Regulation of NF-κB by TNF family cytokines

Abstract

The NF-κB family of inducible transcription factors is activated in response to a variety of stimuli. Amongst the best-characterized inducers of NF-κB are members of the TNF family of cytokines. Research on NF-κB and TNF have been tightly intertwined for more than 25 years. Perhaps the most compelling examples of the interconnectedness of NF-κB and the TNF have come from analysis of knock-out mice that are unable to activate NF-κB. Such mice die embryonically, however, deletion of TNF or TNFR1 can rescue the lethality thereby illustrating the important role of NF-κB as the key regulator of transcriptional responses to TNF. The physiological connections between NF-κB and TNF cytokines are numerous and best explored in articles focusing on a single TNF family member. Instead, in this review, we explore general mechanisms of TNF cytokine signaling, with a focus on the upstream signaling events leading to activation of the so-called canonical and noncanonical NF-κB pathways by TNFR1 and CD40, respectively.

Keywords: CD40; IKK; Inflammation; NF-kappaB; Signaling; TNF.

Figure 1. Schematic representation of NF-κB, IκB and IKK proteins

Key features of members of the NF-κB and IκB protein families and components of the IKK complex are shown. The number of amino acids in each protein is indicated on the right. Phosphorylation (P) and ubiquitination (U) sites on p100 and IκBα proteins that mediate proteasomal degradation and activation of the noncanonical and canonical NF-κB pathways are indicated. Presumed site of p100 cleavage (aa 447) is shown. Phosphorylation sites that mediate IKK kinase activation are indicated. α, β-helical domain; CC, coiled-coil domain; GRR, glycine-rich region; HLH, helix-loop-helix domain; LZ, leucine zipper domain; NBD, NEMO binding domain; RHD, rel homology domain; TAD, transactivation domain; Z, zinc finger domain.

Figure 2. Model of TNFR1 Signaling to Canonical NF-κB

A. Upon binding of TNF trimmers to TNFR1, oligomeric receptor complexes form resulting in recruitment of TRADD to the death domain (DD) of the receptors cytoplasmic tail. RIP1 is recruited through TRADD and TNFR1 through homotypic DD interactions. High affinity binding of TRAF2 trimers to TRADD is augmented by TRAF2/RIP1 interactions. B. TRAF2 trimers recruit cIAP1/2, which in turn recruit the LUBAC complex (HOIP, HOIL and SHARPIN), while RIP1 mediates recruitment of TAK1 and the IKK complex through NEMO. This oligomeric signaling complex is termed complex I and signals to NF-κB and AP-1 and MAPK pathways (not shown). C. Recruitment and induced proximity of TAK1 and the IKK complex supporting phosphorylation and activation of IKK. The ubiquitin ligase cIAP1/2 and LUBAC may facilitate TAK1 and IKK activation through production of linear and/or K63 linked ubiquitination. D. Activated IKK phosphorylates IκBα leading its ubiquitination and degradation and nuclear translocation of p65:p50 NF-κB complexes. DNA bound canonical NF-κB induces transcription of immune response genes as well as genes that protect the cell from TNF induced cell death. E. Cell death pathways are triggered by distinct signaling complexes triggering either apoptosis (complex I) or RIP1/RIP3 kinase dependent necroptosis (Necrosome). F. Negative feedback is mediated by NF-κB dependent resynthesis of IκB proteins as well as other factors including A20, which may facilitate complex I disassembly by disrupting TRAF2/cIAP binding. G. Multiple mechanisms, discussed in the text, act to induce degradation or displacement of DNA bound canonical NF-κB dimers.

Figure 3. Model of CD40 signaling to Noncanonical NF-κB

A. In resting B cells, a TRAF2/TRAF3 heterotrimer mediates constitutive NIK degradation by binding to NIK and the E3 ubiquitin ligases cIAP1/2. B. Upon encountering CD40L expressing CD4 T cells, CD40 dimers are oligomerized by CD40L trimmers resulting in altered affinity and avidity for TRAF proteins. C. Three non-exclusive mechanisms of NIK stabilization are shown. 1. Binding of TRAF2/3 trimers to the CD40 cytoplasmic tail results in decreased affinity for NIK. 2. TRAF3 is displaced from TRAF2 by preferential binding of TRAF1/2 heterotrimers to CD40 and cIAP1/2, resulting in CIAP1/2 mediated TRAF3 ubiquitination and degradation. 3. High affinity binding between TRAF2 and CD40 receptor displaces TRAF3 and alters cIAP1/2 function resulting in cIAP1/2, altered cIAP1/2 function and TRAF3 ubiquitination and degradation. D. Degradation or displacement from TRAF3 results in accumulation of newly synthesized NIK, which undergoes autophosphorylation and activation. NIK phosphorylates IKKα resulting in IKKα activation and phosphorylation of destruction box serines on p100 associated with RelB. Phosphorylated p100 is ubiquitinated and undergoes non-degradative processing by the proteasome yielding p52:RelB dimers that can mediate transcription by binding to DNA κB sites. E. IKKα induced phosphorylation of COOH-terminal serines on NIK may act as a negative feedback mechanism by inducing TRAF3-independent proteosomal degradation of NIK.