The role of C/EBP homologous protein (CHOP) in cell fate decision and hepatic identity gene suppression during ER stress
Creators: name: Liu, Kaihua
Publisher: name: University of Iowa
Year: 2023
Subjects: C/EBP homologous protein
cell death
ER stress
liver
proliferation
public abstract

description: Endoplasmic Reticulum (ER) is the hub of secretary protein processing and synthesis in a cell. Perturbation of ER homeostasis will cause the accumulation of misfolded or unfolded protein in the ER, resulting in ER stress. Upon ER stress, the unfolded protein response (UPR) is activated to help ER match the demand of protein processing. UPR activation is primarily a pro-survival process, but prolonged UPR activation, an indicator of the inability to restore ER homeostasis, is associated with cell death. How cell fate is determined under ER stress is largely unknown. Stress-induced transcription factor CHOP is thought to be a death executioner under ER stress. However, our previous work found that at least in mouse embryonic fibroblasts, CHOP is still expressed at the early phase of stress response under stress conditions where cells are more likely to adapt and proliferate than die, suggesting that CHOP might be involved in regulating cell survival under ER stress. Besides its role in cell fate decision, our previous work also found that in the liver, CHOP contributes to the stress-dependent suppression of metabolic genes, which is part of the gene regulatory network that establishes liver identity, mechanism unknown. My thesis work aims to delineate the role of CHOP in cell fate decision and explore the mechanism of CHOP in hepatic identity gene regulation during ER stress. The first part of my thesis work is set to explore the role of CHOP in cell fate decision under ER stress. CHOP is thought to elicit cell death by directly promoting protein synthesis, further burdening the already stressed ER and eventually killing cells. However, considering the importance of protein synthesis in cell adaptation and proliferation, we wondered whether CHOP plays a role in proliferation under ER stress. To explore the role of CHOP in cell survival under mild stress, we created a genetically modified Chop locus in which its expression can be turned either on or off to better examine the contribution of CHOP to cell fate during ER stress. Surprisingly, we found that while CHOP favored death in some cells, it also stimulated proliferation in others. This outcome arose at least in part from the same promotion of protein synthesis that led to cell death and occurred in the recovery phase of the stress response. Strikingly, this function of CHOP confers a growth advantage on cells expressing CHOP that were exposed to mild ER stress, compared to cells lacking CHOP. Single-cell RNA-seq further elucidated the heterogeneity in stress response in MEFs and validated the role of CHOP in supporting proliferation under mild stress.
description: The second part of my thesis work is to explore the role of CHOP in hepatic identity gene suppression during ER stress. The transgenic mouse model mentioned above enables us to manipulate CHOP expression in the hepatocytes. We performed RNA seq and CHIP seq of CHOP on the liver during ER stress to explore the role of CHOP in hepatic identity gene regulation. The RNA-seq data pointed to an indirect role of CHOP in this process by altering the general ER stress response in the liver. However, the CHIP-seq of CHOP also suggested a potential direct role of CHOP in this process-several key transcription factors that regulate hepatic identity genes are direct targets of CHOP during ER stress. Taken together, the RNA-seq and CHIP-seq data suggest both a direct and indirect role of CHOP in regulating liver identity during ER stress.