糖尿病性视网膜病变术后CRT、SFCT、mALB、MBG、HbA1c水平的变化及相关性

本研究通过观察糖尿病性视网膜病变术后患者黄斑中心凹视网膜厚度,脉络膜厚度,尿微量白蛋白、血糖、糖化血红蛋白的水平,试图了解其差异及相关性。我们选取2016年1月至2017年12月于我院就诊的糖尿病性视网膜病变患者200例,根据其有无视网膜病变、有无肾病、术后有无黄斑水肿分为合并组和未合并组,同时选取100例正常成年人作为对照。观察糖尿病患者和对照组、未合并和合并并发症糖尿病患者的黄斑中心凹视网膜厚度(central retinal thickness, CRT),凹下脉络膜厚度(subfoveal choroidal thickness,SFCT),尿微量白蛋白(microAlbunminuria, mALB)、平均血糖(mean blood glucose, MBG)和糖化血红蛋白(glycated haemoglobin, HbA1c)水平,进一步分析了糖尿病黄斑水肿患者的尿微量白蛋白水平与黄斑中心凹视网膜厚度、脉络膜厚度、血糖、糖化血红蛋白的相关性。研究结果表明,糖尿病组患者的CRT水平较对照组低,SFCT、mALB、MBG和HbA1c水平高于对照组;合并视网膜病变、合并肾病和合并黄斑水肿组患者的CRT水平较未合并组低,SFCT、mALB、MBG和HbA1c水平均高于未合并组;黄斑水肿患者的m ALB水平与CRT水平负相关,与SFCT、MBG和HbA1c水平正相关。本研究得出结论:糖尿病性视网膜病变术后患者黄斑中心凹视网膜厚度(CRT)较薄,脉络膜厚度(SFCT)变厚,且与尿微量白蛋白密切相关。     

Low plasma albumin levels are associated with increased plasma protein glycation and HbA1c in diabetes

Albumin is one of the most abundant plasma proteins and is heavily glycated in diabetes. In this study, we have addressed whether variation in the albumin levels influence glycation of plasma proteins and HbA1c. The study was performed in three systems: (1) streptozotocin (STZ)-induced diabetic mice plasma, (2) diabetic clinical plasma, and (3) in vitro glycated plasma. Diabetic mice and clinical plasma samples were categorized as diabetic high albumin plasma (DHAP) and diabetic low albumin plasma (DLAP) on the basis of their albumin levels. For the in vitro experiment, two albumin levels, high albumin plasma (HAP) and low albumin plasma (LAP), were created by differential depletion of plasma albumin. Protein glycation was studied by using a combination of two-dimensional electrophoresis (2DE), Western blotting, and LC-MS(E). In both mice and clinical experiments, an increased plasma protein glycation was observed in DLAP than in DHAP. Additionally, plasma albumin levels were negatively correlated with HbA1c. The in vitro experiment with differential depletion of albumin mechanistically showed that the low albumin levels are associated with increased plasma protein glycation and that albumin competes for glycation with other plasma proteins.     

Proteome analysis of serum from type 2 diabetics with nephropathy

Diabetic nephropathy (DN) is a renal disease which develops as a consequence of diabetes mellitus. Microalbuminuria is the earliest clinical sign of DN. There are no specific diagnostic biomarkers for type 2 diabetics with nephropathy other than microalbuminuria and macroalbuminuria. However, microalbuminuria does not constitute a sole independent indicator for type 2 diabetics with nephropathy, and thus, another screening method, such as a biomarker assay, is required in order to diagnose it more correctly. Therefore, we have utilized two-dimensional electrophoresis (2-DE) to identify human serum protein markers for the more specific and accurate prediction of progressive nephropathy in type 2 diabetes patients, via comparisons of the serum proteome in three experimental groups: type 2 diabetes patients without microalbuminuria (DM, n = 30), with microalbuminuria (MA, n = 29), and with chronic renal failure (CRF, n = 31). As a result, proteins which were differentially expressed with statistical significance (p < 0.05) in MA and CRF groups as compared to those in DM group were selected and identified by ESI-Q-TOF MS/MS. Among these identified proteins, two proteins which might be useful as diagnostic biomarkers of type 2 diabetics with nephropathy were verified by Western blotting: extracellular glutathione peroxidase (eGPx) and apolipoprotein (ApoE) were found to exhibit a progressive reduction in MA and CRF groups. Notably, eGPx was further verified by ELISA using DM (n = 100) and MA (n = 96) patient samples. Collectively, our results show that the two proteins identified in this study may constitute potential biomarkers for the diagnosis of type 2 diabetics with nephropathy.     

