Comparison between modifications of lens proteins resulted from glycation with methylglyoxal, glyoxal, ascorbic acid, and fructose

Alleviation of albumin glycation-induced diabetic cardiomyopathy by L-Arginine: Insights into Nrf-2 signaling

In hyperglycemia, accelerated glycation and oxidative stress give rise to many diabetic complications, such as diabetic cardiomyopathy (DCM). Glycated human serum albumin (GHSA) has disturbed structural integrity and hampered functional capabilities. When GHSA accumulates around cardiac cells, Nrf-2 is dysregulated, aiding oxidative stress. L-Arginine (L-Arg) is prescribed to patients with diabetes and cardiovascular diseases. This research contributes to the mechanistic insights on antiglycation and antioxidant potential of L-Arg in alleviating DCM. HSA was glycated with methylglyoxal in the presence of L-Arg (20-640 mM). Structural and functional modifications of HSA were studied. L-Arg and HSA, GHSA interactions, and thermodynamics were determined by steady-state fluorescence. H9c2 cardiomyocytes were given treatments of GHSA-L-Arg along with the inhibitor of the receptor of AGEs. Cellular antioxidant levels, detoxification enzyme activities were measured. Gene, protein expressions, and immunofluorescence data examined the activation and nuclear translocation of Nrf-2 during glycation and oxidative stress. L-Arg protected HSA from glycation-induced structural and functional modifications. The binding affinity of L-Arg was more towards HSA (104 M-1). L-Arg, specifically at lower concentration (20 mM), upregulated Nrf-2 gene, protein expressions and facilitated its nuclear translocation by activating Nrf-2 signaling. The study concluded that L-Arg can be of therapeutic advantage in glycation-induced DCM and associated oxidative stress.

Protective effect of colchicine on albumin glycation and cellular oxidative stress: Insights into diabetic cardiomyopathy

The present work elucidates the role of colchicine (COL) on albumin glycation and cellular oxidative stress in diabetic cardiomyopathy (DCM). Human serum albumin (HSA) was glycated with methylglyoxal in the presence of COL (2.5, 3.75, and 5 µM), whereas positive and negative control samples were maintained separately. The effects of COL on HSA glycation, structural and functional modifications in glycated HSA were analyzed using different spectroscopical and fluorescence techniques. Increased fructosamine, carbonyl, and pentosidine formation in glycated HSA samples were inhibited in the presence of COL. Structural conformation of HSA and glycated HSA samples was examined by field emission scanning electron microscopy, circular dichroism, Fourier transform infrared, and proton nuclear magnetic resonance analyses, where COL maintained both secondary and tertiary structures of HSA against glycation. Functional marker assays included ABTS•+ radical scavenging and total antioxidant activities, advanced oxidative protein product formation, and turbidimetry, which showed preserved functional properties of glycated HSA in COL-containing samples. Afterward, rat cardiomyoblast (H9c2 cell line) was treated with glycated HSA-COL complex (400 μg/mL) for examining various cellular antioxidants (nitric oxide, catalase, superoxide dismutase, and glutathione) and detoxification enzymes (aldose reductase, glyoxalase I, and II) levels. All three concentrations of COL exhibited effective anti-glycation properties, enhanced cellular antioxidant levels, and detoxification enzyme activities. The report comprehensively analyzes the potential anti-glycation and properties of COL during its initial assessment.

Current Approach to the Pathogenesis of Diabetic Cataracts

Cataracts remain the first or second leading cause of blindness in all world regions. In the diabetic population, cataracts not only have a 3–5 times higher incidence than in the healthy population but also affect people at a younger age. In patients with type 1 diabetes, cataracts occur on average 20 years earlier than in the non-diabetic population. In addition, the risk of developing cataracts increases with the duration of diabetes and poor metabolic control. A better understanding of the mechanisms leading to the formation of diabetic cataracts enables more effective treatment and a holistic approach to the patient.

