title: Anti-apoptotic signalling by the Dot/Icm secretion system of L. pneumophila.
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.
authors.affiliation: Department of Microbiology, University of Louisville Collage of Medicine, Louisville, KY 40202, USA.
authors.forename: Alaeddin
authors.initials: A
authors.lastname: Abu-Zant
authors.forename: Snake
authors.initials: S
authors.lastname: Jones
authors.forename: Rexford
authors.initials: R
authors.lastname: Asare
authors.forename: Jill
authors.initials: J
authors.lastname: Suttles
authors.forename: Christopher
authors.initials: C
authors.lastname: Price
authors.forename: James
authors.initials: J
authors.lastname: Graham
authors.forename: Yousef Abu
authors.initials: YA
authors.lastname: Kwaik
mesh_terms: D021581:Active Transport, Cell Nucleus; D000818:Animals; D017209:Apoptosis / Q000187:drug effects / Q000235:genetics; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism; D053148:Caspase 3 / Q000378:metabolism; D045744:Cell Line, Tumor; D002467:Cell Nucleus / Q000378:metabolism; D002478:Cells, Cultured; D049452:Green Fluorescent Proteins / Q000235:genetics; D006801:Humans; D051550:I-kappa B Kinase / Q000235:genetics; D016952:Legionella pneumophila / Q000235:genetics / Q000254:growth & development / Q000378:metabolism*; D008264:Macrophages / Q000166:cytology / Q000382:microbiology*; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism; D051379:Mice; D016328:NF-kappa B / Q000378:metabolism*; D015398:Signal Transduction; D051996:Transcription Factor RelA / Q000235:genetics; D020298:U937 Cells; D015854:Up-Regulation
chemical_list: D001426:Bacterial Proteins; D002352:Carrier Proteins; C440002:DotA protein, Legionella pneumophila; D008565:Membrane Proteins; D016328:NF-kappa B; D051996:Transcription Factor RelA; D049452:Green Fluorescent Proteins; D051550:I-kappa B Kinase; D053148:Caspase 3