New insights into NF-kappaB regulation and function

Abstract

NF-kappaB (nuclear factor-kappaB) transcription factors have multiple critical roles in the regulation of immune responses. In unstimulated cells, NF-kappaB proteins are sequestered in the cytoplasm by IkappaB inhibitory proteins. Various immune stimuli induce the IkappaB kinase (IKK) to phosphorylate IkappaBs, triggering their ubiquitination and proteasomal degradation, which permits nuclear translocation of associated NF-kappaB subunits and activation of NF-kappaB target genes. Recent studies have highlighted the importance of dynamic ubiquitination-deubiquitination events in regulating this canonical NF-kappaB signaling pathway. Ubiquitination additionally plays critical roles in activation of the noncanonical pathway that regulates NF-kappaB via signal-induced processing of NF-kappaB2 p100. New research has also identified several novel regulatory proteins that control the transcriptional activity of nuclear NF-kappaB.

Figure 1

Activation of NF-κB (nuclear factor-κB) by canonical and atypical pathways. (a) NF-κB activation by Toll-like receptor (TLRs) and cytokine receptors. The canonical pathway is activated by a large number of agonists [e.g. tumor necrosis fator (TNF), interleukin 1 (IL-1) or microbial ligands such as lipopolysaccharide (LPS)] through the triggering of various cytokine receptors or TLRs and stimulate the IκB kinase (IKK) complex (IKK1-IKK2-NEMO) to phosphorylate IκBα and promote its degradation. The atypical NF-κB pathways involve stimulus-induced proteolysis of the NF-κB2 p100 (the so-called noncanonical pathway) and NF-κB1 p105 precursor proteins. A limited number of agonists induce processing of p100 to p52 via activation of NF-κB–inducing kinase (NIK) and IKK1, resulting in the nuclear translocation of p52-RelB heterodimers. By contrast, the canonical IKK complex triggers p105 proteolysis, which releases associated Rel subunits and also facilitates activation of the p105-associated TPL-2 MEK kinase, which triggers extracellular signal-regulated kinase (ERK) mitogen-activated protein (MAP) kinase activation. (b) Canonical NF-κB activation by T-cell receptor (TCR). Ligation of TCR and the costimulatory molecule CD28 triggers the association of the intermediate CBM signaling complex, composed of Carma1, Bcl10 and MALT1, with the Ubc13-Uve1 ubiquitin conjugating enzyme heterodimer. This mediates K63-linked ubiquitination of NEMO, Bcl10 and MALT1, promoting the interaction of the CBM complex with IKK and transforming growth factor β (TGF-β)-activated kinase 1 (TAK1), which is a required step for the activation of IKK. A Carma1-independent pathway, which stimulates Tak1 catalytic activity and consequently phosphorylation of the IKK2 activation loop, is also necessary for IKK activation. Ub, ubiquitin.

Figure 2

A model of NF-κB–inducing kinase (NIK) regulation and noncanonical NF-κB (nuclear factor-κB) signaling. TNF receptor–associated factor 2 (TRAF2) and TRAF3 physically associate with cellular inhibitor of apoptosis 1 (c-IAP1; or c-IAP2) and NIK, respectively, and form a NIK-degradation complex via TRAF2-TRAF3 dimerization. Within this complex, c-IAP1 or 2 functions as an E3 ubiquitin ligase that mediates NIK polyubiquitination and proteolysis, thereby preventing NIK accumulation and noncanonical NF-κB signaling (a). Induction of noncanonical NF-κB signaling by different inducers involves degradation of c-IAP1 or 2 (b) or TRAF2 and 3 (c), stabilizing NIK, which triggers activation of NF-κB.

Figure 3

Regulation of the transcriptional activity of NF-κB (nuclear factor-κB). (a) In unstimulated cells, homodimers of p50 and p52 actively repress transcription of NF-κB target genes to which they are bound by recruitment of histone deacetylases (HDAC). (b) After ligand stimulation [e.g. via tumor necrosis factor (TNF)], p50-RelA dimers are released from degraded IκBα to translocate into the nucleus and displace repressive p50 and p52 homodimers from NF-κB target gene promoters. Transcriptional activity of RelA is regulated by phosphorylation on multiple sites, of which serine (S) 276 is the best understood. S276 can be phosphorylated by protein kinase A (PKA) and mitogen-and stress-activated protein kinases (MSKs) 1 and 2, inducing binding to CREB-binding protein (CBP) and p300 co-activator complexes, which acetylate adjacent histones and RelA to stimulate target gene transcription. RelA S276 phosphorylation also triggers recruitment of a complex of CDK1 and cyclin T1, which promotes transcriptional elongation. IκB kinase 1 (IKK1)phosphorylation of CBP-p300 stimulates preferential binding to NF-κB complexes. (c) Downregulation of NF-κB activity involves several different mechanisms. NF-κB dimers can be transported back to the cytoplasm as a result of NF-κB–dependent IκBα resynthesis. Alternatively, RelA can be ubiquitinated by ECS^SOCS1 or PDLIM2 E3 ligases, triggering its degradation by the proteasome. c-Rel–mediated transcription is also turned off by ubiquitin-dependent proteolysis in stimulated cells. Ub, ubiquitin.