EP4 receptor as a new target for bronchodilator therapy

Inflammatory profile of eosinophils in asthma-COPD overlap and eosinophilic COPD: a multi-omics study

Introduction

Elevated blood eosinophil levels in patients with chronic obstructive pulmonary disease (COPD) with or without asthma are linked to increased exacerbations and the effectiveness of inhaled corticosteroid treatment. This study aimed to delineate the inflammatory cellular properties of eosinophils in patients with asthma-COPD overlap (ACO) and eosinophilic COPD (eCOPD).

Methods

Eosinophils were isolated from the peripheral blood of healthy volunteers, patients with non-eCOPD, and those with ACO/eCOPD. Multi-omics analysis involving transcriptomics, proteomics, and lipidomics was performed, followed by bioinformatic data analyses. In vitro experiments using eosinophils from healthy volunteers were conducted to investigate the molecular mechanisms underlying cellular alterations in eosinophils.

Results

Proteomics and transcriptomics analyses revealed cellular characteristics in overall COPD patients represented by viral infection (elevated expression of sterol regulatory element-binding protein-1) and inflammatory responses (elevated levels of IL1 receptor-like 1, Fc epsilon receptor Ig, and transmembrane protein 176B). Cholesterol metabolism enzymes were identified as ACO/eCOPD-related factors. Gene Ontology and pathway enrichment analyses demonstrated the key roles of antiviral responses, cholesterol metabolism, and inflammatory molecules-related signaling pathways in ACO/eCOPD. Lipidomics showed the impaired synthesis of cyclooxygenase-derived mediators including prostaglandin E2 (PGE2) in ACO/eCOPD. In vitro assessment confirmed that IL-33 or TNF-α stimulation combined with IL-5 and IFN-γ stimulation induced cellular signatures in eosinophils in ACO/eCOPD. Atorvastatin, dexamethasone, and PGE2 differentially modulated these inflammatory changes.

Discussion

ACO/eCOPD is associated with viral infection and an inflammatory milieu. Therapeutic strategies using statins and inhaled corticosteroids are recommended to control these pathogenic changes.

Comparison of the Genomic Activity of an EP4-Receptor and β2-Adrenoceptor Agonist in BEAS-2B Human Bronchial Epithelial Cells: In Search of Compartmentalized, cAMP-Dependent Gene Expression

It has been proposed that inhaled E-prostanoid 4 (EP4)-receptor agonists could represent a new class of bronchodilators for the treatment of asthma that are as effective as β 2-adrenoceptor agonists. However, the genomic impact of such drugs is unknown despite being potentially deleterious to respiratory health. Herein, we used mRNA-seq to compare the transcriptomic responses produced by 2-[3-[(1R,2S,3R)-3-hydroxy-2-[(E,3S)-3-hydroxy-5-[2-(methoxymethyl)phenyl]pent-1-enyl]-5-oxo-cyclopentyl]sulphanylpropylsulphanyl] acetic acid (ONO-AE1-329; an EP4-receptor agonist) and vilanterol (a β 2-adrenoceptor agonist) in BEAS-2B human airway epithelial cells. We also determined if an increase in cAMP mediated by different G protein-coupled receptors (GPCRs) promoted distinct transcriptional signatures by expanding this inquiry to include the adenosine A2B- and I-prostanoid receptor agonists, 2-[[6-amino-3,5-dicyano-4-[4-(cyclopropylmethoxy)phenyl]-2-pyridinyl]thio]-acetamide (Bay60-6583) and taprostene, respectively. Maximally-effective concentrations of ONO-AE1-329 and vilanterol significantly regulated (q ≤ 0.05; ≥1.5-/≤0.67-fold) 232 and 320 genes, respectively of which 217 were shared. Spearman analysis showed these gene expression changes to be highly rank order correlated, indicating that the functional overlap between the two interventions should be considerable. Unexpectedly, the genomic effects of ONO-AE1-329, vilanterol, Bay 60-6583, and taprostene were also highly rank order correlated. This finding suggests that cAMP generated by any GPCR would initiate the same transcriptional program. Nevertheless, relative to vilanterol, ONO-AE1-329 typically behaved as a partial agonist that varied across transcripts. These data indicate that each ONO-AE1-329-regulated gene differs in sensitivity to cAMP and is defined by a unique receptor occupancy-response relationship. Moreover, if this relatively modest genomic response in BEAS-2B cells is retained in vivo, then inhaled EP4-receptor agonists could represent an alternative, and possibly safer, class of bronchodilators. SIGNIFICANCE STATEMENT: The genomic consequences of β 2-adrenoceptor agonists in asthma are often overlooked despite being potentially harmful to lung health. We determined that ONO-AE1-329, an EP4-receptor agonist and effective bronchodilator, produced gene expression changes in BEAS-2B cells that were typically modest relative to the β 2-adrenoceptor agonist vilanterol. Furthermore, ONO-AE1-329 behaved as a partial agonist that varied across transcripts. If this genomic activity is reproduced in vivo, then EP4-receptor agonists could represent an alternative, and possibly safer, class of bronchodilators.

Pulmonary surfactant and prostaglandin E2 in airway smooth muscle relaxation of human and male guinea pigs

Pulmonary surfactant serves as a barrier to respiratory epithelium but can also regulate airway smooth muscle (ASM) tone. Surfactant (SF) relaxes contracted ASM, similar to β2-agonists, anticholinergics, nitric oxide, and prostanoids. The exact mechanism of surfactant relaxation and whether surfactant relaxes hyperresponsive ASM remains unknown. Based on previous research, relaxation requires an intact epithelium and prostanoid synthesis. We sought to examine the mechanisms by which surfactant causes ASM relaxation. Organ bath measurements of isometric tension of ASM of guinea pigs in response to exogenous surfactant revealed that surfactant reduces tension of healthy and hyperresponsive tracheal tissue. The relaxant effect of surfactant was reduced if prostanoid synthesis was inhibited and/or if prostaglandin E2-related EP2 receptors were antagonized. Atomic force microscopy revealed that human ASM cells stiffen during contraction and soften during relaxation. Surfactant softened ASM cells, similarly to the known bronchodilator prostaglandin E2 (PGE2) and the cell softening was abolished when EP4 receptors for PGE2 were antagonized. Elevated levels of PGE2 were found in cultures of normal human bronchial epithelial cells exposed to pulmonary surfactant. We conclude that prostaglandin E2 and its EP2 and EP4 receptors are likely involved in the relaxant effect of pulmonary surfactant in airways.

Extracellular acidification attenuates bronchial contraction via an autocrine activation of EP2 receptor: Its diminishment in murine experimental asthma

PURPOSE

Extracellular acidification is a major component of tissue inflammation, including airway inflammation in asthmatics. However, its physiological/pathophysiological significance in bronchial function is not fully understood. Currently, the functional role of extracellular acidification on bronchial contraction was explored.

METHODS

Left main bronchi were isolated from male BALB/c mice. Epithelium-removed tissues were exposed to acidic pH under submaximal contraction induced by 10-5 M acetylcholine in the presence or absence of a COX inhibitor indomethacin (10-6 M). Effects of AH6809 (10-6 M, an EP2 receptor antagonist), BW A868C (10-7 M, a DP receptor antagonist) and CAY10441 (3×10-6 M, an IP receptor antagonist) on the acidification-induced change in tension were determined. The release of prostaglandin E2 (PGE2) from epithelium-denuded tissues in response to acidic pH was assessed using an ELISA.

RESULTS

In the bronchi stimulated with acetylcholine, change in the extracellular pH from 7.4 to 6.8 caused a transient augmentation of contraction followed by a sustained relaxing response. The latter inhibitory response was abolished by indomethacin and AH6809 but not by BW A868C or CAY10441. Both indomethacin and AH6809 significantly increased potency and efficacy of acetylcholine at pH 6.8. Stimulation with low pH caused an increase in PGE2 release from epithelium-denuded bronchi. Interestingly, the acidic pH-induced bronchial relaxation was significantly reduced in a murine asthma model that had a bronchial hyperresponsiveness to acetylcholine.

CONCLUSION

Taken together, extracellular acidification could inhibit the bronchial contraction via autocrine activation of EP2 receptors. The diminished acidic pH-mediated inhibition of bronchial tone may contribute to excessive bronchoconstriction in inflamed airways such as asthma.

