BRCA1 negatively regulates the cancer-associated aromatase promoters I.3 and II in breast adipose fibroblasts and malignant epithelial cells

Context: Heterozygous mutations of the breast cancer susceptibility gene 1 (BRCA1) gene that lead to haploinsufficiency increase the risk of breast cancer. The underlying mechanism is unknown. Objective: Because excessive estrogen production increases the risk of breast cancer, we determined whether BRCA1 suppresses aromatase expression and thus local estrogen production in breast adipose fibroblasts ( BAFs) and breast malignant epithelial cells by interacting with the cancer-associated promoter I.3/II region of the aromatase gene. Results: Treatment of BAFs with prostaglandin E-2 or a surrogate hormonal cocktail of dibutyryl (Bt(2)) cAMP plus phorbol 12, 13-diacetate (PDA) significantly reduced BRCA1 levels and induced aromatase mRNA levels. Reduction of BRCA1 in BAFs and in MCF7 and SKBR3 malignant breast epithelial cells using small interfering RNA ( siRNA) or small hairpin RNA significantly increased aromatase mRNA levels and enzyme activity. This effect of BRCA1 was mediated via selective inhibition of aromatase promoters I. 3 and II that are up-regulated by prostaglandin E-2 or Bt(2)cAMP + PDA treatment. Chromatin immunoprecipitation assays revealed that BRCA1 binds directly to the aromatase promoter I.3/II region and that BRCA1 binding is abolished by treatment with Bt(2)cAMP+PDA. Conclusions: Selective inhibition of aromatase expression by BRCA1 binding to the I.3/II tumorigenic promoter region may be an important protective mechanism against breast cancer development.