Characterization of protein C receptor expression in monocytes

The APC-EPCR-PAR1 axis in sickle cell disease

Sickle Cell Disease (SCD) is a group of inherited hemoglobinopathies. Sickle cell anemia (SCA) is caused by a homozygous mutation in the β-globin generating sickle hemoglobin (HbS). Deoxygenation leads to pathologic polymerization of HbS and sickling of erythrocytes. The two predominant pathologies of SCD are hemolytic anemia and vaso-occlusive episodes (VOE), along with sequelae of complications including acute chest syndrome, hepatopathy, nephropathy, pulmonary hypertension, venous thromboembolism, and stroke. SCD is associated with endothelial activation due to the release of danger-associated molecular patterns (DAMPs) such as heme, recurrent ischemia-reperfusion injury, and chronic thrombin generation and inflammation. Endothelial cell activation is mediated, in part, by thrombin-dependent activation of protease-activated receptor 1 (PAR1), a G protein coupled receptor that plays a role in platelet activation, endothelial permeability, inflammation, and cytotoxicity. PAR1 can also be activated by activated protein C (APC), which promotes endothelial barrier protection and cytoprotective signaling. Notably, the APC system is dysregulated in SCD. This mini-review will discuss activation of PAR1 by APC and thrombin, the APC-EPCR-PAR1 axis, and their potential roles in SCD.

Differential expression of endothelial protein C receptor (EPCR) in hematopoietic stem and multipotent progenitor cells in young and old mice

Endothelial protein C receptor (EPCR) has emerged as one of the most conserved and reliable surface markers for the prospective identification and isolation of hematopoietic stem cells (HSCs). Prior studies have consistently demonstrated that EPCR expression enriches HSCs capable of long-term multilineage repopulation in both mouse and human across different hematopoietic tissues, including bone marrow (BM), fetal liver and ex vivo HSC expansion cultures. However, little is known about the expression profiles of EPCR in multipotent progenitor (MPP) populations located immediately downstream of HSCs in the hematopoietic hierarchy and which play a major role in sustaining lifelong blood cell production. Here, we incorporate EPCR antibody detection into a multi-parameter flow cytometric panel, which allows accurate identification of HSCs and five MPP subsets (MPP1-5) in mouse BM. Our data reveal that all MPP populations contain EPCR-expressing cells. Multipotent MPP1 and MPP5 contain higher proportion of EPCR⁺ cells compared to the more lineage-biased MPP2-4. Notably, high expression of EPCR enriches phenotypic HSC and MPP5, but not MPP1. Comparison of EPCR expression profiles between young and old BM reveals ageing mediated expansion of EPCR-expressing cells only in HSCs, but not in any of the MPP populations. Collectively, our study provides a comprehensive characterization of the surface expression pattern of EPCR in mouse HSC and MPP1-5 cells during normal and aged hematopoiesis.

Identification of a broad lipid repertoire associated to the endothelial cell protein C receptor (EPCR)

Evidence is mounting that the nature of the lipid bound to the endothelial cell protein C receptor (EPCR) has an impact on its biological roles, as observed in anticoagulation and more recently, in autoimmune disease. Phosphatidylethanolamine and phosphatidylcholine species dominate the EPCR lipid cargo, yet, the extent of diversity in the EPCR-associated lipid repertoire is still unknown and remains to be uncovered. We undertook mass spectrometry analyses to decipher the EPCR lipidome, and identified species not yet described as EPCR ligands, such as phosphatidylinositols and phosphatidylserines. Remarkably, we found further, more structurally divergent lipids classes, represented by ceramides and sphingomyelins, both in less abundant quantities. In support of our mass spectrometry results and previous studies, high-resolution crystal structures of EPCR in three different space groups point to a prevalent diacyl phospholipid moiety in EPCR’s pocket but a mobile and ambiguous lipid polar head group. In sum, these studies indicate that EPCR can associate with varied lipid classes, which might impact its properties in anticoagulation and the onset of autoimmune disease.

Downregulation of thrombomodulin-thrombin-activated protein C pathway as a mechanism for SARS-CoV-2 induced endotheliopathy and microvascular thrombosis

There is emerging evidence of microvascular thrombosis and thrombotic microangiopathy (TMA) induced by COVID-19, presumably from endothelial injury. Thrombomodulin (TM) is an endothelial glycoprotein that plays a dual role in maintaining healthy endothelium-as a natural anticoagulant by binding thrombin to activate protein C (APC) and a negative regulator of the alternate complement pathway (AP). TM is shed into the plasma as soluble TM (sTM) during endothelial injury. We hypothesize that SARS-CoV-2 spike proteins cause direct microvascular endothelial injury, leading to TM shedding, decreased activation of PC, and consequently, microvascular thrombosis in COVID-19. We conducted this study twofold: 1) in vivo, we assessed endothelial injury (by measuring sTM) and AP activation by quantifying Ba (cleavage product of AP component Factor B) in a cohort of critically ill COVID-19 pediatric patients and the implications on clinical outcomes; and 2)in vitro, we investigated endothelial injury (TM shedding) by SARS-COV-2 spike proteins and the subsequent functional consequence in activated PC (APC) levels and Ba levels. sTM and Ba in plasma samples from SARS-CoV-2 positive patients admitted to Texas Children's Hospital Pediatric Intensive Care Unit (n = 33) and from healthy controls (n = 38) were measured by ELISA. In vitro, confluent glomerular microvascular endothelial cells (GMVECs) were incubated for 48 h in the presence or absence (control) of purified SARS-CoV-2 spike proteins, S1 and S2. TM from the cell lysates while Ba and APC from the cell supernatants were measured by ELISA. sTM and Ba levels were significantly higher in the COVID-19 pediatric patients compared to healthy controls (p < 0.01 and p < 0.001, respectively). Among the COVID-19 patients, elevated sTM was associated with increased vasopressor use (p = 0.01) and elevated Ba was associated with increased duration of mechanical ventilation (p = 0.04). In vitro, surface bound TM and soluble APC were significantly lower in GMVECs after addition of spike proteins (p < 0.05), while Ba was undetectable in both control and spike proteins exposed GMVECs. In conclusion, we provide evidence of endothelial injury in COVID-19 pediatric patients and demonstrate a potential pathway of SARS-CoV-2 induced thrombosis. Decreased surface-bound TM results in lower amount of thrombin-TM complex, hence lesser activation of PC, likely leading to a pro-thrombotic state. These findings in GMVECs could explain the vulnerability of kidneys to COVID-19-induced TMA.

