Advanced glycation end products in uremia

Estimation of Advanced Glycation End Products in Selected Foods and Beverages by Spectrofluorimetry and ELISA

Advanced glycation end products (AGEs) are formed within the body as a part of normal metabolism and are also the by-products of cooking food. The elevated levels of AGEs in the body are considered pathogenic. The modern diets contain high levels of AGEs which are getting incorporated into the body AGEs pool and contribute to post-diabetic and age-related complications. The objective of the present study is to estimate the cross-linked AGEs (AGE-fluorescence) and the more stable carboxymethyl-lysine (CML) by spectrofluorimetry and ELISA in 58 kinds of foods in India. It was evident from the results that the foods cooked at higher temperatures showed high levels of AGEs. Among the studied foods, the highest fluorescence was observed in Biscuits 2 (362 AU), and the highest level of carboxymethyl lysine (CML) was found in Soya milk (659.3 ng/g). However, there was less correlation between the AGE-fluorescence and the CML content of the food samples. Processed food such as tomato sauce, chilli sauce, and cheese, along with western foods like chicken nuggets, pizza, and biscuits like Biscuits 2, are known to contain high levels of AGEs. In the present study a preliminary database of AGE-fluorescence and CML content of 58 foods was developed, which is the first attempt among Indian foods. Furthermore, elaborated database can be developed including maximum consumed foods in India which will help in suggesting a better diet for the diabetic population.

Effects of advanced glycation end products (AGEs) on the differentiation potential of primary stem cells: a systematic review

The formation and accumulation of advanced glycation end products (AGEs) have been associated with aging and the development, or worsening, of many degenerative diseases, such as atherosclerosis, chronic kidney disease, and diabetes. AGEs can accumulate in a variety of cells and tissues, and organs in the body, which in turn induces oxidative stress and inflammatory responses and adversely affects human health. In addition, under abnormal pathological conditions, AGEs create conditions that are not conducive to stem cell differentiation. Moreover, an accumulation of AGEs can affect the differentiation of stem cells. This, in turn, leads to impaired tissue repair and further aggravation of diabetic complications. Therefore, this systematic review clearly outlines the effects of AGEs on cell differentiation of various types of primary isolated stem cells and summarizes the possible regulatory mechanisms and interventions. Our study is expected to reveal the mechanism of tissue damage caused by the diabetic microenvironment from a cellular and molecular point of view and provide new ideas for treating complications caused by diabetes.

Dietary advanced glycation end-products (dAGEs) intake and its relation to sarcopenia and frailty - The Rotterdam Study

Studies on mice have shown a relationship between dietary intake of advanced glycation end-products (dAGEs) and deterioration of musculoskeletal health, but human studies are absent. We investigated the relationship between dietary intake of carboxymethyllysine (dCML) - an AGE prototype - and risk of sarcopenia at baseline and after 5 years of follow-up and a single evaluation of physical frailty in participants from the population-based Rotterdam Study. Appendicular lean mass (ALM) was obtained using insight dual-energy X-ray absorptiometry and hand grip strength (HGS) using a hydraulic hand dynamometer. Subjects with both low ALM and weak HGS were classified as having sarcopenia. Frailty (yes/no) was defined by presence of ≥3 and pre-frailty by presence of 1 or 2 components namely, exhaustion, weakness, slowness, weight loss or low physical activity. dCML was calculated using a food frequency questionnaire and dAGE databases. Logistic regression analysis was used to evaluate the odds of physical frailty and prevalent sarcopenia at baseline and follow-up and incident sarcopenia. 2782 participants with an age 66.4 ± 9.9 years and dCML intake 3.3 ± 1.3 mg/day, had data on sarcopenia at both time points. Of whom 84 had sarcopenia at baseline and 73 developed sarcopenia at follow-up. We observed an association of one SD increase in dCML intake with prevalent sarcopenia at baseline [odds ratio, OR = 1.27 (1.01-1.59)] and no association of dCML with incident sarcopenia at 5-year follow-up [OR = 1.12 (0.86-1.44)]. For frailty we analyzed 3577 participants, of whom 1972 were pre-frail and 158 were frail. We observed no association of dCML with either pre-frailty [OR = 0.99 (0.91-1.07)] or frailty [OR = 1.01 (0.83-1.22)] when non-frail subjects were used as reference. Our results show an association of dAGEs with sarcopenia cross-sectionally but not longitudinally where inconclusive findings are observed possibly due to a very low incidence of sarcopenia. There was no association with frailty cross-sectionally.

Skin autofluorescence, a non-invasive biomarker of advanced glycation end-products (AGEs), is associated with frailty: The Rotterdam study

Background

Accumulation of advanced glycation end-products (AGEs) in tissues has been linked to various age-related disease phenotypes. Therefore, we investigated the potential relationship between skin AGE accumulation and frailty.

Methods

A cross-sectional analysis was performed on 2 521 participants from the Rotterdam Study. Skin AGEs were assessed as skin autofluorescence (SAF) using the AGE reader™. We used 2 approaches to define frailty. Fried’s criteria, including weight loss, weakness, slow gait speed, exhaustion, and low physical activity, were used to define physical frailty (presence of ≥3 components) and prefrailty (presence of ≤2 components). Rockwood’s concept, including 38 deficits from physical and psychosocial health domains, was used to calculate the frailty index (score 0–1). Multinomial logistic and multivariate linear regression were used with SAF as exposure and physical frailty (ordinal) and frailty index (continuous) as outcome adjusting for age, sex, diabetes, renal function, socioeconomic status, and smoking status.

Results

The mean SAF was 2.39 ± 0.49 arbitrary units and the median age was 74.2 (14.0) years. Regarding physical frailty, 96 persons (4%) were frail and 1 221 (48%) were prefrail. Skin autofluorescence was associated with both being prefrail (odds ratio [95% confidence interval] = 1.29 [1.07–1.56]) and frail (1.87 [1.20–2.90]) compared with nonfrail. Regarding the frailty index, the median value was 0.14 (0.10–0.19) and higher SAF was also associated with a higher frailty index (coefficient, B = 0.017 (0.011–0.023]).

Conclusions

Higher skin AGEs are associated with both physical frailty and frailty index. Longitudinal studies are needed to evaluate the causality and the potential of SAF as a biomarker to screen frailty.

