Editors' ChoiceImmunology

Inflammatory Crosstalk

Science Signaling  25 Nov 2008:
Vol. 1, Issue 47, pp. ec404
DOI: 10.1126/scisignal.147ec404

Notch proteins are membrane-bound receptors that are proteolytically processed upon ligand binding. Notch intracellular domain (NICDs), which are released by γ-secretase, translocate to the nucleus, where they bind to the DNA-binding protein RBP-J to activate gene expression. Toll-like receptor (TLR) signaling is critical for immune responses to infections; however, various mechanisms inhibit TLR signaling to avoid uncontrollable inflammation and tissue damage. Hu et al. performed microarray analyses of human macrophages and found a subset of genes whose induction by various TLR ligands was inhibited by interferon-γ (IFN-γ). These genes were known targets of Notch, including the genes encoding the transcriptional repressors Hes1 and Hey1. The authors found that macrophages exhibited basal Notch signaling, as demonstrated by the detection of NICD1 and NICD2 by Western blotting. Inhibitors of γ-secretase blocked this signaling and also inhibited TLR-induced expression of hes1 and hey1. Small inhibitory RNA (siRNA)–mediated knockdown of RBP-J also inhibited TLR-induced expression of Notch-target genes. Although Notch ligands induced the expression of hes1 in macrophages, the extent of induction was less than that observed in response to TLR signaling. Inhibition of NF-κB and p38 mitogen-activated protein kinase signaling (which are activated by TLRs) inhibited TLR-induced expression of Notch-target genes. Together, these data suggested that both Notch and TLR signaling pathways must be functional for optimal expression of Notch-target genes. Further analysis showed that IFN-γ inhibited TLR-induced expression of Notch-target genes by decreasing the basal abundance of NICD2, but not NICD1, in macrophages, without having any effects on the expression of the genes encoding Notch1 and Notch2. Cooperation between Notch and TLR signaling was also relevant for responses to TLR ligands. TLR-induced expression of the gene encoding the proinflammatory cytokine interleukin-6 (IL-6) was inhibited in RBP-J–deficient mouse macrophages compared with that in wild-type (WT) macrophages, and the absence of RBP-J protected mice from endotoxin-induced death. Inflammatory responses to TLR ligands, including the production of IL-6, were higher in chimeric mice that received hey1-deficient or hes1-deficient macrophages compared with those in mice that received WT cells, which suggests a role for these Notch targets in mediating feedback inhibition of TLR-induced proinflammatory cytokine production in vivo. As Hertzog discusses in commentary, these findings suggest that Notch and TLR pathways reciprocally regulate each other’s signaling and have important implications for how these pathways interact during infection and during lymphocyte development, in which Notch signaling plays a role.

X. Hu, A. Y. Chung, I. Wu, J. Foldi, J. Chen, J. D. Ji, T. Tateya, Y. J. Kang, J. Han, M. Gessler, R. Kageyama, L. B. Ivashkiv, Integrated regulation of Toll-like receptor responses by Notch and interferon-γ pathways. Immunity 29, 691–703 (2008). [PubMed]

P. Hertzog, A Notch in the Toll belt. Immunity 29, 663–665 (2008). [PubMed]