Molecular characterization and ocular hypotensive properties of the prostanoid EP2 receptor

The cloning of the genes that encode for prostaglandin (PG) receptors has resolved much of the complexity and controversy in this area by confirming the classification proposed by Coleman, et al. Two issues that remained unresolved were (1) the inability of the EP2 agonist butaprost to interact with the cloned putative EP2 receptor and (2) molecular biological confirmation of a fourth PGE2-sensitive receptor, which was pharmacologically designated EP4. In order to provide clarification, we attempted to clone further PGE2-sensitive receptors. By using a cDNA probe that encodes for the human EP3A receptor, a cDNA clone that encoded for a novel PGE2-sensitive receptor was obtained by screening a human placenta library. This cDNA clone was transfected into COS-7 cells for pharmacological studies. The cDNA clone obtained from human placenta had only about 30% amino acid identity with cDNAs for other PG receptors, including those that encode for the previously proposed murine and human EP2 receptors. Radioligand binding studies on the novel EP receptor expressed in COS-7 cells revealed that selective EP2 agonists such as butaprost, AH 13205, AY 23626 and 19(R)-OH PGE2 all competed with 3H-PGE2 for its binding sites, whereas selective agonists for other PG receptor subtypes had minimal or no effect. This receptor was coupled to adenylate cyclase and EP2 agonists caused dose-related increases in cAMP. It appears that the cDNA described herein encodes for the pharmacologically defined EP2 receptor. Ocular studies revealed that AH 13205 decreased intraocular pressure in normal and ocular hypertensive monkeys by a mechanism that does not appear to involve inhibition of aqueous humor secretion.