4-O-Methylascochlorin Synergistically Enhances 5-Fluorouracil-Induced Apoptosis by Inhibiting the Wnt/β-Catenin Signaling Pathway in Colorectal Cancer Cells

4-O-Methyl-ascochlorin (MAC), a derivative of the prenyl-phenol antibiotic ascochlorin extracted from the fungus Ascochyta viciae, shows anticarcinogenic effects on various cancer cells. 5-Fluorouracil (5-FU) is used to treat colorectal cancer (CRC); however, its efficacy must be enhanced. In this study, we investigated the molecular mechanisms by which MAC acts synergistically with 5-FU to inhibit cell proliferation and induce apoptosis in CRC cells. MAC enhanced the cytotoxic effects of 5-FU by suppressing the Akt/mTOR/p70S6K and Wnt/beta-catenin signaling pathways. It also reduced the viability of 5-FU-resistant (5-FU-R) cells. Furthermore, expression of anti-apoptosis-related proteins and cancer stem-like cell (CSC) markers by 5-FU-R cells decreased in response to MAC. Similar to MAC, the knockdown of CTNNB1 induced apoptosis and reduced expression of mRNA encoding CRC markers in 5-FU-R cells. In summary, these results suggest that MAC and other beta-catenin modulators may be useful in overcoming the 5-FU resistance of CRC cells.