von Willebrand Factor Elevates Plasma Factor VIII Without Induction of Factor VIII Messenger RNA in the Liver

Cellular stress and coagulation factor production: when more is not necessarily better

Remarkably, it has been 40 years since the isolation of the 2 genes involved in hemophilia A (HA) and hemophilia B (HB), encoding clotting factor (F) VIII (FVIII) and FIX, respectively. Over the years, these advances led to the development of purified recombinant protein factors that are free of contaminating viruses from human pooled plasma for hemophilia treatments, reducing the morbidity and mortality previously associated with human plasma-derived clotting factors. These discoveries also paved the way for modified factors that have increased plasma half-lives. Importantly, more recent advances have led to the development and Food and Drug Administration approval of a hepatocyte-targeted, adeno-associated viral vector-mediated gene transfer approach for HA and HB. However, major concerns regarding the durability and safety of HA gene therapy remain to be resolved. Compared with FIX, FVIII is a much larger protein that is prone to misfolding and aggregation in the endoplasmic reticulum and is poorly secreted by the mammalian cells. Due to the constraint of the packaging capacity of adeno-associated viral vector, B-domain deleted FVIII rather than the full-length protein is used for HA gene therapy. Like full-length FVIII, B-domain deleted FVIII misfolds and is inefficiently secreted. Its expression in hepatocytes activates the cellular unfolded protein response, which is deleterious for hepatocyte function and survival and has the potential to drive hepatocellular carcinoma. This review is focused on our current understanding of factors limiting FVIII secretion and the potential pathophysiological consequences upon expression in hepatocytes.

Genetic determinants of VWF clearance and FVIII binding modify FVIII pharmacokinetics in pediatric hemophilia A patients

Factor VIII (FVIII) pharmacokinetic (PK) properties show high interpatient variability in hemophilia A patients. Although previous studies have determined that age, body mass index, von Willebrand factor antigen (VWF:Ag) levels, and ABO blood group status can influence FVIII PK, they do not account for all observed variability. In this study, we aim to describe the genetic determinants that modify the FVIII PK profile in a population of 43 pediatric hemophilia A patients. We observed that VWF:Ag and VWF propeptide (VWFpp)/VWF:Ag, but not VWFpp, were associated with FVIII half-life. VWFpp/VWF:Ag negatively correlated with FVIII half-life in patients with non-O blood type, but no correlation was observed for type O patients, suggesting that von Willebrand factor (VWF) half-life, as modified by the ABO blood group, is a strong regulator of FVIII PK. The FVIII-binding activity of VWF positively correlated with FVIII half-life, and the rare or low-frequency nonsynonymous VWF variants p.(Arg826Lys) and p.(Arg852Glu) were identified in patients with reduced VWF:FVIIIB but not VWF:Ag. Common variants at the VWF, CLEC4M, and STAB2 loci, which have been previously associated with plasma levels of VWF and FVIII, were associated with the FVIII PK profile. Together, these studies characterize the mechanistic basis by which VWF clearance and ABO glycosylation modify FVIII PK in a pediatric population. Moreover, this study is the first to identify non-VWF and non-ABO variants that modify FVIII PK in pediatric hemophilia A patients.

Abnormal von Willebrand factor secretion, factor VIII stabilization and thrombus dynamics in type 2N von Willebrand disease mice

Essentials Type 2N von Willebrand disease involves impaired von Willebrand factor to factor VIII binding. Type 2N von Willebrand disease mutations exhibit qualitative and mild quantitative deficiencies. Type 2N von Willebrand disease mice exhibit unstable venous hemostatic thrombi. The factor VIII-binding ability of von Willebrand factor regulates arteriole thrombosis dynamics.

