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SOCS Proteins, Regulators of Intracellular Signaling

2000; Cell Press; Volume: 13; Issue: 3 Linguagem: Inglês

10.1016/s1074-7613(00)00028-5

1097-4180

X.Peter Chen, Julie A. Losman, Paul B. Rothman,

Medicinal Plant Pharmacodynamics Research

The growth, differentiation, and function of hematopoietic cells are controlled by the coordinated action of the cytokine network. The pathways by which cytokines exert their biologic effects have been under intensive investigation over the past few years. As most cytokine receptors lack a cytoplasmic kinase domain, ligand-dependent tyrosine phosphorylation is mediated by nonreceptor tyrosine kinases. The major tyrosine kinases activated immediately following cytokine stimulation are the Janus kinase (JAK) family. In fact, the JAK-STAT pathway is one of the most important mechanisms by which many cytokines activate gene transcription. When cytokines bind to receptors on the cell surface, they cause receptor oligomerization, which in turn induces JAK kinase activation. The activated JAK kinases, in turn, phosphorylate the cytokine receptors, leading to the recruitment and subsequent activation of other signaling molecules such as the STAT family proteins. The activated STAT proteins translocate into the nucleus and activate transcription of a range of cytokine responsive genes. Although it is clear that the effect of most cytokines is limited in both magnitude and duration, the mechanisms underlying this regulation are not well understood. The most important limitation of cytokine activity occurs through regulated production of the cytokine itself. In addition, several other mechanisms have been demonstrated to control responsiveness to cytokines. Among these, selective expression of cytokine receptors has been well documented as an effective way to regulate cytokine responsiveness. In addition, more complex regulation exists inside the cells, where signals from multiple stimuli converge. Intracellular regulation can occur on the receptor at the plasma membrane, in the cytoplasm, and in the nucleus. Tyrosine phosphatases have been reported to modulate signaling by dephosphorylation of signaling proteins. The PIAS family of proteins has also been implicated in the negative regulation of STAT function (for review, see 19Yasukawa H. Sasaki A. Yoshimura A Annu. Rev. Immunol. 2000; 18: 143-164Crossref PubMed Scopus (502) Google Scholar). Recently, a new family of proteins has been identified as negative inhibitors of cytokine signaling. The first member of the family, CIS, was identified as an immediate early gene induced by multiple cytokines (20Yoshimura A. Ohkubo T. Kiguchi T. Jenkins N.A. Gilbert D.J. Copeland N.G. Hara T. Miyajima A EMBO J. 1995; 14: 2816-2826Crossref PubMed Scopus (598) Google Scholar). The second family member, which can inhibit JAK kinases, was identified independently by three groups. The same protein was given three different names denoting how it was identified: SOCS-1 (suppressor of cytokine signaling-1), JAB (JAK binding protein), and SSI-1 (STAT induced STAT inhibitor-1) (3Endo T.A. Masuhara M. Yokouchi M. Suzuki R. Sakamoto H. Mitsui K. Matsumoto A. Tanimura S. Ohtsubo M. Misawa H. et al.Nature. 1997; 387: 921-924Crossref PubMed Scopus (1192) Google Scholar, 12Naka T. Narazaki M. Hirata M. Matsumoto T. Minamoto S. Aono A. Nishimoto N. Kajita T. Taga T. Yoshizaki K. et al.Nature. 1997; 387: 924-929Crossref PubMed Scopus (1103) Google Scholar, 16Starr R. Willson T.A. Viney E.M. Murray L.J. Rayner J.R. Jenkins B.J. Gonda T.J. Alexander W.S. Metcalf D. Nicola N.A. Hilton D.J Nature. 