Characterization of a functional thrombin receptor. Issues and opportunities

The Tissue Factor Pathway in Cancer: Overview and Role of Heparan Sulfate Proteoglycans

Historically, the only focus on tissue factor (TF) in clinical pathophysiology has been on its function as the initiation of the extrinsic coagulation cascade. This obsolete vessel-wall TF dogma is now being challenged by the findings that TF circulates throughout the body as a soluble form, a cell-associated protein, and a binding microparticle. Furthermore, it has been observed that TF is expressed by various cell types, including T-lymphocytes and platelets, and that certain pathological situations, such as chronic and acute inflammatory states, and cancer, may increase its expression and activity. Transmembrane G protein-coupled protease-activated receptors can be proteolytically cleaved by the TF:FVIIa complex that develops when TF binds to Factor VII (PARs). The TF:FVIIa complex can activate integrins, receptor tyrosine kinases (RTKs), and PARs in addition to PARs. Cancer cells use these signaling pathways to promote cell division, angiogenesis, metastasis, and the maintenance of cancer stem-like cells. Proteoglycans play a crucial role in the biochemical and mechanical properties of the cellular extracellular matrix, where they control cellular behavior via interacting with transmembrane receptors. For TFPI.fXa complexes, heparan sulfate proteoglycans (HSPGs) may serve as the primary receptor for uptake and degradation. The regulation of TF expression, TF signaling mechanisms, their pathogenic effects, and their therapeutic targeting in cancer are all covered in detail here.

Protease activated receptor 2 and matriptase expression in the joints of cats with and without osteoarthritis

OBJECTIVES

The aim of this study was to determine the presence of protease-activated receptor 2 (PAR2) and matriptase proteins and quantify PAR2 and matriptase mRNA expression in the articular cartilage and synovial membrane of cats with and without osteoarthritis (OA).

METHODS

A total of 28 articular cartilage samples from adult cats (14 OA and 14 normal), 10 synovial membranes from adult cats (five OA and five normal) and three cartilage samples from 9-week-old fetal cats were used. The presence of PAR2 and matriptase in the cartilage and synovial membrane of the adult samples was detected by immunohistochemical (IHC) staining, while real-time PCR was used for mRNA expression analyses in all samples.

RESULTS

PAR2 was detected in all OA and normal articular cartilage and synovial membrane samples but confined to only a few superficial chondrocytes in the normal samples. Matriptase was only detected in OA articular cartilage and synovial membrane samples. PAR2 and matriptase mRNA expression were, however, detected in all cartilage and synovial membrane samples. PAR2 and matriptase mRNA expression levels in OA articular cartilage were five (P <0.001) and 3.3 (P <0.001) times higher than that of the healthy group, respectively. There was no significant difference (P = 0.05) in the OA synovial membrane PAR2 and matriptase mRNA expression compared with the normal samples.

CONCLUSIONS AND RELEVANCE

Detection of PAR2 and matriptase proteins and gene expression in feline articular tissues is a novel and important finding, and supports the hypothesis that serine proteases are involved in the pathogenesis of feline OA. The consistent presence of PAR2 and matriptase protein in the cytoplasm of OA chondrocytes suggests a possible involvement of proteases in cartilage degradation. Further investigations into the PAR2 and matriptase pathobiology could enhance our understanding of the proteolytic cascades in feline OA, which might lead to the development of novel therapeutic strategies.

Blockage of protease-activated receptor 2 exacerbates inflammation in high-fat environment partly through autophagy inhibition

Protease-activated receptor 2 (PAR2) regulates inflammatory responses and lipid metabolism. However, its precise role in colitis remains unclear. In this study, we aimed to investigate the function of PAR2 in high-fat diet-fed mice with colitis and its potential role in autophagy. PAR2^+/+ and PAR2^-/- mice were fed a high-fat diet (HFD) for 7 days before colitis induction with dextran sodium sulfate. Deletion of PAR2 and an HFD significantly exacerbated colitis, as shown by increased mortality, body weight loss, diarrhea or bloody stools, colon length shortening, and mucosal damage. Proinflammatory cytokine levels were elevated in HFD-fed PAR2^-/- mice and in cells treated with the PAR2 antagonist GB83, palmitic acid (PA), and a cytokine cocktail (CC). Damaging effects of PAR2 blockage were associated with autophagy regulation by reducing the levels of YAP1, SIRT1, PGC-1α, Atg5, and LC3A/B-I/II. In addition, mitochondrial dysfunction was demonstrated only in cells treated with GB83, PA, and CC. Reduced cell viability and greater induction of apoptosis, as shown by increased levels of cleaved caspase-9, cleaved caspase-3, and cleaved poly(ADP-ribose) polymerase (PARP), were observed in cells treated with GB83, PA, and CC but not in those treated with only PA and CC. Collectively, protective effects of PAR2 were elucidated during inflammation accompanied by a high-fat environment by promoting autophagy and inhibiting apoptosis, suggesting PAR2 as a therapeutic target for inflammatory bowel disease co-occurring with metabolic syndrome.NEW & NOTEWORTHY Deletion of PAR2 with high-fat diet feeding exacerbates colitis in a murine colitis model. Proinflammatory effects of PAR2 blockage in a high-fat environment were associated with an altered balance between autophagy and apoptosis. Increased colonic levels of PAR2 represent as a therapeutic strategy for IBD co-occurring with metabolic syndrome.

Conserved aging-related signatures of senescence and inflammation in different tissues and species

Increasing evidence indicates that chronic inflammation and senescence are the cause of many severe age-related diseases, with both biological processes highly upregulated during aging. However, until now, it has remained unknown whether specific inflammation- or senescence-related genes exist that are common between different species or tissues. These potential markers of aging could help to identify possible targets for therapeutic interventions of aging-associated afflictions and might also deepen our understanding of the principal mechanisms of aging. With the objective of identifying such signatures of aging and tissue-specific aging markers, we analyzed a multitude of cross-sectional RNA-Seq data from four evolutionarily distinct species (human, mouse and two fish) and four different tissues (blood, brain, liver and skin). In at least three different species and three different tissues, we identified several genes that displayed similar expression patterns that might serve as potential aging markers. Additionally, we show that genes involved in aging-related processes tend to be tighter controlled in long-lived than in average-lived individuals. These observations hint at a general genetic level that affect an individual’s life span. Altogether, this descriptive study contributes to a better understanding of common aging signatures as well as tissue-specific aging patterns and supplies the basis for further investigative age-related studies.

