Transglycation--a potential new mechanism for deglycation of Schiff's bases

Glucose-independent racial disparity in HbA1c is evident at onset of type 1 diabetes

OBJECTIVE

Higher levels of HbA1c, independent of blood glucose levels, have been described in Blacks compared to Whites patients with established diabetes. The goal of this study was to determine if glucose-independent racial disparity in HbA1C is evident at diabetes onset.

RESEARCH DESIGN AND METHODS

We conducted a retrospective single-center chart review of 189 youth with new onset Type 1 diabetes (T1D) 60 % Whites and 40 % Blacks. HbA1c, glucose and other biochemistry measures were obtained at presentation in the Emergency Department before initiation of any therapy. HbA1c levels were adjusted for presenting glucose, self-identified race, age, gender, hematocrit, and RDW-CV.

RESULTS

Blacks with T1D had statistically significant higher unadjusted HbA1c (11.9 ± 1.9 vs 11.04 ± 2.0 %, p = 0.004), initial glucose (530.6 ± 230.4 vs 442 ± 211.3 mg/dL, p = 0.0075) and lower pHs (7.28 ± 0.15 vs 7.33 ± 0.12, p = 0.02) compared to white patients. Least squares means of HbA1c remained higher in Black patients even after statistical adjustment for presenting glucose, age, gender, RDW-CV, and pH. In a multiple variable model (R² = 0.38, p < 0.0001) c-peptide was influenced by HCO₃ (p = 0.0035), gender (p = 0.0092), BMI (p < 0.0001), but not race or glucose.

CONCLUSIONS

HbA1c at initial presentation of T1D is higher in young Black patients compared to Whites even after adjustment for glucose, age, gender, and RDW-CV. This racial disparity is consistent with other studies in individuals without diabetes and patients with long-standing diabetes under treatment.

Stochastic non-enzymatic modification of long-lived macromolecules - A missing hallmark of aging

Damage accumulation in long-living macromolecules (especially extracellular matrix (ECM) proteins, nuclear pore complex (NPC) proteins, and histones) is a missing hallmark of aging. Stochastic non-enzymatic modifications of ECM trigger cellular senescence as well as many other hallmarks of aging affect organ barriers integrity and drive tissue fibrosis. The importance of it for aging makes it a key target for interventions. The most promising of them can be AGE inhibitors (chelators, O-acetyl group or transglycating activity compounds, amadorins and amadoriases), glucosepane breakers, stimulators of elastogenesis, and RAGE antagonists.

Dual Glycation-Inflammation Modulation, DPP-IV and Pancraetic Lipase Inhibitory Potentials and Antiproliferative Activity of Novel Fluoroquinolones

Paramount efforts by pharmaceutical industry to identify new targets for obesity-diabetes (Diabesity) pharmacological intervention have led to a number of agents developed and directed at DPP IV [dipeptidyl peptidase IV] enzyme inhibition thereby enhancing endogenous insulinotropic incretins. Besides antioxidative-antiinflammtory molecules that inhibit accumulation of advanced glycation end products (AGEs) can be good candidates for ameliorating diabetic complications. Fluoroquinolones (FQs) have been identified recently as potent inhibitors of pancreatic lipase (PL). The suggested association between obesity and colorectal cancer initiated the evaluation of antiproliferative activity of the new FQs and TFQs against a panel of obesity related colorectal cells (HT29, HCT116, SW620 CACO2 and SW480). The aim of the current study is to examine the potential of newly synthesized FQs and triazolofluoroquinolones (TFQs) derivatives as dual inhibitors for glycation and inflammation, DPP IV inhibitors, PL inhibitors for dual management of obesity and diabetes, as well as antiprolifertaive efficacy against colorectal cancer cell lines. Sulforodamine B (SRB) colorimetric assay revealed that some derivatives exhibited unselective cytotoxity against HT29, HCT116, SW620 CACO2 and SW480. The superior antiglycation activity of the reduced derivatives 4a and 4b over that of aminoguanidine with respective IC50 (μM) values of 3.05±0.33 and 8.51±3.21; none of the tested synthetic compounds could perform equally effectively to Diprotin A, a dose dependent inhibitor of DPP IV. Compounds 4a, 5a, 3b, 4b and 5b demonstrated anti-inflammatory IC50 values exceeding that of indomethacin. Compounds 3a and 4a showed IC50 lower than 10 μM as PL inhibitors. In conclusion, FQ and TFQ derivatives may unveil new antiobesity and anticancer agents in the future. Our research qualifies FQs and TFQs as promising candidates for the development of related α-dicarbonyl scavengers as therapeutic agents to protect cells against carbonyl stress.