1997; 387: 917-921Crossref PubMed Scopus (1739) Google Scholar). For reasons of simplicity, we will use the SOCS nomenclature in this review. Sequence searching in public databases has revealed six other family members that have a central SH2 domain and a C-terminal SOCS box. The N-terminal regions of these proteins are highly variable (Figure 1). Table 1 summarizes different nomenclatures that have been used in the literature for these proteins. In addition to the SH2-containing members, there are at least a dozen other proteins that share the SOCS box motif but not the SH2 domain (4Hilton D.J. Richardson R.T. Alexander W.S. Viney E.M. Willson T.A. Sprigg N.S. Starr R. Nicholson S.E. Metcalf D. Nicola N.A Proc. Natl. Acad. Sci. USA. 1998; 95: 114-119Crossref PubMed Scopus (592) Google Scholar). Instead, these proteins possess other domains that can potentially mediate protein-protein interactions. Together, these twenty proteins form a superfamily of proteins that may require the SOCS box for their function. The non-SH2-containing proteins can be further divided into several subfamilies: WSB for WD-40-repeat-containing proteins, SSB for SPRY domain-containing proteins, and ASB for ankyrin-repeat-containing proteins. In addition, there are two small GTPases and two ESTs of unknown structural class that contain the SOCS box motif (Figure 1).Table 1List of SH2-Containing SOCS Family MembersNomenclature in This ReviewOther NamesCellular TargetCISCIS1cytokine receptors, EPO, etcSOCS-1JAB (SSI-1)JAK kinasesSOCS-2CIS2 (SSI-2)IGF-I receptor?SOCS-3CIS3 (SSI-3)JAK kinases, cytokine receptors?SOCS-4CIS7?SOCS-5CIS6?SOCS-6CIS4?SOCS-7CIS5 (NAP4)Nck, Ash and phospholipase γ? Open table in a new tab The first member of the SOCS family was identified as an immediate early gene induced in response to several cytokines (20Yoshimura A. Ohkubo T. Kiguchi T. Jenkins N.A. Gilbert D.J. Copeland N.G. Hara T. Miyajima A EMBO J. 1995; 14: 2816-2826Crossref PubMed Scopus (598) Google Scholar). It was denoted as CIS for cytokine-inducible SH2-containing protein. CIS has been shown to bind to phosphorylated tyrosines on multiple cytokine receptors. In vitro and in vivo studies support a role for CIS as a negative regulator of STAT5 activation in response to several cytokines, including EPO, IL-2, and IL-3 (for review, see 19Yasukawa H. Sasaki A. Yoshimura A Annu. Rev. Immunol. 2000; 18: 143-164Crossref PubMed Scopus (502) Google Scholar). One mechanism by which CIS may inhibit cytokine signaling is by competing for Stat5 binding sites on the activated receptors and thus preventing recruitment of Stat5 to the receptors (Figure 2). Alternatively, CIS may function as an adaptor protein linking cytokine receptors and other negative regulators. 9Matsumoto A. Seki Y. Kubo M. Ohtsuka S. Suzuki A. Hayashi I. Tsuji K. Nakahata T. Okabe M. Yamada S. Yoshimura A Mol. Cell. Biol. 1999; 19: 6396-6407Crossref PubMed Scopus (210) Google Scholar generated transgenic mice expressing CIS under control of the β-actin promoter. Mice overexpressing CIS are smaller in body size, suggesting defects in growth-hormone signaling. Female mice also manifest defects in the development of mammary glands, most likely due to inhibition of prolactin signaling. Furthermore, IL-2 induced activation of STAT5 is markedly inhibited in T cells from CIS-transgenic mice, while leukemia inhibitory factor induced STAT3 phosphorylation is not affected. These data indicate a certain level of specificity underlying CIS-mediated inhibition of cytokine signaling. Besides cytokines, CIS is also induced by TCR stimulation in T cells (6Li S. Chen S. Xu X. Sundstedt A. Paulsson K.M. Anderson P. Karlsson S. Sjogren H.O. Wang P J. Exp. Med. 2000; 191: 985-994Crossref PubMed Scopus (80) Google Scholar). In the transgenic mice with CIS under control of the β-actin promoter, the numbers of γδ T cells and natural killer (NK) cells are drastically reduced. In addition, differentiation of Th1/Th2 cells is also altered (9Matsumoto A. Seki Y. Kubo