The 1.5 Å crystal structure of human receptor for advanced glycation endproducts (RAGE) ectodomains reveals unique features determining ligand binding

Interaction of the pattern recognition receptor, RAGE with key ligands such as advanced glycation end products (AGE), S100 proteins, amyloid β, and HMGB1 has been linked to diabetic complications, inflammatory and neurodegenerative disorders, and cancer. To help answer the question of how a single receptor can recognize and respond to a diverse set of ligands we have investigated the structure and binding properties of the first two extracellular domains of human RAGE, which are implicated in various ligand binding and subsequent signaling events. The 1.5-Å crystal structure reveals an elongated molecule with a large basic patch and a large hydrophobic patch, both highly conserved. Isothermal titration calorimetry (ITC) and deletion experiments indicate S100B recognition by RAGE is an entropically driven process involving hydrophobic interaction that is dependent on Ca²⁺ and on residues in the C′D loop (residues 54–67) of domain 1. In contrast, competition experiments using gel shift assays suggest that RAGE interaction with AGE is driven by the recognition of negative charges on AGE-proteins. We also demonstrate that RAGE can bind to dsDNA and dsRNA. These findings reveal versatile structural features of RAGE that help explain its ability to recognize of multiple ligands.     

Systems analysis of GLP-1 receptor signaling in pancreatic β-cells

Glucagon-like peptide-1 (GLP-1) elevates intracellular concentration of cAMP ([cAMP]) and facilitates glucose-dependent insulin secretion in pancreatic β-cells. There has been much evidence to suggest that multiple key players such as the GLP-1 receptor, G(s) protein, adenylate cyclase (AC), phosphodiesterase (PDE), and intracellular Ca(2+) concentration ([Ca(2+)]) are involved in the regulation of [cAMP]. However, because of complex interactions among these signaling factors, the kinetics of the reaction cascade as well as the activities of ACs and PDEs have not been determined in pancreatic β-cells. We have constructed a minimal mathematical model of GLP-1 receptor signal transduction based on experimental findings obtained mostly in β-cells and insulinoma cell lines. By fitting this theoretical reaction scheme to key experimental records of the GLP-1 response, the parameters determining individual reaction steps were estimated. The model reconstructed satisfactorily the dynamic changes in [cAMP] and predicted the activities of cAMP effectors, protein kinase A (PKA), and cAMP-regulated guanine nucleotide exchange factor [cAMP-GEF or exchange protein directly activated by cAMP (Epac)] during GLP-1 stimulation. The simulations also predicted the presence of two sequential desensitization steps of the GLP1 receptor that occur with fast and very slow reaction rates. The cross talk between glucose- and GLP-1-dependent signal cascades for cAMP synthesis was well reconstructed by integrating the direct regulation of AC and PDE by [Ca(2+)]. To examine robustness of the signaling system in controlling [cAMP], magnitudes of AC and PDE activities were compared in the presence or absence of GLP-1 and/or the PDE inhibitor IBMX.(1).     

Inhibition of SCD1 impairs palmitate-derived autophagy at the step of autophagosome-lysosome fusion in pancreatic β-cells

Autophagy is indispensable for the proper architecture and flawless functioning of pancreatic β-cells. A growing body of evidence indicates reciprocal communication between autophagic pathways, apoptosis, and intracellular lipids. The way in which elevated levels of free saturated or unsaturated FAs contribute to progressive β-cell failure remains incompletely understood. Stearoyl-CoA desaturase (SCD)1, a key regulatory enzyme in biosynthesis of MUFAs, was shown to play an important role in regulation of β-cell function. Here, we investigated whether SCD1 activity is engaged in palmitate-induced pancreatic β-cell autophagy. We found augmented apoptosis and diminished autophagy upon cotreatment of INS-1E cells with palmitate and an SCD1 inhibitor. Furthermore, we found that additional treatment of the cells with monensin, an inhibitor of autophagy at the step of fusion, exacerbates palmitate-induced apoptosis. Accordingly, diminished SCD1 activity affected the accumulation, composition, and saturation status of cellular membrane phospholipids and neutral lipids. Such an effect was accompanied by aberrant endoplasmic reticulum stress, mitochondrial injury, and decreases in insulin secretion and cell proliferation. Our data reveal a novel mechanism by which the inhibition of SCD1 activity affects autophagosome-lysosome fusion because of perturbations in cellular membrane integrity, thus leading to an aberrant stress response and β-cell failure.     

