No abstract

Aberrant activation of Wnt/␤-catenin signaling and subsequent up-regulation of ␤-catenin response transcription (CRT) is a critical event in the development of human colon cancer. Thus, Wnt/␤-catenin signaling is an attractive target for the development of anticancer therapeutics. In this study, we identified hexachlorophene as an inhibitor of Wnt/␤-catenin signaling from cell-based small-molecule screening. Hexachlorophene antagonized CRT that was stimulated by Wnt3a-conditioned medium by promoting the degradation of ␤-catenin. This degradation pathway is Siah-1 and adenomatous polyposis colidependent, but glycogen synthase kinase-3␤ and F-box ␤-transducin repeat-containing protein-independent. In addition, hexachlorophene represses the expression of cyclin D1, which is a known ␤-catenin target gene, and inhibits the growth of colon cancer cells. Our findings suggest that hexachlorophene attenuates Wnt/␤-catenin signaling through the Siah-1-mediated ␤-catenin degradation.

(−)-Epigallocatechin-3-gallate (EGCG), the major polyphenol in green tea, has been reported to inhibit the Wnt/β-catenin pathway, which is aberrantly up-regulated in colorectal cancers, but its precise mechanism of action remains unclear. Here, we used a sensitive cell-based system to demonstrate that EGCG suppresses β-catenin response transcription (CRT), activated by Wnt3a-conditioned medium (Wnt3a-CM), by promoting the degradation of intracellular β-catenin. EGCG induced β-catenin N-terminal phosphorylation at the Ser33/37 residues and subsequently promoted its degradation; however, this effect was not observed for oncogenic forms of β-catenin. Pharmacological inhibition or depletion of glycogen synthase kinase-3β (GSK-3β) did not abrogate the EGCG-mediated β-catenin degradation. EGCG did not affect the activity and expression of protein phosphatase 2A (PP2A). Consistently, the phosphorylation and degradation of β-catenin was found in adenomatous polyposis coli (APC) mutated colon cancer cells after EGCG treatment. EGCG repressed the expression of cyclin D1 and c-myc, which are β-catenin/T-cell factor-dependent genes, and inhibited the proliferation of colon cancer cells. Our findings suggest that EGCG exerts its cancer-preventive or anticancer activity against colon cancer cells by promoting the phosphorylation and proteasomal degradation of β-catenin through a mechanism independent of the GSK-3β and PP2A.