Beta-cell mitochondria and insulin secretion: messenger role of nucleotides and metabolites

Association between protein undernutrition and diabetes: Molecular implications in the reduction of insulin secretion

Undernutrition is still a recurring nutritional problem in low and middle-income countries. It is directly associated with the social and economic sphere, but it can also negatively impact the health of the population. In this sense, it is believed that undernourished individuals may be more susceptible to the development of non-communicable diseases, such as diabetes mellitus, throughout life. This hypothesis was postulated and confirmed until today by several studies that demonstrate that experimental models submitted to protein undernutrition present alterations in glycemic homeostasis linked, in part, to the reduction of insulin secretion. Therefore, understanding the changes that lead to a reduction in the secretion of this hormone is essential to prevent the development of diabetes in undernourished individuals. This narrative review aims to describe the main molecular changes already characterized in pancreatic β cells that will contribute to the reduction of insulin secretion in protein undernutrition. So, it will provide new perspectives and targets for postulation and action of therapeutic strategies to improve glycemic homeostasis during this nutritional deficiency.

Regulation of autophagy by perilysosomal calcium: a new player in β-cell lipotoxicity

Autophagy is an essential quality control mechanism for maintaining organellar functions in eukaryotic cells. Defective autophagy in pancreatic beta cells has been shown to be involved in the progression of diabetes through impaired insulin secretion under glucolipotoxic stress. The underlying mechanism reveals the pathologic role of the hyperactivation of mechanistic target of rapamycin (mTOR), which inhibits lysosomal biogenesis and autophagic processes. Moreover, accumulating evidence suggests that oxidative stress induces Ca2+ depletion in the endoplasmic reticulum (ER) and cytosolic Ca2+ overload, which may contribute to mTOR activation in perilysosomal microdomains, leading to autophagic defects and β-cell failure due to lipotoxicity. This review delineates the antagonistic regulation of autophagic flux by mTOR and AMP-dependent protein kinase (AMPK) at the lysosomal membrane, and both of these molecules could be activated by perilysosomal calcium signaling. However, aberrant and persistent Ca2+ elevation upon lipotoxic stress increases mTOR activity and suppresses autophagy. Therefore, normalization of autophagy is an attractive therapeutic strategy for patients with β-cell failure and diabetes.

Injectable systems for long-lasting insulin therapy

Insulin therapy is the mainstay to treat diabetes characterizedd by hyperglycemia. However, its short half-life of only 4-6 min limits its effectiveness in treating chronic diabetes. Advances in recombinant DNA technology and protein engineering have led to several insulin analogue products that have up to 42 h of glycemic control. However, these insulin analogues still require once- or twice-daily injections for optimal glycemic control and have poor patient compliance and adherence issues. To achieve insulin release for more than one day, different injectable delivery systems including microspheres, in situ forming depots, nanoparticles and composite systems have been developed. Several of these delivery systems have advanced to clinical trials for once-weekly insulin injection. This review comprehensively summarizes the developments of injectable insulin analogs and delivery systems covering the whole field of injectable long-lasting insulin technologies from prototype design, preclinical studies, clinical trials to marketed products for the treatment of diabetes.

Single cell multiomic analysis reveals diabetes-associated β-cell heterogeneity driven by HNF1A

Broad heterogeneity in pancreatic β-cell function and morphology has been widely reported. However, determining which components of this cellular heterogeneity serve a diabetes-relevant function remains challenging. Here, we integrate single-cell transcriptome, single-nuclei chromatin accessibility, and cell-type specific 3D genome profiles from human islets and identify Type II Diabetes (T2D)-associated β-cell heterogeneity at both transcriptomic and epigenomic levels. We develop a computational method to explicitly dissect the intra-donor and inter-donor heterogeneity between single β-cells, which reflect distinct mechanisms of T2D pathogenesis. Integrative transcriptomic and epigenomic analysis identifies HNF1A as a principal driver of intra-donor heterogeneity between β-cells from the same donors; HNF1A expression is also reduced in β-cells from T2D donors. Interestingly, HNF1A activity in single β-cells is significantly associated with lower Na+ currents and we nominate a HNF1A target, FXYD2, as the primary mitigator. Our study demonstrates the value of investigating disease-associated single-cell heterogeneity and provides new insights into the pathogenesis of T2D.

Biomarkers of aging

Aging biomarkers are a combination of biological parameters to (i) assess age-related changes, (ii) track the physiological aging process, and (iii) predict the transition into a pathological status. Although a broad spectrum of aging biomarkers has been developed, their potential uses and limitations remain poorly characterized. An immediate goal of biomarkers is to help us answer the following three fundamental questions in aging research: How old are we? Why do we get old? And how can we age slower? This review aims to address this need. Here, we summarize our current knowledge of biomarkers developed for cellular, organ, and organismal levels of aging, comprising six pillars: physiological characteristics, medical imaging, histological features, cellular alterations, molecular changes, and secretory factors. To fulfill all these requisites, we propose that aging biomarkers should qualify for being specific, systemic, and clinically relevant.

Vanillic acid potentiates insulin secretion and prevents pancreatic β-cells cytotoxicity under H2O2-induced oxidative stress

BACKGROUND

Oxidative stress is known to impair cellular functions and, therefore, plays a significant role in the pathophysiology of various diseases, including diabetes. The persistently elevated glucose levels may cause enhanced mitochondrial reactive oxygen species generation, which in turn can damage the pancreatic β-cells. In this study, we have investigated the effect of vanillic acid on preventing H₂O₂-induced β-cells death and retaining its insulin secretion potentiating effect in the presence of H₂O₂.

METHODS

The insulin secretion from the BRIN-BD11 cells was quantified using ELISA-based assays. The viability of the cells was assessed by estimated by the [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide] (MTT) colorimetric assay and DAPI staining. The expression levels of apoptotic and antioxidant proteins were estimated by western blot experiments.

RESULTS

Vanillic acid protected pancreatic β-cells viability and function under the H₂O₂ oxidative stress condition. The Erk1/2 activation appears to play an important role in vanillic acid potentiated insulin secretion and protection of the β-cells in the presence of H₂O₂. Vanillic acid pretreated cells exhibited enhanced expression of antioxidant enzymes such as catalase and SOD-2 and reduced the expression of proapoptotic markers such as BAX and BAD. In addition, it also enhanced the expression of oxidative stress-sensitive transcription factor Nrf-2 and cell survival protein Akt.

CONCLUSION

The present study shows that vanillic acid potentiates insulin secretion and protects pancreatic β-cells from H₂O₂-induced oxidative stress.

Liver-Derived S100A6 Propels β-Cell Dysfunction in NAFLD

Nonalcoholic fatty liver disease (NAFLD) is an independent predictor of systemic insulin resistance and type 2 diabetes mellitus (T2DM). However, converse correlates between excess liver fat content and β-cell function remain equivocal. Specifically, how the accumulation of liver fat consequent to the enhanced de novo lipogenesis (DNL) leads to pancreatic β-cell failure and eventually to T2DM is elusive. Here, we have identified that low-molecular-weight calcium-binding protein S100A6, or calcyclin, inhibits glucose-stimulated insulin secretion (GSIS) from β cells through activation of the receptor for the advanced glycation end products and diminution of mitochondrial respiration. Serum S100A6 level is elevated both in human patients with NAFLD and in a high-fat diet-induced mouse model of NAFLD. Although serum S100A6 levels are negatively associated with β-cell insulin secretory capacity in human patients, depletion of hepatic S100A6 improves GSIS and glycemia in mice, suggesting that S100A6 contributes to the pathophysiology of diabetes in NAFLD. Moreover, transcriptional induction of hepatic S100A6 is driven by the potent regulator of DNL, carbohydrate response element-binding protein (ChREBP), and ectopic expression of ChREBP in the liver suppresses GSIS in a S100A6-sensitive manner. Together, these data suggest elevated serum levels of S100A6 may serve as a biomarker in identifying patients with NAFLD with a heightened risk of developing β-cell dysfunction. Overall, our data implicate S100A6 as, to our knowledge, a hitherto unknown hepatokine to be activated by ChREBP and that participates in the hepato-pancreatic communication to impair insulin secretion and drive the development of T2DM in NAFLD.

