Type VIII adenylyl cyclase in rat beta cells: coincidence signal detector/generator for glucose and GLP-1
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The secretory function of pancreatic beta cells is synergistically stimulated by two signalling pathways which mediate the effects of nutrients and hormones such as glucagon-like peptide 1 (GLP-1), glucose-dependent insulinotropic peptide (GIP) or glucagon. These hormones are known to activate adenylyl cyclase in beta cells. We examined the type of adenylyl cyclase that is associated with this synergistic interaction.
Insulin release, cAMP production, adenylyl cyclase activity, mRNA and protein expression were measured in fluorescence-activated cell sorter-purified rat beta cells and in the rat beta-cell lines RINm5F, INS-1 832/13 and INS-1 832/2.
In primary beta cells, glucagon and GLP-1 synergistically potentiate the stimulatory effect of 20 mmol/l glucose on insulin release and cAMP production. Both effects are abrogated in the presence of the L-type Ca2+-channel blocker verapamil. The cAMP-producing activity of adenylyl cyclase in membranes from RINm5F cells is synergistically increased by Ca2+-calmodulin and recombinant GTPγS-activated Gsα-protein subunits. This type of regulation is characteristic for type I and type VIII AC isoforms. Consistent with this functional data, AC mRNA analysis shows abundant expression of type VI AC, four splice variants of type VIII AC and low expression level of type I AC in beta cells. Type VIII AC expression at the protein level was observed using immunoblots of RINm5F cell extracts.
This study identifies type VIII AC in insulin-secreting cells as one of the potential molecular targets for synergism between GLP-1 receptor mediated and glucose-mediated signalling.
KeywordsAdenylyl cyclase beta cells cyclic AMP coincidence detection insulin release
alpha subunit of G-protein stimulatory to adenylyl cyclase
A-kinase anchoring protein
glucagon-like peptide 1
glucose-dependent insulinotropic peptide
protein kinase A
protein phosphatase 2B
We have shown previously that a synergism exists between nutrient signals and cellular cAMP concentrations in the stimulation of insulin release . Stimulation of beta cells with nutrients alone elicits only a partial secretory response that can be potentiated by adding glucagon or other agents which raise intracellular cAMP [2, 3]. In contrast, glucose alone cannot increase cAMP concentration in beta cells, but the sugar has a modest effect on glucagon-induced cAMP production . Subsequent work has shown that glucagon-like peptide 1 (GLP-1) is also a potent stimulus for insulin release that is fully dependent on the presence of glucose . Glucose elicits its best known molecular actions on beta cells via cellular uptake and mitochondrial metabolism, increased ATP to ADP ratios, closure of K+ATP-channels and calcium influx via L-type Ca2+-channels [6, 7, 8]. A less well characterised pathway occurs independently of the closure of K+ATP channels [9, 10, 11]. Pharmacological activation of the cAMP signalling pathway can be achieved by various means such as cAMP analogues , stimulators of adenylyl cyclase (AC) such as forskolin , phosphodiesterase (PDE) inhibitors such as methylxanthines , and peptides that stimulate protein kinase A (PKA) . Effects are partially mediated via PKA activation , requiring subcellular concentration of the kinase in signalling complexes . Recent evidence suggests that another part of cAMP signalling in beta cells is mediated via the cAMP-binding protein GEFII (Epac2) complexed with Rim2 and Rab3 [17, 18]. Two important physiological stimulators of cAMP production in beta cells are glucose-dependent insulinotropic peptide (GIP) and GLP-1 . Their physiological relevance in the incretin response of the gut on glucose-induced insulin secretion has been underlined by mouse models that have targeted disruption of the corresponding receptor genes [20, 21, 22]. Glucagon also increases cAMP in isolated beta cells [13, 19, 23], and can act as a paracrine stimulator of beta cells. At supraphysiological concentrations, it is able to potentiate glucose-induced insulin release from isolated beta cells  as well as in vivo . However, the physiological relevance of glucagon-stimulated cAMP in the beta cell is uncertain, as locally released peptide did not activate beta cell function in the intact perfused rat pancreas .
