Filamin-A is required for the incorporation of tissue factor into cell-derived microvesicles

Regulation of tissue factor activity by interaction with the first PDZ domain of MAGI1

BACKGROUND

Tissue factor (TF) activity is stringently regulated through processes termed encryption. Post-translational modification of TF and its interactions with various protein and lipid moieties allows for a multi-step de-encryption of TF and procoagulant activation. Membrane-associated guanylate kinase-with inverted configuration (MAGI) proteins are known to regulate the localisation and activity of a number of proteins including cell-surface receptors.

METHODS

The interaction of TF with MAGI1 protein was examined as a means of regulating TF activity. MDA-MB-231 cell line was used which express TF and MAGI1, and respond well to protease activated receptor (PAR)2 activation. Proximity ligation assay (PLA), co-immunoprecipitation and pull-down experiments were used to examine the interaction of TF with MAGI1-3 proteins and to investigate the influence of PAR2 activation. Furthermore, by cloning and expressing the PDZ domains from MAGI1, the TF-binding domain was identified. The ability of the recombinant PDZ domains to act as competitors for MAGI1, allowing the induction of TF procoagulant and signalling activity was then examined.

RESULTS

PLA and fluorescence microscopic analysis indicated that TF predominantly associates with MAGI1 and less with MAGI2 and MAGI3 proteins. The interaction of TF with MAGI1 was also demonstrated by both co-immunoprecipitation of TF with MAGI1, and co-immunoprecipitation of MAGI1 with TF. Moreover, activation of PAR2 resulted in reduction in the association of these two proteins. Pull-down assays using TF-cytoplasmic domain peptides indicated that the phosphorylation of Ser253 within TF prevents its association with MAGI1. Additionally, the five HA-tagged PDZ domains of MAGI1 were overexpressed separately, and the putative TF-binding domain was identified as PDZ1 domain. Expression of this PDZ domain in cells significantly augmented the TF activity measured both as thrombin-generation and also TF-mediated proliferative signalling.

CONCLUSIONS

Our data indicate a stabilising interaction between TF and the PDZ-1 domain of MAGI1 and demonstrate that the activation of PAR2 disrupts this interaction. The release of TF from MAGI1 appears to be an initial step in TF de-encryption, associated with increased TF-mediated procoagulant and signalling activities. This mechanism is also likely to lead to further interactions and modifications leading to further enhancement of procoagulant activity, or the release of TF.

Tissue factor in cancer-associated thromboembolism: possible mechanisms and clinical applications

Venous and arterial thromboses, called as cancer-associated thromboembolism (CAT), are common complications in cancer patients that are associated with high mortality. The cell-surface glycoprotein tissue factor (TF) initiates the extrinsic blood coagulation cascade. TF is overexpressed in cancer cells and is a component of extracellular vesicles (EVs). Shedding of TF⁺EVs from cancer cells followed by association with coagulation factor VII (fVII) can trigger the blood coagulation cascade, followed by cancer-associated venous thromboembolism in some cancer types. Secretion of TF is controlled by multiple mechanisms of TF⁺EV biogenesis. The procoagulant function of TF is regulated via its conformational change. Thus, multiple steps participate in the elevation of plasma procoagulant activity. Whether cancer cell-derived TF is maximally active in the blood is unclear. Numerous mechanisms other than TF⁺EVs have been proposed as possible causes of CAT. In this review, we focused on a wide variety of regulatory and shedding mechanisms for TF, including the effect of SARS-CoV-2, to provide a broad overview for its role in CAT. Furthermore, we present the current technical issues in studying the relationship between CAT and TF.

The Role of Platelet-Derived Extracellular Vesicles in Immune-Mediated Thrombosis

Platelet-derived extracellular vesicles (PEVs) play important roles in hemostasis and thrombosis. There are three major types of PEVs described based on their size and characteristics, but newer types may continue to emerge owing to the ongoing improvement in the methodologies and terms used to define various types of EVs. As the literature on EVs is growing, there are continuing attempts to standardize protocols for EV isolation and reach consensus in the field. This review provides information on mechanisms of PEV production, characteristics, cellular interaction, and their pathological role, especially in autoimmune and infectious diseases. We also highlight the mechanisms through which PEVs can activate parent cells in a feedback loop.

