Proteasome inhibition potentiates CYP2E1-mediated toxicity in HepG2 cells

Mitochondrial Dysfunction-Associated Mechanisms in the Development of Chronic Liver Diseases

The liver is a major metabolic organ that performs many essential biological functions such as detoxification and the synthesis of proteins and biochemicals necessary for digestion and growth. Any disruption in normal liver function can lead to the development of more severe liver disorders. Overall, about 3 million Americans have some type of liver disease and 5.5 million people have progressive liver disease or cirrhosis, in which scar tissue replaces the healthy liver tissue. An estimated 20% to 30% of adults have excess fat in their livers, a condition called steatosis. The most common etiologies for steatosis development are (1) high caloric intake that causes non-alcoholic fatty liver disease (NAFLD) and (2) excessive alcohol consumption, which results in alcohol-associated liver disease (ALD). NAFLD is now termed "metabolic-dysfunction-associated steatotic liver disease" (MASLD), which reflects its association with the metabolic syndrome and conditions including diabetes, high blood pressure, high cholesterol and obesity. ALD represents a spectrum of liver injury that ranges from hepatic steatosis to more advanced liver pathologies, including alcoholic hepatitis (AH), alcohol-associated cirrhosis (AC) and acute AH, presenting as acute-on-chronic liver failure. The predominant liver cells, hepatocytes, comprise more than 70% of the total liver mass in human adults and are the basic metabolic cells. Mitochondria are intracellular organelles that are the principal sources of energy in hepatocytes and play a major role in oxidative metabolism and sustaining liver cell energy needs. In addition to regulating cellular energy homeostasis, mitochondria perform other key physiologic and metabolic activities, including ion homeostasis, reactive oxygen species (ROS) generation, redox signaling and participation in cell injury/death. Here, we discuss the main mechanism of mitochondrial dysfunction in chronic liver disease and some treatment strategies available for targeting mitochondria.

The cytotoxic concentration of rosmarinic acid increases MG132-induced cytotoxicity, proteasome inhibition, autophagy, cellular stresses, and apoptosis in HepG2 cells

Rosmarinic acid (RA) is a natural polyphenolic compound derived from many common herbal plants. Although it is known that RA has many important biological activities, its effect on proteasome inhibitor-induced changes in cancer treatment or its effects on any experimental proteasome inhibition model is unknown. The aim of the study was to investigate the effect of RA on MG132-induced cytotoxicity, proteasome inhibition, autophagy, cellular stresses, and apoptosis in HepG2 cells. HepG2 cells were treated with 10, 100, and 1000 µM RA in the presence of MG132 for 24 h; 10 and 100 µM RA did not affect but 1000 µM RA decreased cell viability in HepG2 cells. MG132 caused a significant decrease in cell viability and phosphorylation of mammalian target of rapamycin and a significant increase in levels of polyubiquitinated protein, microtubule-associated proteins 1A/1B light chain 3B-II (LC3B-II), heat shock protein 70 (HSP70), binding immunoglobulin protein (BiP), activating transcription factor 4 (ATF4), protein carbonyl, and cleaved poly(adenosine diphosphate-ribose) polymerase 1 (PARP1); 10 and 100 µM RA did not significantly change these effects of MG132 in HepG2 cells; 1000 µM RA caused a significant decrease in cell viability and a significant increase in polyubiquitinated protein, LC3B-II, HSP70, BiP, ATF4, protein carbonyl, and cleaved PARP1 levels in MG132-treated cells. Our study showed that only 1000 µM RA increased MG132-induced cytotoxicity, proteasome inhibition, autophagy, cellular stresses, and apoptosis in HepG2 cells. According to our results, cytotoxic concentration of RA can potentiate the effects of MG132 in hepatocellular carcinoma treatment.

