Psammomys obesus, a model for environment-gene interactions in type 2 diabetes

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.

Hypothetical molecular interconnection between type 2 diabetes and dyslexia

BACKGROUND

Dyslexia is one of the most common learning disabilities, especially among children. Type 2 diabetes is a metabolic disorder that affects a large population globally, with metabolic disorders. There have been several genes that are identified as causes of Dyslexia, and in recent studies, it has been found out that some of those genes are also involved in several metabolic pathways. For several years, it has been known that type 2 diabetes causes several neurodegenerative disorders, such as Alzheimer's disease and Parkinson's disease. Furthermore, in several studies, it was suggested that type 2 diabetes also has some associations with learning disabilities. This raises the question of whether "Is there a connection between type 2 diabetes and dyslexia?". In this study, this question is elaborated by linking their developmental processes via bioinformatics analysis about these two diseases individually and collectively.

RESULT

The literature review for dyslexia and type two diabetes was completed. As the result of this literature review, the genes that are associated to type 2 diabetes and dyslexia were identified. The biological pathways of dyslexia, and dyslexia associated genes, type 2 diabetes, and type 2 diabetes associated genes were identified. The association of these genes, regarding to their association with pathways were analysed, and using STRING database the gene associations were analysed and identified.

CONCLUSION

The findings of this research included the interaction analysis via gene association, co-expression and protein-protein interaction. These findings clarified the interconnection between dyslexia and type 2 diabetes in molecular level and it will be the beginning of an answer regarding to the relationship between T2D and dyslexia. Finally, by improving the understanding this paper aims to open the way for the possible future approach to examine this hypothesis.

Functional alterations of retinal neurons and vascular involvement progress simultaneously in the Psammomys obesus model of diabetic retinopathy

To investigate the progression of diabetic retinopathy (DR) in a new diurnal animal model, we monitored clinically the DR in Psammomys obesus (P. obesus) during 7 months using electroretinography (ERG) and imaging techniques. After the onset of DR, all ERG components decreased progressively. In scotopic conditions, by 3-months of disease progression, the diabetic P. obesus displayed a significant decrease in amplitude of b-max, b-wave responses, and mixed b-waves. While mixed a-wave decreased between 4 and 7 months. Significant differences of OP2 appeared following 1 month of disease. In photopic conditions, we noticed a decrease in the a-wave at 2 months, while it took more than 5 months in b-wave amplitude. The photopic negative response (PhNR) and the i-wave amplitudes decreased following 4 and 5 months. OP1 and OP2 were the first to be altered and a significant decrease in the amplitude started after 3 months. Finally, 30 Hz-flicker and photopic S-cone were impaired after 2 and 3 months, respectively. The assessment of the eye fundus of the retina revealed an abnormal vascular architecture appeared at Months 6 and 7. In addition, we noticed exudates in the superior periphery of the retina at the same stage. The retina thickness showed a significant reduction at Month 7. Our results indicate that the clinical correlates of human DR are present in diabetic P. obesus. The depressed of ERGs, disruption of retinal architecture, and the appearance of exudates may reflect vascular and neuronal damage throughout the retina as are seen in the advanced stages of human DR.

In Vivo and In Vitro Models of Diabetes: A Focus on Pregnancy

Diabetes in pregnancy is associated with an increased risk of poor outcomes, both for the mother and her offspring. Although clinical and epidemiological studies are invaluable to assess these outcomes and the effectiveness of potential treatments, there are certain ethical and practical limitations to what can be assessed in human studies.Thus, both in vivo and in vitro models can aid us in the understanding of the mechanisms behind these complications and, in the long run, towards their prevention and treatment. This review summarizes the existing animal and cell models used to mimic diabetes, with a specific focus on the intrauterine environment. Summary of this review.

