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