Gp78, an E3 ubiquitin ligase acts as a gatekeeper suppressing nonalcoholic steatohepatitis (NASH) and liver cancer

The role of ubiquitination and deubiquitination in the pathogenesis of non-alcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) is one of the most common chronic liver diseases and is closely associated with metabolic abnormalities. The causes of NAFLD are exceedingly complicated, and it is known that a variety of signaling pathways, endoplasmic reticulum stress, and mitochondrial dysfunction play a role in the pathogenesis of NAFLD. Recent studies have shown that ubiquitination and deubiquitination are involved in the regulation of the NAFLD pathophysiology. Protein ubiquitination is a dynamic and diverse post-translational alteration that affects various cellular biological processes. Numerous disorders, including NAFLD, exhibit imbalances in ubiquitination and deubiquitination. To highlight the significance of this post-translational modification in the pathogenesis of NAFLD and to aid in the development of new therapeutic approaches for the disease, we will discuss the role of enzymes involved in the processes of ubiquitination and deubiquitination, specifically E3 ubiquitin ligases and deubiquitinating enzymes that are important in the regulation of NAFLD.

The emerging role of E3 ubiquitin ligases and deubiquitinases in metabolic dysfunction-associated steatotic liver disease

Metabolic dysfunction-associated steatotic liver disease (MASLD) is the most common chronic liver disease worldwide, with a prevalence as high as 32.4%. MASLD encompasses a spectrum of liver pathologies, ranging from steatosis to metabolic dysfunction-associated steatohepatitis (MASH), fibrosis, and, in some cases, progression to end-stage liver disease (cirrhosis and hepatocellular carcinoma). A comprehensive understanding of the pathogenesis of this highly prevalent liver disease may facilitate the identification of novel targets for the development of improved therapies. E3 ubiquitin ligases and deubiquitinases (DUBs) are key regulatory components of the ubiquitin‒proteasome system (UPS), which plays a pivotal role in maintaining intracellular protein homeostasis. Emerging evidence implicates that aberrant expression of E3 ligases and DUBs is involved in the progression of MASLD. Here, we review abnormalities in E3 ligases and DUBs by (1) discussing their targets, mechanisms, and functions in MASLD; (2) summarizing pharmacological interventions targeting these enzymes in preclinical and clinical studies; and (3) addressing challenges and future therapeutic strategies. This review synthesizes current evidence to highlight the development of novel therapeutic strategies based on the UPS for MASLD and progressive liver disease.

The ubiquitin-proteasome system: A potential target for the MASLD

Metabolic dysfunction-associated steatotic liver disease (MASLD), the most prevalent chronic liver condition globally, lacks adequate and effective therapeutic remedies in clinical practice. Recent studies have increasingly highlighted the close connection between the ubiquitin-proteasome system (UPS) and the progression of MASLD. This relationship is crucial for understanding the disease's underlying mechanism. As a sophisticated process, the UPS govern protein stability and function, maintaining protein homeostasis, thus influencing a multitude of elements and biological events of eukaryotic cells. It comprises four enzyme families, namely, ubiquitin-activating enzymes (E1), ubiquitin-conjugating enzymes (E2), ubiquitin-protein ligases (E3), and deubiquitinating enzymes (DUBs). This review aims to delve into the array of pathways and therapeutic targets implicated in the ubiquitination within the pathogenesis of MASLD. Therefore, this review unveils the role of ubiquitination in MASLD while spotlighting potential therapeutic targets within the context of this disease.

Polyphenols alleviate metabolic disorders: the role of ubiquitin-proteasome system

Metabolic disorders include obesity, nonalcoholic fatty liver disease, insulin resistance and type 2 diabetes. It has become a major health issue around the world. Ubiquitin-proteasome system (UPS) is essential for nearly all cellular processes, functions as a primary pathway for intracellular protein degradation. Recent researches indicated that dysfunctions in the UPS may result in the accumulation of toxic proteins, lipotoxicity, oxidative stress, inflammation, and insulin resistance, all of which contribute to the development and progression of metabolic disorders. An increasing body of evidence indicates that specific dietary polyphenols ameliorate metabolic disorders by preventing lipid synthesis and transport, excessive inflammation, hyperglycemia and insulin resistance, and oxidative stress, through regulation of the UPS. This review summarized the latest research progress of natural polyphenols improving metabolic disorders by regulating lipid accumulation, inflammation, oxidative stress, and insulin resistance through the UPS. In addition, the possible mechanisms of UPS-mediated prevention of metabolic disorders are comprehensively proposed. We aim to provide new angle to the development and utilization of polyphenols in improving metabolic disorders.

Ubiquitination and Metabolic Disease

The increasing incidence of metabolic diseases, including obesity, type 2 diabetes mellitus (T2DM), and non-alcoholic fatty liver disease (NAFLD), in the past decade is a serious concern worldwide. Disruption of cellular protein homeostasis has been considered as a crucial contributor to the pathogenesis of metabolic diseases. To maintain protein homeostasis, cells have evolved multiple dynamic and self-regulating quality control processes to adapt new environmental conditions and prevent prolonged damage. Among them, the ubiquitin proteasome system (UPS), the primary proteolytic pathway for degradation of aberrant proteins via ubiquitination, has an essential role in maintaining cellular homeostasis in response to intracellular stress. Correspondingly, accumulating evidences have shown that dysregulation of ubiquitination can aggravate various metabolic derangements in many tissues, including the liver, skeletal muscle, pancreas, and adipose tissue, and is involved in the initiation and progression of diverse metabolic diseases. In this part, we will summarize the role of ubiquitination in the pathogenesis of metabolic diseases, including obesity, T2DM and NAFLD, and discuss its potential as a therapeutic target.

Cellular stress in the pathogenesis of nonalcoholic steatohepatitis and liver fibrosis

The burden of chronic liver disease is rising substantially worldwide. Fibrosis, characterized by excessive deposition of extracellular matrix proteins, is the common pathway leading to cirrhosis, and limited treatment options are available. There is increasing evidence suggesting the role of cellular stress responses contributing to fibrogenesis. This Review provides an overview of studies that analyse the role of cellular stress in different cell types involved in fibrogenesis, including hepatocytes, hepatic stellate cells, liver sinusoidal endothelial cells and macrophages.

