Tumor Necrosis Factor-α Increases the Steady-state Reduction of Cytochrome bof the Mitochondrial Respiratory Chain in Metabolically Inhibited L929 Cells*

The mechanism of tumor necrosis factor α (TNFα)-induced cytotoxicity in metabolically inhibited cells is unclear, although some studies have suggested that mitochondrial dysfunction and generation of reactive oxygen species may be involved. Here we studied the effect of TNFα on the redox state of mitochondrial cytochromes and its involvement in the generation of reactive oxygen species in metabolically inhibited L929 cells. Treatment with TNFα and cycloheximide (TNFα/CHX) induced mitochondrial cytochrome crelease, increased the steady-state reduction of cytochrome b, and decreased the steady-state reduction of cytochromes cc1andaa3. TNFα/CHX treatment also induced lipid peroxidation, intracellular generation of reactive oxygen species, and cell death. Furthermore, as the cells died mitochondrial morphology changed from an orthodox to a hyperdense and condensed and finally to a swollen conformation. Antimycin A, a mitochondrial respiratory chain complex III inhibitor that binds to cytochrome b, blocked the formation of reactive oxygen species, suggesting that the free radicals are generated at the level of cytochrome b.Moreover, antimycin A, when added after 3 h of TNFα/CHX treatment, arrested the further release of cytochrome cand the cytotoxic response. We propose that the reduced cytochromebpromotes the formation of reactive oxygen species, lipid peroxidation of the cell membrane, and cell death.