Upregulation of glyoxalase I fails to normalize methylglyoxal levels: a possible mechanism for biochemical changes in diabetic mouse lenses

Protection of Polyphenols against Glyco-Oxidative Stress: Involvement of Glyoxalase Pathway

Chronic high glucose (HG) exposure increases methylglyoxal (MGO)-derived advanced glycation end-products (AGEs) and is involved in the onset of pathological conditions, such as diabetes, atherosclerosis and chronic-degenerative diseases. Under physiologic conditions the harmful effects of MGO are contrasted by glyoxalase system that is implicated in the detoxification of Reactive Carbonyl Species (RCS) and maintain the homeostasis of the redox environment of the cell. Polyphenols are the most abundant antioxidants in the diet and present various health benefits. Aims of the study were to investigate the effects of HG-chronic exposure on glyco-oxidation and glyoxalase system in intestinal cells, using CaCo-2 cells. Moreover, we studied the effect of apple polyphenols on glyco-oxidative stress. Our data demonstrated that HG-treatment triggers glyco-oxidation stress with a significant increase in intracellular Reactive Oxygen Species (ROS), lipid peroxidation, AGEs, and increase of Glyoxalase I (GlxI) activity. On the contrary, Glyoxalase II (GlxII) activity was lower in HG-treated cells. We demonstrate that apple polyphenols exert a protective effect against oxidative stress and dicarbonyl stress. The increase of total antioxidant capacity and glutathione (GSH) levels in HG-treated cells in the presence of apple polyphenols was associated with a decrease of GlxI activity.

Dicarbonyls and Advanced Glycation End-Products in the Development of Diabetic Complications and Targets for Intervention

Advanced glycation end-products (AGEs) are non-enzymatic protein and amino acid adducts as well as DNA adducts which form from dicarbonyls and glucose. AGE formation is enhanced in diabetes and is associated with the development of diabetic complications. In the current review, we discuss mechanisms that lead to enhanced AGE levels in the context of diabetes and diabetic complications. The methylglyoxal-detoxifying glyoxalase system as well as alternative pathways of AGE detoxification are summarized. Therapeutic approaches to interfere with different pathways of AGE formation are presented.

Glyoxalase 1 overexpression does not affect atherosclerotic lesion size and severity in ApoE-/- mice with or without diabetes

AIMS

Advanced glycation end-products (AGEs) and their precursors have been associated with the development of atherosclerosis. We recently discovered that glyoxalase 1 (GLO1), the major detoxifying enzyme for AGE precursors, is decreased in ruptured human plaques, and that levels of AGEs are higher in rupture-prone plaques. We here investigated whether overexpression of human GLO1 in ApoE(-/-) mice could reduce the development of atherosclerosis.

METHODS AND RESULTS

We crossed C57BL/6 ApoE(-/-) mice with C57BL/6 GLO1 overexpressing mice (huGLO1(+/-)) to generate ApoE(-/-) (n = 16) and ApoE(-/-) huGLO1(+/-) (n = 20) mice. To induce diabetes, we injected a subset with streptozotocin (STZ) to generate diabetic ApoE(-/-) (n = 8) and ApoE(-/-) huGLO1(+/-) (n = 13) mice. All mice were fed chow and sacrificed at 25 weeks of age. The GLO1 activity was three-fold increased in huGLO1(+/-) aorta, but aortic root lesion size and phenotype did not differ between mice with and without huGLO1(+/-) overexpression. We detected no differences in gene expression in aortic arches, in AGE levels and cytokines, in circulating cells, and endothelial function between ApoE(-/-) mice with and without huGLO1(+/-) overexpression. Although diabetic mice showed decreased GLO1 expression (P < 0.05) and increased lesion size (P < 0.05) in comparison with non-diabetic mice, GLO1 overexpression also did not affect the aortic root lesion size or inflammation in diabetic mice.

CONCLUSION

In ApoE(-/-) mice with or without diabetes, GLO1 overexpression did not lead to decreased atherosclerotic lesion size or systemic inflammation. Increasing GLO1 levels does not seem to be an effective strategy to reduce glycation in atherosclerotic lesions, likely due to increased AGE formation through GLO1-independent mechanisms.

