Periovulatory expression of cholesterol side-chain cleavage cytochrome P-450 in cumulus cells.

Immunocytochemical staining methods were used to examine the appearance of cholesterol side-chain cleavage cytochrome P-450 (P-450scc) in mitochondria of cumulus cells during follicular development. The cumulus-oocyte complexes were isolated from pregnant mare serum gonadotropin (PMSG)/human CG (hCG)-treated 25 day rats and examined in culture. It is shown that P-450scc is not expressed in the cumulus cell earlier than 2-3 h before ovulation. After ovulation, the expression of P-450scc rapidly increased, so that postovulatory cumuli contained ample amounts of the cytochrome. RIA of progestins secreted by the cumulus-oocyte complexes in culture corroborated the immunocytochemical observations. A single administration of LH or PMSG treatment of hypophysectomized rat did not result in P-450scc accumulation. However, this failure of hormonal responses in vivo was not due to lack of available receptors, since both FSH and LH could induce cAMP accumulation and P-450scc when added to isolated cumuli in culture. Therefore, these findings suggest the presence of a putative intraovarian suppressive factor(s) which disappears before ovulation and thus render(s) the cumulus cells permissive for P-450scc responsiveness. An additional intriguing aspect of P-450scc responsiveness to gonadotropins was revealed in experiments showing that 60% of the cultured cumulus complexes failed to accumulate P-450scc in response to hormones, if collected from 21 day animals. Interestingly, those P-450scc negative cumuli were always associated with an oocyte which did not resume meiotic maturation. We may therefore suggest that meiotic incompetence of the oocyte is also accompanied by functional incompetence of its embracing cumulus cells which cannot, for yet unclear reasons, acquire their steroidogenic capacity.