Endoplasmic reticulum stress and protein degradation in chronic liver disease

Si-Wei Xia, Zhi-Min Wang, Su-Min Sun, Ying Su, Zhang-Hao Li, Jiang-Juan Shao, Shan-Zhong Tan, An-Ping Chen, Shi-Jun Wang, Zi-Li Zhang, Feng Zhang, Shi-Zhong Zheng

PII: S1043-6618(20)31526-7
DOI: https://doi.org/10.1016/j.phrs.2020.105218
Reference: YPHRS 105218

To appear in: Pharmacological Research

Received Date: 15 July 2020
Revised Date: 21 September 2020
Accepted Date: 21 September 2020

Please cite this article as: Xia S-Wei, Wang Z-Min, Sun S-Min, Su Y, Li Z-Hao, Shao J-Juan, Tan S-Zhong, Chen A-Ping, Wang S-Jun, Zhang Z-Li, Zhang F, Zheng S-Zhong, Endoplasmic reticulum stress and protein degradation in chronic liver disease, Pharmacological Research (2020), doi: https://doi.org/10.1016/j.phrs.2020.105218

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Endoplasmic reticulum stress and protein degradation in chronic liver disease
Si-Wei Xiaa, Zhi-Min Wanga, Su-Min Suna, Ying Sua, Zhang-Hao Lia, Jiang-Juan Shaoa, Shan-Zhong Tanb, An-Ping Chenc, Shi-Jun Wangd, Zi-
Li Zhang a, Feng Zhanga*, Shi-Zhong Zhenga*

a Jiangsu Key Laboratory for Pharmacology and Safety Evaluation of Chinese Materia Medica, School of Pharmacy, Nanjing University of Chinese Medicine,
Nanjing 210023, China

b Nanjing Hospital Affiliated to Nanjing University of Chinese Medicine, Nanjing 210023, China

c Department of Pathology, School of Medicine, Saint Louis University, MO 63104, USA

d Shandong University of Traditional Chinese Medicine, Jinan 250035, China

Abstract

Endoplasmic reticulum (ER) stress is easily observed in chronic liver disease, which often causes accumulation of unfolded or misfolded proteins in the ER, leading to unfolded protein response (UPR). Regulating protein degradation is an integral part of UPR to relieve ER stress. The major protein degradation system includes the ubiquitin-proteasome system (UPS) and autophagy. All three arms of UPR triggered in response to ER stress can regulate UPS and autophagy. Accumulated misfolded proteins could activate these arms, and then generate various transcription factors to regulate the expression of UPS-related and autophagy-related genes. The protein degradation process regulated by UPR has great significance in many chronic liver diseases, including non-alcoholic fatty liver disease (NAFLD), alcoholic liver disease (ALD), viral hepatitis, liver fibrosis, and hepatocellular carcinoma（HCC）. In mostinstances, the degradation of excessive proteins protects cells with ER stress survival from apoptosis. According to the specific functions of protein degradation in chronic liver disease, choosing to promote or inhibit this process is promising as a potential method for treating chronic liver disease.

Abbreviations:
MA: 3- methyladenine; ALD: alcoholic liver disease; ASK1: apoptosis signal- regulating kinase 1; ATF6: activating transcription factor 6Bip: binding protein for immunoglobulins; BDL: bile duct ligation; CPT1: carnitine palmitoyltransferase Ⅰ; ER: Endoplasmic reticulum; ECM: extracellular matrix; ER: endoplasmic reticulum; ERAD: ER-associated degradation; FFAs: free fatty acids; FoxO1: Forkhead box O1; HBV: Hepatitis B virus; HCC: hepatocellular carcinoma; HCC: hepatocellular carcinoma; HCV: Hepatitis C virus; HFD: high fat diet; HSC: hepatic stellate cell; IRE1: inositol-requiring enzyme 1; NAFLD: non-alcoholic fatty liver disease; NASH: non-alcoholic steatohepatitis; NNMT: N-nicotinamide methyltransferase; PERK:protein kinase R (PKR)-like endoplasmic reticulum kinase; ROS: reactive oxygen species; SRSF3: serine rich splicing factor 3; TβRI: TGF-β receptor type I; TRAF2: tumor necrosis factor receptor-associated factor 2; TUDCA: tauroursodeoxycholic acid; UPR: unfolded protein response; UPS: ubiquitin-proteasome system; (u)XBP1: unspliced XBP1; (s)XBP1: spliced XBP1

Keywords：endoplasmic reticulum stress; unfolded protein response; ubiquitin-proteasome system; autophagy; chronic liver disease
Chemical compounds studied in this article

1. Introduction

Chronic liver disease poses a major threat to public health worldwide.

Numerous studies have shown that the development of the chronic liver disease is often accompanied by ER stress, and studying the interaction between ER stress and protein degradation has excellent significance for the treatment of chronic liver disease[1, 2].
The endoplasmic reticulum is the main site of protein folding and processing.

However, protein folding is an extremely error-prone process, especially under the interference of various injured stimulation, such as reactive oxygen species (ROS) and toxic metabolite. These misfolded proteins will accumulate in the ER and cause ER stress. To reduce the damage of misfolded protein agents, there is a quality control system in the cells, which mainly relieves the stress state in two ways. One is to start the UPR in response to ER stress to reduce the synthesis of new proteins and increase the expression of molecular chaperones that can promote protein folding. The other is to degrade of misfolded proteins through protein degradation systems[3]. The protein degradation process is mainly composed of UPS and autophagy. More importantly, numerous studies have shown that UPR is involved in the regulation of these two systems[4-6]. Interestingly, the UPR can also be activated in the absence of ER stress and before the accumulation of misfolded proteins, especially through the action of secreted factors[7, 8].
In this review, we mainly summarize the regulation of three arms of UPR on the UPS and autophagy and the research progress of these pathways in chronic liver disease. More importantly, we summarized bioactive compounds and treatment strategies for chronic liver disease based on UPR and protein degradation.

2. ER stress-induced unfolded protein response and protein degradation

The ER is an essential part of the cell’s inner membrane system, and it has many functions such as promoting protein folding, regulating Ca2+ concentration, participating in lipid synthesis and transportation. However, protein folding is prone to make mistakes. Accumulation of these misfolded proteins in the ER will further exacerbate the dysfunction of ER and generate ER stress, which in turn triggers the UPR.
Three transmembrane UPR sensors, inositol-requiring enzyme 1 (IRE1), protein kinase R (PKR)-like endoplasmic reticulum kinase (PERK) and activating transcription factor 6 (ATF6), are well known to detect misfolded proteins in the ER lumen and transmit the signal across the ER membrane to the cytosol[9]. UPR relieves the ER stress via reducing the protein translation and increasing the expression of the ER folding enzymes and molecular chaperones. Besides, UPR promotes the degradation of unfolded or misfolded proteins through UPS or autophagy. Binding protein for immunoglobulins (Bip), a member of the heat-shock protein 70 family, is a vital ubiquitous resident of the ER. In physiological condition, Bip binds to three sensors and inhibits its transcriptional activity. In response to ER stress, misfolded proteins bind to Bip and promote it to dissociate from these sensors,then activate these sensors and their downstream signal cascade reaction, which plays a vital role in restoring ER function or inducing apoptosis signals.[10].

UPR regulates UPS and autophagy mainly by activating ubiquitination-related and autophagy-related genes. In most cases, UPS mainly specifically degrades most micromolecule or short-lived soluble proteins in cells, and autophagy mainly degrades excess or damaged intracellular organelles and macromolecule or relatively stable proteins[11]. Ubiquitin, a polypeptide chain consisting of 76 amino acids, is widely distributed in various cells. In cells, when a protein needs to be degraded after completing its function or misfolding, its tyrosine residues are labeled with ubiquitin, which is then specifically recognized and rapidly degraded. Protein ubiquitination involves three consecutive processes, involving ubiquitin activating enzyme E1, ubiquitin conjugating enzyme E2 and ubiquitin protein ligase E3 [12]. After targeted by ubiquitination, the protein will be degraded by UPS or autophagy (Figure 1). UPR activation, ubiquitination and protein degradation in the ER. After translation, proteins are translocated into the lumen of the ER, where they are modified and correctly folded via folding enzymes and chaperones. Then they will be translocated to Golgi for future processing. For the misfolded proteins, they will be recognized by some chaperones (such as glycosyltransferase and Bip). If the protein still misfolds, it will be transported out from the ER and degraded by UPS. Due to the accumulation of misfolded proteins, Bip dissociates from the three sensors and product corresponding transcription factors. These transcription factors regulate the expression of many genes related to adaptive UPR, ERAD and autophagy.

2.1. Unfolded protein response regulated ubiquitin-proteasome system

When a newly synthesized protein is misfolded, it will eventually be reversely transported from the ER to the cytoplasm, and then be ubiquitinated and degraded. This is a method called endoplasmic reticulum-associated degradation (ERAD). The 26S proteasome is the central molecular machinery responsible for ERAD in eukaryotic cells. These subunits are assembled into two complexes: 20S core particles and 19S regulatory particles. The 20S core particle has a highly conserved hollow structure and proteolytic activity and the 19S regulatory particle is responsible for recognizing the ubiquitin-targeted substrate and transposing it to the 20S core particle for degradation. [13]. All three UPR arms triggered by ER stress can regulate UPS mainly through E3 ligase. Researchers investigated the role of the proteasome in the modulation of the proteome of HeLa cells after treatment with two different ER stress inducer, thapsigargin and tunicamycin, finding that more than 64% are targeted by the proteasome[14].
IRE1α signaling is the most actively studied pathway among three UPR arms.

ASB11, a kind of E3 ligase, is activated by XBP1 in response to ER stress, which stimulates BIK ubiquitination and proteolysis of BIK in vitro [15]. IRE1α increased the stability of RNF183, one of the typical ubiquitin E3 ligases, in a manner that did not depend on XBP1 but reduced miR-7 levels[16]. XBP1 itself can also be targeted by ubiquitination. identified Two key ubiquitination sites of K60 and K77 have been identified on XBP1, and inhibiting the ubiquitination degradation of XBP1 may have benefits on relieving ER stress and insulin resistance[17].

The main function of PERK-mediated eIF2α phosphorylation is to reduce new protein synthesis, but it also plays a vital role in the process of ERAD. The reduction of PERK in skeletal muscle promotes proteasomes degradation and autophagy[18].
The activation of PERK in the response of ER stress can promote the phosphorylation of E3 ubiquitin ligases MARCH5, MULAN, and Parkin, then increase the degradation of related substrates[19]. Similarly, eIF2α is also a substrate for proteasome degradation. The E3 ligase HRD1 in renal tubular epithelial cells can promote the ubiquitination and proteasome degradation of eIF2α, protecting cells from apoptosis[20]. Besides, once severe and long-term ER stress is encountered, the ATF4-CHOP signaling will be activated to cause apoptosis[21].

