Attenuation of Atherosclerosis With PAR4 Deficiency: A Potential Role of Hematopoietically Expressed PAR4 in Atherosclerosis—Brief Report

BACKGROUND: Cardiovascular disease is a significant burden globally and, despite current therapeutics, remains the leading cause of death. PAR (protease-activated receptor) 4 is a receptor highly expressed by hematopoietic cells, strongly activated by thrombin, and plays a vital role in platelet activation and aggregation. However, the role of PAR4 in atherothrombotic disease remains understudied. METHODS: Mice on a low-density lipoprotein receptor–deficient ( Ldlr −/− ) background were bred with Par4 -deficient ( Par4 −/− ) mice to create Ldlr −/− /Par4 +/+ and Ldlr −/− /Par4 −/− cousin lines. Mice were fed high-fat (42%) and high-cholesterol (0.2%) Western diet for 12 weeks for all studies. Bone marrow transplant studies were conducted by irradiating Ldlr −/− /Par4 +/+ and Ldlr −/− /Par4 −/− mice with 550 rads (2×, 4 hours apart) and then repopulated with Par4 +/+ or Par4 −/− bone marrow. To determine whether the effects of thrombin were mediated solely by PAR4, the thrombin inhibitor dabigatran was added to the Western diet. RESULTS: We observed higher abundance of PAR4 in arteries with atherosclerosis in mouse and human lesions versus healthy controls. Using a global deletion of PAR4, we observed an attenuation in atherosclerosis versus Par4 +/+ mice. Bone marrow transplant studies demonstrated these effects were due to hematopoietic cells. When observing whether PAR4 activation via thrombin contributes to atherosclerotic development, Ldlr −/− /Par4 −/− mice given dabigatran did not further decrease their atherosclerotic burden. Differences between apoE-deficient ( apoE −/− ) and Ldlr −/− platelets were assessed for changes in reactivity. PAR4 appeared to be acting independent of PAR1, as there were no changes with the addition of dabigatran to Par4 −/− mice. apoE −/− platelets were hyperreactive compared with Ldlr −/− platelets. CONCLUSIONS: We conclude that hematopoietic-derived PAR4 plays a vital role in the development and progression of atherosclerosis. More specifically, we observe thrombin-activated PAR4 contributes to the progression. Specifically, targeting of PAR4 may be a potential therapeutic target for cardiovascular disease.