Type 1 diabetes is thought to occur as a result of the loss of
insulin-producing pancreatic beta cells by an environmentally triggered autoimmune reaction. In rodent models of
diabetes, streptozotocin (STZ), a genotoxic methylating agent that is targeted to the beta cells, is used to trigger the initial cell death. High single doses of STZ cause extensive beta-cell
necrosis, while multiple low doses induce limited apoptosis, which elicits an autoimmune reaction that eliminates the remaining cells. We now show that in mice lacking the
DNA repair enzyme alkylpurine-
DNA-N-glycosylase (APNG), beta-cell
necrosis was markedly attenuated after a single dose of STZ. This is most probably due to the reduction in the frequency of base excision repair-induced strand breaks and the consequent activation of
poly(ADP-ribose) polymerase (PARP), which results in catastrophic
ATP depletion and cell
necrosis. Indeed, PARP activity was not induced in APNG(-/-) islet cells following treatment with STZ in vitro. However, 48 h after STZ treatment, there was a peak of apoptosis in the beta cells of APNG(-/-) mice. Apoptosis was not observed in PARP-inhibited APNG(+/+) mice, suggesting that apoptotic pathways are activated in the absence of significant numbers of
DNA strand breaks. Interestingly, STZ-treated APNG(-/-) mice succumbed to diabetes 8 months
after treatment, in contrast to previous work with
PARP inhibitors, where a high incidence of
beta-cell tumors was observed. In the multiple-low-dose model, STZ induced diabetes in both APNG(-/-) and APNG(+/+) mice; however, the initial peak of apoptosis was 2.5-fold greater in the APNG(-/-) mice. We conclude that APNG substrates are diabetogenic but by different mechanisms according to the status of APNG activity.