The ligand status of the aromatic hydrocarbon receptor modulates transcriptional activation of BRCA-1 promoter by estrogen.
In sporadic breast cancers, BRCA-1 expression is down-regulated in the absence of mutations in the BRCA-1 gene. This suggests that disruption of BRCA-1 expression may contribute to the onset of mammary tumors. Environmental contaminants found in industrial pollution, tobacco smoke, and cooked foods include benzo(a)pyrene [B(a)P] and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), which have been shown to act as endocrine disruptors and tumor promoters. In previous studies, we documented that estrogen (E2) induced BRCA-1 transcription through the recruitment of an activator protein-1/estrogen receptor-alpha (ER alpha) complex to the proximal BRCA-1 promoter. Here, we report that activation of BRCA-1 transcription by E2 requires occupancy of the BRCA-1 promoter by the unliganded aromatic hydrocarbon receptor (AhR). The stimulatory effects of E2 on BRCA-1 transcription are counteracted by (a) cotreatment with the AhR antagonist 3'-methoxy-4'-nitroflavone; (b) transient expression in ER alpha-negative HeLa cells of ER alpha lacking the protein-binding domain for the AhR; and (c) mutation of two consensus xenobiotic-responsive elements (XRE, 5'-GCGTG-3') located upstream of the ER alpha-binding region. These results suggest that the physical interaction between the unliganded AhR and the liganded ER alpha plays a positive role in E2-dependent activation of BRCA-1 transcription. Conversely, we show that the AhR ligands B(a)P and TCDD abrogate E2-induced BRCA-1 promoter activity. The repressive effects of TCDD are paralleled by increased recruitment of the liganded AhR and HDAC1, reduced occupancy by p300, SRC-1, and diminished acetylation of H4 at the BRCA-1 promoter region flanking the XREs. We propose that the ligand status of the AhR modulates activation of the BRCA-1 promoter by estrogen.
Hockings
Jennifer K
JK
Cancer Biology Interdisciplinary Graduate Program, University of Arizona, Tucson, USA.
Thorne
Patricia A
PA
Kemp
Michael Q
MQ
Morgan
Sherif S
SS
Selmin
Ornella
O
Romagnolo
Donato F
DF
D019313:BRCA1 Protein / Q000096:biosynthesis / Q000235:genetics; D001564:Benzo(a)pyrene / Q000378:metabolism / Q000494:pharmacology; D001943:Breast Neoplasms / Q000235:genetics; D004958:Estradiol / Q000494:pharmacology*; D047628:Estrogen Receptor alpha / Q000378:metabolism*; D015972:Gene Expression Regulation, Neoplastic / Q000187:drug effects / Q000502:physiology; D019398:Genes, BRCA1 / Q000187:drug effects / Q000502:physiology*; D006367:HeLa Cells; D006801:Humans; D008024:Ligands; D000072317:Polychlorinated Dibenzodioxins / Q000378:metabolism / Q000494:pharmacology; D011401:Promoter Regions, Genetic / Q000187:drug effects; D012333:RNA, Messenger / Q000096:biosynthesis / Q000235:genetics; D020239:Receptor Cross-Talk / Q000187:drug effects / Q000502:physiology; D018336:Receptors, Aryl Hydrocarbon / Q000037:antagonists & inhibitors / Q000378:metabolism*; D018808:Transcription Factor AP-1 / Q000378:metabolism; D015533:Transcriptional Activation / Q000187:drug effects / Q000502:physiology; D014162:Transfection
D019313:BRCA1 Protein; D047628:Estrogen Receptor alpha; D008024:Ligands; D000072317:Polychlorinated Dibenzodioxins; D012333:RNA, Messenger; D018336:Receptors, Aryl Hydrocarbon; D018808:Transcription Factor AP-1; D001564:Benzo(a)pyrene; D004958:Estradiol