Platelet adhesion to immobilized fibrinogen stimulates the induction of tyrosine phosphorylation of multiple proteins. However, platelet spreading and tyrosine phosphorylation of three proteins, the focal adhesion kinase pp125FAK and proteins of 101 and 105 kD (pp101 and pp105), require a second adenosine diphosphate (ADP)-dependent costimulatory event. In this study we show that protein kinase C (PKC) inhibitors prevented the induction of tyrosine phosphorylation of pp125FAK, pp101 and pp105, and abolished spreading. These inhibitory effects were not observed after treatment of the platelets with the intracellular Ca2+ chelator BAPTA-AM. This suggested that in platelets, PKC regulates spreading and related protein tyrosine phosphorylation. In addition, the inhibitory effects of apyrase, an ADP scavenger, on spreading and tyrosine phosphorylation of pp125FAK, pp101, and pp105, were not observed in the presence of phorbol 12-myristate 13-acetate (PMA). These data implied that in fibrinogen-adherent platelets integrin ligation and an agonist receptor occupancy are required for the functional association of PKC and the alpha IIb beta 3-mediated signaling pathways. Taken together these results show that PKC plays a central role in the transduction of intracellular signals downstream from alpha IIb beta 3 that regulate spreading and pp125FAK phosphorylation.

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