Transcriptional Repression of the Rat Osteocalcin Gene: Role of Two Intronic CCTCCT Motifs1

Abstract

The regulation of osteocalcin gene transcription is complex, involving multiple positive and negative regulators. Previous studies have demonstrated that an intronic sequence, TTTCTTT (+118 to +124) is capable of mediating transcriptional repression of osteocalcin-CAT fusion genes in cells of the osteoblast lineage, by interacting with a specific nuclear protein. Further analyses of intronic sequences have identified a second silencer motif in this region. Two copies of a CCTCCT motif are present within the first intron of the rat osteocalcin gene (+106 to +111 and +135 to +140) and are capable of mediating transcriptional repression of osteocalcin-CAT fusion genes in rat osteosarcoma cells. Transient gene expression assays of wild-type and mutant osteocalcin-CAT fusion genes into ROS 17/2.8 cells demonstrate that mutagenesis of either of these CCTCCT motifs in isolation results in a 1.6-fold increase in CAT activity relative to the parent fusion gene. Moreover, a 5-fold increase in reporter gene activity is observed when both motifs are mutated together. These sequences are also capable of suppressing osteocalcin promoter activity when placed upstream to the osteocalcin promoter. Gel retardation and southwestern analyses demonstrate that the CCTCCT motifs interact with specific proteins present in nuclear extracts from ROS 17/2.8 and UMR 106 osteosarcoma cells but not COS-7 kidney cells. Mutations that abolish suppressor function of this motif markedly impair interactions with this specific nuclear protein. These data demonstrate that at least two different silencer motifs (TTTCTTT and CCTCCT) in the first intron of the rat osteocalcin gene contribute to its transcriptional repression.