Molecular cloning and expression of multiple isoforms of human prostaglandin E receptor EP₃ subtype generated by alternative messenger RNA splicing: Multiple second messenger systems and tissue-specific distributions

Five distinct cDNA clones encoding four different isoforms of human prostaglandin (PG) E receptor EP₃ subtype were isolated from a human kidney cDNA library. Two cDNA clones differed only in their 3'-untranslated regions. The four isoforms, tentatively named EP(3-I), EP(3-II), EP(3-III), and EP(3- IV), which were generated by alternative mRNA splicing, had identical amino acid sequences except for their different carboxyl-terminal tails. Transfection experiments revealed that all the four isoforms show high binding affinities to PGE₂, PGE₁, and M and B28767, an EP₃-specific agonist, whereas their downstream signaling pathways are divergent. M and B28767 increased cAMP concentrations in cells expressing EP(3-II) and EP(3- IV), whereas it inhibited forskolin-induced cAMP accumulations in cells expressing all EP₃ isoforms. M and B28767 also stimulated phosphoinositide turnover in cells expressing EP(3-I) and EP(3-II). Northern blot analysis revealed that the EP₃ gene is expressed in a wide variety of human tissues. The human EP₃ mRNA was present most abundantly in the kidney, pancreas, and uterus. A substantial expression was also detected in the heart, liver, skeletal muscle, small intestine, colon, prostate, ovary, and testis. Furthermore, reverse transcription-polymerase chain reaction analysis demonstrated tissue-specific expressions of the five different EP₃ mRNA species. The present study suggests the presence of the multiple systems of PGE₂/EP₃ isoforms and leads to the better understanding of its physiological and pathophysiological implications in humans.