The pancreatic beta-cell glucose sensor

Berberine protects mice against type 2 diabetes by promoting PPARγ-FGF21-GLUT2-regulated insulin sensitivity and glucose/lipid homeostasis

Type 2 diabetes (T2D) is a chronic, burdensome disease that is characterized by disordered insulin sensitivity and disturbed glucose/lipid homeostasis. Berberine (BBR) has multiple therapeutic actions on T2D, including regulation of glucose and lipid metabolism, improvement of insulin sensitivity and energy expenditure. Recently, the function of BBR on fibroblast growth factor 21 (FGF21) has been identified. However, if BBR ameliorates T2D through FGF21, the underlying mechanisms remain unknown. Herein, we used T2D wild type (WT) and FGF21 global knockout (FKO) mice [mouse T2D model: established by high-fat diet (HFD) feeding plus streptozotocin (STZ) injection], and hepatocyte-specific peroxisome proliferator activated receptor γ (PPARγ) deficient (PPARγHepKO) mice, and cultured human liver carcinoma cells line, HepG2 cells, to characterize the role of BBR in glucose/lipid metabolism and insulin sensitivity. We found that BBR activated FGF21 expression by up-regulating PPARγ expression at the cellular level. Meanwhile, BBR ameliorated glucosamine hydrochloride (Glcn)-induced insulin resistance and increased glucose transporter 2 (GLUT2) expression in a PPARγ/FGF21-dependent manner. In T2D mice, BBR up-regulated the expression of PPARγ, FGF21 and GLUT2 in the liver, and GLUT2 in the pancreas. BBR also reversed T2D-induced insulin resistance, liver lipid accumulation, and damage in liver and pancreas. However, FGF21 deficiency diminished these effects of BBR on diabetic mice. Altogether, our study demonstrates that the therapeutic effects of BBR on T2D were partly accomplished by activating PPARγ-FGF21-GLUT2 signaling pathway. The discovery of this new pathway provides a deeper understanding of the mechanism of BBR for T2D treatment.

β-cell-selective inhibition of DNA damage response signaling by nitric oxide is associated with an attenuation in glucose uptake

Nitric oxide (NO) plays a dual role in regulating DNA damage response (DDR) signaling in pancreatic β-cells. As a genotoxic agent, NO activates two types of DDR signaling; however, when produced at micromolar levels by the inducible isoform of NO synthase, NO inhibits DDR signaling and DDR-induced apoptosis in a β-cell-selective manner. DDR signaling inhibition by NO correlates with mitochondrial oxidative metabolism inhibition and decreases in ATP and NAD+. Unlike most cell types, β-cells do not compensate for impaired mitochondrial oxidation by increasing glycolytic flux, and this metabolic inflexibility leads to a decrease in ATP and NAD+. Here, we used multiple analytical approaches to determine changes in intermediary metabolites in β-cells and non-β-cells treated with NO or complex I inhibitor rotenone. In addition to ATP and NAD+, glycolytic and tricarboxylic acid cycle intermediates as well as NADPH are significantly decreased in β-cells treated with NO or rotenone. Consistent with glucose-6-phosphate residing at the metabolic branchpoint for glycolysis and the pentose phosphate pathway (NADPH), we show that mitochondrial oxidation inhibitors limit glucose uptake in a β-cell-selective manner. Our findings indicate that the β-cell-selective inhibition of DDR signaling by NO is associated with a decrease in ATP to levels that fall significantly below the KM for ATP of glucokinase (glucose uptake) and suggest that this action places the β-cell in a state of suspended animation where it is metabolically inert until NO is removed, and metabolic function can be restored.

Kinetic and data-driven modeling of pancreatic β-cell central carbon metabolism and insulin secretion

Pancreatic β-cells respond to increased extracellular glucose levels by initiating a metabolic shift. That change in metabolism is part of the process of glucose-stimulated insulin secretion and is of particular interest in the context of diabetes. However, we do not fully understand how the coordinated changes in metabolic pathways and metabolite products influence insulin secretion. In this work, we apply systems biology approaches to develop a detailed kinetic model of the intracellular central carbon metabolic pathways in pancreatic β-cells upon stimulation with high levels of glucose. The model is calibrated to published metabolomics datasets for the INS1 823/13 cell line, accurately capturing the measured metabolite fold-changes. We first employed the calibrated mechanistic model to estimate the stimulated cell's fluxome. We then used the predicted network fluxes in a data-driven approach to build a partial least squares regression model. By developing the combined kinetic and data-driven modeling framework, we gain insights into the link between β-cell metabolism and glucose-stimulated insulin secretion. The combined modeling framework was used to predict the effects of common anti-diabetic pharmacological interventions on metabolite levels, flux through the metabolic network, and insulin secretion. Our simulations reveal targets that can be modulated to enhance insulin secretion. The model is a promising tool to contextualize and extend the usefulness of metabolomics data and to predict dynamics and metabolite levels that are difficult to measure in vitro. In addition, the modeling framework can be applied to identify, explain, and assess novel and clinically-relevant interventions that may be particularly valuable in diabetes treatment.

In Vitro Evaluation of Nanoerythrosome Cytotoxicity and Uptake in Pancreatic Endothelial Cells: Implications for β-Cell Imaging Applications

Biomolecule-targeted imaging represents one of the most difficult challenges in medicine. Nanoerythrosomes (NERs) are nanovesicles obtained after lysis of red blood cells, and they are promising tools for drug delivery and imaging. In this work, a formulation based on NERs functionalized with 7-amino-3-methylcoumarin via cross-linking was tested on rat INS-1E and mouse MIN6 β-cells and endothelial MSI cell lines. First, the morphology, size, ζ-potentials, and spectroscopic properties of the aggregates were investigated, highlighting that the functionalization did not significantly affect the nanoparticles' physicochemical features. In vitro, the nanoparticles did not significantly affect the proliferation and function of INS-1E and MIN6 β-cells at different concentrations. Only at the highest concentration tested on the MSI cell line, the formulation inhibited proliferation. Furthermore, NER aggregates were not internalized in both INS-1E and MIN6 cell lines, while a diffuse fluorescence was noticed in the cytosol of the MSI cell line at the highest concentrations. These findings proved that NER formulations might represent a new nanotool for β-cell imaging as a part of a strategy aimed to prevent any intracellular accumulation, thus reducing/avoiding side effects.

OGT Regulates Mitochondrial Biogenesis and Function via Diabetes Susceptibility Gene Pdx1

O-GlcNAc transferase (OGT), a nutrient sensor sensitive to glucose flux, is highly expressed in the pancreas. However, the role of OGT in the mitochondria of β-cells is unexplored. In this study, we identified the role of OGT in mitochondrial function in β-cells. Constitutive deletion of OGT (βOGTKO) or inducible ablation in mature β-cells (iβOGTKO) causes distinct effects on mitochondrial morphology and function. Islets from βOGTKO, but not iβOGTKO, mice display swollen mitochondria, reduced glucose-stimulated oxygen consumption rate, ATP production, and glycolysis. Alleviating endoplasmic reticulum stress by genetic deletion of Chop did not rescue the mitochondrial dysfunction in βOGTKO mice. We identified altered islet proteome between βOGTKO and iβOGTKO mice. Pancreatic and duodenal homeobox 1 (Pdx1) was reduced in in βOGTKO islets. Pdx1 overexpression increased insulin content and improved mitochondrial morphology and function in βOGTKO islets. These data underscore the essential role of OGT in regulating β-cell mitochondrial morphology and bioenergetics. In conclusion, OGT couples nutrient signal and mitochondrial function to promote normal β-cell physiology.

