Hidden complexities in the measurement of fructosyl-lysine and advanced glycation end products for risk prediction of vascular complications of diabetes

Untargeted metabolomic profiling reveals molecular signatures associated with type 2 diabetes in Nigerians

BACKGROUND

Type 2 diabetes (T2D) has reached epidemic proportions globally, including in Africa. However, molecular studies to understand the pathophysiology of T2D remain scarce outside Europe and North America. The aims of this study are to use an untargeted metabolomics approach to identify: (a) metabolites that are differentially expressed between individuals with and without T2D and (b) a metabolic signature associated with T2D in a population of Sub-Saharan Africa (SSA).

METHODS

A total of 580 adult Nigerians from the Africa America Diabetes Mellitus (AADM) study were studied. The discovery study included 310 individuals (210 without T2D, 100 with T2D). Metabolites in plasma were assessed by reverse phase, ultra-performance liquid chromatography and mass spectrometry (RP)/UPLC-MS/MS methods on the Metabolon Platform. Welch's two-sample t-test was used to identify differentially expressed metabolites (DEMs), followed by the construction of a biomarker panel using a random forest (RF) algorithm. The biomarker panel was evaluated in a replication sample of 270 individuals (110 without T2D and 160 with T2D) from the same study.

RESULTS

Untargeted metabolomic analyses revealed 280 DEMs between individuals with and without T2D. The DEMs predominantly belonged to the lipid (51%, 142/280), amino acid (21%, 59/280), xenobiotics (13%, 35/280), carbohydrate (4%, 10/280) and nucleotide (4%, 10/280) super pathways. At the sub-pathway level, glycolysis, free fatty acid, bile metabolism, and branched chain amino acid catabolism were altered in T2D individuals. A 10-metabolite biomarker panel including glucose, gluconate, mannose, mannonate, 1,5-anhydroglucitol, fructose, fructosyl-lysine, 1-carboxylethylleucine, metformin, and methyl-glucopyranoside predicted T2D with an area under the curve (AUC) of 0.924 (95% CI: 0.845-0.966) and a predicted accuracy of 89.3%. The panel was validated with a similar AUC (0.935, 95% CI 0.906-0.958) in the replication cohort. The 10 metabolites in the biomarker panel correlated significantly with several T2D-related glycemic indices, including Hba1C, insulin resistance (HOMA-IR), and diabetes duration.

CONCLUSIONS

We demonstrate that metabolomic dysregulation associated with T2D in Nigerians affects multiple processes, including glycolysis, free fatty acid and bile metabolism, and branched chain amino acid catabolism. Our study replicated previous findings in other populations and identified a metabolic signature that could be used as a biomarker panel of T2D risk and glycemic control thus enhancing our knowledge of molecular pathophysiologic changes in T2D. The metabolomics dataset generated in this study represents an invaluable addition to publicly available multi-omics data on understudied African ancestry populations.

HILIC UPLC/ QTof MS Method Development for the Quantification of AGEs Inhibitors - Trouble Shooting Protocol

OBJECTIVE

The paper reports an attempt to develop and validate a HILIC UPLC/ QTof MS method for quantifying N-ε-carboxymethyl-L-lysine (CML) in vitro, testing N-ε- carboxy[D2]methyl-L-lysine (d2-CML), and N-ε-carboxy[4,4,5,5-D4]methyl-L-lysine (d4-CML) as internal standards.

METHODS

During the method development, several challenging questions occurred that hindered the successful completion of the method. The study emphasizes the impact of issues, generally overlooked in the development of similar analytical protocols. For instance, the use of glassware and plasticware was critical for the accurate quantification of CML. Moreover, the origin of atypical variation in the response of the deuterated internal standards, though widely used in other experimental procedures, was investigated.

RESULT

A narrative description of the systematic approach used to address the various drawbacks during the analytical method development and validation is presented.

CONCLUSION

Reporting those findings can be considered beneficial while bringing an insightful notion about critical factors and potential interferences. Therefore, some conclusion and ideas can be drawn from these trouble-shooting questions, which might help other researchers to develop more reliable bioanalytical methods, or to raise their awareness of stumbling blocks along the way.

