Protease-activated receptor-2-mediated inhibition of ion transport in human bronchial epithelial cells

House dust mite allergens induce Ca2+ signalling and alarmin responses in asthma airway epithelial cells

Type 2 inflammation in asthma develops with exposure to stimuli to include inhaled allergens from house dust mites (HDM). Features include mucus hypersecretion and the formation of pro-secretory ion transport characterised by elevated basal Cl- current. Studies using human sinonasal epithelial cells treated with HDM extract report a higher protease activated receptor-2 (PAR-2) agonist-induced calcium mobilisation that may be related to airway sensitisation by allergen-associated proteases. Herein, this study aimed to investigate the effect of HDM on Ca2+ signalling and inflammatory responses in asthmatic airway epithelial cells. Primary bronchial epithelial cells (hPBECs) from asthma donors cultured at air-liquid interface were used to assess electrophysiological, Ca2+ signalling and inflammatory responses. Differences were observed regarding Ca2+ signalling in response to PAR-2 agonist 2-Furoyl-LIGRLO-amide (2-FLI), and equivalent short-circuit current (Ieq) in response to trypsin and 2-FLI, in ALI-asthma and healthy hPBECs. HDM treatment led to increased levels of intracellular cations (Ca2+, Na+) and significantly reduced the 2-FLI-induced change of Ieq in asthma cells. Apical HDM-induced Ca2+ mobilisation was found to mainly involve the activation of PAR-2 and PAR-4-associated store-operated Ca2+ influx and TRPV1. In contrast, PAR-2, PAR-4 antagonists and TRPV1 antagonist only showed slight impact on basolateral HDM-induced Ca2+ mobilisation. HDM trypsin-like serine proteases were the main components leading to non-amiloride sensitive Ieq and also increased interleukin-33 (IL-33) and thymic stromal lymphopoietin (TSLP) from asthma hPBECs. These studies add further insight into the complex mechanisms associated with HDM-induced alterations in cell signalling and their relevance to pathological changes within asthma.

Protease-Activated Receptor-2 Decreased Zonula Occlidens-1 and Claudin-1 Expression and Induced Epithelial Barrier Dysfunction in Allergic Rhinitis

BACKGROUND

Protease-activated receptor-2 (PAR-2)-modulated tight junctions (TJs) have been suggested to be involved in the pathogenesis of chronic inflammatory diseases. However, immunopathogenesis remains to be investigated among patients with allergic rhinitis (AR).

OBJECTIVE

This study sought to investigate the role of PAR-2 in the modulation of epithelial barrier function and the expression of TJs in the nasal mucosa of AR patients.

METHODS

The expression of TJs and PAR-2 of the nasal mucosa in AR patients and control subjects by immunohistochemistry, quantitative real-time polymerase chain reaction (qRT-PCR), and western blotting. In vitro, Primary human nasal epithelial cells (pHNECs) of AR patients were stimulated by Der p1 to analyze the correlation between PAR-2 and TJs expression. Der p1-induced pHNECs were treated with the PAR-2 agonist SLIGRL-NH2 and antagonist FSLLRY-NH2. Fluorescein isothiocyanate-dextran 4 kDa detection was employed as an indicator of epithelial permeability.

RESULTS

Lower expression levels of TJs in the nasal epithelium of AR patients were observed in comparison with that in control subjects. The PAR-2 level was markedly increased following treatment with 1,000 ng/mL of Der p1 for 24 hours in a cellular model of AR. The expression of PAR-2 was increased in Der p1-induced pHNECs of AR patients and correlated inversely with zonula occlidens (ZO)-1 and claudin-1. Treatment with Der p1 further downregulated TJs expression and promoted an increased epithelial permeability in Der p1-induced pHNECs.

CONCLUSIONS

PAR-2 could downregulate the expression of ZO-1 and claudin-1, which is involved in epithelial barrier dysfunction in AR.

Measurement of novel intestinal secretory and barrier pathways and effects of proteases

The epithelial lining of the gastrointestinal (GI) tract in conjunction with the enteric nervous system (ENS) plays an important role in mediating solute absorption and secretion. A dysregulated ionic movement across the epithelium can result in GI diseases that manifest as either watery diarrhea or constipation. Hirschsprung disease is an example of an ENS disorder characterized by absence of enteric ganglia in distal gut resulting in obstructive phenotype. Receptor rearranged during transfection (RET) gene variants are the most commonly recognized genetic associations with Hirschsprung disease. In this issue of Neurogastroenterology and Motility, Russell et al demonstrate that RET mediates colonic ion transport through modulation of cholinergic nerves. They go on to show inhibition of RET can attenuate accelerated transit in a rat model. Normalizing secretory and absorptive defects has been an attractive therapeutic strategy. In addition to the intrinsic regulation of secretory processes, luminal mediators like bile acids, short-chain fatty acids, and proteases can affect both secretion and barrier function of the intestinal epithelium. Elevated levels of proteases have been identified in a wide range of GI diseases including irritable bowel syndrome. Proteases are known to cause visceral hypersensitivity and barrier disruption in vitro and in animal models. The goals of this review are to describe fundamental concepts related to intestinal epithelial secretion, the utility of Ussing chambers to measure ionic mechanisms and to discuss examples of novel signaling pathways; namely the RET signaling cascade in secretomotor neurons and effects of luminal proteases on barrier and ionic secretion.

Activation of protease-activated receptors (PARs)-1 and -2 promotes alpha-smooth muscle actin expression and release of cytokines from human lung fibroblasts

Previous studies have shown that protease-activated receptors (PARs) play an important role in various physiological processes. In the present investigation, we determined the expression of PARs on human lung fibroblasts (HLF-1) and whether they were involved in cellular differentiation and pro-inflammatory cytokine and prostaglandin (PGE₂) secretion. PAR-1, PAR-2, PAR-3, and PAR-4 were detected in fibroblasts using RT-PCR, immunocytochemistry, and flow cytometry. Increased expression of PAR-4, but not other PARs, was observed in fibroblasts stimulated with phorbol myristate acetate. The archetypical activators of PARs, namely, thrombin and trypsin, as well as PAR-1 and PAR-2 agonist peptides, stimulated transient increases in intracellular Ca²⁺, and promoted increased α-smooth muscle actin expression. The proteolytic and peptidic PAR activators also stimulated the release of IL-6 and IL-8, as well as PGE₂, with a rank order of potency of PAR-1 > PAR-2. The combined stimulation of PAR-1 and PAR-2 resulted in an additive release of both IL-6 and IL-8. In contrast, PAR-3 and PAR-4 agonist peptides, as well as all the PAR control peptides examined, were inactive. These results suggest an important role for PARs associated with fibroblasts in the modulation of inflammation and remodeling in the airway.

