Expression of receptors for advanced glycation end-products in occlusive vascular and renal disease

RAGE is a critical factor of sex-based differences in age-induced kidney damage

Introduction: Advanced glycation end products (AGEs) are a heterogeneous group of molecules with potential pathophysiological effects on the kidneys. Fibrosis together with the accumulation of AGEs has been investigated for its contribution to age-related decline in renal function. AGEs mediate their effects in large parts through their interactions with the receptor for AGEs (RAGE). RAGE is a transmembrane protein that belongs to the immunoglobulin superfamily and has the ability to interact with multiple pro-inflammatory/pro-oxidative ligands. The role of RAGE in aging kidneys has not been fully characterized, especially for sex-based differences.

Methods: Therefore, we analyzed constitutive RAGE knockout (KO) mice in an age- and sex-dependent manner. Paraffin-embedded kidney sections were used for histological analysis and protein expression of fibrosis and damage markers. RNA expression analysis from the kidney cortex was done by qPCR for AGE receptors, kidney damage, and early inflammation/fibrosis factors. FACS analysis was used for immune cell profiling of the kidneys.

Results: Histological analysis revealed enhanced infiltration of immune cells (positive for B220) in aged (>70 weeks old) KO mice in both sexes. FACS analysis revealed a similar pattern of enhanced B-1a cells in aged KO mice. There was an age-based increase in pro-fibrotic and pro-inflammatory markers (IL-6, TNF, TGF-β1, and SNAIL1) in KO male mice that presumably contributed to renal fibrosis and renal damage (glomerular and tubular). In fact, in KO mice, there was an age-dependent increase in renal damage (assessed by NGAL and KIM1) that was accompanied by increased fibrosis (assessed by CTGF). This effect was more pronounced in male KO mice than in the female KO mice. In contrast to the KO animals, no significant increase in damage markers was detectable in wild-type animals at the age examined (>70 weeks old). Moreover, there is an age-based increase in AGEs and scavenger receptor MSR-A2 in the kidneys.

Discussion: Our data suggest that the loss of the clearance receptor RAGE in male animals further accelerates age-dependent renal damage; this could be in part due to an increase in AGEs load during aging and the absence of protective female hormones. By contrast, in females, RAGE expression seems to play only a minor role when compared to tissue pathology.

Bioinformatics and systems biology approach to identify the pathogenetic link of Long COVID and Myalgic Encephalomyelitis/Chronic Fatigue Syndrome

Background

The COVID-19 pandemic, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), is a global crisis. Although many people recover from COVID-19 infection, they are likely to develop persistent symptoms similar to those of myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) after discharge. Those constellations of symptoms persist for months after infection, called Long COVID, which may lead to considerable financial burden and healthcare challenges. However, the mechanisms underlying Long COVID and ME/CFS remain unclear.

Methods

We collected the genes associated with Long COVID and ME/CFS in databases by restricted screening conditions and clinical sample datasets with limited filters. The common genes for Long COVID and ME/CFS were finally obtained by taking the intersection. We performed several advanced bioinformatics analyses based on common genes, including gene ontology and pathway enrichment analyses, protein–protein interaction (PPI) analysis, transcription factor (TF)–gene interaction network analysis, transcription factor–miRNA co-regulatory network analysis, and candidate drug analysis prediction.

Results

We found nine common genes between Long COVID and ME/CFS and gained a piece of detailed information on their biological functions and signaling pathways through enrichment analysis. Five hub proteins (IL-6, IL-1B, CD8A, TP53, and CXCL8) were collected by the PPI network. The TF–gene and TF–miRNA coregulatory networks were demonstrated by NetworkAnalyst. In the end, 10 potential chemical compounds were predicted.

Conclusion

This study revealed common gene interaction networks of Long COVID and ME/CFS and predicted potential therapeutic drugs for clinical practice. Our findings help to identify the potential biological mechanism between Long COVID and ME/CFS. However, more laboratory and multicenter evidence is required to explore greater mechanistic insight before clinical application in the future.

