Signaling pathway

Neurotrophin-induced increase in Signal transducer and activator of transcription 3 (STAT3; acute-phase response factor) activation appears to underly several downstream functions of neurotrophin signalling, such as transcription of immediate early genes, proliferation arrest, and neurite outgrowth.

SH2-STAT3

Src homology 2 (SH2) domain found in signal transducer and activator of transcription (STAT) 3 proteinsSTAT3 encoded by this gene is a member of the STAT protein family. STAT3 mediates the expression of a variety of genes in response to cell stimuli, and plays a key role in many cellular processes such as cell growth and apoptosis. The small GTPase Rac1 regulates the activity of STAT3 and PIAS3 inhibits it. Three alternatively spliced transcript variants encoding distinct isoforms have been described. STAT 3 activation is required for self-renewal of embryonic stem cells (ESCs) and is essential for the differentiation of the TH17 helper T cells. Mutations in the STAT3 gene result in Hyperimmunoglobulin E syndrome and human cancers. STAT3 has been shown to interact with Androgen receptor, C-jun, ELP2, EP300, Epidermal growth factor receptor, Glucocorticoid receptor, HIF1A, Janus kinase 1, KHDRBS1, Mammalian target of rapamycin, MyoD, NDUFA13, NFKB1, Nuclear receptor coactivator 1, Promyelocytic leukemia protein, RAC1, RELA, RET proto-oncogene, RPA2, Src, STAT1, and TRIP10. STAT proteins mediate the signaling of cytokines and a number of growth factors from the receptors of these extracellular signaling molecules to the cell nucleus. STATs are specifically phosphorylated by receptor-associated Janus kinases, receptor tyrosine kinases, or cytoplasmic tyrosine kinases. The phosphorylated STAT molecules dimerize by reciprocal binding of their SH2 domains to the phosphotyrosine residues. These dimeric STATs translocate into the nucleus, bind to specific DNA sequences, and regulate the transcription of their target genes. However there are a number of unphosphorylated STATs that travel between the cytoplasm and nucleus and some STATs that exist as dimers in unstimulated cells that can exert biological functions independent of being activated. There are seven mammalian STAT family members which have been identified: STAT1, STAT2, STAT3, STAT4, STAT5 (STAT5A and STAT5B), and STAT6. There are 6 conserved domains in STAT: N-terminal domain (NTD), coiled-coil domain (CCD), DNA-binding domain (DBD), alpha-helical linker domain (LD), SH2 domain, and transactivation domain (TAD). NTD is involved in dimerization of unphosphorylated STATs monomers and for the tetramerization between STAT1, STAT3, STAT4 and STAT5 on promoters with two or more tandem STAT binding sites. It also plays a role in promoting interactions with transcriptional co-activators such as CREB binding protein (CBP)/p300, as well as being important for nuclear import and deactivation of STATs involving tyrosine de-phosphorylation. CCD interacts with other proteins, such as IFN regulatory protein 9 (IRF-9/p48) with STAT1 and c-JUN with STAT3 and is also thought to participate in the negative regulation of these proteins. Distinct genes are bound to STATs via their DBD domain. This domain is also involved in nuclear translocation of activated STAT1 and STAT3 phosphorylated dimers upon cytokine stimulation. LD links the DNA-binding and SH2 domains and is important for the transcriptional activation of STAT1 in response to IFN-gamma. It also plays a role in protein-protein interactions and has also been implicated in the constitutive nucleocytoplasmic shuttling of unphosphorylated STATs in resting cells. The SH2 domain is necessary for receptor association and tyrosine phosphodimer formation. Residues within this domain may be particularly important for some cellular functions mediated by the STATs as well as residues adjacent to this domain. The TAD interacts with several proteins, namely minichromosome maintenance complex component 5 (MCM5), breast cancer 1 (BRCA1) and CBP/p300. TAD also contains a modulatory phosphorylation site that regulates STAT activity and is necessary for maximal transcription of a number of target genes. The conserved tyrosine residue present in the C-terminus is crucial for dimerization via interaction with the SH2 domain upon the interaction of the ligand with the receptor. STAT activation by tyrosine phosphorylation also determines nuclear import and retention, DNA binding

to specific DNA elements in the promoters of responsive genes, and transcriptional activation of STAT dimers. In addition to the SH2 domain there is a coiled-coil domain, a DNA binding domain, and a transactivation domain in the STAT proteins. In general SH2 domains are involved in signal transduction. They typically bind pTyr-containing ligands via two surface pockets, a pTyr and hydrophobic binding pocket, allowing proteins with SH2 domains to localize to tyrosine phosphorylated sites.

