Growth hormone stimulates the tyrosine phosphorylation of the insulin receptor substrate-1 and its association with phosphatidylinositol 3-kinase in primary adipocytes.

Insulin receptor substrate-1 (IRS-1) is tyrosine-phosphorylated in response to insulin resulting in association with and activation of phosphatidylinositol 3-kinase (PI 3-kinase), thereby initiating some of the effects of insulin. We have recently shown that the insulin-like effects of growth hormone (GH) in adipocytes can be inhibited by the selective PI 3-kinase inhibitor wortmannin (Ridderstråle, M., and Tornqvist, H. (1994) Biochem. Biophys. Res. Commun. 203, 306-310), suggesting a similar role for PI 3-kinase in GH action. Here we show that IRS-1 is tyrosine-phosphorylated in a time- and dose-dependent manner in response to GH in primary rat adipocytes. This phosphorylation coincided with the extent of interaction between IRS-1 and the 85-kDa subunit of PI 3-kinase as evidenced by coimmunoprecipitation. Stimulation with 23 nM GH increased the PI 3-kinase activity associated with IRS1 4-fold. Our data suggest that GH-induced tyrosine phosphorylation of IRS-1 and the subsequent docking of PI 3-kinase are important postreceptor events in GH action. The mechanism for the phosphorylation of IRS-1 induced by GH is unknown, but involvement of JAK2, the only known GH receptor-associated tyrosine kinase, seems possible.