Abstract B37: Targeting Myc-reprogrammed glutamine metabolism for treatment of neuroblastoma

Abstract

Abstract Oncogenic Myc is known to alter cellular metabolism resulting in glutamine addiction, and the MYCN-amplified subclass of neuroblastoma is associated with extremely poor prognosis. Successful identification of therapeutic agents that are effective in neuroblastoma, particularly in tumors with MYCN amplification, could significantly increase patient survival. To explore the effect of inhibiting glutamine metabolism in neuroblastoma, we used the glutamine antagonist DON (6-diazo-5-oxo-norleucine). DON is an irreversible competitive inhibitor which alkylates enzymes that utilize glutamine. Although first studied in the 1950s as a possible cancer chemotherapeutic, DON was associated with severe nausea which limited its use in patients. DON remains a potent inhibitor, and a useful tool to study the effect of modulating glutamine metabolism in vivo. We tested the DON sensitivity of six human neuroblastoma cell lines (Kelly, IMR-32, SK-N-BE2, SK-N-AS, SK-N-FI, and SH-SY5Y) compared to immortalized BJ fibroblast control cells. DON effectively inhibited all neuroblastoma cell lines tested with minimal effect on BJ cells. Furthermore, MycN overexpression in SK-N-AS cells, a cell line which is not MYCN-amplified and expresses low levels of cMyc, was sufficient to make cells significantly more sensitive to DON. Next, we tested the effect of DON treatment on subcutaneous xenografts. For these experiments we used the cell lines SK-N-AS (not MYCN-amplified), SK-N-BE2 (MYCN-amplified with moderate MycN levels), and IMR-32 (MYCN-amplified with high MycN levels). Mice were treated twice per week by intraperitoneal injection with DON (50 or 100mg/kg) or water control. Treatment with 50mg/kg DON significantly inhibited tumor growth of all three cell lines, suggesting neuroblastoma tumors may be broadly sensitive to glutamine inhibitors. IMR-32 tumors were the most sensitive to DON treatment, leading to a 6-fold reduction in tumor size after 2.5 weeks of treatment. Tumor BrdU incorporation decreased following DON treatment, and IMR-32 tumors showed increased levels of cleaved caspase 3, a marker of apoptosis. At the higher 100mg/kg dose, DON treatment led to a more substantial decrease in BrdU incorporation, and at this dose BE2 tumors also showed significantly increased levels of cleaved caspase 3, which were not detected in AS tumors. These results suggest that MYCN-amplified tumors may be particularly sensitive to glutamine inhibition. To examine the mechanism of DON-induced cell death, we treated cells with the pan-caspase inhibitor QVD. QVD treatment for 48 or 72hr significantly rescued both MYCN-amplified cell lines (Kelly, IMR-32) and cells that express high levels of cMyc (SY5Y), while having minimal effect in the remaining neuroblastoma cell lines with low Myc levels. These results confirm that DON induces cell death partially by activating apoptosis in high Myc expressing cell lines. Next, we used shRNA to show that in Kelly cells DON-induced apoptosis occurs by signaling through Bax, but not Bak. Lastly, we tested DON in combination with pro-apoptotic compounds in vitro. We found that DON treatment combined with the Bcl-2 family inhibitor ABT-263 had synergistic effects on cell death in the MYCN-amplified cell lines Kelly and BE2. Our results suggest that combined targeting of glutamine metabolism while reducing the threshold for tumor cell apoptosis may be beneficial for neuroblastoma treatment. Citation Format: Rachelle R. Olsen, Michelle N. Mary-Sinclair, Zhirong Yin, Kevin W. Freeman. Targeting Myc-reprogrammed glutamine metabolism for treatment of neuroblastoma. [abstract]. In: Proceedings of the AACR Special Conference on Myc: From Biology to Therapy; Jan 7-10, 2015; La Jolla, CA. Philadelphia (PA): AACR; Mol Cancer Res 2015;13(10 Suppl):Abstract nr B37.