Differential suppressive effects of low physiological doses of estradiol-17β in vivo on levels of mRNAs encoding steroidogenic acute regulatory protein and three steroidogenic enzymes in previtellogenic ovarian follicles of rainbow trout

Numerous recent reports have demonstrated effects of estrogenic chemicals on reproductive physiology of fish. However, there is little information available on the regulation of ovarian steroidogenesis by physiological levels of endogenous steroids in teleosts. Therefore, we analyzed the levels of mRNAs encoding steroidogenic proteins in ovaries of E2-treated rainbow trout Oncorhynchus mykiss). Previtellogenic (perinucleolar oocyte stage) trout received either blank or E2 implants (0.1 μg, 1 μg or 10 μg/g BW) for 7 days in order to achieve low, medium and high physiological levels of E2 in plasma. Plasma E2 levels were measured using radioimmunoassay. Levels of mRNAs encoding steroidogenic acute regulatory protein (StAR), P450 side-chain cleavage enzyme (P450scc), 3β-hydroxysteroid dehydrogenase (3β-HSD) and P450 aromatase A (P450aromA) in the ovary were analyzed by real-time quantitative PCR. E2 levels in control animals were approximately 0.5 ng/ml. Levels in treated fish were approximately 1 ng/ml (0.1 μg implant), 2.6 ng/ml (1 μg implant) and 90 ng/ml (10 μg implant), within or just above the physiological range of immature and maturing female rainbow trout. StAR mRNA levels were significantly reduced by all E2 treatments. P450scc mRNA levels were not affected, but 3β-HSD and P450arom mRNA levels were significantly decreased by the 1 and 10 μg E2/BW implants. These results indicate that E2, either directly or indirectly, downregulates expression of StAR and major steroidogenic enzyme genes in rainbow trout ovary. Furthermore, expression of the trout StAR gene seems particularly sensitive to E2.