Establishment of 2-mercaptoethanol-dependent differentiated insulin-secreting cell lines

Glucose down-regulates the expression of the peroxisome proliferator-activated receptor-alpha gene in the pancreatic beta -cell

To better understand the action of glucose on fatty acid metabolism in the beta-cell and the link between chronically elevated glucose or fatty acids and beta-cell decompensation in adipogenic diabetes, we investigated whether glucose regulates peroxisomal proliferator-activated receptor (PPAR) gene expression in the beta-cell. Islets or INS(832/13) beta-cells exposed to high glucose show a 60-80% reduction in PPARalpha mRNA expression. Oleate, either in the absence or presence of glucose, has no effect. The action of glucose is dose-dependent in the 6-20 mm range and maximal after 6 h. Glucose also causes quantitatively similar reductions in PPARalpha protein and DNA binding activity of this transcription factor. The effect of glucose is blocked by the glucokinase inhibitor mannoheptulose, is partially mimicked by 2-deoxyglucose, and is not blocked by the 3-O-methyl or the 6-deoxy analogues of the sugar that are not phosphorylated. Chronic elevated glucose reduces the expression levels of the PPAR target genes, uncoupling protein 2 and acyl-CoA oxidase, which are involved in fat oxidation and lipid detoxification. A 3-day exposure of INS-1 cells to elevated glucose results in a permanent rise in malonyl-CoA, the inhibition of fat oxidation, and the promotion of fatty acid esterification processes and causes elevated insulin secretion at low glucose. The results suggest that a reduction in PPARalpha gene expression together with a rise in malonyl-CoA plays a role in the coordinated adaptation of beta-cell glucose and lipid metabolism to hyperglycemia and may be implicated in the mechanism of beta-cell "glucolipotoxicity."

Insulin-secreting pituitary GH3 cells: a potential beta-cell surrogate for diabetes cell therapy

In a companion article, we describe the engineering and characterization of pituitary GH3 cell clones stably transfected with a furin-cleavable human insulin cDNA (InsGH3 cells). This article describes the performance of InsGH3 (clones 1 and 7) cell grafts into streptozotocin (STZ)-induced diabetic nude mice. Subcutaneous implantation of 2 x 10(6) InsGH3 cells resulted in the progressive reversal of hyperglycemia and diabetic symptoms, even though the progressive growth of the transplanted cells (clone 7) eventually led to glycemic levels below the normal mouse range. Proinsulin transgene expression was maintained in harvested InsGH3 grafts that, conversely, lose the expression of the prolactin (PRL) gene. Elevated concentrations of circulating mature human insulin were detected in graft recipients, demonstrating that proinsulin processing by InsGH3 cells did occur in vivo. Histologic analysis showed that transplanted InsGH3 grew in forms of encapsulated tumors composed of cells with small cytoplasms weakly stained for the presence of insulin. Conversely, intense insulin immunoreactivity was detected in graft-draining venules. Compared to pancreatic betaTC3 cells, InsGH3 cells showed in vitro a higher rate of replication, an elevate resistance to apoptosis induced by serum deprivation and proinflammatory cytokines, and significantly higher antiapoptotic Bcl-2 protein levels. Moreover, InsGH3 cells were resistant to the streptozotocin toxicity that, in contrast, reduced betaTC3 cell viability to 50-60% of controls. In conclusion, proinsulin gene expression and mature insulin secretion persisted in transplanted InsGH3 cells that reversed hyperglycemia in vivo. InsGH3 cells might represent a potential beta-cell surrogate because they are more resistant than pancreatic beta cells to different apoptotic insults and might therefore be particularly suitable for encapsulation.

Development and characterization of pituitary GH3 cell clones stably transfected with a human proinsulin cDNA

Successful beta-cell replacement therapy in insulin-dependent (type I) diabetes is hindered by the scarcity of human donor tissue and by the recurrence of autoimmune destruction of transplanted beta cells. Availability of non-beta cells, capable of releasing insulin and escaping autoimmune recognition, would therefore be important for diabetes cell therapy. We developed rat pituitary GH3 cells stably transfected with a furin-cleavable human proinsulin cDNA linked to the rat PRL promoter. Two clones (InsGH3/clone 1 and 7) were characterized in vitro with regard to basal and stimulated insulin release and proinsulin transgene expression. Mature insulin secretion was obtained in both clones, accounting for about 40% of total released (pro)insulin-like products. Immunocytochemistry of InsGH3 cells showed a cytoplasmic granular insulin staining that colocalized with secretogranin II (SGII) immunoreactivity. InsGH3 cells/clone 7 contained and released in vitro significantly more insulin than clone 1. Secretagogue-stimulated insulin secretion was observed in both InsGH3 clones either under static or dynamic conditions, indicating that insulin was targeted also to the regulated secretory pathway. Proinsulin mRNA levels were elevated in InsGH3 cells, being significantly higher than in betaTC3 cells. Moreover, proinsulin gene expression increased in response to various stimuli, thereby showing the regulation of the transfected gene at the transcriptional level. In conclusion, these data point to InsGH3 cells as a potential beta-cell surrogate even though additional engineering is required to instruct them to release insulin in response to physiologic stimulations.

The receptor tyrosine phosphatase-like protein ICA512 binds the PDZ domains of beta2-syntrophin and nNOS in pancreatic beta-cells

Islet cell autoantigen (ICA) 512 of type I diabetes is a receptor tyrosine phosphatase-like protein associated with the secretory granules of neurons and endocrine cells including insulin-secreting beta-cells of the pancreas. Here we show that in a yeast two-hybrid assay its cytoplasmic domain binds beta2-syntrophin, a modular adapter which in muscle cells interacts with members of the dystrophin family including utrophin, as well as the signaling molecule neuronal nitric oxide synthase (nNOS). The cDNA isolated by two-hybrid screening corresponded to a novel beta2-syntrophin isoform with a predicted molecular mass of 28 kDa. This isoform included the PDZ domain, but not the C-terminal region, which in full-length beta2-syntrophin is responsible for binding dystrophin-related proteins. In vitro binding of the beta2-syntrophin PDZ domain to ICA512 required both ICA512's C-terminal region and an internal polypeptide preceding its tyrosine phosphatase-like domain. Immunomicroscopy and co-immunoprecipitations from insulinoma INS-1 cells confirmed the occurrence of ICA512-beta2-syntrophin complexes in vivo. ICA512 also interacted in vitro with the PDZ domain of nNOS and ICA512-nNOS complexes were co-immunoprecipitated from INS-1 cells. Finally, we show that INS-1 cells, like muscle cells, contain beta2-syntrophin-utrophin oligomers. Thus, we propose that ICA512, through beta2-syntrophin and nNOS, links secretory granules with the actin cytoskeleton and signaling pathways involving nitric oxide.

Glucose-6-phosphatase in the insulin secreting cell line INS-1

The glucose-6-phosphatase system of the glucose sensitive insulin secreting rat insulinoma cells (INS-1) was investigated. INS-1 cells contain easily detectable levels of glucose-6-phosphatase enzyme protein (assessed by Western blotting) and have a very significant enzymatic activity. The features of the enzyme (Km and Vmax values, sensitivity to acidic pH, partial latency, and double immunoreactive band) are similar to those of the hepatic form. On the other hand, hardly detectable levels of glucose-6-phosphatase activity and protein were present in the parent glucose insensitive RINm5F cell line. The mRNA of the glucose-6-phosphate transporter was also more abundant in the INS-1 cells. The results support the view that the glucose-6-phosphatase system of the beta-cell is associated with the regulation of insulin secretion.

Molecular targets of a human HNF1 alpha mutation responsible for pancreatic beta-cell dysfunction

The reverse tetracycline-dependent transactivator system was employed in insulinoma INS-1 cells to achieve controlled inducible expression of hepatocyte nuclear factor-1 alpha (HNF1 alpha)-P291fsinsC, the most common mutation associated with subtype 3 of maturity-onset diabetes of the young (MODY3). Nuclear localized HNF1 alpha-P291fsinsC protein exerts its dominant-negative effects by competing with endogenous HNF1 alpha for the cognate DNA-binding site. HNF1 alpha controls multiple genes implicated in pancreatic beta-cell function and notably in metabolism- secretion coupling. In addition to reduced expression of the genes encoding insulin, glucose transporter-2, L-pyruvate kinase, aldolase B and 3-hydroxy-3-methylglutaryl coenzyme A reductase, induction of HNF1 alpha-P291fsinsC also significantly inhibits expression of mitochondrial 2-oxoglutarate dehydrogenase (OGDH) E1 subunit mRNA and protein. OGDH enzyme activity and [(14)C]pyruvate oxidation were also reduced. In contrast, the mRNA and protein levels of mitochondrial uncoupling protein-2 were dramatically increased by HNF1 alpha-P291fsinsC induction. As predicted from this altered gene expression profile, HNF1 alpha-P291fsinsC also inhibits insulin secretory responses to glucose and leucine, correlated with impaired nutrient-evoked mitochondrial ATP production and mitochondrial membrane hyperpolarization. These unprecedented results suggest the molecular mechanism of HNF1 alpha-P291fsinsC causing beta-cell dysfunction.

