ABSTRACT
Monocytic differentiation is orchestrated by complex networks that are not fully understood. This study further elucidates the involvement of transcription factor CCAAT/enhancer-binding protein ß (C/EBPß). Initially, we demonstrated a marked increase in nuclear C/EBPß-liver-enriched activating protein* (LAP*)/liver-enriched activating protein (LAP) levels and LAP/liver-enriched inhibiting protein (LIP) ratios in phorbol 12-myristate 13-acetate (PMA)-treated differentiating THP-1 premonocytic cells accompanied by reduced proliferation. To directly study C/EBPß effects on monocytic cells, we generated novel THP-1-derived (low endogenous C/EBPß) cell lines stably overexpressing C/EBPß isoforms. Most importantly, cells predominantly overexpressing LAP* (C/EBPß-long), but not those overexpressing LIP (C/EBPß-short), exhibited a reduced proliferation, with no effect on morphology. PMA-induced inhibition of proliferation was attenuated in C/EBPß-short cells. In C/EBPß(WT) macrophage-like cells (high endogenous C/EBPß), we measured a reduced proliferation/cycling index compared with C/EBPß(KO). The typical macrophage morphology was only observed in C/EBPß(WT), whereas C/EBPß(KO) stayed round. C/EBPα did not compensate for C/EBPß effects on proliferation/morphology. Serum reduction, an independent approach known to inhibit proliferation, induced macrophage morphology in C/EBPß(KO) macrophage-like cells but not THP-1. In PMA-treated THP-1 and C/EBPß-long cells, a reduced phosphorylation of cell cycle repressor retinoblastoma was found. In addition, C/EBPß-long cells showed reduced c-Myc expression accompanied by increased CDK inhibitor p27 and reduced cyclin D1 levels. Finally, C/EBPß-long and C/EBPß(WT) cells exhibited low E2F1 and cyclin E levels, and C/EBPß overexpression was found to inhibit cyclin E1 promoter-dependent transcription. Our results suggest that C/EBPß reduces monocytic proliferation by affecting the retinoblastoma/E2F/cyclin E pathway and that it may contribute to, but is not directly required for, macrophage morphology. Inhibition of proliferation by C/EBPß may be important for coordinated monocytic differentiation.
