Protein S Is a Cofactor for Platelet and Endothelial Tissue Factor Pathway Inhibitor-alpha but Not for Cell Surface-Associated Tissue Factor Pathway Inhibitor

Objective-Tissue factor pathway inhibitor (TFPI) is produced in 2 isoforms: TFPI alpha, a soluble protein in plasma, platelets, and endothelial cells, and TFPI beta, a glycosylphosphatidylinositol-anchored protein on endothelium. Protein S (PS) functions as a cofactor for TFPI alpha, enhancing the inhibition of factor Xa. However, PS does not alter the inhibition of prothrombinase by TFPI alpha, and PS interactions with TFPI beta are undescribed. Thus, the physiological role and scope of the PS-TFPI system remain unclear. Approach and Results-Here, the cofactor activity of PS toward platelet and endothelial TFPI alpha and endothelial TFPI beta was quantified. PS enhanced the inhibition of factor Xa by TFPI alpha from platelets and endothelial cells and stabilized the TFPI alpha/factor Xa inhibitory complex, delaying thrombin generation by prothrombinase. By contrast, PS did not enhance the inhibitory activity of TFPI beta or a membrane-anchored form of TFPI containing the PS-binding third Kunitz domain (K1K2K3) although PS did function as a cofactor for K1K2K3 enzymatically released from the cell surface. Conclusions-The PS-TFPI anticoagulant system is limited to plasma TFPI alpha and TFPI alpha released from platelets and endothelial cells. PS likely functions to localize solution-phase TFPI alpha to the cell surface, where factor Xa is bound. PS does not alter the activity of membrane-associated TFPI. Because activated platelets release TFPI alpha and PS, the PS-TFPI alpha anticoagulant system may act physiologically to dampen thrombin generation at the platelet surface.