Tissue factor deficiency causes cardiac fibrosis and left ventricular dysfunction

Human Genetic Variation in F3 and Its Impact on Tissue Factor-Dependent Disease

Tissue factor (TF) is the primary initiator of blood coagulation in humans. As improper intravascular TF expression and procoagulant activity underlie numerous thrombotic disorders, there has been longstanding interest in the contribution of heritable genetic variation in F3, the gene encoding TF, to human disease. This review seeks to comprehensively and critically synthesize small case-control studies focused on candidate single nucleotide polymorphisms (SNPs), as well as modern genome-wide association studies (GWAS) seeking to discover novel associations between variants and clinical phenotypes. Where possible, correlative laboratory studies, expression quantitative trait loci, and protein quantitative trait loci are evaluated to glean potential mechanistic insights. Most disease associations implicated in historical case-control studies have proven difficult to replicate in large GWAS. Nevertheless, SNPs linked to F3, such as rs2022030, are associated with increased F3 mRNA expression, monocyte TF expression after endotoxin exposure, and circulating levels of the prothrombotic biomarker D-dimer, consistent with the central role of TF in the initiation of blood coagulation.

Tissue factor cytoplasmic domain exacerbates post-infarct left ventricular remodeling via orchestrating cardiac inflammation and angiogenesis

The coagulation protein tissue factor (TF) regulates inflammation and angiogenesis via its cytoplasmic domain in infection, cancer and diabetes. While TF is highly abundant in the heart and is implicated in cardiac pathology, the contribution of its cytoplasmic domain to post-infarct myocardial injury and adverse left ventricular (LV) remodeling remains unknown.

Methods: Myocardial infarction was induced in wild-type mice or mice lacking the TF cytoplasmic domain (TF∆CT) by occlusion of the left anterior descending coronary artery. Heart function was monitored with echocardiography. Heart tissue was collected at different time-points for histological, molecular and flow cytometry analysis.

Results: Compared with wild-type mice, TF∆CT had a higher survival rate during a 28-day follow-up after myocardial infarction. Among surviving mice, TF∆CT mice had better cardiac function and less LV remodeling than wild-type mice. The overall improvement of post-infarct cardiac performance in TF∆CT mice, as revealed by speckle-tracking strain analysis, was attributed to reduced myocardial deformation in the peri-infarct region. Histological analysis demonstrated that TF∆CT hearts had in the infarct area greater proliferation of myofibroblasts and better scar formation. Compared with wild-type hearts, infarcted TF∆CT hearts showed less infiltration of proinflammatory cells with concomitant lower expression of protease-activated receptor-1 (PAR1) - Rac1 axis. In particular, infarcted TF∆CT hearts displayed markedly lower ratios of inflammatory M1 macrophages and reparative M2 macrophages (M1/M2). In vitro experiment with primary macrophages demonstrated that deletion of the TF cytoplasmic domain inhibited macrophage polarization toward the M1 phenotype. Furthermore, infarcted TF∆CT hearts presented markedly higher peri-infarct vessel density associated with enhanced endothelial cell proliferation and higher expression of PAR2 and PAR2-associated pro-angiogenic pathway factors. Finally, the overall cardioprotective effects observed in TF∆CT mice could be abolished by subcutaneously infusing a cocktail of PAR1-activating peptide and PAR2-inhibiting peptide via osmotic minipumps.

Conclusions: Our findings demonstrate that the TF cytoplasmic domain exacerbates post-infarct cardiac injury and adverse LV remodeling via differential regulation of inflammation and angiogenesis. Targeted inhibition of the TF cytoplasmic domain-mediated intracellular signaling may ameliorate post-infarct LV remodeling without perturbing coagulation.

Cardiac Tissue Factor Regulates Inflammation, Hypertrophy, and Heart Failure in Mouse Model of Type 1 Diabetes

Patients with diabetes have an increased risk of heart failure (HF). Diabetes is highly prevalent in HF with preserved ejection fraction (HFpEF), which is on the rise worldwide. The role of diabetes in HF is less established, and available treatments for HF are not effective in patients with HFpEF. Tissue factor (TF), a transmembrane receptor, plays an important role in immune cell inflammation and atherothrombosis in diabetes. However, its role in diabetes-induced cardiac inflammation, hypertrophy, and HF has not been studied. In this study, we used wild-type (WT), heterozygous, and low-TF (with 1% human TF) mice to determine the role of TF in type 1 diabetes-induced HF. We found significant upregulation of cardiac TF mRNA and protein levels in diabetic WT hearts compared with nondiabetic controls. WT diabetic hearts also exhibited increased inflammation and cardiac hypertrophy versus controls. However, these changes in cardiac inflammation and hypertrophy were not found in low-TF mice with diabetes compared with their nondiabetic controls. TF deficiency was also associated with improved cardiac function parameters suggestive of HFpEF, which was evident in WT mice with diabetes. The TF regulation of inflammation and cardiac remodeling was further dependent on downstream ERK1/2 and STAT3 pathways. In summary, our study demonstrated an important role of TF in regulating diabetes-induced inflammation, hypertrophy, and remodeling of the heart leading to HFpEF.

Differential roles of factors IX and XI in murine placenta and hemostasis under conditions of low tissue factor

The intrinsic tenase complex (FIXa-FVIIIa) of the intrinsic coagulation pathway and, to a lesser extent, thrombin-mediated activation of FXI, are necessary to amplify tissue factor (TF)-FVIIa-initiated thrombin generation. In this study, we determined the contribution of murine FIX and FXI to TF-dependent thrombin generation in vitro. We further investigated TF-dependent FIX activation in mice and the contribution of this pathway to hemostasis. Thrombin generation was decreased in FIX- but not in FXI-deficient mouse plasma. Furthermore, injection of TF increased levels of FIXa-antithrombin complexes in both wild-type and FXI-/- mice. Genetic studies were used to determine the effect of complete deficiencies of either FIX or FXI on the survival of mice expressing low levels of TF. Low-TF;FIX-/y male mice were born at the expected frequency, but none survived to wean. In contrast, low-TF;FXI-/- mice were generated at the expected frequency at wean and had a 6-month survival equivalent to that of low-TF mice. Surprisingly, a deficiency of FXI, but not FIX, exacerbated the size of blood pools in low-TF placentas and led to acute hemorrhage and death of some pregnant dams. Our data indicate that FIX, but not FXI, is essential for survival of low-TF mice after birth. This finding suggests that TF-FVIIa-mediated activation of FIX plays a critical role in murine hemostasis. In contrast, FXI deficiency, but not FIX deficiency, exacerbated blood pooling in low-TF placentas, indicating a tissue-specific requirement for FXI in the murine placenta under conditions of low TF.

A coagulation defect arising from heterozygous premature termination of tissue factor

Tissue factor (TF) is the primary initiator of blood coagulation in vivo and the only blood coagulation factor for which a human genetic defect has not been described. As there are no routine clinical assays that capture the contribution of endogenous TF to coagulation initiation, the extent to which reduced TF activity contributes to unexplained bleeding is unknown. Using whole genome sequencing, we identified a heterozygous frameshift variant (p.Ser117HisfsTer10) in F3, the gene encoding TF, causing premature termination of TF (TFshort) in a woman with unexplained bleeding. Routine hematological laboratory evaluation of the proposita was normal. CRISPR-edited human induced pluripotent stem cells recapitulating the variant were differentiated into vascular smooth muscle and endothelial cells that demonstrated haploinsufficiency of TF. The variant F3 transcript is eliminated by nonsense-mediated decay. Neither overexpression nor addition of exogenous recombinant TFshort inhibited factor Xa or thrombin generation, excluding a dominant-negative mechanism. F3+/- mice provide an animal model of TF haploinsufficiency and exhibited prolonged bleeding times, impaired thrombus formation, and reduced survival following major injury. Heterozygous TF deficiency is present in at least 1 in 25,000 individuals and could limit coagulation initiation in undiagnosed individuals with abnormal bleeding but a normal routine laboratory evaluation.

