Circulating Follistatin Is Liver-Derived and Regulated by the Glucagon-to-Insulin Ratio

CONTEXT

Follistatin is a plasma protein recently reported to increase under conditions with negative energy balance, such as exercise and fasting in humans. Currently, the perception is that circulating follistatin is a result of para/autocrine actions from various tissues. The large and acute increase in circulating follistatin in response to exercise suggests that it may function as an endocrine signal.

OBJECTIVE

We assessed origin and regulation of circulating follistatin in humans.

DESIGN/INTERVENTIONS

First, we assessed arterial-to-venous difference of follistatin over the splanchnic bed at rest and during exercise in healthy humans. To evaluate the regulation of plasma follistatin we manipulated glucagon-to-insulin ratio in humans at rest as well as in cultured hepatocytes. Finally, the impact of follistatin on human islets of Langerhans was assessed.

RESULTS

We demonstrate that in humans the liver is a major contributor to circulating follistatin both at rest and during exercise. Glucagon increases and insulin inhibits follistatin secretion both in vivo and in vitro, mediated via the secondary messenger cAMP in the hepatocyte. Short-term follistatin treatment reduced glucagon secretion from islets of Langerhans, whereas long-term follistatin treatment prevented apoptosis and induced proliferation of rat β cells.

CONCLUSIONS

In conclusion, in humans, the liver secretes follistatin at rest and during exercise, and the glucagon-to-insulin ratio is a key determinant of circulating follistatin levels. Circulating follistatin may be a marker of the glucagon-to-insulin tone on the liver.

IL-13 improves beta-cell survival and protects against IL-1beta-induced beta-cell death

Objectives

IL-13 is a cytokine classically produced by anti-inflammatory T-helper-2 lymphocytes; it is decreased in the circulation of type 2 diabetic patients and impacts positively on liver and skeletal muscle. Although IL-13 can exert positive effects on beta-cell lines, its impact and mode of action on primary beta-cell function and survival remain largely unexplored.

Methods

Beta-cells were cultured for 48 h in the presence of IL-13 alone or in combination with IL-1β or cytokine cocktail (IL-1β, IFNγ, TNFα).

Results

IL-13 protected human and rat beta-cells against cytokine induced death. However, IL-13 was unable to protect from IL-1β impaired glucose stimulated insulin secretion and did not influence NFκB nuclear relocalization induced by IL-1β. IL-13 induced phosphorylation of Akt, increased IRS2 protein expression and counteracted the IL-1β induced regulation of several beta-cell stress response genes.

Conclusions

The prosurvival effects of IL-13 thus appear to be mediated through IRS2/Akt signaling with NFκB independent regulation of gene expression. In addition to previously documented beneficial effects on insulin target tissues, these data suggest that IL-13 may be useful for treatment of type 2 diabetes by preserving beta-cell mass or slowing its rate of decline.

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IL-13 decreases human beta-cells apoptosis.

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IL-13 protects primary beta-cells form cytokine induced apoptosis.

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The IRS2/akt pathway mediates IL-13 protective effects.

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IL-13 modulates the expression of genes involved in stress response.

The skeleton in the closet: actin cytoskeletal remodeling in β-cell function

Over the last few decades, biomedical research has considered not only the function of single cells but also the importance of the physical environment within a whole tissue, including cell-cell and cell-extracellular matrix interactions. Cytoskeleton organization and focal adhesions are crucial sensors for cells that enable them to rapidly communicate with the physical extracellular environment in response to extracellular stimuli, ensuring proper function and adaptation. The involvement of the microtubular-microfilamentous cytoskeleton in secretion mechanisms was proposed almost 50 years ago, since when the evolution of ever more sensitive and sophisticated methods in microscopy and in cell and molecular biology have led us to become aware of the importance of cytoskeleton remodeling for cell shape regulation and its crucial link with signaling pathways leading to β-cell function. Emerging evidence suggests that dysfunction of cytoskeletal components or extracellular matrix modification influences a number of disorders through potential actin cytoskeleton disruption that could be involved in the initiation of multiple cellular functions. Perturbation of β-cell actin cytoskeleton remodeling could arise secondarily to islet inflammation and fibrosis, possibly accounting in part for impaired β-cell function in type 2 diabetes. This review focuses on the role of actin remodeling in insulin secretion mechanisms and its close relationship with focal adhesions and myosin II.

50 years forward: beta cells

Our understanding of beta cell development and function has increased substantially these past 50 years but much remains to be learned before this knowledge can be put to clinical use. A comprehensive business plan will be necessary to develop a detailed molecular and functional blueprint of the beta cell in health and disease based on an integrated approach involving all necessary research disciplines. This blueprint will provide a platform for the development of novel therapeutic strategies for the treatment of both major forms of diabetes, foremost among them beta cell replacement therapy. This is one of a series of commentaries under the banner '50 years forward', giving personal opinions on future perspectives in diabetes, to celebrate the 50th anniversary of Diabetologia (1965-2015).

Short term exposure of beta cells to low concentrations of interleukin-1β improves insulin secretion through focal adhesion and actin remodeling and regulation of gene expression

Type 2 diabetes involves defective insulin secretion with islet inflammation governed in part by IL-1β. Prolonged exposure of islets to high concentrations of IL-1β (>24 h, 20 ng/ml) impairs beta cell function and survival. Conversely, exposure to lower concentrations of IL-1β for >24 h improves these same parameters. The impact on insulin secretion of shorter exposure times to IL-1β and the underlying molecular mechanisms are poorly understood and were the focus of this study. Treatment of rat primary beta cells, as well as rat or human whole islets, with 0.1 ng/ml IL-1β for 2 h increased glucose-stimulated (but not basal) insulin secretion, whereas 20 ng/ml was without effect. Similar differential effects of IL-1β depending on concentration were observed after 15 min of KCl stimulation but were prevented by diazoxide. Studies on sorted rat beta cells indicated that the enhancement of stimulated secretion by 0.1 ng/ml IL-1β was mediated by the NF-κB pathway and c-JUN/JNK pathway acting in parallel to elicit focal adhesion remodeling and the phosphorylation of paxillin independently of upstream regulation by focal adhesion kinase. Because the beneficial effect of IL-1β was dependent in part upon transcription, gene expression was analyzed by RNAseq. There were 18 genes regulated uniquely by 0.1 but not 20 ng/ml IL-1β, which are mostly involved in transcription and apoptosis. These results indicate that 2 h of exposure of beta cells to a low but not a high concentration of IL-1β enhances glucose-stimulated insulin secretion through focal adhesion and actin remodeling, as well as modulation of gene expression.

β-cell failure in type 2 diabetes: postulated mechanisms and prospects for prevention and treatment

OBJECTIVE

This article examines the foundation of β-cell failure in type 2 diabetes (T2D) and suggests areas for future research on the underlying mechanisms that may lead to improved prevention and treatment.

RESEARCH DESIGN AND METHODS

A group of experts participated in a conference on 14-16 October 2013 cosponsored by the Endocrine Society and the American Diabetes Association. A writing group prepared this summary and recommendations.

RESULTS

The writing group based this article on conference presentations, discussion, and debate. Topics covered include genetic predisposition, foundations of β-cell failure, natural history of β-cell failure, and impact of therapeutic interventions.

