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

NF-κB and C/EBPβ 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-κB-associated signalling by C/EBPβ. In C/EBPβko macrophage-like cells stimulated with TNF or LPS a reduced 3κB-dependent transcription was detected compared to the wild type. This was accompanied by elevated nuclear p65 and NF-κB activity in the presence of C/EBPβ. In addition, overexpression of C/EBPβ in HeLa cells increased the nuclear level of coexpressed p65. Remarkably, the constitutive level of IκB-α was significantly higher in C/EBPβko cells; and this higher level was readjusted following stimulus-induced proteolysis. The IκB-α protein stability was comparable in both macrophage-like cell types with a somewhat higher stability in unstimulated C/EBPβko cells. Following stimulation with TNF, higher IκB-α mRNA levels were induced in C/EBPβko cells. The autoregulatory recovery of IκB-α protein following activation was completely blocked by transcriptional inhibition, regardless if C/EBPβ was present. Finally, we showed that C/EBPβ overexpression in HeLa cells blocked TNF-mediated inducibility of the IκB-α promoter. Taken together, our results indicate that regulation of the IκB-α level is one of the underlying mechanisms by which C/EBPβ controls NF-κB-associated signalling. C/EBPβ may belong to the group of proteins in the regulatory machinery which adjusts the IκB-α level in different cell types under various conditions with physiological and pathophysiological implications.