Rheb1 promotes glucose-stimulated insulin secretion in human and mouse β-cells by upregulating GLUT expression

Reduced β-cell mass and impaired β-cell function are primary causes of all types of diabetes. However, the intrinsic molecular mechanism that regulates β-cell growth and function remains elusive. Here, we demonstrate that the small GTPase Rheb1 is a critical regulator of glucose-stimulated insulin secretion (GSIS) in β-cells. Rheb1 was highly expressed in mouse and human islets. In addition, β-cell-specific knockout of Rheb1 reduced the β-cell size and mass by suppressing β-cell proliferation and increasing β-cell apoptosis. However, tamoxifen-induced deletion of Rheb1 in β-cells had no significant effect on β-cell mass and size but significantly impaired GSIS. Rheb1 facilitates GSIS in human or mouse islets by upregulating GLUT1 or GLUT2 expression, respectively, in a mTORC1 signaling pathway-dependent manner. Our findings reveal a critical role of Rheb1 in regulating GSIS in β-cells and identified a new target for the therapeutic treatment of diabetes mellitus.

Selective beta-cell toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin on isolated pancreatic islets

An association between exposure to environmental pollutants and diabetes risk has been repeatedly shown by epidemiological studies. However, the biological basis of this association still need to be clarified. In this research we explored the effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exposure on isolated pancreatic islets. After 1, 6 and 24 h exposure of isolated islets to different concentrations (1-50 nM) of TCDD we assayed: i) cell survival; ii) ultrastructure; iii) glucose-stimulated insulin secretion (GSIS); iv) expression of selected genes. A significant, dose-related increase of both necrosis and apoptosis was observed isolated rat islets after 24 h exposure to TCDD. The electron microscopic analysis revealed, at the same time point, the presence of several ultrastructural alterations (mitochondrial swelling, increased mitophagy, dilation of the endoplasmic reticulum) that, very interestingly, were exclusively observed in beta cells and not in other endocrine cells. Similar results were obtained in isolated human islets. GSIS was rapidly (1 h) and persistently (6 and 24 h) decreased by TCDD exposure even at the smallest concentration (1 nM). TCDD exposure significantly affected gene expression in isolated islets: Glut2, Gck, Bcl-xL, MafA, Pdx1 FoxO1 and IRE1 gene expression was significantly decreased, whereas Puma, DP5, iNOS and Chop gene expression was significantly increased after 6 h exposure to TCDD. In conclusion, our results clearly indicated that pancreatic beta cells represent not only a sensitive but also a specific target of the toxic action of dioxin.

Establishment of a long-term stable β-cell line and its application to analyze the effect of Gcg expression on insulin secretion

A pancreatic β-cell line MIN6 was previously established in our lab from an insulinoma developed in an IT6 transgenic mouse expressing the SV40 T antigen in β-cells. This cell line has been widely used for in vitro analysis of β-cell function, but tends to lose the mature β-cell features, including glucose-stimulated insulin secretion (GSIS), in long-term culture. The aim of this study was to develop a stable β-cell line that retains the characteristics of mature β-cells. Considering that mice derived from a cross between C3H and C57BL/6 strains are known to exhibit higher insulin secretory capacity than C57BL/6 mice, an IT6 male mouse of this hybrid background was used to isolate insulinomas, which were independently cultured. After 7 months of continuous culturing, we obtained the MIN6-CB4 β-cell line, which stably maintains its GSIS. It has been noted that β-cell lines express the glucagon (Gcg) gene at certain levels. MIN6-CB4 cells were utilized to assess the effects of differential Gcg expression on β-cell function. Our data show the functional importance of Gcg expression and resulting basal activation of the GLP-1 receptor in β-cells. MIN6-CB4 cells can serve as an invaluable tool for studying the regulatory mechanisms of insulin secretion, such as the GLP-1/cAMP signaling, in β-cells.

Can insulin secreting pancreatic β-cells provide novel insights into the metabolic regulation of the DNA damage response?

Insulin, produced by pancreatic β-cells, is responsible for the control of whole-body glucose metabolism. Insulin is secreted by pancreatic β-cells in a tightly regulated process that is controlled by the serum level of glucose, glucose sensing and glucose oxidative metabolism. The regulation of intermediary metabolism in β-cells is unique as these cells oxidize glucose to CO₂ on substrate supply while mitochondrial oxidative metabolism occurs on demand, for the production of intermediates or energy production, in most cell types. This review discusses recent findings that the regulation of intermediary metabolism by nitric oxide attenuates the DNA damage response (DDR) and DNA damage-dependent apoptosis in a β-cell selective manner. Specific focus is placed on the mechanisms by which iNOS derived nitric oxide (low micromolar levels) regulates DDR activation via the inhibition of intermediary metabolism. The physiological significance of the association of metabolism, nitric oxide and DDR signaling for cancer biology and diabetes is discussed.

Dietary Flavonoids in the Prevention of T2D: An Overview

Type 2 diabetes (T2D) is a progressive metabolic disease that is increasing in prevalence globally. It is well established that insulin resistance (IR) and a progressive decline in functional β-cell mass are hallmarks of developing T2D. Obesity is a leading pathogenic factor for developing IR. Constant IR will progress to T2D when β-cells are unable to secret adequate amounts of insulin to compensate for decreased insulin sensitivity. Recently, a considerable amount of research has been devoted to identifying naturally occurring anti-diabetic compounds that are abundant in certain types of foods. Flavonoids are a group of polyphenols that have drawn great interest for their various health benefits. Results from many clinical and animal studies demonstrate that dietary intake of flavonoids might be helpful in preventing T2D, although cellular and molecular mechanisms underlying these effects are still not completely understood. This review discusses our current understanding of the pathophysiology of T2D and highlights the potential anti-diabetic effects of flavonoids and mechanisms of their actions.

Pancreatic mitochondrial complex I exhibits aberrant hyperactivity in diabetes

It is well established that NADH/NAD+ redox balance is heavily perturbed in diabetes, and the NADH/NAD+ redox imbalance is a major source of oxidative stress in diabetic tissues. In mitochondria, complex I is the only site for NADH oxidation and NAD+ regeneration and is also a major site for production of mitochondrial reactive oxygen species (ROS). Yet how complex I responds to the NADH/NAD+ redox imbalance and any potential consequences of such response in diabetic pancreas have not been investigated. We report here that pancreatic mitochondrial complex I showed aberrant hyperactivity in either type 1 or type 2 diabetes. Further studies focusing on streptozotocin (STZ)-induced diabetes indicate that complex I hyperactivity could be attenuated by metformin. Moreover, complex I hyperactivity was accompanied by increased activities of complexes II to IV, but not complex V, suggesting that overflow of NADH via complex I in diabetes could be diverted to ROS production. Indeed in diabetic pancreas, ROS production and oxidative stress increased and mitochondrial ATP production decreased, which can be attributed to impaired pancreatic mitochondrial membrane potential that is responsible for increased cell death. Additionally, cellular defense systems such as glucose 6-phosphate dehydrogenase, sirtuin 3, and NQO1 were found to be compromised in diabetic pancreas. Our findings point to the direction that complex I aberrant hyperactivity in pancreas could be a major source of oxidative stress and β cell failure in diabetes. Therefore, inhibiting pancreatic complex I hyperactivity and attenuating its ROS production by various means in diabetes might serve as a promising approach for anti-diabetic therapies.