Genetic disruption of mammalian endoplasmic reticulum-associated protein degradation: Human phenotypes and animal and cellular disease models

Endoplasmic reticulum-associated protein degradation (ERAD) is a stringent quality control mechanism through which misfolded, unassembled and some native proteins are targeted for degradation to maintain appropriate cellular and organelle homeostasis. Several in vitro and in vivo ERAD-related studies have provided mechanistic insights into ERAD pathway activation and its consequent events; however, a majority of these have investigated the effect of ERAD substrates and their consequent diseases affecting the degradation process. In this review, we present all reported human single-gene disorders caused by genetic variation in genes that encode ERAD components rather than their substrates. Additionally, after extensive literature survey, we present various genetically manipulated higher cellular and mammalian animal models that lack specific components involved in various stages of the ERAD pathway.

AMFR dysfunction causes autosomal recessive spastic paraplegia in human that is amenable to statin treatment in a preclinical model

Hereditary spastic paraplegias (HSP) are rare, inherited neurodegenerative or neurodevelopmental disorders that mainly present with lower limb spasticity and muscle weakness due to motor neuron dysfunction. Whole genome sequencing identified bi-allelic truncating variants in AMFR, encoding a RING-H2 finger E3 ubiquitin ligase anchored at the membrane of the endoplasmic reticulum (ER), in two previously genetically unexplained HSP-affected siblings. Subsequently, international collaboration recognized additional HSP-affected individuals with similar bi-allelic truncating AMFR variants, resulting in a cohort of 20 individuals from 8 unrelated, consanguineous families. Variants segregated with a phenotype of mainly pure but also complex HSP consisting of global developmental delay, mild intellectual disability, motor dysfunction, and progressive spasticity. Patient-derived fibroblasts, neural stem cells (NSCs), and in vivo zebrafish modeling were used to investigate pathomechanisms, including initial preclinical therapy assessment. The absence of AMFR disturbs lipid homeostasis, causing lipid droplet accumulation in NSCs and patient-derived fibroblasts which is rescued upon AMFR re-expression. Electron microscopy indicates ER morphology alterations in the absence of AMFR. Similar findings are seen in amfra-/- zebrafish larvae, in addition to altered touch-evoked escape response and defects in motor neuron branching, phenocopying the HSP observed in patients. Interestingly, administration of FDA-approved statins improves touch-evoked escape response and motor neuron branching defects in amfra-/- zebrafish larvae, suggesting potential therapeutic implications. Our genetic and functional studies identify bi-allelic truncating variants in AMFR as a cause of a novel autosomal recessive HSP by altering lipid metabolism, which may potentially be therapeutically modulated using precision medicine with statins.

Neglected PTM in Animal Adipogenesis: E3-mediated Ubiquitination

Ubiquitination is a widespread post-transcriptional modification (PTM) that occurs during protein degradation in eukaryotes and participates in almost all physiological and pathological processes, including animal adipogenesis. Ubiquitination is a cascade reaction regulated by the activating enzyme E1, conjugating enzyme E2, and ligase E3. Several recent studies have reported that E3 ligases play important regulatory roles in adipogenesis. However, as a key influencing factor for the recognition and connection between the substrate and ubiquitin during ubiquitination, its regulatory role in adipogenesis has not received adequate attention. In this review, we summarize the E3s' regulation and modification targets in animal adipogenesis, explain the regulatory mechanisms in lipogenic-related pathways, and further analyze the existing positive results to provide research directions of guiding significance for further studies on the regulatory mechanisms of E3s in animal adipogenesis.

Expression of autocrine motility factor receptor (AMFR) in human breast and lung invasive micropapillary carcinomas

The aim of this study was to evaluate the clinicopathological significance of autocrine motility factor receptor (AMFR) expression in a variety of human invasive micropapillary carcinomas (IMPC). AMFR expression was compared in 111 samples of a variety of human IMPCs which had intrinsic non-micropapillary components and with 26 cases of control pulmonary adenocarcinoma (CPA, carcinoma without an IMPC component) by immunohistochemistry (IHC). In the 137 cases analysed, AMFR expression was significantly elevated in the IMPC components compared to the non-IMPC components (p = .005) and normal tissues (p < .001). AMFR expression was also higher in the IMPC samples compared to their intrinsic non-IMPC components (p = .0234). Between the 69 cases of lung IMPC and 26 cases of CPA, AMFR expression was notably higher in the IMPC components than in the CPA components (p = .0455). However, there was no significant difference between the non-IMPC components in the lung and the CPA components (p = .4584). Moreover, in breast cancer, elevated AMFR expression was not significantly correlated with mixed type or pure type IMPC (p = .5969) or with age, gender, T stage, or lymph node metastasis (LNM). Between IMPC and CPA of the lung, there was no statistical significance in age, T stage, and LNM, where AMFR expression was higher in IMPC (p = .0071). Thus this study demonstrated that AMFR was overexpressed in a variety of human IMPC components compared with non-micropapillary components. This suggests that AMFR expression is a potential new prognostic indicator for different types of human IMPC, which might thus be a new therapeutic target.

Basal Gp78-dependent mitophagy promotes mitochondrial health and limits mitochondrial ROS

Mitochondria are major sources of cytotoxic reactive oxygen species (ROS), such as superoxide and hydrogen peroxide, that when uncontrolled contribute to cancer progression. Maintaining a finely tuned, healthy mitochondrial population is essential for cellular homeostasis and survival. Mitophagy, the selective elimination of mitochondria by autophagy, monitors and maintains mitochondrial health and integrity, eliminating damaged ROS-producing mitochondria. However, mechanisms underlying mitophagic control of mitochondrial homeostasis under basal conditions remain poorly understood. E3 ubiquitin ligase Gp78 is an endoplasmic reticulum membrane protein that induces mitochondrial fission and mitophagy of depolarized mitochondria. Here, we report that CRISPR/Cas9 knockout of Gp78 in HT-1080 fibrosarcoma cells increased mitochondrial volume, elevated ROS production and rendered cells resistant to carbonyl cyanide m-chlorophenyl hydrazone (CCCP)-induced mitophagy. These effects were phenocopied by knockdown of the essential autophagy protein ATG5 in wild-type HT-1080 cells. Use of the mito-Keima mitophagy probe confirmed that Gp78 promoted both basal and damage-induced mitophagy. Application of a spot detection algorithm (SPECHT) to GFP-mRFP tandem fluorescent-tagged LC3 (tfLC3)-positive autophagosomes reported elevated autophagosomal maturation in wild-type HT-1080 cells relative to Gp78 knockout cells, predominantly in proximity to mitochondria. Mitophagy inhibition by either Gp78 knockout or ATG5 knockdown reduced mitochondrial potential and increased mitochondrial ROS. Live cell analysis of tfLC3 in HT-1080 cells showed the preferential association of autophagosomes with mitochondria of reduced potential. Xenograft tumors of HT-1080 knockout cells show increased labeling for mitochondria and the cell proliferation marker Ki67 and reduced labeling for the TUNEL cell death reporter. Basal Gp78-dependent mitophagic flux is, therefore, selectively associated with reduced potential mitochondria promoting maintenance of a healthy mitochondrial population, limiting ROS production and tumor cell proliferation.