De-Palmitoylation of Tissue Factor Regulates Its Activity, Phosphorylation and Cellular Functions

In this study, the role of de-palmitoylation of tissue factor (TF) in the decryption of its activity was explored. TF-tGFP constructs were prepared by mutagenesis-substitution at Cys245 to prevent or mimic palmitolyation. Additionally, to reduce TF de-palmitoylation, the expression of palmitoyl-protein thioesterases (PPT) was suppressed. Other TF mutants were prepared with altered flexibility, hydrophobicity or length of the transmembrane domain. The outcome of these alterations on fXa-generation, fVIIa binding, Ser253 phosphorylation and TF-microvesicle release were assessed in endothelial cells, and the influence on endothelial and MCF-7 cell proliferation and apoptosis was analysed. Preventing TF palmitoylation (TF_Ser245-tGFP), increasing the hydrophobicity (TF_Phe241-tGFP) or lengthening (TF_LongTM-tGFP) of the transmembrane domain enhanced fXa-generation in resting cells compared to cells expressing TF_Wt-tGFP, but fXa-generation was not further increased following PAR2 activation. Extending the available length of the transmembrane domain enhanced the TF-tGFP release within microvesicles and Ser253 phosphorylation and increased cell proliferation. Moreover, prevention of PKCα-mediated Ser253 phosphorylation with Gö6976 did not preclude fXa-generation. Conversely, reducing the hydrophobicity (TF_Ser242-tGFP), shortening (TF_ShortTM-tGFP) or reducing the flexibility (TF_Val225-tGFP) of the transmembrane domain suppressed fXa-generation, fVIIa-HRP binding and Ser253 phosphorylation following PAR2 activation. PPT knock-down or mimicking palmitoylation (TF_Phe245-tGFP) reduced fXa-generation without affecting fVIIa binding. This study has for the first time shown that TF procoagulant activity is regulated through de-palmitoylation, which alters the orientation of its transmembrane domain and is independent of TF phosphorylation. However, Ser253 phosphorylation is facilitated by changes in the orientation of the transmembrane domain and can induce TF-cellular signalling that influences cellular proliferation/apoptosis.

Filamin A Regulates Cardiovascular Remodeling

Filamin A (FLNA) is a large actin-binding cytoskeletal protein that is important for cell motility by stabilizing actin networks and integrating them with cell membranes. Interestingly, a C-terminal fragment of FLNA can be cleaved off by calpain to stimulate adaptive angiogenesis by transporting multiple transcription factors into the nucleus. Recently, increasing evidence suggests that FLNA participates in the pathogenesis of cardiovascular and respiratory diseases, in which the interaction of FLNA with transcription factors and/or cell signaling molecules dictate the function of vascular cells. Localized FLNA mutations associate with cardiovascular malformations in humans. A lack of FLNA in experimental animal models disrupts cell migration during embryogenesis and causes anomalies, including heart and vessels, similar to human malformations. More recently, it was shown that FLNA mediates the progression of myocardial infarction and atherosclerosis. Thus, these latest findings identify FLNA as an important novel mediator of cardiovascular development and remodeling, and thus a potential target for therapy. In this update, we summarized the literature on filamin biology with regard to cardiovascular cell function.

Accumulation of tissue factor in endothelial cells promotes cellular apoptosis through over-activation of Src1 and involves β1-integrin signalling

Accumulation of tissue factor (TF) within cells leads to cellular apoptosis mediated through p38 and p53 pathways. In this study, the involvement of Src1 in the induction of TF-mediated cell apoptosis, and the mechanisms of Src1 activation were investigated. Human coronary artery endothelial cell (HCAEC) were transfected with plasmids to express the wild-type TF (TF_Wt-tGFP), or a mutant (Ser253 → Ala) which is incapable of being released from cells (TF_Ala253-tGFP). The cells were then activated with PAR2-agonist peptide (SLIGKV-NH) and the phosphorylation of Src and Rac, and also the kinase activity of Src were assessed. Transfected cells were also pre-incubated with pp60c Src inhibitor, FAK inhibitor-14, or a blocking anti-β1-integrin antibody prior to activation and the phosphorylation of p38 as well as cellular apoptosis was examined. Finally, cells were co-transfected with the plasmids, together with a Src1-specific siRNA, activated as above and the cellular apoptosis measured. Activation of PAR2 lead to the phosphorylation of Src1 and Rac1 proteins at 60 min regardless of TF expression. Moreover, Src phosphorylation and kinase activity was prolonged up to 100 min in the presence of TF, with a significantly higher magnitude when the non-releasable TF_Ala253-tGFP was expressed in HCAEC. Inhibition of Src with pp60c, or suppression of Src1 expression in cells, reduced p38 phosphorylation and prevented cellular apoptosis. In contrast, inhibition of FAK had no significant influence on Src kinase activity or cellular apoptosis. Finally, pre-incubation of cells with an inhibitory anti-β1-integrin antibody reduced both Src1 activation and cellular apoptosis. Our data show for the first time that the over-activation of Src1 is a mediator of TF-induced cellular apoptosis in endothelial cells through a mechanism that is dependent on its interaction with β1-integrin.

