Abstract
Sphere formation in conditioned serum-free culture medium supplemented with epidermal growth factor and basic fibroblast growth factor (tumorospheres) is considered useful for the enrichment of cancer stem-like cells, also known as tumor-initiating cells. We used a gene expression microarray to investigate the gene expression profile of melanoma cancer stem-like cells (MCSLCs). The results showed that MCSLCs highly expressed the following Notch signaling pathway molecules: Notch3 (NM_008716), Notch4 (NM_010929), Dtx4 (NM_172442), and JAG2 (NM_010588). Immunofluorescence staining showed tumorosphere cells highly expressed Notch4. Notch4(high) B16F10 cells were isolated by FACS, and Western blotting showed that high Notch4 expression is related to the expression of epithelial-mesenchymal transition (EMT)-associated proteins. Reduced invasive and migratory properties concomitant with the downregulation of the EMT markers Twist1, vimentin, and VE-cadherin and the overexpression of E-cadherin was observed in human melanoma A375 and MUM-2B cells. In these cells, Notch4 was also downregulated, both by Notch4 gene knockdown and by application of the γ-secretase inhibitor, DAPT. Mechanistically, the re-overexpression of Twist1 by the transfection of cells with a Twist1 expression plasmid led to an increase in VE-cadherin expression and a decrease in E-cadherin expression. Immunohistochemical analysis of 120 human melanoma tissues revealed a significant correlation between the high expression of Notch4 and the metastasis of melanoma. Taken together, our findings indicate that Notch4+ MCSLCs trigger EMT and promote the metastasis of melanoma cells.