Regulation of HER2 oncogene transcription by a multifunctional coactivator/corepressor complex
Jason P Garee1, Christopher D Chien, Jordan V Li
1Department of Oncology (J.P.G., C.D.C., J.V.L., A.W., A.T.R.), Lombardi Cancer Center, Georgetown University Medical Center, Washington, District of Columbia 20007; and Pediatric Oncology Branch (C.D.C.), National Cancer Institute, Bethesda, Maryland 20892.
Abstract:
Transcription of the HER2 oncogene can be repressed by estrogen (E2). We now show that, a splice isoform of the nuclear receptor coactivator AIB1, AIB1-Δ4, is able to reverse E2 repression of HER2 gene expression in breast cancer cells. The first 224 amino acids of AIB1 that are absent in AIB1-Δ4, bind a co-repressor, ANCO1. Using chromatin immunoprecipitation assay approaches in MCF7 and BT474 cell lines, we demonstrate that AIB1 and AIB1-Δ4 can bind to the E2 regulatory site in the first intron of the HER2 gene, after E2 treatment, but only full-length AIB1 recruits ANCO1. Consistent with E2-induced chromatin repression, the AIB1-ANCO1 complex recruits HDAC3 and HDAC4 to the intronic estrogen response element and the proximal promoter acquires the repressive chromatin mark H3K9me3 and loses H3K4me1. In contrast, AIB1-Δ4 does not recruit ANCO 1, HDAC3, or HDAC4 and the proximal promoter retains activation marks of H3K4me1. In cell lines with low levels of ANCO1 (T47D), E2 does not repress HER2 gene transcription but the repressive response can be restored by overexpression of ANCO1. ANCO1 can also repress other E2-responsive genes, indicating that AIB1, AIB1-Δ4 and ANCO1 are important determinants of endocrine and growth factor responsiveness in breast cancer.
Insights
A novel AIB1 splice variant, AIB1-Δ4, reverses estrogen repression of HER2 gene expression in breast cancer. This occurs because AIB1-Δ4 prevents the recruitment of co-repressors, maintaining gene activation.
Area of Science:
- Molecular Biology
- Cancer Research
- Endocrinology
Background:
- Estrogen (E2) represses transcription of the HER2 oncogene in breast cancer cells.
- The nuclear receptor coactivator AIB1 plays a role in regulating gene expression.
Purpose of the Study:
- To investigate the role of AIB1 splice variants in mediating estrogen's effect on HER2 gene expression.
- To elucidate the mechanism by which AIB1 influences E2-mediated repression of HER2.
Main Methods:
- Chromatin immunoprecipitation assays in MCF7 and BT474 breast cancer cell lines.
- Analysis of AIB1, AIB1-Δ4, and ANCO1 interactions with the HER2 gene locus.
- Assessment of histone modifications (H3K9me3, H3K4me1) at the HER2 promoter.
Main Results:
- AIB1-Δ4, a splice variant lacking the first 224 amino acids of AIB1, reverses E2-induced HER2 repression.
- Full-length AIB1 recruits the co-repressor ANCO1, which in turn recruits HDAC3 and HDAC4, leading to repressive chromatin marks.
- AIB1-Δ4 fails to recruit ANCO1, thereby preventing co-repressor recruitment and maintaining activating chromatin marks.
- Low ANCO1 levels correlate with lack of E2-mediated HER2 repression, which can be restored by ANCO1 overexpression.
Conclusions:
- AIB1-Δ4 acts as a dominant-negative regulator, counteracting E2-mediated HER2 repression.
- The interaction between AIB1, ANCO1, and histone deacetylases is critical for E2-induced HER2 gene silencing.
- AIB1 isoforms and ANCO1 are key regulators of endocrine and growth factor responsiveness in breast cancer.
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