von Willebrand factor and inflammation

Von Willebrand factor (VWF) is a plasma glycoprotein best known for its crucial hemostatic role in serving as a molecular bridge linking platelets to subendothelial components following vascular injury. In addition, VWF functions as chaperone for coagulation factor VIII. In pathological settings, VWF is recognized as a risk factor for both arterial and venous thrombosis, as well as a molecular player that directly promotes the thrombotic process. Recent years have seen the emergence of the concept of immuno-thrombosis by which inflammatory cells participate in thrombotic processes. In return, reports about the involvement of hemostatic proteins or cells (such as platelets) in inflammatory responses have become increasingly common, emphasizing the intricate link between hemostasis and inflammation. However, evidence of a link between VWF and inflammation arose much earlier than these recent developments. At first, VWF was considered only as a marker of inflammation in various pathologies, due to its acute release by the activated endothelium. Later on, a more complex role of VWF in inflammation was uncovered, owing to its capacity to direct the biogenesis of specific endothelial organelles, the Weibel-Palade bodies that contain known inflammation players such as P-selectin. Finally, a more direct link between VWF and inflammation has become apparent with the discovery that VWF is able to recruit leukocytes, either via direct leukocyte binding or by recruiting platelets which in turn will attract leukocytes. This review will focus on these different aspects of the connection between VWF and inflammation, with particular emphasis on VWF-leukocyte interactions.

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

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.

SUMMARY

Background 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.

Circulatory support devices: fundamental aspects and clinical management of bleeding and thrombosis

Circulatory support devices are increasingly being used to overcome cardiac or respiratory failure. Long-term devices are used either as a 'bridge to transplant' to support patients who are unable to wait any longer for a heart transplant, or, more recently, as 'destination therapy' for older patients suffering from end-stage heart failure and who have contraindications to heart transplantation. Short-term support devices for high-risk percutaneous coronary intervention, or as a 'bridge for decision' for patients suffering from refractory cardiogenic shock, have also been developed. The clinical benefit of such assist devices has been demonstrated in several important studies, but, unfortunately, thrombotic and bleeding complications are two major clinical issues in patients requiring these devices. Overcoming these issues is of major importance to allow the safe and broad use of these devices, and to consider them as true alternatives to heart transplantation. The present review focuses on thrombotic and bleeding complications, and describes how the risk of thrombosis and bleeding may vary according to the clinical indication, but also according to the type of device. We describe the current knowledge of the mechanisms underlying the occurrence of these complications, provide some guidance for choosing the most appropriate anticoagulation regimen to prevent their occurrence for each type of device and indication, and provide some recommendations for the management of patients when the complication occurs.

Shear stress-independent binding of von Willebrand factor-type 2B mutants p.R1306Q & p.V1316M to LRP1 explains their increased clearance

BACKGROUND

von Willebrand factor (VWF) is cleared in a shear stress- and macrophage-dependent manner by LRP1. von Willebrand disease (VWD)-type 2B mutants are endocytosed more efficiently than wild-type (wt)-VWF by macrophages.

OBJECTIVE

To investigate if VWD-type 2B mutations in the VWF A1-domain affect LRP1 binding and LRP1-dependent clearance.

METHODS

Recombinant Fc-tagged A1 domain (A1-Fc, A2-Fc, A3-Fc) and full-length VWF (wt or mutants thereof) were tested for binding to LRP1 or a recombinant fragment thereof in a static immunosorbent assay. Mutant and wt-VWF were also compared for clearance in mice lacking macrophage LRP1 (macLRP1(-) ) and control mice (macLRP1(+) ).

RESULTS

We found that A1-Fc but not A2-Fc or A3-Fc binds dose-dependently to LRP1. Binding of A1-Fc to LRP1 was markedly enhanced by the VWD-type 2B mutation p.V1316M. As expected, full-length wt-VWF was unable to bind LRP1 under static conditions unless ristocetin was added. In contrast, the presence of the p.V1316M or p.R1306Q mutation induced spontaneous binding to LRP1 without the need for ristocetin or shear stress. Both mutants were cleared more rapidly than wt-VWF in control macLRP1(+) mice. Surprisingly, deletion of macrophage LRP1 abrogated the increased clearance of the VWF/p.R1306Q and VWF/p.V1316M mutant.

CONCLUSION

The VWF A1-domain contains a binding site for LRP1. Certain VWD-type 2B mutations relieve the need for shear stress to induce LRP1 binding. Enhanced LRP1 binding coincides with a reduced survival of VWF/p.R1306Q and VWF/p.V1316M. Our data provide a rationale for reduced VWF levels in at least some VWD-type 2B patients.

Clearance of von Willebrand factor

Quantitative deficiencies in von Willebrand factor (VWF) are associated with abnormal hemostasis that can manifest in bleeding or thrombotic complications. Consequently, many studies have endeavored to elucidate the mechanisms underlying the regulation of VWF plasma levels. This review focuses on the role of VWF clearance pathways. A summary of recent developments are provided, including results from genetic studies, the relationship between glycosylation and VWF clearance, the contribution of increased VWF clearance to the pathogenesis of von Willebrand disease and the identification of VWF clearance receptors. These different studies converge in their conclusion that VWF clearance is a complex phenomenon that involves multiple mechanisms. Deciphering how such different mechanisms coordinate their role in this process is but one of the remaining challenges. Nevertheless, a better insight into the complex clearance pathways of VWF may help us to better understand the clinical implications of aberrant clearance in the pathogenesis of von Willebrand disease and perhaps other disorders as well as aid in developing alternative therapeutic approaches.

von Willebrand factor: the old, the new and the unknown

von Willebrand factor (VWF) is a protein best known from its critical role in hemostasis. Indeed, any dysfunction of VWF is associated with a severe bleeding tendency known as von Willebrand disease (VWD). Since the first description of the disease by Erich von Willebrand in 1926, remarkable progress has been made with regard to our understanding of the pathogenesis of this disease. The cloning of the gene encoding VWF has allowed numerous breakthroughs, and our knowledge of the epidemiology, genetics and molecular basis of VWD has been rapidly expanding since then. These studies have taught us that VWF is rather unique in terms of its multimeric structure and the unusual mechanisms regulating its participation in the hemostatic process. Moreover, it has become increasingly clear that VWF is a more all-round protein than originally thought, given its involvement in several pathologic processes beyond hemostasis. These include angiogenesis, cell proliferation, inflammation, and tumor cell survival. In the present article, an overview of advances concerning the various structural and functional aspects of VWF will be provided.

