Myeloid Protease-Activated Receptor-2 Contributes to Influenza A Virus Pathology in Mice

Background

Innate immune responses to influenza A virus (IAV) infection are initiated in part by toll-like receptor 3 (TLR3). TLR3-dependent signaling induces an antiviral immune response and an NFκB-dependent inflammatory response. Protease-activated receptor 2 (PAR2) inhibits the antiviral response and enhances the inflammatory response. PAR2 deficiency protected mice during IAV infection. However, the PAR2 expressing cell-types contributing to IAV pathology in mice and the mechanism by which PAR2 contributes to IAV infection is unknown.

Methods

IAV infection was analyzed in global (Par2-/-), myeloid (Par2fl/fl;LysMCre+) and lung epithelial cell (EpC) Par2 deficient (Par2fl/fl;SPCCre+) mice and their respective controls (Par2+/+ and Par2fl/fl). In addition, the effect of PAR2 activation on polyinosinic-polycytidylic acid (poly I:C) activation of TLR3 was analyzed in bone marrow-derived macrophages (BMDM). Lastly, we determined the effect of PAR2 inhibition in wild-type (WT) mice.

Results

After IAV infection, Par2-/- and mice with myeloid Par2 deficiency exhibited increased survival compared to infected controls. The improved survival was associated with reduced proinflammatory mediators and reduced cellular infiltration in bronchoalveolar lavage fluid (BALF) of Par2-/- and Par2fl/fl;LysMCre+ 3 days post infection (dpi) compared to infected control mice. Interestingly, Par2fl/fl;SPCCre+ mice showed no survival benefit compared to Par2fl/fl. In vitro studies showed that Par2-/- BMDM produced less IL6 and IL12p40 than Par2+/+ BMDM after poly I:C stimulation. In addition, activation of PAR2 on Par2+/+ BMDM increased poly I:C induction of IL6 and IL12p40 compared to poly I:C stimulation alone. Importantly, PAR2 inhibition prior to IAV infection protect WT mice.

Conclusion

Global Par2 or myeloid cell but not lung EpC Par2 deficiency was associated with reduced BALF inflammatory markers and reduced IAV-induced mortality. Our study suggests that PAR2 may be a therapeutic target to reduce IAV pathology.

Protease Activated Receptors and Arthritis

The catabolic and destructive activity of serine proteases in arthritic joints is well known; however, these enzymes can also signal pain and inflammation in joints. For example, thrombin, trypsin, tryptase, and neutrophil elastase cleave the extracellular N-terminus of a family of G protein-coupled receptors and the remaining tethered ligand sequence then binds to the same receptor to initiate a series of molecular signalling processes. These protease activated receptors (PARs) pervade multiple tissues and cells throughout joints where they have the potential to regulate joint homeostasis. Overall, joint PARs contribute to pain, inflammation, and structural integrity by altering vascular reactivity, nociceptor sensitivity, and tissue remodelling. This review highlights the therapeutic potential of targeting PARs to alleviate the pain and destructive nature of elevated proteases in various arthritic conditions.

Protease-activated receptor-2 in endosomes signals persistent pain of irritable bowel syndrome

Once activated at the surface of cells, G protein-coupled receptors (GPCRs) redistribute to endosomes, where they can continue to signal. Whether GPCRs in endosomes generate signals that contribute to human disease is unknown. We evaluated endosomal signaling of protease-activated receptor-2 (PAR2), which has been proposed to mediate pain in patients with irritable bowel syndrome (IBS). Trypsin, elastase, and cathepsin S, which are activated in the colonic mucosa of patients with IBS and in experimental animals with colitis, caused persistent PAR2-dependent hyperexcitability of nociceptors, sensitization of colonic afferent neurons to mechanical stimuli, and somatic mechanical allodynia. Inhibitors of clathrin- and dynamin-dependent endocytosis and of mitogen-activated protein kinase kinase-1 prevented trypsin-induced hyperexcitability, sensitization, and allodynia. However, they did not affect elastase- or cathepsin S-induced hyperexcitability, sensitization, or allodynia. Trypsin stimulated endocytosis of PAR2, which signaled from endosomes to activate extracellular signal-regulated kinase. Elastase and cathepsin S did not stimulate endocytosis of PAR2, which signaled from the plasma membrane to activate adenylyl cyclase. Biopsies of colonic mucosa from IBS patients released proteases that induced persistent PAR2-dependent hyperexcitability of nociceptors, and PAR2 association with β-arrestins, which mediate endocytosis. Conjugation to cholestanol promoted delivery and retention of antagonists in endosomes containing PAR2 A cholestanol-conjugated PAR2 antagonist prevented persistent trypsin- and IBS protease-induced hyperexcitability of nociceptors. The results reveal that PAR2 signaling from endosomes underlies the persistent hyperexcitability of nociceptors that mediates chronic pain of IBS. Endosomally targeted PAR2 antagonists are potential therapies for IBS pain. GPCRs in endosomes transmit signals that contribute to human diseases.

Cathepsin S signals via PAR2 and generates a novel tethered ligand receptor agonist

Protease-activated receptor-2 is widely expressed in mammalian epithelial, immune and neural tissues. Cleavage of PAR2 by serine proteases leads to self-activation of the receptor by the tethered ligand SLIGRL. The contribution of other classes of proteases to PAR activation has not been studied in detail. Cathepsin S is a widely expressed cysteine protease that is upregulated in inflammatory conditions. It has been suggested that cathepsin S activates PAR2. However, cathepsin S activation of PAR2 has not been demonstrated directly nor has the potential mechanism of activation been identified. We show that cathepsin S cleaves near the N-terminus of PAR2 to expose a novel tethered ligand, KVDGTS. The hexapeptide KVDGTS generates downstream signaling events specific to PAR2 but is weaker than SLIGRL. Mutation of the cathepsin S cleavage site prevents receptor activation by the protease while KVDGTS retains activity. In conclusion, the range of actions previously ascribed to cysteine cathepsins in general, and cathepsin S in particular, should be expanded to include molecular signaling. Such signaling may link together observations that had been attributed previously to PAR2 or cathepsin S individually. These interactions may contribute to inflammation.

Protease-activated receptor-2 induces migration of pancreatic cancer cells in an extracellular ATP-dependent manner

BACKGROUND

Protease-activated receptor 2 (PAR-2) is a G protein-coupled receptor suggested to play an important role in the proliferation and migration of tumor cells of epithelial origin. However, the role of PAR-2 in the setting of pancreatic cancer remains largely unexplored.

OBJECTIVES

To understand the importance of PAR-2 in pancreatic cancer cell migration.

METHODS AND RESULTS

The present study shows that PAR-2 does not affect pancreatic cancer cell proliferation but significantly induces the migration of pancreatic cancer cells in scratch assays. Interestingly, PAR-2 does not affect migration in a trans-well setting. This apparent discrepancy depends on extracellular ATP release in the scratch assays and the addition of exogenous (ATP)-induced PAR-2-dependent migration in trans-well assays, whereas a specific P2Y11 receptor antagonist prevents PAR-2-driven migration in scratch assays. In the scratch assays, inhibitors of Src, Rac, protein kinase C, mitogen-activated protein kinase kinase, p38, and epidermal growth factor (EGF) receptor blocked PAR-2-driven migration, whereas they did not affect fetal calf serum-driven wound closure.

CONCLUSION

Taken together, PAR-2 activation drives pancreatic cancer cell migration via an EGF-Src-Rac-p38/mitogen-activated protein kinase kinase/EGF1/2 signaling pathway, which is facilitated by extracellular ATP. Targeting the PAR-2/ATP-driven signaling pathway may therefore limit cell migration, which could inhibit pancreatic cancer metastasis.

MicroRNA-34a mediates the autocrine signaling of PAR2-activating proteinase and its role in colonic cancer cell proliferation

The tumor microenvironment is replete with proteinases. As a sensor of proteinases, proteinase activated receptor 2 (PAR2) plays critical roles in tumorigenesis. We showed that PAR2 and its activating proteinase were coexpressed in different colon cancer cell lines, including HT29. Inactivating proteinase or knockdown of PAR2 significantly not only reduced cell proliferation in vitro but also inhibited tumorigenicity of HT29 in vivo. In addition, activation of PAR2 promoted DNA synthesis and upregulated Cyclin D1 activity at both transcriptional and post-transcriptional levels. Further studies showed that miRNA-34a mediated PAR2-induced Cyclin D1 upregulation. Inhibition of miR-34a partially abolished the suppression of Cyclin D1 induced by PAR2 deficiency. In addition, we showed that TGF-β contributed to the regulation of miR-34a by PAR2. Finally, in colorectal carcinoma samples, upregulation of PAR2 and downregulation of miR-34a were significantly correlated with grade and lymphomatic metastasis. Our findings provide the first evidence that miRNA mediates autocrine proteinase signaling-mediated cancer cell proliferation.

