arnej mr a 0804690

T h e n e w e ngl a nd j o u r na l o f m e dic i n e

n engl j med 360;3 nejm.org january 15, 2009268

review article

Mechanisms of Disease

EndometriosisSerdar E. Bulun, M.D.

From the Division of Reproductive Biolo-gy Research, Department of Obstetrics and Gynecology, Feinberg School of Medicine, Northwestern University, Chi-cago. Address reprint requests to Dr. Bu-lun at the Division of Reproductive Biol-ogy Research, Dept. of Obstetrics and Gynecology, Feinberg School of Medi-cine, Northwestern University, 303 E. Su-perior St., Rm. 4-123, Chicago, IL 60611, or at [email protected].

N Engl J Med 2009;360:268-79.Copyright © 2009 Massachusetts Medical Society.

Endometriosis is an estrogen-dependent inflammatory disease that affects 5 to 10% of women of reproductive age in the United States.1 Its defining feature is the presence of endometrium-like tissue in sites outside

the uterine cavity, primarily on the pelvic peritoneum and ovaries. The main clinical features are chronic pelvic pain, pain during intercourse, and infertility.1 Endometri-osis can be the result of diverse anatomical or biochemical aberrations of uterine function. For example, endometriosis commonly develops in young women with vagi-nal obstruction of outflow, possibly because of large quantities of backwashed men-strual tissue that has become implanted on pelvic organs.2 In contrast, endometri-osis can also involve mechanisms that are independent of anatomical abnormalities; for example, the incidence of endometriosis is increased in women who were ex-posed in utero to environmental toxins or potent estrogens such as diethylstilbe-strol.3 As cellular and molecular mechanisms involved in endometriosis are being uncovered, the disease’s classification is evolving from a local disorder to a complex, chronic systemic disease. Endometriosis can be inherited in a polygenic manner; its incidence in relatives of affected women is up to seven times the incidence in women without such a family history.4 There is evidence of linkage to chromosomes 7 and 10, but no relevant genes in these regions appear to have yet been identified.5,6

The three clinically distinct forms of endometriosis are endometriotic implants on the surface of the pelvic peritoneum and ovaries (peritoneal endometriosis), ovar-ian cysts lined by endometrioid mucosa (endometriomas), and a complex solid mass comprised of endometriotic tissue blended with adipose and fibromuscular tissue, residing between the rectum and the vagina (rectovaginal endometriotic nodule). All three types may be variants of the same pathologic process or they can be caused by different mechanisms.7,8 Their common histologic features are the presence of endometrial stromal or epithelial cells, chronic bleeding, and signs of inflamma-tion. These lesions can occur singly or in combination and are associated with an increased risk of infertility or chronic pelvic pain.9,10 The inflammation involved in endometriosis can stimulate nerve endings in the pelvis and thereby cause pain, impair the function of uterine tubes, decrease receptivity of the endometrium, and hinder development of the oocyte and embryo.11,12 Endometriosis can also cause infertility by physically blocking the fallopian tubes.9,10

The treatment of infertility caused by endometriosis is surgical removal of endo-metriotic tissue or assisted reproductive technology, whereas the usual treatment of pain is a combination of medical suppression of ovulation and surgery. Peritoneal implants are resected or vaporized by means of an electric current or laser. Ovarian endometriomas and rectovaginal endometriotic nodules, however, can be removed effectively only with the use of full dissection. Epidemiologic and laboratory data sug-gest a link between ovarian endometriosis and distinct types of ovarian cancer.13,14

Clinical evidence clearly points to a deleterious effect of uninterrupted ovulatory cycles on the development and persistence of endometriosis.15,16 First, symptoms of endometriosis usually appear after menarche and vanish after menopause.17 Occa-

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n engl j med 360;3 nejm.org january 15, 2009 269

sionally, a rectovaginal nodule remains symptom-atic in a postmenopausal woman, suggesting that its persistence is independent of ovarian estrogen secretion.18 Second, multiparity is associated with a decreased risk of endometriosis.17 Third, the disruption of ovulation by analogues of gonad-otropin-releasing hormone (GnRH), oral contra-ceptives, or pro gestins reduces pelvic disease and the associated pain.16

