C. Ghosh et Dk. Lahiri, Increased vulnerability of neuronal cell lines to sodium nitroprusside-mediated toxicity is caused by the decreased level of nitric oxide metabolites, J MOL NEURO, 13(1-2), 1999, pp. 77-92
Nitric oxide (NO) is an unstable radical produced during the oxidative deam
ination catalyzed by NO synthase (NOS) that converts L-arginine to L-citrul
line. NO is also generated nonenzymatically from a group of compounds, call
ed NO donors, such as sodium nitroprusside (SNP). NO directly or through it
s 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 effec
t on the beta-amyloid precursor protein (beta APP). When different cell typ
es 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. T
he 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 th
at the ability of cells to protect against it depends on the levels of nitr
ic oxide metabolites. Cell lines, such as astrocytic and epithelial, that p
roduce more NQx are better protected against the SNP-induced toxicity than
the less NOx-protecting neuronal cell lines. The possibility of differentia
l susceptibility of neurons and astrocytes resulting from the different con
tent of reduced glutathione is also discussed. The release of NOx was preve
nted 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, t
he level of soluble beta APP (sAPP) was decreased, and this decrease was mo
re apparent in neuronal than astrocytic cell lines. Taken together, these r
esults suggest that the SNP-derived NO release is independent of the NOS pa
thway, that various cell types metabolize SNP differently, and that neurona
l cell lines are more vulnerable With SNP treatment with lowered sAPP secre
tion. Since the neuronal cell lines lack a nitric-oxide-generated protectiv
e mechanism, we speculate that these cells may be the first targets of neur
odegeneration by several toxic agents, including the cyanides and peroxynit
rites.