ATF6 is also one of the necessary sensors for UPR. The Nrf1 regulates protein homeostasis in the ER through ATF6 rather than IRE1 and PERK. The ATF6 transcriptional program and ERAD are downregulated in Nrf1 CKO mice[22]. In addition, ATF6 may be a key player in the transport of Ub-targeted proteins from the ER to the proteasome. The Derlin protein family participates in this transit process, both Derlin-2 and Derlin-3 could be upregulated in response to UPR. The induction of Derlin3 depends on ATF6[23, 24].

2.2. Unfolded protein response regulated autophagy

The main degradation process of autophagy takes place in the lysosome. It plays a vital role in various physiological processes such as cleaning up cellular waste, reconstructing cell structure, and maintaining growth and development. There are three main types of autophagy in the cell: macroautophagy, microautophagy, chaperone-mediate autophagy. Among these three types, macroautophagy (hereinafter referred to as autophagy) is the most thoroughly studied autophagy pathway. Autophagy can not only support cell survival by eliminating damaged cellular structures and excess potentially harmful proteins but can also sustain energy homeostasis by recycling of cytosolic components during starvation to compensate for nutrient deprivation. When macroautophagy occurs, excess or damaged organelles and extra proteins in the cell are enveloped by the double-layer membrane to form an autophagosome; then the autophagosome and lysosome combine to form an autophagolysosome where various contents degraded by multiple proteolytic enzymes in[25]. When damaged or excrescent proteins and organelles that cannot enter the proteasome accumulate in the cell, or proteins targeted by ubiquitin escape from ubiquitinated proteins and leading to ER stress, the autophagy will start to help cells clear these wastes. All three typical arms of UPR can regulate autophagy in different ways during the ER stress (Figure 2). Although autophagy caused by ER stress is mostly protective, the occurrence of autophagy does not determine the ultimate fate of the cell. The degree of stress and the duration may still determine the fate of the cell.

UPR-mediated autophagy and partial mechanisms. The ER stress-mediated UPR induces autophagy through IRE1α, PERK, and ATF6 signaling pathway. Activated IRE1α forms a complex with TRAF2 and ASK, which promotes the phosphorylation of Bcl-2 cooperating with activated JNK signaling, leading to the release of Beclin1. Cleaved XBP1 also can promote the transcription of Beclin-1. Interfering with XBP1 may activate FoxO1 and increase the level of autophagy by promoting the expression of ATG7. Activated PERK signaling promotes the expression of ATG12 through the eIF2α/ATF4 signaling. Besides, ATF4 can promote autophagy through CHOP and AMPK/mTOR signaling. Activated ATF6 indirectly promotes autophagy via XBP1 and CHOP signaling.

It has been reported that the XBP1 mRNA splicing could directly bind to the Beclin-1 promoter and promote its transcriptional, leading to the formation of autophagic vesicle and the up-regulation of autophagy markers BECLIN-1 and LC3- Ⅱ. [26]. While another study found that the deletion of XBP1 leads to augmented expression of Forkhead box O1 (FoxO1), a keytranscriptionfactorregulating autophagy in neurons[27]. However, the researcher didn’t find the changes in classical ER stress markers levels in symptomatic animals [27]. A possible assumption is that inhibiting the XBP1 signaling could simultaneously inhibit the ubiquitin-proteasome system, compensatively activating the autophagy. Inhibition of the IRE1α activation or increased autophagy both can alleviate the inflammatory response induced by ER and cell death, indicating that autophagy Imbalance also may activate IRE1 and its downstream transcription factors[28]. Independent of the XBP1 signaling, the kinase domain of IRE1α can recruit tumor necrosis factor receptor- associated factor 2 (TRAF2) and apoptosis signal-regulating kinase 1 (ASK1) on the ER to form IRE1/TRAF2/ASK1 complex, which leads to the phosphorylation of Bcl- 2 and releasing of Beclin-1[29]. Knockout of the IRE1 gene or blocking JNK signaling inhibits cell autophagy, indicating that the IRE1-JNK pathway is essential in the ER stress-mediated autophagy[30].The PERK/eIF2α/ATF4 pathway seems to be the most important arm of induced autophagy-related gene expression. Activated ATF4 is sufficient to up- regulate the expression of more than a dozen autophagy genes such as ATG3,
ATG12, BECN1, MAP1LC3B[31]. CHOP is one of the targets of ATF4, whose upregulation can also be used as a transcription factor to further promote the expression of some autophagy genes such as ATG5 which forming a complex with ATG16 and ATG16L and involving in the elongation process. Interestingly, CHOP seems to be a key signal for the changeover between autophagy and apoptosis[32]. PERK/eIF2α/ATF4 can also activate AMPK by upregulating sestrin-2 (SESN2) and inhibit mTORC1 to activate autophagy[33].

Although studies of ATF6 in autophagy relatively less, ATF6 can also be involved in the induction of ATGs and often have crosstalk with the other two UPR pathways. The cleaved ATF6 translocates into nucleus and binds to ATF/cAMP response elements Sano and ER stress-response elements to activate XBP-1 and CHOP[34, 35]. Besides, ATF6 can up-regulate the expression of DAPK and promote the MRLC-mediated transport of mAtg9 from the paranuclear tissue to the cytoplasm, providing a vesicle can provide a membrane source for autophagy[36].

3. ER stress-induced Protein Degradation and Chronic Liver Disease

Chronic liver disease is a series of pathological changes that occur at the level of cells and molecules after continuous liver damage caused by various injured stimulation. At present, there is no practical way to cure chronic liver disease completely, so it is of considerable significance to find useful targets for treating chronic liver disease. Recently, more and more evidence has shown that ER stress plays an important role in the development of chronic liver disease, and the protein degradation regulated by UPR may have a crucial impact on chronic liver disease.

3.1. Non-alcoholic Fatty Liver Disease

Non-alcoholic fatty liver disease is a metabolic disorder characterized by liver steatosis and insulin resistance. Molecular mechanisms of hepatic lipid accumulation in NAFLD have been well described in another review[37]. Lipid accumulation activates several cellular stress pathways including ER stress and UPR[38, 39].Although increased phosphorylation of eIF2a was observed in NAFLD, no significant change was observed in the downstream target of the PERK signaling. More importantly, compared with NADL, the phosphorylation of JNK is significantly increased in NASH[40]. Initially, UPR may be activated as an adaptive response to changes in hepatic metabolism rather than as a result of stress. The researchers found that the IRE1α-XBP1 pathway may be activated by stress-independent catabolic signals such as glucagon and free fatty acids (FFAs) in hepatic of mice during prolonged fasting or by a ketogenic diet [41]. Although hepatic lipid homeostasis can be improved by IRE1α, XBP1can promote the transcription of many genes that participated in adipogenesis such as C/EBPα and PPARγ[42, 43]. Inhibiting the expression of XBP1 and reflexive activating IRE1α are beneficial to improving hepatic steatosis in mice [44]. However, a critical turning point in NAFLD is the transition from pure steatosis to fatty inflammation, during which many cellular stress pathways are activated, including oxidative stress, ER stress, inflammation, autophagy, and apoptosis[38, 45]. These stress processes may cause irreversible damage to cells. Therefore, inhibiting IRE1α signaling in NASH may be more important due to activated IRE1 can exacerbate inflammatory responses in mice[46].

In high fat diet (HFD) mice, liver proteasome activity is reduced, which causes ER stress, UPR activation and insulin resistance[47]. This suggests that defects in proteasome function may play an important role in NAFLD. XBP1, a deacetylation target of Sirt6, can resist hepatic steatosis after degraded via the UPS in Sirt6 knockout mice [48]. However, the degradation of serine rich splicing factor 3 (SRSF3) in mice promoted hepatic steatosis, fibrosis and inflammation[49]. Due to the degradation of different substrates contribute to conflict results, whether direct target UPS to treat NAFLD remains to be further verified. Excitingly, the promotion of autophagy may have potentially positive implications for the treatment of NAFLD. Rubicon, a Beclin1-interacting negative regulator, mainly inhibits autophagy during autophagosome-lysosome fusion and endocytosis, in the pathogenesis of NAFLD. Rubicon-/-mice induced a decrease in ER stress and autophagy dysfunction and an improvement in liver injury and steatosis under the induction of a high-fat diet [50].

Since autophagy can not only clear misfolded proteins, it also can clear various cellular components such as damaged organelles and excess lipid droplets. Therefore, it is more potential to directly target the autophagy pathway to treat NAFLD than to interfere with the UPR and UPS processes.
In summary, except for regulating the protein degradation, the UPR is involved in lipid metabolism in NAFLD. Targeting UPR to treat NAFLD may cause changes in multiple signal pathways, and the superposition of these signals determines the ultimate development of NAFLD. Although existing evidence is not enough to fully clarify the impact of regulating UPR on the treatment of NAFLD, we believe that severity of NAFLD is closely related to the regulation of UPR. Besides, the appropriate promotion of autophagy may have a positive impact on the treatment of NAFLD, and UPS targeting therapy requires more research.

3.2. Alcoholic Liver Disease

Alcoholic fatty liver disease is a chronic liver disease caused by long-term heavy drinking. The main pathogenesis of AFLD is hepatotoxicity caused by ethanol and its metabolites, leading to a series of pathological changes such as oxidative stress, ER stress and mitochondrial dysfunction in cells during the progress of metabolism [51]. Some studies showed that alcohol can inhibit the activity of methionine synthase, leading to hyperhomocysteinemia, protein folding disorder and ER stress[52]. High-homocysteine-induced ER stress disrupts lipid metabolism via increasing levels of SREBP in mice fed hyperhomocysteinemic diets, leading to enhanced production of triglycerides and cholesterol[53]. Betaine, a methyl donor for the conversion of homocysteine to methionine, can reduce homocysteine and relieve endoplasmic reticulum stress in alcohol-induced steatohepatitis mice[52]. Excessive alcohol consumption impairs fatty acid catabolism predominantly through inhibition of mitochondrial β-oxidation. Alcohol also increases the activity of ACC by inhibiting AMPK in rat hepatoma cell lines, leading to increased lipogenesis via inhibition of carnitine palmitoyltransferase Ⅰ(CPT1) activity and reducing the rate of fatty acid oxidation via increased malonyl-CoA concentration [54]. To summarize, alcohol promotes hepatic lipid accumulation and resultant hepatic steatosis mainly viaincreasing the hepatic capacity for exogenous fatty acid uptake and impairing fatty acid catabolism predominantly. However, alcohol-induced UPR was often transient and independent of ER stress. Although the accumulation of misfolded proteins was observed in the alcohol-fed zebrafish model, most of the downstream targets of UPR are not affected[55, 56].

