Receptor-mediated endocytosis of coagulation factor Xa requires cell surface-bound tissue factor pathway inhibitor

Coagulation factor Xa is a plasma serine protease that catalyzes prothrombin to thrombin conversion, which, in turn, leads to the generation of the fibrin clot. Of the several parameters that govern the plasma level of factor Xa, control of its catabolism is of crucial importance. However, little is known regarding the mechanisms by which factor Xa is catabolized. In the present study we examine the cellular basis for the uptake and degradation of factor Xa. ¹²⁵I-Factor Xa was degraded by hepatoma cells and embryonic fibroblasts via a process which required cell surface-bound tissue factor pathway inhibitor (TFPI), a potent inhibitor of factor Xa. Uptake and degradation of cell surface-bound ¹²⁵I-TFPI was also markedly stimulated in response to factor Xa binding. The intracellular kinetics of ¹²⁵I-factor Xa and cell surface-bound ¹²⁵I-TFPI display a strikingly similar pattern, suggesting that factor Xa and cell surface-bound TFPI are taken up as a bimolecular complex. Using cell lines either deficient in low density lipoprotein receptor-related protein, an endocytic receptor that mediates the degradation of uncomplexed TFPI (Warshawsky, I., Broze, G. J., Jr., and Schwartz, A. L. (1994) Proc. Natl. Acad. Sci. U. S. A. 91, 6664- 6668), or deficient in tissue factor (TF), an integral membrane protein capable of forming quarternary complexes with factor Xa, TFPI, and factor VIIa, we demonstrated that the receptor that mediates the uptake and degradation of factor Xa-TFPI complex was neither low density lipoprotein receptor-related protein nor TF. As the vascular endothelial cell surface retains a substantial pool of TFPI (Sandset, P. M., Alildgaard, U., and Larsen, M. L. (1988) Thromb. Res. 50, 803-813; Novotny, W. F., Brown, S. G., Miletich, J.P., Rader, D. J., and Broze, G. J., Jr. (1991) Blood 78, 387- 393), our data suggest that endothelial cell surface TFPI may be actively involved in the clearance of factor Xa from the circulation via mediated uptake and degradation.