Encyclopedia of Signaling Molecules

2018 Edition
| Editors: Sangdun Choi

Inhibitor of KappaB

Reference work entry
DOI: https://doi.org/10.1007/978-3-319-67199-4_101651

Synonyms

Historical Background

The transcriptional factor NF-κB – a key regulator of cellular events such as cell growth, immune response, and cell survival – was first identified by Dr. David Baltimore’s group (Sen and Baltimore 1986). Inhibitor of kappaB (IκB) can form a complex with NF-κB and control these events. IκB family proteins harbor an ankyrin (ANK) repeat domain, a core motif of NF-κB binding, which was first identified in the cell-cycle gene sequences of yeast and Drosophila in 1987 (Breeden and Nasmyth 1987). IκB family members have been found to have conserved ANK repeat domains, including β-strand and α-helix repeat sequences. On the basis of extensive studies, nine IκB family proteins harboring ANK repeats have been identified, and these IκB proteins are classified into two groups according to their localization in the cytosol or nucleus.

IκB Family Proteins Control NF-κB Signaling

IκB proteins are classified into two groups according to their localization: cytosolic or prototypical (IκB-α, IκB-β, IκB-ε, p100, and p105) and nuclear or atypical (IκB-ζ, IκBNS, Bcl-3, and IκB-η) (Fig. 1). The IκB family of proteins, including the cytosolic and nuclear IκBs, interacts with NF-κB and modulates its transcriptional activity. Interestingly, nuclear IκB family proteins also bind to chromatin and control gene expression.
Inhibitor of KappaB, Fig. 1

Localization of IκB family proteins in the cell. IκB family proteins localize in the nucleus or cytosol. The red bars indicate ANK repeat domains

Prototypical IκB Family Proteins

The first identified IκB family member, IκB-α, is known to have six conserved ANK repeat domains, including one β-strand and two α-helix repeat sequences (Fig. 2) (Baeuerle and Baltimore 1988; Huxford et al. 1998). IκB-α can associate with NF-κB (especially the p50 subunit) through the ANK repeat domain and prevent the translocation of NF-κB from the cytosol to the nucleus (Baeuerle and Baltimore 1988). Activation of cells by various stimuli, including cytokines and bacterial components, leads to the phosphorylation (at Ser32 and Ser36), ubiquitination (at Lys21 and Lys22), and degradation of IκB-α by the proteasome system (Hayden and Ghosh 2008). After the degradation of IκB-α, NF-κB is translocated into the nucleus (Fig. 3). Then, NF-κB controls target gene expression in the nucleus, as well as events such as cell survival, proliferation, and inflammatory responses. Interestingly, NF-κB can bind to the IκB-α promoter region, positively regulate IκB-α gene expression, and then form a complex with it in the cytosol. Furthermore, newly synthesized IκB-α has been found to enter the nucleus, bind NF-κB, and then be exported (Ferreiro and Komives 2010). These events constitute “feedback loops,” which play an important role for ending cell-activation events (Fig. 4).
Inhibitor of KappaB, Fig. 2

Amino acid sequence and structure of human IκB-α.Red boxes indicate α-helixes, and red arrows indicate β-strands (For further information on the figure, please refer to Huxford et al. (1998) and the Uniprot database (http://www.uniprot.org/uniprot/P25963))

Inhibitor of KappaB, Fig. 3

Control of NF-κB signaling by IκB-α.Yellow circles (P) indicate phosphorylation. Gray circles (Ub) indicate ubiquitination

Inhibitor of KappaB, Fig. 4

Feedback loops of NF-κB signaling in the nucleus by IκB-α. IκB-α can be induced by NF-κB, bind to NF-κB, and then be exported

IκB-α-deficient mice were first generated in 1995. These mice exhibited severe runting, skin defects, and death within 8 days (Beg et al. 1995). The deficiency of IκB-α was found to lead to constitutive NF-κB activation and high expression levels of NF-κB target genes, including MIP-2 and VCAM-1. Thus, IκB-α plays an important role in maintaining homeostasis through the control of NF-κB activation.

The second member of IκB family proteins to be identified, IκB-β, can also associate with NF-κB through six ANK repeat domains and control NF-κB transcriptional activities (Zabel and Baeuerle 1990). Furthermore, lipopolysaccharide (LPS) stimulation can induce the degradation of IκB-β (Thompson et al. 1995). IκB-α and IκB-β have similar inhibitory effects against NF-κB signaling; however, IκB-β does not have a positive feedback mechanism like IκB-α (induction, binding, export) (Thompson et al. 1995). Furthermore, IκB-β-deficient mice can grow without any disorders or inflammation and do not have constitutive NF-κB activation.

