Abnormal insulin secretion and glucose metabolism in pancreatic islets from the spontaneously diabetic GK rat

Matching model with mechanism: Appropriate rodent models for studying various aspects of diabetes pathophysiology

Many rodent models are available for preclinical diabetes research making it a challenge for researchers to choose the most appropriate one for their experimental question. To aid in this, models have classically been categorized according to which type of diabetes they represent, and further into whether the model is induced, spontaneous or the result of genetic manipulation. This fails to capture the complexity of pathogenesis seen in diabetes in humans. This includes pathogenesis specifically involving the beta cell, which is no longer considered to be innocuous in the development and progression of diabetes. In this chapter we explore rodent models that incorporate the initiating factors believed to be involved in type 1 diabetes (autoimmunity) and type 2 diabetes (insulin resistance), before further discussing rodents that can be used to model specific mechanisms involved in a failure of functional beta cell mass (impaired beta cell function and beta cell apoptosis). We segregate models of beta cell pathogenesis based on the beta cell stressor predominantly associated with phenotype, but it is important to consider that most rodent models will exhibit more than one beta cell stressor. Similarly, many models exhibit more than one pathogenic mechanism, for example the same model may show insulin resistance, impaired beta cell function as well as beta cell loss. This can complicate interpretation of results and should be considered, and the model thoroughly researched, during the experimental planning stage.

β-cell intrinsic dynamics rather than gap junction structure dictates subpopulations in the islet functional network

Diabetes is caused by the inability of electrically coupled, functionally heterogeneous β-cells within the pancreatic islet to provide adequate insulin secretion. Functional networks have been used to represent synchronized oscillatory [Ca2+] dynamics and to study β-cell subpopulations, which play an important role in driving islet function. The mechanism by which highly synchronized β-cell subpopulations drive islet function is unclear. We used experimental and computational techniques to investigate the relationship between functional networks, structural (gap junction) networks, and intrinsic β-cell dynamics in slow and fast oscillating islets. Highly synchronized subpopulations in the functional network were differentiated by intrinsic dynamics, including metabolic activity and KATP channel conductance, more than structural coupling. Consistent with this, intrinsic dynamics were more predictive of high synchronization in the islet functional network as compared to high levels of structural coupling. Finally, dysfunction of gap junctions, which can occur in diabetes, caused decreases in the efficiency and clustering of the functional network. These results indicate that intrinsic dynamics rather than structure drive connections in the functional network and highly synchronized subpopulations, but gap junctions are still essential for overall network efficiency. These findings deepen our interpretation of functional networks and the formation of functional subpopulations in dynamic tissues such as the islet.

Contribution of animal models to diabetes research: Its history, significance, and translation to humans

Diabetes mellitus is still expanding globally and is epidemic in developing countries. The combat of this plague has caused enormous economic and social burdens related to a lowered quality of life in people with diabetes. Despite recent significant improvements of life expectancy in patients with diabetes, there is still a need for efforts to elucidate the complexities and mechanisms of the disease processes to overcome this difficult disorder. To this end, the use of appropriate animal models in diabetes studies is invaluable for translation to humans and for the development of effective treatment. In this review, a variety of animal models of diabetes with spontaneous onset in particular will be introduced and discussed for their implication in diabetes research.

Rodent models to study type 1 and type 2 diabetes induced human diabetic nephropathy

INTRODUCTION

Diabetic nephropathy (DN), an outcome of prolonged diabetes, has affected millions of people worldwide and every year the incidence and prevalence increase substantially. The symptoms may start with mild manifestations of the disease such as increased albuminuria, serum creatinine levels, thickening of glomerular basement membrane, expansion of mesangial matrix to severe pathological symptoms such as glomerular lesions and tubulointerstitial fibrosis which may further proceed to cardiovascular dysfunction or end-stage renal disease.

PERSPECTIVE

Numerous therapeutic interventions are being explored for the management of DN, however, these interventions do not completely halt the progression of this disease and hence animal models are being explored to identify critical genetic and molecular parameters which could help in tackling the disease. Rodent models which mostly include mice and rats are commonly used experimental animals which provide a wide range of advantages in understanding the onset and progression of disease in humans and also their response to a wide range of interventions helps in the development of effective therapeutics. Rodent models of type 1 and type 2 diabetes induced DN have been developed utilizing different platforms and interventions during the last few decades some of which mimic various stages of diabetes ranging from early to later stages. However, a rodent model which replicates all the features of human DN is still lacking. This review tries to evaluate the rodent models that are currently available and understand their features and limitations which may help in further development of more robust models of human DN.

CONCLUSION

Using these rodent models can help to understand different aspects of human DN although further research is required to develop more robust models utilizing diverse genetic platforms which may, in turn, assist in developing effective interventions to target the disease at different levels.

Experimental Models to Study Diabetes Mellitus and Its Complications: Limitations and New Opportunities

Preclinical biomedical models are a fundamental tool to improve the knowledge and management of diseases, particularly in diabetes mellitus (DM) since, currently, the pathophysiological and molecular mechanisms involved in its development are not fully clarified, and there is no treatment to cure DM. This review will focus on the features, advantages and limitations of some of the most used DM models in rats, such as the spontaneous models: Bio-Breeding Diabetes-Prone (BB-DP) and LEW.1AR1-iddm, as representative models of type 1 DM (DM-1); the Zucker diabetic fatty (ZDF) and Goto-kakizaki (GK) rats, as representative models of type 2 DM (DM-2); and other models induced by surgical, dietary and pharmacological-alloxan and streptozotocin-procedures. Given the variety of DM models in rats, as well as the non-uniformity in the protocols and the absence of all the manifestation of the long-term multifactorial complications of DM in humans, the researchers must choose the one that best suits the final objectives of the study. These circumstances, added to the fact that most of the experimental research in the literature is focused on the study of the early phase of DM, makes it necessary to develop long-term studies closer to DM in humans. In this review, a recently published rat DM model induced by streptozotocin injection with chronic exogenous administration of insulin to reduce hyperglycaemia has also been included in an attempt to mimic the chronic phase of DM in humans.

Translation Animal Models of Diabetic Kidney Disease: Biochemical and Histological Phenotypes, Advantages and Limitations

Animal models play a crucial role in studying the pathogenesis of diseases, developing new drugs, identifying disease risk markers, and improving means of prevention and treatment. However, modeling diabetic kidney disease (DKD) has posed a challenge for scientists. Although numerous models have been successfully developed, none of them can encompass all the key characteristics of human DKD. It is essential to choose the appropriate model according to the research needs, as different models develop different phenotypes and have their limitations. This paper provides a comprehensive overview of biochemical and histological phenotypes, modeling mechanisms, advantages and limitations of DKD animal models, in order to update relevant model information and provide insights and references for generating or selecting the appropriate animal models to fit different experimental needs.

Selectively bred rodent models for studying the etiology of type 2 diabetes: Goto-Kakizaki rats and Oikawa-Nagao mice

Type 2 diabetes (T2D) is a polygenic disease and studies to understand the etiology of the disease have required selectively bred animal models with polygenic background. In this review, we present two models; the Goto-Kakizaki (GK) rat and the Oikawa-Nagao Diabetes-Prone (ON-DP) and Diabetes-Resistant (ON-DR) mouse. The GK rat was developed by continuous selective breeding for glucose tolerance from the outbred Wistar rat around 50 years ago. The main cause of spontaneous hyperglycemia in this model is insulin secretion deficiency from pancreatic β-cells and mild insulin resistance in insulin target organs. A disadvantage of the GK rat is that environmental factors have not been considered in the selective breeding. Hence, the GK rat may not be suitable for elucidating predisposition to diabetes under certain environmental conditions, such as a high-fat diet. Therefore, we recently established two mouse lines with different susceptibilities to diet-induced diabetes, which are prone and resistant to the development of diabetes, designated as the ON-DP and ON-DR mouse, respectively. The two ON mouse lines were established by continuous selective breeding for inferior and superior glucose tolerance after high-fat diet feeding in hybrid mice of three inbred strains. Studies of phenotypic differences between ON-DP and ON-DR mice and their underlying molecular mechanisms will shed light on predisposing factors for the development of T2D in the modern obesogenic environment. This review summarizes the background and the phenotypic differences and similarities of GK rats and ON mice and highlights the advantages of using selectively bred rodent models in diabetes research.