Glucagon-like peptide 1 stimulates insulin gene promoter activity by protein kinase A-independent activation of the rat insulin I gene cAMP response element

Glucagon-like peptide 1 (GLP-1), a hormonal activator of adenyl cyclase, stimulates insulin gene transcription, an effect mediated by the cAMP response element (CRE) of the rat insulin I gene promoter (RIP1). Here we demonstrate that the signaling mechanism underlying stimulatory effects of GLP-1 on insulin gene transcription results from protein kinase A (PKA)-independent activation of the RIP1 CRE. Although GLP-1 stimulates cAMP production in rat INS-1 insulinoma cells, we find accompanying activation of a -410-bp RIP1 luciferase construct (-410RIP1-LUC) to exist independently of this second messenger. GLP-1 produced a dose-dependent stimulation of -410RIP1-LUC (EC50 0.43 nmol/l), an effect reproduced by the GLP-1 receptor agonist exendin-4 and abolished by the antagonist exendin(9-39). Activation of RIP1 by GLP-1 was not affected by cotransfection with dominant-negative Gs alpha, was not blocked by cAMP antagonist Rp-cAMPS, and was insensitive to PKA antagonist H-89. Truncation of -410RIP1-LUC to generate -307-, -206-, and -166-bp constructs revealed 2 segments of RIP1 targeted by GLP-1. The first segment, not regulated by forskolin, was located between -410 and -307 bp of the promoter. The second segment, regulated by both GLP-1 and forskolin, included the CRE and was located between -206 and -166 bp. Consistent with these observations, stimulatory effects of GLP-1 at RIP1 were reduced after introduction of delta-182 and delta-183/180 inactivating deletions at the CRE. The action of GLP-1 at -410RIP1-LUC was also reduced by cotransfection with A-CREB, a genetically engineered isoform of the CRE binding protein CREB, which dimerizes with and prevents binding of basic-region-leucine-zipper (bZIP) transcription factors to the CRE. In contrast, the action of GLP-1 at the CRE was not blocked by cotransfection with M1-CREB, an isoform that lacks a consensus serine residue serving as substrate for PKA-mediated phosphorylation. On the basis of these studies, it is proposed that PKA-independent stimulatory actions of GLP-1 at RIP1 are mediated by bZIP transcription factors related in structure but not identical to CREB.

D-glucose metabolism in normal dispersed islet cells and tumoral INS-1 cells

The metabolism of D-glucose was characterized in both normal dispersed rat islet cells and the 2-mercaptoethanol-dependent insulin-secreting cells of the INS-1 line. The normal and tumoral islet cells differed from one another by the relative magnitude, concentration dependency and hierarchy of the increase in the production of 3HOH from D-[5-(3)H]glucose and 14C-labelled CO2, acidic metabolites and amino acids from D-[U-14C]glucose at increasing concentrations of the hexose. For instance, whilst the paired ratio between D-[U-14C]glucose oxidation and D-[5-(3)H]glucose utilization augmented in a typical sigmoidal manner in normal islet cells exposed to increasing concentrations of D-glucose, it progressively decreased under the same experimental conditions in INS-1 cells. Nevertheless, the absolute values and concentration-response relationship for the increase in ATP generation rate attributable to the catabolism of D-glucose were virtually identical in normal and tumoral cells. These findings indicate that the analogy in the secretory response to D-glucose of normal and INS-1 islet cells, although coinciding with a comparable response to the hexose in terms of ATP generation, contrasts with a vastly different pattern of D-glucose metabolism in these two cell types.

An indirect role for upstream stimulatory factor in glucose-mediated induction of pyruvate kinase and S14 gene expression

Transcription of the L-type pyruvate kinase (L-PK) and S14 genes is induced in hepatocytes in response to increased glucose metabolism. The regulatory sequences of these genes responsible for induction by glucose have been mapped to related E-box containing motifs in the promoters. Similarly, L-PK promoter activity is stimulated in a differentiated pancreatic beta-cell line, INS-1, in response to elevated glucose. By mutational analysis, we demonstrate that the sequence requirements for glucose induction in the INS-1 cell are identical to those observed in the hepatocyte, suggesting that the same transcriptional factor(s) is responsible for regulation of L-PK expression in the two cell types. One nuclear factor that binds to the glucose regulatory sequences of both of these genes is the Upstream Stimulatory Factor (USF), a ubiquitous E-box binding protein. Mice deleted for the USF2 gene display a severely delayed response to carbohydrate feeding (Vallet et al. [26]). This observation, however, does not differentiate between a direct and an indirect role for USF in the process. To gain further insight into the possible involvement of USF in glucose signaling, we have used a recombinant adenoviral construct that expresses a dominant negative form of USF. This dominant negative can dimerize with endogenous USF and is shown to inhibit DNA binding of USF in hepatocytes and INS-1 cells. However, expression of the dominant negative USF did not block the ability of glucose to stimulate L-PK or S14 gene expression in hepatocytes or L-PK promoter activity in INS-1 cells. We conclude that USF does not act by binding to the glucose regulatory sequences of the S14 or L-PK genes and the role of USF in the process of glucose induction is indirect.

Synergistic activation of NF-kappab and inducible isoform of nitric oxide synthase induction by interferon-gamma and tumor necrosis factor-alpha in INS-1 cells

Interferon-gamma (IFN-gamma) is known to exert deleterious effects on pancreatic beta-cells and is implicated in the development of type 1 (autoimmune) diabetes mellitus. In this study, we investigated signaling mechanisms mediating the effects of IFN-gamma in pancreatic beta-cells using a differentiated rat insulin-secreting cell line, INS-1, with special reference to the activation of transcription factors STAT (signal transducers and activators of transcription)1 and NF-kappaB. Exposure of INS-1 cells to 100 IU/ml IFN-gamma for 24 h resulted in significant inhibition of nutrient-induced insulin secretion associated with impaired metabolism. In combination with tumor necrosis factor-alpha (TNF-alpha) (50 ng/ml), IFN-gamma elicited severe cytotoxicity and induced the expression of the inducible isoform of nitric oxide synthase (iNOS) mRNA. IFN-gamma promoted tyrosine phosphorylation and DNA-binding of STAT1 through Janus kinase (JAK)1 activation without apparent phosphorylation of JAK2. TNF-alpha did not affect STAT1 activation, but stimulated DNA-binding and transcriptional activity of NF-kappaB, both of which were further increased by IFN-gamma. These effects of IFN-gamma and TNF-alpha seem physiologically relevant, because either inhibition of STAT1 by the tyrosine kinase inhibitor herbimycin A or that of NF-kappaB by sulfasalazine resulted in the reduction of iNOS mRNA expression. In conclusion, IFN-gamma activates STAT1 and potentiates TNF-alpha-induced NF-kappaB activation in INS-1 cells, thereby inducing iNOS and cell destruction.

IB1 reduces cytokine-induced apoptosis of insulin-secreting cells

IB1/JIP-1 is a scaffold protein that interacts with upstream components of the c-Jun N-terminal kinase (JNK) signaling pathway. IB1 is expressed at high levels in pancreatic beta cells and may therefore exert a tight control on signaling events mediated by JNK in these cells. Activation of JNK by interleukin 1 (IL-1beta) or by the upstream JNK constitutive activator DeltaMEKK1 promoted apoptosis in two pancreatic beta cell lines and decreased IB1 content by 50-60%. To study the functional consequences of the reduced IB1 content in beta cell lines, we used an insulin-secreting cell line expressing an inducible IB1 antisense RNA that lead to a 38% IB1 decrease. Reducing IB1 levels in these cells increased phosphorylation of c-Jun and increased the apoptotic rate in presence of IL-1beta. Nitric oxide production was not stimulated by expression of the IB1 antisense RNA. Complementary experiments indicated that overexpression of IB1 in insulin-producing cells prevented JNK-mediated activation of the transcription factors c-Jun, ATF2, and Elk1 and decreased IL-1beta- and DeltaMEKK1-induced apoptosis. These data indicate that IB1 plays an anti-apoptotic function in insulin-producing cells probably by controlling the activity of the JNK signaling pathway.

Immunodetection of alpha1E voltage-gated Ca(2+) channel in chromogranin-positive muscle cells of rat heart, and in distal tubules of human kidney

The calcium channel alpha1E subunit was originally cloned from mammalian brain. A new splice variant was recently identified in rat islets of Langerhans and in human kidney by the polymerase chain reaction. The same isoform of alpha1E was detected in rat and guinea pig heart by amplifying indicative cDNA fragments and by immunostaining using peptide-specific antibodies. The apparent molecular size of cardiac alpha1E was determined by SDS-PAGE and immunoblotting (218 +/- 6 kD; n = 3). Compared to alpha1E from stably transfected HEK-293 cells, this is smaller by 28 kD. The distribution of alpha1E in cardiac muscle cells of the conducting system and in the cardiomyoblast cell line H9c2 was compared to the distribution of chromogranin, a marker of neuroendocrine cells, and to the distribution of atrial natriuretic peptide (ANP). In serial sections from atrial and ventricular regions of rat heart, co-localization of alpha1E with ANP was detected in atrium and with chromogranin A/B in Purkinje fibers of the conducting system in both rat atrium and ventricle. The kidney is another organ in which natriuretic peptide hormones are secreted. The detection of alpha1E in the distal tubules of human kidney, where urodilatin is stored and secreted, led to the conclusion that the expression of alpha1E in rat heart and human kidney is linked to regions with endocrine functions and therefore is involved in the Ca(2+)-dependent secretion of peptide hormones such as ANP and urodilatin.

