Implication of altered proteasome function in alcoholic liver injury

Metallofullerenol alleviates alcoholic liver damage via ROS clearance under static magnetic and electric fields

Alcoholic liver disease (ALD) is a common chronic redox disease caused by increased alcohol consumption. Abstinence is a major challenge for people with alcohol dependence, and approved drugs have limited efficacy. Therefore, this study aimed to explore a new treatment strategy for ALD using ferroferric oxide endohedral fullerenol (Fe3O4@C60(OH)n) in combination with static magnetic and electric fields (sBE). The primary hepatocytes of 8-9-week-old female BALB/c mice were used to evaluate the efficacy of the proposed combination treatment. A mouse chronic binge ethanol feeding model was established to determine the alleviatory effect of Fe3O4@C60(OH)n on liver injury under sBE exposure. Furthermore, the ability of Fe3O4@C60(OH)n to eliminate •OH was evaluated. Alcohol-induced hepatocyte and mitochondrial damage were reversed in vitro. Additionally, the combination therapy reduced liver damage, alleviated oxidative stress by improving antioxidant levels, and effectively inhibited liver lipid accumulation in animal experiments. Here, we used a combination of magnetic derivatives of fullerenol and sBE to further improve the ROS clearance rate, thereby alleviating ALD. The developed combination treatment may effectively improve alcohol-induced liver damage and maintain redox balance without apparent toxicity, thereby enhancing therapy aimed at ALD and other redox diseases.

TIR8 protects against nonalcoholic steatohepatitis by antagonizing lipotoxicity-induced PPARα downregulation and reducing the sensitivity of hepatocytes to LPS

In up to one-third of nonalcoholic fatty liver disease (NAFLD) patients, simple steatosis progresses to its more severe form, nonalcoholic steatohepatitis (NASH), but the precise mechanisms underlying this transition are not fully understood. Toll/interleukin-1 receptor 8 (TIR8), a conventional innate immune regulator highly expressed in hepatic tissue, has shown potential for ameliorating various inflammation-related disorders. However, its role in NASH pathogenesis, especially its regulatory effects on lipid metabolism and inflammatory responses, is still unclear. Here, using a TIR8 knockout (TIR8KO) mouse model and mass spectrometry analyses, we found that TIR8KO mice displayed aggravated hepatic steatosis and inflammation, whereas TIR8 overexpression attenuated these adverse effects. Ectopic TIR8 expression counteracts free fatty acid (FFA)-induced PPARα inhibition and downstream signaling. A decrease in TIR8 levels in hepatocytes heightened lipopolysaccharide (LPS) sensitivity. Notably, FFA stimulation led to a direct interaction between TIR8 and proteasome subunit alpha type 4 (PSMA4), facilitating TIR8 degradation. These results revealed that TIR8 safeguards PPARα-regulated lipid metabolism and mitigates inflammation induced by external factors during NASH progression. Our study highlights TIR8 as a promising target for NASH therapy, indicating the potential of TIR8 agonists in treatment strategies.

Embryonic alcohol exposure disrupts the ubiquitin-proteasome system

Ethanol (EtOH) is a commonly encountered teratogen that can disrupt organ development and lead to fetal alcohol spectrum disorders (FASDs); many mechanisms of developmental toxicity are unknown. Here, we used transcriptomic analysis in an established zebrafish model of embryonic alcohol exposure (EAE) to identify the ubiquitin-proteasome system (UPS) as a critical target of EtOH during development. Surprisingly, EAE alters 20S, 19S, and 11S proteasome gene expression and increases ubiquitylated protein load. EtOH and its metabolite acetaldehyde decrease proteasomal peptidase activity in a cell type-specific manner. Proteasome 20S subunit β 1 (psmb1hi2939Tg) and proteasome 26S subunit, ATPase 6 (psmc6hi3593Tg), genetic KOs define the developmental impact of decreased proteasome function. Importantly, loss of psmb1 or psmc6 results in widespread developmental abnormalities resembling EAE phenotypes, including growth restriction, abnormal craniofacial structure, neurodevelopmental defects, and failed hepatopancreas maturation. Furthermore, pharmacologic inhibition of chymotrypsin-like proteasome activity potentiates the teratogenic effects of EAE on craniofacial structure, the nervous system, and the endoderm. Our studies identify the proteasome as a target of EtOH exposure and signify that UPS disruptions contribute to craniofacial, neurological, and endodermal phenotypes in FASDs.

Role of Mitochondrial Cytochrome P450 2E1 in Healthy and Diseased Liver

Cytochrome P450 2E1 (CYP2E1) is pivotal in hepatotoxicity induced by alcohol abuse and different xenobiotics. In this setting, CYP2E1 generates reactive metabolites inducing oxidative stress, mitochondrial dysfunction and cell death. In addition, this enzyme appears to play a role in the progression of obesity-related fatty liver to nonalcoholic steatohepatitis. Indeed, increased CYP2E1 activity in nonalcoholic fatty liver disease (NAFLD) is deemed to induce reactive oxygen species overproduction, which in turn triggers oxidative stress, necroinflammation and fibrosis. In 1997, Avadhani’s group reported for the first time the presence of CYP2E1 in rat liver mitochondria, and subsequent investigations by other groups confirmed that mitochondrial CYP2E1 (mtCYP2E1) could be found in different experimental models. In this review, we first recall the main features of CYP2E1 including its role in the biotransformation of endogenous and exogenous molecules, the regulation of its expression and activity and its involvement in different liver diseases. Then, we present the current knowledge on the physiological role of mtCYP2E1, its contribution to xenobiotic biotransformation as well as the mechanism and regulation of CYP2E1 targeting to mitochondria. Finally, we discuss experimental investigations suggesting that mtCYP2E1 could have a role in alcohol-associated liver disease, xenobiotic-induced hepatotoxicity and NAFLD.

