Mechanisms involved in suppression of osteoclast supportive activity by transforming growth factor-β1 via the ubiquitin-proteasome system

Orthodontic treatment requires the regulation of bone remodeling in both compression and tension sides. Transforming growth factor-beta 1 (TGF-beta 1) is an important coupling factor for bone remodeling. However, the mechanism underlying the TGF-beta 1-mediated regulation of the osteoclast-supporting activity of osteoblasts and stromal cells remain unclear. The current study investigated the effect of TGF-beta 1 on receptor activator of nuclear factor kappa-B ligand (RANKL) expression in stromal cells induced by 1 alpha,25(OH)(2)D-3 (D-3) and dexamethasone (Dex). TGF-beta 1 downregulated the expression of RANKL induced by D-3 and Dex in mouse bone marrow stromal lineage, ST2 cells. Co-culture system revealed that TGF-beta 1 suppressed osteoclast differentiation from bone marrow cell induced by D-3 and Dex-activated ST2 cells. The inhibitory effect of TGF-beta 1 on RANKL expression was recovered by inhibiting the interaction between TGF-beta 1 and the TGF-beta type I/activin receptor or by down-regulating of smad2/3 expression. Interestingly, TGF-beta 1 degraded the retinoid X receptor (RXR)-alpha protein which forms a complex with vitamin D receptor (VDR) and regulates transcriptional activity of RANKL without affecting nuclear translocation of VDR and phosphorylation of signal transducer and activator of transcription3 (STAT3). The degradation of RXR-alpha protein by TGF-beta 1 was recovered by a ubiquitin-proteasome inhibitor. We also observed that poly-ubiquitination of RXR-alpha protein was induced by TGF-beta 1 treatment. These results indicated that TGF-beta 1 downregulates RANKL expression and the osteoclast-supporting activity of osteoblasts/stromal cells induced by D-3 and Dex through the degradation of the RXR-alpha protein mediated by ubiquitin-proteasome system.