The effect of oral prostaglandin E1 on airway responsiveness in asthmatic subjects

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.

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.

Pathogenesis of COPD and Asthma

Asthma and COPD remain two diseases of the respiratory tract with unmet medical needs. This review considers the current state of play with respect to what is known about the underlying pathogenesis of these two chronic inflammatory diseases of the lung. The review highlights why they are different conditions requiring different approaches to treatment and provides a backdrop for the subsequent chapters in this volume discussing recent advances in the pharmacology and treatment of asthma and COPD.

The PGE2-EP2-mast cell axis: an antiasthma mechanism

Despite the fact that cyclooxygenase and its products, prostaglandins, have been traditionally associated with the development of inflammation, PGE2 was implicated early on as potentially beneficial in asthma. During the 1970s and 1980s, several studies reported the bronchodilator effect of PGE2 in asthma patients. In parallel, it was being shown to exert an inhibitory effect on mast cells in vitro. In spite of this, data supporting the beneficial role for PGE2 in asthma were scarce and sometimes controversial. Many years later, in vitro and in vivo studies suggested a range of biological activities attributable to PGE2, others than the ability to relax smooth muscle, that potentially explained some of the observed positive effects in asthma. The identification and cloning of the four PGE2 receptors made available new tools with which to fine-tune investigation of the anti-inflammatory, pro-inflammatory, immunoregulatory, and bronchodilation mechanisms of PGE2. Among these, several suggested involvement of mast cells, a cell population known to play a fundamental role in acute and chronic asthma. Indeed, it has been shown that PGE2 prevents human and murine MC activity in vitro through activation of the EP2 receptor, and also that both exogenously administered and endogenous PGE2 inhibit airway MC activity in vivo in mouse models of asthma (likely through an EP2-mediated mechanism as well). In the last few years, we have furthered into the functional connection between PGE2-induced mast cells inhibition and attenuated damage, in asthma and allergy models. The validity of the findings supporting a beneficial effect of PGE2 in different asthma phases, the direct effect of PGE2 on mast cells populations, and the functional implications of the PGE2-MC interaction on airway function are some of the topics addressed in this review, under the assumption that increased understanding of the PGE2-EP2-mast cell axis will likely lead to the discovery of novel antiasthma targets.

Prostaglandin E(2) produced by the lung augments the effector phase of allergic inflammation

Elevated PGE(2) is a hallmark of most inflammatory lesions. This lipid mediator can induce the cardinal signs of inflammation, and the beneficial actions of nonsteroidal anti-inflammatory drugs are attributed to inhibition of cyclooxygenase (COX)-1 and COX-2, enzymes essential in the biosynthesis of PGE(2) from arachidonic acid. However, both clinical studies and rodent models suggest that, in the asthmatic lung, PGE(2) acts to restrain the immune response and limit physiological change secondary to inflammation. To directly address the role of PGE(2) in the lung, we examined the development of disease in mice lacking microsomal PGE(2) synthase-1 (mPGES1), which converts COX-1/COX-2-derived PGH(2) to PGE(2). We show that mPGES1 determines PGE(2) levels in the naive lung and is required for increases in PGE(2) after OVA-induced allergy. Although loss of either COX-1 or COX-2 increases the disease severity, surprisingly, mPGES1(-/-) mice show reduced inflammation. However, an increase in serum IgE is still observed in the mPGES1(-/-) mice, suggesting that loss of PGE(2) does not impair induction of a Th2 response. Furthermore, mPGES1(-/-) mice expressing a transgenic OVA-specific TCR are also protected, indicating that PGE(2) acts primarily after challenge with inhaled Ag. PGE(2) produced by the lung plays the critical role in this response, as loss of lung mPGES1 is sufficient to protect against disease. Together, this supports a model in which mPGES1-dependent PGE(2) produced by populations of cells native to the lung contributes to the effector phase of some allergic responses.

