Regulation of alternative splicing of the receptor for advanced glycation endproducts (RAGE) through G-rich cis-elements and heterogenous nuclear ribonucleoprotein H

Dysfunctional gene splicing in glucose metabolism may contribute to Alzheimer's disease

The glucose metabolism is crucial for sustained brain activity as it provides energy and is a carbon source for multiple biomacromolecules; glucose metabolism decreases dramatically in Alzheimer's disease (AD) and may be a fundamental cause for its development. Recent studies reveal that the alternative splicing events of certain genes effectively regulate several processes in glucose metabolism including insulin receptor, insulin-degrading enzyme, pyruvate kinase M, receptor for advanced glycation endproducts, and others, thereby, influencing glucose uptake, glycolysis, and advanced glycation end-products-mediated signaling pathways. Indeed, the discovery of aberrant alternative splicing that changes the proteomic diversity and protein activity in glucose metabolism has been pivotal in our understanding of AD development. In this review, we summarize the alternative splicing events of the glucose metabolism-related genes in AD pathology and highlight the crucial regulatory roles of splicing factors in the alternative splicing process. We also discuss the emerging therapeutic approaches for targeting splicing factors for AD treatment.

Promoter methylation cooperates with SNPs to modulate RAGE transcription and alter UC risk

Single-nucleotide polymorphisms (SNPs) located in the promoter region of the receptor for advanced glycation end products (RAGE) gene have been linked to the activity of RAGE. However, contrary to our expectation, we previously detected no correlation between SNPs within the RAGE promoter and ulcerative colitis (UC) risk in a case-control study. Here, we investigated the methylation of the RAGE promoter and analyzed the collective contribution of methylation and SNPs to UC risk. We found that RAGE promoter hypomethylation was more common in UC patients compared to controls (70% vs. 30%, respectively), as determined via bisulfite sequencing PCR (BSP) and methylation-specific PCR (MSP). Furthermore, we investigated the cooperativity of promoter methylation and SNPs and found that either of two SNPs (rs1800624 or rs1800625) and promoter methylation jointly contributed to UC risk (30 UC patients vs. 30 controls, P < 0.05). There was no correlation between UC risk and either methylation or SNPs when analyzed separately. This lack of correlation is likely due to promoter methylation repressing gene transcription, whereas SNPs in the RAGE promoter region activate RAGE transcription. We found that variant allele carriers with promoter hypomethylation were at an increased risk for UC (rs1800624, OR = 10, 95% CI: 1.641-60.21, P = 0.009; rs1800625, OR = 4.8, 95% CI: 1.074-21.447, P = 0.039). Furthermore, our data revealed that the RAGE mRNA levels in variant allele carriers with promoter hypomethylation were significantly higher compared to those with promoter hypermethylation (P < 0.05) as well as to those in wild-type allele individuals exhibiting promoter hypomethylation (P < 0.05). We therefore speculate that the methylation status and SNPs present in the RAGE promoter region alter RAGE transcription, thereby impacting UC risk. We also propose that the methylation status and RAGE promoter genotype could jointly serve as clinical biomarkers to assist in UC risk assessment.

The regulation of soluble receptor for AGEs contributes to carbonyl stress in schizophrenia

Our previous study showed that enhanced carbonyl stress is closely related to schizophrenia. The endogenous secretory receptor for advanced glycation end-products (esRAGE) is a splice variant of the AGER gene and is one of the soluble forms of RAGE. esRAGE is considered to be a key molecule for alleviating the burden of carbonyl stress by entrapping advanced glycation end-products (AGEs). In the current study, we conducted genetic association analyses focusing on AGER, in which we compared 212 schizophrenic patients to 214 control subjects. We also compared esRAGE levels among a subgroup of 104 patients and 89 controls and further carried out measurements of total circulating soluble RAGE (sRAGE) in 25 patients and 49 healthy subjects. Although the genetic association study yielded inconclusive results, multiple regression analysis indicated that a specific haplotype composed of rs17846798, rs2071288, and a 63 bp deletion, which were in perfect linkage disequilibrium (r² = 1), and rs2070600 (Gly82Ser) were significantly associated with a marked decrease in serum esRAGE levels. Furthermore, compared to healthy subjects, schizophrenia showed significantly lower esRAGE (p = 0.007) and sRAGE (p = 0.03) levels, respectively. This is the first study to show that serum esRAGE levels are regulated by a newly identified specific haplotype in AGER and that a subpopulation of schizophrenic patients are more vulnerable to carbonyl stress.