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.

Advanced Glycation End Products Are Retained in Decellularized Muscle Matrix Derived from Aged Skeletal Muscle

Decellularized tissues are biocompatible materials that engraft well, but the age of their source has not been explored for clinical translation. Advanced glycation end products (AGEs) are chemical cross-links that accrue on skeletal muscle collagen in old age, stiffening the matrix and increasing inflammation. Whether decellularized biomaterials derived from aged muscle would suffer from increased AGE collagen cross-links is unknown. We characterized gastrocnemii of 1-, 2-, and 20-month-old C57BL/6J mice before and after decellularization to determine age-dependent changes to collagen stiffness and AGE cross-linking. Total and soluble collagen was measured to assess if age-dependent increases in collagen and cross-linking persisted in decellularized muscle matrix (DMM). Stiffness of aged DMM was determined using atomic force microscopy. AGE levels and the effect of an AGE cross-link breaker, ALT-711, were tested in DMM samples. Our results show that age-dependent increases in collagen amount, cross-linking, and general stiffness were observed in DMM. Notably, we measured increased AGE-specific cross-links within old muscle, and observed that old DMM retained AGE cross-links using ALT-711 to reduce AGE levels. In conclusion, deleterious age-dependent modifications to collagen are present in DMM from old muscle, implying that age matters when sourcing skeletal muscle extracellular matrix as a biomaterial.

Exploring the protective effects of schizandrol A in acute myocardial ischemia mice by comprehensive metabolomics profiling integrated with molecular mechanism studies

Schizandrol A (SA) is an bioactive component isolated from the Schisandra chinensis (Turcz.) Baill., which has been used as a remedy to prevent oxidative injury. However, whether the cardioprotective effect of SA is associated with regulating endogenous metabolites remains unclear, thus we performed comprehensive metabolomics profiling in acute myocardial ischemia (AMI) mice following SA treatment. AMI was induced in ICR mice by coronary artery ligation, then SA (6 mg·kg⁻¹·d⁻¹, ip) was administered. SA treatment significantly decreased the infarct size, preserved the cardiac function, and improved the biochemical indicators and cardiac pathological alterations. Moreover, SA (10, 100 M) significantly decreased the apoptotic index in OGD-treated H8c2 cardiomycytes in vitro. By using HPLC-Q-TOF/MS, we conducted metabonomics analysis to screen the significantly changed endogenous metabolites and construct the network in both serum and urine. The results revealed that SA regulated the pathways of glycine, serine and threonine metabolism, lysine biosynthesis, pyrimidine metabolism, arginine and proline metabolism, cysteine and methionine metabolism, valine, leucine and isoleucine biosynthesis under the pathological conditions of AMI. Furthermore, we selected the regulatory enzymes related to heart disease, including ecto-5’-nucleotidase (NT5E), guanidinoacetate N-methyltransferase (GAMT), platelet-derived endothelial cell growth factor (PD-ECGF) and methionine synthase (MTR), for validation. In addition, SA was found to facilitate PI3K/Akt activation and inhibit the expression of NOX2 in AMI mice and OGD-treated H9c2 cells. In conclusion, we have elucidated SA-regulated endogenous metabolic pathways and constructed a regulatory metabolic network map. Furthermore, we have validated the new potential therapeutic targets and underlying molecular mechanisms of SA against AMI, which might provide a reference for its future application in cardiovascular diseases.