A highly potent agonist to protease-activated receptor-2 reveals apical activation of the airway epithelium resulting in Ca2+-regulated ion conductance

The airway epithelium provides a barrier that separates inhaled air and its various particulates from the underlying tissues. It provides key physiological functions in both sensing the environment and initiating appropriate innate immune defenses to protect the lung. Protease-activated receptor-2 (PAR2) is expressed both apically and basolaterally throughout the airway epithelium. One consequence of basolateral PAR2 activation is the rapid, Ca(2+)-dependent ion flux that favors secretion in the normally absorptive airway epithelium. However, roles for apically expressed PAR2 activation have not been demonstrated, in part due to the lack of specific, high-potency PAR2 ligands. In the present study, we used the newly developed PAR2 ligand 2at-LIGRLO(PEG3-Pam)-NH2 in combination with well-differentiated, primary cultured airway epithelial cells from wild-type and PAR2 (-/-) mice to examine the physiological role of PAR2 in the conducting airway after apical activation. Using digital imaging microscopy of intracellular Ca(2+) concentration changes, we verified ligand potency on PAR2 in primary cultured airway cells. Examination of airway epithelial tissue in an Ussing chamber showed that apical activation of PAR2 by 2at-LIGRLO(PEG3-Pam)-NH2 resulted in a transient decrease in transepithelial resistance that was due to increased apical ion efflux. We determined pharmacologically that this increase in ion conductance was through Ca(2+)-activated Cl(-) and large-conductance K(+) channels that were blocked with a Ca(2+)-activated Cl(-) channel inhibitor and clotrimazole, respectively. Stimulation of Cl(-) efflux via PAR2 activation at the airway epithelial surface can increase airway surface liquid that would aid in clearing the airway of noxious inhaled agents.

Proteinase-activated receptor-2 activation participates in allergic sensitization to house dust mite allergens in a murine model

BACKGROUND

Many aeroallergens contain proteinase activity and are able to induce allergic sensitization when presented to mucosal surfaces. Some of these allergens activate proteinase-activated receptor-2 (PAR2 ).

OBJECTIVE

To determine the role of PAR2 activation in a murine house dust mite (HDM) allergy model.

METHODS

We sensitized and challenged PAR2 -deficient mice with HDM, and examined allergic outcomes compared to wild-type animals. To focus on the role of PAR2 in allergic sensitization, we administered a PAR2 blocking antibody to wild-type animals during the sensitization phase and examined the outcomes immediately after sensitization or following subsequent allergen challenge.

RESULTS

We found PAR2 -deficient mice sensitized and challenged with HDM failed to develop airway inflammation, did not produce HDM-specific IgG1 and had less IL-4 mRNA in the lungs than wild-type animals. Prevention of PAR2 activation during sensitization in wild-type mice diminished the levels of Th2 mediators, including IL-4, IL-5 and IL-13, in the lungs. Blocking PAR2 during the sensitization phase also led to decreased manifestations of allergic disease, including airway hyperresponsiveness (AHR) and airway inflammation following subsequent allergen challenge. HDM-induced proliferation of splenocytes obtained from animals sensitized in the presence of PAR2 antibody was reduced relative to those that did not receive antibody. The effect of PAR2 blockade could be transferred to naïve mice through splenic CD4(+) T cells from sensitized mice.

CONCLUSIONS AND CLINICAL RELEVANCE

PAR2 activation plays a key role during the sensitization phase of our HDM allergy model, leading to increased lung cytokine production and augmented lung reactivity. PAR2 activation is a common mechanism for sensitization to a wide variety of allergens and is therefore a potential pharmacological target to prevent allergy.

Protease-activated receptor 2 mediates mucus secretion in the airway submucosal gland

Protease-activated receptor 2 (PAR2), a G protein-coupled receptor expressed in airway epithelia and smooth muscle, plays an important role in airway inflammation. In this study, we demonstrated that activation of PAR2 induces mucus secretion from the human airway gland and examined the underlying mechanism using the porcine and murine airway glands. The mucosa with underlying submucosal glands were dissected from the cartilage of tissues, pinned with the mucosal side up at the gas/bath solution interface of a physiological chamber, and covered with oil so that secretions from individual glands could be visualized as spherical bubbles in the oil. Secretion rates were determined by optical monitoring of the bubble diameter. The Ca(2+)-sensitive dye Fura2-AM was used to determine intracellular Ca(2+) concentration ([Ca(2+)](i)) by means of spectrofluorometry. Stimulation of human tracheal mucosa with PAR2-activating peptide (PAR2-AP) elevated intracellular Ca(2+) and induced glandular secretion equal to approximately 30% of the carbachol response in the human airway. Porcine gland tissue was more sensitive to PAR2-AP, and this response was dependent on Ca(2+) and anion secretion. When the mouse trachea were exposed to PAR2-AP, large amounts of secretion were observed in both wild type and ΔF508 cystic fibrosis transmembrane conductance regulator mutant mice but there is no secretion from PAR-2 knock out mice. In conclusion, PAR2-AP is an agonist for mucus secretion from the airway gland that is Ca(2+)-dependent and cystic fibrosis transmembrane conductance regulator-independent.

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.

Prostaglandin E₂regulation of cystic fibrosis transmembrane conductance regulator activity and airway surface liquid volume requires gap junctional communication

Stimulation of the cystic fibrosis transmembrane conductance regulator (CFTR) by protease-activated receptors (PARs) at the basolateral membranes and by adenosine receptors (ADO-Rs) at the apical membrane maintain airway surface liquid (ASL) volume, which is required to ensure hydrated and clearable mucus. Both pathways involve the release of prostaglandin E₂ (PGE₂) and the stimulation of their basolateral receptors (EP-Rs). We sought to determine whether gap junctions contribute to the coordination of these pathways for modulating CFTR activity and mucus hydration. We used RT-PCR and Western blotting to determine connexin (Cx), CD73, and EP-R expression in a Calu-3 airway epithelial cell line grown on Transwell (Corning Costar, Cambridge, MA) inserts. We used dye coupling to evaluate gap junctional intercellular communication (GJIC). We used Ussing chamber studies and X-Z confocal microscopy to monitor Cl(-) secretion and ASL volume regulation. We found that connexin 43 (Cx43)-mediated GJIC was increased either by endogenous ADO after the hydrolysis of purine nucleotides by CD73 or by the direct activation of ADO-Rs. Inhibition of phospholipase A2 and cyclooxygenase prevented ADO-dependent increases in GJIC, suggesting the involvement of PGE₂. PGE₂ was found to increase GJIC markedly by stimulating EP4-Rs. The modulation of ADO signaling also affected the PAR-dependent activation of CFTR. The reduction of GJIC by CD73 or Cx43 inhibition prevented PAR-evoked CFTR currents in Ussing chambers. The inhibition of GJIC resulted in a failure of PGE₂ to increase ASL volume in Calu-3 cells and in primary cultures of well-differentiated human airway epithelial cells. Thus, gap junctions coordinate a signaling network comprising CFTR, ADO-Rs, PARs, and EP-Rs, and are required for ASL volume homeostasis.