β-Cell Succinate Dehydrogenase Deficiency Triggers Metabolic Dysfunction and Insulinopenic Diabetes

Mitochondrial dysfunction plays a central role in type 2 diabetes (T2D); however, the pathogenic mechanisms in pancreatic β-cells are incompletely elucidated. Succinate dehydrogenase (SDH) is a key mitochondrial enzyme with dual functions in the tricarboxylic acid cycle and electron transport chain. Using samples from human with diabetes and a mouse model of β-cell-specific SDH ablation (SDHBβKO), we define SDH deficiency as a driver of mitochondrial dysfunction in β-cell failure and insulinopenic diabetes. β-Cell SDH deficiency impairs glucose-induced respiratory oxidative phosphorylation and mitochondrial membrane potential collapse, thereby compromising glucose-stimulated ATP production, insulin secretion, and β-cell growth. Mechanistically, metabolomic and transcriptomic studies reveal that the loss of SDH causes excess succinate accumulation, which inappropriately activates mammalian target of rapamycin (mTOR) complex 1-regulated metabolic anabolism, including increased SREBP-regulated lipid synthesis. These alterations, which mirror diabetes-associated human β-cell dysfunction, are partially reversed by acute mTOR inhibition with rapamycin. We propose SDH deficiency as a contributing mechanism to the progressive β-cell failure of diabetes and identify mTOR complex 1 inhibition as a potential mitigation strategy.

Luseogliflozin preserves the pancreatic beta-cell mass and function in db/db mice by improving mitochondrial function

We aimed to determine the mechanism by which the sodium glucose co-transporter 2 inhibitor, luseogliflozin, preserves pancreatic beta-cell mass and function in db/db mice. Six-week-old db/db mice were fed to standard chow or standard chow containing 0.01% luseogliflozin. After 4 weeks, DNA microarray analysis, real-time PCR analysis, and measurement of mitochondrial respiratory capacity and reactive oxygen species (ROS) generation were performed using isolated islets. Immunohistochemistry and electron microscopic analysis were performed using pancreatic tissues. Metabolites extracted from the islets were measured by capillary electrophoresis mass spectrometry. The expression of genes involved in the tricarboxylic acid (TCA) cycle and electron transport chain was upregulated by luseogliflozin. Luseogliflozin improved the mitochondrial complex II-linked oxidative phosphorylation capacity and reduced ROS generation. Mitochondrial morphology was normally maintained by luseogliflozin. Luseogliflozin increased NK6 homeobox 1 (NKX6.1) expression and TCA cycle metabolites. Relief of glucotoxicity by luseogliflozin may involve lower mitochondrial ROS generation and an improvement in complex II-linked mitochondrial respiration. Reducing ROS generation through preventing complex II damage likely increases NKX6.1 expression and ameliorate glucose metabolism in the TCA cycle, contributing to the protection of pancreatic beta-cells. Protection of complex II in pancreatic beta-cells represents a novel therapeutic target for type 2 diabetes.

Adrenomedullin and its receptors are expressed in mouse pancreatic β-cells and suppresses insulin synthesis and secretion

Gestational diabetes mellitus (GDM) is associated with defective pancreatic β-cell adaptation in pregnancy, but the underlying mechanism remains obscure. Our previous studies demonstrated that GDM women display increased plasma adrenomedullin (ADM) levels, and non-obese GDM mice show decreased serum concentrations of insulin and the number of β-cells in pancreas islets. The aims of this study is to examine if ADM and its receptors are expressed in female mouse pancreas, and if so, whether insulin secretion is regulated by ADM in mouse β-cell line, NIT-1 cells and isolated mouse pancreatic islets. Present study shows that ADM and its receptor components CRLR, RAMPs are present in mouse pancreatic islets and co-localized with insulin. The expressions of ADM, CRLR and RAMP2 in islets from pregnant mice are reduced compared to that of non-pregnant mice. NIT-1-β cells express ADM and its receptor mRNA, and glucose dose-dependently stimulates expressions. Furthermore, ADM inhibits NIT-1-β cell growth, and this inhibition is reversed by ADM antagonist, ADM22-52. The glucose-induced insulin secretion was suppressed by ADM in NIT-1-β cells and isolated pancreatic islets from pregnant mice. These inhibitory effects are accompanied by upregulation of endoplasmic reticulum (ER) stress biomarker genes in NIT-1-β cells. This study unveils that reduced ADM and its receptors may play a role in β-cell adaptation during pregnancy, while increased plasma ADM in GDM may contribute to the β-cells dysfunction, and blockade of ADM may reverse β-cell insulin production.

Ribosomal biogenesis regulator DIMT1 controls β-cell protein synthesis, mitochondrial function, and insulin secretion

We previously reported that loss of mitochondrial transcription factor B1 (TFB1M) leads to mitochondrial dysfunction and is involved in the pathogenesis of type 2 diabetes (T2D). Whether defects in ribosomal processing impact mitochondrial function and could play a pathogenetic role in β-cells and T2D is not known. To this end, we explored expression and the functional role of dimethyladenosine transferase 1 homolog (DIMT1), a homolog of TFB1M and a ribosomal RNA (rRNA) methyltransferase implicated in the control of rRNA. Expression of DIMT1 was increased in human islets from T2D donors and correlated positively with expression of insulin mRNA, but negatively with insulin secretion. We show that silencing of DIMT1 in insulin-secreting cells impacted mitochondrial function, leading to lower expression of mitochondrial OXPHOS proteins, reduced oxygen consumption rate, dissipated mitochondrial membrane potential, and a slower rate of ATP production. In addition, the rate of protein synthesis was retarded upon DIMT1 deficiency. Consequently, we found that DIMT1 deficiency led to perturbed insulin secretion in rodent cell lines and islets, as well as in a human β-cell line. We observed defects in rRNA processing and reduced interactions between NIN1 (RPN12) binding protein 1 homolog (NOB-1) and pescadillo ribosomal biogenesis factor 1 (PES-1), critical ribosomal subunit RNA proteins, the dysfunction of which may play a part in disturbing protein synthesis in β-cells. In conclusion, DIMT1 deficiency perturbs protein synthesis, resulting in mitochondrial dysfunction and disrupted insulin secretion, both potential pathogenetic processes in T2D.

A requirement for PAK1 to support mitochondrial function and maintain cellular redox balance via electron transport chain proteins to prevent β-cell apoptosis

OBJECTIVE

p21 (Cdc42/Rac1) activated Kinase 1 (PAK1) is a candidate susceptibility factor for type 2 diabetes (T2D). PAK1 is depleted in the islets from T2D donors, compared to control individuals. In addition, whole-body PAK1 knock out (PAK1-KO) in mice worsens the T2D-like effects of high-fat diet. The current study tested the effects of modulating PAK1 levels only in β-cells.

