Ethanol administration alters the proteolytic activity of hepatic lysosomes

Lipid droplet membrane proteome remodeling parallels ethanol-induced hepatic steatosis and its resolution

Lipid droplets (LDs) are composed of neutral lipids enclosed in a phospholipid monolayer, which harbors membrane-associated proteins that regulate LD functions. Despite the crucial role of LDs in lipid metabolism, remodeling of LD protein composition in disease contexts, such as steatosis, remains poorly understood. We hypothesized that chronic ethanol consumption, subsequent abstinence from ethanol, or fasting differentially affects the LD membrane proteome content and that these changes influence how LDs interact with other intracellular organelles. Here, male Wistar rats were pair-fed liquid control or ethanol diets for 6 weeks, and then, randomly chosen animals from both groups were either refed a control diet for 7 days or fasted for 48 h before euthanizing. From all groups, LD membrane proteins from purified liver LDs were analyzed immunochemically and by MS proteomics. Liver LD numbers and sizes were greater in ethanol-fed rats than in pair-fed control, 7-day refed, or fasted rats. Compared with control rats, ethanol feeding markedly altered the LD membrane proteome, enriching LD structural perilipins and proteins involved in lipid biosynthesis, while lowering LD lipase levels. Ethanol feeding also lowered LD-associated mitochondrial and lysosomal proteins. In 7-day refed (i.e., ethanol-abstained) or fasted-ethanol-fed rats, we detected distinct remodeling of the LD proteome, as judged by lower levels of lipid biosynthetic proteins, and enhanced LD interaction with mitochondria and lysosomes. Our study reveals evidence of significant remodeling of the LD membrane proteome that regulates ethanol-induced steatosis, its resolution after withdrawal and abstinence, and changes in LD interactions with other intracellular organelles.

Autophagy and Liver Diseases

Autophagy plays an important role in the physiology and pathology of the liver. It is involved in the development of many liver diseases such as α-1-antitrypsin deficiency, chronic hepatitis virus infection, alcoholic liver disease, nonalcoholic fatty liver disease, and liver cancer. Autophagy has thus become a new target for the treatment of liver diseases. How to treat liver diseases by regulating autophagy has been a hot topic.

Emerging Roles of Lipophagy in Health and Disease

The term lipophagy is used to describe the autophagic degradation of lipid droplets, the main lipid storage organelles of eukaryotic cells. Ever since its discovery in 2009, lipophagy has emerged as a significant component of lipid metabolism with important implications for organismal health. This review aims to provide a brief summary of our current knowledge on the mechanisms that are responsible for regulating lipophagy and the impact the process has under physiological and pathological conditions.

Lipophagy and Alcohol-Induced Fatty Liver

This review describes the influence of ethanol consumption on hepatic lipophagy, a selective form of autophagy during which fat-storing organelles known as lipid droplets (LDs) are degraded in lysosomes. During classical autophagy, also known as macroautophagy, all forms of macromolecules and organelles are sequestered in autophagosomes, which, with their cargo, fuse with lysosomes, forming autolysosomes in which the cargo is degraded. It is well established that excessive drinking accelerates intrahepatic lipid biosynthesis, enhances uptake of fatty acids by the liver from the plasma and impairs hepatic secretion of lipoproteins. All the latter contribute to alcohol-induced fatty liver (steatosis). Here, our principal focus is on lipid catabolism, specifically the impact of excessive ethanol consumption on lipophagy, which significantly influences the pathogenesis alcohol-induced steatosis. We review findings, which demonstrate that chronic ethanol consumption retards lipophagy, thereby exacerbating steatosis. This is important for two reasons: (1) Unlike adipose tissue, the liver is considered a fat-burning, not a fat-storing organ. Thus, under normal conditions, lipophagy in hepatocytes actively prevents lipid droplet accumulation, thereby maintaining lipostasis; (2) Chronic alcohol consumption subverts this fat-burning function by slowing lipophagy while accelerating lipogenesis, both contributing to fatty liver. Steatosis was formerly regarded as a benign consequence of heavy drinking. It is now recognized as the "first hit" in the spectrum of alcohol-induced pathologies that, with continued drinking, progresses to more advanced liver disease, liver failure, and/or liver cancer. Complete lipid droplet breakdown requires that LDs be digested to release their high-energy cargo, consisting principally of cholesteryl esters and triacylglycerols (triglycerides). These subsequently undergo lipolysis, yielding free fatty acids that are oxidized in mitochondria to generate energy. Our review will describe recent findings on the role of lipophagy in LD catabolism, how continuous heavy alcohol consumption affects this process, and the putative mechanism(s) by which this occurs.

Ethanol withdrawal mitigates fatty liver by normalizing lipid catabolism

We are investigating the changes in hepatic lipid catabolism that contribute to alcohol-induced fatty liver. Following chronic ethanol (EtOH) exposure, abstinence from alcohol resolves steatosis. Here, we investigated the hepatocellular events that lead to this resolution by quantifying specific catabolic parameters that returned to control levels after EtOH was withdrawn. We hypothesized that, after its chronic consumption, EtOH withdrawal reactivates lipid catabolic processes that restore lipostasis. Male Wistar rats were fed control and EtOH liquid diets for 6 wk. Randomly chosen EtOH-fed rats were then fed control diet for 7 days. Liver triglycerides (TG), lipid peroxides, key markers of fatty acid (FA) metabolism, lipophagy, and autophagy were quantified. Compared with controls, EtOH-fed rats had higher hepatic triglycerides, lipid peroxides, and serum free fatty acids (FFA). The latter findings were associated with higher levels of FA transporters (FATP 2, 4, and 5) but lower quantities of peroxisome proliferator-activated receptor-α (PPAR-α), which governs FA oxidation. EtOH-fed animals also had lower nuclear levels of the autophagy-regulating transcription factor EB (TFEB), associated with lower hepatic lipophagy and autophagy. After EtOH-fed rats were refed control diet for 7 days, their serum FFA levels and those of FATPs fell to control (normal) levels, whereas PPAR-α levels rose to normal. Hepatic TG and malondialdehyde levels in EtOH-withdrawn rats declined to near control levels. EtOH withdrawal restored nuclear TFEB content, hepatic lipophagy, and autophagy activity to control levels. EtOH withdrawal reversed aberrant FA metabolism and restored lysosomal function to promote resolution of alcohol-induced fatty liver. NEW & NOTEWORTHY Here, using an animal model, we show mechanisms of reversal of fatty liver and injury following EtOH withdrawal. Our data indicate that reactivation of autophagy and lysosome function through the restoration of transcription factor EB contribute to reversal of fatty liver and injury following EtOH withdrawal.

Hepatitis C Virus-Infected Apoptotic Hepatocytes Program Macrophages and Hepatic Stellate Cells for Liver Inflammation and Fibrosis Development: Role of Ethanol as a Second Hit

Hepatocyte apoptosis is a crucially important mechanism for liver disease pathogenesis, and the engulfment of apoptotic bodies (AB) by non-parenchymal cells serves as a leading mechanism of inflammation and fibrosis progression. Previously, we have shown that hepatitis C virus (HCV) and alcohol metabolites induce massive apoptosis in hepatocytes and the spread of HCV-infection to the neighboring uninfected cells. Here, we hypothesize that the capturing of AB by non-parenchymal cells, macrophages and hepatic stellate cells (HSC) changes their phenotype to promote inflammation and fibrosis. In this regard, we generated AB from Huh7.5^CYP2E1 (RLW) cells also treated with an acetaldehyde-generating system (AGS) and incubated them with human monocyte-derived macrophages (MDMs) and HSC (LX2 cells). Activation of inflammasomes and pro-fibrotic markers has been tested by RT-PCR and linked to HCV expression and AGS-induced lipid peroxidation in RLW cells. After exposure to AB we observed activation of inflammasomes in MDMs, with a higher effect of AB HCV+, further enhanced by incubation of MDMs with ethanol. In HSC, activation of inflammasomes was modest; however, HCV and AGS exposure induced pro-fibrotic changes. We conclude that HCV as well as lipid peroxidation-adducted proteins packaged in AB may serve as a vehicle for delivery of parenchymal cell cargo to non-parenchymal cells to activate inflammasomes and pro-fibrotic genes and promote liver inflammation and fibrosis.

