Degradation products of proteins damaged by glycation, oxidation and nitration in clinical type 1 diabetes

Aldehyde-induced DNA-protein crosslinks- DNA damage, repair and mutagenesis

Aldehyde exposure has been shown to lead to the formation of DNA damage comprising of DNA-protein crosslinks (DPCs), base adducts and interstrand or intrastrand crosslinks. DPCs have recently drawn more attention because of recent advances in detection and quantification of these adducts. DPCs are highly deleterious to genome stability and have been shown to block replication forks, leading to wide-spread mutagenesis. Cellular mechanisms to prevent DPC-induced damage include excision repair pathways, homologous recombination, and specialized proteases involved in cleaving the covalently bound proteins from DNA. These pathways were first discovered in formaldehyde-treated cells, however, since then, various other aldehydes have been shown to induce formation of DPCs in cells. Defects in DPC repair or aldehyde clearance mechanisms lead to various diseases including Ruijs-Aalfs syndrome and AMeD syndrome in humans. Here, we discuss recent developments in understanding how aldehydes form DPCs, how they are repaired, and the consequences of defects in these repair pathways.

Dietary glycation compounds - implications for human health

The term "glycation compounds" comprises a wide range of structurally diverse compounds that are formed endogenously and in food via the Maillard reaction, a chemical reaction between reducing sugars and amino acids. Glycation compounds produced endogenously are considered to contribute to a range of diseases. This has led to the hypothesis that glycation compounds present in food may also cause adverse effects and thus pose a nutritional risk to human health. In this work, the Senate Commission on Food Safety (SKLM) of the German Research Foundation (DFG) summarized data on formation, occurrence, exposure and toxicity of glycation compounds (Part A) and systematically assessed potential associations between dietary intake of defined glycation compounds and disease, including allergy, diabetes, cardiovascular and renal disease, gut/gastrotoxicity, brain/cognitive impairment and cancer (Part B). A systematic search in Pubmed (Medline), Scopus and Web of Science using a combination of keywords defining individual glycation compounds and relevant disease patterns linked to the subject area of food, nutrition and diet retrieved 253 original publications relevant to the research question. Of these, only 192 were found to comply with previously defined quality criteria and were thus considered suitable to assess potential health risks of dietary glycation compounds. For each adverse health effect considered in this assessment, however, only limited numbers of human, animal and in vitro studies were identified. While studies in humans were often limited due to small cohort size, short study duration, and confounders, experimental studies in animals that allow for controlled exposure to individual glycation compounds provided some evidence for impaired glucose tolerance, insulin resistance, cardiovascular effects and renal injury in response to oral exposure to dicarbonyl compounds, albeit at dose levels by far exceeding estimated human exposures. The overall database was generally inconsistent or inconclusive. Based on this systematic review, the SKLM concludes that there is at present no convincing evidence for a causal association between dietary intake of glycation compounds and adverse health effects.

Immunometabolic profiling related with gestational diabetes mellitus: a nested case-control study of CD4+ T cell phenotypes and glycemic traits

AIMS

To investigate immunometabolic associations of CD4+ T cell phenotypes with gestational diabetes mellitus (GDM).

METHODS

A nested case-control study was conducted comprising 53 pairs of GDM patients and matched controls within a prospective cohort. Metabolomic signatures related to both CD4+ T cell phenotypes and glycemic traits among pregnant women were investigated by weighted gene co-expression network analysis (WGCNA). Multivariable-adjusted generalized linear models were used to explore the associations of CD4+ T cell phenotypes and selected metabolites with GDM. Mediation analysis was conducted to evaluate the mediating effect of selected metabolites on the relationship between CD4+ T cell phenotypes and glycemic traits.

RESULTS

Higher levels of Treg cells (OR per SD increment (95%CI): 0.57 (0.34, 0.95), p = 0.031) and increased expression of Foxp3 (OR per SD increment (95%CI): 0.59 (0.35, 0.97), p = 0.039) and GATA3 (OR per SD increment (95%CI): 0.42 (0.25, 0.72), p = 0.002) were correlated with a decreased risk of GDM. Plasma pyruvaldehyde, S-adenosylhomocysteine (SAH), bergapten, and 9-fluorenone mediated the association between Tregs and fasting plasma glucose (FPG), with mediation proportions of 46.9%, 39.6%, 52.4%, and 56.9%, respectively.

CONCLUSIONS

Treg cells and Foxp3 expressions were inversely associated with GDM risk, with potential metabolic mechanisms involving metabolites such as pyruvaldehyde and SAH.

iGly-IDN: Identifying Lysine Glycation Sites in Proteins Based on Improved DenseNet

Lysine glycation is one of the most significant protein post-translational modifications, which changes the properties of the proteins and causes them to be dysfunctional. Accurately identifying glycation sites helps to understand the biological function and potential mechanism of glycation in disease treatments. Nonetheless, the experimental methods are ordinarily inefficient and costly, so effective computational methods need to be developed. In this study, we proposed the new model called iGly-IDN based on the improved densely connected convolutional networks (DenseNet). First, one hot encoding was adopted to obtain the original feature maps. Afterward, the improved DenseNet was adopted to capture feature information with the importance degrees during the feature learning. According to the experimental results, Acc reaches 66%, and Mathews correlation coefficient reaches 0.33 on the independent testing data set, which indicates that the iGly-IDN can provide more effective glycation site identification than the current predictors.

Serum-based ATR-FTIR spectroscopy combined with multivariate analysis for the diagnosis of pre-diabetes and diabetes

Diabetes mellitus (DM) is a metabolic disease with an increasing prevalence that is causing worldwide concern. The pre-diabetes stage is the only reversible stage in the patho-physiological process towards DM. Due to the limitations of traditional methods, the diagnosis and detection of DM and pre-diabetes are complicated, expensive, and time-consuming. Therefore, it would be of great benefit to develop a simple, rapid and inexpensive diagnostic test. Herein, the infrared (IR) spectra of serum samples from 111 DM patients, 111 pre-diabetes patients and 333 healthy volunteers were collected using attenuated total reflection Fourier-transform IR (ATR-FTIR) spectroscopy and this was combined with the multivariate analysis of principal component analysis linear discriminant analysis (PCA-LDA) to develop a discriminant model to verify the diagnostic potential of this approach. The study found that the accuracy of the test model established by ATR-FTIR spectroscopy combined with PCA-LDA was 97%, and the sensitivity and specificity were 100% and 100% in the control group, 94% and 98% in the pre-diabetes group, and 91% and 98% in the DM group, respectively. This indicates that this method can effectively diagnose DM and pre-diabetes, which has far-reaching clinical significance.

Non-cross-linking advanced glycation end products affect prohormone processing

Advanced glycation end products (AGEs) are non-enzymatic post-translational modifications of amino acids and are associated with diabetic complications. One proposed pathomechanism is the impaired processing of AGE-modified proteins or peptides including prohormones. Two approaches were applied to investigate whether substrate modification with AGEs affects the processing of substrates like prohormones to the active hormones. First, we employed solid-phase peptide synthesis to generate unmodified as well as AGE-modified protease substrates. Activity of proteases towards these substrates was quantified. Second, we tested the effect of AGE-modified proinsulin on the processing to insulin. Proteases showed the expected activity towards the unmodified peptide substrates containing arginine or lysine at the C-terminal cleavage site. Indeed, modification with Nε-carboxymethyllysine (CML) or methylglyoxal-hydroimidazolone 1 (MG-H1) affected all proteases tested. Cysteine cathepsins displayed a reduction in activity by ∼50% towards CML and MG-H1 modified substrates. The specific proteases trypsin, proprotein convertases subtilisin-kexins (PCSKs) type proteases, and carboxypeptidase E (CPE) were completely inactive towards modified substrates. Proinsulin incubation with methylglyoxal at physiological concentrations for 24 h resulted in the formation of MG-modified proinsulin. The formation of insulin was reduced by up to 80% in a concentration-dependent manner. Here, we demonstrate the inhibitory effect of substrate-AGE modifications on proteases. The finding that PCSKs and CPE, which are essential for prohormone processing, are inactive towards modified substrates could point to a yet unrecognized pathomechanism resulting from AGE modification relevant for the etiopathogenesis of diabetes and the development of obesity.