Evaluation of HZX-960, a novel DCN1-UBC12 interaction inhibitor, as a potential antifibrotic compound for liver fibrosis

Liver fibrosis is a very common health problem and currently lacks effective treatments. Cullin RING E3 ligases (CRLs) regulate the turnover of ∼20% of mammalian cell proteins. Neddylation, the process by which NEDD8 is covalently attached to cullin proteins through sequential enzymatic reactions, is critical for the activation of CRLs and was recently found to be elevated in liver fibrosis. NEDD8-activating enzyme E1-specific inhibition led to the reduced liver damage characterized by decreased apoptosis, inflammation, and fibrosis. However, the relevance of a co-E3 ligase, DCN1, in liver fibrosis remains unclear. Here, a novel and potent DCN1-UBC12 interaction inhibitor HZX-960 was discovered with an IC50 value of 9.37 nmol/L, which could inhibit the neddylation of cullin3. Importantly, we identified that HZX-960 treatment could attenuate transforming growth factor β-induced liver fibrotic responses by reducing the deposition of collagen I and α-smooth muscle actin, and upregulating cellular NF-E2-related factor 2, hemeoxygenase 1, and NADPH quinone oxidoreductase-1 levels in two hepatic stellate cell lines. Additionally, DCN1 was shown to be unregulated in CCl4-induced mice liver tissue, and liver fibrotic signaling in mice was reduced by HZX-960. Therefore, our data demonstrated that HZX-960 possessed anti-liver fibrosis ability and that DCN1 may be a potential therapeutic target for liver fibrosis treatment.

ER Disposal Pathways in Chronic Liver Disease: Protective, Pathogenic, and Potential Therapeutic Targets

The endoplasmic reticulum is a central player in liver pathophysiology. Chronic injury to the ER through increased lipid content, alcohol metabolism, or accumulation of misfolded proteins causes ER stress, dysregulated hepatocyte function, inflammation, and worsened disease pathogenesis. A key adaptation of the ER to resolve stress is the removal of excess or misfolded proteins. Degradation of intra-luminal or ER membrane proteins occurs through distinct mechanisms that include ER-associated Degradation (ERAD) and ER-to-lysosome-associated degradation (ERLAD), which includes macro-ER-phagy, micro-ER-phagy, and Atg8/LC-3-dependent vesicular delivery. All three of these processes are critical for removing misfolded or unfolded protein aggregates, and re-establishing ER homeostasis following expansion/stress, which is critical for liver function and adaptation to injury. Despite playing a key role in resolving ER stress, the contribution of these degradative processes to liver physiology and pathophysiology is understudied. Analysis of publicly available datasets from diseased livers revealed that numerous genes involved in ER-related degradative pathways are dysregulated; however, their roles and regulation in disease progression are not well defined. Here we discuss the dynamic regulation of ER-related protein disposal pathways in chronic liver disease and cell-type specific roles, as well as potentially targetable mechanisms for treatment of chronic liver disease.

HFD-induced TRAF6 upregulation promotes liver cholesterol accumulation and fatty liver development via EZH2-mediated miR-429/PPARα axis

Despite the increasing prevalence of fatty liver diseases worldwide, the molecular mechanism underlying their pathogenesis remains poorly defined. This study examines the expression and significance of tumor necrosis factor (TNF) receptor-associated factor 6 (TRAF6) in the high-fat diet (HFD)-induced mouse obesity model and the oleic acid/palmitic acid (OA/PA)-induced cell model. After developing these models, we measured the expressions of TRAF6, enhancer of the zeste homolog 2 (EZH2), and peroxisome proliferator activated receptor alpha (PPARα). The expression of TRAF6, EZH2, and PPARα was manipulated to investigate their roles in cholesterol accumulation through evaluating the plasma levels of total cholesterol (TC), high-density lipoprotein cholesterol (HDL-C), and low-density lipoprotein cholesterol (LDL-C). Co-immunoprecipitation (coIP) assay was used to determine the interaction between TRAF6 and EZH2 and chromatin immunoprecipitation (ChIP) assay to detect the enrichment of EZH2 and H3K27me3 in microRNA-429 (miR-429) promoter. We found that HFD resulted in elevated TRAF6 and miR-429 in fatty liver and reduced EZH2 and PPARα. TRAF6 mediated the ubiquitination of EZH2 and increased miR-429 expression, and miR-429 targeted PPARα. TRAF6 increased cholesterol accumulation in liver cells in vitro via the EZH2/miR-429/PPARα axis. Collectively, HFD upregulates TRAF6 and ubiquitinates EZH2 to promote the miR-429-dependent inhibition of PPARα, leading to cholesterol accumulation in liver and the occurrence of fatty liver.

The Cellular Prion Protein: A Promising Therapeutic Target for Cancer

Studies on the cellular prion protein (PrP^C) have been actively conducted because misfolded PrP^C is known to cause transmissible spongiform encephalopathies or prion disease. PrP^C is a glycophosphatidylinositol-anchored cell surface glycoprotein that has been reported to affect several cellular functions such as stress protection, cellular differentiation, mitochondrial homeostasis, circadian rhythm, myelin homeostasis, and immune modulation. Recently, it has also been reported that PrP^C mediates tumor progression by enhancing the proliferation, metastasis, and drug resistance of cancer cells. In addition, PrP^C regulates cancer stem cell properties by interacting with cancer stem cell marker proteins. In this review, we summarize how PrP^C promotes tumor progression in terms of proliferation, metastasis, drug resistance, and cancer stem cell properties. In addition, we discuss strategies to treat tumors by modulating the function and expression of PrP^C via the regulation of HSPA1L/HIF-1α expression and using an anti-prion antibody.

