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Preventing macrophages from going haywire
Activation of the nuclear factor κB (NF-κB) transcription factors occurs through two pathways: canonical and noncanonical. In infected fibroblasts and epithelial cells, the combined activation of both pathways induces a burst of late-phase inflammatory gene expression to help eradicate pathogens. Chatterjee et al. used a combination of mathematical modeling and experiments to show that macrophages are wired differently. In macrophages, combined stimulation of canonical and noncanonical signaling led to the delayed generation of an inhibitor of the canonical pathway, thus preventing the potentially deleterious effects of excessive inflammatory responses by the macrophages. This mathematical approach may lead to the discovery of other mechanism of cell type–specific control of transcription factor activity.
Abstract
The nuclear factor κB (NF-κB) transcription factors coordinate the inflammatory immune response during microbial infection. Pathogenic substances engage canonical NF-κB signaling through the heterodimer RelA:p50, which is subjected to rapid negative feedback by inhibitor of κBα (IκBα). The noncanonical NF-κB pathway is required for the differentiation of immune cells; however, cross-talk between both pathways can occur. Concomitantly activated noncanonical signaling generates p52 from the p100 precursor. The synthesis of p100 is induced by canonical signaling, leading to the formation of the late-acting RelA:p52 heterodimer. This cross-talk prolongs inflammatory RelA activity in epithelial cells to ensure pathogen clearance. We found that the Toll-like receptor 4 (TLR4)–activated canonical NF-κB signaling pathway is insulated from lymphotoxin β receptor (LTβR)–induced noncanonical signaling in mouse macrophage cell lines. Combined computational and biochemical studies indicated that the extent of NF-κB–responsive expression of Nfkbia, which encodes IκBα, inversely correlated with cross-talk. The Nfkbia promoter showed enhanced responsiveness to NF-κB activation in macrophages compared to that in fibroblasts. We found that this hyperresponsive promoter engaged the RelA:p52 dimer generated during costimulation of macrophages through TLR4 and LTβR to trigger synthesis of IκBα at late time points, which prevented the late-acting RelA cross-talk response. Together, these data suggest that, despite the presence of identical signaling networks in cells of diverse lineages, emergent cross-talk between signaling pathways is subject to cell type–specific regulation. We propose that the insulation of canonical and noncanonical NF-κB pathways limits the deleterious effects of macrophage-mediated inflammation.
