INACTIVATION OF THE POLY(ADP-RIBOSE) POLYMERASE GENE AFFECTS OXYGEN RADICAL AND NITRIC-OXIDE TOXICITY IN ISLET CELLS

Activation of the nuclear enzyme poly(ADP-ribose) polymerase (PARP) is an early response of cells exposed to DNA-damaging compounds such as nitric oxide (NO) or reactive oxygen intermediates (ROI). Excessive po ly(ADP ribose) formation by PARP has been assumed to deplete cellular NAD(+) pools and to induce the death of several cell types, including the loss of insulin-producing islet cells in type I diabetes. In the p resent study we used cells from mice with a disrupted and thus inactiv ated PARP gene to provide direct evidence for a causal relationship be tween PARP activation, NAD(+) depletion, and cell death. We found that mutant islet cells do not show NAD(+) depletion after exposure to DNA damaging radicals and are more resistant to the toxicity of both NO a nd ROI. These findings directly prove that PARP activation is responsi ble for most of the loss of NAD(+) following such treatment. The ADP-r ibosylation inhibitor 3-aminobenzamide partially protected islet cells with intact PARP gene but not mutant cells from lysis following eithe r NO or ROI treatment. Hence the protective action of 3-aminobenzamide must be due to inhibition of PARP and does not result from its other pharmacological properties such as oxygen radical scavenging. Finally, by the use of mutant cells an alternative pathway of cell death was d iscovered which does not require PARR activation and NAD(+) depletion, In conclusion, the data prove the causal relationship of PARP activat ion and subsequent islet cell death and demonstrate the existence of a n alternative pathway of cell death pendent of PARP activation and NAD (+) depletion.