Nutritional Models of Type 2 Diabetes Mellitus

In order to better understand the events that precede and precipitate the onset of type 2 diabetes (T2DM), several nutritional animal models have been developed. These models are generated by manipulating the diet of either the animal itself, or its mother during her pregnancy, and in comparison to traditional genetic and knock out models, have the advantage that they more accurately reflect the etiology of human T2DM. This chapter will discuss some of the most widely used nutritional models of T2DM: Diet-induced obesity (DIO) in adult rodents, and studies of offspring of mothers fed a low-protein, high-fat and/or high-sugar diet during pregnancy and/or lactation. Several common mechanisms have been identified through which these nutritional manipulations can lead to metabolic disease, including pancreatic beta-cell dysfunction, impaired insulin signaling in skeletal muscle, and the excess accumulation of visceral adipose tissue and consequent deposition of nonesterified fatty acids in peripheral tissues. In addition, there is an emerging concept that obesity/poor quality diets result in increased production and release of pro-inflammatory cytokines from adipose tissue leading to a state of chronic low-grade inflammation, and that this is likely to represent an important link between obesity/diet and metabolic dysfunction. The following chapter will discuss the most common nutritional models of T2DM in experimental animals, their application, and relationship to human etiology, and will highlight the important insights these models have provided into the pathogenesis of T2DM.

High-Energy Diet and Shorter Light Exposure Drives Markers of Adipocyte Dysfunction in Visceral and Subcutaneous Adipose Depots of Psammomys obesus

Dysfunctional adipose tissue phenotype underpins type 2 diabetes mellitus (T2DM) development. The disruption of circadian rhythms contributes to T2DM development. We investigated the effects of high-energy diet and photoperiod length on visceral and subcutaneous adipose tissue phenotype. Psammomys obesus sand rats exposed to neutral (12 light:12 dark) or short (5 light:19 dark) photoperiod were fed a low- (LE) or high- (HE) energy diet. The HE diet and/or short photoperiod reduced subcutaneous expression of adipocyte differentiation/function markers C/ebpα, Pparδ, Pparγ and Adipoq. Visceral Pparα levels were elevated in the 5:19HE group; however, the HE diet and/or short photoperiod decreased visceral Pparγ and Adipoq expression. 5:19HE animals had elevated Ucp1 yet lower Pgc-1α levels. The HE diet increased visceral Tgf-β1, Ccl2 and Cd68 levels, suggestive of a pro-inflammatory state. Daily visceral rhythms of these genes were affected by a short photoperiod and/or HE diet. The 12:12HE, 5:19LE or 5:19HE animals had a higher proportion of larger adipocytes, indicating increased adipocyte hypertrophy. Collectively, the HE diet and/or shorter light exposure drives a dysfunctional adipose tissue phenotype. Daily rhythms are affected by a short photoperiod and HE diet in a site-specific manner. These findings provide mechanistic insight on the influence of disrupted circadian rhythms and HE diet on adipose tissue phenotype.

Genetic Permissiveness and Dietary Glycemic Load Interact to Predict Type-II Diabetes in the Nile rat (Arvicanthis niloticus)

Objective: The Nile rat (Arvicanthis niloticus) is a superior model for Type-II Diabetes Mellitus (T2DM) induced by diets with a high glycemic index (GI) and glycemic load (GLoad). To better define the age and gender attributes of diabetes in early stages of progression, weanling rats were fed a high carbohydrate (hiCHO) diet for between 2 to 10 weeks. Methods: Data from four experiments compared two diabetogenic semipurified diets (Diet 133 (60:20:20, as % energy from CHO, fat, protein with a high glycemic load (GLoad) of 224 per 2000 kcal) versus Diets 73 MBS or 73 MB (70:10:20 with or without sucrose and higher GLoads of 259 or 295, respectively). An epidemiological technique was used to stratify the diabetes into quintiles of blood glucose (Q1 to Q5), after 2–10 weeks of dietary induction in 654 rats. The related metagenetic physiological growth and metabolic outcomes were related to the degree of diabetes based on fasting blood glucose (FBG), random blood glucose (RBG), and oral glucose tolerance test (OGTT) at 30 min and 60 min. Results: Experiment 1 (Diet 73MBS) demonstrated that the diabetes begins aggressively in weanlings during the first 2 weeks of a hiCHO challenge, linking genetic permissiveness to diabetes susceptibility or resistance from an early age. In Experiment 2, ninety male Nile rats fed Diet 133 (60:20:20) for 10 weeks identified two quintiles of resistant rats (Q1,Q2) that lowered their RBG between 6 weeks and 10 weeks on diet, whereas Q3–Q5 became progressively more diabetic, suggesting an ongoing struggle for control over glucose metabolism, which either stabilized or not, depending on genetic permissiveness. Experiment 3 (32 males fed 70:10:20) and Experiment 4 (30 females fed 60:20:20) lasted 8 weeks and 3 weeks respectively, for gender and time comparisons. The most telling link between a quintile rank and diabetes risk was telegraphed by energy intake (kcal/day) that established the cumulative GLoad per rat for the entire trial, which was apparent from the first week of feeding. This genetic permissiveness associated with hyperphagia across quintiles was maintained throughout the study and was mirrored in body weight gain without appreciable differences in feed efficiency. This suggests that appetite and greater growth rate linked to a fiber-free high GLoad diet were the dominant factors driving the diabetes. Male rats fed the highest GLoad diet (Diet 73MB 70:10:20, GLoad 295 per 2000 kcal for 8 weeks in Experiment 3], ate more calories and developed diabetes even more aggressively, again emphasizing the Cumulative GLoad as a primary stressor for expressing the genetic permissiveness underlying the diabetes. Conclusion: Thus, the Nile rat model, unlike other rodents but similar to humans, represents a superior model for high GLoad, low-fiber diets that induce diabetes from an early age in a manner similar to the dietary paradigm underlying T2DM in humans, most likely originating in childhood.

