Tissue factor in coagulation: Which? Where? When?

Specific tissue factor delivery using a tumor-homing peptide for inducing tumor infarction

Targeting the human blood coagulation-inducing protein tissue factor (TF) to the tumor vasculature to induce infarction and disrupt the blood vessels has proven to be an effective approach for tumor therapy. In this study, we investigated the thrombogenic activity and anti-tumor potential of a novel fusion protein (tTF-CREKA) comprising the extracellular domain of human tissue factor (truncated TF, tTF) and a tumor targeting pentapeptide, Cys-Arg-Glu-Lys-Ala (CREKA). tTF is soluble and inactive in its free state, but when it is targeted to the plasma membrane of both tumor vessel endothelial cells and stromal cells by the CREKA peptide, its native coagulation-inducing activity is restored. Systemic administration of the tTF-CREKA fusion protein into tumor-bearing mice induced tumor-selective intravascular thrombosis and reduced tumor blood perfusion, consequently inhibiting tumor growth. The development of tTF-CREKA introduces a new method for treating a wide spectrum of solid tumors by selectively blocking tumor blood supply.

Synthesis of Phosphatidylserine and Its Stereoisomers: Their Role in Activation of Blood Coagulation

Natural phosphatidylserine (PS), which contains two chiral centers, enhances blood coagulation. However, the process by which PS enhanced blood coagulation is not completely understood. An efficient and flexible synthetic route has been developed to synthesize all of the possible stereoisomers of PS. In this study, we examined the role of PS chiral centers in modulating the activity of the tissue factor (TF)-factor VIIa coagulation initiation complex. Full length TF was relipidated with phosphatidylcholine, and the synthesized PS isomers were individually used to estimate the procoagulant activity of the TF-FVIIa complex via a FXa generation assay. The results revealed that the initiation complex activity was stereoselective and had increased sensitivity to the configuration of the PS glycerol backbone due to optimal protein-lipid interactions.

Thrombosis in Cerebral Aneurysms and the Computational Modeling Thereof: A Review

Thrombosis is a condition closely related to cerebral aneurysms and controlled thrombosis is the main purpose of endovascular embolization treatment. The mechanisms governing thrombus initiation and evolution in cerebral aneurysms have not been fully elucidated and this presents challenges for interventional planning. Significant effort has been directed towards developing computational methods aimed at streamlining the interventional planning process for unruptured cerebral aneurysm treatment. Included in these methods are computational models of thrombus development following endovascular device placement. The main challenge with developing computational models for thrombosis in disease cases is that there exists a wide body of literature that addresses various aspects of the clotting process, but it may not be obvious what information is of direct consequence for what modeling purpose (e.g., for understanding the effect of endovascular therapies). The aim of this review is to present the information so it will be of benefit to the community attempting to model cerebral aneurysm thrombosis for interventional planning purposes, in a simplified yet appropriate manner. The paper begins by explaining current understanding of physiological coagulation and highlights the documented distinctions between the physiological process and cerebral aneurysm thrombosis. Clinical observations of thrombosis following endovascular device placement are then presented. This is followed by a section detailing the demands placed on computational models developed for interventional planning. Finally, existing computational models of thrombosis are presented. This last section begins with description and discussion of physiological computational clotting models, as they are of immense value in understanding how to construct a general computational model of clotting. This is then followed by a review of computational models of clotting in cerebral aneurysms, specifically. Even though some progress has been made towards computational predictions of thrombosis following device placement in cerebral aneurysms, many gaps still remain. Answering the key questions will require the combined efforts of the clinical, experimental and computational communities.

Improving the understanding of plasma kallikrein contribution to arterial thrombus formation using two plant protease inhibitors

The purpose of antithrombotic therapy is the prevention of thrombus formation and/or its extension with a minimum risk of bleeding. The inhibition of a variety of proteolytic processes, particularly those of the coagulation cascade, has been reported as a property of plant protease inhibitors. The role of trypsin inhibitors (TIs) from Delonix regia (Dr) and Acacia schweinfurthii (As), members of the Kunitz family of protease inhibitors, was investigated on blood coagulation, platelet aggregation, and thrombus formation. Different from Acacia schweinfurthii trypsin inhibitor (AsTI), Delonix regia trypsin inhibitor (DrTI) is a potent inhibitor of FXIa with a K_iapp of 1.3 × 10^-9 M. In vitro, both inhibitors at 100 µg corresponding to the concentrations of 21 μM and 15.4 μM of DrTI and AsTI, respectively, increased approximately 2.0 times the activated partial thromboplastin time (aPTT) in human plasma compared to the control, likely due to the inhibition of human plasma kallikrein (huPK) or activated factor XI (FXIa), in the case of DrTI. Investigating in vivo models of arterial thrombus formation and bleeding time, DrTI and AsTI, 1.3 µM and 0.96 µM, respectively, prolonged approximately 50% the time for total carotid artery occlusion in mice compared to the control. In contrast to heparin, the bleeding time in mice treated with the two inhibitors did not differ from that of the control group. DrTI and AsTI inhibited 49.3% and 63.8%, respectively, ex vivo murine platelet aggregation induced by adenosine diphosphate (ADP), indicating that these protein inhibitors prevent arterial thrombus formation possibly by interfering with the plasma kallikrein (PK) proteolytic action on the intrinsic coagulation pathway and its ability to enhance the platelet aggregation activity on the intravascular compartment leading to the improvement of a thrombus.

Endosomal NADPH-oxidase is critical for induction of the tissue factor gene in monocytes and endothelial cells

Antiphospholipid antibodies (aPL) have been shown to induce tissue factor (TF) expression in monocytes and endothelial cells. However, the underlying signal transduction has been more or less elusive in the past. We have recently shown that aPL enter the lysosomal route in monocytes and dendritic cells, and subsequently activate endosomal NADPH-oxidase (NOX). The generation of superoxide which is dismutated to hydrogen peroxide upregulates the intracellular toll like receptors (TLR) 7 and 8, and leads to robust production of inflammatory cytokines. Here we show that induction of TF by aPL follows the same signaling pathway. Inhibition of endosomal NOX by the anion channel blocker niflumic acid or capture of superoxide by the radical scavenger N-acetylcysteine blocks TF induction by aPL. Furthermore, monocytes from mice deficient in NOX2 do not increase TF surface expression in response to aPL, while cells from mice deficient in glutathione peroxidase- 1 (GPx-1) show an increased response. Unexpectedly, also induction of TF by tumour necrosis factor (TNF)⍺ and lipopolysaccharide (LPS) was strongly dependent on the activation of endosomal NOX. While TNF⍺ apparently depends almost fully on endosomal NOX, signalling of LPS is only partially dependent on this pathway. These data provide further insight into the well-known role of reactive oxygen species in the induction of TF expression and suggest that endosomal signalling may represent a central coordinating point in this process.