CONCLUSIONS

β-Cell failure is central to the development and progression of T2D. It antedates and predicts diabetes onset and progression, is in part genetically determined, and often can be identified with accuracy even though current tests are cumbersome and not well standardized. Multiple pathways underlie decreased β-cell function and mass, some of which may be shared and may also be a consequence of processes that initially caused dysfunction. Goals for future research include to 1) impact the natural history of β-cell failure; 2) identify and characterize genetic loci for T2D; 3) target β-cell signaling, metabolic, and genetic pathways to improve function/mass; 4) develop alternative sources of β-cells for cell-based therapy; 5) focus on metabolic environment to provide indirect benefit to β-cells; 6) improve understanding of the physiology of responses to bypass surgery; and 7) identify circulating factors and neuronal circuits underlying the axis of communication between the brain and β-cells.

β-cell failure in type 2 diabetes: postulated mechanisms and prospects for prevention and treatment

OBJECTIVE

This article examines the foundation of β-cell failure in type 2 diabetes (T2D) and suggests areas for future research on the underlying mechanisms that may lead to improved prevention and treatment.

RESEARCH DESIGN AND METHODS

A group of experts participated in a conference on 14-16 October 2013 cosponsored by the Endocrine Society and the American Diabetes Association. A writing group prepared this summary and recommendations.

RESULTS

The writing group based this article on conference presentations, discussion, and debate. Topics covered include genetic predisposition, foundations of β-cell failure, natural history of β-cell failure, and impact of therapeutic interventions.

CONCLUSIONS

β-Cell failure is central to the development and progression of T2D. It antedates and predicts diabetes onset and progression, is in part genetically determined, and often can be identified with accuracy even though current tests are cumbersome and not well standardized. Multiple pathways underlie decreased β-cell function and mass, some of which may be shared and may also be a consequence of processes that initially caused dysfunction. Goals for future research include to (1) impact the natural history of β-cell failure; (2) identify and characterize genetic loci for T2D; (3) target β-cell signaling, metabolic, and genetic pathways to improve function/mass; (4) develop alternative sources of β-cells for cell-based therapy; (5) focus on metabolic environment to provide indirect benefit to β-cells; (6) improve understanding of the physiology of responses to bypass surgery; and (7) identify circulating factors and neuronal circuits underlying the axis of communication between the brain and β-cells.

Cell-type, allelic, and genetic signatures in the human pancreatic beta cell transcriptome

Elucidating the pathophysiology and molecular attributes of common disorders as well as developing targeted and effective treatments hinges on the study of the relevant cell type and tissues. Pancreatic beta cells within the islets of Langerhans are centrally involved in the pathogenesis of both type 1 and type 2 diabetes. Describing the differentiated state of the human beta cell has been hampered so far by technical (low resolution microarrays) and biological limitations (whole islet preparations rather than isolated beta cells). We circumvent these by deep RNA sequencing of purified beta cells from 11 individuals, presenting here the first characterization of the human beta cell transcriptome. We perform the first comparison of gene expression profiles between beta cells, whole islets, and beta cell depleted islet preparations, revealing thus beta-cell–specific expression and splicing signatures. Further, we demonstrate that genes with consistent increased expression in beta cells have neuronal-like properties, a signal previously hypothesized. Finally, we find evidence for extensive allelic imbalance in expression and uncover genetic regulatory variants (eQTLs) active in beta cells. This first molecular blueprint of the human beta cell offers biological insight into its differentiated function, including expression of key genes associated with both major types of diabetes.

Non-muscle myosin IIA is involved in focal adhesion and actin remodelling controlling glucose-stimulated insulin secretion

AIMS/HYPOTHESIS

Actin and focal adhesion (FA) remodelling are essential for glucose-stimulated insulin secretion (GSIS). Non-muscle myosin II (NM II) isoforms have been implicated in such remodelling in other cell types, and myosin light chain kinase (MLCK) and Rho-associated coiled-coil-containing kinase (ROCK) are upstream regulators of NM II, which is known to be involved in GSIS. The aim of this work was to elucidate the implication and regulation of NM IIA and IIB in beta cell actin and FA remodelling, granule trafficking and GSIS.

METHODS

Inhibitors of MLCK, ROCK and NM II were used to study NM II activity, and knockdown of NM IIA and IIB to determine isoform specificity, using sorted primary rat beta cells. Insulin was measured by radioimmunoassay. Protein phosphorylation and subcellular distribution were determined by western blot and confocal immunofluorescence. Dynamic changes were monitored by live cell imaging and total internal reflection fluorescence microscopy using MIN6B1 cells.

RESULTS

NM II and MLCK inhibition decreased GSIS, associated with shortening of peripheral actin stress fibres, and reduced numbers of FAs and insulin granules in close proximity to the basal membrane. By contrast, ROCK inhibition increased GSIS and caused disassembly of glucose-induced central actin stress fibres, resulting in large FAs without any effect on FA number. Only glucose-induced NM IIA reorganisation was blunted by MLCK inhibition. NM IIA knockdown decreased GSIS, levels of FA proteins and glucose-induced extracellular signal-regulated kinase 1/2 phosphorylation.

CONCLUSIONS/INTERPRETATION

Our data indicate that MLCK-NM IIA may modulate translocation of secretory granules, resulting in enhanced insulin secretion through actin and FA remodelling, and regulation of FA protein levels.

Cardiotrophin 1 protects beta cells from apoptosis and prevents streptozotocin-induced diabetes in a mouse model

AIMS/HYPOTHESIS

Cardiotrophin 1 (CT-1) is a recently described cytokine originally isolated from the heart where it has been shown to play an important role in apoptotic protection of cardiomyocytes and heart hypertrophy. Its beneficial properties have also been described in other organs such as liver and neuromuscular tissue. In the present study, we investigated whether CT-1 can confer protection against pro-apoptotic stimuli in pancreatic beta cells, and its role in insulin secretion and diabetes development.

METHODS

The effects of CT-1 on apoptosis and function were studied using MIN6B1 cells and freshly isolated murine pancreatic islets. The impact on the development of diabetes was evaluated in Ct1-null (Ct1 (-/-)) mice (the gene Ct1 is also known as Ctf1) using two streptozotocin (STZ)-induced models of diabetes.

RESULTS

CT-1 has a protective effect in MIN6B1 cells and murine islets under the pro-apoptotic stimulus of serum deprivation, which correlates with the expression of B cell lymphoma-extra large, or following exposure to a mixture of cytokines. In addition, CT-1 enhances glucose-stimulated insulin secretion in MIN6B1 cells and this was repressed by inhibitors of phospholipase C. Furthermore, Ct1 (-/-) mice were more prone to develop diabetes, and their glucose tolerance test showed impaired plasma glucose clearance which correlated with decreased pancreatic insulin secretion.

CONCLUSIONS/INTERPRETATION

The results obtained from both in vitro and in vivo experiments show that CT-1 improves beta cell function and survival, and protects mice against STZ-induced diabetes.

A new paradigm for improved co-ordination and efficacy of European biomedical research: taking diabetes as a model

Today, European biomedical and health-related research is insufficiently well funded and is fragmented, with no common vision, less-than-optimal sharing of resources, and inadequate support and training in clinical research. Improvements to the competitiveness of European biomedical research will depend on the creation of new infrastructures that must be dynamic and free of bureaucracy, involve all stakeholders and facilitate faster delivery of new discoveries from bench to bedside. Taking diabetes research as the model, a new paradigm for European biomedical research is presented, which offers improved co-ordination and common resources that will benefit both academic and industrial clinical research. This includes the creation of a European Council for Health Research, first proposed by the Alliance for Biomedical Research in Europe, which will bring together and consult with all health stakeholders to develop strategic and multidisciplinary research programmes addressing the full innovation cycle. A European Platform for Clinical Research in Diabetes is proposed by the Alliance for European Diabetes Research (EURADIA) in response to the special challenges and opportunities presented by research across the European region, with the need for common standards and shared expertise and data.

