Gamma-secretase inhibitors have been proposed as drugs able to kill
cancer cells by targeting the NOTCH pathway. Here, we investigated two of such inhibitors, the Benzyloxicarbonyl-
Leu-Leu-Nle-CHO (LLNle) and the N-[N-(3,5-difluorophenacetyl)-L-alanyl]-S-phenylglycine t-butyl
ester (
DAPT), to assess whether they were effective in killing human
glioblastoma tumor-initiating cells (GBM
TIC) in vitro. We found that only LLNle was able at the micromolar range to induce the death of GBM
TICs by apoptosis. To determine the cellular processes that were activated in GBM
TICs by treatment with LLNle, we analyzed the amount of the NOTCH intracellular domain and the gene expression profiles following treatment with LLNle,
DAPT, and
DMSO (vehicle). We found that LLNIe, beside inhibiting the generation of the NOTCH intracellular domain, also induces
proteasome inhibition, proteolytic stress, and mitotic arrest in these cells by repressing genes required for
DNA synthesis and mitotic progression and by activating genes acting as mitotic inhibitors.
DNA content flow cytometry clearly showed that cells treated with LLNle undergo arrest in the G(2)-M phases of the cell cycle. We also found that
DAPT and
L-685,458, another selective Notch inhibitor, were unable to kill GBM
TICs, whereas
lactacystin, a pure
proteasome inhibitor, was effective although at a much less extent than LLNle. These data show that LLNle kills GBM
TIC cells by inhibiting the
proteasome activity. We suggest that LLNle, being able to target two relevant pathways for GBM
TIC survival, may have a potential therapeutic value that deserves further investigation in animal models.