Synergistic Effects of DNA-PKcs Inhibition and Radiotherapy in Esophageal Squamous Cell Carcinoma

Esophageal squamous cell carcinoma (ESCC) is characterized by intrinsic radioresistance, primarily mediated by hyperactivation of DNA damage response (DDR) pathways. This repair capacity significantly diminishes radiotherapy efficacy, contributing to poor patient outcomes. Targeting DNA-dependent protein kinase catalytic subunit (DNA-PKcs), a master regulator of non-homologous end joining (NHEJ), represents a promising strategy to overcome radioresistance. Immunohistochemistry (IHC) was performed on paired biopsy specimens from esophageal squamous cell carcinoma (ESCC) patients (n = 20) collected before neoadjuvant radiotherapy and after surgical resection. In vitro, ESCC cell lines (TE13 and Eca9706) were co-treated with DNA-dependent protein kinase catalytic subunit (DNA-PKcs) inhibitors (AZD7648 or NU7741) or PRKDC-targeting siRNA plus irradiation. Subsequent functional assays were conducted to assess cell viability, apoptosis, cell cycle distribution, and underlying molecular mechanisms. In vivo, the combined efficacy of AZD7648 and irradiation was evaluated in xenograft tumor models. Phosphorylated DNA-PKcs (Ser2056) in ESCC tissues was significantly elevated following irradiation. Inhibition of DNA-PKcs markedly enhanced radiosensitivity in both ESCC cell lines and xenograft models, correlating with prolonged G2/M arrest and increased apoptosis. Mechanistically, the combined therapy activated the ATM (Ser1981)-Chk2 (Thr68) axis and concurrently enhanced phosphorylation of Chk1 at Ser345, reinforcing G2/M arrest through Wee1 kinase-mediated inhibitory phosphorylation of CDC2 at Tyr15. Consequently, irreparable DNA double-strand breaks (DSBs) accumulated, triggering Caspase-3-dependent cleavage of PARP and execution of the intrinsic apoptosis pathway. Our study reveals a novel mechanism by which DNA-PKcs inhibition augments radiosensitivity in ESCC. Targeting DNA-PKcs could represent a promising strategy to improve the efficacy of radiotherapy in ESCC, offering a potential therapeutic approach to overcome radioresistance.