Streptozotocin (STZ) selectively destroys insulin-producing beta islet cells of the pancreas providing a model of type I diabetes. Poly(ADP-ribose) polymerase (PARP) is a nuclear enzyme whose overactivation by DNA strand breaks depletes its substrate NAD ؉ and then ATP, leading to cellular death from energy depletion. We demonstrate DNA damage and a major activation of PARP in pancreatic islets of STZ-treated mice. These mice display a 500% increase in blood glucose and major pancreatic islet damage. In mice with homozygous targeted deletion of PARP (PARP ؊͞؊), blood glucose and pancreatic islet structure are normal, indicating virtually total protection from STZ diabetes. Partial protection occurs in PARP ؉͞؊ animals. Thus, PARP activation may participate in the pathophysiology of type I diabetes, for which PARP inhibitors might afford therapeutic benefit.Type I diabetes (insulin-dependent diabetes mellitus) is a chronic metabolic disorder characterized by a loss of pancreatic islet B cell mass, decreased serum insulin, and hyperglycemia. Although the pathogenic mechanisms of this disease have not been fully characterized, genetic, environmental, and autoimmune factors have been postulated. In particular, development of this disorder is postulated to proceed through generation of oxygen radicals during prediabetic pancreatic islet inflammation (1, 2). One possible mechanism is that autoimmune activation of macrophages damages B cells through release of massive amounts of NO after inducible NO synthase activation, as damage elicited when islets are cocultured with macrophages is prevented by inhibition of NO synthase (3).Focal cerebral ischemic damage is also associated with NO release leading to DNA damage elicited by reactive oxygen species, including peroxynitrite formed from NO. Downstream of this DNA damage, the enzyme poly(ADP-ribose) polymerase (PARP, EC 2.4.2.30) is stimulated. Nuclear PARP is activated by DNA fragments to transfer branched chains of up to 200 ADP ribose groups from NAD ϩ to acceptor proteins in the nucleus, including histones and PARP itself. PARP activation plays a role in DNA repair, particularly the base excision repair process (4-8), in response to moderate amounts of DNA damage. With excessive DNA damage, however, PARP is so highly activated that its substrate NAD ϩ is critically depleted (9). NAD ϩ is an important enzyme in energy metabolism, and its depletion results in lower ATP production. As ATP is also consumed in efforts to resynthesize NAD ϩ , cells can die from energy loss. Cerebral ischemic damage is greatly diminished in mice with targeted deletion of PARP (PARP Ϫ͞Ϫ) (10, 11) and in animals treated with PARP inhibitors (12, 13). A role of PARP activation in pancreatic damage is also suggested by protection through PARP inhibition of pancreatic islet cells from NO-mediated killing (14, 15). Furthermore, in vitro pancreatic islet cells from PARP Ϫ͞Ϫ mice are resistant to NAD ϩ depletion after exposure to either NO or other reactive oxygen intermediates generated th...

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