Long-term administration of advanced glycation end-product stimulates the activation of NLRP3 inflammasome and sparking the development of renal injury

The accumulation of advanced glycation end-products (AGEs) and the enhanced interaction of AGE with their cellular receptor (RAGE) have been implicated in the progression of chronic kidney disease. The purpose of this study was to examine whether the AGE/RAGE-induced nephrotoxic effects are associated with inflammasome activation and endothelial dysfunction. Chronic renal injury was examined in BALB/c mice by the long-term administration of carbonyl-AGE for 16 weeks. Endothelial dysfunction was detected by measuring the number of circulating endothelial progenitor cells (EPCs) and the levels of nitric oxide synthase (eNOS) and nitric oxide (NO) in kidneys. Results showed that administration of methylglyoxal-bovine serum albumin (MG-BSA) AGE accelerated renal MG, carboxyethyl lysine, carboxymethyl lysine and malondialdehyde formation and, in parallel, the levels of serum creatinine and blood urea nitrogen (BUN) were significantly increased. Expression of RAGE and NLRP3 inflammasome-related proteins (TXNIP, NLRP3, procaspase-1 and caspase-1) and IL (interleukin)-1β secretion were upregulated, whereas the levels of EPCs, eNOS and NO were lower in MG-BSA-treated mice. This induction by MG-BSA was significantly inhibited by RAGE antagonist. Our results firstly reveal a possible mechanism of AGE-mediated renal dysfunction upon NLRP3 inflammasome activation. Therapeutic blockade of RAGE may ameliorate renal and endothelial functions in subjects under high AGE burden.

The Influence of Glycated Hemoglobin on the Cross Susceptibility Between Type 1 Diabetes Mellitus and Periodontal Disease

BACKGROUND

Periodontal disease is a major complication of type 1 diabetes mellitus (T1DM). The aim of the present study is to investigate the relationship between glycated hemoglobin and circulating levels of interleukin (IL)-6, IL-8, and C-X-C motif chemokine ligand 5 (CXCL5) in non-smoking patients suffering from T1DM, with and without periodontitis. In addition, to determine the effect of advanced glycation end products (AGE) in the presence and absence of Porphyromonas gingivalis lipopolysaccharide (LPS) on IL-6, IL-8, and CXCL5 expression by THP-1 monocytes and OKF6/TERT-2 cells.

METHODS

There were 104 participants in the study: 19 healthy volunteers, 23 patients with periodontitis, 28 patients with T1DM, and 34 patients with T1DM and periodontitis. Levels of blood glucose/glycated hemoglobin (International Federation of Clinical Chemistry [IFCC]) were determined by high-performance liquid chromatography. Levels of IL-6, IL-8, and CXCL5 in plasma were determined by enzyme-linked immunosorbent assay (ELISA). In vitro stimulation of OKF6/TERT-2 cells and THP-1 monocytes was performed with combinations of AGE and P. gingivalis LPS. Changes in expression of IL-6, IL-8, and CXCL5 were monitored by ELISA and real-time polymerase chain reaction.

RESULTS

Patients with diabetes and periodontitis had higher plasma levels of IL-8 than patients with periodontitis alone. Plasma levels of IL-8 correlated significantly with IFCC units, clinical probing depth, and attachment loss. AGE and LPS, alone or in combination, stimulated IL-6, IL-8, and CXCL5 expression in both OKF6/TERT-2 cells and THP-1 monocytes.

CONCLUSIONS

Elevated plasma levels of IL-8 potentially contribute to the cross-susceptibility between periodontitis and T1DM. P. gingivalis LPS and AGE in combination caused significantly greater expression of IL-6, IL-8, and CXCL5 from THP-1 monocytes and OKF6/TERT-2 cells than LPS alone.

