Estrogen Receptor {alpha} Enhances the Rate of Oxidative DNA Damage by Targeting an Equine Estrogen Catechol Metabolite to the Nucleus.

Exposure to estrogens increases the risk of breast and endometrial cancer. It is proposed that the estrogen receptor (ER) may contribute to estrogen carcinogenesis by transduction of the hormonal signal and as a "Trojan horse" concentrating genotoxic estrogen metabolites in the nucleus to complex with DNA, enhancing DNA damage. 4-Hydroxyequilenin (4-OHEN), the major catechol metabolite of equine estrogens present in estrogen replacement formulations, autoxidizes to a redox-cycling quinone that has been shown to cause DNA damage. 4-OHEN was found to be an estrogen of nanomolar potency in cell culture using a luciferase reporter assay and, using a chromatin immunoprecipitation assay, was found to activate ERalpha binding to estrogen-responsive genes in MCF-7 cells. DNA damage was measured in cells by comparing ERalpha(+) versus ERalpha(-) cells and 4-OHEN versus menadione, a reactive oxygen species (ROS)-generating, but non-estrogenic, quinone. 4-OHEN selectively induced DNA damage in ERalpha(+) cells, whereas menadione-induced damage was not dependent on cellular ER status. The rate of 4-OHEN-induced DNA damage was significantly enhanced in ERalpha(+) cells, whereas ER status had no effect on the rate of menadione-induced damage. Imaging of ROS induced by 4-OHEN showed accumulation selective for the nucleus of ERalpha(+) cells within 5 min, whereas in ERalpha(-) or menadione-treated cells, no selectivity was observed. These data support ERalpha acting as a Trojan horse concentrating 4-OHEN in the nucleus to accelerate the rate of ROS generation and thereby amplify DNA damage. The Trojan horse mechanism may be of general importance beyond estrogen genotoxins.