title: Alcohol-induced S-adenosylhomocysteine accumulation in the liver sensitizes to TNF hepatotoxicity: possible involvement of mitochondrial S-adenosylmethionine transport.
abstract: Hepatocytes are resistant to tumor necrosis factor-alpha- (TNF) induced killing/apoptosis under normal circumstances, but primary hepatocytes from rats chronically fed alcohol have increased TNF cytotoxicity. Therefore, there must be mechanism(s) by which alcohol exposure "sensitizes" to TNF hepatotoxicity. Abnormal metabolism of methionine and S-adenosylmethionine (SAM) are well-documented acquired metabolic abnormalities in ALD. S-adenosylhomocysteine (SAH) is the product of SAM in hepatic transmethylation reactions, and SAH hydrolase (SAHH) is the only enzyme to metabolize SAH to homocysteine and adenosine. Our previous studies demonstrated that chronic intracellular accumulation of SAH sensitized hepatocytes to TNF cytotoxicity in vitro. In the current study, we extended our previous observations by further characterizing the effects of chronic alcohol intake on mitochondrial SAM levels in liver and examining its possible involvement in SAH sensitization to TNF hepatotoxicity. Chronic alcohol consumption in mice not only increased cytosolic SAH levels, but also decreased mitochondrial SAM concentration, leading to decreased mitochondrial SAM to SAH ratio. Moreover, accumulation of hepatic SAH induced by administration of 3-deaza-adenosine (DZA-a potent inhibitor of SAHH) enhanced lipopolysaccharide (LPS)/TNF hepatotoxicity in mice in vivo. Inhibition of SAHH by DZA resulted not only in accumulation of cytoplasmic SAH, but also in depletion of the mitochondrial SAM pool. Further studies using mitochondrial SAM transporter inhibitors showed that inhibition of SAM transport into mitochondria sensitized HepG2 cells to TNF cytotoxicity. In conclusion, our results demonstrate that depletion of the mitochondrial SAM pool by SAH, which is elevated during chronic alcohol consumption, plays a critical role in SAH induced sensitization to TNF hepatotoxicity.
authors.affiliation: Department of Medicine, University of Louisville Medical Center, Louisville, KY 40292, United States. z0song02@louisville.edu
authors.forename: Zhenyuan
authors.initials: Z
authors.lastname: Song
authors.forename: Zhanxiang
authors.initials: Z
authors.lastname: Zhou
authors.forename: Ming
authors.initials: M
authors.lastname: Song
authors.forename: Silvia
authors.initials: S
authors.lastname: Uriarte
authors.forename: Theresa
authors.initials: T
authors.lastname: Chen
authors.forename: Ion
authors.initials: I
authors.lastname: Deaciuc
authors.forename: Craig J
authors.initials: CJ
authors.lastname: McClain
mesh_terms: D000818:Animals; D002460:Cell Line; D002851:Chromatography, High Pressure Liquid; D004797:Enzyme-Linked Immunosorbent Assay; D000431:Ethanol / Q000494:pharmacology*; D008099:Liver / Q000187:drug effects* / Q000378:metabolism; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D008930:Mitochondria, Liver / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D012435:S-Adenosylhomocysteine / Q000378:metabolism*; D014409:Tumor Necrosis Factor-alpha / Q000633:toxicity*
chemical_list: D014409:Tumor Necrosis Factor-alpha; D000431:Ethanol; D012435:S-Adenosylhomocysteine