C/EBPbeta enhances NF-kappaB-associated signalling by reducing the level of IkappaB-alpha.

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