Ethanol exposure inhibits hepatocyte lipophagy by inactivating the small guanosine triphosphatase Rab7

Alcohol consumption is a well-established risk factor for the onset and progression of fatty liver disease. An estimated 90% of heavy drinkers are thought to develop significant liver steatosis. For these reasons, an increased understanding of the molecular basis for alcohol-induced hepatic steatosis is important. It has become clear that autophagy, a catabolic process of intracellular degradation and recycling, plays a key role in hepatic lipid metabolism. We have shown that Rab7, a small guanosine triphosphatase known to regulate membrane trafficking, acts as a key orchestrator of hepatocellular lipophagy, a selective form of autophagy in which lipid droplets (LDs) are specifically targeted for turnover by the autophagic machinery. Nutrient starvation results in Rab7 activation on the surface of the LD and lysosomal compartments, resulting in the mobilization of triglycerides stored within the LDs for energy production. Here, we examine whether the steatotic effects of alcohol exposure are a result of perturbations to the Rab7-mediated lipophagic pathway. Rats chronically fed an ethanol-containing diet accumulated significantly higher levels of fat in their hepatocytes. Interestingly, hepatocytes isolated from these ethanol-fed rats contained juxtanuclear lysosomes that exhibited impaired motility. These changes are similar to those we observed in Rab7-depleted hepatocytes. Consistent with these defects in the lysosomal compartment, we observed a marked 80% reduction in Rab7 activity in cultured hepatocytes as well as a complete block in starvation-induced Rab7 activation in primary hepatocytes isolated from chronic ethanol-fed animals. Conclusion: A mechanism is supported whereby ethanol exposure inhibits Rab7 activity, resulting in the impaired transport, targeting, and fusion of the autophagic machinery with LDs, leading to an accumulation of hepatocellular lipids and hepatic steatosis. (Hepatology Communications 2017;1:140-152).

Alcohol, microbiome, life style influence alcohol and non-alcoholic organ damage

This paper is based upon the "8th Charles Lieber's Satellite Symposium" organized by Manuela G. Neuman at the Research Society on Alcoholism Annual Meeting, on June 25, 2016 at New Orleans, Louisiana, USA. The integrative symposium investigated different aspects of alcohol-induced liver disease (ALD) as well as non-alcohol-induced liver disease (NAFLD) and possible repair. We revealed the basic aspects of alcohol metabolism that may be responsible for the development of liver disease as well as the factors that determine the amount, frequency and which type of alcohol misuse leads to liver and gastrointestinal diseases. We aimed to (1) describe the immuno-pathology of ALD, (2) examine the role of genetics in the development of alcoholic hepatitis (ASH) and NAFLD, (3) propose diagnostic markers of ASH and non-alcoholic steatohepatitis (NASH), (4) examine age and ethnic differences as well as analyze the validity of some models, (5) develop common research tools and biomarkers to study alcohol-induced effects, 6) examine the role of alcohol in oral health and colon and gastrointestinal cancer and (7) focus on factors that aggravate the severity of organ-damage. The present review includes pre-clinical, translational and clinical research that characterizes ALD and NAFLD. Strong clinical and experimental evidence lead to recognition of the key toxic role of alcohol in the pathogenesis of ALD with simple fatty infiltrations and chronic alcoholic hepatitis with hepatic fibrosis or cirrhosis. These latter stages may also be associated with a number of cellular and histological changes, including the presence of Mallory's hyaline, megamitochondria, or perivenular and perisinusoidal fibrosis. Genetic polymorphisms of ethanol metabolizing enzymes and cytochrome p450 (CYP) 2E1 activation may change the severity of ASH and NASH. Other risk factors such as its co-morbidities with chronic viral hepatitis in the presence or absence of human deficiency virus were discussed. Dysregulation of metabolism, as a result of ethanol exposure, in the intestine leads to colon carcinogenesis. The hepatotoxic effects of ethanol undermine the contribution of malnutrition to the liver injury. Dietary interventions such as micro and macronutrients, as well as changes to the microbiota have been suggested. The clinical aspects of NASH, as part of the metabolic syndrome in the aging population, have been presented. The symposium addressed mechanisms and biomarkers of alcohol induced damage to different organs, as well as the role of the microbiome in this dialog. The microbiota regulates and acts as a key element in harmonizing immune responses at intestinal mucosal surfaces. It is known that microbiota is an inducer of proinflammatory T helper 17 cells and regulatory T cells in the intestine. The signals at the sites of inflammation mediate recruitment and differentiation in order to remove inflammatory inducers and promote tissue homeostasis restoration. The change in the intestinal microbiota also influences the change in obesity and regresses the liver steatosis. Evidence on the positive role of moderate alcohol consumption on heart and metabolic diseases as well on reducing steatosis have been looked up. Moreover nutrition as a therapeutic intervention in alcoholic liver disease has been discussed. In addition to the original data, we searched the literature (2008-2016) for the latest publication on the described subjects. In order to obtain the updated data we used the usual engines (Pub Med and Google Scholar). The intention of the eighth symposia was to advance the international profile of the biological research on alcoholism. We also wish to further our mission of leading the forum to progress the science and practice of translational research in alcoholism.

Benefits of Alcohol on Arsenic Toxicity in Rats

INTRODUCTION

It has been demonstrated earlier that exposure to ethanol and/or arsenic compounds (such as sodium arsenite) produces toxic effects as shown by both in vitro and in vivo experiments. Chronic exposure of humans to arsenic through drinking water, pesticides or consumption of alcoholic beverages has produced major health problem and concern in recent years. Water being one of the main ingredients for alcohol formation (beer fermentation process) can lead to contamination with arsenic. Thus, people consuming such alcohol are getting continuously exposed to arsenic compounds as well along with alcohol.

AIM

The present study was undertaken to investigate the effect of alcohol co-administration on arsenic induced changes in carbohydrate metabolic status in adult male albino rats.

MATERIALS AND METHODS

Adult male albino rats of Wistar strain (weighing~100g) were divided into three groups (n=8 rats/group) including Control or vehicle treated (C), Arsenic treated (As) and Arsenic treated alcohol co-exposed (As+Alc). Treatment with Sodium-arsenite included intra-peritoneal injection consecutively for 14 days at a dose of 5.55 mg/kg (equivalent to 35% of LD50) per day. Absolute alcohol (15% v/v) was fed at a dose of 0.5 ml/100 g body weight per day for five consecutive days from start of the treatment schedule. Distilled water (D/W) was used as vehicle. Blood Glucose (BG) level, levels of glycogen, Pyruvic Acid (PA), Free Amino Acid Nitrogen (FAAN), total protein, Glutamate Oxalate transaminase (GOT) and Glutamate Pyruvate Transaminase (GPT) activity, and glucose-6-phosphatase (G6Pase) activity were measured in tissues including liver, kidney and muscle.

RESULTS

Treatment with arsenic decreased the levels of BG, liver glycogen and PA, tissue protein and G6Pase activity, GOT activity in liver and muscle, and increased free amino acid content in kidney and muscle, GPT activity in liver and kidney. Alcohol administration to rats co-exposed to arsenic treatment reversed these changes.

CONCLUSION

Thus, it is suggested that combined administration of alcohol with arsenic can result in the suppression of the down-regulating action of arsenic on glucose homeostasis as evidenced by its hypoglycaemic effect and increased gluconeogenesis and transamination in liver.

Acute and Chronic Ethanol Administration Differentially Modulate Hepatic Autophagy and Transcription Factor EB

BACKGROUND

Chronic ethanol (EtOH) consumption decelerates the catabolism of long-lived proteins, indicating that it slows hepatic macroautophagy (hereafter called autophagy) a crucial lysosomal catabolic pathway in most eukaryotic cells. Autophagy and lysosome biogenesis are linked. Both are regulated by the transcription factor EB (TFEB). Here, we tested whether TFEB can be used as a singular indicator of autophagic activity, by quantifying its nuclear content in livers of mice subjected to acute and chronic EtOH administration. We correlated nuclear TFEB to specific indices of autophagy.

METHODS

In acute experiments, we gavaged GFP-LC3(tg) mice with a single dose of EtOH or with phosphate buffered saline (PBS). We fed mice chronically by feeding them control or EtOH liquid diets.