A molecular pathway of synergistic crosstalk between glucose and GLP-1 involves an increase of cytosolic Ca2+, as has recently been illustrated at the transcription factor NFAT (nuclear factor of activated T-cells), which strongly promotes the transcription of the insulin gene . Transcriptional activation increases markedly when the protein is dephosphorylated by calcineurin (PP2B), a Ca2+-calmodulin-dependent protein phosphatase. Both glucose and GLP-1 contributed to the rise in cellular calcium required for transcriptional activation . A second illustration of the concept of synergism is that PP2B and PKA are co-localized in the beta cell in signalling complexes that are anchored by the scaffolding protein AKAP79/150 , an organisation proposed to be relevant for exocytosis. We provide another molecular site of interaction between glucose-and GLP-1 signalling by showing that beta cells express type VIII AC, an isoform that is synergistically activated by Ca2+/calmodulin and Gsα.
Materials and methods
Preparation of tissues and cells and culture of beta cells
All studies involving animal cells or tissues were carried out according to the Belgian regulation of animal welfare and after approval by the institution's commission for animal experiments. Control tissues (brain, liver and lung) were dissected from male Wistar rats (±3 months old) washed in phosphate-buffered saline, frozen in liquid nitrogen and stored at −80°C before adenylyl cyclase (AC) expression analysis. Purified rat alpha cells and beta cells were obtained as described  from male Wistar rat pancreata. Purity of beta cells (>97%) and alpha cells (>95%) was analysed by immunocytochemistry and electron microscopy. Culture of purified beta cells started by 2-h reaggregation at 37°C in a rotary shaking incubator (95%O2/5%CO2; Braun, Melsungen, Germany), followed by 16-h static incubation in suspension cultured dishes (Nunc, Roskilde, Denmark) in Ham's F10 medium (Gibco BRL, Grand Island, N.Y., USA) supplemented with 2 mmol/l glutamine, 10 mmol/l glucose (Merck, Darmstadt, Germany), 1% charcoal treated type V bovine serum albumin (BSA; Boehringer Mannheim, Germany), 0.075 g/l penicillin (Sigma, St Louis, Mo., USA), and 0.1 g/l streptomycin (Sigma). The rat insulinoma cell line RINm5F was grown as confluent cultures in RPMI-1640 medium with L-glutamax (Gibco BRL) supplemented with 10% (v/v) fetal calf serum (Life Technologies, Paisley, UK), 0.1 mg/ml streptomycin, and 0.075 mg/ml penicillin, using plastic culture flasks (Falcon, Becton Dickinson). The medium was changed every 2 days and the cells were detached by 5-min incubation at 37°C in phosphate-buffered saline, 1 mmol/l EDTA and 0.5% BSA. The clonal cell lines INS-1 832/13 and 832/2 (obtained from C.B. Newgard, Duke University, Durham, N.C., USA) expressing human insulin and being, respectively, strongly and weakly responsive to glucose  were cultured as RINm5F cells, except that INS-1 cell growth medium was additionally supplemented with 10 mmol/l HEPES, 50 µmol/l beta-mercaptoethanol, 2 mmol/l glutamine and 1 mmol/l pyruvate, as described previously . The viability of the primary and tumoral beta cells was estimated by neutral red uptake  and was routinely above 90%.
Analysis of cAMP production in purified pancreatic beta cells
Cellular cAMP content from pancreatic beta cells was measured during static incubations as described before  using 5×104 cells per sample. Unless stated otherwise, the PDE inhibitors 3-isobutyl-1-methylxanthine (IBMX; Aldrich-Janssen Chimica, Beerse, Belgium) or Ro 20-1724 (Roche, Basel, Switzerland) were added to prevent cAMP breakdown. The L-type Ca2+-channel blocker verapamil was purchased from Knoll (Brussels, Belgium). The cellular cAMP content was measured after sonication of the cell pellets in 8% trichloroacetic acid and ether extraction. Samples were lyophilized and acetylated before measurement in duplicate via a commercially available 125I-cAMP radioimmunoassay kit (Amersham, Little Chalfont, UK).
Measurement of insulin release
Flow-sorted rat beta cells that were cultured overnight in Ham's F10 medium were used for measurements of insulin release in static incubations  and perifusions . Perifusion experiments were carried out in a multiple microchamber module (Endotronics, Coon Rapids, Minn., USA) using 2.5×105 beta cells. Samples were collected every minute and assayed for immunoreactive insulin with guinea pig anti-insulin serum . Results were expressed as the percentage of the insulin content, measured in each individual batch of rat beta cells by sonicating the Biogel P2-containing beta cells in 2 mmol/l acetic acid with 0.25% BSA. In all experiments, the sum of insulin released in the perifusate was 10% or less of the total insulin content in the cells. Static insulin release experiments on INS-1 cell subclones were carried out essentially as described in . Briefly, the cells were seeded in 24-well plates (Falcon) and grown to confluence. Twenty hours before testing, medium was changed with RPMI-1640 growth medium (including serum and supplements) containing 5 mmol/l glucose. Cells were rinsed twice in Kreb's Ringer bicarbonate HEPES (KRBH) buffer (pH 7.4) with 2 mmol/l glucose, and allowed to pre-incubate for 60 min at 37°C. Insulin release was subsequently measured over 60 min in KRBH with 2 or 20 mmol/l glucose in the presence or absence of 100 µmol/l IBMX.