Relationship between the Structure and Chaperone Activity of Human αA-Crystallin after Its Modification with Diabetes-Associated Oxidative Agents and Protective Role of Antioxidant Compounds

The study was aimed to evaluate the impact of peroxynitrite (PON, oxidative stress agent in diabetes), methylglyoxal (MGO, diabetes-associated reactive carbonyl compound), and their simultaneous application on the structural and functional features of human αA-crystallin (αA-Cry) using various spectroscopy techniques. Additionally, the surface tension and oligomer size distribution of the treated and untreated protein were tested using tensiometric analysis and dynamic light scattering, respectively. Our results indicated that the reaction of PON and MGO with human αA-Cry leads to the formation of new chromophores, alterations in the secondary to quaternary protein structure, reduction in the size of protein oligomers, and significant enhancement in the chaperone activity of αA-Cry. To reverse the effects of the tested compounds, ascorbic acid and glutathione (main components of lens antioxidant defense system) were applied. As expected, the two antioxidant compounds significantly prevented formation of high molecular weight aggregates of αA-Cry (according to SDS-PAGE). Our results suggest that the lens antioxidant defense system, in particular, glutathione, may provide a strong protection against rapid incidence and progression of diabetic cataract by preventing the destructive reactions of highly reactive DM-associated metabolites.

Antiglycation Activity of Triazole Schiff's Bases Against Fructosemediated Glycation: In Vitro and In Silico Study

BACKGROUND

Advanced glycation end products (AGEs) are known to be involved in the pathophysiology of diabetic complications, neurodegenerative diseases, and aging. Preventing the formation of AGEs can be helpful in the management of these diseases.

OBJECTIVES

Two classes of previously synthesized traizole Schiff's bases (4H-1,2,4-triazole-4- Schiff's bases 1-14, and 4H-1,2,4-triazole-3-Schiff's bases 15-23) were evaluated for their in vitro antiglycation activity.

METHODS

In vitro fructose-mediated human serum albumin (HSA) glycation assay was employed to assess the antiglycation activity of triazole Schiff's bases. The active compounds were subjected to cytotoxicity analysis by MTT assay on mouse fibroblast (3T3) cell line. Molecular docking and simulation studies were carried out to evaluate the interactions and stability of compounds with HSA. Anti-hyperglycemic and antioxidant activities of selected non-cytotoxic compounds were evaluated by in vitro α-glucosidase inhibition, and DPPH free radical scavenging assays, respectively.

RESULTS

Compound 1 (IC50=47.30±0.38 µM) from 4H-1,2,4-triazole-4-Schiff's bases has exhibited antiglycation activity comparable to standard rutin (IC50=54.5±0.05 µM) along with a stable RMSD profile in MD simulation studies. Compound 1 also exhibited a potent α-glucosidase inhibitory activity, and moderate antioxidant property. Other derivatives showed a weak antiglycation activity with IC50 values between 248.1-637.7 µM. Compounds with potential antiglycation profile were found to be non-cytotoxic in a cellular assay.

CONCLUSION

The study identifies triazole Schiff's bases active against fructose-mediated glycation of HSA, thus indicates their potential against late diabetic complications due to production of advancedend products (AGEs).

Glycation interferes with natural killer cell function

One hallmark of molecular aging is glycation, better known as formation of so-called advanced glycation end products (AGEs), where reactive carbonyls react with amino-groups of proteins. AGEs accumulate over time and are responsible for various age-dependent diseases and impairments. Two very potent dicarbonyls to generate AGEs are glyoxal (GO) and methylglyoxal (MGO). The plasma level of such dicarbonyls is higher in aging and age-related diseases. Natural killer (NK) cells are cells of the innate immune system and provide a major defense against tumor cells and virus infected cells. They are able to kill modified or infected cells and produce different cytokines to modulate the function of other immune cells. Here we investigated the effect of GO- and MGO-induced glycation on the function of NK cells. Using the human NK cell line NK-92, we could demonstrate that both GO and MGO lead to glycation of cellular proteins, but that MGO interferes much stronger with NK cell function (cytotoxicity) than GO. In addition, glycation of NK cell targets, such as K562 tumor cells, also interferes with their lysis by NK cells. From this data we conclude that glycation acts negatively on NK cells function and reduces their cytotoxic potential towards tumor cells.