Cysteine allows ovarian cancer cells to adapt to hypoxia and to escape from carboplatin cytotoxicity

Ovarian cancer is the second most common gynaecologic malignancy and the main cause of death from gynaecologic cancer, due to late diagnosis and chemoresistance. Studies have reported the role of cysteine in cancer, by contributing for hydrogen sulphide (H2S) generation and as a precursor of glutathione (GSH). However, the role of cysteine in the adaptation to hypoxia and therapy response remains unclear. We used several ovarian cancer cell lines, ES2, OVCAR3, OVCAR8, A2780 and A2780cisR, to clarify cysteine relevance in ovarian cancer cells survival upon hypoxia and carboplatin. Results show that ES2 and OVCAR8 cells presented a stronger dependence on cysteine availability upon hypoxia and carboplatin exposure than OVCAR3 cells. Interestingly, the A2780 cisR, but not A2780 parental cells, benefits from cysteine upon carboplatin exposure, showing that cysteine is crucial for chemoresistance. Moreover, GSH degradation and subsequent cysteine recycling pathway is associated with ovarian cancer as seen in peripheral blood serum from patients. Higher levels of total free cysteine (Cys) and homocysteine (HCys) were found in ovarian cancer patients in comparison with benign tumours and lower levels of GSH were found in ovarian neoplasms patients in comparison with healthy individuals. Importantly, the total and S-Homocysteinylated levels distinguished blood donors from patients with neoplasms as well as patients with benign from patients with malignant tumours. The levels of S-cysteinylated proteins distinguish blood donors from patients with neoplasms and the free levels of Cys in serum distinguish blood from patients with benign tumours from patients with malignant tumours. Herein we disclosed that cysteine contributes for a worse disease prognosis, allowing faster adaptation to hypoxia and protecting cells from carboplatin. The measurement of serum cysteine levels can be an effective tool for early diagnosis, for outcome prediction and follow up of disease progression.

Labile haemoglobin as a glycaemic biomarker for patient-specific monitoring of diabetes: mathematical modelling approach

Diabetes mellitus constitutes a major health problem and its clinical presentation and progression may vary considerably. A number of standardized diagnostic and monitoring tests are currently used for diabetes. They are based on measuring either plasma glucose, glycated haemoglobin or both. Their main goal is to assess the average blood glucose concentration. There are several sources of interference that can lead to discordances between measured plasma glucose and glycated haemoglobin levels. These include haemoglobinopathies, conditions associated with increased red blood cell turnover or the administration of some therapies, to name a few. Therefore, there is a need to provide new diagnostic tools for diabetes that employ clinically accessible biomarkers which, at the same time, can offer additional information allowing us to detect possible conflicting cases and to yield more reliable evaluations of the average blood glucose level concentration. We put forward a biomathematical model to describe the kinetics of two patient-specific glycaemic biomarkers to track the emergence and evolution of diabetes: glycated haemoglobin and its labile fraction. Our method incorporates erythrocyte age distribution and utilizes a large cohort of clinical data from blood tests to support its usefulness for diabetes monitoring.

Effect of (-)-epigallocatechin-3-gallate on glucose-induced human serum albumin glycation

(-)-Epigallocatechin-3-gallate (EGCg) is a naturally occurring polyphenol found in plant-based foods and beverages such as green tea. Although EGCg can eliminate carbonyl species produced by glucose autoxidation and thus can inhibit protein glycation, it is also reported to be a pro-oxidant that stimulates protein glycation in vitro. To better understand the balance between antioxidant and pro-oxidant features of EGCg, we evaluated EGCg-mediated bioactivities in a human serum albumin (HSA)/glucose model by varying three different parameters (glucose level, EGCg concentration, and time of exposure to EGCg). Measurements of glycation-induced fluorescence, protein carbonyls, and electrophoretic mobility showed that the level of HSA glycation was positively related to the glucose level over the range 10-100 mM during a 21-day incubation at 37°C and pH: 7.4. Under mild glycemic pressure (10 mM), long exposure to EGCg enhanced HSA glycation, while brief exposure to low concentrations of EGCg did not. Under high glycemic pressure (100 mM glucose), long exposure to EGCg inhibited glycation. For the first time we showed that brief exposure to EGCg reversed glycation-induced fluorescence, indicating a restorative effect. In conclusion, our research identified glucose level, EGCg concentration, and time of exposure as critical factors dictating EGCg bioactivities in HSA glycation. EGCg did not affect HSA glycation under normal physiological conditions but had a potential therapeutic effect on HSA severely damaged by glycation.

