Role of SOCS2 in growth hormone actions

Transcription

1 Review TRENDS in Endocrinology and Metabolism Vol.16 No.2 March 2005 Role of SOCS2 in growth hormone actions Ann M. Turnley Centre for Neuroscience, The University of Melbourne, Parkville, Victoria 3010, Australia Growth hormone () has numerous effects in the body and is most commonly known for its role in regulating metabolism and body growth. Because is involved in many aspects of cell function, its signaling is tightly controlled by several pathways at both the extracellular and intracellular level. Suppressor of cytokine signaling-2 (SOCS2) is one such intracellular regulator of signal transduction. Expression of SOCS2 is tightly regulated and alteration of its levels leads to marked abnormalities in metabolism and growth. Unexpectedly, and SOCS2 have been recently shown to regulate neural development, neural stem cell differentiation and neuronal growth functions that might have important therapeutic implications for both repairing nervous system injuries and treating neurological disease. Introduction Growth factors, cytokines and hormones control many aspects of biological and cellular function, from the earliest stages of development through to death. Nearly every aspect of cellular function is regulated in some way by external signals, from cell proliferation, migration, adhesion or differentiation to general metabolism. is one such factor that has a myriad of effects. Many, but not all, of the effects of on tissue growth result from circulating released from the pituitary, which often acts by inducing the release of insulin-like growth factor-1 (IGF1) or other factors [1]. There are also extrapituitary sites of release for example, in various areas of the central nervous system (CNS) [2] that have important implications for neural function and are described in more detail below. signaling is regulated at many levels to limit the length and strength of the signal and to produce different responses in distinct types of cell. In this review, I focus on the regulation of signaling by SOCS proteins and in particular on the role of SOCS2 in regulating functions in the CNS. dimerization of STAT molecules [4]. STAT5b and/or STAT5a are activated in many types of cell, but STAT1 and STAT3 are also induced in some cell types. Other pathways and factors that can be activated downstream of signaling include the extracellular-signal-regulated kinase (ERK) and mitogen-activated protein kinase (MAPK) pathway, insulin receptor substrate 1 (IRS-1) and IRS-2, protein kinase C, CCAAT enhancer-binding proteins [3 5], and RhoA and its kinase ROCK [6]. Thus, numerous different signal transduction pathways can be activated downstream of signaling, leading to different biological outcomes, depending on the concentration of and the particular cell type. This variation raises the issue of how different types of cell can respond to stimulation by the same factor with distinct outcomes and relies on how a given cell interprets the stimulus. Each type of cell expresses different intracellular regulators of signal transduction with varying levels and kinetics of expression. Although many of these regulators overlap between cells, the system is exquisitely finely tuned in each cell type to bring about the appropriate response for that cell. Regulation of the signal transduction pathways downstream of stimulation occurs at several levels (Figure 1). First, expression and turnover of the R (a) R expression level (b) Phosphatases (c) Negative feedback loop (d) Proteasome degradation signaling signals by binding to the receptor (R), a member of the class I superfamily of cytokine receptors, that transduces signals primarily through the Janus kinase (JAK) and signal transducer and activator of transcription (STAT) pathway [3]. Activation of the R leads to signal transduction, primarily through activation of JAK2, which results in the phosphorylation and Corresponding author: Turnley, A.M. (turnley@unimelb.edu.au). Available online 27 January 2005 TRENDS in Endocrinology & Metabolism Figure 1. Regulation of receptor signaling. In a cell expressing the R, -induced signaling is regulated at several stages, including (a) the expression level of the receptor and thus the strength of the signal; (b) phosphatase-mediated dephosphorylation of R tyrosines that have been activated by binding to the receptor; (c) inactivation of downstream signaling pathways by a negative feedback loop induced by signaling, such as the induction of SOCS molecules that inhibit JAK/STAT activation; and (d) ligand-induced proteasomal degradation of the R /$ - see front matter Q 2005 Elsevier Ltd. All rights reserved. doi: /j.tem

