DDX3 is a DEAD box RNA helicase with oncogenic properties. RK-33 is developed as a small-molecule inhibitor of DDX3 and showed potent radiosensitizing activity in preclinical tumor models. This study aimed to assess DDX3 as a target in breast cancer and to elucidate how RK-33 exerts its anti-neoplastic effects. High DDX3 expression was present in 35% of breast cancer patient samples and correlated with markers of aggressiveness and shorter survival. With a quantitative proteomics approach, we identified proteins involved in the mitochondrial translation and respiratory electron transport pathways to be significantly downregulated after RK-33 or DDX3 knockdown. DDX3 localized to the mitochondria and DDX3 inhibition with RK-33 reduced mitochondrial translation. As a consequence, oxygen consumption rates and intracellular ATP concentrations decreased and reactive oxygen species (ROS) increased. RK-33 antagonized the increase in oxygen consumption and ATP production observed after exposure to ionizing radiation and reduced DNA repair. Overall, we conclude that DDX3 inhibition with RK-33 causes radiosensitization in breast cancer through inhibition of mitochondrial translation, which results in reduced oxidative phosphorylation capacity and increased ROS levels, culminating in a bioenergetic catastrophe.
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We thank Bob Cole, Tatiana Boronina and Bob O’ Meally of the Johns Hopkins Mass Spectrometry and Proteomics core facility for their help with the proteomics experiments; Tri Nguyen for his help with interpretation of the electron microscopy images; the Dawson Laboratory at the Johns Hopkins School of Medicine for their help with the mitochondrial translation assay; and Beth Rodgers, who kindly provided us with S35-methionine. This work was financially supported by Utrecht University Alexandre Suerman Stipend (MRHvV), the Dutch Cancer Foundation (UU2013-5851; MRHvV), NIH (R01CA166348 to PTT, R01CA193895 to AL, and R01CA140226 and R01CA131250 to VR), FAMRI (VR) and Safeway (VR).
VR and PTT have received a patent for the use of RK-33 as a radiosensitizer (US 8,518,901). VR, GMB and PJvD have received a patent for the use of DDX3 as a cancer biomarker (US 9,322,831). PJvD, PTT and VR are on the advisory board of Natsar Pharmaceuticals.
Supplementary Information accompanies this paper on the Oncogene website
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Heerma van Voss, M., Vesuna, F., Bol, G. et al. Targeting mitochondrial translation by inhibiting DDX3: a novel radiosensitization strategy for cancer treatment. Oncogene 37, 63–74 (2018). https://doi.org/10.1038/onc.2017.308
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