Accumulation of endogenous methylglyoxal impaired insulin signaling in adipose tissue of fructose-fed rats

Methylglyoxal in Cardiometabolic Disorders: Routes Leading to Pathology Counterbalanced by Treatment Strategies

Methylglyoxal (MGO) is the major compound belonging to reactive carbonyl species (RCS) responsible for the generation of advanced glycation end products (AGEs). Its upregulation, followed by deleterious effects at the cellular and systemic levels, is associated with metabolic disturbances (hyperglycemia/hyperinsulinemia/insulin resistance/hyperlipidemia/inflammatory processes/carbonyl stress/oxidative stress/hypoxia). Therefore, it is implicated in a variety of disorders, including metabolic syndrome, diabetes mellitus, and cardiovascular diseases. In this review, an interplay between pathways leading to MGO generation and scavenging is addressed in regard to this system's impairment in pathology. The issues associated with mechanistic MGO involvement in pathological processes, as well as the discussion on its possible causative role in cardiometabolic diseases, are enclosed. Finally, the main strategies aimed at MGO and its AGEs downregulation with respect to cardiometabolic disorders treatment are addressed. Potential glycation inhibitors and MGO scavengers are discussed, as well as the mechanisms of their action.

Pyridoxamine reduces methylglyoxal and markers of glycation and endothelial dysfunction, but does not improve insulin sensitivity or vascular function in abdominally obese individuals: A randomized double-blind placebo-controlled trial

AIM

To investigate the effects of pyridoxamine (PM), a B6 vitamer and dicarbonyl scavenger, on glycation and a large panel of metabolic and vascular measurements in a randomized double-blind placebo-controlled trial in abdominally obese individuals.

MATERIALS AND METHODS

Individuals (54% female; mean age 50 years; mean body mass index 32 kg/m² ) were randomized to an 8-week intervention with either placebo (n = 36), 25 mg PM (n = 36) or 200 mg PM (n = 36). We assessed insulin sensitivity, β-cell function, insulin-mediated microvascular recruitment, skin microvascular function, flow-mediated dilation, and plasma inflammation and endothelial function markers. PM metabolites, dicarbonyls and advanced glycation endproducts (AGEs) were measured using ultra-performance liquid chromatography tandem mass spectrometry. Treatment effects were evaluated by one-way ANCOVA.

RESULTS

In the high PM dose group, we found a reduction of plasma methylglyoxal (MGO) and protein-bound Nδ-(5-hydro-5-methyl-4-imidazolon-2-yl)-ornithine (MG-H1), as compared to placebo. We found a reduction of the endothelial dysfunction marker soluble vascular cell adhesion molecule-1 (sVCAM-1) in the low and high PM dose group and of soluble intercellular adhesion molecule-1 (sICAM-1) in the high PM dose, as compared to placebo. We found no treatment effects on insulin sensitivity, vascular function or other functional outcome measurements.

CONCLUSIONS

This study shows that PM is metabolically active and reduces MGO, AGEs, sVCAM-1 and sICAM-1, but does not affect insulin sensitivity and vascular function in abdominally obese individuals. The reduction in adhesion markers is promising because these are important in the pathogenesis of endothelial damage and atherosclerosis.

Glyoxalase 1 knockdown induces age-related β-cell dysfunction and glucose intolerance in mice

Tight control of glycemia is a major treatment goal for type 2 diabetes mellitus (T2DM). Clinical studies indicated that factors other than poor glycemic control may be important in fostering T2DM progression. Increased levels of methylglyoxal (MGO) associate with complications development, but its role in the early steps of T2DM pathogenesis has not been defined. Here, we show that MGO accumulation induces an age-dependent impairment of glucose tolerance and glucose-stimulated insulin secretion in mice knockdown for glyoxalase 1 (Glo1KD). This metabolic alteration associates with the presence of insular inflammatory infiltration (F4/80-positive staining), the islet expression of senescence markers, and higher levels of cytokines (MCP-1 and TNF-α), part of the senescence-activated secretory profile, in the pancreas from 10-month-old Glo1KD mice, compared with their WT littermates. In vitro exposure of INS832/13 β-cells to MGO confirms its casual role on β-cell dysfunction, which can be reverted by senolytic treatment. These data indicate that MGO is capable to induce early phenotypes typical of T2D progression, paving the way for novel prevention approaches to T2DM.

Dicarbonyl Stress in Diabetic Vascular Disease

Late vascular complications play a prominent role in the diabetes-induced increase in morbidity and mortality. Diabetes mellitus is recognised as a risk factor driving atherosclerosis and cardiovascular mortality; even after the normalisation of blood glucose concentration, the event risk is amplified—an effect called “glycolytic memory”. The hallmark of this glycolytic memory and diabetic pathology are advanced glycation end products (AGEs) and reactive glucose metabolites such as methylglyoxal (MGO), a highly reactive dicarbonyl compound derived mainly from glycolysis. MGO and AGEs have an impact on vascular and organ structure and function, contributing to organ damage. As MGO is not only associated with hyperglycaemia in diabetes but also with other risk factors for diabetic vascular complications such as obesity, dyslipidaemia and hypertension, MGO is identified as a major player in the development of vascular complications in diabetes both on micro- as well as macrovascular level. In diabetes mellitus, the detoxifying system for MGO, the glyoxalase system, is diminished, accounting for the increased MGO concentration and glycotoxic load. This overview will summarise current knowledge on the effect of MGO and AGEs on vascular function.

Methylglyoxal impairs β-adrenergic signalling in primary rat adipocytes

Methylglyoxal (MG) is dicarbonyl aldehyde generated intracellularly from glucose and from some other compounds. Its increased formation is associated with several harmful consequences. In the present study, short-term effects of MG on metabolism of isolated rat adipocytes were determined. Insulin-induced lipogenesis was unchanged by MG. However, epinephrine-stimulated lipolysis was shown to be significantly reduced in adipocytes exposed to 200 µM MG. This inhibitory effect was similar in the presence of low and high concentrations of glucose, and also in the presence of alanine. However, MG failed to affect lipolysis induced by forskolin (activator of adenylate cyclase), dibutyryl-cAMP (activator of PKA) and DPCPX (adenosine A₁ receptor antagonist). It was also revealed that lipolysis was unchanged by MG in fat cells pre-incubated with this compound, and then stimulated with epinephrine alone. Our results suggest that MG may impair β-adrenergic signalling in rat adipocytes due to interaction with epinephrine, and thereby disturbs lipolysis.

Intestinal AMPK modulation of microbiota mediates crosstalk with brown fat to control thermogenesis

The energy-dissipating capacity of brown adipose tissue through thermogenesis can be targeted to improve energy balance. Mammalian 5'-AMP-activated protein kinase, a key nutrient sensor for maintaining cellular energy status, is a known therapeutic target in Type II diabetes. Despite its well-established roles in regulating glucose metabolism in various tissues, the functions of AMPK in the intestine remain largely unexplored. Here we show that AMPKα1 deficiency in the intestine results in weight gain and impaired glucose tolerance under high fat diet feeding, while metformin administration fails to ameliorate these metabolic disorders in intestinal AMPKα1 knockout mice. Further, AMPKα1 in the intestine communicates with brown adipose tissue to promote thermogenesis. Mechanistically, we uncover a link between intestinal AMPKα1 activation and BAT thermogenic regulation through modulating anti-microbial peptide-controlled gut microbiota and the metabolites. Our findings identify AMPKα1-mediated mechanisms of intestine-BAT communication that may partially underlie the therapeutic effects of metformin.

Does AGE-RAGE Stress Play a Role in the Development of Coronary Artery Disease in Obesity?