Electrospray ionization mass spectrometric analyses of phospholipids from INS-1 insulinoma cells: comparison to pancreatic islets and effects of fatty acid supplementation on phospholipid composition and insulin secretion

Insulin secretion by pancreatic islet beta-cells is impaired in diabetes mellitus, and normal beta-cells are enriched in phospholipids with arachidonate as sn-2 substituent. Such molecules may play structural roles in exocytotic membrane fusion or serve as substrates for phospholipases activated by insulin secretagogues. INS-1 insulinoma cells respond to secretagogues and permit the study of effects of culture with free fatty acids on phospholipid composition and secretion. INS-1 cell glycerophosphocholine (GPC) and glycerophosphoethanolamine (GPE) lipids are demonstrated here by electrospray ionization mass spectrometry to contain a lower fraction of molecules with arachidonate and a higher fraction with oleate as sn-2 substituent than native islets. Palmitic acid supplementation induces little change in these INS-1 cell lipids, but supplementation with linoleate or arachidonate induces a large rise in the fraction of INS-1 cell GPC species with polyunsaturated sn-2 substituents and a fall in oleate-containing species to yield a GPC profile similar to native islets. The fraction of GPE lipids comprised of plasmenylethanolamine species with polyunsaturated sn-2 substituents in early-passage INS-1 cells is similar to that of islets, but declines on serial passage. Such molecules might participate in exocytotic membrane fusion, and late-passage INS-1 cells have reduced insulin secretory responses. Arachidonate supplementation induces a rise in the fraction of INS-1 cell GPE lipids with polyunsaturated sn-2 substituents and partially restores responses to insulin secretagogues by late-passage INS-1 cells, but does not further amplify secretion by early-passage cells. Effects of extracellular free fatty acids on beta-cell phospholipid composition and secretory responses could be involved in changes in beta-cell function during the period of hyper-free fatty acidemia that precedes diabetes mellitus.

Stevioside acts directly on pancreatic beta cells to secrete insulin: actions independent of cyclic adenosine monophosphate and adenosine triphosphate-sensitive K+-channel activity

The natural sweetener stevioside, which is found in the plant Stevia rebaudiana Bertoni, has been used for many years in the treatment of diabetes among Indians in Paraguay and Brazil. However, the mechanism for the blood glucose-lowering effect remains unknown. To elucidate the impact of stevioside and its aglucon steviol on insulin release from normal mouse islets and the beta-cell line INS-1 were used. Both stevioside and steviol (1 nmol/L to 1 mmol/L) dose-dependently enhanced insulin secretion from incubated mouse islets in the presence of 16.7 mmol/L glucose (P < .05). The insulinotropic effects of stevioside and steviol were critically dependent on the prevailing glucose concentration, ie, stevioside (1 mmol/L) and steviol (1 micromol/L) only potentiated insulin secretion at or above 8.3 mmol/L glucose (P < .05). Interestingly, the insulinotropic effects of both stevioside and steviol were preserved in the absence of extracellular Ca2+. During perifusion of islets, stevioside (1 mmol/L) and steviol (1 micromol/L) had a long-lasting and apparently reversible insulinotropic effect in the presence of 16.7 mmol/L glucose (P < .05). To determine if stevioside and steviol act directly on beta cells, the effects on INS-1 cells were also investigated. Stevioside and steviol both potentiated insulin secretion from INS-1 cells (P < .05). Neither stevioside (1 to 100 micromol/L) nor steviol (10 nmol/L to 10 micromol/L) influenced the plasma membrane K+ adenosine triphosphate ((K+)ATP)-sensitive channel activity, nor did they alter cyclic adenosine monophosphate (cAMP) levels in islets. In conclusion, stevioside and steviol stimulate insulin secretion via a direct action on beta cells. The results indicate that the compounds may have a potential role as antihyperglycemic agents in the treatment of type 2 diabetes mellitus.

Comparison of amperometric methods for detection of exocytosis from single pancreatic beta-cells of different species

Two methods for amperometric detection of exocytosis at single pancreatic beta-cells were compared. In the first, direct detection of insulin was accomplished using an insulin-sensitive chemically modified electrode. In the second, 5-hydroxytryptamine (5-HT) that had been allowed to accumulate within the beta-cell secretory vesicles was detected with a bare carbon electrode. The goal of the comparison was to determine whether 5-HT secretion was a valid marker of insulin secretion in single beta-cells. To aid in this comparison, some experiments involved simultaneous measurement of insulin and 5-HT at cells previously allowed to accumulate 5-HT. Upon application of common insulin secretagogues, current spikes resulting from detection of 5-HT, insulin, or both compounds were obtained indicative of secretion via exocytosis. The mean area of current spikes obtained from simultaneous measurements equaled the sum of the mean area of insulin and 5-HT measured independently. Additionally, analyses of the number of spikes obtained for detection of insulin, 5-HT, or both compounds were similar for several common secretagogues. These data support the hypothesis that accumulated 5-HT is released from insulin containing secretory vesicles, exclusively. In addition, measurement of insulin and 5-HT from beta-cells of different species was compared to determine whether a species dependence exists for the two methods compared here. Detection of 5-HT results in a similar number of spikes that are equivalent to insulin in frequency and amplitude in human, porcine, and canine beta-cells; however, in mouse and INS-1 beta-cells, 5-HT is more readily detected than insulin.

Overexpression of monocarboxylate transporter and lactate dehydrogenase alters insulin secretory responses to pyruvate and lactate in beta cells

Previous investigations revealed low activities of lactate dehydrogenase (LDH) and plasma membrane monocarboxylate transporters (MCT) in the pancreatic beta cell. In this study the significance of these characteristics was explored by overexpressing type A LDH (LDH-A) and/or type 1 MCT (MCT-1) in the clonal INS-1 beta cells and isolated rat islets. Inducible overexpression of LDH-A resulted in an 87-fold increase in LDH activity in INS-1 cells. Adenovirus-mediated overexpression of MCT-1 increased lactate transport activity 3.7-fold in INS-1 cells. Although overexpression of LDH-A, and/or MCT-1 did not affect glucose-stimulated insulin secretion, LDH-A overexpression resulted in stimulation of insulin secretion even at a low lactate concentration with a concomitant increase in its oxidation in INS-1 cells regardless of MCT-1 co-overexpression. Adenovirus-mediated overexpression of MCT-1 caused an increase in pyruvate oxidation and conferred pyruvate-stimulated insulin release to isolated rat islets. Although lactate did not stimulate insulin secretion from control or MCT-1-overexpressing islets, co-overexpression of LDH-A and MCT-1 evoked lactate-stimulated insulin secretion with a concomitant increase in lactate oxidation in rat islets. These results suggest that low expression of MCT and LDH is requisite to the specificity of glucose in insulin secretion, protecting the organism from undesired hypoglycemic actions of pyruvate and lactate during exercise and other catabolic states.

Modulation of gastrin-releasing peptide (GRP) receptors in insulin secreting cells

Gastrin-releasing peptide (GRP) receptors are present in pancreatic islets, though their regulation is unknown except for homologous desensitization. The modulation of binding of GRP to mouse pancreatic islets and INS-1 cells was studied. At 60 min (steady-state), total binding of [(125)I-Tyr(15)] GRP was 1.62 per cent of total radioactivity per 50 islets; non-specific binding (presence of 1 mM unlabelled GRP(1-27)) was 0.05 to 0.61 per cent of total radioactivity. A preincubation with 1000 nM cholecystokinin (CCK(8)) or with 1000 nM glucose-dependent insulinotropic peptide (GIP) augmented the number of GRP binding sites but not their affinity. [(125)I-Tyr(15)]GRP binding to INS-1 cells was saturable (90 min) and specific with respect to compounds that are not chemically related to GRP (e.g. calcitonin gene-regulated peptide-CGRP and atrial natriuretic peptide-ANP). Displacement studies showed one binding site with a K(d) of 0.39 nM and a B(max) of 13.2 fmoles mg(-1) protein. When the cells were pretreated for 24 h with 10 nM GIP or CCK(8), only GIP but not CCK(8) increased the B(max) of the GRP binding site. The affinity (K(d)) was not changed by either compound. This effect of GIP pretreatment was not affected by downregulating PKC by TPA (phorbol ester; long-term pretreatment). These data indicate that: (1) specific binding sites for GRP are present in mouse pancreatic islets and INS-1 cells; (2) the GRP binding is upregulated by GIP in both islets and INS-1 cells and additionally by CCK(8 ), albeit only in islets; and (3) PKC does not seem to be involved in the up-regulation process. Thus a positive interplay between both the incretins GIP and CCK(8) and the neurotransmitter GRP is obvious.

Bridge-1, a novel PDZ-domain coactivator of E2A-mediated regulation of insulin gene transcription

Proteins in the E2A family of basic helix-loop-helix transcription factors are important in a wide spectrum of physiologic processes as diverse as neurogenesis, myogenesis, lymphopoeisis, and sex determination. In the pancreatic beta cell, E2A proteins, in combination with tissue-specific transcription factors, regulate expression of the insulin gene and other genes critical for beta-cell function. By yeast two-hybrid screening of a cDNA library prepared from rat insulinoma (INS-1) cells, we identified a novel protein, Bridge-1, that interacts with E2A proteins and functions as a coactivator of gene transcription mediated by E12 and E47. Bridge-1 contains a PDZ-like domain, a domain known to be involved in protein-protein interactions. Bridge-1 is highly expressed in pancreatic islets and islet cell lines and the expression pattern is primarily nuclear. The interaction of Bridge-1 with E2A proteins is further demonstrated by coimmunoprecipitation of in vitro-translated Bridge-1 with E12 or E47 and by mammalian two-hybrid studies. The PDZ-like domain of Bridge-1 is required for interaction with the carboxy terminus of E12. In both yeast and mammalian two-hybrid interaction studies, Bridge-1 mutants lacking an intact PDZ-like domain interact poorly with E12. An E12 mutant (E12DeltaC) lacking the carboxy-terminal nine amino acids shows impaired interaction with Bridge-1. Bridge-1 has direct transactivational activity, since a Gal4 DNA-binding domain-Bridge-1 fusion protein transactivates a Gal4CAT reporter. Bridge-1 also functions as a coactivator by enhancing E12- or E47-mediated activation of a rat insulin I gene minienhancer promoter-reporter construct in transient-transfection experiments. Substitution of the mutant E12DeltaC for E12 reduces the coactivation of the rat insulin I minienhancer by Bridge-1. Inactivation of endogenous Bridge-1 in insulinoma (INS-1) cells by expression of a Bridge-1 antisense RNA diminishes rat insulin I promoter activity. Bridge-1, by utilizing its PDZ-like domain to interact with E12, may provide a new mechanism for the coactivation and regulation of transcription of the insulin gene.

