Molecular recognition in the protein C anticoagulant pathway

Association between fatty liver index and blood coagulation markers: a population-based study

BACKGROUND

Population-based studies investigating the association between blood coagulation markers and non-alcoholic fatty liver disease (NAFLD) are rare. Thus, we aimed to investigate the relationship between the Fatty Liver Index (FLI) as a measure of hepatic steatosis and plasma concentrations of antithrombin III, D-dimer, fibrinogen D, protein C, protein S, factor VIII, activated partial thromboplastin time (aPTT), quick value and international thromboplastin time (INR) in the general population.

METHODS

After the exclusion of participants with anticoagulative treatment, 776 participants (420 women and 356 men, aged 54-74 years) of the population-based KORA Fit study with analytic data on hemostatic factors were included in the present analysis. Linear regression models were used to explore the associations between FLI and hemostatic markers, adjusted for sex, age, alcohol consumption, education, smoking status, and physical activity. In a second model, additional adjustments were made for the history of stroke, hypertension, myocardial infarction, serum non-HDL cholesterol levels, and diabetes status. In addition, analyses were stratified by diabetes status.

RESULTS

In the multivariable models (with or without health conditions), significantly positive associations with FLI were obtained for plasma concentrations of D-dimers, factor VIII, fibrinogen D, protein C, protein S, and quick value, while INR and antithrombin III were inversely associated. These associations were weaker in pre-diabetic subjects and largely disappeared in diabetic patients.

CONCLUSION

In this population-based study, an increased FLI is clearly related to changes in the blood coagulation system, possibly increasing the risk of thrombotic events. Due to a generally more pro-coagulative profile of hemostatic factors, such an association is not visible in diabetic subjects.

A Series of 14 Polish Patients with Thrombotic Events and PC Deficiency-Novel c.401-1G>A PROC Gene Splice Site Mutation in a Patient with Aneurysms

Objectives: Protein C (PC) deficiency is an inherited thrombophilia with a prevalence of 0.5% in the general population and 3% in subjects with a first-time deep vein thrombosis (DVT). Here we report a series of 14 PC-deficient Polish patients with comprehensive clinical and molecular characteristics, including long-term follow-up data and a deep mutational analysis of the PROC gene. Patients and Methods: Fourteen unrelated probands (mean ± SD age 43.8 ± 13.0 years) with suspicion of PC deficiency, who experienced thromboembolic events and a majority of whom received anticoagulants (92.8%), were screened for PROC mutations by sequencing the nine PROC exons and their flanking intron regions. Results: Ten probands (71.4%) had missense mutations, two patients (14.3%) carried nonsense variants, and the other two subjects (14.3%) had splice-site mutations, the latter including the c.401-1G>A variant, reported here for the very first time. The proband carrying the c.401-1A allele had a hepatic artery aneurysm with a highly positive family history of aneurysms and the absence of any mutations known to predispose to this vascular anomaly. Conclusion: A novel detrimental PROC mutation was identified in a family with aneurysms, which might suggest yet unclear links of thrombophilia to vascular anomalies, including aneurysms at atypical locations in women. The present case series also supports data indicating that novel oral anticoagulants (NOACs) are effective in PC deficient patients.

Thrombosis and Inflammation—A Dynamic Interplay and the Role of Glycosaminoglycans and Activated Protein C

Hemostasis, thrombosis, and inflammation are tightly interconnected processes which may give rise to thrombo-inflammation, involved in infectious and non-infectious acute and chronic diseases, including cardiovascular diseases (CVD). Traditionally, due to its hemostatic role, blood coagulation is isolated from the inflammation, and its critical contribution in the progressing CVD is underrated, until the full occlusion of a critical vessel occurs. Underlying vascular injury exposes extracellular matrix to deposit platelets and inflammatory cells. Platelets being key effector cells, bridge all the three key processes (hemostasis, thrombosis, and inflammation) associated with thrombo-inflammation. Under physiological conditions, platelets remain in an inert state despite the proximity to the endothelium and other cells which are decorated with glycosaminoglycan (GAG)-rich glycocalyx (GAGs). A pathological insult to the endothelium results in an imbalanced blood coagulation system hallmarked by increased thrombin generation due to losses of anticoagulant and cytoprotective mechanisms, i.e., the endothelial GAGs enhancing antithrombin, tissue factor pathway-inhibitor (TFPI) and thrombomodulin-protein C system. Moreover, the loss of GAGs promotes the release of mediators, such as von Willebrand factor (VWF), platelet factor 4 (PF4), and P-selectin, both locally on vascular surfaces and to circulation, further enhancing the adhesion of platelets to the affected sites. Platelet-neutrophil interaction and formation of neutrophil extracellular traps foster thrombo-inflammatory mechanisms exacerbating the cardiovascular disease course. Therefore, therapies which not only target the clotting mechanisms but simultaneously or independently convey potent cytoprotective effects hemming the inflammatory mechanisms are expected to provide clinical benefits. In this regard, we review the cytoprotective protease activated protein C (aPC) and its strong anti-inflammatory effects thereby preventing the ensuing thrombotic complications in CVD. Furthermore, restoring GAG-like vasculo-protection, such as providing heparin-proteoglycan mimetics to improve regulation of platelet and coagulation activity and to suppress of endothelial perturbance and leukocyte-derived pro-inflammatory cytokines, may provide a path to alleviate thrombo-inflammatory disorders in the future. The vascular tissue-modeled heparin proteoglycan mimic, antiplatelet and anticoagulant compound (APAC), dual antiplatelet and anticoagulant, is an injury-targeting and locally acting arterial antithrombotic which downplays collagen- and thrombin-induced and complement-induced activation and protects from organ injury.

Post-translational modifications of the ligands: Requirement for TAM receptor activation

The Tyro3, Axl, and MerTK (TAM) receptors are three homologous Type I Receptor Tyrosine Kinases that have important homeostatic functions in multicellular organisms by regulating the clearance of apoptotic cells (efferocytosis). Pathologically, TAM receptors are overexpressed in a wide array of human cancers, and often associated with aggressive tumor grade and poor overall survival. In addition to their expression on tumor cells, TAMs are also expressed on infiltrating myeloid-derived cells in the tumor microenvironment, where they appear to act akin to negative immune checkpoints that impair host anti-tumor immunity. The ligands for TAMs are two endogenous proteins, Growth Arrest-Specific 6 (Gas6) and Protein S (Pros1), that function as bridging molecules between externalized phosphatidylserine (PtdSer) on apoptotic cells and the TAM ectodomains. One interesting feature of TAMs biology is that their ligand proteins require specific post-translational modifications to acquire activities. This chapter summarized these important modifications and explained the molecular mechanisms behind such phenomenon. Current evidences suggest that these modifications help Gas6/Pros1 to achieve optimal PtdSer-binding capacities. In addition, this chapter included recent discovery of regulating machineries of PtdSer dynamic across the plasma membrane, as well as their potential impacts in the tumor microenvironment. Taken together, this review highlights the importance of the upstream PtdSer and Gas6 in regulating TAMs' function and hope to provide researchers with new perspectives to inspire future studies of TAM receptors in human disease models.

