Abstract 15641: PKCδ Negatively Regulates Platelet Function in Response to Thrombin via Gp1bα
Background and objective: The protein kinase C (PKC) family is essential for platelet function in response to various platelet agonists including thrombin. Thrombin signaling in platelets is mediated by a specialized family of G-protein-coupled receptors known as protease-activated receptors (PARs) and by GPIbα. Indeed, GPIbα, PAR1, and PAR4 are considered the high-, medium-, and low-affinity thrombin receptors in human platelets, respectively. It has been shown that PKC delta (PKCδ) negatively and positively influences platelet function in response to collagen and PAR stimulation, respectively. However, its importance in response to thrombin stimulation via the high-affinity GPIbα binding site is not known. This study was therefore designed to examine the role of GP1bα in platelet PKCδ signaling and function.
Methods and Results: We found that in human platelets, pre-treatment with a specific PKCδ membrane translocation inhibitor δ(V1-1)TAT significantly potentiated platelet aggregation and activation, as well as PKCδ phosphorylation on Tyr311, in response to a priming concentration of thrombin. However, the responses to high concentrations of thrombin or to the PAR agonist peptides, TRAP1 and TRAP4, were not affected. This potentiation process is GP1bα-dependent, as specific inhibition of the high affinity thrombin-binding site on GPIbα reverses this effect. Moreover, this response seems to involve platelet secondary mediators, since inhibition of ADP activity and to a lesser extent thromboxanegeneration significantly block the po-aggregatory effects of PKCδ inhibition. These results were reproduced using platelets from PKCδ-/- mice and in platelets from WT mice pre-treated with δ(V1-1)TAT. Notably, tail bleeding times and blood platelet counts in PKCδ-/- mice were significantly decreased following low thrombin injection as compared to WT mice, which is indicative of enhanced platelet function.
Conclusion: This study adds new insights into the role of PKCδ in platelet function downstream of GP1bα, where it negatively regulates platelet aggregation and activation in response to a priming concentration of thrombin. Thus, PKCδ may constitute a target in the management of thrombotic events.
- © 2012 by American Heart Association, Inc.