Characterization of the bone sialoprotein (BSP) gene promoter

Bone sialoprotein is a 34 kDa phosphorylated and sulphated glycoprotein that is essentially unique to mineralizing connective tissues. Recent studies on the developmental expression of BSP mRNA and the temporo-spatial appearance of the protein during bone formation in vivo and in vitro have demonstrated that BSP is expressed by differentiated osteoblasts and that it may function in the initial nucleation of hydroxyapatite crystals in de novo bone formation. To study the cell-specific regulation of BSP we have isolated genomic clones that encompass the BSP promoter regions of both the human and rat genes. These promoters are characterized by a highly conserved region (BSP Box) that extends upstream from the transcription start site to nt -370. Within this region the immediate promoter is further characterized by a unique inverted TATA box and an inverted CCAAT box, both of which are required for basal transcriptional activity. The TATA box is overlapped by a vitamin D3 response element (VDRE) which appears to mediate vitamin D suppression of BSP gene transcription by competing with the TATA-binding protein (TBP) for occupancy of the site of the pre-initiation complex formation. Mutation of the inverted TATA box into a normal TATA sequence increases transcription slightly but does not affect the functionality of the VDRE indicating that the orientation of the TATA box is not critical for these functions. Further upstream an AP-1 site, overlapped by a steroid hormone response-like sequence, mediates down-regulation of BSP transcription induced by TPA that is abrogated by a complex interaction between Jun and the glucocorticoid receptor protein induced by dexamethasone. Thus, the characterization of approximately 3 kb of the BSP promoter and approximately 2 kb of the first intron has revealed several sites of transcriptional regulation that are important in regulating BSP expression and, consequently, bone formation.