Short-Term Administration of Astaxanthin Attenuates Retinal Changes in Diet-Induced Diabetic Psammomys obesus

OBJECTIVES

Psammomys obesus is a high-fat diet (HFD)-fed animal model of obesity and type 2 diabetes recently explored as a model of non-proliferative diabetic retinopathy. This study tested the protective effect of the pigment astaxanthin (AST) in the P. obesus diabetic retina.

METHODS

Young adult P. obesus were randomly assigned to two groups. The control group received a normal diet consisting of a plant-based regimen, and the HFD group received an enriched laboratory chow. After 3 months, control and diabetic rodents were administered vehicle or AST, daily for 7 days. Body weight, blood glucose, and plasma pentosidine were assessed. Frozen sections of retinas were immunolabeled for markers of oxidative stress, glial reactivity and retinal ganglion cell bodies, and imaged by confocal microscopy.

RESULTS

Retinal tissue from AST-treated control and HFD-diabetic P. obesus showed a greater expression of the antioxidant enzyme heme oxygenase-1 (HO-1). In retinas of HFD-diabetic AST-treated P. obesus, cellular retinaldehyde binding protein and glutamine synthetase in Müller cells were more intense compared to the untreated HFD-diabetic group. HFD-induced diabetes downregulated the expression of glial fibrillary acidic protein in astrocytes, the POU domain protein 3A in retinal ganglion cells, and synaptophysin throughout the plexiform layers.

DISCUSSION

Our results show that type 2-like diabetes induced by HFD affected glial and neuronal retinal cell homeostasis. AST treatment induced the antioxidant enzyme HO-1 and reduced glial reactivity. These findings suggest that diabetic P. obesus is a useful model of HFD-induced obesity and diabetes to evaluate early neuroglial retinal alterations and antioxidant neuroprotection mechanisms in DR.

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.

The Nile Rat (Arvicanthis niloticus) as a Superior Carbohydrate-Sensitive Model for Type 2 Diabetes Mellitus (T2DM)

Type II diabetes mellitus (T2DM) is a multifactorial disease involving complex genetic and environmental interactions. No single animal model has so far mirrored all the characteristics or complications of diabetes in humans. Since this disease represents a chronic nutritional insult based on a diet bearing a high glycemic load, the ideal model should recapitulate the underlying dietary issues. Most rodent models have three shortcomings: (1) they are genetically or chemically modified to produce diabetes; (2) unlike humans, most require high-fat feeding; (3) and they take too long to develop diabetes. By contrast, Nile rats develop diabetes rapidly (8-10 weeks) with high-carbohydrate (hiCHO) diets, similar to humans, and are protected by high fat (with low glycemic load) intake. This review describes diabetes progression in the Nile rat, including various aspects of breeding, feeding, and handling for best experimental outcomes. The diabetes is characterized by a striking genetic permissiveness influencing hyperphagia and hyperinsulinemia; random blood glucose is the best index of disease progression; and kidney failure with chronic morbidity and death are outcomes, all of which mimic uncontrolled T2DM in humans. Non-alcoholic fatty liver disease (NAFLD), also described in diabetic humans, results from hepatic triglyceride and cholesterol accumulation associated with rising blood glucose. Protection is afforded by low glycemic load diets rich in certain fibers or polyphenols. Accordingly, the Nile rat provides a unique opportunity to identify the nutritional factors and underlying genetic and molecular mechanisms that characterize human T2DM.

