Potential role of activated factor VIII (FVIIIa) in FVIIa/tissue factor-dependent FXa generation in initiation phase of blood coagulation
Factor VIIa/tissue factor (FVIIa/TF) initiates blood coagulation by promoting FXa generation (extrinsic-Xa). Subsequent generation of intrinsic FXa (intrinsic-Xa) amplifies thrombin formation. Previous studies suggested that FVIIa/TF activates FVIII rapidly in immediate coagulation reactions, and FVIIa/TF/FXa activates FVIII prior to thrombin-dependent feedback. We investigated FVIII/FVIIa/TF/FXa relationships in early coagulation mechanisms. Total FXa generated by FVIIa/TF and FVIIa/TF-activated FVIII (FVIIIaVIIa/TF) was 22.6 ± 1.7 nM (1 min); total FXa with FVIIa-inhibitor was 3.4 ± 0.7 nM, whereas FXa generated by FVIIa/TF or FVIII/TF was 10.4 ± 1.1 or 0.74 ± 0.14 nM, respectively. Little Xa was generated by FVIII alone, suggesting that intrinsic-Xa mechanisms were mediated by FVIIIaVIIa/TF and FVIII/TF in the initiation phase. Intrinsic-Xa was delayed somewhat by von Willebrand factor (VWF). FVIII activation by FXa with FVIIa/TF was comparable to activation with Glu-Gly-Arg-inactivated-FVIIa/TF. TF counteracted the inhibitory effects of VWF on FXa-induced FVIII activation mediated by Arg372 cleavage. The FVIII-C2 domain bound to cytoplasmic domain-deleted TF (TF1−243), and VWF blocked this binding by > 80%, indicating an overlap between VWF- and TF1−243-binding site(s) on C2. Overall, these data suggest that FVIII-associated intrinsic-Xa, governed by both FVIIa/TF-induced and FXa-induced FVIII activation mediated by FVIII-TF interactions, together with FVIIa-dependent extrinsic-Xa mechanisms, may be central to the initiation phase of coagulation.
KeywordsFVIIa Tissue factor FVIII FXa generation Initiation phase
This work was partly supported by a Grant-in-Aid for Scientific Research (KAKENHI) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) to KN (15K09663 and 18K07885) and by a grand of Bayer Special Project Awards (KN).
SF; performed experiments, interpreted the data, made the figures, and wrote the paper, KN; designed the research, interpreted the data, wrote the paper, edited the manuscript, and approved the final version to be published, KO; performed experiments and interpreted the data, MS; supervised the studies.
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Conflict of interest
The authors declare that they have no conflicts of interest.
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