Protein kinase C-delta activation and tyrosine phosphorylation in platelets.

Several protein kinase C (PKC) isoforms are expressed in human platelets. We report that PKC-delta is tyrosine phosphorylated within 30 s of platelet activation by thrombin. This correlated with a 2-3-fold increase in the kinase activity of PKC-delta relative to unstimulated platelets. The tyrosine phosphorylated PKC-delta isoform was associated with the platelet particulate (100,000 x g insoluble) fraction. Alpha(IIb)beta3 integrin mediated platelet adhesion to fibrinogen did not significantly affect PKC-delta activity. Tyrosine phosphorylation of PKC-delta was similarly not detected in fibrinogen adherent platelet lysates. Treatment of the platelets with mAb 7E3 prior to the addition of thrombin blocked aggregation having no effect on the thrombin induced PKC-delta activation. We conclude that PKC-delta is activated in platelets by an alpha(IIb)beta3 independent pathway.