Plasmin Enhances Cell Surface Tissue Factor Activity without Increasing Tissue Factor Antigen Levels at the Cell Surfaces of Mesothelial and Endothelial Cells.

Mesothelial cells that line the thoracic and peritoneal cavities play an important role in protecting the heart, lungs and internal organs and keeping the surfaces free of friction and non-adhesive. Although human mesothelial cells in culture express low levels of tissue factor (TF), TF expression was not detected in vivo in the mesothelial lining of normal pleura but was detected in the mesothelium overlying injured or inflamed lung tissue. Inflammation in the lung parenchyma predisposes to the development of inflammatory exudative pleural effusions and pleural loculation, but the mechanism(s) responsible for TF expression in mesothelial cells under such conditions remains unclear. In the present study, we investigated whether plasmin and thrombin, two major proteases present in exudative pleural effusions, induce TF expression in cultured primary human pleural mesothelial cells (HMC). Confluent monolayers of HMC were treated with plasmin (50 nM) and thrombin (5 nM) for varying time periods. TF expression was analyzed by measurement of cell surface TF activity in FX activation assay, TF antigen by Western blotting and TF mRNA by Northern blot analysis. Both plasmin and thrombin increased cell surface TF activity by 3–4 fold. Thrombin markedly increased TF antigen levels. Surprisingly, only a minimal increase in TF antigen levels was seen in the plasmin-treated cells and no increase in TF mRNA was observed in these cells. Interestingly, 75% of the increase in TF activity was observed within 30 min of plasmin treatment. In thrombin-stimulated cells, TF activity increased slowly, reaching maximum at 4–6 h. Treatment of HMC with actinomycin-D or cycloheximide prior to the addition of the protease failed to inhibit the plasmin-induced TF activity whereas they completely attenuated the thrombin-induced TF activity. These data suggest that plasmin and thrombin enhance TF activity in HMC by different mechanisms, and that plasmin-induced TF activity is independent of de novo synthesis of TF. We also examined the effect of plasmin on TF activity in human umbilical vein endothelial cells (HUVEC). Although plasmin had a minimal effect on naive HUVEC, 30 min plasmin treatment increased cell surface TF activity of thrombin-stimulated endothelial cells by more than two-fold. In our earlier studies, we showed that proteases could mobilize intracellular TF to the cell surface of fibroblasts and thus enhance TF activity independent of de novo synthesis of TF. However, FACS analysis and ¹²⁵I-FVIIa binding to cell surface TF revealed no significant differences in TF antigen levels at the surfaces of control and plasmin-stimulated HMC. Since exposure of anionic phospholipids at the outer cell surface membrane can increase cell surface TF activity, we next sought to determine if plasmin increases anionic phospholipids on the outer bilayer of the HMC cell membrane. Prothrombin activation assays showed no increase in anionic phospholipids at the surface in plasmin-treated cells. Although recent studies suggest that disulfide bond formation of cysteines in TF enhances TF procoagulant activity, our recent studies in other cell model systems suggest that this mechanism is unlikely to account for plasmin-mediated induction of cell surface TF activity in HMC. While further studies are required to exclude this possibility, our present data indicate that plasmin enhances cell surface TF activity of HMC and HUVEC by a novel mechanism that is yet to be elucidated.