Lactose Glycation of the Maillard-Type Impairs the Benefits of Caseinate Digest to the Weaned Rats for Intestinal Morphology and Serum Biochemistry

The Maillard reaction between the lactose and milk proteins unavoidably occurs during the thermal treatment of milk. Although the impact of this reaction on protein nutrition and safety has been well-studied, whether a lactose glycation of milk proteins of the Maillard-type might affect the rats in their growth and intestinal morphology needs an investigation. In this study, caseinate and lactose-glycated caseinate were digested using pepsin and trypsin. Afterward, the resultant caseinate digest and glycated caseinate digest (lactose content of 13.5 g/kg of protein) at 100, 200, and 400 mg/kg body weight (BW)/d were assessed for their effects on the female weaned Wistar rats in terms of daily body weight gain, intestinal morphology, digestive and brush-border enzyme activities, as well as serum chemical indices. The results showed that glycated caseinate digest always showed a weaker effect on rat than caseinate digest either at the 0-7 or 0-28 d feeding stage, and more importantly, at the highest dose of 400 mg/kg BW/d, it caused obvious adverse effect on the rats, reflected by lower values of these indices. Compared with caseinate digest, glycated caseinate digest in the rats caused 0.9-15.4% and 10.6-49.7% decreases in average daily gain of BW and small intestinal length, 1.1-21.5% and 2.3-33.3% decreases in villus height and the ratio of villus height to crypt depth of the small intestine, or 0.3-57.6% and 0.2-55.7% decreases in digestive and critical brush-border enzyme activities, respectively. In addition, when the rats were fed with glycated caseinate digest, some serum indices related to oxidative stress status were enhanced dose-dependently. Lactose glycation of the Maillard-type is thus considered as a negative event of the Maillard reaction on milk proteins because this reaction might impair protein benefits to the body.

Dietary AGEs as Exogenous Boosters of Inflammation

Most chronic modern non-transmissible diseases seem to begin as the result of low-grade inflammation extending over prolonged periods of time. The importance of diet as a source of many pro-inflammatory compounds that could create and sustain such a low-grade inflammatory state cannot be ignored, particularly since we are constantly exposed to them during the day. The focus of this review is on specific components of the diet associated with inflammation, specifically advanced glycation end products (AGEs) that form during thermal processing of food. AGEs are also generated in the body in normal physiology and are widely recognized as increased in diabetes, but many people are unaware of the potential importance of exogenous AGEs ingested in food. We review experimental models, epidemiologic data, and small clinical trials that suggest an important association between dietary intake of these compounds and development of an inflammatory and pro-oxidative state that is conducive to chronic diseases. We compare dietary intake of AGEs with other widely known dietary patterns, such as the Mediterranean and the Dietary Approaches to Stop Hypertension (DASH) diets, as well as the Dietary Inflammation Index (DII). Finally, we delineate in detail the pathophysiological mechanisms induced by dietary AGEs, both direct (i.e., non-receptor-mediated) and indirect (receptor-mediated).

Consequences of Dicarbonyl Stress on Skeletal Muscle Proteins in Type 2 Diabetes

Skeletal muscle is the largest organ in the body and constitutes almost 40% of body mass. It is also the primary site of insulin-mediated glucose uptake, and skeletal muscle insulin resistance, that is, diminished response to insulin, is characteristic of Type 2 diabetes (T2DM). One of the foremost reasons posited to explain the etiology of T2DM involves the modification of proteins by dicarbonyl stress due to an unbalanced metabolism and accumulations of dicarbonyl metabolites. The elevated concentration of dicarbonyl metabolites (i.e., glyoxal, methylglyoxal, 3-deoxyglucosone) leads to DNA and protein modifications, causing cell/tissue dysfunctions in several metabolic diseases such as T2DM and other age-associated diseases. In this review, we recapitulated reported effects of dicarbonyl stress on skeletal muscle and associated extracellular proteins with emphasis on the impact of T2DM on skeletal muscle and provided a brief introduction to the prevention/inhibition of dicarbonyl stress.

In vitro and in vivo inhibition of maillard reaction products using amino acids, modified proteins, vitamins, and genistein: A review

Maillard reaction is known to result in loss of nutrients, particularly that of essential amino acids; decrease in digestibility and safety issues due to the development of toxic compounds. Maillard reaction products are also known to cause oxidation of tissues and inflammation, thus increasing the risk of cardiovascular diseases and diabetes. The aim of this review is to present a detailed information about the role of foodborne constituents as antibrowning agents to significantly reduce the harmful compounds like advanced glycation end products (AGEs) during food processing. This review includes strategies involving addition of amino acids, aromatic compounds, vitamins, modification of amino acids, and reducing sugars as antibrowning agents to reduce the AGEs. The role of Food borne functional ingredients such as catechin, epicathechin, luteolin, and ferulic acids as inhibitors of AGEs is also discussed. Among the naturally occurring inhibitors, genistein could be a crucial and safe agent to reduce reactive intermediates. PRACTICAL APPLICATIONS: Maillard reaction leads to changes in food color, protein functionality, protein digestibility, and loss of nutrient from foods. Maillard reaction products (MRPs) is also reported to be associated with various inflammatory conditions and may contribute to the progress of chronic diseases, including diabetes. It is hence necessary to reduce the MRPs, in both food and biological products, to offset this phenomenon. Among the strategies adopted till date, chemical agents could inhibit reactive carbonyl species and reactive oxygen species, but also are known to elicit serious side effects. Dietary flavonoids could be a very good inhibitor of MRPs both in biological and in food systems. It could be suggested that dietary flavonoids and isoflavones can be used as antibrowning agents in food and pharmaceutical industries particularly for targeted and sustained release of hypoglycemic drug in the intestines.

Beyond chronic kidney disease: the diagnosis of Renal Disease in the Elderly as an unmet need. A position paper endorsed by Italian Society of Nephrology (SIN) and Italian Society of Geriatrics and Gerontology (SIGG)

The dramatic increase in prevalence of chronic kidney disease (CKD) with ageing makes the recognition and correct referral of these patients of paramount relevance in order to implement interventions preventing or delaying the development of CKD complications and end-stage renal disease. Nevertheless, several issues make the diagnosis of CKD in the elderly cumbersome. Among these are age related changes in structures and functions of the kidney, which may be difficult to distinguish from CKD, and multimorbidity. Thus, symptoms, clinical findings and laboratory abnormalities should be considered as potential clues to suspect CKD and to suggest screening. Comprehensive geriatric assessment is essential to define the clinical impact of CKD on functional status and to plan treatment. Correct patient referral is very important: patients with stage 4-5 CKD, as well as those with worsening proteinuria or progressive nephropathy (i.e. eGFR reduction > 5 ml/year) should be referred to nephrologist. Renal biopsy not unfrequently may be the key diagnostic exam and should not be denied simply on the basis of age. Indeed, identifying the cause(s) of CKD is highly desirable to perform a targeted therapy against the pathogenetic mechanisms of CKD, which complement and may outperform in efficacy the general measures for CKD.

Dietary Advanced Glycation End Products: Digestion, Metabolism and Modulation of Gut Microbial Ecology

The formation of advanced glycation end products (AGEs) in foods is accelerated with heat treatment, particularly within foods that are cooked at high temperatures for long periods of time using dry heat. The modern processed diet is replete with AGEs, and excessive AGE consumption is thought to be associated with a number of negative health effects. Many dietary AGEs have high molecular weight and are not absorbed in the intestine, and instead pass through to the colon, where they are available for metabolism by the colonic bacteria. Recent studies have been conducted to explore the effects of AGEs on the composition of the gut microbiota as well as the production of beneficial microbial metabolites, in particular, short-chain fatty acids. However, there is conflicting evidence regarding the impact of dietary AGEs on gut microbiota reshaping, which may be due, in part, to the formation of alternate compounds during the thermal treatment of foods. This review summarises the current evidence regarding dietary sources of AGEs, their gastrointestinal absorption and role in gut microbiota reshaping, provides a brief overview of the health implications of dietary AGEs and highlights knowledge gaps and avenues for future study.

