Nrf2/Antioxidant Pathway Mediates Beta Cell Self-Repair After Damage By High-Fat Diet-Induced Oxidative Stress

Many theories have been advanced to better understand why beta cell function and structure relentlessly deteriorate during the course of type 2 diabetes (T2D). These theories include inflammation, apoptosis, replication, neogenesis, autophagy, differentiation, dedifferentiation, and decreased levels of insulin gene regulatory proteins. However, none of these have considered the possibility that endogenous self-repair of existing beta cells may be an important factor. To examine this hypothesis, we conducted studies with female Zucker diabetic fatty rats fed a highfat diet (HFD) for 1, 2, 4, 7, 9, 18, or 28 days, followed by a return to regular chow for 2-3 weeks. Repair was defined as reversal of elevated blood glucose and of inappropriately low blood insulin levels caused by a HFD, as well as reversal of structural damage visualized by imaging studies. We observed evidence of functional beta cell damage after a 9-day exposure to a HFD and then repair after 2-3 weeks of being returned to normal chow (blood glucose [BG] = 348 +/- 30 vs. 126 +/- 3; mg/dl; days 9 vs. 23 day, P < 0.01). After 18-and 28-day exposure to a HFD, damage was more severe and repair was less evident. Insulin levels progressively diminished with 9-day exposure to a HFD; after returning to a regular diet, insulin levels rebounded toward, but did not reach, normal values. Increase in beta cell mass was 4-fold after 9 days and 3-fold after 18 days, and there was no increase after 28 days of a HFD. Increases in beta cell mass during a HFD were not different when comparing values before and after a return to regular diet within the 9-, 18-, or 28-day studies. No changes were observed in apoptosis or beta cell replication. Formation of intracellular markers of oxidative stress, intranuclear translocation of Nrf2, and formation of intracellular antioxidant proteins indicated the participation of HFD/oxidative stress induction of the Nrf2/antioxidant pathway. Flow cytometry-based assessment of beta cell volume, morphology, and insulin-specific immunoreactivity, as well as ultrastructural analysis by transmission electron microscopy, revealed that short-term exposure to a HFD produced significant changes in beta cell morphology and function that are reversible after returning to regular chow. These results suggest that a possible mechanism mediating the ability of beta cells to self-repair after a short-term exposure to a HFD is the activation of the Nrf2/antioxidant pathway.