Proteasome inhibition induces cytokeratin accumulation in vivo

Hepatocellular carcinomas with intracellular hyaline bodies have a poor prognosis

BACKGROUND & AIMS

Mallory-Denk bodies (MDBs) and intracellular hyaline bodies (IHBs) are cytoplasmic inclusions found in a subset of hepatocellular carcinoma (HCC). MDBs are mainly composed of the intermediate filament proteins keratin (K) 8 and K18, the cellular stress- and adapter-protein sequestosome 1/p62 (p62) and ubiquitin, whereas IHBs consist of p62 and/or ubiquitin. Of note, cytoplasmic inclusions containing p62 can serve as markers of suppressed autophagy, which in turn has been associated with poor prognosis. The aim of this study was to evaluate the prognostic significance of p62-containing MDB and IHB in patients with HCC.

METHODS

Ninety resected HCCs were assessed by H&E histology for MDB or IHB, and their presence was confirmed by immunohistochemistry using K8/18, p62 and ubiquitin antibodies. The prognostic impact of inclusions was assessed using Kaplan-Meier and multivariate Cox proportional model.

RESULTS

Mallory-Denk bodies and/or IHB were found in about 50% of HCC. Both types of inclusions were seen in 21%, MDB only in 19% and IHB only in 10% of cases. The presence of MDB in tumours was associated with the steatohepatitic variant of HCC, which also showed fatty change, ballooning of tumour cells, MDBs, inflammation and pericellular fibrosis (P<.001). In contrast, IHBs were not associated with steatohepatitic morphology but were associated with significantly shorter overall survival (P=.006). Multivariate analysis revealed macroscopic vascular invasion (P=.045) and presence of IHB in HCC cells (P=.005) as independently associated with overall survival.

CONCLUSIONS

Intracellular hyaline bodies and macroscopic vascular invasion identify a subset of HCC patients with poor prognosis.

The mechanisms of Mallory-Denk body formation are similar to the formation of aggresomes in Alzheimer's disease and other neurodegenerative disorders

There is a possibility that the aggresomes that form in the brain in neurodegenerative diseases like Alzheimer's disease (AD) and in the liver where aggresomes like Mallory-Denk Bodies (MDB) form, share mechanisms. MDBs can be prevented by feeding mice sadenosylmethionine (SAMe) or betaine. Possibly these proteins could prevent AD. We compared the literature on MDBs and AD pathogenesis, which include roles played by p62, ubiquitin UBB +1, HSPs70, 90, 104, FAT10, NEDD8, VCP/97, and the protein quality control mechanisms including the 26s proteasome, the IPOD and JUNQ and autophagosome pathways.

A feature analysis of lower solubility proteins in three eukaryotic systems

Because misfolded and damaged proteins can form potentially harmful aggregates, all living organisms have evolved a wide variety of quality control mechanisms. However, the timely clearance of aggregation-prone species may not always be achieved, potentially leading to the accumulation of low solubility proteins. At the same time, promiscuity, which can be a driving force for aggregation, is also important to the functionality of certain proteins which have a large number of interaction partners. Considerable efforts have been made towards characterizing why some proteins appear to be more aggregation-prone than others. In this study, we analyze the features of proteins which precipitate following centrifugation in unstressed yeast cells, human SH-SY5Y cells and mouse brain tissue. By normalizing for protein abundance, we devised an approach whereby lower solubility proteins are reliably identified. Our findings indicate that these tend to be longer, low abundance proteins, which contain fewer hydrophobic amino acids. Furthermore, low solubility proteins also contain more low complexity and disordered regions. Overall, we observed an increase in features that link low solubility proteins to functional aggregates. Our results indicate that lower solubility proteins from three biologically distinct model systems share several common traits, shedding light on potentially universal solubility determinants.

BIOLOGICAL SIGNIFICANCE

We set up a novel approach to identify lower solubility proteins in unstressed cells by comparing precipitated proteins with those that remain soluble after centrifugation. By analyzing three eukaryotic model systems in parallel, we were able to identify traits which cross the species barrier, as well as species-specific characteristics. Notably, our analyses revealed a number of primary and secondary structural features that set apart lower solubility proteins, a number of which connected them to a greater potential for promiscuity. This article is part of a Special Issue entitled: Protein dynamics in health and disease. Guest Editors: Pierre Thibault and Anne-Claude Gingras.