Protease-activated receptor (PAR)2, but not PAR1, is involved in collateral formation and anti-inflammatory monocyte polarization in a mouse hind limb ischemia model

AIMS

In collateral development (i.e. arteriogenesis), mononuclear cells are important and exist as a heterogeneous population consisting of pro-inflammatory and anti-inflammatory/repair-associated cells. Protease-activated receptor (PAR)1 and PAR2 are G-protein-coupled receptors that are both expressed by mononuclear cells and are involved in pro-inflammatory reactions, while PAR2 also plays a role in repair-associated responses. Here, we investigated the physiological role of PAR1 and PAR2 in arteriogenesis in a murine hind limb ischemia model.

METHODS AND RESULTS

PAR1-deficient (PAR1-/-), PAR2-deficient (PAR2-/-) and wild-type (WT) mice underwent femoral artery ligation. Laser Doppler measurements revealed reduced post-ischemic blood flow recovery in PAR2-/- hind limbs when compared to WT, while PAR1-/- mice were not affected. Upon ischemia, reduced numbers of smooth muscle actin (SMA)-positive collaterals and CD31-positive capillaries were found in PAR2-/- mice when compared to WT mice, whereas these parameters in PAR1-/- mice did not differ from WT mice. The pool of circulating repair-associated (Ly6C-low) monocytes and the number of repair-associated (CD206-positive) macrophages surrounding collaterals in the hind limbs were increased in WT and PAR1-/- mice, but unaffected in PAR2-/- mice. The number of repair-associated macrophages in PAR2-/- hind limbs correlated with CD11b- and CD115-expression on the circulating monocytes in these animals, suggesting that monocyte extravasation and M-CSF-dependent differentiation into repair-associated cells are hampered.

CONCLUSION

PAR2, but not PAR1, is involved in arteriogenesis and promotes the repair-associated response in ischemic tissues. Therefore, PAR2 potentially forms a new pro-arteriogenic target in coronary artery disease (CAD) patients.

PAR2 knock-out C57Bl6 mice as a model for evaluating metastases of cancer cells: pilot in vivo study of the metastatic potential of B16 melanoma in knock-out (PAR2-/-) animals

Proteinase-activated receptor 2 (PAR-2) is a ubiquitous surface molecule. It belongs to the family of G protein-coupled receptors activated by site-specific proteolysis by trypsin. Altered function of PAR-2 has been described in different malignant tumours, both in vivo and in vitro. In the present study, we investigated differences of metastatic spread of B16 melanoma in knock-out animals compared with C57Bl6 mice. Knock-out mice B6.Cg-F2rl1(tm1Mslb)/J (PAR2-/-) and C57Bl6 controls were subcutaneously inoculated with the B16 melanoma tissue cell line. Fourteen days after inoculation, all primary tumours were removed and histopathologically analysed. After one month, animals in both group started to die. Autopsy showed metastatic spread of the melanoma to various organs in both groups. Our experiment confirmed growth and metastatic spread in both groups of mice. Excised tumours differed in volume and weight; average weight (0.62 g in PAR2-/- and 0.4 g in control animals). Metastatic spread was observed in both groups and reached 80 % in PAR2-/- and 50 % in control animals. While in control mice only lung metastases were observed, local tumour recurrence, renal and lung metastases were observed in PAR2-/- mice. The absence of functional PAR-2 could be an important factor influencing the growth and spread of melanoma in vivo, probably associated with tumour cell migration, invasiveness and metastasis formation.

Proteinase-activated receptors in the endometrium and endometriosis

Proteinase-activated receptors (PARs) are G protein-coupled receptors activated by various proteinases. PARs play important roles in haemostasis, thrombosis, and inflammation. PAR₁ and PAR₂ are expressed in endometrial cells from the eutopic endometrium and endometriotic cells derived from endometriotic lesions. A typical activator of PAR₁, thrombin, and a typical activator of PAR₂, tryptase, are produced in the endometrium as well as endometriotic lesions. PAR₁ activation in endometrial stromal cells induces production of vascular endothelial growth factor and matrix metalloproteinases, and increases activities of tissue-type and urokinase-type plasminogen activator. PAR₂ activation in endometrial stromal cells stimulates interleukin (IL)-8 and stem cell factor production and proliferation of the cells. PAR₁ activation in endometriotic stromal cells induces production of IL-8, monocyte chemotactic protein-1, and cyclooxygenase-2, and proliferation of the cells. PAR₂ activation in endometriotic stromal cells increases secretion of IL-6 and IL-8, and the number of the cells. These findings indicate a wide range of function of PAR₁ and PAR₂ in the endometrium and endometriosis, and suggest PAR₁ and PAR₂ as possible therapeutic targets for endometriosis.

Allergic sensitization enhances anion current responsiveness of murine trachea to PAR-2 activation

Protease-activated receptor 2 (PAR-2) is a G protein-coupled receptor possibly involved in the pathogenesis of asthma. PAR-2 also modulates ion transport in cultured epithelial cells, but these effects in native airways are controversial. The influence of allergic inflammation on PAR-2-induced changes in ion transport has received little attention. Here, we studied immediate changes in transepithelial short circuit current (I (sc)) induced by PAR-2 activation in the tracheas of naive and allergic mice. Activation of PAR-2 with an apically added activation peptide (AP) induced a small increase in I (sc), while a much larger increase was observed following basolateral AP addition. In ovalbumin-sensitized and -challenged animals used as a model of allergic airway inflammation, the effect of basolateral AP addition was enhanced. Responses to basolateral AP in both naive and allergic mice were not decreased by blocking sodium absorption with amiloride or CFTR function with CFTR(inh)172 but were reduced by the cyclooxygenase inhibitor indomethacin and largely blocked (>80%) by niflumic acid, a calcium-activated chloride channels' (CaCC) blocker. Allergic mice also showed an enhanced response to ATP and thapsigargin. There was no change in mRNA expression of Par-2 or of the chloride channels Ano1 (Tmem16a) and Bestrophin 2 in tracheas from allergic mice, while mRNA levels of Bestrophin 1 were increased. In conclusion, basolateral PAR-2 activation in the mouse airways led to increased anion secretion through apical CaCC, which was more pronounced in allergic animals. This could be a protective mechanism aimed at clearing allergens and defending against mucus plugging.

Effect of pertussis toxin and herbimycin A on proteinase-activated receptor 2-mediated cyclooxygenase 2 expression in Helicobacter pylori-infected gastric epithelial AGS cells

Helicobacter pylori (H. pylori) is an important risk factor for chronic gastritis, peptic ulcer, and gastric cancer. Proteinase-activated receptor 2 (PAR2), subgroup of G-protein coupled receptor family, is highly expressed in gastric cancer, and chronic expression of cyclooxygenase-2 (COX-2) plays an important role in H. pylori-associated gastric carcinogenesis and inflammation. We previously demonstrated that H. pylori induced the expression of PAR2 and COX-2 in gastric epithelial cells. Present study aims to investigate whether COX-2 expression induced by H. pylori in Korean isolates is mediated by PAR2 via activation of G(i) protein and Src kinase in gastric epithelial AGS cells. Results showed that H. pylori-induced COX-2 expression was inhibited in the cells transfected with antisense oligonucleotide for PAR2 or treated with Gi protein blocker pertussis toxin, Src kinase inhibitor herbimycin A and soybean trypsin inbitor, indicating that COX-2 expression is mediated by PAR2 through activation of Gi protein and Src kinase in gastric epithelial cells infected with H. pylori in Korean isolates. Thus, targeting the activation of PAR2 may be beneficial for prevention or treatment of gastric inflammation and carcinogenesis associated with H. pylori infection.

