Characterization of a novel glucose-responsive insulin-secreting cell line, BRIN-BD11, produced by electrofusion

A novel insulin-secreting cell line (BRIN-BD11) was established after electrofusion of RINm5F cells with New England Deaconess Hospital rat pancreatic islet cells. Wells of cell fusion mixture with insulin output 5-10 times greater than parent RINm5F cells were subcultured with eventual establishment of clones, including BRIN-BD11. Morphological studies established that these cells grow as monolayers with epithelioid characteristics, maintaining stability in tissue culture for > 50 passages. Culture of these cells for 24 h at 5.6-33.3 mmol/l glucose revealed a 1.8- to 2.0-fold increase of insulin output compared with 1.4 mmol/l glucose. Dynamic insulin release was recorded in response to 16.7 mmol/l glucose, resulting in a rapid threefold insulin secretory peak followed by a sustained output slightly above basal. In acute 20-min tests, 4.2-16.7 mmol/l glucose evoked a stepwise two- to three-fold stimulation of insulin release. 3-Isobutyl-1-methylxanthine (1 mmol/l) served to increase basal and glucose-stimulated insulin release, shifting the threshold from 4.4 to 1.1 mmol/l glucose. Stimulation of insulin secretion with 16.7 mmol/l glucose was abolished by mannoheptulose or diazoxide (15 or 0.5 mmol/l). In contrast, glyceraldehyde (10 mmol/l) and 25 mmol/l K+ evoked 1.7- to 9.0-fold insulin responses. L-Alanine (10 mmol/l) evoked a twofold secretory response, which was potentiated 1.4-fold by increasing the Ca2+ concentration from 1.28 to 7.68 mmol/l. Forskolin (25 mumol/l) and phorbol 12-myristate 13-acetate (10 nmol/l) both increased insulin secretion in the presence of L-alanine (1.4- and 1.8-fold, respectively). Western blotting confirmed that BRIN-BD11 cells expressed the GLUT2 glucose transporter. This, coupled with a high glucokinase/hexokinase ratio in the cells, confirms an intact glucose sensing mechanism. High-performance liquid chromatography analysis demonstrated that insulin was the major product secreted under stimulatory conditions. Collectively, these data indicate that the BRIN-BD11 cell line represents an important stable glucose-responsive insulin-secreting beta-cell line for future studies.

Development and functional characterization of insulin-releasing human pancreatic beta cell lines produced by electrofusion

Three novel human insulin-releasing cell lines designated 1.1B4, 1.4E7, and 1.1E7 were generated by electrofusion of freshly isolated of human pancreatic beta cells and the immortal human PANC-1 epithelial cell line. Functional studies demonstrated glucose sensitivity and responsiveness to known modulators of insulin secretion. Western blot, RT-PCR, and immunohistochemistry showed expression of the major genes involved in proinsulin processing and the pancreatic beta cell stimulus-secretion pathway including PC1/3, PC2, GLUT-1, glucokinase, and K-ATP channel complex (Sur1 and Kir6.2) and the voltage-dependent L-type Ca(2+) channel. The cells stained positively for insulin, and 1.1B4 cells were used to demonstrate specific staining for insulin, C-peptide, and proinsulin together with insulin secretory granules by electron microscopy. Analysis of metabolic function indicated intact mechanisms for glucose uptake, oxidation/utilization, and phosphorylation by glucokinase. Glucose, alanine, and depolarizing concentrations of K(+) were all able to increase [Ca(2+)](i) in at least two of the cell lines tested. Insulin secretion was also modulated by other nutrients, hormones, and drugs acting as stimulators or inhibitors in normal beta cells. Subscapular implantation of the 1.1B4 cell line improved hyperglycemia and resulted in glucose lowering in streptozotocin-diabetic SCID mice. These novel human electrofusion-derived beta cell lines therefore exhibit stable characteristics reminiscent of normal pancreatic beta cells, thereby providing an unlimited source of human insulin-producing cells for basic biochemical studies and pharmacological drug testing plus proof of concept for cellular insulin replacement therapy.

A nuclear magnetic resonance-based demonstration of substantial oxidative L-alanine metabolism and L-alanine-enhanced glucose metabolism in a clonal pancreatic beta-cell line: metabolism of L-alanine is important to the regulation of insulin secretion

