THROMBIN INDUCES THE REDISTRIBUTION AND ACUTE RELEASE OF TISSUE FACTOR PATHWAY INHIBITOR FROM SPECIFIC GRANULES WITHIN HUMAN ENDOTHELIAL-CELLS IN CULTURE

Tissue factor pathway inhibitor (TFPI) is a vascular anticoagulant tha t regulates the tissue factor (TF)-dependent pathway of coagulation. T he majority of intravascular TFPI is thought to be noncovalently bound to the vessel wall. Our immunolocalization studies in cultures of hum an umbilical vein endothelial cells (HUVEC) and immortalized EA.hy926 cells show that TFPI is located in well-defined granules evenly spread over the cell surface and with apical polarization within the cytopla sm. These granules are smaller than and distinct from Weibel-Palade bo dies. Upon treatment of cultured cells with low concentrations of thro mbin (0.01 to 1 NIH U/mL), a marked redistribution of TFPI, occurred w ith patching in focal points and increased exposure of both TFPI antig en and anticoagulant activity on the surface of the stimulated endothe lial cells. This redistribution was paralleled by an acute release of TFPI in the cell medium. EA.hy926 cells responded more readily to thro mbin stimulation than HUVECs. The process was inhibited by both hirudi n and anti-thrombin receptor antibody. Our findings demonstrate a nove l mechanism by which thrombin may exert a negative feedback control on blood coagulation. Therefore, this pathway can be of physiological im portance in controlling TF-mediated thrombin generation.