Recombinant human factor VIIa (rFVIIa) in hemophilia: mode of action and evidence to date

Recombinant activated factor VII (rFVIIa) is a bypassing agent widely used both in the treatment and prevention of hemorrhagic complications due to hemophilia with inhibitor. In such cases, antihemophilic factors cannot be used. The normal physiology of factor VII/ factor VIIa (FVII/FVIIa) in the hemostatic process requires the presence of tissue factor (TF) that links to FVII leading to a FVIIa-TF complex which activates both factor X and factor IX. The therapeutic use of rFVIIa requires high amount of FVIIa. Some studies demonstrate that FVIIa at high doses still requires tissue factor for function, whereas others suggest that FVIIa activates FX directly on the platelet surface, in a TF-independent manner. In the present article, we discuss the arguments supporting both TF-dependent and TF-independent modes of action. Finally, the coexistence of both TF-dependent and TF-independent mechanisms cannot be excluded.

Grape intake reduces thrombin generation and enhances plasma fibrinolysis. Potential role of circulating procoagulant microparticles

Phytochemicals contained in grapes down-regulate several prothrombotic pathways in vitro. We evaluated the effect of grape consumption on coagulation and fibrinolysis in healthy volunteers. Thirty subjects were enrolled: 20 were given grape (5 g/kg body weight/day for 3 weeks), while 10 served as controls. Blood samples were taken at baseline (T0), at the end of the grape diet (T1) and after 4-week wash-out (T2). Grape intake caused a significant decrease of the procoagulant and inflammatory responses of whole blood and/or mononuclear cells to bacterial lipopolysaccharide at both T1 and T2. At plasma level, grape diet decreased thrombin generation at T1 and T2, largely through a reduction in the number and/or activity of procoagulant microparticles. This anticoagulant effect resulted in the formation of clots that were more susceptible to fibrinolysis, mainly because of a lesser activation of thrombin activatable fibrinolysis inhibitor. No difference in any variables was detected in controls at the time points considered. In conclusion, chronic grape consumption induces sustained anticoagulant and profibrinolytic effects with potential benefits for human health.

Structural modulation of factor VIIa by full-length tissue factor (TF1-263): implication of novel interactions between EGF2 domain and TF

Tissue factor (TF)-mediated factor VII (FVII) activation and a subsequent proteolytic TF-FVIIa binary complex formation is the key step initiating the coagulation cascade, with implications in various homeostatic and pathologic scenarios. TF binding allosterically modifies zymogen-like free FVIIa to its highly catalytically active form. As a result of unresolved crystal structure of the full-length TF_1-263-FVIIa binary complex and free FVIIa, allosteric alterations in FVIIa following its binding to full-length TF and the consequences of these on function are not entirely clear. The present study aims to map and identify structural alterations in FVIIa and TF resulting from full-length TF binding to FVIIa and the key events responsible for enhanced FVIIa activity in coagulation. We constructed the full-length TF_1-263-FVIIa membrane bound complex using computational modeling and subjected it to molecular dynamics (MD) simulations. MD simulations showed that TF alters the structure of each domain of FVIIa and these combined alterations contribute to enhanced TF-FVIIa activity. Detailed, domain-wise investigation revealed several new non-covalent interactions between TF and FVIIa that were not found in the truncated soluble TF-FVIIa crystal structure. The structural modulation of each FVIIa domain imparted by TF indicated that both inter and intra-domain communication is crucial for allosteric modulation of FVIIa. Our results suggest that these newly formed interactions can provide additional stability to the protease domain and regulate its activity profile by governing catalytic triad (CT) orientation and localization. The unexplored newly formed interactions between EGF2 and TF provides a possible explanation for TF-induced allosteric activation of FVIIa.

Role of Coagulation Factor Concentrates in the Operating Room

The use of fresh frozen plasma, cryoprecipitate, and platelets has been the mainstay of approaches to correct coagulopathies that can arise in the perioperative setting. Limitations include the time delay from obtaining results of coagulation screens to the availability of thawed fresh frozen plasma and the potential of fluid overload. With advances in both global haemostatic testing and concentrates of coagulation factors, there are increasing opportunities for innovative practice. However, there remains a paucity of studies that can provide good quality, unbiased evidence. These issues are elaborated here to form the basis for future study.

Activated factor IX, factor XI and tissue factor identify patients with permanent atrial fibrillation treated with warfarin who are at risk of ischemic stroke

INTRODUCTION

Previously, we have demonstrated that significant proportions of patients with various cardiovascular diseases have active tissue factor and active factor XIa in their plasma. In the current study, we evaluated active tissue factor and active factors (F)XI and FIX in plasma from patients with atrial fibrillation.

MATERIAL AND METHODS

In 110 consecutive patients with permanent atrial fibrillation receiving warfarin, we determined active tissue factor, together with plasma FIXa and FXIa, using clotting assays by measuring the response to inhibitory monoclonal antibodies.

RESULTS

Sixteen (14.5%) patients had detectable active tissue factor and active FXIa, including 11 subjects with both factors, while FIXa was observed in 28 (25.7%) patients. The three positive groups did not differ from the patients without these factors with regard to demographic and clinical characteristics. Von Willebrand factor was higher in the active tissue factor-positive group (p < 0.0001) and FXIa-positive group (p = 0.0037). Individuals positive for active tissue factor and FXIa had higher plasma interleukin-6 levels (p = 0.0014 and 0.0322, respectively). The presence of active tissue factor, FXIa and FIXa in anticoagulated patients with permanent atrial fibrillation correlated with elevated von Willebrand factor and interleukin-6. During a 3-year follow-up, ischemic stroke (n = 12, 10.9%) occurred more commonly among atrial fibrillation patients who had circulating TF (p = 0.002) or FXIa (p = 0.013).

