ABSTRACT
Microbe-macrophage interactions play a central role in the pathogenesis of many infections. The ability of some bacterial pathogens to induce macrophage apoptosis has been suggested to contribute to their ability to elude innate immune responses and successfully colonize the host. Here, we provide evidence that activation of liver X receptors (LXRs) and retinoid X receptors (RXRs) inhibits apoptotic responses of macrophages to macrophage colony-stimulating factor (M-CSF) withdrawal and several inducers of apoptosis. In addition, combined activation of LXR and RXR protected macrophages from apoptosis caused by infection with Bacillus anthracis, Escherichia coli, and Salmonella typhimurium. Expression-profiling studies demonstrated that LXR and RXR agonists induced the expression of antiapoptotic regulators, including AIM/CT2, Bcl-X(L), and Birc1a. Conversely, LXR and RXR agonists inhibited expression of proapoptotic regulators and effectors, including caspases 1, 4/11, 7, and 12; Fas ligand; and Dnase1l3. The combination of LXR and RXR agonists was more effective than either agonist alone at inhibiting apoptosis in response to various inducers of apoptosis, and it acted synergistically to induce expression of AIM/CT2. Inhibition of AIM/CT2 expression in response to LXR/RXR agonists partially reversed their antiapoptotic effects. These findings reveal unexpected roles of LXRs and RXRs in the control of macrophage survival and raise the possibility that LXR/RXR agonists may be exploited to enhance innate immunity to bacterial pathogens that induce apoptotic programs as a strategy for evading host responses.
Subject(s)
Apoptosis , Macrophages/cytology , Macrophages/metabolism , Receptors, Cytoplasmic and Nuclear/metabolism , Retinoid X Receptors/metabolism , Animals , Apoptosis/drug effects , Apoptosis Regulatory Proteins , Cells, Cultured , DNA-Binding Proteins , Liver X Receptors , Macrophages/drug effects , Macrophages/microbiology , Mice , Mice, Knockout , Orphan Nuclear Receptors , Receptors, Cytoplasmic and Nuclear/deficiency , Receptors, Cytoplasmic and Nuclear/genetics , Receptors, Immunologic/metabolism , Receptors, ScavengerABSTRACT
The nuclear receptor corepressor (NCoR) and the related factor known as silencing mediator of retinoic acid and thyroid hormone receptor (SMRT) are essential components of multiprotein complexes that mediate active repression by unliganded nuclear receptors. Recent studies suggest that NCoR and SMRT can interact with and exert repressive effects on several other classes of DNA-binding transcription factors, but the physiological importance of these interactions has not been established. Here, investigation of endogenous transcriptional programs regulated by NCoR in macrophages reveals that NCoR acts as a transcriptional checkpoint for activator protein (AP)-1-dependent gene networks that regulate diverse biological processes including inflammation, cell migration, and collagen catabolism, with loss of NCoR, resulting in derepression of AP-1 target genes. The NCoR corepressor complex imposes an active block of exchange of c-Jun for c-Jun/c-Fos heterodimers, with targeted deletion of the c-Jun locus, resulting in loss of NCoR complexes from AP-1 target genes under basal conditions. The checkpoint function of NCoR is relieved by signal-dependent phosphorylation of c-Jun, which directs removal of NCoR/HDAC3/TBL1/TBLR1 complexes through recruitment of a specific ubiquitylation complex, as a prerequisite to the default binding of c-Jun/c-Fos heterodimers and transcriptional activation. The requirement for a checkpoint function to achieve the appropriate dynamic range of transcriptional responses to inflammatory signals is likely to be used by other signal-dependent transcription factors that regulate diverse homeostatic and developmental processes.
