Endothelial cells play a critical role in thromboregulation by controlling the assembly of fibrinolytic constituents on the membrane. The assembly system illustrated in FIGURE 6 is characterized by the binding of circulating glu-plasminogen to a membrane receptor (Pathway 1). A membrane-associated protease (possibly plasmin) converts the inactive zymogen into a catalytically more efficient zymogen lys-plasminogen (Pathway 2). T-PA binds to a specific receptor, retains its catalytic activity, and is protected from its natural inhibitor PAI-1. The membrane provides a favorable environment for plasmin generation (Pathway 3) at the vessel surface and contributes to the maintenance of a physiological nonthrombogenic state. The immobilization and surface activation of plasminogen provides an important mechanism for localizing proteolytic activity at the surface of other cells such as macrophages and tumor cells. Lp(a), a plasminogen-like lipoprotein, by competing at the endothelial surface for plasminogen binding down-regulates endothelial cell plasmin generation and may thus promote localized thrombogenesis that over a period of time contributes to progressive atherosclerosis.
