Prostaglandin E2 prevents mast cell activation via the EP2 receptor in human small airways

J Säfholm1, ML Manson1, I Delin1, J Bood1, AC Orre2, P Bergman2, M Al-Ameri2, SE Dahlén1, M Adner1. 1Unit for Experimental Asthma and Allergy Research, Centre for Allergy Research, The Institute of Environmental Medicine, Karolinska Institutet, Stockholm, Sweden, 2Department of Cardiovascular and Thoracic surgery, Karolinska University Hospital, Stockholm, Sweden

Inhaled prostaglandin E2 (PGE2) may inhibit asthmatic responses but the mechanisms and the involved receptors have remained unclear, not the least because of significant species differences. Actually, previous attempts to use the anti-inflammatory properties of PGE2 have been unsuccessful by off-target effects on other receptors. This study in human tissues tested the hypothesis that PGE2 prevents allergic bronchoconstriction via selective activation of inhibitory EP2 receptors on mast cells in the bronchi.

Small bronchi (inner diameter of 0.5-1 mm) were isolated from macroscopically healthy human lung tissue specimens obtained from 56 patients undergoing surgery, stored overnight and used the next day to characterise the effect of PGE2 (0.1-10 μM). Mast cell-dependent contractions were evoked by challenge with a monoclonal anti-human IgE antibody (anti-IgE). The mast cell mediators histamine and cysteinyl-leukotrienes (CysLTs) were measured by EIA in bronchial tissue samples before and after challenge with anti-IgE. Statistical analysis was performed using One-Way ANOVA.

First, a single bolus dose of 51.8 μgml-1 anti-IgE induced a contraction reaching 85% of tissue maximum within 15 minutes. This response was due to release of mast cell mediators as it was completely abolished by the combined pretreatment with the antihistamine mepyramine (1 μM), the TP receptor antagonist SQ-29,548 (1 μM), and the leukotriene biosynthesis inhibitor MK-886 (1 μM). Second, pretreatment with PGE2 concentration- and time-dependently reduced the anti-IgE induced contraction (Figure 1A-B; p<0.05). Third, the inhibitory effect of PGE2 was completely antagonised by the EP2 receptor antagonist PF-04418948 (1 μM) (Figure 1C; p<0.05) but unaffected by the EP4 antagonist ONO-AE3-208 (1 μM). Fourth, contractions evoked by exogenous histamine or LTD4 were unaffected by PGE2, indicating that the effect of PGE2 was exerted at the level of mast cell mediator release. Moreover, the inhibitory effect of PGE2 on mast cell mediator release (histamine and CysLTs) was also due to activation of the EP2 receptor as release was prevented by PF-04418948 (p<0.05). In contrast, the small and incomplete relaxations of bronchi in response to PGE2 were solely due to activation of EP4 receptors.

Figure 1: Response to anti-IgE after (A) 15 min PGE2 treatment, (B) PGE2 treatment over time (C) in presence or absence of EP2 or EP4 receptor antagonists. Data represent mean ± SEM (n:5-9).

This first demonstration of EP2 receptor mediated inhibition of IgE-dependent contractions of isolated small human airways introduces a new selective target for treatment of asthma and other mast cell-driven inflammatory conditions.