Smads and AP-1 activation of TGF-β signaling upregulate transcription of Osteoprotegerin in Cementoblasts to inhibit osteoclastogenesis

Orthodontically induced root resorption (OIRR) is a common side effect during orthodontic tooth treatment (OTM). The function of osteoprotegerin (OPG) is considered to protect cementum from excessive resorption to maintain root integrity. In this study, we observed that the expression of TGF-β1 and OPG was upregulated under the loading of orthodontic force in periodontal tissues. However, the specific molecular mechanisms of TGF-β1-induced OPG expression in cementoblasts are not fully elucidated. This study aims to investigate the effect of Smads and AP-1 stimulated by TGF-β signaling on the transcription of OPG in cementoblasts. In vitro, we demonstrated that TGF-β/Smad and AP-1 signaling involved in TGF-β1-induced extracellular secretion of OPG in conditioned media (CM) from cementoblasts, which further inhibited osteoclastogenesis. Reporter gene plasmids containing OPG promoter sequences of different lengths (0.5–3 kb) were constructed to investigate the potential binding sites of Smads and AP-1. We identified nine binding sites of Smads and AP-1 concentrated in the 0–0.5 and 2.3–3 kb regions of OPG promoter in cementoblasts. ChlP results showed that Smad2/3/4 and c-Jun were bound more to the OPG promoter under TGF-β1 stimulation. In vivo, localized administration of TGF-β1 in the OTM model increased OPG expression, which resulted in the inhibition of OIRR. In summary, TGF-β1-induced Smads and AP-1 can bind to the OPG promoter to promote the transcription, expression, and secretion of OPG in cementoblasts, which inhibits osteoclast differentiation and protects cementum from excessive resorption.