Cancer cell-derived tissue factor-positive extracellular vesicles: biomarkers of thrombosis and survival

PURPOSE OF REVIEW

Tissue factor (TF) is released from cancer cells and tumors in the form of extracellular vesicles (EVs). This review summarizes our current knowledge of the mechanisms of release of TF-positive EVs (TF+EVs) from cancer cells and the effect of these TF+EVs on cultured endothelial cells. In addition, we will summarize the contribution of TF+EVs to thrombosis in mice, and the association between plasma EVTF activity and venous thrombosis as well as survival of cancer patients.

RECENT FINDINGS

The release of TF+EVs from cancer cells is regulated by multiple factors, including hypoxia, epithelial-mesenchymal transition, and various intracellular signaling pathways. Cancer cell-derived, TF+EVs confer procoagulant activity to endothelial cells and induce the expression of adhesion proteins and IL-8. In addition, they contribute to thrombosis by directly activating the coagulation system and by generating thrombin that activates platelets in mouse models. Finally, there is an association between EVTF activity and venous thrombosis in pancreatic cancer patients as well as mortality in cancer patients.

SUMMARY

Cancer cell-derived TF+EVs bind to and activate endothelial cells. In addition, they serve as biomarkers of survival of cancer patients and venous thrombosis in pancreatic cancer patients.

Actin and Actin-Associated Proteins in Extracellular Vesicles Shed by Osteoclasts

Extracellular vesicles (EVs) are shed by all eukaryotic cells and have emerged as important intercellular regulators. EVs released by osteoclasts were recently identified as important coupling factors in bone remodeling. They are shed as osteoclasts resorb bone and stimulate osteoblasts to form bone to replace the bone resorbed. We reported the proteomic content of osteoclast EVs with data from two-dimensional, high resolution liquid chromatography/mass spectrometry. In this article, we examine in detail the actin and actin-associated proteins found in osteoclast EVs. Like EVs from other cell types, actin and various actin-associated proteins were abundant. These include components of the polymerization machinery, myosin mechanoenzymes, proteins that stabilize or depolymerize microfilaments, and actin-associated proteins that are involved in regulating integrins. The selective incorporation of actin-associated proteins into osteoclast EVs suggests that they have roles in the formation of EVs and/or the regulatory signaling functions of the EVs. Regulating integrins so that they bind extracellular matrix tightly, in order to attach EVs to the extracellular matrix at specific locations in organs and tissues, is one potential active role for actin-associated proteins in EVs.

Update from the laboratory: mechanistic studies of pathways of cancer-associated venous thrombosis using mouse models

Cancer patients have an increased risk of venous thromboembolism (VTE). The rate of VTE varies with cancer type, with pancreatic cancer having one of the highest rates, suggesting that there are cancer type-specific mechanisms of VTE. Risk assessment scores, such as the Khorana score, have been developed to identify ambulatory cancer patients at high risk of VTE. However, the Khorana score performed poorly in discriminating pancreatic cancer patients at risk of VTE. Currently, thromboprophylaxis is not recommended for cancer outpatients. Recent clinical trials showed that factor Xa (FXa) inhibitors reduced VTE in high-risk cancer patients but also increased major bleeding. Understanding the mechanisms of cancer-associated thrombosis should lead to the development of safer antithrombotic drugs. Mouse models can be used to study the role of different prothrombotic pathways in cancer-associated thrombosis. Human and mouse studies support the notion that 2 prothrombotic pathways contribute to VTE in pancreatic cancer patients: tumor-derived, tissue factor-positive (TF+) extracellular vesicles (EVs), and neutrophils and neutrophil extracellular traps (NETs). In pancreatic cancer patients, elevated levels of plasma EVTF activity and citrullinated histone H3 (H3Cit), a NET biomarker, are independently associated with VTE. We observed increased levels of circulating tumor-derived TF+ EVs, neutrophils, cell-free DNA, and H3Cit in nude mice bearing human pancreatic tumors. Importantly, inhibition of tumor-derived human TF, depletion of neutrophils, or administration of DNAse I to degrade cell-free DNA (including NETs) reduced venous thrombosis in tumor-bearing mice. These studies demonstrate that tumor-derived TF+ EVs, neutrophils, and cell-free DNA contribute to venous thrombosis in a mouse model of pancreatic cancer.

