Role of basic region leucine zipper transcription factors cyclic AMP response element binding protein (CREB), CREB2, activating transcription factor 2 and CAAT/enhancer binding protein α in cyclic AMP response element‐mediated transcription

The transcription factor cAMP response element binding protein (CREB), a member of the basic region leucine zipper (bZIP) family of proteins, is the major cAMP response element (CRE) binding. Other bZIP proteins, including CREB2, activating transcription factor 2 (ATF2), or CAAT/enhancer binding protein (C/EBP) have been reported to transactivate CRE‐containing genes or to interfere with transactivation by CREB. We have designed a simple transactivation assay using expression of either a constitutively active CREB mutant or a nuclear targeted mutant of the catalytic subunit of cAMP‐dependent protein kinase. In both cases, a striking stimulation of transcription of CRE‐containing reporter genes was observed in noradrenergic locus coeruleus‐like CATH.a cells. In addition, a constitutively active mutant of ATF2 specifically transactivated a secretogranin II promoter/luciferase reporter gene, but had no effect on the tyrosine hydroxylase promoter. In contrast, CREB2 and C/EBPα did not transactivate CRE‐containing reporter genes, indicating that these bZIP proteins target distinct genetic elements. Experiments involving dominant‐negative bZIP mutants revealed that CREB does not heterodimerize with CREB2, ATF2, c‐Jun or C/EBP. Rather, CREB and ATF2 compete for binding to the CRE, and are independently able to up‐regulate transcription of genes containing CRE motifs in their regulatory regions.