Current Molecular Medicine - Volume 7, Issue 8, 2007

In unraveling the biology of the Receptor for Advanced Glycation Endproducts (RAGE), it has become increasingly apparent that the ligands of RAGE stimulate signal transduction through this receptor - leading to cascades of events that, depending on the microenvironment, initiate and sustain chronic cell stress. In certain settings, however, RAGE-dependent signaling may augur repair and resolution of stress, especially where acute injury stimulates rapid generation and removal of RAGE ligands. Although first described as a receptor for the products of nonenzymatic glycation and oxidation of proteins, the Advanced Glycation Endproducts (AGEs) [1] the biology of RAGE blossomed upon the discovery that at least four other classes of molecules might bind this receptor. In addition to AGEs, RAGE binds S100/calgranulins, High Mobility Group-1, Mac-1, and amyloid-β peptide and β-sheet fibrils (Aβ) [2-5]. Together, these ligand families bespeak unifying mechanisms underlying the pathogenesis of chronic disease. Thus, irrespective of the specific etiology, the intriguing upregulation and accumulation of RAGE ligands in tissues beset by chronic disease brings RAGE squarely to the battlefield in disorders such as diabetes, chronic inflammation and autoimmunity, neurodegeneration, tumors and aging. As outlined in the reviews in this series, data in both cell culture and animal models of disease reveal significant protection from chronic injury in RAGE-modified mice, or in animals treated with antagonists of RAGE and its ligands. A key question has been asked many a time; how can this receptor be involved in such distinct settings of chronic stress? We propose that a common thread tying RAGE and its ligands to diverse disorders is the link to the inflammatory response. In each case, there is a plethora of evidence suggesting that monocytes, macrophages, T and B lymphocytes and in the central nervous system, glial cells, all of which express RAGE, contribute to tissue-perturbing signaling mechanisms that upregulate matrix metalloproteinases (MMPs), cytokines and other factors that damage tissue and suppress repair [6]. Further, earlier observations that many of RAGE's ligands, particularly the S100/calgranulins, were “biomarkers” of inflammation, ischemia/reperfusion stress and malignancy may now hold a mechanism-based context - we predict that autocrine and/or paracrine interactions of released RAGE ligands with RAGE-expressing cells amplify tissue stress and, if left unchecked, lead to chronic disease. Articles in this Review Series In this review series, we have gathered a compendium of reports that illustrate the evolving tale of RAGE. The first four articles focus on the discovery and implications of the ligand families of RAGE. Yan, Ramasamy and colleagues share insights on AGEs and RAGE, as well as other ligands, and how this interaction contributes importantly to diabetic complications in the cardiovascular system [7]. Donato provides a timely review on the key question - do all S100/calgranulins bind RAGE? In presenting the argument that the answer is probably “no”, Donato elucidates the effects of RAGE signaling stimulated by at least certain members of this family and their implications in inflammatory and neuronal stress [8]. Rauvala and Rouhaianen critically review what is currently known about HMGB1 and RAGE. In detailing the data indicating that RAGE is a signal transduction receptor for HMGB1, they present evidence on distinct receptors that may also engage this molecule [9]. Chen, Yan and colleagues reflect on the discovery that RAGE binds amyloid-β peptide and β-sheet fibrils. The implications of these species in neurodegeneration disorders such as Alzheimer's disease may reflect the tip of the iceberg in settings wherein such “tangled webs” may form, aggregate and emerge as new oligomeric structures highly capable of stimulating and signaling via RAGE [10]. Following these papers is a review on RAGE and its roles in the inflammatory response. Clynes, Schmidt and colleagues present the evidence - from delayed type hypersensitivity studies in non-diabetic mice - to T cell priming experiments in unique T cell receptor-modified mice and lymphocytes, that RAGE is essential for effective T cell priming in vivo [11]. These data provide definitive evidence linking RAGE to the adaptive immune response. The next articles focus on four specific areas in which RAGE and its ligands have been implicated using both cell culture and in vivo models. Yamamoto, Murakami and colleagues review key studies linking RAGE to diabetic nephropathy wherein experiments using RAGE-modified mice clearly reveal that RAGE is essential for the development and progression of this disorder [12]. Barile and Schmidt review the state of RAGE in diabetic and aginglinked retinopathies [13], and Toth, Martinez and Zochodne review the state of RAGE in diabetic neuropathy [14]. In retinopathy and neuropathy, animal models of diabetes revealed striking upregulation of RAGE in these affected tissues, and that pharmacological and/or genetic deletion of RAGE provided protection against the functional and pathological indices of these two disorders. Lastly, Logsdon, Arumugam and colleagues review the biology of RAGE and its ligands in cancer. Far from being “innocent bystanders” and biomarkers in cancer, evidence is mounting that RAGE may be important in mechanisms linked to tumor growth and metastases [15]. Fascinating questions arise in cancer in the context of tumor and/or host roles for this receptor as detailed by these authors. RAGE & Human Biology RAGE blockade is the subject of ongoing clinical trials, thus, there are no data at this time revealing “efficacy” of targeting this approach in human subjects with chronic diseases. It is too soon. Yet, studies in cells and animals continue to deeply probe the questions of “natural” roles for RAGE. Indeed, we speculate that analogous mechanisms linking RAGE to injury may be evolutionarily conserved pathways that in simpler systems, evoked repair. Is there evidence, though, suggesting links between RAGE and human disease? The answer is an emphatic “yes!” Two major areas of research are ongoing probing these exact concepts. First, “soluble” forms of RAGE have been detected in the plasma of human subjects. Apparently produced by alternative splicing programs yielding “endogenous secretory” or esRAGE, these circulating levels of RAGE appear to be associated with disease states, and perhaps may be mutable in response to therapeutic interventions (recently reviewed in [16]). Second, polymorphisms of RAGE have been uncovered that may, especially upon study in large scale observational trials, shed light on vulnerability to development of chronic diseases such as diabetic complications, neurodegeneration or to autoimmune disorders, and/or to the severity of chronic disease states [17]. Published information on RAGE polymorphisms and cardiovascular disease is presented in the review by Yan and Ramasamy [7]. Perspective Taken together, evidence presented herein links RAGE to the pathogenesis of an array of chronic disease states characterized by upregulation and accumulation of RAGE ligands. Far from reflecting “one ligand - one disease,” emerging evidence points to the family of RAGE ligands as key players in the steps launching and perpetuating chronic disease and tissue damage. Drawing the fine line between injury and repair in the biology of RAGE is an important challenge but one well worth the effort of in-depth and hypothesis-driven experimentation. We predict that solving this puzzle may lead to effective therapies for chronic diseases such as inflammation and autoimmunity, neuronal degeneration, unchecked cellular proliferation and metastasis, and AGEing. ACKNOWLEDGEMENTS Thank you to all the authors and peer reviewers who contributed to the development and refinement of this review series on RAGE. Together, we thank the editors of Current Molecular Medicine for the gracious invitation to prepare this series on RAGE, the molecule to which our lives are dedicated! 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[5] Chavakis, T., Bierhaus, A., Al-Fakhri, N., Schneider, D., Witte, S., Linn, T., Nagashima, M., Morser, J., Arnold, B., Preissner, K.T., Nawroth, P.P. (2003). J. Exp. Med., 198, 1507-1515. [6] Herold, K., Moser, B., Chen, Y., Zeng, S., Yan, S.F., Ramasamy, R., Emond, J., Clynes, R., and Schmidt, A.M. J. Leukoc. Biol., 82, 204- 212. [7] Yan, S.F., D'Agati, V.D., Schmidt, A.M., and Ramasamy, R. (2007). Curr. Mol. Med., In press. [8] Rauvala, H., and Rouhiainen, A. (2007) Curr. Mol. Med., In press. [9] Donato, R. (2007) Curr. Mol. Med., In press. [10] Chen, J.X., Walker, D.G., Schmidt, A.M., Arancio, O., Lue, L.F., and Yan, S.D. (2007). Curr. Mol. Med., In press. [11] Clynes, R., Moser, B., Yan, S.F., Ramasamy, R., Herold, K., and Schmidt, A.M. (2007). Curr. Mol. Med., In press. [12] Yamamoto, H., Watanabe, T., Yamamoto, Y., Yonekura, H., Munesue, S., Harashima, A., Ooe, K., Hossain, S., Saito, H., and Murakami, N. (2007). Curr. Mol. Med., In press. [13] Barile, G.R., and Schmidt, A.M. (2007). Curr. Mol. Med., In press. [14] Toth, C., Martinez, J., and Zochodne, D.W. (2007). Curr. Mol. Med., In press. [15] Logsdon, C., Fuentes, K., Huang, E.H., and Arumugam, T. (2007). Curr. Mol. Med., In press. [16] Geroldi, D., Falcone, C., and Emanuele, E. (2006). Curr. Med. Chem., 13, 1971-1978. [17] Hudson, B.I., Stickland, M.H., and Grant, P.J. (1998). Diabetes, 47, 1155-1157.

