Characterization of Osf1, an osteoblast-specific transcription factor binding to a critical cis-acting element in the mouse Osteocalcin promoters.

To elucidate the mechanisms of osteoblast-specific gene expression we are studying the regulation of osteocalcin, the most osteoblast-specific gene. Previous studies of OG2, one of the two mouse osteocalcin genes, identified two osteoblast-specific cis-acting elements, OSE1 and OSE2, the latter being the binding site of Cbfa1, the only osteoblast-specific transcription factor known to date. Here we show that OSE1 is a cis-acting element as important as OSE2 for the osteoblast-specific expression of OG2 in cell culture and transgenic mice. We also show that OSE1 is present in the promoter of several osteoblast-specific genes including Cbfa1 itself. These biological features demonstrate the importance of OSE1 and led us to further characterize this site and the factor binding to it, provisionally termed Osf1. We first defined the core OSE1 sequence, 5'-TTACATCA-3', which is necessary and sufficient for Osf1 binding to DNA. This sequence has no strong homology to any known transcription factor-binding sites. As a first step in identifying Osf1, we performed an analytical purification of this protein using nuclear extracts from two different osteoblastic cell lines. We purified Osf1 to homogeneity through a five-step procedure including a renaturation experiment and found that its apparent molecular mass is 40 kDa. In conclusion, this study indicates the existence of multiple osteoblast-specific cis-acting elements of equal importance in controlling OG2 promoter activity and provides the first biochemical characterization of Osf1, a novel osteoblast-specific transcription factor.