Demonstration of increased concentrations of circulating glycated insulin in human Type 2 diabetes using a novel and specific radioimmunoassay

Hydroxytyrosol Selectively Affects Non-Enzymatic Glycation in Human Insulin and Protects by AGEs Cytotoxicity

Hydroxytyrosol (HT), the major phenolic compound in olive oil, is attracting increasing interest for its beneficial properties including a notable antioxidant and anti-inflammatory power. In this study, using a combination of biophysical and cell biology techniques, we have tested the role of HT in the formation of advanced glycation end-products (AGEs). AGEs have a key role in clinical sciences as they have been associated to diabetes, neurodegenerative and cardiovascular diseases. In addition, as the incidence of Alzheimer’s disease (AD) is strongly increased in diabetic patients, AGE formation is supposed to be involved in the development of the pathological hallmarks of AD. Our data show that HT selectively inhibits protein glycation reaction in human insulin, and it is able to counteract the AGE-induced cytotoxicity in human neurotypical cells by acting on SIRT1 level and oxidative stress, as well as on inflammatory response. This study identifies new beneficial properties for HT and suggests it might be a promising molecule in protecting against the AGE-induced toxicity, a key mechanism underlying the development and progression of neurodegenerative disorders.

Understanding the Role of Protein Glycation in the Amyloid Aggregation Process

Protein function and flexibility is directly related to the native distribution of its structural elements and any alteration in protein architecture leads to several abnormalities and accumulation of misfolded proteins. This phenomenon is associated with a range of increasingly common human disorders, including Alzheimer and Parkinson diseases, type II diabetes, and a number of systemic amyloidosis characterized by the accumulation of amyloid aggregates both in the extracellular space of tissues and as intracellular deposits. Post-translational modifications are known to have an active role in the in vivo amyloid aggregation as able to affect protein structure and dynamics. Among them, a key role seems to be played by non-enzymatic glycation, the most unwanted irreversible modification of the protein structure, which strongly affects long-living proteins throughout the body. This study provided an overview of the molecular effects induced by glycation on the amyloid aggregation process of several protein models associated with misfolding diseases. In particular, we analyzed the role of glycation on protein folding, kinetics of amyloid formation, and amyloid cytotoxicity in order to shed light on the role of this post-translational modification in the in vivo amyloid aggregation process.

Advanced glycation end products and their adverse effects: The role of autophagy

The critical roles played by advanced glycation endproducts (AGEs) accumulation in diabetes and diabetic complications have gained intense recognition. AGEs interfere with the normal functioning of almost every organ with multiple actions like apoptosis, inflammation, protein dysfunction, mitochondrial dysfunction, and oxidative stress. However, the development of a potential treatment strategy is yet to be established. Autophagy is an evolutionarily conserved cellular process that maintains cellular homeostasis with the degradation and recycling systems. AGEs can activate autophagy signaling, which could be targeted as a therapeutic strategy against AGEs induced problems. In this review, we have provided an overview of the adverse effects of AGEs, and we put forth the notion that autophagy could be a promising targetable strategy against AGEs.

Molecular investigation of glycated insulin-induced insulin resistance via insulin signaling and AGE-RAGE axis

Hyperglycemic condition in diabetes promotes glycation of various plasma proteins including insulin. Glycation of insulin has been reported to reduce its biological activity. Reduced biological activity of glycated insulin could be either due to reduced affinity for the insulin receptor and impaired insulin signaling, or it can act as a ligand for the receptor for advanced glycation end products (RAGE) and activates oxidative stress and pro-inflammatory pathways leading to insulin resistance. This study investigates the effect of glycated insulin on both insulin and RAGE signaling. Glycated insulin treatment to Chinese hamster ovary (CHO-IR-GLUT4) cells stably expressing insulin receptor (IR) and glucose transporter fused with a green fluorescent protein (GLUT4-GFP) resulted in the impairment of insulin signaling, as the phosphorylation of IR and AKT significantly reduced, which affected GLUT4 translocation and glucose uptake. Moreover, it also activated RAGE signaling as observed by increased expression of NADPH oxidase accompanied by an increase in reactive oxygen species (ROS). Immunofluorescence study indicated the translocation of NF-κB to the nucleus upon treatment of glycated insulin. This was associated with increased RAGE expression, Caspase 3, and cell death. Downregulation of RAGE with the losartan treatment restored the impaired insulin signaling and glucose uptake. Additionally, in silico study demonstrated that glycated insulin has reduced binding affinity to insulin receptor and increased binding affinity to RAGE. Overall, this study demonstrates the role of glycated insulin in exacerbating insulin resistance by impairing insulin signaling as well as stimulating AGE-RAGE signaling.

