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.