Biophysical and biochemical studies on glycoxidatively modified human low density lipoprotein

Methylglyoxal (MGO), a reactive dicarbonyl metabolite is a potent arginine directed glycating agent which has implications for diabetes-related complications. Dicarbonyl metabolites are produced endogenously and in a state of misbalance, they contribute to cell and tissue dysfunction through protein and DNA modifications causing dicarbonyl stress. MGO is detoxified by glyoxalase 1 (GLO1) system in the cytoplasm. Reactive oxygen species (ROS) are known to aggravate the glycation process. Both the processes are closely linked, and their combined activity is often referred to as "glycoxidation" process. Glycoxidation of proteins has several consequences such as type 2 diabetes mellitus (T2DM), aging etc. In this study, we have investigated the glycation of low-density lipoprotein (LDL) using different concentrations of MGO for varied incubation time periods. The structural perturbations induced in LDL were analyzed by UV-Vis, fluorescence, circular dichroism spectroscopy, molecular docking studies, polyacrylamide gel electrophoresis, FTIR, thermal denaturation studies, Thioflavin T assay and isothermal titration calorimetry. The ketoamine moieties, carbonyl content and HMF content were quantitated in native and glycated LDL. Simulation studies were also done to see the effect of MGO on the secondary structure of the protein. We report structural perturbations, increased carbonyl content, ketoamine moieties and HMF content in glycated LDL as compared to native analog (native LDL). We report the structural perturbations in LDL upon modification with MGO which could obstruct its normal physiological functions and hence contribute to disease pathogenesis and associated complications.

Protective effect of cyanidin against glucose- and methylglyoxal-induced protein glycation and oxidative DNA damage

Cyanidin, a natural anthocyanin abundant in fruits and vegetables, has shown the health benefits due to its pharmacological properties. However, there was no evidence regarding anti-glycation activity of cyanidin. The aim of the study was to investigate the inhibitory effect of cyanidin on methylglyoxal (MG)- and glucose-induced protein glycation in bovine serum albumin (BSA) as well as oxidative DNA damage. Free radical scavenging activity and the MG-trapping ability of cyanidin were also investigated. The results demonstrated that cyanidin (0.125-1mM) significantly inhibited the formation of fluorescent and non-fluorescent AGEs in BSA/MG and BSA/glucose systems. There was a significantly improved protein thiol in BSA/MG and BSA/glucose when incubated with cyanidin. Correspondingly, cyanidin decreased the level of protein carbonyl content in BSA/glucose system. Moreover, cyanidin (0.5-1mM) prevented lysine/MG-mediated oxidative DNA damage in the absence or presence of copper ion. The results demonstrated that cyanidin showed the MG-trapping ability in a concentration-dependent manner. Cyanidin also reduced superoxide anion and hydroxyl radical generation in lysine/MG system. The mechanism by which cyanidin inhibited protein glycation was the MG-trapping ability and the free radical scavenging activity. The present study suggests that cyanidin might be a promising antiglycation agent for preventing or ameliorating AGEs-mediated diabetic complications.

Cyanidin-3-rutinoside attenuates methylglyoxal-induced protein glycation and DNA damage via carbonyl trapping ability and scavenging reactive oxygen species

BACKGROUND

Advanced glycation end-products (AGEs) play a significant role in the development and progression of vascular complication in diabetes. Anthocyanin has been recently reported to possess antiglycating activity. This study aimed to determine whether a naturally occurring anthocyanin, cyanidin-3-rutinoside (C3R) inhibits methylglyoxal (MG) induced protein glycation and oxidative protein and DNA damage.

METHODS

C3R (0.125-1 mM) was incubated with bovine serum albumin and MG (1 mM) for 2 weeks. The formation of fluorescent AGEs was measured by using spectrofluorometer and thiol group content were used to detect protein oxidative damage. Gel electrophoresis was used to determine whether C3R (0.125-1 mM) reduced DNA strand breakage in a glycation model comprising lysine, MG and/or Cu(2+). The generation of superoxide anions and hydroxyl radicals were detected by the cytochrome c reduction assay and the thiobarbituric acid reactive substances assay. MG-trapping capacity was assessed by high performance liquid chromatography (HPLC).

RESULTS

C3R (0.25-1 mM) reduced the formation of fluorescent AGEs and depleted protein thiol groups in bovine serum albumin mediated by MG. At 1 mM C3R inhibited oxidative DNA damage in the glycation model (p < 0.05) and at 0.5-1 mM prevented Cu(2+) induced DNA strand breakage in the presence of lysine and MG. The findings showed that C3R reduced the formation of superoxide anion and hydroxyl radicals during the glycation reaction of MG with lysine. C3R directly trapped MG in a concentration and time dependent manner (both p < 0.001).

CONCLUSIONS

These findings suggest that C3R protects against MG-induced protein glycation and oxidative damage to protein and DNA by scavenging free radicals and trapping MG.

