Natalia A Osna, Terrence M Donohue Jr, Liver Study Unit, The Omaha Veterans Affairs Medical Center, the Department of Internal Medicine, University of Nebraska Medical Center, Omaha, Nebraska 68105, United States
Supported by the National Institute on Alcohol Abuse and Alcoholism, grant number 5R21 AA015379-02
Correspondence to: Natalia A Osna, MD, PhD, Liver Study Unit, Research Service (151), VA Medical Center, 4101 Woolworth Ave, Omaha, NE 68105, United States. firstname.lastname@example.org
Telephone: +1-402-3468800-3735 Fax: +1-402-4490604
Received: June 30, 2007 Revised: July 17, 2007
The proteasome is a major protein-degrading enzyme, which catalyzes degradation of oxidized and aged proteins, signal transduction factors and cleaves peptides for antigen presentation. Proteasome exists in the equilibrium of 26S and 20S particles. Proteasome function is altered by ethanol metabolism, depending on oxidative stress levels: low oxidative stress induces proteasome activity, while high oxidative stress reduces it. The proposed mechanisms for modulation of proteasome activity are related to oxidative modification of proteasomal proteins with primary and secondary products derived from ethanol oxidation. Decreased proteolysis by the proteasome results in the accumulation of insoluble protein aggregates, which cannot be degraded by proteasome and which further inhibit proteasome function. Mallory bodies, a common signature of alcoholic liver diseases, are formed by liver cells, when proteasome is unable to remove cytokeratins. Proteasome inhibition by ethanol also promotes the accumulation of pro-apoptotic factors in mitochondria of ethanol-metabolizing liver cells that are normally degraded by proteasome. In addition, decreased proteasome function also induces accumulation of the negative regulators of cytokine signaling (I-kB and SOCS), thereby blocking cytokine signal transduction. Finally, ethanol-elicited blockade of interferon type 1 and 2 signaling and decreased proteasome function impairs generation of peptides for MHC class Ⅰ-restricted antigen presentation.
© 2007 WJG. All rights reserved.
Key words: 20S proteasome; 26S proteasome; PA28; CYP2E1; Apoptosis; Liver
Osna NA, Donohue TM Jr. Implication of altered proteasome function in alcoholic liver injury. World J Gastroenterol 2007; 13(37): 4931-4937
The steady state level of proteins depends on their rates of synthesis and degradation. The proteasome is a major protein-degrading enzyme, which has a predominant role in intracellular protein catabolism. This enzyme influences various cell functions, as the enzyme not only hydrolyzes obsolete and oxidized proteins, thereby preventing cellular toxicity, but also regulates signaling events by destroying short-lived signal transduction factors and by controlling apoptotic cell death. Another important function of the proteasome is the generation of peptides for MHC class Ⅰ-restricted antigen presentation. Recently, the proteasome has been shown as a potent regulator of the circadian cycle by its role in degradation of Cryptochrome and Period proteins. Besides proteolysis, the proteasome has non-proteolytic functions, regulating transcription, DNA repair and chromatin remodeling.
PROTEASOME STRUCTURE AND FUNCTIONS
In mammalian cells, proteasome is localized mainly in the cytosol and the nucleus. It plays an important role in proteolytic centers within the cells, such as the centrosome, a perinuclear structure. Targeted proteins are transported to the centrosome for degradation, which is carried out by two proteasome structures, the 26S and 20S proteasomes. The 26S proteasome consists of core proteasome (20S proteasome) and two 19S (PA700) regulatory particles. The 20S proteasome is a large complex, which has four stacks containing 14 different gene products and is organized as outer alpha (a) subunits that “shape” 20S proteasome and beta (b) subunits that constitute the 20S catalytic core subunits (b1, b2 and b5, or Y, Z and X). b subunits have various activities, based on the ability to cleave either hydrophobic bonds (chymotrypsin-like activity), basic bonds (trypsin-like activity), or acidic bonds (caspase-like activity). The 20S proteasome exists in two forms: constitutive proteasome and immunoproteasome. The immunoproteasome subunits, LMP2, LMP7 and MECL-1, replace the constitutive subunits, usually under the influence of IFNg. The immunoproteasome is involved in generation of MHC class Ⅰ-restricted peptides for antigen presentation. The efficiency of peptide cleavage by the immunoproteasome is higher than by the constitutive proteasome. Mice lacking the LMP7 subunit have reduced antigen presentation. Recently, interferon type 1 (IFNa) has been reported as an effective inducer of immunoproteasome in liver cells. Unpublished results from our laboratory confirmed this finding in hepatoma cell lines stimulated with IFNa.
