Copper zinc superoxide dismutase (CuZnSOD) is an essential primary
antioxidant enzyme that converts
superoxide radical to
hydrogen peroxide and molecular
oxygen in the cytoplasm.
Cytosolic glutathione peroxidase (GPx) converts
hydrogen peroxide into water. The overall goal of the present study was to explore the possible role of the
antioxidant enzyme CuZnSOD in expression of the malignant phenotype. We hypothesized that overexpression of CuZnSOD would lead to the suppression of at least part of the human malignant phenotype. To test this hypothesis, human CuZnSOD
cDNA was transfected into U118-9 human
malignant glioma cells. CuZnSOD activity levels increased 1.5-, 2.0-, 2.6-, and 3.5-fold, respectively, in four table transfected cell lines compared with wild type and vector controls. Overexpression of CuZnSOD altered cellular
antioxidant enzyme profiles, including those of
manganese superoxide dismutase,
catalase, and GPx. The transfected clone with the highest CuZnSOD:GPx ratio (3.5) showed a 42% inhibition of
tumor cell growth in vitro. The decreased rate of
tumor cell growth in vitro was strongly correlated with the
enzyme activity ratio of CuZnSOD:GPx.
Glioma cells that stably overexpressed CuZnSOD demonstrated additional suppressive effects on the malignant phenotype when compared with the parental cells and vector controls. These cells showed decreased plating efficiency, elongated cell population doubling time, lower clonogenic fraction in soft
agar, and, more significantly, inhibition of
tumor formation in nude mice. This work suggested that CuZnSOD is a new tumor suppressor gene. Increased intracellular ROS levels were found in cells with high activity ratios of CuZnSOD:GPx. Change in the cellular redox status, especially change attributable to the accumulation of
hydrogen peroxide or other hydroperoxides, is a possible reason to explain the suppression of
tumor growth observed in CuZnSOD-overexpressing cells.