Inhibition of autophagy promotes CYP2E1-dependent toxicity in HepG2 cells via elevated oxidative stress, mitochondria dysfunction and activation of p38 and JNK MAPK

Lipid droplets, autophagy, and ER stress as key (survival) pathways during ischemia-reperfusion of transplanted grafts

Ischemia-reperfusion injury is an event concerning any organ under a procedure of transplantation. The early result of ischemia is hypoxia, which causes malfunction of mitochondria and decrease in cellular ATP. This leads to disruption of cellular metabolism. Reperfusion also results in cell damage due to reoxygenation and increased production of reactive oxygen species, and later by induced inflammation. In damaged and hypoxic cells, the endoplasmic reticulum (ER) stress pathway is activated by increased amount of damaged or misfolded proteins, accumulation of free fatty acids and other lipids due to inability of their oxidation (lipotoxicity). ER stress is an adaptive response and a survival pathway, however, its prolonged activity eventually lead to induction of apoptosis. Sustaining cell functionality in stress conditions is a great challenge for transplant surgeons as it is crucial for maintaining a desired level of graft vitality. Pathways counteracting negative consequences of ischemia-reperfusion are autophagy and lipid droplets (LD) metabolism. Autophagy remove damaged organelles and molecules driving them to lysosomes, digested simpler compounds are energy source for the cell. Mitophagy and ER-phagy results in improvement of cell energetic balance and alleviation of ER stress. This is important in sustaining metabolic homeostasis and thus cell survival. LD metabolism is connected with autophagy as LD are degraded by lipophagy, a source of free fatty acids and glycerol-thus autophagy and LD can readily remove lipotoxic compounds in the cell. In conclusion, monitoring and pharmaceutic regulation of those pathways during transplantation procedure might result in increased/improved vitality of transplanted organ.

Quercetin ameliorates nonalcoholic fatty liver disease (NAFLD) via the promotion of AMPK-mediated hepatic mitophagy

The global incidence of nonalcoholic fatty liver disease (NAFLD) has been surging in recent years, however, no drug is currently approved to treat this disease. Quercetin, a natural flavonoid abundant in plants and fruits, has been reported to alleviate NAFLD, however, the exact molecular mechanism remains unclear. This study aims to further elucidate its potential mechanism of action. The beneficial effects and the underlying mechanism of quercetin in alleviating NAFLD were explored both in vitro and in vivo, by employing chemical inhibitors of autophagosomes (3-methyladenine, 3-MA), autolysosomes (chloroquine, CQ), AMPK (Compound C, CC) and SIRT1 (selisistat, EX-527). The levels of intracellular lipids, reactive oxygen species, mitochondria function, autophagy, and mitophagy were assessed by fluorescent labeling and examined using flow cytometry or confocal microscopy. Key protein expressions of autophagy, mitophagy, and inflammation were also determined. In vivo, quercetin was shown to dose-dependently effectively alleviate NAFLD, but intraperitoneal injection of 3-MA could block the beneficial effects of quercetin on body weight, liver weight, serum ALT/AST, hepatic ROS and inflammation. In vitro, quercetin could reduce intracellular lipids (Nile Red staining) and ROS/DHE accumulation, which could be also blocked by 3-MA or CQ. Furthermore, we found that CC could abrogate the protective effects of quercetin on lipid and ROS accumulation in vitro. Also, CC abolished the proautophagic and anti-inflammatory effects of quercetin, as shown by western blot determination and Lyso-Tracker labeling. Importantly, mitophagy, a specific form of mitochondria-targeted autophagy, was enhanced by quercetin, as demonstrated by PINK1/Parkin protein variation and immunofluorescence colocalization of autophagosomes and mitochondria, which could also be blocked by the intervention of CC. This study demonstrates that quercetin prevents NAFLD through AMPK-mediated mitophagy and suggests that promoting mitophagy via an upregulation of AMPK may be a promising therapeutic strategy against NAFLD.

Ethanol Exposure to Ethanol-Oxidizing HEPG2 Cells Induces Intracellular Protein Aggregation

Background: Aggresomes are collections of intracellular protein aggregates. In liver cells of patients with alcoholic hepatitis, aggresomes appear histologically as cellular inclusions known as Mallory–Denk (M–D) bodies. The proteasome is a multicatalytic intracellular protease that catalyzes the degradation of both normal (native) and abnormal (misfolded and/or damaged) proteins. The enzyme minimizes intracellular protein aggregate formation by rapidly degrading abnormal proteins before they form aggregates. When proteasome activity is blocked, either by specific inhibitors or by intracellular oxidants (e.g., peroxynitrite, acetaldehyde), aggresome formation is enhanced. Here, we sought to verify whether inhibition of proteasome activity by ethanol exposure enhances protein aggregate formation in VL-17A cells, which are recombinant, ethanol-oxidizing HepG2 cells that express both alcohol dehydrogenase (ADH) and cytochrome P450 2E1 (CYP2E1). Methods: We exposed ethanol-non-oxidizing HepG2 cells (ADH−/CYP2E1−) or ethanol-oxidizing VL-17A (ADH+/CYP2E1+) to varying levels of ethanol for 24 h or 72 h. After these treatments, we stained cells for aggresomes (detected microscopically) and quantified their numbers and sizes. We also conducted flow cytometric analyses to confirm our microscopic findings. Additionally, aggresome content in liver cells of patients with alcohol-induced hepatitis was quantified. Results: After we exposed VL-17A cells to increasing doses of ethanol for 24 h or 72 h, 20S proteasome activity declined in response to rising ethanol concentrations. After 24 h of ethanol exposure, aggresome numbers in VL-17A cells were 1.8-fold higher than their untreated controls at all ethanol concentrations employed. After 72 h of ethanol exposure, mean aggresome numbers were 2.5-fold higher than unexposed control cells. The mean aggregate size in all ethanol-exposed VL-17A cells was significantly higher than in unexposed control cells but was unaffected by the duration of ethanol exposure. Co-exposure of cells to EtOH and rapamycin, the latter an autophagy activator, completely prevented EtOH-induced aggresome formation. In the livers of patients with alcohol-induced hepatitis (AH), the staining intensity of aggresomes was 2.2-fold higher than in the livers of patients without alcohol use disorder (AUD). Conclusions: We conclude that ethanol-induced proteasome inhibition in ethanol-metabolizing VL-17A hepatoma cells causes accumulation of protein aggregates. Notably, autophagy activation removes such aggregates. The significance of these findings is discussed.

Adverse Effects of Perfluorooctane Sulfonate on the Liver and Relevant Mechanisms

Perfluorooctane sulfonate (PFOS) is a persistent, widely present organic pollutant. PFOS can enter the human body through drinking water, ingestion of food, contact with utensils containing PFOS, and occupational exposure to PFOS, and can have adverse effects on human health. Increasing research shows that the liver is the major target of PFOS, and that PFOS can damage liver tissue and disrupt its function; however, the exact mechanisms remain unclear. In this study, we reviewed the adverse effects of PFOS on liver tissue and cells, as well as on liver function, to provide a reference for subsequent studies related to the toxicity of PFOS and liver injury caused by PFOS.

