Hepatitis C virus and ethanol alter antigen presentation in liver cells

Astragaloside IV Regulates cGAS-STING Signaling Pathway to Alleviate Immunosuppression Caused by PRRSV Infection

Porcine reproductive and respiratory syndrome virus (PRRSV) poses a global threat to pig health and results in significant economic losses. Impaired innate and adaptive immune responses are evident during PRRSV infection. Cyclic GMP-AMP synthase (cGAS), a classical pattern recognition receptor recognizing mainly intracytoplasmic DNA, induces type I IFN responses through the cGAS-STING signaling pathway. It has also been demonstrated that cGAS-STING is involved in PRRSV infection. This study utilized the qRT-PCR, ELISA, and WB methods to examine the effects of Astragaloside IV (AS-IV) on the regulation of innate immune function and cGAS-STING signaling pathway in porcine alveolar macrophages. The results showed that AS-IV attenuated the decreased innate immune function caused by PRRSV infection, restored the inhibited cGAS-STING signaling pathway, and increased the expression of interferon, ultimately exerting antiviral effects. Moreover, these results suggest that AS-IV may be a promising candidate for a new anti-PRRSV antiviral, and its mechanism of action may provide insights for developing novel antiviral agents.

Effect of alcohol on productivity and quality of adeno-associated virus 2 in HEK293 cells

Investigation of enhancers to improve recombinant adeno-associated virus 2 (rAAV2) productivity by human embryonic kidney 293 cells (HEK293) suspension culture showed that the addition of ethanol improved the productivity and packaged genome integrity of rAAV2. Further optimization showed that adding ethanol in the range of 0.09%-1.11% (v/v) during rAAV2 production effectively improved rAAV2 productivity and quality. In addition, ethanol addition improved cell viability. Furthermore, proteome and pathway analysis of the cells during rAAV2 production showed that the addition of ethanol resulted in the upregulation of pathways related to intercellular signaling, gene expression, cell morphology, intercellular maintenance, and others. In contrast, pathways related to cell death, immunity, and reactions to infection were downregulated. These changes in pathway regulation were responsible for the improvement in rAAV2 productivity, packaged genome integrity, and cell viability during rAAV2 production. The results of this study can be applied to the production of viral vectors for in vivo gene therapy in an inexpensive and safe manner.

Second hits exacerbate alcohol-related organ damage: an update

Chronic and excessive alcohol abuse cause direct and indirect detrimental effects on a wide range of body organs and systems and accounts for ~4% of deaths worldwide. Many factors influence the harmful effects of alcohol. This concise review presents newer insights into the role of select second hits in influencing the progression of alcohol-induced organ damage by synergistically acting to generate a more dramatic downstream biological defect. This review specifically addresses on how a lifestyle factor of high fat intake exacerbates alcoholic liver injury and its progression. This review also provides the mechanistic insights into how increasing matrix stiffness during liver injury promotes alcohol-induced fibrogenesis. It also discusses how hepatotropic viral (HCV, HBV) infections as well as HIV (which is traditionally not known to be hepatotropic), are potentiated by alcohol exposure to promote hepatotoxicity and fibrosis progression. Finally, this review highlights the impact of reactive aldehydes generated during alcohol and cigarette smoke coexposure impair innate antimicrobial defense and increased susceptibility to infections. This review was inspired by the symposium held at the 17th Congress of the European Society for Biomedical research on Alcoholism in Lille, France entitled 'Second hits in alcohol-related organ damage'.

The effects of alcohol on spontaneous clearance of acute hepatitis C virus infection in females versus males

BACKGROUND

Approximately one quarter of persons exposed to hepatitis C virus (HCV) will spontaneously clear infection. We undertook this study to investigate the impact of alcohol on likelihood of HCV spontaneous viral clearance stratified by sex groups.

