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

Mazhar Adli,Albert S Baldwin

doi:10.1074/jbc.m603133200

Mazhar Adli, Albert S Baldwin

Open Access

https://doi.org/10.1074/jbc.m603133200

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Abstract

Nuclear factor kappaB (NF-kappaB) has been studied extensively as an inducible transcriptional regulator of the immune and inflammatory response. NF-kappaB activation downstream of lipopolysaccharide or cytokine stimulation is controlled by the IkappaB kinase complex, which contains IKKalpha and IKKbeta. Significantly, the constitutive activity of NF-kappaB has been implicated as an important aspect of many cancer cells, but mechanisms associated with this activity are poorly understood. An inducible kinase, IKK-i/IKKepsilon, related to the catalytic forms of the IkappaB kinase, has been studied as an anti-viral, innate immune regulator through its ability to control the activity of the transcription factors IRF-3 and IRF-7. Here, we demonstrate that IKK-i/IKKepsilon is expressed in a number of cancer cells and is involved in regulating NF-kappaB activity through its ability to control basal/constitutive, but not cytokine-induced, p65/RelA phosphorylation at Ser-536, a modification proposed to contribute to the transactivation function of NF-kappaB. Knockdown of IKK-i/IKKepsilon or expression of a S536A mutant form of p65 suppresses HeLa cell proliferation. The data indicate a role for IKK-i/IKKepsilon in controlling proliferation of certain cancer cells through regulation of constitutive NF-kappaB activity.

Highlights

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]
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]
IKK⑀ Is Expressed in a Number of Cancer Cells and in SV40 Large T-immortalized 293 Cells—To address a potential role for IKK⑀ in controlling Nuclear factor ␬B (NF-␬B) activity, we explored the expression of IKK⑀ in a variety of cell lines

Summary

Introduction

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]. 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

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