Role of Reactive Oxygen Species in Glucose Metabolism Disorder in Diabetic Pancreatic β-Cells

The dysfunction of pancreatic β-cells plays a central role in the onset and progression of type 2 diabetes mellitus (T2DM). Insulin secretory defects in β-cells are characterized by a selective impairment of glucose stimulation, and a reduction in glucose-induced ATP production, which is essential for insulin secretion. High glucose metabolism for insulin secretion generates reactive oxygen species (ROS) in mitochondria. In addition, the expression of antioxidant enzymes is very low in β-cells. Therefore, β-cells are easily exposed to oxidative stress. In islet studies using a nonobese T2DM animal model that exhibits selective impairment of glucose-induced insulin secretion (GSIS), quenching ROS generated by glucose stimulation and accumulated under glucose toxicity can improve impaired GSIS. Acute ROS generation and toxicity cause glucose metabolism disorders through different molecular mechanisms. Nuclear factor erythroid 2-related factor 2 (Nrf2), a transcription factor, is a master regulator of antioxidant defense and a potential therapeutic target in oxidative stress-related diseases, suggesting the possible involvement of Nrf2 in β-cell dysfunction caused by ROS. In this review, we describe the mechanisms of insulin secretory defects induced by oxidative stress in diabetic β-cells.

Alterations in Energy Metabolism, Mitochondrial Function and Redox Homeostasis in GK Diabetic Rat Tissues Treated with Aspirin

Our recent studies have demonstrated that aspirin treatment prevents inflammatory and oxidative stress-induced alterations in mitochondrial function, improves glucose tolerance and pancreatic endocrine function and preserves tissue-specific glutathione (GSH)-dependent redox homeostasis in Goto-Kakizaki (GK) diabetic rats. In the current study, we have investigated the mechanism of action of aspirin in maintaining mitochondrial bioenergetics and redox metabolism in the liver and kidneys of GK rats. Aspirin reduced the production of reactive oxygen species (ROS) and oxidative stress-induced changes in GSH metabolism. Aspirin treatment also improved mitochondrial respiratory function and energy metabolism, in addition to regulating the expression of cell signaling proteins that were altered in diabetic animals. Ultrastructural electron microscopy studies revealed decreased accumulation of glycogen in the liver of aspirin-treated diabetic rats. Hypertrophic podocytes with irregular fusion of foot processes in the renal glomerulus and detached microvilli, condensed nuclei and degenerated mitochondria observed in the proximal convoluted tubules of GK rats were partially restored by aspirin. These results provide additional evidence to support our previous observation of moderation of diabetic complications by aspirin treatment in GK rats and may have implications for cautious use of aspirin in the therapeutic management of diabetes.

Chronological Appearance of Endocrine and Metabolic Dysfunctions Induced by an Unhealthy Diet in Rats

Background and Objectives: The work was aimed to determine the chronological sequence of events triggered by a fructose-rich diet (FRD) (10% w/v in the drinking water) in normal rats. Material and Methods: Serum parameters, liver and islet markers of metabolism, inflammation and oxidative stress were determined weekly for 21 days. Results: At the end of the first week, rats fed with a FRD showed an early increase in circulating triglycerides, fat liver deposit, and enzymatic activity of liver glucokinase and glucose-6-phosphate dehydrogenase (G6P-DH). After two weeks of such a diet, liver glucose-6-phosphatase (G6Pase) activity and liver oxidative stress markers were significantly increased. Liver sterol regulatory element-binding protein 1c (SREBP1c) mRNA also increased in the second week while their target genes fatty acid synthase (FAS) and glycerol-3-phosphate dehydrogenase (GPAT) enhanced their expression at the third week. Liver and pancreatic inflammation markers also enhanced their gene expression in the last week of treatment. Whereas both control and FRD rats remained normoglycemic throughout the entire period of treatment, blood insulin levels were significantly higher in FRD animals at the third week, thereby evidencing an insulin-resistant state (higher HOMA-IR, HOMA-B and HIS indexes). Pancreatic islets isolated from rats fed with a FRD for 3 weeks also increased glucose-induced insulin secretion (8.3 and 16.7 mM). Conclusions: FRD induces asynchronous changes involving early hypertriglyceridemia together with intrahepatic lipid deposit and metabolic disturbances from week one, followed by enhanced liver oxidative stress, liver and pancreas inflammation, pancreatic β-cell dysfunction, and peripheral insulin-resistance registered at the third week. Knowledge of time-course adaptation mechanisms involved in our rat model could be helpful in developing appropriate strategies to prevent the progression from prediabetes to Type 2 diabetes (T2D) triggered by unhealthy diets.

Effect of Aspirin on Mitochondrial Dysfunction and Stress in the Pancreas and Heart of Goto-Kakizaki Diabetic Rats

Our previous study in Goto-Kakizaki (GK) type 2 diabetic rats provided significant evidence that aspirin treatment improves pancreatic β-cell function by reducing inflammatory responses and improving glucose tolerance. In the present study, we aimed to elucidate the mechanism of action of aspirin on the pathophysiology and progression of type 2 diabetic complications in the heart and pancreas of insulin-resistant GK rats. Aspirin treatment demonstrated a reduction in mitochondrial reactive oxygen species (ROS) production and lipid peroxidation, accompanied by improved redox homeostasis. Furthermore, the recovery of metabolic and mitochondrial functions, as well as cytochrome P450 enzyme activities, which were altered in the pancreas and heart of GK rats, were observed. Aspirin treatment brought the activity of CYP 2E1 to the control level in both tissues, whereas the CYP 3A4 level decreased only in the pancreas. This suggests the tissue-specific differential metabolism of substrates in these rats. The recovery of redox homeostasis could be the key target in the improvement of oxidative-stress-dependent alterations in mitochondrial functions which, in turn, facilitated improved energy metabolism in these tissues in the aspirin-treated GK rats. These results may have implications in determining the therapeutic use of aspirin, either alone or in combination with other clinically approved therapies, in insulin-resistant type 2 diabetes.

Small intestine remodeling in male Goto-Kakizaki rats

BACKGROUND

Obesity is associated with the development of insulin resistance (IR) and type-2 diabetes mellitus (T2DM); however, not all patients with T2DM are obese. The Goto-Kakizaki (GK) rat is an experimental model of spontaneous and non-obese T2DM. There is evidence that the intestine contributes to IR development in GK animals. This information prompted us to investigate small intestine remodeling in this animal model.

METHODS

Four-month-old male Wistar (control) and GK rats were utilized for the present study. After removing the small intestine, the duodenum, proximal jejunum, and distal ileum were separated. We then measured villi and muscular and mucosa layer histomorphometry, goblet cells abundance, total myenteric and submucosal neuron populations, and inflammatory marker expression in the small intestinal segments and intestinal transit of both groups of animals.

KEY RESULTS

We found that the GK rats exhibited decreased intestinal area (p < 0.0001), decreased crypt depth in the duodenum (p = 0.01) and ileum (p < 0.0001), increased crypt depth in the jejunum (p < 0.0001), longer villi in the jejunum and ileum (p < 0.0001), thicker villi in the duodenum (p < 0.01) and ileum (p < 0.0001), thicker muscular layers in the duodenum, jejunum, and ileum (p < 0.0001), increased IL-1β concentrations in the duodenum and jejunum (p < 0.05), and increased concentrations of NF-κB p65 in the duodenum (p < 0.01), jejunum and ileum (p < 0.05). We observed high IL-1β reactivity in the muscle layer, myenteric neurons, and glial cells of the experimental group. GK rats also exhibited a significant reduction in submucosal neuron density in the jejunum and ileum, ganglionic hypertrophy in all intestinal segments studied (p < 0.0001), and a slower intestinal transit (about 25%) compared to controls.

CONCLUSIONS

The development of IR and T2DM in GK rats is associated with small intestine remodeling that includes marked alterations in small intestine morphology, local inflammation, and reduced intestinal transit.

The Hypoglycemic and Renal Protection Properties of Crocin via Oxidative Stress-Regulated NF-κB Signaling in db/db Mice

Background

As the main ingredient of Crocus sativus L. (Iridaceae) extract, crocin- I (CR) has been reported to show various pharmacological activities. The aim of this study was to investigate the hypoglycemic and renal protection properties of CR in db/db mice.

