Activation Domains of CCAAT Enhancer Binding Protein : Regions Required for Native Activity and Prostaglandin E2-Dependent Transactivation of Insulin-Like Growth Factor I Gene Expression in Rat Osteoblasts

In osteoblasts, hormones such as prostaglandin E2 that activate protein kinase A increase the translocation of transcription factor CCAAT/enhancer binding protein (C/EBP ) from the cytoplasm to the nucleus where it rapidly induces IGF-I gene expression. In this study, we identified activation and suppression domains in C/EBP using native and heterologous gene promoter assay systems. We demonstrated functional interactions between C/EBP and trans-gene-expressed cAMP response element binding protein-binding protein, and showed that the ability of C/EBP to promote gene expression was suppressed by viral protein E1A, which blocks the activity of native cAMP response element binding protein-binding protein. Site-directed mutations at serines 62 or 191 within C/EBP reduced its basal transcriptional activity, whereas mutation at serine 191 suppressed the stimulatory effect of prostaglandin E2 on C/EBP function as well as its DNA binding potential. These results are consistent with the location of serine 191 in the DNA binding domain of C/EBP . Our studies provide the first evidence for regions of C/EBP that are important for basal and for hormone-induced transcriptional activity, and for its interactions with other enhancers and suppressers of gene expression. (Molecular Endocrinology 17: 1834–1843, 2003)