sign in sign up
<<
Home , Reactive oxygen species

11 REVIEW

Reactive and nitrogen species generation, defenses, and b- function: a critical role for amino acids

P Newsholme, E Rebelato1, F Abdulkader1, M Krause2,3, A Carpinelli1 and R Curi1 School of Biomedical Sciences, Curtin University, PO Box U1987, Perth, Western Australia 6845, Australia 1Department of and Biophysics, Institute of Biomedical Sciences, University Sao Paulo (USP), Sao Paulo, Brazil 2Biomedical Research Group, Department of Science, Institute of Technology Tallaght, Dublin, Ireland 3UCD Institute for Sport and Health, Dublin, Ireland (Correspondence should be addressed to P Newsholme; Email: [email protected]; M Krause at UCD Institute for Sport and Health; Email: [email protected], [email protected])

Abstract Growing evidence indicates that the regulation of intracellular systems are expressed in b cells at higher levels. Herein, we discuss reactive oxygen species (ROS) and reactive nitrogen species the key mechanisms of ROS/RNS production and their (RNS)levelsisessentialformaintainingnormalb-cell glucose physiological function in pancreatic b cells. We also hypothesize responsiveness. While long-term exposure to high glucose that specific interactions between RNS and ROS may be the induces in b cells, conflicting results have been cause of the vulnerability of pancreatic b cells to oxidative damage. published regarding the impact of ROSon acute glucose exposure In addition, using a hypothetical metabolic model based on the and their role in glucose stimulated secretion (GSIS). data available in the literature, we emphasize the importance of Although b cells are considered to be particularly vulnerable to amino acid availability for GSH synthesis and for the maintenance oxidative damage, as they express relatively low levels of some of b-cell function and viability during periods of metabolic peroxide-metabolizing such as and disturbance before the clinical onset of diabetes. (GSH) peroxidase, other less known GSH-based antioxidant Journal of Endocrinology (2012) 214, 11–20

Introduction target . Being a charged species, cannot freely cross biological membranes but may do so via anion Reactive oxygen species (ROS) such as superoxide anion channels. However, the fate of superoxide in cells and tissues %K (O2 ), (H2O2), and the hydroxyl is mostly determined by the activity of various site-specific % K (OH ) plus the related peroxynitrite (ONOO ) are enzymes (extracellular, cytoplasmic, and mitochondrial), the generally thought to cause cell dysfunction and ultimately (SOD) family, that convert superoxide death due to alteration of i) metabolic pathway activity into molecular oxygen and hydrogen peroxide (Fig. 1). (Newsholme et al. 2007a, 2009b) and/or ii) the structure of H2O2 is an even less reactive species that is uncharged and cellular membranes, DNA, or proteins (Boveris et al. 1972, can diffuse across membranes through aquaporins. Despite its Turrens 1997, Johnson et al. 1999, Chandra et al. 2000, low reactivity, some proteins contain specific cysteine residues Limon-Pacheco & Gonsebatt 2009). However, the half-life that are prone to oxidation by hydrogen peroxide, which are and reactivity of these various species are very different, which critical to hydrogen peroxide-based signaling systems. H2O2 % is an indication of the different biological functions of these can be converted to the OH , a highly reactive species (see (Droge 2002). Fig. 1). Superoxide can also react with nitric oxide (NO), %K K Formation of the O2 can be considered to be the initial resulting in the formation of peroxynitrite (ONOO ), see step for the subsequent formation of other ROS. It is Fig. 1. Cell-associated oxidative damage may not be simply a generated by the single reduction of molecular question of the overall concentration of ROS, but rather the oxygen (O2). Superoxide, compared with to other free type of ROS formed with respect to relative reactivity. radicals, is a poorly reactive species and can exist in solution Hydrogen peroxide is the substrate for the majority of the for a considerable time (and thus diffuse) before reacting with antioxidant systems in the cell. These antioxidant systems other free radicals or with specific clusters of –sulfur in (many of which are enzymes) are required to minimize the

Journal of Endocrinology (2012) 214, 11–20 DOI: 10.1530/JOE-12-0072 0022–0795/12/0214–011 q 2012 Society for Endocrinology Printed in Great Britain Online version via http://www.endocrinology-journals.org

Downloaded from Bioscientifica.com at 09/25/2021 06:56:41AM via free access 12 P NEWSHOLME and others . Amino acids and b-cell redox regulation

Figure 1 ROS/RNS species formation and antioxidant defense. ROS and NO may be generated by appropriate/inappropriate stimuli. These molecules may become toxic unless rapidly removed by the pathways described. GSSG, glutathione disulfide; GSH, glutathione; GSHRED, ; GSHPER, ; CAT, catalase; SOD, superoxide dismutase; ETC, ; NOX, NADPH oxidase; IKK, I k B kinase; NFkB, nuclear factor kB; iNOS, inducible nitric oxide synthase.

% reaction controlling the conversion of H2O2 to HO ,a availability for GSH synthesis and for the maintenance of highly reactive molecule. Superoxide may also chemically b-cell function and viability during periods of metabolic combine with the reactive nitrogen species (RNS), NO in a disturbance before the clinical onset of diabetes. diffusion-controlled reaction, resulting in the formation of K peroxynitrite (ONOO ). When cellular antioxidant defenses are overcome, ROS specificity is lost leading to The impact of ROS in pancreatic b cells reaction with non-readily oxidizable amino acid residues, as well as with unsaturated lipids and DNA, therefore leading to b Cells adequately deal with the physiological challenges of oxidative stress. substrate availability imposed both acutely and chronically Pancreatic b cells are considered to be particularly depending on the nutritional and metabolic states (Schmitt vulnerable to oxidative damage (Lenzen 2008b), as it has et al. 2011). The availability and the type of antioxidant been reported that they express relatively low levels of catalase defenses in these cells are dictated by demand. There is now a and glutathione (GSH) peroxidase, which would contribute consensus that the chronically high circulating levels of to lipotoxicity, glucotoxicity, or a combination termed glucose (glucotoxic concentrations) and/or lipid (glucolipo- glucolipotoxicity in b cells chronically exposed to nutrients, toxic or lipotoxic concentrations) associated with type 2 diabetes favoring (Robertson et al. 1992, Newsholme et al. induce oxidative stress in different cell types (Newsholme 2007a,b, Gehrmann et al. 2010). Interestingly, impairment, or et al.2007a, Gehrmann et al.2010). In , otherwise, of b-cell function may depend on the antioxidant induced by autoimmune b-cell destruction, death is associated system involved in the scavenging process, as specific with cytokine-mediated oxidative stress (Morgan et al. 2007, manipulation in antioxidant defenses may result in different Lenzen 2008a). outcomes (Lenzen 2008b). Oxidative stress is currently viewed as an imbalance Herein, we critically revise the major mechanisms of between pro- and in favor of the former, ROS/RNS production and implications for the physiological which implicates a loss of redox signaling. It can be triggered function of pancreatic b cells. In addition, using a by excessive ROS production as well as by low antioxidant hypothetical metabolic model, based on the data available activities. A well-known source of in the literature, we emphasize the importance of amino acid for reduction of molecular oxygen is the .

