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-hr posttreatment incubation in fresh culture medium promotes apoptosis or necrosis, depending on the oxidant concentration. Addition of 3- aminobenzamide (3AB) during the recovery phase prevented necrosis and caused apoptosis. 3AB did not, however, affect the apoptotic response of cells treated with apogenic concentrations of H₂O₂. Cells exposed for 3 hr to 1.5 mM H₂O₂, while showing some signs of suffering, maintained a normal nuclear organization and good organelle morphology. At the biochemical level, the oxidant promoted the formation of Mb-sized DNA fragments and rapidly depleted both the adenine nucleotide and non-protein sulphydryl pools, which did not recover during posttreatment incubation in the absence or presence of 3AB. These results allow a novel interpretation of the concentration-dependent switch from apoptosis to necrosis. We propose that H₂O₂ activates the apoptotic response at the early times of peroxide exposure and that this process can be completed, or inhibited, during the posttreatment incubation phase. Inhibition of apoptosis leads to necrosis and can be prevented by 3AB via a mechanism independent of inhibition of poly(ADP-ribose)polymerase. As a corollary, the necrotic response promoted by high concentrations of H₂O₂ in U937 cells appears to be the result of specific inhibition of the late steps of apoptosis.