Bortezomib alleviates drug-induced liver injury by regulating CYP2E1 gene transcription

Acute liver failure, i.e., the fatal deterioration of liver function, is the most common indication that emergency liver transplantation is necessary. Moreover, in the USA, drug-induced liver injury (DILI), including acetaminophen (APAP)-induced hepatotoxicity, is the main cause of acute liver failure. Matching a donor for liver transplantation is extremely difficult, and thus the development of a novel therapy for DILI is urgently needed. Following recent approval by the FDA of the proteasomal inhibitor bortezomib, its therapeutic effects on various human diseases, including solid and hematologic malignancies, have been validated. However, the specific action of proteasomal inhibition in cases of DILI had not been elucidated prior to this study. To examine the effects of proteasomal inhibition in DILI experimentally, male C56Bl/6 mice were injected with 1 mg bortezomib/kg before APAP treatment. Bortezomib not only alleviated APAP-induced hepatotoxicity in a time- and dose-dependent manner, it also alleviated CCl4- and thioacetamide-induced hepatotoxicity. We also noted that bortezomib significantly reduced cytochrome P450 2E1 (CYP2E1) expression and activity in the liver, which was accompanied by the induction of endoplasmic reticulum (ER) stress. In addition, bortezomib decreased hepatocyte nuclear factor‑1α-induced promoter activation of CYP2E1 in Hep3B cells. By contrast, another proteasome inhibitor, MG132, did not cause ER stress and did not markedly affect CYP2E1 enzyme activity. Liver injury induced by APAP was aggravated by MG132, possibly via elevation of connexin 32 expression. This study suggests that proteasome inhibition has different effects in cases of DILI depending on the specific inhibitor being used. Furthermore, results from the mouse model indicated that bortezomib, but not MG132, was effective in alleviating DILI. ER stress induced by proteasome inhibition has previously been shown to exert various effects on DILI patients, and thus each available proteasomal inhibitor should be evaluated individually in order to determine its potential for clinical application.

The role of reactive oxygen species (ROS) and cytochrome P-450 2E1 in the generation of carcinogenic etheno-DNA adducts

Exocyclic etheno-DNA adducts are mutagenic and carcinogenic and are formed by the reaction of lipidperoxidation (LPO) products such as 4-hydoxynonenal or malondialdehyde with DNA bases. LPO products are generated either via inflammation driven oxidative stress or via the induction of cytochrome P-450 2E1 (CYP2E1). In the liver CYP2E1 is induced by various compounds including free fatty acids, acetone and ethanol. Increased levels of CYP2E1 and thus, oxidative stress are observed in the liver of patients with non-alcoholic steatohepatitis (NASH) as well as in the chronic alcoholic. In addition, chronic ethanol ingestion also increases CYP2E1 in the mucosa of the oesophagus and colon. In all these tissues CYP2E1 correlates significantly with the levels of carcinogenic etheno-DNA adducts. In contrast, in patients with non-alcoholic steatohepatitis (NASH) hepatic etheno-DNA adducts do not correlate with CYP2E1 indicating that in NASH etheno-DNA adducts formation is predominately driven by inflammation rather than by CYP2E1 induction. Since etheno-DNA adducts are strong mutagens producing various types of base pair substitution mutations as well as other types of genetic damage, it is strongly believed that they are involved in ethanol mediated carcinogenesis primarily driven by the induction of CYP2E1.

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Cytochrome P-450 2E1 is induced following chronic ethanol ingestion.

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CYP2E1 correlates with carcinogenic etheno-DNA formation.

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CYP2E1 and oxidative stress are important mechanisms in alcohol mediated carcinogenesis in the liver, undefined and colon.

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In NASH hepatic etheno-DNA adducts occur but possible due to inflammation.

The role of reactive oxygen species (ROS) and cytochrome P-450 2E1 in the generation of carcinogenic etheno-DNA adducts