Markers of early endothelial dysfunction in intrauterine growth restriction-derived human umbilical vein endothelial cells revealed by 2D-DIGE and mass spectrometry analyses

Intrauterine growth restriction (IUGR) associates with fetal and placental vascular dysfunction, and increased cardiovascular risk later on life. We hypothesize that endothelial cells derived from IUGR umbilical veins present significant changes in the proteome which could be involved in the endothelial dysfunction associated to this conditions. To address this the proteome profile of human umbilical endothelial cells (HUVEC) isolated from control and IUGR pregnancies was compared by 2D-Differential In Gel Electrophoresis (DIGE) and further protein identification by MALDI-TOF MS. Using 2D-DIGE 124 spots were identified as differentially expressed between control and IUGR HUVEC, considering a cut-off of 2 fold change, which represented ∼10% of the total spots detected. Further identification by MALDI-TOF MS and in silico clustering of the proteins showed that those differentially expressed proteins between control and IUGR HUVEC were mainly related with cytoskeleton organization, proteasome degradation, oxidative stress response, mRNA processing, chaperones and vascular function. Finally Principal Component analysis of the identified proteins showed that differentially expressed proteins allow distinguishing between control and IUGR HUVEC based on their proteomic profile. This study demonstrates for the first time that IUGR-derived HUVEC maintained in primary culture conditions present an altered proteome profile, which could reflect an abnormal programming of endothelial function in this fetal condition.

Soluble RAGE and atherosclerosis in youth with type 1 diabetes: a 5-year follow-up study

BACKGROUND

Advanced glycation end products (AGEs) play a role in the development of late complications and atherosclerosis in diabetes by engaging the receptor for advanced glycation end products, RAGE. Receptor binding leads to activation of the vascular endothelium and increased inflammation in the vessel wall. The soluble variants of the receptor, endogenous secretory RAGE (esRAGE) and the cleaved cell-surface part of RAGE, which together comprise soluble RAGE (sRAGE), are suggested to have a protective effect acting as decoys for RAGE. We aimed to test whether high levels of soluble variants of RAGE could be protective against atherosclerosis development.

METHODS

Participants in the prospective atherosclerosis and childhood diabetes study were examined at baseline (aged 8-18) and at follow-up after 5 years. Both sRAGE and esRAGE were measured by immunoassay in 299 patients with type 1 diabetes and 112 healthy controls at baseline and 241 patients and 128 controls at follow-up. The AGEs methylglyoxal-derived hydroimidazolone-1 (MG-H1) and carboxymethyllysine (CML) were measured by immunoassay. The surrogate markers of atherosclerosis assessed were carotid intima-media thickness (cIMT), C-reactive protein (CRP) and Young's modulus, measures of arterial wall thickness, inflammation and arterial stiffness, respectively.

RESULTS

Levels of sRAGE and esRAGE correlated strongly both at baseline and at follow-up in both diabetes patients and controls. With increasing age, mean values of both variants declined, independent of gender, diabetes or pubertal stage. In the diabetes group, multiple regression analysis showed a positive association between both variants of soluble RAGE and cIMT. There was no significant relationship with Young's modulus, but a negative association between sRAGE at baseline and CRP at follow-up. The ratios between the AGEs and the variants of soluble RAGE were increased in diabetes patients compared to controls.

CONCLUSIONS

The results show a possible protective effect of high levels of sRAGE at baseline against inflammation 5 years later, but not on arterial stiffness or wall thickness, in this cohort of adolescents and young adults with T1D.

Relationship between serum levels of endogenous secretory RAGE and blood pressure in male nondiabetic patients with obstructive sleep apnea

The interaction of advanced glycation end products (AGE) and their specific cell-surface receptor (RAGE) has an important role in the pathogenesis of cardiovascular disease and diabetic complications. Two isoforms of C-truncated RAGE, soluble RAGE (sRAGE) and endogenous secretory RAGE (esRAGE), may prevent activation of RAGE signaling by acting as decoys. This study investigated whether serum esRAGE and sRAGE levels are associated with blood pressure in nondiabetic patients with obstructive sleep apnea (OSA). Male nondiabetic patients (n=139) with OSA were enrolled. Serum esRAGE and sRAGE levels were examined using enzyme-linked immunosorbent assay. Three consecutive seated systolic blood pressure (SBP) and diastolic blood pressure (DBP) measurements were obtained at 5-min intervals in the morning. In univariate analysis, there was a significant correlation between serum esRAGE and SBP or DBP, but not between serum sRAGE and SBP or DBP. Multiple regression analysis showed that SBP was independently associated with waist circumference, HbA1c, minimum SaO2 and serum esRAGE, and that DBP was independently associated with low-density lipoprotein cholesterol, apnea-hypopnea index, serum AGE and body mass index, but not with serum esRAGE. These results indicated that serum esRAGE levels were inversely associated with blood pressure, especially SBP, in male nondiabetic patients with OSA. esRAGE may have a protective role against hypertension in patients with OSA, and it may be a novel biomarker for OSA patients at high risk of developing cardiovascular diseases.

