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Synergistic inhibition of the intrinsic factor X activation by protein S and C4b-binding protein
S J Koppelman1, C van't Veer, J J Sixma
1Department of Haematology, University Hospital, Utrecht, The Netherlands.
Insights
C4b-binding protein enhances protein S inhibition of factor X activation by binding to factor VIII. This interaction, mediated by C4b-binding protein's alpha-chain, reveals a new role in coagulation regulation.
Area of Science:
- Biochemistry
- Hematology
- Immunology
Background:
- C4b-binding protein (C4bBP) is known to regulate the protein C anticoagulant pathway by inhibiting protein S cofactor activity.
- Protein S is a crucial anticoagulant that requires cofactor activity for activated protein C.
- The role of C4bBP in direct coagulation regulation beyond the protein C pathway was not fully understood.
Purpose of the Study:
- To investigate a novel role for C4b-binding protein in the regulation of coagulation.
- To elucidate the mechanism by which C4bBP influences factor X activation.
- To determine the specific interactions between C4bBP, protein S, and coagulation factors.
Main Methods:
- Assays to measure factor X activation inhibition by C4bBP and protein S complex.
- Studies using C4bBP variants lacking the beta-chain to assess binding and function.
- Investigation of C4bBP binding to factor VIII using various techniques, including monoclonal antibody inhibition.
- Analysis of C4bBP interaction with thrombin-activated factor VIII and factor VIII in complex with von Willebrand factor.
Main Results:
- The complex of C4bBP and protein S significantly inhibited intrinsic factor X activation by nearly 90%, compared to 50% inhibition by protein S alone.
- C4bBP lacking the beta-chain, which cannot bind protein S, failed to potentiate factor X activation inhibition.
- C4bBP specifically bound to factor VIII and thrombin-activated factor VIII, with the binding site located on the alpha-chain.
- Monoclonal antibodies against the alpha-chain of C4bBP blocked the potentiation of factor X activation inhibition, indicating an interaction with factor VIII.
Conclusions:
- C4b-binding protein plays a significant role in potentiating the inhibition of factor X activation by protein S.
- This potentiation is mediated through the interaction of C4bBP's alpha-chain with factor VIII.
- C4bBP's novel function in regulating the intrinsic pathway of coagulation through factor VIII interaction is established.
Abstract:
The complement protein C4b-binding protein plays an important role in the regulation of the protein C anticoagulant pathway. C4b-binding protein can bind to protein S, thereby inhibiting the cofactor activity of protein S for activated protein C. In this report, we describe a new role for C4b-binding protein in coagulation. We observed inhibition of the intrinsic factor X activating reaction by the complex of C4b-binding protein and protein S. At the plasma concentration of protein S, the factor X activation was inhibited for 50% and addition of C4b-binding protein led to a potentiation of the inhibition to almost 90%. Because C4b-binding protein alone had no effect on the activation of factor X, we hypothesized that binding of C4b-binding protein to protein S was a prerequisite for optimal inhibition of factor X activation. C4b-binding protein lacking the beta-chain, which is unable to bind to protein S, did not potentiate the inhibitory effect of protein S. In an earlier study, we observed that C4b-binding protein increased the binding affinity of protein S for factor VIII. Therefore, a possible interaction of C4b-binding protein with factor VIII was investigated. C4b-binding protein bound to factor VIII and to thrombin activated factor VIII in a saturable and specific way. Also, factor VIII in complex with von Willebrand factor was able to bind C4b-binding protein. The beta-chain of C4b-binding protein was not required for the interaction with factor VIII because C4b-binding protein lacking the beta-chain also bound to factor VIII. Monoclonal antibodies directed against the alpha-chain of C4b-binding protein inhibited the binding to factor VIII, whereas monoclonal antibodies directed against the beta-chain had no effect on the binding to factor VIII. This finding indicates that the binding site for factor VIII on C4b-binding protein is localized on the alpha-chains of C4b-binding protein. The potentiation by C4b-binding protein of the inhibition of the factor X activation by protein S was blocked by a monoclonal antibody directed against the alpha-chain of C4b-binding protein. This finding indicates that the potentiation of the inhibitory effect of protein S was mediated via an interaction of C4b-binding protein with factor VIII. C4b-binding protein did not bind to factor V and was not able to potentiate the inhibitory effect of protein S on prothrombinase activity.(ABSTRACT TRUNCATED AT 400 WORDS)