Role of ataxia telangiectasia mutated in insulin signalling of muscle-derived cell lines and mouse soleus

AIM

Ataxia telangiectasia mutated (ATM) reportedly plays a role in insulin-stimulated activation of Akt in some cell types but not in others. The role of ATM in insulin signalling has not been firmly resolved for skeletal muscle cells, for which Akt phosphorylation is a pivotal step in stimulation of glucose transport. Accordingly, our aim was to determine the role of ATM in insulin effects for cell lines derived from skeletal muscle and for skeletal muscle.

METHODS

We examined insulin effects in L6 myotubes, mouse soleus, C2C12 myotubes and differentiated rhabdomyosarcoma (RD) cells in the presence and absence of a low concentration (1 microm) of the ATM inhibitor KU55933. We also compared insulin signalling in C2C12 cells expressing shRNA against ATM and control cell lines (empty vector; cells expressing non-targeting shRNA).

RESULTS

In L6 myotubes and mouse soleus muscle, KU55933 inhibited insulin-stimulated phosphorylation of the 160 kDa substrate of Akt (AS160) despite no effect on Akt. In contrast, KU55933 prevented insulin-stimulated Akt phosphorylation in C2C12 myotubes. Furthermore, C2C12 myotubes expressing shRNA against ATM displayed reduced insulin-stimulated Akt phosphorylation compared to controls. KU55933 also decreased insulin-stimulated Akt phosphorylation in differentiated RD cells.

CONCLUSION

These model-dependent differences in the role of ATM in insulin action demonstrate a role of ATM in insulin-stimulated phosphorylation of Akt (in C2C12 and RD cells) but also allow the elucidation of a novel, Akt-independent role of ATM (in L6 myotubes and mouse soleus, at the level of AS160) in insulin signalling.

Glucose-dependent blood flow dynamics in murine pancreatic islets in vivo

Pancreatic islets are highly vascularized and arranged so that regions containing beta-cells are distinct from those containing other cell types. Although islet blood flow has been studied extensively, little is known about the dynamics of islet blood flow during hypoglycemia or hyperglycemia. To investigate changes in islet blood flow as a function of blood glucose level, we clamped blood glucose sequentially at hyperglycemic ( approximately 300 mg/dl or 16.8 mM) and hypoglycemic ( approximately 50 mg/dl or 2.8 mM) levels while simultaneously imaging intraislet blood flow in mouse models that express green fluorescent protein in the beta-cells or yellow fluorescent protein in the alpha-cells. Using line scanning confocal microscopy, in vivo blood flow was assayed after intravenous injection of fluorescent dextran or sulforhodamine-labeled red blood cells. Regardless of the sequence of hypoglycemia and hyperglycemia, islet blood flow is faster during hyperglycemia, and apparent blood volume is greater during hyperglycemia than during hypoglycemia. However, there is no change in the order of perfusion of different islet endocrine cell types in hypoglycemia compared with hyperglycemia, with the islet core of beta-cells usually perfused first. In contrast to the results in islets, there was no significant difference in flow rate in the exocrine pancreas during hyperglycemia compared with hypoglycemia. These results indicate that glucose differentially regulates blood flow in the pancreatic islet vasculature independently of blood flow in the rest of the pancreas.

Role of sleep and sleep loss in hormonal release and metabolism

Compared to a few decades ago, adults, as well as children, sleep less. Sleeping as little as possible is often seen as an admirable behavior in contemporary society. However, sleep plays a major role in neuroendocrine function and glucose metabolism. Evidence that the curtailment of sleep duration may have adverse health effects has emerged in the past 10 years. Accumulating evidence from both epidemiologic studies and well-controlled laboratory studies indicates that chronic partial sleep loss may increase the risk of obesity and weight gain. The present chapter reviews epidemiologic studies in adults and children and laboratory studies in young adults indicating that sleep restriction results in metabolic and endocrine alterations, including decreased glucose tolerance, decreased insulin sensitivity, increased evening concentrations of cortisol, increased levels of ghrelin, decreased levels of leptin and increased hunger and appetite. Altogether, the evidence points to a possible role of decreased sleep duration in the current epidemic of obesity. Bedtime extension in short sleepers should be explored as a novel behavioral intervention that may prevent weight gain or facilitate weight loss. Avoiding sleep deprivation may help to prevent the development of obesity, particularly in children.

[Type 2 diabetes mellitus as a subclinical inflammation]

Type 2 diabetes mellitus develops as a combination of genetic background and external factors. The disease development is caused by increased oxidative stress under various metabolic factors and in parallel by complex inflammatory reaction without clinical signs. The resulting subclinical inflammation is a consequence of defensive anti-inflammatory reactions. Such Type 2 diabetes conception brings various possibilities in the treatment and prevention.

A mathematical model of brain glucose homeostasis

BACKGROUND

The physiological fact that a stable level of brain glucose is more important than that of blood glucose suggests that the ultimate goal of the glucose-insulin-glucagon (GIG) regulatory system may be homeostasis of glucose concentration in the brain rather than in the circulation.

METHODS

In order to demonstrate the relationship between brain glucose homeostasis and blood hyperglycemia in diabetes, a brain-oriented mathematical model was developed by considering the brain as the controlled object while the remaining body as the actuator. After approximating the body compartmentally, the concentration dynamics of glucose, as well as those of insulin and glucagon, are described in each compartment. The brain-endocrine crosstalk, which regulates blood glucose level for brain glucose homeostasis together with the peripheral interactions among glucose, insulin and glucagon, is modeled as a proportional feedback control of brain glucose. Correlated to the brain, long-term effects of psychological stress and effects of blood-brain-barrier (BBB) adaptation to dysglycemia on the generation of hyperglycemia are also taken into account in the model.

RESULTS

It is shown that simulation profiles obtained from the model are qualitatively or partially quantitatively consistent with clinical data, concerning the GIG regulatory system responses to bolus glucose, stepwise and continuous glucose infusion. Simulations also revealed that both stress and BBB adaptation contribute to the generation of hyperglycemia.

CONCLUSION

Simulations of the model of a healthy person under long-term severe stress demonstrated that feedback control of brain glucose concentration results in elevation of blood glucose level. In this paper, we try to suggest that hyperglycemia in diabetes may be a normal outcome of brain glucose homeostasis.

Effect of acute changes in glucose concentration on neuronal activity and plasticity in the rat hippocampus

OBJECTIVES

Diabetes mellitus is a major public health concern in many regions of the world, including the Caribbean. Diabetes is associated with multi-system pathology and central nervous system complications have been receiving increasing attention (dementia, cognitive decline and memory loss). While such pathology has been shown to be associated with long term derangement in glucose metabolism, less is known about the effects of acute changes in glucose concentration on neuronal function. This study assesses the effects of acute changes in glucose concentration upon neuronal transmission and neuronal plasticity.

METHODS

We made use of extracellular recordings from hippocampal slices of young adult rats and exposed them to changes in glucose concentration for 60 minutes before assessing synaptic plasticity. Experiments were carried out at both 30 degrees C and 35 degrees C.

RESULTS

At 30 degrees C, glucose concentrations of 30 mM and 4 mM had little effect upon population spike potentials (PSP). However reducing glucose concentration to 2 mM, 1 mM and 0 mM respectively resulted in a progressive decrease in the size of PSP until they were completely abolished. Similar results were observed at 35 degrees C except that 30 mM caused a significant increase in PSP size. Changes in glucose concentration had no effect upon synaptic plasticity at either 30 degrees C or 35 degrees C except below 2 mM glucose.

CONCLUSION

Acute changes in glucose concentration have a limited impact on neuronal transmission unless concentrations drop below 2 mM. However there seems to be little impairment of synaptic plasticity even at very low concentrations of glucose. We suggest that short term acute changes in glucose concentrations may not contribute directly to the cognitive decline associated with diabetes unless extremely severe.

Molecular cloning of a novel secreted peptide, INM02, and regulation of its expression by glucose

We report the identification of a novel secreted peptide, INM02. The mRNA transcript of human INM02 gene is about 3.0 kb. Its open-reading frame contains 762 bps and encodes a protein of 254 amino acids. Northern blot analysis demonstrates that INM02 mRNA is widely expressed in rat tissues, especially with abundant quantities in pancreatic islets, testis, and bladder tissue. We have expressed recombinant INM02 protein and generated rabbit anti-INM02 polyclonal antibodies. We show here that INM02 could be detectable in human serum by ELISA. We also present evidence that INM02 mRNA expression could be regulated by glucose. Experiments on both MIN6 cells and intact isolated islets demonstrate that INM02 mRNA levels are increased more than threefold by high glucose (25 mM) when compared with low glucose (5.5 mM). ELISA analysis shows that secretion of INM02 is significantly augmented by high glucose in vitro. It is speculated that as a novel secreted protein, INM02 is associated with functions of pancreatic islets, especially of beta-cells.