Effects of methanolic extracts from edible plants on endogenous secretory receptor for advanced glycation end products induced by the high glucose incubation in human endothelial cells

Background:

In diabetic populations, endogenous secretory receptor for advanced glycation end products (esRAGE) levels may be related to the degree of diabetic complications or to the protection from diabetic complications.

Objective:

We investigated the impact of 29 methanolic extracts from edible plants on esRAGE production in human umbilical vein endothelial cells (HUVECs) cultured in high (4.5 g/L) glucose.

Materials and Methods:

Edible plants were minced, and extracts were obtained with methanol overnight. The methanolic extracts from 29 edible plants were evaporated in a vacuum. For screening study purposes, HUVECs were seeded in culture dishes (1.5 × 10⁵ cells). Then, HUVECs were incubated with 1 g/L or 4.5 g/L of glucose in SFM CS-C medium treated with methanolic extracts from edible plants (MEEP) for 96 h. Determination of esRAGE production in the cell culture-derived supernatants was performed by colorimetric ELISA. The 8-hydroxydeoxyguanosine (8-OHdG) level was determined by using the 8-OHdG Check ELISA kit. Peroxynitrite-dependent oxidation of 2’, 7’-dichlorodihydrofluorescein to 2’, 7’-dichlorofluorescein was estimated based on the method described by Crow. Because MEEP were methanolic extracts, we measured their total phenolic content (TPC). TPC was measured with a modified version of the Folin–Ciocalteu method.

Results:

The results showed eight extracts increased esRAGE production. The extract from white radish sprouts showed the highest esRAGE production activity, and then eggplant, carrot peel, young sweet corn, Jew's marrow, broad bean, Japanese radish and cauliflower. In order to understand the mechanism of esRAGE production, the eight extracts were examined for DNA damage, peroxynitrite scavenging activity, and TPC in correlation with their esRAGE production. The results showed esRAGE production correlates with the peroxynitrite level and TPC.

Conclusion:

This study supports the utilization of these eight extracts in folk medicine for improved treatment of diabetic complications.

Regulation of RAGE splicing by hnRNP A1 and Tra2β-1 and its potential role in AD pathogenesis

The receptor for advanced glycation end products (RAGE) gene expresses two major alternative splicing isoforms, full-length membrane-bound RAGE (mRAGE) and secretory RAGE (esRAGE). Both isoforms play important roles in Alzheimer's disease (AD) pathogenesis, either via interaction of mRAGE with β-amyloid peptide (Aβ) or inhibition of the mRAGE-activated signaling pathway. In the present study, we showed that heterogeneous nuclear ribonucleoprotein A1 (hnRNP A1) and Transformer2β-1 (Tra2β-1) were involved in the alternative splicing of mRAGE and esRAGE. Functionally, two factors had an antagonistic effect on the regulation. Glucose deprivation induced an increased ratio of mRAGE/esRAGE via up-regulation of hnRNP A1 and down-regulation of Tra2β-1. Moreover, the ratios of mRAGE/esRAGE and hnRNP A1/Tra2β-1 were increased in peripheral blood mononuclear cells from AD patients. The results provide a molecular basis for altered splicing of mRAGE and esRAGE in AD pathogenesis. The receptor for advanced glycation end products (RAGE) gene expresses two major alternative splicing isoforms, membrane-bound RAGE (mRAGE) and secretory RAGE (esRAGE). Both isoforms play important roles in Alzheimer's disease (AD) pathogenesis. Mechanism for imbalanced expression of these two isoforms in AD brain remains elusive. We proposed here a hypothetic model to illustrate that impaired glucose metabolism in AD brain may increase the expression of splicing protein hnRNP A1 and reduce Tra2β-1, which cause the imbalanced expression of mRAGE and esRAGE.

