title: Nucleolar retention of a translational C/EBPalpha isoform stimulates rDNA transcription and cell size.
abstract: The messenger RNA of the intronless CEBPA gene is translated into distinct protein isoforms through the usage of consecutive translation initiation sites. These translational isoforms have distinct functions in the regulation of differentiation and proliferation due to the presence of different N-terminal sequences. Here, we describe the function of an N-terminally extended protein isoform of CCAAT enhancer-binding protein alpha (C/EBPalpha) that is translated from an alternative non-AUG initiation codon. We show that a basic amino-acid motif within its N-terminus is required for nucleolar retention and for interaction with nucleophosmin (NPM). In the nucleoli, extended-C/EBPalpha occupies the ribosomal DNA (rDNA) promoter and associates with the Pol I-specific factors upstream-binding factor 1 (UBF-1) and SL1 to stimulate rRNA synthesis. Furthermore, during differentiation of HL-60 cells, endogenous expression of extended-C/EBPalpha is lost concomitantly with nucleolar C/EBPalpha immunostaining probably reflecting the reduced requirement for ribosome biogenesis in differentiated cells. Finally, overexpression of extended-C/EBPalpha induces an increase in cell size. Altogether, our results suggest that control of rRNA synthesis is a novel function of C/EBPalpha adding to its role as key regulator of cell growth and proliferation.
authors.affiliation: Leibniz Institute for Age Research-Fritz Lipmann Institute, Jena, Germany.
authors.forename: Christine
authors.initials: C
authors.lastname: Müller
authors.forename: Anna
authors.initials: A
authors.lastname: Bremer
authors.forename: Sandra
authors.initials: S
authors.lastname: Schreiber
authors.forename: Sabrina
authors.initials: S
authors.lastname: Eichwald
authors.forename: Cornelis F
authors.initials: CF
authors.lastname: Calkhoven
mesh_terms: D015152:Blotting, Northern; D015153:Blotting, Western; D022763:CCAAT-Enhancer-Binding Protein-alpha / Q000378:metabolism*; D002466:Cell Nucleolus / Q000378:metabolism*; D048429:Cell Size; D047369:Chromatin Immunoprecipitation; D004275:DNA, Ribosomal / Q000235:genetics*; D005822:Genetic Vectors / Q000235:genetics; D018922:HL-60 Cells; D006801:Humans; D047468:Immunoprecipitation; D016086:Lentivirus / Q000235:genetics; D009687:Nuclear Proteins / Q000378:metabolism; D000090243:Nucleophosmin; D010766:Phosphorylation; D035463:Pol1 Transcription Initiation Complex Proteins / Q000378:metabolism; D011401:Promoter Regions, Genetic / Q000235:genetics; D020033:Protein Isoforms / Q000378:metabolism / Q000502:physiology; D012318:RNA Polymerase I / Q000378:metabolism; D012190:Retroviridae / Q000235:genetics; D014158:Transcription, Genetic / Q000235:genetics; D020298:U937 Cells
chemical_list: D022763:CCAAT-Enhancer-Binding Protein-alpha; D004275:DNA, Ribosomal; C000716967:NPM1 protein, human; D009687:Nuclear Proteins; D035463:Pol1 Transcription Initiation Complex Proteins; D020033:Protein Isoforms; C063844:transcription factor UBF; C063660:transcription initiation factor TIF-IB; D000090243:Nucleophosmin; D012318:RNA Polymerase I