Methods

Eight-week-old db/db mice were treated with metformin (Met) (100 mg/kg) and CR (50 mg/kg) for eight weeks.

Results

CR treatment showed hypoglycemic functions indicated by reduced bodyweight, food and water intake, plasma glucose, and serum levels of glycated hemoglobin A1c. Additionally, the CR group showed increased serum levels of insulin and pyruvate kinase, hypolipidemic functions indicated by the suppressed levels of total cholesterol and triglyceride, and enhanced levels of high-density lipoprotein cholesterol, which are also indicators of hypoglycemic functions. The renal protection function of CR was demonstrated by its protection of renal structures and its regulation of potential indicators of nephropathy. The anti-oxidation and anti-inflammation effects of CR were verified by enzyme-linked immunosorbent assay. In the kidneys of db/db mice, CR decreased the expression of phospho-IκBα and phospho-nuclear factor-κB (NF-κB), whereas it enhanced the expression of nuclear respiratory factor 2, manganese superoxide dismutase 1, heme oxygenase-1, and catalase.

Conclusions

The anti-diabetic and anti-diabetic nephritic effects of CR were related to its modulation of oxidative stress-mediated NF-κB signaling.

Effect of resveratrol on SNARE proteins expression and insulin resistance in skeletal muscle of diabetic rats

Objective(s):

Soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex proteins are involved in membrane trafficking. The expression of isoforms of SNAP-23, syntaxin-4, and VAMP-2 is significantly done in skeletal muscles; they control GLUT4 trafficking. It is believed that type 2 diabetes could be caused by the modifications in the expression of SNARE complex proteins. The purpose of this study was to evaluate the effect of resveratrol on the expression of these proteins in type 2 diabetes.

Materials and Methods:

Forty male Wistar rats were selected. Streptozotocin and nicotinamide were applied for the induction of type 2 diabetes. The animals were divided into five groups. Healthy and diabetic groups were set as control; resveratrol (1, 5, and 10 mg/kg body weight) was applied to treat the three groups of diabetic rats for 30 days. Real-time qRT-PCR was applied to evaluate the expression of SNARE complex proteins.

Results:

There is a link between diabetes and insulin resistance and up-regulation of SNARE proteins expression. Resveratrol improved hyperglycemia and insulin resistance along with a non-significant reduction in the expression of SNARE proteins.

Conclusion:

Increased expression of SNARE proteins was possibly a compensatory mechanism in response to insulin resistance in the skeletal muscles of diabetic rats. Resveratrol non-significantly reduced the expression of SNARE proteins by enhancing insulin sensitivity, where this effect was dose-dependent. Thus, higher doses of resveratrol and longer intervention periods could probably be more effective. Another molecular mechanism of the anti-diabetic properties of resveratrol was identified with an effect on the expression of SNARE proteins.

Selectively Bred Diabetes Models: GK Rats, NSY Mice, and ON Mice

The polygenic background of selectively bred diabetes models mimics the etiology of type 2 diabetes. So far, three different rodent models (Goto-Kakizaki rats, Nagoya-Shibata-Yasuda mice, and Oikawa-Nagao mice) have been established in the diabetes research field by continuous selective breeding for glucose tolerance from outbred rodent stocks. The origin of hyperglycemia in these rodents is mainly insulin secretion deficiency from the pancreatic β-cells and mild insulin resistance in insulin target organs. In this chapter, we summarize backgrounds and phenotypes of these rodent models to highlight their importance in diabetes research. Then, we introduce experimental methodologies to evaluate β-cell exocytosis as a putative common defect observed in these rodent models.

Anti-Diabetic Nephropathy Activities of Polysaccharides Obtained from Termitornyces albuminosus via Regulation of NF-κB Signaling in db/db Mice

Termitornyces albuminosus is a kind of traditional Chinese edible fungus rich in nutrients and medicinal ingredients, and it has anti-oxidative, analgesic and anti-inflammatory effects. However, the hypoglycemic and nephroprotective effects of polysaccharides separated from T. albuminosus (PTA) have not been reported. The properties of PTA were analyzed in a BKS.Cg-Dock7^m +/+ Lepr^db/JNju (db/db) mouse model of diabetes. After the administration of PTA for eight weeks, the hypoglycemic and hypolipidemic activities of PTA in the db/db mice were assessed. The results of a cytokine array combined with an enzyme-linked immunosorbent assay confirmed the anti-oxidative and anti-inflammatory activities of PTA. An eight-week administration of PTA caused hypoglycemic and hypolipidemic functioning, as indicated by suppressed plasma glucose levels, as well as the modulation of several cytokines related to glycometabolism, in the sera and kidneys of the mice. PTA treatment also had a protective effect on renal function, restoring renal structures and regulating potential indicators of nephropathy. In the kidneys of the db/db mice, PTA treatment reduced the activation of protein kinase B, the inhibitor of κB kinase alpha and beta, and the inhibitor of κB alpha and nuclear factor-κB (NF-κB). We establish the hypoglycemic, hypolipidemic, and anti-diabetic nephropathy effects of PTA, and we find that the renal protection effects of PTA may be related to anti-inflammatory activity via the regulation of NF-κB signaling.

Erythrocytes From Patients With Type 2 Diabetes Induce Endothelial Dysfunction Via Arginase I

BACKGROUND

Cardiovascular complications are major clinical problems in type 2 diabetes mellitus (T2DM). The authors previously demonstrated a crucial role of red blood cells (RBCs) in control of cardiac function through arginase-dependent regulation of nitric oxide export from RBCs. There is alteration of RBC function, as well as an increase in arginase activity, in T2DM.

OBJECTIVES

The authors hypothesized that RBCs from patients with T2DM induce endothelial dysfunction by up-regulation of arginase.

METHODS

RBCs were isolated from patients with T2DM and age-matched healthy subjects and were incubated with rat aortas or human internal mammary arteries from nondiabetic patients for vascular reactivity and biochemical studies.

RESULTS

Arginase activity and arginase I protein expression were elevated in RBCs from patients with T2DM (T2DM RBCs) through an effect induced by reactive oxygen species (ROS). Co-incubation of arterial segments with T2DM RBCs, but not RBCs from age-matched healthy subjects, significantly impaired endothelial function but not smooth muscle cell function in both healthy rat aortas and human internal mammary arteries. Endothelial dysfunction induced by T2DM RBCs was prevented by inhibition of arginase and ROS both at the RBC and vascular levels. T2DM RBCs induced increased vascular arginase I expression and activity through an ROS-dependent mechanism.

CONCLUSIONS

This study demonstrates a novel mechanism behind endothelial dysfunction in T2DM that is induced by RBC arginase I and ROS. Targeting arginase I in RBCs may serve as a novel therapeutic tool for the treatment of endothelial dysfunction in T2DM.

Noninvasive in vivo Assessment of the Re-endothelialization Process Using Ultrasound Biomicroscopy in the Rat Carotid Artery Balloon Injury Model

OBJECTIVES

Ultrasound biomicroscopy (UBM), or ultra high-frequency ultrasound, is a technique used to assess the anatomy of small research animals. In this study, UBM was used to assess differences in intimal hyperplasia thickness as a surrogate measurement of the re-endothelialization process after carotid artery balloon injury in rats.

METHODS

Ultrasound biomicroscopic data from 3 different experiments and rat strains (Sprague Dawley, Wistar, and diabetic Goto-Kakizaki) were analyzed. All animals were subjected to carotid artery balloon injury and examined with UBM (30-70 MHz) 2 and 4 weeks after injury. Re-endothelialization on UBM was defined as the length from the carotid bifurcation to the most distal visible edge of the intimal hyperplasia. En face staining with Evans blue dye was performed at euthanasia 4 weeks after injury, followed by tissue harvesting for histochemical and immunohistochemical evaluations.

RESULTS

A significant correlation (Spearman r = 0.63; P < .0001) was identified when comparing all measurements of re-endothelialization obtained from UBM and en face staining. The findings revealed a similar pattern for all rat strains: Sprague Dawley (Spearman r = 0.70; P < .0001), Wistar (Spearman r = 0.36; P < .081), and Goto-Kakizaki (Spearman r = 0.70; P < .05). A Bland-Altman test showed agreement between en face staining and UBM. Immunohistochemical staining confirmed the presence of the endothelium in the areas detected as re-endothelialized by the UBM assessment.

