What initiates diabetic retinopathy?
A case for impaired retinal insulin signaling

Thomas W. Gardner (Hershey)

Purpose: To test the hypothesis that the retinal insulin receptor signaling pathway is altered in diabetes.
Methods: Sprague-Dawley rats were made diabetic with streptozotocin and maintained for 4 wks. Retinal proteins were assayed for insulin receptor, PI3-kinase, Akt, mTOR (mammalian target of rapamycin), p70 S6 kinase and glycogen synthase kinase 3ß (GSK-3ß) by phosphorylation of peptide substrates and immunoblotting with phospho-specific and total antibodies.
Insulin stimulated mTOR and p70 S6 kinase phosphorylation were tested in retinal explants under normo- and hyperglycemic conditions. Protein synthesis was determined by 3H-leucine incorporation in R28 retinal neurons grown in the
presence or absence of insulin, hyperglycemia and the mTOR inhibitor, rapamycin.
Results: Insulin receptor, PI3-kinase and Akt activities were reduced 25 – 50% in diabetic rat retinas. The phosphorylation and activity of mTOR, GSK-3ß and p70 S6 kinase were likewise reduced by 45 – 50% in retinas of diabetic rats and the change was partially reversed by insulin therapy. 20 mM glucose and 1.5 mM glucosamine likewise significantly reduced insulin-stimulated mTOR and p70 S6 kinase phosphorylation vs 5 mM glucose. Rapamycin and 20 mM glucose prevented insulin stimulated protein synthesis in retinal neurons.
Conclusion: Experimental diabetes and hyperglycemia reduce the constitutive activity of key enzymes that determine retinal neuronal protein synthesis. These changes are likely to contribute to impaired retinal protein synthesis and accelerated cell death in the pathogenesis of diabetic retinopathy.

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