STAT protein, DNA binding domainSTAT proteins (Signal Transducers and Activators of Transcription) are a family of transcription factors that are specifically activated to regulate gene transcription when cells encounter cytokines and growth factors. This family represents the DNA binding domain of STAT, which has an ig-like fold. STAT proteins also include an SH2 domain pfam00017.

STAT protein, all-alpha domainSTAT proteins (Signal Transducers and Activators of Transcription) are a family of transcription factors that are specifically activated to regulate gene transcription when cells encounter cytokines and growth factors. STAT proteins also include an SH2 domain pfam00017.

STAT protein, protein interaction domainSTAT proteins (Signal Transducers and Activators of Transcription) are a family of transcription factors that are specifically activated to regulate gene transcription when cells encounter cytokines and growth factors. STAT proteins also include an SH2 domain.

Molecular GeneticsHyper-IgE Syndrome

Minegishi et al. (2007) showed that dominant-negative mutations in the STAT3 gene result in the classic multisystem hyper-IgE syndrome (HIES; 147060), a disorder of both immunity and connective tissue. They found that 8 of 15 unrelated nonfamilial HIES patients had heterozygous STAT3 mutations (see, e.g., 102582.0001-102582.0003). None of the parents or sibs of the patients had the mutant allele, suggesting that the 5 different mutations, all of which were located in the STAT3 DNA-binding domain, occurred de novo. All 5 mutants were nonfunctional by themselves and showed dominant-negative effects when coexpressed with wildtype STAT3. 

Holland et al. (2007) likewise found mutations in STAT3 in the hyper-IgE syndrome. They found increased levels of proinflammatory gene transcripts in unstimulated peripheral blood neutrophils and mononuclear cells from patients with HIES as compared with levels in control cells. In vitro cultures of mononuclear cells from patients that were stimulated with

lipopolysaccharide had higher tumor necrosis factor-alpha (TNFA; 191160) levels than did identically treated cells from unaffected individuals. In contrast, the cells from patients with HIES generated lower levels of monocyte chemoattractant protein-1 (MCP1; 158105) in response to the presence of interleukin-6, suggesting a defect in interleukin-6 signaling through its downstream mediators, one of which is STAT3. Holland et al. (2007) identified missense mutations and single-codon in-frame deletions in STAT3 in 50 familial and sporadic cases of HIES. Eighteen discrete mutations, 5 of which were hotspots, were predicted to affect directly the DNA-binding and SRC homology-2 (SH2) domains. 

By flow cytometric and RT-PCR analyses, Ma et al. (2008) demonstrated that HIES patients with heterozygous mutations in STAT3 failed to generate IL17-secreting Th17 cells in vivo and in vitro due to a failure to express sufficient levels of the Th17-specific transcription factor RORGT (602943). Ma et al. (2008) proposed that, because Th17 cells are important in immunity against fungal infections, susceptibility to infections in patients with HIES may be explained by their diminished ability to generate Th17 cells. 

By flow cytometric analysis following mitogen activation of IL17-expressing blood T cells from healthy controls or patients with particular genetic traits affecting various cytokine signaling pathways, de Beaucoudrey et al. (2008)found that there was considerable interindividual variability in IL17 expression in controls and most patient groups. However, dominant-negative mutations in STAT3 in HIES patients and, to a lesser extent, null mutations in IL12B or IL12RB1 (601604) in patients with mendelian susceptibility to mycobacterial disease (see 209950) impaired development of IL17-producing T cells. 

Using flow cytometric analysis, Siegel et al. (2011) demonstrated a significant reduction in central memory (i.e., expressing CD27, 186711, and CD45RO, 151460) CD4 (186940)-positive and CD8 (see 186910)-positive T cells in autosomal dominant HIES

patients that was not due to apoptosis or cell turnover. Stimulation of naive T cells in the presence of IL7 (146660) or IL15 (600554) failed to restore memory cell generation in HIES patients. Impaired differentiation was associated with decreased expression of 2 STAT3-responsive transcription factors, BCL6 (109565) and SOCS3 (604176). Siegel et al. (2011) found that HIES patients had increased risk for reactivation of varicella zoster that was associated with poor CD4-positive T-cell responses. HIES patients also had greater detectable Epstein-Barr virus (EBV) viremia that was associated with compromised T-cell memory to EBV. Siegel et al. (2011) concluded that STAT3 has a specific role in central memory T-cell formation. 

