Molecular cloning and expression of murine and bovine endothelial cell protein C/activated protein C receptor (EPCR). The structural and functional conservation in human, bovine, and murine EPCR

Anti-pig antibodies in swine veterinarian serum: Implications for clinical xenotransplantation

Recent clinical xenotransplantation and human decedent studies demonstrate that clinical hyperacute rejection of genetically engineered porcine organs can be reliably avoided but that antibody mediated rejection (AMR) continues to limit graft survival. We previously identified porcine glycans and proteins which are immunogenic after cardiac xenotransplantation in non-human primates, but the clinical immune response to antigens present in glycan depleted triple knockout (TKO) donor pigs is poorly understood. In this study we use fluorescence barcoded human embryonic kidney cells (HEK) and HEK cell lines expressing porcine glycans (Gal and SDa) or proteins (tetraspanin-29 [CD9], membrane cofactor protein [CD46], protectin, membrane attack complex inhibition factor [CD59], endothelial cell protein C receptor, and Annexin A2) to screen antibody reactivity in human serum from 160 swine veterinarians, a serum source with potential occupational immune challenge from porcine tissues and pathogens. High levels of anti-Gal IgM were present in all samples and lower levels of anti-SDa IgM were present in 41% of samples. IgM binding to porcine proteins, primarily CD9 and CD46, previously identified as immunogenic in pig to non-human primate cardiac xenograft recipients, was detected in 28 of the 160 swine veterinarian samples. These results suggest that barcoded HEK cell lines expressing porcine protein antigens can be useful for screening human patient serum. A comprehensive analysis of sera from clinical xenotransplant recipients to define a panel of commonly immunogenic porcine antigens will likely be necessary to establish an array of porcine non-Gal antigens for effective monitoring of patient immune responses and allow earlier therapies to reverse AMR.

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.

Embryonic vascular establishment requires protein C receptor-expressing endothelial progenitors

Vascular establishment is one of the early events in embryogenesis. It is believed that vessel-initiating endothelial progenitors cluster to form the first primitive vessel. Understanding the molecular identity of these progenitors is crucial in order to elucidate lineage hierarchy. In this study, we identify protein C receptor (Procr) as an endothelial progenitor marker and investigate the role of Procr+ progenitors during embryonic vascular development. Using a ProcrmGFP-2A-lacZ reporter, we reveal a much earlier Procr expression (embryonic day 7.5) than previously acknowledged (embryonic day 13.5). Genetic fate-mapping experiments using ProcrCre and ProcrCreER demonstrate that Procr+ cells give rise to blood vessels throughout the entire embryo proper. Single-cell RNA-sequencing analyses place Procr+ cells at the start of endothelial commitment and maturation. Furthermore, targeted ablation of Procr+ cells results in failure of vessel formation and early embryonic lethality. Notably, genetic fate mapping and scRNA-seq pseudotime analysis support the view that Procr+ progenitors can give rise to hemogenic endothelium. In this study, we establish a Procr expression timeline and identify Procr+ vessel-initiating progenitors, and demonstrate their indispensable role in establishment of the vasculature during embryo development.

Blocking human protein C anticoagulant activity improves clotting defects of hemophilia mice expressing human protein C

We generated novel hemophilia A or B mice expressing human protein C.

Selectively blocking the anticoagulant activity of human activated protein C improves the clotting defects in hemophilia mice.

Hemophilia A and B are hereditary coagulation defects resulting in unstable blood clotting and recurrent bleeding. Current factor replacement therapies have major limitations such as the short half-life of the factors and development of inhibitors. Alternative approaches to rebalance the hemostasis by inhibiting the anticoagulant pathways have recently gained considerable interest. In this study, we tested the therapeutic potential of a monoclonal antibody, HAPC1573, that selectively blocks the anticoagulant activity of human activated protein C (APC). We generated F8^−/− or F9^−/− hemophilia mice expressing human protein C by genetically replacing the murine Proc gene with the human PROC. The resulting PROC^+/+;F8^−/− or PROC^+/+;F9^−/− mice had bleeding characteristics similar to their corresponding F8^−/− or F9^−/− mice. Pretreating the PROC^+/+;F8^−/− mice with HAPC1573 shortened the tail bleeding time. HAPC1573 pretreatment significantly reduced mortality and alleviated joint swelling, similar to those treated with either FVIII or FIX, of either PROC^+/+;F8^−/− or PROC^+/+;F9^−/− mice in a needle puncture–induced knee-joint bleeding model. Additionally, we found that HAPC1573 significantly improved the thrombin generation of PROC^+/+;F8^−/− mice but not F8^−/− mice, indicating that HAPC1573 enhanced the coagulant activity of hemophilia mice by modulating human APC in vivo. We further documented that HAPC1573 inhibited the APC anticoagulant activity to improve the clotting time of human plasma deficient of FVIII, FIX, FXI, FVII, VWF, FV, or FX. These results demonstrate that selectively blocking the anticoagulant activity of human APC may be an effective therapeutic and/or prophylactic approach for bleeding disorders lacking FVIII, FIX, or other clotting factors.

Isolation, Maintenance and Expansion of Adult Hematopoietic Stem/Progenitor Cells and Leukemic Stem Cells

Hematopoietic stem cells (HSCs) are rare, self-renewing cells that perch on top of the hematopoietic tree. The HSCs ensure the constant supply of mature blood cells in a tightly regulated process producing peripheral blood cells. Intense efforts are ongoing to optimize HSC engraftment as therapeutic strategy to treat patients suffering from hematopoietic diseases. Preclinical research paves the way by developing methods to maintain, manipulate and expand HSCs ex vivo to understand their regulation and molecular make-up. The generation of a sufficient number of transplantable HSCs is the Holy Grail for clinical therapy. Leukemia stem cells (LSCs) are characterized by their acquired stem cell characteristics and are responsible for disease initiation, progression, and relapse. We summarize efforts, that have been undertaken to increase the number of long-term (LT)-HSCs and to prevent differentiation towards committed progenitors in ex vivo culture. We provide an overview and compare methods currently available to isolate, maintain and enrich HSC subsets, progenitors and LSCs and discuss their individual advantages and drawbacks.

Human platelets express endothelial protein C receptor, which can be utilized to enhance localization of factor VIIa activity

Essentials Factor VIIa binds activated platelets to promote hemostasis in hemophilia patients with inhibitors. The interactions and sites responsible for platelet-FVIIa binding are not fully understood. Endothelial cell protein C receptor (EPCR) is expressed on activated human platelets. EPCR binding enhances the efficacy of a FVIIa variant and could impact design of new therapeutics.

