Engineering of glucose-stimulated insulin secretion and biosynthesis in non-islet cells

Current progress and emerging technologies for generating extrapancreatic functional insulin-producing cells

Diabetes has been one of the major concerns in recent years, due to the increasing rate of morbidity and mortality worldwide. The available treatment strategies for uncontrolled diabetes mellitus (DM) are pancreas or islet transplantation. However, these strategies are limited due to unavailability of quality pancreas/ islet donors, life-long need of immunosuppression, and associated complications. Cell therapy has emerged as a promising alternative options to achieve the clinical benefits in the management of uncontrolled DM. Since the last few years, various sources of cells have been used to convert into insulin-producing β-like cells. These extrapancreatic sources of cells may play a significant role in β-cell turnover and insulin secretion in response to environmental stimuli. Stem/progenitor cells from liver have been proposed as an alternative choice that respond well to glucose stimuli under strong transcriptional control. The liver is one of the largest organs in the human body and has a common endodermal origin with pancreatic lineages. Hence, liver has been proposed as a source of a large number of insulin-producing cells. The merging of nanotechnology and 3D tissue bioengineering has opened a new direction for producing islet-like cells suitable for in vivo transplantation in a cordial microenvironment. This review summarizes extrapancreatic sources for insulin-secreting cells with reference to emerging technologies to fulfill the future clinical need.

GPR21 Inhibition Increases Glucose-Uptake in HepG2 Cells

GPR21 is a constitutively active, orphan, G-protein-coupled receptor, with in vivo studies suggesting its involvement in the modulation of insulin sensitivity. However, its precise contribution is not fully understood. As the liver is both a major target of insulin signalling and critically involved in glucose metabolism, the aim of this study was to examine the role of GPR21 in the regulation of glucose uptake and production in human hepatocytes. In particular, HepG2 cells, which express GPR21, were adopted as cellular models. Compared with untreated cells, a significant increase in glucose uptake was measured in cells treated with siRNA to downregulate GPR21 expression or with the GPR21-inverse agonist, GRA2. Consistently, a significantly higher membrane translocation of GLUT-2 was measured under these conditions. These effects were accompanied by an increased ratio of phAKT^(Ser473)/tot-AKT and phGSK-3β^(Ser9)/tot-GSK-3β, thus indicating a marked activation of the insulin signalling pathway. Moreover, a significant reduction in ERK activation was observed with GPR21 inhibition. Collectively, these results indicate that GPR21 mediates the negative effects on glucose uptake by the liver cells. In addition, they suggest that the pharmacological inhibition of GPR21 could be a novel strategy to improve glucose homeostasis and counteract hepatic insulin resistance.

High Basolateral Glucose Increases Sodium-Glucose Cotransporter 2 and Reduces Sirtuin-1 in Renal Tubules through Glucose Transporter-2 Detection

Under diabetic conditions, sodium-glucose cotransporter 2 (SGLT2) for glucose uptake in proximal tubules (PTs) increases, whereas NAD⁺-dependent protein deacetylase silent mating type information regulation 2 homolog 1 (Sirtuin-1; SIRT1) for PT survival decreases. Therefore, we hypothesized that increased glucose influx by SGLT2 reduces SIRT1 expression. To test this hypothesis, db/db mice with diabetes and high-glucose (HG)-cultured porcine PT LLC-PK1 cells in a two-chamber system were treated with the SGLT2 inhibitor canagliflozin. We also examined SIRT1 and SGLT2 expression in human kidney biopsies. In db/db mice, SGLT2 expression increased with concomitant decreases in SIRT1, but was inhibited by canagliflozin. For determination of the polarity of SGLT2 and SIRT1 expression, LLC-PK1 cells were seeded into Transwell chambers (pore size, 0.4 µm; Becton Dickinson, Oxford, UK). HG medium was added to either or to both of the upper and lower chambers, which corresponded to the apical and basolateral sides of the cells, respectively. In this system, the lower chamber with HG showed increased SGLT2 and decreased SIRT1 expression. Canagliflozin reversed HG-induced SIRT1 downregulation. Gene silencing and inhibitors for glucose transporter 2 (GLUT2) blocked HG-induced SGLT2 expression upregulation. Gene silencing for the hepatic nuclear factor-1α (HNF-1α), whose nuclear translocation was enhanced by HG, blocked HG-induced SGLT2 expression upregulation. Similarly, gene silencing for importin-α1, a chaperone protein bound to GLUT2, blocked HG-induced HNF-1α nuclear translocation and SGLT2 expression upregulation. In human kidney, SIRT1 immunostaining was negatively correlated with SGLT2 immunostaining. Thus, under diabetic conditions, SIRT1 expression in PTs was downregulated by an increase in SGLT2 expression, which was stimulated by basolateral HG through activation of the GLUT2/importin-α1/HNF-1α pathway.

Towards the Development of a Bioartificial Pancreas: Effects of Poly-l-Lysine on Alginate Beads with BTC3 Cells

A bioartificial tissue construct that consists of insulin-secreting cells entrapped in an alginate/poly-l-lysine (PLL) matrix offers a promising approach for the treatment of type I diabetes. Use of transformed cells has been proposed as a solution to the cell availability problem posed by islets. The growth characteristics of transformed cells in their sequestered environment and the effects of PLL on their metabolic and secretory activities have not yet been characterized. Our data demonstrate that mouse insulinoma βTC3 cells proliferate while they are entrapped in both PLL-free and PLL-coated alginate beads. During this process, cell aggregates develop in the bead periphery, which increase in number and size with time. PLL is crucial for the long-term in vitro structural stability of beads, and it does not appear to affect the metabolic and secretory activities of entrapped βTC3 cells. The implications of these findings in the development of a bioartificial pancreatic construct based on transformed cells are discussed.

Improved function and growth of pancreatic cells in a three-dimensional bioreactor environment

Methods of three-dimensional (3D) cell culture have made significant progress in recent years due to a better understanding of cell to cell interactions and the cell's interface with their surrounding environment. We hypothesized that a microgravity 3D culture system would improve upon the growth and function of a pancreatic progenitor cell population. We developed a rotating wall vessel bioreactor and established a culture system using a pancreatic cell line. Cells in the bioreactors showed robust proliferation, enhanced transcriptional signaling, and improved translation of pancreatic genes compared with two-dimensional static culture. Cells also gained the ability to respond to glucose stimulation, which was not observed in the control cultures. These findings suggest that a 3D microgravity bioreactor environment mimics the niche of the pancreas yielding a cell source with potential for cell-based therapy in the treatment of diabetes.

Characterisation of BHK-21 cells engineered to secrete human insulin

Autoimmune destruction of beta cells in the pancreas leads to type I, or insulin dependent diabetes mellitus (IDDM), through the loss of endogenous insulin production capacity. This paper describes an attempt to generate 'artificial'beta cells using the fibroblast cell line BHK21. Stable transfectants expressing the human preproinsulin (PPI) gene were isolated and characterised. The resulting clone selected for further analysis (BHK-PPI-C16) was capable of secreting 0.12 pmol proinsulin/hr/10(5) cells and maintained a steady cellular proinsulin content of 0.36 +/- 0.04 pmol l(-1). There was no processing of the proinsulin to mature insulin. The cells were unresponsive to glucose but there was increased proinsulin secretion in the presence of agents that stimulated formation of intracellular cAMP. Transfection of cDNAs for the key elements of the glucose sensing apparatus (GLUT2 and glucokinase) led to a subphysiological stimulation of secretion when glucokinase was transfected alone while there was a complete loss of insulin secretion when both components were overexpressed. The deleterious effect on proinsulin secretion observed upon co-expression of the glucose sensing genes may have implications for applications requiring multigene expression in BHK21 cells.

