EXPRESSION OF AROMATASE IN THE OVARY - DOWN-REGULATION OF MESSENGER-RNA BY THE OVULATORY LUTEINIZING-HORMONE SURGE

Aromatase (CYP19) mRNA is induced by follicle-stimulating hormone (FSH ) in granulosa cells of preovulatory follicles and subsequently is rap idly diminished as a consequence of the luteinizing hormone (LH) surge . Primary cultures of rat granulosa cells were used to identify some o f the cellular mechanisms by which FSH increases and LH decreases stea dy-state levels of aromatase mRNA. Induction of aromatase mRNA by FSH was increased by cycloheximide but was blocked by alpha-amanitin and t he C-kinase activators gonadotropin-releasing hormone (GnRH) and phorb ol 12-myrisrate 13-acetate (PMA). In contrast, the decrease in steady- state levels of aromatase mRNA by LH was mimicked by A-kinase (forskol in) and C-kinase (PMA or GnRH) activators. The decrease in aromatase m RNA was associated with decreased amounts of mRNA and protein for ster oidogenic factor-1 (SF-1), a nuclear orphan receptor that binds and tr ans-activates the aromatase promoter, and with the A-kinase subunit ty pe II (RII beta), which is required for mediating cAMP action in these cells. The down-regulation of aromatase, SF-1, and RII beta by each k inase activator and alpha-amanitin was prevented by cycloheximide when the drug was added in combination with the activator. If, however, cy cloheximide was added 2 h after PMA (or LH), the drug did not prevent the rapid loss of mRNA. When granulosa cells were transfected with an aromatase CAT transgene, CAT activity was stimulated 10- to 20-fold by FSH and forskolin but not by PMA. Taken together, these results indic ate that the A-kinase but not the C-kinase pathway can trans-activate the aromatase gene in immature granulosa cells, whereas the C-kinase, as well as A-kinase pathways, mimic the LH surge to decrease aromatase mRNA in preovulatory cells. By increasing degradation of aromatase mR NA and by inhibiting transcription, the LH surge rapidly terminates th e granulosa cell pattern of gene expression while reprogramming the ce lls to express genes associated with ovulation and luteinization. (C) 1997 by Elsevier Science Inc.