CONCLUSIONS

These data suggest that circulating active coagulation factors, in particular TF and FXIa, can be detected despite oral anticoagulation in a significant proportion of patients with atrial fibrillation, and could represent novel markers of persistent prothrombotic alterations predisposing to ischemic stroke.

The Role of Putative Phosphatidylserine-Interactive Residues of Tissue Factor on Its Coagulant Activity at the Cell Surface

Exposure of phosphatidylserine (PS) on the outer leaflet of the cell membrane is thought to play a critical role in tissue factor (TF) decryption. Recent molecular dynamics simulation studies suggested that the TF ectodomain may directly interact with PS. To investigate the potential role of TF direct interaction with the cell surface phospholipids on basal TF activity and the enhanced TF activity following the decryption, one or all of the putative PS-interactive residues in the TF ectodomain were mutated and tested for their coagulant activity in cell systems. Out of the 9 selected TF mutants, five of them -TFS160A, TFS161A, TFS162A, TFK165A, and TFD180A- exhibited a similar TF coagulant activity to that of the wild-type TF. The specific activity of three mutants, TFK159A, TFS163A, and TFK166A, was reduced substantially. Mutation of the glycine residue at the position 164 markedly abrogated the TF coagulant activity, resulting in ~90% inhibition. Mutation of all nine lipid binding residues together did not further decrease the activity of TF compared to TFG164A. A similar fold increase in TF activity was observed in wild-type TF and all TF mutants following the treatment of THP-1 cells with either calcium ionomycin or HgCl2, two agents that are commonly used to decrypt TF. Overall, our data show that a few select TF residues that are implicated in interacting with PS contribute to the TF coagulant activity at the cell surface. However, our data also indicate that TF regions outside of the putative lipid binding region may also contribute to PS-dependent decryption of TF.

Embolic Strokes of Unknown Source and Cryptogenic Stroke: Implications in Clinical Practice

Up to a third of strokes are rendered cryptogenic or of undetermined etiology. This number is specifically higher in younger patients. At times, inadequate diagnostic workups, multiple causes, or an under-recognized etiology contributes to this statistic. Embolic stroke of undetermined source, a new clinical entity particularly refers to patients with embolic stroke for whom the etiology of embolism remains unidentified despite through investigations ruling out established cardiac and vascular sources. In this article, we review current classification and discuss important clinical considerations in these patients; highlighting cardiac arrhythmias and structural abnormalities, patent foramen ovale, paradoxical sources, and potentially under-recognized, vascular, inflammatory, autoimmune, and hematologic sources in relation to clinical practice.

Increased tissue factor and thrombomodulin expression and histopathological changes in placentas of pregnancies with preeclampsia

INTRODUCTION

Preeclampsia has a global frequency of 2-8% and a frequency of 10% in developing countries. In Colombia, preeclampsia causes 42% of maternal mortality. Alterations in placental homeostasis have been proposed to be involved in its pathophysiology. The aim of this study was to compare mRNA and protein levels of tissue factor (F3) and thrombomodulin (THBD) and the histopathological findings of placentas.

MATERIALS AND METHODS

We studied 16 placentas from patients with preeclampsia and 19 term placentas with uncomplicated pregnancy. An expert pathologist, who was masked to the group assignment, conducted an evaluation to determine specific histological changes. Assessments of mRNA and protein levels of F3 and THBD were performed using real-time PCR and ELISA, respectively.

RESULTS

Cases and controls differed in the frequency of decidual arteriopathy (p = 0.027), acute infarction (p = 0.001) and hyperplasia of the syncytiotrophoblast (p = 0.0017). Cases had increased levels of F3 mRNA (p = 0.0124) and protein (p <  0.0001) and THBD mRNA (p <  0.0001) and protein (p <  0.0001).

CONCLUSION

In placenta of patients with preeclampsia, we detected abnormal expression of F3 and THBD with increased protein and mRNA levels. The role of these molecules in the pathogenesis of this disease and in alterations of hemostatic and histopathological aspects of placentas need further studying.

Tissue factor-positive tumor microvesicles activate platelets and enhance thrombosis in mice

ESSENTIALS: Cancer patients have a high rate of venous thrombosis (VT) but the underlying mechanisms are unknown. Tumor-derived, tissue factor-positive microvesicles in platelet activation in vitro and in vivo were studied. Tumor-derived, tissue factor-positive microvesicles enhanced VT in mice. Platelets may contribute to VT in some cancer patients, and this could be prevented with antiplatelet drugs.

BACKGROUND

Cancer patients have an approximately 4-fold increased risk of venous thromboembolism (VTE) compared with the general population, and cancer patients with VTE have reduced survival. Tumor cells constitutively release small membrane vesicles called microvesicles (MVs) that may contribute to thrombosis in cancer patients. Clinical studies have shown that levels of circulating tumor-derived, tissue factor-positive (TF(+) ) MVs in pancreatic cancer patients are associated with VTE. Objectives We tested the hypothesis that TF(+) tumor-derived MVs (TMVs) activate platelets in vitro and in mice.

MATERIALS AND METHODS

We selected two human pancreatic adenocarcinoma cell lines expressing high (BxPc-3) and low (L3.6pl) levels of TF as models to study the effect of TF(+) TMVs on platelets and thrombosis.

RESULTS AND CONCLUSIONS

We found that both types of TF(+) TMVs activated human platelets and induced aggregation in vitro in a TF and thrombin-dependent manner. Further, injection of BxPc-3 TF(+) TMVs triggered platelet activation in vivo and enhanced thrombosis in two mouse models of venous thrombosis in a TF-dependent manner. Importantly, BxPc-3 TF(+) TMV-enhanced thrombosis was reduced in Par4-deficient mice and in wild-type mice treated with clopidogrel, suggesting that platelet activation was required for enhanced thrombosis. These studies suggest that TF(+) TMV-induced platelet activation contributes to thrombosis in cancer patients.

