THROMBIN INDEPENDENT TRANSGLUTAMINASE IN VASCULAR CELLS AND TISSUES MAY PROVIDE AN ALTERNATE PATHWAY TOWARD CLOT STABILIZATION

Abstract

Blood coagulation Factor XIIla (FXIIIa) is a thrombin activated transglutaminase (TG) that is involved in the final step of fibrin stabilization. FXIIIa inhibits fibrinolysis by crosslinking α-2-plasmin inhibitor (α-2-PI) to fibrin. A thrombin-independent TG has been identified in vascular cells and tissues -from human, rabbit, rat, porcine and bovine sources. The vascular TG had several properties similar to the well characterized guinea pig liver TG. Both enzymes had similar molecular weights (80-90 kDa) and similar chromatographic and electrophoretic properties. Both enzymes preferentially crosslinked α-chains of fibrinogen and their TG activities were independent of thrombin treatment. Finally, both enzymes reacted with polyclonal and monoclonal antibodies to guinea pig liver TG. However, the TG from cultured adult bovine aortic endothelial (ABAE) cells exhibited a novel Ca++/Mg++ dependence for enzymatic activity which was distinct from purified liver TG. TG from confluent ABAE cells and rabbit vascular smooth muscle cells had between 4-7 fold higher TG activity compared to rapidly dividing (nonconfluent) cells -from the same passage. The difference in activity was not due to enhanced degradation of TG catalyzed isopeptide bonds by nonconfluent cells Upon examination by immunoblots using anti-TG antibodies, the TG antigen in nonconfluent cells appeared extensively degraded. Furthermore, guanosine-5'-triphosphate (GTP) was nearly 3-fold more inhibitory to TG from confluent cells compared to nonconfluent cells. Proteases, GTP and divalent cation levels may be modulating intracellular TG activity. The TG antigen detected by imm-unohistochemical techniques was predominantly associated with endothelial and smooth muscle cells of arteries, veins, venules and capillaries. TG antigen also codistributed with fibronectin antigen along the hepatic sinusoids. The ABAE cell TG crosslinked α-2-PI to fibrinogen. The modified fibrinogen was 40-fold more resistant to plasminolysis compared to unmodified fibrinogen. In conclusion, the presence of a thrombin-independent TG in blood vessels may provide an alternate pathway to inhibit fibrinolysis and promote clot stabilization.