RAGE regulation and signaling in inflammation and beyond

RAGE is a key molecule in the onset and sustainment of the inflammatory response. New studies indicate that RAGE might represent a new link between the innate and adaptive immune system. RAGE belongs to the superfamily of Ig cell-surface receptors and is expressed on all types of leukocytes promoting activation, migration, or maturation of the different cells. RAGE expression is prominent on the activated endothelium, where it mediates leukocyte adhesion and transmigration. Moreover, proinflammatory molecules released from the inflamed or injured vascular system induce migration and proliferation of SMCs. RAGE binds a large number of different ligands and is therefore considered as a PRR, recognizing a structural motif rather than a specific ligand. In this review, we summarize the current knowledge about the signaling pathways activated in the different cell types and discuss a potential activation mechanism of RAGE, as well as putative options for therapeutic intervention.

Glucose degradation products result in cardiovascular toxicity in a rat model of renal failure

BACKGROUND

It has been shown that glucose degradation products (GDP) generated during heat sterilization of peritoneal dialysis (PD) fluids impair the peritoneal membrane locally, then enter the systemic circulation and cause damage to the remnant kidney. Here we examined in subtotally nephrectomized (SNX) rats whether GDP also affect the cardiovascular system.

MATERIALS AND METHODS

Standard 5/6 nephrectomy was carried out in Sprague-Dawley rats; other rats were sham operated and left untreated for 3 weeks. Through an osmotic mini-pump, SNX+GDP group received GDP intravenously for 4 weeks; the SNX and the sham-operated groups remained without GDP. The experiment was terminated for all groups 7 weeks postoperatively. We analyzed cardiovascular damage by serum analyses and immunohistochemical investigation.

RESULTS

In SNX+GDP animals, expression of the advanced glycation end product (AGE) marker carboxymethyllysine and receptor of AGE (RAGE) were significantly higher in the myocardium and the aorta compared to the SNX rats. We also found significantly higher levels of apoptosis measured by caspase 3 staining in the cardiovascular system in the SNX+GDP group. Moreover, we observed a more pronounced expression of oxidative stress in the SNX+GDP rats compared to the SNX rats. In serum analyses, advanced oxidation protein products and reactive oxygen species were increased, as was immunohistochemical endothelial nitric oxide synthase.

CONCLUSIONS

In addition to local toxic effects, GDP cause systemic toxicity. Here we showed that, in SNX rats, administration of GDP increased cardiovascular damage. In particular, we found increased levels of AGE, RAGE, oxidative stress, and apoptosis. Whether these findings are of clinical relevance has to be further investigated.

Skin-autofluorescence is an independent predictor of graft loss in renal transplant recipients

BACKGROUND

Skin-autofluorescence (skin-AF) noninvasively measures the tissue accumulation of advanced glycation end products (AGEs). AGEs are nephrotoxic and potential effectors of cardiovascular mortality. We investigated whether skin-AF predicted graft loss after kidney transplantation.

METHODS

A total of 302 renal transplant recipients were enrolled at a median time of 6.1 (2.6-12.1) years after transplantation and were subsequently followed up for first occurrence of graft loss (i.e., graft failure or all-cause mortality) for 5.2 (4.6-5.4) years. The association of baseline skin-AF with graft loss was investigated with univariable and multivariable Cox-regression and receiver-operator-characteristic curve analyses.

RESULTS

Baseline skin-AF was 2.7+/-0.8 arbitrary units. Skin-AF predicted graft loss in a univariable Cox regression analysis (Hazard ratios 2.40 [1.75-3.29], P<0.001) and in a multivariable model (Hazard ratios 1.83 [1.22-2.75], P=0.003), adjusted for other identified risk-factors, including patient age, creatinine clearance, protein excretion, high sensitivity C-reactive protein (hsCRP), and human leukocyte antigen-DR mismatching. The area under the receiver-operator-characteristic curve for skin-AF as predictor of graft loss was significantly different from 0.5. Skin-AF was also a significant predictor of graft failure and mortality as separate end points.

CONCLUSIONS

We conclude that skin-AF is an independent predictor of graft loss in kidney transplant recipients. Although skin-AF is not a direct measurement for AGEs, we believe that our results do support the hypothesis that accumulation of AGEs in renal transplant recipients contributes to the development of graft loss.