The complete mitochondrial genome of Psammomys obesus (Rodentia: Muridae)

The fat sand rats (Psammomys obesus) can easily induce obesity and acquire type 2 diabetes mellitus when they are fed with high-carbohydrate diets. P. obesus is often used as an animal model for studies on diabetes and obesity. We described 16,592 bp of P. obesus mtDNA that contains 13 protein-coding genes (PGCs), two rRNA genes (12S rRNA and 16S rRNA), 22 transfer RNA (tRNA) genes, and one control region (D-loop). The complete mitochondrial genome sequence provided here would be useful for further understanding the evolution of ratite and conservation genetics of P. obesus.

Genome sequence of a diabetes-prone rodent reveals a mutation hotspot around the ParaHox gene cluster

The sand rat Psammomys obesus is a gerbil species native to deserts of North Africa and the Middle East, and is constrained in its ecology because high carbohydrate diets induce obesity and type II diabetes that, in extreme cases, can lead to pancreatic failure and death. We report the sequencing of the sand rat genome and discovery of an unusual, extensive, and mutationally biased GC-rich genomic domain. This highly divergent genomic region encompasses several functionally essential genes, and spans the ParaHox cluster which includes the insulin-regulating homeobox gene Pdx1. The sequence of sand rat Pdx1 has been grossly affected by GC-biased mutation, leading to the highest divergence observed for this gene across the Bilateria. In addition to genomic insights into restricted caloric intake in a desert species, the discovery of a localized chromosomal region subject to elevated mutation suggests that mutational heterogeneity within genomes could influence the course of evolution.

Inhibition of Calcium Influx Reduces Dysfunction and Apoptosis in Lipotoxic Pancreatic β-Cells via Regulation of Endoplasmic Reticulum Stress

Lipotoxicity plays an important role in pancreatic β-cell failure during the development of type 2 diabetes. Prolonged exposure of β-cells to elevated free fatty acids level could cause deterioration of β-cell function and induce cell apoptosis. Therefore, inhibition of fatty acids-induced β-cell dysfunction and apoptosis might provide benefit for the therapy of type 2 diabetes. The present study examined whether regulation of fatty acids-triggered calcium influx could protect pancreatic β-cells from lipotoxicity. Two small molecule compounds, L-type calcium channel blocker nifedipine and potassium channel activator diazoxide were used to inhibit palmitic acid-induced calcium influx. And whether the compounds could reduce palmitic acid-induced β-cell failure and the underlying mechanism were also investigated. It was found that both nifedipine and diazoxide protected MIN6 pancreatic β-cells and primary cultured murine islets from palmitic acid-induced apoptosis. Meanwhile, the impaired insulin secretion was also recovered to varying degrees by these two compounds. Our results verified that nifedipine and diazoxide could reduce palmitic acid-induced endoplasmic reticulum stress to generate protective effects on pancreatic β-cells. More importantly, it suggested that regulation of calcium influx by small molecule compounds might provide benefits for the prevention and therapy of type 2 diabetes.

Nutrient induced type 2 and chemical induced type 1 experimental diabetes differently modulate gastric GLP-1 receptor expression

T2DM patients demonstrate reduced GLP-1 receptor (GLP-1R) expression in their gastric glands. Whether induced T2DM and T1DM differently affect the gastric GLP-1R expression is not known. This study assessed extrapancreatic GLP-1R system in glandular stomach of rodents with different types of experimental diabetes. T2DM and T1DM were induced in Psammomys obesus (PO) by high-energy (HE) diet and by streptozotocin (STZ) in Sprague Dawly (SD) rats, respectively. GLP-1R expression was determined in glandular stomach by RT PCR and immunohistomorphological analysis. The mRNA expression and cellular association of the GLP-1R in principal glands were similar in control PO and SD rats. However, nutrient and chemical induced diabetes resulted in opposite alterations of glandular GLP-1R expression. Diabetic PO demonstrated increased GLP-1R mRNA expression, intensity of cellular GLP-1R immunostaining, and frequency of GLP-1R positive cells in the neck area of principal glands compared with controls. In contrast, SD diabetic rats demonstrated decreased GLP-1 mRNA, cellular GLP-1R immunoreactivity, and frequency of GLP-1R immunoreactive cells in the neck area compared with controls. In conclusion, nutrient and chemical induced experimental diabetes result in distinct opposite alterations of GLP-1R expression in glandular stomach. These results suggest that induced T1DM and T2DM may differently modulate GLP-1R system in enteropancreatic axis.

