Protease-activated receptors and prostaglandins in inflammatory lung disease

Abstract

Protease-activated receptors (PARs) are a novel family of G protein-coupled receptors. Signalling through PARs typically involves the cleavage of an extracellular region of the receptor by endogenous or exogenous proteases, which reveals a tethered ligand sequence capable of auto-activating the receptor. A considerable body of evidence has emerged over the past 20 years supporting a prominent role for PARs in a variety of human physiological and pathophysiological processes, and thus substantial attention has been directed towards developing drug-like molecules that activate or block PARs via non-proteolytic pathways. PARs are widely expressed within the respiratory tract, and their activation appears to exert significant modulatory influences on the level of bronchomotor tone, as well as on the inflammatory processes associated with a range of respiratory tract disorders. Nevertheless, there is debate as to whether the principal response to PAR activation is an augmentation or attenuation of airways inflammation. In this context, an important action of PAR activators may be to promote the generation and release of prostanoids, such as prostglandin E₂, which have well-established anti-inflammatory effects in the lung. In this review, we primarily focus on the relationship between PARs, prostaglandins and inflammatory processes in the lung, and highlight their potential role in selected respiratory tract disorders, including pulmonary fibrosis, asthma and chronic obstructive pulmonary disease.

This article is part of a themed issue on Mediators and Receptors in the Resolution of Inflammation. To view this issue visit http://www3.interscience.wiley.com/journal/121548564/issueyear?year=2009

Introduction

Protease-activated receptors (PARs) are a family of G protein-coupled receptors that possess a unique mechanism of activation (Hollenberg and Compton, 2002; Steinhoff et al., 2005). The emergence of PARs as a novel receptor family was stimulated by the discovery that thrombin specifically cleaves the extracellular N-terminal region of its receptor to create a new receptor amino terminus that functions as an activating tethered ligand (Vu et al., 1991). PAR activation can also occur in the absence of proteolytic activity, by synthetic peptides called PAR-activating peptides (PAR-APs) that mimic the final five to seven amino acids of the tethered ligand sequence. At present, four distinct subtypes of PAR have been characterized and designated PAR₁, PAR₂, PAR₃ and PAR₄, in chronological order of their discovery (Rasmussen et al., 1991; Vu et al., 1991; Nystedt et al., 1995a; Ishihara et al., 1997; Kahn et al., 1998a; Xu et al., 1998). Of particular interest, numerous studies have recently demonstrated the involvement of PARs in a wide variety of physiological and pathophysiological processes (see recent review by Ramachandran and Hollenberg, 2008). The actions of PARs and their activating proteinases in the airways have also been studied extensively to determine their role in various lung diseases (Sokolova and Reiser, 2007). Among their actions, PARs induce cyclooxygenase (COX) activation and expression in a variety of cell types, resulting in the synthesis and release of various prostanoids (Cocks et al., 1999). PAR-mediated prostaglandin production represents an attractive target for the development of treatments for inflammatory lung diseases, and is the principal focus of this review.

Expression and signalling of PARs in the lung

The human respiratory tract appears to express all four PAR subtypes. Within the lung, PARs are expressed on many cell types including alveoli, fibroblasts, airway smooth muscle, nerves, epithelial cells, endothelial cells, mesothelial cells, goblet cells, as well as on various leukocytes (Sokolova and Reiser, 2007; Ramachandran and Hollenberg, 2008). Of particular interest, exposure of the lung to inflammatory stimuli may enhance the expression of PARs, as well as of PAR-activating proteases. Proteases that may be present in the respiratory tract and activate PARs include the endogenous enzymes mast cell tryptase (activates PAR₂), trypsin (PAR₁, PAR₂ and PAR₄), chymase (PAR₁) and cathepsin G (PAR₄), as well as exogenous enzymes such as Der p1 (PAR₂) that are inhaled. However, these and other enzymes within the respiratory tract may also inactivate or disarm various PARs by cleaving them at other sites that remove the tethered ligand sequence (Loew et al., 2000).

PAR₁ is the primary cell-surface receptor responsible for thrombin-mediated platelet aggregation in humans (Rasmussen et al., 1991; Vu et al., 1991; Ahn et al., 2000). PAR₁ is also activated by many other proteases, including activated protein C (APC), and by selective PAR₁-APs such as TFLLR-NH₂ (Vu et al., 1991; Ramachandran and Hollenberg, 2008). Non-peptide agonists for PAR₁ are currently unavailable. Nevertheless, several potent and selective non-peptidic PAR₁ antagonists have been developed (Ramachandran and Hollenberg, 2008). One such antagonist, SCH530348, is currently undergoing phase III clinical trials as a preventative medication for atherothrombosis (Clasby et al., 2007; Butler, 2008; Severino et al., 2008).

PAR₁ is expressed on many cell types within the lungs, including airway smooth muscle, epithelial cells, platelets, macrophages, mast cells, CD3⁺ T lymphocytes and fibroblasts (Knight et al., 2001; Hollenberg and Compton, 2002; Lan et al., 2002; Steinhoff et al., 2005; Li and He, 2006; Sokolova and Reiser, 2007; Ramachandran and Hollenberg, 2008). Although PAR₁ expression does not appear to be altered in asthmatic airways, increased expression is seen following exposure of cells to influenza A, cockroach allergen and various PAR-APs (Knight et al., 2001; Lan et al., 2004; Ostrowska et al., 2007; Zhang et al., 2008). PAR₁ expression is decreased in fibroblasts following PGI₂ or prostglandin E₂ (PGE₂) exposure (Sokolova et al., 2005), possibly involving a cAMP-dependent mechanism as demonstrated in vascular smooth muscle cells (Sokolova et al., 2005; Pape et al., 2008). G_q/11 appears to be the primary signalling G protein for PAR₁, although it has also been reported to signal via G_i and G_12/13 pathways depending on cell type (see review by Ramachandran and Hollenberg, 2008).

The cloning and characterization of PAR₁ was followed by the discovery of PAR₂, which was shown to be activated by trypsin but resistant to thrombin (Nystedt et al., 1995a,b;). In addition to endogenous proteases such as trypsin and tryptase, PAR₂ can be activated by exogenous proteases, such as Der p1 from the house dust mite Dermatophagoides Pteronyssinus and by PAR₂-APs such as SLIGKV-NH₂ (Molino et al., 1997; Asokananthan et al., 2002; Page et al., 2003). Selective small-molecule agonists, as well as an antagonist, have recently been developed; however, they have low potency and are not yet widely available (Kelso et al., 2006; Gardell et al., 2008; Seitzberg et al., 2008).