Leptin inhibits insulin secretion induced by cellular cAMP in a pancreatic B cell line (INS-1 cells)

The effect of leptin on insulin secretion is controversial due to conflicting results in the literature. In the present study, we incubated insulin-producing rat insulinoma INS-1 cells for 60 min and examined the effects of recombinant murine leptin (20 nmol/l). We found that leptin (0.1-100 nmol/l) did not affect the insulin response to glucose (1-20 mmol/l). However, when cells were incubated with agents that increase the intracellular content of cAMP, i.e., glucagon-like peptide-1 (100 nmol/l), pituitary adenylate cyclase activating polypeptide (100 nmol/l), forskolin (2.5 micromol/l), dibutyryl-cAMP (1 mmol/l), or 3-isobutyl-1-methylxanthine (100 micromol/l), leptin significantly reduced insulin secretion (by 34-58%, P < 0.05-0.001). In contrast, when insulin secretion was stimulated by the cholinergic agonist carbachol (100 micromol/l) or the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (1 micromol/l), both of which activate protein kinase C, leptin was without effect. We conclude that leptin inhibits insulin secretion from INS-1 cells under conditions in which intracellular cAMP is increased. This suggests that the cAMP-protein kinase A signal transduction pathway is a target for leptin to inhibit insulin secretion in insulin-producing cells.

Glucose regulation of the expression of the glucagon receptor gene

The glucagon receptor gene is a member of a gene family, the expression of which is strongly upregulated by glucose. This review deals with the structure of both the glucagon receptor gene and its promoter. Attention is focused on the glucose regulatory element that we discovered in the promoter of this gene. Regulation by glucose of genes implicated in glucose homeostasis represents one mechanism contributing to the control of fuel utilization. Its deficiency or imbalance could potentially lead to or participate in pathological situations such as diabetes mellitus. On the other hand, the regulatory element of the glucagon receptor gene promoter could be used as a tool for the glucose-regulated expression of other genes. Indeed, an analysis of the glucagon receptor gene promoter demonstrated that only a short fragment of the genomic DNA, easy to subclone, contains all required elements for activation by glucose. Its potential use for gene therapy is also considered, therefore, in this report.

Hydrogen peroxide alters mitochondrial activation and insulin secretion in pancreatic beta cells

The effects of a transient exposure to hydrogen peroxide (10 min at 200 microM H(2)O(2)) on pancreatic beta cell signal transduction and insulin secretion have been evaluated. In rat islets, insulin secretion evoked by glucose (16.7 mM) or by the mitochondrial substrate methyl succinate (5 mM) was markedly blunted following exposure to H(2)O(2). In contrast, the secretory response induced by plasma membrane depolarization (20 mM KCl) was not significantly affected. Similar results were obtained in insulinoma INS-1 cells using glucose (12.8 mM) as secretagogue. After H(2)O(2) treatment, glucose no longer depolarized the membrane potential (DeltaPsi) of INS-1 cells or increased cytosolic Ca(2+). Both DeltaPsi and Ca(2+) responses were still observed with 30 mM KCl despite an elevated baseline of cytosolic Ca(2+) appearing approximately 10 min after exposure to H(2)O(2). The mitochondrial DeltaPsi of INS-1 cells was depolarized by H(2)O(2) abolishing the hyperpolarizing action of glucose. These DeltaPsi changes correlated with altered mitochondrial morphology; the latter was not preserved by the overexpression of the antiapoptotic protein Bcl-2. Mitochondrial Ca(2+) was increased following exposure to H(2)O(2) up to the micromolar range. No further augmentation occurred after glucose addition, which normally raises this parameter. Nevertheless, KCl was still efficient in enhancing mitochondrial Ca(2+). Cytosolic ATP was markedly reduced by H(2)O(2) treatment, probably explaining the decreased endoplasmic reticulum Ca(2+). Taken together, these data point to the mitochondria as primary targets for H(2)O(2) damage, which will eventually interrupt the transduction of signals normally coupling glucose metabolism to insulin secretion.

Relative hypoglycemia and hyperinsulinemia in mice with heterozygous lipoprotein lipase (LPL) deficiency. Islet LPL regulates insulin secretion

Lipoprotein lipase (LPL) provides tissues with fatty acids, which have complex effects on glucose utilization and insulin secretion. To determine if LPL has direct effects on glucose metabolism, we studied mice with heterozygous LPL deficiency (LPL+/-). LPL+/- mice had mean fasting glucose values that were up to 39 mg/dl lower than LPL+/+ littermates. Despite having lower glucose levels, LPL+/- mice had fasting insulin levels that were twice those of +/+ mice. Hyperinsulinemic clamp experiments showed no effect of genotype on basal or insulin-stimulated glucose utilization. LPL message was detected in mouse islets, INS-1 cells (a rat insulinoma cell line), and human islets. LPL enzyme activity was detected in the media from both mouse and human islets incubated in vitro. In mice, +/- islets expressed half the enzyme activity of +/+ islets. Islets isolated from +/+ mice secreted less insulin in vitro than +/- and -/- islets, suggesting that LPL suppresses insulin secretion. To test this notion directly, LPL enzyme activity was manipulated in INS-1 cells. INS-1 cells treated with an adeno-associated virus expressing human LPL had more LPL enzyme activity and secreted less insulin than adeno-associated virus-beta-galactosidase-treated cells. INS-1 cells transfected with an antisense LPL oligonucleotide had less LPL enzyme activity and secreted more insulin than cells transfected with a control oligonucleotide. These data suggest that islet LPL is a novel regulator of insulin secretion. They further suggest that genetically determined levels of LPL play a role in establishing glucose levels in mice.

Lipid rather than glucose metabolism is implicated in altered insulin secretion caused by oleate in INS-1 cells

A comprehensive metabolic study was carried out to understand how chronic exposure of pancreatic beta-cells to fatty acids causes high basal secretion and impairs glucose-induced insulin release. INS-1 beta-cells were exposed to 0.4 mM oleate for 3 days and subsequently incubated at 5 or 25 mM glucose, after which various parameters were measured. Chronic oleate promoted triglyceride deposition, increased fatty acid oxidation and esterification, and reduced malonyl-CoA at low glucose in association with elevated basal O(2) consumption and redox state. Oleate caused a modest (25%) reduction in glucose oxidation but did not affect glucose usage, the glucose 6-phosphate and citrate contents, and the activity of pyruvate dehydrogenase of INS-1 cells. Thus changes in glucose metabolism and a Randle-glucose/fatty acid cycle do not explain the altered secretory properties of beta-cells exposed to fatty acids. The main response of INS-1 cells to chronic oleate, which is to increase the oxidation and esterification of fatty acids, may contribute to cause high basal insulin secretion via increased production of reducing equivalents and/or the generation of complex lipid messenger molecule(s).

Glucagon-like peptide 1 elevates cytosolic calcium in pancreatic beta-cells independently of protein kinase A

Glucagon-like peptide 1 (7-36)amide (GLP-1) is an insulinotropic intestinal peptide hormone with a potential role as antidiabetogenic therapeutic agent. It mediates a potentiation of glucose-induced insulin secretion, by activation of adenylate cyclase and subsequent elevation of cytosolic free calcium, [Ca2+]cyt. We investigated the role of protein kinase A (PKA) in GLP-1 signal transduction, using isolated mouse islets as well as the differentiated beta-cell line INS-1. Two specific inhibitors of PKA, (Rp)-adenosine cyclic 3',5'-phosporothioate (Rp-cAMPS, up to 3 mM) and KT5720 (up to 10 microM), did not inhibit the GLP-1-induced [Ca2+]cyt elevation. Another PKA inhibitor, H-89, reduced the [Ca2+]cyt elevation only when applied at high concentrations (10-40 microM), higher than sufficient for PKA inhibition in many cell types. Furthermore, at these concentrations, H-89 also inhibited presumably PKA-independent processes such as glucose-induced [Ca2+]cyt elevations and intracellular calcium storage. This suggests a PKA-independent action of H-89. Similarly to H-89, the potent but unselective protein kinase inhibitor staurosporine inhibited the GLP-1-induced [Ca2+]cyt elevation only at high concentrations, at which it also inhibited glucose-induced [Ca2+]cyt elevations. The same observations as with GLP-1 were made when adenylate cyclase was stimulated with forskolin, for selective examination of signal transduction downstream of receptor and G protein. Our results suggest that the GLP-1-induced [Ca2+]cyt elevation is mediated independently of PKA and thus belongs to the yet-little-characterized ensemble of effects that are mediated by binding of cAMP to other target proteins.