A critical role of endothelial cell protein C receptor in the intestinal homeostasis in experimental colitis

Crohn’s disease and ulcerative colitis are the two forms of disorders of the human inflammatory bowel disease with unknown etiologies. Endothelial cell protein C receptor (EPCR) is a multifunctional and multiligand receptor, which is expressed on the endothelium and other cell types, including epithelial cells. Here, we report that EPCR is expressed in the colon epithelial cells, CD11c⁺, and CD21⁺/CD35⁺ myeloid cells surrounding the crypts in the colon mucosa. EPCR expression was markedly decreased in the colon mucosa during colitis. The loss of EPCR appeared to associate with increased disease index of the experimental colitis in mice. EPCR^−/− mice were more susceptible to dextran sulfate sodium (DSS)-induced colitis, manifested by increased weight loss, macrophage infiltration, and inflammatory cytokines in the colon tissue. DSS treatment of EPCR^−/− mice resulted in increased bleeding, bodyweight loss, anemia, fibrin deposition, and loss of colon epithelial and goblet cells. Administration of coagulant factor VIIa significantly attenuated the DSS-induced colon length shortening, rectal bleeding, bodyweight loss, and disease activity index in the wild-type mice but not EPCR^−/− mice. In summary, our data provide direct evidence that EPCR plays a crucial role in regulating the inflammation in the colon during colitis.

Activated protein C suppresses osteoclast differentiation via endothelial protein C receptor, protease-activated receptor-1, sphingosine 1-phosphate receptor, and apolipoprotein E receptor 2

INTRODUCTION

Bone remodeling relies on a delicate balance between formation and resorption of bone tissues, processes in which bone-forming osteoblasts and bone-resorbing osteoclasts play central roles. Recently, we reported that anticoagulant activated protein C (APC) promotes osteoblast proliferation, but the role of the blood coagulation system in bone remodeling remains unclear. In this study, to further elucidate the relationship between bone remodeling and blood coagulation, we investigated the effect of APC on osteoclast differentiation.

MATERIALS AND METHODS

Normal human osteoclast precursor cells were cultured in their growth medium including soluble RANKL, M-CSF, and FBS, and on days 4 and 7, the culture medium was replaced with the same medium containing various concentrations of APC, protein C (PC), sphingosine 1-phosphate (S1P) receptor agonist, FTY720, or APC+various substances without FBS. On day 8, TRAP-positive multinucleated cells (≥3 nuclei) were counted manually using a light microscope. The effects of APC on NF-κB and NFATc1 activation were evaluated using specific ELISA.

RESULTS

APC suppressed RANKL-induced osteoclast differentiation, and this APC-induced suppression of osteoclast differentiation was inhibited by zymogen protein C and aprotinin, a serine protease inhibitor. Immunohistochemistry and RT-PCR analyses suggested that endothelial protein C receptor (EPCR) and protease-activated receptor-1 (PAR-1) were expressed in osteoclast precursor cells and osteoclasts. Both anti-PAR-1 antibody and anti-EPCR antibody (RCR-252), which blocks APC binding to EPCR, inhibited the APC-induced suppression of osteoclast differentiation. FTY720 had no effect on osteoclast differentiation. However, FTY 720 and S1P receptor antagonist, VP 23019, inhibited the APC-induced suppression of osteoclast differentiation. On the other hand, recombinant soluble human ApoER2 and anti-human ApoER2 inhibited the APC-induced suppression of osteoclast differentiation. Further, APC had no effect on NF-κB and NFATc1 activation.

CONCLUSIONS

APC suppresses human osteoclast differentiation mainly by inhibiting the formation of multinucleated cells via EPCR, PAR-1, S1P receptor, and ApoER2 in a manner that depends on APC protease activity.

The Role of Thromboinflammation in Delayed Cerebral Ischemia after Subarachnoid Hemorrhage

Delayed cerebral ischemia (DCI) is a major determinant of patient outcome following aneurysmal subarachnoid hemorrhage. Although the exact mechanisms leading to DCI are not fully known, inflammation, cerebral vasospasm, and microthrombi may all function together to mediate the onset of DCI. Indeed, inflammation is tightly linked with activation of coagulation and microthrombi formation. Thromboinflammation is the intersection at which inflammation and thrombosis regulate one another in a feedforward manner, potentiating the formation of thrombi and pro-inflammatory signaling. In this review, we will explore the role(s) of inflammation and microthrombi in subarachnoid hemorrhage (SAH) pathophysiology and DCI, and discuss the potential of targeting thromboinflammation to prevent DCI after SAH.

