Activation of the transcription factor NF-κB depends on inducible phosphorylation and subsequent degradation of NF-κB inhibitors, known as IκBs. Two reports by Anest et al. (Nature DOI: 10.1038/nature01648) and Yamamoto et al. (Nature DOI: 10.1038/nature01576) now describe how IκB kinase (IKK) provides an additional layer of regulation through NF-κB-mediated transcription. They show that a subunit of IκB kinase can phosphorylate histone H3 and may therefore modulate chromatin accessibility at NF-κB-responsive promoters.

Nuclear IKKα is necessary for histone H3 phosphorylation at NF-κb-dependent promoters.

IKK is composed of two catalytic subunits, IKKα and IKKβ, and is crucial for cytokine-induced IκB degradation and subsequent activation of NF-κB. Previous studies have shown that IKKβ is essential for degradation of IκB. In contrast, IKKα, although not necessary for proteolysis of IκB, is still important for NF-κB-dependent transcription. However, exactly how IKKα regulates NF-κB-dependent transcription is unclear. Previous work showing that IKKα shuttles between the nucleus and the cytoplasm hinted at the possibility of a novel nuclear function for IKKα.

Here, both Anest et al. and Yamomoto et al. start by confirming that IKKα is a nuclear protein. To address whether nuclear IKKα regulates cytokine-inducible NF-κB gene transcription, the groups used chromatin immunoprecipitation (ChiP) assays to examine the promoter occupancy of NF-κB target genes. These experiments showed that after cytokine stimulation, IKKα is recruited to NF-κB target-promoters in a NF-κB-dependent manner. Furthermore, they showed that the kinetics of IKKα recruitment parallel phosphorylation of histone H3-Ser 10 at the promoter, an event that is known to correlate with active gene expression. Both studies also showed that IKKα is most probably the physiological kinase responsible for cytokine-induced phosphorylation of histone H3, as phosphorylation of histone H3 at Ser 10 was markedly reduced in IKKα−/− murine embryonic fibroblasts and IKKα directly phosphorylated Ser 10 in vitro. Thus, these findings suggest that IKKα may affect gene expression by regulating chromatin structure at promoters.

Histone phosphorylation at Ser 10 is often accompanied by increased acetylation at histone H3-Lys 14. Indeed, both reports show that in addition to decreased levels of phosphorylated H3-Ser 10 in IKKα−/− cells, levels of H3 acetylation are also reduced at NF-κB-responsive promoters. Furthermore, Yamomoto et al. find that IKKα interacts with the histone acetyl transferase, CBP, but that recruitment of CBP to the promoter is not dependent on IKKα. Collectively, these data suggest that phosphorylation of histone H3 at Ser 10 by IKKα is required for subsequent H3-Lys 14 acetylation by CBP and that both events are necessary for efficient activation of NF-κB-mediated transcription.

It is unclear whether phosphorylation of histone H3 at Ser 10 by IKKα will be important for transcription of all, or only a subset of, NF-κB targets. Whether IKKα will be required for NF-κB-independent gene transcription also remains an open question. Furthermore, Anest et al. find that IKKβ is also recruited to NF-κB promoters, but that IKKβ does not seem to phosphorylate histone H3 at Ser 10, hinting at a potentially novel function for IKKβ in the NF-κB pathway.