Protease-activated receptor signaling in intestinal permeability regulation

Protease-activated receptors (PARs) are a unique class of G-protein-coupled transmembrane receptors, which revolutionized the perception of proteases from degradative enzymes to context-specific signaling factors. Although PARs are traditionally known to affect several vascular responses, recent investigations have started to pinpoint the functional role of PAR signaling in the gastrointestinal (GI) tract. This organ is exposed to the highest number of proteases, either from the gut lumen or from the mucosa. Luminal proteases include the host's digestive enzymes and the proteases released by the commensal microbiota, while mucosal proteases entail extravascular clotting factors and the enzymes released from resident and infiltrating immune cells. Active proteases and, in case of a disrupted gut barrier, even entire microorganisms are capable to translocate the intestinal epithelium, particularly under inflammatory conditions. Especially PAR-1 and PAR-2, expressed throughout the GI tract, impact gut permeability regulation, a major factor affecting intestinal physiology and metabolic inflammation. In addition, PARs are critically involved in the onset of inflammatory bowel diseases, irritable bowel syndrome, and tumor progression. Due to the number of proteases involved and the multiple cell types affected, selective regulation of intestinal PARs represents an interesting therapeutic strategy. The analysis of tissue/cell-specific knockout animal models will be of crucial importance to unravel the intrinsic complexity of this signaling network. Here, we provide an overview on the implication of PARs in intestinal permeability regulation under physiologic and disease conditions.

GPR56/ADGRG1 Activation Promotes Melanoma Cell Migration via NTF Dissociation and CTF-Mediated Gα12/13/RhoA Signaling

GPR56/ADGRG1 is a versatile adhesion G protein-coupled receptor with diverse biological functions. GPR56 expression is variably detected in human melanoma cell lines and correlates inversely with the metastatic potential of melanoma lesions. GPR56 associates with the tetraspanins CD9 and CD81 on the melanoma cell surface. GPR56 activation by immobilized CG4 monoclonal antibody facilitates N-terminal fragment dissociation in a CD9/CD81-dependent manner specifically inducing IL-6 production, which promotes cell migration and invasion. Interestingly, expression of GPR56-C-terminal fragment alone recapitulates the antibody-induced receptor function, implicating a major role for the C-terminal fragment in GPR56 activation and signaling. Analysis of site-directed mutant receptors attests the importance of the conserved N-terminal residues of the C-terminal fragment for its self-activation. Finally, we show that the GPR56-induced signaling in melanoma cells is mediated by the Gα_12/13/RhoA pathway. In summary, the expression and activation of GPR56 may modulate melanoma progression in part by inducing IL-6 production after N-terminal fragment dissociation and C-terminal fragment self-activation.

Inhibition of protease-activated receptor 4 impairs platelet procoagulant activity during thrombus formation in human blood

Essentials The platelet thrombin receptor, PAR4, is an emerging anti-thrombotic drug target. We examined the anti-platelet & anti-thrombotic effects of PAR4 inhibition in human blood. PAR4 inhibition impaired platelet procoagulant activity in isolated cells and during thrombosis. Our study shows PAR4 is required for platelet procoagulant function & thrombosis in human blood.

SUMMARY

Background Thrombin-induced platelet activation is important for arterial thrombosis. Thrombin activates human platelets predominantly via protease-activated receptor (PAR)1 and PAR4. PAR1 has higher affinity for thrombin, and the first PAR1 antagonist, vorapaxar, was recently approved for use as an antiplatelet agent. However, vorapaxar is contraindicated in a significant number of patients, owing to adverse bleeding events. Consequently, there is renewed interest in the role of platelet PAR4 in the setting of thrombus formation. Objectives To determine the specific antiplatelet effects of inhibiting PAR4 function during thrombus formation in human whole blood. Methods and Results We developed a rabbit polyclonal antibody against the thrombin cleavage site of PAR4, and showed it to be a highly specific inhibitor of PAR4-mediated platelet function. This function-blocking anti-PAR4 antibody was used to probe for PAR4-dependent platelet functions in human isolated platelets in the absence and presence of concomitant PAR1 inhibition. The anti-PAR4 antibody alone was sufficient to abolish the sustained elevation of cytosolic calcium level and consequent phosphatidylserine exposure induced by thrombin, but did not significantly inhibit integrin αII b β3 activation, α-granule secretion, or aggregation. In accord with these in vitro experiments on isolated platelets, selective inhibition of PAR4, but not of PAR1, impaired thrombin activity (fluorescence resonance energy transfer-based thrombin sensor) and fibrin formation (anti-fibrin antibody) in an ex vivo whole blood flow thrombosis assay. Conclusions These findings demonstrate that PAR4 is required for platelet procoagulant function during thrombus formation in human blood, and suggest PAR4 inhibition as a potential target for the prevention of arterial thrombosis.

Currently available biomaterials for use in cardiopulmonary bypass

Cardiopulmonary bypass (CPB) represents one of the most important technical innovations in healthcare history, yet the systemic responses to CPB remain a fundamentally unresolved problem. Study of the blood-biomaterial interaction and development of biocompatible materials is intimately related to efforts to optimize patient outcome following CPB. This article reviews the design innovations in biomaterial surfaces that have been introduced into clinical practice in an attempt to ameliorate the detrimental consequences of CPB, contrasting the actual clinical improvements and patient benefits achieved against those predicted on the basis of theory and in vitro testing. Some discussion of the underlying mechanisms of action as presently understood is provided and the current limitations of biomaterial-dependent strategies to improve outcome following CPB are addressed.

A DNA methylation network interaction measure, and detection of network oncomarkers

Epigenetic processes–including DNA methylation–are increasingly seen as having a fundamental role in chronic diseases like cancer. DNA methylation patterns offer a route to develop prognostic measures based directly on DNA measurements, rather than less-stable RNA measurements. A novel DNA methylation-based measure of the co-ordinated interactive behaviour of genes is developed, in a network context. It is shown that this measure reflects well the co-regulatory behaviour linked to gene expression (at the mRNA level) over the same network interactions. This measure, defined for pairs of genes in a single patient/sample, associates with overall survival outcome independent of known prognostic clinical features, in several independent data sets relating to different cancer types. In total, more than half a billion CpGs in over 1600 samples, taken from nine different cancer entities, are analysed. It is found that groups of gene-pair interactions which associate significantly with survival identify statistically significant subnetwork modules. Many of these subnetwork modules are shown to be biologically relevant by strong correlation with pre-defined gene sets, such as immune function, wound healing, mitochondrial function and MAP-kinase signalling. In particular, the wound healing module corresponds to an increase in co-ordinated interactive behaviour between genes for worse prognosis, and the immune module corresponds to a decrease in co-ordinated interactive behaviour between genes for worse prognosis. This measure has great potential for defining DNA-based cancer biomarkers. Such biomarkers could naturally be developed further, by drawing on the rapidly expanding knowledge base of network science.