Exocyclic etheno-DNA adducts are mutagenic and carcinogenic and are formed by the reaction of lipidperoxidation (LPO) products such as 4-hydoxynonenal or malondialdehyde with DNA bases. LPO products are generated either via inflammation driven oxidative stress or via the induction of cytochrome P-450 2E1 (CYP2E1). In the liver CYP2E1 is induced by various compounds including free fatty acids, acetone and ethanol. Increased levels of CYP2E1 and thus, oxidative stress are observed in the liver of patients with non-alcoholic steatohepatitis (NASH) as well as in the chronic alcoholic. In addition, chronic ethanol ingestion also increases CYP2E1 in the mucosa of the oesophagus and colon. In all these tissues CYP2E1 correlates significantly with the levels of carcinogenic etheno-DNA adducts. In contrast, in patients with non-alcoholic steatohepatitis (NASH) hepatic etheno-DNA adducts do not correlate with CYP2E1 indicating that in NASH etheno-DNA adducts formation is predominately driven by inflammation rather than by CYP2E1 induction. Since etheno-DNA adducts are strong mutagens producing various types of base pair substitution mutations as well as other types of genetic damage, it is strongly believed that they are involved in ethanol mediated carcinogenesis primarily driven by the induction of CYP2E1.

The role of cytochrome P450 2E1 in ethanol-mediated carcinogenesis

We and others have shown that chronic alcohol consumption results in the induction of CYP2E1 in the liver. We have also detected for the first time such an induction in the mucosa of the small intestine and the colon. The overall induction of CYP2E1 shows interindividual variations and occurs already following a daily ingestion of 40 g of ethanol after 1 week. CYP2E1 induction is associated with an increased metabolism of ethanol resulting in the generation of reactive oxygen species (ROS) with direct and indirect carcinogenic action. ROS generated by CYP2E1 may lead to lipid peroxidation and lipid peroxidation products such as 4-hydroxynonenal bind to DNA forming highly carcinogenic exocyclic etheno DNA-adducts. The generation of these adducts has been shown in cell cultures in animal experiments as well as in human liver biopsies. CYP2E1 also metabolizes various procarcinogens present in diets and in tobacco smoke to their carcinogenic metabolites. Among these, nitrosamines seem to be the most important carcinogens. CYP2E1 also degrades retinoic acid and retinol to polar metabolites. Metabolism of retinoic acid not only results in the loss of retinoic acid promoting carcinogenesis through an increase in cell proliferation and dedifferentiation but also in generation of polar metabolites with apoptotic properties. We have shown that chlormethiazole is a specific CYP2E1 inhibitor in humans. Chlormethiazole inhibits CYP2E1 activity and thus blocks the formation of DNA adducts in cell cultures, restores retinoic acids in alcohol fed animals and inhibits chemical induced ethanol mediated hepatocarcinogenesis. Thus, there is increasing evidence that CYP2E1 induced by chronic alcohol consumption plays an important role in alcohol mediated carcinogenesis.

Role of oxidative stress in alcohol-induced liver injury

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

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.

Alcohol accelerates loss of muscle and impairs recovery of muscle mass resulting from disuse atrophy

BACKGROUND

Muscle disuse atrophy is observed in patients recovering from trauma and there is an increased risk and severity of injury in patients abusing alcohol (EtOH). However, the interaction of EtOH and disuse on muscle protein balance has not been examined. Therefore, the present study addressed the hypothesis that EtOH accelerates the disuse atrophy and/or impairs the accretion of muscle protein during muscle recovery.

METHODS

To address this aim, disuse atrophy was induced in rats by 3 days of unilateral hindlimb immobilization (casting), using the contralateral leg as control, with EtOH or saline being orally gavaged twice, each day during this period. In a separate study, EtOH-treated rats received Velcade to inhibit proteasomal degradation. Finally, in the last study, rats had 1 limb casted for 5 days, the cast removed, and EtOH or saline gavaged twice daily during a 5-day recovery period. Muscle protein metabolism was assessed using surrogate markers of protein synthesis [i.e., phosphorylation of 4E-binding protein 1 (BP1) and S6 kinase 1 (S6K1)] and protein degradation (i.e., mRNA content of the ubiquitin E3 ligases atrogin-1 and MuRF1).

