Poly(ADP-ribose)polymerase activation determines strain sensitivity to streptozotocin-induced β cell death in inbred mice

Abstract

Streptozotocin (STZ) is believed to induce pancreatic β cell death in mice by depleting the cell of NAD+NADH. The drug is known to cause a greater depletion of β cell NAD+NADH in C57bl/6J mice than in Balb/c mice. To investigate the basis for this strain difference, we compared the effects of streptozotocin on poly(ADP-ribose)polymerase (PARP) activation - the major site of NAD consumption, and on mitochondrial activity - the major site of NAD production. A significant strain difference was demonstrated in STZ- induced PARP activation (fmol NAD incorporated/min/μg DNA ± S.E.M.: Balb/c control 2.28 ± 0.14, Balb STZ 3.11 ± 0.25; C57bl/6J control 2.57 ± 0.29, C57bl/6J STZ 4.17 ± 0.24). In comparison, no strain difference could be demonstrated in hydrogen-peroxide-induced PARP activation: No strain differences could be detected in the activity of STZ-treated islet mitochondria as measured by determining ATP production (pmol/μg protein/h ± S.E.M.: Balb/c control 0.20 ± 0.02, Balb/c STZ 0.15 ± 0.02; C57bl/6J control 0.23 ± 0.03, C57bl/6J STZ 0-154 ± 0.02) or by 3-[4,5- dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT) dye reduction (change in optical density/mg protein ± S.E.M.: Balb/c control 10.19 ± 0.62, Balb/c STZ 6.01 ± 1.17; C57bl/6J control 6.15 ± 0.98, C57bl/6J STZ 5.81 ± 0.96). The strain difference in STZ-induced NAD depletion appears to be due to a difference in NAD consumption and not a difference in a mitochondrial process involved in replacing decreasing NAD concentrations. It is unlikely that a strain difference in the enzymic activity of PARP is responsible for strain differences in the effects of STZ, as no strain differences in hydrogen-peroxide-induced PARP activation could be detected. Thus the greater PARP activation, NAD depletion and β cell death observed in C57bl/6J islets may be due to greater levels of DNA damage or differences in the DNA excision repair processes.