to override this defect in the presence of a truncated AR protein (lack-ing the entire DNA binding domain) compared to ova-ries that are void of all AR protein. Mutation of the DNA binding domain that alters the ability of AR to bind to selective androgen response elements in the SPARKI mouse model presented by Schauwaers et al. did not lead to a female reproductive defect, suggesting that the ovary does not require selective AR binding to response elements as compared to the male SPARKI mice that have a reproductive phenotype and will not be further discussed herein.186

The identification of genes necessary for oocyte-gran-ulosa communication in the ovary as being AR targets690 provides a possible mechanism that is aberrant in AR-null ovaries, but further studies are necessary to confirm this difference and explore the pathways involved. Loss of the DNA binding domain in AR did not affect circulat-ing hormone levels, as the mice had normal serum levels of E2, T, LH, and FSH; however, an increase in intraovar-ian T levels was observed in mice aged 10–12 weeks,185 suggesting alterations in the steroid environment locally may have paracrine effects on ovarian function that was not measured in other AR-null models.183,690

MICE WITH OVARIAN-SPECIFIC DELETION OF AR

The global AR knockout mouse models described herein have offered many insights into the important role of AR in female reproduction, specifically in the ovary. However, these models lack AR in all tissues, includ-ing all parts of the hypothalamic–pituitary–gonadal axis, which could contribute to the ovarian phenotype observed. To circumvent this, two conditional knockout

mouse models have been developed to remove AR spe-cifically in granulosa cells of the ovary.691,692 These mod-els provide a unique way to explore the role(s) of AR in female fertility and ovarian function.

Sen and Hammes used Amhr2-cre to delete exon 2 of AR in the ovary and found that the animals had reduced numbers of offspring born to young dams (2.8 pups in KO versus 7 in WT mice),692 similar to that observed in the global knockout animals.183,290 In these granulosa cell AR conditional knockout (GCARcKO) animals, altered follicle progression was observed in animals aged 9 weeks as well as in animals aged 24 weeks (Table 25.9),692 supporting observations in global knockout ani-mals,183,290 where numbers of corpora lutea were reduced and atretic follicles were increased. These patterns per-sisted in older animals, which also showed an increased length of estrous cycle at 24 weeks of age compared to mice aged 9 weeks old that had a normal estrous cycle.

Several other differences were observed in animals at the two ages examined (9 weeks versus 24 weeks), including differences in ovulation. Mice aged 9 weeks had a reduced number of oocytes ovulated during natu-ral mating, but treatment with exogenous gonadotropins was able to circumvent this defect and cause normal numbers of oocytes to be ovulated from these animals. As the animals aged, however, the ability of exogenous gonadotropins to override this defect was lost, and reduced oocyte numbers were observed from natural mating and superovulation paradigms.692 Failure of the ovary to respond to exogenous gonadotropins concur-rent with a reduction in the total number of offspring born over a long-term fertility study (19 in KO versus 119 in WT mice) supports the POF phenotype observed in the global AR-knockout models.183,185,290

The GCARcKO presented by Sen and Hammes was made using the Amhr2-cre mouse, which has been found to be active in tissues other than granulosa cells includ-ing the pituitary,696 which may complicate the findings. While reduced expression of Ar was not reported in the hypothalamus or the pituitary,692 the heterogeneous population of cells present could mask a reduction in gonadotrope cells that are required for normal female fertility. To eliminate this possibility and explore the classical role of AR in large preantral to antral follicles, Walters et al. used the Amh-cre mouse model to delete exon 3 in the ovary.691 Amh is the ligand for Amhr2 and is expressed specifically in large preantral to antral fol-licles during folliculogenesis demonstrated by crossing the Amh-cre mouse to R26R (ROSA) reporter mice.691 This mouse expresses β-galactosidase activity where Cre recombinase is expressed, and the X-Gal blue stain can be used to localize cells expressing the β-gal indica-tor within tissues. The Amh-cre mouse was crossed with exon3 floxed AR mouse that creates a truncated protein lacking the DNA binding domain.185 The use of this



mouse model allowed Ar/AR to be deleted in a tempo-ral pattern and to examine whether DNA binding activi-ties are necessary during this stage of folliculogenesis for normal ovarian function.