MATERIALS/METHODS

β-cell-specific inducible PAK1 KO (βPAK1-iKO) mice were generated and used with human β-cells and T2D islets to evaluate β-cell function.

RESULTS

βPAK1-iKO mice exhibited glucose intolerance and elevated β-cell apoptosis, but without peripheral insulin resistance. β-cells from βPAK-iKO mice also contained fewer mitochondria per cell. At the cellular level, human PAK1-deficient β-cells showed blunted glucose-stimulated insulin secretion and reduced mitochondrial function. Mitochondria from human PAK1-deficient β-cells were deficient in the electron transport chain (ETC) subunits CI, CIII, and CIV; NDUFA12, a CI complex protein, was identified as a novel PAK1 binding partner, and was significantly reduced with PAK1 knockdown. PAK1 knockdown disrupted the NAD+/NADH and NADP+/NADPH ratios, and elevated ROS. An imbalance of the redox state due to mitochondrial dysfunction leads to ER stress in β-cells. PAK1 replenishment in the β-cells of T2D human islets ameliorated levels of ER stress markers.

CONCLUSIONS

These findings support a protective function for PAK1 in β-cells. The results support a new model whereby the PAK1 in the β-cell plays a required role upstream of mitochondrial function, via maintaining ETC protein levels and averting stress-induced β-cell apoptosis to retain healthy functional β-cell mass.

Mitochondrial gene expression in single cells shape pancreatic beta cells' sub-populations and explain variation in insulin pathway

Mitochondrial gene expression is pivotal to cell metabolism. Nevertheless, it is unknown whether it diverges within a given cell type. Here, we analysed single-cell RNA-seq experiments from human pancreatic alpha (N = 3471) and beta cells (N = 1989), as well as mouse beta cells (N = 1094). Cluster analysis revealed two distinct human beta cells populations, which diverged by mitochondrial (mtDNA) and nuclear DNA (nDNA)-encoded oxidative phosphorylation (OXPHOS) gene expression in healthy and diabetic individuals, and in newborn but not in adult mice. Insulin gene expression was elevated in beta cells with higher mtDNA gene expression in humans and in young mice. Such human beta cell populations also diverged in mitochondrial RNA mutational repertoire, and in their selective signature, thus implying the existence of two previously overlooked distinct and conserved beta cell populations. While applying our approach to human alpha cells, two sub-populations of cells were identified which diverged in mtDNA gene expression, yet these cellular populations did not consistently diverge in nDNA OXPHOS genes expression, nor did they correlate with the expression of glucagon, the hallmark of alpha cells. Thus, pancreatic beta cells within an individual are divided into distinct groups with unique metabolic-mitochondrial signature.

RNA binding protein HuD contributes to β-cell dysfunction by impairing mitochondria dynamics

Imbalanced mitochondrial dynamics in pancreatic β-cells contributes to β-cell dysfunction in diabetes; however, the molecular mechanisms underlying mitochondrial dynamics in the pathology of diabetes are not fully elucidated. We previously reported the reduction of RNA binding protein HuD in pancreatic β-cells of diabetes. Herein, we demonstrate that HuD plays a novel role in the regulation of mitochondrial dynamics by promoting mitochondrial fusion. We show enhanced mitochondrial fragmentation in the pancreas of db/db mice and HuD KO mice. Downregulation of HuD increases the number of cells with fragmented mitochondria and reduces the mitochondrial activity determined by mitochondrial membrane potential and ATP production in mouse insulinoma βTC6 cells. HuD binds to 3'-untraslated region of mitofusin 2 (Mfn2) mRNA and positively regulates its expression. Ectopic expression of Mfn2 in βTC6 cells stably expressing short hairpin RNA against HuD (shHuD) restores HuD-mediated mitochondrial dysfunction. Taken together, our results suggest that HuD regulates mitochondrial dynamics by regulating Mfn2 level and its reduced expression leads to mitochondrial dysfunction in pancreatic β-cells.

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.

Mitochondrial Ca2+ Uptake Relieves Palmitate-Induced Cytosolic Ca2+ Overload in MIN6 Cells

Saturated fatty acids contribute to β-cell dysfunction in the onset of type 2 diabetes mellitus. Cellular responses to lipotoxicity include oxidative stress, endoplasmic reticulum (ER) stress, and blockage of autophagy. Palmitate induces ER Ca2+ depletion followed by notable store-operated Ca2+ entry. Subsequent elevation of cytosolic Ca2+ can activate undesirable signaling pathways culminating in cell death. Mitochondrial Ca2+ uniporter (MCU) is the major route for Ca2+ uptake into the matrix and couples metabolism with insulin secretion. However, it has been unclear whether mitochondrial Ca2+ uptake plays a protective role or contributes to lipotoxicity. Here, we observed palmitate upregulated MCU protein expression in a mouse clonal β-cell, MIN6, under normal glucose, but not high glucose medium. Palmitate elevated baseline cytosolic Ca2+ concentration ([Ca2+]i) and reduced depolarization-triggered Ca2+ influx likely due to the inactivation of voltage-gated Ca2+ channels (VGCCs). Targeted reduction of MCU expression using RNA interference abolished mitochondrial superoxide production but exacerbated palmitate-induced [Ca2+]i overload. Consequently, MCU knockdown aggravated blockage of autophagic degradation. In contrast, co-treatment with verapamil, a VGCC inhibitor, prevented palmitate-induced basal [Ca2+]i elevation and defective [Ca2+]i transients. Extracellular Ca2+ chelation as well as VGCC inhibitors effectively rescued autophagy defects and cytotoxicity. These observations suggest enhanced mitochondrial Ca2+ uptake via MCU upregulation is a mechanism by which pancreatic β-cells are able to alleviate cytosolic Ca2+ overload and its detrimental consequences.

Serotonergic regulation of insulin secretion

The exact physiological role for the monoamine serotonin (5-HT) in modulation of insulin secretion is yet to be fully understood. Although the presence of this monoamine in islets of Langerhans is well established, it is only with recent advances that the complex signalling network in islets involving 5-HT is being unravelled. With more than fourteen different 5-HT receptors expressed in human islets and receptor-independent mechanisms in insulin-producing β-cells, our understanding of 5-HT's regulation of insulin secretion is increasing. It is now widely accepted that failure of the pancreatic β-cell to release sufficient amounts of insulin is the main cause of type 2 diabetes (T2D), an ongoing global epidemic. In this context, 5-HT signalling may be of importance. In fact, 5-HT may serve an essential role in regulating the release of insulin and glucagon, the two main hormones that control glucose and lipid homoeostasis. In this review, we will discuss past and current understanding of 5-HT's role in the endocrine pancreas.

