New insights into Notch1 regulation of the PI3K-AKT-mTOR1 signaling axis: Targeted therapy of gamma-secretase inhibitor resistant T-cell acute lymphoblastic leukemia

T-cell acute lymphoblastic leukemia (T-ALL) is characterized as a high-risk stratified disease associated with frequent relapse, chemotherapy resistance, and a poorer prognostic outlook than B-precursor ALL Many of the challenges in treating T-ALL reflect the lack of prognostic cytogenetic or molecular abnormalities on which to base therapy, including targeted therapy. Notchl activating mutations were identified in more than 50% of T-ALL cases and can be therapeutically targeted with y-secretase inhibitors (GSIs). Mutant Notchl can activate cMyc and PI3K-AKT-mTOR1 signaling in T-ALL In T-ALLs with wild-type phosphatase and tensin homolog deleted on chromosome ten (PTEN), Notchl transcriptionally represses PTEN, an effect reversible by GSIs. Notchl also promotes growth factor receptor (IGF1R and IL7Rot) signaling to PI3K-AKT. Loss of PTEN is common in primary T-ALLs due to mutation or posttranslational inactivation and results in chronic activation of PI3KAKT-mTOR1 signaling, GSI-resistance, and repression of p53-mediated apoptosis. Notchl itself might regulate posttranslational inactivation of PTEN. PP2A is activated by Notch1 in PTEN-null 1-ALL cells, and GSIs reduce PP2A activity and increase phosphorylation of AKT, AMPK, and p70S6K. This review focuses on the central role of the PI3K-AKT-mTOR1 signaling in 1-ALL, including its regulation by Notchl and potential therapeutic interventions, with emphasis on GSI-resistant T-ALL (C) 2013 Elsevier Inc. All rights reserved.