Activation of the KATP channel-independent signaling pathway by the nonhydrolyzable analog of leucine, BCH

Dynamic Uni- and Multicellular Patterns Encode Biphasic Activity in Pancreatic Islets

Biphasic secretion is an autonomous feature of many endocrine micro-organs to fulfill physiological demands. The biphasic activity of islet β-cells maintains glucose homeostasis and is altered in type 2 diabetes. Nevertheless, underlying cellular or multicellular functional organizations are only partially understood. High-resolution noninvasive multielectrode array recordings permit simultaneous analysis of recruitment, of single-cell, and of coupling activity within entire islets in long-time experiments. Using this unbiased approach, we addressed the organizational modes of both first and second phase in mouse and human islets under physiological and pathophysiological conditions. Our data provide a new uni- and multicellular model of islet β-cell activation: during the first phase, small but highly active β-cell clusters are dominant, whereas during the second phase, electrical coupling generates large functional clusters via multicellular slow potentials to favor an economic sustained activity. Postprandial levels of glucagon-like peptide 1 favor coupling only in the second phase, whereas aging and glucotoxicity alter coupled activity in both phases. In summary, biphasic activity is encoded upstream of vesicle pools at the micro-organ level by multicellular electrical signals and their dynamic synchronization between β-cells. The profound alteration of the electrical organization of islets in pathophysiological conditions may contribute to functional deficits in type 2 diabetes.

Butyric Acid and Leucine Induce α-Defensin Secretion from Small Intestinal Paneth Cells

The intestine not only plays a role in fundamental processes in digestion and nutrient absorption, but it also has a role in eliminating ingested pathogenic bacteria and viruses. Paneth cells, which reside at the base of small intestinal crypts, secrete α-defensins and contribute to enteric innate immunity through potent microbicidal activities. However, the relationship between food factors and the innate immune functions of Paneth cells remains unknown. Here, we examined whether short-chain fatty acids and amino acids induce α-defensin secretion from Paneth cells in the isolated crypts of small intestine. Butyric acid and leucine elicit α-defensin secretion by Paneth cells, which kills Salmonella typhimurium. We further measured Paneth cell secretion in response to butyric acid and leucine using enteroids, a three-dimensional ex vivo culture system of small intestinal epithelial cells. Paneth cells expressed short-chain fatty acid receptors, Gpr41, Gpr43, and Gpr109a mRNAs for butyric acid, and amino acid transporter Slc7a8 mRNA for leucine. Antagonists of Gpr41 and Slc7a8 inhibited granule secretion by Paneth cells, indicating that these receptor and transporter on Paneth cells induce granule secretion. Our findings suggest that Paneth cells may contribute to intestinal homeostasis by secreting α-defensins in response to certain nutrients or metabolites.

Glutamine-Elicited Secretion of Glucagon-Like Peptide 1 Is Governed by an Activated Glutamate Dehydrogenase

Glucagon-like peptide 1 (GLP-1), secreted from intestinal L cells, glucose dependently stimulates insulin secretion from β-cells. This glucose dependence prevents hypoglycemia, rendering GLP-1 analogs a useful and safe treatment modality in type 2 diabetes. Although the amino acid glutamine is a potent elicitor of GLP-1 secretion, the responsible mechanism remains unclear. We investigated how GLP-1 secretion is metabolically coupled in L cells (GLUTag) and in vivo in mice using the insulin-secreting cell line INS-1 832/13 as reference. A membrane-permeable glutamate analog (dimethylglutamate [DMG]), acting downstream of electrogenic transporters, elicited similar alterations in metabolism as glutamine in both cell lines. Both DMG and glutamine alone elicited GLP-1 secretion in GLUTag cells and in vivo, whereas activation of glutamate dehydrogenase (GDH) was required to stimulate insulin secretion from INS-1 832/13 cells. Pharmacological inhibition in vivo of GDH blocked secretion of GLP-1 in response to DMG. In conclusion, our results suggest that nonelectrogenic nutrient uptake and metabolism play an important role in L cell stimulus-secretion coupling. Metabolism of glutamine and related analogs by GDH in the L cell may explain why GLP-1 secretion, but not that of insulin, is activated by these secretagogues in vivo.

Pharmacological approach to understanding the control of insulin secretion in human islets

AIMS

To understand better the control of insulin secretion by human β cells and to identify similarities to and differences from rodent models.

METHODS

Dynamic insulin secretion was measured in perifused human islets treated with pharmacological agents of known modes of action.

RESULTS

Glucokinase activation (Ro28-1675) lowered the glucose threshold for stimulation of insulin secretion to 1 mmol/L (G1), augmented the response to G3-G5 but not to G8-G15, whereas tolbutamide remained active in G20, which indicates that not all K_ATP channels were closed by high glucose concentrations. An almost 2-fold greater response to G15 than to supramaximal tolbutamide in G3 or to KCl+diazoxide in G15 vs G3 quantified the contribution of metabolic amplification to insulin secretion. Both disruption (latrunculin-B) and stabilization (jasplakinolide) of microfilaments augmented insulin secretion without affecting metabolic amplification. Tolbutamide-induced insulin secretion was consistently greater in G10 than G3, with a threshold at 1 and maximum at 10 µmol/L tolbutamide in G10, vs 10 and 25 µmol/L in G3. Sulphonylurea effects were thus clearly glucose-dependent. Insulin secretion was also increased by inhibiting K channels other than K_ATP channels: Kv or BK channels (tetraethylammonium), TASK-1 channels (ML-365) and SK4 channels (TRAM-34). Opening K_ATP channels with diazoxide inhibited glucose-induced insulin secretion with half maximum inhibitory concentrations of 9.6 and 24 µmol/L at G7 and G15. Blockade of L-type Ca channels (nimodipine) abolished insulin secretion, whereas a blocker of T-type Ca channels (NNC-55-0396) was ineffective at specific concentrations. Blockade of Na channels (tetrodotoxin) did not affect glucose-induced insulin secretion.

CONCLUSIONS

In addition to sharing a K_ATP channel-dependent triggering pathway and a metabolic amplifying pathway, human and rodent β cells were found to display more similarities than differences in the control of insulin secretion.

Beta Cell Function and the Nutritional State: Dietary Factors that Influence Insulin Secretion

Approximately 366 million people worldwide have been diagnosed with type-2 diabetes (T2D). Chronic insulin resistance, decreased functional β-cell mass, and elevated blood glucose are defining characteristics of T2D. Great advances have been made in understanding the pathogenesis of T2D with respect to the effects of dietary macronutrient composition and energy intake on β-cell physiology and glucose homeostasis. It has been further established that obesity is a leading pathogenic factor for developing insulin resistance. However, insulin resistance may not progress to T2D unless β-cells are unable to secret an adequate amount of insulin to compensate for decreased insulin sensitivity. Therefore, pancreatic β-cell dysfunction plays an important role in the development of overt diabetes. This paper reviews recent research findings on the effects of several micronutrients (zinc, vitamin D, iron, vitamin A), leucine, and the phytochemical, genistein on pancreatic β-cell physiology with emphasis on their effects on insulin secretion, specifically in the context of T2D.

