IKK-i/IKKϵ Controls Constitutive, Cancer Cell-associated NF-κB Activity via Regulation of Ser-536 p65/RelA Phosphorylation

Abstract

The transcription factor nuclear factor-B (NF-B) 2 plays a pivotal role in controlling the expression of a diverse set of genes that contribute to a variety of biological functions, including cell survival, cell proliferation, and immune and inflammatory responses (1). The classic form of NF-B is composed of a heterodimer of the p50 and p65 subunits, which is preferentially localized in the cytoplasm as an inactive complex with inhibitor proteins of the IB family. Following exposure of cells to a variety of stimuli, including inflammatory cytokines and LPS, IBs are phosphorylated by the IKK␣/␤ complex, polyubiquitinated, and subsequently degraded by the 26 S proteasome complex (1-3). Released NF-B complexes then accumulate in the nucleus, where they transcriptionally regulate the expression of genes involved in the immune and inflammatory responses (3).Based on a number of observations, it was assumed that virtually all inducers of NF-B lead to the activation of a single classic IKK␣/␤/␥ complex. However, recent studies demonstrated the existence of distinct IKK complexes that do not contain IKK␣, -␤, or -␥ (4). One of these complexes was described as a PMA-inducible IB kinase complex, with a critical component being an IKK-related kinase designated IKK⑀ (5), which is identical to a kinase named IKK-i identified via its induction downstream of LPS-induced signaling (6). IKK⑀ in turn is closely related to another recently discovered IKK-related kinase designated as TBK1 (TANK-binding kinase 1) (7) or NAK (NF-B activating kinase) (8). TBK1, which is highly homologous to IKK⑀, binds to TANK and TRAF and may form an alternative IKK complex consisting of IKK⑀ and TBK1 (7).IKK⑀ and TBK1 are enzymatically distinct from the homologous enzymes IKK␣ and IKK␤ (9) and have been shown to play important roles in the innate immune response. These kinases function as critical components of the interferon regulatory factor 3 (IRF3) and IRF7 signaling pathways involved in responses to viral infection or dsRNA treatment (10,11). Recent studies demonstrated that embryonic fibroblasts (MEFs) derived from TBK1-deficient (TBK1 Ϫ/Ϫ ) mice show impaired production of NF-B-dependent (12) as well as IRF3-dependent (13) gene expression. It has also been shown that IFN-␤ and IFN-inducible gene expression is defective in TBK1 knock-out cells in response to LPS, poly(I:C), or viral infection (14 -16).The relationships of IKK⑀ and TBK1 with NF-B activation remain enigmatic. Although recent studies defined their roles in IRF3 and IRF7 transcriptional activation (10, 11, 13) and suggested their involvement in 12,17,18), the exact molecular mechanism of NF-B activation by these kinases is not clearly understood. One report (19) indicated that IKK⑀ plays a key role integrating signals induced by pro-inflammatory stimuli by activating CAAT/enhancer-binding protein ␦ whose expression is regulated by NF-B. There is a recent report suggesting that IKK⑀ and TBK1 are among the * This work was supported by National Institutes of Health Grants AI35098, CA...

ABINs inhibit EGF receptor-mediated NF-κB activation and growth of EGF receptor-overexpressing tumour cells

“…Because we found that EGF-induced NF-kB activation in HEK293T cells is inhibited by expression of a dominant-negative IKKb mutant, we analysed the effect of EGF on p65 Ser-536 phosphorylation. Consistent with previously reported data (Adli and Baldwin, 2006), nuclear p65 was already significantly phosphorylated in untreated HEK293T cells. However, EGF did not further increase p65 Ser-536 phosphorylation.…”

Are the IKKs and IKK-related kinases TBK1 and IKK-ɛ similarly activated?

“…However, the role for TANK in the IKK-dependent NF-kB signaling cascades remains controversial because TANK-depleted, knockdown cells do not show any defect in TNF-a-or LPSmediated IKK activation [21,58]. Nevertheless, these observations do not rule out the possibility that TANK might connect upstream kinases such as TBK1 and IKK-e, thereby promoting IKK-independent phosphorylation of the NF-kB proteins p65 [59,60], c-Rel [61] or p52 [62] in response to as yet poorly characterized signals. TANK constitutively binds to IKK-e [63] and TBK1 [64] through its N-terminal domain; this interaction is probably essential for the positive regulation of signal transduction by TANK because recent studies demonstrated that TANK is involved in some TLR-dependent IRF-activating pathways by promoting TBK1 and IKK-e-mediated phosphorylation of IRF3 and IRF7 [20,21].…”

Introducing, OncoTarget

“…In addition p65 Ser536 is the target of many kinases. There were multiple p65 Ser536 kinases have been described including IKKβ, RSK1, IKKα, IKKε and NAK [46][47][48][49]. But none of them was systematically studied in prostate system especially in T/E fusion expressing PCa.…”