Metabolic control of beta-cell function

Environmental and behavioral regulation of HIF-mitochondria crosstalk

Reduced oxygen availability (hypoxia) can lead to cell and organ damage. Therefore, aerobic species depend on efficient mechanisms to counteract detrimental consequences of hypoxia. Hypoxia inducible factors (HIFs) and mitochondria are integral components of the cellular response to hypoxia and coordinate both distinct and highly intertwined adaptations. This leads to reduced dependence on oxygen, improved oxygen supply, maintained energy provision by metabolic remodeling and tapping into alternative pathways and increased resilience to hypoxic injuries. On one hand, many pathologies are associated with hypoxia and hypoxia can drive disease progression, for example in many cancer and neurological diseases. But on the other hand, controlled induction of hypoxia responses via HIFs and mitochondria can elicit profound health benefits and increase resilience. To tackle pathological hypoxia conditions or to apply health-promoting hypoxia exposures efficiently, cellular and systemic responses to hypoxia need to be well understood. Here we first summarize the well-established link between HIFs and mitochondria in orchestrating hypoxia-induced adaptations and then outline major environmental and behavioral modulators of their interaction that remain poorly understood.

Carbohydrate and fat intake associated with risk of metabolic diseases through epigenetics of CPT1A

BACKGROUND

Epigenome-wide association studies identified the cg00574958 DNA methylation site at the carnitine palmitoyltransferase-1A (CPT1A) gene to be associated with reduced risk of metabolic diseases (hypertriglyceridemia, obesity, type 2 diabetes, hypertension, metabolic syndrome), but the mechanism underlying these associations is unknown.

OBJECTIVES

We aimed to elucidate whether carbohydrate and fat intakes modulate cg00574958 methylation and the risk of metabolic diseases.

METHODS

We examined associations between carbohydrate (CHO) and fat (FAT) intake, as percentages of total diet energy, and the CHO/FAT ratio with CPT1A-cg00574958, and the risk of metabolic diseases in 3 populations (Genetics of Lipid Lowering Drugs and Diet Network, n = 978; Framingham Heart Study, n = 2331; and REgistre GIroní del COR study, n = 645) while adjusting for confounding factors. To understand possible causal effects of dietary intake on the risk of metabolic diseases, we performed meta-analysis, CPT1A transcription analysis, and mediation analysis with CHO and FAT intakes as exposures and cg00574958 methylation as the mediator.

RESULTS

We confirmed strong associations of cg00574958 methylation with metabolic phenotypes (BMI, triglyceride, glucose) and diseases in all 3 populations. Our results showed that CHO intake and CHO/FAT ratio were positively associated with cg00574958 methylation, whereas FAT intake was negatively correlated with cg00574958 methylation. Meta-analysis further confirmed this strong correlation, with β = 58.4 ± 7.27, P = 8.98 x 10-16 for CHO intake; β = -36.4 ± 5.95, P = 9.96 x 10-10 for FAT intake; and β = 3.30 ± 0.49, P = 1.48 x 10-11 for the CHO/FAT ratio. Furthermore, CPT1A mRNA expression was negatively associated with CHO intake, and positively associated with FAT intake, and metabolic phenotypes. Mediation analysis supports the hypothesis that CHO intake induces CPT1A methylation, hence reducing the risk of metabolic diseases, whereas FAT intake inhibits CPT1A methylation, thereby increasing the risk of metabolic diseases.

CONCLUSIONS

Our results suggest that the proportion of total energy supplied by CHO and FAT can have a causal effect on the risk of metabolic diseases via the epigenetic status of CPT1A.Study registration at https://www.clinicaltrials.gov/: the Genetics of Lipid Lowering Drugs and Diet Network (GOLDN)-NCT01023750; and the Framingham Heart Study (FHS)-NCT00005121.

The role of α-ketoglutarate and the hypoxia sensing pathway in the regulation of pancreatic β-cell function

Anaplerosis and the associated mitochondrial metabolite transporters generate unique cytosolic metabolic signaling molecules that can regulate insulin release from pancreatic β-cells. It has been shown that mitochondrial metabolites, transported by the citrate carrier (CIC), dicarboxylate carrier (DIC), oxoglutarate carrier (OGC), and mitochondrial pyruvate carrier (MPC) play a vital role in the regulation of glucose-stimulated insulin secretion (GSIS). Metabolomic studies on static and biphasic insulin secretion, suggests that several anaplerotic derived metabolites, including α-ketoglutarate (αKG), are strongly associated with nutrient regulated insulin secretion. Support for a role of αKG in the regulation of insulin secretion comes from studies looking at αKG dependent enzymes, including hypoxia-inducible factor-prolyl hydroxylases (PHDs) in clonal β-cells, and rodent and human islets. This review will focus on the possible link between defective anaplerotic-derived αKG, PHDs, and the development of type 2 diabetes (T2D).

GPCRs, G Proteins, and Their Impact on β-cell Function

Glucose-induced (physiological) insulin secretion from the islet β-cell involves interplay between cationic (i.e., changes in intracellular calcium) and metabolic (i.e., generation of hydrophobic and hydrophilic second messengers) events. A large body of evidence affirms support for novel regulation, by G proteins, of specific intracellular signaling events, including actin cytoskeletal remodeling, transport of insulin-containing granules to the plasma membrane for fusion, and secretion of insulin into the circulation. This article highlights the following aspects of GPCR-G protein biology of the islet. First, it overviews our current understanding of the identity of a wide variety of G protein regulators and their modulatory roles in GPCR-G protein-effector coupling, which is requisite for optimal β-cell function under physiological conditions. Second, it describes evidence in support of novel, noncanonical, GPCR-independent mechanisms of activation of G proteins in the islet. Third, it highlights the evidence indicating that abnormalities in G protein function lead to islet β-cell dysregulation and demise under the duress of metabolic stress and diabetes. Fourth, it summarizes observations of potential beneficial effects of GPCR agonists in preventing/halting metabolic defects in the islet β-cell under various pathological conditions (e.g., metabolic stress and inflammation). Lastly, it identifies knowledge gaps and potential avenues for future research in this evolving field of translational islet biology. Published 2020. Compr Physiol 10:453-490, 2020.

Goldfish adipocytes are pancreatic beta cell-like, glucose-responsive insulin-producing cells

Glucose homeostasis plays a key role in maintaining stable physiological conditions, and its dysfunction causes severe chronic health issues including diabetes. In this study, we have characterized goldfish adipocytes as cells with properties similar to that of pancreatic β-cells: they express considerable high levels of preproinsulin mRNAs, possess the necessary machinery for processing preproinsulin (prohormone convertases 1 and 2, carboxypeptidase E and trypsin) and responding to extracellular glucose (glucokinase and the glucose transporters 1, 2, and 4), produce insulin in a glucose-responsive manner and express key transcription factors typically involved in pancreas development (Pdx1, Neurogenin3, Nkx2.2, Pax6, and FOXO1A). These findings reinforce the feature of fish adipocytes as alternate sources of active insulin, holding the promise that they could eventually be developed as transplantable sources of this vital hormone.

Treatments for diabetes mellitus type II: New perspectives regarding the possible role of calcium and cAMP interaction

Diabetes mellitus (DM) is among the top ten causes of death worldwide. It is considered to be one of the major global epidemics of the 21st century, with a significant impact on public health budgets. DM is a metabolic disorder with multiple etiologies. Its pathophysiology is marked by dysfunction of pancreatic β-cells which compromises the synthesis and secretion of insulin along with resistance to insulin action in peripheral tissues (muscle and adipose). Subjects presenting insulin resistance in DM type 2 often also exhibit increased insulin secretion and hyperinsulinemia. Insulin secretion is controlled by several factors such as nutrients, hormones, and neural factors. Exocytosis of insulin granules has, as its main stimulus, increased intracellular calcium ([Ca⁺²]i) and it is further amplified by cyclic AMP (cAMP). In the event of this hyperfunction, it is very common for β-cells to go into exhaustion leading to failure or death. Several animal studies have demonstrated pleiotropic effects of L-type Ca²⁺ channel blockers (CCBs). In animal models of obesity and diabetes, treatment with CCBs promoted restoration of insulin secretion, glycemic control, and reduction of pancreatic β-cell apoptosis. In addition, hypertensive individuals treated with CCBs presented a lower incidence of DM when compared with other antihypertensive agents. In this review, we propose that pharmacological manipulation of the Ca²⁺/cAMP interaction system could lead to important targets for pharmacological improvement of insulin secretion in DM type 2.

