Selectively targeting prostanoid E (EP) receptor-mediated cell signalling pathways: Implications for lung health and disease.

Arachidonic acid is metabolized by cyclooxygenases (COX-1 and COX-2) into various prostanoids which exert different functions in mammalian physiology. One of these prostanoids, prostaglandin E2 (PGE2), interacts with four different G protein-coupled receptors, named EP1, EP2, EP3 and EP4, to initiate different downstream signalling pathways. Prostanoid receptors are diversely expressed throughout different tissues all over the body and PGE2 is responsible for a large variety of beneficial and disadvantageous effects. We have recently achieved a greater understanding of the biology of prostanoid E receptors and the potential for specific drug targeting with the advent of potent and selective EP receptor agonists and antagonists. This has important implications for lung health and disease as PGE2-mediated EP receptor activation impacts upon migration of airway smooth muscle cells, airway microvascular leak, tone regulation of pulmonary blood vessels, mast cell degranulation, bronchodilatation, cough, angiogenesis and airway inflammation, to name a few. In this review, we overview the EP receptor family and the related signalling pathways, summarize a variety of EP1-4 receptor agonists and antagonists, provide an overview of pharmacological tools used to implicate EP receptor function in the context of respiratory health and disease and finally highlight some of the more selective pharmacological reagents that have recently been developed. The availability of selective pharmacological agonists and antagonists for the distinct EP receptors, as well as the development of specific prostanoid receptor knock-out mice, offer hitherto unattainable opportunities for achieving an in depth understanding of the role and function of PGE2 in respiratory disease and the exciting potential of targeting EP receptors more broadly.

The phosphorylated form of FTY720 activates PP2A, represses inflammation and is devoid of S1P agonism in A549 lung epithelial cells.

Protein phosphatase 2A (PP2A) activity can be enhanced pharmacologically by PP2A-activating drugs (PADs). The sphingosine analog FTY720 is the best known PAD and we have shown that FTY720 represses production of pro-inflammatory cytokines responsible for respiratory disease pathogenesis. Whether its phosphorylated form, FTY720-P, also enhances PP2A activity independently of the sphingosine 1-phosphate (S1P) pathway was unknown. Herein, we show that FTY720-P enhances TNF-induced PP2A phosphatase activity and significantly represses TNF-induced interleukin 6 (IL-6) and IL-8 mRNA expression and protein secretion from A549 lung epithelial cells. Comparing FTY720 and FTY720-P with S1P, we show that unlike S1P, the sphingosine analogs do not induce cytokine production on their own. In fact, FTY720 and FTY720-P significantly repress S1P-induced IL-6 and IL-8 production. We then examined their impact on expression of cyclooxygenase 2 (COX-2) and resultant prostaglandin E2 (PGE2) production. S1P did not increase production of this pro-inflammatory enzyme because COX-2 mRNA gene expression is NF-κB-dependent, and unlike TNF, S1P did not activate NF-κB. However, TNF-induced COX-2 mRNA expression and PGE2 secretion is repressed by FTY720 and FTY720-P. Hence, FTY720-P enhances PP2A activity and that PADs can repress production of pro-inflammatory cytokines and enzymes in A549 lung epithelial cells in a manner devoid of S1P agonism.