Novel players in pancreatic islet signaling: from membrane receptors to nuclear channels

The Glucagon-Like Adipokinetic Hormone in Drosophila melanogaster - Biosynthesis and Secretion

Metabolic homeostasis requires the precise regulation of circulating sugar titers. In mammals, homeostatic control of circulating sugar titers requires the coordinated secretion and systemic activities of glucagon and insulin. Metabolic homeostasis is similarly regulated in Drosophila melanogaster through the glucagon-like adipokinetic hormone (AKH) and the Drosophila insulin-like peptides (DILPs). In flies and mammals, glucagon and AKH are biosynthesized in and secreted from specialized endocrine cells. KATP channels borne on these cells respond to fluctuations in circulating glucose titers and thereby regulate glucagon secretion. The influence of glucagon in the pathogenesis of type 2 diabetes mellitus is now recognized, and a crucial mechanism that regulates glucagon secretion was reported nearly a decade ago. Ongoing efforts to develop D. melanogaster models for metabolic syndrome must build upon this seminal work. These efforts make a critical review of AKH physiology timely. This review focuses on AKH biosynthesis and the regulation of glucose-responsive AKH secretion through changes in CC cell electrical activity. Future directions for AKH research in flies are discussed, including the development of models for hyperglucagonemia and epigenetic inheritance of acquired metabolic traits. Many avenues of AKH physiology remain to be explored and thus present great potential for improving the utility of D. melanogaster in metabolic research.

Using stem cells to produce insulin

INTRODUCTION

Tremendous progress has been made in generating insulin-producing cells from pluripotent stem cells. The best outcome of the refined protocols became apparent in the first clinical trial announced by ViaCyte, based on the implantation of pancreatic progenitors that would further mature into functional insulin-producing cells inside the patient's body.

AREAS COVERED

Several groups, including ours, have contributed to improve strategies to generate insulin-producing cells. Of note, the latest results have gained a substantial amount of interest as a method to create a potentially functional and limitless supply of β-cell to revert diabetes mellitus. This review analyzes the accomplishments that have taken place over the last few decades, summarizes the state-of-art methods for β-cell replacement therapies based on the differentiation of embryonic stem cells into glucose-responsive and insulin-producing cells in a dish and discusses alternative approaches to obtain new sources of insulin-producing cells.

EXPERT OPINION

Undoubtedly, recent events preface the beginning of a new era in diabetes therapy. However, in our opinion, a number of significant hurdles still stand in the way of clinical application. We believe that the combination of the private and public sectors will accelerate the process of obtaining the desired safe and functional β-cell surrogates.

Ionic mechanisms in pancreatic β cell signaling

The function and survival of pancreatic β cells critically rely on complex electrical signaling systems composed of a series of ionic events, namely fluxes of K(+), Na(+), Ca(2+) and Cl(-) across the β cell membranes. These electrical signaling systems not only sense events occurring in the extracellular space and intracellular milieu of pancreatic islet cells, but also control different β cell activities, most notably glucose-stimulated insulin secretion. Three major ion fluxes including K(+) efflux through ATP-sensitive K(+) (KATP) channels, the voltage-gated Ca(2+) (CaV) channel-mediated Ca(2+) influx and K(+) efflux through voltage-gated K(+) (KV) channels operate in the β cell. These ion fluxes set the resting membrane potential and the shape, rate and pattern of firing of action potentials under different metabolic conditions. The KATP channel-mediated K(+) efflux determines the resting membrane potential and keeps the excitability of the β cell at low levels. Ca(2+) influx through CaV1 channels, a major type of β cell CaV channels, causes the upstroke or depolarization phase of the action potential and regulates a wide range of β cell functions including the most elementary β cell function, insulin secretion. K(+) efflux mediated by KV2.1 delayed rectifier K(+) channels, a predominant form of β cell KV channels, brings about the downstroke or repolarization phase of the action potential, which acts as a brake for insulin secretion owing to shutting down the CaV channel-mediated Ca(2+) entry. These three ion channel-mediated ion fluxes are the most important ionic events in β cell signaling. This review concisely discusses various ionic mechanisms in β cell signaling and highlights KATP channel-, CaV1 channel- and KV2.1 channel-mediated ion fluxes.

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.

Improved quantification of islet transplants by magnetic resonance imaging with Resovist

OBJECTIVES

We aimed to develop an accurate and reproducible method to quantify transplanted islets and monitor their functional status in vivo. To support this aim, we investigated the cytotoxic effect of Resovist on islet function and survival.