Molecular effects of advanced glycation end products on cell signalling pathways, ageing and pathophysiology

Advanced glycation end-products (AGEs) are a heterogeneous group of compounds formed by the Maillard chemical process of non- enzymatic glycation of free amino groups of proteins, lipids and nucleic acids. This chemical modification of biomolecules is triggered by endogeneous hyperglycaemic or oxidative stress-related processes. Additionally, AGEs can derive from exogenous, mostly diet-related, sources. Considering that AGE accumulation in tissues correlates with ageing and is a hallmark in several age-related diseases it is not surprising that the role of AGEs in ageing and pathology has become increasingly evident. The receptor for AGEs (RAGE) is a single transmembrane protein being expressed in a wide variety of human cells. RAGE binds a broad repertoire of extracellular ligands and mediates responses to stress conditions by activating multiple signal transduction pathways being mostly responsible for acute and/or chronic inflammation. RAGE activation has been implicated in ageing as well as in a number of age-related diseases, including atherosclerosis, neurodegeneration, arthritis, stoke, diabetes and cancer. Here we present a synopsis of findings that relate to AGEs-reported implication in cell signalling pathways and ageing, as well as in pathology. Potential implications and opportunities for translational research and the development of new therapies are also discussed.

Perspectives on RAGE signaling and its role in cardiovascular disease

RAGE stands for Receptor of Advanced Glycation Endproducts. The two main topics discussed are (1) the nature of RAGE signaling and (2) its role in cardiovascular disease. RAGE may occur in membrane-bound form or in secretory form. RAGE signaling involves multiple ligands: (1) several AGEs (2) amyloid β pecursor protein (APP), (3) high mobility group box 1 (HMGB1), (4) S100A4, (5) S100A8/A9, and (6) S100A12, which are calcium-binding proteins, and (7) S100B, a glial-derived protein. RAGE ligands and various diseases involving RAGE signaling are summarized in tabular form.

RAGE-mediated cell signaling

RAGE (receptor for advanced glycation end products) is a multi-ligand receptor that belongs to the immunoglobulin superfamily of transmembrane proteins. RAGE binds AGEs (advanced glycation end products), HMGB1 (high-mobility group box-1; also designated as amphoterin), members of the S100 protein family, glycosaminoglycans and amyloid β peptides. Recent studies using tools of structural biology have started to unravel common molecular patterns in the diverse set of ligands recognized by RAGE. The distal Ig domain (V1 domain) of RAGE has a positively charged patch, the geometry of which fits to anionic surfaces displayed at least in a proportion of RAGE ligands. Association of RAGE to itself, to HSPGs (heparan sulfate proteoglycans), and to Toll-like receptors in the cell membrane plays a key role in cell signaling initiated by RAGE ligation. Ligation of RAGE activates cell signaling pathways that regulate migration of several cell types. Furthermore, RAGE ligation has profound effects on the transcriptional profile of cells. RAGE signaling has been mainly studied as a pathogenetic factor of several diseases, where acute or chronic inflammation plays a role. Recent studies have suggested a physiological role for RAGE in normal lung function and in neuronal signaling.

The functional -374T/A polymorphism of the receptor for advanced glycation end products may modulate Crohn's disease

The receptor for advanced glycation end products (RAGE) is involved in innate immune mechanisms. Polymorphisms of the RAGE gene have been described as a factor amplifying inflammation in susceptible patients, but the association with Crohn's disease (CD) is not known. The coding RAGE polymorphism G82S (rs2070600) and two promoter polymorphisms, -374T/A (rs1800624) and -429T/C (rs1800625), were studied in two samples from Germany and the United States consisting of 421 and 317 CD patients and 549 and 218 controls, respectively. To test the functional relevance, additional data on serum soluble RAGE (sRAGE), tissue RNA, and protein levels were collected and immunohistochemical stainings of bowel tissue of CD patients and healthy controls as well as models of experimental (dextran sodium sulfate-induced) colitis in RAGE knockout and wild-type mice were performed. The -374T/A RAGE promotor single nucleotide polymorphism (SNP) was negatively associated with CD (odds ratio = 0.708, 95% confidence interval = 0.535-0.938, P = 0.016) and with stenosis (OR = 0.627, P = 0.04) in the German sample. Transmission disequilibrium testing confirmed an undertransmission of the -374A allele. Serum sRAGE levels were higher in patients in complete remission of the -374AA/TA group (1,975 ± 299 pg/ml; -374TT group: 1,310 ± 153 pg/ml SE, P < 0.05) and showed a trend toward decreased levels in CD patients with active disease compared with CD patients in remission. Further in vitro and in vivo studies indicated that an increase of sRAGE ameliorates inflammation. The -429T/C and the G82S polymorphism were not associated with CD. The -374T/A RAGE polymorphism leading to facilitated RAGE gene transcription may to some degree protect from developing a stricturing subphenotype of CD, most likely by increasing levels of sRAGE, which neutralizes proinflammatory mediators.