Although the impacts of oxidative stress in early-stage ALD were determined to be independent of UPR induction, chronic ALD results in increased hepatic ER stress and UPR, contributing to serious oxidative injury, inflammation and apoptosis[57, 58]. The activation of PERK-ATF6 signaling upregulated N- nicotinamide methyltransferase (NNMT), leading to enhanced denove lipogenesis in ALD mouse models[59]. In liver specimens from ALD patients the ER protein Nogo- B, a regulator of Kupffer cell M1 polarization and facilitates ethanol-induced liver injury, can be inhibited by ER stress/CHOP signaling[60]. Of course, UPR signals also regulate alcohol-induced liver injury through protein degradation pathways. A microarray analysis on the liver of rats treated with bortezomib has shown that proteasome inhibitor treatment significantly reduces the mRNA expression of SREBP-1c and the downstream lipogenic enzymes[61]. Alcohol can induce autophagy to degrade the accumulation of damaged mitochondria and lipid droplets in cells, but long-term drinking can inhibit autophagy[62, 63]. Interestingly, rapamycin has been reported to improve alcohol-induced liver pathogenesis by inducing autophagy in acute ethanol binge mouse model[62].

In general, the balance between ER stress and protein degradation in AFLD influenced by many factors including toxic acetaldehyde, excess homocysteine and oxidative stress, and the interactions between these mechanisms need further study. ER stress and protein degradation generally do not play a dominant role in the early stage of ALD. However, in serious ALD, the activated UPR signals increase lipogenesis, inflammation and apoptosis. At this point, inhibiting the activation of UPR or may be beneficial to the treatment of ALD. In addition, proteasome inhibitors and autophagy inducers may have positive effects on the treatment of ALD.

3.3. Viral Hepatitis

There are currently five known hepatitis viruses, of which hepatitis B and C viruses have a long-term infection and may develop into chronic hepatitis[64]. The degree of cytopathy caused by HBV and HCV infection is relatively weak, allowing viruses to escape the surveillance of immune checkpoints and avoid the differentiation of T cells into memory cells to establish long-term Chronic inflammatory environment[65]. Due to increased protein synthesis requirements, viral hepatitis is often accompanied by ER stress. Severe ER stress and activation of UPR downstream signals were observed in both HBV or HCV infected cells. However, there was a significant difference between the results observed at the tissue level and the cell level[66-69]. This may be the result of the local lesion being masked by the global level.

HBV could activate the IRE1/XBP1 pathway and enhance the expression of ERAD-related EDEM1 to limit the amount of protein and reduce stress to protect cells in HepG2 cell lines, which may be one of the reasons that HBV can achieve chronic inflammation[70]. Interestingly, the UPS acts as a double-edged sword in HBV infection[71]. One the one hand, the UPS eliminates viral components of HBV and blocks the viral life cycle. On the other hand, the HBV can manipulate many specific proteins to limit the degradation of viral proteins mediated by the UPS pathways. Besides, HBV can accelerate self-replication by promoting autophagy.

HBV can rely on the non-degradable components of early autophagy as a scaffold and prevent itself from being damaged by autophagy. Using siRNA to interfere with the expression of autophagy-related genes such as ATG5, Atg12 and Atg16L1 can reduce virus production obviously[72]. Similarly, HCV can trigger ER stress and UPR to accelerate liver damage. Clinical data show that although HCV triggers ER stress and activates the three sensors of UPR, it does not seem to have a large effect on genes downstream of UPR[68]. A possible reason for this phenomenon may be that HCV infection is localized. These local reactions are easily covered up by other normal tissues and are not easily observed. To maintain a balance of intracellular stress levels and protein degradation rates, HCV can activate PERK and ATF6 rather than IRE1 in Huh7 hepatoma cells, inducing the upregulated expression of ATG [73]. The upregulated Beclin1 and activating mTOR signaling pathway can also be observed in the autophagy induced by HCV in vitro [74].

In HBV and HCV-infected hepatocytes, the protein degradation pathway is mostly activated to prolong cell survival time and adapt to the requirements of viral replication. Inhibiting protein degradation may become a potential treatment for viral hepatitis. However, considering the complex role of protein degradation in viral hepatitis, the validity of this conclusion needs further verification. Excitingly, the development of small molecules that can catalyze ubiquitin-mediated degradation of target proteins without affecting the overall level of protein degradation may become a new therapeutic strategy in the treatment of viral hepatitis[75, 76].

3.4. Liver Fibrosis

Liver fibrosis is a tissue repair response secondary to various chronic liver injuries. The characteristics of liver fibrosis are the activation of hepatic stellate cells (HSCs) and the deposition of extracellular matrix (ECM). Synthesizing a large amount of ECM may increase the burden on the endoplasmic reticulum. Surprisingly, UPR seems not a necessary event in the early stage of HSCs activation. The immediate early UPS has observed during the activation of primary mouse HSCs and ER stress induction is not sufficient to drive HSC activation independently in a model for quiescence[77]. Similarly, the mRNA of sXBP1 and ATF4 and the phosphorylation level of eIF2α have no significant increase in the early stage of common bile duct ligation (CBDL) mouse model[78]. Therefore, early UPR is activated as a physiological adaptive response independent of ER stress. However, it has been reported that ER stress triggers the activation of HSC isolated from rats through IRE1α/P38/MAPK signaling, which then leads to stellate cell activation[79]. Besides, the activation of PERK promotes liver fibrosis via inducing the degradation of HNRNPA1 and up-regulation of SMAD2 in HSCs. This evidence suggests that the establishment of a chronic hepatic fibrosis environment will induce the change of protective response of UPR to injurious response. Although the expression of CHOP and the mRNA levels of IRE1 were found in the cirrhosis rat induced by dimethylnitrosamine, no evident changes were observed in cleaved ATF6 and phosphorylated PERK protein[80]. Therefore, we hypothesized that IRE1α signaling in liver fibrosis may play a key role, and CHOP may be activated in a way that does not depend on PERK activation.

Protein degradation seems to play a different role at different stages of liver fibrosis. Both MG132 and bortezomib promote apoptosis by blocking NF-κB activation in HSCs [81]. Bortezomib has been reported to inhibit hepatic fibrogenesis in bile duct-ligated (BDL) mice via blocking stellate cell NF-κB activation, inhibiting Kupffer and macrophage, inducing hepatocyte proliferation, and reducing hepatocyte injury[82]. However, in the early period of liver fibrosis, UPS seems to play a positive role in preventing TGF-β-mediated liver fibrosis via the degradation of TGF-β
receptor type I (TβRI) in cirrhotic rat liver tissue[83]. The research in autophagy caused by ER stress in HSCs has also reached two different results. A study showed that XBP1 could drive liver fibrosis through autophagy in HSCs, and blocking autophagy can reduce fibrosis activity in human and mouse HSC lines[84]. One possible reason is that autophagy provides energy for HSC activation by digesting lipid droplets in resting HSCs[85]. In the liver fibrosis induced by ethanol, the researchers found that the IRE1/XBP1 signaling promotes p38MAPK-dependent autophagy and activation of HSCs[79]. The level of intracellular calcium and the expression of UPR-related gene was increased in caffeine-treated LX-2 whose activation is inhibited by the activation of IRE1α and autophagy [86]. However, a recent study showed that impaired autophagy flux caused by TRIB3 still leads to liver fibrosis in mouse models of BDL[87]. These contradictory observations remind us that the effect of protein degradation on liver fibrosis is complex and further studies of protein degradation in liver fibrosis are necessary.

Whether the protein degradation regulated by UPR protects cells from death or promotes cell death may depend on the stage of liver fibrosis. Now, there is a lot of evidence proving that the UPR signals pathway seems to play a significant role in the activation of autophagy, especially IRE1α-XBP1 arm.[79, 84, 88]. Unfortunately, although a significant increase in proteasome content was found in a rat model of acute liver injury, there are still a few related studies about how UPR regulates UPS to affect liver fibrosis [89]. Although most of the current studies suggest that inhibition of UPR and protein degradation can reduce liver fibrosis via promoting the apoptosis of activated HSCs the function of UPR and protein degradation are still intricate in liver fibrosis.

3.5. Hepatocellular carcinoma

Hepatocellular carcinoma, the most common form of primary liver cancer, is the fourth most common cause of cancer-related death in the world. The upregulated ER stress and UPR levels can increase the protein synthesis and processing capacity of HCC cells to meet their own needs. However, if the ER stress level is further increased, it may exceed the repair capacity of the UPR and cause cells to overload and die. In mouse HCC model, the activated IRE1α accelerates the malignant progression of HCC via aggravating the inflammatory induced by the activation of the IKKβ-NF-κB pathway and increasing the proliferation of hepatocyte through maintaining the activation of STAT3[90]. The upregulated level of phosphor-eIF2α and Atf4 mRNA was observed in HCC mice, and the PERK inhibitor disrupts the UPS and reduces tumor growth in vivo[91]. Compared to the other two signals, PERK/eIF2a/ATF4/CHOP signaling is the most significantly activated signals in HCC[91]. The researchers found that the PERK but not IRE1 inhibitor strikingly reduced the viability of HCC cells under stressed conditions[91].

Furthermore, the use of proteasome inhibitors in hepatocellular carcinoma is widespread. A study has shown that proteasome inhibitor MLN2238 can aggravate ER stress and up-regulate P27/P21, leading to cycle arrest and apoptosis in hepatoma carcinoma cells [92]. In another interesting immunotherapy for liver cancer, researchers used bortezomib and hTer-Ad (human telomerase reverse transcriptase promoter-regulated adenovirus) for therapy. They found that BZM disrupts UPR caused by viruses, improving the ER stress-induced apoptosis in vitro and oncolysis in vivo[93]. This evidence suggests that proteasome inhibitors can help treat liver cancer. Sorafenib is a multikinase inhibitor with well-known inhibitory activity of Ser/Thr kinase and receptor tyrosine kinase. It plays an important role in inhibiting tumor growth and angiogenesis[94]. Interestingly, a study has shown that sorafenib inhibits the ubiquitination of proteins, and the combination treatment of sorafenib and proteasome inhibitors can increase the lethality of HCC cells[95]. Besides, sorafenib also induces tumor cell death through the activation of ER stress and autophagy. The activation of autophagy may be one of the main reasons for sorafenib resistance[96]. It was reported that both IRE1 and PERK arms are activated by sorafenib in HepG2 cells, leading to the disruption of the secretory pathway and autophagy[97]. Another study has observed that sorafenib induce ER stress and activated autophagy based on the PERK-ATF4-Beclin1 pathway in HepG2 cells. And a low concentration of melatonin increased the sensitivity of HCC to sorafenib by inhibiting autophagy through this pathway[98].

In general, UPR and protein degradation help to maintain the stability of cellular protein quality. Most studies currently support that in HCC cells with ER stress, the combination of sorafenib and UPR, UPS, or autophagy inhibitors can inhibit the degradation of protein and the induction of apoptosis and reduce of drug resistance, which is a potential treatment for HCC.

4. Targeting ER stress and protein degradation in chronic liver diseases

Serious ER stress and abnormal protein degradation regulated by UPR are crucial factors that aggravate the disease process in chronic liver disease. Targeting the specific UPR and protein degradation pathways may provide opportunities for developing new therapeutic strategies to treat chronic liver disease. Novel therapeutic strategies based on several compounds have exciting intense over the past of years (Table 1). Although exciting results have been obtained in animal models, their clinical value remains to be future verified.