This article deals with the role of AGE (advanced glycation end products)-RAGE (receptor for AGE) stress (AGE/sRAGE) in the development of coronary artery disease (CAD) in obesity. CAD is due to atherosclerosis in coronary artery. The serum/plasma levels of AGE and sRAGE are reduced, while AGE-RAGE stress and expression of RAGE are elevated in obese individuals. However, the levels of AGE are elevated in obese individuals with more than one metabolic syndrome. The increases in the AGE-RAGE stress would elevate the expression and production of atherogenic factors, including reactive oxygen species, nuclear factor-kappa B, cytokines, intercellular adhesion molecule-1, vascular cell adhesion molecule-1, endothelial leukocyte adhesion molecules, monocyte chemoattractant protein-1, granulocyte-macrophage colony-stimulating factor, and growth factors. Low levels of sRAGE would also increase the atherogenic factors. The increases in the AGE-RAGE stress and decreases in the levels of sRAGE would induce development of atherosclerosis, leading to CAD. The therapeutic regimen for AGE-RAGE stress-induced CAD in obesity would include lowering of AGE intake, prevention of AGE formation, degradation of AGE in vivo, suppression of RAGE expression, blockade of AGE-RAGE interaction, downregulation of sRAGE expression, and use of antioxidants. In conclusion, the data suggest that AGE-RAGE stress is involved in the development of CAD in obesity, and the therapeutic interventions to reduce AGE-RAGE would be helpful in preventing, regressing, and slowing the progression of CAD in obesity.

Methylglyoxal induces ambience for cancer promotion in HepG2 cells via Warburg effect and promotes glycation

Methylglyoxal (MGO) is a toxic, highly reactive metabolite derived mainly from glucose and amino acids degradation. MGO is also one of the prime precursors for advanced glycation end products formation. The present research was performed to check whether MGO has any role in the promotion of cancer in HepG2 cells. For this, cells were incubated with MGO (50 µM) for 24 h and subjected to various analyses. Aminoguanidine (200 µM) was positive control. The various biochemical and protein expression studies, relevant to the MGO detoxification system, oxidative stress, and glycolysis were performed. MGO caused the reduction of expression of GLO 1 (27%) and GLO 2 (11%) causing weakening of the innate detoxification system. This is followed by an increase of RAGE (95%), AGEs or methylglyoxal adducts. We also observed hypoxia via estimation of oxygen consumption rate and surplus reactive oxygen species (ROS) (24%). To investigate the off-target effect of MGO we checked its effect on glucose transport, and its associated proteins. Glucose uptake was found to increase (15%) significantly with overexpression of GLUT 1 (35%). We also found a significant increase of glycolytic enzymes such as hexokinase II, phosphofructokinase 1, and lactate dehydrogenase along with lactate production. Observation of surplus ROS and enhanced glycolysis led us to check the expression of HIF 1α which is their downstream signaling pathway. Interestingly HIF 1α was found to increase significantly (35%). It is known that enhanced glycolysis and oxidative stress are catalysts for the overexpression of HIF 1α which in turn creates an ambience for the promotion of cancer. Aminoguanidine was able to prevent the adverse effect of MGO partially. This is the first study to show the potential of MGO for the promotion of cancer in the non-tumorigenic HepG2 cells via the Warburg effect and glycation.

Salt-Sensitivity of Blood Pressure and Insulin Resistance

Salt sensitivity of blood pressure (SSBP) is an independent risk factor for cardiovascular morbidity and mortality that is seen in both hypertensive and normotensive populations. Insulin resistance (IR) strongly correlates with SSBP and affects nearly 50% of salt sensitive people. While the precise mechanism by which IR and SSBP relate remains elusive, several common pathways are involved in the genesis of both processes, including vascular dysfunction and immune activation. Vascular dysfunction associated with insulin resistance is characterized by loss of nitric oxide (NO)-mediated vasodilation and heightened endothelin-1 induced vasoconstriction, as well as capillary rarefaction. It manifests with increased blood pressure (BP) in salt sensitive murine models. Another common denominator in the pathogenesis of insulin resistance, hypertension, and salt sensitivity (SS) is immune activation involving pro-inflammatory cytokines like tumor necrosis factor (TNF)-α, IL-1β, and IL-6. In the last decade, a new understanding of interstitial sodium storage in tissues such as skin and muscle has revolutionized traditional concepts of body sodium handling and pathogenesis of SS. We have shown that interstitial Na⁺ can trigger a T cell mediated inflammatory response through formation of isolevuglandin protein adducts in antigen presenting cells (APCs), and that this response is implicated in salt sensitive hypertension. The peroxisome proliferator-activated receptor γ (PPARγ) is a transcription factor that modulates both insulin sensitivity and BP. PPARγ agonists increase insulin sensitivity and ameliorate salt sensitivity, whereas deficiency of PPARγ results in severe insulin resistance and hypertension. These findings suggest that PPARγ plays a role in the common pathogenesis of insulin sensitivity and salt sensitivity, perhaps via effects on the immune system and vascular function. The goal of this review is to discuss those mechanisms that may play a role in both SSBP and in insulin resistance.

Role of RAGE in obesity-induced adipose tissue inflammation and insulin resistance

Obesity is known to be associated with adipose tissue inflammation and insulin resistance. Importantly, in obesity, the accumulation of proinflammatory macrophages in adipose tissue correlates with insulin resistance. We hypothesized that the receptor for advanced glycation end products (RAGE) and associated ligands are involved in adipose tissue insulin resistance, and that the activation of the AGE–RAGE axis plays an important role in obesity-associated inflammation. C57BL/6J mice (WT) and RAGE deficient (RAGE^−/−) mice were fed a high fat diet (HFD) and subjected to glucose and insulin tolerance tests. Epdidymal adipose tissue (eAT) was collected and adipose stromal vascular cells isolated using flow cytometry. Visceral adipose tissue macrophage polarization was assessed by quantitative real time PCR. Immunoblotting was performed to evaluate the insulin signaling in adipose tissues. In additional studies, cell trafficking was assessed by injecting labeled blood monocytes into recipient mice. RAGE^−/− mice displayed improved insulin sensitivity and glucose tolerance, accompanied by decreased body weight and eAT mass. Exogenous methylglyoxal (MGO) impaired insulin-stimulated AKT signaling in adipose tissues from WT mice fed a normal chow diet, but not in RAGE^−/− mice. In contrast, in obese mice, treatment with MGO did not reduce insulin-induced phosphorylation of AKT in WT-HFD mice. Moreover, insulin-induced AKT phosphorylation was found to be impaired in adipose tissue from RAGE^−/−-HFD mice. RAGE^−/− mice displayed improved inflammatory profiles and evidence for increased adipose tissue browning. This observation is consistent with the finding of reduced plasma levels of FFA, glycerol, IL-6, and leptin in RAGE^−/− mice compared to WT mice. Collectively the data demonstrate that RAGE-mediated adipose tissue inflammation and insulin-signaling are potentially important mechanisms that contribute to the development of obesity-associated insulin resistance.

Early AGEing and metabolic diseases: is perinatal exposure to glycotoxins programming for adult-life metabolic syndrome?

Perinatal early nutritional disorders are critical for the developmental origins of health and disease. Glycotoxins, or advanced glycation end-products, and their precursors such as the methylglyoxal, which are formed endogenously and commonly found in processed foods and infant formulas, may be associated with acute and long-term metabolic disorders. Besides general aspects of glycotoxins, such as their endogenous production, exogenous sources, and their role in the development of metabolic syndrome, we discuss in this review the sources of perinatal exposure to glycotoxins and their involvement in metabolic programming mechanisms. The role of perinatal glycotoxin exposure in the onset of insulin resistance, central nervous system development, cardiovascular diseases, and early aging also are discussed, as are possible interventions that may prevent or reduce such effects.

