Diabetogenic milieus induce specific changes in mitochondrial transcriptome and differentiation of human pancreatic islets

Physiological Fatty Acid-Stimulated Insulin Secretion and Redox Signaling Versus Lipotoxicity

Significance: Type 2 diabetes as a world-wide epidemic is characterized by the insulin resistance concomitant to a gradual impairment of β-cell mass and function (prominently declining insulin secretion) with dysregulated fatty acids (FAs) and lipids, all involved in multiple pathological development. Recent Advances: Recently, redox signaling was recognized to be essential for insulin secretion stimulated with glucose (GSIS), branched-chain keto-acids, and FAs. FA-stimulated insulin secretion (FASIS) is a normal physiological event upon postprandial incoming chylomicrons. This contrasts with the frequent lipotoxicity observed in rodents. Critical Issues: Overfeeding causes FASIS to overlap with GSIS providing repeating hyperinsulinemia, initiates prediabetic states by lipotoxic effects and low-grade inflammation. In contrast the protective effects of lipid droplets in human β-cells counteract excessive lipids. Insulin by FASIS allows FATP1 recruitment into adipocyte plasma membranes when postprandial chylomicrons come late at already low glycemia. Future Directions: Impaired states of pancreatic β-cells and peripheral organs at prediabetes and type 2 diabetes should be revealed, including the inter-organ crosstalk by extracellular vesicles. Details of FA/lipid molecular physiology are yet to be uncovered, such as complex phenomena of FA uptake into cells, postabsorptive inactivity of G-protein-coupled receptor 40, carnitine carrier substrate specificity, the role of carnitine-O-acetyltransferase in β-cells, and lipid droplet interactions with mitochondria. Antioxid. Redox Signal. 42, 566-622.

Effects of different hypoglycaemic drugs on beta-cell function in type 2 diabetes mellitus: a systematic review and network meta-analysis

AIM

This study compared the effects of hypoglycaemic drugs on beta-cell function among type 2 diabetes mellitus (T2DM) patients through a network meta-analysis of randomized controlled trials (RCTs).

METHODS

We searched the PubMed, EMBASE, and Cochrane Library databases for RCTs of different hypoglycaemic drugs as T2DM treatment from database inception to December 1, 2024. The primary outcome was homeostasis model assessment-β (HOMA-β), and the secondary outcome was glycated haemoglobin (HbA1c). Direct and indirect evidence types were combined to calculate weighted mean difference (WMD) and 95% confidence interval (CI) values for the change in (△) HOMA-β and △HbA1c, and to determine surface under the cumulative ranking curve (SUCRA) values.

RESULTS

A total of 58 RCTs involving 16,345 T2DM patients were incorporated into this network meta-analysis. The mean patient age was 66.70 years, and 54.14% were male. For improving HOMA-β, the top treatments were glimepiride + rosiglitazone (WMD = 81.83, 95% CI 45.85-117.82) and glibenclamide + rosiglitazone (WMD = 79.51, 95% CI 40.66-118.36). Acarbose (WMD = 60.90, 95% CI 27.56-94.25) ranked third as monotherapy. For reducing HbA1c, glibenclamide + rosiglitazone was the most efficacious treatment (WMD = - 2.48, 95% CI - 3.67 to - 1.29), followed by metformin + exenatide (WMD = - 1.77, 95% CI - 2.25 to - 1.29) and liraglutide (WMD = - 1.77, 95% CI - 2.33 to - 1.21). The treatment with the highest SUCRA value for HOMA-β improvement was glimepiride + rosiglitazone (95.1%), followed by glibenclamide + rosiglitazone (94.9%). For HbA1c improvement, glibenclamide + rosiglitazone had the highest SUCRA value (97.6%).

CONCLUSIONS

The combination of glimepiride/glibenclamide and rosiglitazone was the most effective hypoglycaemic regimen for protecting beta-cell function and improving glycaemic control in T2DM treatment, possibly due to control of HbA1c and glycotoxicity.

Lipid Dynamics in Pancreatic β-Cells: Linking Physiology to Diabetes Onset

Significance: Glucose-induced lipid metabolism is essential for preserving functional β-cells, and its disruption is linked to type 2 diabetes (T2D) development. Lipids are an integral part of the cells playing an indispensable role as structural components, energy storage molecules, and signals. Recent Advances: Glucose presence significantly impacts lipid metabolism in β-cells, where fatty acids are primarily synthesized de novo and/or are transported from the bloodstream. This process is regulated by the glycerolipid/free fatty acid cycle, which includes lipogenic and lipolytic reactions producing metabolic coupling factors crucial for insulin secretion. Disrupted lipid metabolism involving oxidative stress and inflammation is a hallmark of T2D. Critical Issues: Lipid metabolism in β-cells is complex involving multiple simultaneous processes. Exact compartmentalization and quantification of lipid metabolism and its intermediates, especially in response to glucose or chronic hyperglycemia, are essential. Current research often uses non-physiological conditions, which may not accurately reflect in vivo situations. Future Directions: Identifying and quantifying individual steps and their signaling, including redox, within the complex fatty acid and lipid metabolic pathways as well as the metabolites formed during acute versus chronic glucose stimulation, will uncover the detailed mechanisms of glucose-stimulated insulin secretion. This knowledge is crucial for understanding T2D pathogenesis and identifying pharmacological targets to prevent this disease. Antioxid. Redox Signal. 41, 865-889.