Alcohol-Induced Lysosomal Damage and Suppression of Lysosome Biogenesis Contribute to Hepatotoxicity in HIV-Exposed Liver Cells

Although the causes of hepatotoxicity among alcohol-abusing HIV patients are multifactorial, alcohol remains the least explored "second hit" for HIV-related hepatotoxicity. Here, we investigated whether metabolically derived acetaldehyde impairs lysosomes to enhance HIV-induced hepatotoxicity. We exposed Cytochrome P450 2E1 (CYP2E1)-expressing Huh 7.5 (also known as RLW) cells to an acetaldehyde-generating system (AGS) for 24 h. We then infected (or not) the cells with HIV-1_ADA then exposed them again to AGS for another 48 h. Lysosome damage was assessed by galectin 3/LAMP1 co-localization and cathepsin leakage. Expression of lysosome biogenesis-transcription factor, TFEB, was measured by its protein levels and by in situ immunofluorescence. Exposure of cells to both AGS + HIV caused the greatest amount of lysosome leakage and its impaired lysosomal biogenesis, leading to intrinsic apoptosis. Furthermore, the movement of TFEB from cytosol to the nucleus via microtubules was impaired by AGS exposure. The latter impairment appeared to occur by acetylation of α-tubulin. Moreover, ZKSCAN3, a repressor of lysosome gene activation by TFEB, was amplified by AGS. Both these changes contributed to AGS-elicited disruption of lysosome biogenesis. Our findings indicate that metabolically generated acetaldehyde damages lysosomes and likely prevents their repair and restoration, thereby exacerbating HIV-induced hepatotoxicity.

Novel mechanisms in alcohol neurodevelopmental disorders via BRCA1 depletion and BRCA1-dependent NADPH oxidase regulation

The breast cancer 1 protein (BRCA1) facilitates DNA repair, preventing embryolethality and protecting the fetus from reactive oxygen species (ROS)-induced developmental disorders mediated by oxidatively damaged DNA. Alcohol (ethanol, EtOH) exposure during pregnancy causes fetal alcohol spectrum disorders (FASD), characterized by aberrant behaviour and enhanced ROS formation and proteasomal protein degradation. Herein, ROS-producing NADPH oxidase (NOX) activity was higher in Brca1 +/- vs. +/+ fetal and adult brains, and further enhanced by a single EtOH exposure. EtOH also enhanced catalase and proteasomal activities, while conversely reducing BRCA1 protein levels without affecting Brca1 gene expression. EtOH-initiated adaptive postnatal freezing behaviour was lost in Brca1 +/- progeny. Pretreatment with the free radical spin trap and ROS inhibitor phenylbutylnitrone blocked all EtOH effects, suggesting ROS-dependent mechanisms. This is the first in vivo evidence of NOX regulation by BRCA1, and of EtOH-induced, ROS-mediated depletion of BRCA1, revealing novel mechanisms of BRCA1 protection in FASD.

Ubiquitin pathways regulate the pathogenesis of chronic liver disease

Chronic liver disease (CLD) is considered the leading cause of global mortality. In westernized countries, increased consumption of alcohol and overeating foods with high fat/ high glucose promote progression of CLD such as alcoholic liver disease (ALD) and non-alcoholic liver disease (NAFLD). Accumulating evidence and research suggest that ubiquitin, a 75 amino acid protein, plays crucial role in the pathogenesis of CLD through dynamic post-translational modifications (PTMs) exerting diverse cellular outcomes such as protein degradation through ubiquitin-proteasome system (UPS) and autophagy, and regulation of signal transduction. In this review, we present the function of ubiquitination and latest findings on diverse mechanism of PTMs, UPS and autophagy which significantly contribute to the pathogenesis of alcoholic liver disease (ALD), non-alcoholic fatty liver disease (NAFLD), cirrhosis, and HCC. Despite its high prevalence, morbidity, and mortality, there are only few FDA approved drugs that could be administered to CLD patients. The goal of this review is to present a variety of pathways and therapeutic targets involving ubiquitination in the pathogenesis of CLD. Further, this review summarizes collective views of pharmaceutical inhibition or activation of recent drugs targeting UPS and autophagy system to highlight potential targets and new approaches to treat CLD.