Soluble RAGE and skeletal muscle tissue RAGE expression profiles in lean and obese young adults across differential aerobic exercise intensities

Nearly 40% of Americans have obesity and are at increased risk for developing type 2 diabetes. Skeletal muscle is responsible for >80% of insulin-stimulated glucose uptake that is attenuated by the inflammatory milieu of obesity and augmented by aerobic exercise. The receptor for advanced glycation endproducts (RAGE) is an inflammatory receptor directly linking metabolic dysfunction with inflammation. Circulating soluble isoforms of RAGE (sRAGE) formed either by proteolytic cleavage (cRAGE) or alternative splicing (esRAGE) act as decoys for RAGE ligands, thereby counteracting RAGE-mediated inflammation. We aimed to determine if RAGE expression or alternative splicing of RAGE is altered by obesity in muscle, and whether acute aerobic exercise (AE) modifies RAGE and sRAGE. Young (20-34 yr) participants without [n = 17; body mass index (BMI): 22.6 ± 2.6 kg/m2] and with obesity (n = 7; BMI: 32.8 ± 2.9 kg/m2) performed acute aerobic exercise (AE) at 40%, 65%, or 80% of maximal aerobic capacity (V̇o2max; mL/kg/min) on separate visits. Blood was taken before and 30 min after each AE bout. Muscle biopsy samples were taken before, 30 min, and 3 h after the 80% V̇o2max AE bout. Individuals with obesity had higher total RAGE and esRAGE mRNA and RAGE protein (P < 0.0001). In addition, RAGE and esRAGE transcripts correlated to transcripts of the NF-κB subunit P65 (P < 0.05). There was no effect of AE on total RAGE or esRAGE transcripts, or RAGE protein (P > 0.05), and AE tended to decrease circulating sRAGE in particular at lower intensities of exercise. RAGE expression is exacerbated in skeletal muscle with obesity, which may contribute to muscle inflammation via NF-κB. Future work should investigate the consequences of increased skeletal muscle RAGE on the development of obesity-related metabolic dysfunction and potential mitigating strategies.NEW & NOTEWORTHY This study is the first to investigate the effects of aerobic exercise intensity on circulating sRAGE isoforms, muscle RAGE protein, and muscle RAGE splicing. sRAGE isoforms tended to diminish with exercise, although this effect was attenuated with increasing exercise intensity. Muscle RAGE protein and gene expression were unaffected by exercise. However, individuals with obesity displayed nearly twofold higher muscle RAGE protein and gene expression, which positively correlated with expression of the P65 subunit of NF-κB.

Glyoxalase I is a novel target for the prevention of metabolic derangement

Obesity prevalence in the US has nearly tripled since 1975 and a parallel increase in prevalence of type 2 diabetes (T2D). Obesity promotes a myriad of metabolic derangements with insulin resistance (IR) being perhaps the most responsible for the development of T2D and other related diseases such as cardiovascular disease. The precarious nature of IR development is such that it provides a valuable target for the prevention of further disease development. However, the mechanisms driving IR are numerous and complex making the development of viable interventions difficult. The development of metabolic derangement in the context of obesity promotes accumulation of reactive metabolites such as the reactive alpha-dicarbonyl methylglyoxal (MG). MG accumulation has long been appreciated as a marker of disease progression in patients with T2D as well as the development of diabetic complications. However, recent evidence suggests that the accumulation of MG occurs with obesity prior to T2D onset and may be a primary driving factor for the development of IR and T2D. Further, emerging evidence also suggests that this accumulation of MG with obesity may be a result in a loss of MG detoxifying capacity of glyoxalase I. In this review, we will discuss the evidence that posits MG accumulation because of GLO1 attenuation is a novel target mechanism of the development of metabolic derangement. In addition, we will also explore the regulation of GLO1 and the strategies that have been investigated so far to target GLO1 regulation for the prevention and treatment of metabolic derangement.

Analysis of Signaling Pathways of Necroptotic and Pyroptotic Cell Death in the Hearts of Rats With Type 2 Diabetes Mellitus

Diabetes mellitus is known to produce various cell-damaging events and thereby underlie heart dysfunction and remodeling. However, very little is known about its inflammation-associated pathomechanisms due to necrosis-like cell death. For this purpose, we aimed to investigate signaling pathways of necroptosis and pyroptosis, known to produce plasma membrane rupture with the resultant promotion of inflammation. One-year old Zucker diabetic fatty (ZDF) rats did not exhibit significant heart dysfunction as revealed by echocardiographic measurement. On the other hand, there was a decrease in heart rate due to diabetes. Immunoblotting analysis showed that the left ventricles of ZDF rats overexpress neither the main necroptotic proteins including receptor-interacting protein kinase 3 (RIP3) and mixed lineage domain kinase-like pseudokinase (MLKL), nor the pyroptotic regulators including NLR family pyrin domain containing 3 protein (NLRP3), caspase-1, interleukin-1 beta (IL-1beta and the N-terminal gasdermin D (GSDMD-N). On the other hand, the increased activation of the RIP3 kinase due to phosphorylation was found in such hearts. In summary, we showed for the first time that the activation of cardiac RIP3 is upregulated due to disturbances in glucose metabolism which, however, did not proceed to necrosis-like cell death. These data can indicate that the activated RIP3 might also underlie other pleiotropic, non-necroptotic signaling pathways under basal conditions.

Autoimmunity: A New Focus on Nasal Polyps

Chronic rhinosinusitis with nasal polyps (CRSwNP) has long been considered a benign, chronic inflammatory, and hyperplastic disease. Recent studies have shown that autoimmune-related mechanisms are involved in the pathology of nasal polyps. Activated plasma cells, eosinophils, basophils, innate type 2 lymphocytes, mast cells, and proinflammatory cytokine in polyp tissue indicate the mobilization of innate and adaptive immune pathways during polyp formation. The discovery of a series of autoantibodies further supports the autoimmune nature of nasal polyps. Local homeostasis dysregulation, infection, and chronic inflammation may trigger autoimmunity through several mechanisms, including autoantigens overproduction, microbial translocation, molecular mimicry, superantigens, activation or inhibition of receptors, bystander activation, dysregulation of Toll-Like Receptors (TLRs), epitope spreading, autoantigens complementarity. In this paper, we elaborated on the microbiome-mediated mechanism, abnormal host immunity, and genetic changes to update the role of autoimmunity in the pathogenesis of chronic rhinosinusitis with nasal polyps.

Methylglyoxal products in pre-symptomatic type 1 diabetes

INTRODUCTION

Progression to type 1 diabetes has emerged as a complex process with metabolic alterations proposed to be a significant driver of disease. Monitoring products of altered metabolism is a promising tool for determining the risk of type 1 diabetes progression and to supplement existing predictive biomarkers. Methylglyoxal (MG) is a reactive product produced from protein, lipid, and sugar metabolism, providing a more comprehensive measure of metabolic changes compared to hyperglycemia alone. MG forms covalent adducts on nucleic and amino acids, termed MG-advanced glycation end products (AGEs) that associate with type 1 diabetes.

METHODS

We tested their ability to predict risk of disease and discriminate which individuals with autoimmunity will progress to type 1 diabetes. We measured serum MG-AGEs from 141 individuals without type 1 diabetes and 271 individuals with type 1 diabetes enrolled in the Fr1da cohort. Individuals with type 1 diabetes were at stages 1, 2, and 3.

RESULTS

We examined the association of MG-AGEs with type 1 diabetes. MG-AGEs did not correlate with HbA1c or differ between stages 1, 2, and 3 type 1 diabetes. Yet, RNA MG-AGEs were significantly associated with the rate of progression to stage 3 type 1 diabetes, with lower serum levels increasing risk of progression.

DISCUSSION

MG-AGEs were able to discriminate which individuals with autoantibodies would progress at a faster rate to stage 3 type 1 diabetes providing a potential new clinical biomarker for determining rate of disease progression and pointing to contributing metabolic pathways.

Analysis of Serum Advanced Glycation Endproducts Reveals Methylglyoxal-Derived Advanced Glycation MG-H1 Free Adduct Is a Risk Marker in Non-Diabetic and Diabetic Chronic Kidney Disease

Accumulation of advanced glycation endproducts (AGEs) is linked to decline in renal function, particularly in patients with diabetes. Major forms of AGEs in serum are protein-bound AGEs and AGE free adducts. In this study, we assessed levels of AGEs in subjects with and without diabetes, with normal renal function and stages 2 to 4 chronic kidney disease (CKD), to identify which AGE has the greatest progressive change with decline in renal function and change in diabetes. We performed a cross-sectional study of patients with stages 2-4 CKD, with and without diabetes, and healthy controls (n = 135). Nine protein-bound and free adduct AGEs were quantified in serum. Most protein-bound AGEs increased moderately through stages 2-4 CKD whereas AGE free adducts increased markedly. Methylglyoxal-derived hydroimidazolone MG-H1 free adduct was the AGE most responsive to CKD status, increasing 8-fold and 30-fold in stage 4 CKD in patients without and with diabetes, respectively. MG-H1 Glomerular filtration flux was increased 5-fold in diabetes, likely reflecting increased methylglyoxal glycation status. We conclude that serum MG-H1 free adduct concentration was strongly related to stage of CKD and increased in diabetes status. Serum MG-H1 free adduct is a candidate AGE risk marker of non-diabetic and diabetic CKD.