Exendin-4 regulates GLUT2 expression via the CaMKK/CaMKIV pathway in a pancreatic β-cell line

The GLUT2 glucose transporter plays an important role in glucose-induced insulin secretion in pancreatic β-cells by catalyzing the uptake of glucose into the cell. In this study, we investigated whether exendin-4, a long-acting agonist of glucagon-like peptide-1, mediates stimulatory effects on GLUT2 gene expression through the Ca²+/calmodulin (CaM)-dependent protein kinase IV (CaMKIV) cascade. GLUT2 expression was examined by real-time polymerase chain reaction, Western blot analysis, and a reporter gene assay in rat insulin-secreting INS-1 cells incubated with exendin-4. An increased expression level of GLUT2 protein was noted in response to increasing concentrations of exendin-4, with maximal induction at 10 nmol/L. Real-time polymerase chain reaction analysis similarly revealed a significant increase in the amount of GLUT2 messenger RNA by 10 nmol/L exendin-4. Exendin-4 also stimulated GLUT2 promoter activity in response to increasing exendin-4 concentrations, but failed to do so in the presence of STO-609, a CaMKK inhibitor. We also investigated the effect of the constitutively active form of CaMKK (CaMKKc) on GLUT2 promoter activity. The result is consistent with the observations that CaMKKc/CaMKIV enhanced or up-regulated GLUT2 promoter activity in INS-1 cells. Furthermore, exendin-4 induction of GLUT2 protein expression was significantly suppressed in the cells knocking down the CaMKIV. In summary, activation of the CaMKK/CaMKIV cascade might be required for exendin-4-induced GLUT2 gene transcription, indicating that exendin-4 plays an important role in insulin secretion in pancreatic β-cells.

GLUT2 mutations, translocation, and receptor function in diet sugar managing

Cloned 20 years ago, GLUT2 is a facilitative glucose transporter in the liver, pancreas, intestine, kidney, and brain. It ensures large bidirectional fluxes of glucose in and out the cell due to its low affinity and high capacity. It also transports other dietary sugars, such as fructose and galactose, within the range of physiological concentrations. Sugars and hormones regulate its gene expression. The contribution of GLUT2 to human metabolic diseases previously appeared modest. However, in the past decade, three major features of the GLUT2 protein have been revealed. First, GLUT2 mutations cause the severe but rare Fanconi-Bickel syndrome, mainly characterized by glycogenosis. Recently, a GLUT2 polymorphism has been associated with preferences for sugary food. Second, the GLUT2 location at the cell surface is regulated; this governs cellular activities dependent on glucose in the intestine and possibly those in the liver and pancreas. For instance, GLUT2 translocation from an intracellular pool to the apical membrane after a sugar meal transiently increases sugar uptake by enterocytes (reviewed in 32). Third, GLUT2 functions as a membrane receptor of sugar. Independently of glucose metabolism, GLUT2 detects the presence of extracellular sugar and transduces a signal to modulate cell functions, including beta-cell insulin secretion, renal reabsorption, and intestinal absorption according to the sugar environment. These recent developments are examined here in heath and metabolic disease, highlighting various unanswered questions.

Co-transfection of GK and mhPINS genes into HepG2 cells confers glucose-stimulated insulin secretion

BACKGROUND

The purpose of this study was to construct an 'artificial beta cell' that can exhibit physiologic glucose-stimulated insulin secretion for the treatment of type 1 diabetes.

METHODS

Retroviral vector containing the glucokinase (GK) gene and mutated human proinsulin (mhPINS) gene was constructed. HepG2 cells were first infected with recombinant retrovirus carrying the GK and mhPINS genes, then selectively cultured with G418 to obtain the positive clones. GK and mhPINS gene transcription and expression were identified by radioimmunity, Western blot and RT-PCR techniques. Finally, the dose-response effect of glucose on insulin secretion from those HepG2 cells that expressed both GK and mhPINS genes was tested with HepG2 cells that only expressed the mhPINS gene as a control.

RESULTS

HepG2 cells with transferred GK and mhPINS genes were selectively cultured with G418 and the positive clones were obtained in 3 weeks. Four clones with GK and mhPINS gene expression were selected from 20 positive clones by radioimmunity and Western blot. We picked up one clone with a strong GK and mhPINS gene expression and named it clone Beta. In clone Beta, differences in insulin secretion at 0.5 and 0.75 mmol/L glucose concentrations were not significant (P>0.05) and differences in insulin secretion at 2.0, 3.0, 4.0, 5.0 and 6.0 mmol/L glucose concentrations were not significant (P>0.05), while there were significant differences in insulin secretion at other glucose concentrations(P<0.05). The artificial beta cell, clone Beta, obtained a glucose-stimulated insulin secretion with maximal insulin secretion at 1.75-2.00 mmol/L glucose concentrations.

DISCUSSION

An artificial beta cell that exhibits glucose-stimulated insulin secretion can be constructed successfully.

Engineered enteroendocrine cells secrete insulin in response to glucose and reverse hyperglycemia in diabetic mice

Type 1 diabetes is a metabolic disorder caused by loss of insulin-producing pancreatic beta-cells. Expression of insulin in non-beta-cells to create beta-cell surrogates has been tried to treat type 1 diabetes. Enteroendocrine K cells have characteristics similar to pancreatic beta-cells, such as a glucose-sensing system and insulin-processing proteases. In this study, we genetically engineered an enteroendocrine cell line (STC-1) to express insulin under the control of the glucose-dependent insulinotropic polypeptide promoter. We screened clones and chose one, Gi-INS-7, based on its high production of insulin. Gi-INS-7 cells expressed glucose transporter 2 (GLUT2) and glucokinase (GK) and secreted insulin in response to elevated glucose levels in vitro. To determine whether Gi-INS-7 cells can control blood glucose levels in diabetic mice, we transplanted these cells under the kidney capsule of streptozotocin (STZ)-induced diabetic mice and found that blood glucose levels became normal within 2 weeks of transplantation. In addition, glucose tolerance tests in mice that became normoglycemic after transplantation with Gi-INS-7 cells showed that exogenous glucose was cleared appropriately. These results suggest that engineered K cells may be promising surrogate beta-cells for possible therapeutic use for the treatment of type 1 diabetes.

Inhibition of the intestinal glucose transporter GLUT2 by flavonoids

We tested whether the dominant intestinal sugar transporter GLUT2 was inhibited by intestinal luminal compounds that are inefficiently absorbed and naturally present in foods. Because of their abundance in fruits and vegetables, flavonoids were selected as model compounds. Robust inhibition of glucose and fructose transport by GLUT2 expressed in Xenopus laevis oocytes was produced by the flavonols myricetin, fisetin, the widely consumed flavonoid quercetin, and its glucoside precursor isoquercitrin [corrected]. IC50s for quercetin, myricetin, and isoquercitirin [corrected]were approximately 200- to 1000-fold less than glucose or fructose concentrations, and noncompetitive inhibition was observed. The two other major intestinal sugar transporters, GLUT5 and SGLT1, were unaffected by flavonoids. Sugar transport by GLUT2 overexpressed in pituitary cells and naturally present in Caco-2E intestinal cells was similarly inhibited by quercetin. GLUT2 was detected on the apical side of Caco-2E cells, indicating that GLUT2 was in the correct orientation to be inhibited by luminal compounds. Quercetin itself was not transported by the three major intestinal glucose transporters. Because the flavonoid quercetin, a food component with an excellent pharmacology safety profile, might act as a potent luminal inhibitor of sugar absorption independent of its own transport, flavonols show promise as new pharmacologic agents in the obesity epidemic.

