Lysosomal enzymes promote mitochondrial oxidant production, cytochrome c release and apoptosis

Authors

  • Ming Zhao,

    1. Division of Pathology II, Faculty of Health Sciences, Linköping University, Sweden;
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  • Fernando Antunes,

    1. Division of Pathology II, Faculty of Health Sciences, Linköping University, Sweden;
    2. Grupo de Bioquímica e Biologia Teóricas – Instituto Bento da Rocha Cabral and Department of Chemistry and Biochemistry, Faculty of Sciences, University of Lisbon, Portugal;
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  • John W. Eaton,

    1. Division of Pathology II, Faculty of Health Sciences, Linköping University, Sweden;
    2. James Graham Brown Cancer Center, University of Louisville, Louisville, KY, USA
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  • Ulf T. Brunk

    1. Division of Pathology II, Faculty of Health Sciences, Linköping University, Sweden;
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M. Zhao, Division of Pathology II, Faculty of Health Sciences, Linköping University, SE-581 85 Linköping, Sweden. Fax: +46 13 22 15 29, Tel.: +46 13 22 15 15, E-mail: ming.zhao@inr.liu.se

Abstract

Exposure of mammalian cells to oxidant stress causes early (iron catalysed) lysosomal rupture followed by apoptosis or necrosis. Enhanced intracellular production of reactive oxygen species (ROS), presumably of mitochondrial origin, is also observed when cells are exposed to nonoxidant pro-apoptotic agonists of cell death. We hypothesized that ROS generation in this latter case might promote the apoptotic cascade and could arise from effects of released lysosomal materials on mitochondria. Indeed, in intact cells (J774 macrophages, HeLa cells and AG1518 fibroblasts) the lysosomotropic detergent O-methyl-serine dodecylamide hydrochloride (MSDH) causes lysosomal rupture, enhanced intracellular ROS production, and apoptosis. Furthermore, in mixtures of rat liver lysosomes and mitochondria, selective rupture of lysosomes by MSDH promotes mitochondrial ROS production and cytochrome c release, whereas MSDH has no direct effect on ROS generation by purifed mitochondria. Intracellular lysosomal rupture is associated with the release of (among other constituents) cathepsins and activation of phospholipase A2 (PLA2). We find that addition of purified cathepsins B or D, or of PLA2, causes substantial increases in ROS generation by purified mitochondria. Furthermore, PLA2 − but not cathepsins B or D − causes rupture of semipurified lysosomes, suggesting an amplification mechanism. Thus, initiation of the apoptotic cascade by nonoxidant agonists may involve early release of lysosomal constituents (such as cathepsins B and D) and activation of PLA2, leading to enhanced mitochondrial oxidant production, further lysosomal rupture and, finally, mitochondrial cytochrome c release. Nonoxidant agonists of apoptosis may, thus, act through oxidant mechanisms.

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