Induction of SOCS-3 is insufficient to confer IRS-1 protein degradation in 3T3-L1 adipocytes

Stat3 activation induces insulin resistance via a muscle-specific E3 ubiquitin ligase Fbxo40

Cellular mechanisms causing insulin resistance (IR) in chronic kidney disease (CKD) are poorly understood. One potential mechanism is that CKD-induced inflammation activates the signal transducer and activator of transcription 3 (Stat3) in muscle. We uncovered increased p-Stat3 in muscles of mice with CKD or mice fed high-fat diet (HFD). Activated Stat3 stimulates the expression of Fbxo40, a muscle-specific E3 ubiquitin ligase that stimulates ubiquitin conjugation leading to degradation of insulin receptor substrate 1 (IRS1). Evidence that Stat3 activates Fbxo40 includes 1) potential Stat3 binding sites in Fbxo40 promoters; 2) Stat3 binding to the Fbxo40 promoter; and 3) constitutively active Stat3 stimulating both Fbxo40 expression and its promoter activity. We found that IL-6 activates Stat3 in myotubes, increasing Fbxo40 expression with reduced IRS1 and p-Akt. Knockdown Fbxo40 using siRNA from myotubes results in higher levels of IRS1 and p-Akt despite the presence of IL-6. We treated mice with a small-molecule inhibitor of Stat3 (TTI-101) and found improved glucose tolerance and insulin signaling in skeletal muscles of mice with CKD or fed an HFD. Finally, we uncovered improved glucose tolerance in mice with muscle-specific Stat3 KO versus results in Stat3^f/f mice in response to the HFD. Thus Stat3 activation in muscle increases IR in mice. Inhibition of Stat3 by TTI-101 could be developed into clinical strategies to improve muscle insulin signaling in inflammation and other catabolic diseases.

SOCS-1 rescues IL-1β-mediated suppression of epithelial sodium channel in mouse lung epithelial cells via ASK-1

Background

Acute lung injury (ALI) is characterized by alveolar damage, increased levels of pro-inflammatory cytokines and impaired alveolar fluid clearance. Recently, we showed that the deletion of Apoptosis signal-regulating kinase 1 (ASK1) protects against hyperoxia-induced acute lung injury (HALI) by suppressing IL-1β and TNF-α. Previously, our data revealed that the suppressor of cytokine signaling-1 (SOCS-1) overexpression restores alveolar fluid clearance in HALI by inhibiting ASK-1 and suppressing IL-1β levels. Furthermore, IL-1β is known to inhibit the expression of epithelial sodium channel α-subunit (ENaC) via a p38 MAPK signaling pathway.

Objective

To determine whether SOCS-1 overexpression in MLE-12 cells would protect against IL-1β-mediated depletion of αENaC by suppressing ASK-1 expression.

Methods

We co-transfected MLE-12 cells with SOCS-1 overexpressing plasmid with or without IL-1β in the presence or absence of sodium channel inhibitor, amiloride. We measured potential difference, transepithelial current, resistance, and sodium uptake levels across MLE-12 cells. We studied the effect of ASK-1 depletion, as well as ASK-1 and SOCS-1 overexpression on αENaC expression.

Results

SOCS-1 overexpression sufficiently restored transepithelial current and resistance in MLE-12 cells treated with either IL-1β or amiloride. The αENaC mRNA levels and sodium transport were increased in SOCS-1 overexpressing MLE-12 cells exposed to IL-1β. Depletion of ASK-1 in MLE-12 cells increased αENaC mRNA levels. Interestingly, SOCS-1 overexpression restored αENaC expression in MLE-12 cells in the presence of ASK-1 overexpression.

Conclusion

Collectively, these findings suggest that SOCS-1 may exert its protective effect by rescuing αENaC expression via suppression of ASK-1.

The Novel Functions of High-Molecular-Mass Complexes Containing Insulin Receptor Substrates in Mediation and Modulation of Insulin-Like Activities: Emerging Concept of Diverse Functions by IRS-Associated Proteins

Insulin-like peptides, such as insulin-like growth factors (IGFs) and insulin, induce a variety of bioactivities, such as growth, differentiation, survival, increased anabolism, and decreased catabolism in many cell types and in vivo. In general, IGFs or insulin bind to IGF-I receptor (IGF-IR) or insulin receptor (IR), activating the receptor tyrosine kinase. Insulin receptor substrates (IRSs) are known to be major substrates of receptor kinases, mediating IGF/insulin signals to direct bioactivities. Recently, we discovered that IRSs form high-molecular-mass complexes (referred to here as IRSomes) even without IGF/insulin stimulation. These complexes contain proteins (referred to here as IRSAPs; IRS-associated proteins), which modulate tyrosine phosphorylation of IRSs by receptor kinases, control IRS stability, and determine intracellular localization of IRSs. In addition, in these complexes, we found not only proteins that are involved in RNA metabolism but also RNAs themselves. Thus, IRSAPs possibly contribute to modulation of IGF/insulin bioactivities. Since it is established that disorder of modulation of insulin-like activities causes various age-related diseases including cancer, we could propose that the IRSome is an important target for treatment of these diseases.