β-glycosphingolipids ameliorated non-alcoholic steatohepatitis in the Psammomys obesus model

Liver steatosis is a common characteristic of obesity and type 2 diabetes, and fatty liver disease is increasingly recognized as a major health burden. Accumulating evidence suggests that β-glycosphingolipids play an important role in insulin sensitivity and thus could affect hepatic steatosis. To determine the effect associated with β-glycosphingolipid-mediated amelioration of liver injury, seven groups of Psammomys obesus on a high-energy diet were studied. Animals were treated with daily injections of β-glucosylceramide, β-lactosylceramide, or a combination of both. β-glycosphingolipids ameliorated the hepatic injury manifested by decreased liver enzymes, liver weight, and hepatic fat, and improved liver histology. Administration of both β-glucosylceramide and β-lactosylceramide also decreased interferon (IFN)-γ serum levels. These effects were associated with improved serum cholesterol and triglyceride levels. These data suggest that β-glycosphingolipids ameliorate liver injury in an animal model of nonalcoholic steatohepatitis.

A review of rodent models of type 2 diabetic skeletal fragility

Evidence indicating that adult type 2 diabetes (T2D) is associated with increased fracture risk continues to mount. Unlike osteoporosis, diabetic fractures are associated with obesity and normal to high bone mineral density, two factors that are typically associated with reduced fracture risk. Animal models will likely play a critical role in efforts to identify the underlying mechanisms of skeletal fragility in T2D and to develop preventative treatments. In this review we critically examine the ability of current rodent models of T2D to mimic the skeletal characteristics of human T2D. We report that although there are numerous rodent models of T2D, few have undergone thorough assessments of bone metabolism and strength. Further, we find that many of the available rodent models of T2D have limitations for studies of skeletal fragility in T2D because the onset of diabetes is often prior to skeletal maturation and bone mass is low, in contrast to what is seen in adult humans. There is an urgent need to characterize the skeletal phenotype of existing models of T2D, and to develop new models that more closely mimic the skeletal effects seen in adult-onset T2D in humans.

Beta cell dynamics: beta cell replenishment, beta cell compensation and diabetes

Type 2 diabetes, characterized by persistent hyperglycemia, arises mostly from beta cell dysfunction and insulin resistance and remains a highly complex metabolic disease due to various stages in its pathogenesis. Glucose homeostasis is primarily regulated by insulin secretion from the beta cells in response to prevailing glycemia. Beta cell populations are dynamic as they respond to fluctuating insulin demand. Beta cell replenishment and death primarily regulate beta cell populations. Beta cells, pancreatic cells, and extra-pancreatic cells represent the three tiers for replenishing beta cells. In rodents, beta cell self-replenishment appears to be the dominant source for new beta cells supported by pancreatic cells (non-beta islet cells, acinar cells, and duct cells) and extra-pancreatic cells (liver, neural, and stem/progenitor cells). In humans, beta cell neogenesis from non-beta cells appears to be the dominant source of beta cell replenishment as limited beta cell self-replenishment occurs particularly in adulthood. Metabolic states of increased insulin demand trigger increased insulin synthesis and secretion from beta cells. Beta cells, therefore, adapt to support their physiology. Maintaining physiological beta cell populations is a strategy for targeting metabolic states of persistently increased insulin demand as in diabetes.

