1,25-DIHYDROXYVITAMIN D-3 INHIBITS OSTEOCALCIN EXPRESSION IN MOUSE THROUGH AN INDIRECT MECHANISM

1,25-Dihydroxyvitamin D-3 (1,25-(OH)(2)D-3), a key regulator of minera l metabolism, regulates the expression of several genes that are expre ssed in osteoblasts. In particular, in rat and human osteoblasts, 1,25 -(OH)(2)D-3 increases the expression of Osteocalcin by interacting, th rough a hormone-receptor complex, with a vitamin D-responsive element present in the promoter of the genes. Here we show that in mouse, 1,25 -(OH)(2)D-3 inhibits the expression of both osteocalcin genes, OG1 and OG2. This inhibition was observed in primary osteoblast cultures and in the whole animal. From sequence inspection, DNA transfection experi ments, and DNA binding assays, we could not identify a functional vita min D-responsive element in the promoter of OG2 or in the first 3.3 ki lobases of the OG1 promoter. However, we show that 1,25-(OH)(2)D-3 tre atment of primary osteoblasts abolishes the binding of OSF2, an osteob last-specific activator of transcription that binds to OSE2, a critica l osteoblast specific cis-acting element present in OG1 and OG2 promot ers. Consistent with these DNA binding data, a mutation in OSE2 in the OG2 promoter abrogated the inhibitory effect of 1,25-(OH)(2)D-3 treat ment on this promoter activity. This study illustrates that 1,25-(OH)( 2)D-3 can play different roles in the expression of the same gene in v arious species and indicates that this regulation in mouse occurs thro ugh an indirect mechanism, 1,25-(OH)(2)D-3 acting on a gene geneticall y located upstream of Osteocalcin.