Proteolysis of the endothelial cell protein C receptor by neutrophil proteinase 3

Endothelial Protein C Receptor and Its Impact on Rheumatic Disease

Endothelial Protein C Receptor (EPCR) is a key regulator of the activated protein C anti-coagulation pathway due to its role in the binding and activation of this protein. EPCR also binds to other ligands such as Factor VII and X, γδ T-cells, plasmodium falciparum erythrocyte membrane protein 1, and Secretory group V Phospholipases A2, facilitating ligand-specific functions. The functions of EPCR can also be regulated by soluble (s)EPCR that competes for the binding sites of membrane-bound (m)EPCR. sEPCR is created when mEPCR is shed from the cell surface. The propensity of shedding alters depending on the genetic haplotype of the EPCR gene that an individual may possess. EPCR plays an active role in normal homeostasis, anti-coagulation pathways, inflammation, and cell stemness. Due to these properties, EPCR is considered a potential effector/mediator of inflammatory diseases. Rheumatic diseases such as rheumatoid arthritis and systemic lupus erythematosus are autoimmune/inflammatory conditions that are associated with elevated EPCR levels and disease activity, potentially driven by EPCR. This review highlights the functions of EPCR and its contribution to rheumatic diseases.

Pathogenesis and Therapy of Coagulation Disorders in Severe Acute Pancreatitis

Ischemia superimposed upon pancreatic edema leads to acute necrotizing pancreatitis. One possible mechanism contributing to ischemia is intravascular thrombogenesis since fibrin deposits have been detected in pancreatic capillaries by electron microscope. Current experimental and clinical data provided compelling evidence that the disorders in the blood coagulation system play a critical role in the pathogenesis of severe acute pancreatitis (SAP). This leads to microcirculatory failure of intra- and extrapancreatic organs and multiple organ failure and increases the case fatality rate. However, the mechanism of coagulopathy underlying SAP is not yet clear, although some anticoagulant drugs have entered clinical practice showing improvement in prognosis. Thus, enhanced understanding of the process might improve the treatment strategies with safety and high efficacy. Herein, the pathogenesis of the coagulation system of SAP was reviewed with a focus on the coagulation pathway, intercellular interactions, and complement system, thereby illustrating some anticoagulant therapies and potential therapeutic targets.

Neutrophils and Malaria

Neutrophils are abundant in the circulation and are one of the immune system's first lines of defense against infection. There has been substantial work carried out investigating the role of neutrophils in malaria and it is clear that during infection neutrophils are activated and are capable of clearing malaria parasites by a number of mechanisms. This review focuses on neutrophil responses to human malarias, summarizing evidence which helps us understand where neutrophils are, what they are doing, how they interact with parasites as well as their potential role in vaccine mediated immunity. We also outline future research priorities for these, the most abundant of leukocytes.

A Linear Temporal Increase in Thrombin Activity and Loss of Its Receptor in Mouse Brain following Ischemic Stroke

BACKGROUND

Brain thrombin activity is increased following acute ischemic stroke and may play a pathogenic role through the protease-activated receptor 1 (PAR1). In order to better assess these factors, we obtained a novel detailed temporal and spatial profile of thrombin activity in a mouse model of permanent middle cerebral artery occlusion (pMCAo).

METHODS

Thrombin activity was measured by fluorescence spectroscopy on coronal slices taken from the ipsilateral and contralateral hemispheres 2, 5, and 24 h following pMCAo (n = 5, 6, 5 mice, respectively). Its spatial distribution was determined by punch samples taken from the ischemic core and penumbra and further confirmed using an enzyme histochemistry technique (n = 4). Levels of PAR1 were determined using western blot.

RESULTS

Two hours following pMCAo, thrombin activity in the stroke core was already significantly higher than the contralateral area (11 ± 5 vs. 2 ± 1 mU/ml). At 5 and 24 h, thrombin activity continued to rise linearly (r = 0.998, p = 0.001) and to expand in the ischemic hemisphere beyond the ischemic core reaching deleterious levels of 271 ± 117 and 123 ± 14 mU/ml (mean ± SEM) in the basal ganglia and ischemic cortex, respectively. The peak elevation of thrombin activity in the ischemic core that was confirmed by fluorescence histochemistry was in good correlation with the infarcts areas. PAR1 levels in the ischemic core decreased as stroke progressed and thrombin activity increased.