Antithrombin Resistance Rescues Clotting Defect of Homozygous Prothrombin-Y510N Dysprothrombinemia

A patient with hematuria in our clinic was diagnosed with urolithiasis. Analysis of the patient's plasma clotting time indicated that both activated partial thromboplastin time (52.6 seconds) and prothrombin time (19.4 seconds) are prolonged and prothrombin activity is reduced to 12.4% of normal, though the patient exhibited no abnormal bleeding phenotype and a prothrombin antigen level of 87.9%. Genetic analysis revealed the patient is homozygous for prothrombin Y510N mutation. We expressed and characterized the prothrombin-Y510N variant in appropriate coagulation assays and found that the specificity constant for activation of the mutant zymogen by factor Xa is impaired approximately fivefold. Thrombin generation assay using patient's plasma and prothrombin-deficient plasma supplemented with either wild-type or prothrombin-Y510N revealed that both peak height and time to peak for the prothrombin mutant are decreased; however, the endogenous thrombin generation potential is increased. Further analysis indicated that the thrombin mutant exhibits resistance to antithrombin and is inhibited by the serpin with approximately 12-fold slower rate constant. Protein C activation by thrombin-Y510N was also decreased by approximately 10-fold; however, thrombomodulin overcame the catalytic defect. The Na⁺-concentration-dependence of the amidolytic activities revealed that the dissociation constant for the interaction of Na⁺ with the mutant has been elevated approximately 20-fold. These results suggest that Y510 (Y184a in chymotrypsin numbering) belongs to network of residues involved in binding Na⁺. A normal protein C activation by thrombin-Y510N suggests that thrombomodulin modulates the conformation of the Na⁺-binding loop of thrombin. The clotting defect of thrombin-Y510N appears to be compensated by its markedly lower reactivity with antithrombin, explaining patient's normal hemostatic phenotype.

Protease-activated receptor 1 as a potential therapeutic target for COVID-19

Acute respiratory disease caused by a novel coronavirus (SARS-CoV-2) has spread all over the world, since its discovery in 2019, Wuhan, China. This disease is called COVID-19 and already killed over 1 million people worldwide. The clinical symptoms include fever, dry cough, dyspnea, headache, dizziness, generalized weakness, vomiting, and diarrhea. Unfortunately, so far, there is no validated vaccine, and its management consists mainly of supportive care. Venous thrombosis and pulmonary embolism are highly prevalent in patients suffering from severe COVID-19. In fact, a prothrombotic state seems to be present in most fatal cases of the disease. SARS-CoV-2 leads to the production of proinflammatory cytokines, causing immune-mediated tissue damage, disruption of the endothelial barrier, and uncontrolled thrombogenesis. Thrombin is the key regulator of coagulation and fibrin formation. In severe COVID-19, a dysfunctional of physiological anticoagulant mechanisms leads to a progressive increase of thrombin activity, which is associated with acute respiratory distress syndrome development and a poor prognosis. Protease-activated receptor type 1 (PAR1) is the main thrombin receptor and may represent an essential link between coagulation and inflammation in the pathophysiology of COVID-19. In this review, we discuss the potential role of PAR1 inhibition and regulation in COVID-19 treatment.

C-terminal residues of activated protein C light chain contribute to its anticoagulant and cytoprotective activities

BACKGROUND

Activated protein C (APC) is an important homeostatic blood coagulation protease that conveys anticoagulant and cytoprotective activities. Proteolytic inactivation of factors Va and VIIIa facilitated by cofactor protein S is responsible for APC's anticoagulant effects, whereas cytoprotective effects of APC involve primarily the endothelial protein C receptor (EPCR), protease activated receptor (PAR)1 and PAR3.

OBJECTIVE

To date, several binding exosites in the protease domain of APC have been identified that contribute to APC's interaction with its substrates but potential contributions of the C-terminus of the light chain have not been studied in detail.

METHODS

Site-directed Ala-scanning mutagenesis of six positively charged residues within G142-L155 was used to characterize their contributions to APC's anticoagulant and cytoprotective activities.

RESULTS AND CONCLUSIONS

K151 was involved in protein S dependent-anticoagulant activity of APC with some contribution of K150. 3D structural analysis supported that these two residues were exposed in an extended protein S binding site on one face of APC. Both K150 and K151 were important for PAR1 and PAR3 cleavage by APC, suggesting that this region may also mediate interactions with PARs. Accordingly, APC's cytoprotective activity as determined by endothelial barrier protection was impaired by Ala substitutions of these residues. Thus, both K150 and K151 are involved in APC's anticoagulant and cytoprotective activities. The differential contribution of K150 relative to K151 for protein S-dependent anticoagulant activity and PAR cleavage highlights that binding exosites for protein S binding and for PAR cleavage in the C-terminal region of APC's light chain overlap.

Recurrent superficial venous thrombophlebitis because of mutations in the protein C and fibrinogen genes in a young Argentinian female

: Hypodysfibrinogenemia and protein C deficiency are coagulopathies and in this report, we describe a young patient with both defects confirmed by molecular genetic tests. The patient was a 24-year-old woman referred for recurrent thrombophlebitis and finally deep venous thrombosis. Routine coagulation studies revealed mild decrease of protein C (0.49 IU, reference values 0.7-1.40 IU) and hypodysfibrinogenemia (0.88 g/l and 1.83 g/l for activity and antigen, respectively, reference values 2.0-4.0 g/l). Direct sequencing analyses were performed on FGA, FGB, and FGG genes to confirm hypodysfibrinogenemia and on the protein C gene to confirm protein C deficiency. As a result, the patient was shown to be heterozygous p.Ala82Gly in the FGG gene (Fibrinogen Dunedin) and for compound heterozygous missense mutation in protein C gene. To our knowledge, this is the first report on a case of combined dysfibrinogenemia and protein C deficiency confirmed by molecular genetic tests.

The Cardioprotective Signaling Activity of Activated Protein C in Heart Failure and Ischemic Heart Diseases

Activated protein C (APC) is a vitamin-K dependent plasma serine protease, which functions as a natural anticoagulant to downregulate thrombin generation in the clotting cascade. APC also modulates cellular homeostasis by exhibiting potent cytoprotective and anti-inflammatory signaling activities. The beneficial cytoprotective effects of APC have been extensively studied and confirmed in a number of preclinical disease and injury models including sepsis, type-1 diabetes and various ischemia/reperfusion diseases. It is now well-known that APC modulates downstream cell signaling networks and transcriptome profiles when it binds to the endothelial protein C receptor (EPCR) to activate protease-activated receptor 1 (PAR1) on various cell types. However, despite much progress, details of the downstream signaling mechanism of APC and its crosstalk with other signaling networks are far from being fully understood. In this review, we focus on the cardioprotective properties of APC in ischemic heart disease and heart failure with a special emphasis on recent discoveries related to the modulatory effect of APC on AMP-activated protein kinase (AMPK), PI3K/AKT, and mTORC1 signaling pathways. The cytoprotective properties of APC might provide a novel strategy for future therapies in cardiac diseases.

β-arrestin-2 in PAR-1-biased signaling has a crucial role in endothelial function via PDGF-β in stroke

Thrombin aggravates ischemic stroke and activated protein C (APC) has a neuroprotective effect. Both proteases interact with protease-activated receptor 1, which exhibits functional selectivity and leads to G-protein- and β-arrestin-mediated-biased signal transduction. We focused on the effect of β-arrestin in PAR-1-biased signaling on endothelial function after stroke or high-fat diet (HFD). Thrombin had a rapid disruptive effect on endothelial function, but APC had a slow protective effect. Paralleled by prolonged MAPK 42/44 signaling activation by APC via β-arrestin-2, a lower cleavage rate of PAR-1 for APC than thrombin was quantitatively visualized by bioluminescence video imaging. HFD-fed mice showed lower β-arrestin-2 levels and more severe ischemic injury. The expression of β-arrestin-2 in capillaries and PDGF-β secretion in HFD-fed mice were reduced in penumbra lesions. These results suggested that β-arrestin-2-MAPK-PDGF-β signaling enhanced protection of endothelial function and barrier integrity after stroke.