Unifying mechanisms for the consequences of aging and chronic diabetes are coming to light with the identification that common to both settings is the production and accumulation of the largely irreversible Advanced Glycation Endproducts (AGEs). AGEs impart multiple consequences in the tissues; a key means by which they exert maladaptive effects is via their interaction with and activation of their chief cell surface receptor, Receptor for AGE or RAGE. Although the time course, rate and extent of AGE generation and accumulation in diabetes and aging may be distinct, unifying outcomes of the ligand-RAGE interaction in the vasculature and heart are linked to upregulation of inflammatory and tissue-destructive mechanisms. Consistent with these concepts, administration of the ligand-binding decoy of RAGE, soluble or sRAGE, suppresses early initiation and progression of atherosclerosis in diabetic mice; suppresses exaggerated neointimal expansion consequent to arterial injury; and mitigates the adverse impact of ischemia/reperfusion injury in the heart. Importantly, the RAGE ligand repertoire upregulated in these settings is not limited to AGEs. The key finding that RAGE was a multi-ligand receptor unified the concept that in diabetes and aging, innate and adaptive inflammatory mechanisms contribute to the pathogenesis of tissue injury. We conclude that antagonism of RAGE may reflect a novel and therapeutically logical and safe target in cardiovascular stress induced by aging and chronic diabetes.

