Johan Bood

Prostaglandin E2 inhibits mast cell-dependent bronchoconstriction in human small airways through the E prostanoid subtype 2 receptor.

BACKGROUND Inhaled prostaglandin (PG) E2 might inhibit asthmatic responses, but the mechanisms involved remain undefined. OBJECTIVE We sought to characterize the direct and indirect effects of PGE2 on human small airways with particular reference to the receptors mediating the responses. METHODS Contraction and relaxation were studied in isolated human bronchi with an inner diameter of 1 mm or less. RESULTS Low concentrations of PGE2 (0.01-1 μmol/L) relaxed the bronchi precontracted by histamine. The bronchodilator response was inhibited by the E prostanoid (EP) subtype 4 receptor antagonist ONO-AE3-208 but unaffected by the EP2 receptor antagonist PF-04418948. Higher concentrations of PGE2 (10-100 μmol/L) contracted the small airways. However, the TP receptor agonists U-46,619, PGF2α, and PGD2 were more potent than PGE2. Moreover, the bronchoconstrictor responses to PGE2 and all other tested prostanoids, including the EP1/EP3 receptor agonist 17-phenyl trinor PGE2 and the partial FP receptor agonist AL-8810, were uniformly abolished by the TP receptor antagonist SQ-29,548. In the presence of TP and EP4 antagonists, PGE2 inhibited the mast cell-mediated bronchoconstriction resulting from anti-IgE challenge. Measurement of the release of histamine and cysteinyl leukotrienes documented that this bronchoprotective action of PGE2 was mediated by the EP2 receptor, unrelated to bronchodilation, and increased with time of exposure. CONCLUSION The pharmacology of PGE2 in isolated human small airways was different from its profile in animal models. This first demonstration of powerful EP2 receptor-mediated inhibition of IgE-dependent contractions in human airways introduces a new selective target for the treatment of asthma. This EP2 control of mast cell-mediated bronchoconstriction is presumably exaggerated in patients with aspirin-exacerbated respiratory disease.

The Effect of Omega-3 Fatty Acids on Bronchial Hyperresponsiveness, Sputum Eosinophilia, and Mast Cell Mediators in Asthma

BACKGROUND Omega-3 fatty acid supplements have been reported to inhibit exercise-induced bronchoconstriction (EIB). It has not been determined whether omega-3 supplements inhibit airway sensitivity to inhaled mannitol, a test for bronchial hyperresponsiveness (BHR) and model for EIB in people with mild to moderate asthma. METHODS In a double-blind, crossover trial, subjects with asthma who had BHR to inhaled mannitol (n = 23; 14 men; mean age, 28 years; one-half taking regular inhaled corticosteroids) were randomized to omega-3 supplements (4.0 g/d eicosapentaenoic acid and 2.0 g/d docosahexaenoic acid) or matching placebo for 3 weeks separated by a 3-week washout. The primary outcome was the provoking dose of mannitol (mg) to cause a 15% fall in FEV1 (PD15). Secondary outcomes were sputum eosinophil count, spirometry, Asthma Control Questionnaire (ACQ) score, serum triacylglyceride level, and lipid mediator profile in urine and serum. RESULTS PD15 (geometric mean, 95% CI) to mannitol following supplementation with omega-3s (78 mg, 51-119 mg) was not different from placebo (88 mg, 56-139 mg, P = .5). There were no changes in sputum eosinophils (mean ± SD) in a subgroup of 11 subjects (omega-3, 8.4% ± 8.2%; placebo, 7.8% ± 11.8%; P = .9). At the end of each treatment period, there were no differences in FEV1 % predicted (omega-3, 85% ± 13%; placebo, 84% ± 11%; P = .9) or ACQ score (omega-3, 1.1% ± 0.5%; placebo, 1.1% ± 0.5%; P = .9) (n = 23). Omega-3s caused significant lowering of blood triglyceride levels and expected shifts in serum fatty acids and eicosanoid metabolites, confirming adherence to the supplements; however, no changes were observed in urinary mast cell mediators. CONCLUSIONS Three weeks of omega-3 supplements does not improve BHR to mannitol, decrease sputum eosinophil counts, or inhibit urinary excretion of mast cell mediators in people with mild to moderate asthma, indicating that dietary omega-3 supplementation is not useful in the short-term treatment of asthma. TRIAL REGISTRY ClinicalTrials.gov; No.: NCT00526357; URL: www.clinicaltrials.gov.

Urinary excretion of lipid mediators in response to repeated eucapnic voluntary hyperpnea in asthmatic subjects

Exercise-induced bronchoconstriction displays refractoriness manifested as a decreased response to repeated exercise challenge within hours. The refractoriness may be attenuated by inhibition of the biosynthesis of prostaglandins (PG). The aim of the study was to determine which PGs and other lipid mediators are excreted during the refractory period. First, 16 subjects with mild stable asthma performed two repeated 4-min challenges with eucapnic voluntary hyperpnea (EVH) 1 and 3 h apart. There was a similar degree of refractoriness in both protocols (∼15% protection). The 1-h interval was too short to study mediator excretion because the urinary levels did not return to baseline before the second challenge. With the 3-h protocol, there was increased urinary excretion of cysteinyl-leukotrienes and metabolites of the mast cell product PGD2 after both challenges. Next, another eight subjects performed two 6-min challenges with EVH 3 h apart, which produced a greater bronchoconstrictor response than the 4-min protocol (30.0 ± 5.4 vs. 17.7 ± 1.5%; P = 0.0029) and a greater degree of refractoriness (∼30%). Analysis by ultra-performance liquid chromatography triple quadrupole mass spectrometry confirmed excretion of the bronchoconstrictor cysteinyl-leukotrienes and PGD2 during both challenges. In addition, there was increased excretion of the bronchoprotective PGE2, and also of the main metabolite of PGI2. This is the first report of excretion of PGE2 and PGI2 during the refractory period to EVH challenge, suggesting that they may mediate the refractoriness. Maintained excretion of PGD2 and leukotriene E4 following the repeat challenge argues against mast cell mediator depletion as the mechanism of refractoriness.

Leukotriene E4 induces airflow obstruction and mast cell activation through the cysteinyl leukotriene type 1 receptor

Background Leukotriene (LT) E4 is the final active metabolite among the cysteinyl leukotrienes (CysLTs). Animal studies have identified a distinct LTE4 receptor, suggesting that current cysteinyl leukotriene type 1 (CysLT1) receptor antagonists can provide incomplete inhibition of CysLT responses. Objective We tested this hypothesis by assessing the influence of the CysLT1 antagonist montelukast on responses induced by means of inhalation of LTE4 in asthmatic patients. Methods Fourteen patients with mild intermittent asthma and 2 patients with aspirin‐exacerbated respiratory disease received 20 mg of montelukast twice daily and placebo for 5 to 7 days in a randomized, double‐blind, crossover study (NCT01841164). The PD20 value was determined at the end of each treatment period based on an increasing dose challenge. Measurements included lipid mediators in urine and sputum cells 4 hours after LTE4 challenge. Results Montelukast completely blocked LTE4‐induced bronchoconstriction. Despite tolerating an at least 10 times higher dose of LTE4 after montelukast, there was no difference in the percentage of eosinophils in sputum. Urinary excretion of all major lipid mediators increased after LTE4 inhalation. Montelukast blocked release of the mast cell product prostaglandin (PG) D2, as well as release of PGF2&agr; and thromboxane (Tx) A2, but not increased excretion of PGE2 and its metabolites or isoprostanes. Conclusion LTE4 induces airflow obstruction and mast cell activation through the CysLT1 receptor. Graphical abstract Figure. No Caption available.