The impact of legacy and novel perfluoroalkyl substances on human cytochrome P450: An in vitro study on the inhibitory potential and underlying mechanisms

Per- and polyfluoroalkyl substances (PFASs) are a group of synthetic compounds with a wide range of industrial applications. PFOA and PFOS have been the most extensively studied and have been associated with hepatotoxicity. Recently, the interaction with cytochrome P450 (CYP) has been proposed as a potential key molecular event leading to PFAS-induced hepatotoxicity. In the present study, we aimed to determine a structure-activity relationship between thirteen PFASs and their inhibitory potential on the activities of four CYPs (CYP2E1, CYP2D6, CYP3A4 and CYP2C19). The influence of PFASs (5- 3200 µM) on CYP enzyme activities was measured using the Vivid® P450 metabolism assays. Using the same assays, Michaelis-Menten saturation curves were determined to explore the type of PFAS-induced CYP inhibition. Most PFASs were capable of inhibiting activity of the tested CYPs, as shown by their IC50 values. CYP2E1 is particularly inhibited by 3:1 FTOH, PFOA, and PFOS, whereas CYP2D6 is inhibited by PFHxS, PFHpA, PFOA, PFOS, PFNA, and PFDA. Additionally, CYP3A4 is most strongly inhibited by PFHxS, PFOA, PFOS, PFNA, and PFDA. Finally, CYP2C19 is inhibited by PFBS, PFHxS, PFHpA, PFOA, PFOS, PFNA, and PFDA. Interestingly, PFHxA and PFHxS induced an increase in CYP2E1 activity, whereas 4:2 FTOH strongly induced CYP2D6 activity. The mechanism of inhibition of CYPs by PFASs differed per CYP isoenzyme. CYP3A4 was competitively inhibited by PFBS, PFHxS, PFOS, PFNA and PFDA and non-competitively by PFOA. Additionally, CYP2C19 was competitively inhibited by PFHxA, PFOS and PFNA, whereas PFBS and PFHxS induced a mixed inhibition. Inhibition of CYP2C19 by PFHpA was atypical with an increased Vmax and a decreased Km. Finally, PFHxS competitively inhibited CYP2D6, whereas PFBS, PFOA, PFOS, PFDA and PFNA induced an atypical inhibition. Our results show that CYP inhibition by PFASs appears to be structure-dependent as well as CYP dependent. Inhibition of CYP2D6, CYP2C19 and CYP3A4 increased with increasing chain-lengths between six and nine carbons. The PFTOHs were only able to inhibit CYP2E1 and did not affect any of the other CYPS. Some PFASs remarkably induced the enzyme activity of CYPs. These results indicate that in addition to PFOA and PFOS, multiple novel PFASs may alter drug metabolism by the interference with CYPs.

Irinotecan-Induced Steatohepatitis: Current Insights

The hepatotoxicity of irinotecan is drawing wide concern nowadays due to the widespread use of this chemotherapeutic against various solid tumors, particularly metastatic colorectal cancer. Irinotecan-induced hepatotoxicity mainly manifests as transaminase increase and steatosis with or without transaminase increase, and is accompanied by vacuolization, and lobular inflammation. Irinotecan-induced steatohepatitis (IIS) increases the risk of morbidity and mortality in patients with colorectal cancer liver metastasis (CRCLM). The major risks and predisposing factors for IIS include high body mass index (BMI) or obesity, diabetes, and high-fat diet. Mitochondrial dysfunction and autophagy impairment may be involved in the pathogenesis of IIS. However, there is currently no effective preventive or therapeutic treatment for this condition. Thus, the precise mechanisms underlying the pathogenesis of IIS should be deciphered for the development of therapeutic drugs. This review summarizes the current knowledge and research progress on IIS.

H2O2-mediated autophagy during ethanol metabolism

BACKGROUND

Alcoholic liver disease (ALD) is the most common liver disease worldwide and its underlying molecular mechanisms are still poorly understood. Moreover, conflicting data have been reported on potentially protective autophagy, the exact role of ethanol-metabolizing enzymes and ROS.

METHODS

Expression of LC3B, CYP2E1, and NOX4 was studied in a mouse model of acute ethanol exposure by immunoblotting and immunohistochemistry. Autophagy was further studied in primary mouse hepatocytes and huh7 cells in response to ethanol and its major intermediator acetaldehyde. Experiments were carried out in cells overexpressing CYP2E1 and knock down of NOX4 using siRNA. The response to external H2O2 was studied by using the GOX/CAT system. Autophagic flux was monitored using the mRFP-GFP-LC3 plasmid, while rapamycin and chloroquine served as positive and negative controls.

RESULTS

Acute ethanol exposure of mice over 24 h significantly induced autophagy as measured by LC3B expression but also induced the ROS-generating CYP2E1 and NOX4 enzymes. Notably, ethanol but not its downstream metabolite acetaldehyde induced autophagy in primary mouse hepatocytes. In contrast, autophagy could only be induced in huh7 cells in the presence of overexpressed CYP2E1. In addition, overexpression of NOX4 also significantly increased autophagy, which could be blocked by siRNA mediated knock down. The antioxidant N-acetylcysteine (NAC) also efficiently blocked CYP2E1-and NOX4-mediated induction of autophagy. Finally, specific and non-toxic production of H2O2 by the GOX/CAT system as evidenced by elevated peroxiredoxin (Prx-2) also induced LC3B which was efficiently blocked by NAC. H2O2 strongly increased the autophagic flux as measured by mRFP-GFP-LC3 plasmid.

CONCLUSION

We here provide evidence that short-term ethanol exposure induces autophagy in hepatocytes both in vivo and in vitro through the generation of ROS. These data suggest that suppression of autophagy by ethanol is most likely due to longer alcohol exposure during chronic alcohol consumption with the accumulation of e.g. misfolded proteins.

NADPH-Oxidase 2 Promotes Autophagy in Spinal Neurons During the Development of Morphine Tolerance

Repeated morphine administration results in analgesic tolerance. However, the underlying mechanism of morphine analgesic tolerance remains unclear. NADPH-oxidase 2 (NOX2) is the first discovered NADPH oxidase, which mainly functions to produce reactive oxygen species. Its specific role in morphine tolerance has not been fully investigated. In this work, we found that chronic morphine administration significantly increased the expression of NOX2 in spinal cord. Pretreatment of NOX2 inhibitor blocked the upregulation of NOX2 and autophagy markers, including LC3B and P62, and consequently the development of morphine tolerance. NOX2 and LC3B were both colocalized with NeuN in spinal dorsal horn in morphine-tolerant rats. Our results suggest that the increased autophagy activity in spinal neurons promoted by NOX2 activation contributes to the development of morphine tolerance. NOX2 may be considered as a new therapeutic target for morphine tolerance.

Why Hepatic CYP2E1-Elevation by Itself Is Insufficient for Inciting NAFLD/NASH: Inferences from Two Genetic Knockout Mouse Models

Hepatic cytochrome P450 CYP2E1 is an enzyme engaged in the metabolic biotransformation of various xenobiotics and endobiotics, resulting in both detoxification and/or metabolic activation of its substrates to more therapeutic or toxic products. Elevated hepatic CYP2E1 content is implicated in various metabolic diseases including alcoholic liver disease, nonalcoholic fatty liver disease (NAFLD)/nonalcoholic steatohepatitis (NASH), diabetes and obesity. While hepatic CYP2E1 elevation is considered essential to the pathogenesis of these liver diseases, our findings in two mouse models of E3 ubiquitin ligase genetic ablation fed a regular lab chow diet, argue that it is not sufficient for triggering NAFLD/NASH. Thus, albeit comparable hepatic CYP2E1 elevation and functional stabilization in these two models upon E3 ubiquitin ligase genetic ablation and consequent disruption of its ubiquitin-dependent proteasomal degradation, NAFLD/NASH was only observed in the mouse livers that exhibited concurrent SREBP1c-transcriptional upregulation of hepatic lipogenesis. These findings reinforce the critical complicity of an associated prolipogenic scenario induced by either an inherently upregulated hepatic lipogenesis or a high fat/high carbohydrate diet in CYP2E1-mediated NAFLD/NASH.

Effects of acrylamide on protein degradation pathways in human liver-derived cells and the efficacy of N-acetylcysteine and curcumin

Acrylamide is a harmful chemical, and its metabolism occurs mainly in the liver. Acrylamide can form adducts on proteins. Protein homeostasis is vital for metabolic and secretory functions of the liver. No study has investigated the effect of acrylamide on the ubiquitin-proteasome system (UPS). Also, the effect of acrylamide on autophagy and its regulation is not fully known. We aimed to investigate the effects of acrylamide on the UPS, autophagy, mammalian target of rapamycin (mTOR), and heat shock protein 70 (HSP70) in HepG2 cells as well as to examine the effects of N-acetylcysteine and curcumin on these parameters in acrylamide-treated cells. HepG2 cells were initially treated with variable concentrations of acrylamide (0.01-0.1-1-10 mM) for 24 hours. Then, HepG2 cells were treated with 5 mM N-acetylcysteine and 6.79 µM curcumin in the presence of 10 mM acrylamide for 24 hours. Cell viability was evaluated by 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide assay. Ubiquitinated protein, mTOR, microtubule-associated proteins 1 A/1B light chain 3B-II (LC3B-II), and HSP70 levels were measured by immunoblotting. Acrylamide at 10 mM concentration, without any significant change at lower concentrations, caused an increase in ubiquitinated protein, LC3B-II, and HSP70 levels and a decrease in mTOR phosphorylation. Furthermore, 5 mM N-acetylcysteine caused a decrease in ubiquitinated protein and HSP70 levels; however, 6.79 µM curcumin did not affect 10 mM in acrylamide-treated cells. Our study showed that acrylamide at high concentration inhibits UPS and mTOR, activates autophagy, and increases HSP70 levels in HepG2 cells, and N-acetylcysteine reduces UPS inhibition and HSP70 levels in acrylamide-treated cells.

