title: Activating transcription factor 6 limits intracellular accumulation of mutant α(1)-antitrypsin Z and mitochondrial damage in hepatoma cells.
abstract: α(1)-Antitrypsin is a serine protease inhibitor secreted by hepatocytes. A variant of α(1)-antitrypsin with an E342K (Z) mutation (ATZ) has propensity to form polymers, is retained in the endoplasmic reticulum (ER), is degraded by both ER-associated degradation and autophagy, and causes hepatocyte loss. Constant features in hepatocytes of PiZZ individuals and in PiZ transgenic mice expressing ATZ are the formation of membrane-limited globular inclusions containing ATZ and mitochondrial damage. Expression of ATZ in the liver does not induce the unfolded protein response (UPR), a protective mechanism aimed to maintain ER homeostasis in the face of an increased load of proteins. Here we found that in hepatoma cells the ER E3 ligase HRD1 functioned to degrade most of the ATZ before globular inclusions are formed. Activation of the activating transcription factor 6 (ATF6) branch of the UPR by expression of spliced ATF6(1-373) decreased intracellular accumulation of ATZ and the formation of globular inclusions by a pathway that required HRD1 and the proteasome. Expression of ATF6(1-373) in ATZ-expressing hepatoma cells did not induce autophagy and increased the level of the proapoptotic factor CCAAT/enhancer-binding protein (C/EBP) homologous protein (CHOP) but did not lead to apoptotic DNA fragmentation. Expression of ATF6(1-373) did not cause inhibition of protein synthesis and prevented mitochondrial damage induced by ATZ expression. It was concluded that activation of the ATF6 pathway of the UPR limits ATZ-dependent cell toxicity by selectively promoting ER-associated degradation of ATZ and is thereby a potential target to prevent hepatocyte loss in addition to autophagy-enhancing drugs.
authors: Department of Biochemistry and Molecular Biology, University of Arkansas for Medical Sciences, Little Rock, Arkansas 72205.
authors: Steven E
authors: SE
authors: Smith
authors: Department of Biochemistry and Molecular Biology, University of Arkansas for Medical Sciences, Little Rock, Arkansas 72205.
authors: Susana
authors: S
authors: Granell
authors: Departament de Biologia Cellular, Immunologia i Neurociències, Facultat de Medicina, Institut d'Investigacions Biomèdiquens August Pi i Sunyer (IDIBAPS), Universitat de Barcelona, Barcelona E-08036, Spain.
authors: Laia
authors: L
authors: Salcedo-Sicilia
authors: Dipartimento Universitario Clinico di Scienze Mediche, Chirurgiche e della Salute, Università degli Studi di Trieste, Trieste I-34138, Italy.
authors: Giovanna
authors: G
authors: Baldini
authors: Departament de Biologia Cellular, Immunologia i Neurociències, Facultat de Medicina, Institut d'Investigacions Biomèdiquens August Pi i Sunyer (IDIBAPS), Universitat de Barcelona, Barcelona E-08036, Spain.
authors: Gustavo
authors: G
authors: Egea
authors: Department of Pediatrics, Saint Louis University School of Medicine, St. Louis, Missouri 63104.
authors: Jeff H
authors: JH
authors: Teckman
authors: Department of Biochemistry and Molecular Biology, University of Arkansas for Medical Sciences, Little Rock, Arkansas 72205. Electronic address: gbaldini@uams.edu.
authors: Giulia
authors: G
authors: Baldini
mesh_terms: D051702:Activating Transcription Factor 6 / Q000235:genetics / Q000378:metabolism*; D019943:Amino Acid Substitution; D000818:Animals; D017209:Apoptosis / Q000235:genetics; D001343:Autophagy / Q000235:genetics; D022762:CCAAT-Enhancer-Binding Proteins / Q000235:genetics / Q000378:metabolism; D006528:Carcinoma, Hepatocellular / Q000235:genetics / Q000378:metabolism* / Q000473:pathology; D045744:Cell Line, Tumor; D053938:DNA Fragmentation; D004721:Endoplasmic Reticulum / Q000235:genetics / Q000378:metabolism / Q000473:pathology; D051379:Mice; D008930:Mitochondria, Liver / Q000235:genetics / Q000378:metabolism* / Q000473:pathology; D020125:Mutation, Missense; D009363:Neoplasm Proteins / Q000235:genetics / Q000378:metabolism*; D059748:Proteolysis; D051742:Transcription Factor CHOP / Q000235:genetics / Q000378:metabolism; D056811:Unfolded Protein Response / Q000235:genetics; D000515:alpha 1-Antitrypsin / Q000235:genetics / Q000378:metabolism*
chemical_list: D051702:Activating Transcription Factor 6; C497112:Atf6 protein, mouse; D022762:CCAAT-Enhancer-Binding Proteins; C497250:Ddit3 protein, mouse; D009363:Neoplasm Proteins; D000515:alpha 1-Antitrypsin; D051742:Transcription Factor CHOP