title: 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.
authors.affiliation: Department of Cellular and Molecular Biology, The University of Texas Health Science Center at Tyler, Tyler, Texas, United States of America.
authors.forename: Rit
authors.initials: R
authors.lastname: Vatsyayan
authors.affiliation: Department of Cellular and Molecular Biology, The University of Texas Health Science Center at Tyler, Tyler, Texas, United States of America.
authors.forename: Hema
authors.initials: H
authors.lastname: Kothari
authors.affiliation: Division of Hematology and Oncology, McAllister Heart Institute, Department of Medicine, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, United States of America.
authors.forename: Nigel
authors.initials: N
authors.lastname: Mackman
authors.affiliation: Department of Cellular and Molecular Biology, The University of Texas Health Science Center at Tyler, Tyler, Texas, United States of America.
authors.forename: Usha R
authors.initials: UR
authors.lastname: Pendurthi
authors.affiliation: Department of Cellular and Molecular Biology, The University of Texas Health Science Center at Tyler, Tyler, Texas, United States of America.
authors.forename: L Vijaya Mohan
authors.initials: LV
authors.lastname: Rao
mesh_terms: D000818:Animals; D000991:Antithrombins / Q000097:blood / Q000378:metabolism*; D001779:Blood Coagulation Factors / Q000378:metabolism* / Q000502:physiology; D015942:Factor VIIa / Q000378:metabolism*; D061307:Human Umbilical Vein Endothelial Cells; D006801:Humans; D008810:Mice, Inbred C57BL; D011956:Receptors, Cell Surface / Q000097:blood / Q000378:metabolism* / Q000502:physiology; D013925:Thromboplastin / Q000378:metabolism* / Q000502:physiology
chemical_list: D000991:Antithrombins; D001779:Blood Coagulation Factors; D011956:Receptors, Cell Surface; C091963:activated protein C receptor; D013925:Thromboplastin; D015942:Factor VIIa
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