CULTURED HUMAN OMENTAL TISSUE MICROVASCULAR ENDOTHELIAL CELLS LYSE FIBRIN CLOTS IN CONTRAST TO UMBILICAL VEIN ENDOTHELIAL CELLS
Previously, we have shown that cultured human omental tissue microvascular endothelial cells (HOTMEC) produce about 100-fold more tPA antigen than cultured human umbilical vein endothelial cells (HUVEC). Free plasminogen activator inhibitor (PAI) activity, but not free tissue plasminogen activator (tPA) activity was detected in the conditioned media of both cell types. In the present study, the clot lysis activities of HOTMEC and HUVEC were compared with each other. For this purpose, 2 mg/ml purified human fibrinogen and 0.2 μM purified human plasminogen were added to serum free medium (SFM) on top of a confluent cell layer. After the addition of α -thrombin (1 NIH U/ml; final concentration), a clot generated within 2 min. The clots formed on the surface of HOTMEC were lysed within 6 h, whereas clots on the surface of HUVEC were not lysed within 72 h. To find out, whether tPA is involved in the activation of the fibrinolytic system, studies were performed in the presence of antibodies against tPA or urokinase (uPA). The fibrinolytic capacity of HOTMEC could be blocked by incorporation of anti-tPA IgG into the clot. Clot lysis by HOTMEC was neither affected by the incorporation of anti-uPA IgG nor of exogenous PAI derived from conditioned medium of HUVEC. Incubation of HUVEC with 1000 μl SFM containing 1 NIH U/ml α -thrombin for 1 h and consecutive cultivation in 5% fetal calf serum for 48 h resulted in a 2.5-fold increase in tPA production. The elevated tPA synthesis was accompanied by an equal increase in PAI production so that free PAI activity, measured in conditioned media, did not change after incubation with α -thrombin. In contrast to experiments with HUVEC, tPA and PAI production of HOTMEC were not influenced by α-thrombin. From these studies it is concluded that 1. the fibrinolytic capacity of HOTMEC is stronger than that of HUVEC, 2. tPA released from endothelial cells is protected from inactivation by PAI when fibrin is present, and 3. HOTMEC do not release higher concentrations of tPA and PAI after contact with (X.-thrombin as it is known for HUVEC.