Nucleolar retention of a translational C/EBPα isoform stimulates rDNA transcription and cell size

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/EBP alpha) 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/EBP alpha 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/EBP alpha is lost concomitantly with nucleolar C/EBP alpha immunostaining probably reflecting the reduced requirement for ribosome biogenesis in differentiated cells. Finally, overexpression of extended-C/EBP alpha induces an increase in cell size. Altogether, our results suggest that control of rRNA synthesis is a novel function of C/EBP alpha adding to its role as key regulator of cell growth and proliferation. The EMBO Journal (2010) 29, 897-909. doi: 10.1038/emboj.2009.404; Published online 14 January 2010