ACTIVATION OF SRC FAMILY KINASE-ACTIVITY BY THE G-PROTEIN-COUPLED THROMBIN RECEPTOR IN GROWTH RESPONSIVE FIBROBLASTS

Thrombin stimulates G protein-coupled signaling pathways in target cel ls by proteolytic cleavage of its seven transmembrane domain receptor. Protein tyrosine phosphorylation is also stimulated by the protease v ia poorly defined mechanisms. In human platelets, thrombin has been sh own to activate the nonreceptor tyrosine kinase Src. To elucidate the signal transduction pathways involved in transmission of thrombin's ce llular effects, we have examined the ability of thrombin to activate S rc family tyrosine kinases in a growth-responsive line of lung fibrobl asts (CCL39 cells). We report here that thrombin induces a rapid (less than or equal to 30 s) and transient increase in the kinase activity of Src and Fyn as determined by autophosphorylation in immune complex kinase assays. Activation is mediated by the G protein-coupled thrombi n receptor since a synthetic peptide agonist of the receptor mimics th rombin action. The involvement of one or more G proteins in this respo nse was confirmed by the observation that thrombin's effect is partial ly sensitive to pertussis toxin. Furthermore, both alpha(2)-adrenegic and muscarinic m(1) receptors are able to increase Src kinase activity via pertussis toxin-sensitive and insensitive G proteins, respectivel y. These findings suggest that nonreceptor tyrosine kinases of the Src family may represent a novel effector system linking G protein-couple d receptors to downstream activation of Ras and the mitogen-activated protein kinase cascade.