Abstract 2497: Inhibitors of RNA Pol I and an aminoacyl-tRNA synthetase result in MYC and MYCN downregulation in MYC family protein-driven neuroblastoma cells

Abstract

Abstract We have previously reported histologic variants among unfavorable histology neuroblastomas (NB), having large cell appearance with vesicular nuclei and or prominent nucleolar (PN) formation, indicating hyperactive rRNA synthesis and protein translation. These features appear to be associated with dismal outcome and high-level MYC family protein expression. PN formation could provide critical prognostic and risk-stratification information for NB. At the same time, this observation could open up an opportunity for innovative therapy for aggressive NB. To assess this possibility, we examined the effect of two potent and specific small-molecule inhibitors of RNA Pol I activity and an aminoacyl tRNA synthetase (CX-5461 and Halofuginone, respectively) on growth and MYC family protein expression in NB cell lines. These small-molecule inhibitors inhibited growth of NB cell lines at low to submicromolar concentrations in vitro in 48 hours. Moreover, the inhibitors destabilized MYC and MYCN proteins in NB cells. Interestingly, halofuginone showed a rapid effect on the stability of MYC family proteins at 500 nM-1 uM (<3 hours). In contrast, CX-5461 at 1 uM was less effective at the 3-hour time. At the concentration of 100 nM-250 nM for halofuginone and 1 uM for CX-5461 for up to 48 hours of the drug treatments, MYC and MYCN expression were both downregulated. We further examined the preclinical efficacy of halofuginone in orthotopic Kelly human NB xenografts in mice using a silk film as the drug delivery device. Halofuginone showed a significant growth-suppressive effect on the xenografts, as indicated by the time periods to reach certain tumor sizes being significantly longer in the halofuginone-treated group than those in the control group. Moreover, histologic examination of the xenografts showed a marked suppression of MYCN protein expression in the halofuginone-treated xenograft at day 17 of the drug treatment. Specific inhibition of hyperactive rRNA synthesis and protein translation was shown to be an effective way to suppress MYC/MYCN protein expression and NB growth. Together, MYC-family protein overexpression and PN formation should be included in new NB risk stratification and considered for potential therapeutic targets. Citation Format: Naohiko Ikegaki, Jasmine Zeki, Bill Chiu, Hiroyuki Shimada. Inhibitors of RNA Pol I and an aminoacyl-tRNA synthetase result in MYC and MYCN downregulation in MYC family protein-driven neuroblastoma cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 2497.