The receptor for advanced glycation end products (RAGE) is elevated in women with preeclampsia

A leading theory of the pathophysiology of preeclampsia is that oxidative stress induces vascular endothelial cell dysfunction. Advanced glycation end products (AGEs) form when aldose sugars react nonenzymatically with proteins under conditions of oxidative stress. AGEs are circulating molecules and can generate reactive oxygen species and vascular dysfunction (in diabetes and atherosclerosis) through an association with cell surface receptors (RAGE). RAGE is a multiligand receptor, expressed in vascular tissue, which is upregulated by its own ligands. Insulin resistance and obesity are risk factors for developing preeclampsia, as well as being conditions that would increase RAGE levels. Thus, we hypothesized that women with preeclampsia will have elevated levels of RAGE protein compared with normal pregnant women. Biopsies of nonlaboring myometrium as well as omentum were taken from normal pregnant and preeclamptic women. Nonpregnant samples were obtained at the time of hysterectomy. Tissue sections were immunostained with anti-RAGE as well as anti-alpha-actin and anti-von Willebrand factor (to identify blood vessels and intact endothelial cells). Staining intensity was qualitatively described as well as given an intensity score, with the identity of the section concealed. Nonpregnant myometrial and omental vessels showed very low to undetectable levels of RAGE staining. Pregnancy induced a significant increase in RAGE protein levels in both myometrium and omental vasculature. Blood vessels from women with preeclampsia consistently had intense staining for RAGE in both vessel beds. Thus, our data suggest that since RAGE activation can induce similar pathophysiologic changes to those observed in women with preeclampsia (including NFkappaB activation, increased TNFalpha and endothelin), elevated RAGE protein may be contributing to the vascular dysfunction in preeclampsia.

Activation of tubular epithelial cells in diabetic nephropathy

Previous studies have shown that renal function in type 2 diabetes correlates better with tubular changes than with glomerular pathology. Since advanced glycation end products (AGEs; AGE-albumin) and in particular carboxymethyllysine (CML) are known to play a central role in diabetic nephropathy, we studied the activation of nuclear factor kappaB (NF-kappaB) in tubular epithelial cells in vivo and in vitro by AGE-albumin and CML. Urine samples from healthy control subjects (n = 50) and type 2 diabetic patients (n = 100) were collected and tested for excretion of CML and the presence of proximal tubular epithelial cells (pTECs). CML excretion was significantly higher in diabetic patients than in healthy control subjects (P < 0.0001) and correlated with the degree of albuminuria (r = 0.7, P < 0.0001), while there was no correlation between CML excretion and HbA(1c) (r = 0.03, P = 0.76). Urine sediments from 20 of 100 patients contained pTECs, evidenced by cytokeratin 18 positivity, while healthy control subjects (n = 50) showed none (P < 0.0001). Activated NF-kappaB could be detected in the nuclear region of excreted pTECs in 8 of 20 patients with pTECs in the urine sediment (40%). Five of eight NF-kappaBp65 antigen-positive cells stained positive for interleukin-6 (IL-6) antigen (62%), while only one of the NF-kappaB-negative cells showed IL-6 positivity. pTECs in the urine sediment correlated positively with albuminuria (r = 0.57, P < 0.0001) and CML excretion (r = 0.55, P < 0.0001). Immunohistochemistry in diabetic rat kidneys and a human diabetic kidney confirmed strong expression of NF-kappaB in tubular cells. To further prove an AGE/CML-induced NF-kappaB activation in pTECs, NF-kappaB activation was studied in cultured human pTECs by electrophoretic mobility shift assays (EMSAs) and Western blot. Stimulation of NF-kappaB binding activity was dose dependent and was one-half maximal at 250 nmol/l AGE-albumin or CML and time dependent at a maximum of activation after 4 days. Functional relevance of the observed NF-kappaB activation was demonstrated in pTECs transfected with a NF-kappaB-driven luciferase reporter plasmid and was associated with an increased release of IL-6 into the supernatant. The AGE- and CML-dependent activation of NF-kappaBp65 and NF-kappaB-dependent IL-6 expression could be inhibited using the soluble form of the receptor for AGEs (RAGE) (soluble RAGE [sRAGE]), RAGE-specific antibody, or the antioxidant thioctic acid. In addition transcriptional activity and IL-6 release from transfected cells could be inhibited by overexpression of the NF-kappaB-specific inhibitor kappaBalpha. The findings that excreted pTECs demonstrate activated NF-kappaB and IL-6 antigen and that AGE-albumin and CML lead to a perpetuated activation of NF-kappaB in vitro infer that a perpetuated increase in proinflammtory gene products, such as IL-6, plays a role in damaging the renal tubule.

