1α,25 dihydroxyvitamin D3 rapidly regulates the mouse osteoprotegerin gene through dual pathways

1alpha,25(OH)(2)D-3 rapidly and transiently suppressed OPG gene expression both by accelerating the degradation of mRNA and by suppressing promoter activity. The latter process was mediated through the AP-1 binding site by a reduction in the proportion of phospho-c-Jun in a JNK-independent manner. Introduction: Osteoclastogenesis is regulated by an integrated network of numerous bone metabolic factors, among which 1alpha,25-dihydroxyvitamin D-3 [1alpha,25(OH)(2)D-3] promotes osteoclastogenesis by reciprocally upregulating the expression of RANKL and downregulating that of osteoprotegerin (OPG). Materials and Methods: To analyze the mechanism by which 1alpha,25(OH)(2)D-3 suppresses OPG, we characterized cis-acting elements of the Mouse OPG gene and assessed the post-transcriptional modifications by actinomycin D assays. Results: 1alpha,25(OH)(2)D-3 rapidly and transiently suppressed OPG expression and shortened the half-life of OPG rnRNA; additionally, the c-Jun homodinier bound to the AP-1 binding site (TGACTGA, -293/-287) and maintained steady-state transcription of the OPG gene. Furthermore, mutation of the AP-1 site negated 1alpha,25(OH)(2)D-3-driven OPG Suppression. Moreover, 1alpha,25(OH)(2)D-3 treatment of ST2 cells decreased the amount of phosphorylated C-Jun protein (phospho-c-Jun), while the total amount of c-Jun remained constant; however, the amount of phosphorylated Jun N-terminal kinase (JNK) was nearly unchanged by 1alpha,25(OH)(2)D-3 treatment. Conclusion: Taken together with the observation that the OPG promoter has no consensus negative vitamin D-responsive elements. these data suggest that 1alpha,25(OH)(2)D-3 transrepresses mouse OPG by reducing the proportion of phospho-c-Jun in a JNK-independent manner. Our data indicated that short-term treatment with 1alpha,25(OH)(2)D-3 effectively downregulated OPG expression both by accelerating the degradation of OPG mRNA and by transrepressing the OPG gene through its AP-1 binding site in the catabolic phase. The OPG gene became insensitive to 1alpha,25(OH)(2)D-3 treatment. however, and reverted to its steady-state expression level over time, leading to the anabolic phase of the effect of 1alpha,25(OH)(2)D-3 on bone.