Ravichandran Ramasamy

Mechanisms of Disease: advanced glycation end-products and their receptor in inflammation and diabetes complications

Many important biochemical mechanisms are activated in the presence of high levels of glucose, which occur in diabetes. Elevated levels of glucose accelerate the formation of advanced glycation end-products (AGEs). Via their chief signaling receptor—the AGE-specific receptor (commonly abbreviated as RAGE)—AGEs generate reactive oxygen species and activate inflammatory signaling cascades. Consequently, AGEs have key roles in the pathogenesis of diabetic complications. Two discoveries have advanced our knowledge of the roles of RAGE in inflammation. First, this receptor has multiple ligands and binds not only AGEs but also proinflammatory, calcium-binding S100 proteins (also known as calgranulins) and nuclear high mobility group protein box-1. Second, RAGE is expressed on T lymphocytes, monocytes and macrophages; RAGE expression on T lymphocytes is essential for effective priming of immune responses in vivo. In this Review, we chronicle roles for RAGE in the pathogenesis of diabetic complications and develop the hypothesis that, in addition to RAGEs central role in the inflammatory response, it is critically linked to the pathogenesis of types 1 and 2 diabetes.

Receptor for Advanced-Glycation End Products Key Modulator of Myocardial Ischemic Injury

Background— The beneficial effects of reperfusion therapies have been limited by the amount of ischemic damage that occurs before reperfusion. To enable development of interventions to reduce cell injury, our research has focused on understanding mechanisms involved in cardiac cell death after ischemia/reperfusion (I/R) injury. In this context, our laboratory has been investigating the role of the receptor for advanced-glycation end products (RAGE) in myocardial I/R injury. Methods and Results— In this study we tested the hypothesis that RAGE is a key modulator of I/R injury in the myocardium. In ischemic rat hearts, expression of RAGE and its ligands was significantly enhanced. Pretreatment of rats with sRAGE, a decoy soluble part of RAGE receptor, reduced ischemic injury and improved functional recovery of myocardium. To specifically dissect the impact of RAGE, hearts from homozygous RAGE-null mice were isolated, perfused, and subjected to I/R. RAGE-null mice were strikingly protected from the adverse impact of I/R injury in the heart, as indicated by decreased release of LDH, improved functional recovery, and increased adenosine triphosphate (ATP). In rats and mice, activation of the RAGE axis was associated with increases in inducible nitric oxide synthase expression and levels of nitric oxide, cyclic guanosine monophosphate (cGMP), and nitrotyrosine. Conclusions— These findings demonstrate novel and key roles for RAGE in I/R injury in the heart. The findings also demonstrate that the interaction of RAGE with advanced-glycation end products affects myocardial energy metabolism and function during I/R.

Receptor for advanced glycation end products (RAGE) in a dash to the rescue: inflammatory signals gone awry in the primal response to stress

The multiligand receptor for advanced glycation end products (RAGE) of the Ig superfamily transduces the biological impact of discrete families of ligands, including advanced glycation end products, certain members of the S100/calgranulin family, high mobility group box‐1, Mac‐1 (αMβ2, CD11b/CD18), and amyloid‐β peptide and β‐sheet fibrils. Although structurally dissimilar, at least at the monomeric level, recent evidence suggests that oligomeric forms of these RAGE ligands may be especially apt to activate the receptor and up‐regulate a program of inflammatory and tissue injury‐provoking genes. The challenge in probing the biology of RAGE and its impact in acute responses to stress and the potential development of chronic disease is to draw the line between mechanisms that evoke repair versus those that sustain inflammation and tissue damage. In this review, we suggest the concept that the ligands of RAGE comprise a primal program in the acute response to stress. When up‐regulated in environments laden with oxidative stress, inflammation, innate aging, or high glucose, as examples, the function of these ligand families may be transformed from ones linked to rapid repair to those that drive chronic disease. Identification of the threshold beyond which ligands of RAGE mediate repair versus injury is a central component in delineating optimal strategies to target RAGE in the clinic.

Receptor for Advanced Glycation End Products and Its Ligands: A Journey from the Complications of Diabetes to Its Pathogenesis

Abstract: Many studies have suggested that the expression of RAGE (receptor for advanced glycation end products) is upregulated in human tissues susceptible to the long‐term complications of diabetes. From the kidneys to the macrovessels of the aorta, RAGE expression is upregulated in a diverse array of cell types, from glomerular epithelial cells (podocytes) to endothelial cells, vascular smooth muscle cells, and inflammatory mononuclear phagocytes and lymphocytes. Although RAGE was first described as a receptor for advanced glycation end products (AGEs), the key finding that RAGE was also a signaling receptor for proinflammatory S100/calgranulins and amphoterin, led to the premise that even in euglycemia, ligand‐RAGE interaction propagated inflammatory mechanisms linked to chronic cellular perturbation and tissue injury. Indeed, such considerations suggested that RAGE might even participate in the pathogenesis of type 1 diabetes. Our studies have shown that pharmacological and/or genetic deletion/mutation of the receptor attenuates the development of hyperglycemia in NOD mice; in mice with myriad complications of diabetes, interruption of ligand‐RAGE interaction prevents or delays the chronic complications of the disease in both macro‐ and microvessel structures. Taken together, these findings suggest that RAGE is “at the right place and time” to contribute to the pathogenesis of diabetes and it complications. Studies are in progress to test the premise that antagonism of this interaction is a logical strategy for the prevention and treatment of diabetes.

