Ethanol cytotoxicity to a transfected HepG2 cell line expressing human cytochrome P4502E1

Iron and CYP2E1-dependent oxidative stress and toxicity

Iron plays a critical role in catalyzing the formation of potent oxidants. Increases in iron content enhance oxidative stress, whereas removal of iron deceases such stress. An association between iron and alcoholic liver injury has been proposed. The ability of iron to modulate the biochemical and toxicologic actions of cytochrome P450 2E1 (CYP2E1) has been evaluated by using isolated microsomes and intact liver cells. The ability of different iron complexes to stimulate microsomal lipid peroxidation and hydroxyl radical production during reduced form of nicotinamide adenine dinucleotide phosphate (NADPH)- and reduced form of nicotinamide adenine dinucleotide (NADH)-dependent electron transfer has been characterized. Certain iron complexes have been shown to be effective in promoting lipid peroxidation; others are better catalysts of hydroxyl radical production as a complex pattern has been found. Reactive oxygen production, lipid peroxidation, and interaction with iron chelates have been shown to be enhanced with microsomes isolated from ethanol-treated rats with elevated levels of CYP2E1. This increase was prevented by anti-CYP2E1 immunoglobulin (Ig)G or chemical inhibitors of CYP2E1. Thus, in the presence of iron complexes, microsomes enriched in CYP2E1 are especially reactive in generation of reactive oxygen species. To assess the toxicologic significance of this iron-CYP2E1 interaction, iron (ferric-nitrilotriacetate) was added to HepG2 cells, which were engineered to express the human CYP2E1. Ferric-nitrilotriacetate produced a greater toxicity in the CYP2E1-expressing HepG2 cells than that in control HepG2 cells. This enhanced, synergistic toxicity was blocked by antioxidants and inhibitors of CYP2E1. Mitochondrial membrane potential and ATP levels were decreased, and damage to the mitochondria played a critical role in the CYP2E1-plus-iron-dependent toxicity. These results support the suggestion that low concentrations of iron and polyunsaturated fatty acids can act as priming or sensitizing factors for CYP2E1-induced injury in HepG2 cells and hepatocytes. Such interactions may play a role in alcohol-induced liver injury.

Role of p38 MAPK in CYP2E1-dependent arachidonic acid toxicity

Polyunsaturated fatty acids such as arachidonic acid (AA) play an important role in alcohol-induced liver injury. AA promotes toxicity in rat hepatocytes with high levels of cytochrome P4502E1 (CYP2E1) and in HepG2 E47 cells, which express CYP2E1. The possible role of mitogen-activated protein kinase (MAPK) members in this process was evaluated. SB203580, a p38 MAPK inhibitor, and PD98059, an ERK inhibitor, but not wortmannin a phosphatidylinositol 3-kinase (PI3K) inhibitor, prevented AA toxicity in pyrazole hepatocytes and E47 cells. SB203580 prevented the enhancement of AA toxicity by salicylate. SB203580 neither lowered the levels of CYP2E1 nor affected CYP2E1-dependent oxidative stress. The decrease in mitochondrial membrane potential produced by AA was prevented by SB203580. Treating CYP2E1-induced cells with AA activated p38 MAPK but not ERK or AKT. This activation was blocked by antioxidants. AA increased the translocation of NF-kappaB to the nucleus. Salicylate blocked this translocation, which may contribute to the enhancement of AA toxicity by salicylate. SB203580 restored AA-induced NF-kappaB translocation, which may contribute to protection against toxicity. In conclusion, AA toxicity was related to lipid peroxidation and oxidative stress, and to the activation of p38 MAPK, as a consequence of CYP2E1-dependent production of reactive oxygen species. Activation of p38 MAPK by AA coupled to AA-induced oxidative stress may synergize to cause cell toxicity by affecting mitochondrial membrane potential and by modulation of NF-kappaB activation.

Impaired adaptive resynthesis and prolonged depletion of hepatic mitochondrial DNA after repeated alcohol binges in mice

BACKGROUND & AIMS

A single dose of alcohol causes transient hepatic mitochondrial DNA (mtDNA) depletion in mice followed by increased mtDNA synthesis and an overshoot of mtDNA levels. We determined the effect of repeated alcohol binges on hepatic mtDNA in mice.

METHODS

Ethanol (5 g/kg) was administered by gastric intubation daily for 4 days, and mtDNA levels, synthesis, and integrity were assessed by slot blot hybridization, in organello [3H]deoxythymidine triphosphate incorporation, and long polymerase chain reaction analysis, respectively.

RESULTS

mtDNA levels were decreased for 48 hours after the last dose, with no overshoot phenomenon later on. Two and 24 hours after the fourth dose, long polymerase chain reaction experiments showed DNA lesions that blocked the progress of the polymerases and in organello mtDNA synthesis was decreased, although DNA polymerase gamma activity was unchanged with synthetic templates. Mitochondria exhibited ultrastructural abnormalities, and respiration was impaired 2 and 24 hours after the fourth binge. Cytochrome P450 2E1, mitochondrial generation of peroxides, thiobarbituric acid reactants, and ethane exhalation were increased.

CONCLUSIONS

After repeated doses of ethanol, the accumulation of unrepaired mtDNA lesions (possibly involving lipid peroxidation-induced adducts) blocks the progress of polymerase gamma on mtDNA and prevents adaptive mtDNA resynthesis, causing prolonged hepatic mtDNA depletion.

Cytochrome P450 2E1 expression induces hepatocyte resistance to cell death from oxidative stress

Increased expression of cytochrome P450 2E1 (CYP2E1) occurs in alcoholic liver disease, and leads to the hepatocellular generation of toxic reactive oxygen intermediates (ROI). Oxidative stress created by CYP2E1 overexpression may promote liver cell injury by sensitizing hepatocytes to oxidant-induced damage from Kupffer cell-produced ROI or cytokines. To determine the effect of CYP2E1 expression on the hepatocellular response to injury, stably transfected hepatocytes expressing increased (S-CYP15) and decreased (AN-CYP10) levels of CYP2E1 were generated from the rat hepatocyte line RALA255-10G. S-CYP15 cells had increased levels of CYP2E1 as demonstrated by Northern blot analysis, immunoblotting, catalytic activity, and increased cell sensitivity to death from acetaminophen. Death in S-CYP15 cells was significantly decreased relative to that in AN-CYP10 cells following treatment with hydrogen peroxide and the superoxide generator menadione. S-CYP15 cells underwent apoptosis in response to these ROI, whereas AN-CYP10 cells died by necrosis. This differential sensitivity to ROI-induced cell death was partly explained by markedly decreased levels of glutathione (GSH) in AN-CYP10 cells. However, chemically induced GSH depletion triggered cell death in S-CYP15 but not AN-CYP10 cells. Increased expression of CYP2E1 conferred hepatocyte resistance to ROI-induced cytotoxicity, which was mediated in part by GSH. However, CYP2E1 overexpression left cells vulnerable to death from GSH depletion.

Induction and regulation of hepatocyte apoptosis by oxidative stress

Reactive oxygen intermediates (ROI) have been implicated in the induction of hepatocyte apoptosis that results from a variety of forms of liver injury. Exogenous oxidants induce hepatocyte apoptosis and may mediate death during inflammatory liver injury. Lethal levels of intracellularly generated ROI resulting from hepatotoxin metabolism, or the induction of enzymes in the cytochrome P450 family, are also important inducers of apoptosis. In addition, ROI production may mediate death from a number of diverse factors, including tumor necrosis factor-alpha, bile acids, ischemia, and transforming growth factor-beta1. Oxidants alter many redox-sensitive cellular signaling pathways, including mitogen-activated protein kinases and transcription factors such as activator protein-1 and nuclear factor-kappaB. The mechanisms of oxidant-induced hepatocyte apoptosis remain unclear, but probably involve effects on cell signaling, as well as direct chemical interactions. The delineation of stimulus-specific mechanisms of oxidant-dependent hepatocyte apoptosis is important to the design of effective therapies for a number of forms of liver injury.

