De-regulated expression of components of the Notch signaling pathway is observed in
malignant melanoma. This pathway is activated by catalytic cleavage of the
Notch receptor by γ-
secretase. Phase-I trials with
RO4929097, a potent
gamma secretase inhibitor (GSI), and other agents of this class have demonstrated clinical activity in patients with
melanoma. An understanding of the mechanisms for de novo sensitivity and resistance to this class of drugs would be critical for future
drug development. We treated a panel of
Phosphatase and
Tensin Homolog (PTEN)-null, -mutant and -wild-type human
melanoma cell lines with
RO4929097 and evaluated the efficacy alone and in combination with
chemotherapy. Although cleaved Notch-1 formation was observed in all the cell lines, RO4929097-induced senescence or apoptosis was achieved only in PTEN-wild-type cell lines in which
gamma-secretase inhibition with an induction of PTEN expression and decreased AKT/PKB (
protein kinase B) phosphorylation in addition to transcriptional suppression at the Hairy and enhancer of split-1 (HES1) gene promoter. Overexpression of wild-type PTEN in PTEN-null and -mutant cell lines, and studies with isogenic breast cell lines that differ only in PTEN status, confirmed the importance of PTEN expression for conferring
tumor cell susceptibility to
RO4929097. Furthermore, in PTEN-expressing rapidly accelerated
fibrosarcoma 1 (B-RAF)-mutant
melanoma cells,
RO4929097 enhanced the effect of
temozolomide both in vitro and in vivo. These results indicate that
tumor cell susceptibility to a GSI, whether alone or in combination with
chemotherapy, are reliant upon reducing AKT phosphorylation and hence GSI in combination with
chemotherapy may be useful as a new therapeutic approach in treating PTEN-wild-type
melanoma.