Department of Medicine, Division of Infectious Diseases and Immunology
Adaptor Proteins, Signal Transducing; Animals; Antigens, Differentiation; Cells, Cultured; DNA-Binding Proteins; Escherichia coli K12; Gene Expression Profiling; Gene Expression Regulation; Genetic Markers; Humans; Inflammation; Interferon Regulatory Factor-3; Kidney; Lipopolysaccharides; Macrophage Activation; Macrophages; Mice; Mice, Inbred C57BL; Microarray Analysis; Myeloid Differentiation Factor 88; NF-kappa B; Proteins; Receptors, Immunologic; Signal Transduction; Toll-Like Receptor 4; Transcription Factors; Transfection
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.
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Citation: Physiol Genomics. 2004 Nov 17;19(3):319-30. Epub 2004 Sep 14. Link to article on publisher's site