Overexpression of glucose transporter 2 in GT1-7 cells inhibits AMP-activated protein kinase and agouti-related peptide expression

Glucose transporter 2 (GLUT2) has been proposed as a glucose sensor in pancreatic beta cells. GLUT2 has been found widely expressed in the brain and GLUT2 in the hypothalamus and hindbrain has been suggested to be involved in the central glucose sensing and regulation of glucose homeostasis and food intake. In this study, we overexpressed GLUT2 in GT1-7 neuroblastoma cells and investigated the effect of GLUT2 overexpression on cellular energy status in these cells. Compared with control cells, GLUT2 overexpression resulted in significantly increased cellular ATP levels at 5 and 25 mM glucose concentrations, more inhibition of agouti-related peptide (AgRP) mRNA and AMP-activated protein kinase (AMPK) phosphorylation by glucose, and attenuated stimulation of AgRP mRNA and AMPK by 2-deoxy-d-glucose (2DG), implicating that brain GLUT2 may be important in the regulation of food intake.

Metabolic engineering: advances in modeling and intervention in health and disease

The field of metabolic engineering encompasses a powerful set of tools that can be divided into (a) methods to model complex metabolic pathways and (b) techniques to manipulate these pathways for a desired metabolic outcome. These tools have recently seen increased utility in the medical arena, and this paper aims to review significant accomplishments made using these approaches. The modeling of metabolic pathways has been applied to better understand disease-state physiology in a variety of cellar, subcellular, and organ systems, including the liver, heart, mitochondria, and cancerous cells. Metabolic pathway engineering has been used to generate cells with novel biochemical functions for therapeutic use, and specific examples are provided in the areas of glycosylation engineering and dopamine-replacement therapy. In order to document the potential of applying both metabolic modeling and pathway manipulation, we describe pertinent advances in the field of diabetes research. Undoubtedly, as the field of metabolic engineering matures and is applied to a wider array of problems, new advances and therapeutic strategies will follow.

Hepatic insulin gene therapy in insulin-dependent diabetes mellitus

Insulin-dependent diabetes mellitus (IDDM) is an autoimmune disease resulting in destruction of the pancreatic beta-cells in the islets of Langerhans. Commonly employed treatment of IDDM requires periodic insulin therapy, which is not ideal because of its inability to prevent chronic complications such as nephropathy, neuropathy and retinopathy. Although pancreas or islet transplantation are effective treatments that can reverse metabolic abnormalities and prevent or minimize many of the chronic complications of IDDM, their usefulness is limited as a result of shortage of donor pancreas organs. Gene therapy as a novel field of medicine holds tremendous therapeutic potential for a variety of human diseases including IDDM. This review focuses on the liver-based gene therapy for generation of surrogate pancreatic beta-cells for insulin replacement because of the innate ability of hepatocytes to sense and metabolically respond to changes in glucose levels and their high capacity to synthesize and secrete proteins. Recent advances in the use of gene therapy to prevent or regenerate beta-cells from autoimmune destruction are also discussed.

Engineering physiologically regulated insulin secretion in non-beta cells by expressing glucagon-like peptide 1 receptor

Glucagon-like peptide 1 (GLP-1) is released from neuroendocrine cells in the intestine in the postprandial state and augments glucose-stimulated insulin secretion from pancreatic beta cells. To develop non-beta cells that exhibit physiologically regulated insulin secretion, we coexpressed the GLP-1 receptor and human insulin in primary rat pituitary cells using adenovirus-mediated gene transfer. The transduced cells were analyzed in a perifusion system and after transplantation into mice. Normal pituitary cells do not express the GLP-1 receptor as shown by the absence of GLP-1 receptor mRNA and the inability of GLP-1 to stimulate pituitary hormone secretion. Following transduction with an adenovirus carrying the GLP-1 receptor cDNA, the pituitary cells expressed functional GLP-1 receptors as reflected by the ability of GLP-1 to stimulate secretion of pituitary hormones. When both the GLP-1 receptor and human insulin were introduced, GLP-1 stimulated cosecretion of human insulin and endogenous pituitary hormones. GLP-1 was similar in potency to the hypothalamic-releasing hormones and stimulated hormone secretion in a dose-dependent fashion. In contrast to pancreatic beta cells, the hormone-releasing effect of GLP-1 on transduced pituitary cells was not dependent on the concentration of extracellular glucose. After transplantation of pituitary cells coexpressing human insulin and GLP-1 receptor into mice, enteral glucose stimulated insulin secretion. These results demonstrate a new approach to engineer physiologically regulated insulin secretion by non-beta cells.

Production of recombinant human relaxin 3 in AtT20 cells

Relaxin 3 has been reported recently as a member of the insulin/IGF/relaxin family. To clarify the function of relaxin 3, we prepared recombinant human relaxin 3 using a mouse adrenocorticotrophic hormone (ACTH)-secreting cell line, AtT20. To detect a mature form of recombinant human relaxin 3, a competitive enzyme immunoassay (EIA) was developed using a monoclonal antibody (mAb; HK4-144-10), which was raised for the N-terminal peptide of human relaxin 3 A-chain. We detected immunoreactive (ir-) relaxin 3 in the culture supernatant of AtT20 cells stably transfected with human relaxin 3 cDNA. After treatment with 5 microM forskolin for 3 days, the concentration of the ir-relaxin 3 in the culture supernatant reached 12 nM. Ir-relaxin 3 was purified from the culture supernatant by a combination of various chromatographies. By analyses of N-terminal amino acid sequence and electrospray ionization mass spectrometry (ESI-MS), we confirmed that the purified material was a mature form of human relaxin 3. The recombinant human relaxin 3 thereby obtained increased intracellular cAMP production in THP-1 cells. Our results demonstrate that the expression of relaxin 3 cDNA in AtT20 cells is a useful tool to produce a bioactive and mature form of relaxin 3.

Prospects for the prevention and reversal of type 1 diabetes mellitus

Type 1 (insulin-dependent) diabetes mellitus results from selective immune-mediated destruction of pancreatic islet beta cells. Strategies to prevent or reverse the development of diabetes can be divided into three groups, depending on whether they focus on beta-cell protection, regeneration or replacement. Prevention of immune beta-cell destruction involves either halting the immune attack directed against beta cells or making beta cells better able to withstand immune attack, for example, by making them resistant to free radical damage. The recent identification of beta-cell growth factors and development of stem cell technologies provides an alternative route to the reversal of diabetes, namely beta-cell regeneration. Interestingly, stem cell-derived islets appear to be less sensitive to recurrent immune destruction that is normally seen in response to islet transplantation. The last alternative is beta-cell replacement or substitution. This covers a wide range of interventions including human whole pancreas transplantation, xenotransplantation, genetically modified beta cells, mechanical insulin sensing and delivery devices, and the artificial pancreas. This review describes recent advances in each of these research areas and aims to provide clinicians with an idea of where and when an effective strategy to prevent or reverse diabetes development will become available.

