Lack of significant effects of genistein on the progression of 7,12-dimethylbenz(a)anthracene-induced mammary tumors in ovariectomized Sprague-Dawley rats

Genistein is thought to be one of the possible factors for decreasing the incidence of breast cancer in Asian peoples who take soy-rich diets. However some experimental data suggest that genistein can stimulate breast cancer development via its estrogenic activities. To clarify the influence of genistein on the promotion/progression stage of mammary carcinogenesis, female Sprague-Dawley rats received a single intragastric administration of 7,12-dimethyl-benz(a)anthracene (DMBA). When the incidence of palpable mammary tumors reached about 50%, all animals were then subjected to ovariectomy and divided into tumor-bearing [DMBA-tumor (+)] and no-tumor-bearing [DMBA-tumor (-)] groups, with subgroups of each treated with genistein at concentrations of 0, 25, or 250 ppm in soybean-free diet for 36 wk. At terminal sacrifice, the 25-ppm subgroup of DMBA-tumor(+) had a higher tumor incidence and volume, whereas the 250-ppm subgroup showed lower incidence, number and volume than the 0-ppm subgroup, although differences were not statistically significant. In the DMBA-tumor(-) groups, eventual tumor volumes in the genistein-treated groups were dose dependently smaller than in the 0-ppm subgroup, although again without statistical significance. The present study indicates that genistein does not exert clear inhibitory effects on mammary carcinogenesis in the promotion/progression stage in female rats under ovarian hormone-free conditions.