Molecular Cloning of the Porcine Insulin cDNA Using a Monolayer Culture of Pancreatic Endocrine Cells

Porcine pancreatic endocrine cells are an attractive candidate for islet cell transplantation in view of the immunological properties and structural similarities of porcine insulin to human insulin. We recently established a method of isolation and a primary monolayer culture of porcine pancreatic endocrine cells. In this study, cloning of the porcine insulin cDNA was performed to clarify the genetic background of the purified isolated cells. A homology-based PCR cloning method was employed to determine the sequence using mRNA extracted from the monolayer-forming cells, and the candidate products were then determined by a homology search on the human insulin cDNA. According to the newly identified sequence, rapid amplification of cDNA ends was applied to the 5′ and 3′ ends, and the entire cDNA sequence was determined. Gene and protein expression was confirmed by Northern blotting, immunohistochemistry, and enzyme assay. To examine the transcriptional level, the cultured cells were incubated in a 20 mM D-glucose medium in the presence or absence of 5 μM forskolin. The porcine insulin cDNA exhibited a high homology to the human cDNA and showed 85% matching with the human amino acid sequence. D-Glucose at 20 mM stimulated the insulin secretion and mRNA expression, and further addition of 5 μM forskolin with the glucose was applied as the strongest stimulus in this culture system.

Gut-Brain Glucose Signaling in Energy Homeostasis

Intestinal gluconeogenesis is a recently identified function influencing energy homeostasis. Intestinal gluconeogenesis induced by specific nutrients releases glucose, which is sensed by the nervous system surrounding the portal vein. This initiates a signal positively influencing parameters involved in glucose control and energy management controlled by the brain. This knowledge has extended our vision of the gut-brain axis, classically ascribed to gastrointestinal hormones. Our work raises several questions relating to the conditions under which intestinal gluconeogenesis proceeds and may provide its metabolic benefits. It also leads to questions on the advantage conferred by its conservation through a process of natural selection.

Understanding Metabolic Regulation at a Systems Level: Metabolite Sensing, Mathematical Predictions, and Model Organisms

Metabolic networks are extensively regulated to facilitate tissue-specific metabolic programs and robustly maintain homeostasis in response to dietary changes. Homeostatic metabolic regulation is achieved through metabolite sensing coupled to feedback regulation of metabolic enzyme activity or expression. With a wealth of transcriptomic, proteomic, and metabolomic data available for different cell types across various conditions, we are challenged with understanding global metabolic network regulation and the resulting metabolic outputs. Stoichiometric metabolic network modeling integrated with "omics" data has addressed this challenge by generating nonintuitive, testable hypotheses about metabolic flux rewiring. Model organism studies have also yielded novel insight into metabolic networks. This review covers three topics: the feedback loops inherent in metabolic regulatory networks, metabolic network modeling, and interspecies studies utilizing Caenorhabditis elegans and various bacterial diets that have revealed novel metabolic paradigms.

Glucose and hypothalamic astrocytes: More than a fueling role?

Brain plays a central role in energy homeostasis continuously integrating numerous peripheral signals such as circulating nutrients, and in particular blood glucose level, a variable that must be highly regulated. Then, the brain orchestrates adaptive responses to modulate food intake and peripheral organs activity in order to achieve the fine tuning of glycemia. More than fifty years ago, the presence of glucose-sensitive neurons was discovered in the hypothalamus, but what makes them specific and identifiable still remains disconnected from their electrophysiological signature. On the other hand, astrocytes represent the major class of macroglial cells and are now recognized to support an increasing number of neuronal functions. One of these functions consists in the regulation of energy homeostasis through neuronal fueling and nutrient sensing. Twenty years ago, we discovered that the glucose transporter GLUT2, the canonical "glucosensor" of the pancreatic beta-cell together with the glucokinase, was also present in astrocytes and participated in hypothalamic glucose sensing. Since then, many studies have identified other actors and emphasized the astroglial participation in this mechanism. Growing evidence suggest that astrocytes form a complex network and have to be considered as spatially coordinated and regulated metabolic units. In this review we aim to provide an updated view of the molecular and respective cellular pathways involved in hypothalamic glucose sensing, and their relevance in physiological and pathological states.

Roles of Pyruvate, NADH, and Mitochondrial Complex I in Redox Balance and Imbalance in β Cell Function and Dysfunction

Pancreatic β cells not only use glucose as an energy source, but also sense blood glucose levels for insulin secretion. While pyruvate and NADH metabolic pathways are known to be involved in regulating insulin secretion in response to glucose stimulation, the roles of many other components along the metabolic pathways remain poorly understood. Such is the case for mitochondrial complex I (NADH/ubiquinone oxidoreductase). It is known that normal complex I function is absolutely required for episodic insulin secretion after a meal, but the role of complex I in β cells in the diabetic pancreas remains to be investigated. In this paper, we review the roles of pyruvate, NADH, and complex I in insulin secretion and hypothesize that complex I plays a crucial role in the pathogenesis of β cell dysfunction in the diabetic pancreas. This hypothesis is based on the establishment that chronic hyperglycemia overloads complex I with NADH leading to enhanced complex I production of reactive oxygen species. As nearly all metabolic pathways are impaired in diabetes, understanding how complex I in the β cells copes with elevated levels of NADH in the diabetic pancreas may provide potential therapeutic strategies for diabetes.

Yeast HXK2 gene reverts glucose regulation mutation of penicillin biosynthesis in P. chrysogenum

The mutant Penicillium chrysogenum strain dogR5, derived from strain AS-P-78, does not respond to glucose regulation of penicillin biosynthesis and β-galactosidase, and is partially deficient in D-glucose phosphorilating activity. We have transformed strain dogR5 with the (hexokinase) hxk2 gene from Saccharomyces cerevisiae. Transformants recovered glucose control of penicillin biosynthesis in different degrees, and acquired a hexokinase (fructose phosphorylating) activity absent in strains AS- P-78 and dogR5. Interestingly, they also recovered glucose regulation of β-galactosidase. On the other hand, glucokinase activity was affected in different ways in the transformants; one of which showed a lower activity than the parental dogR5, but normal glucose regulation of penicillin biosynthesis. Our results show that Penicillium chrysogenum AS-P-78 and dogR5 strains lack hexokinase, and suggest that an enzyme with glucokinase activity is involved in glucose regulation of penicillin biosynthesis and β-galactosidase, thus signaling glucose in both primary and secondary metabolism; however, catalytic and signaling activities seem to be independent.

Association of dioxin and other persistent organic pollutants (POPs) with diabetes: epidemiological evidence and new mechanisms of beta cell dysfunction

The worldwide explosion of the rates of diabetes and other metabolic diseases in the last few decades cannot be fully explained only by changes in the prevalence of classical lifestyle-related risk factors, such as physical inactivity and poor diet. For this reason, it has been recently proposed that other "nontraditional" risk factors could contribute to the diabetes epidemics. In particular, an increasing number of reports indicate that chronic exposure to and accumulation of a low concentration of environmental pollutants (especially the so-called persistent organic pollutants (POPs)) within the body might be associated with diabetogenesis. In this review, the epidemiological evidence suggesting a relationship between dioxin and other POPs exposure and diabetes incidence will be summarized, and some recent developments on the possible underlying mechanisms, with particular reference to dioxin, will be presented and discussed.