Application of shielding boronate affinity chromatography in the study of the glycation pattern of haemoglobin

Human haemoglobin (Hb) may appear in a number of glycated species. The glycation pattern of Hb using shielding boronate affinity chromatography (SBAC) has been studied in the present work. SBAC is a novel separation technique, which eliminates nonspecific boronate-protein interactions by introducing a so-called shielding reagent. Two samples from Bio-Rad (Lyphochek)--one from normal persons' blood with relatively low HbA(1c) level (HbL) and the other from diabetic patients' blood with an elevated HbA(1c) level (HbH)--were used for the investigation. Glycated Hb (GHb) was separated from nonglycated Hb species using Tris as the shielding reagent. Two eluted peaks, eluted peak 1 (E1) and eluted peak 2 (E2), were obtained using a linear gradient elution with Tris. Several bands were observed on isoelectric focusing gel, which showed the same migration positions as Hb adducts, such as HbA(0), which is major Hb component containing two alpha chains and two beta chains; HbA(1c), which is post-translational glycation on the N-terminus of the beta chains of HbA(0); Foetal Hb (HbF), consisting of two alpha chains and two gamma chains; and glutathione Hb (also called HbSSG), which is the result from thiol-disulphide interchain exchange during oxidation of the thiol groups of Hb. In both HbL and HbH samples, E2 exhibited slightly higher amounts of HbF than E1. Electrospray-ionisation mass spectrometry showed that: (1) HbL-E1 was glycated with single glucose on both alpha and beta chains while no observable glycated chains were present in HbL-E2; (2) both HbH-E1 and HbH-E2 were glycated with single glucoses on both alpha and beta chains, however, compared with HbH-E1, HbH-E2 showed a higher relative intensity of the glycated beta chain and lower relative intensity of the glycated alpha chain; and (3) the degree of glycation increased with increasing glycation level of the sample. The amount of HbA(1c) presented in the eluted peaks was further determined using enzymatic digestion of glycated Hb by endoproteinase Glu-C and the subsequent separation and analysis of the digested peptides by reversed-phase high-performance liquid chromatography and capillary electrophoresis. The values of HbA(1c)/HbA(0) of the eluted peaks, i.e. HbL-E1, HbL-E2, HbH-E1 and HbH-E2, were 0.27, 0.19, 0.50 and 0.43, respectively. In both HbL and HbH samples, E1 contained higher amounts of HbA(1c) than E2. This study demonstrates the structural heterogeneity of GHb as well as the possibility of using SBAC to detect glycated species of Hb.     

Proteomic Analysis of Glycated Proteins from Streptozotocin-Induced Diabetic Rat Kidney

Glycation of proteins leading to formation of advanced glycation end products (AGEs) has been considered as one of the important causes of diabetic nephropathy. Therefore, in this study, glycated proteins were detected by anti-AGE antibodies from kidney of streptozotocin-induced diabetic rat showing nephropathic symptoms, by using two dimensional electrophoresis and western blot analysis. These glycated proteins were identified and characterized by using combination of peptide mass finger printing and tandem mass spectrometric approaches. Glycated proteins identified included proteins from metabolic pathways, oxidative stress, cell signaling, and transport. Several of the proteins modified by glycation were involved in glucose metabolism. The extent of glycation was higher in diabetes compared to control, in the glycated proteins that were common to both control and diabetic kidney. Two dimensional electrophoresis proteins profiling of glycated proteins suggest that four of the glycated proteins were significantly up regulated in diabetes.     