Role of fatty acids in the pathogenesis of ß-cell failure and Type-2 diabetes

Pancreatic ß-cells are glucose sensors in charge of regulated insulin delivery to the organism, achieving glucose homeostasis and overall energy storage. The latter function promotes obesity when nutrient intake chronically exceeds daily expenditure. In case of ß-cell failure, such weight gain may pave the way for the development of Type-2 diabetes. However, the causal link between excessive body fat mass and potential degradation of ß-cells remains largely unknown and debated. Over the last decades, intensive research has been conducted on the role of lipids in the pathogenesis of ß-cells, also referred to as lipotoxicity. Among various lipid species, the usual suspects are essentially the non-esterified fatty acids (NEFA), in particular the saturated ones such as palmitate. This review describes the fundamentals and the latest advances of research on the role of fatty acids in ß-cells. This includes intracellular pathways and receptor-mediated signaling, both participating in regulated glucose-stimulated insulin secretion as well as being implicated in ß-cell dysfunction. The discussion extends to the contribution of high glucose exposure, or glucotoxicity, to ß-cell defects. Combining glucotoxicity and lipotoxicity results in the synergistic and more deleterious glucolipotoxicity effect. In recent years, alternative roles for intracellular lipids have been uncovered, pointing to a protective function in case of nutrient overload. This requires dynamic storage of NEFA as neutral lipid droplets within the ß-cell, along with active glycerolipid/NEFA cycle allowing subsequent recruitment of lipid species supporting glucose-stimulated insulin secretion. Overall, the latest studies have revealed the two faces of the same coin.

MicroRNA 29 modulates β-cell mitochondrial metabolism and insulin secretion via underlying miR-29-OXPHOS complex pathways

AIM

MicroRNAs (miRNAs) regulate β-cell function, and β-cell mitochondria and insulin secretion are perturbed in diabetes. We aimed to identify key miRNAs regulating β-cell mitochondrial metabolism and novel β-cell miRNA-mitochondrial pathways.

METHODS

TargetScan (http://www.targetscan.org/) was used to predict if 16 miRNAs implicated in β-cell function target 27 cis-eGenes implicated in mitochondrial activity. The expression of candidate miRNAs and insulin secretion after 24 and 1 h pre-incubation in 2.8, 11.1- and 16.7-mM glucose was measured in clonal INS-1 832/13 β-cells. MiR-29 silenced INS-1 832/13 cells were assessed for insulin secretion (glucose, pyruvate, and K+), target cis-eGene expression (Ndufv3 and Ndufa10 components of mitochondrial complex I (CI)), OXPHOS (CI-V) protein expression, and mitochondrial OXPHOS respiration/activity. The expression of differentially expressed miR-29 miRNAs was evaluated in Goto-Kakizaki (GK) rat, db/db mouse and type 2 diabetic (T2D) human islets, as well as NMRI mouse islets cultured under glucolipotoxic conditions.

RESULTS

MiR-29, miR-15 and miR-124 were predicted to regulate ~20 cis-eGenes, while miR-29 alone was predicted to regulate ≥12 of these in rat and human species. MiR-29 expression and insulin secretion were reduced in INS-1 832/13 cells after 24 h in elevated glucose. MiR-29 knockdown increased all tested insulin secretory responses, Nudfv3, Ndufa10, complex I and II expression, and cellular mitochondrial OXPHOS. MiR-29 expression was reduced in db/db islets but increased in GK rat and T2D human islets.

CONCLUSION

We conclude miR-29 is a key miRNA in regulating β-cell mitochondrial metabolism and insulin secretion via underlying miR-29-OXPHOS complex pathways. Furthermore, we infer reduced miR-29 expression compensatorily enhances insulin secretion under glucotoxicity.

Mitochondrial Dysfunction, Oxidative Stress, and Inter-Organ Miscommunications in T2D Progression

Type 2 diabetes (T2D) is a heterogenous disease, and conventionally, peripheral insulin resistance (IR) was thought to precede islet β-cell dysfunction, promoting progression from prediabetes to T2D. New evidence suggests that T2D-lean individuals experience early β-cell dysfunction without significant IR. Regardless of the primary event (i.e., IR vs. β-cell dysfunction) that contributes to dysglycemia, significant early-onset oxidative damage and mitochondrial dysfunction in multiple metabolic tissues may be a driver of T2D onset and progression. Oxidative stress, defined as the generation of reactive oxygen species (ROS), is mediated by hyperglycemia alone or in combination with lipids. Physiological oxidative stress promotes inter-tissue communication, while pathological oxidative stress promotes inter-tissue mis-communication, and new evidence suggests that this is mediated via extracellular vesicles (EVs), including mitochondria containing EVs. Under metabolic-related stress conditions, EV-mediated cross-talk between β-cells and skeletal muscle likely trigger mitochondrial anomalies leading to prediabetes and T2D. This article reviews the underlying molecular mechanisms in ROS-related pathogenesis of prediabetes, including mitophagy and mitochondrial dynamics due to oxidative stress. Further, this review will describe the potential of various therapeutic avenues for attenuating oxidative damage, reversing prediabetes and preventing progression to T2D.

Beta cell lipotoxicity in the development of type 2 diabetes: the need for species-specific understanding

The propensity to develop type 2 diabetes (T2D) is known to have both environmental and hereditary components. In those with a genetic predisposition to T2D, it is widely believed that elevated concentrations of circulatory long-chain fatty acids (LC-FFA) significantly contribute towards the demise of insulin-producing pancreatic β-cells - the fundamental feature of the development of T2D. Over 25 years of research support that LC-FFA are deleterious to β-cells, through a process termed lipotoxicity. However, the work underpinning the theory of β-cell lipotoxicity is mostly based on rodent studies. Doubts have been raised as to whether lipotoxicity also occurs in humans. In this review, we examine the evidence, both in vivo and in vitro, for the pathogenic effects of LC-FFA on β-cell viability and function in humans, highlighting key species differences. In this way, we aim to uncover the role of lipotoxicity in the human pathogenesis of T2D and motivate the need for species-specific understanding.