Prorelaxant E-type Prostanoid Receptors Functionally Partition to Different Procontractile Receptors in Airway Smooth Muscle

Prostaglandin E2 imparts diverse physiological effects on multiple airway cells through its actions on four distinct E-type prostanoid (EP) receptor subtypes (EP1-EP4). Gs-coupled EP2 and EP4 receptors are expressed on airway smooth muscle (ASM), yet their capacity to regulate the ASM contractile state remains subject to debate. We used EP2 and EP4 subtype-specific agonists (ONO-259 and ONO-329, respectively) in cell- and tissue-based models of human ASM contraction-magnetic twisting cytometry (MTC), and precision-cut lung slices (PCLSs), respectively-to study the EP2 and EP4 regulation of ASM contraction and signaling under conditions of histamine or methacholine (MCh) stimulation. ONO-329 was superior (<0.05) to ONO-259 in relaxing MCh-contracted PCLSs (log half maximal effective concentration [logEC50]: 4.9 × 10-7 vs. 2.2 × 10-6; maximal bronchodilation ± SE, 35 ± 2% vs. 15 ± 2%). However, ONO-259 and ONO-329 were similarly efficacious in relaxing histamine-contracted PCLSs. Similar differential effects were observed in MTC studies. Signaling analyses revealed only modest differences in ONO-329- and ONO-259-induced phosphorylation of the protein kinase A substrates VASP and HSP20, with concomitant stimulation with MCh or histamine. Conversely, ONO-259 failed to inhibit MCh-induced phosphorylation of the regulatory myosin light chain (pMLC20) and the F-actin/G-actin ratio (F/G-actin ratio) while effectively inhibiting their induction by histamine. ONO-329 was effective in reversing induced pMLC20 and the F/G-actin ratio with both MCh and histamine. Thus, the contractile-agonist-dependent differential effects are not explained by changes in the global levels of phosphorylated protein kinase A substrates but are reflected in the regulation of pMLC20 (cross-bridge cycling) and F/G-actin ratio (actin cytoskeleton integrity, force transmission), implicating a role for compartmentalized signaling involving muscarinic, histamine, and EP receptor subtypes.

Intranasal EP4 agonist and arginase-1 therapy in a murine model of asthma

Research findings evermore suggest a crucial role of myeloid-derived suppressor cells (MDSCs) in chronic lung diseases including asthma. Previously, we showed that intravenous (IV) treatment with a prostaglandin E2 receptor 4 (EP4) agonist, L-902,688, promoted MDSC suppressive activity. IV therapy with L-902,688 and BCT-100, a human pegylated arginase-1, ameliorated lung inflammatory features in a murine model of asthma. Here, we further investigate the potential therapeutic approach by studying the local therapy effects on the lungs after intranasal (IN) application. Using a two-week model of house dust mite (HDM)-induced murine asthma, the effect of IN treatment with L-902,688 or BCT-100 on in vivo lung function, inflammatory features of asthma and MDSC generation and activation was studied. Our experiments demonstrated increased suppressive activity of pulmonary MDSCs after induction of allergic airway disease. IN treatment with L-902,688 and BCT-100 further enhanced the immunosuppressive activity of pulmonary MDSCs. Additionally, treatment with BCT-100 reduced pulmonary T cell numbers. Asthmatic mice that received IN L-902,688 showed improved in vivo lung function. In conclusion, our results underline the potential of modulating MDSCs systemically or locally as a future therapeutic option in airway inflammatory diseases such as asthma.

Metabolic regulation by prostaglandin E2 impairs lung group 2 innate lymphoid cell responses

BACKGROUND

Group 2 innate lymphoid cells (ILC2s) play a critical role in asthma pathogenesis. Non-steroidal anti-inflammatory drug (NSAID)-exacerbated respiratory disease (NERD) is associated with reduced signaling via EP2, a receptor for prostaglandin E2 (PGE2 ). However, the respective roles for the PGE2 receptors EP2 and EP4 (both share same downstream signaling) in the regulation of lung ILC2 responses has yet been deciphered.

METHODS

The roles of PGE2 receptors EP2 and EP4 on ILC2-mediated lung inflammation were investigated using genetically modified mouse lines and pharmacological approaches in IL-33-induced lung allergy model. The effects of PGE2 receptors and downstream signals on ILC2 metabolic activation and effector function were examined using in vitro cell cultures.

RESULTS

Deficiency of EP2 rather than EP4 augments IL-33-induced mouse lung ILC2 responses and eosinophilic inflammation in vivo. In contrast, exogenous agonism of EP4 and EP2 or inhibition of phosphodiesterase markedly restricts IL-33-induced lung ILC2 responses. Mechanistically, PGE2 directly suppresses IL-33-dependent ILC2 activation through the EP2/EP4-cAMP pathway, which downregulates STAT5 and MYC pathway gene expression and ILC2 energy metabolism. Blocking glycolysis diminishes IL-33-dependent ILC2 responses in mice where endogenous PG synthesis or EP2 signaling is blocked but not in mice with intact PGE2 -EP2 signaling.

CONCLUSION

We have defined a mechanism for optimal suppression of mouse lung ILC2 responses by endogenous PGE2 -EP2 signaling which underpins the clinical findings of defective EP2 signaling in patients with NERD. Our findings also indicate that exogenously targeting the PGE2 -EP4-cAMP and energy metabolic pathways may provide novel opportunities for treating the ILC2-initiated lung inflammation in asthma and NERD.

The role of PGE2 and EP receptors on lung's immune and structural cells; possibilities for future asthma therapy

Asthma is the most common airway chronic disease with treatments aimed mainly to control the symptoms. Adrenergic receptor agonists, corticosteroids and anti-leukotrienes have been used for decades, and the development of more targeted asthma treatments, known as biological therapies, were only recently established. However, due to the complexity of asthma and the limited efficacy as well as the side effects of available treatments, there is an urgent need for a new generation of asthma therapies. The anti-inflammatory and bronchodilatory effects of prostaglandin E2 in asthma are promising, yet complicated by undesirable side effects, such as cough and airway irritation. In this review, we summarize the most important literature on the role of all four E prostanoid (EP) receptors on the lung's immune and structural cells to further dissect the relevance of EP2/EP4 receptors as potential targets for future asthma therapy.

Therapeutic Approaches for Chronic Obstructive Pulmonary Disease (COPD) Exacerbations

Chronic Obstructive Pulmonary Disease (COPD) is a progressive pulmonary disorder underpinned by poorly reversible airflow resulting from chronic bronchitis or emphysema. The prevalence and mortality of COPD continue to increase. Pharmacotherapy for patients with COPD has included antibiotics, bronchodilators, and anti-inflammatory corticosteroids (but with little success). Oral diseases have long been established as clinical risk factors for developing respiratory diseases. The establishment of a very similar microbiome in the mouth and the lung confirms the oral-lung connection. The aspiration of pathogenic microbes from the oral cavity has been implicated in several respiratory diseases, including pneumonia and chronic obstructive pulmonary disease (COPD). This review focuses on current and future pharmacotherapeutic approaches for COPD exacerbation including antimicrobials, mucoregulators, the use of bronchodilators and anti-inflammatory drugs, modifying epigenetic marks, and modulating dysbiosis of the microbiome.

The endocannabinoid anandamide is an airway relaxant in health and disease

Chronic obstructive airway diseases are a global medical burden that is expected to increase in the near future. However, the underlying mechanistic processes are poorly understood so far. Herein, we show that the endocannabinoid anandamide (AEA) induces prominent airway relaxation in vitro and in vivo. In contrast to 2-arachidonlyglycerol-induced airway relaxation, this is mediated by fatty acid amide hydrolase (FAAH)-dependent metabolites. In particular, we identify mouse and also human epithelial and airway smooth muscle cells as source of AEA-induced prostaglandin E2 production and cAMP as direct mediator of AEA-dependent airway relaxation. Mass spectrometry experiments demonstrate reduced levels of endocannabinoid-like compounds in lungs of ovalbumin-sensitized mice indicating a pathophysiological relevance of endocannabinoid signalling in obstructive airway disease. Importantly, AEA inhalation protects against airway hyper-reactivity after ovalbumin sensitization. Thus, this work highlights the AEA/FAAH axis as a critical regulator of airway tone that could provide therapeutic targets for airway relaxation.

A transcriptomics-guided drug target discovery strategy identifies receptor ligands for lung regeneration

Currently, there is no pharmacological treatment targeting defective tissue repair in chronic disease. Here, we used a transcriptomics-guided drug target discovery strategy using gene signatures of smoking-associated chronic obstructive pulmonary disease (COPD) and from mice chronically exposed to cigarette smoke, identifying druggable targets expressed in alveolar epithelial progenitors, of which we screened the function in lung organoids. We found several drug targets with regenerative potential, of which EP and IP prostanoid receptor ligands had the most profound therapeutic potential in restoring cigarette smoke-induced defects in alveolar epithelial progenitors in vitro and in vivo. Mechanistically, we found, using single-cell RNA sequencing analysis, that circadian clock and cell cycle/apoptosis signaling pathways were differentially expressed in alveolar epithelial progenitor cells in patients with COPD and in a relevant model of COPD, which was prevented by prostaglandin E2 or prostacyclin mimetics. We conclude that specific targeting of EP and IP receptors offers therapeutic potential for injury to repair in COPD.

