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 microg/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 (> or = 250 microg/g) increased tumor size in a dose-dependent manner [8.4x the negative control (NC) group in the 250 microg/g group, 12.0x in the 500 microg/g group, 20.2x in the 1,000 microg/g group and 23.2x in the positive control (PC) group]. The percentage of proliferating cells was significantly increased by genistein at and above 250 microg/g (5.3x the NC group in the 250 microg/g, 5.6x in the 500 microg/g, 5.0x in the 1,000 microg/g and 4.8x in the PC group). Expression of pS2 mRNA was also significantly increased with increasing dietary genistein levels (11.25x the NC group in the 500 microg/g group and 15.84x in the 1,000 microg/g group). Total plasma genistein concentrations were between 0.39 and 3.36 micromol/L in mice fed between 125 and 1,000 microg/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.

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Soy-based products consumed in Asian countries are minimally processed whereas in the USA many of the soy foods and soy ingredients are highly processed. Soy foods contain complex mixtures of bioactive compounds, which may interact with one another. The objective of this study was to evaluate the ability of various soy products containing genistin, the glycoside form of genistein, to affect growth of MCF-7 cells transplanted into ovariectomized athymic mice. Products investigated included soy flour, two crude extracts of soy (soy molasses and Novasoy(R)), a mixture of isoflavones and genistin in pure form. Each of the soy flour-processed products was added to the diet to provide equivalent amounts of genistein aglycone equivalents (750 p.p.m.). Tumors in the negative control animals regressed throughout the study while the tumors in the soy flour-fed animals remained basically the same size (neither grew nor regressed). In animals consuming soy molasses, Novasoy(R), mixed isoflavones or genistin alone, tumor growth was stimulated when compared with animals consuming a control diet devoid of soy. These same dietary treatments resulted in increased cellular proliferation. Changes in mRNA expression of gene targets (estrogen responsiveness, cell cycle progression, apoptosis and aromatase activity) in tumors induced by the different diets were evaluated. The relative expression of pS2, progesterone receptor and cyclin D1 was increased in animals consuming the Novasoy(R), mixed isoflavones and genistin. Bcl2 mRNA expression was low in most of the dietary treatment groups compared with positive (estradiol implant) controls. Aromatase expression was not affected in any of the treatment groups. The degree of soy flour processing affects the estrogenicity of products containing a constant amount of genistein. Collectively, these findings suggest that for postmenopausal women with estrogen-dependent breast cancer, the consumption of foods containing soy flour is more advisable than consuming isoflavones in more purified forms.

Genistein (GEN), a soy isoflavone, stimulates growth of estrogen-dependent human tumor cells (MCF-7) in a preclinical mouse model for postmenopausal breast cancer. Antiestrogens and aromatase inhibitors are frontline therapies for estrogen-dependent breast cancer. We have demonstrated that dietary GEN can negate the inhibitory effect of tamoxifen. In this study, we evaluated the interaction of dietary GEN (at 250-1000 p.p.m. in the American Institute of Nutrition 93 growth diet) and an aromatase inhibitor, letrozole (LET), on the growth of tumors in an aromatase-expressing breast cancer xenograft model (MCF-7Ca) in the presence and absence of the substrate androstenedione (AD). Dietary GEN (250 and 500 p.p.m.) or implanted AD stimulated MCF-7Ca tumor growth. Implanted LET inhibited AD-stimulated MCF-7Ca tumor growth. In the presence of AD and LET, dietary GEN (250, 500 and 1000 p.p.m.) reversed the inhibitory effect of LET in a dose-dependent manner. Uterine wet weight, plasma estradiol (E(2)) levels (enzyme-linked immunosorbent assay) and total plasma GEN and LET levels (liquid chromatography-electrospray/tandem mass spectrometry) were measured. Ki-67 (cellular proliferation), aromatase and pS2 protein expression in tumors were evaluated using immunohistochemical (IHC) analysis. In conclusion, dietary GEN increased the growth of MCF-7Ca tumors implanted in ovariectomized mice and could also negate the inhibitory effect of LET on MCF-7Ca tumor growth. These findings are significant because tumors, which express aromatase and synthesize estrogen, are good candidates for aromatase therapy dietary and GEN can reverse the inhibitory effect of LET on tumor growth and adversely impact breast cancer therapy. Caution is warranted for consumption of dietary GEN by postmenopausal women with estrogen-dependent breast cancer taking LET treatment.