Cation-Independent Mannose 6-Phosphate Receptor Deficiency Enhances β-Cell Susceptibility to Palmitate

Palmitate attenuates insulin secretion and reduces the viability of insulin-producing cells. Previous studies identified the aberrant palmitoylation or mispalmitoylation of proteins as one mechanism by which palmitate causes β-cell damage. In this report, we identify a role for lysosomal protein degradation as a mechanism by which β cells defend themselves against excess palmitate. The cation-independent mannose 6-phosphate receptor (CI-MPR) is responsible for the trafficking of mannose 6-phosphate-tagged proteins to lysosomes via Golgi sorting and from extracellular locations through endocytosis. RINm5F cells, which are highly sensitive to palmitate, lack CI-MPR. The reconstitution of CI-MPR expression attenuates the induction of endoplasmic reticulum (ER) stress and the toxic effects of palmitate on RINm5F cell viability. INS832/13 cells express CI-MPR and are resistant to the palmitate-mediated loss of cell viability. The reduction of CI-MPR expression increases the sensitivity of INS832/13 cells to the toxic effects of palmitate treatment. The inhibition of lysosomal acid hydrolase activity by weak base treatment of islets under glucolipotoxic conditions causes islet degeneration that is prevented by the inhibition of protein palmitoylation. These findings indicate that defects in lysosomal function lead to the enhanced sensitivity of insulin-producing cells to palmitate and support a role for normal lysosomal function in the protection of β cells from excess palmitate.

Glucose potentiates β-cell function by inducing Tph1 expression in rat islets

Impaired pancreatic β-cell function is the primary defect in type 2 diabetes. Glucose is an important regulator of β-cell growth and function; however, the mechanisms that are involved in the chronic adaptation of β cells to hyperglycemia remain largely unknown. In the present study, global gene expression patterns revealed that tryptophan hydroxylase 1 (Tph1) was the most profound of genes that are up-regulated in rat islets exposed to high glucose. Calcium and cAMP signals synergistically mediated glucose-stimulated Tph1 transcription in β cells by activating cAMP-responsive element-binding protein and promoting its binding with a Tph1 promoter. Similar to in vitro results, in vivo infusion of high glucose also strongly induced Tph1 expression and serotonin production in rat islets, along with enhanced islet function. Inhibition or knockdown of Tph1 markedly decreased glucose-potentiated insulin secretion. In contrast, overexpression of Tph1 augmented glucose-stimulated insulin secretion in rat islets by up-regulating the expression of genes that are related to islet function. In addition, the long-acting glucagon-like peptide 1 receptor agonist, exendin-4, stimulated Tph1 expression in a glucose-dependent manner. Knockdown of Tph1 inhibited exendin-4-potentiated insulin secretion in rat islets. These findings suggest that Tph1 mediates the compensation of islet function induced by glucose, and that promoting Tph1 expression in pancreatic β cells will provide a new strategy for the treatment of type 2 diabetes mellitus.-Zhang, Y., Deng, R., Yang, X., Xu, W., Liu, Y., Li, F., Zhang, J., Tang, H., Ji, X., Bi, Y., Wang, X., Zhou, L., Ning, G. Glucose potentiates β-cell function by inducing Tph1 expression in rat islets.

Thymoquinone, a bioactive component of Nigella sativa, normalizes insulin secretion from pancreatic β-cells under glucose overload via regulation of malonyl-CoA

Thymoquinone (2-isopropyl-5-methylbenzo-1,4-quinone) is a major bioactive component of Nigella sativa, a plant used in traditional medicine to treat a variety of symptoms, including elevated blood glucose levels in type 2 diabetic patients. Normalization of elevated blood glucose depends on both glucose disposal by peripheral tissues and glucose-stimulated insulin secretion (GSIS) from pancreatic β-cells. We employed clonal β-cells and rodent islets to investigate the effects of thymoquinone (TQ) and Nigella sativa extracts (NSEs) on GSIS and cataplerotic metabolic pathways implicated in the regulation of GSIS. TQ and NSE regulated NAD(P)H/NAD(P)(+) ratios via a quinone-dependent redox cycling mechanism. TQ content was positively correlated with the degree of redox cycling activity of NSE extracts, suggesting that TQ is a major component engaged in mediating NSE-dependent redox cycling. Both acute and chronic exposure to TQ and NSE enhanced GSIS and were associated with the ability of TQ and NSE to increase the ATP/ADP ratio. Furthermore, TQ ameliorated the impairment of GSIS following chronic exposure of β-cells to glucose overload. This protective action was associated with the TQ-dependent normalization of chronic accumulation of malonyl-CoA, elevation of acetyl-CoA carboxylase (ACC), fatty acid synthase, and fatty acid-binding proteins following chronic glucose overload. Together, these data suggest that TQ modulates the β-cell redox circuitry and enhances the sensitivity of β-cell metabolic pathways to glucose and GSIS under normal conditions as well as under hyperglycemia. This action is associated with the ability of TQ to regulate carbohydrate-to-lipid flux via downregulation of ACC and malonyl-CoA.

Diluted serum from calorie-restricted animals promotes mitochondrial β-cell adaptations and protect against glucolipotoxicity

β-cells quickly adjust insulin secretion to oscillations in nutrients carried by the blood, acting as fuel sensors. However, most studies of β-cell responses to nutrients do not discriminate between fuel levels and signaling components present in the circulation. Here we studied the effect of serum from calorie-restricted rats versus serum from rats fed ad libitum, diluted tenfold in the medium, which did not contribute significantly to the pool of nutrients, on β-cell mitochondrial function and dynamics under regular and high-nutrient culture conditions. Insulin secreting beta-cell derived line (INS1) cells incubated with serum from calorie-restricted rats (CR serum) showed higher levels of peroxisome proliferator-activated receptor gamma coactivator 1-α (PGC-1α) and active nitric oxide synthase. The expression of mitofusin-2 (Mfn-2) and optic atrophy 1 (OPA-1), proteins involved in mitochondrial fusion, was increased, while the levels of the mitochondrial fission mediator dynamin related protein 1 (DRP-1) were reduced. Consistent with changes in mitochondrial dynamics protein levels, CR serum treatment increased mitochondrial fusion rates, as well as their length and connectivity. These changes in mitochondrial morphology were associated with prolonged glucose-stimulated insulin secretion and mitochondrial respiration. When combining CR serum and high levels of glucose and palmitate (20 and 0.4 mm, respectively), an in vitro model of type II diabetes, we observed that signaling promoted by CR serum was enough to overcome glucolipotoxicity, as indicated by CR-mediated prevention of mitochondrial fusion arrest and reduced respiratory function in INS1 cells under glucolipotoxicity. Overall, our results provide evidence that non-nutrient factors in serum have a major impact on β-cell mitochondrial adaptations to changes in metabolism.

Calcium modulation of exocytosis-linked plasma membrane potential oscillations in INS-1 832/13 cells

Oscillations in plasma membrane potential initiated by substrate-dependent blockade of ATP-sensitive K+ channels in insulin-secreting INS-1 832/13 are differentially linked to distinct voltage-activated Ca2+ channels and drive exocytosis. Ca2+ feeds back to control oscillation frequency, amplitude and prevalence.