Decreased active von Willebrand factor level owing to shear stress in aortic stenosis patients

BACKGROUND

Aortic stenosis patients often show bleeding complications. Previously, a prolonged platelet function analyzer (PFA-100) closure time was observed with plasma of severe aortic stenosis patients. To elucidate a possible role of circulating preactivated von Willebrand factor (VWF), we determined the level of VWF in its active, platelet-binding conformation in plasma of patients with aortic stenosis.

PATIENTS/METHODS

Sixty-two aortic stenosis patients were included in this study. VWF and related parameters were measured, and the results were related to severity of aortic stenosis.

RESULTS

VWF activation factor, indicating the proportion of circulating VWF able to bind to platelets, correlated negatively with peak transvalvular gradient and PFA-100 closure time. No correlation was observed between ADAMTS13 activity and peak transvalvular gradient or PFA-100 closure time. Both VWF antigen and VWF propeptide levels were significantly higher in patients with mild and moderate aortic stenosis, but not in those with severe stenosis.

CONCLUSIONS

Our data demonstrate that the aortic pressure gradient is inversely associated with VWF activation factor, but not with VWF antigen or VWF multimerization in patients with aortic stenosis. These findings might have implications for the bleeding observed in patients with aortic stenosis.

Factor VIII and von Willebrand factor--too sweet for their own good

SUMMARY

Although factor VIII (FVIII) and von Willebrand factor (VWF) are products of two distinct genes, they circulate in plasma as a tight non-covalent complex. Moreover, they both play a critical role in the haemostatic process, a fact that is illustrated by the severe bleeding tendency associated with the functional absence of either protein. FVIII is an essential cofactor for coagulation factor IX, while VWF is pertinent to the recruitment of platelets to the injured vessel wall under conditions of rapid flow. FVIII and VWF have in common that they are heavily glycosylated: full-length FVIII contains 20 N-linked and at least seven O-linked glycans, while VWF contains 12 N-linked and 10 O-linked glycans. Three decades of research have revealed that the carbohydrate structures of FVIII and VWF contribute to many of the steps that can be distinguished in the life-cycle of these proteins, including biosynthesis/secretion, function and clearance. In this review, several of these aspects will be discussed. In addition, the interaction of the FVIII/VWF complex with two families of carbohydrate-binding proteins, i.e. Galectins and Siglecs, and their potential physiological relevance will be discussed.

von Willebrand factor activation, granzyme-B and thrombocytopenia in meningococcal disease

SUMMARY BACKGROUND

During invasive meningococcal disease, severe thrombocytopenia is strongly associated with a poor outcome.

OBJECTIVES

In order to elucidate the pathophysiological mechanism behind the development of thrombocytopenia, we studied the role of von Willebrand factor (VWF) in meningococcal disease.

PATIENTS/METHODS

Thirty-two children with severe meningococcal disease admitted to our university hospital were included in this study. VWF and related parameters were measured and results were correlated with the development of shock and thrombocytopenia.

RESULTS

At admission, all patients had increased levels of (active) VWF and VWF propeptide. The highest VWF propeptide levels were observed in patients with shock, indicating acute endothelial activation. Although VWF propeptide levels in patients with shock, with or without thrombocytopenia, were similar, increased active VWF was significantly lower in patients with thrombocytopenia as compared with patients without thrombocytopenia. ADAMTS13 was moderately decreased. However, the VWF multimeric pattern was minimally increased. We assume that these findings are explained by VWF consumption and perhaps by granzyme B (GrB). In vitro experiments showed that GrB is able to cleave VWF multimers in plasma, whereas GrB was high in patients with shock, who developed thrombocytopenia.

CONCLUSIONS

Our results demonstrate that consumption of VWF, derived from endothelial cells, could be a key feature of meningococcal disease and primary to the development of thrombocytopenia during shock.

The active conformation of von Willebrand factor in patients with thrombotic thrombocytopenic purpura in remission

BACKGROUND

Functional deficiency of ADAMTS13 in thrombotic thrombocytopenic purpura (TTP) patients is associated with circulating ultralarge von Willebrand factor (VWF) molecules that display spontaneous platelet-binding capacities. Upon remission, however, ADAMTS13 activity does not always return to baseline.

OBJECTIVE

To study ADAMTS13 and VWF-related features in TTP patients in remission.

METHODS

ADAMTS13 activity, anti-ADAMTS13 antibodies, VWF antigen, ultralarge VWF and levels of VWF that circulate in a glycoprotein Ibalpha-binding conformation were determined in plasma samples of 22 acquired TTP patients in remission between 1 month and 6 years after achieving remission. The composition of active multimers was investigated with a novel immunoprecipitation assay based on monoclonal antibody AU/VWF-a12, which specifically recognizes the active conformation of VWF.

RESULTS

ADAMTS13 activity was undetectable in 23% of the patients, even years after they had achieved remission, and lack of ADAMTS13 activity was associated with increased active VWF levels and the presence of ultralarge VWF multimers. Active VWF levels and ultralarge VWF were also associated with blood groups. Results from immunoprecipitation experiments revealed the full range of multimers to be present.

CONCLUSION

ADAMTS13 deficiency and the concurrent presence of ultralarge VWF and increased active VWF levels can be detected in TTP patients for years after they have achieved remission. Immunoprecipitation results suggest that the active conformation of VWF may be present in the lower molecular weight multimers, but future studies are necessary to confirm our findings.

The effect of exercise on von Willebrand factor and ADAMTS-13 in individuals with type 1 and type 2B von Willebrand disease

BACKGROUND

The effect of exercise on von Willebrand factor (VWF) and ADAMTS-13 levels in individuals with von Willebrand disease (VWD) has never been reported.

OBJECTIVES

The aim was to quantify the effect of a standardized exercise protocol on individuals with type 1 and type 2B VWD.

PATIENTS/METHODS

Thirty individuals from three groups (10 controls, 11 with type 1 VWD and 9 with type 2B VWD) completed the Standard Bruce Protocol Treadmill Test. A bleeding questionnaire was administered and blood tests were performed pre- and immediately postexercise. The groups were well matched for age, gender and body mass index (BMI).

RESULTS

There was a correlation in all groups between the metabolic equivalents (METS) achieved and the degree of change of VWF and FVIII:C levels (P < 0.002, Pearson's correlation). There was a significant postexercise increase in VWF:Ag, VWF:RCo, FVIII:C and activated VWF levels in both the control group and in the type 2B VWD group, but not in the type 1 VWD group. Specific to the type 2B VWD group was an increase in the percentage of high molecular weight multimers (P = 0.022), a decrease in the mean platelet count compared with the other groups (P < 0.001) and an increase in the ADAMTS-13 level (P = 0.001).

