The recent discovery of the direct oxidation of
spermine via
spermine oxidase (SMO) as a mechanism through which specific antitumor
polyamine analogues exert their cytotoxic effects has fueled interest in the study of the
polyamine catabolic pathway. A major byproduct of
spermine oxidation is H2O2, a source of toxic
reactive oxygen species. Recent targeted
small interfering RNA studies have confirmed that SMO-produced
reactive oxygen species are directly responsible for oxidative stress capable of inducing apoptosis and potentially mutagenic DNA damage. In the present study, we describe a second catalytically active
splice variant protein of the human
spermine oxidase gene, designated SMO5, which exhibits substrate specificities and affinities comparable to those of the originally identified human
spermine oxidase-1, SMO/PAOh1, and, as such, is an additional source of H2O2. Importantly, overexpression of either of these SMO
isoforms in NCI-H157 human
non-small cell lung carcinoma cells resulted in significant localization of SMO
protein in the nucleus, as determined by confocal microscopy. Furthermore, cell lines overexpressing either SMO/PAOh1 or SMO5 demonstrated increased
spermine oxidation in the nucleus, with accompanying alterations in individual nuclear
polyamine concentrations. This increased oxidation of
spermine in the nucleus therefore increases the production of highly reactive H2O2 in close proximity to
DNA, as well as decreases nuclear
spermine levels, thus altering the protective roles of
spermine in
free radical scavenging and
DNA shielding, and resulting in an overall increased potential for oxidative DNA damage in these cells. The results of these studies therefore have considerable significance both with respect to targeting
polyamine oxidation as an
antineoplastic strategy, and in regard to the potential role of
spermine oxidase in
inflammation-induced
carcinogenesis.