Endothelial cells express a matrix protein which binds activated factor XII in a zinc-independent manner

Recent studies have shown that peptides identified as surface binding regions of high molecular mass kininogen (HK) and factor XII (FXII) inhibit the Zn(2+)-dependent binding of FXII to confluent layers of human umbilical vein endothelial cells (HUVEC). This indicates that negatively charged FXII binding surfaces, such as sulfatides and dextran sulfate, may interfere with the binding of FXII to confluent layer of HUVEC. Upon investigating this hypothesis it was unexpectedly found that sulfatides enhanced a specific binding of FXII to a matrix protein expressed during growth of the endothelial cells and that this binding was independent of the presence of Zn(2+). The function of sulfatides was partly to minimize nonspecific electrostatic binding and partly to induce and enhance autoactivation of FXII generating alphaFXIIa. Western blot analysis of the extracts of the matrix incubated with FXII and sulfatides showed that the binding was specific for alphaFXIIa. The dissociation constant for binding alphaFXIIa was 12.8 +/- 0.4 nM (n = 4). The binding of alphaFXIIa to ECM was mapped to the heavy chain as no binding was observed of the light chain containing the catalytic domain. HK, which previously has been shown to completely abolish the Zn(2+)-dependent binding of FXII to confluent layers of HUVEC, did not inhibit the binding of alphaFXIIa to the matrix but sulfatides enhanced binding of FXII to ECM. This suggests that HK interferes with the binding of FXII to sulfatides and thereby the autoactivation of FXII. Trypsin treatment of the matrix protein completely abolished the binding, and fibronectin but not laminin was found to be a suitable target. The binding of activated FXII to the ECM suggests that FXIIa may be a modulator of cellular adhesion, migration and vascularization.