γ-Tocotrienol inhibits cell viability through suppression of β-catenin/Tcf signaling in human colon carcinoma HT-29 cells

γ-Tocotrienol, a major component of the tocotrienol-rich fraction of palm oil, has been suggested to have antioxidant and anticancer activity as well as potent chemopreventive effects on tumor cells. In this study, the mechanisms underlying γ-tocotrienol-mediated growth inhibition of human carcinoma HT-29 cells were further investigated, especially in correlation with the involvement of β-catenin/T-cell factor (Tcf) signaling pathway. We found that γ-tocotrienol could strongly suppress the transcriptional activity of β-catenin/Tcf signaling pathway in HT-29 cells. γ-Tocotrienol inhibited the expression level of total β-catenin protein but did not significantly affect the phosphorylated β-catenin level. Meanwhile, γ-tocotrienol down-regulated the protein level of nuclear β-catenin and induced its redistribution to cell membrane. Furthermore, γ-tocotrienol suppressed the expression of downstream target genes such as c-myc, cyclin D1 and survivin. The results demonstrated that γ-tocotrienol-inhibited growth and -induced apoptosis in HT-29 cells were accompanied by significant inhibition of β-catenin/Tcf signaling. Blocking the expression of β-catenin with small interfering RNA significantly suppressed the ability of γ-tocotrienol to reduce viability and induce apoptosis in HT-29 cells. Thus, our data suggested that γ-tocotrienol exerts its anticancer activity through β-catenin/Tcf signaling, and β-catenin is a target for γ-tocotrienol in the Wnt/β-catenin signaling pathway.