EXPRESSION OF MANGANESE SUPEROXIDE-DISMUTASE REDUCES TUMOR-CONTROL RADIATION-DOSE - GENE-RADIOTHERAPY

This study investigated the in vitro and in vivo radiation response of tumor cells transfected with human manganese superoxide dismutase (Mn SOD) cDNA. A major objective was to test the potential tumor suppressi ve effect of MnSOD in vivo. Tumor cells studied were an in vitro Line derived from a murine spontaneous fibrosarcoma, FSa-II, which expresse d an undetectable MnSOD activity. These cells were transfected with pS V2-NEO plasmid (NEO line) or cotransfected with MnSOD plasmid plus pSV 2-NEO plasmid (SOD lines) as described previously. The cell lines used were SOD-L and SOD-H, which expressed, respectively, low and high MnS OD activities after transfection, and NEO and parental FSa-II controls . Both SOD-L and SOD-H cell lines were slightly more resistant to ioni zing radiation than were the two control cell lines when irradiated in vitro in the presence of oxygen. The dose-modifying factors calculate d at the survival level of 0.01 were 1.13 and 1.15 for the SOD-L and S OD-H cells, respectively. To investigate potential tumor suppressive e ffects, animal tumors of 4 mm diameter were irradiated in vivo under h ypoxic conditions, and the radiation dose to control one-half of the i rradiated tumors (TCD50) was determined for each tumor. The TCD(50)s o btained on the basis of the tumor control rate in 120 days after irrad iation were substantially lower for the SOD-H and SOD-L tumors compare d 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 u nder hypoxic conditions in vivo and assaying in vitro. Analysis of the se curves suggests that the decrease in the TCD(50)s of SOD tumors is attributable to the reduced tumorigenicity in these tumors. The hypoxi c cell survival curves also showed that SOD did not protect cells from radiation in the absence of oxygen. Electron microscopy showed no mor phological differences between these cells. These results suggest that the fraction of tumorigenic cells could be reduced by expression of M nSOD, resulting in a substantial decrease in the TCD50.