The Rel Family Member P50 Mediates Cytokine-Induced C-Reactive Protein Expression by a Novel Mechanism

Abstract

Transcription of C-reactive protein (CRP) in Hep 3B cells is induced by IL-6, acting through C/EBP isoforms and STAT3. IL-1β, which alone has no effect, greatly enhances IL-6-induced transcription by unknown mechanisms. Because IL-1β activates the NF-κB system, we 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-1β induced a 3-fold increase in CRP expression, and responses to IL-6 and to IL-6 plus IL-1β 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 κB site overlapping 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-1β, when C/EBPβ is greatly activated, in the presence or absence of IL-6. These findings suggest that IL-1β induces nuclear translocation of p50-containing dimers and that p50 interacts with C/EBPβ activated by both IL-6 and IL-1β to induce CRP expression.