Early experiments indicated that islet beta-cells substantially metabolized L-alanine but that insulin secretion was largely unaffected by the amino acid. It was subsequently demonstrated using more intricate studies that L-alanine is a strong stimulus to insulin secretion in the presence of glucose in normal rodent islets and beta-cell lines. Using (13)C nuclear magnetic resonance (NMR), we have demonstrated substantial oxidative metabolism of L-alanine by the clonal beta-cell line BRIN-BD11, with time-dependent increases in production of cellular glutamate and aspartate. Stimulatory effects of L-alanine on insulin secretion were attenuated by the inhibition of beta-cell oxidative phosphorylation using oligomycin. Additionally, we detected substantial production of lactate, alanine, and glutamate from glucose (16.7 mmol/l) after 60 min. On addition of 10 mmol/l L-alanine to a stimulus of 16.7 mmol/l glucose, the utilization rate of glucose increased approximately 2.4-fold. L-Alanine dramatically enhanced NMR-measurable aspects of glucose metabolism (both oxidative and nonoxidative). The enhanced rate of entry of glucose-derived pyruvate into the tricarboxylic acid (TCA) cycle in the presence of alanine may have stimulated rates of generation of key metabolites, including ATP, which affect the insulin secretory process. Thus L-alanine metabolism, in addition to the enhancing effect on glucose metabolism, contributes to the stimulatory effects of this amino acid on insulin secretion in vitro.

Mechanisms of amino acid-induced insulin secretion from the glucose-responsive BRIN-BD11 pancreatic B-cell line

The effects of different classes of amino acids known to be transported and utilized by pancreatic B-cells were examined using the novel glucose-responsive pancreatic B-cell line, BRIN-BD11. Amino acids tested included alpha-aminoisobutyric acid, L-alanine, L-arginine, L-glutamine, glycine, L-leucine, L-lysine, L-proline and L-serine. At non-stimulatory (1.1 mmol/l) glucose, acute incubations with either 1 or 10 mmol/l amino acid evoked 1.3- to 4.7-fold increases of insulin release. Raising glucose to 16.7 mmol/l enhanced the effects of all amino acids except L-glutamine, and increased insulin output at 10 mmol/l compared with 1 mmol/l amino acid. Glyceraldehyde (10 mmol/l) also served to promote 10 mmol/l amino acid-induced insulin secretion with the exceptions of L-arginine, glycine, L-lysine and L-proline. At 16.7 mmol/l glucose, diazoxide (300 mumol/l) significantly decreased the secretory response to all amino acids except L-glutamine. Likewise, verapamil (20 mumol/l) or depletion of extracellular Ca2+ reduced insulin output indicating the importance of Ca2+ influx in the actions of amino acids. These data indicate that BRIN-BD11 cells transport and utilize amino acids, acting in association with glycolysis, K(+)-ATP channels and/or voltage-dependent Ca2+ channels to promote Ca2+ influx and insulin secretion. The response of BRIN-BD11 cells to glucose and amino acids indicates that this is a useful cell line for future research on the mechanisms of nutrient regulation of insulin secretion.

L-arginine is essential for pancreatic β-cell functional integrity, metabolism and defense from inflammatory challenge

In this work, our aim was to determine whether L-arginine (a known insulinotropic amino acid) can promote a shift of β-cell intermediary metabolism favoring glutathione (GSH) and glutathione disulfide (GSSG) antioxidant responses, stimulus-secretion coupling and functional integrity. Clonal BRIN-BD11 β-cells and mouse islets were cultured for 24 h at various L-arginine concentrations (0-1.15  mmol/l) in the absence or presence of a proinflammatory cytokine cocktail (interleukin 1β, tumour necrosis factor α and interferon γ). Cells were assessed for viability, insulin secretion, GSH, GSSG, glutamate, nitric oxide (NO), superoxide, urea, lactate and for the consumption of glucose and glutamine. Protein levels of NO synthase-2, AMP-activated protein kinase (AMPK) and the heat shock protein 72 (HSP72) were also evaluated. We found that L-arginine at 1.15  mmol/l attenuated the loss of β-cell viability observed in the presence of proinflammatory cytokines. L-arginine increased total cellular GSH and glutamate levels but reduced the GSSG/GSH ratio and glutamate release. The amino acid stimulated glucose consumption in the presence of cytokines while also stimulating AMPK phosphorylation and HSP72 expression. Proinflammatory cytokines reduced, by at least 50%, chronic (24 h) insulin secretion, an effect partially attenuated by L-arginine. Acute insulin secretion was robustly stimulated by L-arginine but this effect was abolished in the presence of cytokines. We conclude that L-arginine can stimulate β-cell insulin secretion, antioxidant and protective responses, enabling increased functional integrity of β-cells and islets in the presence of proinflammatory cytokines. Glucose consumption and intermediary metabolism were increased by L-arginine. These results highlight the importance of L-arginine availability for β-cells during inflammatory challenge.

