Control of peripheral benzodiazepine receptor-mediated breast cancer in rats by soy protein

Soy protein is known to have breast tumor suppressing activity. The expression of peripheral benzodiazepine receptors (PBRs), currently renamed as translocator protein (TSPO) and their associated functions, such as nuclear cholesterol uptake and content also have been shown to be increased in breast cancer. Here we investigated whether the breast tumor suppressing effects of soy protein is mediated by down-regulation of PBR expression and function. Breast tumors were induced by gavage administration of a single dose (80 mg/kg) of dimethylbenz[a]anthracene (DMBA) into 50-d old female Sprague Dawley rats, maintained on a standard AIN-76A diet containing either casein or soy protein. Approximately 120 d following DMBA administration, the animals were sacrificed. All tumors were detected by palpation and at autopsy biopsy specimens were taken for histological grading. The ligand binding capacity, expression, and protein levels of PBRs, their nuclear localization and function, such as nuclear cholesterol uptake and content, were significantly increased in the tumors. However, replacement of casein by soy protein in the diet caused a significant decrease in all of these parameters. These data suggest that soy protein inhibits breast tumor development by decreasing the expression of the tumor-promoting gene, which encodes PBRs.