Mechanisms of action of proteinase-activated receptor agonists on human platelets

1 We studied the activation of human platelets by thrombin and proteinase activated receptor (PAR)-activating peptides (PAR-APs) [SFLLRNPNDKYEPF-amide (TRAP), TFLLR-amide (PAR1AP) and AYPGKF-amide (PAR4AP)]. 2 PAR agonist-induced platelet aggregation, glycoprotein (GP) Ib and GPIIb/IIIa surface expression and ADP release were measured by light aggregometry, flow cytometry and chemiluminescence. 3 Aggregation inhibitors, including prostacyclin (PGI(2)), nitric oxide-releasing agent (S-nitroso-glutathione, GSNO), aspirin, apyrase, and phenanthroline were used to study the susceptibility of PAR agonist-induced aggregation to pharmacological inhibition. 4 Thrombin was the most potent platelet agonist, followed by PAR1AP, TRAP and PAR4AP. 5 The aggregatory potencies of PAR-APs were not modified by the aminopeptidase inhibitor, amastatin. 6 Subthreshold concentrations of PAR1AP potentiated the effects of PAR4AP to stimulate maximal aggregation. 7 Both PGI(2) and GSNO reduced PAR agonist-induced aggregation and diminished GPIIb/IIIa upregulation. 8 PAR agonist-induced aggregation was aspirin-insensitive indicating a minor role for TXA(2). 9 In contrast, phenanthroline and apyrase significantly enhanced the anti-aggregatory effects of aspirin against thrombin-, PAR1AP- and TRAP-induced aggregation suggesting the involvement of ADP- and MMP-2-dependent pathways. 10 PAR4AP-induced aggregation (but not PAR1AP-induced aggregation) was entirely ADP-dependent (abolished by apyrase) and resistant to phenanthroline (MMP-2-independent). 11 Thus, the mechanisms of PARI and 4-induced platelet aggregation are distinct and depend differentially on their ability to interact with pathways of aggregation, along with the subsequent activation of GPIIb/IIIa receptors.