A comparative study of amino acid consumption by rat islet cells and the clonal beta-cell line BRIN-BD11 - the functional significance of L-alanine

Evidence has been published that L -alanine may, under appropriate conditions, promote insulin secretion in normal rodent islets and various beta cell lines. Previous results utilising the clonal beta-cell line BRIN-BD11, demonstrated that alanine dramatically elevated insulin release by a mechanism requiring oxidative metabolism. We demonstrate in this paper that addition ofL -alanine had an insulinotropic effect in dispersed primary islet cells. Addition of D -glucose increasedL -alanine consumption in both BRIN-BD11 cells and primary islet cells.L -glutamine consumption in the BRIN-BD11 cell line and primary rat islets was also determined. The consumption rate was in line with that previously reported for cells of the immune system and other glutamine-utilising cells or tIssues. However,L -alanine consumption was at least an order of magnitude higher thanL -glutamine consumption. The metabolism ofL -alanine in the beta-cell may result in stimulation of insulin secretion via generation of metabolic stimulus secretion coupling factors such asL -glutamate.

Pro-inflammatory cytokines increase glucose, alanine and triacylglycerol utilization but inhibit insulin secretion in a clonal pancreatic beta-cell line

We have investigated the effects of prolonged exposure (24 h) to pro-inflammatory cytokines on beta-cell metabolism and insulin secretion using clonal BRIN-BD11 beta cells. Addition of IL-1beta, tumour necrosis factor-alpha and IFN-gamma (at concentrations that did not induce apoptosis) inhibited chronic (24 h) and acute stimulated levels of insulin release (by 59 and 93% respectively), increased cellular glucose and alanine consumption, and also elevated lactate and glutamate release. However, ATP levels and cellular triacylglycerol were decreased while glutathione was increased. We conclude that sub-lethal concentrations of pro-inflammatory cytokines appear to shift beta-cell metabolism away from a key role in energy generation and stimulus-secretion coupling and towards a catabolic state which may be related to cell defence.

L-Alanine induces changes in metabolic and signal transduction gene expression in a clonal rat pancreatic β-cell line and protects from pro-inflammatory cytokine-induced apoptosis

Acute effects of nutrient stimuli on pancreatic β-cell function are widely reported; however, the chronic effects of insulinotropic amino acids, such as L-alanine, on pancreatic β-cell function and integrity are unknown. In the present study, the effects of prolonged exposure (24 h) to the amino acid L-alanine on insulin secretory function, gene expression and pro-inflammatory cytokine-induced apoptosis were studied using clonal BRIN-BD11 cells. Expression profiling of BRIN-BD11 cells chronically exposed to L-alanine was performed using oligonucleotide microarray analysis. The effect of alanine, the iNOS (inducible nitric oxide synthase) inhibitor NMA (NG-methyl-L-arginine acetate) or the iNOS and NADPH oxidase inhibitor DPI (diphenylene iodonium) on apoptosis induced by a pro-inflammatory cytokine mix [IL-1β (interleukin-1β), TNF-α (tumour necrosis factor-α) and IFN-γ (interferon-γ)] was additionally assessed by flow cytometry. Culture for 24 h with 10 mM L-alanine resulted in desensitization to the subsequent acute insulin stimulatory effects of L-alanine. This was accompanied by substantial changes in gene expression of BRIN-BD11 cells. Sixty-six genes were up-regulated >1.8-fold, including many involved in cellular signalling, metabolism, gene regulation, protein synthesis, apoptosis and the cellular stress response. Subsequent functional experiments confirmed that L-alanine provided protection of BRIN-BD11 cells from pro-inflammatory cytokine-induced apoptosis. Protection from apoptosis was mimicked by NMA or DPI suggesting L-alanine enhances intracellular antioxidant generation. These observations indicate important long-term effects of L-alanine in regulating gene expression, secretory function and the integrity of insulin-secreting cells. Specific amino acids may therefore play a key role in β-cell function in vivo.

Role of islet structure and cellular interactions in the control of insulin secretion

Close cellular proximity and correct anatomical arrangement within islets are essential for normal patterns of insulin secretion. Thus, segregation of islets into single cells is associated with a dramatic decline in stimulus secretion-coupling and glucose-induced insulin release. Generation of pseudoislets from clonal islet cell lines provides a useful model to examine islet cell interactions and insulin secretion. Such studies have highlighted the functional importance of cell adhesion molecules and connexins. Pseudoislets comprising insulin-secreting cell lines have been shown to closely mimic primary islets in both size and morphology, displaying a significantly enhanced response to glucose, nutrients and drugs over equivalent monolayer cultures. Here, we consider the influence of islet structure and cellular interactions in the control of insulin secretion. The functional characteristics of pseudoislets derived from clonal beta-cell lines or a combination of alpha-, beta- and delta-cell lines are discussed in light of normal islet function and possible therapeutic application.

