MTOR KINASE INHIBITORS AND APOPTOSIS IN GLIOBLASTOMA

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Abstract

BACKGROUND: Glioblastoma represents the most common primary brain tumor, and among the most lethal of cancers. The axis linking receptor tyrosine kinases and phosphatidylinositol 3' kinase to the mammalian target of rapamycin (mTOR) is activated in a majority of glioblastomas, suggesting mTOR as a prominent target for therapy. A new class of mTOR kinase inhibitors already in clinical trials disrupts signaling through two mTORC1 translational effectors (S6K and eIF4E) whereas clinical allosteric binders (rapamycin and analogues) disrupt only S6K. METHODS: We tested three distinct mTOR kinase inhibitors (KU63794, PP242 and the clinical compound MLN128) against glioma cell lines and primary cultures wild-type or mutant at PTEN. We used viability assays, flow cytometry, western blotting, as well as convection enhanced delivery and gavage to test agents in glioblastoma. RESULTS: All agents blocked viability in a dose dependent manner. PP242 uniquely induced apoptosis. Cell based screening suggested Janus Kinase 2 (JAK2) and protein kinase C alpha (PKCa) as prominent off-targets for PP242. We validated both off-targets. To determine whether these targets contributed to apoptosis, we combined inhibition or knockdown of JAK2 and PKCa. Blocking both kinases drove apoptosis, associated with reduced phosphorylation of Signal Transducer and Activator of Transcription 3 (STAT3). Apoptosis was abrogated by inhibition of mTOR. The EGFR inhibitors erlotinib and lapatanib partially blocked PKC and JAK2. Combining either EGFR inhibitor with a JAK2 inhibitor also drove apoptosis in a PTEN-indepenent manner, associated with blockade of p-STAT3. Current experiments extend this combination in-vivo. CONCLUSIONS: We recently demonstrated that EGFR and EGFRvIII converge to activate STAT signaling in glioblastoma. By dissecting off-target effects underlying the ability of an mTOR kinase inhibitor to drive apoptosis, we identified a combination therapy approach that blocks STAT activation. Use of EGFR and of JAK kinases in combination blocked STAT signaling, and was associated with apoptosis in glioblastoma. Combined therapy with approved JAK and EGFR inhibitors could be translated rapidly to patients. SECONDARY CATEGORY: Tumor Biology.

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