Inhibitory effects of nitric oxide on the expression and activity of aromatase in human granulosa cells

The aim of the present study was to explore the mechanisms by which nitric oxide (NO) may inhibit aromatase activity of human granulosa cells. Ovarian granulosa-luteal cells, obtained from patients undergoing in-vitro fertilization (IVF) were cultured in the presence of NO-related substances. After 24 h of culture, aromatase activity of the cells was significantly inhibited by treatment with the NO donors, SNAP or NOC12 at greater than or equal to 10(-4) M in a dose-dependent manner. Treatment with NO catabolites or a peroxynitrite-releasing compound, SIN1, had no significant influence. Treatment with SNAP at 10(-3) M decreased relative aromatase mRNA values by 72% (P < 0.05) and intracellular cyclic AMP concentrations by 53% (P < 0.01). However, treatment with H89, an inhibitor of protein kinase A, did not inhibit aromatase activity. Since there were no significant effects of NO catabolites or peroxinitrite, the inhibitory action of NO donors on aromatase must be related to NO release. The action of NO is, in part, attributable to the down-regulation of aromatase gene transcription. Although NO decreased intracellular cAMP values, down-regulation of aromatase gene transcription may not be mediated by protein kinase A-dependent mechanisms.