Time-resolved metabolomics analysis of β-cells implicates the pentose phosphate pathway in the control of insulin release

Insulin secretion is coupled with changes in β-cell metabolism. To define this process, 195 putative metabolites, mitochondrial respiration, NADP+, NADPH and insulin secretion were measured within 15 min of stimulation of clonal INS-1 832/13 β-cells with glucose. Rapid responses in the major metabolic pathways of glucose occurred, involving several previously suggested metabolic coupling factors. The complexity of metabolite changes observed disagreed with the concept of one single metabolite controlling insulin secretion. The complex alterations in metabolite levels suggest that a coupling signal should reflect large parts of the β-cell metabolic response. This was fulfilled by the NADPH/NADP+ ratio, which was elevated (8-fold; P<0.01) at 6 min after glucose stimulation. The NADPH/NADP+ ratio paralleled an increase in ribose 5-phosphate (>2.5-fold; P<0.001). Inhibition of the pentose phosphate pathway by trans-dehydroepiandrosterone (DHEA) suppressed ribose 5-phosphate levels and production of reduced glutathione, as well as insulin secretion in INS-1 832/13 β-cells and rat islets without affecting ATP production. Metabolite profiling of rat islets confirmed the glucose-induced rise in ribose 5-phosphate, which was prevented by DHEA. These findings implicate the pentose phosphate pathway, and support a role for NADPH and glutathione, in β-cell stimulus-secretion coupling.

Ficus deltoidea: A Potential Alternative Medicine for Diabetes Mellitus

Ficus deltoidea from the Moraceae family has been scientifically proven to reduce hyperglycemia at different prandial states. In this study, we evaluate the mechanisms that underlie antihyperglycemic action of Ficus deltoidea. The results had shown that hot aqueous extract of Ficus deltoidea stimulated insulin secretion significantly with the highest magnitude of stimulation was 7.31-fold (P < 0.001). The insulin secretory actions of the hot aqueous extract involved K⁺  _ATP channel-dependent and K⁺  _ATP-channel-independent pathway. The extract also has the ability to induce the usage of intracellular Ca²⁺ to trigger insulin release. The ethanolic and methanolic extracts enhanced basal and insulin-mediated glucose uptake into adipocytes cells. The extracts possess either insulin-mimetic or insulin-sensitizing property or combination of both properties during enhancing glucose uptake into such cells. Meanwhile, the hot aqueous and methanolic extracts augmented basal and insulin-stimulated adiponectin secretion from adipocytes cells. From this study, it is suggested that Ficus deltoidea has the potential to be developed as future oral antidiabetic agent.

Enhanced GLP-1- and sulfonylurea-induced insulin secretion in islets lacking leptin signaling

We have previously reported that the absence of leptin signaling in β-cells enhances glucose-stimulated insulin secretion and improves glucose tolerance in vivo. To investigate the relevance of β-cell leptin signaling in the context of postprandial or therapeutic insulin secretion, we examined the cross talk between leptin and glucagon-like peptide (GLP)-1 and sulfonylurea actions. Single and size-matched islets isolated from control or pancreas-specific leptin receptor knockout (pancreas-ObR-KO) mice were treated either with GLP-1 or with glibenclamide. Leptin suppressed GLP-1-stimulated intracellular Ca(2+) concentrations ([Ca(2+)](i)) increase that paralleled the decrease in insulin secretion in controls. In contrast, and as expected, the ObR-KO islets were nonresponsive to leptin, and instead, showed a 2.8-fold greater GLP-1-stimulated [Ca(2+)](i) increase and a 1.7-fold greater insulin secretion. Phosphorylation of cAMP-responsive element binding protein was enhanced, and phosphodiesterase enzymatic activity was suppressed in MIN6 β-cells with ObR knockdown compared with controls. The ObR-KO islets also showed significantly higher glibenclamide-induced insulin secretion compared with control islets, whereas [Ca(2+)](i) was similar to the controls. These data support enhanced insulinotropic effects of glucose, GLP-1, and sulfonylureas in the islets lacking leptin signaling with potential therapeutic implications.

β-Cell-protective effect of 2-aminobicyclo-(2,2,1)-heptane-2-carboxylic acid as a glutamate dehydrogenase activator in db/db mice

2-Aminobicyclo-(2,2,1)-heptane-2-carboxylic acid (BCH) is an activator of glutamate dehydrogenase (GDH), which is a mitochondrial enzyme with an important role in insulin secretion. We investigated the effect of BCH on the high-glucose (HG)-induced reduction in glucose-stimulated insulin secretion (GSIS), the HG/palmitate (PA)-induced reduction in insulin gene expression, and HG/PA-induced β-cell death. We also studied whether long-term treatment with BCH lowers blood glucose and improves β-cell integrity in db/db mice. We evaluated GSIS, insulin gene expression, and DNA fragmentation in INS-1 cells exposed to HG or HG/PA in the presence or absence of BCH. An in vivo study was performed in which 7-week-old diabetic db/db mice were treated with BCH (0.7  g/kg, n = 10) and placebo (n = 10) every other day for 6 weeks. After treatment, an intraperitoneal glucose tolerance test and immunohistological examinations were performed. Treatment with BCH blocked HG-induced GSIS inhibition and the HG/PA-induced reduction in insulin gene expression in INS-1 cells. In addition, BCH significantly reduced HG/PA-induced INS-1 cell death and phospho-JNK level. BCH treatment improved glucose tolerance and insulin secretion in db/db mice. BCH treatment also increased the ratio of insulin-positive β-cells to total islet area (P < 0.05) and reduced the percentage of β-cells expressing cleaved caspase 3 (P < 0.05). In conclusion, the GDH activator BCH improved glycemic control in db/db mice. This anti-diabetic effect may be associated with improved insulin secretion, preserved islet architecture, and reduced β-cell apoptosis.

Reduction of plasma membrane glutamate transport potentiates insulin but not glucagon secretion in pancreatic islet cells

Glutamate is generated during nutrient stimulation of pancreatic islets and has been proposed to act both as an intra- and extra-cellular messenger molecule. We demonstrate that glutamate is not co-secreted with the hormones from intact islets or purified α- and β-cells. Fractional glutamate release was 5-50 times higher than hormone secretion. Furthermore, various hormone secretagogues did not elicit glutamate efflux. Interestingly, epinephrine even decreased glutamate release while increasing glucagon secretion. Rather than being co-secreted with hormones, we show that glutamate is mainly released via plasma membrane excitatory amino acid transporters (EAAT) by uptake reversal. Transcripts for EAAT1, 2 and 3 were present in both rat α- and β-cells. Inhibition of EAATs by L-trans-pyrrolidine-2,4-dicarboxylate augmented intra-cellular glutamate and α-ketoglutarate contents and potentiated glucose-stimulated insulin secretion from islets and purified β-cells without affecting glucagon secretion from α-cells. In conclusion, intra-cellular glutamate-derived metabolite pools are linked to glucose-stimulated insulin but not glucagon secretion.