New insights into the mechanisms of action of aspirin and its use in the prevention and treatment of arterial and venous thromboembolism

The antithrombotic action of aspirin has long been recognized. Aspirin inhibits platelet function through irreversible inhibition of cyclooxygenase (COX) activity. Until recently, aspirin has been mainly used for primary and secondary prevention of arterial antithrombotic events. The aim of this study was to review the literature with regard to the various mechanisms of the newly discovered effects of aspirin in the prevention of the initiation and development of venous thrombosis. For this purpose, we used relevant data from the latest numerous scientific studies, including review articles, original research articles, double-blinded randomized controlled trials, a prospective combined analysis, a meta-analysis of randomized trials, evidence-based clinical practice guidelines, and multicenter studies. Aspirin is used in the prevention of venous thromboembolism (VTE), especially the prevention of recurrent VTE in patients with unprovoked VTE who were treated with vitamin K antagonists (VKAs) or with non-vitamin K antagonist oral anticoagulants (NOACs). Numerous studies have shown that aspirin reduces the rate of recurrent VTE in patients, following cessation of VKAs or NOACs. Furthermore, low doses of aspirin are suitable for long-term therapy in patients recovering from orthopedic or other surgeries. Aspirin is indicated for the primary and secondary prevention as well as the treatment of cardiovascular diseases, including acute coronary syndrome, myocardial infarction, peripheral artery disease, acute ischemic stroke, and transient ischemic attack (especially in atrial fibrillation or mechanical heart valves). Aspirin can prevent or treat recurrent unprovoked VTEs as well as VTEs occurring after various surgeries or in patients with malignant disease. Recent trials have suggested that the long-term use of low-dose aspirin is effective not only in the prevention and treatment of arterial thrombosis but also in the prevention and treatment of VTE. Compared with VKAs and NOACs, aspirin has a reduced risk of bleeding.

pHLIP-mediated targeting of truncated tissue factor to tumor vessels causes vascular occlusion and impairs tumor growth

Occluding tumor blood supply by delivering the extracellular domain of coagulation-inducing protein tissue factor (truncated tissue factor, tTF) to tumor vasculature has enormous potential to eliminate solid tumors. Yet few of the delivery technologies are moved into clinical practice due to their non-specific tissue biodistribution and rapid clearance by the reticuloendothelial system. Here we introduced a novel tTF delivery method by generating a fusion protein (tTF-pHLIP) consisting of tTF fused with a peptide with a low pH-induced transmembrane structure (pHLIP). This protein targets the acidic tumor vascular endothelium and effectively induces local blood coagulation. tTF-pHLIP, wherein pHLIP is cleverly designed to mimic the natural tissue factor transmembrane domain, triggered thrombogenic activity of the tTF by locating it to the endothelial cell surface, as demonstrated by coagulation assays and confocal microscopy. Systemic administration of tTF-pHLIP into tumor-bearing mice selectively induced thrombotic occlusion of tumor vessels, reducing tumor perfusion and impairing tumor growth without overt side effects. Our work introduces a promising strategy for using tTF as an anti-cancer drug, which has great potential value for clinical applications.

Why does aspirin decrease the risk of venous thromboembolism? On old and novel antithrombotic effects of acetyl salicylic acid

It is well established that aspirin, an irreversible inhibitor of platelet cyclooxygenase activity, is effective in secondary prevention of arterial thromboembolic events. The pooled results of the recent randomized, multicenter WARFASA and ASPIRE aspirin trials showed a 32% reduction in the rate of recurrence of venous thromboembolism (VTE) in patients receiving aspirin following VTE. These clinical data support evidence that platelets contribute to the initiation and progression of venous thrombosis and aspirin inhibits thrombin formation and thrombin-mediated coagulant reactions. In addition to the known acetylation of serine 529 residue in platelet cyclooxygenase-1, the postulated mechanisms of aspirin-induced antithrombotic actions also involve the acetylation of other proteins in blood coagulation, including fibrinogen, resulting in more efficient fibrinolysis. This review summarizes current knowledge on the aspirin-induced antithrombotic effects that potentially explain clinical studies showing reduced rates of VTE events in aspirin-treated subjects.

Regulation of platelet activity in a changing redox environment

SIGNIFICANCE

The regulation of platelet function is finely tuned by a balance between the vasculature's redox environment and the oxidative processes that occur in it. The activation of platelets at sites of vascular damage is essential for the maintenance of normal hemostasis. In the extracellular milieu, a normal redox environment is maintained by thiol/disulfide redox couples, which include reduced and oxidized glutathione (GSH/GSSG) and cysteine (Cys/CySS). Oxidative changes in either of the plasma redox potentials are directly linked with risk factors for cardiovascular disease.

RECENT ADVANCES

Many proteins found on the surface of platelets contain cysteine residues that are targets for oxidation. These include platelet-specific integrins and thiol isomerase enzymes that respond to changes in the extracellular redox environment, thus influencing normal platelet responses.

CRITICAL ISSUES

The post-translational modification of critical cysteine thiol groups is linked to alterations in redox potentials and occurs both intracellularly and extracellularly in normal platelet activation. Platelet integrins, in particular, are prime targets for redox modification due to their high cysteine content. Although the role of thiol/disulfide bond exchange in platelet activation is established, the effects of a changing redox environment on platelet reactivity are unclear.

FUTURE DIRECTIONS

A thorough understanding of these mechanisms and how they interact with other platelet signaling events is of the utmost importance for the development of novel therapeutic targets so that we can protect against inappropriate thrombus formation.

New players in haemostasis and thrombosis

The blood coagulation cascade is essential for haemostasis, but excessive activation can cause thrombosis. Importantly, recent studies have identified factors that contribute to thrombosis but not haemostasis. These include factor XII (FXII), tissue factor-positive microparticles (MPs) and neutrophil extracellular traps (NETs). Studies have shown that FXII plays a role in thrombosis but not haemostasis. FXII is activated in vivo by a variety of negatively-charged polyphosphates, which include extracellular RNA, DNA and inorganic polyphosphate (PolyP) that are released during cell damage and infection. These findings have led to the development of nucleic acid-binding polymers as a new class of anticoagulant drug. Other studies have analysed the role of MPs in experimental thrombosis. MPs are small membrane vesicles released from activated or apoptotic cells. We and others have found that tissue factor-positive MPs enhance thrombosis in mouse models and are elevated in the plasma of pancreatic cancer patients. Finally, NETs have been shown to contribute to experimental venous thrombosis in mouse models and are present in human thrombi. NETs are composed of chromatin fibers that are released from neutrophils undergoing cell death. NETs can capture platelets and increase fibrin deposition. The recent advances in our understanding of the factors contributing to thrombosis in animal models provide new opportunities for the development of safer anticoagulant drugs.