Ufmylation and FATylation pathways are downregulated in human alcoholic and nonalcoholic steatohepatitis, and mice fed DDC, where Mallory-Denk bodies (MDBs) form

We previously reported the mechanisms involved in the formation of Mallory-Denk bodies (MDBs) in mice fed DDC. To further provide clinical evidence as to how ubiquitin-like protein (Ubls) modification, gene transcript expression in Ufmylation and FATylation were investigated in human archived formalin-fixed, paraffin-embedded (FFPE) liver biopsies and frozen liver sections from DDC re-fed mice were used. Real-time PCR analysis showed that all Ufmylation molecules (Ufm1, Uba5, Ufc1, Ufl1 and UfSPs) were significantly downregulated, both in DDC re-fed mice livers and patients' livers where MDBs had formed, indicating that gene transcript changes were limited to MDB-forming livers where the protein quality control system was downregulated. FAT10 and subunits of the immunoproteasome (LMP2 and LMP7) were both upregulated as previously shown. An approximate 176- and 5-fold upregulation (respectively) of FAT10 was observed in the DDC re-fed mice liver and in the livers of human alcoholic hepatitis with MDBs present, implying that there was an important role played by this gene. The FAT10-specific E1 and E2 enzymes Uba6 and USE1, however, were found to be downregulated both in patients' livers and in the liver of DDC re-fed mice. Interestedly, the downregulation of mRNA levels was proportionate to MDB abundance in the liver tissues. Our results show the first systematic demonstration of transcript regulation of Ufmylation and FATylation in the liver of patients who form MDBs, where protein quality control is downregulated. This was also shown in the livers of DDC re-fed mice where MDBs had formed.

Cytoskeletal proteins: shaping progression of hepatitis C virus-induced liver disease

Hepatitis C virus (HCV) infection, which results in chronic hepatitis C (CHC) in most patients (70-85%), is a major cause of liver disease and remains a major therapeutic challenge. The mechanisms determining liver damage and the key factors that lead to a high rate of CHC remain imperfectly understood. The precise role of cytoskeletal (CS) proteins in HCV infection remains to be determined. Some studies including our recent study have demonstrated that changes occur in the expression of CS proteins in HCV-infected hepatocytes. A variety of host proteins interact with HCV proteins. Association between CS and HCV proteins may have implications in future design of CS protein-targeted therapy for the treatment for HCV infection. This chapter will focus on the interaction between host CS and viral proteins to signify the importance of this event in HCV entry, replication and transportation.

Proteasome inhibitor treatment reduced fatty acid, triacylglycerol and cholesterol synthesis