Journal of Endocrinology (2012) 214, 11–20 www.endocrinology-journals.org

Downloaded from Bioscientifica.com at 09/25/2021 06:56:41AM via free access Amino acids and b-cell redox regulation . P NEWSHOLME and others 13

The increase in superoxide formation in the electron Numerous papers have described the negative effects of transport chain is associated with a high (inner) mitochondrial NO generation in the b cell including attenuation of glucose- membrane potential. This causes a decrease in the electron stimulated insulin secretion and stimulation of apoptosis, flow through the respiratory chain, increasing the probability originating from the observation that pro-inflammatory of superoxide formation by the retained electrons at various cytokines induced gene and expression of iNOS sites in the mitochondrial respiratory chain. However, an and subsequent NO generation, which was associated with additional specialized enzyme-based system can also generate caspase activation and cell death (Rizzo & Piston 2003). superoxide in a regulated fashion. This enzyme complex, the It is possible that NOX2-derived ROS in the latter NADPH oxidase (NOX), is a member of a family of enzyme conditions may also trigger concomitant nitrosative stress isoforms that are able to transfer electrons from NADPH to (Michalska et al. 2010), thus suggesting that in toxic %K molecular oxygen to generate O2 . NOX activation is more conditions, a key mediator for dysfunction and apoptosis widely associated with efficient killing of pathogens by would be peroxynitrite. , such as macrophages, monocytes, dendritic cells, However, another example of a deleterious pathway that and , that form ROS from NOX2 within the mediates substrate or cytokine toxicity is the formation of phagosomal membrane (Bylund et al. 2010). NOX-derived hydroxyl radicals via the Fenton reaction. In this situation, an ROS have been shown to stimulate mitogenic signaling abnormal increase in mitochondrial electron leakage or in and proliferation (Murrell et al. 1990, Arnold et al. 2001). NOX2 activity could overcome the antioxidant defenses It is now evident that NOX isoforms (NOX1, NOX2, allowing the reaction of H2O2 with transition metals C % and NOX4) are also expressed in pancreatic b cells (Oliveira (i.e. Fe2 ) producing HO . In addition, another toxic et al. 2003, Uchizono et al. 2006), where their function pathway was recently described, which was associated with is related to regulation of insulin secretion and cell integrity cytokine-dependent b-cell death, which was derived from an (as discussed below). interaction between RNS and ROS. This pathway for Interestingly, exposure to high levels of metabolic fuels cytokine-induced b-cell apoptosis involves an interaction in vivo is not necessarily associated with NOX2-derived of mitochondrion-derived hydrogen peroxide with NO, oxidative stress and decreased function of pancreatic b cells. in which the presence of trace metals leads to hydroxyl Islets isolated from Wistar fed a high-fat diet for 13 weeks radical formation resulting in b-cell death (Gurgul-Convey resulted in increased pancreatic islet functionality, associated et al. 2011). with high levels of glucose and GSIS but also low Likewise, the cytokine death pathway involving either K % levels of NOX2 expression and ROS production (Valle et al. ONOO or HO formation may increase ROS production 2011). On the other hand, when Sprague Dawley rats (which via increased NOX2 activity, as the treatment of b cells with a are prone to develop insulin resistance and obesity (Reaven & cytokine mixture (IL1b, TNFa, and INFg) increased the Reaven 1981)) were submitted to a high-fat diet for a longer expression of p47, the key cytosolic regulator protein of period (24 weeks), diabetes ensued that was associated NOX2 (Michalska et al. 2010). Additionally, iNOS and with increased islet ROS and reduced insulin expression NOX2 share a same transcription activator factor, NF-kB, (Yuan et al. 2010). Moreover, human islets isolated from which in turn is increased in concentration by cytokine type 2 diabetic patients demonstrated increased mRNA levels exposure in b cells, thus increasing the possibility of of the p22 subunit of NAD(P)H oxidase (Marchetti et al.1998). nitro-oxidative stress. Therefore, a reduction in islet NOX2 expression and thus Interestingly, the stress pathways driven by either cytokine ROS production, in vivo, may constitute an adaptive response or chronically increased fuel levels appear to share common- of the pancreatic b cell to handle high levels of metabolic ality in the formation of nitro-oxidative species. An increase fuels. However, if the stimulus is maintained for a sufficiently in intracellular oxidative stress triggers an increase in the long period (taking into account the different susceptibility of proteins involved in the formation of NOX2 and iNOS animal models, for instance Wistar vs Sprague Dawley rats), (Michalska et al. 2010), which could provoke a deleterious K % this adaptive response may fail to operate and the system effect, either by ONOO or by HO formation (Fig. 1). would then operate in a positive feedback loop mode, enhancing NOX2 expression and oxidative stress, leading to impaired insulin secretion and overt type 2 diabetes. Physiological function of RNS generated in the Chronic glucose exposure also induces the expression and pancreatic b cell activity of inducible NO synthase (iNOS) in pancreatic b cells (Meidute-Abaraviciene et al. 2009). Moreover, other toxic Numerous papers have described the negative effects of NO stimuli as pro-inflammatory cytokines, saturated non- generation in the b cell, including attenuation of glucose- esterified fatty acids, or direct addition of ROS induce stress stimulated insulin secretion and stimulation of apoptosis, resulting in upregulation of iNOS in the pancreatic b cells, originating from the observation that pro-inflammatory thus causing generation of high levels of NO that are cytokines induced gene and protein expression of iNOS associated with reduced insulin secretion and apoptotic cell and subsequent NO generation, which was associated with death (Michalska et al. 2010). caspase activation and cell death (Te j e d o et al. 1999). www.endocrinology-journals.org Journal of Endocrinology (2012) 214, 11–20

Downloaded from Bioscientifica.com at 09/25/2021 06:56:41AM via free access 14 P NEWSHOLME and others . Amino acids and b-cell redox regulation