Mechanism for antioxidative effects of thiazolidinediones in pancreatic β-cells

Thiazolidinediones (TZDs) are synthetic ligands of peroxisome proliferator-activated receptor-γ (PPARγ), a member of the nuclear receptor superfamily. TZDs are known to increase insulin sensitivity and also to have an antioxidative effect. In this study, we tested whether TZDs protect pancreatic β-cells from oxidative stress, and we investigated the mechanism involved in this process. To generate oxidative stress in pancreatic β-cells (INS-1 and βTC3) or isolated islets, glucose oxidase was added to the media. The extracellular and intracellular reactive oxygen species (ROS) were measured to directly determine the antioxidant effect of TZDs. The phosphorylation of JNK/MAPK after oxidative stress was detected by Western blot analysis, and glucose-stimulated insulin secretion and cell viability were also measured. TZDs significantly reduced the ROS levels that were increased by glucose oxidase, and they effectively prevented β-cell dysfunction. The antioxidative effect of TZDs was abolished in the presence of a PPARγ antagonist, GW9662. Real-time PCR was used to investigate the expression levels of antioxidant genes. The expression of catalase, an antioxidant enzyme, was increased by TZDs in pancreatic β-cells, and the knockdown of catalase significantly inhibited the antioxidant effect of TZDs. These results suggest that TZDs effectively protect pancreatic β-cells from oxidative stress, and this effect is dependent largely on PPARγ. In addition, the expression of catalase is increased by TZDs, and catalase, at least in part, mediates the antioxidant effect of TZDs in pancreatic β-cells.     

The nuclear receptor FXR is expressed in pancreatic β-cells and protects human islets from lipotoxicity

Farnesoid X receptor (FXR) is highly expressed in liver and intestine where it controls bile acid (BA), lipid and glucose homeostasis. Here we show that FXR is expressed and functional, as assessed by target gene expression analysis, in human islets and β-cell lines. FXR is predominantly cytosolic-localized in the islets of lean mice, but nuclear in obese mice. Compared to FXR+/+ mice, FXR−/− mice display a normal architecture and β-cell mass but the expression of certain islet-specific genes is altered. Moreover, glucose-stimulated insulin secretion (GSIS) is impaired in the islets of FXR−/− mice. Finally, FXR activation protects human islets from lipotoxicity and ameliorates their secretory index.     

Iodoacetamide-induced sensitization of the pancreatic β-cells to glucose stimulation

At a glucose concentration of 3mm or less, iodoacetamide had no effect on the release of insulin from microdissected pancreatic islets of ob/ob-mice. At higher glucose concentrations, iodoacetamide exerted both an initial stimulatory and a subsequent inhibitory action. When islets were perifused with 1mm-iodoacetamide and 17mm-glucose the inhibitory action predominated after about 15min of transient stimulation. With decreasing concentrations of iodoacetamide the stimulatory phase was gradually prolonged, and with 0.003-0.1mm-iodoacetamide stimulation only was observed for 75min. Prolonged stimulation was also noted after a short pulse of iodoacetamide. Similar responses to 0.1mm-iodoacetamide were observed with islets from normal mice. With islets from ob/ob-mice the effect of 0.1mm-iodoacetamide was reproduced with 0.1mm-iodoacetate, whereas 0.1mm-acetamide had no apparent effect. Iodoacetamide increased the V(max.) of glucose-stimulated insulin release without altering the apparent K(m) for glucose. Leucine, glibenclamide or theophylline could not replace glucose in this synergistic action with iodoacetamide. Iodoacetamide rather inhibited the insulin-releasing action of theophylline. Iodoacetamide-induced potentiation of the glucose-stimulated insulin release was rapidly and reversibly inhibited by mannoheptulose, adrenaline, or calcium deficiency. The potentiating effect on insulin release was not paralleled by effects on glucose oxidation or on islet fructose 1,6-diphosphate. However, the inhibitory action of iodoacetamide might be explained by inhibition of glycolysis as evidenced by an inhibition of glucose oxidation and a rise of fructose 1,6-diphosphate. The results support our previous hypothesis that thiol reagents can stimulate insulin release by acting on relatively superficial thiol groups in the beta-cell plasma membrane. Glycolysis seems to be necessary in order for iodoacetamide to stimulate in this way.     

