EPIDERMAL GROWTH-FACTOR MODULATES TYROSINE PHOSPHORYLATION OF P130(CAS) - INVOLVEMENT OF PHOSPHATIDYLINOSITOL 3'-KINASE AND ACTIN CYTOSKELETON

Epidermal growth factor (EGF) treatment of Rat-1 cells expressing huma n EGF receptor results in the modification of the tyrosine phosphoryla tion of the p130 Crk-associated substrate (Cas), a novel signaling mol ecule residing in focal adhesions. At low, mitogenic concentrations (< 10 ng/ml), EGF treatment induced a rapid and transient tyrosine phosph orylation of Cas and promoted the formation of a Cas-adapter protein C rk complex in intact cells. The increase in tyrosine phosphorylation o f Cas paralleled an increase in the cellular content of actin stress f ibers and occurred via a pathway that depended on the integrity of the cytoskeleton. Further, phosphatidylinositol 3'-kinase activity was fo und to be required for the EGF-stimulated Cas phosphorylation and acti n polymerization. At high concentrations (>30 ng/ml), EGF treatment re sulted in the tyrosine dephosphorylation of Cas in a time-dependent ma nner with a concomitant decrease in the length and number of actin str ess fibers. Thus, Cas exhibits an unusual bell-shaped dose-response cu rve in response to EGF stimulation, These results demonstrate a novel signaling role for EGF in inducing changes in tyrosine phosphorylation of Cas and Cas-Crk complex formation and suggest that Cas could be a signaling component in EGF-mediated signal transduction.