CONCLUSIONS

Ultrasound biomicroscopy can be used for repeated in vivo assessment of re-endothelialization after carotid artery balloon injury in rats.

Amino acid transporters in the regulation of insulin secretion and signalling

Amino acids are increasingly recognised as modulators of nutrient disposal, including their role in regulating blood glucose through interactions with insulin signalling. More recently, cellular membrane transporters of amino acids have been shown to form a pivotal part of this regulation as they are primarily responsible for controlling cellular and circulating amino acid concentrations. The availability of amino acids regulated by transporters can amplify insulin secretion and modulate insulin signalling in various tissues. In addition, insulin itself can regulate the expression of numerous amino acid transporters. This review focuses on amino acid transporters linked to the regulation of insulin secretion and signalling with a focus on those of the small intestine, pancreatic β-islet cells and insulin-responsive tissues, liver and skeletal muscle. We summarise the role of the amino acid transporter B⁰AT1 (SLC6A19) and peptide transporter PEPT1 (SLC15A1) in the modulation of global insulin signalling via the liver-secreted hormone fibroblast growth factor 21 (FGF21). The role of vesicular vGLUT (SLC17) and mitochondrial SLC25 transporters in providing glutamate for the potentiation of insulin secretion is covered. We also survey the roles SNAT (SLC38) family and LAT1 (SLC7A5) amino acid transporters play in the regulation of and by insulin in numerous affective tissues. We hypothesise the small intestine amino acid transporter B⁰AT1 represents a crucial nexus between insulin, FGF21 and incretin hormone signalling pathways. The aim is to give an integrated overview of the important role amino acid transporters have been found to play in insulin-regulated nutrient signalling.

Profiling and activity screening of Dammarane-type triterpen saponins from Gynostemma pentaphyllum with glucose-dependent insulin secretory activity

The global prevalence of type 2 diabetes is increasing rapidly; consequently there is great need for new and novel therapeutic options. Gynostemma pentaphyllum (GP) is a traditional medicinal plant, mainly present in Southeast Asian countries, that has been reported to exert antidiabetic effects, by stimulating insulin secretion. The specific compound responsible for this effect is however as yet unidentified. Screening for discovery and identification of bioactive compounds of an herbal GP extract, was performed in isolated pancreatic islets from spontaneously diabetic Goto-Kakizaki (GK) rats, a model of type 2 diabetes, and from non-diabetic control Wistar rats. From this herbal extract 27 dammarane-type saponins, including two novel compounds, were isolated and their structure was elucidated by mass spectrometry and NMR spectroscopy. One of the dammarane-type triterpenoid showed a glucose-dependent insulin secretion activity. This compound, gylongiposide I, displays unique abilities to stimulate insulin release at high glucose levels (16.7 mM), but limited effects at a low glucose concentration (3.3 mM). Further studies on this compound, also in vivo, are warranted with the aim of developing a novel anti-diabetic therapeutic with glucose-dependent insulinogenic effect.

Glutamate as intracellular and extracellular signals in pancreatic islet functions

l-Glutamate is one of the most abundant amino acids in the body and is a constituent of proteins and a substrate in metabolism. It is well known that glutamate serves as a primary excitatory neurotransmitter and a critical neuromodulator in the brain. Recent studies have shown that in addition to its pivotal role in neural functions, glutamate plays many important roles in a variety of cellular functions, including those as intracellular and extracellular signals. In pancreatic islets, glutamate is now known to be required for the normal regulation of insulin secretion, such as incretin-induced insulin secretion. In this review, we primarily discuss the physiological and pathophysiological roles of glutamate as intracellular and extracellular signals in the functions of pancreatic islets.

Characterization of the Goto-Kakizaki (GK) Rat Model of Type 2 Diabetes

The Goto-Kakizaki (GK) rat is a model of type 2 diabetes mellitus, produced originally by selective inbreeding for a hyperglycemic trait. These rats are characterized as having insulin resistance and an insulin secretory defect. Pancreatic islets from these mice show abnormal morphology in the distribution of glucagon-secreting α cells and insulin-secreting β cells, which may affect paracrine functions involved in the secretory response. This chapter describes the characterization of GK rat islets using immunofluorescence microscopy and Western blot analyses to demonstrate the effects on islet architecture and how this translates to changes in expression of predominant islet proteins.

Altered Purinergic Receptor Sensitivity in Type 2 Diabetes-Associated Endothelial Dysfunction and Up₄A-Mediated Vascular Contraction

Purinergic signaling may be altered in diabetes accounting for endothelial dysfunction. Uridine adenosine tetraphosphate (Up₄A), a novel dinucleotide substance, regulates vascular function via both purinergic P1 and P2 receptors (PR). Up₄A enhances vascular contraction in isolated arteries of diabetic rats likely through P2R. However, the precise involvement of PRs in endothelial dysfunction and the vasoconstrictor response to Up₄A in diabetes has not been fully elucidated. We tested whether inhibition of PRs improved endothelial function and attenuated Up₄A-mediated vascular contraction using both aortas and mesenteric arteries of type 2 diabetic (T2D) Goto Kakizaki (GK) rats vs. control Wistar (WT) rats. Endothelium-dependent (EDR) but not endothelium-independent relaxation was significantly impaired in both aortas and mesenteric arteries from GK vs. WT rats. Non-selective inhibition of P1R or P2R significantly improved EDR in aortas but not mesenteric arteries from GK rats. Inhibition of A1R, P2X₇R, or P2Y₆R significantly improved EDR in aortas. Vasoconstrictor response to Up₄A was enhanced in aortas but not mesenteric arteries of GK vs. WT rats via involvement of A1R and P2X₇R but not P2Y₆R. Depletion of major endothelial component nitric oxide enhanced Up₄A-induced aortic contraction to a similar extent between WT and GK rats. No significant differences in protein levels of A1R, P2X₇R, and P2Y₆R in aortas from GK and WT rats were observed. These data suggest that altered PR sensitivity accounts for endothelial dysfunction in aortas in diabetes. Modulating PRs may represent a potential therapy for improving endothelial function.

Oral Glucose Tolerance and Tryptophan Metabolism in Non-Obese and Non-Insulin-Dependent Diabetic Goto-Kakizaki Rats Fed High-Tryptophan Diets

We investigated oral glucose tolerance and tryptophan (Trp) metabolism in non-obese and non-insulin-dependent diabetic Goto-Kakizaki (GK) rats fed high-Trp diets. Five-week-old male Wistar and GK rats were fed a 20% casein diet (control diet) or the same diet supplemented with 1%, 2%, 3%, or 5% Trp for 58 d. Oral glucose tolerance tests were performed on Days 14 and 28 of the experimental period. Urine as well as livers and blood were collected on the last day of the experiment. The glucose concentration and the amount of Trp metabolites were measured. On Day 14 of the experiment, the incremental blood glucose concentrations integrated over a period of 2 h (ΔAUC_0-2h) of blood glucose in rats fed the 3% and 5% Trp diets had decreased by 13% and 18%, respectively, compared with that of the control-GK rats. However, no significant differences were found in the rats fed +1% or +2% Trp diets compared with control-GK rats. On Day 28, there were no significant differences found in the ΔAUC_0-2h of blood glucose levels in any group including the control-GK group. On the last day, the concentrations of plasma glucose, total cholesterol, and triglyceride did not show differences in any group. There were no specific phenomena observed in the metabolism of Trp in GK rats even when fed an excess of Trp, compared with that of Wistar rats. Oral Trp administration and its continuous use may not improve blood glucose levels in type 2 diabetic rats.