Multiple Glycation Sites in Blood Plasma Proteins as an Integrated Biomarker of Type 2 Diabetes Mellitus

Type 2 diabetes mellitus (T2DM) is one of the most widely spread metabolic diseases. Because of its asymptomatic onset and slow development, early diagnosis and adequate glycaemic control are the prerequisites for successful T2DM therapy. In this context, individual amino acid residues might be sensitive indicators of alterations in blood glycation levels. Moreover, due to a large variation in the half-life times of plasma proteins, a generalized biomarker, based on multiple glycation sites, might provide comprehensive control of the glycemic status across any desired time span. Therefore, here, we address the patterns of glycation sites in highly-abundant blood plasma proteins of T2DM patients and corresponding age- and gender-matched controls by comprehensive liquid chromatography-mass spectrometry (LC-MS). The analysis revealed 42 lysyl residues, significantly upregulated under hyperglycemic conditions. Thereby, for 32 glycation sites, biomarker behavior was demonstrated here for the first time. The differentially glycated lysines represented nine plasma proteins with half-lives from 2 to 21 days, giving access to an integrated biomarker based on multiple protein-specific Amadori peptides. The validation of this biomarker relied on linear discriminant analysis (LDA) with random sub-sampling of the training set and leave-one-out cross-validation (LOOCV), which resulted in an accuracy, specificity, and sensitivity of 92%, 100%, and 85%, respectively.

Serum Levels of Protein-Bound Methylglyoxal-Derived Hydroimidazolone-1 are Independently Correlated with Asymmetric Dimethylarginine

Asymmetric dimethylarginine (ADMA) is an endogenous inhibitor of nitric oxide synthase, being involved in endothelial dysfunction. Furthermore, ADMA levels have been shown to predict future cardiovascular events in patients with coronary risk factors, such as diabetes and hypertension. We have previously found that glyceraldehyde-derived advanced glycation end products (glycer-AGEs) stimulate ADMA generation in vitro and the levels are associated with ADMA, endothelial dysfunction, and vascular inflammation in humans. However, it remains unclear what structurally distinct glycer-AGEs are independent correlates of ADMA. In this study, we addressed the issue. We measured serum levels of protein-bound and free methylglyoxal-derived hydroimidazolone-1 (MG-H1) and argpyrimidine, two major structurally identified glycer-AGEs by liquid chromatography-tandem mass spectrometry in 128 outpatients, and examined the correlations of these AGEs, vascular stiffness, and inflammation with ADMA. Moreover, we examined whether the changes in serum MG-H1 and argpyrimidine levels after 4-month treatment with oral hypoglycemic agents (OHAs) were associated with those of ADMA in other 44 patients with impaired glucose tolerance or type 2 diabetes. Multiple stepwise regression analysis revealed that protein-bound MG-H1, high-density lipoprotein cholesterol (inversely), high-sensitivity C-reactive protein, and cardio-ankle vascular index were independently correlated with ADMA (R² = 0.259). Treatment with OHAs significantly decreased ADMA levels in 44 glucose-intolerant or type 2 diabetic patients, and the changes in protein-bound MG-H1 levels were positively associated with those in ADMA values (p < 0.05). This study demonstrates that serum levels of protein-bound MG-H1 are independently correlated with ADMA and may be a therapeutic target for cardiovascular disease.

The Role of Advanced Glycation End Products in Aging and Metabolic Diseases: Bridging Association and Causality

Accumulation of advanced glycation end products (AGEs) on nucleotides, lipids, and peptides/proteins are an inevitable component of the aging process in all eukaryotic organisms, including humans. To date, a substantial body of evidence shows that AGEs and their functionally compromised adducts are linked to and perhaps responsible for changes seen during aging and for the development of many age-related morbidities. However, much remains to be learned about the biology of AGE formation, causal nature of these associations, and whether new interventions might be developed that will prevent or reduce the negative impact of AGEs-related damage. To facilitate achieving these latter ends, we show how invertebrate models, notably Drosophila melanogaster and Caenorhabditis elegans, can be used to explore AGE-related pathways in depth and to identify and assess drugs that will mitigate against the detrimental effects of AGE-adduct development.