ATP-DEPENDENT AND INDEPENDENT HYDROLYSIS OF PROTEINS BY PROTEASOME
Degradation of proteins by proteasome may or may not require ATP. The 26S proteasome catalyses protein degradation in an ATP-dependent manner, while the 20S proteasome does it ATP-independently. The most important question is how the proteasome “senses” the proteins, which are to be degraded. 26S proteasome usually recognizes proteins, which are marked with ubiquitin, a small 8.5-kDa protein that is covalently attached to protein substrate. To be attached, ubiquitin is activated by E1, the ubiquitin-activating enzyme (Figure 1). Activated ubiquitin is transferred via a high-energy thiol ester intermediate, E2-ubuquitin-conjugating enzyme, to the protein substrate destined for degradation. Protein covalently attaches multiple ubiquitin molecules that generate polyubiquitin chain binding to internal lysine (lysine 48) residue. If the substrate protein binds to monoubiquitin (i.e one ubiquitin molecule per residue) instead of polyubiquitin (K48), it is targeted for degradation by the lysosome. Linkage through Lys 48, is associated with the ubiquitin-proteasome pathway, while Lys 63 linkage has a role in the inflammatory response, endocytic pathway, and ribosomal protein synthesis. The ubiquitin-conjugated protein is subjected for degradation by the 26S proteasome with a help of E3 ligase. To be ubiquitylated and recognized by proteasome, the substrates usually undergo post-translational modification, such as phosphorylation and oxidation. For recognition by E3, in certain cases proteins may require association with molecular chaperones.
The important functions of the 19S particle in 26S proteasome are to recognize ubiquitylated proteins and to open the aperture of the 20S proteasome a-rings to allow substrate entry into the catalytic core. The 19S particle unfolds ubiquitylated substrates, dissociates from ubiquitin with the help of de-ubiquitylating enzymes for further ubiquitin recycling and inserts the proteins into the 20S proteasome for degradation. This insertion needs ATP hydrolysis and is controlled by ATPases, which forms 19S sub-complexes known as the base and the lid.
Alternatively, degradation of non-ubiquitylated proteasome substrates can be carried out in an ATP- and ubiquitin-independent manner by the 20S proteasome, which requires the gate opening by the activators, PA28 a, b and g (Figure 1). PA28 a and b are predominantly located in the cytosol, whereas PA28 g has a nuclear localization. PA28 enhances hydrolysis of short peptides. In mice, disruption of PA28 a and b genes leads to defective generation of MHC class-restricted peptides, while mice with disrupted PA28 g gene have slower cell proliferation and enhanced susceptibility to apoptosis. Functions of the PA28 are antagonized by proline-rich proteins, PI31 and Pr39, which, in vitro, block proteasome activity[11,12] 20S proteasome can be simultaneously activated by both 19S and PA28 via the two outer rings. This structure is called a hybrid proteasome, which displays the catalytic properties of both the 26S and 20S forms of the proteasome. These hybrid proteasomes generate a unique spectrum of antigenic peptides for presentation in an ubiquitin-dependent as well as ubiquitin-independent ways.
ETHANOL METABOLISM AND LIVER PROTEASOME FUNCTION
The liver is the main site of ethanol metabolism. Hepatocytes express the major ethanol metabolizing enzymes, alcohol dehydrogenase (ADH) and cytochrome P450E1 (CYP2E1), which catalyze the generation of acetaldehyde and reactive oxygen/nitrogen species when liver cells are exposed to ethanol. The level of intracellular oxidative stress depends on the balance between oxidative and protective (antioxidant systems) factors. The level of oxidative stress differentially regulates proteasome function (i.e. low oxidative stress enhances proteasome activity, while high oxidative stress suppresses it).
There is a reciprocal relationship between CYP2E1 and the proteasome. Studies on HepG2 cells that overexpress CYP2E1 showed that ethanol and other CYP2E1 ligands increase the content and activity of CYP2E1 without affecting CYP2E1 mRNA[14,15]. Pulse-chase experiments demonstrated that ethanol and other ligands stabilize the enzyme against proteolysis. The proteasome is responsible for CYP2E1 degradation since treatment with the proteasome inhibitor, PSI, which blocks its chemotrypsin-like activity, caused a dose-dependent increase in the CYP2E1 apoptrotein. Heat shock proteins 70 and 90 provide the communication between CYP2E1 and the proteasome[16,17].