Oxygen microsensor and its application to single cells and mouse pancreatic islets

An oxygen microsensor with a < 3-micron tip diameter was developed for monitoring oxygen levels at single cells and mouse pancreatic islets. The sensor was fabricated by electrochemically recessing an etched Pt wire inside a pulled glass micropipet and then coating with cellulose acetate. This fabrication process was found to be simpler than previous oxygen electrode designs of comparable size. The microsensors had a average sensitivity of 0.59 +/- 0.29 pA/mmHg (mean +/- SD, n = 42), signals that were minimally perturbed by convection, and response times of < 1 s. The electrode was used to measure the oxygen gradient around and inside single mouse islets. The measurements demonstrate that oxygen levels within even the largest islets at maximal glucose stimulation are 67 +/- 1.6 mmHg (mean +/- SD, n = 5), indicating that islets have adequate oxygen supplies by diffusion under tissue culture conditions to support insulin secretion. The electrode was also used to record the dynamics of oxygen level at single islets as a function of glucose concentration. As glucose level was changed from 3 to 10 mM, oxygen level decreased by 15.8 +/- 2.3 mmHg (mean +/- SEM, n = 6) and oscillations with a period of 3.3 +/- 0.6 min (mean +/- SEM, n = 6) appeared in the oxygen level. In islets bathed in quiescent solutions containing 10 mM glucose, similar oscillations could be observed. In addition, in the quiet solutions it was possible to detect faster oscillations with a period of 12.1 +/- 1.7 s (mean +/- SEM, n = 6) superimposed on the slower oscillations. Oxygen consumption could also be observed at single insulinoma cells using the electrode. Individual cells also showed oscillations in oxygen consumption with a period of a few seconds. The results demonstrate that the electrode can be used for dynamic oxygen level recordings in biological microenvironments.

Immunohistochemical detection of alpha1E voltage-gated Ca(2+) channel isoforms in cerebellum, INS-1 cells, and neuroendocrine cells of the digestive system

Polyclonal antibodies were raised against a common and a specific epitope present only in longer alpha1E isoforms of voltage-gated Ca(2+) channels, yielding an "anti-E-com" and an "anti-E-spec" serum, respectively. The specificity of both sera was established by immunocytochemistry and immunoblotting using stably transfected HEK-293 cells or membrane proteins derived from them. Cells from the insulinoma cell line INS-1, tissue sections from cerebellum, and representative regions of gastrointestinal tract were stained immunocytochemically. INS-1 cells expressed an alpha1E splice variant with a longer carboxy terminus, the so-called alpha1Ee isoform. Similarily, in rat cerebellum, which was used as a reference system, the anti-E-spec serum stained somata and dendrites of Purkinje cells. Only faint staining was seen throughout the cerebellar granule cell layer. After prolonged incubation times, neurons of the molecular layer were stained by anti-E-com, suggesting that a shorter alpha1E isoform is expressed at a lower protein density. In human gastrointestinal tract, endocrine cells of the antral mucosa (stomach), small and large intestine, and islets of Langerhans were stained by the anti-E-spec serum. In addition, staining by the anti-E-spec serum was observed in Paneth cells and in the smooth muscle cell layer of the lamina muscularis mucosae. We conclude that the longer alpha1Ee isoform is expressed in neuroendocrine cells of the digestive system and that, in pancreas, alpha1Ee expression is restricted to the neuroendocrine part, the islets of Langerhans. alpha1E therefore appears to be a common voltage-gated Ca(2+) channel linked to neuroendocrine and related systems of the body.

Expression, processing, and secretion of the neuroendocrine VGF peptides by INS-1 cells

The neurotropin-inducible gene vgf is expressed in neuronal and endocrine tissues. It encodes a secretory protein that is proteolytically processed in neuronal cells to low molecular mass polypeptides. In the present report, we show that vgf is expressed in different insulinoma cell lines and in normal rat pancreatic islets. In the insulinoma-derived beta-cell line INS-1, vgf messenger RNA was transcriptionally up-regulated by increased levels ofintracellular cAMP, but not by the addition of glucose (20 mM) or phorbol 12-myristate 13-acetate (100 nM). Furthermore, nerve growth factor failed to stimulate vgf gene expression. In INS-1 cells, the VGF protein was shown to be processed in a post endoplasmic reticulum compartment to produce a peptide profile similar to that seen in neurons. The release of such VGF peptides occurred at a low rate in the absence of secretory stimuli (<2%/h). A 3-fold increase in the rate of release was seen after the addition of glucose (15 mM), a 4-fold increase was seen after (Bu)2cAMP (1 mM), and a 6-fold increase was seen after phorbol 12-myristate 13-acetate (100 nM). These results indicated that insulin-containing cells produce VGF-derived peptides that are released via a regulated pathway in response to insulin secretagogues.

Regulation of prolactin receptor (PRLR) gene expression in insulin-producing cells. Prolactin and growth hormone activate one of the rat prlr gene promoters via STAT5a and STAT5b

Expression of the prolactin receptor (PRLR) gene is increased in pancreatic islets during pregnancy and in vitro in insulin-producing cells by growth hormone (GH) and prolactin (PRL). The 5'-region of the rat PRLR gene contains at least three alternative first exons that are expressed tissue-specifically because of differential promoter usage. We show by reverse transcription-polymerase chain reaction analysis that both exon 1A- and exon 1C-containing PRLR transcripts are expressed in rat islets and that human (h)GH, ovine (o)PRL, and bovine (b)GH increase exon 1A expression 6.5 +/- 0. 8-fold, 6.8 +/- 0.7-fold, and 3.9 +/- 0.7-fold and exon 1C expression 4.8 +/- 0.4-fold, 4.4 +/- 0.6-fold, and 2.5 +/- 0.7-fold, respectively. Expression of exon 1B was not detectable. The transcriptional activities of reporter constructs containing the 1A, 1B, or 1C promoter were found to be 22.8-fold, 2.7-fold, and 8. 0-fold, respectively, above that of a promoterless reporter construct when transfected into the insulin-producing INS-1 cells. The transcriptional activity of the 1A promoter construct was increased 8.9 +/- 1.9-fold by 0.5 microgram/ml hGH. Responsiveness to hGH of the 1A promoter was localized to the region from -225 to +81 with respect to the transcription start site. This region contains the sequence TTCTAGGAA that by gel retardation experiments was shown to bind the transcription factors STAT5a and STAT5b in response to stimulation by hGH, oPRL, or bGH. Mutation of this gamma-activated sequence-like element completely abolished transcriptional induction of the 1A promoter by hGH. Our results suggest that GH and PRL increase the levels of exon 1A- and 1C-containing PRLR mRNA species and furthermore that the transcriptional activity of the 1A promoter is increased via activation of STAT5a and STAT5b.

Bioartificial pancreas: alternative supply of insulin-secreting cells

In this study, insulin secretion function of INS-1 cells immunoisolated in microcapsules was evaluated. Following encapsulation, the immunoisolated INS-1 cells continued to propagate and flourish within the microcapsules during the entire two-month in vitro incubation period. The insulin secretion from encapsulated INS-1 cells following seven days of in vitro culture increased from 1.6 +/- 0.2 ng/2h/10(6) cells in a glucose-free medium to 11.5 +/- 2.1 ng/2h/10(6) cells at 16.7 mM glucose. In vivo, transplants of 1.2 x 10(7) cells into each of six diabetic C57BL/6 mice resulted in the restoration of normoglycemia in all graft recipients for up to 60 days post transplantation. Most capsules recovered from two animals 30 days post transplantation were free of cell overgrowth and physically intact. Immunostaining for insulin of the cells within the recovered capsules clearly indicated the presence of insulin. The presented data demonstrate the potential use of an immunoisolated beta-cell line for the treatment of diabetes.

Secretagogues modulate the calcium concentration in the endoplasmic reticulum of insulin-secreting cells. Studies in aequorin-expressing intact and permeabilized ins-1 cells

The precise regulation of the Ca2+ concentration in the endoplasmic reticulum ([Ca2+]er) is important for protein processing and signal transduction. In the pancreatic beta-cell, dysregulation of [Ca2+]er may cause impaired insulin secretion. The Ca2+-sensitive photoprotein aequorin mutated to lower its Ca2+ affinity was stably expressed in the endoplasmic reticulum (ER) of rat insulinoma INS-1 cells. The steady state [Ca2+]er was 267 +/- 9 microM. Both the Ca2+-ATPase inhibitor cyclopiazonic acid and 4-chloro-m-cresol, an activator of ryanodine receptors, caused an almost complete emptying of ER Ca2+. The inositol 1,4,5-trisphosphate generating agonists, carbachol, and ATP, reduced [Ca2+]er by 20-25%. Insulin secretagogues that raise cytosolic [Ca2+] by membrane depolarization increased [Ca2+]er in the potency order K+ >> glucose > leucine, paralleling their actions in the cytosolic compartment. Glucose, which augmented [Ca2+]er by about 25%, potentiated the Ca2+-mobilizing effect of carbachol, explaining the corresponding observation in cytosolic [Ca2+]. The filling of ER Ca2+ by glucose is not directly mediated by ATP production as shown by the continuous monitoring of cytosolic ATP in luciferase expressing cells. Both glucose and K+ increase [Ca2+]er, but only the former generated whereas the latter consumed ATP. Nonetheless, drastic lowering of cellular ATP with a mitochondrial uncoupler resulted in a marked decrease in [Ca2+]er, emphasizing the requirement for mitochondrially derived ATP above a critical threshold concentration. Using alpha-toxin permeabilized cells in the presence of ATP, glucose 6-phosphate did not change [Ca2+]er, invalidating the hypothesis that glucose acts through this metabolite. Therefore, insulin secretagogues that primarily stimulate Ca2+ influx, elevate [Ca2+]er to ensure beta-cell homeostasis.