Different responses of mouse islets and MIN6 pseudo-islets to metabolic stimulation: a note of caution

MIN6 cells and MIN6 pseudo-islets are popular surrogates for the use of primary beta cells and islets. Even though it is generally agreed that the stimulus-secretion coupling may deviate from that of beta cells or islets, direct comparisons are rare. The present side-by-side comparison of insulin secretion, cytosolic Ca(2+) concentration ([Ca(2+)] i ) and oxygen consumption rate (OCR) points out where similarities and differences exist between MIN6 cells and normal mouse beta cells. In mouse islets and MIN6 pseudo-islets depolarization by 40 mM KCl was a more robust insulinotropic stimulus than 30 mM glucose. In MIN6 pseudo-islets, but not in mouse islets, the response to 30 mM glucose was much lower than to 40 mM KCl and could be suppressed by a preceding stimulation with 40 mM KCl. In MIN6 pseudo-islets, glucose was less effective to raise [Ca(2+)] i than in primary islets. In marked contrast to islets, the OCR response of MIN6 pseudo-islets to 30 mM glucose was smaller than to 40 mM KCl and was further diminished by a preceding stimulation with 40 mM KCl. The same pattern was observed when MIN6 pseudo-islets were cultured in 5 mM glucose. As with insulin secretion memory effects on the OCR remained after wash-out of a stimulus. The differences between MIN6 cells and primary beta cells were generally larger in the responses to glucose than to depolarization by KCl. Thus, the use of MIN6 cells in investigations on metabolic signalling requires particular caution.

CFTR silencing in pancreatic β-cells reveals a functional impact on glucose-stimulated insulin secretion and oxidative stress response

Cystic fibrosis (CF)-related diabetes (CFRD) has become a critical complication that seriously affects the clinical outcomes of CF patients. Although CFRD has emerged as the most common nonpulmonary complication of CF, little is known about its etiopathogenesis. Additionally, whether oxidative stress (OxS), a common feature of CF and diabetes, influences CFRD pathophysiology requires clarification. The main objective of this study was to shed light on the role of the cystic fibrosis transmembrane conductance regulator (CFTR) in combination with OxS in insulin secretion from pancreatic β-cells. CFTR silencing was accomplished in MIN6 cells by stable expression of small hairpin RNAs (shRNA), and glucose-induced insulin secretion was evaluated in the presence and absence of the valuable prooxidant system iron/ascorbate (Fe/Asc; 0.075/0.75 mM) along with or without the antioxidant Trolox (1 mM). Insulin output from CFTR-silenced MIN6 cells was significantly reduced (∼ 70%) at basal and at different glucose concentrations compared with control Mock cells. Furthermore, CFTR silencing rendered MIN6 cells more sensitive to OxS as evidenced by both increased lipid peroxides and weakened antioxidant defense, especially following incubation with Fe/Asc. The decreased insulin secretion in CFTR-silenced MIN6 cells was associated with high levels of NF-κB (the major participant in inflammatory responses), raised apoptosis, and diminished ATP production in response to the Fe/Asc challenge. However, these defects were alleviated by the addition of Trolox, thereby pointing out the role of OxS in aggravating the effects of CFTR deficiency. Our findings indicate that CFTR deficiency in combination with OxS may contribute to endocrine cell dysfunction and insulin secretion, which at least in part may explain the development of CFRD.

Calcium release channel RyR2 regulates insulin release and glucose homeostasis

The type 2 ryanodine receptor (RyR2) is a Ca2+ release channel on the endoplasmic reticulum (ER) of several types of cells, including cardiomyocytes and pancreatic β cells. In cardiomyocytes, RyR2-dependent Ca2+ release is critical for excitation-contraction coupling; however, a functional role for RyR2 in β cell insulin secretion and diabetes mellitus remains controversial. Here, we took advantage of rare RyR2 mutations that were identified in patients with a genetic form of exercise-induced sudden death (catecholaminergic polymorphic ventricular tachycardia [CPVT]). As these mutations result in a "leaky" RyR2 channel, we exploited them to assess RyR2 channel function in β cell dynamics. We discovered that CPVT patients with mutant leaky RyR2 present with glucose intolerance, which was heretofore unappreciated. In mice, transgenic expression of CPVT-associated RyR2 resulted in impaired glucose homeostasis, and an in-depth evaluation of pancreatic islets and β cells from these animals revealed intracellular Ca2+ leak via oxidized and nitrosylated RyR2 channels, activated ER stress response, mitochondrial dysfunction, and decreased fuel-stimulated insulin release. Additionally, we verified the effects of the pharmacological inhibition of intracellular Ca2+ leak in CPVT-associated RyR2-expressing mice, in human islets from diabetic patients, and in an established murine model of type 2 diabetes mellitus. Taken together, our data indicate that RyR2 channels play a crucial role in the regulation of insulin secretion and glucose homeostasis.

Mitochondrial regulation of β-cell function: maintaining the momentum for insulin release

All forms of diabetes share the common etiology of insufficient pancreatic β-cell function to meet peripheral insulin demand. In pancreatic β-cells, mitochondria serve to integrate the metabolism of exogenous nutrients into energy output, which ultimately leads to insulin release. As such, mitochondrial dysfunction underlies β-cell failure and the development of diabetes. Mitochondrial regulation of β-cell function occurs through many diverse pathways, including metabolic coupling, generation of reactive oxygen species, maintenance of mitochondrial mass, and through interaction with other cellular organelles. In this chapter, we will focus on the importance of enzymatic regulators of mitochondrial fuel metabolism and control of mitochondrial mass to pancreatic β-cell function, describing how defects in these pathways ultimately lead to diabetes. Furthermore, we will examine the factors responsible for mitochondrial biogenesis and degradation and their roles in the balance of mitochondrial mass in β-cells. Clarifying the causes of β-cell mitochondrial dysfunction may inform new approaches to treat the underlying etiologies of diabetes.

How the location of superoxide generation influences the β-cell response to nitric oxide

Cytokines impair the function and decrease the viability of insulin-producing β-cells by a pathway that requires the expression of inducible nitric oxide synthase (iNOS) and generation of high levels of nitric oxide. In addition to nitric oxide, excessive formation of reactive oxygen species, such as superoxide and hydrogen peroxide, has been shown to cause β-cell damage. Although the reaction of nitric oxide with superoxide results in the formation of peroxynitrite, we have shown that β-cells do not have the capacity to produce this powerful oxidant in response to cytokines. When β-cells are forced to generate peroxynitrite using nitric oxide donors and superoxide-generating redox cycling agents, superoxide scavenges nitric oxide and prevents the inhibitory and destructive actions of nitric oxide on mitochondrial oxidative metabolism and β-cell viability. In this study, we show that the β-cell response to nitric oxide is regulated by the location of superoxide generation. Nitric oxide freely diffuses through cell membranes, and it reacts with superoxide produced within cells and in the extracellular space, generating peroxynitrite. However, only when it is produced within cells does superoxide attenuate nitric oxide-induced mitochondrial dysfunction, gene expression, and toxicity. These findings suggest that the location of radical generation and the site of radical reactions are key determinants in the functional response of β-cells to reactive oxygen species and reactive nitrogen species. Although nitric oxide is freely diffusible, its biological function can be controlled by the local generation of superoxide, such that when this reaction occurs within β-cells, superoxide protects β-cells by scavenging nitric oxide.

High-throughput luminescent reporter of insulin secretion for discovering regulators of pancreatic Beta-cell function

Defects in insulin secretion play a central role in the pathogenesis of type 2 diabetes, yet the mechanisms driving beta-cell dysfunction remain poorly understood, and therapies to preserve glucose-dependent insulin release are inadequate. We report a luminescent insulin secretion assay that enables large-scale investigations of beta-cell function, created by inserting Gaussia luciferase into the C-peptide portion of proinsulin. Beta-cell lines expressing this construct cosecrete luciferase and insulin in close correlation, under both standard conditions or when stressed by cytokines, fatty acids, or ER toxins. We adapted the reporter for high-throughput assays and performed a 1,600-compound pilot screen, which identified several classes of drugs inhibiting secretion, as well as glucose-potentiated secretagogues that were confirmed to have activity in primary human islets. Requiring 40-fold less time and expense than the traditional ELISA, this assay may accelerate the identification of pathways governing insulin secretion and compounds that safely augment beta-cell function in diabetes.