A Novel Dual Drug Approach That Combines Ivermectin and Dihydromyricetin (DHM) to Reduce Alcohol Drinking and Preference in Mice

Alcohol use disorder (AUD) affects over 18 million people in the US. Unfortunately, pharmacotherapies available for AUD have limited clinical success and are under prescribed. Previously, we established that avermectin compounds (ivermectin [IVM] and moxidectin) reduce alcohol (ethanol/EtOH) consumption in mice, but these effects are limited by P-glycoprotein (Pgp/ABCB1) efflux. The current study tested the hypothesis that dihydromyricetin (DHM), a natural product suggested to inhibit Pgp, will enhance IVM potency as measured by changes in EtOH consumption. Using a within-subjects study design and two-bottle choice study, we tested the combination of DHM (10 mg/kg; i.p.) and IVM (0.5–2.5 mg/kg; i.p.) on EtOH intake and preference in male and female C57BL/6J mice. We also conducted molecular modeling studies of DHM with the nucleotide-binding domain of human Pgp that identified key binding residues associated with Pgp inhibition. We found that DHM increased the potency of IVM in reducing EtOH consumption, resulting in significant effects at the 1.0 mg/kg dose. This combination supports our hypothesis that inhibiting Pgp improves the potency of IVM in reducing EtOH consumption. Collectively, we demonstrate the feasibility of this novel combinatorial approach in reducing EtOH consumption and illustrate the utility of DHM in a novel combinatorial approach.

Dihydromyricetin Protects the Liver via Changes in Lipid Metabolism and Enhanced Ethanol Metabolism

Background

Excess alcohol (ethanol, EtOH) consumption is a significant cause of chronic liver disease, accounting for nearly half of the cirrhosis‐associated deaths in the United States. EtOH‐induced liver toxicity is linked to EtOH metabolism and its associated increase in proinflammatory cytokines, oxidative stress, and the subsequent activation of Kupffer cells. Dihydromyricetin (DHM), a bioflavonoid isolated from Hovenia dulcis, can reduce EtOH intoxication and potentially protect against chemical‐induced liver injuries. But there remains a paucity of information regarding the effects of DHM on EtOH metabolism and liver protection. As such, the current study tests the hypothesis that DHM supplementation enhances EtOH metabolism and reduces EtOH‐mediated lipid dysregulation, thus promoting hepatocellular health.

Methods

The hepatoprotective effect of DHM (5 and 10 mg/kg; intraperitoneal injection) was evaluated using male C57BL/6J mice and a forced drinking ad libitum EtOH feeding model and HepG2/VL‐17A hepatoblastoma cell models. EtOH‐mediated lipid accumulation and DHM effects against lipid deposits were determined via H&E stains, triglyceride measurements, and intracellular lipid dyes. Protein expression of phosphorylated/total proteins and serum and hepatic cytokines was determined via Western blot and protein array. Total NAD⁺/NADH Assay of liver homogenates was used to detect NAD + levels.

Results

DHM reduced liver steatosis, liver triglycerides, and liver injury markers in mice chronically fed EtOH. DHM treatment resulted in increased activation of AMPK and downstream targets, carnitine palmitoyltransferase (CPT)‐1a, and acetyl CoA carboxylase (ACC)‐1. DHM induced expression of EtOH‐metabolizing enzymes and reduced EtOH and acetaldehyde concentrations, effects that may be partly explained by changes in NAD⁺. Furthermore, DHM reduced the expression of proinflammatory cytokines and chemokines in sera and cell models.

Conclusion

In total, these findings support the utility of DHM as a dietary supplement to reduce EtOH‐induced liver injury via changes in lipid metabolism, enhancement of EtOH metabolism, and suppressing inflammation responses to promote liver health.

Phosphodiesterase 4 Inhibition as a Therapeutic Target for Alcoholic Liver Disease: From Bedside to Bench

Alcoholic liver disease (ALD) is a major cause of liver-related mortality. There is still no US Food and Drug Administration-approved therapy for ALD, and therefore, identifying therapeutic targets is needed. Our previous work demonstrated that ethanol exposure leads to up-regulation of cAMP-degrading phosphodiesterase 4 (PDE4) expression, which compromises normal cAMP signaling in monocytes/macrophages and hepatocytes. This effect of ethanol on cAMP signaling contributes to dysregulated inflammatory response and altered lipid metabolism. It is unknown whether chronic alcohol consumption in humans alters hepatic PDE4 expression and cAMP signaling and whether inadequate cAMP signaling plays a pathogenic role in alcohol-induced liver injury. Our present work shows that expression of the PDE4 subfamily of enzymes is significantly up-regulated and cAMP levels are markedly decreased in hepatic tissues of patients with severe ALD. We also demonstrate the anti-inflammatory efficacy of roflumilast, a clinically available PDE4 inhibitor, on endotoxin-inducible proinflammatory cytokine production ex vivo in whole blood of patients with alcoholic hepatitis. Moreover, we demonstrate that ethanol-mediated changes in hepatic PDE4 and cAMP levels play a causal role in liver injury in in vivo and in vitro models of ALD. This study employs a drug delivery system that specifically delivers the PDE4 inhibitor rolipram to the liver to avoid central nervous system side effects associated with this drug. Our results show that PDE4 inhibition significantly attenuates ethanol-induced hepatic steatosis and injury through multiple mechanisms, including reduced oxidative and endoplasmic reticulum stress both in vivo and in vitro. Conclusion: Increased PDE4 plays a pathogenic role in the development of ALD; hence, directed interventions aimed at inhibiting PDE4 might be an effective treatment for ALD.

