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Nitric oxide (NO) is an unstable radical produced during the oxidative deamination catalyzed by NO synthase (NOS) that converts L-arginine to L-citrulline. NO is also generated nonenzymatically from a group of compounds, called NO donors, such as sodium nitroprusside (SNP). NO directly or through its metabolites has been implicated in several disorders, including Alzheimer's disease (AD). Since NO is a highly labile unstable free gas, we measured the stable end products, nitrite and nitrate (NOx). Here, we investigated the effect of SNP-mediated NO release in different cell types and its effect on the beta-amyloid precursor protein (betaAPP). When different cell types were induced with SNP, a significant level of NOx was detected in a time and dose-dependent manner over the spontaneous release of NOx by SNP. The astrocytes, glial, and epithelial cell lines released significantly higher level of NOx as compared to neuronal cells following the exposure of SNP. The latter group of cells was more sensitive to NO-mediated cytotoxicity, as demonstrated by the lactate dehydrogenase assay. The SNP-mediated toxicity is known to be caused by the accumulation of cyanide ions and we report that the ability of cells to protect against it depends on the levels of nitric oxide metabolites. Cell lines, such as astrocytic and epithelial, that produce more NOx are better protected against the SNP-induced toxicity than the less NOx-protecting neuronal cell lines. The possibility of differential susceptibility of neurons and astrocytes resulting from the different content of reduced glutathione is also discussed. The release of NOx was prevented by cotreatment with a NO scavenger and superoxide dismutase but not by a NOS inhibitor. The activity of NOS was decreased when cytosolic extracts were incubated with SNP. In the conditioned medium of SNP-induced cells, the level of soluble betaAPP (sAPP) was decreased, and this decrease was more apparent in neuronal than astrocytic cell lines. Taken together, these results suggest that the SNP-derived NO release is independent of the NOS pathway, that various cell types metabolize SNP differently, and that neuronal cell lines are more vulnerable with SNP treatment with lowered sAPP secretion. Since the neuronal cell lines lack a nitric-oxide-generated protective mechanism, we speculate that these cells may be the first targets of neurodegeneration by several toxic agents, including the cyanides and peroxynitrites.
Amyloid beta-Protein Precursor↗