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 cells by proteolytic cleavage of its seven transmembrane domain receptor. Protein tyrosine phosphorylation is also stimulated by the protease via poorly defined mechanisms. In human platelets, thrombin has been shown to activate the nonreceptor tyrosine kinase Src. To elucidate the signal transduction pathways involved in transmission of thrombin's cellular effects, we have examined the ability of thrombin to activate Src family tyrosine kinases in a growth-responsive line of lung fibroblasts (CCL39 cells). We report here that thrombin induces a rapid (≤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 thrombin receptor since a synthetic peptide agonist of the receptor mimics thrombin action. The involvement of one or more G proteins in this response was confirmed by the observation that thrombin's effect is partially sensitive to pertussis toxin. Furthermore, both α₂-adrenergic and muscarinic m₁ receptors are able to increase Src kinase activity via pertussis toxin-sensitive and -insensitive G proteins, respectively. These findings suggest that nonreceptor tyrosine kinases of the Src family may represent a novel effector system linking G protein-coupled receptors to downstream activation of Ras and the mitogen-activated protein kinase cascade.