Murine tissue factor disulfide mutation causes a bleeding phenotype with sex specific organ pathology and lethality

Tissue factor is highly expressed in sub-endothelial tissue. The extracellular allosteric disulfide bond Cys186-Cys209 of human tissue factor shows high evolutionary conservation and in vitro evidence suggests that it significantly contributes to tissue factor procoagulant activity. To investigate the role of this allosteric disulfide bond in vivo, we generated a C213G mutant tissue factor mouse by replacing Cys213 of the corresponding disulfide Cys190-Cys213 in murine tissue factor. A bleeding phenotype was prominent in homozygous C213G tissue factor mice. Pre-natal lethality of 1/3rd of homozygous offspring was observed between E9.5 and E14.5 associated with placental hemorrhages. After birth, homozygous mice suffered from bleedings in different organs and reduced survival. Homozygous C213G tissue factor male mice showed higher incidence of lung bleedings and lower survival rates than females. In both sexes, C213G mutation evoked a reduced protein expression (about 10-fold) and severely reduced pro-coagulant activity (about 1000-fold). Protein glycosylation was impaired and cell membrane exposure decreased in macrophages in vivo. Single housing of homozygous C213G tissue factor males reduced the occurrence of severe bleeding and significantly improved survival, suggesting that inter-male aggressiveness might significantly account for the sex differences. These experiments show that the tissue factor allosteric disulfide bond is of crucial importance for normal in vivo expression, post-translational processing and activity of murine tissue factor. Although C213G tissue factor mice do not display the severe embryonic lethality of tissue factor knock-out mice, their postnatal bleeding phenotype emphasizes the importance of fully functional tissue factor for hemostasis.

Thrombin receptor PAR4 drives canonical NLRP3 inflammasome signaling in the heart

The deleterious effects of diabetes in the heart are increasingly attributed to inflammatory signaling through the NLRP3 (NOD, LRR and PYD domains-containing protein 3) inflammasome. Thrombin antagonists reduce cardiac remodeling and dysfunction in diabetic mice, in part by suppressing fibrin-driven inflammation. The role of cellular thrombin receptor subtypes in this context is not known. We sought to determine the causal involvement of protease-activated receptors (PAR) in inflammatory signaling of the diabetic heart. Mice with diet-induced diabetes showed increased abundance of pro-caspase-1 and pro-interleukin (IL)-1β in the left ventricle (LV), indicating transcriptional NLRP3 inflammasome priming, and augmented cleavage of active caspase-1 and IL-1β, pointing to canonical NLRP3 inflammasome activation. Caspase-11 activation, which mediates non-canonical NLRP3 inflammasome signaling, was not augmented. Formation of the plasma membrane pore-forming protein N-terminal gasdermin D (GDSMD), a prerequisite for IL-1β secretion, was also higher in diabetic vs. control mouse LV. NLRP3, ASC and IL-18 expression did not differ between the groups, nor did expression of PAR1 or PAR2. PAR3 was nearly undetectable. LV abundance of PAR4 by contrast increased with diabetes and correlated positively with active caspase-1. Genetic deletion of PAR4 in mice prevented the diet-induced cleavage of caspase-1, IL-1β and GDSMD. Right atrial appendages from patients with type 2 diabetes also showed higher levels of PAR4, but not of PAR1 or PAR2, than non-diabetic atrial tissue, along with increased abundance of cleaved caspase-1, IL-1β and GSDMD. Human cardiac fibroblasts maintained in high glucose conditions to mimic diabetes also upregulated PAR4 mRNA and protein, and increased PAR4-dependent IL-1β transcription and secretion in response to thrombin, while PAR1 and PAR2 expressions were unaltered. In conclusion, PAR4 drives caspase-1-dependent IL-1β production through the canonical NLRP3 inflammasome pathway in the diabetic heart, providing mechanistic insights into diabetes-associated cardiac thromboinflammation. The emerging PAR4-selective antagonists may provide a feasible approach to prevent cardiac inflammation in patients with diabetes.

Transglutaminase 2 limits the extravasation and the resultant myocardial fibrosis associated with factor XIII-A deficiency

Background and aims

Transglutaminase (TG) 2 and Factor (F) XIII-A have both been implicated in cardiovascular protection and repair. This study was designed to differentiate between two competing hypotheses: that TG2 and FXIII-A mediate these functions in mice by fulfilling separate roles, or that they act redundantly in this respect.

Methods

Atherosclerosis was assessed in brachiocephalic artery plaques of fat-fed mixed strain apolipoprotein (Apo)e deficient mice that lacked either or both transglutaminases. Cardiac fibrosis was assessed both in the mixed strain mice and also in C57BL/6J Apoe expressing mice lacking either or both transglutaminases.

Results

No difference was found in the density of buried fibrous caps within brachiocephalic plaques from mice expressing or lacking these transglutaminases. Cardiac fibrosis developed in both Apoe/F13a1 double knockout and F13a1 single knockout mice, but not in Tgm2 knockout mice. However, concomitant Tgm2 knockout markedly increased fibrosis, as apparent in both Apoe/Tgm2/F13a1 knockout and Tgm2/F13a1 knockout mice. Amongst F13a1 knockout and Tgm2/F13a1 knockout mice, the extent of fibrosis correlated with hemosiderin deposition, suggesting that TG2 limits the extravasation of blood in the myocardium, which in turn reduces the pro-fibrotic stimulus. The resulting fibrosis was interstitial in nature and caused only minor changes in cardiac function.

Conclusions

These studies confirm that FXIII-A and TG2 fulfil different roles in the mouse myocardium. FXIII-A protects against vascular leakage while TG2 contributes to the stability or repair of the vasculature. The protective function of TG2 must be considered when designing clinical anti-fibrotic therapies based upon FXIII-A or TG2 inhibition.

•

Double transglutaminase 2 and Factor XIII-A knockout exacerbates cardiac fibrosis.

•

Double knockout does not promote the growth of, or destabilise, brachiocephalic plaques.

•

FXIII-A in resident cardiac macrophages does not protect against cardiac fibrosis.

•

FXIII-A in inflammatory macrophages may contribute to protection against fibrosis.

•

Transglutaminase 2 and Factor XIII-A protect against extravasation of blood.

Absence of functional compensation between coagulation factor VIII and plasminogen in double-knockout mice

Plasminogen deficiency is associated with severely compromised fibrinolysis and extravascular deposition of fibrin. In contrast, coagulation factor VIII (FVIII) deficiency leads to prolonged and excessive bleeding. Based on opposing biological functions of plasminogen and FVIII deficiencies, we hypothesized that genetic elimination of FVIII would alleviate the systemic formation of fibrin deposits associated with plasminogen deficiency and, in turn, elimination of plasminogen would limit bleeding symptoms associated with FVIII deficiency. Mice with single and combined deficiencies of FVIII (F8^-/-) and plasminogen (Plg^-/-) were evaluated for phenotypic characteristics of plasminogen deficiency, including wasting disease, shortened lifespan, rectal prolapse, and multiorgan fibrin deposition. Conversely, to specifically examine the role of plasmin-mediated fibrinolysis on bleeding caused by FVIII deficiency, F8^-/- and F8^-/-/Plg^-/- mice were subjected to a bleeding challenge. Mice with a combined deficiency in FVIII and plasminogen displayed no phenotypic differences relative to mice with single FVIII or plasminogen deficiency. Plg^-/- and F8^-/-/Plg^-/- mice exhibited the same penetrance and severity of wasting disease, rectal prolapse, extravascular fibrin deposits, and reduced viability. Furthermore, following a tail vein-bleeding challenge, no significant differences in bleeding times or total blood loss could be detected between F8^-/- and F8^-/-/Plg^-/- mice. Moreover, F8^-/- and F8^-/-/Plg^-/- mice responded similarly to recombinant FVIII (rFVIII) therapy. In summary, the pathological phenotype of Plg^-/- mice developed independently of FVIII-dependent coagulation, and elimination of plasmin-driven fibrinolysis did not play a significant role in a nonmucosal bleeding model in hemophilia A mice.

