Tocotrienols induce apoptosis in breast cancer cell lines via an endoplasmic reticulum stress-dependent increase in extrinsic death receptor signaling

Tocotrienols are naturally occurring forms of vitamin E based on their structural similarity. This study focused on investigating anticancer effects of tocotrienols and the mechanisms of apoptosis induction by tocotrienols in vivo and in vitro. Dietary delivery of gamma-tocotrienol (gamma-T3) suppressed tumor growth in a syngeneic implantation mouse mammary cancer model by inhibiting cell proliferation and inducing apoptosis. In cell culture studies, gamma-T3 inhibited colony formation of a mouse mammary cancer cell line and human breast cancer cell lines. The anti-proliferative effects of tocotrienols were highly correlated with an increase in apoptosis based on Annexin V assessment. Treatment of human MDA-MB-231 and MCF-7 cells with gamma-T3 induced cleavages of PARP as well as caspase-8, -9, and -3. Additional analyses showed that gamma-T3 activated c-Jun NH2-terminal kinase (JNK) and p38 MAPK, and upregulated death receptor 5 (DR5) and C/EBP homologous protein (CHOP), an endoplasmic reticulum (ER) stress marker. Silencing either JNK or p38 MAPK reduced the increase in DR5 and CHOP and partially blocked gamma-T3-induced apoptosis. Both DR5 and CHOP upregulation were required for gamma-T3-induced apoptosis, and DR5 was transcriptionally regulated by CHOP after gamma-T3 treatment. Moreover, gamma-T3 increased the level of other ER-stress markers. Taken together, these results suggest that upregulation of DR5 by gamma-T3 treatment is dependent on JNK and p38 MAPK activation which is mediated by ER-stress.