Function of a genetically modified human liver cell line that stores, processes and secretes insulin

An alternative approach to the treatment of type I diabetes is the use of genetically altered neoplastic liver cells to synthesize, store and secrete insulin. To try and achieve this goal we modified a human liver cell line, HUH7, by transfecting it with human insulin cDNA under the control of the cytomegalovirus promoter. The HUH7-ins cells created were able to synthesize insulin in a similar manner to that which occurs in pancreatic beta cells. They secreted insulin in a regulated manner in response to glucose, calcium and theophylline, the dose-response curve for glucose being near-physiological. Perifusion studies showed that secretion was rapid and tightly controlled. Removal of calcium resulted in loss of glucose stimulation while addition of brefeldin A resulted in a 30% diminution of effect, indicating that constitutive release of insulin occurred to a small extent. Insulin was stored in granules within the cytoplasm. When transplanted into diabetic immunoincompetent mice, the cells synthesized, processed, stored and secreted diarginyl insulin in a rapid regulated manner in response to glucose. Constitutive release of insulin also occurred and was greater than regulated secretion. Blood glucose levels of the mice were normalized but ultimately became subnormal due to continued proliferation of cells. Examination of the HUH7-ins cells as well as the parent cell line for beta cell transcription factors showed the presence of NeuroD but not PDX-1. PC1 and PC2 were also present in both cell types. Thus, the parent HUH7 cell line possessed a number of endocrine pancreatic features that reflect the common endodermal ancestry of liver and pancreas, perhaps as a result of ontogenetic regression of the neoplastic liver cell from which the line was derived. Introduction of the insulin gene under the control of the CMV promoter induced changes in these cells to make them function to some extent like pancreatic beta cells. Our results support the view that neoplastic liver cells can be induced to become substitute pancreatic beta cells and become a therapy for the treatment of type I diabetes.

An insulin-related peptide expressed in 3T3L1 adipocytes is localized in GLUT4 vesicles and secreted in response to exogenous insulin, which augments the insulin-stimulated glucose uptake

If an adipocyte is programmed to secrete insulin, then the insulin released may amplify the insulin action by an autocrine manner. To examine this hypothesis in vitro, we investigated the effects of expressing the preproinsulin gene in 3T3L1 adipocytes on (pro)insulin release and glucose uptake. The human preproinsulin gene was transferred into 3T3L1 adipocytes by infecting the cells with recombinant adenovirus Adex1CA human preproinsulin. Immunocytochemical studies showed that (pro)insulin is associated with vesicular structures that colocalize with GLUT4 vesicles but not with GLUT1 vesicles. We then examined insulin-induced proinsulin release from 3T3L1 adipocytes expressing the insulin gene. The exogenously administered insulin stimulated proinsulin release from these cells in a dose-dependent manner. HPLC determination revealed the existence of mature human insulin in these cells, which suggested the release of mature insulin into the medium. Further we monitored the (pro)insulin release from these cells with confocal laser microscopy using the expression of a fusion protein between insulin and green fluorescent protein (GFP). Time-lapse confocal laser-scanning microscopy revealed that the total number of vesicles containing insulin-GFP was decreased by the addition of 10(-7) M insulin within 1 minute. Finally, we examined the insulin-stimulated glucose uptake by these cells. The data showed that insulin-stimulated glucose uptake increased to about 150% of that of control cells in response to exogenously administered insulin, indicating that the insulin released augmented the insulin-stimulated glucose uptake in an autocrine manner. Thus, the data support our hypothesis, indicating that we could construct the insulin-regulated insulin release system in adipocytes by introducing the preproinsulin gene.

Pancreatic Islet Cell Transplantation

Pancreatic islet cell transplantation as a treatment for diabetes has hitherto been confined to small patient cohorts with limited success. This article summarizes the results of islet cell transplantation before and after the advent of the new 'Edmonton protocol' of immunosuppression and management of the donor pancreas. Adopting this regimen has achieved unprecedented success and renewed interest in this potential cure for diabetes. Central to recent improvements in the technique has been the transplantation of an adequate islet mass. Improved methods to procure, isolate, and purify islets for clinical use are now being adopted as a new 'gold standard'. The use of new immunosuppressive drugs has further improved clinical results. Corticosteroid sparing-based regimens, and agents such as humanized monoclonal antibodies, are likely to form the mainstay of immunosuppressive protocols with the aim of achieving donor-specific tolerance. Alternative sources of islet cells are also required to expand the technique in an era of reduced numbers of donor pancreata. Manipulation of stem cells and xenotransplantation may yet yield sufficient islets to overcome the problem of donor shortage. Islet cell transplantation now forms the basis of a prospective multicenter trial under the aegis of the Immune Tolerance Network. The results of this are awaited, but it appears that islet cell transplantation may yet emerge as an effective treatment option for some members of the diabetic population.

Regulation of secretory granule pH in insulin-secreting cells

Luminal acidification is important for the maturation of secretory granules, yet little is known regarding the regulation of pH within them. A pH-sensitive green fluorescent protein (EGFP) was targeted to secretory granules in RIN1046-38 insulinoma cells by using a construct in which the EGFP gene was preceded by the nucleotide sequence for human growth hormone. Stimulatory levels of glucose doubled EGFP secretion from cell cultures, and potentiators of glucose-induced insulin secretion enhanced EGFP release. Thus this targeted EGFP is useful for population measurements of secretion. However, less than ~4% of total cell EGFP was released after 1.5 h of stimulation. Consequently, when analyzed in single cells, fluorescence of the targeted EGFP acts as an indicator of pH within secretory granules. Glucose elicited a decrease in granule pH, whereas inhibitors of the V-type H(+)-ATPase increased pH and blocked the glucose effect. Granule pH also was modified by effectors of the protein kinase A pathway, with activation eliciting granule alkalinization, suggesting that potentiation of peptide release by cAMP may involve regulated changes in secretory granule pH.

Engineering Vero cells to secrete human insulin

Cell therapy may have the potential for the treatment of Type I diabetes. To date, cells suitable for this purpose have not been developed. This study investigates the feasibility of modifying Vero, a cell line that may be considered safe to implant into humans, for this purpose. Stable Vero transfectants containing full-length human preproinsulin complementary deoxyribonucleic acid (cDNA) were generated using a liposomal transfection reagent. Reverse transcriptase-polymerase chain reaction, immunocytochemistry, Western blotting, and enzyme-linked immunosorbent assays were used to assess the resulting cells. Proinsulin was expressed but was not processed to insulin by these cells. Proinsulin cDNA was genetically modified, resulting in a form that is furin sensitive. The resulting stably transfected Vero clones constitutively release approximately 34%/h (32.68 +/- 2.21 to 35.62 +/- 3.14%) of the product formed, approximately 62% (59.99 +/- 6.45 to 64.64 +/- 4.57%) of which is mature insulin. These Vero transfectants did not exhibit glucose-stimulated insulin secretion. As GLUT2 and glucokinase (GCK) are not constitutively expressed by these cells, human GLUT2 cDNA and GCK cDNA were cotransfected with furin-sensitive preproinsulin cDNA into Vero cells. Insulin and GCK proteins were detected in the cytoplasmic region of the resulting cells, whereas GLUT2 was predominantly expressed in the nucleus. Coexpression of GLUT2 and GCK did not result in glucose-stimulated insulin secretion. The results from this study demonstrate the feasibility of engineering a relatively "safe" nonbeta cell line to produce human insulin. Coexpression of GLUT2 and GCK, at the levels achieved here, is not adequate enough to induce glucose-stimulated insulin secretion in such cells; the subcellular location of transfected components may be relevant.