CONCLUSIONS

There are significant differences in the effects of exercise on individuals with type 1 and type 2B VWD compared with controls. Further clinical studies are necessary to evaluate exercise as a therapeutic option in VWD.

Characterization of the interaction between von Willebrand factor and osteoprotegerin

BACKGROUND AND OBJECTIVE

Osteoprotegerin (OPG), a member of the tumor necrosis-factor receptor superfamily, plays an important role in bone remodeling and is also involved in vascular diseases. OPG is physically associated with von Willebrand factor (VWF), a glycoprotein involved in primary hemostasis, within the Weibel-Palade bodies (WPBs) of endothelial cells and in plasma. The present study aimed to elucidate the molecular mechanisms underlying the interaction between OPG and VWF.

METHODS AND RESULTS

In a solid-phase binding assay, VWF was able to bind specifically to OPG in a calcium-dependent manner. This interaction displayed strong pH dependence with optimal binding occurring at pH 6.5 and was severely impaired by chloride-ion concentrations above 40 mm. Using a series of purified VWF derivatives the functional site that supports VWF interaction with OPG was localized on its Al domain. Fluorescence microscopy on human umbilical vein endothelial cells showed co-localization of VWF and OPG in WPBs. When secretion was induced, OPG remained associated with VWF in extracellular patches of release under biochemical conditions found in blood plasma.

CONCLUSIONS

Our observations demonstrate the existence of an interactive site for OPG within the VWF A1-domain. This study established that the optimal biochemical parameters allowing a complex formation between VWF and OPG are those thought to prevail in the trans-Golgi network. These conditions would allow VWF to act as a cargo targeting OPG to WPBs. Finally, blood environments appear suitable to preserve the complex, which may participate in vascular injury, arterial calcification and inflammation.

Acute activation of the endothelium results in increased levels of active von Willebrand factor in hemolysis, elevated liver enzymes and low platelets (HELLP) syndrome

BACKGROUND

HELLP (hemolysis, elevated liver enzymes and low platelets) syndrome is a severe complication of pre-eclampsia in pregnancy, characterized by microvascular platelet thrombi. Activation of the endothelium is thought to play a key role in pre-eclampsia and HELLP syndrome. Activation of endothelial cells may lead to release of von Willebrand factor (VWF) multimers, which are highly reactive with platelets. Normally, newly released multimers are cleaved by ADAMTS13, resulting in less reactive derivatives.

OBJECTIVE

We hypothesized that HELLP syndrome is characterized by increased amounts of active VWF compared with healthy pregnancy and pre-eclampsia, due to acute activation of endothelial cells. This might contribute to thrombocytopenia and thrombotic microangiopathy.

METHODS

Active VWF and ADAMTS13 activity were measured in healthy pregnant volunteers (n = 9), patients with pre-eclampsia (n = 6) and patients with HELLP syndrome (n = 14) at similar gestational ages. To study the role of endothelial cell activation, the propeptide/mature VWF ratio was determined, and VWF released by cultured endothelial cells was analyzed.

RESULTS

Active VWF levels were increased 2.1-fold in HELLP syndrome compared with healthy pregnant volunteers (P < 0.001) and 1.6-fold compared with patients with pre-eclampsia (P = 0.001). ADAMTS13 activity was moderately decreased in patients with HELLP syndrome compared with healthy pregnant volunteers (P < 0.004), but not compared with patients with pre-eclampsia. The propeptide/mature VWF ratio was increased 1.7-fold compared with healthy pregnant volunteers (P < 0.001) and 1.5-fold compared with patients with pre-eclampsia (P < 0.05). A significant correlation was found between this ratio and the activation factor of VWF (r = 0.68, P < 0.001). The amount of active VWF was increased 1.4-fold in medium of stimulated endothelial cells when compared with non-stimulated cells (P < 0.05).

CONCLUSION

Acute endothelial cell activation in HELLP syndrome and decreased ADAMTS13 activity result in increased amounts of active VWF. This might explain the consumptive thrombocytopenia and thrombotic microangiopathy associated with HELLP syndrome. Inhibition of circulating active VWF could be a potential new approach in the treatment of patients with HELLP syndrome.

Interaction of beta2-glycoprotein I with members of the low density lipoprotein receptor family

The antiphospholipid syndrome (APS) is a non-inflammatory autoimmune disease characterized by arterial and/or venous thrombosis and/or pregnancy morbidity in the presence of autoantibodies that recognize beta2-glycoprotein I (beta2GPI) bound to phospholipids. We have previously demonstrated that dimerization of beta2GPI by autoantibodies induces platelet activation, involving the platelet receptor apolipoprotein E receptor 2' (apoER2') a receptor belonging to the low-density lipoprotein receptor (LDL-R) family. Here, we show that dimeric beta2GPI, but not monomeric beta2GPI, interacts with four other LDL-R family members: the LDL-R related protein (LRP), megalin, the LDL-R and the very-low density lipoprotein receptor (VLDL-R). Interaction between dimeric beta2GPI and LDL-R, apoER2' and VLDL-R was best described with a one-site binding model (half-maximal binding; approximately 20 nm for apoER2' and VLDL-R and approximately 300 nm for LDL-R), whereas the interaction between dimeric beta2GPI and LRP or megalin was best described with a two-site binding model, representing a high- (approximately 3 nm) and a low-affinity site (approximately 0.2 microm). Binding to all receptors tested was unaffected by a tryptophane to serine (W316S) substitution in domain V of beta2GPI, which is known to disrupt the phospholipid binding site of beta2GPI. Also deletion of domain I or II left the interaction with the receptors unaffected. Deletion of domain V, however, significantly decreased the affinity for the receptors. In conclusion, our data show that dimeric beta2GPI can interact with different LDL-R family members. This interaction is dependent on a binding site within domain V of beta2GPI, which does not overlap with the phospholipid-binding site within domain V.

Increased metastatic potential of tumor cells in von Willebrand factor-deficient mice

BACKGROUND

The key role played by von Willebrand factor (VWF) in platelet adhesion suggests a potential implication in various pathologies, where this process is involved. In cancer metastasis development, tumor cells interact with platelets and the vessel wall to extravasate from the circulation. As a potential mediator of platelet-tumor cell interactions, VWF could influence this early step of tumor spread and therefore play a role in cancer metastasis.

OBJECTIVES

To investigate whether VWF is involved in metastasis development.

METHODS

In a first step, we characterized the interaction between murine melanoma cells B16-BL6 and VWF in vitro. In a second step, an experimental metastasis model was used to compare the formation of pulmonary metastatic foci in C57BL/6 wild-type and VWF-null mice following the injection of B16-BL6 cells or Lewis lung carcinoma cells.