Polydeoxyribonucleotide Exerts Protective Effect Against CCl4-Induced Acute Liver Injury Through Inactivation of NF-κB/MAPK Signaling Pathway in Mice

Acute liver injury (ALI) causes life-threatening clinical problem, and its underlying etiology includes inflammation and apoptosis. An adenosine A_2A receptor agonist, polydeoxyribonucleotide (PDRN), exhibits anti-inflammatory and anti-apoptotic effects by inhibiting the secretion of pro-inflammatory cytokines. In the current study, the protective effect of PDRN against carbon tetrachloride (CCl₄)-induced ALI was investigated using mice. For the induction of ALI, mice received intraperitoneal injection of CCl₄ twice over seven days. Mice from the PDRN-treated groups received an intraperitoneal injection of 200 μL saline containing PDRN (8 mg/kg), once a day for seven days, starting on day 1 after the first CCl₄ injection. In order to confirm that the action of PDRN occurs through the adenosine A_2A receptor, 8 mg/kg 3,7-dimethyl-1-propargylxanthine (DMPX), an adenosine A_2A receptor antagonist, was treated with PDRN. Administration of CCl₄ impaired liver tissue and increased the liver index and histopathologic score. The expression of pro-inflammatory cytokines was increased, and apoptosis was induced by the administration of CCl₄. Administration of CCl₄ activated nuclear factor-kappa B (NF-κB) and facilitated phosphorylation of signaling factors in mitogen-activated protein kinase (MAPK). In contrast, PDRN treatment suppressed the secretion of pro-inflammatory cytokines and inhibited apoptosis. PDRN treatment inactivated NF-κB and suppressed phosphorylation of signaling factors in MAPK. As a result, liver index and histopathologic score were reduced by PDRN treatment. When PDRN was treated with DMPX, the anti-inflammatory and anti-apoptotic effect of PDRN disappeared. Therefore, PDRN can be used as an effective therapeutic agent for acute liver damage.

Associations between Huwe1 and autophagy in rat cerebral neuron oxygen‑glucose deprivation and reperfusion injury

Autophagy and the ubiquitin proteasome system (UPS) are two major protein degradation pathways involved in brain ischemia. Autophagy can compensate for UPS impairment-induced cellular dysfunction. HECT, UBA and WWE domain containing E3 ubiquitin protein ligase 1 (Huwe1), an E3 ubiquitin ligase, serves critical roles in nervous system plasticity, regeneration and disease. However, the role of Huwe1 in autophagy in brain ischemia/reperfusion (I/R) injury remains unknown. The aim of the present study was to investigate the crosstalk between autophagy and the UPS in brain ischemia. The present study established an oxygen-glucose deprivation and reperfusion (OGD/R) model in rat primary cortex neurons in vitro. Lentiviral interference was used to silence the expression of Huwe1. An autophagy promoter (rapamycin), an autophagy inhibitor (wortmannin) and a JNK pathway inhibitor (SP600125) were also used in the current study. Cellular autophagy-related proteins, including Beclin-1, autophagy related (ATG) 7, ATG5, ATG3 and microtubule associated protein 1 light chain 3 α, and apoptosis-related proteins, such as P53, cleaved caspase 3, Bax and Bcl2, were detected via western blotting and immunocytochemistry. Neuronal apoptosis was evaluated using a TUNEL assay. The results demonstrated that silencing Huwe1 increased the expression levels of autophagy-related proteins at 24 h after OGD/R. Treatment with a JNK inhibitor or cotreatment with Huwe1 shRNA significantly increased autophagy. Rapamycin increased apoptosis under OGD/R conditions. However, treatment with Huwe1 shRNA decreased the number of TUNEL-positive cells at 24 h after OGD/R. Cotreatment with Huwe1 shRNA and wortmannin alleviated neuronal apoptosis under OGD/R conditions compared with cotreatment with DMSO. Collectively, the present results suggested that silencing Huwe1 was accompanied by a compensatory induction of autophagy under OGD/R conditions. Furthermore, the JNK pathway may be a key mediator of the interaction between Huwe1 and autophagy in response to UPS impairment.

leaf extract protects against H2O2-induced oxidative stress by increasing autophagy in PC12 cells

The effect of Apocynum venetum leaf extract (AVLE) on the nervous system has been widely studied, but its effect on injured neurons is not fully understood. In the present study, the protective effect of AVLE on injured neurons was determined. H2O2 was used to induce oxidative stress in PC12 cells and cell viability assays were used to determine the optimum concentration range of AVLE and its protective effects against oxidative stress. A live-dead assay was performed to confirm the effects of AVLE on oxidative stress. Subsequently, expression of apoptotic proteins including Bax and cleaved-caspase-3 were evaluated to determine whether AVLE affected apoptosis, and reactive oxygen species (ROS) levels were detected to determine the role of AVLE in H2O2 exposure. Furthermore, expression of autophagic proteins including LC3-II and p62 were detected to evaluate the effects of AVLE on autophagic activity, and cells were treated with 3-methyladenine (3-MA), an autophagic inhibitor, to identify the underlying protective mechanism of AVLE. The results showed that the optimum conditions to induce oxidative stress were treatment with 40 µM H2O2 for 2 h, and the suitable range of AVLE concentrations was shown to be 1-100 µg/ml. AVLE improved cell viability in PC12 cells following treatment with H2O2. AVLE reduced the expression of Bax and cleaved-caspase-3, and decreased ROS production. Furthermore, AVLE upregulated LC3-II expression and downregulated p62 expression, whereas treatment with 3-MA increased the levels of ROS and apoptotic proteins. These results suggest that AVLE may protect injured neurons against oxidative stress-induced apoptosis, and this effect may be associated with the reduction of ROS by increasing autophagy.

Hepatoprotective effect of Phellinus linteus mycelia polysaccharide (PL-N1) against acetaminophen-induced liver injury in mouse

Edible and medicinal fungi are one of the major sources for extraction and identification of polysaccharides, which are important biological response modifiers with notable antitumor, hepatoprotective effect and other pharmacological activities. This study aimed to evaluate the hepatoprotective effect of isolated Phellinus linteus polysaccharide (PL-N1) against acetaminophen (APAP) induced liver injury in mice. Mice were treated intragastrical with PL-N1 (10, 50 and 100 mg/kg) and APAP (300 mg/kg) injection. APAP alone caused increased serum aminotransferase levels and changes in hepatic histopathology, promoted oxidative stress by increasing lipid peroxidation and decreasing anti-oxidant enzyme activities, leading to hepatocellular necrosis and reduced liver function. PL-N1 decreased cytochrome P450 2E1 (CYP2E1) expression and hepatic release of cytokines to enhance the level of phase II enzymes. Also, PL-N1 obviously accelerates the metabolism of APAP in the rat model. Molecular docking analysis revealed the α-d-glucopyranosyl exhibit maximum interaction (-8.099) against CYP2E1 as comparably less than standard drug silibinin (-13.767). PL-N1 could be a promising natural substance for ameliorating acute APAP-induced oxidative stress and hepatic injury.

Inhibition of autophagy in theca cells induces CYP17A1 and PAI-1 expression via ROS/p38 and JNK signalling during the development of polycystic ovary syndrome

Polycystic ovary syndrome (PCOS) is a clinical syndrome characterized by hyperandrogenism, oligo/anovulation, and polycystic ovary. Autophagy is an intracellular system that degrades cytosolic proteins and organelles. The relationship between autophagy and PCOS has not been clarified. We found that p62 and ubiquitin were significantly increased in theca cells of women with PCOS using immunohistochemistry. Autophagy inhibition by palmitic acid and chloroquine in bovine theca cells increased p62 and ubiquitin and induced the expression of cytochrome P450 17A1 (CYP17A1) and plasminogen activator inhibitor-1 (PAI-1) mRNA. Furthermore, palmitic acid and chloroquine exposure significantly increased reactive oxygen species (ROS) and activated p38 and c-Jun N-terminal kinase (JNK). Inhibition of p38 and JNK significantly reduced CYP17A1 and PAI-1 mRNA expression. We showed that inhibition of autophagy in theca cells may have contributed to the pathogenesis of PCOS, based on CYP17A1 and PAI-1 mRNA expression via the ROS/p38 and JNK signalling pathways.