The AMHCre+Ar(exon3)f/f females are subfertile, and while the number of offspring born per litter was not significantly altered, an age-dependent decrease in the total number of pups born was observed185 similar to the POF phenotype observed in other AR-null models (Table 25.9).183,185,283,690,692 This provides evidence that AR binding to direct AR target genes in large prean-tral follicles is necessary for normal ovarian function over time. The AMHCre+Ar(exon3)f/f females also have an increased length of estrous cycle at the age of 6 months that is not observed in younger mice,691 supporting the POF phenotype observed and the importance of AR in maintaining normal ovarian health. Altered follicle pro-gression was observed in these mice, and at 3 months of age fewer large preantral and small antral follicles were observed; however, this difference was not noted in older animals.691 While no difference is observed in follicle pro-gression during folliculogenesis in AMHCre+Ar(exon3)f/f females aged 6 months, a significant increase in unhealthy follicles was observed,691 supporting the notation that AR signaling is necessary for normal ovarian health and pre-vention of POF in the ovary.

Communication between granulosa cells and the oocyte are necessary for oocyte health and viability, and disruption of this communication can lead to infertil-ity. Animals lacking functional AR protein have altered expression of several genes implicated in this commu-nication network, including the AR-dependent gene Kitl.690 Pharmacological studies have suggested that T can induce mouse697,698 and porcine389,699,700 oocyte maturation presumably through classical AR signaling in vitro. To directly examine the role of AR expression in oocytes, a conditional knockout mouse model was developed using an oocyte-selective Gdf9-cre crossed with exon 2–floxed AR.692 These mice had normal fer-tility and no overt ovarian phenotype compared to WT controls, demonstrating that AR expression within oocytes is not necessary for female fertility, while granu-losa cell specific expression of the AR nuclear receptor is necessary in vivo.692 Interestingly, when oocytes were removed from immature unprimed females and grown in vitro, DHT was unable to induce oocyte maturation in the absence of AR expression in oocytes.692 However, P-mediated oocyte maturation was not affected by the loss of AR in vitro, demonstrating that these oocytes could still mature, albeit not through an androgen-mediated mechanism. This suggests that AR expression in oocytes is necessary for in vitro maturation in the absence of granulosa cells, while in vivo expression of AR in the granulosa cells is sufficient for oocyte matura-tion, possibly due to P-mediated mechanisms.

Loss of AR in granulosa cells contributes to altered ovarian function including a POF phenotype and loss of ability of the ovary to ovulate oocytes through both natural mating or superovulation depending on the age the animals were tested.183,185,692 While differences were observed depending on the AR-null mouse model exam-ined and the ovulation paradigm presented, the data provide evidence that AR is necessary for normal ovula-tory function of the ovary. Interestingly, a decrease was observed in the number of oocytes naturally ovulated by one granulosa cell–specific AR-null model,692 while no differences were observed in a second model (Table 25.9).691 These differences may be due to the timing of AR deletion or the different flox animals used (i.e., exon 2 versus exon 3). Further study was done on oocytes ovulated from natural mating in the AMHCre+Ar(exon3)f/f females to see if these oocytes could be fertilized and progress through early embryo development in vitro.691 While the number of oocytes ovulated was not differ-ent, the number of fertilized oocytes was reduced (38.6% in KO versus 94.5% in WT), as was the number of the embryos able to process to the two-cell stage of embryo development (35.7% in KO versus 89.5% in WT).691 This supports the necessity of AR expression in granulosa cells to promote oocyte maturation and development, and indicates that DHT-mediated oocyte maturation could be mediated through expression of AR in granu-losa cells. It also provides insight into the inability of some oocytes to respond to in vitro fertilization, which has implications in human health in cases where some women fail to respond to assisted reproductive tech-nologies. The variety of AR-null mouse models to date provides evidence of the importance of AR signaling in normal ovarian development, female fertility, and fol-licle health.

Intraovarian Role of Androgen in Ovarian Function

The intraovarian roles of androgens can be catego-rized into three distinct functions: (1) as substrates for E2 synthesis, (2) as an enhancer of follicle differentiation, and (3) as a stage-specific inhibitor of follicle growth.683 The role of thecal-derived androgens, most notably androstenedione, as substrates for E2 synthesis in granu-losa cells is obviously critical given the profound impor-tance of the latter hormone to reproductive function. While ovarian steroidogenesis is essential for function of the ovary and female fertility, this topic will not be covered here; instead we focus on the additional intra-ovarian actions of androgens, which include their role as activating ligands for AR-mediated effects.