Protease-activated receptor (Par)1 alters bioelectric properties of distal lung epithelia without compromising barrier function

Proteinases contribute to the pathogenesis of various lung diseases, partly through activating cell surface receptors by limited proteolytic cleavage. The authors provide evidence that in primary cultures of distal lung epithelia, basolateral protease-activated receptor 1 activation rapidly reduces transepithelial resistance but does not alter paracellular permeability to small uncharged solutes. Changes in transepithelial resistance were partially blocked by ion transport inhibitors and were completely blocked by placing cells in low chloride buffer. In vivo studies did not reveal enhanced lung permeability in response to pulmonary or intravenous administration of protease-activated receptor 1 activators. This information is relevant as strategies to inhibit protease-activated receptor 1 signaling are considered in order to preserve lung epithelial barrier function.

Protease-activated receptors as drug targets in inflammation and pain

Proteases have been shown to signal to cells through the activation of a novel class of receptors coupled to G proteins: the protease-activated receptors (PARs). Those receptors are expressed in a wide range of cells, which ultimately are all involved in mechanisms of inflammation and pain. Numerous studies have considered the role of PARs in cells, organ systems or in vivo, highlighting the fact that PAR activation results in signs of inflammation. A growing body of evidences discussed here suggests that these receptors, and the proteases that activate them, interfere with inflammation and pain processes. Whether a role for PARs has been clearly defined in inflammatory and pain pathologies is discussed in this review. Further, the pros and cons for considering PARs as targets for the development of therapeutic options for the treatment of inflammation and pain are discussed.

Thrombin promotes release of ATP from lung epithelial cells through coordinated activation of rho- and Ca2+-dependent signaling pathways

Extracellular ATP controls key aspects of lung function via activation of epithelial cell purinergic receptors, but how ATP is released from cells remains poorly understood. To identify mechanistic components upstream of ATP release, we examined the effect of selected G protein coupled-receptor activation on ATP release from lung epithelial cells. The protease-activated receptor (PAR) agonist thrombin elicited a rapid Ca(2+)-dependent release of ATP from A549 cells. In contrast, the P2Y(2) receptor agonist UTP caused negligible ATP release, despite promoting a robust Ca(2+) response. Agonist-elicited ATP release was associated with Rho activation and was reduced in cells transfected with dominant negative mutants of p115-Rho GEF or RhoA, and by inhibitors of Rho kinase (ROCK). However, RhoA activation alone did not promote ATP release if temporally separated from Ca(2+) mobilization. PAR3 was the only PAR subtype detected in A549 cells by reverse transcription-PCR. Transfection of cells with human PAR3 cDNA increased thrombin-promoted ATP release, inositol phosphate formation, and RhoA activation. Conversely, small interference RNA against PAR3 diminished thrombin-evoked responses. Thrombin-elicited ATP release was accompanied by an enhanced cellular uptake of propidium iodide in a Ca(2+)- and ROCK-dependent manner and was inhibited by connexin/pannexin hemichannel blockers. Our data suggest that thrombin promotes ATP release from A549 cells via Rho- and Ca(2+)-dependent activation of connexin/pannexin hemichannels. The relevance of these findings is highlighted by the observation that exposure of primary cultures of well differentiated human bronchial epithelial cells to thrombin resulted in robust ATP release, which was inhibited by ROCK inhibitors and by connexin/pannexin hemichannel blockers.

Proteinase-activated receptor 2 stimulates Na,K-ATPase and sodium reabsorption in native kidney epithelium

Proteinase-activated receptors 2 (PAR2) are expressed in kidney, but their function is mostly unknown. Since PAR2 control ion transport in several epithelia, we searched for an effect on sodium transport in the cortical thick ascending limb of Henle's loop, a nephron segment that avidly reabsorbs NaCl, and for its signaling. Activation of PAR2, by either trypsin or a specific agonist peptide, increased the maximal activity of Na,K-ATPase, its apparent affinity for sodium, the sodium permeability of the paracellular pathway, and the lumen-positive transepithelial voltage, featuring increased NaCl reabsorption. PAR2 activation induced calcium signaling and phosphorylation of ERK1,2. PAR2-induced stimulation of Na,K-ATPase Vmax was fully prevented by inhibition of phospholipase C, of changes in intracellular concentration of calcium, of classical protein kinases C, and of ERK1,2 phosphorylation. PAR2-induced increase in paracellular sodium permeability was mediated by the same signaling cascade. In contrast, increase in the apparent affinity of Na,K-ATPase for sodium, although dependent on phospholipase C, was independent of calcium signaling, was insensitive to inhibitors of classical protein kinases C and of ERK1,2 phosphorylation, but was fully prevented by the nonspecific protein kinase inhibitor staurosporine, as was the increase in transepithelial voltage. In conclusion, PAR2 increases sodium reabsorption in rat thick ascending limb of Henle's loop along both the transcellular and the paracellular pathway. PAR2 effects are mediated in part by a phospholipase C/protein kinase C/ERK1,2 cascade, which increases Na,K-ATPase maximal activity and the paracellular sodium permeability, and by a different phospholipase C-dependent, staurosporine-sensitive cascade that controls the sodium affinity of Na,K-ATPase.

Proteinase-activated receptor-2 promotes allergic sensitization to an inhaled antigen through a TNF-mediated pathway

The reason why particular inhaled Ags induce allergic sensitization while others lead to immune tolerance is unclear. Along with a genetic predisposition to atopy, intrinsic characteristics of these Ags must be important. A common characteristic of many allergens is that they either possess proteinase activity or are inhaled in particles rich in proteinases. Many allergens, such as house dust mite and cockroach allergens, have the potential to activate the proteinase-activated receptor (PAR)-2. In this study, we report that PAR-2 activation in the airways at the same time as exposure to inhaled Ags induces allergic sensitization, whereas exposure to Ag alone induces tolerance. BALB/c mice were administered OVA with a PAR-2 activating peptide intranasally. Upon allergen re-exposure mice developed airway inflammation and airway hyperresponsiveness, as well as OVA-specific T cells with a Th2 cytokine profile when restimulated with OVA in vitro. Conversely, mice given OVA alone or OVA with a PAR-2 control peptide developed tolerance. These tolerant mice did not develop airway inflammation or airway hyperresponsiveness, and developed OVA-specific T cells that secreted high levels of IL-10 when restimulated with OVA in vitro. Furthermore, pulmonary dendritic cell trafficking was altered in mice following intranasal PAR-2 activation. Finally, we showed that PAR-2-mediated allergic sensitization was TNF-dependent. Thus, PAR-2 activation in the airways could be a critical factor in the development of allergic sensitization following mucosal exposure to allergens with serine proteinase activity. Interfering with this pathway may prove to be useful for the prevention or treatment of allergic diseases.