However, it is now clear that enhanced glucose metabolism in integrity that includes insulin secretion (Sun et al. 2010), C the b cell, in response to an increase in glucose availability, AMP, Ca2 , and NO. apart from involving activation of glycolysis, the tricarboxylic Recent studies involving engineered liver cells demon- acid (TCA) cycle, and oxidative phosphorylation to produce strated that expression of either human insulin or the ATP, also triggers additional signaling mechanisms involving b-cell-specific transcription factors PDX1, NeuroD1, the modulation of nitrogen species at intracellular levels and MafA in the Hepa1–6 cell line or primary liver cells via (Rizzo & Piston 2003, Leloup et al. 2009, Gray & Heart adenoviral gene transfer resulted in production and secretion of 2010). Posttranslational modification by S-nitrosylation insulin. However, insulin secretion was not significantly requires the modification of the sulfhydryl side chain of increased in response to high glucose. Interestingly, L-arginine specific cysteine residues within a protein by NO. This stimulated insulin secretion up to threefold, an effect dependent on the production of NO (Muniappan & Ozcan 2007). modification is dynamic, like that of phosphorylation, C although it proceeds in the absence of a protein catalyst and Interactions of NO with K ATP channel activity are is responsive to nutrient availability. A family of NOS concentration dependent and complex. NO released from enzymes, for example, nNOS, eNOS, and iNOS, can donors in the micromole range have been reported to produce synthesize NO endogenously from L-arginine, oxygen, and membrane hyperpolarization and channel opening due to NADPH. iNOS is increased in concentration in response to attenuation of glucose metabolism in b cells (Drews et al. fatty acids, pro-inflammatory cytokines, and L-arginine 2010). NO diminished mitochondrial ATP production by depolarizing DJ (Drews et al. 2010). NO may interact directly (Michalska et al. 2010, Krause et al. 2011). C 2C In human islets and MIN6 pancreatic b cells, with K ATP channels or L-type Ca channels (Drews et al. 2010) presumably via SH-groups in the channel protein. S-nitrosylation of syntaxin 4 was reported to occur in response C to an acute (5 min) glucose stimulation. S-nitrosylation NO suppressed K ATP channel activity in b cells via a cGMP/PKG-dependent pathway but at higher concentrations mapped specifically to Cys141 and facilitated activation of C syntaxin 4 as determined by association with its cognate increased the K ATP current by inhibition of ATP production (Drews et al. 2000). Genetic or pharmacological ablation of v-SNARE VAMP2 (Wiseman et al. 2011). Stimulation of C K channels protected b cells against an oxidant insult MIN6 b cells with inflammatory cytokines TNFa, IL1b, ATP (Drews et al. 2010). The NO donor sodium nitroprusside (SNP) and IFNg for 2 h resulted in an induction of syntaxin 4, attenuated glucose-induced but not glyceraldehyde-induced S-nitrosylation, and stimulation of insulin secretion in the C K channel inhibition (Drews et al.2010), suggesting that absence of glucose (Wiseman et al. 2011). Thus, this site for ATP NO may target a glycolytic step upstream to glyceraldehyde- S-nitrosylation, which is important for physiological pro- 3-phosphate, hence reducing critical glycolytic NADH cesses, may also be involved in dysregulated NO-mediated production. Indeed, it has been reported that SNP inhibits processes that could contribute to the onset of secretory glyceraldehyde-3-phosphate dehydrogenase in insulin-secreting b dysregulation in pancreatic cells subjected to inflammatory RINm5F cells (Drews et al. 2010). stress and/or chronic stimulation. AMP-activated protein kinase (AMPK) has been reported to be a target for NO action in the b cell (Meares et al. 2010). Antioxidants in pancreatic b cells and the role AMPK is critical to b-cell functional integrity as on activation of glucose AMPK can phosphorylate many downstream effectors so as to reduce ATP-consuming processes and promote ATP- Although b cells express low levels of certain antioxidant producing processes (Meares et al.2010). LKB1 can defenses (i.e. SOD, catalase, and GSH peroxidase (Ivarsson phosphorylate AMPK on threonine 172 and, together with et al. 2005, Lenzen 2008b)), it has been previously shown that increased AMP, activates AMPK (Hawley et al. 2005, Hardie they are particularly rich in other peroxidase-based anti- 2007). LKB1 is thought to be the predominant kinase oxidant defenses, such as glutaredoxin and responsible for the activating phosphorylation of AMPK; (Ivarsson et al. 2005). Interestingly, modulation of b-cell however, calmodulin-dependent protein kinase kinase b function may depend on the antioxidant system involved in (CaMKKb) can act as an alternative activator of AMPK, the scavenging process, as specific manipulation in antioxidant independent of cellular energy status but responsive to defenses promoted differential results, e.g. modulation of 2C changes in intracellular Ca (Hawley et al. 2005). either glutaredoxin or thioredoxin levels altered exocytosis in AMPK was recently reported to play a critical role in the the presence of NADPH in a reciprocal fashion (Ivarsson et al. functional recovery of b cells from NO-induced damage. 2005) while mitochondrial SOD overexpression increased Indeed, pro-inflammatory cytokines activated AMPK in a cytokine-induced apoptosis in b cells (Lortz et al. 2005). NO-dependent fashion and AMPK functioned to attenuate While long-term exposure to high glucose induces death and promote the functional recovery of b cells from oxidative stress in b cells, conflicting results have been NO-mediated stress (Meares et al. 2010). Thus, it is possible to published regarding the levels of ROS on acute glucose consider three interdependent mechanisms responsible for exposure and their role in GSIS. Despite evidence from AMPK activation and promotion of b-cell functional mouse cells that ROS enhanced insulin secretion at low

