Integrin-independent tyrosine phosphorylation of p125(fak) in human platelets stimulated by collagen

Collagen fibers or a glycoprotein VI-specific collagen-related peptide (CRP -XL) stimulated tyrosine phosphorylation of the focal adhesion kinase, p125 (fak) (FAK), in human platelets. An integrin alpha (2)beta (2)-specific tri ple-helical peptide ligand, containing the sequence GFOGER (single-letter n omenclature, O = Hyp) was without effect. Antibodies to the alpha (2) and b eta (1) integrin subunits did not inhibit platelet FAK tyrosine phosphoryla tion caused by either collagen fibers or CRP-XL, Tyrosine phoslphorylation of FAK caused by CRP-XL or thrombin, but not that caused by collagen fibers , was partially inhibited by GR144053F, an antagonist of integrin alpha (II b)beta (3). The intracellular Ca2+ chelator, BAPTA, and the protein kinase C inhibitor, Ro31-8220, were each highly effective inhibitors of the FAK ty rosine phosphorylation caused by collagen or CRP-XL. These data suggest tha t, in human platelets, 1) occupation or clustering of the integrin alpha (2 )beta (1) is neither sufficient nor necessary for activation of FAK, 2) the fibrinogen receptor alpha (IIb)beta (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.