Information-Derived Mechanistic Hypotheses for Structural Cardiotoxicity

Adverse events resulting from drug therapy can be a cause of drug withdrawal, reduced and or restricted clinical use, as well as a major economic burden for society. To increase the safety of new drugs, there is a need to better understand the mechanisms causing the adverse events. One way to derive new mechanistic hypotheses is by linking data on drug adverse events with the drugs' biological targets. In this study, we have used data mining techniques and mutual information statistical approaches to find associations between reported adverse events collected from the FDA Adverse Event Reporting System and assay outcomes from ToxCast, with the aim to generate mechanistic hypotheses related to structural cardiotoxicity (morphological damage to cardiomyocytes and/or loss of viability). Our workflow identified 22 adverse event-assay outcome associations. From these associations, 10 implicated targets could be substantiated with evidence from previous studies reported in the literature. For two of the identified targets, we also describe a more detailed mechanism, forming putative adverse outcome pathways associated with structural cardiotoxicity. Our study also highlights the difficulties deriving these type of associations from the very limited amount of data available.

Tissue Factor: An Essential Mediator of Hemostasis and Trigger of Thrombosis

Tissue factor (TF) is the high-affinity receptor and cofactor for factor (F)VII/VIIa. The TF-FVIIa complex is the primary initiator of blood coagulation and plays an essential role in hemostasis. TF is expressed on perivascular cells and epithelial cells at organ and body surfaces where it forms a hemostatic barrier. TF also provides additional hemostatic protection to vital organs, such as the brain, lung, and heart. Under pathological conditions, TF can trigger both arterial and venous thrombosis. For instance, atherosclerotic plaques contain high levels of TF on macrophage foam cells and microvesicles that drives thrombus formation after plaque rupture. In sepsis, inducible TF expression on monocytes leads to disseminated intravascular coagulation. In cancer patients, tumors release TF-positive microvesicles into the circulation that may contribute to venous thrombosis. TF also has nonhemostatic roles. For instance, TF-dependent activation of the coagulation cascade generates coagulation proteases, such as FVIIa, FXa, and thrombin, which induce signaling in a variety of cells by cleavage of protease-activated receptors. This review will focus on the roles of TF in protective hemostasis and pathological thrombosis.

Heterodimers of the transcriptional factors NFATc3 and FosB mediate tissue factor expression for 15(S)-hydroxyeicosatetraenoic acid-induced monocyte trafficking

Tissue factor (TF) is expressed in vascular and nonvascular tissues and functions in several pathways, including embryonic development, inflammation, and cell migration. Many risk factors for atherosclerosis, including hypertension, diabetes, obesity, and smoking, increase TF expression. To better understand the TF-related mechanisms in atherosclerosis, here we investigated the role of 12/15-lipoxygenase (12/15-LOX) in TF expression. 15(S)-hydroxyeicosatetraenoic acid (15(S)-HETE), the major product of human 15-LOXs 1 and 2, induced TF expression and activity in a time-dependent manner in the human monocytic cell line THP1. Moreover, TF suppression with neutralizing antibodies blocked 15(S)-HETE-induced monocyte migration. We also found that NADPH- and xanthine oxidase-dependent reactive oxygen species (ROS) production, calcium/calmodulin-dependent protein kinase IV (CaMKIV) activation, and interactions between nuclear factor of activated T cells 3 (NFATc3) and FosB proto-oncogene, AP-1 transcription factor subunit (FosB) are involved in 15(S)-HETE-induced TF expression. Interestingly, NFATc3 first induced the expression of its interaction partner FosB before forming the heterodimeric NFATc3-FosB transcription factor complex, which bound the proximal AP-1 site in the TF gene promoter and activated TF expression. We also observed that macrophages from 12/15-LOX-/- mice exhibit diminished migratory response to monocyte chemotactic protein 1 (MCP-1) and lipopolysaccharide compared with WT mouse macrophages. Similarly, compared with WT macrophages, monocytes from 12/15-LOX-/- mice displayed diminished trafficking, which was rescued by prior treatment with 12(S)-HETE, in a peritonitis model. These observations indicate that 15(S)-HETE-induced monocyte/macrophage migration and trafficking require ROS-mediated CaMKIV activation leading to formation of NFATc3 and FosB heterodimer, which binds and activates the TF promoter.

Cytoplasmic domain of tissue factor promotes liver fibrosis in mice

AIM

To evaluate the role of tissue factor (TF) and protease activated receptor (PAR)-2 in liver fibrosis.

METHODS

Using CCl₄ administration for eight weeks, we induced hepatic fibrosis in wild-type C57BL/6 mice and in mice with deletion of the cytoplasmic signalling domain of TF (TF^§CT/§CT), deletion of PAR-2 (PAR-2^-/-) and combined deletion of TF signalling domain and PAR-2 (TF^§CT/§CT/PAR-2^-/-). Hepatic fibrosis area was assessed by quantitative imaging of picrosirius red staining. Hepatic collagen content was assessed by hydroxyproline levels. Hepatic stellate cells (αSMA positive) and hepatic macrophages (CD68 positive) were identified by immunohistochemistry. Hepatic gene expression was determined by PCR and liver TGFβ1 content by ELISA.

RESULTS

CCl₄ treated mice with deletion of the PAR-2 gene (PAR-2^-/-) and the cytoplasmic domain of TF (TF^§CT/§CT) developed significantly less hepatic fibrosis, characterised by reduced liver fibrosis area and hydroxyproline content, compared to control wildtype mice treated with CCl₄. The observed reduction in histological fibrosis was accompanied by a significant decrease in the hepatic content of TGFβ, the prototypic fibrogenic cytokine, as well as fewer activated hepatic stellate cells and hepatic macrophages. Deletion of the TF cytoplasmic signalling domain reduced hepatic fibrosis to levels similar to those observed in mice lacking PAR-2 signalling but combined deletion provided no added protection against fibrosis indicating a lack of mutual modulating effects that have been observed in other contexts such as angiogenic responses.

CONCLUSION

Tissue factor cytoplasmic domain is involved in TF-PAR-2 signalling initiating hepatic fibrosis and is a potential therapeutic target, as its deletion would not impact coagulation.

Protease-Activated Receptor 1 Contributes to Angiotensin II-Induced Cardiovascular Remodeling and Inflammation

BACKGROUND

Angiotensin II (Ang II) plays an important role in cardiovascular disease. It also leads to the activation of coagulation. The coagulation protease thrombin induces cellular responses by activating protease-activated receptor 1 (PAR-1). We investigated whether PAR-1 contributes to Ang II-induced cardiovascular remodeling and inflammation.