4.1. Chemical chaperones

Chemical chaperones modulate ER stress by attenuating protein misfolding and aggregation, as well as stabilizing folding intermediates. 4-PBA and tauroursodeoxycholic acid (TUDCA) are the most well-known Chemical chaperones that reduce ER stress. TUDCA has been approved by the US FDA for the treatment of biliary cirrhosis. Both 4-PBA and TUDCA have been shown outstanding in vivo safety profiles[99].Especially, 4-PBA may play a protective role in lipid accumulation and lipotoxicity via activation of autophagy in vitro[100]. In another study, 4-PBA inhibits the activation of IRE1-XBP1 and the expression of ASIC1α and α-SMA, suggesting the potential of 4-PBA in the treatment of liver fibrosis[101]. However, Chemical chaperones play a complex role in HCC. Preventive administration of TUDCA reduces HCC burden in an orthotopic mouse model, while it does not affect tumor progression[102]. Also, 4-PBA inhibits the expression of PERK, ATF4 and CHOP, decreasing apoptosis of HCC cell[103]. The treatment of HCC with chemical chaperone may be problematic as the targeted increase of adaptive UPR may promote cancer cell survival.

4.2. IRE1α signaling

Salicylaldimine analogs, such as 4μ8c and STF-083010 were shown to covalently attach to a lysine residue, inhibiting the RNase activity of IRE1α and blocking RIDD and XBP1 splicing without affecting its kinase activity or ability to dimerize/oligomerize[104, 105]. The NLRP3 inflammasome activation and cell death mediated via IRE1α RNase activity were confirmed in primary mouse hepatocytes by
4μ8c or STF-083010[46]. In a mice model with CCl4-induced liver fibrosis, STF- 083010 protected against liver fibrosis associated with hepatic mi-122[106]. In primary hepatocytes isolated from HFD rat, toyocamycin attenuates hepatocyte lipogenesis and ameliorates NAFLD[107]. An encouraging sign is that a randomized, phase 2 trial suggests that selonsertib, a selective inhibitor of ASK1, may reduce liver fibrosis in patients with NASH and stage 2-3 fibrosis[108].

4.3. PERK signaling

GSK2656157 and GSK2606414 are the most commonly used small molecule PERK inhibitors. It has been reported that GSK2656157 reduced ER stress and caused a decrease in their angiogenic properties, which alleviated BDL-induced liver fibrosis[109]. Besides, GSK2606414 was shown to inhibit palmitic acid-induced JNK activation and cell apoptosis in primary mouse hepatocytes[110]. Recently, FGF21 was identified to protect against hepatic steatosis by limiting the increase in VLDLR levels via attenuation of the eIF2α-ATF4 pathway[111]. Future, FGF21 protects against long term ethanol-induced hepatic damage and may attenuate the progression of ALD[112]. In addition, FGF21 reduces inflammation and fibrosis in thioacetamide- induced liver fibrosis mice[113]. Interestingly, FGF21 seems to play an important role in the progression from NAFL to HCC, suggesting that it should be further studied as a key clinical biomarker[114, 115]. Salubrinal, a novel inhibitor of eIF-2α dephosphorylation, inhibited NF-κB activation, nuclear TG2 expression, and apoptosis only if it was induced by FFAs, but not by ethanol[116]. Besides, guanabenz also has the potential to effectively treat NAFLD and hyperglycemia in obese patients via affecting the expression of genes involved in lipogenesis and fatty
acid β-oxidation in HFD mice[117].

4.4. ATF6 signaling

There are fewer compounds that directly target ATF6 signaling. Ceapins, a class of pyrazole amides, that inhibit ATF6α signaling by selectively preventing transport of ATF6α to the Golgi apparatus[118]. However, its function in chronic liver disease is unclear. In addition, it has been reported that some natural compounds, such as resveratrol and patchouli alcohol, may have positive effects on chronic liver disease through ATF6 signaling[119, 120]. But whether these compounds directly affect
ATF6 remains to be confirmed.

4.5. UPS and autophagy

Compounds that target the process of protein degradation have been widely used in chronic liver disease. Representative proteasome inhibitors, such as MG132 and bortezomib, have made great progress in the treatment of ALD, liver fibrosis and HCC[61, 81, 93, 121]. Similarly, some achievements have been reported in targeted autophagy therapy for chronic liver disease. Rapamycin has shown excellent potential in the treatment of liver ischemia and reperfusion injury[122]. Although a transient reduction in hepatosteatosis has been found in the HFD mice model, long-term rapamycin treatment increases IL-6 production and is unsuitable for the prevention or treatment of obesity-promoted liver cancer[123]. These findings suggest that both hyper- and hypo-activation of mTOR are detrimental to the liver. The autophagy inhibitor, such as 3- methyladenine (3-MA) and LY294002, both inhibit autophagy via blocking autophagosome formation. Chloroquine, one of the only autophagy inhibitors approved for use in the clinic, prevents maturation and fusion of endosomes and lysosomes by changing the PH of the compartment. These compounds are widely used in the treatment of HCC during preclinical studies, especially act as a combination treatment strategy[46, 124, 125]. However, their functions in liver fibrosis are still debatable[126, 127]. In conclusion, it is promising and challenging to target the protein degradation process in the treatment of chronic liver disease.

4.6. Clinical studies targeting UPR and protein degradation

Unfortunately, there are relatively few studies targeting UPR and protein degradation processes in the treatment of chronic liver disease. One major reason is that few drugs are safe enough and directly target these processes. Only a few drugs, such as 4-PBA, TUDCA, bortezomib and rapamycin, have been approved for clinical use. However, most of these marketed drugs are approved to treat other types of diseases such as urea cycle disorders[128], hypertension[117], malarial and rheumatoid[129]. In the field of chronic liver disease, these drugs that target UPR and protein degradation are still in preclinical or clinical research (Table1). Clinical studies have shown that TUDCA improves liver insulin sensitivity and other diseases associated with bile acid metabolism disorders[130]. However, although bortezomib has achieved good results in vitro, it administered alone has minimal single-agent clinical activity in patients with advanced, unresectable HCC[131]. We suspect that one of the possible reasons for the large difference between clinical studies and preclinical studies is that the drug effective concentration on target cells of human is much lower than it on cell and animal models. Besides, the efficacy of sorafenib in combination with mTOR inhibitor for recurrent HCC after liver transplantation has been reported, although levels of autophagy were not measured in the study[132].Overall, current clinical data are insufficient to demonstrate the potential of targeting UPR and protein degradation in the treatment of chronic liver disease. Designing drugs that are safer and more targeted, and conducting larger clinical studies, may be important to solving this problem.

6. Conclusion

Under the influence of various damage factors, unfolded or misfolded proteins that accumulate in the ER will cause ER stress. However, the activation of UPR varies greatly under different disease models (Table2). UPR is the most important repair method in ER stress. It activates the corresponding transcription factors through three stress receptors to inhibit protein synthesis and promote protein degradation, but apoptosis would be activated when this stress state exceeds the repair capacity of UPR. Although the exact mechanism of this transformation is unknown, the activation of JNK signaling and the CHOP signaling play an important role in this process. UPS and autophagy are the main ways of protein degradation in cells. All three arms of UPR can participate in the regulation of the protein degradation process, and there is often a synergy between these arms.
Table2. Downstream signals of UPR under different chronic liver disease models More and more evidence has shown that UPR and protein degradation play an important role in the development of chronic liver disease. Although preclinical studies have achieved gratifying results, defining the relationship between ER stress and protein degradation in chronic liver disease is still a huge challenge in clinical research. Because ER stress in the body is often accompanied by a series of other problems such as oxidative stress, metabolic disorders, and disturbance of calcium homeostasis, it is difficult to determine whether ER stress and UPR signals are the decisive factors affecting protein degradation. Another key question is that UPR signal also plays an important role in apoptosis How does the UPR signal keep a balance between autophagy and apoptosis require more studies. The effects of UPR and protein degradation on chronic liver disease are complex, especially in the different degrees of disease. How to choose a suitable period and target to intervene in the protein degradation induced by UPR requires more experiments to support.

Conflict of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

Funding: This work was supported by the National Natural Science Foundation of
China (31401210, 81870423), the Joint Project of Jiangsu Key Laboratory for Pharmacology and Safety Evaluation of Chinese Materia Medica and Yangtze River

Pharmaceutical (JKLPSE202005), the Major Project of the Natural Science Research of Jiangsu Higher Education Institutions (19KJA310005), and the Project of the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

Human and animal rights

This article does not contain any studies with human or animal subjects.

References
[1] A. Baiceanu, P. Mesdom, M. Lagouge, F. Foufelle, Endoplasmic reticulum proteostasis in hepatic steatosis, Nat Rev Endocrinol 12(12) (2016) 710-722.
[2] T. Sasako, M. Ohsugi, N. Kubota, S. Itoh, Y. Okazaki, A. Terai, T. Kubota, S. Yamashita,

K. Nakatsukasa, T. Kamura, K. Iwayama, K. Tokuyama, H. Kiyonari, Y. Furuta, J. Shibahara,

M. Fukayama, K. Enooku, K. Okushin, T. Tsutsumi, R. Tateishi, K. Tobe, H. Asahara, K. Koike, T. Kadowaki, K. Ueki, Hepatic Sdf2l1 controls feeding-induced ER stress and regulates metabolism, Nat Commun 10(1) (2019) 947.
[3] K. McCaffrey, I. Braakman, Protein quality control at the endoplasmic reticulum, Essays Biochem 60(2) (2016) 227-235.
[4] W. Li, J. Zhu, J. Dou, H. She, K. Tao, H. Xu, Q. Yang, Z. Mao, Phosphorylation of LAMP2A by p38 MAPK couples ER stress to chaperone-mediated autophagy, Nat Commun 8(1) (2017) 1763.
[5] N.J. Lehrbach, G. Ruvkun, Endoplasmic reticulum-associated SKN-1A/Nrf1 mediates a cytoplasmic unfolded protein response and promotes longevity, Elife 8 (2019).
[6] R.M. Schmidt, J.P. Schessner, G.H. Borner, S. Schuck, The proteasome biogenesis regulator Rpn4 cooperates with the unfolded protein response to promote ER stress resistance, Elife 8 (2019).
[7] C.C. Hu, S.K. Dougan, A.M. McGehee, J.C. Love, H.L. Ploegh, XBP-1 regulates signal transduction, transcription factors and bone marrow colonization in B cells, EMBO J 28(11) (2009) 1624-36.
[8] C. Hetz, E. Chevet, S.A. Oakes, Proteostasis control by the unfolded protein response,

Nat Cell Biol 17(7) (2015) 829-38.