Nitric oxide induces ataxia telangiectasia mutated (ATM) protein-dependent γH2AX protein formation in pancreatic β cells

In this study, the effects of cytokines on the activation of the DNA double strand break repair factors histone H2AX (H2AX) and ataxia telangiectasia mutated (ATM) were examined in pancreatic β cells. We show that cytokines stimulate H2AX phosphorylation (γH2AX formation) in rat islets and insulinoma cells in a nitric oxide- and ATM-dependent manner. In contrast to the well documented role of ATM in DNA repair, ATM does not appear to participate in the repair of nitric oxide-induced DNA damage. Instead, nitric oxide-induced γH2AX formation correlates temporally with the onset of irreversible DNA damage and the induction of apoptosis. Furthermore, inhibition of ATM attenuates cytokine-induced caspase activation. These findings show that the formation of DNA double strand breaks correlates with ATM activation, irreversible DNA damage, and ATM-dependent induction of apoptosis in cytokine-treated β cells.

The effect of unhealthy β-cells on insulin secretion in pancreatic islets

Background

Insulin secreted by pancreatic islet β-cells is the principal regulating hormone of glucose metabolism and plays a key role in controlling glucose level in blood. Impairment of the pancreatic islet function may cause glucose to accumulate in blood, and result in diabetes mellitus. Recent studies have shown that mitochondrial dysfunction has a strong negative effect on insulin secretion.

Methods

In order to study the cause of dysfunction of pancreatic islets, a multiple cell model containing healthy and unhealthy cells is proposed based on an existing single cell model. A parameter that represents the function of mitochondria is modified for unhealthy cells. A 3-D hexagonal lattice structure is used to model the spatial differences among β-cells in a pancreatic islet. The β-cells in the model are connected through direct electrical connections between neighboring β-cells.

Results

The simulation results show that the low ratio of total mitochondrial volume over cytoplasm volume per β-cell is a main reason that causes some mitochondria to lose their function. The results also show that the overall insulin secretion will be seriously disrupted when more than 15% of the β-cells in pancreatic islets become unhealthy.

Conclusion

Analysis of the model shows that the insulin secretion can be reinstated by increasing the glucokinase level. This new discovery sheds light on antidiabetic medication.

The GTPase Rab37 Participates in the Control of Insulin Exocytosis

Rab37 belongs to a subclass of Rab GTPases regulating exocytosis, including also Rab3a and Rab27a. Proteomic studies indicate that Rab37 is associated with insulin-containing large dense core granules of pancreatic β-cells. In agreement with these observations, we detected Rab37 in extracts of β-cell lines and human pancreatic islets and confirmed by confocal microscopy the localization of the GTPase on insulin-containing secretory granules. We found that, as is the case for Rab3a and Rab27a, reduction of Rab37 levels by RNA interference leads to impairment in glucose-induced insulin secretion and to a decrease in the number of granules in close apposition to the plasma membrane. Pull-down experiments revealed that, despite similar functional effects, Rab37 does not interact with known Rab3a or Rab27a effectors and is likely to operate through a different mechanism. Exposure of insulin-secreting cells to proinflammatory cytokines, fatty acids or oxidized low-density lipoproteins, mimicking physiopathological conditions that favor the development of diabetes, resulted in a decrease in Rab37 expression. Our data identify Rab37 as an additional component of the machinery governing exocytosis of β-cells and suggest that impaired expression of this GTPase may contribute to defective insulin release in pre-diabetic and diabetic conditions.

Mathematical model of metabolism and electrophysiology of amino acid and glucose stimulated insulin secretion: in vitro validation using a β-cell line

We integrated biological experimental data with mathematical modelling to gain insights into the role played by L-alanine in amino acid-stimulated insulin secretion (AASIS) and in D-glucose-stimulated insulin secretion (GSIS), details important to the understanding of complex β-cell metabolic coupling relationships. We present an ordinary differential equations (ODEs) based simplified kinetic model of core metabolic processes leading to ATP production (glycolysis, TCA cycle, L-alanine-specific reactions, respiratory chain, ATPase and proton leak) and Ca(2+) handling (essential channels and pumps in the plasma membrane) in pancreatic β-cells and relate these to insulin secretion. Experimental work was performed using a clonal rat insulin-secreting cell line (BRIN-BD11) to measure the consumption or production of a range of important biochemical parameters (D-glucose, L-alanine, ATP, insulin secretion) and Ca(2+) levels. These measurements were then used to validate the theoretical model and fine-tune the parameters. Mathematical modelling was used to predict L-lactate and L-glutamate concentrations following D-glucose and/or L-alanine challenge and Ca(2+) levels upon stimulation with a non metabolizable L-alanine analogue. Experimental data and mathematical model simulations combined suggest that L-alanine produces a potent insulinotropic effect via both a stimulatory impact on β-cell metabolism and as a direct result of the membrane depolarization due to Ca(2+) influx triggered by L-alanine/Na(+) co-transport. Our simulations indicate that both high intracellular ATP and Ca(2+) concentrations are required in order to develop full insulin secretory responses. The model confirmed that K(+) ATP channel independent mechanisms of stimulation of intracellular Ca(2+) levels, via generation of mitochondrial coupling messengers, are essential for promotion of the full and sustained insulin secretion response in β-cells.

Metabolome response to glucose in the β-cell line INS-1 832/13

Glucose-stimulated insulin secretion (GSIS) from pancreatic β-cells is triggered by metabolism of the sugar to increase ATP/ADP ratio that blocks the KATP channel leading to membrane depolarization and insulin exocytosis. Other metabolic pathways believed to augment insulin secretion have yet to be fully elucidated. To study metabolic changes during GSIS, liquid chromatography with mass spectrometry was used to determine levels of 87 metabolites temporally following a change in glucose from 3 to 10 mM glucose and in response to increasing concentrations of glucose in the INS-1 832/13 β-cell line. U-[(13)C]Glucose was used to probe flux in specific metabolic pathways. Results include a rapid increase in ATP/ADP, anaplerotic tricarboxylic acid cycle flux, and increases in the malonyl CoA pathway, support prevailing theories of GSIS. Novel findings include that aspartate used for anaplerosis does not derive from the glucose fuel added to stimulate insulin secretion, glucose flux into glycerol-3-phosphate, and esterification of long chain CoAs resulting in rapid consumption of long chain CoAs and de novo generation of phosphatidic acid and diacylglycerol. Further, novel metabolites with potential roles in GSIS such as 5-aminoimidazole-4-carboxamide ribotide (ZMP), GDP-mannose, and farnesyl pyrophosphate were found to be rapidly altered following glucose exposure.

Ablation of TSC2 enhances insulin secretion by increasing the number of mitochondria through activation of mTORC1

Aim

We previously found that chronic tuberous sclerosis protein 2 (TSC2) deletion induces activation of mammalian target of rapamycin Complex 1 (mTORC1) and leads to hypertrophy of pancreatic beta cells from pancreatic beta cell-specific TSC2 knockout (βTSC2^−/−) mice. The present study examines the effects of TSC2 ablation on insulin secretion from pancreatic beta cells.

Methods

Isolated islets from βTSC2^−/− mice and TSC2 knockdown insulin 1 (INS-1) insulinoma cells treated with small interfering ribonucleic acid were used to investigate insulin secretion, ATP content and the expression of mitochondrial genes.

