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

Abstract

Transcription factor CCAAT/enhancer-binding protein δ (C/EBPδ) is normally associated with acute-phase gene expression. However, it is expressed constitutively in primary osteoblast cultures where it increases insulin-like growth factor I synthesis in a cAMP-dependent way. Here we show that the 3' proximal region of the C/EBPδ gene promoter contains a binding sequence for Runt domain factor Runx2, which is essential for osteogenesis. This region of the C/EBPδ promoter directed high reporter gene expression in osteoblasts, and specifically bound Runx2 in osteoblast-derived nuclear extract. C/EBPδ gene promoter activity was reduced by mutating the Runx binding sequence or by co-transfecting with Runx2 antisense expression plasmid, and was enhanced by overexpression of Runx-2. Exposure to prostaglandin E2 increased Runx-dependent gene trans-activation independently of Runx2 binding to DNA. Runx2 bound directly to the carboxyl- terminal region of C/EBPδ itself, and its ability to drive C/EBPδ expression was suppressed when C/EBPδ or its carboxyl-terminal fragment was increased by overexpression. Consistent effects also occurred on C/EBPδ- dependent increases in gene expression driven by synthetic or insulin-like growth factor I gene promoter fragments. These interactions between Runx2 and C/EBPδ, and their activation by prostaglandin E2, provide new evidence for their importance during skeletal remodeling, inflammatory bone disease, or fracture repair.