Over-expression of the glucagon-like peptide-1 receptor on INS-1 cells confers autocrine stimulation of insulin gene promoter activity: a strategy for production of pancreatic beta-cell lines for use in transplantation

To develop transplantable beta-cell lines for the treatment of diabetes mellitus, we have taken advantage of the property of INS-1 cells to synthesize and secrete not only insulin, but also small quantities of the insulinotropic hormone glucagon-like peptide-1 (GLP-1). In INS-1 cells over-expressing the beta-cell GLP-1 receptor (GLP-1-R), we have shown, by radioimmune assay and bioassay of conditioned medium, that an autocrine signaling mechanism of hormone action exists whereby self-secreted GLP-1 acts as a competence factor in support of insulin gene transcription. INS-1 cells also exhibit insulin gene promoter activity, as assayed in cells transfected with a rat insulin gene I promoter-luciferase construct (RIP1-Luc). The GLP-1-R agonist exendin-4 stimulates RIP1-Luc activity in a glucose-dependent manner, an effect mediated by endogenous GLP-1-Rs, and is blocked by the serine/threonine protein kinase inhibitor Ro 31-8220. Over-expression of GLP-1-R in transfected INS-1 cells reduces the threshold for exendin-4 agonist action, whereas basal RIP1-Luc activity increases 2.5-fold in the absence of added agonist. The increase of basal RIP1-Luc activity is a consequence of autocrine stimulation by self-secreted GLP-1 and is blocked by introduction of (1) an inactivating W39A mutation in the N-terminus ligand-binding domain of GLP-1-R or (2) mutations in the third cytoplasmic loop that prevent G protein coupling. No evidence for constitutive ligand-independent signaling properties of the GLP-1-R has been obtained. Over-expression of GLP-1-R increases the potency and efficacy of D-glucose as a stimulator of RIP1-Luc. Thus, INS-1 cells over-expressing the GLP-1-R recapitulate the incretin hormone effect of circulating GLP-1, thereby providing a possible strategy by which beta-cell lines may be engineered for efficient glucose-dependent insulin biosynthesis and secretion.

Adenovirus-mediated preproinsulin gene transfer into adipose tissues ameliorates hyperglycemia in obese diabetic KKA(y) mice

We investigated whether adenovirus-mediated preproinsulin gene transfer into insulin target tissues (adipocytes) ameliorates hyperglycemia in diabetic mice. KKA(y) mice, a genetically obese type 2 diabetic animal model, were treated with a single subcutaneous injection of recombinant adenovirus, Adex1CA-human preproinsulin (Adex1CA-pchi), into the epididymal fat pads. pchi mRNA was expressed only in adipose tissue in which mature insulin was produced. Three days after virus injection these mice showed a marked decrease of blood glucose levels (from about 400 to 200 mg/dl), and an intraperitoneal glucose tolerance test revealed the markedly improved glucose tolerance. There was no significant difference in serum insulin levels between control and recombinant adenovirus-treated KKA(y) mice. The normalized glucose levels in diabetic mice were maintained for at least 2 weeks after the virus injection. This strategy could provide a novel and, most importantly, a simple and convenient gene therapy for obese type 2 diabetes patients.

Non-Invasive monitoring of a bioartificial pancreas in vitro and in vivo

Monitoring biochemical processes relevant to the function, survival, and longevity of tissue-engineered pancreatic constructs is important for the development of an optimum construct design as well as patient care management after implantation. In this report we demonstrate the ability of nuclear magnetic resonance (NMR) techniques to monitor aspects of intracellular metabolism, overall morphology, and distribution of a microencapsulation based bioartificial pancreas in vitro and in vivo.

Glucose-induced toxicity in insulin-producing pituitary cells that coexpress GLUT2 and glucokinase. Implications for metabolic engineering

We have shown that intermediate lobe (IL) pituitary cells can be engineered to produce sufficient amounts of insulin (ins) to cure diabetes in nonobese diabetic mice but, unlike transplanted islets, ILins cells evade immune attack. To confer glucose-sensing capabilities into these cells, they were further modified with recombinant adenoviruses to express high levels of GLUT2 and the beta-cell isoform of glucokinase (GK). Although expression of GLUT2 alone had negligible effects on glucose usage and lactate production, expression of GK alone resulted in approximately 2-fold increase in glycolytic flux within the physiological (3-20 mm) glucose range. GLUT2/GK coexpression further increased glycolytic flux at 20 mm glucose but disproportionately increased flux at 3 mm glucose. Despite enhanced glycolytic fluxes, GLUT2/GK-coexpressing cells showed glucose dose-dependent accumulation of hexose phosphates, depletion of intracellular ATP, and severe apoptotic cell death. These studies demonstrate that glucose-sensing properties can be introduced into non-islet cells by the single expression of GK and that glucose responsiveness can be augmented by the coexpression of GLUT2. However, in the metabolic engineering of surrogate beta cells, it is critical that the levels of the components be closely optimized to ensure their physiological function and to avoid the deleterious consequences of glucose-induced toxicity.

Studies of phospholipid metabolism, proliferation, and secretion of stably transfected insulinoma cells that overexpress group VIA phospholipase A2

A cytosolic 84 kDa Group VIA phospholipase A2 (iPLA2beta) that does not require Ca2+ for catalysis was cloned from Chinese hamster ovary (CHO) cells, murine P388D1 cells, pancreatic islet beta-cells, and other sources. Proposed iPLA2beta functions include participation in phosphatidylcholine (PC) homeostasis by degrading excess PC generated in CHO cells that overexpress CTP:phosphocholine cytidylyltransferase (CT), which catalyzes the rate-limiting step in PC biosynthesis; participation in biosynthesis of arachidonate-containing PC species in P388D1 cells by generating lysophosphatidylcholine (LPC) acceptors for arachidonate incorporation; and participation in signaling events in insulin secretion from islet beta-cells. To further examine iPLA2beta functions in beta-cells, we prepared stably transfected INS-1 insulinoma cell lines that overexpress iPLA2beta activity eightfold compared to parental INS-1 cells or to INS-1 cells transfected with an empty retroviral vector that did not contain iPLA2beta cDNA. The iPLA2beta-overexpressing cells exhibit a twofold increase in CT activity compared to parental cells but little change in rates of [3H]choline incorporation into or disappearance from PC. Electrospray ionization (ESI) tandem mass spectrometric measurements indicate that iPLA2beta-overexpressing cells have 1.5-fold higher LPC levels than parental INS-1 cells but do not exhibit increased rates of [3H]arachidonate incorporation into phospholipids, and incorporation is unaffected by a bromoenol lactone (BEL) suicide substrate inhibitor of iPLA2beta. The rate of appearance of arachidonate-containing phosphatidylethanolamine species visualized by ESI mass spectrometry is also similar in iPLA2beta-overexpressing and parental INS-1 cells incubated with supplemental arachidonic acid, and this process is unaffected by BEL. Compared to parental INS-1 cells, iPLA2beta-overexpressing cells proliferate more rapidly and exhibit amplified insulin secretory responses to a protein kinase C-activating phorbol ester, glucose, and a cAMP analog. These findings suggest that iPLA2beta plays a signaling role in beta-cells that differs from housekeeping functions in PC biosynthesis and degradation in P388D1 and CHO cells.