Bimodal impact of skeletal muscle on pancreatic β-cell function in health and disease

Diabetes is a complex disease that affects many organs directly or indirectly. Type 2 diabetes mellitus is characterized by insulin resistance with a relative deficiency in insulin secretion. It has become apparent that inter-organ communication is of great importance in the pathophysiology of diabetes. Far from being an inert tissue in terms of inter-organ communication, it is now recognized that skeletal muscle can secrete so-called myokines that can impact on the function of distant organs/tissues both favourably and unfavourably. We have proposed that communication between insulin-resistant skeletal muscle and β-cells occurs in diabetes. This is a novel route of communication that we further suggest is modified by the prevailing degree of insulin resistance of skeletal muscle. This review focuses on the various myokines [interleukin-6 (IL-6), tumor necrosis factor-α, CXCL10, follistatin and IL-8] which have been identified either after different types of exercise or in the secretome from control and insulin-resistant human skeletal myotubes. We will also summarize studies on the impact of several myokines on pancreatic β-cell proliferation, survival and function.

In vitro proliferation of adult human beta-cells

A decrease in functional beta-cell mass is a key feature of type 2 diabetes. Glucagon-like peptide 1 (GLP-1) analogues induce proliferation of rodent beta-cells. However, the proliferative capacity of human beta-cells and its modulation by GLP-1 analogues remain to be fully investigated. We therefore sought to quantify adult human beta-cell proliferation in vitro and whether this is affected by the GLP-1 analogue liraglutide.

Human islets from 7 adult cadaveric organ donors were dispersed into single cells. Beta-cells were purified by FACS. Non-sorted cells and the beta-cell enriched (“beta-cells”) population were plated on extracellular matrix from rat (804G) and human bladder carcinoma cells (HTB9) or bovine corneal endothelial ECM (BCEC). Cells were maintained in culture+/−liraglutide for 4 days in the presence of BrdU.

Rare human beta-cell proliferation could be observed either in the purified beta-cell population (0.051±0.020%; 22 beta-cells proliferating out of 84'283 beta-cells counted) or in the non-sorted cell population (0.055±0.011%; 104 proliferating beta-cells out of 232'826 beta-cells counted), independently of the matrix or the culture conditions. Liraglutide increased human beta-cell proliferation on BCEC in the non-sorted cell population (0.082±0.034% proliferating beta-cells vs. 0.017±0.008% in control, p<0.05).

These results indicate that adult human beta-cell proliferation can occur in vitro but remains an extremely rare event with these donors and particular culture conditions. Liraglutide increases beta-cell proliferation only in the non-sorted cell population and only on BCEC. However, it cannot be excluded that human beta-cells may proliferate to a greater extent in situ in response to natural stimuli.

Novel mechanistic link between focal adhesion remodeling and glucose-stimulated insulin secretion

Actin cytoskeleton remodeling is well known to be positively involved in glucose-stimulated pancreatic β cell insulin secretion. We have observed glucose-stimulated focal adhesion remodeling at the β cell surface and have shown this to be crucial for glucose-stimulated insulin secretion. However, the mechanistic link between such remodeling and the insulin secretory machinery remained unknown and was the major aim of this study. MIN6B1 cells, a previously validated model of primary β cell function, were used for all experiments. Total internal reflection fluorescence microscopy revealed the glucose-responsive co-localization of focal adhesion kinase (FAK) and paxillin with integrin β1 at the basal cell surface after short term stimulation. In addition, blockade of the interaction between β1 integrins and the extracellular matrix with an anti-β1 integrin antibody (Ha2/5) inhibited short term glucose-induced phosphorylation of FAK (Tyr-397), paxillin (Tyr-118), and ERK1/2 (Thr-202/Tyr-204). Pharmacological inhibition of FAK activity blocked glucose-induced actin cytoskeleton remodeling and glucose-induced disruption of the F-actin/SNAP-25 association at the plasma membrane as well as the distribution of insulin granules to regions in close proximity to the plasma membrane. Furthermore, FAK inhibition also completely blocked short term glucose-induced activation of the Akt/AS160 signaling pathway. In conclusion, these results indicate 1) that glucose-induced activation of FAK, paxillin, and ERK1/2 is mediated by β1 integrin intracellular signaling, 2) a mechanism whereby FAK mediates glucose-induced actin cytoskeleton remodeling, hence allowing docking and fusion of insulin granules to the plasma membrane, and 3) a possible functional role for the Akt/AS160 signaling pathway in the FAK-mediated regulation of glucose-stimulated insulin secretion.

Interleukin-6 enhances insulin secretion by increasing glucagon-like peptide-1 secretion from L cells and alpha cells

Exercise, obesity and type 2 diabetes are associated with elevated plasma concentrations of interleukin-6 (IL-6). Glucagon-like peptide-1 (GLP-1) is a hormone that induces insulin secretion. Here we show that administration of IL-6 or elevated IL-6 concentrations in response to exercise stimulate GLP-1 secretion from intestinal L cells and pancreatic alpha cells, improving insulin secretion and glycemia. IL-6 increased GLP-1 production from alpha cells through increased proglucagon (which is encoded by GCG) and prohormone convertase 1/3 expression. In models of type 2 diabetes, the beneficial effects of IL-6 were maintained, and IL-6 neutralization resulted in further elevation of glycemia and reduced pancreatic GLP-1. Hence, IL-6 mediates crosstalk between insulin-sensitive tissues, intestinal L cells and pancreatic islets to adapt to changes in insulin demand. This previously unidentified endocrine loop implicates IL-6 in the regulation of insulin secretion and suggests that drugs modulating this loop may be useful in type 2 diabetes.

Hypercholesterolaemia, signs of islet microangiopathy and altered angiogenesis precede onset of type 2 diabetes in the Goto-Kakizaki (GK) rat

AIMS/HYPOTHESIS

The adult non-obese Goto-Kakizaki (GK) rat model of type 2 diabetes, particularly females, carries in addition to hyperglycaemia a genetic predisposition towards dyslipidaemia, including hypercholesterolaemia. As cholesterol-induced atherosclerosis may be programmed in utero, we looked for signs of perinatal lipid alterations and islet microangiopathy. We hypothesise that such alterations contribute towards defective pancreas/islet vascularisation that might, in turn, lead to decreased beta cell mass. Accordingly, we also evaluated islet inflammation and endothelial activation in both prediabetic and diabetic animals.

METHODS

Blood, liver and pancreas were collected from embryonic day (E)21 fetuses, 7-day-old prediabetic neonates and 2.5-month-old diabetic GK rats and Wistar controls for analysis/quantification of: (1) systemic variables, particularly lipids; (2) cholesterol-linked hepatic enzyme mRNA expression and/or activity; (3) pancreas (fetuses) or collagenase-isolated islet (neonates/adults) gene expression using Oligo GEArray microarrays targeted at rat endothelium, cardiovascular disease biomarkers and angiogenesis, and/or RT-PCR; and (4) pancreas endothelial immunochemistry: nestin (fetuses) or von Willebrand factor (neonates).

RESULTS

Systemic and hepatic cholesterol anomalies already exist in GK fetuses and neonates. Hyperglycaemic GK fetuses exhibit a similar percentage decrease in total pancreas and islet vascularisation and beta cell mass. Normoglycaemic GK neonates show systemic inflammation, signs of islet pre-microangiopathy, disturbed angiogenesis, collapsed vascularisation and altered pancreas development. Concomitantly, GK neonates exhibit elevated defence mechanisms.

