Implication of NƐ-(Carboxymethyl)lysine in altered metabolism of low density lipoproteins from patients with type 2 diabetes and cardiovascular diseases

Low-density lipoproteins (LDL), the major cholesterol carriers of human plasma, contain a single copy of apolipoprotein B (apoB). ApoB, a ligand for the cell surface LDL receptor, has an important role in cholesterol metabolism. Biochemical modification of apoB, such as modification with advanced glycation end-products (AGEs) or glycoxidation can dramatically affect the functional integrity of LDL and subsequently impair capacity to be taken up by cell surface LDL receptors. Alternatively, AGE-LDL present in blood circulation due to hyperglycemia can result in increased LDL uptake by macrophages and therefore the increased atherogenic potential of LDL. Diabetic patients have elevated concentrations of AGE-modified LDL (AGELDL). NE-(Carboxymethyl)lysine (CML), the glycoxidation product, is the predominant AGE found in vivo. The purpose of this study was to clarify the role of the glycoxidation product, NE-(Carboxymethyl)lysine, on the metabolism of low-density lipoprotein (LDL) and give rise to increased risks of cardiovascular diseases in Type 2 diabetic patients. In this study LDL from diabetic patients and healthy controls were isolated, labeled with the fluorescence dye, 3, 3'-dioctadecyloxa-carbocyanine perchlorate (DiI), and subjected to modification, including oxidation, glycation, glycoxidation and carbomethylation. The susceptibility of LDL to in vitro oxidation was assessed. Receptor-mediated binding and uptake of fluorescently labeled native, oxidized-, AGE-, glycoxidized-and CML-LDL were studied in HepG2 cells. LDL from diabetic patients and AGE-LDL were more susceptible to copper-mediated oxidation than LDL from controls. The receptor-mediated uptake of fluorescently labeled LDL from Type 2 diabetes with cardiovascular diseases, glycoxidized-LDL and CML-LDL was significantly lower than that for LDL from both diabetics without CVD and healthy control subjects in HepG2 cells. From these results we conclude that glycoxidation and carbomethylation of LDL may be largely responsible for the defective uptake of LDL by the LDL receptor rather than AGE-LDL alone and these findings confirm the previous studies that CML is the major glycoxidation product present in vivo. CML may be considered as a marker of interest for early detection of Type 2 diabetic complications and may represent a target for the glycaemic control in those patients.