Identification of a glucose response element in the promoter of the rat glucagon receptor gene

We cloned the 5' upstream region of the rat glucagon receptor gene, demonstrating that the 5' noncoding domain of the glucagon receptor mRNA contained two untranslated exons of 131 and 166 nucleotides (nt), respectively, separated by two introns of 0.6 and 3.2 kilobase pairs. We also observed an alternative splicing involving the 166-base pair exon. Cloning of up to 2 kilobase pairs of the newly identified genomic domain and transfection of various constructs driving a reporter gene, in pancreatic islet cell line INS-1, uncovered a strong glucose regulation of the promoter activity of plasmids containing up to nucleotide -868, or more, upstream from the transcriptional start point. This promoter activity displayed threshold-like behavior, with low activity of the promoter below 5 mM glucose, and maximal activation as of 10 mM glucose. This glucose regulation was mapped to a highly palindromic 19-nucleotide region between nt -545 and -527. Indeed, deletion or mutation of this sequence abolished the glucose regulation. This domain contained two palindromic "E-boxes" CACGTG and CAGCTG separated by 3 nt, a feature similar to the "L4 box" found in the pyruvate kinase L gene promoter. This is the first description of a G protein-coupled receptor gene promoter regulated by glucose.

Free radical modulation of insulin release in INS-1 cells exposed to alloxan

Generation of free radicals is thought to mediate the cytotoxic action of alloxan on the pancreatic beta-cell. In this investigation, the early effects of alloxan on cell function were studied. When INS-1D insulinoma cells were exposed to alloxan (1 mM) for 45 min followed by a 3-hr recovery period, the drug increased basal insulin release while abolishing the effect of glucose in static incubations. This was associated with impaired stimulation of cellular metabolism by glucose and reduced viability, both monitored colorimetrically with 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT). These alterations were largely counteracted by the antioxidant butylated hydroxyanisol (BHA). Similar changes occurred when glucose was added directly after 5 min of alloxan treatment, whereas KCl-induced secretion was only partially inhibited. In perifusion, alloxan caused transient insulin secretion to 50% of the rates obtained with glucose 30 min later. Under these conditions, epinephrine abolished the stimulation due to both agents. Membrane potential and cytosolic calcium concentrations ([Ca2+]i) were recorded to clarify the action of alloxan. Alloxan-induced insulin release correlated with depolarization of INS-1D cells and a rise in [Ca2+]i. Alloxan did not augment [Ca2+]i in the presence of BHA or the absence of extracellular calcium. Nickel chloride blocked the effect of alloxan on [Ca2+]i, whereas verapamil was ineffective. This suggests that alloxan promotes Ca2+ influx through channels distinct from L-type channels, perhaps through non-selective cation channels. Thus, alloxan causes changes in INS-1D cells prevented by antioxidant treatment, suggesting that free radicals may modulate the ionic permeability leading to functional activation.

Caffeine releases a glucose-primed endoplasmic reticulum Ca2+ pool in the insulin secreting cell line INS-1

Caffeine mobilized an intracellular Ca2+ pool in intact fura-2-loaded INS-1 cells in suspension exposed to high (16 mM) [glucose], while a minor effect was observed with low (2 mM) [glucose]. Cells were kept in a medium containing diaxozide or no Ca2+ to prevent the influx of extracellular Ca2+. The caffeine-sensitive intracellular Ca2+ pool was within the endoplasmic reticulum since it was depleted by the inhibitor of the reticular Ca2+ pumps thapsigargin and the InsP3-dependent agonist carbachol. No effect of caffeine was observed in the parent glucose-insensitive RINmF5 cells. In microsomes from INS-1 but not RINmF5 cells, the type 2 ryanodine receptor was present as revealed by Western blotting. It was concluded that the endoplasmic reticulum of INS-1 cells possesses caffeine-sensitive type 2 ryanodine receptors Ca2+ channels.

Subcellular distribution and function of Rab3A, B, C, and D isoforms in insulin-secreting cells

Insulin-secreting cells express four GTPases of the Rab3 family. After separation of extracts of INS-1 cells on a sucrose density gradient, the bulk of the A, B, and C isoforms was recovered in the fractions enriched in insulin-containing secretory granules. Rab3D was also mainly associated with secretory granules, but a fraction of this isoform was localized on lighter organelles. Analyses by confocal microscopy of immunostained HIT-T15 cells transfected with epitope-tagged constructs confirmed the distribution of the Rab3 isoforms. Transfection of HIT-T15 cells with GTPase-deficient mutants of the Rab3 isoforms decreased nutrient-induced insulin release to different degrees (D>B>A>>C), while overexpression of Rab3 wild types had minor or no effects. Expression of the same Rab3 mutants in PC12 cells provoked an inhibition of K+-stimulated secretion of dense core vesicles, indicating that, in beta-cells and neuroendocrine cells, the four Rab3 isoforms play a similar role in exocytosis. A Rab3A/C chimera in which the carboxyterminal domain of A was replaced with the corresponding region of C inhibited insulin secretion as Rab3A. In contrast, a Rab3C/A chimera containing the amino-terminal domain of C was less potent and reduced exocytosis as Rab3C. This suggests that the degree of inhibition obtained after transfection of the Rab3 isoforms is determined by differences in the variable amino-terminal region.

Characterization of beta cells developed in vitro from rat embryonic pancreatic epithelium

The present study evaluates the development and functional properties of beta cells differentiated in vitro. The authors have previously demonstrated that when E12.5 rat pancreatic rudiments are cultured in vitro in the absence of mesenchyme, the majority of the epithelial cells differentiate into endocrine beta cells. Thus, depletion of the mesenchyme provokes the expansion of endocrine tissue at the expense of exocrine tissue. The potential use of this procedure for the production of beta cells led the authors to characterize the beta cells differentiated in this model and to compare their properties with those of the endocrine cells of the embryonic and adult pancreas. This study shows that the beta cells that differentiate in vitro in the absence of mesenchyme express the homeodomain protein Nkx6.1, a transcription factor that is characteristic of adult mature beta cells. Further, electron microscopy analysis shows that these beta cells are highly granulated, and the ultrastructural analysis of the granules shows that they are characteristic of mature beta cells. The maturity of these granules was confirmed by a double-immunofluorescence study that demonstrated that Rab3A and SNAP-25, two proteins associated with the secretory pathway of insulin, are strongly expressed. Finally, the maturity of the differentiated beta cells in this model was confirmed when the cells responded to stimulation with 16 mM glucose by a 5-fold increase in insulin release. The authors conclude that the beta cells differentiated in vitro from rat embryonic pancreatic rudiments devoid of mesenchyme are mature beta cells.

Expression of adenylyl cyclase subtypes in pancreatic beta-cells

Activation of adenylyl cyclase by Gs-coupled receptors for insulinotropic hormones such as glucagon-like peptide-1 and pituitary adenylate cyclase-activating polypeptide plays a critical role in stimulating glucose-induced insulin secretion. Despite this important role of insulinotropic hormones in the regulation of insulin secretion, little is known about which of the multiple subtypes of adenylyl cyclase are expressed in beta-cells. Here we report the use of PCR primers designed to amplify all subtypes of adenylyl cyclase from cDNA prepared from human and rat islets and from insulin-secreting beta-cell lines. PCR products were cloned and sequenced to identify the subtypes of adenylyl cyclase amplified. Adenylyl cyclase types V and VI, known to couple to Galphas and Gbetagamma in the cAMP signaling pathway, account for all subtypes identified in human islets and INS-1 cells and the majority of subtypes in rat islets and HIT-T15 cells. These findings indicate that pancreatic beta-cells are particularly well suited to transmit signals via Gs-coupled receptors such as that for glucagon-like peptide-1.

Glucose induces glucose 6-phosphatase hydrolytic subunit gene transcription in an insulinoma cell line (INS-1)

Primer extension analysis and RNase protection assays revealed the identity of glucose 6-phosphatase gene transcripts in both the insulinoma cell line INS-1 and hepatic cells. In transient transfection assays of INS-1 cells, using constructs between the human glucose 6-phosphatase gene promoter and a luciferase reporter gene, the reporter gene activity was induced by dexamethasone and dibutyryl cAMP. Furthermore, the promoter was regulated by the glucose concentration in the medium. This effect was dependent on glucose metabolism. The data indicated that glucose 6-phosphatase gene transcription is regulated in a similar way in the insulinoma cell line and in liver.

Leptin inhibits insulin gene transcription and reverses hyperinsulinemia in leptin-deficient ob/ob mice

Leptin controls feeding behavior and insulin secretion from pancreatic beta-cells. Insulin stimulates the production of leptin, thereby establishing an adipoinsular axis. Earlier we identified leptin receptors on pancreatic beta-cells and showed leptin-mediated inhibition of insulin secretion by activation of ATP-sensitive potassium channels. Here we examine transcriptional effects of leptin on the promoter of the rat insulin I gene in rodent beta-cells. A fall in levels of preproinsulin mRNA is detected in vivo in islets of ob/ob mice 24 h after a single injection of leptin, in isolated ob/ob islets treated with leptin in vitro and in the beta-cell line INS-1 on leptin exposure when preproinsulin mRNA expression is stimulated by 25 mM glucose or 10 nM glucagon-like peptide 1. Under these conditions, transcriptional activity of -410 bp of the rat insulin I promoter is inhibited by leptin, whereas transactivation of a 5'-deleted promoter (-307 bp) is not. The -307 sequence contains the known glucose-responsive control elements (E2:A3/4). Constitutive activation of ATP-sensitive potassium channels by diazoxide does not alter leptin inhibition of preproinsulin mRNA levels. Distinct protein-DNA complexes appear on the rat insulin I promoter sequences located between -307 and -410 with nuclear extracts from ob/ob islets in response to leptin, including a signal transducer and activator of transcription (STAT)5b binding site. These results indicate that leptin inhibits transcription of the preproinsulin gene by altering transcription factor binding to sequences upstream from the elements (307 bp) that confer glucose responsivity to the rat insulin I gene promoter. Thus leptin exerts inhibitory effects on both insulin secretion and insulin gene expression in pancreatic beta-cells, but by different cellular mechanisms.