In the present study, the beneficial effects of proteasome inhibitor treatment in reducing ethanol-induced steatosis were investigated. A microarray analysis was performed on the liver of rats injected with PS-341 (Bortezomib, Velcade), and the results showed that proteasome inhibitor treatment significantly reduced the mRNA expression of SREBP-1c, and the downstream lipogenic enzymes, such as fatty acid synthase (FAS) and acetyl-CoA carboxylase (ACC), which catalyzes the carboxylation of acetyl-CoA to malonyl-CoA, the rate-limiting step in fatty acid synthesis. ELOVL6, which is responsible for fatty acids long chain elongation, was also significantly downregulated by proteasome inhibitor treatment. Moreover, PS-341 administration significantly reduced the expression of acyl-glycerol-3-phosphate acyltransferase (AGPAT), and diacylglycerol acyltransferase (DGAT), enzyme involved in triacylglycerol (TAG) synthesis. Finally, PS-341 was found to downregulate the enzyme 3-hydroxy-3-methylglutaryl-CoenzymeA synthase (HMG-CoA synthase) that is responsible for cholesterol synthesis. Proteasome inhibitor was also found to play a role in intestinal lipid adsorption because apolipoproteins A (apoA-I, apoAII, apoA-IV and ApoCIII) were downregulated by proteasome inhibitor treatment, especially ApoA-II that is known to be a marker of alcohol consumption. Proteasome inhibitor treatment also decreased apobec-1 complementation factor (ACF) leading to lower level of editing and production of ApoB protein. Moreover apolipoprotein C-III, a major component of chylomicrons was significantly downregulated. However, lipoprotein lipase (Lpl) and High density lipoprotein binding protein (Hdlbp) mRNA levels were increased by proteasome inhibitor treatment. These results suggested that proteasome inhibitor treatment could be used to reduce the alcohol-enhanced lipogenesis and alcohol-induced liver steatosis. A morphologic analysis, performed on the liver of rats fed ethanol for one month and treated with PS-341, showed that proteasome inhibitor treatment significantly decreased ethanol-induced liver steatosis. SREBP-1c, FAS and ACC were increased by ethanol feeding alone, but were significantly decreased when proteasome inhibitor was administered to rats fed ethanol. Our results also show that both mRNA and protein levels of these lipogenic enzymes, up regulated by ethanol, were then downregulated when proteasome inhibitor was administered to rats fed ethanol. It was also confirmed that alcohol feeding caused an increase in AGPAT and DGAT, which was prevented by proteasome inhibitor treatment of the animal fed ethanol. Chronic alcohol feeding did not affect the gene expression of HMG-CoA synthase. However, PS341 administration significantly reduced the HMG-CoA synthase mRNA levels, confirming the results obtained with the microarray analysis. C/EBP transcription factors alpha (CCAAT/enhancer-binding protein alpha) has been shown to positively regulate SREBP-1c mRNA expression, thus regulating lipogenesis. Proteasome inhibition caused a decrease in C/EBP alpha mRNA expression, indicating that C/EBP downregulation may be the mechanism by which proteasome inhibitor treatment reduced lipogenesis. In conclusion, our results indicate that proteasome activity is not only involved in downregulating fatty acid synthesis and triacylglycerol synthesis, but also cholesterol synthesis and intestinal lipid adsorption. Proteasome inhibitor, administrated at a non-toxic low dose, played a beneficial role in reducing lipogenesis caused by chronic ethanol feeding and these beneficial effects are obtained because of the specificity and reversibility of the proteasome inhibitor used.

Cross β-sheet conformation of keratin 8 is a specific feature of Mallory-Denk bodies compared with other hepatocyte inclusions

BACKGROUND & AIMS

Mallory-Denk bodies (MDBs) are cytoplasmic protein aggregates in hepatocytes in steatohepatitis and other liver diseases. We investigated the molecular structure of keratin 8 (K8) and 18 (K18), sequestosome 1/p62, and ubiquitin, which are the major constituents of MDBs, to investigate their formation and role in disease pathogenesis.

METHODS

Luminescent conjugated oligothiophenes (LCOs), h-HTAA, and p-FTAA are fluorescent amyloid ligands that specifically bind proteins with cross β-sheet conformation. We used LCOs to investigate conformational changes in MDBs in situ in human and murine livers as well as in transfection studies.

RESULTS

LCO analysis showed cross β-sheet conformation in human MDBs from patients with alcoholic and nonalcoholic steatohepatitis or hepatocellular carcinoma, but not in intracellular hyaline bodies, α₁-antitrypsin deficiency, or ground-glass inclusions. LCOs bound to MDBs induced by 3,5-diethoxycarbonyl-1,4-dihydrocollidine feeding of mice at all developmental stages. CHO-K1 cells transfected with various combinations of SQSTM1/p62, ubi, and Krt8/Krt18 showed that K8 was more likely to have cross β-sheet conformation than K18, whereas p62 never had cross β-sheet conformation. The different conformational properties of K8 and K18 were also shown by circular dichroism analysis.

CONCLUSIONS

K8 can undergo conformational changes from predominantly α-helical to cross β-sheet, which would allow it to form MDBs. These findings might account for the observation that krt8⁻/⁻ mice do not form MDBs, whereas its excess facilitates MDB formation. LCOs might be used in diagnosis of liver disorders; they can be applied to formalin-fixed, paraffin-embedded tissues to characterize protein aggregates in liver cells.