Increased proteolysis after single-dose exposure with hepatotoxins in HepG2 cells

Chronic ethanol consumption is associated with increased protein oxidation and decreased proteolysis in the liver. We tested the hypothesis that even single-dose treatment with ethanol or bromotrichloromethane causes increased protein oxidation and a distinct proteolytic response in cultured hepatocytes. HepG2 cells were treated for 30 min with ethanol, H(2)O(2) and bromotrichloromethane at various nontoxic concentrations. Protein degradation was measured in living cells using [35S]-methionine labeling. Protein oxidation, and 20S proteasome activity were measured in cell lysates. Oxidized proteins increased immediately after ethanol, H(2)O(2), and bromotrichloromethane exposure, but a further significant increase 24-h after exposure was observed only following ethanol and bromotrichloromethane treatment. All three reagents caused a significant increase of the overall intracellular proteolysis at rather low concentrations, which could be suppressed by the proteasome inhibitor lactacystin. A decline of proteolysis observed at higher-subtoxic-concentrations was not related to decreased proteasome activity. Preincubation with ketoconazole or 4-methylpyrazole completely prevented the ethanol- and bromotrichloromethane-induced but not the H(2)O(2)-induced protein oxidation and proteolysis, suggesting strongly an enzyme-mediated generation of reactive oxygen species. In conclusion single-dose exposure with ethanol or haloalkanes causes increased protein oxidation followed by an increased proteasome-dependent protein degradation in human liver cells.

Increased cytotoxicity of carbon tetrachloride in a human hepatoma cell line overexpressing cytochrome P450 2E1

Cytotoxic free radicals generated during the metabolism of carbon tetrachloride by cytochrome P450 2E1 (CYP2E1) are thought to cause hepatotoxicity. Here, the cytotoxic effects of carbon tetrachloride in a liver cell line expressing CYP2E1 (HLE/2E1) are compared with those in the mother cell line (HLE). The effects of carbon tetrachloride on the gene expression of HSP70, a potential marker of oxidative stress, were also examined. The viability of HLE/2E1 cells after exposure to carbon tetrachloride was significantly decreased compared with that of HLE cells. Northern blot analysis revealed that the HSP70 mRNA level was significantly increased after carbon tetrachloride treatment in both cell lines, while the magnitude of its increase was much greater in HLE/2E1 cells than in HLE cells. These results suggest that the oxidative stress induced by CYP2E1 plays an important role in the increase in cytotoxicity of carbon tetrachloride in CYP2E1-overexpressing cells.

Ethanol, oxidative stress, and cytokine-induced liver cell injury

Both clinical findings and results of experiments with animal models of alcoholic hepatitis have shown the importance of cytokine-mediated cell-cell interactions in the onset of ethanol-induced liver damage. Proinflammatory cytokines, such as tumor necrosis factor-alpha (TNF-alpha), interleukin (IL)-1 beta (IL-1 beta), and interleukin-6, are released from Kupffer cells or infiltrating neutrophils and macrophages and elicit defensive responses in parenchymal cells, including activation of apoptosis. Reactive oxygen species (ROS) and reactive nitrogen species (RNS), generated in response to cytokine-induced stress signals in parenchymal cells and also by activation of Kupffer cells and inflammatory cells, further mobilize cellular defense mechanisms. When these defensive responses are overwhelmed cells may die by necrosis, further stimulating inflammatory responses and infiltration of neutrophils. Chronic ethanol intake (i.e., many years of heavy alcohol use in human patients, several weeks or months in experimental animals) enhances the damaging consequences of these events through a variety of mechanisms. The formation of cytokines in the liver is stimulated by increasing circulating levels of endotoxin and by enhancing the responsiveness of Kupffer cells to such stimuli. In addition, ethanol promotes oxidative stress, both by increased formation of ROS and by depletion of oxidative defenses in the cell. Furthermore, liver cells from ethanol-treated animals are more susceptible to the cytotoxic effects of TNF-alpha and other cytokines than cells from control animals. Mitochondria play a critical role in the apoptotic response, and alterations in mitochondrial function after chronic ethanol treatment may contribute to enhanced cell death by apoptosis or necrosis. How the shift in the balance of cytokine-induced defensive and damage responses in hepatocytes contributes to the liver injury that occurs in alcoholic hepatitis remains poorly characterized and should be a rewarding area for future studies.

Relationship between acetaldehyde levels and cell survival in ethanol-metabolizing hepatoma cells

We have created a number of recombinant Hep G2 cell lines, designated VA cells, that constitutively express alcohol dehydrogenase. Oxidation of ethanol by the VA cells results in the production and accumulation of acetaldehyde, and a dramatic increase in the nicotinamide adenine dinucleotide, reduced (NADH)/nicotinamide adenine dinucleotide (NAD(+)) ratio (redox-state). It is believed that production of acetaldehyde, and the increase in the redox-state of hepatocytes, are responsible for many of the dysfunctions associated with alcoholic liver disease. When the VA cells were cultured in the presence of ethanol, we observed a dramatic reduction in cell accumulation. This reduction was more pronounced in cells that metabolized ethanol more efficiently. Inhibition of alcohol dehydrogenase activity abolished this reduction, demonstrating that ethanol oxidation was required for this dysfunction. Subsequent investigations indicated that this ethanol oxidation-mediated reduction in cell accumulation was the result of both cytotoxicity and impaired DNA synthesis. To dissociate the increase in the cellular redox-state from acetaldehyde production, VA cells were cultured in the presence of isopropanol. The oxidation of isopropanol results in similar redox changes, but the metabolic by-product of isopropanol oxidation is acetone. The metabolism of isopropanol by VA cells resulted in very little reduction in cell number. Furthermore, treatment of ethanol-metabolizing VA cells with the aldehyde dehydrogenase inhibitor, cyanamide, increased the levels of acetaldehyde and resulted in an additional reduction in cell number. In conclusion, these studies indicated that exposure to acetaldehyde caused cytotoxicity, as well as the ethanol oxidation-mediated reduction in cell number.

Acetaldehyde activates Jun/AP-1 expression and DNA binding activity in human oral keratinocytes

Oral cancer is a significant health problem, particularly among individuals that ingest alcohol in combination with the use of tobacco products. The enhanced development of tobacco-initiated oral cancers by ethanol suggests that ethanol or one of its metabolites may act as a type of tumor promoter. Nevertheless, the mechanisms underlying the ability of ethanol to enhance oral carcinogenesis remain unclear. We hypothesize that acetaldehyde, the first metabolite of ethanol, may activate the expression and/or activity of Jun/AP-1 in oral keratinocytes analogous to the phorbol ester TPA and other tumor promoters in epidermal keratinocytes. To test this hypothesis, we treated HPV immortalized, non-tumorigenic human oral keratinocytes with acetaldehyde at various concentrations and for various times and measured several parameters of Jun/AP-1expression and function. Our results indicated that c-Jun mRNA and protein levels increased in the acetaldehyde treated cells compared to untreated control cells. Moreover, Jun/AP-1 DNA binding activity was rapidly activated by acetaldehyde in a dose-dependent fashion. The increases in Jun protein and AP-1 DNA binding activity were accompanied by increased transactivation of an AP-1 responsive reporter construct as well as increased transcript levels of a candidate AP-1 responsive gene, stromelysin 3. The levels of acetaldehyde employed were minimally toxic to the cells as determined by MTT assays. Thus, acetaldehyde was found to activate the expression and activity of an oncogenic transcription factor in HPV-initiated cells. Taken together, these results suggest that acetaldehyde may participate, at least in part, in the promotion stage of oral carcinogenesis.

Adenovirus-mediated overexpression of catalase in the cytosolic or mitochondrial compartment protects against toxicity caused by glutathione depletion in HepG2 cells expressing CYP2E1

Induction of cytochrome P450 CYP2E1 by ethanol appears to be one of the mechanisms by which ethanol creates a state of oxidative stress. Glutathione (GSH) is a key cellular antioxidant that detoxifies reactive oxygen species. Depletion of GSH, especially mitochondrial GSH, is believed to play a role in the ethanol-induced liver injury. Previous results reported that depletion of GSH by buthionine-(S,R)-sulfoximine (BSO) treatment caused apoptosis and necrosis in HepG2 cells, which overexpress CYP2E1. In the current work, adenoviral infection with vectors that resulted in expression of catalase either in the cytosol or mitochondrial compartments was able to abolish the loss of mitochondrial membrane potential or damage to mitochondria observed in HepG2 cells overexpressing CYP2E1 that were treated with BSO. Loss of cell viability, either necrotic or apoptotic, was also prevented by the catalase overexpression after infection with the adenoviral vectors. The protective effects of catalase were associated with the suppression of the increase in the production of reactive oxygen species and of mitochondrial lipid peroxidation observed after GSH depletion. These results reveal a prominent role for H(2)O(2) as a mediator in the cytotoxicity observed after depletion of GSH in HepG2 cells overexpressing CYP2E1. Damage to mitochondria may be a critical step for cellular toxicity by CYP2E1-derived reactive oxygen species.