In line with these clinical observations are the findings that indicate major roles of the ovarian steroids estrogen and progesterone in the devel-opment of endometriosis. In humans and other primates, estrogen stimulates the growth of en-dometriotic tissue, whereas aromatase inhibitors that block estrogen formation are beneficial, as are antiprogestins, in patients with endometrio-sis.19-21 Levels of nuclear receptors for estrogen and progesterone are strikingly perturbed in en-dometriotic tissue as compared with normal endo-metrium.22-24 Finally, biologically significant quan-tities of progesterone and estrogen are produced locally in endometriotic tissue, through an abnor-mally active steroidogenic cascade that includes aromatase.25 This review deals mainly with ste-roid-related mechanisms of endometriosis.

Cellul a r Or igins

There is no consensus concerning the histologic origin of endometriosis. Sampson proposed that fragments of menstrual endometrium pass back-ward through the fallopian tubes and then be-come implanted on peritoneal surfaces and per-sist there.26 This mechanism was shown in primate models, was observed in humans, and is support-ed by the observation that endometriosis occurs exclusively in species that menstruate (i.e., humans and other primates).27 In contrast, the coelomic-metaplasia hypothesis proposes that the genesis of endometriotic lesions within the peritoneal cav-ity is the differentiation of mesothelial cells into endometrium-like tissue.28 A third hypothesis ar-gues that menstrual tissue from the endometrial cavity travels to other sites through veins or lym-phatic vessels.29 Another proposal is that circu-lating blood cells originating from bone marrow can differentiate into endometriotic tissue at vari-ous sites.30 Proving or disproving these hypoth-eses is challenging because of the difficulty in con-structing clinically relevant models.

Sampson’s implantation hypothesis is a plau-sible mechanism for most endometriotic lesions

but does not explain why endometriosis develops in some, but not most, women. Most women have reflux menstruation into the peritoneal cavity, but endometriosis occurs in only 5 to 10%. One of two mechanisms could explain the successful implan-tation of refluxed endometrium onto the perito-neal surface: molecular defects or immunologic abnormalities (or both).31 In endometriosis, the eutopic endometrium exhibits multiple subtle but biologically important molecular abnormalities, including the activation of oncogenic pathways (e.g., the Wingless-type MMTV integration site family member Wnt or the rat sarcoma viral on-cogene homologue Ras) or biosynthetic cascades favoring increased production of estrogen, cyto-kines, prostaglandins, and metalloproteinases.1,32-36 When the eutopic endometrium, biologically dis-tinct tissue, attaches to mesothelial cells, the mag-nitude of the molecular abnormalities is ampli-fied drastically, enhancing the survival of the implant.37 A possible second mechanism of im-plant survival entails a failure of the immune sys-tem to clear implants from the peritoneal sur-face.1,38,39 Both mechanisms may contribute to the development of endometriosis.

Molecul a r Mech a nisms

There are clear molecular distinctions between en-dometriotic tissue and endometrium, such as the overproduction of estrogen, prostaglandins, and cytokines in endometriotic tissue (Fig. 1).32,40,41 Subtle forms of these abnormalities also occur in endometrium from a woman with endometriosis as compared with endometrium from a woman without the disease (Fig. 1).32,40,41 Moreover, gene-expression profiling of endometrium from women with endometriosis as compared with endometri-um from disease-free women has revealed candi-date genes related to implantation failure, infertil-ity, and progesterone resistance.35,42

Inflammation, a feature of endometriotic tis-sue, is associated with the overproduction of pros-taglandins, metalloproteinases, cytokines, and chemokines.1,32-36,43 Increased levels of acute in-flammatory cytokines such as interleukin-1β, in-terleukin-6, and tumor necrosis factor probably enhance the adhesion of shed endometrial-tissue fragments onto peritoneal surfaces, and prote-olytic membrane metalloproteinases may further promote implantation of the fragments.1,32-36,43 Monocyte chemoattractant protein 1, interleukin-8, and RANTES (regulated upon activation normal