Role of advanced glycation end products in mobility and considerations in possible dietary and nutritional intervention strategies

Advanced glycation end products (AGEs), a group of compounds that are formed by non-enzymatic reactions between carbonyl groups of reducing sugars and free amino groups of proteins, lipids or nucleic acids, can be obtained exogenously from diet or formed endogenously within the body. AGEs accumulate intracellularly and extracellularly in all tissues and body fluids and can cross-link with other proteins and thus affect their normal functions. Furthermore, AGEs can interact with specific cell surface receptors and hence alter cell intracellular signaling, gene expression, the production of reactive oxygen species and the activation of several inflammatory pathways. High levels of AGEs in diet as well as in tissues and the circulation are pathogenic to a wide range of diseases. With respect to mobility, AGEs accumulate in bones, joints and skeletal muscles, playing important roles in the development of osteoporosis, osteoarthritis, and sarcopenia with aging. This report covered the related pathological mechanisms and the potential pharmaceutical and dietary intervention strategies in reducing systemic AGEs. More prospective studies are needed to determine whether elevated serum AGEs and/or skin autofluorescence predict a decline in measures of mobility. In addition, human intervention studies are required to investigate the beneficial effects of exogenous AGEs inhibitors on mobility outcomes.

Dietary Advanced Glycosylation End-Products (dAGEs) and Melanoidins Formed through the Maillard Reaction: Physiological Consequences of their Intake

The main purpose of this review is to clarify whether the consumption of food rich in melanoidins and dietary advanced glycosylation end-products (dAGEs) is harmful or beneficial for human health. There are conflicting results on their harmful effects in the literature, partly due to a methodological issue in how dAGEs are determined in food. Melanoidins have positive functions particularly within the gastrointestinal tract, whereas the intake of dAGEs has controversial physiological consequences. Most of the in vivo intervention trials were done comparing boiled versus roasted diet (low and high dAGE, respectively). However, these studies can be biased by different lipid oxidation and by different calorie density of foods in the two conditions. The attraction that humans have to cooked foods is linked to the benefits they have had during mankind's evolution. The goal for food technologists is to design low-energy-dense products that can satisfy humans' attraction to rewarding cooked foods.

Stirring the Pot: Can Dietary Modification Alleviate the Burden of CKD?

Diet is one of the largest modifiable risk factors for chronic kidney disease (CKD)-related death and disability. CKD is largely a progressive disease; however, it is increasingly appreciated that hallmarks of chronic kidney disease such as albuminuria can regress over time. The factors driving albuminuria resolution remain elusive. Since albuminuria is a strong risk factor for GFR loss, modifiable lifestyle factors that lead to an improvement in albuminuria would likely reduce the burden of CKD in high-risk individuals, such as patients with diabetes. Dietary therapy such as protein and sodium restriction has historically been used in the management of CKD. Evidence is emerging to indicate that other nutrients may influence kidney health, either through metabolic or haemodynamic pathways or via the modification of gut homeostasis. This review focuses on the role of diet in the pathogenesis and progression of CKD and discusses the latest findings related to the mechanisms of diet-induced kidney disease. It is possible that optimizing diet quality or restricting dietary intake could be harnessed as an adjunct therapy for CKD prevention or progression in susceptible individuals, thereby reducing the burden of CKD.

Advanced glycation end products upregulate lysyl oxidase and endothelin-1 in human aortic endothelial cells via parallel activation of ERK1/2-NF-κB and JNK-AP-1 signaling pathways

Endothelial dysfunction involves deregulation of the key extracellular matrix (ECM) enzyme lysyl oxidase (LOX) and the vasoconstrictor protein, endothelin-1 (ET-1), whose gene expression can be modulated by the transcriptional activators nuclear factor kappa B (NF-κB) and activator protein-1 (AP-1). Advanced glycation end products (AGEs) present an aggravating factor of endothelial dysfunction which upon engagement to their receptor RAGE induce upregulation of mitogen-activated protein kinases (MAPKs), leading to NF-κB and AP-1 potentiation. We hypothesized that AGEs could induce NF-κΒ- and AP-1-dependent regulation of LOX and ET-1 expression via the AGE/RAGE/MAPK signaling axis. Western blot, real-time qRT-PCR, FACS analysis and electrophoretic mobility-shift assays were employed in human aortic endothelial cells (HAECs) following treatment with AGE-bovine serum albumin (AGE-BSA) to investigate the signaling pathway towards this hypothesis. Furthermore, immunohistochemical analysis of AGEs, RAGE, LOX and ET-1 expression was conducted in aortic endothelium of a rat experimental model exposed to high- or low-AGE content diet. HAECs exposed to AGE-BSA for various time points exhibited upregulation of LOX and ET-1 mRNA levels in a dose- and time-dependent manner. Exposure of HAECs to AGE-BSA also showed specific elevation of phospho(p)-ERK1/2 and p-JNK levels in a dose- and time-dependent fashion. AGE administration significantly increased NF-κΒ- and AP-1-binding activity to both LOX and ET-1 cognate promoter regions. Moreover, LOX and ET-1 overexpression in rat aortic endothelium upon high-AGE content diet confirmed the functional interrelation of these molecules. Our findings demonstrate that AGEs trigger NF-κΒ- and AP-1-mediated upregulation of LOX and ET-1 via the AGE/RAGE/MAPK signaling cascade in human endothelial cells, thus contributing to distorted endothelial homeostasis by impairing endothelial barrier function, altering ECM biomechanical properties and cell proliferation.

Creatine is a scavenger for methylglyoxal under physiological conditions via formation of N-(4-methyl-5-oxo-1-imidazolin-2-yl)sarcosine (MG-HCr)

Following incubation of methylglyoxal and creatine under physiological conditions, N-(4-methyl-5-oxo-1-imidazolin-2-yl)sarcosine (MG-HCr) was isolated and identified by NMR and mass spectrometry. Due to its rapid formation, MG-HCr represents a specific product following "scavenging" of methylglyoxal by creatine. Using hydrophilic interaction chromatography coupled to mass spectrometry, MG-HCr was analyzed in urine samples of healthy volunteers. Daily MG-HCr excretion of nonvegetarians ranged from 0.35 to 3.84 μmol/24 h urine (median: 0.90 μmol/24 h urine) and of vegetarians from 0.11 to 0.31 μmol/24 h urine (median: 0.19 μmol/24 h urine), indicating that formation of MG-HCr in vivo is influenced by the dietary intake of creatine. The trapping of methylglyoxal by creatine may delay the formation of advanced glycation compounds in vivo and, therefore, could be of special importance in situations in which the body has to deal with pathophysiologically increased amounts of dicarbonyl compounds ("carbonyl stress"), for instance in diabetic patients.