GRANULOSA CELL PROLIFERATION

The follicular response to androgens is dependent upon the stage of growth and differentiation. In preantral,



undifferentiated follicles that are unable to synthesize E2, androgens promote gonadotropin-induced granu-losa cell proliferation and maturation. In large, differen-tiated follicles that have acquired aromatase activity, E2 assumes the role of enhancing FSH-induced granulosa cell proliferation and differentiation, and androgens in excess of that required for conversion to E2 are detri-mental to follicle health, leading to atresia. This paradox of androgen action during follicle growth and differen-tiation explains the contradictory results when compar-ing experimental studies.

Several studies in hypophysectomized rats have shown that coadministration of T inhibits estrogen or gonadotropin-induced granulosa cell proliferation and increased ovarian weight, and promotes degeneration in more mature follicles.422,489,701–703 In contrast, Arm-strong and Papkoff found aromatizable androgens (e.g., T or androstenedione) enhance the promotional effects of FSH in the ovaries of hypophysectomized rats but that the nonaromatizable androgen DHT is inhibi-tory.291 Similar treatment with hCG instead of T has a comparable effect on gonadotropin or E2-induced folli-cle growth in hypophysectomized rats, but is prevented by AR-antagonists, indicating the detrimental effects of hCG are due to stimulation of thecal cell androgen syn-thesis.702,704,705 However, Bley et al. found that granulosa cells from preantral follicles of estrogen-primed rats pro-liferate at comparable rates in response to FSH plus E2 or DHT in vitro496 and that the promotional effects of the latter steroid are blocked by the antiandrogen hydroxy-flutamide.496 Investigations toward the mechanism by which androgens inhibit granulosa cell proliferation indicate that DHT exposure to estrogen-primed rats prior to granulosa cell isolation causes a blunted response to forskolin-induced cyclin-D2 (Ccnd2) expression in vitro, leading to cell-cycle arrest and reduced proliferation.706 DHT is able to exert its inhibitory effect on Ccnd2 expres-sion through AMP-activated protein kinase (AMPK) activation, which in turn inhibits phosphorylation of MAPK and ultimately impacts granulosa cell prolifera-tion downstream of FSH.707,708 AMPK has a role in inhib-iting cell proliferation, and pharmacological activation of the kinase can lead to cell cycle arrest, while blocking AMPK activity prior to treatment with DHT was able to overcome the inhibitory effect normally observed,708 providing insights into the mechanism of action of DHT inhibition on cell cycle progression in estrogen-primed rat granulosa cells. This work also highlights how andro-gens are able to be stimulatory during early stages of fol-liculogenesis and then inhibitory during the latter stages when estrogen signaling is the main driver of follicle development.

The stage-specific effects of androgens during follicu-logenesis described herein may be more apparent in rat ovaries relative to other species. In hypophysectomized

mice, Wang and Greenwald found that T or DHT, in combination with FSH, induces greater levels of DNA synthesis in antral follicles than FSH alone; a similar response is not observed in small- and medium-sized follicles.492 In preantral mouse follicles grown in vitro, antiandrogen antisera reduce FSH-stimulated growth and DNA synthesis; however, these data must be inter-preted with caution since antiandrogen antisera may also remove androgens from the pool of substrates for E2 synthesis. Additional evidence that androgens may promote granulosa cell proliferation in mice comes from the findings of Burns et al. that the AR-antagonist flu-tamide reduces the growth rate of granulosa cell tumors in animals lacking functional inhibins.499 In vitro follicle culture studies demonstrated the importance of AR sig-naling in folliculogenesis, where treating follicles with antiandrogenic compounds reduced follicle growth dur-ing the preantral stage of folliculogenesis,709 demonstrat-ing the importance of androgens in early follicle growth.

In cumulus cells of large antral follicles from porcine ovaries, DHT augments FSH or IGF1-induced prolifera-tion, and this effect is inhibited by flutamide.710 Rhesus monkey ovaries exhibit a strong correlation (r = 0.91) between AR expression and cell proliferation675 and exhibit an increased number of follicles of all stages except large antral following T or DHT treatments.711 Yang and Fortune reported that T was able to stimu-late early follicle maturation in bovine follicles.712 The collective data support the notion that androgens are important in granulosa cell proliferation during early stages, but at latter stages the effect appears to be species dependent.

