A fatty acid found in olive oil may reduce the production of a protein from a gene associated with breast cancer, according to a new study. The findings may explain why a Mediterranean diet rich in olive oil appears to protect against breast cancer.

Researchers found that the oleic acid (search) found in olive oil dramatically reduces the levels of a protein produced by the breast cancer gene Her-2/neu (search), which occurs in more than a fifth of women with breast cancer and is associated with particularly aggressive tumors. Olive oil is the richest natural source of this fatty acid.

Doctors use Her-2/neu to help predict treatment response in women with breast cancer.

"To our knowledge this is the first report that a dietary monounsaturated fatty acid previously suggested to be protective against breast cancer significantly down-regulates the expression of Her-2/neu, cutting it by up to 46 percent, says researcher Javier Menendez, PhD, an assistant professor at the Northwestern University Feinberg School of Medicine, in a news release.

In addition, researchers found that oleic acid’s effect on this breast cancer gene also enhanced the effectiveness of the breast cancer drug Herceptin in at least two ways. First, by promoting the death of breast cancer cells with high levels of this gene, and second, by decreasing the accumulation of a protein thought to foster resistance to Herceptin treatment. This protein also plays a key role in the onset and progression of Her-2/neu breast cancer.

Although previous studies of people in southern Europe who eat a diet rich in olive oil appear to have a lower risk of breast cancer, researchers say that until now very little has been known about any specific mechanisms that may explain olive oil’s potential anticancer effects.

Olive Oil Ingredient May Reduce Breast Cancer Risk

In the study, which appears in today’s edition of the Annals of Oncology, researchers analyzed the effects of oleic acid on breast cancer cells in the laboratory. They also evaluated oleic acid’s effect on the breast cancer drug Herceptin (search).

The study showed that oleic acid decreased the production of proteins produced by the breast cancer gene Her-2/neu by up to 36 percent. Researchers say this anticancer effect was almost as strong as that produced with Herceptin treatment, which reduced expression by up to 48 percent.

When the two treatments were combined, researchers found that the two substances appeared to work even more effectively and reduced protein production by up to 70 percent.

Menendez says the findings should not only help researchers better understand olive oil’s potentially anticancer effects, but they also suggest that dietary changes that increase the amount of olive oil in the diet may be delay or prevent resistance to the drug Herceptin in some breast cancer patients.

By Jennifer Warner, reviewed by Brunilda Nazario, MD

SOURCES: Menendez, J. Annals of Oncology, Jan. 10, 2005. News release, European Society for Medical Oncology.