CONCLUSION

In conclusion, there is a time- and space-related increase in brain thrombin activity in acute ischemic stroke that is closely related to the progression of brain damage. These results may be useful in the development of therapeutic strategies for ischemic stroke that involve the thrombin-PAR1 pathway in order to prevent secondary thrombin related brain damage.

Proteinases, Their Extracellular Targets, and Inflammatory Signaling

Given that over 2% of the human genome codes for proteolytic enzymes and their inhibitors, it is not surprising that proteinases serve many physiologic-pathophysiological roles. In this context, we provide an overview of proteolytic mechanisms regulating inflammation, with a focus on cell signaling stimulated by the generation of inflammatory peptides; activation of the proteinase-activated receptor (PAR) family of G protein-coupled receptors (GPCR), with a mechanism in common with adhesion-triggered GPCRs (ADGRs); and by proteolytic ion channel regulation. These mechanisms are considered in the much wider context that proteolytic mechanisms serve, including the processing of growth factors and their receptors, the regulation of matrix-integrin signaling, and the generation and release of membrane-tethered receptor ligands. These signaling mechanisms are relevant for inflammatory, neurodegenerative, and cardiovascular diseases as well as for cancer. We propose that the inflammation-triggering proteinases and their proteolytically generated substrates represent attractive therapeutic targets and we discuss appropriate targeting strategies.

Activated Neutrophils Are Associated with Pediatric Cerebral Malaria Vasculopathy in Malawian Children

Most patients with cerebral malaria (CM) sustain cerebral microvascular sequestration of Plasmodium falciparum-infected red blood cells (iRBCs). Although many young children are infected with P. falciparum, CM remains a rare outcome; thus, we hypothesized that specific host conditions facilitate iRBC cerebral sequestration. To identify these host factors, we compared the peripheral whole-blood transcriptomes of Malawian children with iRBC cerebral sequestration, identified as malarial-retinopathy-positive CM (Ret+CM), to the transcriptomes of children with CM and no cerebral iRBC sequestration, defined as malarial-retinopathy-negative CM (Ret-CM). Ret+CM was associated with upregulation of 103 gene set pathways, including cytokine, blood coagulation, and extracellular matrix (ECM) pathways (P < 0.01; false-discovery rate [FDR] of <0.05). Neutrophil transcripts were the most highly upregulated individual transcripts in Ret+CM patients. Activated neutrophils can modulate diverse host processes, including the ECM, inflammation, and platelet biology to potentially facilitate parasite sequestration. Therefore, we compared plasma neutrophil proteins and neutrophil chemotaxis between Ret+CM and Ret-CM patients. Plasma levels of human neutrophil elastase, myeloperoxidase, and proteinase 3, but not lactoferrin or lipocalin, were elevated in Ret+CM patients, and neutrophil chemotaxis was impaired, possibly related to increased plasma heme. Neutrophils were rarely seen in CM brain microvasculature autopsy samples, and no neutrophil extracellular traps were found, suggesting that a putative neutrophil effect on endothelial cell biology results from neutrophil soluble factors rather than direct neutrophil cellular tissue effects. Meanwhile, children with Ret-CM had lower levels of inflammation, higher levels of alpha interferon, and upregulation of Toll-like receptor pathways and other host transcriptional pathways, which may represent responses that do not favor cerebral iRBC sequestration.

There were approximately 198 million cases of malaria worldwide in 2013, with an estimated 584,000 deaths occurring mostly in sub-Saharan African children. CM is a severe and rare form of Plasmodium falciparum infection and is associated with high rates of mortality and neurological morbidity, despite antimalarial treatment. A greater understanding of the pathophysiology of CM would allow the development of adjunctive therapies to improve clinical outcomes. A hallmark of CM is cerebral microvasculature sequestration of P. falciparum-infected red blood cells (iRBCs), which results in vasculopathy in some patients. Our data provide a global analysis of the host pathways associated with CM and newly identify an association of activated neutrophils with brain iRBC sequestration. Products of activated neutrophils could alter endothelial cell receptors and coagulation to facilitate iRBC adherence. Future studies can now examine the role of neutrophils in CM pathogenesis to improve health outcomes.

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.