Insulinotropic effect of the non-steroidal compound STX in pancreatic β-cells

The non-steroidal compound STX modulates the hypothalamic control of core body temperature and energy homeostasis. The aim of this work was to study the potential effects of STX on pancreatic β-cell function. 1-10 nM STX produced an increase in glucose-induced insulin secretion in isolated islets from male mice, whereas it had no effect in islets from female mice. This insulinotropic effect of STX was abolished by the anti-estrogen ICI 182,780. STX increased intracellular calcium entry in both whole islets and isolated β-cells, and closed the K(ATP) channel, suggesting a direct effect on β-cells. When intraperitoneal glucose tolerance test was performed, a single dose of 100 μg/kg body weight STX improved glucose sensitivity in males, yet it had a slight effect on females. In agreement with the effect on isolated islets, 100 μg/kg dose of STX enhanced the plasma insulin increase in response to a glucose load, while it did not in females. Long-term treatment (100 μg/kg, 6 days) of male mice with STX did not alter body weight, fasting glucose, glucose sensitivity or islet insulin content. Ovariectomized females were insensitive to STX (100 μg/kg), after either an acute administration or a 6-day treatment. This long-term treatment was also ineffective in a mouse model of mild diabetes. Therefore, STX appears to have a gender-specific effect on blood glucose homeostasis, which is only manifested after an acute administration. The insulinotropic effect of STX in pancreatic β-cells is mediated by the closure of the K(ATP) channel and the increase in intracellular calcium concentration. The in vivo improvement in glucose tolerance appears to be mostly due to the enhancement of insulin secretion from β-cells.     

Glucose-induced period-doubling cascade in the electrical activity of pancreatic β-cells

 In the presence of stimulatory concentrations of glucose, the membrane potential of pancreatic β-cells may experience a transition from periods of rapid spike-like oscillations alternating with a pseudo-steady state to spike-only oscillations. Insulin secretion from β-cells closely correlates the periods of spike-like oscillations. The purpose of this paper is to study the mathematical structure which underlines this transitional stage in a pancreatic β-cell model. It is demonstrated that the transition can be chaotic but becomes more and more regular with increase in glucose. In particular, the system undergoes a reversed period-doubling cascade leading to the spike-only oscillations as the glucose concentration crosses a threshold. The transition interval in glucose concentration is estimated to be extremely small in terms of the rate of change for the calcium dynamics in the β-cells. The methods are based on the theory of unimodal maps and the geometric and asymptotic theories of singular perturbations. Received: 25 October 1996/Revised version: 18 August 1997     

Blockade of sphingosine 1-phosphate receptor 2 signaling attenuates streptozotocin-induced apoptosis of pancreatic β-cells

Sphingosine 1-phosphate (S1P) is a potent sphingolipid mediator that acts through five cognate G protein-coupled receptors (S1P₁-S1P₅) and regulates many critical biological processes. Recent studies indicated that S1P at nanomolar concentrations significantly reduces cytokine-induced apoptosis of pancreatic β-cells in which genes for S1P₁-S1P₄ are co-expressed. However, the S1P receptor subtype(s) involved in this effect remains to be clarified. In this study, we investigated the potential role of S1P₂ in streptozotocin (STZ)-induced apoptosis of pancreatic β-cells and progression of diabetes. S1P₂-deficient (S1P₂^-/-) mice displayed a greater survive ability, lower blood glucose levels, and smaller numbers of TUNEL-positive apoptotic β-cells to administration of a high dose of STZ than wild-type (WT) mice. S1P₂^-/- mice showed higher insulin/glucose ratios (an index of relative insulin deficiency) and larger insulin-positive islet areas to administration of a low dose of STZ than WT mice. Moreover, administration of JTE-013, a S1P₂-specific antagonist, to WT mice ameliorated STZ-induced blood glucose elevation and reduced the incidence of diabetes. Our findings indicate that blockade of S1P₂ signaling attenuates STZ-induced apoptosis of pancreatic β-cells and decreases the incidence of diabetes.     

Inhibition of protein kinase C βII isoform ameliorates methylglyoxal advanced glycation endproduct-induced cardiomyocyte contractile dysfunction

Aims

Accumulation of advanced glycation endproduct (AGE) contributes to diabetic complication including diabetic cardiomyopathy although the precise underlying mechanism still remains elusive. Recent evidence depicted a pivotal role of protein kinase C (PKC) in diabetic complications. To this end, this study was designed to examine if PKCβII contributes to AGE-induced cardiomyocyte contractile and intracellular Ca^2 + aberrations.