Suppressors of cytokine signaling 3 expression in eosinophils: regulation by PGE₂ and Th2 cytokines

Asthma and nonasthmatic eosinophilic bronchitis (NAEB) are respiratory disorders characterized by a predominance of Th2 cells and eosinophilic inflammation. Suppressors of cytokine signaling (SOCS) proteins play an important role in Th2-mediated allergic responses through control of the balance between Th1 and Th2 cells, particularly, SOCS3 and SOCS5. The aim of this study was to analyze SOCS expression in human peripheral blood eosinophils from patients with asthma, NAEB and healthy controls. SOCS expression in eosinophils from subjects was demonstrated by different techniques. Results showed that expression of SOCS3 in eosinophils and CD4 T cells from patients was higher than in healthy subjects. In addition, we demonstrated that prostaglandin E₂ (PGE₂) and Th2 cytokines are able to upregulate SOCS3 production in eosinophils and attenuate its degranulation. In conclusion, eosinophils are able to transcribe and translate SOCS3 protein and can contribute to the regulation of the Th1/Th2 balance through SOCS3 production.

Changes in sputum eicosanoids and inflammatory markers after inhalation challenges with occupational agents

BACKGROUND

An increase in cysteinyl-leukotrienes (LTs) after specific inhalation challenge (SIC) with common allergens in patients with atopic asthma has been shown previously, but there are scarce data with occupational agents. We sought to determine whether there are differences in lower airway inflammatory markers and the production of cytokines and eicosanoids between patients with a positive or negative SIC response to occupational agents.

METHODS

Twenty-six patients with suspected occupational asthma and 13 healthy control subjects were studied. Spirometry, methacholine challenge, and sputum induction were performed at baseline and 24 h after SIC with occupational agents. Several cytokines and inflammatory mediators, including eicosanoids, were measured in sputum.

RESULTS

Twenty-six SICs were carried out with high-molecular-weight or low-molecular-weight agents, and the responses were positive in 18 patients. SIC elicited nine early asthmatic responses, two dual asthmatic responses, and seven isolated late asthmatic responses. Significant increments in sputum eosinophil counts were found only in patients with positive SIC responses compared with baseline values. Interleukin-10 levels were decreased in patients with positive and negative SIC responses compared to those in healthy control subjects. A significant increase (p < 0.05) in the LTC(4)/prostaglandin E(2) (PGE(2)) ratio was observed in patients after positive SIC responses compared to those with negative SIC responses.

CONCLUSIONS

Overexpression of LTC(4), relative underproduction of PGE(2), and greater airway eosinophilia were observed in patients with positive SIC responses.

Increased prostaglandin E2 levels in the airway of patients with eosinophilic bronchitis

BACKGROUND

Eosinophilic bronchitis is a common cause of chronic cough, which like asthma is characterized by sputum eosinophilia, but unlike asthma there is no variable airflow obstruction or airway hyperresponsiveness. We tested the hypothesis that the different airway function in patients with eosinophilic bronchitis and asthma could be caused by an imbalance in the production of bronchoconstrictor (LTC(4)) and bronchoprotective (prostaglandin E(2); PGE(2)) lipid mediators.

METHODS

We measured cytokines levels, proinflammatory mediators and eicosanoids concentration in sputum from 13 subjects with nonasthmatic eosinophilic bronchitis, 13 subjects with asthma, and 11 healthy control subjects. Cytokines mRNA levels were measured by real time PCR, proinflammatory mediators, PGE(2), and LTC(4) were measured by enzyme immunoassays.

