Advanced Glycation End Products Signaling Research Articles

Mechanisms of kidney fibrosis and the role of antifibrotic therapies

Kidney fibrosis is a common observation in human and experimental models of kidney disease and contributes to the progressive loss of kidney function. This review discusses the recent recognition of the role of podocytes in the development of common glomerular disease and focuses on the basis for new antifibrotic therapies. A growing body of evidence indicates that changes in the structure and function of podocytes are involved in the development and progression of kidney disease. The changes include podocyte de-differentiation, podocyte-mediated endothelial dysfunction and podocyte-induced epithelial-mesenchymal transition, all contributing to the development of kidney fibrosis. Furthermore, new antifibrotic strategies aiming at the transforming growth factor-beta, connective tissue growth factor, angiotensin (1-7), and advanced glycation endproducts/receptors advanced glycation endproducts signaling pathways are being developed. Podocytes are recognized to play a key role in the development of kidney fibrosis. New antifibrotic therapies are rapidly progressing toward definitive clinical trials but will need to be tested on top of the existing therapy of renin-angiotensin system inhibition. Novel approaches targeting podocyte function would be a promising approach for early stages of the disease.

Therapeutic Interruption of Advanced Glycation in Diabetic Nephropathy

A major common feature of the chemically disparate compounds that inhibit advanced glycation end product (AGE) accumulation or signaling is their ability to show end-organ protection in experimental models of diabetes complications. The mechanisms by which these AGE- lowering therapies confer their benefits remain unsolved. Is it the reduction in tissue AGE levels per se or the inhibition of downstream signal transduction (as has been described with the soluble receptor for AGE)? Possible modes of action that need to be investigated include the ability of some of these agents to stimulate antioxidant defenses, to lower cholesterol and other lipid levels, and to inhibit low-grade inflammation. To understand these novel mechanisms, further examination of the advanced glycation pathway and, in particular, the diverse action of these agents in ameliorating the development of diabetic complications is needed.

Convergence and amplification of toll-like receptor (TLR) and receptor for advanced glycation end products (RAGE) signaling pathways via high mobility group B1 (HMGB1)

Sustained proinflammatory responses in rheumatoid arthritis, atherosclerosis, and diabetic retinopathy, as well as in cancer, are often associated with increased angiogenesis that contributes to tissue disruption and disease progression. High mobility group B1 (HMGB1) has been recognized as a proinflammatory cytokine and more recently, as a proangiogenic factor. HMGB1 can either be passively released from necrotic cells or actively secreted in response to angiogenic and inflammatory signals. HMGB1 itself may signal through the receptor for advanced glycation end products (RAGE), and via toll-like receptors, TLR2 and TLR4. Activation of these receptors results in the activation of NFkappaB, which induces the upregulation of leukocyte adhesion molecules and the production of proinflammatory cytokines and angiogenic factors in both hematopoietic and endothelial cells, thereby promoting inflammation. Interestingly, HMGB1 seems to be involved in a positive feedback mechanism, that may help to sustain inflammation and angiogenesis in several pathological conditions, thereby contributing to disease progression. Endothelial cells express HMGB1, as well as the receptors RAGE, TLR2, and TLR4, and in diverse pathologies HMGB1 and its receptors are overexpressed. Furthermore, HMGB1-induced signaling can activate NFkappaB, which can subsequently induce the expression of HMGB1 receptors. Thus, HMGB1 can mediate amplification of inflammation and angiogenesis through increased secretion of HMGB1 and increased expression of the receptors it can interact with. In this review, we discuss signaling cascades that HMGB1 can induce via TLRs and RAGE, as well as its contribution to pathologies involving endothelial cells.

RAGE Expression in Rhabdomyosarcoma Cells Results in Myogenic Differentiation and Reduced Proliferation, Migration, Invasiveness, and Tumor Growth

Activation of receptor for advanced glycation end products (RAGE) by its ligand, HMGB1, stimulates myogenesis via a Cdc42-Rac1-MKK6-p38 mitogen-activated protein kinase pathway. In addition, functional inactivation of RAGE in myoblasts results in reduced myogenesis, increased proliferation, and tumor formation in vivo. We show here that TE671 rhabdomyosarcoma cells, which do not express RAGE, can be induced to differentiate on transfection with RAGE (TE671/RAGE cells) but not a signaling-deficient RAGE mutant (RAGEDeltacyto) (TE671/RAGEDeltacyto cells) via activation of a Cdc42-Rac1-MKK6-p38 pathway and that TE671/RAGE cell differentiation depends on RAGE engagement by HMGB1. TE671/RAGE cells also show p38-dependent inactivation of extracellular signal-regulated kinases 1 and 2 and c-Jun NH(2) terminal protein kinase and reduced proliferation, migration, and invasiveness and increased apoptosis, volume, and adhesiveness in vitro; they also grow smaller tumors and show a lower tumor incidence in vivo compared with wild-type cells. Two other rhabdomyosarcoma cell lines that express RAGE, CCA and RMZ-RC2, show an inverse relationship between the level of RAGE expression and invasiveness in vitro and exhibit reduced myogenic potential and enhanced invasive properties in vitro when transfected with RAGEDeltacyto. The rhabdomyosarcoma cell lines used here and C2C12 myoblasts express and release HMGB1, which activates RAGE in an autocrine manner. These data suggest that deregulation of RAGE expression in myoblasts might concur in rhabdomyosarcomagenesis and that increasing RAGE expression in rhabdomyosarcoma cells might reduce their tumor potential.