Engineering cultured insulin-secreting pancreatic B-cell lines

Despite many triumphs, a significant limitation of the usefulness of many of the available B-cell lines for the study of insulin secretion are either inappropriate or lack of responsiveness to glucose. Commonly employed cell lines generated prior to the 1990s following X-ray irradiation (RINm5F cells) or simian virus 40 B-cell transformation (HIT-T15 cells and BTC) fall into this category. More recent success has been achieved with the generation of INS-1 cells and MIN6 cells, but the production of these cell lines owes much to good fortune, dedication and hard work. In the present era, molecular biology techniques provide the opportunity to engineer novel pancreatic B-cell lines which possess many attributes of normal insulin-secreting cells. This review describes the electrofusion of normal NEDH rat pancreatic B-cells with immortal RINm5F cells to create three new glucose-responsive clonal insulin-secreting cells, designated BRIN-BG5, BRIN-BG7 and BRIN-BD11. These cell lines exhibit up to four-fold insulin-secretory responses to depolarization with 25 mmol/l K+, 7.68 mmol/l Ca2+, 10 mmol/l L-alanine, and activation of protein kinase C or adenylate cyclase with 10 nmol/l phorbol- 12-myristate-13-acetate or 25 micromol/l forskolin, respectively. The maximal insulin-secretory response of both BRIN-BG5 and BRIN-BG7 cells to glucose occurred at 8.4 mmol/l (1.9- and 1.8-fold increases, respectively). In contrast, 4.2-16.7 mmol/l glucose evoked a stepwise 2- to 3-fold of insulin release from BRIN-BD11 cells. The superior glucose responsiveness of BRIN-BD11 cells compared with BRIN-BG5 or BRIN-BG7 cells was associated with increased expression of GLUT-2 and a greater contribution of glucokinase to total glucose phosphorylating enzyme activity. Furthermore, BRIN-BD11 cells also showed appropriate responses to a diverse range of modulators of pancreatic B-cell function, including amino acids, neurotransmitters and sulphonylurea drugs. Collectively these observations indicate that genetic modification of insulin-secreting cells by electrofusion (or transfection with cDNA) offers a new avenue for generation of useful clonal glucose-responsive pancreatic B-cell lines for studies of insulin secretion and transplantation in insulin-dependent diabetes mellitus.

Arachidonic acid, palmitic acid and glucose are important for the modulation of clonal pancreatic beta-cell insulin secretion, growth and functional integrity

Insulin-resistant states such as obesity can result in an increase in the function and mass of pancreatic beta-cells, so that insulin secretion is up-regulated and Type II diabetes does not develop. However, expansion of beta-cell mass is not indefinite and may well decrease with time. Changes in circulating concentrations of nutritional factors, such as fatty acids and/or glucose, may lead to a reduction in beta-cell mass in vivo. Few previous studies have attempted to explore the interplay between glucose, amino acids and fatty acids with respect to beta-cell mass and functional integrity. In the present study, we demonstrate that culture of clonal BRIN-BD11 cells for 24 h with the polyunsaturated fatty acid arachidonic acid (AA) increased beta-cell proliferation and enhanced alanine-stimulated insulin secretion. These effects of AA were associated with significant decreases in the cellular consumption of D-glucose and L-alanine as well as decreased rates of production of nitric oxide and ammonia. Conversely 24 h exposure to the saturated fatty acid palmitic acid (PA) was found to decrease beta-cell viability (by increasing apoptosis), increase the intracellular concentration of triacylglycerol (triglyceride), while inhibiting alanine-stimulated insulin secretion. These effects of PA were associated with significant increases in D-glucose and L-glutamine consumption as well as nitric oxide and ammonia production. However, L-alanine consumption was decreased in the presence of PA. The effects of AA, but not PA, were additionally dependent on glucose concentration. These studies indicate that AA may have a critical role in maintaining the appropriate mass and function of islet beta-cells by influencing rates of cell proliferation and insulin secretion. This regulatory effect may be compromised by high circulating levels of glucose and/or PA, both of which are elevated in Type II diabetes and may impact upon dysfunctional and apoptotic intracellular events in the beta-cell.

13C NMR analysis reveals a link between L-glutamine metabolism, D-glucose metabolism and gamma-glutamyl cycle activity in a clonal pancreatic beta-cell line

AIMS/HYPOTHESIS

Pancreatic islet cells and clonal beta-cell lines can metabolise L-glutamine at high rates. The pathway of L-glutamine metabolism has traditionally been described as L-glutamine-->L-glutamate-->2-oxoglutarate-->oxidation in TCA cycle following conversion to pyruvate. Controversially, the metabolism of D-glucose to L-glutamate in beta cells is not widely accepted. However, L-glutamate has been proposed to be a stimulation-secretion coupling factor in glucose-induced insulin secretion. We aimed to investigate the metabolism of glutamine and glucose by using (13)C NMR analysis.

METHODS

BRIN-BD11 cells were incubated in the presence of 16.7 mmol/l [1-(13)C]glucose, 2 mmol/l [2-(13)C]L-glycine or 2 mmol/l [1,2-(13)C]glutamine in the presence or absence of other amino acids or inhibitors. After an incubation period the cellular metabolites were extracted using a PCA extract procedure. After neutralisation, the extracts were prepared for analysis using (13)C-NMR spectroscopy.