METHODS AND RESULTS

PAR-1+/+ (wild-type; WT) and PAR-1-/- mice were infused with Ang II (600 ng/kg/min) for up to 4 weeks. In WT mice, this dose of Ang II did not cause a significant increase in blood pressure but it did cause pathological changes in both the aorta and the heart. Ang II infusion resulted in vascular remodeling of the aorta, demonstrated by a significant increase in medial wall thickening and perivascular fibrosis. Importantly, both parameters were significantly attenuated by PAR-1 deficiency. Furthermore, perivascular fibrosis around coronary vessels was reduced in Ang II-treated PAR-1-/- mice compared to WT mice. In addition, PAR-1 deficiency significantly attenuated Ang II induction of inflammatory cytokines and profibrotic genes in the aortas compared to WT mice. Finally, PAR-1 deficiency had no effect on Ang II-induced heart hypertrophy. However, the heart function measured by fractional shortening was less impaired in PAR-1-/- mice than in WT mice.

CONCLUSION

Our data indicate that PAR-1 plays a significant role in cardiovascular remodeling mediated by a blood pressure-independent action of Ang II.

A combined deficiency of tissue factor and PAR-4 is associated with fatal pulmonary hemorrhage in mice

INTRODUCTION

Mice with a complete absence of tissue factor (TF) die during embryonic development whereas mice with low levels of TF (Low-TF mice) survive to adulthood. Low-TF mice exhibit spontaneous hemorrhage in various organs, including the lung. In contrast, mice can survive without protease-activated receptor (PAR)-4, which is the major thrombin receptor on mouse platelets. We determined the effect of combining a deficiency PAR-4 (primary hemostasis) with a deficiency in TF (secondary hemostasis) on embryonic development and survival of adult mice.

MATERIALS AND METHODS

Low-TF mice (mTF^-/-, hTF^+/+) were crossed with PAR-4^-/- mice to generate heterozygous mice (mTF^+/-, hTF^+/-, PAR-4^+/-). These mice were intercrossed to generate Low-TF mice lacking PAR-4. Mice surviving to wean were genotyped and survival was monitored for 6months.

RESULTS

We observed the expected number of Low-TF,PAR-4^-/- mice at wean indicating survival in utero and after birth. However, an absence of PAR-4 was associated with premature death of all Low-TF,PAR-4^-/- mice in the 6month observational period. This compares with 40% death of the Low-TF,PAR-4^+/+ mice (p=0.003). Low-TF,PAR-4^+/- mice had an intermediate phenotype with 55% of the mice dying within 6months. The primary cause of mortality of Low-TF,PAR-4^-/- mice was pulmonary hemorrhage.

CONCLUSIONS

Low-TF,PAR-4^-/- mice survive into adulthood, but combining a deficiency of primary hemostasis (PAR-4 deficiency) with secondary hemostasis (low levels of TF) leads to premature death primarily due to pulmonary hemorrhage.

Protective and detrimental effects of neuroectodermal cell-derived tissue factor in mouse models of stroke

Within the CNS, a dysregulated hemostatic response contributes to both hemorrhagic and ischemic strokes. Tissue factor (TF), the primary initiator of the extrinsic coagulation cascade, plays an essential role in hemostasis and also contributes to thrombosis. Using both genetic and pharmacologic approaches, we characterized the contribution of neuroectodermal (NE) cell TF to the pathophysiology of stroke. We used mice with various levels of TF expression and found that astrocyte TF activity reduced to ~5% of WT levels was still sufficient to maintain hemostasis after hemorrhagic stroke but was also low enough to attenuate inflammation, reduce damage to the blood-brain barrier, and improve outcomes following ischemic stroke. Pharmacologic inhibition of TF during the reperfusion phase of ischemic stroke attenuated neuronal damage, improved behavioral deficit, and prevented mortality of mice. Our data demonstrate that NE cell TF limits bleeding complications associated with the transition from ischemic to hemorrhagic stroke and also contributes to the reperfusion injury after ischemic stroke. The high level of TF expression in the CNS is likely the result of selective pressure to limit intracerebral hemorrhage (ICH) after traumatic brain injury but, in the modern era, poses the additional risk of increased ischemia-reperfusion injury after ischemic stroke.

Limiting prothrombin activation to meizothrombin is compatible with survival but significantly alters hemostasis in mice

Thrombin-mediated proteolysis is central to hemostatic function but also plays a prominent role in multiple disease processes. The proteolytic conversion of fII to α-thrombin (fIIa) by the prothrombinase complex occurs through 2 parallel pathways: (1) the inactive intermediate, prethrombin; or (2) the proteolytically active intermediate, meizothrombin (fIIa(MZ)). FIIa(MZ) has distinct catalytic properties relative to fIIa, including diminished fibrinogen cleavage and increased protein C activation. Thus, fII activation may differentially influence hemostasis and disease depending on the pathway of activation. To determine the in vivo physiologic and pathologic consequences of restricting thrombin generation to fIIa(MZ), mutations were introduced into the endogenous fII gene, resulting in expression of prothrombin carrying 3 amino acid substitutions (R157A, R268A, and K281A) to limit activation events to yield only fIIa(MZ) Homozygous fII(MZ) mice are viable, express fII levels comparable with fII(WT) mice, and have reproductive success. Although in vitro studies revealed delayed generation of fIIa(MZ) enzyme activity, platelet aggregation by fII(MZ) is similar to fII(WT) Consistent with prior analyses of human fIIa(MZ), significant prolongation of clotting times was observed for fII(MZ) plasma. Adult fII(MZ) animals displayed significantly compromised hemostasis in tail bleeding assays, but did not demonstrate overt bleeding. More notably, fII(MZ) mice had 2 significant phenotypic advantages over fII(WT) animals: protection from occlusive thrombosis after arterial injury and markedly diminished metastatic potential in a setting of experimental tumor metastasis to the lung. Thus, these novel animals will provide a valuable tool to assess the role of both fIIa and fIIa(MZ) in vivo.

Effects of angiotensin II on transient receptor potential melastatin 7 channel function in cardiac fibroblasts

The aim of the present study was to investigate the electrophysiological properties of transient receptor potential melastatin 7 (TRPM7) channels in the cell membranes of cardiac fibroblasts (CFs). CFs obtained from Sprague-Dawley rats were treated with six different concentrations of angiotensin (Ang) II for 12 h. The optimum concentration of Ang II for inducing fibrosis was selected by measuring collagen protein expression levels at each concentration. The optimum concentration of Ang II was then used to treat the CFs at five different time-points, and the TRPM7 current across the CF membranes was recorded along with the collagen protein levels in each group. The optimum Ang II concentration for inducing fibrosis was found to be 1 nmol/l; when it was administered to CFs at 0, 6, 12, 24 and 48 h, the protein expression levels of TRPM7 and collagen III initially increased, reaching maximum levels at 12 h, and then decreased. This result positively correlated with the TRPM7 current levels on either side of the CF membranes: The TRPM7 current under the effect of the optimal Ang II concentration for inducing fibrosis was initially increased and then later decreased. This is one of the key mechanisms by which Ang II mediates the proliferation and apoptosis of CFs. This knowledge may be used to identify treatments for the prevention of cardiac fibrosis.

