Role of endogenous IL-6 in the neonatal expansion and functionality of Wistar rat pancreatic alpha cells

AIMS/HYPOTHESIS

Plasma glucagon concentrations rise sharply during the early postnatal period. This condition is associated with increased alpha cell mass. However, the trophic factors that regulate alpha cell turnover during the perinatal period have not been studied. Macrophage infiltrations are present in the neonatal pancreas, and this cell type releases cytokines such as IL-6. Alpha cells have been identified as a primary target of IL-6 actions. We therefore investigated the physiological relevance of IL-6 to neonatal pancreatic alpha cell maturation.

METHODS

Histochemical analyses were performed to quantify alpha cell mass, replication and apoptosis. Pancreatic Il6 expression was determined by quantitative RT-PCR. The biological effect of IL-6 was tested in two in vivo rat models of IL-6 blockade and chronic undernutrition.

RESULTS

Alpha cell mass increased sharply shortly after birth but decreased significantly after weaning. Pancreatic alpha cell proliferation was as high as 2.5% at the beginning of suckling but diminished with time to 1.2% in adulthood. Similarly, alpha cell neoformation was remarkably high on postnatal day (PN) 4, whereas alpha cell apoptosis was low throughout the neonatal period. Moreover, Il6 mRNA exhibited developmental upregulation in the pancreas of suckling rats, with the highest expression on PN2. Neutralisation of IL-6 reduced alpha cell mass expansion and glucagon production. IL-6 staining was detected within the islets, mainly in the alpha cells. Finally, undernourished neonates showed altered alpha cell number and function and delayed appearance of IL-6 in the pancreas.

CONCLUSIONS/INTERPRETATION

These data point to a potential role for local IL-6 in the regulation of alpha cell growth and function during suckling.

UCP2 regulates the glucagon response to fasting and starvation

Glucagon is important for maintaining euglycemia during fasting/starvation, and abnormal glucagon secretion is associated with type 1 and type 2 diabetes; however, the mechanisms of hypoglycemia-induced glucagon secretion are poorly understood. We previously demonstrated that global deletion of mitochondrial uncoupling protein 2 (UCP2(-/-)) in mice impaired glucagon secretion from isolated islets. Therefore, UCP2 may contribute to the regulation of hypoglycemia-induced glucagon secretion, which is supported by our current finding that UCP2 expression is increased in nutrient-deprived murine and human islets. Further to this, we created α-cell-specific UCP2 knockout (UCP2AKO) mice, which we used to demonstrate that blood glucose recovery in response to hypoglycemia is impaired owing to attenuated glucagon secretion. UCP2-deleted α-cells have higher levels of intracellular reactive oxygen species (ROS) due to enhanced mitochondrial coupling, which translated into defective stimulus/secretion coupling. The effects of UCP2 deletion were mimicked by the UCP2 inhibitor genipin on both murine and human islets and also by application of exogenous ROS, confirming that changes in oxidative status and electrical activity directly reduce glucagon secretion. Therefore, α-cell UCP2 deletion perturbs the fasting/hypoglycemic glucagon response and shows that UCP2 is necessary for normal α-cell glucose sensing and the maintenance of euglycemia.

Sex differences in insulin resistance in GABAB1 knockout mice

AIMS

We have previously demonstrated that the absence of functional GABA B receptors (GABABRs) disturbs glucose homeostasis in GABAB1KO mice. The aim of this work was to extend our studies of these alterations in GABAB1KO mice and investigate the sexual differences therein.

MAIN METHODS

Male and female, GABAB1KO and WT mice were used. Glucose and insulin tolerance tests (GTT and ITT), and insulin and glucagon secretion tests (IST and GST) were performed. Blood glucose, serum insulin and hyperglycemic hormones were determined, and HOMA-IR calculated. Skeletal muscle insulin receptor β subunit (IRβ), insulin receptor substrates 1/2 (IRS1, IRS2) and hexokinase-II levels were determined by Western blot. Skeletal muscle insulin sensitivity was assessed by in vivo insulin-induced Akt phosphorylation (Western blot). Food intake and hypothalamic NPY mRNA expression (by qPCR) were also evaluated.

KEY FINDINGS

Fasted insulin and HOMA-IR were augmented in GABAB1KO males, with no alterations in females. Areas under the curve (AUC) for GTT and ITT were increased in GABAB1KO mice of both genders, indicating compromised insulin sensitivity. No genotype differences were observed in IST, GST or in IRβ, IRS1, IRS2 and hexokinase-II expression. Akt activation was severely impaired in GABAB1KO males while no alterations were observed in females. GABAB1KO mice showed increased food intake and NPY expression.

SIGNIFICANCE

Glucose metabolism and energy balance disruptions were more pronounced in GABAB1KO males, which develop peripheral insulin resistance probably due to augmented insulin secretion. Metabolic alterations in females were milder and possibly due to previously described reproductive disorders, such as persistent estrus.

