title: Death receptor 5 signaling promotes hepatocyte lipoapoptosis.
abstract: Nonalcoholic steatohepatitis is characterized by hepatic steatosis, elevated levels of circulating free fatty acids (FFA), endoplasmic reticulum (ER) stress, and hepatocyte lipoapoptosis. Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) death receptor 5 (DR5) is significantly elevated in patients with nonalcoholic steatohepatitis, and steatotic hepatocytes demonstrate increased sensitivity to TRAIL-mediated cell death. Nonetheless, a role for TRAIL and/or DR5 in mediating lipoapoptotic pathways is unexplored. Here, we examined the contribution of DR5 death signaling to lipoapoptosis by free fatty acids. The toxic saturated free fatty acid palmitate induces an increase in DR5 mRNA and protein expression in Huh-7 human hepatoma cells leading to DR5 localization into lipid rafts, cell surface receptor clustering with subsequent recruitment of the initiator caspase-8, and ultimately cellular demise. Lipoapoptosis by palmitate was not inhibited by a soluble human recombinant DR5-Fc chimera protein suggesting that DR5 cytotoxic signaling is ligand-independent. Hepatocytes from murine TRAIL receptor knock-out mice (DR(-/-)) displayed reduced palmitate-mediated lipotoxicity. Likewise, knockdown of DR5 or caspase-8 expression by shRNA technology attenuated palmitate-induced Bax activation and apoptosis in Huh-7 cells, without altering induction of ER stress markers. Similar observations were verified in other cell models. Finally, knockdown of CHOP, an ER stress-mediated transcription factor, reduced DR5 up-regulation and DR5-mediated caspase-8 activation upon palmitate treatment. Collectively, these results suggest that ER stress-induced CHOP activation by palmitate transcriptionally up-regulates DR5, likely resulting in ligand-independent cytotoxic signaling by this death receptor.
authors.affiliation: Division of Gastroenterology and Hepatology, College of Medicine, Mayo Clinic, Rochester, Minnesota 55905, USA.
authors.forename: Sophie C
authors.initials: SC
authors.lastname: Cazanave
authors.forename: Justin L
authors.initials: JL
authors.lastname: Mott
authors.forename: Steven F
authors.initials: SF
authors.lastname: Bronk
authors.forename: Nathan W
authors.initials: NW
authors.lastname: Werneburg
authors.forename: Christian D
authors.initials: CD
authors.lastname: Fingas
authors.forename: X Wei
authors.initials: XW
authors.lastname: Meng
authors.forename: Niklas
authors.initials: N
authors.lastname: Finnberg
authors.forename: Wafik S
authors.initials: WS
authors.lastname: El-Deiry
authors.forename: Scott H
authors.initials: SH
authors.lastname: Kaufmann
authors.forename: Gregory J
authors.initials: GJ
authors.lastname: Gores
mesh_terms: D000818:Animals; D017209:Apoptosis; D053181:Caspase 8 / Q000235:genetics / Q000378:metabolism; D045744:Cell Line, Tumor; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005234:Fatty Liver / Q000235:genetics / Q000378:metabolism* / Q000473:pathology; D055785:Gene Knockdown Techniques; D022781:Hepatocytes / Q000378:metabolism* / Q000473:pathology; D006801:Humans; D021962:Membrane Microdomains / Q000235:genetics / Q000378:metabolism / Q000473:pathology; D051379:Mice; D018345:Mice, Knockout; D019308:Palmitic Acid / Q000494:pharmacology; D053220:Receptors, TNF-Related Apoptosis-Inducing Ligand / Q000235:genetics / Q000378:metabolism*; D051742:Transcription Factor CHOP / Q000235:genetics / Q000378:metabolism; D014158:Transcription, Genetic / Q000187:drug effects / Q000235:genetics; D015854:Up-Regulation / Q000187:drug effects / Q000235:genetics; D051028:bcl-2-Associated X Protein / Q000235:genetics / Q000378:metabolism
chemical_list: C494826:BAX protein, human; C494827:Bax protein, mouse; C497249:DDIT3 protein, human; C497250:Ddit3 protein, mouse; D004791:Enzyme Inhibitors; D053220:Receptors, TNF-Related Apoptosis-Inducing Ligand; D051028:bcl-2-Associated X Protein; D051742:Transcription Factor CHOP; D019308:Palmitic Acid; C505530:CASP8 protein, human; C505526:Casp8 protein, mouse; D053181:Caspase 8