Protein C deficiency resulting from two mutations in PROC presenting with recurrent venous thromboembolism

Hereditary protein C (PC) deficiency is an autosomal dominant disorder associated with a high risk of venous thromboembolism (VTE). Here we report a case of inherited PC deficiency associated with recurrent deep venous thrombosis. Two mutations were revealed in PROC (c.1152C>G, p.N384K and c.1207G>T, p.G403W) by genetic testing. Results from this case suggest that the inherited PC deficiency due to the PROC mutations may cause recurrent VTE. Long-term anticoagulant therapy may be appropriate for these patients with recurrent VTE and hereditary PC deficiency.

Paradoxical bleeding and thrombotic episodes of dysprothrombinaemia due to a homozygous Arg382His mutation

We have characterised the pathogenic basis of dysprothrombinaemia in a patient exhibiting paradoxical bleeding and thrombotic defects during pregnancy and postpartum. Genetic analysis revealed that the proband is homozygous for the prothrombin Arg382His mutation, possessing only ~1 % clotting activity. The proband experienced severe bleeding episodes during her pregnancy, which required treatment with prothrombin complex concentrates, and then pulmonary embolism and deep-vein thrombosis at 28 days postpartum, which required treatment with LMWH and fresh frozen plasma. Analysis of haemostatic parameters revealed that the subject had elevated FDP and DD and decreased fibrinogen levels, indicating the presence of hyperfibrinolysis. Thrombin generation and clotting assays with the proband's plasma in the presence of soluble thrombomodulin and tissue-type plasminogen activator indicated a defect in activation of both protein C and thrombin activatable fibrinolysis inhibitor (TAFI). Unlike normal plasma, no TAFI activation could be detected in the patient's plasma. The expression and characterisation of recombinant prothrombin Arg382His indicated that zymogen activation by prothrombinase was markedly impaired and the activation of protein C and TAFI by thrombin-Arg382His was impaired 600-fold and 2500-fold, respectively. The recombinant thrombin mutant exhibited impaired catalytic activity toward both fibrinogen and PAR1 as determined by clotting and signalling assays. However, the mutant activated factor XI normally in both the absence and presence of polyphosphates. Arg382 is a key residue on (pro)exosite-1 of prothrombin and kinetic analysis of substrate activation suggested that the poor zymogenic activity of the mutant is due to its inability to bind factor Va in the prothrombinase complex.

Occupancy of human EPCR by protein C induces β-arrestin-2 biased PAR1 signaling by both APC and thrombin

Activation of protease-activated receptor 1 (PAR1) by activated protein C (APC) and thrombin elicits paradoxical cytoprotective and cytotoxic signaling responses in vascular endothelial cells through cleavage of the receptor at Arg-46 and Arg-41 protease recognition sites, respectively. It has been reported that unlike a disruptive G-protein-mediated PAR1 signaling by thrombin, APC induces a protective β-arrestin-2 biased PAR1 signaling by unknown mechanisms. We hypothesize that the occupancy of endothelial protein C receptor (EPCR) by the Gla-domain of protein C/APC is responsible for the β-arrestin-2 biased PAR1 signaling independent of the protease cleavage site. To test this hypothesis, we monitored the signaling specificity of thrombin in endothelial cells in response to lipopolysaccharide (LPS) with or without pretreatment of cells with protein C-S195A. The PAR1-dependent recruitment of β-arrestin-2 in response to LPS by both APC and thrombin was analyzed by functional, gene silencing, and signaling assays. Results indicate that similar to APC, thrombin exerts cytoprotective effects via β-arrestin-2 biased PAR1 signaling. Similar to APC, thrombin triggered β-arrestin-2-dependent recruitment of disheveled 2 (Dvl-2) in PC-S195A pretreated cells. Further studies in HeLa cells transfected with PAR1 constructs revealed that EPCR occupancy initiates β-arrestin-2 biased PAR1 signaling independent of the protease cleavage sites. We demonstrate that EPCR occupancy recruits G-protein coupled receptor kinase 5, thereby inducing β-arrestin-2 biased PAR1 signaling by both APC and thrombin. In support of a physiological relevance for these results, intraperitoneal administration of PC-S195A conferred a cytoprotective effect for thrombin in an in vivo inflammatory model.

A direct thrombin inhibitor suppresses protein C activation and factor Va degradation in human plasma: Possible mechanisms of paradoxical enhancement of thrombin generation

We have demonstrated that antithrombin (AT)-independent thrombin inhibitors paradoxically increase thrombin generation (TG) in human plasma in a thrombomodulin (TM)- and protein C (PC)-dependent manner. We determined the effects of AT-independent thrombin inhibitors on the negative-feedback system, activation of PC and production and degradation of factor Va (FVa), as possible mechanisms underlying the paradoxical enhancement of TG. TG in human plasma containing 10nM TM was assayed by means of the calibrated automated thrombography. As an index of PC activation, plasma concentration of activated PC-PC inhibitor complex (aPC-PCI) was measured. The amounts of FVa heavy chain and its degradation product (FVa(307-506)) were examined by western blotting. AT-independent thrombin inhibitors, melagatran and dabigatran (both at 25-600nM) and 3-30μg/ml active site-blocked thrombin (IIai), increased peak levels of TG. Melagatran, dabigatran and IIai significantly decreased plasma concentration of aPC-PCI complex at 25nM or more, 75nM or more, and 10 and 30μg/ml, respectively. Melagatran (300nM) significantly increased FVa and decreased FVa(307-506). In contrast, a direct factor Xa inhibitor edoxaban preferentially inhibited thrombin generation (≥25nM), and higher concentrations were required to inhibit PC activation (≥150nM) and FVa degradation (300nM). The present study suggests that the inhibitions of protein C activation and subsequent degradation of FVa and increase in FVa by antithrombin-independent thrombin inhibitors may contribute to the paradoxical TG enhancement, and edoxaban may inhibit PC activation and FVa degradation as a result of TG suppression.

Activated protein C upregulates ovarian cancer cell migration and promotes unclottability of the cancer cell microenvironment

The objective of this study was to evaluate the role of activated protein C (aPC), known to be a physiological anticoagulant, in ovarian cancer cell activation as well as in loss of clotting of cancer ascitic fluid. The effect of aPC on an ovarian cancer cell line (OVCAR-3) was tested in regards to i) cell migration and adhesion with the use of adhesion and wound healing assays as well as a droplet test; ii) protein phosphorylation, evaluated by cyto-ELISA; iii) cell cycle modification assessed by flow cytometric DNA quantification; and iv) anticoagulant activity evaluated by the prolongation of partial thromboplastin time (aPTT) of normal plasma in the presence or absence of aPC-treated ovarian cancer cells. In addition, the soluble endothelial protein C receptor (sEPCR) was quantified by ELISA in ascitic fluid of patients with ovarian cancer. Our results showed that in the OVCAR-3 aPC-induced cells i) an increase in cell migration was noted, which was inhibited when anti-endothelial protein C receptor (EPCR) was added to the culture medium and which may act via MEK-ERK and Rho-GTPase pathways; ii) an increase in threonine, and to a lesser extent tyrosine phosphorylation; iii) cell cycle activation (G1 to S/G2); and iv) a 2-3-fold prolongation of aPTT of normal plasma. In the peritoneal fluid, the sEPCR concentration was 71±23 ng/ml. In conclusion, free aPC binds to membrane EPCR in ovarian cancer cells and induces cell migration via MEK-ERK and Rho-GTPase pathways. This binding could also explain the loss of clotting of peritoneal fluids.

