Abstract 235: Oncogene addiction switch from NOTCH to PI3K/AKT requires simultaneous targeting of NOTCH and PI3K pathway inhibition in glioblastoma.
Authors: Saito Norihiko , Fu Jun , Wang Shuzhen , Sulman Erik P. , Lang Frederick F. , Yung Wk Alfred , Koul Dimpy 
Published: August 2014

Abstract
Notch signaling pathway regulates normal stem cells in the brain and glioma stem cells (GSCs) with high Notch activity. However, blocking the proteolytic activation of NOTCH with γ-secretase inhibitors (GSIs) fails to alter the growth of some GSCs as GSIs seem to be active in only a fraction of GSCs lines with constitutive NOTCH activity and limiting the efficacy of this strategy. Linking the molecular aberrations of cancer to drug responses could guide treatment choice and identification of new therapeutic applications. Here we report loss of PTEN as a critical event leading to resistance to NOTCH inhibition, which causes the transfer of “oncogene addiction” from the NOTCH to the phosphoinositol-3 kinase (PI3K)/AKT pathway. This novel observation suggests the need to simultaneous inhibition of both pathways as a means to improve therapeutic efficacy in human GBMs.
We investigated the effects of Notch pathway inhibition in GSC using GSI. Drug cytotoxicity test on 16 GSCs show differential growth response to GSI stratifying GSCs into two groups: responders (6 cell lines) vs non-responders (10 cell lines). Active Notch signaling seems to be important features for the GSC as Notch inhibition only affected GSCs defined as having increased endogenous Notch activity. However in the responder group GSCs with the PTEN mutation seems to be less sensitive to GSI treatment. Here we show that NOTCH regulates the expression of PTEN and the activity of the PI3K/AKT signaling pathway in GSCs since treatment with GSI attenuated Notch intracellular domain (NICD), Hes-1, Hes-3, and Hes-5 and increases PTEN expression. NOTCH regulates PTEN expression via Hes-1 as knockdown of either Notch or Hes1 led to increase expression of PTEN. In contrast, PTEN knockdown decreases the response of GSCs to GSI in PTEN wild type GSCs. Our data shows that PTEN is an important mediator of GSI induced attenuation of cell growth suggesting that there might be a regulatory circuit-linking Notch signaling with PTEN/PI3K/Akt pathway, providing the basis for the design of new therapeutic strategies and suggests the need to simultaneously inhibit both pathways as a means to improve therapeutic efficacy in human GBMs. Our preliminary data reveal the synergistic attenuation of cell growth using combination of GSI and PI3K inhibitors in PTEN mutant GSCs. Our results warrant further studies to test the efficacy of combined PI3K/AKT- and NOTCH-directed therapies in human cancer.
Citation Format: Norihiko Saito, Jun Fu, Shuzhen Wang, Erik P. Sulman, Frederick F. Lang, Wk Alfred Yung, Dimpy Koul. Oncogene addiction switch from NOTCH to PI3K/AKT requires simultaneous targeting of NOTCH and PI3K pathway inhibition in glioblastoma. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 235. doi:10.1158/1538-7445.AM2013-235