Chemoprevention of tocotrienols: the mechanism of antiproliferative effects.

Tocotrienols have been reported as antitumor agents and widely commercialized as an antioxidant dietary supplement. Tocotrienols have more significant biological activity than tocopherols, although serum level of tocotrienols is much lower than that of tocopherols. This may be because intracellular concentration of tocotrienols was revealed to be significantly higher compared with tocopherols, and tocotrienol accumulation is observed in tumor. Previous reports have suggested antiproliferative effect, induction of apoptosis, modulation of cell cycle, antioxidant activity, inhibition of angiogenesis, and suppression of 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase activity as anticarcinogenesis mechanisms of tocotrienols both in vivo and in vitro. Extension of the duration of host survival was observed in tumor-implanted mice treated with tocotrienol. Tocotrienols induce apoptosis mainly via mitochondria-mediated pathway. Cell cycle arrest is due to suppression of cyclin D by tocotrienols. Tocotrienols also inhibit vascularization-reducing proliferation, migration and tube formation. Malignant proliferation demands elevation of HMG CoA reductase activity, and tocotrienols suppress its activity. Tocotrienol treatment decreases oncogene expression and increases the level of tumor suppressors. Only a few clinical trials to determine the effects of tocotrienol on cancer prevention or treatment have been carried out. There is no convincing or probable evidence of the role of tocotrienols in cancer prevention, while alpha-tocopherol has been suggested to have a limited anti-prostate cancer potential. Neither beneficial activity nor adverse effect of tocotrienol has sufficiently been explored so far. The above-mentioned mechanisms of tocotrienols seem to be promising for cancer prevention; however, further clinical studies are warranted to assess the efficacy and safety of tocotrienol.