title: Mn-superoxide dismutase overexpression enhances G2 accumulation and radioresistance in human oral squamous carcinoma cells.
abstract: This study investigates the hypothesis that Mn-superoxide dismutase (MnSOD) influences cancer cell radiosensitivity by regulating the G(2)-checkpoint pathway. Human oral squamous carcinoma cells (SCC25) stably overexpressing MnSOD were irradiated (6 Gy) and assayed for cell survival, cell-cycle phase distributions, and bromodeoxyuridine (BrdU) pulse-chase flow-cytometric measurements of cell-cycle phase transits. Electron paramagnetic resonance (EPR) spectroscopy was used to measure steady-state levels of oxygen-centered free radicals. Glutathione and glutathione disulfide levels were used as indicators of changes in the intracellular redox state. MnSOD overexpression increased radioresistance threefold to fourfold; this increase was associated with twofold to threefold increases in radiation-induced G(2) accumulation. BrdU pulse-chase and flow-cytometric measurements of the percentage of G(1) and relative movement showed no significant changes in G(1) and S transits; however, the percentage of G(2) cells and BrdU-positive cells showed delayed G(2)+M transits in MnSOD-overexpressing irradiated cells. The steady-state levels of oxygen-centered free radicals were not significantly different in vector compared with MnSOD-overexpressing cells, suggesting that the free radical generation is essentially similar. MnSOD overexpression did prevent radiation-induced decreases in total glutathione content, which correlated with radioresistance and enhanced G(2) accumulation. These results support the hypothesis that a "metabolic redox-response" to IR exposure regulates radiosensitivity by altering radiation-induced G(2) accumulation.
authors.affiliation: Free Radical and Radiation Biology Program, Department of Radiation Oncology, University of Iowa, Iowa City, 52242, USA.
authors.forename: Amanda L
authors.initials: AL
authors.lastname: Kalen
authors.forename: Ehab H
authors.initials: EH
authors.lastname: Sarsour
authors.forename: Sujatha
authors.initials: S
authors.lastname: Venkataraman
authors.forename: Prabhat C
authors.initials: PC
authors.lastname: Goswami
mesh_terms: D001973:Bromodeoxyuridine / Q000378:metabolism; D002294:Carcinoma, Squamous Cell / Q000201:enzymology / Q000378:metabolism / Q000473:pathology / Q000532:radiotherapy; D002453:Cell Cycle; D002470:Cell Survival / Q000528:radiation effects; D004578:Electron Spin Resonance Spectroscopy; D005434:Flow Cytometry; D005609:Free Radicals / Q000032:analysis / Q000378:metabolism; D016195:G2 Phase; D015971:Gene Expression Regulation, Enzymologic / Q000528:radiation effects; D015972:Gene Expression Regulation, Neoplastic / Q000528:radiation effects; D005978:Glutathione / Q000032:analysis / Q000378:metabolism; D019803:Glutathione Disulfide / Q000032:analysis / Q000378:metabolism; D006801:Humans; D009062:Mouth Neoplasms / Q000201:enzymology / Q000378:metabolism / Q000473:pathology / Q000532:radiotherapy; D010084:Oxidation-Reduction; D011836:Radiation Tolerance; D013482:Superoxide Dismutase / Q000096:biosynthesis* / Q000235:genetics; D013997:Time Factors
chemical_list: D005609:Free Radicals; D013482:Superoxide Dismutase; D001973:Bromodeoxyuridine; D005978:Glutathione; D019803:Glutathione Disulfide