CLONING AND EXPRESSION OF 3 ISOFORMS OF THE HUMAN EP(3) PROSTANOID RECEPTOR

Functional cDNA clones coding for three isoforms of the human prostaglandin E receptor EP, subtype have been isolated from kidney and uterus cDNA libraries. The three isoforms, designated hEP(3.I), hEP(3.II), and hEP(3.III), have open reading frames corresponding to 390, 388 and 365 amino acids, respectively. They differ only in the length and amino acid composition of their carboxy-terminal regions, beginning at position 360. The human EP, receptor has seven predicted transmembrane spanning domains and therefore belongs to the G-protein-coupled receptor family. The rank order of potency for prostaglandins and related analogs in competition for [H-3]PGE, specific binding to membranes prepared from transfected COS cells was comparable for all three isoforms, and as predicted for the EP(3) receptor, with PGE(2) = PGE(1) >> PGF(2 alpha) = iloprost > PGD(2) >> U44619. In addition, the EP(3)-selective agonist MB28767 was a potent competing ligand with an IC50 value of 0.3 nM, whereas the EP(1)-selective antagonist AH6909 gave IC50 values of 2-7 mu M and the EP(2)-selective agonist butaprost was inactive. In summary, we have cloned three isoforms of the human EP(3) receptor having comparable ligand binding properties.