Osteosarcoma(OS)is the most common type of primary malignant tumors that originate in the bone.Resistance to chemotherapy confers a poor prognosis on OS patients.Dysregulation of the epidermal growth factor receptor(EGFR)signaling has been reported in sarcomas.However,the functional contribution of EGFR hyperactivation to the tumor biology and chemoresistance remains largely unexplored in OS.In this study,we aimed to investigate the role of EGFR in OS progression and in the response of OS to gemcitabine treatment.The EGFR expression was found to be upreg-ulated in fibroblastic OS cell lines.EGFR knockdown suppressed OS cell proliferation,migration,and invasion in vitro and tumor formation in vivo.Conversely,EGFR overexpression promoted the growth and motility of OS cells.In terms of mechanism,the levels of phospho-Akt and phospho-ERK were decreased upon EGFR knockdown but increased as a result of EGFR overexpression,implying a possible involvement of PI3K/Akt and ERK pathways in mediating the effects of EGFR on OS cells.Moreover,the level of phospho-EGFR was increased in OS cells when exposed to gemcitabine treatment.A more profound proliferative inhibition and a higher rate of apoptosis were obtained in OS cells via inducing cell cycle arrest at G1 phase upon gemcitabine treatment combined with EGFR knockdown,as compared to gemcitabine alone.On the contrary,EGFR over-expression counteracted the growth-inhibiting and pro-apoptotic effects of gemcitabine in OS cells.The present study suggests that EGFR promotes tumor progression and contributes to gemcitabine resistance in OS.