FIG. 7.
Molecular mechanisms of peroxynitrite-mediated cell death. A number of pathological conditions are associated with the simultaneous generation of nitric oxide (NO) and O2•−. NO sources are restricted to the activity of the various NO synthases, whereas O2•− arises from multiple sources, including electron leak from the mitochondria, NADPH oxidase, xanthine oxidase, and uncoupling of NO synthases. Once a flux of NO and O2•− is produced simultaeously in close proximity, the generation of peroxynitrite is considerably enhanced. Peroxynitrite-dependent cytotoxicity is then mediated by a myriad of effects including lipid peroxidation, protein nitration and oxidation, DNA oxidative damage, activation of matrix metalloproteinases (MMP), and inactivation of a series of enzymes. Mitochondrial enzymes are particularly vulnerable to attacks by peroxynitrite, leading to reduced ATP formation and induction of mitochondrial permeability transition by opening of the permeability transition pore, which dissipates the mitochondrial membrane potential (Δψm). These events result in cessation of electron transport and ATP formation, mitochondrial swelling, and permeabilization of the outer mitochondrial membrane, allowing the efflux of several proapoptotic molecules, including cytochrome c and apoptosis-inducing factor (AIF). In turn, cytochrome c and AIF activate a series of downstream effectors that eventually lead to the fragmentation of nuclear DNA. In addition to its damaging effects on mitochondria, peroxynitrite inflicts more or less severe oxidative injury to DNA, resulting in DNA strand breakage which in turn activates the nuclear enzyme poly(ADP-ribose) polymerase (PARP). Activated PARP consumes NAD to build-up poly(ADP-ribose) polymers (PAR), which are themselves rapidly metabolized by the activity of poly(ADP-ribose) glycohydrolase (PARG). Some free PAR may exit the nucleus and travel to the mitochondria, where they amplify the mitochondrial efflux of AIF (nuclear to mitochondria cross-talk). Mild damage of DNA activates the DNA repair machinery. On the contrary, once excessive oxidative and nitrosative stress-induced DNA damage occurs, like in various forms of reperfusion injury and other pathophysiological conditions, the cell may be executed by apoptosis in case of moderate PTP opening and PARP activation with preservation of cellular ATP, or by necrosis in the case of widespread PTP opening and PARP overactivation, leading to massive NAD consumption and collapse of cellular ATP. [Derived from Pacher et al. (995) with permission from Elsevier.]