Crosby et al. (2012) described a patient with food allergies, a high score for HIES, and eosinophilic esophagitis. They identified a thr389-to-ile (T389I; 102582.0007) mutation in the patient's STAT3 gene.

Berglund et al. (2013) noted that a feature of autosomal dominant HIES due to STAT3 deficiency is impaired humoral immunity following infection and vaccination. Using microarray analysis, they analyzed STAT3-deficient and normal human naive B cells after stimulation with CD40L (TNFSF5; 300386) alone or with IL21 (605384). The authors observed upregulation of IL2RA (147730) and IL10 (124092) production in normal cells, but not STAT3-deficient cells. IL2 enhanced differentiation of plasma cells and Ig secretion from IL21-stimulated naive B cells. Berglund et al. (2013)concluded that IL21, via STAT3, sensitizes B cells to the stimulatory effects of IL2, which may play an active role in IL21-induced B-cell differentiation. They proposed that lack of this secondary effect of IL21 may amplify humoral immunodeficiency in patients with mutations in STAT3, IL2RG (308380), or IL21R (605383) due to impaired IL21 responsiveness. 

Infantile-Onset Multisystem Autoimmune Disease

In 5 unrelated patients with infantile-onset multisystem autoimmune disease (ADMIO; 615952), Flanagan et al.

(2014)identified 4 different de novo heterozygous missense mutations in the STAT3 gene (102582.0008-102582.0011). The mutation in the first patient was found by exome sequencing, and the mutations in the subsequent patients were found by sequencing the coding exons of the STAT3 gene in 24 individuals with early-onset autoimmune disorder. In vitro functional expression studies showed that all the mutations resulted in a gain of function, with increased STAT3-responsive reporter activity and an increase in cytokine-related function compared to wildtype and compared to dominant-negative inactivating mutations associated with HIES. Samples from 2 patients showed increased cytokine-related function, including decreased regulatory T-cell numbers. 

Somatic Mutations in Large Granular Lymphocytic Leukemia

T-cell large granular lymphocytic leukemia is a rare lymphoproliferative disorder characterized by the expansion of clonal CD3+CD8+ cytotoxic T lymphocytes (CTLs) and often associated with autoimmune disorders and immune-mediated cytopenias (summary by Koskela et al., 2012). Koskela et al. (2012) used next-generation exome sequencing to identify somatic mutations in CTLs from an index patient with large granular lymphocytic leukemia and used targeted resequencing in a well-characterized cohort of 76 patients with this disorder. Mutations in STAT3 were found in 31 of 77 patients (40%) with large granular lymphocytic leukemia. Among these 31 patients, recurrent mutational hotspots included Y640F in 13 (17%), D661V in 7 (9%), D661Y in 7 (9%), and N647I in 3 (4%). All mutations were located in exon 21, encoding the Src homology-2 (SH2) domain, which mediates the dimerization and activation of STAT protein. The amino acid changes resulted in a more hydrophobic protein surface and were associated with phosphorylation of STAT3 and its localization in the nucleus. In vitro functional studies showed that the Y640F and D661V mutations increased the transcriptional activity of STAT3. In the affected patients, downstream target genes of the STAT3 pathway (IFNGR2, 147569; BCL2L1, 600039; and JAK2, 147796) were

upregulated. Patients with STAT3 mutations presented more often with neutropenia and rheumatoid arthritis than did patients without these mutations. 

Associations Pending Confirmation

For discussion of a possible association between variation in the STAT3 gene and Crohn disease, see IBD22 (612380).

For discussion of a possible association between variation in the STAT3 gene and susceptibility to multiple sclerosis, see MS (126200).

The serum response element (SRE) was originally identified within the promoter of the c-fos proto-oncogene and is responsible for the activation of this gene following addition of serum to cultured cells (1). Subsequently an SRE was also identified in a number of other genes whose transcription is enhanced by treatment of cells with serum or growth factors (2, 3). The SRE has a consensus sequence of the form CC A/T6 GG, in which two C residues are followed by a run of six residues that can be either A or T and then followed by a further two G residues. The sequence of the SRE within the c-Fos promoter is illustrated in Figure 1.Figure 1. Structure of the c-Fos SRE and the adjacent binding site for TCF. The SRE is underlined and the TCF site is overlined.