SUMMARY

Background High-dose factor VIIa (FVIIa) is routinely used as an effective bypassing agent to treat hemophilia patients with inhibitory antibodies that compromise factor replacement. However, the mechanism by which FVIIa binds activated platelets to promote hemostasis is not fully understood. FVIIa-DVQ is an analog of FVIIa with enhanced tissue factor (TF)-independent activity and hemostatic efficacy relative to FVIIa. Our previous studies have shown that FVIIa-DVQ exhibits greater platelet binding, thereby suggesting that features in addition to lipid composition contribute to platelet-FVIIa interactions. Objectives Endothelial cell protein C receptor (EPCR) also functions as a receptor for FVIIa on endothelial cells. We therefore hypothesized that an interaction with EPCR might play a role in platelet-FVIIa binding. Methods/results In the present study, we used flow cytometric analyses to show that platelet binding of both FVIIa and FVIIa-DVQ is partially inhibited in the presence of excess protein C or an anti-EPCR antibody. This decreased binding results in a corresponding decrease in the activity of both molecules in FXa and thrombin generation assays. Enhanced binding to EPCR was sufficient to account for the increased platelet binding of FVIIa-DVQ compared with wild-type FVIIa. As EPCR protein expression has not previously been shown in platelets, we confirmed the presence of EPCR in platelets using immunofluorescence, flow cytometry, immunoprecipitation, and mass spectrometry. Conclusions This work represents the first demonstration that human platelets express EPCR and suggests that modulation of EPCR binding could be utilized to enhance the hemostatic efficacy of rationally designed FVIIa analogs.

Expression of MHC class I-related molecules MICA, HLA-E and EPCR shape endothelial cells with unique functions in innate and adaptive immunity

Endothelial cells (ECs) located at the interface of blood and tissues display regulatory activities toward coagulation, inflammation and vascular homeostasis. By expressing MHC class I and II antigens, ECs also contribute to immune responses. In transplantation, graft ECs are both trigger and target of alloimmune responses. ECs express a set of MHC class I-like or structural related molecules such as HLA-E, MHC class I related chain A (MICA) and the endothelial protein C receptor (EPCR) that provide multiple and unique functions to ECs. HLA-E is a low polymorphic ligand for the CD94/NKG2A/C receptors, and triggers HLA-E-restricted CD8+αβT cell responses against viral and bacterial peptides. MICA is a highly polymorphic ligand for NKG2D activating NK and costimulating CD8+T cells and a ligand for tissue-resident Vδ1 γδ T subsets. More intriguing is the role of EPCR, a key regulator of coagulation, as a ligand for a circulating subset of Vδ2- γδ T cells. Coexpression of this set of MHC class I-related molecules that allow ECs to activate a subtle array of immune responses upon stress and infection may also influence transplant outcome. Here, the respective structure, expression, and functions of HLA-E, MICA and EPCR as well as the impact of their polymorphism are reviewed.

Regulation of long-term repopulating hematopoietic stem cells by EPCR/PAR1 signaling

The common developmental origin of endothelial and hematopoietic cells is manifested by coexpression of several cell surface receptors. Adult murine bone marrow (BM) long-term repopulating hematopoietic stem cells (LT-HSCs), endowed with the highest repopulation and self-renewal potential, express endothelial protein C receptor (EPCR), which is used as a marker to isolate them. EPCR/protease-activated receptor-1 (PAR1) signaling in endothelial cells has anticoagulant and anti-inflammatory roles, while thrombin/PAR1 signaling induces coagulation and inflammation. Recent studies define two new PAR1-mediated signaling cascades that regulate EPCR(+) LT-HSC BM retention and egress. EPCR/PAR1 signaling facilitates LT-HSC BM repopulation, retention, survival, and chemotherapy resistance by restricting nitric oxide (NO) production, maintaining NO(low) LT-HSC BM retention with increased VLA4 expression, affinity, and adhesion. Conversely, acute stress and clinical mobilization upregulate thrombin generation and activate different PAR1 signaling that overcomes BM EPCR(+) LT-HSC retention, inducing their recruitment to the bloodstream. Thrombin/PAR1 signaling induces NO generation, TACE-mediated EPCR shedding, and upregulation of CXCR4 and PAR1, leading to CXCL12-mediated stem and progenitor cell mobilization. This review discusses new roles for factors traditionally viewed as coagulation related, which independently act in the BM to regulate PAR1 signaling in bone- and blood-forming progenitor cells, navigating their fate by controlling NO production.

Activation of protein C and thrombin activable fibrinolysis inhibitor on cultured human endothelial cells

ESSENTIALS: It is unknown if thrombin activatable fibrinolysis inhibitor (TAFI) and protein C compete on cells. TAFI and protein C activation on endothelial cells was simultaneously quantified. TAFI and protein C do not compete for activation on endothelial cells. TAFI and protein C are independently recognized by the thrombin-thrombomodulin complex.

BACKGROUND

When bound to thrombomodulin (TM), thrombin is a potent activator of protein C (PC) and thrombin activable fibrinolysis inhibitor (TAFI). By binding PC and presenting it to the thrombin-TM complex, endothelial cell PC receptor (EPCR) enhances PC activation. It is unknown whether PC and TAFI compete for the thrombin-TM complex on endothelial cells.

OBJECTIVE

To compare PC and TAFI activation on the surface of cultured human endothelial cells in the absence or presence of JRK1535 and/or CTM1009, inhibitory antibodies directed against EPCR and TM, respectively, and to determine whether PC and TAFI compete with each other for activation.

METHODS

PC and TAFI activation on endothelial cells were compared, and the effect of PC on TAFI activation and TAFI on PC activation was determined in the absence or presence of JRK1535 and/or CTM1009.

RESULTS

In the absence of antibodies, activation of PC was four-fold faster than that of TAFI. Blocking EPCR with JRK1535 resulted in a 53-fold decrease in PC activation and no effect on TAFI activation. Blocking TM with CTM1009 inhibited both TAFI and PC activation. Neither TAFI nor PC competed with each other in the absence or presence of JRK1535.

CONCLUSIONS

PC and TAFI are concurrently activated in a TM-dependent manner and do not compete for the thrombin-TM complex, raising the possibility that they interact with distinct activation complexes. EPCR selectively enhances PC activation so that PC and TAFI activation kinetics become comparable on endothelial cells.

Vascular immunotargeting to endothelial determinant ICAM-1 enables optimal partnering of recombinant scFv-thrombomodulin fusion with endogenous cofactor

The use of targeted therapeutics to replenish pathologically deficient proteins on the luminal endothelial membrane has the potential to revolutionize emergency and cardiovascular medicine. Untargeted recombinant proteins, like activated protein C (APC) and thrombomodulin (TM), have demonstrated beneficial effects in acute vascular disorders, but have failed to have a major impact on clinical care. We recently reported that TM fused with an scFv antibody fragment to platelet endothelial cell adhesion molecule-1 (PECAM-1) exerts therapeutic effects superior to untargeted TM. PECAM-1 is localized to cell-cell junctions, however, whereas the endothelial protein C receptor (EPCR), the key co-factor of TM/APC, is exposed in the apical membrane. Here we tested whether anchoring TM to the intercellular adhesion molecule (ICAM-1) favors scFv/TM collaboration with EPCR. Indeed: i) endothelial targeting scFv/TM to ICAM-1 provides ~15-fold greater activation of protein C than its PECAM-targeted counterpart; ii) blocking EPCR reduces protein C activation by scFv/TM anchored to endothelial ICAM-1, but not PECAM-1; and iii) anti-ICAM scFv/TM fusion provides more profound anti-inflammatory effects than anti-PECAM scFv/TM in a mouse model of acute lung injury. These findings, obtained using new translational constructs, emphasize the importance of targeting protein therapeutics to the proper surface determinant, in order to optimize their microenvironment and beneficial effects.

