title: Deficiency in the mitochondrial apoptotic pathway reveals the toxic potential of autophagy under ER stress conditions.
abstract: Endoplasmic reticulum (ER) stress-induced cell death is normally associated with activation of the mitochondrial apoptotic pathway, which is characterized by CYCS (cytochrome c, somatic) release, apoptosome formation, and caspase activation, resulting in cell death. In this study, we demonstrate that under conditions of ER stress cells devoid of CASP9/caspase-9 or BAX and BAK1, and therefore defective in the mitochondrial apoptotic pathway, still undergo a delayed form of cell death associated with the activation of caspases, therefore revealing the existence of an alternative stress-induced caspase activation pathway. We identified CASP8/caspase-8 as the apical protease in this caspase cascade, and found that knockdown of either of the key autophagic genes, ATG5 or ATG7, impacted on CASP8 activation and cell death induction, highlighting the crucial role of autophagy in the activation of this novel ER stress-induced death pathway. In line with this, we identified a protein complex composed of ATG5, FADD, and pro-CASP8 whose assembly coincides with caspase activation and cell death induction. Together, our results reveal the toxic potential of autophagy in cells undergoing ER stress that are defective in the mitochondrial apoptotic pathway, and suggest a model in which the autophagosome functions as a platform facilitating pro-CASP8 activation. Chemoresistance, a common problem in the treatment of cancer, is frequently caused by the downregulation of key mitochondrial death effector proteins. Alternate stress-induced apoptotic pathways, such as the one described here, may become of particular relevance for tackling the problem of chemoresistance in cancer cells.
authors.affiliation: a Apoptosis Research Center; NUI Galway ; Galway , Ireland.
authors.forename: Shane
authors.initials: S
authors.lastname: Deegan
authors.forename: Svetlana
authors.initials: S
authors.lastname: Saveljeva
authors.forename: Susan E
authors.initials: SE
authors.lastname: Logue
authors.forename: Karolina
authors.initials: K
authors.lastname: Pakos-Zebrucka
authors.forename: Sanjeev
authors.initials: S
authors.lastname: Gupta
authors.forename: Peter
authors.initials: P
authors.lastname: Vandenabeele
authors.forename: Mathieu J M
authors.initials: MJ
authors.lastname: Bertrand
authors.forename: Afshin
authors.initials: A
authors.lastname: Samali
mesh_terms: D000818:Animals; D017209:Apoptosis; D001343:Autophagy; D000071187:Autophagy-Related Protein 5; D000071193:Autophagy-Related Protein 7; D053148:Caspase 3 / Q000378:metabolism; D053181:Caspase 8 / Q000378:metabolism; D053453:Caspase 9 / Q000378:metabolism; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D000091342:Endoplasmic Reticulum Chaperone BiP; D059865:Endoplasmic Reticulum Stress; D004789:Enzyme Activation; D053200:Fas-Associated Death Domain Protein / Q000378:metabolism; D005347:Fibroblasts / Q000378:metabolism; D005786:Gene Expression Regulation; D057809:HEK293 Cells; D006801:Humans; D051379:Mice; D008869:Microtubule-Associated Proteins / Q000378:metabolism; D008928:Mitochondria / Q000378:metabolism*; D015398:Signal Transduction
chemical_list: C495773:Atg5 protein, mouse; C502661:Atg7 protein, mouse; D000071187:Autophagy-Related Protein 5; D000091342:Endoplasmic Reticulum Chaperone BiP; C505534:Fadd protein, mouse; D053200:Fas-Associated Death Domain Protein; C000717247:HSPA5 protein, human; C000717248:Hspa5 protein, mouse; D008869:Microtubule-Associated Proteins; C505409:Casp3 protein, mouse; C505526:Casp8 protein, mouse; C506434:Casp9 protein, mouse; D053148:Caspase 3; D053181:Caspase 8; D053453:Caspase 9; D000071193:Autophagy-Related Protein 7
keywords: ATG, autophagy related; BAK1, BCL2-antagonist/killer 1; BAX, BCL2-associated X protein; BCL2, B-cell CLL/lymphoma 2; DDIT3, DNA-damage-inducible transcript 3; DISC, death inducing signaling complex; DTT, dithiothreitol; ER, endoplasmic reticulum; FADD, Fas (TNFRSF6)-associated via death domain; GAPDH, glyceraldehyde 3-phosphate dehydrogenase; HSPA5, heat shock 70 kDa protein 5 (glucose-regulated protein, 78 kDa); MAP1LC3 (LC3), microtubule-associated protein 1 light chain 3; MEFs, mouse embryonic fibroblasts; MOMP, mitochondrial outer membrane permeabilization; PARP, poly (ADP-ribose) polymerase; PBS, phosphate-buffered saline; PI, propidium iodide; TNF, tumor necrosis factor; TNFSF10, tumor necrosis factor (ligand) superfamily, member 10; Tg, thapsigargin; Tm, tunicamycin; apoptosis; autophagic cell death; autophagy; caspase; endoplasmic reticulum stress; unfolded protein response