Glucose stimulation of protein acylation in the pancreatic β-cell

AIMS

To determine whether protein acylation plays a role in the effects of glucose on the insulin secreting β-cell.

MAIN METHODS

The measurement of (3)H-palmitate incorporation into protein in the INS 832/13 cell that has a robust and well-characterized biphasic insulin secretory response to stimulation with glucose.

KEY FINDINGS

Stimulating the cells with glucose increased the incorporation of (3)H-palmitic acid into protein by up to 90%. Similarly, 2-aminobicyclo [2.2.1] heptane-2-carboxylic acid (BCH) the non-metabolizable analog of leucine that mimics the stimulatory effect of glucose on insulin secretion also increased the incorporation of (3)H-palmitic acid into protein. Treatment of cell lysates with hydroxylamine substantially reduced the incorporation indicating that most of the incorporation was due to enzymatic palmitoylation of proteins. Cerulenin, a classical inhibitor of protein acylation also substantially reduced the incorporation. Using PAGE and autoradiography a glucose-induced increase in protein palmitoylation and specific glucose-induced increases in the palmitoylation of proteins of 30, 44, 48 and 76kD were identified.

SIGNIFICANCE

The data suggest that protein acylation plays multiple roles in β-cell function.

Leucine metabolism in regulation of insulin secretion from pancreatic beta cells

Leucine, a branched-chain amino acid that must be supplied in the daily diet, plays an important role in controlling protein synthesis and regulating cell metabolism in various cell types. In pancreatic beta cells, leucine acutely stimulates insulin secretion by serving as both metabolic fuel and allosteric activator of glutamate dehydrogenase to enhance glutaminolysis. Leucine has also been shown to regulate gene transcription and protein synthesis in pancreatic islet beta cells via both mTOR-dependent and -independent pathways at physiological concentrations. Long-term treatment with leucine has been shown to improve insulin secretory dysfunction of human diabetic islets via upregulation of certain key metabolic genes. In vivo, leucine administration improves glycemic control in humans and rodents with type 2 diabetes. This review summarizes and discusses the recent findings regarding the effects of leucine metabolism on pancreatic beta-cell function.

Mutation of asparagine 76 in the center of glutamine transporter SNAT3 modulates substrate-induced conductances and Na+ binding

The glutamine transporter SLC38A3 (SNAT3) plays an important role in the release of glutamine from brain astrocytes and the uptake of glutamine into hepatocytes. It is related to the vesicular GABA (gamma-aminobutyric acid) transporter and the SLC36 family of proton-amino acid cotransporters. The transporter carries out electroneutral Na+-glutamine cotransport-H+ antiport. In addition, substrate-induced uncoupled cation currents are observed. Mutation of asparagine 76 to glutamine or histidine in predicted transmembrane helix 1 abolished all substrate-induced currents. Mutation of asparagine 76 to aspartate rendered the transporter Na+-independent and resulted in a gain of a large substrate-induced chloride conductance in the absence of Na+. Thus, a single residue is critical for coupled and uncoupled ion flows in the glutamine transporter SNAT3. Homology modeling of SNAT3 along the structure of the related benzyl-hydantoin permease from Microbacterium liquefaciens reveals that Asn-76 is likely to be located in the center of the membrane close to the translocation pore and forms part of the predicted Na+ -binding site.

Involvement of Ca2+/calmodulin kinase II (CaMK II) in genistein-induced potentiation of leucine/glutamine-stimulated insulin secretion

Genistein has been reported to potentiate glucose-stimulated insulin secretion (GSIS). Inhibitory activity on tyrosine kinase or activation of protein kinase A (PKA) was shown to play a role in the genistein-induced potentiation effect on GSIS. The aim of the present study was to elucidate the mechanism of genistein-induced potentiation of insulin secretion. Genistein augmented insulin secretion in INS-1 cells stimulated by various energy-generating nutrients such as glucose, pyruvate, or leucine/glutamine (Leu/Gln), but not the secretion stimulated by depolarizing agents such as KCl and tolbutamide, or Ca(2+) channel opener Bay K8644. Genistein at a concentration of 50 μM showed a maximum potentiation effect on Leu/Gln-stimulated insulin secretion, but this was not sufficient to inhibit the activity of tyrosine kinase. Inhibitor studies as well as immunoblotting analysis demonstrated that activation of PKA was little involved in genistein-induced potentiation of Leu/Gln-stimulated insulin secretion. On the other hand, all the inhibitors of Ca(2+)/calmodulin kinase II tested, significantly diminished genistein-induced potentiation. Genistein also elevated the levels of [Ca(2+)]i and phospho-CaMK II. Furthermore, genistein augmented Leu/Gln-stimulated insulin secretion in CaMK II-overexpressing INS-1 cells. These data suggest that the activation of CaMK II played a role in genistein-induced potentiation of insulin secretion.

Taurine supplementation enhances nutrient-induced insulin secretion in pancreatic mice islets

BACKGROUND

Taurine (TAU), a naturally occurring sulfur-containing amino acid, is found at high concentrations in plasma and mammalian tissues and regulates osmolarity, ion channel activity, and glucose homeostasis. Several reports have shown that physiological plasma TAU levels seem to be important for adequate beta (beta)-cell function and insulin action, since low concentrations of TAU in the plasma have been reported in the pre-diabetic and diabetic states.

METHODS

Glucose tolerance and insulin sensitivity were investigated in mice supplemented with 2% (w/v) TAU in their drinking water for 30 days, as well as the insulin secretion from isolated islets stimulated by glucose or L-leucine.

RESULTS

TAU-supplemented mice demonstrated improved glucose tolerance and higher insulin sensitivity, compared to controls (CTL). In addition, their islets secreted more insulin in response to high concentrations of glucose and L-leucine. L-[U-(14)C]leucine oxidation was higher in TAU than in CTL islets, whereas D-[U-(14)C]glucose oxidation, ATP levels, glucose transporter (GLUT) 2 and glucokinase (GCK) protein expressions were similar in both types of islets. The L-type beta(2) subunit voltage-sensitive Ca(2+) channel protein, as well as (45)Ca uptake, were significantly higher in TAU-supplemented than CTL islets. In addition, islets from TAU-supplemented mice secreted more glucagon than CTL islets at low glucose.

CONCLUSIONS

TAU supplementation improves glucose tolerance and insulin sensitivity in mice, as well as insulin secretion from isolated islets. The latter effect seems to be, at least in part, dependent on a better Ca(2+) handling by the islets.

Complementary expression of SN1 and SAT2 in the islets of Langerhans suggests concerted action of glutamine transport in the regulation of insulin secretion

Insulin and glucagon secretion from the islets of Langerhans is highly regulated. Although an increased plasma glucose level is the major stimulus for insulin exocytosis, roles for glutamine and glutamate have been suggested. Interestingly, the islet cells display elements associated with synaptic transmission. In the central nervous system (CNS), glutamine transport by SN1 and SAT2 sustain the generation of neurotransmitter glutamate. We hypothesized that the same transporters are essential for glutamine transport into the islet cells and for subsequent formation of glutamate acting as an intracellular signaling molecule. We demonstrate that islet cells express several transporters which can mediate glutamine transport. In particular, we show pronounced expression of SN1 and SAT2 in B-cells and A-cells, respectively. The cell-specific expression of these transporters together with their functional characteristics suggest an important role for glutamine in the regulation of insulin secretion.