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.

Oxidative Stress-Driven Autophagy acROSs Onset and Therapeutic Outcome in Hepatocellular Carcinoma

Reactive oxygen species- (ROS-) mediated autophagy physiologically contributes to management of cell homeostasis in response to mild oxidative stress. Cancer cells typically engage autophagy downstream of ROS signaling derived from hypoxia and starvation, which are harsh environmental conditions that need to be faced for cancer development and progression. Hepatocellular carcinoma (HCC) is a solid tumor for which several environmental risk factors, particularly viral infections and alcohol abuse, have been shown to promote carcinogenesis via augmentation of oxidative stress. In addition, ROS burst in HCC cells frequently takes place after administration of therapeutic compounds that promote apoptotic cell death or even autophagic cell death. The interplay between ROS and autophagy (i) in the disposal of dysfunctional mitochondria via mitophagy, as a tumor suppressor mechanism, or (ii) in the cell survival adaptive response elicited by chemotherapeutic interventions, as a tumor-promoting event, will be depicted in this review in relation to HCC development and progression.

Hydrogen protects against chronic intermittent hypoxia induced renal dysfunction by promoting autophagy and alleviating apoptosis

AIMS

Hydrogen gas (H₂) has a diversity of effects such as anti-apoptotic, anti-inflammatory and anti-oxidative properties. However, molecular mechanism underlying the potential effect of H₂ on chronic intermittent hypoxia (CIH) induced renal injury remains obscure.

MATERIALS AND METHODS

In the present study, adult male Sprague-Dawley rats were randomly allocated into four groups: control (CON) group, CIH group, CIH with H₂ treatment (CIH + H₂) group, and control with H₂ treatment (CON + H₂) group. Oxidative stress, autophagy and endoplasmic reticulum (ER) stress were detected to determine how H₂ affected the renal function of CIH exposed rats.

KEY FINDINGS

We demonstrated that rats who inhale hydrogen gas showed improved renal function, alleviated pathological damage, oxidative stress and apoptosis in CIH rats. Meanwhile, CIH-induced endoplasmic reticulum stress was decreased by H₂ as the expressions of CHOP, caspase-12, and GRP78 were down-regulated. Furthermore, relative higher levels of LC3-II/I ratio and Beclin-1, with decreased expression of p62, were found after H₂ administrated. Inhibition of mTOR may be involved in the upregulation of autophagy by H₂. Finally, increased phosphorylation of p38 and JNK was involved in the CIH-induced pathological process. H₂ could inhibit the activation of p38 and JNK, suggesting H₂ played an active part in resisting renal injury via MAPK.

SIGNIFICANCE

Taken together, our study reveals that H₂ can ameliorate CIH-induced kidney injury by decreasing endoplasmic reticulum stress and activating autophagy through inhibiting oxidative stress-dependent p38 and JNK MAPK activation.

The Activation and Function of Autophagy in Alcoholic Liver Disease

Autophagy is an important lysosome-mediated intracellular degradation pathway required for tissue homeostasis. Dysregulation of liver autophagy is closely associated with different liver diseases including alcoholic liver disease. Studies now indicate that autophagy may be induced or suppressed depending on the amount and the duration of ethanol treatment. Autophagy induced by ethanol serves as a protective mechanism, probably by selective degradation of the damaged mitochondria (mitophagy) and excess lipid droplets (lipophagy) and in turn attenuates alcohol-induced steatosis and liver injury. However, the detailed molecular mechanism of selective targeting of mitochondria and lipid is still unclear. Autophagy may possess other functions that protect hepatocytes from ethanol. Understanding these molecular entities would be essential in order to therapeutically module autophagy for treatment of alcoholic liver disease.

Role of CYP2E1 in Mitochondrial Dysfunction and Hepatic Injury by Alcohol and Non-Alcoholic Substances

Alcoholic fatty liver disease (AFLD) and non-alcoholic fatty liver disease (NAFLD) are two pathological conditions that are spreading worldwide. Both conditions are remarkably similar with regard to the pathophysiological mechanism and progression despite different causes. Oxidative stressinduced mitochondrial dysfunction through post-translational protein modifications and/or mitochondrial DNA damage has been a major risk factor in both AFLD and NAFLD development and progression. Cytochrome P450-2E1 (CYP2E1), a known important inducer of oxidative radicals in the cells, has been reported to remarkably increase in both AFLD and NAFLD. Interestingly, CYP2E1 isoforms expressed in both endoplasmic reticulum (ER) and mitochondria, likely lead to the deleterious consequences in response to alcohol or in conditions of NAFLD after exposure to high fat diet (HFD) and in obesity and diabetes. Whether CYP2E1 in both ER and mitochondria work simultaneously or sequentially in various conditions and whether mitochondrial CYP2E1 may exert more pronounced effects on mitochondrial dysfunction in AFLD and NAFLD are unclear. The aims of this review are to briefly describe the role of CYP2E1 and resultant oxidative stress in promoting mitochondrial dysfunction and the development or progression of AFLD and NAFLD, to shed a light on the function of the mitochondrial CYP2E1 as compared with the ER-associated CYP2E1. We finally discuss translational research opportunities related to this field.

Cilostazol protects hepatocytes against alcohol-induced apoptosis via activation of AMPK pathway

Alcoholic liver disease (ALD) is a worldwide health problem and hepatocyte apoptosis has been associated with the development/progression of ALD. However, no definite effective pharmacotherapy for ALD is currently available. Cilostazol, a selective type III phosphodiesterase inhibitor has been shown to protect hepatocytes from ethanol-induced apoptosis. In the present study, the underlying mechanisms for the protective effects of cilostazol were examined. Primary rat hepatocytes were treated with ethanol in the presence or absence of cilostazol. Cell viability and intracellular cAMP were measured. Apoptosis was detected by Hoechst staining, TUNEL assay, and caspase-3 activity assay. The roles of cAMP and AMP-activated protein kinase (AMPK) pathways in the action of CTZ were explored using pharmacological inhibitors and siRNAs. Liver from mice received ethanol (5 g/kg body weight) by oral gavage following cilostazol treatment intraperitoneally was obtained for measurement of apoptosis and activation of AMPK pathway. Cilostazol inhibited ethanol-induced hepatocyte apoptosis and potentiated the increases in cAMP level induced by forskolin. However, the anti-apoptotic effect of cilostazol was not reversed by an inhibitor of adenylyl cyclase. Interestingly, cilostazol activated AMPK and increased the level of LC3-II, a marker of autophagy. The inhibition of AMPK abolished the effects of cilostazol on LC3-II expression and apoptosis. Moreover, the inhibition of LKB1 and CaMKK2, upstream kinases of AMPK, dampened cilostazol-inhibited apoptosis as well as AMPK activation. In conclusion, cilostazol protected hepatocytes from apoptosis induced by ethanol mainly via AMPK pathway which is regulated by both LKB1 and CaMKK2. Our results suggest that cilostazol may have potential as a promising therapeutic drug for treatment of ALD.

Oxidative Stress and Autophagy

Living in a complex environment, humans are always faced with various external stimuli and internal changes including oxidative stress and tissue damage. To adapt to these stimuli, maintain physiological stability, and ensure survival, cells in the body initiate a series of interactive and regulatory response pathways. For example, increased reactive oxygen species in the body can induce autophagy through a variety of signalling pathways. This section will focus on ROS-mediated regulation of autophagy through PI3K/Akt, AMPK, JNK, ERK, ATG4, and other pathways.

The role of ROS-induced autophagy in hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is a main cause of cancer-related mortality and its etiology is not fully understood. As prominent factors that regulate cellular homeostasis, both reactive oxygen species (ROS) and autophagy are considered to play an essential role in the liver carcinogenesis. However, the crosstalk between ROS and autophagy is not well characterized in the pathogenesis of HCC. This review summarizes the roles of autophagy in ROS-mediated hepatocarcinogenesis and discusses the role of ROS-induced autophagy in HCC cell fate decision following treatment with chemotherapeutic agents in preclinical settings, which may allow the identification of novel strategies for the treatment of HCC.