PAR₂ is expressed on a range of cell types including mesothelial cells of the pleura, bronchial glands, epithelial cells, endothelial cells, smooth muscle cells, nerves and immune cells such as CD3⁺ T lymphocytes, eosinophils, mast cells and neutrophils (Knight et al., 2001; Miotto et al., 2002; Henry, 2006; Li and He, 2006). PAR₂ expression is elevated following exposure to a variety of inflammatory stimuli, including respiratory tract viruses, smoke, bacterial products and allergens (Knight et al., 2001; Ostrowska et al., 2007). PAR₂ signals primarily through activation of G_q/11, but has recently been shown to signal through G protein-independent mechanisms via β-arrestins (Zoudilova et al., 2007).

The discovery of a second platelet thrombin receptor (Connolly et al., 1996) was soon followed by the cloning and characterization of PAR₃ (Ishihara et al., 1997). PAR₃ mRNA is expressed in airway smooth muscle cells, epithelial cells, CD3⁺ T lymphocytes and fibroblasts (Hauck et al., 1999; Shimizu et al., 2000; Li and He, 2006; Ramachandran et al., 2006), and its expression on epithelial cells is increased following exposure to influenza A. Somewhat surprisingly, the corresponding tethered ligand sequence does not activate PAR₃. At first it appeared that proteolytic cleavage of PAR₃ and exposure of its tethered ligand did not induce signalling per se; rather PAR₃ appeared to act as a co-factor for the activation of other PARs (Kahn et al., 1998b). For example, PAR₃ dimerized with PAR₁ and PAR₄ to amplify their signalling (Nakanishi-Matsui et al., 2000; Weiss et al., 2002; McLaughlin et al., 2007). A recent study, however, has shown that thrombin can signal through PAR₃ independently of other PARs, to induce interleukin (IL)-8 release from HEK-293 cells, via ERK1/2 phosphorylation (Ostrowska and Reiser, 2008).

The observation that thrombin could stimulate the aggregation of mouse platelets in the absence of PAR₁ and PAR₃ provided evidence for the existence of a fourth PAR subtype (Kahn et al., 1998b; Xu et al., 1998). Although thrombin activates PAR₄, it is much less potent at PAR₄ than at PAR₁ (Kahn et al., 1998b; Xu et al., 1998). A PAR-AP, HYPGKF-NH₂ also activates PAR₄. No non-peptidic agonists for PAR₄ are available; however, a non-peptidic antagonist has been reported (Wu et al., 2002). A low potency peptide antagonist has been developed, although it exhibits low selectivity and is known to produce non-PAR effects (Hollenberg and Saifeddine, 2001; Hollenberg et al., 2004). P4pal-10 is a high-potency pepducin antagonist for PAR₄, although it also partially inhibits activation of PAR₁ by SFLLRN-NH₂ (Covic et al., 2002a,b; Kuliopulos and Covic, 2003; Hansen et al., 2008).

PAR₄ is widely expressed in the lung, present on endothelial and epithelial cells, airway smooth muscle cells as well as alveolar macrophages (Lan et al., 2000; Shimizu et al., 2000; Asokananthan et al., 2002; Kataoka et al., 2003). PAR₄ expression on bronchial fibroblasts was recently reported to be elevated following exposure to inflammatory stimuli, such as tumour necrosis factor-α (TNF-α) (Ramachandran et al., 2007). PAR₄ signals through a G_q/11-mediated pathway (Xu et al., 1998).

PAR-mediated effects in the lung

PAR-mediated inhibition of airway smooth muscle tone

PAR₂ activators can modulate bronchomotor tone, with the predominant effect being bronchodilatation. For example, intravenous administration of PAR₂-APs attenuated methacholine-, serotonin- and histamine-induced increases in airway resistance in mice, rats and guinea pigs respectively (Cicala et al., 1999; Cocks et al., 1999; Lan et al., 2004). Consistent with this, PAR₂-APs induce concentration-dependent relaxation response in isolated airway preparations from these animals (Cocks et al., 1999; Chow et al., 2000; Lan et al., 2000; Ricciardolo et al., 2000; Kawabata et al., 2004b; Franchi-Micheli et al., 2005). In general, inhibitors of COX such as indomethacin block PAR₂-induced bronchodilatory effects, indicating a prominent mediator role for relaxant prostanoids in this response (Cicala et al., 1999; Cocks et al., 1999; Lan et al., 2004). Direct evidence that PGE₂ was an important mediator in PAR-induced relaxation responses came from studies showing that exposure of murine airways to PAR₂-APs caused concentration-dependent increases in PGE₂ release, which correlated strongly and positively with the magnitude of the relaxation response (Lan et al., 2001). PAR₂-mediated release of PGE₂ is likely to cause bronchodilator responses via activation of airway smooth muscle EP₂ receptors, which signal through G_αs, adenylate cyclase and cAMP (Fortner et al., 2001; Lan et al., 2001).

PAR-mediated production of the anti-inflammatory prostanoid PGE₂

In most organ systems, PGE₂ promotes inflammatory processes, whereas it produces predominantly anti-inflammatory effects in the lung (Vancheri et al., 2004). This section will cover the specific prostaglandins released by PAR subtypes, the mechanisms involved in PAR-mediated generation of prostaglandins and the modulatory effects of these prostaglandins on inflammatory processes in the airways. As indicated above, PAR activators induce the rapid and sustained formation and release of prostanoids from a wide variety of cell and tissue types. For example, PAR₁-APs cause PGE₂ release from human bronchial epithelial cells (Asokananthan et al., 2002) and human lung fibroblasts (Sokolova et al., 2005; Sokolova et al., 2008). Human bronchial airway epithelial cell cultures release PGE₂ following exposure to PAR₂-APs; however, cultured airway smooth muscle cells, while capable of PGE₂ production, do not do so in response to PAR₂ activation (Pang and Knox, 1997; Asokananthan et al., 2002; Chambers et al., 2003; Dulon et al., 2005; Sharma et al., 2006). Tryspin induces PGE₂ release from airway smooth muscle cells independently of PAR₂, suggesting that PAR₄ may induce PGE₂ release from airway smooth muscle (Chambers et al., 2003). PAR₄-APs can also induce PGE₂ release from bronchial epithelial cell cultures (Asokananthan et al., 2002). PAR₁, PAR₂ and PAR₄-APs increased PGE₂ levels in murine isolated tracheal preparations, with PAR₂-APs inducing the largest response (Lan et al., 2001). As PAR₂ expression on airway epithelial cells increases in response to a variety of inflammatory stimuli, PAR₂-mediated generation of PGE₂ may be amplified in inflammatory lung disorders (Knight et al., 2001).