Triggering and amplification of insulin secretion by dimethyl alpha-ketoglutarate, a membrane permeable alpha-ketoglutarate analogue

Cytosolic alpha-ketoglutarate is a potential signalling compound at late steps of stimulus-secretion-coupling in the course of insulin secretion induced by glucose and other fuels. This hypothesis is mainly based on the insulin-releasing effect of the membrane permeable ester dimethyl alpha-ketoglutarate which enters the beta-cell and is cleaved to produce cytosolic monomethyl alpha-ketoglutarate and eventually alpha-ketoglutarate. The present study tested this hypothesis. Insulin release, K(ATP) channel currents, membrane potential, ATP/ADP ratio and fluorescence of NAD(P)H (reduced pyridine nucleotides) were measured in mouse pancreatic islets and beta-cells. At a substimulatory glucose concentration (5 mM), dimethyl alpha-ketoglutarate (15 mM) produced a sustained insulin release, but no change of the islet ATP/ADP ratio and NAD(P)H fluorescence. In the absence of glucose, however, dimethyl alpha-ketoglutarate (15 mM) did not stimulate insulin release although it increased the ATP/ADP ratio and NAD(P)H fluorescence. Insulin secretion induced by a maximally effective concentration of the K(ATP) channel-blocking sulfonylurea glipizide was strongly amplified by dimethyl alpha-ketoglutarate in the presence of 5 mM glucose, but only moderately in the absence of glucose. Dimethyl alpha-ketoglutarate directly inhibited K(ATP) channels in inside-out membrane patches, depolarized the plasma membrane of intact beta-cells and generated action potentials. In conclusion, the stimulation of insulin secretion by extracellularly applied dimethyl alpha-ketoglutarate depends on inhibition of beta-cell K(ATP) channels by direct action of dimethyl alpha-ketoglutarate. The metabolism of alpha-ketoglutarate generated intracellularly by ester cleavage contributes to stimulation of insulin secretion both by indirect K(ATP) channel inhibition (via activation of ATP production) and by an amplifying effect.

Functional significance of repressor element 1 silencing transcription factor (REST) target genes in pancreatic beta cells

AIMS/HYPOTHESIS

The expression of several neuronal genes in pancreatic beta cells is due to the absence of the transcription factor repressor element 1 (RE-1) silencing transcription factor (REST). The identification of these traits and their functional significance in beta cells has only been partly elucidated. Herein, we investigated the biological consequences of a repression of REST target genes by expressing REST in beta cells.

METHODS

The effect of REST expression on glucose homeostasis, insulin content and release, and beta cell mass was analysed in transgenic mice selectively expressing REST in beta cells. Relevant target genes were identified in INS-1E and primary beta cells expressing REST.

RESULTS

Transgenic mice featuring a beta cell-targeted expression of REST exhibited glucose intolerance and reduced beta cell mass. In primary beta cells, REST repressed several proteins of the exocytotic machinery, including synaptosomal-associated protein (SNAP) 25, synaptotagmin (SYT) IV, SYT VII, SYT IX and complexin II; it impaired first and second phases of insulin secretion. Using RNA interference in INS-1E cells, we showed that SYT IV and SYT VII were implicated in the control of insulin release.

CONCLUSIONS/INTERPRETATION

The data document the critical role of REST target genes in pancreatic beta cells. Specifically, we provide evidence that the downregulation of these genes is detrimental for the exocytosis of large dense core vesicles, thus contributing to beta cell dysfunction and impaired glucose homeostasis.

Dimethyl amiloride improves glucose homeostasis in mouse models of type 2 diabetes

Dimethyl amiloride (DMA) enhances insulin secretion in the pancreatic beta-cell. DMA also enhances time-dependent potentiation (TDP) and enables TDP to occur in situations where it is normally absent. As we have demonstrated before, these effects are mediated in part through inhibition of neuronal nitric oxide synthase (nNOS), resulting in increased availability of arginine. Thus both DMA and arginine have the potential to correct the secretory defect in diabetes by enabling or enhancing TDP. In the current study we have demonstrated the ability of these agents to improve blood glucose homeostasis in three mouse models of type 2 diabetes. The pattern of TDP under different conditions indicates that inhibition of NOS is not the only mechanism through which DMA exerts its positive effects. Thus we also have explored another possible mechanism through which DMA enables/enhances TDP, via the activation of mitochondrial alpha-ketoglutarate dehydrogenase.

Studies with leucine, beta-hydroxybutyrate and ATP citrate lyase-deficient beta cells support the acetoacetate pathway of insulin secretion

We hypothesized that contrasting leucine with its non-metabolizable analog 2-aminobicyclo[2,2,1]heptane-2-carboxylic acid (BCH) might provide new information about metabolic pathways involved in insulin secretion. Both compounds stimulate insulin secretion by allosterically activating glutamate dehydrogenase, which enhances glutamate metabolism. However, we found that leucine was a stronger secretagogue in rat pancreatic islets and INS-1 cells. This suggested that leucine's metabolism contributed to its insulinotropism. Indeed, we found that leucine increased acetoacetate and was metabolized to CO(2) in pancreatic islets and increased short chain acyl-CoAs (SC-CoAs) in INS-1 cells. We then used the leucine-BCH difference to study the hypothesis that acyl groups derived from secretagogue carbon can be transferred as acetoacetate, in addition to citrate, from mitochondria to the cytosol where they can be converted to SC-CoAs. Since BCH cannot form sufficient acetoacetate from glutamate, transport of any glutamate-derived acyl groups to the cytosol in BCH-stimulated cells must proceed mainly via citrate. In ATP citrate lyase-deficient INS-1 cells, which are unable to convert citrate into cytosolic acetyl-CoA, insulin release by BCH was decreased and adding beta-hydroxybutyrate or alpha-ketoisocaproate, which increases mitochondrial acetoacetate, normalized BCH-induced insulin release. This strengthens the concept that acetoacetate-transferred acyl carbon can be converted to cytosolic SC-CoAs to stimulate insulin secretion.

Fuel-induced amplification of insulin secretion in mouse pancreatic islets exposed to a high sulfonylurea concentration: role of the NADPH/NADP+ ratio

AIMS/HYPOTHESIS

The aim of this study was to examine whether the cytosolic NADPH/NADP+ ratio of beta cells serves as an amplifying signal in fuel-induced insulin secretion and whether such a function is mediated by cytosolic alpha-ketoglutarate.

METHODS

Pancreatic islets and islet cells were isolated from albino mice by collagenase digestion. Insulin secretion of incubated or perifused islets was measured by ELISA. The NADPH and NADP+ content of incubated islets was determined by enzymatic cycling. The cytosolic Ca2+ concentration ([Ca2+]c) in islets was measured by microfluorimetry and the activity of ATP-sensitive K+ channels in islet cells by patch-clamping.

