BACKGROUND: Studies using purified enzyme preparations, placental microsomes or cell lines have shown that certain phytoestrogens can inhibit the enzymes that convert androgens to estrogens, namely aromatase and 17beta-hydroxysteroid dehydrogenase (HSD) type 1 and type 5. The study aim was to investigate the effects of selected phytoestrogens on aromatase and 17beta-HSD type 1 activity in primary cultures of human granulosa-luteal (GL) cells. METHODS AND RESULTS: GL cells, cultured for 48 h in medium containing 5% fetal calf serum and for a further 24 h in serum-free medium with or without hFSH or hCG, were exposed to steroid substrates during the last 1-4 h of the experiment. The production of progesterone in the presence of pregnenolone or estradiol synthesis from androstenedione, estrone or testosterone showed dose- and time-dependent increases. Whilst hCG priming had no effect on progesterone production, FSH priming induced mean 68 and 56% increases in the production of estradiol from androstenedione (A-dione) and estrone respectively, but had no significant effect on the metabolism of testosterone to estradiol. None of the phytoestrogens investigated had any acute effects on enzyme activity. In contrast, when GL cells were exposed to the compounds for 24 h prior to exposure to steroid substrates for 4 h, 10 mumol/l apigenin and zearalenone significantly inhibited aromatase activity, whilst biochanin A and quercetin had no effect. None of the phytoestrogens inhibited FSH-induced 17beta-HSD type 1 activity, and only quercetin significantly inhibited progesterone production. CONCLUSIONS: The inability of phytoestrogens to acutely inhibit steroidogenic enzymes in human GL cells (as has been shown in cell-free models) suggests that they are either rapidly metabolized to relatively inactive compounds or that the high enzyme activity in human GL cells masks any inhibitory effects of the compounds at the concentration tested.