SOCS proteins in infectious diseases of mammals

As for most biological processes, the immune response to microbial infections has to be tightly controlled to remain beneficial for the host. Inflammation is one of the major consequences of the host's immune response. For its orchestration, this process requires a fine-tuned interplay between interleukins, endothelial cells and various types of recruited immune cells. Suppressors of cytokine signalling (SOCS) proteins are crucially involved in the complex control of the inflammatory response through their actions on various signalling pathways including the JAK/STAT and NF-κB pathways. Due to their cytokine regulatory functions, they are frequent targets for exploitation by infectious agents trying to escape the host's immune response. This review article aims to summarize our current knowledge regarding SOCS family members in the different mammalian species studied so far, and to display their complex molecular interactions with microbial pathogens.

TNFα and SOCS3 regulate IRS-1 to increase retinal endothelial cell apoptosis

Rates of diabetes are reaching epidemic levels. The key problem in both type 1 and type 2 diabetes is dysfunctional insulin signaling, either due to lack of production or due to impaired insulin sensitivity. A key feature of diabetic retinopathy in animal models is degenerate capillary formation. The goal of this present study was to investigate a potential mechanism for retinal endothelial cell apoptosis in response to hyperglycemia. The hypothesis was that hyperglycemia-induced TNFα leads to retinal endothelial cell apoptosis through inhibition of insulin signaling. To test the hypothesis, primary human retinal endothelial cells were grown in normal glucose (5 mM) or high glucose (25 mM) and treated with exogenous TNFα, TNFα siRNA or suppressor of cytokine signaling 3 (SOCS3) siRNA. Cell lysates were processed for Western blotting and ELISA analyses to verify TNFα and SOCS3 knockdown, as well as key pro- and anti-apoptotic factors, IRS-1, and Akt. Data indicate that high glucose culturing conditions significantly increase TNFα and SOCS3 protein levels. Knockdown of TNFα and SOCS3 significantly increases anti-apoptotic proteins, while decreasing pro-apoptotic proteins. Knockdown of TNFα leads to decreased phosphorylation of IRS-1(Ser307), which would promote normal insulin signaling. Knockdown of SOCS3 increased total IRS-1 levels, as well as decreased IR(Tyr960), both of which would inhibit retinal endothelial cell apoptosis through increased insulin signaling. Taken together, our findings suggest that increased TNFα inhibits insulin signaling in 2 ways: 1) increased phosphorylation of IRS-1(Ser307), 2) increased SOCS3 levels to decrease total IRS-1 and increase IR(Tyr960), both of which block normal insulin signal transduction. Resolution of the hyperglycemia-induced TNFα levels in retinal endothelial cells may prevent apoptosis through disinhibition of insulin receptor signaling.

A protein interaction network for Ecm29 links the 26 S proteasome to molecular motors and endosomal components

Ecm29 is a 200-kDa HEAT repeat protein that binds the 26 S proteasome. Genome-wide two-hybrid screens and mass spectrometry have identified molecular motors, endosomal components, and ubiquitin-proteasome factors as Ecm29-interacting proteins. The C-terminal half of human Ecm29 binds myosins and kinesins; its N-terminal region binds the endocytic proteins, Vps11, Rab11-FIP4, and rabaptin. Whereas full-length FLAG-Ecm29, its C-terminal half, and a small central fragment of Ecm29 remain bound to glycerol-gradient-separated 26 S proteasomes, the N-terminal half of Ecm29 does not. Confocal microscopy showed that Ecm-26 S proteasomes are present on flotillin-positive endosomes, but they are virtually absent from caveolin- and clathrin-decorated endosomes. Expression of the small central fragment of Ecm29 markedly reduces proteasome association with flotillin-positive endosomes. Identification of regions within Ecm29 capable of binding molecular motors, endosomal proteins, and the 26 S proteasome supports the hypothesis that Ecm29 serves as an adaptor for coupling 26 S proteasomes to specific cellular compartments.

