Metformin induces apoptosis in TRAIL-resistant colorectal cancer cells

Resistance to chemotherapy drugs, which commonly occurs during the treatment of colorectal cancer (CRC), can lead to tumor recurrence and metastasis, so combinational treatment strategies according to the cancer cell type are urgently needed to overcome drug resistance and increase therapeutic efficiency. To this end, the tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) is a promising anticancer strategy. Some CRC cell lines such as SW620 have low sensitivity to TRAIL, so additional sensitizers are required to make the strategy effective. Therefore, we focused on the apoptotic effect of combinational metformin and TRAIL treatment on TRAIL-resistant SW620 cells. Treatment with TRAIL alone did not induce apoptosis whereas combined treatment with metformin and TRAIL significantly increased it. TRAIL activated caspases through an extrinsic pathway but increased resistance to apoptosis through the protein kinase B or AKT (PKB/AKT)/mammalian target of rapamycin (mTOR) pathway. On the other hand, metformin reduced the inhibitory effect of X-linked inhibitor of apoptosis (XIAP) by blocking the AKT and nuclear factor kappa B (NF-κB) pathways and activated CCAAT-enhancer-binding protein homologous protein (CHOP) via endoplasmic reticulum (ER) stress but without inducing apoptosis. In addition, metformin induced cell-cycle arrest, thereby blocking cell proliferation and growth. These results were also confirmed through an in vivo mouse xenograft CRC model, in which combined treatment with metformin and TRAIL induced tumor cell death, thus demonstrating the anticancer effect of their coadministration. Therefore, cotreatment of metformin and TRAIL could be an effective anticancer treatment strategy for TRAIL-resistant CRC.