Proteasome inhibitor treatment in alcoholic liver disease

Oxidative stress, generated by chronic ethanol consumption, is a major cause of hepatotoxicity and liver injury. Increased production of oxygen-derived free radicals due to ethanol metabolism by CYP2E1 is principally located in the cytoplasm and in the mitochondria, which does not only injure liver cells, but also other vital organs, such as the heart and the brain. Therefore, there is a need for better treatment to enhance the antioxidant response elements. To date, there is no established treatment to attenuate high levels of oxidative stress in the liver of alcoholic patients. To block this oxidative stress, proteasome inhibitor treatment has been found to significantly enhance the antioxidant response elements of hepatocytes exposed to ethanol. Recent studies have shown in an experimental model of alcoholic liver disease that proteasome inhibitor treatment at low dose has cytoprotective effects against ethanol-induced oxidative stress and liver steatosis. The beneficial effects of proteasome inhibitor treatment against oxidative stress occurred because antioxidant response elements (glutathione peroxidase 2, superoxide dismutase 2, glutathione synthetase, glutathione reductase, and GCLC) were up-regulated when rats fed alcohol were treated with a low dose of PS-341 (Bortezomib, Velcade^®). This is an important finding because proteasome inhibitor treatment up-regulated reactive oxygen species removal and glutathione recycling enzymes, while ethanol feeding alone down-regulated these antioxidant elements. For the first time, it was shown that proteasome inhibition by a highly specific and reversible inhibitor is different from the chronic ethanol feeding-induced proteasome inhibition. As previously shown by our group, chronic ethanol feeding causes a complex dysfunction in the ubiquitin proteasome pathway, which affects the proteasome system, as well as the ubiquitination system. The beneficial effects of proteasome inhibitor treatment in alcoholic liver disease are related to proteasome inhibitor reversibility and the rebound of proteasome activity 72 h post PS-341 administration.

Pathogenesis of alcoholic hepatitis: Role of inflammatory signaling and oxidative stress

Inflammatory signaling and oxidative stress are two major components in the pathogenesis of alcoholic hepatitis. Alcohol consumption results in translocation of gut bacteria into the portal system along with lipopolysaccharides that interact with toll-like receptors and results in the production of inflammatory and immunogenic mediators such as tumor necrosis factor-alpha (TNF-α) and interferons. Chronic consumption of alcohol causes priming of this process in which there is enhanced production of cytokines, interferon, interleukins, and TNF-α. Oxidative stress, genetic predisposition, and the unfolded protein response are other contributory mechanisms. Novel therapies aimed at these pathways may prevent, decrease, or delay the complications of alcoholic hepatitis.

Mallory-Denk body pathogenesis revisited

This editorial reviews the recent evidence showing that Mallory-Denk bodies (MDBs) form in hepatocytes as the result of a drug-induced shift from the 26s proteasome formation to the immunoproteasome formation. The shift is the result of changes in gene expression induced in promoter activation, which is induced by the IFNγ and TNFα signaling pathway. This activates TLR 2 and 4 receptors. The TLR signaling pathway stimulates both the induction of a cytokine proinflammatory response and an up regulation of growth factors. The MDB- forming hepatocytes proliferate as a result of the increase in growth factor expression by the MDB- forming cells, which selectively proliferate in response to drug toxicity. All of these mechanisms are induced by drug toxicity, and are prevented by feeding the methyl donors SAMe and betaine, supporting the epigenetic response of MDB formation.

High-content functional screen to identify proteins that correct F508del-CFTR function

Cystic Fibrosis is caused by mutations in CFTR, with a deletion of a phenylalanine at position 508 (F508del-CFTR) representing the most common mutation. The F508del-CFTR protein exhibits a trafficking defect and is retained in the endoplasmic reticulum. Here we describe the development of a high-content screen based on a functional assay to identify proteins that correct the F508del-CFTR defect. Using a HEK293 MSR GripTite cell line that stably expresses F508del-CFTR, we individually co-expressed approximately 450 unique proteins fused to the Cl(-)-sensitive YFP(H148Q/I152L) mutant. We then tested correction of F508del-CFTR function by the CI(-)/l(-) exchange method following stimulation with forskolin/IBMX/genistein, using quantitative recordings in multiple individual cells with a high-content (high-throughput) Cellomics KSR imaging system. Using this approach, we identified several known and novel proteins that corrected F508del-CFTR function, including STAT1, Endothelin 1, HspA4, SAPK substrate protein 1, AP2M1, LGALS3/galectin-3, Trk-fused gene, Caveolin 2, PAP/REG3alpha, and others. The ability of these correctors to rescue F508del-CFTR trafficking was then validated by demonstrating their enhancement of maturation (appearance of band C) and by cell surface expression of F508del-CFTR bearing HA tag at the ectodomain using confocal microscopy and flow cytometry. These data demonstrate the utility of high-content analyses for identifying proteins that correct mutant CFTR and discover new proteins that stimulate this correction. This assay can also be utilized for RNAi screens to identify inhibitory proteins that block correction of F508del-CFTR, small molecule, and peptide screens.

