Regulation of P450 cholesterol side-chain cleavage messenger ribonucleic acid expression and progesterone production in hen granulosa cells

Previous studies have shown that granulosa cells from hen follicles < 8 mm in diameter are steroidogenically inactive. We have proposed that cytochrome P450 cholesterol side-chain cleavage (P450(scc)) enzyme activity within the granulosa layer is first expressed at the time the follicle is selected into the preovulatory hierarchy. The present studies were conducted to evaluate the actions of FSH and of the growth factors epidermal growth factor (EGF), transforming growth factor α (TGFα), and insulin-like growth factor I (IGF- I) in regulating levels of P450(scc) mRNA and initiating progesterone production in cultured granulosa cells from 6- to 8-mm follicles. Levels of P450(scc) mRNA were elevated, and progesterone production initiated, in granulosa cells cultured for 16 h and 8 h, respectively, in serum-free medium containing recombinant human (rh) FSH or the cAMP analog, 8-bromo-cAMP. While both EGF and TGFα blocked the ability of rhFSH to increase P450(scc) mRNA levels and initiate progesterone production, IGF-I failed to alter P450(scc) mRNA levels or progesterone production in either the presence or absence of rhFSH. Results from these studies indicate that, as in mammals, FSH is a primary factor that mediates the initiation of steroidogenesis in hen granulosa cells at the time of follicle selection. Furthermore, we propose that EGF and TGFα are, at least in part, responsible for preventing premature differentiation of granulosa cells in ovarian follicles that have not yet been selected into the preovulatory hierarchy.