Abstract
An inadequate innate immune response appears to contribute to the virulence of Francisella tularensis following pulmonary infection. Studies in mice suggest that this poor response results from suppression of proinflammatory cytokine production early during infection, but the mechanisms involved are not understood. PI3K is known to regulate proinflammatory cytokine expression, but its exact role (positive versus negative) is controversial. We sought to clarify the role of PI3K in regulating proinflammatory signaling and cytokine production during infection with F. tularensis live vaccine strain (LVS). In this study, we demonstrate that the induction of TNF and IL-6 expression by LVS in mouse bone marrow-derived macrophages was markedly enhanced when PI3K activity was inhibited by either of the well-known chemical inhibitors, wortmannin or LY294002. The enhanced cytokine expression was accompanied by enhanced activation of p38 MAPK and ERK1/2, both of which were critical for LVS-induced expression of TNF and IL-6. LVS-induced MAPK activation and cytokine production were TLR2- and MyD88- dependent. PI3K/Akt activation was MyD88-dependent, but was surprisingly TLR2-independent. LVS infection also rapidly induced MAPK phosphatase-1 (MKP-1) expression; PI3K and TLR2 signaling were required. Peak levels of MKP-1 correlated closely with the decline in p38 MAPK and ERK1/2 phosphorylation. These data suggest that infection by LVS restrains the TLR2-triggered proinflammatory response via parallel activation of PI3K, leading to enhanced MKP-1 expression, accelerated deactivation of MAPKs, and suppression of proinflammatory cytokine production. This TLR2-independent inhibitory pathway may be an important mechanism by which Francisella suppresses the host's innate immune response.
Publication types
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Research Support, N.I.H., Extramural
MeSH terms
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Androstadienes / pharmacology
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Animals
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Bacterial Vaccines / genetics
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Bacterial Vaccines / immunology*
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Bacterial Vaccines / metabolism
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Bone Marrow Cells / immunology
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Bone Marrow Cells / metabolism
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Chromones / pharmacology
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Dual Specificity Phosphatase 1 / genetics
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Dual Specificity Phosphatase 1 / immunology
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Dual Specificity Phosphatase 1 / metabolism
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Enzyme Inhibitors / pharmacology
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Francisella tularensis / immunology*
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Francisella tularensis / metabolism
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Immunity, Innate / drug effects
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Immunity, Innate / genetics
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Immunity, Innate / immunology*
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Inflammation / genetics
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Inflammation / immunology
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Inflammation / metabolism
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Interleukin-6 / genetics
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Interleukin-6 / immunology
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Interleukin-6 / metabolism
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Macrophages / immunology
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Macrophages / metabolism
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Mice
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Mice, Mutant Strains
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Mitogen-Activated Protein Kinase 3 / antagonists & inhibitors
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Mitogen-Activated Protein Kinase 3 / genetics
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Mitogen-Activated Protein Kinase 3 / immunology
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Mitogen-Activated Protein Kinase 3 / metabolism
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Morpholines / pharmacology
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Myeloid Differentiation Factor 88 / genetics
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Myeloid Differentiation Factor 88 / immunology
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Myeloid Differentiation Factor 88 / metabolism
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Phosphatidylinositol 3-Kinases / genetics
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Phosphatidylinositol 3-Kinases / immunology*
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Phosphatidylinositol 3-Kinases / metabolism
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Phosphoinositide-3 Kinase Inhibitors
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Phosphorylation / drug effects
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Phosphorylation / genetics
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Phosphorylation / immunology
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Proto-Oncogene Proteins c-akt / genetics
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Proto-Oncogene Proteins c-akt / immunology
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Proto-Oncogene Proteins c-akt / metabolism
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Signal Transduction / drug effects
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Signal Transduction / genetics
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Signal Transduction / immunology*
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Toll-Like Receptor 2 / genetics
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Toll-Like Receptor 2 / immunology*
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Toll-Like Receptor 2 / metabolism
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Tularemia / genetics
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Tularemia / immunology*
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Tularemia / metabolism
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Tularemia / prevention & control
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Tumor Necrosis Factor-alpha / genetics
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Tumor Necrosis Factor-alpha / immunology
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Tumor Necrosis Factor-alpha / metabolism
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Vaccines, Attenuated / genetics
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Vaccines, Attenuated / immunology
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Vaccines, Attenuated / metabolism
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Wortmannin
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p38 Mitogen-Activated Protein Kinases / antagonists & inhibitors
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p38 Mitogen-Activated Protein Kinases / genetics
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p38 Mitogen-Activated Protein Kinases / immunology
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p38 Mitogen-Activated Protein Kinases / metabolism
Substances
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Androstadienes
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Bacterial Vaccines
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Chromones
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Enzyme Inhibitors
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Interleukin-6
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Morpholines
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Myd88 protein, mouse
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Myeloid Differentiation Factor 88
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Phosphoinositide-3 Kinase Inhibitors
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Tlr2 protein, mouse
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Toll-Like Receptor 2
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Tumor Necrosis Factor-alpha
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Vaccines, Attenuated
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2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one
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Proto-Oncogene Proteins c-akt
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Mitogen-Activated Protein Kinase 3
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p38 Mitogen-Activated Protein Kinases
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Dual Specificity Phosphatase 1
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Dusp1 protein, mouse
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Wortmannin