High passage MIN6 cells have impaired insulin secretion with impaired glucose and lipid oxidation

Type 2 diabetes is a metabolic disorder characterized by the inability of beta-cells to secrete enough insulin to maintain glucose homeostasis. MIN6 cells secrete insulin in response to glucose and other secretagogues, but high passage (HP) MIN6 cells lose their ability to secrete insulin in response to glucose. We hypothesized that metabolism of glucose and lipids were defective in HP MIN6 cells causing impaired glucose stimulated insulin secretion (GSIS). HP MIN6 cells had no first phase and impaired second phase GSIS indicative of global functional impairment. This was coupled with a markedly reduced ATP content at basal and glucose stimulated states. Glucose uptake and oxidation were higher at basal glucose but ATP content failed to increase with glucose. HP MIN6 cells had decreased basal lipid oxidation. This was accompanied by reduced expressions of Glut1, Gck, Pfk, Srebp1c, Ucp2, Sirt3, Nampt. MIN6 cells represent an important model of beta cells which, as passage numbers increased lost first phase but retained partial second phase GSIS, similar to patients early in type 2 diabetes onset. We believe a number of gene expression changes occurred to produce this defect, with emphasis on Sirt3 and Nampt, two genes that have been implicated in maintenance of glucose homeostasis.

Mice deficient in GEM GTPase show abnormal glucose homeostasis due to defects in beta-cell calcium handling

Aims and Hypothesis

Glucose-stimulated insulin secretion from beta-cells is a tightly regulated process that requires calcium flux to trigger exocytosis of insulin-containing vesicles. Regulation of calcium handling in beta-cells remains incompletely understood. Gem, a member of the RGK (Rad/Gem/Kir) family regulates calcium channel handling in other cell types, and Gem over-expression inhibits insulin release in insulin-secreting Min6 cells. The aim of this study was to explore the role of Gem in insulin secretion. We hypothesised that Gem may regulate insulin secretion and thus affect glucose tolerance in vivo.

Methods

Gem-deficient mice were generated and their metabolic phenotype characterised by in vivo testing of glucose tolerance, insulin tolerance and insulin secretion. Calcium flux was measured in isolated islets.

Results

Gem-deficient mice were glucose intolerant and had impaired glucose stimulated insulin secretion. Furthermore, the islets of Gem-deficient mice exhibited decreased free calcium responses to glucose and the calcium oscillations seen upon glucose stimulation were smaller in amplitude and had a reduced frequency.

Conclusions

These results suggest that Gem plays an important role in normal beta-cell function by regulation of calcium signalling.

Reduced expression of the LRP16 gene in mouse insulinoma (MIN6) cells exerts multiple effects on insulin content, proliferation and apoptosis

This study assessed the effects of leukemia-related protein 16 (LRP16) on the regulation of pancreatic functions in mouse insulinoma (MIN6) cells. Cells with down-regulated expression of LRP16 were obtained by a shRNA interference strategy. Insulin content and glucose-stimulated insulin secretion (GSIS) were examined by radioimmunoassay. Western blotting was applied to detect protein expression. Glucose-stimulated sub-cellular localization of PDX-1 was immunocytochemically determined. Cell proliferation and apoptosis were detected by flow cytometry. Our results showed that LRP16 regulated insulin content in MIN6 cells by controlling expression of insulin and insulin transcription factors. LRP16 gene silence in MIN6 cells led to reduced cell proliferation and increased apoptosis. The observation of phosphorylation of serine-473 Akt and the localization of PDX-1 to the nucleus under glucose-stimulation exhibited that LRP16 was a component mediating Akt signaling in MIN6 cells. These results suggest that LRP16 plays a key role in maintaining pancreatic β-cell functions and may help us to understand the protective effects of estrogen on the functions of pancreatic β-cells.

Deregulation of CREB signaling pathway induced by chronic hyperglycemia downregulates NeuroD transcription

CREB mediates the transcriptional effects of glucose and incretin hormones in insulin-target cells and insulin-producing β-cells. Although the inhibition of CREB activity is known to decrease the β-cell mass, it is still unknown what factors inversely alter the CREB signaling pathway in β-cells. Here, we show that β-cell dysfunctions occurring in chronic hyperglycemia are not caused by simple inhibition of CREB activity but rather by the persistent activation of CREB due to decreases in protein phophatase PP2A. When freshly isolated rat pancreatic islets were chronically exposed to 25 mM (high) glucose, the PP2A activity was reduced with a concomitant increase in active pCREB. Brief challenges with 15 mM glucose or 30 µM forskolin after 2 hour fasting further increased the level of pCREB and consequently induced the persistent expression of ICER. The excessively produced ICER was sufficient to repress the transcription of NeuroD, insulin, and SUR1 genes. In contrast, when islets were grown in 5 mM (low) glucose, CREB was transiently activated in response to glucose or forskolin stimuli. Thus, ICER expression was transient and insufficient to repress those target genes. Importantly, overexpression of PP2A reversed the adverse effects of chronic hyperglycemia and successfully restored the transient activation of CREB and ICER. Conversely, depletion of PP2A with siRNA was sufficient to disrupt the negative feedback regulation of CREB and induce hyperglycemic phenotypes even under low glucose conditions. Our findings suggest that the failure of the negative feedback regulation of CREB is the primary cause for β-cell dysfunctions under conditions of pathogenic hyperglycemia, and PP2A can be a novel target for future therapies aiming to protect β-cells mass in the late transitional phase of non-insulin dependent type 2 diabetes (NIDDM).

Glucose: an energy currency and structural precursor in articular cartilage and bone with emerging roles as an extracellular signaling molecule and metabolic regulator

In the skeletal system glucose serves as an essential source of energy for the development, growth, and maintenance of bone and articular cartilage. It is particularly needed for skeletal morphogenesis during embryonic growth and fetal development. Glucose is vital for osteogenesis and chondrogenesis, and is used as a precursor for the synthesis of glycosaminoglycans, glycoproteins, and glycolipids. Glucose sensors are present in tissues and organs that carry out bulk glucose fluxes (i.e., intestine, kidney, and liver). The beta cells of the pancreatic islets of Langerhans respond to changes in blood glucose concentration by varying the rate of insulin synthesis and secretion. Neuronal cells in the hypothalamus are also capable of sensing extracellular glucose. Glucosensing neurons use glucose as a signaling molecule to alter their action potential frequency in response to variations in ambient glucose levels. Skeletal muscle and adipose tissue can respond to changes in circulating glucose but much less is known about glucosensing in bone and cartilage. Recent research suggests that bone cells can influence (and be influenced by) systemic glucose metabolism. This focused review article discusses what we know about glucose transport and metabolism in bone and cartilage and highlights recent studies that have linked glucose metabolism, insulin signaling, and osteocalcin activity in bone. These new findings in bone cells raise important questions about nutrient sensing, uptake, storage and processing mechanisms and how they might contribute to overall energy homeostasis in health and disease. The role of glucose in modulating anabolic and catabolic gene expression in normal and osteoarthritic chondrocytes is also discussed. In summary, cartilage and bone cells are sensitive to extracellular glucose and adjust their gene expression and metabolism in response to varying extracellular glucose concentrations.

Chronic methylglyoxal infusion by minipump causes pancreatic beta-cell dysfunction and induces type 2 diabetes in Sprague-Dawley rats

OBJECTIVE

The incidence of high dietary carbohydrate-induced type 2 diabetes is increasing worldwide. Methylglyoxal (MG) is a reactive glucose metabolite and a major precursor of advanced glycation end products (AGEs). MG levels are elevated in diabetic patients. We investigated the effects of chronic administration of MG on glucose tolerance and β-cell insulin secreting mechanism in 12-week-old male Sprague-Dawley rats.

