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Estrogen receptor alpha (ER) and p53 are critical prognostic indicators in breast cancer. Loss of functional p53 is correlated with poor prognosis, ER negativity, and resistance to antiestrogen treatment. Previously, we found that p53 genotype was correlated with ER expression and response to tamoxifen in mammary tumors arising in mouse mammary tumor virus-Wnt-1 transgenic mice. These results lead us to hypothesize that p53 may regulate ER expression. To test this, MCF-7 cells were treated with doxorubicin or ionizing radiation, both of which stimulated a 5-fold increase in p53 expression. ER expression was also increased 4-fold over a 24-h time frame. In cells treated with small interfering RNA (siRNA) targeting p53, expression of both p53 and ER was significantly reduced (&gt;60%) by 24 h. Induction of ER by DNA-damaging agents was p53 dependent as either ionizing radiation or doxorubicin failed to up-regulate ER after treatment with p53-targeting siRNA. To further investigate whether p53 directly regulates transcription of the ER gene promoter, MCF-7 cells were transiently transfected with a wild-type (WT) p53 expression vector along with a luciferase reporter containing the proximal promoter of ER. In cells transfected with WT p53, transcription from the ER promoter was increased 8-fold. Chromatin immunoprecipitation assays showed that p53 was recruited to the ER promoter along with CARM1, CBP, c-Jun, and Sp1 and that this multifactor complex was formed in a p53-dependent manner. These data show that p53 regulates ER expression through transcriptional control of the ER promoter, accounting for their concordant expression in human breast cancer.

Transcriptional Regulation of Estrogen Receptor-α by p53 in Human Breast Cancer Cells