CHOP aggravates hepatocyte apoptosis upon endoplasmic reticulum stress by down-regulating autophagy.

Background: Endoplasmic reticulum (ER) stress-associated apoptosis is involved in various liver diseases, including liver fibrosis, nonalcoholic fatty liver disease, and cirrhosis. Hepatocytes respond to ER stress by eliciting unfolded protein response (UPR) and enhancing autophagy. Autophagy is a pivotal mechanism for sustaining normal ER function, through degradation of damaged ER fragments and removal of abnormal protein aggregates in the ER lumen. Failure to restore ER homeostasis via autophagy is harmful to hepatocytes and contributes to ER stress-associated apoptosis. Recent findings indicated that C/EBP homologous protein (CHOP) could exacerbate ER stress-related apoptosis by down-regulating autophagy, but the underlying mechanism remains elusive.

Aim: To investigate the impact of CHOP on ER stress-induced apoptosis in rat hepatocytes, and the potential molecular mechanisms.

Methods: BRL-3A cells were pre-treated with rapamycin (RAP) and 3-methyladenine, then treated with dithiothreitol (DTT). Growth and apoptotic rates were detected using real-time cellular analysis (RTCA) and flow cytometry, respectively. ER stress-associated molecule levels were determined via western blotting. CHOP, small interfering RNA, and the lentivirus vector system were used to transfect BRL-3A cells and observe the impact of CHOP gene silencing or overexpression on autophagy and apoptosis. Chromatin immunoprecipitation (ChIP) was used to confirm whether CHOP binds directly to ATG12, ATG5, and LC3 promotor regions undergoing ER stress.

Results: ER stress-associated molecules were dramatically upregulated in BRL-3A hepatocytes and hepatocyte apoptosis was augmented. RAP pre-treatment significantly reduced DTT-induced expression of ER stress-associated molecules; conversely, 3-MA pre-treatment promoted DTT-induced levels of ER stress-associated apoptotic molecules. Following the decreased CHOP expression in hepatocytes, the level of autophagy-associated molecules dramatically increased, and DTT-induced hepatocyte apoptosis decreased. However, opposite trends were observed in CHOP overexpression cells. A negative regulation of CHOP on autophagy-associated molecules including ATG12, ATG5, and LC3 in BRL-3A cells upon DTT treatment was detected via ChIP.

Conclusion: CHOP enhancement during ER stress inhibits autophagy and promotes hepatocyte apoptosis; however, the decreased CHOP gene expression could attenuate DTT-induced hepatocyte apoptosis. Overexpression of CHOP could aggravate DTT-induced hepatocyte apoptosis.