Eicosanoid receptors as therapeutic targets for asthma

Eicosanoids comprise a group of oxidation products of arachidonic and 5,8,11,14,17-eicosapentaenoic acids formed by oxygenases and downstream enzymes. The two major pathways for eicosanoid formation are initiated by the actions of 5-lipoxygenase (5-LO), leading to leukotrienes (LTs) and 5-oxo-6,8,11,14-eicosatetraenoic acid (5-oxo-ETE), and cyclooxygenase (COX), leading to prostaglandins (PGs) and thromboxane (TX). A third group (specialized pro-resolving mediators; SPMs), including lipoxin A4 (LXA4) and resolvins (Rvs), are formed by the combined actions of different oxygenases. The actions of the above eicosanoids are mediated by approximately 20 G protein-coupled receptors, resulting in a variety of both detrimental and beneficial effects on airway smooth muscle and inflammatory cells that are strongly implicated in asthma pathophysiology. Drugs targeting proinflammatory eicosanoid receptors, including CysLT1, the receptor for LTD4 (montelukast) and TP, the receptor for TXA2 (seratrodast) are currently in use, whereas antagonists of a number of other receptors, including DP2 (PGD2), BLT1 (LTB4), and OXE (5-oxo-ETE) are under investigation. Agonists targeting anti-inflammatory/pro-resolving eicosanoid receptors such as EP2/4 (PGE2), IP (PGI2), ALX/FPR2 (LXA4), and Chemerin1 (RvE1/2) are also being examined. This review summarizes the contributions of eicosanoid receptors to the pathophysiology of asthma and the potential therapeutic benefits of drugs that target these receptors. Because of the multifactorial nature of asthma and the diverse pathways affected by eicosanoid receptors, it will be important to identify subgroups of asthmatics that are likely to respond to any given therapy.

Cellular and molecular mechanisms of allergic asthma

Asthma is a chronic disease of the airways, which affects more than 350 million people worldwide. It is the most common chronic disease in children, affecting at least 30 million children and young adults in Europe. Asthma is a complex, partially heritable disease with a marked heterogeneity. Its development is influenced both by genetic and environmental factors. The most common, as well as the most well characterized subtype of asthma is allergic eosinophilic asthma, which is characterized by a type 2 airway inflammation. The prevalence of asthma has substantially increased in industrialized countries during the last 60 years. The mechanisms underpinning this phenomenon are incompletely understood, however increased exposure to various environmental pollutants probably plays a role. Disease inception is thought to be enabled by a disadvantageous shift in the balance between protective and harmful lifestyle and environmental factors, including exposure to protective commensal microbes versus infection with pathogens, collectively leading to airway epithelial cell damage and disrupted barrier integrity. Epithelial cell-derived cytokines are one of the main drivers of the type 2 immune response against innocuous allergens, ultimately leading to infiltration of lung tissue with type 2 T helper (T_H2) cells, type 2 innate lymphoid cells (ILC2s), M2 macrophages and eosinophils. This review outlines the mechanisms responsible for the orchestration of type 2 inflammation and summarizes the novel findings, including but not limited to dysregulated epithelial barrier integrity, alarmin release and innate lymphoid cell stimulation.

Myeloid-Derived Suppressor Cells Dampen Airway Inflammation Through Prostaglandin E2 Receptor 4

Emerging evidence suggests a mechanistic role for myeloid-derived suppressor cells (MDSCs) in lung diseases like asthma. Previously, we showed that adoptive transfer of MDSCs dampens lung inflammation in murine models of asthma through cyclooxygenase-2 and arginase-1 pathways. Here, we further dissected this mechanism by studying the role and therapeutic relevance of the downstream mediator prostaglandin E2 receptor 4 (EP4) in a murine model of asthma. We adoptively transferred MDSCs generated using an EP4 agonist in a murine model of asthma and studied the consequences on airway inflammation. Furthermore, pegylated human arginase-1 was used to model MDSC effector activities. We demonstrate that the selective EP4 agonist L-902,688 increased the number and suppressive activity of MDSCs through arginase-1 and nitric oxide synthase-2. These results showed that adoptive transfer of EP4-primed MDSCs, EP4 agonism alone or arginase-1 administration ameliorated lung inflammatory responses and histopathological changes in asthmatic mice. Collectively, our results provide evidence that MDSCs dampen airway inflammation in murine asthma through a mechanism involving EP4.

Pathophysiological regulation of lung function by the free fatty acid receptor FFA4

Increased prevalence of inflammatory airway diseases including asthma and chronic obstructive pulmonary disease (COPD) together with inadequate disease control by current frontline treatments means that there is a need to define therapeutic targets for these conditions. Here, we investigate a member of the G protein-coupled receptor family, FFA4, that responds to free circulating fatty acids including dietary omega-3 fatty acids found in fish oils. We show that FFA4, although usually associated with metabolic responses linked with food intake, is expressed in the lung where it is coupled to G_q/11 signaling. Activation of FFA4 by drug-like agonists produced relaxation of murine airway smooth muscle mediated at least in part by the release of the prostaglandin E₂ (PGE₂) that subsequently acts on EP₂ prostanoid receptors. In normal mice, activation of FFA4 resulted in a decrease in lung resistance. In acute and chronic ozone models of pollution-mediated inflammation and house dust mite and cigarette smoke-induced inflammatory disease, FFA4 agonists acted to reduce airway resistance, a response that was absent in mice lacking expression of FFA4. The expression profile of FFA4 in human lung was similar to that observed in mice, and the response to FFA4/FFA1 agonists similarly mediated human airway smooth muscle relaxation ex vivo. Our study provides evidence that pharmacological targeting of lung FFA4, and possibly combined activation of FFA4 and FFA1, has in vivo efficacy and might have therapeutic value in the treatment of bronchoconstriction associated with inflammatory airway diseases such as asthma and COPD.

Can GPCRs Be Targeted to Control Inflammation in Asthma?

Historically, the drugs used to manage obstructive lung diseases (OLDs), asthma, and chronic obstructive pulmonary disease (COPD) either (1) directly regulate airway contraction by blocking or relaxing airway smooth muscle (ASM) contraction or (2) indirectly regulate ASM contraction by inhibiting the principal cause of ASM contraction/bronchoconstriction and airway inflammation. To date, these tasks have been respectively assigned to two diverse drug types: agonists/antagonists of G protein-coupled receptors (GPCRs) and inhaled or systemic steroids. These two types of drugs "stay in their lane" with respect to their actions and consequently require the addition of the other drug to effectively manage both inflammation and bronchoconstriction in OLDs. Indeed, it has been speculated that safety issues historically associated with beta-agonist use (beta-agonists activate the beta-2-adrenoceptor (β₂AR) on airway smooth muscle (ASM) to provide bronchoprotection/bronchorelaxation) are a function of pro-inflammatory actions of β₂AR agonism. Recently, however, previously unappreciated roles of various GPCRs on ASM contractility and on airway inflammation have been elucidated, raising the possibility that novel GPCR ligands targeting these GPCRs can be developed as anti-inflammatory therapeutics. Moreover, we now know that many GPCRs can be "tuned" and not just turned "off" or "on" to specifically activate the beneficial therapeutic signaling a receptor can transduce while avoiding detrimental signaling. Thus, the fledging field of biased agonism pharmacology has the potential to turn the β₂AR into an anti-inflammatory facilitator in asthma, possibly reducing or eliminating the need for steroids.

Current perspective on eicosanoids in asthma and allergic diseases: EAACI Task Force consensus report, part I

Eicosanoids are biologically active lipid mediators, comprising prostaglandins, leukotrienes, thromboxanes, and lipoxins, involved in several pathophysiological processes relevant to asthma, allergies, and allied diseases. Prostaglandins and leukotrienes are the most studied eicosanoids and established inducers of airway pathophysiology including bronchoconstriction and airway inflammation. Drugs inhibiting the synthesis of lipid mediators or their effects, such as leukotriene synthesis inhibitors, leukotriene receptors antagonists, and more recently prostaglandin D₂ receptor antagonists, have been shown to modulate features of asthma and allergic diseases. This review, produced by an European Academy of Allergy and Clinical Immunology (EAACI) task force, highlights our current understanding of eicosanoid biology and its role in mediating human pathology, with a focus on new findings relevant for clinical practice, development of novel therapeutics, and future research opportunities.

Could Arachidonic Acid-Derived Pro-Resolving Mediators Be a New Therapeutic Strategy for Asthma Therapy?