Protective and pathological roles of tissue factor in the heart

Tissue factor (TF) is expressed in the heart where it is required for haemostasis. High levels of TF are also expressed in atherosclerotic plaques and likely contribute to atherothrombosis after plaque rupture. Indeed, risk factors for atherothrombosis, such as diabetes, hypercholesterolaemia, smoking and hypertension, are associated with increased TF expression in circulating monocytes, microparticles and plasma. Several therapies that reduce atherothrombosis, such as statins, ACE inhibitors, beta-blockers and anti-platelet drugs, are associated with reduced TF expression. In addition to its haemostatic and pro-thrombotic functions, the TF : FVIIa complex and downstream coagulation proteases activate cells by cleavage of protease-activated receptors (PARs). In mice, deficiencies in either PAR-1 or PAR-2 reduce cardiac remodelling and heart failure after ischaemia-reperfusion injury. This suggests that inhibition of coagulation proteases and PARs may be protective in heart attack patients. In contrast, the TF/thrombin/PAR-1 pathway is beneficial in a mouse model of Coxsackievirus B3-induced viral myocarditis. We found that stimulation of PAR-1 increases the innate immune response by enhancing TLR3-dependent IFN-β expression. Therefore, inhibition of the TF/thrombin/PAR-1 pathway in patients with viral myocarditis could have detrimental effects.

CONCLUSION

The TF : FVIIa complex has both protective and pathological roles in the heart.

Coagulation in liver toxicity and disease: role of hepatocyte tissue factor

The liver is the primary source of a number of circulating coagulation factors, and acute liver injury and chronic liver disease are each associated with alterations in blood coagulation. Current views of the connection between liver injury and coagulation extend beyond the impact of liver disease on synthesis of coagulation factors to include a role for coagulation factor activity in the initiation and progression of liver disease. Mechanisms of coagulation initiation in liver disease are not completely understood. Compared to other tissues, liver expresses very low levels of tissue factor (TF). Recent studies indicate that expression of TF by hepatocytes comprises the majority of liver procoagulant activity, and that hepatocyte TF activates coagulation induced by liver injury. This review will briefly cover the expression and regulation of TF by hepatocytes, the role of TF in coagulation triggered by liver toxicity, and the contribution of coagulation activity to the progression of liver disease.

Tissue factor, protease activated receptors and pathologic heart remodelling

Tissue factor is the primary initiator of coagulation cascade and plays an essential role in haemostasis and thrombosis. In addition, tissue factor and coagulation proteases contribute to many cellular responses via activation of protease activated receptors. The heart is an organ with high levels of constitutive tissue factor expression. This review focuses on the role of tissue factor, coagulation proteases and protease activated receptors in heart haemostasis and the pathological heart remodelling associated with myocardial infarction, viral myocarditis and hypertension.

Alternatively spliced tissue factor is not sufficient for embryonic development

Tissue factor (TF) triggers blood coagulation and is translated from two mRNA splice isoforms, encoding membrane-anchored full-length TF (flTF) and soluble alternatively-spliced TF (asTF). The complete knockout of TF in mice causes embryonic lethality associated with failure of the yolk sac vasculature. Although asTF plays roles in postnatal angiogenesis, it is unknown whether it activates coagulation sufficiently or makes previously unrecognized contributions to sustaining integrity of embryonic yolk sac vessels. Using gene knock-in into the mouse TF locus, homozygous asTF knock-in (asTFKI) mice, which express murine asTF in the absence of flTF, exhibited embryonic lethality between day 9.5 and 10.5. Day 9.5 homozygous asTFKI embryos expressed asTF protein, but no procoagulant activity was detectable in a plasma clotting assay. Although the α-smooth-muscle-actin positive mesodermal layer as well as blood islands developed similarly in day 8.5 wild-type or homozygous asTFKI embryos, erythrocytes were progressively lost from disintegrating yolk sac vessels of asTFKI embryos by day 10.5. These data show that in the absence of flTF, asTF expressed during embryonic development has no measurable procoagulant activity, does not support embryonic vessel stability by non-coagulant mechanisms, and fails to maintain a functional vasculature and embryonic survival.

Protease activated receptor-2 contributes to heart failure

Heart failure is a major clinical problem worldwide. Previous studies have demonstrated an important role for G protein-coupled receptors, including protease-activated receptors (PARs), in the pathology of heart hypertrophy and failure. Activation of PAR-2 on cardiomyocytes has been shown to induce hypertrophic growth in vitro. PAR-2 also contributes to myocardial infarction and heart remodeling after ischemia/reperfusion injury. In this study, we found that PAR-2 induced hypertrophic growth of cultured rat neonatal cardiomyocytes in a MEK1/2 and p38 dependent manner. In addition, PAR-2 activation on mouse cardiomyocytes increased expression of the pro-fibrotic chemokine MCP-1. Furthermore, cardiomyocyte-specific overexpression of PAR-2 in mice induced heart hypertrophy, cardiac fibrosis, inflammation and heart failure. Finally, in a mouse model of myocardial infarction induced by permanent ligation of the left anterior descending coronary artery, PAR-2 deficiency attenuated heart remodeling and improved heart function independently of its contribution to the size of the initial infarct. Taken together, our data indicate that PAR-2 signaling contributes to the pathogenesis of hypertrophy and heart failure.

Coagulation and coagulation signalling in fibrosis

Following tissue injury, a complex and coordinated wound healing response comprising coagulation, inflammation, fibroproliferation and tissue remodelling has evolved to nullify the impact of the original insult and reinstate the normal physiological function of the affected organ. Tissue fibrosis is thought to result from a dysregulated wound healing response as a result of continual local injury or impaired control mechanisms. Although the initial insult is highly variable for different organs, in most cases, uncontrolled or sustained activation of mesenchymal cells into highly synthetic myofibroblasts leads to the excessive deposition of extracellular matrix proteins and eventually loss of tissue function. Coagulation was originally thought to be an acute and transient response to tissue injury, responsible primarily for promoting haemostasis by initiating the formation of fibrin plugs to enmesh activated platelets within the walls of damaged blood vessels. However, the last 20years has seen a major re-evaluation of the role of the coagulation cascade following tissue injury and there is now mounting evidence that coagulation plays a critical role in orchestrating subsequent inflammatory and fibroproliferative responses during normal wound healing, as well as in a range of pathological contexts across all major organ systems. This review summarises our current understanding of the role of coagulation and coagulation initiated signalling in the response to tissue injury, as well as the contribution of uncontrolled coagulation to fibrosis of the lung, liver, kidney and heart. This article is part of a Special Issue entitled: Fibrosis: Translation of basic research to human disease.

Alternatively spliced tissue factor and full-length tissue factor protect cardiomyocytes against TNF-α-induced apoptosis

Tissue Factor (TF) is expressed in various cell types of the heart, such as cardiomyocytes. In addition to its role in the initiation of blood coagulation, the TF:FVIIa complex protects cells from apoptosis. There are two isoforms of Tissue Factor (TF): "full length" (fl)TF--an integral membrane protein, and alternatively spliced (as)TF--a protein that lacks a transmembrane domain and can thus be secreted in a soluble form. Whether asTF or flTF affects apoptosis of cardiomyocytes is unknown. In this study, we examined whether asTF or flTF protects murine cardiomyocytes from TNF-α-induced apoptosis. We used murine cardiomyocytic HL-1 cells and primary murine embryonic cardiomyocytes that overexpressed either murine asTF or murine flTF, and stimulated them with TNF-α to initiate cell death. Apoptosis was assessed by annexin-V assay, propidium iodide assay, as well as activation of caspase-3 and -9. In addition, signaling via integrins, Akt, NFκB and Erk1/2, and gene-expression of Bcl-2 family members were analyzed. We here report that overexpression of asTF reduced phosphatidylserine exposure upon TNF-α-stimulation. asTF overexpression led to an increased expression and phosphorylation of Akt, as well as up-regulation of the anti-apoptotic protein Bcl-x(L). The anti-apoptotic effects of asTF overexpression were mediated via α(V)β(3)/Akt/NFκB signaling and were dependent on Bcl-x(L) expression in HL-1 cells. The anti-apoptotic activity of asTF was also observed using primary cardiomyocytes. Analogous yet less pronounced anti-apoptotic sequelae were observed due to overexpression of flTF. Importantly, cardiomyocytes deficient in TF exhibited increased apoptosis compared to wild type cells. We propose that asTF and flTF protect cardiomyocytes against TNF-α-induced apoptosis via activation of specific signaling pathways, and up-regulation of anti-apoptotic members of the Bcl-2 protein family.

