The Cleaved Peptide of PAR1 is a More Potent Stimulant of Platelet-Endothelial Cell Adhesion Than is Thrombin

Purpose: Platelet-endothelial cell adhesion is an important pathologic response to vessel injury or inflammation. On binding to its endothelial or platelet G protein-linked seven-transmembrane domain receptor, protease-activated receptor-1 (PAR1), thrombin releases a 41-amino acid peptide (TR1-40) We examined the effect of TR1-41 on platelet activation and on platelet-endothelial cell adhesion.Methods: A monolayer of confluent human saphenous vein endothelial cells was incubated with washed human platelets. Platelets were stimulated with either TR1-41, TR21-41, scrambled TR1-41, adenosine diphosphate (ADP)-epinephrine (EPI), thrombin, or thrombin receptor activating peptide (TRAP). Platelet activation was identified with flow cytometry. The magnitude of platelet-endothelial cell adhesion was determined with a laser scanning cytometer that scanned the monolayer of endothelial cells and identified fluorescently bound platelets.Results: Maximal thrombin stimulation (0.1 U/mL) induced a threefold increase in platelets bound to endothelial cells compared with buffer alone. Stimulation with TR1-41 (20 mmol/L) tripled the number of platelets bound to endothelial cells compared with thrombin. Scrambled sequence of TR1-41 (20 mmol/L) and TR21-41 (20 mmol/L), neither of which induces platelet activation, had minimal effect on platelet adhesion. Both TRAP (20 mmol/L) and ADP-EPI (20 mmol/L) induced less platelet-endothelial cell adhesion than did thrombin. TR1-41-induced platelet-endothelial cell adhesion was partially blocked by glycoprotein (GP)IIb-IIIa-specific monoclonal antibody, 10E5 (10 mg/mL).Conclusions: TR1-41, the cleaved peptide of PARI, is a more potent stimulant of platelet-endothelial cell adhesion than is thrombin, TRAP, or ADP-EPI, and this adhesion is at least in part mediated by the platelet GPIIb-IIIa receptor.