RESULTS

Using (13)C NMR analysis we have shown that L-glutamine could be metabolised in BRIN-BD11 cells via reactions constituting part of the gamma-glutamyl cycle producing glutathione. Moderate or high activities of the enzymes required for these pathways of metabolism including glutaminase, gamma-glutamyltransferase and gamma-glutamylcysteine synthetase were observed. We additionally report significant D-glucose metabolism to L-glutamate. Addition of the aminotransferase inhibitor, aminooxyacetate, attenuated L-glutamate production from D-glucose.

CONCLUSION/INTERPRETATION

We propose that L-glutamine metabolism is important in the beta cell for generation of stimulus-secretion coupling factors, precursors of glutathione synthesis and for supplying carbon for oxidation in the TCA cycle. D-glucose, under appropriate conditions, can be converted to L-glutamate via an aminotransferase catalysed step.

Glutamine regulates expression of key transcription factor, signal transduction, metabolic gene, and protein expression in a clonal pancreatic beta-cell line

We have investigated the effects of prolonged exposure (24 h) to the amino acid l-glutamine, on gene and protein expression using clonal BRIN-BD11 beta-cells. Expression profiling of BRIN-BD11 cells was performed using oligonucleotide microarray analysis. Culture for 24 h with 10 mM l-glutamine compared with 1 mM resulted in substantial changes in gene expression with 148 genes upregulated more than 1.8-fold, and 18 downregulated more than 1.8-fold, including many genes involved in cellular signaling, metabolism, gene regulation, and the insulin-secretory response. Subsequent functional experiments confirmed that l-glutamine increased the activity of the Ca(2+) regulated phosphatase calcineurin and the transcription factor Pdx1. Additionally, we demonstrated that beta-cell-derived l-glutamate was released into the extracellular medium at high rates. As calcineurin is a regulator of the glutamate N-methyl-d-aspartate (NMDA) receptor activity, we investigated the action of NMDA on nutrient-induced insulin secretion, and demonstrated suppressed insulin release. These observations indicate important long-term effects of l-glutamine in regulating beta-cell gene expression, signaling, and secretory function.

Glycation of insulin in the islets of Langerhans of normal and diabetic animals

The glycation of immunoreactive insulin (IRI) was assessed in extracts of pancreas and islets from control and hyperglycemic animal models. Glycated and nonglycated IRI were separated by affinity chromatography and quantified by radioimmunoassay. Hydrocortisone-treated Wistar rats (80 mg x kg-1 x day-1 and obese hyperglycemic (ob/ob) mice showed significant increases in plasma glucose (P < 0.001), percentage glycated hemoglobin (P < 0.001), plasma IRI (P < 0.01), and total pancreatic IRI content (P < 0.01), compared with their respective controls. These diabetic groups also demonstrated significant increases (P < 0.05) in the percentage of glycated pancreatic IRI above the controls. Streptozotocin-treated (200 mg/kg) Swiss TO mice exhibited significant increases in plasma glucose (P < 0.001), glycated hemoglobin (P < 0.001), and percentage glycated pancreatic IRI (P < 0.05), compared with untreated controls, despite a marked decrease in both plasma IRI (P < 0.001) and total pancreatic IRI content (P < 0.001). Significant elevations in the percentage of glycated IRI were also observed in islets isolated from obese hyperglycemic (ob/ob) mice (P < 0.001), compared with islets from lean controls, and when lean mouse islets were cultured in hyperglycemic media for 24 h (33.3 vs. 5.6 mmol/l D-glucose; P < 0.001). The contribution of glycated plus nonglycated insulin and proinsulin to the total IRI was estimated in lean and obese mouse pancreatic extracts following high-performance liquid chromatography separation. The contribution of proinsulin to the total IRI was approximately 10%. Proinsulin represented 27-28% of the total glycated IRI. These data indicate that the glycation of insulin and proinsulin occurs within the pancreatic islets and is elevated in both insulin-deficient and insulin-resistant diabetic animal models.