Analysis of AC activity in RINm5F membranes
The interaction between Ca2+-calmodulin and preactivated Gsα-protein at the level of AC activity was tested in RINm5F membranes prepared from cells that were washed three times with phosphate-buffered saline. The cell pellet was suspended in 100 µl buffer A (20 mmol/l HEPES at pH 7.5, 5 mmol/l EDTA, 1 mmol/l EGTA, 200 mmol/l sucrose, 100 µg/ml phenylmethylsulfonyl fluoride, 100 µg/ml aprotinin, 10 µg/ml leupeptin, 10 µg/ml pepstatin, 22 µg/ml L-1-tosylamido-2-phenylethyl chloromethyl ketone, 22 µg/ml 1-chloro-3-tosylamido-7-amino-2-heptanone and 3.2 µg/ml lima bean trypsin inhibitor) and sonicated on ice for 1 min (Branson sonifier 250, Danbury, Conn., USA). After centrifugation (500×g for 7 min at 4°C) the supernatant was collected while the cell pellet was sonicated and centrifuged a second time. The pooled supernatant fractions were ultracentrifuged at 110 000×g for 1 h at 4°C. Supernatant and pellet fractions containing cellular nonmembrane and membrane compartments, respectively, were homogenized in buffer B (=buffer A with 2 mmol/l EDTA and without EGTA). Samples were taken for protein content using the Micro BCA protein assay Reagent kit (Pierce, Rockford, Ill., USA). The effects of adding 1 µmol/l of the diterpene forskolin , 40 nmol/l of the recombinant preactivated GTPγS.rGsα, 120 nmol/l bovine brain calmodulin (Calbiochem, La Jolla, Calif., USA), 100 µmol/l EGTA plus 250 µmol/l CaCl2 (resulting in 17 µmol/l free Ca2+) , alone or in combination, were tested on 25 µg protein per reaction. The AC assay mixture and incubation conditions were as described  except that 0.25 mmol/l IBMX was used instead of Ro 20-1724. The reaction was stopped by adding 8% ice cold trichloroacetic acid and total cAMP was determined by radioimmunoassay (Amersham). Recombinant Gsα protein was activated for 1 h with GTPγS (Sigma) as described previously  and unincorporated GTPγS was removed by gel filtration on a Sephadex G-50 MicroSpin column (Amersham-Pharmacia Biotech, Uppsala, Sweden).
PCR and nucleotide sequence analysis of AC-mRNA in beta cells
Total RNA from cells or tissues was extracted using TRIzol Reagent according to the manufacturer's protocol (Gibco BRL). The RNA quality and quantity was determined in UV-160A spectrophotometer (Shimadzu, Kyoto, Japan). First strand cDNA was generated using GeneAmp RNA PCR Core kit (Perkin Elmer, Branchburg, N.J., USA) using random hexamer primers and the manufacturer's protocol. Controls without template were included in each assay; amplified PCR fragments were absent. Specific cDNA sequences were amplified in a Perkin Elmer GeneAmp thermocycler 9600 (Perkin Elmer) using 2.5 U AmpliTaq Gold (Perkin Elmer), 1.5 mmol/l MgCl2, 20 pmole 5′ primer and 20 pmole 3′ primer (spanning AC cDNA fragments with the following nucleotide (nt) sequences: AC I (nt 2791–3294); AC II (nt 2802–3505); AC III (nt 3211–3665); AC IV (nt 1521–1957); AC V (nt 2802–3651); AC VI (nt 3062–3506); AC VII (nt 2944–3343); AC VIII (nt 3789–4267). Primer pairs for RT-PCR analysis were tested for self-complementarity, dimer formation and melting temperature using the Primer Analysis Software Oligo (National Biosciences, Plymouth, Minn., USA). For all eight AC isoforms, amplification consisted of 35 PCR cycles, including five cycles of [94°C—1 min, between 52°C and 68°C—1 min (optimized for each isoform), 72°C—1 min] and 30 cycles of [94°C—0.5 min, between 51°C and 64°C—1 min (optimized for each isoform), 72°C—1.5 min]. To assess integrity and quantity of reverse transcribed cellular RNA, we amplified the same cDNA samples with beta actin-specific primers (V01217; 5′-primer: codons 249–255 and 3′-primer: codons 338–344, yielding a fragment of 284 bp). The degree of cross-contaminating alpha and beta cells in AC signals amplified from the purified cell preparations was assessed via amplification of preproinsulin-1 (nt 238–1051; 10 cycles of 94°C—1 min, 65°C—1 min,72°C—1 min followed by 10 cycles of 94°C—0.5 min,60°C—1 min,72°C—1 min) and preproglucagon (5′-primer: codons 33–39 and 3'-primer: codons 142–149; five cycles of 94°C—1 min,58°C—1 min,72°C—1 min followed by 20 cycles of 94°C—0.5 min,55°C—1 min,72°C—1 min). Amplified fragments were controlled for length by ethidium bromide after electrophoresis on 1% or 1.5% agarose gels and further characterized via DNA sequencing (ABI-Prism 310, Perkin Elmer, Foster City, Calif., USA). The PCR fragments were photographed using a digital image processor (Kodak DC40 camera, Eastman Kodak, Rochester, N.Y., USA) and signal intensities were determined (Biomax 1D image analysis software, Kodak).
