Immunohistochemical detection of advanced glycosylation end products within the vascular lesions and glomeruli in diabetic nephropathy

Insight into the Molecular Mechanism of Diabetic Kidney Disease and the Role of Metformin in Its Pathogenesis

Diabetic kidney disease (DKD) is one of the leading causes of death among patients diagnosed with diabetes mellitus. Despite the growing knowledge about the pathogenesis of DKD, we still do not have effective direct pharmacotherapy. Accurate blood sugar control is essential in slowing down DKD. It seems that metformin has a positive impact on kidneys and this effect is not only mediated by its hypoglycemic action, but also by direct molecular regulation of pathways involved in DKD. The molecular mechanism of DKD is complex and we can distinguish polyol, hexosamine, PKC, and AGE pathways which play key roles in the development and progression of this disease. Each of these pathways is overactivated in a hyperglycemic environment and it seems that most of them may be regulated by metformin. In this article, we summarize the knowledge about DKD pathogenesis and the potential mechanism of the nephroprotective effect of metformin. Additionally, we describe the impact of metformin on glomerular endothelial cells and podocytes, which are harmed in DKD.

UV Fluorescence-Based Determination of Urinary Advanced Glycation End Products in Patients with Chronic Kidney Disease

Advanced glycation end products (AGEs) are a class of proteins or lipids that are non-enzymatically glycated and oxidized after contact with aldose sugars. The accumulation of AGEs results in carbonyl stress, which is characteristic for diabetes mellitus, uremia, atherosclerosis and vascular dysfunction. In recent decades, several innovative methods have been developed to measure the concentration of AGEs in blood or urine. In the present study, we evaluated the use of UV fluorescence as an alternative tool to detect urinary AGEs in four groups of well characterized chronic kidney disease (CKD) patients over a wide range of kidney insufficiency and in a group of healthy subjects. Using an excitation wavelength of 365 nm, the fluorescence spectra of urinary AGEs were recorded in the 400–620 nm emission range. When considering the emission peaks at 440 nm and 490 nm, a significantly higher AGE-specific fluorescence intensity was detected in CKD patients compared to healthy subjects (p < 0.0001 and p = 0.0001, respectively). The urinary creatinine adjusted fluorescence emission spectra in the group of CKD patients with diabetes mellitus were comparable with those of CKD patients without diabetes mellitus. Creatinine-adjusted fluorescence emission spectra were highest in CKD patients with proteinuria, moderate in CKD patients without proteinuria and lowest in healthy controls (p < 0.0001 at both emission wavelengths). In a multiple regression analysis, age, CRP and insulin treatment were predictors of fluorescence intensity at the emission wavelength of 440 nm. Age and insulin treatment were predictors at 490 nm. The presented method is a simple, cheap, alternative method to monitor the AGE-load in the CKD population.

Advanced Glycation End Products Are Associated With Inflammation and Endothelial Dysfunction in HIV

OBJECTIVE

To compare levels of advanced glycation end products (AGEs) between HIV-infected patients and uninfected controls and assess the relationship between AGEs, HIV, inflammation, and endothelial dysfunction.

DESIGN

Cross-sectional study involving 90 individuals (68 HIV+ and 22 healthy controls matched by age and sex).

METHODS

AGE levels were assessed using 3 different modalities: free AGEs were measured in the serum, skin autofluorescence (AF) was determined with a noninvasive reader, and dietary AGEs were estimated using 24-hour dietary recalls. Markers of inflammation, immune activation, and endothelial dysfunction were also measured. Wilcoxon rank-sum and χ tests were used to compare AGEs between groups. Spearman correlations were used to explore relationships between variables while adjusting for different covariates.

RESULTS

Overall, 71% were men and 68% were African American, with a median age of 53 years. Among HIV-infected individuals, all participants were on antiretroviral therapy by design, and most participants (78%) had an undetectable HIV-1 RNA level (≤20 copies/mL). Skin AF and serum AGEs were significantly higher in HIV-infected participants compared with uninfected controls (P < 0.01), whereas no differences in dietary AGEs were found between groups (P = 0.2). In the HIV-infected group, but not in controls, skin AF and circulating AGEs were significantly associated with inflammatory and oxidative markers, and with markers of endothelial dysfunction.

CONCLUSIONS

These results suggest intrinsic production of AGE in HIV-infected individuals. The relationship between serum/skin AGE and inflammatory, oxidative, and cardiovascular markers highlights the potential implications of AGEs in chronic inflammation and endothelial dysfunction in HIV, suggesting a new potential target for HIV-associated heightened inflammation and cardiovascular risk.

Advanced Glycation End Products Predict Loss of Renal Function and Correlate With Lesions of Diabetic Kidney Disease in American Indians With Type 2 Diabetes

We examined associations of advanced glycation end products (AGEs) with renal function loss (RFL) and its structural determinants in American Indians with type 2 diabetes. Data were from a 6-year clinical trial that assessed renoprotective efficacy of losartan. Participants remained under observation after the trial concluded. Glomerular filtration rate (GFR) was measured annually. Kidney biopsies were performed at the end of the trial. Five AGEs were measured in serum collected at enrollment and at kidney biopsy. RFL was defined as ≥40% decline of measured GFR from baseline. Of 168 participants (mean baseline age 41 years, HbA_1c 9.2%, GFR 164 mL/min, and albumin-to-creatinine ratio 31 mg/g), 104 reached the RFL end point during median follow-up of 8.0 years. After multivariable adjustment, each doubling of carboxyethyl lysine (hazard ratio [HR] 1.60 [95% CI 1.08-2.37]) or methylglyoxal hydroimidazolone (HR 1.30 [95% CI 1.02-1.65]) concentration was associated with RFL. Carboxyethyl lysine, carboxymethyl lysine, and methylglyoxal hydroimidazolone correlated positively with cortical interstitial fractional volume (partial r = 0.23, P = 0.03; partial r = 0.25, P = 0.02; and partial r = 0.31, P = 0.003, respectively). Glyoxyl hydroimidazolone and methylglyoxal hydroimidazolone correlated negatively with total filtration surface per glomerulus (partial r = -0.26, P = 0.01; and partial r = -0.21, P = 0.046, respectively). AGEs improve prediction of RFL and its major structural correlates.

