Prostaglandin E2 as mediator and modulator of airway smooth muscle responses

Abstract: Prostaglandin E2 (PGE2) is a lipid mediator produced by virtually every cell of the human body. Because common non-steroidal anti-inflammatory drugs (NSAIDs) inhibit its biosynthesis, PGE2 is usually considered to be a ‘pro-inflammatory’ mediator. The role of PGE2 in the lung and airways has however always been unclear. In particular, the airway responses caused by activation of its four different EP receptors have been debated. Research on the mechanisms involved in the actions of PGE2 has previously been limited by the low potency and selectivity of available pharmacological tools. Recently, a number of potent receptor antagonists and enzyme inhibitors have become available. The aim of this thesis was therefore to characterise airway responses to PGE2 in greater detail, focusing on the role of its receptors on baseline smooth muscle function and during antigen-induced contractions. Alongside investigating PGE2 responses, the newly discovered relaxant effects of bitter tasting substances acting at their respective receptors (TAS2Rs) were examined. The project mainly involved analysis of isometric contractions and relaxations in isolated airways from guinea pigs and humans in organ baths. In addition, mRNA expression of receptors and enzymes was analysed by PCR and prostanoid release was measured by chemical or immunological methods. It was found that all four EP receptors, the two cyclooxygenases (COX-1 and COX-2) and three PGE synthases were expressed in the guinea pig trachea and lung. Exogenous PGE2 induced a bell-shaped concentration-response curve, causing contraction at lower concentrations mediated by the EP1 receptor and relaxation mediated by the EP2 receptor at higher concentrations. The spontaneous airway tone was maintained by biosynthesis of endogenous PGE2, mainly catalysed by COX-2 in the airway epithelium. The level of tone in the preparation was determined by the balance between activation of EP1 and EP2 receptors. The EP1 receptor, but not the EP2 receptor, displayed homologous desensitisation to endogenous PGE2. When the antigen-induced contractions of the guinea pig trachea were studied, it was found that the primary effect of PGE2 was again to maintain the spontaneous airway tone. When this effect was blocked by EP1 and EP2 receptor antagonists, it was revealed that a component of the antigen-response was mediated by prostaglandin D2 and thromboxane A2, acting at the thromboxane TP receptor. In human small airways, PGE2 induced relaxations mediated by EP4 receptors at low concentrations and contractions through TP receptors at higher concentrations. In addition, it was discovered that the IgE-dependent contraction of human bronchi could be abolished by the action of exogenous PGE2 at the EP2 receptor, an effect presumably involving inhibition of mast cell mediator release. The bitter tasting substances chloroquine, denatonium, thiamine, and noscapine caused relaxations of human and guinea pig airways with a greater efficacy than beta-adrenergic bronchodilators. TAS2R3, TAS2R4 and TAS2R10 were expressed in the guinea pig trachea. The EP receptor-mediated tone could be relaxed by chloroquine and noscapine, but not by denatonium and thiamine. Although the mechanisms underlying these powerful relaxations remain unknown, the data support the involvement of several different pathways. In summary, PGE2 causes contractions and relaxations of guinea pig and human airways, but the receptors involved differ between the two species. Furthermore, the data gathered suggest that EP, TP and TAS2R receptors may be potential targets for the development of drugs to treat asthma and other forms of airway obstruction.