Serum harvested from heifers one month post-zeranol implantation stimulates MCF-7 breast cancer cell growth.

Breast cancer is a serious disease in the US. Numerous risk factors have been linked to this disease. The safety of using growth promoters, such as zeranol, remains under debate due to the lack of sufficient in vitro and in vivo evidence. Using CellTiter 96(™) Aqueous Non-Radioactive Cell Proliferation assay, real-time PCR and Western blot analysis, we evaluated the effects of sera harvested from experimental and control heifers before and after one month of zeranol implantation on the growth of human breast cancer cell line MCF-7 as well as the involved mechanisms. We found that sera harvested from the heifers following one month of zeranol implantation were more mitogenically potent in stimulating the proliferation of MCF-7 cells when compared to sera harvested from the same heifers before zeranol implantation and the control heifers. Further investigation found that dextran-coated charcoal suppressed the stimulating effect of the sera on MCF-7 cell growth. The mechanisms involved in the MCF-7 cell proliferation stimulated by zeranol-containing sera may include up-regulation of cyclin D1 and down-regulation of p53 and p21 expression at the mRNA and protein levels in the cells. The results suggest that the consumption of beef products containing biologically active residues of zeranol or its metabolites is a risk linked to breast cancer development. Further investigation is required in order to clarify this critical issue.