Combined therapy with metformin and insulin attenuates systemic and hepatic alterations in a model of high-fat diet-/streptozotocin-induced diabetes

In this study we have explored the pathogenesis of the hepatic alterations which occur in diabetes and the modulation of these complications by the combination of metformin adjunct treatment and insulin monotherapy. For this purpose, diabetic rats were treated with insulin (DM + Ins) or metformin plus insulin (DM + Met + Ins). Biochemical and cardiometabolic parameters were analysed by spectrophotometry. Intravital microscopy was used to study the hepatic microcirculation. In the liver tissue, real-time PCR was used to analyse oxidative stress enzymes, inflammatory markers and receptors for advanced glycation end products (AGE) (RAGE) gene expression. Lipid peroxidation was assessed by thiobarbituric acid reactive species (TBARs) analyses. AGE deposition and RAGE protein expression were studied by fluorescence spectrophotometry and Western blot respectively. Body weight, %HbA1c , urea, total proteins and oxidative stress parameters were found to be similarly improved by insulin or Met + Ins treatments. On the other hand, Met + Ins treatment showed a more pronounced effect on fasting blood glucose level than insulin monotherapy. Fructosamine, uric acid, creatinine, albumin and amylase levels and daily insulin dose requirements were found to be only improved by the combined Met + Ins treatment. Liver, renal and pancreatic dysfunction markers were found to be more positively affected by metformin adjunct therapy when compared to insulin treatment. Liver microcirculation damage was found to be completely protected by Met + Ins treatment, while insulin monotherapy showed no effect. Our results suggest that oxidative stress, microcirculatory damage and glycated proteins could be involved in the aetiology of liver disease due to diabetes. Additionally, metformin adjunct treatment improved systemic and liver injury in induced diabetes and showed a more pronounced effect than insulin monotherapy.

Cell Interactions with Vascular Regenerative MAA-Based Materials in the Context of Wound Healing

In diabetic patients the development of chronic non-healing wounds is a common complication. A methacrylic acid-based biomaterial is a vascular regenerative material that enhances diabetic healing without the use of cells or growth factors. The bioactive nature of this material is thought to be associated with its anionic charge or surface chemistry. Contact between the methacrylic acid-based biomaterial and tissue begins with protein (including complement) adsorption and is followed by interaction of the biomaterial with resident and infiltrating cells in the wound bed (e.g., macrophages and endothelial cells). This results in changes to their surface receptors to activate phosphorylation cascades that lead to differential activation of signalling pathways such as those involving osteopontin and sonic hedgehog. These changes modulate the phenotype of the cells in the wound bed, eventually improving vessel formation and wound healing. Understanding the molecular and cellular mechanisms will have broad implications for biomaterials, not just the methacrylic acid-based material, and will facilitate the advancement of regenerative biomaterials for diverse applications.

Oxidative stress in diabetes: implications for vascular and other complications

In recent decades, oxidative stress has become a focus of interest in most biomedical disciplines and many types of clinical research. Increasing evidence shows that oxidative stress is associated with the pathogenesis of diabetes, obesity, cancer, ageing, inflammation, neurodegenerative disorders, hypertension, apoptosis, cardiovascular diseases, and heart failure. Based on these studies, an emerging concept is that oxidative stress is the “final common pathway” through which the risk factors for several diseases exert their deleterious effects. Oxidative stress causes a complex dysregulation of cell metabolism and cell–cell homeostasis; in particular, oxidative stress plays a key role in the pathogenesis of insulin resistance and β-cell dysfunction. These are the two most relevant mechanisms in the pathophysiology of type 2 diabetes and its vascular complications, the leading cause of death in diabetic patients.

Activity of Wen-Pi-Tang, and Purified Constituents of Rhei Rhizoma and Glycyrrhizae Radix against Glucose-Mediated Protein Damage

Wen-Pi-Tang, an Oriental medical prescription composed of Rhei Rhizoma, Ginseng Radix, Aconiti Tuber, Zingiberis Rhizoma and Glycyrrhizae Radix, is used clinically as a medicine to treat renal failure. This study was conducted to examine the inhibitory activity of the five crude drug components of Wen-Pi-Tang and several pure compounds isolated from Rhei Rhizoma and Glycyrrhizae Radix against the protein glycation reaction. Rhei Rhizoma exerted the most potent activity, Zingiberis Rhizoma and Glycyrrhizae Radix showed relatively moderate activity, whereas Aconiti Tuber and Ginseng Radix showed weak activity. On the other hand, of 20 compounds obtained from Rhei Rhizoma and Glycyrrhizae Radix, tannins, especially rhatannin, RG-tannin and procyanidin B-2 3,3′-di-O-gallate, showed significantly strong activities that were more effective than the positive control, aminoguanidine. Some flavones such as licochalcone A and licochalcone B, and anthraquinones such as emodin and aloe-emodin, also showed inhibitory activity. These findings may help to explain, at least in part, certain pharmacological activities of Wen-Pi-Tang, whose clinical efficacy against renal failure is already recognized.

