Age-dependent reduction in insulin secretion and insulin mRNA in isolated islets from rats

Blockade of cannabinoid 1 receptor improves glucose responsiveness in pancreatic beta cells

Cannabinoid 1 receptors (CB1Rs) are expressed in peripheral tissues, including islets of Langerhans, where their function(s) is under scrutiny. Using mouse β-cell lines, human islets and CB1R-null (CB1R-/- ) mice, we have now investigated the role of CB1Rs in modulating β-cell function and glucose responsiveness. Synthetic CB1R agonists diminished GLP-1-mediated cAMP accumulation and insulin secretion as well as glucose-stimulated insulin secretion in mouse β-cell lines and human islets. In addition, silencing CB1R in mouse β cells resulted in an increased expression of pro-insulin, glucokinase (GCK) and glucose transporter 2 (GLUT2), but this increase was lost in β cells lacking insulin receptor. Furthermore, CB1R-/- mice had increased pro-insulin, GCK and GLUT2 expression in β cells. Our results suggest that CB1R signalling in pancreatic islets may be harnessed to improve β-cell glucose responsiveness and preserve their function. Thus, our findings further support that blocking peripheral CB1Rs would be beneficial to β-cell function in type 2 diabetes.

Animal models of obesity and diabetes mellitus

More than one-third of the worldwide population is overweight or obese and therefore at risk of developing type 2 diabetes mellitus. In order to mitigate this pandemic, safer and more potent therapeutics are urgently required. This necessitates the continued use of animal models to discover, validate and optimize novel therapeutics for their safe use in humans. In order to improve the transition from bench to bedside, researchers must not only carefully select the appropriate model but also draw the right conclusions. In this Review, we consolidate the key information on the currently available animal models of obesity and diabetes and highlight the advantages, limitations and important caveats of each of these models.

Long-term activation of PKA in β-cells provides sustained improvement to glucose control, insulin sensitivity and body weight

Type 2 diabetes is associated with obesity, insulin resistance and β-cell failure. Therapeutic aims are to reduce adiposity, improve insulin sensitivity and enhance β-cell function. However, it has been proposed that chronically increasing insulin release leads to β-cell exhaustion and failure. We previously developed mice to have increased activity of the cAMP-dependent protein kinase (PKA), specifically in β-cells (β-caPKA mice). β-caPKA mice have enhanced acute phase insulin release, which is the primary determinant of the efficacy of glucose clearance. Here these mice were used to determine the sustainability of enhanced insulin secretion, and to characterize peripheral effects of enhanced β-cell function. Increased PKA activity was induced by tamoxifen administration at 10 weeks of age. Male mice were aged to 12 months of age and female mice to 16 months. Glucose control in both male and female β-caPKA mice was significantly improved relative to littermate controls with ad libitum feeding, upon refeeding after fasting, and in glucose tolerance tests. In female mice insulin release was both greater and more rapid than in controls. Female mice were more insulin sensitive than controls. Male and female β-caPKA mice had lower body weights than controls. DEXA analysis of male mice revealed that this was due to reduced adiposity and not due to changes in lean body mass. This study indicates that targeting β-cells to enhance insulin release is sustainable, maintains insulin sensitivity and reduces body weight. These data identify β-cell PKA activity as a novel target for obesity therapies.

GIT2 Acts as a Systems-Level Coordinator of Neurometabolic Activity and Pathophysiological Aging

Aging represents one of the most complicated and highly integrated somatic processes. Healthy aging is suggested to rely upon the coherent regulation of hormonal and neuronal communication between the central nervous system and peripheral tissues. The hypothalamus is one of the main structures in the body responsible for sustaining an efficient interaction between energy balance and neurological activity and therefore likely coordinates multiple systems in the aging process. We previously identified, in hypothalamic and peripheral tissues, the G protein-coupled receptor kinase interacting protein 2 (GIT2) as a stress response and aging regulator. As metabolic status profoundly affects aging trajectories, we investigated the role of GIT2 in regulating metabolic activity. We found that genomic deletion of GIT2 alters hypothalamic transcriptomic signatures related to diabetes and metabolic pathways. Deletion of GIT2 reduced whole animal respiratory exchange ratios away from those related to primary glucose usage for energy homeostasis. GIT2 knockout (GIT2KO) mice demonstrated lower insulin secretion levels, disruption of pancreatic islet beta cell mass, elevated plasma glucose, and insulin resistance. High-dimensionality transcriptomic signatures from islets isolated from GIT2KO mice indicated a disruption of beta cell development. Additionally, GIT2 expression was prematurely elevated in pancreatic and hypothalamic tissues from diabetic-state mice (db/db), compared to age-matched wild type (WT) controls, further supporting the role of GIT2 in metabolic regulation and aging. We also found that the physical interaction of pancreatic GIT2 with the insulin receptor and insulin receptor substrate 2 was diminished in db/db mice compared to WT mice. Therefore, GIT2 appears to exert a multidimensional "keystone" role in regulating the aging process by coordinating somatic responses to energy deficits.