Factor VIIa-induced interaction with integrin controls the release of tissue factor on extracellular vesicles from endothelial cells

Essentials Prothrombotic extracellular vesicles (EV) carry agonist pathway-specific proteomes Agonists for protease activated receptor (PAR) 2 signaling have distinct effects on EV composition PAR2 signaling rapidly generates prothrombotic EV and slowly EV with inactive tissue factor (TF) FVIIa integrin ligation restricts TF incorporation into EV from endothelial cells SUMMARY: Background Cell injury signal-induced activation and release of tissue factor (TF) on extracellular vesicles (EVs) from immune and vessel wall cells propagate local and systemic coagulation initiation. TF trafficking and release on EVs occurs in concert with the release of cell adhesion receptors, including integrin β1 heterodimers, which control trafficking of the TF-activated factor VII (FVIIa) complex. Activation of the TF signaling partner, protease-activated receptor (PAR) 2, also triggers TF release on integrin β1+ EVs from endothelial cells, but the physiological signals for PAR2-dependent EV generation at the vascular interface remain unknown. Objective To define relevant protease ligands of TF contributing to PAR2-dependent release on EVs from endothelial cells. Methods In endothelial cells with balanced expression of TF and PAR2, we evaluated TF release on EVs by using a combination of activity and antigen assays, immunocapture, and confocal imaging. Results and Conclusions PAR2 stimulation generated time-dependent release of distinct TF+ EVs with high coagulant activity (early) and high antigen levels (late). Whereas PAR2 agonist peptide and a stabilized TF-FVIIa-activated FX complex triggered TF+ EV release, stimulation with FVIIa alone promoted cellular retention of TF, despite comparable PAR2 activation. On endothelial cells, FVIIa uniquely induced formation of a complex of TF with integrin α5 β1 . Internalization of TF by FVIIa or anti-TF and activating antibodies against integrin β1 prevented PAR2 agonist-induced release of TF on EVs. These data demonstrate that intracellular trafficking controlled by FVIIa forcing interaction with integrin β1 regulates TF availability for release on procoagulant EVs.

Tissue factor at the crossroad of coagulation and cell signaling

The tissue factor (TF) pathway plays a central role in hemostasis and thrombo-inflammatory diseases. Although structure-function relationships of the TF initiation complex are elucidated, new facets of the dynamic regulation of TF's activities in cells continue to emerge. Cellular pathways that render TF non-coagulant participate in signaling of distinct TF complexes with associated proteases through the protease-activated receptor (PAR) family of G protein-coupled receptors. Additional co-receptors, including the endothelial protein C receptor (EPCR) and integrins, confer signaling specificity by directing subcellular localization and trafficking. We here review how TF is switched between its role in coagulation and cell signaling through thiol-disulfide exchange reactions in the context of physiologically relevant lipid microdomains. Inflammatory mediators, including reactive oxygen species, activators of the inflammasome, and the complement cascade play pivotal roles in TF procoagulant activation on monocytes, macrophages and endothelial cells. We furthermore discuss how TF, intracellular ligands, co-receptors and associated proteases are integrated in PAR-dependent cell signaling pathways controlling innate immunity, cancer and metabolic inflammation. Knowledge of the precise interactions of TF in coagulation and cell signaling is important for understanding effects of new anticoagulants beyond thrombosis and identification of new applications of these drugs for potential additional therapeutic benefits.