SUMMARY

Background von Willebrand factor (VWF) and factor VIII (FVIII) circulate as a non-covalent complex, with VWF serving as the carrier for FVIII. VWF indirectly influences secondary hemostasis by stabilizing FVIII and transporting it to the site of primary hemostasis. Type 2N von Willebrand disease involves impaired binding of VWF to FVIII, resulting in decreased plasma levels of FVIII. Objectives In these studies, we characterize the impact of three type 2N VWD variants (R763A, R854Q, R816W) on VWF secretion, FVIII stabilization and thrombus formation in a murine model. Methods Type 2N VWD mice were generated by hydrodynamic injections of mutant murine VWF cDNAs and the influence of these variants on VWF secretion and FVIII binding was evaluated. In vivo hemostasis and the dynamics of thrombus formation and embolization were assessed using a murine tail vein transection hemostasis model and an intravital thrombosis model in the cremaster arterioles. Results Type 2N VWD variants were associated with decreased VWF secretion using cell and animal-based models. FVIII-binding to type 2N variants was impaired in vitro and was variably stabilized in vivo by expressed or infused 2N variant VWF protein. Both transgenic type 2N VWD and FVIII knockout (KO) mice demonstrated impaired thrombus formation associated with decreased thrombus stability. Conclusions The type 2N VWD phenotype can be recapitulated in a murine model and is associated with both quantitative and qualitative VWF deficiencies and impaired thrombus formation. Patients with type 2N VWD may have normal primary hemostasis formation but decreased thrombus stability related to ineffective secondary hemostasis.

Similarity of coagulation and inflammation despite different surgical revascularization strategies – a prospective randomized trial

Background:

Supposedly, minimized extracorporeal circulation or off-pump revascularization as alternatives to conventional extracorporeal circulation (ONCAB) reduce inflammation and coagulation disturbances.

Methods:

One hundred and twenty coronary artery bypass graft (CABG) patients were prospectively randomized for three surgical techniques. Coagulation and inflammation markers were measured up to 72 hours postoperatively.

Results:

Coagulation factors I, II, V, X, antithrombin III and C-reactive protein did not differ perioperatively between the groups and increased, as did several other markers, 12 to 72 hours postoperatively. Compared to its alternatives, ONCAB showed the most obvious transient increase in thrombin-antithrombin complexes (p<0.0001), D-dimers (p=0.0059), tissue factor pathway inhibitor (p=0.0005), factor VIII (p=0.0041) and tumor necrosis factor α (p=0.0300) during the operation and up to 12 hours postoperatively. Furthermore, ONCAB generated lower leukocyte and platelet counts and higher values of soluble P-selectin and soluble intercellular adhesion molecule 1 at some time points.

Conclusions:

With similarity in pivot coagulation factors, a specific detrimental influence of ONCAB on common coagulation pathways was excluded. Higher perioperative concentrations of products from the coagulation cascade most likely indicate activation of pericardial blood – recirculated only in ONCAB. Furthermore, with only temporary differences in markers of inflammation, the alternatives to ONCAB altogether were without advantage at 72 hours postoperatively. In the general answer to surgical trauma, the part of modern extracorporeal circulation is possibly overestimated. The study is registered at the German Clinical Trial Registry. Registration number DRKS00007580. URL: https://drks-neu.uniklinik-freiburg.de/drks_web/ URL: http://apps.who.int/trialsearch/

Patterns of expression of factor VIII and von Willebrand factor by endothelial cell subsets in vivo

Circulating factor VIII (FVIII) is derived from liver and from extrahepatic sources probably of endothelial origin, but the vascular sites of FVIII production remain unclear. Among organs profiled, only liver and lymph nodes (LNs) show abundant expression of F8 messenger RNA (mRNA). Transcriptomic profiling of subsets of stromal cells, including endothelial cells (ECs) from mouse LNs and other tissues, showed that F8 mRNA is expressed by lymphatic ECs (LECs) but not by capillary ECs (capECs), fibroblastic reticular cells, or hematopoietic cells. Among blood ECs profiled, F8 expression was seen only in fenestrated ECs (liver sinusoidal and renal glomerular ECs) and some high endothelial venules. In contrast, von Willebrand factor mRNA was expressed in capECs but not in LECs; it was coexpressed with F8 mRNA in postcapillary high endothelial venules. Purified LECs and liver sinusoidal ECs but not capECs from LNs secrete active FVIII in culture, and human and mouse lymph contained substantial

FVIII

C activity. Our results revealed localized vascular expression of FVIII and von Willebrand factor and identified LECs as a major cellular source of FVIII in extrahepatic tissues.

