DIFFERENT BASAL NAD LEVELS DETERMINE OPPOSITE EFFECTS OF POLY(ADP-RIBOSYL)POLYMERASE INHIBITORS ON H2O2-INDUCED APOPTOSIS

We have recently described that poly(ADP-ribosyl)polymerase (PARP) inh ibitors rescue U937 cells from apoptosis induced by 1 mM H2O2 oxidativ e stress; PARP activation leads to a reversible drop in NAD level, whi ch could be blocked by PARP inhibitors (Nosseri et al., 1994, Exp. Cel l 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 mM H2O2, although ac tivating PARP, does not lower NAD concentration; puzzlingly, PARP inhi bitors increase (instead of decreasing, as occurs in HN cells) the ext ent of stress-induced apoptosis, leading to a reduced cell survival. N AD concentration could be 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 inhi bitors could rescue LN(+) cells from peroxide-induced apoptosis. H2O2- induced apoptosis is not triggered by NAD depletion, but instead it ta kes place only when NAD levels have been preserved or have recovered: on HN U937, peroxide doses (5 and 10 mM) which lead to necrosis induce an irreversible NAD drop, whereas apoptosis occurs only at lower dose s, where NAD depletion is reversible; on LN cells NAD levels do not dr op even upon 10 mM H2O2 treatment, and these cells die only by apoptos is; moreover, in HN cells apoptosis is not detectable until 8 h posttr eatment, when NAD levels recover, whereas in LN cells, where NAD is al ways present, apoptosis begins to take place as early as 3 h after str ess. (C) 1995 Academic Press, Inc.