A novel Arabidopsis thaliana protein protects tumor cells from tumor necrosis factor-induced apoptosis.

Recently we have cloned and characterized a novel, oxidative stress-induced Arabidopsis thaliana gene (oxy5), and showed that expression of oxy5 protects bacterial cells from death caused by oxidative stress. As oxidative stress is one pathway of TNF cytotoxicity, we investigated whether the encoded protein could also protect human tumor cells from TNF killing. We stably transfected the oxy5 gene into TNF-sensitive HeLa D98 cells (D98/O.5), and found that all examined transfectants were highly TNF-resistant in the absence of the protein synthesis inhibitor cycloheximide. The acquired TNF resistance of these clones was accompanied by a sharp decrease in the intracellular formation of reactive oxygen species, suggesting the activation of antioxidant enzymes like superoxide dismutases (SODs). Indeed, D98/O.5 clones showed an increased manganous superoxide dismutase (MnSOD) mRNA and protein expression in the absence or presence of TNF stimulation, whereas the expression of the Cu/ZnSOD was not affected. Furthermore, the elevated MnSOD expression in the D98/O.5 clones correlated well with an increased antioxidative activity, which was specifically due to MnSOD as measured by the suppression of xanthine oxidase. Our results demonstrate a novel role for a plant-derived protein in resistance to TNF cytotoxicity, and that the Arabidopsis thaliana protein Oxy5 can exert its protective function across evolutionary boundaries through activation of antioxidant enzymes like MnSOD.