Stabilization of CYP2E1 by ethanol, in turn, results in increased production of ethanol metabolites, which block proteasome function. In vitro treatment of VL-17A cells (ADH/CYP2E1-positive HepG2 cells) with 100 mmol/L ethanol significantly reduces proteasome activity[18,19] as well as the ability of crude proteasome preparations (cytosols) from VL-17A cells to hydrolyze HBV peptide cleavage. The effects of ethanol exposure are blocked by treatment with inhibitors of ethanol metabolism and CYP2E1 activity. In vivo, proteasome inhibition is common in rodents intragastrically fed with ethanol[21,22]. The proteasomal chymotrypsin-like activity reduction in intagastrically fed rats was blunted in animals treated with a CYP2E1 inhibitor, chlormethazole. Furthermore, suppression of proteasome activity by ethanol was established in wild-type mice, but not in ethanol-fed CYP2E1 knockout mice. In orally fed mice, the effect on the proteasome depends on blood ethanol levels achieved after chronic ethanol feeding: usually when blood ethanol exceeds 40 mmol/L, there is a suppression of proteasome function. However, lower blood ethanol levels sometimes result in proteasome activation.
The proposed mechanisms for modulation of proteasome activity are related to oxidative modification of proteasomal proteins with primary and secondary products derived from ethanol oxidation. Thus, CYP2E1 activation, reactive oxygen species formation and subsequent adduction of proteasome with protein carbonyls in E47 cells (CYP2E1+ HepG2 cells) caused a decline in the trypsin-like, proteasome activity. Furthermore, in vivo ethanol studies on intragastrically fed rats demonstrated significant induction of CYP2E1 and an increase in 4-hydroxynonenal (4-NHE) concentrations after one month of ethanol feeding, which resulted in decreased proteasome activities. While a 3.5-fold CYP2E1 induction occurred after 15 d of feeding, there was no loss in proteasome activity, indicating that modulation of proteasome activity is a delayed response to CYP2E1-induced proteasome oxidation. Proteasome suppression was attributed to 4-HNE adducts of the 19S (PA700) proteasome regulator specifically an ATPase Rpt4 subunit. It was postulated that this adduction caused disrupted association between the 19S and 20S proteasome particles. In addition, our laboratory discovered the regulating effects of nitration on proteasome function. Nitration is caused by peroxynitrite, a reaction product of superoxide (generated by CYP2E1) and nitric oxide (generated by nitric oxide synthase), which is released as a result of ethanol metabolism and is involved in alcohol-induced liver injury. Massive nitration of 20S proteasome with subsequent 3-NT adduct formation caused a reduction of chymotrypsin-like proteasome activity, primarily due to reduced interaction between PA28 and the 20S proteasome. Interestingly, low levels of nitration activated proteasome function and enhanced proteasome activation by PA28. Similar effects of the peroxynitrite donor, SIN-1, were observed using highly purified proteasome, cultured hepatoma cells and rodent livers. Immunoproteasome has been shown to be more sensitive to the effects of peroxynitrite than the constitutive form of the enzyme. Studies on the 26S proteasome (4-HNE adducts) and on the 20S proteasome (3-NT adducts) revealed that in oxidatively modified proteasome, the ability of 20S proteasome to respond to activating stimuli of its regulators, 19S proteasome and PA28 is limited, which impedes proteasome function. The 26S proteasome is more susceptible to oxidative stress than 20S proteasome.
Another potent oxidant generated by ethanol metabolism, acetaldehyde, which is produced by ADH and CYP2E1 has also been shown to affect proteasome function. In vitro adduction of acetaldehyde to cytosolic proteins inhibits proteasome function in a dose-dependent manner. We also observed suppression of proteasome activity in hepatoma cells treated with 400 mmol/L acetaldehyde (Osna, unpublished observation). Thus, not only CYP2E1, but also ADH by generating ethanol metabolites can contribute to proteasome inhibition.
OXIDATIVE MODIFICATION OF PROTEINS: AGGREGATION AND PROTEASOME FUNCTION
The inability for ubiquitylated proteins to be degraded by proteasome is based not only on impaired proteasome activity but also on the relative susceptibility of substrates to degradation. Proteasomal substrates (proteins) can be modified by oxidation of amino acid residues by intracellular oxidants, such as H2O2 or by covalent binding to reactive species, such as peroxynitrite. Depending on their degree of modification, these substrates become either more or less susceptible to proteasome-catalyzed hydrolysis. Thus, when aconitase was treated with peroxynitrite and hydrogen peroxide, low concentrations of oxidants (up to 2 mmol/L for hydrogen peroxide and up to 1 mmol/L for peroxinitrite) enhanced susceptibility of aconitase to degradation, while higher concentrations of oxidants decreased proteolysis.