Pigment Epithelium-derived Factor Inhibits Advanced Glycation End Product-induced Retinal Vascular Hyperpermeability by Blocking Reactive Oxygen Species-mediated Vascular Endothelial Growth Factor Expression

Pigment epithelium-derived factor (PEDF) is the most potent inhibitor of angiogenesis, suggesting that loss of PEDF contributes to proliferative diabetic retinopathy. However, the role of PEDF against retinal vascular hyperpermeability remains to be elucidated. We investigated here whether and how PEDF could inhibit the advanced glycation end product (AGE) signaling to vascular hyperpermeability. Intravenous administration of AGEs to normal rats not only increased retinal vascular permeability by stimulating vascular endothelial growth factor (VEGF) expression but also decreased retinal PEDF levels. Simultaneous treatments with PEDF inhibited the AGE-elicited VEGF-mediated permeability by down-regulating mRNA levels of p22(phox) and gp91(phox), membrane components of NADPH oxidase, and subsequently decreasing retinal levels of an oxidative stress marker, 8-hydroxydeoxyguanosine. PEDF also inhibited the AGE-induced vascular hyperpermeability evaluated by transendothelial electrical resistance by suppressing VEGF expression. Furthermore, PEDF decreased reactive oxygen species (ROS) generation in AGE-exposed endothelial cells by suppressing NADPH oxidase activity via down-regulation of mRNA levels of p22(PHOX) and gp91(PHOX). This led to blockade of the AGE-elicited Ras activation and NF-kappaB-dependent VEGF gene induction in endothelial cells. These results indicate that the central mechanism for PEDF inhibition of the AGE signaling to vascular permeability is by suppression of NADPH oxidase-mediated ROS generation and subsequent VEGF expression. Substitution of PEDF may offer a promising strategy for halting the development of diabetic retinopathy.

New developments in diabetic neuropathy

Diabetic neuropathy is a debilitating consequence of type 1 and 2 diabetes. Hyperglycemia disrupts the normal function of neurons and their supporting glia at multiple levels. The complexity of this complication, combined with difficulties of delivering therapy to sensory, sympathetic and parasympathetic neurons, contributes to the intractability of this serious diabetic complication. This review summarizes recent reviews examining the state of research on and treatment of diabetic neuropathy and highlights areas of clinical and basic research that may yield new diagnostic and treatment options. Recent reviews summarize the effects of hyperglycemia on the peripheral nervous system as well as diagnosis and treatment of patients with diabetic neuropathy. Advances in the analysis of intraepidermal fiber densities could shorten the time course of clinical trials and extend data analyses to include sympathetic as well as sensory information. Unchecked glucose-mediated oxidative stress and advanced glycation endproduct signaling through receptors for advanced glycation endproducts are implicated in diabetic neuropathy and may serve as new therapeutic targets. The best efforts of countless investigators have yet to find effective treatments either to stop the progression of axonal degeneration and cell death or regrow damaged axons. Basic research into the prevention of oxidative stress caused by excess glucose as well as the prevention of advanced glycation endproduct/receptor for advanced glycation endproduct signaling may offer new therapeutic targets. The use of skin biopsies may aid in early detection of both sensory and autonomic neuropathy, and perhaps in the case of patients with type 2 diabetes, diagnose neuropathy prior to the onset of symptoms.

Receptor for Advanced Glycation End Products (RAGE) Signaling Induces CREB-dependent Chromogranin Expression during Neuronal Differentiation

Receptor for advanced glycation end products (RAGE) mediates neurite outgrowth and cell migration upon stimulation with its ligand, amphoterin. We show here that RAGE-dependent changes in cell morphology are associated with proliferation arrest and changes in gene expression in neuroblastoma cells. Chromogranin B, a component of secretory vesicles in endocrine cells and neurons, was found to be up-regulated by RAGE signaling during differentiation of neuroblastoma cells along with the two other members of the chromogranin family, chromogranin A and secretogranin II. Ligation of RAGE by amphoterin lead to rapid phosphorylation and nuclear localization of cyclic AMP response element-binding protein (CREB), a major regulator of chromogranin expression. Furthermore, inhibition of ERK1/2-Rsk2-dependent CREB phosphorylation efficiently inhibited up-regulation of chromogranin gene expression upon RAGE activation. To further study the effects of RAGE and amphoterin on cellular differentiation, we stimulated embryonic stem cells expressing RAGE or a signaling-deficient mutant of RAGE with amphoterin. Amphoterin was found to promote RAGE-dependent neuronal differentiation of embryonic stem cells characterized by up-regulation of neuronal markers light neurofilament protein and beta-III-tubulin, activation of CREB, and increased expression of chromogranins A and B. These data suggest that RAGE signaling is capable of driving neuronal differentiation involving CREB activation and induction of chromogranin expression.