The S100 protein family comprises at least 25 members which, with the exception of S100G, act as Ca2+-sensor proteins that participate in Ca2+ signal transduction by interacting with target proteins thereby modifying their activities. S100 proteins are expressed in vertebrates exclusively, display a cell-specific distribution, and regulate a large variety of intracellular activities. Some S100 proteins are released by a nonclassical pathway and exert regulatory effects on several cell types. The receptor for advanced glycation end products (RAGE) has been shown to transduce extracellular effects of S100B, S100A4, S100A6, S100A11, S100A12, S100A13 and S100P. However, some S100 proteins can signal by engaging RAGE as well as non- RAGE receptors. Immune cells (i.e., monocytes/macrophages/microglia, neutrophils and lymphocytes), activated endothelial and vascular smooth muscle cells, neurons, astrocytes, chondrocytes and pancreatic tumor cells are the cell types reported to respond to certain S100 proteins via RAGE engagement. In general, relatively high concentrations of S100 proteins are required for activation of RAGE in responsive cells. S100B is unique in that it can engage RAGE in neurons at low and high concentrations with trophic and toxic effects, respectively, and S100A4 stimulates matrix metalloproteinase 13 release from chondrocytes at nanomolar doses in a RAGE-mediated manner. Oligomerization of S100 proteins under the non-reducing, high-Ca2+ conditions found extracellularly appears to play a relevant role in RAGE activation, and binding of at least S100A12 and S100B results in RAGE oligomerization. Thus, S100/RAGE interactions might have important consequences during development and in tissue homeostasis as well as in inflammatory, degenerative and tumor processes.

HMGB1/Amphoterin is a ubiquitous, highly conserved DNA-binding protein that can be also released to the extracellular space by various cell types. Extracellular HMGB1 regulates migratory responses of several cell types through binding to RAGE that communicates with the cytoskeleton to regulate cell motility. HMGB1- induced cell signalling has been associated with mechanisms of several diseases, including cancer, sepsis, rheumatoid arthritis, stroke and atherosclerosis. This article reviews the evidence linking the functional roles of HMGB1 to RAGE signalling. Furthermore, we discuss the molecular and cellular mechanisms that may explain the roles of HMGB1/RAGE in diverse disease processes.

This review focuses on the current findings regarding interaction between amyloid β peptide (Aβ) and receptor for advanced glycation endproducts (RAGE) and its roles in the pathogenesis of Alzheimer's disease (AD). As a ubiquitously expressed cell surface receptor, RAGE mediates the effects of Aβ on microglia, blood-brain barrier (BBB) and neurons through activating different signaling pathways. Data from autopsy brain tissues, in vitro cell cultures and transgenic mouse models suggest that Aβ-RAGE interaction exaggerates neuronal stress, accumulation of Aβ, impaired learning memory, and neuroinflammation. Blockade of RAGE protects against Aβ-mediated cellular perturbation. These findings may have an important therapeutic implication for neurodegenerative disorders relevant to AD.

The family of RAGE ligands, including Advanced Glycation Endproducts (AGEs), S100/calgranulins, High Mobility Group Box-1 (HMGB1) and amyloid β peptide (Aβ) and β-sheet fibrils are highly enriched in immune and inflammatory foci. In parallel, upregulation of Receptor for AGE (RAGE) is noted in diverse forms of inflammation and autoimmunity, based on experiments examining human tissues as well as animal models. Indeed, prior to the demonstration that S100/calgranulins were signal transduction ligands of RAGE, these molecules were considered “biomarkers” of disease and disease activity in disorders such as colitis and arthritis. Premiere roles for RAGE in advancing cellular migration implicate this receptor in targeting immune cells to vulnerable foci. Once engaged, ligand-RAGE interaction in inflammatory and vascular cells amplifies upregulation of inflammatory cytokines, adhesion molecules and matrix metalloproteinases (MMPs). Discerning the primal versus chronic injury-provoking roles for this ligand-receptor interaction is a challenge in delineating the functions of the ligand/RAGE axis. As RAGE is expressed by many of the key cell types linked integrally to the immune response, we propose that the sites and time course of ligand-RAGE stimulation determine the phenotype produced by this axis. Ultimately, drawing the fine line between antagonism versus stimulation of the receptor in health and disease will depend on the full characterization of RAGE in repair versus injury.