Baking, ageing, diabetes: a short history of the Maillard reaction

The reaction of reducing carbohydrates with amino compounds described in 1912 by Louis-Camille Maillard is responsible for the aroma, taste, and appearance of thermally processed food. The discovery that non-enzymatic conversions also occur in organisms led to intensive investigation of the pathophysiological significance of the Maillard reaction in diabetes and ageing processes. Dietary Maillard products are discussed as "glycotoxins" and thus as a nutritional risk, but also increasingly with regard to positive effects in the human body. In this Review we give an overview of the most important discoveries in Maillard research since it was first described and show that the complex reaction, even after over one hundred years, has lost none of its interdisciplinary actuality.

Pathophysiological importance of aggregated damaged proteins

Reactive oxygen species (ROS) are formed continuously in the organism even under physiological conditions. If the level of ROS in cells exceeds the cellular defense capacity, components such as RNA/DNA, lipids, and proteins are damaged and modified, thus affecting the functionality of organelles as well. Proteins are especially prominent targets of various modifications such as oxidation, glycation, or conjugation with products of lipid peroxidation, leading to the alteration of their biological function, nonspecific interactions, and the production of high-molecular-weight protein aggregates. To ensure the maintenance of cellular functions, two proteolytic systems are responsible for the removal of oxidized and modified proteins, especially the proteasome and organelles, mainly the autophagy-lysosomal systems. Furthermore, increased protein oxidation and oxidation-dependent impairment of proteolytic systems lead to an accumulation of oxidized proteins and finally to the formation of nondegradable protein aggregates. Accordingly, the cellular homeostasis cannot be maintained and the cellular metabolism is negatively affected. Here we address the current knowledge of protein aggregation during oxidative stress, aging, and disease.

Dietary influence on urinary excretion of 3-deoxyglucosone and its metabolite 3-deoxyfructose

3-Deoxyglucosone (3-DG), a reactive 1,2-dicarbonyl compound derived from d-glucose in food and in vivo, is an important precursor for advanced glycation endproducts (AGEs). At present, virtually no information about the metabolic transit of dietary 3-DG is available. One possible metabolic pathway of 3-DG during digestion is enzymatic transformation to less reactive compounds such as 3-deoxyfructose (3-DF). To study the handling of dietary 1,2-dicarbonyl compounds by the human body, 24 h urinary excretion of 3-DG and its metabolite, 3-deoxyfructose, was investigated. Urinary 3-DG and 3-DF excretion was monitored for nine healthy volunteers following either a diet with no dietary restrictions or a diet avoiding the ingestion of 3-DG and other Maillard reaction products ("raw food" diet). During the "raw food" diet, the urinary 3-DG and 3-DF excretion decreased approximately to 50% compared to the excretions during the diet with no restrictions. When subjects received a single dose of wild honey (50 g) naturally containing a defined amount of 3-DG (505 μmol), median excretion of 3-DG and 3-DF increased significantly from 4.6 and 77 to 7.5 and 147 μmol/day, respectively. The obtained experimental data for the first time demonstrate a dietary influence on urinary 3-DG and 3-DF levels in healthy human subjects.

Advanced glycation end products of beta2-microglobulin in uremic patients as determined by high resolution mass spectrometry

By using a high resolution top-down and bottom-up approach we identified and characterized the AGEs of beta2-microglobulin (β2-m) formed by incubating the protein in the presence of glucose and of the main reactive carbonyl species. Glucose induced glycation on the N-terminal residue, while glyoxal (GO) and methylglyoxal (MGO) covalently reacted with Arg3. Carboxymethyl (CM-R) and imidazolinone (R-GO) derivatives were identified in the case of GO and carboxyethyl arginine (CE-R) and methyl-imidazolinone (R-MGO) for MGO. Interestingly, α,β-unsaturated aldehydes [4-hydroxy-2-nonenal (HNE); 4-oxo-2-nonenal (ONE); acrolein (ACR)] did not induce any covalent modifications up to 100μM. The different reactivity of β2-m towards the different RCS was then rationalized by molecular modeling studies. The MS method was then applied to fully characterize the AGEs of β2-m isolated from the urine of uremic subjects. CM-R, CE-R and R-MGO were easily identified on Arg3 and their relative abundance in respect to the native protein determined by a semi-quantitative approach. Overall, the AGEs content of urinary β2-m ranged from 0.2 to 1% in uremic subjects. The results here reported offer novel insights and technical achievements for a potential biological role of AGEs-β2-m in pathological conditions.

Advanced glycation end product-induced astrocytic differentiation of cultured neurospheres through inhibition of Notch-Hes1 pathway-mediated neurogenesis

This study aims to investigate the roles of the Notch-Hes1 pathway in the advanced glycation end product (AGE)-mediated differentiation of neural stem cells (NSCs). We prepared pLentiLox3.7 lentiviral vectors that express short hairpin RNA (shRNA) against Notch1 and transfected it into NSCs. Cell differentiation was analyzed under confocal laser-scanning microscopy. The percentage of neurons and astrocytes was quantified by normalizing the total number of TUJ1⁺ (Neuron-specific class III β-tubulin) and GFAP⁺ (Glial fibrillary acidic protein) cells to the total number of Hoechst 33342-labeled cell nuclei. The protein and gene expression of Notch-Hes1 pathway components was examined via western blot analysis and real-time PCR. After 1 week of incubation, we found that AGE-bovine serum albumin (BSA) (400 μg/mL) induced the astrocytic differentiation of cultured neurospheres and inhibited neuronal formation. The expression of Notch-Hes1 pathway components was upregulated in the cells in the AGE-BSA culture medium. Immunoblot analysis indicated that shRNA silencing of Notch1 expression in NSCs significantly increases neurogenesis and suppresses astrocytic differentiation in NSCs incubated with AGE-BSA. AGEs promote the astrocytic differentiation of cultured neurospheres by inhibiting neurogenesis through the Notch-Hes1 pathway, providing a potential therapeutic target for hyperglycemia-related cognitive deficits.

Advanced glycation endproducts in food and their effects on health

Advanced glycation endproducts (AGEs) form by Maillard-reactions after initial binding of aldehydes with amines or amides in heated foods or in living organisms. The mechanisms of formation may include ionic as well as oxidative and radical pathways. The reactions may proceed within proteins to form high-molecular weight (HMW) AGEs or among small molecules to form low-molecular weight (LMW) AGEs. All free amino acids form AGEs, but lysine or arginine side chains dominate AGE formation within proteins. The analysis of AGEs in foods and body fluids is most often performed by ELISA or LC-MS; however, none of the methodologies cover all HMW and LMW AGEs. Most research is, therefore, carried out using 'representative' AGE compounds, most often N(ε)-carboxymethyl-lysine (CML). Only LMW AGEs, including peptide-bound forms, and carbonyls may be absorbed from the gut and contribute to the body burden of AGEs. Some AGEs interact with specific pro- or anti-inflammatory receptors. Most studies on the biological effects of AGEs have been carried out by administering heated foods. The pro-inflammatory and deteriorating biological effects of AGEs in these studies, therefore, need further confirmation. The current review points out several research needs in order to address important questions on AGEs in foods and health.

