THE CAMP RESPONSE ELEMENT-BINDING PROTEIN SYNERGIZES WITH OTHER TRANSCRIPTION FACTORS TO MEDIATE CAMP RESPONSIVENESS

The cAMP responsiveness of the promoter for phosphoenolpyruvate carbox ykinase (EC 4.1.1.32) is mediated by a synergistic interaction between a complex regulatory region, which binds liver-enriched transcription factors, and a typical cAMP response element (CRE). Although a role f or the CRE-binding protein (CREB) in the cAMP-responsiveness of this p romoter has been generally assumed, some uncertainty remains due to th e observations that several C/EBP-related proteins bind with near equa l affinity, relative to CREB, to this particular CRE. Thus, a detailed analysis of the involvement of CREB in this synergism was undertaken in HepG2 cells. Gel mobility shift assays demonstrate that a CRE probe is bound by CREB present in HepG2 cells. Furthermore, we show that a dominant repressor of CREB is able to significantly reduce the cAMP re sponsiveness of the PEPCK promoter in HepG2 cells. Finally, we demonst rate using a GAL4-CREB fusion protein that CREB is able to synergize w ith the liver-enriched factors bound upstream on the PEPCK promoter to mediate a liver-specific response to cAMP. Examination of several mut ant forms of CREB allow us to conclude that the ''synergy'' domain of CREB resides within amino acid residues 83-203, and that residues 83-1 45 can mediate a partial synergistic response. This study establishes that CREB is able to synergize with liver-enriched transcription facto rs to mediate a tissue-specific response to cAMP.