Lupinus mutabilis Extract Exerts an Anti-Diabetic Effect by Improving Insulin Release in Type 2 Diabetic Goto-Kakizaki Rats

Lupinus mutabilis (LM) is a legume part of Bolivian traditional diet that has a nutraceutical property reducing blood glucose levels. The prevalence of type 2 diabetes is increasing worldwide thus; the search for novel anti-diabetic drugs is needed. Based on its traditional use, we evaluated the anti-diabetic effect of LM in the spontaneously diabetic Goto-Kakizaki (GK) rat, a model of type 2 diabetes and in Wistar (W) rats as healthy control. LM seeds hydroethanolic extract, analyzed by gas chromatography-mass spectrometry and high-performance liquid chromatography-high resolution mass spectrometry, is a complex mixture of volatile and non-volatile components. A single oral administration of LM extract (2000 mg/kg b.w.) improved glucose tolerance during the oral glucose tolerance test (OGTT) (30–120 min) in GK and W rats (p < 0.0001). The long-term treatment with LM (1000 mg/kg b.w.), for 21 days, improved the area under the curve (AUC) of glucose during OGTT at day 20, in both GK (p < 0.01) and W rats (p < 0.01). The HbA1c (GK rats, p < 0.05 and W rats, p < 0.0001) and the non-fasting glucose (GK rats, p < 0.05) were also reduced. LM increased both serum insulin levels (2.4-fold in GK rats and 2.5-fold W rats), and the glucose-induced (16.7 mM glucose) insulin release in isolated islets from treated animals (6.7-fold in GK rats, and 6.6-fold in W rats). Moreover, LM (10 mg/mL) stimulated in vitro glucose induced (16.7 mM glucose) insulin release in batch incubated GK and W rat islets (p < 0.0001). In perifused GK rat islets, insulin release in 16.7 mM glucose was increased 95.3-fold compared to untreated islets (p < 0.0001), while no significant differences were found in perifused W rat islets. The LM mechanism of action, evaluated using inhibitory compounds of the insulin secretion pathway, showed that LM-dependent insulin secretion was reduced 42% by diazoxide (p < 0.001), 70% by nifedipine (p < 0.001), 86.7% by H89 (p < 0.0001), 70.8% by calphostine-C (p < 0.0001) and 93% by pertussis toxin (p < 0.0001). A similar effect was observed in W rats islets. Our findings provide evidence that LM has an anti-diabetic effect through stimulation of insulin release. The effect is-dependent on L-type calcium channel, protein kinase A and C systems, and G protein-coupled exocytosis and is partially mediated by K-ATP channels.

Amaranthus caudatus Stimulates Insulin Secretion in Goto-Kakizaki Rats, a Model of Diabetes Mellitus Type 2

Diabetes Mellitus Type 2 prevalence is increasing worldwide; thus efforts to develop novel therapeutic strategies are required. Amaranthus caudatus (AC) is a pseudo-cereal with reported anti-diabetic effects that is usually consumed in food preparations in Bolivia. This study evaluated the anti-diabetic nutraceutical property of an AC hydroethanolic extract that contains mainly sugars and traces of polyphenols and amino acids (as shown by nalysis with liquid chromatography-mass spectrometry (LC-MS) and nuclear magnetic resonance (NMR)), in type 2 diabetic Goto-Kakizaki (GK) rats and healthy Wistar (W) rats. A single oral administration of AC extract (2000 mg/kg body weight) improved glucose tolerance during Oral Glucose Tolerance Tests (OGTT) in both GK rats and in W rats. Long-term treatment (21 days) with AC (1000 mg/kg b.w.) improved the glucose tolerance evaluated by the area under the curve (AUC) of glucose levels during the OGTT, in both GK and W rats. The HbA1c levels were reduced in both GK (19.83%) and W rats (10.7%). This effect was secondary to an increase in serum insulin levels in both GK and W rats and confirmed in pancreatic islets, isolated from treated animals, where the chronic AC exposure increased the insulin production 4.1-fold in GK and 3.7-fold in W rat islets. Furthermore, the effect of AC on in vitro glucose-dependent insulin secretion (16.7 mM glucose) was concentration-dependent up to 50 mg/mL, with 8.5-fold increase in GK and 5.7-fold in W rat islets, and the insulin secretion in perifused GK and W rat islets increased 31 and nine times, respectively. The mechanism of action of AC on insulin secretion was shown to involve calcium, PKA and PKC activation, and G-protein coupled-exocytosis since the AC effect was reduced 38% by nifedipine (L-type channel inhibitor), 77% by H89 (PKA inhibitor), 79% by Calphostine-C (PKC inhibitor) and 20% by pertussis toxin (G-protein suppressor).

Calcium Signaling in the Ventricular Myocardium of the Goto-Kakizaki Type 2 Diabetic Rat

The association between diabetes mellitus (DM) and high mortality linked to cardiovascular disease (CVD) is a major concern worldwide. Clinical and preclinical studies have demonstrated a variety of diastolic and systolic dysfunctions in patients with type 2 diabetes mellitus (T2DM) with the severity of abnormalities depending on the patients' age and duration of diabetes. The cellular basis of hemodynamic dysfunction in a type 2 diabetic heart is still not well understood. The aim of this review is to evaluate our current understanding of contractile dysfunction and disturbances of Ca²⁺ transport in the Goto-Kakizaki (GK) diabetic rat heart. The GK rat is a widely used nonobese, nonhypertensive genetic model of T2DM which is characterized by insulin resistance, elevated blood glucose, alterations in blood lipid profile, and cardiac dysfunction.

Relationship between circulating levels of pancreatic proteolytic enzymes and pancreatic hormones

BACKGROUND

While the close morphological relationship between the exocrine and endocrine pancreas is well established, their functional interaction remains poorly understood. The aim of this study was to investigate the associations between circulating levels of pancreatic proteolytic enzymes and insulin, as well as other pancreatic hormones.

METHODS

Fasting venous blood samples were collected and analyzed for trypsin, chymotrypsin, insulin, glucagon, somatostatin, and pancreatic polypeptide. Linear regression analysis was used in unadjusted and two adjusted (accounting for prediabetes/diabetes, body mass index, smoking, and other covariates) statistical models.

RESULTS

A total of 93 individuals with a history of acute pancreatitis were included in this cross-sectional study. Chymotrypsin was significantly associated with insulin in the two adjusted models (p = 0.005; p = 0.003) and just missed statistical significance in the unadjusted model (p = 0.066). Chymotrypsin was significantly associated with glucagon in both unadjusted (p = 0.025) and adjusted models (p = 0.014; p = 0.015); as well as with somatostatin - in both unadjusted (p = 0.001) and adjusted models (p = 0.001; p = 0.002). Trypsin was not significantly associated with insulin in any of the models but was significantly associated with glucagon in both unadjusted (p < 0.001) and adjusted models (p < 0.001), and pancreatic polypeptide in both unadjusted (p < 0.001) and adjusted (p < 0.001) models.

CONCLUSION

The state of hyperinsulinemia is characterized by a dysfunction of the exocrine pancreas. In particular, chymotrypsin is increased in the state of hyperinsulinemia and trypsin is significantly associated with glucagon and pancreatic polypeptide.

Lipasin/betatrophin is differentially expressed in liver and white adipose tissue without association with insulin resistance in Wistar and Goto-Kakizaki rats

Lipasin is a recently identified lipokine expressed predominantly in liver and in adipose tissue. It was linked to insulin resistance in mice and to type 1 and type 2 diabetes (T1D, T2D) in humans. No metabolic studies concerning lipasin were performed yet in rats. Therefore, we used rat model of T2D and insulin resistance, Goto-Kakizaki (GK) rats, to determine changes of lipasin expression in liver and in white adipose tissue (WAT) over 52 weeks in the relation to glucose tolerance, peripheral tissue insulin sensitivity and adiposity. GK rats were grossly glucose intolerant since the age of 6 weeks and developed peripheral insulin resistance at the age of 20 weeks. Expression of lipasin in the liver did not differ between GK and Wistar rats, declining with age, and it was not related to hepatic triacylglycerol content. In WAT, the lipasin expression was significantly higher in Wistar rats where it correlated positively with adiposity. No such correlation was found in GK rats. In conclusion, lipasin expression was associated neither with a mild age-related insulin resistance (Wistar), nor with severe genetically-based insulin resistance (GK).

