CHARACTERIZATION OF THE LIVER-SPECIFIC COMPONENT OF THE CAMP RESPONSEUNIT IN THE PHOSPHOENOLPYRUVATE CARBOXYKINASE (GTP) GENE PROMOTER

The cAMP response unit of the phosphoenolpyruvate carboxykinase (GTP) (EC 4.1.1.32) gene promoter consists of two independently weak compone nts; the typical cAMP response element and a region of the promoter th at contains three binding sites for CCAAT/enhancer-binding proteins. P revious work from our laboratory indicated that all three binding site s were required for the full response to cAMP. However, in the present study, we demonstrate that the activity of this latter component cann ot be mimicked by multiple copies of other well characterized CCAAT/en hancer-binding protein binding sites. Re examination of the promoter r egion containing the three C/EBP binding sites revealed the presence o f an additional cis-element, which is required to mediate the activati on by cAMP. This DNA sequence binds a protein in HepG2 nuclear extract s that is distinct from CCAAT/enhancer-binding protein and cAMP respon se element-binding protein, and evidence is presented which suggests t hat its identity is activator protein-1. Thus, the robust response of the phosphoenolpyruvate carboxykinase gene promoter to cAMP in liver r equires the involvement of three different transcription factors. Util ization of multiple transcription factors to mediate the activation by cAMP likely allows for a tissue-specific response to this signal, a m echanism whereby to fine-tune the extent of the response, and a mechan ism whereby to integrate signals from separate signaling pathways into a coordinated response.