Studies of insulin secretory responses and of arachidonic acid incorporation into phospholipids of stably transfected insulinoma cells that overexpress group VIA phospholipase A2 (iPLA2beta ) indicate a signaling rather than a housekeeping role for iPLA2beta

A cytosolic 84-kDa group VIA phospholipase A(2) (iPLA(2)beta) that does not require Ca(2+) for catalysis has been cloned from several sources, including rat and human pancreatic islet beta-cells and murine P388D1 cells. Many potential iPLA(2)beta functions have been proposed, including a signaling role in beta-cell insulin secretion and a role in generating lysophosphatidylcholine acceptors for arachidonic acid incorporation into P388D1 cell phosphatidylcholine (PC). Proposals for iPLA(2)beta function rest in part on effects of inhibiting iPLA(2)beta activity with a bromoenol lactone (BEL) suicide substrate, but BEL also inhibits phosphatidate phosphohydrolase-1 and a group VIB phospholipase A(2). Manipulation of iPLA(2)beta expression by molecular biologic means is an alternative approach to study iPLA(2)beta functions, and we have used a retroviral construct containing iPLA(2)beta cDNA to prepare two INS-1 insulinoma cell clonal lines that stably overexpress iPLA(2)beta. Compared with parental INS-1 cells or cells transfected with empty vector, both iPLA(2)beta-overexpressing lines exhibit amplified insulin secretory responses to glucose and cAMP-elevating agents, and BEL substantially attenuates stimulated secretion. Electrospray ionization mass spectrometric analyses of arachidonic acid incorporation into INS-1 cell PC indicate that neither overexpression nor inhibition of iPLA(2)beta affects the rate or extent of this process in INS-1 cells. Immunocytofluorescence studies with antibodies directed against iPLA(2)beta indicate that cAMP-elevating agents increase perinuclear fluorescence in INS-1 cells, suggesting that iPLA(2)beta associates with nuclei. These studies are more consistent with a signaling than with a housekeeping role for iPLA(2)beta in insulin-secreting beta-cells.

Identification of glucokinase mutation in subjects with post-renal transplantation diabetes mellitus

Mutations in the glucokinase (GCK) gene are considered to be a possible cause of maturity-onset diabetes of the young. The purpose of this study was to evaluate the contribution of this gene to the development of post-renal transplantation diabetes mellitus (PTDM). Identification of the GCK mutation was attempted in 58 selected renal allograft recipients with PTDM and 45 normal controls. The exons in the GCK gene were examined using polymerase chain reaction (PCR), followed by an analysis of single-stranded DNA conformational polymorphism (SSCP). The abnormal bands were then confirmed by DNA sequencing analysis. The family members of the patients affected with GCK mutation were also examined. Two of the 58 PTDM patients (3. 4%) were found to have GCK mutations. One had the mutation on exon 5 and the other on intron 7. One control subject had the mutation on intron 9. The mutation on exon 5 was identified as a substitution of CCT (proline) for CTT (leucine) at codon 164, which has never been reported before. The family members of the PTDM patient with a mutation on exon 5 were analyzed by PCR, followed by SSCP, and two of them had the same mutation. The abnormal band seen on SSCP analysis of exon 7 was identified as the C-->T substitution at the 39th nucleotide in intron 7. Two of the family members also displayed the same bands on the SSCP. One of the 45 normal controls had a known polymorphism located at the 8th nucleotide in intron 9. We found a GCK mutation on the exon in subjects with PTDM and we speculate that this mutation may be one of the possible contributing factors of PTDM, although variations of the GCK gene are not common causes of PTDM.

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.

Glucose-responsive gene delivery in pancreatic Islet cells via recombinant adeno-associated viral vectors

PURPOSE

Recent progress in genetic engineering presents the possibility of providing physiologically regulated glucose metabolism in individuals with diabetes. The objective of this study is to explore the feasibility of obtaining glucose dependent gene expression in the pancreatic beta-cell lines via recombinant adeno-associated virus type 2 (rAAV) mediated gene transfer.

METHODS

Two transcription cassettes containing the luciferase gene under the control of the rat insulin I gene promoter and the enhanced green fluorescent protein (EGFP) open reading frame under the control of the immediate early gene promoter of human cytomegalovirus (CMV) were placed in series between the inverted terminal repeats (ITRs) of AAV. The rAAV vectors produced were used to transduce pancreatic beta-cell line grown in the absence or presence of various concentrations of glucose. Luciferase activity assays were performed at 72 hr post-transduction.

RESULTS

Glucose-responsive reporter gene expression was obtained in both calcium phosphate transfected HIT-T15 and betaHC-9 cells, demonstrating regulated luciferase gene expression under control of the insulin gene promoter. At MOI of 100, rAAV-transduced betaHC-9 cells exhibited glucose-dependent luciferase activities, which were approximately 4.3 fold higher than those transfected by the calcium phosphate coprecipitation method at 20 mM glucose.

CONCLUSIONS

Delivery of the insulin gene promoter via rAAV was shown in this study to result in glucose-dependent control of the reporter gene expression. The results suggest that rAAV is an efficient viral vector for gene transfer into the pancreatic islet cells.

Functional glucokinase isoforms are expressed in rat brain

Recently, the description of glucokinase mRNA in certain neuroendocrine cells has opened new ways to characterize this enzyme in the rat brain. In this study, we found glucokinase mRNA and a similar RNA splicing pattern of the glucokinase gene product in rat hypothalamus and pancreatic islets; the mRNA that codes for B1 isoform was the most abundant, with minor amounts of those coding for the B2, P1, P2, P1/B2, and P2/B2 isoforms. Glucokinase gene expression in rat brain gave rise to a protein of 52 kDa with a high apparent Km for glucose and no product inhibition by glucose 6-phosphate, with a contribution to the total glucose phosphorylating activity of between 40 and 14%; the hypothalamus and cerebral cortex were the regions of maximal activity. Low and high Km hexokinases were characterized by several criteria. Also, using RT-PCR analysis we found a glucokinase regulatory protein mRNA similar to that previously reported in liver. These findings indicate that the glucokinase present in rat brain should facilitate the adaptation of this organ to fluctuations in blood glucose concentrations, and the expression of glucokinase and GLUT-2 in the same hypothalamic neurons suggests a role in glucose sensing.

An islet-specific CD8+ T cell hybridoma generated from non-obese diabetic mice recognizes insulin as an autoantigen

Although CD8+ T cells play a major role in beta cell destruction in insulin-dependent diabetes in the non-obese diabetic mouse, the T cell autoantigen(s) recognized by such cells remains to be identified. Therefore, an islet-reactive, CD8+ T cell line was generated from islet-infiltrating cells and hybridized by fusion with a CD8+ alphabeta TCR- BW5147 thymoma. In the presence of islets, none of the 12 CD3+ CD8+ T cell hybridomas isolated secreted IL-2/IL-4 or IFNgamma but three were islet specific, as shown by activation induced cell death. Subclone 4A7.7.15 recognized only islets expressing H-2Kd, demonstrated islet-specific inhibition of proliferation and concomitant partial arrest in the G2/M phase of the cell cycle. Further analysis using a panel of cell lines, expressing H-2Kd, and transfected with the cDNA for various putative autoantigens in type 1 diabetes showed that 4A7.7.15 recognizes insulin as an antigen.

Engineering of a glucose-responsive surrogate cell for insulin replacement therapy of experimental insulin-dependent diabetes

Glucose responsiveness in the millimolar concentration range is a crucial requirement of a surrogate pancreatic beta cell for insulin replacement therapy of insulin-dependent diabetes. Novel insulin-secreting GK cell clones with millimolar glucose responsiveness were generated from an early-passage glucose-unresponsive RINm5F cell line. This line expressed constitutively both the K(ATP) channel and the GLUT2 glucose transporter; but it had a relative lack of glucokinase. Through overexpression of glucokinase, however, it was possible to generate glucose-responsive clones with a glucokinase-to-hexokinase ratio comparable to that of a normal pancreatic beta cell. This aim, on the other hand, was not achieved through overexpression of the GLUT2 glucose transporter. Raising the expression level of this glucose transporter into the range of rat liver, without correcting the glucokinase-to-hexokinase enzyme ratio, did not render the cells glucose responsive. These glucokinase-overexpressing RINm5F cells also stably maintained their molecular and insulin secretory characteristics in vivo. After implantation into streptozotocin diabetic immunodeficient rats, glucokinase-overexpressing cells retained their insulin responsiveness to physiological glucose stimulation under in vivo conditions. These cells represent a notable step toward the future bioengineering of a surrogate beta cell for insulin replacement therapy in insulin-dependent diabetes mellitus.

