Abstract 1139: Translational control of MYC protein stability as a target of small molecules

It is well established that MYC family proteins drive tumor growth and sustain the malignant phenotype. However, MYC family proteins have gained the reputation of being “un-druggable” by small molecule inhibitors. This is because MYC proteins lack intrinsic enzymatic activities or appropriate molecular grooves to which small molecules may bind and inhibit their function. Nonetheless, previous studies suggest that MYC family proteins (MYC, MYCN, MYCL) can be “targetable” indirectly by accelerating their turnover rate at different biochemical pathways that regulate the steady state level of the proteins. These include transcriptional-inhibition of the corresponding genes and acceleration of their proteasome-dependent degradation. The goal of this study is to gain a better understanding of how the MYC family protein turnover is regulated, which in turn might help develop future new drugs against “MYC family-driven” cancers. We have used neuroblastoma, a childhood cancer, as a disease model and identified several small molecules that rapidly destabilize MYC and MYCN proteins in MYC-driven neuroblastoma cells. The small molecules include FCCP, OSU-03012 and Salinomycin. Preliminary data suggest that the common target of these compounds is mitochondria. Recently, we have also found Halofuginone, an FDA approved orphan drug for Scleroderma treatment exhibits a similar MYC-destabilizing activity. At submicromolar concentrations and less than one hour of drug-treatment, Halofuginone rapidly causes significant down-regulation of MYC proteins in neuroblastoma cells. Halofuginone is known to activate the eIF2α pathway, which controls translation. Activation of the eIF2α pathway is initiated by the activation of eIF2a kinases (GCN2, HRI, PERK, PKR) via autophosphorylation by various stress-related stimuli such as reactive oxygen species and misfolded proteins. The activated eIF2α kinases then inactivates eIF2α by phosphorylation, which subsequently halts translation globally. Interestingly, FCCP, OSU-03012 and Salinomycin treatments also caused eIF2α phosphorylation. These observations suggest that activation of the eIF2α pathway results in translational block of the MYC family protein expression in tumor cells. Because half-lives of MYC family proteins are very short, a sudden attenuation in protein translation can cause rapid down-regulation of the proteins. Hence, activation of the eIF2α pathway by small molecules may prove effective for rapid down-regulation of MYC family proteins in MYC-driven cancer cells. Citation Format: Naohiko Ikegaki, Luqman Baloch, Christopher Chu, Joshua Lomahan, Jamie Harris, Bill Chiu. Translational control of MYC protein stability as a target of small molecules. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 1139.