Emerging role of AMP-activated protein kinase in endocrine control of metabolism in the liver

This review summarizes the emerging role of AMP-activated protein kinase (AMPK) in mediating endocrine regulation of metabolic fluxes in the liver. There are a number of hormones which, when acting on the liver, alter AMPK activation. Here we describe those hormones associated with activation and de-activation of AMPK and the potential mechanisms for changes in AMPK activation state. The actions of these hormones, in many cases, are consistent with downstream effects of AMPK signaling thus strengthening the circumstantial case for AMPK-mediated hormone action. In recent years, genetic mouse models have also been used in an attempt to establish the role of AMPK in hormone-stimulated metabolism in the liver. Few experiments have, however, firmly established a causal relationship between hormone action at the liver and AMPK signaling.

N -acetyl-GLP-1: a DPP IV-resistant analogue of glucagon-like peptide-1 (GLP-1) with improved effects on pancreatic β-cell-associated gene expression

Glucagon-like peptide-1(7-36)amide (GLP-1) is a key insulinotropic hormone with the reported potential to differentiate non-insulin secreting cells into insulin-secreting cells. The short biological half-life of GLP-1 after cleavage by dipeptidylpeptidase IV (DPP IV) to GLP-1(9-36)amide is a major therapeutic drawback. Several GLP-1 analogues have been developed with improved stability and insulinotropic action. In this study, the N-terminally modified GLP-1 analogue, N-acetyl-GLP-1, was shown to be completely resistant to DPP IV, unlike native GLP-1, which was rapidly degraded. Furthermore, culture of pancreatic ductal ARIP cells for 72 h with N-acetyl-GLP-1 indicated a greater ability to induce pancreatic beta-cell-associated gene expression, including insulin and glucokinase. Further investigation of the effects of stable GLP-1 analogues on beta-cell differentiation is required to assess their potential in diabetic therapy.

Identification of key residues for the binding of glucagon to the N-terminal domain of its receptor: an alanine scan and modeling study

Glucagon plays an essential role in the glycemia maintenance during fasting, but also aggravates hyperglycemia in diabetic patients. A series of analogues of glucagon were synthesized replacing each amino acid of the C-terminal region (residues 15-29) with alanine. The residues affecting the binding to the glucagon receptor are found to be located on one face of the glucagon helix. Several 3-dimensional models of the N-terminal domain of the glucagon receptor in complex with its ligand peptide were built and used to analyze the peptide-receptor interface in terms of the nature of the peptide residues and the interactions they form with the receptor. The models suggest that glucagon keeps its native helical structure upon binding, and that a large part of the interface formed with the receptor is hydrophobic. We find that in the C-terminal region, F22, V23, M27, and D15 are the most important residues for peptide binding. They bury a large portion of their solvent accessible surface area and make numerous interactions with the receptor mainly of the hydrophobic type.

Regeneration of pancreatic non-β endocrine cells in adult mice following a single diabetes-inducing dose of streptozotocin

The non-β endocrine cells in pancreatic islets play an essential counterpart and regulatory role to the insulin-producing β-cells in the regulation of blood-glucose homeostasis. While significant progress has been made towards the understanding of β-cell regeneration in adults, very little is known about the regeneration of the non-β endocrine cells such as glucagon-producing α-cells and somatostatin producing δ-cells. Previous studies have noted the increase of α-cell composition in diabetes patients and in animal models. It is thus our hypothesis that non-β-cells such as α-cells and δ-cells in adults can regenerate, and that the regeneration accelerates in diabetic conditions. To test this hypothesis, we examined islet cell composition in a streptozotocin (STZ)-induced diabetes mouse model in detail. Our data showed the number of α-cells in each islet increased following STZ-mediated β-cell destruction, peaked at Day 6, which was about 3 times that of normal islets. In addition, we found δ-cell numbers doubled by Day 6 following STZ treatment. These data suggest α- and δ-cell regeneration occurred rapidly following a single diabetes-inducing dose of STZ in mice. Using in vivo BrdU labeling techniques, we demonstrated α- and δ-cell regeneration involved cell proliferation. Co-staining of the islets with the proliferating cell marker Ki67 showed α- and δ-cells could replicate, suggesting self-duplication played a role in their regeneration. Furthermore, Pdx1(+)/Insulin(-) cells were detected following STZ treatment, indicating the involvement of endocrine progenitor cells in the regeneration of these non-β cells. This is further confirmed by the detection of Pdx1(+)/glucagon(+) cells and Pdx1(+)/somatostatin(+) cells following STZ treatment. Taken together, our study demonstrated adult α- and δ-cells could regenerate, and both self-duplication and regeneration from endocrine precursor cells were involved in their regeneration.

