Meiosis-activating sterol and the maturation of isolated mouse oocytes

This study was carried out to examine the effects of the meiosis-activating C-29 sterol, 4,4-dimethyl-5 alpha -cholesta-8,14,24-trien-3 beta -ol (FF-MAS), on mouse oocyte maturation in vitro. Cumulus cell-enclosed oocytes (CEO) and denuded oocytes (DO) from hormonally primed, immature mice were cultured 17-18 h in minimum essential medium (MEM) containing 4 mM hypoxanthine plus increasing concentrations of FF-MAS. The sterol induced maturation in DO with an optimal concentration of 3 etag/ml but was without effect in CEO, even at concentrations as high as 10 mug/ml. Some stimulation of maturation in hypoxanthine-arrested CEO was observed when MEM was replaced by MEM alpha. Interestingly, the sterol suppressed the maturation of hypoxanthine-arrested CEO in MEM upon removal of glucose from the medium. FF-MAS also failed to induce maturation in DO when meiotic arrest was maintained with dibutyryl cAMP (dbcAMP). The rate of maturation in FF-MAS-stimulated, hypoxanthine-arrested DO was slow, as more than 6 h of culture elapsed before significant meiotic induction was observed, and this response required the continued presence of the sterol. Although the oocyte took up radiolabeled lanosterol, such accumulation was restricted by the presence of cumulus cells. In addition, lanosterol failed to augment FSH-induced maturation and was even inhibitory at a high concentration. Moreover, the downstream metabolite, cholesterol, augmented the inhibitory action of dbcAMP on maturation in both CEO and DO. Two inhibitors of 14 alpha -demethylase, ketoconazole, and 14 alpha -ethyl-5 alpha -cholest-7-ene-3 beta ,1 5 alpha -diol that can suppress FF-MAS production from lanosterol failed to block consistently FSH-induced maturation. These results confirm the stimulatory action of FF-MAS on hypoxanthine-arrested DO but do not support a universal meiosis-inducing function for this sterol.