BACKGROUND: Estrogen receptor-α (ERα)-negative breast cancer is clinically 
aggressive and normally does not respond to conventional estrogen 
target-directed therapies. The soybean isoflavone, genistein (GE), has been 
shown to prevent and inhibit breast cancer and recent studies have suggested 
that GE can enhance the anticancer capacity of an estrogen antagonist, tamoxifen 
(TAM), especially in ERα-positive breast cancer cells. However, the role of GE 
in ERα-negative breast cancer remains unknown.
METHODS: We have evaluated the in vitro and in vivo epigenetic effects of GE on 
ERα reactivation by using MTT assay, real-time reverse transcription-polymerase 
chain reaction (RT-PCR) assay, western-blot assay, immunoprecipitation (ChIP) 
assay, immunohistochemistry and epigenetic enzymatic activity analysis. 
Preclinical mouse models including xenograft and spontaneous breast cancer mouse 
models were used to test the efficacy of GE in vivo.
RESULTS: We found that GE can reactivate ERα expression and this effect was 
synergistically enhanced when combined with a histone deacetylase (HDAC) 
inhibitor, trichostatin A (TSA), in ERα-negative MDA-MB-231 breast cancer cells. 
GE treatment also re-sensitized ERα-dependent cellular responses to activator 
17β-estradiol (E2) and antagonist TAM. Further studies revealed that GE can lead 
to remodeling of the chromatin structure in the ERα promoter thereby 
contributing to ERα reactivation. Consistently, dietary GE significantly 
prevented cancer development and reduced the growth of ERα-negative mouse breast 
tumors. Dietary GE further enhanced TAM-induced anti-cancer efficacy due at 
least in part to epigenetic ERα reactivation.
CONCLUSIONS: Our studies suggest that soybean genistein can epigenetically 
restore ERα expression, which in turn increases TAM-dependent anti-estrogen 
therapeutic sensitivity in vitro and in vivo. The results from our studies 
reveal a novel therapeutic combination approach using bioactive soybean product 
and anti-hormone therapy in refractory ERα-negative breast cancer which will 
provide more effective options in breast cancer therapy.