Protease-activated receptor (PAR) 1 but not PAR2 or PAR4 mediates endothelium-dependent relaxation to thrombin and trypsin in human pulmonary arteries

Protease-activated receptors in health and disease

Proteases are signaling molecules that specifically control cellular functions by cleaving protease-activated receptors (PARs). The four known PARs are members of the large family of G protein-coupled receptors. These transmembrane receptors control most physiological and pathological processes and are the target of a large proportion of therapeutic drugs. Signaling proteases include enzymes from the circulation; from immune, inflammatory epithelial, and cancer cells; as well as from commensal and pathogenic bacteria. Advances in our understanding of the structure and function of PARs provide insights into how diverse proteases activate these receptors to regulate physiological and pathological processes in most tissues and organ systems. The realization that proteases and PARs are key mediators of disease, coupled with advances in understanding the atomic level structure of PARs and their mechanisms of signaling in subcellular microdomains, has spurred the development of antagonists, some of which have advanced to the clinic. Herein we review the discovery, structure, and function of this receptor system, highlight the contribution of PARs to homeostatic control, and discuss the potential of PAR antagonists for the treatment of major diseases.

Neutrophil cathepsin G proteolysis of protease-activated receptor 4 generates a novel, functional tethered ligand

Platelet-neutrophil interactions regulate ischemic vascular injury. Platelets are activated by serine proteases that cleave protease-activated receptor (PAR) amino termini, resulting in an activating tethered ligand. Neutrophils release cathepsin G (CatG) at sites of injury and inflammation, which activates PAR4 but not PAR1, although the molecular mechanism of CatG-induced PAR4 activation is unknown. We show that blockade of the canonical PAR4 thrombin cleavage site did not alter CatG-induced platelet aggregation, suggesting CatG cleaves a different site than thrombin. Mass spectrometry analysis using PAR4 N-terminus peptides revealed CatG cleavage at Ser67-Arg68. A synthetic peptide, RALLLGWVPTR, representing the tethered ligand resulting from CatG proteolyzed PAR4, induced PAR4-dependent calcium flux and greater platelet aggregation than the thrombin-generated GYPGQV peptide. Mutating PAR4 Ser67or Arg68 reduced CatG-induced calcium flux without affecting thrombin-induced calcium flux. Dog platelets, which contain a conserved CatG PAR4 Ser-Arg cleavage site, aggregated in response to human CatG and RALLLGWVPTR, while mouse (Ser-Gln) and rat (Ser-Glu) platelets were unresponsive. Thus, CatG amputates the PAR4 thrombin cleavage site by cleavage at Ser67-Arg68 and activates PAR4 by generating a new functional tethered ligand. These findings support PAR4 as an important CatG signaling receptor and suggest a novel therapeutic approach for blocking platelet-neutrophil-mediated pathophysiologies.

Trypsin-induced elevated contractile responses in a rat model of interstitial cystitis/bladder pain syndrome: Involvement of PAR2 and intracellular Ca2+ release pathways

AIMS

Interstitial cystitis/bladder pain syndrome (IC/BPS) is a chronic inflammatory disease with unclear etiology. Different receptors play a role in the pathophysiology including protease activated receptors (PARs). The present study aimed to investigate the subtypes and the effects of PARs on contractility using permeabilized detrusor smooth muscle strips in IC/BPS.

MAIN METHODS

IC/BPS was induced by cyclophosphamide injection. Histopathological analysis, PCR for detecting PAR proteins, western blotting for indicating PAR2 protein expression levels and myograph recording for measuring contractile force were used.

KEY FINDINGS

The present study reveals that in rat bladder PAR1 and PAR2 but not PAR4 were found to be expressed. The first evidence was revealed where trypsin-induced contractions in rat permeabilized detrusor were potentiated in CYP-induced cystitis. Moreover, the functional inhibition of trypsin-induced contractions by selective PAR2 antagonist (ENMD-1068) and the supporting immunoblotting results emphasized that the main PAR subtype involved in IC/BPS model in rat bladder is PAR2. Our data emphasize the prominent role of IP₃ in cystitis pathology besides ryanodine channels. Trypsin-induced Ca²⁺sensitization contractions were also higher in cystitis. Both Rho kinase and protein kinase C played a role in this increased Ca²⁺sensitization situation.

SIGNIFICANCE

The present paper highlights the intracellular pathways that are involved in trypsin-induced contractions mainly via PAR2 in permeabilized bladder detrusor smooth muscle in a rat model of IC/BPS.

Novel Direct Anticoagulants and Atherosclerosis

Coagulation factors can affect cellular processes that include inflammatory signaling by acting on endothelial protease activated receptors, vascular smooth muscle and inflammatory cells beyond the coagulation cascade. This is important in the pathogenesis of atherosclerosis. Accordingly, experimental data points to beneficial effects of coagulation protease inhibitors on the attenuation of atherosclerosis progression in animal models. However, available clinical data do not support the use of anticoagulants as an add-on treatment of atherosclerosis. New clinical studies are needed with a better selection of patients to clarify the role of novel direct anticoagulants in the management of atherosclerosis.

Protease-Activated Receptor-2 Deficiency Attenuates Atherosclerotic Lesion Progression and Instability in Apolipoprotein E-Deficient Mice

Inflammatory mechanisms are involved in the process of atherosclerotic plaque formation and rupture. Accumulating evidence suggests that protease-activated receptor (PAR)-2 contributes to the pathophysiology of chronic inflammation on the vasculature. To directly examine the role of PAR-2 in atherosclerosis, we generated apolipoprotein E/PAR-2 double-deficient mice. Mice were fed with high-fat diet for 12 weeks starting at ages of 6 weeks. PAR-2 deficiency attenuated atherosclerotic lesion progression with reduced total lesion area, reduced percentage of stenosis and reduced total necrotic core area. PAR-2 deficiency increased fibrous cap thickness and collagen content of plaque. Moreover, PAR-2 deficiency decreased smooth muscle cell content, macrophage accumulation, matrix metallopeptidase-9 expression and neovascularization in plaque. Relative quantitative PCR assay using thoracic aorta revealed that PAR-2 deficiency reduced mRNA expression of inflammatory molecules, such as vascular cell adhesion molecule-1, intercellular adhesion molecule-1, tumor necrosis factor (TNF)-α and monocyte chemoattractant protein (MCP)-1. In vitro experiment, we found that PAR-2 deficiency reduced mRNA expression of interferon-γ, interleukin-6, TNF-α and MCP-1 in macrophage under unstimulated and lipopolysaccharide-stimulated conditions. These results suggest that PAR-2 deficiency attenuates the progression and instability of atherosclerotic plaque.

Per a 10 activates human derived epithelial cell line in a protease dependent manner via PAR-2

BACKGROUND

Protease activity of Per a 10 has been shown to modulate dendritic cells toward Th-2 polarization and to induce airway inflammation.

OBJECTIVE

To elucidate the role of serine protease activity of Per a 10 in inducing biochemical responses in epithelial cells.