Metabolic signals trigger glucose-induced inactivation of maltose permease in Saccharomyces

Organisms such as Saccharomyces capable of utilizing several different sugars selectively ferment glucose when less desirable carbon sources are also available. This is achieved by several mechanisms. Glucose down-regulates the transcription of genes involved in utilization of these alternate carbon sources. Additionally, it causes posttranslational modifications of enzymes and transporters, leading to their inactivation and/or degradation. Two glucose sensing and signaling pathways stimulate glucose-induced inactivation of maltose permease. Pathway 1 uses Rgt2p as a sensor of extracellular glucose and causes degradation of maltose permease protein. Pathway 2 is dependent on glucose transport and stimulates degradation of permease protein and very rapid inactivation of maltose transport activity, more rapid than can be explained by loss of protein alone. In this report, we characterize signal generation through pathway 2 using the rapid inactivation of maltose transport activity as an assay of signaling activity. We find that pathway 2 is dependent on HXK2 and to a lesser extent HXK1. The correlation between pathway 2 signaling and glucose repression suggests that these pathways share common upstream components. We demonstrate that glucose transport via galactose permease is able to stimulate pathway 2. Moreover, rapid transport and fermentation of a number of fermentable sugars (including galactose and maltose, not just glucose) are sufficient to generate a pathway 2 signal. These results indicate that pathway 2 responds to a high rate of sugar fermentation and monitors an intracellular metabolic signal. Production of this signal is not specific to glucose, glucose catabolism, glucose transport by the Hxt transporters, or glucose phosphorylation by hexokinase 1 or 2. Similarities between this yeast glucose sensing pathway and glucose sensing mechanisms in mammalian cells are discussed.

Genetic engineering of glucose-stimulated insulin secretion in Chinese hamster ovary cells

To engineer an a non-islet cell capable of glucose-stimulated insulin secretion, a chinese hamster ovary cell line (CHO) was transfected with a mammalian expression vector carrying the human insulin cDNA (pCB/hINS). More proinsulin than insulin was released daily by the stably transformed cell line (CHO-INS). Examination of acid-ethanol extracts confirmed that both insulin and proinsulin were stored. Immunohistochemical analysis of the cells also showed that (pro)insulin was stored. Unlike beta cells, CHO-INS cells did not secrete insulin in response to glucose. To investigate this lack of effect, we examined whether transfection of GLUT2 cDNA, which is ordinarily not expressed in CHO-INS cells, would confer glucose-stimulated insulin secretion. Consequently, we have demonstrated that glucose regulated insulin release occurs in the CHO-INS-GLUT2 cell line and that glucose potentiates the insulin secretory response to non-glucose secretagogues.

Dissociation of early glucocorticoid inhibition of ACTH secretion and glucose uptake in mouse AtT20 D16:16 corticotrophs

Adrenal glucocorticoid hormones rapidly exert powerful effects on neurons, immune and neuroendocrine cells through induction of de novo protein synthesis. In this study, we investigated, using mouse clonal anterior pituitary AtT20 D16:16 corticotrophs, whether (i) glucocorticoids rapidly inhibit glucose transport and (ii) whether this inhibition of glucose transport is directly correlated with early inhibition of ACTH secretion. Glucose uptake in AtT20 D16:16 cells was Na+-independent because the Na+-independent glucose transport inhibitor phloretin (100 microM) completely inhibited specific 14C-deoxygluose (DoG) uptake and replacement of extracellular Na+ with N-methyl D-glucamine+ had no effect. Furthermore, the Na+-independent glucose transporters, GLUTs 1 and 3 were expressed in AtT20 D16:16 cells. The synthetic type II glucocorticoid receptor agonist dexamethasone, rapidly, within 2 h, inhibited DoG uptake into AtT20 D16:16 cells through a mechanism that was dependent on de novo mRNA synthesis. Glucocorticoid inhibition of glucose transport was not correlated with early inhibition of ACTH secretion because removal of glucose from the external medium had no effect on CRF-stimulated ACTH secretion or the efficacy of early glucocorticoid inhibition of ACTH release. Although the Na+-independent glucose transport inhibitor phloretin significantly inhibited CRF-stimulated ACTH release, this effect of phloretin was a result of its potent activation of large conductance calcium-activated potassium (BK) channels. These data suggest that different molecular pathways and/or glucocorticoid induced proteins underlie the mechanism(s) of early glucocorticoid inhibition of glucose uptake and ACTH release, respectively.

Role of the GLUT 2 glucose transporter in the response of the L-type pyruvate kinase gene to glucose in liver-derived cells

Twenty-six different hepatoma cell lines established from cancer-prone transgenic mice exhibited a close correlation between expression of the GLUT 2 glucose transporter and activation of the L-type pyruvate kinase (L-PK) gene by glucose, as judged by Northern blot analyses and transient transfection assays. The L-PK gene and a transfected L-PK construct were silent in GLUT 2(+) cells and active in GLUT 2(-) cells cultured in glucose-free medium. Transfection of GLUT 2(-) cells with a GLUT 2 expression vector restored the inducibility of the L-PK promoter by glucose, mainly by suppressing the glucose-independent activity of this promoter. Culture of GLUT 2(-) cells, in which the L-PK gene is constitutively expressed, in a culture medium using fructose as fuel selected GLUT 2(+) clones in which the L-PK gene responded to glucose. The expression of the L-PK gene in GLUT 2(-) cells cultured in the absence of glucose was correlated with a high intracellular glucose 6-phosphate (Glu-6-P) concentration while under similar culture conditions Glu-6-P concentration was very low in GLUT 2(+) cells. Consequently, a role of GLUT 2 in the glucose responsiveness of glucose-sensitive genes in cultured hepatoma cells could be to allow for Glu-6-P depletion under gluconeogenic culture conditions. In the absence of GLUT 2, glucose endogeneously produced might be unable to be exported from the cells and would be phosphorylated again to Glu-6-P by constitutively expressed hexokinase isoforms, continuously generating the glycolytic intermediates active on the L-PK gene transcription.

Constant delivery of proinsulin by encapsulation of transfected cells

Gene therapy is a potentially excellent approach for the treatment of diabetes instead of pancreas and islet transplantation. However, one difficulty involved in gene therapy for diabetes is a control of insulin/proinsulin production by the cells transfected with insulin cDNA. The purpose of this study is to examine whether control of the proliferation of transfected cells by encapsulation is a feasible approach for the constant delivery of proinsulin to avoid a life-threatening hypoglycemic state. Previously, we established a mouse fibroblast Ltk- cells transfected with human insulin cDNA producing human proinsulin (91 ng/24 hr/10(6) cells). These cells were encapsulated with semipermeable 5% agarose gel and proinsulin production was examined by in vitro long-term culture system. Intraperitoneal implantation into streptozocin (STZ)-induced diabetic mice was performed to investigate in vivo function of the encapsulated cells. The data from the in vitro study demonstrated that encapsulation of 2 x 10(6) transfectants enabled the stable production of proinsulin for over 80 days (204.4 +/- 5.18 ng/ml/day). Implantation of the encapsulated 2 x 10(7) transfectants improved the hyperglycemic state of diabetic mice for 30 days on the mean value of blood glucose concentration (n = 20). Histological analysis revealed pericapsular inflammation at 30 days after implantation and this may result in malfunction of encapsulated cells. Constant production and delivery of proinsulin could be achieved by encapsulating the human insulin cDNA-transfected cells using 5% agarose. Control of the proliferation of transfected cells appears to be an important factor for constant delivery of human proinsulin toward gene therapy of diabetes mellitus.