RESULTS

The median sputum eosinophil count was not statistically different in patients with asthma (7.95%) and eosinophilic bronchitis (15.29%). The levels of mRNA specific to interleukin-5 (IL-5), IL-4, IL-10, IL-13, interferon gamma (IFN-gamma), IL-2, vascular endothelial growth factor and transforming growth factor beta were similar in both conditions. In addition, no differences were found between asthma and eosinophilic bronchitis in proinflammatory cytokines, such as IL-8, IFN-gamma and tumor necrosis factor alpha (TNF-alpha) levels. Sputum cysteinyl-leukotrienes concentration was raised both in eosinophilic bronchitis and asthma patients. We found that induced sputum PGE(2) concentrations were significantly increased in subjects with eosinophilic bronchitis (838.3 +/- 612 pg/ml) when compared with asthmatic (7.54 +/- 2.14 pg/ml) and healthy subjects (4 +/- 1.3 pg/ml).

CONCLUSION

This data suggest that the difference in airway function observed in subjects with eosinophilic bronchitis and asthma could be due to differences in PGE(2) production in the airways.

Involvement of cyclooxygenase-2 and prostaglandins in the molecular pathogenesis of inflammatory lung diseases

Inducible cyclooxygenase (COX-2) and its metabolites have diverse and potent biological actions that are important for both physiological and disease states of lung. The wide variety of prostaglandin (PG) products are influenced by the level of cellular activation, the exact nature of the stimulus, and the specific cell type involved in their production. In turn, the anti- and proinflammatory response of PG is mediated by a blend of specific surface and intracellular receptors that mediate diverse cellular events. The complexity of this system is being at least partially resolved by the generation of specific molecular biological research tools that include cloning and characterization of the enzymes distal to COX-2 and the corresponding receptors to the final cellular products of arachidonic metabolism. The most informative of these approaches have employed genetically modified animals and specific receptor antagonists to determine the exact role of specific COX-2-derived metabolites on specific cell types of the lung in the context of inflammatory models. These data have suggested a number of cell-specific, pathway-specific, and receptor-specific approaches that could lead to effective therapeutic interventions for most inflammatory lung diseases.

The prostaglandin E agonist, misoprostol, inhibits airway IL-5 production in atopic asthmatics

BACKGROUND

Prostaglandin E2 is a potent immunomodulator that inhibits the early and late bronchoconstriction to inhaled allergen, as well as inhibiting the acute allergen-induced release of mediators into the human airway. To determine if the stable prostaglandin E agonist misoprostol could alter the late allergic formation of mediators we measured the appearance of eosinophils and key cytokines in the bronchoalveolar lavage fluid 24 h after allergen instillation.

METHODS

Six atopic asthmatics underwent bronchoscopy, alveolar lavage and antigen instillation followed 24 h later by bronchoalveolar lavage. Eosinophil counts were done, together with measurements of IL-4, IL-5, eotaxin, RANTES and cysteinyl leukotrienes by immunoassay. The study was done in randomized blinded fashion while the volunteers took placebo or 600 microg of misoprostol four times a day (QID).

RESULTS

Misoprostol significantly decreased the appearance of IL-5 late after allergen challenge. Eotaxin levels were reduced, but not statistically significantly. Eosinophil number, RANTES, eosinophil cationic protein and cysteinyl leukotrienes were not altered by misoprostol.

CONCLUSIONS

Misoprostol reduces the formation of IL-5 late after allergen challenge, perhaps by inhibiting eosinophil, mast cell, and/or T lymphocyte production of IL-5. Despite decreases in IL-5 and eotaxin, eosinophils were recruited and activated by allergen.