Glucose homeostasis, obesity and diabetes

Plasma glucose levels are maintained within a narrow range in normal individuals. Both insulin-dependent and insulin-independent processes contribute to fasting and postprandial plasma glucose regulation. The brain and nervous system are insulin independent. Muscle and adipose tissue are responsive to insulin and can use either glucose or ketones and free fatty acids as their primary metabolic fuel. The essential components of metabolic syndrome are obesity, glucose intolerance, insulin resistance, lipid disturbances, and hypertension. The risk of type 2 diabetes increases exponentially as body mass index increases above about 25 kg/m2. The links between obesity and type 2 diabetes include proinflammatory cytokines, insulin resistance, deranged fatty acid metabolism, and cellular processes. Modest weight reduction can improve glycaemic control and reduce diabetes risk. Obesity also leads to hyperinsulinaemia and insulin resistance, with a progressive decrease in insulin secretory function. Ageing is another important risk factor for metabolic disorders, including obesity, impaired glucose tolerance, and type 2 diabetes.

Age-related impairment in insulin release: the essential role of β(2)-adrenergic receptor

In this study, we investigated the significance of β(2)-adrenergic receptor (β(2)AR) in age-related impaired insulin secretion and glucose homeostasis. We characterized the metabolic phenotype of β(2)AR-null C57Bl/6N mice (β(2)AR(-/-)) by performing in vivo and ex vivo experiments. In vitro assays in cultured INS-1E β-cells were carried out in order to clarify the mechanism by which β(2)AR deficiency affects glucose metabolism. Adult β(2)AR(-/-) mice featured glucose intolerance, and pancreatic islets isolated from these animals displayed impaired glucose-induced insulin release, accompanied by reduced expression of peroxisome proliferator-activated receptor (PPAR)γ, pancreatic duodenal homeobox-1 (PDX-1), and GLUT2. Adenovirus-mediated gene transfer of human β(2)AR rescued these defects. Consistent effects were evoked in vitro both upon β(2)AR knockdown and pharmacologic treatment. Interestingly, with aging, wild-type (β(2)AR(+/+)) littermates developed impaired insulin secretion and glucose tolerance. Moreover, islets from 20-month-old β(2)AR(+/+) mice exhibited reduced density of β(2)AR compared with those from younger animals, paralleled by decreased levels of PPARγ, PDX-1, and GLUT2. Overexpression of β(2)AR in aged mice rescued glucose intolerance and insulin release both in vivo and ex vivo, restoring PPARγ/PDX-1/GLUT2 levels. Our data indicate that reduced β(2)AR expression contributes to the age-related decline of glucose tolerance in mice.

Cannabinoids inhibit insulin receptor signaling in pancreatic β-cells

OBJECTIVE

Optimal glucose homeostasis requires exquisitely precise adaptation of the number of insulin-secreting β-cells in the islets of Langerhans. Insulin itself positively regulates β-cell proliferation in an autocrine manner through the insulin receptor (IR) signaling pathway. It is now coming to light that cannabinoid 1 receptor (CB1R) agonism/antagonism influences insulin action in insulin-sensitive tissues. However, the cells on which the CB1Rs are expressed and their function in islets have not been firmly established. We undertook the current study to investigate if intraislet endogenous cannabinoids (ECs) regulate β-cell proliferation and if they influence insulin action.

RESEARCH DESIGN AND METHODS

We measured EC production in isolated human and mouse islets and β-cell line in response to glucose and KCl. We evaluated human and mouse islets, several β-cell lines, and CB1R-null (CB1R(-/-)) mice for the presence of a fully functioning EC system. We investigated if ECs influence β-cell physiology through regulating insulin action and demonstrated the therapeutic potential of manipulation of the EC system in diabetic (db/db) mice.

RESULTS

ECs are generated within β-cells, which also express CB1Rs that are fully functioning when activated by ligands. Genetic and pharmacologic blockade of CB1R results in enhanced IR signaling through the insulin receptor substrate 2-AKT pathway in β-cells and leads to increased β-cell proliferation and mass. CB1R antagonism in db/db mice results in reduced blood glucose and increased β-cell proliferation and mass, coupled with enhanced IR signaling in β-cells. Furthermore, CB1R activation impedes insulin-stimulated IR autophosphorylation on β-cells in a Gα(i)-dependent manner.

CONCLUSIONS

These findings provide direct evidence for a functional interaction between CB1R and IR signaling involved in the regulation of β-cell proliferation and will serve as a basis for developing new therapeutic interventions to enhance β-cell function and proliferation in diabetes.