Physiological concentrations of interleukin-6 directly promote insulin secretion, signal transduction, nitric oxide release, and redox status in a clonal pancreatic β-cell line and mouse islets

Interleukin-6 (IL6) has recently been reported to promote insulin secretion in a glucagon-like peptide-1-dependent manner. Herein, the direct effects of IL6 (at various concentrations from 0 to 1000 pg/ml) on pancreatic β-cell metabolism, AMP-activated protein kinase (AMPK) signaling, insulin secretion, nitrite release, and redox status in a rat clonal β-cell line and mouse islets are reported. Chronic insulin secretion (in μg/mg protein per 24  h) was increased from 128·7±7·3 (no IL6) to 178·4±7·7 (at 100  pg/ml IL6) in clonal β-cells and increased significantly in islets incubated in the presence of 5·5  mM glucose for 2  h, from 0·148 to 0·167±0·003  ng/islet. Pretreatment with IL6 also induced a twofold increase in basal and nutrient-stimulated insulin secretion in subsequent 20 min static incubations. IL6 enhanced both glutathione (GSH) and glutathione disulphide (GSSG) by nearly 20% without changing intracellular redox status (GSSG/GSH). IL6 dramatically increased iNOS expression (by ca. 100-fold) with an accompanying tenfold rise in nitrite release in clonal β-cells. Phosphorylated AMPK levels were elevated approximately twofold in clonal β-cells and mouse islet cells. Calmodulin-dependent protein kinase kinase levels (CaMKK), an upstream kinase activator of AMPK, were also increased by 50% after IL6 exposure (in β-cells and islets). Our data have demonstrated that IL6 can stimulate β-cell-dependent insulin secretion via direct cell-based mechanisms. AMPK, CaMKK (an upstream kinase activator of AMPK), and the synthesis of nitric oxide appear to alter cell metabolism to benefit insulin secretion. In summary, IL6 exerts positive effects on β-cell signaling, metabolism, antioxidant status, and insulin secretion.

Detrimental actions of metabolic syndrome risk factor, homocysteine, on pancreatic beta-cell glucose metabolism and insulin secretion

Elevated plasma homocysteine has been reported in individuals with diseases of the metabolic syndrome including vascular disease and insulin resistance. As homocysteine exerts detrimental effects on endothelial and neuronal cells, this study investigated effects of acute homocysteine exposure on beta-cell function and insulin secretion using clonal BRIN-BD11 beta-cells. Acute insulin release studies in the presence of various test reagents were performed using monolayers of BRIN-BD11 cells and samples assayed by insulin radioimmunoassay. Cellular glucose metabolism was assessed by nuclear magnetic resonance (NMR) analysis following 60-min exposure of BRIN-BD11 cell monolayers to glucose in either the absence or presence of homocysteine. Homocysteine dose-dependently inhibited insulin release at moderate and stimulatory glucose concentrations. This inhibitory effect was reversible at all but the highest concentration of homocysteine. 13C-glucose NMR demonstrated decreased labelling of glutamate from glucose at positions C2, C3 and C4, indicating that the tricarboxylic acid (TCA) cycle-dependent glucose metabolism was reduced in the presence of homocysteine. Homocysteine also dose-dependently inhibited insulinotropic responses to a range of glucose-dependent secretagogues including nutrients (alanine, arginine, 2-ketoisocaproate), hormones (glucagon-like peptide-1 (7-36)amide, gastric inhibitory polypeptide and cholecystokinin-8), neurotransmitter (carbachol), drug (tolbutamide) as well as a depolarising concentration of KCl or elevated Ca2+. Insulin secretion induced by activation of adenylate cyclase and protein kinase C pathways with forskolin and phorbol 12-myristate 13-acetate were also inhibited by homocysteine. These effects were not associated with any adverse action on cellular insulin content or cell viability, and there was no increase in apoptosis/necrosis following exposure to homocysteine. These data indicate that homocysteine impairs insulin secretion through alterations in beta-cell glucose metabolism and generation of key stimulus-secretion coupling factors. The participation of homocysteine in possible beta-cell demise merits further investigation.

Cellular responses of novel human pancreatic β-cell line, 1.1B4 to hyperglycemia

The novel human-derived pancreatic β-cell line, 1.1B4 exhibits insulin secretion and β-cell enriched gene expression. Recent investigations of the cellular responses of this novel cell line to lipotoxicity and cytokine toxicity revealed similarities to primary human β cells. The current study has investigated the responses of 1.1B4 cells to chronic 48 and 72 h exposure to hyperglycemia to probe mechanisms of human β-cell dysfunction and cell death. Exposure to 25 mM glucose significantly reduced insulin content (p<0.05) and glucokinase activity (p<0.01) after 72 h. Basal insulin release was unaffected but acute secretory response to 16.7 mM glucose was impaired (p<0.05). Insulin release stimulated by alanine, GLP-1, KCl, elevated Ca (2+) and forskolin was also markedly reduced after exposure to hyperglycemia (p<0.001). In addition, PDX1 protein expression was reduced by 58% by high glucose (p<0.05). Effects of hyperglycemia on secretory function were accompanied by decreased mRNA expression of INS, GCK, PCSK1, PCSK2, PPP3CB, GJA1, ABCC8, and KCNJ11. In contrast, exposure to hyperglycemia upregulated the transcription of GPX1, an antioxidant enzyme involved in detoxification of hydrogen peroxide and HSPA4, a molecular chaperone involved in ER stress response. Hyperglycemia-induced DNA damage was demonstrated by increased % tail DNA and olive tail moment, assessed by comet assay. Hyperglycemia-induced apoptosis was evident from increased activity of caspase 3/7 and decreased BCL2 protein. These observations reveal significant changes in cellular responses and gene expression in novel human pancreatic 1.1B4 β cells exposed to hyperglycemia, illustrating the usefulness of this novel human-derived cell line for studying human β-cell biology and diabetes.

