Impaired Autophagic Flux by Citalopram Inhibits DR5 Degradation and Increases TRAIL-Mediated Apoptosis

To overcome TRAIL resistance, we tested the antidepressant drug citalopram (CTL) in combination with TRAIL. The resistance of several types of cancer cells to TRAIL impedes TRAIL-induced cancer cell death. In this study, we investigated the role of, and molecular mechanism by which, the antidepressant CTL-induced cell death in TRAIL-resistant lung cancer cells. We found that CTL increased death receptor 5 (DR5) expression levels by impairing autophagic flux and protecting against lysosomal degradation, thereby increasing the TRAIL-induced apoptosis of TRAIL-resistant A549 lung cancer cells. We also found that CTL impaired autophagic flux and promoted the conversion of light chain 3 (LC3)-I to its lipid-conjugated form, LC3-II, thereby inducing autophagosome accumulation. Our hypothesis that impaired autophagic flux plays an important role in the upregulation of DR5 being confirmed when we determined that DR5 upregulation by CTL was markedly decreased in the presence of rapamycin, an autophagy inducer. Further verification of our theory was achieved through experiments pairing CTL with the early-stage autophagy inhibitor 3-methyladenine (3-MA) and the late-stage autophagy inhibitor chloroquine (CQ). CQ inhibits autophagy by impairing autophagosome–lysosome fusion. Both CTL and CQ markedly increased DR5 expression levels and increase TRAIL-induced apoptosis, whereas 3-MA marginally enhanced TRAIL-induced apoptosis and resulted in minimal DR5 expression. In summary, our findings indicate that CTL impairs autophagic flux, resulting in autophagosome accumulation and augmentation of DR5 to potentiate TRAIL-induced apoptosis, suggesting that CTL may act as a therapeutic agent that sensitizes TRAIL-resistant cancer cells to TRAIL-mediated cancer therapy.