METHODS

Per a 10 was inactivated by heat treatment (ΔPer a 10) or AEBSF (iPer a 10). A549 cells were exposed to either enzymatically active/inactive Per a 10. The supernatant was analyzed for the secretion of proinflammatory cytokines by ELISA. Ca(2+) mobilization was analyzed by flow cytometry. A PAR-2 derived synthetic peptide 28GTNRSSKGRSLIGKVDGTSHVTGKGVTC54 was incubated with Per a 10 and the resultant cleaved products were analyzed by LC-MS. PAR-2 activation was inhibited by PAR-2 cleavage inhibiting antibody.

RESULTS

ΔPer a 10 was completely inactivated whereas iPer a 10 showed some residual activity. nPer a 10 having protease activity increased the secretion of IL-6, IL-8 and GMCSF from A549 in a dose and time dependent manner whereas iPer a 10 has reduced cytokine secretion. ΔPer a 10 and rPer a 10 were unable to activate the cells. nPer a 10 mobilized intracellular Ca(2+). nPer a 10 cleaved the PAR-2 derived peptide between arginine and serine residues (36R-S37) to expose PAR-2 ligand SLIGKV, as determined by LC-MS. Incubating with anti-PAR-2 cleavage antibody showed diminished cytokine secretion when treated with nPer a 10.

CONCLUSION

Serine protease activity of Per a 10 activates A549 cells to secrete proinflammatory cytokines by PAR-2 activation and Ca(2+)mobilization and can be exploited therapeutically.

Expression of protease-activated receptor 1 and 2 and anti-tubulogenic activity of protease-activated receptor 1 in human endothelial colony-forming cells

Endothelial colony-forming cells (ECFCs) are obtained from the culture of human peripheral blood mononuclear cell (hPBMNC) fractions and are characterised by high proliferative and pro-vasculogenic potential, which makes them of great interest for cell therapy. Here, we describe the detection of protease-activated receptor (PAR) 1 and 2 amongst the surface proteins expressed in ECFCs. Both receptors are functionally coupled to extracellular signal-regulated kinase (ERK) 1 and 2, which become activated and phosphorylated in response to selective PAR1- or PAR2-activating peptides. Specific stimulation of PAR1, but not PAR2, significantly inhibits capillary-like tube formation by ECFCs in vitro, suggesting that tubulogenesis is negatively regulated by proteases able to stimulate PAR1 (e.g. thrombin). The activation of ERKs is not involved in the regulation of tubulogenesis in vitro, as suggested by use of the MEK inhibitor PD98059 and by the fact that PAR2 stimulation activates ERKs without affecting capillary tube formation. Both qPCR and immunoblotting showed a significant downregulation of vascular endothelial growth factor 2 (VEGFR2) in response to PAR1 stimulation. Moreover, the addition of VEGF (50-100 ng/ml) but not basic Fibroblast Growth Factor (FGF) (25-100 ng/ml) rescued tube formation by ECFCs treated with PAR1-activating peptide. Therefore, we propose that reduction of VEGF responsiveness resulting from down-regulation of VEGFR2 is underlying the anti-tubulogenic effect of PAR1 activation. Although the role of PAR2 remains elusive, this study sheds new light on the regulation of the vasculogenic activity of ECFCs and suggests a potential link between adult vasculogenesis and the coagulation cascade.

Modulation of PAR(1) signalling by benzimidazole compounds

BACKGROUND AND PURPOSE

Recently, a small molecule (Q94) was reported to selectively block PAR(1) /Gα(q) interaction and signalling. Here, we describe the pharmacological properties of Q94 and two analogues that share its benzimidazole scaffold (Q109, Q89). Q109 presents a modest variation from Q94 in the substituent group at the 2-position, while Q89 has quite different groups at the 1- and 2-positions.

EXPERIMENTAL APPROACH

Using human microvascular endothelial cells, we examined intracellular Ca(2+) mobilization and inositol 1,4,5-trisphosphate accumulation as well as isoprenaline- or forskolin-stimulated cAMP production in response to thrombin.

KEY RESULTS

Q89 (10 µM) produced a leftward shift in the thrombin-mediated intracellular Ca(2+) mobilization concentration-response curve while having no effect on the E(max) . Both Q94 (10 µM) and Q109 (10 µM) reduced intracellular Ca(2+) mobilization, leading to a decrease in E(max) and an increase in EC(50) values. Experiments utilizing receptor-specific activating peptides confirmed that Q94 and Q109 were selective for PAR(1) as they did not alter the Ca(2+) response mediated by a PAR(2) activating peptide. Consistent with our Ca(2+) results, micromolar concentrations of either Q94 or Q109 significantly reduced thrombin-induced inositol 1,4,5-trisphosphate production. Neither Q94 nor Q109 diminished the inhibitory effects of thrombin on cAMP production, indicating they inhibit signalling selectively through the G(q) pathway. Our results also suggest the 1,2-disubstituted benzimidazole derivatives act as 'allosteric agonists' of PAR(1) .

CONCLUSIONS AND IMPLICATIONS

The Q94 and Q109 benzimidazole derivatives represent a novel scaffold for the development of new PAR(1) inhibitors and provide a starting point to develop dual signalling pathway-selective positive/negative modulators of PAR(1) .

Haemostatic system in inflammatory bowel diseases: New players in gut inflammation

Inflammation and coagulation constantly influence each other and are constantly in balance. Emerging evidence supports this statement in acute inflammatory diseases, such as sepsis, but it also seems to be very important in chronic inflammatory settings, such as inflammatory bowel disease (IBD). Patients with Crohn’s disease and ulcerative colitis have an increased risk of thromboembolic events, and several abnormalities concerning coagulation components occur in the endothelial cells of intestinal vessels, where most severe inflammatory abnormalities occur. The aims of this review are to update and classify the type of coagulation system abnormalities in IBD, and analyze the strict and delicate balance between coagulation and inflammation at the mucosal level. Recent studies on possible therapeutic applications arising from investigations on coagulation abnormalities associated with IBD pathogenesis will also be briefly presented and critically reviewed.

Structure, function and pathophysiology of protease activated receptors

Discovered in the 1990s, protease activated receptors(1) (PARs) are membrane-spanning cell surface proteins that belong to the G protein coupled receptor (GPCR) family. A defining feature of these receptors is their irreversible activation by proteases; mainly serine. Proteolytic agonists remove the PAR extracellular amino terminal pro-domain to expose a new amino terminus, or tethered ligand, that binds intramolecularly to induce intracellular signal transduction via a number of molecular pathways that regulate a variety of cellular responses. By these mechanisms PARs function as cell surface sensors of extracellular and cell surface associated proteases, contributing extensively to regulation of homeostasis, as well as to dysfunctional responses required for progression of a number of diseases. This review examines common and distinguishing structural features of PARs, mechanisms of receptor activation, trafficking and signal termination, and discusses the physiological and pathological roles of these receptors and emerging approaches for modulating PAR-mediated signaling in disease.

