Evidence for a Second Receptor for Prostacyclin on Human Airway Epithelial Cells That Mediates Inhibition of CXCL9 and CXCL10 Release

Herein we provide evidence for the coexpression of two distinct prostacyclin (PGI(2)) receptors (IP) on BEAS-2B human airway epithelial cells. IP receptor heterogeneity initially was suggested by the finding that the rank orders of potency of PGI(2) and three structurally similar analogs [taprostene, iloprost, 15-deoxy-16-(m-tolyl)-17,18,19,20-tetranorisocarbacyclin (15-deoxy-TIC)] for the inhibition of chemokine (CXCL9 and CXCL10) release and for transcriptional activation/augmentation of cAMP response element and glucocorticoid response element luciferase reporters were distinct. Indeed, PGI(2), taprostene, and iloprost activated both reporters whereas 15-deoxy-TIC was inert. Conversely, 15-deoxy-TIC, PGI(2), and taprostene (but not iloprost) suppressed chemokine release. Further experiments established that iloprost did not antagonize the inhibitory effect taprostene or 15-deoxy-TIC on chemokine output. Likewise, 15-deoxy-TIC failed to antagonize taprostene- and iloprost-induced reporter transactivation. Thus, iloprost- and 15-deoxy-TIC-induced responses were apparently mediated via pharmacologically distinct receptors. In human embryonic kidney 293 cells overexpressing the human recombinant IP receptor receptor, 15-deoxy-TIC was considerably less potent (>10,000-fold) than iloprost and taprostene in promoting cAMP accumulation, yet in BEAS-2B cells, these analogs were equipotent. IP receptor heterogeneity was also supported by the finding that the affinity of the IP receptor antagonist R-3-(4-fluorophenyl)-2-[5-(4-fluorophenyl)-benzofuran-2-yl-methoxycarbonyl-amino] propionic acid (RO3244794) for the receptor mediating inhibition of chemokine release was approximately 10-fold lower than for the receptor mediating both transcriptional outputs. Finally, small interfering RNAs directed against the IP receptor gene, PTGIR, failed to block the suppression of chemokine output induced by taprostene and 15-deoxy-TIC, whereas taprostene-and iloprost-induced transcriptional responses were markedly attenuated. Collectively, these results indicate that PGI(2), taprostene and 15-deoxy-TIC suppress chemokine release from BEAS-2B cells by interacting with a novel IP receptor that we denote here as the IP2 subtype.