A Molecular Model For Transcriptional Regulation of BRCA-1 Expression

by Hockings, Chi-Fan Ku.

Breast cancer is the second leading cause of cancer-related death in women. Mutations in the tumor suppressor gene BRCA-1 confer a high risk of breast tumor development. However, in sporadic breast cancers, which represent 90-95% of breast cancer cases, BRCA-1 expression is downregulated in the absence of mutations in the BRCA-1 gene. This suggests that epigenetic effectors may contribute to disruption of BRCA-1 expression and the onset of mammary tumors. Prototypical environmental contaminants found in industrial pollution, tobacco smoke, and cooked foods include benzo[a]pyrene (B[a] P) and 2,3,7,8tetrachlorodibenzo-p-dioxin (TCDD), which have been shown to alter mammary gland development, act as endocrine disruptors and tumor promoters. Population studies detected accumulation of TCDD in women’s adipose tissue and breast milk. Moreover, sporadic breast tissue exhibited statistically significant higher levels of PAH-DNA adducts. Based on this information, we examined the effect of B[a]P on the tumor suppressor BRCA-1and observed that exposure to B[a]P led to repression of BRCA-1 transcription through a p53-dependent mechanism. We have also demonstrated that 17?estradiol (E2) stimulated the recruitment of ER? and AP-1 family members to a region of the BRCA-1 promoter flanking an AP-1-like site. However, accumulation of p53 prevented E2-mediated BRCA-1 transcription and recruitment of ER?, potentially providing one mechanism of B[a]P-mediated repression. In addition, the effects of B[a]P and TCDD are mediated through binding of the liganded aromatic hydrocarbon receptor (AhR) to dioxin or xenobiotic-responsive 13 elements (XRE). We have evidence that suggests B[a]P and TCDD may modulate repression of E2-stimulated BRCA-1 expression through 1) binding of the liganded AhR to XREs on the BRCA-1 promoter and 2) preventing promoter occupancy by p300 and SRC-1. Taken together, the data presented here suggest that the transcriptional regulation of BRCA-1 is complex and involves modulation of the recruitment of ER?, AhR, p53, and their cofactors. An important implication of these findings is a greater understanding of the role of ER?, AhR, and p53 in regulation of BRCA-1 which could lead to the development of therapeutic strategies that target these interactions to enhance upregulation of BRCA-1 expression in sporadic breast tumors. 14