Diet-induced weight loss reduces postprandial dicarbonyl stress in abdominally obese men: Secondary analysis of a randomized controlled trial

AIMS

Dicarbonyl compounds contribute to the formation of advanced glycation endproducts (AGEs) and the development of insulin resistance and vascular complications. Dicarbonyl stress may already be detrimental in obesity. We evaluated whether diet-induced weight loss can effectively reverse dicarbonyl stress in abdominally obese men.

MATERIALS AND METHODS

Plasma samples were collected from lean (n = 25) and abdominally obese men (n = 52) in the fasting state, and during a mixed meal test (MMT). Abdominally obese men were randomized to 8 weeks of dietary weight loss or habitual diet, followed by a second MMT. The α-dicarbonyls methylglyoxal (MGO), glyoxal (GO) and 3-deoxyglucosone (3-DG) and AGEs were measured by UPLC-MS/MS. Skin autofluorescence (SAF) was measured using the AGE reader. T-tests were used for the cross-sectional analysis and ANCOVA to assess the treatment effect.

RESULTS

Postprandial glucose, MGO and 3-DG concentrations were higher in obese men as compared to lean men (p < 0.05 for all). Fasting dicarbonyls, AGEs, and SAF were not different between lean and obese men. After the weight loss intervention, fasting MGO levels tended to decrease by 25 nmol/L (95%-CI: -51-0.5; p = 0.054). Postprandial dicarbonyls were decreased after weight loss as compared to the control group: iAUC of MGO decreased by 57% (5280 nmol/L∙min; 95%-CI: 33-10526; p = 0.049), of GO by 66% (11,329 nmol/L∙min; 95%-CI: 495-22162; p = 0.041), and of 3-DG by 45% (20,175 nmol/L∙min; 95%-CI: 5351-35000; p = 0.009). AGEs and SAF did not change significantly after weight loss.

CONCLUSION

Abdominal obesity is characterized by increased postprandial dicarbonyl stress, which can be reduced by a weight loss intervention. Registered under ClinicalTrials.gov Identifier no. NCT01675401.

In vitro chronic glycation induces AGEs accumulation reducing insulin-stimulated glucose uptake and increasing GLP1R in adipocytes

Intracellular AGEs accumulation increases RAGE and GLP1R and reduces glucose uptake in adipocytes.

Carbonyl Stress in Red Blood Cells and Hemoglobin

The paper overviews the peculiarities of carbonyl stress in nucleus-free mammal red blood cells (RBCs). Some functional features of RBCs make them exceptionally susceptible to reactive carbonyl compounds (RCC) from both blood plasma and the intracellular environment. In the first case, these compounds arise from the increased concentrations of glucose or ketone bodies in blood plasma, and in the second-from a misbalance in the glycolysis regulation. RBCs are normally exposed to RCC-methylglyoxal (MG), triglycerides-in blood plasma of diabetes patients. MG modifies lipoproteins and membrane proteins of RBCs and endothelial cells both on its own and with reactive oxygen species (ROS). Together, these phenomena may lead to arterial hypertension, atherosclerosis, hemolytic anemia, vascular occlusion, local ischemia, and hypercoagulation phenotype formation. ROS, reactive nitrogen species (RNS), and RCC might also damage hemoglobin (Hb), the most common protein in the RBC cytoplasm. It was Hb with which non-enzymatic glycation was first shown in living systems under physiological conditions. Glycated HbA1c is used as a very reliable and useful diagnostic marker. Studying the impacts of MG, ROS, and RNS on the physiological state of RBCs and Hb is of undisputed importance for basic and applied science.

Downregulation of the Glo1 Gene Is Associated with Reduced Adiposity and Ectopic Fat Accumulation in Spontaneously Hypertensive Rats

Methylglyoxal (MG), a potent precursor of advanced glycation end-products (AGE), is increased in metabolic disorders such as diabetes and obesity. MG and other dicarbonyl metabolites are detoxified by the glyoxalase system in which glyoxalase 1, coded by the Glo1 gene, serves as the rate-limiting enzyme. In this study, we analyzed the effects of Glo1 downregulation on glucose and lipid metabolism parameters in spontaneously hypertensive rats (SHR) by targeting the Glo1 gene (SHR-Glo1^+/− heterozygotes). Compared to SHR wild-type animals, SHR-Glo1^+/− rats showed significantly reduced Glo1 expression and lower GLO1 activity in tissues associated with increased MG levels. In contrast to SHR controls, SHR-Glo1^+/− rats exhibited lower relative weight of epididymal fat, reduced ectopic fat accumulation in the liver and heart, and decreased serum triglycerides. In addition, compared to controls, SHR-Glo1^+/− rats showed reduced serum insulin and increased basal and insulin stimulated incorporation of glucose into white adipose tissue lipids (lipogenesis). Reduced ectopic fat accumulation in the heart was associated with significantly increased pAMPK/AMPK ratio and GLUT4 activity. These results provide evidence that Glo1 downregulation in SHR is associated with reduced adiposity and ectopic fat accumulation, most likely mediated by AMPK activation in the heart.

The occurrence and stability of Maillard reaction products in various traditional Chinese sauces

Traditional Chinese sauces, especially soy sauce, vinegar, and oyster sauce, provide foods with special color, flavor, and taste. However, during their manufacturing process, Maillard reaction products (MRPs) might be formed and cause adverse effects on human health. This study detected the levels of some typical MRPs in these sauces. Results showed that dark soy sauce and some vinegar products were high in 5-hydroxymethylfurfural (HMF). Moreover, dark soy sauce was significantly higher in 5-methylfurfural (MF) and methylglyoxal (MGO) than oyster sauce but lower in glyoxal (GO) than vinegar and oyster sauce, while light soy sauce had significantly higher advanced glycation end products (AGEs) than the rest three types of sauces. Besides, storage stability results indicated that MRPs increased obviously in vinegar and dark soy sauce. Finally, concentration limits/regulations of MRPs in traditional Chinese sauces should be established in the future considering their high frequency of usage and high potential of insalubrity.

Proteomic identification of aerobic glycolysis as a potential metabolic target for methylglyoxal in adipocytes

Obesity is often accompanied by metabolic changes in adipocytes that are closely associated with metabolic disease. Although high sugar consumption contributes to obesity, it may also directly affect adipocytes by increasing the rate of glycolysis and formation of the glycolytic by-product methylglyoxal (MG). MG is a reactive dicarbonyl that irreversibly damages proteins and other cellular components. Although the accumulation of MG is clinically associated with hyperglycemia and diabetic complications, a better understanding of how proteins are regulated by MG is needed to evaluate its role in the pathogenesis of metabolic disease. Because adipocytes rely heavily on glycolysis for glucose disposal, we hypothesized that prolonged MG treatment at nontoxic concentrations would impact the landscape of proteins involved in glucose metabolism. To test this hypothesis, we treated 3T3-L1 adipocytes with MG (100 μmol/L) and used comparative proteomics to assess the effects. We identified 25 differentially expressed proteins in adipocytes treated with MG compared to the control. Our results suggested that MG induced metabolic changes typically associated with aerobic glycolysis, including a lowered expression of proteins involved in oxidative metabolism and increased expression of the glycolytic enzymes L-lactate dehydrogenase and glyceraldehyde-3-phosphate dehydrogenase. The detection of increased lactate secreted into the culture media of adipocytes treated with MG further supported these findings, as did gene expression analysis. In summary, these results indicate MG as a metabolic contributor to aerobic glycolysis in adipocytes, a potential adaptive response to increased glucose flux which over time could lead to permanent metabolic changes.