Journal of Endocrinology (2012) 214, 11–20 www.endocrinology-journals.org

Downloaded from Bioscientifica.com at 09/25/2021 06:56:41AM via free access Amino acids and b-cell redox regulation . P NEWSHOLME and others 15 glucose (Pi et al. 2007, Leloup et al. 2009), both previous and recent results in islets indicate that glucose acutely decreases ROS content in islets, an effect that may involve reduction in mitochondrial ROS production (Martens et al. 2005) and/or increased antioxidant output from the pentose– phosphate pathway (Rebelato et al. 2011). Indeed, the evidence from rat islets indicates that antioxidant production acutely surpasses ROS production on exposure to glucose load, (Martens et al. 2005) and that the control of ROS levels is important for insulin secretion (Zhang et al. 2010). Another class of metabolic substrate, fatty acids, acutely enhanced ROS levels in islets, at least in part through activation of NOX (Morgan et al. 2007, Graciano et al. 2011, Santos et al. 2011). However, the mechanisms underlying this activation and their functional role seem to differ depending on the type of fatty acid analyzed. The palmitic acid activates NOX at high glucose level in a manner independent on fatty acid oxidation (Graciano et al. 2011), whereas oleic acid- induced NOX activation is metabolism-dependent (Santos et al. 2011). Figure 2 The cellular balance of redox state and b-cell function. We wish to present a hypothetical model of the pro-oxidant In this diagram, the lower part represents ROS/RNS production and antioxidant behavior in b cells required for the (increasing levels will result in clockwise direction change). The maintenance of cell function and viability (Fig. 2). Events upper part represents antioxidant (mainly glutathione) production leading to ROS/RNS production are detailed in the lower (increasing levels will result in clockwise direction change). Both arrows can change independently. This figure indicates that in b-cell part of Fig. 2 (increasing levels will result in clockwise function (insulin secretion), ROS/RNS production may increase; direction change). Antioxidant (mainly GSH) production in however, the levels of antioxidants are also increased and surpass response to oxidant challenge is indicated in the upper part of ROS/RNS production, protecting the cells from oxidative damage. Fig. 2 (increasing levels will result in clockwise direction In order to avoid the deleterious effects of free radicals generated during metabolic overload, the antioxidant levels must accompany change). The arrows can change in direction independently. this change overlapping the ROS/RNS production. If the pancreatic b Cells must maintain their function (insulin secretion) under b cell is exposed to prolonged metabolic overload (i.e. hyper- normal physiological conditions, which is accomplished by glycemia/hyperlipemia), ROS and RNS production may overcome the fact that the concentration/activity of antioxidants the antioxidant defenses, thus causing oxidative damage and cell anticipates and adjusts to ROS/RNS production at a level dysfunction. Under this condition, antioxidant production increases to a higher level in order to maintain cell viability, shifting that is able to protect the cells from oxidative damage but b-cell metabolism away from a key role in energy generation and maintain redox signaling (Fig. 2). stimulus–secretion coupling and toward a catabolic state that may If the pancreatic b cell is exposed to prolonged metabolic be related to cell defense. overload (i.e. hyperglycemia/hyperlipidemia), ROS and RNS production may cause oxidative damage and cell toxicity in various cell types and irreversible b-cell dysfunction. Under this condition, antioxidant production dysfunction driven, at least in part, by chronic oxidative may increase to a higher level in order to maintain cell stress caused by continuous hyperglycemia (Numazawa et al. viability, shifting b-cell metabolism away from a key role in 2008). ROS increases levels of protein oxidation, DNA energy generation and stimulus–secretion coupling and oxidation, and . Consequently, oxidative toward a catabolic state, which may be related to cell defense stress originating from hyperglycemia would be a major cause (Fig. 2). As long as the pancreatic b cells receive all relevant of impaired islet function at the level of insulin synthesis macro- and micro-nutrients (i.e. amino acids, vitamins, and and secretion. minerals) for the synthesis of antioxidants (especially GSH), In pancreatic b cells, GSH seems to be the most important they will be able to suppress the harmful effects of increased tool to protect the cells against oxidative damage (Newsholme ROS/RNS generation. et al. 2009a, Krause et al. 2011). GSH is the major low- molecular weight thiol in mammalian cells. The cellular GSH redox buffer is present in cells at millimolar concentrations Proposed model for pancreatic b-cell dysfunction and maintains redox critical to appropriate on the time course of diabetes protein conformation (Schafer & Buettner 2001). Recent studies have demonstrated that an acute reduction in ROS During the progression of type 2 diabetes, deterioration of concentration induced by glucose was correlated with insulin secretion due to pancreatic b-cell dysfunction results increased pentose phosphate pathway activity (Lenzen in progressive hyperglycemia. This can result in glucose 2008b) and increased insulin secretion. As increased pentose www.endocrinology-journals.org Journal of Endocrinology (2012) 214, 11–20

Downloaded from Bioscientifica.com at 09/25/2021 06:56:41AM via free access 16 P NEWSHOLME and others . Amino acids and b-cell redox regulation

phosphate pathway activity will generate NADPH, we would LDH and a-HBDH catalyze the synthesis of a-HB when the like to suggest that GSH levels could have been elevated levels of NADH are high, a condition found in diabetic and under such conditions leading to an acute reduction in ROS obese patients as they have higher fatty acid oxidation rates that concentration but increased insulin secretion. Synthesis of result in NADH production. GSH involves the use of three key amino acids: glutamate, Glutamate, another key amino acid for GSH synthesis, can glycine, and cysteine. Changes in the availability of these be synthesized from glutamine (Newsholme 2001). The likely amino acids may lead to decreased levels of GSH and source of glutamine is the skeletal muscle, which is known to ultimately cell death. The rate-limiting step in de novo increase the release of this amino acid in starvation and synthesis of GSH is a reaction catalyzed by cytosolic pathophysiological conditions such as insulin resistance g-glutamylcysteine synthetase (g-GCS). On the other hand, (conditions that also result in branched chain amino acid L-cysteine availability is also an important factor to maintain release; Wa ng et al. 2011). The continuous uptake of GSH levels (Numazawa et al. 2008). In b cells, the transport of L-glutamine by the liver and by immune cells makes a cystine/cysteine is mediated by a L-cystine/-glutamate significant quantitative contribution to a reduction in plasma K exchanger (system Xc ; Numazawa et al. 2008). Physiological L-glutamine (Krause Mda & de Bittencourt 2008). Indeed, K flux via system Xc involves the entry of L-cystine and exit of plasma glutamine levels are known to be significantly reduced K L-glutamate. L-cystine imported by system Xc is reduced to in type 2 diabetes mellitus (T2DM) (Menge et al. 2010). form L-cysteine and is used as a substrate for GSH synthesis. If Glycine, the third key amino acid for GSH synthesis, is the activity of the L-cystine/L-glutamate exchanger is found decreased in the plasma of insulin-resistant ob/ob mice restricted to reduce the levels of intracellular GSH, b cells (Altmaier et al. 2008). The possible reasons for the decreased become susceptible to oxidative stress (Numazawa et al. 2008). levels of glycine are increased glycine uptake by the liver and Importantly, excess extracellular glutamate reverses the also that elevated glucagon levels can lead to increased K activity of the glutamate/cystine antiporter system Xc , oxidation of this amino acid (Gall et al. 2010). Serine, an thus depleting the cells of cysteine (Murphy et al. 1989). Thus, important amino acid for glycine and cysteine synthesis, is extracellular concentrations of glutamate can determine the known to be reduced in T2DM, confirming the importance availability of cysteine for the intracellular synthesis of GSH. of these amino acids for GSH synthesis under oxidative stress L-Glutamate can be released by pancreatic a and b cells in conditions (Gall et al. 2010). pathophysiological conditions such as pro-inflammatory Islet structure and thus its function display considerable challenge (Kiely et al. 2007), a condition likely to occur in diversity between vertebrate species. Moreover, islet structure diabetes. Therefore, we propose that under low-grade is not static but undergoes changes in response to normal inflammation and metabolic overload, both a and b cells physiological and pathophysiological conditions, including will release glutamate to the extracellular islet environment, diabetes. In general, human islets present with fewer b cells causing accumulation of this amino acid and the inhibition of and more a cells than rodent islets (for further information see cysteine uptake leading to the failure of GSH production, Steiner et al. (2010)). Another important difference between resulting in oxidative stress and finally cell death (Fig. 3A). human and rodent islets is the fact that human b cells do not Under these conditions, L-arginine and L-glutamine avail- express the key urea cycle enzyme ornithine transcarbamylase; ability are important for GSH synthesis as they participate thus, the ornithine produced from arginase activity may be in intracellular glutamate production (Kiely et al. 2007, converted to glutamate and GSH or used for polyamine Krause et al. 2011). synthesis, with profound implications for b-cell functional A physiological model is illustrated in Fig. 3B. Hyper- integrity (Ohtani et al. 2009). glycemia and dyslipidemia are features of diabetes and The pro-inflammatory state associated with T2DM may pre-diabetes. They can induce oxidative stress in many tissues lead to activation of immune cells, such as macrophages. and, if associated with obesity, can lead to the production of These cells, in the presence of pro-inflammatory cytokines, pro-inflammatory cytokines that will culminate in a chronic can secrete other cytokines, such as IL1b (which causes pro-inflammatory state and further oxidative damage dysfunctional effects in b cells), but they can also secrete the (Newsholme et al. 2009a). Under this condition, the liver enzyme arginase (Krause et al. 2011). Increased levels of will increase GSH production (Lord & Bralley 2008). arginase and the subsequent degradation of extracellular Increased synthesis of cysteine is a key regulatory mechanism L-arginine to urea and ornithine may explain the low levels for GSH synthesis, and the oxidative stress state can induce the of this amino acid in T2DM (Pieper & Dondlinger 1997, activation of cysteine formation from and serine Krause et al. 2011). (Lord & Bralley 2008). Cystathionase catalyses the conversion The repercussions of reduced availability of amino acids for of cystathionine into cysteine and also forms a by-product pancreatic b cells are many: pro-inflammatory cytokines called a-ketobutyrate (a-KB) that can be converted into (from the plasma or from infiltrate macrophages) can stimulate a-hydroxybutyrate (a-HB) in a reaction catalyzed by lactate glutamate release through specific glutamate transporters (and dehydrogenase (LDH) and/or by hydroxybutyrate dehydro- many other effects) that will culminate in the reversal or genase (a-HBDH). Indeed, when a-HB levels are elevated, this inhibition of cysteine transporters leading to lower intra- condition can predict diabetes development (Gall et al. 2010). cellular cystine/cysteine levels. Secretion of arginase from

