Background Ionizing radiation poses a challenge to the healing of bone defects. Radiation therapy and accidental exposure to gamma-ray (γ-ray) radiation inhibit bone formation and increase the risk of fractures. Cortical bone-derived stem cells (CBSCs) are essential for osteogenic lineages, bone maintenance, and repair. This study aimed to investigate the effects of melatonin on postradiation CBSCs and bone defects. Methods CBSCs were extracted from C57/BL6 mice and were identified by flow

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... etry. The effects of exogenous melatonin on the self-renewal and osteogenic capacity of postradiation CBSCs were detected in vitro. The underlying mechanisms in terms of genomic stability, apoptosis and oxidative stress-related signaling were further analyzed by western blotting, flow cytometry and immunofluorescence. Finally, the effects of melatonin on healing in postradiation bone defects were evaluated in vivo by micro-CT and immunohistochemical analysis. Results The radiation-induced reduced self-renewal and osteogenic capacity were partially reversed in postradiation CBSCs treated with melatonin. Melatonin maintained the genomic stability and apoptosis of postradiation CBSCs, and intracellular oxidative stress was decreased significantly while antioxidant-related enzymes were enhanced. Western blotting verified the anti-inflammatory effect of melatonin by downregulating the levels of IL-6 and TNF-α via extracellular regulated kinase (ERK)/nuclear factor erythroid 2-related factor 2 (NRF2)/heme oxygenase 1 (HO-1) signaling, distinct from its antioxidant effect via NRF2 signaling. In vivo experiments demonstrated that the newly formed bone in the melatonin plus Matrigel group had higher trabecular bone volume per tissue volume (BV/TV) and bone mineral density (BMD) values, and lower levels of IL-6 and TNF-α than those in the irradiation and the Matrigel groups. Conclusions This study suggested the potential of melatonin to protect CBSCs against γ-ray radiation and to assist the healing of postradiation bone defects.