ACTIVATION OF PI-3-KINASE IN 3T3-L1 ADIPOCYTES BY ASSOCIATION WITH INSULIN-RECEPTOR SUBSTRATE-1

Activation of PI 3-kinase in 3T3-L1 adipocytes by association with ins ulin receptor substrate-1. Am. J. Physiol. 266 (Endocrinol. Metab. 29) : E486-E494, 1994. - Insulin treatment of adipocytes causes the rapid phosphorylation of the insulin receptor substrate-1 (IRS-1) on tyrosin e. The phosphotyrosine [Tyr(P)] form of IRS-1 then complexes with the enzyme phosphatidylinositol (PI) S-kinase. In this study, we have inve stigated the effect of this association on PI S-kinase activity in 3T3 -L1 adipocytes. Insulin stimulated cytosolic PI S-kinase activity abou t sevenfold. This stimulation was maximal after 1 min of exposure of c ells to insulin, persisted for at least 1 h, and occurred over the ran ge of insulin concentrations that saturate its receptor. By means of i mmunoprecipitation of IRS-1, it was shown that virtually all of the en hanced activity was due to PI 3-kinase complexed with IRS-1. Moreover, the purified Tyr(P) form of IRS-1, either isolated from 3T3-L1 adipoc ytes or obtained by phosphorylation of the recombinant protein with th e insulin receptor, markedly stimulated the activity of purified rat l iver PI 3-kinase. These results show that the association of Tyr(P) IR S-1 with PI 3-kinase activates the enzyme and thereby can explain the elevation of PI 3,4-bisphosphate and PI 3,4,5-trisphosphate in vivo ob served upon treatment of adipocytes with insulin.