Melatonin up‐regulates bone marrow mesenchymal stem cells osteogenic action but suppresses their mediated osteoclastogenesis via MT2‐inactivated NF‐κB pathway

Background and Purpose Melatonin is a neurohormone involved in bone homeostasis. Melatonin directs bone remodelling and the role of bone marrow mesenchymal stem cells (BMMSCs) in the regulating melatonin‐mediated bone formation–resorption balance remains undefined. Experimental Approach Osteoporosis models were established and bone tissue and serum were collected to test the effects of melatonin on bone homeostasis. Melatonin receptors were knocked down, the NF‐κB signalling pathway and receptor activator of NF‐κB ligand (RANKL) expression were investigated. Communication between bone marrow mesenchymal stem cells and osteoclasts was detected with direct‐contact or indirect‐contact system. Key Results Bone loss and microstructure disorder in mice were reversed after melatonin treatment, as a result of anabolic and anti‐resorptive effects. In vitro, a physiological (low) concentration of melatonin promoted the bone marrow mesenchymal stem cells, osteogenic lineage commitment and extracellular mineralization but had no impact on extracellular matrix synthesis. After MT knockdown, especially MT2, the positive effects of melatonin on osteogenesis were attenuated. The canonical NF‐κB signalling pathway was the first discovered downstream signalling pathway after MT receptor activation and was found to be down‐regulated by melatonin during osteogenesis. Melatonin suppressed BMMSC‐mediated osteoclastogenesis by inhibiting RANKL production in BMMSCs and this effect only occurred when BMMSCs and osteoclast precursors were co‐cultured in an indirect‐contact manner. Conclusion and Implications Our work suggests that melatonin plays a crucial role in bone balance, significantly accelerates the osteogenic differentiation of bone marrow mesenchymal stem cells by suppressing the MT2‐dependent NF‐κB signalling pathway, and down‐regulates osteoclastogenesis via RANKL paracrine secretion.