Vascular PAR-1: activity and antagonism

Despite major advances in antiplatelet therapies, recurrent cardiovascular events remain high after acute coronary syndrome. Furthermore, incremental benefits achieved in the reduction of atherothrombotic events have almost always been at the expense of hemorrhagic side effects. Thrombin is the most potent platelet activating factor known and it makes important interactions with the endothelium and vascular smooth muscle with proinflammatory, proatherogenic effects. Distinct from its activity within the coagulation cascade, thrombin mediates these effects via protease-activated receptor type 1 (PAR-1) in man. This review discusses the role of PAR-1 in the vasculature and the development of novel PAR-1 antagonists. These drugs may provide important antiatherothrombotic effects without attendant bleeding complications and could represent a major breakthrough for the treatment of cardiovascular diseases.

Thrombin activation of proteinase-activated receptor 1 potentiates the myofilament Ca2+ sensitivity and induces vasoconstriction in porcine pulmonary arteries

BACKGROUND AND PURPOSE

Thrombus formation is commonly associated with pulmonary arterial hypertension (PAH). Thrombin may thus play an important role in the pathogenesis and pathophysiology of PAH. Hence, we investigated the contractile effects of thrombin and its mechanism in pulmonary artery.

EXPERIMENTAL APPROACH

The cytosolic Ca(2+) concentrations ([Ca(2+)](i)), 20 kDa myosin light chain (MLC20) phosphorylation and tension development were evaluated using the isolated porcine pulmonary artery.

KEY RESULTS

Thrombin induced a sustained contraction in endothelium-denuded strips obtained from different sites of a pulmonary artery, ranging from the main pulmonary artery to the intrapulmonary artery. In the presence of endothelium, thrombin induced a transient relaxation. The contractile effect of thrombin was abolished by either a protease inhibitor or a proteinase-activated receptor 1 (PAR(1)) antagonist, while it was mimicked by PAR(1)-activating peptide (PAR(1)AP), but not PAR(4)AP. The thrombin-induced contraction was associated with a small elevation of [Ca(2+)](i) and an increase in MLC20 phosphorylation. Thrombin and PAR(1)AP induced a greater increase in tension for a given [Ca(2+)](i) elevation than that obtained with high K(+)-depolarization. They also induced a contraction at a fixed Ca(2+) concentration in alpha-toxin-permeabilized preparations.

CONCLUSIONS AND IMPLICATIONS

The present study revealed a unique property of the pulmonary artery. In contrast to normal arteries of the systemic circulation, thrombin induces a sustained contraction in the normal pulmonary artery, by activating PAR(1) and thereby increasing the sensitivity of the myofilament to Ca(2+). This responsiveness of the pulmonary artery to thrombin may therefore contribute to the pathogenesis and pathophysiology of PAH.

Parstatin: a cryptic peptide involved in cardioprotection after ischaemia and reperfusion injury

AIMS

Thrombin activates protease-activated receptor 1 by proteolytic cleavage of the N-terminus. Although much research has focused on the activated receptor, little is known about the 41-amino acid N-terminal peptide (parstatin). We hypothesized that parstatin would protect the heart against ischaemia-reperfusion injury.

METHODS AND RESULTS

We assessed the protective role of parstatin in an in vivo and in vitro rat model of myocardial ischaemia-reperfusion injury. Parstatin treatment before, during, and after ischaemia decreased infarct size by 26%, 23%, and 18%, respectively, in an in vivo model of ischaemia-reperfusion injury. Parstatin treatment immediately before ischaemia decreased infarct size by 65% and increased recovery in ventricular function by 23% in an in vitro model. We then assessed whether parstatin induced cardioprotection by activation of a Gi-protein-dependent pathway. Gi-protein inactivation by pertussis toxin completely abolished the cardioprotective effects. The cardioprotective effects were also abolished by inhibition of nitric oxide synthase (NOS), extracellular signal-regulated kinases 1/2 (ERK1/2), p38 mitogen-activated protein kinase (p38 MAPK), and K(ATP) channels in vitro. Furthermore, parstatin increased coronary flow and decreased perfusion pressure in the isolated heart. The vasodilatory properties of parstatin were confirmed in rat coronary arterioles.

CONCLUSION

A single treatment of parstatin administered prior to ischaemia confers immediate cardioprotection by recruiting the Gi-protein activation pathway including p38 MAPK, ERK1/2, NOS, and K(ATP) channels. Parstatin exerts effects on both the cardiomyocytes and the coronary circulation to induce cardioprotection. This suggests a potential therapeutic role of parstatin in the treatment of cardiac injury resulting from ischaemia and reperfusion.

Distinct roles of protease-activated receptors in signal transduction regulation of endothelial nitric oxide synthase

Protease-activated receptors (PARs), such as PAR1 and PAR2, have been implicated in the regulation of endothelial NO production. We hypothesized that PAR1 and PAR2 distinctly regulate the activity of endothelial NO synthase through the selective phosphorylation of a positive regulatory site, Ser(1179), and a negative regulatory site, Thr(497), in bovine aortic endothelial cells. A selective PAR1 ligand, TFLLR, stimulated the phosphorylation of endothelial NO synthase at Thr(497). It had a minimal effect on Ser(1179) phosphorylation. In contrast, a selective PAR2 ligand, SLIGRL, stimulated the phosphorylation of Ser(1179) with no noticeable effect on Thr(497). Thrombin has been shown to transactivate PAR2 through PAR1. Thus, thrombin, as well as a peptide mimicking the PAR1 tethered ligand, TRAP, stimulated phosphorylation of both sites. Also, thrombin and SLIGRL, but not TFLLR, stimulated cGMP production. A G(q) inhibitor blocked thrombin- and SLIGRL-induced Ser(1179) phosphorylation, whereas it enhanced thrombin-induced Thr(497) phosphorylation. In contrast, a G(12/13) inhibitor blocked thrombin- and TFLLR-induced Thr(497) phosphorylation, whereas it enhanced the Ser(1179) phosphorylation. Although a Rho-kinase inhibitor, Y27632, blocked the Thr(497) phosphorylation, other inhibitors that targeted Rho-kinase failed to block TFLLR-induced Thr(497) phosphorylation. These data suggest that PAR1 and PAR2 distinctly regulate endothelial NO synthase phosphorylation and activity through G(12/13) and G(q), respectively, delineating the novel signaling pathways by which the proteases act on protease-activated receptors to potentially modulate endothelial functions.