Stroke and Cancer- A Complicated Relationship

The interrelationship between stroke and cancer is complex. Cancer and stroke may occur independently in a given patient, or cancer may directly or indirectly lead to stroke via: hypercoaguability, non-bacterial thrombotic endocarditis (NBTE), direct tumor compression of blood vessels, or treatment-related effects which potentiate stroke. Patients with cryptogenic stroke are relatively common, and under the right circumstances, may provide an opportunity to screen for occult malignancy. In this review, we discuss relevant data linking stroke and cancer as well as propose a testable algorithm for cancer screening in the patient with cryptogenic stroke. Future directions should focus on validating patient-care algorithms in prospective clinical trials to provide an evidence base for this important issue.

Anticoagulant effects of statins and their clinical implications

There is evidence indicating that statins (3-hydroxy-methylglutaryl coenzyme A reductase inhibitors) may produce several cholesterol-independent antithrombotic effects. In this review, we provide an update on the current understanding of the interactions between statins and blood coagulation and their potential relevance to the prevention of venous thromboembolism (VTE). Anticoagulant properties of statins reported in experimental and clinical studies involve decreased tissue factor expression resulting in reduced thrombin generation and attenuation of pro-coagulant reactions catalysed by thrombin, such as fibrinogen cleavage, factor V and factor XIII activation, as well as enhanced endothelial thrombomodulin expression, resulting in increased protein C activation and factor Va inactivation. Observational studies and one randomized trial have shown reduced VTE risk in subjects receiving statins, although their findings still generate much controversy and suggest that the most potent statin rosuvastatin exerts the largest effect.

Tumor-derived tissue factor-positive microparticles and venous thrombosis in cancer patients

Patients with cancer have an increased risk for venous thrombosis. Interestingly, different cancer types have different rates of thrombosis, with pancreatic cancer having one of the highest rates. However, the mechanisms responsible for the increase in venous thrombosis in patients with cancer are not understood. Tissue factor (TF) is a transmembrane receptor and primary initiator of blood coagulation. Tumor cells express TF and spontaneously release TF-positive microparticles (MPs) into the blood. MPs are small membrane vesicles that are highly procoagulant. It has been proposed that these circulating tumor-derived, TF-positive MPs may explain the increased rates of venous thrombosis seen in patients with cancer. In animal models, increased levels of tumor-derived, TF-positive MPs are associated with activation of coagulation. Moreover, these MPs bind to sites of vascular injury and enhance thrombosis. We and others have found that patients with cancer have elevated levels of circulating TF-positive MPs. These MPs are derived from tumors because they express tumor markers and are decreased by tumor resection. Importantly, several studies have shown that increased levels of TF-positive MPs correlate with venous thrombosis in patients with cancer. Taken together, these results suggest that TF-positive MPs may be a useful biomarker to identify patients with cancer who are at high risk for thrombosis.

Revisited role of microparticles in arterial and venous thrombosis

Microparticles (MPs) represent a heterogeneous population of submicronic vesicles that are released in response to cell activation or apoptosis. MPs harbor a large repertoire of cell surface receptors and mRNA and biological activities representative of their parent cells and related to their involvement in many biological functions. Although MP generation is a physiological response, a dramatic increase in circulating MPs is detectable in a variety of thrombosis-associated disorders compared with healthy individuals. In this review, we will discuss a new vision of MPs as complex and ambivalent structures that express both activators and inhibitors of coagulation but also convey fibrinolytic properties. After summarizing our current knowledge about the role of MPs in venous and arterial thrombosis, this review will explore how this new vision of MPs influences their definition as emergent biomarkers in thrombotic diseases. Among the studies that have aimed to establish a link between thrombosis and MPs, a few studies have demonstrated a predictive value of MPs. So far, it is unclear whether this limited causative association is the result of current technical concerns and limited standardization or has to be integrated into a more complex vision of the role of MPs as key systems for regulating the balance between coagulation and fibrinolysis.

Defibrotide: properties and clinical use of an old/new drug

The drug named defibrotide (DFT) has been studied for many years. It has been shown to possess many activities: profibrinolytic, antithrombotic-thrombolytic, antiischemic (heart, liver, kidney, skin, brain), antishock, antiatherosclerotic, antirejection and anti-angiogenic. The previously displayed activities, as antithrombotic, profibrinolytic and anti-inflammatory, suggested its use in vascular disorders, as in the treatment of peripheral obliterative arterial disease and in thrombophlebitis. Some years after, the use of DFT in hepatic veno-occlusive disease has been also proposed. Even if DFT was considered for long time a multi-target drug, now it could be considered on the whole as a drug able to protect endothelium against activation. The present work reviews the more important experimental and clinical studies performed to detect DFT effects.

Host cell invasion by Staphylococcus aureus stimulates the shedding of microvesicles

During severe sepsis, microvesicles that are positive for tissue factor (TF) are at increased levels within blood and in pulmonary lavage. These microvesicles potentially disperse TF, the major initiator of the coagulation cascade, throughout multiple organ systems, initiating fibrin deposition and resultant ischemia. The source of these microvesicles has remained incompletely defined. Although TF(+) microvesicles are shed from cells that express nascent TF transcript in response to injury, recent findings revealed that circulating, full-length TF protein is detectable prior to these nascent transcripts. This finding suggested that the protein is released from constitutive sources as an acute response. We examined whether Staphylococcus aureus, the Gram-positive bacteria that is emerging as one of the most common etiologic agents in sepsis, is capable of stimulating the release of TF(+) microvesicles from a pulmonary cell line that constitutively expresses TF protein. We found that host cell invasion stimulated an acute release of TF(+) microvesicles and that these microvesicles mediated the transfer of the protein to TF-negative endothelial cells. We also found that transfer was inhibited by cholesterol-lowering simvastatin. Taken together, our findings reveal that S. aureus pathogenesis extends to the acute release of TF(+) microvesicles and that inhibiting dispersal by this mechanism may provide a therapeutic target.