Myricitrin alleviates methylglyoxal-induced mitochondrial dysfunction and AGEs/RAGE/NF-κB pathway activation in SH-SY5Y cells

Advanced glycation end products (AGEs) have been identified in age-related intracellular protein deposits of neurodegenerative diseases. Methylglyoxal (MGO), a dicarbonyl metabolite, is a major precursor of AGEs which have been linked to the development of neurodegenerative diseases. Myricitrin, a flavanoid isolated from the root bark of Myrica cerifera, attenuated 6-OHDA-induced mitochondrial dysfunction and had a potential anti-Parkinson's disease in our previous investigation. The aims of this study were to investigate the protective effects of myricitrin against MGO-induced injury in SH-SY5Y cells and also to look for the possible mechanisms. The results showed that exposure of SH-SY5Y cells to MGO caused decreases of cell viability, intracellular ATP, mitochondrial redox activity, and mitochondrial membrane potential and an increase in reactive oxygen species generation. However, these mitochondrial dysfunctions were alleviated by co-treatment with myricitrin. Additionally, myricitrin was capable of inhibiting AGEs formation, blocking RAGE expression, and inhibiting NF-κB activation and translocation triggered by MGO in SH-SY5Y cells. Our results suggest that myricitrin alleviates MGO-induced mitochondrial dysfunction, and the possible mechanism is through modulating the AGEs/RAGE/NF-κB pathway. In summary, myricitrin might offer a promising therapeutic strategy to reduce the neurotoxicity of reactive dicarbonyl compounds, providing a potential benefit agent with age-related neurodegenerative diseases.

Chaperone peptides of α-crystallin inhibit epithelial cell apoptosis, protein insolubilization, and opacification in experimental cataracts

α-Crystallin is a member of the small heat-shock protein (sHSP) family and consists of two subunits, αA and αB. Both αA- and αB-crystallin act as chaperones and anti-apoptotic proteins. Previous studies have identified the peptide (70)KFVIFLDVKHFSPEDLTVK(88) in αA-crystallin and the peptide (73)DRFSVNLDVKHFSPEELKVK(92) in αB-crystallin as mini-chaperones. In the human lens, lysine 70 (Lys(70)) of αA and Lys(92) of αB (in the mini-chaperone sequences) are acetylated. In this study, we investigated the cellular effects of the unmodified and acetyl mini-chaperones. The αA- and αB-crystallin peptides inhibited stress-induced aggregation of four client proteins, and the αA-acetyl peptide was more effective than the native peptide against three of the client proteins. Both the acetyl and native crystallin peptides inhibited stress-induced apoptosis in two mammalian cell types, and this property was directly related to the inhibition of cytochrome c release from mitochondria and the activity of caspase-3 and -9. In organ-cultured rat lenses, the peptides inhibited calcimycin-induced epithelial cell apoptosis. Intraperitoneal injection of the peptides inhibited cataract development in selenite-treated rats, which was accompanied by inhibition of oxidative stress, protein insolubilization, and caspase activity in the lens. These inhibitory effects were more pronounced for acetyl peptides than native peptides. A scrambled αA-crystallin peptide produced no such effects. The results suggest that the α-crystallin chaperone peptides could be used as therapeutic agents to treat cataracts and diseases in which protein aggregation and apoptosis are contributing factors.

Altered glyoxalase 1 expression in psychiatric disorders: cause or consequence?

Glyoxalase 1 is an enzyme, shown to protect against dicarbonyl glycation and the formation of advanced glycation end products. Recent findings suggest glyoxalase 1 as a molecular marker of psychiatric disorders. In clinical studies aberrant expression of glyoxalase 1 was shown to be involved in major depression, panic disorders and schizophrenia. In mouse models glyoxalase 1 was identified as a molecular marker of trait anxiety. However, anxiety-related behaviour in mice was inconsistently reported to correlate with elevated or reduced expression of glyoxalase 1. As yet, those findings were considered contradicting and the contribution of glyoxalase 1 to the aetiology of psychiatric disorders remained elusive. This review summarizes recent clinical and animal studies. In order to unravel the role of glyoxalase 1 in mental disease, findings are discussed with a particular focus on dicarbonyl substrate concentration. Prevailing the impact of dicarbonyl substrates on anxiety-related behaviour over the influence of glyoxalase 1 expression may consolidate findings that have been considered inconsistent. Taken together, this report suggests that physiological concentration of dicarbonyl compounds may differentiate a remedy from a poison.

