Abstract 1670: Targeting PI3K/mTOR/MAPK pathway in inhibition of cell growth and dissemination of glioblastoma multiforme

Abstract

Abstract Activation of aberrant PI3K/AKT/mTOR (mammalian target of rapamycin) have been shown in tumorigenesis of glioblastoma multiforme (GBM), suggesting that inhibition of PI3K/mTOR may have therapeutic advantages. mTOR, which is deregulated in GBM, exists in two distinct mutiprotein complexes, mTORC1 and mTORC2, that regulates cell survival, growth, and motility. However, the inhibitors of mTOR, such as Rapamycin (RAPA) or its analogues that work via allosteric mechanism have provided limited clinical benefits to date. The aim of this study is to provide evidence that a combined inhibition of mTORC1/2 (via allosteric or ATP-competitive inhibition) with MAPK inhibitors would effectively suppress GBM growth and dissemination. We observed a significant number of GBM tumors showed increased expression of pAKTSer473 and pmTORSer2448, as assessed by immunohistochemistry. Inhibition of mTOR using RAPA suppressed pAKTSer473 levels. 12-O- Tetradecanoylphorbol-13-acetate (TPA) or insulin induced activation of pAKTSer473 was suppressed by pre-treatment with RAPA. Unlike this allosteric inhibitor of mTOR, ATP-competitive inhibitors of mTORC1/2, pp242, showed a complete suppression of pAKTSer473 in a dose-dependent manner. Also, pre-treatment with pp242 partially restored the insulin induced activation of pAKTSer473. The downstream substrate of mTORC1, p70S6KThr389 was noticeably suppressed by pp242 (2.2µM) and remained suppressed even after the treatment with TPA or insulin. Also, treatment with TPA or insulin significantly enhanced the activity of both p-p70S6K85and p-p70S6K70, which was completely abrogated by pretreatment with pp242, and partially by RAPA. Downstream from p70S6KThr389, S6 kinase (pS6KSer235/236 mTORC1 substrate), was suppressed in a dose dependent manner by pp242. We observed that PP242 completely inhibited the phosphorylation of a protein that controls protein synthesis, the mTORC1 substrate, eukaryotic initiation factor 4E binding protein-1 (4EBP1), whereas RAPA had partial effect. Analyses of extra-cellular signal regulated kinases (ERK1/2) showed that pp242 or RAPA suppressed the levels of pERK1/2Thr202/Tyr204. Significance of these altered signaling pathways by pp242 and RAPA correlated with cell proliferation and motility. Cell proliferation was suppressed by pp242 in a dose-dependent manner while RAPA also suppressed cell viability albeit was not as pronounced. Importantly, in the presence of MEK1/2 inhibitor, U0126, the pp242 noticeably suppressed cell growth as compared to RAPA. Moreover, the pp242 was effective in inhibiting the GBM cell migration. Taken together, these results suggest novel mTORC1/2/MAPK inhibitors suppress GBM growth and dissemination, which underscores the potential of a combined treatment strategy in GBM. . 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 1670. doi:10.1158/1538-7445.AM2011-1670