Anti-apoptotic Signalling by the Dot/Icm Secretion System of L. Pneumophila

Cell Microbiol. 2007 Jan;9(1):246-64. doi: 10.1111/j.1462-5822.2006.00785.x. Epub 2006 Aug 15.

Abstract

The Dot/Icm type IV secretion system of Legionella pneumophila triggers robust activation of caspase-3 during early and exponential stages of proliferation within human macrophages, but apoptosis is delayed till late stages of infection, which is novel. As caspase-3 is the executioner of the cell, we tested the hypothesis that L. pneumophila triggers anti-apoptotic signalling within the infected human macrophages to halt caspase-3 from dismantling the cells. Here we show that during early and exponential replication, L. pneumophila-infected human monocyte-derived macrophages (hMDMs) exhibit a remarkable resistance to induction of apoptosis, in a Dot/Icm-dependent manner. Microarray analyses and real-time PCR reveal that during exponential intracellular replication, L. pneumophila triggers upregulation of 12 anti-apoptotic genes that are linked to activation of the nuclear transcription factor kappa-B (NF-kappaB). Our data show that L. pneumophila induces a Dot/Icm-dependent sustained nuclear translocation of the p50 and p65 subunits of NF-kappaB during exponential intracellular replication. Bacterial entry is essential both for the anti-apoptotic phenotype of infected hMDMs and for nuclear translocation of the p65. Using p65-/- and IKKalpha-/- beta-/- double knockout mouse embryonic fibroblast cell lines, we show that nuclear translocation of NF-kappaB is required for the resistance of L. pneumophila-infected cells to apoptosis-inducing agents. In addition, the L. pneumophila-induced nuclear translocation of NF-kappaB requires the activity of IKKalpha and/or IKKbeta. We conclude that although the Dot/Icm secretion system of L. pneumophila elicits an early robust activation of caspase-3 in human macrophages, it triggers a strong anti-apoptotic signalling cascade mediated, at least in part by NF-kappaB, which renders the cells refractory to external potent apoptotic stimuli.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Active Transport, Cell Nucleus
  • Animals
  • Apoptosis* / drug effects
  • Apoptosis* / genetics
  • Bacterial Proteins / genetics
  • Bacterial Proteins / metabolism
  • Carrier Proteins / genetics
  • Carrier Proteins / metabolism
  • Caspase 3 / metabolism
  • Cell Line, Tumor
  • Cell Nucleus / metabolism
  • Cells, Cultured
  • Green Fluorescent Proteins / genetics
  • Humans
  • I-kappa B Kinase / genetics
  • Legionella pneumophila / genetics
  • Legionella pneumophila / growth & development
  • Legionella pneumophila / metabolism*
  • Macrophages / cytology
  • Macrophages / microbiology*
  • Membrane Proteins / genetics
  • Membrane Proteins / metabolism
  • Mice
  • NF-kappa B / metabolism*
  • Signal Transduction*
  • Transcription Factor RelA / genetics
  • U937 Cells
  • Up-Regulation

Substances

  • Bacterial Proteins
  • Carrier Proteins
  • DotA protein, Legionella pneumophila
  • Membrane Proteins
  • NF-kappa B
  • Transcription Factor RelA
  • Green Fluorescent Proteins
  • I-kappa B Kinase
  • Caspase 3