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

Abstract

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.

Keywords: arteries; atherosclerosis; blood platelets; cause of death; thrombin.