RESULTS

Ethanol alone did not decrease muscle weight in the uncasted muscle. However, the loss of mass of immobilized muscle from EtOH-gavaged rats was 80% greater than in the animals not receiving EtOH. This atrophic response was not associated with a change in Akt, 4E-BP1 or S6K1 phosphorylation among groups. In contrast, immobilization alone increased both atrogin-1 and MuRF1 mRNA, and EtOH further increased their expression in immobilized muscle. The proteasome inhibitor Velcade attenuated atrophy produced by EtOH + disuse. When administered during the recovery period, EtOH prevented the normal accretion of muscle mass. This EtOH effect was associated with increased atrogin-1 mRNA, a reduction in 4E-BP1 and S6 phosphorylation, and an increased AMP-activated kinase phosphorylation.

CONCLUSIONS

Based on the changes in these surrogate markers, our data suggest that EtOH accelerates disuse atrophy by stimulating ubiquitin-mediated proteolysis, and blunts repletion of muscle protein during recovery from disuse by increasing proteolysis and decreasing protein synthesis.

Role of the proteasome in ethanol-induced liver pathology

The ubiquitin-proteasome system has come to be known as a vital constituent of mammalian cells. The proteasome is a large nonlysosomal enzyme that acts in concert with an 8.5 kDa polypeptide called ubiquitin and a series of conjugating enzymes, known as E1, E2 and E3, that covalently bind multiple ubiquitin moieties in a polyubiquitin chain to protein substrates in a process called ubiquitylation. The latter process targets protein substrates for unfolding and degradation by the 26S proteasome. This enzyme system specifically recognizes and degrades polyubiquitylated proteins, many of which are key proteins involved in cell cycle regulation, apoptosis, signal transduction, and antigen presentation. The 26S proteasome contains a cylinder-shaped 20S catalytic core that, itself, degrades proteins in an ATP- and ubiquitin-independent manner. The 20S form is actually the predominant enzyme form in mammalian cells. Proteolysis by the constitutive 20S proteasome is vital in removing oxidized, misfolded and otherwise modified proteins. Such degradation is critical as a means of cellular detoxification, as intracellular accumulation of damaged and misfolded proteins is potentially lethal. Studies have shown that inhibition of proteasome activity can lead to cell death. Ethanol and its metabolism cause partial inhibition of the proteasome. This leads to a number of pleiotropic effects that can affect a variety of cellular processes. This critical review describes important aspects of ethanol metabolism and its influence on the proteasome. The review will summarize recent findings on: (1) the interactions between the proteasome and the ethanol metabolizing enzyme, CYP2E1; (2) the dynamics of proteasome inhibition by ethanol in animal models and cultured cells; (3) ethanol-elicited suppression of proteasome activity and its effect on signal transduction; (4) The role of proteasome inhibition in cytokine production by liver cells; and (5) ethanol elicited suppression of peptide hydrolysis and the potential effects on antigen presentation. While the principal focus is on alcohol-induced liver injury, the authors foresee that the findings presented in this review will prompt further research on the role of this proteolytic system in other tissues injured by excessive alcohol consumption.

CYP2E1 induced by ethanol causes oxidative stress, proteasome inhibition and cytokeratin aggresome (Mallory body-like) formation

The role of oxidative stress in alcoholic liver disease and cytokeratin aggresome formation is the focus of this in vitro study. HepG2 cells transduced to over express CYP2E1 (E47) and control HepG2 cells (C34) were first treated with arachidonic acid, then Fe-NAT, and finally with ethanol. In the E47 ethanol-treated cells, CYP2E1 was induced and a higher level of reactive oxygen species and carbonyl proteins were generated. The proteasome activity decreased significantly in the E47 ethanol-treated cells. This inhibition was prevented when CYP2E1 was inhibited by DAS. Microarray analysis showed gene expression down regulation of the proteasome subunit, as well as ubiquitin pathway proteins in the E47 ethanol-treated cells. 4-Hydroxynonenal (4-HNE) adducts were increased in the E47 cells treated with ethanol. Furthermore, the immunoprecipitated 4-HNE modified proteins from these cells stained positive with antibodies to the proteasome subunit alpha 6. These results indicate that the ethanol induced CYP2E1 generates oxidative stress that is responsible for the decrease in proteasome activity. Cytokeratin 8 and 18 were induced by ethanol treatment of E47 cells and polyubiquitinated forms of these proteins were found in the polyubiquitin smear upon Western blots analysis. Cytokeratin aggresomes and Mallory body-like inclusions formed in the ethanol-treated E47 cells, indicating that the ubiquitinated cytokeratins accumulated as a result of the inhibition of the proteasome by ethanol treatment when oxidation of ethanol induced oxidative stress. This is the first report where ethanol caused Mallory body-like cytokeratin inclusions in transformed human liver cells in vitro.