T-cell expressed and secreted) attract the granu-locytes, natural killer cells, and macrophages that are typical of endometriosis.1,32-36,44,45 Autoregu-latory positive-feedback loops ensure further ac-cumulation of these immune cells, cytokines, and chemokines in established lesions.46

In patients with endometriosis, inflammatory and immune responses, angiogenesis, and apop-tosis are altered in favor of the survival and replen-ishment of endometriotic tissue.38,39,47-49 These basic pathologic processes depend in part on es-trogen or progesterone. Excessive formation of estrogen and prostaglandin and the development of progesterone resistance have emerged as clini-cally useful points of study because the therapeu-tic targeting of aromatase in the estrogen biosyn-thetic pathway, cyclooxygenase-2 (COX-2) in the

prostaglandin pathway, or the progesterone recep-tor reduces pelvic pain or laparoscopically visible endometriosis or both (Fig. 1).9,20,50,51 These three critical targets have been linked through specific epigenetic markers (hypomethylation) that cause overexpression of the nuclear receptors steroido-genic factor 1 (SF1) and estrogen receptor β.24,52

Estrogen Formation in Endometriosis

Estrogen production plays a key role in endometri-osis. Its inhibition by GnRH analogues, oral con-traceptives, progestins, and aromatase inhibitors reduces pelvic disease and pain (Fig. 2A).16 Ste-roidogenic acute regulatory protein (STAR) facili-tates the initial step of estrogen formation (i.e., the entry of cytosolic cholesterol into the mitochon-drion). Then, five proteins catalyzing six enzymatic

A Endometrium in disease-free women

B Endometrium in women with endometriosis

C Ectopic endometriotic tissue

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Figure 1. Normal Endometrium and Endometriosis.

In normal endometrial tissue (Panel A), activity of the enzyme cyclooxygenase-2 (COX-2), and thus production of pros-taglandin E2 (PGE2), is low. Estrogen is not produced locally, owing to the absence of aromatase. During the luteal phase, the progesterone-dependent 17β-hydroxysteroid dehydrogenase 2 (HSD17B2) enzyme catalyzes the conversion of the biologically active estradiol to estrone that is less estrogenic. In the endometrium of women with endometriosis (Panel B), there is a subtle increase in COX-2 activity and detectable aromatase activity. In ectopic endometriotic tissue (Panel C), full-blown molecular abnormalities include high COX-2 and aromatase levels. Increased PGE2 formation in endometrial and endometriotic tissues can cause severe menstrual cramps and chronic pelvic pain. Tissue estradiol levels should be high, because estradiol is overproduced by aromatase and is not metabolized owing to deficient HSD17B2 activity. Increasing enzyme activity is denoted by increasing thickness of arrows. Modified from Bulun et al.25





A Sources of estradiol in endometriotic tissue

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Figure 2. Molecular Distinctions between Endometriotic Tissue and Endometrium.

Panel A shows the three sources of estradiol, biologically active estrogen, in endometriotic tissue. The first sources are follicle-stimulating hormone (FSH) and luteinizing hormone (LH), which induce the expression of ovarian ste-roidogenic genes, including aromatase, for biosynthesis of estradiol. Ovarian secretion of estradiol can be reduced through suppression of FSH and LH by gonadotropin-releasing hormone (GnRH) analogues, combination oral con-traceptives, or progestins. The second source of estrogen is the estradiol that arises from aromatase activity in fat or skin. The third source of estradiol is local production in endometriotic tissue. Aromatase inhibition in peripheral tissue (fat and skin) and endometriotic tissue stops estradiol biosynthesis and is therapeutic in endometriosis. As shown in Panel B, high levels of local estradiol and prostaglandin E2 (PGE2) are maintained in endometriotic tissue by autoregulatory positive-feedback mechanisms (indicated by plus signs) that involve nuclear receptors (steroido-genic factor 1 [SF1] and estrogen receptor β [ER-β]), enzymatic pathways, cytokines, and growth factors. COX-2 de-notes cyclooxygenase-2, HSD17B1 17β-hydroxysteroid dehydrogenase 1, HSD3B2 3β-hydroxysteroid dehydrogenase 2, SCC side-chain cleavage enzyme, STAR steroidogenic acute regulatory protein, and VEGF vascular endothelial growth factor. Modified from Bulun et al.25





