Differential transcription of steroidogenic enzymes in the equine primary corpus luteum during diestrus and early pregnancy

In pregnant mares, eCG stimulates luteal androgen and estrogen production, increasing plasma concentrations 2- to 3-fold. To study how these changes are regulated, we examined the expression of mRNA for the steroidogenic enzymes 3β-hydroxysteroid dehydrogenase (3β-HSD), cytochrome P450 17α-hydroxylase/17,20-lyase (P450(17α)), and cytochrome P450 aromatase (P450(arom)) in equine primary corpora lutea using Northern blot analyses. Three equine specific cDNAs were generated by reverse transcriptase polymerase chain reaction. When compared to human, bovine, and rat sequences, the nucleotide identities were 82%, 84%, and 76%, respectively, for 3β-HSD cDNA (843 base pairs [bp]); 79%, 80% and 66% for P450(17α) cDNA (541 bp); and 80%, 83% and 75% for P450(arom) cDNA (289 bp). The P450(17α) cDNA sequence demonstrated 99.6% nucleotide identity with the previously published sequence for equine testicular P450(17α). Luteal tissue samples were collected at three times: diestrus (Days 8-10), early pregnancy before the onset of eCG secretion (Days 29-35), and early pregnancy after the onset of eCG secretion (Days 42-45). Although no significant changes were observed in 3β-HSD expression, P450(17α) and P450(arom) demonstrated stage-specific transcriptional regulation. Steady-state levels of P450(17α) mRNA were similar during diestrus and early pregnancy before the onset of eCG secretion but increased significantly after the onset of eCG secretion. Cytochrome P450(arom) mRNA levels decreased significantly after the onset of eCG secretion. Steady-state levels of P450(arom) mRNA were highest in luteal tissue collected during pregnancy before the onset of eCG secretion and intermediate during diestrus. Secretion of eCG appears to increase luteal estrogen synthesis by a transcriptional up-regulation of P450(17α) expression. These data suggest that availability of aromatizable androgens may be rate-limiting in luteal estrogen synthesis before the onset of eCG secretion.