Regulation of cytotoxic drug-induced apoptosis by death receptor 5 in human colon cancer cells

5-Fluorouracil(5-FU)is a cytotoxic drug that is widely used for the treatment of colon cancer. However, 5-FU-resistance is still a common issue which needs to be overcome. Understanding the molecular pathway of 5-FU-induced apoptosis will help designing more effective cancer treatments. Chemotherapeutic drugs such as 5-FU are thought to trigger cell death via the intrinsic apoptosis pathway which involves caspase-9 activation as one of the initiating molecular events. However, our group recently found the new apoptotic pathways which are involved in 5-FU-induced apoptosisin which caspase-8 is an essential factor for 5-FU-induced cell death with caspase-9 being dispensable for this process. Aside from caspase-8, the death receptor TRAIL-R2 (DR5), which is usually involved in the extrinsic pathway of apoptosis, was found to be required for the execution of 5-FU-triggered cell death. Moreover,following 5-FU treatment, DR5 is not responsible for caspase-8 activation, but for the activation of c-Jun N-terminal kinases (JNK) without the involvement of its natural ligand TRAIL. In the current study, factors associated with DR5 that are involved in JNK activation are identified. Also, the involvement ofTRAF2, FADD, caspase-8, caspase-10 and RIP1 in JNK activation were examined. It was found that JNK is activated in HCT116, but not in DR5, caspase-8 and FADD knockdown cells after 5-FU treatment. Moreover, the absence of caspase-10, TRAF2 and RIP1 does not affect JNK activation.Also it was discovered that similar to 5-FU, when cells are treated with etoposide,another cytotoxic drug, JNK is also activated and initiated by DR5 and mediated by FADD and caspase-8. Thus, in the present study, the factors that are involved in activating JNK after 5-FU and etoposide treatment have been identified. The obtained results further clarify the mechanism of cell death in colon cancer cells after 5-FU and etoposide treatment which constitutes a novel regulating pathway of apoptosis.