METHODS

Pooled data from an international collaboration of prospective observational studies of incident HIV and HCV infection in high-risk cohorts (the InC3 Study) was restricted to 411 persons (or 560.7 person-years of observation) with documented acute HCV infection and data regarding alcohol use. The predictor of interest was self-reported alcohol use at or after estimated date of incident HCV infection and the outcome was HCV spontaneous clearance. Sex stratified Cox proportional hazards models were used to evaluate the association between alcohol and spontaneous clearance, adjusting for age, race/ethnicity, and IFNL4 genotype.

RESULTS

The median age was 28.5 years, 30.4% were women, 87.2% were white, and 71.8% reported alcohol use at or after incident infection. There were 89 (21.6%) cases of spontaneous clearance observed, 39 (31.2%) among women and 50 (17.5%) in men (p<0.01). Overall, spontaneous clearance occurred less frequently among participants who drank alcohol compared to those who did not drink (18.9% v. 28.5%, p=0.03). After adjustment for other covariates, alcohol was significantly and independently associated with lower relative hazards for spontaneous clearance of HCV in women (AHR=0.35; 95% CI: 0.19-0.66; p=0.001) but not in men (AHR=0.63; 95% CI: 0.36-1.09; p=0.10).

CONCLUSION

Results indicate that abstaining from drinking alcohol may increase the likelihood of spontaneous clearance among women.

Alcohol use disorder and its impact on chronic hepatitis C virus and human immunodeficiency virus infections

Alcohol use disorder (AUD) and hepatitis C virus (HCV) infection frequently co-occur. AUD is associated with greater exposure to HCV infection, increased HCV infection persistence, and more extensive liver damage due to interactions between AUD and HCV on immune responses, cytotoxicity, and oxidative stress. Although AUD and HCV infection are associated with increased morbidity and mortality, HCV antiviral therapy is less commonly prescribed in individuals with both conditions. AUD is also common in human immunodeficiency virus (HIV) infection, which negatively impacts proper HIV care and adherence to antiretroviral therapy, and liver disease. In addition, AUD and HCV infection are also frequent within a proportion of patients with HIV infection, which negatively impacts liver disease. This review summarizes the current knowledge regarding pathological interactions of AUD with hepatitis C infection, HIV infection, and HCV/HIV co-infection, as well as relating to AUD treatment interventions in these individuals.

Activity-based profiling of the proteasome pathway during hepatitis C virus infection

Hepatitis C virus (HCV) infection often leads to chronic hepatitis, liver cirrhosis, and hepatocellular carcinoma. The stability of the HCV proteins is controlled by ubiquitin-dependent and ubiquitin-independent proteasome pathways. Many viruses modulate proteasome function for their propagation. To examine the interrelationship between HCV and the proteasome pathways we employed a quantitative activity-based protein profiling method. Using this approach we were able to quantify the changes in the activity of several proteasome subunits and found that proteasome activity is drastically reduced by HCV replication. The results imply a link between the direct downregulation of the activity of this pathway and chronic HCV infection.

Influence of host resistance on viral adaptation: hepatitis C virus as a case study

Genetic and cellular studies have shown that the host’s innate and adaptive immune responses are an important correlate of viral infection outcome. The features of the host’s immune response (host resistance) reflect the coevolution between hosts and pathogens that has occurred over millennia, and that has also resulted in a number of strategies developed by viruses to improve fitness and survival within the host (viral adaptation). In this review, we discuss viral adaptation to host immune pressure via protein–protein interactions and sequence-specific mutations. Specifically, we will present the “state of play” on viral escape mutations to host T-cell responses in the context of the hepatitis C virus, and their influence on infection outcome.

Update on Alcohol and Viral Hepatitis

Alcohol consumption is often associated with viral hepatitis. Although alcohol is known to worsen viral liver disease, the interactions between alcohol and viral hepatitis are not fully understood. Molecular alterations in the liver due to alcohol and viral hepatitis include effects on viral replication, increased oxidative stress, cytotoxicity, and a weakened immune response. Clinically, alcohol enhances disease progression and favors induction of primitive liver neoplasm. The use of new antivirals for hepatitis C and well-established drugs for hepatitis B will determine how viral hepatitis can be controlled in a large percentage of these patients. However, alcohol-related liver disease continues to represent a barrier for access to antivirals, and it remains an unresolved health issue.