Factor VIII and von Willebrand factor interaction: biological, clinical and therapeutic importance

The interaction of factor VIII (FVIII) with von Willebrand Factor (VWF) is of direct clinical significance in the diagnosis and treatment of patients with haemophilia A and von Willebrand disease (VWD). A normal haemostatic response to vascular injury requires both FVIII and VWF. It is well-established that in addition to its role in mediating platelet to platelet and platelet to matrix binding, VWF has a direct role in thrombin and fibrin generation by acting as a carrier molecule for the cofactor FVIII. Recent studies show that the interaction affects not only the biology of both FVIII and VWF, and the pathology of haemophilia and VWD, but also presents opportunities in the treatment of haemophilia. This review details the mechanisms and the molecular determinants of FVIII interaction with VWF, and the role of FVIII-VWF interaction in modulating FVIII interactions with other proteases, cell types and cellular receptors. The effect of defective interaction of FVIII with VWF as a result of mutations in either protein is discussed.

The protein structure and effect of factor VIII

Factor VIII (FVIII) is a key component of the fluid phase of the blood coagulation system. The proteases efficiently cleave FVIII at three sites, two within the heavy and one within the light chain resulting in alteration of its covalent structure and conformation and yielding the active cofactor, FVIIIa. FVIIIa is a trimer composed of A1, A2 and A3-C1-C2 subunits. The role of FVIIIa is to markedly increase the catalytic efficiency of factor IXa in the activation of factor X. Variants of these factors frequently also lead to severe bleeding disorders.

Interaction of dicaproyl phosphatidylserine with recombinant factor VIII and its impact on immunogenicity

Replacement therapy with exogenous recombinant factor VIII (rFVIII) to control bleeding episodes results in the development of inhibitory antibodies in 15% to 30% of hemophilia A patients. The inhibitory antibodies are mainly directed against specific and universal immunodominant epitopes located in the C2 domain. Previously we have shown that complexation of O-phospho-L-serine (phosphatidylserine head group) with the phospholipid binding region of the C2 domain can lead to an overall reduction in the immunogenicity of rFVIII. Here, we have investigated the hypothesis that dicaproyl phosphatidylserine, a short-chain water-soluble phospholipid, can reduce the immunogenicity of rFVIII. Circular dichroism and fluorescence spectroscopy studies suggest that dicaproyl phosphatidylserine interacts with rFVIII, causing subtle changes in the tertiary and secondary structure of the protein. Sandwich enzyme-linked immunosorbent assay studies indicate that dicaproyl phosphatidylserine probably interacts with the phospholipid binding region of the C2 domain. The immunogenicity of FVIII-dicaproyl phosphatidylserine complexes prepared at concentrations above and below the critical micellar concentrations of the lipid were evaluated in hemophilia A mice. Our results suggest that micellar dicaproyl phosphatidylserine may be useful to reduce the immunogenicity of rFVIII preparations.

Elevated plasma factor VIII in a mouse model of low-density lipoprotein receptor-related protein deficiency

It has been established that low-density lipoprotein receptor-related protein (LRP) is involved in the cellular uptake and degradation of coagulation factor VIII (FVIII) in vitro. To address the physiologic role of LRP in regulating plasma FVIII in vivo, we used cre/loxP-mediated conditional LRP- deficient mice (MX1cre(+)LRP(flox/flox)). Upon inactivation of the LRP gene, MX1cre(+)LRP(flox/flox) mice had significantly higher plasma FVIII as compared with control LRP(flox/flox) mice (3.4 and 2.0 U/mL, respectively; P <.001). Elevated plasma FVIII levels in MX1cre(+)LRP(flox/flox) mice coincided with increased plasma von Willebrand factor (VWF) (2.0 and 1.6 U/mL for MX1cre(+)LRP(flox/flox) and control LRP(flox/flox) mice, respectively; P <.05). Elevation of plasma FVIII and VWF persisted for at least 6 weeks after inactivation of the LRP gene. Upon comparing plasma FVIII and VWF in individual mice, we observed an increase of the FVIII/VWF ratio in MX1cre(+)LRP(flox/flox) mice as compared with control LRP(flox/flox) mice. Administration of either a vasopressin analog or an endotoxin resulted in increased plasma VWF, but not FVIII. In clearance experiments, MX1cre(+)LRP(flox/flox) mice displayed a 1.5-fold prolongation of FVIII mean residence time. Adenovirus-mediated overexpression of the 39-kDa receptor-associated protein (RAP) in normal mice resulted in a 3.5-fold increase of plasma FVIII. These data confirm that the regulation of plasma FVIII in vivo involves a RAP-sensitive mechanism. Surprisingly, plasma FVIII in MX1cre(+)LRP(flox/flox) mice increased 2-fold after RAP gene transfer. We propose that RAP-sensitive determinants other than hepatic LRP contribute to the regulation of plasma FVIII in vivo.