The paradoxical effect of acetaminophen on CYP3A4 activity and content in transfected HepG2 cells

HepG2 cell lines that constitutively and stably express human CYP3A4 were constructed in order to study enzyme interactions with CYP3A4 as the only P450 present. CYP3A4 activity and content were assessed by the metabolism of fentanyl, a CYP3A substrate, and Western blots. Northern blots were used to examine the effects of acetaminophen (APAP) on CYP3A4-mRNA. The HepG2 cell lines' CYP3A4 activity was stable over time. High concentrations of APAP inhibited CYP3A4 activity. At lower concentrations, APAP produced a dose-dependent increase in CYP3A4 activity and content. No increases in CYP3A4-mRNA were seen. Incubation with cycloheximide caused a decrease in fentanyl metabolism secondary to a decrease in P450 levels that was prevented by the coincubation with APAP. Additionally, human microsomal CYP3A4 was stabilized by APAP against cytosol-mediated degradation. In our models, APAP appears to increase CYP3A4 activity. This increase appears to be via substrate stabilization. This is the first report that APAP can increase CYP3A4 activity and content in transfected HepG2 cells.

Role of calcium and calcium-activated proteases in CYP2E1-dependent toxicity in HEPG2 cells

The objective of this work was to investigate whether CYP2E1- and oxidative stress-dependent toxicity in HepG2 cells is mediated by an increase of cytosolic Ca2+ and activation of Ca2+-modulated processes. HepG2 cells expressing CYP2E1 (E47 cells) or control cells not expressing CYP2E1 (C34 cells) were preloaded with arachidonic acid (AA, up to 10 microm) and, after washing, incubated with iron-nitrilotriacetic acid (up to 100 microm) for variable periods (up to 12 h). Toxicity was greater in E47 cells than in C34 cells at all times and combinations of iron/AA tested. Cytosolic calcium increased with incubation time in both cell lines, but the increase was higher in E47 cells than in C34 cells. The rise in calcium was an early event and preceded the developing toxicity. Toxicity in E47 cells and the increase in Ca2+ were inhibited by omission of Ca2+ from the extracellular medium, and toxicity was restored by reincorporation of Ca2+. An inhibitor of Ca2+ release from intracellular stores did not prevent the toxicity or the increase in Ca2+, reflecting a role for the influx of extracellular Ca2+ in the toxicity. Reactive oxygen production was similar in media with or without calcium, indicating that calcium was not modulating CYP2E1-dependent oxidative stress. Toxicity, lipid peroxidation, and the increase of Ca2+ in E47 cells exposed to iron-AA were inhibited by alpha-tocopherol. E47 cells (but not C34 cells) exposed to iron-AA showed increased calpain activity in situ (40-fold). The toxicity in E47 cells mirrored calpain activation and was inhibited by calpeptin, suggesting that calpain activation plays a causal role in toxicity. These results suggest that CYP2E1-dependent toxicity in this model depends on the activation of lipid peroxidation, followed by an increased influx of extracellular Ca2+ and activation of Ca2+-dependent proteases.

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.

S-Adenosylmethionine: a control switch that regulates liver function

Genome sequence analysis reveals that all organisms synthesize S-adenosylmethionine (AdoMet) and that a large fraction of all genes is AdoMet-dependent methyltransferases. AdoMet-dependent methylation has been shown to be central to many biological processes. Up to 85% of all methylation reactions and as much as 48% of methionine metabolism occur in the liver, which indicates the crucial importance of this organ in the regulation of blood methionine. Of the two mammalian genes (MAT1A, MAT2A) that encode methionine adenosyltransferase (MAT, the enzyme that makes AdoMet), MAT1A is specifically expressed in adult liver. It now appears that growth factors, cytokines, and hormones regulate liver MAT mRNA levels and enzyme activity and that AdoMet should not be viewed only as an intermediate metabolite in methionine catabolism, but also as an intracellular control switch that regulates essential hepatic functions such as regeneration, differentiation, and the sensitivity of this organ to injury. The aim of this review is to integrate these recent findings linking AdoMet with liver growth, differentiation, and injury into a comprehensive model. With the availability of AdoMet as a nutritional supplement and evidence of its beneficial role in various liver diseases, this review offers insight into its mechanism of action.

A Genetically engineered cell-based system for detecting metabolism-mediated toxicity

Xenobiotics undergoing bioactivation by CYP450 enzymes form reactive metabolites that may exert direct metabolism-mediated toxicity. An in vitro model was developed to study the direct toxic effects that follow the metabolic activation of chemicals. The model uses monolayer cultures of genetically engineered NIH-3T3 or V79 cells that express individual human or rat CYP450 isoforms, respectively. Following exposure to 1,3-dichloropropanol or cyclophosphamide, basal cytotoxicity endpoints, including neutral red uptake and Alamar Blue( reduction were used to assess changes in cell number and functional viability resulting from the formation of metabolites. Cell lines that express cytochrome P450 enzymes metabolised the test compounds, leading to increased toxicity compared with that observed in the control cell line. The use of specific inhibitors confirmed that the formation of reactive metabolites was CYP450-isoform dependent. These results indicate that a panel of genetically engineered cell lines expressing various cytochrome P450 enzyme isoforms can be used to reveal measurable metabolising capabilities, and could become a useful tool for the detection and possible determination of CYP450 isoforms in human liver metabolism-mediated toxicity.

Ethanol-induced apoptosis in hepatoma cells proceeds via intracellular Ca(2+) elevation, activation of TLCK-sensitive proteases, and cytochrome c release

Ethanol is known to induce apoptosis in hepatocytes. However, intracellular signaling events of ethanol-induced death are still only partially understood. We studied such processes in ethanol-induced apoptosis in HepG2 cells as a model system for human liver cells. We determined the incidence of apoptosis by DNA fragmentation and tested the effects of various known inhibitors. Ethanol induces apoptosis in HepG2 cells in a dose- and time-dependent manner as well as in rat primary hepatocytes. This effect was not mediated through the death receptor CD95 and the tumor necrosis factor receptors. It was efficiently inhibited by the caspase inhibitor N-benzyloxycarbonyl-Asp-Glu-Val-Asp-fluoromethylketone (zVAD-fmk), the Ca(2+) chelator EGTA, and the serine protease inhibitor N-p-tosyl-l-lysine chloromethyl ketone (TLCK). Upon ethanol treatment, the intracellular calcium ion concentration was increased and cytochrome c was released from the mitochondria, and caspases were activated. EGTA and TLCK could inhibit cytochrome c release from the mitochondria. Furthermore, overexpression of Bcl-x(L) saved cells from ethanol-induced apoptosis. These data suggest that ethanol-induced apoptosis in liver cells is initiated by the intracellular Ca(2+) elevation in the cytoplasm and activation of TLCK-sensitive serine proteases. Our data provide new insight into ethanol-induced apoptosis in liver cells and may lead to therapeutic strategies to prevent liver damage.

Alcoholic liver disease and apoptosis

This article represents the proceedings of a symposium at the 2000 ISBRA Meeting in Yokohama, Japan. The chairs were Carol A. Casey and Amin Nanji. The presentations were (1) Mechanisms of apoptosis in alcoholic liver disease, by Amin A. Nanji; (2) Impaired receptor-mediated endocytosis: Its role in alcoholic apoptosis, by Carol A. Casey; (3) Toxicity of ethanol in HepG2 cells that express CYP2E1, by Arthur I. Cederbaum; (4) Mitochondrial regulation of ethanol-induced hepatocyte apoptosis, by M. Adachi; and (5) Apoptosis in alcoholic hepatitis, by T. Takahashi.

How is the liver primed or sensitized for alcoholic liver disease?