Induction of multinucleation by beta-glucosyl Yariv reagent in regenerated cells from Marchantia polymorpha protoplasts and involvement of arabinogalactan proteins in cell plate formation

Arabinogalactan proteins (AGPs) are abundant plant cell surface proteoglycans widely distributed in plant species. Since high concentrations of beta-glucosyl Yariv reagent (betaglcY), which binds selectively to AGPs, inhibited cell division of protoplast-regenerated cells of the liverwort Marchantia polymorpha L. (Shibaya and Sugawara in Physiol Plant 130:271-279, 2007), we investigated the mechanism underlying the inability of the cells to divide normally by staining nuclei, cell walls and beta-1,3-glucan. Microscopic observation showed that the diameter of regenerated cells cultured with betaglcY was about 2.8-fold larger than that of cells cultured without betaglcY. The cells cultured with betaglcY were remarkably multinucleated. These results indicated that betaglcY did not inhibit mitosis but induced multinucleation. In the regenerated cells cultured with low concentrations of betaglcY (5 and 1 microg ml(-1)), the cell plate was stained strongly by betaglcY, suggesting abundant AGPs in the forming cell plate. In these cell plates, beta-1,3-glucan was barely detectable or not detected. In multinucleated cells, cell plate-like fragments, which could not reach the cell wall, were frequently observed and they were also stained strongly by betaglcY. Our results indicated that AGPs might have an important role in cell plate formation, and perturbation of AGPs with betaglcY might result in remarkable multinucleation in protoplast-regenerated cells of M. polymorpha.

Lifelong enhanced diabetes susceptibility and obesity after temporary intrahypothalamic hyperinsulinism during brain organization

Newborn male Wistar-rats received bilateral intrahypothalamic insulin-agar-implants on the 2nd or 8th day of life. In male control animals only the insulin-free indifferent agar-vehicle was implanted at the same age. In both experimental groups with temporary intrahypothalamic hyperinsulinism during brain organization the following results were obtained: 1) Higher body weight gain starting at the end of the hypothalamic differentiation period and continuing during juvenile life until adulthood, resulting in increased relative body weight as a sign of obesity; 2) A tendency to basal hyperinsulinaemia in juvenile and adult age; 3) Impaired glucose tolerance in adulthood; 4) Increased diabetes susceptibility to a single "subdiabetogenic" dose of streptozotocin in adult age. In view of these and previous observations a teratogenetic role of high insulin concentrations during the organization of glucoregulatory hypothalamic structures is hypothesized and the possible relevance of such hyperinsulinism as a predisposing factor for a lifelong enhanced diabetes and/or obesity risk is suggested.

Impaired insulin exocytosis in neural cell adhesion molecule-/- mice due to defective reorganization of the submembrane F-actin network

The neural cell adhesion molecule (NCAM) is required for cell type segregation during pancreatic islet organogenesis. We have investigated the functional consequences of ablating NCAM on pancreatic beta-cell function. In vivo, NCAM(-/-) mice exhibit impaired glucose tolerance and basal hyperinsulinemia. Insulin secretion from isolated NCAM(-/-) islets is enhanced at glucose concentrations below 15 mM but inhibited at higher concentrations. Glucagon secretion from pancreatic alpha-cells evoked by low glucose was also severely impaired in NCAM(-/-) islets. The diminution of insulin secretion is not attributable to defective glucose metabolism or glucose sensing (documented as glucose-induced changes in intracellular Ca(2+) and K(ATP)-channel activity). Resting K(ATP) conductance was lower in NCAM(-/-) beta-cells than wild-type cells, and this difference was abolished when F-actin was disrupted by cytochalasin D (1 muM). In wild-type beta-cells, the submembrane actin network disassembles within 10 min during glucose stimulation (30 mM), an effect not seen in NCAM(-/-) beta-cells. Cytochalasin D eliminated this difference and normalized insulin and glucagon secretion in NCAM(-/-) islets. Capacitance measurements of exocytosis indicate that replenishment of the readily releasable granule pool is suppressed in NCAM(-/-) alpha- and beta-cells. Our data suggest that remodeling of the submembrane actin network is critical to normal glucose regulation of both insulin and glucagon secretion.

Advanced glycation end products inhibit glucose-stimulated insulin secretion through nitric oxide-dependent inhibition of cytochrome c oxidase and adenosine triphosphate synthesis

Advanced glycation end products (AGEs) are implicated in diabetic complications. However, their role in beta-cell dysfunction is less clear. In this study we examined the effects of AGEs on islet function in mice and in isolated islets. AGE-BSA or BSA was administered ip to normal mice twice a day for 2 wk. We showed that AGE-BSA-treated mice exhibited significantly higher glucose levels and lower insulin levels in response to glucose challenge than did BSA-treated mice, although there were no significant differences in insulin sensitivity and islet morphology between two groups. Glucose-stimulated insulin secretion by islets of the AGE-BSA-treated mice or AGE-BSA-treated normal islets was significantly lower than that by islets isolated from the BSA-treated mice or BSA-treated normal islets. Furthermore, AGE treatment of islet beta-cells inhibited ATP production, and glimepiride, a sulfonylurea derivative, restored glucose-stimulated insulin secretion. Further investigation indicated that AGEs inhibited cytochrome c oxidase activity by inducing the expression of inducible nitric oxide synthase (iNOS). Blocking the formation of nitric oxide with an iNOS selective inhibitor aminoguanidine reversed the inhibitory effects of AGEs on ATP production and insulin secretion. We conclude that AGEs inhibit cytochrome c oxidase and ATP production, leading to the impairment of glucose-stimulated insulin secretion through iNOS-dependent nitric oxide production.

Glucose and auxin signaling interaction in controlling Arabidopsis thaliana seedlings root growth and development

Background

Plant root growth and development is highly plastic and can adapt to many environmental conditions. Sugar signaling has been shown to affect root growth and development by interacting with phytohormones such as gibberellins, cytokinin and abscisic acid. Auxin signaling and transport has been earlier shown to be controlling plant root length, number of lateral roots, root hair and root growth direction.

Principal Findings

Increasing concentration of glucose not only controls root length, root hair and number of lateral roots but can also modulate root growth direction. Since root growth and development is also controlled by auxin, whole genome transcript profiling was done to find out the extent of interaction between glucose and auxin response pathways. Glucose alone could transcriptionally regulate 376 (62%) genes out of 604 genes affected by IAA. Presence of glucose could also modulate the extent of regulation 2 fold or more of almost 63% genes induced or repressed by IAA. Interestingly, glucose could affect induction or repression of IAA affected genes (35%) even if glucose alone had no significant effect on the transcription of these genes itself. Glucose could affect auxin biosynthetic YUCCA genes family members, auxin transporter PIN proteins, receptor TIR1 and members of a number of gene families including AUX/IAA, GH3 and SAUR involved in auxin signaling. Arabidopsis auxin receptor tir1 and response mutants, axr2, axr3 and slr1 not only display a defect in glucose induced change in root length, root hair elongation and lateral root production but also accentuate glucose induced increase in root growth randomization from vertical suggesting glucose effects on plant root growth and development are mediated by auxin signaling components.

Conclusion

Our findings implicate an important role of the glucose interacting with auxin signaling and transport machinery to control seedling root growth and development in changing nutrient conditions.

Nutritional and lactational effects on follicular development in the pig

In sows, follicular development is inhibited during lactation, and weaning the piglets allows recruitment and selection of follicles that will undergo preovulatory maturation and ovulate. Lactation inhibits GnRH secretion, and in turn LH secretion, through neuroendocrine stimuli induced by suckling. Pituitary response to GnRH and the sensitivity of the hypothalamo-pituitary unit to oestradiol positive feedback are also reduced. The impact of lactation on the reproductive axis is further complicated by the physiological and metabolic adaptations that are developed for milk production and that depend on nutrient intake, nutrient needs and body reserves. A strongly catabolic state during lactation amplifies the inhibition of LH secretion, thereby inducing a delay of oestrus and ovulation after weaning. Nevertheless, post-weaning ovulation is less delayed nowadays than in the 1970's or 80's. Nutritional deficiency has also deleterious effects on embryo survival, which are likely related to alterations in follicular growth and maturation. The physiological mechanisms by which information on the metabolic changes is transmitted to the hypothalamus-pituitary-ovary axis are not fully understood in the sow. Glucose, insulin and leptin are the most likely signals informing the hypothalamus of the metabolic state, yet their roles have not been definitely established. At the ovarian level, folliculogenesis is likely to be altered by the reduction in insulin and IGF-I concentrations induced by nutritional deficiency. More knowledge is needed at the intrafollicular level to better understand nutritional effects on follicular development, and also on occyte quality and embryo development.

