Pancreatic beta-cell death, regeneration and insulin secretion: roles of poly(ADP-ribose) polymerase and cyclic ADP-ribose.

In the early 1980s, we proposed a unifying model for beta-cell damage (The OKAMOTO model), in which poly(ADP-ribose) synthetase/polymerase (PARP) activation plays an essential role in the consumption of NAD+, which leads to energy depletion and necrotic cell death. In 1984, we demonstrated that the administration of PARP inhibitors to 90% depancreatized rats induces islet regeneration. From the regenerating islet-derived cDNA library we isolated Reg (Regenerating Gene) and demonstrated that Reg protein induces beta-cell replication via the Reg receptor and ameliorates experimental diabetes. More recently, we showed that the combined addition of IL-6 and dexamethasone induces the Reg gene expression in beta-cells and that PARP inhibitors enhance the expression. In 1993, we found that cyclic ADP-ribose (cADPR), a product synthesized from NAD+, is a second messenger for intracellular Ca2+ mobilization for insulin secretion by glucose, and proposed a novel mechanism of insulin secretion, the CD38-cADPR signal system. Therefore, PARP inhibitors prevent beta-cell necrosis, induce beta-cell replication and maintain insulin secretion.