2 54 Review TRENDS in Endocrinology and Metabolism Vol.16 No.2 March 2005 are regulated by ligand binding, which leads either to receptor internalization in a ubiquitin-dependent manner [7,8], or to R cleavage resulting in the production of -binding proteins via the metalloproteinase TACE [9]. Second, activation of R leads to JAK-induced phosphorylation of tyrosine residues in various intracellular domains of the receptor. These tyrosine residues then become docking sites for molecules that dictate which signal transduction pathway is followed. The length of time that the tyrosine residues remain phosphorylated also controls the strength and type of response elicited by the cell. For example, mechanisms exist to limit the duration of tyrosine phosphorylation, including dephosphorylation by phosphatases such as SH2-domain-containing protein tyrosine phosphatase (SHP2) [10] and SHP1 [11]. The R phosphorylation state is thus a balance between phosphorylation as a consequence of binding and dephosphorylation as a consequence of phosphatase activity. Third, the range of proteins that can bind to the phosphorylated tyrosines also alters over the course of a given signal transduction event. There are, of course, binding proteins that lead to activation of the appropriate signal transduction pathway; however, there are also proteins that can block the binding of the effector molecules to shut down or to modify the signaling, and it is the balance between these molecules that determines the final outcome of a given signaling event. These blocking molecules can be constitutively expressed; alternatively, they can be induced by the signaling event such that they can form a negative feedback loop to shut down the signaling that induced their expression. One such family of molecules is the SOCS family that regulates the JAK/STAT pathway. proteosomal degradation of their targets, providing another level of complexity in their actions [16]. Mounting evidence suggests that several SOCS family members can regulate signal transduction pathways other than the JAK/STAT pathway, including those of receptor tyrosine kinases such as the IGF1 receptor [17,18], Kit [19] and the insulin receptor [20 22]. In addition, they have been reported to interact with both the GTPaseactivating protein p120 RasGAP to enhance Ras activation [23] and the guanine nucleotide exchange factor Vav [24]. Although much of the work on SOCS function has concentrated on their regulation of the JAK/STAT pathway, it is likely that they have a much wider range of activities and that we have only just scratched the surface in terms of what these other functions might be. SOCS2 is an interesting molecule in this regard, because not only does it regulate signaling through the JAK/STAT pathway [25], but it also seems to regulate phosphorylation and signaling of the epidermal growth factor receptor [26,27]. SOCS2 and regulation of signaling Because signals primarily through the JAK/STAT pathway, it is a prime candidate for regulation by members of the SOCS family. Indeed, it can be regulated by SOCS1, SOCS2 or SOCS3 (Figure 2). Although the SOCS2-null mutant mouse has an overgrowth phenotype, consistent with its inability to downregulate signaling [28], in vitro studies have indicated that SOCS2 is not as potent as SOCS1 or SOCS3 in blocking action [29]. Furthermore, SOCS1 knockout mice, if treated to prevent premature death owing to interferon The SOCS family Members of the SOCS family are primarily considered to be negative regulators of cytokine signaling that switch off signaling as part of a negative feedback loop. Although in some cells they are expressed at basal levels, in many cells their expression is induced by activation of the same JAK/STAT pathway that they then inhibit [12 14]. Indeed, this feature of SOCS molecules is suggested by two of their alternative names that are used less commonly: cytokine-inducible SH2-containing protein (CIS) and STAT-induced STAT inhibitor (SSI). Some members of the SOCS family are also referred to as Janus-binding (JAB) proteins. The SOCS family comprises eight members, SOCS1 SOCS7 and CIS. This family, named after a novel domain in their carboxy (C)-terminal end the SOCS box forms part of a wider family of proteins containing the same domain [15]. Their expression varies in different tissues; in addition, the particular JAK/STAT pathway that they regulate varies between SOCS family members and depends on the cell type in which the SOCS are expressed. To complicate matters further, factors that induce the expression of a particular SOCS protein in one tissue do not necessarily do so in another. Furthermore, some members of the SOCS family are involved in promoting Nucleus JAK SOCS3 Transcription SOCS1 SOCS2 STAT5 TRENDS in Endocrinology & Metabolism Figure 2. Inhibition of signal transduction by SOCS molecules. SOCS1, SOCS2 and SOCS3 can each inhibit signaling downstream of R activation; however, they do so by different mechanisms. SOCS1 binds to JAK and inhibits its ability to phosphorylate the receptor, whereas SOCS3 binds to membrane-proximal phosphorylated tyrosines on the R. Although the mechanism of SOCS2 action has not been elucidated in full, SOCS2 binds to membrane-distal phosphorylated tyrosines that might also be STAT5-binding sites, such that SOCS2 might block STAT5 binding and thus inhibit the phosphorylation, dimerization and transcriptional activation of STAT5.

