Metformin, a widely used agent in the treatment of type 2 diabetes, stopped a number of natural and man-made chemicals from stimulating the growth of breast cancer cells in a recent study (PLoS One. 2011;6[11]:e28068; www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0028068).

Persons with diabetes are at high risk for cancers of the liver, pancreas, endometrium, colon, bladder, and breast. Previous research had shown that long-term use of metformin, which inhibits transcription of gluconeogenesis genes, lowers the risk of some diabetes-related tumors, but not all studies have demonstrated this reponse.

To test whether metformin reduces breast cancer risk, the investigators heading the current project grew miniature human breast tumors known as mammospheres. The mammospheres, which activated the Oct4A stem cell gene, were exposed to natural estrogen as well as man-made chemicals known to promote tumors or disrupt the endocrine system.

The estrogen and chemicals caused the mammospheres to increase in number and size; however, the addition of metformin dramatically reduced cell count and size, despite the fact that the estrogen and each of the chemicals enhanced growth by different means.

Although the mechanism by which metformin achieved these results remains unclear, the findings suggest that the medication may be a potential preventive treatment for cancer.