Thrombin-mediated increases in cytosolic [Ca2+] involve different mechanisms in human pulmonary artery smooth muscle and endothelial cells

Thrombin is a procoagulant inflammatory agonist that can disrupt the endothelium-lumen barrier in the lung by causing contraction of endothelial cells and promote pulmonary cell proliferation. Both contraction and proliferation require increases in cytosolic Ca(2+) concentration ([Ca(2+)](cyt)). In this study, we compared the effect of thrombin on Ca(2+) signaling in human pulmonary artery smooth muscle (PASMC) and endothelial (PAEC) cells. Thrombin increased the [Ca(2+)](cyt) in both cell types; however, the transient response was significantly higher and recovered quicker in the PASMC, suggesting different mechanisms may contribute to thrombin-mediated increases in [Ca(2+)](cyt) in these cell types. Depletion of intracellular stores with cyclopiazonic acid (CPA) in the absence of extracellular Ca(2+) induced calcium transients representative of those observed in response to thrombin in both cell types. Interestingly, CPA pretreatment significantly attenuated thrombin-induced Ca(2+) release in PASMC; this attenuation was not apparent in PAEC, indicating that a PAEC-specific mechanism was targeted by thrombin. Treatment with a combination of CPA, caffeine, and ryanodine also failed to abolish the thrombin-induced Ca(2+) transient in PAEC. Notably, thrombin-induced receptor-mediated calcium influx was still observed in PASMC after CPA pretreatment in the presence of extracellular Ca(2+). Ca(2+) oscillations were triggered by thrombin in PASMC resulting from a balance of extracellular Ca(2+) influx and Ca(2+) reuptake by the sarcoplasmic reticulum. The data show that thrombin induces increases in intracellular calcium in PASMC and PAEC with a distinct CPA-, caffeine-, and ryanodine-insensitive release existing only in PAEC. Furthermore, a dynamic balance between Ca(2+) influx, intracellular Ca(2+) release, and reuptake underlie the Ca(2+) transients evoked by thrombin in some PASMC. Understanding of such mechanisms will provide an important insight into thrombin-mediated vascular injury during hypertension.

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

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

Characterization of agonist-induced vasoconstriction in mouse pulmonary artery

In recent years, transgenic mouse models have been developed to examine the underlying cellular and molecular mechanisms of lung disease and pulmonary vascular disease, such as asthma, pulmonary thromboembolic disease, and pulmonary hypertension. However, there has not been systematic characterization of the basic physiological pulmonary vascular reactivity in normal and transgenic mice. This represents an intellectual "gap", since it is important to characterize basic murine pulmonary vascular reactivity in response to various contractile and relaxant factors to which the pulmonary vasculature is exposed under physiological conditions. The present study evaluates excitation- and pharmacomechanical-contraction coupling in pulmonary arteries (PA) isolated from wild-type BALB/c mice. We demonstrate that both pharmaco- and electromechanical coupling mechanisms exist in mice PA. These arteries are also reactive to stimulation by alpha(1)-adrenergic agonists, serotonin, endothelin-1, vasopressin, and U-46619 (a thromboxane A(2) analog). We conclude that the basic vascular responsiveness of mouse PA is similar to those observed in PA of other species, including rat, pig, and human, albeit on a different scale and to varying amplitudes.

A novel therapeutic target in various lung diseases: Airway proteases and protease-activated receptors

Protease-activated receptors (PAR), which are G protein-coupled receptors, have 4 members, PAR-1 to PAR-4. PARs are activated by proteolysis of a peptide bond at the N-terminal domain of the receptor. PARs are widely distributed throughout the airways. Their activity is modulated by airway proteases of endogenous and exogenous origin, which can either activate or disable the receptors. The regulation of PAR activity by proteases is important under pathological conditions when the activity of proteases is increased. Moreover, various inflammatory mediators, such as cytokines, growth factors, or prostanoids, alter the PAR expression level. Elevated PAR levels are observed in various lung disorders, and their significance in the development of pathological situations in the lung is currently intensively investigated. Consequences of PAR activation can be either beneficial or deleterious, depending on the PAR subtype. PAR-1 has been shown to be an important player in the development of pulmonary fibrosis. Thus, PAR-1 represents an exciting target for clinical intervention in fibrotic diseases. PAR-2 contributes to allergic airway inflammation. However, the question whether the impact of PAR-2 is beneficial or deleterious is still under intensive discussion. Therefore, precise information concerning the participation of PAR-2 in various lesions is required. Moreover, it is necessary to generate selective PAR- and organ-targeted approaches for treating the diseases. A thorough understanding of PAR-induced cellular events and the consequences of receptor blockade may help in the development of novel therapeutic strategies targeted to prevent lung destruction and to avoid deterioration of conditions of patients with inflammatory or fibrotic lung diseases.

Mandatory role of proteinase-activated receptor 1 in experimental bladder inflammation

Background

In general, inflammation plays a role in most bladder pathologies and represents a defense reaction to injury that often times is two edged. In particular, bladder neurogenic inflammation involves the participation of mast cells and sensory nerves. Increased mast cell numbers and tryptase release represent one of the prevalent etiologic theories for interstitial cystitis and other urinary bladder inflammatory conditions. The activity of mast cell-derived tryptase as well as thrombin is significantly increased during inflammation. Those enzymes activate specific G-protein coupled proteinase-activated receptors (PAR)s.

Four PARs have been cloned so far, and not only are all four receptors highly expressed in different cell types of the mouse urinary bladder, but their expression is altered during experimental bladder inflammation. We hypothesize that PARs may link mast cell-derived proteases to bladder inflammation and, therefore, play a fundamental role in the pathogenesis of cystitis.

Results

Here, we demonstrate that in addition to the mouse urinary bladder, all four PA receptors are also expressed in the J82 human urothelial cell line. Intravesical administration of PAR-activating peptides in mice leads to an inflammatory reaction characterized by edema and granulocyte infiltration. Moreover, the inflammatory response to intravesical instillation of known pro-inflammatory stimuli such as E. coli lipopolysaccharide (LPS), substance P, and antigen was strongly attenuated by PAR1-, and to a lesser extent, by PAR2-deficiency.

Conclusion

Our results reveal an overriding participation of PAR1 in bladder inflammation, provide a working model for the involvement of downstream signaling, and evoke testable hypotheses regarding the role of PARs in bladder inflammation. It remains to be determined whether or not mechanisms targeting PAR1 gene silencing or PAR1 blockade will ameliorate the clinical manifestations of cystitis.

Distinct roles for PAR1- and PAR2-mediated vasomotor modulation in human arterial and venous conduits

BACKGROUND

Patency rates after coronary artery bypass grafting (CABG) are better if the internal mammary artery (IMA) is used rather than the greater saphenous vein (GSV), and may be related to the endothelial release of vasodilators antagonizing vascular contraction. It has recently been shown that a family of protease-activated receptors (PARs) modulate endothelium-dependent vasodilatation.

OBJECTIVE AND METHODS

The aim of this study was to evaluate the presence and functional role of protease-activated receptor 1 (PAR1) and protease-activated receptor 2 (PAR2) in mediating vascular tone in IMAs and GSVs from patients undergoing CABG by means of real time-PCR and isometric tension measurements.

RESULTS

PAR1 mRNA levels were higher than those of PAR2 mRNA in both vessels. A selective PAR2-activating peptide (PAR2-AP), SLIGKV-NH(2) (0.01-100 micromol L(-1)), failed to induce vasorelaxation in precontracted IMA and GSV rings, whereas the selective PAR1-AP, TFLLR-NH(2) (0.001 to 10 micromol L(-1)), caused greater endothelium-dependent relaxation in the IMAs (pD(2) values 7.25 +/- 0.6 vs. 7.86 +/- 0.42, P < 0.05; E(max) values 56.2 +/- 17.3% vs. 29.7 +/- 13.4%, P < 0.001). Preincubation with TNFalpha (3 nmol L(-1)) induced vasorelaxation in IMAs in response to PAR2-AP (P < 0.05 vs. non-stimulated vessels); the response to PAR1-AP was unchanged. The relaxation induced by both PAR-APs was NO- and endothelium-dependent.