Subsequent studies demonstrated that the SRE acts by binding a ubiquitous 67-kDa protein known as the serum response factor (SRF). Although the binding of SRF

to the SRE is essential for the response to growth factors in serum, it is not sufficient for this to occur. Thus, further studies identified a 62-kDa protein known as ternary complex factor (TCF), which could only bind to the SRE in partnership with SRF and was required for SRE function (4). Subsequent studies identified TCF as a member of the ets family of DNA-binding proteins (5). Although a binding site for this factor is located adjacent to the SRE in the c-Fos promoter (Fig. 1), TCF cannot bind to this site unless it has also formed a protein-protein interaction with SRF bound at the SRE. Hence binding of TCF to its binding site is dependent on the prior association of SRF with the SRE.

Many eukaryotic genes have a conserved promoter sequence called the TATA box, located 25 to 35 base pairs upstream of the transcription start site. Transcription factors bind to the TATA box and initiate the formation of the RNA polymerase transcription complex, which promotes transcription.http://www.nature.com/scitable/definition/promoter-259

STATs (Signal Transducers and Activators of Transcription) are a family of cytoplasmic proteins with SH2 (Src Homology-2) domains that act as signal messengers and transcription factors and participate in normal cellular responses to Cytokines and GFs (Growth Factors). STATs are activated via the tyrosine phosphorylation cascade after ligand binding and stimulation of the Cytokine Receptor–Kinase complex and Growth Factor-Receptor complex like the EGF (Epidermal Growth Factor), FGF (Fibroblast Growth Factor), PDGF (Platelet-Derived Growth Factor), GCSF (Granulocyte Colony Stimulating Factor), IL-6 (Interleukin-6), CNTF (Ciliary Neurotrophic Factor), OSM (Oncostatin-M), LIF (Leukemia Inhibitory Factor), CSF1R (Colony Stimulating Factor-1 Receptor), c-kit, Insulin receptor, c-Met and GPCRs (G-Protein Coupled Receptors): AgtR2 (Angiotensin-II Receptor). Ligands signaling through the same class of receptor complexes usually activates the same set of STAT factors, e.g. all IL-6-type cytokines activate STAT3 and STAT1 (Ref.1). The activated STATs are subsequently translocated to the nucleus where they bind to specific DNA sites as homo

or heterodimers to stimulate transcription of the responsive genes. Among the STAT proteins known to date, STAT3 has been implicated in transduction of the cellular signals that are involved in development of cardiac hypertrophy. The signal-transducing receptor protein GP130 stimulates the JAK (Janus-family tyrosine kinases)-STAT3 pathway and is associated with the regulation of cardiac growth and development (Ref.2).

STAT3 induce gene activation in response to Cytokine Receptor stimulation. Following tyrosine phosphorylation, STAT3 proteins dimerize, translocate to the nucleus, and activate specific target genes like the cis element ISRE (IFN-stimulated Response Element) thereby initiating transcription of several IFN-inducible genes. This transcriptional activation by STAT3 proteins requires the recruitment of coactivators such as CBP (CREB-binding Protein)/p300. STAT3 proteins recognize a conserved element in the promoter of p21/WAF1 (Wildtype p53-Activated Fragment-1) and increase the mRNA expression of this cell cycle regulatory gene. Effectively, STAT3 activates several other genes involved in cell cycle progression such as Fos, Cyclin-D, CDC25A, c-Myc or Pim1 and up-regulates antiapoptotic genes such as BCL2 (B-Cell CLL/Lymphoma-2), BCLXL and Beta2-Macroglobulin (Ref.3). Thus, many STAT3 target genes are key components of the regulation of cell cycle progression from G1 to S phase. Accordingly, STAT3 activation is often associated with cell growth or transformation, and disruption of the STAT3 gene causes embryonic lethality. Following IL-6 stimulation, transcriptional cofactor NCOA/SRC1a interacts with STAT3 and potentiates its transcriptional activity through its CBP/p300-interacting domain AD1 (Ref.4). Pathways other than JAK kinases involving mTOR (mammalian Target of Rapamycin) or p70S6 kinase, MAPK (Mitogen Activated Protein Kinase), p38, and MEK (MAPK/ERK Kinase) signaling cascades also lead to phosphorylation and activation of STAT3 (Ref.5).