Lysine glycation of apolipoprotein A-I impairs its anti-inflammatory function in type 2 diabetes mellitus

Apolipoprotein A-I (apoA-I), the major protein compontent of high-density lipoprotein (HDL), exerts many anti-atherogenic functions. This study aimed to reveal whether nonenzymatic glycation of specific sites of apoA-I impaired its anti-inflammatory effects in type 2 diabetes mellitus (T2DM). LC-MS/MS was used to analyze the specific sites and the extent of apoA-I glycation either modified by glucose in vitro or isolated from T2DM patients. Cytokine release in THP-1 monocyte-derived macrophages was tested by ELISA. Activation of NF-kappa B pathway was detected by western blot. The binding affinity of apoA-I to THP-1 cells was measured using ¹²⁵I-labeled apoA-I. We identified seven specific lysine (Lys, K) residues of apoA-I (K12, K23, K40, K96, K106, K107 and K238) that were susceptible to be glycated either in vitro or in vivo. Glycation of apoA-I impaired its abilities to inhibit the release of TNF-α and IL-1β against lipopolysaccharide (LPS) in THP-1 cells. Besides, the glycation levels of these seven K sites in apoA-I were inversely correlated with its anti-inflammatory abilities. Furthermore, glycated apoA-I had a lower affinity to THP-1 cells than native apoA-I had. We generated mutant apoA-I (K107E, M-apoA-I) with a substitution of glutamic acid (Glu, E) for lysine at the 107th site, and found that compared to wild type apoA-I (WT-apoA-I), M-apoA-I decreased its anti-inflammatory effects in THP-1 cells. We also modeled the location of these seven K residues on apoA-I which allowed us to infer the conformational alteration of glycated apoA-I and HDL. In summary, glycation of these seven K residues altered the conformation of apoA-I and consequently impaired the protective effects of apoA-I, which may partly account for the increased risk of cardiovascular disease (CVD) in diabetic subjects.

Studies of advanced glycation end products and oxidation biomarkers for type 2 diabetes

Advanced glycation end products (AGEs) are formed upon nonenzymatic reactions of sugars or their metabolites with proteins and other cellular constituents. Many AGEs are long lived. Recent findings suggest that AGEs may predict diabetes and its complications and thus may warrant further study. The objective of this study was to assess the validity of our experimental procedures for measuring AGEs in stored blood sample and to conduct a pilot study for developing AGE biomarkers for diabetes and/or age-related changes of glucose metabolism. We conducted a reliability study of the samples and methods using liquid chromatography-tandem mass spectrometry (LC-MS)/MS assays for 10 AGEs (including methylglyoxal-derived hydroimidazolone (MG-H1), glucosepane (GSP) and two oxidation measures, in stored repository blood samples from the Nurses' Health Study and the Health Professionals Follow-up Study. We also analyzed data relating blood GSP levels to type 2 diabetes status in a case-control study (25 cases and 15 controls). Among the AGEs, GSP, and MG-H1 showed the highest reliability across the various measures: reliability in duplicate samples and stability with delayed processing and storage over 1-2 year period. Furthermore, plasma GSP was associated with older age (P = 0.04) and type 2 diabetes status (age-adjusted P = 0.0475). Our findings suggest that analysis of these AGEs may be developed as biomarkers for diabetes and/or age-related changes of glucose metabolism. © 2018 BioFactors, 44(3):281-288, 2018.

Kidney, heart and brain: three organs targeted by ageing and glycation

Advanced glycation end-product (AGE) is the generic term for a heterogeneous group of derivatives arising from a non-enzymatic reaction between reducing sugars and proteins. In recent years, evidence has accumulated that incriminates AGEs in pathogenic processes associated with both chronic hyperglycaemia and age-related diseases. Regardless of their exogenous or endogenous origin, the accumulation of AGEs and their derivatives could promote accelerated ageing by leading to protein modifications and activating several inflammatory signalling pathways via AGE-specific receptors. However, it remains to be demonstrated whether preventing the accumulation of AGEs and their effects is an important therapeutic option for successful ageing. The present review gives an overview of the current knowledge on the pathogenic role of AGEs by focusing on three AGE target organs: kidney, heart and brain. For each of these organs we concentrate on an age-related disease, each of which is a major public health issue: chronic kidney disease, heart dysfunction and neurodegenerative diseases. Even though strong connections have been highlighted between glycation and age-related pathogenesis, causal links still need to be validated. In each case, we report evidence and uncertainties suggested by animal or epidemiological studies on the possible link between pathogenesis and glycation in a chronic hyperglycaemic state, in the absence of diabetes, and with exogenous AGEs alone. Finally, we present some promising anti-AGE strategies that are currently being studied.