Renal and urinary levels of endothelial protein C receptor correlate with acute renal allograft rejection

The Endothelial Protein C Receptor (EPCR) is expressed on leukocytes, on endothelium of large blood vessels and to a lesser extent on capillaries. Membrane bound EPCR plays an important role in the activation of protein C which has anticoagulant, anti-inflammatory and cytoprotective effects. After cleavage by a protease EPCR is also found as a soluble protein. Acute rejection of kidney allografts can be divided in T-cell-mediated rejection (TCMR) and antibody-mediated (ABMR) rejection. The latter is characterized by strong activation of coagulation. Currently no reliable non-invasive biomarkers are available to monitor rejection.

Renal biopsies were available from 81 renal transplant patients (33 without rejection, 26 TCMR and 22 ABMR), we had access to mRNA material, matched plasma and urine samples for a portion of this cohort. Renal EPCR expression was assessed by RT-PCR and immunostaining. Plasma and urine sEPCR levels were measured by ELISA.

ABMR patients showed higher levels of EPCR mRNA than TCMR patients. EPCR expression on glomeruli was significantly elevated in ABMR patients than in TCMR or control patients. In the peritubular capillaries EPCR expression was higher in ABMR patients than in control patients. EPCR expression was higher in tubules and arteries of rejection patients than in control patients. Plasma sEPCR levels did not differ. Urine sEPCR levels were more elevated in the ABMR group than in patients with TCMR or without rejection. ROC analysis demonstrated that urinary sEPCR is appropriate to discriminate between ABMR patients and TCMR or control patients. We conclude that urinary sEPCR could be a novel non-invasive biomarker of antibody mediated rejection in renal transplantation.

Mac-1 directly binds to the endothelial protein C-receptor: a link between the protein C anticoagulant pathway and inflammation?

OBJECTIVE

The endothelial protein C-receptor (EPCR) is an endothelial transmembrane protein that binds protein C and activated protein C (APC) with equal affinity, thereby facilitating APC formation. APC has anticoagulant, antiapoptotic and antiinflammatory properties. Soluble EPCR, released by the endothelium, may bind activated neutrophils, thereby modulating cell adhesion. EPCR is therefore considered as a possible link between the anticoagulant properties of protein C and the inflammatory response of neutrophils. In the present study, we aimed to provide proof of concept for a direct binding of EPCR to the β2-integrin Mac-1 on monocytic cells under static and physiological flow conditions.

MEASUREMENTS AND MAIN RESULTS

Under static conditions, human monocytes bind soluble EPCR in a concentration dependent manner, as demonstrated by flow cytometry. Binding can be inhibited by specific antibodies (anti-EPCR and anti-Mac-1). Specific binding was confirmed by a static adhesion assay, where a transfected Mac-1 expressing CHO cell line (Mac-1+ cells) bound significantly more recombinant EPCR compared to Mac-1+ cells blocked by anti-Mac-1-antibody and native CHO cells. Under physiological flow conditions, monocyte binding to the endothelium could be significantly blocked by both, anti-EPCR and anti-Mac-1 antibodies in a dynamic adhesion assay at physiological flow conditions. Pre-treatment of endothelial cells with APC (drotrecogin alfa) diminished monocyte adhesion significantly in a comparable extent to anti-EPCR.

CONCLUSIONS

In the present study, we demonstrate a direct binding of Mac-1 on monocytes to the endothelial protein C receptor under static and flow conditions. This binding suggests a link between the protein C anticoagulant pathway and inflammation at the endothelium side, such as in acute vascular inflammation or septicaemia.

Immunogenetics of the NKG2D ligand gene family

NKG2D ligands (NKG2DLs) are a group of major histocompatibility complex (MHC) class I-like molecules, the expression of which is induced by cellular stresses such as infection, tumorigenesis, heat shock, tissue damage, and DNA damage. They act as a molecular danger signal alerting the immune system for infected or neoplastic cells. Mammals have two families of NKG2DL genes: the MHC-encoded MIC gene family and the ULBP gene family encoded outside the MHC region in most mammals. Rodents such as mice and rats lack the MIC family of ligands. Interestingly, some mammals have NKG2DL-like molecules named MILL that are phylogenetically related to MIC, but do not function as NKG2DLs. In this paper, we review our current knowledge of the MIC, ULBP, and MILL gene families in representative mammalian species and discuss the origin and evolution of the NKG2DL gene family.

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.

Is EPCR a multi-ligand receptor? Pros and cons

In the last decade, the endothelial cell protein C/activated protein C receptor (EPCR) has received considerable attention. The role initially attributed to EPCR, i.e. the enhancement of protein C (PC) activation by the thrombin-thrombomodulin complex on the surface of the large vessels, although important, did not go beyond the haemostasis scenario. However, the discovery of the cytoprotective, anti-inflammatory and anti-apoptotic features of the activated PC (APC) and the required involvement of EPCR for APC to exert such actions did place the receptor in a privileged position in the crosstalk between coagulation and inflammation. The last five years have shown that PC/APC are not the only molecules able to interact with EPCR. Factor VII/VIIa (FVII/VIIa) and factor Xa (FXa), two other serine proteases that play a central role in haemostasis and are also involved in signalling processes influencing wound healing, tissue remodelling, inflammation or metastasis, have been reported to bind to EPCR. These observations have paved the way for an exploration of unsuspected new roles for the receptor. This review aims to offer a new image of EPCR in the light of its extended panel of ligands. A brief update of what is known about the APC-evoked EPCR-dependent cell signalling mechanisms is provided, but special care has been taken to assemble all the information available about the interaction of EPCR with FVII/VIIa and FXa.

Thrombomodulin and its role in inflammation

The goal is to provide an extensive review of the physiologic role of thrombomodulin (TM) in maintaining vascular homeostasis, with a focus on its anti-inflammatory properties. Data were collected from published research. TM is a transmembrane glycoprotein expressed on the surface of all vascular endothelial cells. Expression of TM is tightly regulated to maintain homeostasis and to ensure a rapid and localized hemostatic and inflammatory response to injury. By virtue of its strategic location, its multidomain structure and complex interactions with thrombin, protein C (PC), thrombin activatable fibrinolysis inhibitor (TAFI), complement components, the Lewis Y antigen, and the cytokine HMGB1, TM exhibits a range of physiologically important anti-inflammatory, anti-coagulant, and anti-fibrinolytic properties. TM is an essential cofactor that impacts on multiple biologic processes. Alterations in expression of TM and its partner proteins may be manifest by inflammatory and thrombotic disorders. Administration of soluble forms of TM holds promise as effective therapies for inflammatory diseases, and infections and malignancies that are complicated by disseminated intravascular coagulation.