M. Ohtsuka S. Suzuki A. Hayashi I. Tsuji K. Nakahata T. Okabe M. Yamada S. Yoshimura A Mol. Cell. Biol. 1999; 19: 6396-6407Crossref PubMed Scopus (210) Google Scholar). The mechanisms underlying these abnormalities are not clear. To investigate the specific role of CIS in the T cell lineage, transgenic mice selectively overexpressing CIS in CD4+ T cells were created (6Li S. Chen S. Xu X. Sundstedt A. Paulsson K.M. Anderson P. Karlsson S. Sjogren H.O. Wang P J. Exp. Med. 2000; 191: 985-994Crossref PubMed Scopus (80) Google Scholar). Forced expression of CIS promotes TCR-mediated proliferation and prolongs survival of CD4+ T cells after TCR stimulation. The authors proposed that CIS may exert these effects by regulating MAP kinase activation. Analysis of the CIS null mice will be needed to determine the physiologic role of CIS in cytokine signaling and T cell activation. SOCS-1 was initially identified as an inhibitor of IL-6 induced differentiation of murine monocytic leukemic M1 cells, as a JAK binding protein, and as a protein with an SH2 domain similar to that of STAT proteins (3Endo T.A. Masuhara M. Yokouchi M. Suzuki R. Sakamoto H. Mitsui K. Matsumoto A. Tanimura S. Ohtsubo M. Misawa H. et al.Nature. 1997; 387: 921-924Crossref PubMed Scopus (1192) Google Scholar, 12Naka T. Narazaki M. Hirata M. Matsumoto T. Minamoto S. Aono A. Nishimoto N. Kajita T. Taga T. Yoshizaki K. et al.Nature. 1997; 387: 924-929Crossref PubMed Scopus (1103) Google Scholar, 16Starr R. Willson T.A. Viney E.M. Murray L.J. Rayner J.R. Jenkins B.J. Gonda T.J. Alexander W.S. Metcalf D. Nicola N.A. Hilton D.J Nature. 1997; 387: 917-921Crossref PubMed Scopus (1739) Google Scholar). Indeed, SOCS-1 has been shown to bind to all four JAK kinases through its central SH2 domain and to inhibit their kinase activity in vitro. When the SOCS-1 protein is overexpressed in cell lines, it can inhibit STAT activation induced by several cytokines, including interferons, IL-6, IL-4, and LIF (for review, see 19Yasukawa H. Sasaki A. Yoshimura A Annu. Rev. Immunol. 2000; 18: 143-164Crossref PubMed Scopus (502) Google Scholar) (Figure 2). In addition to inhibiting JAK kinases, SOCS-1 has also been shown to suppress Tec kinase, as well as signaling downstream of receptor tyrosine kinases (for review, see 19Yasukawa H. Sasaki A. Yoshimura A Annu. Rev. Immunol. 2000; 18: 143-164Crossref PubMed Scopus (502) Google Scholar). 2De Sepulveda P. Ilangumaran S. Rottapel R J. Biol. Chem. 2000; 275: 14005-14008Crossref PubMed Scopus (147) Google Scholar have recently reported that SOCS-1 may inhibit Vav function and suppress c-kit-mediated proliferation induced by steel factor. As SOCS-1 protein levels appear to be tightly regulated, however, these observations using overexpression systems should be interpreted cautiously. SOCS-1 mRNA is present at the highest levels in the thymus and spleen (16Starr R. Willson T.A. Viney E.M. Murray L.J. Rayner J.R. Jenkins B.J. Gonda T.J. Alexander W.S. Metcalf D. Nicola N.A. Hilton D.J Nature. 1997; 387: 917-921Crossref PubMed Scopus (1739) Google Scholar, 8Marine J.C. Topham D.J. McKay C. Wang D. Parganas E. Stravopodis D. Yoshimura A. Ihle J.N Cell. 1999; 98 (b): 609-616Abstract Full Text Full Text PDF PubMed Scopus (436) Google Scholar), suggesting a role for SOCS-1 in the immune system. To investigate the physiological role of SOCS-1, mice lacking the SOCS-1 gene have been generated by several groups. The SOCS-1-deficient mice display growth retardation and die within 3 weeks of birth with fatty degeneration of the liver and monocytic infiltration of several organs. The thymus of these mice is markedly smaller than those of wild-type littermates. Lymphocytes from the SOCS-1−/− mice undergo accelerated apoptosis associated with increased levels of the proapoptotic protein Bax (13Naka T. Matsumoto