Dihydromyricetin modulates p62 and autophagy crosstalk with the Keap-1/Nrf2 pathway to alleviate ethanol-induced hepatic injury

Increasing evidence has demonstrated that dihydromyricetin (DMY) contains highly effective antioxidative, anti-inflammatory, anti-microbial and anti-diabetic properties. Nevertheless, the underlying hepatoprotective mechanisms of DMY have infrequently been reported thus far. In the present study, C57BL/6 mice were fed with the Lieber-DeCarli diet containing alcohol or isocaloric maltose dextrin as a control diet with or without DMY (75 and 150mg/kg/d bw) for 6 weeks. DMY significantly attenuated hepatic enzyme release, hepatic lipid peroxidation and triglyceride deposition induced by chronic alcohol exposure. In addition, DMY dramatically attenuated the alcohol-triggered elevation of the level of inflammatory cytokines and partially recovered hepatic pathological changes. Notably, DMY remarkably modified aberrant expression of CYP2E1, Keap-1 and HO-1 in the liver and simultaneously ameliorated disordered nuclear localization of NF-κB and Nrf2 to exert its hepatoprotective effects. Further mechanistic exploration suggested that DMY activated Nrf2, possibly mediated through the autophagy pathway. Analysis of the crosstalk among p62, Keap-1 and Nrf2 demonstrated that the p62 upregulation caused by DMY contributes to a positive feedback loop in Nrf2 activation. In summary, DMY likely modulates p62 and autophagy crosstalk with the Keap-1/Nrf2 pathway to alleviate liver steatosis and the inflammatory response in the pathological progression of ALD.

Autophagy in alcohol-induced multiorgan injury: mechanisms and potential therapeutic targets

Autophagy is a genetically programmed, evolutionarily conserved intracellular degradation pathway involved in the trafficking of long-lived proteins and cellular organelles to the lysosome for degradation to maintain cellular homeostasis. Alcohol consumption leads to injury in various tissues and organs including liver, pancreas, heart, brain, and muscle. Emerging evidence suggests that autophagy is involved in alcohol-induced tissue injury. Autophagy serves as a cellular protective mechanism against alcohol-induced tissue injury in most tissues but could be detrimental in heart and muscle. This review summarizes current knowledge about the role of autophagy in alcohol-induced injury in different tissues/organs and its potential molecular mechanisms as well as possible therapeutic targets based on modulation of autophagy.

Utilization of causal reasoning of hepatic gene expression in rats to identify molecular pathways of idiosyncratic drug-induced liver injury

Drug-induced liver injury (DILI) represents a leading cause of acute liver failure. Although DILI can be discovered in preclinical animal toxicology studies and/or early clinical trials, some human DILI reactions, termed idiosyncratic DILI (IDILI), are less predictable, occur in a small number of individuals, and do not follow a clear dose-response relationship. The emergence of IDILI poses a critical health challenge for patients and a financial challenge for the pharmaceutical industry. Understanding the cellular and molecular mechanisms underlying IDILI is key to the development of models that can assess potential IDILI risk. This study used Reverse Causal Reasoning (RCR), a method to assess activation of molecular signaling pathways, on gene expression data from rats treated with IDILI or pharmacologic/chemical comparators (NON-DILI) at the maximum tolerated dose to identify mechanistic pathways underlying IDILI. Detailed molecular networks involved in mitochondrial injury, inflammation, and endoplasmic reticulum (ER) stress were found in response to IDILI drugs but not negative controls (NON-DILI). In vitro assays assessing mitochondrial or ER function confirmed the effect of IDILI compounds on these systems. Together our work suggests that using gene expression data can aid in understanding mechanisms underlying IDILI and can guide in vitro screening for IDILI. Specifically, RCR should be considered for compounds that do not show evidence of DILI in preclinical animal studies positive for mitochondrial dysfunction and ER stress assays, especially when the therapeutic index toward projected human maximum drug plasma concentration is low.

Convergent mechanisms for dysregulation of mitochondrial quality control in metabolic disease: implications for mitochondrial therapeutics

Mitochondrial dysfunction is associated with a broad range of pathologies including diabetes, ethanol toxicity, metabolic syndrome and cardiac failure. It is now becoming clear that maintaining mitochondrial quality through a balance between biogenesis, reserve capacity and mitophagy is critical in determining the response to metabolic or xenobiotic stress. In diseases associated with metabolic stress, such as Type II diabetes and non-alcoholic and alcoholic steatosis, the mitochondria are subjected to multiple 'hits' such as hypoxia and oxidative and nitrative stress, which can overwhelm the mitochondrial quality control pathways. In addition, the underlying mitochondrial genetics that evolved to accommodate high-energy demand, low-calorie supply environments may now be maladapted to modern lifestyles (low-energy demand, high-calorie environments). The pro-oxidant and pro-inflammatory environment of a sedentary western lifestyle has been associated with modified redox cell signalling pathways such as steatosis, hypoxic signalling, inflammation and fibrosis. These data suggest that loss of mitochondrial quality control is intimately associated with the aberrant activation of redox cell signalling pathways under pathological conditions. In the present short review, we discuss evidence from alcoholic liver disease supporting this concept, the insights obtained from experimental models and the application of bioenergetic-based therapeutics in the context of maintaining mitochondrial quality.

