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 E2 or a surrogate hormonal cocktail of dibutyryl (Bt2) 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 E2 or Bt2cAMP + 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 Bt2cAMP + 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.