Effects of olmesartan on arterial stiffness in rats with chronic renal failure

Background

It has been suggested that the antioxidant properties of olmesartan (OLM), an angiotensin II type 1 receptor (AT₁R) blocker, contribute to renal protection rather than blood pressure lowering effects despite the fact that causal relationships between hypertension and renal artery disease exist. This study aimed to examine the hypothesis whether the antioxidative activities of OLM were correlated to arterial stiffness, reactive oxygen species and advanced glycation end products (AGEs) formation in rats with chronic renal failure (CRF).

Methods

CRF rats were induced by 5/6 nephrectomy and randomly assigned to an OLM (10 mg/day) group or a control group. Hemodynamic states, oxidative stress, renal function and AGEs were measured after 8 weeks of OLM treatment.

Results

All the hemodynamic derangements associated with renal and cardiovascular dysfunctions were abrogated in CRF rats receiving OLM. Decreased cardiac output was normalized compared to control (p <0.05). Mean aortic pressure, total peripheral resistance and left ventricular weight/body weight ratio were reduced by 21.6% (p <0.05), 28.2% (p <0.05) and 27.2% ((p <0.05). OLM also showed beneficial effects on the oscillatory components of the ventricular after-load, including 39% reduction in aortic characteristic impedance (p < 0.05), 75.3% increase in aortic compliance (p <0.05) and 50.3% increase in wave transit time (p < 0.05). These results implied that OLM attenuated the increased systolic load of the left ventricle and prevented cardiac hypertrophy in CRF rats. Improved renal function was also reflected by increases in the clearances of BUN (28.7%) and serum creatinine (SCr, 38.8%). In addition to these functional improvements, OLM specifically reduced the levels of malondialdehyde (MDA) equivalents in aorta and serum by 14.3% and 25.1%, as well as the amount of AGEs in the aortic wall by 32% (p < 0.05) of CRF rats.

Conclusion

OLM treatment could ameliorate arterial stiffness in CRF rats with concomitant inhibition of MDA and AGEs levels through the reduction of oxidative stress in aortic wall.

Prognostic potential and tumor growth-inhibiting effect of plasma advanced glycation end products in non-small cell lung carcinoma

The plasma fluorescence related to the standard fluorescence of advanced glycation end products (AGEs) is a simple measurable blood parameter for distinct diseases but its importance in human cancer, including non-small cell lung carcinoma (NSCLC), is unknown. Plasma samples of 70 NSCLC patients who underwent resection surgery of the tumor were analyzed for the distinct AGE-related fluorescence at 370 nm excitation/440 nm emission. In a retrospective study, we tested the prognostic relevance of this AGE-related plasma fluorescence. The effect of circulating AGEs on the NSCLC growth was studied experimentally in vitro and in vivo. NSCLC patients with high (> median) AGE-related plasma fluorescence were characterized by a later reoccurrence of the tumor after curative surgery and a higher survival rate compared with patients with low plasma fluorescence (25% versus 47% 5-y survival, P = 0.011). Treating NSCLC cell spheroids with patients' plasma showed an inverse correlation between the growth of spheroids in vitro and the individual AGE-related fluorescence of each plasma sample. To confirm the impact of circulating AGEs on the NSCLC progression, we studied the NSCLC growth in mice whose circulating AGE level was elevated by AGE-rich diet. In vivo tumorigenicity assays demonstrated that mice with higher levels of circulating AGEs developed smaller tumors than mice with normal AGE levels. The AGE-related plasma fluorescence has prognostic relevance for NSCLC patients in whom the tumor growth-inhibiting effect of circulating AGEs might play a critical role.

Urea kinetics and intermittent dialysis prescription in small animals

Hemodialysis improves survival for animals with acute kidney injury beyond what would be expected with conventional management of the same animals. Clinical evidence and experience in human patients suggest a role for earlier intervention with renal replacement to avoid the morbidity of uremia and to promote better metabolic stability and recovery. For a large population of animal patients, it is the advanced standard for the management of acute and chronic uremia, life-threatening poisoning, and fluid overload for which there is no alternative therapy.

Lysyl oxidase interacts with AGE signalling to modulate collagen synthesis in polycystic ovarian tissue

Connective tissue components – collagen types I, III and IV – surrounding the ovarian follicles undergo drastic changes during ovulation. Abnormal collagen synthesis and increased volume and density of ovarian stroma characterize the polycystic ovary syndrome (PCOS). During the ovulatory process, collagen synthesis is regulated by prolyl hydroxylase and lysyl oxidase (LOX) activity in ovarian follicles. LOX catalyzes collagen and elastin cross-linking and plays essential role in coordinating the control of ovarian extracellular matrix (ECM) during follicular development. We have recently shown accumulation of advanced glycation end products (AGEs), molecules that stimulate ECM production and abnormal collagen cross-linking, in ovarian tissue. However, the possible link between LOX and AGEs-induced signalling in collagen production and stroma formation in ovarian tissue from PCOS remains elusive. The present study investigates the hypothesis of AGE signalling pathway interaction with LOX gene activity in polycystic ovarian (PCO) tissue. We show an increased distribution and co-localization of LOX, collagen type IV and AGE molecules in the PCO tissue compared to control, as well as augmented expression of AGE signalling mediators/effectors, phospho(p)-ERK, phospho(p)-c-Jun and nuclear factor κB (NF-κB) in pathological tissue. Moreover, we demonstrate binding of AGE-induced transcription factors, NF-κB and activator protein-1 (AP-1) on LOX promoter, indicating a possible involvement of AGEs in LOX gene regulation, which may account for the documented increase in LOX mRNA and protein levels compared to control. These findings suggest that deposition of excess collagen in PCO tissue that induces cystogenesis may, in part, be due to AGE-mediated stimulation of LOX activity.

Glycolaldehyde-modified β-lactoglobulin AGEs are unable to stimulate inflammatory signaling pathways in RAGE-expressing human cell lines

SCOPE

Advanced glycation endproducts (AGEs) are suspected to stimulate inflammatory signaling pathways in target tissues via activation of the receptor for AGEs. Endotoxins are generally recognized as potential contamination of AGE preparations and stimulate biological actions that are very similar as or identical to those induced by AGEs.

