Oncogenic mutations in metabolic genes and associated oncometabolite accumulation support cancer progression but can also restrict cellular functions needed to cope with DNA damage. For example, gain-of-function mutations in isocitrate dehydrogenase (IDH) and the resulting accumulation of the oncometabolite D-2-hydroxyglutarate (D-2-HG) enhanced the sensitivity of cancer cells to inhibition of poly(ADP-ribose)-polymerase (PARP)1 and radiotherapy (RT). In our hand, inhibition of the mitochondrial citrate transport protein (SLC25A1) enhanced radiosensitivity of cancer cells and this was associated with increased levels of D-2-HG and a delayed repair of radiation-induced DNA damage. Here we aimed to explore the suggested contribution of D-2-HG-accumulation to disturbance of DNA repair, presumably homologous recombination (HR) repair, and enhanced radiosensitivity of cancer cells with impaired SLC25A1 function. Genetic and pharmacologic inhibition of SLC25A1 (SLC25A1i) increased D-2-HG-levels and sensitized lung cancer and glioblastoma cells to the cytotoxic action of ionizing radiation (IR). SLC25A1i-mediated radiosensitization was abrogated in MEFs with a HR-defect. D-2-HG-accumulation was associated with increased DNA damage and delayed resolution of IR-induced γH2AX and Rad51 foci. Combining SLC25A1i with PARP- or the catalytic subunit of DNA-dependent protein kinase (DNA-PKcs)-inhibitors further potentiated IR-induced DNA damage, delayed DNA repair kinetics resulting in radiosensitization of cancer cells. Importantly, proof of concept experiments revealed that combining SLC25A1i with IR without and with PARPi also reduced tumor growth in the chorioallantoic membrane (CAM) model in vivo. Thereby SLC25A1i offers an innovative strategy for metabolic induction of context-dependent lethality approaches in combination with RT and clinically relevant inhibitors of complementary DNA repair pathways.

Radiotherapy (RT) is applied in 45-60% of all cancer patients either alone or in multimodal therapy concepts comprising surgery, RT and chemotherapy. However, despite technical innovations approximately only 50% are cured, highlight a high medical need for innovation in RT practice. RT is a multidisciplinary treatment involving medicine and physics, but has always been successful in integrating emerging novel concepts from cancer and radiation biology for improving therapy outcome. Currently, substantial improvements are expected from integration of precision medicine approaches into RT concepts. Altered metabolism is an important feature of cancer cells and a driving force for malignant progression. Proper metabolic processes are essential to maintain and drive all energy-demanding cellular processes, e.g. repair of DNA double-strand breaks (DSBs). Consequently, metabolic bottlenecks might allow therapeutic intervention in cancer patients. Increasing evidence now indicates that oncogenic activation of metabolic enzymes, oncogenic activities of mutated metabolic enzymes, or adverse conditions in the tumor microenvironment can result in abnormal production of metabolites promoting cancer progression, e.g. 2-hyroxyglutarate (2-HG), succinate and fumarate, respectively. Interestingly, these so-called "oncometabolites" not only modulate cell signaling but also impact the response of cancer cells to chemotherapy and RT, presumably by epigenetic modulation of DNA repair. Here we aimed to introduce the biological basis of oncometabolite production and of their actions on epigenetic regulation of DNA repair. Furthermore, the review will highlight innovative therapeutic opportunities arising from the interaction of oncometabolites with DNA repair regulation for specifically enhancing the therapeutic effects of genotoxic treatments including RT in cancer patients.

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Tumor hypoxia and hypoxic adaptation of cancer cells represent major barriers to successful cancer treatment. We revealed that improved antioxidant capacity contributes to increased radioresistance of cancer cells with tolerance to chronic-cycling severe hypoxia/reoxygenation stress. We hypothesized, that the improved tolerance to oxidative stress will increase the ability of cancer cells to cope with ROS-induced damage to free deoxy-nucleotides (dNTPs) required for DNA replication and may thus contribute to acquired resistance of cancer cells in advanced tumors to antineoplastic agents inhibiting the nucleotide-sanitizing enzyme MutT Homologue-1 (MTH1), ionizing radiation (IR) or both. Therefore, we aimed to explore potential differences in the sensitivity of cancer cells exposed to acute and chronic-cycling hypoxia/reoxygenation stress to the clinically relevant MTH1-inhibitor TH1579 (Karonudib) and to test whether a multi-targeting approach combining the glutathione withdrawer piperlongumine (PLN) and TH1579 may be suited to increase cancer cell sensitivity to TH1579 alone and in combination with IR. Combination of TH1579 treatment with radiotherapy (RT) led to radiosensitization but was not able to counteract increased radioresistance induced by adaptation to chronic-cycling hypoxia/reoxygenation stress. Disruption of redox homeostasis using PLN sensitized anoxia-tolerant cancer cells to MTH1 inhibition by TH1579 under both normoxic and acute hypoxic treatment conditions. Thus, we uncover a glutathione-driven compensatory resistance mechanism towards MTH1-inhibition in form of increased antioxidant capacity as a consequence of microenvironmental or therapeutic stress.

Metabolic rewiring is the result of the growing demands and proliferation of cancer cells, leading to alterations of biological activities and reactions to treatments of cancer cells. The mitochondrial citrate transport protein SLC25A1 is involved in metabolic reprogramming offering a strategy to induce metabolic bottlenecks with relevance for radiosensitization by accumulation of the oncometabolite D-2-hydroxyglutarate (D-2HG) upon SLC25A1 inhibition (SLC25A1i). Previous studies have uncovered the comparative effects of SLC25A1i or cell permeable D-2HG (octyl-D-2HG) treatments on DNA damage induction and repair as well as on energy metabolism and cellular function crucial for long-term survival of irradiated cells. Here, α-ketoglutarate (αKG), the precursor of D-2HG, potentiated the effects observed upon SLC25A1i on DNA damage repair, cell function and long-term survival in vitro and in vivo, thus making NCI-H460 cancer cells more vulnerable to ionizing radiation. However, αKG treatment alone could hardly exert its effect on the respective phenotypes. Additionally, nicotinamide (NAM) supplementation, as the precursor of NAD (including NAD+ and NADH), was counteracting the effects of SLC25A1i or the combination of SLC25A1i with αKG, emphasizing a potential importance of NAD+/NADH equilibrium on cellular activities relevant for survival of irradiated cancer cells upon SLC25A1i. Furthermore, inhibition of histone lysine demethylases (KDMs), as a major factor affected upon SLC25A1i, by JIB04 treatment alone or in combination with αKG supplementation phenocopied the wide-ranging effects on mitochondrial and cellular function induced upon SLC25A1i. Collectively, αKG-supplementation potentiated the effects on cellular processes observed upon SLC25A1i and increased cellular need for NAD to re-balance the cellular state and ensure survival upon irradiation. Future studies will uncover the underlying metabolic reprogramming induced by SLC25A1i offering novel therapeutic strategies for cancer treatment.