CONCLUSION

These data show that functionally active PAR1 and PAR2 are present in IMAs and GSVs, and that inflammatory stimuli selectively enhance endothelium-dependent relaxation to PAR2-AP in IMAs.

Upregulation of Na+/Ca2+ exchanger contributes to the enhanced Ca2+ entry in pulmonary artery smooth muscle cells from patients with idiopathic pulmonary arterial hypertension

A rise in cytosolic Ca(2+) concentration ([Ca(2+)](cyt)) in pulmonary artery smooth muscle cells (PASMC) is a trigger for pulmonary vasoconstriction and a stimulus for PASMC proliferation and migration. Multiple mechanisms are involved in regulating [Ca(2+)](cyt) in human PASMC. The resting [Ca(2+)](cyt) and Ca(2+) entry are both increased in PASMC from patients with idiopathic pulmonary arterial hypertension (IPAH), which is believed to be a critical mechanism for sustained pulmonary vasoconstriction and excessive pulmonary vascular remodeling in these patients. Here we report that protein expression of NCX1, an NCX family member of Na(+)/Ca(2+) exchanger proteins is upregulated in PASMC from IPAH patients compared with PASMC from normal subjects and patients with other cardiopulmonary diseases. The Na(+)/Ca(2+) exchanger operates in a forward (Ca(2+) exit) and reverse (Ca(2+) entry) mode. By activating the reverse mode of Na(+)/Ca(2+) exchange, removal of extracellular Na(+) caused a rapid increase in [Ca(2+)](cyt), which was significantly enhanced in IPAH PASMC compared with normal PASMC. Furthermore, passive depletion of intracellular Ca(2+) stores using cyclopiazonic acid (10 microM) not only caused a rise in [Ca(2+)](cyt) due to Ca(2+) influx through store-operated Ca(2+) channels but also mediated a rise in [Ca(2+)](cyt) via the reverse mode of Na(+)/Ca(2+) exchange. The upregulated NCX1 in IPAH PASMC led to an enhanced Ca(2+) entry via the reverse mode of Na(+)/Ca(2+) exchange, but did not accelerate Ca(2+) extrusion via the forward mode of Na(+)/Ca(2+) exchange. These observations indicate that the upregulated NCX1 and enhanced Ca(2+) entry via the reverse mode of Na(+)/Ca(2+) exchange are an additional mechanism responsible for the elevated [Ca(2+)](cyt) in PASMC from IPAH patients.

Thrombin activates AMP-activated protein kinase in endothelial cells via a pathway involving Ca2+/calmodulin-dependent protein kinase kinase beta

AMP-activated protein kinase (AMPK) is a sensor of cellular energy state in response to metabolic stress and other regulatory signals. AMPK is controlled by upstream kinases which have recently been identified as LKB1 or Ca2+/calmodulin-dependent protein kinase kinase beta (CaMKKbeta). Our study of human endothelial cells shows that AMPK is activated by thrombin through a Ca2+-dependent mechanism involving the thrombin receptor protease-activated receptor 1 and Gq-protein-mediated phospholipase C activation. Inhibition of CaMKK with STO-609 or downregulation of CaMKKbeta using RNA interference decreased thrombin-induced AMPK activation significantly, indicating that CaMKKbeta was the responsible AMPK kinase. In contrast, downregulation of LKB1 did not affect thrombin-induced AMPK activation but abolished phosphorylation of AMPK with 5-aminoimidazole-4-carboxamide ribonucleoside. Thrombin stimulation led to phosphorylation of acetyl coenzyme A carboxylase (ACC) and endothelial nitric oxide synthase (eNOS), two downstream targets of AMPK. Inhibition or downregulation of CaMKKbeta or AMPK abolished phosphorylation of ACC in response to thrombin but had no effect on eNOS phosphorylation, indicating that thrombin-stimulated phosphorylation of eNOS is not mediated by AMPK. Our results underline the role of Ca2+ as a regulator of AMPK activation in response to a physiologic stimulation. We also demonstrate that endothelial cells possess two pathways to activate AMPK, one Ca2+/CaMKKbeta dependent and one AMP/LKB1 dependent.

Molecular Biology of Chronic Thromboembolic Pulmonary Hypertension

Recent efforts have seen major advances in elucidating the mechanisms underlying pulmonary arterial hypertension. However, chronic thromboembolic pulmonary hypertension (CTEPH) often has been excluded from these studies. Consequently, whereas the clinical, radiographic, and hemodynamic characteristics of CTEPH have been well described, there remains a deficit in our understanding of the cellular, molecular, and genetic mechanisms underlying CTEPH. Furthermore, although prior venous thromboembolism may act as the inciting event, it is still unclear what predisposes some patients to develop CTEPH. CTEPH has two major pathogenic components. The first is the primary obstruction of central pulmonary arteries by accumulation of thrombotic material. The second is characterized by severe pulmonary vascular remodeling, similar to that seen in idiopathic pulmonary arterial hypertension. Other articles in this series describe the pathological, surgical, and therapeutic aspects of CTEPH. Here, we review the potential molecular and cellular mechanisms that may contribute to the pathogenesis of CTEPH.

Effect of acetic acid or trypsin application on rat colonic motility in vitro and modulation by two synthetic fragments of chromogranin A

The hypothesis that Chromogranin A (CgA)-derived peptides are involved in mechanisms modulating altered colonic motility was tested. Rat distal colonic strips were studied using an organ bath technique. Acetic acid (AA)-induced effects were characterized on spontaneous mechanical activities (SMA) in the presence of CgA4-16 or CgA47-66. In preparations with mucosa, AA induced a transient hyperactivity followed by a decrease in tone. The first phase is sensitive to tetrodotoxin (TTX) and capsaicin. The second phase was sensitive to BAYK8644 but insensitive to L-nitro-arginine-methyl-ester (L-Name)/apamin together. CgA4-16 or CgA47-66 alone produced no change on SMA. The administration of CgA4-16 prior to AA increased the duration of the excitatory component and reduced tone inhibition. CgA47-66 prior to AA only decreased duration of the excitatory phase. In preparations without mucosa, AA decreased tone. This effect was sensitive to BAYK8644 and CgA4-16. Trypsin decreased basal tone. This effect was suppressed by TTX, BAYK8644 or L-Name/apamin and were reduced by CgA4-16. AA-induced effects on rat colonic motility in vitro may be mediated through activation of primary afferents and an action at L-Type calcium channels. CgA-derived peptides are shown to decrease AA-induced effects on motility.

