MMP and PAR1 expression in atherothrombotic disease. Specific MMPs and PAR1 and their cellular sources in an atherosclerotic blood vessel. Instability of the fibrous cap overlaying the plaque can lead to subendothelial collagen exposure and subsequent platelet activation. Platelets express MMP-1, -2, -3, and -14^13,60,61  and exposure of naive platelets to collagen activates MMP-1 from the proMMP1 zymogen. Endothelial cells express a number of MMPs, including MMP-1, -2, -9, -13, and -14.¹⁸  Macrophages and smooth muscle cells (SMCs) are activated (Mφ foam cells and SMCs with red nuclei) by the proinflammatory state resulting from cholesterol deposition in the plaque body and secrete MMP-1, -2,-9, and -13, among others.^18,26