Wnt signalling at the crossroads of nutritional regulation

The ability to sense and respond to nutritional cues is among the most fundamental processes that support life in living organisms. At the cellular level, a number of biochemical mechanisms have been proposed to mediate cellular glucose sensing. These include ATP-sensitive potassium channels, AMP-activated protein kinase, activation of PKC (protein kinase C), and flux through the hexosamine pathway. Less well known is how cellularly heterogenous organs couple nutrient availability to prioritization of cell autonomous functions and appropriate growth of the entire organ. Yet what is clear is that when such mechanisms fail or become inappropriately active they can lead to dire consequences such as diabetes, metabolic syndromes, cardiovascular diseases and cancer. In this issue of the Biochemical Journal, Anagnostou and Shepherd report the identification of an important link between cellular glucose sensing and the Wnt/beta-catenin signalling pathway in macrophages. Their data strongly indicate that the Wnt/beta-catenin pathway of Wnt signalling is responsive to physiological concentrations of nutrients but also suggests that that this system could be inappropriately activated in the diabetic (hyperglycaemic) or other metabolically compromised pathological states. This opens the exciting possibility that organ-selective modulation of Wnt signalling may become an attractive therapeutic target to treat these diseases.

Role of glucose in cloned mouse embryo development

Cloned mouse embryos display a marked preference for glucose-containing culture medium, with enhanced development to the blastocyst stage in glucose-containing medium attributable mainly to an early beneficial effect during the first cell cycle. This early beneficial effect of glucose is not displayed by parthenogenetic, fertilized, or tetraploid nuclear transfer control embryos, indicating that it is specific to diploid clones. Precocious localization of the glucose transporter SLC2A1 to the cell surface, as well as increased expression of glucose transporters and increased uptake of glucose at the one- and two-cell stages, is also seen in cloned embryos. To examine the role of glucose in early cloned embryo development, we examined glucose metabolism and associated metabolites, as well as mitochondrial ultrastructure, distribution, and number. Clones prepared with cumulus cell nuclei displayed significantly enhanced glucose metabolism at the two-cell stage relative to parthenogenetic controls. Despite the increase in metabolism, ATP content was reduced in clones relative to parthenotes and fertilized controls. Clones at both stages displayed elevated concentrations of glycogen compared with parthenogenetic controls. There was no difference in the number of mitochondria, but clone mitochondria displayed ultrastructural alterations. Interestingly, glucose availability positively affected mitochondrial structure and localization. We conclude that cloned embryos may be severely compromised in terms of ATP-dependent processes during the first two cell cycles and that glucose may exert its early beneficial effects via positive effects on the mitochondria.

[Effects of glucose and its modulation by insulin on proliferation and differentiation of rat dental follicle cells in vitro]

PURPOSE

To investigate the effects of glucose and its modulation by insulin on proliferation and differentiation of rat dental follicle cells (DFCs) in vitro.

METHODS

The rat dental follicle cells from 7-day neonatal rat were cultured. The effects of different doses of glucose concentration (5.5, 16.5, and 49.5mmol/L), alone with insulin(6mug/ml), on rat dental follicle cells was examined in vitro. The DFCs proliferation and alkaline phosphatase (ALP) were examined after 1,3,7,9 days and 1-7 days of culture, respectively. The data were analyzed using SPSS13.0 software package.

RESULTS

At 1,3 day, DFCs proliferation greatly enhanced at 16.5mmol/L glucose, and decreased at 49.5mmol/L glucose. Insulin treatment greatly decreased the proliferation of 16.5mmol/L glucose and increased the proliferation of 49.5mmol/L glucose. With high concentrations of glucose, cell proliferation greatly increased at 7,9 day, insulin treatment greatly increased the proliferation of DFCs. High concentrations of glucose greatly increased the DFCs ALP activity in 1-7 days. Insulin treatment greatly decreased DFCs ALP activity with high concentrations of glucose (P<0.05).

CONCLUSION

This study suggests that insulin contains the deleterious effect of acute high concentrations of glucose on rat DFCs proliferation and differentiation.

ChREBP: a glucose-activated transcription factor involved in the development of metabolic syndrome

Excess carbohydrate intake leads to fat accumulation and insulin resistance. Glucose and insulin coordinately regulate de novo lipogenesis from glucose in the liver, and insulin activates several transcription factors including SREBP1c and LXR, while those activated by glucose remain unknown. Recently, a carbohydrate response element binding protein (ChREBP), which binds to the carbohydrate response element (ChoRE) in the promoter of rat liver type pyruvate kinase (LPK), has been identified. The target genes of ChREBP are involved in glycolysis, lipogenesis, and gluconeogenesis. Although the regulation of ChREBP remains unknown in detail, the transactivity of ChREBP is partly regulated by a phosphorylation/dephosphorylation mechanism. During fasting, protein kinase A and AMP-activated protein kinase phosphorylate ChREBP and inactivate its transactivity. During feeding, xylulose-5-phosphate in the hexose monophosphate pathway activates protein phosphatase 2A, which dephosphorylates ChREBP and activates its transactivity. ChREBP controls 50% of hepatic lipogenesis by regulating glycolytic and lipogenic gene expression. In ChREBP (-/-) mice, liver triglyceride content is decreased and liver glycogen content is increased compared to wild-type mice. These results indicate that ChREBP can regulate metabolic gene expression to convert excess carbohydrate into triglyceride rather than glycogen. Furthermore, complete inhibition of ChREBP in ob/ob mice reduces the effects of the metabolic syndrome such as obesity, fatty liver, and glucose intolerance. Thus, further clarification of the physiological role of ChREBP may be useful in developing treatments for the metabolic syndrome.

Regulation by glucose of oscillatory electrical activity and 5-HT/insulin release from single mouse pancreatic islets in absence of functional K(ATP) channels

The glucose sensitivity of bursting electrical activity and pulsatile insulin release from pancreatic islets was determined in absence of functional K(ATP) channels. Membrane potential, [Ca(2+)](i) and 5-HT/insulin release were measured by intracellular recording, fura-2 fluorescence and 5-HT amperometry, respectively. Single mouse islets, bathed in tolbutamide or glibenclamide and high extracellular Ca(2+) (Ca(2+)(o)), displayed bursting activity and concomitant fast [Ca(2+)](i) and 5-HT/insulin oscillations. Sulphonylurea block of K(ATP) channel current was unaffected by raising Ca(2+)(o). Raising glucose or alpha-ketoisocaproic acid (KIC) concentration from 3 to 30 mM increased spiking activity and burst plateau duration. Staurosporine did not impair glucose potentiation of electrical activity, ruling out the involvement of serine/threonine kinases. Glucose enhanced both [Ca(2+)](i) and 5-HT/insulin oscillatory activity, causing a approximately 3-fold increase in overall 5-HT release rate. Cells lacking bursting activity in high Ca(2+)(o) and low glucose (or KIC) developed a pattern of intensified spiking in response to 11 mM glucose. It is concluded that beta-cells exhibit graded oscillatory electrical and secretory responses to glucose in absence of functional K(ATP) channels. This suggests that, under physiological conditions, early glucose sensing may involve other channels besides the K(ATP) channel.

CaV1.2 rather than CaV1.3 is coupled to glucose-stimulated insulin secretion in INS-1 832/13 cells

In clonal beta-cell lines and islets from different species, a variety of calcium channels are coupled to glucose-stimulated insulin secretion. The aim of this study was to identify the voltage-gated calcium channels that control insulin secretion in insulinoma (INS)-1 832/13 cells. The mRNA level of Ca(V)1.2 exceeded that of Ca(V)1.3 and Ca(V)2.3 two-fold. Insulin secretion, which rose tenfold in response to 16.7 mM glucose, was completely abolished by 5 microM isradipine that blocks Ca(V)1.2 and Ca(V)1.3. Similarly, the increase in intracellular calcium in response to 15 mM glucose was decreased in the presence of 5 microM isradipine, and the frequency of calcium spikes was decreased to the level seen at 2.8 mM glucose. By contrast, inhibition of Ca(V)2.3 with 100 nM SNX-482 did not significantly affect insulin secretion or intracellular calcium. Using RNA interference, Ca(V)1.2 mRNA and protein levels were knocked down by approximately 65% and approximately 34% respectively, which reduced insulin secretion in response to 16.7 mM glucose by 50%. Similar reductions in calcium currents and cell capacitance were seen in standard whole-cell patch-clamp experiments. The remaining secretion of insulin could be reduced to the basal level by 5 microM isradipine. Calcium influx underlying this residual insulin secretion could result from persisting Ca(V)1.2 expression in transfected cells since knock-down of Ca(V)1.3 did not affect glucose-stimulated insulin secretion. In summary, our results suggest that Ca(V)1.2 is critical for insulin secretion in INS-1 832/13 cells.

