Characterization of the liver-specific component of the cAMP response unit 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 components; the typical cAMP response element and a region of the promoter that contains three binding sites for CCAAT/enhancer-binding proteins. Previous work from our laboratory indicated that all three binding sites were required for the full response to cAMP. However, in the present study, we demonstrate that the activity of this latter component cannot be mimicked by multiple copies of other well characterized CCAAT/enhancer-binding protein binding sites. Re-examination of the premoter region containing the three C/EBP binding sites revealed the presence of an additional cis-element, which is required to mediate the activation by cAMP. This DNA sequence binds a protein in HepG2 nuclear extracts that is distinct from CCAAT/enhancer-binding protein and cAMP response element-binding protein, and evidence is presented which suggests that its identity is activator protein-1. Thus, the robust response of the phosphoenolpyruvate carboxykinase gene promoter to cAMP in liver requires the involvement of three different transcription factors. Utilization of multiple transcription factors to mediate the activation by cAMP likely allows for a tissue-specific response to this signal, a mechanism whereby to fine-tune the extent of the response, and a mechanism whereby to integrate signals from separate signaling pathways into a coordinated response.