Rodent models of diabetic nephropathy: their utility and limitations

Diabetic nephropathy is the most common cause of end-stage renal disease. Therefore, novel therapies for the suppression of diabetic nephropathy must be developed. Rodent models are useful for elucidating the pathogenesis of diseases and testing novel therapies, and many type 1 and type 2 diabetic rodent models have been established for the study of diabetes and diabetic complications. Streptozotocin (STZ)-induced diabetic animals are widely used as a model of type 1 diabetes. Akita diabetic mice that have an Ins2+/C96Y mutation and OVE26 mice that overexpress calmodulin in pancreatic β-cells serve as a genetic model of type 1 diabetes. In addition, db/db mice, KK-Ay mice, Zucker diabetic fatty rats, Wistar fatty rats, Otsuka Long-Evans Tokushima Fatty rats and Goto-Kakizaki rats serve as rodent models of type 2 diabetes. An animal model of diabetic nephropathy should exhibit progressive albuminuria and a decrease in renal function, as well as the characteristic histological changes in the glomeruli and the tubulointerstitial lesions that are observed in cases of human diabetic nephropathy. A rodent model that strongly exhibits all these features of human diabetic nephropathy has not yet been developed. However, the currently available rodent models of diabetes can be useful in the study of diabetic nephropathy by increasing our understanding of the features of each diabetic rodent model. Furthermore, the genetic background and strain of each mouse model result in differences in susceptibility to diabetic nephropathy with albuminuria and the development of glomerular and tubulointerstitial lesions. Therefore, the validation of an animal model reproducing human diabetic nephropathy will significantly facilitate our understanding of the underlying genetic mechanisms that contribute to the development of diabetic nephropathy. In this review, we focus on rodent models of diabetes and discuss the utility and limitations of these models for the study of diabetic nephropathy.

A Novel Diphenylthiosemicarbazide Is a Potential Insulin Secretagogue for Anti-Diabetic Agen

Insulin secretagogues are used for treatment of type 2 diabetes. We attempted to discover novel small molecules to stimulate insulin secretion by using in silico similarity search using sulfonylureas as query, followed by measurement of insulin secretion. Among 38 compounds selected by in silico similarity search, we found three diphenylsemicarbazides and one quinolone that stimulate insulin secretion. We focused on compound 8 (C8), which had the strongest insulin-secreting effect. Based on the structure-activity relationship of C8-derivatives, we identified diphenylthiosemicarbazide (DSC) 108 as the most potent secretagogue. DSC108 increased the intracellular Ca²⁺ level in MIN6-K8 cells. Competitive inhibition experiment and electrophysiological analysis revealed sulfonylurea receptor 1 (SUR1) to be the target of DSC108 and that this diphenylthiosemicarbazide directly inhibits ATP-sensitive K⁺ (K_ATP) channels. Pharmacokinetic analysis showed that DSC108 has a short half-life in vivo. Oral administration of DSC108 significantly suppressed the rises in blood glucose levels after glucose load in wild-type mice and improved glucose tolerance in the Goto-Kakizaki (GK) rat, a model of type 2 diabetes with impaired insulin secretion. Our data indicate that DSC108 is a novel insulin secretagogue, and is a lead compound for development of a new anti-diabetic agent.

Exercise-induced alterations in pancreatic oxidative stress and mitochondrial function in type 2 diabetic Goto-Kakizaki rats

Progressive metabolic complications accompanied by oxidative stress are the hallmarks of type 2 diabetes. The precise molecular mechanisms of the disease complications, however, remain elusive. Exercise-induced nontherapeutic management of type 2 diabetes is the first line of choice to control hyperglycemia and diabetes associated complications. In this study, using 11-month-old type 2 Goto-Kakizaki (GK) rats, we have investigated the effects of exercise on mitochondrial metabolic and oxidative stress in the pancreas. Our results showed an increase in theNADPHoxidase enzyme activity and reactive oxygen species (ROS) production inGKrats, which was inhibited after exercise. Increased lipid peroxidation and protein carbonylation andSODactivity were also inhibited after exercise. Interestingly, glutathione (GSH) level was markedly high in the pancreas ofGKdiabetic rats even after exercise. However,GSH-peroxidase andGSH-reductase activities were significantly reduced. Exercise also induced energy metabolism as observed by increased hexokinase and glutamate dehydrogenase activities. A significant decrease in the activities of mitochondrial ComplexesII/IIIandIVwere observed in theGKrats. Exercise improved only ComplexIVactivity suggesting increased utilization of oxygen. We also observed increased activities of cytochrome P450s in the pancreas ofGKrats which was reduced significantly after exercise.SDS-PAGEresults have shown a decreased expression ofNF-κB, Glut-2, andPPAR-ϒ inGKrats which was markedly increased after exercise. These results suggest differential oxidative stress and antioxidant defense responses after exercise. Our results also suggest improved mitochondrial function and energy utilization in the pancreas of exercisingGKrats.

Adenylate cyclase 3: a new target for anti-obesity drug development

Obesity has become epidemic worldwide, and abdominal obesity has a negative impact on health. Current treatment options on obesity, however, still remain limited. It is then of importance to find a new target for anti-obesity drug development based upon recent molecular studies in obesity. Adenylate cyclase 3 (ADCY3) is the third member of adenylyl cyclase family and catalyses the synthesis of cAMP from ATP. Genetic studies with candidate gene and genome-wide association study approaches have demonstrated that ADCY3 genetic polymorphisms are associated with obesity in European and Chinese populations. Epigenetic studies have indicated that increased DNA methylation levels in the ADCY3 gene are involved in the pathogenesis of obesity. Furthermore, biological analyses with animal models have implicated that ADCY3 dysfunction resulted in increased body weight and fat mass, while reduction of body weight is partially explained by ADCY3 activation. In this review, we describe genomic and biological features of ADCY3, summarize genetic and epigenetic association studies of the ADCY3 gene with obesity and discuss dysfunction and activation of ADCY3. Based upon all data, we suggest that ADCY3 is a new target for anti-obesity drug development. Further investigation on the effectiveness of ADCY3 activator and its delivery approach to treat abdominal obesity has been taken into our consideration.

Integrator of Stress Responses Calmodulin Binding Transcription Activator 1 (Camta1) Regulates miR-212/miR-132 Expression and Insulin Secretion

Altered microRNA profiles have been demonstrated in experimental models of type 2 diabetes, including in islets of the diabetic Goto-Kakizaki (GK) rat. Our bioinformatic analysis of conserved sequences in promoters of microRNAs, previously observed to be up-regulated in GK rat islets, revealed putative CGCG-core motifs on the promoter of the miR-212/miR-132 cluster, overexpression of which has been shown to increase insulin secretion. These motifs are possible targets of calmodulin binding transcription activators Camta1 and Camta2 that have been recognized as integrators of stress responses. We also identified putative NKE elements, possible targets of NK2 homeobox proteins like the essential islet transcription factor Nkx2-2. As Camtas can function as co-activators with NK2 proteins in other tissues, we explored the role of Camta1, Camta2, and Nkx2-2 in the regulation of the miR-212/miR-132 cluster and insulin secretion. We demonstrate that exposure of control Wistar or GK rat islets to 16.7 mm glucose increases miR-212/miR-132 expression but significantly less so in the GK rat. In addition, Camta1, Camta2, and Nkx2-2 were down-regulated in GK rat islets, and knockdown of Camta1 reduced miR-212/miR-132 promoter activity and miR-212/miR-132 expression, even under cAMP elevation. Knockdown of Camta1 decreased insulin secretion in INS-1 832/13 cells and Wistar rat islets but increased insulin content. Furthermore, knockdown of Camta1 reduced K(+)-induced insulin secretion and voltage-dependent Ca(2+) currents. We also demonstrate Camta1 and Nkx2-2 protein interaction. These results indicate that Camta1 is required not only for expression of the miR-212/miR-132 cluster but at multiple levels for regulating beta cell insulin content and secretion.

β-Cell glutamate signaling: Its role in incretin-induced insulin secretion

Insulin secretion from the pancreatic β-cell (referred to as β-cell hereafter) plays a central role in glucose homeostasis. Impaired insulin secretion is a major factor contributing to the development of diabetes and, therefore, is an important target for treatment of the disease. Cyclic adenosine monophosphate is a key second messenger in β-cells that amplifies insulin secretion. Incretins released by the gut potentiate insulin secretion through cyclic adenosine monophosphate signaling in β-cells, which is the basis for the incretin-based diabetes therapies now being used worldwide. Despite its importance, the interaction between glucose metabolism and incretin/cyclic adenosine monophosphate signaling in β-cells has long been unknown. A recent study showed that cytosolic glutamate produced by glucose metabolism in β-cells is a key signal in incretin-induced insulin secretion. Here we review the physiological and pathophysiological roles of β-cell glutamate signaling in incretin-induced insulin secretion.