Flow cytometric analysis of glyoxalase-1 expression in human leukocytes

Altered glyoxalase-1 (GLO-1) activity and expression is associated with the development of late diabetic complications, malignancy and oxidative stress- and aging-related diseases. In the present study, we developed a flow cytometry method for GLO-1 detection in human leukocytes isolated from peripheral blood samples to investigate GLO-1 expression in leukocyte subsets from type 1 and 2 diabetes mellitus patients (n = 11) and healthy subjects (n = 8). The flow cytometry analysis of GLO-1 in leukocytes showed that expression index of GLO-1-positive cells was slightly increased in mononuclear leukocytes from diabetic patients. This result correlated with the increase in GLO-1 activity in the whole blood samples of type 2 diabetes patients. In conclusion, the present study demonstrates that flow cytometry is suitable for the detection of the GLO-1 enzyme in human leukocytes and that this method could be used to investigate the fast adaptation of the glyoxalase system related to the pathogenesis of late complications of diabetes mellitus and other glycation stress-related disorders.

Inhibition of methylglyoxal-mediated protein modification in glyoxalase I overexpressing mouse lenses

Objective. Here we tested the role of Glo I in the prevention of advanced glycation end product (AGE) formation in transgenic mouse lenses. Methods. A transgenic animal line that expressed high levels of human Glo I in the lens was developed from the C57B6 mouse strain. The role of Glo I in the inhibition of MGO-AGE formation was tested in organ-cultured lenses. Results. Organ culture of Wt and Glo I lenses with 5 mM D, L-glyceraldehyde (GLD) enhanced MGO by 29-fold and 17-fold in Wt lenses and Glo I lenses, respectively. Argpyrimidine levels were 192 ± 73 pmoles/mg protein, and hydroimidazolone levels were 22 ± 0.7 units/μg protein in GLD-incubated Wt lenses. In Glo I lenses, formation of AGEs was significantly inhibited; the argpyrimidine levels were 82 ± 18 pmoles/mg protein, and the HI levels were 2.6 ± 2.3 units/μg protein. Incubation of Wt lens proteins with 5 mM ribose for 7 days resulted in the formation of pentosidine. However, the levels were substantially higher in Glo I lens proteins incubated with ribose. Conclusion. Our study provides direct evidence that Glo I activity plays an important role in the regulation of AGE synthesis in the lens; while Glo I activity blocks the formation of MGO-AGEs, it might promote the formation of sugar-derived AGEs.

Posttranslational modification of human glyoxalase 1 indicates redox-dependent regulation

BACKGROUND

Glyoxalase 1 (Glo1) and glyoxalase 2 (Glo2) are ubiquitously expressed cytosolic enzymes that catalyze the conversion of toxic alpha-oxo-aldehydes into the corresponding alpha-hydroxy acids using L-glutathione (GSH) as a cofactor. Human Glo1 exists in various isoforms; however, the nature of its modifications and their distinct functional assignment is mostly unknown.

METHODOLOGY/PRINCIPAL FINDINGS

We characterized native Glo1 purified from human erythrocytes by mass spectrometry. The enzyme was found to undergo four so far unidentified posttranslational modifications: (i) removal of the N-terminal methionine 1, (ii) N-terminal acetylation at alanine 2, (iii) a vicinal disulfide bridge between cysteine residues 19 and 20, and (iv) a mixed disulfide with glutathione on cysteine 139. Glutathionylation of Glo1 was confirmed by immunological methods. Both, N-acetylation and the oxidation state of Cys(19/20), did not impact enzyme activity. In contrast, glutathionylation strongly inhibited Glo1 activity in vitro. The discussed mechanism for enzyme inhibition by glutathionylation was validated by molecular dynamics simulation.