RESEARCH DESIGN AND METHODS

MG (60 mg/kg/day) or 0.9% saline was administered by continuous infusion with a minipump for 28 days. We performed glucose and insulin tolerance tests and measured adipose tissue glucose uptake and insulin secretion from isolated pancreatic islets. We also used cultured INS-1E cells, a pancreatic β-cell line, for molecular studies. Western blotting, quantitative PCR, immunohistochemistry, and transferase-mediated dUTP nick-end labeling (TUNEL) assay were performed.

RESULTS

In rats treated with MG and MG + l-buthionine sulfoximine (BSO), MG levels were significantly elevated in plasma, pancreas, adipose tissue, and skeletal muscle; fasting plasma glucose was elevated, whereas insulin and glutathione were reduced. These two groups also had impaired glucose tolerance, reduced GLUT-4, phosphoinositide-3-kinase activity, and insulin-stimulated glucose uptake in adipose tissue. In the pancreatic β-cells, MG and MG + BSO reduced insulin secretion, pancreatic duodenal homeobox-1, MafA, GLUT-2, and glucokinase expression; increased C/EBPβ, nuclear factor-κB, MG-induced AGE, N(ε)-carboxymeythyllysine, and receptor for AGEs expression; and caused apoptosis. Alagebrium, an MG scavenger and an AGE-breaking compound, attenuated the effects of MG.

CONCLUSIONS

Chronic MG induces biochemical and molecular abnormalities characteristic of type 2 diabetes and is a possible mediator of high carbohydrate-induced type 2 diabetes.

Immortalized neurons for the study of hypothalamic function

The hypothalamus is a vital part of the central nervous system: it harbors control systems implicated in regulation of a wide range of homeostatic processes, including energy balance and reproduction. Structurally, the hypothalamus is a complex neuroendocrine tissue composed of a multitude of unique neuronal cell types that express a number of neuromodulators, including hormones, classical neurotransmitters, and specific neuropeptides that play a critical role in mediating hypothalamic function. However, neuropeptide and receptor gene expression, second messenger activation, and electrophysiological and secretory properties of these hypothalamic neurons are not yet fully defined, primarily because the heterogeneity and complex neuronal architecture of the neuroendocrine hypothalamus make such studies challenging to perform in vivo. To circumvent this problem, our research group recently generated embryonic- and adult-derived hypothalamic neuronal cell models by utilizing the novel molecular techniques of ciliary neurotrophic factor-induced neurogenesis and SV40 T antigen transfer to primary hypothalamic neuronal cell cultures. Significant research with these cell lines has demonstrated their value as a potential tool for use in molecular genetic analysis of hypothalamic neuronal function. Insights gained from hypothalamic immortalized cells used in conjunction with in vivo models will enhance our understanding of hypothalamic functions such as neurogenesis, neuronal plasticity, glucose sensing, energy homeostasis, circadian rhythms, and reproduction. This review discusses the generation and use of hypothalamic cell models to study mechanisms underlying the function of individual hypothalamic neurons and to gain a more complete understanding of the overall physiology of the hypothalamus.

Hyperinsulinism and diabetes: genetic dissection of beta cell metabolism-excitation coupling in mice

The role of metabolism-excitation coupling in insulin secretion has long been apparent, but in recent years, in parallel with studies of human hyperinsulinism and diabetes, genetic manipulation of proteins involved in glucose transport, metabolism, and excitability in mice has brought the central importance of this pathway into sharp relief. We focus on these animal studies and how they provide important insights into not only metabolic and electrical regulation of insulin secretion, but also downstream consequences of alterations in this pathway and the etiology and treatment of insulin-secretion diseases in humans.

The early steps of glucose signalling in yeast

In the presence of glucose, yeast undergoes an important remodelling of its metabolism. There are changes in the concentration of intracellular metabolites and in the stability of proteins and mRNAs; modifications occur in the activity of enzymes as well as in the rate of transcription of a large number of genes, some of the genes being induced while others are repressed. Diverse combinations of input signals are required for glucose regulation of gene expression and of other cellular processes. This review focuses on the early elements in glucose signalling and discusses their relevance for the regulation of specific processes. Glucose sensing involves the plasma membrane proteins Snf3, Rgt2 and Gpr1 and the glucose-phosphorylating enzyme Hxk2, as well as other regulatory elements whose functions are still incompletely understood. The similarities and differences in the way in which yeasts and mammalian cells respond to glucose are also examined. It is shown that in Saccharomyces cerevisiae, sensing systems for other nutrients share some of the characteristics of the glucose-sensing pathways.

The Botrytis cinerea hexokinase, Hxk1, but not the glucokinase, Glk1, is required for normal growth and sugar metabolism, and for pathogenicity on fruits

Hexose kinases play a central role in the initiation of sugar metabolism of living organisms and have also been implicated in carbon catabolite repression in yeasts and plants. In this study, the genes encoding glucokinase (Glk1) and hexokinase (Hxk1) from the plant-pathogenic ascomycete Botrytis cinerea were isolated and functionally characterized. Glk1-deficient mutants were indistinguishable from the wild-type in all growth parameters tested. In contrast, Deltahxk1 mutants lacking Hxk1 showed a pleiotropic growth defect. On artificial media, vegetative growth was retarded, and conidia formation strongly reduced. No or only marginal growth of Deltahxk1 mutants was observed when fructose, galactose, sucrose or sorbitol were used as carbon sources, and fructose inhibited growth of the mutant in the presence of other carbon sources. B. cinerea mutants containing hxk1 alleles with point mutations leading to enzymically inactive enzymes showed phenotypes similar to the Deltahxk1 disruption mutant, indicating that loss of hexose phosphorylation activity of Hxk1 is solely responsible for the pleiotropic growth defect. Virulence of the Deltahxk1 mutants was dependent on the plant tissue: on leaves, lesion formation was only slightly retarded compared to the wild-type, whereas only small lesions were formed on apples, strawberries and tomatoes. The low virulence of Deltahxk1 mutants on fruits was correlated with their high contents of sugars, in particular fructose. Heterologous expression of Hxk1 and Glk1 in yeast allowed their enzymic characterization, revealing kinetic properties similar to other fungal hexokinases and glucokinases. Both Deltaglk1 and Deltahxk1 mutants showed normal glucose repression of secreted lipase 1 activity, indicating that, in contrast to yeast, B. cinerea hexose kinases are not involved in carbon catabolite repression.

Characterization of regulatory non-catalytic hexokinases in Aspergillus nidulans

Hexokinases catalyse the first step in glucose metabolism and play a role in glucose sensing in mammals, plants and fungi. We describe a new class of hexokinases that appear to be solely regulatory in function. The Aspergillus nidulans hxkD gene (formerly named xprF) encodes a hexokinase-like protein. We constructed hxkDDelta gene disruption mutants which showed increased levels of extracellular protease in response to carbon starvation. The hxkDDelta mutations are not completely recessive, indicating that the level of the gene product is critical. Transcript levels of hxkD increase during carbon starvation and this response is not dependent on functional HxkD. A gene encoding a second atypical hexokinase (HxkC) was identified. The hxkCDelta gene disruption mutant exhibits a phenotype similar, but not identical, to hxkDDelta mutants. As with hxkD, mutations in hxkC are suppressed by loss-of-function mutations in xprG, which encodes a putative transcriptional activator involved in the response to nutrient limitation. We show that GFP-tagged HxkD was found only in nuclei suggesting a regulatory role for HxkD. GFP-tagged HxkC was associated with mitochondria. Homologs of hxkC and hxkD are conserved in multi-cellular fungi. Genes encoding atypical hexokinases are present in many genome sequence databases. Thus, non-catalytic hexokinases may be widespread.