Dominant-negative suppression of HNF-1alpha function results in defective insulin gene transcription and impaired metabolism-secretion coupling in a pancreatic beta-cell line

Mutations in the hepatocyte nuclear factor-1alpha (HNF-1alpha) have been linked to subtype 3 of maturity-onset diabetes of the young (MODY3), which is characterized by a primary defect in insulin secretion. The role of HNF-1alpha in the regulation of pancreatic beta-cell function was investigated. Gene manipulation allowed graded overexpression of HNF-1alpha and controlled dominant-negative suppression of HNF-1alpha function in insulinoma INS-1 cells. We show that HNF-1alpha is essential for insulin gene transcription, as demonstrated by a pronounced decrease in insulin mRNA expression and in insulin promoter activity under dominant-negative conditions. The expression of genes involved in glucose transport and metabolism including glucose transporter-2 and L-type pyruvate kinase is also regulated by HNF-1alpha. Loss of HNF-1alpha function leads to severe defects in insulin secretory responses to glucose and leucine, resulting from impaired glucose utilization and mitochondrial oxidation. The nutrient-evoked ATP production and subsequent changes in plasma membrane potential and intracellular Ca2+ were diminished by suppression of HNF-1alpha function. These results suggest that HNF-1alpha function is essential for maintaining insulin storage and nutrient-evoked release. The defective mitochondrial oxidation of metabolic substrates causes impaired insulin secretion, indicating a molecular basis for the diabetic phenotype of MODY3 patients.

Development of rat pancreatic islet cells in vitro

Islet cell lines were produced by retroviral transduction of SV-40 T antigen to monolayer cultures of neonatal rat islets. One line, RN0-11, showed evolution of phenotypes in vitro. It evolved from a non-hormone secreting cell line to an insulin secreting line. It further developed glucagon producing capability before it lost all hormone producing phenotypes. At passage 8, RN0-11 cells secreted a small amount of insulin 25 ng/10(6) cells/24 hours. They were unresponsive to glucose and secreted 1.7-1.9 ng of insulin/10(6) cells/2 hours under various levels of glucose. At passage 16, they secreted 2,577 ng of insulin/10(6) cells/24 hours and responded to glucose stimulation in static incubation. The insulin secreted by these cells at 0, 2.8, 5.5, 11.1, 16.7, and 27.7 mM of glucose was 6.46 +/- 2.56, 17.74 +/- 2.66, 32.24 +/- 0.58, 30.66 +/- 1.59, 33.55 +/- 4.83, and 20.95 +/- 2.17 ng/10(6) cells/2 hours respectively. The responsiveness to glucose and the ability to secrete insulin diminished as cells were passaged in culture, and by passage 35 no insulin was detectable in medium under any level of glucose tested. Northern blot analyses also showed corresponding changes of insulin expression in these cells at different passages. In addition, glucagon was detectable at passage 14 by immunocytochemistry and at passage 16 by Northern blot analysis. By passage 35, no insulin or glucagon expression was detectable by Northern blot analysis or immunocytochemistry. Immunocytochemical staining of these cells at passage 14 showed insulin-positive and glucagon-positive cells and cells positive for both insulin and glucagon. Presence of insulin and glucagon in the same cells suggests single clonality of the cell line. The evolution of RN0-11 cells in vitro provides an opportunity to study the development of islet cells.

AMP-activated protein kinase is activated by low glucose in cell lines derived from pancreatic beta cells, and may regulate insulin release

The role of the AMP-activated protein kinase (AMPK) cascade in the glucose-sensitive pancreatic beta cell lines HIT-T15 and INS-1 was addressed. In both cell types, removal of glucose leads to a >5-fold activation of AMPK activity. Activation of AMPK was due to phosphorylation, since the effect was reversed by protein phosphatase treatment of the extracts, and was restored by re-addition of MgATP and the purified upstream kinase. When the effects of different concentrations of medium glucose were examined, insulin secretion and AMPK activity were inversely related, and varied over the same concentration range. The activation in response to glucose removal appeared to be due to changes in the concentration of the known regulators of the cascade, i.e. AMP and ATP, since AMPK activation was associated with a large increase in the cellular AMP/ATP ratio, and the two parameters varied over the same range of glucose concentrations. In late-passage HIT-T15 cells that had lost the glucose-dependent insulin secretion response, both AMPK activity and the AMP/ATP ratio also became insensitive to the extracellular glucose concentration. Treatment of INS-1 cells, but not HIT-T15 cells, with AICA riboside (5-aminoimidazole-4-carboxamide riboside) results in accumulation of the ribotide, ZMP (AICA riboside monophosphate), and activation of AMPK. AICA riboside treatment of INS-1 cells, and of isolated rat islets, had both inhibitory and stimulatory effects on insulin secretion. These results show that in beta cell lines the AMP-activated protein kinase, like its yeast homologue the SNF1 complex, can respond to the level of glucose in the medium, and may be involved in regulating insulin release.

Cysteine-string proteins regulate exocytosis of insulin independent from transmembrane ion fluxes

Cysteine-string proteins (Csps) are vesicle proteins involved in exocytosis of synaptic vesicles in Drosophila and modulation of presynaptic calcium influx. As both the contribution of calcium channel regulation to the role of Csp in exocytosis and a function of Csp outside the nervous system are unknown, we studied its function in endocrine exocytosis from large dense core vesicles (LDCVs) using insulin-secreting pancreatic beta-cells. Csps were expressed in primary and derived beta-cell lines on insulin-containing LDCVs. Suppression of Csp expression reduced not only depolarisation induced insulin release but also exocytosis in permeabilised cells directly stimulated by Ca2+. Thus, Csp is a secretory granule protein and is required for endocrine exocytosis independent of the modulation of transmembrane calcium fluxes.

Prolonged depletion of guanosine triphosphate induces death of insulin-secreting cells by apoptosis

Inhibitors of IMP dehydrogenase, such as mycophenolic acid (MPA) and mizoribine, which deplete cellular GTP, are used clinically as immunosuppressive drugs. The prolonged effect of such agents on insulin-secreting beta-cells (HIT-T15 and INS-1) was investigated. Both MPA and mizoribine inhibited mitogenesis, as reflected by [3H]thymidine incorporation. Cell number, DNA and protein contents, and cell (metabolic) viability were decreased by about 30%, 60%, and 80% after treatment of HIT cells with clinically relevant concentrations (e.g. 1 microg/ml) of MPA for 1, 2, and 4 days, respectively. Mizoribine (48 h) similarly induced the death of HIT cells. INS-1 cells also were damaged by prolonged MPA treatment. MPA-treated HIT cells displayed a strong and localized staining with a DNA-binding dye (propidium iodide), suggesting condensation and fragmentation of DNA, which were confirmed by detection of DNA laddering in multiples of about 180 bp. DNA fragmentation was observed after 24-h MPA treatment and was dose dependent (29%, 49%, and 70% of cells were affected after 48-h exposure to 1, 3, and 10 microg/ml MPA, respectively). Examination of MPA-treated cells by electron microscopy revealed typical signs of apoptosis: condensed and marginated chromatin, apoptotic bodies, cytosolic vacuolization, and loss of microvilli. MPA-induced cell death was almost totally prevented by supplementation with guanosine, but not with adenosine or deoxyguanosine, indicating a specific effect of GTP depletion. An inhibitor of protein isoprenylation (lovastatin, 10-100 microM for 2-3 days) induced cell death and DNA degradation similar to those induced by sustained GTP depletion, suggesting a mediatory role of posttranslationally modified GTP-binding proteins. Indeed, impeding the function of G proteins of the Rho family (via glucosylation using Clostridium difficile toxin B), although not itself inducing apoptosis, potentiated cell death induced by MPA or lovastatin. These findings indicate that prolonged depletion of GTP induces beta-cell death compatible with apoptosis; this probably involves a direct impairment of GTP-dependent RNA-primed DNA synthesis, but also appears to be modulated by small GTP-binding proteins. Treatment of intact adult rat islets (the beta-cells of which replicate slowly) induced a modest, but definite, death by apoptosis over 1- to 3-day periods. Thus, more prolonged use of the new generation of immunosuppressive agents exemplified by MPA might have deleterious effects on the survival of islet or pancreas grafts.

