Thrombin‐mediated thromboxane and 12‐HETE formation in human platelets is differentially regulated through PAR1 and PAR4

Abstract

Thrombin‐regulated platelet activation mediates a diverse range of vascular effects which may result in thrombus formation and stroke. Thromboxane (TxB2) is thought to play a crucial role in the reinforcement of platelet activation following thrombin activation via TxB2 formation in the platelet. We demonstrate that PAR1 and PAR4 work synergistically but through unique signaling pathways following activation in order to generate both TxB2 as well as 12‐HETE. To investigate the signaling differences, we examined multiple biochemical and physiological steps involved in platelet activation including Rap1, aggregation, Ca2+ mobilization and formation of 12‐HETE and TxB2. Inhibition of PIP2 and PIP3 significantly altered PAR1‐, but not PAR4‐mediated platelet aggregation and resulted in inhibition of TxB2, but not 12‐HETE. Additionally, the time course for PAR1‐mediated TxB2 formation was faster compared to PAR4. Inhibiting PAR4 activation with YD3 only partially blocked thrombin‐mediated effects confirming the synergistic involvement of both PAR1 and PAR4 in this process. A better understanding of regulation of platelet function through PAR signaling pathways is crucial to developing a better class of anti‐platelet therapies with a lower profile for bleeding as a side effect. This study was supported in part by grants P50 HL081009 (to H.E.H.) and R00 HL089457 (to M.H.) from National Institutes of Health.