CONCLUSION/SIGNIFICANCE

It is shown for the first time that Glo1 activity directly can be regulated by an oxidative posttranslational modification that was found in the native enzyme, i.e., glutathionylation. Inhibition of Glo1 by chemical reaction with its co-factor and the role of its intramolecular disulfides are expected to be important factors within the context of redox-dependent regulation of glucose metabolism in cells.

Methylglyoxal-mediated anxiolysis involves increased protein modification and elevated expression of glyoxalase 1 in the brain

Methylglyoxal (MG) is a highly reactive metabolite that forms adducts with basic amino acid side chains in proteins. MG is degraded by glyoxalase1 (GLO1), an enzyme shown to be differentially expressed in several mouse models of anxiety-related behavior. As yet, molecular mechanisms by which altered GLO1 expression influences emotionality have not been elucidated. Here we report that both MG concentration and protein modification are altered in brain tissue of a mouse model for trait anxiety, with elevated levels in low anxiety-related behavior relative to high anxiety-related behavior animals. Accordingly, repeated intracerebroventricular injections of MG mediated anxiolysis in inbred high anxiety-related behavior and outbred CD1 mice. We found that anxiolytic-like properties of MG were independent of GLO1 expression. In contrast, antidepressant-like properties of intracerebroventricular MG were suppressed in CD1 mice carrying extra copies of the GLO1 gene. Moreover, MG treatment increased expression of GLO1 only in CD1 mice that did not have extra copies of GLO1. Taken together, these results suggest that the MG levels in brain are negatively correlated with anxiety. Thereby, we identified a novel molecular mechanism for anxiety-related behavior in mice that may help to elucidate genesis of psychiatric disorders in humans.

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.

Enhanced methylglyoxal formation in the erythrocytes of hemodialyzed patients

Methylglyoxal (MG) contributes significantly to the carbonyl stress in uremia; however, the reason for its increased concentration is not clear. Thus, the present study was aimed to investigate the formation and degradation of MG in the erythrocytes of hemodialyzed (HD) patients with end-stage renal disease. In 22 nondiabetic patients on long-term HD, erythrocyte MG and d-lactate levels, glyoxalase activities, and whole blood reduced glutathione content were determined. The data were compared with those from 22 healthy controls. Erythrocyte MG and d-lactate production were also investigated in vitro under normoglycemic (5 mmol/L) and hyperglycemic (50 mmol/L) conditions. The erythrocyte MG levels were elevated (P < .001) in the HD patients. The blood reduced glutathione content and glyoxalase I activity were similar to the control levels, but the glyoxalase II activity was significantly (P < .005) increased. In the normoglycemic in vitro model, production of both MG (P < .001) and d-lactate (P < .002) was significantly enhanced in the HD erythrocytes relative to the controls. During hyperglycemia, the MG formation and degradation rates were further increased (P < .001). The present study demonstrated an increased formation of MG in the erythrocytes of HD patients. This seemed to be related to a glucose metabolism disturbance of the cells. The degradation system of MG was also activated; still, it was not able to counteract the high rate of MG formation. The alterations and imbalance of these metabolic processes may contribute to the carbonyl overload and stress in the HD patients.

Cell cycle arrest by kynurenine in lens epithelial cells

PURPOSE

Indolemine 2,3-dioxygenase (IDO)-mediated oxidation of tryptophan produces kynurenines (KYNs), which may play a role in cataract formation. The molecular mechanisms by which KYNs cause cellular changes are poorly understood. The effects of KYNs on mouse lens epithelial cells by overexpression of human IDO were investigated.

METHODS

Lens epithelial cells (mLECs) derived from human IDO-overexpressing hemizygous transgenic (hemTg) and wild-type (Wt) mice were used. IDO activity was measured by quantifying kynurenine (KYN) by HPLC. KYN-mediated protein modifications were detected by immunocytochemistry and measured by ELISA. Cell proliferation and apoptosis were measured with commercially available kits. Cell distribution between cell cycle phases was examined with flow cytometric analysis. Immunoprecipitation followed by LC/MS was used to identify kynurenine-modified proteins.