Protein C Thr315Ala variant results in gain of function but manifests as type II deficiency in diagnostic assays

Protein C (PC) is a vitamin K-dependent plasma glycoprotein, which upon activation by thrombin in complex with thrombomodulin (TM), regulates the coagulation cascade through a feedback loop inhibition mechanism. PC deficiency is associated with an increased risk of venous thromboembolism (VTE). A recent cohort study aimed at establishing a normal PC range identified a healthy PC-deficient subject whose PC antigen level of 65% and activity levels of 50% (chromogenic assay) and 36% (clotting assay) were markedly low. The proband has a negative family history of VTE. Genetic analysis revealed the proband has a heterozygous missense mutation in which Thr-315 of the PC heavy chain has been substituted with Ala. We expressed this mutant in HEK-293 cells and purified it to homogeneity. A similar decrease in both anticoagulant and anti-inflammatory activities of the activated protein C mutant was observed in plasma- and cell-based assays. Interestingly, we discovered if functional assays were coupled to PC activation by the thrombin-TM complex, the variant exhibits improved activities in all assays. Sequence analysis revealed Thr-315 is a consensus N-linked glycosylation site for Asn-313 and that its elimination significantly (∼four- to fivefold) improves the maximum velocity of PC activation by the thrombin-TM complex, explaining the basis for the proband's negative VTE pedigree.

Molecular control of capillary morphogenesis and maturation by recognition and remodeling of the extracellular matrix: functional roles of endothelial cells and pericytes in health and disease

This review addresses fundamental mechanisms underlying how capillaries form in three-dimensional extracellular matrices and how endothelial cells (ECs) and pericytes co-assemble to form capillary networks. In addition to playing a critical role in supplying oxygen and nutrients to tissues, recent work suggests that blood vessels supply important signals to facilitate tissue development. Here, we hypothesize that another major function of capillaries is to supply signals to suppress major disease mechanisms including inflammation, infection, thrombosis, hemorrhage, edema, ischemic injury, fibrosis, autoimmune disease and tumor growth/progression. Capillary dysfunction plays a key pathogenic role in many human diseases, and thus, this suppressing function may be attenuated and central toward the initiation and progression of disease. We describe how capillaries form through creation of EC-lined tube networks and vascular guidance tunnels in 3D extracellular matrices. Pericytes recruit to the abluminal EC tube surface within these tunnel spaces, and work together to assemble the vascular basement membrane matrix. These processes occur under serum-free conditions in 3D collagen or fibrin matrices and in response to five key growth factors which are stem cell factor, interleukin-3, stromal-derived factor-1α, fibroblast growth factor-2 and insulin. In addition, we identified a key role for EC-derived platelet-derived growth factor-BB and heparin-binding epidermal growth factor in pericyte recruitment and proliferation to promote EC-pericyte tube co-assembly and vascular basement membrane matrix deposition. A molecular understanding of capillary morphogenesis and maturation should lead to novel therapeutic strategies to repair capillary dysfunction in major human disease contexts including cancer and diabetes.

Drug-Like Protein-Protein Interaction Modulators: Challenges and Opportunities for Drug Discovery and Chemical Biology

[Formula: see text] Fundamental processes in living cells are largely controlled by macromolecular interactions and among them, protein-protein interactions (PPIs) have a critical role while their dysregulations can contribute to the pathogenesis of numerous diseases. Although PPIs were considered as attractive pharmaceutical targets already some years ago, they have been thus far largely unexploited for therapeutic interventions with low molecular weight compounds. Several limiting factors, from technological hurdles to conceptual barriers, are known, which, taken together, explain why research in this area has been relatively slow. However, this last decade, the scientific community has challenged the dogma and became more enthusiastic about the modulation of PPIs with small drug-like molecules. In fact, several success stories were reported both, at the preclinical and clinical stages. In this review article, written for the 2014 International Summer School in Chemoinformatics (Strasbourg, France), we discuss in silico tools (essentially post 2012) and databases that can assist the design of low molecular weight PPI modulators (these tools can be found at www.vls3d.com). We first introduce the field of protein-protein interaction research, discuss key challenges and comment recently reported in silico packages, protocols and databases dedicated to PPIs. Then, we illustrate how in silico methods can be used and combined with experimental work to identify PPI modulators.

High coagulation factor levels and low protein C levels contribute to enhanced thrombin generation in patients with diabetes who do not have macrovascular complications

AIMS

A prothrombotic state characterized by activation of the coagulation system has been implicated in the pathogenesis of vascular complications in diabetes mellitus. Recently, a thrombin generation assay was introduced as a laboratory assessment of global hemostatic potential. We used this thrombin generation assay to investigate global hemostatic potential in patients with diabetes who did not have macrovascular complications.

METHODS

This study was a prospective case-control study comparing 89 patients with diabetes with 49 healthy controls. The thrombin generation assay was conducted with the calibrated automated thrombogram using tissue factor with or without the addition of thrombomodulin, giving values for lag time, endogenous thrombin potential, and peak thrombin.

RESULTS

Patients with diabetes showed hypercoagulability, as detected by the thrombin generation assay, compared with healthy controls. Correspondingly, high levels of coagulation factors (II, V, VII, VIII, and X) and low levels of anticoagulant (protein C) were major contributing factors in this hypercoagulability. Interestingly, a high blood glucose level was correlated with shortened clotting time, reflecting the association between hyperglycemia and hypercoagulability. Patients who were taking statins or angiotensin receptor blockers showed decreased endogenous thrombin potential ratio and increased protein C levels, suggesting relative hypocoagulability.

CONCLUSIONS

Patients with diabetes showed hypercoagulability, high levels of coagulation factors, and low levels of protein C. Further study is required to investigate how this hemostatic potential may be used to guide physicians toward more effective management of hemostatic complications.

Plasma endothelial protein C receptor influences innate immune response in ovarian cancer by decreasing the population of natural killer and TH17 helper cells

In spite of the growing importance of endothelial protein C receptor/active protein C (EPCR/aPC) in tumor biology, their impact on immunological homeostasis remains largely unexplored. The objective of this study was to assess whether soluble plasma endothelial protein C receptor (sEPCR), which is a regulator of circulating aPC, is involved in innate immune response in cancer patients. In the Ovcar-3 ovarian cancer line, the role of aPC in secretion of cytokines was analyzed. In parallel, in 33 patients, with a diagnosis of ovarian epithelial cancer, sEPCR was quantified, blood immune cell phenotypes were determined by flow cytometry and plasma cytokines were evaluated using a protein array. Spearman’s rank correlation coefficients (r) and coefficient significance was determined by a statistical hypothesis test (α=0.05). Our results show that i) aPC induced the secretion of several cytokines in Ovcar-3 cells; ii) 61% of patients exhibited a concentration of plasma sEPCR well above the baseline (normal plasma level, 100±28 ng/ml); iii) comparing immune cell phenotypes in patients having a normal level of sEPCR with those having a high level of sEPCR, it was found that sEPCR levels were correlated with high intensity of cells expressing CD45ra, CD3, CD8, CD25 and low intensity of cells expressing CD56 (NK cells), CD294 (TH2 cells), IL-2, IL-10, IL-17a (TH17 cells), IL-21 (TH21 cells) and CD29 markers (r ≥0.60); and iv) high levels of sEPCR correlate with high levels of plasma bioactive proteins such as insulin-like growth factor-2 (IGFII), IL-13rα, macrophage inflammatory protein (MIP1α) and matrix metalloproteinase-7 (MMP-7) that have already been proposed as biomarkers for ovarian cancer and particularly those with poor prognosis. In conclusion, sEPCR produced by ovarian cancer cells, by modulating circulating aPC, influences the secretory behavior of tumor cells (cytokines and interleukins). Consequently, sEPCR in turn acts on the innate immune response by decreasing effector cells such as natural killer and T helper cells (TH2, TH17 and TH21).

