Suppressor of Cytokine Signaling 3 Expression and Insulin Resistance in Skeletal Muscle of Obese and Type 2 Diabetic Patients Jennifer Rieusset, 1 Karim Bouzakri, 1 Emmanuel Chevillotte, 1 Nade ` ge Ricard, 1 Delphine Jacquet, 2 Jean-Philippe Bastard, 3 Martine Laville, 1,4 and Hubert Vidal 1,4 Interleukin-6 (IL-6) could be a possible mediator of insulin resistance. We investigated whether IL-6 could inhibit insulin signaling in human skeletal myotubes and whether suppressor of cytokine signaling 3 (SOCS-3) could be related to insulin resistance in vivo in humans. IL-6 inhibited insulin signaling and induced SOCS-3 expression in differentiated myotubes. SOCS-3 mRNA levels were significantly increased in the skeletal mus- cle of type 2 diabetic patients compared with control subjects and correlated with reduced insulin-stimulated glucose uptake. In contrast, SOCS-3 mRNA levels were reduced in muscle of obese nondiabetic subjects com- pared with type 2 diabetic patients, despite similar circulating concentrations of IL-6. Increased SOCS-3 mRNA levels in diabetes were not attributable to hyper- glycemia, as type 1 diabetic patients had normal SOCS-3 mRNA expression in muscle. However, the combination of high glucose and IL-6 levels in type 2 diabetic pa- tients may induce SOCS-3 expression, as has been seen in human muscle cells. In subcutaneous adipose tissue, SOCS-3 mRNA levels were increased in obese individu- als and strongly correlated with IL-6 expression, sup- porting a paracrine effect of IL-6 on SOCS-3 expression in fat. Taken together, our results showed that SOCS-3 expression in human skeletal muscle in vivo is not related to insulin resistance in the presence of elevated IL-6 concentrations and suggest that cytokine action could differ in type 2 diabetic patients and nondiabetic obese subjects. Diabetes 53:2232–2241, 2004 O besity and type 2 diabetes are important meta- bolic disorders characterized by impaired insu- lin action (1,2). However, the relations among obesity, insulin resistance, and type 2 diabetes are not well understood. A large body of evidence demon- strates that obesity is associated with a chronic state of low-grade inflammation (3), which could be one of the important parameters in the development of insulin resis- tance (4,5). Indeed, elevated levels of proinflammatory cytokines can produce insulin resistance (6); furthermore, it has been clearly demonstrated that adipose tissue is able to synthesize and secrete several cytokines such as leptin, tumor necrosis factor- (TNF-), or interleukin-6 (IL-6) (7,8). These proinflammatory cytokines have also been shown to be elevated in type 2 diabetes (9,10), suggesting that adipocytokines might be a possible link among insulin resistance, diabetes, and obesity (5). Among these cyto- kines, IL-6 presents the strongest correlation with insulin resistance and type 2 diabetes in humans (9 –13). The mechanisms of cytokine-induced insulin resistance are not clearly defined. One possible mechanism is the serine phosphorylation of insulin receptor substrate 1 (IRS-1) by cytokine-activated kinases and the subsequent direct inhibitory effect on the insulin-signaling cascade (14 –16). An alternative mechanism is that cytokines in- duce the expression of cellular proteins, such as members of the suppressor of cytokine signaling (SOCS) family, that inhibit insulin receptor signal transduction (17–19). The SOCS proteins are induced by various cytokines, and participate in a classic feedback loop to modulate cytokine action (20). It has been demonstrated that SOCS proteins can also play a role in the negative regulation of the signaling of insulin and IGF-I receptors (17,18,21). Thus, SOCS proteins might be good candidates for cytokine- mediated insulin resistance. Few data exist regarding the possible implication of the SOCS family in cytokine action in human skeletal muscle, which is the major site of insulin-dependent glucose disposal. Therefore, this study was conducted to 1) char- acterize the effect of IL-6 on insulin signaling and the expression of SOCS members in vitro in human differen- tiated myotubes and 2) determine the relations among IL-6 plasma levels, SOCS expression, and insulin sensitivity in obese nondiabetic subjects and type 2 diabetic patients. From the 1 INSERM U449/INRA U1235 and Centre de Recherche en Nutrition Humaine de Lyon, IFR 62, Faculte ´ de Me ´ decine Rene ´ Laennec, Universite ´ Claude Bernard Lyon-1, Lyon, France; the 2 INSERM U457, Ho ˆ pital Robert Debre ´ , Paris, France; the 3 Service de Biochimie et Hormonologie, Ho ˆ pital Tenon, Assistance Publique-Ho ˆ pitaux de Paris and INSERM U402, Paris, France; and the 4 Service d’Endocrinologie, Diabe ´ tologie et Nutrition, Ho ˆ pital Edouard Herriot, Lyon, France. Address correspondence and reprint requests to Dr. Jennifer Rieusset, INSERM U449/INRA U1235, Faculte ´ de Me ´ decine Rene ´ Laennec, Rue G. Paradin, F-69372 Lyon, Cedex 08, France. E-mail: jennifer.rieusset@univ- lyon1.fr. Received for publication 20 January 2004 and accepted in revised form 28 May 2004. CIS, cytokine-inducible SH-2– containing protein; FBS, fetal bovine serum; HPRT-1, hypoxanthine phosphoribosyltransferase 1; IL-6, interleukin-6; IRS-1, insulin receptor substrate 1; PI, phosphatidylinositol; PKB, protein kinase B; SOCS, suppressor of cytokine signaling; TNF-, tumor necrosis factor-. © 2004 by the American Diabetes Association. 2232 DIABETES, VOL. 53, SEPTEMBER 2004