Update on Alcohol and Viral Hepatitis

Alcohol consumption is often associated with viral hepatitis. Although alcohol is known to worsen viral liver disease, the interactions between alcohol and viral hepatitis are not fully understood. Molecular alterations in the liver due to alcohol and viral hepatitis include effects on viral replication, increased oxidative stress, cytotoxicity, and a weakened immune response. Clinically, alcohol enhances disease progression and favors induction of primitive liver neoplasm. The use of new antivirals for hepatitis C and well-established drugs for hepatitis B will determine how viral hepatitis can be controlled in a large percentage of these patients. However, alcohol-related liver disease continues to represent a barrier for access to antivirals, and it remains an unresolved health issue.

Influence of HIV and HCV on T cell antigen presentation and challenges in the development of vaccines

Some of the central challenges for developing effective vaccines against HIV and hepatitis C virus (HCV) are similar. Both infections are caused by small, highly mutable, rapidly replicating RNA viruses with the ability to establish long-term chronic pathogenic infection in human hosts. HIV has caused 60 million infections globally and HCV 180 million and both viruses may co-exist among certain populations by virtue of common blood-borne, sexual, or vertical transmission. Persistence of both pathogens is achieved by evasion of intrinsic, innate, and adaptive immune defenses but with some distinct mechanisms reflecting their differences in evolutionary history, replication characteristics, cell tropism, and visibility to mucosal versus systemic and hepatic immune responses. A potent and durable antibody and T cell response is a likely requirement of future HIV and HCV vaccines. Perhaps the single biggest difference between the two vaccine design challenges is that in HCV, a natural model of protective immunity can be found in those who resolve acute infection spontaneously. Such spontaneous resolvers exhibit durable and functional CD4⁺ and CD8⁺ T cell responses (Diepolder et al., 1995; Cooper et al., 1999; Thimme et al., 2001; Grakoui et al., 2003; Lauer et al., 2004; Schulze Zur Wiesch et al., 2012). However, frequent re-infection suggests partial or lack of protective immunity against heterologous HCV strains, possibly indicative of the degree of genetic diversity of circulating HCV genotypes and subtypes. There is no natural model of protective immunity in HIV, however, studies of “elite controllers,” or individuals who have durably suppressed levels of plasma HIV RNA without antiretroviral therapy, has provided the strongest evidence for CD8⁺ T cell responses in controlling viremia and limiting reservoir burden in established infection. Here we compare and contrast the specific mechanisms of immune evasion used by HIV and HCV, which subvert adaptive human leukocyte antigen (HLA)-restricted T cell immunity in natural infection, and the challenges these pose for designing effective preventative or therapeutic vaccines.

Ethanol and hepatitis C virus suppress peptide-MHC class I presentation in hepatocytes by altering proteasome function

BACKGROUND

Previously, we reported that exposure of hepatitis C virus (HCV) core-expressing ethanol (EtOH)-metabolizing cells to EtOH significantly suppresses proteasome activity which exists as 26S (20S and 19S) and as an unassociated 20S particle. The replacement of the constitutive proteasomal subunits with immunoproteasome (IPR) favors antigen processing. Here, we examined the effects of EtOH consumption by HCV core transgenic mice on proteasome activity in hepatocytic lysates and in partially purified 26S proteasome and the impact of these changes on antigen presentation.

METHODS

HCV (-) and HCV (+) core transgenic mice were fed chow diet with or without 20% (v/v) EtOH in water for 4 weeks. Following the feeding regimen, hepatocytes were isolated and examined for chymotrypsin-like proteasome activity, oxidative stress, and the presentation of SIINFEKL-H2Kb complex. Additionally, the constitutive proteasome and IPR were purified for further analysis and identification of proteasome-interacting proteins (PIPs).

