Diabetic retinopathy risk correlates with intracellular concentrations of the glycoxidation product N(ε)-(carboxymethyl) lysine independently of glycohaemoglobin concentrations

Abstract

Aims/hypothesis. We investigated whether either the amount of diabetes- induced intracellular oxidative stress or the concentration of hyperglycaemia-induced advanced glycation endproducts is associated with the risk of diabetic retinopathy. Methods. We measured concentrations of the glycoxidation product N(ε)-(carboxymethyl)lysine and two non-oxidation- dependent advanced glycation endproducts (methylglyoxal-derived and 3- deoxyglucosone-derived) in CD45RA+ T-cells from 21 Type I (insulin- dependent) diabetic patients with and without diabetic retinopathy and from age-matched nondiabetic control subjects. Results. Intracellular concentrations of both oxidation-dependent N(ε)(carboxymethyl)lysine and oxidation-independent advanced glycation endproducts were increased in memory T-cells from diabetic patients. N(ε)(carboxymethyl)lysine: diabetic median- 24176 arbitrary units/mg protein (95% confidence interval 18690-34099 arbitrary units/mg protein); nondiabetic-9088 arbitrary units/mg protein (confidence interval 6994-10696 arbitrary units/mg protein; p < 0.0001). Methylglyoxal-derived advanced glycation end products: diabetic-5430 arbitrary units/mg protein (confidence interval 3458-13610); nondiabetic-271 arbitrary units/mg protein (confidence interval 61-760 arbitrary units/mg protein; p< 0.0001). 3-Deoxyglucosone-derived advanced glycation end products: diabetic-8070 arbitrary units/mg protein (confidence interval 7049- 16551 arbitrary units/mg protein); nondiabetic-1479 arbitrary units/mg protein (confidence interval 1169-3170; p< 0.0001). Only N(ε)- (carboxymethyl)lysine concentrations, however, inversely correlated with the duration of retinopathy-free diabetes (r = -0.51; p < 0.02). Diabetes- dependent N(ε)-(carboxymethyl)lysine accumulation did not correlate with age, diabetes duration, or averaged glycohaemoglobin concentrations. In vitro experiments with menadione and lymphocytes confirmed that N(ε)- (carboxymethyl)lysine concentrations reflect intracellular oxidative stress. Conclusion/interpretation. Monitoring intracellular concentrations of increased oxidative stress in long-lived CD45RA+ lymphocytes by markers such as N(ε)-(carboxymethyl)lysine possibly identifies a subgroup of patients at high risk for microvascular complications.