steps (side-chain cleavage enzyme, 3β-hydroxy-steroid dehydrogenase 2, 17-hydroxylase–17-20-lyase, aromatase, and 17β-hydroxysteroid dehy-drogenase 1) convert cholesterol to biologically active estradiol (Fig. 2B).25 The key step, the con-version of C19 steroids to estrogens, is catalyzed by aromatase, inhibition of which effectively elim-inates all estrogen production (Fig. 2B).25

Origins of Estrogen in EndometriosisThree major sites in the body produce estrogen in women with endometriosis. First, estradiol se-creted by the ovary reaches endometriotic tissue through the circulation. Moreover, follicular rup-ture during each ovulation causes spillage of large amounts of estradiol directly onto pelvic implants. Second, aromatase in adipose tissue and skin cat-alyzes the conversion of circulating androstene-dione to estrone that is subsequently converted to estradiol, and this estrone and estradiol can enter the circulation and reach sites of endometriosis. Another source of estradiol is cholesterol, which is converted to estradiol locally in endometriosis because endometriotic tissue expresses a complete set of steroidogenic genes, including aromatase.25 Peripheral or local aromatase activity, or both, may be particularly important in the persistence of en-dometriosis — the use of a combination of a GnRH agonist plus an aromatase inhibitor is superior to the use of a GnRH agonist alone for long-lasting relief of pain (Fig. 2A).53

Estrogen Production and InflammationCell survival and inflammation are responsible for chronic pelvic pain and infertility, the primary symptoms of endometriosis. Estrogen enhances the survival or persistence of endometriotic tissue, whereas prostaglandins and cytokines mediate pain, inflammation, and infertility.54,55 A link be-tween inflammation and estrogen production in endometriosis was uncovered in a feedback cycle that favors the overexpression of key steroidogenic genes (most notably aromatase), overexpression of COX-2, and continuous local production of es-tradiol and prostaglandin E2 in endometriotic tissue.41,56-58

Prostaglandin E2 and Biosynthesis of EstradiolProstaglandin E2 coordinately stimulates the ex-pression of all steroidogenic genes necessary to en-able the endometriotic stromal cell to synthesize estradiol from cholesterol.25 Among the six ste-

roidogenic genes, the regulation of two — STAR and CYP19A1, the aromatase gene — has been char-acterized extensively (Fig. 2B).25

Endometriotic and endometrial stromal cells express each of the prostaglandin E2 receptor sub-types, EP1, EP2, EP3, and EP4.59 Activation of the EP2 receptor raises the intracellular levels of cyclic AMP (cAMP), which is responsible for inducing the expression of STAR and CYP19A1 by way of pros-taglandin E2 in endometriotic stromal cells.25,57,59 Prostaglandin E2 or a cAMP analogue increases STAR and aromatase levels and activity in endo-metriotic cells but not in endometrial stromal cells.25,41,57 Thus, steroidogenesis dependent on prostaglandin E2 and cAMP in endometriotic stro-mal cells requires downstream effectors, whereas inhibitory mechanisms or a lack of stimulatory effectors downstream of cAMP account for the absence of steroidogenesis in normal endometrial stromal cells.