3 Review TRENDS in Endocrinology and Metabolism Vol.16 No.2 March hyperresponsiveness [30], do not have an overgrowth phenotype, suggesting that, although SOCS1 is a potent blocker of signaling in vitro, it seems to have no major function in vivo. Of course, this observation could be a result of functional compensation by SOCS3: the role of SOCS3 on has not been examined in vivo because the SOCS3-null mutation is embryonic lethal [31]. The mechanism by which each of these SOCS molecules regulates signaling is different and might help to explain their differential effectiveness in vitro and in vivo (Figure 2). Whereas SOCS1 binds to JAK, thereby inhibiting phosphorylation of the R, SOCS3 binds to phosphorylated tyrosines in the juxtamembrane region of the R [29]. Although the precise mechanism by which SOCS2 regulates activation of STAT5 downstream of signaling remains to be determined, it seems to involve the competitive binding of SOCS2 to the STAT5- and SHP2-binding sites on the R [29,32,33]. Thus, the effectiveness of STAT5 activation or downregulation might depend on the balance of SOCS2 and SHP2 binding to these sites. Functional consequences of altering SOCS2 expression in mice The role of SOCS2 in regulating signaling in vivo is obviously complex, as assessed by a comparison of the phenotypes of SOCS2-deficient (knockout) and SOCS2- overexpressing transgenic phenotypes. Although SOCS2 knockout mice appear normal at birth, they show an overgrowth phenotype after a few weeks [28]. A similar phenotype is observed in a naturally occurring mouse mutant, the high growth mouse, which has a chromosomal deletion encompassing the region containing the gene encoding SOCS2 [34]. The enlarged body growth of these mice is likely to be a consequence of their failure to downregulate effectively -induced activation of STAT5. Cultured hepatocytes from SOCS2 knockout mice show prolonged activation of STAT5 in response to stimulation [25]. Furthermore, if SOCS2 knockout mice are crossed with STAT5b-deficient mice, they show an attenuated overgrowth phenotype, indicating that regulation of -induced STAT5 activation is an important factor in the SOCS2-mediated regulation of body growth. The enlarged body growth of the SOCS2 knockout mice results from an increase in the size of most organs and bones. The mice also have collagen deposition in skin and show a decrease in the production of the urinary protein MUP a characteristic of dysregulated and/or IGF1 signaling. Given that one of the main mechanisms by which mediates its effects is through the induction of IGF1 and that SOCS2 has been shown to bind to the IGF1 receptor [17], it is possible that SOCS2 also acts by regulating IGF1 signaling. IGF1 signaling does not, however, seem to be altered in SOCS2-deficient embryonic fibroblasts [25]. Because the deletion of SOCS2 was found to cause hyperresponsiveness and gigantism in SOCS2 knockout mice, it was thought that transgenic mice that overexpressed SOCS2 in all tissues of the body would show the opposite effect. Interestingly, however, SOCS2 transgenic mice, in which SOCS2 expression is driven by the human ubiquitin C promoter, show a similar overgrowth phenotype to that of SOCS2-deficient mice [33]. Thus, SOCS2, it turns out, can either enhance or suppress signaling. Such a role might have been suspected, given that different groups have reported conflicting results concerning the effect of SOCS2 on signaling in vitro. For example, SOCS2 has been reported to inhibit signaling moderately [29], to enhance signaling [32,35], or to have a dual role in which it inhibits signaling at low doses and enhances it at higher doses [33,36]. Overall, it would seem that normal physiological levels of SOCS2 inhibit signaling, as assessed by the overgrowth phenotype in SOCS2 knockout mice. When overexpressed, by contrast, SOCS2 