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

Summary Essentials Factor VIIa is cleared principally as a complex with antithrombin. Enzyme/serpin complexes are preferred ligands for the scavenger-receptor LRP1. Factor VIIa/antithrombin but not factor VIIa alone is a ligand for LRP1. Macrophage-expressed LRP1 contributes to the clearance of factor VIIa/antithrombin. SummaryBackground Recent findings point to activated factor VII (FVIIa) being cleared predominantly (± 65% of the injected protein) as part of a complex with the serpin antithrombin. FVIIa–antithrombin complexes are targeted to hepatocytes and liver macrophages. Both cells lines abundantly express LDL receptor-related protein 1 (LRP1), a scavenger receptor mediating the clearance of protease–serpin complexes. Objectives To investigate whether FVIIa–antithrombin is a ligand for LRP1. Methods Binding of FVIIa and pre-formed FVIIa–antithrombin to purified LRP1 Fc-tagged cluster IV (rLRP1-cIV/Fc) and to human and murine macrophages was analyzed. FVIIa clearance was determined in macrophage LRP1 (macLRP1)-deficient mice. Results Solid-phase binding assays showed that FVIIa–antithrombin bound in a specific, dose-dependent and saturable manner to rLRP1-cIV/Fc. Competition experiments with human THP1 macrophages indicated that binding of FVIIa but not of FVIIa–antithrombin was reduced in the presence of annexin-V or anti-tissue factor antibodies, whereas binding of FVIIa–antithrombin but not FVIIa was inhibited by the LRP1-antagonist GST-RAP. Additional experiments revealed binding of both FVIIa and FVIIa–antithrombin to murine control macrophages. In contrast, no binding of FVIIa–antithrombin to macrophages derived from macLRP1-deficient mice could be detected. Clearance of FVIIa–antithrombin but not of active site-blocked FVIIa was delayed 1.5-fold (mean residence time of 3.3 ± 0.1 h versus 2.4 ± 0.2 h) in macLRP1-deficient mice. The circulatory presence of FVIIa was prolonged to a similar extent in macLRP1-deficient mice and in control mice. Conclusions Our data show that FVIIa–antithrombin but not FVIIa is a ligand for LRP1, and that LRP1 contributes to the clearance of FVIIa–antithrombin in vivo.