Paraquat (PQ) is a well-known pneumotoxicant that exerts its toxic effect b
y elevating intracellular levels of superoxide. In addition, production of
pro-inflammatory cytokines has possibly been linked to PQ-induced inflammat
ory processes through reactive oxygen species (ROSs) and nitric oxide (NO).
However, the role of NO in PQ-induced cell injury has been controversial.
To explore this problem, we examined the effect of NO on A549 cells by expo
sing them to the exogenous NO donor NOC18 or to cytokines; tumor necrosis f
actor-alpha, interleukin-1 beta and interferon-gamma, as well as PQ. Althou
gh the exogenous NO donor on its own had no effect on the release of lactat
e dehydrogenase (LDH), remarkable release was observed when the cells were
exposed to high concentrations of NOC18 and PQ. This cellular damage caused
by 1 mM NOC18 plus 0.2 mM PQ was ascertained by phase contrast microscopy.
On the other hand, NO derived from 25-50 muM NOC18 added into the medium i
mproved the MTT reduction activity of mitochondria, suggesting a beneficial
effect of NO on the cells. Incubation of A549 cells with cytokines increas
ed in inducible NO synthase (iNOS) expression and nitrite accumulation, res
ulting in LDH release. PQ further potentiated this release. The increase in
nitrite levels could be completely prevented by NOS inhibitors, while the
leakage of LDH was not attenuated by the inhibition of NO production with t
hem. On the other hand, ROS scavenging enzymes, superoxide dismutase and ca
talase, inhibited the leakage of LDH, whereas they had no effect on the inc
rease in the nitrite level. These results indicate that superoxide, not NO,
played a kev role in the cellular damage caused by PQ/cytokines. Our in vi
tro models demonstrate that NO has both beneficial and deleterious actions,
depending on the concentrations produced and model system used.