Construction of recombinant adeno-associated virus vector containing the rat preproinsulin II gene

We have investigated a possible delivery system for the rat preproinsulin II gene (rI2) utilising a recombinant adeno-associated virus (rAAV) vector system, with the long-term goal of engineering stably infected insulin-producing cell lines. The rAAV vector was chosen because it is a safe and nonpathogenic method for gene transfer. The plasmid pBC12BI (ATCC) was purified and digested with restriction enzymes SepI and StuI to release a fragment containing the Rous sarcoma virus long terminal repeat (RSV-LTR) promoter-driven rat preproinsulin II gene (rI2). Subsequently, the RSV-rI2 gene fragment was cloned into the BamHI site of rAAV vector plasmid pWP-19 to produce the rI2 recombinant plasmid designated pLP-1. The pWP-19 also encodes the AAV inverted terminal repeats for integration and replication and the herpes virus thymidine kinase promoter-driven gene for neomycin resistance (neoR). The cell line 293 (ATCC) was then cotransfected with pLP-1 and helper plasmid pAAV/AD, which is required for viral replication. The rAAV genome, now containing rI2, was rescued using adenovirus and packaged into mature AAV virions termed vLP-1. Finally, human pancreatic adenocarcinoma cells (HPAC; ATCC) were exposed to vLP-1, selected for G418 resistance, and screened for insulin production. Successful rescue was confirmed by Southern blot analysis using the rI2 gene probe derived from the original plasmid. The final titer of 1.25 x 10(9) particles/ ml was determined by DNA slot blots using pLP-1 as the standard, HPAC cells were infected with vLP-1 (termed HPAC/rI2). Integration of the rI2 genome in G418-resistant clones was confirmed by Southern blot analysis and again after 6 months in culture by amplification of the rI2 gene by PCR. Insulin gene transcription was confirmed by RT-PCR. We have developed a rAAV-mediated gene transfer system for the rat preproinsulin II gene. Successful transduction and stable integration of rI2 into HPAC was achieved. Production of insulin by HPAC/rI2 was confirmed by RIA and RT-PCR, validating this system as an effective approach to experimental gene therapy.

Overexpressed syntaxin 1A/HPC-1 inhibits insulin secretion via a regulated pathway, but does not influence glucose metabolism and intracellular Ca2+ in insulinoma cell line beta TC3 cells

We have previously established a stable beta TC3 cell line that overexpresses syntaxin 1A, designated beta TC-hpc1 cells, in which glucose-stimulated insulin release was decreased. Using beta TC-hpc1 cells, we aimed to determine whether syntaxin 1A functions in the regulatory or constitutive pathway of insulin release. We therefore examined the secretion of phorbol-12-myristate-13-acetate (TPA)-stimulated newly synthesized proinsulin/insulin and total immunoreactive insulin. beta TC3 and beta TC-hpc1 cells were simultaneously pulse-labeled with 3H-leucine for 30 min in 11 mM glucose and chased for 1 h in one of a number of different concentrations of TPA in 11 mM glucose. Total immunoreactive insulin release (IRI) by both cell types during the chase period was markedly increased by the addition of TPA in a dose-dependent manner; however, the IRI from beta TC-hpc1 cells was lower than that from beta TC3 cells. The secretion of newly synthesized proinsulin/insulin from both cell types, which in beta TC3 cells is thought to occur via a constitutive pathway, was in the same range under any condition. Thus, the evidence indicates that syntaxin 1A preferentially functions in the regulated insulin release pathway in beta TC3 cells. In order to clarify the effect of overexpressed syntaxin 1A on glucose metabolism and intracellular Ca2+ we analyzed the glucose transport system, glucose phosphorylation activity, and cytosolic concentration of free Ca2+ ([Ca2+]i). 2-Deoxy-glucose uptake and the content of GLUT1 protein in the plasma membrane fractions of beta TC-hpc1 cells were not different from those of beta TC3 cells. Radiometric assays of glucose phosphorylation activity showed that there were no differences in hexokinase activity and glucokinase activity between beta TC3 and beta TC-hpc1 cells. [Ca2+]i measured by using fura 2 demonstrated that there was no difference in [Ca2+]i between beta TC3 and beta TC-hpc 1 cells under glucose-stimulated conditions. The present experiments indicate that syntaxin 1A plays a central role in a late step of the regulatory insulin release pathway without a change in glucose metabolism and [Ca2+]i in beta TC3 cells.

Differential metabolic effects of adenovirus-mediated glucokinase and hexokinase I overexpression in rat primary hepatocytes

The first step of glucose metabolism is the phosphorylation of glucose, catalyzed by the hexokinase family of enzymes. To address the metabolic impact of increasing glucose phosphorylation capacity in liver, rat primary hepatocytes were treated with recombinant adenoviruses containing the cDNAs encoding either rat liver glucokinase (AdCMV-GKL) or rat hexokinase I (AdCMV-HKI). Maximal glucose phosphorylation in AdCMV-GKL- and AdCMV-HKI-treated hepatocytes was increased 7.1 +/- 1.2- and 6.3 +/- 0.8-fold, respectively, over hepatocytes treated with an adenovirus expressing beta-galactosidase. Glucose usage (measured with 3 and 20 m 2-[3H]glucose and 5-[3H]glucose) was significantly increased in AdCMV-GKL-treated cells preincubated in 1 or 25 mM glucose. Treatment of hepatocytes with AdCMV-HKI also caused enhanced glucose utilization, but the increases were smaller and were less apparent in cells preincubated in high (25 mM) glucose. AdCMV-GKL-treated hepatocytes incubated for 48 h in the presence of variable glucose concentrations had glycogen levels that were maximally 15.0 +/- 0. 6-fold greater than levels in corresponding control cells. AdCMV-HKI-treated hepatocytes incubated under similar conditions had unchanged glycogen levels relative to controls. In AdCMV-GKL-treated cells, lactate output was increased to a maximum of 3.0 +/- 0.4-fold (at 25 mM glucose), glucose oxidation was increased 3.5 +/- 0.3-fold, and triglyceride production was unchanged relative to untreated cells. Among these three parameters, only lactate production was increased in AdCMV-HKI-treated cells, and then only at low glucose concentrations. We conclude that overexpression of glucokinase has potent effects on glucose storage and utilization in hepatocytes and that these effects are not matched by overexpression of hexokinase I.

Insulin-secreting non-islet cells are resistant to autoimmune destruction

Transgenic nonobese diabetic mice were created in which insulin expression was targeted to proopiomelanocortin-expressing pituitary cells. Proopiomelanocortin-expressing intermediate lobe pituitary cells efficiently secrete fully processed, mature insulin via a regulated secretory pathway, similar to islet beta cells. However, in contrast to the insulin-producing islet beta cells, the insulin-producing intermediate lobe pituitaries are not targeted or destroyed by cells of the immune system. Transplantation of the transgenic intermediate lobe tissues into diabetic nonobese diabetic mice resulted in the restoration of near-normoglycemia and the reversal of diabetic symptoms. The absence of autoimmunity in intermediate lobe pituitary cells engineered to secrete bona fide insulin raises the potential of these cell types for beta-cell replacement therapy for the treatment of insulin-dependent diabetes mellitus.