Progesterone inhibits the induction of aromatase activity in rat granulosa cells in vitro.

The development of ovulatory follicles is suppressed during reproductive states with high circulating levels of progesterone. We have investigated the effects of progesterone on estradiol secretion by rat granulosa cells in vitro to determine if progesterone has direct effects on follicular aromatization. Relatively undifferentiated granulosa cells were obtained from hypophysectomized (HPX), diethylstilbestrol-treated immature rats and differentiated granulosa cells were obtained from preovulatory follicles on the morning of proestrus. Progesterone suppressed, in a dose-dependent fashion, the accumulation of estradiol when HPX or proestrous cells were cultured with follicle-stimulating hormone (FSH) (300 ng/ml) and testosterone (0.5 microM), but granulosa cells from proestrous rats were less sensitive to the inhibitory effects of progesterone. When aromatase was first induced in cells from HPX rats by culturing them for 1, 2, or 3 days with FSH, the cells were less susceptible to the inhibitory effects of progesterone during the subsequent 3 days of culture. The inhibitory effects of progesterone on estradiol secretion appeared irreversible when cells were exposed to progesterone for longer than 1 day. The effects of progesterone on HPX granulosa cells were specific since the progesterone precursor, pregnenolone, was as inhibitory as progesterone, while the progesterone metabolite, 17 alpha-hydroxyprogesterone, had no effect. When granulosa cells from proestrous follicles were cultured with testosterone and graded doses of progesterone, a slight inhibition of estradiol production occurred. This inhibition was much less than that observed when FSH was present. Therefore, the inhibitory effects of progesterone are exerted primarily on the induction of aromatase by FSH, rather than on the functioning of existing aromatase. Whole proestrous follicles were also cultured with graded doses of progesterone in the presence or absence of FSH and/or testosterone. In those experiments progesterone increased estradiol on the first day of culture, but inhibited the effects of FSH in maintaining estradiol secretion later in culture. These experiments have shown that progesterone exerts a specific and irreversible inhibitory effect on estradiol production by granulosa cells. Less differentiated granulosa cells appear to be more sensitive to the inhibitory effects of progesterone. These results suggest that in vivo progesterone may directly suppress development of ovulatory follicles by inhibiting aromatase in granulosa cells of developing follicles.