Tissue factor mediates microvesicles shedding from MDA-MB-231 breast cancer cells

Extracellular vesicles, such as microvesicles (MVs), were identified as important players in tumor progression and acquisition of an aggressive phenotype. Tissue factor (TF) is a transmembrane protein that initiates the blood coagulation cascade. In tumor cells, TF has been associated with aggressiveness and cancer progression. Previous studies demonstrate that TF is incorporated into MVs secreted by tumor cells; however, it is unknown whether TF is actively involved in the release of MVs. Here, we investigated the influence of TF expression on the release of MVs. TF silencing was achieved through CRISPR/Cas9 approaches in the human breast cancer cell line, MDA-MB-231. TF knockout in MDA-MB-231 cells efficiently reduced TF-dependent signaling and procoagulant activity. Remarkably, silencing of TF caused a significant decrease in the number of MVs released by MDA-MB-231 cells. We also observed an increase in actin-positive membrane projections in TF knockout cells and a reduction in RhoA expression when compared to TF-expressing cells. Treatment of MDA-MB-231 cells with the RhoA-ROCK signaling pathway inhibitor, fasudil, significantly reduced the release of MVs. Taken together, our results suggest a novel and relevant role for TF in tumor biology by playing an active role in the MVs secretion.

Accumulation of tissue factor in endothelial cells induces cell apoptosis, mediated through p38 and p53 activation

We previously reported that high levels of tissue factor (TF) can induce cellular apoptosis in endothelial cells. In this study, TF-mediated mechanisms of induction of apoptosis were explored. Endothelial cells were transfected to express wild-type TF. Additionally, cells were transfected to express Asp253-substituted, or Ala253-substitued TF to enhance or prevent TF release, respectively. Alternatively, cells were pre-incubated with TF-rich and TF-poor microvesicles. Cell proliferation, apoptosis and the expression of cyclin D1, p53, bax and p21 were measured following activation of cells with PAR2-agonist peptide. Greatest levels of cell proliferation and cyclin D1 expression were observed in cells expressing wild-type or Asp253-substituted TF. In contrast, increased cellular apoptosis was observed in cells expressing Ala253-substituted TF, or cells pre-incubated with TF-rich microvesicles. The level of p53 protein, p53-phosphorylation at ser33, p53 nuclear localisation and transcriptional activity, but not p53 mRNA, were increased in cells expressing wild-type and Ala253-substituted TF, or in cells pre-incubated with TF-rich microvesicles. However, the expression of bax and p21 mRNA, and Bax protein were only increased in cells pre-incubated with TF-rich microvesicle and in cells expressing Ala253-substituted TF. Inhibition of the transcriptional activity of p53 using pifithrin-α suppressed the expression of Bax. Finally, siRNA- mediated suppression of p38α, or inhibition using SB202190 significantly reduced the p53 protein levels, p53 nuclear localisation and transcriptional activity, suppressed Bax expression and prevented cellular apoptosis. In conclusion, accumulation of TF within endothelial cells, or sequestered from the surrounding can induce cellular apoptosis through mechanisms mediated by p38, and involves the stabilisation of p53.

Peptidyl-prolyl isomerase 1 (Pin1) preserves the phosphorylation state of tissue factor and prolongs its release within microvesicles

The exposure and release of TF is regulated by post-translational modifications of its cytoplasmic domain. Here, the potential of Pin1 to interact with the cytoplasmic domain of TF, and the outcome on TF function was examined. MDA-MB-231 and transfected-primary endothelial cells were incubated with either Pin1 deactivator Juglone, or its control Plumbagin, as well as transfected with Pin1-specific or control siRNA. TF release into microvesicles following activation, and also phosphorylation and ubiquitination states of cellular-TF were then assessed. Furthermore, the ability of Pin1 to bind wild-type and mutant forms of overexpressed TF-tGFP was investigated by co-immunoprecipitation. Additionally, the ability of recombinant or cellular Pin1 to bind to peptides of the C-terminus of TF, synthesised in different phosphorylation states was examined by binding assays and spectroscopically. Finally, the influence of recombinant Pin1 on the ubiquitination and dephosphorylation of the TF-peptides was examined. Pre-incubation of Pin1 with Juglone but not Plumbagin, reduced TF release as microvesicles and was also achievable following transfection with Pin1-siRNA. This was concurrent with early ubiquitination and dephosphorylation of cellular TF at Ser253. Pin1 co-immunoprecipitated with overexpressed wild-type TF-tGFP but not Ser258→Ala or Pro259→Ala substituted mutants. Pin1 did interact with Ser258-phosphorylated and double-phosphorylated TF-peptides, with the former having higher affinity. Finally, recombinant Pin1 was capable of interfering with the ubiquitination and dephosphorylation of TF-derived peptides. In conclusion, Pin1 is a fast-acting enzyme which may be utilised by cells to protect the phosphorylation state of TF in activated cells prolonging TF activity and release, and therefore ensuring adequate haemostasis.

