Abstract PO-022: Inhibition of the citrate carrier SLC25A1 affects repair of radiation-induced DNA damage, increases radiosensitivity, and enhances lethality of IR in combination with DNA repair defects or DSB inhibitors

Abstract Introduction: Balancing redox-homeostasis and energy metabolism allows cancer cells exposed to chemotherapy or radiotherapy to ensure DNA repair and survival. Our work revealed that adaptive changes in antioxidant defense and cancer cell metabolism in adverse environments enhance cancer cell radioresistance and create exploitable therapeutic metabolic dependencies for overcoming adaptive radioresistance1-4. Here, we aimed to decipher the mechanisms behind enhances lethality of ionizing radiation (IR) upon SLC25A1-inhibition (SLC25A1i) beyond disturbance of mitochondrial metabolism. Therefore, we explored a potential impact of SLC25A1i on the suggested accumulation of D-2-hydroxyglutarate (D2HG) and the repair of IR-induced lethal DNA lesions with a focus on homologous recombination (HR) repair. We hypothesized that metabolic disturbance of HR by SLC25A1i might offer opportunities for context-dependent lethality approaches. Methods: We used genetic (SLC25A1 RNAi) or pharmacologic SLC25A1i (small molecule SLC25A1 inhibitor CTPi2) to determine the effects of SLC25A1i alone or in combination with IR on 2HG-accumulation, cellular function, radiosensitivity and DNA repair kinetics in lung (A549, NCI-H460) and glioblastoma (U87-MG, T98G) cell lines. Isogenic MEF cell lines with defects in HR repair or non-homologous end joining (NHEJ) DSB repair were used to explore potential differences in the radiosensitizing effects of SLC25A1i in cells with defects in specific DSB repair pathways. Finally, we analyzed the potential of SLC25A1i to induce context-dependent lethality of IR with clinically relevant inhibitors of poly(ADP-ribose)-polymerase (PARP) or of the catalytic subunit of DNA-dependent protein kinase (DNA-PKcs) in cancer cells. Results: Genetic and pharmacologic SLC25A1i increased D2HG-levels, and sensitized lung cancer and glioblastoma cells to the cytotoxic action of IR. Radiosensitization was abrogated in MEFs with a HR defect. Accumulation of D2HG was associated with increased DNA damage and delayed resolution of IR-induced gH2AX and Rad51 foci. Combining SLC25A1i with PARP- or DNA-PKcs-inhibitors further delayed DNA repair kinetics of IR-induced DNA damage and enhanced the radiation-induced eradication of clonogenic tumor cells. Conclusion: The metabolic effects of SLC25A1i involve disturbance of energy metabolism and inhibition of HR execution. SLC25A1i increases the efficacy of IR in cancer cells but not in MEFs with a defect in HR. SLC25A1i might offer opportunities for the metabolic induction of HRness and thus increased lethality of radiotherapy in cells with defects in end-joining pathways or in combination with clinically relevant DSB repair inhibitors in cancer cells without or weak HR defects. 1 Matschke et al., Antioxid Redox Signal 2016,25:89-107, 2 Matschke et al., Radiat Oncol 2016,11(1):75, 3 Hlouschek et al., Cancer Lett. 2018 Dec 28;439:24-38, 4 Hlouschek et al., Front Oncol. 2018 May 25;8:170 Supported by grants of the DFG (GRK1739/2, MA 8970/1-1) and Mildred Scheel-Stiftung (70112711). Citation Format: Christian Kalthoff, Kexu Xiang, Corinna Muench, Julian Hlouschek, Verena Jendrossek, Johann Matschke. Inhibition of the citrate carrier SLC25A1 affects repair of radiation-induced DNA damage, increases radiosensitivity, and enhances lethality of IR in combination with DNA repair defects or DSB inhibitors [abstract]. In: Proceedings of the AACR Virtual Special Conference on Radiation Science and Medicine; 2021 Mar 2-3. Philadelphia (PA): AACR; Clin Cancer Res 2021;27(8_Suppl):Abstract nr PO-022.