RESULTS

mLECs derived from hemTg animals exhibited considerable IDO immunoreactivity and enzyme activity, which were barely detectable in Wt mLECs. KYN and KYN-mediated protein modification were detected in hemTg but not in Wt mLECs; the modified proteins were myosin II and alpha/gamma-actin. HemTg mLECs displayed reduced viability and proliferation. Cell cycle analysis of hemTg mLEC cultures showed approximately a twofold increase in cells at G(2)/M or in both phases, relative to Wt mLECs. Blocking IDO activity with 1-methyl-d,l-tryptophan in hemTg mLECs prevented KYN formation, KYN-mediated protein modification, and G(2)/M arrest.

CONCLUSIONS

Excess IDO activity in mLECs results in KYN production, KYN-mediated modification of myosin II and alpha/gamma-actin, and cell cycle perturbation. Modification of myosin II and gamma-actin by KYN may interfere with cytokinesis, leading to defective epithelial cell division and thus a decreased number of fiber cells.

High-performance liquid chromatographic determination of glyoxal and methylglyoxal in urine by prederivatization to lumazinic rings using in serial fast scan fluorimetric and diode array detectors

Glyoxal and methylglyoxal are two important markers of oxidative stress and both are involved in the evaluation of several diseases. A new HPLC method for determining glyoxal and methylglyoxal in urine was developed. The method is based on the reaction of alpha-dialdehydes, glyoxal and methylglyoxal, with 5,6-diamino-2,4-hydroxypyrimidine sulfate in basic medium to form highly fluorescent lumazine derivatives. Creatinine was also included in the method even though it does not react with the reagent. The derivatives and creatinine are separated on a C(18) reversed-phase column with a mobile phase consisting of acetonitrile:citrate buffer, pH 6.0 (3:97 v/v). The flow rate was 1.0mLmin(-1) and the effluent was monitored photometrically at 250 nm for determination of creatinine and fluorimetrically at 500 nm (exciting at 330 nm) for determination of glyoxal and methylglyoxal derivatives. Recording time of the separation is less than 10 min. Determination of the analytes is performed in urine after incubation of the sample, with the reagent in alkaline medium, for 30 min at 60 degrees C. Urinary levels of glyoxal and methylglyoxal, expressed as glyoxal/creatinine and methylglyoxal/creatinine ratios, in healthy young women and men were determined. For women, values of 0.80+/-0.37 and 0.60+/-0.22 microg/mg of creatinine were found for glyoxal and methylglyoxal, respectively. For men, values of 0.63+/-0.15 and 0.49+/-0.05 microg/mg of creatinine were found for glyoxal and methylglyoxal, respectively. These results were also related to the body mass index of each individual.

Quantitative trait loci for carbohydrate and total energy intake on mouse chromosome 17: congenic strain confirmation and candidate gene analyses (Glo1, Glp1r)

Quantitative trait loci (QTL) for carbohydrate (Mnic1) and total energy (Kcal2) intake on proximal mouse chromosome 17 were identified previously from a C57BL/6J (B6) X CAST/Ei (CAST) intercross. Here we report that a new congenic strain developed in our laboratory has confirmed this complex locus by recapitulating the original linked phenotypes: B6.CAST-17 homozygous congenic mice consumed more carbohydrate (27%) and total energy (17%) compared with littermate wild-type mice. Positional gene candidates with relevance to carbohydrate metabolism, glyoxalase I (Glo1) and glucagon-like peptide-1 receptor (Glp1r), were evaluated. Glo1 expression was upregulated in liver and hypothalamus of congenic mice when compared with B6 mice. Analyses of Glp1r mRNA and protein expression revealed tissue-specific strain differences in pancreas (congenic>B6) and stomach (B6>congenic). These results suggest the possibility of separate mechanisms for enhanced insulin synthesis and gastric accommodation in the presence of high carbohydrate intake and larger food volume, respectively. Sequence analysis of Glp1r found a G insert at nt position 1349, which results in earlier termination of the open reading frame, thus revealing an error in the public sequence. Consequently, the predicted length of GLP-1R is 463 aa compared with 489 aa, as previously reported. Also, we found a polymorphism in Glp1r between parental strains that alters the amino acid sequence. Variation in Glp1r could influence nutrient intake in this model through changes in the regulatory or protein coding regions of the gene. These congenic mice offer a powerful tool for investigating gene interactions in the control of food intake.