Potential role of skeletal muscle glucose metabolism on the regulation of insulin secretion

Pancreatic beta cells sense glucose flux and release as much insulin as required in order to maintain glycaemia within a narrow range. Insulin secretion is regulated by many factors including glucose, incretins, and sympathetic and parasympathetic tones among other physiological factors. To identify the mechanisms linking obesity-related insulin resistance with impaired insulin secretion represents a central challenge. Recently, it has been argued that a crosstalk between skeletal muscle and the pancreas may regulate insulin secretion. Considering that skeletal muscle is the largest organ in non-obese subjects and a major site of insulin- and exercise-stimulated glucose disposal, it appears plausible that muscle might interact with the pancreas and modulate insulin secretion for appropriate peripheral intracellular glucose utilization. There is growing evidence that muscle can secrete so-called myokines that can have auto/para/endocrine actions. Although it is unclear in which direction they act, interleukin-6 seems to be a possible muscle-derived candidate protein mediating such inter-organ communication. We herein review some of the putative skeletal muscle-derived factors mediating this interaction. In addition, the evidence coming from in vitro, animal and human studies that support such inter-organ crosstalk is thoroughly discussed.

Control of insulin secretion by cytochrome C and calcium signaling in islets with impaired metabolism

The aim of the study was to assess the relative control of insulin secretion rate (ISR) by calcium influx and signaling from cytochrome c in islets where, as in diabetes, the metabolic pathways are impaired. This was achieved either by culturing isolated islets at low (3 mm) glucose or by fasting rats prior to the isolation of the islets. Culture in low glucose greatly reduced the glucose response of cytochrome c reduction and translocation and ISR, but did not affect the response to the mitochondrial fuel α-ketoisocaproate. Unexpectedly, glucose-stimulated calcium influx was only slightly reduced in low glucose-cultured islets and was not responsible for the impairment in glucose-stimulated ISR. A glucokinase activator acutely restored cytochrome c reduction and translocation and ISR, independent of effects on calcium influx. Islets from fasted rats had reduced ISR and cytochrome c reduction in response to both glucose and α-ketoisocaproate despite normal responses of calcium. Our data are consistent with the scenario where cytochrome c reduction and translocation are essential signals in the stimulation of ISR, the loss of which can result in impaired ISR even when calcium response is normal.

Assessment of the metabolic pathways associated with glucose-stimulated biphasic insulin secretion

Biphasic glucose-stimulated insulin secretion involves a rapid first phase followed by a prolonged second phase of insulin secretion. The biochemical pathways that control these 2 phases of insulin secretion are poorly defined. In this study, we used a gas chromatography mass spectroscopy-based metabolomics approach to perform a global analysis of cellular metabolism during biphasic insulin secretion. A time course metabolomic analysis of the clonal β-cell line 832/13 cells showed that glycolytic, tricarboxylic acid, pentose phosphate pathway, and several amino acids were strongly correlated to biphasic insulin secretion. Interestingly, first-phase insulin secretion was negatively associated with L-valine, trans-4-hydroxy-L-proline, trans-3-hydroxy-L-proline, DL-3-aminoisobutyric acid, L-glutamine, sarcosine, L-lysine, and thymine and positively with L-glutamic acid, flavin adenine dinucleotide, caprylic acid, uridine 5'-monophosphate, phosphoglycerate, myristic acid, capric acid, oleic acid, linoleic acid, and palmitoleic acid. Tricarboxylic acid cycle intermediates pyruvate, α-ketoglutarate, and succinate were positively associated with second-phase insulin secretion. Other metabolites such as myo-inositol, cholesterol, DL-3-aminobutyric acid, and L-norleucine were negatively associated metabolites with the second-phase of insulin secretion. These studies provide a detailed analysis of key metabolites that are either negatively or positively associated with biphasic insulin secretion. The insights provided by these data set create a framework for planning future studies in the assessment of the metabolic regulation of biphasic insulin secretion.

In vitro toxicity and antidiabetic activity of a newly developed polyherbal formulation (MAC-ST/001) in streptozotocin-induced diabetic Wistar rats

The present study was designed to investigate the hypoglycemic effect of an aqueous extract of MAC-ST/001 (a new polyherbal formulation) which was given once daily to rats at different doses. The animals were divided into diabetic and nondiabetic control groups. The duration of each experiment lasted from 1 week to 1 month, and the results were compared with that of the standard hypoglycemic drug glibenclamide (10 mg/kg), which was given once daily. In this study, biochemical and histopathological parameters were studied in streptozotacin (STZ) (single intraperitoneal injection of 55 mg/kg)-induced diabetic rats. The diabetic rats showed a significant (p < 0.05 and p < 0.01) decrease in their body weight and serum amylase with marked elevation in blood glucose, serum cholesterol, blood urea nitrogen, creatinine, alkaline phosphatase, and serum transaminases (AST and ALT) after 1 week till the 28th day of diabetes. Cytotoxicity of MAC-ST/001 formulation was also studied on C2C12, 3T3-L1, and HepG2 cells through MTT assay. Histological examination of the liver and pancreas of normal control, diabetic control, and drug-treated rats revealed significant results. Finally, it was concluded that administration of this MAC-ST/001 extract reversed most blood and tissue changes caused by STZ-induced diabetes in rats.

Mitochondrial dynamics in the regulation of nutrient utilization and energy expenditure

Mitochondrial fusion, fission, and mitophagy form an essential axis of mitochondrial quality control. However, quality control might not be the only task carried out by mitochondrial dynamics. Recent studies link mitochondrial dynamics to the balance between energy demand and nutrient supply, suggesting changes in mitochondrial architecture as a mechanism for bioenergetic adaptation to metabolic demands. By favoring either connected or fragmented architectures, mitochondrial dynamics regulates bioenergetic efficiency and energy expenditure. Placement of bioenergetic adaptation and quality control as competing tasks of mitochondrial dynamics might provide a new mechanism, linking excess nutrient environment to progressive mitochondrial dysfunction, common to age-related diseases.

Streptozotocin-nicotinamide-induced diabetes in the rat. Characteristics of the experimental model

Administration of both streptozotocin (STZ) and nicotinamide (NA) has been proposed to induce experimental diabetes in the rat. STZ is well known to cause pancreatic B-cell damage, whereas NA is administered to rats to partially protect insulin-secreting cells against STZ. STZ is transported into B-cells via the glucose transporter GLUT2 and causes DNA damage leading to increased activity of poly(ADP-ribose) polymerase (PARP-1) to repair DNA. However, exaggerated activity of this enzyme results in depletion of intracellular NAD(+) and ATP, and the insulin-secreting cells undergo necrosis. The protective action of NA is due to the inhibition of PARP-1 activity. NA inhibits this enzyme, preventing depletion of NAD(+) and ATP in cells exposed to STZ. Moreover, NA serves as a precursor of NAD(+) and thereby additionally increases intracellular NAD(+) levels. The severity of diabetes in experimental rats strongly depends on the doses of STZ and NA given to these animals. Therefore, in diabetic rats, blood glucose may be changed in a broad range--from slight hyperglycemia to substantial hyperglycemia compared with control animals. Similarly, blood insulin may be only slightly decreased or substantial hypoinsulinemia may be induced. In vitro studies demonstrated that the insulin-secretory response to glucose is attenuated in STZ-NA-induced diabetic rats compared with control animals. This is due to reduced B-cell mass as well as metabolic defects in the insulin-secreting cells. Results of numerous experiments have demonstrated that this model of diabetes is useful in studies of different aspects of diabetes.

