A study of cross-talk between the expression mechanisms of osteoclast differentiation factor (ODF) and osteoclastogenesis inhibitory factor (OCIF)

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Bone is a dynamic tissue that is formed and remodeled by continuously occurring bone formation and resorption. Osteoblasts are involved not only in bone formation, but also in bone resorption via inducing osteoclast differentiation factor (ODF)/receptor activator of NF-kB ligand (RANKL) and its antagonist, osteoclastogenesis inhibitory factor OCIF)/osteoprotegerin (OPG) in osteoblasts. In the present study, we examined a potential regulatory mechanism of RANKL and OPG expression by bone-resorbing hormones in osteoblasts. Both 1,25(OH)_2D_3 and parathyroid hormone (PTH) induced RANKL mRNA expression and inhibited OPG mRNA expression. These effects of PTH reached a peak within 3 h, then sharply decreased thereafter, whereas those of 1,25(OH)_2D_3 gradually increased up to 10 h and were kept until 24 h. The effects of 1,25(OH)_2D_3 were blocked by adding cychloheximide but not MAP kinase inhibitors, suggesting that dc novo protein synthesis is essential for the l,25(OH)_2D_3 effects. Target genes of 1,25(OH)_2D_3 were analyzed by microarray analysis. Vitamin D receptor (VDR), interleukin 4 receptor a (IL4Ra) and a RNA helicase (Ddx21), were up-regulated more than 2 fold, and serum/glucocorticoid- regulated kinase (Sgk) was down-regulated more than 0.5 fold in osteoblastic cells by 1,25(OH)_2D_3. Transient over-expression of VDR, IL4Ra or Ddx21 in osteoblasts decreased the levels of RANKL mRNA induced by 1,25(OH)_2D_3. VDR increased the basal level of RANKL mRNA. However, none of them affected OPG mRNA levels irrespective of the presence and absence of 1,25(OH)_2D_3. Taken together, these results suggest that VDR, IL4Ra and Ddx21 are involved in the RANKL expression induced by 1,25(OH)_2D_3. Different transcription factors may regulate mRNA expression of RANKL and OPG in response to 1,25(OH)_2D_3.