Different Basal NAD Levels Determine Opposite Effects of Poly(ADP-Ribosyl)Polymerase Inhibitors on H 2O 2-Induced Apoptosis

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We have recently described that poly(ADP-ribosyl)-polymerase (PARP) inhibitors rescue U937 cells from apoptosis induced by 1 m M H 2O 2 oxidative stress; PARP activation leads to a reversible drop in NAD level, which could be blocked by PARP inhibitors (Nosseri et al., 1994, Exp. Cell Res. 212, 367-373). A phenotypic variant of U937 is characterized by a lower basal NAD level (low NAD, LN U937, as opposed to the original high NAD, HN U937). In LN cells treatment with 1 m M H 2O 2, although activating PARP, does not lower NAD concentration; puzzlingly, PARP inhibitors increase (instead of decreasing, as occurs in HN cells) tile extent of stress-induced apoptosis, leading to a reduced cell survival. NAD concentration could he increased in LN cells by adding nicotinamide (5- and 25-fold increase) to the culture medium. These cells (LN +) behaved as HN U937: oxidative stress induced a NAD drop, the extent of which is dependent on the cells' basal NAD level; moreover, PARP inhibitors could rescue LN + cells from peroxide-induced apoptosis. H 2O 2-induced apoptosis is not triggered by NAD depletion, but instead it takes place only when NAD levels have been preserved or have recovered: on HN U937, peroxide doses (5 and 10 m M) which lead to necrosis induce an irreversible NAD drop, whereas apoptosis occurs only at lower doses, where NAD depletion is reversible; on LN cells NAD levels do not drop even upon 10 m M H 2O 2 treatment, and these cells die only by apoptosis; moreover, in HN cells apoptosis is not detectable until 8 h posttreatment, when NAD levels recover, whereas in LN cells, where NAD is always present, apoptosis begins to take place as early as 3 h after stress.