RESULTS

Compared with PBS-gavaged controls, livers of EtOH-gavaged mice exhibited greater autophagosome (AV) numbers, a higher incidence of AV-lysosome co-localization, and elevated levels of free GFP, all indicating enhanced autophagy, which correlated with a higher nuclear content of TFEB. Compared with pair-fed controls, livers of EtOH-fed mice exhibited higher AV numbers, but had lower lysosome numbers, lower AV-lysosome co-localization, higher P62/SQSTM1 levels, and lower free GFP levels. The latter findings correlated with lower nuclear TFEB levels in EtOH-fed mice. Thus, enhanced autophagy after acute EtOH gavage correlated with a higher nuclear TFEB content. Conversely, chronic EtOH feeding inhibited hepatic autophagy, associated with a lower nuclear TFEB content.

CONCLUSIONS

Our findings suggest that the effect of acute EtOH gavage on hepatic autophagy differs significantly from that after chronic EtOH feeding. Each regimen distinctly affects TFEB localization, which in turn, regulates hepatic autophagy and lysosome biogenesis.

Hepatocyte-specific Hmgb1 Deletion

High mobility group box 1 (HMGB1) plays a key role in human health and disease. Currently, three different labs in the USA use Cre-lox technology to create mice with a hepatocyte-specific Hmgb1 deletion (HMGB1-HC-KO mice) by backcrossing Hmgb1 (flox)/(flox) mice to Alb-Cre mice. This mouse strain has a different phenotype following exposure to several stressors.

Ethanol-induced oxidant stress modulates hepatic autophagy and proteasome activity

In this review, we describe research findings on the effects of alcohol exposure on two major catabolic systems in liver cells: the ubiquitin-proteasome system (UPS) and autophagy. These hydrolytic systems are not unique to liver cells; they exist in all eukaryotic tissues and cells. However, because the liver is the principal site of ethanol metabolism, it sustains the greatest damage from heavy drinking. Thus, the focus of this review is to specifically describe how ethanol oxidation modulates the activities of the UPS and autophagy and the mechanisms by which these changes contribute to the pathogenesis of alcohol-induced liver injury. Here, we describe the history and the importance of cellular hydrolytic systems, followed by a description of each catabolic pathway and the differential modulation of each by ethanol exposure. Overall, the evidence for an involvement of these catabolic systems in the pathogenesis of alcoholic liver disease is quite strong. It underscores their importance, not only as effective means of cellular recycling and eventual energy generation, but also as essential components of cellular defense.

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.

Autophagy modulation as a potential therapeutic target for liver diseases

Autophagy is a critical intracellular pathway which maintains cellular function by lysosomal degradation of damaged proteins and organelles besides elimination of invading pathogens. Its primary function is to prevent cell death. Autophagy has diverse physiological functions namely; starvation adaptation, prevention of tumorigenesis, energy homeostasis, intracellular quality control and degradation of abnormal intracellular protein aggregates. Understanding the molecular mechanisms of autophagy has given key insights into the pathogenesis of various diseases like Non Alcoholic Steato-Hepatitis, Hepatitis B and C infections, Alpha-1 antitrypsin deficiency and hepatocellular carcinoma. Pharmacological modulation of autophagy may have a therapeutic potential in management of these liver diseases.

Multilevel regulation of autophagosome content by ethanol oxidation in HepG2 cells

Acute and chronic ethanol administration increase autophagic vacuole (i.e., autophagosome; AV) content in liver cells. This enhancement depends on ethanol oxidation. Here, we used parental (nonmetabolizing) and recombinant (ethanol-metabolizing) Hep G2 cells to identify the ethanol metabolite that causes AV enhancement by quantifying AVs or their marker protein, microtubule-associated protein 1 light chain 3-II (LC3-II). The ethanol-elicited rise in LC3-II was dependent on ethanol dose, was seen only in cells that expressed alcohol dehydrogenase (ADH) and was augmented in cells that coexpressed cytochrome CYP2E1 (P450 2E1). Furthermore, the rise in LC3-II was inversely related to a decline in proteasome activity. AV flux measurements and colocalization of AVs with lysosomes or their marker protein Lysosomal-Associated Membrane Protein 1 (LAMP1) in ethanol-metabolizing VL-17A cells (ADH (+) /CYP2E1 (+) ) revealed that ethanol exposure not only enhanced LC3-II synthesis but also decreased its degradation. Ethanol-induced accumulation of LC3-II in these cells was similar to that induced by the microtubule inhibitor, nocodazole. After we treated cells with either 4-methylpyrazole to block ethanol oxidation or GSH-EE to scavenge reactive species, there was no enhancement of LC3-II by ethanol. Furthermore, regardless of their ethanol-metabolizing capacity, direct exposure of cells to acetaldehyde enhanced LC3-II content. We conclude that both ADH-generated acetaldehyde and CYP2E1-generated primary and secondary oxidants caused LC3-II accumulation, which rose not only from enhanced AV biogenesis, but also from decreased LC3 degradation by the proteasome and by lysosomes.

CYP2E1-catalyzed alcohol metabolism: role of oxidant generation in interferon signaling, antigen presentation and autophagy

Cytochrome P450 2E1 (CYP2E1) is one of two major enzymes that catalyze ethanol oxidation in the liver. CYP2E1 is also unique because it is inducible, as its hepatic content rises after continuous (chronic) ethanol administration, thereby accelerating the rate of ethanol metabolism and affording greater tolerance to heavy alcohol consumption. However, the broad substrate specificity of CYP2E1 and its capacity to generate free radicals from alcohol and other hepatotoxins, places CYP2E1 as a central focus of not only liver toxicity, but also as an enzyme that regulates cytokine signaling, antigen presentation, and macromolecular degradation, all of which are crucial to liver cell function and viability. Here, we describe our own and other published work relevant to the importance of CYP2E1-catalyzed ethanol oxidation and how this catalysis affects the aforementioned cellular processes to produce liver injury.

Autophagy as a stress-response and quality-control mechanism: implications for cell injury and human disease

Autophagy, a vital catabolic process that degrades cytoplasmic components within the lysosome, is an essential cytoprotective response to pathologic stresses that occur during diseases such as cancer, ischemia, and infection. In addition to its role as a stress-response pathway, autophagy plays an essential quality-control function in the cell by promoting basal turnover of long-lived proteins and organelles, as well as by selectively degrading damaged cellular components. This homeostatic function protects against a wide variety of diseases, including neurodegeneration, myopathy, liver disease, and diabetes. This review discusses our current understanding of these two principal functions of autophagy and describes in detail how alterations in autophagy promote human disease.

Autophagy in alcohol-induced liver diseases

Alcohol is the most abused substance worldwide and a significant source of liver injury; the mechanisms of alcohol-induced liver disease are not fully understood. Significant cellular toxicity and impairment of protein synthesis and degradation occur in alcohol-exposed liver cells, along with changes in energy balance and modified responses to pathogens. Autophagy is the process of cellular catabolism through the lysosomal-dependent machinery, which maintains a balance among protein synthesis, degradation, and recycling of self. Autophagy is part of normal homeostasis and it can be triggered by multiple factors that threaten cell integrity, including starvation, toxins, or pathogens. Multiple factors regulate autophagy; survival and preservation of cellular integrity at the expense of inadequately folded proteins and damaged high-energy generating intracellular organelles are prominent targets of autophagy in pathological conditions. Coincidentally, inadequately folded proteins accumulate and high-energy generating intracellular organelles, such as mitochondria, are damaged by alcohol abuse; these alcohol-induced pathological findings prompted investigation of the role of autophagy in the pathogenesis of alcohol-induced liver damage. Our review summarizes the current knowledge about the role and implications of autophagy in alcohol-induced liver disease.

Early growth response-1 contributes to steatosis development after acute ethanol administration

BACKGROUND

Previous work demonstrated that the transcription factor, early growth response-1 (Egr-1), participates in the development of steatosis (fatty liver) after chronic ethanol (EtOH) administration. Here, we determined the extent to which Egr-1 is involved in fatty liver development in mice subjected to acute EtOH administration.

METHODS

In acute studies, we treated both wild-type and Egr-1 null mice with either EtOH or phosphate-buffered saline (PBS) by gastric intubation. At various times after treatment, we harvested sera and livers and quantified endotoxin, indices of liver injury, steatosis, and hepatic Egr-1 content. In chronic studies, groups of mice were fed liquid diets containing either EtOH or isocaloric maltose-dextrin for 7 to 8 weeks.