Configuration of electrofusion-derived human insulin-secreting cell line as pseudoislets enhances functionality and therapeutic utility

Formation of pseudoislets from rodent cell lines has provided a particularly useful model to study homotypic islet cell interactions and insulin secretion. This study aimed to extend this research to generate and characterize, for the first time, functional human pseudoislets comprising the recently described electrofusion-derived insulin-secreting 1.1B4 human β-cell line. Structural pseudoislets formed readily over 3-7 days in culture using ultra-low-attachment plastic, attaining a static size of 100-200 μm in diameter, corresponding to ~6000 β cells. This was achieved by decreases in cell proliferation and integrity as assessed by BrdU ELISA, 3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide, and lactate dehydrogenase assays. Insulin content was comparable between monolayers and pseudoislets. However, pseudoislet formation enhanced insulin secretion by 1·7- to 12·5-fold in response to acute stimulation with glucose, amino acids, incretin hormones, or drugs compared with equivalent cell monolayers. Western blot and RT-PCR showed expression of key genes involved in cell communication and the stimulus-secretion pathway. Expression of E-Cadherin and connexin 36 and 43 was greatly enhanced in pseudoislets with no appreciable connexin 43 protein expression in monolayers. Comparable levels of insulin, glucokinase, and GLUT1 were found in both cell populations. The improved secretory function of human 1.1B4 cell pseudoislets over monolayers results from improved cellular interactions mediated through gap junction communication. Pseudoislets comprising engineered electrofusion-derived human β cells provide an attractive model for islet research and drug testing as well as offering novel therapeutic application through transplantation.

Islet-intrinsic effects of CFTR mutation

Cystic fibrosis-related diabetes (CFRD) is the most significant extra-pulmonary comorbidity in cystic fibrosis (CF) patients, and accelerates lung decline. In addition to the traditional view that CFRD is a consequence of fibrotic destruction of the pancreas as a whole, emerging evidence may implicate a role for cystic fibrosis transmembrane-conductance regulator (CFTR) in the regulation of insulin secretion from the pancreatic islet. Impaired first-phase insulin responses and glucose homeostasis have also been reported in CF patients. CFTR expression in both human and mouse beta cells has been confirmed, and recent studies have shown differences in endocrine pancreatic morphology from birth in CF. Recent experimental evidence suggests that functional CFTR channels are required for insulin exocytosis and the regulation of membrane potential in the pancreatic beta cell, which may account for the impairments in insulin secretion observed in many CF patients. These novel insights suggest that the pathogenesis of CFRD is more complicated than originally thought, with implications for diabetes treatment and screening in the CF population. This review summarises recent emerging evidence in support of a primary role for endocrine pancreatic dysfunction in the development of CFRD. Summary • CF is an autosomal recessive disorder caused by mutations in the CFTR gene • The vast majority of morbidity and mortality in CF results from lung disease. However CFRD is the largest extra-pulmonary co-morbidity and rapidly accelerates lung decline • Recent experimental evidence shows that functional CFTR channels are required for normal patterns of first phase insulin secretion from the pancreatic beta cell • Current clinical recommendations suggest that insulin is more effective than oral glucose-lowering drugs for the treatment of CFRD. However, the emergence of CFTR corrector and potentiator drugs may offer a personalised approach to treating diabetes in the CF population.

Physiological regulation of the pancreatic β-cell: functional insights for understanding and therapy of diabetes

Knowledge about the sites and actions of the numerous physiological and pharmacological factors affecting insulin secretion and pancreatic beta-cell function has been derived from the use of bioengineered insulin-producing cell lines. Application of an innovative electrofusion approach has generated novel glucose-responsive insulin-secreting cells for pharmaceutical and experimental research, including popular BRIN-BD11 beta-cells. This review gives an overview of the establishment and core characteristics of clonal electrofusion-derived BRIN-BD11 beta-cells. As discussed, BRIN-BD11 cells have facilitated studies aimed at dissecting important pathways by which nutrients and other bioactive molecules regulate the complex mechanisms regulating insulin secretion, and highlight the future potential of novel and diverse bioengineering approaches to provide a cell-based insulin-replacement therapy for diabetes. Clonal BRIN-BD11 beta-cells have been instrumental in: (a) characterization of K(ATP) channel-dependent and -independent actions of nutrients and established and emerging insulinotropic antidiabetic drugs, and the understanding of drug-induced beta-cell desensitization; (b) tracing novel metabolic and beta-cell secretory pathways, including use of state-of-the-art NMR approaches to provide new insights into the relationships between glucose and amino acid handling and insulin secretion; and (c) determination of the chronic detrimental actions of nutrients and the diabetic environment on pancreatic beta-cells, including the recent discovery that homocysteine, a risk factor for metabolic syndrome, may play a role in the progressive demise of insulin secretion and pancreatic beta-cell function in diabetes. Collectively, the studies discussed in this review highlight the importance of innovative experimental beta-cell physiology in the discovery and characterization of new and improved drugs and therapeutic strategies to help tackle the emerging diabetes epidemic.

