Dietary supplementation with isolated soy protein reduces metastasis of mammary carcinoma cells in mice.

The present study investigated the effect of dietary supplementation with isolated soy protein (ISP) on pulmonary metastasis of carcinoma cells from primary mammary tumors induced by orthotopic injection of 4526 murine mammary carcinoma cells in female BALB/c mice. Three diets were compared: a basal AIN-93G diet and the basal diet supplemented with 10% or 20% ISP. After three weeks on the experimental diets, each mouse was injected 4 x 10(5) carcinoma cells into the right inguinal mammary fat pad. The primary tumors were excised when they reached a size of 1.0 cm in diameter. After surgery, mice were maintained on their respective diets for another three weeks. At necropsy, the incidence of metastasis, the number and size of macroscopic tumors, and the number of microscopic tumors in the lungs were determined. The incidence of mice with macroscopically visible tumors was 93%, 76%, and 67%, and the median number of macroscopic tumor was 5, 2, and 1 for the control, 10%, and 20% ISP groups (P < or = 0.05, 20% ISP vs. control). The median cross-sectional area of the macroscopic tumors was 0.93 mm2, 0.80 mm2, and 0.31 mm2, and the median volume was 0.73 mm3, 0.56 mm3, and 0.14 mm3 for the control, 10%, and 20% ISP groups (P < or = 0.01, 20% ISP vs. control). Histological examination revealed fewer microscopically detectable tumors in the ISP groups compared with the control. These results demonstrated that dietary supplementation with ISP reduced pulmonary metastasis of carcinoma cells from primary mammary tumors in BALB/c mice.