Asthma represents one of the leading chronic diseases worldwide and causes a high global burden of death and disability. In asthmatic patients, the exacerbation and chronification of the inflammatory response are often related to a failure in the resolution phase of inflammation. We reviewed the role of the main arachidonic acid (AA) specialized pro-resolving mediators (SPMs) in the resolution of chronic lung inflammation of asthmatics. AA is metabolized by two classes of enzymes, cyclooxygenases (COX), which produce prostaglandins (PGs) and thromboxanes, and lypoxygenases (LOX), which form leukotrienes and lipoxins (LXs). In asthma, two primary pro-resolving derived mediators from COXs are PGE₂ and the cyclopentenone prostaglandin15-Deoxy-Delta-12,14-PGJ₂ (15d-PGJ₂) while from LOXs are the LXA₄ and LXB₄. In different models of asthma, PGE₂, 15d-PGJ₂, and LXs reduced lung inflammation and remodeling. Furthermore, these SPMs inhibited chemotaxis and function of several inflammatory cells involved in asthma pathogenesis, such as eosinophils, and presented an antiremodeling effect in airway epithelial, smooth muscle cells and fibroblasts in vitro. In addition, PGE₂, 15d-PGJ₂, and LXs are all able to induce macrophage reprogramming to an alternative M2 pro-resolving phenotype in vitro and in vivo. Although PGE₂ and LXA₄ showed some beneficial effects in asthmatic patients, there are limitations to their clinical use, since PGE₂ caused side effects, while LXA₄ presented low stability. Therefore, despite the strong evidence that these AA-derived SPMs induce resolution of both inflammatory response and tissue remodeling in asthma, safer and more stable analogs must be developed for further clinical investigation of their application in asthma treatment.

Airway relaxation mechanisms and structural basis of osthole for improving lung function in asthma

Overuse of β2-adrenoceptor agonist bronchodilators evokes receptor desensitization, decreased efficacy, and an increased risk of death in asthma patients. Bronchodilators that do not target β2-adrenoceptors represent a critical unmet need for asthma management. Here, we characterize the utility of osthole, a coumarin derived from a traditional Chinese medicine, in preclinical models of asthma. In mouse precision-cut lung slices, osthole relaxed preconstricted airways, irrespective of β2-adrenoceptor desensitization. Osthole administered in murine asthma models attenuated airway hyperresponsiveness, a hallmark of asthma. Osthole inhibited phosphodiesterase 4D (PDE4D) activity to amplify autocrine prostaglandin E₂ signaling in airway smooth muscle cells that eventually triggered cAMP/PKA-dependent relaxation of airways. The crystal structure of the PDE4D complexed with osthole revealed that osthole bound to the catalytic site to prevent cAMP binding and hydrolysis. Together, our studies elucidate a specific molecular target and mechanism by which osthole induces airway relaxation. Identification of osthole binding sites on PDE4D will guide further development of bronchodilators that are not subject to tachyphylaxis and would thus avoid β2-adrenoceptor agonist resistance.

Prostanoid Receptors of the EP4-Subtype Mediate Gene Expression Changes in Human Airway Epithelial Cells with Potential Anti-Inflammatory Activity

There is a clear, unmet clinical need to identify new drugs to treat individuals with asthma, chronic obstructive pulmonary disease (COPD), and idiopathic pulmonary fibrosis (IPF) in whom current medications are either inactive or suboptimal. In preclinical models, EP₄-receptor agonists display efficacy, but their mechanism of action is unclear. In this study, using human bronchial epithelial cells as a therapeutically relevant drug target, we hypothesized that changes in gene expression may play an important role. Several prostanoid receptor mRNAs were detected in BEAS-2B cells, human primary bronchial epithelial cells (HBECs) grown in submersion culture and HBECs grown at an air-liquid interface with PTGER4 predominating. By using the activation of a cAMP response element reporter in BEAS-2B cells as a surrogate of gene expression, Schild analysis determined that PTGER4 mRNAs encoded functional EP₄-receptors. Moreover, inhibitors of phosphodiesterase 4 (roflumilast N-oxide [RNO]) and cAMP-dependent protein kinase augmented and attenuated, respectively, reporter activation induced by 2-[3-[(1R,2S,3R)-3-hydroxy-2-[(E,3S)-3-hydroxy-5-[2-(methoxymethyl)phenyl]pent-1-enyl]-5-oxo-cyclopentyl]sulphanylpropylsulphanyl] acetic acid (ONO-AE1-329), a selective EP₄-receptor agonist. ONO-AE1-329 also enhanced dexamethasone-induced activation of a glucocorticoid response element reporter in BEAS-2B cells, which was similarly potentiated by RNO. In each airway epithelial cell variant, numerous genes that may impart therapeutic benefit in asthma, COPD, and/or IPF were differentially expressed by ONO-AE1-329, and those changes were often augmented by RNO and/or dexamethasone. We submit that an EP₄-receptor agonist, either alone or as a combination therapy, may be beneficial in individuals with chronic lung diseases in whom current treatment options are inadequate. SIGNIFICANCE STATEMENT: Using human bronchial epithelial cells as a therapeutically relevant drug target, we report that EP₄-receptor activation promoted gene expression changes that could provide therapeutic benefit in individuals with asthma, COPD, and IPF in whom current treatment options are ineffective or suboptimal.

Prostaglandin D2 strengthens human endothelial barrier by activation of E-type receptor 4

Life-threatening inflammatory conditions such as acute respiratory distress syndrome or sepsis often go hand in hand with severe vascular leakage. During inflammation, endothelial cell integrity and intact barrier function are crucial to limit leukocyte and plasma extravasation. Prostaglandin D₂ (PGD₂) is a potent inflammatory lipid mediator with vasoactive properties. Previous studies suggest that PGD₂ is involved in the regulation of endothelial barrier function; however, it is unclear whether this is also true for primary human pulmonary microvascular endothelial cells. Furthermore, as PGD₂ is a highly promiscuous ligand, we set out to determine which receptors are important in human pulmonary endothelial cells. In the current study, we found that PGD₂ and the DP1 agonist BW245c potently strengthened pulmonary and dermal microvascular endothelial cell barrier function and protected against thrombin-induced barrier disruption. Yet surprisingly, these effects were mediated only to a negligible extent via DP1 receptor activation. In contrast, we observed that the EP4 receptor was most important and mediated the barrier enhancement by PGD₂ and BW245c. Stimulation with PGE₂ or PGD₂ reduced AKT phosphorylation which could be reversed by prior blockade of EP4 receptors. These data demonstrate a novel mechanism by which PGD₂ may modulate inflammation and emphasizes the role of EP4 receptors in human endothelial cell function.

International Union of Basic and Clinical Pharmacology. CIX. Differences and Similarities between Human and Rodent Prostaglandin E2 Receptors (EP1-4) and Prostacyclin Receptor (IP): Specific Roles in Pathophysiologic Conditions

Prostaglandins are derived from arachidonic acid metabolism through cyclooxygenase activities. Among prostaglandins (PGs), prostacyclin (PGI2) and PGE2 are strongly involved in the regulation of homeostasis and main physiologic functions. In addition, the synthesis of these two prostaglandins is significantly increased during inflammation. PGI2 and PGE2 exert their biologic actions by binding to their respective receptors, namely prostacyclin receptor (IP) and prostaglandin E2 receptor (EP) 1-4, which belong to the family of G-protein-coupled receptors. IP and EP1-4 receptors are widely distributed in the body and thus play various physiologic and pathophysiologic roles. In this review, we discuss the recent advances in studies using pharmacological approaches, genetically modified animals, and genome-wide association studies regarding the roles of IP and EP1-4 receptors in the immune, cardiovascular, nervous, gastrointestinal, respiratory, genitourinary, and musculoskeletal systems. In particular, we highlight similarities and differences between human and rodents in terms of the specific roles of IP and EP1-4 receptors and their downstream signaling pathways, functions, and activities for each biologic system. We also highlight the potential novel therapeutic benefit of targeting IP and EP1-4 receptors in several diseases based on the scientific advances, animal models, and human studies. SIGNIFICANCE STATEMENT: In this review, we present an update of the pathophysiologic role of the prostacyclin receptor, prostaglandin E2 receptor (EP) 1, EP2, EP3, and EP4 receptors when activated by the two main prostaglandins, namely prostacyclin and prostaglandin E2, produced during inflammatory conditions in human and rodents. In addition, this comparison of the published results in each tissue and/or pathology should facilitate the choice of the most appropriate model for the future studies.