Immunoblotting of type VIII AC protein
Membranes were prepared from cells or tissues as described in "analysis of AC activity". Membrane and non-membrane protein fractions were treated for 5 min at 56°C in 5% SDS, 80 mmol/l TRIS at pH 6.8, 5 mmol/l EDTA, 10% glycerol, 5% beta mercaptoethanol, 1 mmol/l PMSF and bromophenol blue. The samples were further incubated for 10 min at room temperature after adding N-ethylmaleimide (final concentration 50 mmol/l) before loading on 6% polyacrylamide SDS gels. After electrophoresis and blotting, the nitrocellulose membranes (Schleicher and Schuell, Dassel, Germany) were blocked for 1 h at room temperature in TBST/M (5% milk, 10 mmol/l Tris.HCl pH 7.4, 150 mmol/l NaCl, 1% Tween20) and incubated overnight in TBST/M with affinity purified, primary polyclonal antibody (D88AP; dilution 1/250) directed against an oligopeptide corresponding to residues 1229–1248 of type VIII AC . Binding of the primary antibody was detected using Western Blot Chemiluminiscence Reagent Plus (NEN Life Science Products, Boston, Mass., USA) after incubation with donkey anti rabbit F(ab')2 HRP conjugate (diluted1/2500 in TBST/M; Amersham-Pharmacia Biotech) as secondary antibody. As a quality control for the immunoreactivity, the antibody was used for immunodetection of type VIII AC protein in membranes from HEK 293 cells transfected with pCMV5-neo vector in which the full length type VIII AC cDNA was subcloned (HEK 293/AC-VIII-A in Fig. 6) . Furthermore, preincubation of the primary antibody with excess of the respective antigenic peptide completely abolished the immunoreactive signal.
Results of functional tests are presented as means ± SEM of at least three independent experiments; in each experiment duplicate samples were incubated in parallel. The statistical significance of differences was assessed by two-tailed unpaired Student's t tests and confirmed by non-parametric Wilcoxon tests when appropriate or by ANOVA as mentioned, accepting p values of 0.05 or less as significant.
Receptor-mediated cAMP accumulation in beta cells is potentiated by glucose in a calcium-dependent manner
Interaction between glucose, glucagon and acetylcholine (1 µmol/l) in the regulation of cAMP production and insulin release from beta cells
1.4 mmol/l glucose
1.4 mmol/l glucose + 10 nmol/l glucagon
20 mmol/l glucose
20 mmol/l glucose + 10 nmol/l glucagon
The L-type Ca2+-channel blocker verapamil abrogates the synergism between GLP-1 and glucose upon insulin release
AC activity in RINm5F cells is activated by both Gsα and Ca2+-calmodulin
AC type VIII is expressed in insulin-producing cells
In vitro studies on rat pancreatic beta cells have shown that the insulinotropic effect of glucose is markedly potentiated by AC activators such as glucagon  or GLP-1 . The molecular mechanism that is responsible for this potentiation is still unclear. Proposed sites are K+ATP-channels , L-type Ca2+ channels , PKA anchoring site AKAP79/150 allowing crosstalk with PP2B  and target proteins involved in exocytosis [15, 18]. Glucose is known to increase cAMP concentrations in isolated islets [40, 41] and in purified beta cells when these are co-incubated with glucagon . It is therefore conceivable that glucose and GLP-1 interact at the level of cAMP production. We propose that synergistic interaction between glucose and GLP-1 signalling pathways also exists for Ca2+-calmodulin-activated type VIII AC, an isotype that is expressed in insulin-producing beta cells. This organization allows beta cells to integrate a variety of signals induced by nutrients and hormones to ensure an appropriate physiological response.
