Suppressor of Cytokine Signaling 7 Inhibits Prolactin, Growth Hormone, and Leptin Signaling by Interacting with STAT5 or STAT3 and Attenuating Their Nuclear Translocation

Abstract

We report here the role of one of the less studied members of the family of suppressors of cytokine signaling (SOCS), namely SOCS-7, in cytokine signaling. We demonstrate that SOCS-7 inhibits prolactin (PRL), growth hormone (GH), or leptin (LEP) signaling mediated through STAT3 and STAT5 in a dose-dependent manner. SOCS-7 also attenuated STAT3 and STAT5 signaling induced by overexpression of JH1, the catalytic subdomain of JAK2. Since SOCS-7 interacted with phosphorylated STAT3 or STAT5, we assumed that SOCS-7 acts at the level of STAT proteins. Indeed, we showed that SOCS-7 inhibits PRL-and leptin-induced STAT5 and STAT3 phosphorylation and prevented the nuclear translocation of activated STAT3. Taken together, our results indicate that SOCS-7 is a physiological dysregulator of PRL, leptin, and probably also GH signaling and that its mode of action is a novel variation of SOCS protein inhibition of cytokine-inducible STAT-mediated signal transduction.One of the main pathways for the termination of cytokineactivated JAK 1 /STAT signaling is mediated by a family of proteins discovered nearly a decade ago, the suppressors of cytokine signaling (SOCS) (1-4). These proteins' modulatory activity is not restricted to the JAK/STAT pathway; it has also been reported for activities mediated by receptor tyrosine kinases, such as c-Kit and insulin receptor substrate (IRS)-1 to -4 (5-7). The current paradigm attributes the main function of SOCS to the targeting of the affected proteins for ubiquitination and subsequent degradation by the proteasome, although additional mechanisms are also involved (4 -6). The SOCS family consists of eight proteins, termed SOCS-1 through -7 and CIS (cytokineinducible SH2 protein). Whereas CIS and SOCS-1 through -3 have been studied extensively, others, in particular SOCS-7, have been given much less attention. The latter protein was first discovered by Matuoka et al. (8), who cloned its partial cDNA sequence and called it NAP-4 due to its ability to interact with the adaptor proteins Nck, Grb2 (Ash), and Phospholipase C-␥-1. More recently, full sequences of mouse, rat, and human SOCS-7 have been reported (GenBank TM NM_138657 for mouse; XP_213443 for rat; and see Ref. 9 for human). The NAP-4 sequence lacks the 127 N-terminal amino acids of SOCS-7 but has an additional segment of 34 amino acids (exon 4). SOCS-7 is constitutively expressed in several tissues, most strongly in the testis and brain (8, 10, 11), and its expression in leukocytes is stimulated by prolactin (PRL), growth hormone (GH), and interleukin-6 (10). Recently, SOCS-7 (and SOCS-6) has been shown to interact with IRS-2 and IRS-4 (11). However, to date, no report concerning the possible biological role of SOCS-7 has been published aside from our recent work showing that SOCS-7 interacts with the cytoskeleton proteins actin and vinexin and is present in cell membranes (9). This latter finding prompted us to study whether SOCS-7 affects biological activities mediated through the membrane-embedded receptors of three cytokines: GH ...

Inhibition of Preproinsulin Gene Expression by Leptin Induction of Suppressor of Cytokine Signaling 3 in Pancreatic β-Cells

“…SOCS3 induced by leptin binds to the leptin receptor-associated kinase JAK2 in a leptin-dependent manner in vitro, antagonizes kinase activity, and thereby attenuates proximal leptin signaling through the JAK-STAT pathway (17). Intriguingly, another SOCS protein, SOCS7, has recently been shown to act as an inhibitor of leptin-activated STAT3 and STAT5 signaling by abolishing their translocation to the nucleus (35). We found that SOCS molecules may also represent candidate mediators of leptin-dependent inhibition of insulin promoter activity in pancreatic ␤-cells.…”

Crystal structure of the SOCS2–elongin C–elongin B complex defines a prototypical SOCS box ubiquitin ligase

“…Direct target specificity for GHR appears to be restricted to the SH2 domains of SOCS3, which binds pY333 and pY338, and SOCS2 and CIS1, which target the membrane-distal pY487 and pY595 sites (1). SOCS2 is uniquely identified as a primary GHR inhibitor in vivo by its overgrowth knockout phenotype in mice (5); other SOCS family knockout mice show no overgrowth phenotype or are growth-retarded (25)(26)(27). The binding affinity of SOCS2 for the primary pY595 site is typical for physiological SH2-ligand interactions determined by isothermal titration calorimetry (ITC) and 5-fold higher than for the erythropoietin receptor (Table 2).…”

“…For example, SOCS1 can inhibit JAK2 through either its N-terminal kinase inhibitory region or SH2 domains and also mediate its degradation by means of the SOCS box (17). Downstream inhibition of STAT3 and STAT5 may also be effected by SOCS7 to suppress multiple cytokine pathways (25). Direct target specificity for GHR appears to be restricted to the SH2 domains of SOCS3, which binds pY333 and pY338, and SOCS2 and CIS1, which target the membrane-distal pY487 and pY595 sites (1).…”

Dynamic Trafficking of STAT5 Depends on an Unconventional Nuclear Localization Signal

“…Negative regulation of STAT5 is known to be mediated by tyrosine phosphatases and suppressors of cytokine signaling (SOCS) (Aoki and Matsuda, 2000;Aoki and Matsuda, 2002;Chen et al, 2004;Cornish et al, 2003;Hoyt et al, 2007;Martens et al, 2005). Another possible means of negative regulation is STAT5 export from the nucleus.…”