Journal of Endocrinology (2012) 214, 11–20 www.endocrinology-journals.org

Downloaded from Bioscientifica.com at 09/25/2021 06:56:41AM via free access Amino acids and b-cell redox regulation . P NEWSHOLME and others 17

Figure 3 Hypothetical models for the failure of b-cell antioxidant production with respect to the time course of diabetes. (A) In b cells and pancreatic islets of Langerhans: failure in GSH synthesis can result in changes at redox state, loss of b-cell function, and cell death. GSH synthesis involves three key amino acids: glutamate, glycine, and cysteine. Changes in the availability of these amino acids may lead to decreased levels of GSH and cell death. L-glutamine and L-arginine are also important for intracellular glutamate synthesis. Extracellular glutamate plays an essential role on the b-cell function/survival. This amino acid is an inhibitor of cysteine transport. Increased levels of glutamate (from b and/or a cells) can be induced by pro-inflammatory cytokines and metabolic overload, leading to a blockage of cysteine uptake. When the level of cystine/cysteine falls, glutathione levels decrease, leading to oxidative stress and cell death. (B) In vivo model: hyperglycemia and dyslipidemia are features of diabetes and pre-diabetes. They can induce oxidative stress in many tissues and, if associated with obesity (adipose expansion), can lead to the production of pro-inflammatory cytokines that will culminate in a pro- inflammatory state (low-grade inflammation) and further oxidative damage. Under this condition, the liver is requested to increase glutathione production. Increased synthesis of cysteine is a key regulatory mechanism for GSH synthesis, and the oxidative stress state can induce the activation of cysteine formation from methionine and serine. Cystathionase (1) catalyses the conversion of cystathionine into cysteine and also forms a by-product called a-KB that can be converted into a-HB in a reaction catalyzed by LDH and/or by a-HBDH. Indeed, a-HB levels are higher and also predict diabetes development (see text for reference) and prove that this pathway is very active in diabetes and pre-diabetes. Glutamate, another key amino acid for GSH synthesis, can be synthesized from glutamine. The likely source of glutamine is the skeletal muscle, which is known to increase the release of this amino acid especially in a state of an insulin resistance state (a condition that is also known to induce BCAA release). Continuous uptake of L-glutamine by the liver (and by immunological cells) can lead to a fall in the amino acid plasma concentration (as indicated by the arrow). Glycine, the third key amino acid for GSH synthesis, is found decreased in the plasma of diabetic subjects. The reasons for this are as follows: increased glycine uptake by the liver and elevated glucagon levels can lead to increased oxidation of this amino acid. Serine, an important amino acid for glycine and cysteine synthesis, is decreased in type 2 diabetes. The pro-inflammatory state may lead to activation of immunological cells, such as macrophages. These cells, in the presence of pro-inflammatory cytokines, can secrete inflammatory factors, such as IL1b (with several effects in b cells), but they can also secrete the enzyme arginase. Increased levels of arginase and the degradation of L-arginine to urea and ornithine may explain the lower levels of this amino acid in type 2 diabetic patients. Repercussion for the pancreatic b cells: pro-inflammatory cytokines (from the plasma or from infiltrate macrophages) can stimulate glutamate secretion (and many other effects) that will culminate in the inhibition of the cystine leading to lower intracellular glutamate exchanger/cysteine levels. Secretion of arginase from infiltrated macrophages depletes L-arginine in the islet microenvironment leading to the lack of this amino acid. Together, lower arginine, glutamine, cysteine, and glycine lead to the decreased intracellular availability of these amino acids, essential for glutathione synthesis. A lower antioxidant defense culminates into oxidative stress, cell dysfunction, and b-cell death. *The key amino acids for GSH synthesis. www.endocrinology-journals.org Journal of Endocrinology (2012) 214, 11–20

Downloaded from Bioscientifica.com at 09/25/2021 06:56:41AM via free access 18 P NEWSHOLME and others . Amino acids and b-cell redox regulation

islet-infiltrated macrophages would deplete L-arginine in the Technology Tallaght, Dublin, Ireland; UCD School of Biomolecular and islet microenvironment leading to the lack of this amino acid. Biomedical Science; Department of Physiology and Biophysics, Institute of Together, lower arginine, glutamine, cysteine, and glycine Biomedical Sciences, University Sao Paulo (USP), Sao Paulo, Brazil; and UCD Institute for Sport and Health and the TSR: Strand III – Core Research lead to the decreased availability of amino acids for GSH Strengths Enhancement Scheme (Ireland) for supporting their work. synthesis. The resulting compromised antioxidant defenses culminate in oxidative stress and cellular dysfunction finally result in b-cell death. References