Understanding the role of glycation in the pathology of various non-communicable diseases along with novel therapeutic strategies

Glycation refers to carbonyl group condensation of the reducing sugar with the free amino group of protein, which forms Amadori products and advanced glycation end products (AGEs). These AGEs alter protein structure and function by configuring a negative charge on the positively charged arginine and lysine residues. Glycation plays a vital role in the pathogenesis of metabolic diseases, brain disorders, aging, and gut microbiome dysregulation with the aid of 3 mechanisms: (i) formation of highly reactive metabolic pathway-derived intermediates, which directly affect protein function in cells, (ii) the interaction of AGEs with its associated receptors to create oxidative stress causing the activation of transcription factor NF-κB, and (iii) production of extracellular AGEs hinders interactions between cellular and matrix molecules affecting vascular and neural genesis. Therapeutic strategies are thus required to inhibit glycation at different steps, such as blocking amino and carbonyl groups, Amadori products, AGEs-RAGE interactions, chelating transition metals, scavenging free radicals, and breaking crosslinks formed by AGEs. The present review focused on explicitly elaborating the impact of glycation-influenced molecular mechanisms in developing and treating noncommunicable diseases.

Methylglyoxal and Its Adducts: Induction, Repair, and Association with Disease

Metabolism is an essential part of life that provides energy for cell growth. During metabolic flux, reactive electrophiles are produced that covalently modify macromolecules, leading to detrimental cellular effects. Methylglyoxal (MG) is an abundant electrophile formed from lipid, protein, and glucose metabolism at intracellular levels of 1-4 μM. MG covalently modifies DNA, RNA, and protein, forming advanced glycation end products (MG-AGEs). MG and MG-AGEs are associated with the onset and progression of many pathologies including diabetes, cancer, and liver and kidney disease. Regulating MG and MG-AGEs is a potential strategy to prevent disease, and they may also have utility as biomarkers to predict disease risk, onset, and progression. Here, we review recent advances and knowledge surrounding MG, including its production and elimination, mechanisms of MG-AGEs formation, the physiological impact of MG and MG-AGEs in disease onset and progression, and the latter in the context of its receptor RAGE. We also discuss methods for measuring MG and MG-AGEs and their clinical application as prognostic biomarkers to allow for early detection and intervention prior to disease onset. Finally, we consider relevant clinical applications and current therapeutic strategies aimed at targeting MG, MG-AGEs, and RAGE to ultimately improve patient outcomes.

On the Prediction of In Vitro Arginine Glycation of Short Peptides Using Artificial Neural Networks

One of the hallmarks of diabetes is an increased modification of cellular proteins. The most prominent type of modification stems from the reaction of methylglyoxal with arginine and lysine residues, leading to structural and functional impairments of target proteins. For lysine glycation, several algorithms allow a prediction of occurrence; thus, making it possible to pinpoint likely targets. However, according to our knowledge, no approaches have been published for predicting the likelihood of arginine glycation. There are indications that arginine and not lysine is the most prominent target for the toxic dialdehyde. One of the reasons why there is no arginine glycation predictor is the limited availability of quantitative data. Here, we used a recently published high-quality dataset of arginine modification probabilities to employ an artificial neural network strategy. Despite the limited data availability, our results achieve an accuracy of about 75% of correctly predicting the exact value of the glycation probability of an arginine-containing peptide without setting thresholds upon whether it is decided if a given arginine is modified or not. This contribution suggests a solution for predicting arginine glycation of short peptides.

AGEomics Biomarkers and Machine Learning-Realizing the Potential of Protein Glycation in Clinical Diagnostics

Protein damage by glycation, oxidation and nitration is a continuous process in the physiological system caused by reactive metabolites associated with dicarbonyl stress, oxidative stress and nitrative stress, respectively. The term AGEomics is defined as multiplexed quantitation of spontaneous modification of proteins damage and other usually low-level modifications associated with a change of structure and function—for example, citrullination and transglutamination. The method of quantitation is stable isotopic dilution analysis liquid chromatography—tandem mass spectrometry (LC-MS/MS). This provides robust quantitation of normal and damaged or modified amino acids concurrently. AGEomics biomarkers have been used in diagnostic algorithms using machine learning methods. In this review, I describe the utility of AGEomics biomarkers and provide evidence why these are close to the phenotype of a condition or disease compared to other metabolites and metabolomic approaches and how to train and test algorithms for clinical diagnostic and screening applications with high accuracy, sensitivity and specificity using machine learning approaches.

The Role of Advanced Glycation End Products and Its Soluble Receptor in Kidney Diseases

Patients with chronic kidney disease (CKD) are more prone to oxidative stress and chronic inflammation, which may lead to an increase in the synthesis of advanced glycation end products (AGEs). Because AGEs are mostly removed by healthy kidneys, AGE accumulation is a result of both increased production and decreased kidney clearance. On the other hand, AGEs may potentially hasten decreasing kidney function in CKD patients, and are independently related to all-cause mortality. They are one of the non-traditional risk factors that play a significant role in the underlying processes that lead to excessive cardiovascular disease in CKD patients. When AGEs interact with their cell-bound receptor (RAGE), cell dysfunction is initiated by activating nuclear factor kappa-B (NF-κB), increasing the production and release of inflammatory cytokines. Alterations in the AGE-RAGE system have been related to the development of several chronic kidney diseases. Soluble RAGE (sRAGE) is a decoy receptor that suppresses membrane-bound RAGE activation and AGE-RAGE-related toxicity. sRAGE, and more specifically, the AGE/sRAGE ratio, may be promising tools for predicting the prognosis of kidney diseases. In the present review, we discuss the potential role of AGEs and sRAGE as biomarkers in different kidney pathologies.

Emerging Glycation-Based Therapeutics-Glyoxalase 1 Inducers and Glyoxalase 1 Inhibitors

The abnormal accumulation of methylglyoxal (MG) leading to increased glycation of protein and DNA has emerged as an important metabolic stress, dicarbonyl stress, linked to aging, and disease. Increased MG glycation produces inactivation and misfolding of proteins, cell dysfunction, activation of the unfolded protein response, and related low-grade inflammation. Glycation of DNA and the spliceosome contribute to an antiproliferative and apoptotic response of high, cytotoxic levels of MG. Glyoxalase 1 (Glo1) of the glyoxalase system has a major role in the metabolism of MG. Small molecule inducers of Glo1, Glo1 inducers, have been developed to alleviate dicarbonyl stress as a prospective treatment for the prevention and early-stage reversal of type 2 diabetes and prevention of vascular complications of diabetes. The first clinical trial with the Glo1 inducer, trans-resveratrol and hesperetin combination (tRES-HESP)-a randomized, double-blind, placebo-controlled crossover phase 2A study for correction of insulin resistance in overweight and obese subjects, was completed successfully. tRES-HESP corrected insulin resistance, improved dysglycemia, and low-grade inflammation. Cell permeable Glo1 inhibitor prodrugs have been developed to induce severe dicarbonyl stress as a prospective treatment for cancer-particularly for high Glo1 expressing-related multidrug-resistant tumors. The prototype Glo1 inhibitor is prodrug S-p-bromobenzylglutathione cyclopentyl diester (BBGD). It has antitumor activity in vitro and in tumor-bearing mice in vivo. In the National Cancer Institute human tumor cell line screen, BBGD was most active against the glioblastoma SNB-19 cell line. Recently, potent antitumor activity was found in glioblastoma multiforme tumor-bearing mice. High Glo1 expression is a negative survival factor in chemotherapy of breast cancer where adjunct therapy with a Glo1 inhibitor may improve treatment outcomes. BBGD has not yet been evaluated clinically. Glycation by MG now appears to be a pathogenic process that may be pharmacologically manipulated for therapeutic outcomes of potentially important clinical impact.