Reactive oxygen species stimulate insulin secretion in rat pancreatic islets: studies using mono-oleoyl-glycerol

Chronic exposure (24–72 hrs) of pancreatic islets to elevated glucose and fatty acid leads to glucolipoxicity characterized by basal insulin hypersecretion and impaired glucose-stimulated insulin secretion (GSIS). Our aim was to determine the mechanism for basal hypersecretion of insulin. We used mono-oleoyl-glycerol (MOG) as a tool to rapidly increase lipids in isolated rat pancreatic ß-cells and in the clonal pancreatic ß-cell line INS-1 832/13. MOG (25–400 µM) stimulated basal insulin secretion from ß-cells in a concentration dependent manner without increasing intracellular Ca²⁺ or O₂ consumption. Like GSIS, MOG increased NAD(P)H and reactive oxygen species (ROS). The mitochondrial reductant ß-hydroxybutyrate (ß-OHB) also increased the redox state and ROS production, while ROS scavengers abrogated secretion. Diazoxide (0.4 mM) did not prevent the stimulatory effect of MOG, confirming that the effect was independent of the K_ATP-dependent pathway of secretion. MOG was metabolized to glycerol and long-chain acyl-CoA (LC-CoA), whereas, acute oleate did not similarly increase LC-CoA. Inhibition of diacylglycerol kinase (DGK) did not mimic the effect of MOG on insulin secretion, indicating that MOG did not act primarily by inhibiting DGK. Inhibition of acyl-CoA synthetase (ACS) reduced the stimulatory effect of MOG on basal insulin secretion by 30% indicating a role for LC-CoA. These data suggest that basal insulin secretion is stimulated by increased ROS production, due to an increase in the mitochondrial redox state independent of the established components of GSIS.

Banting lecture 2011: hyperinsulinemia: cause or consequence?

The Banting Medal for Scientific Achievement Award is the American Diabetes Association's highest scientific award and honors an individual who has made significant, long-term contributions to the understanding of diabetes, its treatment, and/or prevention. The award is named after Nobel Prize winner Sir Frederick Banting, who codiscovered insulin treatment for diabetes. Dr. Barbara E. Corkey received the American Diabetes Association's Banting Medal for Scientific Achievement at the Association's 71st Scientific Sessions, 24-28 June 2011, San Diego, California. She presented the Banting Lecture, "Hyperinsulinemia: Cause or Consequence?" on Sunday, 26 June 2011.

Signal integration at the level of ion channel and exocytotic function in pancreatic β-cells

Whole body energy balance is ensured by the exquisite control of insulin secretion, the dysregulation of which has serious consequences. Although a great deal has been learned about the control of insulin secretion from pancreatic β-cells in the past 30 years, there remains much to be understood about the molecular mechanisms and interactions that underlie the precise control of this process. Numerous molecular interactions at the plasma membrane mediate the excitatory and amplifying events involved in insulin secretion; this includes interactions between ion channels, signal transduction machinery, and exocytotic proteins. The present Perspectives article considers evidence that key membrane and membrane-associated proteins essential to insulin secretion are regulated in concert as a functional unit, ensuring an integrated excitatory and exocytotic response to the signals that control insulin secretion.

The level of menadione redox-cycling in pancreatic β-cells is proportional to the glucose concentration: role of NADH and consequences for insulin secretion

Pancreatic β-cells release insulin in response to elevation of glucose from basal (4-7mM) to stimulatory (8-16mM) levels. Metabolism of glucose by the β-cell results in the production of low levels of reactive oxygen intermediates (ROI), such as hydrogen peroxide (H(2)O(2)), a newly recognized coupling factor linking glucose metabolism to insulin secretion. However, high and toxic levels of H(2)O(2) inhibit insulin secretion. Menadione, which produces H(2)O(2) via redox cycling mechanism in a dose-dependent manner, was investigated for its effect on β-cell metabolism and insulin secretion in INS-1 832/13, a rat β-cell insulinoma cell line, and primary rodent islets. Menadione-dependent redox cycling and resulting H(2)O(2) production under stimulatory glucose exceeded several-fold those reached at basal glucose. This was paralleled by a differential effect of menadione (0.1-10μM) on insulin secretion, which was enhanced at basal, but inhibited at stimulatory glucose. Redox cycling of menadione and H(2)O(2) formation was dependent on glycolytically-derived NADH, as inhibition of glycolysis and application of non-glycogenic insulin secretagogues did not support redox cycling. In addition, activity of plasma membrane electron transport, a system dependent in part on glycolytically-derived NADH, was also inhibited by menadione. Menadione-dependent redox cycling was sensitive to the NQO1 inhibitor dicoumarol and the flavoprotein inhibitor diphenylene iodonium, suggesting a role for NQO1 and other oxidoreductases in this process. These data may explain the apparent dichotomy between the stimulatory and inhibitory effects of H(2)O(2) and menadione on insulin secretion.

Specific glucose-induced control of insulin receptor substrate-2 expression is mediated via Ca2+-dependent calcineurin/NFAT signaling in primary pancreatic islet β-cells

OBJECTIVE

Insulin receptor substrate-2 (IRS-2) plays an essential role in pancreatic islet β-cells by promoting growth and survival. IRS-2 turnover is rapid in primary β-cells, but its expression is highly regulated at the transcriptional level, especially by glucose. The aim was to investigate the molecular mechanism on how glucose regulates IRS-2 gene expression in β-cells.

RESEARCH DESIGN AND METHODS

Rat islets were exposed to inhibitors or subjected to adenoviral vector-mediated gene manipulations and then to glucose-induced IRS-2 expression analyzed by real-time PCR and immunoblotting. Transcription factor nuclear factor of activated T cells (NFAT) interaction with IRS-2 promoter was analyzed by chromatin immunoprecipitation assay and glucose-induced NFAT translocation by immunohistochemistry.

RESULTS

Glucose-induced IRS-2 expression occurred in pancreatic islet β-cells in vivo but not in liver. Modulating rat islet β-cell Ca(2+) influx with nifedipine or depolarization demonstrated that glucose-induced IRS-2 gene expression was dependent on a rise in intracellular calcium concentration derived from extracellular sources. Calcineurin inhibitors (FK506, cyclosporin A, and a peptide calcineurin inhibitor [CAIN]) abolished glucose-induced IRS-2 mRNA and protein levels, whereas expression of a constitutively active calcineurin increased them. Specific inhibition of NFAT with the peptide inhibitor VIVIT prevented a glucose-induced IRS-2 transcription. NFATc1 translocation to the nucleus in response to glucose and association of NFATc1 to conserved NFAT binding sites in the IRS-2 promoter were demonstrated.

CONCLUSIONS

The mechanism behind glucose-induced transcriptional control of IRS-2 gene expression specific to the islet β-cell is mediated by the Ca(2+)/calcineurin/NFAT pathway. This insight into the IRS-2 regulation could provide novel therapeutic means in type 2 diabetes to maintain an adequate functional mass.

The central role of calcium in the effects of cytokines on beta-cell function: implications for type 1 and type 2 diabetes

The appropriate regulation of intracellular calcium is a requirement for proper cell function and survival. This review focuses on the effects of proinflammatory cytokines on calcium regulation in the insulin-producing pancreatic beta-cell and how normal stimulus-secretion coupling, organelle function, and overall beta-cell viability are impacted. Proinflammatory cytokines are increasingly thought to contribute to beta-cell dysfunction not only in type 1 diabetes (T1D), but also in the progression of type 2 diabetes (T2D). Cytokine-induced disruptions in calcium handling result in reduced insulin release in response to glucose stimulation. Cytokines can alter intracellular calcium levels by depleting calcium from the endoplasmic reticulum (ER) and by increasing calcium influx from the extracellular space. Depleting ER calcium leads to protein misfolding and activation of the ER stress response. Disrupting intracellular calcium may also affect organelles, including the mitochondria and the nucleus. As a chronic condition, cytokine-induced calcium disruptions may lead to beta-cell death in T1D and T2D, although possible protective effects are also discussed. Calcium is thus central to both normal and pathological cell processes. Because the tight regulation of intracellular calcium is crucial to homeostasis, measuring the dynamics of calcium may serve as a good indicator of overall beta-cell function.