Atypical IκB Family Proteins

Nuclear IκB family proteins have a nuclear localization signal and localize only in the nucleus. The most classic nuclear IκB family protein, Bcl-3, was identified in chromosome 19 in chronic lymphocytic leukemia (Ohno et al. 1990). Bcl-3 has seven ANK repeat domains (Fig. 2), and it forms a complex with NF-κB (p50 subunit) and removes p50 from its DNA-binding site (Franzoso et al. 1993). Bcl-3 ChIP sequence experiments have shown that Bcl-3 can be enriched near the p50-binding site (Jackman et al. 2012). Thus, Bcl-3 might control transcription in cooperation with the NF-κB subunit p50.

Bcl-3-deficient mice were first generated in 1997, and they demonstrated normal growth without any disorders or inflammation (Franzoso et al. 1997). In another study, Bcl-3-deficient mice were found to be less sensitive to dextran-sodium sulfate–induced colitis (Massoumi et al. 2006). Bcl-3 deficiency increases influences UVB-induced apoptosis of the skin. In macrophages, Bcl-3 deficiency leads to the production of large amounts of IL-10 in response LPS stimulation. Interestingly, IL-10 stimulation in macrophages induces Bcl-3 expression, and it negatively regulates LPS-induced TNF-α expression (Kuwata et al. 2003).

The second nuclear IκB family protein, IκB-ζ (also called MAIL), has been identified in macrophages in response to LPS stimulation (Kitamura et al. 2000). IκB-ζ has five ANK repeat domains. It forms a complex with the NF-κB subunit p50 and controls IL-6 expression in macrophages (Yamamoto et al. 2004). In contrast, IκB-ζ negatively regulates NF-κB-dependent ELAM1 gene expression. Interestingly, only IκB-ζ has a unique motif, called a transcriptional activation domain, which controls IL-17A expression without NF-κB transcriptional activity (Fig. 5).
Inhibitor of KappaB, Fig. 5

Gene expression control by IκB-ζ

In 2004, IκB-ζ-deficient mice were developed for the first time (Yamamoto et al. 2004). Further analysis demonstrated IκB-ζ-deficient mice to show skin inflammation, especially around the eye (Fig. 6) (Okuma et al. 2013). Mechanistically, the deficiency of IκB-ζ in keratinocytes leads to apoptosis, thereby activating T-lymphocytes. Thus, Z-VAD, an antiapoptotic drug, can prevent skin inflammation in IκB-ζ-deficient mice (Okuma et al. 2013).
Inhibitor of KappaB, Fig. 6

An IκB-ζ-deficient mouse showing inflammation. (a) An 8-week-old IκB-ζ-deficient mouse showing inflammation around the eye. The picture was kindly provided by Dr. Muta Tatsushi (Tohoku University). (b, c) A cartoon of cell death–induced inflammation through lymphocyte activation

The third member of nuclear IκB family proteins, IκBNS, was first identified in T cells, and the structure matched that of IκB-ζ by 43% (Fiorini et al. 2002). IκBNS also has six ANK repeat domains and forms a complex with the p50 subunit (Fig. 7).
Inhibitor of KappaB, Fig. 7

An IκB-ζ-deficient mouse showing inflammation. IκBNS has six ANK repeat domains and nuclear localization signal but no transactivation domain

IκBNS-deficient mice grow for over 6 months without any disorders or inflammation. In macrophages, IκBNS deficiency leads to high levels of IL-6 in response to LPS stimulation. Therefore, IκBNS-deficient mice have a high sensitivity to LPS-induced endotoxin shock. Another report showed that IκBNS-deficient mice are resistant to experimental autoimmune encephalomyelitis, and that they have low levels of Th17 cell differentiation (Kobayashi et al. 2014). A master regulator of Th17, RORγt, is upregulated by NF-κB signaling. However, IκBNS-deficient T cells show low RORγt expression even in the presence of TGF-β+IL-6 stimulation. Thus, IκBNS in T cells might control NF-κB activity and RORγt expression.

Summary

NF-κB signaling plays crucial roles in many cellular events via gene regulation. IκB family proteins are major signaling molecules that control NF-κB signaling. They all have ANK repeat domains and are able to form complexes with NF-κB. Initially, prototypical (cytosolic) IκB family proteins control NF-κB signaling. Subsequently, regulation by nuclear IκB family proteins follows. Importantly, nuclear IκB family proteins can bind to DNA and control gene regulation by NF-κB. However, each nuclear IκB family protein has different target genes and different roles in transcriptional activity. Thus, understanding of the NF-κB signaling pathway is important for understanding many cellular events, including cell survival, development, and homeostasis.

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Copyright information

© Springer International Publishing AG 2018

Authors and Affiliations

  1. 1.Department of Immunology, Graduate School of MedicineAkita UniversityAkitaJapan