Aims/hypothesis: A strong positive correlation has been found between lipid peroxidation product and vascular endothelial growth factor concentrations in the vitreous of patients with proliferative diabetic retinopathy. To establish a causal relation between diabetes-associated enhanced oxidative stress and vascular endothelial growth factor production, we evaluated two antioxidants, DL-alpha-lipoic acid and taurine, on retinal vascular endothelial growth factor protein and mRNA expression and on parameters of oxidative stress in streptozotocin-diabetic rats.
Methods: Our experiments were on control rats and streptozotocin-diabetic rats with a 6-week duration of diabetes, treated with or without DL-alpha-lipoic acid (100 mg x kg(-1) x d(-1), i.p.) or taurine (1% in the diet) starting from induction of diabetes. Vascular endothelial growth factor protein in retinal homogenates was assessed by sandwich ELISA with an affinity-purified polyclonal antibody and vascular endothelial growth factor mRNA by ribonuclease protection assay. Retinal lipid peroxidation products i.e. malondialdehyde plus 4-hydroxyalkenals were quantified with N-methyl-2-phenylindole. Retinal reduced and oxidized glutathione, ascorbate, dehydroascorbate, and sorbitol pathway intermediates were measured spectrofluorometrically, and taurine by reverse-phase HPLC.
Results: Vascular endothelial growth factor protein concentration (means +/- SD) was increased in diabetic rats compared with control rats (33+/-7 vs 19+/-5 pg/mg total protein, p < 0.01) This increase was attenuated by taurine (26+/-8, p < 0.05) and prevented by DL-alpha-lipoic acid (21+/-4, p < 0.01). Vascular endothelial growth factor mRNA abundance was reduced by 1.4-fold in diabetic rats compared with control rats and this decrease was attenuated but not completely prevented by both antioxidants. Malondialdehyde plus 4-hydroxyalkenal concentration was increased in diabetic rats compared with control rats, and both antioxidants arrested accumulation of lipid peroxidation products. Taurine, reduced glutathione, oxidized glutathione, ascorbate, dehydroascorbate and sorbitol pathway intermediate concentrations as well as oxidized glutathione/reduced glutathione and dehydroascorbate/ascorbate ratios were similar in control and diabetic rats treated with or without taurine.
Conclusion/interpretation: Oxidative stress is directly involved in up regulation of vascular endothelial growth factor protein in the retina during early diabetes.