A study has reported that the beef and lamb growth promoter zeranol increases estrogen production in breast fat cells, thereby potentially increasing breast cancer cell growth. The study was designed to investigate whether obesity increases the risk of breast cancer induced by zeranol and suppressed by (-)-gossypol (gossypol). Zeranol is a mycotoxin with potent estrogenic activity. This endocrine disruptor is used in the U.S. beef industry as a non-steroidal growth promoter. Gossypol, a polyphenol extracted from cottonseed, has been shown to be chemopreventive. Obesity has been shown to be associated with an increased risk of estrogen-dependent breast cancer. Concerns have been raised that zeranol may be a possible contributor to an increased incidence of human breast cancer, based in part on previous work by the authors. In the present study, the aromatase expression of three types of cells isolated from fat tissues were determined. Aromatase activity is a process by which androgen is converted to estrogen in the body. The authors also investigated the proliferation of primary cultured human normal breast pre-adipocyte cells.

Breast pre-adipocyte cells were found to express higher aromatase than other breast cell types. Zeranol enhanced the cell division-inducing activity of leptin and increased aromatase expression in breast pre-adipocytes. Furthermore, gossypol was found to counteracted zeranol- and leptin-induced cell proliferation and aromatase expression. The authors conclude that bioactive zeranol metabolites contained in meat produced from zeranol-implanted beef cattle may increase estrogen biosynthesis in obese individuals by increasing aromatase expression and estrogen production. This in turn will promote cell sensitivity and increase breast cancer cell growth, according to the authors.