Environmental toxin exposure in polycystic ovary syndrome women and possible ovarian neoplastic repercussion

Purpose: Over the last two decades, increasing attention has been paid to environmental toxins and their effects on the female reproductive system. Endocrine disrupting chemicals (EDCs) are exogenous substances or mixtures that can mimic the action of steroid hormones and interfere with their metabolism. Advanced glycation end products (AGEs) are proinflammatory molecules that can interact with cell surface receptors and mediate the triggering of proinflammatory pathways and oxidative stress. The purpose of this review is to explore the effects of environmental toxin exposure in the pathogenesis of both polycystic ovary syndrome (PCOS) and OC (ovarian cancer), considered separately, and also to evaluate possible neoplastic ovarian repercussion after exposure in patients diagnosed with PCOS.Materials and methods: We searched PubMed for articles published in the English language with the use of the following MeSH search terms: "polycystic ovary syndrome" and "ovarian cancer" combined with "endocrine disruptors". Titles and abstracts were examined and full articles that met the selection criteria were retrieved. A manual search of review articles and cross-references completed the search.Results: Extensive data from different studies collected in recent years concerning the effects of EDC/AGE exposure have confirmed their role in the pathophysiology of both PCOS and OC. They favor PCOS/OC development through different mechanisms that finally lead to hormonal and metabolic disruption and epigenetic modifications.Conclusions: Environmental toxin exposure in PCOS women could favor neoplastic transformation by exacerbating and potentiating some PCOS features. Further research, although difficult, is needed in order to prevent further diffusion of these substances in the environment, or at least to provide adequate information to the population considered at risk.

Methylglyoxal, a Highly Reactive Dicarbonyl Compound, in Diabetes, Its Vascular Complications, and Other Age-Related Diseases

The formation and accumulation of methylglyoxal (MGO), a highly reactive dicarbonyl compound, has been implicated in the pathogenesis of type 2 diabetes, vascular complications of diabetes, and several other age-related chronic inflammatory diseases such as cardiovascular disease, cancer, and disorders of the central nervous system. MGO is mainly formed as a byproduct of glycolysis and, under physiological circumstances, detoxified by the glyoxalase system. MGO is the major precursor of nonenzymatic glycation of proteins and DNA, subsequently leading to the formation of advanced glycation end products (AGEs). MGO and MGO-derived AGEs can impact on organs and tissues affecting their functions and structure. In this review we summarize the formation of MGO, the detoxification of MGO by the glyoxalase system, and the biochemical pathways through which MGO is linked to the development of diabetes, vascular complications of diabetes, and other age-related diseases. Although interventions to treat MGO-associated complications are not yet available in the clinical setting, several strategies to lower MGO have been developed over the years. We will summarize several new directions to target MGO stress including glyoxalase inducers and MGO scavengers. Targeting MGO burden may provide new therapeutic applications to mitigate diseases in which MGO plays a crucial role.

Effects of Maternal Fructose Intake on Perinatal ER-Stress: A Defective XBP1s Nuclear Translocation Affects the ER-stress Resolution

Endoplasmic reticulum (ER) homeostasis is crucial to appropriate cell functioning, and when disturbed, a safeguard system called unfolded protein response (UPR) is activated. Fructose consumption modifies ER homeostasis and has been related to metabolic syndrome. However, fructose sweetened beverages intake is allowed during gestation. Therefore, we investigate whether maternal fructose intake affects the ER status and induces UPR. Thus, administrating liquid fructose (10% w/v) to pregnant rats partially activated the ER-stress in maternal and fetal liver and placenta. In fact, a fructose-induced increase in the levels of pIRE1 (phosphorylated inositol requiring enzyme-1) and its downstream effector, X-box binding protein-1 spliced form (XBP1s), was observed. XBP1s is a key transcription factor, however, XBP1s nuclear translocation and the expression of its target genes were reduced in the liver of the carbohydrate-fed mothers, and specifically diminished in the fetal liver and placenta in the fructose-fed mothers. These XBP1s target genes belong to the ER-associated protein degradation (ERAD) system, used to buffer ER-stress and to restore ER-homeostasis. It is known that XBP1s needs to form a complex with diverse proteins to migrate into the nucleus. Since methylglyoxal (MGO) content, a precursor of advanced glycation endproducts (AGE), was augmented in the three tissues in the fructose-fed mothers and has been related to interfere with the functioning of many proteins, the role of MGO in XBP1s migration should not be discarded. In conclusion, maternal fructose intake produces ER-stress, but without XBP1s nuclear migration. Therefore, a complete activation of UPR that would resolve ER-stress is lacking. A state of fructose-induced oxidative stress is probably involved.

Dicarbonyl Stress at the Crossroads of Healthy and Unhealthy Aging

Dicarbonyl stress occurs when dicarbonyl metabolites (i.e., methylglyoxal, glyoxal and 3-deoxyglucosone) accumulate as a consequence of their increased production and/or decreased detoxification. This toxic condition has been associated with metabolic and age-related diseases, both of which are characterized by a pro-inflammatory and pro-oxidant state. Methylglyoxal (MGO) is the most reactive dicarbonyl and the one with the highest endogenous flux. It is the precursor of the major quantitative advanced glycated products (AGEs) in physiological systems, arginine-derived hydroimidazolones, which accumulate in aging and dysfunctional tissues. The aging process is characterized by a decline in the functional properties of cells, tissues and whole organs, starting from the perturbation of crucial cellular processes, including mitochondrial function, proteostasis and stress-scavenging systems. Increasing studies are corroborating the causal relationship between MGO-derived AGEs and age-related tissue dysfunction, unveiling a previously underestimated role of dicarbonyl stress in determining healthy or unhealthy aging. This review summarizes the latest evidence supporting a causal role of dicarbonyl stress in age-related diseases, including diabetes mellitus, cardiovascular disease and neurodegeneration.

Diabetes Mellitus as a Risk Factor for Parkinson's Disease: a Molecular Point of View

Type 2 diabetes mellitus (T2DM) is a metabolic disorder characterized by elevated concentrations of glucose in the blood. The chronic hyperglycemic state accounts for most of the vascular complications associated to the disease and the prevalent mechanism proposed is related to the glycating chemistry mediated by methylglyoxal (MG), which accumulates in T2DM. In recent years, a higher risk of Parkinson's disease (PD) onset in people affected by T2DM has become evident, but the molecular mechanisms underlying the interplay between T2DM and PD are still unknown. The oxidative chemistry of dopamine and its reactivity towards the protein α-Synuclein (aS) has been associated to the pathogenesis of PD. Recently, aS has also been described to interact with MG. Interestingly, MG and the dopamine oxidation products share both structural similarity and chemical reactivity. The ability of MG to spread over the site of its production and react with aS could represent the rationale to explain the higher incidence of PD in T2DM-affected people and may open opportunities for the development of novel strategies to antagonize the raise of PD.