Oxidative stress and diabetic complications

Oxidative stress plays a pivotal role in the development of diabetes complications, both microvascular and cardiovascular. The metabolic abnormalities of diabetes cause mitochondrial superoxide overproduction in endothelial cells of both large and small vessels, as well as in the myocardium. This increased superoxide production causes the activation of 5 major pathways involved in the pathogenesis of complications: polyol pathway flux, increased formation of AGEs (advanced glycation end products), increased expression of the receptor for AGEs and its activating ligands, activation of protein kinase C isoforms, and overactivity of the hexosamine pathway. It also directly inactivates 2 critical antiatherosclerotic enzymes, endothelial nitric oxide synthase and prostacyclin synthase. Through these pathways, increased intracellular reactive oxygen species (ROS) cause defective angiogenesis in response to ischemia, activate a number of proinflammatory pathways, and cause long-lasting epigenetic changes that drive persistent expression of proinflammatory genes after glycemia is normalized ("hyperglycemic memory"). Atherosclerosis and cardiomyopathy in type 2 diabetes are caused in part by pathway-selective insulin resistance, which increases mitochondrial ROS production from free fatty acids and by inactivation of antiatherosclerosis enzymes by ROS. Overexpression of superoxide dismutase in transgenic diabetic mice prevents diabetic retinopathy, nephropathy, and cardiomyopathy. The aim of this review is to highlight advances in understanding the role of metabolite-generated ROS in the development of diabetic complications.

Impact of uremia, diabetes, and peritoneal dialysis itself on the pathogenesis of peritoneal sclerosis: a quantitative study of peritoneal membrane morphology

BACKGROUND AND OBJECTIVES

Peritoneal interstitial fibrosis and hyalinizing vasculopathy were induced by peritoneal dialysis and other associated conditions (e.g., uremia). A quantitative method for peritoneal biopsy evaluation is required to investigate possible causative factors and severity of the peritoneal dialysis-related peritoneal alterations.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Peritoneal biopsy specimens from 173 uremic (before peritoneal dialysis) and 80 peritoneal dialysis patients with or without impaired ultrafiltration capacity were evaluated by average peritoneal thickness of submesothelial compact zone measured at five randomly selected points of peritoneum and by lumen/vessel diameter ratio at postcapillary venule.

RESULTS

The average peritoneal thickness was increased in uremic patients and progressively thickened as the duration of peritoneal dialysis prolonged. The lumen/vessel diameter ratio was lower in uremia than normal and progressively decreased as the duration of peritoneal dialysis prolonged. In pre-peritoneal dialysis peritoneum, patients with diabetes showed significant decrease in lumen/vessel diameter ratio compared with patients without diabetes. The average peritoneal thickness was significantly higher in patients with impaired ultrafiltration capacity than in patients with maintained ultrafiltration capacity; however, no significant difference was observed in the postcapillary venule thickness and lumen/vessel diameter ratio between the two groups.

CONCLUSIONS

The average peritoneal thickness and lumen/vessel diameter ratio were useful morphologic parameters to quantify the severity of the peritoneal alterations in uremic and peritoneal dialysis patients. Uremia and diabetes had an impact on the pathogenesis of peritoneal sclerosis in pre-peritoneal dialysis peritoneum. Peritoneal dialysis treatment itself had a much stronger impact on the progression of peritoneal sclerosis.

Accumulation of Advanced Glycation End Products, Measured as Skin Autofluorescence, in Renal Disease

Advanced glycation end products (AGEs) accumulate during renal failure and dialysis. Kidney transplantation is thought to reverse this accumulation by restoring renal function. Using a noninvasive and validated autofluorescence reader, we evaluated AGE levels in 285 transplant recipients (mean age, 52 years; range, 41 to 60 years), 32 dialysis patients (mean age, 56 years; range, 43 to 65 years), and 231 normal control subjects (mean age, 51 years; range, 40 to 65 years). Measurements in transplant recipients were performed for a mean of 73 months (range, 32 to 143 months) after transplantation. Dialysis patients were on dialysis therapy for a mean of 42 months (range, 17 to 107 months). Fluorescence was significantly increased in dialysis patients compared with normal control subjects (2.8 vs. 2.0 arbitrary units [a.u.], P < .0001). Although fluorescence levels were significantly decreased in transplant recipients compared with dialysis patients (2.5 vs. 2.8 a.u., P < .0001), fluorescence in transplant recipients was higher than in controls (2.5 vs. 2.0 a.u., P < .0001). In transplant recipients, fluorescence correlated positively with the duration of dialysis prior to transplantation (R = 0.21, P < .0001), and negatively with creatinine clearance (R = -0.34, P < .0001). No correlation was found between time after transplantation and fluorescence in transplant recipients (R = -0.10, P = .10). Fluorescence in dialysis patients was positively correlated with duration of dialysis (R = 0.36, P = .042). Our results, like those of others, suggest that kidney transplantation does not fully correct increased AGE levels found in dialysis patients. The increased AGE levels in kidney transplant recipients cannot be explained by the differences in renal function alone. The availability of a simple, noninvasive method (AGE-Reader) to measure AGE accumulation may be used to monitor AGE accumulation in a clinical setting as well as in a study setting.

Advanced glycation end products in kidney transplant patients: a putative role in the development of chronic renal transplant dysfunction

Chronic renal transplant dysfunction is one of the leading causes of graft failure in kidney transplantation. A complex interplay of both alloantigen-related and alloantigen-unrelated risk factors is believed to underlie its development. We propose that advanced glycation end products (AGEs) are involved in the development of chronic renal transplant dysfunction. AGE formation is associated with different alloantigen-unrelated risk factors for chronic renal transplant dysfunction, such as recipient age, diabetes, proteinuria, hypertension, and hyperlipidemia. In vitro studies have shown that AGEs induce the expression of various mediators associated with chronic renal transplant dysfunction. Furthermore, AGE-induced renal damage has been found in multiple experimental studies. This renal damage shows similarity to the damage found in chronic renal transplant dysfunction. Together, several lines of evidence support a role of AGEs in the development of chronic renal transplant dysfunction and suggest that preventive therapy with AGE inhibitors may be helpful in preserving renal function in transplant recipients.

