Protein synthesis inhibitors prevent both spontaneous and hormone-dependent maturation of isolated mouse oocytes

The present study was carried out to examine the role of protein synthesis in mouse oocyte maturation in vitro. In the first part of this study, the effects of cycloheximide (CX) were tested on spontaneous meiotic maturation when oocytes were cultured in inhibitor‐free medium. CX reversibly suppressed maturation of oocytes as long as maturation was either initially prevented by the phosphodiesterase inhibitor, 3‐isobutyl‐1‐methyl‐xanthine (IBMX), or delayed by follicle‐stimulating hormone (FSH). In the second part of this study, the actions of protein synthesis inhibitors were tested on hormone‐induced maturation. CEO were maintained in meiotic arrest for 21–22 h with hypoxanthine, and germinal vesicle breakdown (GVB) was induced with follicle‐stimulating hormone (FSH). Three different protein synthesis inhibitors [CX, emetine (EM), and puromycin (PUR)] each prevented the stimulatory action of FSH on GVB in a dose‐dependent fashion. This was accompanied by a dose‐dependent suppression of 3H‐leucine incorporation by oocyte‐cumulus cell complexes. The action of these inhibitors on FSH‐ and epidermal growth factor (EGF)‐induced GVB was next compared. All three drugs lowered the frequency of GVB in the FSH‐treated groups, below even that of the control (drug + hypoxanthine); the drugs maintained meiotic arrest at the control frequencies in the EGF‐treated groups. Puromycin aminonucleoside, an analog of PUR with no inhibitory action on protein synthesis, had no effect. The three inhibitors also suppressed the stimulatory action of FSH on oocyte maturation when meiotic arrest was maintained with the cAMP analog, dbcAMP. In a time course experiment, it was determined that CX could prevent FSH‐induced maturation if added as late as 3 h after the onset of culture, but the drug was less effective in EGF‐treated cultures. It is concluded that a protein with a rapid turnover rate is involved in the spontaneous maturation of mouse oocytes and that de novo protein synthesis is a requirement for hormone induction of GVB in vitro.