This article represents the proceedings of a symposium at the 2000 ISBRA Meeting in Yokohama, Japan. The chairs were Hidekazu Tsukamoto and Yoshiyuki Takei. The presentations were (1) Tribute to Professor Rajendar K. Chawla, by Craig J. McClain; (2) Dysregulated TNF signaling in alcoholic liver disease, by Craig J. McClain, S. Joshi-Barve, D. Hill, J Schmidt, I. Deaciuc, and S. Barve; (3) The role of mitochondria in ethanol-mediated sensitization of the liver, by Anna Colell, Carmen Garcia-Ruiz, Neil Kaplowitz, and Jose C. Fernandez-Checa; (4) A peroxisome proliferator (bezafibrate) can prevent superoxide anion release into hepatic sinusoid after acute ethanol administration, by Hirokazu Yokoyama, Yukishige Okamura, Yuji Nakamura, and Hiromasa Ishii; (5) S-adenosylmethionine affects tumor necrosis factor-alpha gene expression in macrophages, by Rajendar K. Chawla, S. Barve, S. Joshi-Barve, W. Watson, W. Nelson, and C. McClain; (6) Iron, retinoic acid and hepatic macrophage TNFalpha gene expression in ALD, by Hidekazu Tsukamoto, Min Lin, Mitsuru Ohata, and Kenta Motomura; and (7) Role of Kupffer cells and gut-derived endotoxin in alcoholic liver injury, by N. Enomoto, K. Ikejima, T. Kitamura, H. Oide, Y. Takei, M. Hirose, B. U. Bradford, C. A. Rivera, H. Kono, S. Peter, S. Yamashina, A. Konno, M. Ishikawa, H. Shimizu, N. Sato, and R. Thurman.

Use of cultured cells in assessing ethanol toxicity and ethanol-related metabolism

This article represents the proceedings of a symposium at the 2000 ISBRA Meeting in Yokohama, Japan. The chairs were Terrence M. Donohue, Jr, and Dahn L. Clemens. The presentations were (1) Characterization of single and double recombinant hepatoma cells that express ethanol-metabolizing enzymes, by Terrence M. Donohue, Jr; (2) Inhibition of cell growth by ethanol metabolism, by Dahn L. Clemens; (3) Use of transfected HeLa cells to study the genesis of alcoholic fatty liver, by Andrea Galli and David Crabb; (4) CYP2E1-mediated oxidative stress induces COL1A2 mRNA in hepatic stellate cells and in a coculture system of HepG2 and stellate cells, by Natalia Nieto; (5) Transforming growth factor-alpha secreted from ethanol-exposed hepatocytes contributes to development of alcoholic hepatic fibrosis, by Junji Kato; and (6) Effect of ethanol on Fas-dependent caspase-3 activation and apoptosis in CD4+ T cells, by Shirish S. Barve.

Establishment of the transformants expressing human cytochrome P450 subtypes in HepG2, and their applications on drug metabolism and toxicology

Transformants with stable expression of a series of human cytochrome P450 (CYP) subtypes in the human hepatic cell line, HepG2, were established. These transformants are designated Hepc/1A1.4, Hepc/1A2.9, Hepc/2A6L.14, Hepc/2B6.68, Hepc/2C8.46, Hepc/2C9.1, Hepc/2C19.12, Hepc/2D6.39, Hepc/2E1.3-8 and Hepc/3A4.2-30, which stably expressed human CYP1A1, CYP1A2, CYP2A6, CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP2E1 and CYP3A4, respectively. The expression of the CYP subtypes in the transformants was confirmed by both determination of enzyme activities and the reverse transcriptase polymerase chain reaction (RT-PCR) procedure. The apparent K(m) values of the expressed CYP subtypes for their specific substrates were close to those of human liver microsomes. In addition to their CYP activities, these transformants retained glucuronide- and sulfate-conjugating activities. Furthermore, the activities of CYP2C9, CYP2D6 and CYP3A4 were inhibited by their specific inhibitors. The cytotoxicity of acetaminophen (APAP), cyclophosphamide (CPA) and benz[a]anthracene (BA) were analyzed by CYP-expressing transformants. The cytotoxicity depended on the expression of CYP subtypes and increased in a dose-dependent manner. These results show the metabolic activation of APAP, CPA and BA by the specific CYP subtypes expressed in the transformants and demonstrate the usefulness of these transformants for in vitro metabolic and toxicological studies in human liver.

Apoptosis and mitochondrial damage in INS-1 cells treated with alloxan

To evaluate the participation of mitochondrial damage, oxygen radicals and cell death in diabetes mellitus, we designed a way to investigate INS-1 cells, rat pancreatic beta-cell line, to die by treatment with alloxan which generate reactive oxygen species (ROS). Incubation of INS-1 cells with alloxan for 24 h resulted in a decrease in viability of cells as well as inhibition of glucose-stimulated insulin release; this could be prevented by antioxidants, vitamin E and butylated hydroxyanisol (BHA). The formation of a DNA ladder and the distribution of phosphatidylserine at the external surface of plasma membrane were observed as indicators of apoptosis in the cells treated with alloxan at concentrations below 0.5 mM. The formation of DNA ladder was prevented by vitamin E, BHA and catalase, suggesting that the ROS is involved in the process of apoptosis in INS-1 cells treated with alloxan. Lower levels of intracellular ATP, collapse of mitochondrial membrane potential and release of cytochrome c from mitochondria were also observed in INS-1 cells treated with alloxan, suggesting that alloxan caused the damage of mitochondria in cells and was related to the process of apoptosis. In contrast, rat liver RLC-18 cells treated with alloxan were not observed in the decrease of viability. It follows from the present study that mitochondrial damages by ROS generated from alloxan is linked to apoptosis in INS-1 cells.

Current concepts in the pathogenesis of alcoholic liver injury

Alcoholic liver disease (ALD) develops as a consequence of priming and sensitizing mechanisms rendered by cross-interactions of primary mechanistic factors and secondary risk factors. This concept, albeit not novel, is becoming widely accepted by the field, and more research is directed toward identifying and characterizing the interfaces of the cross-interactions to help understand individual predisposition to the disease. Another pivotal development is the beginning of cell type-specific research to elucidate specific contributions not only of hepatocytes, but also of hepatic macrophages, liver-associated lymphocytes, sinusoidal endothelial cells, and hepatic stellate cells to sensitizing and priming mechanisms. In particular, the critical role of hepatic macrophages has been highlighted and the priming mechanisms concerning this paracrine effect have been proposed. Glutathione depletion in hepatocyte mitochondria is considered the most important sensitizing mechanism. One of the contributing factors is decreased methionine metabolism. Remaining key questions include how altered methionine metabolism contribute to the pathogenesis of ALD; how cross-talk among nonparenchymal liver cells or between nonparenchymal cells and hepatocytes leads to ALD; how dysfunctional mitochondria determine the type of cell death in ALD; and what secondary factors are critical for the development of advanced ALD such as alcoholic hepatitis and cirrhosis.

Use of cultured cells in assessing ethanol toxicity and ethanol-related metabolism

This article represents the proceedings of a symposium at the 2000 ISBRA Meeting in Yokohama, Japan. The chairs were Terrence M. Donohue, Jr, and Dahn L. Clemens. The presentations were (1) Characterization of single and double recombinant hepatoma cells that express ethanol-metabolizing enzymes, by Terrence M. Donohue, Jr; (2) Inhibition of cell growth by ethanol metabolism, by Dahn L. Clemens; (3) Use of transfected HeLa cells to study the genesis of alcoholic fatty liver, by Andrea Galli and David Crabb; (4) CYP2E1-mediated oxidative stress induces COL1A2 mRNA in hepatic stellate cells and in a coculture system of HepG2 and stellate cells, by Natalia Nieto; (5) Transforming growth factor-alpha secreted from ethanol-exposed hepatocytes contributes to development of alcoholic hepatic fibrosis, by Junji Kato; and (6) Effect of ethanol on Fas-dependent caspase-3 activation and apoptosis in CD4+ T cells, by Shirish S. Barve.

CYP2E1-mediated modulation of valproic acid-induced hepatocytotoxicity

OBJECTIVES

To determine the cytotoxicity of valproic acid (VPA) and its metabolite, 4-ene-valproic acid (4-ene-VPA) in human hepatoblastoma cells (Hep G2), and to study the modulatory effect of cytochrome P450 2E1 induction in this model.

