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B. Menon, M. Kanwar, R. A. Kowluru; Role of Glyceraldehyde 3-Phosphate Dehydrogenase in the Failure of Diabetic Retinopathy to Halt After Reversal of Hyperglycemia. Invest. Ophthalmol. Vis. Sci. 2008;49(13):4908.
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Progression of diabetic retinopathy does not halt after re-institution of good glycemic control (GC) that had preceded a period of poor glycemic control (PC). Inhibition of glyceraldehyde 3-phosphate dehydrogenase (GAPDH) by poly(ADP-ribose) polymerase is postulated to regulate the major pathways implicated in the damage of endothelial cells in diabetes. The purpose of the study is to evaluate the role of GAPDH in the persistence of diabetic retinopathy after reinstitution of GC.
A group of streptozotocin- diabetic rats were maintained in poor glycemic control (PC, glycated hemoglobin, GHb>12.0%) for 6 months followed by GC (GHb<6.5%) for additional 6 months (PC→GC), and the other group was maintained in PC for the entire 12 months. At the end of the experiment retina was isolated and nuclear and cytosolic fractions were prepared by high speed ultracentrifugation. The enzyme activity of GAPDH was analyzed using a spectrophotometric method and its mRNA and protein expressions by quantitative real-time PCR and western blot technique respectively. Ribosylation was determined by first immunoprecipitating GAPDH in retina samples followed by immunoblot analysis using poly(ADP) ribose antibody.
GAPDH protein expression was decreased by 20% in the retina of diabetic rats and its enzyme activity in the cytosolic and nuclear fractions by 20% and 70% respectively. This was paralleled by reduction in the gene expressions of GAPDH in both cytosolic and nuclear fractions compared to the values obtained from the age-matched normal rats. Ribosylation of retinal GAPDH was increased by 25% in PC group. However, re-institution of GC that had followed six months of PC did not produce any beneficial effects; GAPDH enzyme activity remained subnormal in both nuclear and cytosolic fractions. In the same PC→GC rats, retinal GAPDH ribosylation also remained elevated by about 25 % compared to the normal rats.
Reversal of hyperglycemia has no significant effect on the inactivation of GAPDH, and sustained ribosylation of GAPDH appears to be one of the possible mechanisms responsible for the failure of GC to inhibit its inactivation. Thus, failure to halt progression of diabetic retinopathy after reestablishment of GC could be, in part, due to continued inactivation of GAPDH.
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