Integrin-independent Tyrosine Phosphorylation of p125fak in Human Platelets Stimulated by Collagen

Abstract

Collagen fibers or a glycoprotein VI-specific collagen-related peptide (CRP-YL) stimulated tyrosine phosphorylation of the focal adhesion kinase, p125fak (FAK), in human platelets. An integrin α 2β1-specific triple-helical peptide ligand, containing the sequence GFOGER (single-letter nomenclature, O = Hyp) was without effect. Antibodies to the α2 and β1 integrin subunits did not inhibit platelet FAK tyrosine phosphorylation caused by either collagen fibers or CRP-XL. Tyrosine phosphorylation of FAK caused by CRP-XL or thrombin, but not that caused by collagen fibers, was partially inhibited by GR144053F, an antagonist of integrin αIIbβ 3. The intracellular Ca2+ chelator, BAPTA, and the protein kinase C inhibitor, Ro31-8220, were each highly effective inhibitors of the FAK tyrosine phosphorylation caused by collagen or CRP-XL. These data suggest that, in human platelets, 1) occupation or clustering of the integrin α2β1 is neither sufficient nor necessary for activation of FAK, 2) the fibrinogen receptor αIIbβ 3 is not required for activation of FAK by collagen fibers, and 3) both intracellular Ca2+ and protein kinase C activity are essential intermediaries of FAK activation.