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1,25-dihydroxyvitamin D3 inhibits Osteocalcin expression in mouse through an indirect mechanism
1Department of Molecular Genetics, University of Texas M. D. Anderson Cancer Center, Houston 77030, USA.
Abstract:
1,25-Dihydroxyvitamin D3 (1,25-(OH)2D3), a key regulator of mineral metabolism, regulates the expression of several genes that are expressed in osteoblasts. In particular, in rat and human osteoblasts, 1,25-(OH)2D3 increases the expression of Osteocalcin by interacting, through 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)2D3 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 experiments, and DNA binding assays, we could not identify a functional vitamin D-responsive element in the promoter of OG2 or in the first 3.3 kilobases of the OG1 promoter. However, we show that 1,25-(OH)2D3 treatment of primary osteoblasts abolishes the binding of OSF2, an osteoblast-specific activator of transcription that binds to OSE2, a critical osteoblast-specific cis-acting element present in OG1 and OG2 promoters. Consistent with these DNA binding data, a mutation in OSE2 in the OG2 promoter abrogated the inhibitory effect of 1,25-(OH)2D3 treatment on this promoter activity. This study illustrates that 1,25-(OH)2D3 can play different roles in the expression of the same gene in various species and indicates that this regulation in mouse occurs through an indirect mechanism, 1,25-(OH)2D3 acting on a gene genetically located upstream of Osteocalcin.
Insights
In mice, 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) inhibits osteocalcin gene expression by preventing transcription factor binding, unlike in rats and humans. This occurs indirectly, impacting genes upstream of osteocalcin.
Area of Science:
- Molecular Biology
- Genetics
- Endocrinology
Background:
- 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) is crucial for mineral metabolism and osteoblast gene regulation.
- In rats and humans, 1,25-(OH)2D3 enhances osteocalcin expression via vitamin D-responsive elements.
- Osteocalcin is a key protein in bone mineralization and remodeling.
Purpose of the Study:
- To investigate the effect of 1,25-(OH)2D3 on osteocalcin gene expression in mice.
- To elucidate the regulatory mechanism of 1,25-(OH)2D3 on osteocalcin genes (OG1 and OG2) in mouse osteoblasts.
Main Methods:
- Primary osteoblast cultures and whole-animal studies in mice.
- DNA transfection assays and DNA binding experiments.
- Analysis of transcription factor binding to cis-acting elements (OSE2) in gene promoters.
Main Results:
- 1,25-(OH)2D3 inhibited both OG1 and OG2 expression in mouse osteoblasts and in vivo.
- No functional vitamin D-responsive element was identified in the promoters of OG1 or OG2.
- 1,25-(OH)2D3 treatment reduced the binding of OSF2 to the OSE2 element in OG1 and OG2 promoters.
- Mutation of the OSE2 element abolished the inhibitory effect of 1,25-(OH)2D3.
Conclusions:
- 1,25-(OH)2D3 exhibits species-specific regulation of osteocalcin gene expression.
- In mice, 1,25-(OH)2D3 inhibits osteocalcin expression through an indirect mechanism involving OSF2 binding to OSE2.
- This regulation highlights the complexity of vitamin D signaling in bone metabolism.