Activation domains of CCAAT enhancer binding protein δ: Regions required for native activity and prostaglandin E2-dependent transactivation of insulin-like growth factor I gene expression in rat osteoblasts

Abstract

In osteoblasts, hormones such as prostaglandin E2 that activate protein kinase A increase the translocation of transcription factor CCAAT/enhancer binding protein δ (C/EBPδ) 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/EBPδ using native and heterologous gene promoter assay systems. We demonstrated functional interactions between C/EBPδ and trans-gene-expressed cAMP response element binding protein-binding protein, and showed that the ability of C/EBPδ 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/EBPδ reduced its basal transcriptional activity, whereas mutation at serine 191 suppressed the stimulatory effect of prostaglandin E2 on C/EBPδ 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/EBPδ. Our studies provide the first evidence for regions of C/ EBPδ that are important for basal and for hormone-induced transcriptional activity, and for its interactions with other enhancers and suppressers of gene expression.