Inhibition of Wnt Signaling by Silymarin in Human Colorectal Cancer Cells

Silymarin from milk thistle (Silybum marianum) has been reported to show an anti-cancer activity. In previous study, we reported that silymarin induces cyclin D1 proteasomal degradation through NF-kappa B-mediated threonine-286 phosphorylation. However, mechanism for the inhibition of Wnt signaling by silymarin still remains unanswered. Thus, we investigated whether silymarin affects Wnt signaling in human colorectal cancer cells to elucidate the additional anti-cancer mechanism of silymarin. Transient transfection with a TOP and FOP FLASH luciferase construct indicated that silymarin suppressed the transcriptional activity of beta-catenin/TCF. Silymarin treatment resulted in a decrease of intracellular beta-catenin protein but not mRNA. The inhibition of proteasome by MG132 and GSK3 beta inhibition by SB216763 blocked silymarin-mediated downregulation of beta-catenin. In addition, silymarin increased phosphorylation of beta-catenin and a point mutation of S33Y attenuated silymarin-mediated beta-catenin downregulation. In addition, silymarin decreased TCF4 and increased Axin expression in both protein and mRNA level. From these results, we suggest that silymarin-mediated downregulation of beta-catenin and TCF4 may result in the inhibition of Wnt signaling in human colorectal cancer cells.