Impact of altered methylation in cytokine signaling and proteasome function in alcohol and viral-mediated diseases

Data from several laboratories have shown that ethanol (EtOH) feeding impairs many essential methylation reactions that contribute to alcoholic liver disease (ALD). EtOH is also a comorbid factor in the severity of hepatitis C virus-induced liver injury. The presence of viral proteins further exacerbates the methylation defects to disrupt multiple pathways that promote the pathogenesis of liver disease. This review is a compilation of presentations that linked the methylation reaction defects with proteasome inhibition, decreased antigen presentation, and impaired interferon (IFN) signaling in the hepatocytes and dysregulated TNFα expression in macrophages. Two therapeutic modalities, betaine and S-adenosylmethionine, can correct methylation defects to attenuate many EtOH-induced liver changes, as well as improve IFN signaling pathways, thereby overcoming viral treatment resistance.

Ethanol and hepatitis C virus suppress peptide-MHC class I presentation in hepatocytes by altering proteasome function

BACKGROUND

Previously, we reported that exposure of hepatitis C virus (HCV) core-expressing ethanol (EtOH)-metabolizing cells to EtOH significantly suppresses proteasome activity which exists as 26S (20S and 19S) and as an unassociated 20S particle. The replacement of the constitutive proteasomal subunits with immunoproteasome (IPR) favors antigen processing. Here, we examined the effects of EtOH consumption by HCV core transgenic mice on proteasome activity in hepatocytic lysates and in partially purified 26S proteasome and the impact of these changes on antigen presentation.

METHODS

HCV (-) and HCV (+) core transgenic mice were fed chow diet with or without 20% (v/v) EtOH in water for 4 weeks. Following the feeding regimen, hepatocytes were isolated and examined for chymotrypsin-like proteasome activity, oxidative stress, and the presentation of SIINFEKL-H2Kb complex. Additionally, the constitutive proteasome and IPR were purified for further analysis and identification of proteasome-interacting proteins (PIPs).

RESULTS

EtOH significantly decreased proteasome activity in hepatocytes of HCV (+) mice, and this finding correlated with oxidative stress and dysregulated methylation reactions. In isolated 26S proteasome, EtOH suppressed proteasome activity equally in HCV (+) and HCV (-) mice. EtOH feeding caused proteasome instability and lowered the content of both constitutive and IPR subunits in the 20S proteasome. In addition, the level of other PIPs, PA28 and UCHL5, were also suppressed after EtOH exposure. Furthermore, in EtOH-fed mice and, especially, in HCV (+) mice, the presentation of SIINFEKL-H2Kb complex in hepatocytes was also decreased.

CONCLUSIONS

Proteasomal dysfunction induced by EtOH feeding and exacerbated by the presence of HCV structural proteins led to suppression of SIINFEKL-H2Kb presentation in hepatocytes.

Proteomic analysis reveals that treatment with tocotrienols reverses the effect of H₂O₂ exposure on peroxiredoxin expression in human lymphocytes from young and old individuals

Vitamin E has been suggested to modulate age-associated changes by altering the redox balance resulting in altered gene and/or protein expression. Here we have utilized proteomics to determine whether such regulation in protein expression occurs in human lymphocytes from two different age groups stressed with H₂O₂ and then treated with vitamin E in the form of tocotrienol-rich fraction (TRF). In this study, lymphocytes obtained from young (30-49 years old) and old (>50 years old) volunteers were first challenged with 1 mM H₂O₂. They were then treated by exposure to 50, 100 and 200 μg/ml TRF. Two-dimensional gel electrophoresis followed by MALDI-TOF/TOF (matrix-assisted laser desorption/ionization time-of-flight/time-of-flight) tandem mass spectrometry was then performed on whole-cell protein extracts to identify proteins that have changed in expression. A total of 24 proteins were found to be affected by H₂O₂ and/or TRF treatment. These included proteins that were related to metabolism, antioxidants, structural proteins, protein degradation and signal transduction. Of particular interest was the regulation of a number of proteins involved in stress response--peroxiredoxin-2, peroxiredoxin-3 and peroxiredoxin-6-all of which were shown to be down-regulated with H₂O₂ exposure. The effect was reversed following TRF treatment. The expression of peroxiredoxin-2 and peroxiredoxin-6 was confirmed by quantitative reverse transcriptase polymerase chain reaction. These results suggested that TRF directly influenced the expression dynamics of the peroxiredoxin-2, thus improving the cells ability to resist damage caused by oxidative stress.

Essential Amino Acid Supplementation Decreases Liver Damage Induced by Chronic Ethanol Consumption in Rats

The liver sustains the greatest damage from ethanol (EtOH) abuse. EtOH and its metabolites impair hepatocyte metabolism, causing intracellular accumulation of proteins and lipids and increasing radical oxygen species production. These processes are toxic to the mitochondrial respiratory chain and to mitochondrial DNA. We have recently shown that supplementating the diet of rodents with an essential amino acid-enriched mixture (EAAem) significantly increases mitochondrial mass and number in cardiac and skeletal muscles and improves mitochondrial function in aged animals. Thus, in this study we sought to test whether EAAem supplementation could reduce EtOH-induced liver damage. Groups of adult male Wistar rats were fed a standard diet and water ad libitum (the control group), drinking water with 20% EtOH (the EtOH group), or drinking water with 20% EtOH and EAAem supplementation (1.5 g/kg/day) (the EtOH+EAAem group) for 2 months. The blood EtOH concentration was measured, and markers for fat (Oil-Red-O), mitochondria (Grp75, Cyt-c-ox), endoplasmic reticulum (Grp78), and inflammation (Heme Oxigenase 1, iNOS, and peroxisomes) were analyzed in the liver of animals in the various experimental groups. EAAem supplementation in EtOH-drinking rats ameliorated EtOH-induced changes in liver structure by limiting steatosis, recruiting more mitochondria and peroxisomes mainly to perivenous hepatocytes, stimulating or restoring antioxidant markers, limiting the expression of inflammatory processes, and reducing ER stress. Taken together, these results suggest that EAAem supplementation may represent a promising strategy to prevent and treat EtOH-induced liver damage.

