EP2 and EP4 receptor antagonists: Impact on cytokine production and 2-adrenergic receptor desensitization in human airway smooth muscle

Prostaglandin E-2 (PGE(2)) is a key prostanoid known to have both proinflammatory and anti-inflammatory impact in the context of chronic respiratory diseases. We hypothesize that these opposing effects may be the result of different prostanoid E (EP) receptor-mediated signaling pathways. In this study, we focus on two of the four EP receptors, EP2 and EP4, as they are known to induce cyclic adenosine monophosphate (cAMP)-dependent signaling pathways. Using primary human airway smooth muscle (ASM) cells, we first focussed on the PGE(2)-induced production of two cAMP-dependent proinflammatory mediators: interleukin 6 (IL-6) and cyclo-oxygenase 2 production. We show that PGE(2)-induced IL-6 protein secretion occurs via an EP2-mediated pathway, in a manner independent of receptor-mediated effects on messenger RNA (mRNA) expression and temporal activation kinetics of the transcription factor cAMP response element binding. Moreover, stimulation of ASM with PGE(2) did not establish a positive, receptor-mediated, feedback loop, as mRNA expression for EP2 and EP4 receptors were not upregulated and receptor antagonists were without effect. Our studies revealed that the EP2, but not the EP4, receptor is responsible for (2)-adrenergic desensitization induced by PGE(2). We demonstrate that PGE(2)-induced heterologous receptor desensitization responsible for tachyphylaxis to short- (salbutamol) or long- (formoterol) (2)-agonists (measured by cAMP release) can be reversed by the EP2 receptor antagonist PF-04418948. Importantly, this study highlights that inhibiting the EP2 receptor restores (2)-adrenergic receptor function in vitro and offers an attractive novel therapeutic target for treating infectious exacerbations in people suffering from chronic respiratory diseases in the future.