Soluble endothelial protein C receptor (sEPCR) is likely a biomarker of cancer-associated hypercoagulability in human hematologic malignancies

Elevated plasma level of soluble endothelial protein C receptor (sEPCR) may be an indicator of thrombotic risk. The present study aims to correlate leukemia-associated hypercoagulability to high level plasma sEPCR and proposes its measurement in routine clinical practice. EPCR expressions in leukemic cell lines were determined by flow cytometry, immunocytochemistry, and reverse transcription polymerase chain reaction (RT-PCR). EPCR gene sequence of a candidate cell line HL-60 was also determined. Plasma samples (n = 76) and bone marrow aspirates (n = 72) from 148 patients with hematologic malignancies and 101 healthy volunteers were analyzed by enzyme-linked immunosorbent assay (ELISA) via a retrospective study for sEPCR and D-dimer. All leukemic cell lines were found to express EPCR. Also, HL-60 EPCR gene sequence showed extensive similarities with the endothelial reference gene. All single nucleotide polymorphisms (SNPs) originally described and some new SNPs were revealed in the promoter and intronic regions. Among these patients 67% had plasma sEPCR level higher than the controls (100 ± 28 ng/mL), wherein 16.3% patients had experienced a previous thrombotic event. These patients were divided into: group-1 (n = 45) with amount of plasmatic sEPCR below 100 ng/mL, group-2 (n = 45) where the concentration of sEPCR was between 100 and 200, and group-3 (n = 20) higher than 200 ng/mL. The numbers of thrombotic incidence recorded in each group were four, six, and eight, respectively. These results reveal that EPCR is expressed not only by a wide range of human malignant hematological cells but also the detection of plasma sEPCR levels provides a powerful insight into thrombotic risk assessment in cancer patients, especially when it surpasses 200 ng/mL.

Endothelial protein C receptor expressed by ovarian cancer cells as a possible biomarker of cancer onset

Coagulation disorders often accompany cancer onset and evolution, which, if not properly managed, could have grave consequences. Endothelial protein C is an important regulator of homeostasis and acts through its high affinity binding to its transmembrane receptor (EPCR). Soluble (sEPCR) which results from the proteolytic cleavage of the membrane bound form can trap activated endothelial protein C and deprive it of its anti-coagulant function. In this study, the expression of EPCR and its soluble form (sEPCR) released into plasma as a result of proteolytic cleavage were investigated in ovarian, breast, lung and colorectal cancer biopsies, as well as in ascitic cell clusters and peritoneal fluid from ovarian cancer samples. In parallel, breast, ovarian, lung and colorectal cancer cell lines were investigated for the expression of EPCR. The integrity of the EPCR gene sequence as well gene haplotypes were ascertained in the established cancer cell lines in order to understand their eventual regulatory functions. The results from the present study indicate that in cancer patients, the levels of sEPCR are significantly higher than the normal range compared to healthy volunteers. The increase in the levels of sEPCR parallels the increase in CA125, showing a close correlation. Therefore, the detection of sEPCR in cancer and during the post-treatment period could be taken into account as an additional marker that could re-inforce the one obtained using CA125 alone as a marker of cancer cell mass.

Genetic background analysis of protein C deficiency demonstrates a recurrent mutation associated with venous thrombosis in Chinese population

Background

Protein C (PC) is one of the most important physiological inhibitors of coagulation proteases. Hereditary PC deficiency causes a predisposition to venous thrombosis (VT). The genetic characteristics of PC deficiency in the Chinese population remain unknown.

Methods

Thirty-four unrelated probands diagnosed with hereditary PC deficiency were investigated. PC activity and antigen levels were measured. Mutation analysis was performed by sequencing the PROC gene. In silico analyses, including PolyPhen-2, SIFT, multiple sequence alignment, splicing prediction, and protein molecular modeling were performed to predict the consequences of each variant identified. One recurrent mutation and its relative risk for thrombosis in relatives were analyzed in 11 families. The recurrent mutation was subsequently detected in a case (VT patients)-control study, and the adjusted odds ratio (OR) for VT risk was calculated by logistic regression analysis.

Results

A total of 18 different mutations, including 12 novel variants, were identified. One common mutation, PROC c.565C>T (rs146922325:C>T), was found in 17 of the 34 probands. The family study showed that first-degree relatives bearing this variant had an 8.8-fold (95%CI = 1.1–71.6) increased risk of venous thrombosis. The case-control (1003 vs. 1031) study identified this mutation in 5.88% patients and in 0.87% controls, respectively. The mutant allele conferred a high predisposition to venous thrombosis (adjusted OR = 7.34, 95%CI = 3.61–14.94). The plasma PC activity and antigen levels in heterozygotes were 51.73±6.92 U/dl and 75.17±4.84 U/dl, respectively.

Conclusions

This is the first study on the genetic background of PC deficiency in the Chinese population. The PROC c.565C>T mutation is the most frequent cause of PC deficiency as well as a prevalent risk factor for VT in Chinese individuals. The inclusion of this variant in routine thrombophilic detection may improve the diagnosis and prevention of venous thrombosis.

Melagatran, a direct thrombin inhibitor, but not edoxaban, a direct factor Xa inhibitor, nor heparin aggravates tissue factor-induced hypercoagulation in rats

There are concerns that some anticoagulants can paradoxically increase thrombogenesis under certain circumstances. We have shown that low-dose administration of a direct thrombin inhibitor, melagatran, significantly worsens the coagulation status induced by tissue factor injection in rats. We compared the effect of inhibition of thrombin and factor Xa for their potential to aggravate tissue factor-induced coagulation in rats. Hypercoagulation was induced by the injection of 2.8 U/kg tissue factor after administration of melagatran, heparin and edoxaban in rats. Blood samples were collected 10min after tissue factor injection. Platelet numbers, thrombin-antithrombin complex concentrations and plasma compound concentrations were measured. Though a high dose of melagatran (1mg/kg, i.v.) suppressed platelet consumption and thrombin-antithrombin complex generation induced by tissue factor, lower doses of melagatran (0.01, 0.03 and 0.1mg/kg, i.v.) significantly enhanced platelet consumption and thrombin-antithrombin complex generation. In addition, although melagatran (3mg/kg, i.v.) improved coagulation status when tissue factor was given 5min after the drug administration, and 2, 4 and 8h after melagatran dosing, it deteriorated coagulation status. These results were well explained by the plasma melagatran concentration. Low concentrations (15-234ng/ml) of melagatran aggravated coagulation status whereas it was mended by high concentrations (1190ng/ml or more) of the compound. In contrast, edoxaban and heparin did not show any exacerbation under these examination conditions. These results show that subtherapeutic concentrations of melagatran are associated with coagulation pathway activation, whereas factor Xa inhibition with edoxaban has a low risk of paradoxical hypercoagulation.