N epsilon-carboxymethyllysine in brain aging, diabetes mellitus, and Alzheimer's disease

Oxidative stress has been implicated in the pathophysiology of Alzheimer's disease (AD) and diabetes mellitus (DM). N epsilon-carboxymethyllysine (CML) is an advanced glycation end product (AGE) recently found to be associated with oxidative stress mechanisms. Using immunocytochemical methods we examined the distribution of CML in brain tissue from AD and DM subjects and aging controls. CML reactivity was present in the cytoplasm of neurons, but there were marked differences in the intensity of expression, number of cells, and topographical distribution. CML expression was higher in hippocampus than in frontal and temporal cortex. In the hippocampus, neuronal and, to an extent, glial expression was more marked in CA3 and CA4 than in CA1 and CA2. In AD, CML was found to be coexpressed with tau protein, showing the similar neurofibrillary tangle shape, as well as in neuritic plaques but not in the core of amyloid plaques. We noted an increasing degree of CML expression such that the highest reactivity was evident in those with both AD and DM, followed by AD, DM, and aging controls. There was an inverse relationship between Braak staging and topography of CML expression. Although DM cases did not show Abeta deposition or neurofibrillary tangles, these findings suggest increased CML expression is not limited to AD. Nonetheless, high CML expression in AD with coexistent DM suggests there are additive effects compared with AD alone. It is plausible that the microangiopathy also associated with DM could worsen AD pathogenesis.

Therapeutic potential of breakers of advanced glycation end product-protein crosslinks

Long-lived structural proteins, collagen and elastin, undergo continual non-enzymatic crosslinking during aging and in diabetic individuals. This abnormal protein crosslinking is mediated by advanced glycation end products (AGEs) generated by non-enzymatic glycosylation of proteins by glucose. The AGE-derived protein crosslinking of structural proteins contributes to the complications of long-term diabetes such as nephropathy, retinopathy, and neuropathy. AGE-crosslinks have also been implicated in age-related cardiovascular diseases. Potential treatment strategies for these AGE-derived complications include prevention of AGE-formation and breaking of the existing AGE-crosslinks. The therapeutic potential of the AGE-inhibitor, pimagedine (aminoguanidine), has been extensively investigated in animal models and in Phase 3 clinical trials. This review presents the pre-clinical and clinical studies using ALT-711, a highly potent AGE-crosslink breaker that has the ability to reverse already-formed AGE-crosslinks. Oral administration of ALT-711 has resulted in a rapid improvement in the elasticity of stiffened myocardium in experimental animals. Topical administration of ALT-711 was effective in improving the skin hydration of aged rats. The therapeutic potential of crosslink breakers for cardiovascular complications and dermatological alterations associated with aging and diabetes is discussed.

The determination of AGE-peptides by flow injection assay, a practical marker of diabetic nephropathy

BACKGROUND

Increasing evidence has suggested that advanced glycation end products (AGEs) might play a central role in the pathogenesis of diabetic complications. Serum AGEs concentration may serve as a useful marker for monitoring pathological processes and progression of diabetic complications.

METHODS

A flow injection assay (FIA) system was developed using high performance liquid chromatography (HPLC) to detect low molecular mass AGEs (AGE-peptides, AGE-P). Serum from diabetic patients (n=126), normal controls (n=54) and diabetic mice (n=20) and matched controls (n=20) were collected.

RESULTS

The coefficient of variance for intra-assay and inter-assay were 1.2% and 6.3%, respectively. The range of recoveries was 94.9-101.9%. The serum AGE-P concentration was significantly increased both in diabetic patients (2.976+/-0.247 vs. 1.385+/-0.131 U/ml, P<0.0001) and mice (6.71+/-0.50 vs. 2.49+/-0.10 U/ml, P<0.0001) than their respective controls. Concentration of AGE-P was positively correlated with serum creatinine (Scr) (r=0.7133, P<0.0001), 24-h urinary protein (24-h UPro) (r=0.8704, P<0.0001) and urinary albumin excretion (UAE) (r=0.5989, P<0.0001).

CONCLUSIONS

The present study suggested that FIA might be a reliable method for measuring the serum AGE-P. Furthermore, our results supported the notion that AGE-P might be a valuable marker for predicting the severity of diabetic nephropathy (DN).

Therapeutic potential of AGE inhibitors and breakers of AGE protein cross-links

Glucose and other reducing sugars react non-enzymatically with proteins leading to the formation of advanced glycosylation end products (AGEs) and AGE-derived protein cross-linking. Formation of AGEs is a normal physiological process, which is accelerated under the hyperglycaemic condition in diabetes. Under normal conditions, AGEs build up slowly and accumulate as one ages. Numerous studies have indicated that AGEs contribute to the pathological events leading to diabetic complications, such as age-related diseases, including nephropathy, retinopathy, vasculopathy and neuropathy. Potential therapeutic approaches to prevent these complications include pharmacological inhibition of AGE formation and disruption of pre-formed AGE-protein cross-links. Studies using animal models and preliminary clinical trials have shown the ability of the AGE-inhibitor, pimagedine and the cross-link breaker, ALT-711, to reduce the severity of pathologies of advanced glycosylation. These agents offer potential treatments for glucose-derived complications of diabetes and ageing.