Coupling of metabolic, second messenger pathways and insulin granule dynamics in pancreatic beta-cells: a computational analysis

Insulin secretory responses to nutrient stimuli and hormonal modulators in pancreatic beta-cells are controlled by a variety of secondary messengers. We have analyzed numerous mechanisms responsible for regulated exocytosis in these cells and present an integrated mathematical model of cytosolic Ca²⁺, cAMP and granule dynamics. The insulin-containing granules in the beta-cell were divided into four classes: a large "reserve" granule pool, a smaller pool of the morphologically docked granules that is chemically 'primed' for release or the "readily releasable pool", and a pool of "restless newcomer granules" that undergoes preferential exocytosis. The model incorporates glucose and other aspects of metabolism, the cAMP amplifying pathway, insulin granule dynamics and the exocyst concept for granule binding. The values of most of the model parameters were inferred from available experimental data. The model can generate both the fast first phase and slow biphasic insulin secretion found experimentally in response to a step increase of membrane potential or of glucose. The numerical simulations have also reproduced a variety of experimental conditions, such as periodic stimulation by high K⁺ and the potentiation induced in islets by pre-incubation with cAMP pathway activators. The explicit incorporation of Ca²⁺ channels, Ca²⁺ and cAMP dynamics allows the model to be further connected to current models for calcium and metabolic dynamics and provides an interpretation of the roles of the triggering and amplifying pathways of glucose-stimulated insulin secretion. The model may be important in the identification of pharmacological targets for improving insulin secretion in type 2 diabetes.

Reduced cytochrome C is an essential regulator of sustained insulin secretion by pancreatic islets

Influx of calcium is an essential but insufficient signal in sustained nutrient-stimulated insulin secretion, and increased metabolic rate of the beta cell is also required. The aim of the study was to test the hypothesis that the reduced state of cytochrome c is a metabolic co-factor necessary for insulin secretion, over and above its participation in the ATP-generating function of electron transport/oxidative phosphorylation. We found that nutrient stimulation of insulin secretion by isolated rat islets was strongly correlated with reduced cytochrome c, and agents that acutely and specifically reduced cytochrome c led to increased insulin secretion, even in the face of decreased oxygen consumption and calcium influx. In contrast, neither sites 1 nor 4 of the electron transport chain were both necessary and essential for the stimulation of insulin secretion to occur. Importantly, stimulation of islets with glucose, α-ketoisocaproate, or glyceraldehyde resulted in the appearance of cytochrome c in the cytosol, suggesting a pathway for the regulation of exocytotic machinery by reduction of cytochrome c. The data suggest that the metabolic factor essential for sustained calcium-stimulated insulin secretion to occur is linked to reduction and translocation of cytochrome c.

Influence of catch-up growth on islet function and possible mechanisms in rats

OBJECTIVE

The purpose of this study was to examine how catch-up growth modulated islet function and what the detailed mechanisms were, especially at various stages and in different forms, of catch-up growth.

METHODS

We examined the modulation of islet function during catch-up growth by employing an oral glucose tolerance test and gained some insight into the possible mechanisms involved by measuring general physiologic parameters, pancreatic morphometry, insulin content, and the state of chronic oxidative stress. Correlation analyses were used to assess the correlation of insulin/glucose incremental area ratio to other parameters.

RESULTS

The catch-up growth groups resulted in damage to islet function as shown by an increased insulin/glucose incremental area ratio (P ≤ 0.05), smaller relative area of β-cells (P ≤ 0.05), larger relative area of α-cells (P ≤ 0.05), lower insulin content (P ≤ 0.05), increased nitric oxide and malondialdehyde concentrations, and decreased superoxide dismutase concentration (P ≤ 0.05, respectively). With time these changes became increasingly unmarked.

CONCLUSION

Catch-up growth in different stages and forms induces varying degrees of islet dysfunction, possibly by corresponding changes in general physiologic parameters, pancreatic morphometry, insulin content, and the state of chronic oxidative stress.

Amino acid-induced gene expression profiling in clonal β-cell line INS-1E cells

BACKGROUND

There is abundant evidence that glucotoxicity and lipotoxicity contribute to impaired β-cell function in type 2 diabetes. Interestingly, amino acid (AA) derangement is also a characteristic part of the diabetic state. The acute effects of AA on pancreatic β-cell function have been widely explored; however, to our knowledge, the chronic effects of AA, e.g. proline (Pro), homocysteine (Hcy), and leucine (Leu), on pancreatic β-cell function and integrity have not yet been studied. We aimed to investigate global alterations in β-cell gene expression after long-term exposure of clonal INS-1E cells to elevated level of specific AA in vitro.

METHODS

Global gene expression profiling was performed to characterize genes differently modified by Pro, Hcy, and Leu, respectively, in INS-1E cells.

RESULTS

Gene expression profiling revealed significant changes in INS-1E cell mRNAs involved in the control of several aspects of β-cell function, e.g. epigenetic regulation of gene expression, metabolism, innate and adaptive immune responses, cellular signalling, protein synthesis, apoptosis, and cellular stress response. After 72 h, INS-1E cells were differentially regulated (≥1.5- or ≤ -1.5-fold) by Pro (295 transcripts), Hcy (301 transcripts), and Leu (701 transcripts). It appears that Hcy effects changes opposite to those induced by Leu and/or Pro.

CONCLUSIONS

AA appears to participate in and to influence many physiological processes including those involved in cholesterol metabolism, immune responses, and oxidative phosphorylation. Whether such events promote the β-cell dysfunction and the β-cell failure in diabetes remains to be elucidated. Our data strongly indicate that AA elevation may take part in the progressive development of type 2 diabetes.

The dicarboxylate carrier plays a role in mitochondrial malate transport and in the regulation of glucose-stimulated insulin secretion from rat pancreatic beta cells

AIMS/HYPOTHESIS

We have previously described a strong correlation between pyruvate cycling and insulin secretion. We have also demonstrated a particularly important role for a pyruvate-isocitrate cycling pathway involving the mitochondrial citrate/isocitrate carrier (CIC) and cytosolic NADP-dependent isocitrate dehydrogenase. CIC requires cytosolic malate as a counter-substrate during citrate and isocitrate export. Thus, considering that the mitochondrial dicarboxylate carrier (DIC) provides an important source of cytosolic malate, we investigated the potential role of DIC in control of glucose-stimulated insulin secretion (GSIS).

METHODS

We used pharmacological and small interfering RNA (siRNA) tools to assess the role of DIC in insulin release in clonal insulin-secreting 832/13 cells and isolated rat islets.

RESULTS

Butylmalonate, an inhibitor of malate transport, reduced cytosolic malate and citrate levels, and inhibited GSIS in a dose-dependent manner in 832/13 cells. Suppression of DIC expression resulted in inhibition of GSIS by 5% to 69%, the extent of inhibition of insulin secretion being proportional to the level of Dic (also known as Slc25a10) gene knockdown. The most effective siRNA duplex against Dic did not affect glucose utilisation, glucose oxidation or ATP/ADP ratio, but did suppress glucose-induced increments of the NADPH/NADP(+) ratio. Confirmation of our results in primary cultures of isolated rat islets showed that butylmalonate and an adenovirus expressing an siRNA against Dic-inhibited GSIS.

CONCLUSIONS/INTERPRETATION

Malate transport by DIC may play an important role in GSIS, possibly by providing cytosolic malate as a counter-substrate for citrate and/or isocitrate export by CIC. These studies also suggest that malate transport by DIC is (1) a critical component of NADPH production mediated by pyruvate-cycling and (2) regulates GSIS.

Regulation of glucose- and mitochondrial fuel-induced insulin secretion by a cytosolic protein histidine phosphatase in pancreatic beta-cells

We report localization of a cytosolic protein histidine phosphatase (PHP; approximately 16 kDa) in INS 832/13 cells, normal rat islets, and human islets. siRNA-mediated knockdown of PHP markedly reduced glucose- or mitochondrial fuel-induced but not KCl-induced insulin secretion. siRNA-mediated knockdown of PHP also attenuated mastoparan-induced insulin secretion, suggesting its participation in G protein-sensitive signaling steps, leading to insulin secretion. Functional assays revealed that the beta-cell PHP catalyzes the dephosphorylation of ATP-citrate lyase (ACL). Silencing of PHP expression markedly reduced ACL activity, suggesting functional regulation of ACL by PHP in beta-cells. Coimmunoprecipitation studies revealed modest effects of glucose on the interaction between PHP and ACL. Confocal microscopic evidence indicated that glucose promotes association between ACL and nm23-H1, a known kinase histidine kinase, but not between PHP and ACL. Furthermore, metabolic viability of INS 832/13 cells was resistant to siRNA-PHP, suggesting no regulatory roles of PHP in cell viability. Finally, long-term exposure (24 h) of INS 832/13 cells or rat islets to high glucose (30 mM) increased the expression of PHP. Such increases in PHP expression were also seen in islets derived from the Zucker diabetic fatty rat compared with islets from the lean control animals. Together, these data implicate regulatory roles for PHP in a G protein-sensitive step involved in nutrient-induced insulin secretion. In light of the current debate on putative regulatory roles of ACL in insulin secretion, additional studies are needed to precisely identify the phosphoprotein substrate(s) for PHP in the cascade of events leading to nutrient-induced insulin secretion.

