Nuclear Receptors

Delivering a Toxic Message

Science Signaling  31 Mar 2009:
Vol. 2, Issue 64, pp. ec111
DOI: 10.1126/scisignal.264ec111

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 α (ERα) 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 ERα-positive and ERα-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 ERα, and, in a chromatin immunoprecipitation assay, 4-OHEN increased the binding of ERα to an ER target gene promoter. With multiple assays for DNA damage, ERα-positive cells were found more sensitive to DNA damage caused by exposure to 4-OHEN than were ERα-negative cells, and the addition of estrogen receptor antagonists only decreased the damage in ERα-positive cells. Furthermore, the rate of DNA damage formation was increased in ERα-positive cells exposed to 4-OHEN compared with that in ERα-negative cells. Indeed, damage was detected in less than 8 minutes in the ERα-positive cells, whereas no damage was detectable in that time frame in ERα-negative cells. Thus, the ERα appears to actively deliver 4-OHEN to the nucleus, whereas in ERα-negative cells, the effect takes longer because of a passive diffusion of the metabolite. ERα-positive cells also exhibited rapid production of nuclear-localized reactive oxygen species when exposed to 4-OHEN, whereas ERα-negative cells showed reactive oxygen species throughout the cell. Thus, ERα 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 α 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]