Results

Activation of mTORC1 increased mitochondrial DNA expression, mitochondrial density and ATP production in pancreatic beta cells of βTSC2^−/− mice. In TSC2 knockdown INS-1 cells, mitochondrial DNA expression, mitochondrial density and ATP production were increased compared with those in control INS-1 cells, consistent with the phenotype of βTSC2^−/− mice. TSC2 knockdown INS-1 cells also exhibited augmented insulin secretory response to glucose. Rapamycin inhibited mitochondrial DNA expression and ATP production as well as insulin secretion in response to glucose. Thus, βTSC2^−/− mice exhibit hyperinsulinemia due to an increase in the number of mitochondria as well as enlargement of individual beta cells via activation of mTORC1.

Conclusion

Activation of mTORC1 by TSC2 ablation increases mitochondrial biogenesis and enhances insulin secretion from pancreatic beta cells.

Control of precursor maturation and disposal is an early regulative mechanism in the normal insulin production of pancreatic β-cells

The essential folding and maturation process of proinsulin in β-cells is largely uncharacterized. To analyze this process, we improved approaches to immunoblotting, metabolic labeling, and data analysis used to determine the proportion of monomers and non-monomers and changes in composition of proinsulin in cells. We found the natural occurrence of a large proportion of proinsulin in various non-monomer states, i.e., aggregates, in normal mouse and human β-cells and a striking increase in the proportion of proinsulin non-monomers in Ins2^+/Akita mice in response to a mutation (C96Y) in the insulin 2 (Ins2) gene. Proinsulin emerges in monomer and abundant dual-fate non-monomer states during nascent protein synthesis and shows heavy and preferential ATP/redox-sensitive disposal among secretory proteins during early post-translational processes. These findings support the preservation of proinsulin's aggregation-prone nature and low relative folding rate that permits the plentiful production of non-monomer forms with incomplete folding. Thus, in normal mouse/human β-cells, proinsulin's integrated maturation and degradation processes maintain a balance of natively and non-natively folded states, i.e., proinsulin homeostasis (PIHO). Further analysis discovered the high susceptibility of PIHO to cellular energy and calcium changes, endoplasmic reticulum (ER) and reductive/oxidative stress, and insults by thiol reagent and cytokine. These results expose a direct correlation between various extra-/intracellular influences and (a)typical integrations of proinsulin maturation and disposal processes. Overall, our findings demonstrated that the control of precursor maturation and disposal acts as an early regulative mechanism in normal insulin production, and its disorder is crucially linked to β-cell failure and diabetes pathogenesis.

Long-term exposure of INS-1 rat insulinoma cells to linoleic acid and glucose in vitro affects cell viability and function through mitochondrial-mediated pathways

Obesity with excessive levels of circulating free fatty acids (FFAs) is tightly linked to the incidence of type 2 diabetes. Insulin resistance of peripheral tissues and pancreatic β-cell dysfunction are two major pathological changes in diabetes and both are facilitated by excessive levels of FFAs and/or glucose. To gain insight into the mitochondrial-mediated mechanisms by which long-term exposure of INS-1 cells to excess FFAs causes β-cell dysfunction, the effects of the unsaturated FFA linoleic acid (C 18:2, n-6) on rat insulinoma INS-1 β cells was investigated. INS-1 cells were incubated with 0, 50, 250 or 500 μM linoleic acid/0.5% (w/v) BSA for 48 h under culture conditions of normal (11.1 mM) or high (25 mM) glucose in serum-free RPMI-1640 medium. Cell viability, apoptosis, glucose-stimulated insulin secretion, Bcl-2, and Bax gene expression levels, mitochondrial membrane potential and cytochrome c release were examined. Linoleic acid 500 μM significantly suppressed cell viability and induced apoptosis when administered in 11.1 and 25 mM glucose culture medium. Compared with control, linoleic acid 500 μM significantly increased Bax expression in 25 mM glucose culture medium but not in 11.1 mM glucose culture medium. Linoleic acid also dose-dependently reduced mitochondrial membrane potential (ΔΨm) and significantly promoted cytochrome c release from mitochondria in both 11.1 mM glucose and 25 mM glucose culture medium, further reducing glucose-stimulated insulin secretion, which is dependent on normal mitochondrial function. With the increase in glucose levels in culture medium, INS-1 β-cell insulin secretion function was deteriorated further. The results of this study indicate that chronic exposure to linoleic acid-induced β-cell dysfunction and apoptosis, which involved a mitochondrial-mediated signal pathway, and increased glucose levels enhanced linoleic acid-induced β-cell dysfunction.

Intracellular coenzymes as natural biomarkers for metabolic activities and mitochondrial anomalies

Mitochondria play a pivotal role in energy metabolism, programmed cell death and oxidative stress. Mutated mitochondrial DNA in diseased cells compromises the structure of key enzyme complexes and, therefore, mitochondrial function, which leads to a myriad of health-related conditions such as cancer, neurodegenerative diseases, diabetes and aging. Early detection of mitochondrial and metabolic anomalies is an essential step towards effective diagnoses and therapeutic intervention. Reduced nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (FAD) play important roles in a wide range of cellular oxidation-reduction reactions. Importantly, NADH and FAD are naturally fluorescent, which allows noninvasive imaging of metabolic activities of living cells and tissues. Furthermore, NADH and FAD autofluorescence, which can be excited using distinct wavelengths for complementary imaging methods and is sensitive to protein binding and local environment. This article highlights recent developments concerning intracellular NADH and FAD as potential biomarkers for metabolic and mitochondrial activities.

Low-level arsenic impairs glucose-stimulated insulin secretion in pancreatic beta cells: involvement of cellular adaptive response to oxidative stress

BACKGROUND

Chronic exposure of humans to inorganic arsenic, a potent environmental oxidative stressor, is associated with incidence of type 2 diabetes (T2D). A key driver in the pathogenesis of T2D is impairment of pancreatic beta-cell function, with the hallmark of beta-cell function being glucose-stimulated insulin secretion (GSIS). Reactive oxygen species (ROS) derived from glucose metabolism serve as one of the metabolic signals for GSIS. Nuclear factor-erythroid 2-related factor 2 (Nrf2) is a central transcription factor regulating cellular adaptive response to oxidative stress.

OBJECTIVES

We tested the hypothesis that activation of Nrf2 and induction of antioxidant enzymes in response to arsenic exposure impedes glucose-triggered ROS signaling and thus GSIS.

METHODS AND RESULTS

Exposure of INS-1(832/13) cells to low levels of arsenite led to decreased GSIS in a dose- and time-dependent fashion. Consistent with our hypothesis, a significantly enhanced Nrf2 activity, determined by its nuclear accumulation and induction of its target genes, was observed in arsenite-exposed cells. In keeping with the activation of Nrf2-mediated antioxidant response, intracellular glutathione and intracellular hydrogen peroxide-scavenging activity was dose dependently increased by arsenite exposure. Although the basal cellular peroxide level was significantly enhanced, the net percentage increase in glucose-stimulated intracellular peroxide production was markedly inhibited in arsenite-exposed cells. In contrast, insulin synthesis and the consensus GSIS pathway, including glucose transport and metabolism, were not significantly reduced by arsenite exposure.

CONCLUSIONS

Our studies suggest that low levels of arsenic provoke a cellular adaptive oxidative stress response that increases antioxidant levels, dampens ROS signaling involved in GSIS, and thus disturbs beta-cell function.

