ELEVATION OF MANGANESE SUPEROXIDE-DISMUTASE GENE-EXPRESSION BY THIOREDOXIN

Manganese superoxide dismutase (MnSOD) is a mitochondrial enzyme that dismutates potentially toxic superoxide radical into hydrogen peroxide and dioxygen. This enzyme is critical for protection against cellular injury due to elevated partial pressures of oxygen. Thioredoxin (TRX) is a potent protein disulfide reductase found in most organisms that participates in many thiol-dependent cellular reductive processes and plays an important role in antioxidant defense, signal transduction, a nd regulation of cell growth and proliferation. Here we describe induc tion of manganese superoxide dismutase by thioredoxin. MnSOD mRNA and activity were increased dramatically by low concentrations of TRX (28 mu M). Elevation of MnSOD mRNA by TRX was inhibited by actinomycin D, but not cycloheximide, occurring both in cell lines and primary human lung microvascular endothelial cells. mRNAs for other antioxidant enzy mes including copper-zinc superoxide dismutase and catalase were not e levated, demonstrating specificity of induction of MnSOD by TRX. Thiol oxidation by diamide or alkylation by chlorodinitrobenzene inhibited MnSOD induction, further indicating a requirement for reduced TRX. Bec ause both oxidized and reduced thioredoxin (28 mu M) induced MnSOD mRN A, the intracellular redox status of externally added Escherichia coli oxidized TRX was determined. About 45% of internalized E. coli TRX wa s reduced, with 8% in fully reduced form and about 37% in partially re duced form, However, when TRX reductase and nicotinamide adenine dinuc leotide (NADPH) were added to the extracellular medium with TRX, more than 80% of E. coli TRX was found to be in a fully reduced state in hu man adenocarcinoma (A549) cells. Although lower concentrations of oxid ized TRX (7 mu M) did not induce MnSOD mRNA, this concentration of TRX , when reduced by NADPH and TRX reductase, increased MnSOD mRNA six-fo ld. In additional studies, MCF-7 cells stably transfected with the hum an TRX gene had elevated expression of MnSOD mRNA relative to vector-t ransfected controls, Thus, both endogenously produced and exogenously added TRX elevate MnSOD gene expression. These findings suggest a nove l mechanism involving reduced TRX in regulation of MnSOD.