Melatonin promotes the BMP9-induced osteogenic differentiation of mesenchymal stem cells by activating the AMPK/β-catenin signalling pathway.BACKGROUND
 Mesenchymal stem cells (MSCs) play a crucial role in maintaining the dynamic balance of bone metabolism.  Melatonin may have a regulatory effect on bone metabolism by regulating the lineage commitment and differentiation signalling pathways of MSCs.  Among the BMP families, the osteogenesis of BMP9 is considered to be one of the strongest in MSCs.  Here, we explored whether melatonin and BMP9 act synergistically on MSC osteogenic differentiation.


 The C3H10T1/2 osteogenic differentiation function induced by melatonin synergizes with BMP9, as detected by the expression of osteogenic markers at different periods.  The result was further confirmed by foetal limb explant culture and in vivo stem cell implantation experiments.  The effects of the AMPK/β-catenin pathway on the osteogenic differentiation of C3H10T1/2 cells were evaluated by Western blotting.


 Melatonin combined with BMP9 significantly enhanced the expression of osteogenic markers at different periods in C3H10T1/2 cells, effectively enhancing BMP9-induced bone formation in cultured foetal explants and ectopic bone formation in vivo in stem cell transplantation experiments.  Melatonin increases the expression of BMP9 in C3H10T1/2 cells and induces Smad1/5/8 translocation from the cytoplasm to the nucleus.  In addition, melatonin and BMP9 synergistically promote AMPK and β-catenin phosphorylation, which can be largely eliminated by AMPK siRNA pretreatment.


 CONCLUSIONS
 Melatonin and BMP9 in C3H10T1/2 cells synergistically promote osteogenic differentiation at least in part by activating the AMPK/β-catenin signalling pathway.
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