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Repression of BRCA1 through a feedback loop involving p53
T K MacLachlan1, B C Dash, D T Dicker
1Laboratory of Molecular Oncology and Cell Cycle Regulation, Howard Hughes Medical Institute, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104, USA.
Abstract:
The BRCA1 and p53 tumor suppressors have been shown to interact and cooperate to activate transcription of p53-responsive genes. In this study, we show that BRCA1 is initially up-regulated, followed by a reduction to below basal levels in response to treatment with the DNA-damaging agents adriamycin and mitomycin C, and that the reduction of BRCA1 expression is dependent on the presence of wild-type p53. Elimination of p53 by expression of human papilloma virus E6 resulted in an inability to down-regulate BRCA1 in response to adriamycin. Ectopic expression of p53 resulted in a rapid decrease in BRCA1 protein and RNA levels and BRCA1 promoter-driven luciferase activity even in null p21 cells deficient in p53-dependent G(1) arrest. ATM(-)(/-) lymphoblastoid cells were deficient in their ability to reduce BRCA1 protein in response to DNA damage, whereas the wild-type counterparts reduced BRCA1 protein levels after exposure to adriamycin. These results, in conjunction with others, suggest a loop wherein BRCA1 initially participates in accumulation of p53 protein, whereas later p53 acts to reduce BRCA1 expression.
Insights
BRCA1 is initially increased then decreased by DNA damage, a process requiring wild-type p53. This study reveals a feedback loop where p53 regulates BRCA1 expression.
Area of Science:
- Molecular Biology
- Cancer Biology
- Genetics
Background:
- BRCA1 and p53 are crucial tumor suppressors known to interact.
- Their cooperative role in activating p53-responsive genes is established.
- The dynamic regulation of BRCA1 by DNA damage and p53 is not fully understood.
Purpose of the Study:
- To investigate the regulation of BRCA1 expression by DNA-damaging agents.
- To determine the role of wild-type p53 in BRCA1 down-regulation.
- To elucidate the interplay between BRCA1 and p53 in response to DNA damage.
Main Methods:
- Treatment of cells with DNA-damaging agents (adriamycin, mitomycin C).
- Manipulation of p53 levels via human papilloma virus E6 and ectopic expression.
- Assessment of BRCA1 protein and RNA levels.
- Luciferase reporter assays to measure BRCA1 promoter activity.
- Use of ATM-deficient and wild-type lymphoblastoid cells.
Main Results:
- BRCA1 expression is initially upregulated, then downregulated below basal levels following DNA damage.
- BRCA1 reduction is dependent on the presence of wild-type p53.
- Elimination of p53 prevents BRCA1 downregulation.
- Ectopic p53 expression rapidly decreases BRCA1 levels and promoter activity.
- ATM-deficient cells show impaired BRCA1 reduction after DNA damage.
Conclusions:
- A feedback loop exists where BRCA1 initially aids p53 accumulation.
- Subsequently, p53 acts to reduce BRCA1 expression.
- This p53-mediated downregulation of BRCA1 is linked to DNA damage response pathways, including ATM.