T. Narazaki M. Fujimoto M. Morita Y. Ohsawa Y. Saito H. Nagasawa T. Uchiyama Y. Kishimoto T Proc. Natl. Acad. Sci. USA. 1998; 95: 15577-15582Crossref PubMed Scopus (256) Google Scholar). In addition, these mice exhibited a progressive loss of maturing B lymphocytes in the bone marrow, spleen, and peripheral blood (13Naka T. Matsumoto T. Narazaki M. Fujimoto M. Morita Y. Ohsawa Y. Saito H. Nagasawa T. Uchiyama Y. Kishimoto T Proc. Natl. Acad. Sci. USA. 1998; 95: 15577-15582Crossref PubMed Scopus (256) Google Scholar, 17Starr R. Metcalf D. Elefanty A.G. Brysha M. Willson T.A. Nicola N.A. Hilton D.J. Alexander W.S Proc. Natl. Acad. Sci. USA. 1998; 95: 14395-14399Crossref PubMed Scopus (367) Google Scholar). More recently, 11Morita Y. Naka T. Kawazoe Y. Fujimoto M. Narazaki M. Nakagawa R. Fukuyama H. Nagata S. Kishimoto T Proc. Natl. Acad. Sci. USA. 2000; 97: 5405-5410Crossref PubMed Scopus (163) Google Scholar have demonstrated that murine embryonic fibroblasts lacking SOCS-1 are more sensitive to TNFα induced cell death, providing a possible mechanism for the increased apoptosis in SOCS-1-deficient lymphocytes. The pathology caused by disruption of SOCS-1 is similar to that seen in wild-type mice subjected to daily administration of IFNγ from birth. This observation prompted two groups to generate mice lacking both the SOCS-1 and IFNγ genes. The complex diseases observed in the SOCS-1−/− mice are eliminated in mice deficient in both SOCS-1 and IFNγ (1Alexander W.S. Starr R. Fenner J.E. Scott C.L. Handman E. Sprigg N.S. Corbin J.E. Cornish A.L. Darwiche R. Owczarek C.M. et al.Cell. 1999; 98: 597-608Abstract Full Text Full Text PDF PubMed Scopus (625) Google Scholar, 8Marine J.C. Topham D.J. McKay C. Wang D. Parganas E. Stravopodis D. Yoshimura A. Ihle J.N Cell. 1999; 98 (b): 609-616Abstract Full Text Full Text PDF PubMed Scopus (436) Google Scholar). In addition, SOCS-1−/− mice are hyperresponsive to viral infection and yield macrophages with an enhanced IFNγ-dependent capacity to kill L. major parasites (1Alexander W.S. Starr R. Fenner J.E. Scott C.L. Handman E. Sprigg N.S. Corbin J.E. Cornish A.L. Darwiche R. Owczarek C.M. et al.Cell. 1999; 98: 597-608Abstract Full Text Full Text PDF PubMed Scopus (625) Google Scholar). Taken together, these data suggest that SOCS-1 is a critical regulator of cellular sensitivity to IFNγ, balancing the beneficial immunological function with the potentially lethal effects of IFNγ. Intriguingly, lymphoid stem cells from mice lacking SOCS-1 can confer this lethality to sublethally irradiated JAK3-deficient mice (8Marine J.C. Topham D.J. McKay C. Wang D. Parganas E. Stravopodis D. Yoshimura A. Ihle J.N Cell. 1999; 98 (b): 609-616Abstract Full Text Full Text PDF PubMed Scopus (436) Google Scholar). Since the recipient mice are otherwise normal except for defects in the lymphoid lineage, this experiment demonstrates that the lethality in SOCS-1-deficient mice can be mediated by lymphocytes on a background in which the SOCS-1 gene is present in other cell types. Furthermore, introducing RAG2 deficiency into the SOCS-1 null mice can rescue the lethality caused by SOCS-1 deficiency (8Marine J.C. Topham D.J. McKay C. Wang D. Parganas E. Stravopodis D. Yoshimura A. Ihle J.N Cell. 1999; 98 (b): 609-616Abstract Full Text Full Text PDF PubMed Scopus (436) Google Scholar). These results suggest a critical role of lymphocytes in causing the lethality. The mechanism by which the SOCS-1-deficient lymphoid cells exert the lethal effect is still unknown. It has been postulated that loss of SOCS-1 may result in aberrant development of T or NK cells, which then produce excessive amounts of IFNγ and cause lethality. One prediction from this model is that the serum levels of IFNγ are higher in SOCS-1-deficient mice than in wild-type littermates. However, it remains controversial