seems to enhance signaling, again as assessed by the overgrowth phenotype. To explain these dual effects, Greenhalgh et al. [33] have proposed a mechanism in which SOCS2 binds to Tyr595 on the R as the preferred high-affinity site. This site is also a binding site for SHP2 [10] and is presumed to be the main site used by SOCS2 to inhibit signaling. At higher concentrations, SOCS2 might also bind to other tyrosines on the R, such as Tyr487 or Tyr332, which are SOCS3-binding sites [29,32]. Because SOCS3 is more potent than SOCS2 at inhibiting R signaling in vitro, blocking binding by SOCS3 might enhance R signal transduction. SOCS2 and in the nervous system One of the unexpected results of the studies of SOCS2 knockout mice has been the finding that SOCS2 and have important roles in the development of the nervous system, particularly with regard to neural differentiation. SOCS2 is highly expressed in the nervous system during development and shows a peak of expression in the mouse at embryonic day 14 that coincides with the peak of neuronal generation [37]. Notably, one of the few organs that does not show an increase in size in the SOCS2 knockout mice is the brain [28]. Although their brain size is normal, however, the mice show a range of neural abnormalities [38,39], the most pronounced of which is a 25 30% decrease in neuronal density in the cortex. They also show alterations in their percentages of interneuron subtypes and increases in the soma size of some neuronal populations. When neural stem cells are cultured from SOCS2 knockout mice and induced to differentiate, they produce 50% fewer neurons and an increased number of astroglial cells (Figure 3), whereas SOCS2-overexpressing neural stem cells show an increase in the percentage of neurons generated [39]. Unlike in the rest of the body, therefore, a lack of SOCS2 expression and overexpression of SOCS2 in cells of the CNS lead to opposite effects on neuronal density and seem to be due to a respective enhancement and inhibition of signaling. Although conventionally has not been considered to be a main factor in the development of the nervous system, the neural phenotype of the SOCS2 mutant mice suggested that might be involved in this process. Expression of [40] and the R [39,41,42] has been reported at various stages of brain development, and some studies have shown neural cells to produce in vitro [39,43], although these studies have provided little

4 56 Review TRENDS in Endocrinology and Metabolism Vol.16 No.2 March 2005 (a) Wildtype (b) SOCS2 knockout Astrocytes Neurons (c) (d) Figure 3. Deletion of SOCS2 inhibits neuronal differentiation and enhances astrocyte differentiation. As compared with wild-type cells (a,c), neural stem cells derived from SOCS2 knockout mice (b,d) that have been induced to differentiate in vitro produce fewer neurons (a,b) and more astrocytes (c,d). Neurons were immunostained for the neuronal marker biii-tubulin and astrocytes were immunostained for the astrocyte marker glial fibrillary acidic protein. Scale bar (d),100mm. Images have been adapted from reference [39]. indication of the role that might have. The results suggest, however, that any effect of on development of the nervous system is likely to be caused by produced locally rather than by circulating, pituitary-derived, which is not present at high levels until puberty in the mouse. This idea is supported by the analysis of R knockout mice [44]: the brains of these mice are normal size at birth but, like the rest of the body, are decreased in size by adulthood relative to the brains of wild-type mice. Furthermore, analysis of their neuroanatomy has shown that knockout of the R has the opposite effect to knockout of SOCS2; that is, the R knockout mice show an increase in neuronal density [38]. This observation indicates that neuronal