Na+ cotransport by metabolizable and nonmetabolizable amino acids stimulates a glucose-regulated insulin-secretory response

The involvement of Na+ in insulin-secretory responses to metabolizable and nonmetabolizable amino acids known to be cotransported with Na+, were examined using islet-derived BRIN-BD11 cells. At stimulatory (16.7 mM) glucose, 10 mM of l-alanine, alpha-aminoisobutyric acid (AIB) or l-proline stimulated 1.3- to 10. 4-fold (p < 0.01) insulin-secretory responses. In each case, these effects were significantly greater than those observed at nonstimulatory (1.1 mM) glucose (p < 0.01). While, tetrodotoxin blockade of voltage-dependent Na+ channels exerted no significant effect on insulin release, Na/K pump blockade with ouabain significantly promoted the amino acid-induced effects (p < 0.05). Replacement of extracellular Na+ with equimolar N-methyl-d-glucamine+ and omission of extracellular K+ or Ca2+ were all effective in removing the actions of each amino acid, confirming the critical role of ionic fluxes in the secretory responses to these amino acids. Collectively these results demonstrate that metabolizable and nonmetabolizable amino acids can induce glucose-dependent insulin-secretory responses by modulating electrogenic Na+ transport.

Comparison of the secretory properties of four insulin-secreting cell lines

The insulin-secretory responsiveness of four popular and widely used insulin-secreting cells lines (RINm5F, HIT-T15, INS-1 and BRIN-BD11 cells) to a range of stimuli including glucose, amino acids, neurotransmitters, peptide hormones and sulphonylureas was studied. Differences were seen in the pattern of responsiveness of the cell lines to the various modulators of insulin release. While these studies revealed that INS-1 cells had the highest insulin content, only BRIN-BD11 cells exhibited a significant step-wise insulin secretory response to increasing glucose concentrations. BRIN-BD11 cells also showed pronounced insulin responses to leucine, KIC, L-arginine, L-alanine and palmitic acid. All the cell lines tested gave significant responses to the neurotransmitters carbachol and glibenclamide with increased insulin release. A comparison was made between the functional characteristics of the various cell lines with those of freshly isolated rat islets. This illustrated the general value of each cell line as a model for studies of insulin secretion. Electrofusion-derived BRIN-BD11 cells appeared to closely mimic the glucose sensitivity and overall secretory performance of normal rat islets.

ATP-sensitive potassium channels and efaroxan-induced insulin release in the electrofusion-derived BRIN-BD11 beta-cell line

The properties of ATP-sensitive K+ (K(ATP)) channels were explored in the electrofusion-derived, glucose-responsive, insulin-secreting cell line BRIN-BD11 using patch-clamp techniques. In intact cells, K(ATP) channels were inhibited by glucose, the sulfonylurea tolbutamide, and the imidazoline compounds efaroxan and phentolamine. Each of these agents initiated insulin secretion and potentiated the actions of glucose. K(ATP) channels were blocked by ATP in a concentration-dependent manner and activated by ADP in the presence of ATP. In both intact cells and excised inside-out patches, the K(ATP) channel agonists diazoxide and pinacidil activated channels, and both compounds inhibited insulin secretion evoked by glucose, tolbutamide, and imidazolines. The mechanisms of action of imidazolines were examined in more detail. Pre-exposure of BRIN-BD11 cells to either efaroxan or phentolamine selectively inhibited imidazoline-induced insulin secretion but not the secretory responses of cells to glucose, tolbutamide, or a depolarizing concentration of KCl. These conditions did not result in the loss of depolarization-dependent rises in intracellular Ca2+ ([Ca2+]i), K(ATP) channel operation, or the actions of either ATP or efaroxan on K(ATP) channels. Desensitization of the imidazoline receptor following exposure to high concentrations of efaroxan, however, was found to result in an increase in SUR1 protein expression and, as a consequence, an upregulation of K(ATP) channel density. Our data provide 1) the first characterization of K(ATP) channels in BRIN-BD11 cells, a novel insulin-secreting cell line produced by electrofusion techniques, and 2) a further analysis of the role of imidazolines in the control of insulin release.