Alcohol and hepatitis C virus--interactions in immune dysfunctions and liver damage

Hepatitis C virus infection affects 170 million people worldwide, and the majority of individuals exposed to HCV develop chronic hepatitis leading to progressive liver damage, cirrhosis, and hepatocellular cancer. The natural history of HCV infection is influenced by genetic and environmental factors of which chronic alcohol use is an independent risk factor for cirrhosis in HCV-infected individuals. Both the hepatitis C virus and alcohol damage the liver and result in immune alterations contributing to both decreased viral clearance and liver injury. This review will capture the major components of the interactions between alcohol and HCV infection to provide better understanding for the molecular basis of the dangerous combination of alcohol use and HCV infection. Common targets of HCV and alcohol involve innate immune recognition and dendritic cells, the critical cell type in antigen presentation and antiviral immunity. In addition, both alcohol and HCV affect intracellular processes critical for hepatocyte and immune cell functions including mitochondrial and proteasomal activation. Finally, both chronic alcohol use and hepatitis C virus infection increase the risk of hepatocellular cancer. The common molecular mechanisms underlying the pathological interactions between alcohol and HCV include the modulation of cytokine production, lipopolysaccharide (LPS)-TLR4 signaling, and reactive oxygen species (ROS) production. LPS-induced chronic inflammation is not only a major cause of progressive liver injury and fibrosis, but it can also contribute to modification of the tissue environment and stem cells to promote hepatocellular cancer development. Alteration of these processes by alcohol and HCV produces an environment of impaired antiviral immune response, greater hepatocellular injury, and activation of cell proliferation and dedifferentiation.

Effects of ethanol on the proteasome interacting proteins

Proteasome dysfunction has been repeatedly reported in alcoholic liver disease. Ethanol metabolism end-products affect the structure of the proteasome, and, therefore, change the proteasome interaction with its regulatory complexes 19S and PA28, as well as its interacting proteins. Chronic ethanol feeding alters the ubiquitin-proteasome activity by altering the interaction between the 19S and the 20S proteasome interaction. The degradation of oxidized and damaged proteins is thus decreased and leads to accumulation of insoluble protein aggregates, such as Mallory-Denk bodies. Ethanol also affects the immunoproteasome formation. PA28a/b interactions with the 20S proteasome are decreased in the proteasome fraction isolated from the liver of rats fed ethanol chronically, thus affecting the cellular antigen presentation and defense against pathogenic agents. Recently, it has been shown that ethanol also affects the proteasome interacting proteins (PIPs). Interaction of the proteasome with Ecm29 and with deubiquitinating enzymes Rpn11, UCH37, and Usp14 has been found to decrease. However, the two UBL-ubiquitin-associated domain (UBA) PIPs p62 and valosin-containing protein are upregulated when the proteasome is inhibited. The increase of these UBL-UBA proteins, as well as the increase in Hsp70 and Hsp25 levels, compensated for the proteasome failure and helped in the unfolding/docking of misfolded proteins. Chronic alcohol feeding to rats causes a significant inhibition of the proteasome pathway and this inhibition results from a decreases of the interaction between the 20S proteasome and the regulatory complexes, PIPs, and the ubiquitin system components.

Alcohol-induced alterations of the hepatocyte cytoskeleton

The hepatocyte cytoskeleton consists of three filamentous networks: microtubules, actin microfilaments and keratin intermediate filaments. Because of the abundance of the proteins that comprise each system and the central role each network plays in a variety of cellular processes, the three filament systems have been the focus of a host of studies aimed at understanding the progression of alcohol-induced liver injury. In this review, we will briefly discuss the hepatic organization of each cytoskeletal network and highlight some components of each system. We will also describe what is known about ethanol-induced changes in the dynamics and distributions of each cytoskeletal system and discuss what is known about changes in protein expression levels and post-translational modifications. Finally, we will describe the possible consequences of these cytoskeletal alterations on hepatocyte function and how they might contribute to the progression of liver disease.

Chronic ethanol consumption results in atypical liver injury in copper/zinc superoxide dismutase deficient mice

BACKGROUND

Ethanol metabolism increases production of reactive oxygen species, including superoxide (O2(.-)) in the liver, resulting in significant oxidative stress, which causes cellular damage. Superoxide dismutase (SOD) is an antioxidant enzyme that converts superoxide to less toxic intermediates, preventing accumulation. Because the absence of SOD would confer less resistance to oxidative stress, we determined whether damage to hepatic proteolytic systems was greater in SOD(-/-) than in SOD(+/+) mice after chronic ethanol feeding.

METHODS

Female wild-type (SOD(+/+)) and Cu/Zn-SOD knockout (SOD(-/-)) mice were pair-fed ethanol and control liquid diets for 24 days, after which liver injury was assessed.