The nuclear receptor SF1, which is present in endometriotic tissue and absent in endometrium, is the key transcription factor that mediates the expression — dependent on prostaglandin E2 and cAMP — of STAR, CYP19A1, and possibly other steroidogenic genes in endometriotic stromal cells. In prostaglandin E2–treated endometriotic cells, SF1 binds coordinately and assembles an enhancer transcriptional complex recruited to the promoters of the STAR and CYP19A1 genes to induce their expression (Fig. 2B).25

The absence of SF1 in endometrial cells is a major cause of the lack of responsiveness of ste-roidogenic genes to prostaglandin E2. In addition, the redundancy of transcriptional inhibitors of STAR and CYP19A1 gene promoters in endometrial cells constitute a fail-safe system for silencing these steroidogenic genes. These repressors are chicken ovalbumin upstream promoter–transcrip-tion factor (COUP-TF), the Wilms’ tumor 1 tran-scription factor (WT1), and CCAAT/enhancer bind-ing protein β (C/EBPβ). Levels of WT1 and C/EBPβ are much higher in normal endometrium than in endometriotic tissue. In the absence of SF1, a tran-scriptional complex composed of repressors binds the steroidogenic promoters and suppresses them in endometrial cells.25

In summary, on induction of prostaglandin E2, coordinated recruitment of SF1 to the promoters of the essential steroidogenic genes is the key event for estradiol synthesis in endometriotic stromal cells. A suitable therapeutic target among the ste-





roidogenic enzymes is aromatase, because its inhi-bition blocks all estradiol biosynthesis. Aromatase inhibitors diminish or eradicate endometriotic implants and associated pain that is refractory to currently available treatments.19

Prostaglandin Production in Endometriosis

Prostaglandins, locally produced hormones in-volved in inflammation and pain, are important in the pathogenesis of endometriosis. In particu-lar, prostaglandin E2 and prostaglandin F2α are produced in excess in uterine and endometriotic tissues of women with endometriosis.60 The vaso-constrictive properties of prostaglandin F2α, to-gether with its ability to cause uterine contractions, contributes to dysmenorrhea, whereas prostaglan-din E2 can induce pain directly.60 These prosta-glandins are clinically relevant because the reduc-tion of prostaglandin formation by nonselective COX inhibitors (e.g., naproxen sodium and ibu-profen) decreases pelvic pain associated with en-dometriosis.16,50 Care must be taken in long-term administration of nonselective COX inhibitors be-cause they have gastrointestinal side effects. Se-lective COX-2 inhibitors with milder side effects (e.g., rofecoxib and valdecoxib) were approved by the Food and Drug Administration for the treat-ment of primary dysmenorrhea, and their long-term administration could potentially reduce the chronic pelvic pain associated with endometriosis. The use of COX-2 inhibitors has, however, been limited by their link to an increased risk of car-diovascular disease.50

Cyclooxygenase catalyzes the conversion of arachidonic acid (released by phospholipase A2) to prostaglandin H2 in most cells, including those in myometrium, endometrium, and endometriotic tissue.61 Of the two isoforms, COX-1 is generally responsible for basal prostaglandin synthesis, whereas COX-2 is important in inflammation. The coupling of COX-dependent synthesis of pros-taglandin H2 to its metabolism by enzymes down-stream is orchestrated in a cell-specific fashion. Uterine cells are rich in prostaglandin F syn-thase and microsomal prostaglandin E synthase, which catalyze the conversion of prostaglandin H2 to prostaglandin F2α and prostaglandin E2, respectively.60

Excessive prostaglandin E2 production during inflammation through coordinated induction of multiple enzymes, in particular COX-2 and mi-crosomal prostaglandin E synthase, is a concept

in development.61,62 Endometriotic stromal cells produce large quantities of prostaglandin E2, which induce local estrogen biosynthesis and pel-vic pain.41,56,57,63 COX-2 is up-regulated to a greater degree in endometriotic stromal cells as compared with endometrial stromal cells; moreover, its ex-pression is also increased in the endometrium of women with endometriosis as compared with that of disease-free women.64,65 Expression of micro-somal prostaglandin E synthase has been shown in both benign and malignant endometrium.66-68 Thus, increased synthesis of prostaglandin E2 in endometriotic tissue may be due to coordinated hyperactivity of COX-2 and microsomal prosta-glandin E synthase.

