ATM Inhibitor KU-55933 Overcomes Ferroptosis-Resistance in Human Cancer Cell Lines

Ferroptosis is an iron-dependent form of regulated cell death, and its resistance represents a major obstacle in cancer therapy. In this study, we evaluated the responsiveness of human lung and renal cancer cell lines to ferroptosis inducers and investigated whether pharmacological inhibition of ataxia-telangiectasia mutated (ATM) kinase could overcome ferroptosis resistance. While A549, ACHN, and Caki-1 cells exhibited dose-dependent sensitivity to erastin, RSL3, and FIN56, PC-9 and Caki-2 cells were relatively resistant. Notably, pharmacological inhibition of ATM using KU-55933 significantly sensitized these resistant cells to all tested ferroptosis inducers. This combinatorial treatment induced a DNA damage response, as evidenced by increased γ-H2AX expression, and reduced cell viability in a ferroptosis-dependent manner, which was reversed by ferroptosis inhibitors (ferrostatin-1 and liproxstatin-1) or an iron chelator, deferoxamine. Mechanistically, ATM inhibition disrupted glutathione (GSH) homeostasis in resistant cells. Although single treatments had minimal effects, the combination of erastin and KU-55933 led to a marked depletion of total GSH and reduced GSH, an increase in oxidized glutathione (GSSG), and a significant reduction in the GSH/GSSG ratio. These findings indicate that ATM inhibition promotes ferroptosis by impairing redox balance rather than directly modulating ferroptosis-inducing pathways. Collectively, our results demonstrate that pharmacological ATM inhibition by KU-55933 overcomes ferroptosis resistance by inducing a DNA damage response and disrupting GSH-dependent redox homeostasis. Targeting ATM may represent a promising therapeutic strategy to sensitize resistant cancers to ferroptosis-based treatments.