Protein C anticoagulant system--anti-inflammatory effects

3K3A-activated protein C stimulates postischemic neuronal repair by human neural stem cells in mice

Activated protein C (APC) is a blood protease with anticoagulant activity and cell-signaling activities mediated by the activation of protease-activated receptor 1 (F2R, also known as PAR1) and F2RL1 (also known as PAR3) via noncanonical cleavage. Recombinant variants of APC, such as the 3K3A-APC (Lys191-193Ala) mutant in which three Lys residues (KKK191-193) were replaced with alanine, and/or its other mutants with reduced (>90%) anticoagulant activity, engineered to reduce APC-associated bleeding risk while retaining normal cell-signaling activity, have shown benefits in preclinical models of ischemic stroke, brain trauma, multiple sclerosis, amyotrophic lateral sclerosis, sepsis, ischemic and reperfusion injury of heart, kidney and liver, pulmonary, kidney and gastrointestinal inflammation, diabetes and lethal body radiation. On the basis of proof-of-concept studies and an excellent safety profile in humans, 3K3A-APC has advanced to clinical trials as a neuroprotectant in ischemic stroke. Recently, 3K3A-APC has been shown to stimulate neuronal production by human neural stem and progenitor cells (NSCs) in vitro via a PAR1-PAR3-sphingosine-1-phosphate-receptor 1-Akt pathway, which suggests the potential for APC-based treatment as a strategy for structural repair in the human central nervous (CNS) system. Here we report that late postischemic treatment of mice with 3K3A-APC stimulates neuronal production by transplanted human NSCs, promotes circuit restoration and improves functional recovery. Thus, 3K3A-APC-potentiated neuronal recruitment from engrafted NSCs might offer a new approach to the treatment of stroke and related neurological disorders.

Vascular Procr+ stem cells: Finding new branches while looking for the roots

Generation and growth of the blood vasculature network is a highly synchronized process, requiring coordinated efforts of endothelial cells and pericytes to maintain blood vessel integrity and regeneration. In a recent paper published in Cell Research, Yu et al. identified and characterized bipotent Procr-expressing vascular endothelial stem cells, which give rise to both endothelial cells and pericytes.

Recombinant Thrombomodulin (Solulin) Ameliorates Early Intestinal Radiation Toxicity in a Preclinical Rat Model

Intestinal radiation toxicity occurs during and after abdominopelvic radiotherapy. Endothelial cells play a significant role in modulating radiation-induced intestinal damage. We demonstrated that the endothelial cell surface receptor thrombomodulin (TM), a protein with anticoagulant, anti-inflammatory and antioxidant properties, mitigates radiation-induced lethality in mice. The goal of this study was to determine whether recombinant TM (Solulin) can protect the intestine from toxicity in a clinically relevant rat model. A 4 cm loop of rat small bowel was exposed to fractionated 5 Gy X radiation for 9 consecutive days. The animals were randomly assigned to receive daily subcutaneous injections of vehicle or Solulin (3 mg/kg/day or 10 mg/kg/day) for 27 days starting 4 days before irradiation. Early intestinal injury was assessed two weeks after irradiation by quantitative histology, morphometry, immunohistochemistry and luminol bioluminescence imaging. Solulin treatment significantly ameliorated intestinal radiation injury, made evident by a decrease in myeloperoxidase (MPO) activity, transforming growth factor beta (TGF-β) immunoreactivity, collagen-I deposition, radiation injury score (RIS) and intestinal serosal thickening. These findings indicate the need for further development of Solulin as a prophylactic and/or therapeutic agent to mitigate radiation-induced intestinal damage.

Physiological cerebrovascular remodeling in response to chronic mild hypoxia: A role for activated protein C

Activated protein C (APC) is a serine protease that promotes favorable changes in vascular barrier integrity and post-ischemic angiogenic remodeling in animal models of ischemic stroke, and its efficacy is currently being investigated in clinical ischemic stroke trials. Interestingly, application of sub-clinical chronic mild hypoxia (CMH) (8% O2) also promotes angiogenic remodeling and increased tight junction protein expression, suggestive of enhanced blood-brain barrier (BBB) integrity, though the role of APC in mediating the influence of CMH has not been investigated. To examine this potential link, we studied CMH-induced cerebrovascular remodeling after treating mice with two different reagents: (i) a function-blocking antibody that neutralizes APC activity, and (ii) exogenous recombinant murine APC. While CMH promoted endothelial proliferation, increased vascular density, and upregulated the angiogenic endothelial integrins α5β1 and αvβ3, these events were almost completely abolished by functional blockade of APC. Consistent with these findings, addition of exogenous recombinant APC enhanced CMH-induced endothelial proliferation, expansion of total vascular area and further enhanced the CMH-induced right-shift in vessel size distribution. Taken together, our findings support a key role for APC in mediating physiological remodeling of cerebral blood vessels in response to CMH.

Disseminated intravascular coagulation - new pathophysiological concepts and impact on management

INTRODUCTION

Disseminated intravascular coagulation (DIC) is an intermediary mechanism of disease which develops secondary to many causes including sepsis, trauma and malignancies. This review attempts to summarise the new pathophysiological developments and the impact they have on the current and future management of DIC.

AREAS COVERED

Several publications detailing the pathophysiology of DIC and the clinical management were identified using a pubmed search. Expert commentary: In recent years, on the initiatives of the international society of thrombosis and haemostasis, important advances have been made on the diagnostic aspect of DIC. In addition, several researchers have focused on the pathophysiology of the condition which is likely to provide better diagnostic markers and targeted therapy. However, some confusion still exists in the definition and management of DIC since various specialists understands the mechanisms involved in DIC from different perspectives.

Non Activated Protein C Supplementation in Septic Pediatric Hematological Patients

The purpose of the study was to examine safety and efficacy of non-activated Protein C (PC) supplementation in our cohort of septic pediatric hematological patients. We conducted a retrospective study of 22 septic patients receiving human plasma-derived PC concentrate from 2008 to 2015 at our Pediatric Oncology Center (Bari, Italy). The Surviving sepsis campaign definitions for sepsis, severe sepsis and septic shock were used to define the patients' septic status. For each patient, we calculated Lansky performance status scale (LPSS) and a risk score defined the Hematologic risk score (HRS) that we created in 2007. Patients were defined as High risk for severe sepsis/septic shock in case of HRS>3. HRS<3 identified low risk patients. Baseline serum PC levels, PC administration dosage and duration and days until a 20% improvement in LPSS. Observed baseline serum PC levels (bPC) blood concentrations ranged from 31 to 80%. Patients received PC supplementation in case of low age-related bPC levels or >10% PC concentration decrease within 12 hours from the first evaluation. All patients received 80 U/kg/day PC, intravenously, every twenty-four hours. No drug-related adverse event was observed. The observed sepsis-related mortality rate in our cohort was 9%. PC supplementation in our cohort appeared to be safe, and, probably due to prompt PC administration, we observed an overall mortality that was much lower than expected mortality in cancer severe septic patients.