Effects of advanced glycation end products on the proliferation and fibronectin production of smooth muscle cells

The aim of this study was to evaluate the effects of advanced glycation end-products (AGEs) on the proliferative activity and fibronectin production of smooth muscle cells (SMCs). AGE-bovine serum albumin (AGE-BSA) was prepared by incubation with D-glucose at 37 degrees C for 60 days. Cultured SMCs were obtained from explants isolated from porcine abdominal aorta and used between passages 3 and 10. The proliferative activity of SMCs was examined by MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide) assay and by incorporation of 3H-thymidine into DNA. Fibronectin production was assessed by competitive ELISA assay for both fibronectin secreted into the culture medium (M-FN) and cell-associated fibronectin (C-FN), i.e., both intra- and peri-cellular fibronectin. Theassay revealed that AGE-BSA did not produce any change in optical density (A570) of SMCs at concentrations of up to 20 microg/ml, but decreased that of SMCs at a concentration of 40 microg/ml. The addition of PDGF (5 ng/ml) induced an increase in 3H-thymidine incorporation into DNA of quiescent SMCs, while the addition of AGE-BSA (20 microg/ml) had no effect. In contrast, AGE-BSA significantly increased C-FN of SMCs (30.8+/-8.58 ng/microg TP), compared to unmodified BSA (16.5+/-4.19 ng/microg TP). However, no difference in M-FN levels was observed between cells treated with AGE-BSA and unmodified BSA. The addition of anti-transforming growth factor (TGF)-beta antibody restored the levels of C-FN in SMCs cultured in 20 microg/ml of AGE-BSA, suggesting that TGF-beta might act as an intermediate factor in AGE-induced fibronectin production by SMCs. Our results suggest that interaction of AGE-modified proteins with SMCs may play a role in the development of atherosclerosis in diabetic and non-diabetic patients.

N-phenacylthiazolium bromide decreases renal and increases urinary advanced glycation end products excretion without ameliorating diabetic nephropathy in C57BL/6 mice

AIMS

Advanced glycation end products (AGE), which form from the non-enzymatic reaction of proteins and sugars, have been implicated in the pathogenesis of diabetic nephropathy. Recently, a compound [N-phenacylthiazolium bromide (PTB)] has been described which cleaves alpha,beta-dicarbonyl compounds. In the present study we used diabetic C57BL/6 mice to determine if PTB altered renal AGE levels and reduced diabetic glomerulosclerosis.

METHODS

Mice with stable hyperglycaemia induced by streptozotocin were given daily subcutaneous injections of either PTB (10 microg/g) or saline for 12 weeks. Renal-collagen bound AGE and urinary AGE-peptides were measured by ELISA using an anti-AGE-RNase antibody. Renal collagen-released Nepsilon(carboxymethyl)lysine (CML) and pentosidine were determined by high pressure liquid chromatography (HPLC). Glomerular lesions (volume and mesangial/total surface area) were evaluated by computer-assisted image analysis. We determined urinary protein/creatinine ratio as a functional parameter. AGE localization was examined by immunohistochemistry using the anti-AGE-RNase antibody.

RESULTS

Renal collagen-bound AGE were decreased and urinary AGE excretion was increased in PTB-treated diabetic mice. However, collagen-released CML and pentosidine were similar in both groups. Glomerular histology and morphometric analysis revealed also no differences between PTB-and saline-treated diabetic mice. The urinary protein/creatinine ratio was unaffected by PTB-treatment. AGE staining by anti-AGE-RNase antibody was present in Bowman's capsules, glomerular basement membranes and cortical tubules. It was decreased in all structures in PTB-treated diabetic mice.

CONCLUSION

In summary, PTB decreased renal AGE accumulation but did not ameliorate glomerular lesions or proteinuria. Thus, cleavage of AGE by PTB is not sufficient to prevent development of diabetic nephropathy in C57BL/6 mice.

Glycation during human dermal intrinsic and actinic ageing: an in vivo and in vitro model study

BACKGROUND

Non-enzymatic glycation occurring in normal human skin plays an important part in ageing. OBJECTIVES To visualize and quantify, in human subjects, the extent of glycation during human dermal intrinsic and actinic ageing, and to develop a reliable reproducible in vitro model for evaluating the efficacy of potential inhibitors of glycation.

METHODS

By immunohistochemistry using a monoclonal antibody recognizing carboxymethyl lysine, an advanced glycation end-product (AGE) (first objective), and by incubating dead de-epidermized dermis (DED) with glucose to simulate ageing-induced glycation in a human dermal equivalent model (second objective).

RESULTS

We found that glycation of the dermis generally arises after 35 years, then increases rapidly with intrinsic ageing. We also noticed an enhancement of glycation by solar irradiation that occurred via glycation of the elastic fibre network or solar elastosis tissue. In the model, production of AGEs appeared in a time-dependent way, mimicking glycation observed in vivo during chronological ageing. Irradiation of DED before incubation with glucose strongly enhanced induction of AGEs, corresponding to the effect of solar irradiation on AGEs observed in vivo.

CONCLUSIONS

These results confirm a marked increase of AGEs during intrinsic ageing in normal human skin and also suggest that glycation is enhanced in photoaged skin.