Pharmacology and Therapeutics of Bronchodilators Revisited

Bronchodilators remain the cornerstone of the treatment of airway disorders such as asthma and chronic obstructive pulmonary disease (COPD). There is therefore considerable interest in understanding how to optimize the use of our existing classes of bronchodilator and in identifying novel classes of bronchodilator drugs. However, new classes of bronchodilator have proved challenging to develop because many of these have no better efficacy than existing classes of bronchodilator and often have unacceptable safety profiles. Recent research has shown that optimization of bronchodilation occurs when both arms of the autonomic nervous system are affected through antagonism of muscarinic receptors to reduce the influence of parasympathetic innervation of the lung and through stimulation of β ₂-adrenoceptors (β ₂-ARs) on airway smooth muscle with β ₂-AR-selective agonists to mimic the sympathetic influence on the lung. This is currently achieved by use of fixed-dose combinations of inhaled long-acting β ₂-adrenoceptor agonists (LABAs) and long-acting muscarinic acetylcholine receptor antagonists (LAMAs). Due to the distinct mechanisms of action of LAMAs and LABAs, the additive/synergistic effects of using these drug classes together has been extensively investigated. More recently, so-called "triple inhalers" containing fixed-dose combinations of both classes of bronchodilator (dual bronchodilation) and an inhaled corticosteroid in the same inhaler have been developed. Furthermore, a number of so-called "bifunctional drugs" having two different primary pharmacological actions in the same molecule are under development. This review discusses recent advancements in knowledge on bronchodilators and bifunctional drugs for the treatment of asthma and COPD. SIGNIFICANCE STATEMENT: Since our last review in 2012, there has been considerable research to identify novel classes of bronchodilator drugs, to further understand how to optimize the use of the existing classes of bronchodilator, and to better understand the role of bifunctional drugs in the treatment of asthma and chronic obstructive pulmonary disease.

Bioactive lipids, inflammation and chronic diseases

Endogenous bioactive lipids are part of a complex network that modulates a plethora of cellular and molecular processes involved in health and disease, of which inflammation represents one of the most prominent examples. Inflammation serves as a well-conserved defence mechanism, triggered in the event of chemical, mechanical or microbial damage, that is meant to eradicate the source of damage and restore tissue function. However, excessive inflammatory signals, or impairment of pro-resolving/anti-inflammatory pathways leads to chronic inflammation, which is a hallmark of chronic pathologies. All main classes of endogenous bioactive lipids - namely eicosanoids, specialized pro-resolving lipid mediators, lysoglycerophopsholipids and endocannabinoids - have been consistently involved in the chronic inflammation that characterises pathologies such as cancer, diabetes, atherosclerosis, asthma, as well as autoimmune and neurodegenerative disorders and inflammatory bowel diseases. This review gathers the current knowledge concerning the involvement of endogenous bioactive lipids in the pathogenic processes of chronic inflammatory pathologies.

Novel airway smooth muscle-mast cell interactions and a role for the TRPV4-ATP axis in non-atopic asthma

Mast cell–airway smooth muscle (ASM) interactions play a major role in the immunoglobulin (Ig)E- dependent bronchoconstriction seen in asthma but less is known about IgE-independent mechanisms of mast cell activation. Transient receptor potential cation channel, subfamily V, member 4 (TRPV4) activation causes contraction of human ASM via the release of cysteinyl leukotrienes (cysLTs) but the mechanism is unknown. The objective of the present study was to investigate a role for IgE-independent, mast cell–ASM interaction in TRPV4-induced bronchospasm.

A technique not previously applied to respiratory research now uncovers important IgE-independent mechanisms involved in human mast cell–airway smooth muscle interactions that may be responsible for the bronchospasm associated with non-atopic asthmahttp://bit.ly/2U1n5nT

Transforming Growth Factor-β1 Decreases β2-Agonist-induced Relaxation in Human Airway Smooth Muscle

Helper T effector cytokines implicated in asthma modulate the contractility of human airway smooth muscle (HASM) cells. We have reported recently that a profibrotic cytokine, transforming growth factor (TGF)-β1, induces HASM cell shortening and airway hyperresponsiveness. Here, we assessed whether TGF-β1 affects the ability of HASM cells to relax in response to β₂-agonists, a mainstay treatment for airway hyperresponsiveness in asthma. Overnight TGF-β1 treatment significantly impaired isoproterenol (ISO)-induced relaxation of carbachol-stimulated, isolated HASM cells. This single-cell mechanical hyporesponsiveness to ISO was corroborated by sustained increases in myosin light chain phosphorylation. In TGF-β1-treated HASM cells, ISO evoked markedly lower levels of intracellular cAMP. These attenuated cAMP levels were, in turn, restored with pharmacological and siRNA inhibition of phosphodiesterase 4 and Smad3, respectively. Most strikingly, TGF-β1 selectively induced phosphodiesterase 4D gene expression in HASM cells in a Smad2/3-dependent manner. Together, these data suggest that TGF-β1 decreases HASM cell β₂-agonist relaxation responses by modulating intracellular cAMP levels via a Smad2/3-dependent mechanism. Our findings further define the mechanisms underlying β₂-agonist hyporesponsiveness in asthma, and suggest TGF-β1 as a potential therapeutic target to decrease asthma exacerbations in severe and treatment-resistant asthma.

Transient receptor potential ankyrin-1 causes rapid bronchodilation via nonepithelial PGE2

Transient receptor potential ankyrin-1 (TRPA1) is a ligand-gated cation channel that responds to endogenous and exogenous irritants. TRPA1 is expressed on multiple cell types throughout the lungs, but previous studies have primarily focused on TRPA1 stimulation of airway sensory nerves. We sought to understand the integrated physiological airway response to TRPA1 stimulation. The TRPA1 agonists allyl isothiocyanate (AITC) and cinnamaldehyde (CINN) were tested in sedated, mechanically ventilated guinea pigs in vivo. Reproducible bronchoconstrictions were induced by electrical stimulation of the vagus nerves. Animals were then treated with intravenous AITC or CINN. AITC and CINN were also tested on isolated guinea pig and mouse tracheas and postmortem human trachealis muscle strips in an organ bath. Tissues were contracted with methacholine, histamine, or potassium chloride and then treated with AITC or CINN. Some airways were pretreated with TRPA1 antagonists, the cyclooxygenase inhibitor indomethacin, the EP₂ receptor antagonist PF 04418948, or tetrodotoxin. AITC and CINN blocked vagally mediated bronchoconstriction in guinea pigs. Pretreatment with indomethacin completely abolished the airway response to TRPA1 agonists. Similarly, AITC and CINN dose-dependently relaxed precontracted guinea pig, mouse, and human airways in the organ bath. AITC- and CINN-induced airway relaxation required TRPA1, prostaglandins, and PGE₂ receptor activation. TRPA1-induced airway relaxation did not require epithelium or tetrodotoxin-sensitive nerves. Finally, AITC blocked airway hyperreactivity in two animal models of allergic asthma. These data demonstrate that stimulation of TRPA1 causes bronchodilation of intact airways and suggest that the TRPA1 pathway is a potential pharmacological target for bronchodilation.

The Biology of Prostaglandins and Their Role as a Target for Allergic Airway Disease Therapy

Prostaglandins (PGs) are a family of lipid compounds that are derived from arachidonic acid via the cyclooxygenase pathway, and consist of PGD₂, PGI₂, PGE₂, PGF₂, and thromboxane B₂. PGs signal through G-protein coupled receptors, and individual PGs affect allergic inflammation through different mechanisms according to the receptors with which they are associated. In this review article, we have focused on the metabolism of the cyclooxygenase pathway, and the distinct biological effect of each PG type on various cell types involved in allergic airway diseases, including asthma, allergic rhinitis, nasal polyposis, and aspirin-exacerbated respiratory disease.

The future of bronchodilation: looking for new classes of bronchodilators

Available bronchodilators can satisfy many of the needs of patients suffering from airway disorders, but they often do not relieve symptoms and their long-term use raises safety concerns. Therefore, there is interest in developing new classes that could help to overcome the limits that characterise the existing classes.At least nine potential new classes of bronchodilators have been identified: 1) selective phosphodiesterase inhibitors; 2) bitter-taste receptor agonists; 3) E-prostanoid receptor 4 agonists; 4) Rho kinase inhibitors; 5) calcilytics; 6) agonists of peroxisome proliferator-activated receptor-γ; 7) agonists of relaxin receptor 1; 8) soluble guanylyl cyclase activators; and 9) pepducins. They are under consideration, but they are mostly in a preclinical phase and, consequently, we still do not know which classes will actually be developed for clinical use and whether it will be proven that a possible clinical benefit outweighs the impact of any adverse effect.It is likely that if developed, these new classes may be a useful addition to, rather than a substitution of, the bronchodilator therapy currently used, in order to achieve further optimisation of bronchodilation.

Bronchodilation induced by PGE2 is impaired in Group III pulmonary hypertension

BACKGROUND AND PURPOSE

In patients with pulmonary hypertension (PH) associated with lung disease and/or hypoxia (Group III), decreased pulmonary vascular tone and tissue hypoxia is therapeutically beneficial. PGE₂ and PGI₂ induce potent relaxation of human bronchi from non-PH (control) patients via EP₄ and IP receptors, respectively. However, the effects of PGE₂ /PGI₂ and their mimetics on human bronchi from PH patients are unknown. Here, we have compared relaxant effects of several PGI₂ -mimetics approved for treating PH Group I with several PGE₂ -mimetics, in bronchial preparations derived from PH Group III and control patients.