At least four hormones, present in high quan-tities in endometriotic tissue, induce COX-2 expres-sion and prostaglandin E2 production in endo-metriotic and uterine cells (Fig. 2B). The cytokine interleukin-1β or prostaglandin E2 itself (in an autocrine manner) induces COX-2 expression in endometriotic and endometrial stromal cells.69,70 The angiogenic factor vascular endothelial growth factor or estradiol rapidly induces, through estro-gen receptor β, COX-2 expression in uterine en-dothelial and endometriotic stromal cells.69-71 These redundant mechanisms maintain large quantities of prostaglandin E2 in endometriotic tissue (Fig. 2B).

Thus, cytokines, angiogenic factors, prosta-glandin E2 itself, and estradiol all induce COX-2 expression in patients with endometriosis and thereby ensure production of large quantities of prostaglandin E2. Estrogen receptor β mediates the induction of COX-2 by estradiol in endometri-otic tissue.

Progesterone Resistance in Endometriosis

In contrast to the clearly unfavorable effect of es-trogen on endometriosis, the role of progesterone has remained ambiguous for the following three reasons. First, the protective role of progesterone with regard to endometrial cancer, in which there is epithelial-cell proliferation, has been inappro-priately attributed to endometriosis, which consists primarily of stromal cells with a low rate of apop-tosis and little differentiation.49 Paradoxically, pro-gesterone induces the transient proliferation of stromal cells in normal endometrium during the secretory phase. Second, progestin-based treat-ments for pain are at best variably effective, espe-cially in patients who have previously received other





forms of treatment.72,73 Progestins probably reduce pain by suppressing or attenuating ovulation. A di-rect effect of progestins on endometriotic tissue cannot be ruled out, however. Third, a spectrum of antiprogestins with a mix of agonist and an-

tagonist properties (selective progesterone-receptor modulators) may reduce endometriosis-associated pelvic pain more effectively than progestins.74,75 To add a further twist, endometriotic tissue pro-duces large quantities of progesterone and con-tains much lower levels of progesterone receptors than endometrium.23,57 These seemingly disparate observations make it difficult to form a unified hypothesis regarding the role of progesterone in endometriosis.

Estrogen and progesterone are essential and sufficient to control all endometrial function by regulating expression of hundreds to thousands of genes during the menstrual cycle.76 Indeed, the administration of estradiol and progesterone is sufficient to prepare the endometrium for implan-tation in postmenopausal women undergoing do-nor embryo transfer.77 Progesterone induces the differentiation of endometrial stromal cells (called decidualization) and epithelial cells (resulting in the secretory phenotype). Molecular markers of progesterone action include increased production of epithelial glycodelin (a glycoprotein produced by the secretory endometrium during the luteal phase) and stromal prolactin in endometrium.76,78,79 Progesterone, however, induces much lower levels of prolactin expression in endometriotic cells than in endometrial stromal cells, suggesting that pro-gesterone resistance may be at play in endome-triosis.80

In the endometrium, progesterone exerts an antiestrogenic effect, in part by inducing 17β- hydroxy steroid dehydrogenase 2 (HSD17B2), which catalyzes the conversion of biologically potent es-tradiol to the much less estrogenic estrone.81-84 Progesterone acts by way of progesterone receptors in endometrial stromal cells to increase formation of retinoic acid, which in turn induces HSD17B2 expression in endometrial epithelial cells, in a paracrine fashion.85-87 Endometriotic stromal cells, however, fail to respond to progesterone and thus do not produce retinoic acid (Fig. 3).88 In endo-metriotic tissue, this lack of retinoic acid leads to the lack of epithelial HSD17B2 and the failure to inactivate estradiol.88,89 In combination with high estradiol production due to aberrant aro-matase activity, this additional defect contributes to the abnormally high levels of estradiol in en-dometriotic tissue.80

Gene-expression profiles of the endometrium of women with and those without endometriosis have shown that a number of genes targeted by

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Figure 3. Disrupted Paracrine Action of Progesterone in Endometriotic Tissue.

