Factor XI, but not prekallikrein, blocks high molecular weight kininogen binding to human umbilical vein endothelial cells.

Previous studies on the interaction of high molecular weight kininogen (HK) with endothelial cells have reported a large number of binding sites (106-107 sites/cell) with differing relative affinities (KD = 7-130 nm) and have implicated various receptors or receptor complexes. In this study, we examined the binding of HK to human umbilical vein endothelial cells (HUVEC) with a novel assay system utilizing HUVEC immobilized on microcarrier beads, which eliminates the detection of the high affinity binding sites found nonspecifically in conventional microtiter well assays. We report that HK binds to 8.5 x 104 high affinity (KD = 21 nm) sites per HUVEC, i.e. 10-100-fold fewer than previously reported. Although HK binding is unaffected by the presence of a physiological concentration of prekallikrein, factor XI abrogates HK binding to HUVEC in a concentration-dependent manner. Disruption of the naturally occurring complex between factor XI and HK by the addition of a 31-amino acid peptide mimicking the factor XI-binding site on HK restored HK binding to HUVEC. Furthermore, HK inhibited thrombin-stimulated von Willebrand factor release by HUVEC but not thrombin receptor activation peptide (SFLLRN-amide)-stimulated von Willebrand factor release. Factor XI restored the ability of thrombin to stimulate von Willebrand factor release in the presence of low HK concentrations. These results suggest that free HK, or HK in complex with prekallikrein but not in complex with factor XI, interacts with the endothelium and can maintain endothelial cell quiescence by preventing endothelial stimulation by thrombin.