RAGE deficiency does not affect non-alcoholic steatohepatitis and atherosclerosis in Western type diet-fed Ldlr-/- mice

SMTP-44D Inhibits Atherosclerotic Plaque Formation in Apolipoprotein-E Null Mice Partly by Suppressing the AGEs-RAGE Axis

SMTP-44D has been reported to have anti-oxidative and anti-inflammatory reactions, including reduced expression of receptor for advanced glycation end products (RAGE) in experimental diabetic neuropathy. Although activation of RAGE with its ligands, and advanced glycation end products (AGEs), play a crucial role in atherosclerotic cardiovascular disease, a leading cause of death in diabetic patients, it remains unclear whether SMTP-44D could inhibit experimental atherosclerosis by suppressing the AGEs–RAGE axis. In this study, we investigated the effects of SMTP-44D on atherosclerotic plaque formation and expression of AGEs in apolipoprotein-E null (Apoe−/−) mice. We further studied here whether and how SMTP-44D inhibited foam cell formation of macrophages isolated from Apoe−/− mice ex vivo. Although administration of SMTP-44D to Apoe−/− mice did not affect clinical or biochemical parameters, it significantly decreased the surface area of atherosclerotic lesions and reduced the atheromatous plaque size, macrophage infiltration, and AGEs accumulation in the aortic roots. SMTP-44D bound to immobilized RAGE and subsequently attenuated the interaction of AGEs with RAGE in vitro. Furthermore, foam cell formation evaluated by Dil-oxidized low-density lipoprotein (ox-LDL) uptake, and gene expression of RAGE, cyclin-dependent kinase 5 (Cdk5) and CD36 in macrophages isolated from SMTP-44D-treated Apoe−/− mice were significantly decreased compared with those from saline-treated mice. Gene expression levels of RAGE and Cdk5 were highly correlated with each other, the latter of which was also positively associated with that of CD36. The present study suggests that SMTP-44D may inhibit atherosclerotic plaque formation in Apoe−/− mice partly by blocking the AGEs-RAGE-induced ox-LDL uptake into macrophages via the suppression of Cdk5-CD36 pathway.

Advanced glycation endproducts in diabetes-related macrovascular complications: focus on methylglyoxal

Diabetes is associated with vascular injury and the onset of macrovascular complications. Advanced glycation endproducts (AGEs) and the AGE precursor methylglyoxal (MGO) have been identified as key players in establishing the relationship between diabetes and vascular injury. While most research has focused on the link between AGEs and vascular injury, less is known about the effects of MGO on vasculature. In this review, we focus on the mechanisms linking AGEs and MGO to the development of atherosclerosis. AGEs and MGO are involved in many stages of atherosclerosis progression. However, more research is needed to determine the exact mechanisms underlying these effects. Nevertheless, AGEs and MGO could represent valid therapeutic targets for the macrovascular complications of diabetes.

Loss of GPR40 in LDL receptor-deficient mice exacerbates high-fat diet-induced hyperlipidemia and nonalcoholic steatohepatitis

GPR40, a G protein-coupled receptor for free fatty acids (FFAs), is considered as a therapeutic target for type 2 diabetes mellitus (T2DM) since GPR40 activation in pancreatic beta cells enhances glucose-stimulated insulin secretion. Nonalcoholic fatty liver disease (NAFLD) is a common complication of T2DM or metabolic syndrome (MetS). However, the role of GPR40 in NAFLD associated with T2DM or MetS has not been well established. Given that it is known that cholesterol and FFAs are critically involved in the pathogenesis of nonalcoholic steatohepatitis (NASH) and LDL receptor (LDLR)-deficient mice are a good animal model for human hyperlipidemia including high cholesterol and FFAs, we generated GPR40 and LDLR double knockout (KO) mice in this study to determine the effect of GPR40 KO on hyperlipidemia-promoted NASH. We showed that GPR40 KO increased plasma levels of cholesterol and FFAs in high-fat diet (HFD)-fed LDLR-deficient mice. We also showed that GPR40 KO exacerbated HFD-induced hepatic steatosis, inflammation and fibrosis. Further study demonstrated that GPR40 KO led to upregulation of hepatic CD36 and genes involved in lipogenesis, fatty acid oxidation, fibrosis and inflammation. Finally, our in vitro mechanistic studies showed that while CD36 was involved in upregulation of proinflammatory molecules in macrophages by palmitic acid (PA) and lipopolysaccharide (LPS), GPR40 activation in macrophages exerts anti-inflammatory effects. Taken together, this study demonstrated for the first time that loss of GPR40 in LDLR-deficient mice exacerbated HFD-induced hyperlipidemia, hepatic steatosis, inflammation and fibrosis potentially through a CD36-dependent mechanism, suggesting that GPR40 may play a beneficial role in hyperlipidemia-associated NASH in LDLR-deficient mice.

