Inactivation of Factor VIIa by Antithrombin In Vitro, Ex Vivo and In Vivo: Role of Tissue Factor and Endothelial Cell Protein C Receptor

Abstract

Recent studies have suggested that antithrombin (AT) could act as a significant physiologic regulator of FVIIa. However, in vitro studies showed that AT could inhibit FVIIa effectively only when it was bound to tissue factor (TF). Circulating blood is known to contain only traces of TF, at best. FVIIa also binds endothelial cell protein C receptor (EPCR), but the role of EPCR on FVIIa inactivation by AT is unknown. The present study was designed to investigate the role of TF and EPCR in inactivation of FVIIa by AT in vivo. Low human TF mice (low TF, ∼1% expression of the mouse TF level) and high human TF mice (HTF, ∼100% of the mouse TF level) were injected with human rFVIIa (120 µg kg−1 body weight) via the tail vein. At varying time intervals following rFVIIa administration, blood was collected to measure FVIIa-AT complex and rFVIIa antigen levels in the plasma. Despite the large difference in TF expression in the mice, HTF mice generated only 40–50% more of FVIIa-AT complex as compared to low TF mice. Increasing the concentration of TF in vivo in HTF mice by LPS injection increased the levels of FVIIa-AT complexes by about 25%. No significant differences were found in FVIIa-AT levels among wild-type, EPCR-deficient, and EPCR-overexpressing mice. The levels of FVIIa-AT complex formed in vitro and ex vivo were much lower than that was found in vivo. In summary, our results suggest that traces of TF that may be present in circulating blood or extravascular TF that is transiently exposed during normal vessel damage contributes to inactivation of FVIIa by AT in circulation. However, TF’s role in AT inactivation of FVIIa appears to be minor and other factor(s) present in plasma, on blood cells or vascular endothelium may play a predominant role in this process.

The lipid peroxidation product 4-hydroxy-2-nonenal induces tissue factor decryption via ROS generation and the thioredoxin system

“…The addition of anti-TF immunoglobulin G attenuated the increased activation of FX in HNE-treated cells as well as basal FX activation in control cells (supplemental Figure 2). As expected from our earlier investigation, 29 HNE treatment did not alter TF antigen levels (supplemental Figure 3). These data confirm that HNE-induced increase in FX activation in THP-1 cells stems from TF decryption.…”

Activated factor VII–antithrombin complex predicts mortality in patients with stable coronary artery disease: a cohort study

“…S1). TF expression contributes to FVIIa‐AT complex formation in a mouse model , and plasma levels of FVIIa‐AT have been reported to directly correlate with TF mRNA expression . The assay of this complex could indirectly reflect TF–FVIIa interaction and TF exposure.…”

LDL receptor‐related protein 1 contributes to the clearance of the activated factor VII–antithrombin complex

“…Apart from TF, EPCR, asialoglycoprotein receptor and the cubilin/megalin receptor complex have also been reported as receptors. Importantly, EPCR does not seem to modulate the clearance of FVIIa , whereas the asialoglycoprotein receptor is probably involved in the clearance of FVIIa molecules that are hyposialylated . Regarding the cubilin/megalin receptor, this complex is known to be involved in the recovery of proteins in the renal system .…”

“…Recently, this view has been changed by the discovery that, in contrast to in vitro conditions, the presence of FVIIa in circulating blood actually enhances complex formation with antithrombin by a so far unrecognized mechanism . The amount of FVIIa that is complexed to antithrombin following intravenous injection increases up to 60% to > 90% in both humans and mice . Complex formation with antithrombin is compatible with the shorter presence of FVIIa activity than of FVIIa antigen in the circulation .…”