Ubiquitin is essential for ATP-dependent protein degradation by the proteasome. Under certain pathological conditions or due to genetic predisposition, defective ubiquitin + 1 (Ub + 1) is generated by a dinucleotide deletion in Ub mRNA. Proteins ubiquitylated with Ub + 1 are the substrates for proteasomal degradation, but at high concentrations of Ub + 1, the degradation is inhibited. Ub + 1 forms tight noncovalent interactions with target proteins and cannot be removed from the substrate (Luders et al, 2003). The accumulation of Ub + 1-labeled undegraded proteins is observed in neurological disorders (e.g. Huntington’s Disease, Alzheimer’s Disease and Down’s Syndrome).
Decreased proteolysis by the proteasome also results in the accumulation of insoluble protein aggregates, which cannot be degraded by proteasome and which further inhibit proteasome function. In patients with alcoholic cirrhosis, the aggregates are known as Mallory bodies or so-called alcoholic hyaline[35,36] and are a common signature of alcoholic liver diseases. Mallory bodies are formed by liver cells when proteasomes are unable to remove cytokeratins, mainly cytokeratins 7 and 19. Experimental studies revealed that the blockade of proteasome with specific inhibitors or by ethanol metabolism induces overexpression of heat shock proteins and Mallory body formation, which was observed both in vivo and in vitro[38,39]. Ub + 1 has also been detected in Mallory bodies, which is a common histological signature of alcoholic liver disease. In vitro delivery of Ub + 1 to mouse hepatocytes caused formation of aggresomes and Mallory body formation.
THE UBIQUITIN-PROTEASOME SYSTEM AND ETHANOL-INDUCED CELL DEATH
Proteasome function controls cell death and survival under the normal and pathological conditions. Thus, suppression of proteasome function with specific proteasome inhibitors results in increased cell death. The link between the proteasome function and cell survival is supported by the fact of degradation of certain cell cycle controlling and pro- or anti-apoptotic factors by the 26S proteasome. Because apoptotic events are downstream from proteasome function and ethanol suppresses proteasome activity, it may be possible that proteasome inhibition would promote the accumulation of pro-apoptotic factors in mitochondria of ethanol-metabolizing liver cells. Ultimately, it might stimulate liver cell apoptosis.
Inhibition of the 26S proteasome causes hepatotoxicity in vitro and in vivo. In Hep G2 cells transfected with CYP2E1, the amount of cell death by apoptotic mechanism depended on actual CYP2E1 activity. We also observed elevation of caspase 3 activity in VL-17A cells exposed to 100 mmol/L ethanol. In addition, a proteasome inhibitor, MG132, injected to mice 4 h prior to TNFa injection, sensitized liver cells to TNFa-induced apoptosis. In ethanol non-metabolizing cells, there is a negative feedback between chemotrypsin-like activity of proteasome and caspase-3 activation, when cells were treated with staurosporin. Furthermore, thymocytes, which normally are rich with proteasome, have declined proteasome activity when they underwent apoptosis after dexamethasone treatment.
Ethanol promotes cell death by inducing both apoptotic and necrotic events. Mitochondria, the primary site of energy (ATP) generation, are critical for induction of cell death by ethanol. Ethanol metabolism causes mitochondrial dysfunction, including impairment of fatty acid b-oxidation, inhibition of mitochondrial respiration and damage of mitochondrial DNA. The opening of mitochondrial pores (mitochondria permeability transition, MTP), normally blocked by anti-apoptotic factor, Bcl-2, results in release of cytochrome C that initiates the further death events. The scenario of the death events depends on ATP levels. In the case of ATP depletion, cells die via necrosis. However, if ATP levels are not changed, cytochrome C release causes activation of the mediators of apoptosis, caspases, and subsequent DNA fragmentation. Certain pro-apoptotic factors (Bid, Bax) closely associated with the mitochondrial membrane are degraded by the proteasome. Thus, ethanol plays a dual role in regulating cell death: on one hand, it has a tendency to deplete ATP and facilitates necrosis; on the other hand, due to the proteasome suppression, it promotes the accumulation of pro-apoptotic factors in mitochondria of ethanol metabolizing liver cells and initiates apoptosis.
UBIQUITIN-PROTEASOME INVOLVEMENT IN ALCOHOL-INDUCED TISSUE INFLAMMATION
The outcome of acute inflammation depends on the balance of pro- and anti-inflammatory events initiated by mediators of inflammation, which can be “turned on or off” via certain signal transduction pathways. To be effective and non-toxic, the signals must be short enough to allow switching to the next stage of the process (such as the transition from inflammation to regeneration). These changes in gene activation repertoire occur with the help of transduction factors, which, after their job is done, are ubiquitylated and degraded by the proteasome. Alcohol consumption seems to interfere with the transduction of pro-inflammatory signals.