A Novel Glycated Hemoglobin A1c-Lowering Traditional Chinese Medicinal Formula,Identified by Translational Medicine Study

Diabetes is a chronic metabolic disorder that has a significant impact on the health care system. The reduction of glycated hemoglobin A1c is highly associated with the improvements of glycemic control and diabetic complications. In this study, we identified a traditional Chinese medicinal formula with a HbA1c-lowering potential from clinical evidences. By surveying 9,973 diabetic patients enrolled in Taiwan Diabetic Care Management Program, we found that Chu-Yeh-Shih-Kao-Tang (CYSKT) significantly reduced HbA1c values in diabetic patients. CYSKT reduced the levels of HbA1c and fasting blood glucose, and stimulated the blood glucose clearance in type 2 diabetic mice. CYSKT affected the expressions of genes associated with insulin signaling pathway, increased the amount of phosphorylated insulin receptor in cells and tissues, and stimulated the translocation of glucose transporter 4. Moreover, CYSKT affected the expressions of genes related to diabetic complications, improved the levels of renal function indexes, and increased the survival rate of diabetic mice. In conclusion, this was a translational medicine study that applied a “bedside-to-bench” approach to identify a novel HbA1c-lowering formula. Our findings suggested that oral administration of CYSKT affected insulin signaling pathway, decreased HbA1c and blood glucose levels, and consequently reduced mortality rate in type 2 diabetic mice.     

Hemoglobin autofluorescence as potential long-term glycemic marker in the rat animal model

Diabetes is a serious disease whose patients often require long-term care. Blood glucose and intermediate glycation product of glycated hemoglobin (HbA1c) are, at best, surrogate biomarkers of disease progression. There is indication that advanced glycation end products (AGEs) better reflect diabetic risks. In this study, we explored the use of red blood cells (RBCs) and lysed hemoglobin (Hb) autofluorescence (AF) as potential biomarkers of diabetic complication. AF spectra measured under 370 nm excitation reveals that both RBC and Hb fluorescence in the 420 to 600 nm region. At early time points following diabetic induction in rats, AF increase in lysed Hb is more dramatic compared to that of RBCs. Moreover, we found significance variance of Hb autofluorescence despite relatively constant HbA1c levels. Furthermore, we found that although a correlation exists between AGE autofluorescence and HbA1c levels, the lack of complete correspondence suggests that the rate of AGE production differs significantly among different rats. Our results suggest that with additional development, both RBC and Hb autofluorescence from lysed RBCs may be used act long-term glycemic markers for diabetic complications in patients.      

Plasma protein advanced glycation end products, carboxymethyl cysteine, and carboxyethyl cysteine, are elevated and related to nephropathy in patients with diabetes

In Diabetes Mellitus (DM), glucose and the aldehydes glyoxal and methylglyoxal modify free amino groups of lysine and arginine of proteins forming advanced glycation end products (AGEs). Elevated levels of these AGEs are implicated in diabetic complications including nephropathy. Our objective was to measure carboxymethyl cysteine (CMC) and carboxyethyl cysteine (CEC), AGEs formed by modification of free cysteine sulfhydryl groups of proteins by these aldehydes, in plasma proteins of patients with diabetes, and investigate their association with the albumin creatinine ratio (ACR, urine albumin (mg)/creatinine (mmol)), an indicator of nephropathy. Blood was collected from forty-two patients with type 1 and 2 diabetes (18–36 years) and eighteen individuals without diabetes (17–35 years). A liquid chromatography-mass spectrophotometric method was developed to measure plasma protein CMC and CEC levels. Values for ACR and hemoglobin A1C (HbA1C) were obtained. Mean plasma CMC (μg/l) and CEC (μg/l) were significantly higher in DM (55.73 ± 29.43, 521.47 ± 239.13, respectively) compared to controls (24.25 ± 10.26, 262.85 ± 132.02, respectively). In patients with diabetes CMC and CEC were positively correlated with ACR, as was HbA1C. Further, CMC or CEC in combination with HbA1C were better predictors of nephropathy than any one of these variables alone. These results suggest that glucose, glyoxal, and methylglyoxal may all be involved in the etiology of diabetic nephropathy.     

Proteolytic degradation by cathepsin D of glycated hemoglobin from diabetes patients gives rise to hemorphin-7 peptides

Previous studies showed a significantly reduced level of hemorphins in the serum of diabetes patients. In order to elucidate the biochemical mechanisms responsible for this anomaly, the influence of hemoglobin glycation on hemorphin generation was studied. The glycation of hemoglobin occurs in the blood of diabetes patients and this could modify its enzymatic digestion and the resulting proteolytic products. Several samples of hemoglobin were obtained from the blood of type 1 diabetes patients (n = 8) and normal healthy control subjects (n = 2). The glycated hemoglobin samples were classified on the basis of their HbA1c values expressed as a percentage of total hemoglobin. Four solutions of glycated hemoglobin characterized by HbA1c values of 6%, 9.1%, 10.7% and 12.1% were treated with cathepsin D and the hemorphins obtained following the proteolysis were compared to controls. It was found that hemorphins were produced whatever the level of glycation of hemoglobin and also that the degree of glycation had no effect on the quantity of hemorphins released. Thus the alteration of hemoglobin does not seem to be the essential reason for the decrease in hemorphin concentrations in the sera of diabetic patients.     