Use of CYP2E1-Transfected Human Liver Cell Lines in Elucidating the Actions of Ethanol

This article represents the proceedings of a symposium at the 2004 RSA Meeting held in Vancouver, Canada. The chairs were Arthur I. Cederbaum and Raj Lakshman. The presentations were (1) ethanol regulates 2,6-sialyltransferase (2,6-ST) gene expression posttranscriptionally by the interaction of a cytosolic binding protein with 2,6-ST mRNA in CYP2E1- and ADH-transfected HepG2 cells, by Raj Lakshman; (2) nature versus nurture: HepG2-E47 cells as a tool to investigate mechanisms of ethanol-mediated potentiation of cell killing, by Jan B. Hoek; (3) ethanol up-regulates profibrogenic connective tissue growth factor gene expression in HepG2 cells via cytochrome P-450 2E1-mediated ethanol oxidation, by Masahiro Konishi; (4) role of calcium and calcium-activated enzymes in CYP2E1-dependent toxicity, by Arthur I Cederbaum; (5) the use of cell lines to characterize the role of CYP2E1 in the metabolism of farnesol, by Dennis Koop; and (6) studies with HepG2 cells that express the two major ethanol-metabolizing enzymes, by Terrence M. Donohue.

The effect of CYP2E1-dependent oxidant stress on activity of proteasomes in HepG2 cells

A reduction in proteasome activity and accumulation of oxidized proteins may play a role in alcoholic liver disease. The current study assessed proteasome peptidase activities and oxidative modifications of proteasomes during oxidative stress generated by CYP2E1. The model of toxicity by arachidonic acid (AA) and iron [ferric-nitrilotriacetate (Fe-NTA)] in HepG2 cells overexpressing CYP2E1 (E47 cells) and control C34 cells was used. AA/Fe-NTA treatment decreased trypsin-like (T-L) activity of the proteasome in E47 cells but not in C34 cells. This inhibition was abolished by antioxidants. Chymotrypsin-like activity of the proteasome was increased in E47 cells, and activity was not altered by AA/Fe-NTA treatment. There were no changes in content of subunits of 20S proteasomes or 19S regulator ATPase subunits S4 and p42 by AA/Fe-NTA treatment. An increased content of the PA28alpha subunit of the 11S regulator of proteasomes was detected in E47 cells. In proteasome pellets, the decline of T-L activity was accompanied by increased content of carbonyl adducts, suggesting oxidative modification of proteasomes. Higher levels of ubiquitinated, 3-nitrotyrosine- and 4-hydroxynonenal-modified proteins and lower levels of free ubiquitin were detected in untreated E47 cells in comparison with C34 cells. Accumulation of protein cross-linked, detergent-insoluble aggregates was increased with AA/Fe-NTA treatment in E47 cells. Thus, reactive oxygen species generated upon CYP2E1-dependent oxidative stress mediated a decline in T-L proteasome function, increased carbonyl adducts in proteasomes, and promoted protein aggregate formation; this may alter the balance among protein oxidation, ubiquitination, and degradation.

Decreased proteasome activity is associated with increased severity of liver pathology and oxidative stress in experimental alcoholic liver disease

BACKGROUND

Because of its role in degrading the bulk of intracellular proteins and eliminating damaged proteins, the proteasome is important in maintaining cell viability. Previously, we showed a 35-40% decrease in proteasome peptidase activity when ethanol was administered to rats by intragastric infusion. We hypothesized that this reduction was caused by ethanol-elicited oxidative stress, the degree of which varies depending on the method of ethanol administration. This study examined the relationship of proteasome activity and content with ethanol-induced oxidative stress and the degree of liver injury.

