The role of suppressor of cytokine signaling-3 in interleukin-6 dependent insulin resistance

Insulin resistance, or the smaller than normal response to a given dose of insulin, is the cardinal defect associated with type 2 diabetes. The underlying cause of insulin resistance is unknown. Type 2 diabetes and insulin resistance have recently been hypothesized to represent an acute phase response. It is well known that obesity is a major contributing factor to insulin resistance. Recent observations indicate that obese patients and animals have elevated circulating levels of the proinflammatory cytokines, particularly IL-6. There is little known about the effect of IL-6 on the cellular response to insulin. I show in this thesis that IL-6 can inhibit insulin signal transduction at the cellular level and that the major mechanism by which IL-6 causes insulin resistance is through the induction of a member of the Suppressor of Cytokine Signaling Family (SOCS), SOCS-3 and its direct interaction with the insulin receptor.; Using the yeast-two hybrid system, it has been observed that several of the SOCS proteins interact with the insulin receptor. SOCS-1, -3, or -6 associate with and inhibit insulin receptor signal transduction in liver cell lines when ectopically expressed. The SOCS family of proteins are induced by the proinflammatory cytokines in multiple tissues, including the liver, and are attractive candidates for negative regulators of insulin signal transduction.; In this study the ability of the proinflammatory cytokine, IL-6 to directly inhibit insulin action at the cellular level was evaluated in liver cell models. Pretreatment of liver for 60–90 min with IL-6 blocks insulin signal transduction by approximately 50%, effectively inducing insulin resistance at the cellular level. To determine the molecular mechanism through which IL-6 acts, the expression of the SOCS family of proteins was monitored. It was observed in the human heptocarcinoma cell line, HepG2, and in whole animals, that SOCS-3 was elevated in response to IL-6 treatment. The elevation of SOCS-3 coincided with the onset of insulin resistance in the liver. This observation suggested that there was a strong correlation between SOCS-3 expression and insulin resistance.; To determine whether SOCS-3 mediates IL-6 dependent insulin resistance, a dominant negative SOCS construct containing a point mutation in the kinase inhibitory region, SOCS-1 F59D, was used. This construct can antagonize the action of both SOCS-1 and SOCS-3. Ectopic expression of this construct alone had no effect on insulin signal transduction though it associated with the insulin receptor similarly to wildtype SOCS. When co-expressed with SOCS-3, the dominant negative SOCS-1 F59D blocked the inhibitory effect of wildtype SOCS-3 on insulin receptor signal transduction. Most critically, the ectopic expression of SOCS-1 F59D completely blocked IL-6-induced insulin resistance in the McA RH7777 hepatoma cell line. These observations suggest a critical role of SOCS-3 in mediating IL-6-induced insulin resistance and provide a novel therapeutic target for the treatment of type 2 diabetes and insulin resistance.