ERK activation and autophagy impairment are central mediators of irinotecan-induced steatohepatitis

OBJECTIVE

Preoperative chemotherapy with irinotecan is associated with the development of steatohepatitis, which increases the risk of perioperative morbidity and mortality for liver surgery. The molecular mechanisms of this chemotherapeutic complication are widely unknown.

DESIGN

Mechanisms of irinotecan-induced steatohepatitis were studied in primary human hepatocytes in vitro, in mice treated with irinotecan and in liver specimens from irinotecan-treated compared with control patients.

RESULTS

Irinotecan dose-dependently induced lipid accumulation and pro-inflammatory gene expression in hepatocytes. This was accompanied by an impairment of mitochondrial function with reduced expression of carnitine palmitoyltransferase I and an induction of acyl-coenzyme A oxidase-1 (ACOX1), oxidative stress and extracellular signal-regulated kinase (ERK) activation. ERK inhibition prevented irinotecan-induced pro-inflammatory gene expression but had only a slight effect on lipid accumulation. However, irinotecan also induced an impairment of the autophagic flux mediated by alkalisation of lysosomal pH. Re-acidification of lysosomal pH abolished irinotecan-induced autophagy impairment and lipid accumulation. Also in mice, irinotecan treatment induced hepatic ACOX1 expression, ERK phosphorylation and inflammation, as well as impairment of autophagy and significant steatosis. Furthermore, irinotecan-treated patients revealed higher hepatic ERK activity, expression of pro-inflammatory genes and markers indicative for a shift to peroxisomal fatty acid oxidation and an impaired autophagic flux. Pretreatment with the multityrosine kinase inhibitor sorafenib did not affect autophagy impairment and steatosis but significantly reduced ERK phosphorylation and inflammatory response in irinotecan-treated hepatocytes and murine livers.

CONCLUSIONS

Irinotecan induces hepatic steatosis via autophagy impairment and inflammation via ERK activation. Sorafenib appears as a novel therapeutic option for the prevention and treatment of irinotecan-induced inflammation.

Activation of Ca2+-sensing receptor as a protective pathway to reduce Cadmium-induced cytotoxicity in renal proximal tubular cells

Cadmium (Cd), as an extremely toxic metal could accumulate in kidney and induce renal injury. Previous studies have proved that Cd impact on renal cell proliferation, autophagy and apoptosis, but the detoxification drugs and the functional mechanism are still in study. In this study, we used mouse renal tubular epithelial cells (mRTECs) to clarify Cd-induced toxicity and signaling pathways. Moreover, we proposed to elucidate the prevent effect of activation of Ca²⁺ sensing receptor (CaSR) by Calcimimetic (R-467) on Cd-induced cytotoxicity and underlying mechanisms. Cd induced intracellular Ca²⁺ elevation through phospholipase C-inositol 1, 4, 5-trisphosphate (PLC) followed stimulating p38 mitogen-activated protein kinases (MAPK) activation and suppressing extracellular signal-regulated kinase (ERK) activation, which leaded to increase apoptotic cell death and inhibit cell proliferation. Cd induced p38 activation also contribute to autophagic flux inhibition that aggravated Cd induced apoptosis. R-467 reinstated Cd-induced elevation of intracellular Ca²⁺ and apoptosis, and it also increased cell proliferation and restored autophagic flux by switching p38 to ERK pathway. The identification of the activation of CaSR-mediated protective pathway in renal cells sheds light on a possible cellular protective mechanism against Cd-induced kidney injury.

Cytochrome P450s and Alcoholic Liver Disease

Alcohol consumption causes liver diseases, designated as Alcoholic Liver Disease (ALD). Because alcohol is detoxified by alcohol dehydrogenase (ADH), a major ethanol metabolism system, the development of ALD was initially believed to be due to malnutrition caused by alcohol metabolism in liver. The discovery of the microsomal ethanol oxidizing system (MEOS) changed this dogma. Cytochrome P450 enzymes (CYP) constitute the major components of MEOS. Cytochrome P450 2E1 (CYP2E1) in MEOS is one of the major ROS generators in liver and is considered to be contributive to ALD. Our labs have been studying the relationship between CYP2E1 and ALD for many years. Recently, we found that human CYP2A6 and its mouse analog CYP2A5 are also induced by alcohol. In mice, the alcohol induction of CYP2A5 is CYP2E1-dependent. Unlike CYP2E1, CYP2A5 protects against the development of ALD. The relationship of CYP2E1, CYP2A5, and ALD is a major focus of this review.

ULK1-regulated autophagy: A mechanism in cellular protection for ALDH2 against hyperglycemia

Mitochondrial aldehyde dehydrogenase 2 (ALDH2), an important enzyme in the elimination of toxic aldehydes, is involved in cardioprotection against diabetes mellitus. This study was designed to examine the mechanism behind ALDH2-offered protection against high glucose exposure with a focus on autophagy. H9C2 cells were cultured with normal or high glucose medium in the presence or absence of the ALDH2 agonist Alda-1. GFP-LC3 puncta and immunofluorescence were employed to assess autophagosome formation. Western blotting was applied to evaluate autophagy protein markers Atg5, LC3, p62, ULK1 phosphorylation and ALDH2. JC-1 staining was used to monitor mitochondrial membrane potential and mitochondrial injury. CCK-8 and TUNEL assays were employed for apoptosis and cell viability. Our results indicated that high glucose promoted cell death and decreased cell viability. Levels of autophagy protein marker Atg5, and LC3B were decreased and level of p62 was elevated in hyperglycemic condition, the effects of which were reversed by ALHD2. High glucose lowered mitochondrial membrane potential, the effect of which was accentuated by ULK1 knock-down. All these high glucose-induced responses were negated by Alda-1 along with upregulated autophagy. The autophagy inhibitor 3-MA and lysosomal inhibitor bafilomycin A1 cancelled off whereas autophagy inducer rapamycin mimicked the Alda-1-offered protection against high glucose. High glucose suppressed phosphorylation of ULK1, the effect of which was mitigated by Alda-1. Knock-down of ULK1 using siRNA negated Alda-1-induced upregulation of autophagosome accumulation and LC3 expression. High glucose-dampened autophagy was also confirmed using GFP-LC3 puncta, and immunofluorescence. Taken together, these data suggested that ULK1 played a crucial role in ALDH2-offered protective effect against high glucose exposure-induced cardiomyocyte injury through regulation of autophagy.

Autophagy protects gastric mucosal epithelial cells from ethanol-induced oxidative damage via mTOR signaling pathway

Alcohol abuse is an important cause of gastric mucosal epithelial cell injury and gastric ulcers. A number of studies have demonstrated that autophagy, an evolutionarily conserved cellular mechanism, has a protective effect on cell survival. However, it is not known whether autophagy can protect gastric mucosal epithelial cells against the toxic effects of ethanol. In the present study, gastric mucosal epithelial cells (GES-1 cells) and Wistar rats were treated with ethanol to detect the adaptive response of autophagy. Our results demonstrated that ethanol exposure induced gastric mucosal epithelial cell damage, which was accompanied by the downregulation of mTOR signaling pathway and activation of autophagy. Suppression of autophagy with pharmacological agents resulted in a significant increase of GES-1 cell apoptosis and gastric mucosa injury, suggesting that autophagy could protect cells from ethanol toxicity. Furthermore, we evaluated the cellular oxidative stress response following ethanol treatment and found that autophagy induced by ethanol inhibited generation of reactive oxygen species and degradation of antioxidant and lipid peroxidation. In conclusion, these findings provide evidence that ethanol can activate autophagy via downregulation of the mTOR signaling pathway, serving as an adaptive mechanism to ameliorate oxidative damage induced by ethanol in gastric mucosal epithelial cells. Therefore, modifying autophagy may provide a therapeutic strategy against alcoholic gastric mucosa injury. Impact statement The effect and mechanism of autophagy on ethanol-induced cell damage remain controversial. In this manuscript, we report the results of our study demonstrating that autophagy can protect gastric mucosal epithelial cells against ethanol toxicity in vitro and in vivo. We have shown that ethanol can activate autophagy via downregulation of the mTOR signaling pathway, serving as an adaptive mechanism to ameliorate ethanol-induced oxidative damage in gastric mucosal epithelial cells. This study brings new and important insights into the mechanism of alcoholic gastric mucosal injury and may provide an avenue for future therapeutic strategies.

