One of the mechanisms by which the innate immune system senses the invasion of pathogenic microorganisms is through the Toll-like receptors (TLRs), which recognize specific molecular patterns that are present in microbial components. Stimulation of different TLRs induces distinct patterns of gene expression, which not only leads to the activation of innate immunity but also instructs the development of antigen-specific acquired immunity. Here, we review the rapid progress that has recently improved our understanding of the molecular mechanisms that mediate TLR signalling.

Toll-like receptor signalling

The discovery that mammalian cells have the ability to synthesize the free radical nitric oxide (NO) has stimulated an extraordinary impetus for scientific research in all the fields of biology and medicine. Since its early description as an endothelial-derived relaxing factor, NO has emerged as a fundamental signaling device regulating virtually every critical cellular function, as well as a potent mediator of cellular damage in a wide range of conditions. Recent evidence indicates that most of the cytotoxicity attributed to NO is rather due to peroxynitrite, produced from the diffusion-controlled reaction between NO and another free radical, the superoxide anion. Peroxynitrite interacts with lipids, DNA, and proteins via direct oxidative reactions or via indirect, radical-mediated mechanisms. These reactions trigger cellular responses ranging from subtle modulations of cell signaling to overwhelming oxidative injury, committing cells to necrosis or apoptosis. In vivo, peroxynitrite generation represents a crucial pathogenic mechanism in conditions such as stroke, myocardial infarction, chronic heart failure, diabetes, circulatory shock, chronic inflammatory diseases, cancer, and neurodegenerative disorders. Hence, novel pharmacological strategies aimed at removing peroxynitrite might represent powerful therapeutic tools in the future. Evidence supporting these novel roles of NO and peroxynitrite is presented in detail in this review.

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Nitric Oxide and Peroxynitrite in Health and Disease

The transcription factor NF-κB regulates multiple aspects of innate and adaptive immune functions and serves as a pivotal mediator of inflammatory responses. NF-κB induces the expression of various pro-inflammatory genes, including those encoding cytokines and chemokines, and also participates in inflammasome regulation. In addition, NF-κB plays a critical role in regulating the survival, activation and differentiation of innate immune cells and inflammatory T cells. Consequently, deregulated NF-κB activation contributes to the pathogenic processes of various inflammatory diseases. In this review, we will discuss the activation and function of NF-κB in association with inflammatory diseases and highlight the development of therapeutic strategies based on NF-κB inhibition.

/pdf/nf-kb-signaling-in-inflammation-r4c2bxufoi.pdf

NF-κB signaling in inflammation

Shared principles in NF-kappaB signaling

The transcription factor NF-kappaB has been the focus of intense investigation for nearly two decades. Over this period, considerable progress has been made in determining the function and regulation of NF-kappaB, although there are nuances in this important signaling pathway that still remain to be understood. The challenge now is to reconcile the regulatory complexity in this pathway with the complexity of responses in which NF-kappaB family members play important roles. In this review, we provide an overview of established NF-kappaB signaling pathways with focus on the current state of research into the mechanisms that regulate IKK activation and NF-kappaB transcriptional activity.

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Signaling to NF-kappaB.

In mammals, the canonical nuclear factor kappaB (NF-kappaB) signaling pathway activated in response to infections is based on degradation of IkappaB inhibitors. This pathway depends on the IkappaB kinase (IKK), which contains two catalytic subunits, IKKalpha and IKKbeta. IKKbeta is essential for inducible IkappaB phosphorylation and degradation, whereas IKKalpha is not. Here we show that IKKalpha is required for B cell maturation, formation of secondary lymphoid organs, increased expression of certain NF-kappaB target genes, and processing of the NF-kappaB2 (p100) precursor. IKKalpha preferentially phosphorylates NF-kappaB2, and this activity requires its phosphorylation by upstream kinases, one of which may be NF-kappaB-inducing kinase (NIK). IKKalpha is therefore a pivotal component of a second NF-kappaB activation pathway based on regulated NF-kappaB2 processing rather than IkappaB degradation.

https://science.sciencemag.org/content/sci/293/5534/1495.full.pdf

Activation by IKKα of a Second, Evolutionary Conserved, NF-κB Signaling Pathway

NF-κB-Inducing Kinase Regulates the Processing of NF-κB2 p100

IκB kinase (IKK) is a key mediator of NF-κB activation induced by various immunological signals. In T cells and most other cell types, the primary target of IKK is a labile inhibitor of NF-κB, IκBα, which is responsible for the canonical NF-κB activation. Here, we show that in T cells infected with the human T-cell leukemia virus (HTLV), IKKα is targeted to a novel signaling pathway that mediates processing of the nfκb2 precursor protein p100, resulting in active production of the NF-κB subunit, p52. This pathogenic action is mediated by the HTLV-encoded oncoprotein Tax, which appears to act by physically recruiting IKKα to p100, triggering phosphorylation-dependent ubiquitylation and processing of p100. These findings suggest a novel mechanism by which Tax modulates the NF-κB signaling pathway.

/pdf/retroviral-oncoprotein-tax-induces-processing-of-nf-kb2-p100-2o1w5ya507.pdf

Retroviral oncoprotein Tax induces processing of NF‐κB2/p100 in T cells: evidence for the involvement of IKKα

NF-κB-inducing Kinase and IκB Kinase Participate in Human T-cell Leukemia Virus I Tax-mediated NF-κB Activation

NF-kappa B plays a pivotal role in normal T-cell activation and may also mediate human T-cell leukemia virus (HTLV)-induced T-cell transformation. Activation of NF-kappa B by both T-cell costimulatory signals and the HTLV Tax protein involves stimulation of I kappa B kinase (IKK). As a genetic approach to dissect the intermediate steps involved in NF-kappa B activation in human T cells, we performed somatic cell mutagenesis to isolate signaling-defective mutant Jurkat T-cell lines. One of the mutant cell lines was shown to have a specific blockade in the IKK signaling pathway but remained competent in the c-Jun N-terminal kinase and MAP kinase pathways. Interestingly, this mutant cell line lacks expression of IKK gamma, a non-catalytic component of the IKK complex. Expression of exogenous IKK gamma in the mutant cells restored NF-kappa B activation by both the T-cell costimulation agents and Tax. These findings provide genetic evidence for the requirement of IKK gamma in NF-kappa B signaling triggered by both T-cell costimulatory signals and HTLV-I Tax protein.

Somatic mutagenesis studies of NF-κB signaling in human T cells: Evidence for an essential role of IKKγ, in NP-κB activation by T-cell costimulatory signals and HTLV-I Tax protein

Gutian Xiao

Papers

Activation by IKKα of a Second, Evolutionary Conserved, NF-κB Signaling Pathway

NF-κB-Inducing Kinase Regulates the Processing of NF-κB2 p100

Retroviral oncoprotein Tax induces processing of NF‐κB2/p100 in T cells: evidence for the involvement of IKKα

NF-κB-inducing Kinase and IκB Kinase Participate in Human T-cell Leukemia Virus I Tax-mediated NF-κB Activation

Somatic mutagenesis studies of NF-κB signaling in human T cells: Evidence for an essential role of IKKγ, in NP-κB activation by T-cell costimulatory signals and HTLV-I Tax protein