Small G proteins in islet beta-cell function

Glucose-stimulated insulin secretion from the islet beta-cell involves a sequence of metabolic events and an interplay between a wide range of signaling pathways leading to the generation of second messengers (e.g., cyclic nucleotides, adenine and guanine nucleotides, soluble lipid messengers) and mobilization of calcium ions. Consequent to the generation of necessary signals, the insulin-laden secretory granules are transported from distal sites to the plasma membrane for fusion and release of their cargo into the circulation. The secretory granule transport underlies precise changes in cytoskeletal architecture involving a well-coordinated cross-talk between various signaling proteins, including small molecular mass GTP-binding proteins (G proteins) and their respective effector proteins. The purpose of this article is to provide an overview of current understanding of the identity of small G proteins (e.g., Cdc42, Rac1, and ARF-6) and their corresponding regulatory factors (e.g., GDP/GTP-exchange factors, GDP-dissociation inhibitors) in the pancreatic beta-cell. Plausible mechanisms underlying regulation of these signaling proteins by insulin secretagogues are also discussed. In addition to their positive modulatory roles, certain small G proteins also contribute to the metabolic dysfunction and demise of the islet beta-cell seen in in vitro and in vivo models of impaired insulin secretion and diabetes. Emerging evidence also suggests significant insulin secretory abnormalities in small G protein knockout animals, further emphasizing vital roles for these proteins in normal health and function of the islet beta-cell. Potential significance of these experimental observations from multiple laboratories and possible avenues for future research in this area of islet research are highlighted.

Fat accumulation in Caenorhabditis elegans is mediated by SREBP homolog SBP-1

Research into the metabolism of fats may reveal potential targets for developing pharmaceutical approaches to obesity and related disorders. Such research may be limited, however, by the cost and time involved in using mammalian subjects or developing suitable cell lines. To determine whether invertebrates could be used to carry out such research more efficiently, we investigated the ability of Caenorhabditis elegans (C. elegans) to accumulate body fat following the consumption of excess calories and the mechanisms it uses to metabolize fat. C. elegans worms were grown on media containing various sugars and monitored for changes in body fat and expression of sbp-1, a homolog of the mammalian transcription factor SREBP-1c, which facilitates fat storage in mammals. The fat content increased markedly in worms exposed to glucose. In situ analysis of gene expression in transgenic worms carrying the GFP-labeled promoter region of sbp-1 revealed that sbp-1 mRNA was strongly expressed in the intestine. An sbp-1 knockdown caused a reduction in body size, fat storage, and egg-laying activity. RT-PCR analysis revealed a considerable decrease in the expression of fatty acid synthetic genes (including elo-2, fat-2, and fat-5) and a considerable increase of starvation-inducible gene acs-2. Normal egg-laying activity and acs-2 expression were restored on exposure to a polyunsaturated fatty acid. These findings suggest that SBP-1 and SREBP regulate the amount and composition of fat and response to starvation in a similar manner. Thus, C. elegans may be an appropriate subject for studying the metabolism of fats.

Islet protein profiling

Islet protein profiling is defined as generation of extended protein expression data sets from islets or islet cells. Islets from rodent control and animal models of type 1 and type 2 diabetes mellitus and healthy humans and insulin- and glucagon-producing cell lines have been used. Protein profiling entails separation, differential expression determination, identification and expression analysis. Protein/peptide separation is either gel-based or by chromatography. Differential expression is based on comparison of visualized spots/proteins between gels or by sample labelling in gel-free systems. Identification of proteins is made by tryptic fragmentation of proteins, fragment mass determination and mass comparison with protein databases. Analysis of expression data sets interprets the complex protein changes into cellular mechanisms to generate hypotheses. The importance of such protein expression sets to elucidate islet cellular events is evidenced by the observation that only about 50% of the differentially expressed proteins and transcripts showed concordance when measured in parallel. Using protein profiling, different areas related to islet dysfunction in type 1 and type 2 diabetes mellitus have been addressed, including dysfunction induced by elevated levels of glucose and fatty acids and cytokines. Because islets from individuals with type 1 or type 2 diabetes mellitus have not yet been protein profiled, islets from rat (BB-DP) and mouse (NOD, ob/ob, MKR) models of the disease have been used, and mechanisms responsible for islet dysfunction delineated offering avenues of intervention.

The glucose transporter 2 undergoes plasma membrane endocytosis and lysosomal degradation in a secretagogue-dependent manner

In beta-cells of the pancreas, the glucose transporter (GLUT)-2 facilitative glucose transporter protein is localized to the plasma membrane and functions as part of the glucose sensing mechanism for the stimulation of insulin secretion. We observed that expressed GLUT2 protein in the cultured Min6B1 cell line undergoes enhanced endocytosis at high extracellular glucose concentrations that stimulate insulin secretion. Moreover, the internalized GLUT2 protein undergoes rapid degradation induced by chronic high-glucose or arginine stimulation but does not undergo plasma membrane recycling or accumulation in any microscopically apparent intracellular membrane compartment. The rapid degradation of GLUT2 was prevented by lysosomal inhibition (chloroquine) concomitant with the accumulation of GLUT2 in endomembrane structures. In contrast, neither endocytosis nor the lack of internal membrane localized GLUT2 remained completely unaffected by proteosomal inhibition (lactacystin) or an heat shock protein-90 inhibitor (geldanamycin). Moreover, the endocytosis and degradation of GLUT2 was specific for beta-cells because expression of GLUT2 in 3T3L1 adipocytes remained cell surface localized and did not display a rapid rate of degradation. Together, these data demonstrate that hyperglycemia directly affects beta-cell function and activates a trafficking pathway that results in the rapid endocytosis and degradation of the cell surface GLUT2 glucose transporter.

Glucose, regulator of survival and phenotype of pancreatic beta cells

The key role of glucose in regulating insulin release by the pancreatic beta cell population is not only dependent on acute stimulus-secretion coupling mechanisms but also on more long-term influences on beta cell survival and phenotype. Glucose serves as a major survival factor for beta cells via at least three actions: it prevents an oxidative redox state, it suppresses a mitochondrial apoptotic program that is triggered at reduced mitochondrial metabolic activity and it induces genes needed for the cellular responsiveness to glucose and to growth factors. Glucose-regulated pathways may link protein synthetic and proliferative activities, making glucose a permissive factor for beta cell proliferation, in check with metabolic needs. Conditions of inadequate glucose metabolism in beta cells are not only leading to deregulation of acute secretory responses but should also be considered as causes for increased apoptosis and reduced formation of beta cells, and loss of their normal differentiated state.