Methylglyoxal and Its Adducts: Induction, Repair, and Association with Disease

Metabolism is an essential part of life that provides energy for cell growth. During metabolic flux, reactive electrophiles are produced that covalently modify macromolecules, leading to detrimental cellular effects. Methylglyoxal (MG) is an abundant electrophile formed from lipid, protein, and glucose metabolism at intracellular levels of 1-4 μM. MG covalently modifies DNA, RNA, and protein, forming advanced glycation end products (MG-AGEs). MG and MG-AGEs are associated with the onset and progression of many pathologies including diabetes, cancer, and liver and kidney disease. Regulating MG and MG-AGEs is a potential strategy to prevent disease, and they may also have utility as biomarkers to predict disease risk, onset, and progression. Here, we review recent advances and knowledge surrounding MG, including its production and elimination, mechanisms of MG-AGEs formation, the physiological impact of MG and MG-AGEs in disease onset and progression, and the latter in the context of its receptor RAGE. We also discuss methods for measuring MG and MG-AGEs and their clinical application as prognostic biomarkers to allow for early detection and intervention prior to disease onset. Finally, we consider relevant clinical applications and current therapeutic strategies aimed at targeting MG, MG-AGEs, and RAGE to ultimately improve patient outcomes.

The mechanisms of ferroptosis and its role in alzheimer’s disease

Alzheimer’s disease (AD) accounts for two-thirds of all dementia cases, affecting 50 million people worldwide. Only four of the more than 100 AD drugs developed thus far have successfully improved AD symptoms. Furthermore, these improvements are only temporary, as no treatment can stop or reverse AD progression. A growing number of recent studies have demonstrated that iron-dependent programmed cell death, known as ferroptosis, contributes to AD-mediated nerve cell death. The ferroptosis pathways within nerve cells include iron homeostasis regulation, cystine/glutamate (Glu) reverse transporter (system xc⁻), glutathione (GSH)/glutathione peroxidase 4 (GPX4), and lipid peroxidation. In the regulation pathway of AD iron homeostasis, abnormal iron uptake, excretion and storage in nerve cells lead to increased intracellular free iron and Fenton reactions. Furthermore, decreased Glu transporter expression leads to Glu accumulation outside nerve cells, resulting in the inhibition of the system xc⁻ pathway. GSH depletion causes abnormalities in GPX4, leading to excessive accumulation of lipid peroxides. Alterations in these specific pathways and amino acid metabolism eventually lead to ferroptosis. This review explores the connection between AD and the ferroptosis signaling pathways and amino acid metabolism, potentially informing future AD diagnosis and treatment methodologies.

Glycation and a Spark of ALEs (Advanced Lipoxidation End Products) - Igniting RAGE/Diaphanous-1 and Cardiometabolic Disease

Obesity and non-alcoholic fatty liver disease (NAFLD) are on the rise world-wide; despite fervent advocacy for healthier diets and enhanced physical activity, these disorders persist unabated and, long-term, are major causes of morbidity and mortality. Numerous fundamental biochemical and molecular pathways participate in these events at incipient, mid- and advanced stages during atherogenesis and impaired regression of established atherosclerosis. It is proposed that upon the consumption of high fat/high sugar diets, the production of receptor for advanced glycation end products (RAGE) ligands, advanced glycation end products (AGEs) and advanced lipoxidation end products (ALEs), contribute to the development of foam cells, endothelial injury, vascular inflammation, and, ultimately, atherosclerosis and its consequences. RAGE/Diaphanous-1 (DIAPH1) increases macrophage foam cell formation; decreases cholesterol efflux and causes foam cells to produce and release damage associated molecular patterns (DAMPs) molecules, which are also ligands of RAGE. DAMPs stimulate upregulation of Interferon Regulatory Factor 7 (IRF7) in macrophages, which exacerbates vascular inflammation and further perturbs cholesterol metabolism. Obesity and NAFLD, characterized by the upregulation of AGEs, ALEs and DAMPs in the target tissues, contribute to insulin resistance, hyperglycemia and type two diabetes. Once in motion, a vicious cycle of RAGE ligand production and exacerbation of RAGE/DIAPH1 signaling ensues, which, if left unchecked, augments cardiometabolic disease and its consequences. This Review focuses on RAGE/DIAPH1 and its role in perturbation of metabolism and processes that converge to augur cardiovascular disease.

