Epidermal Growth Factor Modulates Tyrosine Phosphorylation of p130Cas

Abstract

Epidermal growth factor (EGF) treatment of Rat-1 cells expressing human EGF receptor results in the modification of the tyrosine phosphorylation of the p130 Crk-associated substrate (Cas), a novel signaling mole- cule residing in focal adhesions. At low, mitogenic con- centrations (<10 ng/ml), EGF treatment induced a rapid and transient tyrosine phosphorylation of Cas and pro- moted the formation of a Cas-adapter protein Crk com- plex in intact cells. The increase in tyrosine phosphoryl- ation of Cas paralleled an increase in the cellular content of actin stress fibers and occurred via a path- way that depended on the integrity of the cytoskeleton. Further, phosphatidylinositol 3?-kinase activity was found to be required for the EGF-stimulated Cas phos- phorylation and actin polymerization. At high concen- trations (>30 ng/ml), EGF treatment resulted in the ty- rosine dephosphorylation of Cas in a time-dependent manner with a concomitant decrease in the length and number of actin stress fibers. Thus, Cas exhibits an un- usual bell-shaped dose-response curve in response to EGF stimulation. These results demonstrate a novel sig- naling 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. The