Metabolism of cancer cells commonly responds to irradiation by a transient early mitochondrial shutdown

Extracellular release of damaged mitochondria induced by prehematopoietic stem cell transplant conditioning exacerbates GVHD

ABSTRACT

Despite therapeutic advancements, graft-versus-host disease (GVHD) is a major complication of hematopoietic stem cell transplantation (HSCT). In current models of GVHD, tissue injury induced by cytotoxic conditioning regimens, along with translocation of microbes expressing pathogen-associated molecular patterns, result in activation of host antigen-presenting cells (APCs) to stimulate alloreactive donor T lymphocytes. Recent studies have demonstrated that in many pathologic states, tissue injury results in the release of mitochondria from the cytoplasm to the extracellular space. We hypothesized that extracellular mitochondria, which are related to archaebacteria, could also trigger GVHD by stimulation of host APCs. We found that clinically relevant doses of radiation or busulfan induced extracellular release of mitochondria by various cell types, including cultured intestinal epithelial cells. Conditioning-mediated mitochondrial release was associated with mitochondrial damage and impaired quality control but did not affect the viability of the cells. Extracellular mitochondria directly stimulated host APCs to express higher levels of major histocompatibility complex II (MHC-II), costimulatory CD86, and proinflammatory cytokines, resulting in increased donor T-cell activation, and proliferation in mixed lymphocyte reactions. Analyses of plasma from both experimental mice and a cohort of children undergoing HSCT demonstrated that conditioning induced extracellular mitochondrial release in vivo. In mice undergoing MHC-mismatched HSCT, administration of purified syngeneic extracellular mitochondria increased host APC activation and exacerbated GVHD. Our data suggest that pre-HSCT conditioning results in extracellular release of damaged mitochondria, which increase alloreactivity and exacerbate GVHD. Therefore, decreasing the extracellular release of damaged mitochondria after conditioning could serve as a novel strategy for GVHD prevention.

Metabolic reprogramming-driven homologous recombination and TCA cycle dysregulation contribute to poor prognoses in lung adenocarcinoma

Increasing evidence has shown that homologous recombination (HR) and metabolic reprogramming are essential for cellular homeostasis. These two processes are independent as well as closely intertwined. Nevertheless, they have rarely been reported in lung adenocarcinoma (LUAD). We analysed the genomic, immune microenvironment and metabolic microenvironment features under different HR activity states. Using cell cycle, EDU and cell invasion assays, we determined the impacts of si-SHFM1 on the LUAD cell cycle, proliferation and invasion. The levels of isocitrate dehydrogenase (IDH) and α-ketoglutarate dehydrogenase (α-KGDH) were determined by ELISA in the NC and si-SHFM1 groups of A549 cells. Finally, cell samples were used to extract metabolites for HPIC-MS/MS to analyse central carbon metabolism. We found that high HR activity was associated with a poor prognosis in LUAD, and HR was an independent prognostic factor for TCGA-LUAD patients. Moreover, LUAD samples with a high HR activity presented low immune infiltration levels, a high degree of genomic instability, a good response status to immune checkpoint blockade therapy and a high degree of drug sensitivity. The si-SHFM1 group presented a significantly higher proportion of cells in the G0/G1 phase, lower levels of DNA replication, and significantly lower levels of cell migration and both TCA enzymes. Our current results indicated that there is a strong correlation between HR and the TCA cycle in LUAD. The TCA cycle can promote SHFM1-mediated HR in LUAD, raising their activities, which can finally result in a poor prognosis and impair immunotherapeutic efficacy.

