Molecular mechanism and functional implications of thrombin-mediated tyrosine phosphorylation of PKCδ in platelets

Abstract

AbstractThrombin has been known to cause tyrosine phosphorylation of protein kinase C δ (PKCδ) in platelets, but the molecular mechanisms and function of this tyrosine phosphorylation is not known. In this study, we investigated the signaling pathways used by protease-activated receptors (PARs) to cause tyrosine phosphorylation of PKCδ and the role of this event in platelet function. PKCδ was tyrosine phosphorylated by either PAR1 or PAR4 in a concentration- and time-dependent manner in human platelets. In particular, the tyrosine 311 residue was phosphorylated downstream of PAR receptors. Also the tyrosine phosphorylation of PKCδ did not occur in Gαq-deficient mouse platelets and was inhibited in the presence of a phospholipase C (PLC) inhibitor U73122 and calcium chelator BAPTA (5,5′-dimethyl-bis(o-aminophenoxy)ethane-N, N, N ′, N ′-tetraacetic acid), suggesting a role for Gαq pathways and calcium in this event. Both PAR1 and PAR4 caused a time-dependent activation of Src (pp60c-src) tyrosine kinase and Src tyrosine kinase inhibitors completely blocked the tyrosine phosphorylation of PKCδ. Inhibition of tyrosine phosphorylation or the kinase activity of PKCδ dramatically blocked PAR-mediated thromboxane A2 generation. We conclude that thrombin causes tyrosine phosphorylation of PKCδ in a calcium- and Src-family kinase–dependent manner in platelets, with functional implications in thromboxane A2 generation.