The role of oxidative stress in the long-term glycation of LDL

Effects of Nutrients on Platelet Function: A Modifiable Link between Metabolic Syndrome and Neurodegeneration?

Metabolic syndrome increases the risk of vascular dementia and other neurodegenerative disorders. Recent studies underline that platelets play an important role in linking peripheral with central metabolic and inflammatory mechanisms. In this narrative review, we address the activation of platelets in metabolic syndrome, their effects on neuronal processes and the role of the mediators (e.g., serotonin, platelet-derived growth factor). Emerging evidence shows that nutritional compounds and their metabolites modulate these interactions-specifically, long chain fatty acids, endocannabinoids and phenolic compounds. We reviewed the role of activated platelets in neurovascular processes and nutritional compounds in platelet activation.

Lipids and Lipoproteins in Health and Disease: Focus on Targeting Atherosclerosis

Despite advances in pharmacotherapy, intervention devices and techniques, residual cardiovascular risks still cause a large burden on public health. Whilst most guidelines encourage achieving target levels of specific lipids and lipoproteins to reduce these risks, increasing evidence has shown that molecular modification of these lipoproteins also has a critical impact on their atherogenicity. Modification of low-density lipoprotein (LDL) by oxidation, glycation, peroxidation, apolipoprotein C-III adhesion, and the small dense subtype largely augment its atherogenicity. Post-translational modification by oxidation, carbamylation, glycation, and imbalance of molecular components can reduce the capacity of high-density lipoprotein (HDL) for reverse cholesterol transport. Elevated levels of triglycerides (TGs), apolipoprotein C-III and lipoprotein(a), and a decreased level of apolipoprotein A-I are closely associated with atherosclerotic cardiovascular disease. Pharmacotherapies aimed at reducing TGs, lipoprotein(a), and apolipoprotein C-III, and enhancing apolipoprotein A-1 are undergoing trials, and promising preliminary results have been reported. In this review, we aim to update the evidence on modifications of major lipid and lipoprotein components, including LDL, HDL, TG, apolipoprotein, and lipoprotein(a). We also discuss examples of translating findings from basic research to potential therapeutic targets for drug development.

Uremic Toxicity of Advanced Glycation End Products in CKD

Advanced glycation end products (AGEs), a heterogeneous group of compounds formed by nonenzymatic glycation reactions between reducing sugars and amino acids, lipids, or DNA, are formed not only in the presence of hyperglycemia, but also in diseases associated with high levels of oxidative stress, such as CKD. In chronic renal failure, higher circulating AGE levels result from increased formation and decreased renal clearance. Interactions between AGEs and their receptors, including advanced glycation end product-specific receptor (RAGE), trigger various intracellular events, such as oxidative stress and inflammation, leading to cardiovascular complications. Although patients with CKD have a higher burden of cardiovascular disease, the relationship between AGEs and cardiovascular disease in patients with CKD is not fully characterized. In this paper, we review the various deleterious effects of AGEs in CKD that lead to cardiovascular complications and the role of these AGEs in diabetic nephropathy. We also discuss potential pharmacologic approaches to circumvent these deleterious effects by reducing exogenous and endogenous sources of AGEs, increasing the breakdown of existing AGEs, or inhibiting AGE-induced inflammation. Finally, we speculate on preventive and therapeutic strategies that focus on the AGE-RAGE axis to prevent vascular complications in patients with CKD.

Advanced glycation end product accumulation: a new enemy to target in chronic kidney disease?

PURPOSE OF REVIEW

The critical role of advanced glycation end products (AGEs) in the progression of chronic diseases and their complications has recently become more apparent. This review summarizes the recent contributions to the field of AGEs in chronic kidney disease (CKD).

RECENT FINDINGS

Over the past 3 decades, AGEs have been implicated in the progression of CKD, and specifically diabetic nephropathy. Although numerous in-vitro and in-vivo studies highlight the detrimental role of AGEs accumulation in tissue injury, few prospective human studies or clinical trials show that inhibiting this process ameliorates disease. Nonetheless, recent studies have focused on the novel mechanisms that contribute to end-organ injury as a result of AGEs accumulation, as well as novel targets of therapy in kidney disease.

SUMMARY

As the prevalence and the incidence of CKD rises in the United States, it is essential to identify therapeutic strategies that either delay the progression of CKD or improve mortality in this population. The focus of this review is on highlighting the recent studies that advance our current understanding of the mechanisms mediating AGEs-induced CKD progression, as well as novel treatment strategies that have the potential to abrogate this disease process.

VIDEO ABSTRACT

http://links.lww.com/CONH/A12.

