Abstract 20120: Protease-activated Receptor 4 Deficiency Reduces Atherosclerosis

Abstract

Objective: Platelet activation is emerging as a critical process in both the formation and propagation of atherosclerosis. The coagulation protease thrombin activates platelets by cleavage of protease-activated receptor 4 (PAR4) in mice. Recent studies have demonstrated direct thrombin inhibitors reduce atherosclerosis in hypercholesterolemic mice. However, PAR4 deficiency had no effect on atherosclerosis in the apolipoprotein E (Apoe-/-) mouse model. The objective of this study was to re-examine the role of PAR4 deficiency in atherosclerosis using the alternative low-density lipoprotein receptor deficient mouse model. Methods and Results: To examine the effects of PAR4 deficiency on early atherosclerotic formation, low-density lipoprotein receptor deficient (Ldlr-/-) mice that were either PAR4+/+ (n = 11) or PAR4-/- (n = 12) were fed a fat-enriched diet (21% milk fat) and infused with angiotensin II (AngII; 1,000 ng/kg/min) for 28 days. PAR4 deficiency reduced AngII-induced aortic root atherosclerosis (+/+: 107 ± 22.1; -/-: 28.3 ± 7.50 μm2; P = 0.007). Bone marrow transplantation experiments were performed to determine the cellular source of PAR4 in atherosclerosis. Preliminary results demonstrate this reduction is dependent upon hematopoietic-derived PAR4 (+/+ into +/+: 230 ± 41; +/+ into -/-: 194 ± 23; -/- into +/+: 78 ± 18; and -/- into -/-: 68 ± 13 μm2; n = 4 each group; P < 0.02 +/+ hematopoietic versus -/- hematopoietic). PAR4 deficiency had no effects on total plasma cholesterol concentrations, lipoprotein-cholesterol distributions, or AngII increased systolic blood pressure. To determine the effects of a longer interval of diet-induced atherosclerosis, Ldlr-/-/PAR4+/+ (n = 12) or Ldlr-/-/PAR4-/- (n = 12) were fed a fat-enriched diet for 12 weeks. PAR4 deficiency attenuated aortic root atherosclerosis (+/+: 299 ± 15; -/-: 188 ± 18 μm2; P = 0.001) with no effects on total plasma cholesterol concentrations or lipoprotein distributions. Conclusion: We demonstrated that PAR4 deficiency resulted in an early (AngII: 74%) and late (Diet-induced: 37%) reduction in aortic root atherosclerosis. These results suggest that thrombin activation of PAR4 on platelets contributes to atherosclerosis in the Ldlr-/- model, but not the Apoe-/- model.