Transcriptional and translational regulation of CCAAT/Enhancer Binding Protein β expression by P53

Abstract

The transcription factor CCAAT/enhancer‐binding protein β (C/EBPβ) is highly expressed in various tissues during development, including liver, adipose tissue, blood cells, lung and the endocrine pancreas. Two isoforms of C/EBPβ namely liver‐enriched activator protein (LAP) and liver‐enriched inhibitor protein (LIP) translated from the same mRNA have been described. The LAP and LIP isoforms share the same C‐terminal DNA binding domain except LAP has an extra N‐terminal activation domain. Probably due to its higher affinity for its DNA cognate sequences, the LIP can counteract the effects of LAP isoforms even at substoichiometric levels. In vitro and in vivo experiments with cultured cells and knockout mice demonstrated that C/EBPβ play a critical role in the regulation of myofibroblast differentiation and bleomycin induced pulmonary fibrosis. However, the regulatory mechanism of C/EBPβ gene expression is unclear. Analysis of the C/EBPβ promoter sequence identified a conserved P53 response element (P53RE) in the C/EBPβ gene promoter. Gel shift mobility and chromatin immunoprecipitation assays confirmed that P53 binds to the P53RE. Promoter reporter assays and real time PCR analysis of samples from fibroblasts transiently transfected with P53 or dominant negative P53 expression plasmids indicated that P53 was a repressor of C/EBPβ gene transcription. Western blot analysis with protein extracts from similarly transfected fibroblasts or P53 deficient fibroblasts showed that P53 differentially regulated translation of the C/EBPβ LAP and LIP isoforms through the regulation of eIF4E and eIF4E‐BP1. Further studies with constructs containing the LAP 5′ untranslated region or the LIP 5′ untranslated region showed that the 5′ untranslated region is important in differential control of C/EBPβ LAP and LIP translation. Thus P53 could regulate C/EBPβ gene expression at both transcriptional and translational levels.Support or Funding InformationThis work was supported by grants HL052285 and HL112880 from the National Institute of Health.