[Interaction between environment and genetic background in type 2 diabetes: lessons from animal models]

The respective roles of predisposing genetic factors and environmental factors in the development of type 2 diabetes (T2D) in obese subjects is poorly documented. Rodent models have been set up in an attempt to better understand of the differential effect of a prolonged metabolic stress induced by a high fat diet on glycaemic control according to the genetic background. In utero growth retardation resulting from a hypocaloric diet in pregnant rats induces a dramatic alteration of the development of islet cells leading to diabetes and insulin secretory defects in adult age. Experimentally induced diabetes in rodents results in hyperglycaemia and hyperinsulinemia in the fetus related to accelerated endocrine pancreas maturation responsible for the onset of diabetes in the adult. Deranged metabolic environment during fetal life may therefore further contribute to the onset of diabetes in the adult. Normal mouse strains with different genetic backgrounds show a wide range of responses to a high fat diet, with strains resistant to the diet and other more or less sensitive to the diet, the most sensitive exhibiting obesity diabetes and, insulin deficiency. The inability of the β cell to respond to the increased insulin demand related to insulin resistance seems to be pivotal in the pathophysiologic process and a new notion is emerging: "nutritional genetics" which studies the influence of nutrients on gene expression.

The expression of the beta cell-derived autoimmune ligand for the killer receptor nkp46 is attenuated in type 2 diabetes

NK cells rapidly kill tumor cells, virus infected cells and even self cells. This is mediated via killer receptors, among which NKp46 (NCR1 in mice) is prominent. We have recently demonstrated that in type 1 diabetes (T1D) NK cells accumulate in the diseased pancreas and that they manifest a hyporesponsive phenotype. In addition, we found that NKp46 recognizes an unknown ligand expressed by beta cells derived from humans and mice and that blocking of NKp46 activity prevented diabetes development. Here we investigated the properties of the unknown NKp46 ligand. We show that the NKp46 ligand is mainly located in insulin granules and that it is constitutively secreted. Following glucose stimulation the NKp46 ligand translocates to the cell membrane and its secretion decreases. We further demonstrate by using several modalities that the unknown NKp46 ligand is not insulin. Finally, we studied the expression of the NKp46 ligand in type 2 diabetes (T2D) using 3 different in vivo models and 2 species; mice and gerbils. We demonstrate that the expression of the NKp46 ligand is decreased in all models of T2D studied, suggesting that NKp46 is not involved in T2D.

Obesity, insulin resistance and diabetes in the sand rat exposed to a hypercaloric diet; possible protective effect for IL1-β

It is well established that, upon changing their natural desert low caloric (succulent halophilic plants) to a regular laboratory high caloric diet, sand rats undergo various phenotypic changes depending on their genetic background and including obesity and various degrees of insulin resistance. Our aim was to investigate the acute effects of Interleukin-1β (IL-1β) and Interferon-γ (IFN-γ) on glucose-induced insulin secretion in normal lean sand rats maintained on their natural diet and in obese insulin resistant normoglycemic or type 2 diabetic animals after a 9-month high caloric diet. Animals were fed either a low or a high caloric diet; after 9 months, pancreatic islets were isolated and incubated in the presence of increasing cytokine concentrations. At the end of the high-energy diet, animals were all over-weight, and probably due to a different genetic background, they displayed either insulin resistance, hyperinsulinemia and normoglycemia or a marked type-2 diabetic state. Pancreatic islets from obese insulin resistant normoglycemic animals were much more sensitive and responsive to IL-1β when compared to lean controls. The cytokine was inefficient in diabetic islets. In conclusion, the markedly increased insulinotropic effect of IL-1β in obese diabetes-resistant sand rat could participate and be involved in pancreatic β-cell hyperactivity that compensates for insulin resistance and thereby prevent the development of type 2 diabetes in these animals.

The plasma 5'-AMP acts as a potential upstream regulator of hyperglycemia in type 2 diabetic mice

Increased plasma free fatty acid (FFA) level is a hallmark of type 2 diabetes. However, the underlying molecular basis for FFA-caused hyperglycemia remains unclear. Here we identified plasma 5'-adenosine monophosphate (pAMP) markedly elevated in the plasma of type 2 diabetic mice. High levels of FFAs induced damage in vein endothelial cells and contributed to an increase in pAMP. Administration of synthetic 5'-AMP caused hyperglycemia and impaired insulin action in lean wild-type mice. 5'-AMP elevated blood glucose in mice deficient in adenosine receptors with equal efficiency as wild-type mice. The function of pAMP was initiated by the elevation of cellular adenosine levels, directly stimulating G-6-Pase enzyme activity, attenuating insulin-dependent GLUT4 translocation in skeletal muscle, and displaying a rapid and steep increase in blood glucose and a decrease in hepatic glycogen level. It was followed by an increase in the gene expression of hepatic Foxo1 and its targeting gene Pepck and G6Pase, which was similar to diabetic phenotype in db/db mice. Our results suggest that pAMP is a potential upstream regulator of hyperglycemia in type 2 diabetes.