NF-κB in the Radiation Response of A549 Non-Small Cell Lung Cancer Cells to X-rays and Carbon Ions under Hypoxia

Cellular hypoxia, detectable in up to 80% of non-small cell lung carcinoma (NSCLC) tumors, is a known cause of radioresistance. High linear energy transfer (LET) particle radiation might be effective in the treatment of hypoxic solid tumors, including NSCLC. Cellular hypoxia can activate nuclear factor κB (NF-κB), which can modulate radioresistance by influencing cancer cell survival. The effect of high-LET radiation on NF-κB activation in hypoxic NSCLC cells is unclear. Therefore, we compared the effect of low (X-rays)- and high (12C)-LET radiation on NF-κB responsive genes' upregulation, as well as its target cytokines' synthesis in normoxic and hypoxic A549 NSCLC cells. The cells were incubated under normoxia (20% O2) or hypoxia (1% O2) for 48 h, followed by irradiation with 8 Gy X-rays or 12C ions, maintaining the oxygen conditions until fixation or lysis. Regulation of NF-κB responsive genes was evaluated by mRNA sequencing. Secretion of NF-κB target cytokines, IL-6 and IL-8, was quantified by ELISA. A greater fold change increase in expression of NF-κB target genes in A549 cells following exposure to 12C ions compared to X-rays was observed, regardless of oxygenation status. These genes regulate cell migration, cell cycle, and cell survival. A greater number of NF-κB target genes was activated under hypoxia, regardless of irradiation status. These genes regulate cell migration, survival, proliferation, and inflammation. X-ray exposure under hypoxia additionally upregulated NF-κB target genes modulating immunosurveillance and epithelial-mesenchymal transition (EMT). Increased IL-6 and IL-8 secretion under hypoxia confirmed NF-κB-mediated expression of pro-inflammatory genes. Therefore, radiotherapy, particularly with X-rays, may increase tumor invasiveness in surviving hypoxic A549 cells.

AMPK-mediated CD47 H3K4 methylation promotes phagocytosis evasion of glioma stem cells post-radiotherapy

Radiotherapy alters the tumor microenvironment and reprograms cellular metabolism. Transition of tumor cell phenotypes contributes to post-radiotherapy tumor recurrence. Low radiosensitivity of glioma stem cells is one of the reasons for radiotherapy failure. Here, we found that radiotherapy resulted in a higher proportion of infiltration of inflammatory macrophages in glioma non-stem cell grafts compared with that in glioma stem cell-transplanted tumors in a mouse model, where immunosuppressive macrophages dominated in the tumor microenvironment. In radioresistant glioma stem cells, ionizing radiation upregulated CD47 expression by AMP-activated protein kinase (AMPK), resulting in the inhibition of phagocytosis and the promotion of M2-like polarization in macrophages. Ionizing radiation promoted H3K4 methylation on CD47 promotor by downregulating KDM5A. Hyper-phosphorylated retinoblastoma protein RB maintained its dissociation status with KDM5A following AMPK activation, which inhibited the demethylated function of KDM5A. In contrast, in radiosensitive glioma non-stem cells, RB S807/S811 hypo-phosphorylation contributed to the binding of RB with KDM5A, which suppressed H3K4 methylation on CD47 promotor. In addition, ionizing radiation promoted H3K27 acetylation on CD47 promotor by HDAC7 in glioma stem cells. These data suggested that glioma stem cells reprogrammed the tumor immune microenvironment by epigenetic editing to escape macrophage phagocytosis after ionizing radiation. Targeting CD47 might be a potential strategy to sensitize glioblastoma to radiotherapy.

Inhibitors of Rho/MRTF/SRF Transcription Pathway Regulate Mitochondrial Function

RhoA-regulated gene transcription by serum response factor (SRF) and its transcriptional cofactor myocardin-related transcription factors (MRTFs) signaling pathway has emerged as a promising therapeutic target for pharmacological intervention in multiple diseases. Altered mitochondrial metabolism is one of the major hallmarks of cancer, therefore, this upregulation is a vulnerability that can be targeted with Rho/MRTF/SRF inhibitors. Recent advances identified a novel series of oxadiazole-thioether compounds that disrupt the SRF transcription, however, the direct molecular target of these compounds is unclear. Herein, we demonstrate the Rho/MRTF/SRF inhibition mechanism of CCG-203971 and CCG-232601 in normal cell lines of human lung fibroblasts and mouse myoblasts. Further studies investigated the role of these molecules in targeting mitochondrial function. We have shown that these molecules hyperacetylate histone H4K12 and H4K16 and regulate the genes involved in mitochondrial function and dynamics. These small molecule inhibitors regulate mitochondrial function as a compensatory mechanism by repressing oxidative phosphorylation and increasing glycolysis. Our data suggest that these CCG molecules are effective in inhibiting all the complexes of mitochondrial electron transport chains and further inducing oxidative stress. Therefore, our present findings highlight the therapeutic potential of CCG-203971 and CCG-232601, which may prove to be a promising approach to target aberrant bioenergetics.