Parkinsonism-associated protein DJ-1/Park7 is a major protein deglycase that repairs methylglyoxal- and glyoxal-glycated cysteine, arginine, and lysine residues

Glycation is an inevitable nonenzymatic covalent reaction between proteins and endogenous reducing sugars or dicarbonyls (methylglyoxal, glyoxal) that results in protein inactivation. DJ-1 was reported to be a multifunctional oxidative stress response protein with poorly defined function. Here, we show that human DJ-1 is a protein deglycase that repairs methylglyoxal- and glyoxal-glycated amino acids and proteins by acting on early glycation intermediates and releases repaired proteins and lactate or glycolate, respectively. DJ-1 deglycates cysteines, arginines, and lysines (the three major glycated amino acids) of serum albumin, glyceraldehyde-3-phosphate dehydrogenase, aldolase, and aspartate aminotransferase and thus reactivates these proteins. DJ-1 prevented protein glycation in an Escherichia coli mutant deficient in the DJ-1 homolog YajL and restored cell viability in glucose-containing media. These results suggest that DJ-1-associated Parkinsonism results from excessive protein glycation and establishes DJ-1 as a major anti-glycation and anti-aging protein.

Glycated proteome: from reaction to intervention

Glycation, a nonenzymatic reaction between reducing sugars and proteins, is a proteome wide phenomenon, predominantly observed in diabetes due to hyperglycemia. Glycated proteome of plasma, kidney, lens, and brain are implicated in the pathogenesis of various diseases, including diabetic complications, neurodegenerative diseases, cancer, and aging. This review discusses the strategies to characterize protein glycation, its functional implications in different diseases, and intervention strategies to protect the deleterious effects of protein glycation.

Proteome wide reduction in AGE modification in streptozotocin induced diabetic mice by hydralazine mediated transglycation

The non-enzymatic reaction between glucose and protein can be chemically reversed by transglycation. Here we report the transglycation activity of hydralazine using a newly developed MALDI-TOF-MS based assay. Hydralazine mediated transglycation of HbA1c, plasma proteins and kidney proteins was demonstrated in streptozotocin (STZ) induced diabetic mice, as evidenced by decrease in protein glycation, as well as presence of hydralazine-glucose conjugate in urine of diabetic mice treated with hydralazine. Hydralazine down regulated the expression of Receptor for Advanced Glycation End products (RAGE), NADPH oxidase (NOX), and super oxide dismutase (SOD). These findings will provide a new dimension for developing intervention strategies for the treatment of glycation associated diseases such as diabetes complications, atherosclerosis, and aging.

Labile A1C is inversely correlated with the hemoglobin glycation index in children with type 1 diabetes

OBJECTIVE

We hypothesized that labile A1C (LA1C) is directly correlated with stable A1C (SA1C) and between-patient differences in SA1C, which are independent of mean blood glucose (MBG).

RESEARCH DESIGN AND METHODS

We measured SA1C, LA1C, MBG, and a single clinic capillary glucose (CCG) from 152 pediatric patients with type 1 diabetes. Patients were grouped as high, moderate, or low glycators by hemoglobin glycation index (HGI).

RESULTS

LA1C and SA1C were correlated with CCG and MBG. LA1C was not correlated with SA1C (r = 0.06, P = 0.453). LA1C level was significantly associated with glycator group status (P < 0.0019) and CCG (P < 0.0001). Adjusted LA1C levels were highest in the low-HGI patients and lowest in the high-HGI group.

CONCLUSIONS

A conventional model of SA1C being directly correlated with LA1C concentration was not confirmed. Between-patient differences in SA1C at the same MBG may be due to complex intracellular factors influencing formation of SA1C from LA1C.

Red cell life span heterogeneity in hematologically normal people is sufficient to alter HbA1c

Although red blood cell (RBC) life span is a known determinant of percentage hemoglobin A1c (HbA1c), its variation has been considered insufficient to affect clinical decisions in hematologically normal persons. However, an unexplained discordance between HbA1c and other measures of glycemic control can be observed that could be, in part, the result of differences in RBC life span. To explore the hypothesis that variation in RBC life span could alter measured HbA1c sufficiently to explain some of this discordance, we determined RBC life span using a biotin label in 6 people with diabetes and 6 nondiabetic controls. Mean RBC age was calculated from the RBC survival curve for all circulating RBCs and for labeled RBCs at multiple time points as they aged. In addition, HbA1c in magnetically isolated labeled RBCs and in isolated transferrin receptor-positivereticulocytes was used to determine the in vivo synthetic rate of HbA1c. The mean age of circulating RBCs ranged from 39 to 56 days in diabetic subjects and 38 to 60 days in nondiabetic controls. HbA1c synthesis was linear and correlated with mean whole blood HbA1c (R(2) = 0.91). The observed variation in RBC survival was large enough to cause clinically important differences in HbA1c for a given mean blood glucose.

Frequency of HbA1c discordance in estimating blood glucose control

PURPOSE OF REVIEW

HbA1c is a trusted standard for monitoring glycemic control and predicting complications. However, there are emerging issues complicating the interpretation of HbA1c that the clinician caring for patients with diabetes needs to consider.