Main methods

Adult rat cardiomyocytes were incubated with methylglyoxal-AGE (MG-AGE) in the absence or presence of the PKCβII inhibitor LY333531 for 12 h. Contractile and intracellular Ca^2 + properties were assessed using an IonOptix system including peak shortening (PS), maximal velocity of shortening/relengthening (± dL/dt), time-to-PS (TPS), time-to-90% relengthening (TR₉₀), rise in intracellular Ca^2 + Fura-2 fluorescence intensity and intracellular Ca^2 + decay. Oxidative stress, O₂⁻ production and mitochondrial integrity were examined using TBARS, fluorescence imaging, aconitase activity and Western blotting.

Key findings

MG-AGE compromised contractile and intracellular Ca^2 + properties including reduced PS, ± dL/dt, prolonged TPS and TR₉₀, decreased electrically stimulated rise in intracellular Ca^2 + and delayed intracellular Ca^2 + clearance, the effects of which were ablated by the PKCβII inhibitor LY333531. Inhibition of PKCβII rescued MG-AGE-induced oxidative stress, O₂⁻ generation, cell death, apoptosis and mitochondrial injury (reduced aconitase activity, UCP-2 and PGC-1α). In vitro studies revealed that PKCβII inhibition-induced beneficial effects were replicated by the NADPH oxidase inhibitor apocynin and were mitigated by the mitochondrial uncoupler FCCP.

Significance

These findings implicated the therapeutic potential of specific inhibition of PKCβII isoform in the management of AGE accumulation-induced myopathic anomalies.     

Inhibition of advanced glycation by flavonoids. A nutritional implication for preventing diabetes complications?

Advanced glycation of collagens contributes to development of micro- and macrovascular complications in diabetes. Since flavonoids are potent natural antioxidants, it was interesting to examine their effect on the formation of a cross-linking advanced glycation endproduct, pentosidine, in collagen incubated with glucose. Monomeric flavonoids (25 and 250 microM) markedly reduced pentosidine/hydroxyproline values in a concentration- and structure-dependent manner. Procyanidin oligomers from grape seed were more active than pine bark procyanidin oligomers. Oligomers are known to be cleaved into monomers in the gastric milieu and monomeric flavonoids to be absorbed and recovered at micromolar concentrations (with a long plasmatic half-life) in extracellular fluids, in contact with collagens. In conclusion, flavonoids are very potent inhibitors of pentosidine formation in collagens, active at micromolar concentrations; these concentrations might be achieved in plasma of diabetic patients after oral intake of flavonoids.     

The role of TRPM2 in pancreatic β-cells and the development of diabetes

TRPM2 is a Ca²⁺-permeable non-selective cation channel that can be activated by adenosine dinucleotides, hydrogen peroxide, or intracellular Ca²⁺. The protein is expressed in a wide variety of cells, including neurons in the brain, immune cells, endocrine cells, and endothelial cells. This channel is also well expressed in β-cells in the pancreas. Insulin secretion from pancreatic β-cells is the primary mechanism by which the concentration of blood glucose is reduced. Thus, impairment of insulin secretion leads to hyperglycemia and eventually causes diabetes. Glucose is the principal stimulator of insulin secretion. The primary pathway involved in glucose-stimulated insulin secretion is the ATP-sensitive K⁺ (K_ATP) channel to voltage-gated Ca²⁺ channel (VGCC)-mediated pathway. Increases in the intracellular Ca²⁺ concentration are necessary for insulin secretion, but VGCC is not sufficient to explain [Ca²⁺]_i increases in pancreatic β-cells and the resultant secretion of insulin. In this review, we focus on TRPM2 as a candidate for a [Ca²⁺]_i modulator in pancreatic β-cells and its involvement in insulin secretion and development of diabetes. Although further analyses are needed to clarify the mechanism underlying TRPM2-mediated insulin secretion, TRPM2 could be a key player in the regulation of insulin secretion and could represent a new target for diabetes therapy.     

Expression,purification and fluorine-18 radiolabeling of recombinant S100A4: a potential probe for molecular imaging of receptor for advanced glycation endproducts in vivo?

