Using culture dishes, Trosko and colleagues grew miniature human breast tumors -- mammospheres -- that activated the stem cell gene Oct4A. Then the mammospheres were exposed to natural estrogen, a known growth factor and potential breast tumor promoter, and man-made chemicals that are known to promote tumors or disrupt the endocrine system.
The study, published in the journal PLoS One, found that estrogen and the chemicals caused the mammospheres to increase in numbers and size.
However, with metformin added, the numbers and size of the mammospheres were dramatically reduced. While each of the chemicals enhanced growth by different means, metformin seemed to inhibit their stimulated growth in all cases, Trosko said.
"While future studies are needed to understand the exact mechanism by which metformin works to reduce the growth of breast cancers, this study reveals the need to determine if the drug might be used as a preventive drug and for individuals who have no indication of any existing cancers," Trosko said.