Potential Contributory Role of H-Ras, a Small G-Protein, in the Development of Retinopathy in Diabetic Rats

Abstract

Hyperglycemia is thought to be the underlying factor in the development of diabetic retinopathy, but the mechanisms involved remain partially understood. Diabetes increases oxidative stress, and reactive oxygen species affect the interactions between a small-molecularweight G-protein, H-Ras, and several of its effector proteins. The purpose of this study was to examine the regulatory role of H-Ras in glucose-induced apoptosis of retinal endothelial cells. The expressions of H-Ras and its effector protein (Raf-1) were compared in the retina obtained from diabetic rats (2-8 months' duration) and age-matched normal rats and in retinal endothelial cells exposed to high-glucose medium. The effect of inhibition of H-Ras function on glucose-induced capillary cell death and the potential involvement of oxidative stress in diabetes-induced activation of H-Ras were also determined. The expressions of H-Ras and Raf-1 were increased in the retina in diabetes, and antioxidant therapy prevented diabetes-induced increased protein and mRNA expressions. The inhibitors of Ras farnesylation inhibited glucose-induced nitric oxides and apoptosis in isolated retinal endothelial cells. Thus, the results suggest that functional activation of H-Ras might be one of the signaling steps involved in glucoseinduced capillary cell apoptosis and supports the role of H-Ras in the development of retinopathy in diabetes.

Effect of Long-Term Administration of α-Lipoic Acid on Retinal Capillary Cell Death and the Development of Retinopathy in Diabetic Rats

“…Accelerated apoptosis of endothelial cells can result in capillary closure because the increased turnover can prematurely exhaust the cell's replicative capability and because pericytes probably do not replicate in the adults (25), their accelerated death is likely to result in pericyte ghosts. Histopathology of diabetic retinopathy takes over a year to develop in rats (17,26), but apoptosis is a rapidly consummated phenomenon, and the cell contains fragmented DNA for only a few hours (27). Thus, a small number of TUNEL-positive cells observed in diabetic retina (8,9) along with defective endothelial replication may well be sufficient to account for the pericyte loss and formation of acellular capillaries.…”

“…The rats were weighed two times a week, and their food consumption was measured once every week to calculate the amount of ␣-lipoic acid consumed. GHb was measured at 2 months of diabetes, and every 3 months thereafter, using affinity columns (kit 442-B; Sigma Chemicals) (17). At the end of the experiment (11 months' duration), the rats were killed by overdose of pentobarbital, and the eyes were removed.…”

“…Administration of ␣-lipoic acid to diabetic rats has been shown to inhibit parameters of oxidative stress in various organs of diabetic rats, including kidney, nerve, and retina (10). ␣-Lipoic acid is also shown to inhibit diabetes-induced activation of a smallmolecular weight G-protein in the retina (H-Ras), increased levels of vascular endothelial growth factor, and leukostaisis (10,17,18), and these abnormalities have been postulated to play roles in the pathogenesis of retinopathy in diabetes. However, the effects of ␣-lipoic acid on retinal capillary cell apoptosis and the development of retinopathy are not known.…”

Role of matrix metalloproteinase-2 and -9 in the development of diabetic retinopathy

“…The PI3K/Akt and ERK1/2 pathways have been shown to regulate MMP-2 and -9 expressions in diabetes including diabetic retina [56,57]. Diabetes activates a small molecular weight G-protein, H-Ras, in the retina, and its capillary cells is implicated in the apoptosis of retinal capillary cells [54,57]. It was shown that H-ras regulate MMP-9 induction; hence, its activation is associated with increased vascular permeability in diabetes.…”

“…Results of a recent research suggest that the pathways activated by Ras and many of its intermediates play a crucial role in the diabetes-associated complications including retinopathy [23,[49][50][51]. Glucose-induced biochemical alterations induce ROS, often activate Raf-1-MAPK/ERK kinase (MEK)-ERK cascade via Ras, and activate a cascade of signaling pathways leading to cellular dysfunction or death [52][53][54][55]. The PI3K/Akt and ERK1/2 pathways have been shown to regulate MMP-2 and -9 expressions in diabetes including diabetic retina [56,57].…”