Data concerning the pathophysiological role of extracellular S100A4, a member of the multigenic family of Ca²⁺-modulated S100 proteins, and its interaction with the receptor for advanced glycation endproducts (RAGE) or other putative receptors in tumorigenesis, metastasis, and inflammatory processes in vivo are scarce. One reason is the shortage of suitable radiotracer methods. We report a novel methodology using recombinant human S100A4 as potential probe for molecular imaging and functional characterization of this interaction. Therefore, human S100A4 was cloned as GST fusion protein in the bacterial expression vector pGEX-6P-1 and expressed in E. coli strain BL21. Purified recombinant human S100A4 was radiolabeled with the positron emitter fluorine-18 (¹⁸F) by conjugation with N-succinimidyl-4-[¹⁸F]fluorobenzoate ([¹⁸F]SFB). The radioligand [¹⁸F]fluorobenzoyl-S100A4 (¹⁸F-S100A4) was used in cell binding experiments in RAGE-bearing human melanoma cells and endothelial cells in vitro, and in both biodistribution experiments and small animal positron emission tomography (PET) studies in normal rats in vivo. The cellular association and tissue-specific distribution of ¹⁸F-S100A4 in vitro and in vivo correlated well with the protein expression and anatomical localization of RAGE, e.g., in the vascular system and in lung. Compared to other S100 RAGE radioligands, the overall findings of this study indicate that extracellular S100A4 in vivo shows only a moderate interaction with RAGE and, furthermore, exhibits a substantially faster metabolic degradation. On the other hand, the approach allows the use of quantitative small animal PET and provides a novel probe to both delineate functional expression and differentiate multiligand interaction of RAGE under normal and pathophysiological conditions in rodent models of disease.     

Regulation of K(ATP) channel by 17β-estradiol in pancreatic β-cells

ATP-sensitive potassium channels (K_ATP) regulate electrical activity and insulin secretion in pancreatic β-cells. When glucose concentration increases, the [ATP]/[ADP] ratio rises closing K_ATP channels, and the membrane potential depolarizes, triggering insulin secretion. This pivotal role of K_ATP channels is used not only by glucose but also by neurotransmitters, hormones and other physiological agents to modulate electrical and secretory β-cell response.In recent years, it has been demonstrated that estrogens and estrogen receptors are involved in glucose homeostasis, and that they can modulate the electrical activity and insulin secretion of pancreatic β-cells. The hormone 17β-estradiol (E2), at physiological levels, is implicated in maintaining normal insulin sensitivity for β-cell function. Long term exposure to E2 increases insulin content, insulin gene expression and insulin release via the estrogen receptor α (ERα), while rapid responses to E2 can regulate K_ATP channels increasing cGMP levels through the estrogen receptor β (ERβ) and type A guanylate cyclase receptor (GC-A). This review summarizes the main actions of 17β-estradiol on K_ATP channels and the subsequent insulin release in pancreatic β-cells.     

Electrophysiology of pancreatic β-cells in intact mouse islets of Langerhans

When exposed to intermediate glucose concentrations (6–16 mol/l), pancreatic β-cells in intact islets generate bursts of action potentials (superimposed on depolarised plateaux) separated by repolarised electrically silent intervals. First described more than 40 years ago, these oscillations have continued to intrigue β-cell electrophysiologists. To date, most studies of β-cell ion channels have been performed on isolated cells maintained in tissue culture (that do not burst). Here we will review the electrophysiological properties of β-cells in intact, freshly isolated, mouse pancreatic islets. We will consider the role of ATP-regulated K⁺-channels (K_ATP-channels), small-conductance Ca²⁺-activated K⁺-channels and voltage-gated Ca²⁺-channels in the generation of the bursts. Our data indicate that K_ATP-channels not only constitute the glucose-regulated resting conductance in the β-cell but also provide a variable K⁺-conductance that influence the duration of the bursts of action potentials and the silent intervals. We show that inactivation of the voltage-gated Ca²⁺-current is negligible at voltages corresponding to the plateau potential and consequently unlikely to play a major role in the termination of the burst. Finally, we propose a model for glucose-induced β-cell electrical activity based on observations made in intact pancreatic islets.     

Stimulation of the insulin secretory mechanism following barium accumulation in pancreatic β-cells

Electrothermal atomic absorption spectroscopy was employed for measuring barium in β-cell-rich pancreatic islets microdissected from ob/ob-mice. Both the uptake and efflux of barium displayed two distinct phases. There was a 4-fold accumulation of barium into intracellular stores when its extracellular concentration was 0.26 mM. Unlike divalent cations with more extensive intracellular accumulation, the washout of Ba²⁺ was not inhibited by d-glucose. Ba²⁺ served as a substitute for Ca²⁺ both in maintaining the glucose metabolism after removal of extracellular Ca²⁺ and making it possible for glucose to stimulate insulin release. Furthermore, Ba²⁺ elicited insulin release in the absence of glucose and other secretagogues. The latter effect was reversible and was markedly potentiated under conditions known to increase the β-cell content of cyclic AMP. It is likely that the observed actions of Ba²⁺ are mediated by Ca²⁺, since Ca²⁺-dependent regulatory proteins, such as calmodulin, apparently cannot bind Ba²⁺ specifically.