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Tocopherols and tocotrienols represent the two subgroups within the vitamin E family of compounds. However, tocotrienols are significantly more potent than tocopherols in suppressing EGF-dependent neoplastic mammary epithelial cell growth in vitro. However, the exact intracellular mechanism mediating the growth inhibitory effects of tocotrienols had not been determined. Since activation of the PI3K/PDK/Akt signaling pathway is important for mediating EGF-induced mitogenesis in these cells, studies were conducted to investigate the effects of α-tocopherol and γ-tocotrienol on the activity of the specific intracellular signaling components within the PI3K/PDK/Akt mitogenic signaling pathways. Highly malignant mouse +SA mammary epithelial cells were grown in culture and maintained on serum-free media containing 10 ng/ml EGF as a mitogen. Treatment with 0-10 μM γ-tocotrienol resulted in a dose-responsive decrease, whereas treatment with 0-500 μM α-tocopherol had no affect on EGF-induced +SA cell growth over a 5-day culture period. Studies also showed that treatment with growth inhibitory doses of γ-tocotrienol caused a corresponding decrease, whereas as treatment with very high doses of α-tocopherol had no effect on the in intracellular levels of phospho-PDK1 (active) and phosphor-Akt (active) as determined by Western blot analysis. Although these results strongly suggested that the inhibitory effects of γ-tocotrienol on +SA cell growth is mediated through suppression of PI3K/PDK/Akt mitogenic signaling, they do not indicate the exact site action along this pathway that is targeted by γ-tocotrienol. Additional studies were conducted to determine the direct effect of γ-tocotrienol on the enzymatic activity of isolated components within the PI3K/PDK/Akt mitogenic signaling pathway. Treatment with growth inhibitory doses of γ-tocotrienol or very high doses of α-tocopherol was found to have little or no direct effect on isolated Akt activity, as determined with an Akt (PKB) kinase activity ELISA assay kit. However, additional studies showed that treatment with similar doses of γ-tocotrienol caused a corresponding 40% reduction in PI3K activity, while high doses of α-tocopherol had no direct effect on PI3K activity, as determined with a PI3K activity ELISA assay kit. In summary, these results demonstrate that the antiproliferative effects of γ-tocotrienol on the highly malignant +SA mammary epithelial cells is mediated, at least in part, through the direct inhibition of PI3K activity, and subsequent suppression of the PI3K/PDK/Akt mitogenic signaling pathway. Supported by NIH grant CA86833.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]