Differential effects on glial activation by a direct versus an indirect thrombin inhibitor

Thrombin is a potent regulator of brain function in health and disease, modulating glial activation and brain inflammation. Thrombin inhibitors, several of which are in clinical use as anti-coagulants, can reduce thrombin-dependent neuroinflammation in pathological conditions. However, their effects in a healthy CNS are largely unknown. In adult healthy mice, we compared the effects of treatment by the direct thrombin inhibitor dabigatran etexilate (DE), to those of warfarin, which acts by preventing vitamin K recycling essential for coagulation. After 4weeks, warfarin increased both astrocyte GFAP and microglia Iba-1 staining throughout the CNS; whereas DE reduced expression of both markers. Warfarin, but not DE, reduced sulfatide levels; and warfarin showed longer lasting changes in cerebellar gene expression. DE also reduced glial activation in a mouse model of Alzheimer's disease, although no changes in amyloid plaque burden were observed. These results suggest that treatment with direct thrombin inhibitors may be preferable to those agents which reduce vitamin K levels and have the potential to increase glial activation.

LXR agonist T0901317 upregulates thrombomodulin expression in glomerular endothelial cells by inhibition of nuclear factor‑κB

Dysfunction of glomerular endothelial cells (GECs) induces a variety of symptoms, including proteinuria, inflammation, vascular diseases, fibrosis and thrombosis. Thrombomodulin (TM) acts as a vasoprotective molecule on the surface of the vascular endothelial cells to maintain the homeostasis of the endothelial microenvironment by suppressing cellular proliferation, adhesion and inflammatory responses. Liver X receptor (LXR), a nuclear receptor (NR) and a bile acid‑activated transcription factor, regulates metabolism and cholesterol transport, vascular tension and inflammation. Previous studies indicated that TM expression is upregulated by various NRs; however, it is unclear whether pharmacological modulation of LXR may affect TM expression and GEC function. The current study revealed that LXR activation by its agonist, T0901317, upregulates the expression and activity of TM. This effect was mediated specifically through LXR‑α, and not through LXR‑β. Additionally, T0901317 treatment inhibited nuclear factor‑κB (NF‑κB) signaling and the secretion of high glucose‑induced proinflammatory mediators, including tumor necrosis factor‑α and interleukin‑1β in GECs. Co‑immunoprecipitation experiments determined that treatment with T0901317 enhances the interaction between LXR‑α and the transcriptional coactivator, p300, in GEC extracts. The present findings suggest that NF‑κB may be a negative regulator of TM expression, and its removal may contribute to TM gene expression, particularly when in competition with the T0901317‑enhanced formation of the LXR/p300 complex. Therefore, LXR may be a novel molecular target for manipulating TM in GECs, which may advance the treatment of endothelial cell‑associated diseases.

The pathobiology of pig-to-primate xenotransplantation: a historical review

The immunologic barriers to successful xenotransplantation are related to the presence of natural anti-pig antibodies in humans and non-human primates that bind to antigens expressed on the transplanted pig organ (the most important of which is galactose-α1,3-galactose [Gal]), and activate the complement cascade, which results in rapid destruction of the graft, a process known as hyperacute rejection. High levels of elicited anti-pig IgG may develop if the adaptive immune response is not prevented by adequate immunosuppressive therapy, resulting in activation and injury of the vascular endothelium. The transplantation of organs and cells from pigs that do not express the important Gal antigen (α1,3-galactosyltransferase gene-knockout [GTKO] pigs) and express one or more human complement-regulatory proteins (hCRP, e.g., CD46, CD55), when combined with an effective costimulation blockade-based immunosuppressive regimen, prevents early antibody-mediated and cellular rejection. However, low levels of anti-non-Gal antibody and innate immune cells and/or platelets may initiate the development of a thrombotic microangiopathy in the graft that may be associated with a consumptive coagulopathy in the recipient. This pathogenic process is accentuated by the dysregulation of the coagulation-anticoagulation systems between pigs and primates. The expression in GTKO/hCRP pigs of a human coagulation-regulatory protein, for example, thrombomodulin, is increasingly being associated with prolonged pig graft survival in non-human primates. Initial clinical trials of islet and corneal xenotransplantation are already underway, and trials of pig kidney or heart transplantation are anticipated within the next few years.

Haplotypes of the endothelial protein C receptor (EPCR) gene are not associated with severe malaria in Tanzania

Background

Endothelial protein C receptor (EPCR) was recently identified as a key receptor for Plasmodium falciparum erythrocyte membrane protein 1 mediating sequestration of P. falciparum-infected erythrocytes in patients suffering from severe malaria. Soluble EPCR (sEPCR) inhibits binding of P. falciparum to EPCR in vitro and increased levels of sEPCR have been associated with the H3 haplotype of the EPCR encoding PROCR gene. It has been hypothesized that elevated sEPCR levels, possibly linked to the PROCR H3 genetic variant, may confer protection against severe forms of malaria. This study determined the frequencies of PROCR haplotypes H1–4 and plasma levels of sEPCR in a Tanzanian study population to investigate a possible association with severe malaria.

Methods

Study participants were children under 5 years of age admitted at the Korogwe District Hospital (N = 143), and diagnosed as having severe malaria (N = 52; including cerebral malaria N = 17), uncomplicated malaria (N = 24), or an infection other than malaria (N = 67). In addition, blood samples from 71 children living in nearby villages were included. The SNPs defining the haplotypes of PROCR gene were determined by post-PCR ligation detection reaction-fluorescent microsphere assay.

Results

Individuals carrying at least one H3 allele had significantly higher levels of sEPCR than individuals with no H3 alleles (P < 0.001). No difference in the frequency of H3 was found between the non-malaria patients, malaria patients or the village population (P > 0.1). Plasma levels of sEPCR differed between these three groups, with higher sEPCR levels in the village population compared to the hospitalized patients (P < 0.001) and higher levels in malaria patients compared to non-malaria patients (P = 0.001). However, no differences were found in the distribution of H3 (P = 0.2) or levels of sEPCR (P = 0.8) between patients diagnosed with severe and uncomplicated malaria.