CONCLUSIONS/INTERPRETATION

These data suggest an autoinflammatory disease, triggered by in utero programming of cholesterol-induced islet microangiopathy interacting with chronic hyperglycaemia in GK rats. During the perinatal period, GK rats show also a marked deficient islet vascularisation in conjunction with decreased beta cell mass.

DIAMAP: a road map for diabetes research in Europe

The DIAMAP Project, which has drawn up a road map for diabetes research in Europe, has now concluded, and the results are available in the form of a report and searchable databases. The DIAMAP road maps provide strategic guidance for diabetes research activity and investment in Europe, with the person with diabetes and a broad approach to research being integral to the process.

Bimodal effect on pancreatic β-cells of secretory products from normal or insulin-resistant human skeletal muscle

OBJECTIVE

Type 2 diabetes is characterized by insulin resistance with a relative deficiency in insulin secretion. This study explored the potential communication between insulin-resistant human skeletal muscle and primary (human and rat) β-cells.

RESEARCH DESIGN AND METHODS

Human skeletal muscle cells were cultured for up to 24 h with tumor necrosis factor (TNF)-α to induce insulin resistance, and mRNA expression for cytokines was analyzed and compared with controls (without TNF-α). Conditioned media were collected and candidate cytokines were measured by antibody array. Human and rat primary β-cells were used to explore the impact of exposure to conditioned media for 24 h on apoptosis, proliferation, short-term insulin secretion, and key signaling protein phosphorylation and expression.

RESULTS

Human myotubes express and release a different panel of myokines depending on their insulin sensitivity, with each panel exerting differential effects on β-cells. Conditioned medium from control myotubes increased proliferation and glucose-stimulated insulin secretion (GSIS) from primary β-cells, whereas conditioned medium from TNF-α-treated insulin-resistant myotubes (TMs) exerted detrimental effects that were either independent (increased apoptosis and decreased proliferation) or dependent on the presence of TNF-α in TM (blunted GSIS). Knockdown of β-cell mitogen-activated protein 4 kinase 4 prevented these effects. Glucagon-like peptide 1 protected β-cells against decreased proliferation and apoptosis evoked by TMs, while interleukin-1 receptor antagonist only prevented the latter.

CONCLUSIONS

Taken together, these data suggest a possible new route of communication between skeletal muscle and β-cells that is modulated by insulin resistance and could contribute to normal β-cell functional mass in healthy subjects, as well as the decrease seen in type 2 diabetes.

Focal adhesion remodeling is crucial for glucose-stimulated insulin secretion and involves activation of focal adhesion kinase and paxillin

OBJECTIVE

Actin cytoskeleton remodeling is known to be involved in glucose-stimulated insulin secretion (GSIS). We have observed glucose-stimulated changes at the β-cell basal membrane similar to focal adhesion remodeling in cell migration. This led us to study the role of two key focal adhesion proteins, focal adhesion kinase (FAK) and paxillin, in GSIS.

RESEARCH DESIGN AND METHODS

All studies were performed using rat primary β-cells or isolated islets. Protein phosphorylation and subcellular localization were determined by Western blotting and confocal immunofluorescence, respectively. Insulin was measured by radioimmunoassay. Both siRNA and pharmacological approaches were used to assess the role of FAK and paxillin in glucose-stimulated focal adhesion remodeling and insulin secretion.

RESULTS

Glucose stimulation of β-cells in monolayer significantly increased phosphorylation of FAK and paxillin as well as cell surface area. This coincided with the appearance at the basal membrane of numerous shorter actin filopodial extensions, containing not only phosphorylated paxillin, FAK, and extracellular signal-related kinase 1/2 but also two SNARE proteins, synaptosomal-associated protein 25 and syntaxin 1, indicating involvement in exocytosis. SR7037 completely inhibited this sequence of events, indicating the requirement of increased cytosolic Ca²(+). Furthermore, knockdown of paxillin significantly decreased GSIS, as did inhibition of glucose-induced FAK phosphorylation by compound Y15. Key findings were confirmed in β-cells within the natural setting of islets.

CONCLUSIONS

Glucose-stimulated remodeling of focal adhesions and phosphorylation of FAK and paxillin are involved in full development of GSIS, indicating a previously unknown role for focal adhesion remodeling in pancreatic β-cell function.

Regulation of insulin secretion by phosphatidylinositol-4,5-bisphosphate

The role of PIP(2) in pancreatic beta cell function was examined here using the beta cell line MIN6B1. Blocking PIP(2) with PH-PLC-GFP or PIP5KIgamma RNAi did not impact on glucose-stimulated secretion although susceptibility to apoptosis was increased. Over-expression of PIP5KIgamma improved cell survival and inhibited secretion with accumulation of endocytic vacuoles containing F-actin, PIP(2), transferrin receptor, caveolin 1, Arf6 and the insulin granule membrane protein phogrin but not insulin. Expression of constitutively active Arf6 Q67L also resulted in vacuole formation and inhibition of secretion, which was reversed by PH-PLC-GFP co-expression. PIP(2) co-localized with gelsolin and F-actin, and gelsolin co-expression partially reversed the secretory defect of PIP5KIgamma-over-expressing cells. RhoA/ROCK inhibition increased actin depolymerization and secretion, which was prevented by over-expressing PIP5KIgamma, while blocking PIP(2) reduced constitutively active RhoA V14-induced F-actin polymerization. In conclusion, although PIP(2) plays a pro-survival role in MIN6B1 cells, excessive PIP(2) production because of PIP5KIgamma over-expression inhibits secretion because of both a defective Arf6/PIP5KIgamma-dependent endocytic recycling of secretory membrane and secretory membrane components such as phogrin and the RhoA/ROCK/PIP5KIgamma-dependent perturbation of F-actin cytoskeleton remodelling.

Cytokine production by islets in health and diabetes: cellular origin, regulation and function

Islets produce a variety of cytokines and chemokines in response to physiologic and pathologic stimulation by nutrients. The cellular source of these inflammatory mediators includes alpha-, beta-, endothelial-, ductal- and recruited immune cells. Islet-derived cytokines promote alpha- and beta-cell adaptation and repair in the short term. Eventually, chronic metabolic stress can induce a deleterious autoinflammatory process in islets leading to insulin secretion failure and type 2 diabetes. Understanding the specific role of islet derived cytokines and chemokines has opened the door to targeted clinical interventions aimed at remodeling islet inflammation from destruction to adaptation. In this article, we review the islet cellular origin of various cytokines and chemokines and describe their regulation and respective roles in physiology and diabetes.

Current status of islet cell replacement and regeneration therapy

CONTEXT

Beta cell mass and function are decreased to varying degrees in both type 1 and type 2 diabetes. In the future, islet cell replacement or regeneration therapy may thus offer therapeutic benefit to people with diabetes, but there are major challenges to be overcome.

EVIDENCE ACQUISITION

A review of published peer-reviewed medical literature on beta-cell development and regeneration was performed. Only publications considered most relevant were selected for citation, with particular attention to the period 2000-2009 and the inclusion of earlier landmark studies.

EVIDENCE SYNTHESIS

Islet cell regenerative therapy could be achieved by in situ regeneration or implantation of cells previously derived in vitro. Both approaches are being explored, and their ultimate success will depend on the ability to recapitulate key events in the normal development of the endocrine pancreas to derive fully differentiated islet cells that are functionally normal. There is also debate as to whether beta-cells alone will assure adequate metabolic control or whether it will be necessary to regenerate islets with their various cell types and unique integrated function. Any approach must account for the potential dangers of regenerative therapy.