Coincidence detection of simultaneous stimuli is crucial in the central nervous system and requires molecular integrators that allow crosstalk between different signal transduction systems. One example proceeds via voltage-dependent Ca2+-channels and calmodulin-regulated AC isoforms . Type I and type VIII AC isoforms are particularly well adapted to fulfill this role since they are synergistically activated by Ca2+-calmodulin and Gsα [30, 32, 42, 43]. Consistent with their functional importance in the brain is the observation that mice rendered deficient in type I AC by targeted gene disruption present learning defects as a result of impaired long-term potentiation in hippocampal neurons  and in the cerebellum . Type VIII AC is particularly abundant in hippocampal neurons  which exhibit a large capacity for long-term potentiation and altered synaptic plasticity. Accordingly, type VIII AC knock-out mice show defects in the anxiety response after repetitive exposure to stress .
A synergistic regulation by glucose and glucagon was observed at the juncture of cellular cAMP accumulation in flow-sorted beta cells, on condition that inhibitors of cAMP breakdown such as IBMX or Ro-20-1724 were present. This suggests that glucagon- and glucose-induced signalling interact at the level of cAMP production rather than at the point of cAMP breakdown. Glucagon and GLP-1 acutely amplify the glucose-dependent signal to a similar extent, so that their common target of interaction is likely to be distal from the activated receptors. Previous work on whole islets emphasised the importance of calcium for islet adenylyl cyclase activity. Two studies [47, 48] have shown that homogenates of rat isolated islets contain Ca2+-calmodulin-dependent AC activity. We show that the Ca2+-calmodulin effects interact synergistically with that of Gsα. As such, our data provide an explanation for the glucose potentiation on hormone-induced cAMP accumulation in intact beta cells . Our observation that the L-type Ca2+-channel blocker verapamil  can abrogate the potentiating effect of glucose on GLP-1-induced intracellular cAMP accumulation supports the idea that the opening of L-type Ca2+-channels is required for the effect of glucose on AC activity in intact beta cells. Already at low glucose concentration, we observe a small, but borderline significant difference between cAMP accumulation in the presence and absence of verapamil subsequent to GLP-1 stimulation. It can be speculated that this effect is mediated by direct GLP-1 action on L-type Ca2+ channels. Activation of the GLP-1 receptor has been observed before to cause opening of L-type voltage-dependent Ca2+ channels under non-stimulatory glucose conditions [49, 50, 51] and to increase electrical activity by slowing Ca2+ channel inactivation . The GLP-1 induced increase in intracellular Ca2+ concentrations may be able to stimulate Ca2+-calmodulin-regulated AC isoforms (type I and VIII AC) resulting in the production of cAMP , which can be antagonized by adding the L-type Ca2+-channel blocker verapamil with the preservation of the Gs-protein stimulated cAMP production by ACs. The effect of glucose on glucagon receptor cAMP production was not mimicked or amplified by acetylcholine. This indicates that mobilization of Ca2+ from intracellular stores is not as effective as influx through L-type Ca2+-channels in the activation of AC, consistent with existing literature .
Amplification of cDNA using the type VIII AC primer set resulted in strong signals both with primary flow-sorted rat beta cells and the rat beta cell lines RINm5F or INS-1. While it can be argued for FACS-purified beta cells that the PCR signals are derived from the few non-beta cells that are present in these preparations , this possibility is highly unlikely. First, the signal intensity of type VIII AC mRNA in FACS-purified alpha cells was around 20% of the signal intensity of flow sorted beta cells. Consequently, the type VIII AC signal in pure beta cells cannot be explained by the 5% or fewer alpha cells present in this cell preparation. Second, type VIII AC expression was observed in three different beta cell lines, with strong (INS-1 832/13), moderate (INS-1 832/2) and no (RINm5F) glucose responsiveness. This indicates that type VIII AC is not directly responsible for conveying glucose-responsiveness to a beta cell, but direct evidence involving gene-overexpression or gene-silencing will be required to further address this issue.