Anti-proliferative effect of pro-inflammatory cytokines in cultured beta cells is associated with extracellular signal-regulated kinase 1/2 pathway inhibition: protective role of glucagon-like peptide -1

Pancreatic beta-cell homeostasis is a balance between programmed cell death (apoptosis) and regeneration. Although autoimmune diabetes mellitus type 1 (DM1) is the most-studied cause of beta-cell mass loss by pro-inflammatory cytokine-induced apoptosis, influences of a pro-inflammatory environment on beta-cell regenerative response have been poorly studied. In this study, we assess the anti-proliferative effect of pro-inflammatory cytokines and glucose concentration on rat pancreatic beta cells and the potential protective role of glucagon-like peptide (GLP-1). Apoptotic and proliferating islet cells were stained using the DeadEnd Fluorimetric TUNEL System and 5-bromo-2'-deoxyuridine label respectively, in the presence-absence of varying concentrations of glucose, pro-inflammatory cytokines, and GLP-1. The potential signaling pathways involved were evaluated by western blot. Considerable anti-proliferative effects of pro-inflammatory cytokines interleukin (IL)-1beta, interferon (IFN)-gamma, and tumour necrosis factor-alpha (TNF-alpha) were observed. The effects were synergistic and independent of glucose concentration, and appeared to be mediated by the inhibition of extracellular signal-regulated kinase 1/2 (ERK1/2) activation, the signaling pathway involved in beta-cell replication. GLP-1 completely reversed the cytokine-induced inhibition of ERK phosphorylation and increased beta-cell proliferation threefold in cytokine-treated cultures. While pro-inflammatory cytokines reduced islet cell ERK1/2 activation and beta-cell proliferation in pancreatic islet culture, GLP-1 was capable of reversing this effect. These data suggest a possible pharmacological application of GLP-1 in the treatment of early stage DM1, to prevent the loss of pancreatic beta cells as well as to delay the development of overt diabetes.

Inhibition of NADPH oxidase-related oxidative stress-triggered signaling by honokiol suppresses high glucose-induced human endothelial cell apoptosis

Angiopathy is a major complication of diabetes. Abnormally high blood glucose is a crucial risk factor for endothelial cell damage. Nuclear factor-kappaB (NF-kappaB) has been demonstrated as a mediated signaling in hyperglycemia or oxidative stress-triggered apoptosis of endothelial cells. Here we explored the efficacy of honokiol, a small molecular weight natural product, on NADPH oxidase-related oxidative stress-mediated NF-kappaB-regulated signaling and apoptosis in human umbilical vein endothelial cells (HUVECs) under hyperglycemic conditions. The methods of morphological Hoechst staining and annexin V/propidium iodide staining were used to detect apoptosis. Submicromolar concentrations of honokiol suppressed the increases of NADPH oxidase activity, Rac-1 phosphorylation, p22(phox) protein expression, and reactive oxygen species production in high glucose (HG)-stimulated HUVECs. The degradation of IkappaBalpha and increase of NF-kappaB activity were inhibited by honokiol in HG-treated HUVECs. Moreover, honokiol (0.125-1 microM) also suppressed HG-induced cyclooxygenase (COX)-2 upregulation and prostaglandin E(2) production in HUVECs. Honokiol could reduce increased caspase-3 activity and the subsequent apoptosis and cell death triggered by HG. These results imply that inhibition of NADPH oxidase-related oxidative stress by honokiol suppresses the HG-induced NF-kappaB-regulated COX-2 upregulation, apoptosis, and cell death in HUVECs, which has the potential to be developed as a therapeutic agent to prevent hyperglycemia-induced endothelial damage.

Imaging cyclic AMP changes in pancreatic islets of transgenic reporter mice

Cyclic AMP (cAMP) and Ca(2+) are two ubiquitous second messengers in transduction pathways downstream of receptors for hormones, neurotransmitters and local signals. The availability of fluorescent Ca(2+) reporter dyes that are easily introduced into cells and tissues has facilitated analysis of the dynamics and spatial patterns for Ca(2+) signaling pathways. A similar dissection of the role of cAMP has lagged because indicator dyes do not exist. Genetically encoded reporters for cAMP are available but they must be introduced by transient transfection in cell culture, which limits their utility. We report here that we have produced a strain of transgenic mice in which an enhanced cAMP reporter is integrated in the genome and can be expressed in any targeted tissue and with tetracycline induction. We have expressed the cAMP reporter in beta-cells of pancreatic islets and conducted an analysis of intracellular cAMP levels in relation to glucose stimulation, Ca(2+) levels, and membrane depolarization. Pancreatic function in transgenic mice was normal. In induced transgenic islets, glucose evoked an increase in cAMP in beta-cells in a dose-dependent manner. The cAMP response is independent of (in fact, precedes) the Ca(2+) influx that results from glucose stimulation of islets. Glucose-evoked cAMP responses are synchronous in cells throughout the islet and occur in 2 phases suggestive of the time course of insulin secretion. Insofar as cAMP in islets is known to potentiate insulin secretion, the novel transgenic mouse model will for the first time permit detailed analyses of cAMP signals in beta-cells within islets, i.e. in their native physiological context. Reporter expression in other tissues (such as the heart) where cAMP plays a critical regulatory role, will permit novel biomedical approaches.

High glucose-induced upregulation of Rho/Rho-kinase via platelet-derived growth factor receptor-beta increases migration of aortic smooth muscle cells

Small GTPase Rho and Rho-kinase, the target protein of Rho, play an important role in atherosclerosis. In diabetic macroangiopathy, one of the major pathogenic changes is the migration of vascular smooth muscle cells (SMCs). Platelet-derived growth factor (PDGF) is known to stimulate the migration of SMCs. In the current study, we have investigated the involvement of the Rho/Rho-kinase pathway in the increased migration of cultured human aortic SMCs under a high glucose condition. PDGF stimulated the activation and the protein level of Rho. The protein level of PDGF receptor-beta (PDGFR-beta) was increased under the high glucose condition concomitant with the increased protein level and activation of Rho. The increased protein level and activity of Rho were suppressed by an anti-PDGF neutralizing antibody or a PDGFR-beta inhibitor, AG1433, under the high glucose condition. Furthermore, high glucose significantly increased the migration of SMCs. A specific inhibitor of Rho-kinase, Y-27632, or anti-PDGF neutralizing antibody inhibited increased migration of SMCs under the high glucose condition. The protein levels of Rho were increased in aortae of diabetic rats, which were abolished by the treatment of Imatinib, the inhibitor of PDGFR. These observations indicate that the upregulation of the PDGFR-beta / Rho / Rho-kinase pathway increases the migration of SMCs under the high glucose condition. The inhibition of Rho/Rho-kinase may be a new target for the treatment of diabetic macroangiopathy.

High glucose levels upregulate upstream stimulatory factor 2 gene transcription in mesangial cells

Previously, we demonstrated that upstream stimulatory factor 2 (USF2) mediates high glucose-induced thrombospondin1 (TSP1) gene expression and TGF-beta activity in glomerular mesangial cells and plays a role in diabetic renal complications. In the present studies, we further determined the molecular mechanisms by which high glucose levels regulate USF2 gene expression. In primary rat mesangial cells, we found that glucose treatment time and dose-dependently up-regulated USF2 expression (mRNA and protein). By using cycloheximide to block the de novo protein synthesis, similar rate of USF2 degradation was found under either normal glucose or high glucose conditions. USF2 mRNA stability was not altered by high glucose treatment. Furthermore, high glucose treatment stimulated USF2 gene promoter activity. By using the luciferase-promoter deletion assay, site-directed mutagenesis, and transactivation assay, we identified a glucose-responsive element in the USF2 gene promoter (-1,740 to -1,620, relative to the transcription start site) and demonstrated that glucose-induced USF2 expression is mediated through a cAMP-response element-binding protein (CREB)-dependent transactivation of the USF2 promoter. Furthermore, siRNA-mediated CREB knock down abolished glucose-induced USF2 expression. Taken together, these data indicate that high glucose levels up-regulate USF2 gene transcription in mesangial cells through CREB-dependent transactivation of the USF2 promoter.

Omentin-1, a novel adipokine, is decreased in overweight insulin-resistant women with polycystic ovary syndrome: ex vivo and in vivo regulation of omentin-1 by insulin and glucose

OBJECTIVE

Polycystic ovary syndrome (PCOS) is associated with insulin resistance and obesity. Recent studies have shown that plasma omentin-1 levels decrease with obesity. Currently, no data exist on the relative expression and regulation of omentin-1 in adipose tissue of women with PCOS. The objective of this study was to assess mRNA and protein levels of omentin-1, including circulating omentin-1, in omental adipose tissue of women with PCOS and matched control subjects. Ex vivo and in vivo regulation of adipose tissue omentin-1 was also studied.

