Poly(ADP-ribose) polymerase inhibition prevents both apoptotic-like delayed neuronal death and necrosis after H₂O₂ injury

Toxic reactive oxygen species (ROS) such as hydrogen peroxide, nitric oxide, superoxide, and the hydroxyl radical are generated in a variety of neuropathological conditions and cause significant DNA damage. We determined the effects of 3-aminobenzamide (AB), an inhibitor of the DNA repair enzyme poly(ADP-ribose) polymerase (PARP), on cell death in differentiated PC12 cells, a model of sympathetic neurons, after H₂O₂ injury. Exposure to 0.5 mM H₂O₂ resulted in a significant decrease in intracellular NAD(H), NADP(H), and ATP levels. This injury resulted in the death of 90% of the cells with significant necrosis early (2 h) after injury and increased apoptosis (12-24 h after injury), as measured by PS exposure and the presence of cytoplasmic oligonudeosomal fragments. Treatment with 2.5 mM AB restored pyridine nudeotide and ATP levels and ameliorated cell death (65% versus 90%) by decreasing the extent of both necrosis and apoptosis. Interestingly, we observed that H₂O₂-induced injury caused a delayed cell death exhibiting features of apoptosis but in which caspase-3 like activity was absent. Moreover, pretreatment with AB restored caspase-3-like activity. Our results suggest that apoptosis and necrosis are both triggered by PARP overactivation, and that maintenance of cellular energy levels after injury by inhibiting PARP shifts cell death from necrosis to apoptosis.