RESULTS

EtOH significantly decreased proteasome activity in hepatocytes of HCV (+) mice, and this finding correlated with oxidative stress and dysregulated methylation reactions. In isolated 26S proteasome, EtOH suppressed proteasome activity equally in HCV (+) and HCV (-) mice. EtOH feeding caused proteasome instability and lowered the content of both constitutive and IPR subunits in the 20S proteasome. In addition, the level of other PIPs, PA28 and UCHL5, were also suppressed after EtOH exposure. Furthermore, in EtOH-fed mice and, especially, in HCV (+) mice, the presentation of SIINFEKL-H2Kb complex in hepatocytes was also decreased.

CONCLUSIONS

Proteasomal dysfunction induced by EtOH feeding and exacerbated by the presence of HCV structural proteins led to suppression of SIINFEKL-H2Kb presentation in hepatocytes.

Hepatitis C virus-mediated inhibition of cathepsin S increases invariant-chain expression on hepatocyte surface

Hepatocytes are the main source of hepatitis C virus (HCV) replication and contain the maximum viral load in an infected person. Chronic HCV infection is characterized by weak cellular immune responses to viral proteins. Cathepsin S is a lysosomal cysteine protease and controls HLA-DR-antigen complex presentation through the degradation of the invariant chain. In this study, we examined the effect of HCV proteins on cathepsin S expression and found it to be markedly decreased in dendritic cells (DCs) exposed to HCV or in hepatocytes expressing HCV proteins. The downregulation of cathepsin S was mediated by HCV core and NS5A proteins involving inhibition of the transcription factors interferon regulatory factor 1 (IRF-1) and upstream stimulatory factor 1 (USF-1) in gamma interferon (IFN-γ)-treated hepatocytes. Inhibition of cathepsin S by HCV proteins increased cell surface expression of the invariant chain. In addition, hepatocytes stably transfected with HCV core or NS5A inhibited HLA-DR expression. Together, these results suggested that HCV has an inhibitory role on cathepsin S-mediated major histocompatibility complex (MHC) class II maturation, which may contribute to weak immunogenicity of viral antigens in chronically infected humans.

The peritoneal macrophage inflammatory profile in cirrhosis depends on the alcoholic or hepatitis C viral etiology and is related to ERK phosphorylation

BACKGROUND

The development of ascites in cirrhotic patients generally heralds a deterioration in their clinical status. A differential gene expression profile between alcohol- and hepatitis C virus (HCV)-related cirrhosis has been described from liver biopsies, especially those associated with innate immune responses. The aim of this work was to identify functional differences in the inflammatory profile of monocyte-derived macrophages from ascites in cirrhotic patients of different etiologies in an attempt to extrapolate studies from liver biopsies to immune cells in ascites. To this end 45 patients with cirrhosis and non-infected ascites, distributed according to disease etiology, HCV (n=15) or alcohol (n=30) were studied. Cytokines and the cell content in ascites were assessed by ELISA and flow cytometry, respectively. Cytokines and ERK phosphorylation in peritoneal monocyte-derived macrophages isolated and stimulated in vitro were also determined.

RESULTS

A different pattern of leukocyte migration to the peritoneal cavity and differences in the primed status of macrophages in cirrhosis were observed depending on the viral or alcoholic etiology. Whereas no differences in peripheral blood cell subpopulations could be observed, T lymphocyte, monocyte and polymorphonuclear cell populations in ascites were more abundant in the HCV than the alcohol etiology. HCV-related cirrhosis etiology was associated with a decreased inflammatory profile in ascites compared with the alcoholic etiology. Higher levels of IL-10 and lower levels of IL-6 and IL-12 were observed in ascitic fluid from the HCV group. Isolated peritoneal monocyte-derived macrophages maintained their primed status in vitro throughout the 24 h culture period. The level of ERK1/2 phosphorylation was higher in ALC peritoneal macrophages at baseline than in HCV patients, although the addition of LPS induced a greater increase in ERK1/2 phosphorylation in HCV than in ALC patients.