The present observations do not infer that AC type VIII is the only or even the main adenylyl cyclase contributing to overall cellular cAMP accumulation. In fact, our RT-PCR analysis clearly identifies the presence of several isoforms in RNA prepared from purified primary beta cells. It is possible that the rather weak type III, IV, and V AC signals are due to contaminating non-beta cells, because signals were much stronger in FACS-purified alpha cells, respecting the beta to alpha signal intensity ratio that was also observed for glucagon. Amongst the five Ca2+-calmodulin-regulated AC isoforms, only type I and type VIII are synergistically activated by Ca2+-calmodulin and Gsα . Previous expression analyses in isolated islets [54, 55] suggested that the AC isoforms III, V and VI are also present in beta cells. From a regulatory perspective, the possible presence of type III AC in beta cells seems interesting. In isolated membranes, type III AC can be stimulated by Ca2+-calmodulin on condition that other activators, e.g. forskolin or activated Gsα are present at the same time . However, (i) this stimulation is additive rather than synergistic; (ii) in intact cells, type III AC is inhibited by raised cytosolic [Ca2+], possibly as a consequence of the activation of Ca2+-calmodulin-dependent kinases or phosphatases . It was reported that the spontaneously diabetic rat strain Goto-Kakizaki has defects both in the KATP-dependent and -independent pathways of glucose stimulation , abnormal glucose activation of exocytosis  as well as increased expression of AC type III  and type VIII in islets  and increased cAMP generation after forskolin stimulation . Congruently, interaction between glucose and glucagon (or related peptides) can be disturbed in other models of Type 2 diabetes .
Besides overall cellular expression, subcellular localisation in signalling complexes could be crucial for the integrated response of beta cells to glucose and GLP-1. Co-localisation of Ca2+-sensitive AC and capacitative Ca2+-entry channels was found in embryonic kidney cells [62, 63], as well as in cardiac myocytes  and in parotid cells . It will be a challenge for future studies to study such co-localisation in beta cells, since this spatial organization may contribute to foci of cellular cAMP formation when cells are exposed to hormone as observed in other cell types [66, 67]. In RINm5F cells, cAMP signalling complexes with relevance for exocytosis have been observed recently . Important questions that remain to be answered are: (i) whether GLP-1 receptors of primary beta cells in situ are preferentially coupled to type VIII AC isoforms; (ii) if the cAMP that is thus formed preferentially signals to protein complexes involved in regulation of exocytosis.
Is type VIII AC the only site at which cAMP-generating receptors and glucose interact synergistically? As was mentioned above, other molecular targets of interaction have been proposed [16, 27], following crosstalk between cAMP and Ca2+. In isolated mouse islets that were studied under depolarizing conditions in the presence of diazoxide (i.e. under high tonic intracellular [Ca2+]) the synergism between glucose and GLP-1 is still present . We suggest that at least one of the molecuar targets are regulated by other mediators than Ca2+, possibly by signals generated by cataplerosis .
In conclusion, our study provides evidence for the expression at RNA and protein level of the type VIII AC gene in pancreatic beta cells, as well as its functional activity in isolated membranes or in intact cells. The regulatory properties of this enzyme make it a well adapted molecular integrator of nutrient and hormonal stimuli that control insulin release.
The authors wish to thank E. Quartier, A. Van Breusegem and V. Berger for technical assistance. We are grateful to the personnel of the Department of Metabolism and Endocrinology for providing purified rat islet alpha and beta cells and to L. Kaufman for help with the statistical analysis. The clonal cell lines INS1 832/13 and 832/2  were generously supplied by Dr. C. Newgard, S. Stedman Center for Nutritional and Metabolic Studies, Duke University, Durham, N.C., USA. This study was supported by grant No. 9.0130.99 from the Flemish Fund for Scientific Research (FWO Vlaanderen), the Ministerie van de Vlaamse Gemeenschap, Departement Onderwijs (Geconcerteerde Onderzoeksactie 1807), and the Research Council of the Vrije Universiteit Brussel. S.A. Hinke is Visiting Postdoctoral Fellow at the FWO Vlaanderen.