RhoA efficiently modulate STAT3 transcriptional activity by inducing its simultaneous tyrosine and serine phosphorylation via Src Family of Kinases and JAK2. The JNK (c-Jun N-terminal Kinase)/ERK (Extracellular-Signal Regulated Kinase) Pathway mediates serine phosphorylation (Ser727) and cooperation of both tyrosine as well as serine phosphorylation is necessary for full activation of STAT3 (Ref.5). The Type-I Interferon (IFN-Alpha/Beta) promotes the DNA-binding activity of the transcription factors including STAT3, which is involved in the induction of NF-KappaB (Nuclear Factor-KappaB) DNA-binding activity and in the induction of antiviral and antiproliferative activity. STAT3 is also activated in response to the small guanine nucleotide-binding protein Rac1. The Rac functions in growth factor-induced activation of STAT3 in two ways. It apparently helps localize STAT3 to kinase complexes at the cell surface through Ras and also promotes activation of kinases, like MLKs (Mixed-Lineage Kinases), JAK2, TYK2 that phosphorylate STAT3 at Tyr705 (Ref.6). The SOCS (Suppressor of Cytokine Signaling) family of proteins negatively regulates the receptor-associated JAK-STAT3 pathway of transcriptional activation.

STATs have been implicated in programming gene expression in biological events such as embryonic development, programmed cell death, organogenesis, innate immunity, adaptive immunity and cell growth regulation in many organisms. Abnormal activity of certain STAT family members, particularly STAT3 is associated with a wide variety of human malignancies, including lymphomas; leukemias; mycoses fungoides and multiple myeloma (Ref.7). Thus, STAT3 activate either of two gene expression programs, one for growth promotion and one for growth arrest. Recently inhibition of the STAT3 signaling pathway using the JAK-selective inhibitor, AG490, and a dominant negative STAT3 (STAT3-Beta) significantly suppressed the growth of ovarian and breast cancer cell lines. These results suggest that inhibition of STAT3 signaling may provide a potential therapeutic approach for treating ovarian and breast cancers (Ref.8).

References:

1. Abell K, Watson CJThe Jak/Stat pathway: a novel way to regulate PI3K activity.Cell Cycle. 2005 Jul; 4(7):897-900. Epub 2005 Jul 11.

2. Ji JD, Kim HJ, Rho YH, Choi SJ, Lee YH, Cheon HJ, Sohn J, Song GGInhibition of IL-10-induced STAT3 activation by 15-deoxy-Delta12,14-prostaglandin J2.Rheumatology (Oxford). 2005 Aug; 44(8):983-8. Epub 2005 Apr 19.

3. Barre B, Avril S, Coqueret OOpposite Regulation of Myc and p21waf1 Transcription by STAT3 Proteins.J Biol Chem. 2003 Jan 31; 278(5): 2990-6.

4. Giraud S, Bienvenu F, Avril S, Gascan H, Heery DM, Coqueret O

Functional interaction of STAT3 transcription factor with the coactivator NcoA/SRC1a.J Biol Chem. 2002 Mar 8; 277(10): 8004-11.

5. Aznar S, Valeron PF, del Rincon SV, Perez LF, Perona R, Lacal JCSimultaneous tyrosine and serine phosphorylation of STAT3 transcription factor is involved in Rho A GTPase oncogenic transformation.Mol Biol Cell. 2001 Oct; 12(10): 3282-94.

6. Simon AR, Vikis HG, Stewart S, Fanburg BL, Cochran BH, Guan KLRegulation of STAT3 by direct binding to the Rac1 GTPase.Science. 2000 Oct 6; 290(5489): 144-7.

7. Shen Y, Devgan G, Darnell JE Jr, Bromberg JFConstitutively activated Stat3 protects fibroblasts from serum withdrawal and UV-induced apoptosis and antagonizes the proapoptotic effects of activated Stat1.Proc Natl Acad Sci U S A. 2001 Feb 13; 98(4): 1543-8.

8. Yu X, Kennedy RH, Liu SJJAK2/STAT3, not ERK1/2 pathway mediates interleukin-6-elicited inducible NOS activation and decrease in contractility in adult ventricular myocytes.J Biol Chem. 2003 Feb 20 [epub ahead of print]