PAR-mediated PGE₂ production occurs via a complex signalling pathway, which has been studied in greatest detail in cultures of A549 cells (a transformed human airway epithelial cell line) (Kawao et al., 2005), and in mouse isolated tracheal preparations (Kawabata et al., 2004b). In these systems, PAR₂-APs induce a rapid, transient increase in PGE₂ levels, which is thought to sequentially involve the activation of PAR₂, an increase in [Ca²⁺]_i, activation of cytosolic phospholipase A₂, release of arachidonic acid and downstream processing of arachidonic acid by COX-1 and prostaglandin E synthase (PGES). This is followed by a second, sustained phase of PGE₂ generation involving COX-1-dependent up-regulation of COX-2 and microsomal prostaglandin E synthase-1 (mPGES-1) (Kawabata et al., 2004b; Kawabata and Kawao, 2005; Kawao et al., 2005; Nagataki et al., 2007; Sekiguchi et al., 2007; Hsieh et al., 2008; Rastogi et al., 2008; Wang et al., 2008). PAR₂-mediated production of PGE₂ appears to be dependent on mPGES-1 but not mPGES-2 or cPGES (Nagataki et al., 2007). Figure 1 shows a simplified representation of PAR₂-mediated generation of PGE₂.

Figure 1.

PAR₂-mediated generation of PGE₂ in the airways.

PGE₂ induces an extensive array of effects within the respiratory tract ( Table 1), affecting the activity of most structural and inflammatory cells. These effects are mediated by E prostanoid receptors, a family of four G protein-coupled receptors (EP₁–EP₄) linked to G_q/11 (EP₁ receptors), G_s (EP₂ and EP₄ receptors) and G_i/o (EP₃ receptors) (Alexander et al., 2008). As highlighted by Vancheri and coworkers (Vancheri et al., 2004) and the data presented in Table 1, PGE₂ appears to have a role in limiting the immune-inflammatory response and tissue repair processes. These findings indicate that PAR-mediated production of PGE₂ may be beneficial in instances when dysregulated inflammatory and tissue repair processes contribute to disease.

Table 1.

Summary of effects produced by prostglandin E₂ (PGE₂) in isolated cells of the respiratory tract, in animal models of airway disease, and in humans with airway disease

	Response to PGE2	EP receptor (species if not human)	Reference
In vitro effects of PGE2 on
Airway smooth muscle	Relaxation	EP2	Norel et al. (1999)
	↓Proliferation	EP2	Burgess et al. (2004); Kassel et al. (2008)
	↓Migration	?	Goncharova et al. (2003)
	↓RANTES, ICAM, GM-CSF, Il-8, eotaxin, MCP-1	?	Ammit et al. (2000); Lazzeri et al. (2001); Wuyts et al. (2003); Kaur et al. (2008)
	↑VEGF, G-CSF, Il-6	EP2/EP4	Ammit et al. (2000); Bradbury et al. (2005); Clarke et al. (2005)
Epithelial	↑Cilia beat frequency	?	Bonin et al. (1992); Schuil et al. (1995); Haxel et al. (2001)
	↑Cl-channel conductance	EP4 (frog, cow)	Clayton et al. (2005); Palmer et al. (2006); Joy and Cowley (2008); Seto et al. (2008)
	↑Na+ transport	EP1/EP2 (frog)	Berk et al. (2004)
	↑Mucin secretion	EP4	Kook Kim et al. (2006)
	↑MUC5A/8 expression	EP4	Cho et al. (2005); Kook Kim et al. (2006); Song et al. (2009)
	↑Rate of wound closure	EP1/EP4/EP2?	Savla et al. (2001)
	↓Il-8 & ET-1 secretion	EP3/EP4	Pelletier et al. (2001); Hattori et al. (2008)
	↑Il-6 release	EP2/EP4	Tavakoli et al. (2001)
Submucosal gland	↑Ionic currents & secretory function	? (pig)	Liu et al. (2005)
	↑Sensitivity to acetylcholine	EP2 (pig)	Liu and Farley (2007)
Cholinergic nerve	↓Acetylcholine release	EP3 (guinea pig, dog)	Deckers et al. (1989); Zhao et al. (1994); Spicuzza et al. (1998); Clarke et al. (2004)
Sensory nerve (pulmonary C-fibre afferents)	↑Sensitivity to chemical stimulants	EP2 (rat)	Ho et al. (2000); Kwong and Lee (2002; 2005;)
Alveolar type II	↑Surfactant secretion	EP1 (rat)	Marino and Rooney (1980); Morsy et al. (2001)
	↑Na+ uptake	EP3	Mukhopadhyay et al. (1998)
Pulmonary endothelial	↑Barrier function		Birukova et al. (2007)
Alveolar macrophage	↓Phagocytosis	EP2 (mouse/rat)	Canning et al. (1991); Aronoff et al. (2004); Canetti et al. (2007); Brock et al. (2008); Lee et al. (2009); Medeiros et al. (2009)
	↓Bacterial killing	EP2/EP4	Serezani et al. (2007)
	↓Mitochondrial inner membrane perturbation and necrosis	EP2	Chen et al. (2008)
	↓TNF-α	EP2/EP4	Ratcliffe et al. (2007)
	↑Il-10 & NO release	? (rat)	Menard et al. (2007)
Fibroblast	↓proliferation	EP2	Bitterman et al. (1986); Liu et al. (2004); Huang et al. (2007); Huang et al. (2008)
	↓Collagen production	EP2	Saltzman et al. (1982); Fine et al. (1989); Liu et al. (2004); Huang et al. (2007); Huang et al. (2008)
	↓Fibroblast to myofibroblast transition	?	Kolodsick et al. (2003)
	↓Myofibroblast differentiation		Dunkern et al. (2007)
	↓Migration	EP2	Kohyama et al. (2001); White et al. (2005)
	↓Smoke-induced apoptosis	EP2	Sugiura et al. (2007)
Mast cell	↓Migration	EP3?	Duffy et al. (2008)
	↓Histamine release	EP2	Drury et al. (1998); Kay et al. (2006); Duffy et al. (2008)
Dendritic cell	↑Podosome dissolution		van Helden et al. (2008)
	↑Migration	EP2/EP4	Legler et al. (2006)
	↑Maturation	EP2/EP4	Kubo et al. (2004)
	↓CCL3 and CCL4	EP2/EP4 (mouse)	Jing et al. (2003)
	↑Resistance to apoptosis	EP2/EP4	Baratelli et al. (2005a)
	↓TNF-α release & antigen presentation	? (mouse)	Kambayashi et al. (2001)
T cells	↓Proliferation	?	Jarvinen et al. (2008)
	↑Il-10 expression	?	Benbernou et al. (1997)
Treg	↑Inhibitory function & differentiation	?	Baratelli et al. (2005b)
	↑Expansion	?	Garg et al. (2008)
Eosinophils	↓Migration	EP2/EP4	Sturm et al. (2008)
	↓Release from bone marrow	? (guinea pig)	Sturm et al. (2008)
	↓Degranulation	EP2/EP4	Sturm et al. (2008)
	↓PAF-induced aggregation	EP2 (guinea pig)	Teixeira et al. (1997)
Neutrophils	↓Chemotaxis	EP2?	Armstrong (1995)
B cells	↓Proliferation	EP4 (mouse)	Murn et al. (2008)
	Promotes differentiation and Il-4 and LPS-driven class switching to IgE	EP2/EP4 (mouse)	Fedyk and Phipps (1996)
In vivo effects of PGE2 in
Normal animals or subjects	Bronchodilatation	EP2 (mouse)	Mathe and Hedqvist (1975); Smith et al. (1975); Sheller et al. (2000); Tilley et al. (2003)
Asthmatics (allergic)	Bronchodilatation		Smith et al. (1975)
	↓Early and late response to allergen		Pavord et al. (1993); Gauvreau et al. (1999)
	↓Airway hyperresponsiveness and sputum eosinophils		Gauvreau et al. (1999)
	↓BAL PGD2, ↓BAL eosinophils		Hartert et al. (2000)
Asthmatic (exercise)	↓Exercise-induced bronchoconstriction		Melillo et al. (1994)
Allergic inflammation	↓Allergen-induced bronchoconstriction	EP2/EP4 (guinea pig)	Martin et al. (2002); Tanaka et al. (2005)
	↓BAL eosinophils	? (mouse, rat)	Martin et al. (2002); De Campo and Henry (2005); Sturm et al. (2008)
	↓BAL LTs, ↓T cell cytokine expression	? (rat)	Martin et al. (2002)
Bleomycin-induced fibrosis	PGE2 synthetic analogue 16,16-dimethyl-PGE2↓infiltration by leukocytes, ↓myeloperoxidase activity, ↓Il-1, TNF-α and nitrotyrosine, ↓lung edema & injury, ↓collagen deposition, ↓weight loss and mortality	? (mouse)	Failla et al. (2009)

