Abstract
Oxidative stress is a state in which the accumulation of reactive oxygen species exceeds the capacity of cellular antioxidant systems. Both apoptosis and necrosis are observed under oxidative stress, and we have reported that these two forms of cell death are induced in H2O2-stimulated HeLa cells depending on the concentration of H2O2. Weak H2O2 stimulation induces apoptosis, while strong H2O2 stimulation induces necrosis. However, the detailed mechanisms controlling the switching between these forms of cell death depending on the level of oxidative stress remain elusive. Here, we found that NAD+ metabolism is a key factor in determining the form of cell death in H2O2-stimulated HeLa cells. Under both weak and strong H2O2 stimulation, intracellular nicotinamide adenine dinucleotide (NAD+) was depleted to a similar extent by poly (ADP-ribose) (PAR) polymerase 1 (PARP1)-dependent consumption. However, the intracellular NAD+ concentration recovered under weak H2O2 stimulation but not under strong H2O2 stimulation. NAD+ recovery was mediated by nicotinamide (NAM) phosphoribosyltransferase (NAMPT)-dependent synthesis via the NAD+ salvage pathway, which was suggested to be impaired only under strong H2O2 stimulation. Furthermore, downstream of NAD+, the dynamics of the intracellular ATP concentration paralleled those of NAD+, and ATP-dependent caspase-9 activation via apoptosome formation was thus impaired under strong H2O2 stimulation. Collectively, these findings suggest that NAD+ dynamics balanced by PARP1-dependent consumption and NAMPT-dependent production are important to determine the form of cell death activated under oxidative stress.