Development of an HbA1c-Based Conversion Equation for Estimating Glycated Albumin in a Korean Population with a Wide Range of Glucose Intolerance

Background

Compared to the golden standard glycation index of HbA1c, glycated albumin (GA) has potentials for assessing insulin secretory dysfunction and glycemic fluctuation as well as predicting diabetic vascular complications. However, the reference ranges of GA and a conversion equation need to be clearly defined. We designed this study to determine the reference ranges in patients with normal glucose tolerance (NGT) based on conventional measures of glycemic status and to devise a conversion equation for calculating HbA1c and GA in a Korean population.

Methodology/Principal Findings

In this multicenter, retrospective, cross-sectional study, we recruited antidiabetic drug-naïve patients with available glycemic variables including HbA1c, GA, and fasting plasma glucose regardless of glucose status. For the reference interval of serum GA, 5^th to 95^th percentile value of GA in subjects with NGT was adopted. The conversion equation between HbA1c and GA was devised using an estimating regression model with unknown break-points method. The reference range for GA was 9.0–14.0% in 2043 subjects. The 95^th percentile responding values for FPG, and HbA1c were approximately 5.49 mmol/l, and 5.6%, respectively. The significant glycemic turning points were 5.868% HbA1c and 12.2% GA. The proposed conversion equation for below and above the turning point were GA (%) = 6.960+0.8963 × HbA1c (%) and GA (%) = −9.609+3.720 × HbA1c (%), respectively.

Conclusions/Significance

These results should be helpful in future studies on the clinical implications of high GA relative to HbA1c and the clinical implementation of diabetes management.     

Comprehensive identification of glycated peptides and their glycation motifs in plasma and erythrocytes of control and diabetic subjects

Nonenzymatic glycation of proteins sets the stage for formation of advanced glycation end-products and development of chronic complications of diabetes. In this report, we extended our previous methods on proteomics analysis of glycated proteins to comprehensively identify glycated proteins in control and diabetic human plasma and erythrocytes. Using immunodepletion, enrichment, and fractionation strategies, we identified 7749 unique glycated peptides, corresponding to 3742 unique glycated proteins. Semiquantitative comparisons showed that glycation levels of a number of proteins were significantly increased in diabetes and that erythrocyte proteins were more extensively glycated than plasma proteins. A glycation motif analysis revealed that some amino acids were favored more than others in the protein primary structures in the vicinity of the glycation sites in both sample types. The glycated peptides and corresponding proteins reported here provide a foundation for potential identification of novel markers for diabetes, hyperglycemia, and diabetic complications in future studies.     

Monitoring of early and advanced glycation in relation to the occurrence of microvascular complications in children and adolescents with type 1 diabetes mellitus

The authors aimed to evaluate if the monitoring of serum advanced glycation end-products (s-AGEs) could help to predict a development of diabetic complications. Clinical and biochemical parameters including fructosamine (FAM), glycated hemoglobin (HbA1c) and serum AGEs were investigated in children and adolescents with 1 type diabetes with (+DC) and without (-DC) complications. FAM levels (in mmol/l) were significantly elevated in +DC diabetic group compared to -DC one (3.043+/-0.459 vs. 2.614+/-0.430; p<0.001) or to controls (3.043+/-0.459 vs. 1.620+/-0.340; p<0.001) as well as in -DC compared to controls (2.614+/-0.430 vs. 1.620+/-0.340; p<0.001). HbA1c (in %) were significantly elevated in +DC diabetic group compared to -DC one (10.48+/-1.83 vs. 8.41+/-1.19; p<0.001) or to controls (10.48+/-1.83 vs. 5.0+/-0.38, p<0.001) and also in -DC compared to controls (8.41+/-1.19 vs. 5.0+/-0.38; p<0.001). Serum AGEs levels (in A. U.) were significantly higher in +DC group than in -DC (73.0+/-14.09 vs. 65.8+/-9.05; p<0.05) and in group +DC than in controls (73.0+/-14.09 vs. 60.17+/-13.78; p<0.05), whereas there was no difference between -DC and controls. FAM correlated with HbA1c in both diabetic groups (+DC: r=0.374; p<0.05; -DC: r=0.719; p<0.001), but not in controls. Serum AGEs were correlated with HbA1c (r=0.478; p=0.003) in +DC, but not in -DC or controls. Enhanced serum AGEs levels show that they could be not only an attendant phenomenon of microangiopathies, but also a predictor of their development.     