whether the serum levels of IFNγ are elevated in SOCS-1-deficient mice (1Alexander W.S. Starr R. Fenner J.E. Scott C.L. Handman E. Sprigg N.S. Corbin J.E. Cornish A.L. Darwiche R. Owczarek C.M. et al.Cell. 1999; 98: 597-608Abstract Full Text Full Text PDF PubMed Scopus (625) Google Scholar, 8Marine J.C. Topham D.J. McKay C. Wang D. Parganas E. Stravopodis D. Yoshimura A. Ihle J.N Cell. 1999; 98 (b): 609-616Abstract Full Text Full Text PDF PubMed Scopus (436) Google Scholar). This contradiction may result from variations in mouse strains or the reagents used to detect IFNγ. In summary, the available data suggest that SOCS-1 may have two essential functions that, when disrupted, result in perinatal lethality. One role of SOCS-1 may be to regulate T cell differentiation and function. The other role may be to modulate responsiveness to IFNγ in both lymphoid and nonlymphoid cells. More studies are necessary to understand the precise mechanism by which SOCS-1 protein exerts these effects. Although SOCS-1−/− mice show few phenotypes associated with altered signaling by other cytokines, it is possible that some defects may only manifest themselves late in life or only under certain circumstances. Indeed, a role for SOCS-1 as a negative regulator of IL-4 signaling has been suggested based on observations that IL-4 induced Stat6 activation is prolonged in SOCS-1-deficient mice (13Naka T. Matsumoto T. Narazaki M. Fujimoto M. Morita Y. Ohsawa Y. Saito H. Nagasawa T. Uchiyama Y. Kishimoto T Proc. Natl. Acad. Sci. USA. 1998; 95: 15577-15582Crossref PubMed Scopus (256) Google Scholar). More detailed analysis of these mice will be needed to clarify the in vivo function of SOCS-1 in other cytokine signaling pathways, as well as in the cross-regulation of different cytokines. SOCS-1 has been shown to bind to and inhibit all four JAK kinases (Figure 2). JAK kinases are activated through transphosphorylation of a critical tyrosine within the activation loop, and SOCS-1 has been shown to bind to this phosphorylated tyrosine in an SH2 domain–dependent manner. Extensive mutational analysis has revealed that 12 amino acids N-terminal to the SH2 domain (extended SH2 subdomain) of SOCS-1 are required for binding to the activation loop of JAKs. The extended SH2 subdomains are conserved among the SOCS family members and the STATs. An additional 12 amino acids N-terminal to the extended SH2 subdomain (termed the kinase inhibitory region) also contribute to high-affinity binding to the kinase domain and are indispensable for inhibition of JAK kinase activity (15Nicholson S.E. Willson T.A. Farley A. Starr R. Zhang J.G. Baca M. Alexander W.S. Metcalf D. Hilton D.J. Nicola N.A EMBO J. 1999; 18: 375-385Crossref PubMed Scopus (359) Google Scholar, 18Yasukawa H. Misawa H. Sakamoto H. Masuhara M. Sasaki A. Wakioka T. Ohtsuka S. Imaizumi T. Matsuda T. Ihle J.N. Yoshimura A EMBO J. 1999; 18: 1309-1320Crossref PubMed Scopus (585) Google Scholar). 18Yasukawa H. Misawa H. Sakamoto H. Masuhara M. Sasaki A. Wakioka T. Ohtsuka S. Imaizumi T. Matsuda T. Ihle J.N. Yoshimura A EMBO J. 1999; 18: 1309-1320Crossref PubMed Scopus (585) Google Scholar proposed that the kinase inhibitory region of SOCS-1 may mimic the activation loop of JAK and act as a pseudosubstrate. Thus, SOCS-1 appears to inhibit JAK kinase activity by binding to the activation loop and preventing the access of substrates and/or ATP to the catalytic pocket. Although neither the N-terminal 50 amino acids nor the C-terminal SOCS box is required for inhibition of JAK kinase activity in these in vitro studies, these domains may be important for the in vivo function of SOCS-1. Expression of SOCS-1 protein appears to be controlled at both the transcriptional and posttranscriptional levels. mRNA levels of SOCS-1 have been shown to be increased upon cytokine stimulation (16Starr R. Willson T.A. Viney E.M. Murray L.J. Rayner J.R. Jenkins B.J. Gonda T.J. Alexander W.S. Metcalf D. Nicola N.A. Hilton D.J Nature. 