Conclusion

Frequencies of SNPs determining PROCR haplotypes were in concordance with other African studies. The PROCR H3 allele was associated with higher levels of sEPCR, confirming earlier findings, however, in this Tanzanian population; neither PROCR haplotype nor level of sEPCR was associated with severe malaria, however, larger studies are needed to confirm these findings.

Investigating the Pathogenesis of Severe Malaria: A Multidisciplinary and Cross-Geographical Approach

More than a century after the discovery of Plasmodium spp. parasites, the pathogenesis of severe malaria is still not well understood. The majority of malaria cases are caused by Plasmodium falciparum and Plasmodium vivax, which differ in virulence, red blood cell tropism, cytoadhesion of infected erythrocytes, and dormant liver hypnozoite stages. Cerebral malaria coma is one of the most severe manifestations of P. falciparum infection. Insights into its complex pathophysiology are emerging through a combination of autopsy, neuroimaging, parasite binding, and endothelial characterizations. Nevertheless, important questions remain regarding why some patients develop life-threatening conditions while the majority of P. falciparum-infected individuals do not, and why clinical presentations differ between children and adults. For P. vivax, there is renewed recognition of severe malaria, but an understanding of the factors influencing disease severity is limited and remains an important research topic. Shedding light on the underlying disease mechanisms will be necessary to implement effective diagnostic tools for identifying and classifying severe malaria syndromes and developing new therapeutic approaches for severe disease. This review highlights progress and outstanding questions in severe malaria pathophysiology and summarizes key areas of pathogenesis research within the International Centers of Excellence for Malaria Research program.

Diabetes increases the risk of deep-vein thrombosis and pulmonary embolism. A population-based cohort study

We evaluated the effects of diabetes on the risks of developing deep-vein thrombosis (DVT) and pulmonary embolism (PE) in a nationwide, population-based cohort study in Taiwan. The patients with newly diagnosed type 2 diabetes mellitus (T2DM) were identified, and DM-free controls were randomly selected from the general population and frequency-matched according to age, sex, and index year by using the records of the Longitudinal Health Insurance Database between 2000 and 2011. Both cohorts were followed up until the end of 2011 to measure the incidence of DVT and PE. We analysed the risks of DVT and PE using Cox proportional-hazards regression models. The overall incidence of VTE was higher in the T2DM patients than in the controls (12.0 vs 7.51 per 10,000 person-years). The T2DM patients exhibited a 1.44-fold adjusted hazard ratio (aHR) of VTE development compared with the controls (95% confidence interval [CI] = 1.27-1.63). The risks of DVT (aHR = 1.43, 95% CI = 1.23-1.65) and PE (aHR = 1.52, 95% CI = 1.22-1.90) were greater in the T2DM than those in the controls. The T2DM patients had a substantially higher risk of DVT (aHR = 5.10, 95% CI = 3.12-8.32) and PE (aHR = 7.50, 95% CI = 3.29-17.1) development than the controls did in adults aged 49 years and younger. In conclusion, the longitudinal nationwide cohort study indicated that T2DM patients carried greater risks of developing VTE than did the general population.

Cell painting with an engineered EPCR to augment the protein C system

The protein C (PC) system conveys beneficial anticoagulant and cytoprotective effects in numerous in vivo disease models. The endothelial protein C receptor (EPCR) plays a central role in these pathways as cofactor for PC activation and by enhancing activated protein C (APC)-mediated protease-activated receptor (PAR) activation. During inflammatory disease, expression of EPCR on cell membranes is often diminished thereby limiting PC activation and APC's effects on cells. Here a caveolae-targeting glycosylphosphatidylinositol (GPI)-anchored EPCR (EPCR-GPI) was engineered to restore EPCR's bioavailability via "cell painting." The painting efficiency of EPCR-GPI on EPCR-depleted endothelial cells was time- and dose-dependent. The EPCR-GPI bioavailability after painting was long lasting since EPCR surface levels reached 400 % of wild-type cells after 2 hours and remained > 200 % for 24 hours. EPCR-GPI painting conveyed APC binding to EPCR-depleted endothelial cells where EPCR was lost due to shedding or shRNA. EPCR painting normalised PC activation on EPCR-depleted cells indicating that EPCR-GPI is functional active on painted cells. Caveolin-1 lipid rafts were enriched in EPCR after painting due to the GPI-anchor targeting caveolae. Accordingly, EPCR painting supported PAR1 and PAR3 cleavage by APC and augmented PAR1-dependent Akt phosphorylation by APC. Thus, EPCR-GPI painting achieved physiological relevant surface levels on endothelial cells, restored APC binding to EPCR-depleted cells, supported PC activation, and enhanced APC-mediated PAR cleavage and cytoprotective signalling. Therefore, EPCR-GPI provides a novel tool to restore the bioavailability and functionality of EPCR on EPCR- depleted and -deficient cells.

Serum sEPCR levels are elevated in patients with Alzheimer's disease

Blood-brain barrier impairment and endothelial cell dysfunction have both been implicated in the pathogenesis of Alzheimer's disease (AD). The presence of vascular risk factors is also known to increase the risk of AD. Soluble endothelial protein C receptor (sEPCR) should thus produce procoagulant and proinflammatory effects. Serum sEPCR levels have been found to be associated with several diseases. To date, no reports have been published regarding serum sEPCR levels in AD. In this study, we found that serum sEPCR levels were significantly increased in patients with AD when compared to control participants (P = .0005). There was no significant difference between patients with mild cognitive impairment (MCI) and healthy controls (P = .055) or between patients with AD and MCI (P = .054). Importantly, our results also indicate that the degree of cognitive impairment is significantly correlated with serum sEPCR levels in all patients and healthy controls. These findings suggest that serum sEPCR levels could be a potential candidate for a biomarker panel for AD diagnosis.

Characterization of mice harboring a variant of EPCR with impaired ability to bind protein C: novel role of EPCR in hematopoiesis

Mutation of the PC-binding domain of EPCR results in viable mice that exhibit procoagulant and proinflammatory phenotype when challenged. EPCRR84A/R84A mice develop splenomegaly as a result of BM failure, suggesting that EPCR plays an important role in hematopoiesis.