胰岛素联合六味地黄丸治疗老年早期糖尿病肾病患者的临床疗效研究

目的:探讨胰岛素联合六味地黄丸治疗老年早期糖尿病肾病患者的临床疗效。方法:收集我院收治的老年早期糖尿病肾病患者100例,随机分为对照组和实验组,每组各50例,对照组患者血糖情况给予胰岛素1次~2次/d,实验组患者在此基础上给予六味地黄丸1丸/次,2次/d,口服。连续4周。治疗结束后,对患者血清糖基化血红蛋白(Hb A1c)、同型半胱氨酸(HCY)、胱抑素C(Cys-C)水平的变化及临床疗效进行检测并比较。结果:与治疗前相比,两组患者的Hb A1c、HCY、Cys-C均下降(P0.05);与对照组相比,实验组患者Hb A1c、HCY、Cys-C水平较低(P0.05),实验组患者治疗总有效率较高(P0.05)。结论:胰岛素联合六味地黄丸能够有效改善老年早期糖尿病肾病患者的糖稳态及肾功能,临床疗效较好。     

Nonenzymatic glycation of immunoglobulins leads to an impairment of immunoreactivity

Incubation of purified human and rabbit immunoglobulin G with glucose leads to covalent incorporation of the sugar into the protein, depending on glucose concentration, incubation time and pH. Furthermore, the level of glycated immunoglobulin G from normal and diabetic subjects has been determined using the thiobarbituric acid reaction. The median for glycated immunoglobulin G, expressed as mmol 5-hydroxymethylfurfural per mol IgG, obtained from 20 normal and 29 diabetic subjects was 62 and 107, respectively. Glucose incubation of immunoglobulin G purified from rabbit anti-human-transferrin serum, from human anti-varicella/zoster virus serum and from human anti-lues-spirochete serum, respectively, leads to a marked decrease in biological activity, as determined in a micro complement fixation test. Inactivation of specific antibody was dependent on incubation time and glucose concentration employed. Loss in complement-fixing activity was observed at glycation levels well comparable to those found in diabetics.     

Role of glycaemic control in development of microalbuminuria in patients with insulin dependent diabetes.

OBJECTIVE--To ascertain which factors determine the progression from very low rates of albumin excretion to persistent microalbuminuria in patients with insulin dependent diabetes mellitus. DESIGN--A 10 year prospective study of a cohort of diabetic patients. SETTING--Outpatient department of the Portsmouth District Hospitals. SUBJECTS--97 patients with insulin dependent diabetes mellitus who were initially free of microalbuminuria and hypertension. MAIN OUTCOME MEASURE--Urinary albumin: creatinine ratio. RESULTS--Eight of the 97 patients had developed microalbuminuria (urinary albumin:creatinine ratio > 3 mg/mmol in three consecutive early morning samples) by the 10 year follow up. The group who developed microalbuminuria had higher baseline log10 plasma glucose concentrations (mean (SD), 1.210 (0.122) v 0.984 (0.196) mmol/l, P < 0.001) and glycated haemoglobin concentrations (1.112% (0.069%) v 0.997% (0.076%), P < 0.001) and a younger age at onset of diabetes (10.0 (5.5) v 15.6 (7.8) years, P < 0.05). There was no difference in baseline duration of diabetes, smoking, sex, insulin dose, body mass index, serum creatinine concentration, or systolic, diastolic, or mean arterial blood pressure between the two groups. Multiple linear regression analysis showed that urinary albumin:creatinine ratio at 10 years was influenced by initial albumin:creatinine ratio (P = 0.006), initial glycated haemoglobin concentration (P = 0.002), and duration of diabetes (P = 0.045). Genotype for angiotensin converting enzyme was not related to the development of microalbuminuria nor, in a larger group of patients, the presence of any degree of diabetic nephropathy. CONCLUSION--In patients with insulin dependent diabetes mellitus the progression of minimal albuminuria and the development of microalbuminuria is determined primarily by poor long term glycaemic control. There is a weaker relation with longer duration of disease and younger age at onset of diabetes, but blood pressure does not seem to be implicated. Gene polymorphism for angiotensin converting enzyme is not linked to the development of microalbuminuria or established diabetic nephropathy.     