Acute Hypoxia Does Not Alter Tumor Sensitivity to FLASH Radiation Therapy

PURPOSE

Tumor hypoxia is a major cause of treatment resistance, especially to radiation therapy at conventional dose rate (CONV), and we wanted to assess whether hypoxia does alter tumor sensitivity to FLASH.

METHODS AND MATERIALS

We engrafted several tumor types (glioblastoma [GBM], head and neck cancer, and lung adenocarcinoma) subcutaneously in mice to provide a reliable and rigorous way to modulate oxygen supply via vascular clamping or carbogen breathing. We irradiated tumors using a single 20-Gy fraction at either CONV or FLASH, measured oxygen tension, monitored tumor growth, and sampled tumors for bulk RNAseq and pimonidazole analysis. Next, we inhibited glycolysis with trametinib in GBM tumors to enhance FLASH efficacy.

RESULTS

Using various subcutaneous tumor models, and in contrast to CONV, FLASH retained antitumor efficacy under acute hypoxia. These findings show that in addition to normal tissue sparing, FLASH could overcome hypoxia-mediated tumor resistance. Follow-up molecular analysis using RNAseq profiling uncovered a FLASH-specific profile in human GBM that involved cell-cycle arrest, decreased ribosomal biogenesis, and a switch from oxidative phosphorylation to glycolysis. Glycolysis inhibition by trametinib enhanced FLASH efficacy in both normal and clamped conditions.

CONCLUSIONS

These data provide new and specific insights showing the efficacy of FLASH in a radiation-resistant context, proving an additional benefit of FLASH over CONV.

α-Ketoglutarate supplementation and NAD+ modulation enhance metabolic rewiring and radiosensitization in SLC25A1 inhibited cancer cells

Metabolic rewiring is the result of the increasing demands and proliferation of cancer cells, leading to changes in the biological activities and responses to treatment of cancer cells. The mitochondrial citrate transport protein SLC25A1 is involved in metabolic reprogramming offering a strategy to induce metabolic bottlenecks relevant to radiosensitization through the accumulation of the oncometabolite D-2-hydroxyglutarate (D-2HG) upon SLC25A1 inhibition (SLC25A1i). Previous studies have revealed 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, which are crucial for the 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, rendering NCI-H460 cancer cells more vulnerable to ionizing radiation. However, αKG treatment alone had little effect on these phenotypes. In addition, supplementation with nicotinamide (NAM), a precursor of NAD (including NAD+ and NADH), counteracted the effects of SLC25A1i or the combination of SLC25A1i with αKG, highlighting a potential importance of the NAD+/NADH balance on cellular activities relevant to the 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 broad effects on mitochondrial and cellular function induced by SLC25A1i. Taken together, αKG supplementation potentiated the effects on cellular processes observed upon SLC25A1i and increased the cellular demand for NAD to rebalance the cellular state and ensure survival after irradiation. Future studies will elucidate the underlying metabolic reprogramming induced by SLC25A1i and provide novel therapeutic strategies for cancer treatment.