Homozygous carriers of the G allele of rs4664447 of the glucagon gene (GCG) are characterised by decreased fasting and stimulated levels of insulin, glucagon and glucagon-like peptide (GLP)-1

AIMS/HYPOTHESIS

The glucagon gene (GCG) encodes several hormones important for energy metabolism: glucagon, oxyntomodulin and glucagon-like peptide (GLP)-1 and -2. Variants in GCG may associate with type 2 diabetes, obesity and/or related metabolic traits.

METHODS

GCG was re-sequenced as a candidate gene in 865 European individuals. Twenty-nine variants were identified. Four variants that were considered to have a likelihood for altered functionality: rs4664447, rs7581952, Ile158Val and Trp169Ter, were genotyped in 17,584 Danes.

RESULTS

When examined in 5,760 treatment-naive individuals, homozygous carriers of the low frequency (minor allele frequency 2.3%) G allele of rs4664447, predicted to disrupt an essential splice enhancer binding site, had lower levels of fasting plasma glucose (mean ± SD, 4.8 ± 1.2 vs 5.5 ± 0.8 mmol/l, p = 0.004); fasting serum insulin (22 ± 14 vs 42 ± 27 pmol/l, p = 0.04); glucose-stimulated serum insulin (159 ± 83 vs 290 ± 183 pmol/l, p = 0.01) and adult height (165 ± 10 vs 172 ± 9 cm, p = 0.0009) compared with A allele carriers. During oral glucose tolerance and hyperglycaemic arginine stimulation tests, the plasma AUC for GLP-1 (730 ± 69 vs 1,334 ± 288 pmol/l × min, p = 0.0002) and basal and stimulated levels of serum insulin and plasma glucagon were ∼50% decreased (p < 0.001) among three homozygous carriers compared with nine matched wild-type carriers. rs7581952, Ile158Val and Trp169Ter (where 'Ter' indicates 'termination') variants of GCG did not significantly associate or co-segregate with the metabolic traits examined.

CONCLUSIONS/INTERPRETATION

Re-sequencing of GCG revealed a low frequency intronic variant, rs4664447, and follow-up physiological studies suggest that this variant in homozygous form may cause decreased fasting and stimulated levels of insulin, glucagon and GLP-1. Overall, our findings suggest that variation in GCG has no major impact on carbohydrate metabolism in the study populations examined.

Normal glucagon signaling and β-cell function after near-total α-cell ablation in adult mice

OBJECTIVE

To evaluate whether healthy or diabetic adult mice can tolerate an extreme loss of pancreatic α-cells and how this sudden massive depletion affects β-cell function and blood glucose homeostasis.

RESEARCH DESIGN AND METHODS

We generated a new transgenic model allowing near-total α-cell removal specifically in adult mice. Massive α-cell ablation was triggered in normally grown and healthy adult animals upon diphtheria toxin (DT) administration. The metabolic status of these mice was assessed in 1) physiologic conditions, 2) a situation requiring glucagon action, and 3) after β-cell loss.

RESULTS

Adult transgenic mice enduring extreme (98%) α-cell removal remained healthy and did not display major defects in insulin counter-regulatory response. We observed that 2% of the normal α-cell mass produced enough glucagon to ensure near-normal glucagonemia. β-Cell function and blood glucose homeostasis remained unaltered after α-cell loss, indicating that direct local intraislet signaling between α- and β-cells is dispensable. Escaping α-cells increased their glucagon content during subsequent months, but there was no significant α-cell regeneration. Near-total α-cell ablation did not prevent hyperglycemia in mice having also undergone massive β-cell loss, indicating that a minimal amount of α-cells can still guarantee normal glucagon signaling in diabetic conditions.

CONCLUSIONS

An extremely low amount of α-cells is sufficient to prevent a major counter-regulatory deregulation, both under physiologic and diabetic conditions. We previously reported that α-cells reprogram to insulin production after extreme β-cell loss and now conjecture that the low α-cell requirement could be exploited in future diabetic therapies aimed at regenerating β-cells by reprogramming adult α-cells.

Catalase rs769214 SNP in elderly malnutrition and during renutrition: is glucagon to blame?

Impaired glucose tolerance is common during aging. The transcription factor PAX6 is involved in glucose homeostasis. Computational promoter sequence analysis of the catalase gene highlighted a putative PAX6 binding site on the rs769214 polymorphism A allele. Creation of this binding site has been suggested to explain renutrition inefficiency in malnourished elderly patients. Our aim was to evaluate the link between the rs769214 polymorphism of the catalase gene and glucose homeostasis in malnourished elderly patients at inclusion and during renutrition. Thirty-three malnourished elderly Caucasian inpatients were recruited. Nutritional and inflammatory statuses were assessed and a multiplex adipokine analysis was conducted at inclusion and discharge from the Geriatric Nutritional Care Unit at Charles-Foix Hospital (Ivry-sur-Seine, France). Serum glucagon, PAI-1, and TNF-α levels were significantly lower in the A-allele carriers at inclusion. During renutrition, A-allele carriers exhibited increased serum glucagon, PAI-1, and TNF-α variation. After renutrition, levels of these parameters were similar for A-allele carriers and G-allele carriers. A logistic ordinal multivariate regression analysis linked only variation of glucagon to rs769214 SNP. These results support a role for catalase SNP in the efficiency of renutrition in malnourished elderly patients via the modulation of glucagon secretion, probably involving PAX6.