Conclusions and perspectives Altmaier E, Ramsay SL, Graber A, Mewes HW, Weinberger KM & Suhre K 2008 Bioinformatics analysis of targeted metabolomics – uncovering old Weconclude that pancreatic b-cell function and viability depends and new tales of diabetic mice under medication. Endocrinology 149 on the balance between ROS/RNS and GSH production in 3478–3489. (doi:10.1210/en.2007-1747) Arnold RS, Shi J, Murad E, Whalen AM, Sun CQ, Polavarapu R, order to maintain normal cell function and insulin secretion but Parthasarathy S, Petros JA & Lambeth JD 2001 Hydrogen peroxide mediates without causing oxidative damage. This balance must be the cell growth and transformation caused by the mitogenic oxidase Nox1. maintained even in conditions of metabolic overload, when PNAS 98 5550–5555. (doi:10.1073/pnas.101505898) GSH synthesis plays a crucial role for the protection of the b cell. Boveris A, Oshino N & Chance B 1972 The cellular production of hydrogen Under normal physiological conditions if the b cells receive all peroxide. Biochemical Journal 128 617–630. the required amino acids for GSH synthesis, this balance is Bylund J, Brown KL, Movitz C, Dahlgren C & Karlsson A 2010 Intracellular generation of superoxide by the NADPH oxidase: how, where, maintained. However, during the development of T2DM when and what for? Free Radical Biology & Medicine 49 1834–1845. (doi:10.1016/j. the availability of amino acids (such as cysteine, arginine, freeradbiomed.2010.09.016) glutamine,andglycine)isreduced,GSHsynthesisdecreases, Chandra J, Samali A & Orrenius S 2000 Triggering and modulation of resulting in reduced b-cell protection and loss of integrity. We apoptosis by oxidative stress. Free Radical Biology & Medicine 29 323–333. here propose that a key cause of b-cell failure, during the (doi:10.1016/S0891-5849(00)00302-6) development of diabetes, is the reduced levels of critical amino Drews G, Kramer C & Krippeit-Drews P 2000 Dual effect of NO on K(C)(ATP) current of mouse pancreatic B-cells: stimulation by acids required for GSH formation. Supplementation with these deenergizing mitochondria and inhibition by direct interaction with the key amino acids may be a novel approach to support the channel. Biochimica et Biophysica Acta 1464 62–68. (doi:10.1016/S0005- maintenance and function of b cells in the face of nutrient 2736(99)00242-4) overload, thus allowing adequate glycemic control to prevail. Drews G, Krippeit-Drews P & Dufer M 2010 Oxidative stress and beta-cell dysfunction. Pflu¨gers Archiv: European Journal of Physiology 460 703–718. (doi:10.1007/s00424-010-0862-9) Declaration of interest Droge W 2002 Free radicals in the physiological control of cell function. Physiological Reviews 82 47–95. (doi:10.1152/physrev.00018.2001) The authors declare that there is no conflict of interest that could be perceived Gall WE, Beebe K, Lawton KA, Adam KP,Mitchell MW,Nakhle PJ, Ryals JA, as prejudicing the impartiality of the review reported. Milburn MV,Nannipieri M, Camastra S et al. 2010 Alpha-hydroxybutyrate is an early biomarker of insulin resistance and glucose intolerance in a nondiabetic population. PLoS ONE 5 e10883. (doi:10.1371/journal.pone. Funding 0010883) Gehrmann W, Elsner M & Lenzen S 2010 Role of metabolically generated This work was supported by the School of Biomedical Sciences, Curtin reactive oxygen species for lipotoxicity in pancreatic beta-cells. Diabetes, University, Perth, WA, Australia; FAPESP; the Department of Science, Obesity & Metabolism 12 (Suppl 2) 149–158. (doi:10.1111/j.1463-1326. Institute of Technology Tallaght, Dublin, Ireland; the UCD School of 2010.01265.x) Biomolecular and Biomedical Science; Department of Physiology and Graciano MF, Santos LR, Curi R & Carpinelli AR 2011 NAD(P)H oxidase Biophysics, Institute of Biomedical Sciences, University Sao Paulo (USP), participates in the palmitate-induced superoxide production and insulin Sao Paulo, Brazil; and UCD Institute for Sport and Health and the TSR: secretion by rat pancreatic islets. Journal of Cellular Physiology 226 Strand III – Core Research Strengths Enhancement Scheme (Ireland). 1110–1117. (doi:10.1002/jcp.22432) Gray JP & Heart E 2010 Usurping the mitochondrial supremacy: extramitochondrial sources of reactive oxygen intermediates and their role Author contribution statement in beta cell metabolism and insulin secretion. Toxicology Mechanisms and Methods 20 167–174. (doi:10.3109/15376511003695181) P N provided the initial ideas and stimulus for writing this review, which was Gurgul-Convey E, Mehmeti I, Lortz S & Lenzen S 2011 Cytokine toxicity in conceived during a period of academic leave at the University Sao Paulo, Brazil. insulin-producing cells is mediated by nitro-oxidative stress-induced P N, E R, F A, and M K contributed ideas and input for writing and revision of formation in mitochondria. Journal of Molecular Medicine 89 the text of the review while M K additionally provided all Figures. A Carpinelli 785–798. (doi:10.1007/s00109-011-0747-1) and R Curi provided critical comment on the content of the review. Hardie DG 2007 AMP-activated/SNF1 protein kinases: conserved guardians of cellular energy. Nature Reviews. Molecular Cell Biology 8 774–785. (doi:10.1038/nrm2249) Acknowledgements Hawley SA, Pan DA, Mustard KJ, Ross L, Bain J, Edelman AM, Frenguelli BG & Hardie DG 2005 Calmodulin-dependent protein kinase kinase-beta is an The authors thank the School of Biomedical Sciences, Curtin University, alternative upstream kinase for AMP-activated protein kinase. Perth, WA, Australia; FAPESP; the Department of Science, Institute of Cell Metabolism 2 9–19. (doi:10.1016/j.cmet.2005.05.009)

Journal of Endocrinology (2012) 214, 11–20 www.endocrinology-journals.org

Downloaded from Bioscientifica.com at 09/25/2021 06:56:41AM via free access Amino acids and b-cell redox regulation . P NEWSHOLME and others 19