Effect of diabetes and aminoguanidine therapy on renal advanced glycation end-product binding

BACKGROUND

Advanced glycation end-products (AGEs) have been implicated in the pathogenesis of diabetic nephropathy, and aminoguanidine (AG) has been shown to decrease the accumulation of AGEs in the diabetic kidney.

METHODS

This study investigates changes in AGE binding associated with diabetes in the rat kidney using in vitro and in vivo autoradiographic techniques. Male Sprague-Dawley rats were randomized into control and diabetic groups with and without AG treatment and were sacrificed after three weeks. Frozen kidney sections (20 microm) were incubated with [125I]-AGE-RNase or [125I]-AGE-BSA. To localize the AGE binding site, in vivo autoradiography was performed by injection of 15 microCi of [125I]-AGE-BSA into the abdominal aorta of the rat.

RESULTS

Low-affinity binding sites specific for AGEs in the renal cortex (IC50 = 0.28 microm) were detected by in vitro autoradiography. There was a significant increase in [125I]-AGE binding in the diabetic kidney, which was prevented by AG treatment. Emulsion autoradiography revealed that binding was localized primarily to proximal tubules in the renal cortex. Renal AGE levels, as assessed by fluorescence or by radioimmunoassay, were increased after three weeks of diabetes. This increase was attenuated by AG therapy.

CONCLUSIONS

AGE binding sites are present within the proximal tubules of the kidney and appear to be modulated by endogenous AGE levels. It remains to be determined if these binding sites represent receptors involved in clearance of AGEs or are linked to pathogenic pathways that lead to the development of diabetic nephropathy.

[Non-enzymatic glycation and oxidative stress in chronic illnesses and diabetes mellitus]

New approaches in biochemistry and molecular biology have increased the knowledge on the pathophysiology of chronic diseases as late diabetic complications, Alzheimer's disease, arteriosclerosis and vascular disease by defining the concept of "AGE-formation and oxidative stress." Nonenzymatic glycation, in which reducing sugars are covalently bound to free aminogroups of macromolecules, results in the formation of Advanced Glycation End products (AGEs) which accumulate during aging and at accelerated rate during the course of diabetes. Glycation accompanying oxidation processes support AGE-formation. AGE-formation changes the physicochemical properties of proteins, lipids and nucleic acids. In addition, binding of AGEs to specific surface receptors induces cellular signalling and cell activation. Interaction of AGEs with one of the receptors, RAGE, generates intracellular oxidative stress, which results in activation of the transcription factor NF-kappa B and subsequent gene expression, which might be relevant in late diabetic complications.

CONCLUSION

Knowledge of the basis molecular mechanisms allows to understand the interplay of different inducers such as redicals, cytokines, AGE-proteins and amyloid-beta-peptids and to define oxidative stress as a "common endpoint" of cell dysfunction. With respect to therapeutic options it is now possible not only to optimize blood glycemic control, but also to design drugs such as AGE-inhibitors and AGE-"cross-link" breakers. In addition patients with chronic disease associated with increased oxidative stress ay benefit from an antioxidant rich (and AGE protein poor?) nutrition.

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.