METHODS

Cells were exposed to VPA or 4-ene-VPA in the presence of either ethanol (EtOH), or EtOH combined with disulphiram (DS). Some cells were exposed to alpha-fluoro-VPA or to alpha-fluoro-4-ene-VPA in the absence of CYP2E1 inducers. Apoptosis and necrosis were measured by analyzing 6000 cells per sample using transmission electron microscopy, while cytokine release and apoptosis were quantitated by ELISA.

RESULTS

VPA + EtOH increased VPA cytotoxicity. 4-ene-VPA + EtOH significantly increased toxicity, while DS + EtOH significantly reduced this toxicity. Alpha-fluorinated analogues reduced cytotoxicity compared to the corresponding VPA compounds. Neither VPA nor alpha-fluorinated VPA increased the release of IL-6 or TNF-alpha in media. A significant increase in the release of TNF-alpha was observed in cells exposed to 4-ene-VPA that further increased with EtOH exposure.

CONCLUSIONS

Cells exposed to 4-ene-VPA experience greater cytotoxicity than those treated with VPA. Cytochrome P450 2E1 inducers enhance toxicity in VPA-exposed cells, while alpha-fluorination of VPA diminishes cytotoxicity by directly interfering with the beta-oxidation of the 4-ene-VPA metabolite.

How is the liver primed or sensitized for alcoholic liver disease?

This article represents the proceedings of a symposium at the 2000 ISBRA Meeting in Yokohama, Japan. The chairs were Hidekazu Tsukamoto and Yoshiyuki Takei. The presentations were (1) Tribute to Professor Rajendar K. Chawla, by Craig J. McClain; (2) Dysregulated TNF signaling in alcoholic liver disease, by Craig J. McClain, S. Joshi-Barve, D. Hill, J Schmidt, I. Deaciuc, and S. Barve; (3) The role of mitochondria in ethanol-mediated sensitization of the liver, by Anna Colell, Carmen Garcia-Ruiz, Neil Kaplowitz, and Jose C. Fernandez-Checa; (4) A peroxisome proliferator (bezafibrate) can prevent superoxide anion release into hepatic sinusoid after acute ethanol administration, by Hirokazu Yokoyama, Yukishige Okamura, Yuji Nakamura, and Hiromasa Ishii; (5) S-adenosylmethionine affects tumor necrosis factor-alpha gene expression in macrophages, by Rajendar K. Chawla, S. Barve, S. Joshi-Barve, W. Watson, W. Nelson, and C. McClain; (6) Iron, retinoic acid and hepatic macrophage TNFalpha gene expression in ALD, by Hidekazu Tsukamoto, Min Lin, Mitsuru Ohata, and Kenta Motomura; and (7) Role of Kupffer cells and gut-derived endotoxin in alcoholic liver injury, by N. Enomoto, K. Ikejima, T. Kitamura, H. Oide, Y. Takei, M. Hirose, B. U. Bradford, C. A. Rivera, H. Kono, S. Peter, S. Yamashina, A. Konno, M. Ishikawa, H. Shimizu, N. Sato, and R. Thurman.

Alcoholic liver disease and apoptosis

This article represents the proceedings of a symposium at the 2000 ISBRA Meeting in Yokohama, Japan. The chairs were Carol A. Casey and Amin Nanji. The presentations were (1) Mechanisms of apoptosis in alcoholic liver disease, by Amin A. Nanji; (2) Impaired receptor-mediated endocytosis: Its role in alcoholic apoptosis, by Carol A. Casey; (3) Toxicity of ethanol in HepG2 cells that express CYP2E1, by Arthur I. Cederbaum; (4) Mitochondrial regulation of ethanol-induced hepatocyte apoptosis, by M. Adachi; and (5) Apoptosis in alcoholic hepatitis, by T. Takahashi.

Identification of a possible association between carbon tetrachloride-induced hepatotoxicity and interleukin-8 expression

Hepatotoxicants can elicit liver damage by various mechanisms that can result in cell necrosis and death. The changes induced by these compounds can vary from gross alterations in DNA repair mechanisms, protein synthesis, and apoptosis, to more discrete changes in oxidative damage and lipid peroxidation. However, little is known of the changes in gene expression that are fundamental to the mechanisms of hepatotoxicity. We have used DNA microarray technology to identify gene transcription associated with the toxicity caused by the hepatotoxicant carbon tetrachloride. Labeled poly A+ RNA from cultured human hepatoma cells (HepG2) exposed to carbon tetrachloride for 8 hours was hybridized to a human microarray filter. We found that 47 different genes were either upregulated or downregulated more than 2-fold by the hepatotoxicant compared with dimethyl formamide, a chemical that does not cause liver cell damage. The proinflammatory cytokine interleukin-8 (IL-8) was upregulated over 7-fold compared with control on the array, and this was subsequently confirmed at 1 hour and 8 hours by Northern blot analyses. We also found that carbon tetrachloride caused a time-dependent increase in interleukin-8 protein release in HepG2 cells, which was paralleled by a decrease in cell viability. These data demonstrate that carbon tetrachloride causes a rapid increase in IL-8 mRNA expression in HepG2 cells and that this increase correlates with a later and significant increase in the levels of interleukin-8 protein. These results illustrate the potential of microarray technology in the identification of novel gene changes associated with toxic processes.

Adenovirus-mediated overexpression of catalase in the cytosolic or mitochondrial compartment protects against cytochrome P450 2E1-dependent toxicity in HepG2 cells

Cytochrome P450 2E1 (CYP2E1) is an effective producer of reactive oxygen species such as superoxide radical and hydrogen peroxide, which may contribute to the development of alcohol liver disease or cytotoxicity. To investigate the protective role of catalase against CYP2E1-dependent cytotoxicity, E47 cells, a transfected HepG2 cell line overexpressing CYP2E1, were infected with adenoviral vectors containing human catalase cDNA (AdCat) and catalase cDNA with a mitochondrial leader sequence (AdmCat). Forty-eight hours after infection with AdCat or AdmCat at a multiplicity of infection of 100, intracellular catalase protein was increased >2-fold compared with uninfected E47 cells and E47 cells infected with empty adenoviral vector (AdNull) as determined by Western blotting and catalase activity measurements. Overexpression of catalase in the cytosol (AdCat) and in mitochondria (AdmCat) was confirmed by confocal microscopy. Cell death caused by arachidonic acid plus iron was considerably suppressed in both AdCat- and AdmCat-infected E47 cells as determined by assays of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide absorbance, lactate dehydrogenase release, and morphology changes. AdCat- and AdmCat-infected cells were also more resistant to the loss of mitochondrial membrane potential and to the increase in lipid peroxidation induced by arachidonic acid and iron. This study indicates that catalase in the cytosol and catalase in mitochondria are capable of protecting HepG2 cells expressing CYP2E1 against cytotoxicity induced by oxidants that promote lipid peroxidation and suggests the possibility that such agents may be useful in protecting against the development of alcohol liver injury.

CYP2E1-dependent toxicity and up-regulation of antioxidant genes

Induction of cytochrome P450 2E1 (CYP2E1) by ethanol appears to be one of the central pathways by which ethanol generates a state of oxidative stress. Glutathione (GSH) is critical in preserving the proper cellular redox balance and for its role as a cellular protectant. The goal of the present study was to characterize the GSH homeostasis in human hepatocarcinoma cells (HepG2-E47 cells) that overexpress CYP2E1. Toxicity in the E47 cells was markedly enhanced after GSH depletion by buthionine sulfoximine (BSO) treatment. The antioxidant trolox partially prevented the apoptosis and necrosis, while diallylsulfide, a CYP2E1 inhibitor, was fully protective. Damage to mitochondria appears to play a role in the CYP2E1- and BSO-dependent toxicity. CYP2E1-overexpressing cells showed increases in total GSH levels, GSH synthetic rate and in gamma-glutamylcysteine synthetase (GCS) mRNA. This GCS increase was due to transcriptional activation of the GCS gene and could be blocked by certain antioxidants. Activity, protein and mRNA levels for other antioxidants such as catalase, alpha- and microsomal glutathione transferases were also increased in the E47 cells. Up-regulation of these antioxidant genes may reflect an adaptive mechanism to remove CYP2E1-derived oxidants. These oxidants are diffusable and were able to elevate collagen type I protein in a co-culture system consisting of the E47 cells + rat hepatic stellate cells. Such interactions between CYP2E1, mitochondria and altered GSH homeostasis, and elevation of collagen levels, may play a role in alcohol-induced liver injury.

