Mechanistic insights into Nrf2- and PARP1-mediated radioresistance of glioblastoma stem cells under photon, proton, and carbon ion irradiation: An in vitro study.

Abstract

Glioblastoma multiforme (GBM) exhibits strong resistance to radiotherapy, partly driven by glioblastoma stem-like cells (GSCs) with enhanced redox homeostasis and DNA repair capacity. This study evaluated whether targeting Nrf2-mediated antioxidant signaling and PARP1-dependent DNA repair enhances GSC radiosensitivity to different radiation modalities. Pharmacological inhibition of Nrf2 (ML385, 6 µmol/L) or PARP1 (olaparib, 5 µmol/L) reduced tumorsphere formation to 74.5 ± 10% and 58.56 ± 14.5% of control levels, respectively, while combined treatment further reduced formation to 51 ± 11% and sphere size to 29% of control. Western blotting confirmed effective pathway inhibition, with complete suppression of PARP activity and approximately 30% reduction in Nrf2 downstream proteins (SOD1, PRDX2, and NQO1). Dose-response analysis showed D₅₀ values of 5.03 ± 0.09 Gy (photons), 2.96 ± 0.91 Gy (protons), and 2.04 ± 0.47 Gy (carbon ions), corresponding to RBE₅₀ values of 1, 1.70 ± 0.55, and 2.46 ± 0.57, respectively. ML385 enhanced radiosensitivity to photons and protons and showed a similar radiosensitizing trend following carbon-ion irradiation, whereas olaparib showed its strongest effect with photons and limited effects with protons and carbon ions. Combined treatment produced a greater reduction in radiation survival than either inhibitor alone under selected conditions, particularly following photon irradiation. Nrf2 inhibition reduced downstream antioxidant proteins and increased late apoptotic/necrotic fraction, while PARP1 inhibition was associated with altered DNA damage persistence. Combined inhibition further increased γ-H2AX foci at selected time points following proton irradiation, consistent with delayed or incomplete repair of radiation-induced DNA damage. These findings support Nrf2 and PARP1 as potential regulators of GSC radioresistance and provide a rationale for further investigation of their therapeutic targeting in combination with radiotherapy.

Key Findings

• Pharmacological inhibition of Nrf2 or PARP1 reduced glioblastoma stem-like cell tumorsphere formation and size, with combined treatment showing greater effects.
• Nrf2 inhibition enhanced radiosensitivity to photon and proton irradiation and showed a radiosensitizing trend with carbon ions, while PARP1 inhibition was most effective with photons.
• Combined Nrf2 and PARP1 inhibition increased DNA damage persistence and apoptosis in glioblastoma stem cells following radiation, indicating impaired repair mechanisms.

Clinical Significance

Citation

Salma Rima, Hammad Mira, Hariri Mehranet al.. Mechanistic insights into Nrf2- and PARP1-mediated radioresistance of glioblastoma stem cells under photon, proton, and carbon ion irradiation: An in vitro study. Biomedicine & pharmacotherapy = Biomedecine & pharmacotherapie. 2026-Sep-05.