The Rel family member p50 mediates cytokine-induced C-reactive protein expression by a novel mechanism

Transcription of C-reactive protein (CRP) in Hep 3B cells is induced by IL-6, acting through C/EBP isoforms and STAT3. IL-1 beta, which alone has no effect, greatly enhances IL-6-induced transcription by unknown mechanisms. Because IL-1 beta activates the NF-kappa B system, me explored the effects of overexpressed Rel family members on CRP expression, Unexpectedly, transactivation assays in transiently transfected Hep 3B cells showed p50 overexpression to markedly induce CRP transcription, acting in a region 3' to -86, In the presence of overexpressed p50, IL-1 beta induced a 3-fold increase in CRP expression, and responses to IL-6 and to IL-6 plus IL-1 beta were 4-fold greater than seen in cells without p50 overexpression. In contrast, overexpressed p65 abolished CRP induction by p50 and by cytokines, EMSA studies demonstrated that recombinant p50 bound to a nonconsensus kappa B site over-lapping the proximal C/EBP binding site on the CRP promoter. Mutation of a polypyrimidine tract in the p50-binding site inhibited the transactivating effect of cytokines. P50- but not p65-containing dimers were found in nuclei of Hep 3B cells 18 h after stimulation with IL-1 beta, when C/EBP beta is greatly activated, in the presence or absence of IL-6, These findings suggest that IL-1 beta induces nuclear translocation of p50-containing dimers and that p50 interacts with C/EBP beta activated by both IL-6 and IL-1 beta to induce CRP expression.