Uremic serum and solutes increase post-vascular interventional thrombotic risk through altered stability of smooth muscle cell tissue factor

BACKGROUND

Stent thrombosis (ST), a postinterventional complication with a mortality rate of 50%, has an incidence that rises precipitously in patients at risk. Chronic renal failure and end-stage renal disease have emerged as particularly strong ST risk factors, yet the mechanism remains elusive. Tissue factor (TF) is a crucial mediator of injury-related thrombosis and has been implicated for ST. We posit that uremia modulates TF in the local vessel wall to induce postinterventional thrombosis in patients with end-stage renal disease.

METHODS AND RESULTS

As a model of the de-endothelialized, postinterventional state, we exposed primary human vascular smooth muscle cells (vSMCs) pretreated with uremic serum (obtained from ESRD patients on hemodialysis) to coronary-like blood flow. vSMC TF expression, activity, stability, and posttranslational modification were examined after vSMCs were treated with uremic serum or solutes. We found significantly greater clot formation after uremic serum exposure, which was substantially reduced with the prior treatment with anti-TF neutralizing antibody. Uremic sera induced 2- to 3-fold higher TF expression and activity in vSMCs independent of diabetes mellitus. Relevant concentrations of isolated uremic solutes such as indole-3-acetic acid (3.5 μg/mL), indoxyl sulfate (25 μg/mL), and uric acid (80 μg/mL) recapitulated these effects in cell culture and the flow loop model. We show further that TF undergoes ubiquitination at baseline and that uremic serum, indole-3-acetic acid, and indoxyl sulfate significantly prolong TF half-life by inhibiting its ubiquitination.

CONCLUSIONS

The uremic milieu is profoundly thrombogenic and upregulates vSMC TF levels by increasing TF stability and decreasing its ubiquitination. Together, these data demonstrate for the first time that the posttranslational regulation of TF in uremia may have a causative role in the increased ST risk observed in uremic patients. These data suggest that interventions that reduce vSMC TF may help to prevent ST and that uremic solutes should be considered as novel risk factors for ST in patients with chronic renal failure.

Extracellular vesicles in physiological and pathological conditions

Body fluids contain surprising numbers of cell-derived vesicles which are now thought to contribute to both physiology and pathology. Tools to improve the detection of vesicles are being developed and clinical applications using vesicles for diagnosis, prognosis, and therapy are under investigation. The increased understanding why cells release vesicles, how vesicles play a role in intercellular communication, and how vesicles may concurrently contribute to cellular homeostasis and host defense, reveals a very complex and sophisticated contribution of vesicles to health and disease.

Skin extracts from 2 Italian table grapes (Italia and Palieri) inhibit tissue factor expression by human blood mononuclear cells

Grape and its products such as red wine and grape juice have well-known antithrombotic properties, which have been attributed to their high content in polyphenolic compounds. Most studies on the mechanisms underlying these beneficial effects, among which the suppression of tissue factor (TF) synthesis in blood mononuclear cells (MNC) and vascular endothelium is a prominent one, have been performed with purified polyphenols, while little is known about the effect of fresh grapes which contain a multitude of phytochemicals whose interaction may lead to different cell responses. In this study, we investigated the effect of grape skin extracts (GSEs) on TF expression in isolated blood MNC and in whole blood. Alcoholic extracts from skins of 2 grape varieties (Palieri and Italia) inhibited TF expression in lipopolysaccharide (LPS)-stimulated MNC in a concentration-dependent manner with ≥90% inhibition of TF activity and antigen at 6 μg/mL of gallic acid equivalents. Noteworthy, GSEs were also able to inhibit the appearance of TF in whole blood challenged with LPS. The 2 grape varieties displayed a fairly similar TF-inhibiting capacity despite marked differences in phenolic profile. When selected purified polyphenols were tested, their ability to inhibit TF expression was markedly lower as compared to grape extracts, whereas a mixture of some representative polyphenols was much more efficient, supporting the occurrence of a synergistic effect. Given the key role of cell TF in thrombotic diseases, the inhibition of MNC-mediated clotting activation, if confirmed by in vivo studies, might represent an important antithrombotic mechanism.

PRACTICAL APPLICATION

Our data indicate that the combination of different polyphenols, as in grape extracts, is much more efficient than the single constituents, a finding that might be useful as starting point for the development of new antithrombotic nutraceutics. In addition, our study validated a simple, inexpensive, and physiologically relevant in vitro method on whole blood that allows the evaluation of one of the most important antithrombotic activities of food and food-derived products. The simplicity of the method makes it suitable also for screening purposes in large-scale studies.

Antithrombotic activity of 12 table grape varieties. Relationship with polyphenolic profile

The synthesis of tissue factor (TF) by monocytes/macrophages activated by inflammatory agents is of utmost importance in the pathogenesis of thrombotic diseases and substances inhibiting TF synthesis represent novel and promising antithrombotics. We investigated the effect of 12 table grape varieties (white, red and black) on TF synthesis and the possible relation with the phenolic profile. The ability of grape skin extracts (GSEs) to inhibit TF was evaluated in whole blood and isolated mononuclear cells challenged with endotoxin. TF expression was assayed by functional and immunological assays. All GSEs inhibited TF synthesis but with a different efficiency, red grapes being the most active. By correlation analysis, the compounds showing the strongest association with TF-inhibiting activity were quercetin and cyanidin. However, no single polyphenol was able to inhibit TF synthesis as efficiently as the crude grape extracts, unless it was combined with at least another compound, suggesting a synergism.

Predictive potential of haemostatic biomarkers for venous thromboembolism in cancer patients

Venous thromboembolism (VTE) is a common problem in cancer patients. However, the rates of VTE vary widely between different types of malignancies. Furthermore, patient- and treatment-related risk factors contribute to the risk of VTE. The prediction of the individual risk of VTE and the identification of patients with cancer that might benefit from thromboprophylaxis is a major clinical challenge, because the pathogenesis of cancer-associated VTE is multifactorial. Therefore, recent studies have focused on identification of predictive biomarkers for VTE. As there is a close interrelation between cancer and the haemostatic system, which leads to activation of haemostasis and fibrinolysis, biomarkers of the haemostatic system seem to be promising in predicting risk of developing VTE in cancer patients. Some candidate biomarkers or laboratory tests of the haemostatic system including platelet count, soluble P-selectin, tissue factor (TF) and TF-bearing microparticles, coagulation factor VIII, D-dimer, prothrombin fragment 1+2 and the thrombin generation assay have been reported to predict cancer-associated VTE. In addition, it has been shown that risk-scoring models, incorporating clinical parameters and biomarkers, allow risk stratification of cancer patients into groups at high- and at low risk of VTE. The accuracy of a previously established risk scoring model can be improved when it is expanded and biomarkers of the haemostatic system are added. The benefit of a targeted thromboprophylaxis for primary prevention of VTE in cancer patients based on risk assessment by measuring biomarkers or applying risk scoring models has to be shown in interventional clinical trials.