Factor X/Xa elicits protective signaling responses in endothelial cells directly via PAR-2 and indirectly via endothelial protein C receptor-dependent recruitment of PAR-1

We recently demonstrated that the Gla domain-dependent interaction of protein C with endothelial protein C receptor (EPCR) leads to dissociation of the receptor from caveolin-1 and recruitment of PAR-1 to a protective signaling pathway. Thus, the activation of PAR-1 by either thrombin or PAR-1 agonist peptide elicited a barrier-protective response if endothelial cells were preincubated with protein C. In this study, we examined whether other vitamin K-dependent coagulation protease zymogens can modulate PAR-dependent signaling responses in endothelial cells. We discovered that the activation of both PAR-1 and PAR-2 in endothelial cells pretreated with factor FX (FX)-S195A, but not other procoagulant protease zymogens, also results in initiation of protective intracellular responses. Interestingly, similar to protein C, FX interaction with endothelial cells leads to dissociation of EPCR from caveolin-1 and recruitment of PAR-1 to a protective pathway. Further studies revealed that, FX activated by factor VIIa on tissue factor bearing endothelial cells also initiates protective signaling responses through the activation of PAR-2 independent of EPCR mobilization. All results could be recapitulated by the receptor agonist peptides to both PAR-1 and PAR-2. These results suggest that a cross-talk between EPCR and an unknown FX/FXa receptor, which does not require interaction with the Gla domain of FX, recruits PAR-1 to protective signaling pathways in endothelial cells.

Endothelial function and its implications for cardiovascular and renal disease in systemic lupus erythematosus

Vascular manifestations associated with systemic lupus erythematosus (SLE) span a broad range, including vasculopathy. An understudied pathway of this morbidity is a repair component. Recent studies have elevated the anti-injury biomarkers, adiponectin and membrane endothelial protein C receptor (EPCR), for consideration with roles to antagonize premature atherosclerosis and SLE nephritis, respectively. For example, adiponectin was found to serve as an independent predictor of carotid plaque, and its elevations were persistent over more than one visit. Unexpectedly, this biomarker was present despite clinical quiescence. In vasculopathy as a comorbidity to SLE nephritis, the persistent expression of membrane EPCR at peritubular capillaries may represent a response to the local cues of a deficit of active protein C. Under conditions of unresolved morbidity, higher levels of adiponectin and membrane EPCR may represent a physiologic attempt to limit further endothelial damage, and the observed increase in plaque and progression of SLE nephritis represent an overwhelming of this reparative process by disease-provoking stimuli.

Factor VIIa interaction with endothelial cells and endothelial cell protein C receptor

Plasma coagulation factor VIIa (FVIIa) initiates the coagulation cascade by binding to its cofactor, tissue factor (TF) on cell surfaces, which eventually leads to fibrin deposition and platelet activation. Recent studies showed that FVIIa also binds to endothelial cell protein C receptor (EPCR), a known cellular receptor for anticoagulant protein C\activated protein C, on the endothelium. The present article reviews our current knowledge of FVIIa interaction with EPCR and discusses the potential significance of this interaction in hemostasis, treatment of bleeding disorders with pharmacological doses of FVIIa and FVIIa clearance.

Recombinant expression of biologically active murine soluble EPCR

Endothelial cell protein C receptor (EPCR) downregulates the coagulation system and prevents thrombosis by binding to protein C/activated protein C (APC) and factor VII/activated factor VII (VIIa). Recombinant APC and factor VIIa have been shown to be useful in a variety of clinical conditions. Murine models could prove extremely helpful in order to study in vivo actions of these drugs. It is therefore crucial to demonstrate the interaction between these and murine EPCR. We expressed the extracellular region of the murine EPCR in a yeast expression system and obtained a colony of Pichia pastoris that produce high amounts of recombinant extracellular murine EPCR, which we purified by liquid chromatography to homogeneity. The analysis of the interaction of EPCR with APC and factor VIIa was carried out using surface plasmon resonance technology. Murine EPCR binds to APC and factor VIIa with similar affinity than human EPCR. As for human EPCR, the binding is Ca2+ dependent while Mg2+ ions optimize it. In conclusion, we succeeded in establishing a system to produce enough recombinant soluble murine EPCR to perform biochemical studies. Murine EPCR binds to human APC and factor VIIa, which opens up new possibilities for characterizing the in vivo effect of APC and factor VII by using murine models.

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

BACKGROUND

The endothelial cell protein C receptor (EPCR) presents protein C to the thrombin:thrombomodulin complex on the endothelium of large vessels, and enhances the generation of activated protein C (APC) and activation of protease-activated receptor-1. A previous report has demonstrated binding of soluble (s) EPCR to activated neutrophils via surface proteinase 3 (PR3).

METHODS

We now report further characterization of this interaction. Activated neutrophils and purified PR3 both decrease endothelial cell (EC) surface EPCR, suggestive of its proteolysis.

RESULTS

When added to purified recombinant sEPCR, PR3 produced multiple cleavages, with early products including 20 kDa N-terminal and C-terminal (after Lys(176)) fragments. The binding of active site blocked PR3 to sEPCR was studied by surface plasmon resonance. Estimates of the K(D) of 18.5-102 nM were obtained with heterogeneous binding, suggestive of more than a single interaction site.

CONCLUSIONS

This work demonstrates PR3 binding to and proteolysis of EPCR and suggests a mechanism by which anticoagulant and cell protective pathways can be down-regulated during inflammation.

Novel role of the human alveolar epithelium in regulating intra-alveolar coagulation

Intra-alveolar fibrin deposition is a common response to localized and diffuse lung infection and acute lung injury (ALI). We hypothesized that the alveolar epithelium modulates intra-alveolar fibrin deposition through activation of protein C. Our objectives [corrected] were to determine whether components of the protein C activation pathway are present in the alveolar compartment in ALI and whether alveolar epithelium is a potential source. In patients with ALI, pulmonary edema fluid levels of endothelial protein C receptor (EPCR) were higher than plasma, suggesting a source in the lung. To determine whether alveolar epithelial cells are a potential source, protein C activation by A549, small airway epithelial, and primary human alveolar epithelial type II cells was measured. All three cell types express thrombomodulin (TM) and EPCR, and activate protein C on the cell surface. Activation of protein C was inhibited by cytomix (TNF-alpha, IL-1beta, and IFN-gamma). Release of EPCR and TM into the conditioned medium was inhibited by the metalloproteinase inhibitors tumor necrosis factor protease inhibitor (TAPI) and GM6001, indicating that the shedding of EPCR and TM from the alveolar epithelium is mediated by a metalloproteinase. These findings provide new evidence that the alveolar epithelium can modulate the protein C pathway and thus could be an important determinant of alveolar fibrin deposition. Local fibrin deposition may be a fundamental mechanism for the lung to localize and confine injury, thus limiting the risk of dissemination of injury or infection to the systemic circulation.

Multifunctional specificity of the protein C/activated protein C Gla domain

Activated protein C (APC) has potent anticoagulant and anti-inflammatory properties that are mediated in part by its interactions with its cofactor protein S and the endothelial cell protein C receptor (EPCR). The protein C/APC Gla domain is implicated in both interactions. We sought to identify how the protein C Gla domain enables specific protein-protein interactions in addition to its conserved role in phospholipid binding. The human prothrombin Gla domain, which cannot bind EPCR or support protein S cofactor activity, has 22/45 residues that are not shared with the human protein C Gla domain. We hypothesized that the unique protein C/APC Gla domain residues were responsible for mediating the specific interactions. To assess this, we generated 13 recombinant protein C/APC variants incorporating the prothrombin residue substitutions. Despite anticoagulant activity similar to wild-type APC in the absence of protein S, APC variants APC(PT33-39) (N33S/V34S/D35T/D36A/L38D/A39V) and APC(PT36/38/39) (D36A/L38D/A39V) were not stimulated by protein S, whereas APC(PT35/36) (D35T/D36A) exhibited reduced protein S sensitivity. Moreover, PC(PT8/10) (L8V/H10K) displayed negligible EPCR affinity, despite normal binding to anionic phospholipid vesicles and factor Va proteolysis in the presence and absence of protein S. A single residue variant, PC(PT8), also failed to bind EPCR. Factor VIIa, which also possesses Leu-8, bound soluble EPCR with similar affinity to wild-type protein C, collectively confirming Leu-8 as the critical residue for EPCR recognition. These results reveal the specific Gla domain residues responsible for mediating protein C/APC molecular recognition with both its cofactor and receptor and further illustrate the multifunctional potential of Gla domains.