Degradation of STAT5 proteins in 3T3-L1 adipocytes is induced by TNF-{alpha} and cycloheximide in a manner independent of STAT5A activation

Tumor necrosis factor-alpha (TNF-alpha) is a multifunctional cytokine that has been implicated as a causative factor in obesity-linked insulin resistance. It is commonly accepted that macrophage-derived TNF-alpha acts in a paracrine manner on adjacent adipocytes to inhibit the expression of various adipocyte genes and to attenuate insulin signaling. Several studies have revealed that signal transducer and activator of transcription (STAT)5 proteins are modulated during adipogenesis and can modulate the transcription of some adipocyte genes. In this study, we demonstrate that TNF-alpha treatment, in the presence of cycloheximide, also results in the rapid turnover of STAT5A and STAT5B in a process that is independent of STAT5 activation by tyrosine phosphorylation. In addition, STAT5B is more labile than STAT5A under these conditions, suggesting that the COOH terminus of STAT5 may be involved in the turnover of each protein. Initial characterization of the TNF-alpha and cycloheximide-mediated degradation of STAT5 indicates that inhibition of the proteasome stabilizes both forms of STAT5 in the presence of TNF-alpha. In addition, the use of an NF-kappaB inhibitor results in the stabilization of STAT5A in the presence of TNF-alpha and cycloheximide, indicating that the degradation of STAT5 proteins under these conditions may involve the NF-kappaB pathway. STAT5 proteins are abundantly expressed in mature adipocytes and are normally extremely stable proteins under a wide range of conditions. However, our results demonstrate that the potentiation of TNF-alpha-mediated signaling in the presence of cyclohexmide is associated with a significant increase in the degradation of STAT5 proteins in 3T3-L1 adipocytes.

Interleukin-1beta-induced insulin resistance in adipocytes through down-regulation of insulin receptor substrate-1 expression

Inflammation is associated with obesity and insulin resistance. Proinflammatory cytokines produced by adipose tissue in obesity could alter insulin signaling and action. Recent studies have shown a relationship between IL-1beta level and metabolic syndrome or type 2 diabetes. However, the ability of IL-1beta to alter insulin signaling and action remains to be explored. We demonstrated that IL-1beta slightly increased Glut 1 translocation and basal glucose uptake in 3T3-L1 adipocytes. Importantly, we found that prolonged IL-1beta treatment reduced the insulin-induced glucose uptake, whereas an acute treatment had no effect. Chronic treatment with IL-1beta slightly decreased the expression of Glut 4 and markedly inhibited its translocation to the plasma membrane in response to insulin. This inhibitory effect was due to a decrease in the amount of insulin receptor substrate (IRS)-1 but not IRS-2 expression in both 3T3-L1 and human adipocytes. The decrease in IRS-1 amount resulted in a reduction in its tyrosine phosphorylation and the alteration of insulin-induced protein kinase B activation and AS160 phosphorylation. Pharmacological inhibition of ERK totally inhibited IL-1beta-induced down-regulation of IRS-1 mRNA. Moreover, IRS-1 protein expression and insulin-induced protein kinase B activation, AS160 phosphorylation, and Glut 4 translocation were partially recovered after treatment with the ERK inhibitor. These results demonstrate that IL-1beta reduces IRS-1 expression at a transcriptional level through a mechanism that is ERK dependent and at a posttranscriptional level independently of ERK activation. By targeting IRS-1, IL-1beta is capable of impairing insulin signaling and action, and could thus participate in concert with other cytokines, in the development of insulin resistance in adipocytes.

Modulation and lack of cross-talk between signal transducer and activator of transcription 5 and Suppressor of cytokine signaling-3 in insulin and growth hormone signaling in 3T3-L1 adipocytes

OBJECTIVE

To examine the role of signal transducer and activator of transcription (STAT) 5 and suppressor of cytokine signaling (SOCS)-3 in the cross-talk between growth hormone and insulin (INS) signaling in fat cells.

RESEARCH METHODS AND PROCEDURES

Fully differentiated 3T3-L1 adipocytes were exposed to INS, growth hormone (GH), or both of these growth factors, and the activation of STAT5 proteins and mitogen-activated protein kinase was examined using phospho-specific antibodies. The induction of SOCS-3 mRNA was assessed by Northern blot analysis. INS-stimulated glucose transport was also measured.

RESULTS

We observed that GH, not INS, induced STAT5 activation in adipocytes in a manner that was independent of extracellular signal-regulated kinase (ERK) activation or new protein synthesis. GH strongly induced SOCS-3 mRNA expression, whereas INS had a much less potent effect on SOCS-3 mRNA expression. Because SOCS-3 has been implicated in the attenuation of GH and INS signaling, we examined the cross-talk between these signaling pathways. GH pretreatment of adipocytes inhibited GH signaling. Similarly, INS pretreatment inhibited INS signaling. However, INS did not block the GH-induced activation of STAT5, and GH did not block the INS induction of ERK activity or of increased glucose uptake. We observed that neither new protein synthesis nor activation of ERKs 1 and 2 were required for the inhibition of GH signaling.

DISCUSSION

These results demonstrate that blocking the induction of the SOCS-3 protein has no effect on the attenuation of GH signaling and support recent studies suggesting that SOCS proteins have additional functions. In addition, these studies demonstrate that GH-induced SOCS-3 expression is insufficient to inhibit INS-induced glucose uptake in adipocytes.