RESULTS

In vitro adhesion assays revealed that VWF is able to promote a dose-dependent adhesion of B16-BL6 cells via its Arg-Gly-Asp (RGD) sequence. In the experimental metastasis model, we found a significant increase in the number of pulmonary metastatic foci in VWF-null mice compared with the wild-type mice, a phenotype that could be corrected by restoring VWF plasma levels. We also showed that increased survival of the tumor cells in the lungs during the first 24 h in the absence of VWF was the cause of this increased metastasis.

CONCLUSION

These findings suggest that VWF plays a protective role against tumor cell dissemination in vivo. Underlying mechanisms remain to be investigated.

Cysteine-mutations in von Willebrand factor associated with increased clearance

BACKGROUND

von Willebrand disease (VWD) is a bleeding disorder caused by the decrease of functional von Willebrand factor (VWF). Low levels of VWF can result from decreased synthesis, impaired secretion, increased clearance or combinations thereof. Several mutations lead to impaired synthesis or secretion of VWF, however, little is known about the survival of VWF in the circulation.

OBJECTIVES

To evaluate the effect of several VWF mutations on VWF clearance.

PATIENTS/METHODS

The effect of three cysteine-mutations (C1130F, C1149R or C2671Y) on the in vivo survival of VWF was studied in patients carrying these mutations and in a VWF-deficient mice model.

RESULTS

In patients carrying these mutations, we observed increased propeptide/mature VWF ratios and rapid disappearance of VWF from the circulation after desmopressin treatment. Detailed analysis of in vivo clearance of recombinant VWF in a VWF-deficient mice model revealed a fourfold increased clearance rate of the mutants. The mutations C1130F, C1149R and C2671Y are each associated with reduced survival of VWF in the circulation. Detailed analysis of the recombinant mutant VWF demonstrated that increased clearance was not due to increased proteolysis by ADAMTS-13. We did not identify functional or structural characteristics that the mutant proteins have in common and could be associated with the phenomenon of increased clearance.

CONCLUSIONS

Cysteine-mutations in VWF may result in reduced in vivo survival. The observation that various mutations are associated with increased in vivo clearance may have major implications for the therapeutic strategies that rely on the rise of endogenous VWF after desmopressin administration.

The von Willebrand factor self-association is modulated by a multiple domain interaction

BACKGROUND

Platelet adhesion and aggregation at sites of vascular injury exposed to rapid blood flow require von Willebrand factor (VWF). VWF becomes immobilized by binding to subendothelial components or by a self-association at the interface of soluble and surface-bound VWF.

OBJECTIVES

As this self-association has been demonstrated only under shear conditions, our first goal was to determine whether the same interaction could be observed under static conditions. Furthermore, we wanted to identify VWF domain(s) important for this self-association.

RESULTS

Biotinylated VWF (b-VWF) interacted dose-dependently and specifically with immobilized VWF in an enzyme-linked immunosorbent assay (ELISA) assay, showing that shear is not necessary to induce the VWF self-association. Whereas anti-VWF monoclonal antibodies (mAbs) had no effect on the self-association, the proteolytic VWF-fragments SpII(1366-2050) and SpIII(1-1365) inhibited the b-VWF-VWF interaction by 70 and 80%, respectively. Moreover, a specific binding of b-VWF to immobilized Sp-fragments was demonstrated. Finally, both biotinylated SpII and SpIII were able to bind specifically to both immobilized SpII and SpIII. Similar results were observed under flow conditions, which confirmed the functional relevance of our ELISA system.

CONCLUSION

We have developed an ELISA binding assay in which a specific VWF self-association under static conditions can be demonstrated. Our results suggest a multiple domain interaction between immobilized and soluble VWF.

Functional mapping of anti-factor IX inhibitors developed in patients with severe hemophilia B

Development of inhibitory antibodies is a serious complication of treatment with repeated factor IX infusions in a minority of patients with hemophilia B. Such antibodies detected in 8 patients have been characterized. Typing studies revealed that patients' immune response toward factor IX is highly heterogeneous and involves immunoglobulin G (IgG) antibodies, preferentially IgG1 and IgG4. The preservation of the sequence and the 3-dimensional orientation of the amino acids constituting one epitope are highly important for the assembly of an antibody-antigen complex. To localize the epitopes on the factor IX molecule, an original approach was designed using a set of factor X chimeras carrying regions of factor IX. Results showed that some patients' antibodies were directed against both the domain containing the gamma-carboxy glutamic acid residues (Gla domain) and the protease domain of factor IX. In contrast, no binding was observed to the epidermal growth factor-like domains or to the activation peptide. Functional characterization showed that the purified IgG from patients' serum inhibited the factor VIIIa-dependent activation of factor X. Moreover, patients' IgG directed against the Gla domain inhibited the binding of factor IX to phospholipids as well as the binding of factor VIII light chain to factor IXa. These data demonstrate that inhibitors appearing in patients with severe hemophilia B display specificity against restricted functional domains of factor IX.

Activation of factor IX zymogen results in exposure of a binding site for low-density lipoprotein receptor-related protein

The interaction between the endocytic receptor low density lipoprotein receptor-related protein (LRP) and either coagulation factor IX or its active derivative factor IXa was studied. Purified factor IX was unable to associate with LRP when analyzed by surface plasmon resonance. By contrast, factor XIa-mediated conversion of factor IX into factor IXa resulted in reversible dose- and calcium-dependent binding to LRP. Active-site blocking of factor IXa did not affect binding to LRP, whereas LRP binding was efficiently inhibited in the presence of heparin or antibodies against factor IX or LRP. The factor IXa-LRP interaction could be described by a 2-site binding model with equilibrium dissociation constants of 27 nmol/L and 69 nmol/L. Consistent with this model, it was observed that factor IXa binds to 2 different recombinant receptor fragments of LRP (denoted cluster II and cluster IV) with equilibrium dissociation constants of 227 nmol/L and 53 nmol/L, respectively. The amount of factor IXa degraded by LRP-deficient cells was 35% lower than by LRP-expressing cells, demonstrating that LRP contributes to the transport of factor IXa to the intracellular degradation pathway. Because ligand binding to LRP is often preceded by binding to proteoglycans, the contribution of proteoglycans to the catabolism of factor IXa was addressed by employing proteoglycan-deficient cells. Degradation of factor IXa by proteoglycan-deficient cells proceeded at a 83% lower rate than wild-type cells. In conclusion, the data presented here indicate that both LRP and proteoglycans have the potential to contribute to the catabolism of factor IXa.