Metabolic impact of glucagon deficiency

Multiple bioactive peptides are produced from proglucagon encoded by glucagon gene (Gcg). Glucagon is produced in islet α-cells through processing by prohormone convertase 2 (Pcsk2) and exerts its action through the glucagon receptor (Gcgr). Although it is difficult to produce a genetic model that harbours isolated glucagon deficiency without affecting the production of other peptides derived from proglucagon, three different animal models that harbour deficiencies in glucagon signalling have been generated by gene targeting strategy. Although both Pcsk2(-/-) and Gcgr(-/-) mice display lower blood glucose levels, homozygous glucagon-GFP knock-in mice (Gcg(gfp/gfp) ) display normoglycaemia despite complete glucagon deficiency. In Gcg(gfp/gfp) mice, the metabolic impact of glucagon deficiency is probably ameliorated by lower plasma insulin levels and glucagon-independent mechanisms that maintain gluconeogenesis. As both Pcsk2(-/-) and Gcgr(-/-) mice exhibit increased production of glucagon-like peptide-1 (GLP-1), which is absent in Gcg(gfp/gfp), GLP-1 is the likely cause of the difference in metabolic impact of glucagon deficiency in these animal models. Although all the three models display islet 'α'-cell hyperplasia, the mechanisms involved remain to be elucidated. Studies using Pcsk2(-/-), Gcgr(-/-) and Gcg(gfp/gfp) mice, especially in combination with α-cell ablation models such as pancreas-specific aristaless-related homeobox (ARX) knockout mice, should further clarify the physiological and pathological roles of glucagon in the regulation of metabolism and the control of islet cell differentiation and proliferation.

Growth factor signalling in the regulation of α-cell fate

Glucagon plays critical roles in regulating glucose homeostasis, mainly by counteracting the effects of insulin. Consequently, the dysregulated glucagon secretion that is evident in type 2 diabetes has significant implications in the pathophysiology of the disease. Glucagon secretion from pancreatic α-cells has been suggested to be modulated by blood glucose, signals from the nervous system and endocrine components. In addition to these regulators, intraislet factors acting in a paracrine manner from neighbouring β-cells are emerging as central modulator(s) of α-cell biology. One of the most important of these paracrine factors, insulin, modulates glucagon secretion. Indeed, the α-cell-specific insulin receptor knockout (αIRKO) mouse manifests hypersecretion of glucagon in the postprandial stage and exhibits defective secretion in fasting-induced hypoglycaemia, together mimicking the α-cell defects observed in type 2 diabetes. Interestingly, αIRKO mice display a progressive increase in β-cell mass and a concomitant decrease in α-cells. Lineage trace analyses reveal that the new β-cells originate, in part, from the insulin receptor-deficient α-cells indicating a critical role for α-cell insulin signalling in determining β-cell origin. Our studies also reveal that glucagon-like peptide-1 (GLP-1) treatment of αIRKO mice suppresses glucagon secretion despite absence of functional insulin receptors precluding a role for insulin in GLP-1 action on α-cells in this model. These findings highlight the significance of insulin signalling in the regulation of α-cell biology.

Incretin hormones and the expanding families of glucagon-like sequences and their receptors

Peptide hormones encoded by the proglucagon (Gcg) and glucose-dependent insulinotropic polypeptide (Gip) genes are evolutionarily related glucagon-like sequences and act through a subfamily of G-protein-coupled receptors. A better understanding of the evolutionary history of these hormones and receptors should yield insight into their biological functions. The availability of a large number of near-complete vertebrate genome sequences is a powerful resource to address questions concerning the evolution of sequences; here, we utilize these resources to examine the evolution of glucagon-like sequences and their receptors. These studies led to the discovery of novel genes for a glucagon receptor-like receptor (Grlr) and a glucagon-like sequence (exendin) in vertebrates. Both exendin and GRLR have ancient origins, early in vertebrate evolution, but have been lost on the ancestral lineage leading to extant mammals. We also show that exendin and GRLR are both expressed in the brain of the chicken and Xenopus tropicals, results that suggest that the products of these genes function in this tissue. The lack of exendin or Grlr genes in mammals suggests that other genes may have acquired the functions of exendin and Grlr during mammalian evolution.

