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* The UCLA Center for Human Nutrition and The David Geffen School of Medicine at UCLA, Los Angeles, California 90095
To whom requests for reprints should be addressed at 1 The UCLA Center for Human Nutrition, 13–145 Warren Hall, 900 Veteran Avenue, Los Angeles, CA 90095-1742. E-mail: dmharris{at}ucla.edu
Increased intake of phytoestrogens may be associated with a lower risk of cancer in the breast and several other sites, although there is controversy surrounding this activity. One of the mechanisms proposed to explain the activity of phytoestrogens is their ability to bind and activate human estrogen receptor 
(ER) and human estrogen receptor ß (ERß). Nine phytoestrogens were tested for their ability to transactivate ER or ERß at a range of doses. Mammary adenocarcinoma (MCF-7) cells were co-transfected with either ER or ERß, and an estrogen-response element was linked to a luciferase reporter gene. Dose-dependent responses were compared with the endogenous ligand 17ß-estradiol. Purified genistein, daidzein, apigenin, and coumestrol showed differential and robust transactivation of ER- and ERß-induced transcription, with an up to 100-fold stronger activation of ERß. Equol, naringenin, and kaempferol were weaker agonists. When activity was evaluated against a background of 0.5 nM 17ß-estradiol, the addition of genistein, daidzein, and resveratrol superstimulated the system, while kaempferol and quercetin were antagonists at the highest doses. This transfection assay provides an excellent model to evaluate the activation of ER and ERß by different phytoestrogens in a breast cancer context and can be used as a screening bioassay tool to evaluate the estrogenic activity of extracts of herbs and foods.
Key Words: estrogen receptor ß • phytoestrogens • isoflavonoids • MCF-7 • bioassay
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