SOCS-1 and SOCS-3 Block Insulin Signaling by Ubiquitin-mediated Degradation of IRS1 and IRS2

Abstract

Inflammation associates with peripheral insulin resistance, which dysregulates nutrient homeostasis and leads to diabetes. Inflammation induces the expression of SOCS proteins. We show that SOCS1 or SOCS3 targeted IRS1 and IRS2, two critical signaling molecules for insulin action, for ubiquitin-mediated degradation. SOCS1 or SOCS3 bound both recombinant and endogenous IRS1 and IRS2 and promoted their ubiquitination and subsequent degradation in multiple cell types. Mutations in the conserved SOCS box of SOCS1 abrogated its interaction with the elongin BC ubiquitin-ligase complex without affecting its binding to IRS1 or IRS2. The SOCS1 mutants also failed to promote the ubiquitination and degradation of either IRS1 or IRS2. Adenoviralmediated expression of SOCS1 in mouse liver dramatically reduced hepatic IRS1 and IRS2 protein levels and caused glucose intolerance; by contrast, expression of the SOCS1 mutants had no effect. Thus, SOCS-mediated degradation of IRS proteins, presumably via the elongin BC ubiquitin-ligase, might be a general mechanism of inflammation-induced insulin resistance, providing a target for therapy.Insulin and insulin-like growth factors exert many biological effects through receptor-mediated tyrosine phosphorylation of insulin receptor substrates (IRS 1 proteins), including IRS1, IRS2, IRS3, and IRS4 (1, 2). IRS proteins coordinate multiple signals through the PI 3-kinase 3 Pkb/Akt and the Grb2/Sos 3 Ras cascades (1, 2). In mice, IRS1 mediates the effects of insulin and IGF1 on somatic cell growth, whereas IRS2 is essential for nutrient homeostasis (3, 4). Mice lacking IRS1 or IRS2 display peripheral insulin resistance, which is a major determinant of type 2 diabetes (5-10). However, IRS1Ϫ/Ϫ mice never develop diabetes because of life-long compensatory hyperinsulinemia, whereas disruption of IRS2 causes severe glucose intolerance and diabetes (4). Dysregulation of IRS2 is especially problematic, because it regulates transcription factors in ␤-cells that are essential for glucose sensing and insulin secretion (1, 11). IRS proteins also integrate heterologous signals that negatively regulate the insulin-signaling cascade. Proinflammatory cytokines or insulin activate the c-Jun Nterminal kinase (JNK), which promotes serine phosphorylation of IRS1 and IRS2 that inhibits coupling to the activated insulin receptor (12). In addition, IRS proteins are decreased in people and rodents with diabetes (13-16). Our recent studies reveal that IRS proteins are ubiquitinated and subsequently degraded by the 26 S proteasome during insulin stimulation or during cellular stress, but the mechanisms that recruit ubiquitinligases to the IRS proteins are unknown (17,18).Insulin resistance is a common consequence of physiological stress, owing at least in part to the production of proinflammatory cytokines during infection or injury, pregnancy, growth and aging, or chronic obesity (19 -22). Many proinflammatory cytokines up-regulate suppressors of cytokine signaling (SOCS) proteins, including eight isoforms that c...

SOCS proteins in regulation of receptor tyrosine kinase signaling

“…One possibility is that SOCS1 limits the availability of downstream signaling proteins either through ubiquitin-mediated degradation or by competing for the same binding site in the receptor. For example, SOCS1 regulates insulin signaling by ubiquitin-mediated degradation of the INSR substrates IRS1 and IRS2 [28], while it can associate with INSR in response to insulin [29]. Furthermore, SOCS1 binding is not dependent on INSR-pY960, a site required for both IRS1 and IRS2 interaction but overlaps with the alternative IRS2 binding site in the catalytic loop [29].…”

“…Since the pY960 is the major binding site for IRS1 and IRS2 in the INSR [29], it is possible that SOCS3 competes with INSR substrates resulting in reduced phosphorylation of IRS1 or IRS2 [53]. SOCS3 also binds to IRS1 and IRS2, and abrogates insulin signaling by ubiquitin-mediated degradation of these two substrate proteins [28]. Therefore SOCS3 inhibits INSR signaling by two different mechanisms: by destabilization of IRS1 and IRS2 and by abrogation binding of IRS proteins to the receptor.…”

An immune origin of type 2 diabetes?

“…This protein associates with the insulin receptor and suppresses insulindependent receptor autophosporylation and IRS-1 phosphorylation [100]. SOCS-3 also binds to IRS-1 and IRS-2, leading to their ubiquitination and proteasomal degradation [104]. Immune mediators also affect insulin sensitivity indirectly by modulating the regulatory function of fat, nerve or other cells, e.g.…”

SOCS-mediated downregulation of mutant Jak2 (V617F, T875N and K539L) counteracts cytokine-independent signaling

“…The members of this protein family contain a central SH2 domain, as well as a C-terminal domain called SOCS-Box, which is required for proteasomal degradation of SOCS-binding partners (De Sepulveda et al, 2000;Frantsve et al, 2001;Kamizono et al, 2001;Rui et al, 2002;Ungureanu et al, 2002). Also, both SOCS1 and SOCS3 have been shown to interact directly with Jaks and thereby inhibit the phosphorylation of cytokine receptors, STATs and the Jaks themselves.…”