In endometriotic tissue, the paracrine action of progesterone is disrupted by the decreased progesterone receptor (PR) levels in the stromal cell. In contrast, in the endometrium, progesterone induces the expression of epi-thelial 17β-hydroxysteroid dehydrogenase 2 (HSD17B2) by activating strom-al PRs. These receptors mediate the formation of retinoic acid (RA) and other unknown paracrine factors, which induce the binding to, and thus ac-tivation of, the HSD17B2 promoter by a transcriptional activator complex of the transcription factors Sp1 and Sp3, retinoic acid receptor α (RARα), and retinoid X receptor α (RXRα). The outcome is a decrease in local biological-ly active estrogen, estradiol. The term mRNA denotes messenger RNA.





progesterone are deregulated during implantation, at which time the endometrium is exposed to the highest levels of progesterone.35,42 For example, the prototypic gene responsive to progesterone, glycodelin, is down-regulated in the endometrium of women with endometriosis as compared with women without endometriosis.35 These findings suggest that eutopic endometrium of women with endometriosis also exhibits progesterone resis-tance.35,90

Progesterone resistance is explicable by the ex-tremely low progesterone-receptor levels in endo-metriotic tissue.23 In endometrium, levels of the progesterone receptor isoforms, PR-B and PR-A, progressively increase during the proliferative phase, peak immediately before ovulation, and diminish after ovulation, suggesting that estradiol stimulates progesterone-receptor levels.23 In con-trast, PR-B is undetectable, and PR-A is markedly reduced, in endometriotic tissues. Moreover, defi-ciency of the cochaperone FK506-binding pro-tein of 52 kD, a peptidyl prolyl isomerase neces-sary for progesterone-receptor function in the endometrium of baboons with endometriosis, may contribute to progesterone resistance.91

Epigenetic Changes in Endometriosis

Levels of the nuclear receptors SF1, estrogen re-ceptors α and β, and progesterone receptors are much different in endometriotic tissue than in endometrium. The virtual absence of the orphan nuclear receptor SF1 in endometrium and its pres-ence at levels more than 12,000 times higher, in endometriotic tissue, is in part determined by a classic CpG (cytosine–phosphate–guanine) island at its promoter that is heavily methylated in en-dometrial stromal cells and unmethylated in en-dometriotic stromal cells.52 In endometrial cells, the silencer-type transcription factor methyl-CpG-binding domain protein 2 is recruited to the meth-ylated SF1 promoter and prevents its interaction with transcriptional activators (Fig. 4). In endo-metriotic cells, the transcription factor, upstream stimulatory factor 2, binds to the unmethylated SF1 promoter and activates it.92 Upstream stimu-latory factor 2 levels are much higher in endometri-otic tissue than in the endometrium. Thus, differ-ential SF1 expression in endometriotic tissue as compared with endometrium is primarily con-trolled by an epigenetic mechanism that permits the binding of activator complexes, rather than in-hibitor complexes, to its promoter.52,92

Among estrogen receptors, estrogen receptor β levels in endometriotic tissue are 142 times the levels in endometrium, whereas estrogen recep-tor α levels are 9 times the levels in endometri-um.24 Hypomethylation of a CpG island at the promoter region of the estrogen receptor β gene causes high levels of expression of the gene in endometriotic stromal cells, and hypermethylation silences it in endometrial stromal cells (Fig. 4). Estrogen receptor β in endometriotic stromal cells occupies the estrogen receptor α promoter and down-regulates its activity, thus favoring the sup-pression of estrogen receptor α levels.24 The high ratio of estrogen receptor β levels and estrogen receptor α levels in endometriotic stromal cells in turn leads to increased estrogen receptor β bind-

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Figure 4. Epigenetic Changes in Endometriotic Tissue.