METHODS

The average pixel number for a single Resovist-labeled islet was measured. To determine Resovist cytotoxicity, DNA fragmentation, adenosine diphosphate-adenosine triphosphate ratio, ion channel activity, and in vivo islet function were evaluated. To quantitatively monitor the fate of islet transplant, we transplanted Resovist-labeled islets into syngeneic C57BL/6 mice for magnetic resonance imaging analysis.

RESULTS

The average pixel volume for a medium-sized islet (100-150 μm in diameter) was determined from the contrast signal void of magnetic resonance image. Toxicological analysis showed that Resovist did not affect islet at concentrations up to 40 times the labeling dose. In the quantitative analysis, the number of contrast spots did not correlated with the number of transplanted islets, whereas our newly adopted measure showed a significant correlation.

CONCLUSIONS

Islet transplant survival may be safely and accurately monitored using magnetic resonance imaging with the Resovist. We found in this study that pixel number may correlate more closely than the number of contrast spots with the number of islets transplanted.

Metabolic impact of estrogen signalling through ERalpha and ERbeta

Estrogens, acting on both estrogen receptors alpha (ERalpha) and beta (ERbeta) are recognized as important regulators of glucose homeostasis and lipid metabolism. ERs belong to the family of nuclear hormone receptors which mainly act as ligand activated transcription factors. Both ERs are expressed in metabolic tissue such as adipose tissue, skeletal muscle, liver and pancreas, as well as in the central nervous system. Expression pattern of both ERs differ between species, sexes, and specific tissues. The present review will focus on the key effects of ERs on glucose- and lipid metabolism. It appears that ERalpha mainly mediates beneficial metabolic effects of estrogens such as anti-lipogenesis, improvement of insulin sensitivity and glucose tolerance, and reduction of body weight/fat mass. In contrast, ERbeta activation seems to be detrimental for the maintenance of regular glucose and lipid homeostasis. Metabolic actions of both receptors in relevant tissues will be discussed.

The atrial natriuretic peptide and guanylyl cyclase-A system modulates pancreatic beta-cell function

Atrial natriuretic peptide (ANP) and its guanylyl cyclase-A (GC-A) receptor are being involved in metabolism, although their role in the endocrine pancreas is still greatly unknown. The aim of this work is to study a possible role for the ANP/GC-A system in modulating pancreatic beta-cell function. The results presented here show a direct effect of the GC-A receptor in regulating glucose-stimulated insulin secretion (GSIS) and beta-cell mass. GC-A activation by its natural ligand, ANP, rapidly blocked ATP-dependent potassium (K(ATP)) channel activity, increased glucose-elicited Ca(2+) signals, and enhanced GSIS in islets of Langerhans. The effect in GSIS was inhibited in islets from GC-A knockout (KO) mice. Pancreatic islets from GC-A KO mice responded to increasing glucose concentrations with enhanced insulin secretion compared with wild type (WT). Remarkably, islets from GC-A KO mice were smaller, presented lower beta-cell mass and decreased insulin content. However, glucose-induced Ca(2+) response was more vigorous in GC-A KO islets, and basal K(ATP) channel activity in GC-A KO beta-cells was greatly diminished compared with WT. When protein levels of the two K(ATP) channel constitutive subunits sulfonylurea receptor 1 and Inward rectifier potassium channel 6.2 were measured, both were diminished in GC-A KO islets. These alterations on beta-cell function were not associated with disruption of glucose tolerance or insulin sensitivity in vivo. Glucose and insulin tolerance tests were similar in WT and GC-A KO mice. Our data suggest that the ANP/GC-A system may have a modulating effect on beta-cell function.

Sexual dimorphic evolution of metabolic programming in non-genetic non-alimentary mild metabolic syndrome model in mice depends on feed-back mechanisms integrity for pro-opiomelanocortin-derived endogenous substances

Previously, we showed that our post-natal handling model induces pro-opiomelanocortin-derived (POMC) endogenous systems alterations in male mice at weaning. These alterations last up to adult age, and are at the basis of adult hormonal and metabolic conditions similar to mild metabolic syndrome/type-2 diabetes. Here, we evaluate how sex influences post-natal programming in these metabolic conditions. Subjects are adult control (non-handled) female (NHF) and male (NHM) CD-1 mice; adult post-natal handled female (HF) and male (HM) mice. Handling consists of daily maternal separation (10 min) plus sham injection, from birth to weaning (21 days). In adult handled males (90-days old) we find not only POMC-derived hormones alterations (enhanced basal plasma corticosterone (+91%) and ACTH (+109%)) but also overweight (+5.4%), fasting hyperglycemia (+40%), hypertriglyceridemia (+21%), enhanced brain mRNA expression of hydroxysteroid(11-beta)dehydrogenase type-1 (HSD11B1) (+49%), and decreased mRNA-HSD11B2 (-39%). Conversely, uric acid, creatinine, HDL(C), total cholesterol, glucose and insulin incremental area under-the-curve are not affected. In females, post-natal handling does not produce both hormonal and dysmetabolic diabetes-like changes; but handling enhances n3- and n6-poly-unsaturated, and decreases saturated fatty acids content in erythrocyte membrane composition in HF versus NHF. In conclusion, for the first time we show that female sex in mice exerts effective protection against the hypothalamus-pituitary-adrenal homeostasis disruption induced by our post-natal handling model on POMC cleavage products; endocrine disruption is in turn responsible for altered metabolic programming in male mice. The role of sex hormones is still to be elucidated.