Protease activated receptor signaling is required for African trypanosome traversal of human brain microvascular endothelial cells

Background

Using human brain microvascular endothelial cells (HBMECs) as an in vitro model for how African trypanosomes cross the human blood-brain barrier (BBB) we recently reported that the parasites cross the BBB by generating calcium activation signals in HBMECs through the activity of parasite cysteine proteases, particularly cathepsin L (brucipain). In the current study, we examined the possible role of a class of protease stimulated HBMEC G protein coupled receptors (GPCRs) known as protease activated receptors (PARs) that might be implicated in calcium signaling by African trypanosomes.

Methodology/Principal Findings

Using RNA interference (RNAi) we found that in vitro PAR-2 gene (F2RL1) expression in HBMEC monolayers could be reduced by over 95%. We also found that the ability of Trypanosoma brucei rhodesiense to cross F2RL1-silenced HBMEC monolayers was reduced (39%–49%) and that HBMECs silenced for F2RL1 maintained control levels of barrier function in the presence of the parasite. Consistent with the role of PAR-2, we found that HBMEC barrier function was also maintained after blockade of Gα_q with Pasteurella multocida toxin (PMT). PAR-2 signaling has been shown in other systems to have neuroinflammatory and neuroprotective roles and our data implicate a role for proteases (i.e. brucipain) and PAR-2 in African trypanosome/HBMEC interactions. Using gene-profiling methods to interrogate candidate HBMEC pathways specifically triggered by brucipain, several pathways that potentially link some pathophysiologic processes associated with CNS HAT were identified.

Conclusions/Significance

Together, the data support a role, in part, for GPCRs as molecular targets for parasite proteases that lead to the activation of Gα_q-mediated calcium signaling. The consequence of these events is predicted to be increased permeability of the BBB to parasite transmigration and the initiation of neuroinflammation, events precursory to CNS disease.

Human African trypanosomiasis, or sleeping sickness, occurs when single-cell trypanosome protozoan parasites spread from the blood to brain over the blood-brain barrier (BBB). This barrier is composed of brain microvascular endothelial cells (BMECs) especially designed to keep pathogens out. Safe drugs for treating sleeping sickness are lacking and alternative treatments are urgently required. Using our human BMEC BBB model, we previously found that a parasite protease, brucipain, induced calcium activation signals that allowed this barrier to open up to parasite crossing. Because human BMECs express protease-activated receptors (PARs) that trigger calcium signals in BMECs, we hypothesized a functional link between parasite brucipain and BMEC PARs. Utilizing RNA interference to block the production of one type of PAR called PAR-2, we hindered the ability of trypanosomes to both open up and cross human BMECs. Using gene-profiling methods to interrogate candidate BMEC pathways specifically triggered by brucipain, several pathways that potentially link brain inflammatory processes were identified, a finding congruent with the known role of PAR-2 as a mediator of inflammation. Overall, our data support a role for brucipain and BMEC PARs in trypanosome BBB transmigration, and as potential triggers for brain inflammation associated with the disease.

Proteinase-activated receptors in the gastrointestinal system: a functional linkage to prostanoids

Proteinase-activated receptors (PARs), G protein-coupled receptors, play critical roles in the alimentary system. Increasing evidence suggests that endogenous prostaglandins (PGs) mediate some of PARs' gastrointestinal functions. Systemic administration of the PAR1 agonist protects against gastric mucosal injury through PG formation in rats. PGs also appear to contribute, at least in part, to enhancement of gastric mucosal blood flow and suppression of gastric acid secretion by PAR1 activation. There is also evidence for involvement of PGs in modulation of gastrointestinal motility by PAR1 or PAR2. Importantly, modulation of ion transport by PAR1 or PAR2 in the intestinal mucosal epithelium is largely mediated by PGs. Studies using gastric and intestinal mucosal epithelial cell lines imply that the PAR1-triggered formation of PGs involves multiple signaling pathways including Src, EGF receptor trans-activation and activation of MAP kinases. Collectively, a functional linkage of PAR1 and/or PAR2 to PGs is considered important in the gastrointestinal system.

Role of protease activated receptor-2 in tumor advancement of ovarian cancers

BACKGROUND

Protease activated receptor-2 (PAR-2) has been implicated in cellular proliferation, invasion and metastasis with angiogenesis in various tumors. This prompted us to study the role of PAR-2 in tumor advancement of ovarian cancers.

MATERIALS AND METHODS

Forty-eight patients underwent surgery for ovarian cancers. In ovarian cancers, PAR-2 histoscores and mRNA levels were determined by immunohistochemistry and real-time reverse transcription-polymerase chain reaction, respectively. Patient prognosis was analysed with a 36-month survival rate. Microvessel counts were determined by immunohistochemistry for CD31 and factor VIII-related antigen and the rate of cell proliferation was determined by immunohistochemistry for Ki67.

RESULTS

Immunohistochemical staining revealed distribution of PAR-2, dominantly in cancer cells and faintly in stromal cells of the tumor. PAR-2 histoscores in cancer cells and mRNA levels both significantly increased in ovarian cancers with clinical stages (I < II < III < IV, P < 0.05), regardless of histopathological type. The 36-month survival rate of 24 patients with high PAR-2 was poor (58%), while that of the other 24 patients with low PAR-2 was significantly higher (83%). There were significant correlations between PAR-2 histoscores in cancer cells and mRNA levels with microvessel counts and with the rate of cell proliferation in ovarian cancers.

CONCLUSIONS

PAR-2 was up-regulated during ovarian cancer progression. Therefore, PAR-2 might work on tumor advancement of ovarian cancers via angiogenic activity and is considered to be a novel prognostic indicator in ovarian cancers.

Differential regulation of class IA phosphoinositide 3-kinase catalytic subunits p110 alpha and beta by protease-activated receptor 2 and beta-arrestins

PAR-2 (protease-activated receptor 2) is a GPCR (G-protein-coupled receptor) that can elicit both G-protein-dependent and -independent signals. We have shown previously that PAR-2 simultaneously promotes Galphaq/Ca2+-dependent activation and beta-arrestin-1-dependent inhibition of class IA PI3K (phosphoinositide 3-kinase), and we sought to characterize further the role of beta-arrestins in the regulation of PI3K activity. Whereas the ability of beta-arrestin-1 to inhibit p110alpha (PI3K catalytic subunit alpha) has been demonstrated, the role of beta-arrestin-2 in PI3K regulation and possible differences in the regulation of the two catalytic subunits (p110alpha and p110beta) associated with p85alpha (PI3K regulatory subunit) have not been examined. In the present study we have demonstrated that: (i) PAR-2 increases p110alpha- and p110beta-associated lipid kinase activities, and both p110alpha and p110beta are inhibited by over-expression of either beta-arrestin-1 or -2; (ii) both beta-arrestin-1 and -2 directly inhibit the p110alpha catalytic subunit in vitro, whereas only beta-arrestin-2 directly inhibited p110beta; (iii) examination of upstream pathways revealed that PAR-2-induced PI3K activity required the small GTPase Cdc (cell-division cycle)42, but not tyrosine phosphorylation of p85; and (iv) beta-arrestins inhibit PAR-2-induced Cdc42 activation. Taken together, these results indicated that beta-arrestins could inhibit PAR-2-stimulated PI3K activity, both directly and through interference with upstream pathways, and that the two beta-arrestins differ in their ability to inhibit the p110alpha and p110beta catalytic subunits. These results are particularly important in light of the growing interest in PAR-2 as a pharmacological target, as commonly used biochemical assays that monitor G-protein coupling would not screen for beta-arrestin-dependent signalling events.

Proteinase-activated receptor-2 exerts protective and pathogenic cell type-specific effects in Alzheimer's disease

The proteinase-activated receptors (PARs) are a novel family of G protein-coupled receptors, and their effects in neurodegenerative diseases remain uncertain. Alzheimer's disease (AD) is a neurodegenerative disorder defined by misfolded protein accumulation with concurrent neuroinflammation and neuronal death. We report suppression of proteinase-activated receptor-2 (PAR2) expression in neurons of brains from AD patients, whereas PAR2 expression was increased in proximate glial cells, together with up-regulation of proinflammatory cytokines and chemokines and reduced IL-4 expression (p < 0.05). Glial PAR2 activation increased expression of formyl peptide receptor-2 (p < 0.01), a cognate receptor for a fibrillar 42-aa form of beta-amyloid (Abeta(1-42)), enhanced microglia-mediated proinflammatory responses, and suppressed astrocytic IL-4 expression, resulting in neuronal death (p < 0.05). Conversely, neuronal PAR2 activation protected human neurons against the toxic effects of Abeta(1-42) (p < 0.05), a key component of AD neuropathogenesis. Amyloid precursor protein-transgenic mice, displayed glial fibrillary acidic protein and IL-4 induction (p < 0.05) in the absence of proinflammatory gene up-regulation and neuronal injury, whereas PAR2 was up-regulated at this early stage of disease progression. PAR2-deficient mice, after hippocampal Abeta(1-42) implantation, exhibited enhanced IL-4 induction and less neuroinflammation (p < 0.05), together with improved neurobehavioral outcomes (p < 0.05). Thus, PAR2 exerted protective properties in neurons, but its activation in glia was pathogenic with secretion of neurotoxic factors and suppression of astrocytic anti-inflammatory mechanisms contributing to Abeta(1-42)-mediated neurodegeneration.