Synthesis and biological evaluation of direct thrombin inhibitors bearing 4-(piperidin-1-yl)pyridine at the P1 position with potent anticoagulant activity

The design and synthesis of a new class of nonpeptide direct thrombin inhibitors, built on the structure of 1-(pyridin-4-yl)piperidine-4-carboxamide, are described. Starting from a strongly basic 1-amidinopiperidine derivative (6) showing poor thrombin (fIIa) and factor Xa (fXa) inhibition activities, anti-fIIa activity and artificial membrane permeability were considerably improved by optimizing the basic P1 and the X-substituted phenyl P4 binding moieties. Structure-activity relationship studies, usefully complemented with molecular modeling results, led us to identify compound 13b, which showed excellent fIIa inhibition (Ki = 6 nM), weak anti-Xa activity (Ki = 5.64 μM), and remarkable selectivity over other serine proteases (e.g., trypsin). Compound 13b showed in vitro anticoagulant activity in the low micromolar range and significant membrane permeability. In mice (ex vivo), 13b demonstrated anticoagulant effects at 2 h after oral dosing (100 mg·kg(-1)), with a significant 43% prolongation of the activated partial thromboplastin time (aPTT), over controls (P < 0.05).

Oral antithrombotic effects of acylhydrazone derivatives

AIM

In the search for new antithrombotic drug candidates, the synthesis and anti-platelet activity of a new series of N-acylhydrazones that were designed as thrombin inhibitors has been previously described. The aim of this work was to further characterize the effects of these compounds on thrombin-induced platelet aggregation and induced thrombosis in vivo.

METHODS

In this work, four compounds were tested, LASSBio-693, 694, 743 and 752, on platelet aggregation induced by thrombin, ADP and TRAP-4A. These compounds were further tested using a mouse pulmonary thromboembolism model induced by collagen (500 µg/kg) and norepinephrine (80 µg/kg) or thrombin (2,000 UI), and a deep venous thrombosis model.

RESULTS

At 200 µM, the compounds showed between 36% and 82% inhibition (for L-743 and L-752, respectively) of thrombin-induced platelet aggregation. The receptor agonist of PAR-4, TRAP-4A (250 µM), was used and inhibition between 43% and 77% was observed for each compound (200 µM).Compounds LASSBio-752 and 743 were the most effective in the venous thrombosis model, increasing the survival of the treated animals to 63% and 46%, respectively, in the model of collagen-induced thromboembolism and increasing to 80% (both) in the thrombin-induced model. LASSBio 743 was more effective for deep vein thrombosis, reducing the weight of the thrombus by approximately 70%.

CONCLUSION

All compounds were administered orally and have shown effective antithrombotic action independently of the thrombotic stimulus. These results indicate that compounds LASSBio-743 and 752 are potential candidates for the treatment of cardiovascular diseases.

Non-conventional hemostatic risk factors for coronary heart disease in individuals with spinal cord injury

STUDY DESIGN

Review.

OBJECTIVES

In subjects with spinal cord injury (SCI), there is strong evidence for platelet hyperactivity, which may stimulate atherosclerosis and coronary heart disease (CHD). The literature was reviewed.

BACKGROUND

Individuals with SCI develop premature CHD. In addition to the conventional risk factors associated with CHD, there are pathologic hematological factors involved in atherogenesis that are similar to those that have been demonstrated in individuals with diabetes, and these hematological factors might affect individuals with SCI. One such hematological factor, platelet aggregation, is essential for the development of CHD, which results from thrombus formation in the coronary vasculature. Prostacyclin (PGI(2)) is a potent inhibitor of platelet aggregation and is thought to have a beneficial role in inhibiting atherogenesis; therefore, it is possible that individuals with SCI have impaired PGI(2) receptor function.

METHODS

We reviewed the literature by conducting a search using PubMed (1970-2007).

RESULTS

Acute thrombosis is emerging as an important factor in the etiology of CHD and therefore could mediate the risk of CHD in persons with SCI, in addition to previously known risk factors such as hyperlipidemia, hypertension, hyperlipidemia, diabetes mellitus and hyperinsulinemia. Because PGI(2) may retard atherogenesis through its inhibitory effects on platelet function, we discuss the effects of PGI(2) on platelets in persons with SCI in this review.

CONCLUSIONS

Subjects with chronic SCI develop abnormal platelet function, resulting in the production of atherogenic and thrombogenic factors for the following reasons: (1) the PGI(2) and insulin receptors on their platelets are impaired; (2) thrombin generation and platelet-derived growth factor release are elevated; (3) insulin-induced nitric oxide production by platelets is markedly impaired; and (4) a circulating antibody (immunoglobulin G (IgG)) blocks the antithrombotic effect of both insulin and PGI(2) receptors. Thus, this IgG molecule is thought to be one of the pathological mediators of the increased incidence of CHD in individuals with SCI.

Protease-activated receptor 2 mediates interleukin-8 and intercellular adhesion molecule-1 expression in response to Aggregatibacter actinomycetemcomitans

INTRODUCTION

We investigated the mechanisms by which extracts of Aggregatibacter actinomycetemcomitans affect the inflammatory response in gingival epithelial cells.

METHODS

Human gingival cells (Ca9-22) were cultured in bacterial extracts prepared from A. actinomycetemcomitans ATCC 29522. The cells were pretreated with protease inhibitors or transfected with small interfering RNA (siRNA) specific for protease-activated receptor 2 (PAR-2).

RESULTS

The pretreatment of cells with serine protease inhibitors significantly inhibited A. actinomycetemcomitans extract-induced expression of interleukin-8 (IL-8) and intercellular adhesion molecule-1 (ICAM-1) at both the messenger RNA and protein levels. In addition, A. actinomycetemcomitans extract-induced IL-8 and ICAM-1 expression was significantly decreased in PAR-2/siRNA-transfected cells.

CONCLUSIONS

A. actinomycetemcomitans extract-induced IL-8 and ICAM-1 expression in gingival epithelial cells is mediated by PAR-2.

Pediatric critical care management of septic shock prior to acute kidney injury and renal replacement therapy

A high index of suspicion for bacterial sepsis and recognition of the potential for rapid deterioration is essential for impacting patient outcome. Meningococcemia produces a stereotypical clinical and biochemical constellation of profound septic shock and purpura fulminans with marked inflammatory disturbance and a complex disruption of coagulation. Meningococcal infections preferentially affect infants and young children, but adolescents are also at risk. Aggressive fluid resuscitation, hemodynamic management, and clinical monitoring are based on understanding of pathophysiologic disturbances typical of the pediatric cardiovascular response and guided by evidence-based guidelines. Appropriate antibiotic choice is important, and corticosteroid use may be beneficial. A variety of efforts to manipulate the coagulation abnormalities may be considered, although evidence is lacking. Extracorporeal support remains a consideration both for the failing cardiorespiratory systems but also potentially for the use of plasma exchange. A team approach between the intensivist and subspecialist is important in managing the frequent multiorgan complications seen with meningococcemia.