CONCLUSIONS

Islet cell regenerative therapy may one day offer an improved treatment of diabetes and potentially a cure. However, the various approaches are at an early stage of preclinical development and should not be offered to patients until shown to be safe as well as more efficacious than existing therapy.

Silencing mitogen-activated protein 4 kinase 4 (MAP4K4) protects beta cells from tumor necrosis factor-alpha-induced decrease of IRS-2 and inhibition of glucose-stimulated insulin secretion

Obesity and type 2 diabetes present partially overlapping phenotypes with systemic inflammation as a common feature, raising the hypothesis that elevated cytokine levels may contribute to peripheral insulin resistance as well as the decreased beta cell functional mass observed in type 2 diabetes. In healthy humans, TNF-alpha infusion induces skeletal muscle insulin resistance. We now explore the impact of TNF-alpha on primary beta cell function and the underlying signaling pathways. Human and rat primary beta cells were sorted by FACS and cultured for 24 h +/- 20 ng/ml TNF-alpha to explore the impact on apoptosis, proliferation, and short-term insulin secretion (1 h, 2.8 mm glucose followed by 1 h, 16.7 mm glucose at the end of the 24-h culture period) as well as key signaling protein phosphorylation and expression. Prior exposure to TNF-alpha for 24 h inhibits glucose-stimulated insulin secretion from primary beta cells. This is associated with a decrease in glucose-stimulated phosphorylation of key proteins in the insulin signaling pathway including Akt, AS160, and other Akt substrates, ERK as well as the insulin receptor. Strikingly, TNF-alpha treatment decreased IRS-2 protein level by 46 +/- 7% versus control, although mRNA expression was unchanged. While TNF-alpha treatment increased MAP4K4 mRNA expression by 33 +/- 5%, knockdown of MAP4K4 by siRNA-protected beta cells against the detrimental effects of TNF-alpha on both insulin secretion and signaling. We thus identify MAP4K4 as a key upstream mediator of TNF-alpha action on the beta cell, making it a potential therapeutic target for preservation of beta cell function in type 2 diabetes.

Signaling pathways implicated in the stimulation of beta-cell proliferation by extracellular matrix

Laminin-5-rich extracellular matrix derived from 804G cells (804G-ECM) induces spreading, improves glucose-stimulated insulin secretion, and increases survival and proliferation of rat pancreatic beta-cells. The aim of the study was to determine growth signaling pathways activated by ECM with a particular focus on Ca(2+)-dependent transcription factors. 804G-ECM increased rat beta-cell proliferation, and this stimulation was glucose and Ca(2+) dependent. NF-kappaB nuclear translocation as well as IkappaBalpha gene expression were also Ca(2+) dependent. Inhibition of NF-kappaB almost completely blocked 804G-ECM-stimulated beta-cell proliferation as did the soluble IL-1 receptor antagonist IL-1Ra. 804G-ECM-induced proliferation was also blocked by cyclosporin A and the VIVIT peptide, suggesting involvement of nuclear factor of activated T cells (NFAT)/calcineurin. Use of selective inhibitors further implicated other pathways in this process. Inhibition of phosphatidylinositol 3-kinase and protein kinase A both prevented beta-cell replication stimulated by 804G-ECM. Conversely, inhibition of MAPK, c-Jun N-terminal kinase, p38, and glycogen synthase kinase-3beta increased beta-cell proliferation on 804G-ECM. Our results suggest that Ca(2+) entry, which is necessary for increased beta-cell proliferation on 804G-ECM, is also involved in 804G-ECM-induced NF-kappaB activity. It is proposed that increased cytosolic Ca(2+) leads to activation of the transcription factors NFAT and NF-kappaB that in turn increase beta-cell proliferation. Activation of phosphatidylinositol 3-kinase by 804G-ECM also increases proliferation possibly by synergistic coactivation of NFAT via inhibition of glycogen synthase kinase-3beta, whereas IL-1beta may amplify the process by feed-forward activation of NF-kappaB. Conversely, inhibition of the MAPK pathway increased beta-cell proliferation, indicating a counterregulatory restraining role for this signaling pathway.

Role of the Rho-ROCK (Rho-associated kinase) signaling pathway in the regulation of pancreatic beta-cell function

Extracellular matrix has a beneficial impact on beta-cell spreading and function, but the underlying signaling pathways have yet to be fully elucidated. In other cell types, Rho, a well-characterized member of the family of Rho GTPases, and its effector Rho-associated kinase (ROCK), play an important role as downstream mediators of outside in signaling from extracellular matrix. Therefore, a possible role of the Rho-ROCK pathway in beta-cell spreading, actin cytoskeleton dynamics, and function was investigated. Rho was inhibited using a new cell-permeable version of C3 transferase, whereas the activity of ROCK was repressed using the specific ROCK inhibitors H-1152 and Y-27632. Inhibition of Rho and of ROCK increased spreading and improved both short-term and prolonged glucose-stimulated insulin secretion but had no impact on basal secretion. Inhibition of this pathway led to a depolymerization of the actin cytoskeleton. Furthermore, the impact of the inhibition of ROCK on stimulated insulin secretion was acute and reversible, suggesting that rapid signaling such as phosphorylation is involved. Finally, quantification of the activity of RhoA indicated that the extracellular matrix represses RhoA activity. Overall these results show for the first time that the Rho-ROCK signaling pathway contributes to the stabilization of the actin cytoskeleton and inhibits glucose-stimulated insulin secretion in primary pancreatic beta-cells. Furthermore, they indicate that inhibition of this pathway might be one of the mechanisms by which the extracellular matrix exerts its beneficial effects on pancreatic beta-cell function.

Cell therapy for type 2 diabetes: is it desirable and can we get it?

The functional mass of beta-cells is decreased in type 2 diabetes. Replacing missing beta-cells or triggering their regeneration may thus allow for improved treatment of type 2 diabetes, to the extent that this is combined with therapy for improved insulin sensitivity. Although progress has been made in deriving beta-cell-like cells from stem or precursor cells in vitro, these cannot yet be obtained in sufficient quantities or well enough differentiated to envisage their therapeutic use in beta-cell replacement therapy. Likewise, our very limited understanding of beta-cell regeneration in adult man does not yet allow for development of a valid strategy for kick-starting such a process in individuals with type 2 diabetes, whether by bona fide neogenesis or self-replication of existing beta-cells. Regardless of how beta-cell mass is restored in type 2 diabetes, it will be important to prevent any renewed decrease thereafter. Current understanding suggests that islet inflammation as well as signals from (insulin-resistant/inflamed) adipose tissue and skeletal muscle contribute towards decreased beta-cell mass in type 2 diabetes. It will likely be important to protect newly formed or implanted beta-cells from these negative influences to ensure their long-term survival.

Small interfering RNA-mediated suppression of proislet amyloid polypeptide expression inhibits islet amyloid formation and enhances survival of human islets in culture

OBJECTIVE

Islet amyloid, formed by aggregation of the beta-cell peptide islet amyloid polypeptide (IAPP; amylin), is a pathological characteristic of pancreatic islets in type 2 diabetes. Toxic IAPP aggregates likely contribute to the progressive loss of beta-cells in this disease. We used cultured human islets as an ex vivo model of amyloid formation to investigate whether suppression of proIAPP expression would inhibit islet amyloid formation and enhance beta-cell survival and function.

RESEARCH DESIGN AND METHODS

Islets from cadaveric organ donors were transduced with a recombinant adenovirus expressing a short interfering RNA (siRNA) designed to suppress human proIAPP (Ad-hProIAPP-siRNA), cultured for 10 days, and then assessed for the presence of islet amyloid, beta-cell apoptosis, and beta-cell function.