Metformin attenuates myocardium dicarbonyl stress induced by chronic hypertriglyceridemia

Reactive dicarbonyls stimulate production of advanced glycation endproducts, increase oxidative stress and inflammation and contribute to the development of vascular complications. We measured concentrations of dicarbonyls - methylglyoxal (MG), glyoxal (GL) and 3-deoxyglucosone (3-DG) - in the heart and kidney of a model of metabolic syndrome - hereditary hypertriglyceridemic rats (HHTg) and explored its modulation by metformin. Adult HHTg rats were fed a standard diet with or without metformin (300 mg/kg b.w.) and dicarbonyl levels and metabolic parameters were measured. HHTg rats had markedly elevated serum levels of triacylglycerols (p<0.001), FFA (p<0.01) and hepatic triacylglycerols (p<0.001) along with increased concentrations of reactive dicarbonyls in myocardium (MG: p<0.001; GL: p<0.01; 3-DG: p<0.01) and kidney cortex (MG: p<0.01). Metformin treatment significantly reduced reactive dicarbonyls in the myocardium (MG: p<0.05, GL: p<0.05, 3-DG: p<0.01) along with increase of myocardial concentrations of reduced glutathione (p<0.01) and glyoxalase 1 mRNA expression (p<0.05). Metformin did not have any significant effect on dicarbonyls, glutathione or on glyoxalase 1 expression in kidney cortex. Chronically elevated hypertriglyceridemia was associated with increased levels of dicarbonyls in heart and kidney. Beneficial effects of metformin on reactive dicarbonyls and glyoxalase in the heart could contribute to its cardioprotective effects.

A Hypothesis: Moderate Consumption of Alcohol Contributes to Lower Prevalence of Type 2 Diabetes Due to the Scavenging of Alpha-Dicarbonyls by Dietary Polyphenols

The world is experiencing an epidemic of type-2-diabetes mellitus (T2DM). This has led to increased morbidity and mortality, explosive growth in health care budgets, and an even greater adverse, if indirect, impact on societies and economies of affected countries. While genetic susceptibility to T2DM is a major determinant of its prevalence, changes in lifestyles also play a role. One such change has been a transition from traditional diets characterized by low caloric and high nutrient density to calorie-rich but nutrient-poor Western diets. Given this, one solution to the epidemic of T2DM would be to abandon Western diets and revert to traditional eating patterns. However, traditional diets cannot provide enough calories for the increasing global population, so transition from traditional to Western foodstuffs appears to be irreversible. Consequently, the only practical solution to problems caused by these changes is to modify Western diets, possibly by supplementing them with functional foods containing nutrients that would compensate for these dietary deficits. I present in this study a hypothesis to explain why shifts from traditional to Western diets have been so problematic and to suggest nutrients that may counteract these adverse effects. I postulate that the components of traditional diets that may compensate for deficiencies of Westerns diets are scavengers of reactive α-dicarbonyls produced as unavoidable by-products of glucose and lipid metabolism. Most important among these scavengers are some plant secondary metabolites: polyphenols, phlorotannins, and carotenoids. They are found in alcoholic beverages and are abundant in seasonings, cocoa, coffee, tea, whole grains, pigmented vegetables, fruits, and berries.

Involvement of exogenous 3‑deoxyglucosone in β‑cell dysfunction induces impaired glucose regulation

β‑cell dysfunction is the primary cause of type 2 diabetes mellitus (T2DM). 1,2‑dicarbonyl compounds, such as 3‑deoxyglucosone (3DG) have been reported to increase the risk of T2DM. Abnormal elevation of plasma 3DG may impair β‑cell function and thereby, it is linked to T2DM. Previous findings suggest that exogenous 3DG may serve an important role in the development of pre‑diabetes. In the present study, the authors examine whether exogenous 3DG induces impaired glucose regulation in mice by decreasing β‑cell function involving of accumulation of plasma 3DG. At two weeks following administration of 3DG, fasting blood glucose (FBG) levels, oral glucose tolerance (by a glucose meter) and plasma levels of 3DG (by HPLC) and insulin (by radioimmunoassay) were measured. Glucose‑stimulated insulin secretion in cultured pancreas islets and INS‑1 cells was measured by radioimmunoassay. Western blotting was used to examine the expression of the key molecules of the insulin‑PI3K signaling pathway. 3DG treatment increased FBG and fasting blood insulin levels, reduced oral glucose tolerance in conjunction with decreased ∆Ins30‑0/∆G30‑0. In 3DG‑treated mice, an increase in the plasma 3DG level was observed, which was most likely the mechanism for decreased β‑cell function. This idea was further supported by these results that non‑cytotoxic 3DG concentration obviously decreased glucose‑stimulated insulin secretion in cultured pancreas islets and INS‑1 cells exposure to high glucose (25.5 mM). 3DG decreased the expression of GLUT2 and phosphorylation of IRS‑1, PI3K‑p85 and Akt in high glucose‑induced INS‑1 cells. To the best of the authors' knowledge, the present study is the first to demonstrate that exogenous 3DG induced normal mice to develop IGR, resulting from β‑cell dysfunction. Exogenous 3DG administration increased plasma 3DG levels, which participates in inducing β‑cell dysfunction, at least in part, through impairing IRS‑1/PI3K/GLUT2 signaling.

Methylglyoxal-induced glycation changes adipose tissue vascular architecture, flow and expansion, leading to insulin resistance

Microvascular dysfunction has been suggested to trigger adipose tissue dysfunction in obesity. This study investigates the hypothesis that glycation impairs microvascular architecture and expandability with an impact on insulin signalling. Animal models supplemented with methylglyoxal (MG), maintained with a high-fat diet (HFD) or both (HFDMG) were studied for periepididymal adipose (pEAT) tissue hypoxia and local and systemic insulin resistance. Dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) was used to quantify blood flow in vivo, showing MG-induced reduction of pEAT blood flow. Increased adipocyte size and leptin secretion were observed only in rats feeding the high-fat diet, without the development of hypoxia. In turn, hypoxia was only observed when MG was combined (HFDMG group), being associated with impaired activation of the insulin receptor (Tyr1163), glucose intolerance and systemic and muscle insulin resistance. Accordingly, the adipose tissue angiogenic assay has shown decreased capillarization after dose-dependent MG exposure and glyoxalase-1 inhibition. Thus, glycation impairs adipose tissue capillarization and blood flow, hampering its expandability during a high-fat diet challenge and leading to hypoxia and insulin resistance. Such events have systemic repercussions in glucose metabolism and may lead to the onset of unhealthy obesity and progression to type 2 diabetes.

Fructose and NAFLD: The Multifaceted Aspects of Fructose Metabolism

Among various factors, such as an unhealthy diet or a sedentarity lifestyle, excessive fructose consumption is known to favor nonalcoholic fatty liver disease (NAFLD), as fructose is both a substrate and an inducer of hepatic de novo lipogenesis. The present review presents some well-established mechanisms and new clues to better understand the pathophysiology of fructose-induced NAFLD. Beyond its lipogenic effect, fructose intake is also at the onset of hepatic inflammation and cellular stress, such as oxidative and endoplasmic stress, that are key factors contributing to the progression of simple steatosis to nonalcoholic steatohepatitis (NASH). Beyond its hepatic effects, this carbohydrate may exert direct and indirect effects at the peripheral level. Excessive fructose consumption is associated, for example, with the release by the liver of several key mediators leading to alterations in the communication between the liver and the gut, muscles, and adipose tissue and to disease aggravation. These multifaceted aspects of fructose properties are in part specific to fructose, but are also shared in part with sucrose and glucose present in energy–dense beverages and foods. All these aspects must be taken into account in the development of new therapeutic strategies and thereby to better prevent NAFLD.

