Phytoestrogens induce differential estrogen receptor alpha- or beta-mediated responses in transfected breast cancer cells

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 alpha (ER alpha) and human estrogen receptor beta (ER beta). Nine phytoestrogens were tested for their ability to transactivate ER alpha or ER beta at a range of doses. Mammary adenocarcinoma (MCF-7) cells were co-transfected with either ER alpha or ER beta, and an estrogen-response element was linked to a luciferase reporter gene. Dose-dependent responses were compared with the endogenous ligand 17 beta-estradiol. Purified genistein, daidzein, apigenin, and coumestrol showed differential and robust transactivation of ER alpha- and ER beta-induced transcription, with an up to 100-fold stronger activation of ER beta. Equol, naringenin, and kaempferol were weaker agonists. When activity was evaluated against a background of 0.5 nM 17 beta-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 alpha and ER beta 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.