Antithrombin-independent thrombin inhibitors, but not direct factor Xa inhibitors, enhance thrombin generation in plasma through inhibition of thrombin-thrombomodulin-protein C system

There is increasing concern that some anticoagulants can paradoxically increase thrombogenesis under certain circumstances. Previously, we demonstrated that at certain doses a direct thrombin inhibitor, melagatran, worsens the coagulation status induced by tissue factor (TF) injection in a rat model. We utilised an in vitro thrombin generation (TG) assay to determine if direct thrombin inhibitors could enhance TG in human plasma, and whether inhibition of the negative-feedback system [thrombin-thrombomodulin (TM)-protein C] contributed to the TG enhancement. TG in human plasma was assayed by means of the calibrated automated thrombography. In this assay, direct factor Xa (FXa) inhibitors such as edoxaban and antithrombin (AT)-dependent anticoagulants such as heparin did not increase, but simply suppressed TG. AT-independent thrombin inhibitors (melagatran, lepirudin, and active site blocked thrombin (IIai)) increased peak levels of TG (2.0, 1.6, and 2.2-fold, respectively) in the presence of 12 nM recombinant human soluble TM (rhsTM). Melagatran and lepirudin at higher concentrations began to suppress TG. In the absence of rhsTM, the enhancement of peak TG by melagatran decreased to 1.2-fold. Furthermore, in protein C-deficient plasma, AT-independent thrombin inhibitors failed to enhance TG. In addition, a human protein C neutralising antibody increased the peak height of TG in the presence of rhsTM. These results suggest that AT-independent thrombin inhibitors may activate thrombogenesis by suppression of the thrombin-induced negative-feedback system through inhibition of protein C activation. In contrast, direct FXa inhibitors are more useful than AT-independent thrombin inhibitors in terms of lower possibility of activation of the coagulation pathway.

Protein C mutation (A267T) results in ER retention and unfolded protein response activation

Background

Protein C (PC) deficiency is associated with a high risk of venous thrombosis. Recently, we identified the PC-A267T mutation in a patient with PC deficiency and revealed by in vitro studies decreased intracellular and secreted levels of the mutant. The aim of the present study was to characterize the underlying mechanism(s).

Methodology/Principal Findings

CHO-K1 cells stably expressing the wild-type (PC-wt) or the PC mutant were generated. In order to examine whether the PC mutant was subjected to increased intracellular degradation, the cells were treated with several inhibitors of various degradation pathways and pulse-chase experiments were performed. Protein-chaperone complexes were analyzed by treating the cells with a cross-linker followed by Western blotting (WB). Expression levels of the immunoglobulin-binding protein (BiP) and the phosphorylated eukaryotic initiation factor 2α (P-eIF2α), both common ER stress markers, were determined by WB to examine if the mutation induced ER stress and unfolded protein response (UPR) activation. We found no major differences in the intracellular degradation between the PC variants. The PC mutant was retained in the endoplasmic reticulum (ER) and had increased association with the Grp-94 and calreticulin chaperones. Retention of the PC-A267T in ER resulted in UPR activation demonstrated by increased expression levels of the ER stress markers BiP and P-eIF2α and caused also increased apoptotic activity in CHO-K1 cells as evidenced by elevated levels of DNA fragmentation.

Conclusions/Significance

The reduced intracellular level and impaired secretion of the PC mutant were due to retention in ER. In contrast to other PC mutations, retention of the PC-A267T in ER resulted in minor increased proteasomal degradation, rather it induced ER stress, UPR activation and apoptosis.

Anticoagulant effect of polyphenols-rich extracts from black chokeberry and grape seeds

Blood coagulation consists of a series of zymogens that can be converted by limited proteolysis to active enzymes leading to the generation of thrombin. Fresh plasma and human thrombin was incubated with extracts from berries of Aronia melanocarpa or seeds of Vitis vinifera (0.5; 5; 50 μg/ml). The in vitro experiments showed that both extracts prolonged clotting time and decreased the maximal velocity of fibrin polymerization in human plasma. Moreover thrombin incubation with both extracts results in the inhibition of amidolytic activity of this enzyme. It gives hopes for development of diet supplements, which may be preventing thrombosis in pathological states.

Functional characterization of the protein C A267T mutation: evidence for impaired secretion due to defective intracellular transport

Background

Activated protein C (PC) is a serine protease that regulates blood coagulation by inactivating coagulation factors Va and VIIIa. PC deficiency is an autosomally inherited disorder associated with a high risk of recurrent venous thrombosis. The aim of the study was to explore the mechanisms responsible for severe PC deficiency in a patient with the protein C A267T mutation by in-vitro expression studies.

Results

Huh7 and CHO-K1 cells were transiently transfected with expression vectors containing wild-type (WT PC) and mutated PC (A267T PC) cDNAs. PC mRNA levels were assessed by qRT-PCR and the PC protein levels were measured by ELISA. The mRNA levels of WT PC and A267T PC were similar, while the intracellular protein level of A267T PC was moderately decreased compared to WT PC. The secretion of A267T PC into the medium was severely impaired. No differences in molecular weights were observed between WT and A267T PC before and after treatment with endo-β-N-acetylglucosaminidase. Proteasomal and lysosomal degradations were examined using lactacystin and bafilomycin, respectively, and revealed that A267T PC was slightly more susceptible for proteasomal degradation than WT PC. Intracellular co-localization analysis indicated that A267T PC was mainly located in the endoplasmic reticulum (ER), whereas WT PC was observed in both ER and Golgi.

Conclusions

In contrast to what has been reported for other PC mutants, intracellular degradation of A267T PC was not the main/dominant mechanism underlying the reduced intracellular and secretion levels of PC. Our results indicate that the A267T mutation most likely caused misfolding of PC, which might lead to increased retention of the mutated PC in ER.

Druggable pockets and binding site centric chemical space: a paradigm shift in drug discovery

Detection, comparison and analyses of binding pockets are pivotal to structure-based drug design endeavors, from hit identification, screening of exosites and de-orphanization of protein functions to the anticipation of specific and non-specific binding to off- and anti-targets. Here, we analyze protein-ligand complexes and discuss methods that assist binding site identification, prediction of druggability and binding site comparison. The full potential of pockets is yet to be harnessed, and we envision that better understanding of the pocket space will have far-reaching implications in the field of drug discovery, such as the design of pocket-specific compound libraries and scoring functions.

Expression of protein C (PC), protein S (PS) and thrombomodulin (TM) in human colorectal cancer

INTRODUCTION

Colorectal cancer (CRC) is often complicated by thromboembolic episodes. It has been recognized that blood coagulation proteins play a role in cancer progression. An important inhibitory mechanism is provided by the protein C (PC) system consisting of PC, protein S (PS) and thrombomodulin (TM). Recently, novel biological activities have been ascribed to the PC system that do not relate to their hemostatic functions, eg. in angiogenesis, apoptosis and inflammation.

OBJECTIVES

The purpose of the study was to elucidate the solid phase interactions between CRC tissue and components of the PC system that may contribute to tumor progression.

MATERIAL AND METHODS

CRC tissues were obtained at surgical resection during treatment of 66 patients. Immunohistochemical studies were performed using polyclonal antibodies against PC, PS and TM. A semiquantitative analysis of the protein expression was also performed.

RESULTS

Weak expression of PC was observed in cancer cells of two-thirds of the specimens examined, while in 3/66 cases there was no staining for PC in cancer cells. One fourth of CRCs exhibited strong expression of PC. The presence of PS was demonstrated in 64/66 cases of CRC. However, its expression was irregular in terms of intensity of staining and percentage of cancer cells exhibiting protein expression. Weak expression of TM was demonstrated in two thirds of the cases examined, while a strong TM staining was revealed in one third of colon cancers.

CONCLUSION

Heterogeneous expression of the PC system components in CRC tissue may point to their biological activity modulating tumor growth.

