Background/Aims: Signaling through thrombin receptor, protease-activated receptor 1 (PAR1), induces activation of platelets, endothelial cells, and immune cells. High PAR1 expression already in early lesions, suggests a contribution in the pathophysiology of atherosclerosis. The exact mechanisms remain unknown.

Methods and results: Apolipoprotein E knockout (ApoEko) mice fed an atherogenic diet for 16 weeks develop lesions within aortic arch. The addition of vorapaxar (10mg/kg of western diet) reduced atherosclerosis (10% aortic arch, p=0.009; 30% aortic sinus, p<0.001). Direct PAR1 inhibition reduced the aortic and endothelial expression of VCAM-1, E-selectin, and tissue factor. Vorapaxar treatment reduced lipid retention within aortic plaques of ApoEko mice by 10% (p=0.0063). Activation of PAR1 with thrombin increased lipid accumulation by 56% in THP-1 cells, an effect that could be abrogated by vorapaxar. We observed that inhibition of PAR1 with vorapaxar led to a reduction of CD68 positive cells that infiltrated the aortic plaque by 14% (p=0.0022). The predominance of inflammatory macrophages, as indicated by an increased ratio of CD80+/CD206+ infiltrating cells, was reduced by directly targeting PAR1. In line with these observations, circulating classical Ly6Chigh monocytes as well as the ratio of classical/non-classical (Ly6Chigh/Ly6Clow) monocytes dropped under the addition of the PAR1-inhibitor. Vorapaxar also led to a reduction of the aortic transcription of inflammation associated cytokines and chemokines. A reduction of the necrotic core size (p=0.0066) and apoptotic plaque area (p=0.0025), as observed in vorapaxar treated mice, underline the shift towards an anti-inflammatory immunological micromilieu. The net thrombogenic potential of plaques from vorapaxar treated mice was reduced.

Conclusion: PAR1 is critically involved in important pathophysiological steps of de-novo atherogenesis. We uncovered that the direct PAR1-inhibtor vorapaxar possess pleiotropic, anti-inflammatory and atheroprotective effects.