Neutrophil elastase and proteinase-3 trigger G protein-biased signaling through proteinase-activated receptor-1 (PAR1)

Neutrophil proteinases released at sites of inflammation can affect tissue function by either activating or disarming signal transduction mediated by proteinase-activated receptors (PARs). Because PAR1 is expressed at sites where abundant neutrophil infiltration occurs, we hypothesized that neutrophil-derived enzymes might also regulate PAR1 signaling. We report here that both neutrophil elastase and proteinase-3 cleave the human PAR1 N terminus at sites distinct from the thrombin cleavage site. This cleavage results in a disarming of thrombin-activated calcium signaling through PAR1. However, the distinct non-canonical tethered ligands unmasked by neutrophil elastase and proteinase-3, as well as synthetic peptides with sequences derived from these novel exposed tethered ligands, selectively stimulated PAR1-mediated mitogen-activated protein kinase activation. This signaling was blocked by pertussis toxin, implicating a Gαi-triggered signal pathway. We conclude that neutrophil proteinases trigger biased PAR1 signaling and we describe a novel set of tethered ligands that are distinct from the classical tethered ligand revealed by thrombin. We further demonstrate the function of this biased signaling in regulating endothelial cell barrier integrity.

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.

Neutrophil proteinase 3 acts on protease-activated receptor-2 to enhance vascular endothelial cell barrier function

OBJECTIVE

The principle role of the vascular endothelium is to present a semi-impermeable barrier to soluble factors and circulating cells, while still permitting the passage of leukocytes from the bloodstream into the tissue. The process of diapedesis involves the selective disruption of endothelial cell junctions, which could compromise vascular integrity. It is therefore somewhat surprising that neutrophil transmigration does not significantly impair endothelial barrier function. We examined whether neutrophils might secrete factors that promote vascular integrity during the latter stages of neutrophil transmigration, in particular, the role of neutrophil serine proteinase 3 (PR3).

METHODS AND RESULTS

Endothelial cells were treated with PR3 either in its soluble form or in a complex form with cell surface NB1. We observed that PR3 mediated the enhancement of endothelial cell junctional integrity and that this required its proteolytic activity, as well as endothelial cell expression of the protease-activated receptor-2. Importantly, PR3 suppressed the vascular permeability changes and disruption of junctional proteins induced by the action of protease-activated receptor-1 agonists.

CONCLUSIONS

These findings establish the potential for neutrophil-derived PR3 to play a role in reestablishing vascular integrity after leukocyte transmigration and in protecting endothelial cells from protease-activated receptor-1-induced permeability changes that occur during thrombotic and inflammatory events.

PR3 and elastase alter PAR1 signaling and trigger vWF release via a calcium-independent mechanism from glomerular endothelial cells

Neutrophil proteases, proteinase-3 (PR3) and elastase play key roles in glomerular endothelial cell (GEC) injury during glomerulonephritis. Endothelial protease-activated receptors (PARs) are potential serine protease targets in glomerulonephritis. We investigated whether PAR1/2 are required for alterations in GEC phenotype that are mediated by PR3 or elastase during active glomerulonephritis. Endothelial PARs were assessed by flow cytometry. Thrombin, trypsin and agonist peptides for PAR1 and PAR2, TFLLR-NH(2) and SLIGKV-NH(2,) respectively, were used to assess alterations in PAR activation induced by PR3 or elastase. Endothelial von Willebrand Factor (vWF)release and calcium signaling were used as PAR activation markers. Both PR3 and elastase induced endothelial vWF release, with elastase inducing the highest response. PAR1 peptide induced GEC vWF release to the same extent as PR3. However, knockdown of PARs by small interfering RNA showed that neither PAR1 nor PAR2 activation caused PR3 or elastase-mediated vWF release. Both proteases interacted with and disarmed surface GEC PAR1, but there was no detectable interaction with cellular PAR2. Neither protease induced a calcium response in GEC. Therefore, PAR signaling and serine protease-induced alterations in endothelial function modulate glomerular inflammation via parallel but independent pathways.