Autophagy activation by rapamycin eliminates mouse Mallory-Denk bodies and blocks their proteasome inhibitor-mediated formation

The proteasomal and lysosomal/autophagy pathways in the liver and other tissues are involved in several biological processes including the degradation of misfolded proteins. Exposure of hepatocyte cell lines to proteasome inhibitors (PIs) results in the formation of inclusions that resemble Mallory-Denk bodies (MDBs). Keratins are essential for MDB formation and keratin 8 (K8)-overexpressing transgenic mice are predisposed to MDB formation. We tested the hypothesis that PIs induce MDBs in vivo and that autophagy participates in MDB turnover. The effect of the PI bortezomib (which is used to treat some malignancies) on MDB formation was tested in K8-overexpressing mice and in cultured cells. Inclusion formation was examined using immune and conventional electron microscopy (EM). Bortezomib induced MDB-like inclusions composed of keratins, ubiquitin, and p62 in cultured cells. Short-term exposure to bortezomib induced similar inclusions in K8-overexpressing but not in nontransgenic mice, without causing liver injury. In bortezomib-treated mice, autophagy was activated in hepatocytes as determined by EM and biochemical analysis. Further activation of autophagy by rapamycin (Rap) decreased the number of inclusions in bortezomib-treated K8 transgenic mice significantly. Rap also led to resorption of spontaneously formed MDBs in aging K8-overexpressing mice. Immune EM demonstrated K8-positive and ubiquitin-positive structures in autophagic vacuoles in the mouse liver.

CONCLUSION

PIs alone are sufficient to induce MDBs in susceptible animals, while Rap-mediated activation of autophagy prevents MDB formation and causes MDB resorption. These findings suggest that some patients treated with PIs may become predisposed to MDB formation. Autophagy provides a potential cellular mechanism for the resorption of cytoplasmic inclusions.

The Mrj co-chaperone mediates keratin turnover and prevents the formation of toxic inclusion bodies in trophoblast cells of the placenta

Defects in protein-folding and -degradation machinery have been identified as a major cause of intracellular protein aggregation and of aggregation-associated diseases. In general, it remains unclear how these aggregates are harmful to normal cellular function. We demonstrate here that,in the developing placenta of the mouse, the absence of the Mrj (Dnajb6)co-chaperone prevents proteasome degradation of keratin 18 (K18; Krt18)intermediate filaments, resulting in the formation of keratin inclusion bodies. These inclusions in chorionic trophoblast cells prevent chorioallantoic attachment during placental development. We show further that keratin-deficient embryos undergo chorioallantoic attachment and that, by genetically reducing keratin expression in Mrj-/-conceptuses, chorioallantoic attachment was rescued. Therefore, the chorioallantoic attachment phenotype in Mrj mutants is not due to a deficiency of the normal keratin cytoskeleton, but rather is cytotoxicity caused by keratin aggregates that disrupt chorion trophoblast cell organization and function.

2005 Trophoblast Research Award Lecture: Defects in the keratin cytoskeleton disrupt normal murine placental development and trophoblast cell function

The keratin cytoskeleton is present in all trophoblast cell subtypes of the mouse and human placenta and is required to maintain the structural integrity of these cells. Recently, various genetic mouse models have shown that a normal keratin network is necessary for placental development. Keratin-deficiency leads to trophoblast giant cell fragility, breaking the barrier between the conceptus and the maternal blood circulation. Alternatively, keratin aggregation prevents chorioallantoic attachment, a key developmental milestone required for the formation of the labyrinth within the mouse placenta. These models give us insight into cytokeratin function in human trophoblast cell subtypes and suggest that defects in the keratin cytoskeleton may result in intrauterine growth restriction or miscarriage.

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.

