Multiple pathways of neuronal death induced by DNA-damaging agents, NGF deprivation, and oxidative stress

Here, we compare the pathways by which DNA-damaging agents, NGF deprivation, and superoxide dismutase 1 (SOD1) depletion evoke apoptosis of sympathetic neurons. Previous work raised the hypothesis that cell cycle signaling plays a required role in neuronal apoptosis elicited by NGF deprivation and the DNA-damaging agent camptothecin. To test this hypothesis, we extended our investigation of DNA-damaging agents to cytosine arabinoside (AraC) and UV irradiation. As with NGF deprivation and camptothecin treatment, the cyclin-dependent kinase inhibitors flavopiridol and olomoucine protected neurons from apoptosis induced by AraC and UV treatment. These observations support the model that camptothecin, AraC, and UV treatment cause DNA damage, which leads to apoptosis by a mechanism that, as in the case of NGF deprivation, includes activation of cell cycle components. Flavopiridol and olomoucine, however, had no effect on death induced by SOD1 depletion, suggesting that CDKs do not play a role in this paradigm of neuronal death. To compare further the mechanisms of death evoked by NGF withdrawal, SOD1 depletion, and DNA-damaging agents, we investigated their responses to inhibitors of cysteine aspartases, elements of apoptotic pathways. The V-ICEinh and BAF, two peptide inhibitors of cysteine aspartases, protected neurons in all three death paradigms. In contrast, the cysteine aspartase inhibitory peptide zVAD-fmk conferred protection from NGF withdrawal and SOD1 depletion, but not DNA-damaging agents, whereas acYVAD-cmk protected only from SOD1 depletion. Taken together, these findings indicate that three different apoptotic stimuli activate separate pathways of death in the same neuron type.