| Abstract
| - Binding of estrogen receptor (ER) to estrogen response element (ERE) induces gene activation andis an important step in estrogen-induced biological effects. Here, we investigated the effects of somedietary phytoestrogens such as the isoflavones genistein and daidzein, its metabolite equol, and thecoumestane coumestrol on the binding rate of ERα and ERβ to ERE by a nonradioactive real-timemethod, the Biacore Technology. ERα and ERβ were able to bind to ERE immobilized on the surfaceof a sensor chip even in the absence of estrogens. 17β-Estradiol and phytoestrogens induced anincrease in ER binding to ERE in a concentration-dependent manner. 17β-Estradiol was a morepotent activator of binding than the phytoestrogens studied. The concentrations of 17β-estradiolinducing an increase in the binding response of ERα and ERβ to ERE by 50% (EC(50)) as comparedto 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 (>300μM) 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 anddaidzein preferentially activate the binding of ERβ to ERE. The endogenous hormone 17β-estradiolas well as coumestrol and daidzein metabolite equol activate the binding of ERβ to ERE only slightlymore effectively than the binding of ERα to ERE. Thus, the effect of daidzein can be changed froma specific activator of ERβ to an activator of both ER isotypes α and β in humans who are able toconvert daidzein to equol. While the results of the measurements with ERα were in line with thebinding affinities of compounds tested for ER, there was a distinct difference between our resultsand the binding affinities of phytoestrogens for the ERβ. This leads to the conclusion thatphytoestrogens differ not only in their binding affinities for the ER, but also in their potential to increasethe rate of receptor binding to the ERE. Keywords: Estrogen receptor; phytoestrogens; estrogen response element; biacore; surface plasmonresonance
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