Soybean phytochemicals inhibit the growth of transplantable human prostate carcinoma and tumor angiogenesis in mice

Author: Zhou JR, Gugger ET, Tanaka T, Guo Y, Blackburn GL, Clinton SK
Publication: J Nutr. 1999 Sep;129(9):1628-35

There exist large geographic differences in prostate cancer rates between people form the Western world and Asian countries. These differences may be caused by dietary variables, such as the high soy consumption in Asian countries. Only few animal studies have been conducted to investigate the relationship between soy isoflavones and prostate cancer tumorigenesis. Some earlier studies have illustrated that soy isoflavones can reduce the growth of prostate cancer, other have shown no effect. That is why more in vivo tests are required.

The purpose of this study was to characterize the ability of soy protein to inhibit the growth of human prostate cancer cells in mice.

In vitro tests demonstrate that soy isoflavones, genistein and daidzein, inhibit the growth of human prostate cancer cells LNCaP, DU-145 and PC-3, but only in high concentrations (supraphysiologic concentrations). Genistein inhibited the LNCaP cell growth by 33% at 25 micromol/L and 50% at 50 micromol/L. At 50 micromol/L genistein inhibited the growth of DU-145 and PC-3 by 23% and 34%, respectively. Daidzein is less potent than genistein in inhibiting the growth of the LNCaP and DU-145 cells. The in vitro growth of endothelial cells was inhibited by soy isoflavones at much lower concentrations.

For the vivo test, forty-eight male SCID mice were inoculated with LNCaP cells and were divided in 6 groups based on protein source (casein or soy protein) and three levels of soy isoflavones concentrate (0.0%, 0.2% and 1.0%). Soy protein isolate containing 2 mg isoflavones per gram was used as soy protein. The soy isoflavones concentrate was made from soybeans and contained 170 mg aglycone isoflavones equivalents per gram (79.2 mg genistein, 70.4 mg daidzein and 20.4 g glycitein).

The following reductions in tumor volumes were observed:

  • 11% by soy protein
  • 19% by 0.2% soy isoflavones concentrate
  • 28% by soy protein and 0.2% soy isoflavones concentrate
  • 30% by 1.0 % soy isoflavones concentrate
  • 40% by soy protein and 1.0% soy isoflavones concentrate.

The measurement of the microvessel density, which is a measurement of tumor angiogenesis, showed that mice fed with soy products have a reduced tumor vascularity.

The data suggest that dietary soy products may inhibit experimental prostate cancer growth. Soy products seem to have a direct effect on cancer cell growth and indirectly by influencing cancer neovasculature. This study provides proof that isoflavones containing soy products should be developed further for the prevention and treatment of prostate cancer.