尿微量白蛋白检测在社区糖尿病患者管理中的临床意义

目的:观察尿微量白蛋白在糖尿病患者中的改变情况,探讨提高社区糖尿病肾病早期诊断检出率的方法。方法:选择社区2型糖尿病患者73例,根据空腹血糖控制情况及糖尿病病程时长分别分组,比较不同组间患者尿微量白蛋白水平及阳性病例检出率,同时进行相关性分析。结果:MAlb在不同血糖控制情况患者中,血糖水平与MAlb数值、阳性检出率具有正相关性(P<0.05);血糖控制良好患者与其他血糖控制情况患者相比,差异具有(高度)统计学意义(P<0.01;P<0.05);MAlb在不同病程患者中,病程时间与MAlb数值、阳性检出率具有正相关性(P<0.05);≤5年组患者与5～10年组和≥10年组相比,差异具有(高度)统计学意义(P<0.01;P<0.05)。结论:尿微量白蛋白(MAlb)可作为早期糖尿病肾病检测的重要指标,能够提示肾脏病变的发展,定期检查能够提高早期糖尿病肾病的检出率。     

Glycation Isotopic Labeling with 13C-Reducing Sugars for Quantitative Analysis of Glycated Proteins in Human Plasma

Non-enzymatic glycation of proteins is a post-translational modification produced by a reaction between reducing sugars and amino groups located in lysine and arginine residues or in the N-terminal position. This modification plays a relevant role in medicine and food industry. In the clinical field, this undesired role is directly linked to blood glucose concentration and therefore to pathological conditions derived from hyperglycemia (>11 mm glucose) such as diabetes mellitus or renal failure. An approach for qualitative and quantitative analysis of glycated proteins is here proposed to achieve the three information levels for their complete characterization. These are: 1) identification of glycated proteins, 2) elucidation of sugar attachment sites, and 3) quantitative analysis to compare glycemic states. Qualitative analysis was carried out by tandem mass spectrometry after endoproteinase Glu-C digestion and boronate affinity chromatography for isolation of glycated peptides. For this purpose, two MS operational modes were used: higher energy collisional dissociation-MS2 and CID-MS3 by neutral loss scan monitoring of two selective neutral losses (162.05 and 84.04 Da for the glucose cleavage and an intermediate rearrangement of the glucose moiety). On the other hand, quantitative analysis was based on labeling of proteins with [¹³C₆]glucose incubation to evaluate the native glycated proteins labeled with [¹²C₆]glucose. As glycation is chemoselective, it is exclusively occurring in potential targets for in vivo modifications. This approach, named glycation isotopic labeling, enabled differentiation of glycated peptides labeled with both isotopic forms resulting from enzymatic digestion by mass spectrometry (6-Da mass shift/glycation site). The strategy was then applied to a reference plasma sample, revealing the detection of 50 glycated proteins and 161 sugar attachment positions with identification of preferential glycation sites for each protein. A predictive approach was also tested to detect potential glycation sites under high glucose concentration.Among post-translational modifications (PTMs)1 of proteins, non-enzymatic glycation is one of the less frequently studied in proteomics. Glycated proteins are formed by a non-enzymatic reaction between reducing carbohydrates (e.g. glucose, fructose, ribose, or derivatives such as ascorbic acid) with amino groups located in the N-terminal position or in lysine and arginine residues. It is worth emphasizing the differences between glycation and glycosylation. The latter is enzymatically catalyzed by glycosyltransferase and occurs in specific protein side chains such as asparagine (N-linked), serine and threonine (O-linked), and the C termini of cell surface proteins (1). Glycosylation is involved in many biological processes in contrast to glycation, which is a completely undesired modification from a clinical point of view.Because of the crucial role of glucose as an energy source in humans, it is the main circulating sugar and thus the most relevant molecule in terms of protein glycation. The mechanisms involved in glycation are illustrated in Fig. 1 for glucose as the reducing sugar (2). The process starts with the formation of the Schiff base by a condensation reaction between the carbonyl group of the reducing sugar and the amino group of the protein. The next step is the conversion of the thermodynamically unstable Schiff base into the Amadori compound that is considered as the first glycation level. Finally, the Amadori compound undergoes a series of dehydration and fragmentation reactions, generating a variety of carbonyl compounds such as methylglyoxal, glyoxal, glucosones, deoxyglucosones, and dehydroascorbate (3). These carbonyl compounds are generally more reactive than the original carbohydrate and act as propagators by reactions with free amino groups, leading to the formation of a variety of heterogeneous structures irreversibly formed and commonly known as advanced glycation end products. The impact of glycation encompasses alterations of the structure, function, and turnover of proteins (4). Evidently, the effects on biological function will depend on the extent of glycation. From a clinical point of view, the detection of this PTM at the initial stage would be helpful for both prognostic and diagnostic purposes.Open in a separate windowFig. 