Abstract 5250: PI3K inhibitors prime neuroblastoma cells for chemotherapy-induced apoptosis apoptosis in vitro and in vivo by shifting the balance towards pro-apoptotic Bcl-2 proteins and increased mitochondrial perturbations
Authors: Bender Ariane , Opel Daniela , Friedman Lori , Debatin Klaus-Michael , Fulda Simone 
Published: February 2011

Abstract
Aberrant activation of the PI3K/Akt/mTOR survival cascade is a characteristic feature of many cancers and is associated with treatment resistance and poor prognosis. We recently identified activation of Akt as a novel predictor of poor outcome in neuroblastoma, indicating that PI3K/Akt presents a clinically relevant novel target for therapeutic intervention. In the present study, we therefore investigated whether targeting aberrant PI3K signaling by small molecule inhibitors could increase the efficacy of chemotherapy in neuroblastoma.
Here, we provide first evidence that the PI3K inhibitor PI103 synergistically increases apoptosis upon treatment with various cytotoxic drugs that are currently used in clinical protocols of neuroblastoma including Doxorubicin, Etoposide, Topotecan, Cisplatin, Vincristine and Taxol. Mechanistic studies reveal that PI103 cooperates with Doxorubicin to reduce Mcl-1 expression and Bim phosphorylation and to upregulate Noxa and Bim levels. Accordingly, knockdown of Mcl-1 enhances Doxorubicin-induced apoptosis, while silencing of Noxa reduces Doxorubicin-mediated cell death, underscoring the functional relevance of the PI103-mediated modulation of Mcl-1 and Noxa for apoptosis sensitization. This shifted ratio of pro- and anti-apoptotic Bcl-2 proteins results in increased Bax conformational change, loss of mitochondrial membrane potential, cytochrome c release, caspase activation and caspase-dependent apoptosis upon combined treatment with PI103 and Doxorubicin. Overexpression of Bcl-2 to block mitochondrial apoptosis also prevents Bax conformational change, mitochondrial perturbations, caspase activation and apoptosis, thus confirming the central role of the mitochondrial pathway for chemosensitization. Interestingly, the broad range caspase inhibitor zVAD.fmk blocks caspase activation and apoptosis, while it does not interfere with Bax activation or mitochondrial outer membrane permeabilization. These findings place mitochondrial events upstream of caspase activation. PI103 and Doxorubicin cooperate to induce apoptosis in patients’ derived primary neuroblastoma cells, supporting the clinical relevance of the results. Importantly, the combination treatment with PI103 and Doxorubicin is superior compared to either agent alone to suppress tumor growth in an in vivo model of neuroblastoma. By demonstrating that PI3K inhibitors such as PI103 prime neuroblastoma cells for chemotherapy-induced apoptosis in vitro and in vivo, these findings have important clinical implications for the development of targeted therapies to increase chemosensitivity of neuroblastoma.
Note: This abstract was not presented at the AACR 101st Annual Meeting 2010 because the presenter was unable to attend.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 5250.