CONCLUSIONS

The macrophage inflammatory status is higher in ascites of alcohol-related cirrhotic patients than in HCV-related patients, which could be related with differences in bacterial translocation episodes or regulatory T cell populations. These findings should contribute to identifying potential prognostic and/or therapeutic targets for chronic liver diseases of different etiology.

Roles of the two distinct proteasome pathways in hepatitis C virus infection

Hepatitis C virus (HCV) infection often causes chronic hepatitis, liver cirrhosis, and hepatocellular carcinoma. The development of a HCV cell culture system enabled us to investigate its whole HCV life cycle and develop a better understanding of the pathogenesis of this virus. Post-translational modification plays a crucial role in HCV replication and in the maturation of viral particles. There is growing evidence also suggesting that the ubiquitin-proteasome pathway and the ubiquitin-independent proteasome pathway are involved in the stability control of HCV proteins. Many viruses are known to manipulate the proteasome pathways to modulate the cell cycle, inhibit apoptosis, evade the immune system, and activate cell signaling, thereby contributing to persistent infection and viral carcinogenesis. The identification of functional interactions between HCV and the proteasome pathways will therefore shed new light on the life cycle and pathogenesis of HCV. This review summarizes the current knowledge on the involvement of the ubiquitin-dependent and -independent proteasome pathways in HCV infection and discusses the roles of these two distinct mechanisms in HCV pathogenesis.

Oxidative stress, endogenous antioxidants, alcohol, and hepatitis C: pathogenic interactions and therapeutic considerations

Hepatitis C virus (HCV) is a blood-borne pathogen that was identified as an etiologic agent of non-A, non-B hepatitis in 1989. HCV is estimated to have infected at least 170 million people worldwide. The majority of patients infected with HCV do not clear the virus and become chronically infected, and chronic HCV infection increases the risk for hepatic steatosis, cirrhosis, and hepatocellular carcinoma. HCV induces oxidative/nitrosative stress from multiple sources, including inducible nitric oxide synthase, the mitochondrial electron transport chain, hepatocyte NAD(P)H oxidases, and inflammation, while decreasing glutathione. The cumulative oxidative burden is likely to promote both hepatic and extrahepatic conditions precipitated by HCV through a combination of local and more distal effects of reactive species, and clinical, animal, and in vitro studies strongly point to a role of oxidative/nitrosative stress in HCV-induced pathogenesis. Oxidative stress and hepatopathogenesis induced by HCV are exacerbated by even low doses of alcohol. Alcohol and reactive species may have other effects on hepatitis C patients such as modulation of the host immune system, viral replication, and positive selection of HCV sequence variants that contribute to antiviral resistance. This review summarizes the current understanding of redox interactions of HCV, outlining key experimental findings, directions for future research, and potential applications to therapy.

Redox signaling and the innate immune system in alcoholic liver disease

The development of alcoholic liver disease (ALD) is a complex process involving both parenchymal and nonparenchymal cells resident in the liver. Although the mechanisms for ALD are not completely understood, it is clear that increased oxidative stress, and activation of the innate immune system are essential elements in the pathophysiology of ALD. Oxidative stress from ethanol exposure results from increased generation of reactive oxygen species and decreased hepatocellular antioxidant activity, including changes in the thioredoxin/peroxiredoxin family of proteins. Both cellular and circulating components of the innate immune system are activated by exposure to ethanol. For example, ethanol exposure enhances toll-like receptor-4 (TLR-4)-dependent cytokine expression by Kupffer cells, likely due, at least in part, to dysregulation of redox signaling. Similarly, complement activation in response to ethanol leads to increased production of the anaphylatoxins, C3a and C5a, and activation C3a receptor and C5a receptor. Complement activation thus contributes to increased inflammatory cytokine production and can influence redox signaling. Here we will review recent progress in understanding the interactions between oxidative stress and innate immunity in ALD. These data illustrate that ethanol-induced oxidative stress and activation of the innate immune system interact dynamically during ethanol exposure, exacerbating ethanol-induced liver injury.

