Progestins block cholesterol synthesis to produce meiosis-activating sterols

The resumption of meiosis is regulated by meiosis-preventing and meiosis-ac tivating substances in testes and ovaries. Certain C-29 precursors of chole sterol are present at elevated levels in gonadal tissue, but the mechanism by which these meiosis-activating sterols (MAS) accumulate has remained an unresolved question. Here we report that progestins alter cholesterol synth esis in HepG2 cells and rat testes to increase levels of major MAS (FF-MAS and T-MAS), These C-29 sterols accumulated as a result of inhibition of Del ta 24-reduction and 4 alpha -demethylation, Progesterone, pregnenolone, and 17 alpha -OH-pregnenolone were potent inhibitors of Delta 24-reduction in an in vitro cell assay and led to the accumulation of desmosterol, a Delta5 ,24 sterol precursor of cholesterol, A markedly different effect was observ ed for 17 alpha -OH-progesterone, which caused the accumulation of sterols associated with inhibition of 4 alpha -demethylation. The flux of C-13-acet ate into lathosterol and cholesterol was decreased by progestins as measure d by isotopomer spectral analysis, whereas newly synthesized MAS accumulate d, The combined evidence that MAS concentrations can be regulated by physio logical levels of progestins and their specific combination provides a plau sible explanation for the elevated concentration of MAS in gonads and sugge sts a new role for progestins in fertility.