Trypsin IV or Mesotrypsin and p23 Cleave Protease-activated Receptors 1 and 2 to Induce Inflammation and Hyperalgesia

Although principally produced by the pancreas to degrade dietary proteins in the intestine, trypsins are also expressed in the nervous system and in epithelial tissues, where they have diverse actions that could be mediated by protease-activated receptors (PARs). We examined the biological actions of human trypsin IV (or mesotrypsin) and rat p23, inhibitor-resistant forms of trypsin. The zymogens trypsinogen IV and pro-p23 were expressed in Escherichia coli and purified to apparent homogeneity. Enteropeptidase cleaved both zymogens, liberating active trypsin IV and p23, which were resistant to soybean trypsin inhibitor and aprotinin. Trypsin IV cleaved N-terminal fragments of PAR(1), PAR(2), and PAR(4) at sites that would expose the tethered ligand (PAR(1) = PAR(4) > PAR(2)). Trypsin IV increased [Ca(2+)](i) in transfected cells expressing human PAR(1) and PAR(2) with similar potencies (PAR(1), 0.5 microm; PAR(2), 0.6 microm). p23 also cleaved fragments of PAR(1) and PAR(2) and signaled to cells expressing these receptors. Trypsin IV and p23 increased [Ca(2+)](i) in rat dorsal root ganglion neurons that responded to capsaicin and which thus mediate neurogenic inflammation and nociception. Intraplantar injection of trypsin IV and p23 in mice induced edema and granulocyte infiltration, which were not observed in PAR (-/-)(1)(trypsin IV) and PAR (-/-)(2) (trypsin IV and p23) mice. Trypsin IV and p23 caused thermal hyperalgesia and mechanical allodynia and hyperalgesia in mice, and these effects were absent in PAR (-/-)(2) mice but maintained in PAR (-/-)(1) mice. Thus, trypsin IV and p23 are inhibitor-resistant trypsins that can cleave and activate PARs, causing PAR(1)- and PAR(2)-dependent inflammation and PAR(2)-dependent hyperalgesia.

Protease activated receptor 2 and epidermal growth factor receptor are involved in the regulation of human sperm motility

AIM

To investigate mechanisms of tryptase-induced reduction of sperm motility and explore whether epidermal growth factor receptor (EGF-R) and protease activated receptor 2 (PAR-2)- associated pathways are involved.

METHODS

Fresh semen was collected from healthy donors (n = 15). Semen parameters and quality were assessed in accordance with the World Health Organization (WHO) criteria. Swim-up sperm were fixed and subjected to immunocytochemistry and immunoelectronmicroscopy with specific antibodies directed against PAR-2 and EGF-R. Protein extractions from swim-up spermatozoa were analyzed by Western blotting with antibodies for both receptors. Motility of spermatozoa was evaluated by computer-assisted semen analysis.

RESULTS

Immunocytochemistry found PAR-2 and EGF-R in approximately 30% of examined human ejaculated spermatozoa. Both receptors were localized in the plasma membrane. Like tryptase, the PAR-2 synthetic agonist SLIGKV reduced sperm motility, and this effect was inhibited by application of two specific EGF-R pathway blockers (AG1478 and PD168393).

CONCLUSION

The observed reduction of sperm motility by tryptase through the PAR-2 receptor involves EGF-R pathways.

The role of calcium in protease-activated receptor-induced secretion of chemokine GRO/CINC-1 in rat brain astrocytes

Our recent data showed that activation of protease-activated receptor (PAR)-1 and PAR-2 in rat astrocytes not only evokes calcium signaling, but also regulates the release of the chemokine growth-regulated oncogene/cytokine-induced neutrophil chemoattractant-1 (GRO/CINC-1), a counterpart of the human GRO. This chemokine provides a feedback to protect astrocytes from toxic insults. Activated PAR-1 and PAR-2 were strong stimuli to induce the release of GRO/CINC-1. The effect was comparable to that induced by TNF-alpha. However, the role of calcium in the PAR-induced GRO/CINC-1 secretion remains unknown. Here, we found that the pharmacological blockade of either calcium release from the intracellular stores, or influx from the extracellular space, increased PAR-1- and PAR-2-induced GRO/CINC-1 secretion. Under calcium-free conditions, the basal mRNA level of GRO/CINC-1 was clearly increased. Further studies revealed that the intracellular GRO/CINC-1 protein level was slightly increased by treatment with thrombin or TRag in calcium-free conditions. However, the amount of protein synthesized was largely reduced in the absence of extracellular calcium as compared to that under normal calcium conditions. Importantly, we found that the intracellularly formed GRO/CINC-1 was not secreted into the cell culture supernatant under calcium-free conditions. These data suggest a dual role of calcium. On the one side, an increase in cytosolic calcium negatively regulates PAR-induced GRO/CINC-1 gene expression in rat astrocytes, but on the other side, the basal level of calcium is the pre-requisite for GRO/CINC-1 protein synthesis and secretion.

Indirect activation of the epithelial Na+ channel by trypsin

We tested the hypothesis that the serine protease trypsin can indirectly activate the epithelial Na(+) channel (ENaC). Experiments were carried out in Xenopus oocytes and examined the effects on the channel formed by all three human ENaC subunits and that formed by Xenopus epsilon and human beta and gamma subunits (epsilonbetagammaENaC). Low levels of trypsin (1-10 ng/ml) were without effects on the oocyte endogenous conductances and were specifically used to test the effects on ENaC. Addition of 1 ng/ml trypsin for 60 min stimulated the amiloride-sensitive human ENaC conductance (g(Na)) by approximately 6-fold. This effect on the g(Na) was [Na(+)]-independent, thereby ruling out an interaction with channel feedback inhibition by Na(+). The indirect nature of this activation was confirmed in cell-attached patch clamp experiments with trypsin added to the outside of the pipette. Trypsin was comparatively ineffective at activating epsilonbetagammaENaC, a channel that exhibited a high spontaneous open probability. These observations, in combination with surface binding experiments, indicated that trypsin indirectly activated membrane-resident channels. Activation by trypsin was also dependent on catalytic activity of this protease but was not accompanied by channel subunit proteolysis. Channel activation was dependent on downstream activation of G-proteins and was blocked by G-protein inhibition by injection of guanyl-5'-yl thiophosphate and by pre-stimulation of phospholipase C. These data indicate a receptor-mediated activation of ENaC by trypsin. This trypsin-activated receptor is distinct from that of protease-activated receptor-2, because the response to trypsin was unaffected by protease-activated receptor-2 overexpression or knockdown.

Activation of proteinase-activated receptor-2 by human kallikrein-related peptidases

Proteinase-activated receptor-2 (PAR2) is a seven transmembrane spanning, G-protein-coupled receptor, present on the membrane of many cell types including keratinocytes. In skin, PAR2 is suggested to play a regulatory role during inflammation, epidermal barrier function, and pruritus. PAR2 is activated by trypsin-like proteases by a unique mechanism where cleavage of the receptor leads to the release of a small peptide, which activates the receptor as a tethered ligand. The endogenous activators of PAR2 on keratinocytes have not been identified as of yet. Potential candidates are kallikrein-related peptidases (KLKs) expressed by epidermal cells. Therefore, the ability of four human skin-derived KLKs was examined with regard to their capacity to activate PAR2 in vitro. PAR2 cleavage was followed by immunofluorescence analysis and functional activation by measurements of changes in intracellular calcium levels. We found that KLK5 and KLK14, but neither KLK7 nor KLK8, induced PAR2 signalling. We conclude that certain, but not all, epidermal KLKs are capable of activating PAR2. We could also show the coexpression of KLK14 and PAR2 receptor in inflammatory skin disorders. These in vitro results suggest that KLKs may take part in PAR2 activation in the epidermis and thereby in PAR2-mediated inflammatory responses, including epidermal barrier repair and pruritus. The role of KLKs in PAR2 activation in vivo remains to be elucidated.