The promoters of two nuclear receptors, steroidogenic factor 1 (SF1) and estrogen receptor β (ER-β), are heavily methylated and thus silenced in en-dometrial stromal cells. A lack of promoter methylation is associated with promoter activation and pathologic overexpression of these nuclear recep-tors in endometriotic stromal cells. SF1 mediates prostaglandin E2 (PGE2)–dependent induction of estradiol production. ER-β suppresses estrogen re-ceptor α (ER-α) and progesterone receptors (PRs), leading to progesterone resistance and deficient inactivation of estradiol. Consequently, in endo-metriotic tissue, estradiol and PGE2 are produced in large quantities and enhance cell survival and inflammation. The corepressor and coactivator are hypothetical. COX-2 denotes cyclooxygenase-2, CpG a cytosine–phos-phate–guanine sequence, HSD17B2 17β-hydroxysteroid dehydrogenase 2, MeCP2 methyl-CpG-binding domain protein 2, and RA retinoic acid.





ing to the progesterone-receptor promoter and mediates the down-regulation of expression of progesterone receptors (Fig. 4).

Conclusions

Increased cell survival, inflammation, and defi-cient differentiation in endometriosis have been linked to a stromal-cell defect involving the exces-sive formation of estrogen and prostaglandin, as well as progesterone resistance, all of which origi-nate from two distinct epigenetic changes that af-fect the transcription factors SF1 and estrogen re-ceptor β. The CpG islands occupying the promoters of the genes encoding SF1 and estrogen receptor β are heavily methylated and silence these genes in

endometrium, whereas the lack of methy lation re-sults in extraordinarily high levels of SF1 and es-trogen receptor β in endometriotic tissue (Fig. 4). In response to the exposure of endometriotic cells to prostaglandin E2, SF1 coordinately binds to the promoters of multiple steroidogenic genes, includ-ing that of aromatase, and causes the formation of large quantities of estradiol. Estradiol acts by way of estrogen receptor β to stimulate COX-2, leading to the overproduction of prostaglandin E2. Thus, inflammation and estrogen are linked in a feed-back cycle involving the overexpression of genes that encode the aromatase and COX-2 enzymes and continuous formation of the products of aro-matase and COX-2 — estradiol and prostaglandin E2 — in endometriotic tissue. Moreover, estrogen

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Cross section of a 5-week, 46,XX embryo

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Wolffianduct

Aorta

environmental effects

Gonadalridge

Genitalridge

Origin ofmüllerian duct

Hindgut

Epigenetic aberrations in progenitor cells

Hypomethylation of SF1 and ER-β promoters

Subtle differentiation defects of uterus,ovaries, or pelvic peritoneum

Endometriotic phenotype

Genetic or

Location ofcross section

Figure 5. Speculated Changes in DNA Methylation from Genetic or Environmental Factors.

During the embryonic differentiation of the female genital tract, various environmental or genetic factors may cause changes in DNA methylation, shown here for a 5-week embryo. If these epigenetic alterations pathologically affect the expression of critical genes such as steroidogenic factor 1 (SF1) or estrogen receptor β (ER-β) in progenitor cells destined to form various pelvic tissues, the ensuing molecular abnormalities may predispose the adult woman to endometriosis.





receptor β suppresses progesterone-receptor lev-els, resulting in progesterone resistance and dis-ruption of a paracrine pathway that inactivates es-tradiol. Large amounts of estradiol accumulate, owing to its increased formation and deficient inactivation in endometriotic tissue (Fig. 4).

This working model (Fig. 4) is clinically rele-vant because the targeting of aromatase, COX-2, estrogen receptor β, or progesterone receptors re-duces pelvic pain and ablates visible endometriotic tissue.93 Finally, I speculate that genetic predis-position or the exposure to environmental toxins

of fetal progenitor cells destined to form adult female pelvic organs may result in epigenetic events, including promoter hypomethylation and overexpression of SF1 and estrogen receptor β, that can play critical roles in the pathogenesis of endometriosis (Fig. 5).

Supported in part by grants from the National Institutes of Health (HD38691 and HD40093) and from the Friends of Prentice.

Dr. Bulun reports receiving consultation fees from Meditrina Pharmaceuticals, GlaxoSmithKline, and Novartis. No other po-tential conflict of interest relevant to this article was reported.

I thank Dr. Sherman Elias for helpful discussions.

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