Reactive oxygen species (ROS) are thought to be involved in many forms of programmed cell death. The role of ROS in cell death caused by oxidative glutamate toxicity was studied in an immortalized mouse hippocampal cell line (HT22). The causal relationship between ROS production and glutathione (GSH) levels, gene expression, caspase activity, and cytosolic Ca2+ concentration was examined. An initial 5–10-fold increase in ROS after glutamate addition is temporally correlated with GSH depletion. This early increase is followed by an explosive burst of ROS production to 200–400-fold above control values. The source of this burst is the mitochondrial electron transport chain, while only 5–10% of the maximum ROS production is caused by GSH depletion. Macromolecular synthesis inhibitors as well as Ac-YVAD-cmk, an interleukin 1β–converting enzyme protease inhibitor, block the late burst of ROS production and protect HT22 cells from glutamate toxicity when added early in the death program. Inhibition of intracellular Ca2+ cycling and the influx of extracellular Ca2+ also blocks maximum ROS production and protects the cells. The conclusion is that GSH depletion is not sufficient to cause the maximal mitochondrial ROS production, and that there is an early requirement for protease activation, changes in gene expression, and a late requirement for Ca2+ mobilization.

Oxidative stress is implicated in a number of neurological disorders including stroke, Parkinson's disease, and Alzheimer's disease. To study the effects of oxidative stress on neuronal cells, we have used an immortalized mouse hippocampal cell line (HT-22) that is particularly sensitive to glutamate. In these cells, glutamate competes for cystine uptake, leading to a reduction in glutathione and, ultimately, cell death. As it has been reported that protein kinase C activation inhibits glutamate toxicity in these cells and is also associated with the inhibition of apoptosis in other cell types, we asked if glutamate toxicity was via apoptosis. Morphologically, glutamate-treated cells underwent plasma membrane blebbing and cell shrinkage, but no DNA fragmentation was observed. At the ultrastructural level, there was damage to mitochondria and other organelles although the nuclei remained intact. Protein and RNA synthesis inhibitors as well as certain protease inhibitors protected the cells from glutamate toxicity. Both the macromolecular synthesis inhibitors and the protease inhibitors had to be added relatively soon after the addition of glutamate, suggesting that protein synthesis and protease activation are early and distinct steps in the cell death pathway. Thus, the oxidative stress brought about by treatment with glutamate initiates a series of events that lead to a form of cell death distinct from either necrosis or apoptosis. Key Words: Apoptosis-Caspase-Glutamate toxicity-Protein kinase C-Glutathione. J. Neurochem. 71, 95-105 (1998).Glutamate is thought to cause the neuronal cell death Evidence for the involvement of a second pathway

Extensive nerve cell death occurs during the development of the central nervous system as well as in episodes of trauma and in neurodegenerative disease. The mechanistic details of how these cells die are poorly understood. Here we describe a unique oxidative stress-induced programmed cell death pathway called oxytosis, and outline pharmacological approaches which interfere with its execution. Oxidative glutamate toxicity, in which exogenous glutamate inhibits cystine uptake through the cystine/glutamate antiporter leading to a depletion of glutathione, is used as an example of oxytosis. It is shown that there is a sequential requirement for de novo macromolecular synthesis, lipoxygenase activation, reactive oxygen species production, and the opening of cGMP-gated channels which allow the influx of extracellular calcium. The translation initiation factor elF2alpha plays a central role in this pathway by regulating the levels of glutathione. Finally, examples are given in which the reduction in glutathione, the production of reactive oxygen species, and calcium influx can be experimentally manipulated to prevent cell death. Data are reviewed which suggest that oxytosis may be involved in nerve cell death associated with nervous system trauma and disease.

Oxidative stress and highly specific decreases in glutathione (GSH) are associated with nerve cell death in Parkinson's disease. Using an experimental nerve cell model for oxidative stress and an expression cloning strategy, a gene involved in oxidative stress–induced programmed cell death was identified which both mediates the cell death program and regulates GSH levels. Two stress-resistant clones were isolated which contain antisense gene fragments of the translation initiation factor (eIF)2α and express a low amount of eIF2α. Sensitivity is restored when the clones are transfected with full-length eIF2α; transfection of wild-type cells with the truncated eIF2α gene confers resistance. The phosphorylation of eIF2α also results in resistance to oxidative stress. In wild-type cells, oxidative stress results in rapid GSH depletion, a large increase in peroxide levels, and an influx of Ca2+. In contrast, the resistant clones maintain high GSH levels and show no elevation in peroxides or Ca2+ when stressed, and the GSH synthetic enzyme γ-glutamyl cysteine synthetase (γGCS) is elevated. The change in γGCS is regulated by a translational mechanism. Therefore, eIF2α is a critical regulatory factor in the response of nerve cells to oxidative stress and in the control of the major intracellular antioxidant, GSH, and may play a central role in the many neurodegenerative diseases associated with oxidative stress.

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