Protein kinase C-δ activation and tyrosine phosphorylation in platelets

Several protein kinase C (PKC) isoforms are expressed in human platelets. We report that PKC-δ 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-δ relative to unstimulated platelets. The tyrosine phosphorylated PKC-δ isoform was associated with the platelet particulate (100 000xg insoluble) fraction. α(IIb)β₃ integrin mediated platelet adhesion to fibrinogen did not significantly affect PKC-δ activity. Tyrosine phosphorylation of PKC-δ 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-δ activation. We conclude that PKC-δ is activated in platelets by an α(IIb)β₃ independent pathway. Copyright (C) 1998 Federation of European Biochemical Societies.