Abstract
Targeting endoplasmic reticulum (ER) stress is being investigated for its anticancer effect in various cancers, including cervical cancer. However, the molecular pathways whereby ER stress mediates cell death remain to be fully elucidated. In this study, we confirmed that ER stress triggered by compounds such as brefeldin A (BFA), tunicamycin (TM), and thapsigargin (TG) leads to the induction of the unfolded protein response (UPR) in cervical cancer cell lines, which is characterized by elevated levels of inositol-requiring kinase 1α, glucose-regulated protein-78, and C/EBP homologous protein, and swelling of the ER observed by transmission electron microscope (TEM). We found that BFA significantly increased autophagy in tumor cells and induced TC-1 tumor cell death in a dose-dependent manner. BFA increased punctate staining of LC3 and the number of autophagosomes observed by TEM in TC-1 and HeLa cells. The autophagic flux was also assessed. Bafilomycin, which blocked degradation of LC3 in lysosomes, caused both LC3I and LC3II accumulation. BFA initiated apoptosis of TC-1 tumor cells through activation of the caspase-12/caspase-3 pathway. At the same time, BFA enhanced the phosphorylation of IκBα protein and translocation into the nucleus of NF-κB p65. Quinazolinediamine, an NF-κB inhibitor, attenuated both autophagy and apoptosis induced by BFA; meanwhile, it partly enhances survival of cervical cancer cells following BFA treatment. In conclusion, our results indicate that the cross-talk between ER stress, autophagy, apoptosis, and the NF-κB pathways controls the fate of cervical cancer cells. Careful evaluation should be given to the addition of an NF-κB pathway inhibitor to treat cervical cancer in combination with drugs that induce ER stress-mediated cell death.
Highlights
Endoplasmic reticulum (ER) stress is associated with the progression of cancer
Our studies showed that induction of ER stress led to the activation of the unfolded protein response (UPR) in cervical tumor cells, which was characterized by elevated levels of IRE1a, GRP-78, and the swelling ER
QNZ, a NF-κB pathway inhibitor, decreased the autophagy and apoptosis, and attenuated cervical tumor cell death induced by brefeldin A (BFA) (Figure 7b)
Summary
Endoplasmic reticulum (ER) stress is associated with the progression of cancer. Cellular adaptation to ER stress is mediated by the unfolded protein response (UPR).[1,2] The UPR is mainly induced by three signaling sensors, inositol requiring enzyme 1α (IRE1α), protein kinase R-like ER kinase (PERK), and activating transcription factor 6α. The autophagic flux was assessed, and bafilomycin decreased the degradation of LC3 in lysosomes, which in turn caused both LC3I and LC3II accumulation in TC-1 tumor cells and HeLa cells (Figure 3i) These results showed that BFA can promote autophagy in cervical cancer cells, suggesting that autophagy is the preferred route for degradation of proteins during UPR activation. We found that blocking the NF-κB pathway with QNZ attenuated the induction of LC3II following treatment with BFA in cervical tumor cells (Figures 6a and b), suggesting that QNZ partly inhibits autophagy-induced BFA. QNZ enhanced activation of the CHOP pathway in TC-1 tumor cells and Atg5+/+ and Atg5 − / − MEF cells (Supplementary Figure 2) These results indicate that blocking NF-κB pathway activity by QNZ inhibited autophagy and apoptosis, partly enhancing survival of cervical cancer cells following BFA treatment
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