Mycolactone-Dependent Depletion of Endothelial Cell Thrombomodulin Is Strongly Associated with Fibrin Deposition in Buruli Ulcer Lesions

A well-known histopathological feature of diseased skin in Buruli ulcer (BU) is coagulative necrosis caused by the Mycobacterium ulcerans macrolide exotoxin mycolactone. Since the underlying mechanism is not known, we have investigated the effect of mycolactone on endothelial cells, focussing on the expression of surface anticoagulant molecules involved in the protein C anticoagulant pathway. Congenital deficiencies in this natural anticoagulant pathway are known to induce thrombotic complications such as purpura fulimans and spontaneous necrosis. Mycolactone profoundly decreased thrombomodulin (TM) expression on the surface of human dermal microvascular endothelial cells (HDMVEC) at doses as low as 2ng/ml and as early as 8hrs after exposure. TM activates protein C by altering thrombin’s substrate specificity, and exposure of HDMVEC to mycolactone for 24 hours resulted in an almost complete loss of the cells’ ability to produce activated protein C. Loss of TM was shown to be due to a previously described mechanism involving mycolactone-dependent blockade of Sec61 translocation that results in proteasome-dependent degradation of newly synthesised ER-transiting proteins. Indeed, depletion from cells determined by live-cell imaging of cells stably expressing a recombinant TM-GFP fusion protein occurred at the known turnover rate. In order to determine the relevance of these findings to BU disease, immunohistochemistry of punch biopsies from 40 BU lesions (31 ulcers, nine plaques) was performed. TM abundance was profoundly reduced in the subcutis of 78% of biopsies. Furthermore, it was confirmed that fibrin deposition is a common feature of BU lesions, particularly in the necrotic areas. These findings indicate that there is decreased ability to control thrombin generation in BU skin. Mycolactone’s effects on normal endothelial cell function, including its ability to activate the protein C anticoagulant pathway are strongly associated with this. Fibrin-driven tissue ischemia could contribute to the development of the tissue necrosis seen in BU lesions.

Buruli ulcer (BU) is a neglected tropical disease that is most common in West Africa and parts of Australia, but has been reported from over 30 countries worldwide. The symptoms are painless ulcers of the skin caused by a bacterial infection. The bacteria, Mycobacterium ulcerans, produce a macrolide toxin called mycolactone. In this manuscript, we have studied the effects of mycolactone on endothelial cells, specialised cells that line blood vessels and form capillaries. One of the most important functions of these cells is to prevent blood from clotting inside the vessels. We show that mycolactone reduces the ability of cultured endothelial cells to anticoagulate blood, by blocking the expression of a protein called thrombomodulin. We went on to examine samples of BU patient skin and found that thrombomodulin is also reduced here, and that in contrast to normal skin large amounts of fibrin (one of the main constituents of blood clots) were present. This means that it may be useful to consider whether anticoagulants might improve the response to antibiotics and thereby improve treatment outcomes for BU patients.

Activated protein C modulates the proinflammatory activity of dendritic cells

Background

Previous studies have demonstrated the beneficial activity of activated protein C in allergic diseases including bronchial asthma and rhinitis. However, the exact mechanism of action of activated protein C in allergies is unclear. In this study, we hypothesized that pharmacological doses of activated protein C can modulate allergic inflammation by inhibiting dendritic cells.

Materials and methods

Dendritic cells were prepared using murine bone marrow progenitor cells and human peripheral monocytes. Bronchial asthma was induced in mice that received intratracheal instillation of ovalbumin-pulsed dendritic cells.

Results

Activated protein C significantly increased the differentiation of tolerogenic plasmacytoid dendritic cells and the secretion of type I interferons, but it significantly reduced lipopolysaccharide-mediated maturation and the secretion of inflammatory cytokines in myeloid dendritic cells. Activated protein C also inhibited maturation and the secretion of inflammatory cytokines in monocyte-derived dendritic cells. Activated protein C-treated dendritic cells were less effective when differentiating naïve CD4 T-cells from Th1 or Th2 cells, and the cellular effect of activated protein C was mediated by its receptors. Mice that received adoptive transfer of activated protein C-treated ovalbumin-pulsed dendritic cells had significantly less airway hyperresponsiveness, significantly decreased lung concentrations of Th1 and Th2 cytokines, and less plasma concentration of immunoglobulin E when compared to control mice.

Conclusion

These results suggest that dendritic cells mediate the immunosuppressive effect of activated protein C during allergic inflammation.

Activated protein C: biased for translation

The homeostatic blood protease, activated protein C (APC), can function as (1) an antithrombotic on the basis of inactivation of clotting factors Va and VIIIa; (2) a cytoprotective on the basis of endothelial barrier stabilization and anti-inflammatory and antiapoptotic actions; and (3) a regenerative on the basis of stimulation of neurogenesis, angiogenesis, and wound healing. Pharmacologic therapies using recombinant human and murine APCs indicate that APC provides effective acute or chronic therapies for a strikingly diverse range of preclinical injury models. APC reduces the damage caused by the following: ischemia/reperfusion in brain, heart, and kidney; pulmonary, kidney, and gastrointestinal inflammation; sepsis; Ebola virus; diabetes; and total lethal body radiation. For these beneficial effects, APC alters cell signaling networks and gene expression profiles by activating protease-activated receptors 1 and 3. APC's activation of these G protein-coupled receptors differs completely from thrombin's activation mechanism due to biased signaling via either G proteins or β-arrestin-2. To reduce APC-associated bleeding risk, APC variants were engineered to lack >90% anticoagulant activity but retain normal cell signaling. Such a neuroprotective variant, 3K3A-APC (Lys191-193Ala), has advanced to clinical trials for ischemic stroke. A rich data set of preclinical knowledge provides a solid foundation for potential translation of APC variants to future novel therapies.

EPCR-dependent PAR2 activation by the blood coagulation initiation complex regulates LPS-triggered interferon responses in mice

Infection and inflammation are invariably associated with activation of the blood coagulation mechanism, secondary to the inflammation-induced expression of the coagulation initiator tissue factor (TF) on innate immune cells. By investigating the role of cell-surface receptors for coagulation factors in mouse endotoxemia, we found that the protein C receptor (ProcR; EPCR) was required for the normal in vivo and in vitro induction of lipopolysaccharide (LPS)-regulated gene expression. In cultured bone marrow-derived myeloid cells and in monocytic RAW264.7 cells, the LPS-induced expression of functionally active TF, assembly of the ternary TF-VIIa-Xa initiation complex of blood coagulation, and the EPCR-dependent activation of protease-activated receptor 2 (PAR2) by the ternary TF-VIIa-Xa complex were required for the normal LPS induction of messenger RNAs encoding the TLR3/4 signaling adaptor protein Pellino-1 and the transcription factor interferon regulatory factor 8. In response to in vivo challenge with LPS, mice lacking EPCR or PAR2 failed to fully initiate an interferon-regulated gene expression program that included the Irf8 target genes Lif, Iigp1, Gbp2, Gbp3, and Gbp6. The inflammation-induced expression of TF and crosstalk with EPCR, PAR2, and TLR4 therefore appear necessary for the normal evolution of interferon-regulated host responses.