Mallory body (cytokeratin aggresomes) formation is prevented in vitro by p38 inhibitor

Microarray analysis of livers from mice fed diethyl-1,4-dihydro-2,4,6-trimethyl-3,5-pyridinedicarboxylate (DDC) to induce Mallory body (MB) cytokeratin aggresome formation showed that gene expression for cellular adhesion molecules, cytokeratins, kinases and aggresome forming proteins were upregulated, when MBs were formed in vivo. This response was enhanced when the DDC was refed (mice fed DDC for 10 weeks followed by DDC withdrawal for 1 month, then refed DDC for 7 days). Immunofluorescent antibody staining of the MBs that formed showed that MAPK p38 was colocalized with ubiquitin and p62 in the MBs. To investigate further the mechanisms of MB formation, primary cultures derived from DDC primed mice and their controls were incubated for 6 days. Liver cells cultured for 3 h and 6 days were used for microarray analysis. At 3 h, there were no MBs formed, but MBs were numerous after 6 days of culture. At 3 h, the expression of a large number of genes was different when the control, and the DDC primed hepatocytes were compared, which indicates that the primed hepatocytes were phenotypically changed. The gene expression of many kinases including p38 was upregulated after 6 days where the gene expression of cytokeratins, adhesion molecules and aggresome forming proteins were upregulated when MBs formed. An inhibitor of p38 phosphorylation (SB202190) completely prevented MB formation. Western blot showed that phosphorylated p38 MAPK and total p38 were absent in vitro after the p38 inhibitor treatment. Immunostaining of 6-day DDC-primed hepatocyte cultures stained with antibodies to p62 and phospho-p38 MAPK showed that phosphorylated p38 MAPK was concentrated within the MBs. Antibodies to specific serine phosphorylated sites 73 and 431, located in cytokeratin 8, localized to Mallory bodies in vivo, indicating that cytokeratin 8 was hyperphosphorylated. The data supported the concept that MBs form as the result of hyperphosphorylation of cytokeratin 8 by p38.

Proteome analysis of human liver carcinoma Huh7 cells harboring hepatitis C virus subgenomic replicon

Chronic infection by hepatitis C virus (HCV) is closely correlated with serious liver diseases. Although considerable progress has been made during recent years, the mechanism of replication and pathogenesis of HCV infection are still elusive. We have applied proteomic techniques in this work to globally analyze the protein expression profiles of a human liver cell lines Huh7 in absence and presence of HCV replication, aiming at elucidating the components of HCV replication and the cellular responses to HCV replication. The protein mixtures of three subcellular fractions from Huh7 and Huh7-HCV were separated by 2-DE under various pH gradients. Differentially expressed spots were identified by MALDI-TOF MS, followed by database searching. A total of 179 comparative proteins were identified unambiguously, including proteins associated with host cytoskeleton, intracellular traffic, oxidative and ER stress, proteasome degradation, translation, apoptosis, proliferation, etc. Host proteins known to interact with HCV proteins, such as HSP27, alpha-actinin, nucleolin and eukaryotic initiation factor 4A-I, were elevated in Huh7-HCV cells. Our study provides the global information of proteomic alteration of Huh7 cells in the presence of HCV replication and the clues for further understanding of the mechanism of HCV replication and pathogenesis.

Increased oxidation and degradation of cytosolic proteins in alcohol-exposed mouse liver and hepatoma cells

We recently developed a sensitive method using biotin-N-maleimide (biotin-NM) as a probe to positively identify oxidized mitochondrial proteins. In this study, biotin-NM was used to identify oxidized cytosolic proteins in alcohol-fed mouse livers. Alcohol treatment for 6 wk elevated the levels of CYP2E1 and nitrotyrosine, a marker of oxidative stress. Markedly increased levels of oxidized proteins were detected in alcohol-fed mouse livers compared to pair-fed controls. The biotin-NM-labeled oxidized proteins from alcohol-exposed mouse livers were subsequently purified with streptavidin-agarose and resolved on 2-DE. More than 90 silver-stained protein spots that displayed differential intensities on 2-D gels were identified by MS. Peptide sequence analysis revealed that many enzymes or proteins involved in stress response, chaperone activity, intermediary metabolism, and antioxidant defense systems such as peroxiredoxin were oxidized after alcohol treatment. Smaller fragments of many proteins were repeatedly detected only in alcohol-fed mice, indicating that many oxidized proteins after alcohol exposure were degraded. Immunoblot results showed that the level of oxidized peroxiredoxin (inactivated) was markedly increased in the alcohol-exposed mouse livers and ethanol-sensitive hepatoma cells compared to the corresponding controls. Our results may explain the underlying mechanism for cellular dysfunction and increased susceptibility to other toxic agents following alcohol-mediated oxidative stress.

