Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), a member of the tumor necrosis factor superfamily, is in clinical trials for cancer therapy, but its anticancer potential is limited by the development of resistance. We investigated the ability of tocotrienol (T3), an unsaturated vitamin E present in palm oil, rice bran, barley, oats, and wheat germ, to sensitize tumor cells to TRAIL. Results from esterase staining, colony formation, caspase activation, and sub-G(1) cell cycle arrest revealed that gamma-T3 can sensitize human colon cancer cells to TRAIL. When examined for the mechanism, we found that gamma-T3 significantly downregulated the expression of antiapoptotic proteins (c-IAP2 and Bcl-xL). We also found that gamma-T3, but not tocopherol, induced the expression of the TRAIL receptors death receptor (DR)-4 and DR5. This induction was not cell type specific, as upregulation was also found in pancreatic, kidney, and leukemic cells. Upregulation of DRs by gamma-T3 required the production of reactive oxygen species (ROS), and sequestering of ROS abolished both upregulation of the receptors and potentiation of TRAIL-induced apoptosis. Induction of DRs by gamma-T3 also required activation of extracellular signal-regulated kinase 1 (ERK1), as silencing of ERK1 by specific siRNA abrogated the upregulation of TRAIL receptors. Further, induction of DRs by gamma-T3 required the expression of p53 and Bax, as no induction of the receptors was found in colon cancer cells with deletion of these genes. Overall, our results show that gamma-T3 sensitizes tumor cells to TRAIL by upregulating DRs through the ROS/ERK/p53 pathway and by downregulating cell survival proteins.