Involvement of low-density lipoprotein receptor-related protein (LRP) in the clearance of factor VIII in von Willebrand factor-deficient mice

Factor VIII is tightly noncovalently linked to von Willebrand factor (vWF) in plasma with a stoichiometry of 1:50, and vWF deficiency results in secondary factor VIII deficiency, with accelerated clearance of factor VIII from the circulation. We used a murine model of severe von Willebrand disease (vWF knockout mice) to study the effect of a recombinant vWF/pro-vWF preparation (rpvWF) on factor VIII survival and to investigate whether low-density lipoprotein receptor-related protein (LRP) might be involved in the in vivo clearance of factor VIII in the absence of vWF. vWF-deficient mice received 70 U/kg rpvWF in the first series of experiments, and in a second series, 80 mg/kg receptor-associated protein (RAP) as a recombinant fusion protein to block the action of LRP. Factor VIII levels were measured at time 0, or 1 or 3 hours after administration of rpvWF or RAP. RAP induced a sustained rise in factor VIII levels comparable to that induced by rpvWF. In a third series, the preadministration of RAP resulted in a slower disappearance of factor VIII antigen (measured by an enzyme-linked immunosorbent assay specific for human factor VIII) after infusion of recombinant factor VIII. These findings suggest that the accelerated clearance of factor VIII seen in the absence of vWF may be a result of the involvement of LRP in factor VIII metabolism. (Blood. 2000;95:1703-1708)

Hydrophobic contact between the two epidermal growth factor-like domains of blood coagulation factor IX contributes to enzymatic activity

The three-dimensional structure of activated factor IX comprises multiple contacts between the two epidermal growth factor (EGF)-like domains. One of these is a salt bridge between Glu(78) and Arg(94), which is essential for binding of factor IXa to its cofactor factor VIII and for factor VIII-dependent factor X activation (Christophe, O. D., Lenting, P. J., Kolkman, J. A., Brownlee, G. G., and Mertens, K. (1998) J. Biol. Chem. 273, 222-227). We now addressed the putative hydrophobic contact at the interface between the EGF-like domains. Recombinant factor IX chimeras were constructed in which hydrophobic regions Phe(75)-Phe(77) and Lys(106)-Val(108) were replaced by the corresponding sites of factor X and factor VII. Activated factor IX/factor X chimeras were indistinguishable from normal factor IXa with respect to factor IXa enzymatic activity. In contrast, factor IXa(75-77)/factor VII displayed approximately 2-fold increased factor X activation in the presence of factor VIII, suggesting that residues 75-77 contribute to cofactor-dependent factor X activation. Activation of factor X by factor IX(106-108)/factor VII was strongly decreased, both in the absence and presence of factor VIII. Activity could be restored by simultaneous substitution of the hydrophobic sites in both EGF-like domains for factor VII residues. These data suggest that factor IXa enzymatic activity requires hydrophobic contact between the two EGF-like domains.

Interaction between factor VIII and LDL receptor-related protein. Modulation of coagulation?

Recent reports suggest that the multifunctional receptor low-density lipoprotein receptor-related protein (LRP) may contribute to the regulation of blood coagulation by mechanisms that differ from the simple removal of protease/inhibitor complexes from the circulation. This possibility became apparent from the observation that LRP is involved in down-regulation of Tissue Factor expression at the surface of monocytes and fibroblasts. Furthermore, coagulation Factor VIII and activated Factor IX (Factor IXa) have been identified as proteins that are able to bind to LRP. In the present review, the potential contribution of LRP to the regulation of the coagulation cascade through these novel pathways is discussed, with particular reference to the interaction between LRP and coagulation Factor VIII.

Electron crystallography of human blood coagulation factor VIII bound to phospholipid monolayers

Coagulation factor VIII binds to negatively charged platelets prior to assembly with the serine protease, factor IXa, to form the factor X-activating enzyme (FX-ase) complex. The macromolecular organization of membrane-bound factor VIII has been studied by electron crystallography for the first time. For this purpose two-dimensional crystals of human factor VIII were grown onto phosphatidylserine-containing phospholipid monolayers, under near to physiological conditions (pH and salt concentration). Electron crystallographic analysis revealed that the factor VIII molecules were organized as monomers onto the lipid layer, with unit cell dimensions: a = 81.5A, b = 67.2 A, gamma = 66.5 degrees, P1 symmetry. Based on a homology-derived molecular model of the factor VIII (FVIII) A domains, the FVIII projection structure solved at 15-A resolution presents the A1, A2, and A3 domain heterotrimer tilted approximately 65 degrees relative to the membrane plane. The A1 domain is projecting on top of the A3, C1, and C2 domains and with the A2 domain protruding partially between A1 and A3. This organization of factor VIII allows the factor IXa protease and epidermal growth factor-like domain binding sites (localized in the A2 and A3 domains, respectively) to be situated at the appropriate position for the binding of factor IXa. The conformation of the lipid-bound FVIII is therefore very close to that for the activated factor VIIIa predicted in the FX-ase complex.

Surface loop 199-204 in blood coagulation factor IX is a cofactor-dependent site involved in macromolecular substrate interaction

In factor IX residues 199-204 encompass one of six surface loops bordering its substrate-binding groove. To investigate the contribution of this loop to human factor IX function, a series of chimeric factor IX variants was constructed, in which residues 199-204 were replaced by the corresponding sequence of factor VII, factor X, or prothrombin. The immunopurified and activated chimeras were indistinguishable from normal factor IXa in hydrolyzing a small synthetic substrate, indicating that this region is not involved in the interaction with substrate residues on the N-terminal side of the scissile bond. In contrast, replacement of loop 199-204 resulted in a 5-25-fold reduction in reactivity toward the macromolecular substrate factor X. This reduction was due to a combination of increased K(m) and reduced k(cat). In the presence of factor VIIIa the impaired reactivity toward factor X was largely restored for all factor IXa variants, resulting in a more pronounced stimulation by factor VIIIa compared with normal factor IXa (3 to 5 x 10(4)-fold versus 5 x 10(3)-fold). Inhibition by antithrombin was only slightly affected for the factor IXa variant with the prothrombin loop sequence, whereas factor IXa variants containing the analogous residues of factor VII or factor X were virtually insensitive to antithrombin inhibition. In the presence of heparin, however, all chimeric factor IXa variants formed complexes with antithrombin. Thus the cofactors heparin and factor VIIIa have in common that they both alleviate the deleterious effects of mutations in the factor IX loop 199-204. Collectively, our data demonstrate that loop 199-204 plays an important role in the interaction of factor IXa with macromolecular substrates.