Ethanol inhibits antigen presentation by dendritic cells

Previous studies suggest that altered virus-specific T-cell responses observed during chronic ethanol exposure may be due to abnormal functioning of dendritic cells (DCs). Here we explored the effects of ethanol on exogenous antigen presentation by DCs. BALB/c, C57BL/6, and CBA/caj mice were fed ethanol or an isocaloric control diet for 8 weeks. The splenic DC population was expanded using an Flt3L expression plasmid via tail vein injection. DCs were purified and assessed for antigen presentation and processing and for peptide-major histocompatibility complex class I and II (MHCI and MHCII) formation on the cell surface. Interleukin-2 (IL-2) was measured as an indicator of antigen-specific T-cell activation by DCs in coculture. Antigen processing and peptide-MHCII complexes were evaluated by flow cytometry. We observed that ethanol not only suppressed allogeneic peptide presentation to T cells by DCs but also altered presentation of exogenous ovalbumin (OVA) peptide 323-339 to an OVA-specific DO11 T-cell line as well as to OVA-sensitized primary T cells. Smaller amounts of peptide-MHCII complexes were found on the DCs isolated from the spleens of ethanol-fed mice. In contrast to MHCII presentation, cross-presentation of exogenous OVA peptide via MHCI by DCs remained intact. More importantly, ethanol-exposed DCs had reduced B7-DC and enhanced ICOS-L (inhibitory) costimulatory molecule expression. Ethanol inhibits exogenous and allogeneic antigen presentation and affects the formation of peptide-MHCII complexes, as well as altering costimulatory molecule expression on the cell surface. Therefore, DC presentation of peptides in a favorable costimulatory protein environment is required to subsequently activate T cells and appears to be a critical target for the immunosuppressive effects of ethanol.

Innate immunity in alcoholic liver disease

Excessive alcohol consumption is a leading cause of chronic liver disease in the Western world. Alcohol-induced hepatotoxicity and oxidative stress are important mechanisms contributing to the pathogenesis of alcoholic liver disease. However, emerging evidence suggests that activation of innate immunity involving TLR4 and complement also plays an important role in initiating alcoholic steatohepatitis and fibrosis, but the role of adaptive immunity in the pathogenesis of alcoholic liver disease remains obscure. Activation of a TLR4-mediated MyD88-independent (TRIF/IRF-3) signaling pathway in Kupffer cells contributes to alcoholic steatohepatitis, whereas activation of TLR4 signaling in hepatic stellate cells promotes liver fibrosis. Alcohol consumption activates the complement system in the liver by yet unidentified mechanisms, leading to alcoholic steatohepatitis. In contrast to activation of TLR4 and complement, alcohol consumption can inhibit natural killer cells, another important innate immunity component, contributing to alcohol-mediated acceleration of viral infection and liver fibrosis in patients with chronic viral hepatitis. Understanding of the role of innate immunity in the pathogenesis of alcoholic liver disease may help us identify novel therapeutic targets to treat this disease.

