Effect of soy protein isolate and conjugated linoleic acid on the growth of Dunning R‐3327‐AT‐1 rat prostate tumors

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Abstract
BACKGROUND Epidemiologic and animal model studies suggest that consumption of soy isoflavones may be associated with reduced risk of prostate cancer (PC). In addition, animal model studies suggest that conjugated linoleic acid (CLA), a natural positional isomer of linoleic acid, inhibits tumor growth in various models, including models of PC. METHODS Based on the above‐mentioned data, the objective of the present study was to test the hypothesis that supplementation of the diet with combinations of isoflavone‐rich soy protein isolate and CLA would act to inhibit the growth of androgen‐independent R‐3327‐AT‐1 rat prostate tumor cells inoculated ectopically into male Copenhagen rats. RESULTS The results of this study indicate that neither an isoflavone‐rich soy protein isolate (SPI), nor CLA inhibit the in vivo growth and development of prostate tumor cells when administered in the diet either singly or in combination. Moreover, at the highest concentrations SPI and CLA (i.e., 20% SPI, 1% CLA), there was a statistically significant increase in tumors volume over controls. Administration of SPI at 10% in the diet also enhanced tumor growth, whereas at 5%, SPI exerted no measurable effect. CLA administration alone had no observable effects on AT‐1 tumor growth. CONCLUSION These results, in an established rat model, suggest caution in using isoflavone‐rich SPI in human studies involving advanced hormone‐refractory prostate cancer until further investigation of these effects are completed. Prostate 54: 169–180, 2003.