1997; 387: 917-921Crossref PubMed Scopus (1739) Google Scholar, 12Naka T. Narazaki M. Hirata M. Matsumoto T. Minamoto S. Aono A. Nishimoto N. Kajita T. Taga T. Yoshizaki K. et al.Nature. 1997; 387: 924-929Crossref PubMed Scopus (1103) Google Scholar). In fact, the promoter region of SOCS-1 contains binding sequences for Stat3 and Stat6 (12Naka T. Narazaki M. Hirata M. Matsumoto T. Minamoto S. Aono A. Nishimoto N. Kajita T. Taga T. Yoshizaki K. et al.Nature. 1997; 387: 924-929Crossref PubMed Scopus (1103) Google Scholar). Moreover, 8Marine J.C. Topham D.J. McKay C. Wang D. Parganas E. Stravopodis D. Yoshimura A. Ihle J.N Cell. 1999; 98 (b): 609-616Abstract Full Text Full Text PDF PubMed Scopus (436) Google Scholar have suggested that the expression of SOCS-1 in thymocytes is also developmentally regulated. More interestingly, the levels of SOCS-1 appears to be controlled by modulation of its protein stability. 14Narazaki M. Fujimoto M. Matsumoto T. Morita Y. Saito H. Kajita T. Yoshizaki K. Naka T. Kishimoto T Proc. Natl. Acad. Sci. USA. 1998; 95: 13130-13134Crossref PubMed Scopus (214) Google Scholar first reported that deletion of the SOCS box decreased the expression level of SOCS-1 protein in M1 cells, suggesting that the SOCS box may protect the protein from degradation. More recently, two groups have independently identified Elongin BC as binding partner of the SOCS box (5Kamura T. Sato S. Haque D. Liu L. Kaelin Jr., W.G. Conaway R.C. Conaway J.W Genes Dev. 1998; 12: 3872-3881Crossref PubMed Scopus (487) Google Scholar, 21Zhang J.G. Farley A. Nicholson S.E. Willson T.A. Zugaro L.M. Simpson R.J. Moritz R.L. Cary D. Richardson R. Hausmann G. et al.Proc. Natl. Acad. Sci. USA. 1999; 96: 2071-2076Crossref PubMed Scopus (507) Google Scholar). In one report, it was demonstrated that cotransfection of Elongin BC stabilizes SOCS-1 in 293T cells (5Kamura T. Sato S. Haque D. Liu L. Kaelin Jr., W.G. Conaway R.C. Conaway J.W Genes Dev. 1998; 12: 3872-3881Crossref PubMed Scopus (487) Google Scholar). In the other report, 21Zhang J.G. Farley A. Nicholson S.E. Willson T.A. Zugaro L.M. Simpson R.J. Moritz R.L. Cary D. Richardson R. Hausmann G. et al.Proc. Natl. Acad. Sci. USA. 1999; 96: 2071-2076Crossref PubMed Scopus (507) Google Scholar suggested that binding of Elongin BC may target SOCS proteins (along with their binding partners) to the proteasome for degradation. Interestingly, the SOCS box motif that binds to Elongin BC structurally resembles the α domain of the von Hippel-Lindau (VHL) tumor-suppressor gene product and the F box proteins. Both VHL and the F box proteins have been shown to be part of cellular complexes consisting of Elongin BC, Cullins, and Rbx1 (for review, see 19Yasukawa H. Sasaki A. Yoshimura A Annu. Rev. Immunol. 2000; 18: 143-164Crossref PubMed Scopus (502) Google Scholar). Rbx1 is a ring-finger-containing protein that is conserved from yeast to human. The yeast homolog of Rbx1 has been shown to be a key component of the ubiquitin ligase E3 complex. It is therefore possible that SOCS-1, by binding to the activated JAK kinases, may target JAK to the proteasome in an Elongin BC–dependent manner. Thus, interaction with Elongin BC may not only control the levels of SOCS-1 protein but may also contribute to SOCS-1 function. In addition to CIS and SOCS-1, SOCS-3 is another family member that has been shown to suppress cytokine signaling. The mechanism by which SOCS-3 inhibit cytokine induced STAT activation is not completely understood. Although SOCS-3 can bind to JAK kinases, it fails to inhibit JAK kinase activity in vitro. Moreover, mutation of the arginine in the conserved FLVRDS motif within the SH2 domain of SOCS-3 does not abolish its ability to inhibit LIF-induced STAT activation (15Nicholson S.E. Willson T.A. Farley A. Starr R. Zhang J.G. Baca M. Alexander W.S. Metcalf D. Hilton D.J. Nicola N.A EMBO J. 1999; 18: 375-385Crossref PubMed Scopus (359) Google Scholar). SOCS-3 mRNA is induced by a number of cytokines, and overexpression studies have implicated SOCS-3 in the signaling of growth hormone, leptin, CNTF, and IL-2 (for review, see 19Yasukawa H. Sasaki A. Yoshimura A Annu. Rev. Immunol. 