RelA repression of RelB activity induces selective gene activation downstream of TNF receptors

TNF-alpha is a potent proinflammatory cytokine that regulates immune and inflammatory responses and programmed cell death. TNF-a stimulation causes nuclear translocation of several NF-kappa B dimers, including ReIA/p50 and ReIB/p50. However, contrary to ReIA, ReIB entering the nucleus in response to TNF-alpha cannot bind to DNA in mouse embryonic fibroblasts, strongly suggesting that ReIB DNA-binding activity is modulated by additional nuclear mechanisms. Here, we demonstrate that TNF-alpha promotes the association of ReIA with RelB in the nucleus and that TNF-alpha-induced ReIA/ReIB heterodimers do not bind to KB sites. Remarkably, we show that ReIA serine-276, the phosphorylation of which is induced by TNF receptor ligation, is crucial for ReIA/ReIB complex formation and subsequent inhibition of ReIB DNA binding. In the absence of RelA phosphorylation on serine-276, TNF-alpha stimulation leads to a strong increase in the expression of endogenous NF-kappa B-responsive genes, such as Bcl-xL, whose transcriptional up-regulation is mainly controlled by RelB. Our findings demonstrate that ReIA has a major regulatory role serving to dampen ReIB activity in response to TNF-alpha and define a previously unrecognized mechanism that represents an essential step leading to selective NF-kappa B target gene expression.