Glyoxal induced glycative insult suffered by immunoglobulin G and fibrinogen proteins: A comparative physicochemical characterization to reveal structural perturbations

Glycation of proteins results in structural alteration, functional deprivation, and generation of advanced glycation end products (AGEs). Reactive oxygen species (ROS) that are generated during in vivo autoxidation of glucose induces glycoxidation of intermediate glycation-adducts, which in turn give rise to aldehyde and/or ketone groups containing dicarbonyls or reactive carbonyl species (RCS). RCS further reacts non-enzymatically and starts the glycation-oxidation vicious cycle, thus exacerbating oxidative, carbonyl, and glycative stress in the physiological system. Glyoxal (GO), a reactive dicarbonyl that generates during glycoxidation and lipid peroxidation, contributes to glycation. This in vitro physicochemical characterization study focuses on GO-induced glycoxidative damage suffered by immunoglobulin G (IgG) and fibrinogen proteins. The structural alterations were analyzed by UV-vis, fluorescence, circular dichroism, and Fourier transform infrared (FT-IR) spectroscopy. Ketoamines, protein carbonyls, hydroxymethylfurfural (HMF), free lysine, free arginine, carboxymethyllysine (CML), and protein aggregation were also quantified. Structural perturbations, increased concentration of ketoamines, protein carbonyls, HMF, and malondialdehyde (MDA) were reported in glycated proteins. The experiment results also validate increased oxidative stress and AGEs formation i.e. IgG-AGEs and Fib-AGEs. Thus, we can conclude that AGEs formation during GO-mediated glycation of IgG and fibrinogen could hamper normal physiology and might play a significant role in the pathogenesis of diabetes-associated secondary complications.

BERT-Kgly: A Bidirectional Encoder Representations From Transformers (BERT)-Based Model for Predicting Lysine Glycation Site for Homo sapiens

As one of the most important posttranslational modifications (PTMs), protein lysine glycation changes the characteristics of the proteins and leads to the dysfunction of the proteins, which may cause diseases. Accurately detecting the glycation sites is of great benefit for understanding the biological function and potential mechanism of glycation in the treatment of diseases. However, experimental methods are expensive and time-consuming for lysine glycation site identification. Instead, computational methods, with their higher efficiency and lower cost, could be an important supplement to the experimental methods. In this study, we proposed a novel predictor, BERT-Kgly, for protein lysine glycation site prediction, which was developed by extracting embedding features of protein segments from pretrained Bidirectional Encoder Representations from Transformers (BERT) models. Three pretrained BERT models were explored to get the embeddings with optimal representability, and three downstream deep networks were employed to build our models. Our results showed that the model based on embeddings extracted from the BERT model pretrained on 556,603 protein sequences of UniProt outperforms other models. In addition, an independent test set was used to evaluate and compare our model with other existing methods, which indicated that our model was superior to other existing models.

Plasma advanced glycation end products and the subsequent risk of microvascular complications in type 1 diabetes in the DCCT/EDIC

INTRODUCTION

To assess impact of glycemic control on plasma protein-bound advanced glycation end products (pAGEs) and their association with subsequent microvascular disease.

RESEARCH DESIGN AND METHODS

Eleven pAGEs were measured by liquid chromatography-mass spectrometry in banked plasma from 466 participants in the Diabetes Control and Complications Trial/Epidemiology of Diabetes Interventions and Complications (DCCT/EDIC) study at three time points (TPs): DCCT year 4 (TP1) and year 8 (TP2) and EDIC year 5/6 (TP3). Correlation coefficients assessed cross-sectional associations, and Cox proportional hazards models assessed associations with subsequent risk of microvascular complications through EDIC year 24.

RESULTS

Glucose-derived glycation products fructose-lysine (FL), glucosepane (GSPN) and carboxymethyl-lysine (CML) decreased with intensive glycemic control at both TP1 and TP2 (p<0.0001) but were similar at TP3, and correlated with hemoglobin A1c (HbA1c). At TP1, the markers were associated with the subsequent risk of several microvascular outcomes. These associations did not remain significant after adjustment for HbA1c, except methionine sulfoxide (MetSOX), which remained associated with diabetic kidney disease. In unadjusted models using all 3 TPs, glucose-derived pAGEs were associated with subsequent risk of proliferative diabetic retinopathy (PDR, p<0.003), clinically significant macular edema (CSME, p<0.015) and confirmed clinical neuropathy (CCN, p<0.018, except CML, not significant (NS)). Adjusted for age, sex, body mass index, diabetes duration and mean updated HbA1c, the associations remained significant for PDR (FL: p<0.002, GSPN: p≤0.02, CML: p<0.003, pentosidine: p<0.02), CMSE (CML: p<0.03), albuminuria (FL: p<0.02, CML: p<0.03) and CCN (FL: p<0.005, GSPN : p<0.003).

CONCLUSIONS

pAGEs at TP1 are not superior to HbA1c for risk prediction, but glucose-derived pAGEs at three TPs and MetSOX remain robustly associated with progression of microvascular complications in type 1 diabetes even after adjustment for HbA1c and other factors.

Influence of Chlorhexidine and Cetylpyridine on Periodontal Status and Indicators of Oxidative Stress in Patients with Type 1 Diabetes

Objective: One of the treatment goals in type 1 diabetes and periodontitis is to address chronic inflammation to prevent the development of neurovascular complications. The aim of this study was to assess the local anti-inflammatory effects of chlorhexidine digluconate and cetylpyridine chloride on periodontal status and indicators of oxidative stress in saliva in patients with type 1 diabetes. Materials and Methods: A total of 42 subjects aged 27 (interquartile range, IQR 22–35) years, with type 1 diabetes for a duration of 12 (IQR 9–18) years, and glycated hemoglobin 8.05 (IQR 7.1–9.4)% were included. Patients were examined twice—initially, and after 14 days of using toothpaste with chlorhexidine and cetylpyridine. Clinical examination of gingival tissues was performed. Certain oxidative stress markers (TP, TEAC, TBARS, AOPP) were measured in the saliva samples. Results: There were significant changes in clinical indicators of periodontal status before and after the application of the toothpaste (API before 0.35 (0.24–0.65) vs. API after 0.265 (0.18–0.39), p = 0.03; SBI before 0.07 (0.04–0.15) vs. SBI after 0.035 (0-0.06), p = 0.002; GI before 0.88 (0.46–1) vs. GI after 0.67 (0.25–1), p = 0.0008). The concentration of saliva TBARS decreased (p = 0.00005) and TEAC increased (p = 0.09). Conclusion: Proper oral hygiene supported by antibacterial chemicals may improve the periodontal status and reduce inflammation.

Reversal of Insulin Resistance in Overweight and Obese Subjects by trans-Resveratrol and Hesperetin Combination-Link to Dysglycemia, Blood Pressure, Dyslipidemia, and Low-Grade Inflammation

The dietary supplement, trans-resveratrol and hesperetin combination (tRES-HESP), induces expression of glyoxalase 1, countering the accumulation of reactive dicarbonyl glycating agent, methylglyoxal (MG), in overweight and obese subjects. tRES-HESP produced reversal of insulin resistance, improving dysglycemia and low-grade inflammation in a randomized, double-blind, placebo-controlled crossover study. Herein, we report further analysis of study variables. MG metabolism-related variables correlated with BMI, dysglycemia, vascular inflammation, blood pressure, and dyslipidemia. With tRES-HESP treatment, plasma MG correlated negatively with endothelial independent arterial dilatation (r = -0.48, p < 0.05) and negatively with peripheral blood mononuclear cell (PBMC) quinone reductase activity (r = -0.68, p < 0.05)-a marker of the activation status of transcription factor Nrf2. For change from baseline of PBMC gene expression with tRES-HESP treatment, Glo1 expression correlated negatively with change in the oral glucose tolerance test area-under-the-curve plasma glucose (ΔAUGg) (r = -0.56, p < 0.05) and thioredoxin interacting protein (TXNIP) correlated positively with ΔAUGg (r = 0.59, p < 0.05). Tumor necrosis factor-α (TNFα) correlated positively with change in fasting plasma glucose (r = 0.70, p < 0.001) and negatively with change in insulin sensitivity (r = -0.68, p < 0.01). These correlations were not present with placebo. tRES-HESP decreased low-grade inflammation, characterized by decreased expression of CCL2, COX-2, IL-8, and RAGE. Changes in CCL2, IL-8, and RAGE were intercorrelated and all correlated positively with changes in MLXIP, MAFF, MAFG, NCF1, and FTH1, and negatively with changes in HMOX1 and TKT; changes in IL-8 also correlated positively with change in COX-2. Total urinary excretion of tRES and HESP metabolites were strongly correlated. These findings suggest tRES-HESP counters MG accumulation and protein glycation, decreasing activation of the unfolded protein response and expression of TXNIP and TNFα, producing reversal of insulin resistance. tRES-HESP is suitable for further evaluation for treatment of insulin resistance and related disorders.