Animal models of hemophilia

The X-linked bleeding disorder hemophilia is caused by mutations in coagulation factor VIII (hemophilia A) or factor IX (hemophilia B). Unless prophylactic treatment is provided, patients with severe disease (less than 1% clotting activity) typically experience frequent spontaneous bleeds. Current treatment is largely based on intravenous infusion of recombinant or plasma-derived coagulation factor concentrate. More effective factor products are being developed. Moreover, gene therapies for sustained correction of hemophilia are showing much promise in preclinical studies and in clinical trials. These advances in molecular medicine heavily depend on availability of well-characterized small and large animal models of hemophilia, primarily hemophilia mice and dogs. Experiments in these animals represent important early and intermediate steps of translational research aimed at development of better and safer treatments for hemophilia, such a protein and gene therapies or immune tolerance protocols. While murine models are excellent for studies of large groups of animals using genetically defined strains, canine models are important for testing scale-up and for long-term follow-up as well as for studies that require larger blood volumes.

Tissue factor, blood coagulation, and beyond: an overview

Emerging evidence shows a broad spectrum of biological functions of tissue factor (TF). TF classical role in initiating the extrinsic blood coagulation and its direct thrombotic action in close relation to cardiovascular risks have long been established. TF overexpression/hypercoagulability often observed in many clinical conditions certainly expands its role in proinflammation, diabetes, obesity, cardiovascular diseases, angiogenesis, tumor metastasis, wound repairs, embryonic development, cell adhesion/migration, innate immunity, infection, pregnancy loss, and many others. This paper broadly covers seminal observations to discuss TF pathogenic roles in relation to diverse disease development or manifestation. Biochemically, extracellular TF signaling interfaced through protease-activated receptors (PARs) elicits cellular activation and inflammatory responses. TF diverse biological roles are associated with either coagulation-dependent or noncoagulation-mediated actions. Apparently, TF hypercoagulability refuels a coagulation-inflammation-thrombosis circuit in “autocrine” or “paracrine” fashions, which triggers a wide spectrum of pathophysiology. Accordingly, TF suppression, anticoagulation, PAR blockade, or general anti-inflammation offers an array of therapeutical benefits for easing diverse pathological conditions.

Life-threatening hemorrhage and prolonged wound healing are remarkable phenotypes manifested by complete plasminogen activator inhibitor-1 deficiency in humans

BACKGROUND

 Plasminogen activator inhibitor-1 (PAI-1) is the primary physiological regulator of urokinase plasminogen activator (uPA) and tissue plasminogen activator (tPA) activity. A number of studies have shown that elevated levels of PAI-1 are related to pathological states such as an increased risk of arterial thrombotic events and a poor prognosis for cancer patients; however, there are few reports about PAI-1 deficiency in humans because the disorder is very rare.

OBJECTIVE

 To understand the in vivo impact of a complete PAI-1 deficiency, Serpine1(-/-) mice were generated; a number of in vivo studies have been conducted to elucidate the function of PAI-1 using Serpine1(-/-) mice. The phenotypes demonstrated in Serpine1(-/-) mice, however, were quite different from those in humans. Therefore, it is necessary to find out and analyze SERPINE1 deficiency in humans.

PATIENT AND METHODS

 The patient is a 47-year-old woman who has had multiple episodes of major bleeding. Although most of the patient's blood coagulation factors were functionally normal, her PAI-1 antigen levels were undetectable. Therefore, DNA sequencing of the SERPINE1 gene were analyzed.

RESULTS

 The proband had a homozygous 1-bp duplication (C) at exon 3 (c.356dupC; p.Ile120AspfsX42). Both wild-type PAI-1 (42.7 kDa) and mutated (Mut) PAI-1 (14.7kDa) were expressed in COS-1 cells, although the level of Mut PAI-1 expressed in the cell lysates was much lower. Wild-type PAI-1 was observed in the culture supernatant, whereas no Mut PAI-1 was detected in the supernatant.

CONCLUSIONS

 Considering the results of the present study, the translation of mouse studies to humans must be performed with great care.

Hemophilic synovitis: factor VII and the potential role of extravascular factor VIIa

The initiation of coagulation via tissue factor/factor VIIa is relatively weak in normal synovium and joint tissues, so that hemophilic patients with additional deficiency of the intrinsic pathway of coagulation are especially at risk for joint bleeding and the development of hemophilic synovitis. The inflamed joint that results from recurrent bleeding-induced injury, however, may be an environment in which pharmacologic doses of factor VIIa have potentially greater procoagulant action than in the uninjured joint. There is accumulating evidence suggesting that coagulation factors in the extravascular space, and not only circulating plasma factors, have the potential to contribute to hemostatic protection of joints. The potential role of extravascular factor VIIa is considered.

Differential induction of cellular proliferation, hypertrophy and apoptosis in H9c2 cardiomyocytes by exogenous tissue factor

Recent evidence has shown that prolonged exposure to exogenous tissue factor (TF) can alter the cellular functions of cardiomyocytes resulting in cardiac dysfunction. The effect of TF may arise from local inflammation within or in the vicinity of the heart. The aim of this study was to investigate the effect of TF on cardiomyocyte proliferation and growth. H9c2 rat cardiomyocytes were exposed to a range of concentrations of recombinant TF (rTF) (1.3-52 ng/ml) for up to 10 days and the outcome on cell proliferation and induction of apoptosis measured. At lower concentrations examined (1.3 ng/ml), rTF had a proliferative influence on the H9c2 cells. In contrast, elevated concentrations of rTF (52 ng/ml) induced cellular apoptosis as indicated by increased caspase-3 activity and nuclear localisation of p53. Moreover, incubation with intermediate concentrations of rTF (13 ng/ml) resulted in an initial increase in proliferation but subsequently, led to cellular apoptosis by day 7 of the incubation. In order to determine if these effects induced hypertrophic cell growth, expression of mechano-growth factor (MGF) was analysed. Incubation of cells with rTF resulted in enhanced expression of MGF particularly at the intermediate concentrations of rTF (13 ng/ml) as well as mean cellular transverse diameter. In addition, there was a rapid increase in the expression of atrial natriuretic factor (ANF) in the cells, on incubation with rTF but diminished rapidly when exposed to higher concentrations of rTF. These data indicate that exposure to increasing concentrations of rTF can accelerate the rate of cardiomyocyte turnover which may ultimately lead to depletion of viable cells within the heart. Moreover, at lower concentrations of rTF, the induction of cell proliferation together with hypertrophic markers indicates that rTF may contribute to the induction and progression of cardiac hypertrophy.