Beneficial effect of Azadirachta indica on advanced glycation end-product in streptozotocin-diabetic rat

CONTEXT

Both oxidation and hyperglycemia cause increased glycation and the formation of advanced glycation end-products (AGEs) which underlie the complications of diabetes.

OBJECTIVE

The goal of this article is to determine the effect of the chloroform extract from leaves of Azadirachta indica A. Juss; (Meliaceae) (AI) on the formation of glycated protein.

MATERIALS AND METHODS

Chloroform extract was subjected to in vitro bioassays to evaluate advanced glycation end-products formation. Bovine serum albumin (BSA)-glucose, BSA-methylglyoxal, Amadori-rich protein, glycated hemoglobin, oxidation, and glycation of LDL were determined. Doses of AI of 200 mg/kg/d by oral gavage were administered once daily for 30 d, at streptozotocin-induced diabetic rats. After this period, renal damage (TBARS), glucose, methylglyoxal, glycolaldehyde, and tail tendon collagen were investigated.

RESULTS AND DISCUSSION

AI exhibits protective action in BSA against glycation formation, GHb, protein levels, and LDL against glycation and oxidation. The renal glucose level decreases a 3.9 mg/g wet tissue. TBA-reactive substance showed a significant decrease to 1.82 mmol/mg protein. In addition, AI showed inhibitory activity against AGEs formation, methylglyoxal, and glycolaldehyde levels in kidney. Treatment with AI in rat tail tendon produced a reduction in cross-linking of collagen proteins. The antiglycation activities of A. indica were attributed in part to their antioxidant activity. AI alleviated oxidative stress under diabetic conditions through the inhibition of lipid peroxidation prevents the onset renal damage.

CONCLUSION

We found that A. indica is an inhibitor AGE formation, and oxidative stress with a renoprotective effect, which are considered to play important roles in diabetic kidney disease.

Evaluation of antidiabetic, antioxidant and antiglycating activities of the Eysenhardtia polystachya

Background:

Many diseases are associated with oxidative stress caused by free radicals. The aim of the present study was to evaluate the antidiabetic, antioxidant and antiglycation properties of Eysenhardtia polystachya (EP) bark methanol-water extract.

Materials and Methods

: The antioxidant capacities were evaluated by studying in vitro the scavenging of DPPH and ABTS free radical, reactive oxygen species such as RO₂, O₂·^-, H₂O₂, OH^., H₂O₂, ONOO-, NO, HOCl,¹ O₂, chelating ability, ORAC, β-carotene-bleaching and lipid peroxidation. The antiglycation activities of EP were evaluated by haemoglobin, bovine serum albumin (BSA)-glucose, BSA-methylglyoxal and BSA-glucose assays. Oral administration of EP at the doses of 100 mg/kg, 200 mg/kg and 400 mg/g was studied in normal, glucose-loaded and antidiabetic effects on streptozotocin-induced mildly diabetic (MD) and severely diabetic (SD) mice.

Results:

EP showed Hdonor activity, free radical scavenging activity, metal chelating ability and lipid peroxidation Antioxidant activity may be attributed to the presence of phenolic and flavonoid compounds. EP is an inhibitor of fluorescent AGE, methylglyoxal and the glycation of haemoglobin. In STZ-induced diabetic mice, EP reduced the blood glucose, increased serum insulin, body weight, marker enzymes of hepatic function, glycogen, HDL, GK and HK while there was reduction in the levels of triglyceride, cholesterol, TBARS, LDL and G6Pase.

Conclusions:

Eysenhardtia polystachya possesses considerable antioxidant activity with reactive oxygen species (ROS) scavenging activity and demonstrated an anti-AGEs and hepatoprotective role, inhibits hyperglycemic, hyperlipidemic and oxidative stress indicating that these effects may be mediated by interacting with multiple targets operating in diabetes mellitus.

Inhibition of Advanced Glycation End-Product Formation by Origanum majorana L. In Vitro and in Streptozotocin-Induced Diabetic Rats

The development of AGE inhibitors is considered to have therapeutic potential in patients with diabetes diseases. The aim of the present study was investigate the effect of methanolic extract of the leaves of Origanum majorana (OM) used as spice in many countries on AGEs formation. In vitro studies indicated a significant inhibitory effects on the formation of AGEs. Their antiglycation activities were not only brought about by their antioxidant activities but also related to their trapping abilities of reactive carbonyl species such as methylglyoxal, an intermediate reactive carbonyl of AGE formation. The results demonstrate that OM have significant effects on in vitro AGE formation, and the glycation inhibitory activity was more effectively than those obtained using as standard antiglycation agent aminoguanidine. OM is a potent agent for protecting LDL against oxidation and glycation. Treatment of streptozotocin-diabetic mice with OM and glibenclamide for 28 days had beneficial effects on renal metabolic abnormalities including glucose level and AGEs formation. Diabetic mice showed increase in tail tendon collagen, glycated collagen linked fluorescence and reduction in pepsin digestion. Treatment with OM improved these parameters when compared to diabetic control and glibenclamide.