New isatin derivative inhibits neurodegeneration by restoring insulin signaling in brain

Diabetes is associated with neurodegeneration. Glycation ensues in diabetes and glycated proteins cause insulin resistance in brain resulting in amyloid plaques and NFTs. Also glycation enhances gliosis by promoting neuroinflammation. Currently there is no therapy available to target neurodegenration in brain therefore, development of new therapy that offers neuroprotection is critical. The objective of this study was to evaluate mechanistic effect of isatin derivative URM-II-81, an anti-glycation agent for improvement of insulin action in brain and inhibition of neurodegenration. Methylglyoxal induced stress was inhibited by treatment with URM-II-81. Also, Ser473 and Ser9 phosphorylation of Akt and GSK-3β respectively were restored by URM-II-81. Effect of URM-II-81 on axonal integrity was studied by differentiating Neuro2A using retinoic acid. URM-II-81 restored axonal length in MGO treated cells. Its effects were also studied in high fat and low dose streptozotocin induced diabetic mice where it reduced RBG levels and inhibited glycative stress by reducing HbA1c. URM-II-81 treatment also showed inhibition of gliosis in hippocampus. Histological analysis showed reduced NFTs in CA3 hippocampal region and restoration of insulin signaling in hippocampii of diabetic mice. Our findings suggest that URM-II-81 can be developed as a new therapeutic agent for treatment of neurodegenration.

The role of miR-190a in methylglyoxal-induced insulin resistance in endothelial cells

Methylglyoxal (MGO) is a reactive dicarbonyl produced as by-product of glycolysis, and its formation is heightened in hyperglycaemia. MGO plasma levels are two-fold to five-fold increased in diabetics and its accumulation promotes the progression of vascular complications. Impairment of endothelium-derived nitric oxide represents a common feature of endothelial dysfunction in diabetics. We previously demonstrated that MGO induces endothelial insulin resistance. Increasing evidence shows that high glucose and MGO modify vascular expression of several microRNAs (miRNAs), suggesting their potential role in the impairment of endothelial insulin sensitivity. The aim of the study is to investigate whether miRNAs may be involved in MGO-induced endothelial insulin resistance in endothelial cells. MGO reduces the expression of miR-190a both in mouse aortic endothelial cells (MAECs) and in aortae from mice knocked-down for glyoxalase-1. miR-190a inhibition impairs insulin sensitivity, whereas its overexpression prevents the MGO-induced insulin resistance in MAECs. miR-190a levels are not affected by the inhibition of ERK1/2 phosphorylation. Conversely, ERK1/2 activation is sustained by miR-190a inhibitor and the MGO-induced ERK1/2 hyper-activation is reduced by miR-190a mimic transfection. Similarly, protein levels of the upstream KRAS are increased by both MGO and miR-190a inhibitor, and these levels are reduced by miR-190a mimic transfection. Interestingly, silencing of KRAS is able to rescue the MGO-impaired activation of IRS1/Akt/eNOS pathway in response to insulin. In conclusion, miR-190a down-regulation plays a role in MGO-induced endothelial insulin resistance by increasing KRAS. This study highlights miR-190a as new candidate for the identification of strategies aiming at ameliorating vascular function in diabetes.

Polycystic ovary syndrome and environmental toxins

Polycystic ovary syndrome (PCOS) is the most common, heterogeneous, and multifactorial endocrine disorder in premenopausal women. The pathophysiology of this endocrinopathy is still unclear; however, the heterogeneity of its features within ethnic races, geographic location, and families suggests that environment and lifestyle are of prime importance. This work is mainly focused on the possible role of the most common and studied environmental toxins for this syndrome in the pathogenesis of PCOS. Plasticizers, such as bisphenol A (BPA) or phthalates, which belong to the categories of endocrine disrupting chemicals (EDCs) and advanced glycation end products (AGEs), affect humans' health in everyday, industrialized life; therefore special attention should be paid to such exposure. Timing of exposure to EDCs is crucial for the intensity of adverse health effects. It is now evident that fetuses, infants, and/or young children are the most susceptible groups, especially in the early development periods. Prenatal exposure to EDCs that mimic endogenous hormones may contribute to the altered fetal programming and in consequence lead to PCOS and other adverse health effects, potentially transgenerationally. Acute or prolonged exposure to EDCs and AGEs through different life cycle stages may result in destabilization of the hormonal homeostasis and lead to disruption of reproductive functions. They may also interfere with metabolic alterations such as obesity, insulin resistance, and compensatory hyperinsulinemia that can exacerbate the PCOS phenotype and contribute to PCOS consequences such as type 2 diabetes and cardiovascular disease. Since wide exposure to environmental toxins and their role in the pathophysiology of PCOS are supported by extensive data derived from diverse scientific models, protective strategies and strong recommendations should be considered to reduce human exposure to protect present and future generations from their adverse health effects.

Alagebrium attenuates methylglyoxal induced oxidative stress and AGE formation in H9C2 cardiac myocytes

AIM

Diabetes mellitus associated cardiovascular complications are a leading cause of morbidity and mortality worldwide. Methylglyoxal (MG) is a reactive ketoaldehyde and a byproduct of glucose metabolism and an inducer of advanced glycation endproducts (AGEs). Alagebrium (ALA) is an AGEs crosslink breaker, however, the effects of ALA on MG levels and its consequences in cultured rat cardiomyocytes are not known. The aim of the present study was to examine the effect of high glucose and MG on cultured rat cardiomyocytes and to investigate whether ALA could prevent any deleterious effects of high glucose and MG in these cells.

MAIN METHODS

MG levels were determined by HPLC. The expression of different genes was measured by RT-PCR. Oxidative stress and AGEs formation was determined by DCF probe and immunocytochemistry respectively.

KEY FINDINGS

High glucose- and MG treated- cardiomyocytes developed a significant increase in MG, and the expression for caspase-3, Bax, RAGE and NF-KB, which were all attenuated after pretreatment with ALA. A significant increase in reactive oxygen species generation and AGEs formation in high glucose- and MG treated- cultured cardiomyocytes was also observed, which was attenuated after pretreatment with ALA.

SIGNIFICANCE

ALA may have a preventive role against the deleterious effects of high glucose and MG in the heart. Prevention of dicarbonyl-induced AGEs, by safer and specific scavengers of MG is an attractive therapeutic option.

Methylglyoxal Impairs Insulin Secretion of Pancreatic β-Cells through Increased Production of ROS and Mitochondrial Dysfunction Mediated by Upregulation of UCP2 and MAPKs

Methylglyoxal (MG) is a highly reactive glucose metabolic intermediate and a major precursor of advanced glycation end products. MG level is elevated in hyperglycemic disorders such as diabetes mellitus. Substantial evidence has shown that MG is involved in the pathogenesis of diabetes and diabetic complications. We investigated the impact of MG on insulin secretion by MIN6 and INS-1 cells and the potential mechanisms of this effect. Our study demonstrates that MG impaired insulin secretion by MIN6 or ISN-1 cells in a dose-dependent manner. It increased reactive oxygen species (ROS) production and apoptosis rate in MIN6 or ISN-1 cells and inhibited mitochondrial membrane potential (MMP) and ATP production. Furthermore, the expression of UCP2, JNK, and P38 as well as the phosphorylation JNK and P38 was increased by MG. These effects of MG were attenuated by MG scavenger N-acetyl cysteine. Collectively, these data indicate that MG impairs insulin secretion of pancreatic β-cells through increasing ROS production. High levels of ROS can damage β-cells directly via JNK/P38 upregulation and through activation of UCP2 resulting in reduced MMP and ATP production, leading to β-cell dysfunction and impairment of insulin production.

Dietary Advanced Glycation End Products Consumption as a Direct Modulator of Insulin Sensitivity in Overweight Humans: A Study Protocol for a Double-Blind, Randomized, Two Period Cross-Over Trial

BACKGROUND

Advanced glycation end products (AGEs) are formed during the processing, storage, and cooking of foods. As part of a western diet, AGEs are consumed in excess and impair glucose metabolism in patients with type 2 diabetes. In the absence of diabetes, AGE-mediated decreases in insulin sensitivity and signaling have been postulated. However, randomized studies to test this relationship in humans are limited.

