Modulation of CCAAT/enhancer-binding protein-alpha gene expression by metabolic signals in rodent adipocytes

The transcription factor CCAAT/enhancer-binding protein-alpha (C/EBP alpha) is a positive modulator of transcription for several adipocyte-specific ge nes that play a role in energy metabolism. However, there is little informa tion available regarding the regulation of its expression by metabolic sign als. Exposure to insulin for 5-24 h attenuated C/EBP alpha expression when 3T3-L1 adipocytes were incubated in 24 mM glucose, but not in 5.7 mar gluco se. Nuclear run-on transcription assays indicated a transcriptional repress ion of C/EBP alpha: gene, but not that of C/EBP beta. Glucosamine, a produc t of the hexosamine pathway, in the presence of low glucose mimicked high g lucose's ability to reduce C/EBP alpha messenger RNA expression in insulin- treated cells. Similar results were obtained with xylitol, an activator of the pentose phosphate pathway. There was no correlation between the accumul ation of hexosamine pathway metabolites (e.g. UDP-N-acetylhexosamines) and/ or changes in intracellular protein glycosylation with the ability of high glucose, glucosamine, or xylitol to down-regulate C/EBP alpha gene expressi on. None of these treatments caused a reduction in intracellular ATP levels . Stable transfection of 3T3-L1 cells with the 5'-flanking 468-bp sequence of the mouse C/EBP alpha gene fused to luciferase demonstrated that promote r activity was also reduced by these nutrients. Of interest, treatment of r ats with glucose or glucosamine led to a reduction in C/EBP alpha messenger RNA levels in epididymal, but not omental, fat. Taken together, these resu lts suggest that metabolic signals serve to down-regulate C/EBP alpha expre ssion both in vitro and in vivo.