Identification of an Osteoblastic Silencer Element in the First Intron of the Rat Osteocalcin Gene

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Abstract
The osteocalcin gene has been used as a model for studying the regulation of gene expression by 1,25-dihydroxyvitamin D3, as well as for examining factors which contribute to osteoblast-specific regulation of gene expression. Most of these studies have focused on transactivation. We report the identification of a sequence in the first intron of the rat osteocalcin gene which suppresses the expression of osteocalcin−CAT fusion genes approximately 10-fold in ROS 17/2.8 and UMR 106 osteosarcoma cells. Mutation of a TTTCTTT motif in the first intron abolishes this suppression. The silencing effect of this motif is also observed after bone morphogenic protein-2 (BMP-2)-induced expression of the osteoblastic phenotype in the MLB13MYC clone 17 cell line. Mutation of the splice donor site does not affect suppression by these sequences in ROS 17/2.8 cells. When multimerized and placed upstream of the native osteocalcin promoter, these sequences retain their ability to mediate transcriptional repression. Electrophoresis mobility shift analysis demonstrates a specific protein−DNA interaction with the TTTCTTT motif in nuclear extracts from ROS 17/2.8, UMR 106, and MLB13MYC clone 17 cells but not those from COS-7 kidney cells. The mutation of this motif, which abolishes suppressing activity in the native context, also abolishes binding. The presence and activity of this suppressor in cells of the osteoblast lineage suggest that it is expressed with other cell-specific transcriptional regulators of the osteocalcin gene, coordinately regulating expression of this gene in bone cells.

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