Hepatic adenylate cyclase 3 is upregulated by Liraglutide and subsequently plays a protective role in insulin resistance and obesity

OBJECTIVE

Recent studies have demonstrated that adenylate cyclase 3 (AC3) has a protective role in obesity. This gene resides at the pathway with glucagon-like peptide (GLP)-1. Liraglutide is a GLP-1 analog and has independent glucose and body weight (BW)-reducing effects. In the present study, we aimed to examine whether hepatic AC3 activity was regulated by Liraglutide and to further understand the effect of AC3 in reduction of BW and insulin resistance.

SUBJECTS

The diabesity and obese mice were induced from db/db and C57BL/6 J mice, respectively, by high-fat diet. Liraglutide (0.1 mg kg(-1) per 12 h) was given to the mice twice daily for 12 weeks. C57BL/6 J mice fed with chow diet and obese or diabesity mice treated with saline were used as the controls. Hepatic AC3 gene expression at mRNA and protein levels was analyzed with real-time reverse transcription-PCR and western blot. Fasting blood glucose and serum insulin levels were measured and followed insulin resistance index (HOMA-IR) was evaluated according to the homeostasis model assessment.

RESULTS

After administration of Liraglutide, BW and HOMA-IR in obese and diabesity mice were decreased, whereas hepatic AC3 mRNA and protein expression levels were upregulated. The AC3 gene expression was negatively correlated with BW, HOMA-IR and the area ratio of hepatic fat deposition in the liver.

CONCLUSIONS

The present study thus provides the evidence that hepatic AC3 gene expression is upregulated by Liraglutide. The reduction of BW and improvement of insulin resistance with Liraglutide may be partially explained by AC3 activation.

In vitro effects of bis(1,2-dimethyl-3-hydroxy-4-pyridinonato)oxidovanadium(IV), or VO(dmpp)2, on insulin secretion in pancreatic islets of type 2 diabetic Goto-Kakizaki rats

Vanadium compounds have been explored as therapy of diabetes, and most studies have focussed on insulin mimetic effects, i.e. reducing hyperglycemia by improving glucose sensitivity and thus glucose uptake in sensitive tissues. We have recently shown that bis(1,2-dimethyl-3-hydroxy-4-pyridinonato)oxidovanadium(IV), VO(dmpp)2, has promising effects when compared to another vanadium compound, bis(maltolato)oxidovanadium(IV), BMOV, and insulin itself, in isolated adipocytes and in vivo in Goto-Kakizaki (GK) rats, an animal model of hereditary type 2 diabetes (T2D).We now have investigated in GK rats whether VO(dmpp)2 also modulates another important defect in T2D, impaired insulin secretion. VO(dmpp)2, but not BMOV, stimulated insulin secretion from isolated GK rat pancreatic islets at high, 16.7mM, but not at low–normal, 3.3 mM, glucose concentration. Mechanistic studies demonstrate that the insulin releasing effect of VO(dmpp)2 is due to its interaction with several steps in the stimulus-secretion coupling for glucose, including islet glucose metabolism and K-ATP channels, L-type Ca2+ channels, modulation by protein kinases A and C, as well as the exocytotic machinery. In conclusion, VO(dmpp)2 exhibits properties of interest for treatment of the insulin secretory defect in T2D, in addition to its well-described insulin mimetic activity.

Expression of Protein Kinase C Isoforms in Pancreatic Islets and Liver of Male Goto-Kakizaki Rats, a Model of Type 2 Diabetes

Protein kinase C (PKC) is a family of protein kinases controlling protein phosphorylation and playing important roles in the regulation of metabolism. We have investigated expression levels of PKC isoforms in pancreatic islets and liver of diabetic Goto-Kakizaki (GK) rats with and without insulin treatment to evaluate their association with glucose homeostasis. mRNA and protein expression levels of PKC isoforms were assessed in pancreatic islets and liver of Wistar rats and GK rats with or without insulin treatment. PKCα and PKCζ mRNA expressions were down-regulated in islets of GK compared with Wistar rats. PKCα and phosphorylated PKCα (p-PKCα) protein expressions were decreased in islets of GK compared with insulin-treated GK and Wistar rats. PKCζ protein expression in islets was reduced in GK and insulin-treated GK compared with Wistar rats, but p-PKCζ was decreased only in GK rats. Islet PKCε mRNA and protein expressions were lower in GK compared with insulin-treated GK and Wistar rats. In liver, PKCδ and PKCζ mRNA expressions were decreased in both GK and insulin-treated GK compared with Wistar rats. Hepatic PKCζ protein expression was diminished in both GK rats with and without insulin treatment compared with Wistar rats. Hepatic PKCε mRNA expression was down-regulated in insulin-treated GK compared with GK and Wistar rats. PKCα, PKCε, and p-PKCζ expressions were secondary to hyperglycaemia in GK rat islets. Hepatic PKCδ and PKCζ mRNA expressions were primarily linked to hyperglycaemia. Additionally, hepatic PKCε mRNA expression could be under control of insulin.

Evaluation of Antidiabetic Effects of the Traditional Medicinal Plant Gynostemma pentaphyllum and the Possible Mechanisms of Insulin Release

Aims. To evaluate the antidiabetic effects of Gynostemma pentaphyllum (GP) in Goto-Kakizaki (GK) rat, an animal model of type 2 diabetes, and to investigate the mechanisms of insulin release. Methods. Oral glucose tolerance test was performed and plasma insulin levels were measured. Results. An oral treatment with GP (0.3 g/kg of body weight daily) for two weeks in GK rats improved glucose tolerance versus placebo group (P < 0.01). Plasma insulin levels were significantly increased in the GP-treated group. The insulin release from GP-treated GK rats was 1.9-fold higher as compared to the control group (P < 0.001). GP stimulated insulin release in isolated GK rat islets at high glucose. Opening of ATP-sensitive potassium (K-ATP) channels by diazoxide and inhibition of calcium channels by nifedipine significantly decreased insulin response to GP. Furthermore, the protein kinase A (PKA) inhibitor H89 decreased the insulin response to GP (P < 0.05). In addition, GP-induced insulin secretion was decreased after preincubation of GK islets with pertussis toxin to inhibit exocytotic G_e proteins (P < 0.05). Conclusion. The antidiabetic effect of GP is associated with the stimulation of insulin release from the islets. GP-induced insulin release is partly mediated via K-ATP and L-type Ca²⁺ channels, the PKA system and also dependent on pertussis toxin sensitive G_e-protein.

Glutamate acts as a key signal linking glucose metabolism to incretin/cAMP action to amplify insulin secretion

Incretins, hormones released by the gut after meal ingestion, are essential for maintaining systemic glucose homeostasis by stimulating insulin secretion. The effect of incretins on insulin secretion occurs only at elevated glucose concentrations and is mediated by cAMP signaling, but the mechanism linking glucose metabolism and cAMP action in insulin secretion is unknown. We show here, using a metabolomics-based approach, that cytosolic glutamate derived from the malate-aspartate shuttle upon glucose stimulation underlies the stimulatory effect of incretins and that glutamate uptake into insulin granules mediated by cAMP/PKA signaling amplifies insulin release. Glutamate production is diminished in an incretin-unresponsive, insulin-secreting β cell line and pancreatic islets of animal models of human diabetes and obesity. Conversely, a membrane-permeable glutamate precursor restores amplification of insulin secretion in these models. Thus, cytosolic glutamate represents the elusive link between glucose metabolism and cAMP action in incretin-induced insulin secretion.