A Supportive Role of Mesenchymal Stem Cells on Insulin-Producing Langerhans Islets with a Specific Emphasis on The Secretome

Type 1 Diabetes (T1D) is a chronic autoimmune disease characterized by a gradual destruction of insulin-producing β-cells in the endocrine pancreas due to innate and specific immune responses, leading to impaired glucose homeostasis. T1D patients usually require regular insulin injections after meals to maintain normal serum glucose levels. In severe cases, pancreas or Langerhans islet transplantation can assist in reaching a sufficient β-mass to normalize glucose homeostasis. The latter procedure is limited because of low donor availability, high islet loss, and immune rejection. There is still a need to develop new technologies to improve islet survival and implantation and to keep the islets functional. Mesenchymal stem cells (MSCs) are multipotent non-hematopoietic progenitor cells with high plasticity that can support human pancreatic islet function both in vitro and in vivo and islet co-transplantation with MSCs is more effective than islet transplantation alone in attenuating diabetes progression. The beneficial effect of MSCs on islet function is due to a combined effect on angiogenesis, suppression of immune responses, and secretion of growth factors essential for islet survival and function. In this review, various aspects of MSCs related to islet function and diabetes are described.

A Defect in Mitochondrial Complex III but Not in Complexes I or IV Causes Early β-Cell Dysfunction and Hyperglycemia in Mice

Mitochondrial metabolism and oxidative respiration are crucial for pancreatic β-cell function and stimulus secretion coupling. Oxidative phosphorylation (OxPhos) produces ATP and other metabolites that potentiate insulin secretion. However, the contribution of individual OxPhos complexes to β-cell function is unknown. We generated β-cell-specific, inducible OxPhos complex knock-out (KO) mouse models to investigate the effects of disrupting complex I, complex III, or complex IV on β-cell function. Although all KO models had similar mitochondrial respiratory defects, complex III caused early hyperglycemia, glucose intolerance, and loss of glucose-stimulated insulin secretion in vivo. However, ex vivo insulin secretion did not change. Complex I and IV KO models showed diabetic phenotypes much later. Mitochondrial Ca2+ responses to glucose stimulation 3 weeks after gene deletion ranged from not affected to severely disrupted, depending on the complex targeted, supporting the unique roles of each complex in β-cell signaling. Mitochondrial antioxidant enzyme immunostaining increased in islets from complex III KO, but not from complex I or IV KO mice, indicating that severe diabetic phenotype in the complex III-deficient mice is causing alterations in cellular redox status. The present study highlights that defects in individual OxPhos complexes lead to different pathogenic outcomes.

ARTICLE HIGHLIGHTS

Mitochondrial metabolism is critical for β-cell insulin secretion, and mitochondrial dysfunction is involved in type 2 diabetes pathogenesis. We determined whether individual oxidative phosphorylation complexes contribute uniquely to β-cell function. Compared with loss of complex I and IV, loss of complex III resulted in severe in vivo hyperglycemia and altered β-cell redox status. Loss of complex III altered cytosolic and mitochondrial Ca2+ signaling and increased expression of glycolytic enzymes. Individual complexes contribute differently to β-cell function. This underscores the role of mitochondrial oxidative phosphorylation complex defects in diabetes pathogenesis.

Small RNAs derived from tRNA fragmentation regulate the functional maturation of neonatal β cells

tRNA-derived fragments (tRFs) are an emerging class of small non-coding RNAs with distinct cellular functions. Here, we studied the contribution of tRFs to the regulation of postnatal β cell maturation, a critical process that may lead to diabetes susceptibility in adulthood. We identified three tRFs abundant in neonatal rat islets originating from 5' halves (tiRNA-5s) of histidine and glutamate tRNAs. Their inhibition in these islets reduced β cell proliferation and insulin secretion. Mitochondrial respiration was also perturbed, fitting with the mitochondrial enrichment of nuclear-encoded tiRNA-5^HisGTG and tiRNA-5^GluCTC. Notably, tiRNA-5 inhibition reduced Mpc1, a mitochondrial pyruvate carrier whose knock down largely phenocopied tiRNA-5 inhibition. tiRNA-5^HisGTG interactome revealed binding to Musashi-1, which was essential for the mitochondrial enrichment of tiRNA-5^HisGTG. Finally, tiRNA-5s were dysregulated in the islets of diabetic and diabetes-prone animals. Altogether, tiRNA-5s represent a class of regulators of β cell maturation, and their deregulation in neonatal islets may lead to diabetes susceptibility in adulthood.

Research progress on the mechanism of beta-cell apoptosis in type 2 diabetes mellitus

Type 2 diabetes mellitus(T2DM) is regarded as one of the most severe chronic metabolic diseases worldwide, which poses a great threat to human safety and health. The main feature of T2DM is the deterioration of pancreatic beta-cell function. More and more studies have shown that the decline of pancreatic beta-cell function in T2DM can be attributable to beta-cell apoptosis, but the exact mechanisms of beta-cell apoptosis in T2DM are not yet fully clarified. Therefore, in this review, we will focus on the current status and progress of research on the mechanism of pancreatic beta-cell apoptosis in T2DM, to provide new ideas for T2DM treatment strategies.