Tissue factor and thrombomodulin expression on endothelial cell-seeded collagen modules for tissue engineering

The creation of functional tissue engineering constructs to repair or replace diseased tissues requires a well-formed vasculature network within the construct and the endothelial cells lining that vascular bed must display a nonthrombogenic phenotype. A new approach to tissue engineering involves the assembly of smaller components (modules fabricated at the hundred micron scale) into larger constructs. The modules, collagen gel containing the particular tissue cell of interest, are covered with endothelial cells prior to assembly so that the interconnected channels that are formed are lined with endothelial cells, creating a mimic of a vascular network. Here, we confirmed (using confocal microscopy primarily) that the human umbilical vein endothelial cells, seeded on collagen gel modules without a second embedded cell and without flow, bore the molecular markers of low thrombogenicity. Two days, after seeding on the modules, endothelial cells displayed the typical cobblestone morphology, formed tight cell-cell junctions and covered the whole module surface. Immunofluorescence staining showed that at both 2 days and 7 days after seeding, only a few cells expressed tissue factor while this number was dramatically increased after TNFalpha stimulation. On the other hand, thrombomodulin was expressed by the majority of seeded cells and expression was reduced after TNFalpha stimulation.

Endothelial protein C receptor (CD201) explicitly identifies hematopoietic stem cells in murine bone marrow

The hematopoietic stem cell (HSC) is a unique cell type found in bone marrow, which has the capacity for both self-renewal and differentiation into all blood lineages. The identification of genes expressed specifically in HSCs may help identify gene products vital to the control of self-renewal and/or differentiation, as well as antigens capable of forming the basis for improved methods of stem cell isolation. In previous studies, we identified a number of genes that appeared to be differentially expressed in murine bone marrow-derived HSCs, using microarray technology. We report here that one of those genes, encoding the murine endothelial protein C receptor (EPCR), is expressed at high levels within the bone marrow in HSCs. Bone marrow cells isolated on the basis of EPCR expression alone are highly enriched for hematopoietic reconstitution activity, showing levels of engraftment in vivo comparable to that of stem cells purified using the most effective conventional methods. Moreover, evaluation of cell populations first enriched for stem cell activity by conventional methods and subsequently fractionated on the basis of EPCR expression indicates that stem cell activity is always associated with EPCR-expressing cells. Based on our findings, we believe EPCR represents the first known marker that 'explicitly' identifies hematopoietic stem cells within murine bone marrow.

C-reactive protein decreases expression of thrombomodulin and endothelial protein C receptor in human endothelial cells

BACKGROUND

C-reactive protein (CRP) is associated with atherosclerosis and thrombosis. However, it is unclear whether CRP has direct effects on the antithrombogenic properties of endothelial cells. The objective of the present study was to determine the effect of CRP on the expression of thrombomodulin (TM) and the endothelial protein C receptor (EPCR) in human endothelial cells.

METHODS

Human coronary artery endothelial cells (HCAECs) were treated with CRP in a dose- and time-dependent manner. The messenger RNA levels of TM and EPCR were determined by real-time polymerase chain reaction. Anti-CD32 antibody and curcumin were used to block the potential effects of CRP.

RESULTS

In HCAECs, CRP (10 and 25 microg/mL) significantly reduced TM messenger RNA levels by 18 and 30%, respectively, compared with controls (P < .05). This effect was also confirmed in other types of human endothelial cells from umbilical veins and skin microvessels. The cells treated with CRP (10 and 25 microg/mL) showed significant reductions of EPCR mRNA levels by 34% and 33%, respectively (P < .05). Anti-CD32 antibody partially blocked CRP-induced downregulation of TM and EPCR in HCAECs. Furthermore, curcumin (5 and 10 microM) in combination with CRP (10 microg/mL) significantly increased TM mRNA levels by 45 and 100%, respectively, and increased EPCR mRNA levels by 24 and 45%, respectively, compared with those in CRP-treated cells (P < .05).

CONCLUSIONS

CRP significantly decreases the expression of TM and EPCR in human endothelial cells, thereby promoting thrombogenic conditions. This effect is partially mediated by CD32. Curcumin completely blocks CRP-induced downregulation of TM and EPCR in HCAECs.

Genetic variants in the endothelial protein C receptor gene: reaching significance

Since the discovery of activated protein C (APC) resistance, mutations and genetic variants in the protein C pathway have been put into the spotlight of research in the pathophysiology of haemostasis. Although none of the more recently discovered mutations in the components of the natural anticoagulant pathways have equalled the clear-cut effect of factor V Leiden in thrombosis and its epidemiological importance, some recent work in several genes of the pathway have shown promising results in linking genotype and phenotype which could explain certain mechanisms in respective thrombosis associated pathologies. The discovery of a specific endothelial protein C receptor (EPCR) and the deciphering of its importance in the biology of protein C, opened up the possibility that genetic variants in the EPCR gene (PROCR) could play a role in the risk for thrombosis (1, 2). The EPCR functions to enhance the activation of anticoagulant protein C by the thrombin-thrombomodulin complex, and it is essential for the interaction of protein C with protease activated receptor 1. In this way, the EPCR acts in the anticoagulant, profibrinolytic and anti-inflammatory response (1, 3, 4). EPCR knock-out mice show early embryonic lethality (5), although a small amount of EPCR is sufficient to maintain the animals alive and fertile (6) …

Selective modulation of protein C affinity for EPCR and phospholipids by Gla domain mutation

Uniquely amongst vitamin K-dependent coagulation proteins, protein C interacts via its Gla domain both with a receptor, the endothelial cell protein C receptor (EPCR), and with phospholipids. We have studied naturally occurring and recombinant protein C Gla domain variants for soluble (s)EPCR binding, cell surface activation to activated protein C (APC) by the thrombin-thrombomodulin complex, and phospholipid dependent factor Va (FVa) inactivation by APC, to establish if these functions are concordant. Wild-type protein C binding to sEPCR was characterized with surface plasmon resonance to have an association rate constant of 5.23 x 10(5) m(-1).s(-1), a dissociation rate constant of 7.61 x 10(-2) s(-1) and equilibrium binding constant (K(D)) of 147 nm. It was activated by thrombin over endothelial cells with a K(m) of 213 nm and once activated to APC, rapidly inactivated FVa. Each of these interactions was dramatically reduced for variants causing gross Gla domain misfolding (R-1L, R-1C, E16D and E26K). Recombinant variants Q32A, V34A and D35A had essentially normal functions. However, R9H and H10Q/S11G/S12N/D23S/Q32E/N33D/H44Y (QGNSEDY) variants had slightly reduced (< twofold) binding to sEPCR, arising from an increased rate of dissociation, and increased K(m) (358 nm for QGNSEDY) for endothelial cell surface activation by thrombin. Interestingly, these variants had greatly reduced (R9H) or greatly enhanced (QGNSEDY) ability to inactivate FVa. Therefore, protein C binding to sEPCR and phospholipids is broadly dependent on correct Gla domain folding, but can be selectively influenced by judicious mutation.