Hypoxia Modulates Radiosensitivity and Response to Different Radiation Qualities in A549 Non-Small Cell Lung Cancer (NSCLC) Cells

Hypoxia-induced radioresistance reduces the efficacy of radiotherapy for solid malignancies, including non-small cell lung cancer (NSCLC). Cellular hypoxia can confer radioresistance through cellular and tumor micro-environment adaptations. Until recently, studies evaluating radioresistance secondary to hypoxia were designed to maintain cellular hypoxia only before and during irradiation, while any handling of post-irradiated cells was carried out in standard oxic conditions due to the unavailability of hypoxia workstations. This limited the possibility of simulating in vivo or clinical conditions in vitro. The presence of molecular oxygen is more important for the radiotoxicity of low-linear energy transfer (LET) radiation (e.g., X-rays) than that of high-LET carbon (12C) ions. The mechanisms responsible for 12C ions' potential to overcome hypoxia-induced radioresistance are currently not fully understood. Therefore, the radioresistance of hypoxic A549 NSCLC cells following exposure to X-rays or 12C ions was investigated along with cell cycle progression and gene expression by maintaining hypoxia before, during and after irradiation. A549 cells were incubated under normoxia (20% O2) or hypoxia (1% O2) for 48 h and then irradiated with X-rays (200 kV) or 12C ions (35 MeV/n, LET ~75 keV/µm). Cell survival was evaluated using colony-forming ability (CFA) assays immediately or 24 h after irradiation (late plating). DNA double-strand breaks (DSBs) were analyzed using γH2AX immunofluorescence microscopy. Cell cycle progression was determined by flow cytometry of 4',6-diamidino-2-phenylindole-stained cells. The global transcription profile post-irradiation was evaluated by RNA sequencing. When hypoxia was maintained before, during and after irradiation, hypoxia-induced radioresistance was observed only in late plating CFA experiments. The killing efficiency of 12C ions was much higher than that of X-rays. Cell survival under hypoxia was affected more strongly by the timepoint of plating in the case of X-rays compared to 12C ions. Cell cycle arrest following irradiation under hypoxia was less pronounced but more prolonged. DSB induction and resolution following irradiation were not significantly different under normoxia and hypoxia. Gene expression response to irradiation primarily comprised cell cycle regulation for both radiation qualities and oxygen conditions. Several PI3K target genes involved in cell migration and cell motility were differentially upregulated in hypoxic cells. Hypoxia-induced radioresistance may be linked to altered cell cycle response to irradiation and PI3K-mediated changes in cell motility and migration in A549 cells rather than less DNA damage or faster repair.

DNA damage-induced senescence is associated with metabolomic reprogramming in breast cancer cells

Senescence due to exogenous and endogenous stresses triggers metabolic reprogramming and is associated with many pathologies, including cancer. In solid tumors, senescence promotes tumorigenesis, facilitates relapse, and changes the outcomes of anti-cancer therapies. Hence, cellular and molecular mechanisms regulating senescent pathways make attractive therapeutic targets. Cancer cells undergo metabolic reprogramming to sustain the growth-arrested state of senescence. In the present study, we aimed to understand the metabolic reprogramming in MCF-7 breast tumor cells in response to two independent inducers of DNA damage-mediated senescence, including ionizing radiation and doxorubicin. Increased DNA double-strand breaks, as demonstrated by γH2AX staining, showed a senescence phenotype, with expression of senescence-associated β-galactosidase accompanied by the upregulation of p21 and p16 in both groups. Further, untargeted analysis of the senescence-related extracellular metabolome profile of MCF-7 cells showed significantly reduced concentrations of carnitine and pantothenic acid and increased levels of S-adenosylhomocysteine in doxorubicin-treated cells, indicating the accumulation of ROS mediated DNA damage and impaired mitochondrial membrane potential. Similarly, a significant decline in the creatine level was observed in radiation-exposed cells, suggesting an increase in oxidative stress-mediated DNA damage. Our study, therefore, provides key effectors of the metabolic changes in doxorubicin and radiation-induced early senescence in MCF-7 breast cancer cells.