Phytometabolomic analysis of boiled rhizome of Nymphaea nouchali (Burm. f.) using UPLC-Q-TOF-MSE, LC-QqQ-MS & GC-MS and evaluation of antihyperglycemic and antioxidant activities

Phytometabolomic analysis of Nymphaea nouchali (Burm. F.) boiled rhizome was carried out utilizing UPLC-Q-TOF-MS^E, LC-QqQ-MS and GC-MS techniques and evaluated for antihyperglycemic and antioxidative stress potentials. Metabolomic analysis revealed presence of multiple antidiabetic and antioxidant compounds. Boiled rhizome powder exhibited potent antihyperglycemic activity against sugar-induced postprandial hyperglycemia in rats plausibly due to the presence of intestinal α-glucosidase inhibitory and augmenting cellular glucose uptake activities. It also prevented hyperglycemia-induced hemoglobin and insulin glycation. Rhizome displayed potent reducing power, effectively scavenged various reactive oxygen species. It displayed antioxidative stress potential in assuaging H₂O₂ induced erythrocyte hemolysis and antioxidant activity by inhibiting membrane lipid peroxidation. Boiled rhizome was also found to preserve the loss of cellular antioxidants under H₂O₂ induced oxidative stress and disturbances caused to mitochondrial membrane potential. This is the first research reporting boiled N. nouchali rhizome as an ideal food material to manage the cause of hyperglycemia and resultant oxidative stress.

Wnt Glycation Inhibits Canonical Signaling

Glycation occurs as a non-enzymatic reaction between amino and thiol groups of proteins, lipids, and nucleotides with reducing sugars or α-dicarbonyl metabolites. The chemical reaction underlying is the Maillard reaction leading to the formation of a heterogeneous group of compounds named advanced glycation end products (AGEs). Deleterious effects have been observed to accompany glycation such as alterations of protein structure and function resulting in crosslinking and accumulation of insoluble protein aggregates. A substantial body of evidence associates glycation with aging. Wnt signaling plays a fundamental role in stem cell biology as well as in regeneration and repair mechanisms. Emerging evidence implicates that changes in Wnt/β-catenin pathway activity contribute to the aging process. Here, we investigated the effect of glycation of Wnt3a on its signaling activity. Methods: Glycation was induced by treatment of Wnt3a-conditioned medium (CM) with glyoxal (GO). Effects on Wnt3a signaling activity were analyzed by Topflash/Fopflash reporter gene assay, co-immunoprecipitation, and quantitative RT-PCR. Results: Our data show that GO-treatment results in glycation of Wnt3a. Glycated Wnt3a suppresses β-catenin transcriptional activity in reporter gene assays, reduced binding of β-catenin to T-cell factor 4 (TCF-4) and extenuated transcription of Wnt/β-catenin target genes. Conclusions: GO-induced glycation impairs Wnt3a signaling function.

Glycated Insulin Exacerbates the Cytotoxicity of Human Islet Amyloid Polypeptides: a Vicious Cycle in Type 2 Diabetes

The aggregation of human islet amyloid polypeptide (hIAPP) is one of the triggering factors of type 2 diabetes mellitus (T2DM). hIAPP is cosynthesized, costored, and cosecreted with insulin in pancreatic β-cells, and insulin inhibits hIAPP aggregation. In T2DM patients, long-term hyperglycemia causes glycation of near 10% of total insulin. The glycation not only modifies insulin but also cross-links insulin into oligomers. However, the effect of glycated human insulin on hIAPP aggregation is unknown. In this study, four physiologically relevant monosaccharides, methylglyoxal, glucose, fructose, and ribose were used to glycate human insulin and two C-terminus truncated insulin analogues. Glycated insulin monomers or low molecular weight oligomers such as dimers significantly exacerbated the cytotoxicity of hIAPP. Notably, glycation-induced cross-linking of insulin inhibited the aggregation, membrane disruption, and cytotoxicity of hIAPP, which was corroborated by a control study using EGS-induced cross-linking of insulin or lysozyme. Removal of B29^Lys on the C terminus of the insulin B chain not only abolished glycation-induced cross-linking but also attenuated the aggravation effect of glycated insulin on hIAPP cytotoxicity. Taken together, this study reveals a vicious cycle in T2DM, that hyperglycemia-driven insulin glycation exacerbates the cytotoxicity of hIAPP, which accelerates β-cells death and further deteriorates T2DM.

Antibodies to post-translationally modified insulin in type 1 diabetes

AIM/HYPOTHESIS

Insulin is the most specific beta cell antigen and a potential primary autoantigen in type 1 diabetes. Insulin autoantibodies (IAAs) are the earliest marker of beta cell autoimmunity; however, only slightly more than 50% of children and even fewer adults newly diagnosed with type 1 diabetes are IAA positive. The aim of this investigation was to determine if oxidative post-translational modification (oxPTM) of insulin by reactive oxidants associated with islet inflammation generates neoepitopes that stimulate an immune response in individuals with type 1 diabetes.