Role of advanced glycation endproducts and potential therapeutic interventions in dialysis patients

It has been nearly 100 years since the first published report of advanced glycation end products (AGEs) by the French chemist Maillard. Since then, our understanding of AGEs in diseased states has dramatically changed. Especially in the last 25 years, AGEs have been implicated in complications related to aging, neurodegenerative diseases, diabetes, and chronic kidney disease. Although AGE formation has been well characterized by both in vitro and in vivo studies, few prospective human studies exist demonstrating the role of AGEs in patients on chronic renal replacement therapy. As the prevalence of end-stage renal disease (ESRD) in the United States rises, it is essential to identify therapeutic strategies that either delay progression to ESRD or improve morbidity and mortality in this population. This article reviews the role of AGEs, especially those of dietary origin, in ESRD patients as well as potential therapeutic anti-AGE strategies in this population.

Mechanisms of LDL oxidation

BACKGROUNDS

Many lines of evidence suggest that oxidized low-density lipoprotein (LDL) is implicated in the pathogenesis of atherosclerotic vascular diseases. This review summarizes a diversity of mechanisms proposed for LDL oxidation serving for the so-called "LDL oxidation hypothesis of atherogenesis".

METHODS AND RESULTS

We investigated the literature and our research results related to mechanisms of LDL oxidation and its atherogenesis. LDL oxidation is catalyzed by transition metal ions and several free radicals, and LDL is also oxidized by some oxidizing enzymes. In this way, LDL can be converted to a form that is recognized specifically by and with high affinity to macrophage scavenger receptors, leading to foam cell formation, the defining characteristic of fatty streak lesions.

CONCLUSIONS

Several pathways are involved in the promotion of LDL oxidation in vitro and in vivo, but it would appear that the physiologically relevant mechanisms of LDL oxidation are still imperfectly understood. The underlying mechanisms of LDL oxidation must be further explored to reveal appropriate ways for the diagnosis and treatment of atherosclerosis and its relevant diseases.

Linking diabetes and atherosclerosis

Cardiovascular diseases are the major causes of morbidity and mortality in people with diabetes. Macroangiopathy in diabetes is manifested by more accelerated and progressive atherosclerosis, which is more widely distributed. The pathogenesis of this accelerated atherosclerosis is multifactorial and includes very complex interactions. Several abnormalities - such as hyperglycemia, dyslipidemia, hypertension, endothelial dysfunction, renin-angiotensin system activation and chronic subclinical inflammation - all appear to play important roles in the development of diabetes-induced atherosclerosis. Treatment of the residual risk, other than glycemia, blood pressure and low-density lipoprotein cholesterol, remains important as the rate of diabetes increases worldwide. A synergistic multifactorial approach against both conventional cardiovascular risk factors and emerging risk factors, such as vasoactive systems, the AGE-RAGE axis, novel proteins, such as TRAIL, and the complement system, as well as oxidative stress and inflammation, may be a promising way to prevent macrovascular disease in diabetes. In this review we focus on the major causes and mechanisms of atherosclerotic disease in patients with diabetes and highlight emerging targets for therapeutic intervention.

Glycation as an atherogenic modification of LDL

PURPOSE OF REVIEW

To highlight the potential importance of glycation as an atherogenic modification of LDL in both diabetic and nondiabetic people.

RECENT FINDINGS

Small dense LDL which is known to be most closely associated with atherogenesis is more susceptible to glycation than more buoyant LDL. Glycation and oxidation of LDL appear to be intimately associated.

SUMMARY

Glycation of LDL occurs chiefly due to the nonenzymatic reaction of glucose and its metabolites with the free amino groups of lysine in which LDL is rich. Higher concentrations of glycated LDL are present in diabetic than in nondiabetic individuals, but even in the latter, there is generally more circulating glycated LDL than oxidatively modified LDL. Probably, oxidation and glycation of LDL are at least partially interdependent, but both prevent LDL receptor-mediated uptake and promote macrophage scavenger receptor uptake. The recognition that LDL glycation is at least as important as oxidation in atherogenesis may lead to improvements in our understanding of its mechanism and how to prevent it.

Comparative binding of 125I-and 99mTc-d-labeled native and glycated low-density lipoprotein to human microvascular endothelial cells-potential for atherosclerosis imaging?