RESEARCH DESIGN AND METHODS

Real-time RT-PCR and Western blotting were used to assess mRNA and protein expression of omentin-1. Plasma omentin-1 was measured by enzyme-linked immunosorbent assay. The effects of d-glucose, insulin, and gonadal and adrenal steroids on adipose tissue omentin-1 were analyzed ex vivo. The in vivo effects of insulin (hyperinsulinemia) on omentin-1 levels were also assessed by a prolonged insulin-glucose infusion.

RESULTS

In addition to decreased plasma omentin-1 levels in women with PCOS (P < 0.05), compared with control subjects, there was significantly lower levels of omentin-1 mRNA (P < 0.01) and protein (P < 0.05) in omental adipose tissue of women with PCOS (P < 0.01). Furthermore, in omental adipose tissue explants, insulin and glucose significantly dose-dependently decreased omentin-1 mRNA expression, protein levels, and secretion into conditioned media (P < 0.05, P < 0.01). Also, hyperinsulinemic induction in healthy subjects significantly reduced plasma omentin-1 levels (P < 0.01).

CONCLUSIONS

Our novel findings reveal that omentin-1 is downregulated by insulin and glucose. These may, in part, explain the decreased omentin-1 levels observed in our overweight women with PCOS.

Adaptive landscapes and emergent phenotypes: why do cancers have high glycolysis?

Investigating the causes of increased aerobic glycolysis in tumors (Warburg Effect) has gone in and out of fashion many times since it was first described almost a century ago. The field is currently in ascendance due to two factors. Over a million FDG-PET studies have unequivocally identified increased glucose uptake as a hallmark of metastatic cancer in humans. These observations, combined with new molecular insights with HIF-1alpha and c-myc, have rekindled an interest in this important phenotype. A preponderance of work has been focused on the molecular mechanisms underlying this effect, with the expectation that a mechanistic understanding may lead to novel therapeutic approaches. There is also an implicit assumption that a mechanistic understanding, although fundamentally reductionist, will nonetheless lead to a more profound teleological understanding of the need for altered metabolism in invasive cancers. In this communication, we describe an alternative approach that begins with teleology; i.e. adaptive landscapes and selection pressures that promote emergence of aerobic glycolysis during the somatic evolution of invasive cancer. Mathematical models and empirical observations are used to define the adaptive advantage of aerobic glycolysis that would explain its remarkable prevalence in human cancers. These studies have led to the hypothesis that increased consumption of glucose in metastatic lesions is not used for substantial energy production via Embden-Meyerhoff glycolysis, but rather for production of acid, which gives the cancer cells a competitive advantage for invasion. Alternative hypotheses, wherein the glucose is used for generation of reducing equivalents (NADPH) or anabolic precursors (ribose) are also discussed.

[Significant increase of glucose transport activity in breast cancer]

OBJECTIVE

To study the expression level and significance of glucose transporter 1 (Glut-1) in normal breast tissue, adenosis, adenoma and breast carcinoma.

METHODS

A total of 147 cases of female breast tissue samples, including 92 cases of invasive ductal carcinoma, 26 cases of breast fibroadenoma, 24 cases of breast adenosis and 5 cases of normal breast tissues, were collected for quantitative detection of the expression of Glut-1 protein by immunohistochemistry (EnVision method) and Western blot, and its mRNA by reverse transcriptase-polymerase chain reaction (RT-PCR).

RESULTS

In normal breast tissue and benign lesions of the breast, Glut-1 was undetectable or only weakly detectable in cytoplasm of ductal and acinar epithelia. In contrast, the intensity of Glut-1 staining was significantly higher in invasive ductal carcinomas (P = 0.0002) with protein expression predominantly in cellular membrane and lesser in cytoplasm. Western blot and RT-PCR analyses showed that the expression of Glut-1 protein and mRNA were significantly increased in invasive ductal carcinoma than fibroadenoma (P =0.001 for protein; P <0.05 for mRNA) and adenosis (P =0.001 for protein; P < 0.05 for mRNA). There was a significant difference among groups (P = 0.0002 for protein; P = 0.0001 for mRNA).

CONCLUSIONS

Glucose transport activity, as indicated by Glut-1 protein and its mRNA expression, significantly increases in breast carcinoma than non-cancerous lesions. The over-expression of Glut-1 in breast carcinoma is tightly coupled with tumor cell proliferation, invasion and metastasis, implying that Glut-1 may serve as a new marker in the early diagnosis and prognostication of breast malignancy as well as a new therapeutic target.

Metabolic transformation of rabbit skeletal muscle cells in primary culture in response to low glucose

We have investigated the mechanism of the changes in the profile of metabolic enzyme expression that occur in association with fast-to-slow transformation of rabbit skeletal muscle. The hypotheses assessed are: do 1) lowered intracellular ATP concentration or 2) reduction of the muscular glycogen stores act as triggers of metabolic transformation? We find that 3 days of decreased cytosolic ATP content have no impact on the investigated metabolic markers, whereas incubation of the cells with little or no glucose leads to decreases in glycogen in conjunction with decreases in glyceraldehyde-3-phosphate dehydrogenase (GAPDH) promoter activity, GAPDH mRNA and specific GAPDH enzyme activity (indicators of the anaerobic glycolytic pathway), and furthermore to increases in mitochondrial acetoacetyl-CoA thiolase (MAT, also known as ACAT) promoter activity, peroxisome proliferator-activated receptor gamma coactivator 1alpha (PGC-1alpha) expression and citrate synthase (CS) specific enzyme activity (all indicators of oxidative metabolic pathways). The AMP-activated protein kinase (AMPK) activity under these conditions is reduced compared to controls. In experiments with two inhibitors of glycogen degradation we show that the observed metabolic transformation caused by low glucose takes place even if intracellular glycogen content is high. These findings for the first time provide evidence that metabolic adaptation of skeletal muscle cells from rabbit in primary culture can be induced not only by elevation of intracellular calcium concentration or by a rise of AMPK activity, but also by reduction of glucose supply. Contrary to expectations, neither an increase in phospho-AMPK nor a reduction of muscular glycogen content are crucial events in the glucose-dependent induction of metabolic transformation in the muscle cell culture system studied.

Methylglyoxal and high glucose co-treatment induces apoptosis or necrosis in human umbilical vein endothelial cells

Hyperglycemia and elevation of methylglyoxal (MG) are symptoms of diabetes mellitus (DM). We previously showed that high glucose (HG; 30 mM) or MG (50-400 microM) could induce apoptosis in mammalian cells, but these doses are higher than the physiological concentrations of glucose and MG in the plasma of DM patients. The physiological concentration of MG and glucose in the normal blood circulation is about 1 microM and 5 mM, respectively. Here, we show that co-treatment with concentrations of MG and glucose comparable to those seen in the blood circulation of DM patients (5 microM and 15-30 mM, respectively) could cause cell apoptosis or necrosis in human umbilical vein endothelial cells (HUVECs) in vitro. HG/MG co-treatment directly increased the reactive oxygen species (ROS) content in HUVECs, leading to increases in intracellular ATP levels, which can control cell death through apoptosis or necrosis. Co-treatment of HUVECs with 5 microM MG and 20 mM glucose significantly increased cytoplasmic free calcium levels, activation of nitric oxide synthase (NOS), caspase-3 and -9, cytochrome c release, and apoptotic cell death. In contrast, these apoptotic biochemical changes were not detected in HUVECs treated with 5 microM MG and 30 mM glucose, which appeared to undergo necrosis. Pretreatment with nitric oxide (NO) scavengers could inhibit 5 microM MG/20 mM glucose-induced cytochrome c release, decrease activation of caspase-9 and caspase-3, and increase the gene expression and protein levels of p53 and p21, which are known to be involved in apoptotic signaling. Inhibition of p53 protein expression using small interfering RNA (siRNA) blocked the activation of p21 and the cell apoptosis induced by 5 microM MG/20 mM glucose. In contrast, inhibition of p21 protein expression by siRNA prevented apoptosis in HUVECs but had no effect on p53 expression. These results collectively suggest that the treatment dosage of MG and glucose could determine the mode of cell death (apoptosis vs. necrosis) in HUVECs, and both ROS and NO played important roles in MG/HG-induced apoptosis of these cells.