Ethanol-mediated CYP1A1/2 induction in rat skeletal muscle tissue

The causes of non-trauma-mediated rhabdomyolysis are not well understood. It has been speculated that ethanol-associated rhabdomyolysis may be attributed to ethanol induction of skeletal muscle cytochrome P450(s), causing drugs such as acetaminophen or cocaine to be metabolized to myotoxic compounds. To examine this possibility, the hypothesis that feeding ethanol induces cytochrome P450 in skeletal muscle was tested. To this end, rats were fed an ethanol-containing diet and skeletal muscle tissue was assessed for induction of CYP2E1 and CYP1A1/2 by immunohistochemical procedures; liver was examined as a positive control tissue. Enzymatic assays and Western blot analyses were also performed on these tissues. In one feeding system, ethanol-containing diets induced CYP1A1/2 in soleus, plantaris, and diaphragm muscles, with immunohistochemical staining predominantly localized to capillaries surrounding myofibers. Antibodies to CYP2E1 did not react with skeletal muscle tissue from animals receiving a control or ethanol-containing diet. However, neither skeletal muscle CYP1A1/2 nor CYP2E1 was induced when ethanol diets were administered by a different feeding system. Ethanol consumption can induce some cytochrome P450 isoforms in skeletal muscle tissue; however, the mechanism of CYP induction is apparently complex and appears to involve factors in addition to ethanol, per se.

Establishment of a human hepatoma cell line, HLE/2E1, suitable for detection of p450 2E1-related cytotoxicity

By transfection of an expression vector of human cytochrome P450 2E1 (CYP2E1) into a human hepatoma cell line (HLE), a new cell line (HLE/2E1) that stably expresses activity of CYP2E1 has been established. The HLE/2E1 cell line expressed a higher level of CYP2E1 messenger ribonucleic acid than did the mother HLE cell line. CYP2E1 enzyme activity determined by a p-nitrophenol oxidation assay was also higher in HLE/2E1 cells than in HLE cells. In addition, the enzyme activity of the HLE/2E1 cells was increased by ethanol treatment. Exposure to acetaminophen (APAP) or buthionine sulfoximine (BSO) caused a greater decrease in viability of the HLE/2E1 cells than that of the HLE cells, as determined by the 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide assay. The cytotoxicity of APAP or BSO to HLE/2EI cells was inhibited by the addition of ethanol or vitamin E. However, the cytotoxicity of both APAP and BSO was enhanced by 24-h preincubation of HLE/2E1 cells with ethanol. These results show that this cell line provides a useful model for studying catalytic properties of CYP2E1 and cytotoxic mechanisms of chemicals metabolized by CYP2E1.

Differential in vivo effects of alpha-naphthoflavone and beta-naphthoflavone on CYP1A1 and CYP2E1 in rat liver, lung, heart, and kidney

Male Sprague-Dawley rats were treated intraperitoneally with corn oil, the aryl hydrocarbon receptor (AHR) agonist beta-naphthoflavone (betaNF), or the relatively weak AHR agonist alpha-naphthoflavone (alphaNF). Animals treated with betaNF experienced a significant loss (12%) of total body mass over 5 days and a dramatic elevation of CYP1A1 mRNA in all of the organs studied. Treatment with alphaNF had no significant effect on body mass after 5 days and caused only minor increases of liver, kidney, and heart CYP1A1 mRNA. In contrast, lung CYP1A1 mRNA was increased by alphaNF treatment to levels comparable to that seen with betaNF treatment. CYP2E1 mRNA levels were also elevated in liver, lung, kidney, and heart in response to betaNF treatment, whereas alphaNF was without effect. Large increases of CYP1Al-dependent 7-ethoxyresorufin O-deethylation (EROD) activity occurred with microsomes prepared from the tissues of betaNF-treated animals. Comparatively small changes were associated with alphaNF treatment, with the exception of lung, where EROD activity was increased to approximately 60% of that with betaNF treatment. CYP2E1-dependent p-nitrophenol hydroxylase (PNP) activity was also increased by betaNF treatment in microsomes prepared from kidney (3.1-fold), whereas alphaNF was without effect. In contrast, alphaNF or betaNF treatment caused significant decreases of lung microsomal PNP (72% and 27% of corn oil control, respectively) and 7-pentoxyresorufin O-deethylation (48% and 17% of corn oil control, respectively) activities, indicating that PNP activity may be catalyzed by P450 isoforms other than CYP2E1 in rat lung. We conclude that betaNF and alphaNF have differential effects on the expression and catalytic activity of CYP1A1 and CYP2E1, depending upon the organ studied. These changes most likely occur as a result of the direct actions of these compounds as AHR agonists, in addition to secondary effects associated with AHR-mediated toxicity.

The alcohol-inducible form of cytochrome P450 (CYP 2E1): role in toxicology and regulation of expression

Cytochrome P450 (CYP) 2E1 catalyzes the metabolism of a wide variety of therapeutic agents, procarcinogens, and low molecular weight solvents. CYP2E1-catalyzed metabolism may cause toxicity or DNA damage through the production of toxic metabolites, oxygen radicals, and lipid peroxidation. CYP2E1 also plays a role in the metabolism of endogenous compounds including fatty acids and ketone bodies. The regulation of CYP2E1 expression is complex, and involves transcriptional, post-transcriptional, translational, and post-translational mechanisms. CYP2E1 is transcriptionally activated in the first few hours after birth. Xenobiotic inducers elevate CYP2E1 protein levels through both increased translational efficiency and stabilization of the protein from degradation, which appears to occur primarily through ubiquitination and proteasomal degradation. CYP2E1 mRNA and protein levels are altered in response to pathophysiologic conditions by hormones including insulin, glucagon, growth hormone, and leptin, and growth factors including epidermal growth factor and hepatocyte growth factor, providing evidence that CYP2E1 expression is under tight homeostatic control.

Enhanced DNA binding and activation of transcription factors NF-kappa B and AP-1 by acetaldehyde in HEPG2 cells

Because transcription factors NF-kappaB and activator protein-1 (AP-1) are known to regulate gene expression, we have analyzed the role of acetaldehyde in the activation of NF-kappaB and AP-1 in HepG2 cells. Binding activity and transactivation of NF-kappaB and AP-1 were determined by gel retardation assays and transfection of a luciferase reporter construct controlled by kappaB and AP-1 binding sites, respectively. Acetaldehyde enhanced the DNA binding of NF-kappaB and AP-1 by 1 and 4 h, respectively, increasing the kappaB- and AP-1-dependent luciferase expression. Supershift assays revealed the presence of NF-kappaB heterodimers p65/p50 and p50/p52, whereas nuclear c-Jun levels correlated with the DNA binding of AP-1. The enhanced binding of NF-kappaB to DNA by acetaldehyde in intact cells was accompanied by the proteolytic degradation of IkappaB-alpha. However, the addition of acetaldehyde to cytostolic extracts from untreated Hep G2 cells did not affect the DNA binding of AP-1 but activated the NF-kappaB heterodimer p65/p50 in the absence of IkappaB-alpha degradation. Preincubation of HepG2 cells with protein kinase C inhibitors abolished the enhanced DNA binding of NF-kappaB and AP-1 caused by acetaldehyde. Hence, these findings uncover a previously unrecognized role for acetaldehyde in the activation of NF-kappaB and AP-1, which may be of relevance in the alcohol-induced liver disease.