METHODS AND RESULTS

In our study, we used glycolaldehyde-modified β-lactoglobulin preparations as model AGEs and employed two methods to remove endotoxin using either affinity columns or extraction with Triton X-114 (TX-114). Affinity column-purified AGEs retained their ability to stimulate inflammatory signaling as measured by mRNA expression of inflammatory cytokines in the human lung epithelial cell line Beas2b. However, glycolaldehyde-modified AGEs purified by extraction with TX-114 did not show any stimulation of mRNA expression of inflammatory cytokines. The presence of a cell stimulating endotoxin-like activity was demonstrated in the detergent phase after extraction with TX-114, thus indicating that not AGEs but a lipophilic contamination was responsible for the stimulation of inflammatory signaling.

CONCLUSION

Our results demonstrate that glycolaldehyde-modified AGEs are unable to induce inflammatory signaling in receptor for AGE-expressing cells. The observed cell-activating activity can be ascribed to an endotoxin-like lipophilic contamination present in AGE preparations and affinity column purification was insufficient to remove this contamination.

Advanced glycation end-products and the kidney

BACKGROUND

Advanced glycation end-products (AGEs) are increased in situations with hyperglycemia and oxidative stress such as diabetes mellitus. They are products of nonenzymatic glycation and oxidation of proteins and lipids. The kidney plays an important role in clearance and metabolism of AGEs.

METHODS

Medline and other relevant databases were searched. In addition, key review articles were scanned for relevant original publication. Finally, original data from our research group were also included.

RESULTS

Kidney podocytes and endothelial cells express specific receptors for AGEs. Their activation leads to multiple pathophysiological effects including hypertrophy with cell cycle arrest and apoptosis, altered migration, and generation of proinflammatory cytokines. AGEs have been primarily implicated in the pathophysiology of diabetic nephropathy and diabetic microvascular complications. AGEs are also involved in other primary renal diseases as well as in the development and progression of atherosclerosis. However, serum or plasma concentrations of AGEs do not correlate well with cardiovascular events in patients with chronic kidney disease (CKD). This is likely due to the fact that serum concentrations failed to correlate with AGEs deposited in target tissues. Several inhibitors of the AGE-RAGE axis are currently tested for various indications.

CONCLUSION

AGEs and their receptors are involved in the pathogenesis of vascular and kidney disease. The role of circulating AGEs as biomarkers for cardiovascular risk estimation is questionable. Whether putative inhibitors of AGEs will get the maturity for its therapeutic use in the future remains open.

Identification and quantification of methylglyoxal as the dominant antibacterial constituent of Manuka (Leptospermum scoparium) honeys from New Zealand

The 1,2-dicarbonyl compounds 3-deoxyglucosulose (3-DG), glyoxal (GO), and methylglyoxal (MGO) were measured as the corresponding quinoxalines after derivatization with orthophenylendiamine using RP-HPLC and UV-detection in commercially available honey samples. Whereas for most of the samples values for 3-DG, MGO, and GO were comparable to previously published data, for six samples of New Zealand Manuka (Leptospermum scoparium) honey very high amounts of MGO were found, ranging from 38 to 761 mg/kg, which is up to 100-fold higher compared to conventional honeys. MGO was unambigously identified as the corresponding quinoxaline via photodiodearry detection as well as by means of mass spectroscopy. Antibacterial activity of honey and solutions of 1,2-dicarbonyl towards Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) were analyzed using an agar well diffusion assay. Minimum concentrations needed for inhibition of bacterial growth (minimum inhibitory concentration, MIC) of MGO were 1.1 mM for both types of bacteria. MIC for GO was 6.9 mM (E. coli) or 4.3 mM (S. aureus), respectively. 3-DG showed no inhibition in concentrations up to 60 mM. Whereas most of the honey samples investigated showed no inhibition in dilutions of 80% (v/v with water) or below, the samples of Manuka honey exhibited antibacterial activity when diluted to 15-30%, which corresponded to MGO concentrations of 1.1-1.8 mM. This clearly demonstrates that the pronounced antibacterial activity of New Zealand Manuka honey directly originates from MGO.

Immunogenicity of advanced glycation end products in diabetic patients and in nephropathic non-diabetic patients on hemodialysis or after renal transplantation

Advanced glycation end products (AGE) increase as a consequence of diabetic hyperglycemia and, in nephropathic patients, following renal function loss. Protein-bound AGE behave as immunogens, inducing formation of specific antibodies (Ab-AGE). In this work AGE immunogenicity was studied in 42 diabetic patients, 26 nephropathic patients on hemodialysis and 26 patients with end-stage renal disease who underwent kidney transplantation and in 20 normal subjects. Non-oxidation-derived AGE (nox-AGE), oxidation-derived AGE (ox-AGE) and Ab-AGE were measured by competitive or direct enzyme-linked immunosorbent assay (ELISA) and circulating immune complexes (CIC) by C1q ELISA. Nox- AGE increased significantly in all patient groups (p < or = 0.05 to < or = 0.0001) except in patients on hemodialysis for less than 6 yr. Ox-AGE were only significantly increased in patients transplanted more than 3 yr previously (p < 0.05). Ab-AGE were significantly lower than controls in both diabetic groups and in patients on hemodialysis for more than 6 yr (p < 0.005 to < 0.0001) and not unlike controls in the other groups. These results demonstrate that hemodialysis or renal transplantation can, initially, reduce either nox- or ox-AGE levels, which however go back to being high in time. Renal transplantation fails to normalize nox-AGE. More importantly, plasma Ab-AGE levels are reduced or unchanged in all patient groups in comparison with controls, despite higher circulating AGE levels. This suggests the importance of tissue-bound AGE as Ab-AGE targets. Additional interventions are needed to control AGE levels in treated nephropathic patients. The search and quantification of specific Ab-AGE would give more meaningful results if performed over specific tissue specimens.

Dicarbonyl-mediated protein modifications affect matrix metalloproteinase (MMP) activity

Advanced age is linked with an increased incidence of epithelial tumours (carcinomas) including lung tumours. However, a slowing rate of the increase of age-specific cancer incidence is demonstrated at very advanced ages, and elderly patients also develop less invasive and metastatic tumours than their younger counterparts. Matrix metalloproteinases (MMPs) are commonly upregulated in the stromal compartment of the carcinoma tissue and are believed to promote invasion and metastasis. As the increased serum and tissue level of advanced glycation end products (AGEs) is a characteristic feature of old humans, our study focused on the impact of AGEs on the activity of MMPs released from lung fibroblasts (WI- 38). The collagen gel zymography technique showed the primary presence of MMP-2 in the conditioned medium of the WI-38 fibroblasts, which was even higher in senescent WI-38 fibroblasts. Subsequent treatment of the WI- 38 conditioned medium with the dicarbonyl compound glyoxal, a highly reactive precursor of the AGE formation, resulted in a dose-dependent reduction of the MMP-2 activity. Therefore, our study suggests that the age-associated increase in AGEs might be one potential host factor responsible for the less invasiveness of tumours at very advanced age.

