Osteosarcoma (OS) is the most common primary malignancy of bone and is usually associated with poor
prognosis due to its high incidence of metastasis and chemoresistance. Molecular pathogenesis of OS is poorly
understood. We previously showed that OS cells are refractory to BMP9-induced osteogenesis and respond favorably to
proliferation and tumor growth. Here we investigate if Notch signaling mediates the BMP9-promoted cell proliferation
and tumor growth of human osteosarcoma (OS). We find that the expression of Notch1, Notch2, Notch3, DLL1, JAG1
and JAG2 is readily detected in most of the tested OS cell lines. BMP9-promoted OS cell proliferation, migration, and cell
cycle S/G2 progression are effectively inhibited by a dominant-negative mutant of Notch1 (dnNotch1) or the γ-secretase
inhibitor Compound E (ComE). Furthermore, BMP9-promoted tumor growth and osteolytic lesions in vivo are
significantly inhibited by dnNotch1. BMP9 up-regulates the expression of Notch1, Notch3, DLL1, and JAG1 in OS cells.
Accordingly, BMP9 stimulation induces a nuclear accumulation of NICD, which is blocked by ComE. Our results
demonstrate that BMP9-promoted OS proliferation and tumor growth is at least in part mediated by Notch signaling,
suggesting that osteogenic BMPs may function as upstream regulators of Notch signaling in OS tumorigenesis. Thus,
pharmacologic intervention of Notch signaling may be explored as a new therapeutic strategy for human OS tumors.