Innate immunity in alcoholic liver disease

Excessive alcohol consumption is a leading cause of chronic liver disease in the Western world. Alcohol-induced hepatotoxicity and oxidative stress are important mechanisms contributing to the pathogenesis of alcoholic liver disease. However, emerging evidence suggests that activation of innate immunity involving TLR4 and complement also plays an important role in initiating alcoholic steatohepatitis and fibrosis, but the role of adaptive immunity in the pathogenesis of alcoholic liver disease remains obscure. Activation of a TLR4-mediated MyD88-independent (TRIF/IRF-3) signaling pathway in Kupffer cells contributes to alcoholic steatohepatitis, whereas activation of TLR4 signaling in hepatic stellate cells promotes liver fibrosis. Alcohol consumption activates the complement system in the liver by yet unidentified mechanisms, leading to alcoholic steatohepatitis. In contrast to activation of TLR4 and complement, alcohol consumption can inhibit natural killer cells, another important innate immunity component, contributing to alcohol-mediated acceleration of viral infection and liver fibrosis in patients with chronic viral hepatitis. Understanding of the role of innate immunity in the pathogenesis of alcoholic liver disease may help us identify novel therapeutic targets to treat this disease.

Effects of ethanol on the proteasome interacting proteins

Proteasome dysfunction has been repeatedly reported in alcoholic liver disease. Ethanol metabolism end-products affect the structure of the proteasome, and, therefore, change the proteasome interaction with its regulatory complexes 19S and PA28, as well as its interacting proteins. Chronic ethanol feeding alters the ubiquitin-proteasome activity by altering the interaction between the 19S and the 20S proteasome interaction. The degradation of oxidized and damaged proteins is thus decreased and leads to accumulation of insoluble protein aggregates, such as Mallory-Denk bodies. Ethanol also affects the immunoproteasome formation. PA28a/b interactions with the 20S proteasome are decreased in the proteasome fraction isolated from the liver of rats fed ethanol chronically, thus affecting the cellular antigen presentation and defense against pathogenic agents. Recently, it has been shown that ethanol also affects the proteasome interacting proteins (PIPs). Interaction of the proteasome with Ecm29 and with deubiquitinating enzymes Rpn11, UCH37, and Usp14 has been found to decrease. However, the two UBL-ubiquitin-associated domain (UBA) PIPs p62 and valosin-containing protein are upregulated when the proteasome is inhibited. The increase of these UBL-UBA proteins, as well as the increase in Hsp70 and Hsp25 levels, compensated for the proteasome failure and helped in the unfolding/docking of misfolded proteins. Chronic alcohol feeding to rats causes a significant inhibition of the proteasome pathway and this inhibition results from a decreases of the interaction between the 20S proteasome and the regulatory complexes, PIPs, and the ubiquitin system components.

Impaired methylation as a novel mechanism for proteasome suppression in liver cells

The proteasome is a multi-catalytic protein degradation enzyme that is regulated by ethanol-induced oxidative stress; such suppression is attributed to CYP2E1-generated metabolites. However, under certain conditions, it appears that in addition to oxidative stress, other mechanisms are also involved in proteasome regulation. This study investigated whether impaired protein methylation that occurs during exposure of liver cells to ethanol, may contribute to suppression of proteasome activity. We measured the chymotrypsin-like proteasome activity in Huh7CYP cells, hepatocytes, liver cytosols and nuclear extracts or purified 20S proteasome under conditions that maintain or prevent protein methylation. Reduction of proteasome activity of hepatoma cell and hepatocytes by ethanol or tubercidin was prevented by simultaneous treatment with S-adenosylmethionine (SAM). Moreover, the tubercidin-induced decline in proteasome activity occurred in both nuclear and cytosolic fractions. In vitro exposure of cell cytosolic fractions or highly purified 20S proteasome to low SAM:S-adenosylhomocysteine (SAH) ratios in the buffer also suppressed proteasome function, indicating that one or more methyltransferase(s) may be associated with proteasomal subunits. Immunoblotting a purified 20S rabbit red cell proteasome preparation using methyl lysine-specific antibodies revealed a 25kDa proteasome subunit that showed positive reactivity with anti-methyl lysine. This reactivity was modified when 20S proteasome was exposed to differential SAM:SAH ratios. We conclude that impaired methylation of proteasome subunits suppressed proteasome activity in liver cells indicating an additional, yet novel mechanism of proteasome activity regulation by ethanol.