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Cadmium (Cd) is a toxic heavy metal that represents an occupational hazard and environmental pollutant toxic heavy metal, which can cause osteoporosis following accumulation in the body. The purpose of this study was to investigate the effect of Cd on bone tissue osteoclast differentiation in vivo. Female BALB/c mice were randomly divided into three groups and given drinking water with various concentrations of Cd (0, 5, and 25 mg/L) for 16 weeks, after which the mice were sacrificed after collecting urine and blood. The level of Cd, calcium (Ca), phosphorus (P), trace elements, and some biochemical indicators were measured, and the bone was fixed in a 4% formaldehyde solution for histological observation. Bone marrow cells were isolated to determine the expression of osteoclast‐associated mRNA and proteins. Cd was increased in the blood, urine, and bone in response to Cd in drinking water in a dose‐dependent manner. The content of iron (Fe), manganese (Mn), and zinc (Zn) was significantly increased, whereas Ca and P were decreased in bone compared to the control group. Cd affected the histological structure of the bone, and induced the upregulation and downregulation of tartrate‐resistant acid phosphatase 5b (TRACP‐5b) and estradiol in the serum, respectively. Cd had no significant effect on the alkaline phosphatase activity in the serum. The expression of osteoclast marker proteins, including TRACP, cathepsin K, matrix metalloprotein 9, and carbonic anhydrases were all increased in the Cd‐treated bone marrow cells. Cd significantly increased the expression of receptor activator of nuclear factor kappa B ligand (RANKL), but had lower effect on the expression of osteoprotegerin (OPG) in both bone marrow cells and bone tissue. Thus, Cd exposure destroyed the bone microstructure, promoted the formation of osteoclasts in the bone tissue, and accelerated bone resorption, in which the OPG/RANKL pathway may play an important role. [ABSTRACT FROM AUTHOR]

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 FROM AUTHOR]