THE PHOSPHATIDYLINOSITOL 3-KINASE SERINE KINASE PHOSPHORYLATES IRS-1 - STIMULATION BY INSULIN AND INHIBITION BY WORTMANNIN

Phosphatidylinositol 3-kinase (PI 3-kinase) is a heterodimer composed of an 85-kDa subunit that binds tyrosyl-phosphorylated proteins via it s SH2 domains and a 110-kDa catalytic subunit. Expression and mutagene sis experiments have shown that the 110-kDa subunit is a dual specific ity kinase that possesses both lipid and serine kinase activities. Exc ept for the 85- and 110-kDa subunits of PI 3-kinase, however, no endog enous substrates for the serine kinase have been identified. The resul ts of the present study show that another target of this kinase is the insulin receptor substrate, IRS-1. Serine phosphorylation of IRS-1 as well as the 85-kDa subunit of PI 3-kinase was demonstrated in immunop recipitates of PI 3-kinase and IRS-1 isolated from rat adipocytes incu bated with insulin. In adipocytes incubated in the absence of insulin, only the serine phosphorylation of p85 was observed in immunoprecipit ates of PI 3-kinase. Both the serine and lipid kinase activities of PI 3-kinase were abolished by the fungal metabolite Wortmannin. Wortmann in also partially inhibited the ability of insulin to stimulate glucos e transport and inhibit lipolysis in fat cells. These data raise the p ossibility that the serine kinase activity of PI 3-kinase is involved in insulin signaling. They also suggest that inhibition of the lipid o r serine kinase activities of PI 3-kinase could explain the effect of Wortmannin to diminish insulin action.