Many important pro-inflammatory and proliferative signals are regulated via the NF-kB pathway. Activation of this pathway occurs by cytokines (such as tumor necrosis factor alpha, TNFa), phosphorylation of cytoplasmic inhibitory protein IkB and subsequent translocation of NF-kB to nucleus to initiate gene transcription. IkB can be “marked” by ubiquitin upon phosphorylation and then is degraded by proteasome; alternatively, non-phosphorylated IkB is modified by small ubiquitin-like modifier (SUMO) and is not targeted for degradation. In ethanol non-metabolizing cells, ethanol enhances NF-kB activation; however, in ethanol metabolizing cells, IkB dissociation is blocked and NF-kB activation does not happen. Acetaldehyde, a product of ethanol metabolism, is responsible for IkB stabilization and non-degradation by the proteasome. Acetaldehyde combines with the product of lipid peroxidation, malondialdehyde, and form the adducts with numerous proteins (malondialdehyde-acetaldehyde adducts, MAA). The MAA adducts reduce the susceptibility of lysozyme to proteasome degradation. MAA-modified proteins can serve as the targets of autoimmune reactions, enhancing liver injury.
In liver, ethanol treatment modulates activity of various signal transduction pathways and induces certain downstream events. Thus, feeding of animals for 16 wk with ethanol diet causes up-regulation of chemokines and neutrophil infiltration in the liver. It also induces elevation of IL-8 levels, which results in suppression of the 26S proteasome, due to activated JNK/AP1 pathways.
The Jak-STAT pathway, which plays a critical role in inflammation, proliferation, repair, and antiviral defense and usually supports proteasome activation, is also regulated by ethanol metabolism. Thus, acute ethanol exposure inhibited STAT3 activation by leptin in human Huh7 hepatoma cells. Inhibition of MAP kinase, protein kinase A and protein kinase C partially enhance IFN-induced STAT1 phosphorylation reduced by ethanol. We also reported inhibition of STAT1 phosphorylation in VL-17A hepatoma cells, which was attributed to ethanol-mediated induction of SOCS1, due to the limited degradation of SOCS1 by suppressed proteasome (Figure 2). Peroxynitrite formation with subsequent nitration of tyrosine residues seems to play a critical role in this process. Similarly, inhibition of proteasome has also been shown to inhibit IL-6 signaling via stabilization of SOCS3, which is supposed to be proteasome dependent as well.
Except liver tissue, some other types of tissues (brain, heart) induce the inflammation in response to ethanol and its metabolism. In these tissues, ethanol metabolism occurs predominantly via CYP2E1 and catalase, a peroxisomal enzyme, which catalyze ethanol oxidation by using H2O2 as a hydrogen acceptor. Hepatocytes transfected with catalase, are also protected from lipid peroxidation after glutathione depletion. In all ethanol-affected tissues, ethanol can also undergo non-oxidative metabolism and form fatty acid ethyl esters, which also induce mitochondrial dysfunction, inhibit protein synthesis and toxically affect cultured cells.
ETHANOL-ELICITED IMPAIRMENT OF PROTEASOME FUNCTION AND MHC CLASS I-RESTRICTED ANTIGEN PRESENTATION
Processing of peptides for MHC classⅠ-restricted antigen presentation is necessary for elimination of liver cells that express viral and other non-self proteins. At the most downstream level, these non-self proteins are cleaved by the proteasome in the cytoplasm[10,53]. Further N-terminal cleavage is carried by aminopeptidases. These cleavages are regulated by IFNg via the Jak-STAT1 pathway. Generated 8-10 amino acid peptides are presented on the cell surface, for recognition by cytotoxic T-lymphocytes (CTLs) and elimination. Escaping surveillance of infected cells by CTLs ultimately causes chronic persistence of viral infection in the liver, as is often observed in case of viral hepatitis B and C. Recently, we have shown that ethanol treatment of VL-17A cells suppresses proteasome and leucine aminopeptidase functions, which results in a partial blockade of HBV peptide hydrolysis and suppression of IFNg-induced expression of PA28 and immunoproteasome (Figure 3). Ethanol-elicited impairment in IFNg signaling is partially responsible for the latter event. This situation might be observed in chronically HBV-infected alcohol consumers, who are unable to clear viral infection in the liver.