[The dual (joker) function of glucose: study of its association with aging and glucose metabolism disorders]

Relation was studied between generation of glucose-induced reactive oxygen species (ROS), which appear to be related to DNA damage (genotoxic effect, G), and insulin secretion (endocrine or hormonal effect, H) in women of different ages (one group under 45 and the other one over 45; n=25 and n=14, respectively). The healthy women in those two groups were compared with patients in whom we had found an impaired glucose tolerance (IGT) or type 2 diabetes mellitus (DM) (n=17, mean age 57.3 +/- 2.7). The hormonal effect of glucose was more pronounced in the senior group, and especially in group with IGT, if compared with the younger group. Genotoxic effect of glucose was discovered more frequently in the younger group, mainly in smoking women. Comparison of G/H effects showed that the evaluation of glucose-induced genotoxity (GIGT) was more frequent in the IGT group than in the senior group (p < 0.05). No difference was detected in the GIGT frequency values in the two healthy groups. It may therefore be concluded that GIGT did not increase within the ambit of ageing studied in this work, while it increased in the IGT group. It is possible that the high frequency of the G effect in the IGT group could be a marker of oxidative stress and/or predisposition to complications in DM. The dual (joker) function of glucose and the prevalence of G effects over H effects may be of use in choosing the method of correction for each particular case.

Reduced expression of the KATP channel subunit, Kir6.2, is associated with decreased expression of neuropeptide Y and agouti-related protein in the hypothalami of Zucker diabetic fatty rats

The link between obesity and diabetes is not fully understood but there is evidence to suggest that hypothalamic signalling pathways may be involved. The hypothalamic neuropeptides, pro-opiomelanocortin (POMC), neuropeptide Y (NPY) and agouti-related protein (AGRP) are central to the regulation of food intake and have been implicated in glucose homeostasis. Therefore, the expression of these genes was quantified in hypothalami from diabetic Zucker fatty (ZDF) rats and nondiabetic Zucker fatty (ZF) rats at 6, 8, 10 and 14 weeks of age. Although both strains are obese, only ZDF rats develop pancreatic degeneration and diabetes over this time period. In both ZF and ZDF rats, POMC gene expression was decreased in obese versus lean rats at all ages. By contrast, although there was the expected increase in both NPY and AGRP expression in obese 14-week-old ZF rats, the expression of NPY and AGRP was decreased in 6-week-old obese ZDF rats with hyperinsulinaemia and in 14-week-old rats with the additional hyperglycaemia. Therefore, candidate genes involved in glucose, and insulin signalling pathways were examined in obese ZDF rats over this age range. We found that expression of the ATP-sensitive potassium (K(ATP)) channel component, Kir6.2, was decreased in obese ZDF rats and was lower compared to ZF rats in each age group tested. Furthermore, immunofluorescence analysis showed that Kir6.2 protein expression was reduced in the dorsomedial and ventromedial hypothalamic nuclei of 6-week-old prediabetic ZDF rats compared to ZF rats. The Kir6.2 immunofluorescence colocalised with NPY throughout the hypothalamus. The differences in Kir6.2 expression in ZF and ZDF rats mimic those of NPY and AGRP, which could infer that the changes occur in the same neurones. Overall, these data suggest that chronic changes in hypothalamic Kir6.2 expression may be associated with the development of hyperinsulinaemia and hyperglycaemia in ZDF rats.

Common variants of the novel type 2 diabetes genes CDKAL1 and HHEX/IDE are associated with decreased pancreatic beta-cell function

OBJECTIVE

Type 2 diabetes is characterized by impaired pancreatic beta-cell function and decreased insulin sensitivity. Genome-wide association studies have identified common, novel type 2 diabetes susceptibility loci within the FTO, CDKAL1, CDKN2A/CDKN2B, IGF2BP2, HHEX/IDE, and SLC30A8 gene regions. Our objective was to explore the relationships between the diabetes-associated alleles and measures of beta-cell function and whole-body insulin sensitivity.

RESEARCH DESIGN AND METHODS

A total of 1,276 healthy subjects of European ancestry were studied at 19 centers. Indexes of beta-cell function (including 30-min insulin response and glucose sensitivity) were derived from a 75-g oral glucose tolerance test, and whole-body insulin sensitivity (M/I) was assessed by hyperinsulinemic-euglycemic clamp. Genotype/phenotype relationships were studied by linear trend analysis correcting for age, sex, and recruitment center.

RESULTS

CDKAL1 and HHEX/IDE diabetes-associated alleles were both associated with decreased 30-min insulin response (both P = 0.0002) and decreased pancreatic beta-cell glucose sensitivity (P = 9.86 x 10(-5) and 0.009, respectively), and these relationships remained after correction for M/I. The FTO susceptibility allele showed a weak but consistent association with increased adiposity, which in turn was linked to a decrease in M/I. However, none of the other novel diabetes susceptibility alleles were associated with insulin sensitivity.

CONCLUSIONS

CDKAL1 and HHEX/IDE diabetes-associated alleles are associated with decreased pancreatic beta-cell function, including decreased beta-cell glucose sensitivity that relates insulin secretion to plasma glucose concentration. We confirmed the association between the FTO allele and increased adiposity, but none of the other novel susceptibility alleles were associated with whole-body insulin sensitivity.

Developmental Changes in Extracellular Matrix Messenger RNAs in the Mouse Placenta During the Second Half of Pregnancy: Possible Factors Involved in the Regulation of Placental Extracellular Matrix Expression1

Expression of procollagens (Col1a1/2, Col3a1, Col4a1/2, Col5a1/2) and fibronectin 1 (Fn1) in the mouse fetal placental tissue was examined during the second half of pregnancy. Ribonuclease protection assays (RPAs) revealed that levels of these mRNAs noticeably increased between Days 10 and 14 of pregnancy, and they remained at relatively constant levels thereafter. In situ hyridization showed that Col1a1 and Col4a1 mainly localized in the labyrinth, whereas Fn1 was expressed mainly in the spongiotrophoblast. Since members of the transforming growth factor-beta (TGFB) superfamily are involved in the regulation of extracellular matrix (ECM) expression in various tissues, mRNA levels of TGFB family members and their binding proteins were also examined by RPAs. Transforming growth factor-beta1-3 (Tgfb1-3), activin subunits (Inhba, Inhbb), follistatin (Fst), and follistatin-like 3 (Fstl3) were expressed in the placenta, whereas significant expression of myostatin (Mstn) was not detected. Although the expression patterns of Tgfb1-3 and Inhba in the placenta suggest possible involvement of TGFBs and activin A in the regulation of placental ECM expression, neither TGFBs nor activin A affected ECM mRNA levels in vitro. On the other hand, hypoxia significantly decreased Col1a1/2 and Col4a1/2 mRNAs in cultured placental cells, and a high-glucose condition significantly increased Col1a1 and Col3a1 mRNAs. Fn1 expression was increased under the high-glucose condition, although hypoxia also increased Fn1 expression to a lesser degree. These data suggest that an increase in oxygen tension and nutrient supply during placentation rather than TGFB family members may be responsible for the increase in the placental ECM mRNA expression.

Prolactin stimulates proliferation of the glucose-dependent beta-cell line INS-1 via different IRS-proteins

CONTEXT

Prolactin is one of the most potent growth stimulating growth hormones of pancreatic beta cells.

OBJECTIVE

We investigated the role of prolactin on the proliferation of the beta-cell line INS-1.

DESIGN

In particular, we investigated the involvement of intracellular signal transduction molecules in prolactin-dependent upregulation of INS-1 growth.

SETTING

The effect of prolactin on the growth of INS-1 cells was assessed in vitro under various feeding conditions.

MAIN OUTCOME MEASURES

Cell proliferation was measured in the pancreatic beta-cell line INS-1 using 3H-thymidine incorporation. The activation of mitogenic signaling proteins was assessed by co-immunoprecipitation, immunoblot analysis and in proliferation assays using specific protein inhibitors.

RESULTS

Prolactin (0.5-2 nM) increased INS-1 cell proliferation in the presence of 3-24 mM glucose up to 48 fold, having a maximum in the presence of physiological glucose concentrations (6 mM). Prolactin activated the JAK2/STAT5 pathway and phosphatidylinositol-3'-kinase (PI3'K) in the presence of all the glucose concentrations used (3-15 mM). At low glucose concentrations (3 mM), PI3'K activation occurred through IRS-2 phosphorylation whereas, in the presence of physiological glucose concentration IRS4 and at high glucose concentrations (15 mM), IRS-1 triggered a proliferative effect. PI3'K activation was essential for prolactin and glucose stimulated INS-1 cell proliferation. Co-stimulation with different growth factors (IGF-I, growth hormone) in addition to prolactin and glucose had no additive effects.

CONCLUSION

These results define prolactin as an important hormone. mediating glucose-dependent pancreatic beta-cell proliferation primarily by the activation of PI3'K-dependent signaling pathways.

