Sensitizing TRAIL-resistant A549 lung cancer cells and enhancing TRAIL-induced apoptosis with the antidepressant amitriptyline

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a cytokine with the potential to induce cancer cell-specific apoptosis with minimal toxicity to normal cells. Therefore, the resistance of certain cancer cells to TRAIL is a major concern and agents that can either enhance TRAIL capabilities or overcome TRAIL resistance are necessary for the development of cancer treatments. The present study investigated whether the antidepressant drug amitriptyline could sensitize TRAIL-resistant A549 lung cancer cells and enhance TRAIL-induced apoptosis. Antidepressants are usually prescribed to cancer patients to relieve emotional distress, such as depression or dysthymia. The present study revealed for the first time, to the best of our knowledge, that amitriptyline increased death receptor (DR) 4 and 5 expression, a requirement for TRAIL-induced cell death. Genetic inhibitors of DR4 and DR5 significantly reduced amitriptyline-enhanced TRAIL-mediated apoptosis. Additionally, the present study explored whether blocking autophagy increased DR4 and DR5 expression. Blocking autophagy flux with the final stage autophagy inhibitor chloroquine (CQ) also upregulated DR4 and DR5 expression. TRAIL in combination with amitriptyline or CQ significantly increased the expression of apoptosis-indicator proteins cleaved caspase-8 and caspase-3. The expression levels of LC3-II and p62 were significantly higher in amitriptyline-treated cells, which confirmed that amitriptyline blocks autophagy by inhibiting the fusion of autophagosomes with lysosomes. Overall, the present results contributed to understanding the mechanism responsible for the synergistic anticancer effect of amitriptyline and TRAIL and also presented a novel mechanism involved in DR4 and DR5 upregulation.