Accelerated Degradation of Caspase-8 Protein Correlates with TRAIL Resistance in a DLD1 Human Colon Cancer Cell Line

The tumor-selective cytotoxic effect of tumor necro-sis factor–related apoptosis-inducing ligand (TRAIL) makes TRAIL an attractive candidate as an anticancer agent. However, resistance to TRAIL poses a challenge in anticancer therapy with TRAIL. Therefore, character-izing the mechanisms of resistance and developing strategies to overcome the resistance are important steps toward successful TRAIL-mediated cancer ther-apy. In this study, we investigated mechanisms of ac-quired TRAIL resistance in a colon cancer DLD1 cell line. Compared with the TRAIL-susceptible DLD1 cell line, TRAIL-resistant DLD1/TRAIL-R cells have a low level of caspase-8 protein, but not its mRNA. Suppres-sion of caspase-8 expression by siRNA in parental DLD1 cells led to TRAIL resistance. Restoration of caspase-8 protein expression by stable transfection rendered the DLD1/TRAIL-R cell line fully sensitive to TRAIL protein, suggesting that the low level of caspase-8 protein expression might be the culprit in TRAIL resistance in DLD1/TRAIL-R cells. Sequencing analysis of the caspase-8 coding region revealed a missense mutation that is present in both TRAIL-sensitive and TRAIL-resistant DLD1 cells. Subsequent study showed that the degradation of caspase-8 protein was accelerated in DLD1/TRAIL-R cells com-pared to parental DLD1 cells. Thus, accelerated degradation of caspase-8 protein is one of the mech-anisms that lead to TRAIL resistance