Hyperpolarization of murine small caliber mesenteric arteries by activation of endothelial proteinase-activated receptor 2

Activation of endothelial proteinase-activated receptor 2 (PAR-2) relaxes vascular smooth muscle (VSM) and causes hypotension by nitric oxide (NO)-prostanoid-dependent and -independent mechanisms. We investigated whether endothelium-dependent hyperpolarization of VSM was the mechanism whereby resistance caliber arteries vasodilated independently of NO. VSM membrane potentials and isometric tension were measured concurrently to correlate the electrophysiological and mechanical changes in murine small caliber mesenteric arteries. In uncontracted arteries, the PAR-2 agonist, SLIGRL-NH2 (0.1 to 10 micromol/L), hyperpolarized the VSM membrane potential only in endothelium-intact arterial preparations. This response was unaltered by treatment of arteries with inhibitors of NO synthases (L-NAME), soluble guanylyl cyclase (ODQ), and cyclooxygenases (indomethacin). L-NAME, ODQ, and indomethacin also failed to inhibit SLIGRL-NH2-induced hyperpolarization and of cirazoline-contracted mesenteric arteries. However, in blood vessels that were depolarized and contracted with 30 mmol/L KCl, the effects of the SLIGRL-NH2 on membrane potential and tension were not observed. SLIGRL-NH2-induced hyperpolarization and relaxation was inhibited completely by the combination of apamin plus charybdotoxin, but only partially inhibited after treatment with the combination of barium plus ouabain, suggesting an important role for SKCa and IKCa channels and a lesser role for Kir channels and Na+/K+ ATPases in the hyperpolarization response. We concluded that activation of endothelial PAR-2 hyperpolarized the vascular smooth muscle (VSM) cells of small caliber arteries, without requiring the activation of NO synthases, cyclooxygenases, or soluble guanylyl cyclase. Indeed, this hyperpolarization may be a primary mechanism for PAR-2-induced hypotension in vivo.

Protease-activated receptors in hemostasis, thrombosis and vascular biology

The coagulation cascade and protease-activated receptors (PARs) together provide an elegant mechanism that links mechanical information in the form of tissue injury to cellular responses. These receptors appear to largely account for the cellular effects of thrombin and can mediate signaling to other trypsin-like proteases. An important role for PARs in hemostasis and thrombosis is established in animal models, and studies in knockout mice and nonhuman primates raise the question of whether PAR inhibition might offer an appealing new approach to the prevention and treatment of thrombosis. PARs may also trigger inflammatory responses to tissue injury. For example, PAR activation on endothelial cells and perhaps sensory afferents can trigger local accumulation of leukocytes and platelets and transudation of plasma. However, panoply of signaling systems and cell types orchestrates inflammatory responses, and efforts to define the relative importance and roles of PARs in various inflammatory processes are just beginning. Lastly, roles for PARs in blood vessel formation and other processes during embryonic development are emerging, and whether these reflect new roles for the coagulation cascade and/or PAR signaling to other proteases remains to be explored.

Effects of thrombin, PAR-1 activating peptide and a PAR-1 antagonist on umbilical artery resistance in vitro

Background

The non-thrombotic effects of thrombin in cardiovascular tissues, as mediated via the protease activated receptors (PARs), and particularly PAR-1, have been the focus of much recent research. The aims of this study were to evaluate the effects of thrombin, a specific PAR-1 activating peptide (PAR1-AP), and a PAR-1 antagonist on human umbilical artery tone in vitro.

Methods

Human umbilical artery samples were obtained from 17 women at term. Arterial rings were suspended under physiologic conditions for isometric recording. The in vitro effects of thrombin (0.5 units/mL to 3 units/mL), PAR1-AP TFLLR-NH2 [10(-9) to 10(-6) M], and PAR-1 antagonist (N-trans cinnamoyl- p-fluoroPhe-p-guanidinoPhe-Leu-Arg-Orn-NH2) [10(-9) M to 10(-5) M] on umbilical artery tone were measured.

Results

Both thrombin and TFLLR-NH2 exerted a potent cumulative vasodilatory effect on human umbilical artery resistance (P < 0.001). The mean net maximal inhibition (MMI) for thrombin was 53.05% (n = 6; SEM = 1.43) at tissue bath concentration of 3 units/mL. The MMI with TFLLR-NH2 was 61.50 % (n = 6; SEM = 1.43) at bath concentration of 10(-6) M. In comparison to vehicle control, the PAR-1 antagonist did not show a significant relaxant or contractile effect (P > 0.05).

Conclusion

These findings highlight a potential role for thrombin and PAR-1 receptors in vascular regulation of feto-placental blood flow in normal pregnancy, and in association with the vascular lesions associated with IUGR and pre-eclampsia.

Proteinase-activated receptors: transducers of proteinase-mediated signaling in inflammation and immune response

Serine proteinases such as thrombin, mast cell tryptase, trypsin, or cathepsin G, for example, are highly active mediators with diverse biological activities. So far, proteinases have been considered to act primarily as degradative enzymes in the extracellular space. However, their biological actions in tissues and cells suggest important roles as a part of the body's hormonal communication system during inflammation and immune response. These effects can be attributed to the activation of a new subfamily of G protein-coupled receptors, termed proteinase-activated receptors (PARs). Four members of the PAR family have been cloned so far. Thus, certain proteinases act as signaling molecules that specifically regulate cells by activating PARs. After stimulation, PARs couple to various G proteins and activate signal transduction pathways resulting in the rapid transcription of genes that are involved in inflammation. For example, PARs are widely expressed by cells involved in immune responses and inflammation, regulate endothelial-leukocyte interactions, and modulate the secretion of inflammatory mediators or neuropeptides. Together, the PAR family necessitates a paradigm shift in thinking about hormone action, to include proteinases as key modulators of biological function. Novel compounds that can modulate PAR function may be potent candidates for the treatment of inflammatory or immune diseases.

Protease-activated receptors: contribution to physiology and disease

Proteases acting at the surface of cells generate and destroy receptor agonists and activate and inactivate receptors, thereby making a vitally important contribution to signal transduction. Certain serine proteases that derive from the circulation (e.g., coagulation factors), inflammatory cells (e.g., mast cell and neutrophil proteases), and from multiple other sources (e.g., epithelial cells, neurons, bacteria, fungi) can cleave protease-activated receptors (PARs), a family of four G protein-coupled receptors. Cleavage within the extracellular amino terminus exposes a tethered ligand domain, which binds to and activates the receptors to initiate multiple signaling cascades. Despite this irreversible mechanism of activation, signaling by PARs is efficiently terminated by receptor desensitization (receptor phosphorylation and uncoupling from G proteins) and downregulation (receptor degradation by cell-surface and lysosomal proteases). Protease signaling in tissues depends on the generation and release of proteases, availability of cofactors, presence of protease inhibitors, and activation and inactivation of PARs. Many proteases that activate PARs are produced during tissue damage, and PARs make important contributions to tissue responses to injury, including hemostasis, repair, cell survival, inflammation, and pain. Drugs that mimic or interfere with these processes are attractive therapies: selective agonists of PARs may facilitate healing, repair, and protection, whereas protease inhibitors and PAR antagonists can impede exacerbated inflammation and pain. Major future challenges will be to understand the role of proteases and PARs in physiological control mechanisms and human diseases and to develop selective agonists and antagonists that can be used to probe function and treat disease.