EXPERIMENTAL APPROACH

Relaxation of bronchial muscle was assessed in samples isolated from control and PH Group III patients. Expression of prostanoid receptors was analysed by western blot and real-time PCR, and endogenous PGE₂ , PGI₂ , and cAMP levels were determined by ELISA.

KEY RESULTS

Maximal relaxations induced by different EP₄ receptor agonists (PGE₂ , L-902688, and ONO-AE1-329) were decreased in human bronchi from PH patients, compared with controls. However, maximal relaxations produced by PGI₂ -mimetics (iloprost, treprostinil, and beraprost) were similar for both groups of patients. Both EP₄ and IP receptor protein and mRNA expressions were significantly lower in human bronchi from PH patients. cAMP levels significantly correlated with PGI₂ but not with PGE₂ levels.

CONCLUSION AND IMPLICATIONS

The PGI₂ -mimetics retained maximal bronchodilation in PH Group III patients, whereas bronchodilation induced by EP₄ receptor agonists was decreased. Restoration of EP₄ receptor expression in airways of PH Group III patients with respiratory diseases could bring additional therapeutic benefit.

Current state and future prospect of the therapeutic strategy targeting cysteinyl leukotriene metabolism in asthma

Asthma is an allergic disorder with dominant type 2 airway inflammation, and its prevalence is increasing worldwide. Inhalation of corticosteroids is the primary treatment for asthma along with add-on drugs, including long-acting β2 agonists and/or cysteinyl leukotriene (cys-LT) receptor antagonists, in patients with poorly controlled asthma. Cys-LTs are composed of leukotriene C4 (LTC₄), LTD₄, and LTE₄, which are enzymatically metabolized from arachidonic acid. These molecules act as inflammatory mediators through different types of high-affinity receptors, namely, CysLT₁, CysLT₂, and CysLT₃ (also named as GPR99). CysLT₁ antagonists possessing anti-inflammatory and bronchodilatory effects can be orally administered to patients with asthma. Recently, molecular biology-based studies have revealed the mechanism of inflammatory responses via other receptors, such as CysLT₂ and CysLT₃, as well as the importance of upstream inflammatory regulators, including type 2 cytokines (e.g., interleukins 4 and 5), in controlling cys-LT metabolism. These findings indicate the therapeutic potential of pharmacological agents targeting cys-LT metabolism-related receptors and enzymes, and antibody drugs neutralizing or antagonizing type 2 cytokines. This review focuses on the current state and future prospect of the therapeutic strategy targeting cys-LT metabolism.

Mannitol triggers mast cell-dependent contractions of human small bronchi and prostacyclin bronchoprotection

BACKGROUND

Clinical research supports that exercise-induced bronchoconstriction (EIB) is caused by hyperosmolar triggering of mast cells. The reaction can be mimicked by inhalation of mannitol, but it has paradoxically previously not been possible to replicate this mode of action of mannitol in isolated airways.

OBJECTIVE

We sought to establish an ex vivo model of EIB in human small bronchi.

METHODS

Small bronchi (inner diameter, 0.5-2 mm) from macroscopically healthy human lung tissue were obtained from 48 patients and mounted in organ baths. Contractions and mediator release were analyzed after challenge with hyperosmolar mannitol (850 mOsm).

RESULTS

Ten minutes of exposure to mannitol caused a small initial contraction (12% ± 1% of maximum) that was followed by a second and much larger contraction (maximum effect [E_max], 47% ± 5%) when mannitol was washed out. The mast cell stabilizer cromolyn reduced the second contraction (E_max, 27% ± 3%). Furthermore, this main contraction was abolished by the combination of antagonists of histamine and cysteinyl leukotrienes in the presence of indomethacin. Mannitol increased the release of the mast cell mediators histamine (9.0-fold), cysteinyl leukotrienes (4.5-fold), and prostaglandin (PG) D₂ (5.4-fold), as well as PGE₂ (6.3-fold) and the prostacyclin metabolite 6-keto PGF_1α (5.7-fold). In contrast, indomethacin alone enhanced the bronchoconstriction (E_max, 68% ± 6%). Likewise, receptor antagonists for PGE₂ (EP₂ and EP₄) and prostacyclin (IP) also enhanced the mannitol-induced bronchoconstriction (E_max, 67% ± 5%, 66% ± 4%, and 68% ± 3%, respectively). In bronchi precontracted by carbachol, the IP receptor agonist cicaprost induced profound relaxation.

CONCLUSION

This new protocol established an in vitro model for studies of EIB in isolated human bronchi. The IP receptor might be a new target for asthma treatment.

Cooperativity of E-prostanoid receptor subtypes in regulating signaling and growth inhibition in human airway smooth muscle

Prostaglandin E2 (PGE₂) is produced in the airway during allergic lung inflammation and both promotes and inhibits features of asthma pathology. These mixed effects relate to 4 E-prostanoid (EP) receptor subtypes (EP1, 2, 3 and 4) expressed at different levels on different resident and infiltrating airway cells. Although studies have asserted both EP2 and EP4 expression in human airway smooth muscle (HASM), a recent study asserted EP4 to be the functionally dominant EP subtype in HASM. Herein, we employ recently-developed subtype-selective ligands to investigate singular or combined EP2 and EP4 receptor activation in regulating HASM signaling and proliferation. The subtype specificity of ONO-AE1-259-01 (EP2 agonist) and ONO-AE1-329 (EP4 agonist) was first demonstrated in human embryonic kidney 293 cells stably expressing different EP receptor subtypes. EP receptor knockdown and subtype-selective antagonists demonstrated EP2 and EP4 receptor responsiveness in HASM cells to the specific ONO compounds, whereas PGE₂ appeared to preferentially signal via the EP4 receptor. Both singular EP2 and EP4 receptor agonists inhibited HASM proliferation, and combined EP2 and EP4 receptor agonism exhibited positive cooperativity in both chronic G_s-mediated signaling and inhibiting HASM proliferation. These findings suggest both EP2 and EP4 are functionally important in HASM, and their combined targeting optimally inhibits airway smooth muscle proliferation.-Michael, J. V. Gavrila, A., Nayak, A. P., Pera, T., Liberato, J. R., Polischak, S. R., Shah, S. D., Deshpande, D. A., Penn, R. B. Cooperativity of E-prostanoid receptor subtypes in regulating signaling and growth inhibition in human airway smooth muscle.

Prostaglandins in asthma and allergic diseases

Prostaglandins are synthesized through the metabolism of arachidonic acid via the cyclooxygenase pathway. There are five primary prostaglandins, PGD₂, PGE₂, PGF₂, PGI₂, and thromboxane B₂, that all signal through distinct seven transmembrane, G-protein coupled receptors. The receptors through which the prostaglandins signal determines their immunologic or physiologic effects. For instance, the same prostaglandin may have opposing properties, dependent upon the signaling pathways activated. In this article, we will detail how inhibition of cyclooxygenase metabolism and regulation of prostaglandin signaling regulates allergic airway inflammation and asthma physiology. Possible prostaglandin therapeutic targets for allergic lung inflammation and asthma will also be reviewed, as informed by human studies, basic science, and animal models.

New targets for resolution of airway remodeling in obstructive lung diseases

Airway remodeling (AR) is a progressive pathological feature of the obstructive lung diseases, including asthma and chronic obstructive pulmonary disease (COPD). The pathology manifests itself in the form of significant, progressive, and (to date) seemingly irreversible changes to distinct respiratory structural compartments. Consequently, AR correlates with disease severity and the gradual decline in pulmonary function associated with asthma and COPD. Although current asthma/COPD drugs manage airway contraction and inflammation, none of these effectively prevent or reverse features of AR. In this review, we provide a brief overview of the features and putative mechanisms affecting AR. We further discuss recently proposed strategies with promise for deterring or treating AR.

EP4 Agonist L-902,688 Suppresses EndMT and Attenuates Right Ventricular Cardiac Fibrosis in Experimental Pulmonary Arterial Hypertension

Right ventricular (RV) hypertrophy is characterized by cardiac fibrosis due to endothelial–mesenchymal transition (EndMT) and increased collagen production in pulmonary arterial hypertension (PAH) patients, but the mechanisms for restoring RV function are unclear. Prostanoid agonists are effective vasodilators for PAH treatment that bind selective prostanoid receptors to modulate vascular dilation. The importance of prostanoid signaling in the RV is not clear. We investigated the effects of the EP4-specific agonist L-902,688 on cardiac fibrosis and TGF-β-induced EndMT. EP4-specific agonist treatment reduced right ventricle fibrosis in the monocrotaline (MCT)-induced PAH rat model. L-902,688 (1 µM) attenuated TGF-β-induced Twist and α-smooth muscle actin (α-SMA) expression, but these effects were reversed by AH23848 (an EP4 antagonist), highlighting the crucial role of EP4 in suppressing TGF-β-induced EndMT. These data indicate that the selective EP4 agonist L-902,688 attenuates RV fibrosis and suggest a potential approach to reducing RV fibrosis in patients with PAH.