Cigarette smoke causes a bioenergetic crisis in RPE cells involving the downregulation of HIF-1α under normoxia

Age-related macular degeneration (AMD) is the most common blinding disease in the elderly population. However, there are still many uncertainties regarding the pathophysiology at the molecular level. Currently, impaired energy metabolism in retinal pigment epithelium (RPE) cells is discussed as one major hallmark of early AMD pathophysiology. Hypoxia-inducible factors (HIFs) are important modulators of mitochondrial function. Moreover, smoking is the most important modifiable risk factor for AMD and is known to impair mitochondrial integrity. Therefore, our aim was to establish a cell-based assay that enables us to investigate how smoking affects mitochondrial function in conjunction with HIF signaling in RPE cells. For this purpose, we treated a human RPE cell line with cigarette smoke extract (CSE) under normoxia (21% O2), hypoxia (1% O2), or by co-treatment with Roxadustat, a clinically approved HIF stabilizer. CSE treatment impaired mitochondrial integrity, involving increased mitochondrial reactive oxygen species, disruption of mitochondrial membrane potential, and altered mitochondrial morphology. Treatment effects on cell metabolism were analyzed using a Seahorse Bioanalyzer. Mitochondrial respiration and ATP production were impaired in CSE-treated cells under normoxia. Surprisingly, CSE-treated RPE cells also exhibited decreased glycolytic rate under normoxia, causing a bioenergetic crisis, because two major metabolic pathways that provide ATP were impaired by CSE. Downregulation of glycolytic rate was HIF-dependent because HIF-1α, the α-subunit of HIF-1, was downregulated by CSE on the protein level, especially under normoxia. Moreover, hypoxia incubation and treatment with Roxadustat restored glycolytic flux. Taken together, our in vitro model provides interesting insights into HIF-dependent regulation of glycolysis under normoxic conditions, which will enable us to investigate signaling pathways involved in RPE metabolism in health and disease.

Nitric Oxide-Scavenging, Anti-Migration Effects, and Glycosylation Changes after Hemin Treatment of Human Triple-Negative Breast Cancer Cells: A Mechanistic Study

The enhanced expression of nitric oxide (•NO) synthase predicts triple-negative breast cancer outcome and its resistance to different therapeutics. Our earlier work demonstrated the efficiency of hemin to scavenge the intra- and extracellular •NO, proposing its potency as a therapeutic agent for inhibiting cancer cell migration. In continuation, the present work evaluates the effects of •NO on the migration of MDA-MB-231 cells and how hemin modulates the accompanied cellular behavior, focusing on the corresponding expression of cellular glycoproteins, migration-associated markers, and mitochondrial functions. We demonstrated for the first time that while •NO induced cell migration, hemin contradicted that by •NO-scavenging. This was in combination with modulation of the •NO-enhanced glycosylation patterns of cellular proteins with inhibition of the expression of specific proteins involved in the epithelial-mesenchymal transition. These effects were in conjunction with changes in the mitochondrial functions related to both •NO, hemin, and its nitrosylated product. Together, these results suggest that hemin can be employed as a potential anti-migrating agent targeting •NO-scavenging and regulating the expression of migration-associated proteins.

Rewiring of mitochondrial metabolism in therapy-resistant cancers: permanent and plastic adaptations

Deregulation of tumor cell metabolism is widely recognized as a "hallmark of cancer." Many of the selective pressures encountered by tumor cells, such as exposure to anticancer therapies, navigation of the metastatic cascade, and communication with the tumor microenvironment, can elicit further rewiring of tumor cell metabolism. Furthermore, phenotypic plasticity has been recently appreciated as an emerging "hallmark of cancer." Mitochondria are dynamic organelles and central hubs of metabolism whose roles in cancers have been a major focus of numerous studies. Importantly, therapeutic approaches targeting mitochondria are being developed. Interestingly, both plastic (i.e., reversible) and permanent (i.e., stable) metabolic adaptations have been observed following exposure to anticancer therapeutics. Understanding the plastic or permanent nature of these mechanisms is of crucial importance for devising the initiation, duration, and sequential nature of metabolism-targeting therapies. In this review, we compare permanent and plastic mitochondrial mechanisms driving therapy resistance. We also discuss experimental models of therapy-induced metabolic adaptation, therapeutic implications for targeting permanent and plastic metabolic states, and clinical implications of metabolic adaptations. While the plasticity of metabolic adaptations can make effective therapeutic treatment challenging, understanding the mechanisms behind these plastic phenotypes may lead to promising clinical interventions that will ultimately lead to better overall care for cancer patients.

