Abstract
The levels of the antioxidant enzyme manganese superoxide dismutase (Mn-SOD) are frequently decreased in tumor cells and increased in normal cells upon treatment with ionizing radiation. We studied Mn-SOD at different stages during the neoplastic conversion of radiation-initiated Syrian hamster embryo HDR-3 cells. Mn-SOD activity and the concentration of Mn-SOD protein and mRNA increased gradually during the malignant transformation of HDR-3 cells after radiation exposure; fully neoplastic cells showed greater Mn-SOD levels than preneoplastic and normal 84-3 cells. Inhibitors of superoxide (SO) anion production (thenoyltrifluoroacetone and rotenone) decreased the concentration of Mn-SOD mRNA, raising the possibility that the generation of SO radicals participated in the upregulation of Mn-SOD in cells transformed by exposure to radiation. We observed an increase in the concentration of tumor necrosis factor alpha (TNF alpha) in HDR-3 cells relative to mock-irradiated cells. Together with the observation that TNF alpha stimulates the production of SO by mitochondria and increases the level of Mn-SOD mRNA in other experimental systems, our results suggest that as normal 84-3 cells undergo malignant transformation induced by ionizing radiation they produce TNF alpha, to which the cells respond by increasing the concentration of Mn-SOD mRNA and protein and the activity of the enzyme.
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