Changes in steroidogenic enzyme and steroidogenic acute regulatory protein messenger RNAs in ovarian follicles during ovarian development of rainbow trout (Oncorhynchus mykiss)

In salmonid fishes, estradiol-17β (E2) and 17α,20β-dihydroxy-4-pregnen-3-one (17,20β-P) are the major steroid hormones controlling oocyte growth (vitellogenesis) and final maturation (resumption of meiosis). The aim of this study was to determine changes in mRNAs encoding ovarian steroidogenic enzymes and steroidogenic acute regulatory protein (StAR) during ovarian development in female rainbow trout. We analyzed the levels of mRNAs encoding the enzymes P450 side-chain cleavage enzyme (P450scc), 3β-hydroxysteroid dehydrogenase (3β-HSD), 17α-hydroxylase/C17-C20 lyase (P450c17), aromatase (P450arom), and carbonyl reductase-like 20β-hydroxysteroid dehydrogenase (20β-HSD), and StAR in developing ovarian follicles of rainbow trout by Northern blot, in relation to the pattern of serum E2 and 17,20β-P levels. Serum E2 levels were elevated during vitellogenesis and decreased prior to an ovulatory increase in 17,20β-P. Transcripts for P450scc and P450c17 increased in late vitellogenic follicles, then decreased in post-ovulatory follicles. In contrast, P450arom transcripts were abundant during vitellogenesis and then declined as vitellogenesis was completed and were barely detectable in post-ovulatory follicles. 3β-HSD mRNA levels increased in late vitellogenic follicles and were maintained at high levels in post-ovulatory follicles. 20β-HSD and StAR mRNA levels were very low during vitellogenesis, and then strongly increased during late vitellogenesis to a peak in post-ovulatory follicles. These results indicate that the expression of genes encoding steroidogenic enzymes and StAR change dynamically, dependent on the developmental stages of rainbow trout follicles. The acquisition of the ability of later stage follicles to rapidly produce large quantities of 17,20β-P appears to be supported by a preparatory increase in mRNAs encoding StAR and other steroidogenic enzymes.