Tocotrienols belong to the
vitamin E family of chemicals known to have potent anti-proliferative and apoptotic activities against a variety of
cancer cells with little to no comparable influence on the normal cells. Whether
tocotrienols control the expression of phase II
antioxidant enzymes in the context of their anti-carcinogenic mechanisms has not been investigated. The present studies were performed to test whether the differential growth inhibition resulting from exposure to α-, γ- and δ-
tocotrienols in
estrogen receptor-positive human MCF-7 and
estrogen receptor-negative MDA-MB-231
breast cancer cells might be accompanied by changes in phase II
antioxidant enzymes. Cell proliferation and clonogenicity in both cell lines were significantly inhibited by γ- and δ-
tocotrienols with little affect when cells were similarly exposed to α-
tocotrienol, at doses up to 10 μM. The expression and activity of several
antioxidant enzymes in 10 μM
tocotrienol-treated cells were determined by Western blot and biochemical assays. In MDA-MB-231 cells, δ- was more active than α- or γ-
tocotrienols in up-regulating
glutathione peroxidase; however, the three
tocotrienols had comparable activity in inducing
thioredoxin. In MCF-7 cells, expression of
quinone reductase 2 and
thioredoxin was increased by γ- and δ-
tocotrienols, whereas
quinone reductase 1 was unaffected by exposure to the
tocotrienols. The
tocotrienols also did not affect the expression and activity of
superoxide dismutase in both MCF-7 and MDA-MB-231 cells, but increased
catalase activity concomitant with slight reduction in the
catalase expression. In MDA-MB-231 cells, treatment by
tocotrienols led to several fold increase of NRF2 expression marked by corresponding decrease in KEAP1 levels. By contrast, no significant change in NRF2 and KEAP1 levels was observed in MCF-7 cells. These studies demonstrate that different
tocotrienols show distinct and selective activity in regulating the NRF2-KEAP1, in coordination with the induced expression of cytoprotective oxidative stress modulatory genes and regulation of proliferation in
breast cancer cells.