Mitochondrial DNA in β-Cells Is a Sensitive Target for Damage by Nitric Oxide

Increasing evidence indicates that nitric oxide (NO) may play a role in immune-mediated injury to β-cells. One site for the action of this agent is the mitochondrion. Although the exact targets for damage within this organelle have yet to be fully elucidated, a potential location for injury is mitochondrial DNA (mtDNA). Therefore, experiments were initiated to evaluate damage to mtDNA caused by NO. Both exogenous NO generation (spermine/NO adduct [sper/NO]) and endogenous production of NO (IL-1β) were studied. To study the effects of exogenously produced NO, neonatal rat islet cells in monolayers were exposed to varying doses of sper/NO for 30 min. Total cellular DNA was isolated and treated with alkali to produce strand breaks at abasic sites resulting from exposure to NO. Damage to mtDNA was evaluated using a quantitative Southern blot technique. The results showed that sper/NO caused dose-dependent damage to mtDNA. Additionally, mtDNA was found to be more sensitive to injury generated by either source than a similarly sized fragment of nuclear DNA. To evaluate the effects of endogenously produced NO, β-cell cultures were treated with IL-1β for 18 h. Other cultures were treated with IL-1β and an inhibitor of the inducible form of nitric oxide synthase, aminoguanidine. DNA was evaluated as described for the sper/NO studies. IL-1β caused appreciable damage to mtDNA, and this damage was reduced in mtDNA from cultures treated with IL-1β and aminoguanidine. These studies show that mtDNA is a sensitive target for NO generated both endogenously and exogenously and that this DNA is more vulnerable to NO-induced damage than nuclear DNA.