Collagen, convulxin, and thrombin stimulate aggregation-independent tyrosine phosphorylation of CD31 in platelets - Evidence for the involvement of Src family kinases

Platelet endothelial cell adhesion molecule-1 (CD31) is a 130-kDa glycoprot ein receptor present on the surface of platelets, neutrophils, monocytes, c ertain T-lymphocytes, and vascular endothelial cells, CD31 is involved in a dhesion and signal transduction and is implicated in the regulation of a nu mber of cellular processes. These include transendothelial migration of leu kocytes, integrin regulation, and T-cell function, although its function in platelets remains unclear, In this study, we demonstrate the ability of th e platelet agonists collagen, convulxin, and thrombin to induce tyrosine ph osphorylation of CD31, Furthermore, we show that this event is independent of platelet aggregation and secretion and is accompanied by an increase in surface expression of CD31, A kinase capable of phosphorylating CD31 was de tected in CD31 immunoprecipitates, and its activity was increased following activation of platelets. CD31 tyrosine phosphorylation was reduced or abol ished by the Src family kinase inhibitor PP2, suggesting a role for these e nzymes. In accordance with this, each of the Src family members expressed i n platelets, namely Fyn, Lyn, Src, Yes, and Hck, was shown to co-immunoprec ipitate with CD31. The involvement of Src family kinases in this process wa s confirmed through the study of mouse platelets deficient in Fyn.