Dietary advanced glycation end products--a risk to human health? A call for an interdisciplinary debate

Physiological consequences resulting from protein-bound Maillard compounds in foods must be discussed carefully. This was the idea behind the debate, which is put for discussion by the papers by Sebekova and Somoza, who argued for the motion that dietary advanced glycation end products (AGEs) are a health risk, and by Ames, who provided evidence against the motion. In this two excellent reviews, numerous arguments based on papers published in high-impact journals are given for each of the opinions. The fact that no final conclusion can be drawn, may reflect the need for a more comprehensive examination of this issue in the future. For a deeper understanding of biological consequences resulting from heated foods, the relationships between well-defined biological effects and well-characterized chemical structures must be studied. Prerequisite for this is profound chemistry--pure compounds, exact concentrations, and unambiguous analytical techniques. A real "risk assessment" is much too complex than to leave it up to one discipline alone. It must be a comprehensive and interdisciplinary approach, joining the resources of biology, medicine, and chemistry.

Isolation and determination of alpha-dicarbonyl compounds by RP-HPLC-DAD in green and roasted coffee

Glyoxal, methylglyoxal, and diacetyl formed as Maillard reaction products in heat-treated food were determined in coffee extracts (coffee brews) obtained from green beans and beans with different degrees of roast. The compounds have been reported to be mutagenic in vitro and genotoxic in experimental animals in a number of papers. More recently, alpha-dicarbonyl compounds have been implicated in the glycation process. Our data show that small amounts of glyoxal and methylglyoxal occur naturally in green coffee beans. Their concentrations increase in the early phases of the roasting process and then decline. Conversely, diacetyl is not found in green beans and forms later in the roasting process. Therefore, light and medium roasted coffees had the highest glyoxal and methylglyoxal content, whereas dark roasted coffee contained smaller amounts of glyoxal, methylglyoxal, and diacetyl. For the determination of coffee alpha-dicarbonyl compounds, a reversed-phase high performance liquid chromatography with a diode array detector (RP-HPLC-DAD) method was devised that involved the elimination of interfering compounds, such as chlorogenic acids, by solid phase extraction (SPE) and their derivatization with 1,2-diaminobenzene to give quinoxaline derivatives. Checks of SPE and derivatization conditions to verify recovery and yield, respectively, resulted in rates of 100%. The results of the validation procedure showed that the proposed method is selective, precise, accurate, and sensitive.

Accumulation of free adduct glycation, oxidation, and nitration products follows acute loss of renal function

Glycation, oxidation, and nitration of endogenous proteins occur spontaneously and these modifications are also present in foods. Increased levels of these chemical changes are associated with chronic renal failure; however, little is known about acute kidney failure. We measured these modifications of plasma protein and related free adducts in plasma following bilateral nephrectomy and bilateral ureteral obstruction. Advanced glycation end-product (AGE) residues of plasma protein were increased 3 h post-surgery, and thereafter slowly decreased in all groups, reflecting changes in plasma protein synthesis and transcapillary flow post-surgery. Ureteral ligation increased oxidation and nitration adduct residues. There were, however, marked increases in AGE, dityrosine, or 3-nitrotyrosine free adducts in both nephrectomized and ureter-ligated rats compared to rats that had undergone sham operations. There were lower modified adduct concentrations in the ureter-ligated compared to the nephrectomized rats, reflecting residual glomerular filtration and tubular removal. There was no increase in glycated, oxidized, and nitrated proteins. Glyoxal and methylglyoxal were also increased in both renal failure models. Our study shows that the acute loss of renal function and urinary excretion leads to the accumulation of AGE, oxidation, and nitration free adducts in the plasma.

Analysis and biological properties of amino acid derivates formed by Maillard reaction in foods

Maillard reaction products (MRPs), especially early stage MRPs and melanoidins, are currently gaining a lot of attention due to their reported health-promoting properties and their potential to be used as functional food ingredients. It is often not clear which specific biological function is assigned to which MRP, due to the large amount of MRPs formed during the reaction and difficulties in their purification and identification. This paper provides an overview of amino acid derivatives such as Amadori compounds, carboxymethyllysine, pyrraline, cross-linking products and melanoidins, which can be formed by Maillard reaction in foods, their biological properties and the analytical tools commonly employed for their determination.

Transepithelial flux of early and advanced glycation compounds across Caco-2 cell monolayers and their interaction with intestinal amino acid and peptide transport systems

Maillard products arise from condensation reactions between amino acids or proteins with reducing sugars during food processing. As ubiquitous components of human food, these early or advanced glycation products may be subject to intestinal absorption. The present study was performed to investigate the intestinal uptake of Maillard products and to determine whether they are substrates for peptide and amino acid transporters expressed at the apical membrane of Caco-2 cells. At a concentration of 10 mM, N(epsilon)-(carboxymethyl)-L-lysine, N(alpha)-hippuryl-N(epsilon)-(1-deoxy-D-fructosyl)-L-lysine, N(alpha)-hippuryl-N(epsilon)-(carboxymethyl)-L-lysine and N(epsilon)-(1-deoxy-D-fructosyl)-L-lysine inhibited the [(14)C]glycylsarcosine uptake mediated by the H(+)-peptide co-transporter PEPT1 by 13 to 45%. For N(epsilon)-(1-deoxy-D-fructosyl)-L-lysine, an inhibitory constant of 8.7 mM was determined, reflecting a low affinity to PEPT1 in comparison with natural dipeptides. Uptake of L-[(3)H]lysine was weakly affected by N(epsilon)-(carboxymethyl)-L-lysine, N(alpha)-hippuryl-L-lysine and N(alpha)-hippuryl-N(epsilon)-(carboxymethyl)-L-lysine but strongly inhibited by N(epsilon)-(1-deoxy-D-fructosyl)-L-lysine (81%). None of the Maillard products was able to inhibit the uptake of L-[(3)H]leucine by more than 15%. We also studied the transepithelial flux of Maillard products across Caco-2 cell monolayers cultured on permeable filters. The flux rates of Maillard products ranged from 0.01 to 0.3%/cm(2) per h and were shown to be much lower than those of carrier substrates such as glycylsarcosine, L-proline and the space marker [(14)C]mannitol. We conclude that the Maillard products investigated in the present study are neither transported by PEPT1 nor by carriers for neutral amino acids. The low transepithelial flux measured for these compounds most probably occurs by simple diffusion.

Significance of Proximal Tubular Metabolism of Advanced Glycation End Products in Kidney Diseases

Advanced glycation end products (AGEs) are formed by the nonenzymatic Maillard reaction between sugars and proteins. Low-molecular weight AGEs are filtered by renal glomeruli and then reabsorbed and metabolized by proximal tubule cells (PTCs). High-molecular weight AGEs are also delivered to PTCs in proteinuric states. In patients with diabetes, AGE generation is increased, and the actions of AGEs on PTCs are likely involved in the pathogenesis of diabetic nephropathy. In patients with chronic renal failure (CRF), reduced renal metabolism of AGEs likely accounts for the accumulation of AGEs in serum, leading to uremic complications including dialysis-related amyloidosis. AGE precursors such as reactive carbonyl compounds also accumulate in the sera of patients with CRF. It is likely that PTCs take up AGEs and AGE precursors via specific endocytotic receptors or transporters. Megalin is a multiligand endocytotic receptor that is abundantly expressed on PTCs. There is evidence that megalin is involved in the cellular uptake and degradation of AGEs. We previously reported a cell therapy model involving implantation of megalin-expressing cells into experimental mice with renal failure for elimination of uremic toxin proteins. Further studies are needed to clarify the molecular mechanisms of the metabolism of AGEs and their precursors to develop a strategy for the treatment of diabetic nephropathy and uremic complications of CRF.

