Tocotrienols Inhibited Growth and Induced Apoptosis in Human HeLa Cells Through the Cell Cycle Signaling Pathway

Tocotrienols of palm oil have been shown to possess potent neuroprotective, antioxidative, anticancer, and cholesterol-lowering activities. In this study, the authors examined the antiproliferative effects of alpha-, gamma- and delta-tocotrienols (alpha T3, gamma T3, and delta T3), and alpha-tocopherol (alpha T) in human cervical carcinoma (HeLa) cells. Their mechanism(s) of action on cell cycle signaling pathway were also investigated. Results showed that the antiproliferative effect of alpha T3 (IC50: 3.19 +/- 0.05 mu M) and gamma T3 (IC50: 2.85 +/- 0.07 mu M) was more potent than delta T3 (IC50: > 100 mu M) and alpha T (IC50: 69.46 +/- 3.01 mu M). Both alpha T3 and gamma T3 also demonstrated a dose-dependent and time-dependent induction of cell death. They caused cell cycle arrest at G2/M phase and triggered apoptosis as displayed by the externalization of annexin V-targeted phosphatidylserine and accumulation of sub-G1 peak. At a concentration of 3 mu M, alpha T3 downregulated the expression of cyclin D3, p16, and CDK6, while having no effect on cyclin D1, p15, p21, p27, and CDK4 expression. However, gamma T3 showed no effect on these proteins. The induction of HeLa cell apoptosis by alpha T3 and gamma T3 appeared to be associated with the expression of IL-6, but not the other cytokines (IFN-gamma, IL-2, and IL-10). Taken together, the results suggest that alpha T3 and gamma T3 are more effective than delta T3 and alpha T in inhibiting HeLa cell proliferation, and their mode of action could be through the upregulation of IL-6, and the downregulation of cyclin D3, p16, and CDK6 expression in the cell cycle signaling pathway.