Native (n), glycated (g), and glycoxidated (go) low-density lipoproteins (LDL) were labeled with 125I or 99mTc, and the labeling efficiency and binding were assessed for potential use of these LDL compounds in imaging analysis of atherosclerotic lesions (PPAR-gamma receptors) by determining the number of specific receptors for nLDL, gLDL or goLDL on human microvascular endothelial cells as well as the KDs using either 125I-or 99mTc-labeled LDLs. The specific activity of labeled gLDL and goLDL was much higher (for goLDL 20 times higher) than that of nLDL. Gel filtration of labeled LDLs revealed, however, that 99mTc-g/goLDL is significantly degraded by the labeling reaction. No fragmentation was observed for 99mTc-nLDL and all the 125I-labeled LDL forms. Binding studies using both 125I-and 99mTc-nLDL indicated a weak binding affinity (KD 10- 7mol/L) to human microvascular endothelial cells. The binding affinity of 125I-g/goLDL to these cells was significantly higher (KD 10- 9mol/L) and could be increased further by preactivation of the endothelial cells using TNFalpha. Incubation with 99mTc-goLDL, however, did not result in specific binding of the ligand, possibly as a consequence of the fragmentation of the lipoprotein during the labeling. Scatchard transformation of the binding data with 99mTc-gLDL revealed the presence of only a few binding sites. This was in contrast to the results obtained with 125I-labeled gLDL, which revealed a much higher membrane density of scavenger receptors for this ligand. We conclude that for in vitro binding studies as well as for potential in vivo imaging, only 125I-labeled goLDL should be used, whereas nLDL may be applied as 125I-or 99mTc-labeled ligand.

Effect of simvastatin on the oxidation of native and modified lipoproteins

Modified (oxidized) low-density lipoprotein (LDL) plays a significant role in atherosclerosis by accumulation in arteries. Also, glycated LDL, such as in diabetics, are increasing the risk for atherosclerosis, due to an increased oxidizability as compared to native LDL. For these reasons, the potential inhibition of such modifications is of clinical importance. We investigated the influence of simvastatin on oxidation of native and modified LDL as well as high-density lipoprotein (HDL), which plays a protective role in atherosclerosis. Quantitative assessment of the oxidation end-product malondialdehyde (MDA) revealed the highest inhibitory rate for HDL at concentrations of 1.6 microg/ml and 0.8 microg/ml by 30.3% and 20.4%, at 6 h and 4 h, respectively. At 24 h, the inhibition was still persisting amounting to 27.9% and 20.3%, respectively. For native LDL, we found less inhibition of oxidation at a concentration of 1.6 microg/ml amounting to 19.2% and 11.5%, for 4 h and 6 h, respectively. Similar effects were found at a concentration of 0.8 microg/ml. For modified, glycated LDL, the most pronounced effect was found at a concentration of 1.6 microg/ml amounting to 22.4% for the period of 2-24 h of oxidation. For glycoxidated LDL, the inhibition of oxidation was less expressed amounting to 10.1% for the period of 2-6 h at the same concentration. The influence of simvastatin on lag time (protection from oxidation) by diene conjugation was also investigated. At the highest concentration of simvastatin (1.6 microg/ml), we found a prolongation of lag time from 73 min to 99 min for native LDL, for glycoxidated LDL 60 min to 89 min and for HDL 54 min to 64 min. For glycated LDL, only a small decrease of lag time (66 min versus 71 min) at same concentration was observed. For glycated and glycoxidated LDL, we found a moderate increase in relative electrophoretic mobility (REM) by 2.0 and 2.3, respectively, but no changes in the presence of simvastatin were observed. These data show that simvastatin besides its lipid-lowering action has also significant antioxidative properties.

Troglitazone Inhibits Long-Term Glycation and Oxidation of Low-Density Lipoprotein