GRANULOSA CELL DIFFERENTIATION

Fully differentiated preovulatory follicles in mamma-lian ovaries are distinctly characterized by a large fluid-filled antrum, significantly increased aromatase (CYP19) activity, and acquisition of LH responsiveness. The absolute requirement of FSH signaling in this process is illustrated by the absence of all three phenotypes in mice null for FSH action.420,512,513 E2 is also required for full FSH-induced follicle differentiation. However, there is substantial evidence that androgens may be equally efficient as E2 in augmenting certain responses to FSH, most especially the induction of aromatase activity and antrum formation.683 Both T and DHT enhance FSH induction of aromatase activity in hypophysectomized rat ovaries or isolated granulosa cells in vitro in a dose-dependent fashion.291,713 Furthermore, pharmacological inhibition of ovarian androgen synthesis in rats simulta-neously reduces aromatase activity that can be restored by exogenous T.714

Fitzpatrick et al. found that T, and DHT to a lesser extent, significantly enhances FSH induction of Cyp19 expression and aromatase activity in granulosa cells from



untreated hypophysectomized rats442 (Figure 25.19). This finding was supported by work done in in vitro cultured granulosa cells isolated from rats where T was able to increase Cyp19 and P450scc expression. This regulation was found to be dependent on liver receptor homolog-1 (Lrh1) expression that is induced by T and not DHT through AR binding to Lrh1 promoter regu-latory region715 and provides evidence that T, and not DHT, affects the expression of genes necessary for estro-gen production during folliculogenes. Furthermore, Lrh1 expression is also induced by FSH in granulosa cells,715 providing a potential mechanism of action for synergy necessary for differentiation of granulosa cells in the ovary.

Interestingly, this effect is lost in granulosa cells iso-lated from estrogen-primed hypophysectomized rats,442 supporting the hypothesis that early growing follicles are more sensitive to androgen action and that estrogens downregulate AR expression as part of the process of follicle differentiation. Similar data has been produced in mice. For example, AR antagonists are able to inhibit FSH-induced E2 synthesis in individually cultured murine follicles if included at the beginning of the culture period prior to any indications of differentiation.716 Simi-larly, E2 output by large antral murine follicles in vitro is unaffected by DHT.492 In marmoset ovaries, granulosa cells isolated from small (0.5–1 mm) follicles are highly responsive to the enhancing effects of T or DHT on FSH-induced aromatase activity, but this response is lost in granulosa cells from larger (>2 mm) follicles whereupon DHT exposure becomes inhibitory.717,718 In contrast, 3 or 10 days of T treatment in rhesus monkeys has no effect on ovarian CYP19 expression.684

The majority of data indicate that healthy preantral follicles are responsive to T and require the steroid for the initiation of Cyp19 expression by FSH in granulosa cells. Since androgen synthesis in thecal cells precedes the acquisition of aromatase activity in the accompany-ing granulosa cells, this mechanism of androgen/FSH synergism allows for more efficient induction of Cyp19 expression than would occur with FSH alone. Once the granulosa cells acquire sufficient aromatase activity, thecal-derived androgens become more important as substrates and ER-mediated E2 actions continue the role of synergizing with FSH to promote follicle differentia-tion (Figure 25.25). Hence, the hallmark of a healthy pre-ovulatory follicle in rodent ovaries may be the presence of substantial aromatase activity and E2 output, and decreased AR expression and androgen sensitivity.

The mechanism by which androgens augment FSH action on granulosa cells is unclear but may differ from that hypothesized for E2. Whereas ER-mediated E2 actions are believed to enhance the amount and effec-tiveness of FSH-stimulated intracellular cAMP with-out obvious changes in FSH-receptor levels, evidence

