SUMMARYWe described previously a screening protocol in Drosophila melanogaster that allows us to identify small molecules that increase the killing effect of ionizing radiation in vivo in a multicellular context. The ability of this screen to identify agents that enhance the effect of radiation in human cancer models has been validated in published proof-of-concept studies. Here we describe an agent, identified by screening through two National Cancer Institute (NCI) small molecule libraries in Drosophila, that increases the effect of radiation. This agent, Bouvardin (NSC 259968), inhibits the elongation step of protein synthesis. We find that Bouvardin enhances the killing effect of X-rays in both Drosophila larvae and in human cancer cells. More detailed analysis showed that Bouvardin also increases the effect of radiation in clonogenic assays and in human cancer xenografts in mice. Finally, we present data that Bouvardin can also increase the efficacy of taxol. Regulation of translation is important to cancer biology. Current therapies target every aspect of cancer cell proliferation from growth factor signaling to cell division, with the exception of translation elongation. Our identification of Bouvardin as an enhancer of radio- and chemo-therapeutic agents suggests that targeting this niche has the potential to improve existing cancer therapies.

Protein synthesis is essential for growth, proliferation and survival of cells. Translation factors are overexpressed in many cancers and in preclinical models, their experimental inhibition has been shown to inhibit cancer growth. Differential regulation of translation also occurs upon exposure to cancer-relevant stressors such as hypoxia and ionizing radiation. The failure to regulate translation has been shown to interfere with recovery after genotoxic stress. These findings suggest that modulation of translation, alone or in conjunction with genotoxins, may be therapeutic in oncology. Yet, only two drugs that directly inhibit translation are FDA-approved for oncology therapies used today. We have previously identified the protein synthesis inhibitor, bouvardin in a screen for small molecule enhancers of ionizing radiation in Drosophila melanogaster. Bouvardin was independently identified in a screen for selective inhibitors of engineered human breast cancer stem cells. Here we report the effect of bouvardin treatment in preclinical models of head and neck cancer (HNC) and glioma, two cancer types for which radiation therapy is the most common treatment. Our data show that bouvardin treatment blocked translation elongation on human ribosomes and suggest that it did so by blocking the dissociation of elongation factor 2 from the ribosome. Bouvardin and radiation enhanced the induction of clonogenic death in HNC and glioma cells, although by different mechanisms. Bouvardin treatment enhanced the radiation-induced antitumor effects in HNC tumor xenografts in mice. These data suggest that inhibition of translation elongation, particularly in combination with radiation treatment, may be a promising treatment option for cancer.

In this review, we will summarize the data from different model systems that illustrate the need for proteome-wide analyses of the biological consequences of ionizing radiation (IR). IR remains one of three main therapy choices for oncology, the others being surgery and chemotherapy. Understanding how cells and tissues respond to IR is essential for improving therapeutic regimes against cancer. Numerous studies demonstrating the changes in the transcriptome following exposure to IR, in diverse systems, can be found in the scientific literature. However, the limitation of our knowledge is illustrated by the fact that the number of transcripts that change after IR exposure is approximately an order of magnitude lower than the number of transcripts that re-localize to or from ribosomes under similar conditions. Furthermore, changes in the post-translational modifications of proteins (phosphorylation, acetylation as well as degradation) are profoundly important for the cellular response to IR. These considerations make proteomics a highly suitable tool for mechanistic studies of the effect of IR. Strikingly such studies remain outnumbered by those utilizing proteomics for diagnostic purposes such as the identification of biomarkers for the outcome of radiation therapy. Here we will discuss the role of the ribosome and translational regulation in the survival and preservation of cells and tissues after exposure to ionizing radiation. In doing so we hope to provide a strong incentive for the study of proteome-wide changes following IR exposure.

An emerging interest in oncology is to tailor treatment to particular cancer genotypes, i.e. oncogenic mutations present in the tumor, and not the tissue of cancer incidence. Integral to such a practice is the idea that the same oncogenic mutation(s) produces similar outcomes in different tissues. To test this idea experimentally, we studied tumors driven by a combination of RasV12 and scrib1 mutations in Drosophila larvae. We found that tumors induced in tissues of neural ectodermal and mesodermal origins behaved similarly in every manner examined: cell cycle checkpoints, apoptosis, cellular morphology, increased aneuploidy and response to Taxol. We conclude that oncogenic effects override tissue-specific differences, at least for the mutations, tissues, and phenotypes studied herein.