Subject(s)
Macrophage Activation/physiology , Nuclear Proteins/metabolism , Repressor Proteins/metabolism , Transcription Factor AP-1/metabolism , Animals , Cells, Cultured , Genes, jun , Macrophage Activation/genetics , Mice , Mice, Knockout , Models, Biological , Nuclear Proteins/deficiency , Nuclear Proteins/genetics , Nuclear Receptor Co-Repressor 1 , Recombinant Proteins/genetics , Recombinant Proteins/metabolism , Repressor Proteins/genetics , Signal Transduction , Transcription Factor AP-1/genetics , Transcription, Genetic , TransfectionABSTRACT
The molecular mechanisms involved in regulating the balance between cellular proliferation and differentiation remain poorly understood. Members of the Ets-domain family of transcription factors are candidates for proteins that might differentially regulate cell cycle control and cell type-specific genes during the differentiation of myeloid progenitor cells. The Ets repressor PE-1/METS has been suggested to contribute to growth arrest during terminal macrophage differentiation by repressing Ets target genes involved in Ras-dependent proliferation. An important feature of this regulatory model is that PE-1/METS is itself induced by the program of macrophage differentiation elicited by M-CSF. Here, we present evidence that the PE-1/METS gene is a transcriptional target of the cyclic AMP response element-binding protein-1 (CREB-1). CREB-1 expression is dramatically up-regulated during macrophage differentiation and phosphorylation of CREB-1 and the related factor CREM-1 are stimulated by M-CSF in a SAPK2/p38-dependent manner. Chromatin immunoprecipitation experiments demonstrate that CREB-1/CREM-1 are recruited to the PE-1/METS promoter as well as to the promoters of other genes that are up-regulated during terminal macrophage differentiation. Overexpression of CREB-1 stimulates the activities of the PE-1/METS, and macrosialin promoters, while expression of a dominant negative form of CREB-1 during macrophage differentiation inhibits expression of the PE-1/METS and macrosialin genes. Inhibition of CREB function also results in reduced expression of CD54 and impaired cell adhesion. Taken together, these findings reveal new roles of CREB-1/CREM-1 as regulators of macrophage differentiation.
Subject(s)
DNA-Binding Proteins/physiology , Macrophages/cytology , Oncogene Proteins/physiology , Repressor Proteins , Transcription Factors/physiology , Adenoviridae/genetics , Animals , Antigens, CD/genetics , Antigens, Differentiation, Myelomonocytic/genetics , Base Sequence , Blotting, Western , Bone Marrow Cells/cytology , Cell Adhesion , Cell Differentiation , Cell Division , Cell Nucleus/metabolism , Chromatin/metabolism , Cloning, Molecular , Cyclic AMP Response Element Modulator , Cyclic AMP Response Element-Binding Protein , DNA/chemistry , DNA-Binding Proteins/metabolism , Flow Cytometry , Genes, Dominant , Humans , Intercellular Adhesion Molecule-1/biosynthesis , Macrophages/metabolism , Mice , Models, Genetic , Molecular Sequence Data , Mutation , Oncogene Proteins/metabolism , Phosphorylation , Precipitin Tests , Promoter Regions, Genetic , Protein Structure, Tertiary , Proto-Oncogene Proteins c-ets , Reverse Transcriptase Polymerase Chain Reaction , Time Factors , Transfection , U937 Cells , Up-RegulationABSTRACT
Defining the molecular mechanisms that coordinately regulate proliferation and differentiation is a central issue in development. Here, we describe a mechanism in which induction of the Ets repressor METS/PE1 links terminal differentiation to cell cycle arrest. Using macrophages as a model, we provide evidence that METS/PE1 blocks Ras-dependent proliferation without inhibiting Ras-dependent expression of cell type-specific genes by selectively replacing Ets activators on the promoters of cell cycle control genes. Antiproliferative effects of METS require its interaction with DP103, a DEAD box-containing protein that assembles a novel corepressor complex. Functional interactions between the METS/DP103 complex and E2F/ pRB family proteins are also necessary for inhibition of cellular proliferation, suggesting a combinatorial code that directs permanent cell cycle exit during terminal differentiation.