Troglitazone (T) is a member of a new class of antidiabetic drugs termed thiazolidinediones (TZDs), which has previously been used as an anti-diabetic agent. In this study we investigated the influence of T, a ligand for PPAR-gamma receptor, on copper-catalyzed or cell-mediated oxidation of native, glycated, and glycoxidated low-density lipoprotein (LDL). A dose-dependent inhibition of copper-mediated low-density lipoprotein-oxidation, as monitored by the formation of oxidation-specific fluorescence, was observed for both native and glycated low-density lipoprotein. At the concentration of 20 microg/mL the inhibition amounted from 14.7% to 64.7% by all low-density lipoprotein forms. For glycated low-density lipoprotein we obtained the highest oxidation rate, but the most pronounced inhibition by T was found for glycoxidated low-density lipoprotein (goLDL). Inhibitory effects of T were also investigated by measurement of relative electrophoretic mobility (REM) in the concentration range of 0 to 20 microg/mL. The inhibition of 4h oxidation of native low-density lipoprotein was found in the entire concentration range, but significance was seen at 10 microg/mL. The long-term glycation and glycoxidation of low-density lipoprotein as measured by 5-hydroxymethyl-2-furaldehyde (5-HMF) formation and binding of fructosamine was found to be inhibited by T. In endothelial cell-mediated oxidation of low-density lipoprotein cytotoxicity of T in the concentration range of 0 to 160 microg/mL during 2 to 24 h oxidation was investigated. In the non-cytotoxic concentration range of 5 to 20 microg/mL, a significantly reduced liberation of isoprostane 8-epi-PGF2alpha during 24 h cell-mediated oxidation of low-density lipoprotein and its modifications was found. This inhibitory action of T was most significant in the case of goLDL and amounted to approximately 20% to 60% inhibition at 5 to 20 microg/mL T, respectively. In the concentration range of 40 to 160 microg/mL, however, T showed an increasing cytotoxic action, as evidenced by loss of cell adhesion, loss of cellular protein, morphological changes, and cell disintegration as well as by strongly enhanced troglitazone-mediated isoprostane 8-IP liberation (fivefold to sixfold). T may be used as a model to explore the thiazolidinediones' mechanism on oxidation in a more general aspect for treatment for T2DM, because T is not clinically available.

Protective action on human LDL against oxidation and glycation by four organosulfur compounds derived from garlic

Human LDL were used to study the protective action of four organosulfur compounds (diallyl sulfide, DAS; diallyl disulfide, DADS; S-ethylcysteine, SEC; N-acetylcysteine, NAC) derived from garlic against oxidation and glycation. The four organosulfur compounds significantly inhibited superoxide production by xanthine-xanthine oxidase (P < 0.05) and showed marked copper-chelating capability. DAS and DADS exhibited greater antioxidant activities against copper- and amphotericin B-induced LDL oxidation (P < 0.05) than SEC and NAC. However, SEC and NAC were more effective in sparing LDL alpha-tocopherol (P < 0.05). When oxidation was minimized, SEC was the most powerful agent against LDL glycation (P < 0.05); however, DADS was superior to other agents in suppressing both oxidation and glycation when LDL oxidation occurred simultaneously with glycation. These results suggest that the four organosulfur compounds derived from garlic are potent agents for protecting LDL against oxidation and glycation, and that they may benefit patients with diabetes mellitus or cardiovascular diseases by preventing complications.

Glycated high-density lipoprotein regulates reactive oxygen species and reactive nitrogen species in endothelial cells

Nonenzymatic glycosylation of plasma proteins may contribute to the excess risk of developing atherosclerosis in patients with diabetes mellitus. Although it is believed that high-density lipoprotein (HDL) is glycosylated at an increased level in diabetic individuals, little is known about a possible linkage between glycated HDL and endothelial dysfunction in diabetes. To clarify whether glucose-modified HDL affects the function of endothelial cells, we first examined herein the level of H(2)O(2) generation from cultured human aortic endothelial cells (HAECs) exposed to a glycated oxidized HDL (gly-ox-HDL) prepared in vitro. Incubation for 48 hours with 100 microg/mL of gly-ox-HDL induced significant release of H(2)O(2) from cells and gly-ox-HDL-induced H(2)O(2) formation was inhibited in the presence of diphenyleneiodonium, an inhibitor of NADPH oxidase. In addition, stimulation of HAECs with gly-ox-HDL for 48 hours elicited a marked downregulation of catalase and Cu(2+), Zn(2+)-superoxide dismutase (CuZn-SOD), suggesting H(2)O(2) formation by gly-ox-HDL to be due to a disturbance involving oxidant and antioxidant enzymes in the cells. Treatment of HAECs with gly-ox-HDL attenuated the expression of endothelial nitric oxide synthase (eNOS), but not inducible nitric oxide synthase (iNOS), and this was followed by decreased production of nitric oxide (NO) by the cells. Furthermore, in vitro experiments with glycated HDL (gly-HDL) in the presence of 2 mmol/L EDTA and Cu(2+)-oxidized HDL suggested the effect of gly-HDL on endothelial function to be possibly potentiated by additional oxidative modification. Taking all of the above findings together, gly-ox-HDL may lead to the deterioration of vascular function through altered production of reactive oxygen species and reactive nitrogen species in endothelial cells.