Misoprostol therapeutics revisited

Misoprostol, a prostaglandin E1 analog, is a racemate of four stereoisomers. On administration it rapidly de-esterifies to its active form, misoprostolic acid. Misoprostolic acid is 85% albumin bound and has a half-life of approximately 30 minutes. It is excreted in urine as inactive metabolites. No significant drug interactions have been reported. Besides its gastrointestinal protective and uterotonic activities, misoprostol regulates various immunologic cascades. It inhibits platelet-activating factor and leukocyte adherence, and modulates adhesion molecule expression. It protects against gut irradiation injury, experimental gastric cancer, enteropathy, and constipation. It improves nutrient absorption in cystic fibrosis. Misoprostol has utility in acetaminophen and ethanol hepatotoxicity, hepatitis, and fibrosis. It is effective in asthmatics and aspirin-sensitive asthmatic and allergic patients. It lowers cholesterol and severity of peripheral vascular diseases, prolongs survival of cardiac and kidney transplantation, synergizes cyclosporine, and protects against cyclosporine-induced renal damage. It works against drug-induced renal damage, interstitial cystitis, lupus nephritis, and hepatorenal syndrome. It is useful in periodontal disease and dental repair. Misoprostol enhances glycosoaminoglycan synthesis in cartilage after injury. It prevents ultraviolet-induced cataracts and reduces intraocular pressure in glaucoma and ocular hypertension. It synergizes antiinflammatory and analgesic effects of diclofenac or colchicine and has been administered to treat trigeminal neuralgic pain. It reduces chemotherapy-induced hair loss and recovery time from burn injury, and is effective in treating sepsis, multiple sclerosis, and pancreatitis.

Prostanoid receptors involved in the relaxation of human bronchial preparations

1. Iloprost and cicaprost (IP-receptor agonists) induced relaxations in the histamine- (50 microM) contracted human bronchial preparations (pD2 values, 6.63+/-0.12 and 6.86+/-0.08; Emax values, 90+/-04 and 65+/-08% of the papaverine response for iloprost (n=6) and cicaprost (n=3), respectively). 2. Prostaglandin E2 (PGE2) and misoprostol (EP-receptor agonist) relaxed the histamine-contracted human bronchial preparations (pD2 values, 7.13+/-0.07 and 6.33+/-0.28; Emax values, 67+/-04 and 57+/-08% of the papaverine response for PGE2 (n=14) and misoprostol (n=4), respectively). In addition, both relaxations were inhibited by AH6809 (DP/EP1/EP2-receptor antagonist; 3 microM; n=5-6). 3. The PGE2-induced relaxations of human bronchial preparations were not modified by treatment with AH23848B (TP/EP4-receptor antagonist; 30 microM; n=4). 4. The contracted human bronchial preparations were significantly relaxed by prostaglandin D2 (PGD2) or by BW245C a DP-receptor agonist. However, these responses did not exceed 40% of the relaxation induced by papaverine. In addition, the relaxations induced by PGD2 were significantly inhibited by treatment with a DP-receptor antagonist BWA868C (0.1 microM; n=3). 5. These data suggest that the relaxation of human isolated bronchial preparations induced by prostanoids involved IP-, EP2- and to a lesser extent DP-receptors but not EP4-receptor.

Protective effects of inhaled PGE2 on allergen-induced airway responses and airway inflammation

Inhalation of prostaglandin E2 (PGE2) had been reported to prevent allergen-induced bronchoconstrictor responses; however, the effects of inhaled PGE2 on allergen-induced airway inflammation or hyperresponsiveness after allergen are unknown. This study examined the effects of inhaled PGE2 on allergen-induced airway responses and inflammation. Eight mild asthmatics with a dual airway response to inhaled allergen were recruited into a double-blind randomized crossover study comparing the effects of inhaled PGE2 (100 microgram) or placebo, on allergen-induced changes in FEV1 measured for 7 h, induced sputum inflammatory cells, obtained at baseline, 7 and 24 h, and methacholine airway responsiveness measured at 24 h after challenge. Inhaled PGE2 attenuated the allergen-induced early fall in FEV1 from 24.4 +/- 3.6% after placebo to 10.3 +/- 2.5% after PGE2 (p = 0.002), the late fall in FEV1 from 21.2 +/- 2.7% after placebo to 12.6 +/- 3.6% after PGE2 (p = 0.03), allergen-induced methacholine airway hyperresponsiveness (p = 0.03) and allergen-induced increases in percent sputum eosinophils from 36.3 +/- 8.8% after placebo to 21.0 +/- 7.3% after PGE2 (p = 0.01), percentage of EG2+ cells (p = 0.02), and percentage of metachromatic cells (p = 0.02). These results indicate that inhaled PGE2 attenuates allergen-induced airway responses, hyperresponsiveness, and inflammation, when given immediately before inhaled allergen.