Lipid-Induced Adaptations of the Pancreatic Beta-Cell to Glucotoxic Conditions Sustain Insulin Secretion

Over the last decades, lipotoxicity and glucotoxicity emerged as established mechanisms participating in the pathophysiology of obesity-related type 2 diabetes in general, and in the loss of β-cell function in particular. However, these terms hold various potential biological processes, and it is not clear what precisely they refer to and to what extent they might be clinically relevant. In this review, we discuss the basis and the last advances of research regarding the role of free fatty acids, their metabolic intracellular pathways, and receptor-mediated signaling related to glucose-stimulated insulin secretion, as well as lipid-induced β-cell dysfunction. We also describe the role of chronically elevated glucose, namely, glucotoxicity, which promotes failure and dedifferentiation of the β cell. Glucolipotoxicity combines deleterious effects of exposures to both high glucose and free fatty acids, supposedly provoking synergistic defects on the β cell. Nevertheless, recent studies have highlighted the glycerolipid/free fatty acid cycle as a protective pathway mediating active storage and recruitment of lipids. Finally, we discuss the putative correspondence of the loss of functional β cells in type 2 diabetes with a natural, although accelerated, aging process.

Electrogenic sodium bicarbonate cotransporter NBCe1 regulates pancreatic β cell function in type 2 diabetes

Pancreatic β cell failure in type 2 diabetes mellitus (T2DM) is attributed to perturbations of the β cell's transcriptional landscape resulting in impaired glucose-stimulated insulin secretion. Recent studies identified SLC4A4 (a gene encoding an electrogenic Na+-coupled HCO3- cotransporter and intracellular pH regulator, NBCe1) as one of the misexpressed genes in β cells of patients with T2DM. Thus, in the current study, we set out to test the hypothesis that misexpression of SLC4A4/NBCe1 in T2DM β cells contributes to β cell dysfunction and impaired glucose homeostasis. To address this hypothesis, we first confirmed induction of SLC4A4/NBCe1 expression in β cells of patients with T2DM and demonstrated that its expression was associated with loss of β cell transcriptional identity, intracellular alkalinization, and β cell dysfunction. In addition, we generated a β cell-selective Slc4a4/NBCe1-KO mouse model and found that these mice were protected from diet-induced metabolic stress and β cell dysfunction. Importantly, improved glucose tolerance and enhanced β cell function in Slc4a4/NBCe1-deficient mice were due to augmented mitochondrial function and increased expression of genes regulating β cell identity and function. These results suggest that increased β cell expression of SLC4A4/NBCe1 in T2DM plays a contributory role in promotion of β cell failure and should be considered as a potential therapeutic target.

Mechanisms of Beta-Cell Apoptosis in Type 2 Diabetes-Prone Situations and Potential Protection by GLP-1-Based Therapies

Type 2 diabetes (T2D) is characterized by chronic hyperglycemia secondary to the decline of functional beta-cells and is usually accompanied by a reduced sensitivity to insulin. Whereas altered beta-cell function plays a key role in T2D onset, a decreased beta-cell mass was also reported to contribute to the pathophysiology of this metabolic disease. The decreased beta-cell mass in T2D is, at least in part, attributed to beta-cell apoptosis that is triggered by diabetogenic situations such as amyloid deposits, lipotoxicity and glucotoxicity. In this review, we discussed the molecular mechanisms involved in pancreatic beta-cell apoptosis under such diabetes-prone situations. Finally, we considered the molecular signaling pathways recruited by glucagon-like peptide-1-based therapies to potentially protect beta-cells from death under diabetogenic situations.

Mitochondrial Carriers Regulating Insulin Secretion Profiled in Human Islets upon Metabolic Stress

Chronic exposure of β-cells to nutrient-rich metabolic stress impairs mitochondrial metabolism and its coupling to insulin secretion. We exposed isolated human islets to different metabolic stresses for 3 days: 0.4 mM oleate or 0.4 mM palmitate at physiological 5.5 mM glucose (lipotoxicity), high 25 mM glucose (glucotoxicity), and high 25 mM glucose combined with 0.4 mM oleate and/or palmitate (glucolipotoxicity). Then, we profiled the mitochondrial carriers and associated genes with RNA-Seq. Diabetogenic conditions, and in particular glucotoxicity, increased expression of several mitochondrial solute carriers in human islets, such as the malate carrier DIC, the α-ketoglutarate-malate exchanger OGC, and the glutamate carrier GC1. Glucotoxicity also induced a general upregulation of the electron transport chain machinery, while palmitate largely counteracted this effect. Expression of different components of the TOM/TIM mitochondrial protein import system was increased by glucotoxicity, whereas glucolipotoxicity strongly upregulated its receptor subunit TOM70. Expression of the mitochondrial calcium uniporter MCU was essentially preserved by metabolic stresses. However, glucotoxicity altered expression of regulatory elements of calcium influx as well as the Na⁺/Ca²⁺ exchanger NCLX, which mediates calcium efflux. Overall, the expression profile of mitochondrial carriers and associated genes was modified by the different metabolic stresses exhibiting nutrient-specific signatures.

Understanding Dietary Intervention-Mediated Epigenetic Modifications in Metabolic Diseases

The global prevalence of metabolic disorders, such as obesity, diabetes and fatty liver disease, is dramatically increasing. Both genetic and environmental factors are well-known contributors to the development of these diseases and therefore, the study of epigenetics can provide additional mechanistic insight. Dietary interventions, including caloric restriction, intermittent fasting or time-restricted feeding, have shown promising improvements in patients' overall metabolic profiles (i.e., reduced body weight, improved glucose homeostasis), and an increasing number of studies have associated these beneficial effects with epigenetic alterations. In this article, we review epigenetic changes involved in both metabolic diseases and dietary interventions in primary metabolic tissues (i.e., adipose, liver, and pancreas) in hopes of elucidating potential biomarkers and therapeutic targets for disease prevention and treatment.

Differential sensitivity of human islets from obese versus lean donors to chronic high glucose or palmitate

BACKGROUND

Due to the shortage of multi-organ donors, human pancreatic islet transplantation has now been extended to islets originating from obese subjects. In this study, our aim is to compare the respective sensitivity of human islets from lean vs obese donors to chronic high glucose or high palmitate.