The mitochondrial protease OMA1 acts as a metabolic safeguard upon nuclear DNA damage

The metabolic plasticity of mitochondria ensures cell development, differentiation, and survival. The peptidase OMA1 regulates mitochondrial morphology via OPA1 and stress signaling via DELE1 and orchestrates tumorigenesis and cell survival in a cell- and tissue-specific manner. Here, we use unbiased systems-based approaches to show that OMA1-dependent cell survival depends on metabolic cues. A metabolism-focused CRISPR screen combined with an integrated analysis of human gene expression data found that OMA1 protects against DNA damage. Nucleotide deficiencies induced by chemotherapeutic agents promote p53-dependent apoptosis of cells lacking OMA1. The protective effect of OMA1 does not depend on OMA1 activation or OMA1-mediated OPA1 and DELE1 processing. OMA1-deficient cells show reduced glycolysis and accumulate oxidative phosphorylation (OXPHOS) proteins upon DNA damage. OXPHOS inhibition restores glycolysis and confers resistance against DNA damage. Thus, OMA1 dictates the balance between cell death and survival through the control of glucose metabolism, shedding light on its role in cancerogenesis.

Biological Mechanisms to Reduce Radioresistance and Increase the Efficacy of Radiotherapy: State of the Art

Cancer treatment with ionizing radiation (IR) is a well-established and effective clinical method to fight different types of tumors and is a palliative treatment to cure metastatic stages. Approximately half of all cancer patients undergo radiotherapy (RT) according to clinical protocols that employ two types of ionizing radiation: sparsely IR (i.e., X-rays) and densely IR (i.e., protons). Most cancer cells irradiated with therapeutic doses exhibit radio-induced cytotoxicity in terms of cell proliferation arrest and cell death by apoptosis. Nevertheless, despite the more tailored advances in RT protocols in the last few years, several tumors show a relatively high percentage of RT failure and tumor relapse due to their radioresistance. To counteract this extremely complex phenomenon and improve clinical protocols, several factors associated with radioresistance, of both a molecular and cellular nature, must be considered. Tumor genetics/epigenetics, tumor microenvironment, tumor metabolism, and the presence of non-malignant cells (i.e., fibroblast-associated cancer cells, macrophage-associated cancer cells, tumor-infiltrating lymphocytes, endothelial cells, cancer stem cells) are the main factors important in determining the tumor response to IR. Here, we attempt to provide an overview of how such factors can be taken advantage of in clinical strategies targeting radioresistant tumors.

Hypoxia aggravates ferroptosis in RPE cells by promoting the Fenton reaction

Oxidative stress and hypoxia in the retinal pigment epithelium (RPE) have long been considered major risk factors in the pathophysiology of age-related macular degeneration (AMD), but systematic investigation of the interplay between these two risk factors was lacking. For this purpose, we treated a human RPE cell line (ARPE-19) with sodium iodate (SI), an oxidative stress agent, together with dimethyloxalylglycine (DMOG) which leads to stabilization of hypoxia-inducible factors (HIFs), key regulators of cellular adaptation to hypoxic conditions. We found that HIF stabilization aggravated oxidative stress-induced cell death by SI and iron-dependent ferroptosis was identified as the main cell death mechanism. Ferroptotic cell death depends on the Fenton reaction where H₂O₂ and iron react to generate hydroxyl radicals which trigger lipid peroxidation. Our findings clearly provide evidence for superoxide dismutase (SOD) driven H₂O₂ production fostering the Fenton reaction as indicated by triggered SOD activity upon DMOG + SI treatment as well as by reduced cell death levels upon SOD2 knockdown. In addition, iron transporters involved in non-transferrin-bound Fe²⁺ import as well as intracellular iron levels were also upregulated. Consequently, chelation of Fe²⁺ by 2'2-Bipyridyl completely rescued cells. Taken together, we show for the first time that HIF stabilization under oxidative stress conditions aggravates ferroptotic cell death in RPE cells. Thus, our study provides a novel link between hypoxia, oxidative stress and iron metabolism in AMD pathophysiology. Since iron accumulation and altered iron metabolism are characteristic features of AMD retinas and RPE cells, our cell culture model is suitable for high-throughput screening of new treatment approaches against AMD.

