The induction of macrophage gene expression by LPS predominantly utilizes Myd88-independent signaling cascades.
Myeloid differentiation protein-88 (MyD88) is a signal adaptor protein required for cytokine production following engagement of Toll-like receptors (TLRs) by their cognate ligands. Activation of both TLR-3 and TLR-4, however, can engage signaling events independent of MyD88 expression. The relative importance of these MyD88-dependent and -independent signaling pathways in the macrophage response to lipopolysaccharide (LPS) is unknown. Here we define these events using microarray expression profiling of LPS-stimulated macrophages taken from MyD88-null and wild-type mice. Of the 1,055 genes found to be LPS responsive, only 21.5% were dependent on MyD88 expression, with MyD88-independent genes constituting 74.7% of the genetic response. This MyD88-independent gene expression was predominantly transcriptionally regulated, as it was unaffected by cycloheximide blockade of new protein synthesis. A previously undescribed group of LPS-regulated genes (3.8%), whose induction or repression was significantly greater in the absence of MyD88, was also identified by these studies. The regulation of these genes suggested that MyD88 could serve as a molecular brake, constraining gene activity in a subset of LPS-responsive genes. The findings generated with LPS stimulation were recapitulated by exposure of macrophages to live Escherichia coli. These expression-profiling studies redefine the current dogma of TLR-4 signaling and establish that MyD88, although essential for some of the best-characterized macrophage responses to LPS, is not required for the regulation of the majority of genes engaged by macrophage exposure to endotoxin or live bacteria.
Björkbacka
Harry
H
Lipid Metabolism Unit, Department of Molecular Biology, Massachusetts General Hospital, Boston 02114, USA.
Fitzgerald
Katherine A
KA
Huet
François
F
Li
Xiaoman
X
Gregory
James A
JA
Lee
Melinda A
MA
Ordija
Christine M
CM
Dowley
Nicole E
NE
Golenbock
Douglas T
DT
Freeman
Mason W
MW
D048868:Adaptor Proteins, Signal Transducing; D000818:Animals; D000943:Antigens, Differentiation / Q000235:genetics* / Q000502:physiology; D002478:Cells, Cultured; D004268:DNA-Binding Proteins / Q000378:metabolism; D048168:Escherichia coli K12 / Q000276:immunology; D020869:Gene Expression Profiling / Q000379:methods; D005786:Gene Expression Regulation / Q000502:physiology*; D005819:Genetic Markers / Q000235:genetics; D006801:Humans; D007249:Inflammation / Q000235:genetics / Q000378:metabolism; D050838:Interferon Regulatory Factor-3; D007668:Kidney / Q000737:chemistry / Q000166:cytology / Q000196:embryology / Q000378:metabolism; D008070:Lipopolysaccharides / Q000276:immunology*; D008262:Macrophage Activation / Q000502:physiology*; D008264:Macrophages / Q000737:chemistry / Q000166:cytology / Q000378:metabolism* / Q000502:physiology; D051379:Mice; D008810:Mice, Inbred C57BL; D046228:Microarray Analysis / Q000379:methods; D053594:Myeloid Differentiation Factor 88; D016328:NF-kappa B / Q000378:metabolism; D011506:Proteins / Q000235:genetics; D011971:Receptors, Immunologic / Q000172:deficiency / Q000235:genetics* / Q000378:metabolism / Q000502:physiology; D015398:Signal Transduction / Q000235:genetics* / Q000502:physiology; D051197:Toll-Like Receptor 4; D014157:Transcription Factors / Q000378:metabolism; D014162:Transfection; D000090123:Viperin Protein
D048868:Adaptor Proteins, Signal Transducing; D000943:Antigens, Differentiation; D004268:DNA-Binding Proteins; D005819:Genetic Markers; D050838:Interferon Regulatory Factor-3; D008070:Lipopolysaccharides; D053594:Myeloid Differentiation Factor 88; D016328:NF-kappa B; D011506:Proteins; D011971:Receptors, Immunologic; D051197:Toll-Like Receptor 4; D014157:Transcription Factors; C494232:IRF3 protein, human; C494233:Irf3 protein, mouse; C507779:MYD88 protein, human; C507780:Myd88 protein, mouse; C493487:Tlr4 protein, mouse; C417529:Rsad2 protein, mouse; D000090123:Viperin Protein