PAR-2 activation and LPS synergistically enhance inflammatory signaling in airway epithelial cells by raising PAR expression level and interleukin-8 release

Protease-activated receptors (PARs) are involved in the contribution of airway epithelial cells to the development of inflammation by release of pro- and anti-inflammatory mediators. Here, we evaluated in epithelial cells the influence of LPS and continuous PAR activation on PAR expression level and the release of the proinflammatory chemokine IL-8. We studied primary human small airway epithelial cells and two airway epithelial cell lines, A549 and HBE cells. LPS specifically upregulated expression of PAR-2 but not of PAR-1. Exposure of epithelial cells to PAR-1 or PAR-2 agonists increased the PAR-1 expression level. The PAR-2 agonist exhibited higher potency than PAR-1 activators. However, the combined exposure of epithelial cells to LPS and PAR agonists abrogated the PAR-1 upregulation. The PAR-2 expression level was also upregulated after exposure to PAR-1 or PAR-2 agonists. This elevation was higher than the effect of PAR agonists on the PAR-1 level. In contrast to the PAR-1 level, the PAR-2 level remained elevated under concomitant stimulation with LPS and PAR-2 agonist. Furthermore, activation of PAR-2, but not of PAR-1, caused production of IL-8 from the epithelial cells. Interestingly, both in the epithelial cell line and in primary epithelial cells, there was a potentiation of the stimulation of the IL-8 synthesis and release by PAR-2 agonist together with LPS. In summary, these results underline the important role of PAR-2 in human lung epithelial cells. Moreover, our study shows an intricate interplay between LPS and PAR agonists in affecting PAR regulation and IL-8 production.

Agonists of protease-activated receptors 1 and 2 stimulate electrolyte secretion from mouse gallbladder

Cholecystitis is one of the most common gastrointestinal diseases. Inflammation induces the activation of proteases that can signal to cells by cleaving protease-activated receptors (PARs) to induce hemostasis, inflammation, pain, and repair. However, the distribution of PARs in the gallbladder is unknown, and their effects on gallbladder function have not been fully investigated. We localized immunoreactive PAR(1) and PAR(2) to the epithelium, muscle, and serosa of mouse gallbladder. mRNA transcripts corresponding to PAR(1) and PAR(2), but not PAR(4), were detected by RT-PCR and sequencing. Addition of thrombin and a PAR(1)-selective activating peptide (TFLLRN-NH(2)) to the serosal surface of mouse gallbladder mounted in an Ussing chamber stimulated an increase in short-circuit current in wild-type but not PAR(1) knockout mice. Similarly, serosally applied trypsin and PAR(2) activating peptide (SLIGRL-NH(2)) increased short-circuit current in wild-type but not PAR(2) knockout mice. Proteases and activating peptides strongly inhibited electrogenic responses to subsequent stimulation with the same agonist, indicating homologous desensitization. Removal of HCO(3)(-) ions from the serosal buffer reduced responses to thrombin and trypsin by >80%. Agonists of PAR(1) and PAR(2) increase intracellular Ca(2+) concentration in isolated and cultured gallbladder epithelial cells. The COX-2 inhibitor meloxicam and an inhibitor of CFTR prevented the stimulatory effect of PAR(1) but not PAR(2). Thus PAR(1) and PAR(2) are expressed in the epithelium of the mouse gallbladder, and serosally applied proteases cause a HCO(3)(-) secretion. The effects of PAR(1) but not PAR(2) depend on generation of prostaglandins and activation of CFTR. These mechanisms may markedly influence fluid and electrolyte secretion of the inflamed gallbladder when multiple proteases are generated.

The signaling pathway involved in neutrophil elastase stimulated MUC1 transcription

We previously reported that neutrophil elastase (NE) stimulated MUC1 gene expression in A549 lung epithelial cells through binding of Sp1 to the MUC1 promoter element. The current study was undertaken to elucidate the complete signaling pathway leading to Sp1 activation. Using a combination of pharmacologic inhibitors, dominant-negative mutant, RNA interference, and soluble receptor blocking techniques, we identified a protein kinase Cdelta (PKCdelta) --> dual oxidase 1 (Duox1) --> reactive oxygen species (ROS) --> TNF-alpha-converting enzyme (TACE) --> TNF-alpha --> TNF receptor (TNFR)1 --> extracellular signal-regulated kinase (ERK)1/2 --> Sp1 pathway as responsible for NE-activated MUC1 transcription. This cascade was identical up to the point of TACE with the signaling pathway previously reported for NE-stimulated MUC5AC production. However, unlike the MUC5AC pathway, TNF-alpha, TNFR1, ERK1/2, and Sp1 were unique components of the MUC1 pathway. Given the anti-inflammatory role of MUC1 during airway bacterial infection, up-regulation of MUC1 by inflammatory mediators such as NE and TNF-alpha suggests a crucial role for MUC1 in the control of excessive inflammation during airway bacterial infection.

Inflammatory conditions increase expression of protease-activated receptor-2 by human airway smooth muscle cells in culture

The protease-activated receptor-2 (PAR-2) has been implicated in airway inflammation and bronchial hyperresponsiveness. We wondered whether inflammatory conditions may upregulate PAR-2 expression by the human airway smooth muscle. To do so, we treated human airway smooth muscle cells (HASMC) in primary culture with interleukin-1beta (IL-1beta), a pro-inflammatory and asthma-associated cytokine. Cells were starved for 24 h and incubated with or without IL-1beta. Online fluorescent polymerase chain reaction after reverse transcription quantified PAR-2 mRNA, and Western blotting measured PAR-2 protein expression. PAR-2 was constitutively expressed by HASMC in primary culture, and IL-1beta (10 U/mL) time dependently elevated PAR-2 mRNA with a maximum of 4.7-fold after 1.5 h (P < 0.01), and PAR-2 protein expression with a maximum of 1.5-fold after 24 h (P < 0.01). The concentration dependence of the IL-1beta effect (0.1-30 U/mL) confirmed a maximal increase of PAR-2 expression at 10 U/mL. Our study clearly shows that IL-1beta upregulates PAR-2 mRNA and protein expression by HASMC in culture. This increased expression of PAR-2 in inflammatory conditions may have functional consequences in the bronchial dysfunction of asthmatic airways.

Identification of novel VHL targets that are associated with the development of renal cell carcinoma

von Hippel-Lindau (VHL) disease is a dominantly inherited family cancer syndrome characterized by the development of retinal and central nervous system haemangioblastomas, renal cell carcinoma (RCC) and phaeochromocytoma. Specific germline VHL mutations may predispose to haemangioblastomas, RCC and phaeochromocytoma to a varying extent. Although dysregulation of the hypoxia-inducible transcription factor-2 and JunB have been linked to the development of RCC and phaeochromocytoma, respectively, the precise basis for genotype-phenotype correlations in VHL disease have not been defined. To gain insights into the pathogenesis of RCC in VHL disease we compared gene expression microarray profiles in a RCC cell line expressing a Type 1 or Type 2B mutant pVHL (RCC-associated) to those of a Type 2A or 2C mutant (not associated with RCC). We identified 19 differentially expressed novel VHL target genes linked to RCC development. Eight targets were studied in detail by quantitative real-time polymerase chain reaction (three downregulated and five upregulated by wild-type VHL) and for six genes the effect of VHL inactivation was mimicked by hypoxia (but hypoxic-induction of smooth muscle alpha-actin 2 was specific for a RCC cell line). The potential role of four RCC-associated VHL target genes was assessed in vitro. NB thymosin beta (TMSNB) and proteinase-activated receptor 2 (PAR2) (both downregulated by wt pVHL) increased cell growth and motility in a RCC cell line, but aldehyde dehydrogenase (ALDH)1 and ALDH7 had no effect. These findings implicate TMSNB and PAR2 candidate oncogenes in the pathogenesis of VHL-associated RCC.

