Abstract

Addition of insulin growth factor-I (IGF-I) to quiescent Swiss 3T3 cells rapidly induced tyrosine phosphorylation of the p130Crk-associated substrate (p130(Cas)), a novel adaptor protein localized at focal adhesions. Half-maximal effect was obtained at 0. 6 nM. IGF-I also promoted the formation of a complex between p130(Cas) and c-Crk and elicited a parallel increase in the tyrosine phosphorylation of p125(Fak) and paxillin. IGF-I-induced p130(Cas), p125(Fak), and paxillin tyrosine phosphorylation could be dissociated from mitogen-activated protein kinase kinase, p70(S6K), and protein kinase C activation. In contrast, the structurally unrelated phosphatidylinositol 3-kinase inhibitors wortmannin and LY294002 markedly attenuated the increase in tyrosine phosphorylation of p130(Cas), p125(Fak), and paxillin induced by IGF-I. Cytochalasin D, which disrupts the network of actin microfilaments, completely prevented tyrosine phosphorylation of p130(Cas), p125(Fak), and paxillin and the formation of a p130(Cas). Crk complex in response to IGF-I. Thus, our results identified a phosphatidylinositol 3-kinase-dependent pathway that requires the integrity of the actin cytoskeleton to induce tyrosine phosphorylation of p130(Cas), p125(Fak), and paxillin in response to IGF-I and suggest that tyrosine phosphorylation of these focal adhesion proteins, together with the recruitment of c-Crk into a complex with p130(Cas), may play a novel role in IGF-I signal transduction.