Hindlimb casting decreases muscle mass in part by proteasome-dependent proteolysis but independent of protein synthesis

The hypothesis of the present study was that rats subjected to short-term unilateral hindlimb immobilization would incur skeletal muscle wasting and concomitant alterations in protein synthesis, controllers of translation, and indexes of protein degradation. Rats were unilaterally casted for 1, 3, or 5 days to avoid complications associated with other disuse models. In the casted limb, gastrocnemius wet weight decreased 12% after 3 days and thereafter remained constant. In contrast, the contralateral control leg displayed a steady growth rate over time. The rate of protein synthesis and translational efficiency were unchanged in the immobilized muscle at day 5. The total amount and phosphorylation state of regulators of translational initiation and elongation were unaltered. The mRNA contents of polyubiquitin and the ubiquitin ligases muscle atrophy F-box (MAFbx)/Atrogin-1 and muscle RING finger 1 (MuRF1) were elevated in immobilized muscle at all time points, with peak expression occurring at day 3. Daily injection of the type II glucocorticoid receptor antagonist RU-486 did not prevent decreases in gastrocnemius wet weight nor increases in mRNA for MAFbx/Atrogin-1 and MuRF1. However, in vivo administration of the proteasome inhibitor Velcade prevented 53% of wet weight loss associated with 3 days of immobilization. These data suggest that the loss of skeletal muscle mass in this model of disuse appears to be glucocorticoid independent, can be partially rescued with a potent proteasome inhibitor, and is associated with enhanced mRNA expression of multiple factors that contribute to ubiquitin- proteasome-dependent degradation and are likely to control the remodeling of immobilized skeletal muscle during atrophy.

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.

The role of laminin–integrin signaling in triggering MB formation. An in vivo and in vitro study

It is still unclear as to how hepatocytes perceive external factors and transduce the signals which initiate MB formation. To investigate this phenomenon, the model of MB formation in liver in vivo and in primary culture of hepatocytes derived from drug-primed mice was used. Control mice were fed the control diet (group 1). MBs were induced in the livers of mice fed diethyl-1, 4-dihydro-2, 4, 6-trimethyl-3, 5-pyridinedicarboxylate (DDC) for 10 weeks (group 2). The induced MBs completely disappeared after the withdrawal of DDC for 4 weeks (group 3). Newly formed MBs were numerous after DDC was refed for 1 week (group 4). Relative mRNA abundance was determined by quantitative real-time RT-PCR in the liver from the mice. The expression of integrin alpha(6) and beta(2) was significantly increased in the livers of DDC-treated (group 2) and drug refed mice (group 4), when compared with the livers from controls (group 1) and DDC-withdrawn (group 3) mice. The increased mRNA of these two integrin genes was associated with the increased expression of laminin (a ligand for integrin alpha(6)beta(1) and alpha(6)beta(4)), Icam1 (a ligand of alphaLbeta2), Src, MEKK1, and ERK1. Primary cultures of isolated DDC-primed hepatocytes (group 4 mice were withdrawn from DDC-CMZ for 4-6 weeks) produced significantly more MBs on laminin-coated coverslips compared with plastic uncoated, fibronectin-, collagen-, or fibrinogen-coated coverslips. U0126, an inhibitor of MEK1 protein, significantly reduced the phosphorylated forms of ERK1/2 and MB formation in vitro. In conclusion, the current study revealed an association between MB formation and integrin-mediated signaling in vivo. The data indicate that laminin-integrin signaling which activates ERK, triggered MB formation in vitro, and an inhibitor of the signaling cascade reduced MB formation.