Evaluation of the effect of recombinant thrombomodulin on a lipopolysaccharide-induced murine sepsis model

To evaluate the effect of recombinant human thrombomodulin (rTM) on sepsis, the levels of nucleosome as well as high-mobility group box 1 (HMGB1) and cytokines in sera and peritoneal fluids were measured in a mouse model of lipopolysaccharide (LPS)-induced sepsis after administration of rTM. C57BL/6 mice were intraperitoneally injected with LPS (15 mg/kg; Escherichia coli O111:B4) with or without the intravenous administration of rTM (3 mg/kg; 30 min prior to or 2 h after LPS injection). The survival rates were evaluated and levels of tumor necrosis factor (TNF)-α, interleukin (IL)-6, monocyte chemoattractant protein (MCP)-1, HMGB1 and nucleosome in sera and peritoneal fluids were analyzed by ELISA. Administration of rTM prior to or after LPS improved the survival rate of septic mice. In addition, rTM administered prior to or after LPS suppressed the level of pro-inflammatory cytokine TNF-α in sera at 1-3 h after LPS injection, whereas only the administration of rTM after LPS suppressed the levels of HMGB1 and nucleosome (late-phase mediators of sepsis) (9-12 h) in sera after the LPS injection. Furthermore, administration of rTM prior to or after LPS suppressed the level of TNF-α in the peritoneal fluids at 1-3 h after LPS injection, whereas only the administration of rTM after LPS suppressed the levels of IL-6 and MCP-1 in the peritoneal fluids at 6-9 h after LPS injection. These observations indicated that administration of rTM significantly improves the survival rate and suppresses the increased levels of TNF-α, IL-6, MCP-1, HMGB1 and nucleosome in the LPS-induced septic shock model. Thus, rTM may exert a protective action on sepsis and reduce mortality, possibly by reducing not only the levels of cytokines and chemokine but also the levels of late-phase mediators of sepsis.

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.

The role of leukocytes in thrombosis

In recent years, the traditional view of the hemostatic system as being regulated by a coagulation factor cascade coupled with platelet activation has been increasingly challenged by new evidence that activation of the immune system strongly influences blood coagulation and pathological thrombus formation. Leukocytes can be induced to express tissue factor and release proinflammatory and procoagulant molecules such as granular enzymes, cytokines, and damage-associated molecular patterns. These mediators can influence all aspects of thrombus formation, including platelet activation and adhesion, and activation of the intrinsic and extrinsic coagulation pathways. Leukocyte-released procoagulant mediators increase systemic thrombogenicity, and leukocytes are actively recruited to the site of thrombus formation through interactions with platelets and endothelial cell adhesion molecules. Additionally, phagocytic leukocytes are involved in fibrinolysis and thrombus resolution, and can regulate clearance of platelets and coagulation factors. Dysregulated activation of leukocyte innate immune functions thus plays a role in pathological thrombus formation. Modulation of the interactions between leukocytes or leukocyte-derived procoagulant materials and the traditional hemostatic system is an attractive target for the development of novel antithrombotic strategies.

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

Background

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

Methods

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

Results

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

Conclusion

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

A Double-Chambered Protein Nanocage Loaded with Thrombin Receptor Agonist Peptide (TRAP) and γ-Carboxyglutamic Acid of Protein C (PC-Gla) for Sepsis Treatment

New protein nanocages are designed bearing two functional proteins, γ-carboxyglutamic acid of protein C (PC-Gla) and thrombin receptor agonist peptide (TRAP), and have an anti-septic response. These nanoparticles reduce sepsis-induced organ injury and septic mortality in vivo. Noting that there are currently no medications for severe sepsis, these results show that novel nanoparticles can be used to treat sepsis.

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

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

Barrier enhancing signals in pulmonary edema

Increased endothelial permeability and reduction of alveolar liquid clearance capacity are two leading pathogenic mechanisms of pulmonary edema, which is a major complication of acute lung injury, severe pneumonia, and acute respiratory distress syndrome, the pathologies characterized by unacceptably high rates of morbidity and mortality. Besides the success in protective ventilation strategies, no efficient pharmacological approaches exist to treat this devastating condition. Understanding of fundamental mechanisms involved in regulation of endothelial permeability is essential for development of barrier protective therapeutic strategies. Ongoing studies characterized specific barrier protective mechanisms and identified intracellular targets directly involved in regulation of endothelial permeability. Growing evidence suggests that, although each protective agonist triggers a unique pattern of signaling pathways, selected common mechanisms contributing to endothelial barrier protection may be shared by different barrier protective agents. Therefore, understanding of basic barrier protective mechanisms in pulmonary endothelium is essential for selection of optimal treatment of pulmonary edema of different etiology. This article focuses on mechanisms of lung vascular permeability, reviews major intracellular signaling cascades involved in endothelial monolayer barrier preservation and summarizes a current knowledge regarding recently identified compounds which either reduce pulmonary endothelial barrier disruption and hyperpermeability, or reverse preexisting lung vascular barrier compromise induced by pathologic insults.

Endothelial protein C receptor polymorphisms and risk of severe sepsis in critically ill patients

Purpose

Endothelial protein C receptor (EPCR) is expressed mainly in endothelial cells and is involved in regulation of the cytoprotective and anticoagulant pathways of protein C. We assessed whether haplotypes in the EPCR gene modify the risk of severe sepsis and/or septic shock (SS/SS) development in critically ill patients.

Methods

Three polymorphisms in the EPCR gene were genotyped in 389 Caucasian critically ill patients, hospitalized in the intensive care units of two major hospitals in Athens, Greece. Multivariate logistic regression analysis controlling for age, acute physiology and chronic health evaluation (APACHE) II and sequential organ failure assessment (SOFA) scores, sex, and diagnosis was performed to determine the effect of haplotypes H1 and H3 in the EPCR gene on the development of SS/SS.

Results

H2 carriers versus all other genotypes combined had a nonsignificant excess of SS/SS (p = 0.087). SS/SS occurred in 38.8 % of critically ill patients carrying minor alleles belonging to both H1 and H3 haplotypes, in 58.0 % of H1 carriers, 64.3 % of H3 carriers, and 65.2 % of patients carrying all common alleles (H2). Compared with H2 carriers, the odds ratios (OR) for developing SS/SS were 0.34 [95 % confidence interval (CI) 0.16–0.76, p = 0.008] for simultaneous H1 and H3 carriers, 0.65 (95 % CI 0.37–1.13, p = 0.123) for H1 carriers, and 0.82 (95 % CI 0.39–1.70, p = 0.590) for H3 carriers.

Conclusions

Our results indicate that simultaneous carriers of minor alleles belonging to both the H1 and H3 haplotypes may be at reduced risk of developing SS/SS in this cohort of critically ill patients.

