Runt domain factor (Runx)-dependent effects on CCAAT/enhancer-binding protein delta expression and activity in osteoblasts

Transcription factor CCAAT/enhancer-binding protein delta (C/EBP delta) is normally associated with acute-phase gene expression. However, it is expres sed constitutively in primary osteoblast cultures where it increases insuli n-like growth factor I synthesis in a cAMP-dependent way. Here we show that the 3' proximal region of the C/EBP delta gene promoter contains a binding sequence for Hunt domain factor Runx2, which is essential for osteogenesis . This region of the C/EBP delta promoter directed high reporter gene expre ssion in osteoblasts, and specifically bound Runx2 in osteoblast-derived nu clear extract. C/EBP delta gene promoter activity was reduced by mutating t he Runx binding sequence or by co-transfecting with Runx2 antisense express ion plasmid, and was enhanced by overexpression of Runx-2. Exposure to pros taglandin E-2 increased Runx-dependent gene transactivation independently o f Runx2 binding to DNA. Runx2 bound directly to the carboxyl-terminal regio n of C/EBP delta itself, and its ability to drive C/EBP delta expression wa s suppressed when C/EBP delta or its carboxyl-terminal fragment was increas ed by overexpression. Consistent effects also occurred on C/EBP delta-depen dent increases in gene expression driven by synthetic or insulin-like growt h factor I gene promoter fragments. These interactions between Runx2 and C/ EBP delta, and their activation by prostaglandin E-2, provide new evidence for their importance during skeletal remodeling, inflammatory bone disease, or fracture repair.