Immunohistochemical distribution of the receptor for advanced glycation end products in neurons and astrocytes in Alzheimer's disease

Advanced glycation end products (AGE) and the receptor for AGE (RAGE) have been implicated in the chronic complications of diabetes mellitus (DM), and have been reported to play an important role in the pathogenesis of Alzheimer's disease (AD). In this study, we established a polyclonal anti-RAGE antibody, and examined the immunohistochemical localization of amyloid beta protein (Abeta), AGE, and RAGE in neurons and astrocytes from patients with AD and DM. Our anti-RAGE antibody recognized full-length RAGE (50 kd) and N-terminal RAGE (35 kd) in human brain tissue. Abeta-, AGE-, and RAGE-positive granules were identified in the perikaryon of hippocampal neurons (especially from CA3 and CA4) in all subjects. The distribution and staining pattern of these immunopositive granules showed good concordance with each antibody. In AD, most astrocytes contained both AGE-and RAGE-positive granules and their distribution was almost the same. Abeta-positive granules were less common, but Abeta-, AGE-, and RAGE-positive granules were colocalized in one part of a single astrocyte. In DM patients and control cases, AGE-and RAGE-positive astrocytes were very rare. These finding support the hypothesis that glycated Abeta is taken up via RAGE and is degraded through the lysosomal pathway in astrocytes. In addition to the presence of AGE, the process of AGE degradation and receptor-mediated reactions may contribute to neuronal dysfunction and promote the progression of AD.

Glycochelates and the etiology of diabetic peripheral neuropathy

People with diabetes are prone to develop peripheral vascular and nerve abnormalities which, in extreme cases, can lead to limb amputations. Although numerous theories have been advanced for these complications, no firm explanation is yet available. Recently, evidence has appeared suggesting that these vascular and nerve abnormalities may involve transition metals; administration of chelators such as desferrioxamine has been shown to prevent or actually reverse slowed peripheral nerve conduction and neuronal blood flow, as well as impaired endothelium-dependent arterial relaxation. Here, we argue that (i) the heavily glycated proteins known to accumulate in people with diabetes gain an increased affinity for transition metals such as iron and copper, (ii) as a result, proteins such as elastin and collagen within the arterial wall-which are known to be particularly heavily glycosylated in diabetes-may accumulate bound metal, especially copper, (iii) the bound metal causes the catalytic destruction of endothelium derived relaxing factor (nitric oxide or a derivative thereof), thereby engendering a state of chronic vasoconstriction. The resulting impairment of blood flow to peripheral nerves restricts the delivery of oxygen and nutrients and, in extremis, nerve death eventuates. If this hypothesis is proved correct, there are important implications for the development of novel pharmaceuticals for the treatment of diabetic peripheral neuropathy.

The use of optical sensors to understand cellular interactions with renal cells

Optical biosensor technology has revolutionized the assessment of receptor binding, enabling the characterization of low affinity interactions in real time. We report the application of the LAsys Optical Biosensor to the investigation of the affinity and specificity of the putative proximal tubular scavenging receptor for protein reabsorption and the specificity of AGE-modified protein interactions with primary human mesangial cells. Using the LLCPK cell line, the carboxy-methyl dextran cuvette surface and five different proteins (ranging in size and charge), we have shown that there is evidence to support the existence of a single scavenging receptor for all the proteins tested. The proteins competed with each other differing only in their relative binding affinity for the common receptor. We have also shown that human mesangial cells can bind to AGE-modified human serum albumin (AGE-HSA) immobilized onto the carboxylate surfaced planar cuvette and that binding can be inhibited using increasing concentrations of soluble AGE-HSA. However, increasing concentrations of soluble Non-AGE modified HSA can also inhibit binding to a similar extent which implies that there is relatively little AGE-receptor (RAGE) expression on cultured primary human mesangial cells. These results demonstrate the exciting potential of this technology as a tool to explore cellular interactions with renal cells.

The role of oxidative stress and NF-kappaB activation in late diabetic complications

A common endpoint of hyperglycemia dependent cellular changes is the generation of reactive oxygen intermediates (ROIs) and the presence of elevated oxidative stress. Therefore, oxidative stress is supposed to play an important role in the development of late diabetic complications. Formation of advanced glycation end products (AGE's) due to elevated nonenzymatic glycation of proteins, lipids and nucleic acids is accompanied by oxidative, radical-generating reactions and thus represents a major source for oxygen free radicals under hyperglycemic conditions. Once formed, AGE's can influence cellular function by binding to several binding sites including the receptor for AGE's, RAGE. Binding of AGE's (and other ligands) to RAGE results in generation of intracellular oxidative stress and subsequent activation of the redox-sensitive transcription factor NF-kappaB in vitro and in vivo. Consistently, activation of NF-kappaB in diabetic patients correlates with the quality of glycemic control and can be reduced by treatment with the antioxidant alpha-lipoic acid. The development of techniques allowing for a tissue culture independent measurement of NF-kappaB activation in patients with diabetes mellitus gives insights into the molecular mechanisms linking diabetes mellitus and hyperglycemia with formation of advanced glycated endproducts and generation of oxidative stress finally resulting in oxidative stress mediated cellular activation.

Advanced glycation end products in Alzheimer's disease and other neurodegenerative diseases

Advanced glycation end products (AGEs) have been implicated in the chronic complications of diabetes mellitus and have been reported to play an important role in the pathogenesis of Alzheimer's disease. In this study, we examined the immunohistochemical localization of AGEs, amyloid beta protein (A beta), apolipoprotein E (ApoE), and tau protein in senile plaques, neurofibrillary tangles (NFTs), and cerebral amyloid angiopathy (CAA) in Alzheimer's disease and other neurodegenerative diseases (progressive supranuclear palsy, Pick's disease, and Guamanian amyotrophic lateral sclerosis/Parkinsonism-dementia complex). In most senile plaques (including diffuse plaques) and CAA from Alzheimer's brains, AGE and ApoE were observed together. However, approximately 5% of plaques were AGE positive but A beta negative, and the vessels without CAA often showed AGE immunoreactivity. In Alzheimer's disease, AGEs were mainly present in intracellular NFTs, whereas ApoE was mainly present in extracellular NFTs. Pick's bodies in Pick's disease and granulovacuolar degeneration in various neurodegenerative diseases were also AGE positive. In non-Alzheimer neurodegenerative diseases, senile plaques and NFTs showed similar findings to those in Alzheimer's disease. These results suggest that AGE may contribute to eventual neuronal dysfunction and death as an important factor in the progression of various neurodegenerative diseases, including Alzheimer's disease.

