Sci. Signal., 31 March 2009
Nuclear Receptors Delivering a Toxic Message
Nancy R. Gough
Science Signaling, AAAS, Washington, DC 20005, USA
The increased health risks associated with estrogen replacement therapy in postmenopausal women are so high that a large-scale study of its effects was halted because of them. It is well known that long-term exposure to estrogen therapy is associated with increased risk of cancer in estrogen-responsive tissues. Estrogens may act as a transcriptional regulator to promote cancer, but they may also promote cancer by causing DNA damage through the production of reactive metabolites that produce DNA adducts. Wang et al. propose that estrogen receptor (ER) serves as a "Trojan horse," delivering reactive estrogen metabolites to the nucleus, where they generate DNA damage. To support this hypothesis, the authors studied the effects of the equine estrogen equilenin in ER-positive and ER-negative cells. Conjugated horse estrogens are commonly used in estrogen replacement preparations, and horse equilenin is oxidized to 4-hydroxyequilenin (4-OHEN), which then auto-oxidizes to the reactive 4-OHEN-o-quinone, known to cause DNA damage. In a reporter gene assay, 4-OHEN stimulated the transcriptional activity of ER, and, in a chromatin immunoprecipitation assay, 4-OHEN increased the binding of ER to an ER target gene promoter. With multiple assays for DNA damage, ER-positive cells were found more sensitive to DNA damage caused by exposure to 4-OHEN than were ER-negative cells, and the addition of estrogen receptor antagonists only decreased the damage in ER-positive cells. Furthermore, the rate of DNA damage formation was increased in ER-positive cells exposed to 4-OHEN compared with that in ER-negative cells. Indeed, damage was detected in less than 8 minutes in the ER-positive cells, whereas no damage was detectable in that time frame in ER-negative cells. Thus, the ER appears to actively deliver 4-OHEN to the nucleus, whereas in ER-negative cells, the effect takes longer because of a passive diffusion of the metabolite. ER-positive cells also exhibited rapid production of nuclear-localized reactive oxygen species when exposed to 4-OHEN, whereas ER-negative cells showed reactive oxygen species throughout the cell. Thus, ER may deliver toxic metabolites to the nucleus, where they can cause DNA damage.
Z. Wang, G. T. Wijewickrama, K.-w. Peng, B. M. Dietz, L. Yuan, R. B. van Breemen, J. L. Bolton, G. R. J. Thatcher, Estrogen receptor enhances the rate of oxidative DNA damage by targeting an equine estrogen catechol metabolite to the nucleus. J. Biol. Chem. 284, 8633–8642 (2009). [Abstract] [Full Text]
Citation: N. R. Gough, Delivering a Toxic Message. Sci. Signal. 2, ec111 (2009).
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