RESULTS

Both 30 mmol/l glucose and 10 mmol/l alpha-ketoisocaproate stimulated insulin secretion and elevated the NADPH/NADP+ ratio of islets preincubated in the absence of fuel. The increase in the NADPH/NADP+ ratio was abolished in the presence of 2.7 micromol/l glipizide (closing all ATP-sensitive K+ channels). However, alpha-ketoisocaproate, but not glucose, still stimulated insulin secretion. That glipizide did not inhibit alpha-ketoisocaproate-induced insulin secretion was not the result of elevated [Ca2+]c, as glucose caused a more marked [Ca2+]c increase. Insulin release triggered by glipizide alone was moderately amplified by dimethyl alpha-ketoglutarate (which is cleaved to produce cytosolic alpha-ketoglutarate), but there was no indication of a signal function of cytosolic alpha-ketoglutarate.

CONCLUSIONS/INTERPRETATION

The results strongly suggest that the NADPH/NADP+ ratio in the beta cell cytosol does not serve as an amplifying signal in fuel-induced insulin release. The study supports the view that amplification results from the intramitochondrial production of citrate by citrate synthase and from the associated export of citrate into the cytosol.

Inhibitory role of Src family tyrosine kinases on Ca2+-dependent insulin release

Both neurotransmitter release and insulin secretion occur via regulated exocytosis and share a variety of similar regulatory mechanisms. It has been suggested that Src family tyrosine kinases inhibit neurotransmitter release from neuronal cells (H. Ohnishi, S. Yamamori, K. Ono, K. Aoyagi, S. Kondo, and M. Takahashi. Proc Natl Acad Sci USA 98: 10930-10935, 2001). Thus the potential role of Src family kinases in the regulation of insulin secretion was investigated in this study. Two structurally different inhibitors of Src family kinases, SU-6656 and PP2, but not the inactive compound, PP3, enhanced Ca2+-induced insulin secretion in both rat pancreatic islets and INS-1 cells in a concentration-dependent and time-dependent manner. Furthermore, Src family kinase-mediated insulin secretion appears to be dependent on elevated intracellular Ca2+ and independent of glucose metabolism, the ATP-dependent K+ channel, adenylyl cyclase, classical PKC isoforms, extracellular signal-regulated kinase 1/2, and insulin synthesis. The sites of action for Src family kinases seem to be distal to the elevation of intracellular Ca2+ level. These results indicate that one or more Src family tyrosine kinases exert a tonic inhibitory role on Ca2+-dependent insulin secretion.

Inhibition of insulin secretion by cerulenin might be due to impaired glucose metabolism

BACKGROUND

Cerulenin, an inhibitor of protein acylation, has been used as a tool to study the potential role of protein acylation in a variety of activities in different cells, and in stimulus-secretion coupling in pancreatic islets and clonal beta-cells.

METHODS

In the present study we investigated its effects on stimulated insulin secretion, glucose metabolism and utilization, oxygen consumption and ATP levels.

RESULTS

In isolated rat pancreatic islets, cerulenin pre-treatment (100 microM) inhibited insulin secretion in response to glucose, and to the non-hydrolysable analogue of leucine, aminobicyclo-[2,2,1]heptane-2-carboxylic acid (BCH). These data are in accord with the hypothesis that protein acylation could be involved in the stimulation of insulin secretion. However, we also found that cerulenin profoundly decreased glucose oxidation, glucose utilization, oxygen consumption and ATP levels. Consequently, decreased metabolism provides an alternative mechanism to inhibition of protein acylation that could explain the inhibition of insulin secretion by cerulenin.

CONCLUSIONS

Inhibition of insulin secretion by cerulenin can no longer be taken as evidence in favour of a role for protein acylation in the control of insulin release. As protein acylation is known to be involved in the normal functioning of proteins in stimulus-secretion coupling and exocytosis, more direct approaches to understand its role(s) are required.

Fatty acid signaling in the beta-cell and insulin secretion

Fatty acids (FAs) and other lipid molecules are important for many cellular functions, including vesicle exocytosis. For the pancreatic beta-cell, while the presence of some FAs is essential for glucose-stimulated insulin secretion, FAs have enormous capacity to amplify glucose-stimulated insulin secretion, which is particularly operative in situations of beta-cell compensation for insulin resistance. In this review, we propose that FAs do this via three interdependent processes, which we have assigned to a "trident model" of beta-cell lipid signaling. The first two arms of the model implicate intracellular metabolism of FAs, whereas the third is related to membrane free fatty acid receptor (FFAR) activation. The first arm involves the AMP-activated protein kinase/malonyl-CoA/long-chain acyl-CoA (LC-CoA) signaling network in which glucose, together with other anaplerotic fuels, increases cytosolic malonyl-CoA, which inhibits FA partitioning into oxidation, thus increasing the availability of LC-CoA for signaling purposes. The second involves glucose-responsive triglyceride (TG)/free fatty acid (FFA) cycling. In this pathway, glucose promotes LC-CoA esterification to complex lipids such as TG and diacylglycerol, concomitant with glucose stimulation of lipolysis of the esterification products, with renewal of the intracellular FFA pool for reactivation to LC-CoA. The third arm involves FFA stimulation of the G-protein-coupled receptor GPR40/FFAR1, which results in enhancement of glucose-stimulated accumulation of cytosolic Ca2+ and consequently insulin secretion. It is possible that FFA released by the lipolysis arm of TG/FFA cycling is partly "secreted" and, via an autocrine/paracrine mechanism, is additive to exogenous FFAs in activating the FFAR1 pathway. Glucose-stimulated release of arachidonic acid from phospholipids by calcium-independent phospholipase A2 and/or from TG/FFA cycling may also be involved. Improved knowledge of lipid signaling in the beta-cell will allow a better understanding of the mechanisms of beta-cell compensation and failure in diabetes.

Anaplerosis via pyruvate carboxylase is required for the fuel-induced rise in the ATP:ADP ratio in rat pancreatic islets

AIMS/HYPOTHESIS

The molecular mechanisms of insulin release are only partially known. Among putative factors for coupling glucose metabolism to insulin secretion, anaplerosis has lately received strong support. The anaplerotic enzyme pyruvate carboxylase is highly expressed in beta cells, and anaplerosis influences insulin secretion in beta cells. By inhibiting pyruvate carboxylase in rat islets, we aimed to clarify the hitherto unknown metabolic events underlying anaplerotic regulation of insulin secretion.

METHODS

Phenylacetic acid (5 mmol/l) was used to inhibit pyruvate carboxylase in isolated rat islets, which were then assessed for insulin secretion, fuel oxidation, ATP:ADP ratio, respiration, mitochondrial membrane potential, exocytosis and ATP-sensitive K(+) channel (K(ATP)-channel) conductance.