Not all references to PGE₂-induced responses are cited, with preference given to studies that have used human cells, tissues or subjects. Animal studies are cited where human studies have not been reported, or to indicate which E-prostanoid (EP) receptor subtype mediated the response.

PAR-mediated production of pro-inflammatory cytokines

PAR activation may also promote inflammatory processes in the respiratory tract, as indicated by reports that PAR-APs induce the release of pro-inflammatory cytokines and mediators in cell cultures relevant to airways disease. For example, PAR₁, PAR₂ and PAR₄-APs induced IL-6 and IL-8 release from airway epithelial cells (Asokananthan et al., 2002). Additionally, PAR₁- and PAR₂-APs, but not PAR₃- or PAR₄-APs, induced IL-6, but not IL-13, release from cultured human T cells (Li and He, 2006). PAR₁ activators have also been reported to induce CCL2, IL-1β and TNF-α release from macrophages (Naldini et al., 1998; 2002; Riewald et al., 2002; Colognato et al., 2003), and PAR₂-APs induced MMP-9 release from primary cultures of human airway epithelial cells (Vliagoftis et al., 2000). The PAR₂-AP, SLIGKV-NH₂, induced GM-CSF mRNA expression but did not increase protein expression in human fibroblasts. SLIGKV-NH₂ also increased cell surface expression of VCAM-1 on primary human bronchial fibroblasts (Ramachandran et al., 2006). Furthermore, PAR₂ activation promoted eosinophil degranulation and superoxide production (Miike et al., 2001), and may promote IL-8 release (Temkin et al., 2002).

PAR-mediated modulation of allergic, eosinophilic inflammation

Consistent with PAR-mediated production of pro-inflammatory cytokines in vitro, several in vivo studies have demonstrated a pro-inflammatory role for PARs in the airways. For example, mice lacking PAR₂ exhibit decreased inflammation in an allergen challenge model, whereas overexpression of PAR₂ was associated with increased inflammation (Schmidlin et al., 2002; Takizawa et al., 2005). In subsequent studies, co-administration of PAR₂-APs caused increased cell influx, IL-5, IL-13 and TNF-α in bronchoalveolar lavage (BAL) fluid, while lowering IL-10, in allergen-sensitized and challenged mice (Ebeling et al., 2005; 2007;). In these latter studies, a single dose of PAR₂-AP did not induce an inflammatory response; however, multiple doses did. Other studies have suggested that PAR₂ may induce airway inflammation through a neuropeptide-dependent mechanism; however, this requires further investigation given that PAR₂-APs are known to activate NK₁ receptors (Su et al., 2005; Abey et al., 2006).

On the contrary, there is also a significant body of evidence indicating that PAR activators suppress inflammatory processes within the lung. Of particular relevance to the current review, the anti-inflammatory effects produced by PAR activators appear to be mediated by prostaglandins such as PGE₂ (Asokananthan et al., 2002; Henry, 2006). For example, intratracheally administered PAR₂-APs reduced bronchoconstriction, airway hyperresponsiveness and eosinophil influx in a murine model of allergic inflammation (De Campo and Henry, 2005). This PAR-mediated effect was suppressed by inhibitors of COX and mimicked by PGE₂ (De Campo and Henry, 2005). Administration of PAR₂-AP to antigen-sensitized and challenged rabbits was also associated with reduced bronchoconstriction, airway hyperresponsiveness and eosinophilia (D'Agostino et al., 2007), although the role of COX and PGE₂ is uncertain. Nevertheless, this latter study showed a marked increase in IL-10 mRNA from CD4⁺ T cells recovered from PAR₂-AP-treated animals, as well as decreased interferon (IFN)-γ and IL-2 production to control levels (D'Agostino et al., 2007), consistent with an anti-inflammatory response.