Selective inhibition of histamine-evoked Ca2+ signals by compartmentalized cAMP in human bronchial airway smooth muscle cells

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β₂-adrenoceptors, via cAMP and PKA, inhibit histamine-evoked Ca²⁺ signals in human bronchial airway smooth muscle cells.

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Responses to other Ca²⁺-mobilizing receptors are unaffected or minimally affected by cAMP.

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There is no consistent relationship between the amounts of cAMP produced by different stimuli and inhibition of histamine-evoked Ca²⁺ release.

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Local delivery of cAMP within hyperactive signaling junctions stimulates PKA.

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PKA inhibits an early step in the signaling pathway activated by H₁ histamine receptors.

Intracellular Ca²⁺ and cAMP typically cause opposing effects on airway smooth muscle contraction. Receptors that stimulate these pathways are therapeutic targets in asthma and chronic obstructive pulmonary disease. However, the interactions between different G protein-coupled receptors (GPCRs) that evoke cAMP and Ca²⁺ signals in human bronchial airway smooth muscle cells (hBASMCs) are poorly understood. We measured Ca²⁺ signals in cultures of fluo-4-loaded hBASMCs alongside measurements of intracellular cAMP using mass spectrometry or [³H]-adenine labeling. Interactions between the signaling pathways were examined using selective ligands of GPCRs, and inhibitors of Ca²⁺ and cAMP signaling pathways. Histamine stimulated Ca²⁺ release through inositol 1,4,5-trisphosphate (IP₃) receptors in hBASMCs. β₂-adrenoceptors, through cAMP and protein kinase A (PKA), substantially inhibited histamine-evoked Ca²⁺ signals. Responses to other Ca²⁺-mobilizing stimuli were unaffected by cAMP (carbachol and bradykinin) or minimally affected (lysophosphatidic acid). Prostaglandin E₂ (PGE₂), through EP₂ and EP₄ receptors, stimulated formation of cAMP and inhibited histamine-evoked Ca²⁺ signals. There was no consistent relationship between the inhibition of Ca²⁺ signals and the amounts of intracellular cAMP produced by different stimuli. We conclude that β-adrenoceptors, EP₂ and EP₄ receptors, through cAMP and PKA, selectively inhibit Ca²⁺ signals evoked by histamine in hBASMCs, suggesting that PKA inhibits an early step in H₁ receptor signaling. Local delivery of cAMP within hyperactive signaling junctions mediates the inhibition.

Prostaglandin E2 suppresses human group 2 innate lymphoid cell function

Background

Group 2 innate lymphoid cells (ILC2s) are involved in the initial phase of type 2 inflammation and can amplify allergic immune responses by orchestrating other type 2 immune cells. Prostaglandin (PG) E₂ is a bioactive lipid that plays protective roles in the lung, particularly during allergic inflammation.

Objective

We set out to investigate how PGE₂ regulates human ILC2 function.

Methods

The effects of PGE₂ on human ILC2 proliferation and intracellular cytokine and transcription factor expression were assessed by means of flow cytometry. Cytokine production was measured by using ELISA, and real-time quantitative PCR was performed to detect PGE₂ receptor expression.

Results

PGE₂ inhibited GATA-3 expression, as well as production of the type 2 cytokines IL-5 and IL-13, from human tonsillar and blood ILC2s in response to stimulation with a combination of IL-25, IL-33, thymic stromal lymphopoietin, and IL-2. Furthermore, PGE₂ downregulated the expression of IL-2 receptor α (CD25). In line with this observation, PGE₂ decreased ILC2 proliferation. These effects were mediated by the combined action of E-type prostanoid receptor (EP) 2 and EP4 receptors, which were specifically expressed on ILC2s.

Conclusion

Our findings reveal that PGE₂ limits ILC2 activation and propose that selective EP2 and EP4 receptor agonists might serve as a promising therapeutic approach in treating allergic diseases by suppressing ILC2 function.

Prostanoid EP4 agonist L-902,688 activates PPARγ and attenuates pulmonary arterial hypertension

Prostacyclin agonists that bind the prostacyclin receptor (IP) to stimulate cAMP synthesis are effective vasodilators for the treatment of idiopathic pulmonary arterial hypertension (IPAH), but this signaling may occur through nuclear peroxisome proliferator-activated receptor-γ (PPARγ). There is evidence of scant IP and PPARγ expression but stable prostanoid EP₄ receptor (EP₄) expression in IPAH patients. Both IP and EP₄ functionally couple with stimulatory G protein (G_s), which activates signal transduction. We investigated the effect of an EP₄-specific agonist on pulmonary arterial remodeling and its regulatory mechanisms in pulmonary arterial smooth muscle cells (PASMCs). Immunoblotting evealed IP, EP₄, and PPARγ expression in human pulmonary arterial hypertension (PAH) and monocrotaline (MCT)-induced PAH rat lung tissue. Isolated PASMCs from MCT-induced PAH rats (MCT-PASMCs) were treated with L-902,688, a selective EP₄ agonist, to investigate the anti-vascular remodeling effect. Scant expression of IP and PPARγ but stable expression of EP₄ was observed in IPAH patient lung tissues and MCT-PASMCs. L-902,688 inhibited IP-insufficient MCT-PASMC proliferation and migration by activating PPARγ in a time- and dose-dependent manner, but these effects were reversed by AH-23848 (an EP₄ antagonist) and H-89 [a protein kinase A (PKA) inhibitor], highlighting the crucial role of PPARγ in the activity of this EP₄ agonist. L-902,688 attenuated pulmonary arterial remodeling in hypoxic PAH mice and MCT-induced PAH rats; therefore, we conclude that the selective EP₄ agonist L-902,688 reverses vascular remodeling by activating PPARγ. This study identified a novel EP₄-PKA-PPARγ pathway, and we propose EP₄ as a potential therapeutic target for PAH.

Activating prostaglandin E2 receptor subtype EP4 increases secreted mucin from airway goblet cells

Prostaglandin E2 (PGE2) is a ligand of the E-type prostanoid receptors, EP1-4. PGE2 secretion is increased in the airways of patients with asthma by secretory phospholipases A₂, which also increases MUC5AC mucin in goblet cells. We hypothesized that PGE2 would also increase MUC5AC mRNA and secreted protein through specific EP receptor activation. We sought to assess the effect of specific EP receptor activation on MUC5AC secretion from ciliated-enriched cells or goblet-enriched cells induced by IL-13. We develop an enriched goblet cell epithelium by growing normal human bronchial epithelial cells at air liquid interface for 14 days in the presence of IL-13. We examined exposure to 4 specific EP receptor agonists at 24 h and 14 days in cells grown with or without IL-13 exposure, and measured MUC5AC mRNA and secreted protein, as well as airway culture morphology, and EP receptor expression. In ciliated-enriched cells grown in the absence of IL-13, the EP4 receptor agonist modestly increased both MUC5AC mRNA and secretion (p < 0.001, 241% increase of transcripts and p < 0.01, 86% increase of secreted protein) but did not visibly change cell morphology. In goblet-enriched cells grown in the presence of IL-13, the EP4 receptor agonist greatly increased both MUC5AC mRNA and protein (p < 0.001, 315% increase of transcripts and 92% increase of secreted protein). Specific activation of the other EP receptor had no effect on secreted mucin. EP4 receptor mRNA and protein were significantly increased in goblet-enriched cells, while the other receptor mRNA were decreased. We conclude that PGE2 stimulates airway mucin production predominantly by EP4 receptor activation in association with increased EP4 receptor expression. This may contribute to mucus hypersecretion as seen in severe asthma.