Ethanol potentiates tumor necrosis factor-alpha cytotoxicity in hepatoma cells and primary rat hepatocytes by promoting induction of the mitochondrial permeability transition

In the present study, tumor necrosis factor-alpha (TNF-alpha) cytotoxicity is shown to be potentiated by ethanol exposure in vitro in the human hepatoma cell line, HepG2, and in rat primary hepatocytes. Exposure of HepG2 cells and primary hepatocytes for 48 hours to concentrations of ethanol ranging between 50 and 100 mmol/L significantly increased TNF-alpha cytotoxicity compared with cells treated with TNF-alpha alone. The cell killing was associated with, and dependent on, the development of the mitochondrial permeability transition (MPT). Two inhibitors of MPT pore opening, cyclosporin A and bongkrekic acid, prevented TNF-alpha cytotoxicity in the presence of ethanol. In addition to inhibiting cell death caused by TNF-alpha, blockade of MPT pore opening prevented mitochondrial depolarization, cytochrome c redistribution from the mitochondria to the cytosol, caspase 3 activation, and oligonucleosomal DNA fragmentation. Unlike the potentiation of TNF-alpha cytotoxicity by the translational inhibitor cycloheximide, ethanol promoted TNF-alpha-induced cell killing by a mechanism that was independent of caspase-8 activity. HepG2 cells overexpressing cytochrome-P4502E1 were even more sensitized by ethanol to induction of the MPT by TNF-alpha and the resultant cytotoxicity than wild-type HepG2 cells. In addition, primary hepatocytes isolated from chronically ethanol-fed rats showed enhanced susceptibility to TNF-alpha cytotoxicity compared with their isocalorically matched controls. Again as with the HepG2 cells, inhibiting MPT pore opening prevented the cytotoxicity of TNF-alpha in the primary hepatocytes isolated from ethanol-fed animals.

Mitochondrial permeability transition induced by 1-hydroxyethyl radical

Impairment of mitochondrial functions has been found in ethanol-induced liver injury. Ethanol can be oxidized to the 1-hydroxyethyl radical (HER) by rat liver microsomal systems. Experiments were carried out to evaluate the ability of HER to cause mitochondrial swelling as an indicator of the mitochondrial permeability transition (MPT). Electron spin resonance (ESR) spectroscopy was used to detect HER and to study its interaction with mitochondria. The ESR signal intensity of the spin adduct formed from alpha-(4-pyridyl-1-oxide) N-tert-butylnitrone (POBN) and HER generated from either a thermic decomposition of 1,1'-dihydroxyazoethane (DHAE) or a Fenton reaction system containing ethanol was markedly diminished by the addition of mitochondria, indicating an interaction between HER and mitochondria. Exposure of rat liver mitochondria to HER generated from either system caused swelling, as reflected by a decrease in absorbance at 540 nm, in a HER concentration-dependent and a cyclosporin A-sensitive manner. Mitochondrial swelling was also induced in the Fenton reaction system without ethanol. The DHAE-dependent generation of HER in mitochondrial suspension resulted in a decrease of membrane protein thiols and collapse of the membrane potential (delta psi). The swelling induced by HER was prevented by glutathione and vitamin E, but not by superoxide dismutase. Catalase did not prevent the swelling caused by the acetaldehyde/hydroxylamine O-sulfonate (HOS) system, but was inhibitory in the Fenton reaction system with or without ethanol. These results indicate that HER, as well as hydroxyl radical, can induce the MPT, and suggest the possibility that the collapse of delta psi caused by HER may, at least in part, contribute to impairment of mitochondrial function caused by ethanol and in ethanol-induced liver injury.

Ethanol and arachidonic acid produce toxicity in hepatocytes from pyrazole-treated rats with high levels of CYP2E1

Ethanol and polyunsaturated fatty acids such as arachidonic acid were shown to be toxic and cause apoptosis in HepG2 cells which express CYP2E1 but not in control HepG2 cell lines. The goal of the current study was to extend the observations made with the HepG2 cells to non-transformed, intact hepatocytes. Rats were treated with pyrazole to increase CYP2E1 levels, hepatocytes were isolated and placed into culture and treated for varying time points with ethanol or arachidonic acid. Comparisons were made to hepatocytes from saline-treated rats, with low CYP2E1 content. Incubation with ethanol (100 mM) or especially arachidonic acid (60 microM) resulted in loss of viability of hepatocytes from the pyrazole-treated rats, without any effect on the hepatocytes from the saline-treated rats. The toxicity appeared to be apoptotic in nature and was prevented by diallyldisulfide, an inhibitor of CYP2E1. Toxicity was reduced by trolox, an antioxidant. The treatment with ethanol or arachidonic acid resulted in release of cytochrome c into the cytosol fraction, and activation of caspase 3 (but not caspase 1) in hepatocytes from the pyrazole-treated rats but not hepatocytes from the saline-treated rats. The activation of caspase 3 was prevented by diallyldisulfide, by trolox, and by DEVD-fmk. The latter also prevented the toxicity produced by ethanol or arachidonic acid. These results extend previous observations found with HepG2 cells expressing CYP2E1 to intact hepatocytes and suggest that release of cytochrome c and activation of caspase 3 play a role in the overall pathway by which CYP2E1 contributes towards the hepatotoxic actions of ethanol and polyunsaturated fatty acids.

Differential role of ethanol and acetaldehyde in the induction of oxidative stress in HEP G2 cells: effect on transcription factors AP-1 and NF-kappaB

The oxidative metabolism of ethanol by the cytochrome P450 2E1 (CYP2E1) has been recognized to contribute to the ethanol-induced deleterious effects through the induction of oxidative stress. This study compared the effect of ethanol and acetaldehyde in the induction of oxidative stress and activation of transcription factors nuclear factor-kappaB (NF-kappaB) and activating protein 1 (AP-1) in HepG2 cells, which do not express CYP2E1, and HepG2 cells transfected with CYP2E1 (E47 cells). Neither ethanol (80 mmol/L) nor acetaldehyde (25-200 micromol/L) caused oxidative stress in HepG2 cells, an effect that was independent of blocking reduced glutathione (GSH) synthesis with buthionine-L-sulfoximine (BSO). However, BSO preincubation caused an overproduction of peroxides and activation of NF-kappaB and AP-1 in E47 cells even in the absence of ethanol. Furthermore, the incubation of E47 cells with ethanol (80 mmol/L for up to 5 days) depleted cellular GSH stores in both cytosol and mitochondria, reflecting the induction of oxidative stress. Ethanol activated NF-kappaB and AP-1 in E47 cells, an effect that was prevented by 4-methylpyrazole, potentiated by cyanamide, and attenuated by trolox C. Interestingly, however, despite the inability of acetaldehyde to induce oxidative stress in HepG2, acetaldehyde activated NF-kappaB and AP-1; in contrast, ethanol failed to activate these transcription factors in HepG2. Thus, our findings indicate that activation of NF-kappaB and AP-1 by ethanol and acetaldehyde occurs through distinct mechanisms. CYP2E1 is indispensable in the induction of oxidative stress from ethanol, whereas the activation of NF-kappaB and AP-1 by acetaldehyde is independent of oxidative stress.

NADPH-dependent microsomal electron transfer increases degradation of CYP2E1 by the proteasome complex: role of reactive oxygen species

Increased levels of cytochrome P450 2E1 (CYP2E1) produced by low-molecular-weight compounds is mostly due to stabilization of the enzyme against proteolytic degradation. CYP2E1, in the absence of substrate or ligand, normally has a short half-life, but the factors which regulate CYP2E1 turnover or trigger its rapid degradation are not known. Since CYP2E1 is active in producing reactive oxygen species, experiments were carried out to evaluate whether reactive oxygen species modulated the degradation of CYP2E1. CYP2E1 present in human liver microsomes was very stable. Addition of the cytosol fraction produced degradation of CYP2E1, and this was enhanced when NADPH was present in the reaction system. Antioxidants or iron chelators which prevent lipid peroxidation, prevented the degradation of CYP2E1 by the cytosolic fraction. Similarly, diphenyleneiodonium chloride, which inhibits NADPH-dependent electron transfer, prevented the degradation of CYP2E1, as did 4-methylpyrazole, a ligand which increases the level of CYP2E1. If microsomes were first incubated with NADPH for 30 min, followed by the addition of these agents, there was no protection against CYP2E1 degradation. Lactacystin, an inhibitor of the proteasome, decreased the degradation of CYP2E1. In intact HepG2 cells transduced to express CYP2E1, proteasome inhibitors elevated steady-state levels of CYP2E1. Steady-state levels of CYP2E1 were increased by about 50% when the cells were incubated with trolox. Trolox decreased the rate of loss of CYP2E1 protein when the cells were treated with cycloheximide. These results suggest that NADPH-dependent production of reactive oxygen species may result in oxidative modification of CYP2E1, followed by rapid degradation of the labilized CYP2E1 by the proteasome complex. It is interesting to speculate that one consequence of the high rates of production of reactive oxygen species by CYP2E1 is its own labilization and subsequent rapid degradation, and this may be a regulatory mechanism to prevent high levels of the enzyme from accumulating within the cell.