Time-dependent mechanisms in beta-cell glucose sensing

The relation between plasma glucose and insulin release from pancreatic beta-cells is not stationary in the sense that a given glucose concentration leads to a specific rate of insulin secretion. A number of time-dependent mechanisms appear to exist that modify insulin release both on a short and a longer time scale. Typically, two phases are described. The first phase, lasting up to 10 min, is a pulse of insulin release in response to fast changes in glucose concentration. The second phase is a more steady increase of insulin release over minutes to hours, if the elevated glucose concentration is sustained. The paper describes the glucose sensing mechanism via the complex dynamics of the key enzyme glucokinase, which controls the first step in glucose metabolism: phosphorylation of glucose to glucose-6-phosphate. Three time-dependent phenomena (mechanisms) are described. The fastest, corresponding to the first phase, is a delayed negative feedback regulating the glucokinase activity. Due to the delay, a rapid glucose increase will cause a burst of activity in the glucose sensing system, before the glucokinase is down-regulated. The second mechanism corresponds to the translocation of glucokinase from an inactive to an active form. As the translocation is controlled by the product(s) of the glucokinase reaction rather than by the substrate glucose, this mechanism gives a positive, but saturable, feedback. Finally, the release of the insulin granules is assumed to be enhanced by previous glucose exposure, giving a so-called glucose memory to the beta-cells. The effect depends on the insulin release of the cells, and this mechanism constitutes a second positive, saturable feedback system. Taken together, the three phenomena describe most of the glucose sensing behaviour of the beta-cells. The results indicate that the insulin release is not a precise function of the plasma glucose concentration. It rather looks as if the beta-cells just increase the insulin production, until the plasma glucose has returned to normal. This type of integral control has the advantage that the precise glucose sensitivity of the beta-cells is not important for normal glucose homeostasis.

Glucokinase in chicken (Gallus gallus). Partial cDNA cloning, immunodetection and activity determination

Chickens are more hyperglycaemic and insulin-resistant than mammals, and in efforts to understand their glucose metabolism we investigated whether glucokinase (GK) is present in chicken liver or pancreas. This enzyme plays a major role in glucose-sensing in mammals and we have examined whether it also contributes to glucose homeostasis in chickens. Using RT-PCR, we cloned and sequenced a partial cDNA fragment (750 bp) from liver and pancreas that showed a high degree of identity with mammalian GK. Using antibodies directed towards human GK, we immunodetected a 50 kDa band in chicken liver and pancreas. The molecular mass of the band and its specific interaction with the antibody suggest that this protein corresponds to a chicken homologue of human GK. We also determined by spectrophotometry a glucokinase-like activity in crude liver homogenates with an apparent half saturating concentration for glucose of 8.6 mM. GK gene and protein expression did not differ between fed and 24 h fasted states but GK-like activity was significantly increased in fed chickens. In conclusion, our study provides evidence for the presence of GK gene and protein in chicken liver and pancreas and shows that the liver enzyme is active.

2,3,7,8-Tetrachlorodibenzo-p-dioxin-induced impairment of glucose-stimulated insulin secretion in isolated rat pancreatic islets

We have explored the effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) administration on the secretory function of isolated rat pancreatic islets. Twenty-four hours after TCDD administration (1 microg/kg b.w., i.p.), rats showed no significant differences in plasma glucose, insulin, triglycerides and leptin levels whereas plasma-free fatty acids were significantly increased with respect to untreated controls. In isolated islets, DNA and protein content were unchanged, whereas insulin content was significantly decreased in TCDD-treated rats. Incubation with different concentrations of glucose demonstrated a significant impairment of glucose-stimulated insulin secretion in islets isolated from TCDD-treated rats, whereas insulin release was better preserved upon alpha-ketoisocaproate stimulation. A significant reduction of [3H]-2-deoxy-glucose uptake was observed in pancreatic tissue of TCDD-treated rats, whereas no significant reduction in GLUT-2 protein levels was detectable by immunoblotting in islets from TCDD-treated rats. We concluded that low-dose TCDD could rapidly induce significant alterations of the pancreatic endocrine function in the rat.

Fructose normalizes specific counterregulatory responses to hypoglycemia in patients with type 1 diabetes

We have previously reported that specific counterregulatory responses to hypoglycemia were augmented by an infusion of fructose in nondiabetic humans. We hypothesized that this effect was due to the interaction of a "catalytic" dose of fructose with the regulatory protein for glucokinase in glucose-sensing cells that drive counterregulation. To examine whether fructose could restore counterregulatory responses in type 1 diabetic patients with defective counterregulation, we performed stepped hypoglycemic clamp studies (5.0, 4.4, 3.9, and 3.3 mmol/l glucose steps, 50 min each) in eight intensively treated patients (HbA(1c) 6.4 +/- 0.7%) on two separate occasions: without (control) or with coinfusion of fructose (1.2 mg . kg(-1) . min(-1)). Fructose induced a resetting of the glycemic threshold for secretion of epinephrine to higher plasma glucose concentrations (from 3.3 +/- 0.1 to 3.9 +/- 0.1 mmol/l; P = 0.001) and markedly augmented the increment in epinephrine (by 56%; P < 0.001). The amplification of epinephrine responses was specific; plasma norepinephrine, glucagon, growth hormone, and cortisol were unaffected. Hypoglycemia-induced endogenous glucose production ([3-(3)H]-glucose) rose by 90% (P < 0.001) in the fructose studies, compared with -2.0% (NS) in control. In concert, the glucose infusion rates during the 3.9- and 3.3-mmol/l steps were significantly lower with fructose (2.3 +/- 0.6 and 0.0 +/- 0.0 vs. 5.9 +/- 1.15 and 3.9 +/- 1.0 micromol . kg(-1) . min(-1), respectively; P < 0.001 for both), indicating the more potent counterregulatory response during fructose infusion. We conclude that infusion of fructose nearly normalizes the epinephrine and endogenous glucose production responses to hypoglycemia in type 1 diabetic patients with impaired counterregulation, suggesting that defects in these responses may be dependent on glucokinase-mediated glucose sensing.

The cellular fate of glucose and its relevance in type 2 diabetes

Type 2 diabetes is a complex disorder with diminished insulin secretion and insulin action contributing to the hyperglycemia and wide range of metabolic defects that underlie the disease. The contribution of glucose metabolic pathways per se in the pathogenesis of the disease remains unclear. The cellular fate of glucose begins with glucose transport and phosphorylation. Subsequent pathways of glucose utilization include aerobic and anaerobic glycolysis, glycogen formation, and conversion to other intermediates in the hexose phosphate or hexosamine biosynthesis pathways. Abnormalities in each pathway may occur in diabetic subjects; however, it is unclear whether perturbations in these may lead to diabetes or are a consequence of the multiple metabolic abnormalities found in the disease. This review is focused on the cellular fate of glucose and relevance to human type 2 diabetes.

Hepatic insulin gene therapy in insulin-dependent diabetes mellitus

Insulin-dependent diabetes mellitus (IDDM) is an autoimmune disease resulting in destruction of the pancreatic beta-cells in the islets of Langerhans. Commonly employed treatment of IDDM requires periodic insulin therapy, which is not ideal because of its inability to prevent chronic complications such as nephropathy, neuropathy and retinopathy. Although pancreas or islet transplantation are effective treatments that can reverse metabolic abnormalities and prevent or minimize many of the chronic complications of IDDM, their usefulness is limited as a result of shortage of donor pancreas organs. Gene therapy as a novel field of medicine holds tremendous therapeutic potential for a variety of human diseases including IDDM. This review focuses on the liver-based gene therapy for generation of surrogate pancreatic beta-cells for insulin replacement because of the innate ability of hepatocytes to sense and metabolically respond to changes in glucose levels and their high capacity to synthesize and secrete proteins. Recent advances in the use of gene therapy to prevent or regenerate beta-cells from autoimmune destruction are also discussed.