Effect of thrombin peptide 508 (TP508) on bone healing during distraction osteogenesis in rabbit tibia

Thrombin-related peptide 508 (TP508) accelerates bone regeneration during distraction osteogenesis (DO). We have examined the effect of TP508 on bone regeneration during DO by immunolocalization of Runx2 protein, a marker of osteoblast differentiation, and of osteopontin (OPN) and bone sialoprotein (BSP), two late markers of the osteoblast lineage. Distraction was performed in tibiae of rabbits over a period of 6 days. TP508 (30 or 300 μg) or vehicle was injected into the distraction gap at the beginning and end of the distraction period. Two weeks after active distraction, tissue samples were harvested and processed for immunohistochemical analysis. We also tested the in vitro effect of TP508 on Runx2 mRNA expression in osteoblast-like (MC3T3-E1) cells by polymerase chain reaction analysis. Runx2 and OPN protein were observed in preosteoblasts, osteoblasts, osteocytes of newly formed bone, blood vessel cells and many fibroblast-like cells of the soft connective tissue. Immunostaining for BSP was more restricted to osteoblasts and osteocytes. Significantly more Runx2- and OPN-expressing cells were seen in the group treated with 300 μg TP508 than in the control group injected with saline or with 30 μg TP508. However, TP508 failed to increase Runx2 mRNA levels significantly in MC3T3-E1 cells after 2–3 days of exposure. Our data suggest that TP508 enhances bone regeneration during DO by increasing the proportion of cells of the osteoblastic lineage. Clinically, TP508 may shorten the healing time during DO; this might be of benefit when bone regeneration is slow.

Is There Still a Role for Aprotinin in Cardiac Surgery?

Cardiac surgery is associated with a systemic inflammatory response and systemic coagulopathy, which can result in significant organ dysfunction and bleeding. Aprotinin, a serine protease inhibitor, can limit systemic inflammation, and has been associated with myocardial, pulmonary and cerebral protection in addition to its proven haemostatic efficacy. Data are currently conflicting regarding the haemostatic efficacy of aprotinin relative to alternative agents including tranexamic acid. Recent studies have demonstrated aprotinin usage is associated with increased rates of thrombotic and renal complications, but these findings are at odds with the majority of studies relating to aprotinin safety to date. The lack of adequately powered, randomised studies evaluating aprotinin and alternative agents limits drawing conclusions about the complete use or disuse of aprotinin presently and requires individualised patient selection based on bleeding risk and co-morbidities for its usage.

Aprotinin and the protease-activated receptor 1 thrombin receptor: antithrombosis, inflammation, and stroke reduction

Cardiopulmonary bypass, although remaining an indispensable asset in cardiac surgery, especially in more complex and repeat operations, is associated with significant thrombin generation in the bypass circuit, leading to the activation of platelets, the coagulation system, an inflammatory response, and perioperative stroke. Recent clinical studies and meta-analyses of clinical trials in coronary artery bypass grafting surgery have confirmed that aprotinin not only reduces transfusion requirements in cardiac surgery but also confers significant protection against platelet dysfunction, activation of the systemic inflammatory response, and perioperative stroke when administered at the full (or "Hammersmith") dose. This article reviews research from several independent groups to propose a novel mechanism through which the antithrombotic, anti-inflammatory, and neuroprotective mechanism might be mediated, via protection of the high-affinity thrombin receptor protease-activated receptor 1 (PAR1).

Upregulation of proteinase-activated receptors and hypercontractile responses precede development of arterial lesions after balloon injury

Thrombin and other proteinases exert vascular effects by activating the proteinase-activated receptors (PARs). The expression of PARs has been shown to be upregulated after balloon injury and in human arteriosclerosis. However, the relationship between the receptor upregulation and the alteration of vasomotor function remains to be elucidated. We herein demonstrated that the contractile responses to the PAR-1 and PAR-2 agonist were markedly enhanced in the rabbit femoral arteries after balloon injury. Neointimal thickening was established 4 wk after the injury. No histological change was observed in the sham operation, where the saphenous artery was ligated without any balloon injury. The contractile response to K+depolarization was significantly attenuated 1 wk after the injury and then partly recovered after 4 wk. Thrombin, PAR-1-activating peptide, trypsin, and PAR-2-activating peptide induced no significant contraction in the control. All these stimulants induced enhanced responses 1 wk after balloon injury. Such enhanced responses were seen 4 wk after the injury, except for thrombin. There was no change in the Ca2+sensitivity of the contractile apparatus as evaluated in the permeabilized preparations. PAR-1-activating peptide (100 μmol/l), but no other stimulants, induced an enhanced contraction in the sham operation. The expression of PAR-1 and PAR-2 slightly increased after the sham operation, whereas it markedly and significantly increased after balloon injury. Our observations suggest that balloon injury induced the receptor upregulation, thereby enhancing the contractile response before the establishment of vascular lesions. The local inflammation associated with the sham operation may also contribute to the receptor upregulation.

Protease-activated receptor-2 activation: a major role in the pathogenesis of Porphyromonas gingivalis infection

We have investigated the specific contribution of protease-activated receptor-2 (PAR(2)) to host defense during Porphyromonas gingivalis infection. Culture supernatants from P. gingivalis strains 33277 and W50 provoked Ca(2+) mobilization in cells transfected with PAR(2) (PAR(2)-KNRK) and desensitized the subsequent responses to PAR(2)-selective agonist. In addition, culture supernatants of P. gingivalis E8 (RgpA/RgpB double knockout) did not cause calcium response in PAR(2)-KNRK cells, evidencing the involvement of the arginine-specific cysteine proteases RgpA and RgpB in PAR(2) activation by P. gingivalis. Injection of P. gingivalis into mouse subcutaneous chambers provoked an increased proteolytic activity, which was inhibited by serine protease inhibitors. Fluids collected from chambers of P. gingivalis-injected mice were able to activate PAR(2) and this activation was inhibited by serine protease inhibitors. P. gingivalis inoculation into subcutaneous chambers of wild-type mice induced an inflammatory response that was inhibited by a serine protease inhibitor and was significantly reduced in PAR(2)-deficient mice. Finally, mice orally challenged with P. gingivalis developed alveolar bone loss, which was significantly reduced in PAR(2)-deficient mice at 42 and 60 days after P. gingivalis infection. We conclude that PAR(2) is activated on P. gingivalis infection, in which it plays an important role in the host inflammatory response.

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.

