Factor VIIa interaction with EPCR modulates the hemostatic effect of rFVIIa in hemophilia therapy: Mode of its action

Abstract

Recent studies established that clotting factor VIIa (FVIIa) binds endothelial cell protein C receptor (EPCR). It has been speculated that FVIIa interaction with EPCR might augment the hemostatic effect of rFVIIa in therapeutic conditions. The present study is carried out to investigate the mechanism by which FVIIa interaction with EPCR contributes to the hemostatic effect of rFVIIa in hemophilia therapy. Active-site inhibited FVIIa, which is capable of binding to EPCR but has no ability to activate factor X, reduced the concentration of rFVIIa required to correct the bleeding following the saphenous vein injury in mouse hemophilia model systems. Higher doses of rFVIIa were required to restore hemostasis in EPCR overexpressing hemophilia mice compared to hemophilia mice expressing normal levels of EPCR. Administration of FVIII antibody induced only mild hemophilic bleeding in EPCR-deficient mice, which was corrected completely with a low dose of rFVIIa. Administration of therapeutic concentrations of rFVIIa increased plasma protein C levels in EPCR overexpressing mice, indicating the displacement of protein C from EPCR by rFVIIa. EPCR levels did not significantly alter the bioavailability of rFVIIa in plasma. Overall, our data indicate that EPCR levels influence the hemostatic effect of rFVIIa in treating hemophilia. Our present findings suggest that FVIIa displacement of anticoagulant protein C from EPCR that results in down-regulation of activated protein C generation and not the direct effect of EPCR-FVIIa on FX activation is the mechanism by which FVIIa interaction with EPCR contributes to the hemostatic effect of rFVIIa in hemophilia therapy.

Graphical abstract

Figure 1.

Proteolytically inactive FVIIa binding to EPCR augments the hemostatic activity of a low dose of rFVIIa in hemophilia A (Hem A) mice. (A) Average hemostatic times of wild-type and hemophilia A mice following saphenous vein incision and the effect of varying doses of rFVIIa in restoring hemostasis in hemophilia A mice. Saline or varying doses of rFVIIa (1, 4, or 10 mg/kg body weight) was administered to hemophilia A mice via the tail vein. Five minutes after rFVIIa administration, mice were subjected to saphenous vein incision. Average time to achieve hemostasis was determined as described in “Materials and methods.” *P < .05 compared with hemophilia mice not receiving rFVIIa. (B) Administration of a pharmacological concentration of FVIIa_AI promotes the hemostatic effect of a low dose of rFVIIa. Hemophilia A mice were injected with saline, a low dose of rFVIIa (1 mg/kg), FVIIa_AI (10 mg/kg), or both FVIIa_AI (10 mg/kg) and rFVIIa (1 mg/kg). Five minutes following rFVIIa administration, mice were subjected to saphenous vein incision and the average time to achieve hemostasis was determined (n = 4-7 mice/group). ***P < .001. (C) FVIIa_AI downregulation of APC generation. Wild-type mice were administered with saline or FVIIa_AI (10 mg/kg) via the tail vein. After obtaining the blood sample (pre), mice were injected with saline or thrombin (6 U/kg) via the tail vein to induce activation of protein C in vivo. After 10 minutes, the blood was obtained (post). APC levels in plasma were measured as described earlier. Note: the data shown in panel A are essentially similar to that reported in our earlier publication and was reproduced here to illustrate the rationale behind using 1 mg/kg rFVIIa dose in experiments described in panel B. Hemophilia A mice used in this experiment were in the B6/129S genetic background. All other experiments were performed with mice in the C57BL/6J genetic background. NS, not significant.

Figure 2.

A low concentration of rFVIIa, 0.25 mg/kg, restores hemostasis in FVIII antibody-induced hemophilia mice administered with proteolytically inactive FVIIa or EPCR blocking antibodies (BL. Ab). (A,B) Wild-type C57BL/6J mice were administered intravenously with saline (Con) or FVIII mAb (1 mg/kg) to induce hemophilia. Two hours after administering FVIII antibody, the mice were injected with saline (0), 0.25, 1.0, or 4.0 mg/kg rFVIIa or FVII_AI, or 0.25 mg/kg rFVIIa plus FVII_AI (0.25 or 1.0 mg/kg) via the tail vein. Five minutes following rFVIIa administration, the bleeding was initiated by the saphenous vein incision and average time to achieve hemostasis (A) and blood loss (B) were determined. (C-D) FVIII antibody-induced hemophilia mice were injected with EPCR blocking or non-blocking antibodies (4 mg/kg) ± 0.25 mg/kg rFVIIa. Following the saphenous vein injury, the average time to achieve hemostasis (C) and blood loss (D) were determined as described in “Materials and methods” (n = 5-10 mice/group). *P < .05; **P < .01 ***P < .001. Note: The data shown for Con and Ab-induced hemophilia with 0, 0.25, and 1.0 mg/kg of rFVIIa (C-D) were the same as that of shown in panels A and B, respectively, except for minor differences in the number of animals in some groups.

