Physiological concentrations of dietary genistein dose-dependently stimulate growth of estrogen-dependent human breast cancer (MCF-7) tumors implanted in athymic nude mice.

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.

Dietary genistin stimulates growth of estrogen-dependent breast cancer tumors similar to that observed with genistein.

The estrogenic soy isoflavone, genistein, stimulates growth of estrogen-dependent human breast cancer (MCF-7) cells in vivo. Genistin is the glycoside form of genistein and the predominant form found in plants. It is generally believed that genistin is metabolized to the aglycone genistein in the lower gut. However, it is unclear if the rate of metabolism of genistin to genistein is sufficient to produce a level of genistein capable of stimulating estrogen-dependent breast cancer cell growth. Our hypothesis was that dietary genistin would stimulate tumor growth similar to that observed with genistein in athymic mice. To test this hypothesis, genistin or genistein was fed to athymic mice containing xenografted estrogen-dependent breast tumors (MCF-7). Mice were fed either genistein at 750 p.p.m. (parts per milllion) or genistin at 1200 p.p.m., which provides equal molar concentrations of aglycone equivalents in both diets. Tumor size was measured weekly for 11 weeks. At completion of the study, half of the animals per treatment group were killed and tumors collected for evaluation of cellular proliferation and estrogen-responsive pS2 gene expression. Incorporation of bromo-deoxyuridine into cellular DNA was utilized as an indicator of cellular proliferation. Dietary genistin resulted in increased tumor growth, pS2 expression and cellular proliferation similar to that observed with genistein. The remaining mice were switched to diets free of genistin and genistein. When mice were placed on isoflavone free diets, tumors regressed over a span of 9 weeks. Next, we examined how effectively and where metabolism of genistin to genistein occurred in the digestive tract. We present evidence that demonstrates conversion of genistin to its aglycone form genistein begins in the mouth and then continues in the small intestine. Both human saliva and the intestinal cell-free extract from mice converted genistin to genistein. In summary, the glycoside genistin, like the aglycone genistein, can stimulate estrogen-dependent breast cancer cell growth in vivo. Removal of genistin or genistein from the diet caused tumors to regress.

Soy diets containing varying amounts of genistein stimulate growth of estrogen-dependent (MCF-7) tumors in a dose-dependent manner.

We have demonstrated that the isoflavone, genistein, stimulates growth of estrogen-dependent human breast cancer (MCF-7) cells in vivo (C. Y. Hsieh et al., Cancer Res., 58: 3833-3838, 1998). The isoflavones are a group of phytoestrogens that are present in high concentrations in soy. Whether consumption of genistein from soy protein will have similar effects on estrogen-dependent tumor growth as pure genistein has not been investigated in the athymic mouse tumor implant model. Depending on processing, soy protein isolates vary widely in concentrations of genistein. We hypothesize that soy isolates containing different concentrations of genistein will stimulate the growth of estrogen-dependent cells in vivo in a dose-dependent manner. To test this hypothesis we conducted experiments in which these soy protein isolates were fed to athymic mice implanted s.c. with estrogen-dependent tumors. Genistein content (aglycone equivalent) of the soy isolate diets were 15, 150, or 300 ppm. Positive (with 17beta-estradiol pellet implant) and negative (no 17beta-estradiol) control groups received casein-based (isoflavone-free) diets. Tumor size was measured weekly. At completion of the study animals were killed and tumors collected for evaluation of cellular proliferation and estrogen-dependent gene expression. Incorporation of bromodeoxyuridine into cellular DNA was used as an indicator of cell proliferation, and pS2 mRNA was used as an estrogen-responsive gene. Soy protein diets containing varying amounts of genistein increased estrogen-dependent tumor growth in a dose-dependent manner. Cell proliferation was greatest in tumors of animals given estrogen or dietary genistein (150 and 300 ppm). Expression of pS2 was increased in tumors from animals consuming dietary genistein (150 and 300 ppm). Here we present new information that soy protein isolates containing increasing concentrations of genistein stimulate the growth of estrogen-dependent breast cancer cells in vivo in a dose-dependent manner.