FoxO3a Serves as a Biomarker of Oxidative Stress in Human Lens Epithelial Cells under Conditions of Hyperglycemia

Background

Forkhead box ‘O’ transcription factors (FoxOs) are implicated in the pathogenesis of type2 diabetes and other metabolic diseases. Abnormal activity of FoxOs was reported in the glucose and insulin metabolism. Expression of FoxO proteins was reported in ocular tissues; however their function under hyperglycemic conditions was not examined.

Methods

Human lens epithelial cell line was used to study the function of FoxO proteins. Immunofluorescence, flow cytometry and Western blotting were employed to detect the FoxO proteins under the conditions of hyperglycemia.

Results

In this study we examined the role of FoxO3a in hyperglycemia-induced oxidative stress in human lens epithelial cells. FoxO3a protein expression was elevated in a dose- and time-dependent fashion after high glucose treatment. Anti-oxidant defense mechanisms of the lens epithelial cells were diminished as evidenced from loss of mitochondrial membrane integrity and lowered MnSOD after 72 h treatment with high glucose. Taken together, FoxO3a acts as a sensitive indicator of oxidative stress and cell homeostasis in human lens epithelial cells during diabetic conditions.

Conclusion

FoxO3a is an early stress response protein to glucose toxicity in diabetic conditions.

Deleterious effects of reactive aldehydes and glycated proteins on macrophage proteasomal function: possible links between diabetes and atherosclerosis

People with diabetes experience chronic hyperglycemia and are at a high risk of developing atherosclerosis and microvascular disease. Reactions of glucose, or aldehydes derived from glucose (e.g. methylglyoxal, glyoxal, or glycolaldehyde), with proteins result in glycation that ultimately yield advanced glycation end products (AGE). AGE are present at elevated levels in plasma and atherosclerotic lesions from people with diabetes, and previous in vitro studies have postulated that the presence of these materials is deleterious to cell function. This accumulation of AGE and glycated proteins within cells may arise from either increased formation and/or ineffective removal by cellular proteolytic systems, such as the proteasomes, the major multi-enzyme complex that removes proteins within cells. In this study it is shown that whilst high glucose concentrations fail to modify proteasome enzyme activities in J774A.1 macrophage-like cell extracts, reactive aldehydes enhanced proteasomal enzyme activities. In contrast BSA, pre-treated with high glucose for 8 weeks, inhibited both the chymotrypsin-like and caspase-like activities. BSA glycated using methylglyoxal or glycolaldehyde, also inhibited proteasomal activity though to differing extents. This suppression of proteasome activity by glycated proteins may result in further intracellular accumulation of glycated proteins with subsequent deleterious effects on cellular function.

Fructose-induced structural and functional modifications of hemoglobin: implication for oxidative stress in diabetes mellitus

Increased fructose concentration in diabetes mellitus causes fructation of several proteins. Here we have studied fructose-induced modifications of hemoglobin. We have demonstrated structural changes in fructose-modified hemoglobin (Fr-Hb) by enhanced fluorescence emission with excitation at 285 nm, more surface accessible tryptophan residues by using acrylamide, changes in secondary and tertiary structures by CD spectroscopy, and increased thermolability by using differential scanning calorimetry in comparison with those of normal hemoglobin, HbA(0). Release of iron from hemoglobin is directly related with the extent of fructation. H2O2-induced iron release from Fr-Hb is significantly higher than that from HbA(0). In the presence of H2O2, Fr-Hb degrades arachidonic acid, deoxyribose and plasmid DNA more efficiently than HbA(0), and these processes are significantly inhibited by desferrioxamine or mannitol. Thus increased iron release from Fr-Hb may cause enhanced formation of free radicals and oxidative stress in diabetes. Compared to HbA(0), Fr-Hb exhibits increased carbonyl formation, an index of oxidative modification. Functional modification in Fr-Hb has also been demonstrated by its decreased peroxidase activity and increased esterase activity in comparison with respective HbA(0) activities. Molecular modeling study reveals Lys 7alpha, Lys 127alpha and Lys 66beta to be the probable potential targets for fructation in HbA(0).