Mitochondria are an essential mediator of nitric oxide/cyclic guanosine 3',5'-monophosphate blocking of glucose depletion induced cytotoxicity in human HepG2 cells

It is well known that glucose is a major energy source in tumors and that mitochondria are specialized organelles required for energy metabolism. Previous studies have revealed that nitric oxide (NO) protects against glucose depletion-induced cytotoxicity in mouse liver cells and in rat hepatocytes, but the detailed mechanism is not well understood. Therefore, we investigated the involvement of mitochondria in the NO protective effect in human hepatoma HepG2 cells. In this study, we showed that glucose depletion resulted in a time-dependent decrease in intracellular NO and in the protein expression of NO synthases. This glucose depletion-induced decrease in NO was blocked by NO donors. Next, we showed that the cytoprotective effect of NO is via a cyclic guanosine 3',5'-monophosphate-dependent pathway. Additionally, SNP blocked a glucose depletion-induced decrease in mitochondrial mass, mitochondrial DNA copies, and ATP level in HepG2 cells. Moreover, glucose depletion decreased the expression of various mitochondrial proteins, including cytochrome c, complex I (NADH dehydrogenase), complex III (cytochrome c reductase), and heat shock protein 60; these glucose depletion-induced effects were blocked by SNP. Furthermore, we found that rotenone and antimycin A (mitochondria complex I and III inhibitors, respectively) blocked SNP cytoprotection against glucose depletion-induced cytotoxicity. Taken together, our results indicated that the mitochondria serve as an important cellular mediator of NO during protection against glucose deprivation-induced damage.

Glucagon-like peptide-1 (GLP-1) receptors expressed on nerve terminals in the portal vein mediate the effects of endogenous GLP-1 on glucose tolerance in rats

Glucagon-like peptide-1 (GLP-1) is an intestinal hormone that is secreted during meal absorption and is essential for normal glucose homeostasis. However, the relatively low plasma levels and rapid metabolism of GLP-1 raise questions as to whether direct endocrine action on target organs, such as islet cells, account for all of its effects on glucose tolerance. Recently, an alternative neural pathway initiated by sensors in the hepatic portal region has been proposed to mediate GLP-1 activity. We hypothesized that visceral afferent neurons in the portal bed express the GLP-1 receptor (GLP-1r) and regulate glucose tolerance. Consistent with this hypothesis, GLP-1r mRNA was present in the nodose ganglia, and nerve terminals innervating the portal vein contained the GLP-1r. Rats given an intraportal infusion of the GLP-1r antagonist, [des-His(1),Glu(9)] exendin-4, in a low dose, had glucose intolerance, with a 53% higher glucose excursion compared with a vehicle-infused control group. Infusion of [des-His(1),Glu(9)] exendin-4 at an identical rate into the jugular vein had no effect on glucose tolerance, demonstrating that this dose of GLP-1r antagonist did not affect blood glucose due to spillover into the systemic circulation. These studies demonstrate that GLP-1r are present on nerve terminals in the hepatic portal bed and that GLP-1 antagonism localized to this region impairs glucose tolerance. These data are consistent with an important component of neural mediation of GLP-1 action.

Proliferation, hyperplasia, neogenesis, and neoplasia in the islets of Langerhans

Pancreatic disease is responsible for significant morbidity and mortality as a result of pancreatic carcinoma and diabetes mellitus. Regulation of endocrine cell mass is thought to have a central role in the pathogenesis of both these diseases. Islet cell proliferation, hypertrophy, neogenesis, and apoptosis are the main determinants of endocrine cell mass in the pancreas, and their understanding has been improved by new clues of their genetic and molecular basis. Beta cells have attracted most research interest because of potential implications in the treatment of diabetes mellitus and hypoglycemic disorders. The processes that operate during pancreatic adaptation to a changing hormonal milieu are important in pancreatic carcinogenesis. There is evidence that somatostatin and its receptors are fundamental regulators of endocrine cell mass and are involved in islet tumorigenesis.

Endocrine regulation of energy metabolism by the skeleton

The regulation of bone remodeling by an adipocyte-derived hormone implies that bone may exert a feedback control of energy homeostasis. To test this hypothesis we looked for genes expressed in osteoblasts, encoding signaling molecules and affecting energy metabolism. We show here that mice lacking the protein tyrosine phosphatase OST-PTP are hypoglycemic and are protected from obesity and glucose intolerance because of an increase in beta-cell proliferation, insulin secretion, and insulin sensitivity. In contrast, mice lacking the osteoblast-secreted molecule osteocalcin display decreased beta-cell proliferation, glucose intolerance, and insulin resistance. Removing one Osteocalcin allele from OST-PTP-deficient mice corrects their metabolic phenotype. Ex vivo, osteocalcin can stimulate CyclinD1 and Insulin expression in beta-cells and Adiponectin, an insulin-sensitizing adipokine, in adipocytes; in vivo osteocalcin can improve glucose tolerance. By revealing that the skeleton exerts an endocrine regulation of sugar homeostasis this study expands the biological importance of this organ and our understanding of energy metabolism.

Effects of ghrelin on glucose-sensing and gastric distension sensitive neurons in rat dorsal vagal complex

Ghrelin has been identified as the endogenous ligand of the growth hormone secretagogue receptor (GHS-R). Recent studies have shown that site-specific injection of ghrelin directly into the dorsal vagal complex (DVC) of rats is equally as sensitive in its orexigenic response to ghrelin as the arcuate nucleus of the hypothalamus (ARC). It is as yet unclear how circulating ghrelin would gain access to and influence the activity of the neurons in the DVC in which GHS receptors are expressed. In the present study, neuronal activity was recorded extracellularly in the DVC of anesthetized rats in order to examine the effects of ghrelin on the glucosensing neurons and the gastric distension (GD) sensitive neurons. The 82 neurons were tested with glucose, of which 26 were depressed by glucose and identified as glucose-inhibited (glucose-INH) neurons; 11 were activated and identified as glucose-excited (glucose-EXC) neurons. Of 26 glucose-inhibited neurons examined for response to ghrelin, 23 were depressed, 1 was activated, and 2 failed to respond to ghrelin. Nine of 11 glucose-excited neurons were suppressed by ghrelin application, and the responses are abolished by the pretreatment with the GHS-R antagonist, [D-Lys-3]-GHRP-6. In addition, of 47 DVC neurons examined for responses to gastric distension (GD), 25 were excited (GD-EXC), 18 were inhibited (GD-INH). 18 out of the 25 GD-EXC neurons were excited, whereas 15 out of 18 GD-INH neurons were suppressed by ghrelin. In conclusion, the activity of the glucosensing neurons in the DVC can be modulated by ghrelin, the primary effect of ghrelin on the glucose-INH and glucose-EXC neurons was inhibitory. Two distinct population of GD-sensitive neurons exist in the rat DVC: GD-EXC neurons are activated by ghrelin; the GD-INH neurons are suppressed by ghrelin. There is a diversity of effects of ghrelin on neuronal activity within the DVC, it is as yet unclear how this diversity in ghrelin's effects on cellular excitability contributes to ghrelin biological actions to influence food intake and gastric motility.

High level glucose increases mutagenesis in human lymphoblastoid cells

Epidemiological data have suggested an increased cancer rates in diabetic patients, for which the underlying mechanism is poorly understood. We studied whether high level of glucose (HG) treatment that mimic the hyperglycemic condition in diabetes mellitus is mutagenic. Mutagenesis studies were carried out at both hypoxanthine phosphoribosyltransferase (hprt) and thymidine kinase (tk) loci. Role of p53 in HG-induced mutagenesis was also investigated by using human lymphoblastoid cell lines derived from same donor but differs in p53 statuses; TK6 has wild-type p53, NH32 has null p53, and WTK1 has mutant p53 (ile237). In addition, we studied the influence of antioxidant treatment on HG-induced mutagenesis. Mutation fractions at both loci increased significantly in all three lines at 21 and 28 days after HG treatments. At tk locus, the increase of a class of mutants with normal growth rate is mainly responsible for the overall increased mutant fraction. Compared to TK6 cells, both NH32 and WTK1 cells showed an early onset of mutagenesis. Treatment of cells with antioxidant N-acetyl-L-cysteine partially reduced HG induced mutagenesis. This study is the first to indicate that HG is able to induce gene mutation which may be one of the important mechanisms of diabetes-associated carcinogenesis.

Impedance spectroscopy of solutions at physiological glucose concentrations

Impedance spectroscopy has been proposed as possible approach for non-invasive glycaemia monitoring. However, few quantitative data are reported about impedance variations related to glucose concentration variations, especially below the MHz band. Furthermore, it is not clear whether glucose directly affects the impedance parameters or only indirectly by inducing biochemical phenomena. We investigated the impedance variations in glucose-water, glucose-sodium chloride, and glucose-blood samples, for increasing glucose values (up to 300 mg/dl). In all the frequency range (0.1-10(7) Hz) glucose-water samples showed impedance modulus increases for increasing glucose values (up to 135%). In blood and sodium chloride samples the impedance modulus showed only slight variations (2% and 1.4%), but again in wide frequency ranges. Therefore: i) glucose directly affects the impedance parameters of solutions; ii) effects are more relevant at frequencies below the MHz band; iii) the influence on the impedance is decreased in high conductivity solutions, but still clearly present.