suggests that androgens may act at a site prior to adeny-lyl cyclase. For example, neither T nor DHT synergize with (Bu)2-cAMP to induce aromatase activity in rat granulosa cells719 whereas E2 does.442,720 Instead, AR-mediated androgen actions may largely act by increasing the level of FSH receptor in granulosa cells of preantral follicles. T can also restore the losses in FSH receptor and responsiveness that occur in rat granulosa cells cultured without gonadotropins, and this effect is inhibited by AR antagonists.719 Evidence that androgens increase granu-losa FSH-receptor levels comes from reports that ovaries of 10-day-old and prepubertal AR-null females exhibit significantly reduced Fshr mRNA levels, even 48 h after PMSG treatment in the latter age group.183 Similarly, Weil et al. found that 3 or 10 days of T treatment in adult rhesus monkeys increases FSH-receptor expression in follicles of all stages except primary.684 In human small antral follicles cultured in vitro, a positive correlation was observed in AR expression, androgen concentrations in follicular fluid, and FSHR expression721 in small follicles demonstrating an association with AR and androgens in immature granulosa cells, and suggests that in human ovaries this association is important for normal follicle development.

AR-mediated androgen actions may also be involved in antrum formation. Murray et al. found DHT poten-tiates suboptimal doses of FSH to stimulate increased follicle diameter of murine follicles in culture, and this promotional effect is inhibited by AR antagonist.716 T or DHT are also reported to be almost as effective as E2 in augmenting FSH-induced increases in the num-ber of antral follicles in the ovaries of hypophysecto-mized mice.492 However, the ovaries of global AR-null mice do not show differences in the number of antral follicles,183,185,690 while in the granulosa-specific AR-null mice a reduction in the number of antral follicles was observed at all ages,692 or only in younger mice (3 months).691 The conflicting reports of antral forma-tion in AR-null mice demonstrate the complexity of this developmental process in the ovary. Therefore, FSH-dependent antrum formation may be enhanced by either receptor-mediated androgen or estrogen actions, congruent with the process being part of the transition period during which follicles become less sensitive to androgens and more dependent on estrogens.

Not all FSH-dependent processes during follicle dif-ferentiation are enhanced by androgens. LH-receptor expression and LH responsiveness by granulosa cells occurs only in preovulatory follicles during the final stages of folliculogenesis, just prior to the LH surge. Unlike the above processes of follicle differentiation, FSH stimulation of LH-receptor expression in pre-ovulatory granulosa cells is facilitated by estrogen or aromatizable androgens only515,548,549 while DHT has no effect or may even be inhibitory.515,549 These data



are consistent with the mutually exclusive pattern of AR and LH receptor in preovulatory follicles of rodent ovaries, as AR expression is limited to the cumulus cells667 while LH receptor is predominantly localized to mural and antral granulosa cells.722–724 In addition, in vivo studies demonstrating that AR antagonists have no effect on LH-receptor expression in the ovaries of FSH-stimulated, DES-primed hypophysectomized rats suggest that AR-mediated actions are not required to promote or maintain LH-receptor expression in preovulatory follicles.725 The specific requirement of ER-mediated E2 actions to augment FSH-stimulated LH-receptor expression in preovulatory granulosa cells provides a mechanism by which only those fol-licles that acquire sufficient aromatase activity and are suitable to ovulate acquire the capacity to respond to the LH surge.

THECAL CELL STEROIDOGENESIS

There are few studies on the role of androgen sig-naling in thecal cell function and steroidogenesis despite these cells being the primary source of andro-gen synthesis. Several nonsteroidal paracrine factors are known to modulate thecal cell steroidogenesis and have been more thoroughly studied.476,566 The limited available evidence suggests that AR-mediated andro-gen actions may negatively regulate thecal cell ste-roidogenesis, thereby forming an autocrine regulatory feedback loop. Mahesh and colleagues have shown in enriched thecal/interstitial cell cultures from rats that AR activation by nonaromatizable agonists attenuates hCG or hCG/IFG-1–stimulated increases in andro-stenedione synthesis by 32% and 40%, respectively, via selective inhibition of Cyp17 expression.726 Simi-lar experiments demonstrated that AR-antagonists enhance hCG-induced androstenedione synthesis, fur-ther indicating a receptor-mediated effect of androgens on thecal cell steroidogenesis.726 The majority of recent studies published to date focus on the transcriptional regulation of genes involved in androgen biosynthesis in theca cells,687,688 but not on the autocrine action of the synthesized androgen(s) within the cells. In fact, a recent review entitled “Theca: The forgotten cell of the ovarian follicle” described the various factors that are known to regulate steroidogenesis and androgen production,727 which also highlights the need for more focused experiments looking at theca cell functions of AR. The use of conditional knockout mouse models to explore the role of AR specifically in theca cells could also shine light on this important question. Therefore, ER-mediated E2 actions (see the section Estrogen Receptor Signaling in Ovarian Function) and AR-mediated androgen actions may both contribute to maintaining homeostasis of androgen synthesis in the thecal cells of developing follicles.