METHODS

Rats were given ethanol or isocaloric dextrose-containing liquid diets by intragastric infusion for 1 month. The diets contained medium-chain triglycerides (MCT), palm oil (PO), corn oil (CO), or fish oil (FO) as the principal source of fat.

RESULTS

Rats given ethanol and MCT exhibited no significant liver pathology, whereas cumulative pathology scores in ethanol-fed rats given PO, CO, or FO were 2.5, 5.4 and 7.0, respectively, indicating that ethanol and FO caused the greatest liver damage. The severity of liver pathology in the last three groups of animals correlated with levels of lipid peroxides and serum 8-isoprostanes. Alpha smooth muscle actin, an indicator of stellate cell activation, was increased relative to controls in the livers of all ethanol-fed rats except FO-fed animals, in which both control and ethanol-fed rats had similar levels of this protein. In livers of CO and FO ethanol-fed rats, proteasome chymotrypsin-like activity was decreased by 55-60%, but there was no quantitative alteration in 20S proteasome subunit content. In contrast, ethanol affected neither proteasome activity nor its content in MCT- and PO-treated animals.

CONCLUSIONS

Our findings indicate that the severity of liver injury and ethanol-induced oxidative stress is associated with a reduction in proteasome catalysis.

Proteasome inhibition induces cytokeratin accumulation in vivo

Chronic ethanol ingestion leads to inhibition of proteasomal activity. As a consequence, proteins accumulate in liver cells. Cytokeratin accumulation as seen in alcoholic hepatitis could lead to the formation of Mallory bodies. In order to study the phenomenon of cytokeratin accumulation in liver cells, rats were fed ethanol or dextrose for 1 month and some were given the proteasome inhibitor, PS-341, to augment the inhibitory effect of ethanol feeding. This was done to study the involvement of proteasome inhibition in the process of cytokeratin accumulation. There was a marked increase in the accumulation of polyubiquitinated proteins, and heat shock proteins (hsp) 25 and 70 in the liver of rats treated with PS-341. Similarly, cytokeratin-8 (CK-8) levels were markedly increased in the liver homogenates of rats fed ethanol when given PS-341. When normal mouse cultured hepatocytes were transfected with cytokeratin-18 (CK-18) tagged with red fluorescent protein (RFP), CK-18 aggresomes formed because proteasome was overloaded. These data provide new evidence that proteasome inhibition is involved in cytokeratin accumulation, when aggresomes are formed in tissue culture. Accumulation of cytokeratin in this way may ultimately lead to Mallory body formation as seen in alcoholic hepatitis.

Inhibition of proteasome function leads to NF-kappaB-independent IL-8 expression in human hepatocytes

Breakdown of cellular proteins is a highly regulated process, and the ubiquitin-proteasome pathway is the major proteolytic system in the cell. It regulates the levels of numerous proteins that control gene expression and cell division, as well as responses to stress and inflammation. Recent studies have reported abnormalities in proteasome function in alcoholic liver disease (ALD). Moreover, a direct relation has been reported between impaired proteasome function and oxidative stress in experimental models of ALD. Neutrophil infiltration is a hallmark of ALD, and activated neutrophils are thought to play a role in the pathology of ALD. As a potent neutrophil chemoattractant and activator, interleukin 8 (IL-8) likely plays a key mechanistic role in many forms of liver injury. In this study, we evaluated the effects of inhibition of proteasome function on expression and release of IL-8 by human fetal hepatocytes and hepatoma cells. Our data demonstrate that inhibition of proteasome function in hepatocytes leads to apoptotic cell death. Decreased hepatocyte survival coincides with enhanced expression of IL-8, both at the protein and the messenger RNA (mRNA) levels. This increase in IL-8 is independent of nuclear factor kappaB (NF-kappaB) activation and is associated with an increase in c-Jun N-terminal kinase (JNK) and activator protein-1 (AP-1) activity. In conclusion, hepatocytes dying because of inhibition of proteasome function produce massive quantities of the proinflammatory chemokine IL-8, possibly resulting in neutrophil infiltration, increased inflammation, and liver injury.