Genetic induction of a releasable pool of factor VIII in human endothelial cells

Although it is known that factor VIII (FVIII) plasma levels increase rapidly in response to a number of stimuli, the biological stimuli behind this release is less clear. Previously, we showed that FVIII can traffic together with von Willebrand factor (vWF) into storage granules in a pituitary tumor cell line, AtT-20; however, AtT-20 cells could not be used to address the release or functional activity of released FVIII. To investigate the regulated secretion of stored FVIII, endothelial cells with intact agonist-stimulated release pathways were used. Human umbilical vein endothelial cells (HUVECs) were transduced with retroviral FVIII construct [hFVIII(V)] to create a FVIII/vWF storage pool. Immunofluorescent staining of transduced cells demonstrated FVIII in Weibel-Palade bodies. In contrast, the transduction of hFVIII(V) into HT-1080 and HepG2 cells displayed FVIII only in the cytoplasm. We studied the regulated release of both FVIII and vWF from endothelial cells after agonist-induced stimulation and demonstrated a parallel release of FVIII and vWF proteins. This released FVIII was functionally active. Hence, endothelial cells transduced with hFVIII(V) store FVIII together with vWF in Weibel-Palade bodies, creating a releasable storage pool of both proteins. Because FVIII secretion can be physiologically regulated by agonists in culture, this may explain the pharmacological agonist-induced release of FVIII by drugs such as desmopressin in vivo and suggests vascular endothelium as a reasonable target of gene therapy of hemophilia A.

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.

Advances toward gene therapy for hemophilia at the millennium

Hemophilia A and hemophilia B are both X chromosome-linked recessive bleeding disorders that affect about 1 in 5000 males and result from a deficiency in the coagulation factors VIII and IX, respectively. Severely affected individuals require frequent administration of factor VIII or factor IX preparations derived from human plasma, and more recently, from recombinant DNA technology. Although these preparations have greatly reduced the contamination with blood-borne pathogens, there still exist significant limitations with protein replacement therapy. As we elucidate more about the expression, structure, and function of these coagulation factors new avenues will open for the development of novel genetically improved coagulation factors. Several aspects of the hemophilias A and B make these diseases attractive candidates for gene therapy. These advantages include the following: (1) these proteins are readily delivered into the circulation from a variety of different cell types; (2) low levels of expression would significantly improve the management of bleeding episodes in these patients; (3) it is unlikely that regulated expression of these proteins will be required; and (4) there are excellent animal models for these diseases. Although progress with gene transfer of factor IX has proceeded at a greater rate than factor VIII, success with both molecules has demonstrated partial persistent correction in mouse and canine models of hemophilia A and B. The information gained from these animal studies has provided new insights into gene therapeutic approaches for genetic diseases. In addition, several gene therapy clinical studies for the treatment of hemophilia A and B were initiated in 1999.

Expression of factor VIII by murine liver sinusoidal endothelial cells

Factor VIII (fVIII) is the procoagulant plasma glycoprotein that is missing or decreased in hemophilia A. The cellular origin of fVIII synthesis is controversial. Liver transplantation cures hemophilia A, demonstrating that the liver is a major site of fVIII synthesis. We detected fVIII mRNA in purified populations of murine liver sinusoidal endothelial cells (LSECs) and hepatocytes, but not Kupffer cells. LSECs and hepatocytes contained comparable numbers of fVIII mRNA (40 and 70 transcripts per cell, respectively) by quantitative competitive reverse transcriptase-polymerase chain reaction analysis. There was not detectable mRNA for factor IX, a hepatocyte marker, in the LSEC preparation, nor was there detectable mRNA for von Willebrand factor, an endothelial cell marker, in the hepatocyte preparation. This excludes the possibility that detectable fVIII mRNA is due to cross-contamination in the hepatocyte or LSEC preparations. Primary cultures of LSECs were established in which fVIII mRNA levels were indistinguishable from purified LSECs. LSECs secreted active fVIII into the culture medium. This finding represents the first demonstration of homologous expression of fVIII mRNA and protein in cell culture and should facilitate studies of fVIII gene regulation. Additionally, LSECs potentially are targets for a fVIII transgene during gene therapy of hemophilia A.