An endocrine-exocrine switch in the activity of the pancreatic homeodomain protein PDX1 through formation of a trimeric complex with PBX1b and MRG1 (MEIS2)

HOX proteins and some orphan homeodomain proteins form complexes with either PBX or MEIS subclasses of homeodomain proteins. This interaction can increase the binding specificity and transcriptional effectiveness of the HOX partner. Here we show that specific members of both PBX and MEIS subclasses form a multimeric complex with the pancreatic homeodomain protein PDX1 and switch the nature of its transcriptional activity. The two activities of PDX1 are exhibited through the 10-bp B element of the transcriptional enhancer of the pancreatic elastase I gene (ELA1). In pancreatic acinar cells the activity of the B element requires other elements of the ELA1 enhancer; in beta-cells the B element can activate a promoter in the absence of other enhancer elements. In acinar cell lines the activity is mediated by a complex comprising PDX1, PBX1b, and MRG1 (MEIS2). In contrast, beta-cell lines are devoid of PBX1b and MRG1, so that a trimeric complex does not form, and the beta-cell-type activity is mediated by PDX1 without PBX1b and MRG1. The presence of specific nuclear isoforms of PBX and MEIS is precisely regulated in a cell-type-specific manner. The beta-cell-type activity can be detected in acinar cells if the B element is altered to retain binding of PDX1 but prevent binding of the PDX1-PBX1b-MRG1 complex. These observations suggest that association with PBX and MEIS partners controls the nature of the transcriptional activity of the organ-specific PDX1 transcription factor in exocrine versus endocrine cells.

Desensitization of mitochondrial Ca2+ and insulin secretion responses in the beta cell

The role of mitochondria in the desensitization of insulin secretion was investigated. In rat pancreatic beta cells, both insulin secretion and mitochondrial [Ca2+] increases were desensitized following two challenges with the mitochondrial substrate methyl succinate. In the beta cell line INS-1, similar results were observed when a 5-min interval separated two 5-min pulses. In contrast, ATP generation monitored in luciferase-expressing INS-1 cells was stimulated to the same extent during both exposures to methyl succinate. Succinate, like alpha-glycerophosphate, activates the electron transport chain at complex II. As a consequence, the mitochondrial membrane hyperpolarizes, promoting ATP synthesis and Ca2+ influx into the mitochondria through the uniporter. The mitochondrial desensitization was further studied in permeabilized INS-1 cells. Increasing extramitochondrial [Ca2+] from 100 to 500 nM enhanced succinate oxidation 4-fold. At 500 nM Ca2+, 1 mM succinate caused a blunted mitochondrial [Ca2+] increase upon the second, compared with the first, stimulation. These effects were mimicked by alpha-glycerophosphate, and there was cross-desensitization between the two compounds. Succinate hyperpolarized the mitochondrial membrane during both the first and second applications. This suggests that the uniporter itself, rather than the respiratory chain, is desensitized. These results emphasize the key role of the mitochondria not only in the stimulation of insulin secretion, but also in its desensitization.

Ca2+ depletion from granules inhibits exocytosis. A study with insulin-secreting cells

The secretory compartment is characterized by low luminal pH and high Ca2+ content. Previous studies in several cell types have shown that the size of the acidic Ca2+ pool, of which secretory granules represent a major portion, could be estimated by applying first a Ca2+ ionophore followed by agents that collapse acidic pH gradients. In the present study we have employed this protocol in the insulin-secreting cell line Ins-1 to determine whether the Ca2+ trapped in the secretory granules plays a role in exocytosis. The results demonstrate that a high proportion of ionophore-mobilizable Ca2+ in Ins-1 cells resides in the acidic compartment. The latter pool, however, does not significantly contribute to the [Ca2+]i changes elicited by thapsigargin and the inositol trisphosphate-producing agonist carbachol. By monitoring membrane capacitance at the single cell level or by measuring insulin release in cell populations, we show that Ca2+ mobilization from nonacidic Ca2+ pools causes a profound and long lasting increase in depolarization-induced secretion, whereas breakdown of granule pH had no significant effect. In contrast, releasing Ca2+ from the acidic pool markedly reduces secretion. It is suggested that a high Ca2+ concentration in the secretory compartment is needed to sustain optimal exocytosis.

Insulin-like growth factor I (IGF-I)-stimulated pancreatic beta-cell growth is glucose-dependent. Synergistic activation of insulin receptor substrate-mediated signal transduction pathways by glucose and IGF-I in INS-1 cells

Nutrients and certain growth factors stimulate pancreatic beta-cell mitogenesis, however, the appropriate mitogenic signal transduction pathways have not been defined. In the glucose-sensitive pancreatic beta-cell line, INS-1, it was found that glucose (6-18 mM) independently increased INS-1 cell proliferation (>20-fold at 15 mM glucose). Insulin-like growth factor I (IGF-I)-induced INS-1 cell proliferation was glucose-dependent only in the physiologically relevant concentration range (6-18 mM glucose). The combination of IGF-I and glucose was synergistic, increasing INS-1 cell proliferation >50-fold at 15 mM glucose + 10 nM IGF-I. Glucose metabolism and phosphatidylinositol 3'-kinase (PI 3'-kinase) activation were necessary for both glucose and IGF-I-stimulated INS-1 cell proliferation. IGF-I and 15 mM glucose increased tyrosine phosphorylation mediated recruitment of Grb2/mSOS and PI 3'-kinase to IRS-2 and pp60. Glucose and IGF-I also induced Shc association with Grb2/mSOS. Glucose (3-18 mM) and IGF-I, independently of glucose, activated mitogen-activated protein kinase but this did not correlate with IGF-I-induced beta-cell proliferation. In contrast, p70(S6K) was activated with increasing glucose concentration (between 6 and 18 mM), and potentiated by IGF-I in the same glucose concentration range which correlated with INS-1 cell proliferation rate. Thus, glucose and IGF-I-induced beta-cell proliferation were mediated via a signaling mechanism that was facilitated by mitogen-activated protein kinase but dependent on IRS-mediated induction of PI 3'-kinase activity and downstream activation of p70(S6K). The glucose dependence of IGF-I mediated INS-1 cell proliferation emphasizes beta-cell signaling mechanisms are rather unique in being tightly linked to glycolytic metabolic flux.

Secretory-granule dynamics visualized in vivo with a phogrin-green fluorescent protein chimaera

To image the behaviour in real time of single secretory granules in neuroendocrine cells we have expressed cDNA encoding a fusion construct between the dense-core secretory-granule-membrane glycoprotein, phogrin (phosphatase on the granule of insulinoma cells), and enhanced green fluorescent protein (EGFP). Expressed in INS-1 beta-cells and pheochromocytoma PC12 cells, the chimaera was localized efficiently (up to 95%) to dense-core secretory granules (diameter 200-1000 nm), identified by co-immunolocalization with anti-(pro-)insulin antibodies in INS-1 cells and dopamine beta-hydroxylase in PC12 cells. Using laser-scanning confocal microscopy and digital image analysis, we have used this chimaera to monitor the effects of secretagogues on the dynamics of secretory granules in single living cells. In unstimulated INS-1 beta-cells, granule movement was confined to oscillatory movement (dithering) with period of oscillation 5-10 s and mean displacement <1 microm. Both elevated glucose concentrations (30 mM), and depolarization of the plasma membrane with K+, provoked large (5-10 microm) saltatory excursions of granules across the cell, which were never observed in cells maintained at low glucose concentration. By contrast, long excursions of granules occurred in PC12 cells without stimulation, and occurred predominantly from the cell body towards the cell periphery and neurite extensions. Purinergic-receptor activation with ATP provoked granule movement towards the membrane of PC12 cells, resulting in the transfer of fluorescence to the plasma membrane consistent with fusion of the granule and diffusion of the chimaera in the plasma membrane. These results illustrate the potential use of phogrin-EGFP chimeras in the study of secretory-granule dynamics, the regulation of granule-cytoskeletal interactions and the trafficking of a granule-specific transmembrane protein during the cycle of exocytosis and endocytosis.

Use of a soluble tetrazolium compound to assay metabolic activation of intact beta cells

Although assessments of metabolic activation are central to studies of beta-cell function, available techniques are tedious, insensitive, and/or require cell disruption. We have investigated the use of a new water-soluble tetrazolium salt, MTS (3-[4,5,dimethylthiazol-2-yl]-5-[3-carboxymethoxy-phenyl]-2-[4- sulfophenyl]-2H-tetrazolium, inner salt), in the presence of phenazine methosulfate (PMS), an intermediate electron acceptor that amplifies its signal (fluorescence at 490 nm). During static incubations of glucose-responsive (HIT-T15 or INS-1) dispersed beta cells with increasing glucose concentrations, there was a progressive increase in MTS reduction, with a maximum signal-to-noise (S/N) ratio of 24 with HIT-T15 cells and 10 with INS-1 cells. This was associated with, but not attributable to, parallel increases in insulin secretion. Pure mitochondrial fuels (alpha-ketoisocaproate [KIC], methyl pyruvate [MP], or L-glutamine [GLN] + L-leucine [LEU]) also increased the reduction of MTS in INS-1 cells (6.5-, 4.8-, and 14.4-fold, respectively), but generally less than glucose, suggesting a major role of glycolysis in the signal induced by glucose. Inhibitors of glucose metabolism (mannoheptulose [MH], lodoacetate [IA], or 2-deoxyglucose [2-DG]) markedly reduced the glucose-stimulated MTS signal. In comparison to another tetrazolium compound, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT), MTS assay provided a better S/N ratio with glucose or other nutrient secretagogues. Extant theory holds that activation of mitochondrial dehydrogenases by increments in Ca2+ influx couples glycolysis to mitochondrial oxidation of glucose-derived fuels. However, reduction of fuel-induced calcium influx (by Ca2+-free medium or diazoxide [DZX]) or direct stimulation of calcium influx (by 40 mmol/L K+) failed to significantly modulate the signal, arguing against this theory. We conclude that the MTS assay is a facile test that reflects the global metabolic function of insulin-secreting beta cells. Furthermore, since this assay does not require disruption of cells to solubilize the formazan product, and therefore also allows concomitant measurement of insulin secretion, it offers considerable advantages over earlier methods.