Huang-Lian-Jie-Du-Tang supplemented with Schisandra chinensis Baill. and Polygonatum odoratum Druce improved glucose tolerance by potentiating insulinotropic actions in islets in 90% pancreatectomized diabetic rats

We investigated to determine what effects, if any, the respective water extracts of Radix scutellariae (RS), Fructus schisandrae chinensis (FSC), Huang-Lian-Jie-Du-Tang (HLJDT), and HLJDT supplemented with FSC, and Rhizoma Polygonati odorati (HLJDT-M) would have on glucose tolerance by modulating glucose-stimulated insulin secretion, beta-cell mass, and morphometry in 90% pancreatectomized (Px) diabetic rats fed high-fat diets. Through the elevation of intracellular cAMP levels, FSC RS, HLJDT, and HLJDT-M increased insulin secretion in Min6 cells and GLP-1 secretion in NCI-H716 cells. After an 8-week period of treatment, it was found that HLJDT-M improved glucose tolerance in an oral glucose tolerance test in Px rats. HLJDT-M also potentiated first- and second-phase insulin secretion, but RS and HLJDT elevated only the second phase at hyperglycemic clamp. RS and HLJDT increased beta-cell mass by hyperplasia and hypertrophy, while HLJDT-M increased it only by hyperplasia. The rise in hyperplasia was associated with elevated IRS2 and PDX-1 expression in the islets. In conclusion, HLJDT-M worked as an anti-diabetic prescription by enhancing insulinotropic actions in diabetic rats.

The GTPase RalA regulates different steps of the secretory process in pancreatic beta-cells

Background

RalA and RalB are multifuntional GTPases involved in a variety of cellular processes including proliferation, oncogenic transformation and membrane trafficking. Here we investigated the mechanisms leading to activation of Ral proteins in pancreatic β-cells and analyzed the impact on different steps of the insulin-secretory process.

Methodology/Principal Findings

We found that RalA is the predominant isoform expressed in pancreatic islets and insulin-secreting cell lines. Silencing of this GTPase in INS-1E cells by RNA interference led to a decrease in secretagogue-induced insulin release. Real-time measurements by fluorescence resonance energy transfer revealed that RalA activation in response to secretagogues occurs within 3–5 min and reaches a plateau after 10–15 min. The activation of the GTPase is triggered by increases in intracellular Ca²⁺ and cAMP and is prevented by the L-type voltage-gated Ca²⁺ channel blocker Nifedipine and by the protein kinase A inhibitor H89. Defective insulin release in cells lacking RalA is associated with a decrease in the secretory granules docked at the plasma membrane detected by Total Internal Reflection Fluorescence microscopy and with a strong impairment in Phospholipase D1 activation in response to secretagogues. RalA was found to be activated by RalGDS and to be severely hampered upon silencing of this GDP/GTP exchange factor. Accordingly, INS-1E cells lacking RalGDS displayed a reduction in hormone secretion induced by secretagogues and in the number of insulin-containing granules docked at the plasma membrane.

Conclusions/Significance

Taken together, our data indicate that RalA activation elicited by the exchange factor RalGDS in response to a rise in intracellular Ca²⁺ and cAMP controls hormone release from pancreatic β-cell by coordinating the execution of different events in the secretory pathway.

Maternal low-protein diet alters pancreatic islet mitochondrial function in a sex-specific manner in the adult rat

Mitochondrial dysfunction may be a long-term consequence of a poor nutritional environment during early life. Our aim was to investigate whether a maternal low-protein (LP) diet may program mitochondrial dysfunction in islets of adult progeny before glucose intolerance ensues. To address this, pregnant Wistar rats were fed isocaloric diets containing either 20% protein (control) or 8% protein (LP diet) throughout gestation. From birth, offspring received the control diet. The mitochondrial function was analyzed in islets of 3-mo-old offspring. Related to their basal insulin release, cultured islets from both male and female LP offspring presented a lower response to glucose challenge and a blunted ATP production compared with control offspring. The expression of malate dehydrogenase as well as the subunit 6 of the ATP synthase encoded by mitochondrial genome (mtDNA) was lower in these islets, reducing the capacity of ATP production through the Krebs cycle and oxidative phosphorylation. However, mtDNA content was unchanged in LP islets compared with control. Several consequences of protein restriction during fetal life were more marked in male offspring. Only LP males showed an increased reactive oxygen species production associated with a higher expression of mitochondrial subunits of the electron transport chain NADH-ubiquinone oxireductase subunit 4L, an overexpression of peroxisome proliferator-activated receptor-gamma and uncoupling protein-2, and a strongly reduced beta-cell mass. In conclusion, mitochondrial function is clearly altered in islets from LP adult offspring in a sex-specific manner. That may provide a cellular explanation for the earlier development of glucose intolerance in male than in female offspring of dams fed an LP diet.

Mitochondrial glutamate carrier GC1 as a newly identified player in the control of glucose-stimulated insulin secretion

The SLC25 carrier family mediates solute transport across the inner mitochondrial membrane, a process that is still poorly characterized regarding both the mechanisms and proteins implicated. This study investigated mitochondrial glutamate carrier GC1 in insulin-secreting beta-cells. GC1 was cloned from insulin-secreting cells, and sequence analysis revealed hydropathy profile of a six-transmembrane protein, characteristic of mitochondrial solute carriers. GC1 was found to be expressed at the mRNA and protein levels in INS-1E beta-cells and pancreatic rat islets. Immunohistochemistry showed that GC1 was present in mitochondria, and ultrastructural analysis by electron microscopy revealed inner mitochondrial membrane localization of the transporter. Silencing of GC1 in INS-1E beta-cells, mediated by adenoviral delivery of short hairpin RNA, reduced mitochondrial glutamate transport by 48% (p < 0.001). Insulin secretion at basal 2.5 mM glucose and stimulated either by intermediate 7.5 mM glucose or non-nutrient 30 mM KCl was not modified by GC1 silencing. Conversely, insulin secretion stimulated with optimal 15 mM glucose was reduced by 23% (p < 0.005) in GC1 knocked down cells compared with controls. Adjunct of cell-permeant glutamate (5 mM dimethyl glutamate) fully restored the secretory response at 15 mM glucose (p < 0.005). Kinetics of insulin secretion were investigated in perifused isolated rat islets. GC1 silencing in islets inhibited the secretory response induced by 16.7 mM glucose, both during first (-25%, p < 0.05) and second (-33%, p < 0.05) phases. This study demonstrates that insulin-secreting cells depend on GC1 for maximal glucose response, thereby assigning a physiological function to this newly identified mitochondrial glutamate carrier.

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.

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

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

Matrix alkalinization: a novel mitochondrial signal for sustained pancreatic beta-cell activation

Nutrient secretagogues activate mitochondria of the pancreatic beta-cell through the provision of substrate, hyperpolarisation of the inner mitochondrial membrane and mitochondrial calcium rises. We report that mitochondrial matrix pH, a parameter not previously studied in the beta-cell, also exerts an important control function in mitochondrial metabolism. During nutrient stimulation matrix pH alkalinises, monitored by the mitochondrial targeted fluorescent pH-sensitive protein mtAlpHi or (31)P-NMR inorganic phosphate chemical shifts following saturation transfer. Compared with other cell types, the resting mitochondrial pH was surprisingly low, rising from pH 7.25 to 7.7 during nutrient stimulation of rat beta-cells. As cytosolic alkalinisation to the nutrient was of much smaller amplitude, the matrix alkalinisation was accompanied by a pronounced increase of the DeltapH across the inner mitochondrial membrane. Furthermore, matrix alkalinisation closely correlates with the cytosolic ATP net increase, which is also associated with elevated ATP synthesis rates in mitochondria. Preventing DeltapH increases in permeabilised cells abrogated substrate-driven ATP synthesis. We propose that the mitochondrial pH and DeltapH are key determinants of mitochondrial energy metabolism and metabolite transport important for cell activation.