Eupatilin induces Sestrin2-dependent autophagy to prevent oxidative stress

Eupatilin (5,7-dihydroxy-3,4,6-trimethoxyflavone) has many pharmacological activities including anti-inflammation, anti-oxidant and anti-cancer effects. Autophagy is the basic cellular machinery involving the digestion of damaged cellular components. In the present study, we investigated the protection effects of eupatilin against arachidonic acid (AA) and iron-induced oxidative stress in HepG2 cells and tried to elucidate the molecular mechanisms responsible. Eupatilin increased cell viability against AA + iron in a concentration-dependent manner and prevented mitochondrial dysfunction and reactive oxygen species (ROS) production. In addition, AA + iron increased the levels of pro-apoptotic proteins and these changes were prevented by eupatilin. Eupatilin also induced autophagy, as evidenced by the accumulation of microtubule-associated protein 1 light chain3-II and the detection of autophagic vacuoles. Furthermore, the protective effects of eupatilin on mitochondrial dysfunction and ROS production were significantly abolished by autophagy inhibitors. Eupatilin also increased the mRNA level of sestrin-2 and its promoter-driven reporter gene activity, which resulted in the up-regulation of sestrin-2 protein. Finally, gene silencing using sestrin-2 siRNA and the ectopic expression of recombinant adenoviral sestrin-2 indicated that sestrin-2 induction by eupatilin was required for autophagy-mediated cytoprotection against AA + iron. Our results suggest that eupatilin activates sestrin-2-dependent autophagy, thereby preventing oxidative stress induced by AA + iron.

Reversal of ethanol-induced hepatotoxicity by cinnamic and syringic acids in mice

Ethanol was used to induce acute hepatotoxicity in mice. Effects of cinnamic acid (CA) and syringic acid (SA) post-intake for hepatic recovery from alcoholic injury was investigated. Ethanol treated mice were supplied by CA or SA at 40 or 80 mg/kg BW/day for 5 days. Results showed that ethanol stimulated protein expression of CYP2E1, p47^phox, gp91^phox, cyclooxygenase-2 and nuclear factor kappa B in liver. CA or SA post-intake restricted hepatic expression of these molecules. Ethanol suppressed nuclear factor erythroid 2-related factor (Nrf2) expression, and CA or SA enhanced Nrf2 expression in cytosolic and nuclear fractions. Ethanol increased the release of reactive oxygen species, oxidized glutathione, interleukin-6, tumor necrosis factor-alpha, nitric acid and prostaglandin E₂. CA or SA lowered hepatic production of these oxidative and inflammatory factors. Histological data revealed that ethanol administration caused obvious foci of inflammatory cell infiltration, and CA or SA post-intake improved hepatic inflammatory infiltration. These findings support that cinnamic acid and syringic acid are potent nutraceutical agents for acute alcoholic liver disease therapy. However, potential additive or synergistic benefits of cinnamic and syringic acids against ethanol-induced hepatotoxicity need to be investigated.

Identification of cytochrome CYP2E1 as critical mediator of synergistic effects of alcohol and cellular lipid accumulation in hepatocytes in vitro

Clinical studies propose a causative link between the consumption of alcohol and the development and progression of liver disease in obese individuals. However, it is incompletely understood how alcohol and obesity interact and whether the combined effects are additive or synergistic. In this study, we developed an in vitro model to address this question. Lipid accumulation in primary human hepatocytes was induced by incubation with oleic acid. Subsequently, steatotic and control hepatocytes were incubated with up to 50 mM alcohol. This alcohol concentration on its own revealed only minimal effects but significantly enhanced oleate-induced lipogenesis and cellular triglyceride content compared to control cells. Similarly, lipid peroxidation, oxidative stress and pro-inflammatory gene expression as well as CYP2E1 levels and activity were synergistically induced by alcohol and steatosis. CYP2E1 inhibition blunted these synergistic pathological effects. Notably, alcohol and cellular steatosis also induced autophagy in a synergistic manner, and also this was mediated via CYP2E1. Further induction of autophagy ameliorated the joint effects of alcohol and oleic acid on hepatocellular lipid accumulation and inflammatory gene expression while inhibition of autophagy further enhanced the dual pathological effects. Further analyses revealed that the joint synergistic effect of alcohol and steatosis on autophagy was mediated via activation of the JNK-pathway. In summary, our data indicate that alcohol induces not only pathological but also protective mechanisms in steatotic hepatocytes via CYP2E1. These findings may have important implications on the prognosis and treatment of alcoholic liver disease particularly in obese individuals.

CHIP(-/-)-Mouse Liver: Adiponectin-AMPK-FOXO-Activation Overrides CYP2E1-Elicited JNK1-Activation, Delaying Onset of NASH: Therapeutic Implications

Genetic ablation of C-terminus of Hsc70-interacting protein (CHIP) E3 ubiquitin-ligase impairs hepatic cytochrome P450 CYP2E1 degradation. Consequent CYP2E1 gain of function accelerates reactive O2 species (ROS) production, triggering oxidative/proteotoxic stress associated with sustained activation of c-Jun NH2-terminal kinase (JNK)-signaling cascades, pro-inflammatory effectors/cytokines, insulin resistance, progressive hepatocellular ballooning and microvesicular steatosis. Despite this, little evidence of nonalcoholic fatty liver disease (NAFLD)/nonalcoholic steatohepatitis (NASH) was found in CHIP(-/-)-mice over the first 8-9-months of life. We herein document that this lack of tissue injury is largely due to the concurrent up-regulation and/or activation of the adiponectin-5'-AMP-activated protein kinase (AMPK)-forkhead box O (FOXO)-signaling axis stemming from at the least three synergistic features: Up-regulated expression of adipose tissue adiponectin and its hepatic adipoR1/adipoR2 receptors, stabilization of hepatic AMPKα1-isoform, identified herein for the first time as a CHIP-ubiquitination substrate (unlike its AMPKα2-isoform), as well as nuclear stabilization of FOXOs, well-known CHIP-ubiquitination targets. Such beneficial predominance of the adiponectin-AMPK-FOXO-signaling axis over the sustained JNK-elevation and injurious insulin resistance in CHIP(-/-)-livers apparently counteracts/delays rapid progression of the hepatic microvesicular steatosis to the characteristic macrovesicular steatosis observed in clinical NASH and/or rodent NASH-models.

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.

Glutathione Depletion Induces Spermatogonial Cell Autophagy

The development and survival of male germ cells depend on the antioxidant capacity of the seminiferous tubule. Glutathione (GSH) plays an important role in the antioxidant defenses of the spermatogenic epithelium. Autophagy can act as a pro-survival response during oxidative stress or nutrient deficiency. In this work, we evaluated whether autophagy is involved in spermatogonia-type germ cell survival during severe GSH deficiency. We showed that the disruption of GSH metabolism with l-buthionine-(S,R)-sulfoximine (BSO) decreased reduced (GSH), oxidized (GSSG) glutathione content, and GSH/GSSG ratio in germ cells, without altering reactive oxygen species production and cell viability, evaluated by 2',7'-dichlorodihydrofluorescein (DCF) fluorescence and exclusion of propidium iodide assays, respectively. Autophagy was assessed by processing the endogenous protein LC3I and observing its sub-cellular distribution. Immunoblot and immunofluorescence analysis showed a consistent increase in LC3II and accumulation of autophagic vesicles under GSH-depletion conditions. This condition did not show changes in the level of phosphorylation of AMP-activated protein kinase (AMPK) or the ATP content. A loss in S-glutathionylated protein pattern was also observed. However, inhibition of autophagy resulted in decreased ATP content and increased caspase-3/7 activity in GSH-depleted germ cells. These findings suggest that GSH deficiency triggers an AMPK-independent induction of autophagy in germ cells as an adaptive stress response.