METHODS

Human islets were isolated from pancreases harvested from brain-dead multi-organ donors. Islets were cultured during 72 hours in the presence of moderate (16.7 mmol/L) or high (28 mmoL/L) glucose concentrations, or glucose (5.6 mmoL/L) and palmitate (0.4 mmoL/L), before measurement of their response to glucose.

RESULTS

We first observed a greater insulin response in islets from obese donors under both basal and high-glucose conditions, confirming their hyperresponsiveness to glucose. When islets from obese donors were cultured in the presence of moderate or high glucose concentrations, insulin response to glucose remained unchanged or was slightly reduced, as opposed to that observed in lean subjects. Moreover, culturing islets from obese donors with high palmitate also induced less reduction in insulin response to glucose than in lean subjects. This partial protection of obese islets is associated with less induction of inducible nitric oxide synthase in islets, together with a greater expression of the transcription factor forkhead box O1 (FOXO1).

CONCLUSIONS

Our data suggest that in addition to an increased sensitivity to glucose, islets from obese subjects can be considered as more resistant to glucose and fatty acid excursions and are thus valuable candidates for transplantation.

AMPK Profiling in Rodent and Human Pancreatic Beta-Cells under Nutrient-Rich Metabolic Stress

Chronic exposure of pancreatic β-cells to elevated nutrient levels impairs their function and potentially induces apoptosis. Like in other cell types, AMPK is activated in β-cells under conditions of nutrient deprivation, while little is known on AMPK responses to metabolic stresses. Here, we first reviewed recent studies on the role of AMPK activation in β-cells. Then, we investigated the expression profile of AMPK pathways in β-cells following metabolic stresses. INS-1E β-cells and human islets were exposed for 3 days to glucose (5.5–25 mM), palmitate or oleate (0.4 mM), and fructose (5.5 mM). Following these treatments, we analyzed transcript levels of INS-1E β-cells by qRT-PCR and of human islets by RNA-Seq; with a special focus on AMPK-associated genes, such as the AMPK catalytic subunits α1 (Prkaa1) and α2 (Prkaa2). AMPKα and pAMPKα were also evaluated at the protein level by immunoblotting. Chronic exposure to the different metabolic stresses, known to alter glucose-stimulated insulin secretion, did not change AMPK expression, either in insulinoma cells or in human islets. Expression profile of the six AMPK subunits was marginally modified by the different diabetogenic conditions. However, the expression of some upstream kinases and downstream AMPK targets, including K-ATP channel subunits, exhibited stress-specific signatures. Interestingly, at the protein level, chronic fructose treatment favored fasting-like phenotype in human islets, as witnessed by AMPK activation. Collectively, previously published and present data indicate that, in the β-cell, AMPK activation might be implicated in the pre-diabetic state, potentially as a protective mechanism.

Coloured Rice Phenolic Extracts Increase Expression of Genes Associated with Insulin Secretion in Rat Pancreatic Insulinoma β-cells

Glucose-induced oxidative stress is associated with the overproduction of reactive oxygen species (ROS), which may dysregulate the expression of genes controlling insulin secretion leading to β-cell dysfunction, a hallmark of type 2 diabetes mellitus (T2DM). This study investigated the impact of coloured rice phenolic extracts (CRPEs) on the expression of key genes associated with β-cell function in pancreatic β-cells (INS-1E). These genes included glucose transporter 2 (Glut2), silent mating type information regulation 2 homolog 1 (Sirt1), mitochondrial transcription factor A (Tfam), pancreatic/duodenal homeobox protein 1 (Pdx-1) and insulin 1 (Ins1). INS-1E cells were cultured in high glucose (25 mM) to induce glucotoxic stress conditions (HGSC) and in normal glucose conditions (NGC-11.1 mM) to represent normal β-cell function. Cells were treated with CRPEs derived from two coloured rice cultivars, Purple and Yunlu29-red varieties at concentrations ranged from 50 to 250 µg/mL. CRPEs upregulated the expression of Glut2, Sirt1 and Pdx-1 significantly at 250 µg/mL under HGSC. CRPEs from both cultivars also upregulated Glut2, Sirt1, Tfam, Pdx-1 and Ins1 markedly at 250 µg/mL under NGC with Yunlu29 having the greatest effect. These data suggest that CRPEs may reduce β-cell dysfunction in T2DM by upregulating the expression of genes involved in insulin secretion pathways.

Chronic fructose renders pancreatic β-cells hyper-responsive to glucose-stimulated insulin secretion through extracellular ATP signaling

Fructose is widely used as a sweetener in processed food and is also associated with metabolic disorders, such as obesity. However, the underlying cellular mechanisms remain unclear, in particular, regarding the pancreatic β-cell. Here, we investigated the effects of chronic exposure to fructose on the function of insulinoma cells and isolated mouse and human pancreatic islets. Although fructose per se did not acutely stimulate insulin exocytosis, our data show that chronic fructose rendered rodent and human β-cells hyper-responsive to intermediate physiological glucose concentrations. Fructose exposure reduced intracellular ATP levels without affecting mitochondrial function, induced AMP-activated protein kinase activation, and favored ATP release from the β-cells upon acute glucose stimulation. The resulting increase in extracellular ATP, mediated by pannexin1 (Panx1) channels, activated the calcium-mobilizer P2Y purinergic receptors. Immunodetection revealed the presence of both Panx1 channels and P2Y1 receptors in β-cells. Addition of an ectonucleotidase inhibitor or P2Y1 agonists to naïve β-cells potentiated insulin secretion stimulated by intermediate glucose, mimicking the fructose treatment. Conversely, the P2Y1 antagonist and Panx1 inhibitor reversed the effects of fructose, as confirmed using Panx1-null islets and by the clearance of extracellular ATP by apyrase. These results reveal an important function of ATP signaling in pancreatic β-cells mediating fructose-induced hyper-responsiveness.