1.Scheme of glycation process.The kinetics of the initial glycation process is governed by the formation of the Amadori compound, a slow process under human physiological conditions (37 °C; ∼5 mm blood glucose concentration in healthy subjects) (5). However, the reaction kinetics is enhanced under prolonged hyperglycemia exposure, which is one of the pathological mechanisms involved. In contrast to physiological glucose concentration, chronic supraphysiological glucose concentration (>10 mm) negatively affects a large number of organs and tissues, such as pancreas, eyes, liver, muscles, adipose tissues, brain, heart, kidneys, and nerves. Glucose toxicity is the main cause of diabetic complications, which are often observed only several years after the development of the illness (6, 7). However, chronic hyperglycemia can also increase the development rate of early diabetic states by affecting the secretion capacity of pancreatic cells, which in turn increases blood glucose concentration. This vicious circle finally leads to the total incapacity of β-cells to secrete insulin (8, 9). Thus, glycation has often been related to chronic complications of diabetes mellitus, renal failure, and degenerative changes occurring in the course of aging (10–12).Glycation of proteins is one of the potential mechanisms expected to be involved in glucotoxicity because of clinical evidence. Calvo et al. (13–15) have evaluated the non-enzymatic glycation rate of high density lipoprotein in type 1 and 2 diabetic patients. The authors isolated glycated apolipoprotein A-I (apoA-I) from diabetic patients and compared its lipid binding properties with those of apoA-I from healthy subjects. They found that apoA-I glycation promotes a decrease in the stability of the lipid-apolipoprotein interaction and also in its self-association. Therefore, the structural cohesion of high density lipoprotein molecules is seriously affected by glycation of apoA-I. In vivo studies in mice proved that glycated insulin exhibits a reduced ability to stimulate glucose oxidation by the isolated mouse diaphragm muscle. This observation was in concordance with previous studies suggesting that glycation of insulin decreases its potency to stimulate lipogenesis in isolated rat adipocytes. This is consistent with the observation that glycated insulin displayed a significantly reduced ability to lower plasma glucose concentrations in mice. These and other studies clearly indicated that glycation results in a significant impairment of insulin action to regulate plasma glucose homeostasis (16).The glycemic control of clinical patients is currently assessed indirectly with the conventional test of glycated hemoglobin (HbA1c). HbA1c is a long term indicator of the patient glycemic state because of the erythrocyte lifespan (∼120 days). HbA1c concentration represents the memory effect of blood glucose concentrations over the previous 8–12 weeks (17–20). Other measurements indicative of short term glucose perturbation are needed to understand its potential biological effect. It should also be taken into account that any protein could be potentially glycated. Because of the continuous exposition to glucose, the concentrations of HbA1c and glycated human serum albumin in plasma from healthy subjects have been estimated around 5–7 and 15%, respectively (21, 22). Therefore, the development of methods for the identification and quantification of glycated proteins as well as for prediction of new potential targets under different conditions is crucial to elucidate their biological effect.Recently, Metz and co-workers (23–25) proposed several approaches for the characterization of glycated proteins. These approaches are based on bottom-up work flows characterized by the implementation of selective and sensitive steps for the enrichment and isolation of glycated proteins and/or peptides with boronate affinity chromatography (BAC) and data-dependent mass spectrometry methods. Nevertheless, these approaches have been focused on qualitative analysis only. Therefore, it is clear that there is a demand for quantitative methods for the analysis of glycated proteins to evaluate the glycemic control of clinical samples or to compare patient glycemic states.A method for quantitative analysis of glycated proteins is presented here. This method is based on differential labeling of proteins with isotopically labeled sugars (¹³C-sugars), named glycation isotopic labeling (GIL). The labeling step is performed by natural incubation under physiological conditions mimicking the in vivo glycation process. By this procedure, only preferential glycation targets are labeled because of the chemoselectivity of this process. After labeling, this approach can be implemented in any proteomics work flow based on MS detection and relative quantitation of the two isotopic forms. In this study, the approach was implemented in the analysis of non-enzymatic glycation sites in the human plasma proteome.     

