Pro-inflammatory cytokines including interleukin-1 (IL-1), interleukin-6 (IL-6), and tumor necrosis factor-alpha (TNFa) are elevated in obesity and type 2 diabetes and have been shown to antagonize insulin action in cell and animal models. While TNFa has been strongly implicated in obesity-dependent insulin resistance in skeletal muscle and adipose tissue, the role of IL-6, TNFa, and IL-1 in hepatic insulin resistance is less understood. Recently, a family of eight cytokine-induced tyrosine kinase inhibitors called Suppressors of Cytokine Signaling (SOCS) have been described. We have now demonstrated in HepG2 cells that IL-6 induces expression of SOCS-3 in a temporal pattern that parallels its inhibitory effects on insulin receptor (IR) signal transduction. Ectopically expressed SOCS-3 also inhibits IR signaling in HepG2 cells. Importantly, when induced by IL-6, endogenous SOCS-3 complexes with the IR in these cells. The objective of this proposal is to develop experimental support for the hypothesis that cytokine-induced SOCS proteins (SOCS-3 being the prototype) are antagonists of IR signal transduction in the liver and contribute to insulin resistance. With the long term goal of defining the mechanism by which cytokines contribute to insulin resistance and type 2 diabetes, the following aims will be pursued: Specific Aim #1: Characterize the effect of cytokine (IL-1, IL-6 and TNFa)-dependent induction of SOCS-3 on insulin receptor signal transduction in primary hepatocytes, HepG2 cells, and mouse models. Determine if SOCS-3 expression is necessary and/or sufficient for IL-6-dependent inhibition of IR signaling (using RNAi, dominant negative mutants, and transcriptional repression) in cells and animal models. Specific Aim #2: Define the molecular mechanism by which SOCS-3 inhibits IR signal transduction. Deletion and point mutations of SOCS-3 and IR will be constructed and their impact on SOCS-IR interactions and IL-6-mediated IR inhibition will be examined using structure-function analysis. This project focuses on the poorly understood antagonism by cytokines (especially IL-6) of IR signaling in the liver. SOCS proteins may potentially be an important contributors to regulation of insulin signaling and a possible target for therapeutic intervention in the treatment of insulin resistance and type 2 diabetes.