Inhibition of AP‐1 activation in DMBA‐induced rat breast tumors by dietary soy protein

Soy protein is beneficial for the control of breast cancer; however the molecular mechanism is not clear. This study examines the effects of soy protein on MAPK signaling in rat breast cancer model. Tumor was developed by gavage administration of single dose of DMBA (80 mg/kg) into 50‐day old female rats, maintained on a standard AIN‐76A diet containing either casein (20%) or soy protein (20%). After 122 days of DMBA administration, the animals were sacrificed. Tumors were detected by palpation and at autopsy biopsy specimens were graded. The casein group had 20% grade I, 60% grade II and 20% grade III adenocarcinoma. Soy protein group had 100% grade I tumors. The expression of the AP‐1 family of transcription factors in tumors was analyzed by Western blot. JNK activation/phosphorylation was increased in tumors. The increase was more in casein group than that in soy protein group. This in turn increased the c‐jun expression in tumors in both groups. However, c‐fos expression in tumors was increased only in casein group and not in soy protein group. The c‐fos expression in the soy protein group was at the basal level. Since c‐fos and c‐jun dimerization is necessary to activate AP‐1, the activation of AP‐1 will be higher in tumors from casein group than that in tumors from soy protein group. Hence soy protein may impart its beneficial effect on cancer progression by regulating AP‐1 activation. (Supported by Grant DAMD‐17‐03‐1‐0352)