differentiation is potentiated when signaling is absent during neural development, such as in the R knockout mouse, and inhibited when signaling is excessive, such as in the SOCS2 knockout mouse. The mechanism by which regulates neuronal differentiation has been elucidated by analyses of neural stem cell cultures derived from wild-type and SOCS2 knockout mice. As mentioned above, SOCS2 knockout neural stem cells produce 50% fewer neurons than wildtype stem cells, but if action is blocked in these cultures either by adding somatostatin to inhibit the release of from cells in the culture or by adding a blocking antibody to then neuronal differentiation of SOCS2 knockout cultures is increased to wild-type levels. Furthermore, the sensitivity of the neural stem cells to stimulation has been demonstrated by adding exogenous to cultures in which endogenous production is inhibited by somatostatin; addition of at concentrations as low as 1 ng/ml inhibits the production of neurons by SOCS2 knockout stem cells, whereas inhibition of SOCS2-expressing wild-type cells requires 100 times the concentration of to be effective [39]. Expression of the basic helix loop helix transcription factor Neurogenin-1 (Ngn1) is required for neuronal differentiation [45], and has been shown to block neuronal differentiation via the inhibition of this factor [39]. Within 1 h of its administration, has been found to induce the downregulation of Ngn1 protein, and this downregulation is even more pronounced by 4 h. By contrast, overexpression of SOCS2 blocks the -induced decrease in Ngn1 expression [39]. Analysis of Ngn1 levels in the developing nervous system of SOCS2 knockout mice has shown that there is a marked decrease in both Ngn1 expression and the number of cells that express it, leading to the observed decrease in neuronal density in the adult brain. These results indicate that SOCS2, via its regulation of signaling, is a chief factor in the developing nervous system. A model outlining the role of and SOCS2 in neural differentiation is shown in Figure 4. The effect on neurogenesis is not the only role that SOCS2 seems to have in the nervous system. In situ hybridization analysis has shown that, although SOCS2 is expressed at moderate levels in neural progenitor and/or stem cells in the developing brain, it is expressed at higher levels in newly formed neurons that have migrated to the cortex [37]. This observation suggests that SOCS2 might have a role in neuronal function in addition to its role in regulating differentiation. One of the main functions of the new neurons produced during development is to extend processes to other neurons to form the wiring of the nervous system. Indeed, in vitro analyses of SOCS2- overexpressing neural cells has shown that SOCS2 promotes the extension of neurites from neurons,

5 Review TRENDS in Endocrinology and Metabolism Vol.16 No.2 March Neuron Progenitor cell SOCS2 neg/low Cytokines? Ngn1 Stem cell SOCS2 high SOCS2 neg/low Progenitor cell Ngn1 SOCS2 high SOCS2 neg Astrocyte TRENDS in Endocrinology & Metabolism Figure 4. Model of the regulation of neural stem cell differentiation by SOCS2 and. Neural stem cells generally express negative to low levels of SOCS2, but in some cells expression of SOCS2 is increased to high levels, possibly by cytokines. Progenitor cells that express high levels of SOCS2 can block signals from, resulting in an increase in expression of the basic helix loop helix transcription factor Ngn1, which is required for neuronal differentiation. Progenitor cells that do not express high levels of SOCS2 are unable to block signaling and therefore have low levels of Ngn1, leading to astrocyte differentiation. increasing both their number and length [26,27]. Thus, SOCS2 has a role in both neuronal differentiation and neuronal function. Perspective: where to from