Activation of NLRP3 signaling contributes to cadmium-induced bone defects, associated with autophagic flux obstruction

Cadmium (Cd) is a widespread environmental and industrial pollutant to cause various bone metabolic diseases. Our former study reported that Cd promoted adipogenesis and inhibited osteogenic differentiation of primary bone marrow-derived mesenchymal stem cells (BMSCs) by NF-κB inflammation signaling and oxidative stress, and Cd-induced osteoporosis of long bone and compromised repair of cranial bone defect in vivo. However, the underlying mechanisms of Cd-induced bone damage remain elusive. In this study, we used Sprague Dawley (SD) rat and NLRP3-knockout mouse models to elucidate the exact effects and molecular mechanisms of Cd-induced bone damage and aging. Herein we found that the exposure of Cd preferentially targeted a few specific tissues such as bone and kidney. Cd triggered NLRP3 inflammasome pathways and the accumulation of autophagosomes of primary BMSCs, and also Cd stimulated the differentiation and bone resorption function of primary osteoclasts. Moreover, Cd not only activated ROS/NLRP3/caspase-1/p20/IL-1β pathways, but also influenced Keap1/Nrf2/ARE signaling. The data revealed that autophagy dysfunction and NLRP3 pathways synergistically mediated the impairments of Cd in bone tissues. Loss of NLRP3 function partially alleviated Cd-induced osteoporosis and craniofacial bone defect in the NLRP3-knockout mouse model. Furthermore, we characterized the protective effects and potential therapeutic targets of the combined treatment of anti-aging agents (rapamycin+melatonin+NLRP3 selective inhibitor MCC950) on Cd-induced bone damage and inflammatory aging. These results illuminate that ROS/NLRP3 pathways and autophagic flux obstruction are involved in the Cd-induced toxic actions of bone tissues. Collectively, our study unveils some therapeutic targets and the regulatory mechanism to prevent Cd-caused bone rarefaction. The findings improve the mechanistic understanding of environmental Cd exposure-caused bone metabolism disorders and tissue damage.