Tissue factor in cancer progression and angiogenesis

Constitutive expression of tissue factor (TF) by cancer cells triggers local and systemic activation of the coagulation cascade and is a major cause of cancer-associated thrombosis. Primary breast cancer biopsies show a marked upregulation of TF and protease activated receptor (PAR) 2, as well as increased TF cytoplasmic domain phosphorylation that is correlated with cancer relapse. TF signaling involving PAR2 and integrins has multiple effects on angiogenesis and tumor progression. The non-coagulant, alternatively spliced form of TF retains an integrin-binding site and, upon deposition into the tumor stroma, stimulates angiogenesis by ligating endothelial integrins alpha(v)beta(3) and alpha(6)beta(1). On tumor cells, full-length TF is constitutively associated with laminin-binding beta(1) integrins that support TF-VIIa-PAR2 signaling leading to upregulation of pro-angiogenic and immune modulatory cytokines and growth factors. Deficiency of PAR2, but not of the thrombin receptor PAR1, delays spontaneous breast cancer development and the angiogenic switch in mice. In addition, human xenograft breast cancer growth and angiogenesis is suppressed by selective antibody inhibition of TF-VIIa-PAR2 signaling, but not by blocking TF initiated coagulation. Thus, interruption of TF signaling represents a potential anti-angiogenic strategy that does not carry an increased risk of bleeding associated with prolonged inhibition of the TF coagulation pathway.

Vascular haemostasis

SUMMARY

While the majority of this session will deal with selected inherited vascular abnormalities that may manifest as a haemorrhagic disorder, the initial discussion by Dr Key will focus on the interplay between the vessel wall and components of the coagulation system, with a focus on haemophilia A and B. Although it is generally accepted that physiological haemostasis is triggered by contact of blood with tissue factor (TF), there remains some controversy regarding the cellular origin of TF in vivo. In addition, the initiation and propagation of thrombin generation are highly dependent on the balance of pro- and anticoagulant functions of endothelium, a profile that varies significantly throughout the vasculature. Drs De Paepe and Malfait address heritable collagen disorders such as the Ehlers-Danlos syndromes (EDS), a heterogeneous group of diseases involving the skin, ligaments and joints, blood vessels and internal organs. Most EDS subtypes are caused by mutations in genes encoding fibrillar collagens, or in genes coding for enzymes involved in posttranslational modifications of collagens. Accurate biochemical and molecular testing is now available for most EDS subtypes and can direct genetic counselling and medical management for these disorders. Dr Shovlin reviews recent developments in hereditary haemorrhagic telengiectasia (HHT), a frequently undiagnosed disorder characterized by arteriovenous malformations in multiple organs. These abnormal blood vessels are the result of mutations in one of a number of genes whose protein products influence TGF-beta signalling in vascular endothelial cells. Several HHT management guidelines have been published and are discussed.

Tissue factor in cardiovascular disease pathophysiology and pharmacological intervention

Tissue factor (TF) is the major trigger of the coagulation cascade and thereby crucially involved in the maintenance of vascular hemostasis. By binding factor VIIa, the resulting TF:VIIa complex activates the coagulation factors IX and X ultimately leading to fibrin and clot formation. In the vessel wall, TF expression and activity is detectable in vascular smooth muscle cells and fibroblasts and, at a much lower level, in endothelial cells and can be induced by various stimuli including cytokines. In addition, TF is found in the bloodstream in circulating cells such as monocytes, in TF containing microparticles, and as a soluble splicing isoform. Besides its well-known extracellular role as a trigger of coagulation, TF also functions as a transmembrane receptor, and TF-dependent intracellular signaling events regulate the expression of genes involved in cellular responses such as proliferation and migration. TF indeed appears to be involved in the pathogenesis of neointima formation and tumor growth, and increased levels of TF have been detected in patients with cardiovascular risk factors or coronary artery disease as well as in those with cancer. Therefore, pharmacological or genetic inhibition of TF may be an attractive target for the treatment of cardiovascular disease and cancer. Different strategies for inhibition of TF have been developed such as inhibition of TF synthesis and blockade of TF action. Clinical applications of such strategies need to be tested in appropriate trials, in particular for evaluating the advantages of targeted versus systemic delivery of the inhibitors.

Plasminogen activator inhibitor-1 (PAI-1) is cardioprotective in mice by maintaining microvascular integrity and cardiac architecture

Although the involvement of plasminogen activator inhibitor-1 (PAI-1) in fibrotic diseases is well documented, its role in cardiac fibrosis remains controversial. The goal of this study was to determine the effect of a PAI-1 deficiency (PAI-1(-/-)) on the spontaneous development of cardiac fibrosis. PAI-1(-/-) mice developed pervasive cardiac fibrosis spontaneously with aging, and these mice displayed progressively distorted cardiac architecture and markedly reduced cardiac function. To mechanistically elucidate the role of PAI-1 in cardiac fibrosis, 12-week-old mice were chosen to study the biologic events leading to fibrosis. Although fibrosis was not observed at this early age, PAI-1(-/-) hearts presented with enhanced inflammation, along with increased microvascular permeability and hemorrhage. A potent fibrogenic cytokine, transforming growth factor-beta (TGF-beta), was markedly enhanced in PAI-1(-/-) heart tissue. Furthermore, the expression levels of several relevant proteases associated with tissue remodeling were significantly enhanced in PAI-1(-/-) hearts. These results suggest that PAI-1 is cardioprotective, and functions in maintaining normal microvasculature integrity. Microvascular leakage in PAI-1(-/-) hearts may provoke inflammation, and predispose these mice to cardiac fibrosis. Therefore, a PAI-1 deficiency contributes to the development of cardiac fibrosis by increasing vascular permeability, exacerbating local inflammation, and increasing extracellular matrix remodeling, an environment conducive to accelerated fibrosis.

A modified thrombin generation test for investigating very low levels of factor VIII activity in hemophilia A

Discrepancies between low levels of FVIII:C and clinical symptoms in severe hemophilia A are well-known. We have recently demonstrated that levels of FVIII:C < 0.2 IU/dl were consistent with clinical phenotype by clot waveform analysis, suggesting that precise measurement of very low levels of FVIII:C was clinically important. Thrombin generation tests (TGTs) triggered by tissue factor (TF) have been recently utilized to monitor coagulation function in hemophilia A. We examined whether TGT was useful for evaluating hemophilia A patients with very low levels of FVIII:C. TGTs in 40 hemophilia A plasmas with FVIII:C < 0.2-17 IU/dl (measured by clot waveform analysis using MDA-II) were performed using TF and/or ellagic acid (ELG). The lagtime in ELG-TGT at very low levels of FVIII:C was shortened dose-dependently, whilst this parameter in TF-TGT was not significantly affected. Other parameters (endogenous thrombin potential, peak thrombin, time to peak) correlated with FVIII:C levels to some extent in both assays (r = 0.4-0.7). Using a TF/ELG mixture in TGT, however, the correlation coefficients increased to ~0.85. TGT parameters correlated well with levels of FVIII:C > 0.2 IU/dl, although the lagtime was not especially informative. We conclude that modified TGT, using a TF/ELG mixture as the trigger, is useful for monitoring coagulation function at very low levels of FVIII:C in hemophilia A.

The role of tissue factor and tissue factor pathway inhibitor in health and disease states

OBJECTIVE

To review the veterinary and human literature on the role of tissue factor (TF) and tissue factor pathway inhibitor (TFPI) in health and disease states.

DATA SOURCES

Original research articles and scientific reviews from both human and veterinary literature were searched for relevance to TF and TFPI.

HUMAN DATA SYNTHESIS

Interest in both TF and TFPI has grown widely over the last several years. The impact TF plays in coagulation, inflammation, angiogenesis, tumor metastasis, and cellular signaling has become apparent. Treatment with TFPI for severe sepsis has been examined and is still currently under investigation. Inhibition of the TF pathway is being studied as an aid in the treatment of neoplasia. The important physiologic and pathophysiologic role these molecules play has only begun to be understood.