Role of type Ialpha phosphatidylinositol-4-phosphate 5-kinase in insulin secretion, glucose metabolism, and membrane potential in INS-1 beta-cells

Insulin secretion from beta-cells is regulated by a complex signaling network. Our earlier study has reported that Rac1 participates in glucose- and cAMP-induced insulin secretion probably via maintaining a functional actin structure for recruitment of insulin granules. Type Ialpha phosphatidylinositol-4-phosphate 5-kinase (PIP5K-Ialpha) is a downstream effector of Rac1 and a critical enzyme for synthesis of phosphatidylinositol-4,5-bisphosphate (PIP(2)). By using an RNA interference technique, PIP5K-Ialpha in INS-1 beta-cells could be specifically knocked down by 70-75%. PIP5K-Ialpha knockdown disrupted filamentous actin structure and caused changes in cell morphology. In addition, PIP(2) content in the plasma membrane was reduced and the glucose effect on PIP(2) was abolished but without affecting glucose-induced formation of inositol 1,4,5-trisphosphate. At basal conditions (2.8 mM glucose), PIP5K-Ialpha knockdown doubled insulin secretion, elevated glucose metabolic rate, depolarized resting membrane potential, and raised cytoplasmic free Ca(2+) levels ([Ca(2+)](i)). The total insulin release at high glucose was increased upon PIP5K-Ialpha knockdown. However, the percent increment of insulin secretion by high glucose and forskolin over the basal release was significantly reduced, an effect more apparent on the late phase of insulin secretion. Metabolism and [Ca(2+)](i) rises at high glucose were also attenuated in cells after PIP5K-Ialpha knockdown. In contrast, PIP5K-Ialpha knockdown had no effect on cell growth and viability. Taken together, our data suggest that PIP5K-Ialpha may play an important role in both the proximal and distal steps of signaling cascade for insulin secretion in beta-cells.

Evidence of diminished glucose stimulation and endoplasmic reticulum function in nonoscillatory pancreatic islets

Pulsatility is a fundamental feature of pancreatic islets and a hallmark of hormone secretion. Isolated pancreatic islets endogenously generate rhythms in secretion, metabolic activity, and intracellular calcium ([Ca(2+)](i)) that are important to normal physiological function. Few studies have directly compared oscillatory and nonoscillatory islets to identify possible differences in function. We investigated the hypothesis that the loss of these oscillations is a leading indicator of islet dysfunction by comparing oscillatory and nonoscillatory mouse islets for multiple parameters of function. Nonoscillatory islets displayed elevated basal [Ca(2+)](i) and diminished [Ca(2+)](i) response and insulin secretory response to 3-28 mm glucose stimulation compared with oscillatory islets, suggesting diminished glucose sensitivity. We investigated several possible mechanisms to explain these differences. No differences were observed in mitochondrial membrane potential, estimated ATP-sensitive potassium channel and L-type calcium channel activity, or cell death rates. Nonoscillatory islets, however, showed a reduced response to the sarco(endo)plasmic reticulum calcium ATPase inhibitor thapsigargin, suggesting a disruption in calcium homeostasis in the endoplasmic reticulum (ER) compared with oscillatory islets. The diminished ER calcium homeostasis among nonoscillatory islets was also consistent with the higher cytosolic calcium levels observed in 3 mm glucose. Inducing mild damage with low-dose proinflammatory cytokines reduced islet oscillatory capacity and produced similar effects on glucose-stimulated [Ca(2+)](i), basal [Ca(2+)](i), and thapsigargin response observed among untreated nonoscillatory islets. Our data suggest the loss of oscillatory capacity may be an early indicator of diminished islet glucose sensitivity and ER dysfunction, suggesting targets to improve islet assessment.

A role for von Hippel-Lindau protein in pancreatic beta-cell function

OBJECTIVE

The Vhlh gene codes for the von Hippel-Lindau protein (VHL), a tumor suppressor that is a key player in the cellular response to oxygen sensing. In humans, a germline mutation in the VHL gene leads to the von Hippel-Lindau disease, a familial syndrome characterized by benign and malignant tumors of the kidney, central nervous system, and pancreas.

RESEARCH DESIGN AND METHODS

We use Cre-lox recombination to eliminate Vhlh in adult mouse pancreatic beta-cells. Morphology of mutant islets is assessed by immunofluorescence analysis. To determine the functional state of Vhlh(-/-) islets, insulin secretion is measured in vivo and in vitro, and quantitative PCR is used to identify changes in gene expression.

RESULTS

Loss of VHL in beta-cells leads to a severe glucose-intolerant phenotype in adult animals. Although VHL is not required for beta-cell specification and development, it is critical for beta-cell function. Insulin production is normal in beta-cells lacking VHL; however, insulin secretion in the presence of high concentrations of glucose is impaired. Furthermore, the loss of VHL leads to dysregulation of glycolytic enzymes, pointing to a perturbation of the intracellular energy homeostasis.

CONCLUSIONS

We show that loss of VHL in beta-cells leads to defects in glucose homeostasis, indicating an important and previously unappreciated role for VHL in beta-cell function. We believe that the beta-cell-specific Vhlh-deficient mice might be a useful tool as a "genetic hypoxia" model, to unravel the possible link between hypoxia signaling and impairment of beta-cell function.

Prolonged L-alanine exposure induces changes in metabolism, Ca2+ handling and desensitization of insulin secretion in clonal pancreatic β-cells

Acute insulin-releasing actions of amino acids have been studied in detail, but comparatively little is known about the β-cell effects of long-term exposure to amino acids. The present study examined the effects of prolonged exposure of β-cells to the metabolizable amino acid L-alanine. Basal insulin release or cellular insulin content were not significantly altered by alanine culture, but acute alanine-induced insulin secretion was suppressed by 74% (P<0.001). Acute stimulation of insulin secretion with glucose, KCl or KIC (2-oxoisocaproic acid) following alanine culture was not affected. Acute alanine exposure evoked strong cellular depolarization after control culture, whereas AUC (area under the curve) analysis revealed significant (P<0.01) suppression of this action after culture with alanine. Compared with control cells, prior exposure to alanine also markedly decreased (P<0.01) the acute elevation of [Ca2+]i (intracellular [Ca2+]) induced by acute alanine exposure. These diminished stimulatory responses were partially restored after 18 h of culture in the absence of alanine, indicating reversible amino-acid-induced desensitization. 13C NMR spectra revealed that alanine culture increased glutamate labelling at position C4 (by 60%; P<0.01), as a result of an increase in the singlet peak, indicating increased flux through pyruvate dehydrogenase. Consistent with this, protein expression of the pyruvate dehydrogenase kinases PDK2 and PDK4 was significantly reduced. This was accompanied by a decrease in cellular ATP (P<0.05), consistent with diminished insulin-releasing actions of this amino acid. Collectively, these results illustrate the phenomenon of β-cell desensitization by amino acids, indicating that prolonged exposure to alanine can induce reversible alterations to metabolic flux, Ca2+ handling and insulin secretion.

Differential modulation of L-type calcium channel subunits by oleate

Nonesterified fatty acids such as oleate and palmitate acutely potentiate insulin secretion from pancreatic islets in a glucose-dependent manner. In addition, recent studies show that fatty acids elevate intracellular free Ca(2+) and increase voltage-gated Ca(2+) current in mouse beta-cells, although the mechanisms involved are poorly understood. Here we utilized a heterologous system to express subunit-defined voltage-dependent L-type Ca(2+) channels (LTCC) and demonstrate that beta-cell calcium may increase in part from an interaction between fatty acid and specific calcium channel subunits. Distinct functional LTCC were assembled in both COS-7 and HEK-293 cells by expressing either one of the EYFP-tagged L-type alpha(1)-subunits (beta-cell Cav1.3 or lung Cav1.2) and ERFP-tagged islet beta-subunits (ibeta(2a) or ibeta(3)). In COS-7 cells, elevations in intracellular Ca(2+) mediated by LTCC were enhanced by an oleate-BSA complex. To extend these findings, Ca(2+) current was measured in LTCC-expressing HEK-293 cells that revealed an increase in peak Ca(2+) current within 2 min after addition of the oleate complex, with maximal potentiation occurring at voltages <0 mV. Both Cav1.3 and Cav1.2 were modulated by oleate, and the presence of different auxiliary beta-subunits resulted in differential augmentation. The potentiating effect of oleate on Cav1.2 was abolished by the pretreatment of cells with triacsin C, suggesting that long-chain CoA synthesis is necessary for Ca(2+) channel modulation. These results show for the first time that two L-type Ca(2+) channels expressed in beta-cells (Cav1.3 and Cav1.2) appear to be targeted by nonesterified fatty acids. This effect may account in part for the acute potentiation of glucose-dependent insulin secretion by fatty acids.

