A Novel Combination Approach Targeting an Enhanced Protein Synthesis Pathway in MYC-driven (Group 3) Medulloblastoma

Abstract

The MYC oncogene is frequently amplified in patients with medulloblastoma, particularly in group 3 patients, who have the worst prognosis. mTOR signaling-driven deregulated protein synthesis is very common in various cancers, including medulloblastoma, that can promote MYC stabilization. As a transcription factor, MYC itself is further known to regulate transcription of several components of protein synthesis machinery, leading to an enhanced protein synthesis rate and proliferation. Thus, inhibiting enhanced protein synthesis by targeting the MYC and mTOR pathways together may represent a highly relevant strategy for the treatment of MYC-driven medulloblastoma. Here, using siRNA and smallmolecule inhibitor approaches, we evaluated the effects of combined inhibition of MYC transcription and mTOR signaling on medulloblastoma cell growth/survival and associated molecular mechanism(s) in MYC-amplified (group 3) medulloblastoma cell lines and xenografts. Combined inhibition of MYC and mTOR synergistically suppressed medulloblastoma cell growth and induced G 1 cellcycle arrest and apoptosis. Mechanistically, the combined inhibition significantly downregulated the expression levels of key target proteins of MYC and mTOR signaling. Our results with RNAsequencing revealed that combined inhibition synergistically modulated global gene expression including MYC/mTOR components. In addition, the combination treatment significantly delayed tumor growth and prolonged survival of MYC-amplified medulloblastoma xenografted mice by downregulating expression of MYC and the key downstream components of mTOR signaling, compared with single-agent therapy. Together, our findings demonstrated that dual inhibition of MYC (transcription) and mTOR (translation) of the protein synthesis pathway can be a novel therapeutic approach against MYC-driven medulloblastoma.

Synergistic efficacy of inhibiting MYCN and mTOR signaling against neuroblastoma

“…MTOR signaling is one of the key oncogenic pathways which is often deregulated in MYC-driven cancers, including NB [ 20 ]. Moreover, it has been shown that MYC proteins and the mTOR pathway cooperate with each other at the transcription and translation levels, respectively, to elevate overall protein synthesis rate, leading to increased cell proliferation and cancer progression [ 21 , 22 , 27 ]. These pathways, either working individually or together, can cause cancer progression.…”

“…While our data show that although individual or concomitant inhibition of BET/mTOR has greater antitumor efficacies against MYCN-amplified NB cells, the combination also efficiently inhibits cell growth and induces apoptosis in non-MYCN-amplified NB cells, pointing to a broader relevance of our combined approach in NB. Because non-MYCN-amplified NB cell lines, including SK-N-AS, express MYC [ 29 ] which is a target for combined BET/mTOR in medulloblastoma cells [ 22 ]. MYC may be a secondary target for these inhibitors in the context of NB.…”

“…Our study also revealed that inhibition of MYCN/mTOR not only inhibits cell growth and survival in NB cells, but also chemosensitizes NB cells. Beyond NB, this therapeutic approach can be of broader relevance for therapy of MYC/MYCN-addicted cancers, as we have previously shown a synergistic antitumor efficacy of BET/mTOR inhibitors in MYC-driven medulloblastoma [ 22 ]. While further studies using appropriate in vivo models are needed to evaluate this combination, our study highlights a basis for considering this combination approach as a new therapy for NB.…”

“…Effects of inhibitors on NB Cell growth/viability were assessed using the MTT assay as previously described [ 22 , 23 ].…”

“…Effects of inhibitors on NB spheres were performed using the neurosphere/sphere assay as previously described [ 22 ].…”