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.

Activated protein C differentially regulates both viability and differentiation of osteoblasts mediated by bisphosphonates

Activated protein C (APC) is a cytoprotective anticoagulant that can promote cutaneous healing. We examined the effect of APC on viability and differentiation of the osteoblastic line, MG63, in the presence and absence of bisphosphonates (BPs). Osteoblasts were cultured and treated for 24 or 48 h with Alendronate (Aln), Zoledronate (Zol) or Pamidronate (Pam) at concentrations ranging from 10⁻⁴ to 10⁻⁶ ℳ. Cell differentiation was measured using type 1 collagen production, Alizarin red staining and alkaline phosphatase activity, whereas cell viability was assessed using MTT and crystal violet assays. All three BPs induced MG63 cell death in a dose- and time-dependent manner. Pam- and Zol-related cell death was prevented by APC treatment; however, cell death induced by Aln was accelerated by APC. APC induced MG63 cell differentiation that was enhanced by Aln, but inhibited by Pam or Zol. Endothelial protein C receptor (EPCR) was expressed by MG63 cells and mediated the protective effect of APC on Zol-induced viability. In summary, we have demonstrated that (1) APC favorably regulates MG63 viability and differentiation toward bone growth, (2) APC differentially regulates the effects of specific BPs and (3) at least part of the effects of APC is mediated through EPCR. These findings highlight the potential importance of the PC pathway in bone physiology and provide strong evidence that APC may influence bone cells and has potential to be a therapeutic drug for bone regeneration, depending on concurrent BP treatment.

Treatment with activated protein C (aPC) is protective during the development of myocardial fibrosis: an angiotensin II infusion model in mice

Aims

Myocardial fibrosis contributes to the development of heart failure. Activated Protein C (aPC) is a circulating anticoagulant with anti-inflammatory and cytoprotective properties. Using a model of myocardial fibrosis second to Angiotensin II (AngII) infusion, we investigated the novel therapeutic function aPC in the development of fibrosis.

Methods and Results

C57Bl/6 and Tie2-EPCR mice were infused with AngII (2.0 µg/kg/min), AngII and aPC (0.4 µg/kg/min) or saline for 3d. Hearts were harvested and processed for analysis or used for cellular isolation. Basic histology and collagen deposition were assessed using histologic stains. Transcript levels of molecular mediators were analyzed by quantitative RT-PCR. Mice infused with AngII exhibited multifocal areas of myocardial cellular infiltration associated with significant collagen deposition compared to saline control animals (p<0.01). AngII-aPC infusion inhibited this cellular infiltration and the corresponding collagen deposition. AngII-aPC infusion also inhibited significant expression of the pro-fibrotic cytokines TGF-β1, CTGF and PDGF found in AngII only infused animals (p<0.05). aPC signals through its receptor, EPCR. Using Tie2-EPCR animals, where endothelial cells over-express EPCR and exhibit enhanced aPC-EPCR signaling, no significant reduction in cellular infiltration or fibrosis was evident with AngII infusion suggesting aPC-mediate protection is endothelial cell independent. Isolated infiltrating cells expressed significant EPCR transcripts suggesting a direct effect on infiltrating cells.

Conclusions

This data indicates that aPC treatment abrogates the fibrogenic response to AngII. aPC does not appear to confer protection by stimulating the endothelium but by acting directly on the infiltrating cells, potentially inhibiting migration or activation.

Activated protein C inhibits pancreatic islet inflammation, stimulates T regulatory cells, and prevents diabetes in non-obese diabetic (NOD) mice

Activated protein C (aPC) is a natural anticoagulant with strong cyto-protective and anti-inflammatory properties. aPC inhibits pancreatic inflammation and preserves functional islets after intraportal transplantation in mice. Whether aPC prevents the onset or development of type 1 diabetes (T1D) is unknown. In this study, when human recombinant aPC was delivered intraperitoneally, twice weekly for 10 weeks (from week 6 to 15) to non-obese diabetic (NOD) mice, a model for T1D, the incidence of diabetes was reduced from 70% (saline control) to 7.6% by 26 weeks of age. Islets of aPC-treated mice exhibited markedly increased expression of insulin, aPC/protein C, endothelial protein C receptor, and matrix metalloproteinase (MMP)-2 when examined by immunostaining. The insulitis score in aPC-treated mice was 50% less than that in control mice. T regulatory cells (Tregs) in the spleen, pancreatic islets, and pancreatic lymph nodes were increased 37, 53, and 59%, respectively, in NOD mice following aPC treatment. These Tregs had potent suppressor function and, after adoptive transfer, delayed diabetes onset in NOD.severe combined immunodeficiency mice. The culture of NOD mouse spleen cells with aPC reduced the secretion of inflammatory cytokines interleukin (IL)-1β and interferon-γ but increased IL-2 and transforming growth factor-β1, two cytokines required for Treg differentiation. In summary, our results indicate that aPC prevents T1D in the NOD mouse. The aPC mechanism of action is complex, involving induction of Treg differentiation, inhibition of inflammation, and possibly direct cyto-protective effects on β cells.

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.

Activated protein C targets CD8+ dendritic cells to reduce the mortality of endotoxemia in mice

Activated protein C (aPC) therapy reduces mortality in adult patients with severe sepsis. In mouse endotoxemia and sepsis models, mortality reduction requires the cell signaling function of aPC, mediated through protease-activated receptor-1 (PAR1) and endothelial protein C receptor (EPCR; also known as Procr). Candidate cellular targets of aPC include vascular endothelial cells and leukocytes. Here, we show that expression of EPCR and PAR1 on hematopoietic cells is required in mice for an aPC variant that mediates full cell signaling activity but only minimal anticoagulant function (5A-aPC) to reduce the mortality of endotoxemia. Expression of EPCR in mature murine immune cells was limited to a subset of CD8+ conventional dendritic cells. Adoptive transfer of splenic CD11chiPDCA-1- dendritic cells from wild-type mice into animals with hematopoietic EPCR deficiency restored the therapeutic efficacy of aPC, whereas transfer of EPCR-deficient CD11chi dendritic cells or wild-type CD11chi dendritic cells depleted of EPCR+ cells did not. In addition, 5A-aPC inhibited the inflammatory response of conventional dendritic cells independent of EPCR and suppressed IFN-gamma production by natural killer-like dendritic cells. These data reveal an essential role for EPCR and PAR1 on hematopoietic cells, identify EPCR-expressing dendritic immune cells as a critical target of aPC therapy, and document EPCR-independent antiinflammatory effects of aPC on innate immune cells.

