Hyperosmotic Stimulus Down-regulates 1&#1345;, 25-dihydroxyvitamin D<SUB>3</SUB>- induced Osteoclastogenesis by Suppressing the RANKL Expression in a Co-culture System

Hyperosmotic Stimulus Down-regulates 1&#1345;, 25-dihydroxyvitamin D<SUB>3</SUB>- induced Osteoclastogenesis by Suppressing the RANKL Expression in a Co-culture System

The hyperosmotic stimulus is regarded as a mechanical factor for bone remodeling. However, whether the hyperosmotic stimulus affects 1&#1345;, 25-dihydroxyvitamin D₃ (1&#1345;,25(OH)₂D₃)-induced osteoclastogenesis is not clear. In the present study, the effect of the hyperosmotic stimulus on 1&#1345;,25(OH)₂D₃- induced osteoclastogenesis was investigated in an osteoblast-preosteoclast co-culture system. Serial doses of sucrose were applied as a mechanical force. These hyperosmotic stimuli significantly evoked a reduced number of 1&#1345;,25(OH)₂D₃-induced tartrate-resistant acid phosphatase-positive multinucleated cells and 1&#1345;,25(OH)₂D₃-induced bone-resorbing pit area in a co-culture system. In osteoblastic cells, receptor activator of nuclear factor &#1354;B ligand (RANKL) and Runx2 expressions were down-regulated in response to 1&#1345;,25(OH)₂D₃. Knockdown of Runx2 inhibited 1&#1345;,25(OH)₂D₃-induced RANKL expression in osteoblastic cells. Finally, the hyperosmotic stimulus induced the overexpression of TonEBP in osteoblastic cells. These results suggest that hyperosmolarity leads to the down-regulation of 1&#1345;,25(OH)₂D₃- induced osteoclastogenesis, suppressing Runx2 and RANKL expression due to the TonEBP overexpression in osteoblastic cells.