Endothelial cell protein C receptor: a multiliganded and multifunctional receptor

Endothelial cell protein C receptor (EPCR) was first identified and isolated as a cellular receptor for protein C on endothelial cells. EPCR plays a crucial role in the protein C anticoagulant pathway by promoting protein C activation. In the last decade, EPCR has received wide attention after it was discovered to play a key role in mediating activated protein C (APC)-induced cytoprotective effects, including antiapoptotic, anti-inflammatory, and barrier stabilization. APC elicits cytoprotective signaling through activation of protease activated receptor-1 (PAR1). Understanding how EPCR-APC induces cytoprotective effects through activation of PAR1, whose activation by thrombin is known to induce a proinflammatory response, has become a major research focus in the field. Recent studies also discovered additional ligands for EPCR, which include factor VIIa, Plasmodium falciparum erythrocyte membrane protein, and a specific variant of the T-cell receptor. These observations open unsuspected new roles for EPCR in hemostasis, malaria pathogenesis, innate immunity, and cancer. Future research on these new discoveries will undoubtedly expand our understanding of the role of EPCR in normal physiology and disease, as well as provide novel insights into mechanisms for EPCR multifunctionality. Comprehensive understanding of EPCR may lead to development of novel therapeutic modalities in treating hemophilia, inflammation, cerebral malaria, and cancer.

A hereditary bleeding disorder resulting from a premature stop codon in thrombomodulin (p.Cys537Stop)

In this study, we describe a novel thrombomodulin (TM) mutation (c.1611C>A) that codes for a change from cysteine 537 to a premature stop codon (p.Cys537Stop). Three members of a family with a history of posttraumatic bleeding were identified to be heterozygous for this TM mutation. All coagulation screening tests, coagulation factor assays, and platelet function test results were within normal limits. However, the endogenous thrombin potential was markedly reduced at low-tissue factor concentration, and failure to correct with normal plasma indicated the presence of a coagulation inhibitor. Plasma TM levels were highly elevated (433-845 ng/ml, normal range 2-8 ng/ml, equating to 5 to 10 nM), and the addition of exogenous protein C further decreased thrombin generation. The mutation, p.Cys537Stop, results in a truncation within the carboxyl-terminal transmembrane helix. We predict that as a consequence of the truncation, the variant TM is shed from the endothelial surface into the blood plasma. This would promote systemic protein C activation and early cessation of thrombin generation within a developing hemostatic clot, thereby explaining the phenotype of posttraumatic bleeding observed within this family.

Protease-activated receptor signalling by coagulation proteases in endothelial cells

Endothelial cells express several types of integral membrane protein receptors, which upon interaction and activation by their specific ligands, initiate a signalling network that links extracellular cues in circulation to various biological processes within a plethora of cells in the vascular system. A small family of G-protein coupled receptors, termed protease-activated receptors (PAR1-4), can be specifically activated by coagulation proteases, thereby modulating a diverse array of cellular activities under various pathophysiological conditions. Thrombin and all vitamin K-dependent coagulation proteases, with the exception of factor IXa for which no PAR signalling has been attributed, can selectively activate cell surface PARs on the vasculature. Thrombin can activate PAR1, PAR3 and PAR4, but not PAR2 which can be specifically activated by factors VIIa and Xa. The mechanistic details of the specificity of PAR signalling by coagulation proteases are the subject of extensive investigation by many research groups worldwide. However, analysis of PAR signalling data in the literature has proved to be challenging since a single coagulation protease can elicit different signalling responses through activation of the same PAR receptor in endothelial cells. This article is focused on briefly reviewing the literature with respect to determinants of the specificity of PAR signalling by coagulation proteases with special emphasis on the mechanism of PAR1 signalling by thrombin and activated protein C in endothelial cells.

Inflammation-associated activation of coagulation and immune regulation by the protein C pathway

The inflammation-induced activation of the protein C pathway provides negative feedback inhibition of coagulation and exerts coagulation-independent anti-inflammatory and cytoprotective effects. The balance between these activities of aPC modulates the outcome of diverse inflammatory diseases such as encephalitis, diabetes, and sepsis; and is affected by naturally occurring aPC-resistance of coagulation factor V Leiden.

The Northwick Park Heart Study: evidence from the laboratory

The Northwick Park Heart Study (NPHS) has shown associations of high plasma fibrinogen and factor VII (FVIIc) levels with the risk of death from coronary heart disease (CHD). The finding for fibrinogen has been confirmed in many other studies. Whereas one further study has found a similar prospective association for FVIIc, several have not. Experimental studies have demonstrated the impact that the coagulation activity of fibrinogen and FVIIc have on the progression and phenotype of atherosclerotic lesions. FVIIc-driven thrombin generation and fibrin formation within the vessel wall are important determinants of both plaque (in)stability and atherothrombosis. In blood, local concentrations of FVIIc and thrombin may be sufficient to allow interactions between these serine proteases and protease-activated receptors, to drive cellular inflammatory reactions that further promote these processes. Local fibrinogen concentrations dictate fibrin clot structure and resistance to fibrinolysis. Within the atherosclerotic plaque, coagulation reactions driven by proinflammatory stimuli may initially support lesion stability (as part of wound healing), but, with advanced inflammation, thrombin and fibrin generation diminish because of proteolytic activity contributing to plaque instability. The NPHS findings have proved controversial, but, in the light of current knowledge, a reappraisal of the importance of FVIIc and fibrinogen in atherosclerosis, atherothrombosis and CHD is justified. Hypercoagulability, reflected in turn by thrombin generation capacity, and local concentrations of coagulation proteins, including FVIIc and fibrinogen, is linked to plaque phenotype, and even minute local concentrations of fibrinogen and proteases such as FVIIc may affect thrombin generation capacity.