Glucagon gene expression in the endocrine pancreas: the role of the transcription factor Pax6 in α-cell differentiation, glucagon biosynthesis and secretion

The glucagon gene is expressed in α-cells of the pancreas, L cells of the intestine and the hypothalamus. The determinants of the α-cell-specific expression of the glucagon gene are not fully characterized, although Arx, Pax6 and Foxa2 are critical for α-cell differentiation and glucagon gene expression; in addition, the absence of the β-cell-specific transcription factors Pdx1, Pax4 and Nkx6.1 may allow for the glucagon gene to be expressed. Pax6, along with cMaf and MafB, binds to the DNA control element G(1) which confers α-cell specificity to the promoter and to G(3) and potently activates glucagon gene transcription. In addition, to its direct role on the transcription of the glucagon gene, Pax6 controls several transcription factors involved in the activation of the glucagon gene such as cMaf, MafB and NeuroD1/Beta2 as well as different steps of glucagon biosynthesis and secretion. We conclude that Pax6 independently of Arx and Foxa2 is critical for α-cell function by coordinating glucagon gene expression as well as glucagon biosynthesis and secretion.

Novel insight into glucagon receptor action: lessons from knockout and transgenic mouse models

Using knockout and transgenic technology, genetically modified animal models allowed us to understand the role of glucagon signalling in metabolism. Mice with a global deletion of the glucagon receptor gene (Gcgr) were designed using gene targeting. The phenotype of Gcgr(-/-) mouse provided important clues about the role of Gcgr in foetal growth, pancreatic development and glucose and lipid homeostasis. The lack of Gcgr activation was associated with: (i) hypoglycaemic pregnancies, poor foetal growth and increased foetal-neonatal demise; (ii) altered cytoarchitecture of pancreatic islets; (iii) altered glucose, lipid and hormonal milieu; (iv) reduced gastric emptying; (v) altered body composition and protection from diet-induced obesity; (vi) altered energy state; (vii) impaired hepatocyte survival; (viii) altered metabolic response to prolonged fasting and exercise and (ix) prevented development of diabetes in insulin-deficient mice. In contrast, mice overexpressing the Gcgr on pancreatic β-cells displayed an increase insulin secretion, pancreatic insulin content and β-cell mass, and partially protected against hyperglycaemia and impaired glucose tolerance when fed a high-fat diet. These findings suggest that glucagon signalling plays a significant role in the regulation of glucose and lipid homeostasis. Treatment options designed to block Gcgr activation may have negative implications in the treatment of diabetes.

Endothelium-derived Netrin-4 supports pancreatic epithelial cell adhesion and differentiation through integrins α2β1 and α3β1

BACKGROUND

Netrins have been extensively studied in the developing central nervous system as pathfinding guidance cues, and more recently in non-neural tissues where they mediate cell adhesion, migration and differentiation. Netrin-4, a distant relative of Netrins 1-3, has been proposed to affect cell fate determination in developing epithelia, though receptors mediating these functions have yet to be identified.

METHODOLOGY/PRINCIPAL FINDINGS

Using human embryonic pancreatic cells as a model of developing epithelium, here we report that Netrin-4 is abundantly expressed in vascular endothelial cells and pancreatic ductal cells, and supports epithelial cell adhesion through integrins α2β1 and α3β1. Interestingly, we find that Netrin-4 recognition by embryonic pancreatic cells through integrins α2β1 and α3β1 promotes insulin and glucagon gene expression. In addition, full genome microarray analysis revealed that fetal pancreatic cell adhesion to Netrin-4 causes a prominent down-regulation of cyclins and up-regulation of negative regulators of the cell cycle. Consistent with these results, a number of other genes whose activities have been linked to developmental decisions and/or cellular differentiation are up-regulated.

CONCLUSIONS/SIGNIFICANCE

Given the recognized function of blood vessels in epithelial tissue morphogenesis, our results provide a mechanism by which endothelial-derived Netrin-4 may function as a pro-differentiation cue for adjacent developing pancreatic cell populations expressing adhesion receptors α2β1 and α3β1 integrins.

Leptin downregulates expression of the gene encoding glucagon in alphaTC1-9 cells and mouse islets

AIMS/HYPOTHESIS

Leptin, released by adipocytes, can modulate glucose homeostasis through direct actions on pancreatic alpha and beta cells. Although this hormone rapidly regulates alpha cell exocytosis, its long-term effects on glucagon gene expression are currently unknown.

METHODS

We analysed glucagon mRNA levels and protein content in alphaTC1-9 cells and isolated mouse islets cultured with leptin, as well as in islets from mice treated in vivo with leptin. We also studied the involvement of the signal transducers and activators of transcription (STAT) pathway by western blot, immunofluorescence and interference RNA.

RESULTS

Leptin incubation (0.0625-18.75 nmol/l) for 24 h inhibited glucagon gene expression in alphaTC1-9 cells. This inhibitory effect was also observed in isolated mouse islets cultured with leptin, as well as in islets from mice treated with leptin for 5 days. In contrast, no changes were detected in islets from db/db mice, which lack leptin receptors. Leptin treatment also reduced the glucagon protein content in alphaTC1-9 cells and mouse islets. Moreover, leptin induced phosphorylation of STAT3 and its translocation to the nucleus, which was confirmed by western blot analysis in alphaTC1-9 cells and by immunofluorescence in isolated alpha cells. Interestingly, the effect of leptin on glucagon mRNA levels was significantly reduced by Stat3 knockdown. In contrast, pharmacological inhibition of the phosphoinositide 3-kinase pathway did not affect leptin actions.

