MEIOSIS-ACTIVATING STEROL PROMOTES RESUMPTION OF MEIOSIS IN MOUSE OOCYTES CULTURED IN-VITRO IN CONTRAST TO RELATED OXYSTEROLS

The sterol 4,4-dimethyl-5 alpha-cholesta-8,14,24-trien-3 beta-ol (FF-M AS [follicular-fluid meiosis-activating sterol]) from human follicular fluid has recently been identified as a compound that induces the res umption of meiosis. FF-MAS and various oxysterols have been reported t o transactivate the orphan receptor LXR alpha. The objective was to de termine the biological activity of synthetic FF-MAS on the resumption of meiosis and final maturation of mouse oocytes in vitro. In order to evaluate whether LXR alpha might mediate FF-MAS action on the oocyte, we compared the capability of various compounds to activate LXR alpha -dependent transcription and to induce resumption of meiosis in the oo cyte assay. Ovaries were isolated from immature mice primed with FSH 4 8 h before collection. Naked oocytes (NkO) and cumulus enclosed oocyte s (CEO) were isolated from follicles. The oocytes were cultured in two groups, NkO and CEO, respectively, in media containing either 3 mM hy poxanthine, 5 mu M IBMX, or 0.100 mM dbcAMP to maintain the oocytes in the germinal vesicle stage. The resumption of meiosis was assessed by the frequency of germinal vesicle breakdown (GVBD) after 24 h of in v itro culture. FF-MAS overcame the meiotic inhibition by hypoxanthine i n both the NkO group and CEO group in a dose-dependent manner within t he concentration range 0.07-7 mu M. FF-MAS displayed similar potency i n all inhibitory agents used. Also, FF-MAS significantly increased the formation of polar bodies in both the CEO and NkO group. The oxystero ls 22(R)-hydroxycholesterol (a potent ligand for the LXR alpha recepto r), 16-hydroxycholesterol, 25-hydroxycholesterol, and 27-hydroxycholes terol, as well as cholesterol, were tested without any significant eff ect on maturation compared to that of controls. Oxysterols and FF-MAS were observed to activate LXR alpha. In conclusion, the results report ed here clearly demonstrate that synthetic FF-MAS exclusively is capab le of mediating resumption of meiosis in vitro in both NkO and CEO irr espective of the inhibitory substance used. In contrast, the oxysterol s and cholesterol had no significant biological activity on this oocyt e function, and consequently we found no correlation between LXR alpha activation and meiosis stimulation.