The regulation of permeability barrier homeostasis

Disruption of the permeability barrier stimulates a repair response that leads to the restoration of barrier function. Previous studies demonstrated that changes in ions, particularly calcium, and cytokines are positive signals, whereas serine protease activation of proteinase-activated receptor 2 is a negative signal regulating barrier recovery. Ikeyama and colleagues provide data that the nitric oxide signaling pathway also regulates barrier homeostasis.

Protease-activated receptor-2 protects against pancreatitis by stimulating exocrine secretion

BACKGROUND

Protease-activated receptor-2 (PAR-2) is present in the pancreas, where it has been shown to play a protective role during pancreatitis. However, the mechanism by which it protects against pancreatitis still remains to be elucidated. Acute pancreatitis is associated with premature zymogen activation and a blockage in digestive enzyme secretion.

AIM

To examine the effects of PAR-2 activation on the severity of pancreatitis, and to determine whether its protective effects are mediated by affecting either premature activation or secretory blockage, or both.

RESULTS

The results confirmed that PAR-2 -/- mice have more severe pancreatitis than wild-type mice. Interestingly, intrapancreatic trypsin levels in the PAR-2 knockouts remained high after 6 h of pancreatitis, whereas they reverted to normal in the wild types. During pancreatitis, PAR-2 mRNA levels were upregulated in wild-type mice in response to supramaximal caerulein administration. Further, after a single injection of supramaximal caerulein, PAR-2 mRNA levels were also elevated, reaching a peak at 3 h. Stimulating PAR-2 with trypsin or the PAR-2-activating peptide, serine-leucine-isoleucine-glycine-arginine-leucine (SLIGRL), induced significantly more secretion from the acini of these caerulein-sensitised mice compared with the controls. PAR-2 activation also reversed the inhibition of secretion observed in both the caerulein and arginine models.

CONCLUSIONS

Trypsin released during the early stages of pancreatitis activates PAR-2 receptors on the acinar cells and stimulates secretion from these cells. Thus, PAR-2 activation may decrease pancreatic injury and limit the severity of pancreatitis by allowing extracellular trypsin to act as a secretagogue.

Pancreatic protease-activated receptors: friend and foe

PAR2 activation may protect the acinar cell by enhancing secretion, but may still leave you in pain

Protease-activated receptor 2 mediates human beta-defensin 2 and CC chemokine ligand 20 mRNA expression in response to proteases secreted by Porphyromonas gingivalis

The oral pathogen Porphyromonas gingivalis secretes proteases such as Arg-gingipain B (RgpB) that activate protease-activated receptors (PARs). Human beta-defensins (hBDs) and the macrophage inflammatory protein 3alpha/CC chemokine ligand 20 (CCL20) produced by epithelial cells are antimicrobial peptides that provide cytokine function and play an important role in innate immunity. The aim of the present study was to determine whether specific members of the PAR family mediate the expression of these innate immunity markers in gingival epithelial cells (GECs) when exposed to P. gingivalis cell-free culture supernatant or purified RgpB. hBD-2 mRNA in GECs was induced in response to supernatant and purified RgpB from P. gingivalis (P = 0.02 and P = 0.016, respectively). This effect was abrogated by the protease inhibitor tosyl-l-lysine chloromethyl ketone (TLCK) (P < 0.05). In response to P. gingivalis supernatant and to purified RgpB, the hBD-2 mRNA expression was significantly decreased in PAR-2 gene knockdown cells, whereas no change was detected in PAR-1 gene knockdown cells. CCL20 mRNA expression also increased in response to the supernatant of P. gingivalis, and this effect was blocked by the protease inhibitor, TLCK (P = 0.05 and P = 0.024, respectively), and was blocked in PAR-2 gene knockdown cells. Our data indicate that hBD-2 and CCL20 mRNA up-regulation by P. gingivalis supernatant and purified RgpB was mediated via PAR-2, but not via PAR-1, and that proteases play a role in the regulation of innate immune responses in GECs. GECs use PARs to recognize P. gingivalis and mediate cell responses involved in innate immunity.

Role of capsaicin-sensitive sensory nerves in protease-activated receptor-2-mediated contraction of rat urinary bladder

The capsaicin receptor, transient receptor potential vanilloid receptor 1 (TRPV1), colocalizes with protease-activated receptors (PARs) in the urinary bladder, but functional interaction between TRPV1 and PARs remains to be clarified. In the present study, the role of capsaicin-sensitive sensory neurons in contraction induced by activation of PAR-2 was investigated in the rat urinary bladders. The contractile responses to trypsin and the PAR-2 agonist 2-furoyl-LIGRL-NH(2) were significantly attenuated by either desensitization of sensory neurons achieved by capsaicin (10 microM) or by the TRPV1 antagonist capsazepine (30 microM). These results suggest that stimulation of capsaicin-sensitive sensory fibers contributes, at least in part, to PAR-2-mediated bladder contractions in rats.

Mold allergen, pen C 13, induces IL-8 expression in human airway epithelial cells by activating protease-activated receptor 1 and 2

Allergenic serine proteases are important in the pathogenesis of asthma. One of these, Pen c 13, is the immunodominant allergen produced by Penicillium citrinum. Many serine proteases induce cytokine expression, but whether Pen c 13 does so in human respiratory epithelial cells is not known. In this study, we investigated whether Pen c 13 caused IL-8 release and activated protease-activated receptors (PARs) in airway epithelial cells. In airway-derived A549 cells and normal human airway epithelial cells, Pen c 13 induced IL-8 release in a dose-dependent manner. Pen c 13 also increased IL-8 release in a time-dependent manner in A549 cells. Pen c 13 cleaved PAR-1 and PAR-2 at their activation sites. Treatment with Pen c 13 induced intracellular Ca(2+) mobilization and desensitized the cells to the action of other proteases and PAR-1 and PAR-2 agonists. Moreover, Pen c 13-mediated IL-8 release was significantly decreased in Ca(2+)-free medium and was abolished by the protease inhibitors, PMSF and 4-(2-aminoethyl) benzenesulfonyl fluoride. Blocking Abs against the cleavage sites of PAR-1 and PAR-2, but not of PAR-4, inhibited Pen c 13-induced IL-8 production, as did inhibition of phospholipase C. Pen c 13 induced IL-8 expression via activation of ERK 1/2, and not of p38 and JNK. In addition, treatment of A549 cells or normal human airway epithelial cells with Pen c 13 increased phosphorylation of ERK 1/2 by a Ca(2+)-dependent pathway. These finding show that Pen c 13 induces IL-8 release in airway epithelial cells and that this is dependent on PAR-1 and PAR-2 activation and intracellular calcium.

Proteinase-activated receptor-2 (PAR2): a tumor suppressor in skin carcinogenesis

The proteinase-activated receptor PAR(2) has been demonstrated to modulate tumor growth, invasion and metastasis in various tissues. However, the role of PAR(2) in cutaneous cancerogenesis is still unknown. Here we could show a protective role of PAR(2) in the development of epidermal skin tumors: we established a mouse skin tumor model using chemically induced carcinogenesis. Tumors started to appear after eight weeks. After 13 weeks, PAR(2)-deficient mice showed a significantly increased number of skin tumors (14 per animal on the average) in contrast to the wild type (eight tumors per mouse). Analysis of possible signal transduction pathways activated upon PAR(2) stimulation in HaCaT keratinocytes showed an involvement of extracellular signal-regulated kinase 1/2 and profound epidermal growth factor receptor transactivation, leading to secretion of the tumor-suppressing factor transforming growth factor-beta1. Thus, our results provide early experimental evidence for a tumor-protective role of PAR(2).

