THE INCREASE IN H2O2-INDUCED APOPTOSIS BY ADP-RIBOSYLATION INHIBITORSIS RELATED TO CELL BLEBBING

HN and LN are two phenotypic variants of the U937 monocytic cell line which differ in their basal NAD content; they respond in an opposite w ay to oxidative stress in the presence of the poly(ADP-ribosyl)polymer ase (PARP) inhibitors 3-aminobenzamide (3ABA) and nicotinamide (NA): t he inhibitors protect HN cells from stress-induced apoptosis, while th ey enhance it on LN cells (Coppola et al., 1995, Exp. Cell Res. 221, 4 62-469). These opposite effects are due to two overlapping and contras ting phenomena occurring in LN cells, as shown by the hi-modal respons e of stressed LN cells to increasing 3ABA doses. Indeed H2O2-induced a poptosis is enhanced only at high 3ABA concentrations (i.e., sufficien t to inhibit also mono-ADP-ribosylations); lower 3ABA concentrations, which specifically inhibit PARP, also protect LN U937 from stress-indu ced apoptosis. Unlike HN U937, H2O2-induced apoptosis in LN cells is a ccompanied by cell blebbing. High 3ABA doses strongly enhance blebbing , leading to cellular fragmentation. Blebbing could be blocked by inte rfering with actin polymerization with cytochalasin B and D: this elim inated the increase in apoptosis due to 3ABA, suggesting that it is in deed the consequence of excess blebbing. This is supported by the unus ual finding that in U937 LN stressed in the presence of SABA or NA, bl ebbing, usually a late event in apoptosis, may even precede its onset. (C) 1995 Academic Press, Inc.