Abstract 3392: Regulation and maintenance of cancer stem cells of glioblastoma multiforme: The role of mTOR/MAPK signaling pathways

Glioblastoma multiforme (GBM), the most aggressive primary brain tumor, remains uniformly incurable despite current treatment modalities. 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. The coexistence within the same tumors of cells with different phenotypes and isolation of CSC from gliomas, suggests that the neural stem and/or progenitor cells as the cell of origin for GBM. The use of targeted therapies for GBM has failed due to the presence of chemo-resistant CSCs. Mammalian target of Rapamycin (mTOR) is deregulated in GBM, implying that the mTOR inhibition may have therapeutic potential in suppression of CSC. In addition, studies have shown that MAPK plays an important role in differentiation. We aim to test the hypothesis that: 1) CSC growth and self-renewal in GBM is regulated by mTOR and MAPK, 2) differentiating agents, along with other inhibitors can target CSCs. For this study, we utilized primary tumor and GBM cell line (LN18, U373 U87), possessing CSC properties, as evidenced by neurosphere formation. We observed the presence of neural stem cell marker, nestin in a large number of GBM tumors. Neurospheres expressed the markers of embryonic stem cells such as, SOX2 and nanog, in addition to neural stem cell marker, nestin. Treatment of U87 cells with all-trans retinoic acid (ATRA) decreased the expression of the stem cell markers nanog and nestin, as treatment with ATRA enhanced the differentiation of the CSCs. Further analysis suggested that MAPKinase inhibitor, U0126, increased the expression of nanog and nestin. In addition, inhibition of mTOR altered the expression of nanog and nestin, implying mTOR had effect on maintenance of CSC of GBM. Formation of neurosphere was suppressed by treatment with ATRA. Furthermore, neurophere formation was suppressed by allosteric as well as active-site inhibitor of mTOR, rapamycin (RAPA) and pp242, respectively. The proliferation of neurosphere, as measured by neurosphere diameter was also suppressed by ATRA, RAPA and U0126 treatments. While ATRA decreased neurophere diameter by inducing differentiation, the inhibitors of mTOR and MAPK did so by suppressing the activation of down-stream targets, S6Kinase. The effect of these treatments on the CSCs was further elucidated by scratch migration assay. The migration of LN18 and U373 stem cells was significantly inhibited by ATRA, RAPA, and U0126. These results provide further evidence that a significant number of GBMs express stem cell markers. Moreover, the CSC can be targeted by specific mTOR or MAPK inhibition. Furthermore, suppression of the MAPK pathway by U0126, as made evident by the increased expression of nanog and nestin, suggests the role of this pathway in maintenance of pleuripotency of stem cells. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 3392. doi:10.1158/1538-7445.AM2011-3392