Synergistic inhibitory effects of genistein and tamoxifen on human dysplastic and malignant epithelial breast cells in vitro.

OBJECTIVE: Genistein is a phytoestrogen with in vitro anticancerogenic activity. We examined in vitro the effects of genistein alone, or in combination with estradiol and tamoxifen, on the growth of human dysplastic and malignant epithelial breast cell lines.
METHODS: Dysplastic breast cell lines (MCF-10A(1), MCF-ANeoT, MCF-T(6)3B) and cell lines of breast cancer (MCF-7, MDA-231, MDA-435) were cultured as monolayers in RPMI 1640 medium supplemented with 10% fetal bovine serum, and L-glutamine. After preincubation of 20 h, genistein (1, 2.5, 5, 7.5 and 10 microg/ml) alone or in combination with estrogen or tamoxifen was added to the cultured cells. The cells were treated continuously for 72 h and then the growth rate was assessed colorimetrically. Stepwise multiple linear regression analysis was used to evaluate the effect of genistein, tamoxifen, and estradiol on cell proliferation.
RESULTS: Genistein had a significant (dose-dependent) inhibitory effect on the proliferation of both dysplastic (P<0.0001) and malignant (P<0.0001) cells. The growth inhibition was significantly higher P<0.0001 in dysplastic cells compared to the cancer cells. Addition of tamoxifen to genistein further inhibited the proliferation of both cell types, reflecting a synergistic antiproliferative effect on dysplastic cells P<0.0001 and an additive growth inhibition effect P<0.0003 on malignant cells. Estradiol significantly (P=0.005) stimulated the growth of dysplastic cell lines while a significant (P=0.003) antiproliferative effect on growth of the malignant cells was observed. The concentration of estrogen receptor (ER) had no significant effect on growth rates and did not modulate the effects of genistein or tamoxifen.
CONCLUSIONS: Genistein (1-10 microg/ml) inhibits the growth of dysplastic and malignant epithelial breast cancer cells in vitro and the addition of tamoxifen (10(-6), 10(-7)M) has a synergistic/additive inhibitory effect. These effects are not modulated by the presence of ER.