Targeting Scavenger Receptors in Inflammatory Disorders and Oxidative Stress

Oxidative stress and inflammation cannot be considered as diseases themselves; however, they are major risk factors for the development and progression of the pathogenesis underlying many illnesses, such as cancer, neurological disorders (including Alzheimer's disease and Parkinson's disease), autoimmune and metabolic disorders, etc. According to the results obtained from extensive studies, oxidative stress-induced biomolecules, such as advanced oxidation protein products, advanced glycation end products, and advanced lipoxidation end products, are critical for an accelerated level of inflammation and oxidative stress-induced cellular damage, as reflected in their strong affinity to a wide range of scavenger receptors. Based on the limitations of antioxidative and anti-inflammatory molecules in practical applications, targeting such interactions between harmful molecules and their cellular receptors/signaling with advances in gene engineering technology, such as CRISPR or TALEN, may prove to be a safe and effective alternative. In this review, we summarize the findings of recent studies focused on the deletion of scavenger receptors under oxidative stress as a development in the therapeutic approaches against the diseases linked to inflammation and the contribution of advanced glycation end products (AGEs), advanced lipid peroxidation products (ALEs), and advanced oxidation protein products (AOPPs).

Deletion of RAGE fails to prevent hepatosteatosis in obese mice due to impairment of other AGEs receptors and detoxifying systems

Advanced glycation endproducts (AGEs) are involved in several diseases, including NAFLD and NASH. RAGE is the main receptor mediating the pro-inflammatory signalling induced by AGEs. Therefore, targeting of RAGE has been proposed for prevention of chronic inflammatory diseases. However, the role of RAGE in the development of NAFLD and NASH remains poorly understood. We thus aimed to analyse the effect of obesity on AGEs accumulation, AGE-receptors and AGE-detoxification, and whether the absence of RAGE might improve hepatosteatosis and inflammation, by comparing the liver of lean control, obese (LeptrDb-/-) and obese RAGE-deficient (RAGE-/- LeptrDb-/-) mice. Obesity induced AGEs accumulation and RAGE expression with hepatosteatosis and inflammation in LeptrDb-/-, compared to lean controls. Despite the genetic deletion of RAGE in the LeptrDb-/- mice, high levels of intrahepatic AGEs were maintained accompanied by decreased expression of the protective AGE-receptor-1, impaired AGE-detoxifying system glyoxalase-1, and increased expression of the alternative AGE-receptor galectin-3. We also found sustained hepatosteatosis and inflammation as determined by persistent activation of the lipogenic SREBP1c and proinflammatory NLRP3 signalling pathways. Thus, RAGE targeting is not effective in the prevention of NAFLD in conditions of obesity, likely due to the direct liver specific crosstalk of RAGE with other AGE-receptors and AGE-detoxifying systems.

Glucose-Dependent Insulinotropic Polypeptide Suppresses Foam Cell Formation of Macrophages through Inhibition of the Cyclin-Dependent Kinase 5-CD36 Pathway