Endothelial protein C receptor-expressing hematopoietic stem cells reside in the perisinusoidal niche in fetal liver

Hematopoietic stem cells (HSCs) are maintained in specialized niches in adult bone marrow. However, niche and HSC maintenance mechanism in fetal liver (FL) still remains unclear. Here, we investigated the niche and the molecular mechanism of HSC maintenance in mouse FL using HSCs expressing endothelial protein C receptor (EPCR). The antiapoptotic effect of activated protein C (APC) on EPCR(+) HSCs and the expression of protease-activated receptor 1 (Par-1) mRNA in these cells suggested the involvement of the cytoprotective APC/EPCR/Par-1 pathway in HSC maintenance. Immunohistochemistry revealed that EPCR(+) cells were localized adjacent to, or integrated in, the Lyve-1(+) sinusoidal network, where APC and extracellular matrix (ECM) are abundant, suggesting that HSCs in FL were maintained in the APC- and ECM-rich perisinusoidal niche. EPCR(+) HSCs were in a relatively slow cycling state, consistent with their high expression levels of p57 and p18. Furthermore, the long-term reconstitution activity of EPCR(+) HSCs decreased significantly after short culture but not when cocultured with feeder layer of FL-derived Lyve-1(+) cells, which suggests that the maintenance of the self-renewal activity of FL HSCs largely depended on the interaction with the perisinusoidal niche. In conclusion, EPCR(+) HSCs resided in the perisinusoidal niche in mouse FL.

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.

The protein C pathway in tissue inflammation and injury: pathogenic role and therapeutic implications

Inflammation and coagulation are closely linked interdependent processes. Under physiologic conditions, the tissue microcirculation functions in anticoagulant and anti-inflammatory fashions. However, when inflammation occurs, coagulation is also set in motion and actively participates in enhancing inflammation. Recently, novel and unexpected roles of hemostasis in the humoral and cellular components of innate immunity have been described. In particular, the protein C system, besides its well-recognized role in anticoagulation, plays a crucial role in inflammation. Indeed, the protein C system is now emerging as a novel participant in the pathogenesis of acute and chronic inflammatory diseases, such as sepsis, asthma, inflammatory bowel disease, atherosclerosis, and lung and heart inflammation, and may emerge as unexpected therapeutic targets for intervention.

The protein C pathway in tissue inflammation and injury: pathogenic role and therapeutic implications

Inflammation and coagulation are closely linked interdependent processes. Under physiologic conditions, the tissue microcirculation functions in anticoagulant and anti-inflammatory fashions. However, when inflammation occurs, coagulation is also set in motion and actively participates in enhancing inflammation. Recently, novel and unexpected roles of hemostasis in the humoral and cellular components of innate immunity have been described. In particular, the protein C system, besides its well-recognized role in anticoagulation, plays a crucial role in inflammation. Indeed, the protein C system is now emerging as a novel participant in the pathogenesis of acute and chronic inflammatory diseases, such as sepsis, asthma, inflammatory bowel disease, atherosclerosis, and lung and heart inflammation, and may emerge as unexpected therapeutic targets for intervention.

Endothelial protein C receptor is expressed in rat cortical and hippocampal neurons and is necessary for protective effect of activated protein C at glutamate excitotoxicity

Activated protein C (APC) is an anticoagulant and anti-inflammatory factor that acts via endothelial protein C receptor (EPCR). Interestingly, APC also exhibits neuroprotective activities. In the present study, we demonstrate for the first time expression of EPCR, the receptor for APC, in rat cortical and hippocampal neurons. Moreover, exposing the neurons to glutamate excitotoxicity we studied the functional consequence of the expression of EPCR. By cytotoxicity assay we showed that EPCR was necessary for the APC-mediated protective effect in both neuronal cell types in culture. The effect of APC was abrogated in the presence of blocking EPCR antibodies. Analysis of neuronal death by cell labelling with dyes which allow distinguishing living and dead cells confirmed that the anti-apoptotic effect of APC was dependent on both EPCR and protease-activated receptor-1. Thus, we suggest that binding of APC to EPCR on neurons and subsequent activation of protease-activated receptor-1 by the complex of APC-EPCR promotes survival mechanisms after exposure of neurons to damaging factors.

Factor VIIa binding to endothelial cell protein C receptor

Recent studies have shown that factor VIIa (FVIIa) binds specifically to endothelial protein C receptor (EPCR), a known cellular receptor for protein C and activated protein C, on the endothelium. The formation of FVIIa:EPCR complexes neither supports the activation of coagulation nor modulates tissue factor-initiated coagulation. However, FVIIa interaction with EPCR, particularly at pharmacological concentrations of FVIIa, may impair EPCR-dependent protein C activation and activated protein C-mediated cell signaling by competing directly with them for binding to EPCR. FVIIa binding to EPCR may also contribute to FVIIa clearance. This review summarizes recent data on FVIIa interaction with EPCR and discusses potential physiological significance and consequences of the interaction.

Protein C concentrate to restore physiological values in adult septic patients

OBJECTIVE

To describe the efficacy and safety of protein C (PC) concentrate to restore physiological values in adult septic patients having clinical contraindications to activated PC.

DESIGN

Case series (pilot study).

SETTING

Three adult ICUs of a University Hospital.

PATIENTS AND PARTICIPANTS

Twenty adult patients affected by severe sepsis or septic shock with plasma values of PC < 50%.