Thus, a puzzling, yet not uncommon trait of research investigating the role of PARs in inflammatory processes in the respiratory system is the existence of seemingly contradictory observations, with some reports demonstrating that PAR-APs induce inflammatory responses, and others demonstrating overt anti-inflammatory effects. Although there is unlikely to be single reason that adequately explains these inconsistencies, a contributing factor is likely to be the lack of subtype-selective small-molecule agonists and antagonists for PARs, often necessitating the use of indirect methods to determine the role of PARs in these processes.

As PAR-APs are typically between five and seven amino acids long, the development of small-molecule lead compounds that selectively bind to PARs is problematic. Thus, there is a current paucity of agents capable of selectively activating and inhibiting these receptors. Until recently, this has necessitated the use of one or more of the following approaches to explore PARs: (i) use of PAR-activating proteases; (ii) use of relatively selective but low-potency PAR-APs; and (iii) manipulation of PAR expression.

Although numerous proteases are well-established activators of PARs, their use in characterizing subtypes of PARs is often limited. One particular limitation is that proteases frequently possess a multitude of effects – being capable of activating more than one PAR subtype as well as inducing PAR-independent effects. For example, thrombin can activate PAR₁, PAR₃ and PAR₄, as well as induce smooth muscle proliferation in a PAR-independent fashion (Tran and Stewart, 2002). The currently used PAR-APs are typically more selective than proteases, but most lack absolute subtype specificity and may activate non-PAR receptors (Abey et al., 2006). Furthermore, aminopeptidases can degrade most PAR-APs, which limits their effectiveness. An exception is the aminopeptidase-resistant PAR₂-AP 2-furoyl-LIGRLO-NH₂ (Kawabata et al., 2004a). Gene knockout and overexpression approaches have been used to investigate the role of PARs, although the ubiquitous expression of PARs, and their central role in platelet aggregation, coagulation and inflammation makes clear interpretation of findings difficult. The development of potent, subtype-selective, small-molecule ligands for PARs will provide valuable information on the roles of these receptors, and may help clarify current inconsistencies in the airway PAR literature.

Potential role of PARs in asthma

Asthma is a chronic inflammatory airway disease, characterized by shortness of breath and repeated wheezing episodes (Masoli et al., 2004; Hamid and Tulic, 2009). Allergic asthma involves an inappropriately large immune response to one or more inhaled allergens (Busse and Lemanske, 2001). In asthmatic individuals, otherwise innocuous stimuli trigger a Th₂ cell-driven immune response frequently involving antigen-specific IgE production, release of mast cell-derived mediators and recruitment of eosinophils to the airways. Allergen-induced inflammation is typically associated with acute bronchoconstriction, airway hyperresponsiveness and eventually, airway remodelling. The current mainstay of asthma treatment remains glucocorticoids as a preventative medication, with short- or long-acting β₂-adrenoceptor agonists as a reliever from acute attacks (Adcock et al., 2008). Recently, newer medications such as anti-leukotrienes and IgE inhibitors have been used in the clinical management of asthma (Holgate and Polosa, 2008). While many of the current medications are useful in managing the clinical symptoms of asthma, they are not curative and the search for better asthma treatments remains an important focus.

Increased immunization, antibiotic use, altered diet and decreased pathogen exposure contribute to an immune system that is more susceptible to developing allergies (Strachan, 1989; Anderson et al., 2001; Kaiser, 2004; Devereux, 2006). Of the immune cells involved in allergic asthma, the CD4⁺ T cells appear to play a central role in the development and maintenance of allergic sensitization (Fischer et al., 2007; Anderson, 2008; Burchell et al., 2008; Pucci and Incorvaia, 2008). CD4⁺ T cells can be broadly categorized into four subsets, namely, T helper type 1 (Th₁), Th₂, Th₁₇ and regulatory T cells (T_reg). Subtype selection of undifferentiated T cells (Th₀) is controlled by the cytokine signals present upon stimulation of the Th₀ cell (Murphy and Reiner, 2002; Curiel, 2007; McGeachy and Cua, 2008; Korn et al., 2009). These cells serve varied and distinct functions in immune responses (Broide, 2008). Th₁ cells are primarily responsible for coordinating the immune response to intracellular infections such as viruses (Murphy and Reiner, 2002). Th₂ cells are primarily involved in the destruction of extracellular pathogens such as helminth parasites (Fallon and Mangan, 2007). Th₁₇ cells have not been studied as extensively due to their recent discovery; however, an increasing body of evidence suggests their involvement in asthma and allergy (Infante-Duarte et al., 2000; Korn et al., 2008; Lochner et al., 2008; McKinley et al., 2008; Oboki et al., 2008). T_reg cells suppress inflammation and are involved in maintaining peripheral tolerance (Cohn, 2008; Vignali et al., 2008). A polarization towards Th₂-type responses appears central to the pathogenesis of asthma (Salvi et al., 2001; O'Byrne et al., 2004; Strickland et al., 2006; Galli et al., 2008; Oboki et al., 2008; Pucci and Incorvaia, 2008; Broide, 2009; Hamid and Tulic, 2009). While recent evidence suggests that Th₂ cells are not the only T helper lineage involved in allergic asthma, they play a significant role and therefore, inhibiting their actions may prove useful in asthma (Holt et al., 2005; Fischer et al., 2007; Holt and Sly, 2007; Caramori et al., 2008; Schmidt-Weber, 2008). Very few controlled clinical trials have determined the effect of currently used medications on Th₂ cell responses (Caramori et al., 2008).

PAR activation may alter CD4⁺ cell polarization in the airways and inhibit allergic inflammation through a variety of mechanisms, including via production of PGE₂. While PGE₂ favours Th₁ differentiation in vitro, the converse appears to be true in vivo (Betz and Fox, 1991; Snijdewint et al., 1993; Martin et al., 2002; Nagamachi et al., 2007). Administering PGE₂in vivo appears to inhibit Th₂ activation and cytokine expression (Martin et al., 2002), and inhibits T cell proliferation through EP₂ receptors (Nataraj et al., 2001). PGE₂ also inhibits transendothelial migration of lymphocytes into the airways, likely via increased intracellular cAMP (Pober et al., 1993; Oppenheimer-Marks et al., 1994; Panettieri et al., 1995). Consistent with this, mice deficient in COX-1 or COX-2 show increased inflammation upon allergen challenge, and inhibitors of COX-1 or COX-2 produce similar effects (Gavett et al., 1999; Stokes Peebles et al., 2002; Carey et al., 2003). Epidemiological studies have also shown that frequent use of COX inhibitors may increase the risk of developing asthma and allergy (Allmers, 2005). Furthermore, a recent study has shown that exposing mice to indomethacin during allergic sensitization increases primary and memory Th₂ responses in vivo (Zhou et al., 2008). Thus, the activation and up-regulation of COX may well increase prostaglandin levels in the lung, thereby reducing Th₂ polarization and inflammation.