The light chain of factor VIII comprises a binding site for low density lipoprotein receptor-related protein

In the present study, the interaction between the endocytic receptor low density lipoprotein receptor-related protein (LRP) and coagulation factor VIII (FVIII) was investigated. Using purified components, FVIII was found to bind to LRP in a reversible and dose-dependent manner (K(d) approximately 60 nM). The interaction appeared to be specific because the LRP antagonist receptor-associated protein readily inhibited binding of FVIII to LRP (IC(50) approximately 1 nM). In addition, a 12-fold molar excess of the physiological carrier of FVIII, i.e. von Willebrand factor (vWF), reduced the binding of FVIII to LRP by over 90%. Cellular degradation of (125)I-labeled FVIII by LRP-expressing cells ( approximately 8 fmol/10(5) cells after a 4.5-h incubation) was reduced by approximately 70% in the presence of receptor-associated protein. LRP-directed antibodies inhibited degradation to a similar extent, indicating that LRP indeed contributes to binding and transport of FVIII to the intracellular degradation pathway. Degradation of FVIII was completely inhibited by vWF. Because vWF binding by FVIII involves its light chain, LRP binding to this subunit was studied. In ligand blotting experiments, binding of FVIII light chain to LRP could be visualized. More detailed analysis revealed that FVIII light chain interacts with LRP with moderate affinity (k(on) approximately 5 x 10(4) M(-1) s(-1); k(off) approximately 2.5 x 10(-3) s(-1); K(d) approximately 50 nM). Furthermore, experiments using recombinant FVIII C2 domain showed that this domain contributes to the interaction with LRP. In contrast, no association of FVIII heavy chain to LRP could be detected under the same experimental conditions. Collectively, our data demonstrate that in vitro LRP is able to bind FVIII at the cell surface and to mediate its transport to the intracellular degradation pathway. FVIII-LRP interaction involves the FVIII light chain, and FVIII-vWF complex formation plays a regulatory role in LRP binding. Our findings may explain the beneficial effect of vWF on the in vivo survival of FVIII.

Regions 301-303 and 333-339 in the catalytic domain of blood coagulation factor IX are factor VIII-interactive sites involved in stimulation of enzyme activity

The contribution of the Factor IX catalytic domain to Factor VIIIa binding has been evaluated by functional analysis of Factor IX variants with substitutions in alpha-helix region 333-339 and region 301-303. These regions were found to play a prominent role in Factor VIIIa-dependent stimulation of Factor X activation, but do not contribute to the high-affinity interaction with Factor VIIIa light chain. We propose that complex assembly between Factor IXa and Factor VIIIa involves multiple interactive sites that are located on different domains of these proteins.

Blood coagulation factor IX residues Glu78 and Arg94 provide a link between both epidermal growth factor-like domains that is crucial in the interaction with factor VIII light chain

Recently, we established that mutations at calcium-binding sites within the first epidermal growth factor (EGF)-like domain of activated factor IX affect its interaction with factor VIIIa (Lenting, P. J., Christophe, O. D., ter Maat, H., Rees, D. J. G., and Mertens, K. (1996) J. Biol. Chem. 271, 25332-25337). In the present study, we have investigated the functional role of residue Glu78, which is not involved in calcium binding. Glu78 is also located in the first EGF-like domain and, when mutated to Lys, is associated with severe hemophilia B. Because Glu78 is conserved in related vitamin K-dependent proteins, it is difficult to understand how a mutation at this position is associated with factor IX-specific function. In this study, we addressed the hypothesis that Glu78 exerts its biological activity by interacting with another residue. One candidate was found to be the second EGF-like domain residue, Arg94, which is also associated with severe hemophilia B when mutated. We constructed a series of mutants that included mutations at position 78 alone (Glu78 to Lys/Glu78 to Asp) or at both positions 78 and 94 (Glu78 to Lys and Arg94 to Asp). The functional parameters of immunopurified and activated mutants were compared with normal activated factor IX. Mutants were indistinguishable from normal factor IXa in cleaving the synthetic substrate CH3SO2-Leu-Gly-Arg-p-nitroanilide or activating factor X in the absence of factor VIIIa. In contrast, in the presence of factor VIIIa, factor IXa Glu78 to Asp and factor IXa Glu78 to Lys/Arg94 to Asp were stimulated to the same extent as normal factor IXa, whereas factor IXa Glu78 to Lys was markedly less stimulated (140-fold versus 2,000-fold). This suggests that residues 78 and 94 should carry an opposite charge for a normal interaction of factor IXa to factor VIIIa. This hypothesis was confirmed in inhibition studies employing synthetic peptides comprising the factor IXa-binding motifs of factor VIII heavy (Ser558-Gln565) or light chain (Glu1811-Lys1818) and in direct binding studies. We propose that residues 78 and 94 link both EGF-like domains and thereby maintain the integrity of the factor VIII light chain binding site.

Ca2+ binding to the first epidermal growth factor-like domain of human blood coagulation factor IX promotes enzyme activity and factor VIII light chain binding

Ca2+ binding to the first epidermal growth factor (EGF)-like domain of factor IX is known to be required for biological activity, but the mechanism by which Ca2+ contributes to factor IX function has remained unclear. We have studied recombinant factor IX mutants which lack Ca2+ binding to the first EGF-like domain, due to a replacement of Asp64 by Glu, Lys, or Val. The purified mutants (factors IX D64E, D64K, and D64V), were compared to plasma-derived and recombinant wild-type factor IX with regard to a number of metal-ion dependent functional parameters. In the presence of Mg2+, the activated mutants were indistinguishable from normal factor IXa in hydrolyzing the synthetic substrate CH3-SO2-Leu-Gly-Arg-p-nitroanilide. Replacing Mg2+ by Ca2+ further stimulated the activity of normal factor IXa but not of mutant factor IXa. In factor VIII-independent factor X activation, factor IXa D64K and D64E displayed reduced catalytic activity compared to normal factor IXa (apparent kcat/Km approximately 1, 2, and 4 x 10(3) M-1 s-1, respectively). In the presence of factor VIIIa, factor X activation rates by normal and mutant factor IXa were stimulated by factor VIIIa to a different extent ( approximately700- and 200-fold, respectively), indicating that Asp64 replacements affect the interaction with factor VIIIa. This possibility was addressed in inhibition studies employing synthetic peptides comprising the factor IXa-binding motifs of factor VIII heavy or light chains. Whereas the heavy chain peptide (Ser558-Gln565) inhibited factor VIII-dependent factor X activation by normal and mutant factor IXa with similar efficiency, the light chain peptide (Lys1804-Lys1818) inhibited normal factor IXa 2-3-fold more efficiently than did mutant factor IXa. This indicates that the reduced response to factor VIIIa may be due to impaired binding of mutant factor IXa to the factor VIII light chain. This was further explored in direct binding studies. In the presence of Mg2+, normal and mutant factor IXa were similar in binding to the factor VIII light chain. However, in the presence of Ca2+, factor IXa mutants were less efficient than normal factor IXa, which was illustrated by a 4-5-fold lower affinity than normal factor IXa for factor VIII light chain. Collectively, our data demonstrate that a number of factor IXa functions, including enzymatic activity and assembly into the factor IXa-factor VIIIa complex, are dependent on Ca2+ binding to the first EGF-like domain of factor IX.

