FUNCTIONAL OVARIAN AND PLACENTAL ISOFORMS OF PORCINE AROMATASE

Functional isoforms of porcine aromatase cytochrome P-450 were cloned from placenta, and ovarian theca interna and granulosa tissues, and full length cDNAs were expressed in vitro. Porcine theca and granulosa expressed an identical form of P-450arom. This ovarian cDNA encoded for a protein of 501 amino acids, two amino acids shorter at the N-terminal end than placental P-450arom isoform (503 residues). Overall, the two isoforms exhibited 93% nucleotide and 87% amino acid identity with each other, and both were highly homologous, at the nucleotide and amino acid levels, to human and bovine P-450arom, also 503 amino acid proteins. Analysis of the predicted amino acid sequence further suggested that the regions of the cDNAs, corresponding to presumed exons III, V and IX, assuming conservation of intron-exon boundaries with the human P-450arom gene, were conserved in the porcine placental and ovarian enzymes, while sequence variance occurred in all other putative exons. In vitro expression indicated that the cDNA encoding porcine placental P-450arom was almost 10-fold more active in the synthesis of estrone from androstenedione than was the-ovarian isoform which synthesized more 19OH-androstenedione than estrone. Western analysis of transfected Cos1 cells suggested that the differences in activity were not due to levels of expression of the cDNAs since similar levels of immunodetectable protein were observed in cells transfected with each construct. Both isoforms were sensitive to inhibition of activity by the specific aromatase inhibitors, 4OH-androstenedione and CGS16949A. In addition, activity of the enzyme encoded by the ovarian P-450arom cDNA was suppressed by etomidate, an inhibitor of cytochrome P-450 11 beta-hydroxylase, but the placental P-450arom isoform was not. These functional differences were consistent with observations made in similar experiments involving P-450arom activity in freshly homogenized tissues. These data provide evidence of the existence of distinct, intraspecies isoforms of P-450arom, the first described in any species and suggest that pigs possess a unique mechanism for regulating androgen metabolism.