Role of prostaglandin E2 in bronchoconstriction-triggered cough response in guinea pigs

A feature of cough variant asthma is a heightened cough response to bronchoconstriction. The mediators of this response are unknown. This study was designed to elucidate the role of lipid mediators in bronchoconstriction-triggered cough response in an experimental animal model. We examined the influence of bronchoconstriction on cell components and mediators including prostaglandin E₂ (PGE₂) in bronchoalveolar lavage fluid (BALF). We studied the cough response to bronchoconstriction (CRB) by measuring the correlation between the increase in enhanced pause (Penh), an index of bronchoconstriction, and cough counts induced by methacholine (Mch) inhalation in conscious guinea pigs. We then examined the effects of intraperitoneal pretreatment with 16, 16-dimethyl-prostaglandin E₂ (dm-PGE₂) on CRB and cough counts. The total number of cells and cell components in the BALF were not influenced by bronchoconstriction. While levels of PGE₂, prostaglandin I₂, and cysteinyl leukotrienes were significantly increased, levels of prostaglandin D₂, thromboxane B₂, and substance P in the BALF were not. Dm-PGE₂ significantly decreased the Mch-induced increase in Penh. Following bronchoconstriction by additional Mch inhalation, dm-PGE₂ produced an increase in CRB and cough counts in a dose-dependent manner. Additionally, the heightened CRB following dm-PGE₂ treatment was suppressed by pretreatment with PGE₂ receptor (E-prostanoid EP) -1 and EP-3 antagonists in a dose-dependent manner, but not by EP-2 and EP-4 antagonists. The EP-1 antagonist also decreased cough counts. These results suggest that PGE₂ acts as an exacerbating factor for bronchoconstriction-triggered cough. EP1 and EP3 may provide new therapeutic targets for cough variant asthma.

Lipid Mediators of Allergic Disease: Pathways, Treatments, and Emerging Therapeutic Targets

Bioactive lipids are critical regulators of inflammation. Over the last 75 years, these diverse compounds have emerged as clinically-relevant mediators of allergic disease pathophysiology. Animal and human studies have demonstrated the importance of lipid mediators in the development of asthma, allergic rhinitis, urticaria, anaphylaxis, atopic dermatitis, and food allergy. Lipids are critical participants in cell signaling events which influence key physiologic (bronchoconstriction) and immune phenomena (degranulation, chemotaxis, sensitization). Lipid-mediated cellular mechanisms including: (1) formation of structural support platforms (lipid rafts) for receptor signaling complexes, (2) activation of a diverse family of G-protein coupled receptors, and (3) mediating intracellular signaling cascades by acting as second messengers. Here, we review four classes of bioactive lipids (platelet activating factor, the leukotrienes, the prostanoids, and the sphingolipids) with special emphasis on lipid synthesis pathways and signaling, atopic disease pathology, and the ongoing development of atopy treatments targeting lipid mediator pathways.

Imbalance of endogenous prostanoids in moderate-to-severe asthma

BACKGROUND

Inhalation studies suggested "protective" roles of exogenous prostaglandin E₂, but the clinical relevance of endogenous prostanoids in asthma is poorly known. The objective of this study is to measure sputum levels of prostanoids in asthmatic patients to correlate with clinical indices.

METHODS

Mild (n = 41) or moderate-to-severe (19) asthmatics and 27 normal controls were examined for pulmonary function (FEV₁ and mid-forced expiratory flow), sputum cell differentials, and sputum levels of prostaglandins D₂, E₂, F_2α, and thromboxane B₂ measured by sandwich enzyme immunoassay.

RESULTS

Each prostanoid level did not differ among the three groups. Sputum number of bronchial epithelial cells was greater in moderate-to-severe asthmatics than in the other two groups, suggesting epithelial desquamation. Levels of prostaglandin F_2α, D₂, and thromboxane B₂ positively correlated with the severity of airflow obstruction in the 60 asthmatic patients, whereas prostaglandin E₂ levels were unrelated to pulmonary function. The ratio of combined "contractile" prostanoids (prostaglandin D₂/prostaglandin F_2α/thromboxane B₂) to prostaglandin E₂ was 2.5-fold greater in moderate-to-severe asthmatics than in controls (p = 0.001) or in mild asthmatics (p = 0.0002) but did not differ between the latter two groups. In the two asthmatic groups combined, this ratio positively correlated with the sputum number of epithelial cells. The combined "contractile" prostanoids levels positively correlated with prostaglandin E₂ levels in controls and in mild asthmatics but not in moderate-to-severe asthmatics.

CONCLUSIONS

An imbalance in production, breakdown, or both between prostaglandin E₂ and other prostanoids possibly due to epithelial damage may be involved in the pathogenesis of moderate-to-severe asthma.

Cyclooxygenase 2: its regulation, role and impact in airway inflammation

Cyclooxygenase 2 (COX-2: official gene symbol - PTGS2) has long been regarded as playing a pivotal role in the pathogenesis of airway inflammation in respiratory diseases including asthma. COX-2 can be rapidly and robustly expressed in response to a diverse range of pro-inflammatory cytokines and mediators. Thus, increased levels of COX-2 protein and prostanoid metabolites serve as key contributors to pathobiology in respiratory diseases typified by dysregulated inflammation. But COX-2 products may not be all bad: prostanoids can exert anti-inflammatory/bronchoprotective functions in airways in addition to their pro-inflammatory actions. Herein, we outline COX-2 regulation and review the diverse stimuli known to induce COX-2 in the context of airway inflammation. We discuss some of the positive and negative effects that COX-2/prostanoids can exert in in vitro and in vivo models of airway inflammation, and suggest that inhibiting COX-2 expression to repress airway inflammation may be too blunt an approach; because although it might reduce the unwanted effects of COX-2 activation, it may also negate the positive effects. Evidence suggests that prostanoids produced via COX-2 upregulation show diverse actions (and herein we focus on prostaglandin E2 as a key example); these can be either beneficial or deleterious and their impact on respiratory disease can be dictated by local concentration and specific interaction with individual receptors. We propose that understanding the regulation of COX-2 expression and associated receptor-mediated functional outcomes may reveal number of critical steps amenable to pharmacological intervention. These may prove invaluable in our quest towards future development of novel anti-inflammatory pharmacotherapeutic strategies for the treatment of airway diseases.

The anti-inflammatory effects of PGE2 on human lung macrophages are mediated by the EP4 receptor

BACKGROUND AND PURPOSE

PGE₂ inhibits cytokine generation from human lung macrophages. However, the EP receptor that mediates this beneficial anti-inflammatory effect of PGE₂ has not been defined. The aim of this study was to identify the EP receptor by which PGE₂ inhibits cytokine generation from human lung macrophages. This was determined by using recently developed EP receptor ligands.

EXPERIMENTAL APPROACH

The effects of PGE₂ and EP-selective agonists on LPS-induced generation of TNF-α and IL-6 from macrophages were evaluated. The effects of EP₂ -selective (PF-04852946, PF-04418948) and EP₄ -selective (L-161,982, CJ-042794) receptor antagonists on PGE₂ responses were studied. The expression of EP receptor subtypes by human lung macrophages was determined by RT-PCR.

KEY RESULTS

PGE₂ inhibited LPS-induced and Streptococcus pneumoniae-induced cytokine generation from human lung macrophages. Analysis of mRNA levels indicated that macrophages expressed EP₂ and EP₄ receptors. L-902,688 (EP₄ receptor-selective agonist) was considerably more potent than butaprost (EP₂ receptor-selective agonist) as an inhibitor of TNF-α generation from macrophages. EP₂ receptor-selective antagonists had marginal effects on the PGE₂ inhibition of TNF-α generation, whereas EP₄ receptor-selective antagonists caused rightward shifts in the PGE₂ concentration-response curves.

CONCLUSIONS AND IMPLICATIONS

These studies demonstrate that the EP₄ receptor is the principal receptor that mediates the anti-inflammatory effects of PGE₂ on human lung macrophages. This suggests that EP₄ receptor agonists could be effective anti-inflammatory agents in human lung disease.

Role of EP2 and EP4 receptors in airway microvascular leak induced by prostaglandin E2

BACKGROUND AND PURPOSE

Airway microvascular leak (MVL) involves the extravasation of proteins from post-capillary venules into surrounding tissue. MVL is a cardinal sign of inflammation and an important feature of airway inflammatory diseases such as asthma. PGE2, a product of COX-mediated metabolism of arachidonic acid, binds to four receptors, termed EP1–4. PGE2 has a wide variety of effects within the airway, including modulation of inflammation, sensory nerve activation and airway tone. However, the effect of PGE2 on airway MVL and the receptor/s that mediate this have not been described.

EXPERIMENTAL APPROACH

Evans Blue dye was used as a marker of airway MVL, and selective EP receptor agonists and antagonists were used alongside EP receptor-deficient mice to define the receptor subtype involved.

KEY RESULTS

PGE2 induced significant airway MVL in mice and guinea pigs. A significant reduction in PGE2-induced MVL was demonstrated in Ptger2−/− and Ptger4−/− mice and in wild-type mice pretreated simultaneously with EP2 (PF-04418948) and EP4 (ER-819762) receptor antagonists. In a model of allergic asthma, an increase in airway levels of PGE2 was associated with a rise in MVL; this change was absent in Ptger2−/− and Ptger4−/− mice.

CONCLUSIONS AND IMPLICATIONS

PGE2 is a key mediator produced by the lung and has widespread effects according to the EP receptor activated. Airway MVL represents a response to injury and under ‘disease’ conditions is a prominent feature of airway inflammation. The data presented highlight a key role for EP2 and EP4 receptors in MVL induced by PGE2.