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Expression of manganese superoxide dismutase reduces tumor control radiation dose: gene-radiotherapy
M Urano1, M Kuroda, R Reynolds
1Department of Radiation Medicine, University of Kentucky Medical Center, Lexington 40536-0084, USA.
Abstract:
This study investigated the in vitro and in vivo radiation response of tumor cells transfected with human manganese superoxide dismutase (MnSOD) cDNA. A major objective was to test the potential tumor suppressive effect of MnSOD in vivo. Tumor cells studied were an in vitro line derived from a murine spontaneous fibrosarcoma, FSa-II, which expressed an undetectable MnSOD activity. These cells were transfected with pSV2-NEO plasmid (NEO line) or cotransfected with MnSOD plasmid plus pSV2-NEO plasmid (SOD lines) as described previously. The cell lines used were SOD-L and SOD-H, which expressed, respectively, low and high MnSOD activities after transfection, and NEO and parental FSa-II controls. Both SOD-L and SOD-H cell lines were slightly more resistant to ionizing radiation than were the two control cell lines when irradiated in vitro in the presence of oxygen. The dose-modifying factors calculated at the survival level of 0.01 were 1.13 and 1.15 for the SOD-L and SOD-H cells, respectively. To investigate potential tumor suppressive effects, animal tumors of 4 mm diameter were irradiated in vivo under hypoxic conditions, and the radiation dose to control one-half of the irradiated tumors (TCD50) was determined for each tumor. The TCD50S obtained on the basis of the tumor control rate in 120 days after irradiation were substantially lower for the SOD-H and SOD-L tumors compared to the NEO tumors. They were 22.9, 28.6, and 47.5 Gy for SOD-H, SOD-L and NEO tumors, respectively. To analyze these data, survival curves were obtained for hypoxic cells by irradiating NEO and SOD-H tumors under hypoxic conditions in vivo and assaying in vitro. Analysis of these curves suggests that the decrease in the TCD50S of SOD tumors is attributable to the reduced tumorigenicity in these tumors. The hypoxic cell survival curves also showed that SOD did not protect cells from radiation in the absence of oxygen. Electron microscopy showed no morphological differences between these cells. These results suggest that the fraction of tumorigenic cells could be reduced by expression of MnSOD, resulting in a substantial decrease in the TCD50.
Insights
Human manganese superoxide dismutase (MnSOD) expression reduced tumor growth and radiation resistance in vivo. This suggests MnSOD can suppress tumors by decreasing tumorigenic cell fraction, impacting radiation therapy effectiveness.
Area of Science:
- Oncology
- Radiation Biology
- Molecular Biology
Background:
- The role of manganese superoxide dismutase (MnSOD) in tumor radioresistance and suppression is not fully understood.
- Tumor cells often exhibit varying levels of MnSOD activity, influencing their response to radiation and growth potential.
Purpose of the Study:
- To investigate the in vitro and in vivo radiation response of tumor cells engineered to express human manganese superoxide dismutase (MnSOD).
- To evaluate the potential tumor suppressive effects of MnSOD in vivo.
Main Methods:
- Murine fibrosarcoma cells (FSa-II) were transfected with MnSOD cDNA (SOD lines) or a control plasmid (NEO line).
- In vitro irradiation studies assessed cell survival in the presence of oxygen.
- In vivo studies determined the radiation dose required to control 50% of tumors (TCD50) under hypoxic conditions.
Main Results:
- MnSOD-transfected cells (SOD-L, SOD-H) showed slightly increased radioresistance in vitro under oxygenated conditions (DMFs 1.13-1.15).
- In vivo, MnSOD-expressing tumors exhibited significantly lower TCD50 values (22.9-28.6 Gy) compared to control tumors (47.5 Gy) under hypoxia.
- Hypoxic cell survival curves indicated MnSOD did not protect cells from radiation in the absence of oxygen, suggesting reduced tumorigenicity as the primary mechanism for lower TCD50.
Conclusions:
- Expression of MnSOD in tumor cells can lead to reduced tumorigenicity and a substantial decrease in the in vivo radiation dose required for tumor control.
- MnSOD may act as a tumor suppressor by reducing the fraction of tumorigenic cells, offering potential therapeutic implications in radiation oncology.