Apoptosis and necrosis following exposure of U937 cells to increasing concentrations of hydrogen peroxide: The effect of the poly(ADP-ribose)polymerase inhibitor 3-aminobenzamide

A 3-hr exposure of U937 cells to hydrogen peroxide (H2O2) followed by a 6-h r posttreatment incubation in fresh culture medium promotes apoptosis or ne crosis, depending on the oxidant concentration. Addition of 3-aminobenzamid e (3AB) during the recovery phase prevented necrosis and caused apoptosis. 3AB did not, however, affect the apoptotic response of cells treated with a pogenic concentrations of H2O2. Cells exposed for 3 hr to 1.5 mM H2O2, whil e showing some signs of suffering, maintained a normal nuclear organization and good organelle morphology. At the biochemical level, the oxidant promo ted the formation of Mb-sized DNA fragments and rapidly depleted both the a denine nucleotide and non-protein sulphydryl pools, which did not recover d uring posttreatment incubation in the absence or presence of 3AB. These res ults allow a novel interpretation of the concentration-dependent switch fro m apoptosis to necrosis. We propose that H2O2 activates the apoptotic respo nse at the early times of peroxide exposure and that this process can be co mpleted, or inhibited, during the posttreatment incubation phase. Inhibitio n of apoptosis leads to necrosis and can be prevented by 3 AB via a mechani sm independent of inhibition of po Ly(ADP-ribose) polymerase. As a corollar y, the necrotic response promoted by high concentrations of H2O2 in U937 ce lls appears to be the result of specific inhibition of the late steps of ap optosis. BIOCHEM PHARMACOL 58;11:1743-1750, 1999. (C) 1999 Elsevier Science Inc.