Unifying the mechanism of recombinant FVIIa action: dose dependence is regulated differently by tissue factor and phospholipids

Recombinant factor VIIa (rFVIIa) is used for treatment of hemophilia patients with inhibitors, as well for off-label treatment of severe bleeding in trauma and surgery. Effective bleeding control requires supraphysiological doses of rFVIIa, posing both high expense and uncertain thrombotic risk. Two major competing theories offer different explanations for the supraphysiological rFVIIa dosing requirement: (1) the need to overcome competition between FVIIa and FVII zymogen for tissue factor (TF) binding, and (2) a high-dose-requiring phospholipid-related pathway of FVIIa action. In the present study, we found experimental conditions in which both mechanisms contribute simultaneously and independently to rFVIIa-driven thrombin generation in FVII-deficient human plasma. From mathematical simulations of our model of FX activation, which were confirmed by thrombin-generation experiments, we conclude that the action of rFVIIa at pharmacologic doses is dominated by the TF-dependent pathway with a minor contribution from a phospholipid-dependent mechanism. We established a dose-response curve for rFVIIa that is useful to explain dosing strategies. In the present study, we present a pathway to reconcile the 2 major mechanisms of rFVIIa action, a necessary step to understanding future dose optimization and evaluation of new rFVIIa analogs currently under development.

Decryption of tissue factor

Tissue factor (TF) is a transmembrane protein which, in complex with factor (F)VIIa, initiates blood coagulation. Numerous studies have determined TF epitopes and individual amino acids which play an important role in the TF/FVIIa complex formation and its activity towards natural substrates. However the subject of cell-surface TF activity remains controversial. It has been almost commonly accepted that TF on the cell surface has low (if any) activity, i.e. is encrypted and requires specific conditions/reagents to become active, i.e. decrypted. One of the leading theories suggests that cell membrane lipid composition plays a crucial role in TF decryption, whereas another assigns the key role to the formation of the Cys(186)-Cys(209) disulfide bond. Despite a number of studies published from several laboratories, the role of this bond in the activity of the TF/FVIIa complex remains elusive and controversial. One of the causes of this controversy could be related to the lack of specificity of the reagents used for the cell treatment leading to possible alterations in other cell surface proteins and cell membrane environment. In conclusion, the influence of the Cys(186)-Cys(209) this bond on cell surface TF function remains unclear.

Tissue factor controversies

Tissue factor plays a primary role in both hemorrhage control and thrombosis depending upon whether its presentation is extravascular or intravascular. The molecular architecture and function of the tissue factor molecule and its role in the activations of factor IX and factor X have been elegantly elucidated but controversies prevail with respect to distinctions between tissue factor sources and tissue factor "activity." This presentation will review data on the architecture and functions of the tissue factor-factor VIIa complex and discuss the elements of the controversies associated with tissue factor presentation in both normal and pathologic milieu.

Tissue factor encryption and decryption: facts and controversies

Tissue factor (TF)-initiated coagulation plays a critical role in both hemostasis and thrombosis. It is generally believed that most of the tissue factor expressed on cell surfaces is maintained in a cryptic, i.e., coagulantly inactive state and an activation step (decryption) is required for the expression of maximum TF procoagulant activity. However, what exactly constitutes cryptic or procoagulant TF, molecular differences between these two forms and mechanisms that are responsible for transformation from one to the other form are not entirely clear and remain highly controversial, thus are a matter of ongoing debate. This brief review discusses pertinent literature on TF encryption/decryption with specific emphasis on the role of membrane phospholipids and reduction/oxidation of the TF Cys186-Cys209 disulfide bond in regulating TF activity at cell surfaces.

Active tissue factor and activated factor XI in patients with acute ischemic cerebrovascular events

BACKGROUND

  Elevated factor (F)XI and tissue factor (TF) have been reported to occur in patients with acute ischemic stroke (AIS). We sought to investigate whether circulating activated FXI (FXIa) and TF on admission can predict clinical outcomes in patients with acute cerebrovascular events.

MATERIALS AND METHODS

  In the observational study, we evaluated 205 consecutive patients aged 70 years or less within the first 72 h of acute event, including 140 with AIS and 65 with transient ischemic attack (TIA). Plasma TF and FXIa activity were determined on admission in clotting assays by measuring the response to inhibitory monoclonal antibodies.

RESULTS

Active TF and FXIa activity were detected in 58 (28·9%) and 132 (64·4%) patients on admission, respectively. Active TF was detected in 45 of the 136 AIS patients with available TF levels (33·1%) and 13 of the 65 patients with acute TIA (20%; 0·05). Corresponding values for FXIa were 99 of the 140 (70·7%) and 33 of the 65 (50·8%; P= 0·006), respectively. Patients with detectable TF were more frequently women and hypertensive, while subjects with detectable FXIa had more often diabetes and higher levels of fibrinogen, C-reactive protein and interleukin-6 (all P < 0·05). Patients with detectable FXIa but not TF had higher National Institutes of Health Stroke Scale score, higher modified Rankin scale score and lower Barthel Index at discharge (all P < 0·05).

CONCLUSIONS

Circulating active TF and FXIa occur frequently in acute cerebrovascular ischemic events. Active FXIa in plasma might be useful as a novel risk marker of worse functional outcomes in patients with acute cerebrovascular events.

Tissue factor: mechanisms of decryption

It is generally believed that only a small fraction of the tissue factor (TF) found on cell surfaces is active whereas the vast majority is cryptic in coagulation. It is unclear how cryptic TF differs from the coagulant active TF or potential mechanisms involved in transformation of cryptic TF to the coagulant active form. Exposure of phosphatidylserine (PS) in response to various chemical or pathophysiological stimuli has been considered as the most potent inducer of TF decryption. In addition to PS, TF self-association and association with specialized membrane domains may also play a role in TF decryption. It has been suggested recently that protein disulfide isomerase regulates TF decryption through its oxidoreductase activity by targeting Cys186-Cys209 disulfide bond in TF extracellular domain or regulating the PS equilibrium at the plasma membrane. However, this hypothesis requires further validation to become an accepted mechanism. In this article, we critically review literature on TF encryption/decryption with specific emphasis on recently published data and provide our perspective on this subject.