Molecular inflammation hypothesis of aging based on the anti-aging mechanism of calorie restriction

Accumulating evidence strongly suggests that oxidative stress underlies aging processes. Research provides consistent evidence that calorie restriction (CR) reduces age-related oxidative stress and has anti-inflammatory properties. However, information is lacking on the molecular mechanism that would better define the interrelation of reactive oxygen species and nitrogen species and the pro-inflammatory states of the aging process. In this review, the biochemical and molecular bases of the inflammatory process in the aging process are analyzed to delineate the molecular inflammation hypothesis of aging. The key players involved in the proposed hypothesis are the age-related upregulation of NF-kappa B, IL-1 beta, IL-6, TNFalpha, cyclooxygenase-2, and inducible NO synthase, all of which are attenuated by CR. Furthermore, age-related NF kappa B activation is associated with phosphorylation by I kappa B kinase/NIK and MAPKs, while CR blocked these activation processes. The modulation of these factors provides molecular insights of the anti-inflammatory action of CR in relation to the aging process. Based on available finding and our recent supporting evidence, we prefer to use "molecular inflammation" to emphasize the importance of the molecular reaction mechanisms and their aberrance, predisposing to fully expressed chronic inflammatory phenomena. It was further proposed that CR's major force of the regulation of redox-sensitive inflammation may well be its life-prolonging action.

Corneal advanced glycation end products increase in patients with proliferative diabetic retinopathy

OBJECTIVE

To evaluate corneal advanced glycation end product (AGE) fluorescence in patients with diabetes and in healthy control subjects.

RESEARCH DESIGN AND METHODS

Corneal autofluorescence was measured in 26 eyes of 26 patients with type 2 diabetes (mean age 57.0 years; mean disease duration 12.2 years; mean HbA1c 7.1%) and 13 eyes of 13 healthy age-matched control subjects (mean age 57.9 years). The patients with type 2 diabetes were divided into the following groups: patients without diabetic retinopathy (DR), patients without proliferative diabetic retinopathy (PDR), and patients with PDR. Corneal autofluorescence was measured by fluorophotometry with the wavelength that is characteristic of AGE fluorescence (excitation and emission 360-370 nm and 430-450 nm, respectively). We defined peak corneal autofluorescence levels as corneal AGE fluorescence values. We compared the corneal AGE fluorescence values in the four groups.

RESULTS

In the PDR group (11.9 +/- 3.9 arbitrary units [mean +/- SD]), the corneal AGE fluorescence values were significantly higher compared with the control subjects (6.9 +/- 1.3 arbitrary units), the patients without DR (7.4 +/- 2.1 arbitrary units), and the patients without PDR (6.9 +/- 2.2 arbitrary units) (P < 0.05).

CONCLUSIONS

We found that corneal AGEs may increase in patients with diabetes and PDR compared with control subjects, patients without DR, and patients without PDR. In the patients with PDR, increased corneal AGEs may play a role in diabetic keratopathy.

Calcium antagonists as inhibitors of in vitro low density lipoprotein oxidation and glycation

The peroxidation step in lipid transformation is considered to be essential in the pathogenesis of atherosclerosis. Calcium antagonists (CA) appear to have antioxidant effects in addition to their potent vasorelaxant properties. In the present study, we compared the antioxidative efficacy of CA (amlodipine, lacidipine, nifedipine, isradipine, diltiazem, and semotiadil) in the copper-catalysed oxidation of low-density lipoprotein (LDL) with that of glycated(g)/glycoxidated(go) LDL. This issue is of great importance when considering the potential therapeutic use of antioxidant drugs in diabetes-associated vasculopathy. Oxidation of native LDL was inhibited most efficiently (>90%) by lacidipine and semotiadil in the concentration range 10(-4)-10(-3) M. We found, however, a dramatic decrease in antioxidant activity towards g/goLDL as compared to native LDL in all the CA tested. Only lacidipine significantly inhibited copper-mediated oxidation of g/goLDL in the whole concentration range tested (10(-5) M-10(-3) M). This probably resulted from the increased auto-oxidative potential introduced by early and advanced glycation end products (AGE) into the g/goLDL. We noted that coincubation of LDL with 10(-3) M CA and 0.5 M glucose under oxidative/non-oxidative conditions partially or fully restored the antioxidant capacity of the different CA to inhibit the subsequent copper-catalysed oxidation of the modified LDL. This is a clear indication that CA inhibit glycative or glycoxidative LDL changes during the preceding long-term glycation period. The notion that both oxidative changes and long-term glycation effects were reduced by CA was corroborated by fluorescence analysis, AGE-ELISA, quantitation of lipid peroxidation, and thiobarbituric acid reactive substance (TBARS) measurement of long-term g/goLDL. The strongest antioxidative effects during long-term glycation of LDL were seen with isradipine, lacidipine, nifedipine, and semotiadil. Diltiazem was the only CA that could not prevent TBARS formation in LDL during the long-term glycation period. In contrast, Amadori product formation, as measured by the generation of fructosamines, was not significantly reduced by any CA tested. Thus CA, like other antioxidants, significantly retard AGE formation, while initial glycation reactions, such as Amadori product formation, are only weakly inhibited.

