Tissue Factor-Dependent Cellular Responses of Factor VII/VIIa in the Absence and Presence of Factor X.

Factor VIIa (FVIIa) when bound to tissue factor (TF) triggers both coagulation and intra-cellular signaling. The dose-response of TF/VIIa-mediated factor X (FX) activation and gene regulation by TF-expressing cells were measured and interpreted in relation to plasma in vivo conditions. EC50s in the range 10–30 pM for FVIIa-mediated FX activation were measured with LPS-stimulated monocytes, vascular smooth muscle cells and MDA-MB-231 cells indicating that saturation of FX activation required less than 100 pM FVIIa. Compared to a plasma concentration for FVII/FVIIa of 10 nM this ensures that TF exposure is the determining factor for the coagulation response. This notion was further substantiated by data showing that FVII is readily activated to FVIIa on the TF-expressing cell surface, and that the FX activation kinetics mediated by FVII was similar to that of FVIIa. In MDA-MB-231 cells FVIIa induced TF-dependent signal transduction as measured by IL-8, CXCL-1 and CSF-2 mRNA. Stimulation reached saturation at about 10 nM FVIIa in the presence or absence of 100 nM FX. Furthermore, the maximal level of FVIIa-induced mRNA was similar with or without FX. FX enhanced TF/FVIIa-stimulated mRNA induction at low FVIIa concentrations, whereas a physiological level of FX or FXa (100 nM) alone was insufficient to induce a significant cellular response. In conclusion our results indicate that optimal conditions for triggering both coagulation and signaling prevail already at normal plasma concentration of FVII/FVIIa and FX, and that TF exposure is the determining factor for both responses.