Intracellular signaling mechanisms mediating the antiproliferative and apoptotic effects of gamma-tocopherol in neoplastic mammary epithelial cells

Tocotrienols, a subgroup within the vitamin E family of compounds, display potent antiproliferative and apoptotic activity against neoplastic mammary epithelial cells at treatment doses that have little or no effect on normal cell growth and function. Recent studies have shown that treatment with a growth inhibitory, but non-cytotoxic dose (4 micromolar) of gamma-tocotrienol inhibits phosphatidylinositol-3-kinase-dependent kinase (PI3K)/PI3K-dependent kinase 1 (PDK-1)/mitogenic signaling over a 2-3 day period following treatment exposure, and these effects were not found to be associated with an increased in either phosphatase and tensin homologue deleted from chromosome 10 (PTEN) or protein phosphatase type 2A (PP2A) phosphatase activity. In addition, this treatment caused a large decrease in NFkappaB transcriptional activity, apparently by suppressing IkappaB-kinase (IKK)-alpha/beta activation, an enzyme associated with inducing NFkappaB activation. Since Akt and NFkappaB are intimately involved in mammary tumor cell proliferation and survival, these findings strongly suggest that the antiproliferative effects of gamma-tocotrienol result, at least in part, from a reduction in Akt and NFkappaB activity. In contrast, treatment with 20 micromolar gamma-tocotrienol (cytotoxic dose) resulted in caspase-8 and -3 activation and apoptosis. It was also shown that this same treatment caused a rapid and large decrease in PI3K/PDK/Akt signaling within 2-4 h following treatment exposure, and a corresponding decrease in intracellular levels of FLIP, an antiapoptotic protein that inhibits caspase-8 activation. In summary, both the antiproliferative and apoptotic effects of gamma-tocotrienol appear to be mediated by a reduction in the PI3K/PDK-1/Akt signaling, an important pathway associated with cell proliferation and survival in neoplastic mammary epithelial cells.