Glucose-dependent insulinotropic polypeptide (GIP) has been reported to have an atheroprotective property in animal models. However, the effect of GIP on macrophage foam cell formation, a crucial step of atherosclerosis, remains largely unknown. We investigated the effects of GIP on foam cell formation of, and CD36 expression in, macrophages extracted from GIP receptor-deficient (Gipr^−/−) and Gipr^+/+ mice and cultured human U937 macrophages by using an agonist for GIP receptor, [D-Ala²]GIP(1–42). Foam cell formation evaluated by esterification of free cholesterol to cholesteryl ester and CD36 gene expression in macrophages isolated from Gipr^+/+ mice infused subcutaneously with [D-Ala²]GIP(1–42) were significantly suppressed compared with vehicle-treated mice, while these beneficial effects were not observed in macrophages isolated from Gipr^−/− mice infused with [D-Ala²]GIP(1–42). When macrophages were isolated from Gipr^+/+ and Gipr^−/− mice, and then exposed to [D-Ala²]GIP(1–42), similar results were obtained. [D-Ala²]GIP(1–42) attenuated ox-LDL uptake of, and CD36 gene expression in, human U937 macrophages as well. Gene expression level of cyclin-dependent kinase 5 (Cdk5) was also suppressed by [D-Ala²]GIP(1–42) in U937 cells, which was corelated with that of CD36. A selective inhibitor of Cdk5, (R)-DRF053 mimicked the effects of [D-Ala²]GIP(1–42) in U937 cells. The present study suggests that GIP could inhibit foam cell formation of macrophages by suppressing the Cdk5-CD36 pathway via GIP receptor.

The Role of Oxidative Stress in NAFLD-NASH-HCC Transition-Focus on NADPH Oxidases

A peculiar role for oxidative stress in non-alcoholic fatty liver disease (NAFLD) and its transition to the inflammatory complication non-alcoholic steatohepatitis (NASH), as well as in its threatening evolution to hepatocellular carcinoma (HCC), is supported by numerous experimental and clinical studies. NADPH oxidases (NOXs) are enzymes producing reactive oxygen species (ROS), whose abundance in liver cells is closely related to inflammation and immune responses. Here, we reviewed recent findings regarding this topic, focusing on the role of NOXs in the different stages of fatty liver disease and describing the current knowledge about their mechanisms of action. We conclude that, although there is a consensus that NOX-produced ROS are toxic in non-neoplastic conditions due to their role in the inflammatory vicious cycle sustaining the transition of NAFLD to NASH, their effect is controversial in the neoplastic transition towards HCC. In this regard, there are indications of a differential effect of NOX isoforms, since NOX1 and NOX2 play a detrimental role, whereas increased NOX4 expression appears to be correlated with better HCC prognosis in some studies. Further studies are needed to fully unravel the mechanisms of action of NOXs and their relationships with the signaling pathways modulating steatosis and liver cancer development.

Increase of α-dicarbonyls in liver and receptor for advanced glycation end products on immune cells are linked to nonalcoholic fatty liver disease and liver cancer

Hepatocellular carcinoma (HCC) is the most common primary malignancy of the liver with a very poor prognosis and constantly growing incidence. Among other primary risks of HCC, metabolic disorders and obesity have been extensively investigated over recent decades. The latter can promote nonalcoholic fatty liver disease (NAFLD) leading to the inflammatory form of nonalcoholic steatohepatitis (NASH), that, in turn, promotes HCC. Molecular determinants of this pathogenic progression, however, remain largely undefined. In this study, we have focussed on the investigation of α-dicarbonyl compounds (α-dC), highly reactive and tightly associated with overweight-induced metabolic disorders, and studied their potential role in NAFLD and progression toward HCC using murine models. NAFLD was induced using high-fat diet (HFD). Autochthonous HCC was induced using transposon-based stable intrahepatic overexpression of oncogenic NRAS^G12V in mice lacking p19^Arf tumor suppressor. Our study demonstrates that the HFD regimen and HCC resulted in strong upregulation of α-dC in the liver, heart, and muscles. In addition, an increase in α-dC was confirmed in sera of NAFLD and NASH patients. Furthermore, higher expression of the receptor for advanced glycation products (RAGE) was detected exclusively on immune cells and not on stroma cells in livers of mice with liver cancer progression. Our work confirms astable interplay of liver inflammation, carbonyl stress mediated by α-dC, and upregulated RAGE expression on CD8⁺ Tand natural killer (NK) cells in situ in NAFLD and HCC, as key factors/determinants in liver disease progression. The obtained findings underline the role of α-dC and RAGE⁺CD8⁺ Tand RAGE⁺ NK cells as biomarkers and candidates for a local therapeutic intervention in NAFLD and malignant liver disease.

