Transcription factor c-Rel enhances C-reactive protein expression by facilitating the binding of C/EBPβ to the promoter

Abstract

Induction of C-reactive protein (CRP) synthesis in hepatocytes by cytokines occurs at the transcriptional level. In Hep3B cells, the transcription factors C/EBPβ, STAT3, and Rel p50 have been shown to participate in this process. A C/EBP binding site centered at −53 and an overlapping nonconsensus κB site on the promoter are critical for CRP expression. We have previously found that an oligonucleotide containing a κB site diminished binding of C/EBPβ to the C/EBP site, suggesting that unidentified Rel proteins present in Hep3B nuclei facilitate the formation of C/EBPβ-complexes. The current studies were undertaken to determine which of the five Rel proteins, p50/p65/p52/c-Rel/RelB, play such a role. Mutation of the nonconsensus κB site did not abolish binding of C/EBPβ to its binding site, indicating that this site was not necessary for the formation of C/EBPβ-complexes. Depletion of Rel proteins from Hep3B nuclei led to decreased formation of C/EBPβ-complexes on a CRP promoter-derived oligonucleotide that contained only the intact C/EBP binding site but not the nonconsensus κB site. This finding indicates that Rel proteins are involved in the binding of C/EBPβ to its binding site by a κB site-independent mechanism. Electrophoretic mobility shift assays (EMSAs) revealed that it was c-Rel that facilitated formation of C/EBPβ-complexes and that c-Rel bound directly to C/EBPβ-complexes formed on the C/EBP site. Cotransfection of c-Rel enhanced the induction of CRP promoter-driven luciferase activity and enhanced endogenous CRP expression in cells transfected with C/EBPβ. We conclude that c-Rel regulates CRP expression without the requirement of binding to a κB site, and binds directly to C/EBPβ to facilitate the binding of C/EBPβ to the CRP promoter.