High Glucose Impairs Insulin Signaling in Retina

Abstract

Abstract: : Purpose: We have previously shown that diabetes impairs constitutive retinal insulin receptor signaling and accelerates retinal cell death. We hypothesized that elevated glucose alters retinal insulin signal transduction. Methods: Streptozotocin–induced diabetic rats were treated with insulin or phloridzin, a specific inhibitor of renal glucose uptake. Retinal explants from normal rats and R28 retinal neurons were treated with insulin (10nM) under normo (5mM)– and high glucose (25mM) conditions. The effects of pioglitazone, an insulin–sensitizing PPAR–γ agonist, on insulin stimulated R28 cells cultured in high glucose were examined. The insulin receptor and downstream kinases were analyzed by immunoblotting with phosphorylation–specific antibodies and enzymatic assays. Protein synthesis was measured by ³⁵S–methionine incorporation in R28 cells. Cell death was measured by TUNEL staining. Results: In retinal exlants, high glucose reduced insulin–stimulated retinal Akt and p70S6 kinase phosphorylation. High glucose also inhibited insulin–stimulated retinal cell protein synthesis in R28 retinal neurons. Furthermore, pioglitazone normalized the high glucose–induced reduction of insulin–stimulated Akt and p70S6K phosphorylation. Reducing blood glucose levels with insulin or phloridzin in diabetic rats significantly improved retinal insulin receptor kinase activity and reduced TUNEL–positive cells in rat retina. Conclusions: High glucose reduces insulin–stimulated pro–survival signal transduction and protein synthesis in retinal cells. These results strongly suggest that glucose–induced defective insulin signaling in the retina may contribute to features of early diabetic retinopathy such as increased apoptosis.

Keywords: diabetic retinopathy • signal transduction • cell death/apoptosis