Assessment of temperature effects on β-aggregation of native and glycated albumin by FTIR spectroscopy and PAGE: Relations between structural changes and antioxidant properties

Structural modifications of bovine serum albumin (BSA) induced by heating, and the involvement of glycation of albumin in such processing were studied by using Fourier transform infrared spectroscopy (FTIR) and polyacrylamide gel electrophoresis (PAGE). For native BSA, heating treatments gave rise to beta structures which were amplified to the detriment of alpha-helix form, and which were associated with increased aggregation. A very high correlation was obtained between FTIR Amide I band evolution and aggregation rate parameters, showing the contribution of beta-form in aggregates formation. We further assessed the effect of glycation on protein sensibility to heating treatments. A reduction of conformational changes and aggregation processes was demonstrated for the glycated form of the protein. The antioxidant properties of albumin were evaluated using two different techniques assessing metal binding and free radical neutralizing capacities of the protein. Associations between structural changes in BSA induced by the thermal treatment and its antioxidant activities were established.

Evidence for inactivation of cysteine proteases by reactive carbonyls via glycation of active site thiols

Hyperglycaemia, triose phosphate decomposition and oxidation reactions generate reactive aldehydes in vivo. These compounds react non-enzymatically with protein side chains and N-terminal amino groups to give adducts and cross-links, and hence modified proteins. Previous studies have shown that free or protein-bound carbonyls inactivate glyceraldehyde-3-phosphate dehydrogenase with concomitant loss of thiol groups [Morgan, Dean and Davies (2002) Arch. Biochem. Biophys. 403, 259-269]. It was therefore hypothesized that modification of lysosomal cysteine proteases (and the structurally related enzyme papain) by free and protein-bound carbonyls may modulate the activity of these components of the cellular proteolytic machinery responsible for the removal of modified proteins and thereby contribute to a decreased removal of modified proteins from cells. It is shown that MGX (methylglyoxal), GO (glyoxal) and glycolaldehyde, but not hydroxyacetone and glucose, inhibit catB (cathepsin B), catL (cathepsin L) and catS (cathepsin S) activity in macrophage cell lysates, in a concentration-dependent manner. Protein-bound carbonyls produced similar inhibition with both cell lysates and intact macrophage cells. Inhibition was also observed with papain, with this paralleled by loss of the active site cysteine residue and formation of the adduct species S-carboxymethylcysteine, from GO, in a concentration-dependent manner. Inhibition of autolysis of papain by MGX, along with cross-link formation, was detected by SDS/PAGE. Treatment of papain and catS with the dialdehyde o-phthalaldehyde resulted in enzyme inactivation and an intra-molecular active site cysteine-lysine cross-link. These results demonstrate that reactive aldehydes inhibit cysteine proteases by modification of the active site cysteine residue. This process may contribute to the accumulation of modified proteins in tissues of people with diabetes and age-related pathologies, including atherosclerosis, cataract and Alzheimer's disease.

2-ammonio-6-(3-oxidopyridinium-1-yl)hexanoate (OP-lysine) is a newly identified advanced glycation end product in cataractous and aged human lenses

Post-translational modifications of proteins take place during the aging of human lens. The present study describes a newly isolated glycation product of lysine, which was found in the human lens. Cataractous and aged human lenses were hydrolyzed and fractionated using reverse-phase and ion-exchange high performance liquid chromatography (HPLC). One of the nonproteinogenic amino acid components of the hydrolysates was identified as a 3-hydroxypyridinium derivative of lysine, 2-ammonio-6-(3-oxidopyridinium-1-yl)hexanoate (OP-lysine). The compound was synthesized independently from 3-hydroxypyridine and methyl 2-[(tert-butoxycarbonyl)amino]-6-iodohexanoate. The spectral and chromatographic properties of the synthetic OP-lysine and the substance isolated from hydrolyzed lenses were identical. HPLC analysis showed that the amounts of OP-lysine were higher in water-insoluble compared with water-soluble proteins and was higher in a pool of cataractous lenses compared with normal aged lenses, reaching 500 pmol/mg protein. The model incubations showed that an anaerobic reaction mixture of Nalpha-tert-butoxycarbonyllysine, glycolaldehyde, and glyceraldehyde could produce the Nalpha-t-butoxycarbonyl derivative of OP-lysine. The irradiation of OP-lysine with UVA under anaerobic conditions in the presence of ascorbate led to a photochemical bleaching of this compound. Our results argue that OP-lysine is a newly identified glycation product of lysine in the lens. It is a marker of aging and pathology of the lens, and its formation could be considered as a potential cataract risk-factor based on its concentration and its photochemical properties.