RESULTS

We found that the glucose-provoked rise in ATP:ADP ratio was suppressed by inhibition of pyruvate carboxylase. In contrast, fuel oxidation, respiration and mitochondrial membrane potential, as well as Ca(2+)-induced exocytosis and K(ATP)-channel conductance in single cells, were unaffected. Insulin secretion induced by alpha-ketoisocaproic acid was suppressed, whereas methyl-succinate-stimulated secretion remained unchanged. Perifusion of rat islets revealed that inhibition of anaplerosis decreased both the second phase of insulin secretion, during which K(ATP)-independent actions of fuel secretagogues are operational, as well as the first and K(ATP)-dependent phase.

CONCLUSIONS/INTERPRETATION

Our results are consistent with the concept that anaplerosis via pyruvate carboxylase determines pyruvate cycling, which has previously been shown to correlate with glucose responsiveness in clonal beta cells. These processes, controlled by pyruvate carboxylase, seem crucial for generation of an appropriate ATP:ADP ratio, which may regulate both phases of fuel-induced insulin secretion.

Dual mechanism of the potentiation by glucose of insulin secretion induced by arginine and tolbutamide in mouse islets

Glucose induces insulin secretion (IS) and also potentiates the insulin-releasing action of secretagogues such as arginine and sulfonylureas. This potentiating effect is known to be impaired in type 2 diabetic patients, but its cellular mechanisms are unclear. IS and cytosolic Ca(2+) concentration ([Ca(2+)](i)) were measured in mouse islets during perifusion with 3-15 mmol/l glucose (G3-G15, respectively) and pulse or stepwise stimulation with 1-10 mmol/l arginine or 5-250 micromol/l tolbutamide. In G3, arginine induced small increases in [Ca(2+)](i) but no IS. G7 alone only slightly increased [Ca(2+)](i) and IS but markedly potentiated arginine effects on [Ca(2+)](i), which resulted in significant IS (already at 1 mmol/l). For each arginine concentration, both responses further increased at G10 and G15, but the relative change was distinctly larger for IS than [Ca(2+)](i). At all glucose concentrations, tolbutamide dose dependently increased [Ca(2+)](i) and IS with thresholds of 25 micromol/l for [Ca(2+)](i) and 100 micromol/l for IS at G3 and of 5 micromol/l for both at G7 and above. Between G7 and G15, the effect of tolbutamide on [Ca(2+)](i) increased only slightly, whereas that on IS was strongly potentiated. The linear relationship between IS and [Ca(2+)](i) at increasing arginine or tolbutamide concentrations became steeper as the glucose concentration was raised. Thus glucose augmented more the effect of each agent on IS than that on [Ca(2+)](i). In conclusion, glucose potentiation of arginine- or tolbutamide-induced IS involves increases in both the rise of [Ca(2+)](i) and the action of Ca(2+) on exocytosis. This dual mechanism must be borne in mind to interpret the alterations of the potentiating action of glucose in type 2 diabetic patients.

Insulin secretion in the conscious mouse is biphasic and pulsatile

Islets in most species respond to increased glucose with biphasic insulin secretion, marked by a sharp first-phase peak and a slowly rising second phase. Mouse islets in vitro, however, lack a robust second phase. To date, this observation has not been extended in vivo. We thus compared insulin secretion from conscious mice with isolated mouse islets in vitro. The arterial plasma insulin response to a hyperglycemic clamp was measured in conscious mice 1 wk after surgical implantation of carotid artery and jugular vein catheters. Mice were transfused using clamps with blood from a donor mouse to maintain blood volume, allowing frequent arterial sampling. When plasma glucose in vivo was raised from approximately 5 to approximately 13 mM, insulin rose to a first-phase peak of 403+/-73% above basal secretion (n=5), followed by a rising second phase of mean 289+/- 41%. In contrast, perifused mouse islets ( approximately 75 islets/trial) responded with a similar first phase of 508+/- 94% (n=4) but a smaller and virtually flat second phase of 169+/- 9% (n=4, P<0.05). Furthermore, the slope of the second-phase response differed significantly from zero in mice (2.63+/-0.39%/min, P<0.01), in contrast to perifused islets (0.18+/- 0.14%/min, P>0.30). Mice also displayed pulsatile patterns in insulin concentration (period: 4.2+/- 0.4 min, n=8). Conscious mice thus responded to increased glucose with biphasic and pulsatile insulin secretion, as in other species. The robust second phase observed in vivo suggests that the processes needed to generate second-phase insulin secretion may be abrogated by islet isolation.

In beta-cells, mitochondria integrate and generate metabolic signals controlling insulin secretion

Pancreatic beta-cells are unique neuroendocrine cells displaying the peculiar feature of responding to nutrients, principally glucose, as primary stimulus. This requires translation of a metabolic substrate into intracellular messengers recognized by the exocytotic machinery. Central to this signal transduction mechanism, mitochondria integrate and generate metabolic signals, thereby coupling glucose recognition to insulin secretion. In response to a glucose rise, nucleotides and metabolites are generated by mitochondria and participate, together with cytosolic calcium, to the stimulation of insulin exocytosis. This review describes the mitochondrion-dependent pathways of regulated insulin secretion. In particular, importance of cataplerotic and anaplerotic processes is discussed, with special attention to the mitochondrial enzyme glutamate dehydrogenase. Mitochondrial defects, such as mutations and reactive oxygen species production, are presented in the context of beta-cell failure in the course of type 2 diabetes.

Selective loss of glucose-induced amplification of insulin secretion in mouse pancreatic islets pretreated with sulfonylurea in the absence of fuels

AIMS/HYPOTHESIS

The beta cell metabolism of glucose, and some other fuels, initiates insulin secretion by closure of ATP-sensitive K+ channels and amplifies the secretory response via unknown metabolic intermediates. The aim of this study was to further characterise the mechanism responsible for the metabolic amplification of insulin secretion.

MATERIALS AND METHODS

Pancreatic islets were isolated from albino mice by collagenase digestion. Insulin secretion in perifused islets was determined by ELISA. Bioluminometry was used to determine the ATP and ADP content of the incubated islets.

RESULTS

After perifusing islets for 60 min with 2.7 micromol/l glipizide (closing all ATP-sensitive K+ channels) in the absence of any fuel, perifusion with a test medium containing 2.7 micromol/l glipizide plus 30 mmol/l glucose did not enhance insulin secretion. However, test media supplemented with 2.7 micromol/l glipizide plus either 10 mmol/l alpha-ketoisocaproate or 10 mmol/l 2-aminobicyclo[2,2,1]heptane-2-carboxylic acid amplified the glipizide-induced insulin secretion. In pancreatic islets preincubated for 60 min with 2.7 micromol/l glipizide in the absence of any fuel, 40 min incubations in the presence of 2.7 micromol/l glipizide plus 30 mmol/l glucose or plus 10 mmol/l alpha-ketoisocaproate produced an increase in the ATP content, no change in the ADP content and a rather small increase in the ATP:ADP ratio. The corresponding effects of glucose and alpha-ketoisocaproate were similar.

CONCLUSIONS/INTERPRETATION

These results suggest that metabolic amplification of fuel-induced insulin secretion is not mediated by changes in the beta cell content of ATP and ADP, but might be due to export of citrate cycle intermediates to the beta cell cytosol.

