Antiosteoclastic bone resorption activity of osteoprotegerin via enhanced AKT/mTOR/ULK1‐mediated autophagic pathway. Issue 3 (18th September 2019) - Zhao, Hongyan
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. Abstract : Osteoprotegerin (OPG) treatment activates phosphatidylinositol‐4, 5‐bisphosphate 3‐kinase (PI3K), leading to the downregulation of phosphorylated protein kinase B (AKT) and mammalian target of rapamycin (mTOR) and upregulation of phosphorylated autophagy activating kinase (ULK). This activates the autophagy flow, which plays a critical role in the osteoclastic bone resorption.