here? Regulation of signaling by SOCS2 has important outcomes for the regulation of body size and metabolism. The mechanism by which SOCS2 can either inhibit or potentiate signal transduction needs to be more fully elucidated, particularly in terms of how SOCS2 can interact with other SOCS molecules and phosphatases to modify signaling outcomes. Regulation of signaling by SOCS2 also seems to have important consequences for development of the CNS. Furthermore, analyses of neuronal differentiation and neurite outgrowth in adult neural stem cells have produced similar results to those observed during neural development, which suggests that signaling and SOCS2 might be also important in adult neurogenesis and in repair of the nervous system after disease or injury. Because, the R and -binding protein are upregulated in the nervous system after brain injury [46,47], the ability to regulate signaling could have important therapeutic consequences. Acknowledgements A.M.T. is a C.R. Roper Fellow and would like to acknowledge support from the National Health and Medical Research Council of Australia (Project ) and the BHP Community Trust. References 1 Butler, A.A. and Le Roith, D. (2001) Control of growth by the somatropic axis: growth hormone and the insulin-like growth factors have related and independent roles. Annu. Rev. Physiol. 63, Harvey, S. and Hull, K. (2003) Neural growth hormone: an update. J. Mol. Neurosci. 20, Zhu, T. et al. (2001) Signal transduction via the growth hormone receptor. Cell. Signal. 13, Herrington, J. and Carter-Su, C. (2001) Signaling pathways activated by the growth hormone receptor. Trends Endocrinol. Metab. 12, Piwien-Pilipuk, G. et al. (2002) Growth hormone signal transduction. J. Pediatr. Endocrinol. Metab. 15, Ling, L. and Lobie, P.E. (2004) RhoA/ROCK activation by growth hormone abrogates p300/histone deacetylase 6 repression of Stat5- mediated transcription. J. Biol. Chem. 279, Strous, G.J. et al. (1996) The ubiquitin conjugation system is required for ligand-induced endocytosis and degradation of the growth hormone receptor. EMBO J. 15, Strous, G.J. et al. (1997) Growth hormone-induced signal tranduction depends on an intact ubiquitin system. J. Biol. Chem. 272, Baumann, G. and Frank, S.J. (2002) Metalloproteinases and the modulation of signaling. J. Endocrinol. 174, Stofega, M.R. et al. (2000) Mutation of the SHP-2 binding site in growth hormone () receptor prolongs -promoted tyrosyl phosphorylation of receptor, JAK2, and STAT5B. Mol. Endocrinol. 14, Ram, P.A. and Waxman, D.J. (1997) Interaction of growth hormone-activated STATs with SH2-containing phosphotyrosine phosphatase SHP-1 and nuclear JAK2 tyrosine kinase. J. Biol. Chem. 272, Krebs, D.L. and Hilton, D.J. (2001) SOCS proteins: negative regulators of cytokine signaling. Stem Cells 19, Fujimoto, M. and Naka, T. (2003) Regulation of cytokine signaling by SOCS family molecules. Trends Immunol. 24, Greenhalgh, C.J. and Hilton, D.J. (2001) Negative regulation of cytokine signaling. J. Leukoc. Biol. 70, Hilton, D.J. et al. (1998) Twenty proteins containing a C-terminal SOCS box form five structural classes. Proc. Natl. Acad. Sci. U. S. A. 95, Kile, B.T. et al. (2002) The SOCS box: a tale of destruction and degradation. Trends Biochem. Sci. 27, Dey, B.R. et al. (1998) Interaction of human suppressor of cytokine signaling (SOCS)-2 with the insulin-like growth factor-i receptor. J. Biol. Chem. 273, Dey, B.R. et al. (2000) Suppressor of cytokine signaling (SOCS)-3 protein interacts with the insulin-like growth factor-i receptor. Biochem. Biophys. Res. Commun. 278, De Sepulveda, P. et al. (1999) Socs1 binds to multiple signalling proteins and suppresses steel factor-dependent proliferation. EMBO J. 18, Mooney, R.A. et al. (2001) Suppressors of cytokine signaling-1 and

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