RESULTS

Compared with controls, acute EtOH-treated mice showed a rapid, transient elevation in serum endotoxin beginning 30 minutes after treatment. One hour postgavage, livers from EtOH-treated mice exhibited a robust elevation of both Egr-1 mRNA and protein. By 3 hours postgavage, liver triglyceride increased in EtOH-treated mice as did lipid peroxidation. Acute EtOH treatment of Egr-1-null mice showed no Egr-1 expression, but these animals still developed elevated triglycerides, although significantly lower than EtOH-fed wild-type littermates. Despite showing decreased fatty liver, EtOH-treated Egr-1 null mice exhibited greater liver injury. After chronic EtOH feeding, steatosis and liver enlargement were clearly evident, but there was no indication of elevated endotoxin. Egr-1 levels in EtOH-fed mice were equal to those of pair-fed controls.

CONCLUSIONS

Acute EtOH administration induced the synthesis of Egr-1 in mouse liver. However, despite its robust increase, the transcription factor had a smaller, albeit significant, function in steatosis development after acute EtOH treatment. We propose that the rise in Egr-1 after acute EtOH is an hepatoprotective adaptation to acute liver injury from binge drinking that is triggered by EtOH metabolism and elevated levels of endotoxin.

Proteasome activity and autophagosome content in liver are reciprocally regulated by ethanol treatment

The proteasome and autophagy are two major intracellular protein degradation pathways and the regulation of each by ethanol metabolism affects cellular integrity. Using acute and chronic ethanol feeding to mice in vivo, and precision-cut rat liver slices (PCLS) ex vivo, we examined whether ethanol treatment altered these proteolytic pathways. In acute studies, we gave C57Bl/6 mice either ethanol or phosphate-buffered saline (PBS) by gastric intubation and sacrificed them 12h later. PCLS were exposed to 0 or 50mM ethanol for 12 and 24h with or without 4-methylpyrazole (4MP). In chronic studies we pair-fed control and ethanol liquid diets for 4-6 weeks to transgenic mice, expressing the green fluorescent protein (GFP) fused to the autophagic marker, microtubule associated protein-1 light chain 3 (LC3). Acute ethanol administration elevated autophagosomes (AVs), as judged by a 1.5-fold increase in LC3II content over PBS-gavaged control mice. Hepatic proteasome activity was unaffected by this treatment. Compared with controls, ethanol exposure for 12 and 24h to PCLS inhibited proteasome activity by 1.5- to 3-fold and simultaneously enhanced AVs by 2- to 5-fold. The decrease in proteasome activity and the rise in AVs both depended on ethanol oxidation as its inhibition by 4-methylpyrazole (4MP) blocked both proteasome inhibition and AV induction. Hepatocytes from mice chronically consuming ethanol exhibited a 1.6-fold decline in proteasome activity, and a 4-fold rise in GFP-LC3 puncta compared with pair-fed control mice. When we exposed hepatocytes from these animals to MG262, a proteasome inhibitor, LC3II puncta per cell further increased 2- to 5-fold over untreated cells.

CONCLUSION

Our findings demonstrate that ethanol metabolism generates oxidants, the levels of which differentially influence the activities of the proteasome and autophagy.

Involvement of autophagy in alcoholic liver injury and hepatitis C pathogenesis

This review describes the principal pathways of macroautophagy (i.e. autophagy), microautophagy and chaperone-mediated autophagy as they are currently known to occur in mammalian cells. Because of its crucial role as an accessory digestive organ, the liver has a particularly robust autophagic activity that is sensitive to changes in plasma and dietary components. Ethanol consumption causes major changes in hepatic protein and lipid metabolism and both are regulated by autophagy, which is significantly affected by hepatic ethanol metabolism. Ethanol exposure enhances autophagosome formation in liver cells, but suppresses lysosome function. Excessive ethanol consumption synergizes with hepatitis C virus (HCV) to exacerbate liver injury, as alcohol-consuming HCV patients frequently have a longer course of infection and more severe manifestations of chronic hepatitis than abstinent HCV patients. Alcohol-elicited exacerbation of HCV infection pathogenesis is related to modulation by ethanol metabolism of HCV replication. Additionally, as part of this mechanism, autophagic proteins have been shown to regulate viral (HCV) replication and their intracellular accumulation. Because ethanol induces autophagosome expression, enhanced levels of autophagic proteins may enhance HCV infectivity in liver cells of alcoholics and heavy drinkers.

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.

Role of autophagy in liver physiology and pathophysiology

Autophagy is a highly conserved intracellular degradation pathway by which bulk cytoplasm and superfluous or damaged organelles are enveloped by double membrane structures termed autophagosomes. The autophagosomes then fuse with lysosomes for degradation of their contents, and the resulting amino acids can then recycle back to the cytosol. Autophagy is normally activated in response to nutrient deprivation and other stressors and occurs in all eukaryotes. In addition to maintaining energy and nutrient balance in the liver, it is now clear that autophagy plays a role in liver protein aggregates related diseases, hepatocyte cell death, steatohepatitis, hepatitis virus infection and hepatocellular carcinoma. In this review, I discuss the recent findings of autophagy with a focus on its role in liver pathophysiology.

Autophagy and ethanol-induced liver injury

The majority of ethanol metabolism occurs in the liver. Consequently, this organ sustains the greatest damage from ethanol abuse. Ethanol consumption disturbs the delicate balance of protein homeostasis in the liver, causing intracellular protein accumulation due to a disruption of hepatic protein catabolism. Evidence indicates that ethanol or its metabolism impairs trafficking events in the liver, including the process of macroautophagy, which is the engulfment and degradation of cytoplasmic constituents by the lysosomal system. Autophagy is an essential, ongoing cellular process that is highly regulated by nutrients, endocrine factors and signaling pathways. A great number of the genes and gene products that govern the autophagic response have been characterized and the major metabolic and signaling pathways that activate or suppress autophagy have been identified. This review describes the process of autophagy, its regulation and the possible mechanisms by which ethanol disrupts the process of autophagic degradation. The implications of autophagic suppression are discussed in relation to the pathogenesis of alcohol-induced liver injury.

S-adenosyl-L-methionine co-administration prevents the ethanol-elicited dissociation of hepatic mitochondrial ribosomes in male rats

BACKGROUND

Chronic ethanol feeding to male rats has been shown to result in decreased mitochondrial translation, depressed respiratory complex levels and mitochondrial respiration rates. In addition, ethanol consumption has been shown to result in an increased dissociation of mitoribosomes. S-adenosyl-L-methionine (SAM) is required for the assembly and subsequent stability of mitoribosomes and is depleted during chronic ethanol feeding. The ability of dietary SAM co-administration to prevent these ethanol-elicited lesions was investigated.

METHODS

Male Sprague-Dawley rats were fed a nutritionally adequate liquid diet with ethanol comprising 36% of the calories according to a pair-fed design for 28 days. For some animals, SAM was supplemented in the diet at 200 mg/l. Liver mitochondria were prepared and mitoribosomes isolated. Respiration rates, ATP levels, respiratory complex levels, and the extent of mitoribosome dissociation were determined.

RESULTS

Twenty-eight days of ethanol feeding were found to result in decreased SAM content, depressed respiration, and increased mitoribosome dissociation. No changes in mitochondrial protein content; levels of respiratory complexes I, III, and V; complex I activities; and ATP levels were detected. Co-administration of SAM in the diet was found to prevent ethanol-induced SAM depletion, respiration decreases and mitoribosome dissociation.

CONCLUSIONS

Taken together, these findings suggest (1) that mitoribosome dissociation precedes respiratory complex depressions in alcoholic animals and (2) that dietary supplementation of SAM prevents some of the early mitochondrial lesions associated with chronic ethanol consumption.