Protein glycation - biomarkers of metabolic dysfunction and early-stage decline in health in the era of precision medicine

Protein glycation provides a biomarker in widespread clinical use, glycated hemoglobin HbA1c (A1C). It is a biomarker for diagnosis of diabetes and prediabetes and of medium-term glycemic control in patients with established diabetes. A1C is an early-stage glycation adduct of hemoglobin with glucose; a fructosamine derivative. Glucose is an amino group-directed glycating agent, modifying N-terminal and lysine sidechain amino groups. A similar fructosamine derivative of serum albumin, glycated albumin (GA), finds use as a biomarker of glycemic control, particularly where there is interference in use of A1C. Later stage adducts, advanced glycation endproducts (AGEs), are formed by the degradation of fructosamines and by the reaction of reactive dicarbonyl metabolites, such as methylglyoxal. Dicarbonyls are arginine-directed glycating agents forming mainly hydroimidazolone AGEs. Glucosepane and pentosidine, an intense fluorophore, are AGE covalent crosslinks. Cellular proteolysis of glycated proteins forms glycated amino acids, which are released into plasma and excreted in urine. Development of diagnostic algorithms by artificial intelligence machine learning is enhancing the applications of glycation biomarkers. Investigational glycation biomarkers are in development for: (i) healthy aging; (ii) risk prediction of vascular complications of diabetes; (iii) diagnosis of autism; and (iv) diagnosis and classification of early-stage arthritis. Protein glycation biomarkers are influenced by heritability, aging, decline in metabolic, vascular, renal and skeletal health, and other factors. They are applicable to populations of differing ethnicities, bridging the gap between genotype and phenotype. They are thereby likely to find continued and expanding clinical use, including in the current era of developing precision medicine, reporting on multiple pathogenic processes and supporting a precision medicine approach.

The Role of Glyoxalase in Glycation and Carbonyl Stress Induced Metabolic Disorders

Glycation refers to the covalent binding of sugar molecules to macromolecules, such as DNA, proteins, and lipids in a non-enzymatic reaction, resulting in the formation of irreversibly bound products known as advanced glycation end products (AGEs). AGEs are synthesized in high amounts both in pathological conditions, such as diabetes and under physiological conditions resulting in aging. The body's anti-glycation defense mechanisms play a critical role in removing glycated products. However, if this defense system fails, AGEs start accumulating, which results in pathological conditions. Studies have been shown that increased accumulation of AGEs acts as key mediators in multiple diseases, such as diabetes, obesity, arthritis, cancer, atherosclerosis, decreased skin elasticity, male erectile dysfunction, pulmonary fibrosis, aging, and Alzheimer's disease. Furthermore, glycation of nucleotides, proteins, and phospholipids by α-oxoaldehyde metabolites, such as glyoxal (GO) and methylglyoxal (MGO), causes potential damage to the genome, proteome, and lipidome. Glyoxalase-1 (GLO-1) acts as a part of the anti-glycation defense system by carrying out detoxification of GO and MGO. It has been demonstrated that GLO-1 protects dicarbonyl modifications of the proteome and lipidome, thereby impeding the cell signaling and affecting age-related diseases. Its relationship with detoxification and anti-glycation defense is well established. Glycation of proteins by MGO and GO results in protein misfolding, thereby affecting their structure and function. These findings provide evidence for the rationale that the functional modulation of the GLO pathway could be used as a potential therapeutic target. In the present review, we summarized the newly emerged literature on the GLO pathway, including enzymes regulating the process. In addition, we described small bioactive molecules with the potential to modulate the GLO pathway, thereby providing a basis for the development of new treatment strategies against age-related complications.

A role for estrogen in skin ageing and dermal biomechanics

The skin is the body's primary defence against the external environment, preventing infection and desiccation. Therefore, alterations to skin homeostasis, for example with skin ageing, increase susceptibility to skin disease and injury. Skin biological ageing is uniquely influenced by a combination of intrinsic and extrinsic (primarily photoageing) factors, with differential effects on skin structure and function. Interestingly, skin architecture rapidly changes following the menopause, as a direct result of reduced circulating 17β-estradiol. The traditional clinical benefit of estrogens are supported by recent experimental data, where 17β-estradiol supplementation prevents age-related decline in the skin's structural and mechanical properties. However, the off-target effects of 17β-estradiol continue to challenge therapeutic application. Here we discuss how ageing alters the physiological and structural properties of the dermal extracellular matrix, and explore how estrogen receptor-targeted therapies may restore the mechanical defects associated with skin ageing.

Biochemical Regulation of the Glyoxalase System in Response to Insulin Signaling

Methylglyoxal (MG) is a reactive glycation metabolite and potentially induces dicarbonyl stress. The production of MG in cells is increased along with an increase in carbohydrate metabolism. The efficiency of the glyoxalase system, consisting of glyoxalase 1 (GlxI) and glyoxalase 2 (GlxII), is crucial for turning the accumulated MG into nontoxic metabolites. Converting MG-glutathione hemithioacetal to S-d-lactoylglutathione by GlxI is the rate-determining step of the enzyme system. In this study, we found lactic acid accumulated during insulin stimulation in cells, however, cellular MG and S-d-lactoylglutathione also increased due to the massive flux of glycolytic intermediates. The insulin-induced accumulation of MG and S-d-lactoylglutathione were efficiently removed by the treatment of metformin, possibly via affecting the glyoxalase system. With the application of isotopic ¹³C₃-MG, the flux of MG from extracellular and intracellular origins was dissected. While insulin induced an influx of extracellular MG, metformin inhibited the trafficking of MG across the plasma membrane. Therefore, metformin could maintain the extracellular MG by means of reducing the secretion of MG rather than facilitating the scavenging. In addition, metformin may affect the glyoxalase system by controlling the cellular redox state through replenishing reduced glutathione. Overall, alternative biochemical regulation of the glyoxalase system mediated by insulin signaling or molecules like biguanides may control cellular MG homeostasis.

Uremic Toxins, Oxidative Stress, Atherosclerosis in Chronic Kidney Disease, and Kidney Transplantation

Patients with chronic kidney disease (CKD) are at a high risk for cardiovascular disease (CVD), and approximately half of all deaths among patients with CKD are a direct result of CVD. The premature cardiovascular disease extends from mild to moderate CKD stages, and the severity of CVD and the risk of death increase with a decline in kidney function. Successful kidney transplantation significantly decreases the risk of death relative to long-term dialysis treatment; nevertheless, the prevalence of CVD remains high and is responsible for approximately 20-35% of mortality in renal transplant recipients. The prevalence of traditional and nontraditional risk factors for CVD is higher in patients with CKD and transplant recipients compared with the general population; however, it can only partly explain the highly increased cardiovascular burden in CKD patients. Nontraditional risk factors, unique to CKD patients, include proteinuria, disturbed calcium, and phosphate metabolism, anemia, fluid overload, and accumulation of uremic toxins. This accumulation of uremic toxins is associated with systemic alterations including inflammation and oxidative stress which are considered crucial in CKD progression and CKD-related CVD. Kidney transplantation can mitigate the impact of some of these nontraditional factors, but they typically persist to some degree following transplantation. Taking into consideration the scarcity of data on uremic waste products, oxidative stress, and their relation to atherosclerosis in renal transplantation, in the review, we discussed the impact of uremic toxins on vascular dysfunction in CKD patients and kidney transplant recipients. Special attention was paid to the role of native and transplanted kidney function.

ABC-Gly: Identifying Protein Lysine Glycation Sites with Artificial Bee Colony Algorithm

Background:

Glycation is a nonenzymatic post-translational modification process by attaching a sugar molecule to a protein or lipid molecule. It may impair the function and change the characteristic of the proteins which may lead to some metabolic diseases. In order to understand the underlying molecular mechanisms of glycation, computational prediction methods have been developed because of their convenience and high speed. However, a more effective computational tool is still a challenging task in computational biology.

Methods:

In this study, we showed an accurate identification tool named ABC-Gly for predicting lysine glycation sites. At first, we utilized three informative features, including position-specific amino acid propensity, secondary structure and the composition of k-spaced amino acid pairs to encode the peptides. Moreover, to sufficiently exploit discriminative features thus can improve the prediction and generalization ability of the model, we developed a two-step feature selection, which combined the Fisher score and an improved binary artificial bee colony algorithm based on the support vector machine. Finally, based on the optimal feature subset, we constructed an effective model by using the Support Vector Machine on the training dataset.

Results:

The performance of the proposed predictor ABC-Gly was measured with the sensitivity of 76.43%, the specificity of 91.10%, the balanced accuracy of 83.76%, the Area Under the receiveroperating characteristic Curve (AUC) of 0.9313, a Matthew’s Correlation Coefficient (MCC) of 0.6861 by 10-fold cross-validation on training dataset, and a balanced accuracy of 59.05% on independent dataset. Compared to the state-of-the-art predictors on the training dataset, the proposed predictor achieved significant improvement in the AUC of 0.156 and MCC of 0.336.

