Nitric Oxide Mediates Apoptosis Induction Selectively in Transformed Fibroblasts Compared to Nontransformed Fibroblasts

Nitric oxide (NO) mediates apoptosis induction in fibroblasts with constitutive src or induced ras oncogene expression, whereas nontransformed parental cells and revertants are not affected. This direct link between the transformed phenotype and sensitivity to NO-mediated apoptosis induction seems to be based on the recently described extracellular superoxide anion generation by transformed cells, as NO-mediated apoptosis induction in transformed cells is inhibited by extracellular superoxide dismutase (SOD), by SOD mimetics and by apocynin, an inhibitor of NADPH oxidase. Furthermore, nonresponsive nontransformed cells can be rendered sensitive for NO-mediated apoptosis induction when they are supplemented with xanthine oxidase/xanthine as an extracellular source for superoxide anions. As superoxide anions and NO readily interact in a diffusion-controlled reaction to generate peroxynitrite, peroxynitrite seems to be the responsible apoptosis inducer in NO-mediated apoptosis induction. In line with this conclusion, NO-mediated apoptosis induction in superoxide anion-generating transformed cells is inhibited by the peroxynitrite scavengers ebselen and FeTPPS. Moreover, direct application of peroxynitrite induces apoptosis both in transformed and nontransformed cells, indicating that peroxynitrite is no selective apoptosis inducer per se, but that selective apoptosis induction in transformed cells by NO is achieved through selective peroxynitrite generation. The interaction of NO with target cell derived superoxide anions represents a novel concept for selective apoptosis induction in transformed cells. This mechanism may be the basis for selective apoptosis induction by natural antitumor systems (like macrophages, natural killer cells, granulocytes) that utilize NO for antitumor action. Apoptosis induction mediated by NO involves mitochondrial depolarization and is blocked by Bcl-2 overexpression.