Broad expression of fructose-1,6-bisphosphatase and phosphoenolpyruvate carboxykinase provide evidence for gluconeogenesis in human tissues other than liver and kidney

The importance of renal and hepatic gluconeogenesis in glucose homeostasis is well established, but the cellular localization of the key gluconeogenic enzymes liver fructose-1,6-bisphosphatase (FBPase) and cytosolic phosphoenolpyruvate carboxykinase (PEPCK) in these organs and the potential contribution of other tissues in this process has not been investigated in detail. Therefore, we analyzed the human tissue localization and cellular distribution of FBPase and PEPCK immunohistochemically. The localization analysis demonstrated that FBPase was expressed in many tissues that had not been previously reported to contain FBPase activity (e.g., prostate, ovary, suprarenal cortex, stomach, and heart). In some multicellular tissues, this enzyme was detected in specialized areas such as epithelial cells of the small intestine and prostate or lung pneumocytes II. Interestingly, FBPase was also present in pancreas and cortex cells of the adrenal gland, organs that are involved in the control of carbohydrate and lipid metabolism. Although similar results were obtained for PEPCK localization, different expression of this enzyme was observed in pancreas, adrenal gland, and pneumocytes type I. These results show that co-expression of FBPase and PEPCK occurs not only in kidney and liver, but also in a variety of organs such as the small intestine, stomach, adrenal gland, testis, and prostate which might also contribute to gluconeogenesis. Our results are consistent with published data on the expression of glucose-6-phosphatase in the human small intestine, providing evidence that this organ may play an important role in the human glucose homeostasis.

In vivo imaging of the dynamics of glucose uptake in the cytosol of COS-7 cells by fluorescent nanosensors

Glucose homeostasis is a function of glucose supply, transport across the plasma membrane, and metabolism. To monitor glucose dynamics in individual cells, a glucose nanosensor was developed by flanking the Escherichia coli periplasmic glucose/galactose-binding protein with two different green fluorescent protein variants. Upon binding of substrate the FLIPglu-170n sensor showed a concentration-dependent decrease in fluorescence resonance energy transfer between the attached chromophores with a binding affinity for glucose of 170 nm. Fluorescence resonance energy transfer measurements with different sugars indicated a broad selectivity for monosaccharides. An affinity mutant with a Kd of approximately 600 microM was generated, which showed higher substrate specificity, and thus allowed specific monitoring of reversible glucose dynamics in COS-7 cells in the physiological range. At external glucose concentrations between 0.5 and 10 mM, reflecting typical blood levels, free cytosolic glucose concentrations remained at approximately 50% of external levels. The removal of glucose lead to reduced glucose levels in the cell, demonstrating reversibility and visualizing homeostasis. Glucose levels dropped even in the presence of the transport inhibitor cytochalasin B, indicating rapid metabolism. Consistently, the addition of 2-deoxyglucose, which is not recognized by the sensor, affects glucose uptake and metabolism rates. Within the physiological range, glucose utilization, i.e. hexokinase activity, was not limiting. Furthermore, the results show that in COS-7 cells, cytosolic glucose concentrations can vary over at least two orders of magnitude. The glucose nanosensor provides a novel tool with numerous scientific, medical, and environmental applications.

Differential gene expression in well-regulated and dysregulated pancreatic beta-cell (MIN6) sublines

To identify genes involved in regulated insulin secretion, we have established and characterized two sublines derived from the mouse pancreatic beta-cell line MIN6, designated B1 and C3. They have a similar insulin content, but differ in their secretory properties. B1 responded to glucose in a concentration- and cell confluence-dependent manner, whereas C3 did not. B1 cells were stimulated by phorbol 12-myristate 13-acetate, leucine, arginine, glibenclamide, isobutylmethylxanthine, and KCl, whereas C3 did not respond (leucine, arginine, and glibenclamide) or responded to a lesser extent (isobutylmethylxanthine, phorbol 12-myristate 13-acetate, and KCl). Although intracellular Ca(2+) rose in response to glucose in B1 but not C3 cells, KCl increased intracellular Ca(2+) in a similar manner in both sublines. GLUT-1, GLUT-2, Kir6.2, and SUR1 expression was not significantly different between B1 and C3 cells, whereas E-cadherin was more abundantly expressed in B1 cells. A more complete list of differentially expressed genes was established by suppression subtractive hybridization and high density (Affymetrix) oligonucleotide microarrays. Genes were clustered according to known or putative function. Those involved in metabolism, intracellular signaling, cytoarchitecture, and cell adhesion are of potential interest. These two sublines should be useful for identification of the genes and mechanisms involved in regulated insulin secretion of the pancreatic beta-cell.

Connexin 36 controls synchronization of Ca2+ oscillations and insulin secretion in MIN6 cells

Cx36 is the predominant connexin isoform expressed by pancreatic beta-cells. However, little is known about the role of this protein in the functioning of insulin-secreting cells. To address this question, we searched for a cell line expressing Cx36 and having glucose-induced insulin secretion comparable to that of primary beta-cells. By evaluating Cx36 expression in MIN6, betaTC3, RIN2A, INS1, and HIT cell lines, which differ in their sensitivity to glucose, we found that wild-type MIN6 cells fit these requirements. Therefore, we stably transfected MIN6 cells with a cDNA coding for a Cx36 antisense sequence to study the role of Cx36 in these cells. Independent clones of MIN6 cells were obtained that had a markedly reduced Cx36 expression. Loss of Cx36 decreased functional gap junctional conductance in these clones. This alteration impaired the synchronization of glucose-induced [Ca(2+)](i) oscillations and insulin secretion in response to glucose, to secretagogues that increase [cAMP](i), and to depolarizing conditions. These data provide the first evidence that Cx36-made channels 1) mediate functional coupling in MIN6 cells, 2) provide for synchronous [Ca(2+)](i) oscillations, and 3) are necessary for proper insulin secretion in response to metabolizable and nonmetabolizable secretagogues.

The biochemistry and medical significance of the flavonoids

Flavonoids are plant pigments that are synthesised from phenylalanine, generally display marvelous colors known from flower petals, mostly emit brilliant fluorescence when they are excited by UV light, and are ubiquitous to green plant cells. The flavonoids are used by botanists for taxonomical classification. They regulate plant growth by inhibition of the exocytosis of the auxin indolyl acetic acid, as well as by induction of gene expression, and they influence other biological cells in numerous ways. Flavonoids inhibit or kill many bacterial strains, inhibit important viral enzymes, such as reverse transcriptase and protease, and destroy some pathogenic protozoans. Yet, their toxicity to animal cells is low. Flavonoids are major functional components of many herbal and insect preparations for medical use, e.g., propolis (bee's glue) and honey, which have been used since ancient times. The daily intake of flavonoids with normal food, especially fruit and vegetables, is 1-2 g. Modern authorised physicians are increasing their use of pure flavonoids to treat many important common diseases, due to their proven ability to inhibit specific enzymes, to simulate some hormones and neurotransmitters, and to scavenge free radicals.