Air pollution-associated procoagulant changes: the role of circulating microvesicles

BACKGROUND

Epidemiological studies suggest an association between exposure to particulate matter (PM) in air pollution and the risk of venous thromboembolism (VTE).

OBJECTIVES

To investigate the underlying pathophysiological pathways linking PM exposure and VTE.

PATIENTS AND METHODS

We assessed potential associations between PM exposure and coagulation and inflammation parameters, including circulating microvesicles, in a group of 233 patients with diabetes.

RESULTS

The numbers of circulating blood platelet-derived and annexin V-binding microvesicles were inversely associated with the current levels of PM(2.5) or PM(10), measured on the day of sampling. Recent past exposure to PM(10), up to 1 week prior to blood sampling, estimated at the patients' residential addresses, was associated with elevated high-sensitivity C-reactive protein (CRP), leukocytes and fibrinogen, as well as with tissue factor (TF)-dependent procoagulant changes in thrombin generation assays. When longer windows of past exposure were considered, up to 1 year preceding blood sampling, procoagulant changes were evident from the strongly increased numbers of red blood cell-derived circulating microvesicles and annexin V-binding microvesicles, but they no longer associated with TF. Past PM exposure was never associated with activated partial thromboplastin time (aPTT), prothrombin time (PT), or factor (F) VII, FVIII, FXII or D-dimers. Residential distance to a major road was only marginally correlated with procoagulant changes in FVIII and thrombin generation.

CONCLUSIONS

Increases in the number of microvesicles and in their procoagulant properties, rather than increases in coagulation factors per se, seem to contribute to the risk of VTE, developing during prolonged exposure to air pollutants.

Tissue factor and atherosclerosis: not only vessel wall-derived TF, but also platelet-associated TF

In the last ten years the contribution of both vessel wall-derived tissue factor (TF) and platelets to atherosclerosis has been revisited. At the beginning of the 2000 a circulating blood-borne TF has been proposed to sustain coagulation activation and propagation on the edge of a growing thrombus. Concomitantly with the observation that platelets not only contribute to thrombus formation, but also take part to the onset of the atherosclerotic lesion, evidences have been provided that they express functionally active TF, making them able to trigger the coagulation cascade.

Activated factor XI and tissue factor in aortic stenosis: links with thrombin generation

In our previous studies, we showed that a significant proportion of patients with various cardiovascular diseases have active tissue factor (TF) and factor (F)XIa in their plasma. The objective of the present study was to evaluate these two proteins in plasma from patients with aortic stenosis and establish their relationship with the severity of the disease. Fifty-four consecutive patients with aortic stenosis, including 38 (70.4%) severe aortic stenosis patients, were studied. Plasma FXIa and TF activity were determined in clotting assays by measuring the response to inhibitory monoclonal antibodies. TF activity was detectable in plasma from 14 of 54 patients (25.9%), including 13 of 38 with severe aortic stenosis (34.2%) and one of 16 (6.25%) with moderate aortic stenosis (P=0.052). FXIa activity was found in 12 (22.2%) patients, mostly in individuals with severe aortic stenosis (11 of 38, 28.9%, P=0.067). All 12 patients with circulating FXIa had active TF in their plasma as well. Severe aortic stenosis patients with detectable TF had higher maximal (111±20 vs. 97±16 mmHg, P=0.02) and mean (61±12 vs. 53±8 mmHg, P=0.02) transvalvular gradient, compared with those without such activity in plasma. In severe aortic stenosis patients with detectable active TF, prothrombin fragment 1.2, a thrombin generation marker, was higher than that in patients without TF (375±122 vs. 207±64 pM, P<0.001). Detectable FXIa and TF activity was observed for the first time in aortic stenosis patients, primarily in severe ones. This activity correlates with thrombin generation in those patients.

Stimulation of blood mononuclear cells with bacterial virulence factors leads to the release of pro-coagulant and pro-inflammatory microparticles

Severe infectious diseases remain a major and life-threatening health problem. In serious cases a systemic activation of the coagulation cascade and hypovolemic shock are critical complications that are associated with high mortality rates. Here we report that blood mononuclear cells, stimulated with M1 protein of Streptococcus pyogenes or other bacterial virulence factors, produce not only pro-coagulant, but also pro-inflammatory microparticles (MPs). Our results also show that activation of the contact system on MPs contributes to these two effects. Phosphatidylserine (PS) plays an important role in these processes as its upregulation on MPs allows an assembly and activation of the contact system. This in turn results in stabilization of the tissue factor-induced clot and a processing of high-molecular-weight kininogen by plasma kallikrein followed by the release of bradykinin, a potent vascular mediator. Pro-coagulant monocyte-derived MPs were identified in plasma samples from septic patients and further analysis of MPs from these patients revealed that their pro-coagulant activity is dependent on the tissue factor- and contact system-driven pathway.

Pathogenesis and pathophysiology of pneumococcal meningitis

Pneumococcal meningitis continues to be associated with high rates of mortality and long-term neurological sequelae. The most common route of infection starts by nasopharyngeal colonization by Streptococcus pneumoniae, which must avoid mucosal entrapment and evade the host immune system after local activation. During invasive disease, pneumococcal epithelial adhesion is followed by bloodstream invasion and activation of the complement and coagulation systems. The release of inflammatory mediators facilitates pneumococcal crossing of the blood-brain barrier into the brain, where the bacteria multiply freely and trigger activation of circulating antigen-presenting cells and resident microglial cells. The resulting massive inflammation leads to further neutrophil recruitment and inflammation, resulting in the well-known features of bacterial meningitis, including cerebrospinal fluid pleocytosis, cochlear damage, cerebral edema, hydrocephalus, and cerebrovascular complications. Experimental animal models continue to further our understanding of the pathophysiology of pneumococcal meningitis and provide the platform for the development of new adjuvant treatments and antimicrobial therapy. This review discusses the most recent views on the pathophysiology of pneumococcal meningitis, as well as potential targets for (adjunctive) therapy.