The Notch signaling pathway is a highly conserved signaling pathway that was originally characterized for its function in influencing cell fate and differentiation. However, more recent evidence has well established the role of Notch activation in several cancers. Active Notch has been shown to cooperate with viral oncoproteins in the transformation of human cell lines. Despite these observations, the role of Notch signaling in human melanoma development and melanocyte transformation remains unknown. Here, we show that the Notch signaling pathway is activated in melanoma cells but not in normal melanocytes. Lentiviral mediated expression of active Notch1 in melanocytes results in altered morphology, enhanced growth, focus formation, and changes in gene expression. Constitutive Notch1 activation enhances melanoma cell proliferation in vitro and in vivo in SCID mice. Inhibition of the Notch signaling pathway suppresses the growth of melanoma cells both in vitro and in vivo while sparing normal melanocytes. Activation of Notch1 signaling also enables melanoma cells from primary lesions to acquire metastatic capability. Furthermore, the oncogenic effect of Notch1 on primary melanoma cells is mediated by β-catenin, which is upregulated following Notch1 activation. MCAM, a cell adhesion molecule that promotes melanoma progression and metastasis, is upregulated by constitutive Notch activation in both melanocytes and melanoma cells. We identified a CSL binding consensus sequence within MCAM\u27s promoter and illustrate that CSL is capable of binding an oligonucleotide based on the MCAM promoter sequence in vitro. Finally, expression of active Notch1 in normal melanocytes induces anchorage independent growth in vitro, a hallmark of transformed cells. Our data illustrate that active Notch1 protein alone is capable of conferring transforming properties to normal melanocytes in vitro and promoting the progression of primary melanoma
