The CCAAT displacement protein/cut homeodomain protein represses osteocalcin gene transcription and forms complexes with the retinoblastoma protein-related protein p107 and cyclin A(1)

Developmental control of bone tissue-specific genes requires positive and n egative regulatory factors to accommodate physiological requirements for th e expression or suppression of the encoded proteins, Osteocalcin (OC) gene transcription is restricted to the late stages of osteoblast differentiatio n, OC gene expression is suppressed in nonosseous cells and osteoprogenitor cells and during the early proliferative stages of bone cell differentiati on, The rat OC promoter contains a homeodomain recognition motif within a h ighly conserved multipartite promoter element (OC box I) that contributes t o tissue-specific transcription, In this study, we demonstrate that the CCA AT displacement protein (CDP), a transcription factor related to the cat ho meodomain protein in Drosophila melanogaster, may regulate bone-specific ge ne transcription in immature proliferating osteoblasts, Using gel shift com petition assays and DNase I footprinting, vpe show that CDP/cut recognizes two promoter elements (TATA and OC box I) of the bone-related rat OC gene, Overexpression of CDP/cut in ROS 17/2.8 osteosarcoma cells results in repre ssion of OC promoter activity; this repression is abrogated by mutating OC box I. Gel shift immunoassays show that CDP/cut forms a proliferation-speci fic protein/DNA complex in conjunction with cyclin A and p107, a member of the retinoblastoma protein family of tumor suppressors, Our findings sugges t that CDP/cut may represent an important component of a cell signaling mec hanism that provides cross-talk between developmental and cell cycle-relate d transcriptional regulators to suppress bone tissue-specific genes during proliferative stages of osteoblast differentiation.