Role of proteinase-activated receptor-2 on cyclooxygenase-2 expression in H. pylori-infected gastric epithelial cells

Proteinase-activated receptor-2 (PAR-2) belongs to a novel subfamily of G protein-coupled receptors with seven-transmembrane domains. PAR-2 is activated by serine proteases, such as trypsin, mast cell tryptase, and allergic or bacterial proteases. The presence of trypsin has been shown in human stomach. Cyclooxygenase-2 (COX-2) is induced by inflammatory cytokines, growth factors, gastrin, and reactive oxygen species in gastric epithelial cells, which may lead to mutagenesis and subsequent metaplasia, dysplasia, and cancer formation. We investigated whether PAR-2 is activated in H. pylori (HP99)-infected cells, which is related to COX-2 induction in gastric epithelial cells. After treatment of H. pylori to AGS (gastric adenocarcinoma) cells at a bacteria/cell ratio of 100:1, we determine the expression and the activation of PAR-2 and the expression of COX-2. The same experiments were performed in the cells treated with PAR-2 agonist peptide. mRNA and protein expression of PAR-2 and COX-2 were determined by reverse transcriptase-polymerase chain reaction (RT-PCR) and Western blotting. PAR-2 activation was assessed by increase in intracellular calcium level. As a result, H. pylori induced the activation and expression of PAR-2 as well as COX-2 expression. PAR-2 agonist peptide augmented H. pylori-induced COX-2 expression in AGS cells. H. pylori induces COX-2 expression, which is mediated by both activation and expression of PAR-2 in gastric epithelial cells.

Proteinase-activated receptors 1 and 2 in rat olfactory system: layer-specific regulation of multiple signaling pathways in the main olfactory bulb and induction of neurite retraction in olfactory sensory neurons

Proteinase-activated receptors (PARs) are a family of four G protein-coupled receptors that are widely distributed in the CNS and involved in neural cell proliferation, differentiation and survival. The olfactory system undergoes continuous neurogenesis throughout life and may represent a critical target of PAR cellular actions. In the present study we investigated the functional activity of PAR1 and PAR2 in microdissected tissue preparations of olfactory nerve-glomerular layer (ON-GL), external plexiform layer (EPL) and granule cell layer (GRL) of the rat main olfactory bulb and in primary cultures of olfactory neuroepithelial cells. Activation of either PAR1 or PAR2 regulated multiple signaling pathways, including activation of pertussis-toxin sensitive Gi/o proteins, inhibition of cyclic AMP formation, stimulation of Gq/11-mediated phosphoinositide (PI) hydrolysis, phosphorylation of Ca2+/calmodulin-dependent protein kinase II and activation of the monomeric G protein Rho, predominantly in ON-GL, whereas only activation of Rho was detected in the deeper layers. Olfactory nerve lesion by nasal irrigation with ZnSO4 induced a marked decrease of PAR signaling in ON-GL. In primary cultures of olfactory neurons, double immunofluorescence analysis showed the localization of PAR1 and PAR2 in cells positive for olfactory-marker protein and neuron-specific enolase. Cell exposure to either nanomolar concentrations of thrombin and trypsin or PAR-activating peptides caused rapid neurite retraction. This study provides the first characterization of the laminar distribution of PAR1 and PAR2 signaling in rat olfactory bulb, demonstrates the presence of the receptors in olfactory sensory neurons and suggests a role of PARs in olfactory sensory neuron neuritogenesis.

Tissue factor trafficking in fibroblasts: involvement of protease-activated receptor-mediated cell signaling

Tissue factor (TF) is the cellular receptor for clotting factor VIIa (FVIIa), and the formation of TF-FVIIa complexes on cell surfaces triggers the activation of the coagulation cascade and the cell signaling. Our recent studies have shown that a majority of TF resides in various intracellular compartments, predominantly in the Golgi, and that FVIIa binding to cell surface TF induces TF endocytosis and mobilizes the Golgi TF pool to translocate it to the cell surface. This present study is aimed to elucidate the mechanisms involved in TF endocytosis and its mobilization from the Golgi. Activation of protease-activated receptor 1 (PAR1) and PAR2 by specific peptide agonists and proteases, independent of FVIIa, mobilized TF from the Golgi store and increased the cell surface expression of TF. Blocking PAR2 activation, but not PAR1, with neutralizing antibodies fully attenuated the FVIIa-induced TF mobilization. Consistent with these data, silencing the PAR2 receptor, and not PAR1, abrogated the FVIIa-mediated TF mobilization. In contrast to their effect on TF mobilization, PAR1 and PAR2 activation, in the absence of FVIIa, had no effect on TF endocytosis. However, PAR2 activation is found to be critical for the FVIIa-induced TF endocytosis. Overall the data herein provide novel insights into the role of PARs in regulating cell surface TF expression.

The emergence of proteinase-activated receptor-2 as a novel target for the treatment of inflammation-related CNS disorders

The signalling molecules that are involved in inflammatory pathways are now thought to play a part in many disorders of the central nervous system (CNS). In common with peripheral chronic inflammatory diseases such a rheumatoid arthritis and ulcerative colitis, evidence now exists for the involvement of inflammatory cytokines, for example tumour necrosis factor (TNF) and interleukins (IL), in neurological disorders. A common factor observed with the up-regulation of these cytokines in peripheral inflammatory diseases, is the increased expression of the proteinase-activated receptor (PAR) subtype PAR-2. Indeed, recent evidence suggests that targeting PAR-2 helps reduce joint swelling observed in animal models of arthritis. So could targeting this receptor prove to be useful in treating those CNS disorders where inflammatory processes are thought to play an intrinsic role? The aim of this review is to summarize the emerging data regarding the role of PAR-2 in neuroinflammation and ischaemic injury and discuss its potential as an exciting new target for the prevention and/or treatment of CNS disorders.

Persistence of PAR-2 vasodilation despite endothelial dysfunction in BPH/2 hypertensive mice

This study investigated relaxation of vascular smooth muscle by acetylcholine, bradykinin and protease-activated receptor 2 (PAR-2) to characterise endothelial dysfunction in spontaneously hypertensive mice (BPH/2). We hypothesised that PAR-2 induced vasodilation would be preserved in BPH/2 despite the presence of hypertension and impaired vasodilator responses to acetylcholine and bradykinin. Mean arterial blood pressure (MAP), heart rate and locomotor activity were assessed in conscious mice over 24-h periods by radiotelemetry. Relaxation responses of small mesenteric arteries to acetylcholine, bradykinin and the PAR-2 agonist, 2-furoyl-LIGRLO-amide (2fly), were assessed using wire myographs. MAP and heart rate of BPH/2 were 15 and 18%, respectively, higher than in controls (BPN/3). BPH/2 also exhibited increased locomotor activity. Maximal relaxations of arteries by acetylcholine and bradykinin in BPH/2 were reduced by 25-50% relative to BPN/3. In contrast, relaxation responses to 2fly were only slightly (6%), albeit significantly, reduced. Sodium nitroprusside-induced relaxations were not different between strains. Treatment of BPH/2 arteries with inhibitors of calcium-activated K(+) channels was sufficient to block persistent 2fly- and residual ACh- and bradykinin-induced relaxations, whereas NO synthase inhibitor failed to inhibit these relaxations. In BPH/2 mice, vascular smooth muscle relaxation by PAR-2 is well preserved despite the presence of hypertension and impaired vasodilation responses to acetylcholine and bradykinin.

Mechanisms of protease-activated receptor 2-evoked hyperexcitability of nociceptive neurons innervating the mouse colon

Agonists of protease-activated receptor 2 (PAR(2)) evoke hyperexcitability of dorsal root ganglia (DRG) neurons by unknown mechanisms. We examined the cellular mechanisms underlying PAR(2)-evoked hyperexcitability of mouse colonic DRG neurons to determine their potential role in pain syndromes such as visceral hyperalgesia. Colonic DRG neurons were identified by injecting Fast Blue and DiI retrograde tracers into the mouse colon. Using immunofluorescence, we found that DiI-labelled neurons contained PAR(2) immunoreactivity, confirming the presence of receptors on colonic neurons. Whole-cell current-clamp recordings of acutely dissociated neurons demonstrated that PAR(2) activation with a brief application (3 min) of PAR(2) agonists, SLIGRL-NH(2) and trypsin, evoked sustained depolarizations (up to 60 min) which were associated with increased input resistance and a marked reduction in rheobase (50% at 30 min). In voltage clamp, SLIGRL-NH(2) markedly suppressed delayed rectifier I(K) currents (55% at 10 min), but had no effect on the transient I(A) current or TTX-resistant Na(+) currents. In whole-cell current-clamp recordings, the sustained excitability evoked by PAR(2) activation was blocked by the PKC inhibitor, calphostin, and the ERK(1/2) inhibitor PD98059. Studies of ERK(1/2) phosphorylation using confocal microscopy demonstrated that SLIGRL-NH(2) increased levels of immunoreactive pERK(1/2) in DRG neurons, particularly in proximity to the plasma membrane. Thus, activation of PAR(2) receptors on colonic nociceptive neurons causes sustained hyperexcitability that is related, at least in part, to suppression of delayed rectifier I(K) currents. Both PKC and ERK(1/2) mediate the PAR(2)-induced hyperexcitability. These studies describe a novel mechanism of sensitization of colonic nociceptive neurons that may be implicated in conditions of visceral hyperalgesia such as irritable bowel syndrome.