Protease-activated receptor regulation of Cl- secretion in Calu-3 cells requires prostaglandin release and CFTR activation

Human lung epithelial (Calu-3) cells were used to investigate the effects of protease-activated receptor (PAR) stimulation on Cl(-) secretion. Quantitative RT-PCR (QRT-PCR) showed that Calu-3 cells express PAR-1, -2, and -3 receptor mRNAs, with PAR-2 mRNA in greatest abundance. Addition of either thrombin or the PAR-2 agonist peptide SLIGRL to the basolateral solution of monolayers mounted in Ussing chambers produced a rapid increase in short-circuit current (I(sc): thrombin, 21 +/- 2 microA; SLIGRL, 83 +/- 22 microA), which returned to baseline within 5 min after stimulation. Pretreatment of monolayers with the cell-permeant Ca(2+)-chelating agent BAPTA-AM (50 microM) abolished the increase in I(sc) produced by SLIGRL. When monolayers were treated with the cyclooxygenase inhibitor indomethacin (10 microM), nearly complete inhibition of both the thrombin- and SLIGRL-stimulated I(sc) was observed. In addition, basolateral treatment with the PGE(2) receptor antagonist AH-6809 (25 microM) significantly inhibited the effects of SLIGRL on I(sc). QRT-PCR revealed that Calu-3 cells express mRNAs for CFTR, the Ca(2+)-activated KCNN4 K(+) channel, and the KCNQ1 K(+) channel subunit, which, in association with KCNE3, is known to be regulated by cAMP. Stimulation with SLIGRL produced an increase in apical Cl(-) conductance that was blocked in cells expressing short hairpin RNAs designed to target CFTR. These results support the conclusion that PAR stimulation of Cl(-) secretion occurs by an indirect mechanism involving the synthesis and release of prostaglandins. In addition, PAR-stimulated Cl(-) secretion requires activation of CFTR and at least two distinct K(+) channels located in the basolateral membrane.

PAR2 activation interrupts E-cadherin adhesion and compromises the airway epithelial barrier: protective effect of beta-agonists

The airway epithelium is an important barrier between the environment and subepithelial tissues. The epithelium is also divided into functionally restricted apical and basolateral domains, and this restriction is dependent on the elements of the barrier. The protease-activated receptor-2 (PAR2) receptor is expressed in airway epithelium, and its activation initiates multiple effects including enhanced airway inflammation and reactivity. We hypothesized that activation of PAR2 would interrupt E-cadherin adhesion and compromise the airway epithelial barrier. The PAR2-activating peptide (PAR2-AP, SLIGRL) caused an immediate approximately 50% decrease in the transepithelial resistance of primary human airway epithelium that persisted for 6-10 min. The decrease in resistance was accompanied by an increase in mannitol flux across the epithelium and occurred in cystic fibrosis transmembrane conductance receptor (CFTR) epithelium pretreated with amiloride to block Na and Cl conductances, confirming that the decrease in resistance represented an increase in paracellular conductance. In parallel experiments, activation of PAR2 interrupted the adhesion of E-cadherin-expressing L cells and of primary airway epithelial cells to an immobilized E-cadherin extracellular domain, confirming the hypothesis that activation of PAR2 interrupts E-cadherin adhesion. Selective interruption of E-cadherin adhesion with antibody to E-cadherin decreased the transepithelial resistance of primary airway epithelium by >80%. Pretreatment of airway epithelium or the E-cadherin-expressing L cells with the long-acting beta-agonist salmeterol prevented PAR2 activation from interrupting E-cadherin adhesion and compromising the airway epithelial barrier. Activation of PAR2 interrupts E-cadherin adhesion and compromises the airway epithelial barrier.

Ion transport regulated by protease-activated receptor 2 in human airway Calu-3 epithelia

We examined the mechanisms underlying anion secretion mediated by protease-activated receptor 2 (PAR2) and its role in the regulation of ion transport, using polarized human airway Calu-3 cells. PAR2 stimulation by trypsin and a PAR2-activating peptide (PAR2AP), especially from the basolateral aspect, caused transient Cl(-) secretion due to cytosolic Ca(2+) mobilization. Antagonists of PI-PLC (U73122, ET-18-OCH(3)) and inositol 1,4,5-triphosphate (xestospongin C (Xest C)) were without effect on the PAR2AP-mediated Cl(-) secretion, whereas it was attenuated by D609 (a PC-PLC inhibitor) and phorbol 12-myristate 13 acetate (PMA, a PKC activator). Even 30 min after removal of PAR2AP after a 10-min-exposure, cells were still poorly responsive to PAR2 stimulation, but the reduced responsiveness was upregulated by a PKC inhibitor, GF109203X (GFX). Pretreatment with PAR2AP did not affect responses to anion secretagogues, such as isoproterenol, forskolin, thapsigargin, 1-ethyl-2-benzimdazolinone, and adenosine, but ATP-induced responses were significantly reduced. Nystatin permeabilization studies revealed that the presence of PAR2AP prevented ATP-induced increments in basolateral membrane K(+) conductance without affecting apical membrane Cl(-) conductance. ATP-elicited Ca(2+) mobilization, which was sensitive to D609 and PMA, was inhibited by the pretreatment with PAR2AP, and this inhibition was blunted by the presence of GFX. Collectively, stimulation of PAR2 generates a brief response of Cl(-) secretion through PC-PLC-mediated pathway, followed by not only auto-desensitization of PAR2 itself but also cross-desensitization of a PC-PLC-coupled purinoceptor. The two types of desensitization seem likely to have PKC-mediated downregulation of PC-PLC in common.

The protease-activated receptor2 (PAR2)-prostaglandin E2-prostanoid EP receptor axis: a potential bronchoprotective unit in the respiratory tract?

Protease-activated receptor2 (PAR2) is a subtype of G protein-coupled receptor that is widely expressed within the respiratory tract. Stimulation of PAR2 by proteases such as trypsin and tryptase, or by small peptidic activators induces a complex array of effects within the airways. One such PAR2-mediated effect by basal airway epithelial cells is the generation of prostaglandin E2. Prostaglandin E2 produces a raft of anti-inflammatory effects within the airways, principally through the activation of the prostanoid EP2 and EP3 receptor subtypes. This article reviews the PAR2-prostaglandin E2-prostanoid EP receptor axis and discusses approaches through which its activation may provide beneficial effects in respiratory disease.