Elevating PGI₂ levels represents another distinct pathway by which PARs could alter T helper cell polarization in the airways. Activation of endothelial PAR₁- and PAR₂-APs promotes PGI₂ release (Syeda et al., 2006), which may alter Th₂ immune function by stimulating Th₂ cells to release IL-10, an anti-inflammatory cytokine that serves to suppress immune responses (Jaffar et al., 2002). Additionally, PGI₂ markedly inhibits CCL17-induced chemotaxis of Th₂ cells, perhaps by inhibiting CCR4 and/or CCR8 signalling, resulting in fewer Th₂ cells being recruited to the airways following allergen challenge (Jaffar et al., 2007). Furthermore, pre-treating Th₂ cells with PGI₂ before adoptive transfer markedly decreased inflammation in mice following allergen challenge (Jaffar et al., 2007). Interestingly, Th₂ cells exhibit increased expression of prostanoid IP receptors compared with Th₁ cells. This allows for selective inhibition of Th₂ cells by PGI₂, which may reduce the ratio of Th₂ cells present in the airways (Jaffar et al., 2002). Thus, PAR-mediated production of PGI₂ may selectively inhibit Th₂ immune responses.

IP receptor-deficient mice exhibit increased eosinophil, lymphocyte and neutrophil influx in an allergen challenge model. These IP-null mice also had increased total and antigen-specific serum levels of IgE as well as total IgG. Allergen challenge of isolated spleenocytes from these IP-null mice resulted in increased IL-4 compared with wild-type mice (Takahashi et al., 2002). These findings are supported by studies using an IP receptor agonist, iloprost, in an allergen challenge model. Iloprost decreased allergen-induced BAL fluid levels of eosinophils, lymphocytes, IL-4, IL-5 and IL-13; and these effects were abolished by an IP receptor antagonist. Additionally, iloprost inhibited allergen-induced increases in airway resistance, as well as decreases in compliance (Idzko et al., 2007). Iloprost also altered dendritic cell (DC) function. DCs incubated with iloprost and then adoptively transferred to mice showed markedly decreased Th₂-like responses upon subsequent allergen challenge compared with vehicle-treated DCs. Iloprost treatment of DCs prior to adoptive transfer inhibited allergen-induced BAL numbers of macrophages, lymphocytes and eosinophils; as well as inhibiting IL-4, IL-5 and IL-13; and increasing IL-10 and IFN-γ production (Idzko et al., 2007). Thus, PGI₂ decreases Th₂ cell mediator release, DC induction of Th₂ cell differentiation and allergen-induced cell influx. Additionally, PGI₂ increases levels of the anti-inflammatory cytokine IL-10. Thus, agents capable of increasing airway PGI₂ levels, such as PARs, may prove to be useful in the treatment of asthma.

In addition to inhibition of Th₂ cells, it is likely that induction of T_reg would reduce airway inflammation in allergic asthma (McGee and Agrawal, 2006; Bohle et al., 2007; Adcock et al., 2008; Burchell et al., 2008; Jin et al., 2008). In this context, PAR₂ activation induced the maturation of immature murine DCs (iDCs) into mature DCs (mDCs) (Fields et al., 2003). In addition, recent evidence suggests that PGE₂ may induce immune tolerance via induction of T_reg (Li et al., 2008; Muthuswamy et al., 2008). Pulmonary iDCs exposed to PGE₂ during maturation resulted in mDCs that attracted T_reg with increased affinity (Muthuswamy et al., 2008). This effect was mediated by PGE₂-induced hypersecretion of CCL22, a proposed selective T_reg chemokine (Curiel et al., 2004; Muthuswamy et al., 2008). IFN-α ablated this effect, suggesting that if viral infection were concomitant, an effective immune response would still develop (Muthuswamy et al., 2008). Neither PGE₂ nor lipopolysaccharide (LPS) alone induced nearly as strong an effect; co-stimulation was necessary for hypersecretion of CCL22. This suggests a potential mechanism to explain the allergy resistance conferred by early life exposure to LPS (Cochran et al., 2002).

Potential role of PARs in neutrophilic inflammation

Elevated numbers of neutrophils are a characteristic feature of numerous inflammatory lung diseases, including chronic obstructive pulmonary disease (COPD) and acute respiratory distress syndrome (ARDS), as well as certain forms of asthma (Pettersen and Adler, 2002; Jeffery, 2004; Kamath et al., 2005; Cepkova and Matthay, 2006; Noma et al., 2008). Signals for the influx of neutrophils into the lung are likely to include LTB₄ and IL-8, whose levels become elevated in response to inflammatory stimuli such as bacterial LPS. While the exact effects of LPS exposure in asthma remain to be elucidated, it appears to be involved in the development and severity of asthma (Michel, 2003). LPS exposure early in life may provide some protection against developing asthma; however, in established asthma, LPS exposure levels appear to be correlated with disease severity (Lapa e Silva et al., 2000; Cochran et al., 2002; Jung et al., 2006; Kim et al., 2007). Inhibiting neutrophil influx in asthmatics may be useful as neutrophils cause significant damage to the airways and are associated with severe and treatment-resistant forms of the disease (Delclaux et al., 1997; Pettersen and Adler, 2002). Additionally, ARDS and COPD are also characterized by intense neutrophilia and may benefit from agents that reduce neutrophil influx into the lung. In this context, activators of PAR₂ reduced the airway neutrophilia associated with LPS exposure in mice (Moffatt et al., 2002; Saleh et al., 2008). PGE₂ also inhibited LPS-induced neutrophilia, indicating this product as a likely mediator of PAR₂-AP-induced inhibition of LPS-induced neutrophilia (Goncalves de Moraes et al., 1996; Saleh et al., 2008). Isolated bronchi from LPS-treated rats showed increased relaxation in response to PAR₂-APs, as well as increased PGE₂ release (Morello et al., 2005). Thus PAR₂-mediated reductions in LPS-induced neutrophilia may be mediated by PGE₂; however, this remains to be shown experimentally. As effective medications for both ARDS and COPD are lacking, exploration of the possible benefits of novel therapeutic strategies such as activators of PARs is opportune.