Astrocyte activation in vivo during graded photic stimulation

Astrocytes have important roles in control of extracellular environment, de novo synthesis of neurotransmitters, and regulation of neurotransmission and blood flow. All of these functions require energy, suggesting that astrocytic metabolism should rise and fall with changes in neuronal activity and that brain imaging can be used to visualize and quantify astrocytic activation in vivo. A unilateral photic stimulation paradigm was used to test the hypothesis that graded sensory stimuli cause progressive increases in the uptake coefficient of [2-(14)C]acetate, a substrate preferentially oxidized by astrocytes. The acetate uptake coefficient fell in deafferented visual structures and it rose in intact tissue during photic stimulation of conscious rats; the increase was highest in structures with monosynaptic input from the eye and was much smaller in magnitude than the change in glucose utilization (CMR(glc)) by all cells. The acetate uptake coefficient was not proportional to stimulus rate and did not correlate with CMR(glc) in resting or activated structures. Simulation studies support the conclusions that acetate uptake coefficients represent mainly metabolism and respond to changes in metabolism rate, with a lower response at high rates. A model portraying regulation of acetate oxidation illustrates complex relationships among functional activation, cation levels, and astrocytic metabolism.

Downregulation of TRPC6 protein expression by high glucose, a possible mechanism for the impaired Ca2+ signaling in glomerular mesangial cells in diabetes

The present study was performed to investigate whether transient receptor potential (TRPC)6 participated in Ca(2+) signaling of glomerular mesangial cells (MCs) and expression of this protein was altered in diabetes. Western blots and real-time PCR were used to evaluate the expression level of TRPC6 protein and mRNA, respectively. Cell-attached patch-clamp and fura-2 fluorescence measurements were utilized to assess angiotensin II (ANG II)-stimulated membrane currents and Ca(2+) responses in MCs. In cultured human MCs, high glucose significantly reduced expression of TRPC6 protein, but there was no effect on either TRPC1 or TRPC3. The high glucose-induced effect on TRPC6 was time and dose dependent with the maximum effect observed on day 7 and at 30 mM glucose, respectively. In glomeruli isolated from streptozotocin-induced diabetic rats, TRPC6, but not TRPC1, was markedly reduced compared with the glomeruli of control rats. Furthermore, TRPC6 mRNA in MCs was also significantly decreased by high glucose as early as 1 day after treatment with maximal reduction on day 4. Patch-clamp experiments showed that ANG II-stimulated membrane currents in MCs were significantly attenuated or enhanced by knockdown or overexpression of TRPC6, respectively. Fura-2 fluorescence measurements revealed that the ANG II-induced Ca(2+) influxes were markedly inhibited in MCs with TRPC6 knockdown, reminiscent of the impaired Ca(2+) entry in response to ANG II in high glucose-treated MCs. These results suggest that the TRPC6 protein expression in MCs was downregulated by high glucose and the deficiency of TRPC6 protein might contribute to the impaired Ca(2+) signaling of MCs seen in diabetes.

Regulation of photosynthesis in the unicellular acidophilic red alga Galdieria sulphuraria

Extremophilic organisms are gaining increasing interest because of their unique metabolic capacities and great biotechnological potential. The unicellular acidophilic and mesothermophilic red alga Galdieria sulphuraria (074G) can grow autotrophically in light as well as heterotrophically in the dark. In this paper, the effects of externally added glucose on primary and secondary photosynthetic reactions are assessed to elucidate mixotrophic capacities of the alga. Photosynthetic O2 evolution was quantified in an open system with a constant supply of CO2 to avoid rapid volatilization of dissolved inorganic carbon at low pH levels. In the presence of glucose, O2 evolution was repressed even in illuminated cells. Ratios of variable to maximum chlorophyll fluorescence (Fv/Fm) and 77 K fluorescence spectra indicated a reduced photochemical efficiency of photosystem II. The results were corroborated by strongly reduced levels of the photosystem II reaction centre protein D1. The downregulation of primary photosynthetic reactions was accompanied by reduced levels of the Calvin Cycle enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). Both effects depended on functional sugar uptake and are thus initiated by intracellular rather than extracellular glucose. Following glucose depletion, photosynthetic O2 evolution of illuminated cells commenced after 15 h and Rubisco levels again reached the levels of autotrophic cells. It is concluded that true mixotrophy, involving electron transport across both photosystems, does not occur in G. sulphuraria 074G, and that heterotrophic growth is favoured over autotrophic growth if sufficient organic carbon is available.

Phospholipase A2 is important for glucose induction of rhythmic Ca2+ signals in pancreatic beta cells

OBJECTIVES

Pancreatic beta cells respond to glucose stimulation with pulses of insulin release generated by oscillatory rises of the cytoplasmic Ca2+ concentration ([Ca2+]i). The observation that exposure to external ATP and other activators of cytoplasmic phospholipase A2 (cPLA2) rapidly induces rises of [Ca2+]i similar to ordinary oscillations made it important to analyze whether suppression of the cPLA2 activity affects glucose-induced [Ca2+]i rhythmicity in pancreatic beta cells.

METHODS

Ratiometric fura-2 technique was used for measuring [Ca2+]i in single beta cells and small aggregates prepared from ob/ob mouse islets.

RESULTS

Testing the effects of different inhibitors of cPLA2 in the presence of 20 mM glucose, it was found that N-(p-amylcinnamoyl)anthranilic acid (ACA) removed the oscillations at a concentration of 25 microM, arachidonyl trifluoromethyl ketone (AACOCF3) at 10 microM, and bromoenol lactone (BEL) at 10 to 15 microM. Withdrawal of ACA and BEL resulted in reappearance of the oscillations. Suppression of the arachidonic acid production by addition of 5 microM of the diacylglycerol lipase inhibitor 1,6-bis-(cyclohexyloximinocarbonylamino)-hexane (RHC 80267) effectively removed the [Ca2+]i oscillations, an effect reversed by removal of the inhibitor or addition of 100 microM tolbutamide. Suppression of the arachidonic acid production had a restrictive influence also on the transients of [Ca2+]i supposed to synchronize the beta-cell rhythmicity. Although less sensitive than the oscillations, most transients disappeared during exposure to 50 microM ACA or 35 microM RHC 80267.

CONCLUSIONS

The results support the idea that cyclic variations of cPLA2 activity are important for the generation and synchronization of the beta-cell [Ca2+]i oscillations responsible for pulsatile release of insulin.

Can Myoglobin Expression in Pancreatic Beta Cells Improve Insulin Secretion Under Hypoxia? An Exploratory Study With Transgenic Porcine Islets

The feasibility of myoglobin (Mb)-facilitated oxygen transport in improving porcine islet survival under hypoxia was investigated. Discrete groups of islets were transfected with replication-defective adenoviral vector Ad5 respiratory syncitial virus (RSV) to induce expression of Mb or green fluorescent protein (GFP). Native islets served as the controls. In vitro studies at 37 degrees C assessed islet insulin secretion efficacy: (i) to a glucose challenge from 30 to 300 mg/dL at fixed pO2; and (ii) at variable oxygen tensions ranging from 5 to 40 mm Hg over 12 h. The transfection was effective in initiating islet expression of Mb or GFP. Low Mb-expression levels equivalent to 2% the Mb concentration in a muscle cell (0.25 ng of Mb per islet) were documented, with no statistical improvement in insulin secretion. A surprising side note is that insulin secretion was impaired in islets expressing GFP. Improved Mb expression is essential to determine the feasibility of enhancing islet survival under hypoxia.

Regulation of gene expression by glucose

PURPOSE OF THE REVIEW

In addition to its metabolic function, glucose modulates gene expression which is crucial in adapting cells to variations in glycaemia. We summarize recent advances in our understanding of regulation of gene expression by glucose.

RECENT FINDINGS

In-vivo and in-vitro experiments demonstrated that glucose regulates the transcription of genes encoding not only lipogenic and glycolytic enzymes but also proteins involved in global cell functions. The molecular mechanisms have begun to be elucidated, and the transcription factor carbohydrate responsive element-binding protein has emerged as a key actor, at least in liver. More recently, other candidates have been proposed, such as liver X receptors. In pathological situations, altered glycaemic control, as observed in diabetes mellitus, is associated with increased risk for microvascular and macrovascular complications. Recent findings suggest that changes in gene expression occurring in response to hyperglycaemia represent a novel component of glucotoxicity.

SUMMARY

Until recently, the direct transcriptional effects of glucose were underestimated, and insulin was considered to be the major regulator of gene expression in response to glycaemic variation. The recent discovery and characterization of transcription factors mediating the glucose response demonstrate that glucose, like fatty acids and other key nutrients, can directly control gene expression.