Acute and chronic ethanol consumption differentially impact pathways limiting hepatic protein synthesis

This review identifies the various pathways responsible for modulating hepatic protein synthesis following acute and chronic alcohol intoxication and describes the mechanism(s) responsible for these changes. Alcohol intoxication induces a defect in global protein synthetic rates that is localized to impaired translation of mRNA at the level of peptide-chain initiation. Translation initiation is regulated at two steps: formation of the 43S preinitiation complex [controlled by eukaryotic initiation factors 2 (eIF2) and 2B (eIF2B)] and the binding of mRNA to the 40S ribosome (controlled by the eIF4F complex). To date, alcohol-induced alterations in eIF2 and eIF2B content and activity are best investigated. Ethanol decreases eIF2B activity when ingested either acutely or chronically. The reduced eIF2B activity most likely is a consequence of twofold increased phosphorylation of the alpha-subunit of eIF2 on Ser(51) following acute intoxication. The increase in eIF2alpha phosphorylation after chronic alcohol consumption is the same as that induced by acute ethanol intoxication, and protein synthesis is not further reduced by long-term alcohol ingestion despite additional reduced expression of initiation factors and elongation factors. eIF2alpha phosphorylation alone appears sufficient to maximally inhibit hepatic protein synthesis. Indeed, pretreatment with Salubrinal, an inhibitor of eIF2alpha(P) phosphatase, before ethanol treatment does not further inhibit protein synthesis or increase eIF2alpha phosphorylation, suggesting that acute ethanol intoxication causes maximal eIF2alpha phosphorylation elevation and hepatic protein synthesis inhibition. Ethanol-induced inhibition of hepatic protein synthesis is not rapidly reversed by cessation of ethanol consumption. In conclusion, sustained eIF2alpha phosphorylation is a hallmark of excessive alcohol intake leading to inhibition of protein synthesis. Enhanced phosphorylation of eIF2alpha represents a unique response of liver to alcohol intoxication, because the ethanol-induced elevation of eIF2alpha(P) is not observed in skeletal muscle or heart.

Proteasome activation by hepatitis C core protein is reversed by ethanol-induced oxidative stress

BACKGROUND & AIMS

The proteasome is a major cellular proteinase. Its activity is modulated by cellular oxidants. Hepatitis C core protein and ethanol exposure both cause enhanced oxidant generation. The aim was to investigate whether core protein, by its ability to generate oxidants, alters proteasome activity and whether these alterations are further affected by ethanol exposure.

METHODS

These interactions were examined in Huh-7 cell lines that expressed inducible HCV core protein and/or constitutive cytochrome P450 2E1 (CYP2E1) and as purified components in a cell-free system. Chymotrypsin-like proteasome activity was measured fluorometrically.

RESULTS

Proteasome activity in core-positive 191-20 cells was 20% higher than that in core-negative cells and was enhanced 3-fold in CYP2E1-expressing L14 cells. Exposure of core-positive cells to glutathione ethyl ester, catalase, or the CYP2E1 inhibitor diallyl sulfide partially reversed the elevation of proteasome activity in core-positive cells, whereas ethanol exposure suppressed proteasome activity. The results indicate that proteasome activity was up-regulated by low levels of core-induced oxidative stress but down-regulated by high levels of ethanol-elicited stress. These findings were partially mimicked in a cell-free system. Addition of core protein enhanced the peptidase activity of purified 20S proteasome containing the proteasome activator PA28 and was further potentiated by addition of liver mitochondrial and/or microsome fractions. However, proteasome activation was significantly attenuated when fractions were obtained from ethanol-fed animals.

CONCLUSIONS

HCV core protein interacts with PA28, mitochondrial, and endoplasmic reticulum proteins to cause low levels of oxidant stress and proteasome activation, which is dampened during ethanol metabolism when oxidant generation is higher.

Autophagy in the liver

A great part of our current understanding of mammalian macroautophagy is derived from studies of the liver. The term "autophagy" was introduced by Christian de Duve in part based on ultrastructural changes in rat liver following glucagon injection. Subsequent morphological, biochemical, and kinetics studies of autophagy in the liver defined the basic process of autophagosome formation, maturation, and degradation and the regulation of autophagy by hormones, phosphoinositide 3-kinases, and mammalian target of rapamycin. It is now clear that macroautophagy in the liver is important for the balance of energy and nutrients for basic cell functions, the removal of misfolded proteins resulting from genetic mutations or pathophysiological stimulations, and the turnover of major subcellular organelles such as mitochondria, endoplasmic reticulum, and peroxisomes under both normal and pathophysiological conditions. Disturbance of autophagy function in the liver could thus have a major impact on liver physiology and liver disease.

Effects of chronic alcohol drinking on receptor-binding, internalization, and degradation of human immunodeficiency virus 1 envelope protein gp120 in hepatocytes

Although alcohol drinking increases susceptibility to human immunodeficiency virus (HIV) infection, possible mechanisms underlying the effects of alcohol are not yet known. Since the HIV envelope protein gp120 plays a key role in progression of HIV infection, the aim of the present study was to evaluate the toxicity and degradation of gp120 in hepatocytes isolated from liver of alcohol-preferring rats drinking either 15% ethanol in water or pure water for 70 days. The hypothesis was that alcohol drinking augmented the toxicity, but suppressed degradation of gp120. Hepatocytes from water-drinking rats (C-cells) or ethanol-drinking rats (Et-cells) were treated with laptacystin, anti-CD4 antibodies, CCR5 antagonist, or mannose, followed by [(125)I]gp120 or native gp120. At predetermined intervals, control (C) and ethanol exposed (Et) cells were analyzed for toxicity and degradation of gp120. In C-cells, [(125)I]gp120 binding and internalization peaked within 5-45 min and remained elevated for up to 10h and then decreased gradually. In Et-cells, [(125)I]gp120 binding peaked comparably to C-cells, but the binding remained to the peak level throughout the experimental period. C-cells exhibited the lysosomal/ubiquitin-mediated degradation of intracellular gp120, resulting in released gp120 fragments into the incubation medium that suppressed gp120-CD4 binding, improved cell viability, and inhibited gp120-induced apoptosis. Ethanol drinking suppressed gp120 degradation in and release of gp120 fragments from hepatocytes. The incubation medium of Et-cells did not suppress gp120-CD4 binding or the gp120-mediated apoptosis in hepatocytes. Thus, chronic alcohol drinking augmented the adverse effects of gp120 possibly by suppressing its degradation in hepatocytes. The present observation also suggests that a number of CCR5 or ubiquitin-based therapeutic drugs may not be effective in suppressing HIV infection in alcohol-drinking subjects.

Lysosomal leakage and lack of adaptation of hepatoprotective enzyme contribute to enhanced susceptibility to ethanol-induced liver injury in female rats

BACKGROUND

Women exhibit greater liver damage than men after chronic alcohol consumption. Similar findings are reported in animal models. Here, we determined whether differential liver injury occurred in male and female rats after feeding these animals liquid diets containing either ethanol or isocaloric dextrose with fish oil as the sole source of lipid.

METHODS

Control and ethanol liquid diets containing fish oil were pair-fed to male and female rats for 8 weeks. Liver damage was evaluated by triglyceride accumulation, lipid peroxide formation, serum transaminases, histological evaluation, and the activities of selected lysosomal and hepatoprotective enzymes.

RESULTS

Fatty liver was detected after ethanol feeding in both genders, but in female rats, triglyceride levels were 60% higher, lipid peroxides were 2-fold higher, and inflammatory cells were more evident than in males. A 2-fold elevation of cathepsin B in hepatic cytosol fractions, indicating lysosomal leakage, was detected in ethanol-fed female rats but no such elevation was observed in males. The basal activity of the hepatoprotective enzyme, betaine-homocysteine methyltransferase was 4-fold higher in livers of control male rats than females, and the enzyme activity was further elevated in ethanol-fed male rats but not in females.

CONCLUSIONS

Thus, female rats given ethanol in a diet containing fish oil exhibited more severe liver damage than males. We propose that this difference results, in part, from a greater tendency by females to accumulate hepatic fat, thereby enhancing the potential for oxidative stress, which in turn leads to hepatic inflammation. In addition, our findings indicate that female rats have a higher susceptibility to liver damage because of a reduced capacity for hepatoprotection.

L-Buthionine (S,R) sulfoximine depletes hepatic glutathione but protects against ethanol-induced liver injury

BACKGROUND

L-Buthionine (S,R) sulfoximine (BSO) is an inhibitor of glutathione biosynthesis and has been used as an effective means of depleting glutathione from cells and tissues. Here we investigated whether treatment with BSO enhanced ethanol-induced liver injury in mice.