RESULTS

Thioflavine S-positive amyloid deposits were clearly present after 10 days of culture. Transduction with Ad-hProIAPP-siRNA reduced proIAPP expression by 75% compared with nontransduced islets as assessed by Western blot analysis of islet lysates 4 days after transduction. siRNA-mediated inhibition of IAPP expression decreased islet amyloid area by 63% compared with nontransduced cultured islets. Cell death assessed by transferase-mediated dUTP nick-end labeling staining was decreased by 50% in transduced cultured human islets, associated with a significant increase in islet insulin content (control, 100 +/- 4 vs. +Ad-siRNA, 153 +/- 22%, P < 0.01) and glucose-stimulated insulin secretion (control, 222 +/- 33 vs. +Ad-siRNA, 285 +/- 21 percent basal, P < 0.05).

CONCLUSIONS

These findings demonstrate that inhibition of IAPP synthesis prevents amyloid formation and beta-cell death in cultured human islets. Inhibitors of IAPP synthesis may have therapeutic value in type 2 diabetes.

Increased interleukin (IL)-1beta messenger ribonucleic acid expression in beta -cells of individuals with type 2 diabetes and regulation of IL-1beta in human islets by glucose and autostimulation

CONTEXT

Elevated glucose levels impair islet function and survival, and it has been proposed that intraislet expression of IL-1beta contributes to glucotoxicity.

OBJECTIVE

The objective was to investigate IL-1beta mRNA expression in near-pure beta-cells of patients with type 2 diabetes (T2DM) and study the regulation of IL-1beta by glucose in isolated human islets.

METHODS

Laser capture microdissection was performed to isolate beta-cells from pancreas sections of 10 type 2 diabetic donors and nine controls, and IL-1beta mRNA expression was analyzed using gene arrays and PCR. Cultured human islets and fluorescence-activated cell sorter-purified human beta-cells were used to study the regulation of IL-1beta expression by glucose and IL-1beta.

RESULTS

Gene array analysis of RNA from beta-cells of individuals with T2DM revealed increased expression of IL-1beta mRNA. Real-time PCR confirmed increased IL-1beta expression in six of 10 T2DM samples, with minimal or no expression in nine control samples. In cultured human islets, IL-1beta mRNA and protein expression was induced by high glucose and IL-1beta autostimulation and decreased by the IL-1 receptor antagonist IL-1Ra. The glucose response was negatively correlated with basal IL-1beta expression levels. Autostimulation was transient and nuclear factor-kappaB dependent. Glucose-induced IL-1beta was biologically active and stimulated IL-8 release. Low picogram per milliliter concentrations of IL-1beta up-regulated inflammatory factors IL-8 and IL-6.

CONCLUSION

Evidence that IL-1beta mRNA expression is up-regulated in beta-cells of patients with T2DM is presented, and glucose-promoted IL-1beta autostimulation may be a possible contributor.

Dual effect of cell-cell contact disruption on cytosolic calcium and insulin secretion

Cell-to-cell interactions play an important role in insulin secretion. Compared with intact islets, dispersed pancreatic beta-cells show increased basal and decreased glucose-stimulated insulin secretion. In this study, we used mouse MIN6B1 cells to investigate the mechanisms that control insulin secretion when cells are in contact with each other or not. RNAi-mediated silencing of the adhesion molecule E-cadherin in confluent cells reduced glucose-stimulated secretion to the levels observed in isolated cells but had no impact on basal secretion. Dispersed cells presented high cytosolic Ca(2+) activity, depolymerized cytoskeleton and ERK1/2 activation in low glucose conditions. Both the increased basal secretion and the spontaneous Ca(2+) activity were corrected by transient removal of Ca(2+) or prolonged incubation of cells in low glucose, a procedure that restored the ability of dispersed cells to respond to glucose (11-fold stimulation). In conclusion, we show that dispersed pancreatic beta-cells can respond robustly to glucose once their elevated basal secretion has been corrected. The increased basal insulin secretion of dispersed cells is due to spontaneous Ca(2+) transients that activate downstream Ca(2+) effectors, whereas engagement of cell adhesion molecules including E-cadherin contributes to the greater secretory response to glucose seen in cells with normal intercellular contacts.

Rab GTPase-activating protein AS160 is a major downstream effector of protein kinase B/Akt signaling in pancreatic beta-cells

OBJECTIVE

Protein kinase B/Akt plays a central role in beta-cells, but little is known regarding downstream Akt substrates in these cells. Recently, Rab GTPase-activating protein AS160, a substrate of Akt, was shown to be involved in insulin modulation of GLUT4 trafficking in skeletal muscle and adipose tissue. The aim of this study was to investigate the expression and potential role of AS160 in beta-cells.

RESEARCH DESIGN AND METHODS

AS160 mRNA expression was measured in mouse and human islets and fluorescence-activated cell sorted beta-cells and compared in islets from control subjects versus individuals with type 2 diabetes. For knockdown experiments, transformed mouse insulin-secreting MIN6B1 cells were transfected with pSUPER-GFP plasmid encoding a small hairpin RNA against insulin receptor substrate (IRS)-2, AS160, or a negative control. Primary mouse islet cells were transfected with AS160 small interfering RNA.

RESULTS

AS160 was expressed in human and mouse pancreatic beta-cells and phosphorylated after glucose stimulation. AS160 mRNA expression was downregulated in pancreatic islets from individuals with type 2 diabetes. In MIN6B1 cells, glucose induced phosphorylation of Akt and AS160, and this was mediated by insulin receptor/IRS-2/phosphatidylinositol 3-kinase independently of changes in cytosolic Ca(2+). Knockdown of AS160 resulted in increased basal insulin secretion, whereas glucose-stimulated insulin release was abolished. Furthermore, beta-cells with decreased AS160 showed increased apoptosis and loss of glucose-induced proliferation.

CONCLUSIONS

This study shows for the first time that AS160, previously recognized as a key player in insulin signaling in skeletal muscle and adipose tissue, is also a major effector of protein kinase B/Akt signaling in the beta-cell.

Increasing GLP-1-induced beta-cell proliferation by silencing the negative regulators of signaling cAMP response element modulator-alpha and DUSP14

OBJECTIVE

Glucagon-like peptide-1 (GLP-1) is a growth and differentiation factor for mature beta-cells and their precursors. However, the overall effect of GLP-1 on increasing beta-cell mass in both in vivo and in vitro conditions is relatively small, and augmenting this effect would be beneficial for the treatment or prevention of type 1 and type 2 diabetes. Here, we searched for cellular mechanisms that may limit the proliferative effect of GLP-1 and tested whether blocking them could increase beta-cell proliferation.

RESEARCH DESIGN AND METHODS

We examined GLP-1-regulated genes in beta TC-Tet cells by cDNA microarrays. To assess the effect of some of these gene on cell proliferation, we reduced their expression using small heterogenous RNA in beta-cell lines and primary mouse islets and measured [(3)H]thymidine or 5'-bromo-2'-deoxyuridine incorporation.

RESULTS

We identified four negative regulators of intracellular signaling that were rapidly and strongly activated by GLP-1: the regulator of G-protein-signaling RGS2; the cAMP response element-binding protein (CREB) antagonists cAMP response element modulator (CREM)-alpha and ICERI; and the dual specificity phosphatase DUSP14, a negative regulator of the mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase 1/2 (ERK1/2) pathway. We show that knockdown of CREMalpha or DUSP14 or expression of a dominant-negative form of DUSP14 increased beta-cell line proliferation and enhanced the GLP-1-induced proliferation of primary beta-cells.