The endothelial cell protein C receptor: cell surface conductor of cytoprotective coagulation factor signaling

Increasing evidence links blood coagulation proteins with the regulation of acute and chronic inflammatory disease. Of particular interest are vitamin K-dependent proteases, which are generated as a hemostatic response to vascular injury, but can also initiate signal transduction via interactions with vascular receptors. The endothelial cell protein C receptor (EPCR) is a multi-ligand vitamin K-dependent protein receptor for zymogen and activated forms of plasma protein C and factor VII. Although the physiological role of the EPCR-FVII(a) interaction is not well-understood, protein C binding to EPCR facilitates rapid generation of APC in response to excessive thrombin generation, and is a central requirement for the multiple signal-transduction cascades initiated by APC on both vascular endothelial and innate immune cells. Exciting recent studies have highlighted the emerging role of EPCR in modulating the cytoprotective properties of APC in a number of diverse inflammatory disorders. In this review, we describe the structure-function relationships, signal transduction pathways, and cellular interactions that enable EPCR to modulate the anticoagulant and anti-inflammatory properties of its vitamin K-dependent protein ligands, and examine the relevance of EPCR to both thrombotic and inflammation-associated disease.

Mechanisms and clinical implications of thrombosis in paroxysmal nocturnal hemoglobinuria

Paroxysmal nocturnal hemoglobinuria (PNH) is a rare acquired disease characterized by a clone of blood cells lacking glycosyl phosphatidylinositol (GPI)-anchored proteins at the cell membrane. Deficiency of the GPI-anchored complement inhibitors CD55 and CD59 on erythrocytes leads to intravascular hemolysis upon complement activation. Apart from hemolysis, another prominent feature is a highly increased risk of thrombosis. Thrombosis in PNH results in high morbidity and mortality. Often, thrombosis occurs at unusual locations, with the Budd–Chiari syndrome being the most frequent manifestation. Primary prophylaxis with vitamin K antagonists reduces the risk but does not completely prevent thrombosis. Eculizumab, a mAb against complement factor C5, effectively reduces intravascular hemolysis and also thrombotic risk. Therefore, eculizumab treatment has dramatically improved the prognosis of PNH. The mechanism of thrombosis in PNH is still unknown, but the highly beneficial effect of eculizumab on thrombotic risk suggests a major role for complement activation. Additionally, a deficiency of GPI-anchored proteins involved in hemostasis may be implicated.

Controlling coagulation dysregulation in xenotransplantation

PURPOSE OF REVIEW

Deletion of the α1,3-galactosyltransferase (GalT) gene in pigs has removed a major xenoantigen but has not eliminated the problem of dysregulated coagulation and vascular injury. Rejecting GalT knockout organ xenografts almost invariably show evidence of thrombosis and platelet sequestration, and primate recipients frequently develop consumptive coagulopathy. This review examines recent findings that illuminate potential mechanisms of this current barrier to successful xenotransplantation.

RECENT FINDINGS

The coagulation response to xenotransplantation differs depending on the type of organ and quite likely the distinct vasculatures. Renal xenografts appear more likely to initiate consumptive coagulopathy than cardiac xenografts, possibly reflecting differential transcriptional responses. Liver xenografts induce rapid and profound thrombocytopenia resulting in recipient death within days due to bleeding; ex-vivo data suggest that liver endothelial cells and hepatocytes are responsible for platelet consumption by a coagulation-independent process.It has been proposed that expression of recipient tissue factor on platelets and monocytes is an important trigger of consumptive coagulopathy. Finally, pigs transgenic for human anticoagulants and antithrombotics are slowly but surely coming on line, but have not yet been rigorously tested to date.

SUMMARY

Successful control of coagulation dysregulation in xenotransplantation may require different combinatorial pharmacological and genetic strategies for different organs.

Early rise in circulating endothelial protein C receptor correlates with poor outcome in severe sepsis

PURPOSE

The endothelial protein C receptor (EPCR) negatively regulates the coagulopathy and inflammatory response in sepsis. Mechanisms controlling the expression of cell-bound and circulating soluble EPCR (sEPCR) are still unclear. Moreover, the clinical impact of EPCR shedding and its potential value to predict sepsis progression and outcome remain to be established.

METHODS

We investigated the time course of plasma sEPCR over the 5 first days (D) of severe sepsis in 40 patients.

RESULTS

No significant difference was observed when comparing sEPCR at admission (D1) to healthy volunteers and to patients with vasculitis. We report that the kinetics profile of plasma sEPCR in patients was almost stable at the onset of sepsis with no change from D1 to D4 and then a significant decrease at D5. This pattern of release was consistently observed whatever the level of sEPCR at D1. Characteristics of patients or of infections (except Gram negative) had no or little critical influence on the sEPCR profile. However, we found that sEPCR kinetics was clearly associated with patient's outcome (D28 survival). We demonstrate that a significant but moderate (<15% of basal level) and transient increase in sEPCR level at D2 is associated with poor outcome at D28.

