Bisphenol a binds to the low-affinity estrogen binding site

Environmental estrogens are suspected of being involved in the current increase in the incidence of human reproductive malfunctions, such as a decrease in male reproductive capacity and an increased incidence of breast cancer in women. The influences of these compounds have been proposed to be mediated through binding to macromolecules, such as estrogen receptor alpha or beta. In this study we examined whether the low-affinity Type II estrogen binding site (Type II EBS), originally identified in the rat uterus, is a possible mediator of environmental estrogens such as bisphenol A (BPA). Analysis of BPA's binding to an enriched fraction of Type II EBS, using a competition assay, indicated that BPA was able to compete with estradiol in binding to this site. At a concentration of 10-15 microM (comparable to that required to induce uterine proliferation), BPA inhibited the binding of estradiol to Type II EBS by greater than 50%. The binding affinity of BPA for the Type II EBS was only 8-10-fold lower than that of the synthetic estrogen diethylstilbestrol. The binding of BPA to Type II EBS appeared specific to BPA, in that endosulfan, another environmental estrogen, failed to displace estradiol from the site. A comparison of the relative binding affinities of BPA for rat uterine estrogen receptor alpha to that of the Type II EBS implies that BPA preferentially binds to the Type II EBS.