Protease-Activated Receptor-2 Peptides Activate Neurokinin-1 Receptors in the Mouse Isolated Trachea

Protective roles for protease-activated receptor-2 (PAR(2)) in the airways including activation of epithelial chloride (Cl(-)) secretion are based on the use of presumably PAR(2)-selective peptide agonists. To determine whether PAR(2) peptide-activated Cl(-) secretion from mouse tracheal epithelium is dependent on PAR(2), changes in ion conductance across the epithelium [short-circuit current (I(SC))] to PAR(2) peptides were measured in Ussing chambers under voltage clamp. In addition, epithelium- and endothelium-dependent relaxations to these peptides were measured in two established PAR(2) bioassays, isolated ring segments of mouse trachea and rat thoracic aorta, respectively. Apical application of the PAR(2) peptide SLIGRL caused increases in I(SC), which were inhibited by three structurally different neurokinin receptor-1 (NK(1)R) antagonists and inhibitors of Cl(-) channels but not by capsaicin, the calcitonin gene-related peptide (CGRP) receptor antagonist CGRP(8-37), or the nonselective cyclooxygenase inhibitor indomethacin. Only high concentrations of trypsin caused an increase in I(SC) but did not affect the responses to SLIGRL. Relaxations to SLIGRL in the trachea and aorta were unaffected by the NK(1)R antagonist nolpitantium (SR 140333) but were abolished by trypsin desensitization. The rank order of potency for a range of peptides in the trachea I(SC) assay was 2-furoyl-LIGRL > SLCGRL > SLIGRL = SLIGRT > LSIGRL compared with 2-furoyl-LIGRL > SLIGRL > SLIGRT > SLCGRL (LSIGRL inactive) in the aorta relaxation assay. In the mouse trachea, PAR(2) peptides activate both epithelial NK(1)R coupled to Cl(-) secretion and PAR(2) coupled to prostaglandin E(2)-mediated smooth muscle relaxation. Such a potential lack of specificity of these commonly used peptides needs to be considered when roles for PAR(2) in airway function in health and disease are determined.

Endogenous protease activation of ENaC: effect of serine protease inhibition on ENaC single channel properties

Endogenous serine proteases have been reported to control the reabsorption of Na⁺ by kidney- and lung-derived epithelial cells via stimulation of electrogenic Na⁺ transport mediated by the epithelial Na⁺ channel (ENaC). In this study we investigated the effects of aprotinin on ENaC single channel properties using transepithelial fluctuation analysis in the amphibian kidney epithelium, A6. Aprotinin caused a time- and concentration-dependent inhibition (84 ± 10.5%) in the amiloride-sensitive sodium transport (I_Na) with a time constant of 18 min and half maximal inhibition constant of 1 μM. Analysis of amiloride analogue blocker–induced fluctuations in I_Na showed linear rate–concentration plots with identical blocker on and off rates in control and aprotinin-inhibited conditions. Verification of open-block kinetics allowed for the use of a pulse protocol method (Helman, S.I., X. Liu, K. Baldwin, B.L. Blazer-Yost, and W.J. Els. 1998. Am. J. Physiol. 274:C947–C957) to study the same cells under different conditions as well as the reversibility of the aprotinin effect on single channel properties. Aprotinin caused reversible changes in all three single channel properties but only the change in the number of open channels was consistent with the inhibition of I_Na. A 50% decrease in I_Na was accompanied by 50% increases in the single channel current and open probability but an 80% decrease in the number of open channels. Washout of aprotinin led to a time-dependent restoration of I_Na as well as the single channel properties to the control, pre-aprotinin, values. We conclude that protease regulation of I_Na is mediated by changes in the number of open channels in the apical membrane. The increase in the single channel current caused by protease inhibition can be explained by a hyperpolarization of the apical membrane potential as active Na⁺ channels are retrieved. The paradoxical increase in channel open probability caused by protease inhibition will require further investigation but does suggest a potential compensatory regulatory mechanism to maintain I_Na at some minimal threshold value.

Apical trypsin increases ion transport and resistance by a phospholipase C-dependent rise of Ca2+

We investigated the mechanisms by which serine proteases alter lung fluid clearance in rat lungs and vectorial ion transport in airway and alveolar epithelial cells. Inhibition of endogenous protease activity by intratracheal instillation of soybean trypsin inhibitor (SBTI) or α1-antitrypsin decreased amiloride-sensitive lung fluid clearance across rat fluid-filled lungs; instillation of trypsin partially restored this effect. Gelatin zymography demonstrated SBTI-inhibitable trypsin-like activity in rat lung lavage fluid. Apical trypsin and human neutrophil elastase, but not agonists of protease activated receptors, increased Na+and Cl−short-circuit currents ( Isc) and transepithelial resistance ( RTE) across human bronchial and nasal epithelial cells and rat alveolar type II cells, mounted in Ussing chambers, for at least 2 h. The increase in Iscwas fully reversed by amiloride and glibenclamide. The increase in RTEwas not prevented by ouabain, suggesting that trypsin decreased paracellular conductance. Apical trypsin also induced a transient increase in intracellular Ca2+in human airway cells; treatment of these cells with BAPTA-AM mitigated the trypsin-induced increases of intracellular Ca2+and of Iscand RTE. Increasing intracellular Ca2+in airway cells with either ionomycin or thapsigargin reproduced the increase in Isc, whereas inhibitors of phospholipase C (PLC) prevented the increases in both Ca2+and Isc. These data indicate trypsin-like proteases and elastase, either present in lung cells or released by inflammatory cells into the alveolar space, play an important role in the clearance of alveolar fluid by increasing ion transport and paracellular resistance via a PLC-initiated rise of intracellular Ca2+.

Control of ion transport in mammalian airways by protease activated receptors type 2 (PAR-2)

Protease-activated receptors (PARs) are widely distributed in human airways. They couple to G- proteins and are activated after proteolytic cleavage of the N terminus of the receptor. Evidence is growing that PAR subtype 2 plays a pivotal role in inflammatory airway diseases, such as allergic asthma or bronchitis. However, nothing is known about the effects of PAR-2 on electrolyte transport in the native airways. PAR-2 is expressed in airway epithelial cells, where they are activated by mast cell tryptase, neutrophil proteinase 3, or trypsin. Recent studies produced conflicting results about the functional consequence of PAR-2 stimulation. Here we report that stimulation of PAR-2 receptors in mouse and human airways leads to a change in electrolyte transport and a shift from absorption to secretion. Although PAR-2 appears to be expressed on both sides of the epithelium, only basolateral stimulation results in inhibition of amiloride sensitive Na+ conductance and stimulation of both luminal Cl- channels and basolateral K+ channels. The present data indicate that these changes occur through activation of phospholipase C and increase in intracellular Ca2+, which activates basolateral SK4 K+ channels and luminal Ca2+-dependent Cl- channels. In addition, the present data suggest a PAR-2 mediated release of prostaglandin E2, which may contribute to the secretory response. In conclusion, these results provide further evidence for a role of PAR-2 in inflammatory airway disease: stimulation of these receptors may cause accumulation of airway surface liquid, which, however, may help to flush noxious stimuli away from the affected airways.

