Hierarchical regulation of steroid synthesis in ovarian follicles drives progesterone surge in cloudy catshark

Résumé fourni par la source

We previously demonstrated that two major hormonal phases, testosterone (T)-phase and progesterone (P4)-phase, which are marked by high plasma T and P4, respectively, appeared in each egg-laying cycle of cloudy catshark. In the present study, the mechanisms underlying the cyclical fluctuations of sex steroids were investigated by follicular steroid measurement and gene expression analyses in vivo and in vitro. In the ovary of catshark, a hierarchical pattern of follicles of increasing sizes is present. Estradiol (E2) was produced by early vitellogenic follicles, while T was produced by mid- to late-vitellogenic follicles. The production of T was downregulated in the follicles at P4-phase, concomitant with the significant decreases in expression of cyp17a1 gene encoding P450c17, an enzyme converting P4 to androgen. Meanwhile, P4 was exclusively produced by preovulatory F1 follicles only at P4-phase, which was related in time to a marked upregulation of steroidogenic acute regulatory protein 2 (star2), suggesting that star2 is a key regulatory factor for inducing the P4 surge. Our newly established in vitro follicle culture system confirmed the secretion of E2, T and P4 from follicles at the different hierarchical stages. These findings provide clear evidence that follicular hierarchy is an important driver of the cyclical secretion of steroid hormones by follicles, and of circulating steroids in plasma, in the cloudy catshark.

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