METHODS

Female C57Bl/6 mice were pair fed with control and ethanol-containing liquid diets in which ethanol was 29.2% of total calories. During the final 7 days of pair feeding, groups of control-fed and ethanol-fed mice were given 0, 5 or 7.6 mM BSO in the liquid diets.

RESULTS

Compared with controls, ethanol given alone decreased total liver glutathione. This effect was exacerbated in mice given ethanol with 7.6 mM BSO, causing a 72% decline in hepatic glutathione. While ethanol alone caused no decrease in mitochondrial glutathione, inclusion of 7.6 mM BSO caused a 2-fold decline compared with untreated controls. L-Buthionine (S,R) sulfoximine did not affect ethanol consumption, but serum ethanol levels in BSO-treated mice were nearly 6-fold lower than in mice given ethanol alone. The latter decline in serum ethanol was associated with a significant elevation in the specific activities of cytochrome P450 2E1 and alcohol dehydrogenase in livers of BSO-treated animals. Ethanol consumption caused a 3.5-fold elevation in serum alanine aminotransferase levels but the enzyme fell to control levels when BSO was included in the diet. L-Buthionine (S,R) sulfoximine administration also attenuated ethanol-induced steatosis, prevented the leakage of lysosomal cathepsins into the cytosol, and prevented the ethanol-elicited decline in proteasome activity.

CONCLUSIONS

L-Buthionine (S,R) sulfoximine, administered with ethanol, significantly depleted hepatic glutathione, compared with controls. However, despite the decrease in hepatic antioxidant levels, liver injury by ethanol was alleviated, due, in part, to a BSO-elicited acceleration of ethanol metabolism.

Ethanol consumption alters expression and colocalization of bile salt export pump and multidrug resistance protein 2 in the rat

Chronic ethanol consumption elicits detrimental changes of liver metabolism. By employing a 12-week-long feeding regimen, we investigated the effects of chronic ethanol consumption on the expression and localization of bile salt export pump (Bsep), a major canalicular exporter of bile salts, and multidrug resistance protein 2 (Mrp2), a canalicular organic anion transporter, in the rat liver. RT-PCR, confocal immunofluorescence microscopy, immunoblotting, and quantitative colocalization analysis were used to examine their gene and protein expression, and changes in the distribution of antigenic sites. Bsep mRNA was upregulated, while Mrp2 mRNA responded by downregulation. In agreement with mRNA, the expression of Bsep protein increased, while the expression of Mrp2 protein responded with a decrease, suggesting that the expression of both of them is transcriptionally regulated. Confocal immunofluorescence microscopy showed disruption of the colocalization of Bsep and Mrp2 proteins at the hepatocyte canalicular membrane and their relocation intracellularly. Quantitative colocalization analysis of Bsep and Mrp2 proteins revealed a steady decrease in the degree of colocalization and Mrp2 expression, indicating that although the properties of both transporters are affected, Mrp2 is altered more. These findings provide evidence that ethanol alters Bsep and Mrp2 canalicular transporters in the rat liver, at both the mRNA and protein levels. Mrp2 shows deeper involvement. Eight weeks appears to be a critical time point in this process.

Recombinant Hep G2 cells that express alcohol dehydrogenase and cytochrome P450 2E1 as a model of ethanol-elicited cytotoxicity

HepG2 cells were transfected with recombinant plasmids, one carrying the murine alcohol dehydrogenase (ADH) gene and the other containing the gene encoding human cytochrome P450 2E1 (CYP2E1). One of recombinant clones called VL-17A exhibited ADH and CYP2E1 specific activities comparable to those in isolated rat hepatocytes. VL-17A cells oxidized ethanol and generated acetaldehyde, the levels of which depended upon the initial ethanol concentration. Compared with unexposed VL-17A cells, ethanol exposure increased the cellular redox (lactate:pyruvate ratio) and caused cell toxicity, indicated by increased leakage of lactate dehydrogenase into the medium,. Exposure of VL-17A cells to 100mM ethanol significantly elevated caspase 3 activity, an indicator of apoptosis, but this ethanol concentration did not affect caspase 3 activity in parental HepG2 cells. Because ethanol consumption causes a decline in hepatic protein catabolism, we examined the influence of ethanol exposure on proteasome activity in HepG2, VL-17A, E-47 (CYP2E1(+)) and VA-13 (ADH(+)) cells. Exposure to 100mM ethanol caused a 25% decline in the chymotrypsin-like activity of the proteasome in VL-17A cells, but the enzyme was unaffected in the other cell types. This inhibitory effect on the proteasome was blocked when ethanol metabolism was blocked by 4-methyl pyrazole. We conclude that recombinant VL-17A cells, which express both ADH and CYP2E1 exhibit hepatocyte-like characteristics in response to ethanol. Furthermore, the metabolism of ethanol by these cells via ADH and CYP2E1 is sufficient to bring about an inhibition of proteasome activity that may lead to apoptotic cell death.

Ethanol feeding enhances age-related deterioration of the rat hepatic mitochondrion

Chronic ethanol feeding damages the hepatic mitochondrion by increasing mitochondrial DNA (mtDNA) oxidation, lowering mtDNA yields and impairing mitochondrial respiration. These effects are also seen during aging. By employing a 21-day chronic feeding regimen, we investigated the effects of ethanol consumption on mtDNA content and mitochondrial respiration in 2-, 12-, and 24-mo-old male rats. Aging resulted in decreased mtDNA content, increased mtDNA damage (as indicated by inhibition of Taq polymerase progression), and a decline in state 3 respiration; effects that were further exacerbated by ethanol feeding. Additionally, ethanol consumption caused an increase in the levels of citrate synthase while not impacting mitochondrial protein content. In conclusion, ethanol and aging combine to cause deterioration in the structural and functional integrity of the hepatic mitochondrion. The additive effects of aging and ethanol feeding may have serious consequences for hepatic energy metabolism in aged animals, and their detrimental combination may serve as one of the molecular mechanisms underlying the progression of alcoholic liver disease.

The effects of acetaldehyde in vitro on proteasome activities and its potential involvement after alcoholization of rats by inhalation of ethanol vapours

BACKGROUND/AIMS

Some models of chronic ethanol administration resulted in decreased proteasome activities. The mechanisms still remain speculative. In the present study, we tested another model of alcoholization with high blood alcohol levels (BALs) and high acetaldehyde fluxes as well as the in vitro effect of acetaldehyde on proteasome. Methods/

RESULTS

Ethanol vapour chronically inhaled by adult Wistar rats up to a specific protocol, can reach high BALs (200 mg/dl) with significant circulating acetaldehyde levels. After 4 weeks of ethanol intoxication, although cytochrome CYP2E1 was increased, liver lipid peroxidation remained unchanged when protein carbonyls augmented selectively for high molecular weight with a decrease of the proteasome activities in ethanol rats. Several aldehydes inhibit proteasome function; we specifically explored the effects of acetaldehyde, the first alcohol metabolite. Adduction of acetaldehyde in vitro to cytosolic proteins inhibits proteasome in a dose-dependent manner. Acetaldehyde adducted to purified proteasome also exhibits a decrease in its activities. Furthermore, an acetaldehyde-adducted protein, i.e. bovine serum albumin (BSA) is less degraded than a native BSA by purified proteasome. These findings suggest that acetaldehyde, if overproduced, can inhibit proteasome activities and reduce the proteolysis of acetaldehyde-adducted proteins.

CONCLUSIONS

Our study, for the first time, provided the evidence that acetaldehyde by itself inhibits proteasome activities. As the chronic inhalation model used in this study is not associated with an overt lipid peroxidation, one can suggest that high BALs and their subsequent high acetaldehyde fluxes contribute to impairment of proteasome function and accumulation of carbonylated proteins. This early phenomenon may have relevance in experimental alcohol liver disease.

