Evidence for Redox Regulation of Cytochrome c Release during Programmed Neuronal Death: Antioxidant Effects of Protein Synthesis and Caspase Inhibition

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Abstract
Sympathetic neurons die by apoptosis when they are deprived of nerve growth factor (NGF). Activation of caspases by cytochrome c released from mitochondria is central to this death. In this report we present evidence that cellular redox state regulates cytochrome c redistribution in these neurons. An increase of mitochondrial-produced reactive oxygen species (ROS) occurred in rat sympathetic neurons in cell culture within 3 hr of NGF withdrawal. Caspase inhibitors blocked this ROS burst. By 6 hr after NGF deprivation, glutathione (GSH) levels had increased, neutralizing elevated hydrogen peroxide levels and returning cellular redox state to basal levels. By 12 hr after deprivation, ROS levels had again increased and remained elevated during the rest of the apoptotic process. The later ROS burst appeared to have both caspase-dependent and caspase-independent components and was coincident with the period of cytochrome c release. Inhibition of protein synthesis with cycloheximide (CHX) and treatment with the antioxidant compound, N-acetyl-l-cysteine (l-NAC), blocked both the early and late ROS bursts by increasing cellular GSH levels (Ratan et al., 1994; Tan et al., 1998). Both compounds, and a membrane-permeant form of GSH, also inhibited cytochrome c release and death. Treatment of NGF, CHX-,l-NAC-, and GSH-saved cells with hydrogen peroxide caused rapid cytochrome c release. These data suggest a role for cellular redox state in regulating cytochrome c release during apoptosis induced by NGF withdrawal.