Ethanol exposure creates oxidative stress in liver cells and that bi-phasically affects proteasome activity: low oxidative stress up-regulates proteasome function, while high oxidative stress suppresses it. Both acetaldehyde and CYP2E1-induced ROS and RNS contribute to this inhibition. Suppression of proteasome function results in enhanced cell death, mainly due to increased apoptosis. It also induces inflammation, by modulating various signal transduction pathways. Low proteasome activity interferes with efficient MHC class I-restricted presentation of antigenic peptides, which lowers antiviral defense.
1 Gatfield D, Schibler U. Physiology. Proteasomes keep the circadian clock ticking. Science 2007; 316: 1135-1136 PubMed
2 Demartino GN, Gillette TG. Proteasomes: machines for all reasons. Cell 2007; 129: 659-662 PubMed
3 Bader N, Jung T, Grune T. The proteasome and its role in nuclear protein maintenance. Exp Gerontol 2007 PubMed
4 Nandi D, Tahiliani P, Kumar A, Chandu D. The ubiquitin-proteasome system. J Biosci 2006; 31: 137-155 PubMed
5 Rivett AJ, Hearn AR. Proteasome function in antigen presentation: immunoproteasome complexes, Peptide production, and interactions with viral proteins. Curr Protein Pept Sci 2004; 5: 153-161 PubMed
6 Duan X, Hisaeda H, Shen J, Tu L, Imai T, Chou B, Murata S, Chiba T, Tanaka K, Fehling HJ, Koga T, Sueishi K, Himeno K. The ubiquitin-proteasome system plays essential roles in presenting an 8-mer CTL epitope expressed in APC to corresponding CD8+ T cells. Int Immunol 2006; 18: 679-687 PubMed
7 Shin EC, Seifert U, Kato T, Rice CM, Feinstone SM, Kloetzel PM, Rehermann B. Virus-induced type I IFN stimulates generation of immunoproteasomes at the site of infection. J Clin Invest 2006; 116: 3006-3014 PubMed
8 Mukhopadhyay D, Dasso M. Modification in reverse: the SUMO proteases. Trends Biochem Sci 2007; 32: 286-295 PubMed
9 Ciechanover A. The ubiquitin proteolytic system: from a vague idea, through basic mechanisms, and onto human diseases and drug targeting. Neurology 2006; 66: S7-S19 PubMed
10 Goldberg AL, Cascio P, Saric T, Rock KL. The importance of the proteasome and subsequent proteolytic steps in the generation of antigenic peptides. Mol Immunol 2002; 39: 147-164 PubMed
11 Zaiss DM, Standera S, Kloetzel PM, Sijts AJ. PI31 is a modulator of proteasome formation and antigen processing. Proc Natl Acad Sci USA 2002; 99: 14344-14349 PubMed
12 DeMartino GN, Slaughter CA. The proteasome, a novel protease regulated by multiple mechanisms. J Biol Chem 1999; 274: 22123-22126 PubMed
13 Rechsteiner M, Realini C, Ustrell V. The proteasome activator 11 S REG (PA28) and class I antigen presentation. Biochem J 2000; 345: 1-15 PubMed
14 Yang MX, Cederbaum AI. Characterization of cytochrome P4502E1 turnover in transfected HepG2 cells expressing human CYP2E1. Arch Biochem Biophys 1997; 341: 25-33 PubMed
15 Yang MX, Cederbaum AI. Glycerol increases content and activity of human cytochrome P-4502E1 in a transduced HepG2 cell line by protein stabilization. Alcohol Clin Exp Res 1997; 21: 340-347 PubMed
16 Goasduff T, Cederbaum AI. CYP2E1 degradation by in vitro reconstituted systems: role of the molecular chaperone hsp90. Arch Biochem Biophys 2000; 379: 321-330 PubMed
17 Morishima Y, Peng HM, Lin HL, Hollenberg PF, Sunahara RK, Osawa Y, Pratt WB. Regulation of cytochrome P450 2E1 by heat shock protein 90-dependent stabilization and CHIP-dependent proteasomal degradation. Biochemistry 2005; 44: 16333-16340 PubMed
18 Osna NA, Clemens DL, Donohue TM Jr. Interferon gamma enhances proteasome activity in recombinant Hep G2 cells that express cytochrome P4502E1: modulation by ethanol. Biochem Pharmacol 2003; 66: 697-710 PubMed