Oligoubiquitination of tissue factor on Lys255 promotes Ser253-dephosphorylation and terminates TF release

Restriction of tissue factor (TF) activity at the cell surface and TF release are critical for prevention of excessive coagulation. This study examined the regulation of TF dephosphorylation and its release through ubiquitination. A plasmid containing the sequence to express the tandem protein TF-tGFP was mutated to include an arginine-substitution at Lys255 within TF. MDA-MB-231 cell line, and HCAEC endothelial cells were transfected and subsequently activated with PAR2-agonist peptide. The wild-type and mutant TF-tGFP were immunoprecipitated from the cell lysates and the ubiquitination and phosphorylation state of TF examined. Analysis of the proteins showed that arginine-substitution of Lys255 within TF prevented its ubiquitination while the wild-type TF-tGFP was oligoubiquitinated. The TF-associated oligoubiquitin chain was estimated to contain up to 4 ubiquitin units, with the linkage formed between Lys63 of one ubiquitin unit, and the C-terminus of the next unit. The Lys255→Arg substitution of TF-tGFP prolonged the phosphorylation of Ser253 within TF, compared to the wild-type TF-tGFP, lengthened the presence of TF-tGFP at the cell surface and extended the duration of TF-tGFP release from cells following PAR2 activation. A biotinylated 19-mer peptide corresponding to the C-terminus of TF (TFc) was used as substrate to show that the ubiquitination of TF was mediated by the Ube2D family of E2-enzymes and involved Mdm2. Moreover, double-phosphorylation of TFc was prerequisite for ubiquitination, with subsequent dephosphorylation of Ser253 by phosphatase PP2A. In conclusion, oligoubiquitination of Lys255 within TF permits PP2A to bind and dephosphorylate Ser253 and occurs to terminate TF release and contain its activity.

Targeting clotting proteins in cancer therapy - progress and challenges

Cancer-associated thrombosis remains a significant complication in the clinical management of cancer and interactions of the hemostatic system with cancer biology continue to be elucidated. Here, we review recent progress in our understanding of tissue factor (TF) regulation and procoagulant activation, TF signaling in cancer and immune cells, and the expanding roles of the coagulation system in stem cell niches and the tumor microenvironment. The extravascular functions of coagulant and anti-coagulant pathways have significant implications not only for tumor progression, but also for the selection of appropriate target specific anticoagulants in the therapy of cancer patients.

Analysis of the potential of cancer cell lines to release tissue factor-containing microvesicles: correlation with tissue factor and PAR2 expression

BACKGROUND

Despite the association of cancer-derived circulating tissue factor (TF)-containing microvesicles and hypercoagulable state, correlations with the incidence of thrombosis remain unclear.

METHODS

In this study the upregulation of TF release upon activation of various cancer cell lines, and the correlation with TF and PAR2 expression and/or activity was examined. Microvesicle release was induced by PAR2 activation in seventeen cell lines and released microvesicle density, microvesicle-associated TF activity, and phoshpatidylserine-mediated activity were measured. The time-course for TF release was monitored over 90 min in each cell line. In addition, TF mRNA expression, cellular TF protein and cell-surface TF activities were quantified. Moreover, the relative expression of PAR2 mRNA and cellular protein were analysed. Any correlations between the above parameters were examined by determining the Pearson's correlation coefficients.

RESULTS

TF release as microvesicles peaked between 30-60 min post-activation in the majority of cell lines tested. The magnitude of the maximal TF release positively correlated with TF mRNA (c = 0.717; p < 0.001) and PAR2 mRNA (c = 0.770; p < 0.001) expressions while the percentage increase correlated with PAR2 mRNA (c = 0.601; p = 0.011) and protein (c = 0.714; p < 0.001). There was only a weak correlation between resting TF release, and microvesicle release. However, TF release in resting cells did not significantly correlate with any of the parameters examined. Furthermore, TF mRNA expression correlated with PAR2 mRNA expression (c = 0.745; p < 0.001).

