Phytoestrogens Modulate Binding Response of Estrogen Receptors α and β to the Estrogen Response Element

Binding of estrogen receptor (ER) to estrogen response element (ERE) induces gene activation and is an important step in estrogen-induced biological effects. Here, we investigated the effects of some dietary phytoestrogens such as the isoflavones genistein and daidzein, its metabolite equol, and the coumestane coumestrol on the binding rate of ERα and ERβ to ERE by a nonradioactive real-time method, the Biacore Technology. ERα and ERβ were able to bind to ERE immobilized on the surface of a sensor chip even in the absence of estrogens. 17β-Estradiol and phytoestrogens induced an increase in ER binding to ERE in a concentration-dependent manner. 17β-Estradiol was a more potent activator of binding than the phytoestrogens studied. The concentrations of 17β-estradiol inducing an increase in the binding response of ERα and ERβ to ERE by 50% (EC(50)) as compared to unliganded ER were 0.03 and 0.01 μM, respectively. Regarding the efficacy of activation of ERα, from the most to the least effective compound, the sequence and the EC(50) were as follows: 17β-estradiol (0.03 μM) > coumestrol (0.2 μM) > equol (3.5 μM) > genistein (15 μM) > daidzein (>300 μM) and for ERβ 17β-estradiol (0.01 μM) > coumestrol (0.025 μM) > genistein (0.03 μM) > daidzein (0.35 μM) > equol (0.4 μM). The ratios EC(50)α/EC(50)β were calculated to be for 17β-estradiol, 3; coumestrol, 8; equol, 8.8; genistein, 500; daidzein > 850. These ratios indicate that genistein and daidzein preferentially activate the binding of ERβ to ERE. The endogenous hormone 17β-estradiol as well as coumestrol and daidzein metabolite equol activate the binding of ERβ to ERE only slightly more effectively than the binding of ERα to ERE. Thus, the effect of daidzein can be changed from a specific activator of ERβ to an activator of both ER isotypes α and β in humans who are able to convert daidzein to equol. While the results of the measurements with ERα were in line with the binding affinities of compounds tested for ER, there was a distinct difference between our results and the binding affinities of phytoestrogens for the ERβ. This leads to the conclusion that phytoestrogens differ not only in their binding affinities for the ER, but also in their potential to increase the rate of receptor binding to the ERE. Keywords: Estrogen receptor; phytoestrogens; estrogen response element; biacore; surface plasmon resonance