The interaction of C-Rel with C/EBPbeta enhances C/EBPbeta binding to the C-reactive protein gene promoter.

C-reactive protein (CRP) is a plasma protein primarily synthesized in the liver following inflammatory stimuli as part of the acute phase response. Expression of CRP is tightly regulated in hepatocytes. Normally very little CRP mRNA is transcribed, but inflammatory stimuli are followed by a dramatic increase in mRNA synthesis and accumulation. Interleukins -6 and 1 (IL-6 and IL-1) are believed to be the major cytokines responsible for induction of acute phase protein biosynthesis. We previously demonstrated that in vivo c-Rel plays a novel regulatory role in that it appears to be in complex with C/EBPbeta when C/EBPbeta is bound to the CRP gene promoter following cytokine stimulation, but is not itself bound to DNA. In this study we found that recombinant c-Rel((1-300)) (truncated c-Rel protein missing the transactivation domain) increased the affinity of recombinant C/EBPbeta for a CRP-derived C/EBP site (-53) at least 10-fold. This effect was independent of a previously described p50 binding site at -43 and of binding of c-Rel to DNA. C/EBPbeta and c-Rel((1-300)) were found to physically interact in solution, and overexpression of c-Rel (either full length or truncated (1-300)) in the presence of overexpressed C/EBPbeta stimulated CRP transcription. We conclude that c-Rel((1-300)) binding to C/EBPbeta increases the affinity of C/EBPbeta for the C/EBP binding site at -53 on the CRP promoter, and that the transactivation domain of c-Rel is not necessary for this effect, which depends on protein: protein contacts with C/EBPbeta.