Lipopolysaccharide accelerates caspase-independent but cathepsin B-dependent death of human lung epithelial cells

Authors

  • Peter S. Tang,

    1. Division of Cellular and Molecular Biology, University Health Network Toronto General Research Institute, Toronto, Ontario, Canada
    2. Institute of Medical Science, University of Toronto, Toronto, Ontario, Canada
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  • Melanie E. Tsang,

    1. Division of Cellular and Molecular Biology, University Health Network Toronto General Research Institute, Toronto, Ontario, Canada
    2. Institute of Medical Science, University of Toronto, Toronto, Ontario, Canada
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  • Monika Lodyga,

    1. Division of Cellular and Molecular Biology, University Health Network Toronto General Research Institute, Toronto, Ontario, Canada
    2. Institute of Medical Science, University of Toronto, Toronto, Ontario, Canada
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  • Xiao-Hui Bai,

    1. Division of Cellular and Molecular Biology, University Health Network Toronto General Research Institute, Toronto, Ontario, Canada
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  • Abigale Miller,

    1. Division of Cellular and Molecular Biology, University Health Network Toronto General Research Institute, Toronto, Ontario, Canada
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  • Bing Han,

    1. Division of Cellular and Molecular Biology, University Health Network Toronto General Research Institute, Toronto, Ontario, Canada
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  • Mingyao Liu

    Corresponding author
    1. Division of Cellular and Molecular Biology, University Health Network Toronto General Research Institute, Toronto, Ontario, Canada
    2. Institute of Medical Science, University of Toronto, Toronto, Ontario, Canada
    • Professor of Surgery, Medicine and Physiology, University of Toronto; 101 College Street, TMDT 2-814, Toronto, Ont., Canada, M5G 1L7.
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  • Peter S. Tang and Melanie E. Tsang contributed equally to this work.

Abstract

Caspase-independent cell death has drawn increasing attention. In the present study, we found that lipopolysaccharide (LPS) accelerated spontaneous death of human lung epithelial A549 cells in a serum- and cell density-dependent manner: while serum starvation has been demonstrated to induce apoptosis in the same cell line, LPS-induced cell death was only observed in the presence of serum; in addition, the cell death was not observed when the cells were seeded at 10- or 100-fold lower density. The apoptotic features were demonstrated by TUNEL assay, DNA laddering and Annexin V staining. However, treatment of cells with two commonly used pan-caspase inhibitors, zVAD.fmk or BOC-D.fmk, failed to block cell death. In contrast, two cathepsin B inhibitors, Ca074-Me or N-1845, reduced cell death significantly. A time-dependent activation of cathepsin B, but not caspase 3, was observed in both control and LPS-treated cells. Although LPS did not further activate cathepsin B or its release, it increased expression and translocation of apoptosis inducing factor from mitochondria to the nucleus, and increased release of cytochrome c from mitochondria. LPS-induced cell death was significantly attenuated by either N-acetyl-L-cysteine or pyrrolidine-dithiocarbamate, both free radical scavengers. Disruption of lipid raft formation with filipin or methyl-β-cyclodextrin also reduced apoptosis significantly, suggesting that lipid raft-dependent signaling is essential. These data imply that confluent cells undergo spontaneous cell death mediated by cathepsin B; LPS may accelerate this caspase-independent cell death through release of mitochondrial contents and reactive oxygen species. J. Cell. Physiol. 209: 457–467, 2006. © 2006 Wiley-Liss, Inc.

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