INSULIN STIMULATION OF PHOSPHATIDYLINOSITOL 3-KINASE ACTIVITY AND ASSOCIATION WITH INSULIN-RECEPTOR SUBSTRATE-1 IN LIVER AND MUSCLE OF THE INTACT RAT

Growth factors stimulate the enzyme phosphatidylinositol (PI) 3-kinase in cells in culture. Insulin rapidly stimulates tyrosine phosphorylation of its endogenous substrate, insulin receptor substrate 1 (IRS-1), and in vitro IRS-1 associates with PI 3-kinase, thus activating the enzyme. We have examined whether insulin is capable of stimulating the PI 3-kinase pathway in two physiological target tissues for the actions of insulin in vivo, liver and skeletal muscle. After intraportal injection of insulin into anesthetized rats, there was a 2-fold stimulation of total hepatic PI 3-kinase activity in liver and muscle extracts and a 10- to 20-fold increase in PI 3-kinase activity immunoprecipitated with anti-IRS-1 antibodies. Stimulation of PI 3-kinase was accompanied by an association between this enzyme and IRS-1 as detected by immunoprecipitation of liver and muscle extracts with anti-IRS-1 antibodies and Western blotting with antibodies to the 85-kDa subunit of PI 3-kinase. Immunoprecipitation with anti-p85 antibodies and phosphotyrosine immunoblotting revealed no tyrosine phosphorylation of PI 3-kinase, but demonstrated co-precipitation of tyrosine-phosphorylated IRS-1, as well as another phosphotyrosine protein of approximately 135-140 kDa. Thus, IRS-1 phosphorylation plays a significant role in the activation of PI 3-kinase in vivo by insulin.