C-terminal part of AgRP stimulates insulin secretion through calcium release in pancreatic beta Rin5mf cells

Agouti-related protein (AgRP) is an orexigenic peptide which is composed of three parts; the amino (N)-terminus, the middle part, and the carboxyl (C)-terminus. AgRP has been implicated in various cell signaling, but the precise role of each parts are currently unclear. In this study, we have attempted to determine which part of AgRP was critical for insulin secretion. We have found that the C-terminus of AgRP specifically increases the intracellular calcium concentration in pancreatic beta Rin5mf cells in a PLC-dependent manner, whereas the middle part and C-terminus have little effects on calcium release. This calcium response can be observed in the freshly isolated primary beta cells also. Moreover, amperometric measurement reveals that the C-terminus of AgRP increases the rate of exocytosis in Rin5mf cells. We further show that this region of AgRP is responsible for insulin secretion in a PLC-dependent manner. Taken together, these results indicate that the C-terminus of AgRP can participate in the insulin secretion in pancreatic beta cells, through the modulation of calcium release.

Perspective: emerging evidence for signaling roles of mitochondrial anaplerotic products in insulin secretion

The importance of mitochondrial biosynthesis in stimulus secretion coupling in the insulin-producing beta-cell probably equals that of ATP production. In glucose-induced insulin secretion, the rate of pyruvate carboxylation is very high and correlates more strongly with the glucose concentration the beta-cell is exposed to (and thus with insulin release) than does pyruvate decarboxylation, which produces acetyl-CoA for metabolism in the citric acid cycle to produce ATP. The carboxylation pathway can increase the levels of citric acid cycle intermediates, and this indicates that anaplerosis, the net synthesis of cycle intermediates, is important for insulin secretion. Increased cycle intermediates will alter mitochondrial processes, and, therefore, the synthesized intermediates must be exported from mitochondria to the cytosol (cataplerosis). This further suggests that these intermediates have roles in signaling insulin secretion. Although evidence is quite good that all physiological fuel secretagogues stimulate insulin secretion via anaplerosis, evidence is just emerging about the possible extramitochondrial roles of exported citric acid cycle intermediates. This article speculates on their potential roles as signaling molecules themselves and as exporters of equivalents of NADPH, acetyl-CoA and malonyl-CoA, as well as alpha-ketoglutarate as a substrate for hydroxylases. We also discuss the "succinate mechanism," which hypothesizes that insulin secretagogues produce both NADPH and mevalonate. Finally, we discuss the role of mitochondria in causing oscillations in beta-cell citrate levels. These parallel oscillations in ATP and NAD(P)H. Oscillations in beta-cell plasma membrane electrical potential, ATP/ADP and NAD(P)/NAD(P)H ratios, and glycolytic flux are known to correlate with pulsatile insulin release. Citrate oscillations might synchronize oscillations of individual mitochondria with one another and mitochondrial oscillations with oscillations in glycolysis and, therefore, with flux of pyruvate into mitochondria. Thus citrate oscillations may synchronize mitochondrial ATP production and anaplerosis with other cellular oscillations.

Stimulation of insulin release by glucose is associated with an increase in the number of docked granules in the beta-cells of rat pancreatic islets

Electron microscopy and quantitative stereological techniques were used to study the dynamics of the docked granule pool in the rat pancreatic beta-cell. The mean number of granules per beta-cell was 11,136. After equilibration in RPMI containing 5.6 mmol/l glucose, 6.4% of the granules (approximately 700) were docked at the plasma membrane (also measured as [means +/- SE] 4.3 +/- 0.6 docked granules per 10 microm of plasma membrane at the perimeter of the cell sections). After a 40-min exposure to 16.7 mmol/l glucose, 10.2% of the granules (approximately 1,060) were docked (6.4 +/- 0.8 granules per 10 microm of plasma membrane). Thus, the docked pool increased by 50% during stimulation with glucose. Islets were also exposed to 16.7 mmol/l glucose in the absence or presence of 10 micromol/l nitrendipine. In the absence and presence of nitrendipine, there were 6.1 +/- 0.7 and 6.3 +/- 0.6 granules per 10 microm of membrane, respectively. Thus, glucose increased granule docking independently of increased [Ca2+]i and exocytosis. The data suggest a limit to the number of docking sites. As the rate of docking exceeded the rate of exocytosis, docking is not rate limiting for insulin release. Only with extremely high release rates, glucose stimulation after a 4-h incubation with a high concentration of fatty acid-free BSA, was the docked granule pool reduced in size.

Anaplerotic input is sufficient to induce time-dependent potentiation of insulin release in rat pancreatic islets

Nutrients that induce biphasic insulin release, such as glucose and leucine, provide acetyl-CoA and anaplerotic input in the beta-cell. The first phase of release requires increased ATP production leading to increased intracellular Ca(2+) concentration ([Ca(2+)](i)). The second phase requires increased [Ca(2+)](i) and anaplerosis. There is strong evidence to indicate that the second phase is due to augmentation of Ca(2+)-stimulated release via the K(ATP) channel-independent pathway. To test whether the phenomenon of time-dependent potentiation (TDP) has similar properties to the ATP-sensitive K(+) channel-independent pathway, we monitored the ability of different agents that provide acetyl-CoA and anaplerotic input or both of these inputs to induce TDP. The results show that anaplerotic input is sufficient to induce TDP. Interestingly, among the agents tested, the nonsecretagogue glutamine, the nonhydrolyzable analog of leucine aminobicyclo[2.2.1]heptane-2-carboxylic acid, and succinic acid methyl ester all induced TDP, and all significantly increased alpha-ketoglutarate levels in the islets. In conclusion, anaplerosis that enhances the supply and utilization of alpha-ketoglutarate in the tricarboxylic acid cycle appears to play an essential role in the generation of TDP.

Hypothesis: one rate-limiting step controls the magnitude of both phases of glucose-stimulated insulin secretion

The biphasic secretory response of pancreatic beta-cells to abrupt and sustained exposure to glucose is well documented. Some of the ATP-sensitive K(+) (K(ATP)) channel-dependent mechanisms underlying the first phase of insulin release are known; the mechanisms underlying the second phase are less well known. The hypothesis we propose is that one rate-limiting step, controlling the conversion of granules in a readily releasable (RR) docked granule pool to an immediately releasable (IR) pool, is responsible for the magnitude of both phases of release. Furthermore, we propose that the K(ATP) channel-independent signaling pathway regulates this rate-limiting step. The size of the IR pool of granules that constitutes the first phase is determined under resting conditions by the forward and reverse rates of conversion of granules in the RR and IR pools. The resulting equilibrium position determines the maximum number of beta-cell granules available for release during the first phase upon exposure to glucose. At the nadir between the two phases, the IR pool has been depleted so that the rate of granule release is equal to the low forward rate for the conversion of RR to IR granules. After the nadir, the forward rate is accelerated during the rising portion of the second phase until it reaches a maximum rate at the plateau.