Aldose Reductase and AGE-RAGE Pathways: Key Players in Myocardial Ischemic Injury

Abstract: Cardiovascular disease represents the major cause of morbidity and mortality in patients with diabetes mellitus. The impact of cardiac disease includes increased sensitivity of diabetic myocardium to ischemic episodes and diabetic cardiomyopathy, manifested as a subnormal functional response of the diabetic heart independent of coronary artery disease. In this context, we were to our knowledge the first to demonstrate that diabetes increases glucose flux via the first and key enzyme, aldose reductase, of the polyol pathway, resulting in impaired glycolysis under normoxic and ischemic conditions in diabetic myocardium. Our laboratory has been investigating the role of the polyol pathway in mediating myocardial ischemic injury in diabetics. Furthermore, the influence of the aldose reductase pathway in facilitating generation of key potent glycating compounds has led us to investigate the impact of advanced glycation end products (AGEs) in myocardial ischemic injury in diabetics. The potent impact of increased flux via the aldose reductase pathway and the increased AGE interactions with its receptor (RAGE) resulting in cardiac dysfunction will be discussed in this chapter.

Stopping the Primal RAGE Reaction in Myocardial Infarction : Capturing Adaptive Responses to Heal the Heart?

The report of Andrassy and colleagues1 in the current issue of Circulation adds to the growing body of evidence that the receptor for advanced glycation end products (RAGE) and its ligands, particularly high mobility group box-1 (HMGB1), are central mediators of ischemia/reperfusion (I/R) injury in the heart.1–4 A major cause of injury, especially in the reperfusion phase, is the influx of inflammatory cells into the stressed heart. Andrassy and colleagues show that infiltrating leukocytes express proinflammatory HMGB1 and that HMGB1 plays fundamental roles in injury responses in the I/R heart. In homeostasis, HMGB1 is largely a nuclear protein. In stress conditions, HMGB1 may be released from injured cells, particularly on necrosis. The chief receptor for HMGB1, RAGE, is expressed in multiple cell types in the I/R heart, such as inflammatory cells, and also in cardiomyocytes and vascular cells (endothelial cells and smooth muscle cells).2 Although HMGB1 may interact with distinct receptors beyond RAGE (for example, toll receptors),5 the current study of Andrassy and colleagues reveals major roles for RAGE in transducing the effects of HMGB1 in the heart, as administration of recombinant HMGB1 or antagonists of this molecule had significant proinjury effects in the wild-type mouse heart, but no additive effects were noted in mice devoid of RAGE. The biology of RAGE, however, is complex and extends beyond HMGB1 in the injured heart.nnArticle p 3216 nnIn addition to HMGB1, other ligands of RAGE likely play pivotal roles in the response to ischemia or I/R in the heart. For example, rapid generation of pre–advanced glycation end products (AGE) species (methylglyoxal) and AGEs in transient occlusion/reperfusion of the LAD coronary artery has been illustrated.2 In hypoxia, in both the murine heart and in isolated primary murine endothelial cells, AGE-immunoreactive species are central mediators of regulation of …

Inflammatory stress in primary venous and aortic endothelial cells of type 1 diabetic mice

Objective: The progression of diabetes is associated with profound endothelial dysfunction. We tested the hypothesis that cellular stress would be detectable in ECs retrieved from arterial and venous vessels of diabetic mice. Method: We describe a method for direct isolation of well-characterised aortic and venous ECs from mice in which cells are not subjected to propagation in culture. Results: Gene expression profiling, confirmed by real-time PCR, revealed a progressive increase in markers of injury within two main gene families, EC activation and EC apoptosis, in aortic and venous ECs recovered from diabetic versus non-diabetic mice. In short-term diabetes, Il1b mRNA transcripts were higher in aortic and venous ECs of diabetic mice versus controls. In long-term diabetes, casp-1 mRNA transcripts were higher in aortic and venous ECs of diabetic mice versus controls. Conclusion: These data suggest that diabetes imparts diffuse endothelial perturbation in the arterial and venous endothelium.

Glycation and RAGE: Common Links in the Pathogenesis of Microvascular and Macrovascular Complications of Diabetes

ABSTRACT Experimental evidence is mounting to link the receptor for advanced glycation endproducts (RAGE) to the pathogenesis of complications in diabetes. The multiligand nature of RAGE, a defining property of this receptor, has led to the elucidation that hyperglycemia-stimulated cellular activation ensues, not only consequent to the formation of advanced glycation endproducts (AGEs), which signal through RAGE, but also via the recruitment and activation of inflammatory cells to AGE-laden tissues in diabetes. Such activated inflammatory cells may release the proinflammatory ligands of RAGE—S100/calgranulins and high mobility group box-1 —thereby contributing to the initiation and amplification of perturbation in both micro-and macrovascular tissues in diabetes. Strong support for the role of RAGE in the pathogenesis of diabetes-related complications has been shown by antagonizing the ligand-RAGE axis in animal models, and, as well, by testing the impact of RAGE deletion in diabetic mice. This review summarizes current strategies by which RAGE antagonism has been tested in animal models and highlights results of recent studies in which homozygous RAGE-null models are protected, at least in part, from the adverse consequences of diabetes.Taken together, these findings suggest that RAGE antagonism may be a novel approach to the treatment of diabetic complications.