Thrombomodulin-protein C-EPCR system: integrated to regulate coagulation and inflammation

Late in the 18th century, William Hewson recognized that the formation of a clot is characteristic of many febrile, inflammatory diseases (Owen C. A History of Blood Coagulation. Rochester, Minnesota: Mayo Foundation; 2001). Since that time, there has been steady progress in our understanding of coagulation and inflammation, but it is only in the past few decades that the molecular mechanisms linking these 2 biologic systems have started to be delineated. Most of these can be traced to the vasculature, where the systems most intimately interact. Thrombomodulin (TM), a cell surface-expressed glycoprotein, predominantly synthesized by vascular endothelial cells, is a critical cofactor for thrombin-mediated activation of protein C (PC), an event further amplified by the endothelial cell protein C receptor (EPCR). Activated PC (APC), in turn, is best known for its natural anticoagulant properties. Recent evidence has revealed that TM, APC, and EPCR have activities that impact not only on coagulation but also on inflammation, fibrinolysis, and cell proliferation. This review highlights recent insights into the diverse functions of this complex multimolecular system and how its components are integrated to maintain homeostasis under hypercoagulable and/or proinflammatory stress conditions. Overall, the described advances underscore the usefulness of elucidating the relevant molecular pathways that link both systems for the development of novel therapeutic and diagnostic targets for a wide range of inflammatory diseases.

Protective role of activated protein C in lung and airway remodeling

Recent studies have implicated the protein C pathway in the mechanism of lung and airway remodeling. The effector enzyme of this pathway is activated protein C (APC). Clinical studies have shown that APC generation is decreased in patients with lung injury and airway inflammation and that this decrease is associated with increased collagen deposition in the lung. In line with these findings, low APC activity has been observed in the bronchoalveolar lavage fluid in animal models of lung injury and airway inflammation. Treatment with APC significantly inhibits the development of lung fibrosis in bleomycin-induced lung injury and the development of airway hyperresponsiveness and allergic inflammation in ovalbumin-induced bronchial asthma. APC may protect the lung from fibrosis and airway remodeling by suppressing activation of coagulation, decreasing the secretion of inflammatory cytokines and platelet-derived growth factor, and promoting fibrinolysis. APC inhibits the expression of cytokines by decreasing the nuclear translocation of signal transducer and activator of transcription 6 and the nuclear factor-kappaB family of transcription factors. In view of its multiple functions, APC constitutes a potential therapeutic agent for inflammatory disorders of the lung and airways.

Structure and functions of the endothelial cell protein C receptor

The endothelial cell protein C receptor (EPCR) plays a critical role in augmenting protein C activation by the thrombin-thrombomodulin complex and in modulating the functions of the protein C pathway to aid in preventing organ damage due to various challenges. EPCR exhibits a sequence and three-dimensional homology with the major histocompatibility class 1/CD1 family of proteins. This family of proteins is characterized by having a deep groove that is usually used in antigen presentation. In the case of CD1c and CD1d, this groove is filled with a lipid antigen, usually a glycolipid. Like the CD1 series, EPCR has a lipid in the corresponding groove. In this case, the lipid is usually phosphatidylcholine, but it may be phosphatidylethanolamine. The bound lipid contributes to protein C binding, but its structure suggests a role in maintaining EPCR structure rather than contributing directly to protein C binding. Potential roles for EPCR in hematopoiesis are suggested by the finding that EPCR is located on hematopoietic stem cells at reasonably high concentrations. The structure and the lipid antigen suggest that EPCR may be involved in preventing autoimmunity, which would be consistent with findings in CD1d knockout mice. Complete deletion of EPCR function results in embryonic death, at least in part due to placental thrombosis. In adult animals, the anticoagulant and anti-inflammatory responses to endotoxin increase with increasing EPCR expression. Some of the anti-inflammatory activity is likely to be due to EPCR's interactions with the integrin Mac-1 (CD11b/CD18) on leukocytes, an interaction that probably limits tight adhesion of leukocytes to activated endothelium. Thus, available data suggest a potential role of EPCR in hematopoiesis, autoimmunity, and the control of both the coagulation and inflammation responses to infection and trauma.

Inhibition of staurosporine-induced apoptosis of endothelial cells by activated protein C requires protease-activated receptor-1 and endothelial cell protein C receptor

In a model of staurosporine-induced apoptosis using EAhy926 endothelial cells, inhibition of apoptosis by activated protein C was dose-dependent and required the enzyme's active site, implicating activated protein C-mediated proteolysis. Consistent with this implication, both protease-activated receptor-1 (PAR-1) and endothelial cell protein C receptor (EPCR) were required for the anti-apoptotic effects of activated protein C.

Activated protein C inhibits the expression of platelet-derived growth factor in the lung

The natural anticoagulant-activated protein C may inhibit inflammation and fibrosis in the lung. Platelet-derived growth factor is involved in the pathogenesis of lung fibrosis. This study assessed the effect of activated protein C on platelet-derived growth factor expression in human cell lines and in an in vivo model of lung fibrosis. Activated protein C significantly inhibited the secretion and expression of platelet-derived growth factor in human lung cell lines, primary bronchial epithelial cells, and macrophages. In vitro studies also showed that the endothelial activated protein C receptor is expressed by lung epithelial cells and macrophages, and that this receptor and the proteolytic activity of activated protein are implicated in the inhibition of platelet-derived growth factor expression. In the in vivo model of lung fibrosis, intratracheal administration of activated protein C decreased the expression of platelet-derived growth factor and suppressed the development of lung fibrosis. Concomitant intratracheal administration of activated protein C and anti-endothelial activated protein C receptor or anti-platelet-derived growth factor suppressed the inhibitory activity of activated protein C in vivo. In brief, this study describes a novel biological function of activated protein C that may further explain its inhibitory activity on lung inflammation and fibrosis.

Disruption of the endothelial cell protein C receptor gene in mice causes placental thrombosis and early embryonic lethality

The endothelial cell protein C receptor (EPCR) is a type 1 transmembrane protein found primarily on endothelium that binds both protein C and activated protein C with similar affinity. EPCR augments the activation of protein C by the thrombin-thrombomodulin complex. To determine the physiological importance of EPCR, we generated EPCR-deficient mice by homologous targeting in embryonic stem cells. Genotyping of progeny obtained from EPCR(+/-) interbreeding indicated that EPCR(-/-) embryos died on or before embryonic day 10.5 (E10.5). Reverse transcriptase-PCR confirmed the absence of EPCR mRNA in EPCR(-/-) embryos. EPCR(-/-) embryos removed from extra-embryonic membranes and tissues at day E7.5 and cultured in vitro developed beyond E10.5, suggesting a role for EPCR in the normal function of the placenta and/or at the materno-embryonic interface. Immunohistochemistry revealed the lack of EPCR in trophoblast giant cells of EPCR(-/-) embryos. These cells, which normally express EPCR, are in direct contact with the maternal circulation and its clotting factors. In EPCR(-/-) embryos, greatly increased fibrin deposition was detected around these cells. To prevent this fibrin deposition, EPCR(+/-)-crossed female mice received a daily subcutaneous injection of enoxaparin through pregnancy. Although some EPCR(-/-) embryos were rescued from midgestational lethality, this regimen yielded no EPCR(-/-) pups. We conclude that EPCR is essential for normal embryonic development. Moreover, EPCR plays a key role in preventing thrombosis at the maternal-embryonic interface.

