Abstract 5153: Cancer stem cells of glioblastoma multiforme: regulation by the mTOR pathway

Glioblastoma multiforme (GBM), the most aggressive primary brain tumor, remains uniformly fatal despite current surgical, chemotherapeutic, and radiation treatment regimens. Recurrence of GBM has been associated with therapy resistant cancer stem cells (CSC), which possess the ability to generate tumors of the same genotype and phenotype as parent tumors within in vivo models. Recent evidence has suggested that neural stem and/or progenitor cells are the cell type of origin for GBM. In addition, numerous proto-oncogenes and tumor suppressor genes altered in GBM have been implicated in the self-renewal of normal stem cells, including Bmi1, Gfi1, PTEN, and p53. Aberrant PTEN/AKT signaling has been associated with the tumorigenesis of GBM. The downstream signaling target mTOR (mammalian target of Rapamycin), that exists in two distinct multicomplex proteins (mTORC1 and mTORC2), is a critical effector that is deregulated in GBM, and implies that the inhibition of mTOR may have therapeutic potential. However, inhibitors of mTOR such as Rapamycin (RAPA) or its analogs have thus far been ineffective in GBM treatment due to the activation of mitogenic pathways via feedback loops. The molecular mechanisms of CSC regulation by RAPA remain unknown. We aim to test the hypothesis that mTOR regulates CSC growth and self-renewal in GBM, and also understand the contribution of differentiation agents in targeting CSCs. For this study, we utilized primary tumor and cell line derived CSCs as evidenced by neurosphere formation. The results demonstrated that RAPA significantly suppressed neurosphere proliferation as evidenced by the inhibition of neurosphere diameter and frequency. As with mTOR inhibition, the differentiation agents arsenic trioxide (ATO), all-trans retinoic acid (ATRA), and histone deacetylase inhibitor suberoylanilide hydroxamic acid (SAHA) suppressed the formation of new neurospheres. Fluorescence-activated cell sorting (FACS) analysis revealed that RAPA halted GBM cells in S-phase in a time-dependent manner. In addition, a novel mTORC1 and mTORC2 inhibitor, KU0063794, significantly suppressed cell proliferation greater than RAPA alone. Also, RAPA dramatically suppressed colony formation. Prolonged mTOR inhibition with RAPA caused a shift towards mTORC2 and activation in ERK1/2 which was partially abrogated by upstream PI3K inhibition. Moreover, mTOR inhibition, but not MAPK inhibition, reduced the nuclear expression of embryonic stem cell marker Nanog. Finally, mTOR inhibition influenced cellular motility with respect to mTORC1 and mTORC2 activation. These results highlight that mTOR inhibition affects CSC proliferation and self-renewal in GBM and possibly suppresses tumorigenesis. Despite current clinical limitations, mTOR inhibitors may be effective in understanding aberrant signaling in CSCs, and aid in the design of future targeted therapies for GBM. 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 5153.