PROTEIN-KINASE-C REGULATES TYROSINE PHOSPHORYLATION OF PP125FAK IN PLATELETS ADHERENT TO FIBRINOGEN

Platelet adhesion to immobilized fibrinogen stimulates the induction o f tyrosine phosphorylation of multiple proteins. However, platelet spr eading and tyrosine phosphorylation of three proteins, the focal adhes ion kinase pp125FAK and proteins of 101 and 105 kD (pp101 and pp105), require a second adenosine diphosphate (ADP)-dependent costimulatory e vent. In this study we show that protein kinase C (PKC) inhibitors pre vented the induction of tyrosine phosphorylation of pp125FAK, pp101 an d pp105, and abolished spreading. These inhibitory effects were not ob served after treatment of the platelets with the intracellular Ca2+ ch elator BAPTA-AM. This suggested that in platelets, PKC regulates sprea ding and related protein tyrosine phosphorylation. In addition, the in hibitory effects of apyrase, an ADP scavenger, on spreading and tyrosi ne phosphorylation of pp125FAK, pp101. and pp105, were not observed in the presence of phorbol 12-myristate 13-acetate (PMA). These data imp lied that in fibrinogen-adherent platelets integrin ligation and an ag onist receptor occupancy are required for the functional association o f PKC and the alpha(IIb)beta(3)-mediated signaling pathways. Taken tog ether these results show that PKC plays a central role in the transduc tion of intracellular signals downstream from alpha(IIb)beta(3) that r egulate spreading and pp(125)FAK phosphorylation. (C) 1996 by The Amer ican Society of Hematology.