Eicosapentaenoic acid attenuates dexamethasome-induced apoptosis by inducing adaptive autophagy via GPR120 in murine bone marrow-derived mesenchymal stem cells

Long-term use of glucocorticoids is a widespread clinical problem, which currently has no effective solution other than discontinuing the use. Eicosapentaenoic acid (EPA), an omega-3 long chain polyunsaturated fatty acid (n-3 PUFA), which is largely contained in fish or fish oil, has been reported to promote cell viability and improve bone metabolism. However, little is known about the effects of EPA on dexamethasome (Dex)-induced cell apoptosis. In this study, we showed that EPA-induced autophagy of murine bone marrow-derived mesenchymal stem cells (mBMMSCs). Meanwhile, EPA, but not arachidonic acid (AA), markedly inhibited Dex-induced apoptosis and promoted the viability of mBMMSCs. We also observed that EPA-induced autophagy was modulated by GPR120, but not GPR40. Further experiments showed that the mechanism of EPA-induced autophagy associated with GPR120 modulation involved an increase in the active form of AMP-activated protein kinase and a decrease in the activity of mammalian target of RAPA. The protective effect of EPA on Dex-induced apoptosis via GPR120-meditated induction of adaptive autophagy was supported by in vivo experiments. In summary, our findings may have important implications in developing future strategies to use EPA in the prevention and therapy of the side effects induced by long-term Dex-abuse.

Autophagy in ethanol-exposed liver disease

Ethanol metabolism in hepatocytes causes the generation of reactive oxygen species, endoplasmic reticulum stress and alterations in mitochondrial energy and REDOX metabolism. In ethanol-exposed liver disease, autophagy not only acts as a cleanser to remove damaged organelles and cytosolic components, but also selectively clears specific targets such as lipid droplets and damaged mitochondria. Moreover, ethanol appears to play a role in protecting hepatocytes from apoptosis at certain concentrations. This article describes the evidence, function and potential mechanism of autophagy in ethanol-exposed liver disease and the controversy surrounding the effects of ethanol on autophagy.

Autophagy: a new target for nonalcoholic fatty liver disease therapy

Nonalcoholic fatty liver disease (NAFLD) has gained importance in recent decades due to drastic changes in diet, especially in Western countries. NAFLD occurs as a spectrum from simple hepatic steatosis, steatohepatitis to cirrhosis, and even hepatocellular carcinoma. Although the molecular mechanisms underlying the development of NAFLD have been intensively investigated, many issues remain to be resolved. Autophagy is a cell survival mechanism for disposing of excess or defective organelles, and has become a hot spot for research. Recent studies have revealed that autophagy is linked to the development of NAFLD and regulation of autophagy has therapeutic potential. Autophagy reduces intracellular lipid droplets by enclosing them and fusing with lysosomes for degradation. Furthermore, autophagy is involved in attenuating inflammation and liver injury. However, autophagy is regarded as a double-edged sword, as it may also affect adipogenesis and adipocyte differentiation. Moreover, it is unclear as to whether autophagy protects the body from injury or causes diseases and even death, and the association between autophagy and NAFLD remains controversial. This review is intended to discuss, comment, and outline the progress made in this field and establish the possible molecular mechanism involved.

CD40 Negatively Regulates ATP-TLR4-Activated Inflammasome in Microglia

During acute brain injury and/or sterile inflammation, release of danger-associated molecular patterns (DAMPs) activates pattern recognition receptors (PRRs). Microglial toll-like receptor (TLR)-4 activated by DAMPs potentiates neuroinflammation through inflammasome-induced IL-1β and pathogenic Th17 polarization which critically influences brain injury. TLR4 activation accompanies increased CD40, a cognate costimulatory molecule, involved in microglia-mediated immune responses in the brain. During brain injury, excessive release of extracellular ATP (DAMPs) is involved in promoting the damage. However, the regulatory role of CD40 in microglia during ATP-TLR4-mediated inflammasome activation has never been explored. We report that CD40, in the absence of ATP, synergizes TLR4-induced proinflammatory cytokines but not IL-1β, suggesting that the response is independent of inflammasome. The presence of ATP during TLR4 activation leads to NLRP3 inflammasome activation and caspase-1-mediated IL-1β secretion which was inhibited during CD40 activation, accompanied with inhibition of ERK1/2 and reactive oxygen species (ROS), and elevation in p38 MAPK phosphorylation. Experiments using selective inhibitors prove indispensability of ERK 1/2 and ROS for inflammasome activation. The ATP-TLR4-primed macrophages polarize the immune response toward pathogenic Th17 cells, whereas CD40 activation mediates Th1 response. Exogenous supplementation of IFN-γ (a Th1 cytokine and CD40 inducer) results in decreased IL-1β, suggesting possible feedback loop mechanism of inflammasome inhibition, whereby IFN-γ-mediated increase in CD40 expression and activation suppress neurotoxic inflammasome activation required for Th17 response. Collectively, the findings indicate that CD40 is a novel negative regulator of ATP-TLR4-mediated inflammasome activation in microglia, thus providing a checkpoint to regulate excessive inflammasome activation and Th17 response during DAMP-mediated brain injury.

Scutellaria baicalensis Georgi extract protects against alcohol‑induced acute liver injury in mice and affects the mechanism of ER stress

The aims of the present study were to examine the hepatoprotective effect of Scutellaria baicalensis Georgi extract (Scutellariae Radix extract; SRE) against acute alcohol-induced liver injury in mice, and investigate the mechanism of endoplasmic reticulum (ER) stress. High performance liquid chromatography was used for the phytochemical analysis of SRE. Animals were administered orally with 50% alcohol (12 ml/kg) 4 h following administration of doses of SRE every day for 14 days, with the exception of normal control group. The protective effect was investigated by measuring the levels of aspartate transaminase (AST), alanine transferase (ALT) and triglyceride (TG) in the serum, and the levels of glutathione (GSH) and malondialdehyde (MDA) in liver tissues. The levels of glucose-related protein 78 (GRP78) were detected using immunohistochemical localization and an enzyme-linked immunosorbent assay. Hepatocyte apoptosis was assessed using terminal-deoxynucleoitidyl transferase mediated nick end labeling. The SRE contained 31.2% baicalin. Pretreatment with SRE had a marked protective effect by reversing the levels of biochemical markers and levels of GRP78 in a dose-dependent manner. The results of the present study demonstrated that pretreatment with SRE exerted a marked hepatoprotective effect by downregulating the expression of GRP78, which is a marker of ER stress.

Protective role of autophagy in methionine-choline deficient diet-induced advanced nonalcoholic steatohepatitis in mice

The methionine choline-deficient (MCD) diet leads to severe liver injury similar to human nonalcoholic steatohepatitis (NASH). Autophagy has emerged as a critical lysosomal pathway that maintains cell function and survival through the degradation of cellular components such as organelles and proteins. The goal of this study was to elucidate the role of autophagy in MCD-induced steatosis, fibrosis, inflammation, mitochondrial dysfunction, and endoplasmic reticulum (ER) stress in mice. Mice were fed with MCD diet and treated with rapamycin (an autophagy enhancer) or chloroquine (an autophagy inhibitor) for 10 weeks. Liver injury was evaluated biochemically and histologically together with hepatic gene expression analysis. Autophagic flux was impaired in livers of mice fed with MCD diet, evidenced by reduced ratio of LC3-II/LC3-I and increased protein expression of p62. It was found that autophagy activation by rapamycin attenuated MCD-induced steatosis, fibrosis, inflammation, mitochondrial dysfunction, and ER stress. By contrast, MCD mice treated with chloroquine developed more liver injury. In conclusions, the autophagic pathway plays an important protective role in MCD-induced advanced NASH. Thus, pharmacological promotion of autophagy may provide a novel therapeutic strategy for treatment of NASH.

