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

Functional cDNA clones coding for three isoforms of the human prostagl andin E receptor EP, subtype have been isolated from kidney and uterus cDNA libraries. The three isoforms, designated hEP(3.I), hEP(3.II), a nd hEP(3.III), have open reading frames corresponding to 390, 388 and 365 amino acids, respectively. They differ only in the length and amin o acid composition of their carboxy-terminal regions, beginning at pos ition 360. The human EP, receptor has seven predicted transmembrane sp anning domains and therefore belongs to the G-protein-coupled receptor family. The rank order of potency for prostaglandins and related anal ogs in competition for [H-3]PGE, specific binding to membranes prepare d from transfected COS cells was comparable for all three isoforms, an d as predicted for the EP(3) receptor, with PGE(2) = PGE(1) >> PGF(2 a lpha) = 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 o f 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 hav ing comparable ligand binding properties.