Positive cooperativity between the thrombin and bradykinin B2 receptors enhances arachidonic acid release

Bradykinin (BK) or kallikreins activate B2 receptors (R) that couple Galpha(i) and Galpha(q) proteins to release arachidonic acid (AA) and elevate intracellular Ca2+ concentration ([Ca2+]i). Thrombin cleaves the protease-activated-receptor-1 (PAR1) that couples Galpha(i), Galpha(q), and Galpha(12/13) proteins. In Chinese hamster ovary cells stably transfected with human B2R, thrombin liberated little AA, but it significantly potentiated AA release by B2R agonists. We explored mechanisms of cooperativity between constitutively expressed PAR1 and B2R. We also examined human endothelial cells expressing both Rs constitutively. The PAR1 agonist hexapeptide (TRAP) was as effective as thrombin. Inhibitors of components of Galpha(i), Galpha(q), and Galpha(12/13) signaling pathways, and a protein kinase C (PKC)-alpha inhibitor, Gö-6976, blocked potentiation, while phorbol, an activator, enhanced it. Several inhibitors, including a RhoA kinase inhibitor, a [Ca2+]i antagonist, and an inositol-(1,3,4)-trisphosphate R antagonist, reduced mobilization of [Ca2+]i by thrombin and blocked potentiation of AA release by B2R agonists. Because either a nonselective inhibitor (isotetrandrine) of phospholipase A2 (PLA2) or a Ca2+-dependent PLA2 inhibitor abolished potentiation of AA release by thrombin, while a Ca2+-independent PLA2 inhibitor did not, we concluded that the mechanism involves Ca2+-dependent PLA2 activation. Both thrombin and TRAP modified activation and phosphorylation of the B2R induced by BK. In lower concentrations they enhanced it, while higher concentrations inhibited phosphorylation and diminished B2R activation. Protection of the NH2-terminal Ser1-Phe2 bond of TRAP by an aminopeptidase inhibitor made this peptide much more active than the unprotected agonist. Thus PAR1 activation enhances AA release by B2R agonists through signal transduction pathway.

Design and Synthesis of Novel Biologically Active Thrombin Receptor Non-Peptide Mimetics Based on the Pharmacophoric Cluster Phe/Arg/NH2 of the Ser42-Phe-Leu-Leu-Arg46 Motif Sequence: Platelet Aggregation and Relaxant Activities

The identification of the thrombin receptor has promoted the interest for the development of new therapeutic agents capable of selectively inhibiting unwanted biological effects of thrombin on various cell types. In this study we have designed and synthesized two series of new thrombin receptor antagonists based on the thrombin receptor motif sequence S42FLLR46, one possessing two (Phe/Arg) pharmacophoric groups and the other possessing three (Phe/Arg/NH2). N-(6-Guanidohexanoyl)-N'-(phenylacetyl)piperazine (1), N-(phenylacetyl)-4-(6-guanidohexanoylamidomethyl)piperidine (2), and N-(phenylacetyl)-3-(6-guanidohexanoylamido)pyrrolidine (3) (group A) carry the two pharmacophoric side chains of Phe and Arg residues incorporated on three different templates (piperazine, 4-aminomethylpiperidine, and 3-aminopyrrolidine). Compounds with three pharmacophoric groups (group B) were built similarly to group A using the same templates with the addition of an extra methylamino group leading to (S)-N-(6-guanidohexanoyl)-N'-(2-amino-3-phenylpropionyl)piperazine (4), (S)-N-(2-amino-3-phenylpropionyl)-4-(6-guanidohexanoylamidomethyl)piperidine (5), and (S)-N-(2-amino-3-phenylpropionyl)-3-(6-guanidohexanoylamido)pyrrolidine (6). Compounds were able to inhibit thrombin-induced human platelet activation even at low concentrations. In particular, among compounds in group A, compound 3 was found to be the most powerful thrombin receptor activation inhibitor, showing an IC50 of approximately 0.11 mM on platelet aggregation assay. Among compounds in group B, compound 4 was the most powerful to inhibit thrombin-induced platelet aggregation, showing an IC50 of approximately 0.09 mM. All compounds were also found to act as agonists in the rat aorta relaxation assay. Interestingly, the order of potency of these compounds as agonists of the endothelial thrombin receptor was the inverse of the order of potency of the same compounds as antagonists of the platelet thrombin receptor. Such compounds that are causing vasodilation while simultaneously inhibiting platelet aggregation would be very useful in preventing the installation of atherosclerotic lesions and deserve further investigation as potential drugs for treating cardiovascular diseases. The above findings coupled with computational analysis molecular dynamics experiments support also our hypothesis that a cluster of phenyl, guanidino, and amino groups is responsible for thrombin receptor triggering and activation.

Localization of heparin cofactor II in injured human skin: a potential role in wound healing

The physiologic function of the serpin heparin cofactor II (HCII) is not fully understood. We have hypothesized that HCII functions as an extravascular inhibitor of thrombin. Thrombin formed at a site of injury has been hypothesized to contribute to migration and proliferation of fibroblasts and smooth muscle cells involved in wound healing. To begin to test our hypothesis, we examined the immunohistochemical localization of HCII in human skin and compared it to that of the closely related serpin, antithrombin (ATIII). In skin specimens with acute wounds, there was diffuse HCII and ATIII staining in areas of hemorrhage. In healing skin wounds ATIII was primarily associated with mast cells, while HCII was associated with mononuclear phagocytes in the dermis. Blood monocytes isolated from healthy donors also stained for HCII protein. However, in situ hydridization and RT-PCR studies failed to show significant HCII mRNA expression either in macrophages in wounded skin or in peripheral blood leukocytes. HCII localization is not due to nonspecific uptake of plasma proteins, since ATIII had a very different distribution in wounded skin. These findings support the notion that HCII could function as an extravascular thrombin inhibitor and might play a role in the regulation of wound healing.

A critical review of the methods for cleavage of fusion proteins with thrombin and factor Xa

Expression and purification of proteins in recombinant DNA systems is a powerful and widely used technique. Frequently there is the need to express the protein of interest as a fusion protein or chimeric protein. Fusion protein technology is frequently used to attach a "signal" which can be used for subsequent localization of the protein or a "carrier" which can be used to deliver a "therapeutic" such as a radioactive molecule to a specific site. In addition to these applications, fusion protein technology can be employed for several other useful purposes. Of these, the most frequent reason is to provide a 'tag' or 'handle' which will aid in the purification of the protein. Another useful purpose is to improve the expression or folding of the protein of interest. In these latter two situations, it is often necessary to remove the fusion partner before the recombinant protein of interest can be used for further studies. This removal process involves the insertion of a unique amino acid sequence that is susceptible to cleavage by a highly specific protease. Thrombin and factor Xa are the most frequently used proteases for this application. The purpose of this review is to discuss the application of thrombin and factor Xa for the cleavage of fusion proteins. It is emphasized that while these enzymes are quite specific for cleavage at the inserted cleavage site, proteolysis can frequently occur at other site(s) in the protein of interest. It is necessary to characterize the protein of interest after cleavage from the affinity label to assure that there are no changes in the covalent structure of the protein of interest. Examples are presented which describe the proteolysis of the protein of interest by either factor Xa or thrombin.

