Diabetes Induces Accumulation of Carbonylated Proteins in the Rat Retinal Pigment Epithelium Independently of Oxidative Stress.

Diabetic retinopathy (DR) is a major complication of diabetes mellitus. Growing evidence shows that hyperglycemia causes not only microvascular damage but also retinal neural dysfunction. Although different metabolic pathways have been implicated, the exact mechanism behind retinal degeneration remains unclear. Hyperglycemic stimuli have been shown to reduce the function of the retinal blood-barrier (BRB) in both diabetic humans and animals. As part of the BRB, the retinal pigment epithelium (RPE) plays a key role in retinal function by regulating the flow of metabolites and ions between the choroidal blood supply and the outer retina, and by supporting photoreceptor cell functions. Therefore, RPE dysfunction can lead to retinal injury. To understand the role of RPE in DR, we studied oxidative stress in the RPE at the early onset of streptozotocin-induced diabetes in rats. We found a 60% increase in lipoperoxidation at 45 days of diabetes, along with a 50% reduction in ascorbic acid content. Oxidized proteins were significantly increased after 20 and 45 days of diabetes induction, and changes in cell-cell contacts were observed. Despite these findings, superoxide dismutase activity was greatly increased at 45 days of diabetes, while Nrf2 expression and levels of total and reduced glutathione, key regulators of cellular antioxidant capacity, were similar in control and diabetic rat RPE. Moreover, the increase in oxidized proteins was not affected by the antioxidant quercetin nor by the NOS inhibitor L-NAME. These findings suggest that protein carbonylation may impair protein function or turnover, which in turn leads to RPE damage.