A family of MHC class I-like genes located in the vicinity of the mouse leukocyte receptor complex

Some members of the major histocompatibility complex (MHC) class I gene family are encoded outside the MHC. Here we describe a family of mouse class I-like genes mapping to the vicinity of the leukocyte receptor complex (LRC) on chromosome 7. This family, which we call Mill (MHC class I-like located near the LRC), has two members designated Mill1 and Mill2. Both genes are predicted to encode membrane glycoproteins with domain organization essentially similar to that of MHC class I heavy chains. The following features of Mill are noteworthy. (i) The deduced MILL proteins lack most of the residues known to be involved in the docking of peptides in classical MHC class I molecules. (ii) Among the known members of the class I gene family, MILL1 and MILL2 are related most closely to MICA/MICB encoded in the human MHC. (iii) Unlike all other known members of the class I gene family, Mill1 and Mill2 have an exon between those coding for the signal peptide and the alpha1 domain. (iv) Mill1 has a more restricted expression profile than Mill2. (v) The gene orthologous to Mill1 or Mill2 apparently is absent in the human. (vi) Mill1 and Mill2 show a limited degree of polymorphism in laboratory mice. The observation that the Mill family is related most closely to the MIC family, together with its apparent absence in the human, suggests its involvement in innate immunity.

Recombinant human activated protein C attenuates the inflammatory response in endothelium and monocytes by modulating nuclear factor-kappaB

OBJECTIVE

To review the anti-inflammatory and anti-apoptotic properties of drotrecogin alfa (activated) (recombinant human activated protein C), emphasizing its modulatory effects on endothelial nuclear factor-kappaB. We propose a broad anti-inflammatory effect of drotrecogin alfa (activated), acting on both endothelium and monocytes.

DATA SOURCES

A selected review of the published literature on nuclear factor-kappaB, severe sepsis, and the use of drotrecogin alfa (activated) in clinical and preclinical models, together with data derived from preclinical gene profiling of model systems.

DATA EXTRACTION AND SYNTHESIS

Data from the PROWESS trial support the preclinical evidence of an antithrombotic effect of drotrecogin alfa (activated). Anti-inflammatory effects through reduction of thrombin generation and through thrombin-independent mechanisms in mononuclear and endothelial cells are reviewed. Inhibition of apoptosis is used as an example of the protective effect of drotrecogin alfa (activated) on endothelial and mononuclear cell dysfunction.

CONCLUSIONS

Drotrecogin alfa (activated) acts as a modulator of nuclear factor-kappaB to aid in the host immune response in endothelium and monocytes. Extrapolation of gene array findings to explain apoptosis in endothelium and monocytes, coupled with emerging preclinical reports, provides evidence to support the role of drotrecogin alfa (activated) in modulating nuclear factor-kappaB.

Association of MR1 protein, an MHC class I-related molecule, with beta(2)-microglobulin

MR1 is a major histocompatibility complex (MHC) class I-related gene conserved among mammals, and its predicted amino acid sequence is relatively closer to the classical MHC class I molecules among several divergent class I molecules. However, as its molecular nature and function have not yet been clarified, we set out in this study to establish transfected P388 murine cell lines that stably produce a large number of MR1 proteins and conducted analyses to investigate the molecular nature of MR1. Immunoprecipitation and Western blot analyses with specific antisera revealed that the MR1 protein can associate with beta(2)-microglobulin, suggesting its molecular form of a typical class I heterodimer composed of a heavy and a light chain (beta(2)-microglobulin), like the classical MHC class I molecules.

Plasma levels of endothelial protein C receptor respond to anticoagulant treatment

The endothelial protein C receptor (EPCR) facilitates protein C activation and plays a protective role in the response to Escherichia coli-mediated sepsis in primates. Previously, a soluble form of EPCR (sEPCR) in human plasma was characterized, and several studies indicated that generation of sEPCR is regulated by inflammatory mediators, including thrombin-mediated up-regulation of surface metalloproteolytic activity in vitro. This study addressed the question of whether plasma sEPCR levels reflect changes in thrombin generation in patients undergoing anticoagulant treatment. The sEPCR levels in patients treated with coumarin-type oral anticoagulants were significantly lower than those in healthy asymptomatic adult volunteers (105.3 +/- 70.8 ng/mL [n = 55] versus 165.8 +/- 115.8 ng/mL [n = 200]; P <.0001). A similar decline in plasma sEPCR levels was found in patients treated with unfractionated heparin. In healthy volunteers, sEPCR levels declined to about 100 ng/mL within 3 days after initiation of an 8-day period of warfarin administration and increased within 2 days after its cessation. Plasma sEPCR levels returned to pretreatment values within 1 week, and the changes in plasma sEPCR levels mirrored changes in values for international normalized ratios. A similar decline in sEPCR levels with time was observed in 7 patients beginning treatment with warfarin for a thrombotic disorder. Prothrombin fragment 1 + 2 levels also decreased in volunteers and patients given warfarin. These results show that plasma sEPCR levels decline in response to treatment with anticoagulants whose mechanism of action is known to decrease in vivo thrombin production.

Characterization of protein C receptor expression in monocytes

Many sequelae associated with endotoxaemic-induced shock result from excessive production of the cytokine mediators, tumour necrosis factor alpha (TNF-alpha), interleukin 1 (IL-1) and IL-6 from lipopolysaccharide (LPS)-activated monocytes. Protein C (PC)/activated protein C (APC) has potent cytokine-modifying properties and is protective in animal models and human clinical trials of sepsis. The precise mechanism by which this anti-inflammatory response is achieved remains unknown; however, the recently described endothelial protein C receptor (EPCR) appears to be essential for this function. The pivotal role that monocytes play in the pathophysiology of septic shock led us to investigate the possible expression of a protein C receptor on the monocyte membrane. We used similarity algorithms to screen human sequence databases for paralogues of the EPCR but found none. However, using reverse transcription-polymerase chain reaction (RT-PCR), we detected an mRNA transcribed in primary human monocytes and THP1 cells that was identical to human EPCR mRNA. We also used immunocytochemical analysis to demonstrate the expression of a protein C receptor on the surface of monocytes encoded by the same gene as EPCR. These results confirm a new member of the protein C pathway involving primary monocytes. Further characterization will be necessary to compare and contrast its biological properties with those of EPCR.

Recombinant human activated protein C: a system modulator of vascular function for treatment of severe sepsis

OBJECTIVE

To review the mechanisms of action and rationale for the use of recombinant human activated protein C in the treatment of severe sepsis. Specifically, we focus on the mechanisms of action in the protein C pathway that converge to modulate the pathophysiology of severe inflammatory disease and sepsis. This analysis includes a discussion of the role of activated protein C in directly modulating cell system biology, independent of antithrombotic activity.