Transition metals bind to glycated proteins forming redox active "glycochelates": implications for the pathogenesis of certain diabetic complications

The present investigations arose from our interest in the possibility that some structures which arise secondary to protein glycation might bind transition metals such as iron and copper. In support of this we find that, when glycated, three different proteins--albumin, gelatin (a soluble collagen fragment) and elastin--all gain a substantial affinity for the transition metals iron and copper. The glycated proteins bind at least three times as much iron as do the non-glycated proteins. Similarly, glycated albumin and gelatin also bind 2-3 times as much copper. Furthermore, at least in the case of copper bound to glycated albumin, the bound metal retains redox activity and participates in the catalytic oxidation of ascorbic acid. Should similar "glycochelates" form in vivo in diabetics, reactions mediated by these chelates may be involved in certain complications of diabetes.

Distribution of advanced glycation end products in the cerebellar neurons of dogs

Nonenzymatically glycated proteins and their advanced stage, the 'advanced glycation end products' (AGEs), have been detected in long-lived proteins and protein deposits in human and animal tissues. They are thought to be associated with normal aging and particularly with the pathogenesis of diabetic complications and Alzheimer's disease. AGEs accumulate in human neurons in an age-dependent manner and, in Alzheimer's disease patients, particularly in amyloid plaques and neurofibrillary tangles. In this study, we demonstrate AGE immunoreactivity in the canine brain, particularly in cerebellar Purkinje cells and brainstem neurons. In addition, distinct AGE-positive granules can be detected in the Purkinje cells which accumulate in an age-dependent manner. Staining with PAS and oil-red suggests that these AGE-positive granules contain the protein, but not the lipid constituents associated with lipofuscin. Our results show that the pattern of AGE distribution in the canine cerebellum resembles the situation in the human brain, but that the time course of AGE formation is much faster in dogs reflecting their much shorter life span.

Oxidative stress and lipids in diabetes: a role in endothelium vasodilator dysfunction?

Endothelial dysfunction is a key feature of diabetes mellitus and is thought to be the major cause of vascular complications associated with the disease. The vascular endothelium demonstrates impaired synthesis of vasodilators and increased release of procoagulants and vasoconstrictors, defects which theoretically could explain the increased incidence of atherosclerosis and hypertension found within this patient group. The pathways mediating endothelial cell layer dysfunction are unknown, although many candidates have been proposed. This review concentrates on the hypothesis that increased oxidative stress combined with abnormal plasma lipid composition leads to reduced synthesis of endothelial vasodilators and hence endothelial dysfunction. Free radical generation is undoubtedly raised in diabetes but the evidence for decreased antioxidant status is debatable. The role of antioxidant and lipid-lowering therapy is considered, but few studies have directly investigated the effect of treatment on vascular function. Concern arises from individual studies of vitamin E in diabetic animals which have proved deleterious. Current literature implies that a combination therapy of vitamin E and vitamin C may be beneficial, but this needs to be investigated further in both animal and human diabetes.

Age-dependent accumulation of advanced glycation end-products in adult Drosophila melanogaster

Advanced glycation end-product(s) (AGE) are formed in biological systems when reducing sugars react with amino groups on proteins. Long-lived proteins such as collagen and lens crystallins are known to be susceptible to AGE modification and may play a major role in the development of diabetes and other age-related pathologies. It has been previously suggested that AGE formation might affect the lifespan of experimental animals. Our study is the first to examine the effect of AGE accumulation on the life span of an organism, Drosophila melanogaster. We found that Drosophila melanogaster maintained at 24 degrees C accumulate significant AGE over their lifespan. Young flies (10 days old) had 44% less AGE than senescent flies (75 days old). We were able to reduce AGE accumulation in Drosophila melanogaster by raising the flies on a medium containing a known AGE inhibitor, aminoguanidine HCl. Reduction of AGE in flies failed to increase their mean lifespan, and high concentrations (40 mM) reduced the mean life span, which suggests that aminoguanidine is toxic at levels near those required for inhibition of AGE formation. However, the common fruit fly, Drosophila melanogaster, provides a simple model system to study the age-dependent accumulation of glycated proteins and their inhibition by novel compounds.

Cardiac disease in chronic uremia: pathophysiology

In chronic uremia, apart from frequent coronary lesions, further abnormalities of the heart recently reported include (1) left ventricular hypertrophy, not completely explained by hypertension, (2) interstitial myocardial fibrosis, for which parathyroid hormone is a permissive factor, (3) reduced myocardial perfusion reserve, secondary to functional and structural changes of intramyocardial arteries and to reduced capillary density, (4) abnormalities of myocardial metabolism, which act in concert with restriction of blood flow by microvascular abnormalities to reduce ischemic tolerance. Such metabolic abnormalities include diminished responsiveness to beta-adrenergic stimulation, abnormal control of intracellular calcium concentration, impaired maintenance of energy-rich nucleotide concentrations under conditions of ischemia, impaired insulin-mediated glucose uptake, and abnormalities of myocardial oxidative metabolism.