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Glutamate is derived from the malate-aspartate shuttle upon glucose stimulation

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Shuttle-derived glutamate is crucial for incretin-induced insulin secretion

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Cytosolic glutamate is transported into insulin granules via cAMP/PKA signaling

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Glutamate production by glucose is defective in incretin-unresponsive β cells

Oral administration of soybean peptide Vglycin normalizes fasting glucose and restores impaired pancreatic function in Type 2 diabetic Wistar rats

Vglycin, a natural 37-residue polypeptide isolated from pea seeds in which six half-cysteine residues are embedded in three pairs of disulfide bonds, is resistant to digestive enzymes and has antidiabetic potential. To investigate the pharmacological activity of Vglycin in vivo and to examine the mechanisms involved, the therapeutic effect of Vglycin in diabetic rats was examined. Diabetes was induced in Wistar rats by high-fat diet and multiple streptozotocin intraperitoneal injections. Diabetic rats were treated daily with Vglycin for 4 weeks. Body weight, food intake, fasting plasma glucose and insulin levels were assayed weekly. Glucose and insulin tolerance tests were conducted on Day 29. Subsequently, levels of p-Akt in the liver and pancreas and cleaved PARP, Pdx-1 and insulin in the pancreas were detected by immunoblotting. The morphology of the pancreas and the insulin expression in the pancreas were analyzed by hematoxylin-eosin staining and immunohistochemistry, respectively. Furthermore, human liver-derived cell lines were used to explore the in vitro effects of Vglycin on insulin sensitivity and glucose uptake. Chronic treatment with Vglycin normalized fasting glucose levels in diabetic rats. The improvement in glucose homeostasis and the increased insulin sensitivity mediated by restored insulin signaling likely contributed to decreased food intake and reduced body weight. Vglycin protected pancreatic cells from damage by streptozotocin. Although insulin synthesis and secretion in impaired β-cell were not significantly elevated, islets morphology was improved in the Vglycin-treated groups. These results suggest that Vglycin could be useful in Type 2 diabetes for restoring impaired insulin signaling, glucose tolerance and pancreatic function.

The complement inhibitor CD59 regulates insulin secretion by modulating exocytotic events

Type 2 diabetes is triggered by reduced insulin production, caused by genetic and environmental factors such as inflammation originating from the innate immune system. Complement proteins are a component of innate immunity and kill non-self cells by perforating the plasma membrane, a reaction prevented by CD59. Human pancreatic islets express CD59 at very high levels. CD59 is primarily known as a plasma membrane protein in membrane rafts, but most CD59 protein in pancreatic β cells is intracellular. Removing extracellular CD59 disrupts membrane rafts and moderately stimulates insulin secretion, whereas silencing intracellular CD59 markedly suppresses regulated secretion by exocytosis, as demonstrated by TIRF imaging. CD59 interacts with the exocytotic proteins VAMP2 and Syntaxin-1. CD59 expression is reduced by glucose and in rodent diabetes models but upregulated in human diabetic islets, potentially reflecting compensatory reactions. This unconventional action of CD59 broadens the established view of innate immunity in type 2 diabetes.

Glucose-induced inhibition of insulin secretion

Increase in glucose is known to elevate the concentration of cytoplasmic Ca(2+) ([Ca(2+) ]i ) in pancreatic β-cells and stimulate insulin secretion. However, rise of glucose can also lower [Ca(2+) ]i and inhibit insulin release. In the present review, we examine the mechanisms for this inhibition and highlight its importance for the healthy β-cell and the development of diabetes. It is possible to distinguish between 60 and 90 s of prompt inhibition and the late inhibition seen after the first-phase peak of insulin release. The introductory inhibition is characteristic of the healthy β-cell and mediated by sequestration of [Ca(2+) ]i in the endoplasmic reticulum. This inhibition is easily seen in studies of isolated islets but too brief to be detected in a conventional intravenous glucose tolerance test. Coupled to simultaneous rise of glucagon, the introductory suppression of insulin release is the starting point for the antiphase relation between the subsequent insulin and glucagon pulses. Another effect of the initial suppression is to increase the pool of readily releasable granules responsible for the first-phase release of insulin. The presence of late inhibition of insulin release is an indicator of β-cell dysfunction. Patients with type 2 diabetes often respond to intravenous bolus injection of glucose with 5-10 min of late suppression of circulating insulin.

GPR40 protein levels are crucial to the regulation of stimulated hormone secretion in pancreatic islets. Lessons from spontaneous obesity-prone and non-obese type 2 diabetes in rats

The role of islet GPR40 protein in the pathogenesis of diabetes is unclear. We explored the influence of GPR40 protein levels on hormone secretion in islets from two rat models of spontaneous type 2 diabetes displaying either hyperlipidaemia or hyperglycaemia. GPR40 expression was analysed by confocal microscopy, Western blot and qPCR in islets from preobese Zucker (fa/fa) rats, diabetic Goto-Kakizaki (GK) rats, and controls. Confocal microscopy of control islets showed expression of GPR40 protein in insulin, glucagon and somatostatin cells. GPR40 expression was strongly increased in islets of hyperlipidaemic fa/fa rats and coincided with a concentration-related increase in palmitate-induced release of insulin and glucagon and its inhibition of somatostatin release. Conversely, hyperglycaemic GK islets displayed an extremely faint expression of GPR40 as did high-glucose-cultured control islets. This was reflected in abolished palmitate-induced hormone response in GK islets and high-glucose-cultured control islets. The palmitate antagonist rosiglitazone promoted reappearance of GPR40 in high-glucose-cultured islets and served as partial agonist in glucose-stimulated insulin release. GPR40 protein is abundantly expressed in pancreatic islets and modulates stimulated hormone secretion. Mild hyperlipidaemia in obesity-prone diabetes creates increased GPR40 expression and increased risk for an exaggerated palmitate-induced insulin response and lipotoxicity, a metabolic situation suitable for GPR40 antagonist treatment. Chronic hyperglycaemia creates abrogated GPR40 expression and downregulated insulin release, a metabolic situation suitable for GPR40 agonist treatment to avoid glucotoxicity. GPR40 protein is interactively modulated by both free fatty acids and glucose and is a promising target for pharmacotherapy in different variants of type 2 diabetes.

Antidiabetic Effect of Oral Borapetol B Compound, Isolated from the Plant Tinospora crispa, by Stimulating Insulin Release

Aims. To evaluate the antidiabetic properties of borapetol B known as compound 1 (C1) isolated from Tinospora crispa in normoglycemic control Wistar (W) and spontaneously type 2 diabetic Goto-Kakizaki (GK) rats. Methods. The effect of C1 on blood glucose and plasma insulin was assessed by an oral glucose tolerance test. The effect of C1 on insulin secretion was assessed by batch incubation and perifusion experiments using isolated pancreatic islets. Results. An acute oral administration of C1 improved blood glucose levels in treated versus placebo groups with areas under glucose curves 0–120 min being 72 ± 17 versus 344 ± 10 mmol/L (P < 0.001) and 492 ± 63 versus 862 ± 55 mmol/L (P < 0.01) in W and GK rats, respectively. Plasma insulin levels were increased by 2-fold in treated W and GK rats versus placebo group at 30 min (P < 0.05). C1 dose-dependently increased insulin secretion from W and GK isolated islets at 3.3 mM and 16.7 mM glucose. The perifusions of isolated islets indicated that C1 did not cause leakage of insulin by damaging islet beta cells (P < 0.001). Conclusion. This study provides evidence that borapetol B (C1) has antidiabetic properties mainly due to its stimulation of insulin release.

An Aqueous Extract of Radix Astragali, Angelica sinensis, and Panax notoginseng Is Effective in Preventing Diabetic Retinopathy

Diabetic retinopathy (DR), in which inflammation has been implicated playing important roles, is one of the most common diabetes complications. Dang Gui Bu Xue Tang (DBT), an aqueous extract of Radix Astragali and Radix Angelica sinensis, is a classical prescription in Traditional Chinese Medicine for treating inflammation and ischemic diseases. Here, we investigated the effects of a modified recipe of DBT, with addition of Panax notoginseng, in treating diabetic retinopathy. An aqueous extract of Radix Astragali, Radix Angelica sinensis, and Panax notoginseng (RRP) was given to Goto-Kakizaki (GK) rats and streptozotocin-induced Sprague-Dawley (SD) rats. Leukostasis, vascular leakage, and acellular capillaries in retinal vasculature of animals were determined. Expression of retinal inflammatory biomarkers was assessed. We found that RRP reduced leukostasis, acellular capillaries, and vascular leakage compared to diabetic control rats. We also found that RRP decreased the expression of inflammatory factors including IL-1β, IL-6, TNF-α, NF-κB, MCP-1, ICAM-1, or VCAM-1 in the retinas of GK rats and reversed high glucose-induced inhibition of endothelial cell migration and proliferation in vitro. We conclude that RRP has a potent effect in preventing the pathogenesis and/or progression of DR and thus may serve as a promising nontoxic therapeutic approach of DR.