Regulation of cytochrome P450 aromatase messenger ribonucleic acid and activity by steroids and gonadotropins in rat granulosa cells

Abstract

Estradiol (E) biosynthesis by the cytochrome P450 aromatase (P450arom) enzyme system increases as preovulatory follicles develop and is subsequently reduced by the ovulatory LH surge. To determine the specific effects of gonadotropins and steroids on expression of P450arom in rat granulosa cells, steady state levels of messenger (m) RNA were examined in vivo and in vitro, with the latter also being related to aromatase enzyme activity and cAMP production. P450arom mRNA and activity were induced in granulosa cells by FSH alone in a dose-, time-, and stage-dependent manner. E enhanced the effects of FSH in vivo and in vitro. The synergistic effect of E with FSH (50 ng/ml) was observed in the absence/presence of serum and was mimicked by a similar concentration (20 nM) of testosterone, dihydrotestosterone, or dexamethasone. In contrast, ovulatory doses of LH (500 ng/ml) or forskolin (10 microM) but not concentrations of progesterone reached in preovulatory follicles (100-1000 nM) acted on differentiated (FSH + E) granulosa cells to cause a rapid loss of P450arom mRNA. Whereas cycloheximide prevented the LH/cAMP-mediated decrease in P450arom mRNA in the differentiated cells, enzyme activity remained unaltered during the same 6-h period. Thus, expression of aromatase mRNA in rat granulosa cells is induced primarily by low FSH/cAMP, enhanced by physiological doses of several steroids (except progesterone), and, once induced, can be rapidly inhibited by elevated gonadotropin/cAMP via a pathway requiring protein synthesis.