Cyotomedical therapy for insulinopenic diabetes using microencapsulated pancreatic beta cell lines

Current therapy for type 1 diabetes mellitus involves a daily regimen of multiple subcutaneous or intramuscular injections of recombinant human insulin. To achieve long-term insulin delivery in vivo, we investigated the applicability of cytomedical therapy using beta TC6 cells or MIN6 cells, both of which are murine pancreatic beta cell lines that secrete insulin in a subphysiologically or physiologically regulated manner, respectively. We examined this therapy in the insulinopenic diabetic mice intraperitoneally injected with beta TC6 cells or MIN6 cells microencapsulated within alginate-poly(L)lysine-alginate membranes (APA-beta TC6 cells or APA-MIN6 cells). The diabetic mice treated with APA-beta TC6 cells fell into hypoglycemia, whereas those injected with APA-MIN6 cells maintained normal blood glucose concentrations for over 2 months without developing hypoglycemia. In addition, we also conducted an oral glucose tolerance test using these mice. The blood glucose concentrations of normal and of diabetic mice injected with APA-MIN6 cells similarly changed over time, although the blood insulin concentration increased later in the injected diabetic mice than in the former. These results suggest that cytomedicine utilizing microencapsulated pancreatic beta cell lines with a physiological glucose sensor may be a beneficial and safe therapy with which to treat diabetes mellitus.

Human liver-derived cells stably modified for regulated proinsulin secretion function as bioimplants in vivo

BACKGROUND

Insulin deficiency is currently treated with pharmacological insulin secretagogues, insulin injections or islet transplants. Secondary failure of pharmacological agents is common; insulin injections often fail to achieve euglycemic control; and islet transplants are rare. Non-beta cells capable of regulated insulin secretion in vivo could be a functional cure for diabetes. Hepatocytes are good candidates, being naturally glucose-responsive, protein-secreting cells, while the liver is positioned to receive direct nutrient signals that regulate insulin production.

METHODS

Human liver-derived Chang cells were modified with a plasmid construct in which a bifunctional promoter comprising carbohydrate response elements and the human metallothionein IIA promoter controlled human proinsulin cDNA expression. Secretory responses of stable cell clones were characterized in vitro and in vivo by proinsulin radioimmunoassay.

RESULTS

Transfected Chang cells secreted 5-8 pmol proinsulin/10(6) cells per 24 h in continuous passage for at least a year in response to 5-25 mM glucose and 10-90 microM zinc in vitro. Glucose and zinc synergistically increased proinsulin production by up to 30-fold. Non-glucose secretagogues were also active. Glucose transporter 2 (GLUT2) and glucokinase cDNA co-transfection enhanced glucose responsiveness. Intraperitoneally implanted Chang cells secreted proinsulin in scid and Balb/c mice. Serum proinsulin levels were further increased 1.3-fold (p<0.05) after glucose and 1.4- to 1.6-fold (p<0.005) after zinc administration in vivo.

CONCLUSIONS

These results are the first to demonstrate stable proinsulin production in a human liver-derived cell line with activity in vitro and in vivo and provide a basis for engineering hepatocytes as in vivo bioimplants for future diabetes treatment.

Incorporation of beta cell redifferentiation therapy into a lipoprivic strategy for reversing type 2 diabetes

Type 2 diabetes (non-insulin-dependent diabetes mellitus, NIDDM), at least in the majority of patients characterized by insulin resistance and increased visceral fat, appears to be precipitated by the exposure of tissues to excessive levels of free fatty acids; this can contribute to the muscle insulin resistance, excessive hepatic gluconeogenesis, and beta cell dysfunction that collaborate to impair glycemic control. The resultant hyperglycemia, in turn, exacerbates the insulin resistance and beta cells dysfunction. The failure of glucose-stimulated insulin secretion (GSIS) in beta cells helps to sustain the elevations of serum glucose and free fatty acids, which in turn reinforce the failure of GSIS, possibly by inhibiting expression of the transcription factor IDX-1; NIDDM thus represents a vicious cycle that is not easily broken. A new strategy for achieving rapid loss of body fat - hepatothermic therapy (HT), an integrated approach involving exercise training, low-fat, low-glycemic-index food choices, and a supplementation program that promotes hepatic fatty acid oxidation - shows promise for alleviating the excessive fat exposure at the root of the diabetic syndrome, as well as the underlying insulin resistance syndrome responsible for increased macrovascular risk. However, when HT proves incapable of breaking the vicious cycle sustaining beta cell dysfunction, a supplementary strategy, beta cell redifferentiation therapy (BRT), may be required. BRT consists of a protocol in which near-normoglycemia is maintained for several weeks through use of intensive insulin therapy (e.g. artificial pancreas) or other effective measures, during which time beta cell GSIS can be expected to substantially recover owing to relief from glucolipotoxicity. Clinical experience demonstrates that this improved beta cell function, in certain cases, can persist for months or years after temporary BRT. A portion of the improved glycemic control achieved with very low calorie diets in NIDDM is reflective of improved GSIS, presumably consequent to a sustained reduction in diurnal glycemia. Long-lived analogs of glucagon-like peptide-1 (GLP-1) may find a key role in BRT; this incretin hormone not only potentiates GSIS, but also appears to increase the expression and activity of IDX-1 in beta cells, thus promoting beta cell redifferentiation. If HT is instituted prior to and following BRT to alleviate the FFA overexposure that initially precipitated the diabetic syndrome, it seems likely that the benefits of BRT will be conserved in the long term, thus enabling a reversal of NIDDM - in other words, maintenance of normoglycemia without medication. Since NIDDM is inherently preventable, its reversal should be the fundamental goal of diabetes therapy.

Transactivation of the mouse sulfonylurea receptor I gene by BETA2/NeuroD

The sulfonylurea receptor 1 (SUR1) plays a key role in regulation of insulin secretion in pancreatic beta-cells. In this study we investigated the mechanism for tissue-specific expression of the SUR1 gene. A -138/-20 fragment exhibited basal promoter activity while the -660/-20 fragment contained a regulatory element for tissue-specific expression of the mouse SUR1 gene. A pancreatic beta-cell-specific transcription factor, BETA2 (beta-cell E box transcription factor)/NeuroD, enhanced the promoter activity of the -660/-20 fragment in cooperation with E47. Coexpression of a dominant negative mutant of BETA2/NeuroD, BETA2(1-233), repressed the promoter activity of the -660/-20 fragment. BETA2/NeuroD bound specifically to the E3 element located at -141. The E3 sequence in a heterologous context conferred transactivation by BETA2/NeuroD in HeLa and HIT cells. Mutation of E3 eliminated the stimulatory effect of BETA2/NeuroD. Unlike BETA2/NeuroD, neurogenin 3 (ngn3) could not activate the E3 element in HeLa cells. Overexpression of ngn3 concomitantly increased expression of BETA2/NeuroD and SUR1 in HIT cells but not in HeLa cells. These results indicate that BETA2/NeuroD induces tissue-specific expression of the SUR1 gene through the E3 element. These results also suggest that E3 is specific for BETA2/NeuroD, and the stimulatory effect of ngn3 in HIT cells may require factors specifically expressed in HIT cells.

Human pancreatic glucokinase (GlkB) complements the glucose signalling defect of Saccharomyces cerevisiae hxk2 mutants

Human pancreatic glucokinase (GlkB, hexokinase IV) has been expressed in Saccharomyces cerevisiae. The recombinant protein showed similar enzyme kinetics to those described for the original enzyme. When expressed in hxk2 yeast mutants, GlkB complemented both the glucose induction and the glucose repression defects present in the mutant. It was also functional in regulating the activity of the Snf1 kinase complex in response to glucose, participating in the regulation of the Reg1/Glc7 phosphatase complex, as its yeast counterpart.