Expression and function of the endothelial protein C receptor in human neutrophils

Activation of protein C by thrombin bound to thrombomodulin is enhanced by endothelial protein C receptor. This pathway may inhibit inflammation. We investigated effects of protein C and activated protein C on neutrophils as well as whether an endothelial protein C receptor is involved in mediating protein C effects. Neutrophils were from venous blood of healthy donors. Cell migration, respiratory burst, phagocytic activity, and apoptosis were studied by micropore filter assays and fluorometry. Receptor expression was investigated by reverse transcriptase-polymerase chain reaction (PCR) for mRNA, sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and autoradiography of immunoprecipitated receptor protein, and fluorescence-activated cell-sorter scanner (FACS) analysis using the anti-endothelial protein C receptor antibody RCR-252. Neither protein C nor activated protein C induced migration, yet both of them inhibited neutrophil chemotaxis triggered by interleukin-8, formyl-Met-Leu-Phe, antithrombin, or C5a. A protein C activation-blocking antibody against endothelial protein C receptor diminished inhibitory effects of protein C or activated protein C on migration. No effect of either protein C preparation was seen in neutrophil's respiratory burst, bacterial phagocytosis, or apoptosis assays. Endothelial protein C receptor immunoreactivity was confirmed on neutrophils by FACS. De novo synthesis is suggested by endothelial protein C receptor mRNA expression as demonstrated by reverse transcriptase PCR and immunoprecipitation SDS-PAGE analyses. Data suggest that an endothelial protein C receptor is expressed by human neutrophils whose active site ligation with either protein C or activated protein C arrests directed cell migration. Inhibitory effects of these components of the protein C pathway on neutrophil function may play a role in the protein C-based treatment of severe sepsis.

Endothelial protein C receptor-dependent inhibition of human eosinophil chemotaxis by protein C

BACKGROUND

Eosinophil infiltration is a characteristic feature of allergic inflammation. Allergic responses are associated with local activation of the coagulation pathway and accumulation of fibrin.

OBJECTIVE

We tested whether protein C and activated protein C (APC), which are endogenous anti-inflammatory coagulation inhibitors, affect eosinophil function.

METHODS

Eosinophils were from venous blood of healthy donors. Cell migration and apoptosis were studied by using micropore filter assays and fluorometry, respectively. Receptor expression was investigated by means of RT-PCR and SDS-PAGE of immunoprecipitated protein.

RESULTS

Protein C and APC had no significant chemotactic effects on eosinophils. Eosinophils pretreated with protein C or APC showed significantly reduced migration toward chemoattractants. No effect of either protein C preparation was seen in eosinophil apoptosis assays. The inhibiting effect on migration was reversed by an antibody against the endothelial protein C receptor (EPCR). Synthesis of EPCR by eosinophils is suggested by demonstration of receptor mRNA expression and detection of metabolically labeled receptor protein.

CONCLUSIONS

Data suggest that an EPCR is expressed by eosinophils whose activation with protein C or APC arrests directed migration. Protein C-affected eosinophil chemotaxis is a novel thrombin-independent component of the protein C pathway.

Inhibition of arterial thrombosis by a protease-activated receptor 1 antagonist, FR171113, in the guinea pig

The antiplatelet and antithrombotic effects of FR171113, 3-(4-chlorophenyl)-2-(2,4-dichlorobenzoylimino)-5-(methoxycarbonyl methylene)-1,3-thiazolidin-4-one, a non-peptide protease-activated receptor 1 (PAR1) antagonist, were evaluated in guinea pigs. FR171113 inhibited Ser-Phe-Leu-Leu-Arg-Asn-NH2 (a synthetic PAR1 agonist peptide)-induced and thrombin-induced aggregation of guinea pig platelets in a concentration-dependent manner in vitro (IC50=1.5 and 0.35 microM, respectively). Subcutaneous administration of FR171113 (0.1-3.2 mg/kg) produced a dose-dependent inhibition of platelet aggregation ex vivo. The ED50 value of FR171113 for platelet aggregation was 0.49 mg/kg s.c. However, FR171113 did not have an inhibitory effect on ADP- or collagen-induced platelet aggregation in vitro and ex vivo. One hour after FR171113 treatment at 1.0 mg/kg s.c., significant inhibition of arterial thrombosis without a prolongation of thrombin time or coagulation time was seen in the FeCl3-induced carotid artery thrombosis model in guinea pigs. Furthermore, FR171113 did not prolong bleeding time even at 32 mg/kg s.c., which is a much higher dose than that required in the thrombosis model. These observations indicate that FR171113 has desirable antiplatelet effects both in vitro and in vivo and that its in vivo antithrombotic activity is efficacious without causing a prolongation of bleeding time.

What is all that thrombin for?

The hemostatic process initiated by the exposure of tissue factor to blood is a threshold limited reaction which occurs in two distinct phases. During an initiation phase, small amounts of factor (F)Xa, FIXa and thrombin are generated. The latter activates the procofactors FV and FVIII to the activated cofactors which together with their companion serine proteases form the intrinsic FX activator (FVIIIa-FIXa) and prothrombinase (FVa-FXa) which generate the bulk of FXa and thrombin during a propagation phase. The clotting process (fibrin formation) occurs at the inception of the propagation phase when only 5-10 nM thrombin has been produced. Consequently, the vast majority (greater than 95%) of thrombin is produced after clotting during the propagation phase of thrombin generation. The blood of individuals with either hemophilia A or hemophilia B has no ability to generate the intrinsic FXase, and hence is unable to support the propagation phase of the reaction. Since clot based assays conclude before the propagation phase they are not sensitive to hemophilia A and B. The inception and magnitude of the propagation phase of thrombin generation is influenced by genetic polymorphisms associated with thrombotic and hemorrhagic disease, by the natural abundance of pro- and anticoagulants in healthy individuals and by pharmacologic interventions which influence thrombotic pathology. Therefore, it is our suspicion that the performance of the entire process of thrombin generation from initiation through propagation and termination phases of the reaction are relevant with respect to both hemorrhagic and thrombotic pathology.

Proteinase-activated receptor-1 (PAR-1) activation contracts the isolated human renal artery in vitro

1. The in vitro motor function of protease-activated recepter-1 (PAR-1), PAR-2 and PAR-4 and the presence by immunohistochemistry of PAR-1 in the human renal artery have been investigated. 2. Thrombin and the human PAR-1 (SFLLRN-NH(2)) activating peptide, but not the PAR-1 reverse peptide (NRLLFS-NH(2)), contracted both endothelial-intact and endothelial-denuded human renal artery strips, whereas no relaxation was observed either in strips non-precontracted or precontracted with phenylephrine. Maximum contraction by thrombin or SFLLRN-NH(2) was about 60% of phenylephrine. However, thrombin was approximately 1000-fold more potent than SFLLRN-NH(2). 3. PAR-1 desensitisation, using repeated applications of SFLLRN-NH(2), almost completely blocked the response to thrombin. The contractile effect produced by thrombin and SFLLRN-NH(2) was not affected by nitric oxide synthase inhibition, but was significantly reduced by cyclooxygenase blockade. 4. Trypsin, the PAR-2 (SLIGKV-NH(2) and SLIGRL-NH(2)) and PAR-4 (GYPGQV-NH(2) and AYPGKF-NH(2)) activating peptides did not produce any significant contraction or relaxation. 5. In agreement with the motor function data immunohistochemistry showed specific staining patterns for PAR-1 in the human renal artery. 6. Combined, these studies would suggest a possible role for PAR-1 in renal vascular homeostasis.