METHODS

oxPTM of insulin was generated using ribose and various reactive oxygen species. Modifications were analysed by SDS-PAGE, three-dimensional fluorescence and MS. Autoreactivity to oxPTM insulin (oxPTM-INS) was observed by ELISA and western blotting, using sera from participants with type 1 or type 2 diabetes and healthy controls as probes. IAA was measured using the gold-standard radiobinding assay (RBA).

RESULTS

MS of oxPTM-INS identified chlorination of Tyr16 and Tyr26; oxidation of His5, Cys7 and Phe24; and glycation of Lys29 and Phe1 in chain B. Significantly higher binding to oxPTM-INS vs native insulin was observed in participants with type 1 diabetes, with 84% sensitivity compared with 61% sensitivity for RBA. oxPTM-INS autoantibodies and IAA co-existed in 50% of those with type 1 diabetes. Importantly 34% of those with diabetes who were IAA negative were oxPTM-INS positive. Altogether, 95% of participants with type 1 diabetes presented with autoimmunity to insulin by RBA, oxPTM-INS or both. Binding to oxPTM-INS was directed towards oxPTM-INS fragments with slower mobility than native insulin.

CONCLUSION/INTERPRETATION

These data suggest that oxPTM-INS is a potential autoantigen in individuals with new-onset type 1 diabetes.

Insulin glycation coupled with liposomal lipid peroxidation and microglial cell death

Type 2 diabetes is characterized jointly by hyperglycemia and hyperinsulinemia, which make insulin prone to glycation then fibrillation.

Emerging applications of metabolomic and genomic profiling in diabetic clinical medicine

Clinical and epidemiological metabolomics provides a unique opportunity to look at genotype-phenotype relationships as well as the body\x{2019}s responses to environmental and lifestyle factors. Fundamentally, it provides information on the universal outcome of influencing factors on disease states and has great potential in the early diagnosis, therapy monitoring, and understanding of the pathogenesis of disease. Diseases, such as diabetes, with a complex set of interactions between genetic and environmental factors, produce changes in the body\x{2019}s biochemical profile, thereby providing potential markers for diagnosis and initiation of therapies. There is clearly a need to discover new ways to aid diagnosis and assessment of glycemic status to help reduce diabetes complications and improve the quality of life. Many factors, including peptides, proteins, metabolites, nucleic acids, and polymorphisms, have been proposed as putative biomarkers for diabetes. Metabolomics is an approach used to identify and assess metabolic characteristics, changes, and phenotypes in response to influencing factors, such as environment, diet, lifestyle, and pathophysiological states. The specificity and sensitivity using metabolomics to identify biomarkers of disease have become increasingly feasible because of advances in analytical and information technologies. Likewise, the emergence of high-throughput genotyping technologies and genome-wide association studies has prompted the search for genetic markers of diabetes predisposition or susceptibility. In this review, we consider the application of key metabolomic and genomic methodologies in diabetes and summarize the established, new, and emerging metabolomic and genomic biomarkers for the disease. We conclude by summarizing future insights into the search for improved biomarkers for diabetes research and human diagnostics.

Role of Advanced Glycation End Products in Hypertension and Atherosclerosis: Therapeutic Implications

The vascular diseases, hypertension and atherosclerosis, affect millions of individuals worldwide, and account for a large number of deaths globally. A better understanding of the mechanism of these conditions will lead to more specific and effective therapies. Hypertension and atherosclerosis are both characterized by insulin resistance, and we suggest that this plays a major role in their etiology. The cause of insulin resistance is not known, but may be a result of a combination of genetic and lifestyle factors. In insulin resistance, alterations in glucose and lipid metabolism lead to the production of excess aldehydes including glyoxal and methylglyoxal. These aldehydes react non-enzymatically with free amino and sulfhydryl groups of amino acids of proteins to form stable conjugates called advanced glycation end products (AGEs). AGEs act directly, as well as via receptors to alter the function of many intra- and extracellular proteins including antioxidant and metabolic enzymes, calcium channels, lipoproteins, and transcriptional and structural proteins. This results in endothelial dysfunction, inflammation and oxidative stress. All these changes are characteristic of hypertension and atherosclerosis. Human and animal studies have demonstrated that increased AGEs are also associated with these conditions. A pathological role for AGEs is substantiated by studies showing that therapies that attenuate insulin resistance and/or lower AGEs, are effective in decreasing oxidative stress, lowering blood pressure, and attenuating atherosclerotic vascular changes. These interventions include lipoic acid and other antioxidants, AGE breakers or soluble receptors of AGEs, and aldehyde-binding agents like cysteine. Such therapies may offer alternative specific means to treat hypertension and atherosclerosis. An adjunct therapy may be to implement lifestyle changes such as weight reduction, regular exercise, smoking cessation, and increasing dietary intake of fruits and vegetables that also decrease insulin resistance as well as oxidative stress.

