Abstract 5410: First, second, and third generation mTOR pathways inhibitors for treatment of glioblastoma
Authors: Dominguez Jose F. , Rosberger Haylen , Garell Paul , Gandhi Chirag D. , Jhanwar-Uniyal Meena 
Published: June 2022

Abstract
The Mechanistic Target of Rapamycin (mTOR), which functions in two distinct multiprotein complexes, mTORC1 and mTORC2, is a downstream signaling kinase in the PI3K/AKT pathway. mTOR is often deregulated in glioblastoma (GB), the most aggressive primary brain tumor in adults, due to the frequent loss of the tumor suppressor, PTEN. mTORC1 and mTORC2 control cell growth, proliferation, migration, survival, and stem cell regulation in response to nutrients and growth factors. Nutrient-sensitive mTORC1 regulates protein synthesis through the downstream substrate, 4EBP1 (also called EIF4EBP1), p70S6K1, and p70S6K2. On the other hand, growth factor-sensitive mTORC2 is associated with Rapamycin-insensitivity. The presence of stem cells within the tumor mass and the surrounding areas precludes the possibility of the elimination of GB. Furthermore, the substrate of activated AKT, the proline-rich AKT substrate of 40 kDa (PRAS40), is a surrogate marker of activated AKT in response to mTOR inhibition therapy in GB patients that regulates mTORC1. The aim of this study is to evaluate the efficacy of first (Rapamycin (RAPA)), second (Torin1, Torin2, and XL388), and third (Rapalink-1) mTOR inhibitors to suppress cell growth, dissemination, overcome drug resistance of GB cells, and alter self-renewal properties of GB stem cells. Results demonstrated: a significant number of tumors showed an increased expression of pAKTSer473 and pmTORSer2448, suggesting an overactivation of the mTOR pathway; co-expression of mTOR and stem cell marker, nestin, was evident in GB tumors, implying its role in stem cell regulation; Torin2, but not Torin1 or XL388, suppressed mTORC1 activity completely as shown by the reduced expression of its pS6KSer235/236 in a dose-dependent manner; Torin1, Torin2, and XL388 inhibited the phosphorylation of 4E-BP1 and PRAS40; Torin1, Torin2, and XL388 suppressed dephosphorylation of pAKTSer473; functional analysis revealed that RAPA, Torin1, Torin2, XL388, and Rapalink-1 effectively suppressed proliferation and S-phase entry; RAPA, Torin1, Torin2 and Rapalink-1 inhibited GB cell migration, while XL388 was ineffective; RAPA, Torin1, Torin2, and Rapalink-1 suppressed the self-renewal of GB stem cells and inhibited their proliferation, XL388 remained ineffective; drug resistance analysis revealed that Torin2 was most effective in eradicating GB tumor cells. These results suggested that RAPA, Torin1, Torin2, and Rapalink-1, but not XL388, are useful in suppressing the mTORC1 and mTORC2 activities, thereby inhibiting GB cell proliferation, dissemination, and inhibiting stem cell self-renewal. Therefore, the use of these mTOR inhibitors presents a promising treatment strategy for Glioblastoma.
Citation Format: Jose F. Dominguez, Haylen Rosberger, Paul Garell, Chirag D. Gandhi, Meena Jhanwar-Uniyal. First, second, and third generation mTOR pathways inhibitors for treatment of glioblastoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 5410.