Oxidative stress is a major contributor to the pathophysiology of diabetes. To combat oxidative stress, the transcription factor nuclear factor erythroid-2-related factor 2 (Nrf2) promotes the expression of an array of antioxidant gene products. However, in both the retina of diabetic patients and in preclinical diabetes models, there is a failure to properly upregulate Nrf2 activity. We previously demonstrated that retinal REDD1 protein expression is enhanced by diabetes-induced hyperglycemia and necessary for both the rise in retinal reactive oxygen species and the development of visual dysfunction in diabetic mice. In the present study, we investigated the hypothesis that signaling events downstream of REDD1 act to limit the retinal Nrf2 antioxidant response to the diabetic metabolic environment. We found that REDD1 ablation not only enhanced retinal Nrf2 activity, but also prevented the suppressive effect of streptozotocin-induced diabetes as compared to wild type mice. Nuclear Nrf2 protein expression and activity were enhanced in REDD1 knockout human MIO-M1 retinal Müller cells in culture, independent of a change in Nrf2 mRNA abundance. REDD1 deletion prevented oxidative stress in response to hyperglycemic conditions, and this protective effect was absent upon Nrf2 knockdown. REDD1 suppressed Nrf2 stability independent of Keap1 by promoting GSK3-mediated nuclear exclusion and proteasomal degradation via the ubiquitin ligase adapter β transducin repeat containing protein (β-TrCP). In the retina of diabetic REDD1-deficient mice, enhanced GSK3 phosphorylation was associated with a decrease in oxidative stress as compared to diabetic wild type mice. Pharmacological inhibition was used to suppress GSK3 activity in both cells in culture and in the retina of diabetic mice. Remarkably, GSK3 inhibition prevented the suppressive effect on Nrf2. Overall these findings support therapeutic approaches targeting REDD1 to prevent diabetes-induced visual dysfunction.
W.P. Miller: None. A. Toro: None. J. Giordano: None. M.D. Dennis: None.
American Diabetes Association/Pathway to Stop Diabetes (1-14-INI-04 to M.D.D.); National Eye Institute (1F31EY031199-01)