CONCLUSIONS/INTERPRETATION

Our results demonstrate that leptin can regulate glucagon gene expression in alpha cells via a STAT3 pathway, and are important for understanding the role of leptin in glucose homeostasis.

Long-term enteral administration of melatonin reduces plasma insulin and increases expression of pineal insulin receptors in both Wistar and type 2-diabetic Goto-Kakizaki rats

This paper represents an essential aspect of recent investigations into the functional and clinical implications of insulin-melatonin interrelationships. The aim of the study was to analyze whether melatonin reduces insulin secretion in an animal in a manner comparable to the pattern observed in previous in vitro experiments; to this end, we used two models: Wistar and type 2-diabetic Goto-Kakizaki (GK) rats. Thirty-two Wistar and 32 GK rats were divided into two subgroups of 16 rats each; each subgroup was treated either with or without melatonin. The daily administration of melatonin, starting in 8-wk-old rats, was adjusted to 2.5 mg/kg body weight. Melatonin was given daily during the dark period for 12 hr. After 9 wk of treatment, the rats were sacrificed in the middle of the dark period. Melatonin administration strongly enhanced the plasma melatonin level and diminished the expression of pancreatic melatonin receptor-mRNA, whereas the expression of pineal AA-NAT and HIOMT was unchanged. Furthermore, the experiments showed in agreement with recent in vitro results of pancreatic islets that plasma insulin levels were diminished after melatonin treatment. However, the pineal insulin receptor expression was increased after melatonin administration. The pancreatic expression of glucagon, GLUT2, and glucokinase was decreased in GK rats, whereas the glucose levels, as well as the parameters of glucose sensing, GLUT2-mRNA, and glucokinase-mRNA, were unchanged after melatonin administration in both Wistar and GK rats. In summary, the results show that melatonin administration decreases plasma insulin levels in vivo and, furthermore, that an insulin-melatonin antagonism exists.

Multiple endocrine neoplasia type 1 deletion in pancreatic alpha-cells leads to development of insulinomas in mice

The pancreatic alpha- and beta-cells are critical components in regulating blood glucose homeostasis via secretion of glucagon and insulin, respectively. Both cell types are typically localized in the islets of Langerhans. However, little is known about the roles of paracrine interactions that contribute to their physiological functions. The lack of suitable cell lines to study alpha- and beta-cells interactions have led us to develop an alpha-cell-specific Cre-expressing transgenic line utilizing a glucagon promoter sequence, the Glu-Cre transgenic mouse. Here, we demonstrate that the Glu-Cre could specifically and efficiently excise floxed target genes in adult islet alpha-cells. We further showed that deletion of the tumor suppressor gene, multiple endocrine neoplasia type 1 (Men1), in alpha-cells led to tumorigenesis. However, to our surprise, the lack of Men1 in alpha-cells did not result in glucagonomas but rather beta-cell insulinomas. Because deletion of the Men1 alleles was only present in alpha-cells, our data suggested that cross communication between alpha- and beta-cells contributes to tumorigenesis in the absence of Men1. Together, we believed that the new model systems described here will allow future studies to decipher cellular interactions between islet alpha- and beta-cells in a physiological context.

Correlated evolution of androgen receptor and aromatase revisited

Conserved interactions among proteins or other molecules can provide strong evidence for coevolution across their evolutionary history. Diverse phylogenetic methods have been applied to identify potential coevolutionary relationships. In most cases, these methods minimally require comparisons of orthologous sequences and appropriate controls to separate effects of selection from the overall evolutionary relationships. In vertebrates, androgen receptor (AR) and cytochrome p450 aromatase (CYP19) share an affinity for androgenic steroids, which serve as receptor ligands and enzyme substrates. In a recent study, Tiwary and Li (Tiwary BK, Li W-H. 2009. Parallel evolution between aromatase and androgen receptor in the animal kingdom. Mol Biol Evol. 26:123-129) reported that AR and CYP19 displayed a signature of ancient and conserved interactions throughout all the Eumetazoa (i.e., cnidarians, protostomes, and deuterostomes). Because these findings conflicted with a number of previous studies, we reanalyzed the data set used by Tiwary and Li. First, our analyses demonstrate that the invertebrate genes used in the previous analysis are not orthologous sequences but instead represent a diverse set of nuclear receptors and CYP enzymes with no confirmed or hypothesized relationships with androgens. Second, we show that 1) their analytical approach, which measures correlations in evolutionary distances between proteins, potentially led to spurious significant relationships due simply to conserved domains and 2) control comparisons provide positive evidence for a strong influence of evolutionary history. We discuss how corrections to this method and analysis of key taxa (e.g., duplications in the teleost fish and suiform lineages) can inform investigations of the coevolutionary relationships between AR and aromatase.