Diabetes causes marked inhibition of mitochondrial metabolism in pancreatic β-cells

Diabetes is a global health problem caused primarily by the inability of pancreatic β-cells to secrete adequate levels of insulin. The molecular mechanisms underlying the progressive failure of β-cells to respond to glucose in type-2 diabetes remain unresolved. Using a combination of transcriptomics and proteomics, we find significant dysregulation of major metabolic pathways in islets of diabetic βV59M mice, a non-obese, eulipidaemic diabetes model. Multiple genes/proteins involved in glycolysis/gluconeogenesis are upregulated, whereas those involved in oxidative phosphorylation are downregulated. In isolated islets, glucose-induced increases in NADH and ATP are impaired and both oxidative and glycolytic glucose metabolism are reduced. INS-1 β-cells cultured chronically at high glucose show similar changes in protein expression and reduced glucose-stimulated oxygen consumption: targeted metabolomics reveals impaired metabolism. These data indicate hyperglycaemia induces metabolic changes in β-cells that markedly reduce mitochondrial metabolism and ATP synthesis. We propose this underlies the progressive failure of β-cells in diabetes.

An 8-week diet high in cereal fiber and coffee but free of red meat does not improve beta-cell function in patients with type 2 diabetes mellitus: a randomized controlled trial

BACKGROUND

Higher dietary intake of fibers and coffee, but lower red meat intake is associated with reduced risk for type 2 diabetes in epidemiological studies. We hypothesized that a calorie-restricted diet, which is high in fiber and coffee, but free of red meat, improves beta-cell function in patients with T2D.

METHODS

In a randomized parallel-group pilot trial, obese type 2 diabetes patients were randomly allocated to consume either a diet high in cereal fiber and coffee, but free of red meat (n = 17) (L-RISK) or a diet low in fiber, free of coffee but high in red meat (n = 20) (H-RISK) for 8 weeks. Insulin secretion was assessed from glucagon stimulation tests (GST) and mixed-meal tolerance tests (MMTT) before and after dietary intervention.

RESULTS

Both diets resulted in comparable reduction of fasting concentrations of insulin (H-RISK -28% vs. L-RISK -32%, both p < 0.01), C-peptide (H-RISK -26% vs. L-RISK -30%, both p < 0.01) and blood glucose (H-RISK -6.8%, p < 0.05 vs. L-RISK -10%, p < 0.01). Gastric inhibitory peptide (GIP) secretion increased by 24% after 8 weeks in the L-RISK only (p < 0.01). However, GST and MMTT showed no differences in insulin secretion after intervention.

CONCLUSIONS

Calorie restriction independent of the intake of fiber, coffee or meat failed to improve beta-cell function, but improved GIP secretion in obese patients with type 2 diabetes.

TRIAL REGISTRATION

Registration at Clinicaltrials.gov, Identifier number: NCT01409330, Registered 4 August 2011 - Retrospectively registered.

Human primitive brain displays negative mitochondrial-nuclear expression correlation of respiratory genes

Oxidative phosphorylation (OXPHOS), a fundamental energy source in all human tissues, requires interactions between mitochondrial (mtDNA)- and nuclear (nDNA)-encoded protein subunits. Although such interactions are fundamental to OXPHOS, bi-genomic coregulation is poorly understood. To address this question, we analyzed ∼8500 RNA-seq experiments from 48 human body sites. Despite well-known variation in mitochondrial activity, quantity, and morphology, we found overall positive mtDNA-nDNA OXPHOS genes' co-expression across human tissues. Nevertheless, negative mtDNA-nDNA gene expression correlation was identified in the hypothalamus, basal ganglia, and amygdala (subcortical brain regions, collectively termed the "primitive" brain). Single-cell RNA-seq analysis of mouse and human brains revealed that this phenomenon is evolutionarily conserved, and both are influenced by brain cell types (involving excitatory/inhibitory neurons and nonneuronal cells) and by their spatial brain location. As the "primitive" brain is highly oxidative, we hypothesized that such negative mtDNA-nDNA co-expression likely controls for the high mtDNA transcript levels, which enforce tight OXPHOS regulation, rather than rewiring toward glycolysis. Accordingly, we found "primitive" brain-specific up-regulation of lactate dehydrogenase B (LDHB), which associates with high OXPHOS activity, at the expense of LDHA, which promotes glycolysis. Analyses of co-expression, DNase-seq, and ChIP-seq experiments revealed candidate RNA-binding proteins and CEBPB as the best regulatory candidates to explain these phenomena. Finally, cross-tissue expression analysis unearthed tissue-dependent splice variants and OXPHOS subunit paralogs and allowed revising the list of canonical OXPHOS transcripts. Taken together, our analysis provides a comprehensive view of mito-nuclear gene co-expression across human tissues and provides overall insights into the bi-genomic regulation of mitochondrial activities.

DNA methylation in the pathogenesis of type 2 diabetes in humans

Background

Type 2 diabetes (T2D) is a multifactorial, polygenic disease caused by impaired insulin secretion and insulin resistance. Genome-wide association studies (GWAS) were expected to resolve a large part of the genetic component of diabetes; yet, the single nucleotide polymorphisms identified by GWAS explain less than 20% of the estimated heritability for T2D. There was subsequently a need to look elsewhere to find disease-causing factors. Mechanisms mediating the interaction between environmental factors and the genome, such as epigenetics, may be of particular importance in the pathogenesis of T2D.