Increased risk of venous thrombosis by AB alleles of the ABO blood group and Factor V Leiden in a Brazilian population

Most cases of a predisposition to venous thrombosis are caused by resistance to activated protein C, associated in 95% of cases with the Factor V Leiden allele (FVL or R506Q). Several recent studies report a further increased risk of thrombosis by an association between the AB alleles of the ABO blood group and Factor V Leiden. The present study investigated this association with deep vein thrombosis (DVT) in individuals treated at the Hemocentro de Pernambuco in northeastern Brazil. A case-control comparison showed a significant risk of thrombosis in the presence of Factor V Leiden (OR = 10.1), which was approximately doubled when the AB alleles of the ABO blood group were present as well (OR = 22.3). These results confirm that the increased risk of deep vein thrombosis in the combined presence of AB alleles and Factor V Leiden is also applicable to the Brazilian population suggesting that ABO blood group typing should be routinely added to FVL in studies involving thrombosis.

Antiphospholipid antibodies: paradigm in transition

OBJECTIVES

This is a critical review of anti-phospholipid antibodies (aPL). Most prior reviews focus on the aPL syndrome (APS), a thrombotic condition often marked by neurological disturbance. We bring to attention recent evidence that aPL may be equally relevant to non-thrombotic autoimmune conditions, notably, multiple sclerosis and ITP.

ORGANIZATION

After a brief history, the recent proliferation of aPL target antigens is reviewed. The implication is that many more exist. Theories of aPL in thrombosis are then reviewed, concluding that all have merit but that aPL may have more diverse pathological consequences than now recognized. Next, conflicting results are explained by methodological differences. The lupus anticoagulant (LA) is then discussed. LA is the best predictor of thrombosis, but why this is true is not settled. Finally, aPL in non-thrombotic disorders is reviewed.

CONCLUSION

The current paradigm of aPL holds that they are important in thrombosis, but they may have much wider clinical significance, possibly of special interest in neurology.

Biochemical comparison of seven commercially available prothrombin complex concentrates

AIMS

A number of prothrombin complex concentrates (PCCs) are commercially available but they differ in terms of composition. We performed a series of studies to compare the biochemical properties of seven PCCs.

METHODS

The following products were investigated: Beriplex P/N, Octaplex, S-TIM 4, PPSB Solvent Detergent, Uman Complex DI, Kaskadil and Cofact. Assays were performed to investigate levels of coagulation factors and their inhibitors, activated coagulation factors and heparin. The thrombin inhibitory capacity of each PCC was determined. Protein content was assessed using the Lowry method and sodium dodecyl sulphate-polyacrylamide gel electrophoresis.

RESULTS

The data indicated little difference between most of the products in their levels of factors II, VII, IX and X, with the exception of Uman Complex which had no detectable factor VII. In all cases, the measured levels of coagulation factors were broadly similar to those labelled. Beriplex P/N showed the greatest capacity for thrombin inhibition, a reflection of the observed high levels of the coagulation inhibitors protein C, protein S, protein Z, and small amounts of antithrombin III and heparin in this product. All of the PCCs tested were negative for activated coagulation factors. Purity (i.e. therapeutic protein as a percentage of total protein) was highest in Beriplex P/N, and the second purest product was Uman Complex.

CONCLUSION

This in vitro study showed considerable differences between PCCs in terms of coagulation inhibitory capacity and purity.

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.

Protein S modulates the anticoagulant action of recombinant human activated protein C: a comparison between neonates and adults

Recombinant human-activated protein C (rhAPC, Drotrecogin alpha (activated), Xigris) has been shown to reduce organ damage and decrease mortality in severe sepsis. Since protein S (PS) serves as a potentiating cofactor of activated protein C and since PS levels are low in neonatal plasma, we hypothesized that the anticoagulant effect of rhAPC would be decreased in cord plasma compared to adult plasma. We demonstrate that the anticoagulant action of 0.3 microg ml(-1) rhAPC (5 nmol l(-1)) was decreased in cord plasma compared to adult plasma, and dose dependently increased in cord plasma in the presence of increasing activities of PS. Correspondingly, the anticoagulant action of rhAPC decreased in adult plasma in the presence of decreasing activities of PS. The low anticoagulant action of rhAPC in cord compared to adult plasma is attributable to low neonatal levels of PS, and as previously shown, to low neonatal levels of TFPI and AT. Our laboratory experiments do not allow definite conclusions for clinical situations. However, we speculate that the anticoagulant efficacy of rhAPC is impaired in neonates and in clinical situations associated with consumption and/or inhibition of PS, AT, and TFPI, such as severe sepsis.

Proteolytic cleavage of protein S during the hemostatic response

BACKGROUND

Protein S is a vitamin K-dependent protein with anticoagulant properties. It contains a so-called thrombin-sensitive region (TSR), which is susceptible to cleavage by coagulation factor Xa (FXa) and thrombin. Upon cleavage, the anticoagulant activity of protein S is abolished.

OBJECTIVE

The aim of the present study was to determine whether protein S is cleaved within the TSR during activation of the coagulation system under near physiological conditions.

RESULTS

In a reconstituted coagulation system containing apart from protein S only procoagulant constituents and synthetic phospholipid vesicles, protein S was cleaved at Arg60 by the FXa generated (3 mol min(-1) mol(-1) enzyme). FXa-catalyzed cleavage of protein S, however, was inhibited by factor Va and prothrombin by more than 70%. During clotting of recalcified citrated plasma in the presence of a synthetic lipid membrane, no FXa-catalyzed proteolysis of protein S was observed. Substituting platelets for phospholipid vesicles resulted both in the reconstituted system and in plasma in cleavage of the TSR. Cleavage was at Arg60 and was observed upon platelet activation, irrespective of the presence of FXa (13 pmol min(-1) 10(-8) platelets). No cleavage by thrombin was observed in either the reconstituted coagulation system or clotting plasma.

CONCLUSION

These findings suggest that in vivo the anticoagulant activity of protein S is not down-regulated by FXa or thrombin during activation of coagulation. Our results rather suggest a role for a platelet protease in down-regulating the anticoagulant activity of protein S during the hemostatic response.

Molecular recognition mechanisms of thrombin

Thrombin is the final protease generated in the blood coagulation cascade, and is the only factor capable of cleaving fibrinogen to create a fibrin clot. Unlike every other coagulation protease, thrombin is composed solely of its serine protease domain, so that once formed it can diffuse freely to encounter a large number of potential substrates. Thus thrombin serves many functions in hemostasis through the specific cleavage of at least a dozen substrates. The solution of the crystal structure of thrombin some 15 years ago revealed a deep active site cleft and two adjacent basic exosites, and it was clear that thrombin must utilize these unique features in recognizing its substrates. Just how this occurs is still being investigated, but recent data from thrombin mutant libraries and crystal structures combine to paint the clearest picture to date of the molecular determinants of substrate recognition by thrombin. In almost all cases, both thrombin exosites are involved, either through direct interaction with the substrate protein or through indirect interaction with a third cofactor molecule. The purpose of this article is to summarize recent biochemical and structural data in order to provide insight into the thrombin molecular recognition events at the heart of hemostasis.

Recombinant human activated protein C, heparin and melagatran in umbilical cord versus adult plasma

AIM

We investigated the anticoagulant effects of recombinant human activated protein C (rhAPC), unfractionated heparin (UH) and melagatran (a new direct thrombin inhibitor [DTI]), when administered individually and in combinations of rhAPC with either UH or melagatran, in umbilical cord and adult plasma. rhAPC is a promising candidate treatment to improve the outcome of severe sepsis in neonates and adults; the DTI melagatran represents a potential advance in antithrombotic therapy.

METHODS

The anticoagulant efficacy of these drugs was measured using the standard coagulation assays activated partial thromboplastin time (aPTT) and prothrombin time (PT).

