Synthesis of components of the fibrinolytic system by human peripheral blood monocytes
One feature of the atherosclerotic plaque is deposition of fibrin, suggesting an imbalance in the fibrinolytic system. Monocytes are the major infiltrating cell type during atherogenesis and can contribute to the balance of the fibrinolytic system by production of PAI-2, PAI-1, and u-PA. Production of these proteins by monocytes was investigated in this study. Monocytes synthesized PAI-2 in excess of PAI-1. PAI-2 expression was increased by thrombin. Upregulation of secreted and intracellular PAI-2 was maximal at a thrombin concentration of 1 U/ml. The increase in PAI-2 antigen was verified at the mRNA level. Thrombin upregulation of PAI-2 synthesis was comparable to LPS stimulation of PAI-2, although the dose effect of thrombin and LPS differed. Monocytes responded to native LDL, which is central to atherogenesis, by an upregulation in PAI-2 antigen and mRNA. The increase in PAI-2 was not dependent on modification of native LDL by monocytes, as assessed by incorporation of antioxidants. Stimulation of PAI-2 by monocytes following treatment with modified LDL was variable. Production of PAI-1, PAI-1 and u-PA from monocytes, macrophages and foam cells was compared. Macrophages and foam cells did not respond to thrombin or LPS by an upregulation in PAI-2 and all PAI-2 was intracellular. Intracellular PAI-2 in all cell types was not an active inhibitor of u-PA, unlike secreted PAI-2. Reduction or detergent treatment generated active PAI-2. The accumulation of intracellular PAI-2 was promoted by the presence of fetal calf serum in the culture medium. The upregulation in PAI-2 represents a potential imbalance in the fibrinolytic system, since no u-PA was detected. This may contribute to persistence of fibrin deposits in a local environment such as the atherosclerotic plaque.