Noncanonical NF-κB Signaling Is Limited by Classical NF-κB Activity

Abstract

Precise regulation of nuclear factor κB (NF-κB) signaling is crucial for normal immune responses, and defective NF-κB activity underlies a range of immunodeficiencies. NF-κB is activated through two signaling cascades: the classical and noncanonical pathways. The classical pathway requires inhibitor of κB kinase β (IKKβ) and NF-κB essential modulator (NEMO), and hypomorphic mutations in the gene encoding NEMO (ikbkg) lead to inherited immunodeficiencies, collectively termed NEMO-ID. Noncanonical NF-κB activation requires NF-κB–inducing kinase (NIK) and IKKα, but not NEMO. We found that noncanonical NF-κB was basally active in peripheral blood mononuclear cells from NEMO-ID patients, and that noncanonical NF-κB signaling was similarly enhanced in cell lines lacking functional NEMO. NIK, which normally undergoes constitutive degradation, was aberrantly present in resting NEMO-deficient cells, and regulation of its abundance was rescued by reconstitution with full-length NEMO, but not a mutant NEMO protein unable to physically associate with IKKα or IKKβ. Binding of NEMO to IKKα was not required for ligand-dependent stabilization of NIK or noncanonical NF-κB signaling. Rather, an intact and functional IKK complex was essential to suppress basal NIK activity in unstimulated cells. Despite interacting with IKKα and IKKβ to form an IKK complex, NEMO mutants associated with immunodeficiency failed to rescue classical NF-κB signaling or reverse the accumulation of NIK. Together, these findings identify a crucial role for classical NF-κB activity in the suppression of basal noncanonical NF-κB signaling.

The non-canonical NF-κB pathway is induced by cytokines in pancreatic beta cells and contributes to cell death and proinflammatory responses in vitro

“…The canonical and non-canonical NF-κB pathways were initially believed to fulfil different functions, but recent studies have revealed crosstalk between them [13,19]. Thus, the increase in NFKB2 expression by the canonical pathway positively regulates the non-canonical pathway by providing a greater amount of precursor p100 for p52 generation [35].…”

“…The IKK complex containing NEMO and IKKβ plays an important role in the maintenance of the low basal levels of NIK, thus suppressing basal non-canonical NF-κB signalling [19]. IL-1β mediates IKKβ degradation in beta cells, leading to IKKα homodimer activation [8].…”

“…However, crosstalk between the canonical and non-canonical pathways might exist [13,19]. Therefore, the aims of this study were to investigate whether proinflammatory cytokines associated with the pathogenesis of type 1 diabetes activate the non-canonical pathway in beta cells and, if so, to examine the regulation of this pathway and the consequences of its activation on NF-κB proinflammatory and apoptotic responses in these cells.…”

Negative feedback regulation of NF-κB-inducing kinase is proteasome-dependent but does not require cellular inhibitors of apoptosis

“…We have previously shown that TRAF3 is dispensable for active NIK turnover [29]. However, since cIAP1/2 are the only E3 ubiquitin ligases known to target NIK directly, we hypothesized that cIAP1/2 might play a TRAF3-independent role in the turnover of active NIK.…”

The NF-κB subunit RelB controls p100 processing by competing with the kinases NIK and IKK1 for binding to p100

“…In the second mechanism, NIK activity is reduced by IKK1, which stimulates NIK degradation (26,27). To test whether either or both of these pathways was functional in our system, we transfected HEK 293T cells with a plasmid encoding NIKDN324 and found that this mutant was also unstable in the absence of p100 but was stabilized in the presence of p100 (Fig.…”