Effect of prostaglandin E2 on eicosanoid release by human bronchial biopsy specimens from normal and inflamed mucosa

BACKGROUND

Eicosanoids such as prostaglandin E2 (PGE2), thromboxane A2 (TXA2), and peptidoleukotrienes (pLT) are known to be biologically highly active lipid mediators, especially in human lung epithelium. PGE2 is thought to have mostly bronchoprotective effects, whereas pLT and TXA2 are bronchoconstrictive. This study was undertaken to assess the release and interaction of eicosanoids in human bronchial biopsy specimens of normal and inflamed mucosa.

METHODS

Bronchial biopsy specimens were obtained from 16 patients, seven controls without signs of inflammation and nine patients with severe inflammatory processes in the epithelium. The release of pLT, TXA2 (measured as TXB2), and PGE2 was investigated using a "functional in vitro test" and the addition of several stimuli.

RESULTS

Specimens incubated with arachidonic acid released higher amounts of pLT, TXB2, and PGE2 than unstimulated specimens. Preincubation with PGE2 revealed significant inhibition of arachidonic acid-induced release of pLT and TXB2 (> 50%). The inhibitory effect was higher in normal than in inflamed epithelium.

CONCLUSIONS

Exogenous PGE2 has inhibitory effects on the release of pLT and TXB2 in human bronchial biopsy specimens. This finding could explain the bronchoprotective effect of inhaled PGE2 in normal subjects and asthmatic subjects as direct eicosanoid interactions. It also supports the concept of PGE2 as a bronchoprotective endogenous substance. The complex effects of PGE2 as a modulating mediator in inflammation may be worth investigating.

Effect of inhaled prostaglandin E2 on bronchial reactivity to sodium metabisulphite and methacholine in patients with asthma

Inhaled frusemide protects against the bronchoconstrictor response to a wide range of stimuli that cause bronchoconstriction by indirect mechanisms. One possible explanation for this protection relates to the known ability of frusemide to enhance synthesis of prostaglandin E2 (PGE2). Studies in vitro suggest that PGE2 might protect against indirectly acting bronchoconstrictor challenges rather than those that act directly on airway smooth muscle, though little is known about the effects of PGE2 in vivo. The effect of inhaled PGE2 on the bronchoconstrictor response to inhaled sodium metabisulphite (a stimulus with an indirect action) and methacholine (which acts directly on airway smooth muscle) was studied in nine patients with asthma. Subjects were studied on four days, inhaling PGE2 (100 micrograms) or placebo in a double blind fashion followed immediately by a cumulative dose challenge with sodium metabisulphite or methacholine. The response to the constrictor stimuli was measured as the provocative dose causing a 20% fall in FEV1 (PD20). There was no significant change in FEV1 after inhaled PGE2 compared with placebo, nor any significant change in the response to methacholine; the geometric mean methacholine PD20 was 0.9 mumol after PGE2 and 0.56 mumol after placebo (mean difference 0.7 (95% confidence limits--0.1, 1.5) doubling doses). PGE2, however, protected against sodium metabisulphite, the geometric mean sodium metabisulphite PD20 being 11.8 mumol after PGE2 and 1.8 mumol after placebo (mean difference 2.5 (95% CL 1.9, 3.1) doubling doses). PGE2 conferred significantly greater protection against sodium metabisulphite than methacholine (mean difference 1.8 (95% CL 0.8, 2.8) doubling doses). This suggests that PGE2, like frusemide, has an inhibitory effect on pathways relevant to the bronchoconstriction induced by sodium metabisulphite, with little or no effect on those relevant to methacholine.