Osteosarcoma, with its high metastatic potential, is the most prevalent malignant bone tumor in children and adolescents. Melatonin possesses multiple tumor-suppressing properties for a myriad of tumors, but little is known about the effects of melatonin on osteosarcoma metastasis. In this study, we demonstrated that melatonin elicited very low cytotoxicity and significantly inhibited cellular motility, migration, and invasion in human osteosarcoma U2OS and HOS cells. Moreover, using RNA sequencing technology, we revealed that melatonin repressed C-C motif chemokine ligand 24 (CCL24) gene expression in U2OS cells. Manipulation of CCL24 levels influenced the motility of osteosarcoma cells as cell migration and invasion were enhanced by the addition of recombinant human CCL24 and attenuated by the silencing of CCL24. Moreover, melatonin increased and decreased the activation of extracellular signal-regulated kinase (ERK) 1/2 and c-Jun N-terminal kinase (JNK) 1/2, respectively, in a dose-dependent manner in U2OS and HOS cells while exerting no evident influence on the level and activation of p38, Akt, FAK, steroid receptor coactivator, or Raf. In further functional experiments, the use of JNK inhibitors (SP600125 and DN-JNK) confirmed that the pharmaceutic inhibition of JNK augmented the melatonin-mediated CCL24 suppression and migration of U2OS cells. Overall, our results revealed that melatonin attenuated chemokine CCL24 levels through inhibition of the JNK pathway to hinder human osteosarcoma cell invasion, thereby highlighting the therapeutic potential of melatonin for osteosarcoma metastasis.
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