Acute pancreatitis: proteinase-activated receptor-2 as Dr. Jekyll and Mr. Hyde

"Proteinase-activated" receptor-2 (PAR-2) is a G protein-coupled transmembrane receptor with seven transmembrane domains activated by trypsin. It has been shown in the pancreatic tissue that PAR-2 is involved in duct/acinary cells secretion, arterial tonus regulation and capillary liquid content turnover under physiological conditions. These above mentioned structures play an important role during the development of acute pancreatitis and are profoundly influenced by a high concentration of trypsin enzyme after its secretion into the interstitial tissue from the basolateral aspect of acinar cells. Among the other factors, it is the increase of interstitial trypsin concentration followed rapidly by PAR-2 action on pancreatic vascular smooth muscle cells that initiates ischemic changes in pancreatic parenchyma and that finally leads to necrosis of the pancreas. Consequent reperfusion perpetuates changes leading to the acute pancreatitis development. On the contrary, PAR-2 action on both exocrine and duct structures seems to play locally a protective role during acute pancreatitis development. Moreover, PAR-2 action is not confined to the pancreas but it contributes to the systemic vascular endothelium and immune cell activation that triggers the systemic inflammatory response syndrome (SIRS) contributing to an early high mortality rate in severe disease.

Potential physiological and pathophysiological roles for protease-activated receptor-2 in the kidney (Review Article)

The protease-activated receptor-2 (PAR-2), the second of four members of a unique subfamily of G-protein coupled receptors, is abundantly expressed in the kidney. In a similar manner to other PAR cleavage of its extracellular N-terminus exposes a tethered ligand, SLIGKV in humans, which acts as an intramolecular ligand to activate itself. In the kidney, PAR-2 expression has been variably reported in collecting duct cells, mesangial cells, interstitial fibroblasts, vascular endothelial cells, vascular smooth muscle cells and proximal tubular cells. Despite this renal expression data, the function of PAR-2 in the kidney remains unknown. More than 15 different mammalian serine proteases have been shown to activate PAR-2 in an in vitro setting, but it is still unclear which of these are physiologically relevant activators of PAR-2 in specific tissues. Their identification could provide novel therapeutic targets. PAR-2 activates a number of down-stream signalling molecules that include protein kinase C, extracellular signal regulated kinase and nuclear factor kappa-B. Proteases that can activate PAR-2 are generated and released from cells during injury, inflammation and malignancy and can thus signal to cells under these conditions. Potential physiological and pathophysiological roles for PAR-2 in the kidney include the regulation of inflammation, blood flow, and ion transport and tissue protection, repair and fibrosis. In this review the potential roles of PAR-2 in the kidney are highlighted and discussed.

Protease-activated receptor-1 (PAR1) and PAR2 but not PAR4 mediate relaxations in lower esophageal sphincter

Protease-activated receptor-1 (PAR1), PAR2 and PAR4 activation can alter the gastrointestinal motility. To investigate effects mediated by PARs in the lower esophageal sphincter, we measured contraction or relaxation of transverse strips from the guinea-pig lower esophageal sphincter caused by PAR1 (TFLLR-NH2 and SFLLRN-NH2), PAR2 (SLIGKV-NH2 and SLIGRL-NH2) and PAR4 peptide agonists (GYPGKF-NH2, GYPGQV-NH2 and AYPGKF-NH2) as well as PAR protease activators (thrombin and trypsin). In resting lower esophageal sphincter strips, TFLLR-NH2 and SFLLRN-NH2 caused moderate concentration-dependent relaxation whereas thrombin did not cause any relaxation or contraction. Furthermore, in carbachol-contracted strips, TFLLR-NH2 and SFLLRN-NH2 caused marked whereas thrombin caused mild concentration-dependent relaxation. These indicate the existence of PAR1 mediating relaxation. Similarly, in resting lower esophageal sphincter strips, trypsin caused moderate concentration-dependent relaxation whereas SLIGRL-NH2 and SLIGKV-NH2 did not cause any relaxation or contraction. In addition, in carbachol-contracted strips, trypsin caused marked whereas SLIGRL-NH2 and SLIGKV-NH2 caused mild concentration-dependent relaxation. These indicate the existence of PAR2 mediating relaxation. The relaxant response of thrombin, TFLLR-NH2, trypsin and SLIGKV-NH2 was insensitive to atropine or tetrodotoxin, suggesting a direct effect. The relaxant response of trypsin was not affected by apamin, charybdotoxin, indomethacin and capsaicin but was attenuated by NG-nitro-L-arginine methyl ester, indicating involvement of NO. FSLLR-NH2, a PAR1 control peptide, and VKGILS-NH2, a PAR2 control peptide, as well as all three PAR4 peptide agonists, GYPGKF-NH2, GYPGQV-NH2 and AYPGKF-NH2, did not cause any relaxation or contraction. Taken together, these results demonstrate that PAR1 and PAR2 but not PAR4 mediate relaxations in the guinea-pig lower esophageal sphincter.

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.

Up-Regulated PAR-2-Mediated Salivary Secretion in Mice Deficient in Muscarinic Acetylcholine Receptor Subtypes

Protease-activated receptor-2 (PAR-2) is expressed in the salivary glands and is expected to be a new target for the treatment of exocrine dysfunctions, such as dry mouth; however, the salivary secretory mechanism mediated by PAR-2 remains to be elucidated. Therefore, mechanism of the PAR-2-mediated salivary secretion was investigated in this study. We found that a PAR-2 agonist peptide, SLIGRL-OH, induced salivary flow in vivo and dose-dependent increase in [Ca(2+)](i) submandibular gland (SMG) acinar cells in wild-type (WT) mice and mice lacking M(3) or both M(1) and M(3) muscarinic acetylcholine receptors (mAChRs), whereas secretions in PAR-2 knockout (PAR-2KO) mice were completely abolished. The saliva composition secreted by SLIGRL-OH was similar to that secreted by mAChR stimulation. Ca(2+) imaging in WT acinar cells and beta-galactosidase staining in PAR-2KO mice, in which the beta-galactosidase gene (LacZ) was incorporated into the disrupted gene, revealed a nonubiquitous, sporadic distribution of PAR-2 in the SMG. Furthermore, compared with the secretion in WT mice, PAR-2-mediated salivary secretion and Ca(2+) response were enhanced in mice lacking M(3) or both M(1) and M(3) mAChRs, in which mAChR-stimulated secretion and Ca(2+) response in acinar cells were severely impaired. Although the mechanism underlying the enhanced PAR-2-mediated salivary secretion in M(3)-deficient mice is not clear, the result suggests the presence of some compensatory mechanism involving PAR-2 in the salivary glands deficient in cholinergic activation. These results indicate that PAR-2 present in the salivary glands mediates Ca(2+)-dependent fluid secretion, demonstrating potential usefulness of PAR-2 as a target for dry mouth treatment.

Agonist of proteinase-activated receptor 2 increases painful behavior produced by alpha, beta-methylene adenosine 5'-triphosphate

Proteinase-activated receptor (PAR) 2 is expressed in a subset of primary afferent neurons and is involved in inflammatory nociception. The P2X3 ion channel is localized on nociceptors of sensory neurons. Using immunohistochemistry, we showed that many P2X3s are co-expressed with the PAR2 in rat dorsal root ganglia neurons. Nocifensive behavior induced by alphabeta-methylene adenosine 5'-triphosphate (ATP) injection to the hind paw was significantly augmented after the application of PAR2 agonists. Fos expression induced by the alphabeta-methylene ATP injection in dorsal horn neurons was also increased after the pre-application of PAR2 agonists. These findings indicate that PAR2 agonists may potentiate the sensitivity of P2X3 ion channel to noxious stimuli, and the interaction between PAR2 and P2X3 may be an important mechanism underlying inflammatory pain.