Conclusion:

The detailed analysis results indicated that our predictor may serve as a powerful complementary tool to other existing methods for predicting protein lysine glycation. The source code and datasets of the ABC-Gly were provided in the Supplementary File 1.

Reading patterns of proteome damage by glycation, oxidation and nitration: quantitation by stable isotopic dilution analysis LC-MS/MS

Liquid chromatography-tandem mass spectrometry (LC-MS/MS) provides a high sensitivity, high specificity multiplexed method for concurrent detection of adducts formed by protein glycation, oxidation and nitration, also called AGEomics. Combined with stable isotopic dilution analysis, it provides for robust quantitation of protein glycation, oxidation and nitration adduct analytes. It is the reference method for such measurements. LC-MS/MS has been used to measure glycated, oxidized and nitrated amino acids - also called glycation, oxidation and nitration free adducts, with a concurrent quantitation of the amino acid metabolome in physiological fluids. Similar adduct residues in proteins may be quantitated with prior exhaustive enzymatic hydrolysis. It has also been applied to quantitation of other post-translation modifications, such as citrullination and formation of Nε-(γ-glutamyl)lysine crosslink by transglutaminases. Application to cellular and extracellular proteins gives estimates of the steady-state levels of protein modification by glycation, oxidation and nitration, and measurement of the accumulation of glycation, oxidation and nitration adducts in cell culture medium and urinary excretion gives an indication of flux of adduct formation. Measurement of glycation, oxidation and nitration free adducts in plasma and urine provides for estimates of renal clearance of free adducts. Diagnostic potential in clinical studies has been enhanced by the combination of estimates of multiple adducts in optimized diagnostic algorithms by machine learning. Recent applications have been in early-stage detection of metabolic, vascular and renal disease, and arthritis, metabolic control and risk of developing vascular complication in diabetes, and a blood test for autism.

Confirmation of the Cardioprotective Effect of MitoGamide in the Diabetic Heart

Purpose

HFpEF (heart failure with preserved ejection fraction) is a major consequence of diabetic cardiomyopathy with no effective treatments. Here, we have characterized Akita mice as a preclinical model of HFpEF and used it to confirm the therapeutic efficacy of the mitochondria-targeted dicarbonyl scavenger, MitoGamide.

Methods and Results

A longitudinal echocardiographic analysis confirmed that Akita mice develop diastolic dysfunction with reduced E peak velocity, E/A ratio and extended isovolumetric relaxation time (IVRT), while the systolic function remains comparable with wild-type mice. The myocardium of Akita mice had a decreased ATP/ADP ratio, elevated mitochondrial oxidative stress and increased organelle density, compared with that of wild-type mice. MitoGamide, a mitochondria-targeted 1,2-dicarbonyl scavenger, exhibited good stability in vivo, uptake into cells and mitochondria and reactivity with dicarbonyls. Treatment of Akita mice with MitoGamide for 12 weeks significantly improved the E/A ratio compared with the vehicle-treated group.

Conclusion

Our work confirms that the Akita mouse model of diabetes replicates key clinical features of diabetic HFpEF, including cardiac and mitochondrial dysfunction. Furthermore, in this independent study, MitoGamide treatment improved diastolic function in Akita mice.

Electronic supplementary material

The online version of this article (10.1007/s10557-020-07086-7) contains supplementary material, which is available to authorized users.

Effects of Exogenous Dietary Advanced Glycation End Products on the Cross-Talk Mechanisms Linking Microbiota to Metabolic Inflammation

Heat-processed diets contain high amounts of advanced glycation end products (AGEs). Here we explore the impact of an AGE-enriched diet on markers of metabolic and inflammatory disorders as well as on gut microbiota composition and plasma proteins glycosylation pattern. C57BL/6 mice were allocated into control diet (CD, n = 15) and AGE-enriched diet (AGE-D, n = 15) for 22 weeks. AGE-D was prepared replacing casein by methylglyoxal hydroimidazolone-modified casein. AGE-D evoked increased insulin and a significant reduction of GIP/GLP-1 incretins and ghrelin plasma levels, altered glucose tolerance, and impaired insulin signaling transduction in the skeletal muscle. Moreover, AGE-D modified the systemic glycosylation profile, as analyzed by lectin microarray, and increased Nε-carboxymethyllysine immunoreactivity and AGEs receptor levels in ileum and submandibular glands. These effects were associated to increased systemic levels of cytokines and impaired gut microbial composition and homeostasis. Significant correlations were recorded between changes in bacterial population and in incretins and inflammatory markers levels. Overall, our data indicates that chronic exposure to dietary AGEs lead to a significant unbalance in incretins axis, markers of metabolic inflammation, and a reshape of both the intestinal microbiota and plasma protein glycosylation profile, suggesting intriguing pathological mechanisms underlying AGEs-induced metabolic derangements.

High fractional excretion of glycation adducts is associated with subsequent early decline in renal function in type 1 diabetes

Increased protein glycation, oxidation and nitration is linked to the development of diabetic nephropathy. We reported levels of serum protein glycation, oxidation and nitration and related hydrolysis products, glycation, oxidation and nitration free adducts in patients with type 1 diabetes (T1DM) during onset of microalbuminuria (MA) from the First Joslin Kidney Study, a prospective case-control study of patients with T1DM with and without early decline in GFR. Herein we report urinary excretion of the latter analytes and related fractional excretion values, exploring the link to MA and early decline in GFR. We recruited patients with T1DM and normoalbuminuria (NA) (n = 30) or new onset MA with and without early GFR decline (n = 22 and 33, respectively) for this study. We determined urinary protein glycation, oxidation and nitration free adducts by stable isotopic dilution analysis liquid chromatography-tandem mass spectrometry (LC-MS/MS) and deduced fractional excretion using reported plasma levels and urinary and plasma creatinine estimates. We found urinary excretion of pentosidine was increased ca. twofold in patients with MA, compared to normoalbuminuria (0.0442 vs 0.0103 nmol/mg creatinine, P < 0.0001), and increased ca. threefold in patients with early decline in GFR, compared to patients with stable GFR (0.0561 vs 0.0176 nmol/mg creatinine, P < 0.01). Urinary excretion of all other analytes was unchanged between the study groups. Remarkably, fractional excretions of 6 lysine and arginine-derived glycation free adducts were higher in patients with early decline in GFR, compared to those with stable GFR. Impaired tubular reuptake of glycation free adducts by lysine and arginine transporter proteins in patients with early GFR decline is likely involved. We conclude that higher fractional excretions of glycation adducts are potential biomarkers for early GFR decline in T1DM and MA. Measurement of these analytes could provide the basis for identifying patients at risk of early decline in renal function to target and intensify renoprotective treatment.

Potential Markers of Dietary Glycemic Exposures for Sustained Dietary Interventions in Populations without Diabetes

There is considerable interest in dietary and other approaches to maintaining blood glucose concentrations within the normal range and minimizing exposure to postprandial hyperglycemic excursions. The accepted marker to evaluate the sustained maintenance of normal blood glucose concentrations is glycated hemoglobin A1c (HbA1c). However, although this is used in clinical practice to monitor glycemic control in patients with diabetes, it has a number of drawbacks as a marker of efficacy of dietary interventions that might beneficially affect glycemic control in people without diabetes. Other markers that reflect shorter-term glycemic exposures have been studied and proposed, but consensus on the use and relevance of these markers is lacking. We have carried out a systematic search for studies that have tested the responsiveness of 6 possible alternatives to HbA1c as markers of sustained variation in glycemic exposures and thus their potential applicability for use in dietary intervention trials in subjects without diabetes: 1,5-anhydroglucitol (1,5-AG), dicarbonyl stress, fructosamine, glycated albumin (GA), advanced glycated end products (AGEs), and metabolomic profiles. The results suggest that GA may be the most promising for this purpose, but values may be confounded by effects of fat mass. 1,5-AG and fructosamine are probably not sensitive enough to the range of variation in glycemic exposures observed in healthy individuals. Use of measures based on dicarbonyls, AGEs, or metabolomic profiles would require further research into possible specific molecular species of interest. At present, none of the markers considered here is sufficiently validated and sensitive for routine use in substantiating the effects of sustained variation in dietary glycemic exposures in people without diabetes.