RESULTS

Administered individually, rhAPC, UH and melagatran dose-dependently prolonged aPTT to a significantly greater extent in umbilical cord than in adult plasma. Melagatran alone, but not rhAPC or UH alone, dose-dependently prolonged the PT in both umbilical cord and adult plasma. Combining rhAPC with either UH or melagatran significantly augmented aPTT prolongation in both umbilical cord and adult plasma.

CONCLUSION

Our results, which facilitate estimation of rhAPC and melagatran dose requirements in umbilical cord plasma, may be of benefit in critically sick newborns with severe sepsis.

Anti-protein S antibodies following a varicella infection: detection, characterization and influence on thrombin generation

Postinfectious purpura fulminans is a rare disease. Varicella is one of the precipitating conditions and we recently observed such a case. The 4-year-old child was found to have a severe transient protein S deficiency. By enzyme-linked immunosorbent assay and surface plasmon resonance we first demonstrated that anti-protein S antibodies were present and also transient. Next we characterized the epitopes against which these antibodies were directed and found that they predominantly recognized the N-terminal part of protein S. Finally we showed by thrombography a transient dramatic hypercoagulable state as a result of thrombin being unregulated by the dynamic protein C inhibitory system: in vitro thrombin generation, in response to a low concentration of tissue factor, was almost insensitive to activated protein C up to 25 nmol L(-1) on day 4 while it was normally sensitive on day 42. For the first time, we demonstrated a temporal relationship between protein S deficiency, antibodies to protein S and hypercoagulability, thus supporting the pathogenic role of these antibodies.

Different antithrombotic properties of factor Xa inhibitor and thrombin inhibitor in rat thrombosis models

We compared the antithrombotic properties of a factor Xa inhibitor (DX-9065a) with those of a thrombin inhibitor (melagatran) in a rat disseminated intravascular coagulation model and a rat venous thrombosis model. Rat disseminated intravascular coagulation and venous thrombosis models were produced by injection of tissue factor and platinum wire placement, respectively. DX-9065a exerted antithrombotic effects dose dependently in both models. Melagatran was also effective in the venous thrombosis model, whereas it showed an aggravation in the disseminated intravascular coagulation model at low but not high doses. In the in vitro study, DX-9065a decreased the C(max) of the thrombin generation curve in plasma irrespective of whether protein C was present or not. However, melagatran increased the C(max) at low concentrations when protein C was present. This increase was not detected in protein C-deficient plasma. These results suggest that, unlike DX-9065a, melagatran in low doses aggravates disseminated intravascular coagulation by increasing thrombin generation, which may be partly due to suppression of negative feedback by activated protein C.

The anticoagulant protein C pathway

The anticoagulant protein C system regulates the activity of coagulation factors VIIIa and Va, cofactors in the activation of factor X and prothrombin, respectively. Protein C is activated on endothelium by the thrombin-thrombomodulin-EPCR (endothelial protein C receptor) complex. Activated protein C (APC)-mediated cleavages of factors VIIIa and Va occur on negatively charged phospholipid membranes and involve protein cofactors, protein S and factor V. APC also has anti-inflammatory and anti-apoptotic activities that involve binding of APC to EPCR and cleavage of PAR-1 (protease-activated receptor-1). Genetic defects affecting the protein C system are the most common risk factors of venous thrombosis. The protein C system contains multi-domain proteins, the molecular recognition of which will be reviewed.

New horizons in the analysis of circulating cell-derived microparticles

Analysis of circulating cell-derived microparticles (MP) is becoming more refined and clinically useful. This review, stemming from lectures given at Tokyo late 2003, does not repeat prior reviews but focuses on new horizons. A major theme is the rising recognition of platelets and their MP (PMP) as key mediators of inflammation/immunity. Among the major concepts developed are that (i) many so-called soluble markers of inflammation are in reality MP-bound; (ii) PMP and other MP appear to serve important signaling and immune functions including antigen presentation. In conclusion, MP analysis is poised to enter the mainstream of clinical testing, measuring specific antigens rather than gross levels. However, more research is needed to decisively establish their functions, and international standards are needed to allow comparing results from different laboratories.

Factor V Leiden: a disorder of factor V anticoagulant function

PURPOSE OF REVIEW

Activated protein C (APC) resistance, which is often associated with the factor V R506Q (FV Leiden) mutation, is a common risk factor for venous thrombosis. Study of the mechanism of APC resistance has revealed that coagulation FV stimulates the APC-catalysed inactivation of FVIIIa, and that this anticoagulant function of FV is impaired in FV Leiden. The present review covers the discovery, the physiological significance and the structural requirements of the APC-cofactor activity of FV.

RECENT FINDINGS

Recent in vitro and in vivo experiments indicate that the anticoagulant activity of FV is physiologically relevant and that FV plays a major role in the maintenance of the haemostatic balance. Quantitative and functional defects of the APC-cofactor activity of FV lead to increased thrombin generation and are associated with a prothrombotic state. Although the structural requirements for the expression of the APC-cofactor activity of FV are now beginning to be unravelled, the underlying molecular mechanism remains elusive.

SUMMARY

The APC-cofactor activity of FV and its impairment in FV Leiden can explain the different thrombosis risks associated with heterozygosity, homozygosity and pseudo-homozygosity for FV Leiden. Elucidation of the molecular mechanism of the anticoagulant function of factor V may provide novel targets for the design of antithrombotic drugs.

Structural Mechanism for Inactivation and Activation of CAD/DFF40 in the Apoptotic Pathway

CAD/DFF40 is responsible for the degradation of chromosomal DNA into nucleosomal fragments and subsequent chromatin condensation during apoptosis. It exists as an inactive complex with its inhibitor ICAD/DFF45 in proliferating cells but becomes activated upon cleavage of ICAD/DFF45 into three domains by caspases in dying cells. The molecular mechanism underlying the control and activation of CAD/DFF40 was unknown. Here, the crystal structure of activated CAD/DFF40 reveals that it is a pair of molecular scissors with a deep active-site crevice that appears ideal for distinguishing internucleosomal DNA from nucleosomal DNA. Ensuing studies show that ICAD/DFF45 sequesters the nonfunctional CAD/DFF40 monomer and is also able to disassemble the functional CAD/DFF40 dimer. This capacity requires the involvement of the middle domain of ICAD/DFF45, which by itself cannot remain bound to CAD/DFF40 due to low binding affinity for the enzyme. Thus, the consequence of the caspase-cleavage of ICAD/DFF45 is a self-assembly of CAD/DFF40 into the active dimer.

Coagulation, inflammation, and the risk of neonatal white matter damage

Indicators of coagulation activation are sometimes increased in the blood of newborns and adults who have a systemic inflammatory response. These coagulation factors have the ability to exacerbate inflammation, which in turn can promote coagulation. Therapies directed solely at coagulation factors and therapies directed solely at inflammation factors have not proved effective in reducing mortality in adults with a systemic inflammatory response syndrome and multi-organ dysfunction (SIRS/MOD). On the other hand, the only therapy that has reduced mortality in SIRS/MOD is activated protein C, which has both anti-coagulation and anti-inflammatory effects. This and other observations support the view that activated coagulation factors enhance inflammation. Since newborns at risk of cerebral white matter damage and cerebral palsy are more likely than their peers to have a systemic inflammatory response, which is sometimes accompanied by elevated blood levels of coagulation factors, we suggest that activated coagulation factors contribute to the occurrence of cerebral white matter damage by exacerbating inflammatory phenomena, rather than by occluding cerebral blood vessels.