Role of poly(ADP-ribose) polymerase (PARP) cleavage in apoptosis - Caspase3-resistant PARP mutant increases rates of apoptosis in transfected cells

An early transient burst of poly(ADP-ribosyl)ation of nuclear proteins was recently shown to be required for apoptosis to proceed in various cell line s (Simbulan-Rosenthal, C., Rosenthal, D., Iyer, S., Boulares, H., and Smuls on, M, (1998) J. Biol. Chem. 273, 13703-13712) followed by cleavage of poly (ADP-ribose) polymerase (PARP), catalyzed by caspase-3, This inactivation o f PARP has been proposed to prevent depletion of NAD (a PARP substrate) and ATP, which are thought to be required for later events in apoptosis, The r ole of PARP cleavage in apoptosis has now been investigated in human osteos arcoma cells and PARP -/- fibroblasts stably transfected with a vector enco ding a caspase-3-resistant PARP mutant. Expression of this mutant PARP incr eased the rate of staurosporine and tumor necrosis factor-alpha-induced apo ptosis, at least in part by reducing the time interval required for the ons et of caspase-3 activation and internucleosomal DNA fragmentation, as well as the generation of 50-kilobase pair DNA breaks, thought to be associated with early chromatin unfolding. Overexpression of wild-type PARP in osteosa rcoma cells also accelerated the apoptotic process, although not to the sam e extent as that apparent in cells expressing the mutant PARP, These effect s of the mutant and wild-type enzymes might be due to the early and transie nt poly(ADP-ribose) synthesis in response to DNA breaks, and the accompanyi ng depletion of NAD apparent in the transfected cells. The accelerated NAD depletion did not seem to interfere with the later stages of apoptosis, The se results indicate that PARP activation and subsequent cleavage have activ e and complex roles in apoptosis.