Respiratory tract virus-induced exacerbations of asthma are typically associated with airway neutrophilia (Dougherty and Fahy, 2009). PAR activators can modulate host responses to respiratory tract viral infection, although the role of prostaglandins in these responses is unknown. For example, a recent study using cultured human monocytes revealed that PAR₂-APs were able to increase IFN-γ-induced effects, resulting in lower titres of influenza A virus, indicating a potentially protective role of PAR₂ activation during the progression of influenza A virus infection (Feld et al., 2008). There are no published reports of the in vivo effects of PAR activation on the time-course of a respiratory tract viral infection. Nevertheless, influenza A virus infection in mice has been associated with elevated epithelial PAR expression, and augmented PAR₂-mediated inhibition of methacholine-induced bronchoconstriction (Lan et al., 2004). In non-respiratory systems, PAR₁-APs have been shown to increase the viral infectivity of herpes simplex virus in human foreskin fibroblasts and human umbilical vein endothelial cells; however, neither PAR₂ nor PAR₄-APs increased viral infectivity (Sutherland et al., 2007).

Changes in the levels or activity of the enzymes involved in the synthesis of prostaglandins can alter the host response to respiratory tract viral infection. Carey and coworkers (2005) revealed that deficiency of COX-1 is associated with an enhanced inflammatory response and earlier increases in the levels of the pro-inflammatory cytokines TNF-α and IL-1β (Carey et al., 2005). In contrast, deficiency of COX-2 resulted in reduced inflammation and pro-inflammatory cytokine release, reduced morbidity and enhanced survival (Carey et al., 2005). Thus, COX-1 and COX-2 appear to exert important but contrasting effects on the host immune response to influenza viral infection, which may be due to altered production of prostaglandins and leukotrienes. In a related study, mice that overexpress PGI₂ synthase selectively in bronchial epithelium had less respiratory syncytial virus-induced illness (Hashimoto et al., 2004). In contrast, IP receptor-null mice showed increased mortality, weight loss and viral titers (Hashimoto et al., 2004). This is consistent with earlier reports that exogenous PGI₂ administration greatly enhanced survival of mice exposed to viral infection and interestingly, this effect was reduced by COX inhibition (Zavagno et al., 1987). As PARs increase prostaglandin production, more research is justified to examine the in vivo effects of PAR activators during viral infection of the lungs.

Potential role of PARs in pulmonary fibrosis

Pulmonary fibrosis is a characteristic pathologic feature of many lung diseases, including asthma, ARDS, COPD and idiopathic pulmonary fibrosis (Wynn, 2007). Fibrosis is difficult to reverse pharmacologically and is a major factor in the morbidity and mortality associated with these lung diseases (Rogliani et al., 2008). Pulmonary fibrosis is a complex process involving varying extents of epithelial and endothelial injury, a state of hypercoagulation, fibroblast activation and differentiation, epithelial-mesenchymal transition, fibrocyte recruitment, extracellular matrix deposition, angiogenesis and aberrant repair mechanisms (Scotton and Chambers, 2007). A key cell-type involved in extracellular matrix deposition is the myofibroblast, whose numbers are elevated in fibrotic disease due to increased proliferation and decreased apoptosis. IL-4, IL-5 and IL-13 are important cytokines in pulmonary fibrosis as they induce release of active transforming growth factor-β (TGF-β), a potent fibrotic agent, capable of causing fibroblast proliferation and inflammatory cell recruitment through MCP-1 activation of CCR2 (Strutz, 2001; Szardening-Kirchner et al., 2008). Along with these potent fibrotic effects, however, TGF-β is also involved in T_reg cell differentiation (Huber and Schramm, 2006; Wahl, 2007; Chen et al., 2008). T_reg cells release IL-10, which suppresses inflammation and inhibits fibrosis, making their induction an attractive target for the treatment of fibrosis (Holsti et al., 2004; Nakagome et al., 2006; Couper et al., 2008). The pathogenesis, aetiology and regulation of pulmonary fibrosis have recently been expertly reviewed (Wilson and Wynn, 2009). Here we will focus on the role of PARs and prostaglandins in pulmonary fibrosis.

Activation of the coagulation cascade, with the resultant generation of coagulation proteases such as thrombin, plays a central role in acute and chronic phases of fibrotic lung disease. For example, continuous infusion of a direct thrombin inhibitor significantly reduced lung collagen accumulation in a bleomycin model of pulmonary fibrosis (Howell et al., 2002). A prominent role for PAR₁ in this model was subsequently established in studies showing that gene knockout of PAR₁ was protective against bleomycin-induced fibrosis (Howell et al., 2005). It is not entirely clear how thrombin promotes fibrosis, although it induces PAR₁-dependent fibroblast to myofibroblast proliferation dependent upon PKC-α (Bogatkevich et al., 2001), and inhibits apoptosis of fibroblasts through a PKC-ε-dependent mechanism (Bogatkevich et al., 2005). Recent studies indicate that PAR₄ may also play a profibotic role. Stimulation of epithelial PAR₄ with thrombin or a PAR₄ AP-induced epithelial-mesenchymal transition, as evidenced by changes in cell morphology and changes in the expression of epithelial (e-cadherin) and myofibroblast (α-smooth muscle actin) markers (Ando et al., 2007). In contrast, there is evidence that PAR₄ may suppress pulmonary fibrosis by countering PAR₁-stimulated proliferation of fibroblasts. In these studies, exposure of human primary bronchial fibroblasts to pro-inflammatory stimuli induced expression of functional PAR₄ on fibroblasts (Ramachandran et al., 2007). In these TNF-α-stimulated fibroblasts, thrombin no longer induced proliferation, and a PAR₄-AP caused a reduction in fibroblast cell number (Ramachandran et al., 2007). In this setting, a specific PAR₁-AP retained its mitogenic effects, indicating that thrombin activation of PAR₄ appears to suppress thrombin-mediated PAR₁ signalling (Ramachandran et al., 2007). Furthermore, induction of PAR₄ expression enables cathespin G signalling, a proteinase that silences PAR₁ and PAR₂ but activates PAR₄. Interestingly, in TNF-α-treated fibroblasts, tryptase silenced PAR₂, rather than activating it (Ramachandran et al., 2007).

Intratracheal administration of APC, a coagulation cascade inhibitory protein, is protective in bleomycin-induced pulmonary fibrosis (Yasui et al., 2001). This is of particular interest because APC can activate PAR₁ via its coreceptor, the endothelial cell protein C receptor (Riewald et al., 2002). Furthermore, this group has introduced the concept that when activating PAR₁, APC can stimulate signalling pathways distinct from those activated by thrombin, that is, the paradoxical condition that these two key coagulation proteases can mediate opposite effects on endothelial biology through the same receptor, PAR₁ (Riewald and Ruf 2005; Schuepbach et al., 2008). Thus, stimulation of PAR₁ could inhibit or enhance fibrotic effects, depending on the method of activation.

