The insulin and insulin-like growth factor-I receptor substrate IRS-1 associates with and activates phosphatidylinositol 3-kinase in vitro.

Phosphatidylinositol 3-kinase (PtdIns-3-kinase) is thought to participate in the transductional cascade used by several tyrosine kinase receptors including the insulin-like growth factor (IGF)-I receptor and the insulin receptor. The major insulin receptor cellular substrate IRS-1 (pp185) has been proposed as a possible link between the insulin receptor and PtdIns-3-kinase. In this study we show that both insulin and IGF-I treatment of murine fibroblasts transfected with insulin or IGF-I receptors increase PtdIns-3-kinase activity immunoprecipitated with an antibody directed against the 85-kDa subunit of PtdIns-3-kinase. Whereas only a small amount of PtdIns-3-kinase is found associated with the insulin and IGF-I receptor, a considerable PtdIns-3-kinase activity is immunoprecipitated by an antibody raised against IRS-1. Additionally, insulin and IGF-I stimulation of murine fibroblasts expressing insulin or IGF-I receptors induce tyrosine phosphorylation of IRS-1 and its association with PtdIns-3-kinase. Since IRS-1 seems to be the connection between PtdIns-3-kinase and insulin or IGF-I receptor, we used reconstitution experiments to characterize the implication of IRS-1 in the activation of PtdIns-3-kinase. We show that immunoaffinity-purified IRS-1 can be phosphorylated by ligand-stimulated insulin and IGF-I receptors and that this phosphorylation allows the association of IRS-1 with PtdIns-3-kinase. The interaction between PtdIns-3-kinase and IRS-1 phosphorylated by the insulin or the IGF-I receptor results in the activation of PtdIns-3-kinase. In conclusion, our results demonstrate that IRS-1 is a key component in the signal transduction pathway of PtdIns-3-kinase activation induced by insulin and IGF-I.