CAAT/Enhancer Binding Protein Activates an Enhancer in the Glutamine Synthetase Distal 5′-Flanking Sequence

The glutamine synthetase (GS) gene is expressed at high levels in several cell types, including astrocytes, pericentral hepatocytes, and adipocytes. During hormone-mediated adipocyte differentiation of 3T3-L1 cells, GS gene expression increases several hundred fold. We previously reported that elements in the distal 5′-flanking sequence and intron-1 participate in establishing the temporal pattern of GS transcription during adipocyte differentiation. To examine the role of the distal 5′-flanking region in regulating adipocyte-specific GS expression, GS–CAT fusion genes were constructed and analyzed in transiently transfected 3T3-L1 cells. In this way, adipocyte differentiation-responsive enhancer activity was localized to a 422-bp sequence that occurs about 3.5 kb upstream from the transcription start site. This sequence includes several putative C/EBP binding sites and is activated by ectopic expression of C/EBPα in NIH-3T3 cells. Thus, our data indicate that C/EBPα has the capacity to activate functional C/EBP sites in the GS gene distal 5′-flanking region.