Overexpression of catalase in cytosolic or mitochondrial compartment protects HepG2 cells against oxidative injury

HepG2 cells were transfected with vectors containing human catalase cDNA and catalase cDNA with a mitochondrial leader sequence to allow comparison of the effectiveness of catalase overexpressed in the cytosolic or mitochondrial compartments to protect against oxidant-induced injury. Overexpression of catalase in cytosol and in mitochondria was confirmed by Western blot, and activity measurement and stable cell lines were established. The intracellular level of H(2)O(2) induced by exogenously added H(2)O(2) or antimycin A was lower in C33 cell lines overexpressing catalase in the cytosol and mC5 cell lines overexpressing catalase in the mitochondria as compared with Hp cell lines transfected with empty vector. Cell death caused by H(2)O(2), antimycin A, and menadione was considerably suppressed in both the mC5 and C33 cell lines. C33 and mC5 cells were also more resistant to apoptosis induced by H(2)O(2) and to the loss of mitochondrial membrane potential induced by H(2)O(2) and antimycin A. In view of the comparable protection by catalase overexpressed in the cytosol versus the mitochondria, catalase produced in both cellular compartments might act as a sink to decompose H(2)O(2) and move diffusable H(2)O(2) down its concentration gradient. The present study suggests that catalase in cytosol and catalase in mitochondria are capable of protecting HepG2 cells against cytotoxicity or apoptosis induced by oxidative stress.

Ethanol-induced apoptosis to stable HepG2 cell lines expressing human cytochrome P-4502E1

In a previous study (Wu and Cederbaum, J. Biol. Chem. 271:23914-23919, 1996), ethanol was shown to be cytotoxic to HepG2 cells, which were transduced to express human cytochrome P-4502E1 (CYP2E1) but not to control HepG2 cells. The goal of the current study was to evaluate whether this toxicity was apoptotic in nature. Incubation of CYP2E1-expressing HepG2 cells with 100 mM ethanol for 2 days produced morphological changes and DNA fragmentation (in situ labeling, flow cytometry, and DNA ladder formation) indicative of apoptosis. No changes were observed in the control HepG2 cells that do not express CYP2E1. Ethanol-induced apoptosis was also observed in HepG2 cells transiently transfected to express CYP2E1. The ethanol-induced apoptosis was prevented by 4-methylpyrazole, an inhibitor of ethanol oxidation by CYP2E1, and by trolox, an antioxidant that prevents lipid peroxidation. Ethanol treatment of the cells expressing CYP2E1 resulted in increased activities of caspases 1 and 3. An inhibitor of these caspases prevented the ethanol-induced apoptosis in the stable cell lines and the transiently transfected cell lines. Ethanol did not cause apoptosis in a HepG2 cell line overexpressing bcl-2 plus CYP2E1, but did cause apoptosis in cell lines expressing CYP2E1 in the absence of bcl-2. These experiments demonstrate that ethanol can produce apoptosis in HepG2 cells that express CYP2E1. Increased production of reactive oxygen species and lipid peroxidation can be associated with apoptotic cell death. The prevention of the ethanol-induced apoptosis by 4-methylpyrazole and by trolox suggests that production of a prooxidative state as a consequence of ethanol oxidation by CYP2E1 results in eventual activation of caspases such as caspases 1 and 3, which can trigger the apoptotic process.

Cytotoxicity of short-chain alcohols

Ethanol and other short-chain alcohols elicit a number of cellular responses that are potentially cytotoxic and, to some extent, independent of cell type. Aberrations in phospholipid and fatty acid metabolism, changes in the cellular redox state, disruptions of the energy state, and increased production of reactive oxygen metabolites have been implicated in cellular damage resulting from acute or chronic exposure to short-chain alcohols. Resulting disruptions of intracellular signaling cascades through interference with the synthesis of phosphatidic acid, decreases in phosphorylation potential and lipid peroxidation are mechanisms by which solvent alcohols can affect the rate of cell proliferation and, consequently, cell number. Nonoxidative metabolism of short-chain alcohols, including phospholipase D-mediated synthesis of alcohol phospholipids, and the synthesis of fatty acid alcohol esters are additional mechanisms by which alcohols can affect membrane structure and compromise cell function.

Involvement of oxidative stress in tumor cytotoxic activity of hepatocyte growth factor/scatter factor

In this study, we show that N-acetylcysteine (NAC), a precursor of glutathione and an intracellular free radical scavenger, almost completely prevented hepatocyte growth factor (HGF)-suppressed growth of Sarcoma 180 and Meth A cells, and HGF-induced apoptosis, assessed by DNA fragmentation, and increase in caspase-3 activity, in Sarcoma 180 cells. The reduced form of glutathione also prevented HGF-suppressed growth of the cells as effective as NAC. Ascorbic acid partially prevented the effect of HGF, but other antioxidants such as superoxide dismutase, catalase, and vitamin E, and the free radical spin traps N-t-butyl-alpha-phenylnitrone and 3,3,5, 5-tetramethyl-1-pyrroline-1-oxide did not have protective effects. HGF caused morphological changes of the cells, many cells showing condensation and rounding, and enhanced the generation of intracellular reactive oxygen species (ROS) as judged by flow cytometric analysis using 2',7'-dichlorofluorescein diacetate. NAC completely prevented both HGF-induced morphological changes and the enhancement of ROS generation in the cells. However, NAC did not prevent the HGF-induced scattering of Madin-Darby canine kidney cells. To our knowledge, this is the first report that HGF stimulates the production of ROS, and our results suggest the involvement of oxidative stress in the mechanism by which HGF induces growth suppression of tumor cells.

High-level expression of rat class I alcohol dehydrogenase is sufficient for ethanol-induced fat accumulation in transduced HeLa cells

The mechanisms by which ethanol causes fatty liver are complex. Reducing equivalents generated during ethanol oxidation inhibit tricarboxylic acid cycle activity and fatty acid oxidation. In addition, ethanol inhibits lipoprotein export and increases fatty acid uptake and lipid peroxidation. To test the role that alcohol metabolism by alcohol dehydrogenase (ADH) has on cellular lipid metabolism, a cell line expressing rat ADH was generated by transducing HeLa cells with an ADH-expressing retrovirus. The cells expressed high levels of ADH protein and had ADH activity similar to that of liver. Exposure of the cells to 20 mmol/L ethanol for 24 hours led to substantial accumulation of free fatty acids and triacylglycerol in the transduced, but not wild-type, HeLa cells. The rate of synthesis of saponifiable lipid was increased significantly by ethanol under these conditions. Ethanol exposure also promoted triacylglycerol accumulation when the cells were incubated with linoleic acid. This was associated with a decrease in the rate at which the cells oxidized 1-[14-C]-linoleic acid. Fat accumulation was not prevented by including alpha-tocopherol in the medium, arguing against a role for lipid peroxidation. However, the presence of methylene blue completely prevented the fat accumulation. This was associated with a return of the elevated lactate/pyruvate ratio toward normal. These data suggest that generation of reducing equivalents by ADH was sufficient to cause fat accumulation in this cell model.

Cholesterol hydroperoxides in erythrocyte membranes of alcoholic patients

Evidence for the presence of 5alpha-hydroperoxycholest-6-en-3beta-ol (cholesterol 5alpha-hydroperoxide, Ch 5alpha-OOH) and 7alpha- and 7beta-hydroperoxycholest-5-en-3beta-ols (cholesterol 7-hydroperoxides: Ch 7alpha-OOH and Ch 7beta-OOH, respectively) in human erythrocyte membrane was found. Blood samples were collected from alcoholic patients and healthy volunteers (controls), and their cholesterol hydroperoxides were analyzed by high-performance liquid chromatography postcolumn chemiluminescence and roughly identified by liquid chromatography-mass spectrometry. Ch 7alpha-OOH and Ch 7beta-OOH were present in each sample, being significantly higher in alcoholic samples than in control samples. Ch 5alpha-OOH was present in some alcoholic samples, but not in the control ones. The accumulation of cholesterol hydroperoxides suggests enhanced lipid peroxidation by active oxygen species and/or a reduced elimination system for lipid peroxide in alcoholic patients.

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.