Phytoestrogens bind weakly to oestrogen receptors and can initiate oestrogen-dependent transcription. They are promoted as natural alternatives to HRT and yet epidemiological evidence suggests that they may protect against breast and prostate cancer. Studies in cell-free preparations have shown that phytoestrogens can inhibit the activity of aromatase and that the inhibition is, at least partly, competitive with androgen substrates. The question as to whether chronic exposure to phytoestrogens may alter the expression of aromatase and thereby modulate the conversion of androgens to oestrogens has not been addressed.
We have investigated the effects of three isoflavones, genistein, diadzein and biochanin A, two flavones, quercetin and apigenin and the mycotoxin zearalenone on mRNA and protein expression of aromatase after exposure of human GL cells to these phytoestrogens for 48 h. Real time RT-PCR was used to quantitate aromatase mRNA levels and normalized against two house-keeping genes beta-actin and GAPDH. The cellular concentration of aromatase was quantified with Western blots that were sequentially exposed to antibodies against aromatase and beta-actin. For all experiments the conversion of testosterone to oestradiol over a 4h period, prior to using cells for mRNA or protein expression, was measured. Thus biological activity was assessed in the same cells as either mRNA/protein expression was measured.
Our preliminary results show that relatively high concentrations of phytoestrogens (10 micromolar) inhibit the expression of aromatase and that this is reflected in a reduced ability of GL cells to convert testosterone to oestradiol. Lower doses of phytoestrogens were ineffective in this respect. This is the first study to identify inhibitory effects of phytoestrogens on the mRNA and protein expression of aromatase in primary cultures of human cells. This may be of significance in the epidemiological evidence that diets high in phytoestrogens may protect against breast cancer.
01 - 03 Nov 2004
Society for Endocrinology