AGE-RAGE Axis Stimulates Oxidized LDL Uptake into Macrophages through Cyclin-Dependent Kinase 5-CD36 Pathway via Oxidative Stress Generation

Advanced glycation end products (AGEs) are localized in macrophage-derived foam cells within atherosclerotic lesions, which could be associated with the increased risk of atherosclerotic cardiovascular disease under diabetic conditions. Although foam cell formation of macrophages has been shown to be enhanced by AGEs, the underlying molecular mechanism remains unclear. Since cyclin-dependent kinase 5 (Cdk5) is reported to modulate inflammatory responses in macrophages, we investigated whether Cdk5 could be involved in AGE-induced CD36 gene expression and foam cell formation of macrophages. AGEs significantly increased Dil-oxidized low-density lipoprotein (ox-LDL) uptake, and Cdk5 and CD36 gene expression in U937 human macrophages, all of which were inhibited by DNA aptamer raised against RAGE (RAGE-aptamer). Cdk5 and CD36 gene expression levels were correlated with each other. An antioxidant, N-acetyl-l-cysteine, mimicked the effects of RAGE-aptamer on AGE-exposed U937 cells. A selective inhibitor of Cdk5, (R)-DRF053, attenuated the AGE-induced Dil-ox-LDL uptake and CD36 gene expression, whereas anti-CD36 antibody inhibited the Dil-ox-LDL uptake but not Cdk5 gene expression. The present study suggests that AGEs may stimulate ox-LDL uptake into macrophages through the Cdk5–CD36 pathway via RAGE-mediated oxidative stress.

Danger signals in liver injury and restoration of homeostasis

Damage-associated molecular patterns are signalling molecules involved in inflammatory responses and restoration of homeostasis. Chronic release of these molecules can also promote inflammation in the context of liver disease. Herein, we provide a comprehensive summary of the role of damage-associated molecular patterns as danger signals in liver injury. We consider the role of reactive oxygen species and reactive nitrogen species as inducers of damage-associated molecular patterns, as well as how specific damage-associated molecular patterns participate in the pathogenesis of chronic liver diseases such as alcohol-related liver disease, non-alcoholic steatohepatitis, liver fibrosis and liver cancer. In addition, we discuss the role of damage-associated molecular patterns in ischaemia reperfusion injury and liver transplantation and highlight current studies in which blockade of specific damage-associated molecular patterns has proven beneficial in humans and mice.

Aging-induced aberrant RAGE/PPARα axis promotes hepatic steatosis via dysfunctional mitochondrial β oxidation

Non‐alcoholic fatty liver disease (NAFLD), characterized by an increase in hepatic triglyceride (TG) content, is the most common liver disease worldwide. Aging has been shown to increase susceptibility to NAFLD; however, the underlying molecular mechanism remains poorly understood. In the present study, we examined hepatic TG content and gene expression profiles in body weight‐matched young (3 months old), middle‐aged (10 months old), and old (20 months old) C57BL/6 mice and found that TGs were markedly accumulated while mitochondrial β‐oxidation‐related genes, including PPARα, were downregulated in the liver of old mice. In addition, advanced glycation end product receptor (RAGE), a key regulator of glucose metabolism, was upregulated in the old mice. Mechanistically, suppression of RAGE upregulated PPARα and its downstream target genes, which in turn led to reduced TG retention. Finally, we found that hepatic RAGE expression was increased in aging patients, a finding that correlated with decreased PPARα levels. Taken together, our findings demonstrate that the upregulation of RAGE may play a critical role in aging‐associated liver steatosis.

The role of soluble receptor for advanced glycation end-products (sRAGE) in the general population and patients with diabetes mellitus with a focus on renal function and overall outcome

Isoforms of the receptor for advanced glycation end-product (RAGE) protein, which lack the transmembrane and the signaling (soluble RAGE or sRAGE) domains are hypothesized to counteract the detrimental action of the full-length receptor by acting as a decoy, and they provide a potential tool to treat RAGE-associated diseases. Multiple studies have explored the relationship between sRAGE and endogenous secretory RAGE and its polymorphism and obesity, metabolic syndrome, atherosclerosis, kidney function, and increased mortality in the general population. In addition, sRAGE may be a key player in the pathogenesis of diabetes mellitus and its microvascular (e.g. kidney disease) as well as macrovascular (e.g. cardiovascular disease) complications. In this review, we focus on the role of sRAGE as a biomarker in these specific areas. As there is a lack of an underlying unifying hypothesis about how sRAGE changes according to the disease condition or risk factor, there is a call to incorporate all three players of the AGE-RAGE axis into a new universal biomarker/risk marker: (AGE + RAGE)/sRAGE. However, the measurement of RAGE in humans is not practical as it is a cell-bound receptor for which tissue is required for analysis. A high AGE/sRAGE ratio may be a valuable alternative and practical universal biomarker/risk marker for diseases associated with the AGE-RAGE axis, irrespective of low or high serum sRAGE concentrations.

