B26

Tocotrienols, a subclass of vitamin E compounds, display potent anticancer and apoptotic activity with little or no effect on normal cell growth and function. However, little if any attempt has been made to examine its influence on carcinogenesis, which often are inhibited by protective phase II and antioxidant enzymes. Determining the chemopreventive mechanism of tocotrienols will provide essential information necessary for understanding the potential health benefits of these compounds in reducing the risk of breast cancer in women. The primary aim of this study is to test the potential chemoprotective effects of tocotrienols by studying the relationship between antioxidant-enzyme defence responses and cellular growth suppression in human MCF-7 (estrogen receptor-positive) and MDA-MB-231 (estrogen receptor-negative) breast cancer cells. Comparing the effects of α-, γ-, and δ-tocotrienols on breast cancer cell proliferation, we observed that γ- and δ-tocotrienols significantly induced stronger growth inhibition in both cell lines at 10 µM concentration and α-tocotrienol had no effect on the growth of breast caner cells at tested concentrations. Hence, further studies are conducted using 10 µM concentrations. The expression and activity of antioxidant defenses were assessed using Western blot analysis and biochemical assays, respectively. In MDA-MB-231 cells, δ-tocotrienol was significantly more effective in increasing the expression of thioredoxin and glutathione peroxidase than α- or γ-tocotrienols. All three tocotrienols had no effect on the expression and activity of superoxide dismutase in both MCF-7 and MDA-MB-231 cells. In MCF-7 cells, a moderate decrease in the expression of catalase was observed by α- and γ-tocotrienol treatment, but no difference in the expression of catalase was seen in MDAMB-231 cells. In MCF-7 cells, the activity of phase 2 detoxifying enzyme, glutathione-S-transferase was significantly induced by γ- and δ-tocotrienol, but the activity or expression of quinone reductase 1 was not altered by tocotrienols. The effects of tocotrienols on Nrf2 and KEAP1 expression were also examined by Western blot analysis. In MDA-MB-231 cells, treatment with α-, γ- and δ-tocotrienol lead to 2.6-, 1.9 and 3.2-fold increase, respectively, in Nrf2 protein level with concomitant decrease in KEAP1 expression. In MCF-7 cells, no significant changes in Nrf2 and KEAP1 levels were detected. The induction of antioxidant defence responses and phase 2 enzymes is more pronounced in the estrogen receptor-negative MDA-MB-231 cells, wherein response is prominent with γ- and δ-tocotrienol treatment. These studies demonstrate for the first time that the ability of a specific form of tocotrienols selectively regulates the Nrf2-KEAP1 system, which in turn induces the expression of cytoprotective genes. (Supported by a grant BCTR0600943 from the Susan G Komen Breast Cancer Foundation)

Sixth AACR International Conference on Frontiers in Cancer Prevention Research-- Dec 5-8, 2007; Philadelphia, PA