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

OBJECTIVE

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

DATA SOURCES

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

DATA EXTRACTION AND SYNTHESIS

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

CONCLUSIONS

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

Potent, low molecular weight thrombin receptor antagonists

Several benzimidazole derivatives have been identified as potent thrombin receptor (PAR-1) antagonists as represented by compound 1h, which showed an IC(50) of 33 nM.

Prognostic value of plasminogen activator inhibitor type 1 mRNA in microdissected glomeruli from transplanted kidneys

BACKGROUND

Plasminogen activator inhibitor type 1 (PAI-1) exerts antifibrinolytic and profibrotic activities. Inside the glomerulus, PAI-1 is mainly synthesized by mesangial cells. We hypothesized that thrombin, via its receptor protease activated receptor type 1 (PAR-1), present on the membrane of glomerular cells, is an important mediator of PAI-1 synthesis.

METHODS

Using the technique of Peten et al., we microdissected the glomeruli of 23 kidney transplanted patients admitted in our department from 1993 to 1997, and we followed-up these patients for up to 5 years, with sometimes iterative renal biopsies. With this technique, we also microdissected the glomeruli of three patients who have had a nephrectomy for cancer (control patients). We investigated mRNA expression of the PAI-1, the thrombin receptor PAR-1, the alpha2 chain of type IV (alpha2 IV) collagen, and of a housekeeping gene (cyclophilin) by reverse transcription-polymerase chain reaction. The results were correlated with the renal function and the histological findings classified into acute rejection (9 biopsies), chronic rejection (22 biopsies), or normal (8 biopsies).

RESULTS

A significant up-regulation of PAI-1 and alpha2 IV collagen mRNA was observed in acute rejection (P<0.05) when compared to normal kidneys. A positive correlation exists between alpha2 IV collagen mRNA level and the degree of cellular infiltration. A negative correlation was found between the level of mRNA of PAR-1 and the degree of vascular thrombosis (P=0.005) and glomerulosclerosis (P=0.04). A positive correlation was found between the degradation of renal function and the mRNA level of PAI-1 at the time of the renal biopsy (P<0.05).

CONCLUSIONS

These results suggest that glomerular PAI-1 mRNA may be predictive of the long-term renal graft function.

Statistical molecular design, parallel synthesis, and biological evaluation of a library of thrombin inhibitors

A library of thrombin inhibitors has been designed using statistical molecular design. An aromatic scaffold was used, with three varied positions corresponding to three pockets at the active site of thrombin (the S-, P-, and D-pockets). The selection was performed in the building block space, and previously acquired data were included in the design procedure. The design resulted in six, four, and six building blocks for the first (S), second (P), and third (D) pockets, respectively. A second round of selection applied to the combined selected building blocks resulted in a subset of 18 compounds. The selected library was synthesized in parallel and biologically evaluated. The compounds were analyzed with respect to their inhibition (pIC(50)) of thrombin; membrane permeability, estimated by migration behavior in micellar media (CE log k') and pK(a); and specificity with respect to inhibition (K(i)) of trypsin. Multivariate QSAR studies of the responses yielded valuable results and information that could only be found using statistical molecular design in combination with multivariate analysis.

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

OBJECTIVE

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

DATA SOURCES/STUDY SELECTION

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

DATA EXTRACTION AND SYNTHESIS

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

CONCLUSIONS

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

Thrombin regulates insulin-like growth factor-1 receptor transcription in vascular smooth muscle: characterization of the signaling pathway

We have previously demonstrated that thrombin upregulation of insulin-like growth factor-1 receptor (IGF-1R) is essential for thrombin-induced mitogenic signaling. To characterize the mechanisms involved, we studied transcription of the IGF-1R gene in rat aortic smooth muscle cells. Thrombin markedly increased IGF-1R mRNA levels, peaking at 3 hours (112+/-7% above control). This effect was mimicked by the hexapeptide SFFLRN (that functions as a tethered ligand) and was blocked by the thrombin inhibitor hirudin. Nuclear run-on assays indicated that thrombin stimulated IGF-1R gene transcription by 2.1-fold, and this was confirmed with the use of actinomycin D. Thrombin-mediated upregulation of IGF-1R mRNA and protein levels was protein kinase C independent but was completely inhibited by the protein tyrosine kinase inhibitor genistein and by the antioxidants N-acetyl-L-cysteine and pyrrolidinedithiocarbamate, suggesting the involvement of reactive oxygen species. The thrombin-induced increase in IGF-1R mRNA was inhibitable by diphenyleneiodonium chloride but not by other inhibitors of cellular oxidase systems, suggesting that NAD(P)H oxidase was necessary for the increase. Furthermore, inhibitors of the epidermal growth factor receptor kinase, Janus kinase-2 kinase, and Src kinase did not block the effect. Thus, thrombin transcriptionally regulates the IGF-1R gene via a redox-sensitive protein tyrosine kinase-dependent pathway that does not require protein kinase C activation. In view of our prior data indicating that IGF-1R density is a critical determinant of vascular smooth muscle cell growth, our findings have particular relevance to understanding mechanisms whereby growth factors such as thrombin regulate vascular proliferation in vivo.

Novel platelet inhibitors

Platelet-inhibitory drugs are of proven benefit to individuals who suffer from atherosclerotic cardiovascular disease. Despite substantial effort to identify more potent platelet-inhibitory agents, aspirin, an irreversible inhibitor of platelet cyclooxygenase activity, remains the standard against which other drugs are judged. Drugs that appear to be at least as efficacious as aspirin in specific clinical settings include the thienopyridines ticlopidine and clopidogrel, specific inhibitors of ADP-stimulated platelet function, and the phosphodiesterase 3 inhibitor cilostazol. Ligand binding to the platelet integrin alphaIIbbeta3 (GPIIb-IIIa), a prerequisite for platelet thrombus formation, has been a prominent target for drug development. Currently, three types of alphaIIbbeta3 antagonists are available: the monoclonal antibody Fab fragment abciximab, cyclic peptides based on the Arg-Gly-Asp (RGD) or related amino acid motifs, and RGD-based peptidomimetics. The efficacy of each type of alphaIIbbeta3 antagonist in the setting of acute coronary artery disease has been confirmed in multicenter clinical trials.