Direct Vascular Effects of Protease-Activated Receptor Type 1 Agonism In Vivo in Humans

Background— Protease-activated receptor type 1 (PAR-1) has been proposed as the principal thrombin receptor in humans, although its actions in vivo have not been defined. The aim of the present study was to determine the direct vascular actions of PAR-1 agonism in humans.

Methods and Results— Dorsal hand vein diameter was measured by the Aellig technique in 14 healthy volunteers during local intravenous SFLLRN (PAR-1 agonist; 0.05 to 15 nmol/min) and SLIGKV (PAR-2 agonist; 1.6 to 160 nmol/min) infusions. The venous effects of SFLLRN were further assessed in the presence or absence of norepinephrine or the glycoprotein IIb/IIIa antagonist tirofiban. Forearm blood flow was measured by venous occlusion plethysmography in 16 volunteers during infusion of SFLLRN (1 to 50 nmol/min), SLIGKV (160 to 800 nmol/min), and the endothelium-dependent vasodilator bradykinin (100 to 1000 pmol/min). Platelet-monocyte binding (a sensitive measure of platelet activation) and plasma tissue plasminogen activator (tPA), plasminogen-activator inhibitor 1, and von Willebrand factor concentrations were measured at intervals throughout the study. SFLLRN caused dose-dependent venoconstriction ( P <0.001) that was unaffected by norepinephrine or tirofiban co-infusion. In forearm resistance vessels, SFLLRN increased forearm blood flow ( P <0.001), tPA release ( P <0.001), and platelet-monocyte binding ( P <0.0001) without affecting plasma plasminogen-activator inhibitor 1 or von Willebrand factor concentrations. SLIGKV caused venous ( P <0.001) and arterial ( P <0.01) dilatation without tPA release.

Conclusions— We have demonstrated that PAR-1 agonism causes platelet activation, venous constriction, arterial dilatation, and tPA release in vivo in humans. These unique and contrasting effects provide important insights into the physiological and pathophysiological role of thrombin in the human venous and arterial circulations.

[Roles of protease-activated receptor-2 (PAR-2), a G protein-coupled receptor, in modulation of exocrine gland functions]

Protease-activated receptor-2 (PAR-2), a G protein-coupled receptor, is activated by proteolytic unmasking of the N-terminal extracellular tethered ligand that presumably binds to the extracellular loop 2 of the receptor itself. PAR-2 is widely distributed in the mammalian body and plays various roles in biological events in the cardiovascular, respiratory, alimentary, and central neurons systems. PAR-2-activating peptides administered systemically to mice and rats trigger prompt salivation in vivo. In an in vitro study, PAR-2 agonists including the endogenous PAR-2 activator trypsin induce secretion of amylase and mucin from isolated rat parotid glands and sublingual glands, respectively. PAR-2-activating peptides administered systemically also modulate pancreatic exocrine secretion in vivo as well as in vitro. In the gastric mucosa, PAR-2 stimulation enhances secretion of mucus and pepsinogen and suppresses acid secretion. Tear secretion can also be caused by PAR-2-related peptides in PAR-2-dependent and -independent manners. PAR-2 thus plays a general or key role in the regulation of exocrine secretion. This review focuses on the physiologic and/or pathophysiologic roles of PAR-2 in glandular exocrine secretion. The possibility of PAR-2 as a target for drug development is also discussed.

Constitutive expression of functionally active protease-activated receptors 1 and 2 in human conjunctival epithelial cells

Protease-activated receptors (PARs) are G-protein-coupled receptors which initiate inflammatory responses when activated by specific serine proteases. This study was conducted to examine whether human conjunctival epithelial cells (HCECs) express functionally active PAR1 and PAR2 using Chang conjunctival epithelial cells as in vitro model. We performed RT-PCR and immunofluorescence analyses to determine the expression of PAR1 and PAR2, and monitored the production of IL-6 after activating HCECs with PAR1 activating agents (thrombin or TFLLRN) or PAR2 activating agents (tryptase, trypsin, or SLIGKV). The results show that HCECs constitutively express PAR1 and PAR2 mRNA and proteins, and produce significant amounts of IL-6 when incubated with specific PAR-activating enzymes or agonist peptides. Thrombin- and tryptase-induced HCEC activation was blocked by PAR1 and PAR2 neutralizing antibodies, respectively, and by specific enzyme inhibitors. The constitutive expression of PAR1 and PAR2, and their activation by thrombin and tryptase, respectively, may have important implications in ocular inflammation.

Activation of mouse protease-activated receptor-2 induces lymphocyte adhesion and generation of reactive oxygen species

BACKGROUND AND PURPOSE

Protease-activated receptor-2 (PAR-2) is expressed on lymphocytes and endothelial cells, and plays a significant role in inflammatory reactions. Since leukocyte-endothelial cell interaction and reactive oxygen species (ROS) generation are hallmarks of the development of inflammation, the effects of PAR-2 activation by trypsin on lymphocyte adhesion and ROS generation was examined utilising PAR-2 wild type and knockout (PAR-2-/-) mice.

EXPERIMENTAL APPROACH

Lymphocyte adhesion to the luminal surface of mouse isolated aortae was measured using 51Cr-labelled leukocytes and ROS generation from isolated lymphocytes was quantified using chemiluminescence.

KEY RESULTS

Trypsin induced adhesion of lymphocytes when added exogenously to the endothelial surface of the aorta for 30 min. Similarly, increased lymphocyte adhesion was also observed when mice were injected with trypsin intravenously 24 h prior to the adhesion assay, an effect which was partly ICAM-1 mediated. Trypsin also increased ROS generation from isolated mouse lymphocytes in a dose-dependent manner. The increase in lymphocyte adhesion and ROS production in response to trypsin were abolished in PAR-2-/- mice indicating a PAR-2 dependent mechanism. Superoxide dismutase had a greater inhibitory effect in PAR-2-/- mice compared to wild type mice when lymphocytes were stimulated with PMA but not trypsin.

CONCLUSIONS AND IMPLICATIONS

The present study indicates that activation of PAR-2 may be an important factor in modulating lymphocyte adhesion and ROS generation. The results have implications for developing anti-inflammatory strategies.

Protease-activated receptor 2 sensitizes TRPV1 by protein kinase Cepsilon- and A-dependent mechanisms in rats and mice

Proteases that are released during inflammation and injury cleave protease-activated receptor 2 (PAR2) on primary afferent neurons to cause neurogenic inflammation and hyperalgesia. PAR2-induced thermal hyperalgesia depends on sensitization of transient receptor potential vanilloid receptor 1 (TRPV1), which is gated by capsaicin, protons and noxious heat. However, the signalling mechanisms by which PAR2 sensitizes TRPV1 are not fully characterized. Using immunofluorescence and confocal microscopy, we observed that PAR2 was colocalized with protein kinase (PK) Cepsilon and PKA in a subset of dorsal root ganglia neurons in rats, and that PAR2 agonists promoted translocation of PKCepsilon and PKA catalytic subunits from the cytosol to the plasma membrane of cultured neurons and HEK 293 cells. Subcellular fractionation and Western blotting confirmed this redistribution of kinases, which is indicative of activation. Although PAR2 couples to phospholipase Cbeta, leading to stimulation of PKC, we also observed that PAR2 agonists increased cAMP generation in neurons and HEK 293 cells, which would activate PKA. PAR2 agonists enhanced capsaicin-stimulated increases in [Ca2+]i and whole-cell currents in HEK 293 cells, indicating TRPV1 sensitization. The combined intraplantar injection of non-algesic doses of PAR2 agonist and capsaicin decreased the latency of paw withdrawal to radiant heat in mice, indicative of thermal hyperalgesia. Antagonists of PKCepsilon and PKA prevented sensitization of TRPV1 Ca2+ signals and currents in HEK 293 cells, and suppressed thermal hyperalgesia in mice. Thus, PAR2 activates PKCepsilon and PKA in sensory neurons, and thereby sensitizes TRPV1 to cause thermal hyperalgesia. These mechanisms may underlie inflammatory pain, where multiple proteases are generated and released.