Kinetics of factor VIII light-chain cleavage by thrombin and factor Xa. A regulatory role of the factor VIII heavy-chain region Lys713-Arg740

Activation and limited proteolysis of factor VIII have been investigated with respect to the role of the heavy-chain region Lys713-Arg740. The kinetics of factor VIII activation have been analyzed in a system consisting of human factor VIII, factor IXa, factor X phospholipids, and thrombin or factor Xa. Plasma-derived factor VIII is activated by thrombin with a second-order rate constant of 3.3 +/- 0.3 x 10(6) M-1 s-1, which proved to be slightly higher than for activation by factor Xa. The second-order rate constant of activation by thrombin of plasma-derived factor VIII in the presence of a monoclonal antibody against the sequence Lys713-Arg740 is markedly reduced. The same result was obtained for activation by thrombin and factor Xa of factor VIII with a deletion including the sequence Lys713-Arg740, des-(713-1637)-factor VIII. This suggests that the region Lys713-Arg740 promotes factor VIII activation by both thrombin and factor Xa. Since factor VIII activation is associated with proteolysis, cleavage of factor VIII heavy and light chains was analyzed quantitatively. These studies indicated that heavy-chain cleavage of des-(713-1637)-factor VIII is similar to that of plasma-derived factor VIII. In contrast, cleavage of the light chain of des-(713-1637)-factor VIII is clearly reduced. Furthermore, the secondorder rate constant (0.2 +/- 0.1 x 10(6) M-1 s-1) of des-(713-1637)-factor VIII light-chain cleavage by thrombin was reduced tenfold compared with that of plasma-derived factor VIII. Proteolysis by factor Xa yielded similar results. The rate of des-(713-1637)-factor VIII light-chain cleavage by thrombin is similar to that of isolated light-chain, but isolated light-chain is cleaved by factor Xa 20-fold more efficiently than the light chain in des-(713-1637)-factor VIII. We conclude that activation of factor VIII by both thrombin and factor Xa is closely associated with light-chain cleavage. Furthermore, within the factor VIII heterodimer, the heavy-chain sequence Lys713-Arg740 promotes both activation and light-chain proteolysis.

The sequence Glu1811-Lys1818 of human blood coagulation factor VIII comprises a binding site for activated factor IX

In previous studies have shown that the interaction between factor IXa and VIII involves the light chain of factor VIII and that this interaction inhibited by the monoclonal antibody CLB-CAg A against the factor VIII region Gln1778-Asp1840 (Lenting, P.J., Donath, M.J.S.H., van Mourik, J.A., and Mertens, K. (1994) J. Biol. Chem. 269, 7150-7155). Employing distinct recombinant factor VIII fragments, we now have localized the epitope of this antibody more precisely between the A3 domain residues Glu1801 and Met1823. Hydropathy analysis indicated that this region is part of a major hydrophilic exosite within the A3 domain. The interaction of factor IXa with this exosite was studied by employing overlapping synthetic peptides encompassing the factor VII region Tyr1786-Ala1834. Factor IXa binding was found to be particularly efficient to peptide corresponding to the factor VIII sequences Lys1804-Lys1818 and Glu1811-Gln1820. The same peptides proved effective in binding antibody CLB-CAg A. Further analysis revealed that peptides Lys1804-Lys1818 and Glu1811-Gln1820 interfere with binding of factor IXa to immobilized factor VIII light chain (Ki approximately 0.2 mM and 0.3 mM, respectively). Moreover, these peptides inhibit factor X activation by factor IXa in the presence of factor VIIIa (Ki approximately 0.2 mM and 0.3 mM, respectively) but not in its absence. Equilibrium binding studies revealed that these two peptides bind to the factor IX zymogen and its activated form, factor IXa, with the same affinity (apparent Kd approximately 0.2 mM), whereas the complete factor VIII light chain displays preferential binding to factor IXa. In conclusion, our results demonstrate that peptides consisting of the factor VIII light chain residues Lys1804-Lys1818 and Glu1811-Gln1820 share a factor IXa binding site that is essential for the assembly of the factor X-activating factor IXa-factor VIIIa complex. We propose that the overlapping sequence Glu1811-Lys1818 comprises the minimal requirements for binding to activated factor IX.

Characterization of des-(741-1668)-factor VIII, a single-chain factor VIII variant with a fusion site susceptible to proteolysis by thrombin and factor Xa

A factor VIII variant has been characterized in which the heavy chain is directly fused to the light chain. Des-(741-1668)-factor VIII lacks the processing site at Arg1648, as Arg740 of the heavy chain is fused to Ser1669 of the light chain. The sequence of the fusion site is similar to that of other cleavage sites in factor VIII. The fusion site of des-(741-1668)-factor VIII was readily cleaved by both thrombin and factor Xa, and the same result was obtained for heavy chain cleavage. In contrast, des-(741-1668)-factor VIII cleavage by thrombin at position Arg1689 proceeded at a lower rate than the analogous cleavage by factor Xa, which presumably takes place at position Arg1721. The rate of cleavage at position Arg1689 by thrombin was also lower than that at the other processing sites. When des-(741-1668)-factor VIII was activated by thrombin, initial rates of factor Xa formation were similar to the rates obtained when plasma-derived factor VIII was activated by thrombin or factor Xa. Remarkably, activation of des-(741-1668)-factor VIII proceeded at a higher rate by factor Xa than by thrombin. These results indicate that factor VIII activation is strongly associated with cleavage at position Arg1689 or Arg1721. For the interaction between des-(741-1668)-factor VIII and von Willebrand factor, a Kd value of (0.8 +/- 0.3) x 10(-10) M was determined, which is similar to that of heterodimeric factor VIII. The affinity of single-chain des-(741-1668)-factor VIII for factor IXa was found to be 27 +/- 6 nM. The in vivo recovery and half-life of des-(741-1668)-factor VIII were assessed in guinea pigs. Upon infusion of des-(741-1668)-factor VIII at a dosage of 50 units/kg body weight, a rise of 1.0 +/- 0.3 unit/ml in factor VIII activity was obtained. The same recovery was determined for wild-type factor VIII. The half-life of des-(741-1668)-factor VIII was found to be 3 +/- 1 h, compared with 4 +/- 2 h for heterodimeric recombinant factor VIII. In conclusion, des-(741-1668)-factor VIII displays normal activity, is readily cleaved by thrombin and factor Xa at its fusion site, binds with high affinity to von Willebrand factor and factor IXa, and behaves like heterodimeric recombinant factor VIII in guinea pigs. By virtue of these properties, des-(741-1668)-factor VIII may prove useful for the treatment of bleeding episodes in patients with haemophilia A.

