Survival and proliferation of cells expressing caspase-uncleavable poly(ADP-ribose) polymerase in response to death-inducing DNA damage by an alkylating agent

To determine whether caspase-3-induced cleavage of poly(ADP-ribose) polymer ase (PARP), a DNA damage-sensitive enzyme, alters the balance between survi val and death of the cells following DNA damage, we created stable cell lin es that express either caspase-uncleavable mutant or wild type PARP in the background of PARP (-/-) fibroblasts. The survival and apoptotic responses of these cells were compared after exposure to N-methyl-N'-nitro-N-nitrosog uanidine (MNNG), a DNA-damaging agent that activates PARP, or to tumor necr osis factor-alpha, which causes apoptosis without initial DNA damage. In re sponse to MNNG, the cells with caspase-uncleavable PARP were very resistant to loss of viability or induction of apoptosis. Most significantly, simila r to 25% of these cells survived and retained clonogenicity at a level of D NA damage that eliminated the cells with wild type PARP or PARP (-/-) cells . Expression of caspase-uncleavable PARP could not protect the cells from d eath induced by tumor necrosis factor, although there was a slower progress ion of apoptotic events in these cells. Therefore, one of the functions for cleavage of PARP during apoptosis induced by alkylating agents is to preve nt survival of the extensively damaged cells.