The Journal of Nutrition Vol. 127 No. 3 March 1997, pp. 544S-548S
Copyright ©1997 by the American Society for Nutritional Sciences

Inhibition of Proliferation of Estrogen Receptor-Negative MDA-MB-435 and -Positive MCF-7 Human Breast Cancer Cells by Palm Oil Tocotrienols and Tamoxifen, Alone and in Combination

Najla Guthrie, Abdul Gapor^*, Ann F. Chambers, and Kenneth K. Carroll

Centre for Human Nutrition, Department of Biochemistry, The University of Western Ontario, London, Ontario N6A 5C1, Canada; ^* Palm Oil Research Institute of Malaysia, 50720, Kuala Lumpur, Malaysia; and  Department of Oncology, The University of Western Ontario, London Regional Cancer Centre, London, Ontario N6A 4L6, Canada

Tocotrienols are a form of vitamin E, having an unsaturated isoprenoid side-chain rather than the saturated side-chain of tocopherols. The tocotrienol-rich fraction (TRF) from palm oil contains -tocopherol and a mixture of -, - and -tocotrienols. Earlier studies have shown that tocotrienols display anticancer activity. We previously reported that TRF, -, - and -tocotrienols inhibited proliferation of estrogen receptor-negative MDA-MB-435 human breast cancer cells with 50% inhibitory concentrations (IC₅₀) of 180, 90, 30 and 90 µg/mL, respectively, whereas -tocopherol had no effect at concentrations up to 500 µg/mL. Further experiments with estrogen receptor-positive MCF-7 cells showed that tocotrienols also inhibited their proliferation, as measured by [³H] thymidine incorporation. The IC₅₀s for TRF, -tocopherol, -, - and -tocotrienols were 4, 125, 6, 2 and 2 µg/mL, respectively. Tamoxifen, a widely used synthetic antiestrogen inhibits the growth of MCF-7 cells with an IC₅₀ of 0.04 µg/mL. We tested 1:1 combinations of TRF, -tocopherol and the individual tocotrienols with tamoxifen in both cell lines. In the MDA-MB-435 cells, all of the combinations were found to be synergistic. In the MCF-7 cells, only 1:1 combinations of - or -tocotrienol with tamoxifen showed a synergistic inhibitory effect on the proliferative rate and growth of the cells. The inhibition by tocotrienols was not overcome by addition of excess estradiol to the medium. These results suggest that tocotrienols are effective inhibitors of both estrogen receptor-negative and -positive cells and that combinations with tamoxifen should be considered as a possible improvement in breast cancer therapy.