A peptide analogue of thrombin receptor-activating peptide inhibits thrombin and thrombin-receptor-activating peptide-induced vascular smooth muscle cell proliferation

The serine protease thrombin, in addition to its pivotal role in the coagulation cascade, plays an important role in the development of atherosclerosis and restenosis by inducing smooth cell proliferation. Thrombin exerts its cellular effects mainly by cleaving its own receptor, leaving a new NH2-terminus that can act as a tethered ligand to activate the thrombin receptor. Peptides derived from the new NH2-terminus are able to fully activate thrombin receptor and mimic cellular effects of thrombin. Peptides with structural similarities to the tethered ligand have been tested for their ability to prevent thrombin- and tethered ligand-induced platelet aggregation and thrombus formation. We synthesized a peptide with multiple alanine substitutions in both critical and noncritical residues of tethered ligand that specifically inhibited platelet aggregation induced by thrombin and thrombin receptor-activating peptide and prevented thrombus formation in a rabbit thrombosis model. In the present study we demonstrate that this peptide inhibited only thrombin- and tethered ligand-induced human vascular smooth muscle cell proliferation as determined by (3H)-thymidine incorporation and has no effect on platelet-derived growth factor and serum-induced smooth muscle cell proliferation. The inhibitory effect of this peptide is dependent on the concentration of the antagonist used and length of preincubation time. The possible mechanism by which this peptide exerts its inhibitory effect may by desensitizing the thrombin receptor. The results of the present study suggest that apart from being antithrombotic, tethered ligand antagonist peptides can also act as antiatherosclerotic or antirestenotic agents.

Thrombin inhibition by HCII in the presence of elastase-cleaved HCII and thrombin-HCII complex

The rate of thrombin inhibition by heparin cofactor II (HCII) is facilitated by heparin or dermatan sulfate in vitro. The distributions of these glycosaminoglycans (GAGs) in vivo are not the same; heparin-like substance is rich on the surface of endothelial cells and dermatan sulfate is relatively dominant in the extravascular region. When inflammation takes place, at least two other possible existent forms of HCII, the complexed form with thrombin and the cleaved form by leukocyte elastase, are assumed to be present at relatively high concentrations in a local circumstance. We examined the interactions of HCII with the two forms of HCII on thrombin inhibition in the presence of the GAGs. By HCII in complex with thrombin or cleaved by leukocyte elastase, the affinity of HCII moiety for heparin increases and that for dermatan sulfate decreases. The two forms possibly occur at relatively high concentrations in a local pathological situation, although the heparin cofactor activity for thrombin inhibition by HCII decreases and dermatan sulfate determines the cofactor activity. These results indicate efficient thrombin inhibitory activity of HCII in the extravascular region.

Inhibition of cellular action of thrombin by N3-cyclopropyl-7-[[4-(1-methylethyl)phenyl]methyl]-7H-pyrrolo[3, 2-f]quinazoline-1,3-diamine (SCH 79797), a nonpeptide thrombin receptor antagonist

A growing body of evidence suggests an important contribution of the cellular actions of thrombin to thrombosis and restenosis following angioplasty. Recently we reported on SCH 79797 (N3-cyclopropyl-7-¿[4-(1-methylethyl)phenyl]methyl¿-7H-pyrrolo[3, 2-f]quinazoline-1,3-diamine) and its analogs as new potent, nonpeptide thrombin receptor antagonists. This study further characterizes the biochemical and pharmacological actions of pyrroloquinazoline inhibitors of protease activated receptor-1 (PAR-1) in human platelets and coronary artery smooth muscle cells (hCASMC). SCH 79797 and its N-methyl analog (SCH 203099) inhibited binding of a high-affinity thrombin receptor-activating peptide ([(3)H]haTRAP, Ala-Phe(p-F)-Arg-ChA-HArg-[(3)H]Tyr-NH(2)) to PAR-1 with IC(50) values of 70 and 45 nM, respectively. SCH 79797 inhibited [(3)H]haTRAP binding in a competitive manner. SCH 79797 and SCH 203099 inhibited alpha-thrombin- and haTRAP-induced aggregation of human platelets, but did not inhibit human platelet aggregation induced by the tethered ligand agonist for protease-activated receptor-4 (PAR-4), gamma-thrombin, ADP, or collagen. SCH 203099 inhibited surface expression of P-selectin induced by haTRAP and thrombin, and it did not increase P-selectin expression or prevent thrombin cleavage of the receptor. Thrombin and TFLLRNPNDK-NH(2) (TK), a PAR-1-selective agonist, produced transient increases in cytosolic free Ca(2+) concentration ([Ca(2+)](i)) in hCASMC. This increase in [Ca(2+)](i) was inhibited effectively by SCH 79797. However, the Ca(2+) transients induced by SLIGKV-NH(2,) a PAR-2-selective agonist, were not inhibited by SCH 79797. Thrombin- and TK-stimulated [(3)H]thymidine incorporation also was inhibited completely by SCH 79797. The results of this study demonstrate that SCH 79797 and SCH 203099 are potent, selective antagonists of PAR-1 in human platelets and hCASMC. These data also suggest that the thrombin stimulation of Ca(2+) transients and mitogenesis in hCASMC is mediated primarily through activation of PAR-1.

Inhibition of arterial thrombosis by a peptide ligand of the thrombin receptor

Thrombin plays an important role in promoting arterial thrombosis by platelet activation and by catalyzing fibrin formation. Use of thrombin inhibitors that block both the platelet-activating and fibrin formation properties of thrombin are associated with hemostasis. This problem might be overcome by developing agents that block only the platelet-activating property of thrombin. Because the platelet-activating property of thrombin is mediated by the thrombin receptor, antagonists of the thrombin receptor might be efficacious and potentially safer with regard to bleeding complications. We investigated whether a peptide ligand (AFLARAA) of the thrombin receptor that blocked alpha-thrombin and thrombin receptor activating peptide-induced platelet aggregation could inhibit thrombosis. A partially occlusive thrombus was generated by application of electric current in rabbit carotid artery. In control animals, the artery was completely occluded within 42+/-12 min after the current was discontinued. When the thrombin receptor activating peptide antagonist was given (100 micromol/kg as an IV bolus followed by 900 micromol/kg infusion for a period of 180 min) starting at the time the current was stopped, blood flow remained patent throughout the infusion period and for an additional 60 min after the infusion was stopped. The antithrombotic effect of the antagonist peptide was not associated with increased bleeding tendency, as judged by the amount of blood adsorbed by a gauze pad placed in a surgical incision extending to the muscle tissue and by a standard template bleeding time. These results indicate that thrombin receptor antagonist peptides can be used as antithrombotic agents.

The current excitement in bioinformatics-analysis of whole-genome expression data: how does it relate to protein structure and function?

Whole-genome expression profiles provide a rich new data-trove for bioinformatics. Initial analyses of the profiles have included clustering and cross-referencing to 'external' information on protein structure and function. Expression profile clusters do relate to protein function, but the correlation is not perfect, with the discrepancies partially resulting from the difficulty in consistently defining function. Other attributes of proteins can also be related to expression-in particular, structure and localization-and sometimes show a clearer relationship than function.