OBJECTIVE

The primary aim of this trial is to determine whether dietary consumption of AGEs will decrease insulin sensitivity in healthy overweight adults. A secondary aim is to determine the effects of dietary AGEs on insulin secretion, circulating soluble receptor for AGEs (sRAGE), and inflammation markers.

METHODS

Overweight, but otherwise healthy, non-diabetic adults (N=20) aged 18-50 years old will complete a randomized cross-over design intervention study alternating low and high (4-fold increase) AGE diets (2-week duration). At baseline, participants will undergo a medical review including an intravenous glucose tolerance test (IVGTT), a hyperinsulinemic-euglycemic clamp, and anthropometric measures and questionnaires assessing diet, physical activity, and general wellness. Each test diet will be followed for 14 days, followed by a 4-week washout period before commencement of the second alternate dietary period. Energy, macronutrient, and AGE intake will be calculated for each dietary period. Additionally, the AGE content of foods used in the study will be measured by ultra performance liquid chromatography mass spectrometry. All measurements will be repeated at the beginning and end of each dietary period. Primary and secondary outcomes will be expressed as a change over the dietary period for insulin sensitivity, secretion, anthropometric parameters, sRAGE, and inflammation markers and compared by paired t test and analysis of variance (ANOVA).

RESULTS

The study will be completed in early 2016.

CONCLUSION

The proposed trial will provide much needed clinical evidence on the impact of excess dietary AGE consumption on insulin sensitivity and will indicate whether lowering dietary AGE intake can improve insulin sensitivity and/or secretion, thereby decreasing risk for type 2 diabetes.

TRIAL REGISTRATION

Clinicaltrials.gov NCT00422253; https://clinicaltrials.gov/ct2/show/NCT00422253 (Archived by Webcite at http://www.webcitation.org/6ZXLhT89c).

The role of methylglyoxal and the glyoxalase system in diabetes and other age-related diseases

The formation and accumulation of advanced glycation endproducts (AGEs) are related to diabetes and other age-related diseases. Methylglyoxal (MGO), a highly reactive dicarbonyl compound, is the major precursor in the formation of AGEs. MGO is mainly formed as a byproduct of glycolysis. Under physiological circumstances, MGO is detoxified by the glyoxalase system into D-lactate, with glyoxalase I (GLO1) as the key enzyme in the anti-glycation defence. New insights indicate that increased levels of MGO and the major MGO-derived AGE, methylglyoxal-derived hydroimidazolone 1 (MG-H1), and dysfunctioning of the glyoxalase system are linked to several age-related health problems, such as diabetes, cardiovascular disease, cancer and disorders of the central nervous system. The present review summarizes the mechanisms through which MGO is formed, its detoxification by the glyoxalase system and its effect on biochemical pathways in relation to the development of age-related diseases. Although several scavengers of MGO have been developed over the years, therapies to treat MGO-associated complications are not yet available for application in clinical practice. Small bioactive inducers of GLO1 can potentially form the basis for new treatment strategies for age-related disorders in which MGO plays a pivotal role.

Relationship of Advanced Glycation End Products With Cardiovascular Disease in Menopausal Women

Cardiovascular disease (CVD) represents the most significant cause of death in postmenopausal women. Advanced glycation end products (AGEs) are formed by nonenzymatic modification of proteins, lipids, and nucleic acids by glucose. This review focuses on the contribution of AGEs and their receptors to the development of CVD in menopause. Advanced glycation end products circulate and activate the proinflammatory endothelial cell surface receptor called RAGE, bind to the extracellular matrix of the cardiovascular system, or bind to the circulating anti-inflammatory soluble form of RAGE (sRAGE). Data emerging from human and animal studies suggest that AGEs and both receptors (RAGE and sRAGE) are implicated in the pathophysiology of CVD. Particular emphasis has been given to the role of AGE-RAGE axis in oxidative stress, inflammation, endothelial cell toxicity, and progression of atherosclerosis in menopause. Data accruing from human and animal studies suggest that RAGE expression level and circulating sRAGE level are associated with estradiol and are correlated with CVD risk factors, such as adiposity, dyslipidemia, insulin resistance, diabetes, and metabolic syndrome. By recognizing the impact of AGEs on atherosclerosis, pharmacological strategies targeting the AGE-RAGE pathway hold therapeutic potential for CVD in menopausal women.

Measurement of methylglyoxal by stable isotopic dilution analysis LC-MS/MS with corroborative prediction in physiological samples

This protocol describes a method for the detection and quantification of methylglyoxal (MG), the major physiological substrate of the cytosolic glyoxalase system. Accumulation of MG, also called dicarbonyl stress, is implicated in tissue damage in aging and disease. Measurement of MG is important in physiological studies, in the development of glyoxalase 1 (Glo1) inducer and inhibitor therapeutics, and in the characterization of medical products, especially dialysis fluids, and of thermally processed foods and beverages. MG can be derivatized with 1,2-diaminobenzene (DB), resulting in an adduct that can be detected using liquid chromatography-tandem mass spectrometry (LC-MS/MS). Quantification is achieved by stable isotopic dilution analysis with [(13)C3]MG. Pre-analytic processing at ambient temperature, under acidic conditions with peroxidase inhibition, avoids artifactual overestimation of MG. Estimates obtained from physiological samples can be validated by kinetic modeling of in situ rates of protein glycation by MG for confirmation of the results. This procedure was developed for the analysis of cultured cells, plasma and animal tissue samples, and it can also be used to analyze plant material. Experimental measurement requires 4.5 h for sample batch pre-analytic processing and 30 min per sample for LC-MS/MS analysis.

Methylglyoxal impairs endothelial insulin sensitivity both in vitro and in vivo

AIMS/HYPOTHESIS

Insulin exerts a direct action on vascular cells, thereby affecting the outcome and progression of diabetic vascular complications. However, the mechanism through which insulin signalling is impaired in the endothelium of diabetic individuals remains unclear. In this work, we have evaluated the role of the AGE precursor methylglyoxal (MGO) in generating endothelial insulin resistance both in cells and in animal models.

METHODS

Time course experiments were performed on mouse aortic endothelial cells (MAECs) incubated with 500 μmol/l MGO. The glyoxalase-1 inhibitor S-p-bromobenzylglutathione-cyclopentyl-diester (SpBrBzGSHCp2) was used to increase the endogenous levels of MGO. For the in vivo study, an MGO solution was administrated i.p. to C57BL/6 mice for 7 weeks.

RESULTS

MGO prevented the insulin-dependent activation of the IRS1/protein kinase Akt/endothelial nitric oxide synthase (eNOS) pathway, thereby blunting nitric oxide (NO) production, while extracellular signal-regulated kinase (ERK1/2) activation and endothelin-1 (ET-1) release were increased by MGO in MAECs. Similar results were obtained in MAECs treated with SpBrBzGSHCp2. In MGO- and SpBrBzGSHCp2-exposed cells, inhibition of ERK1/2 decreased IRS1 phosphorylation on S616 and rescued insulin-dependent Akt activation and NO generation, indicating that MGO inhibition of the IRS1/Akt/eNOS pathway is mediated, at least in part, by ERK1/2. Chronic administration of MGO to C57BL/6 mice impaired whole-body insulin sensitivity and induced endothelial insulin resistance.

CONCLUSIONS/INTERPRETATION

MGO impairs the action of insulin on the endothelium both in vitro and in vivo, at least in part through an ERK1/2-mediated mechanism. These findings may be instrumental in developing novel strategies for preserving endothelial function in diabetes.