Glycation Products as Markers and Predictors of the Progression of Diabetic Complications

The structure of a growing number of glycation and advanced glycation end products has been elucidated. Measuring these products can be used to assess cumulative glycemic and glycoxidative damage in diabetes and other chronic conditions. The predictive power of a given glycation product can be tested in large prospective studies that evaluate the risk of developing diabetic micro- and macrovascular disease over years following the quantitative determination of that marker. This article provides a comprehensive review of the field, comparing the merits of each marker, whether in skin, serum, or other tissue. Several conclusions are drawn, one of which identifies skin glycation products as powerful predictors of the risk of developing diabetic complications.

An oral load of the early glycation compound lactuloselysine fails to accumulate in the serum of uraemic patients

BACKGROUND

It has been hypothesized that in renal failure, exogenous glycation compounds from food accumulate and play a major pathogenetic role when renal excretion is impaired.

METHODS

To address this, a diet containing a defined amount of the lysine Amadori product (AP) lactuloselysine was used. Plasma concentrations and cumulative urinary excretion of AP were assessed in 16 healthy subjects, 12 renal failure patients and 6 continuous ambulatory peitoneal dialysis (CAPD) patients. Amadori product was measured as furosine using reverse phase high performance liquid chromatography (RP-HPLC) after acid hydrolysis.

RESULTS

A diet low in glycation compounds significantly decreased excretion of APs in healthy subjects. In healthy individuals, ingestion of lactuloselysine bound to food proteins caused only a minor acute increase (8.24+/-1.11 mg/day, 2% of the administered dose) of AP excretion in the urine; in patients with renal failure not yet on dialysis, the increase in AP excretion in the urine was significantly less (4.0+/-0.51 mg/day) and the same was true in CAPD patients (0.21+/-0.09 mg/day). The plasma concentration of total APs, i.e. the sum of APs as free amino acids and residues bound to plasma proteins, did not change in any of the three groups, however.

CONCLUSION

Dietary APs do not accumulate in the blood even in advanced renal failure. The amount of APs measured as furosine excreted in the urine is significantly less, however, in renal failure and CAPD patients compared with healthy subjects. Although the findings exclude accumulation of lactuloselysine in renal failure, they do not generally exclude accumulation of other food-derived advanced glycation end products (AGEs).

Protein-bound advanced glycation endproducts (AGEs) as bioactive amino acid derivatives in foods

The Maillard reaction or nonenzymatic browning is of outstanding importance for the formation of flavour and colour of heated foods. Corresponding reactions, also referred to as "glycation", are known from biological systems, where the formation of advanced glycation endproducts (AGEs) shall play an important pathophysiological role in diabetes and uremia. In this review, pathways leading to the formation of individual protein-bound lysine and arginine derivatives in foods are described and nutritional consequences resulting from this posttranslational modifications of food proteins are discussed.

Renal accumulation of pentosidine in non-diabetic proteinuria-induced renal damage in rats

BACKGROUND

Advanced glycation end-products (AGEs) contribute to the pathogenesis of diabetic glomerulopathy. The role of AGEs in non-diabetic renal damage is not well characterized. First, we studied whether renal AGE accumulation occurs in non-diabetic proteinuria-induced renal damage and whether this is ameliorated by renoprotective treatment. Secondly, we investigated whether renal AGE accumulation was due to intrarenal effects of local protein trafficking.

METHODS

Pentosidine was measured (by high-performance liquid chromatography) in rats with chronic bilateral adriamycin nephropathy (AN), untreated and treated with lisinopril. Age-matched healthy rats served as negative controls. Secondly, we compared renal pentosidine in mild proteinuric and non-proteinuric kidneys of unilateral AN and in age-matched controls at 12 and 30 weeks. Intrarenal localization of pentosidine was studied by immunohistochemistry.

RESULTS

Renal pentosidine was elevated in untreated AN (0.14+/-0.04 micromol/mol valine) vs healthy controls (0.04+/-0.01 micromol/mol valine, P<0.01). In lisinopril-treated AN, pentosidine was lower (0.09+/-0.02 micromol/mol valine) than in untreated AN (P<0.05). In unilateral proteinuria, pentosidine was similar in non-proteinuric and proteinuric kidneys. After 30 weeks of unilateral proteinuria, pentosidine was increased in both kidneys (0.26+/-0.10 micromol/mol valine) compared with controls (0.18+/-0.06 micromol/mol valine, P<0.05). Pentosidine (AN, week 30) was also increased compared with AN at week 12 (0.16+/-0.06 micromol/mol valine, P<0.01). In control and diseased kidneys, pentosidine was present in the collecting ducts. In proteinuric kidneys, in addition, pentosidine was present in the brush border and cytoplasm of dilated tubular structures, i.e. at sites of proteinuria-induced tubular damage.

CONCLUSION

Pentosidine accumulates in non-diabetic proteinuric kidneys in damaged tubules, and renoprotective treatment by angiotensin-converting enzyme (ACE) inhibitors inhibits AGE accumulation, supporting a relationship between abnormal renal protein trafficking, proteinuria-induced tubular damage and tubular pentosidine accumulation. Future studies, applying specific AGE inhibitors, should be conducted to provide insight into the pathophysiological significance of renal AGEs in non-diabetic renal disease.

Metabolite analysis in positron emission tomography studies: examples from food sciences

Substances of various chemical structures can be labelled with appropriate positron emitting isotopes and applied as tracer compounds in PET examinations. Using dynamic data acquisition protocols, time-activity curves of radioactivity uptake in organs can be derived and the measurements of tissue tracer concentrations can be translated into quantitative values of tissue function. However, analysis of metabolites of these tracers regarding their nature and distribution in the living organism is an essential need for the quantitative analysis of PET measurements. In addition, metabolite analysis contributes to the interpretation of the images obtained as well as to the identification of pathological changes in metabolic pathways. This paper reports on representative examples of radiolabelled compounds which might be of importance in food science (e.g., amino acids, polyphenols, and model compounds for advanced glycation end products (AGEs)). Typical procedures of analysis (radio-HPLC, radio-TLC) including pre-analytical sample preparation are described. Specific challenges of the method, e.g., trace amounts of radiolabelled compounds and the influence of the often very short half-lives of positron-emitting nuclides used are highlighted. Representative results of analyses of plasma, urine, and tissue samples are presented and discussed in terms of the metabolic fate of the tracers.