Figure 3.

EPCR levels modulate the hemostatic effect of rFVIIa in hemophilia. Wild-type, Tie2-EPCR, and EPCR-deficient mice were administered intravenously with saline or FVIII mAb (1 mg/kg) to induce hemophilia. Two hours after administering the antibody, the hemophilia-induced mice were injected with saline (0), 0.25, 1.0, or 4.0 mg/kg rFVIIa via the tail vein. Five minutes following rFVIIa administration, the bleeding was initiated by the saphenous vein incision and average time to achieve hemostasis and blood loss were determined. At the end of the experimental period, blood from a group of mice receiving saline or FVIII mAb but not treated with rFVIIa was obtained by cardiac puncture to isolate plasma to measure FVIII clotting activity. (A) FVIII clotting activity levels; (B) average time to achieve hemostasis; (C) blood loss (n = 9-12 mice/group). (D) Protein C levels in plasma of wild-type mice and Tie2-EPCR mice administered with varying concentrations of rFVIIa. Plasmas were obtained from mice groups (B) following the completion of bleeding analysis. *P < .05; **P < .01; ***P < .001. When performing the statistical comparison between rFVIIa effectiveness in restoring hemostasis in Ab-induced hemophilic wild-type and Tie2-EPCR mice, each dose of rFVIIa was individually compared between wild-type and Tie2-EPCR mice. Note: The data shown for Con and Ab-induced hemophilia with 0, 0.25, and 1.0 mg/kg of rFVIIa in wild-type mice (B-C) were essentially the same as shown in Figure 2A-B, respectively, except for minor differences in the number of animals in some groups.

Figure 4.

Comparison of the hemostatic effect of rFVIIa in FVIII^−/−and Tie2-EPCR/FVIII^−/−mice. FVIII^−/− mice or Tie2-EPCR/FVIII^−/− mice were administered with 0, 1, or 4 mg/kg rFVIIa via the tail vein and subjected to the saphenous vein incision. The average time to achieve hemostasis (A) and blood loss (B) were measured as described in methods (n = 6 to 10 mice/group). * P < .05; ** P < .01; *** P < .001; ns, not statistically significant difference.

Figure 5.

The relevance of FVIIa and EPCR interaction to the hemostatic effect of rFVIIa in hemophilia therapy. When high concentrations of rFVIIa were administered to hemophilia patients with inhibitors, rFVIIa, along with plasma zymogen FVII, binds to tissue factor (TF) at the injury site (1). The formation of the TF-FVIIa complex initiates the activation of FX to FXa and also FIX to FIXa. Once traces of FXa are generated, tissue factor pathway inhibitor (TFPI) binds FXa and TFPI-FXa forms a quaternary complex with TF-FVIIa, inhibiting further activation of FX by TF-FVIIa (not shown). The initial FXa generated by TF-FVIIa also associates with factor Va (FVa) on the endothelial cell surface to form the prothrombinase complex, which activates prothrombin (FII) to thrombin. The resultant thrombin promotes the activation of protein C bound to the EPCR on the endothelium and also activates platelets. EPCR-mediated APC generation inactivates FVa and blocks further thrombin generation by the prothrombinase complex on the endothelium. However, rFVIIa, when present at high concentrations, can activate FX to FXa on the surface of thrombin-activated platelets, independent of TF (2). FXa generated by FVIIa on activated platelets forms appreciable levels of the prothrombinase complex that could lead to thrombin burst capable of restoring hemostasis in hemophilia patients. rFVIIa, in addition to activating FX, also displaces protein C from EPCR by directly competing with protein C to bind to the EPCR (3). FVIIa displacement of protein C from the EPCR reduces APC generation. Reduced APC levels would diminish the extent of FVa inactivation by APC, which would lead to sustained thrombin generation on the endothelium. This, in addition to rFVIIa directly activating FX, contributes to rFVIIa hemostatic effect in hemophilia therapy.