Multi-Omics Integration Highlights the Role of Ubiquitination in CCl4-Induced Liver Fibrosis

Liver fibrosis is the excessive accumulation of extracellular matrix proteins that occurs in chronic liver disease. Ubiquitination is a post-translational modification that is crucial for a plethora of physiological processes. Even though the ubiquitin system has been implicated in several human diseases, the role of ubiquitination in liver fibrosis remains poorly understood. Here, multi-omics approaches were used to address this. Untargeted metabolomics showed that carbon tetrachloride (CCl4)-induced liver fibrosis promotes changes in the hepatic metabolome, specifically in glycerophospholipids and sphingolipids. Gene ontology analysis of public deposited gene array-based data and validation in our mouse model showed that the biological process "protein polyubiquitination" is enriched after CCl4-induced liver fibrosis. Finally, by using transgenic mice expressing biotinylated ubiquitin (bioUb mice), the ubiquitinated proteome was isolated and characterized by mass spectrometry in order to unravel the hepatic ubiquitinated proteome fingerprint in CCl4-induced liver fibrosis. Under these conditions, ubiquitination appears to be involved in the regulation of cell death and survival, cell function, lipid metabolism, and DNA repair. Finally, ubiquitination of proliferating cell nuclear antigen (PCNA) is induced during CCl4-induced liver fibrosis and associated with the DNA damage response (DDR). Overall, hepatic ubiquitome profiling can highlight new therapeutic targets for the clinical management of liver fibrosis.

Unraveling the regulatory role of endoplasmic-reticulum-associated degradation in tumor immunity

During malignant transformation and cancer progression, tumor cells face both intrinsic and extrinsic stress, endoplasmic reticulum (ER) stress in particular. To survive and proliferate, tumor cells use multiple stress response pathways to mitigate ER stress, promoting disease aggression and treatment resistance. Among the stress response pathways is ER-associated degradation (ERAD), which consists of multiple components and steps working together to ensure protein quality and quantity. In addition to its established role in stress responses and tumor cell survival, ERAD has recently been shown to regulate tumor immunity. Here we summarize current knowledge on how ERAD promotes protein degradation, regulates immune cell development and function, participates in antigen presentation, exerts paradoxical roles on tumorigenesis and immunity, and thus impacts current cancer therapy. Collectively, ERAD is a critical protein homeostasis pathway intertwined with cancer development and tumor immunity. Of particular importance is the need to further unveil ERAD's enigmatic roles in tumor immunity to develop effective targeted and combination therapy for successful treatment of cancer.

RETREG1/FAM134B mediated autophagosomal degradation of AMFR/GP78 and OPA1 -a dual organellar turnover mechanism

Turnover of cellular organelles, including endoplasmic reticulum (ER) and mitochondria, is orchestrated by an efficient cellular surveillance system. We have identified a mechanism for dual regulation of ER and mitochondria under stress. It is known that AMFR, an ER E3 ligase and ER-associated degradation (ERAD) regulator, degrades outer mitochondrial membrane (OMM) proteins, MFNs (mitofusins), via the proteasome and triggers mitophagy. We show that destabilized mitochondria are almost devoid of the OMM and generate "mitoplasts". This brings the inner mitochondrial membrane (IMM) in the proximity of the ER. When AMFR levels are high and the mitochondria are stressed, the reticulophagy regulatory protein RETREG1 participates in the formation of the mitophagophore by interacting with OPA1. Interestingly, OPA1 and other IMM proteins exhibit similar RETREG1-dependent autophagosomal degradation as AMFR, unlike most of the OMM proteins. The "mitoplasts" generated are degraded by reticulo-mito-phagy - simultaneously affecting dual organelle turnover.Abbreviations: AMFR/GP78: autocrine motility factor receptor; BAPTA: 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid; BFP: blue fluorescent protein; CCCP: carbonyl cyanide m-chlorophenyl hydrazone; CNBr: cyanogen bromide; ER: endoplasmic reticulum; ERAD: endoplasmic-reticulum-associated protein degradation; FL: fluorescence, GFP: green fluorescent protein; HA: hemagglutinin; HEPES: 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid; IMM: inner mitochondrial membrane; LIR: LC3-interacting region; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; MFN: mitofusin, MGRN1: mahogunin ring finger 1; NA: numerical aperature; OMM: outer mitochondrial membrane; OPA1: OPA1 mitochondrial dynamin like GTPase; PRNP/PrP: prion protein; RER: rough endoplasmic reticulum; RETREG1/FAM134B: reticulophagy regulator 1; RFP: red fluorescent protein; RING: really interesting new gene; ROI: region of interest; RTN: reticulon; SEM: standard error of the mean; SER: smooth endoplasmic reticulum; SIM: structured illumination microscopy; SQSTM1/p62: sequestosome 1; STED: stimulated emission depletion; STOML2: stomatin like 2; TOMM20: translocase of outer mitochondrial membrane 20; UPR: unfolded protein response.

Cytochrome P450 endoplasmic reticulum-associated degradation (ERAD): therapeutic and pathophysiological implications

The hepatic endoplasmic reticulum (ER)-anchored cytochromes P450 (P450s) are mixed-function oxidases engaged in the biotransformation of physiologically relevant endobiotics as well as of myriad xenobiotics of therapeutic and environmental relevance. P450 ER-content and hence function is regulated by their coordinated hemoprotein syntheses and proteolytic turnover. Such P450 proteolytic turnover occurs through a process known as ER-associated degradation (ERAD) that involves ubiquitin-dependent proteasomal degradation (UPD) and/or autophagic-lysosomal degradation (ALD). Herein, on the basis of available literature reports and our own recent findings of in vitro as well as in vivo experimental studies, we discuss the therapeutic and pathophysiological implications of altered P450 ERAD and its plausible clinical relevance. We specifically (i) describe the P450 ERAD-machinery and how it may be repurposed for the generation of antigenic P450 peptides involved in P450 autoantibody pathogenesis in drug-induced acute hypersensitivity reactions and liver injury, or viral hepatitis; (ii) discuss the relevance of accelerated or disrupted P450-ERAD to the pharmacological and/or toxicological effects of clinically relevant P450 drug substrates; and (iii) detail the pathophysiological consequences of disrupted P450 ERAD, contributing to non-alcoholic fatty liver disease (NAFLD)/non-alcoholic steatohepatitis (NASH) under certain synergistic cellular conditions.

