Abstract 15433: Thrombin Activation of Platelet Protease-activated Receptor 4 Augments Atherosclerosis

Objective: Platelet activation has been shown to play a critical role in both formation and propagation of atherosclerosis. Protease-activated receptors (PARs) 1 and 4 mediate signaling by the coagulation protease thrombin. While PAR1 and 4 mediate thrombin activation on human platelets, mouse platelets contain PAR4 with PAR3 acting as a cofactor. Recent studies have demonstrated direct thrombin inhibitors (DTIs) reduced atherosclerosis in hypercholesterolemic mice. In this study, we examined the effect of PAR1 and 4 deficient on atherosclerosis using low-density lipoprotein receptor deficient (Ldlr -/- ) mice. Methods and Results: Ldlr -/- mice that were PAR1 +/+ , PAR1 -/- , PAR4 +/+ , or PAR4 -/- (n = 5-20 each group) were fed a fat and cholesterol-enriched diet for 12 weeks. PAR4 deficiency attenuated aortic root (65% decrease; P = 0.001) and aortic arch (71% decrease; P = 0.001) atherosclerosis with no effects on total plasma cholesterol concentrations or lipoprotein distributions. These reductions were attributable to hematopoietic-derived PAR4 from analysis of bone marrow experiments. To determine if PAR4 mediated all effects of thrombin signaling, Ldlr -/- /PAR4 +/+ or Ldlr -/- /PAR4 -/- mice were fed a fat and cholesterol-enriched diet, supplemented with placebo or the DTI dabigatran etexilate (30 g/kg diet) for 12 weeks. Dabigatran etexilate administration reduced atherosclerosis in PAR4 +/+ mice but not in PAR4 -/- mice. PAR1 deficiency had no effect on atherosclerotic burden. Conclusion: We demonstrated whole body and hematopoietic PAR4 deficiency resulted in decreased aortic root and sinus atherosclerosis. PAR1 deficiency had no effects on atherosclerosis. Thrombin inhibition did not alter atherosclerosis in PAR4 deficient mice. Together, these results suggest that thrombin activation of PAR4 on platelets contributes to atherosclerosis.