Why is glycated LDL more sensitive to oxidation than native LDL? A comparative study

It is well established that oxidative modification of low-density lipoprotein (LDL) plays a causal role in human atherogenesis and the risk of atherosclerosis is increased in patients with diabetes mellitus. To examine the influence of different agents which may influence LDL-glycation and oxidation, experiments including glycation with glucose, glucose 6-phosphate, metal chelators (EDTA) and antioxidants (BHT) were performed. The influence of time dependence on the glycation process and the alteration of the electrophoretic mobility of LDL under diverse glycation and/or oxidation conditions was also investigated. The formation of conjugated dienes and levels of lipid peroxides in these different LDL-modifications were estimated. The copper-induced oxidation of LDL in vitro was determined by measurement of thiobarbituric acid reactive substances (TBARS) and expressed as nmol MDA/mg of LDL protein. We found that glycated LDL is more prone to oxidation than native LDL. Using native LDL, the maximal oxidation effect was found to reach a value of 49.72 nmol MDA/mg protein after 8 h. The maximum oxidation of the 31 days, glycated LDL with glucose was 71.76 nmol MDA/mg protein amounting to 144.33% of the value found for native LDL. In the case of glucose 6-phosphate glycation, the maximum oxidation under the same conditions amounted to 173.77% of the value found for native LDL. To measure the extent of glycation, fluorescence of advanced glycation end products (AGEs) was determined (370 nm excitation and 440 nm emission). The most potent glycation agent was glucose 6-phosphate leading to the formation of very high amounts of AGEs. This process was promoted in the absence of EDTA, which prevents the oxidative cleavage of modified Amadori products (ketoamines) to AGEs. We therefore conclude that both processes, glycation and oxidation, result in the modification of LDL. The lower the glycation-rate (+/- EDTA) as measured by relative fluorescence units RFU (generation of AGEs), the lower the additional oxidation rate after glycation as measured by TBARS (generation of MDA equivalents). Glycation and/or oxidation change the electrophoretic mobility of LDL.

Protection of endogenous vitamin E and beta-carotene by aminoguanidine upon oxidation of human low-density lipoproteins by *OH/O(2)*-

This study was designed to evaluate the antioxidant effect of aminoguanidine toward human low-density lipoproteins (LDLs) initiated by oxygenated free radicals (*OH/O(2)*-) generated by gamma radiolysis. Initial radiolytic yields related to the markers of lipid peroxidation [i.e. decrease in endogenous alpha-tocopherol and beta-carotene, formation of thiobarbituric acid-reactive substances (TBARS) and conjugated dienes] were determined in 3 g liter(-1) LDLs (expressed as total LDL concentration) in the absence and presence of 10 different concentrations of aminoguanidine (from 0.04 to 5 mmol liter(-1)). Fluorescence and relative electrophoretic mobility of oxidized LDLs were also studied as markers that indirectly reflect the attack of the protein moiety of LDLs (namely apolipoprotein B). Our data clearly showed the inhibitory effect of aminoguanidine on lipid peroxidation induced in LDLs by *OH/O(2)*- in a concentration-dependent manner. This effect probably resulted from a scavenging activity of aminoguanidine toward *OH. In contrast, aminoguanidine did not appear to react significantly with O(2)*-, which resulted in a poor residual lipid peroxidation. Our data led us to determine an optimum [aminoguanidine]/[LDL] ratio ranging from 250 to 500 to obtain the best in vitro protection of LDLs under our experimental conditions. It is also of great interest that aminoguanidine was able to protect endogenous alpha-tocopherol and beta-carotene of LDLs upon *OH/O(2)*(-)-induced oxidation.