The effect of glycation on the chemical and enzymatic stability of the endogenous opioid peptide, leucine–enkephalin, and related fragments

Nonenzymatic glycation is a posttranslational modification of peptides and proteins by sugars, which, after a cascade of reactions, leads to the formation of a complex family of irreversibly changed adducts implicated in the pathogenesis of human diseases. The stability of the Amadori compounds, the last reversible intermediates, determines the further course of the reaction. To provide information concerning the fate of glycated opioid peptides introduced into human circulation, the enzymatic (80% human serum) and chemical (phosphate buffer) stability of three Amadori compounds related to the endogenous opioid pentapeptide, leucine-enkephalin (Tyr-Gly-Gly-Phe-Leu), and to its N-terminal fragments: N-(1-deoxy-D-fructos-1-yl)-l-tyrosyl-glycyl-glycyl-L-phenylalanyl-L-leucine, N-(1-deoxy-D-fructos-1-yl)-L-tyrosyl-glycyl-glycine, and N-(1-deoxy-D-fructos-1-yl)-L-tyrosine were investigated. The results obtained in human serum indicate that N-terminal glycation of leucine-enkephalin significantly enhances proteolytic stability. While leucine-enkephalin itself was rapidly degraded (t1/2 = 14.8 min), the glycated-derivative was slowly converted (t1/2 = 14 h) to the corresponding Amadori /compound of Tyr-Gly-Gly and Phe-Leu. In phosphate buffer, the rate of hydrolysis of the Amadori compounds depends on the structure and length of the peptide moiety as well as on the concentration of the phosphate buffer. The hydrolysis patterns for the Amadori compounds in phosphate buffer and in human serum were not the same and appear to be specific for each substrate.

Modulation of insulin action

Insulin is a key hormone regulating the control of metabolism and the maintenance of normoglycaemia and normolipidaemia. Insulin acts by binding to its cell surface receptor, thus activating the receptor's intrinsic tyrosine kinase activity, resulting in receptor autophosphorylation and phosphorylation of several substrates. Tyrosine phosphorylated residues on the receptor itself and on subsequently bound receptor substrates provide docking sites for downstream signalling molecules, including adapters, protein serine/threonine kinases, phosphoinositide kinases and exchange factors. Collectively, those molecules orchestrate the numerous insulin-mediated physiological responses. A clear picture is emerging of the way in which insulin elicits several intracellular signalling pathways to mediate its physiologic functions. A further challenge, being pursued by several laboratories, is to understand the molecular mechanisms that underlie insulin action at the peripheral level, deregulation of which ultimately leads to hyperglycaemia and Type 2 diabetes. We review how circulating factors such as insulin itself, TNF-alpha, interleukins, fatty acids and glycation products influence insulin action through insulin signalling molecules themselves or through other pathways ultimately impinging on the insulin-signalling pathway. Understanding how the mechanism by which molecular insulin action is modulated by these factors will potentially provide new targets for pharmacological agents, to enable the control of altered glucose and lipid metabolism and diabetes.

Effects of nateglinide on the secretion of glycated insulin and glucose tolerance in type 2 diabetes

AIMS

Glycation of insulin has been demonstrated within pancreatic beta-cells and the resulting impaired bioactivity may contribute to insulin resistance in diabetes. We used a novel radioimmunoassay to evaluate the effect of nateglinide on plasma concentrations of glycated insulin and glucose tolerance in type 2 diabetes.

METHODS

Ten patients (5 M/5 F, age 57.8+/-1.9 years, HbA(1c) 7.6+/-0.5%, fasting plasma glucose 9.4+/-1.2 mmol/l, creatinine 81.6+/-4.5 microM/l) received oral nateglinide 120 mg or placebo, 10 min prior to 75 g oral glucose in a random, single blind, crossover design, 1 week apart. Blood samples were taken for glycated insulin, glucose, insulin and C-peptide over 225 min.

RESULTS

Plasma glucose and glycated insulin responses were reduced by 9% (P=0.005) and 38% (P=0.047), respectively, following nateglinide compared with placebo. Corresponding AUC measures for insulin and C-peptide were enhanced by 36% (P=0.005) and 25% (P=0.007) by nateglinide.

CONCLUSIONS

Glycated insulin in type 2 diabetes is reduced in response to the insulin secretagogue nateglinide, resulting in preferential release of native insulin. Since glycated insulin exhibits impaired biological activity, reduced glycated insulin release may contribute to the antihyperglycaemic action of nateglinide.