DISCUSSION AND CONCLUSIONS

In conclusion, our data suggest that TF and PAR2 mRNA, and PAR2 protein are better indicators of the ability of cancer cells to release TF and may constitute more accurate predictors of risk of thrombosis.

High-level secretion of tissue factor-rich extracellular vesicles from ovarian cancer cells mediated by filamin-A and protease-activated receptors

Thromboembolic events occur frequently in ovarian cancer patients. Tissue factor (TF) is often overexpressed in tumours, including ovarian clear-cell carcinoma (CCC), a subtype with a generally poor prognosis. TF-coagulation factor VII (fVII) complexes on the cell surface activate downstream coagulation mechanisms. Moreover, cancer cells secrete extracellular vesicles (EVs), which act as vehicles for TF. We therefore examined the characteristics of EVs produced by ovarian cancer cells of various histological subtypes. CCC cells secreted high levels of TF within EVs, while the high-TF expressing breast cancer cell line MDA-MB-231 shed fewer TF-positive EVs. We also found that CCC tumours with hypoxic tissue areas synthesised TF and fVII in vivo, rendering the blood of xenograft mice bearing these tumours hypercoagulable compared with mice bearing MDA-MB-231 tumours. Incorporation of TF into EVs and secretion of EVs from CCC cells exposed to hypoxia were both dependent on the actin-binding protein, filamin-A (filA). Furthermore, production of these EVs was dependent on different protease-activated receptors (PARs) on the cell surface. These results show that CCC cells could produce large numbers of TF-positive EVs dependent upon filA and PARs. This phenomenon may be the mechanism underlying the increased incidence of venous thromboembolism in ovarian cancer patients.

Extracellular vesicles, tissue factor, cancer and thrombosis - discussion themes of the ISEV 2014 Educational Day

Although the association between cancer and venous thromboembolism (VTE) has long been known, the mechanisms are poorly understood. Circulating tissue factor–bearing extracellular vesicles have been proposed as a possible explanation for the increased risk of VTE observed in some types of cancer. The International Society for Extracellular Vesicles (ISEV) and International Society on Thrombosis and Haemostasis (ISTH) held a joint Educational Day in April 2014 to discuss the latest developments in this field. This review discusses the themes of that event and the ISEV 2014 meeting that followed.

Characterization of physical properties of tissue factor-containing microvesicles and a comparison of ultracentrifuge-based recovery procedures

Microvesicles were isolated from the conditioned media of 3 cell lines (MDA-MB-231, AsPC-1 and A375) by ultracentrifugation at a range of relative centrifugal forces, and the tissue factor (TF) protein and activity, microvesicle number, size distribution and relative density compared. Also, by expressing TF-tGFP in cells and isolating the microvesicles, the relative density of TF-containing microvesicles was established. Nanoparticle tracking analysis (NTA) indicated that the larger-diameter microvesicles (>200 nm) were primarily sedimented at 100,000g and possessed TF-dependent thrombin and factor Xa generation potential, while in the absence of factor VII, all microvesicles possessed some thrombin generation capacity. Immuno-precipitation of TF-containing microvesicles followed by NTA also indicated the range of these microvesicles to be 200–400 nm. Analysis of the microvesicles by gradient density centrifugation showed that lower-density (<1.1 g/ml) microvesicles were mainly present in the samples recovered at 100,000g and were associated with TF antigen and activity. Analysis of these fractions by NTA confirmed that these fractions were principally composed of the larger-diameter microvesicles. Similar analysis of microvesicles from healthy or patient plasma supported those obtained from conditioned media indicating that TF activity was mainly associated with lower-density microvesicles. Furthermore, centrifugation of healthy plasma, supplemented with TF-tGFP-containing microvesicles, resulted in 67% retrieval of the fluorescent microvesicles at 100,000g, but only 26% could be recovered at 20,000g. Pre-centrifugation of conditioned media or plasma at 10,000g improved the speed and yield of recovered TF-containing microvesicles by subsequent centrifugation at either 20,000g or 100,000g. In conclusion, TF appears to be associated with low-density (1.03–1.08 g/ml), larger-diameter (200–350 nm) microvesicles.