Nutrient-stimulated insulin secretion in mouse islets is critically dependent on intracellular pH

BACKGROUND: Many mechanistic steps underlying nutrient-stimulated insulin secretion (NSIS) are poorly understood. The influence of intracellular pH (pHi) on insulin secretion is widely documented, and can be used as an investigative tool. This study demonstrates previously unknown effects of pHi-alteration on insulin secretion in mouse islets, which may be utilized to correct defects in insulin secretion. METHODS: Different components of insulin secretion in mouse islets were monitored in the presence and absence of forced changes in pHi. The parameters measured included time-dependent potentiation of insulin secretion by glucose, and direct insulin secretion by different mitochondrial and non-mitochondrial secretagogues. Islet pHi was altered using amiloride, removal of medium Cl-, and changing medium pH. Resulting changes in islet pHi were monitored by confocal microscopy using a pH-sensitive fluorescent indicator. To investigate the underlying mechanisms of the effects of pHi-alteration, cellular NAD(P)H levels were measured using two-photon excitation microscopy (TPEM). Data were analyzed using Student's t test. RESULTS: Time-dependent potentiation, a function normally absent in mouse islets, can be unmasked by a forced decrease in pHi. The optimal range of pHi for NSIS is 6.4-6.8. Bringing islet pHi to this range enhances insulin secretion by all mitochondrial fuels tested, reverses the inhibition of glucose-stimulated insulin secretion (GSIS) by mitochondrial inhibitors, and is associated with increased levels of cellular NAD(P)H. CONCLUSIONS: Pharmacological alteration of pHi is a potential means to correct the secretory defect in non-insulin dependent diabetes mellitus (NIDDM), since forcing islet pHi to the optimal range enhances NSIS and induces secretory functions that are normally absent.

Insulin secretion profiles are modified by overexpression of glutamate dehydrogenase in pancreatic islets

AIMS/HYPOTHESIS

Glutamate dehydrogenase (GDH) is a mitochondrial enzyme playing a key role in the control of insulin secretion. However, it is not known whether GDH expression levels in beta cells are rate-limiting for the secretory response to glucose. GDH also controls glutamine and glutamate oxidative metabolism, which is only weak in islets if GDH is not allosterically activated by L-leucine or (+/-)-2-aminobicyclo-[2,2,1]heptane-2-carboxylic acid (BCH).

METHODS

We constructed an adenovirus encoding for GDH to overexpress the enzyme in the beta-cell line INS-1E, as well as in isolated rat and mouse pancreatic islets. The secretory responses to glucose and glutamine were studied in static and perifusion experiments. Amino acid concentrations and metabolic parameters were measured in parallel.

RESULTS

GDH overexpression in rat islets did not change insulin release at basal or intermediate glucose (2.8 and 8.3 mmol/l respectively), but potentiated the secretory response at high glucose concentrations (16.7 mmol/l) compared to controls (+35%). Control islets exposed to 5 mmol/l glutamine at basal glucose did not increase insulin release, unless BCH was added with a resulting 2.5-fold response. In islets overexpressing GDH glutamine alone stimulated insulin secretion (2.7-fold), which was potentiated 2.2-fold by adding BCH. The secretory responses evoked by glutamine under these conditions correlated with enhanced cellular metabolism.

CONCLUSIONS/INTERPRETATION

GDH could be rate-limiting in glucose-induced insulin secretion, as GDH overexpression enhanced secretory responses. Moreover, GDH overexpression made islets responsive to glutamine, indicating that under physiological conditions this enzyme acts as a gatekeeper to prevent amino acids from being inappropriate efficient secretagogues.

Overexpression of constitutively activated glutamate dehydrogenase induces insulin secretion through enhanced glutamate oxidation

Glutamate dehydrogenase (GDH) catalyzes reversible oxidative deamination of l-glutamate to alpha-ketoglutarate. Enzyme activity is regulated by several allosteric effectors. Recognition of a new form of hyperinsulinemic hypoglycemia, hyperinsulinism/hyperammonemia (HI/HA) syndrome, which is caused by gain-of-function mutations in GDH, highlighted the importance of GDH in glucose homeostasis. GDH266C is a constitutively activated mutant enzyme we identified in a patient with HI/HA syndrome. By overexpressing GDH266C in MIN6 mouse insulinoma cells, we previously demonstrated unregulated elevation of GDH activity to render the cells responsive to glutamine in insulin secretion. Interestingly, at low glucose concentrations, basal insulin secretion was exaggerated in such cells. Herein, to clarify the role of GDH in the regulation of insulin secretion, we studied cellular glutamate metabolism using MIN6 cells overexpressing GDH266C (MIN6-GDH266C). Glutamine-stimulated insulin secretion was associated with increased glutamine oxidation and decreased intracellular glutamate content. Similarly, at 5 mmol/l glucose without glutamine, glutamine oxidation also increased, and glutamate content decreased with exaggerated insulin secretion. Glucose oxidation was not altered. Insulin secretion profiles from GDH266C-overexpressing isolated rat pancreatic islets were similar to those from MIN6-GDH266C, suggesting observation in MIN6 cells to be relevant in native beta-cells. These results demonstrate that, upon activation, GDH oxidizes glutamate to alpha-ketoglutarate, thereby stimulating insulin secretion by providing the TCA cycle with a substrate. No evidence was obtained supporting the hypothesis that activated GDH produced glutamate, a recently proposed second messenger of insulin secretion, by the reverse reaction, to stimulate insulin secretion.

A signaling role of glutamine in insulin secretion

Children with hypoglycemia due to recessive loss of function mutations of the beta-cell ATP-sensitive potassium (K(ATP)) channel can develop hypoglycemia in response to protein feeding. We hypothesized that amino acids might stimulate insulin secretion by unknown mechanisms, because the K(ATP) channel-dependent pathway of insulin secretion is defective. We therefore investigated the effects of amino acids on insulin secretion and intracellular calcium in islets from normal and sulfonylurea receptor 1 knockout (SUR1-/-) mice. Even though SUR1-/- mice are euglycemic, their islets are considered a suitable model for studies of the human genetic defect. SUR1-/- islets, but not normal islets, released insulin in response to an amino acid mixture ramp. This response to amino acids was decreased by 60% when glutamine was omitted. Insulin release by SUR1-/- islets was also stimulated by a ramp of glutamine alone. Glutamine was more potent than leucine or dimethyl glutamate. Basal intracellular calcium was elevated in SUR1-/- islets and was increased further by glutamine. In normal islets, methionine sulfoximine, a glutamine synthetase inhibitor, suppressed insulin release in response to a glucose ramp. This inhibition was reversed by glutamine or by 6-diazo-5-oxo-l-norleucine, a non-metabolizable glutamine analogue. High glucose doubled glutamine levels of islets. Methionine sulfoximine inhibition of glucose stimulated insulin secretion was associated with accumulation of glutamate and aspartate. We hypothesize that glutamine plays a critical role as a signaling molecule in amino acid- and glucose-stimulated insulin secretion, and that beta-cell depolarization and subsequent intracellular calcium elevation are required for this glutamine effect to occur.