Ethanol-induced oxidative stress suppresses generation of peptides for antigen presentation by hepatoma cells

Abstract

Processing of peptides for antigen presentation is catalyzed by antigen-trimming enzymes, including the proteasome and leucine aminopeptidase. Oxidative stress suppresses proteasome function. We hypothesized that in liver cells, processing of antigenic peptides is altered by ethanol metabolism. To address this issue, soluble extracts of ethanol-metabolizing VL-17A cells treated with 100 mM ethanol or left untreated were incubated with C-extended or N-extended 18-27 HBV core peptides. T he proteasome is a multicatalytic enzyme crucially important for proteolysis of oxidized and misfolded proteins, peptides and short-lived signal transduction factors. In vivo, the proteasome exists in equilibrium as 2 particles: the 20S proteasome degrades substrate proteins with no prior ubiquitylation and the 26S form degrades ubiquitylated proteins in an ATP-dependent manner.Both 20S and 26S proteasomes generate peptides for MHC class I-restricted antigen presentation. 1,2 Processing of antigens is a necessary step for recognition by cytotoxic T lymphocytes (CTLs) of virally infected cells. To be recognized by CTLs, "nonself" antigens are processed to peptides of 8-10 amino acids, which then bind to MHC class I molecules in the endoplasmic reticulum. 3 Initial processing of antigenic proteins in the cytoplasm is catalyzed by the proteasome and/or leucine aminopeptidase (LAP). 4 Both enzymes are activated by the proinflammatory cytokine interferon gamma (IFN␥), 5 which induces formation of the immunoproteasome that cleaves antigenic proteins to uniformly sized peptides for presentation.The foregoing series of events occurs after hepatitis B or C infections during which CTLs eliminate HBV-infected or HCV-infected hepatocytes are based on ability of immune system to clear infected hepatocytes. Epidemiological studies have provided supporting evidence of

CYP2E1 and oxidative liver injury by alcohol

“…A rather interesting HepG2 cell culture model in which CYP2E1 and alcohol dehydrogenase are both expressed has been effectively utilized to study ethanol-CYP2E1-proteasome interactions, interferon gamma induction of the proteasome and interferon gamma signaling and antigen processing (173)(174)(175). The use of these combined cell lines as a model of ethanol-elicited cytotoxicity has recently been reported (176).…”

Proteasome Activation by Hepatitis C Core Protein Is Reversed by Ethanol-Induced Oxidative Stress

“…Because core protein induces oxidant formation and because the proteasome is sensitive to their levels in cells 8-10 , we postulated that core protein regulates proteasome activity. These properties of core protein may be further potentiated by ethanol exposure, as ethanol metabolism suppresses proteasome activity 11,12 . Suppression of proteasome activity may enhance disease progression in HCV-infected alcohol-consuming patients, as it is known that ethanol consumption exacerbates the clinical course of HCV infection in these individuals 13, 14 .…”

Multilevel regulation of autophagosome content by ethanol oxidation in HepG2 cells

“…The chymotrypsin-like activity of the 20S proteasome was assayed as described previously. 36 Activities of CTSB and CTSL were assayed fluorometrically as published.…”