RECENT FINDINGS

There is increasing evidence of the potential for mismatches between HbA1c and other measures of glycemia which require some caution in HbA1c interpretation. We have attempted to characterize the nature of these discordances by developing the concept of the 'glycation gap' in which differences in protein glycation occurring in the intracellular space (HbA1c) versus in the extracellular space (measured as fructosamine) are compared. We have evidence that HbA1c results discordant from other measures of glycemic control may be secondary to previously unappreciated physiological variables, including erythrocyte lifespan, in hematologically normal individuals. We relate these findings to a number of HbA1c interpretation issues important for diabetes care: factors affecting hemoglobin glycation, relationship of HbA1c to glycemia, standardization of the HbA1c assay, and HbA1c relation to complications.

SUMMARY

There are an increasing number of clinical circumstances in which there are nuances to HbA1c interpretation such that standard norms for assessment of glycemic control or complication risk may need to be modified.

Fructosamine-3-kinase-related-protein phosphorylates glucitolamines on the C-4 hydroxyl: Novel substrate specificity of an enigmatic enzyme

Fructosamine-3-kinase (FN3K) phosphorylates fructosamines to fructosamine-3-phosphates. Recent data from FN3K-knockout mouse indicate that this phosphorylation results in deglycation of proteins modified by non-enzymatic glycation process. A homolog of FN3K, the FN3K-related-protein (FN3KRP) displays 65% amino acid sequence identity with FN3K and is highly conserved in evolution. However, FN3KRP does not phosphorylate substrates of FN3K such as fructoselysine and its physiological function remains unknown. We observed that human erythrocytes that contain both enzymes phosphorylate N-methylglucamine (meglumine) to two products. One of these is meglumine-3-phosphate (Meg3P), an activity consistent with the known substrate specificity of FN3K. Here, we identify the second product as meglumine-4-phosphate (Meg4P) and show that it is produced specifically by FN3KRP. While it is unlikely that meglumine is the physiological target of FN3KRP, this novel specificity, along with FN3KRPs known phosphorylation of some ketosamines on the C-3 hydroxyl may prove useful in identifying the physiological substrates of this kinase.

α-Thiolamines such as cysteine and cysteamine act as effective transglycating agents due to formation of irreversible thiazolidine derivatives

Non-enzymatic glycation of proteins and some phospholipids is considered to be an important factor in the genesis of diabetic complications. While this process has been viewed traditionally as entirely non-enzymatic and unidirectional, the discovery of fructosamine-3-phosphate (FN3K) and identification of FN3K-mediated deglycation mechanisms have made it apparent that non-enzymatic glycation is not unidirectional and that it can be reversed by deglycation reactions. While FN3K operates on ketosamines, the second intermediate in the non-enzymatic glycation cascade, we recently identified another potential deglycation mechanism that can operate on Schiff bases, the first intermediates of the non-enzymatic glycation process. The initial step in this postulated deglycation process is a transglycation reaction between a L.M.W. intracellular nucleophiles and a macromolecule-bound aldosamines, which regenerate unmodified proteins or phospholipids with a concomitant production of aldose-nucleophile transglycation byproducts. In vitro, transglycation occurs readily with amino acids, polyamines, thiols and thiolamines. There are indications that this reaction also occurs in vivo since in an initial GC/MS analysis of human urine we detected significant amounts of a transglycation product, glucose-cysteine (G-Cys), which was markedly increased in diabetics. Despite these encouraging early data, it is not yet clear to what extent transglycation is important in vivo and which intracellular nucleophiles are most relevant to this process. As discussed by us previously in this journal, one likely candidate for this role is glutathione since it is distributed universally and since there are well described mechanisms for removal of S-linked glutathione adducts from cells by the multi-drug-resistance (MDR) pumps. In this paper we report on another class of likely transglycating agents, alpha-thiolamines such as cysteine and cysteamine. While concentrations of these compounds in tissues are significantly lower than those of GSH, they react with Schiff bases more rapidly than GSH and, most significantly they form stable and irreversible thiazolidine products such as glucose-cysteine (G-Cys) and glucose-cysteamine (G-Ctm) that can subsequently be removed from cells. The possibility that alpha-thiolamines may play a physiological role as deglycating agents in vivo is very attractive since it suggests a possible strategy for inhibiting nonenzymatic glycation and diabetic complications that could be readily implemented through nutritional or pharmacological approaches. Such intervention is eminently feasible since there are at least three thiolamines already approved for human use. These include cysteamine used for the treatment of cystinosis; N-acetylcysteine utilized as a mucolytic and antioxidant agent, in the therapy of acetaminophen poisoning and radiocontrast-induced nephrotoxicity; and penicillamine used for treatment of Wilson's disease. Consequently, determining whether these compounds have the expected anti-glycating effects in vivo should be relatively straightforward.