Tocotrienols, a subgroup within the
vitamin E family of compounds, have been shown to display potent anticancer activity and inhibit preneoplastic and neoplastic mammary epithelial cell proliferation at treatment doses that have little or no effect on normal cell growth and function. However, the specific intracellular mechanisms mediating the antiproliferative effects of
tocotrienols are presently unknown. Because Akt and
nuclear factor kappaB (NFkappaB) are intimately involved in mammary
tumor cell proliferation and survival, studies were conducted to determine the effects of
gamma-tocotrienol on Akt and NFkappaB activity in neoplastic +SA mammary epithelial cells in vitro. Treatment with 0-8 microM
gamma-tocotrienol for 0-3 days caused a dose-responsive inhibition in +SA cell growth and mitotic activity, as determined by MTT colorimetric assay and
proliferating cell nuclear antigen immunocytochemical staining, respectively. Studies also showed that treatment with 4 microM
gamma-tocotrienol, a dose that inhibited +SA cell growth by more than 50% compared with that of untreated control cells, decreased intracellular levels of activated phosphotidylinositol 3-kinase-dependent
kinase (PI3K)-dependent
kinase 1 (phospho-PDK-1) and Akt, and reduced phospho-Akt
kinase activity. Furthermore, these effects were not found to be associated with an increase in either
phosphatase and
tensin homologue deleted from chromosome 10 (PTEN) or
protein phosphatase type 2A
phosphatase activity. In addition,
gamma-tocotrienol treatment was shown to decrease NFkappaB transcriptional activity, apparently by suppressing the activation of
IkappaB-kinase-alpha/beta, an
enzyme associated with inducing NFkappaB activation. In summary, these findings demonstrate that the antiproliferative effects of
gamma-tocotrienol result, at least in part, from a reduction in Akt and NFkappaB activity in neoplastic +SA mammary epithelial cells.