2000; 18: 143-164Crossref PubMed Scopus (502) Google Scholar). 7Marine J.C. McKay C. Wang D. Topham D.J. Parganas E. Nakajima H. Pendeville H. Yasukawa H. Sasaki A. Yoshimura A. Ihle J.N Cell. 1999; 98 (a): 617-627Abstract Full Text Full Text PDF PubMed Scopus (303) Google Scholar used transgenic and gene disruption approaches to investigate the role of SOCS-3 in vivo. Disruption of SOCS-3 gene results in embryonic lethality at 12–16 days associated with marked erythrocytosis. These results indicate that, while SOCS-3 is not absolutely required for bone marrow erythropoiesis and normal lymphoid development, it plays an essential role during fetal liver erythropoiesis. It is not clear whether SOCS-3 also plays an essential role in the regulation of cytokine signaling after birth. SOCS-2-deficient mice have also been generated (10Metcalf D. Greenhalgh C.J. Viney E. Willson T.A. Starr R. Nicola N.A. Hilton D.J. Alexander W.S Nature. 2000; 405: 1069-1073Crossref PubMed Scopus (388) Google Scholar). Most organs in the SOCS-2−/− mice appear normal, and no defects in the hematopoietic system are observed. Strikingly, the SOCS-2-deficient mice are much larger in body size than their wild-type littermates. Deregulation of growth hormone and insulin-like growth factor-I (IGF-1) signaling is observed in the SOCS-2−/− mice, suggesting a possible role for SOCS-2 in regulating signaling by these two cytokines during postnatal growth. There have been scant reports on the function of other SOCS family members. SOCS-5 mRNA is expressed in many tissues and is induced by IL-6 in the liver (4Hilton D.J. Richardson R.T. Alexander W.S. Viney E.M. Willson T.A. Sprigg N.S. Starr R. Nicholson S.E. Metcalf D. Nicola N.A Proc. Natl. Acad. Sci. USA. 1998; 95: 114-119Crossref PubMed Scopus (592) Google Scholar). SOCS-7 (also termed NAP4) has been shown to bind to Nck, Ash (Grb2), and phospholipase γ (for review, see 19Yasukawa H. Sasaki A. Yoshimura A Annu. Rev. Immunol. 2000; 18: 143-164Crossref PubMed Scopus (502) Google Scholar). The mechanism by which cytokine signaling is regulated has been a focus of research in the past few years. The SOCS family proteins have recently emerged as important regulators of cytokine signaling. Although the first two family members, CIS and SOCS-1, were identified as cytokine-inducible inhibitors of JAK-STAT activation, it is not clear whether other family members also act similarly. Gene disruption experiments have suggested a critical role for SOCS-1 in the regulation of IFNγ signaling and T cell development. SOCS-2 and SOCS-3 have also been shown to be important in postnatal growth and in fetal liver erythropoiesis, respectively. The precise mechanisms by which these proteins exert their effects remain to be determined. In addition, the function of the conserved SOCS box will be another focus of research. Although the SOCS box has been proposed to be involved in the ubiquitylation of proteins, there has been no direct evidence supporting a role for SOCS proteins in cellular ubiquitylation. Overall, identification of the SOCS family has opened a new field of research in signal transduction. Studies of these proteins will not only increase our understanding of immune regulation by the cytokine network but may also lead to development of specific pharmacological inhibitors of cytokine signaling.