Ca²⁺/CaM/CaMK signaling is involved in cadmium-induced osteoclast differentiation

Environmental cadmium (Cd) pollution can ultimately lead to chronic toxicity via food consumption. Previous studies have demonstrated that long-term low-dose Cd exposure decreases bone mineral density and bone mineralization. Cd may increase receptor activator of nuclear factor-κ B ligand (RANKL) expression by osteoclasts, and inhibit the expression of osteoprotegerin. However, the molecular mechanism underlying Cd toxicity toward osteoclasts is unclear. In this study, bone marrow monocytes were isolated from C57BL/6 mice and treated with macrophage colony-stimulating factor and RANKL to induce the formation of osteoclasts. The results show that low-dose Cd exposure induced osteoclast differentiation. Cd also increased the intracellular calcium concentration of osteoclasts by triggering release of calcium ions from the endoplasmic reticulum into the cytoplasm. Furthermore, the elevation of intracellular calcium levels was shown to activate the calmodulin (CaM)/calmodulin-dependent protein kinase (CaMK) pathway. NFATc1 is a downstream protein of CaM/CaMK signaling, as well as a key player in osteoclast differentiation. Overall, we conclude that Cd activates the CaM/CaMK/NFATc1 pathway and regulates osteoclast differentiation by increasing intracellular calcium concentration. Our data provide new insights into the mechanisms underlying osteoclast differentiation following Cd exposure. This study provides a theoretical basis for future investigations into the therapeutic application of CaMK inhibitors in osteoporosis induced by Cd exposure.