OVULATION

Over the past decade there have been few studies aimed at determining a role for androgen signaling in follicle rupture and ovulation, the majority of these from the AR-null mouse models (Table 25.9). This is espe-cially surprising since Mori et al. demonstrated in 1977 that inhibition of androgen action at the time of ovula-tion is detrimental to follicle rupture in the rat.728 These studies employed anti-T and anti-P antisera during induced ovulation in immature or hypophysectomized rats to demonstrate that: (1) acute treatment with anti-T antisera at the time or shortly after hCG-induced ovula-tion leads to a dose-dependent reduction in the number of ovulated oocytes, and (2) inhibition of ovulation by acute treatment with an anti-P antisera at the time of hCG treatment is rescued by concurrent treatment with T or DHT but not E2.728 In a similar study, Peluso et al. demonstrated that treatment of immature rats with the AR antagonists, cyproterone acetate or flutamide, 3 h prior to hCG-induced ovulation drastically reduces the number of ovulated oocytes and prevents expansion of the cumulus–oocyte complex.729 Similar findings are reported in mice.730 The development of AR-null mouse models has provided some insight into the role of androgen signaling in ovulation. An ovulatory defect in AR-null mice is suggested by the presence of a normal number of preovulatory follicles but few corpora lutea in the ovaries,183 as well as reduced circulating P lev-els.689 Furthermore, gonadotropin-induced ovulation of immature AR-null females indicates a severely reduced response in terms of recoverable oocytes in the ovi-ducts, and may be attributable to their failure to exhibit hCG-induced expression of P receptor, hyaluronyl synthetase-2, and tumor necrosis factor-α-stimulated gene 6.183 Reduced ovulatory rates were also observed in another AR-null mouse model lacking the DNA bind-ing domain after normal mating,185 however ovulation rates after treatment with exogenous gonadotropins were normal. A similar finding was observed in granu-losa cell–specific AR-null mouse model where oocytes collected after natural mating are reduced but animals respond to exogenous gonadotropins. Interestingly, when these animals were 6 months of age, they didn’t ovulate at all.692 These mouse models suggest that AR plays a role in normal ovulation, but stimulation with excess gonadotropins can overcome these deficiencies only in young mice. Further work to study the role of androgen signaling in other mammalian species includ-ing primates and humans could provide insight into possible fertility treatments.

FOLLICLE ATRESIA

Follicle atresia in the mammalian ovary has been thoroughly described in several reviews.565,680,731 Fol-licular atresia is a complex hormonally driven process



involving both putative “survival” factors, e.g., insu-lin-like growth factor-1, epidermal growth factor, basic fibroblast growth factor, E2, and activin; and putative atretogenic factors, e.g., tumor necrosis factor-α, GnRH, and androgens.565,680,731 The respective action of each of these factors on atresia is dependent on the follicular stage,731 for example, epidermal growth factor and basic fibroblast growth factor begin to act early on primor-dial stage follicles, whereas insulin-like growth factor-1 affects early antral follicles.

Circumstantial evidence of the actions of androgen includes the numerous observations that atretic follicles consistently exhibit an increased androgen:estrogen ratio within the intrafollicular fluid.565 Supporting experimental evidence includes reports that low doses of hCG (to induce endogenous androgen production), T, or DHT following 4 days of E2 treatment in hypophy-sectomized rats lead to increased numbers of atretic fol-licles within 24 h.705,732 The detrimental effects of hCG or exogenous androgens are inhibited by co-treatment with anti-T antisera or AR antagonists, indicating the involvement of androgen receptor in this process.705,732 Interestingly, large (>150 μm) follicles are more sus-ceptible to the atretogenic effects of androgens than small (<150 μm) follicles, indicating that androgens are largely atretogenic in late-stage follicles.705,732 In simi-lar experiments on hypophysectomized rats, Billig et al. demonstrated that the follicular atresia that follows the withdrawal of 2 days of continuous estrogen (DES) treat-ment is enhanced by subsequent T exposure but pre-vented by E2.733 Conversely, Flaws et al. show follicular atresia induced by the endocrine-disrupting chemical methoxychlor.734

Additionally, AR-null mouse models also have increased atretic follicles,183,185,690 demonstrating that AR signaling plays a role in follicle atresia; how-ever, further studies are needed to understand the mechanism.

