THE EFFECTS OF A SOYBEAN EXTRACT ON TUMOR-GROWTH AND METASTASIS

In vitro genistein (soybean-derived isoflavone/ phytoestrogen) has bee n reported to suppress angiogenesis. We have produced a soybean extrac t (SE) enriched in phytoestrogens which are heat stable. Heat steriliz ation (hs) of SE destroys soy protease inhibitor activity and increase s isoflavone concentrations. This study was performed to determine the effects of SE with and without heat sterilization on tumor growth and metastasis. Sixty female Lewis rats injected s.c. with mammary tumor (MAC-33) were randomized to receive i.p. injection of SE (18 mg), hsSE (18 mg), or saline (control) 5 times per week for 30 days. After 30 d ays the rats were sacrificed to determine carcass weight, tumor weight , tumor volume, liver weight, hepatic protease activity (HPA), and num ber and size of lung metastasis. Comparing either SE or hsSE to contro ls, significant increases were found in tumor weight (g) (109.4 +/- 26 .3, 114.9 +/- 19.7, 86.8 +/- 23.2), tumor volume (cm(3)) (55.6 +/- 14. 6, 60.1 +/- 12.0, 45.5 +/- 14.4), and tumor: carcass ratio (0.59, 0.62 , 0.47). Data expressed as mean +/- SD (SE, hsSE, control, respectivel y) was analyzed by ANOVA. There was a significant increase in the numb er of lung metastasis (31%; Kruskal-Wallis test) in the animals receiv ing hsSE. Additionally, there was a significant decrease in HPA (26.9% ) in the animals receiving SE. Soy protease inhibitors are not respons ible for the increase in tumor growth and number of lung metastases se en with SE. Our results suggest that a heat stable component of SE, pe rhaps the isoflavones, promotes tumor growth and lung metastasis in vi vo.