Ubiquitin-Like Post-Translational Modifications (Ubl-PTMs): Small Peptides with Huge Impact in Liver Fibrosis

Liver fibrosis is characterized by the excessive deposition of extracellular matrix proteins including collagen that occurs in most types of chronic liver disease. Even though our knowledge of the cellular and molecular mechanisms of liver fibrosis has deeply improved in the last years, therapeutic approaches for liver fibrosis remain limited. Profiling and characterization of the post-translational modifications (PTMs) of proteins, and more specifically NEDDylation and SUMOylation ubiquitin-like (Ubls) modifications, can provide a better understanding of the liver fibrosis pathology as well as novel and more effective therapeutic approaches. On this basis, in the last years, several studies have described how changes in the intermediates of the Ubl cascades are altered during liver fibrosis and how specific targeting of particular enzymes mediating these ubiquitin-like modifications can improve liver fibrosis, mainly in in vitro models of hepatic stellate cells, the main fibrogenic cell type, and in pre-clinical mouse models of liver fibrosis. The development of novel inhibitors of the Ubl modifications as well as novel strategies to assess the modified proteome can provide new insights into the overall role of Ubl modifications in liver fibrosis.

Induction via Functional Protein Stabilization of Hepatic Cytochromes P450 upon gp78/Autocrine Motility Factor Receptor (AMFR) Ubiquitin E3-Ligase Genetic Ablation in Mice: Therapeutic and Toxicological Relevance

The hepatic endoplasmic reticulum (ER)-anchored monotopic proteins, cytochromes P450 (P450s), are enzymes that metabolize endobiotics (physiologically active steroids and fatty acids), as well as xenobiotics including therapeutic/chemotherapeutic drugs, nutrients, carcinogens, and toxins. Alterations of hepatic P450 content through synthesis, inactivation, or proteolytic turnover influence their metabolic function. P450 proteolytic turnover occurs via ER-associated degradation (ERAD) involving ubiquitin (Ub)-dependent proteasomal degradation (UPD) as a major pathway. UPD critically involves P450 protein ubiquitination by E2/E3 Ub-ligase complexes. We have previously identified the ER-polytopic gp78/AMFR (autocrine motility factor receptor) as a relevant E3 in CYP3A4, CYP3A23, and CYP2E1 UPD. We now document that liver-conditional genetic ablation of gp78/AMFR in male mice disrupts P450 ERAD, resulting in statistically significant stabilization of Cyp2a5 and Cyp2c, in addition to that of Cyp3a and Cyp2e1. More importantly, we establish that such stabilization is of the functionally active P450 proteins, leading to corresponding statistically significant enhancement of their drug-metabolizing capacities. Our findings, with clinically relevant therapeutic drugs (nicotine, coumarin, chlorzoxazone, and acetaminophen) and the prodrug (tamoxifen) as P450 substrates, reveal that P450 ERAD disruption could influence therapeutic drug response and/or toxicity, warranting serious consideration as a potential source of clinically relevant drug-drug interactions (DDIs). Because gp78/AMFR is not only an E3 Ub-ligase, but also a cell-surface prometastatic oncogene that is upregulated in various malignant cancers, our finding that hepatic gp78/AMFR knockout can enhance P450-dependent bioactivation of relevant cancer chemotherapeutic prodrugs is of therapeutic relevance and noteworthy in prospective drug design and development. SIGNIFICANCE STATEMENT: The cell-surface and ER transmembrane protein gp78/AMFR, a receptor for the prometastatic autocrine motility factor (AMF), as well as an E3 ubiquitin-ligase involved in the ER-associated degradation (ERAD) of not only the tumor metastatic suppressor KAI1 but also of hepatic cytochromes P450, is upregulated in various human cancers, enhancing their invasiveness, metastatic potential, and poor prognosis. Liver-specific gp78/AMFR genetic ablation results in functional protein stabilization of several hepatic P450s and consequently enhanced drug and prodrug metabolism, a feature that could be therapeutically exploited in the bioactivation of chemotherapeutic prodrugs through design and development of novel short-term gp78/AMFR chemical inhibitors.

Disruption of gene SPL35, encoding a novel CUE domain-containing protein, leads to cell death and enhanced disease response in rice

Lesion mimic mutants that exhibit spontaneous hypersensitive response (HR)-like necrotic lesions are ideal experimental systems for elucidating molecular mechanisms involved in plant cell death and defence responses. Here we report identification of a rice lesion mimic mutant, spotted leaf 35 (spl35), and cloning of the causal gene by TAIL-PCR strategy. spl35 exhibited decreased chlorophyll content, higher accumulation of H₂ O₂ , up-regulated expression of defence-related marker genes, and enhanced resistance to both fungal and bacterial pathogens of rice. The SPL35 gene encodes a novel CUE (coupling of ubiquitin conjugation to ER degradation) domain-containing protein that is predominantly localized in cytosol, ER and unknown punctate compartment(s). SPL35 is constitutively expressed in all organs, and both overexpression and knockdown of SPL35 cause the lesion mimic phenotype. SPL35 directly interacts with the E2 protein OsUBC5a and the coatomer subunit delta proteins Delta-COP1 and Delta-COP2 through the CUE domain, and down-regulation of these interacting proteins also cause development of HR-like lesions resembling those in spl35 and activation of defence responses, indicating that SPL35 may be involved in the ubiquitination and vesicular trafficking pathways. Our findings provide insight into a role of SPL35 in regulating cell death and defence response in plants.