Hydrogen sulfide releasing aspirin, ACS14, attenuates high glucose-induced increased methylglyoxal and oxidative stress in cultured vascular smooth muscle cells

Hydrogen sulfide is a gasotransmitter with vasodilatory and anti-inflammatory properties. Aspirin is an irreversible cyclooxygenase inhibitor anti-inflammatory drug. ACS14 is a novel synthetic hydrogen sulfide releasing aspirin which inhibits cyclooxygenase and has antioxidant effects. Methylglyoxal is a chemically active metabolite of glucose and fructose, and a major precursor of advanced glycation end products formation. Methylglyoxal is harmful when produced in excess. Plasma methylglyoxal levels are significantly elevated in diabetic patients. Our aim was to investigate the effects of ACS14 on methylglyoxal levels in cultured rat aortic vascular smooth muscle cells. We used cultured rat aortic vascular smooth muscle cells for the study. Methylglyoxal was measured by HPLC after derivatization, and nitrite+nitrate with an assay kit. Western blotting was used to determine NADPH oxidase 4 (NOX4) and inducible nitric oxide synthase (iNOS) protein expression. Dicholorofluorescein assay was used to measure oxidative stress. ACS14 significantly attenuated elevation of intracellular methylglyoxal levels caused by incubating cultured vascular smooth muscle cells with methylglyoxal (30 µM) and high glucose (25 mM). ACS14, but not aspirin, caused a significant attenuation of increase in nitrite+nitrate levels caused by methylglyoxal or high glucose. ACS14, aspirin, and sodium hydrogen sulfide (NaHS, a hydrogen sulfide donor), all attenuated the increase in oxidative stress caused by methylglyoxal and high glucose in cultured cells. ACS14 prevented the increase in NOX4 expression caused by incubating the cultured VSMCs with MG (30 µM). ACS14, aspirin and NaHS attenuated the increase in iNOS expression caused by high glucose (25 mM). In conclusion, ACS14 has the novel ability to attenuate an increase in methylglyoxal levels which in turn can reduce oxidative stress, decrease the formation of advanced glycation end products and prevent many of the known deleterious effects of elevated methylglyoxal. Thus, ACS14 has the potential to be especially beneficial for diabetic patients pending further in vivo studies.

A vascular piece in the puzzle of adipose tissue dysfunction: mechanisms and consequences

In the last years, several studies unravelled many aspects of adipose tissue pathophysiology in metabolic diseases. Some studies suggested hypoxia as one of such aspects, despite the exact mechanisms and pathophysiological significance is still partially unknown. Adipose tissue was shown to be hypoxic in obesity, mainly resulting from adipocyte hypertrophy, leading to increased activation of inflammatory pathways. In animal and cell models, hypoxia-induced inflammation was shown to lead to endocrine alterations and dysmetabolism. However, recent evidences suggest that instead of a simple low oxygenation theory, adipose tissue microvasculature may be regulated by a series of factors, including vasoactive factors like angiotensin II, angiogenesis and glycation, among others. This review summarizes the current knowledge about the role of these factors in the regulation of adipose tissue irrigation and the functional consequences of adipose tissue microvascular dysfunction.

Consumption of a diet low in advanced glycation end products for 4 weeks improves insulin sensitivity in overweight women

OBJECTIVE High-heat cooking of food induces the formation of advanced glycation end products (AGEs), which are thought to impair glucose metabolism in type 2 diabetic patients. High intake of fructose might additionally affect endogenous formation of AGEs. This parallel intervention study investigated whether the addition of fructose or cooking methods influencing the AGE content of food affect insulin sensitivity in overweight individuals. RESEARCH DESIGN AND METHODS Seventy-four overweight women were randomized to follow either a high- or low-AGE diet for 4 weeks, together with consumption of either fructose or glucose drinks. Glucose and insulin concentrations-after fasting and 2 h after an oral glucose tolerance test-were measured before and after the intervention. Homeostasis model assessment of insulin resistance (HOMA-IR) and insulin sensitivity index were calculated. Dietary and urinary AGE concentrations were measured (liquid chromatography tandem mass spectrometry) to estimate AGE intake and excretion. RESULTS When adjusted for changes in anthropometric measures during the intervention, the low-AGE diet decreased urinary AGEs, fasting insulin concentrations, and HOMA-IR, compared with the high-AGE diet. Addition of fructose did not affect any outcomes. CONCLUSIONS Diets with high AGE content may increase the development of insulin resistance. AGEs can be reduced by modulation of cooking methods but is unaffected by moderate fructose intake.

Up-regulation of aldolase A and methylglyoxal production in adipocytes

BACKGROUND AND PURPOSE

We previously reported that up-regulation of aldolase B, a key enzyme in fructose metabolism, was mainly responsible for vascular methylglyoxal (MG) overproduction under different pathological conditions. Here we investigated whether aldolase A, an enzyme of the glycolytic pathway, also caused MG overproduction in insulin-sensitive adipocytes.

EXPERIMENTAL APPROACH

The relative contributions of different metabolic pathways or enzymes to MG generation were evaluated in cultured 3T3-L1 adipocytes.

KEY RESULTS

Glucose (25 mM) had no effect on aldolase A gene expression, but insulin (100 nM) up-regulated aldolase A mRNA and protein levels in the absence or presence of 25 mM glucose in adipocytes. Treatment with insulin increased levels of basal or glucose (25 mM)-induced MG and glucose 6-phosphate. However, insulin, glucose (25 mM) or their combination had no effect on cellular levels of sorbitol and fructose, but down-regulated gene expression of aldolase B to a similar extent, when compared with the control group. Incubation of 3T3-L1 adipocytes with fructose, acetone, acetol, threonine or glycine (25 mM), with or without insulin did not alter cellular MG levels. The elevated MG levels induced by insulin, glucose (25 mM) or their combination in adipocytes was completely reduced by siRNA knock down of aldolase A or application of 2-deoxy-D-glucose (a non-specific inhibitor of glucose uptake and glycolysis), but not by knock down of aldolase B.

CONCLUSION AND IMPLICATIONS

Insulin enhanced MG overproduction in insulin-sensitive adipocytes by up-regulating aldolase A, a mechanism that could be involved in the development of insulin resistance and obesity.

Dietary fructose feeding increases adipose methylglyoxal accumulation in rats in association with low expression and activity of glyoxalase-2

Methylglyoxal is a precursor to advanced glycation endproducts that may contribute to diabetes and its cardiovascular-related complications. Methylglyoxal is successively catabolized to d-lactate by glyoxalase-1 and glyoxalase-2. The objective of this study was to determine whether dietary fructose and green tea extract (GTE) differentially regulate methylglyoxal accumulation in liver and adipose, mediated by tissue-specific differences in the glyoxalase system. We fed six week old male Sprague-Dawley rats a low-fructose diet (10% w/w) or a high-fructose diet (60% w/w) containing no GTE or GTE at 0.5% or 1.0% for nine weeks. Fructose-fed rats had higher (P < 0.05) adipose methylglyoxal, but GTE had no effect. Plasma and hepatic methylglyoxal were unaffected by fructose and GTE. Fructose and GTE also had no effect on the expression or activity of glyoxalase-1 and glyoxalase-2 at liver or adipose. Regardless of diet, adipose glyoxalase-2 activity was 10.8-times lower (P < 0.05) than adipose glyoxalase-1 activity and 5.9-times lower than liver glyoxalase-2 activity. Adipose glyoxalase-2 activity was also inversely related to adipose methylglyoxal (r = −0.61; P < 0.05). These findings suggest that fructose-mediated adipose methylglyoxal accumulation is independent of GTE supplementation and that its preferential accumulation in adipose compared to liver is due to low constitutive expression of glyoxalase-2.