SHC ADAPTER PROTEINS ARE KEY TRANSDUCERS OF MITOGENIC SIGNALING MEDIATED BY THE G-PROTEIN-COUPLED THROMBIN RECEPTOR

The serine protease thrombin activates G protein signaling systems tha t lead to Ras activation and, in certain cells, proliferation, Whereas the steps leading to Ras activation by G protein-coupled receptors ar e not well defined, the mechanisms of Ras activation by receptor tyros ine kinases have recently been elucidated biochemically and geneticall y, The present study was undertaken to determine whether common signal ing components are used by these two distinct classes of receptors, He re we report that the adaptor protein She, is phosphorylated on tyrosi ne residues following stimulation of the thrombin receptor in growth-r esponsive CCL39 fibroblasts, She phosphorylation by thrombin or the th rombin receptor agonist peptide is maximal by 15 min and persists for greater than or equal to 2 h, Following thrombin stimulation, phosphor ylated She is recruited to Grb2 complexes, One or more pertussis toxin -insensitive proteins appear to mediate this effect, since (i) pertuss is toxin pre-treatment of cells does not blunt the action of thrombin and (ii) She phosphorylation on tyrosine can be stimulated by the musc arinic mi receptor, She phosphorylation does not appear to involve pro tein kinase C, since the addition of 4-beta-phorbol-12,13-dibutyrate h as no effect, Rather, thrombin-induced She phosphorylation is enhanced in cells depleted of phorbol ester-sensitive protein kinase C isoform s, Expression of mutant She proteins defective in Grb2 binding display s a dominant-negative effect on thrombin-stimulated p44 MAP kinase act ivation, gene induction and cell growth, From these data, we conclude that She represents a crucial point of convergence between signaling p athways activated by receptor tyrosine kinases and G protein-coupled r eceptors.