Dual MTORC1/2 Inhibition Compromises Cell Defenses Against Exogenous Stress Potentiating Obatoclax-induced Cytotoxicity in Atypical Teratoid/rhabdoid Tumors

Atypical teratoid/rhabdoid tumors (AT/RT) are the most common malignant brain tumors of infancy and have a dismal 4-year event-free survival (EFS) of 37%. We have previously shown that mTOR activation contributes to AT/RT's aggressive growth and poor survival. Targeting the mTOR pathway with the dual mTORC1/2 inhibitor TAK-228 slows tumor growth and extends survival in mice bearing orthotopic xenografts. However, responses are primarily cytostatic with limited durability. The aim of this study is to understand the impact of mTOR inhibitors on AT/RT signaling pathways and design a rational combination therapy to drive a more durable response to this promising therapy. We performed RNASeq, gene expression studies, and protein analyses to identify pathways disrupted by TAK-228. We find that TAK-228 decreases the expression of the transcription factor NRF2 and compromises AT/RT cellular defenses against oxidative stress and apoptosis. The BH3 mimetic, Obatoclax, is a potent inducer of oxidative stress and apoptosis in AT/RT. These complementary mechanisms of action drive extensive synergies between TAK-228 and Obatoclax slowing AT/RT cell growth and inducing apoptosis and cell death. Combination therapy activates the integrative stress response as determined by increased expression of phosphorylated EIF2α, ATF4, and CHOP, and disrupts the protective NOXA.MCL-1.BIM axis, forcing stressed cells to undergo apoptosis. Combination therapy is well tolerated in mice bearing orthotopic xenografts of AT/RT, slows tumor growth, and extends median overall survival. This novel combination therapy could be added to standard upfront therapies or used as a salvage therapy for relapsed disease to improve outcomes in AT/RT.