19 Donohue TM, Osna NA, Clemens DL. Recombinant Hep G2 cells that express alcohol dehydrogenase and cytochrome P450 2E1 as a model of ethanol-elicited cytotoxicity. Int J Biochem Cell Biol 2006; 38: 92-101 PubMed
20 Osna NA, White RL, Todero S, McVicker BL, Thiele GM, Clemens DL, Tuma DJ, Donohue TM Jr. Ethanol-induced oxidative stress suppresses generation of peptides for antigen presentation by hepatoma cells. Hepatology 2007; 45: 53-61 PubMed
21 Donohue TM Jr, Zetterman RK, Zhang-Gouillon ZQ, French SW. Peptidase activities of the multicatalytic protease in rat liver after voluntary and intragastric ethanol administration. Hepatology 1998; 28: 486-491 PubMed
22 Fataccioli V, Andraud E, Gentil M, French SW, Rouach H. Effects of chronic ethanol administration on rat liver proteasome activities: relationship with oxidative stress. Hepatology 1999; 29: 14-20 PubMed
23 Gouillon Z, Lucas D, Li J, Hagbjork AL, French BA, Fu P, Fang C, Ingelman-Sundberg M, Donohue TM Jr, French SW. Inhibition of ethanol-induced liver disease in the intragastric feeding rat model by chlormethiazole. Proc Soc Exp Biol Med 2000; 224: 302-308 PubMed
24 Bardag-Gorce F, Yuan QX, Li J, French BA, Fang C, Ingelman-Sundberg M, French SW. The effect of ethanol-induced cytochrome p4502E1 on the inhibition of proteasome activity by alcohol. Biochem Biophys Res Commun 2000; 279: 23-29 PubMed
25 Kessova IG, Cederbaum AI. The effect of CYP2E1-dependent oxidant stress on activity of proteasomes in HepG2 cells. J Pharmacol Exp Ther 2005; 315: 304-312 PubMed
26 Bardag-Gorce F, Li J, French BA, French SW. The effect of ethanol-induced CYP2E1 on proteasome activity: the role of 4-hydroxynonenal. Exp Mol Pathol 2005; 78: 109-115 PubMed
27 McKim SE, Gabele E, Isayama F, Lambert JC, Tucker LM, Wheeler MD, Connor HD, Mason RP, Doll MA, Hein DW, Arteel GE. Inducible nitric oxide synthase is required in alcohol-induced liver injury: studies with knockout mice. Gastroenterology 2003; 125: 1834-1844 PubMed
28 Osna NA, Haorah J, Krutik VM, Donohue TM Jr. Peroxynitrite alters the catalytic activity of rodent liver proteasome in vitro and in vivo. Hepatology 2004; 40: 574-582 PubMed
29 Amici M, Lupidi G, Angeletti M, Fioretti E, Eleuteri AM. Peroxynitrite-induced oxidation and its effects on isolated proteasomal systems. Free Radic Biol Med 2003; 34: 987-996 PubMed
30 Reinheckel T, Sitte N, Ullrich O, Kuckelkorn U, Davies KJ, Grune T. Comparative resistance of the 20S and 26S proteasome to oxidative stress. Biochem J 1998; 335: 637-642 PubMed
31 Rouach H, Andraud E, Aufrere G, Beauge F. The effects of acetaldehyde in vitro on proteasome activities and its potential involvement after alcoholization of rats by inhalation of ethanol vapours. Alcohol Alcohol 2005; 40: 359-366 PubMed
32 Grune T, Blasig IE, Sitte N, Roloff B, Haseloff R, Davies KJ. Peroxynitrite increases the degradation of aconitase and other cellular proteins by proteasome. J Biol Chem 1998; 273: 10857-10862 PubMed
33 de Pril R, Fischer DF, Roos RA, van Leeuwen FW. Ubiquitin-conjugating enzyme E2-25K increases aggregate formation and cell death in polyglutamine diseases. Mol Cell Neurosci 2007; 34: 10-19 PubMed
34 van Leeuwen FW, Gerez L, Benne R, Hol EM. +1 Proteins and aging. Int J Biochem Cell Biol 2002; 34: 1502-1505 PubMed
35 Chedid A, Mendenhall CL, Gartside P, French SW, Chen T, Rabin L. Prognostic factors in alcoholic liver disease. VA Cooperative Study Group. Am J Gastroenterol 1991; 86: 210-216 PubMed
36 Bardag-Gorce F, French BA, Lue YH, Nguyen V, Wan YJ, French SW. Mallory bodies formed in proteasome-depleted hepatocytes: an immunohistochemical study. Exp Mol Pathol 2001; 70: 7-18 PubMed