Effect of forkhead box O1 (FOXO1) on beta cell development in the human fetal pancreas

AIMS/HYPOTHESIS

Recent studies have demonstrated that in adult murine beta cells the forkhead box O1 (FOXO1) transcription factor regulates proliferation and stress resistance. However, the role of FOXO1 during pancreatic development remains largely unknown. The present study aimed to characterise the expression of the FOXO1 transcription factor in the early to mid-gestation human fetal pancreas and to understand its role in islet cell development.

METHODS

Human (8-21 week fetal age) pancreases were examined using immunohistological, quantitative RT-PCR and western blotting. Isolated human (18-21 week) fetal islet epithelial cell clusters were treated with insulin or glucose, or transfected with FOXO1 small interfering RNA (siRNA).

RESULTS

Nuclear and cytoplasmic FOXO1 were widely produced during human fetal endocrine pancreatic development, co-localising in cells with the transcription factors pancreatic and duodenal homeobox 1 (PDX-1) and neurogenin 3 (NGN3) as well as cytokeratin 19 (CK19), insulin and glucagon. Treatment with exogenous insulin (50 nmol/l) induced the nuclear exclusion of FOXO1 in both cytokeratin 19 (CK19)(+) (p < 0.01) and insulin(+) cells (p < 0.05) in parallel with increased phospho-Akt (p < 0.05) production. siRNA knockdown of FOXO1 significantly increased the number of NGN3(+) (p < 0.01) and NK6 homeobox 1 (NKX6-1)(+) (p < 0.05) cells in parallel with increases in insulin gene expression (p < 0.03) and C-peptide(+) cells (p < 0.05) and reduced levels of hairy and enhancer of split 1 (HES1) (p < 0.01).

CONCLUSIONS/INTERPRETATION

Our results indicate that FOXO1 may negatively regulate beta cell differentiation in the human fetal pancreas by controlling critical transcription factors, including NGN3 and NKX6-1. These data suggest that the manipulation of FOXO1 levels may be a useful tool for improving cell-based strategies for the treatment of diabetes.

[Expression of glucagon in gestational and diet-induced hyperlipidemic gestational rats]

AIM

To observe the difference in the expression of glucagon in normal and in diet-induced hyperlipidemic gestational rats.

METHODS

Thirty-two female SD rats were divided into control group, hyperlipidemia group, gestation group and hyperlipidemic-gestation group. Fourteen days after gestation, real-time quantitative PCR was performed to detect the expression of preglucagon mRNA and Western blot was employed to detect the expression of glucagon in pancreas in different groups.

RESULTS

The glucose peak of OGTT was 30 minutes after glucose load in all the rats. The insulin peak of OGTT was 15 minutes after glucose load in controls, while the insulin peak was 30 minutes after glucose load in other groups. Fasting insulin in both gestation rats and hyperlipidemic-gestation rats was higher than that in controls(21.68+/-2.55 vs 14.35+/-0.86 mIU/L, P<0.05; 25.76+/-3.31 vs 14.35+/-0.86 mIU/L, P<0.01). Insulin AUC in both hyperlipidemic-gestation rats and hyperlipidemia rats was also higher than that in controls(204.60+/-79.06 vs 129.71+/-11.33 mIU/L, P<0.05; 230.25+/-13.19 vs 129.71+/-11.33 mIU/L, P<0.05). PG relative mRNA levels in gestation rats and in hyperlipidemic-gestation rats were respectively 1.46 fold and 1.77 fold higher than that in controls (P<0.05 and P<0.01)and 1.54 fold higher than that in hyperlipidemia rats (P=0.01). Glucagon in gestation rats and hyperlipidemic-gestation rats was respectively 2.57 fold and 3.44 fold higher than that in controls (both P<0.01)and 2.9 fold higher than that in hyperlipidemia rats(P<0.01).

CONCLUSION

Both hyperlipidemia and gestation may delay the releasing peak of insulin after glucose load. In normal gestation it is mainly reflected by fasting insulin resistance and in hyperlipidemic-gestation it is reflected by both fasting and glucose load insulin resistance. The rising pancreatic glucagons may partly lead to the increased gestational insulin resistance.