Influence of acetylsalicylic acid on oxidation of native and glycated low-density lipoprotein

It is generally accepted that oxidation of low-density lipoproteins (LDL) is a causal factor in the development of atherosclerosis. Non-enzymatic glycosylation of LDL, i.e."glycation", plays a central role in late complications of diabetes mellitus and may initiate and/or accelerate the oxidation process. Therefore, the inhibition of this processes is of major therapeutic relevance. The influence of acetylsalicylic acid (ASA) on the oxidation of native and glycated LDL was studied in vitro. LDL (0.25 mg protein/ml ) was oxidatively modified with 5.0 microM CuSO4. Only at "supratherapeutical" ASA concentrations in the range 0.06-2.0 mg /ml we found a significant concentration-dependent inhibition of LDL oxidation both for native and glycated LDL, which was from 0.2 mg/ml upwards significantly more marked for native LDL than for glycated LDL. The maximal inhibitory effect occurred at 2.0 mg/ml with 89.6% inhibition of LDL-oxidation for native LDL and 64.4% for glycated LDL. At 0.2 mg/ml ASA the respective inhibitory values were 38.5% and 31.0%. For glycated LDL the ASA doses of maximal- and approximately 50%-inhibition, as found for native LDL, were chosen to investigate the inhibitory effect on 2,4,8 and 24 hours oxidation of glycated LDL to monitor the time-dependency of inhibition by ASA. This revealed that ASA only delayed, not permanently inhibited LDL oxidation.

The effects of glycation/glycoxidation on the liberation of 8-epi-PGF2alpha from low density lipoprotein during its in vitro oxidation

In individuals suffering from diabetes mellitus, low-density lipoprotein (LDL) can undergo glycoxidation, both the surface protein and the unsaturated fatty acids in the particle core experiencing oxidative damage. In these patients, plasma levels of glycated(g)/glycoxidated(go) low-density lipoprotein and of 8-epi-PGF2alpha (8-IP), a relatively stable peroxidation product of arachidonic acid, are increased. This study reports on the modified oxidation of gLDL and goLDL by human umbilical vein endothelial cells or by copper cations in the absence of cells. In both systems, glycated LDL was found to be more easily oxidized than either nLDL or goLDL. In addition, liberation of 8-IP from glycoxidated LDL is significantly reduced, because a large amount of 8-IP is already formed during long-term glycation of LDL in the absence of metal chelators, i.e. during glycoxidation. From these in vitro results we conclude, that 1) gLDL is more prone to cell-mediated oxidation than native or goLDL and 2) the increased in vivo plasma levels of 8-IP observed in diabetes mellitus could be due to prolonged LDL-glycoxidation liberating continuously significant amounts of 8-IP.

Effects of PGI2 and analogues (taprostene, iloprost) on oxidation of native and glycated LDL

Oxidation and glycation of low-density lipoprotein (LDL) has been claimed to play a central role in the pathogenesis of atherosclerosis. Therefore, the inhibition of this processes is of major therapeutic importance. In the present paper the influence of prostaglandin (PG)I2, and its stable analogues taprostene and iloprost on copper-induced oxidation of native, glycated and glycoxidated LDL was investigated. The results show, that the most pronounced effect on inhibition of native LDL-oxidation was obtained by taprostene in the whole concentration range tested (0.2 microg-10 microg/ml) reaching a maximal inhibition of 95% at 10 microg/ml. Examining glycoxidated LDL the inhibitory effect on oxidation was less pronounced reaching only about 10%. In case of glycated LDL, however, no significant inhibitory effect on oxidation was seen. Iloprost was effective as inhibitory agent against oxidation of native LDL at concentrations of 10 and 20 microg/ml, showing a maximal inhibition of 86% at a concentration of 20 microg/ml. Iloprost was ineffective on oxidation of glycated and glycoxidated LDL. Examining the extremely short-lived PGI2 itself, no significant inhibitory effect on oxidation of native, glycated or glycoxidated LDL, however, was seen. This finding might be of relevance for patients with diabetes mellitus, showing a decreased endogenous PGI2-production in particular those with bad metabolic control and high concentrations of circulating advanced glycosylation end products (AGEs).

Binding of long-term glycated low density lipoprotein and AGE-albumin by peripheral monocytes and endothelial cells

Modification of low density lipoprotein (LDL) and plasma or tissue proteins by non-enzymatic glycation culminating in the formation of advanced glycation endproducts (AGEs) is one of the essential pathomechanisms leading to diabetes-associated long-term complications. We compared binding of glycated, glycoxidated and oxidated LDL by peripheral monocytes in activated and quiescent form. Interaction via specific receptors was different for glycated as compared to (glyc)oxidated LDL-modifications. In addition, binding of glycated LDL to quiescent and activated human umbilical vein endothelial cells was studied. In patients with insulin-dependent diabetes mellitus (IDDM), AGE-binding was significantly increased as compared to healthy individuals. Specific and non-specific monocyte binding mechanisms were detected, and both were significantly increased in IDDM patients. Specific and non-specific binding strategies possibly act in concert to eliminate circulating AGEs, which are instrumental in the development and progress of microangiopathic and macroangiopathic complications of diabetes mellitus.