Role of protein kinase C delta in regulating platelet functional responses

Platelets upon activation change their shape, aggregate and secrete alpha and dense granule contents among which ADP acts as a feedback activator. Different Protein Kinase C (PKC) isoforms have specific non-redundant roles in mediating platelet responses including secretion and thrombus formation. Protein Kinase C delta (PKCdelta), a novel class of PKC isoform requiring diacyl glycerol but not calcium for its activation, has been shown to play an important role in several pathological processes. The aims of our current study are 1) to identify possible PKCdelta specific substrates in platelets, 2) evaluate a novel PKCdelta selective inhibitor and 3) to investigate the role of PKCdelta in ADP-induced platelet activation. We show that Protein kinase D2 (PKD2) is the major isoform of PKD that is expressed in human as well as murine platelets and is a specific substrate for PKCdelta in platelets. CGX1037 was identified and characterized as a selective small molecule inhibitor of PKCdelta in platelets. Furthermore, by using PKCdelta knock out murine platelets, we showed that PKCdelta plays a functional role in mediating ADP-induced thromboxane generation and classical PKC isoforms &agr;/beta regulate tyrosine phosphorylation on PKCdelta and subsequent thromboxane generation through a tyrosine kinase, Lyn and a tyrosine phosphatase, Shp1 suggesting an important role of PKCdelta to agonistinduced platelet activation.