Effects of Streptozotocin Exposure In Vitro on the Replication and Repair of DNA in Fetal Rat Pancreatic Islet Cells

Abstract

It was recently proposed that the role for poly(ADP-ribose) synthetase during DNA repair was exerted via a depletion of cellular NAD in order to slow down energy-requiring processes in the cell, notably DNA replication. This would enable the cell to more efficiently repair damaged DNA. In the present study fetal rat pancreatic islet cells were exposed to 2.2 mM streptozotocin (SZ). Using [3H]thymidine labeling and autoradiographic techniques, it was found that cellular nuclear silver grain counts, an index of DNA repair synthesis, were doubled after SZ exposure. However, autoradiographically measured DNA replication remained unaffected. Cellular NAD + NADH contents were reduced by 60% in the SZ-treated islets. Estimates of the poly(ADP-ribose) synthetase activity showed that this was doubled in islets exposed to SZ. Immediately after the SZ treatment the islets exhibited a 35% reduction in insulin secretion in response to 16.7 mM glucose. Taken together, the present findings do not favor the suggested role for poly(ADP-ribose) synthetase during DNA repair. Rather we observed an increased activity of this enzyme, a lowered cellular NAD + NADH content, and an increased rate of DNA repair synthesis concomitant with an unchanged DNA replicative rate in the SZ-exposed islets.