Biological Evidence That SOCS-2 Can Act Either as an Enhancer or Suppressor of Growth Hormone Signaling

Abstract

Suppressor of cytokine signaling (SOCS)-2 is a member of a family of intracellular proteins implicated in the negative regulation of cytokine signaling. The generation of SOCS-2-deficient mice, which grow to one and a half times the size of their wild-type littermates, suggests that SOCS-2 may attenuate growth hormone (GH)signaling. In vitro studies indicate that, while SOCS-2 can inhibit GH action at low concentrations, at higher concentrations it may potentiate signaling. To determine whether a similar enhancement of signaling is observed in vivo or alternatively whether increased SOCS-2 levels repress growth in vivo, we generated and analyzed transgenic mice that overexpress SOCS-2 from a human ubiquitin C promoter. These mice are not growth-deficient and are, in fact, significantly larger than wild-type mice. The overexpressed SOCS-2 was found to bind to endogenous GH receptors in a number of mouse organs, while phosphopeptide binding studies with recombinant SOCS-2 defined phosphorylated tyrosine 595 on the GH receptor as the site of interaction. Together, the data implicate SOCS-2 as having dual effects on GH signaling in vivo.The suppressor of cytokine signaling (SOCS) 1 proteins are a family of eight SH2 domain-containing proteins, comprising cytokine-inducible SH2 domain-containing protein (CIS) and SOCS-1-7. Studies in many laboratories have implicated SOCS proteins in the attenuation of cytokine action through inhibition of the Janus kinase (JAK)/signal transducer and activators of transcription (STAT) signal transduction pathway. SOCS proteins operate as part of a classical negative feedback loop, in which activation of cytokine signaling leads to their expression. Once produced, SOCS proteins bind to key components of the signaling apparatus to prevent further signal transduction and possibly target them for degradation via a conserved C-terminal motif, called the SOCS Box, that recruits ubiquitin ligases (reviewed in Refs. 1-3).While in vitro studies have suggested that SOCS proteins may be promiscuous in their activity, gene deletion studies in mice have highlighted their importance in a limited number of signaling pathways. SOCS-1 is a key regulator of interferon ␥ signaling, T-cell homeostasis, and lactation (4 -6), while SOCS-3 is thought to play a crucial role in placental function (7). CIS-deficient mice are reported to have no phenotype, although CIS transgenic mice display growth retardation and defects in mammary development which are accompanied by reductions in STAT5 phosphorylation (8). Interestingly, this phenotype has similarities to those observed in STAT5a-and STAT5b-deficient mice (9 -11).SOCS-2-deficient animals exhibit accelerated post-natal growth resulting in a 30 -50% increase in body weight by 12 weeks of age, significant increases in bone and body lengths, thickening of the skin due to collagen deposition, and increases in internal organ size (12). This phenotype has striking similarities to those of insulin-like growth factor (IGF)-I and growth hormone (GH) transgenic mic...

SOCS2 Induces Neurite Outgrowth by Regulation of Epidermal Growth Factor Receptor Activation

“…Regulation of STAT5 activation by SOCS2 has also been demonstrated in overexpression studies, although it is not as potent as other SOCS family members, such as SOCS1 and SOCS3 (32). Although the precise mechanism by which SOCS2 regulates STAT5 activation downstream of GH signaling remains to be determined, it appears to involve SOCS2 competitively binding to the STAT5 and SHP2 binding sites on the GH receptor (18,32,33). Here we show that SOCS2 can also inhibit STAT5 activation independently of GH stimulation.…”

“…Given that total EGFR levels were similar in SOCS2-overexpressing and control cells, it does not appear that SOCS2 blocks EGFR degradation, although conversely, SOCS1 and SOCS3 have been proposed to negatively regulate EGFR activation, possibly by promoting EGFR degradation (45). SOCS3 has been shown to interact with the binding sites for the tyrosine phosphatase SHP2 on the gp130 cytokine receptor (46,47) and the leptin receptor (48), and SOCS2 binds to the SHP2 site, Tyr 595 , of the GH receptor (18). We show here that SOCS2 binds to the EGFR and that overexpression of SHP2, but not SHP1, inhibited both the SOCS2-induced neurite outgrowth and the constitutive phosphorylation of the EGFR.…”

“…Overexpression of SHP2 Phosphatase Blocks SOCS2-induced Neurite Outgrowth-One possible mechanism by which SOCS2 overexpression could result in constitutive EGFR phosphorylation was by binding to a phosphatase site on the EGFR and blocking phosphatase activity, as has been shown for SOCS2 and the GH receptor (18). If this was the case, then overexpression of the phosphatase should allow more effective competition with SOCS2 for the binding site on the receptor, allowing dephosphorylation to occur.…”

“…These neurons were differentiated from neural stem cells derived from the central nervous system of transgenic mice that overexpress SOCS2 driven off the human ubiquitin C promoter (SOCS2Tg), such that SOCS2 is expressed in most cell types (18). SOCS2Tg and wild-type neurons were immunostained for the neuronal marker ␤III-tubulin (Fig.…”

Ezrin mediates c-Myc actions in prostate cancer cell invasion

“…Lysates were then assayed for luciferase activity using Promega Luciferase Assay Substrate. Galactosidase activity was measured using spectrophotometric assay as described (Greenhalgh et al, 2002). Luciferase activity was normalized to b-galactosidase activity.…”