Blood glucose regulation during prolonged, submaximal, continuous exercise: a guide for clinicians

Management of many chronic diseases now includes regular exercise as part of a viable treatment plan. Exercise in the form of prolonged, submaximal, continuous exercise (SUBEX; i.e., approximately 30 min to 1 h, approximately 40-70% of maximal oxygen uptake) is often prescribed due to its relatively low risk, the willingness of patients to undertake, its efficacy, its affordability, and its ease of prescription. Specifically, patients who are insulin resistant or that have type 2 diabetes mellitus may benefit from regular exercise of this type. During this type of exercise, muscles dramatically increase glucose uptake as the liver increases both glycogenolysis and gluco-neogenesis. While a redundancy of mechanisms is at work to maintain blood glucose concentration ([glucose]) during this type of exercise, the major regulator of blood glucose is the insulin/glucagon response. At exercise onset, blood [glucose] transiently rises before beginning to decline after approximately 30 min, causing a subsequent decline in blood [insulin] and rise in blood glucagon. This leads to many downstream effects, including an increase in glucose output from the liver to maintain adequate glucose in the blood to fuel both the muscles and the brain. Finally, when analyzing blood [glucose], consideration should be given to nutritional status (postabsorptive versus postprandial) as well as both what the analyzer measures and the type of sample used (plasma versus whole blood). In view of both prescribing exercise to patients as well as designing studies that perturb glucose homeostasis, it is imperative that clinicians and researchers alike understand the controls of blood glucose homeostasis during SUBEX.

ROS signaling, oxidative stress and Nrf2 in pancreatic beta-cell function

This review focuses on the emerging evidence that reactive oxygen species (ROS) derived from glucose metabolism, such as H(2)O(2), act as metabolic signaling molecules for glucose-stimulated insulin secretion (GSIS) in pancreatic beta-cells. Particular emphasis is placed on the potential inhibitory role of endogenous antioxidants, which rise in response to oxidative stress, in glucose-triggered ROS and GSIS. We propose that cellular adaptive response to oxidative stress challenge, such as nuclear factor E2-related factor 2 (Nrf2)-mediated antioxidant induction, plays paradoxical roles in pancreatic beta-cell function. On the one hand, induction of antioxidant enzymes protects beta-cells from oxidative damage and possible cell death, thus minimizing oxidative damage-related impairment of insulin secretion. On the other hand, the induction of antioxidant enzymes by Nrf2 activation blunts glucose-triggered ROS signaling, thus resulting in reduced GSIS. These two premises are potentially relevant to impairment of beta-cells occurring in the late and early stage of Type 2 diabetes, respectively. In addition, we summarized our recent findings that persistent oxidative stress due to absence of uncoupling protein 2 activates cellular adaptive response which is associated with impaired pancreatic beta-cell function.

17beta-Estradiol modulates apoptosis in pancreatic beta-cells by specific involvement of the sulfonylurea receptor (SUR) isoform SUR1

Apoptosis of pancreatic beta-cells is an important factor in the pathophysiology of diabetes. Previously, we have shown that the "phytoestrogen" resveratrol can induce beta-cell apoptosis dependent on the expression of sulfonylurea receptor (SUR) 1, the regulatory subunit of pancreatic ATP-sensitive K(+) channels. Here, we investigate whether 17beta-estradiol also influences beta-cell apoptosis in a SUR1-dependent manner. Therefore, islets from wild type or SUR1 knock-out mice, clonal beta-cells, or HEK293 cells expressing different SUR forms were treated with 17beta-estradiol or estrone. Different apoptotic parameters were determined and estrogen binding to SUR was analyzed. In murine islets, 17beta-estradiol treatment resulted in significant apoptotic changes, which in their nature (either apoptotic or anti-apoptotic) were dependent on the age of the animal. These effects were not observed in SUR1 knock-out mice. Furthermore, 17beta-estradiol, which specifically binds to SUR, induced enhanced apoptosis in SUR1-expressing HEK293 cells and clonal beta-cells, whereas apoptosis in recombinant cells expressing SUR2A or SUR2B (cardiac or vascular SUR-isoforms) or sham-transfected control cells was significantly lower. The apoptotic potency of 17beta-estradiol was much higher than that of resveratrol or estrone. SUR1-specific 17beta-estradiol-induced apoptosis was either abolished by the mutation M1289T in transmembrane helix 17 of SUR1 or clearly enhanced by two mutations in nucleotide binding fold 2 (R1379C, R1379L). In conclusion, 17beta-estradiol treatment modulates beta-cell apoptosis under specific involvement of SUR1 in an age-dependent manner. 17beta-Estradiol-induced apoptosis can be influenced by certain SUR1 mutations. These findings may contribute to the understanding of pathophysiological changes in beta-cell mass and could, for instance, provide interesting aspects concerning the etiology of gestational diabetes.