While PAR₁ activation is typically associated with the development of fibrosis, the prostanoids PGE₂ and PGI₂ are potent anti-fibrotic agents. Suppression of COX activity increases bleomycin-induced fibrosis in mice (Keerthisingam et al., 2001; Bonner et al., 2002), and CCR2-null mice are protected from bleomycin-induced fibrosis, due to increased PGE₂ production from airway epithelial cells (Moore et al., 2001; 2003; Lama et al., 2002). EP₂ receptor activation by PGE₂ inhibits fibroblast proliferation and migration, transition to myofibroblast and collagen synthesis (Kolodsick et al., 2003; Moore et al., 2005; White et al., 2005). PGI₂ also appears to be anti-fibrotic, with IP receptor-null mice being more susceptible to bleomycin-induced lung fibrosis (Lovgren et al., 2006). Interestingly, in these studies, E-prostanoid receptor-null and mPGES-1-null mice did not exhibit any increase in bleomycin-induced fibrosis (Lovgren et al., 2006). Consistent with this, no increase in bleomycin-induced fibrosis was observed in COX-2-null mice, despite lung dysfunction (Card et al., 2007).

PAR activation induces PGE₂ release from fibroblasts, which down-regulate PAR₁ expression on these cells via a negative feedback loop (Sokolova et al., 2005; 2008;). PAR₂-mediated generation of PGE₂ by airway epithelial cells (Lan et al., 2001) may also suppress fibroblast PAR₁ expression. Together, these findings indicate that PAR-mediated PGE₂ production within the airways may inhibit fibrosis through a number of mechanisms – indirectly via PGE₂-mediated suppression of fibroblast PAR₁ expression and directly by inhibiting fibroblast function as described above.

In summary, it appears that antagonists of PAR₁ may yield effective therapies in fibrosis. The role of other PAR subtypes is less clear with preliminary data suggesting that PAR₂-mediated generation of PGE₂ and PGI₂ inhibits fibrosis. Thus, a combination of PAR₁ antagonists, PAR₂-APs and protease inhibitors may be beneficial in suppressing fibrosis, although the specific pharmacological agents required to fully test this hypothesis are still in the development stage.

PAR interactions with respiratory system pharmacotherapies

Glucocorticoids are the mainstay therapy for inflammatory airway diseases and are likely to remain so for the foreseeable future. Thus, it is necessary for any new medications to maintain therapeutic activity when co-administered with glucocorticoids. As glucocorticoids suppress many of the pathways of PGE₂ production, the interaction between dexamethasone and PAR₂-APs has been investigated. While dexamethasone suppressed SLIGRL-induced increases in PGE₂ in cell culture, long- or short-term dexamethasone did not affect the PGE₂-dependent, SLIGRL-induced relaxation of isolated tracheal preparations (Saleh et al., 2008). Furthermore, dexamethasone pre-treatment did not ablate PAR₂-AP-induced reductions in LPS-induced neutrophilia in mice (Saleh et al., 2008). Interestingly, glucocorticoids were able to suppress PAR₂-mediated MMP-9 release from epithelial cells in vitro; however, this effect has not been examined in vivo (Vliagoftis et al., 2000). This raises the possibility that glucocorticoids may be able to suppress PAR-mediated inflammatory pathways, but not anti-inflammatory pathways, although additional in vivo studies are required to confirm this. Although the interaction between PAR₂-APs and glucocorticoids has not been evaluated in other models of airway inflammation, these findings indicate that agents capable of increasing endogenous PGE₂ production are likely to retain their effectiveness in the presence of glucocorticoids.

Other therapeutic agents also modulate PAR-mediated PGE₂ production in the airways. Inhibition of prostaglandin metabolism by the thiazolidinedione compound rosiglitazone (Cho and Tai, 2002) augmented PAR₂-mediated increases in PGE₂ levels and relaxation of isolated tracheal preparations (Henry et al., 2005). Whether these effects persist in vivo is unknown, but warrants further investigation. Rosiglitazone is more widely recognized as an activator of PPAR-γ, and this class of drug alters a variety of inflammatory processes within the airways (Belvisi et al., 2006; Ward and Tan, 2007). Further studies are required to investigate the potentially useful anti-inflammatory effects produced by combinations of glucocorticoids, PARs and PPAR-γ agonists (Belvisi et al., 2006; Usami et al., 2006).

Conclusions

While PARs are capable of inducing a wide range of inflammatory processes and cytokines within the lung, PAR-mediated prostaglandin production represents a distinct anti-inflammatory pathway. In the case of allergic inflammation, activation of PARs on epithelial cells causes PGE₂ production, resulting in lowered inflammatory cell recruitment and airway hyperresponsiveness, with PAR₂ being the most prevalent inducer of PGE₂ release (De Campo and Henry, 2005). Thus, inhaled agonists of PAR₂ may prove to be useful agents in the treatment of allergic airway inflammation. Additionally, the inhibition of neutrophil recruitment by PAR₂ is also likely to be prostaglandin-dependent. Coupled to their other anti-inflammatory properties, this effect would likely prove useful in the treatment of inflammatory airway diseases characterized by intense neutrophilia, such as ARDS, COPD and some forms of asthma (Moffatt et al., 2002). Additionally, in vivo retention of anti-inflammatory effects upon concomitant glucocorticoid treatment makes PAR-mediated prostaglandin production an even more attractive target for the development of inflammatory airway disease treatments (Saleh et al., 2008). Further investigation is therefore justified to determine whether PAR-mediated anti-inflammatory effects are retained in other models of airway inflammation in vivo. Additionally, the emerging role of PGI₂ in lung inflammation and its production by PARs warrant further investigation.

Glossary

Abbreviations:

AC

adenylate cyclase

ARDS

acute respiratory distress syndrome

BAL

bronchoalveolar lavage

COPD

chronic obstructive pulmonary disease

COX

cyclooxygenase

DC

dendritic cell

IFN

interferon

IL

interleukin

LPS

lipopolysaccharide

mPGES

microsomal prostaglandin E synthase

NK

neurokinin

PAR

protease-activated receptor

PAR-AP

PAR-activating peptide

PGE₂

prostglandin E₂

PGI₂

prostglandin I₂

PGT

prostaglandin transporter

TGF-β

transforming growth factor-β

TNF-α

tumour necrosis factor-α

VCAM

vascular cell adhesion molecule