1α,25-(OH) 2 -D 3 promotes the autophagy during osteoclastogenesis by enhancing RANKL-RANK-TRAF6 signaling

As the active form of vitamin D3, 1α,25-(OH) -D promotes receptor activator for nuclear factor-κB ligand (RANKL)-induced autophagy in osteoclast precursors (OCPs). However, the relationship between 1α,25-(OH) -D and RANKL signaling is still unknown. This study aimed to explore whether 1α,25-(OH) -D regulates OCP autophagy and osteoclastogenesis through RANKL signaling. Our results showed that 1α,25-(OH) -D directly decreased OCP autophagy while significantly enhancing the ability of RANKL to promote OCP autophagy. Moreover, 1α,25-(OH) -D not only promoted the expression of key signaling proteins in OCPs induced by RANKL but also enhanced the coimmunoprecipitation levels of RANK and TRAF6. Notably, 1α,25-(OH) -D significantly enhanced the autophagic activity and osteoclast differentiation of RANK-positive OCPs but did not affect the autophagic activity or osteoclast differentiation of RANK-negative OCPs. More importantly, 1α,25-(OH) -D had no effect on autophagy or osteoclastogenesis in TRAF6-silenced OCPs. Overall, 1α,25-(OH) -D could upregulate RANKL-RANK-TRAF6 signaling in OCPs, thereby promoting OCP autophagy and osteoclastogenesis.