Direct stimulation of ovarian progesterone-metabolizing enzyme by gonadotropin-releasing hormone in cultured granulosa cells.

The mechanism by which gonadotropin-releasing hormone (GnRH) inhibits ovarian progesterone production was investigated by studying the GnRH modulation of 20 alpha-hydroxysteroid dehydrogenase activity in cultured rat granulosa cells. The granulosa cells obtained from hypophysectomized, estrogen-treated rats were treated with various hormones in vitro and 20 alpha-hydroxysteroid dehydrogenase activity determined by measuring the conversion of radiolabeled 20 alpha-hydroxy-pregn-4-en-3-one to progesterone. Treatment with follicle-stimulating hormone (FSH) increased the 20 alpha-hydroxysteroid dehydrogenase activity of granulosa cells to 1.3-fold of the control value. Concomitant treatment with 10(-8) M GnRH further enhanced the FSH stimulation of enzyme activity to 2.6-fold of the control value. The increases in 20 alpha-hydroxysteroid dehydrogenase activity were associated with increases in maximum velocity, while the Km values remained unchanged. In contrast, prolactin (PRL) treatment of FSH-primed granulosa cells resulted in an 88% decrease in 20 alpha-hydroxysteroid dehydrogenase activity as compared to untreated controls. Concomitant treatment with GnRH partially blocked this inhibitory effect. Furthermore, incubation of granulosa cells for 2 days with GnRH alone resulted in a 2.7-fold increase in 20 alpha-hydroxysteroid dehydrogenase activity, as compared to untreated controls. In all experiments, the effect of GnRH was blocked by concomitant treatment with a GnRH antagonist. These results suggest that one mechanism by which GnRH inhibits FSH- and PRL-stimulated progesterone production in granulosa cells is through the stimulation of the activity of 20 alpha-hydroxysteroid dehydrogenase which converts progesterone to a less active metabolite.