Ethanol consumption decreases the synthesis of the mannose 6-phosphate/insulin-like growth factor II receptor but does not decrease its messenger RNA

The mannose 6-phosphate/insulin-like growth factor II receptor (M6P/IGF-IIR) is a protein that facilitates the transport of acid hydrolases into the lysosome. We have shown that chronic ethanol consumption lowers the M6P/IGF-IIR content in rat hepatocytes. Here, we determined the steady-state level of mRNA encoding M6P/IGF-IIR, as well as the rate of receptor synthesis, to ascertain whether the ethanol-elicited reduction in receptor protein content is related to changes in either or both of these parameters. Rats were pair-fed the normal carbohydrate (NC) or low carbohydrate high-fat (LC) liquid diets containing either ethanol or isocaloric maltose-dextrin for 7-8 weeks. RNA was isolated from hepatocytes and from whole livers of these animals and subjected to reverse transcription-polymerase chain reaction (RT-PCR) to determine the mRNA levels encoding M6P/IGF-IIR. Hepatocytes isolated from these animals were also radiolabeled with Pro-mix L-[35S] in vitro cell labeling mix to measure incorporation into total cellular protein and the immunoprecipitated M6P/IGF-IIR protein. The steady-state levels of M6P/IGF-IIR mRNA in both hepatocytes and whole livers from ethanol-fed rats were the same as those from their respective controls regardless of whether they were fed the NC or the LC diets. Hepatocytes from ethanol-fed rats showed a 36% lower rate of total protein synthesis and an even greater reduction (70%) in receptor synthesis. When the relative rate of receptor synthesis was calculated, hepatocytes from ethanol-fed rats had a 53% lower relative rate of receptor synthesis compared with controls. Autoradiographic analysis of the immunoprecipitated receptor protein from ethanol-fed rats also indicated a 79% decline in the total M6P/IGF-IIR protein synthetic rate compared with pair-fed controls. We conclude that the ethanol-elicited reduction of M6P/IGF-IIR content was, in part, related to a concomitant reduction of receptor protein synthesis but not to a decline in its mRNA level. Thus, the ethanol-elicited decline in receptor protein synthesis may be due to defective M6P/IGF-IIR mRNA translation.

Chronic ethanol administration decreases the ligand binding properties and the cellular content of the mannose 6-phosphate/insulin-like growth factor II receptor in rat hepatocytes

We have shown previously that chronic ethanol administration impairs the maturation of lysosomal enzymes in rat hepatocytes. The mannose 6-phosphate/insulin-like growth factor II receptor (M6P/IGF-IIR) is a protein that facilitates the transport of lysosomal enzymes into the lysosome. Therefore, we examined whether ethanol consumption altered the ligand binding properties and the cellular content of M6P/IGF-IIR. Rats were pair-fed liquid diets containing either ethanol (36% of calories) or isocaloric maltose-dextrin for either 1 week or 5-7 weeks. Hepatocytes prepared from these animals were examined for receptor-ligand binding and receptor content. One week of ethanol feeding had no significant effect on ligand [radioiodinated pentamannose phosphate conjugated to bovine serum albumin ((125)I-PMP-BSA)] binding to hepatocytes, but cells from rats fed ethanol for 5-7 weeks bound less (125)I-PMP-BSA than pair-fed controls. Scatchard plot analysis revealed that the number of (125)I-PMP-BSA binding sites in hepatocytes from ethanol-fed rats was 49% lower than that of controls. (125)I-PMP-BSA binding by perivenular (PV) and periportal (PP) hepatocytes from ethanol-fed rats was, respectively, 40 and 48% lower than their controls, but there was no significant difference between these two types of hepatocytes. Ligand blot analysis using (125)I-insulin-like growth factor II ((125)I-IGF-II) also showed that the receptor in lysates of hepatocytes from ethanol-fed rats bound 26-27% less ligand than controls. Similarly, immunoblot analysis of cell lysates from ethanol-fed rats revealed 62% lower levels of immunoreactive M6P/IGF-IIR than controls. Feeding rats a low carbohydrate-ethanol diet did not exacerbate the reduction in M6P/IGF-IIR-ligand binding nor did it reduce the levels of immunoreactive receptor. Our findings indicate that chronic ethanol consumption lowers M6P/IGF-IIR activity and content in hepatocytes. This reduction may account, in part, for the impaired processing and delivery of acid hydrolases to lysosomes previously observed in ethanol-fed rats.

The ubiquitin-proteasome system and its role in ethanol-induced disorders

Some of the most fundamental yet important cellular activities such as cell division and gene expression are controlled by short-lived regulatory proteins. The levels of these proteins are controlled by their rates of degradation. Similarly, protein catabolism plays a crucial role in prolonging cellular life by destroying damaged proteins that are potentially cytotoxic. A major player in these catabolic reactions is the ubiquitin-proteasome system, a novel proteolytic system that has become the primary proteolytic pathway in eukaryotic cells. Ubiquitin-mediated proteolysis is now regarded as the major pathway by which most intracellular proteins are destroyed. Equally important, from a toxicological standpoint, is that the ubiquitin-proteasome system is also widely considered to be a cellular defense mechanism, since it is involved in the removal of damaged proteins generated by adduct formation and oxidative stress. This review describes the history and the components of the ubiquitin-proteasome system, its regulation and its role in pathological states, with the major emphasis on ethanol-induced organ injury. The available literature cited here deals mainly with the effects of ethanol consumption on the ubiquitin-proteasome pathway in the liver. However, since this proteolytic system is an essential pathway in all cells it is an attractive experimental model and therapeutic target in extrahepatic organs such as the brain and heart that are also affected by excessive alcohol consumption.

Diverse effects of ethanol on the pathway of inducible prostaglandin E2 production in macrophages

The effects of ethanol on inducible prostaglandin production in RAW macrophages were investigated. Indomethacin (1 microM) or cycloheximide (1 microM) abolished prostaglandin E2 (PGE2) production induced by lipopolysaccharide (LPS, 1 microg/ml). Ethanol at concentrations from 100 mM to 600 mM concentration-dependently inhibited inducible PGE2 production, while ethanol only at higher concentrations (400 mM or more) showed cytotoxity to the cells. Cyclooxygenase-2 (COX-2) activity, estimated by transformation of exogenous arachidonic acid into PGE2, was not affected by ethanol (100-400 mM). LPS-induced expression of COX-2 mRNA was inhibited by ethanol (50-400 mM). On the other hand, protein expression of COX-2 by LPS was significantly increased by ethanol (100-400 mM). Ethanol alone at concentrations up to 600 mM did not induce expression of COX-2 protein. In a medium containing arachidonic acid (1 microM), ethanol at a low concentration (100 mM) did not significantly affect LPS-induced PGE2 production. These results suggest that ethanol shows diverse effects on the pathway of inducible PGE2 production in macrophages. Finally, ethanol may suppress utilization of arachidonic acid, resulting in reduction of inducible PGE2 production. Further study is needed to elucidate the mechanism of dissociation of ethanol effects on protein and mRNA expression.

Ethanol increases superoxide anion production stimulated with 4beta-phorbol 12-myristate 13-acetate in human polymorphonuclear leukocytes. Involvement of protein kinase C

Stimulation of human polymorphonuclear leukocytes (PMNs) with PMA initiates a cascade of events leading to the production and release of superoxide anion (O-2), a major component in anti-bacterial defense. Generation of O-2 by PMA-stimulated PMNs occurs through the translocation and activation of protein kinase C (PKC). In this study, using freshly isolated PMNs, we examined the effect of ethanol on this response to PMA. Our results show that the basal production of O-2 was not affected by ethanol. In contrast, the response induced by PMA was potentiated by ethanol. This potentiation was observed even at high doses of PMA (200 nM) which alone had stimulated the O-2 response maximally. This enhanced response was not due to an increase of PMA uptake by PMNs. The maximal effect was obtained when the cells were preincubated with 80 mM of ethanol before PMA stimulation. Measurement of PKC activity in the cytosolic and membrane fractions showed that pretreatment of PMNs with ethanol increased twofold the PMA-stimulated PKC activity in the membrane fraction. Furthermore, Western blot analysis verified that this increase in PKC activity in the membrane fraction was linked to an increase in the translocation of PKC-alpha and -beta isoforms to the membrane. These results suggest that ethanol potentiates PMA-induced O-2 production through increasing PKC translocation and activity in PMNs.