37 French SW. The mechanism of organ injury in alcoholics: implications for therapy. Alcohol Alcohol Suppl 1991; 1: 57- 63 PubMed
38 Bardag-Gorce F, Riley N, Nguyen V, Montgomery RO, French BA, Li J, van Leeuwen FW, Lungo W, McPhaul LW, French SW. The mechanism of cytokeratin aggresome formation: the role of mutant ubiquitin (UBB+1). Exp Mol Pathol 2003; 74: 160-167 PubMed
39 Bardag-Gorce F, French BA, Nan L, Song H, Nguyen SK, Yong H, Dede J, French SW. CYP2E1 induced by ethanol causes oxidative stress, proteasome inhibition and cytokeratin aggresome (Mallory body-like) formation. Exp Mol Pathol 2006; 81: 191-201 PubMed
40 Cederbaum AI, Wu D, Mari M, Bai J. CYP2E1-dependent toxicity and oxidative stress in HepG2 cells. Free Radic Biol Med 2001; 31: 1539-1543 PubMed
41 McClain C, Barve S, Joshi-Barve S, Song Z, Deaciuc I, Chen T, Hill D. Dysregulated cytokine metabolism, altered hepatic methionine metabolism and proteasome dysfunction in alcoholic liver disease. Alcohol Clin Exp Res 2005; 29: 180S-188S PubMed
42 Beyette J, Mason GG, Murray RZ, Cohen GM, Rivett AJ. Proteasome activities decrease during dexamethasone- induced apoptosis of thymocytes. Biochem J 1998; 332: 315-320 PubMed
43 Naujokat C, Hoffmann S. Role and function of the 26S proteasome in proliferation and apoptosis. Lab Invest 2002; 82: 965-980 PubMed
44 Ulrich HD, Vogel S, Davies AA. SUMO keeps a check on recombination during DNA replication. Cell Cycle 2005; 4: 1699-1702 PubMed
45 Novitskiy G, Ravi R, Potter JJ, Rennie-Tankersley L, Wang L, Mezey E. Effects of acetaldehyde and TNF alpha on the inhibitory kappa B-alpha protein and nuclear factor kappa B activation in hepatic stellate cells. Alcohol Alcohol 2005; 40: 96-101 PubMed
46 Tuma DJ. Role of malondialdehyde-acetaldehyde adducts in liver injury. Free Radic Biol Med 2002; 32: 303-308 PubMed
47 Degawa-Yamauchi M, Uotani S, Yamaguchi Y, Takahashi R, Abe T, Kuwahara H, Yamasaki H, Eguchi K. Ethanol inhibits leptin-induced STAT3 activation in Huh7 cells. FEBS Lett 2002; 525: 116-120 PubMed
48 Nguyen VA, Chen J, Hong F, Ishac EJ, Gao B. Interferons activate the p42/44 mitogen-activated protein kinase and JAK-STAT (Janus kinase-signal transducer and activator transcription factor) signalling pathways in hepatocytes: differential regulation by acute ethanol via a protein kinase C-dependent mechanism. Biochem J 2000; 349: 427-434 PubMed
49 Osna NA, Clemens DL, Donohue TM Jr. Ethanol metabolism alters interferon gamma signaling in recombinant HepG2 cells. Hepatology 2005; 42: 1109-1117 PubMed
50 Zhang JG, Farley A, Nicholson SE, Willson TA, Zugaro LM, Simpson RJ, Moritz RL, Cary D, Richardson R, Hausmann G, Kile BJ, Kent SB, Alexander WS, Metcalf D, Hilton DJ, Nicola NA, Baca M. The conserved SOCS box motif in suppressors of cytokine signaling binds to elongins B and C and may couple bound proteins to proteasomal degradation. Proc Natl Acad Sci USA 1999; 96: 2071-2076 PubMed
51 Wu D, Cederbaum AI. Oxidative stress mediated toxicity exerted by ethanol-inducible CYP2E1. Toxicol Appl Pharmacol 2005; 207: 70-76 PubMed
52 Wu H, Cai P, Clemens DL, Jerrells TR, Ansari GA, Kaphalia BS. Metabolic basis of ethanol-induced cytotoxicity in recombinant HepG2 cells: role of nonoxidative metabolism. Toxicol Appl Pharmacol 2006; 216: 238-247 PubMed
53 Teoh CY, Davies KJ. Potential roles of protein oxidation and the immunoproteasome in MHC class I antigen presentation: the 'PrOxI' hypothesis. Arch Biochem Biophys 2004; 423: 88-96 PubMed
54 York IA, Goldberg AL, Mo XY, Rock KL. Proteolysis and class I major histocompatibility complex antigen presentation. Immunol Rev 1999; 172: 49-66 PubMed
S- Editor Ma N L- Editor Alpini GD E- Editor Li JL