Nash Fellowship Progress Report

Project Title: Active-Site mTOR Inhibitors and Oncolytic Viruses Against Malignant Gliomas

Project Summary:

More effective treatments are needed for patients diagnosed with malignant gliomas (MGs). We want to determine the efficacy of novel active-site mTOR inhibitors (asTORis), and combine them with oncolytic viruses, for the treatment of MGs. Two novel therapies have shown potential against MGs: 1) mTOR inhibition principally using the small molecule rapamycin or derivatives, and 2) oncolytic viral therapies. However, none of these therapies alone appear sufficient, as resistance to rapamycin occurs, and the type-I interferon (IFN) responses of MGs greatly limit viral oncolysis. Nonetheless, combining the two therapies together results in a synergistic improvement in the outcome of animals bearing MGs. Recently, several novel asTORis with potency higher than rapamycin were described. Therefore combining asTORis with oncolytic viruses could offer an even greater therapeutic benefit on MGs. Our goal is to understand the regulation of mTOR signalling and determine the synergistic factors between mTOR inhibition and oncolytic viruses for the treatment of brain tumours.

Project Findings and Update:

We have previously demonstrated that the kinase mTOR (mammalian target of rapamycin) and the downstream targets of mTORC1 (complex 1), the eukaryotic initiation factor 4E (eIF4E) – binding proteins (4E-BPs), and the ribosomal S6 kinases (S6Ks), are implicated in the induction of type-I interferon (IFN). We have found that rapamycin (a drug that specifically inhibit mTORC1) limits the innate immune response of brain tumour cells, thus allowing increased viral infection and specific destruction of brain tumours in an immunocompetent rat model. We now find that the asTORi, PP242, inhibits type-I IFN induction better than rapamycin, and that the inhibition of IFN signalling by PP242 enhances HSV-1 replication in MG cell-lines. We discovered that the elevated activity of eIF4E (a proto-oncogene) in glioma cell lines is the reason for HSV-1 increased gene-expression in the presence of PP242. Accordingly, over-expression of eIF4E in normal fibroblasts results also in productive HSV1 infection in the presence of PP242. Overall, our findings reveal the important roles that mTOR/4E-BPs/eIF4E play in type-I IFN signalling and the translation of HSV-1 mRNAs, and propose that a combination therapy of asTORi with HSV-1 for the treatment of cancers with elevated eIF4E expression could be beneficial. We will now be verifying this approach in mouse models of brain cancers.

Secondly, we reported recently that the anti-neoplastic effects of asTORis are mediated by the downstream targets of mTORC1, the 4E-BPs. By inhibiting mTORC1 signaling, asTORis promote the association of 4E-BPs with eIF4E which leads to the suppression of eIF4E function. Since eIF4E levels are frequently upregulated in brain cancer, and the anti-proliferative effects of asTORi are mediated by 4E-BP-dependent inhibition of eIF4E function, we are testing whether cancer cells, with elevated levels of eIF4E, and thus altered eIF4E:4E-BP stoichiometry, are resistant to the anti-neoplastic effects of asTORis. We find that high expression of eIF4E confers an increased resistance of cells to the anti-proliferative effects of asTORis INK-302 (PP242) and Torin1. Conversely, depletion of eIF4E levels by RNAi potentiates the anti-neoplastic effects of PP242 in vitro and in vivo. Anti-sense therapy against eIF4E is presently evaluated in clinical trials against cancers. Our data indicate that combining this strategy with targeted mTOR inhibition could have increased therapeutic benefits against cancer including brain tumours which often present with elevated eIF4E activity.

Finally, we have studied the regulation of innate immunity and type-I IFN induction by the specific phosphorylation of eIF4E. We submitted a study showing that genetically modified mice and mouse embryonic fibroblasts MEFs (in which the phosphorylation site on eIF4E has been mutated and therefore can no longer be phosphorylated) are more resistant to viral infection. We find that the phosphorylation of eIF4E is critical for the translation of IκBα, the repressor of NF- κB, resulting in enhanced NF- κB activity, and IFN production. Specific inhibitors of MNK1/2, the kinases that directly phosphorylate eIF4E are presently tested for their effect on innate immunity, inflammation, and cancer therapy. We are also interested in pursuing their effect in combination with oncolytic viruses VSVΔM51 and HSV-1.

Impact and Relevance:

There are a number of ongoing clinical trials including phase III trials using oncolytic viruses for the treatment of cancers. Furthermore, asTORis are presently undergoing extensive clinical developments against cancers. The synergistic combination of these inhibitors and oncolytic viruses may represent an exciting strategy to target brain tumours. Furthermore, our findings on the regulation of innate immunity and cancer progression by mTOR signalling and the protein synthesis machinery provide important implications for the understanding and treatment of viral and neoplastic diseases.

Watch for more details about Dr. Alain's research when his project concludes in 2012.

Check out other outcomes from research funded by Brain Tumour Foundation of Canada here.