CONCLUSION

Severe sepsis, at the onset, only triggers moderate quantitative changes in plasma sEPCR levels. Our findings suggest that in severe sepsis, an early (at D2), transient but significant increase in circulating sEPCR may be detrimental suggesting that sEPCR could provide an early biological marker of sepsis outcome.

Activated protein C action in inflammation

Activated protein C (APC) is a natural anticoagulant that plays an important role in coagulation homeostasis by inactivating the procoagulation factor Va and VIIIa. In addition to its anticoagulation functions, APC also has cytoprotective effects such as anti-inflammatory, anti-apoptotic, and endothelial barrier protection. Recently, a recombinant form of human APC (rhAPC or drotrecogin alfa activated; known commercially as 'Xigris') was approved by the US Federal Drug Administration for treatment of severe sepsis associated with a high risk of mortality. Sepsis, also known as systemic inflammatory response syndrome (SIRS) resulting from infection, is a serious medical condition in critical care patients. In sepsis, hyperactive and dysregulated inflammatory responses lead to secretion of pro- and anti-inflammatory cytokines, activation and migration of leucocytes, activation of coagulation, inhibition of fibrinolysis, and increased apoptosis. Although initial hypotheses focused on antithrombotic and profibrinolytic functions of APC in sepsis, other agents with more potent anticoagulation functions were not effective in treating severe sepsis. Furthermore, APC therapy is also associated with the risk of severe bleeding in treated patients. Therefore, the cytoprotective effects, rather than the anticoagulant effect of APC are postulated to be responsible for the therapeutic benefit of APC in the treatment of severe sepsis.

Coagulation and innate immune responses: can we view them separately?

The horseshoe crab is often referred to as a "living fossil," representative of the oldest classes of arthropods, almost identical to species in existence more than 500 million years ago. Comparative analyses of the defense mechanisms used by the horseshoe crab that allowed it to survive mostly unchanged throughout the millennia reveal a common ancestry of the coagulation and innate immune systems that are totally integrated-indeed, almost inseparable. In human biology, we traditionally view the hemostatic pathways and those regulating innate immune responses to infections and tissue damage as entirely separate entities. But are they? The last couple of decades have revealed a remarkable degree of interplay between these systems, and the linking cellular and molecular mechanisms are rapidly being delineated. In this review, we present some of the major points of intersection between coagulation and innate immunity. We attempt to highlight the potential impact of these findings by identifying recently established paradigms that will hopefully result in the emergence of new strategies to treat a range of inflammatory and hemostatic disorders.

Regulation of endothelial protein C receptor shedding by cytokines is mediated through differential activation of MAP kinase signaling pathways

The endothelial protein C receptor (EPCR) plays a pivotal role in coagulation, inflammation, cell proliferation, and cancer, but its activity is markedly changed by ectodomain cleavage and release as the soluble protein (sEPCR). In this study we examined the mechanisms involved in the regulation of EPCR shedding in human umbilical endothelial cells (HUVEC). Interleukin-1beta (IL-1beta) and tumor necrosis factor-alpha (TNF-alpha), but not interferon-gamma and interleukin-6, suppressed EPCR mRNA transcription and cell-associated EPCR expression in HUVEC. The release of sEPCR induced by IL-1beta and TNF-alpha correlated with activation of p38 MAPK and c-Jun N-terminal kinase (JNK). EPCR shedding was also induced by phorbol 12-myristate 13-acetate, ionomycin, anisomycin, thiol oxidants or alkylators, thrombin, and disruptors of lipid rafts. Both basal and induced shedding of EPCR was blocked by the metalloproteinase inhibitors, TAPI-0 and GM6001, and by the reduced non-protein thiols, glutathione, dihydrolipoic acid, dithiothreitol, and N-acetyl-l-cysteine. Because other antioxidants and scavengers of reactive oxygen species failed to block the cleavage of EPCR, a direct suppression of metalloproteinase activity seems responsible for the observed effects of reduced thiols. In summary, the shedding of EPCR in HUVEC is effectively regulated by IL-1beta and TNF-alpha, and downstream by MAP kinase signaling pathways and metalloproteinases.

