Glucose Suppresses Nrf2 Translocation and Increases Glutathione Levels in Diabetic Rat Hearts

Diabetic hearts may be very susceptible to mitochondrial damage during hyperglycemic conditions. These hyperglycemic events are often acute, dietary-derived, and increase oxidant production from a variety of pathways primarily from cellular substrate metabolism. Furthermore, the renin angiotensin system is inappropriately elevated during metabolic dysfunctions and contributes to the production of oxidants. Increases in oxidants may impair mitochondrial enzyme function, further contributing to metabolic derangement. Nuclear factor erythriod-2- related factor 2 (Nrf2), responsible for induction of an array of antioxidant genes including those required for glutathione (GSH) synthesis, initiates gene expression after its translocation to the nucleus. To ascertain the effects of a glucose bolus on the diabetic mitochondria and cardiac antioxidants, rats were gavaged with glucose and dissected at T0, T180, and T360 minutes. We hypothesized that an acute elevation in plasma glucose facilitates Nrf2 nuclear translocation and subsequent expression of Nrf2-associated genes. Three groups (n = 6-8/group) of 25 week old rats were utilized: 1) LETO (lean strain-control), 2) type II diabetic OLETF, and 3) OLETF + angiotensin receptor blocker (ARB; 10 mg olmesartan/kg/d x 8 wks). Nrf2 translocation was 45% lower in OLETF rats compared to LETO at T0, and ARB treatment increased Nrf2 translocation by 36% compared to OLETF at T0. Glucose suppressed Nrf2 translocation at T180 and T360 (38% and 58%, respectively) in both OLETF and ARB. LETO GSH levels remained stable, while levels increased 125% in OLETF by T360. ARB treatment had no effect over the measurement period. Glutathione cysteine ligase regulatory (GCLm) subunit protein expression increased 78% in OLETF compared to LETO at T180, while ARB treatment lowered expression by 30% despite the elevations in GSH levels. Complex I activity was lowered 32% in OLETF compared to LETO at T0, while ARB treatment had no effect. Complex II activity was increased 34% at T180 and returned to baseline at T360, while ARB treatment remained 44% higher than OLETF. These data suggest that ARB treatment improves Nrf2 translocation and complex II activity in type II diabetic rat hearts. However, GSH levels increase in diabetic hearts independent of AT1 signaling.