CONCLUSIONS

Together, our data show that 1) the cAMP/protein kinase A/CREB and MAPK/ERK1/2 pathways can additively control beta-cell proliferation, 2) beta-cells have evolved several mechanisms limiting GLP-1-induced cellular proliferation, and 3) blocking these mechanisms increases the positive effect of GLP-1 on beta-cell mass.

Munc 18-1 and granuphilin collaborate during insulin granule exocytosis

Munc 18-1 is a member of the Sec/Munc family of syntaxin-binding proteins known to bind to the plasma membrane Q-SNARE syntaxin1 and whose precise role in regulated exocytosis remains controversial. Here, we show that Munc 18-1 plays a positive role in regulated insulin secretion from pancreatic beta cells. Munc 18-1 depletion caused a loss in the secretory capacity of both transiently transfected INS 1E cells and a stable clone with tetracycline-regulated Munc 18-1 RNA interference. In addition, Munc 18-1-depleted cells exhibited defective docking of insulin granules to the plasma membrane and accumulated insulin in the trans Golgi network. Furthermore, glucose stimulation after Munc 18-1 depletion resulted in the rapid formation of autophagosomes. In contrast, overexpression of Munc 18-1 had no effect on insulin secretion. Although there was no detectable interaction between Munc 18-1 and Munc-18-interacting protein 1 or calcium/calmodulin-dependent serine protein kinase, Munc 18-1 associated with the granular protein granuphilin. This association was regulated by glucose and was required for the specific interaction of insulin granules with syntaxin1. We conclude that Munc 18-1 and granuphilin collaborate in the docking of insulin granules to the plasma membrane in an initial fusion-incompetent state, with Munc 18-1 subsequently playing a positive role in a later stage of insulin granule exocytosis.

Proliferation of sorted human and rat beta cells

AIMS/HYPOTHESIS

The aim of the study was to determine whether purified beta cells can replicate in vitro and whether this is enhanced by extracellular matrix (ECM) and growth factors.

METHODS

Human beta cells were purified by FACS by virtue of their high zinc content using Newport Green, and excluding ductal and dead cells. Rat beta cells were sorted by autofluorescence or using the same method developed for human cells. Cells were plated on poly-L-lysine or ECMs from rat or human bladder carcinoma cells or bovine corneal ECM and incubated in the presence of BrdU with or without growth factors.

RESULTS

The newly developed method for sorting human beta cells yields a population containing 91.4 +/- 2.8% insulin-positive cells with a low level of spontaneous apoptosis and a robust secretory response to glucose. Beta cells from 8-week-old rats proliferated in culture and this was increased by ECM. Among growth factors, only human growth hormone (hGH) and the glucagon-like peptide-1 analogue liraglutide enhanced proliferation of rat beta cells, with a significant increase on both poly-L-lysine and ECM. By contrast, sorted adult human beta cells from 16 donors aged 48.9 +/- 14.3 years (range 16-64 years) failed to replicate demonstrably in vitro regardless of the substratum or growth factors used.

CONCLUSIONS/INTERPRETATION

These findings indicate that, in our conditions, the fully differentiated human adult insulin-producing beta cell was unable to proliferate in vitro. This has important implications for any attempt to expand cells from pancreases of donors of this age group. By contrast, the rat beta cells used here were able to divide in vitro, and this was enhanced by ECM, hGH and liraglutide.

Induction of CXCL1 by extracellular matrix and autocrine enhancement by interleukin-1 in rat pancreatic beta-cells

As we showed previously, the extracellular matrix (ECM) derived from rat bladder carcinoma cells (804G-ECM) has positive effects on rat primary beta-cell function and survival in vitro. The aim of this study was to define beta-cell genes induced by this ECM with a specific focus on cytokines. Analysis of differential gene expression by oligonucleotide microarrays, RT-PCR, and in situ hybridization was performed to identify cytokine mRNA induced by this matrix. Four cytokines were overexpressed on 804G-ECM compared with poly-L-lysine: C-X-C motif ligand 1 (CXCL1), CXCL2, interferon-inducible protein-10, and IL-1beta. A time-course experiment indicated that maximal induction by 804G-ECM of CXCL1/2 and interferon-inducible protein-10 occurred at 4 h. Stimulation of CXCL1 release by beta-cells on 804G-ECM was confirmed at the protein level. Moreover, secreted CXCL1 was shown to be functionally active by attracting rat granulocytes. Preventing the interaction of beta1 integrins and laminin-5 (a major component of 804G-ECM) with specific antibodies resulted in a 40-50% inhibition of CXCL1 expression. Using the nuclear factor-kappaB pathway inhibitor Bay 11-7082 it is demonstrated that CXCL1 expression and secretion are dependent on nuclear factor-kappaB activation. IL-1 secreted by beta-cells plated on 804G-ECM was found to be a key soluble mediator because treatment of cells with the IL-1 receptor antagonist significantly reduced both CXCL1 gene expression and secretion. It is concluded that ECM induces expression of cytokines including CXCL1 with amplification by IL-1 acting via a positive autocrine feedback loop.

Dynamin is functionally coupled to insulin granule exocytosis

The insulin granule integral membrane protein marker phogrin-green fluorescent protein was co-localized with insulin in Min6B1 beta-cell secretory granules but did not undergo plasma membrane translocation following glucose stimulation. Surprisingly, although expression of a dominant-interfering dynamin mutant (Dyn/K44A) inhibited transferrin receptor endocytosis, it had no effect on phogringreen fluorescent protein localization in the basal or secretagogue-stimulated state. By contrast, co-expression of Dyn/K44A with human growth hormone as an insulin secretory marker resulted in a marked inhibition of human growth hormone release by glucose, KCl, and a combination of multiple secretagogues. Moreover, serial pulse depolarization stimulated an increase in cell surface capacitance that was also blocked in cells expressing Dyn/K44A. Similarly, small interference RNA-mediated knockdown of dynamin resulted in marked inhibition of glucose-stimulated insulin secretion. Together, these data suggest the presence of a selective kiss and run mechanism of insulin release. Moreover, these data indicate a coupling between endocytosis and exocytosis in the regulation of beta-cell insulin secretion.

Differential expression of E-cadherin at the surface of rat beta-cells as a marker of functional heterogeneity

The aim of this study was to assess whether the expression of E-cadherin at the surface of rat beta-cells is regulated by insulin secretagogues and correlates with insulin secretion. When cultured under standard conditions, virtually all beta-cells expressed E-cadherin observed by immunofluorescence, but heterogeneous staining was observed. Using fluorescence-activated cell sorting (FACS), two beta-cell sub-populations were sorted: one that was poorly labeled ('ECad-low') and another that was highly labeled ('ECad-high'). After 1-h stimulation with 16.7 mM glucose, insulin secretion (reverse hemolytic plaque assay) from individual ECad-high beta-cells was higher than that from ECad-low beta-cells. Ca2+-dependent beta-cell aggregation was increased at 16.7 mM glucose when compared with 2.8 mM glucose. E-cadherin at the surface of beta-cells was increased after 18 h at 11.1 and 22.2 mM glucose when compared with 2.8 mM glucose, with the greatest increase at 22.2 mM glucose + 0.5 mM isobutylmethylxanthine (IBMX). While no labeling was detected on freshly trypsinized cells, the proportion of stained cells increased in a time-dependent manner during culture for 1, 3, and 24 h. This recovery was faster when cells were incubated at 16.7 vs 2.8 mM glucose. Cycloheximide inhibited expression of E-cadherin at 2.8 mM glucose, but not at 16.7 mM, while depolymerization of actin by either cytochalasin B or latrunculin B increased surface E-cadherin at low glucose. In conclusion, these results show that expression of E-cadherin at the surface of islet beta-cells is controlled by secretagogues including glucose, correlates with insulin secretion, and can serve as a surface marker of beta-cell function.