Protective mechanisms of activated protein C in severe inflammatory disorders

The protein C system is an important natural anticoagulant mechanism mediated by activated protein C (APC) that regulates the activity of factors VIIIa and Va. Besides well-defined anticoagulant properties, APC also demonstrates anti-inflammatory, anti-apoptotic and endothelial barrier-stabilizing effects that are collectively referred to as the cytoprotective effects of APC. Many of these beneficial effects are mediated through its co-receptor endothelial protein C receptor, and the protease-activated receptor 1, although exact mechanisms remain unclear and are likely pleiotropic in nature. Increased insight into the structure-function relationships of APC facilitated design of APC variants that conserve cytoprotective effects and reduce anticoagulant features, thereby attenuating the risk of severe bleeding with APC therapy. Impairment of the protein C system plays an important role in acute lung injury/acute respiratory distress syndrome and severe sepsis. The pathophysiology of both diseases states involves uncontrolled inflammation, enhanced coagulation and compromised fibrinolysis. This leads to microvascular thrombosis and organ injury. Administration of recombinant human APC to correct the dysregulated protein C system reduced mortality in severe sepsis patients (PROWESS trial), which stimulated further research into its mechanisms of action. Several other clinical trials evaluating recombinant human APC have been completed, including studies in children and less severely ill adults with sepsis as well as a study in acute lung injury. On the whole, these studies have not supported the use of APC in these populations and challenge the field of APC research to search for additional answers.

Alternative mRNA is favored by the A3 haplotype of the EPCR gene PROCR and generates a novel soluble form of EPCR in plasma

The endothelial cell protein C receptor also exists in soluble form in plasma (sEPCR), resulting from ADAM17 cleavage. Elevated sEPCR levels are observed in subjects carrying the A3 haplotype, which is characterized by a Ser219Gly substitution in the transmembrane domain, rendering the receptor more sensitive to cleavage. Because sEPCR production is not completely blocked by metalloprotease inhibition, we looked for another mechanism. Comparing mRNA expression patterns and levels in A3 and non-A3 cells from 32 human umbilical cord veins, we detected a truncated mRNA in addition to the full-length mRNA. This truncated mRNA was 16 times more abundant in A3 human umbilical vein endothelial cells than in non-A3 human umbilical vein endothelial cells and encoded a protein lacking the transmembrane domain. We stably expressed a recombinant form of this protein (rEPCRisoform) and a protein mimicking the plasma sEPCR (rEPCRsol). Functional studies of the purified recombinant proteins revealed that the rEPCRisoform bound to recombinant protein C with similar affinity than rEPCRsol and that it also inhibited the anticoagulant activity of APC. Trace amounts of the EPCR isoform were found in the plasma of A3 subjects. These results suggest that the sEPCRisoform could contribute to the regulatory effect of sEPCR in plasma.

Proteinases and signalling: pathophysiological and therapeutic implications via PARs and more

Proteinases like thrombin, trypsin and tissue kallikreins are now known to regulate cell signaling by cleaving and activating a novel family of G-protein-coupled proteinase-activated receptors (PARs 1-4) via exposure of a tethered receptor-triggering ligand. On their own, short synthetic PAR-selective PAR-activating peptides (PAR-APs) mimicking the tethered ligand sequences can activate PARs 1, 2 and 4 and cause physiological responses both in vitro and in vivo. Using the PAR-APs as sentinel probes in vivo, it has been found that PAR activation can affect the vascular, renal, respiratory, gastrointestinal, musculoskeletal and nervous systems (both central and peripheral nervous system) and can promote cancer metastasis and invasion. In general, responses triggered by PARs 1, 2 and 4 are in keeping with an innate immune inflammatory response, ranging from vasodilatation to intestinal inflammation, increased cytokine production and increased or decreased nociception. Further, PARs have been implicated in a number of disease states, including cancer and inflammation of the cardiovascular, respiratory, musculoskeletal, gastrointestinal and nervous systems. In addition to activating PARs, proteinases can cause hormone-like effects by other signalling mechanisms, like growth factor receptor activation, that may be as important as the activation of PARs. We, therefore, propose that the PARs themselves, their activating serine proteinases and their associated signalling pathways can be considered as attractive targets for therapeutic drug development. Thus, proteinases in general must now be considered as 'hormone-like' messengers that can signal either via PARs or other mechanisms.