A new study has reported that meat produced from beef cattle after zeranol implantation may be a risk factor for breast cancer. Zeranol is a mycotoxin with strong estrogenic activity (i.e., it is an endocrine disruptor). Zeranol is widely used as a non-steroidal anabolic growth promoter by the U.S. beef and lamb industry. Consumption of beef from cattle implanted with zeranol has been reported to be a possible risk factor for breast cancer.

The study was designed to investigate the effect of zeranol on both cancerous and normal breast cells. In the study, the authors investigated the effect of serum (the watery portion of blood) from rats implanted with zeranol on the proliferation of MCF-7 hormone receptor positive (ER+/PR+) human breast cancer cells and human breast epithelial cells (the type of cell from which breast cancer typically develops). Rats were implanted with zeranol pellets and blood was drawn 110 days after implantation. The effects of the serum from the zeranol rats on mRNA expression of cell cycle regulating gene (cyclin D1) and the tumor suppressor genes p53 and p21 was evaluated.

The serum derived from the zeranol rats was found to significantly promote the proliferation of MCF-7 breast cancer cells and normal human breast epithelial cells compared to serum from non-implanted control rats. Serum from zeranol rats was also observed to increase cyclin D1 (which is important for cell cycle progression) and reduce p53 and p21 (which are tumor suppressors that help prevent cancer) expression in normal human breast cells compared with control serum. The authors conclude that bioactive zeranol metabolites contained in meat produced from cattle after zeranol implantation may be a risk factor for breast cancer.