The Receptor for Advanced Glycation Endproducts (RAGE) Contributes to Severe Inflammatory Liver Injury in Mice

Background: The receptor for advanced glycation end products (RAGE) is a multiligand receptor involved in a number of processes and disorders. While it is known that RAGE-signaling can contribute to toxic liver damage and fibrosis, its role in acute inflammatory liver injury and septic multiorgan failure is yet undefined. We examined RAGE in lipopolysaccharide (LPS) induced acute liver injury of D-galN sensitized mice as a classical model for tumor necrosis factor alpha (TNF-α) dependent inflammatory organ damage.

Methods: Mice (Rage–/– and C57BL/6) were intraperitoneally injected with D-galN (300 mg/kg) and LPS (10 μg/kg). Animals were monitored clinically, and cytokines, damage associated molecular pattern molecules (DAMPs) as well as liver enzymes were determined in serum. Liver histology, hepatic cytokines as well as RAGE mRNA expression were analyzed. Cellular activation and functionality were evaluated by flow cytometry both in bone marrow- and liver-derived cells.

Results: Genetic deficiency of RAGE significantly reduced the mortality of mice exposed to LPS/D-galN. Hepatocyte damage markers were reduced in Rage–/– mice, and liver histopathology was less severe. Rage–/– mice produced less pro-inflammatory cytokines and DAMPs in serum and liver. While immune cell functions appeared normal, TNF-α production by hepatocytes was reduced in Rage–/– mice.

Conclusions: We found that RAGE deletion attenuated the expression of pro-inflammatory cytokines and DAMPs in hepatocytes without affecting cellular immune functions in the LPS/D-galN model of murine liver injury. Our data highlight the importance of tissue-specific RAGE-signaling also in acute inflammatory liver stress contributing to sepsis and multiorgan failure.

CD8+ T cells regulate liver injury in obesity-related nonalcoholic fatty liver disease

Nonalcoholic steatohepatitis (NASH) has increased in Western countries due to the prevalence of obesity. Current interests are aimed at identifying the type and function of immune cells that infiltrate the liver and key factors responsible for mediating their recruitment and activation in NASH. We investigated the function and phenotype of CD8⁺ T cells under obese and nonobese NASH conditions. We found an elevation in CD8 staining in livers from obese human subjects with NASH and cirrhosis that positively correlated with α-smooth muscle actin, a marker of hepatic stellate cell (HSC) activation. CD8⁺ T cells were elevated 3.5-fold in the livers of obese and hyperlipidemic NASH mice compared with obese hepatic steatosis mice. Isolated hepatic CD8⁺ T cells from these mice expressed a cytotoxic IL-10-expressing phenotype, and depletion of CD8⁺ T cells led to significant reductions in hepatic inflammation, HSC activation, and macrophage accumulation. Furthermore, hepatic CD8⁺ T cells from obese and hyperlipidemic NASH mice activated HSCs in vitro and in vivo. Interestingly, in the lean NASH mouse model, depletion and knockdown of CD8⁺ T cells did not impact liver inflammation or HSC activation. We demonstrated that under obese/hyperlipidemia conditions, CD8⁺ T cell are key regulators of the progression of NASH, while under nonobese conditions they play a minimal role in driving the disease. Thus, therapies targeting CD8⁺ T cells may be a novel approach for treatment of obesity-associated NASH.NEW & NOTEWORTHY Our study demonstrates that CD8⁺ T cells are the primary hepatic T cell population, are elevated in obese models of NASH, and directly activate hepatic stellate cells. In contrast, we find CD8⁺ T cells from lean NASH models do not regulate NASH-associated inflammation or stellate cell activation. Thus, for the first time to our knowledge, we demonstrate that hepatic CD8⁺ T cells are key players in obesity-associated NASH.