INTERVENTIONS

Patients were treated with PC concentrate (Ceprotin ((R))--Baxter) with a starting bolus followed by a continuous infusion for 72 h [3 IU/(kg h)].

MEASUREMENTS AND RESULTS

PC activity, WBC, platelets, D: -Dimer, fibrinogen, PT, aPTT, AT III, lactate, Sepsis-related Organ Failure Assessment (SOFA), Disseminated Intravascular Coagulation (DIC) score, adverse events, and mortality were measured. Baseline plasma PC activity was 34.5 +/- 9.1%. PC concentrate normalized the PC activity in all patients within 48 h, and then remained stable for the following days. At baseline, several patients showed abnormal PT, aPTT, platelets values, and lactate levels. During the study period, there was a significant increase of platelets, fibrinogen, PT, AT III, and a significant decrease of D: -Dimer, aPTT, DIC score, and lactate. No adverse reactions (hemorrhage or thrombosis) were observed. Mortality at 28 days was 35%.

CONCLUSIONS

Our pilot study shows that the administration of PC concentrate to patients having contraindications to the treatment with activated PC was safe and possibly useful to control the coagulopathy triggered and sustained by sepsis. A randomized, double blind study in patients with severe sepsis and contraindications to activated PC administration would be advisable to state the safety and the possible role of this product in the treatment of severe sepsis.

Activated protein C mediates a healing phenotype in cultured tenocytes

Tendon injuries cause considerable morbidity in the general adult population. The tenocytes within the tendon have the full capacity to heal the tendon intrinsically. Activated protein C (APC) plays an important role in coagulation and inflammation and more recently has been shown to promote cutaneous wound healing. In this study we examined whether APC can induce a wound healing phenotype in tenocytes. Sheep tenocytes were treated with APC, endothelial protein C receptor (EPCR) blocking antibody (RCR252) and/or EPCR small interfering (si)RNA. Cell proliferation and migration were measured by crystal violet assay and a scratch wounding assay, respectively. The expression of EPCR, matrix metalloproteinase (MMP)-2, type I collagen and MAP kinase activity were detected by real time PCR, zymography, immunofluorescence, immunohistochemistry and Western blotting. APC stimulated proliferation, MMP-2 activity and type I collagen deposition in a dose-dependent manner and promoted migration of cultured tenocytes. APC dose-dependently stimulated phosphorylated (P)-ERK2 and inhibited P-p38. Interestingly, tenocytes expressed EPCR protein, which was up-regulated by APC. When tenocytes were pre-treated with RCR252 or EPCR siRNA the effect of APC on proliferation, MMP-2 and type 1 collagen synthesis and MAP kinases was blocked. APC promotes the growth, MMP-2 activity, type I collagen deposition and migration of tenocytes. Furthermore, EPCR is expressed by tenocytes and mediates the actions of APC, at least partly by signalling through selective MAP kinases. These data implicate APC as a potential healing agent for injured tendons.

Endothelial protein C receptor is overexpressed in rheumatoid arthritic (RA) synovium and mediates the anti-inflammatory effects of activated protein C in RA monocytes

OBJECTIVES

(1) To investigate whether inflammatory synovial tissues from patients with rheumatoid arthritis (RA) express endothelial protein C receptor (EPCR) and (2) to determine the major cell type(s) that EPCR is associated with and whether EPCR functions to mediate the effects of activated protein C (APC) on these cells.

METHODS

EPCR, CD68 and PC/APC in synovial tissues were detected by immunostaining and in situ PCR. Monocytes were isolated from peripheral blood of patients with RA and treated with APC, lipopolysaccharide (LPS), and/or EPCR blocking antibody RCR252. Cells and supernatants were collected for RT-PCR, western blotting, enzyme-linked immuosorbent assay and chemotaxis assay.

RESULTS

EPCR was expressed by both OA and RA synovial tissues but was markedly increased in RA synovium. EPCR was colocalised with PC/APC mostly on CD68 positive cells in synovium. In RA monocytes, APC upregulated EPCR expression and reduced monocyte chemoattractant protein-1-induced chemotaxis of monocytes by approximately 50%. APC also completely suppressed LPS-stimulated NF-kappaB activation and attenuated TNF-alpha protein by more than 40% in RA monocytes. The inhibitory effects of APC were reversed by RCR252, indicating that EPCR is required.

CONCLUSIONS

Our results demonstrate for the first time that EPCR is expressed by synovial tissues, particularly in RA, where it co-localises with PC/APC on monocytes/macrophages. In addition, APC inhibits the migration and activation of RA monocytes via EPCR. These inhibitory effects on RA monocytes suggest that PC pathway may have a beneficial therapeutic effect in RA.

Differential regulation of matrix metalloproteinase 2 and matrix metalloproteinase 9 by activated protein C: relevance to inflammation in rheumatoid arthritis

OBJECTIVE

To investigate the in vitro effect of activated protein C (APC), a natural anticoagulant and novel antiinflammatory agent, on the regulation of the gelatinases matrix metalloproteinase 2 (MMP-2) and MMP-9.

METHODS

Synovial fibroblasts and peripheral blood monocytes isolated from patients with rheumatoid arthritis (RA) or osteoarthritis (OA) and Mono Mac6 cells were used in this study. After treatment, cells and culture supernatants were collected for zymography, enzyme-linked immunosorbent assay, reverse transcription-polymerase chain reaction, and Western blot analysis.

RESULTS

Fibroblasts and monocytes from RA patients produced substantially more MMP-9 than did those from OA patients; however, there was no difference in MMP-2 production. The addition of recombinant APC markedly reduced MMP-9 at the gene and protein levels. In contrast, APC up-regulated and activated MMP-2. Using a blocking antibody to the endothelial protein C receptor (EPCR), we showed that the inhibition of MMP-9 by APC was EPCR-dependent. Furthermore, APC directly suppressed the production of tumor necrosis factor (TNF) and the activation of NF-kappaB and MAP kinase p38, and inhibitors of NF-kappaB or p38 reduced the production of MMP-9, suggesting that APC inhibits MMP-9 by blocking TNF, NF-kappaB, and p38. Thus, APC acts on MMP-9 by binding to EPCRs on the cell surface and, subsequently, inhibiting the intracellular activation of the proinflammatory signaling molecules NF-kappaB and p38.

