Figure 5.

Overexpression of p53 in MCF-7 cells prevents estrogen-induced accumulation of ERα at the AP-1 site and BRCA-1 protein. (A) Bands are PCR products following ChIP assay for ERα. Inputs are control bands generated by PCR from cross-linked chromatin. (B) Western blot analysis with antibodies for p53 and BRCA-1 documents that p53 protein levels are overexpressed in MCF-7 cells transfected with p53WT, whereas E₂-induced BRCA-1 protein is repressed. (A and B) MCF-7 cells were cultured for 24 hours in DMEM or DMEM plus 10 nM E₂. (C) Proposed model that integrates E₂ regulation of BRCA-1 expression with G₁-phase to S-phase transition. Our data suggest that E₂ stimulates the recruitment of an ERα/p300 transcription complex to an AP-1 site in the BRCA-1 promoter. In turn, increased BRCA-1 levels may activate G₁/S-phase checkpoints to allow time for DNA repair [40] and to block ERα transcriptional activity [23]. However, accumulation of p53 interferes with the recruitment of ERα, leading to transcriptional repression of BRCA-1.