[9] A.E. Frakes, A. Dillin, The UPR(ER): Sensor and Coordinator of Organismal Homeostasis, Mol Cell 66(6) (2017) 761-771.
[10] J. Behnke, M.J. Feige, L.M. Hendershot, BiP and its nucleotide exchange factors Grp170 and Sil1: mechanisms of action and biological functions, J Mol Biol 427(7) (2015) 1589-608.
[11] T. Nam, J.H. Han, S. Devkota, H.W. Lee, Emerging Paradigm of Crosstalk between Autophagy and the Ubiquitin-Proteasome System, Mol Cells 40(12) (2017) 897-905.
[12] M. Gundogdu, H. Walden, Structural basis of generic versus specific E2-RING E3 interactions in protein ubiquitination, Protein Sci 28(10) (2019) 1758-1770.
[13] X. Wang, P. Cimermancic, C. Yu, A. Schweitzer, N. Chopra, J.L. Engel, C. Greenberg,

A.S. Huszagh, F. Beck, E. Sakata, Y. Yang, E.J. Novitsky, A. Leitner, P. Nanni, A. Kahraman,

X. Guo, J.E. Dixon, S.D. Rychnovsky, R. Aebersold, W. Baumeister, A. Sali, L. Huang, Molecular Details Underlying Dynamic Structures and Regulation of the Human 26S Proteasome, Mol Cell Proteomics 16(5) (2017) 840-854.
[14] B. Fabre, I. Livneh, T. Ziv, A. Ciechanover, Identification of proteins regulated by the proteasome following induction of endoplasmic reticulum stress, Biochem Biophys Res Commun 517(2) (2019) 188-192.
[15] F.Y. Chen, M.Y. Huang, Y.M. Lin, C.H. Ho, S.Y. Lin, H.Y. Chen, M.C. Hung, R.H. Chen, BIK ubiquitination by the E3 ligase Cul5-ASB11 determines cell fate during cellular stress, J Cell Biol 218(9) (2019) 3002-3018.
[16] Y. Wu, X. Li, J. Jia, Y. Zhang, J. Li, Z. Zhu, H. Wang, J. Tang, J. Hu, Transmembrane E3 ligase RNF183 mediates ER stress-induced apoptosis by degrading Bcl-xL, Proc Natl Acad

Sci U S A 115(12) (2018) E2762-E2771.

[17] H. Sun, G. Wei, H. Liu, D. Xiao, J. Huang, J. Lu, J. Miao, J. Liu, S. Chen, Inhibition of XBP1s ubiquitination enhances its protein stability and improves glucose homeostasis, Metabolism 105 (2019) 154046.
[18] Y.S. Gallot, K.R. Bohnert, A.R. Straughn, G. Xiong, S.M. Hindi, A. Kumar, PERK regulates skeletal muscle mass and contractile function in adult mice, FASEB J 33(2) (2019) 1946-1962.
[19] T. Toyofuku, Y. Okamoto, T. Ishikawa, S. Sasawatari, A. Kumanogoh, LRRK2 regulates endoplasmic reticulum-mitochondrial tethering through the PERK-mediated ubiquitination pathway, EMBO J 39(2) (2020) e100875.
[20] Y. Huang, Y. Sun, Y. Cao, H. Sun, M. Li, H. You, D. Su, Y. Li, X. Liang, HRD1 prevents apoptosis in renal tubular epithelial cells by mediating eIF2alpha ubiquitylation and degradation, Cell Death Dis 8(12) (2017) 3202.
[21] H.L. Guo, H.M. Hassan, P.P. Ding, S.J. Wang, X. Chen, T. Wang, L.X. Sun, L.Y. Zhang,

Z.Z. Jiang, Pyrazinamide-induced hepatotoxicity is alleviated by 4-PBA via inhibition of the PERK-eIF2alpha-ATF4-CHOP pathway, Toxicology 378 (2017) 65-75.
[22] L. Baird, T. Tsujita, E.H. Kobayashi, R. Funayama, T. Nagashima, K. Nakayama, M. Yamamoto, A Homeostatic Shift Facilitates Endoplasmic Reticulum Proteostasis through Transcriptional Integration of Proteostatic Stress Response Pathways, Mol Cell Biol 37(4) (2017).
[23] B.N. Lilley, H.L. Ploegh, Multiprotein complexes that link dislocation, ubiquitination, and extraction of misfolded proteins from the endoplasmic reticulum membrane, Proc Natl Acad

Sci U S A 102(40) (2005) 14296-301.

[24] Y. Oda, T. Okada, H. Yoshida, R.J. Kaufman, K. Nagata, K. Mori, Derlin-2 and Derlin-3 are regulated by the mammalian unfolded protein response and are required for ER- associated degradation, J Cell Biol 172(3) (2006) 383-93.
[25] I. Dikic, Z. Elazar, Mechanism and medical implications of mammalian autophagy, Nat Rev Mol Cell Biol 19(6) (2018) 349-364.
[26] A. Margariti, H. Li, T. Chen, D. Martin, G. Vizcay-Barrena, S. Alam, E. Karamariti, Q. Xiao, A. Zampetaki, Z. Zhang, W. Wang, Z. Jiang, C. Gao, B. Ma, Y.G. Chen, G. Cockerill, Y. Hu, Q. Xu, L. Zeng, XBP1 mRNA splicing triggers an autophagic response in endothelial cells through BECLIN-1 transcriptional activation, J Biol Chem 288(2) (2013) 859-72.
[27] R.L. Vidal, A. Figueroa, F.A. Court, P. Thielen, C. Molina, C. Wirth, B. Caballero, R. Kiffin,

J. Segura-Aguilar, A.M. Cuervo, L.H. Glimcher, C. Hetz, Targeting the UPR transcription factor XBP1 protects against Huntington’s disease through the regulation of FoxO1 and autophagy, Hum Mol Genet 21(10) (2012) 2245-62.
[28] H. Wan, Q. Wang, X. Chen, Q. Zeng, Y. Shao, H. Fang, X. Liao, H.S. Li, M.G. Liu, T.L. Xu, M. Diao, D. Li, B. Meng, B. Tang, Z. Zhang, L. Liao, WDR45 contributes to neurodegeneration through regulation of ER homeostasis and neuronal death, Autophagy 16(3) (2020) 531-547.
[29] C. Liu, D.Y. Yan, C. Wang, Z. Ma, Y. Deng, W. Liu, B. Xu, IRE1 signaling pathway mediates protective autophagic response against manganese-induced neuronal apoptosis in vivo and in vitro, Sci Total Environ 712 (2020) 136480.
[30] C. Shen, J. Yan, O.F. Erkocak, X.F. Zheng, X.D. Chen, Nitric oxide inhibits autophagy via
suppression of JNK in meniscal cells, Rheumatology (Oxford) 53(6) (2014) 1022-33.
[31] W. B’Chir, A.C. Maurin, V. Carraro, J. Averous, C. Jousse, Y. Muranishi, L. Parry, G. Stepien, P. Fafournoux, A. Bruhat, The eIF2alpha/ATF4 pathway is essential for stress- induced autophagy gene expression, Nucleic Acids Res 41(16) (2013) 7683-99.
[32] H. Matsumoto, S. Miyazaki, S. Matsuyama, M. Takeda, M. Kawano, H. Nakagawa, K. Nishimura, S. Matsuo, Selection of autophagy or apoptosis in cells exposed to ER-stress depends on ATF4 expression pattern with or without CHOP expression, Biol Open 2(10) (2013) 1084-90.
[33] H.J. Kim, Y. Joe, S.K. Kim, S.U. Park, J. Park, Y. Chen, J. Kim, J. Ryu, G.J. Cho, Y.J. Surh, S.W. Ryter, U.H. Kim, H.T. Chung, Carbon monoxide protects against hepatic steatosis in mice by inducing sestrin-2 via the PERK-eIF2alpha-ATF4 pathway, Free Radic Biol Med 110 (2017) 81-91.
[34] F.J. Guo, Z. Xiong, X. Lu, M. Ye, X. Han, R. Jiang, ATF6 upregulates XBP1S and inhibits ER stress-mediated apoptosis in osteoarthritis cartilage, Cell Signal 26(2) (2014) 332-42.
[35] I. Hirsch, M. Weiwad, E. Prell, D.M. Ferrari, ERp29 deficiency affects sensitivity to apoptosis via impairment of the ATF6-CHOP pathway of stress response, Apoptosis 19(5) (2014) 801-15.
[36] Y. Zhou, S. Zhang, C. Dai, S. Tang, X. Yang, D. Li, K. Zhao, X. Xiao, Quinocetone triggered ER stress-induced autophagy via ATF6/DAPK1-modulated mAtg9a trafficking, Cell Biol Toxicol 32(2) (2016) 141-52.
[37] D.H. Ipsen, J. Lykkesfeldt, P. Tveden-Nyborg, Molecular mechanisms of hepatic lipid accumulation in non-alcoholic fatty liver disease, Cell Mol Life Sci 75(18) (2018) 3313-3327.
[38] P. Bozaykut, A. Sahin, B. Karademir, N.K. Ozer, Endoplasmic reticulum stress related molecular mechanisms in nonalcoholic steatohepatitis, Mech Ageing Dev 157 (2016) 17-29.
[39] X.H. Fun, G. Thibault, Lipid bilayer stress and proteotoxic stress-induced unfolded protein response deploy divergent transcriptional and non-transcriptional programmes, Biochim Biophys Acta Mol Cell Biol Lipids 1865(1) (2020) 158449.
[40] P. Puri, F. Mirshahi, O. Cheung, R. Natarajan, J.W. Maher, J.M. Kellum, A.J. Sanyal, Activation and dysregulation of the unfolded protein response in nonalcoholic fatty liver disease, Gastroenterology 134(2) (2008) 568-76.
[41] M. Shao, B. Shan, Y. Liu, Y. Deng, C. Yan, Y. Wu, T. Mao, Y. Qiu, Y. Zhou, S. Jiang, W. Jia, J. Li, J. Li, L. Rui, L. Yang, Y. Liu, Hepatic IRE1alpha regulates fasting-induced metabolic adaptive programs through the XBP1s-PPARalpha axis signalling, Nat Commun 5 (2014) 3528.
[42] Y.M. Cho, T.M. Kim, D. Hun Kim, D. Hee Kim, S.W. Jeong, O.J. Kwon, miR-148a is a downstream effector of X-box-binding protein 1 that silences Wnt10b during adipogenesis of 3T3-L1 cells, Exp Mol Med 48 (2016) e226.
[43] M. Ahmed, T.H. Lai, J.S. Hwang, S. Zada, T.M. Pham, D.R. Kim, Transcriptional Regulation of Autophagy Genes via Stage-Specific Activation of CEBPB and PPARG during Adipogenesis: A Systematic Study Using Public Gene Expression and Transcription Factor Binding Datasets, Cells 8(11) (2019).
[44] J.S. So, K.Y. Hur, M. Tarrio, V. Ruda, M. Frank-Kamenetsky, K. Fitzgerald, V. Koteliansky, A.H. Lichtman, T. Iwawaki, L.H. Glimcher, A.H. Lee, Silencing of lipid metabolism genes through IRE1alpha-mediated mRNA decay lowers plasma lipids in mice, Cell Metab
16(4) (2012) 487-99.
[45] G.S. Hotamisligil, Endoplasmic reticulum stress and the inflammatory basis of metabolic disease, Cell 140(6) (2010) 900-17.
[46] C. Lebeaupin, D. Vallee, D. Rousseau, S. Patouraux, S. Bonnafous, G. Adam, F. Luciano, C. Luci, R. Anty, A. Iannelli, S. Marchetti, G. Kroemer, S. Lacas-Gervais, A. Tran, P. Gual, B. Bailly-Maitre, Bax inhibitor-1 protects from nonalcoholic steatohepatitis by limiting inositol-requiring enzyme 1 alpha signaling in mice, Hepatology 68(2) (2018) 515-532.
[47] T. Otoda, T. Takamura, H. Misu, T. Ota, S. Murata, H. Hayashi, H. Takayama, A. Kikuchi,