Characteristics and functions of {alpha}-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors expressed in mouse pancreatic {alpha}-cells

Pancreatic islet cells use neurotransmitters such as l-glutamate to regulate hormone secretion. We determined which cell types in mouse pancreatic islets express ionotropic glutamate receptor channels (iGluRs) and describe the detailed biophysical properties and physiological roles of these receptors. Currents through iGluRs and the resulting membrane depolarization were measured with patch-clamp methods. Ca(2+) influx through voltage-gated Ca(2+) channels and Ca(2+)-evoked exocytosis were detected by Ca(2+) imaging and carbon-fiber microamperometry. Whereas iGluR2 glutamate receptor immunoreactivity was detected using specific antibodies in immunocytochemically identified mouse alpha- and beta-cells, functional iGluRs were detected only in the alpha-cells. Fast application of l-glutamate to cells elicited rapidly activating and desensitizing inward currents at -60 mV. By functional criteria, the currents were identified as alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors. They were activated and desensitized by AMPA, and were activated only weakly by kainate. The desensitization by AMPA was inhibited by cyclothiazide, and the currents were blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Islet iGluRs showed nonselective cation permeability with a low Ca(2+) permeability (P(Ca)/P(Na) = 0.16). Activation of the AMPA receptors induced a sequence of cellular actions in alpha-cells: 1) depolarization of the membrane by 27 +/- 3 mV, 2) rise in intracellular Ca(2+) mainly mediated by voltage-gated Ca(2+) channels activated during the membrane depolarization, and 3) increase of exocytosis by the Ca(2+) rise. In conclusion, iGluRs expressed in mouse alpha-cells resemble the low Ca(2+)-permeable AMPA receptor in brain and can stimulate exocytosis.

Single pancreatic beta cells co-express multiple islet hormone genes in mice

AIMS/HYPOTHESIS

It is widely accepted that production of insulin, glucagon, somatostatin and pancreatic polypeptide in islet cells is specific to beta, alpha, delta and pancreatic polypeptide cells, respectively. We examined whether beta cells express other genes encoding islet hormones.

METHODS

Nested RT-PCR was performed on single beta cells of transgenic mice with green fluorescent protein (GFP) driven by mouse insulin I promoter (MIP-GFP).

RESULTS

Only 55% of adult beta cells expressed the insulin gene alone, while others expressed two or more islet hormone genes; 4% expressed all four hormone genes. In embryonic and neonatal cells, 60% to 80% of GFP(+) cells co-expressed pancreatic polypeptide and insulin genes in contrast to 29% in adult. To clarify cell fate, we conducted lineage tracing using rat insulin II promoter-cre mice crossed with reporter mice Gt(ROSA)26Sor-loxP-flanked STOP-cassette-GFP. All GFP(+) cells expressed insulin I and II genes, and showed similar heterogeneity of co-expression to that seen in MIP-GFP mice. Although we report expression of other hormone genes in a significant proportion of beta cells, our lineage tracing results demonstrate that after inducing InsII (also known as Ins2) expression, beta cell progenitors do not redifferentiate to non-beta cells.

CONCLUSIONS/INTERPRETATION

This study shows co-expression of multiple hormone genes in beta cells of adult mice as well as in embryos and neonates. This finding could: (1) represent residual expression from beta cell precursors; (2) result from alternative developmental pathways for beta cells; or (3) denote the differentiation potential of these cells. It may be linked to functional heterogeneity. This heterogeneity in gene expression may provide a means to characterise the functional, cellular and developmental heterogeneity seen in beta cells.

Placental stem cell differentiation into islets of Langerhans-like glucagon-secreting cells

BACKGROUND AND PURPOSE

For the past few years, in an attempt to find new sources of cells that may be used in cell therapy, numerous researchers have highlighted the particular properties of mesenchymal stem cells. Mesenchymal stem cells can be isolated from adult tissues such as the bone marrow or adipose tissue, but also from other organs such as the human placenta. Our study focuses adult stem cells isolated from the chorionic villi in an attempt to differentiate them into islets of Langerhans in order to study their differentiation potential, as a future background for cell therapy.

EXPERIMENTAL DESIGN

Full-term placentas were prelevated from volunteer women that have just delivered a normal pregnancy. After a mechanical fragmentation of the placenta, the chorion fragments are transferred in a dish with dispase before the enzyme is inactivated using fetal calf serum. The cell suspension is filtered in order to obtain a single-cell suspension. After the adherence of the first cells, the proliferation rate increased progressively and cell morphology is kept the same for several passages. In order to correctly differentiate placental stem cells into glucagon-secreting cells, we used a culture method on a scaffold with sequential exposure to different growth factors. The underlying substrate used contained type IV collagen, chytosan, Matrigel and laminin. Molecular biology techniques were carried out to investigate the gene expression of the stem cells.

RESULTS

Our results show that exendin-4 is able to induce the differentiation of placental stem cells into glucagon-secreting cells. We also notice the absence of the insulin gene, a conclusion that may be explained by the fact that our phenotype is a partial one, incomplete, closer to islet cell progenitors than to insulin-producing progenitors.

CONCLUSIONS

The identification of the placenta as a valid source for stem cells has important practical advantages because it is easily accessible, it raises no ethical issues and cells are easily to isolate in a large enough number to use. The future knowledge and manipulation of the signaling pathways that determines the dramatic phenotype shift may provide the basis for efficient cell differentiation, with great impact on regenerative medicine and tissue engineering.