Analyzing structural alterations of mitochondrial intermembrane space superoxide scavengers cytochrome-c and SOD1 after methylglyoxal treatment

Mitochondria are quantitatively the most important sources of reactive oxygen species (ROS) which are formed as by-products during cellular respiration. ROS generation occurs when single electrons are transferred to molecular oxygen. This leads to a number of different ROS types, among them superoxide. Although most studies focus on ROS generation in the mitochondrial matrix, the intermembrane space (IMS) is also important in this regard. The main scavengers for the detoxification of superoxide in the IMS are Cu, Zn superoxide dismutase (SOD1) and cytochrome-c. Similar to ROS, certain reactive carbonyl species are known for their high reactivity. The consequences are deleterious modifications to essential components compromising cellular functions and contributing to the etiology of severe pathological conditions like cancer, diabetes and neurodegeneration. In this study, we investigated the susceptibility of SOD1 and cytochrome-c to in vitro glycation by the dicarbonyl methylglyoxal (MGO) and the resulting effects on their structure. We utilized experimental techniques like immunodetection of the MGO-mediated modification 5-hydro-5-methylimidazolone, differential scanning calorimetry, fluorescence emission and circular dichroism measurements. We found that glycation of cytochrome-c leads to monomer aggregation, an altered secondary structure (increase in alpha helical content) and slightly more compact folding. In addition to structural changes, glycated cytochrome-c displays an altered thermal unfolding behavior. Subjecting SOD1 to MGO does not influence its secondary structure. However, similar to cytochrome-c, subunit aggregation is observed under denaturating conditions. Furthermore, the appearance of a second peak in the calorimetry diagram indirectly suggests de-metallation of SOD1 when high MGO levels are used. In conclusion, our data demonstrate that MGO has the potential to alter several structural parameters in important proteins of energy metabolism (cytochrome-c) and antioxidant defense (cytochrome-c, SOD1).

The Role of Dietary Advanced Glycation End Products in Metabolic Dysfunction

Advanced glycation end products (AGEs) are a heterogeneous group of molecules produced, non-enzymatically, from the interaction between reducing sugars and the free amino groups of proteins, nucleic acids, and lipids. AGEs are formed as a normal consequence of metabolism but can also be absorbed from the diet. They have been widely implicated in the complications of diabetes affecting cardiovascular health, the nervous system, eyes, and kidneys. Increased levels of AGEs are also detrimental to metabolic health and may contribute to the metabolic abnormalities induced by the Western diet, which is high in processed foods and represents a significant source of AGEs. While increased AGE levels are a consequence of diabetic hyperglycaemia, AGEs themselves activate signaling pathways, which compromise insulin signaling and pancreatic β-cell function, thus, contributing to the development of type 2 diabetes mellitus (T2DM). Furthermore, AGEs may also contribute to the obesogenic effects of the Western diet by promoting hypothalamic inflammation and disrupting the central control of energy balance. Here, the role of dietary AGEs in metabolic dysfunction is reviewed with a focus on the mechanisms underpinning their detrimental role in insulin resistance, pancreatic β-cell dysfunction, hypothalamic control of energy balance, and the pathogenesis of T2DM and obesity.

Urinary Metabolomic Markers of Protein Glycation, Oxidation, and Nitration in Early-Stage Decline in Metabolic, Vascular, and Renal Health

Glycation, oxidation, nitration, and crosslinking of proteins are implicated in the pathogenic mechanisms of type 2 diabetes, cardiovascular disease, and chronic kidney disease. Related modified amino acids formed by proteolysis are excreted in urine. We quantified urinary levels of these metabolites and branched-chain amino acids (BCAAs) in healthy subjects and assessed changes in early-stage decline in metabolic, vascular, and renal health and explored their diagnostic utility for a noninvasive health screen. We recruited 200 human subjects with early-stage health decline and healthy controls. Urinary amino acid metabolites were determined by stable isotopic dilution analysis liquid chromatography-tandem mass spectrometry. Machine learning was applied to optimise and validate algorithms to discriminate between study groups for potential diagnostic utility. Urinary analyte changes were as follows: impaired metabolic health-increased N ε -carboxymethyl-lysine, glucosepane, glutamic semialdehyde, and pyrraline; impaired vascular health-increased glucosepane; and impaired renal health-increased BCAAs and decreased N ε -(γ-glutamyl)lysine. Algorithms combining subject age, BMI, and BCAAs discriminated between healthy controls and impaired metabolic, vascular, and renal health study groups with accuracy of 84%, 72%, and 90%, respectively. In 2-step analysis, algorithms combining subject age, BMI, and urinary N ε -fructosyl-lysine and valine discriminated between healthy controls and impaired health (any type), accuracy of 78%, and then between types of health impairment with accuracy of 69%-78% (cf. random selection 33%). From likelihood ratios, this provided small, moderate, and conclusive evidence of early-stage cardiovascular, metabolic, and renal disease with diagnostic odds ratios of 6 - 7, 26 - 28, and 34 - 79, respectively. We conclude that measurement of urinary glycated, oxidized, crosslinked, and branched-chain amino acids provides the basis for a noninvasive health screen for early-stage health decline in metabolic, vascular, and renal health.

Quantitation of free glycation compounds in saliva

In the course of the Maillard reaction, which occurs during heating of food but also under physiological condition, a broad spectrum of reaction products is formed. Among them, the advanced glycation endproducts (AGEs) Nε-carboxymethyllysine (CML), pyrraline (Pyr), methylglyoxal-derived hydroimidazolone 1 (MG-H1) and Nε-carboxyethyllysine (CEL) are the quantitatively dominating compounds during later reaction stages. Those dietary glycation compounds are under discussion as to be associated with chronic inflammation and the pathophysiological consequences of diseases such as diabetes. In the present study, the concentration of individual glycation compounds in saliva was monitored for the first time and related to their dietary uptake. Fasting saliva of 33 metabolically healthy subjects was analyzed with HPLC-MS/MS. The observed levels of individual glycation compounds ranged from 0.5 to 55.2 ng/ml and differed both intra- and interindividually. Patterns did not correlate with subject-related features such as vegetarianism or sports activities, indicating that dietary intake may play an important role. Therefore, six volunteers were asked to eat a raw food diet free of glycation compounds for two days. Within two days, salivary Pyr was lowered from median 1.7 ng/ml to a minimum level below the limit of detection, and MG-H1 decreased from 3.6 to 1.7 ng/ml in in a time-dependent manner after two days. Salivary CML and CEL concentrations were not affected. Therefore, measuring Pyr and MG-H1 in saliva is a suitable diagnostic tool to monitor the dietary intake and metabolic transit of glycation compounds present in heated foods.

Divergent Changes in Plasma AGEs and sRAGE Isoforms Following an Overnight Fast in T1DM

Advanced glycation end products (AGEs) promote the development of diabetic complications through activation of their receptor (RAGE). Isoforms of soluble RAGE (sRAGE) sequester AGEs and protect against RAGE-mediated diabetic complications. We investigated the effect of an overnight fast on circulating metabolic substrates, hormones, AGEs, and sRAGE isoforms in 26 individuals with type 1 diabetes (T1DM). Blood was collected from 26 young (18–30 years) T1DM patients on insulin pumps before and after an overnight fast. Circulating AGEs were measured via LC-MS/MS and sRAGE isoforms were analyzed via ELISA. Glucose, insulin, glucagon, and eGFR_cystatin-c decreased while cortisol increased following the overnight fast (p < 0.05). AGEs (CML, CEL, 3DG-H, MG-H1, and G-H1) decreased (21–58%, p < 0.0001) while total sRAGE, cleaved RAGE (cRAGE), and endogenous secretory RAGE (esRAGE) increased (22–24%, p < 0.0001) following the overnight fast. The changes in sRAGE isoforms were inversely related to MG-H1 (rho = −0.493 to −0.589, p < 0.05) and the change in esRAGE was inversely related to the change in G-H1 (rho = −0.474, p < 0.05). Multiple regression analyses revealed a 1 pg/mL increase in total sRAGE, cRAGE, or esRAGE independently predicted a 0.42–0.52 nmol/L decrease in MG-H1. Short-term energy restriction via an overnight fast resulted in increased sRAGE isoforms and may be protective against AGE accumulation.

The LepRdb/db mice model for studying glycation in the context of diabetes

BACKGROUND

Early (furosine) and advanced (carboxymethyllysine, CML) products of glycation (AGEs) have been reported as increased in plasma, tissues, and organs of diabetic people, indicating a direct link between glycation and type 2 diabetes (T2D). While murine models present some of the characteristics observed in diabetic humans, their pertinence as models of glycation, particularly for T2D, remains poorly described. The aim of this study was to characterize and compare glycation in several organs of two commonly studied murine models of T2D using stable isotope dilution liquid chromatography tandem mass spectrometry (LC-MS/MS).