Capillary LC-MS for high sensitivity metabolomic analysis of single islets of Langerhans

Reversed-phase, packed capillary liquid chromatography interfaced by electrospray ionization to mass spectrometry was explored as an analytical method for determination of metabolites in microscale tissue samples using single islets of Langerhans as a model system. With the use of a 75 microm inner diameter column coupled to a quadrupole ion trap mass spectrometer in full scan mode, detection limits of 0.1-33 fmol were achieved for glycoloytic and tricarboxylic acid cycle metabolites. Reproducible processing of islets for analysis with little loss of metabolites was performed by rapid freezing followed by methanol-water extraction. The method yielded 20 microL of extract of which just 15 nL was injected suggesting the potential for performing multiple assays on the same islet. Approximately 200 presumed metabolites could be detected, of which 22 were identified by matching retention times and MS/MS spectra to standards. Relative standard deviations for peak detection was from 7 to 18% and was unaffected by storage for up to 11 days. The method was used to detect changes in metabolism associated with increasing extracellular islet glucose concentration from 3 to 20 mM yielding results largely consistent with known metabolism of islets. Because most previous studies of islet metabolism have only observed a few compounds at once and require far more tissue, this measurement method represents a significant advance for studies of metabolism of islets and other microscale samples.

PKCepsilon mediates glucose-regulated insulin production in pancreatic beta-cells

Endocrine cells produce large amounts of one or more peptides. The post-translational control of selective production of a single protein is often unknown. We used 3 unrelated approaches to diminish PKCepsilon in rat islets to evaluate its role in preferential glucose-mediated insulin production. Transfection with siRNA (siR-PKCepsilon) or expression of inactive PKCepsilon (PKCepsilon-KD) resulted in a significant reduction in insulin response to glucose (16.7 mmol/l). Glucose stimulation resulted in concentration of PKCepsilon in the perinuclear region, an area known to be rich in ER-Golgi systems, associated with insulin-containing structures. ss'COP1 (RACK2) is the anchoring protein for PKCepsilon. Glucose-stimulated proinsulin production was diminished by 50% in islets expressing PKCepsilon-KD, and 60% in islets expressing RACK2 binding protein (epsilonV1-2); total protein biosynthesis was not affected. In islets expressing epsilonV1-2, a chase period following glucose stimulus resulted in a reduced proinsulin conversion to mature insulin. We propose that PKCepsilon plays a specific role in mediating the glucose-signal into insulin production: binding to ss'COP1 localizes the activated enzyme to the RER where it modulates the shuttling of proinsulin to the TGN. Subsequently the enzyme may be involved in anterograde trafficking of the prohormone or in its processing within the TGN.

The efficiency of cellular energy transduction and its implications for obesity

We assess the existence, mechanism, and functions of less-than-maximal coupling efficiency of mitochondrial oxidative phosphorylation and its potential as a target for future antiobesity interventions. Coupling efficiency is the proportion of oxygen consumption used to make adenosine triphosphate (ATP) and do useful work. High coupling efficiency may lead to fat deposition; low coupling efficiency to a decrease in fat stores. We review obligatory and facultative energy expenditure and the role of a futile cycle of proton pumping and proton leak across the mitochondrial inner membrane in dissipating energy. Basal proton conductance is catalyzed primarily by the adenine nucleotide translocase but can be mimicked by chemical uncouplers. Inducible proton conductance is catalyzed by specific uncoupling proteins. We discuss the opportunities and pitfalls of targeting these processes as a treatment for obesity by decreasing coupling efficiency and increasing energy expenditure, either directly or through central mechanisms of energy homeostasis.

Emerging roles for protein histidine phosphorylation in cellular signal transduction: lessons from the islet beta-cell

Protein phosphorylation represents one of the key regulatory events in physiological insulin secretion from the islet β-cell. In this context, several classes of protein kinases (e.g. calcium-, cyclic nucleotide- and phospholipid-dependent protein kinases and tyrosine kinases) have been characterized in the β-cell. The majority of phosphorylated amino acids identified include phosphoserine, phosphothreonine and phosphotyrosine. Protein histidine phosphorylation has been implicated in the prokaryotic and eukaryotic cellular signal transduction. Most notably, phoshohistidine accounts for 6% of total protein phosphorylation in eukaryotes, which makes it nearly 100-fold more abundant than phosphotyrosine, but less abundant than phosphoserine and phosphothreonine. However, very little is known about the number of proteins with phosphohistidines, since they are highly labile and are rapidly lost during phosphoamino acid identification under standard experimental conditions. The overall objectives of this review are to: (i) summarize the existing evidence indicating the subcellular distribution and characterization of various histidine kinases in the islet β-cell, (ii) describe evidence for functional regulation of these kinases by agonists of insulin secretion, (iii) present a working model to implicate novel regulatory roles for histidine kinases in the receptor-independent activation, by glucose, of G-proteins endogenous to the β-cell, (iv) summarize evidence supporting the localization of protein histidine phosphatases in the islet β-cell and (v) highlight experimental evidence suggesting potential defects in the histidine kinase signalling cascade in islets derived from the Goto-Kakizaki (GK) rat, a model for type 2 diabetes. Potential avenues for future research to further decipher regulatory roles for protein histidine phosphorylation in physiological insulin secretion are also discussed.

Amino acid metabolism, insulin secretion and diabetes

In addition to the primary stimulus of glucose, specific amino acids may acutely and chronically regulate insulin secretion from pancreatic beta-cells in vivo and in vitro. Mitochondrial metabolism is crucial for the coupling of glucose, alanine, glutamine and glutamate recognition with exocytosis of insulin granules. This is illustrated by in vitro and in vivo observations discussed in the present review. Mitochondria generate ATP (the main coupling messenger in insulin secretion) and other factors that serve as sensors for the control of the exocytotic process. The main factors that mediate the key amplifying pathway over the Ca(2+) signal in nutrient-stimulated insulin secretion are nucleotides (ATP, GTP, cAMP and NADPH), although metabolites have also been proposed, such as long-chain acyl-CoA derivatives and glutamate. In addition, after chronic exposure, specific amino acids may influence gene expression in the beta-cell, which have an impact on insulin secretion and cellular integrity. Therefore amino acids may play a direct or indirect (via generation of putative messengers of mitochondrial origin) role in insulin secretion.

Diminished phosphodiesterase-8B potentiates biphasic insulin response to glucose

cAMP activates multiple signal pathways, crucial for the pancreatic beta-cells function and survival and is a major potentiator of insulin release. A family of phosphodiesterases (PDEs) terminate the cAMP signals. We examined the expression of PDEs in rat beta-cells and their role in the regulation of insulin response. Using RT-PCR and Western blot analyses, we identified PDE3A, PDE3B, PDE4B, PDE4D, and PDE8B in rat islets and in INS-1E cells and several possible splice variants of these PDEs. Specific depletion of PDE3A with small interfering (si) RNA (siPDE3A) led to a small (67%) increase in the insulin response to glucose in INS-1E cells but not rat islets. siPDE3A had no effect on the glucagon-like peptide-1 (10 nmol/liter) potentiated insulin response in rat islets. Depletion in PDE8B levels in rat islets using similar technology (siPDE8B) increased insulin response to glucose by 70%, the potentiation being of similar magnitude during the first and second phase insulin release. The siPDE8B-potentiated insulin response was further increased by 23% when glucagon-like peptide-1 was included during the glucose stimulus. In conclusion, PDE8B is expressed in a small number of tissues unrelated to glucose or fat metabolism. We propose that PDE8B, an 3-isobutyl-1-methylxanthine-insensitive cAMP-specific phosphodiesterase, could prove a novel target for enhanced insulin response, affecting a specific pool of cAMP involved in the control of insulin granule trafficking and exocytosis. Finally, we discuss evidence for functional compartmentation of cAMP in pancreatic beta-cells.