A weak TATA box is a prerequisite for glucocorticoid-dependent repression of the osteocalcin gene

The TATA box element is not only important for establishing basal levels of transcription, but it can also be used to modulate cell type or stage specific gene activity. In the case of the human osteocalcin gene, which is transcriptionally repressed by glucocorticoids, a specific binding element for the glucocorticoid receptor (GR) overlaps a noncanonical TATA box. In the present study, the relevance and function of the TATA element in glucocorticoid-mediated repression of the human osteocalcin gene was characterized. Mutating this noncanonical TATA box into a consensus TATA box within the context of the osteocalcin promoter greatly decreased hormone-dependent transcriptional repression by GR. TATA-binding protein (TBP) bound this mutated element much more strongly suggesting a physiologically relevant role for the weak osteocalcin TATA element in the regulation of this bone specific gene. The optimization of the putative transcription factor IIB recognition site did not affect the level of GR-mediated repression. Our results support a model wherein competitive DNA binding of GR and TBP for their overlapping sites explains conditional repression of the osteocalcin gene by glucocorticoids.