Cleavage at arginine 145 in human blood coagulation factor IX converts the zymogen into a factor VIII binding enzyme

The transition of the factor IX zymogen into the enzyme factor IXa beta was investigated. For this purpose, the activation intermediate factors IX alpha and IXa alpha were purified after cleavage of the Arg145-Ala146 and Arg180-Val181 bonds, respectively. These intermediates were compared for a number of functional properties with factor IXa beta, which is cleaved at both positions. Factor IXa alpha was equal to factor IXa beta in hydrolyzing the synthetic substrate CH3SO2-Leu-Gly-Arg-p-nitroanilide (kcat/Km approximately 120 s-1 M-1) but was less efficient in factor X activation. Factor IX alpha was incapable of generating factor Xa but displayed reactivity toward p-nitrophenol p-guanidinobenzoate and the peptide substrate. The catalytic efficiency, however, was 4-fold lower compared with factor IXa alpha and factor IXa beta. Factor IX alpha and factor IXa beta had similar affinity for the inhibitor benzamidine (Ki approximately 2.5 mM), and amidolytic activity of both species was inhibited by Glu-Gly-Arg-chloromethyl ketone and antithrombin III. Unlike factor IXa beta, factor IX alpha was unable to form SDS stable complexes with antithrombin III. Moreover, inhibition of factor IXa beta and factor IX alpha by Glu-Gly-Arg-chloromethyl ketone followed distinct pathways, because factor IX alpha was inhibited in a nonirreversible manner and displayed only minor incorporation of the dansylated inhibitor into its catalytic site. These data demonstrate that the catalytic site of factor IX alpha differs from that of the fully activated factor IXa beta. Factor IX and its derivatives were also compared with regard to complex assembly with factor VIII in direct binding studies employing the immobilized factor VIII light chain. Factor IX alpha and factor IXa beta displayed a 30-fold higher affinity for the factor VIII light chain (Kd approximately 12 nM) than the factor IX zymogen. Factor IXa alpha showed lower affinity (Kd approximately 50 nM) than factor IX alpha and factor IXa beta, which may explain the lower efficiency of factor X activation by factor IXa alpha. Collectively, our data indicate that cleavage of the Arg180-Val181 bond develops full amidolytic activity but results in suboptimal binding to the factor VIII light chain. With regard to cleavage of the Arg145-Ala146 bond, we have demonstrated that this results in the transition of the factor IX zymogen into an enzyme that lacks proteolytic activity.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of cleavage of the light chain at positions Arg1689 or Arg1721 in subunit interaction and activation of human blood coagulation factor VIII

The role of Factor VIII light chain cleavage in Factor VIII activation and subunit interaction was investigated. Purified Factor VIII was dissociated into its separate subunits, and the isolated light chain was cleaved by thrombin at position Arg1689 or by Factor Xa at position Arg1721. These Factor VIII light chain derivatives then were used for reconstitution with purified Factor VIII heavy chain to obtain heterodimers that were exclusively cleaved within the light chain. Intact and cleaved light chain could effectively be reassociated with heavy chain, with concomitant regain of Factor VIII cofactor function. The association rate constant of Factor Xa-cleaved light chain was found to be 3-fold lower than that of thrombin-cleaved or intact light chain, suggesting a role of the region Ser1690-Arg1721 in subunit assembly. Dissociation rate constants, however, were independent of Factor VIII light chain cleavage. Low ionic strength was observed to promote association but to destabilize the Factor VIII heterodimer. At high ionic strength, Factor VIII dissociation was extremely slow (kappa off approximately 10(-5) s-1) for all Factor VIII light chain derivatives, indicating that Factor VIII light chain cleavage is not related to Factor VIII dissociation. Furthermore, Factor VIII light chain cleavage does not affect enzyme-cofactor assembly, since the various light chain derivatives proved equally efficient in binding to Factor IXa (Kd approximately 15 nM). Studies in a purified Factor X-activating system demonstrated that thrombin and Factor Xa activate Factor VIII to the same extent. However, Factor Xa differed from thrombin in that it cleaved at Arg1721 rather than at Arg1689. Reassociated heterodimers of Factor VIII heavy chain and intact light chain did not promote Factor X activation. In contrast, heterodimers that contained cleaved light chain exhibited substantial Factor VIIIa activity. These data demonstrate that a single cleavage at either Arg1689 or Arg1721 converts the inactive Factor VIII heterodimer into an active cofactor of Factor IXa.

Identification of a binding site for blood coagulation factor IXa on the light chain of human factor VIII

The interaction between human factor IXa and factor VIII or its constituent units was investigated. Equilibrium binding studies were performed employing factor VIII light chain that was immobilized on a monoclonal antibody. Factor VIII light chain was observed to bind factor IXa with high affinity (Kd = 14.8 +/- 3.2 nM) and approximately 1:1 stoichiometry. Optimal interaction required NaCl concentrations below 0.2 M and the presence of Ca2+ ions. Factor VIII light chain in solution effectively inhibited binding of factor IXa to the immobilized light chain (Ki = 10.9 +/- 1.9 nM). The isolated factor VIII light chain and the factor VIII heterodimer were equally effective in factor IXa binding, demonstrating that this interaction did not require the factor VIII heavy chain. Factor Xa and activated Protein C were found to be inefficient (Ki > or = 1.2 microM) in competing with factor IXa, indicating that the high affinity for factor VIII light chain was unique for factor IXa. The factor IXa-factor VIII light chain interaction was inhibited by von Willebrand factor, but this effect was abolished by cleavage of the factor VIII light chain by thrombin. An antibody that inhibits von Willebrand factor-factor VIII complex formation did not compete for factor IXa binding. In contrast, association of factor IXa with the factor VIII light chain was inhibited by an antibody directed against the factor VIII region Gln1778-Asp1840. We propose that this sequence provides a factor IXa binding site and that its exposure requires dissociation of the factor VIII-von Willebrand factor complex.