Expression of Gp78/Autocrine Motility Factor Receptor and Endocytosis of Autocrine Motility Factor in Human Thyroid Cancer Cells

Gp78/autocrine motility factor receptor (Gp78/AMFR) is a cancer-associated endoplasmic reticulum-localized E3 ubiquitin ligase and also the cell surface receptor for autocrine motility factor (AMF). The study objective was to determine the association between Gp78/AMFR and AMF endocytosis in thyroid cancer cells. Gp78/AMFR expression and AMF internalization were measured in differentiated thyroid cancer (DTC) and anaplastic thyroid cancer (ATC) cell lines and in freshly resected human papillary thyroid cancers (PTC) relative to benign thyroid tissue. Spheroid-like aggregates generated from explants of cancer, goiter, and collateral thyroid tissue were assessed for expression of cancer stem cell markers, surface Gp78/AMFR and AMF endocytosis. DTC cell lines showed elevated total and surface Gp78/AMFR and AMF internalization relative to ATC lines. Gp78/AMFR, Oct-4 and Sox-2 protein expression, Gp78/AMFR surface expression and AMF internalization were elevated in PTC-derived aggregates relative to fibroblasts. Elevated levels of Gp78/AMFR expression and AMF internalization in PTC were associated with expression of cancer stem cell markers. Gp78/AMFR expression and AMF uptake are more closely associated with DTC compared to benign thyroid lesions or ATC and with PTC-derived cancer stem-like cells.

GP78 Cooperates with Dual-Specificity Phosphatase 1 To Stimulate Epidermal Growth Factor Receptor-Mediated Extracellular Signal-Regulated Kinase Signaling

GP78 is an autocrine motility factor (AMF) receptor (AMFR) with E3 ubiquitin ligase activity that plays a significant role in tumor cell proliferation, motility, and metastasis. Aberrant extracellular signal-regulated kinase (ERK) activation via receptor tyrosine kinases promotes tumor proliferation and invasion. The activation of GP78 leads to ERK activation, but its underlying mechanism is not fully understood. Here, we show that GP78 is required for epidermal growth factor receptor (EGFR)-mediated ERK activation. On one hand, GP78 interacts with and promotes the ubiquitination and subsequent degradation of dual-specificity phosphatase 1 (DUSP1), an endogenous negative regulator of mitogen-activated protein kinases (MAPKs), resulting in ERK activation. On the other hand, GP78 maintains the activation status of EGFR, as evidenced by the fact that EGF fails to induce EGFR phosphorylation in GP78-deficient cells. By the regulation of both EGFR and ERK activation, GP78 promotes cell proliferation, motility, and invasion. Therefore, this study identifies a previously unknown signaling pathway by which GP78 stimulates ERK activation via DUSP1 degradation to mediate EGFR-dependent cancer cell proliferation and invasion.

Oncogenic Activation of Nrf2, Though as a Master Antioxidant Transcription Factor, Liberated by Specific Knockout of the Full-Length Nrf1α that Acts as a Dominant Tumor Repressor

Liver-specific knockout of Nrf1 in the mouse leads to spontaneous development of non- alcoholic steatohepatitis with dyslipidemia, and then its deterioration results in hepatoma, but the underlying mechanism remains elusive to date. A similar pathological model is reconstructed here by using human Nrf1α-specific knockout cell lines. Our evidence has demonstrated that a marked increase of the inflammation marker COX2 definitely occurs in Nrf1α^−/− cells. Loss of Nrf1α leads to hyperactivation of Nrf2, which results from substantial decreases in Keap1, PTEN and most of 26S proteasomal subunits in Nrf1α^−/− cells. Further investigation of xenograft model mice showed that malignant growth of Nrf1α^−/−-derived tumors is almost abolished by silencing of Nrf2, while Nrf1α^+/+-tumor is markedly repressed by an inactive mutant (i.e., Nrf2^−/−ΔTA), but largely unaffected by a priori constitutive activator (i.e., caNrf2^ΔN). Mechanistic studies, combined with transcriptomic sequencing, unraveled a panoramic view of opposing and unifying inter-regulatory cross-talks between Nrf1α and Nrf2 at different layers of the endogenous regulatory networks from multiple signaling towards differential expression profiling of target genes. Collectively, Nrf1α manifests a dominant tumor-suppressive effect by confining Nrf2 oncogenicity. Though as a tumor promoter, Nrf2 can also, in turn, directly activate the transcriptional expression of Nrf1 to form a negative feedback loop. In view of such mutual inter-regulation by between Nrf1α and Nrf2, it should thus be taken severe cautions to interpret the experimental results from loss of Nrf1α, Nrf2 or both.

Design, synthesis, and anticancer activity evaluation of irreversible allosteric inhibitors of the ubiquitin-conjugating enzyme Ube2g2

The RING finger-dependent ubiquitin ligase (E3) gp78, known as the tumor autocrine motility factor receptor, contributes to tumor progression. The protein interacts with its cognate ubiquitin-conjugating enzyme (E2), Ube2g2, via its RING domain and a unique region called G2BR that strongly binds to E2. The binding of G2BR to Ube2g2 allosterically enhances the binding of RING to E2, and the binding of RING triggers the departure of G2BR from E2 also in an allosteric fashion. Targeting these allosteric events, we developed a family of inhibitors that irreversibly block E2-E3 interactions and thereby eliminate the tumorigenic effect of gp78. One among 19 compounds screened with the NCI 60 tumor cell lines exhibited outstanding anticancer activities. At 10 μM, it caused >50% growth inhibition to 40% of the cell lines; at 100 μM, it showed lethiferous activity against most cell lines.

SREBP-regulated lipid metabolism: convergent physiology - divergent pathophysiology

Cellular lipid metabolism and homeostasis are controlled by sterol regulatory-element binding proteins (SREBPs). In addition to performing canonical functions in the transcriptional regulation of genes involved in the biosynthesis and uptake of lipids, genome-wide system analyses have revealed that these versatile transcription factors act as important nodes of convergence and divergence within biological signalling networks. Thus, they are involved in myriad physiological and pathophysiological processes, highlighting the importance of lipid metabolism in biology. Changes in cell metabolism and growth are reciprocally linked through SREBPs. Anabolic and growth signalling pathways branch off and connect to multiple steps of SREBP activation and form complex regulatory networks. In addition, SREBPs are implicated in numerous pathogenic processes such as endoplasmic reticulum stress, inflammation, autophagy and apoptosis, and in this way, they contribute to obesity, dyslipidaemia, diabetes mellitus, nonalcoholic fatty liver disease, nonalcoholic steatohepatitis, chronic kidney disease, neurodegenerative diseases and cancers. This Review aims to provide a comprehensive understanding of the role of SREBPs in physiology and pathophysiology at the cell, organ and organism levels.