Dominant-negative effects of a novel mutated Ins2 allele causes early-onset diabetes and severe beta-cell loss in Munich Ins2C95S mutant mice

The novel diabetic mouse model Munich Ins2(C95S) was discovered within the Munich N-ethyl-N-nitrosourea mouse mutagenesis screen. These mice exhibit a T-->A transversion in the insulin 2 (Ins2) gene at nucleotide position 1903 in exon 3, which leads to the amino acid exchange C95S and loss of the A6-A11 intrachain disulfide bond. From 1 month of age onwards, blood glucose levels of heterozygous Munich Ins2(C95S) mutant mice were significantly increased compared with controls. The fasted and postprandial serum insulin levels of the heterozygous mutants were indistinguishable from those of wild-type littermates. However, serum insulin levels after glucose challenge, pancreatic insulin content, and homeostasis model assessment (HOMA) beta-cell indices of heterozygous mutants were significantly lower than those of wild-type littermates. The initial blood glucose decrease during an insulin tolerance test was lower and HOMA insulin resistance indices were significantly higher in mutant mice, indicating the development of insulin resistance in mutant mice. The total islet volume, the volume density of beta-cells in the islets, and the total beta-cell volume of heterozygous male mutants was significantly reduced compared with wild-type mice. Electron microscopy of the beta-cells of male mutants showed virtually no secretory insulin granules, the endoplasmic reticulum was severely enlarged, and mitochondria appeared swollen. Thus, Munich Ins2(C95S) mutant mice are considered a valuable model to study the mechanisms of beta-cell dysfunction and death during the development of diabetes.

The Fas pathway is involved in pancreatic beta cell secretory function

Pancreatic beta cell mass and function increase in conditions of enhanced insulin demand such as obesity. Failure to adapt leads to diabetes. The molecular mechanisms controlling this adaptive process are unclear. Fas is a death receptor involved in beta cell apoptosis or proliferation, depending on the activity of the caspase-8 inhibitor FLIP. Here we show that the Fas pathway also regulates beta cell secretory function. We observed impaired glucose tolerance in Fas-deficient mice due to a delayed and decreased insulin secretory pattern. Expression of PDX-1, a beta cell-specific transcription factor regulating insulin gene expression and mitochondrial metabolism, was decreased in Fas-deficient beta cells. As a consequence, insulin and ATP production were severely reduced and only partly compensated for by increased beta cell mass. Up-regulation of FLIP enhanced NF-kappaB activity via NF-kappaB-inducing kinase and RelB. This led to increased PDX-1 and insulin production independent of changes in cell turnover. The results support a previously undescribed role for the Fas pathway in regulating insulin production and release.

Pro-survival role of gelsolin in mouse beta-cells

We have previously shown that the Ca(2+)-dependent actin-severing protein gelsolin plays an important role in regulated insulin secretion. The aim of this study was to determine the role of gelsolin in beta-cell survival as it has been shown to play a dual role in apoptosis in other cell types. MIN6 subclones B1 and C3, shown previously to express gelsolin at different levels (B1>>C3 cells), were used for this purpose. We demonstrate that B1 cells have lower levels of apoptosis and active caspase-3 when compared with C3 cells, in both standard (25 mmol/l glucose and 15% FCS) and deprived (5 mmol/l glucose and 1% FCS) conditions. Overexpression of gelsolin resulted in a decrease in the percentage of terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL)(+) and active caspase-3(+) cells. Conversely, knockdown of gelsolin by RNA interference in B1 cells caused an increase in the number of TUNEL(+) and active caspase-3(+) cells. Finally, the anti-apoptotic role of gelsolin was confirmed in purified primary mouse beta-cells where overexpression of gelsolin resulted in a decrease in the percentage of TUNEL(+) cells. In summary, our results show for the first time that gelsolin plays a pro-survival role in pancreatic beta-cells.

Impaired NH2-terminal processing of human proislet amyloid polypeptide by the prohormone convertase PC2 leads to amyloid formation and cell death

Islet amyloid, formed by aggregation of islet amyloid polypeptide (IAPP; amylin), is a pathological characteristic of the pancreas in type 2 diabetes and may contribute to the progressive loss of beta-cells in this disease. We tested the hypothesis that impaired processing of the IAPP precursor proIAPP contributes to amyloid formation and cell death. GH3 cells lacking the prohormone convertase 1/3 (PC1/3) and IAPP and with very low levels of prohormone convertase 2 (PC2) were transduced with adenovirus (Ad) expressing human or rat (control) proIAPP linked to green fluorescent protein, with or without Ad-PC2 or Ad-PC1/3. Expression of human proIAPP increased the number of transferase-mediated dUTP nick-end labeling (TUNEL)-positive cells 96 h after transduction (+hIAPP 8.7 +/- 0.4% vs. control 3.0 +/- 0.4%; P < 0.05). COOH-terminal processing of human proIAPP by PC1/3 increased (hIAPP+PC1/3 10.4 +/- 0.7%; P < 0.05), whereas NH(2)-terminal processing of proIAPP by addition of PC2 markedly decreased (hIAPP+PC2 5.5 +/- 0.5%; P < 0.05) the number of apoptotic GH3 cells. Islets from mice lacking PC2 and with beta-cell expression of human proIAPP (hIAPP(+/+)/PC2(-/-)) developed amyloid associated with beta-cell death during 2-week culture. Rescue of PC2 expression by ex vivo transduction with Ad-PC2 restored NH(2)-terminal processing to mature IAPP and decreased both the extent of amyloid formation and the number of TUNEL-positive cells (-PC2 26.5 +/- 4.1% vs. +PC2 16.1 +/- 4.3%; P < 0.05). These findings suggest that impaired NH(2)-terminal processing of proIAPP leads to amyloid formation and cell death and that accumulation of the NH(2)-terminally extended human proIAPP intermediate may be a critical initiating step in amyloid formation.

Blockade of beta1 integrin-laminin-5 interaction affects spreading and insulin secretion of rat beta-cells attached on extracellular matrix

When attached on a matrix produced by a rat bladder carcinoma cell line (804G matrix), rat pancreatic beta-cells spread in response to glucose and secrete more insulin compared with cells attached on poly-l-lysine. The aim of this study was to determine whether laminin-5 and its corresponding cell receptor beta1 integrin are implicated in these phenomena. By using specific blocking antibodies, we demonstrated that laminin-5 is the component present in 804G matrix responsible for the effect of 804G matrix on beta-cell function and spreading. When expression of two well-known laminin-5 ligands, beta1 and beta4 integrin, was assessed by Western blot and RT-PCR, only the beta1 integrin was detected in beta-cells. Anti-beta1 integrin antibody reduced the spreading of beta-cells on 804G matrix. Blockade of the interaction between beta1 integrins and laminin-5 resulted in a reduction in glucose-stimulated insulin secretion. Blocking anti-beta1 integrin antibody also inhibited focal adhesion kinase phosphorylation induced by 804G matrix. In conclusion, anti-beta1 integrin and -laminin-5 antibodies interfere with spreading of beta-cells, resulting in decreased insulin secretion in response to glucose. Our findings indicate that outside-in signaling via engagement of beta1 integrins by laminin-5 is an important component of normal beta-cell function.