RESULTS

Ethanol-fed SOD(-/-) mice had 4-fold higher blood ethanol, 2.8-fold higher alanine aminotransferase levels, 20% higher liver weight, a 1.4-fold rise in hepatic protein levels, and 35 to 70% higher levels of lipid peroxides than corresponding wild-type mice. While wild-type mice exhibited fatty liver after ethanol administration, SOD(-/-) mice showed no evidence of ethanol-induced steatosis, although triglyceride levels were elevated in both groups of knockout mice. Ethanol administration caused no significant change in proteasome activity, but caused lysosomal leakage in livers of SOD(-/-) mice but not in wild-type mice. Alcohol dehydrogenase activity was reduced by 50 to 60% in ethanol-fed SOD(-/-) mice compared with all other groups. Additionally, while ethanol administration induced cytochrome P450 2E1 (CYP2E1) activity in wild-type mice, it caused no such induction in SOD(-/-) mice. Unexpectedly, ethanol feeding significantly elevated total and mitochondrial levels of glutathione in SOD knockout mice compared with wild-type mice.

CONCLUSION

Ethanol-fed SOD(-/-) mice exhibited lower alcohol dehydrogenase activity and lack of CYP2E1 inducibility, thereby causing decreased ethanol metabolism compared with wild-type mice. These and other atypical responses to ethanol, including the absence of ethanol-induced steatosis and enhanced glutathione levels, appear to be linked to enhanced oxidative stress due to lack of antioxidant enzyme capacity.

Impaired methylation as a novel mechanism for proteasome suppression in liver cells

The proteasome is a multi-catalytic protein degradation enzyme that is regulated by ethanol-induced oxidative stress; such suppression is attributed to CYP2E1-generated metabolites. However, under certain conditions, it appears that in addition to oxidative stress, other mechanisms are also involved in proteasome regulation. This study investigated whether impaired protein methylation that occurs during exposure of liver cells to ethanol, may contribute to suppression of proteasome activity. We measured the chymotrypsin-like proteasome activity in Huh7CYP cells, hepatocytes, liver cytosols and nuclear extracts or purified 20S proteasome under conditions that maintain or prevent protein methylation. Reduction of proteasome activity of hepatoma cell and hepatocytes by ethanol or tubercidin was prevented by simultaneous treatment with S-adenosylmethionine (SAM). Moreover, the tubercidin-induced decline in proteasome activity occurred in both nuclear and cytosolic fractions. In vitro exposure of cell cytosolic fractions or highly purified 20S proteasome to low SAM:S-adenosylhomocysteine (SAH) ratios in the buffer also suppressed proteasome function, indicating that one or more methyltransferase(s) may be associated with proteasomal subunits. Immunoblotting a purified 20S rabbit red cell proteasome preparation using methyl lysine-specific antibodies revealed a 25kDa proteasome subunit that showed positive reactivity with anti-methyl lysine. This reactivity was modified when 20S proteasome was exposed to differential SAM:SAH ratios. We conclude that impaired methylation of proteasome subunits suppressed proteasome activity in liver cells indicating an additional, yet novel mechanism of proteasome activity regulation by ethanol.

Focus on the liver: alcohol use, highly active antiretroviral therapy, and liver disease in HIV-infected patients

Since the introduction of highly active antiretroviral therapy (HAART) in the 1990 s, liver disease is emerging as a major cause of morbidity and mortality among HIV-infected patients. This is attributed to a variety of factors, including HAART hepatotoxicity, coinfection with hepatitis B and C virus (HBV and HCV, respectively), and alcohol abuse. Several studies have examined the effects of HAART and HCV/HBV coinfection on liver toxicity. However, the impact of alcohol consumption as a cofactor for hepatotoxicity in HIV patients is only beginning to be understood. Similar to the general population, alcohol use is common in the HIV population but is often overlooked by health care providers. Approximately 25 percent of recently diagnosed HIV patients are alcohol dependent; moreover, alcohol dependence has been associated with HIV treatment failure. Alcohol/HAART interactions appear crucial for the development of liver disease in HIV patients. Recent research has shown that alcohol abuse is associated with severe hepatotoxicity in patients on HAART. Importantly, alcoholic- and HAART-induced liver disease share many potential mechanisms of injury, including altered metabolism of certain signaling molecules (i.e., cytokines) and dysfunction of some cell components (i.e., proteasomes and mitochondria).

Chronic ethanol feeding affects proteasome-interacting proteins

Studies on alcoholic liver injury mechanisms show a significant inhibition of the proteasome activity. To investigate this phenomenon, we isolated proteasome complexes from the liver of rats fed ethanol chronically, and from the liver of their pair-fed controls, using a non-denaturing multiple centrifugations procedure to preserve proteasome-interacting proteins (PIPs). ICAT and MS/MS spectral counting, further confirmed by Western blot, showed that the levels of several PIPs were significantly decreased in the isolated ethanol proteasome fractions. This was the case of PA28alpha/beta proteasome activator subunits, and of three proteasome-associated deubiquitinases, Rpn11, ubiquitin C-terminal hydrolase 14, and ubiquitin carboxyl-terminal hydrolase L5. Interestingly, Rpn13 C-terminal end was missing in the ethanol proteasome fraction, which probably altered the linking of ubiquitin carboxyl-terminal hydrolase L5 to the proteasome. 20S proteasome and most 19S subunits were however not changed but Ecm29, a protein known to stabilize the interactions between the 20S and its activators, was decreased in the isolated ethanol proteasome fractions. It is proposed that ethanol metabolism causes proteasome inhibition by several mechanisms, including by altering PIPs and proteasome regulatory complexes binding to the proteasome.

