Tissue factor pathway inhibitor; its structure, function and clinical significance

Tissue factor pathway inhibitor (TFPI) is a protease inhibitor with three tandem Kunitz-type inhibitory domains (K1, K2 and K3), which inhibits coagulation factor Xa via K2 domain and factor VIIa-tissue factor complex via K1 domain. Thus TFPI blocks the initial steps of the extrinsic coagulation pathway. TFPI is present in human plasma in three forms, i.e., an LDL/VLDL-associated form, an HDL-associated form and a free form. But the majority of in vivo TFPI is associated with the vascular endothelial cells. The interaction of TFPI with heparin has been demonstrated. The heparin binding site of TFPI (HBS-1) locates in its C-terminal basic portion. We found another heparin-binding site in K3 domain of TFPI, using each of the three Kunitz domains prepared by the limited cleavage of TFPI and also synthetic peptides mimicking the amino acid sequence of a Kunitz domain. These heparin binding sites of TFPI will play an important role for the association with endothelial cells, interacting with heparin sulphate on the surface. We have established a method to measure TFPI activities of lipoprotein-associated forms and a free form of TFPI in plasma and a method to measure a free form of TFPI antigen. We, then, applied these methods for the analysis of hypercholesterolemic patients. Relationship between plasma TFPI levels and endothelial cells-associated TFPI level will be discussed.