Mitochondrial DNA repair enzymes targeted to insulin secreting cells enhance cellular survival, and glycerol metabolism protects mice in streptozotocin induced type one diabetes

Reactive oxygen species (ROS) can be highly reactive and damaging to macromolecules such as mitochondrial DNA (mtDNA) (Balaban et al., 2005). One of the interests in our lab is the increase in ROS during the progression of diabetes mellitus and the damaging effect this may have on mtDNA. For this work, we asked whether targeting mtDNA repair proteins to insulin secreting cells would protect these cells from oxidative DNA damage. In initial studies, an insulin secreting cell line (INS-1) was stably transfected with a plasmid coding for a mitochondrial targeted DNA repair enzyme. The repair enzymes used were either human 8-oxoguanine DNA glycosylase (hOGG1) or E. coli's Endonuclease III (EndoIII). The successful targeting of the hOGG1 repair enzyme resulted in increased DNA repair and enhanced cellular viability following oxidative stress. For the next experiments, the protein transduction domain (PTD) of the HIV-1 TAT protein was fused to E. coli's Endonuclease III DNA repair glycosylase (MTS-EndoIII-Tat). Primary rat beta cells that were treated with fusion protein showed a trend toward enhanced DNA repair following oxidative stress. For the final experiments, we used fusion protein in an animal model of type I diabetes. In this study, diabetes was induced in Balb/c mice using multiple low-dose (MLD) streptozotocin (STZ) treatment. Although the TAT-fusion protein did not appear to lower blood glucose in the STZ treated mice, the glycerol containing buffer treatment significantly lowered blood glucose. When the animals were treated with the metabolically inactive form of glycerol, 1, 3-propanediol, the animals were not protected from STZ treatment, suggesting that the glycerol was able to protect by increasing the metabolic activity within the beta cells. In conclusion, we found that targeting mitochondrial DNA repair proteins to insulin producing cells can enhance mtDNA repair and viability, and surprisingly, we found that glycerol is protective in a mouse model of type I diabetes.