Developing a Method to Estimate the Downstream Metabolite Signals from Hyperpolarized [1-13C]Pyruvate

Hyperpolarized carbon-13 MRI has the advantage of allowing the study of glycolytic flow in vivo or in vitro dynamically in real-time. The apparent exchange rate constant of a metabolite dynamic signal reflects the metabolite changes of a disease. Downstream metabolites can have a low signal-to-noise ratio (SNR), causing apparent exchange rate constant inconsistencies. Thus, we developed a method that estimates a more accurate metabolite signal. This method utilizes a kinetic model and background noise to estimate metabolite signals. Simulations and in vitro studies with photon-irradiated and control groups were used to evaluate the procedure. Simulated and in vitro exchange rate constants estimated using our method were compared with the raw signal values. In vitro data were also compared to the Area-Under-Curve (AUC) of the cell medium in ¹³C Nuclear Magnetic Resonance (NMR). In the simulations and in vitro experiments, our technique minimized metabolite signal fluctuations and maintained reliable apparent exchange rate constants. In addition, the apparent exchange rate constants of the metabolites showed differences between the irradiation and control groups after using our method. Comparing the in vitro results obtained using our method and NMR, both solutions showed consistency when uncertainty was considered, demonstrating that our method can accurately measure metabolite signals and show how glycolytic flow changes. The method enhanced the signals of the metabolites and clarified the metabolic phenotyping of tumor cells, which could benefit personalized health care and patient stratification in the future.

Accumulation of oncometabolite D-2-Hydroxyglutarate by SLC25A1 inhibition: A metabolic strategy for induction of HR-ness and radiosensitivity

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.

Targeting AKT-Dependent Regulation of Antioxidant Defense Sensitizes AKT-E17K Expressing Cancer Cells to Ionizing Radiation

Aberrant activation of the phosphatidyl-inositol-3-kinase/protein kinase B (AKT) pathway has clinical relevance to radiation resistance, but the underlying mechanisms are incompletely understood. Protection against reactive oxygen species (ROS) plays an emerging role in the regulation of cell survival upon irradiation. AKT-dependent signaling participates in the regulation of cellular antioxidant defense. Here, we were interested to explore a yet unknown role of aberrant activation of AKT in regulating antioxidant defense in response to IR and associated radiation resistance.

We combined genetic and pharmacologic approaches to study how aberrant activation of AKT impacts cell metabolism, antioxidant defense, and radiosensitivity. Therefore, we used TRAMPC1 (TrC1) prostate cancer cells overexpressing the clinically relevant AKT-variant AKT-E17K with increased AKT activity or wildtype AKT (AKT-WT) and analyzed the consequences of direct AKT inhibition (MK2206) and inhibition of AKT-dependent metabolic enzymes on the levels of cellular ROS, antioxidant capacity, metabolic state, short-term and long-term survival without and with irradiation.

TrC1 cells expressing the clinically relevant AKT1-E17K variant were characterized by improved antioxidant defense compared to TrC1 AKT-WT cells and this was associated with increased radiation resistance. The underlying mechanisms involved AKT-dependent direct and indirect regulation of cellular levels of reduced glutathione (GSH). Pharmacologic inhibition of specific AKT-dependent metabolic enzymes supporting defense against oxidative stress, e.g., inhibition of glutathione synthase and glutathione reductase, improved eradication of clonogenic tumor cells, particularly of TrC1 cells overexpressing AKT-E17K.

We conclude that improved capacity of TrC1 AKT-E17K cells to balance antioxidant defense with provision of energy and other metabolites upon irradiation compared to TrC1 AKT-WT cells contributes to their increased radiation resistance. Our findings on the importance of glutathione de novo synthesis and glutathione regeneration for radiation resistance of TrC1 AKT-E17K cells offer novel perspectives for improving radiosensitivity in cancer cells with aberrant AKT activity by combining IR with inhibitors targeting AKT-dependent regulation of GSH provision.