Protease-activated receptor 2-dependent phosphorylation of the tissue factor cytoplasmic domain

Tissue factor (TF) is the physiological activator of the coagulation cascade that plays pathophysiological roles in metastasis, angiogenesis, and inflammation. Downstream in coagulation, thrombin is the central protease that signals through G protein-coupled, protease-activated receptors (PARs). However, the TF-VIIa-Xa complex upstream in coagulation also activates PAR1 and 2. Here, we address the question of whether signaling of the TF initiation complex is a relevant pathway that leads to TF cytoplasmic domain phosphorylation. In heterologous expression systems and primary endothelial cells, we demonstrate that the ternary TF-VIIa-Xa complex induces TF phosphorylation specifically by activating PAR2 but not through PAR1 signaling. In addition, TF cytoplasmic domain phosphorylation is induced only by TF-dependent signaling but not by other coagulation factors in endothelial cells. Phosphorylation of the Pro-directed kinase target site Ser258 is dependent on prior phosphorylation of Ser253 by protein kinase C (PKC) alpha. TF phosphorylation is somewhat delayed and coincides with sustained PKCalpha activation downstream of PAR2 but not PAR1 signaling. Phosphatidylcholine-dependent phospholipase C is the major pathway that leads to prolonged PKCalpha recruitment downstream of PAR2. Thus, PAR2 signaling specifically phosphorylates TF in a receptor cross-talk that distinguishes upstream from downstream coagulation protease signaling.

Differential Expression of Protease-Activated Receptors 1, 2, and 4 on Human Endothelial Cells from Different Vascular Sites

OBJECTIVE

Protease-activated receptors (PARs) mediate DNA synthesis in endothelial cells when activated by serine proteases. However, despite the existence of heterogeneity among endothelial cells from each tissue, the responses to PAR-1, PAR-2, and PAR-4 activation are poorly defined and compared between endothelial cells from different sites. The aim of this study was to investigate whether PAR-mediated DNA synthesis differed in various endothelial cell types.

METHODS

We examined the incorporation of BrdU by human pulmonary artery endothelial cells (HPAECs), human aortic endothelial cells (HAECs), and human umbilical vein endothelial cells (HUVECs).

RESULTS

When the endothelial cells were treated with the selective PAR-1-activating peptide, SFLLRN, HAECs showed the highest BrdU incorporation rate (182 +/- 28%). In contrast, treatment with the PAR-2-activating peptide, SLIGKV, resulted in the highest BrdU incorporation rate (173 +/- 37%) in HPAECs, when pretreated with TNF-alpha. The PAR-4-activating peptide, GYPGQV, induced DNA synthesis in HPAECs and HAECs, but not in HUVECs.

CONCLUSION

These findings suggest that each PAR preferentially targets an endothelial cell type, and thus plays a distinct role in diverse physiological or pathological conditions.

Proinflammatory role of proteinase-activated receptor-2 in humans and mice during cutaneous inflammation in vivo

Proteinase-activated receptor-2 belongs to a new subfamily of G-protein-coupled receptors. Its precise role during inflammation and the underlying mechanisms is still unclear. Our study establishes that PAR-2 plays a direct proinflammatory role during cutaneous inflammation in mice and humans in vivo. In a model of experimentally induced allergic (ACD) and toxic (ICD) contact dermatitis (CD) we show that ear swelling responses, plasma extravasation, and leucocyte adherence were significantly attenuated in PAR-2 null mutant (PAR-2-/-) mice compared with wild-type (PAR-2+/+) mice, especially at early stages. The proinflammatory effects by PAR-2 activation were significantly diminished using nitric oxide-synthase inhibitors, while NF-kappaB and neuropeptides appear to play a minor role in these mechanisms. PAR-2-mediated up-regulation of E-selectin and cell adhesion molecule ICAM-1; enhanced plasma extravasation was observed in humans and mice and of interleukin-6 in mice in vivo. Thus, PAR-2 may be a beneficial therapeutic target for the treatment of inflammatory skin diseases.

Antiangiogenic property of human thrombin

Using protein chromatography, we purified and identified human prothrombin from human plasma as antiangiogenic. Prothrombin significantly inhibited endothelial cell tube formation in vitro at 10 microg/ml. Importantly, it also inhibited bFGF-induced angiogenesis in Matrigel-plug assays performed in mice. The proteolytic activity of thrombin appeared to be critical for the antiangiogenic activity of prothrombin. For example, thrombin exhibited inhibitory effects on endothelial cell tube formation in vitro at 10 U/ml. Addition of lepirudin, a specific inhibitor of thrombin, completely blocked prothrombin's and thrombin's antiangiogenic effects in vitro. We also assessed the importance of thrombin receptors in angiogenesis. Using small peptides that activate different protease-activated receptors (PARs), we showed that activation of PAR-1 led to inhibition of endothelial cell tube formation in vitro and bFGF-induced angiogenesis in vivo. Collectively, our data suggest that thrombin's proteolytic activity can be antiangiogenic.

Extracellular mediators in atherosclerosis and thrombosis: lessons from thrombin receptor knockout mice

It is well appreciated that thrombin as well as other proteases can act as signaling molecules that specifically regulate cells by cleaving and activating members of a novel class of protease-activated receptors (PARs). The utility of gene knockout strategies to define and better comprehend the physiological role of specific proteins is perhaps best exemplified in the field of thrombin receptors. The development of PAR knockout mice has provided the unique opportunity to identify and characterize new members of this novel family of GPCRs, evaluate the interaction of PARs jointly expressed in common cells and tissues, and better understand the role of PARs in thrombosis, restenosis, vascular remodeling, angiogenesis, and inflammation. Presently, 4 members of the PAR family have been cloned and identified. In this review, we examine experimental evidence gleaned from PAR-/- mouse models as well as how the use of PAR-/- mice has provided insights toward understanding the physiological role of thrombin in cells of the vascular system and vascular pathology.

Involvement of nitric oxide and tachykinins in the effects induced by protease-activated receptors in rat colon longitudinal muscle

(1) The aim of the present study was to verify a possible involvement of nitric oxide (NO) and of tachykinins in the contractile and relaxant effects caused by the activation of protease-activated receptor (PAR)-1 and PAR-2 in the longitudinal muscle of rat colon. (2) Mechanical responses to the PAR-1 activating peptides, SFLLRN-NH(2) (10 nM-10 micro M) and TFLLR-NH(2) (10 nM-10 micro M), and to the PAR-2-activating peptide, SLIGRL-NH(2) (10 nM-10 micro M), were examined in vitro in the absence and in the presence of different antagonists. (3) The relaxation induced by SFLLRN-NH(2), TFLLR-NH(2) and SLIGRL-NH(2) was antagonised by the inhibitor of NO synthase L-N(omega)-nitroarginine methyl ester (300 micro M), or by the inhibitor of the guanylyl cyclase, 1-H-oxodiazol-[1,2,4]-[4,3-a]quinoxaline-1-one (10 micro M). (4) The contractile responses to PAR-1 and PAR-2 activation were concentration-dependently attenuated by SR140333 (0.1-1 micro M), NK(1) receptor antagonist, or by SR48968 (0.1-1 micro M), NK(2) receptor antagonist. The combined pretreatment with SR140333 (1 micro M) and SR48968 (1 micro M) produced additive suppressive effects on the contractile responses to PAR activation. Pretreatment of the preparation with capsaicin (10 micro M) markedly reduced the contractions evoked by SFLLRN-NH(2), TFLLR-NH(2) and SLIGRL-NH(2), while omega-conotoxin GVIA (0.2 micro M) had no effect. (5) The present results suggest that in rat colonic longitudinal muscle, PAR-1 and PAR-2 activation can evoke (i) relaxation through the production of NO or (ii) contraction through the release of tachykinins, likely, from sensory nerves. These actions may contribute to motility disturbances during intestinal trauma and inflammation.

