RNA interference-mediated knockdown of Smad1 inhibits receptor activator of nuclear factor κB ligand expression induced by BMP-2 in primary osteoblasts

•We investigated the role of Smad1 in BMP-2-mediated osteoclast differentiation.•1,25(OH)2D3 and BMP-2 treatment enhanced osteoclast formation and RANKL expression.•Smad1 knockdown reduced RANKL mRNA and VDR protein expression.•Costimulation with 1,25(OH)2D3 and BMP-2 enhanced nuclear localization of VDR.•Induction of BMP-2/Smad1 signaling ultimately enhanced osteoclast differentiation.

ObjectiveBMP-2 induces osteoblast differentiation and activates osteoclast formation. Here, we investigated the role of Smad1, a molecule that signals downstream of BMP-2, in mediating the effects of BMP-2 on osteoclast differentiation induced by 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) in osteoblasts.DesignThe effects of 1,25(OH)2D3 and BMP-2 in osteoclasts were examined using polymerase chain reaction and Western blotting to measure changes in target gene and protein expression. Immunostaining was carried out to investigate the localization of the vitamin D receptor (VDR) in the nucleus in response to BMP-2.ResultsStimulation with both 1,25(OH)2D3 and BMP-2 resulted in significantly greater osteoclast formation and receptor activator of nuclear factor κB ligand (RANKL) mRNA expression compared to stimulation with 1,25(OH)2D3 alone. In addition, expression of the VDR protein was increased, enhancing the activity of 1,25(OH)2D3. Interestingly, knockdown of Smad1 resulted in reduced osteoclast formation, RANKL mRNA expression, and VDR protein expression compared with control cells. Costimulation with 1,25(OH)2D3 and BMP-2 enhanced VDR localization in the nucleus.ConclusionsWe found that BMP-2 induced Smad1 activation, thereby influencing the localization of VDR in the nucleus in the presence of 1,25(OH)2D3 and resulting in increased RANKL mRNA expression. These effects ultimately resulted in enhanced osteoclast differentiation.