The team from Northwestern University also found that when they treated breast cancer cells that overexpressed Her-2/neu with GLA there was a 30- to 40-fold increased response in the cells to the drug Herceptin (trastuzumab), a commonly used treatment for this cancer.
"In our tests, treating the cancer cell lines with both GLA and Herceptin led to a synergistic increase in apoptosis [cell death] and reduced cancer growth," said lead author Ruth Lupu, director of Evanston Northwestern Healthcare Breast Cancer Translational Research Program.
She added that although further studies are necessary before GLA can enter clinical trials, "these findings may reveal a previously unrecognized way of influencing the poor outcome of Her-2/neu-positive cancer patients".
Breast cancer patients with Her-2/neu-positive tumours have an aggressive form of the disease and a poor prognosis, according to the authors of the study published in the 2 November issue of the Journal of the National Cancer Institute.
Co-investigator Javier Menendez added that GLA's inhibition of Her-2/neu works in a different manner from that of Herceptin.
"While Herceptin attempts to neutralize thousands of Her-2/neu molecules commonly found in the surface of overexpressing cancer cells, GLA would be more efficient to reduce Her-2/neu levels by preventing the transcription of few Her-2/neu gene copies," Menendez explained.
Menendez previously showed that oleic acid, the major fatty acid in olive oil, suppresses over-expression of the Her-2/neu gene and, like GLA, boosts the effectiveness of Herceptin.
"Considering that activation and overexpression of the Her-2/neu oncogene are crucial events in the cause, progression and cell sensitivity to various treatments in breast cancer, results of the study reveal a valuable means by which an inexpensive herbal medicine might regulate breast cancer cell growth, metastasis formation and response to chemotherapies and endocrine therapies," Lupu said.
GLA exerts selective toxic effects on cancer cells without affecting normal cells. Menendez's earlier research showed that supplementation with GLA sensitizes breast cancer cells to some chemotherapeutic drugs, such as paclitaxel (Taxol), docetaxel (Taxotere) and vinorelbine (Navelbine).
Lupu and Menendez recently demonstrated that GLA also enhances the efficacy of anti-estrogens, such as tamoxifen and Faslodex.
"Since overexpression of Her-2/neu generally confers resistance to chemo- and endocrine therapies, our current findings can explain why GLA increases the efficacy of breast cancer treatments," Menendez said.
Besides evening primrose oil, other sources of GLAs include borage oil and black current seed oil.