The p105/50 NF-kappaB pathway is essential for Mallory body formation

To determine if nuclear factor-kappaB (NF-kB) plays a role in Mallory body (MB) formation, quantitative real-time RT-PCR assay was used to measure liver NF-kappaB1/p105 mRNA levels in 4 different groups of mice. Group 1: mice given IP saline for 15 weeks; group 2: mice fed diethyl 1,4-dihydro-2,4,6,-trimethyl-3,5-pyridinedicarboxylate (DDC) for 10 weeks when MBs were formed; group3: mice fed DDC 10 weeks, then withdrawn 5 weeks when MBs disappeared; group 4: mice fed DDC 10 weeks, withdrawn 4 weeks, then fed DDC+chlormethiazole (CMZ) for 1 week when MBs again formed. The mRNA for p105 NF-kappaB expression was significantly increased in the livers of mice treated with DDC (group 2) and DDC+CMZ (group 4) compared with the control livers (group 1) as well as the drug-withdrawal livers (group 3). Primary cultures of hepatocytes from drug-primed mice (the group 4 mice were withdrawn for another 4 weeks when the MBs had disappeared) were studied. The hepatocytes from drug-primed mice were MB free when isolated and used for primary culture. MBs began to form spontaneously within their cytoplasm after 2-3 days of culture. The NF-kappaB inhibitor (NF-kappaBi), a cell-permeable quinazoline compound that acts as a potent inhibitor of NF-kappaB transcriptional activation, was added to the medium 3 h after planting the cultures of liver cells. No MBs formed in the cells treated with 10 microM, 1 microM, and 0.1 microM NF-kappaBi for 6 days. MBs still formed in the cells treated with 10 nM NF-kappaBi for 6 days. Both DDC-primed and normal control liver cells began to enlarge and elongate after a few hours of culture. In contrast, the cells treated with NF-kappaBi stayed polyhedral in shape just as they appeared prior to culturing. The level of NF-kappaB1/p105 mRNA significantly increased in DDC-primed hepatocytes after 24 h of culture and in normal control hepatocytes after 48 h of culture. In DDC-primed hepatocytes, NF-kappaBi 0.1 muM treatment for 6 days significantly decreased mRNA expression of Src, p105/NF-kappaB1, ERK1, MEKK1, and JNK1/2. In normal control liver cells, NF-kappaBi treatment decreased mRNA expression of Src and JNK1 and stimulated the mRNA expression of p105/NF-kappaB1 and Junk2. NF-kappaBi treatment significantly decreased the total ERK1/2 protein and further decreased the phosphorylated (activated) form of ERK1/2 in the cultured hepatocytes. The results indicate that the p105 NF-kappaB pathway which putatively regulates ERK at both the transcriptional and post-translational levels regulates MB formation by way of changes in gene expression.

RNA interference of VCP/p97 increases Mallory body formation

In the present report, valosin-containing protein (VCP) was present in Mallory bodies (MBs). To determine if VCP plays a role in MB formation, primary cultured hepatocytes from drug-primed mice that spontaneously form MBs in vitro were studied. The results were compared with control normal hepatocytes. Gene-specific FITC-labeled gripNA (gVCP) was added to the medium of the primary cultures to inhibit the expression of VCP. gVCP increased MB formation by 230% in drug-primed mouse hepatocytes compared with primed liver cells where no VCP oligos were added. Blocking VCP expression induced both multiple small ubiquitin (Ub) and cytokeratin (CK) aggregates to form within the cytoplasm in normal mouse hepatocytes. Inhibition of VCP expression in both drug-primed and control hepatocytes caused a decrease in proteasome chymotrypsin-like (ChT-L) activity. Overexpression of VCP was achieved by transfecting the hepatocytes with a plasmid containing green fluorescent protein (GFP)-fused VCP (pVCP-GFP). Overexpressed VCP was located in both the cytoplasm and nucleus of pVCP-GFP overexpressing drug-primed hepatocytes. VCP was also concentrated within MBs. MB formation was not decreased by the overexpression of VCP in the cells. These results indicate that VCP plays an important role in inducing MB formation, probably through its molecular chaperone function in the ubiquitin-proteasome system (UPS).

p62 is involved in the mechanism of Mallory body formation

p62 is a scaffolding protein that binds to polyubiquitin. It is involved in the degradation of proteins by the proteasome. To determine if p62 is critical in the development of Mallory bodies (MBs), primary culture hepatocytes from drug-primed mice were studied and the results were compared with normal hepatocytes. Gene-specific gripNA (gp62) was added to the medium of the primary cultures of the hepatocytes to inhibit the expression of p62. Overexpression of p62 was achieved by transfecting the hepatocytes with a plasmid containing green fluorescent protein (GFP) fused p62 (p62-GFP). Gp62 dramatically inhibited MB formation by 94% in drug-primed hepatocytes. The cells transfected with gp62 had decreased protein levels of p62, ubiquitin (Ub), and cytokeratin 8 (CK8). Overexpression of p62 accelerated and enhanced MB formation by 339% in drug-primed hepatocytes. Overexpression of p62 in normal mouse hepatocytes induced MB-like aggresomes that were stained by Ub but not by CK8. The results indicate that p62 is involved in the mechanism of MB formation.