DATA SOURCES/STUDY SELECTION

Published research and review articles relating to the protein C pathway, recombinant human protein C, and the role of protein C in sepsis. Data were also derived from broad gene profiling in model systems of endothelial dysfunction.

DATA EXTRACTION AND SYNTHESIS

Relevant studies were included to support discussion of the unique mechanistic aspect of protein C and its role in the pathogenesis of severe sepsis. We discuss the potential of activated protein C as a unique system modulator for the treatment of severe sepsis and other systemic inflammatory responses that result in microvascular coagulopathy, endothelial dysfunction, and vascular bed failure.

CONCLUSIONS

The protein C pathway plays a unique role in modulating vascular function. As an antithrombotic/profibrinolytic agent, it plays a clear role in maintaining vascular patency. Moreover, it has anti-inflammatory properties and appears to play a unique role as an antiapoptotic and endothelial cell survival factor. In states of systemic inflammatory activation, loss of protein C due to consumptive processes results in a compromised ability to modulate coagulation as well as inflammatory and cell survival functions. This compromise leads to vascular dysfunction, end-organ failure, and death. Replacement with recombinant human activated protein C offers a system-modulating approach to improved outcome.

Identification of the protein C/activated protein C binding sites on the endothelial cell protein C receptor. Implications for a novel mode of ligand recognition by a major histocompatibility complex class 1-type receptor

The endothelial cell protein C receptor (EPCR) is an endothelial cell-specific transmembrane protein that binds both protein C and activated protein C (APC). EPCR regulates the protein C anticoagulant pathway by binding protein C and augmenting protein C activation by the thrombin-thrombomodulin complex. EPCR is homologous to the MHC class 1/CD1 family, members of which contain two alpha-helices that sit upon an 8-stranded beta-sheet platform. In this study, we identified 10 residues that, when mutated to alanine, result in the loss of protein C/APC binding (Arg-81, Leu-82, Val-83, Glu-86, Arg-87, Phe-146, Tyr-154, Thr-157, Arg-158, and Glu-160). Glutamine substitutions at the four N-linked carbohydrate attachment sites of EPCR have little affect on APC binding, suggesting that the carbohydrate moieties of EPCR are not critical for ligand recognition. We then mapped the epitopes for four anti-human EPCR monoclonal antibodies (mAbs), two of which block EPCR/Fl-APC (APC labeled at the active site with fluorescein) interactions, whereas two do not. These epitopes were localized by generating human-mouse EPCR chimeric proteins, since the mAbs under investigation do not recognize mouse EPCR. We found that 5 of the 10 candidate residues for protein C/APC binding (Arg-81, Leu-82, Val-83, Glu-86, Arg-87) colocalize with the epitope for one of the blocking mAbs. Three-dimensional molecular modeling of EPCR indicates that the 10 protein C/APC binding candidate residues are clustered at the distal end of the two alpha-helical segments. Protein C activation studies on 293 cells that coexpress EPCR variants and thrombomodulin demonstrate that protein C binding to EPCR is necessary for the EPCR-dependent enhancement in protein activation by the thrombin-thrombomodulin complex. These studies indicate that EPCR has exploited the MHC class 1 fold for an alternative and possibly novel mode of ligand recognition. These studies are also the first to identify the protein C/APC binding region of EPCR and may provide useful information about molecular defects in EPCR that could contribute to cardiovascular disease susceptibility.

Characterization and regulation of the 5'-flanking region of the murine endothelial protein C receptor gene

The protein C pathway plays a critical role in the negative regulation of blood coagulation. The nucleotide sequence of the murine endothelial protein C receptor (mEPCR) gene was determined for 8.8 kilobase pairs of the genomic structure and 3.4 kilobase pairs of the 5'-flanking region. RNase protection assay revealed six major transcription start sites clustered at -100 to -109 upstream of the translation initiation site. A series of 5'-promoter deletion fragments were fused to a luciferase reporter gene and transiently transfected into bovine aortic endothelium. Deletion of the sequence from -220 to -180 dramatically reduced luciferase expression in bovine aortic endothelial cells. This region of the murine endothelial protein C receptor gene contains one AP4 site and one SP1 site. Mutations in the core sequence of the AP4 and SP1 sites impaired both nuclear protein binding and luciferase expression. These results suggest important roles for AP4 and SP1 in the constitutive expression of mEPCR. A thrombin response element (CCCACCCC) was found to mediate the induction of mEPCR by thrombin in cell culture. Transgenic mice were developed expressing green fluorescent protein driven by the -350 to -1 or -1080 to -1 promoter. Thrombin up-regulated mEPCR and the transgene in vivo.

Endotoxin and thrombin elevate rodent endothelial cell protein C receptor mRNA levels and increase receptor shedding in vivo

The endothelial cell protein C receptor (EPCR) facilitates protein C activation by the thrombin-thrombomodulin complex. Protein C activation has been shown to be critical to the host defense against septic shock. In cell culture, tumor necrosis factor- (TNF-) down-regulates EPCR expression, raising the possibility that EPCR might be down-regulated in septic shock. We examined EPCR mRNA and soluble EPCR levels in mice and rats challenged with lethal dose 95 levels of endotoxin. Toxic doses of TNF- failed to alter EPCR mRNA levels in mice. Rather than EPCR mRNA levels falling in response to endotoxin, as predicted from cell-culture experiments, they rose approximately 3-fold 6 hours after exposure to endotoxin before returning toward baseline levels at 24 hours after exposure. Soluble EPCR levels rose approximately 4-fold. Infusion of hirudin, a specific thrombin inhibitor, before endotoxin exposure almost completely blocked the increase in EPCR mRNA and soluble EPCR. Consistent with the idea that the responses were mediated by thrombin, thrombin infusion (5 U/kg of body weight for 3 hours) resulted in an approximately 2-fold increase in EPCR mRNA and soluble EPCR. Incubation of rat endothelial cells with thrombin or murine protease-activated receptor 1 agonist peptide resulted in a 2-fold increase in EPCR mRNA. These results indicate that thrombin plays a major role in up-regulating EPCR mRNA and shedding in vivo.

Structural prediction and analysis of endothelial cell protein C/activated protein C receptor

The endothelial cell receptor (EPCR) for protein C (PC)/activated protein C (APC) is a 221 amino-acid residues long transmembrane glycoprotein with unclear physiological function. To facilitate future studies and to rationalize recently reported experimental data about this protein, we have constructed three-dimensional models of human, bovine and mouse EPCR using threading and comparative model building. EPCR is homologous to CD1/MHC class I molecules. It consists of two domains, which are similar to the alpha1 and alpha2 domains of MHC class I molecules, whereas the alpha3 domain of MHC is replaced in EPCR by a transmembrane region followed by a short cytosolic tail. The alpha1 and alpha2 domains of CD1/MHC proteins form a groove, which binds short peptides. These domains are composed of an eight-stranded antiparallel beta-pleated sheet with two long antiparallel alpha-helices. The distance between the helical segments dictates the width of the groove. The cleft in EPCR appears to be relatively narrow and it is lined with hydrophobic/aromatic and polar residues with a few charged amino acids. Analysis of the human EPCR model predicts that (a) the protein does not contain any calcium binding pockets; (b) C101 and C169 form a buried disulphide bridge, while C97 is free, and buried in the core of the molecule; and (c) four potential glycosylation sites are solvent exposed.