The Notch signaling pathway is required in both nonneoplastic neural stem cells and embryonal
brain tumors, such as
medulloblastoma, which are derived from such cells. We investigated the effects of Notch pathway inhibition on
medulloblastoma growth using pharmacologic inhibitors of
gamma-secretase. Notch blockade suppressed expression of the pathway target Hes1 and caused cell cycle exit, apoptosis, and differentiation in
medulloblastoma cell lines. Interestingly, viable populations of better-differentiated cells continued to grow when Notch activation was inhibited but were unable to efficiently form soft-
agar colonies or
tumor xenografts, suggesting that a cell fraction required for
tumor propagation had been depleted. It has recently been hypothesized that a small population of stem-like cells within
brain tumors is required for the long-term propagation of neoplastic growth and that CD133 expression and Hoechst
dye exclusion (side population) can be used to prospectively identify such
tumor-forming cells. We found that Notch blockade reduced the CD133-positive cell fraction almost 5-fold and totally abolished the side population, suggesting that the loss of
tumor-forming capacity could be due to the depletion of stem-like cells. Notch signaling levels were higher in the stem-like cell fraction, providing a potential mechanism for their increased sensitivity to inhibition of this pathway. We also observed that apoptotic rates following Notch blockade were almost 10-fold higher in primitive
nestin-positive cells as compared with
nestin-negative ones. Stem-like cells in
brain tumors thus seem to be selectively vulnerable to agents inhibiting the Notch pathway.