CONCLUSION

APC appears to be the first physiologic agent to inhibit the production of proinflammatory MMP-9, yet increase antiinflammatory MMP-2 activity. Our results provide the initial evidence that APC may be beneficial in the prevention of inflammation and joint destruction in RA.

Activated protein C

Protein C is a vitamin K-dependent plasma protein zymogen whose genetic mild or severe deficiencies are linked with risk for venous thrombosis or neonatal purpura fulminans, respectively. Studies over past decades showed that activated protein C (APC) inactivates factors (F) Va and VIIIa to down-regulate thrombin generation. More recent basic and preclinical research on APC has characterized the direct cytoprotective effects of APC that involve gene expression profile alterations, anti-inflammatory and anti-apoptotic activities and endothelial barrier stabilization. These actions generally require endothelial cell protein C receptor (EPCR) and protease activated receptor-1. Because of these direct cytoprotective actions, APC reduces mortality in murine endotoxemia and severe sepsis models and provides neuroprotective benefits in murine ischemic stroke models. Furthermore, APC reduces mortality in patients with severe sepsis (PROWESS clinical trial). Although much remains to be clarified about mechanisms for APC's direct effects on various cell types, it is clear that APC's molecular features that determine its antithrombotic action are partially distinct from those providing cytoprotective actions because we have engineered recombinant APC variants with selective reduction or retention of either anticoagulant or cytoprotective activities. Such APC variants can provide relatively enhanced levels of either cytoprotective or anticoagulant activities for various therapeutic applications. We speculate that APC variants with reduced anticoagulant action but normal cytoprotective actions hold the promise of reducing bleeding risk because of attenuated anticoagulant activity while reducing mortality based on direct cytoprotective effects on cells.

Effects of anticoagulant strategies on activation of inflammation and coagulation

Acute inflammatory events, such as those that occur in sepsis, lead to dysregulation of the coagulation cascade. The hemostatic imbalance in sepsis, characterized by the excessive activation of procoagulant pathways and the impairment of anticoagulant activity, leads to disseminated intravascular coagulation and results in microvascular thrombosis, tissue hypoperfusion and, ultimately, multiple organ failure and death. Furthermore, natural anti-inflammatory mechanisms of the endogenous anticoagulants are diminished by the impaired coagulation. Supportive strategies aiming at inhibiting activation of coagulation and inflammation by treatment with exogenous anticoagulants have been found to be beneficial in experimental and initial clinical studies. This review summarizes the available experimental and clinical data regarding the interaction between coagulation and inflammation, focusing on the two anticoagulants which are in clinical use, antithrombin and activated protein C. Identification of the different biological mechanisms of the two endogenous anticoagulants might help to determine target patient populations as well as to develop new anticoagulant analogs that differ in there respective effects in coagulation and inflammation.

Lipopolysaccharide down-regulates the thrombomodulin expression of peripheral blood monocytes: effect of serum on thrombomodulin expression in the THP-1 monocytic cell line

Thrombomodulin has a central role in the regulation of coagulation through its abilities to promote generation of the potent anticoagulant activated protein C. Because little is known about monocyte thrombomodulin expression and its regulatory mechanism by lipopolysaccharide, we investigated the effect of lipopolysaccharide on monocyte's thrombomodulin expression. Lipopolysaccharide reduced the surface thrombomodulin expression of human peripheral blood monocytes in a dose-dependent and time-dependent manner, regardless of the addition of serum. The surface thrombomodulin activity was comparably decreased in monocytes incubated with lipopolysaccharide. Blocking nuclear factor-kappaB by MG132 or aurine tricarboxylic acid effectively inhibited the lipopolysaccharide-induced surface thrombomodulin down-regulation of monocytes. Lipopolysaccharide inactivation by polymyxin B in the supernatants from the lipopolysaccharide-stimulated cultures still reduced the surface thrombomodulin expression of monocytes, suggesting a role for soluble mediators in the down-regulation of thrombomodulin. The lipopolysaccharide-induced thrombomodulin surface expression and the mRNA levels of the monocytic leukemic cell line (THP-1) were decreased in serum-depleted culture, while they were increased in medium containing 10% serum. We conclude that lipopolysaccharide down-regulates the thrombomodulin expression of monocytes and that nuclear factor-kappaB is a critical mediator of the repression of thrombomodulin by lipopolysaccharide. Regulation of the THP-1 thrombomodulin expression by lipopolysaccharide depends on the presence of serum.

The cytoprotective protein C pathway

Protein C is best known for its mild deficiency associated with venous thrombosis risk and severe deficiency associated with neonatal purpura fulminans. Activated protein C (APC) anticoagulant activity involves proteolytic inactivation of factors Va and VIIIa, and APC resistance is often caused by factor V Leiden. Less known is the clinical success of APC in reducing mortality in severe sepsis patients (PROWESS trial) that gave impetus to new directions for basic and preclinical research on APC. This review summarizes insights gleaned from recent in vitro and in vivo studies of the direct cytoprotective effects of APC that include beneficial alterations in gene expression profiles, anti-inflammatory actions, antiapoptotic activities, and stabilization of endothelial barriers. APC's cytoprotection requires its receptor, endothelial cell protein C receptor, and protease-activated receptor-1. Because of its pleiotropic activities, APC has potential roles in the treatment of complex disorders, including sepsis, thrombosis, and ischemic stroke. Although much about molecular mechanisms for APC's effects on cells remains unclear, it is clear that APC's structural features mediating anticoagulant actions and related bleeding risks are distinct from those mediating cytoprotective actions, suggesting the possibility of developing APC variants with an improved profile for the ratio of cytoprotective to anticoagulant actions.

