Physiological Concentrations of Dietary Genistein Dose-Dependently Stimulate Growth of Estrogen-Dependent Human Breast Cancer (MCF-7) Tumors Implanted in Athymic Nude Mice

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Abstract
Previously our laboratory has shown that the soy isoflavone, genistein, stimulates growth of human breast cancer (MCF-7) cells in vivo and in vitro. In this study, the dose-response analysis of genistein at the physiologically achievable concentration range between 125 and 1,000 μg/g in the diet was conducted in ovariectomized athymic nude mice implanted with MCF-7 cells. We hypothesized that genistein at this concentration range can stimulate dose-dependently the breast tumor growth, cell proliferation and an estrogen-responsive pS2 gene induction. Tumor size and body weight were monitored weekly. At completion of the study, we analyzed cellular proliferation of tumors using incorporation of BrdU, pS2 expression of tumors using a Northern blot analysis and total genistein level in plasma using liquid chromatography–isotope dilution mass spectrometry (LC-ES/MS). Dietary genistein (≥250 μg/g) increased tumor size in a dose-dependent manner [8.4× the negative control (NC) group in the 250 μg/g group, 12.0× in the 500 μg/g group, 20.2× in the 1,000 μg/g group and 23.2× in the positive control (PC) group]. The percentage of proliferating cells was significantly increased by genistein at and above 250 μg/g (5.3× the NC group in the 250 μg/g, 5.6× in the 500 μg/g, 5.0× in the 1,000 μg/g and 4.8× in the PC group). Expression of pS2 mRNA was also significantly increased with increasing dietary genistein levels (11.25× the NC group in the 500 μg/g group and 15.84× in the 1,000 μg/g group). Total plasma genistein concentrations were between 0.39 and 3.36 μmol/L in mice fed between 125 and 1,000 μg/g genistein. In conclusion, dietary treatment with genistein at physiological concentrations produces blood levels of genistein sufficient to stimulate estrogenic effects, such as breast tumor growth, cellular proliferation and pS2 expression in athymic mice in a dose-responsive manner similar to that seen in vitro.