Gp78 E3 Ubiquitin Ligase: Essential Functions and Contributions in Proteostasis

As per the requirement of metabolism and fitness, normal cellular functions are controlled by several proteins, and their interactive molecular and signaling events at multiple levels. Protein quality control (PQC) mechanisms ensure the correct folding and proper utilization of these proteins to avoid their misfolding and aggregation. To maintain the optimum environment of complex proteome PQC system employs various E3 ubiquitin ligases for the selective degradation of aberrant proteins. Glycoprotein 78 (Gp78) is an E3 ubiquitin ligase that prevents multifactorial deleterious accumulation of different misfolded proteins via endoplasmic reticulum-associated degradation (ERAD). However, the precise role of Gp78 under stress conditions to avoid bulk misfolded aggregation is unclear, which can act as a crucial resource to establish the dynamic nature of the proteome. Present article systematically explains the detailed molecular characterization of Gp78 and also addresses its various cellular physiological functions, which could be crucial to achieving protein homeostasis. Here, we comprehensively represent the current findings of Gp78, which shows its PQC roles in different physiological functions and diseases; and thereby propose novel opportunities to better understand the unsolved questions for therapeutic interventions linked with different protein misfolding disorders.

Role of HSPA1L as a cellular prion protein stabilizer in tumor progression via HIF-1α/GP78 axis

The cellular prion protein (PrP^C) is associated with metastasis, tumor progression and recurrence; however, the precise mechanisms underlying its action is not well understood. Our study found that PrP^C degradation decreased tumor progression in colorectal cancer (CRC). In a CRC cell line and human CRC tissue exposed to hypoxia, induced heat-shock 70-kDa protein-1-like (HSPA1L) expression stabilized hypoxia-inducible factor-1α (HIF-1α) protein and promoted PrP^C accumulation and tumorigenicity in vivo. PrP^C was degraded via the proteasome pathway mediated by the ubiquitin-protein E3 ligase glycoprotein 78 (GP78), which interacts directly with PrP^C. However, hypoxia-induced HSPA1L interacted with GP78 and inhibited its functions. HSPA1L knockdown facilitated the interaction of GP78 and PrP^C, thereby increasing PrP^C ubiquitination. Thus, GP78 was identified as the ubiquitinase for PrP^C, thereby revealing an essential mechanism that controls PrP^C levels in CRC. Our results suggest that the HSPA1L/HIF-1α/GP78 axis has a crucial role in PrP^C accumulation during tumor progression.

Role of glycoprotein 78 and cidec in hepatic steatosis

Hepatic glycoprotein (gp78), a membrane-anchored E3 ubiquitin ligase, has been reported to be involved in regulating lipid and energy metabolism in animals, and cell death-inducing DFFA-like effector c (cidec) has emerged as an important regulator of metabolism, which has been implicated in the process of fat differentiation. Nonalcoholic fatty liver disease is a metabolic disorder associated with hepatic steatosis. In the present study, to investigate the role of gp78 and cidec in hepatic steatosis, an in vitro cell culture model of hepatic steatosis was established, using the AML12 mouse hepatocyte cell line to assess the protein expression of gp78. The results of Oil Red O staining, phase contrast microscopy and triglyceride content detection experiments indicated that the overexpression of gp78 induced lipid accumulation, whereas gp78-knockdown led to a reduction in lipid accumulation in the AML12 cells. The increased expression of gp78 was associated with steatosis. The expression of cidec was consistent with gp78, and the colocalization of gp78 and cidec was observed on the surface of lipid droplets using immunofluorescence analysis. Furthermore, an interaction between gp78 and cidec was detected using coimmunoprecipitation analysis, and this interaction promoted lipid accumulation. Based on these data, it was hypothesized that gp78 is a regulator of hepatic steatosis, and that it may be a putative molecular mediator in metabolic diseases.

Inhibition of the sterol regulatory element-binding protein pathway suppresses hepatocellular carcinoma by repressing inflammation in mice

Obesity is a critical risk factor for hepatocellular carcinoma (HCC). However, it remains unknown whether inhibition of de novo lipid biosynthesis can suppress HCC. In this study, we blocked the sterol regulatory element-binding protein (SREBP) pathway, one of the key determinants of lipid homeostasis, by ablating 78-kDa cell-surface glycoprotein or SREBP cleavage-activating protein in hepatocytes, as well as by administering a chemical compound called betulin. We found that either genetically or pharmacologically inhibiting the SREBP pathway dramatically reduced diethylnitrosamine-induced HCC progression by down-regulating tumor-promoting cytokines, including interleukin (IL)-6, tumor necrosis factor alpha, and IL-1β.

CONCLUSION

Inhibition of de novo lipid biosynthesis by suppressing the SREBP pathway prevents HCC. This study identifies a previously underappreciated role of the SREBP pathway in HCC and suggests a novel metabolic strategy to control liver cancer. (Hepatology 2017;65:1936-1947).

Sodium 4-phenylbutyric acid prevents murine acetaminophen hepatotoxicity by minimizing endoplasmic reticulum stress

BACKGROUND

Acetaminophen (APAP) overdose induces severe oxidative stress followed by hepatocyte apoptosis/necrosis. Previous studies have indicated that endoplasmic reticulum (ER) stress is involved in the cell death process. Therefore, we investigated the effect of the chemical chaperone 4-phenyl butyric acid (PBA) on APAP-induced liver injury in mice.

METHODS

Eight-week-old male C57Bl6/J mice were given a single intraperitoneal (i.p.) injection of APAP (450 mg/kg body weight), following which some were repeatedly injected with PBA (120 mg/kg body weight, i.p.) every 3 h starting at 0.5 h after the APAP challenge. All mice were then serially euthanized up to 12 h later.

RESULTS

PBA treatment dramatically ameliorated the massive hepatocyte apoptosis/necrosis that was observed 6 h after APAP administration. PBA also significantly prevented the APAP-induced increases in cleaved activating transcription factor 6 and phosphorylation of c-Jun N-terminal protein kinase and significantly blunted the increases in mRNA levels for binding immunoglobulin protein, spliced X-box binding protein-1, and C/EBP homologous protein. Moreover, PBA significantly prevented APAP-induced Bax translocation to the mitochondria, and the expression of heme oxygenase-1 mRNA and 4-hydroxynonenal. By contrast, PBA did not affect hepatic glutathione depletion following APAP administration, reflecting APAP metabolism.