Diet-induced diabetes in the sand rat (Psammomys obesus)

Insulin deficiency is the underlying cause of hyperglycemia in type 2 diabetes. The gerbil Psammomys obesus (P. obesus) is a naturally insulin resistant rodent with tendency to develop diet-induced hyperglycemia associated with obesity. P. obesus does not exhibit hyperglycemia in its natural desert habitat, feeding on low caloric vegetation. However, when fed regular laboratory chow containing higher caloric density, the animals develop moderate obesity and hyperglycemia. Diabetes development and progression is very fast in P. obesus. The animals reach the irreversible hypoinsulinemic stage of the disease, in which a marked reduction of β-cell mass is apparent, within 4-6 weeks of high caloric diet. The present review describes the P. obesus of the Hebrew University colony, with emphasis on its use for the study of β-cell dysfunction in type 2 diabetes.

Upregulation of alpha cell glucagon-like peptide 1 (GLP-1) in Psammomys obesus--an adaptive response to hyperglycaemia?

AIMS/HYPOTHESIS

The hormone glucagon-like peptide 1 (GLP-1) is released in response to a meal from the intestinal L-cells, where it is processed from proglucagon by the proconvertase (PC)1/3. In contrast, in the adult islets proglucagon is processed to glucagon by the PC2 enzyme. The aim of the study was to evaluate if, during the development of diabetes, alpha cells produce GLP-1 that, in turn, might trigger beta cell growth.

METHODS

Beta cell mass, GLP-1 and insulin levels were measured in the gerbil Psammomys obesus (P. obesus), a rodent model of nutritionally induced diabetes. Furthermore, the presence of biologically active forms of GLP-1 and PC1/3 in alpha cells was demonstrated by immunofluorescence, and the release of GLP-1 from isolated P. obesus, mouse and human islets was investigated.

RESULTS

During the development of diabetes in P. obesus, a significant increase in GLP-1 was detected in the portal vein (9.8 ± 1.5 vs 4.3 ± 0.7 pmol/l, p < 0.05), and in pancreas extracts (11.4 ± 2.2 vs 5.1 ± 1.3 pmol/g tissue, p < 0.05). Freshly isolated islets from hyperglycaemic animals released more GLP-1 following 24 h culture than islets from control animals (28.2 ± 4.4 pmol/l vs 5.8 ± 2.4, p < 0.01). GLP-1 release was increased from healthy P. obesus islets following culture in high glucose for 6 days (91 ± 9.1 pmol/l vs 28.8 ± 6.6, p < 0.01). High levels of GLP-1 were also found to be released from human islets. PC1/3 colocalised weakly with alpha cells.

CONCLUSIONS/INTERPRETATION

GLP-1 release from alpha cells is upregulated in P. obesus during the development of diabetes. A similar response is seen in islets exposed to high glucose, which supports the hypothesis that GLP-1 released from alpha cells promotes an increase in beta cell mass and function during metabolic challenge such as diabetes.

Inhibition of the nuclear factor-κB pathway prevents beta cell failure and diet induced diabetes in Psammomys obesus

Background

High doses of anti-inflammatory drugs, such as aspirin and salicylates, improve glucose metabolism in insulin resistant and type 2 diabetic patients. It has also been shown that the glucose lowering effect is related to the unspecific ability of these drugs to inhibit inhibitor kinaseβ (IKKβ). In this study we have investigated the effect of a selective IKKβ-inhibitor on beta cell survival and the prevention of diet induced type 2 diabetes in the gerbil Psammomys obesus (P. obesus).

Methodology/Principal Findings

P. obesus were fed a diabetes inducing high energy diet for one month in the absence or presence of the IKKβ-inhibitor. Body mass, blood glucose, HbA_1C, insulin production and pancreatic insulin stores were measured. The effects on beta cell survival were also studied in INS-1 cells and primary islets. The cells were exposed to IL-1β and subsequently reactive oxygen species, insulin release and cell death were measured in the absence or presence of the IKKβ-inhibitor. In primary islets and beta cells, IL-1β induced the production of reactive oxygen species, reduced insulin production and increased beta cell death, which were all reversed by pre-treatment with the IKKβ-inhibitor. In P. obesus the IKKβ-inhibitor prevented the development of hyperglycaemia and hyperinsulinaemia, and maintained pancreatic insulin stores with no effect on body weight.