Ivarsson R, Quintens R, Dejonghe S, Tsukamoto K, in ’t Veld P, Renstrom E and activity of a phagocyte-like NADPH oxidase in rat pancreatic islets & Schuit FC 2005 Redox control of exocytosis: regulatory role of and a clonal beta cell line. Diabetologia 50 359–369. (doi:10.1007/s00125- NADPH, thioredoxin, and glutaredoxin. Diabetes 54 2132–2142. 006-0462-6) (doi:10.2337/diabetes.54.7.2132) Muniappan L & Ozcan S 2007 Induction of insulin secretion in engineered Johnson FB, Sinclair DA & Guarente L 1999 Molecular biology of aging. Cell liver cells by nitric oxide. BMC Physiology 7 11. (doi:10.1186/1472- 96 291–302. (doi:10.1016/S0092-8674(00)80567-X) 6793-7-11) Kiely A, McClenaghan NH, Flatt PR & Newsholme P 2007 Pro- Murphy TH, Miyamoto M, Sastre A, Schnaar RL & Coyle JT 1989 inflammatory cytokines increase glucose, alanine and triacylglycerol Glutamate toxicity in a neuronal cell line involves inhibition of cystine utilization but inhibit insulin secretion in a clonal pancreatic beta-cell line. transport leading to oxidative stress. Neuron 2 1547–1558. (doi:10.1016/ Journal of Endocrinology 195 113–123. (doi:10.1677/JOE-07-0306) 0896-6273(89)90043-3) Krause Mda S & de Bittencourt PI Jr 2008 Type 1 diabetes: can exercise Murrell GA, Francis MJ & Bromley L 1990 Modulation of fibroblast impair the autoimmune event? The L-arginine/glutamine coupling proliferation by oxygen free radicals. Biochemical Journal 265 659–665. hypothesis Cell Biochemistry and Function 26 406–433. (doi:10.1002/cbf. Newsholme P 2001 Why is L-glutamine metabolism important to cells of the 1470) in health, postinjury, surgery or infection? Journal of Krause MS, McClenaghan NH, Flatt PR, Homem de Bittencourt PI, Nutrition 131 2515S–2522S (discussion 2523S–2514S). Murphy C & Newsholme P 2011 L-Arginine is essential for Newsholme P, Haber EP, Hirabara SM, Rebelato EL, Procopio J, Morgan D, pancreatic {beta}-cell functional integrity, metabolism and defense from Oliveira-Emilio HC, Carpinelli AR & Curi R 2007a Diabetes associated inflammatory challenge. Journal of Endocrinology 211 87–97. (doi:10.1530/ cell stress and dysfunction: role of mitochondrial and non-mitochondrial JOE-11-0236) ROS production and activity. Journal of Physiology 583 9–24. (doi:10.1113/ Leloup C, Tourrel-Cuzin C, Magnan C, Karaca M, Castel J, Carneiro L, jphysiol.2007.135871) Colombani AL, Ktorza A, Casteilla L & Penicaud L 2009 Mitochondrial Newsholme P, Keane D, Welters HJ & Morgan NG 2007b Life and death reactive oxygen species are obligatory signals for glucose-induced insulin decisions of the pancreatic beta-cell: the role of fatty acids. Clinical Science secretion. Diabetes 58 673–681. (doi:10.2337/db07-1056) 112 27–42. (doi:10.1042/CS20060115) Lenzen S 2008a The mechanisms of alloxan- and streptozotocin-induced Newsholme P,Homem De Bittencourt PI, O’ Hagan C, De Vito G, Murphy C diabetes. Diabetologia 51 216–226. (doi:10.1007/s00125-007-0886-7) & Krause MS 2009a Exercise and possible molecular mechanisms Lenzen S 2008b Oxidative stress: the vulnerable beta-cell. Biochemical Society of protection from vascular disease and diabetes: the central role of Transactions 36 343–347. (doi:10.1042/BST0360343) ROS and nitric oxide. Clinical Science 118 341–349. (doi:10.1042/ Limon-Pacheco J & Gonsebatt ME 2009 The role of antioxidants and CS20090433) antioxidant-related enzymes in protective responses to environmentally Newsholme P, Morgan D, Rebelato E, Oliveira-Emilio HC, Procopio J, induced oxidative stress. Mutation Research 674 137–147. (doi:10.1016/ Curi R & Carpinelli A 2009b Insights into the critical role of NADPH j.mrgentox.2008.09.015) oxidase(s) in the normal and dysregulated pancreatic beta cell. Lord RS & Bralley JA 2008 Clinical applications of urinary organic acids. Diabetologia 52 Part I: detoxification markers. Alternative Medicine Review 13 205–215. 2489–2498. (doi:10.1007/s00125-009-1536-z) Lortz S, Gurgul-Convey E, Lenzen S & Tiedge M 2005 Importance of Numazawa S, Sakaguchi H, Aoki R, Taira T & Yoshida T 2008 Regulation of mitochondrial superoxide dismutase expression in insulin-producing cells the susceptibility to oxidative stress by cysteine availability in pancreatic for the toxicity of reactive oxygen species and proinflammatory cytokines. beta-cells. American Journal of Physiology. Cell Physiology 295 C468–C474. Diabetologia 48 1541–1548. (doi:10.1007/s00125-005-1822-3) (doi:10.1152/ajpcell.00203.2008) Marchetti P, Lupi R, Lorenzetti M, Giannarelli R, Del Guerra S, Tellini C, Ohtani M, Mizuno I, Kojima Y,Ishikawa Y,Sodeno M, Asakura Y,Samejima K Coppelli A, Lencioni C, Marselli L, Carmellini M et al. 1998 Pancreatic & Oka T 2009 Spermidine regulates insulin synthesis and cytoplasmic glucagon damages isolated human islet function. Transplantation Proceedings Ca(2C) in mouse beta-TC6 insulinoma cells. Cell Structure and Function 34 30 397. (doi:10.1016/S0041-1345(97)01338-9) 105–113. (doi:10.1247/csf.09008) Martens GA, Cai Y, Hinke S, Stange G, Van de Casteele M & Pipeleers D Oliveira HR, Verlengia R, Carvalho CR, Britto LR, Curi R & Carpinelli AR 2005 Glucose suppresses superoxide generation in metabolically responsive 2003 Pancreatic beta-cells express phagocyte-like NAD(P)H oxidase. pancreatic beta cells. Journal of Biological Chemistry 280 20389–20396. Diabetes 52 1457–1463. (doi:10.2337/diabetes.52.6.1457) (doi:10.1074/jbc.M411869200) Pi J, Bai Y,Zhang Q, Wong V,Floering LM, Daniel K, Reece JM, Deeney JT, Meares GP, Hughes KJ, Jaimes KF, Salvatori AS, Rhodes CJ & Corbett JA Andersen ME, Corkey BE et al. 2007 Reactive oxygen species as a signal in 2010 AMP-activated protein kinase attenuates nitric oxide-induced beta- glucose-stimulated insulin secretion. Diabetes 56 1783–1791. (doi:10.2337/ cell death. Journal of Biological Chemistry 285 3191–3200. (doi:10.1074/jbc. db06-1601) M109.047365) Pieper GM & Dondlinger LA 1997 Plasma and vascular tissue arginine are Meidute-Abaraviciene S, Mosen H, Lundquist I & Salehi A 2009 decreased in diabetes: acute arginine supplementation restores Imidazoline-induced amplification of glucose- and carbachol-stimulated -dependent relaxation by augmenting cGMP production. insulin release includes a marked suppression of islet nitric oxide generation Journal of Pharmacology and Experimental Therapeutics 283 684–691. in the mouse. Acta Physiologica 195 375–383. (doi:10.1111/j.1748-1716. Reaven EP & Reaven GM 1981 Structure and function changes in the 2008.01896.x) endocrine pancreas of aging rats with reference to the modulating effects of Menge BA, Schrader H, Ritter PR, Ellrichmann M, Uhl W, Schmidt WE & exercise and caloric restriction. Journal of Clinical Investigation 68 75–84. Meier JJ 2010 Selective amino acid deficiency in patients with impaired (doi:10.1172/JCI110256) glucose tolerance and type 2 diabetes. Regulatory Peptides 160 75–80. Rebelato E, Abdulkader F, Curi R & Carpinelli AR 2011 Control of the (doi:10.1016/j.regpep.2009.08.001) intracellular redox state by glucose participates in the insulin secretion Michalska M, Wolf G, Walther R & Newsholme P 2010 Effects of mechanism. PLoS ONE 6 e24507. (doi:10.1371/journal.pone.0024507) pharmacological inhibition of NADPH oxidase or iNOS on pro- Rizzo MA & Piston DW 2003 Regulation of beta cell glucokinase by S- inflammatory cytokine, palmitic acid or H2O2-induced mouse islet or nitrosylation and association with nitric oxide synthase. Journal of Cell clonal pancreatic beta-cell dysfunction. Bioscience Reports 30 445–453. Biology 161 243–248. (doi:10.1083/jcb.200301063) (doi:10.1042/BSR20090138) Robertson RP,Zhang HJ, Pyzdrowski KL & Walseth TF 1992 Preservation of Morgan D, Oliveira-Emilio HR, Keane D, Hirata AE, Santos da Rocha M, insulin mRNA levels and insulin secretion in HIT cells by avoidance of Bordin S, Curi R, Newsholme P & Carpinelli AR 2007 Glucose, chronic exposure to high glucose concentrations. Journal of Clinical palmitate and pro-inflammatory cytokines modulate production Investigation 90 320–325. (doi:10.1172/JCI115865) www.endocrinology-journals.org Journal of Endocrinology (2012) 214, 11–20

