THE EFFECT OF HEPARIN ON THE REGULATION OF FACTOR VIIA-TISSUE FACTOR ACTIVITY BY TISSUE FACTOR PATHWAY INHIBITOR

Tissue factor pathway inhibitor (TFPI) inhibits factor VIIa-tissue fac tor and factor Xa through its first and second Kunitz-type domains, re spectively. Previous studies have shown that heparin binds to TFPI at two sites and enhances the inhibitory activity of TFPI towards factor Xa. We have studied the effect of heparin of the TFPI-mediated inhibit ion of factor VIIa-relipidated tissue factor amiodolytic activity and factor VIIa-tissue factor proteolytic activity towards factor IX and f actor X on a human bladder carcinoma cell line, J82. The inhibitory ac tivity of full-length TFPI on factor Wa-tissue factor amidolytic and p roteolytic activities was greatly enhanced by heparin, whereas the inh ibitory activity of a truncated form of TFPI lacking the third Kunitz- type domain and carboxy-terminal tail (TFP1-161) was not affected by h eparin. Optimal inhibition of factor VIIa-tissue factor by TFPI was ob served at 0.1 U/ml heparin. Heparin enhanced the inhibitory activity o f TFPI towards factor Xa even in the presence of factor VIIa-tissue fa ctor complexes in solution phase. In addition, heparin augmented the a bility of TFPI to inhibit factor VIIa-tissue factor amidolytic activit y in the presence of active-site mutated factor X (S376A factor X). Ou r collective results suggest that heparin may play a significant role in augmenting the physiological regulation of factor VIIa-tissue facto r activity by TFPI.