VETERINARY DATA SYNTHESIS

There is a paucity of publications that discuss the importance of TF and TFPI in veterinary medicine. An enhanced understanding of the TF pathway in human medicine, in experimental animal models treating sepsis with TFPI, and in animal models demonstrating the proangiogenic properties of TF provides relevance to veterinary medicine.

CONCLUSION

It is apparent that TF and TFPI are important in health and disease. An enhanced understanding of the physiologic and pathophysiologic roles of these factors provides better insight into coagulation, inflammation, angiogenesis, disseminated intravascular coagulation, and tumor metastasis. This greater understanding may provide for the development of therapeutics for sepsis, disseminated intravascular coagulation, and neoplasia.

The role of tissue factor and factor VIIa in hemostasis

Tissue factor (TF) is a transmembrane receptor for Factor VII/VIIa (FVII/VIIa). It is constitutively expressed by cells surrounding blood vessels. The endothelium physically separates this potent "activator" from its circulating ligand FVII/FVIIa and prevents inappropriate activation of the clotting cascade. Breakage of the endothelial barrier leads to exposure of extravascular TF and rapid activation of the clotting cascade. TF is also expressed in certain tissues, such as the heart and brain, and provides additional hemostatic protection to these tissues. Small amounts of TF are also present in blood in the form of microparticles, which are small membrane vesicles derived from activated and apoptotic cells. Levels of microparticle TF increase in a variety of diseases, such as sepsis and cancer, and this so-called "blood-borne" TF may contribute to thrombosis associated with these diseases. Recombinant FVIIa has been developed as an effective hemostatic drug for the treatment of hemophilia patients with inhibitory antibodies. In addition, it is used for patients with bleeding that do not respond to conventional therapy. However, the mechanism by which recombinant FVIIa restores hemostasis has not been clearly defined. In conclusion, the TF:FVIIa complex is essential for hemostasis and recombinant FVIIa is an effective hemostatic drug.

Severe deficiency of coagulation Factor VII results in spontaneous cardiac fibrosis in mice

Mice genetically modified to produce low levels (approximately 1% of wild-type) of coagulation FVII presented with echocardiographic evidence of heart abnormalities. Decreases in ventricular size and reductions in systolic and diastolic functions were found, suggestive of a restrictive cardiomyopathy and consistent with an infiltrative myopathic process. Microscopic analysis of mouse hearts showed severe patchy fibrosis in the low-FVII mice. Haemosiderin deposition was discovered in hearts of these mice, along with increases in inflammatory cell number, ultimately resulting in widespread collagen deposition. Significant increases in mRNA levels of TGFbeta, TNFalpha and several matrix metalloproteinases in low-FVII mice, beginning at early ages, supported a state of cardiac remodelling associated with the fibrotic pathology. Mechanistic time-course studies suggested that cardiac fibrosis in low-FVII mice originated from bleeding in heart tissue, resulting in the recruitment of leukocytes, which released inflammatory mediators and induced collagen synthesis and secretion. These events led to necrosis of cardiomyocytes and collagen deposition, characteristics of cardiac fibrosis. The results of this study demonstrated that haemorrhagic and inflammatory responses to a severe FVII deficiency resulted in the development of cardiac fibrosis, observed echocardiographically as a restrictive cardiomyopathy, with compromised ventricular diastolic and systolic functions.

Vascular smooth muscle-derived tissue factor is critical for arterial thrombosis after ferric chloride-induced injury

Tissue factor (TF) initiates coagulation, regulates hemostasis, and plays a critical role in mediating arterial thrombosis. TF is up-regulated in vascular smooth muscle cells (VSMCs) in atherosclerosis and arterial injury. To examine the biologic role of VSMC-derived TF, we crossed TF(flox/flox) mice with SM22alphaCre(+/-) mice. TF mRNA and activity were decreased in the aortic media of TF-deficient mice by 96% and 94.8%, respectively. There were no differences in TF activity measured in plasma or concentrated microparticles. TF-deficient mice were generated with the expected frequency, showed no evidence of bleeding or increased mortality, and had similar activated partial thromboplastin and tail vein bleeding times. Thrombus-mediated flow reduction in response to ferric chloride injury of the carotid arteries was significantly attenuated in VSMC-specific TF-deficient. Stable occlusion was seen in 11 of 12 wild-type mice, but in only 6 of 16 VSMC-specific TF-deficient mice (P = .001). These data suggest that VSMC-derived TF is critical in a macrovascular model of arterial thrombosis. This mouse model should be valuable in determining the contribution of VSMC-derived TF in other TF-mediated phenomena, such as restenosis.

Genetic elimination of prothrombin in adult mice is not compatible with survival and results in spontaneous hemorrhagic events in both heart and brain

Mice carrying a conditional prothrombin knockout allele (fII(lox)) were established to develop an experimental setting for exploring the importance of thrombin in the maintenance of vascular integrity, the inflammatory response, and disease processes in adult animals. In the absence of Cre-mediated recombination, homozygous fII(lox/lox) mice or compound heterozygous mice carrying one fII(lox) allele and one constitutive-null allele were viable. Young adults exhibited neither spontaneous bleeding events nor diminished reproductive success. However, the induction of Cre recombinase in fII(lox) mice using the poly I:C-inducible Mx1-Cre system resulted in the rapid and near-complete recombination of the fII(lox) allele within the liver, the loss of circulating prothrombin, and profound derangements in coagulation function. Consistent with the notion that thrombin regulates coagulation and inflammatory pathways, an additional early consequence of reducing prothrombin was impaired antimicrobial function in mice challenged with Staphylococcus aureus peritonitis. However, life expectancy in unchallenged adults genetically depleted of prothrombin was very short ( approximately 5-7 days). The loss of viability was associated with the development of severe hemorrhagic events within multiple tissues, particularly in the heart and brain. Unlike the constitutive loss of either clotting or platelet function alone, the conditional loss of prothrombin is uniformly not compatible with maintenance of hemostasis or long-term survival.

Downregulation of tissue factor (TF) by RNA interference induces apoptosis and impairs cell survival of primary endothelium and tumor cells

Tissue factor (TF) has been implicated in the thrombotic complications seen during vascular rejection of allografts and may contribute to intimal hyperplasia in chronic allograft vasculopathy. Downregulation of endothelial TF expression post-transplantation could therefore be of therapeutic value. Lentivirus-mediated RNA interference was used in primary endothelial cells (EC) to investigate its effects on TF protein expression and functional activity. Lentivirus-mediated expression of a TF-specific short-interfering (si) RNA with green fluorescent protein as a reporter gene (siRNATF-GFP) resulted in a 42 +/- 3.9% reduction in EC surface-expressed TF as compared with cells expressing a scrambled siRNATF sequence (P = 0.025). The TF content in EC lysates was reduced from 6.85 +/- 1.99 ng to 3.05 +/- 0.82 ng (P = 0.006). Factor X (FX) activation was not impaired on the apical EC surface. The subendothelial matrix of ECs with low TF expression showed significantly reduced TF activity compared with non-transduced cells or with cells harboring the empty vector. ECs expressing siRNATF-GFP exhibited reduced reporter gene (GFP) expression and cell density and an altered morphology. Transfection of control cells with high (J82 cells) or low (MiaPaCa-2 cells) TF expression with siRNATF oligonucleotides caused apoptosis of the J82 but not of the MiaPaCa-2 cells. Thus, lentivirus-mediated RNA interference reduces the TF expression of activated ECs but does not affect FX activation by TF/FVIIa expressed on the apical surface. The downregulation has nevertheless substantial negative effects on the viability of ECs and TF-expressing control cells. These findings imply that certain levels of TF are required for the maintained viability and growth of endothelium and TF-expressing tumor cells.