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.

Physiology and pathophysiology of proteinase-activated receptors (PARs): PARs in the respiratory system: cellular signaling and physiological/pathological roles

Proteinase-activated receptors (PARs), a family of G protein-coupled receptors, are widely distributed in the mammalian body, playing a variety of physiological/pathophysiological roles. In the respiratory systems, PARs, particularly PAR-2 and PAR-1, are expressed in the epithelial and smooth muscle cells. In addition to the G(q/11)-mediated activation of the phospholipase C beta pathway, epithelial PAR activation causes prompt and/or delayed prostanoid formation, leading to airway smooth muscle relaxation and/or modulation of an inflammatory process. PAR-2 present in the epithelium and smooth muscle is considered primarily pro-inflammatory in the respiratory system, although PAR-2 may also be anti-inflammatory under certain conditions. In the lung epithelial cells, PAR-2 can also be activated by exogenous proteinases including house dust mite allergens, in addition to various possible endogenous agonist proteinases. Clinical evidence also suggests possible involvement of PARs, particularly PAR-2, in respiratory diseases. PARs thus appear to play critical roles in the respiratory systems, and the agonists/antagonists for PARs may serve as the novel therapeutic strategy for treatment of certain respiratory diseases including asthma.

Assembly of adherens junctions is required for sphingosine 1-phosphate-induced matriptase accumulation and activation at mammary epithelial cell-cell contacts

Sphingosine 1-phosphate (S1P), a bioactive phospholipid, simultaneously induces actin cytoskeletal rearrangements and activation of matriptase, a membrane-associated serine protease in human mammary epithelial cells. In this study, we used a monoclonal antibody selective for activated, two-chain matriptase to examine the functional relationship between these two S1P-induced events. Ten minutes after exposure of 184 A1N4 mammary epithelial cells to S1P, matriptase was observed to accumulate at cell-cell contacts. Activated matriptase first began to appear as small spots at cell-cell contacts, and then its deposits elongated along cell-cell contacts. Concomitantly, S1P induced assembly of adherens junctions and subcortical actin belts. Matriptase localization was observed to be coincident with markers of adherens junctions at cell-cell contacts but likely not to be incorporated into the tightly bound adhesion plaque. Disruption of subcortical actin belt formation and prevention of adherens junction assembly led to prevention of accumulation and activation of the protease at cell-cell contacts. These data suggest that S1P-induced accumulation and activation of matriptase depend on the S1P-induced adherens junction assembly. Although MAb M32, directed against one of the low-density lipoprotein receptor class A domains of matriptase, blocked S1P-induced activation of the enzyme, the antibody had no effect on S1P-induced actin cytoskeletal rearrangement. Together, these data indicate that actin cytoskeletal rearrangement is necessary but not sufficient for S1P-induced activation of matriptase at cell-cell contacts. The coupling of matriptase activation to adherens junction assembly and actin cytoskeletal rearrangement may serve to ensure tight control of matriptase activity, restricted to cell-cell junctions of mammary epithelial cells.

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.

Altered expression and in vivo lung function of protease-activated receptors during influenza A virus infection in mice

Protease-activated receptors (PARs) are widely distributed in human airways, and recent evidence indicates a role for PARs in the pathophysiology of inflammatory airway disease. To further investigate the role of PARs in airway disease, we determined the expression and function of PARs in a murine model of respiratory tract viral infection. PAR-1, PAR-2, PAR-3, and PAR-4 mRNA and protein were expressed in murine airways, and confocal microscopy revealed colocalization of PAR-2 and cyclooxygenase (COX)-2 immunostaining in basal tracheal epithelial cells. Elevated levels of PAR immunostaining, which was particularly striking for PAR-1 and PAR-2, were observed in the airways of influenza A/PR-8/34 virus-infected mice compared with sham-infected mice. Furthermore, increased PAR-1 and PAR-2 expression was associated with significant changes in in vivo lung function responses. PAR-1 agonist peptide potentiated methacholine-induced increases in airway resistance in anesthetized sham-infected mice (and in indomethacin-treated, virus-infected mice), but no such potentiation was observed in virus-infected mice. PAR-2 agonist peptide transiently inhibited methacholine-induced bronchoconstriction in sham-infected mice, and this effect was prolonged in virus-infected mice. These findings suggest that during viral infection, the upregulation of PARs in the airways is coupled to increased activation of COX and enhanced generation of bronchodilatory prostanoids.

Role of protease-activated receptors in airway function: a target for therapeutic intervention?

Protease-activated receptors (PARs) are G-protein-coupled, seven transmembrane domain receptors that act as cellular enzyme sensors. These receptors are activated by the proteolytic cleavage at the amino terminus, enabling interaction between the newly formed "tethered ligand" and the second extracellular loop of the receptor to confer cellular signalling. PARs can also be activated by small peptides that mimic the tethered ligand. In the respiratory tract, PARs may be regulated by endogenous proteases, such as airway trypsin and mast cell tryptase, as well as exogenous proteases, including inhaled aeroallergens such as those from house dust mite faecal pellets. Immunoreactive PARs have been identified in multiple cell types of the respiratory tract, and PAR activation has been reported to stimulate cellular mitogenesis and to promote tissue inflammation. Activation of PARs concurrently stimulates the release of bronchorelaxant and anti-inflammatory mediators, which may serve to induce cytoprotection and to minimise tissue trauma associated with severe chronic airways inflammation. Furthermore, airway inflammatory responses are associated with increased epithelial PAR expression and elevated concentrations of PAR-activating, and PAR-inactivating, proteases in the extracellular space. On this basis, PARs are likely to play a regulatory role in airway homeostasis, and may participate in respiratory inflammatory disorders, such as asthma and chronic obstructive pulmonary disease. Further studies focussing on the effects of newly developed PAR agonists and antagonists in appropriate models of airway inflammation will permit better insight into the role of PARs in respiratory pathophysiology and their potential as therapeutic targets.