New insights into modulation of thrombin formation

Thrombin is a pleiotropic enzyme that regulates hemostasis and nonhemostatic functions, including an array of actions within and on the vasculature. Physiologically, thrombin generation serves mainly to protect against thrombosis, but also to maintain vascular endothelial integrity. This protective effect is mediated in part through generation of anticoagulant enzymes, including activated protein C, formed on the action of thrombin on the endothelial receptor thrombomodulin. Partly, thrombin's vascular effects are effectuated through interaction with protease-activated receptors on various cell types. Pathophysiologically, downregulation and shedding of anticoagulant-acting receptors such as thrombomodulin and endothelial protein C receptor may contribute to a shift in activities of thrombin towards thrombogenic and proinflammatory actions. This shift may typically occur in the process of atherosclerosis, leading to a proatherogenic direction of the effects of thrombin. Therapeutically, the long-term inhibition of thrombin may create new ways of reducing atherosclerosis burden, altering the plaque phenotype.

The coagulation system and its function in early immune defense

Blood coagulation has a Janus-faced role in infectious diseases. When systemically activated, it can cause serious complications associated with high morbidity and mortality. However, coagulation is also part of the innate immune system and its local activation has been found to play an important role in the early host response to infection. Though the latter aspect has been less investigated, phylogenetic studies have shown that many factors involved in coagulation have ancestral origins which are often combined with anti-microbial features. This review gives a general overview about the most recent advances in this area of research also referred to as immunothrombosis.

Functional Analysis of Two Haplotypes of the Human Endothelial Protein C Receptor Gene

Objective—

To confirm the effect of the endothelial protein receptor gene ( PROCR ) haplotypes H1 and H3 on venous thromboembolism (VTE), to study their effect on endothelial protein C receptor (EPCR) expression in human umbilical vein endothelial cells, and to investigate the functionality of H1 tagging single-nucleotide polymorphisms in an in vitro model.

Approach and Results—

Protein C (PC), activated PC, and soluble EPCR (sEPCR) levels were measured in 702 patients with VTE and 518 healthy individuals. All subjects were genotyped for PROCR H1 and H3. Human umbilical vein endothelial cells isolated from 111 umbilical cords were used to study the relation between PROCR haplotypes, PROCR mRNA, cellular distribution of EPCR, and rate of PC activation. Finally, the functionality of the intragenic PROCR H1 single-nucleotide polymorphisms was analyzed using a luciferase-based method. We confirmed that individuals carrying H1 have reduced VTE risk, increased plasma activated PC levels, and reduced plasma sEPCR levels and that individuals with the H3H3 genotype have an increased VTE risk and increased plasma sEPCR levels. In cultured human umbilical vein endothelial cells, H1 is associated with increased membrane-bound EPCR, increased rate of PC activation, and reduced sEPCR in conditioned medium, but does not significantly influence PROCR mRNA levels. In contrast, H3 is associated with reduced membrane-bound EPCR and increased sEPCR in human umbilical vein endothelial cell–conditioned medium, higher levels of a truncated mRNA isoform, and a lower rate of PC activation. Finally, we identified the g.2132T>C single-nucleotide polymorphism in intron 1 as an intragenic H1-specific functional single-nucleotide polymorphism.

Conclusions—

These results support a protective role of PROCR H1 against VTE and an increased risk of VTE associated with the H3 haplotype.

Switch from protective to adverse inflammation during influenza: viral determinants and hemostasis are caught as culprits

Influenza viruses cause acute respiratory infections, which are highly contagious and occur as seasonal epidemic and sporadic pandemic outbreaks. Innate immune response is activated shortly after infection with influenza A viruses (IAV), affording effective protection of the host. However, this response should be tightly regulated, as insufficient inflammation may result in virus escape from immunosurveillance. In contrast, excessive inflammation may result in bystander lung tissue damage, loss of respiratory capacity, and deterioration of the clinical outcome of IAV infections. In this review, we give a comprehensive overview of the innate immune response to IAV infection and summarize the most important findings on how the host can inappropriately respond to influenza.

Changes in plasma levels of natural anticoagulants in disseminated intravascular coagulation: high prognostic value of antithrombin and protein C in patients with underlying sepsis or severe infection

BACKGROUND

Dysfunctional natural anticoagulant systems enhance intravascular fibrin formation in disseminated intravascular coagulation (DIC), and plasma levels of natural anti coagulants can be used in the diagnosis and prognosis of DIC. Herein, the diagnostic value of 4 natural anticoagulants was assessed, and the prognostic value of antithrombin and protein C were validated in a large population.

METHODS

Part 1 study included 126 patients with clinically suspected DIC and estimated plasma levels of 4 candidate anticoagulant proteins: antithrombin, protein C, protein S, and protein Z. Part 2 comprised 1,846 patients, in whom plasma antithrombin and protein C levels were compared with other well-known DIC markers according to the underlying dis eases. The 28-day mortality rate was used to assess prognostic outcome.

RESULTS

Antithrombin and protein C showed higher areas under the ROC curve than protein S and protein Z. In part 2 of the study, antithrombin and protein C levels significantly correlated with DIC score, suggesting that these factors are good indicators of DIC severity. Antithrombin and protein C showed significant prognostic power in Kaplan-Meier analyses. In patients with sepsis/severe infection, antithrombin and protein C showed higher hazard ratios than D-dimer. Platelet count showed the highest hazard ratio in patients with hemato logic malignancy. In patients with liver disease, the hazard ratio for antithrombin levels was significantly high.

CONCLUSIONS

Decreased plasma anticoagulant levels reflect florid consumption of the physiologic defense system against DIC-induced hypercoagulation. Plasma antithrombin and protein C levels are powerful prognostic markers of DIC, especially in patients with sepsis/severe infection.

Differential contribution of FXa and thrombin to vascular inflammation in a mouse model of sickle cell disease

Activation of coagulation and vascular inflammation are prominent features of sickle cell disease (SCD). Previously, we have shown that inhibition of tissue factor (TF) attenuates activation of coagulation and vascular inflammation in mouse models of SCD. In this study, we examined the mechanism by which coagulation proteases enhance vascular inflammation in sickle BERK mice. To specifically investigate the contribution of FXa and thrombin, mice were fed chow containing either rivaroxaban or dabigatran, respectively. In addition, we used bone marrow transplantation to generate sickle mice deficient in either protease activated receptor-1 (PAR-1) or protease activated receptor-2 (PAR-2) on nonhematopoietic cells. FXa inhibition and PAR-2 deficiency in nonhematopoietic cells attenuated systemic inflammation, measured by plasma levels of interleukin-6 (IL-6). In contrast, neither thrombin inhibition nor PAR-1 deficiency in nonhematopoietic cells affected plasma levels of IL-6 in sickle mice. However, thrombin did contribute to neutrophil infiltration in the lung, independently of PAR-1 expressed by nonhematopoietic cells. Furthermore, the TF-dependent increase in plasma levels of soluble vascular cell adhesion molecule-1 in sickle mice was not mediated by FXa or thrombin. Our data indicate that TF, FXa, and thrombin differentially contribute to vascular inflammation in a mouse model of SCD.

