Insulin-like growth factor-I stimulates tyrosine phosphorylation of endogenous c-Crk

Crk, a cellular homolog of v-crk, is an SH2 and SH3 domain-containing adaptor protein related to Grb2 and Nck, two proteins which have been shown to be involved in growth factor signal transduction. Crk proteins have recently been found to associate with two guanine nucleotide releasing proteins, mSos and C3G, and thus appear to lie on the Ras pathway. We investigated whether Crk is a target for the insulin-like growth factor I (IGF-I) receptor tyrosine kinase. We show that IGF-I stimulates tyrosine phosphorylation of Crk II via stimulation of endogenous IGF-I receptors in both 293 cells and NIH-3T3 cells. IGF-I stimulated tyrosine phosphorylation of Crk II in a dose- and time-dependent manner. In 293 cells, which express both IGF-I and insulin receptors, insulin also induced a dose-dependent tyrosine phosphorylation of Crk II, but with somewhat reduced sensitivity, compared to IGF-I. In NIH 3T3 cells, IGF-I also stimulated tyrosine phosphorylation of a 45- kDa protein which co-immunoprecipitated with Crk II. These findings indicate that Crk II is an endogenous substrate of the IGF-I receptor tyrosine kinase and provide the first demonstration that a mitogenic growth factor induces tyrosine phosphorylation of endogenous c-Crk.