T. Kanamori, K.R. Shima, F. Lan, T. Takeda, S. Kurita, K. Ishikura, Y. Kita, K. Iwayama, K. Kato, M. Uno, Y. Takeshita, M. Yamamoto, K. Tokuyama, S. Iseki, K. Tanaka, S. Kaneko, Proteasome dysfunction mediates obesity-induced endoplasmic reticulum stress and insulin resistance in the liver, Diabetes 62(3) (2013) 811-24.
[48] I.H. Bang, O.K. Kwon, L. Hao, D. Park, M.J. Chung, B.C. Oh, S. Lee, E.J. Bae, B.H. Park, Deacetylation of XBP1s by sirtuin 6 confers resistance to ER stress-induced hepatic steatosis, Exp Mol Med 51(9) (2019) 107.
[49] D. Kumar, M. Das, C. Sauceda, L.G. Ellies, K. Kuo, P. Parwal, M. Kaur, L. Jih, G.K. Bandyopadhyay, D. Burton, R. Loomba, O. Osborn, N.J. Webster, Degradation of splicing factor SRSF3 contributes to progressive liver disease, J Clin Invest 129(10) (2019) 4477- 4491.
[50] S. Tanaka, H. Hikita, T. Tatsumi, R. Sakamori, Y. Nozaki, S. Sakane, Y. Shiode, T. Nakabori, Y. Saito, N. Hiramatsu, K. Tabata, T. Kawabata, M. Hamasaki, H. Eguchi, H. Nagano, T. Yoshimori, T. Takehara, Rubicon inhibits autophagy and accelerates hepatocyte
apoptosis and lipid accumulation in nonalcoholic fatty liver disease in mice, Hepatology 64(6) (2016) 1994-2014.
[51] M. Shinohara, C. Ji, N. Kaplowitz, Differences in betaine-homocysteine methyltransferase expression, endoplasmic reticulum stress response, and liver injury between alcohol-fed mice and rats, Hepatology 51(3) (2010) 796-805.
[52] C. Ji, N. Kaplowitz, Betaine decreases hyperhomocysteinemia, endoplasmic reticulum stress, and liver injury in alcohol-fed mice, Gastroenterology 124(5) (2003) 1488-99.
[53] G.H. Werstuck, S.R. Lentz, S. Dayal, G.S. Hossain, S.K. Sood, Y.Y. Shi, J. Zhou, N. Maeda, S.K. Krisans, M.R. Malinow, R.C. Austin, Homocysteine-induced endoplasmic reticulum stress causes dysregulation of the cholesterol and triglyceride biosynthetic pathways, J Clin Invest 107(10) (2001) 1263-73.
[54] M. You, M. Matsumoto, C.M. Pacold, W.K. Cho, D.W. Crabb, The role of AMP-activated protein kinase in the action of ethanol in the liver, Gastroenterology 127(6) (2004) 1798-808.
[55] D.L. Howarth, A.M. Vacaru, O. Tsedensodnom, E. Mormone, N. Nieto, L.M. Costantini,

E.L. Snapp, K.C. Sadler, Alcohol disrupts endoplasmic reticulum function and protein secretion in hepatocytes, Alcohol Clin Exp Res 36(1) (2012) 14-23.
[56] O. Tsedensodnom, A.M. Vacaru, D.L. Howarth, C. Yin, K.C. Sadler, Ethanol metabolism and oxidative stress are required for unfolded protein response activation and steatosis in zebrafish with alcoholic liver disease, Dis Model Mech 6(5) (2013) 1213-26.
[57] J.J. Galligan, R.L. Smathers, C.T. Shearn, K.S. Fritz, D.S. Backos, H. Jiang, C.C. Franklin, D.J. Orlicky, K.N. Maclean, D.R. Petersen, Oxidative Stress and the ER Stress Response in a Murine Model for Early-Stage Alcoholic Liver Disease, J Toxicol 2012 (2012)
207594.
[58] J.J. Galligan, K.S. Fritz, D.S. Backos, C.T. Shearn, R.L. Smathers, H. Jiang, K.N. MacLean, P.R. Reigan, D.R. Petersen, Oxidative stress-mediated aldehyde adduction of GRP78 in a mouse model of alcoholic liver disease: functional independence of ATPase activity and chaperone function, Free Radic Biol Med 73 (2014) 411-20.
[59] Q. Song, Y. Chen, J. Wang, L. Hao, C. Huang, A. Griffiths, Z. Sun, Z. Zhou, Z. Song, ER stress-induced upregulation of NNMT contributes to alcohol-related fatty liver development, J Hepatol (2020).
[60] J.K. Park, M. Shao, M.Y. Kim, S.K. Baik, M.Y. Cho, T. Utsumi, A. Satoh, X. Ouyang, C. Chung, Y. Iwakiri, An endoplasmic reticulum protein, Nogo-B, facilitates alcoholic liver disease through regulation of kupffer cell polarization, Hepatology 65(5) (2017) 1720-1734.
[61] J. Oliva, S.W. French, J. Li, F. Bardag-Gorce, Proteasome inhibitor treatment reduced fatty acid, triacylglycerol and cholesterol synthesis, Exp Mol Pathol 93(1) (2012) 26-34.
[62] W.X. Ding, M. Li, X. Chen, H.M. Ni, C.W. Lin, W. Gao, B. Lu, D.B. Stolz, D.L. Clemens,
X.M. Yin, Autophagy reduces acute ethanol-induced hepatotoxicity and steatosis in mice, Gastroenterology 139(5) (2010) 1740-52.
[63] P.G. Thomes, R.A. Ehlers, C.S. Trambly, D.L. Clemens, H.S. Fox, D.J. Tuma, T.M. Donohue, Multilevel regulation of autophagosome content by ethanol oxidation in HepG2 cells, Autophagy 9(1) (2013) 63-73.
[64] S. Lanini, A. Ustianowski, R. Pisapia, A. Zumla, G. Ippolito, Viral Hepatitis: Etiology, Epidemiology, Transmission, Diagnostics, Treatment, and Prevention, Infect Dis Clin North Am 33(4) (2019) 1045-1062.
[65] S. Piconese, I. Cammarata, V. Barnaba, Viral hepatitis, inflammation, and cancer: A lesson for autoimmunity, J Autoimmun 95 (2018) 58-68.
[66] B. Li, B. Gao, L. Ye, X. Han, W. Wang, L. Kong, X. Fang, Y. Zeng, H. Zheng, S. Li, Z. Wu, L. Ye, Hepatitis B virus X protein (HBx) activates ATF6 and IRE1-XBP1 pathways of unfolded protein response, Virus Res 124(1-2) (2007) 44-9.
[67] C. Lazar, M. Uta, N. Branza-Nichita, Modulation of the unfolded protein response by the human hepatitis B virus, Front Microbiol 5 (2014) 433.
[68] S. McPherson, E.E. Powell, H.D. Barrie, A.D. Clouston, M. McGuckin, J.R. Jonsson, No evidence of the unfolded protein response in patients with chronic hepatitis C virus infection, J Gastroenterol Hepatol 26(2) (2011) 319-27.
[69] E. Merquiol, D. Uzi, T. Mueller, D. Goldenberg, Y. Nahmias, R.J. Xavier, B. Tirosh, O. Shibolet, HCV causes chronic endoplasmic reticulum stress leading to adaptation and interference with the unfolded protein response, PLoS One 6(9) (2011) e24660.
[70] C. Lazar, A. Macovei, S. Petrescu, N. Branza-Nichita, Activation of ERAD pathway by human hepatitis B virus modulates viral and subviral particle production, PLoS One 7(3) (2012) e34169.
[71] F. Kong, H. You, D. Kong, K. Zheng, R. Tang, The interaction of hepatitis B virus with the ubiquitin proteasome system in viral replication and associated pathogenesis, Virol J 16(1) (2019) 73.
[72] T. Doring, L. Zeyen, C. Bartusch, R. Prange, Hepatitis B Virus Subverts the Autophagy Elongation Complex Atg5-12/16L1 and Does Not Require Atg8/LC3 Lipidation for Viral Maturation, J Virol 92(7) (2018).
[73] J. Wang, R. Kang, H. Huang, X. Xi, B. Wang, J. Wang, Z. Zhao, Hepatitis C virus core protein activates autophagy through EIF2AK3 and ATF6 UPR pathway-mediated MAP1LC3B and ATG12 expression, Autophagy 10(5) (2014) 766-84.
[74] S. Shrivastava, J. Bhanja Chowdhury, R. Steele, R. Ray, R.B. Ray, Hepatitis C virus upregulates Beclin1 for induction of autophagy and activates mTOR signaling, J Virol 86(16) (2012) 8705-12.
[75] M. de Wispelaere, G. Du, K.A. Donovan, T. Zhang, N.A. Eleuteri, J.C. Yuan, J. Kalabathula, R.P. Nowak, E.S. Fischer, N.S. Gray, P.L. Yang, Small molecule degraders of the hepatitis C virus protease reduce susceptibility to resistance mutations, Nat Commun 10(1) (2019) 3468.
[76] L. Hou, J. Zhao, S. Gao, T. Ji, T. Song, Y. Li, J. Wang, C. Geng, M. Long, J. Chen, H. Lin, X. Cai, Y. Cang, Restriction of hepatitis B virus replication by c-Abl-induced proteasomal degradation of the viral polymerase, Sci Adv 5(2) (2019) eaau7130.
[77] I. Mannaerts, L.F.R. Thoen, N. Eysackers, F.J. Cubero, S. Batista Leite, I. Coldham, I. Colle, C. Trautwein, L.A. van Grunsven, Unfolded protein response is an early, non-critical event during hepatic stellate cell activation, Cell Death Dis 10(2) (2019) 98.
[78] A. Paridaens, S. Raevens, L. Devisscher, E. Bogaerts, X. Verhelst, A. Hoorens, H. Van Vlierberghe, L.A. van Grunsven, A. Geerts, I. Colle, Modulation of the Unfolded Protein Response by Tauroursodeoxycholic Acid Counteracts Apoptotic Cell Death and Fibrosis in a Mouse Model for Secondary Biliary Liver Fibrosis, Int J Mol Sci 18(1) (2017).
[79] V. Hernandez-Gea, M. Hilscher, R. Rozenfeld, M.P. Lim, N. Nieto, S. Werner, L.A. Devi,
S.L. Friedman, Endoplasmic reticulum stress induces fibrogenic activity in hepatic stellate
cells through autophagy, J Hepatol 59(1) (2013) 98-104.
[80] T. Jiang, L. Wang, X. Li, J. Song, X. Wu, S. Zhou, Inositol-requiring enzyme 1-mediated endoplasmic reticulum stress triggers apoptosis and fibrosis formation in liver cirrhosis rat models, Mol Med Rep 11(4) (2015) 2941-6.
[81] A. Anan, E.S. Baskin-Bey, S.F. Bronk, N.W. Werneburg, V.H. Shah, G.J. Gores, Proteasome inhibition induces hepatic stellate cell apoptosis, Hepatology 43(2) (2006) 335- 44.
[82] A. Anan, E.S. Baskin-Bey, H. Isomoto, J.L. Mott, S.F. Bronk, J.H. Albrecht, G.J. Gores, Proteasome inhibition attenuates hepatic injury in the bile duct-ligated mouse, Am J Physiol Gastrointest Liver Physiol 291(4) (2006) G709-16.
[83] Y. Cai, C.H. Zhou, D. Fu, X.Z. Shen, Overexpression of Smad ubiquitin regulatory factor 2 suppresses transforming growth factor-beta mediated liver fibrosis, J Dig Dis 13(6) (2012) 327-34.
[84] R.S. Kim, D. Hasegawa, N. Goossens, T. Tsuchida, V. Athwal, X. Sun, C.L. Robinson, D. Bhattacharya, H.I. Chou, D.Y. Zhang, B.C. Fuchs, Y. Lee, Y. Hoshida, S.L. Friedman, The XBP1 Arm of the Unfolded Protein Response Induces Fibrogenic Activity in Hepatic Stellate Cells Through Autophagy, Sci Rep 6 (2016) 39342.
[85] V. Hernandez-Gea, Z. Ghiassi-Nejad, R. Rozenfeld, R. Gordon, M.I. Fiel, Z. Yue, M.J. Czaja, S.L. Friedman, Autophagy releases lipid that promotes fibrogenesis by activated hepatic stellate cells in mice and in human tissues, Gastroenterology 142(4) (2012) 938-46.
[86] Y. Li, Y. Chen, H. Huang, M. Shi, W. Yang, J. Kuang, J. Yan, Autophagy mediated by endoplasmic reticulum stress enhances the caffeine-induced apoptosis of hepatic stellate
cells, Int J Mol Med 40(5) (2017) 1405-1414.
[87] X.W. Zhang, J.C. Zhou, D. Peng, F. Hua, K. Li, J.J. Yu, X.X. Lv, B. Cui, S.S. Liu, J.M. Yu,
F. Wang, C.C. Jin, Z.N. Yang, C.X. Zhao, X.Y. Hou, B. Huang, Z.W. Hu, Disrupting the TRIB3-SQSTM1 interaction reduces liver fibrosis by restoring autophagy and suppressing exosome-mediated HSC activation, Autophagy (2019) 1-15.
[88] Y.H. Huang, Y.L. Yang, F.C. Huang, M.M. Tiao, Y.C. Lin, M.H. Tsai, F.S. Wang, MicroRNA-29a mitigation of endoplasmic reticulum and autophagy aberrance counteracts in obstructive jaundice-induced fibrosis in mice, Exp Biol Med (Maywood) 243(1) (2018) 13-21.
[89] M.D. Lewis, B.J. Roberts, Role of CYP2E1 activity in endoplasmic reticulum ubiquitination, proteasome association, and the unfolded protein response, Arch Biochem Biophys 436(2) (2005) 237-45.
[90] Y. Wu, B. Shan, J. Dai, Z. Xia, J. Cai, T. Chen, S. Lv, Y. Feng, L. Zheng, Y. Wang, J. Liu,
J. Fang, D. Xie, L. Rui, J. Liu, Y. Liu, Dual role for inositol-requiring enzyme 1alpha in promoting the development of hepatocellular carcinoma during diet-induced obesity in mice, Hepatology 68(2) (2018) 533-546.
[91] Y.P. Vandewynckel, D. Laukens, E. Bogaerts, A. Paridaens, A. Van den Bussche, X. Verhelst, C. Van Steenkiste, B. Descamps, C. Vanhove, L. Libbrecht, R. De Rycke, B.N. Lambrecht, A. Geerts, S. Janssens, H. Van Vlierberghe, Modulation of the unfolded protein response impedes tumor cell adaptation to proteotoxic stress: a PERK for hepatocellular carcinoma therapy, Hepatol Int 9(1) (2015) 93-104.
[92] G. Augello, M. Modica, A. Azzolina, R. Puleio, G. Cassata, M.R. Emma, C. Di Sano, A. Cusimano, G. Montalto, M. Cervello, Preclinical evaluation of antitumor activity of the proteasome inhibitor MLN2238 (ixazomib) in hepatocellular carcinoma cells, Cell Death Dis 9(2) (2018) 28.
[93] B. Boozari, B. Mundt, N. Woller, N. Struver, E. Gurlevik, P. Schache, A. Kloos, S. Knocke, M.P. Manns, T.C. Wirth, S. Kubicka, F. Kuhnel, Antitumoural immunity by virus- mediated immunogenic apoptosis inhibits metastatic growth of hepatocellular carcinoma, Gut 59(10) (2010) 1416-26.
[94] C. Mendez-Blanco, F. Fondevila, A. Garcia-Palomo, J. Gonzalez-Gallego, J.L. Mauriz, Sorafenib resistance in hepatocarcinoma: role of hypoxia-inducible factors, Exp Mol Med 50(10) (2018) 134.
[95] Y. Honma, M. Harada, Sorafenib enhances proteasome inhibitor-mediated cytotoxicity via inhibition of unfolded protein response and keratin phosphorylation, Exp Cell Res 319(14) (2013) 2166-78.
[96] S. Lu, Y. Yao, G. Xu, C. Zhou, Y. Zhang, J. Sun, R. Jiang, Q. Shao, Y. Chen, CD24 regulates sorafenib resistance via activating autophagy in hepatocellular carcinoma, Cell Death Dis 9(6) (2018) 646.
[97] P. Yi, A. Higa, S. Taouji, M.G. Bexiga, E. Marza, D. Arma, C. Castain, B. Le Bail, J.C. Simpson, J. Rosenbaum, C. Balabaud, P. Bioulac-Sage, J.F. Blanc, E. Chevet, Sorafenib- mediated targeting of the AAA(+) ATPase p97/VCP leads to disruption of the secretory pathway, endoplasmic reticulum stress, and hepatocellular cancer cell death, Mol Cancer Ther 11(12) (2012) 2610-20.
[98] B. Zhou, Q. Lu, J. Liu, L. Fan, Y. Wang, W. Wei, H. Wang, G. Sun, Melatonin Increases the Sensitivity of Hepatocellular Carcinoma to Sorafenib through the PERK-ATF4-Beclin1

