2 RECEPTOR SYSTEMS ARE INVOLVED IN THE PLASMA-CLEARANCE OF TISSUE FACTOR PATHWAY INHIBITOR IN-VIVO

Tissue factor pathway inhibitor (TFPI) is a potent inhibitor of the bl ood coagulation factor VIIa-tissue factor complex, as well as a direct inhibitor of factor Xa. Intravenously administered TFPI is rapidly cl eared from circulation predominantly via liver. We previously reported that the low density lipoprotein receptor related protein (LRP), a mu ltifunctional endocytic receptor, mediates the uptake and degradation of TFPI in hepatoma cells. This process is inhibited by a 39-kDa recep tor-associated protein which binds to LRP and regulates its ligand bin ding activity. However, a distinct, low affinity binding site (perhaps heparin sulfate proteoglycans, HSPGs) on the endothelium and liver is thought to be responsible for the majority of TFPI cell surface bindi ng. In the current study, we investigated the role of LRP and this sec ond binding site in the clearance of I-125-TFPI in, vivo using competi tors and inhibitors of the receptors. Mice overexpressing the 39-kDa p rotein via adenoviral-mediated gene transfer displayed diminished plas ma clearance of I-125-TFPI. Blockade of cell surface HSPGs sites by in cubation with the positively charged molecule, protamine, inhibited I- 125-TFPI binding to the hepatoma cells in vitro. In addition, preadmin istration of protamine in vivo prolonged the plasma clearance of I-125 -TFPI in a dose-dependent manner. However, a dramatic increase of the plasma half-life of I-125-TFPI and virtual elimination of I-125-TFPI c learance was observed in mice overexpressing the 39-kDa protein and ad ministered protamine. Taken together, our results suggest that two rec eptor mechanisms are involved in the clearance of TFPI in vivo.