Inhibition of cytochrome P-450 C17 enzyme by a GnRH agonist in ovarian follicles

PARBORELL, FERNANDA; IRUSTA, GRISELDA; TESONE, MARTA

Am J Physiol Endocrinol Metab

Año: 2007 vol. 292 p. 1456 - 1464

Our objective was to study the direct action of a GnRH-I agonist, leuprolideacetate (LA), on ovarian steroidogenesis in preovulatory follicles obtained from equine chorionic gonadotropin (eCG)-treated rats. Previously, we havedemonstrated an inhibitory effect of LA on steroidogenesis and folliculardevelopment. In this study, we tested the hypothesis that gonadotropin-releasing hormone (GnRH) exerts its negative effect on follicular development by inhibitingthecal cytochrome P-450 C17 (P450C17) alpha-hydroxylase expression and,consequently, androgen synthesis. Studies were carried out in prepubertal female rats injected with either eCG (control) or eCG plus LA (LA) and killed atdifferent time points. Immunohistochemical studies indicated that LA inducedsteroidogenic acute regulatory protein (StAR) expression mainly in theca cells ofpreantral and antral follicles. In addition, serum progesterone levels increased significantly (P < 0.05), whereas those of androsterone decreased (P < 0.05)after 8 h of LA treatment. This inhibition caused by LA seemed to be aconsequence of the decreased expression of follicular P450C17 alpha-hydroxylase, as demonstrated by Western blot and RT-PCR techniques. In vitro studies usingfollicles isolated from 48-h-eCG-treated rats and cultured with LA showed asignificant (P < 0.05) inhibition of FSH-induced androsterone follicular content as well as P450C17 alpha-hydroxylase protein levels, as determined by Westernanalysis. However, LA increased StAR protein expression in these follicleswithout significant changes in P450scc enzyme levels. Taking all these findingsinto account, we suggest that GnRH-I exerts a direct inhibitory action ongonadotropin-induced follicular development by decreasing the temporal expressionof the P450C17 enzyme and, consequently, androgen production, thus reducing thesupply of estrogens available to developing follicles.