Immunology Multiple Mechanisms of Inflammation

Elizabeth M. Adler

Science Signaling, AAAS, Washington, DC 20005, USA

Toll-like receptors (TLRs) recognize pathogen-associated molecular patterns to initiate signaling pathways leading to activation of genes that encode products critical to the inflammatory response. Stimulation of many TLR target genes requires the removal of the nuclear receptor corepressor (NCoR). Huang et al. used chromatin immunoprecipitation (ChIP) to assay the disappearance of NCoR from the promoter of the gene encoding inducible nitric oxide synthase (inos) in macrophages [primary bone marrow–derived macrophages (BMDMs) or RAW264.7 macrophages] and found that clearance was more rapid after TLR2 stimulation [with Pam3CSK4 (Pam3)] than after TLR4 stimulation [with lipopolysaccharide (LPS)]. Moreover, NCoR clearance in response to LPS—but not Pam3—was sensitive to proteasomal inhibition and depended on c-Jun phosphorylation. Subsequent investigation implicated nuclear factor B (NF-B) signaling in TLR4-mediated NCoR clearance, and Ca²⁺-calmodulin–dependent protein kinase II (CaMKII)–dependent signaling in TLR2-mediated NCoR clearance. LPS stimulated recruitment of the NF-B p65 subunit to the inos promoter to NCoR clearance and failed to stimulate NCoR clearance in fibroblasts or BMDMs lacking p65. TLR4 signaling stimulated translocation of inhibitor of B kinase epsilon (IKK) to the nucleus, its interaction with p65, and its recruitment to the inos promoter. IKK phosphorylated c-Jun in vitro; moreover, IKK knockdown decreased LPS-dependent c-Jun phosphorylation and NCoR clearance. TLR2 signaling, however, stimulated activation of CaMKII, its recruitment to the inos promoter, and its phosphorylation of the transducin β–like protein 1 (TBLR1) component of the NCoR complex. TLR2-mediated NCoR clearance, which did not require c-Jun phosphorylation, was delayed by chelation of intracellular calcium and by knockdown of CaMKII or TBLR1, treatments that switched NCoR clearance by TLR2 signaling to a TLR4-like mode. Activation of liver X receptors (LXR) has anti-inflammatory activity. Previous studies have identified LXR effects on LPS- but not Pam3-mediated signaling; here, the authors showed that inhibition of CaMKII signaling enabled an LXR agonist to inhibit TLR2-dependent inos induction, whereas increasing cell calcium interfered with its ability to attenuate the response to TLR4. Despite their common involvement in the inflammatory response, different TLRs recognize distinct microbial components; thus, these distinct pathways to NCoR clearance may enable the activation of pathogen-specific responses and disease-specific programs of inflammation.

W. Huang, S. Ghisletti, V. Perissi, M. G. Rosenfeld, C. K. Glass, Transcriptional integration of TLR2 and TLR4 signaling at the NCoR derepression checkpoint. Mol. Cell 35, 48–57 (2009).[PubMed]