Curcumin inhibits proliferation and invasion of osteosarcoma cells through inactivation of Notch-1 signaling. The Notch signaling pathway plays critical roles in human cancers, including osteosarcoma, suggesting that the discovery of specific agents targeting Notch would be extremely valuable for osteosarcoma. Curcumin, a naturally occurring phenolic compound found in curcuma longa, has been shown to inhibit proliferation and induce apoptosis of osteosarcoma cells in vitro and tumor growth in xenotransplant or orthotransplant models. However, the precise molecular mechanisms by which curcumin exerts its antitumor activity remain unclear. Here we used multiple molecular approaches, such as the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay, the invasion assay, gene transfection, real-time RT-PCR, western blot and gelatin zymography, to investigate whether the downregulation of Notch-1 contributes to curcumin-induced inhibition of proliferation and invasion in osteosarcoma cells. The results showed that curcumin caused marked inhibition of osteosarcoma cell growth and G2/M phase cell cycle arrest. This was associated with concomitant attenuation of Notch-1 and downregulation of its downstream genes, such as matrix metalloproteinases, resulting in the inhibition of osteosarcoma cell invasion through Matrigel. We also found that specific downregulation of Notch-1 via small-interfering RNA prior to curcumin treatment resulted in enhanced inhibition of cell growth and invasion. These results suggest that antitumor activity of curcumin is mediated through a novel mechanism involving inactivation of the Notch-1 signaling pathway. Our data provide the first evidence that the downregulation of Notch-1 by curcumin may be an effective approach for the treatment of osteosarcoma.© 2012 The Authors Journal compilation © 2012 FEBS.