Pathway, Int J Biol Sci 15(9) (2019) 1905-1920.
[99] U. Ozcan, E. Yilmaz, L. Ozcan, M. Furuhashi, E. Vaillancourt, R.O. Smith, C.Z. Gorgun,
G.S. Hotamisligil, Chemical chaperones reduce ER stress and Selonsertib restore glucose homeostasis in a mouse model of type 2 diabetes, Science 313(5790) (2006) 1137-40.
[100] A.U. Nissar, L. Sharma, M.A. Mudasir, L.A. Nazir, S.A. Umar, P.R. Sharma, R.A. Vishwakarma, S.A. Tasduq, Chemical chaperone 4-phenyl butyric acid (4-PBA) reduces hepatocellular lipid accumulation and lipotoxicity through induction of autophagy, J Lipid Res 58(9) (2017) 1855-1868.
[101] L. Zuo, Y. Zhu, L. Hu, Y. Liu, Y. Wang, Y. Hu, H. Wang, X. Pan, K. Li, N. Du, Y. Huang, PI3-kinase/Akt pathway-regulated membrane transportation of acid-sensing ion channel 1a/Calcium ion influx/endoplasmic reticulum stress activation on PDGF-induced HSC Activation, J Cell Mol Med 23(6) (2019) 3940-3950.
[102] Y.P. Vandewynckel, D. Laukens, L. Devisscher, A. Paridaens, E. Bogaerts, X. Verhelst,

A. Van den Bussche, S. Raevens, C. Van Steenkiste, M. Van Troys, C. Ampe, B. Descamps,

C. Vanhove, O. Govaere, A. Geerts, H. Van Vlierberghe, Tauroursodeoxycholic acid dampens oncogenic apoptosis induced by endoplasmic reticulum stress during hepatocarcinogen exposure, Oncotarget 6(29) (2015) 28011-25.
[103] J. Liu, P. Han, M. Li, W. Yan, J. Liu, J. He, J. Gong, Y. Wang, D. Tian, Histidine-rich calcium binding protein promotes growth of hepatocellular carcinoma in vitro and in vivo, Cancer Sci 106(10) (2015) 1288-95.
[104] M. Sanches, N.M. Duffy, M. Talukdar, N. Thevakumaran, D. Chiovitti, M.D. Canny, K. Lee, I. Kurinov, D. Uehling, R. Al-awar, G. Poda, M. Prakesch, B. Wilson, V. Tam, C.
Schweitzer, A. Toro, J.L. Lucas, D. Vuga, L. Lehmann, D. Durocher, Q. Zeng, J.B. Patterson,
F. Sicheri, Structure and mechanism of action of the hydroxy-aryl-aldehyde class of IRE1 endoribonuclease inhibitors, Nat Commun 5 (2014) 4202.
[105] A. Carlesso, C. Chintha, A.M. Gorman, A. Samali, L.A. Eriksson, Merits and pitfalls of conventional and covalent docking in identifying new hydroxyl aryl aldehyde like compounds as human IRE1 inhibitors, Sci Rep 9(1) (2019) 3407.
[106] Q.Q. Chen, C. Zhang, M.Q. Qin, J. Li, H. Wang, D.X. Xu, J.Q. Wang, Inositol-Requiring Enzyme 1 Alpha Endoribonuclease Specific Inhibitor STF-083010 Alleviates Carbon Tetrachloride Induced Liver Injury and Liver Fibrosis in Mice, Front Pharmacol 9 (2018) 1344.
[107] I. Takahara, Y. Akazawa, M. Tabuchi, K. Matsuda, H. Miyaaki, Y. Kido, Y. Kanda, N. Taura, K. Ohnita, F. Takeshima, Y. Sakai, S. Eguchi, M. Nakashima, K. Nakao, Toyocamycin attenuates free fatty acid-induced hepatic steatosis and apoptosis in cultured hepatocytes and ameliorates nonalcoholic fatty liver disease in mice, PLoS One 12(3) (2017) e0170591.
[108] R. Loomba, E. Lawitz, P.S. Mantry, S. Jayakumar, S.H. Caldwell, H. Arnold, A.M. Diehl,
C.S. Djedjos, L. Han, R.P. Myers, G.M. Subramanian, J.G. McHutchison, Z.D. Goodman,
N.H. Afdhal, M.R. Charlton, G.-U.-. Investigators, The ASK1 inhibitor selonsertib in patients with nonalcoholic steatohepatitis: A randomized, phase 2 trial, Hepatology 67(2) (2018) 549- 559.
[109] E. Loeuillard, H. El Mourabit, L. Lei, S. Lemoinne, C. Housset, A. Cadoret, Endoplasmic reticulum stress induces inverse regulations of major functions in portal myofibroblasts during liver fibrosis progression, Biochim Biophys Acta Mol Basis Dis 1864(12) (2018) 3688-3696.
[110] S. Win, T.A. Than, B.H. Le, C. Garcia-Ruiz, J.C. Fernandez-Checa, N. Kaplowitz, Sab

