The Forkhead Box M1 Protein Regulates the Transcription of the Estrogen Receptor α in Breast Cancer Cells*

In this study, we have identified the Forkhead transcription factor FoxM1 as a physiological regulator of estrogen receptor α (ERα) expression in breast carcinoma cells. Our survey of a panel of 16 different breast cell lines showed a good correlation (13/16) between FoxM1 expression and expression of ERα at both protein and mRNA levels. We have also demonstrated that ectopic expression of FoxM1 in two different estrogen receptor-positive breast cancer cell lines, MCF-7 and ZR-75–30, led to up-regulation of ERα expression at protein and transcript levels. Furthermore, treatment of MCF-7 cells with the MEK inhibitor U0126, which blocks ERK1/2-dependent activation of FoxM1, also repressed ERα expression. Consistent with this, silencing of FoxM1 expression in MCF-7 cells using small interfering RNA resulted in the almost complete abrogation of ERα expression. We also went on to show that FoxM1 can activate the transcriptional activity of human ERα promoter primarily through two closely located Forkhead response elements located at the proximal region of the ERα promoter. Chromatin immunoprecipitation and biotinylated oligonucleotide pulldown assays have allowed us to confirm these Forkhead response elements as important for FoxM1 binding. Further co-immunoprecipitation experiments showed that FoxO3a and FoxM1 interact in vivo. Together with the chromatin immunoprecipitation and biotinylated oligonucleotide pulldown data, the co-immunoprecipitation results also suggest the possibility that FoxM1 and FoxO3a cooperate to regulate ERα gene transcription.

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