Downloaded from Bioscientifica.com at 09/25/2021 06:56:41AM via free access 20 P NEWSHOLME and others . Amino acids and b-cell redox regulation

Santos LR, Rebelato E, Graciano MF,Abdulkader F,Curi R & Carpinelli AR Valle MM, Graciano MF, Lopes de Oliveira ER, Camporez JP, Akamine EH, 2011 Oleic acid modulates metabolic substrate channeling during glucose- Carvalho CR, Curi R & Carpinelli AR 2011 Alterations of NADPH stimulated insulin secretion via NAD(P)H oxidase. Endocrinology 152 oxidase activity in rat pancreatic islets induced by a high-fat diet. Pancreas 40 3614–3621. (doi:10.1210/en.2011-0127) 390–395. (doi:10.1097/MPA.0b013e31820569d0) Schafer FQ & Buettner GR 2001 Redox environment of the cell as viewed Wang TJ, Larson MG, Vasan RS, Cheng S, Rhee EP, McCabe E, Lewis GD, through the redox state of the glutathione disulfide/glutathione couple. Free Fox CS, Jacques PF,Fernandez C et al. 2011 Metabolite profiles and the risk Radical Biology & Medicine 30 1191–1212. (doi:10.1016/S0891-5849 of developing diabetes. Nature Medicine 17 448–453. (doi:10.1038/nm. (01)00480-4) 2307) Schmitt H, Lenzen S & Baltrusch S 2011 Glucokinase mediates coupling of Wiseman DA, Kalwat MA & Thurmond DC 2011 Stimulus-induced glycolysis to mitochondrial metabolism but not to beta cell damage at high S-nitrosylation of syntaxin 4 impacts insulin granule exocytosis. glucose exposure levels. Diabetologia 54 1744–1755. (doi:10.1007/s00125- Journal of Biological Chemistry 286 16344–16354. (doi:10.1074/jbc.M110. 011-2133-5) 214031) Steiner DJ, Kim A, Miller K & Hara M 2010 Pancreatic islet plasticity: Yuan H, Lu Y, Huang X, He Q, Man Y, Zhou Y, Wang S & Li J 2010 interspecies comparison of islet architecture and composition. Islets 2 Suppression of NADPH oxidase 2 substantially restores glucose-induced 135–145. (doi:10.4161/isl.2.3.11815) dysfunction of pancreatic NIT-1 cells. FEBS Journal 277 5061–5071. Sun G, Tarasov AI, McGinty J, McDonald A, da Silva Xavier G, Gorman T, (doi:10.1111/j.1742-4658.2010.07911.x) Marley A, French PM, Parker H, Gribble F et al. 2010 Ablation of AMP-activated protein kinase alpha1 and alpha2 from mouse pancreatic Zhang Z, Liew CW, Handy DE, Zhang Y, Leopold JA, Hu J, Guo L, beta cells and RIP2.Cre neurons suppresses insulin release in vivo. Kulkarni RN, Loscalzo J & Stanton RC 2010 High glucose inhibits Diabetologia 53 924–936. (doi:10.1007/s00125-010-1692-1) glucose-6-phosphate dehydrogenase, leading to increased oxidative stress Tejedo J, Bernabe JC, Ramirez R, Sobrino F & Bedoya FJ 1999 NO induces a and beta-cell apoptosis. FASEB Journal 24 1497–1505. (doi:10.1096/fj.09- cGMP-independent release of cytochrome c from mitochondria which 136572) precedes caspase 3 activation in insulin producing RINm5F cells. FEBS Letters 459 238–243. (doi:10.1016/S0014-5793(99)01255-7) Turrens JF 1997 Superoxide production by the mitochondrial respiratory chain. Bioscience Reports 17 3–8. (doi:10.1023/A:1027374931887) Received in final form 17 April 2012 Uchizono Y, Takeya R, Iwase M, Sasaki N, Oku M, Imoto H, Iida M & Accepted 30 April 2012 Sumimoto H 2006 Expression of isoforms of NADPH oxidase components in rat pancreatic islets. Life Sciences 80 133–139. (doi:10.1016/j.lfs.2006. Made available online as an Accepted Preprint 08.031) 30 April 2012

Journal of Endocrinology (2012) 214, 11–20 www.endocrinology-journals.org

Downloaded from Bioscientifica.com at 09/25/2021 06:56:41AM via free access