Roles of endoplasmic reticulum stress and autophagy on H(2)O(2)-induced oxidative stress injury in HepG2 cells

Endoplasmic reticulum stress (ERS) can be induced by a variety of physiological and pathological factors including oxidative stress, which triggers the unfolded protein response to deal with ERS. Autophagy has been hypothesized to be a means for tumor cells to increase cell survival under conditions of hypoxia, metabolic stress and even chemotherapy. Although they may function independently from each other, there are also interactions between responses to oxidative stress injury induced by pathologic and pharmacological factors. The aim of the present study was to investigate the effects of ERS and autophagy on H(2)O(2)-induced oxidative stress injury in human HepG2 hepatoblastoma cells. It was demonstrated that exposure of HepG2 cells to H(2)O(2) decreased cell viability and increased reactive oxygen species (ROS) levels in a dosage-dependent manner. In addition, apoptosis and autophagy rates were elevated and reduced following cell exposure to H(2)O(2) + the ERS inducer Tunicamycin (TM), and to H(2)O(2) + the ERS inhibitor Salubrinal (SAL), compared with the cells treated with H(2)O(2) alone, respectively. Further studies revealed that TM enhanced the expression of ERS-related genes including glucose-regulated protein-78/binding immunoglobulin protein, inositol-requiring kinase-I and activating transcription factor 6 and C/EBP-homologous protein 10, which were attenuated by SAL compared with cells exposed to H(2)O(2) alone. The data from the present study also demonstrated that LC3II/LC3-I and p62, members of autophagy-related genes, were increased and decreased in cells treated with H(2)O(2) + TM compared with cells treated with H(2)O(2), respectively, indicating that autophagy was stimulated by ERS. Furthermore, a reduction in the levels of pro caspase-3 and pro caspase-9, and elevation level of caspase-12 were observed in cells exposed to H(2)O(2) + TM compared with cells treated with H(2)O(2), respectively, suggesting apoptosis induced by H(2)O(2) was enhanced by ERS or autophagy triggered by H(2)O(2). The above results suggest that the ERS inducer may be a potential target for pharmacological intervention targeted to ERS or autophagy to enhance oxidative stress injury of tumor cells induced by antitumor drugs.