Proteomic profiling of nonenzymatically glycated proteins in human plasma and erythrocyte membranes

Nonenzymatic glycation of peptides and proteins by d-glucose has important implications in the pathogenesis of diabetes mellitus, particularly in the development of diabetic complications. In this work, we report the first proteomics-based characterization of nonenzymatically glycated proteins in human plasma and erythrocyte membranes from individuals with normal glucose tolerance, impaired glucose tolerance, and type 2 diabetes mellitus. Phenylboronate affinity chromatography was used to enrich glycated proteins and glycated tryptic peptides from both human plasma and erythrocyte membranes. The enriched peptides were subsequently analyzed by liquid chromatography coupled with electron transfer dissociation-tandem mass spectrometry, resulting in the confident identification of 76 and 31 proteins from human plasma and erythrocyte membranes, respectively. Although most of the glycated proteins could be identified in samples from individuals with normal glucose tolerance, slightly higher numbers of glycated proteins and more glycation sites were identified in samples from individuals with impaired glucose tolerance and type 2 diabetes mellitus.     

Effects of glycated albumin on mesangial cells: evidence for a role in diabetic nephropathy

Nonenzymatically glycated proteins are preferentially transported across the glomerular filtration barrier, and the glomerular mesangium in diabetes is bathed with serum containing increased concentrations of glycated albumin. We investigated effects of glycated albumin on mesangial cells, which are involved in diabetic nephropathy. [³H]-thymidine incorporation was significantly inhibited when murine mesangial cells were grown in culture media containing human serum that had been nonenzymatically glycated by incubation for 4 days with 28 mM glucose. This inhibition was reversed when monoclonal antibodies that selectively react with Amadori products of glycated albumin were added to the culture media. Purified glycated albumin containing Amadori adducts of the glycation reaction induced significant inhibition of thymidine incorporation and stimulation of Type IV collagen secretion compared with cells cultured in the presence of purified nonglycated albumin. These changes were prevented when monoclonal antibodies specifically reactive with fructosyl-lysine epitopes in glycated albumin were added to the cultures. The antibodies had no effect on growth or collagen production in the presence of nonglycated albumin. The results provide the first evidence directly implicating Amadori adducts in glycated albumin in the pathogenesis of diabetic nephropathy, which is characterized by decreased cellularity in association with expansion of the mesangial matrix.     

Increased serum levels of interleukin-18 in patients with diabetic nephropathy

In the present study we measured interleukin-18 (IL-18) and tumour necrosis factor-alpha (TNF-alpha) levels by enzyme linked immunosorbent assay (ELISA) in sera from 65 diabetic [30 with type 1 insulin dependent diabetes mellitus (IDDM) and 35 with type 2 non-insulin dependent diabetes mellitus (NIDDM)] patients and 15 healthy volunteers, to investigate their associations with metabolic parameters and to elucidate their roles in the pathogenesis of diabetic complications especially diabetic nephropathy. Levels of IL-18 and TNF-alpha were significantly higher in both IDDM and NIDDM individuals as compared to the control group. Similarly, their levels in patients with diabetic nephropathy increased gradually according to the clinical stage of the disease, being highest in macroalbuminuric stage. Correlation analyses showed that the serum IL-18 and TNF-alpha concentration were positively correlated with each other and positively with fasting plasma glucose (FPG), 2h postprandial glucose, glycosylated hemoglobin (HbA1c), triglyceride, and urinary albumin levels and negative correlation between TNF-alpha and high density lipoprotein cholesterol (HDL-C) were also found in diabetic subjects. High serum levels of IL-18 and TNF-alpha suggested that they might play a role in the pathogenesis of DM and in the development of nephropathy in diabetic patients whether of type 1 or 2.