METHODS

Defining parameters of type 2 diabetes including body weight, fasting glycaemia, and glucose intolerance were measured in three different C57BL6 mouse models of T2D-the genetic LepR^db/db (db/db) model and two diet-induced obesity (DIO) models-and their respective controls. Furosine, free, and protein-bound CML were quantified in kidneys, lungs, heart, and liver by LC-MS/MS.

RESULTS

The obesity, hyperglycaemia, and glucose intolerance in db/db mice was accompanied by an increase of furosine and protein-bound CML levels in all organs relative to controls. The DIO models took several months to become obese, exhibited less severe hyperglycaemia and glucose intolerance, while glycation products were not significantly different between these groups (with the exception of furosine in liver and CML in lungs).

CONCLUSIONS

The db/db model better reflected the characteristics of human T2D compared with the DIO models and exhibited greater formation and accumulation of both furosine and protein-bound CML in all of the organs tested here.

Gold Nanoparticle-Based Detection of Low Molecular Weight AGEs from In Vitro Glycated Haemoglobin A0 Samples

Protein glycation is a major biochemical event that takes place in the plasma of diabetic patients due to increased sugar levels. Extensive glycation leads to the formation of advanced glycation end products (AGEs) that is well known for having detrimental effects on diabetic patients. In the current work, we have glycated the physiologically important protein Haemoglobin A0 in vitro to study AGE formation and activity by using them as a template for gold nanoparticle (GNPs) synthesis. It was found that the surface plasmon resonance of synthesised GNPs showed high correlation with the extent of glycation. On fractionation, the glycated Haemoglobin A0 segregated into two distinct population of products, one consisting of proteinaceous, cross-linked larger fragments of Haemoglobin A0 and a second population of non-proteinaceous low molecular weight AGEs. Only low molecular weight AGEs contributed to synthesis of GNPs upon using the fractions as a template, substantiating the principle of proposed GNP-based assay. Owing to its physiological importance, AGEs can be used as a diagnostic means for diabetes and its associated complications. In this study, we have employed the high reactivity of AGEs for the development of a GNP-based novel colorimetric sensor to enable their detection. Our proposed GNP-based sensing could have high clinical significance in detecting diabetes and its associated complexities.

Compensatory mechanisms for methylglyoxal detoxification in experimental & clinical diabetes

OBJECTIVES

The deficit of Glyoxalase I (Glo1) and the subsequent increase in methylglyoxal (MG) has been reported to be one the five mechanisms by which hyperglycemia causes diabetic late complications. Aldo-keto reductases (AKR) have been shown to metabolize MG; however, the relative contribution of this superfamily to the detoxification of MG in vivo, particularly within the diabetic state, remains unknown.

METHODS

CRISPR/Cas9-mediated genome editing was used to generate a Glo1 knock-out (Glo1-/-) mouse line. Streptozotocin was then applied to investigate metabolic changes under hyperglycemic conditions.

RESULTS

Glo1-/- mice were viable and showed no elevated MG or MG-H1 levels under hyperglycemic conditions. It was subsequently found that the enzymatic efficiency of various oxidoreductases in the liver and kidney towards MG were increased in the Glo1-/- mice. The functional relevance of this was supported by the altered distribution of alternative detoxification products. Furthermore, it was shown that MG-dependent AKR activity is a potentially clinical relevant pathway in human patients suffering from diabetes.

CONCLUSIONS

These data suggest that in the absence of GLO1, AKR can effectively compensate to prevent the accumulation of MG. The combination of metabolic, enzymatic, and genetic factors, therefore, may provide a better means of identifying patients who are at risk for the development of late complications caused by elevated levels of MG.

Cardiac troponins may be irreversibly modified by glycation: novel potential mechanisms of cardiac performance modulation

Dynamic movements of the cardiac troponin complex are an important component of the cardiac cycle. Whether cardiac troponins are subjected to irreversible advanced glycation end-product (AGE) modification is unknown. This study interrogated human and rat cardiac troponin-C, troponin-I and troponin-T to identify endogenous AGE modifications using mass spectrometry (LC-MS/MS). AGE modifications were detected on two amino acid residues of human troponin-C (Lys₆, Lys₃₉), thirteen troponin-I residues (Lys₃₆, Lys₅₀, Lys₅₈, Arg₇₉, Lys₁₁₇, Lys₁₂₀, Lys₁₃₁, Arg₁₄₈, Arg₁₆₂, Lys₁₆₄, Lys₁₈₃, Lys₁₉₃, Arg₂₀₄), and three troponin-T residues (Lys₁₀₇, Lys₁₂₅, Lys₂₂₇). AGE modifications of three corresponding troponin-I residues (Lys₅₈, Lys₁₂₀, Lys₁₉₄) and two corresponding troponin-T residues (Lys₁₀₇, Lys₂₂₇) were confirmed in cardiac tissue extracts from an experimental rodent diabetic model. Additionally, novel human troponin-I phosphorylation sites were detected (Thr₁₁₉, Thr₁₂₃). Accelerated AGE modification of troponin-C was evident in vitro with hexose sugar exposure. This study provides the first demonstration of the occurrence of cardiac troponin complex AGE-modifications. These irreversible AGE modifications are situated in regions of the troponin complex known to be important in myofilament relaxation, and may be of particular pathological importance in the pro-glycation environment of diabetic cardiomyopathy.

Analysis and Chemistry of Novel Protein Oxidation Markers in Vivo

Proteins continually undergo spontaneous oxidation reactions, which lead to changes in structure and function. The quantitative assessment of protein oxidation adducts provides information on the level of exposure to reactive precursor compounds with a high oxidizing potential and reactive oxygen species (ROS). In the present work, we introduce N⁶-(2-hydroxyethyl)lysine as a novel marker based on the ratio of glycolaldehyde and its oxidized form glyoxal. The high analytical potential was proven with a first set of patients undergoing hemodialysis versus healthy controls, in comparison with well-established parameters for oxidative stress. In vitro experiments with N¹- t-BOC-lysine and N¹- t-BOC-arginine enlightened the mechanistic relationship of glycolaldehyde and glyoxal. Oxidation was strongly dependent on the catalytic action of the ε-amino moiety of lysine. Investigations on the formation of N⁶-carboxymethyl lysine revealed glycolaldehyde-imine as the more reactive precursor, even though an additional oxidative step is required. As a result, a novel and very effective alternative mechanism was unraveled.

Expression of macrophage migration inhibitory factor in footpad skin lesions with diabetic neuropathy

Background

Diabetic neuropathy originating in distal lower extremities is associated with pain early in the disease course, overwhelming in the feet. However, the pathogenesis of diabetic neuropathy remains unclear. Macrophage migration inhibitory factor has been implicated in the onset of neuropathic pain and the development of diabetes. Objective of this study was to observe pain syndromes elicited in the footpad of diabetic neuropathy rat model and to assess the contributory role of migration inhibitory factor in the pathogenesis of diabetic neuropathy.

Methods

Diabetic neuropathy was made in Sprague Dawley rats by streptozotocin. Pain threshold was evaluated using von Frey monofilaments for 24 weeks. On comparable experiment time after streptozotocin injection, all footpads were prepared for following procedures; glutathione assay, terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end labeling staining, immunohistochemistry staining, real-time reverse transcription polymerase chain reaction, and Western blot. Additionally, human HaCaT skin keratinocytes were treated with methylglyoxal, transfected with migration inhibitory factor/control small interfering RNA, and prepared for real-time reverse transcription polymerase chain reaction and Western blot.

Results

As compared to sham group, pain threshold was significantly reduced in diabetic neuropathy group, and glutathione was decreased in footpad skin, simultaneously, cell death was increased. Over-expression of migration inhibitory factor, accompanied by low expression of glyoxalase-I and intraepidermal nerve fibers, was shown on the footpad skin lesions of diabetic neuropathy. But, there was no significance in expression of neurotransmitters and inflammatory mediators such as transient receptor potential vanilloid 1, mas-related G protein coupled receptor D, nuclear factor kappa B, tumor necrosis factor-alpha, and interleukin-6 between diabetic neuropathy group and sham group. Intriguingly, small interfering RNA-transfected knockdown of the migration inhibitory factor gene in methylglyoxal-treated skin keratinocytes increased expression of glyoxalase-I and intraepidermal nerve fibers in comparison with control small interfering RNA-transfected cells, which was decreased by induction of methylglyoxal.

Conclusions

Our findings suggest that migration inhibitory factor can aggravate diabetic neuropathy by suppressing glyoxalase-I and intraepidermal nerve fibers on the footpad skin lesions and provoke pain. Taken together, migration inhibitory factor might offer a pharmacological approach to alleviate pain syndromes in diabetic neuropathy.