Genistein Prevents BRCA1 CpG Methylation and Proliferation in Human Breast Cancer Cells with Activated Aromatic Hydrocarbon Receptor

Background: Previous studies have suggested a causative role for agonists of the aromatic hydrocarbon receptor (AhR) in the etiology of breast cancer 1, early-onset (BRCA-1)–silenced breast tumors, for which prospects for treatment remain poor. Objectives: We investigated the regulation of BRCA1 by the soy isoflavone genistein (GEN) in human estrogen receptor α (ERα)–positive Michigan Cancer Foundation-7 (MCF-7) and ERα-negative sporadic University of Arizona Cell Culture-3199 (UACC-3199) breast cancer cells, respectively, with inducible and constitutively active AhR. Methods: In MCF-7 cells, we analyzed the dose- and time-dependent effects of GEN and (–)-epigallocatechin-3-gallate (EGCG) control, selected as prototype dietary DNA methyltransferase (DNMT) inhibitors, on BRCA-1 expression after AhR activation with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and in TCDD-washout experiments. We compared the effects of GEN and EGCG on BRCA1 cytosine-phosphate-guanine (CpG) methylation and cell proliferation. Controls for DNA methylation and proliferation were changes in expression of DNMT-1, cyclin D1, and p53, respectively. In UACC-3199 cells, we compared the effects of GEN and α-naphthoflavone (αNF; 7,8-benzoflavone), a synthetic flavone and AhR antagonist, on BRCA1 expression and CpG methylation, cyclin D1, and cell growth. Finally, we examined the effects of GEN and αNF on BRCA1, AhR-inducible cytochrome P450 (CYP)-1A1 (CYP1A1) and CYP1B1, and AhR mRNA expression. Results: In MCF-7 cells, GEN exerted dose- and time-dependent preventative effects against TCDD-dependent downregulation of BRCA-1. After TCDD washout, GEN rescued BRCA-1 protein expression while reducing DNMT-1 and cyclin D1. GEN and EGCG reduced BRCA1 CpG methylation and cell proliferation associated with increased p53. In UACC-3199 cells, GEN reduced BRCA1 and estrogen receptor-1 (ESR1) CpG methylation, cyclin D1, and cell growth while inducing BRCA-1 and CYP1A1. Conclusions: Results suggest preventative effects for GEN and EGCG against BRCA1 CpG methylation and downregulation in ERα-positive breast cancer cells with activated AhR. GEN and flavone antagonists of AhR may be useful for reactivation of BRCA1 and ERα via CpG demethylation in ERα-negative breast cancer cells harboring constitutively active AhR.