Role of oxidative stress in alcohol-induced liver injury

Reactive oxygen species (ROS) are highly reactive molecules that are naturally generated in small amounts during the body's metabolic reactions and can react with and damage complex cellular molecules such as lipids, proteins, or DNA. Acute and chronic ethanol treatments increase the production of ROS, lower cellular antioxidant levels, and enhance oxidative stress in many tissues, especially the liver. Ethanol-induced oxidative stress plays a major role in the mechanisms by which ethanol produces liver injury. Many pathways play a key role in how ethanol induces oxidative stress. This review summarizes some of the leading pathways and discusses the evidence for their contribution to alcohol-induced liver injury. Special emphasis is placed on CYP2E1, which is induced by alcohol and is reactive in metabolizing and activating many hepatotoxins, including ethanol, to reactive products, and in generating ROS.

Nuclear effects of ethanol-induced proteasome inhibition in liver cells

Alcohol ingestion causes alteration in several cellular mechanisms, and leads to inflammation, apoptosis, immunological response defects, and fibrosis. These phenomena are associated with significant changes in the epigenetic mechanisms, and subsequently, to liver cell memory. The ubiquitin-proteasome pathway is one of the vital pathways in the cell that becomes dysfunctionial as a result of chronic ethanol consumption. Inhibition of the proteasome activity in the nucleus causes changes in the turnover of transcriptional factors, histone modifying enzymes, and therefore, affects epigenetic mechanisms. Alcohol consumption has been associated with an increase in histone acetylation and a decrease in histone methylation, which leads to gene expression changes. DNA and histone modifications that result from ethanol-induced proteasome inhibition are key players in regulating gene expression, especially genes involved in the cell cycle, immunological responses, and metabolism of ethanol. The present review highlights the consequences of ethanol-induced proteasome inhibition in the nucleus of liver cells that are chronically exposed to ethanol.

Alcohol and the liver

PURPOSE OF REVIEW

To update the reader with advances in epidemiology, genetics, detection, pathogenesis and therapy of alcohol-related liver disease.

RECENT FINDINGS

Ill-health due to alcohol abuse is improving in some nations but deteriorating in others. Oxidative and nitrosative stress are key to the pathogenesis of alcoholic liver disease, and there is now greater emphasis than previously on their development and role of cytochrome P450 2E1, on mitochondrial stress and disruption, (including elucidation of mitochondrial protection mechanisms) disturbance of signaling pathways and involvement of extrahepatic mediators like adiponectin. Treatment of alcoholic liver disease has stagnated, but transplantation is still favored and debated for end-stage cirrhosis.

SUMMARY

Basic and clinical research into the mechanisms of alcoholic liver disease is making headway, but has yet to produce safe and effective therapies for alcoholic hepatitis and for reversing cirrhosis.

Acute alcohol intoxication increases atrogin-1 and MuRF1 mRNA without increasing proteolysis in skeletal muscle

Acute alcohol intoxication decreases muscle protein synthesis, but there is a paucity of data on the ability of alcohol to regulate muscle protein degradation. Furthermore, various types of atrophic stimuli appear to regulate ubiquitin-proteasome-dependent proteolysis by increasing the muscle-specific E3 ligases atrogin-1 and MuRF1 (i.e., "atrogenes"). Therefore, the present study was designed to test the hypothesis that acute alcohol intoxication increases atrogene expression leading to an elevated rate of muscle protein breakdown. In male rats, the intraperitoneal injection of alcohol dose- and time-dependently increased atrogin-1 and MuRF1 mRNA in gastrocnemius, the latter of which was most pronounced. A comparable change was absent in the soleus and heart. The ability of in vivo-administered ethanol to increase atrogene expression was independent of the route of alcohol administration (intraperitoneal vs. oral), as well as of nutritional status (fed vs. fasted) and gender (male vs. female). The increase in atrogin-1 and MuRF1 was independent of alcohol metabolism, and the overproduction of endogenous glucocorticoids and could not be prevented by maintaining the circulating concentration of insulin-like growth factor-I. Despite marked changes in atrogene expression, acute alcohol in vivo did not alter the release of either 3-methylhistidine (MH) or tyrosine from the isolated perfused hindlimb, suggesting that the rate of muscle proteolysis remains unchanged. Moreover, alcohol did not increase the directly determined rate of protein degradation in isolated epitrochlearis muscles or cultured myocytes. Finally, no increase in atrogene expression or 3-MH release was detected in muscle from rats fed an alcohol-containing diet. Our results indicate that although acute alcohol intoxication increases atrogin-1 and MuRF1 mRNA preferentially in fast-twitch skeletal muscle, this change was not associated with increased rates of muscle proteolysis. Therefore, the loss of muscle mass/protein in response to chronic alcohol abuse appears to result primarily from a decrement in muscle protein synthesis, not an increase in degradation.