As is diabetes itself, diabetic angiopathy is a multi-factorial disease. Advanced glycation endproducts (AGE) cause vascular cell derangement characteristic of diabetes, and this is mainly mediated by their interaction with receptor for AGE (RAGE). When made diabetic, RAGE-overexpressing transgenic mice exhibited exacerbation of the indices of nephropathy, and this was prevented by the inhibition of AGE formation. On the other hand, RAGE-deficient animals showed amelioration of diabetic nephropathy. Accordingly, AGE and RAGE should be regarded as environmental and cellular accounts and as a potential therapeutic target for diabetic nephropathy. In effect, substances that inhibit the formation of AGE, break preformed AGE, change metabolic flows away from glycation, antagonize RAGE, and capture RAGE ligands have been proven as effective remedies against this life-threatening disease.

RAGE, the receptor for advanced glycation endproducts (AGEs), is a multiligand signal transduction receptor of the immunoglobulin superfamily of cell surface molecules that has been implicated in the pathogenesis of diabetic complications, neurodegenerative diseases, inflammatory disorders, and cancer. These diverse biologic disorders reflect the multiplicity of ligands capable of cellular interaction via RAGE that include, in addition to AGEs, amyloid-beta (Aβ) peptide, the S100/calgranulin family of proinflammatory cytokines, and amphoterin, a member of the High Mobility Group Box (HMGB) DNA-binding proteins. In the retina, RAGE expression is present in neural cells, the vasculature, and RPE cells, and it has also been detected in pathologic cellular retinal responses including epiretinal and neovascular membrane formation. Ligands for RAGE, in particular AGEs, have emerged as relevant to the pathogenesis of diabetic retinopathy and age-related macular disease. While the understanding of RAGE and its role in retinal dysfunction with aging, diabetes mellitus, and/or activation of pro-inflammatory pathways is less complete compared to other organ systems, increasing evidence indicates that RAGE can initiate and sustain significant cellular perturbations in the inner and outer retina. For these reasons, antagonism of RAGE interactions with its ligands may be a worthwhile therapeutic target in such seemingly disparate, visually threatening retinal diseases as diabetic retinopathy, age-related macular degeneration, and proliferative vitreoretinopathy.

Longstanding diabetes mellitus targets kidney, retina, and blood vessels, but its impact upon the nervous system is another important source of disability. Diabetic peripheral neuropathy is a serious complication of inadequately treated diabetes leading to sensory loss, intractable neuropathic pain, loss of distal leg muscles, and impairment of balance and gait. Diabetes has been implicated as a cause of brain atrophy, white matter abnormalities, and cognitive impairment and a risk factor for dementia. Recent studies have incriminated advanced glycation end products (AGEs) and their receptor (RAGE) in the pathogenesis of diabetic nervous system complications. The availability of RAGE knockout mice and a competitive decoy for AGEs, soluble RAGE (sRAGE), has advanced our knowledge of the RAGE-mediated signalling pathways within the nervous system. They also provide hope for a future novel intervention for the prevention of diabetes-associated neurological complications. This review will discuss current knowledge of diabetes- and RAGE-mediated neurodegeneration, involving the distal-most level of epidermal nerve fibers in skin, major peripheral nerve trunks, dorsal root ganglia, spinal cord, and brain.

The receptor for advanced glycation end-products (RAGE) is a multifunctional receptor with multiple ligands that is known to play a key role in several diseases, including diabetes, arthritis, and Alzheimer's disease. Recent evidence indicates that this receptor also has an important role in cancer. RAGE ligands, which include the S100/calgranulins and high-mobility group box 1 (HMGB1) ligands, are expressed and secreted by cancer cells and are associated with increased metastasis and poorer outcomes in a wide variety of tumors. These ligands can interact in an autocrine manner to directly activate cancer cells and stimulate proliferation, invasion, chemoresistance, and metastasis. RAGE ligands derived from cancer cells can also influence a variety of important cell types within the tumor microenvironment, including fibroblasts, leukocytes, and vascular cells, leading to increased fibrosis, inflammation, and angiogenesis. Several of the cells in the tumor microenvironment also produce RAGE ligands. Most of the cancer-promoting effects of RAGE ligands are the result of their interaction with RAGE. However, these ligands also often have separate intracellular roles, and some may interact with other extracellular targets, so it is not currently possible to assign all of their effects to RAGE activation. Despite these complications, the bulk of the evidence supports the premise that the ligand-RAGE axis is an important target for therapeutic intervention in cancer.