Glucocorticoid Receptor Signaling in Ovarian Function

While the ovary is not considered a classic target tis-sue of glucocorticoids, altered glucocorticoid production can affect reproduction and fertility throughout the HPG axis. Within the ovary, glucocorticoid signaling through GR can modulate steroidogenesis/gametogenesis and will briefly be discussed here. GR is expressed in the rat ovary, and examination of whole ovary expression shows that levels do not change upon treatment with exogenous gonadotropins.735 Further examination of GR expression in isolated granulosa cells shows no signifi-cant alterations in expression within these cells, although GR expression appears to be elevated in corpora lutea.735

Glucocorticoid signaling in the ovary acts to inhibit LH action and steroid biosynthesis during folliculogene-sis.736 Treatment of granulosa cells with cortisol or dexa-methasone was able to inhibit FSH-stimulated increases in aromatase activity in vitro, while P production was increased compared to control-treated cells. This finding suggests that GR signaling may affect steroidogenesis of the ovarian steroids in a differential manner.737 Fur-thermore, this finding suggests that GR signaling in the ovary is important for P function and ovulation. Dexa-methasone was also found to stimulate FSH-mediated increases in P production in porcine granulosa cells,738 suggesting a key role for GR signaling during late stages of ovulation and for corpora lutea function.

The expression of GR in granulosa cells does not change significantly during early follicle development; however, the treatment of cells with agonist shows dif-ferential activity depending on the stage of differentiation of the cells. To circumvent the action of glucocorticoids, expression of two metabolizing enzymes has been found to have stage-specific expression in the ovary.736 The expression of the type I and type II 11β-hydroxysteroid dehydrogenase (11HSD 1 and 11HSD 2) modulates the action of glucocorticoids by regulating their concentra-tions within tissues.739 This is important in the ovary during folliculogenesis since glucocorticoids are not syn-thesized within the tissue itself. During FSH-stimulated follicle growth, 11Hsd1 is expressed and acts to suppress glucocorticoid action, then as the granulosa cells become luteinized after LH stimulation, 11Hsd2 expression is increased.735,740 This acts to increase the action of gluco-corticoid action on the ovary during luteinization and for-mation of the corpora lutea, which also shows an increase in GR expression, demonstrating the importance of GR signaling during this process. Interestingly, expression of 11Hsd2 is reduced during luteal regression,741 demon-strating the importance of GR activity specifically in the corpora lutea. The evidence to date suggests that GR sig-naling is important in ovarian function, although further work is needed to define the precise activity and neces-sity of this signaling mechanism specifically in the ovary.

SUMMARY

The salient aspects of the previous discussion are sum-marized in a model of sex steroid and steroid receptor action during folliculogenesis. Although certain postu-lated mechanisms and pathways portrayed in this model are supported by substantial evidence, other aspects are much more speculative and remain to be demonstrated. Regardless, the evidence is solid that the receptor-medi-ated steroid pathways exist in the ovary and their intra-ovarian functions are critical to female fertility.

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CONCLUSION

Within this fairly extensive review we have endeav-ored to describe as comprehensively as possible what is currently known regarding the important functions of estrogen, progesterone, androgen, and glucocorti-coid receptors in ovarian and uterine tissues. Clearly, an abundance of data has been obtained that has greatly advanced our understanding of the roles of these recep-tors since early descriptions more than five decades ago. Nevertheless, many open questions remain for further elaboration. At the level of the steroid receptors them-selves, questions remain regarding how the receptors mediate the spectrum of responses, and what roles tissue and gene specificity of distinct activation domains on the receptors play. Additionally, better understanding of the interactions of receptors with chromatin, which are often quite far from regulated transcripts, and how the inter-actions lead to changes in transcription, are challeng-ing questions to approach. We can hope to learn more about temporal patterns of changes in the organization of nuclear structures initiated by steroid receptors. At the whole animal level, what remains is to piece together advances gleaned from diverse models and methods into a comprehensive description of the coordinated sig-naling between receptors for different steroids in cells and tissues that culminate in an optimal environment for fertilization, implantation, gestation, and parturition.

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