title: Decrease in intracellular superoxide sensitizes Bcl-2-overexpressing tumor cells to receptor and drug-induced apoptosis independent of the mitochondria.
abstract: At least two mechanisms of early cytosolic acidification during apoptotic signaling have been described, one that involves caspase 8 activation downstream of receptor ligation and another dependent on mitochondria-derived hydrogen peroxide during merocil-induced apoptosis. Here, we show that Bcl-2 inhibits both mechanisms of acidification. Moreover, Bcl-2 overexpression resulted in a slightly elevated constitutive level of superoxide anion and pH in CEM leukemia cells. Interestingly, decreasing intracellular superoxide concentration with an inhibitor of the beta-nicotinamide adenine dinucleotide phosphate oxidase or by transient transfection with a dominant-negative form of the guanosine triphosphate-binding protein Rac1 resulted in a significant increase in the sensitivity of CEM/Bcl-2 cells to CD95- or merocil-induced apoptosis. This increase in sensitivity was a direct result of a significant increase in caspase 8 activation and caspase 8-dependent acidification in the absence of caspase 9 activity or cytochrome c release. These findings suggest a mechanism of switching from mitochondria-dependent to mitochondria-independent death signaling in the same cell, provided the intracellular milieu is permissive for upstream caspase 8 activation, and could have implications for favorably tailoring tumor cells for drug treatment even when the mitochondrial pathway is compromised by Bcl-2.
authors.affiliation: 1Department of Biochemistry, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore.
authors.forename: M-V
authors.initials: MV
authors.lastname: Clément
authors.forename: J L
authors.initials: JL
authors.lastname: Hirpara
authors.forename: S
authors.initials: S
authors.lastname: Pervaiz
mesh_terms: D017209:Apoptosis / Q000187:drug effects / Q000502:physiology*; D053181:Caspase 8; D053453:Caspase 9; D020169:Caspases / Q000378:metabolism; D045744:Cell Line, Tumor; D015536:Down-Regulation / Q000502:physiology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006801:Humans; D006861:Hydrogen Peroxide / Q000494:pharmacology; D007424:Intracellular Fluid / Q000378:metabolism*; D008928:Mitochondria / Q000187:drug effects / Q000378:metabolism*; D019255:NADPH Oxidases / Q000037:antagonists & inhibitors / Q000378:metabolism; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000187:drug effects / Q000235:genetics / Q000378:metabolism*; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology; D013481:Superoxides / Q000378:metabolism*; D013858:Thiobarbiturates / Q000494:pharmacology; D019014:fas Receptor / Q000378:metabolism* / Q000494:pharmacology; D020744:rac GTP-Binding Proteins / Q000235:genetics / Q000378:metabolism
chemical_list: D004791:Enzyme Inhibitors; D019253:Proto-Oncogene Proteins c-bcl-2; D013858:Thiobarbiturates; D019014:fas Receptor; D013481:Superoxides; C092110:merocil; D006861:Hydrogen Peroxide; D019255:NADPH Oxidases; C505530:CASP8 protein, human; C506433:CASP9 protein, human; D053181:Caspase 8; D053453:Caspase 9; D020169:Caspases; D020744:rac GTP-Binding Proteins