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.

Protease-activated receptor 2-mediated vasodilatation in humans in vivo: role of nitric oxide and prostanoids

BACKGROUND

Systemic hypotension as a consequence of vascular dysfunction is a well-recognized and important feature of critical illness. Although serine protease activation has been implicated as a cause of vascular dysfunction in systemic inflammation, the mechanism is unknown. Recently, a class of receptors with an entirely novel mechanism of action, protease-activated receptors (PARs), has been identified that would explain the link between protease activation and systemic hypotension. Our aim was to test the hypothesis that in vivo activation of protease-activated receptor 2 (PAR-2) in humans would mediate vasodilatation.

METHODS AND RESULTS

For these first-in-human studies, an activating peptide for the human PAR-2 receptor was synthesized and administered to healthy volunteers. Using both the dorsal hand vein technique and forearm plethysmography, we studied the effects of PAR-2 activation in human blood vessels and investigated the mechanism of vasodilation. Activation of PAR-2 receptors in vivo dilated human blood vessels in a dose-dependent manner, and the effects were reduced by inhibition of both nitric oxide and prostanoid synthesis

CONCLUSIONS

These findings demonstrate that serine protease activity can cause human vasodilation and provide a possible explanation of why serine protease activation in critical illness is associated with vascular dysfunction.

Protease-activated receptor (PAR)-1 and PAR-2 participate in the cell growth of alveolar capillary endothelium in primary lung adenocarcinomas

BACKGROUND

Cell growth can be induced via elicitation of protease-activated receptors (PAR) with serine proteases such as thrombin and trypsin.

METHODS

To understand whether PAR are involved in tumor vessel formation in the neoplastic cell-bearing alveolar walls, immunohistochemical and reverse transcriptase-polymerase chain reaction analyses were performed using the lung tissues from 16 patients with primary lung adenocarcinomas.

RESULTS

In microdissected tumor alveolar walls, the expressions of PAR-1 and PAR-2 mRNA were increased by 10-fold (P < 0.05) and 16-fold (P < 0.01), respectively, as compared with normal alveolar walls. Confocal microscopy revealed that tumor capillary endothelial cells in alveolar walls lost thrombomodulin expression. Instead, the expression of PAR-2 often became obvious at the normal border. Both PAR-1 and PAR-2 were expressed in the microvessel endothelial cells in tumors. Trypsin mRNA was expressed in 7 of the 16 cancer cell-bearing tissue specimens in contrast to 1 of the 14 normal alveolar walls. Immunohistochemically, trypsin was positive in the neoplastic cells from 10 patients and in lung adenocarcinoma cell lines (A549, HLC-1, LC-2, and PC-14). An in vitro assay showed a significant increase in idoxuridine (IdU) or bromodeoxyuridine uptake in human pulmonary artery endothelial cells and human umbilical cord vein endothelial cells after treatments with alpha-thrombin or activating peptides; SFLLRN for PAR-1 and SLIGKV for PAR-2, respectively.

CONCLUSIONS

Thus, proliferation of alveolar capillary endothelial cells is initialized in part by PAR activation with serum thrombin and neoplastic cell-released trypsin. These results suggest a synergistic effect of PAR with vascular endothelial growth factor in alveolar angiogenesis.

Aprotinin: Antithrombotic and Vasoactive Mechanisms of Action

Aprotinin is a serine protease inhibitor that has been in clinical use since the late 1980s to reduce blood loss in patients undergoing cardiopulmonary bypass surgery. Its hemostatic mechanism of action is mediated predominantly through inhibition of plasmin, thus exerting a net antifibrinolytic effect. Compared to other antifibrinolytics, however, aprotinin provides an additional patient benefit at the level of improved platelet function and diminished inflammatory response to bypass. Recent work on platelets has identified a cell-associated target for aprotinin: the thrombin-receptor, protease-activated receptor 1. Selective blockade of the protease-activated receptor 1 limits thrombin-induced activation and consequent “exhaustion” of platelets in the bypass circuit, while maintaining the hemostatic activity of platelets in the pericardial cavity in response to nonproteolytic agonists, such as collagen, adenosine diphosphate and epinephrine. While no specific cellular receptors have as yet been identified to explain the antiinflammatory and vasoactive properties of aprotinin, awareness is growing that serine protease-sensitive receptors belonging to the protease-activated receptor family (1-4) may represent important aprotinin targets, since these receptors are expressed by all major cells of the vasculature and act as sensors of the coagulation, inflammatory and vasoactive pathways activated by major surgery or trauma. The possibility is discussed that endothelial protease-activated receptor 2, whose natural ligands are trypsin, tryptase and the ternary tissue factor-Vlla-Xa complex, may be targeted by aprotinin.

Enzymatic activation of endothelial protease-activated receptors is dependent on artery diameter in human and porcine isolated coronary arteries

Protease-activated receptor (PAR)-mediated vascular relaxations have been compared in coronary arteries of different diameters isolated from both humans and pigs. Thrombin, trypsin, and the PAR1-activating peptide, TFLLR, all caused concentration-dependent relaxation of both large (epicardial; approximately 2 mm internal diameter) and small (intramyocardial; approximately 200 microm internal diameter) human coronary arteries. EC(50) values for thrombin (0.006 u ml(-1) in epicardial, 1.69 u ml(-1) in intramyocardial) and trypsin (0.02 u ml(-1) in epicardial, 1.05 u ml(-1) in intramyocardial) were significantly (P<0.01) greater in intramyocardial arteries. By contrast, EC(50) values for TFLLR were not different between epicardial (0.35 microM) and intramyocardial (0.43 microM) arteries. In porcine coronary arteries, EC(50) values for relaxations to thrombin (0.03 u ml(-1) in epicardial 0.17 u ml(-1) in intramyocardial) were also significantly (P<0.01) greater in the smaller arteries. EC(50) values for both TFLLR and the PAR2-activating peptide, SLIGKV, were not different between the two different-sized pig coronary arteries. PAR1-immunoreactivity was localized to the endothelium of human epicardial and intramyocardial arteries and both PAR1- and PAR2-immunoreactivity was observed in endothelial cells of equivalent porcine arteries. These findings indicate that enzymatic activation of endothelial cell PARs in human (PAR1) and porcine (PAR1 and PAR2) coronary arteries is markedly reduced in intramyocardial arteries when compared with epicardial arteries, suggesting increased regulation of PAR-mediated vascular responses in resistance-type arteries.