['In a previous study, we found that melatonin inhibits MG-63 osteosarcoma cell proliferation; however, the underlying mechanisms remain elusive. Mitogen-activated protein kinase (MAPK) and Akt signaling pathways play key roles in the anticancer effects of melatonin.', "The present study investigated whether MAPK and Akt signaling pathways are involved in melatonin's antiproliferative actions on the human MG-63 osteosarcoma cells.", 'Western blot analysis confirmed that melatonin significantly inhibited phosphorylation of ERK1/2 but not p38, JNK, or Akt. The expression of ERK1/2, p38, JNK, and Akt was not altered by melatonin. PD98059 and melatonin alone, and especially in combination, significantly inhibited cell proliferation. The changes included G1 and G2/M phase arrest of the cell cycle, and a downregulation of the expression at both the protein and mRNA levels of cyclin D1 and CDK4 (related to the G1 phase) and of cyclin B1 and CDK1 (related to the G2/M phase) as measured by flow cytometry after propidium iodide staining, and both western blot and real-time PCR, respectively. Furthermore, the combination of PD98059 and melatonin synergistically and markedly augmented the action of either agent alone. Co-immunoprecipitation further confirmed that there was an interaction between p-ERK1/2 and cyclin D1, CDK4, cyclin B1, or CDK1, which was blunted in the presence of melatonin or PD98059.'] Is inhibition of ERK1/2 Signaling Pathway Involved in Melatonin 's Antiproliferative Effect on Human MG-63 Osteosarcoma Cells?