CONCLUSIONS

PBA prevents APAP-induced liver injury even when an APAP challenge precedes its administration. The underlying mechanism of action most likely involves the prevention of ER stress-induced apoptosis/necrosis in the hepatocytes during APAP intoxication.

Hepatic cytochromes P450: structural degrons and barcodes, posttranslational modifications and cellular adapters in the ERAD-endgame

The endoplasmic reticulum (ER)-anchored hepatic cytochromes P450 (P450s) are enzymes that metabolize endo- and xenobiotics i.e. drugs, carcinogens, toxins, natural and chemical products. These agents modulate liver P450 content through increased synthesis or reduction via inactivation and/or proteolytic degradation, resulting in clinically significant drug-drug interactions. P450 proteolytic degradation occurs via ER-associated degradation (ERAD) involving either of two distinct routes: Ubiquitin (Ub)-dependent 26S proteasomal degradation (ERAD/UPD) or autophagic lysosomal degradation (ERAD/ALD). CYP3A4, the major human liver/intestinal P450, and the fast-turnover CYP2E1 species are degraded via ERAD/UPD entailing multisite protein phosphorylation and subsequent ubiquitination by gp78 and CHIP E3 Ub-ligases. We are gaining insight into the nature of the structural determinants involved in CYP3A4 and CYP2E1 molecular recognition in ERAD/UPD [i.e. K48-linked polyUb chains and linear and/or "conformational" phosphodegrons consisting either of consecutive sequences on surface loops and/or disordered regions, or structurally-assembled surface clusters of negatively charged acidic (Asp/Glu) and phosphorylated (Ser/Thr) residues, within or vicinal to which, Lys-residues are targeted for ubiquitination]. Structural inspection of select human liver P450s reveals that such linear or conformational phosphodegrons may indeed be a common P450-ERAD/UPD feature. By contrast, although many P450s such as the slow-turnover CYP2E1 species and rat liver CYP2B1 and CYP2C11 are degraded via ERAD/ALD, little is known about the mechanism of their ALD-targeting. On the basis of our current knowledge of ALD-substrate targeting, we propose a tripartite conjunction of K63-linked Ub-chains, P450 structural "LIR" motifs and selective cellular "cargo receptors" as plausible P450-ALD determinants.

Hepatocellular carcinoma and non-alcoholic steatohepatitis: The state of play

Hepatocellular carcinoma (HCC) is now the fifth cancer of greatest frequency and the second leading cause of cancer related deaths worldwide. Chief amongst the risks of HCC are hepatitis B and C infection, aflatoxin B1 ingestion, alcoholism and obesity. The latter can promote non-alcoholic fatty liver disease (NAFLD), that can lead to the inflammatory form non-alcoholic steatohepatitis (NASH), and can in turn promote HCC. The mechanisms by which NASH promotes HCC are only beginning to be characterized. Here in this review, we give a summary of the recent findings that describe and associate NAFLD and NASH with the subsequent HCC progression. We will focus our discussion on clinical and genomic associations that describe new risks for NAFLD and NASH promoted HCC. In addition, we will consider novel murine models that clarify some of the mechanisms that drive NASH HCC formation.

Hepatocellular carcinoma mouse models: Hepatitis B virus-associated hepatocarcinogenesis and haploinsufficient tumor suppressor genes

The multifactorial and multistage pathogenesis of hepatocellular carcinoma (HCC) has fascinated a wide spectrum of scientists for decades. While a number of major risk factors have been identified, their mechanistic roles in hepatocarcinogenesis still need to be elucidated. Many tumor suppressor genes (TSGs) have been identified as being involved in HCC. These TSGs can be classified into two groups depending on the situation with respect to allelic mutation/loss in the tumors: the recessive TSGs with two required mutated alleles and the haploinsufficient TSGs with one required mutated allele. Hepatitis B virus (HBV) is one of the most important risk factors associated with HCC. Although mice cannot be infected with HBV due to the narrow host range of HBV and the lack of a proper receptor, one advantage of mouse models for HBV/HCC research is the numerous and powerful genetic tools that help investigate the phenotypic effects of viral proteins and allow the dissection of the dose-dependent action of TSGs. Here, we mainly focus on the application of mouse models in relation to HBV-associated HCC and on TSGs that act either in a recessive or in a haploinsufficient manner. Discoveries obtained using mouse models will have a great impact on HCC translational medicine.

Deacetylation of HSPA5 by HDAC6 leads to GP78-mediated HSPA5 ubiquitination at K447 and suppresses metastasis of breast cancer

Heat-shock protein 5 (HSPA5) is a marker for poor prognosis in breast cancer patients and has an important role in cancer progression, including promoting drug resistance and metastasis. In this study, we identify that the specific lysine residue 447 (K447) of HSPA5 could be modified with polyubiquitin for subsequent degradation through the ubiquitin proteasomal system, leading to the suppression of cell migration and invasion of breast cancer. We further found that GP78, an E3 ubiquitin ligase, interacted with the C-terminal region of HSPA5 and mediated HSPA5 ubiquitination and degradation. Knock down of GP78 significantly increased the expression of HSPA5 and enhanced migration/invasive ability of breast cancer cells. Knock down of histone deacetylase-6 (HDAC6) increased the acetylation of HSPA5 at lysine residues 353 (K353) and reduced GP78-mediated ubiquitination of HSPA5 at K447 and then increased cell migration/invasion. In addition, we demonstrate that E3 ubiquitin ligase GP78 preferentially binds to deacetylated HSPA5. Notably, the expression levels of GP78 inversely correlated with HSPA5 levels in breast cancer patients. Patients with low GP78 expression significantly correlated with invasiveness of breast cancer, advanced tumor stages and poor clinical outcome. Taken together, our results provide new mechanistic insights into the understanding that deacetylation of HSPA5 by HDAC6 facilitates GP78-mediated HSPA5 ubiquitination and suggest that post-translational regulation of HSPA5 protein is critical for HSPA5-mediated metastatic properties of breast cancer.