Effects of glucose and alpha-tocopherol on low-density lipoprotein oxidation and glycation

Design and Investigation of New Water-Soluble Forms of α-Tocopherol with Antioxidant and Antiglycation Activity Using Amphiphilic Copolymers of N-Vinylpyrrolidone

Water-soluble forms of α-tocopherol (TP) as an effective antioxidant were obtained by encapsulating it into nanoparticles (NPs) of amphiphilic copolymers of N-vinylpyrrolidone with triethylene glycol dimethacrylate (CPL1-TP) and N-vinylpyrrolidone with hexyl methacrylate and triethylene glycol dimethacrylate (CPL2-TP) synthesized by radical copolymerization in toluene. The hydrodynamic radii of NPs loaded with TP (3.7 wt% per copolymers) were typically ca. 50 or 80 nm depending on copolymer composition, media, and temperature. Characterization of NPs was accomplished by transmission electron microscopy (TEM), IR-, and ¹H NMR spectroscopy. Quantum chemical modeling showed that TP molecules are capable to form hydrogen bonds with donor groups of the copolymer units. High antioxidant activity of both obtained forms of TP has been found by the thiobarbituric acid reactive species and chemiluminescence assays. CPL1-TP and CPL2-TP effectively inhibited the process of spontaneous lipid peroxidation as well as α-tocopherol itself. The IC₅₀ values of luminol chemiluminescence inhibition were determined. Antiglycation activity against vesperlysine and pentosidine-like AGEs of TP water-soluble forms was shown. The developed NPs of TP are promising as materials with antioxidant and antiglycation activity and can be used in various biomedical applications.

The association between blood glucose and oxidized lipoprotein(a) in healthy young women

Background

Oxidized lipoproteins play important roles in the atherosclerotic processes. Oxidized lipoprotein(a) (oxLp(a)) may be more potent in atherosclerotic pathophysiology than native Lp(a), a cardiovascular disease-relevant lipoprotein. Increased blood glucose concentrations can induce oxidative modification of lipoproteins. The aim of this study was to investigate the association between circulating oxLp(a) and cardiometabolic variables including blood glucose in healthy volunteers within the normal range of blood glucose.

Methods

Several cardiometabolic variables and serum oxLp(a) (using an ELISA system) were measured among 70 healthy females (mean age, 22 years).

Results

Lp(a) and glucose were significantly and positively correlated with oxLp(a) in simple correlation test. Furthermore, a multiple linear regression analysis showed oxLp(a) to have a weakly, but significantly positive and independent correlation with only blood glucose (β = 0.269, P < 0.05).

Conclusions

These results suggest that increased glucose may enhance the oxidization of Lp(a) even at normal glucose levels.

Low-density-lipoprotein (LDL)-bound flavonoids increase the resistance of LDL to oxidation and glycation under pathophysiological concentrations of glucose in vitro

The higher susceptibility of low-density lipoprotein (LDL) to oxidation and glycation in diabetes has been shown to be related to poor glycemic control. The aim of this study was to determine whether LDL-bound flavonoids attenuate high-glucose (HG)-mediated LDL oxidation and glycation. For this purpose, human plasma was preincubated with individual flavonoids for 3 h, followed by sequential ultracentrifugation and extensive dialysis to remove unbound flavonoid samples. Enriched LDL was subsequently isolated and challenged for its resistance to oxidation and glycation. Results showed that glucose (5-30 mM) dose-dependently accelerates copper (Cu(2+))-mediated LDL oxidative modification. The enrichment of flavonoids such as luteolin, naringenin, and kaempferol significantly increased the resistance of LDL to oxidation and prevented endogenous alpha-tocopherol consumption caused by HG/Cu(2+) (p < 0.05). The long-term glycation of LDL, which was measured by advanced glycation endproducts (AGEs)-related fluorescence and boronate affinity chromatography, was found to be inhibited by LDL-bound flavonoids in the following order: rutin > luteolin > quercetin > kaempferol > naringenin > catechin approximately EC > naringin. Moreover, a solid-phase extraction system with HPLC-diode array detection provided evidence that flavonoids were bound to LDL particles to a certain extent concurrently facilitating the lipoprotein antioxidant and antiglycation activities. In conclusion, this study supports the hypothesis that HG promoted oxidative and glycative modifications of LDL. This is the first study to show that the introduction of flavonoids into LDL particles protects the lipoprotein against glycotoxin-mediated adverse effects.

Acarbose ameliorates atherogenecity of low-density lipoprotein in patients with impaired glucose tolerance

Acarbose, an alpha-glucosidase inhibitor, is administered to control blood glucose levels. The drug also reduces the risk of cardiovascular disease, but the underlying mechanism is still to be elucidated. We therefore hypothesized that treatment with acarbose ameliorates the atherogenecity of low-density lipoprotein (LDL), a key molecule in atherogenesis. Patients with impaired glucose tolerance were or were not treated with acarbose (acarbose-treated group [n = 20] and control group [n = 20], respectively) for 3 months under dietary therapy. The oxidative susceptibility of LDL was determined by measuring lag time for the formation of dienes in the presence of CuSO(4). The lag time was significantly longer in the acarbose-treated group than in the control group before treatment. Moreover, the density gradient lipoprotein separation and disk polyacrylamide gel electrophoresis analyses showed that acarbose reduced the amount of small dense LDL, a more atherogenic and oxidatively susceptible form of LDL. We also found that the fatty acid composition of LDL changed after the treatment: polyunsaturated (omega-3) fatty acid, a beneficial substance for preventing cardiovascular disease, was significantly increased, whereas saturated fatty acids and triglyceride were decreased in the LDL of the acarbose-treated group. The present findings suggest that acarbose treatment reduces the risk of cardiovascular diseases by ameliorating the atherogenecity of LDL.

Recent advances in vitamin E metabolism and deficiency

Alpha-, beta-, gamma- and delta-tocopherol are present in many foods and are, in the absence of fat malabsorption, well absorbed from the gut. Their anti-oxidant property is well known and protects arteries and capillaries as well as blood lipids and nervous tissue against oxidative stress. In contrast to beta-, gamma- and delta-tocopherol, alpha-tocopherol is preferentially conserved by the discriminating action of the liver alpha-tocopherol transfer protein, which also maintains plasma alpha-tocopherol concentration within a range of 20 to 40 microM. In the circulation, alpha-tocopherol, in association with the transfer-protein, is assembled into the very low-density lipoprotein and low-density lipoprotein particles and released for use by the peripheral tissues. Recent data suggest that alpha-tocopherol is not only an anti-oxidant but also a regulator of gene expression through its binding to nuclear receptors. The precise mechanism of regulating gene expression, however, is still unknown. The four tocopherols are ultimately degraded by omega-oxidation and subsequent beta-oxidations followed by the elimination of the metabolites in the bile and in the urine. Patients with a defect of the alpha-tocopherol transfer protein are unable to maintain their alpha-tocopherol reserves and progressively lose tendon reflexes and have signs and symptoms of spinocerebellar ataxia while plasma vitamin E level drops below 2 microg/ml.