DNA Strand Breaks and Poly(ADP-ribose) Synthetase Activation in Pancreatic Islets—A New Aspect to Development of Insulin-dependent Diabetes and Pancreatic B-cell Tumors
Alloxan and streptozotocin, which produce diabetes mellitus in experimental animals, have been known to inhibit various functions of pancreatic islets including proinsulin synthesis. However, little is known about the mechanisms underlying the action of these agents in pancreatic islets. Our recent in vivo and in vitro study using rats and isolated islets showed that one of the primary targets of the diabetogenic agents is the DNA of pancreatic islets. The first step is the generation of hydroxyl radical by alloxan which attacks DNA to produce strand breaks. In the case of streptozotocin, the alkylating activity of this compound may be causally related to its ability to induce DNA strand breaks. Subsequently, the fragmented DNA activates poly(ADP-ribose) synthetase which depletes cellular NAD. Since NAD is the most abundant of cellular coenzymes and participates in many biological reactions in mammalian cells, the reduction in intracellular NAD to such a nonphysiological level may severely affect islet cell functions including proinsulin synthesis. These results, in turn, raise the possibility that insulin-dependent diabetes may be preventable by inhibiting the occurrence of DNA strand breaks or the poly(ADP-ribose) synthetase. In fact, by poly(ADP-ribose) synthetase inhibitors such as nicotinamide and picolinamide, alloxan- and streptozotocin-induced NAD depletion was completely prevented, and B-cell functions including proinsulin synthesis proceeded normally. However, poly(ADP-ribose) synthetase inhibitors did not prevent the DNA strand breaks at all. Therefore, B-cells may survive with the residual DNA damage within their genome. About one year after the combined administration to rats of alloxan or streptozotocin with poly(ADP-ribose) synthetase inhibitors, diabetes did not develop but islet B-cell tumors were found frequently. This suggests that insulindependent diabetes and B-cell tumors are closely related with respect to their developmental processes. In other words, DNA breaks initiate two kinds of pathological state in B-cells, one is degeneratively and the other is oncogenically expressed.