Development of diabetes mellitus in aging transgenic mice following suppression of pancreatic homeoprotein IDX-1

Monogenic forms of diabetes can result from mutations in genes encoding transcription factors. Mutations in the homeodomain transcription factor IDX-1, a critical regulator of pancreas development and insulin gene transcription, confer a strong predisposition to the development of diabetes mellitus in humans. To investigate the role of IDX-1 expression in the pathogenesis of diabetes, we developed a model for the inducible impairment of IDX-1 expression in pancreatic beta cells in vivo by engineering an antisense ribozyme specific for mouse IDX-1 mRNA under control of the reverse tetracycline transactivator (rtTA). Doxycycline-induced impairment of IDX-1 expression reduced activation of the Insulin promoter but activated the Idx-1 promoter, suggesting that pancreatic beta cells regulate IDX-1 transcription to maintain IDX-1 levels within a narrow range. In transgenic mice that express both rtTA and the antisense ribozyme construct, impaired IDX-1 expression elevated glycated hemoglobin levels, diminished glucose tolerance, and decreased insulin/glucose ratios. Metabolic phenotypes induced by IDX-1 deficiency were observed predominantly in male mice over 18 months of age, suggesting that cellular mechanisms to protect IDX-1 levels in pancreatic beta cells decline with aging. We propose that even in the absence of Idx-1 gene mutations, pathophysiological processes that decrease IDX-1 levels are likely to impair glucose tolerance. Therapeutic strategies to attain normal glucose homeostasis by restoring normal IDX-1 levels may be of particular importance for older individuals with diabetes mellitus.

Insulin promoter factor-1 gene mutation linked to early-onset type 2 diabetes mellitus directs expression of a dominant negative isoprotein

The homeodomain transcription factor insulin promoter factor-1 (IPF-1) is required for development of the pancreas and also mediates glucose-responsive stimulation of insulin gene transcription. Earlier we described a human subject with pancreatic agenesis attributable to homozygosity for a cytosine deletion in codon 63 of the IPF-1 gene (Pro63fsdelC). Pro63fsdelC resulted in the premature truncation of an IPF-1 protein which lacked the homeodomain required for DNA binding and nuclear localization. Subsequently, we linked the heterozygous state of this mutation with type 2 diabetes mellitus in the extended family of the pancreatic agenesis proband. In the course of expressing the mutant IPF-1 protein in eukaryotic cells, we detected a second IPF-1 isoform, recognized by COOH- but not NH2-terminal-specific antisera. This isoform localizes to the nucleus and retains DNA-binding functions. We provide evidence that internal translation initiating at an out-of-frame AUG accounts for the appearance of this protein. The reading frame crosses over to the wild-type IPF-1 reading frame at the site of the point deletion just carboxy proximal to the transactivation domain. Thus, the single mutated allele results in the translation of two IPF-1 isoproteins, one of which consists of the NH2-terminal transactivation domain and is sequestered in the cytoplasm and the second of which contains the COOH-terminal DNA-binding domain, but lacks the transactivation domain. Further, the COOH-terminal mutant IPF-1 isoform does not activate transcription and inhibits the transactivation functions of wild-type IPF-1. This circumstance suggests that the mechanism of diabetes in these individuals may be due not only to reduced gene dosage, but also to a dominant negative inhibition of transcription of the insulin gene and other beta cell-specific genes regulated by the mutant IPF-1.

Glucagon-like peptide-1 can reverse the age-related decline in glucose tolerance in rats

Wistar rats develop glucose intolerance and have a diminished insulin response to glucose with age. The aim of this study was to investigate if these changes were reversible with glucagon-like peptide-1 (GLP-1), a peptide that we have previously shown could increase insulin mRNA and total insulin content in insulinoma cells. We infused 1.5 pmol/ kg-1.min-1 GLP-1 subcutaneously using ALZET microosmotic pumps into 22-mo-old Wistar rats for 48 h. Rat infused with either GLP-1 or saline were then subjected to an intraperitoneal glucose (1 g/kg body weight) tolerance test, 2 h after removing the pump. 15 min after the intraperitoneal glucose, GLP-1-treated animals had lower plasma glucose levels (9.04+/-0.92 mmol/liter, P < 0.01) than saline-treated animals (11.61+/-0.23 mmol/liter). At 30 min the plasma glucose was still lower in the GLP-1-treated animals (8.61+/-0.39 mmol/liter, P < 0.05) than saline-treated animals (10.36+/-0.43 mmol/liter). This decrease in glucose levels was reflected in the higher insulin levels attained in the GLP-1-treated animals (936+/-163 pmol/liter vs. 395+/-51 pmol/liter, GLP-1 vs. saline, respectively, P < 0.01), detected 15 min after glucose injection. GLP-1 treatment also increased pancreatic insulin, GLUT2, and glucokinase mRNA in the old rats. The effects of GLP-1 were abolished by simultaneous infusion of exendin [9-39], a specific antagonist of GLP-1. GLP-1 is therefore able to reverse some of the known defects that arise in the beta cell of the pancreas of Wistar rats, not only by increasing insulin secretion but also by inducing significant changes at the molecular level.