Insulin is the primary hormone regulating glucose homeostasis. It binds to and activates the insulin receptor (IR), which subsequently tyrosine phosphorylates and activates multiple cell signaling proteins including IRS proteins, APS and SH2-B. Defects in activation of IR and/or its downstream signaling molecules result in insulin resistance, which is associated with and may be a driving force for type 2 diabetes. The long-term goal of my research program is to elucidate the molecular mechanisms of insulin signaling and resistance. We recently identified SH2-B as a binding protein for IR as well as for JAK2, a cytosolic tyrosine kinase required for cytokine action. SH2-B binds to JAK2 via its SH2 domain, and enhances JAK2 kinase activity; however, its role in insulin action is unclear. To study the physiological function of SH2-B in vivo, we generated mutant mice lacking SH2-B. Initial inspection revealed that the mutant mice homozygous for the SH2-B null allele develop insulin resistance and type 2 diabetes. Moreover, hepatic IRS2 is reduced significantly in SH2-B deficient mice. We hypothesize that SH2-B enhances IR activation and IRS2 expression independently, and that both contribute to maintaining normal insulin sensitivity in animals. To test this hypothesis, we shall determine whether deletion of SH2-B impairs insulin-stimulated IR activation and the subsequent phosphorylation of its substrates and activation of downstream pathways, and whether reintroduction of recombinant SH2-B can rescues normal insulin signaling and physiological responses in SH2-B deficient cells and tissues. We shall determine whether SH2-B activates IRS2 promoter independent of insulin stimulation. We shall use a variety of methods to identify molecular mechanisms by which SH2-B potentiates IR activation and promotes IRS2 expression. Thus, the Specific Aims of this proposal are to: 1. Determine whether SH2-B directly enhances insulin physiological responses and signal transduction in mice, tissues and cultured cells. 2. Determine whether and how SH2-B enhances IR activation in mice, tissues and cultured cells. 3. Determine whether and how SH2-B enhances the IRS2 expression in mice, tissues and cultured cells. The results of this proposal will lead to identification of SH2-B-initiated signaling events that may serve as targets for drug intervention of insulin resistance and type 2 diabetes.