Lipopolysaccharide: An Endotoxin or an Exogenous Hormone?

Conventional models of the pathogenesis of sepsis assume that microorganisms or their products are necessarily injurious to the host. In contrast, an evolutionary perspective suggests that host-microbial interactions are a symbiotic model and that disease results from the disruption of a mutually beneficial homeostatic state. Lipopolysaccharide (LPS) from gram-negative bacteria is a prototypical trigger of sepsis and a target for the development of novel therapeutics. The biological mechanisms underlying the recognition of, and response to, LPS are more characteristic of a hormone than of a toxin. All mammals carry endogenous stores of LPS and express dedicated carrier proteins, a cellular receptor complex, and mechanisms that specifically antagonize the response to LPS. Disruption of any component of this complex recognition system jeopardizes host defenses against infection with exogenous microorganisms. Thus, LPS is not less an endotoxin than an exohormone, and its neutralization may potentially result in either benefit or harm.

Antidiabetic sulphonylureas activate mitochondrial permeability transition in rat skeletal muscle

Antidiabetic sulphonylureas can bind to various intracellular organelles including mitochondria. The aim of this study was to monitor the influence of antidiabetic sulphonylureas on membrane permeability in mitochondria isolated from rat skeletal muscle. The effects of glibenclamide (and other sulphonylurea derivatives) on mitochondrial function were studied by measuring mitochondrial swelling, mitochondrial membrane potential, respiration rate and Ca2+ transport into mitochondria. We observed that glibenclamide induced mitochondrial swelling (EC50 = 8.2 +/- 2.5 microM), decreased the mitochondrial membrane potential and evoked Ca2+ efflux from the mitochondrial matrix. These effects were blocked by 2 microM cyclosporin A, an inhibitor of the mitochondrial permeability transition. Moreover, 30 microM glibenclamide accelerated the respiratory rate in the presence of glutamate/malate, substrates of complex I of the mitochondrial respiratory chain. In conclusion, we postulate that the antidiabetic sulphonylureas activate the mitochondrial permeability transition in skeletal muscle by increasing its sensitivity to Ca2+.

Low doses of bisphenol A and diethylstilbestrol impair Ca2+ signals in pancreatic alpha-cells through a nonclassical membrane estrogen receptor within intact islets of Langerhans

Glucagon, secreted from pancreatic alpha-cells integrated within the islets of Langerhans, is involved in the regulation of glucose metabolism by enhancing the synthesis and mobilization of glucose in the liver. In addition, it has other extrahepatic effects ranging from lipolysis in adipose tissue to the control of satiety in the central nervous system. In this article, we show that the endocrine disruptors bisphenol A (BPA) and diethylstilbestrol (DES), at a concentration of 10(-9) M, suppressed low-glucose-induced intracellular calcium ion ([Ca2+]i) oscillations in alpha-cells, the signal that triggers glucagon secretion. This action has a rapid onset, and it is reproduced by the impermeable molecule estradiol (E2) conjugated to horseradish peroxidase (E-HRP). Competition studies using E-HRP binding in immunocytochemically identified alpha-cells indicate that 17beta-E2, BPA, and DES share a common membrane-binding site whose pharmacologic profile differs from the classical ER. The effects triggered by BPA, DES, and E2 are blocked by the G alpha i- and G alpha o-protein inhibitor pertussis toxin, by the guanylate cyclase-specific inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one, and by the nitric oxide synthase inhibitor N-nitro-L-arginine methyl ester. The effects are reproduced by 8-bromo-guanosine 3',5'-cyclic monophosphate and suppressed in the presence of the cGMP-dependent protein kinase inhibitor KT-5823. The action of E2, BPA, and DES in pancreatic alpha-cells may explain some of the effects elicited by endocrine disruptors in the metabolism of glucose and lipid.