Autophagy Protects against CYP2E1/Chronic Ethanol-Induced Hepatotoxicity

Autophagy is an intracellular pathway by which lysosomes degrade and recycle long-lived proteins and cellular organelles. The effects of ethanol on autophagy are complex but recent studies have shown that autophagy serves a protective function against ethanol-induced liver injury. Autophagy was found to also be protective against CYP2E1-dependent toxicity in vitro in HepG2 cells which express CYP2E1 and in vivo in an acute alcohol/CYPE1-dependent liver injury model. The goal of the current report was to extend the previous in vitro and acute in vivo experiments to a chronic ethanol model to evaluate whether autophagy is also protective against CYP2E1-dependent liver injury in a chronic ethanol-fed mouse model. Wild type (WT), CYP2E1 knockout (KO) or CYP2E1 humanized transgenic knockin (KI), mice were fed an ethanol liquid diet or control dextrose diet for four weeks. In the last week, some mice received either saline or 3-methyladenine (3-MA), an inhibitor of autophagy, or rapamycin, which stimulates autophagy. Inhibition of autophagy by 3-MA potentiated the ethanol-induced increases in serum transaminase and triglyceride levels in the WT and KI mice but not KO mice, while rapamycin prevented the ethanol liver injury. Treatment with 3-MA enhanced the ethanol-induced fat accumulation in WT mice and caused necrosis in the KI mice; little or no effect was found in the ethanol-fed KO mice or any of the dextrose-fed mice. 3-MA treatment further lowered the ethanol-decrease in hepatic GSH levels and further increased formation of TBARS in WT and KI mice, whereas rapamycin blunted these effects of ethanol. Neither 3-MA nor rapamycin treatment affected CYP2E1 catalytic activity or content or the induction CYP2E1 by ethanol. The 3-MA treatment decreased levels of Beclin-1 and Atg 7 but increased levels of p62 in the ethanol-fed WT and KI mice whereas rapamycin had the opposite effects, validating inhibition and stimulation of autophagy, respectively. These results suggest that autophagy is protective against CYP2E1-dependent liver injury in a chronic ethanol-fed mouse model. We speculate that autophagy-dependent processes such as mitophagy and lipophagy help to minimize ethanol-induced CYP2E1-dependent oxidative stress and therefore the subsequent liver injury and steatosis. Attempts to stimulate autophagy may be helpful in lowering ethanol and CYP2E1-dependent liver toxicity.

Autophagy: A new therapeutic target for liver fibrosis

Hepatic fibrosis is a wound-healing response to liver injury and the result of imbalance of extracellular matrix (ECM) accumulation and degradation. The relentless production and progressive accumulation of ECM can lead to end-stage liver disease. Although significant progress has been achieved in elucidating the mechanisms of fibrogenesis, effective anti-fibrotic strategies are still lacking. Autophagy is an intracellular process of self-digestion of defective organelles to provide material recycling or energy for cell survival. Autophagy has been implicated in the pathophysiology of many human disorders including hepatic fibrosis. However, the exact relationships between autophagy and hepatic fibrosis are not totally clear and need further investigations. A new therapeutic target for liver fibrosis could be developed with a better understanding of autophagy.

Translational Implications of the Alcohol-Metabolizing Enzymes, Including Cytochrome P450-2E1, in Alcoholic and Nonalcoholic Liver Disease

Fat accumulation (hepatic steatosis) in alcoholic and nonalcoholic fatty liver disease is a potentially pathologic condition which can progress to steatohepatitis (inflammation), fibrosis, cirrhosis, and carcinogenesis. Many clinically used drugs or some alternative medicine compounds are also known to cause drug-induced liver injury, which can further lead to fulminant liver failure and acute deaths in extreme cases. During liver disease process, certain cytochromes P450 such as the ethanol-inducible cytochrome P450-2E1 (CYP2E1) and CYP4A isozymes can be induced and/or activated by alcohol and/or high-fat diets and pathophysiological conditions such as fasting, obesity, and diabetes. Activation of these P450 isozymes, involved in the metabolism of ethanol, fatty acids, and various drugs, can produce reactive oxygen/nitrogen species directly and/or indirectly, contributing to oxidative modifications of DNA/RNA, proteins and lipids. In addition, aldehyde dehydrogenases including the mitochondrial low Km aldehyde dehydrogenase-2 (ALDH2), responsible for the metabolism of acetaldehyde and lipid aldehydes, can be inactivated by various hepatotoxic agents. These highly reactive acetaldehyde and lipid peroxides, accumulated due to ALDH2 suppression, can interact with cellular macromolecules DNA/RNA, lipids, and proteins, leading to suppression of their normal function, contributing to DNA mutations, endoplasmic reticulum stress, mitochondrial dysfunction, steatosis, and cell death. In this chapter, we specifically review the roles of the alcohol-metabolizing enzymes including the alcohol dehydrogenase, ALDH2, CYP2E1, and other enzymes in promoting liver disease. We also discuss translational research opportunities with natural and/or synthetic antioxidants, which can prevent or delay the onset of inflammation and liver disease.

Globular Adiponectin Causes Tolerance to LPS-Induced TNF-α Expression via Autophagy Induction in RAW 264.7 Macrophages: Involvement of SIRT1/FoxO3A Axis

Adiponectin, an adipokine predominantly produced from adipose tissue, exhibited potent anti-inflammatory properties. In particular, it inhibits production of pro-inflammatory cytokines, including tumor necrosis factor-α (TNF-α), in macrophages. Autophagy, an intracellular self-digestion process, has been recently shown to regulate inflammatory responses. In the present study, we investigated the role of autophagy induction in the suppression of Lipopolysaccharide (LPS) -induced TNF-α expression by globular adiponectin (gAcrp) and its potential mechanisms. Herein, we found that gAcrp treatment increased expression of genes related with autophagy, including Atg5 and microtubule-associated protein light chain (LC3B), induced autophagosome formation and autophagy flux in RAW 264.7 macrophages. Similar results were observed in primary macrophages isolated peritoneum of mice. Interestingly, inhibition of autophagy by pretreatment with Bafilomycin A1 or knocking down of LC3B gene restored suppression of TNF-α expression, tumor necrosis factor receptor- associated factor 6 (TRAF6) expression and p38MAPK phosphorylation by gAcrp, implying a critical role of autophagy induction in the development of tolerance to LPS-induced TNF-α expression by gAcrp. We also found that knocking-down of FoxO3A, a forkhead box O member of transcription factor, blocked gAcrp-induced expression of LC3II and Atg5. Moreover, gene silencing of Silent information regulator 1 (SIRT1) blocked both gAcrp-induced nuclear translocation of FoxO3A and LC3II expression. Finally, pretreatment with ROS inhibitors, prevented gAcrp-induced SIRT1 expression and further generated inhibitory effects on gAcrp-induced autophagy, indicating a role of ROS production in gAcrp-induced SIRT1 expression and subsequent autophagy induction. Taken together, these findings indicate that globular adiponectin suppresses LPS-induced TNF-α expression, at least in part, via autophagy activation. Furthermore, SIRT1-FoxO3A axis plays a crucial role in gAcrp-induced autophagy in macrophages.

Inhibition of soluble epoxide hydrolase attenuates hepatic fibrosis and endoplasmic reticulum stress induced by carbon tetrachloride in mice

Liver fibrosis is a pathological condition in which chronic inflammation and changes to the extracellular matrix lead to alterations in hepatic tissue architecture and functional degradation of the liver. Inhibitors of the enzyme soluble epoxide hydrolase (sEH) reduce fibrosis in the heart, pancreas and kidney in several disease models. In this study, we assess the effect of sEH inhibition on the development of fibrosis in a carbon tetrachloride (CCl4)-induced mouse model by monitoring changes in the inflammatory response, matrix remolding and endoplasmic reticulum stress. The sEH inhibitor 1-trifluoromethoxyphenyl-3-(1-propionylpiperidin-4-yl) urea (TPPU) was administered in drinking water. Collagen deposition in the liver was increased five-fold in the CCl4-treated group, and this was returned to control levels by TPPU treatment. Hepatic expression of Col1a2 and 3a1 mRNA was increased over fifteen-fold in the CCl4-treated group relative to the Control group, and this increase was reduced by 50% by TPPU treatment. Endoplasmic reticulum (ER) stress observed in the livers of CCl4-treated animals was attenuated by TPPU treatment. In order to support the hypothesis that TPPU is acting to reduce the hepatic fibrosis and ER stress through its action as a sEH inhibitor we used a second sEH inhibitor, trans-4-{4-[3-(4-trifluoromethoxy-phenyl)-ureido]-cyclohexyloxy}-benzoic acid (t-TUCB), and sEH null mice. Taken together, these data indicate that the sEH may play an important role in the development of hepatic fibrosis induced by CCl4, presumably by reducing endogenous fatty acid epoxide chemical mediators acting to reduce ER stress.