Scope of Review

This review summarizes knowledge of the impact of epigenetics on the pathogenesis of T2D in humans. In particular, the review will focus on alterations in DNA methylation in four human tissues of importance for the disease; pancreatic islets, skeletal muscle, adipose tissue, and the liver. Case–control studies and studies examining the impact of non-genetic and genetic risk factors on DNA methylation in humans will be considered. These studies identified epigenetic changes in tissues from subjects with T2D versus non-diabetic controls. They also demonstrate that non-genetic factors associated with T2D such as age, obesity, energy rich diets, physical activity and the intrauterine environment impact the epigenome in humans. Additionally, interactions between genetics and epigenetics seem to influence the pathogenesis of T2D.

Conclusions

Overall, previous studies by our group and others support a key role for epigenetics in the growing incidence of T2D.

Identification of the molecular dysfunction caused by glutamate dehydrogenase S445L mutation responsible for hyperinsulinism/hyperammonemia

Congenital hyperinsulinism/hyperammonemia (HI/HA) syndrome gives rise to unregulated protein-induced insulin secretion from pancreatic beta-cells, fasting hypoglycemia and elevated plasma ammonia levels. Mutations associated with HI/HA were identified in the Glud1 gene, encoding for glutamate dehydrogenase (GDH). We aimed at identifying the molecular causes of dysregulation in insulin secretion and ammonia production conferred by the most frequent HI/HA mutation Ser445Leu. Following transduction with adenoviruses carrying the human GDH-wild type or GDH-S445L-mutant gene, immunoblotting showed efficient expression of the transgenes in all the investigated cell types. Enzymatic activity tested in INS-1E beta-cells revealed that the mutant was much more sensitive to the allosteric activator ADP, rendering it highly responsive to substrates. INS-1E cells expressing either the wild type or mutant GDH responded similarly to glucose stimulation regarding mitochondrial activation and insulin secretion. However, at basal glucose glutamine stimulation increased mitochondrial activity and insulin release only in the mutant cells. In mouse and human islets, expression of mutant GDH resulted in robust elevation of insulin secretion upon glutamine stimulation, not observed in control islets. Hepatocytes expressing either the wild type or mutant GDH produced similar levels of ammonia when exposed to glutamine, although alanine response was strongly elevated with the mutant form. In conclusion, the GDH-S445L mutation confers hyperactivity to this enzyme due to higher sensitivity to ADP allosteric activation. This renders beta-cells responsive to amino acid stimulation, explaining protein-induced hypoglycemia secondary to non-physiological insulin release. Hepatocytes carrying mutant GDH produced more ammonia upon alanine exposure, which underscores hyperammonemia developed by the patients.

Temporal Transcriptomic and Proteomic Landscapes of Deteriorating Pancreatic Islets in Type 2 Diabetic Rats

Progressive reduction in β-cell mass and function comprise the core of the pathogenesis mechanism of type 2 diabetes. The process of deteriorating pancreatic islets, in which a complex network of molecular events is involved, is not yet fully characterized. We used RNA sequencing and tandem mass tag-based quantitative proteomics technology to measure the temporal mRNA and protein expression changes of pancreatic islets in Goto-Kakizaki (GK) rats from 4 to 24 weeks of age. Our omics data set outlines the dynamics of the molecular network during the deterioration of GK islets as two stages: The early stage (4-6 weeks) is characterized by anaerobic glycolysis, inflammation priming, and compensation for insulin synthesis, and the late stage (8-24 weeks) is characterized by inflammation amplification and compensation failure. Further time course analysis allowed us to reveal 5,551 differentially expressed genes, a large portion of which have not been reported before. Our comprehensive and temporal transcriptome and proteome data offer a valuable resource for the diabetes research community and for quantitative biology.

Upregulation of UCP2 in beta-cells confers partial protection against both oxidative stress and glucotoxicity

Deterioration of pancreatic beta-cells plays a critical role in the development of type 2 diabetes. Among the various stressors contributing to these deleterious effects, glucotoxicity and superoxides have been proposed as major players. In this context, the mitochondrial uncoupling protein UCP2 is regularly associated with the stress response. In the present study, we tested the effects of UCP2 upregulation in mouse islets with beta-cell specific overexpression of UCP2 (RIP-UCP2). Islets were subjected to both chronic glucotoxicity (7 days at 30 mM glucose) and acute oxidative stress (200 µM H₂O₂ for 10 min). Increased UCP2 expression did not alter mitochondrial potential and ATP generation but protected against glucotoxic effects. Glucose-stimulated insulin secretion was altered by both glucotoxicity and oxidative stress, in particular through higher basal insulin release at non-stimulatory glucose concentrations. The secretory response to glucose stimulation was partially preserved in beta-cells overexpressing UCP2. The higher rate of cell death induced by chronic high glucose exposure was lower in RIP-UCP2 islets. Finally, superoxide production was reduced by high glucose, both under acute and chronic conditions, and not modified by UCP2 overexpression. In conclusion, upregulation of UCP2 conferred protective effects to the stressed beta-cell through mechanisms not directly associated with superoxide production.

•

UCP2 upregulation protects pancreatic ß-cells against glucotoxicity.

•

High glucose reduces superoxide production in pancreatic islets.

•

UCP2 upregulation does not change superoxide production.

•

UCP2 upregulation protects ß-cells against oxidative stress.

