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Antiosteoclastic bone resorption activity of osteoprotegerin via enhanced AKT/mTOR/ULK1-mediated autophagic pathway |
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| Authors: | Hongyan Zhao Ziqiang Sun Yonggang Ma Ruilong Song Yan Yuan Jianchun Bian Jianhong Gu Zongping Liu |
| Institution: | 1. College of Veterinary Medicine, Yangzhou University, Yangzhou, Jiangsu, China
Jiangsu Co-Innovation Center for Prevention and Control of Important Animal Infectious Diseases and Zoonoses, Yangzhou University, Yangzhou, Jiangsu, China Joint International Research Laboratory of Agriculture and Agri-Product Safety, The Ministry of Education of China, Yangzhou University, Yangzhou, Jiangsu, China Hongyan Zhao and Ziqiang Sun contributed equally to this study.;2. College of Veterinary Medicine, Yangzhou University, Yangzhou, Jiangsu, China Jiangsu Co-Innovation Center for Prevention and Control of Important Animal Infectious Diseases and Zoonoses, Yangzhou University, Yangzhou, Jiangsu, China Joint International Research Laboratory of Agriculture and Agri-Product Safety, The Ministry of Education of China, Yangzhou University, Yangzhou, Jiangsu, China;3. College of Veterinary Medicine, Yangzhou University, Yangzhou, Jiangsu, China |
| Abstract: | Autophagy plays a critical role in the maintenance of bone homeostasis. Osteoprotegerin (OPG) is an inhibitor of osteoclast-mediated bone resorption. However, whether autophagy is involved in the antiosteoclastogenic effects of OPG remains unclear. The present study aimed to investigate the potential mechanism of autophagy during OPG-induced bone resorption via inhibition of osteoclasts differentiated from bone marrow-derived macrophages in BALB/c mice. The results showed that after treatment with receptor activator of nuclear factor-κΒ ligand and macrophage colony-stimulating factor for 3 days, TRAP+ osteoclasts formed, representing the resting state of autophagy. These osteoclasts were treated with OPG and underwent autophagy, as demonstrated by LC3-II accumulation, acidic vesicular organelle formation, and the presence of autophagosomes. The levels of autophagy-related proteins, LC3-II increased and P62 decreased at 3 hr in OPG-treated osteoclasts. The viability, differentiation, and bone resorption activity of osteoclasts declined after OPG treatment. Treatment with OPG and chloroquine, an autophagy inhibitor, attenuated OPG-induced inhibition of osteoclastic bone resorption, whereas rapamycin (RAP), an autophagy inducer, enhanced OPG-induced inhibition of differentiation, survival, and bone resorption activity of osteoclasts. Furthermore, OPG reduced the amount of phosphorylated(p) protein kinase B (AKT) and pmTOR and increased the level of pULK, in a dose-dependant manner. LY294002, a phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K)/AKT pathway inhibitor, attenuated the decline in pAKT, but enhanced the decline in pmTOR and the increase in pULK1 following OPG treatment. RAP enhanced the OPG-induced increase in pULK1. The PI3K inhibitor 3-methyladenine partly blocked OPG-induced autophagy. Thus, the results revealed that OPG inhibits osteoclast bone resorption by inducing autophagy via the AKT/mTOR/ULK1 signaling pathway. |
| Keywords: | AKT/mTOR/ULK1 signaling pathway autophagy bone resorption osteoclast osteoprotegerin |