title: TLR4, but not TLR2, signals autoregulatory apoptosis of cultured microglia: a critical role of IFN-beta as a decision maker.
abstract: TLRs mediate diverse signaling after recognition of evolutionary conserved pathogen-associated molecular patterns such as LPS and lipopeptides. Both TLR2 and TLR4 are known to trigger a protective immune response as well as cellular apoptosis. In this study, we present evidence that TLR4, but not TLR2, mediates an autoregulatory apoptosis of activated microglia. Brain microglia underwent apoptosis upon stimulation with TLR4 ligand (LPS), but not TLR2 ligands (Pam(3)Cys-Ser-Lys(4), peptidoglycan, and lipoteichoic acid). Based on studies using TLR2-deficient or TLR4 mutant mice and TLR dominant-negative mutants, we also demonstrated that TLR4, but not TLR2, is necessary for microglial apoptosis. The critical difference between TLR2 and TLR4 signalings in microglia was IFN regulatory factor-3 (IRF-3) activation, followed by IFN-beta expression: while TLR4 agonist induced the activation of IRF-3/IFN-beta pathway, TLR2 did not. Nevertheless, both TLR2 and TLR4 agonists strongly induced NF-kappaB activation and NO production in microglia. Neutralizing Ab against IFN-beta attenuated TLR4-mediated microglial apoptosis. IFN-beta alone, however, did not induce a significant cell death. Meanwhile, TLR2 activation induced microglial apoptosis with help of IFN-beta, indicating that IFN-beta production following IRF-3 activation determines the apoptogenic action of TLR signaling. TLR4-mediated microglial apoptosis was mediated by MyD88 and Toll/IL-1R domain-containing adaptor-inducing IFN-beta, and was associated with caspase-11 and -3 activation rather than Fas-associated death domain protein/caspase-8 pathway. Taken together, TLR4 appears to signal a microglial apoptosis via autocrine/paracrine IFN-beta production, which may act as an apoptotic sensitizer.
authors.affiliation: Department of Pharmacology, Kyungpook National University School of Medicine, Daegu, Korea.
authors.forename: Dae Young
authors.initials: DY
authors.lastname: Jung
authors.forename: Heasuk
authors.initials: H
authors.lastname: Lee
authors.forename: Bo-Young
authors.initials: BY
authors.lastname: Jung
authors.forename: Jiyeon
authors.initials: J
authors.lastname: Ock
authors.forename: Myung-Shik
authors.initials: MS
authors.lastname: Lee
authors.forename: Won-Ha
authors.initials: WH
authors.lastname: Lee
authors.forename: Kyoungho
authors.initials: K
authors.lastname: Suk
mesh_terms: D048868:Adaptor Proteins, Signal Transducing / Q000502:physiology; D000818:Animals; D000943:Antigens, Differentiation / Q000502:physiology; D000950:Antigens, Ly / Q000096:biosynthesis; D017209:Apoptosis / Q000235:genetics / Q000276:immunology*; D053148:Caspase 3; D020169:Caspases / Q000502:physiology; D053471:Caspases, Initiator; D002460:Cell Line; D004268:DNA-Binding Proteins / Q000378:metabolism; D006801:Humans; D050838:Interferon Regulatory Factor-3; D016899:Interferon-beta / Q000096:biosynthesis / Q000502:physiology*; D019169:Jurkat Cells; D018950:Lipopolysaccharide Receptors / Q000096:biosynthesis; D051218:Lymphocyte Antigen 96; D051379:Mice; D008809:Mice, Inbred C3H; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D017628:Microglia / Q000166:cytology* / Q000276:immunology* / Q000378:metabolism; D053594:Myeloid Differentiation Factor 88; D011971:Receptors, Immunologic / Q000819:agonists / Q000096:biosynthesis / Q000235:genetics / Q000502:physiology*; D015398:Signal Transduction / Q000235:genetics / Q000276:immunology*; D051195:Toll-Like Receptor 2; D051197:Toll-Like Receptor 4; D014157:Transcription Factors / Q000378:metabolism
chemical_list: D048868:Adaptor Proteins, Signal Transducing; D000943:Antigens, Differentiation; D000950:Antigens, Ly; D004268:DNA-Binding Proteins; C494232:IRF3 protein, human; D050838:Interferon Regulatory Factor-3; C494233:Irf3 protein, mouse; D018950:Lipopolysaccharide Receptors; C411728:Ly96 protein, mouse; D051218:Lymphocyte Antigen 96; C507779:MYD88 protein, human; C507780:Myd88 protein, mouse; D053594:Myeloid Differentiation Factor 88; D011971:Receptors, Immunologic; C493486:Tlr2 protein, mouse; C493487:Tlr4 protein, mouse; D051195:Toll-Like Receptor 2; D051197:Toll-Like Receptor 4; D014157:Transcription Factors; D016899:Interferon-beta; C505407:CASP3 protein, human; C505409:Casp3 protein, mouse; C101136:Casp4 protein, mouse; D053148:Caspase 3; D020169:Caspases; D053471:Caspases, Initiator