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. ? ?