(Sh3bp5) dependence of JNK mediated inhibition of mitochondrial respiration in palmitic acid induced hepatocyte lipotoxicity, J Hepatol 62(6) (2015) 1367-74.
[111] M. Zarei, E. Barroso, X. Palomer, J. Dai, P. Rada, T. Quesada-Lopez, J.C. Escola-Gil,

L. Cedo, M.R. Zali, M. Molaei, R. Dabiri, S. Vazquez, E. Pujol, A.M. Valverde, F. Villarroya, Y. Liu, W. Wahli, M. Vazquez-Carrera, Hepatic regulation of VLDL receptor by PPARbeta/delta and FGF21 modulates non-alcoholic fatty liver disease, Mol Metab 8 (2018) 117-131.
[112] B.N. Desai, G. Singhal, M. Watanabe, D. Stevanovic, T. Lundasen, F.M. Fisher, M.L. Mather, H.G. Vardeh, N. Douris, A.C. Adams, I.A. Nasser, G.A. FitzGerald, J.S. Flier, C. Skarke, E. Maratos-Flier, Fibroblast growth factor 21 (FGF21) is robustly induced by ethanol and has a protective role in ethanol associated liver injury, Mol Metab 6(11) (2017) 1395- 1406.
[113] H. Kang, E. Seo, J.M. Park, N.Y. Han, H. Lee, H.S. Jun, Effects of FGF21-secreting adipose-derived stem cells in thioacetamide-induced hepatic fibrosis, J Cell Mol Med 22(10) (2018) 5165-5169.
[114] G. Singhal, G. Kumar, S. Chan, F.M. Fisher, Y. Ma, H.G. Vardeh, I.A. Nasser, J.S. Flier,

E. Maratos-Flier, Deficiency of fibroblast growth factor 21 (FGF21) promotes hepatocellular carcinoma (HCC) in mice on a long term obesogenic diet, Mol Metab 13 (2018) 56-66.
[115] C. Yang, W. Lu, T. Lin, P. You, M. Ye, Y. Huang, X. Jiang, C. Wang, F. Wang, M.H. Lee, S.C. Yeung, R.L. Johnson, C. Wei, R.Y. Tsai, M.L. Frazier, W.L. McKeehan, Y. Luo, Activation of Liver FGF21 in hepatocarcinogenesis and during hepatic stress, BMC Gastroenterol 13 (2013) 67.
[116] T.F. Kuo, H. Tatsukawa, T. Matsuura, K. Nagatsuma, S. Hirose, S. Kojima, Free fatty
acids induce transglutaminase 2-dependent apoptosis in hepatocytes via ER stress- stimulated PERK pathways, J Cell Physiol 227(3) (2012) 1130-7.
[117] S. Yoshino, Y. Iwasaki, S. Matsumoto, T. Satoh, A. Ozawa, E. Yamada, S. Kakizaki,
J.A.O. Trejo, Y. Uchiyama, M. Yamada, M. Mori, Administration of small-molecule guanabenz acetate attenuates fatty liver and hyperglycemia associated with obesity, Sci Rep 10(1) (2020) 13671.
[118] C.M. Gallagher, P. Walter, Ceapins inhibit ATF6alpha signaling by selectively preventing transport of ATF6alpha to the Golgi apparatus during ER stress, Elife 5 (2016).
[119] R. Zhou, L. Yi, X. Ye, X. Zeng, K. Liu, Y. Qin, Q. Zhang, M. Mi, Resveratrol Ameliorates Lipid Droplet Accumulation in Liver Through a SIRT1/ ATF6-Dependent Mechanism, Cell Physiol Biochem 51(5) (2018) 2397-2420.
[120] X. Wu, N. Xu, M. Li, Q. Huang, J. Wu, Y. Gan, L. Chen, H. Luo, Y. Li, X. Huang, Z. Su,
Y. Liu, Protective Effect of Patchouli Alcohol Against High-Fat Diet Induced Hepatic Steatosis by Alleviating Endoplasmic Reticulum Stress and Regulating VLDL Metabolism in Rats, Front Pharmacol 10 (2019) 1134.
[121] L. Zhang, J. Chen, D. Ning, Q. Liu, C. Wang, Z. Zhang, L. Chu, C. Yu, H.F. Liang, B. Zhang, X. Chen, FBXO22 promotes the development of hepatocellular carcinoma by regulating the ubiquitination and degradation of p21, J Exp Clin Cancer Res 38(1) (2019) 101.
[122] T. Jiang, F. Zhan, Z. Rao, X. Pan, W. Zhong, Y. Sun, P. Wang, L. Lu, H. Zhou, X. Wang, Combined ischemic and rapamycin preconditioning alleviated liver ischemia and reperfusion injury by restoring autophagy in aged mice, Int Immunopharmacol 74 (2019) 105711.
[123] A. Umemura, E.J. Park, K. Taniguchi, J.H. Lee, S. Shalapour, M.A. Valasek, M. Aghajan, H. Nakagawa, E. Seki, M.N. Hall, M. Karin, Liver damage, inflammation, and enhanced tumorigenesis after persistent mTORC1 inhibition, Cell Metab 20(1) (2014) 133-44.
[124] Z. Chang, G. Shi, J. Jin, H. Guo, X. Guo, F. Luo, Y. Song, X. Jia, Dual PI3K/mTOR inhibitor NVP-BEZ235-induced apoptosis of hepatocellular carcinoma cell lines is enhanced by inhibitors of autophagy, Int J Mol Med 31(6) (2013) 1449-56.
[125] X. Huang, G. Gan, X. Wang, T. Xu, W. Xie, The HGF-MET axis coordinates liver cancer metabolism and autophagy for chemotherapeutic resistance, Autophagy 15(7) (2019) 1258- 1279.
[126] J.Q. Ma, Y.Z. Sun, Q.L. Ming, Z.K. Tian, H.X. Yang, C.M. Liu, Ampelopsin attenuates carbon tetrachloride-induced mouse liver fibrosis and hepatic stellate cell activation associated with the SIRT1/TGF-beta1/Smad3 and autophagy pathway, Int Immunopharmacol 77 (2019) 105984.
[127] S. Tan, Y. Lu, M. Xu, X. Huang, H. Liu, J. Jiang, B. Wu, beta-Arrestin1 enhances liver fibrosis through autophagy-mediated Snail signaling, FASEB J 33(2) (2019) 2000-2016.
[128] R.C. Rubenstein, P.L. Zeitlin, A pilot clinical trial of oral sodium 4-phenylbutyrate (Buphenyl) in deltaF508-homozygous cystic fibrosis patients: partial restoration of nasal epithelial CFTR function, Am J Respir Crit Care Med 157(2) (1998) 484-90.
[129] V.R. Solomon, H. Lee, Chloroquine and its analogs: a new promise of an old drug for effective and safe cancer therapies, Eur J Pharmacol 625(1-3) (2009) 220-33.
[130] M. Kars, L. Yang, M.F. Gregor, B.S. Mohammed, T.A. Pietka, B.N. Finck, B.W. Patterson, J.D. Horton, B. Mittendorfer, G.S. Hotamisligil, S. Klein, Tauroursodeoxycholic Acid
may improve liver and muscle but not adipose tissue insulin sensitivity in obese men and women, Diabetes 59(8) (2010) 1899-905.
[131] G.P. Kim, M.R. Mahoney, D. Szydlo, T.S. Mok, R. Marshke, K. Holen, J. Picus, M. Boyer, H.C. Pitot, J. Rubin, P.A. Philip, A. Nowak, J.J. Wright, C. Erlichman, An international, multicenter phase II trial of bortezomib in patients with hepatocellular carcinoma, Invest New Drugs 30(1) (2012) 387-94.
[132] C. Gomez-Martin, J. Bustamante, J.F. Castroagudin, M. Salcedo, E. Garralda, M. Testillano, I. Herrero, A. Matilla, B. Sangro, Efficacy and safety of sorafenib in combination with mammalian target of rapamycin inhibitors for recurrent hepatocellular carcinoma after liver transplantation, Liver Transpl 18(1) (2012) 45-52.
[133] W.L. van der Woerd, C.G. Wichers, A.L. Vestergaard, J.P. Andersen, C.C. Paulusma,
R.H. Houwen, S.F. van de Graaf, Rescue of defective ATP8B1 trafficking by CFTR correctors as a therapeutic strategy for familial intrahepatic cholestasis, J Hepatol 64(6) (2016) 1339-47.
[134] B. Wang, H. Yang, Y. Fan, Y. Yang, W. Cao, Y. Jia, Y. Cao, K. Sun, Z. Pang, H. Du, 3- Methyladenine ameliorates liver fibrosis through autophagy regulated by the NF-kappaB signaling pathways on hepatic stellate cell, Oncotarget 8(64) (2017) 107603-107611.
[135] P.Y. Ke, S.S. Chen, Activation of the unfolded protein response and autophagy after hepatitis C virus infection suppresses innate antiviral immunity in vitro, J Clin Invest 121(1) (2011) 37-56.
[136] Y. Li, Y. Xia, X. Cheng, D.E. Kleiner, S.M. Hewitt, J. Sproch, T. Li, H. Zhuang, T.J. Liang, Hepatitis B Surface Antigen Activates Unfolded Protein Response in Forming Ground Glass Hepatocytes of Chronic Hepatitis B, Viruses 11(4) (2019).
[137] S.W. Chan, P.A. Egan, Hepatitis C virus envelope proteins regulate CHOP via induction of the unfolded protein response, FASEB J 19(11) (2005) 1510-2.
[138] B. San-Miguel, I. Crespo, D.I. Sanchez, B. Gonzalez-Fernandez, J.J. Ortiz de Urbina,

M.J. Tunon, J. Gonzalez-Gallego, Melatonin inhibits autophagy and endoplasmic reticulum stress in mice with carbon tetrachloride-induced fibrosis, J Pineal Res 59(2) (2015) 151-62.
[139] J. Tang, Y.S. Guo, Y. Zhang, X.L. Yu, L. Li, W. Huang, Y. Li, B. Chen, J.L. Jiang, Z.N. Chen, CD147 induces UPR to inhibit apoptosis and chemosensitivity by increasing the transcription of Bip in hepatocellular carcinoma, Cell Death Differ 19(11) (2012) 1779-90.