Recruitment of IκBα to the hes1 promoter is associated with transcriptional repression

The NF-κB pathway plays a pivotal role in proliferation, differentiation, apoptosis, and immune responses in mammals. The NF-κB inhibitor, IκB, has classically been characterized for its ability to sequester NF-κB transcription factors in the cytoplasm. Nevertheless, a nuclear fraction of IκBα has consistently been detected and associated with repression of nuclear NF-κB. Now we show that IκBα physically associates with different repression elements such as nuclear corepressors and histone acetyltransferases and deacetylases (HDACs). More remarkably, chromatin immunoprecipitation experiments demonstrate that IκBα is recruited to the promoter regions of the Notch-target gene, hes1, together with HDAC1 and -5, whereas we did not detect IκBα associated with classical NF-κB target genes such as IL6 and RANTES. TNF-α treatment results in a temporary release of IκBα from the hes1 promoter that correlates with increased histone acetylation and transcriptional activation. In addition, we demonstrate that both IκB kinase-α and -β are simultaneously recruited to the hes1 promoter in response to TNF-α, coinciding with a maximum of IκBα release and gene activation. Moreover, TNF-α-dependent histone H3 acetylation, release of IκBα from the hes1 promoter, and hes1 mRNA synthesis are affected in IKK-α^-/- mouse embryonic fibroblasts. We propose that IκBα plays a previously undescribed role in regulating the recruitment of repression elements to specific promoters. Recruitment of IKKs to the nucleus in response to TNF-α may induce chromatin-associated IκBα release and gene activation. These findings provide additional insight in the cross-talk between NF-κB and other signaling pathways.