Endotoxin, the lipopolysaccharide component of gram-negative bacteria, is a
common contaminant of plasmid DNA preparations. The present study investig
ated the effect of endotoxin on gene transfection efficiency and the role o
f reactive oxygen species (ROS) in this process. Gene transfection studies
were performed in various cell types with cytomegalovirus-luciferase as a r
eporter plasmid and cationic liposome as a transfecting agent. The presence
of endotoxin in plasmid DNA preparations severely limited transgene expres
sion in macrophages but had little or no effect in other cell types tested.
This decreased transfection was dependent on ROS-mediated cellular toxicit
y induced by endotoxin. Neutralizing the endotoxin by the addition of polym
yxin B effectively increased transfection efficiency and reduced toxicity.
Electron spin resonance studies confirmed the formation of ROS in endotoxin
-treated cells and their inhibition by free radical scavengers. The ROS sca
venger N-t-butyl-alpha -phenylnitrone, the H2O2 scavenger catalase, and the
. OH scavenger sodium formate effectively inhibited endotoxin-induced effe
cts, whereas the O-2(-) scavenger superoxide dismutase had lesser effects.
These results indicate that multiple oxidative species are involved in the
transfection inactivation process and that . OH formed by H2O2-dependent, m
etal-catalyzed Fenton reaction play a major role in this process.