Synergistic efficacy of inhibiting MYCN and mTOR signaling against neuroblastoma.

BACKGROUND: Neuroblastoma (NB) patients with MYCN amplification or overexpression respond poorly to current therapies and exhibit extremely poor clinical outcomes. PI3K-mTOR signaling-driven deregulation of protein synthesis is very common in NB and various other cancers that promote MYCN stabilization. In addition, both the MYCN and mTOR signaling axes can directly regulate a common translation pathway that leads to increased protein synthesis and cell proliferation. However, a strategy of concurrently targeting MYCN and mTOR signaling in NB remains unexplored. This study aimed to investigate the therapeutic potential of targeting dysregulated protein synthesis pathways by inhibiting the MYCN and mTOR pathways together in NB. METHODS: Using small molecule/pharmacologic approaches, we evaluated the effects of combined inhibition of MYCN transcription and mTOR signaling on NB cell growth/survival and associated molecular mechanism(s) in NB cell lines. We used two well-established BET (bromodomain extra-terminal) protein inhibitors (JQ1, OTX-015), and a clinically relevant mTOR inhibitor, temsirolimus, to target MYCN transcription and mTOR signaling, respectively. The single agent and combined efficacies of these inhibitors on NB cell growth, apoptosis, cell cycle and neurospheres were assessed using MTT, Annexin-V, propidium-iodide staining and sphere assays, respectively. Effects of inhibitors on global protein synthesis were quantified using a fluorescence-based (FamAzide)-based protein synthesis assay. Further, we investigated the specificities of these inhibitors in targeting the associated pathways/molecules using western blot analyses. RESULTS: Co-treatment of JQ1 or OTX-015 with temsirolimus synergistically suppressed NB cell growth/survival by inducing G1 cell cycle arrest and apoptosis with greatest efficacy in MYCN-amplified NB cells. Mechanistically, the co-treatment of JQ1 or OTX-015 with temsirolimus significantly downregulated the expression levels of phosphorylated 4EBP1/p70-S6K/eIF4E (mTOR components) and BRD4 (BET protein)/MYCN proteins. Further, this combination significantly inhibited global protein synthesis, compared to single agents. Our findings also demonstrated that both JQ1 and temsirolimus chemosensitized NB cells when tested in combination with cisplatin chemotherapy. CONCLUSIONS: Together, our findings demonstrate synergistic efficacy of JQ1 or OTX-015 and temsirolimus against MYCN-driven NB, by dual-inhibition of MYCN (targeting transcription) and mTOR (targeting translation). Additional preclinical evaluation is warranted to determine the clinical utility of targeted therapy for high-risk NB patients.

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

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 small-molecule 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 G1 cell-cycle arrest and apoptosis. Mechanistically, the combined inhibition significantly downregulated the expression levels of key target proteins of MYC and mTOR signaling. Our results with RNA-sequencing 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.