Effects of chronic ethanol administration on rat liver proteasome activities: relationship with oxidative stress

We previously reported that ethanol elicits an increased protein oxidation in the liver of rats receiving chronic ethanol by continuous intragastric infusion (Tsukamoto-French method). This accumulation of oxidized proteins could result from a decrease in the cytosolic proteolysis, related specifically to alkaline protease and its major components, the proteasomes. Because several studies suggest that intracellular proteolysis depends on the severity of oxidative stress, we investigated the cytosolic proteolytic activity under two chronic ethanol treatment paradigms associated with varying degrees of oxidative stress. For 4 weeks, male rats received chronic ethanol by continuous intragastric infusion or by oral administration (10% ethanol ad libitum as sole drinking fluid). A significant decrease was evident for alkaline protease activity as well as for sodium dodecyl sulfate (SDS)-activated latent 20S proteasome (chymotrypsine-like [ChT-L] and peptidylglutamyl peptide hydrolase [PGPH] activities) in the liver of rats receiving ethanol by continuous intragastric infusion. Free radical production and related processes appeared to be contributing events in proteolysis inhibition, because phenethyl isothiocyanate (PIC), an inhibitor of cytochrome P4502E1 (CYP2E1), reduced the inhibition of the ethanol-related ChT-L activity. Moreover, the lipid peroxidation level was inversely correlated with ChT-L activity. In contrast, no such changes were observed in ChT-L and PGPH activities or in cellular free radical targets following the oral ad libitum consumption of 10% ethanol. It appears, thus, that only the alcohol treatment paradigm associated with an overt oxidative stress produced a significant inhibition of the proteasome activity. The mechanisms of proteasome inhibition could involve the formation of an endogenous inhibitor such as protein aggregates or aldehyde-derivative peptides. Whatever the mechanism, the inhibition of cytosolic proteolysis and the subsequent accumulation of damaged proteins may be involved in the oxidatively challenged alcoholic livers and play a pathogenic role in experimental alcoholic liver disease.

Contrasting effects of acute and chronic ethanol administration on rat liver tyrosine aminotransferase

We compared the effects of acute and chronic ethanol administration on the activity and synthesis of tyrosine aminotransferase (TAT) in rat liver. In acute experiments, chow-fed rats received a single dose of either ethanol (6 g/kg body wt.) or saline. In chronic studies, rats were pair-fed liquid diets containing either ethanol (36 % of calories) or isocaloric maltose-dextrin for 6-8 weeks. In rats acutely fed ethanol, the relative rate of TAT synthesis was more than twofold higher than in saline-treated controls. In rats subjected to chronic ethanol administration, both the TAT activity and synthesis rate were the same as in pair-fed controls, but both these parameters in the two groups were equal to those in animals given acute ethanol acutely. These findings indicate that whereas acute ethanol administration was associated with a stimulation of TAT synthesis, long-term ethanol administration was not. The data suggest that ethanol itself does not directly induce TAT. Rather, enzyme synthesis is regulated by one or more endogenous secondary effector(s) whose production is influenced differently by acute or chronic ethanol feeding.

Ethanol-exposed central neurons fail to migrate and undergo apoptosis

Prenatal exposure of human brain to ethanol impairs neuronal migration and differentiation and causes mental retardation. The present results indicate that the adverse effects of ethanol on brain development may be partly due to the ethanol-induced disturbance of neuronal interaction with laminin, a protein involved in neuronal migration and axon guidance. This report shows that physiological concentrations (IC50 = 28 mM) of ethanol inhibit neurite outgrowth and neuronal migration of the rat cerebellar granule neurons on a laminin substratum. The ethanol-treated granule neurons undergo apoptosis, degrade their laminin substratum, and appear to release and bind increased amounts of the B2-chain-derived peptides along their surfaces. A protease inhibitor aprotinin, and the NMDA receptor channel, and voltage-gated calcium channel antagonist MK801 partially protect cerebellar granule neurons from ethanol-induced neurotoxicity. These results imply that ethanol-treated granule neurons resemble the granule neurons of the homozygous weaver mouse cerebellum with respect to their apoptosis, laminin expression, and partial rescue by approtinin and MK-801. Thus, ethanol may influence neuronal survival and neurite outgrowth via molecular pathways similar to those involved in neuronal death in other neurodegenerative processes of the central nervous system.

Ethanol consumption alters trafficking of lysosomal enzymes and affects the processing of procathepsin L in rat liver

In order to determine whether ethanol consumption alters the targeting of hepatic lysosomal enzymes to their organelles, we examined the sedimentation properties of lysosomal hydrolases in ethanol-fed rats and their pair-fed controls. Rats were fed a liquid diet containing either ethanol (36% of calories) or isocaloric maltose dextrin for one to five wk. Liver extracts were fractionated by Percoll density gradient centrifugation and fractions obtained were analyzed for the distribution of lysosomal marker enzymes. Heavy lysosomes were further purified from these gradients and the activity of specific hydrolases was determined. Compared with those from controls, isolated lysosomes from ethanol-fed rats showed a 20-50% reduction in the activity of lysosomal acid phosphatase and beta-galactosidase. Decreased intralysosomal hydrolase activity in ethanol-fed rats was associated with a significant redistribution of these enzymes as well as those of cathepsins B and L to lighter fractions of Percoll density gradients. This indicated an ethanol-elicited shift of these enzymes to lower density cellular compartments. In order to determine whether ethanol administration affects the synthesis and proteolytic maturation of hepatic procathepsin L, we conducted immunoblot analyses to quantify the steady-state levels of precursor and mature forms of cathepsin L in hepatic post-nuclear fractions. Ethanol administration caused a significant elevation in the steady-state level of the 39 kDa cathepsin L precursor relative to its 30 kDa intermediate and 25 kDa mature product. These results were confirmed by pulse-chase experiments using isolated hepatocytes exposed to [35S]methionine. Hepatocytes from both control and ethanol-fed rats incorporated equal levels of radioactivity into procathepsin L. However, during the chase period, the ratios of the 39 kDa procathepsin L to its 30 kDa intermediate and 25 kDa mature product in cells from ethanol-fed rats were 1.5-3-fold higher than those in controls. These results demonstrate that ethanol consumption caused a marked impairment in the processing of procathepsin L to mature enzyme, without affecting its synthesis. Taken together, our findings suggest that chronic ethanol consumption caused a deficiency in intralysosomal enzyme content by altering the trafficking and processing of these hydrolases into lysosomes.

Association of alcoholism with the N-glycosylation polymorphism of pseudodeficient human arylsulfatase A

The IIIa and IIIb electrophoretic variants of arylsulfatase A (EC 3.1.6.8) are 12 times more prevalent in alcoholic than in nonalcoholic populations. These variant enzymes, found in a subset of alcoholics, possess the pseudodeficient Asn350-Ser mutation of arylsulfatase A and, consequently, lack an N-linked glycan unit. These genetically determined variants of arylsulfatase A show reduced intracellular half-life, and cells from such individuals possess reduced enzymic activity. We propose that this polymorphism is an underlying genetic and biochemical factor contributing to the neuropathology and/or addiction pathway of this disease.

Ethanol consumption reduces the proteolytic capacity and protease activities of hepatic lysosomes

Chronic ethanol consumption causes decreased hepatic protein degradation, resulting in protein accumulation within hepatocytes. In this investigation, we sought to determine whether chronic ethanol feeding alters the degradative capacity and protease activities of isolated hepatic lysosomes. Male Sprague-Dawley-derived rats were fed a liquid diet containing either ethanol (36% of calories) or isocaloric maltose-dextrin for 1-5 wk. Hepatic lysosomes were isolated by differential centrifugation and purified through Percoll gradients. Lysosomes obtained from livers of ethanol-fed rats degraded both endogenous protein substrates and the exogenously added radioactive substrate, 125I-RNase A, 26-42% more slowly than lysosomes from pair fed controls. The ethanol-elicited reduction in proteolytic capacity appeared to result in part, from a deficiency of the lysosomal cathepsins B, L, and H. Compared with controls, the specific activities of these enzymes were 31-45% lower in lysosomes from ethanol-fed rats. Immunoblot analyses also revealed that the intralysosomal as well as the intracellular content of cathepsin B was significantly lower in ethanol-fed rats. In contrast, ethanol consumption did not affect the cellular quantity of cathepsin L but lowered its amount in isolated lysosomes. Our findings suggest that chronic ethanol consumption causes a deficiency in lysosomal cathepsins by altering their biosynthesis and/or their trafficking into lysosomes.