Modulation of Protein C Activation by Histones, Platelet Factor 4, and Heparinoids

Objective—

Histones are detrimental in late sepsis. Both activated protein C (aPC) and heparin can reverse their effect. Here, we investigated whether histones can modulate aPC generation in a manner similar to another positively charged molecule, platelet factor 4, and how heparinoids (unfractionated heparin or oxygen-desulfated unfractionated heparin with marked decrease anticoagulant activity) may modulate this effect.

Approach and Results—

We measured in vitro and in vivo effects of histones, platelet factor 4, and heparinoids on aPC formation, activated partial thromboplastin time, and murine survival. In vitro, histones and platelet factor 4 both affect thrombin/thrombomodulin aPC generation following a bell-shaped curve, with a peak of >5-fold enhancement. Heparinoids shift these curves rightward. Murine aPC generation studies after infusions of histones, platelet factor 4, and heparinoids supported the in vitro data. Importantly, although unfractionated heparin and 2-O, 3-O desulfated heparin both reversed the lethality of high-dose histone infusions, only mice treated with 2-O, 3-O desulfated heparin demonstrated corrected activated partial thromboplastin times and had significant levels of aPC.

Conclusions—

Our data provide a new contextual model of how histones affect aPC generation, and how heparinoid therapy may be beneficial in sepsis. These studies provide new insights into the complex interactions controlling aPC formation and suggest a novel therapeutic interventional strategy.

Pathophysiologic mechanisms in septic shock

The systemic inflammatory response that occurs in the septic patient as a result of an infectious insult affects multiple organs and systems, causing numerous physiological derangements. Alterations in phagocytic, lymphocytic and endothelial cell function and immune regulation are evident, leading to heterogeneity in a host's response to a septic challenge. In addition, the normal hemostatic balance shifts toward a procoagulant state through alterations in tissue factor, antithrombin, protein C and the inhibition of fibinolysis, which can result in thrombus formation and paradoxical hemostatic failure. In an effort to diagnose sepsis and predict outcomes, biomarkers such as C-reactive protein, pro-calcitonin, pro- and anti-inflammatory cytokines have been investigated with varying results. Targeted therapies for sepsis, most notably Xigris (recombinant human activated protein C), have proven unsuccessful and treatment continues to remain reliant on source control, antibiotics and supportive interventions, specifically early goal-directed therapy. This brief review gives an overview of the immunopathologic and coagulopathic alterations that occur in sepsis, soluble inflammatory mediators as potential diagnostic and prognostic biomarkers, and the clinical management of the septic patient.

Activated protein C and its potential applications in prevention of islet β-cell damage and diabetes

Activated protein C (APC) is derived from its precursor, protein C (PC). Originally thought to be synthesized exclusively by the liver, recent reports have shown that PC is also produced by many other cells including pancreatic islet β cells. APC functions as a physiological anticoagulant with anti-inflammatory, anti-apoptotic, and barrier-stabilizing properties. APC exerts its protective effects via an intriguing mechanism requiring combinations of endothelial PC receptor, protease-activated receptors, epidermal growth factor receptor, Tie2 or CD11b, depending on cell types. Diabetes is a chronic condition resulted from the body's inability to produce and/or properly use insulin. The prevalence of diabetes has risen dramatically and has become one of the major causes of premature mortality and morbidity worldwide. Diabetes prevention is an ideal approach to reduce this burden. Type 1 and type 2 diabetes are the major forms of diabetes mellitus, and both are characterized by an autoimmune response, intraislet inflammation, β-cell apoptosis, and progressive β-cell loss. Protecting β-cell from damage is critical in both prevention and treatment of diabetes. Recent in vitro and animal studies show that APC's strong anti-inflammatory and anti-apoptotic properties are beneficial in preventing β-cell destruction and diabetes in the NOD mouse model of type 1 diabetes. Future preventive and therapeutic uses of APC in diabetes look very promising.

The endothelial protein C receptor and malaria

In this issue of Blood, Moxon et al provide novel insight into the pathogenesis of cerebral malaria, linking loss of the endothelial protein C receptor (EPCR) on brain vessels, caused by cytoadherent infected erythrocytes, with localized coagulation, inflammation, and disruption of endothelial barrier function.

Dose-dependent differential effects of thrombin in allergic bronchial asthma

BACKGROUND

Apart from its role in the coagulation system, thrombin plays an important role in the inflammatory response through its protease-activated receptors (PARs). However, the role of thrombin in the immune response is not clear.

OBJECTIVE

To evaluate whether thrombin has a modulatory role in allergic bronchial asthma.

METHODS

Bronchial asthma was induced in mice by intraperitoneal sensitization and inhalation challenge with ovalbumin. Thrombin or its inhibitors were administered by inhalation before each allergen challenge.

RESULTS

Mice with low but sustained coagulation activation had reduced allergic inflammation, and allergic asthma was inhibited by low doses of thrombin but worsened by high doses. Allergic asthma was worsened by antithrombin, argatroban, hirudin, and anti-thrombomodulin antibody. Mice with a higher level of an inhibitor of both thrombin and activated protein C had worse disease. Heterozygous PAR-1 mice had less allergic inflammation, but PAR-1 agonist worsened it. Allergic bronchial inflammation was worsened in mice that received adoptive transfer of PAR-1 agonist-treated Th2 cells as compared with controls. Low levels of thrombin suppressed the maturation and secretion of cytokines in dendritic cells, but high levels enhanced this.

CONCLUSIONS

The effects of thrombin on allergic asthma are dose-dependent, with detrimental effects at high doses and protective effects at low doses. These data demonstrate that thrombin modulates the outcome in allergic bronchial asthma.

