Metabolic Constraints Limit DNA Repair Following Radiation Exposure
Abstract
Purpose
Radiotherapy is central in lung cancer treatment, but many tumors are intrinsically radioresistant. A mechanism of radioresistance in lung cancer is mutations in the KEAP1 gene, which is present in a significant number (15-20%) of lung cancers. This leads to constitutive activation of cellular stress-defense system that mitigates radiation-induced damage. Although this protects the tumor, it also creates a strong dependence on glutamine to maintain redox balance and energy production. This metabolic defect provides an opportunity to enhance radiation response in radioresistant KEAP1 mutated cancers.
Methods
We tested whether blocking glutamine metabolism could sensitize KEAP1-mutant lung cancer cells (H460, A549) to radiation. Cells were treated with an inhibitor of glutaminase (GLS1), the enzyme that converts glutamine into downstream metabolic fuels, and then exposed to photons or protons. We measured oxidative stress, mitochondrial function, total DNA damage, and the activation of major DNA repair pathways. We also tested combinations with drugs that inhibit DNA repair signaling.
Results
Inhibiting glutamine metabolism strongly radiosensitized KEAP1-mutant cells for photons and protons. This was accompanied by elevated mitochondrial oxidative stress, loss of mitochondrial membrane potential, and reduced ATP production, indicating a collapse in cellular energy supply. Despite increased total DNA damage, cells failed to form specific DNA repair structures relating to homologous recombination (HR), suggesting that these repair processes were disabled.
Conclusion
Our results indicate that KEAP1-mutant lung cancer cells treated with a GLS1 inhibitor are significantly radiosensitized. We propose that either bioenergetic crisis prevents fueling of HR processes or that impaired TCA cycle anaplerosis reduces α-ketoglutarate levels, which is an important co-factor for chromatin remodeling, preventing DNA end resection and resulting in Rad51 foci formation. GLS1 inhibitors, which are currently in clinical trials, could be a powerful tool to combat radioresistance in a significant population of lung cancer patients.