Malic enzyme is present in mouse islets and modulates insulin secretion

AIMS/HYPOTHESIS

The pyruvate-malate shuttle is a metabolic cycle in pancreatic beta cells and is important for beta cell function. Cytosolic malic enzyme (ME) carries out an essential step in the shuttle by converting malate to pyruvate and generating NADPH. In rat islets the pyruvate-malate shuttle may regulate insulin secretion and it has been shown to play a critical role in adaptation to obesity and insulin resistance. However, ME has not been demonstrated in mouse islets and three reports indicate that mouse islets contain no ME activity. If mouse islets lack ME, rat and mouse islets must regulate insulin secretion by different mechanisms.

METHODS

We measured ME activity by a fluorometric enzymatic assay and Me mRNA by real-time PCR. ME activity was also measured in streptozotocin-treated mouse islets. FACS-purified beta cells were obtained from MIP-GFP mouse islets, agouti-L obese mouse islets and mouse beta cell line MIN-6. Insulin secretion and NADPH/NADP(+) ratios were measured in Me siRNA-treated beta cells.

RESULTS

ME activity and Me mRNA were present in C57BL/6 mouse islets. ME activity was reduced in streptozotocin-treated mouse islets. ME activity was also measurable in FACS-purified mouse beta cells. In addition, ME activity was significantly increased in obese agouti-L mouse islets and the mouse MIN-6 cell line. Me siRNA inhibited ME activity and reduced glucose-stimulated insulin secretion and also inhibited NADPH products.

CONCLUSIONS/INTERPRETATION

Mouse islets contain ME, which plays a significant role in regulating insulin secretion.

Heterogeneous expression of SNARE proteins SNAP-23, SNAP-25, Syntaxin1 and VAMP in human parathyroid tissue

In regulated exocytosis synaptosomal-associated protein of 25kDa (SNAP-25) is one of the key-players in the formation of SNARE (soluble N-ethylmaleimide-sensitive fusion attachment protein receptor) complex and membrane fusion. SNARE proteins are essentially expressed in neurons, neuroendocrine and endocrine cells. Whether parathyroid cells express these proteins is not known. In this study, we have examined the expression of the SNARE protein SNAP-25 and its cellular homologue SNAP-23, as well as syntaxin1 and VAMP (vesicle-associated membrane protein) in samples of normal parathyroid tissue, chief cell adenoma, and parathyroid carcinoma, using immunohistochemistry and Western blot analysis. SNAP-23 and VAMP were evenly expressed in all studied parathyroid tissues using immunohistochemistry and/or Western blot analysis. SNAP-25 (and Syntaxin1) was not expressed in normal parathyroid tissue, but in approximately 20% of chief cell adenomas, and in approximately 45% of parathyroid carcinoma samples. It is likely that the SNARE proteins SNAP-23 and VAMP play a role in the stimulus-secretion coupling and exocytosis of parathyroid hormone as these proteins were expressed in all of the parathyroid samples we studied. In particular, preferential expression of SNAP-23 rather than SNAP-25 provides an explanation of the high level of PTH secretion that occurs under conditions of low cytoplasmic free Ca(2+) concentration (around 0.1micromol/l). SNAP-25 (and Syntaxin1) appears to be a tumour-specific protein(s) in parathyroid tissues since its expression was restricted to pathological tissues.

Noncoding miRNAs as key controllers of pancreatic β-cell functions

miRNAs, a recently discovered family of small noncoding RNAs, are emerging as major controllers of gene expression and key determinants of pancreatic β-cell function. These 19-22-nucleotide molecules govern gene expression by partially pairing to 3´-untranslated regions of target mRNAs and by inhibiting their translation. The elucidation of the role of miRNAs promises to unravel new aspects of β-cell biology and to clarify the mechanisms leading to defective insulin secretion in diabetes mellitus. This information is expected to favor the design of new approaches for preserving functional β-cells in prediabetic stages and the development of strategies for engineering insulin-secreting cells capable of replacing endogenous β-cells in diabetic patients.

Reactive oxygen species as a signal in glucose-stimulated insulin secretion

One of the unique features of beta-cells is their relatively low expression of many antioxidant enzymes. This could render beta-cells susceptible to oxidative damage but may also provide a system that is sensitive to reactive oxygen species as signals. In isolated mouse islets and INS-1(832/13) cells, glucose increases intracellular accumulation of H2O2. In both models, insulin secretion could be stimulated by provision of either exogenous H2O2 or diethyl maleate, which raises intracellular H2O2 levels. Provision of exogenous H2O2 scavengers, including cell permeable catalase and N-acetyl-L-cysteine, inhibited glucose-stimulated H2O2 accumulation and insulin secretion (GSIS). In contrast, cell permeable superoxide dismutase, which metabolizes superoxide into H2O2, had no effect on GSIS. Because oxidative stress is an important risk factor for beta-cell dysfunction in diabetes, the relationship between glucose-induced H2O2 generation and GSIS was investigated under various oxidative stress conditions. Acute exposure of isolated mouse islets or INS-1(832/13) cells to oxidative stressors, including arsenite, 4-hydroxynonenal, and methylglyoxal, led to decreased GSIS. This impaired GSIS was associated with increases in a battery of endogenous antioxidant enzymes. Taken together, these findings suggest that H2O2 derived from glucose metabolism is one of the metabolic signals for insulin secretion, whereas oxidative stress may disturb its signaling function.

Involvement of Ca2+, CaMK II and PKA in EGb 761-induced insulin secretion in INS-1 cells

EGb 761, a standardized form of Ginkgo biloba L. (Ginkgoaceae) leaf extract, was recently reported to increase pancreatic beta-cell function. To determine whether EGb 761 elicits insulin secretion directly, we treated INS-1 rat beta cells with EGb 761 and then measured insulin release. Treatment of EGb 761 (50 microg/ml) significantly stimulated insulin secretion in INS-1 cells, compared with untreated control (p<0.05) and the stimulatory effect of EGb 761 on insulin secretion was dose-dependent. To elucidate the mechanism of EGb 761-induced insulin secretion, we investigated the involvement of calcium. The treatment with nifedipine, an L-type calcium channel blocker, prevented EGb 761-induced insulin secretion and furthermore, EGb 761 itself elevated [Ca(2+)](i), suggesting the involvement of calcium in this process. To identity the protein kinases involved in EGb 761-induced insulin secretion, INS-1 cells were treated with different kinase inhibitors and their effects on EGb 761-induced secretion were investigated. KN62 and H89, calium/calmodulin kinase (CaMK) II and protein kinase A (PKA) inhibitor, respectively, significantly reduced EGb 761-induced insulin secretion. Immunoblotting studies showed an increase in the phosphorylated-forms of CaMK II and of PKA substrates after EGb 761 treatment. Our data suggest that EGb 761-induced insulin secretion is mediated by [Ca(2+)](i) elevation and subsequent activation of CaMK II and PKA.