Inhibition of endogenous activated protein C attenuates experimental autoimmune encephalomyelitis by inducing myeloid-derived suppressor cells

Activated protein C (PC) is an anticoagulant involved in the interactions between the coagulation and immune systems. Activated PC has broad anti-inflammatory effects that are mediated through its ability to modulate leukocyte function and confer vascular barrier protection. We investigated the influence of activated PC on the pathogenesis of experimental autoimmune encephalomyelitis (EAE), the animal model for multiple sclerosis. We modulated activated PC levels in the circulation during EAE induction through systemic administration of a mAb against PC/activated PC (anti-PC). We initially hypothesized that inhibition of activated PC may result in a heightened inflammatory environment, leading to increased EAE pathogenesis. Contrary to this hypothesis, mice treated with anti-PC Ab (anti-PC mice) exhibited attenuated EAE. Interestingly, despite reduced disease severity and minimal pathogenic conditions in the CNS, anti-PC mice exhibited considerable leukocyte infiltration in the brain, comparable to control mice with severe EAE. Furthermore, CD4(+) T cells were diminished in the periphery of anti-PC mice, whereas various CD11b(+) populations were increased, notably the myeloid-derived suppressor cells (MDSCs), a CD11b(+) subset characterized as potent T cell suppressors. MDSCs from anti-PC mice exhibited increased expression of T cell suppressive factors and effectively inhibited T cell proliferation. Overall, our findings show that activated PC inhibition affected EAE pathogenesis at multiple fronts, specifically increasing vascular barrier permeability, as evidenced by considerable leukocyte infiltration in the brain. Additionally, inhibition of activated PC modulated the functional responses of CD11b(+) cells, leading to the expansion and increased activation of MDSCs, which are suppressive to the CD4(+) T cells required for EAE progression, thereby resulting in attenuated EAE.

Structural and functional studies of γ-carboxyglutamic acid domains of factor VIIa and activated Protein C: role of magnesium at physiological calcium

Crystal structures of factor (F) VIIa/soluble tissue factor (TF), obtained under high Mg(2+) (50mM Mg(2+)/5mM Ca(2+)), have three of seven Ca(2+) sites in the γ-carboxyglutamic acid (Gla) domain replaced by Mg(2+) at positions 1, 4, and 7. We now report structures under low Mg(2+) (2.5mM Mg(2+)/5mM Ca(2+)) as well as under high Ca(2+) (5mM Mg(2+)/45 mM Ca(2+)). Under low Mg(2+), four Ca(2+) and three Mg(2+) occupy the same positions as in high-Mg(2+) structures. Conversely, under low Mg(2+), reexamination of the structure of Gla domain of activated Protein C (APC) complexed with soluble endothelial Protein C receptor (sEPCR) has position 4 occupied by Ca(2+) and positions 1 and 7 by Mg(2+). Nonetheless, in direct binding experiments, Mg(2+) replaced three Ca(2+) sites in the unliganded Protein C or APC. Further, the high-Ca(2+) condition was necessary to replace Mg4 in the FVIIa/soluble TF structure. In biological studies, Mg(2+) enhanced phospholipid binding to FVIIa and APC at physiological Ca(2+). Additionally, Mg(2+) potentiated phospholipid-dependent activations of FIX and FX by FVIIa/TF and inactivation of activated factor V by APC. Since APC and FVIIa bind to sEPCR involving similar interactions, we conclude that under the low-Mg(2+) condition, sEPCR binding to APC-Gla (or FVIIa-Gla) replaces Mg4 by Ca4 with an attendant conformational change in the Gla domain ω-loop. Moreover, since phospholipid and sEPCR bind to FVIIa or APC via the ω-loop, we predict that phospholipid binding also induces the functional Ca4 conformation in this loop. Cumulatively, the data illustrate that Mg(2+) and Ca(2+) act in concert to promote coagulation and anticoagulation.

Mechanisms of anticoagulant and cytoprotective actions of the protein C pathway

The protein C pathway provides multiple important functions to maintain a regulated balance between hemostasis and host defense systems in response to vascular and inflammatory injury. The anticoagulant protein C pathway is designed to regulate coagulation, maintain the fluidity of blood within the vasculature, and prevent thrombosis, whereas the cytoprotective protein C pathway prevents vascular damage and stress. The cytoprotective activities of activated protein C (APC) include anti-apoptotic activity, anti-inflammatory activity, beneficial alterations of gene expression profiles, and endothelial barrier stabilization. These cytoprotective activities of APC, which require the endothelial protein C receptor (EPCR) and protease-activated receptor-1 (PAR1), have been a major research focus. Recent insights, such as non-canonical activation of PAR1 at Arg46 by APC and biased PAR1 signaling, provided better understanding of the molecular mechanisms by which APC elicits cytoprotective signaling through cleavage of PAR1. The discovery and development of anticoagulant-selective and cytoprotective-selective APC mutants provided unique opportunities for preclinical research that has been and may continue to be translated to clinical research. New mechanisms for the regulation of EPCR functionality, such as modulation of EPCR-bound lipids that affect APC's cytoprotective activities, may provide new research directions to improve the efficacy of APC to convey its cytoprotective activities to cells. Moreover, emerging novel functions for EPCR expand the roles of EPCR beyond mediating protein C activation and APC-induced PAR1 cleavage. These discoveries increasingly develop our understanding of the protein C pathway, which will conceivably expand its physiological implications to many areas in the future.

Endothelial protease nexin-1 is a novel regulator of A disintegrin and metalloproteinase 17 maturation and endothelial protein C receptor shedding via furin inhibition

OBJECTIVE

Human protein C is a plasma serine protease that plays a key role in hemostasis, and activated protein C (aPC) is known to elicit protective responses in vascular endothelial cells. This cytoprotective activity requires the interaction of the protease with its cell membrane receptor, endothelial protein C receptor. However, the mechanisms regulating the beneficial cellular effects of aPC are not well known. We aimed to determine whether a serine protease inhibitor called protease nexin-1 (PN-1) or serpinE2, expressed by vascular cells, can modulate the effect of aPC on endothelial cells.

APPROACH AND RESULTS

We found that vascular barrier protective and antiapoptotic activities of aPC were reduced both in endothelial cells underexpressing PN-1 and in endothelial cells whose PN-1 function was blocked by a neutralizing antibody. Our in vitro data were further confirmed in vivo. Indeed, we found that vascular endothelial growth factor-mediated hyperpermeability in the skin of mice was markedly reduced by local intradermal injection of aPC in wild-type mice but not in PN-1-deficient mice. Furthermore, we demonstrated a previously unknown protective role of endothelial PN-1 on endothelial protein C receptor shedding. We provided evidence that PN-1 inhibits furin, a serine protease that activates a disintegrin and metalloproteinase 17 involved in the shedding of endothelial protein C receptor. We indeed evidenced a direct interaction between PN-1 and furin in endothelial cells.

CONCLUSIONS

Our results thus demonstrate an original role of PN-1 as a furin convertase inhibitor, providing new insights for understanding the regulation of endothelial protein C receptor-dependent aPC endothelial protective effects.