Advanced glycation end products in serum predict changes in the kidney morphology of patients with insulin-dependent diabetes mellitus

The biochemical mechanisms that cause the development and progression of diabetic nephropathy are unknown. Advanced glycation end products (AGEs) might play a role, as shown by increased levels of tissue-bound and circulating AGEs that correlate with the severity of diabetic nephropathy. The aim of the present study was to investigate if circulating AGEs predict the progression of morphological pathology in patients with diabetic nephropathy. We have developed an immunoassay to determine serum levels of AGEs. In a prospective clinical trial of young insulin-dependent diabetes mellitus (IDDM) patients with microalbuminuria, kidney biopsies were taken at baseline and after 24 to 36 months. The biopsies were analyzed for structural changes in the glomeruli by quantitative morphometry (electron microscopy). We have retrospectively analyzed serum AGEs. The mean serum level of AGEs at the start of the study was 18.7 U/mL (95% confidence interval [CI], 16.9 to 20.5). A positive correlation between serum AGE levels at the start of study and changes from baseline to follow-up study in basement membrane thickness (r = .56, P < .02) and matrix/glomerular volume fraction (r = .57, P < .02) was demonstrated. In a stepwise regression analysis with changes in the matrix/glomerular volume fraction as the dependent variable, serum AGE levels at the start of the study proved to be a significant independent variable (P < .02), whereas the mean hemoglobin A1c (HbA1c) or HbA1c at the start was not. This study shows that serum AGEs predict the progression of early morphological kidney damage during 2.5 years in patients with IDDM.

Lipoprotein Modification by Advanced Glycosylation Endproducts (AGEs): Role in Atherosclerosis

Recent progress in our understanding of advanced glycosylation reactions in vivo has affirmed the hypothesis that these products play an important role in the evolution of both diabetic and nondiabetic vascular disease. Utilizing newly developed advanced glycosylation end-products (AGE)-specific enzyme-linked immunosorbent assay (ELISA) techniques, AGEs have been identified to be present on a variety of vascular wall, lipoprotein, and lipid constituents. Vascular wall AGEs contribute to vascular pathology by increasing vascular permeability, enhancing subintimal protein and lipoprotein deposition, and inactivating nitric oxide. Lipid-linked AGEs present in low-density lipoprotein (LDL) also have been shown to initiate oxidative modification, promoting oxidation reactions that may proceed without the involvement of free metals or other radical generating systems. AGE-specific ELISA analysis has demonstrated a significantly increased level of AGE-modified LDL in the plasma of diabetic patients when compared to normal controls. AGE-modification impairs LDL-receptor-mediated clearance mechanisms in vivo and may contribute to elevated LDL levels in patients with diabetes. This concept has been substantiated further by the recent clinical observations that administration of the advanced glycosylation inhibitor aminoguanidine to diabetic patients significantly decreases circulating LDL levels. (Trends Cardiovasc Med 1997;7:39-47). © 1997, Elsevier Science Inc.

Advanced glycosylation end-products and heat shock proteins accumulate in the basophilic degeneration of the myocardium and the corpora amylacea of the glia

Using monospecific antibody for the advanced glycosylation end-products (AGEP), it was revealed that the AGEP localized in the basophilic degeneration of the myocardium and the corpora amylacea of the glia. The stability of the proteins that constitute those degenerative deposits suggests that they would be ideal substrates for non-enzymatic glycation, a process that occurs over a long time under a high glucose content, and ultimately results in the formation of the AGEP. Such deposits also exhibited evidence of stress reactions: the accumulation of HSP72, heme oxygenase-1 and ubiquitin. As recent studies have shown that AGEP-modified proteins aggregate and that they generate reactive oxygen intermediates, the accumulation of such heat shock proteins may reflect the oxidative stress concomitant with AGEP accumulation, and thereby promote their cellular dysfunction. Hereby, it is proposed that the age-related increase in the AGEP, that is, a fundamental aging process, is involved in the formation of the basophilic degeneration in the myocardium and the corpora amylacea of the glia.

Protein cross-linking by the Maillard reaction. Isolation, characterization, and in vivo detection of a lysine-lysine cross-link derived from methylglyoxal

The Maillard reaction, initiated by nonenzymatic glycosylation of amino groups on proteins by reducing sugars, has been studied for its potential role in aging and the complications of diabetes. One of the major consequences of the advanced Maillard reaction in proteins is the formation of covalently cross-linked aggregates. The chemical nature of the cross-linking structures is largely unknown. Recently, methylglyoxal has been shown to be a potential glycating agent in vivo and suggested to be a common intermediate in the Maillard reaction involving glucose. Methylglyoxal can form enzymatically or nonenzymatically from glycolytic intermediates and by retro-aldol cleavage of sugars. Its elevation in tissues in diabetes and its high potency to glycate and cross-link proteins led us to investigate the chemical nature of its advanced Maillard products. Using an approach in which a synthetic model peptide was reacted with methylglyoxal, we isolated and purified a cross-linked peptide dimer. Characterization of this dimer revealed that the peptides are linked through epsilon amino groups of lysine residues. The actual cross-link was shown to be a methylimidazolium, formed from the reaction of two lysines and two methylglyoxal molecules. We have named this cross-link imidazolysine. Imidazolysine was detected in proteins by high performance liquid chromatography using a postcolumn derivatization method. Proteins incubated with methylglyoxal showed a time-dependent formation of imidazolysine. Quantification of imidazolysine in human serum proteins revealed a significant increase (p < 0.05) in diabetic samples (mean +/- S.D., 313.8 +/- 52.7 pmol/mg protein) when compared with normal samples (261.3 +/- 50.4). These values correlated with glycohemoglobin (p < 0.05). These results provide chemical evidence for protein cross-linking by dicarbonyl compounds in vivo.