Superoxide radical-initiated apoptotic signalling pathway in selenite-treated HepG₂ cells: Mitochondria serve as the main target

The exact role of superoxide radicals (O₂^·-) in apoptosis is still a matter of debate. The main objective of the present study is to evaluate the apoptotic signalling pathway initiated by O₂^·-. The reductive reaction of sodium selenite with glutathione was used as the intracellular O₂^·--generating system. When cells were exposed to 5 to 25 μM selenite, a temporal pattern of apoptotic events was observed following the elevation of O₂^·-, in which cytochrome c release and mitochondrial depolarization preceded caspase-3 activation and DNA fragmentation. The simultaneous treatment with N-acetylcysteine and 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl markedly reduced O₂^·- level and suppressed the mitochondrial changes and the downstream apoptotic events. Moreover, pretreatment with cyclosporin A plus trifluoperazine, two mitochondrial permeability transition (MPT) inhibitors, was capable of attenuating O₂^·--mediated cytochrome c release and mitochondrial depolarization, and subsequently inhibiting apoptosis. Thus, the present results provide convincing evidence that O₂^·- generated from the reductive reaction of selenite with GSH is capable of triggering a mitochondria-dependent apoptotic pathway. Such knowledge may not only help to obtain a better understanding of the apoptotic effect of selenite per se, but of the role of O₂^·- in initiation and execution of apoptosis.