Targeting mitochondrial translation by inhibiting DDX3: a novel radiosensitization strategy for cancer treatment

Abstract

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.

Targeting RNA helicases in cancer: The translation trap

“…Genetic alterations in DDX3X are in stark contrast with the reports on overexpression of DDX3 in several cancers as compared to the normal tissue of origin[47]. High DDX3 expression correlated with high grade and worse overall survival in breast[48] and lung cancer[49]. DDX3 mutations were not frequently detected in genome wide mutation analyses in these cancer types.…”

“…Interestingly, DDX3, DDX5 and RHA were detected by mass spectrometry in the immunoprecipitate of mitochondriolus’ proteins GRSF1 and DDX28 after crosslinking[73, 74]. DDX3 does localize to the mitochondria[48] and inhibition of DDX3 with the small molecule inhibitor RK-33 resulted in decreased mitochondrial translation, and hereby decreased synthesis of OXPHOS complexes. As a result, decreased OXPHOS capacity and increased ROS production were observed, culminating in bioenergetic catastrophe in cancer cells[48].…”

“…DDX3 does localize to the mitochondria[48] and inhibition of DDX3 with the small molecule inhibitor RK-33 resulted in decreased mitochondrial translation, and hereby decreased synthesis of OXPHOS complexes. As a result, decreased OXPHOS capacity and increased ROS production were observed, culminating in bioenergetic catastrophe in cancer cells[48]. Future studies are needed to further evaluate the specific role of DDX3 in mitochondrial translation facilitation.…”

“…In addition, RK-33 was found to work as a radiosensitizer in a human xenograft model of prostate cancer [97]. RK-33 most likely specifically sensitizes cancer cells to radiation therapy by inhibiting double strand break repair[49] via blockage of the required increase in OXPHOS by inhibiting mitochondrial translation[48]. RK-33 also sensitized breast cancer cells to PARP inhibitors[106].…”

“…Mitochondrial mRNAs (mt-mRNA) are translated by the mitochondrial translation complex called the mitoribosome. Assembly of the 28S small and 39S large mitoribosome subunits occurs in a distinct mitochondrial area called the mitochondriolus[73], and is facilitated by DDX28[73], DHX30[74] and possibly DDX3[48]. …”