Sirtuin 6 in preosteoclasts suppresses age- and estrogen deficiency-related bone loss by stabilizing estrogen receptor α

Emerging evidence indicates that estrogen receptor (ER) α is an important modulator of bone homeostasis, which occurs partly by promoting osteoclast apoptosis. Sirtuin 6 (Sirt6) is an anti-aging molecule, and its deficiency in mice results in skeletal malformations associated with progeroid features. However, the effects of Sirt6 on ERα function in osteoclasts, and thus on aging- or estrogen deficiency-induced bone loss, have not been studied. Here, we show that myeloid-specific deletion of Sirt6 led to decreased ERα protein level and apoptotic cell death in preosteoclasts. Consequently, myeloid Sirt6 KO mice showed aggravated cancellous bone loss both with aging and following an ovariectomy compared to wild-type littermates. In contrast, Sirt6 transgenic mice were protected from ovariectomy-induced bone loss. Mechanistically, Sirt6 deacetylated ERα protein to prevent its proteasomal degradation, in which lysine 171 and lysine 299 were critical residues. Sirt6-mediated ERα stabilization promoted transcription of Fas ligand in preosteoclasts, resulting in apoptosis of osteoclasts. Finally, the level of Sirt6 in human preosteoclasts was correlated positively with bone density and ERα but negatively with age. In conclusion, our results suggest that deacetylation and upregulation of ERα by Sirt6 in preosteoclasts prevent bone loss by inhibiting osteoclast-mediated bone resorption. Activation of Sirt6 in preosteoclasts may provide a new therapeutic approach to attenuate osteoporosis in older or postmenopausal patients.