Mechanisms of the amplifying pathway of insulin secretion in the β cell

Pancreatic islet β cells secrete insulin in response to nutrient secretagogues, like glucose, dependent on calcium influx and nutrient metabolism. One of the most intriguing qualities of β cells is their ability to use metabolism to amplify the amount of secreted insulin independent of further alterations in intracellular calcium. Many years studying this amplifying process have shaped our current understanding of β cell stimulus-secretion coupling; yet, the exact mechanisms of amplification have been elusive. Recent studies utilizing metabolomics, computational modeling, and animal models have progressed our understanding of the metabolic amplifying pathway of insulin secretion from the β cell. New approaches will be discussed which offer in-roads to a more complete model of β cell function. The development of β cell therapeutics may be aided by such a model, facilitating the targeting of aspects of the metabolic amplifying pathway which are unique to the β cell.

Is a β cell a β cell?

PURPOSE OF REVIEW

This report examines recent publications identifying phenotypic and functional heterogeneity among pancreatic β cells and investigating their potential roles in normal and abnormal islet function. The development of new methods and tools for the study of individual islet cells has produced a surge of interest in this topic.

RECENT FINDINGS

Studies of β cell maturation and pregnancy-induced proliferation have identified changes in serotonin and transcription factors SIX2/3 expression as markers of temporal heterogeneity. Structural and functional heterogeneity in the form of functionally distinct 'hub' and 'follower' β cells was found in mouse islets. Heterogeneous expression of Fltp (in mouse β cells) and ST8SIA1 and CD9 (in human β cells) were associated with distinct functional potential. Several impressive reports describing the transcriptomes of individual β cells were also published in recent months. Some of these reveal previously unknown β cell subpopulations.

SUMMARY

A wealth of information on functional and phenotypic heterogeneity has been collected recently, including the transcriptomes of individual β cells and the identities of functionally distinct β cell subpopulations. Several studies suggest the existence of two broad categories: a more proliferative but less functional and a less proliferative but more functional β cell type. The identification of functionally distinct subpopulations and their association with type 2 diabetes underlines the potential clinical importance of these investigations.

NADPH oxidase-2 does not contribute to β-cell glucotoxicity in cultured pancreatic islets from C57BL/6J mice

High glucose-induced oxidative stress and increased NADPH oxidase-2 (NOX2) activity may contribute to the progressive decline of the functional β-cell mass in type 2 diabetes. To test that hypothesis, we characterized, in islets from male NOX2 knockout (NOX2-KO) and wild-type (WT) C57BL/6J mice cultured for up to 3 weeks at 10 or 30 mmol/l glucose (G10 or G30), the in vitro effects of glucose on cytosolic oxidative stress using probes sensing glutathione oxidation (GRX1-roGFP2), thiol oxidation (roGFP1) or H₂O₂ (roGFP2-Orp1), on β-cell stimulus-secretion coupling events and on β-cell apoptosis. After 1-2 days of culture in G10, the glucose stimulation of insulin secretion (GSIS) was ∼1.7-fold higher in NOX2-KO vs. WT islets at 20-30 mmol/l glucose despite similar rises in NAD(P)H and intracellular calcium concentration ([Ca²⁺]_i) and no differences in cytosolic GRX1-roGFP2 oxidation. After long-term culture at G10, roGFP1 and roGFP2-Orp1 oxidation and β-cell apoptosis remained low, and the glucose-induced rises in NAD(P)H, [Ca²⁺]_i and GSIS were similarly preserved in both islet types. After prolonged culture at G30, roGFP1 and roGFP2-Orp1 oxidation increased in parallel with β-cell apoptosis, the glucose sensitivity of the NADPH, [Ca²⁺]_i and insulin secretion responses increased, the maximal [Ca²⁺]_i response decreased, but maximal GSIS was preserved. These responses were almost identical in both islet types. In conclusion, NOX2 is a negative regulator of maximal GSIS in C57BL/6J mouse islets, but it does not detectably contribute to the in vitro glucotoxic induction of cytosolic oxidative stress and alterations of β-cell survival and function.

Integrative Genomics Outlines a Biphasic Glucose Response and a ChREBP-RORγ Axis Regulating Proliferation in β Cells

Glucose is an important inducer of insulin secretion, but it also stimulates long-term adaptive changes in gene expression that can either promote or antagonize the proliferative potential and function of β cells. Here, we have generated time-resolved profiles of enhancer and transcriptional activity in response to glucose in the INS-1E pancreatic β cell line. Our data outline a biphasic response with a first transcriptional wave during which metabolic genes are activated, and a second wave where cell-cycle genes are activated and β cell identity genes are repressed. The glucose-sensing transcription factor ChREBP directly activates first wave enhancers, whereas repression and activation of second wave enhancers are indirect. By integrating motif enrichment within late-regulated enhancers with expression profiles of the associated transcription factors, we have identified multiple putative regulators of the second wave. These include RORγ, the activity of which is important for glucose-induced proliferation of both INS-1E and primary rat β cells.

Beta-cell mitochondrial carriers and the diabetogenic stress response

Mitochondria play a central role in pancreatic beta-cells by coupling metabolism of the secretagogue glucose to distal events of regulated insulin exocytosis. This process requires transports of both metabolites and nucleotides in and out of the mitochondria. The molecular identification of mitochondrial carriers and their respective contribution to beta-cell function have been uncovered only recently. In type 2 diabetes, mitochondrial dysfunction is an early event and may precipitate beta-cell loss. Under diabetogenic conditions, characterized by glucotoxicity and lipotoxicity, the expression profile of mitochondrial carriers is selectively modified. This review describes the role of mitochondrial carriers in beta-cells and the selective changes in response to glucolipotoxicity. In particular, we discuss the importance of the transfer of metabolites (pyruvate, citrate, malate, and glutamate) and nucleotides (ATP, NADH, NADPH) for beta-cell function and dysfunction. This article is part of a Special Issue entitled: Mitochondrial Channels edited by Pierre Sonveaux, Pierre Maechler and Jean-Claude Martinou.