Cross-sectional Analysis of AGE-CML, sRAGE, and esRAGE with Diabetes and Cardiometabolic Risk Factors in a Community-Based Cohort

BACKGROUND

Advanced glycation end products (AGEs) and their receptors are regarded as central to the development of diabetic complications, but associations with diabetes and cardiometabolic outcomes in previous studies are mixed.

METHODS

Using ELISA assays, we measured N(6)-carboxymethyllysine (AGE-CML), soluble receptor for AGEs (sRAGE), and endogenous secreted receptor for AGEs (esRAGE) in 1874 participants from the Atherosclerosis Risk in Communities study. We conducted a cross-sectional analysis to evaluate associations of these biomarkers with demographics, diabetes, hyperglycemia, cardiometabolic measures, and genetic variants in the gene encoding RAGE, AGER (advanced glycosylation end-product specific receptor).

RESULTS

After adjustment for demographics and body mass index (BMI), there were no significant differences in AGE-CML, sRAGE, or esRAGE by diabetes or hemoglobin A_1c. Black race and AGER genetic variants were strongly associated with lower sRAGE and esRAGE even after adjustment [percent difference (95% CI) in black vs whites in sRAGE: -29.17 (-34.86 to -23.48), esRAGE: -26.97 (-33.11 to -20.84); with rs2070600 in sRAGE: -30.13 (-40.98 to -19.29), and esRAGE: -30.32 (-42.42 to -18.21); with rs2071288 in sRAGE: -20.03 (-34.87 to -5.18), and esRAGE: -37.70 (-55.75 to -19.65)]. Estimated glomerular filtration rate and albuminuria significantly correlated with sRAGE and esRAGE. BMI and C-reactive protein significantly negatively correlated with AGE-CML, sRAGE, and esRAGE. AGE-CML was modestly correlated with fructosamine and glycated albumin.

CONCLUSIONS

AGE-CML, sRAGE, and esRAGE were more related to genetic, kidney, and inflammatory measures than to diabetes in this community-based population. Our results suggest that, when measured by ELISA, these biomarkers lack specificity and are of limited value in evaluating the role of these compounds in diabetes.

Improved Glycemic Control and Vascular Function in Overweight and Obese Subjects by Glyoxalase 1 Inducer Formulation

Risk of insulin resistance, impaired glycemic control, and cardiovascular disease is excessive in overweight and obese populations. We hypothesized that increasing expression of glyoxalase 1 (Glo1)-an enzyme that catalyzes the metabolism of reactive metabolite and glycating agent methylglyoxal-may improve metabolic and vascular health. Dietary bioactive compounds were screened for Glo1 inducer activity in a functional reporter assay, hits were confirmed in cell culture, and an optimized Glo1 inducer formulation was evaluated in a randomized, placebo-controlled crossover clinical trial in 29 overweight and obese subjects. We found trans-resveratrol (tRES) and hesperetin (HESP), at concentrations achieved clinically, synergized to increase Glo1 expression. In highly overweight subjects (BMI >27.5 kg/m(2)), tRES-HESP coformulation increased expression and activity of Glo1 (27%, P < 0.05) and decreased plasma methylglyoxal (-37%, P < 0.05) and total body methylglyoxal-protein glycation (-14%, P < 0.01). It decreased fasting and postprandial plasma glucose (-5%, P < 0.01, and -8%, P < 0.03, respectively), increased oral glucose insulin sensitivity index (42 mL ⋅ min(-1) ⋅ m(-2), P < 0.02), and improved arterial dilatation Δbrachial artery flow-mediated dilatation/Δdilation response to glyceryl nitrate (95% CI 0.13-2.11). In all subjects, it decreased vascular inflammation marker soluble intercellular adhesion molecule-1 (-10%, P < 0.01). In previous clinical evaluations, tRES and HESP individually were ineffective. tRES-HESP coformulation could be a suitable treatment for improved metabolic and vascular health in overweight and obese populations.