Protease-activated receptor-2-mediated Ca2+ signaling in guinea pig tracheal epithelial cells

The protease-activated receptor-2 (PAR-2), a G protein-coupled receptor activated by trypsin, contributes to the pathogenesis of inflammatory disease including asthma. Here, we examined the mechanisms by which stimulation of PAR-2 induces an increase in intracellular Ca2+ concentration ([Ca2+]i) in guinea pig tracheal epithelial cells. Trypsin (0.01-3 units/ml) dose-dependently induced a transient increase in [Ca2+]i, the increase being blocked by soybean trypsin inhibitor (SBTI 1 microM). An increase in [Ca2+]i was also induced by an agonist peptide for PAR-2 (SLIGRL-NH2, 0.001-10 microM) but not by thrombin (3 units/ml, an activator for PAR-1, PAR-3 or PAR-4). Repeated or cross stimulation of trypsin or SLIGRL-NH2 caused marked desensitization of the [Ca2+]i response. These responses of [Ca2+]i to trypsin and SLIGRL-NH2 were attenuated by a phospholipase C inhibitor, U-73122, and a Ca2+-ATPase inhibitor, thapsigargin (100 nM), while removal of Ca2+ and a L-type Ca2+-channel blocker, verapamil, were without significant effects. Further, trypsin was without effect on the rate of fura 2 quenching by Mn2+ entry as an indicator of Ca2+ influx. Thus, stimulation of PAR-2 appears to increase [Ca2+]i through the mobilization of Ca2+ from intracellular stores probably via phospholipase Cbeta-linked generation of a second messenger.

Paneth cell trypsin is the processing enzyme for human defensin-5

The antimicrobial peptide human alpha-defensin 5 (HD5) is expressed in Paneth cells, secretory epithelial cells in the small intestine. Unlike other characterized defensins, HD5 is stored in secretory vesicles as a propeptide. The storage quantities of HD5 are approximately 90 450 microg per cm2 of mucosal surface area, which is sufficient to generate microbicidal concentrations in the intestinal lumen. HD5 peptides isolated from the intestinal lumen are proteolytically processed forms--HD5(56-94) and HD5(63-94)--that are cleaved at the Arg55-Ala56 and Arg62-Thr63 sites, respectively. We show here that a specific pattern of trypsin isozymes is expressed in Paneth cells, that trypsin colocalizes with HD5 and that this protease can efficiently cleave HD5 propeptide to forms identical to those isolated in vivo. By acting as a prodefensin convertase in human Paneth cells, trypsin is involved in the regulation of innate immunity in the small intestine.

House dust mite allergen exerts no direct proinflammatory effects on human keratinocytes

BACKGROUND

Dermatophagoides pteronyssinus is a trigger of atopic dermatitis. Many D pteronyssinus allergens are proteases that can elicit airway inflammation by stimulating the release of cytokines and chemokines by bronchial epithelial cells.

OBJECTIVE

We sought to investigate whether D pteronyssinus allergens can exert a similar activity on skin keratinocytes.

METHODS

Primary cultures of keratinocytes from healthy subjects or patients with atopic dermatitis and normal human bronchial epithelial cells were compared for cytokine production in response to D pteronyssinus extract.

RESULTS

Keratinocytes, but not bronchial epithelial cells, displayed a modest dose-dependent release of IL-1alpha and IL-1 receptor antagonist but no induction of their mRNA after exposure to D pteronyssinus. However, D pteronyssinus also degraded these cytokines. On the other hand, D pteronyssinus extract induced bronchial epithelial cells, but not keratinocytes, to increased expression of IL-8/CXCL8 and GM-CSF mRNA and protein. These effects were efficiently abrogated by a mixture of cysteine and serine protease inhibitors. Both IL-8 and GM-CSF were fully resistant to D pteronyssinus proteolytic attack. No induction of monocyte chemoattractant protein 1/CCL2, RANTES/CCL5, or IFN-gamma-induced protein of 10 kd/CXCL10 was detected in either cell type. Only bronchial epithelial cells expressed protease-activated receptor (PAR) 4 mRNA, whereas PAR-1, PAR-2, and PAR-3 mRNA was found in both cell types. D pteronyssinus did not affect PAR mRNA signals.

CONCLUSIONS

Although D pteronyssinus can cause proteolysis-dependent release of cytokines from keratinocytes, it appears incapable of activating de novo expression of cytokines and chemokines, arguing against a direct proinflammatory activity of house dust mite on the skin.

Basolateral PAR-2 receptors mediate KCl secretion and inhibition of Na+ absorption in the mouse distal colon

Proteinase-activated receptor-2 (PAR-2) may participate in epithelial ion transport regulation. Here we examined the effect of mouse activating peptide (mAP), a specific activator of PAR-2, on electrogenic transport of mouse distal colon using short-circuit current (I(SC)) measurements. Under steady-state conditions, apical application of amiloride (100 microM) revealed a positive I(SC) component of 74.3 +/- 6.8 microA x cm(-2) indicating the presence of Na+ absorption, while apical Ba2+ (10 mM) identified a negative I(SC) component of 26.2 +/- 1.8 microA x cm(-2) consistent with K+ secretion. Baseline Cl- secretion was minimal. Basolateral addition of 20 microM mAP produced a biphasic I(SC) response with an initial transient peak increase of 11.2 +/- 0.9 microA x cm(-2), followed by a sustained fall to a level 31.2 +/- 2.6 microA x cm(-2) (n = 43) below resting I(SC). The peak response was due to Cl- secretion as it was preserved in the presence of amiloride but was largely reduced in the presence of basolateral bumetanide (20 microM) or in the absence of extracellular Cl-. The secondary decline of I(SC) was also attenuated by bumetanide and by Ba2+, indicating that it is partly due to a stimulation of K+ secretion. In addition, the amiloride-sensitive I(SC) was slightly reduced by mAP, suggesting that inhibition of Na+ absorption also contributes to the I(SC) decline. Expression of PAR-2 in mouse distal colon was confirmed using RT-PCR and immunocytochemistry. We conclude that functional basolateral PAR-2 is present in mouse distal colon and that its activation stimulates Cl- and K+ secretion while inhibiting baseline Na+ absorption.

Activation of ion secretion via proteinase-activated receptor-2 in human colon

Proteinase-activated receptor (PAR) type 2 (PAR-2) has been shown to mediate ion secretion in cultured epithelial cells and rat jejunum. With the use of a microUssing chamber, we demonstrate the role of PAR-2 for ion transport in native human colonic mucosa obtained from 30 normal individuals and 11 cystic fibrosis (CF) patients. Trypsin induced Cl(-) secretion when added to the basolateral but not luminal side of normal epithelia. Activation of Cl(-) secretion by trypsin was inhibited by indomethacin and was further increased by cAMP in normal tissues but was not present in CF colon, indicating the requirement of luminal CF transmembrane conductance regulator. Effects of trypsin were largely reduced by low Cl(-), by basolateral bumetanide, and in the presence of barium or clotrimazole, but not by tetrodotoxin. Furthermore, trypsin-induced secretion was inhibited by the Ca(2+)-ATPase inhibitor cyclopiazonic acid and in low-Ca(2+) buffer. The effects of trypsin were almost abolished by trypsin inhibitor. Thrombin, an activator of PAR types 1, 3, and 4, had no effects on equivalent short-circuit currents. The presence of PAR-2 in human colon epithelium was confirmed by RT-PCR and additional experiments with PAR-2-activating peptide. PAR-2-mediated intestinal electrolyte secretion by release of mast cell tryptase and potentiation of PAR-2 expression by tumor necrosis factor-alpha may contribute to the hypersecretion observed in inflammatory processes such as chronic inflammatory bowel disease.