C/EBPβ enhances NF-κB-associated signalling by reducing the level of IκB-α

NF-kappa B and C/EBP beta proteins are involved in the regulation of genes which play a role in inflammation, immunity and malignant processes. The present study focuses on the question of how these systems cross talk upstream of the promoter level and investigates the regulation of NF-kappa B-associated signalling by C/EBP beta. In C/EBP beta(ko) macrophage-like cells stimulated with TNF or LIPS a reduced 3 kappa B-dependent transcription was detected compared to the wild type. This was accompanied by elevated nuclear p65 and NF-kappa B activity in the presence of C/EBP beta. In addition, overexpression of C/EBP beta in HeLa cells increased the nuclear level of coexpressed p65. Remarkably, the constitutive level of I kappa B-alpha was significantly higher in C/EBP beta(ko) cells; and this higher level was readjusted following stimulus-induced proteolysis. The I kappa B-alpha protein stability was comparable in both macrophage-like cell types with a somewhat higher stability in unstimulated C/EBP beta(ko) cells. Following stimulation with TNF, higher I kappa B-alpha mRNA levels were induced in C/EBP beta(ko) cells. The autoregulatory recovery of I kappa B-alpha protein following activation was completely blocked by transcriptional inhibition, regardless if C/EBP beta was present. Finally, we showed that C/EBP beta overexpression in HeLa cells blocked TNF-mediated inducibility of the I kappa B-alpha promoter. Taken together, our results indicate that regulation ofthe I kappa B-alpha level is one ofthe underlying mechanisms by which C/EBP beta controls NF-kappa B-associated signalling. C/EBP beta may belong to the group of proteins in the regulatory machinery which adjusts the I kappa B-alpha level in different cell types under various conditions with physiological and pathophysiological implications. (C) 2009 Elsevier Inc. All rights reserved.