Coagulation interventions in experimental human endotoxemia

Recognition of the link between coagulation activation and inflammation has led to the hypothesis that anticoagulants may be effective in the treatment of septic patients by altering the inflammatory response. However, only limited methodologies exist that can be used in human volunteers to mimic the physiologic alterations observed in critically ill patients. The human endotoxemia model represents a model of inflammation-induced tissue factor triggered coagulation activation. As it permits elucidation of a key player in this proinflammatory and procoagulant response, it serves as a useful tool to investigate novel therapeutics in a standardized setting. The aim of this review is to focus on coagulation interventions in the human endotoxemia model.

Modulation of monocyte function by activated protein C, a natural anticoagulant

Activated protein C is the first effective biological therapy for the treatment of severe sepsis. Although activated protein C is well established as a physiological anticoagulant, emerging data suggest that it also exerts anti-inflammatory and antiapoptotic effects. In this study, we investigated the ability of activated protein C to modulate monocyte apoptosis, inflammation, phagocytosis, and adhesion. Using the immortalized human monocytic cell line THP-1, we demonstrated that activated protein C inhibited camptothecin-induced apoptosis in a dose-dependent manner. The antiapoptotic effect of activated protein C requires its serine protease domain and is dependent on the endothelial cell protein C receptor and protease-activated receptor-1. In primary blood monocytes from healthy individuals, activated protein C inhibited spontaneous apoptosis. With respect to inflammation, activated protein C inhibited the production of TNF, IL-1beta, IL-6, and IL-8 by LPS-stimulated THP-1 cells. Activated protein C did not influence the phagocytic internalization of Gram-negative and Gram-positive bioparticles by THP-1 cells or by primary blood monocytes. Activated protein C also did not affect the expression of adhesion molecules by LPS-stimulated blood monocytes nor the ability of monocytes to adhere to LPS-stimulated endothelial cells. We hypothesize that the protective effect of activated protein C in sepsis reflects, in part, its ability to prolong monocyte survival in a manner that selectively inhibits inflammatory cytokine production while maintaining phagocytosis and adherence capabilities, thereby promoting antimicrobial properties while limiting tissue damage.

Detecting cryptic epitopes created by nanoparticles

As potential applications of nanotechnology and nanoparticles increase, so too does the likelihood of human exposure to nanoparticles. Because of their small size, nanoparticles are easily taken up into cells (by receptor-mediated endocytosis), whereupon they have essentially free access to all cellular compartments. Similarly to macroscopic biomaterial surfaces (that is, implants), nanoparticles become coated with a layer of adsorbed proteins immediately upon contact with physiological solutions (unless special efforts are taken to prevent this). The process of adsorption often results in conformational changes of the adsorbed protein, which may be affected by the larger curvature of nanoparticles compared with implant surfaces. Protein adsorption may result in the exposure at the surface of amino acid residues that are normally buried in the core of the native protein, which are recognized by the cells as "cryptic epitopes." These cryptic epitopes may trigger inappropriate cellular signaling events (as opposed to being rejected by the cells as foreign bodies). However, identification of such surface-exposed epitopes is nontrivial, and the molecular nature of the adsorbed proteins should be investigated using biological and physical science methods in parallel with systems biology studies of the induced alterations in cell signaling.

Endothelial protein C receptor and protease-activated receptor-1 mediate induction of a wound-healing phenotype in human keratinocytes by activated protein C

Activated protein C (APC) is a natural anticoagulant and inhibitor of inflammation that can stimulate keratinocyte wound repair in vitro and promote wound healing in vivo. The signaling mechanisms, however, are unknown and a keratinocyte receptor for APC has not been identified. Here, we show that cultured human keratinocytes from neonatal foreskins express the endothelial protein C receptor (EPCR). EPCR was also strongly expressed by lower epidermal layers of neonatal foreskin as determined by immunohistochemistry. In cultured keratinocytes, EPCR expression was upregulated by the addition of APC and inhibited by tumor necrosis factor-alpha. Addition of APC stimulated cell proliferation, production of matrix metalloproteinase-2, activation of ERK and p38 kinase signaling pathways, and expression of protease-activated receptor (PAR)-1. A monoclonal antibody, RCR252, which blocks APC binding to EPCR, or a blocking antibody to PAR-1, abolished APC's effects on keratinocytes. In summary, this study demonstrates that EPCR, a major receptor of protein C pathway, is expressed by human keratinocytes, and facilitates APC's function on keratinocytes via activation of PAR-1 pathway. Our findings highlight a possible new role for the protein C pathway in skin physiology and help elucidate the mechanisms of action by which APC promotes wound healing.

Endothelial protein C receptor-dependent inhibition of migration of human lymphocytes by protein C involves epidermal growth factor receptor

The protein C pathway is an important regulator of the blood coagulation system. Protein C may also play a role in inflammatory and immunomodulatory processes. Whether protein C or activated protein C affects lymphocyte migration and possible mechanisms involved was tested. Lymphocyte migration was studied by micropore filter assays. Lymphocytes that were pretreated with protein C (Ceprotin) or activated protein C (Xigris) significantly reduced their migration toward IL-8, RANTES, MCP-1, and substance P, but not toward sphingosine-1-phosphate. The inhibitory effects of protein C or activated protein C were reversed by Abs against endothelial protein C receptor and epidermal growth factor receptor. Evidence for the synthesis of endothelial protein C receptor by lymphocytes is shown by demonstration of receptor mRNA expression and detection of endothelial protein C receptor immunoreactivity on the cells' surface. Data suggest that an endothelial protein C receptor is expressed by lymphocytes whose activation with protein C or activated protein C arrests directed migration. Exposure of lymphocytes to protein C or activated protein C stimulates phosphorylation of Tyr845 of epidermal growth factor receptor, which may be relevant for cytoprotective effects of the protein C pathway.