Conclusions/Significance

Inhibition of IKKβ activity prevents diet-induced diabetes in P. obesus and inhibits IL-1β induced reactive oxygen species, loss of insulin production and beta cell death in vitro.

Neuronal nitric oxide synthase protects the pancreatic beta cell from glucolipotoxicity-induced endoplasmic reticulum stress and apoptosis

AIMS/HYPOTHESIS

Cytokines stimulate nitric oxide production in pancreatic beta cells, leading to endoplasmic reticulum (ER) stress and apoptosis. Treatment of beta cells with glucose and NEFA induces nitric oxide synthase (NOS) as well as ER stress. However, the role of NO in glucolipotoxicity-induced ER stress in beta cells is not clear.

METHODS

We studied the effect of high glucose and palmitate levels on NOS isoform production in rat and Psammomys obesus islets and in insulinoma-1E beta cells. The effects of neuronal NOS (nNOS) inhibition by small interfering RNA or by N (omega)-nitro-L-arginine methyl ester (L-NAME) on beta cell function, ER stress and apoptosis under conditions of glucolipotoxicity were investigated.

RESULTS

Overnight incubation of rat and P. obesus islets at 22.2 mmol/l glucose with 0.5 mmol/l palmitate induced the production of nNOS but not inducible NOS (iNOS), in contrast with the robust stimulation of iNOS by cytokines. NOS inhibition by L-NAME did not prevent the decrease in glucose-stimulated insulin secretion and proinsulin biosynthesis or the depletion of islet insulin content observed under conditions of glucolipotoxicity. Moreover, treatment of beta cells with palmitate and L-NAME together resulted in marked activation of the IRE1alpha and PERK pathways of the unfolded protein response. This was associated with increased JNK phosphorylation and apoptosis in islets and beta cells. Moreover, partial nNos knockdown increased JNK phosphorylation and CHOP production, leading to apoptosis.

CONCLUSIONS/INTERPRETATION

In beta cells subjected to glucolipotoxic conditions, chronic inhibition of NOS exacerbates ER stress and activates JNK. Therefore, induction of nNOS is an adaptive response to glucolipotoxicity that protects beta cells from stress and apoptosis.

N-acetyltransferase 2 (Nat2) polymorphism in the sand rat Psammomys obesus

Human arylamine N-acetyltransferase 1 (NAT1) and its homologue in rodents (Nat2) are polymorphic xenobiotic metabolizing enzymes and also seem to play a role in endogenous metabolism. NAT1 and Nat2 polymorphism was associated to cancers under xenobiotic procarcinogens metabolism as well as under endogenous substrate metabolism. This study investigated the p-aminobenzoic acid (PABA) -Nat2 catalytic activity and its polymorphism in liver homogenates of adult sand rats Psammomys obesus Cretzschmar, 1828. These Saharian sand rats develop high incidence of spontaneous cancers under standard laboratory diet. The average value of PABA-Nat2 specific activity tested in nine sand rats was significant (2.96 ± 2.16 nmoles/min/mg). The N-acetylation exhibited a bimodal distribution. There was a significant difference (p<0.01) between PABA-Nat2 activity in the fast acetylators group (4.10 ± 1.67 nmol/min/mg) and slow acetylators group (0.7 ± 0.27 nmol/min/mg). The percentage of the fast acetylator group was 66.66%. These results support the presence of Nat2 polymorphism in the liver of the strain sand rats Psammomys obesus. This strain is useful for investigating the role of Nat2 polymorphisms in susceptibility to cancers related to arylamine carcinogen exposures as well as to endogenous substrate metabolism.

Balancing needs and means: the dilemma of the beta-cell in the modern world

The insulin resistance of type 2 diabetes mellitus (T2DM), although important for its pathophysiology, is not sufficient to establish the disease unless major deficiency of beta-cell function coexists. This is demonstrated by the fact that near-physiological administration of insulin (CSII) achieved excellent blood glucose control with doses similar to those used in insulin-deficient type 1 diabetics. The normal beta-cell adapts well to the demands of insulin resistance. Also in hyperglycaemic states some degree of adaptation does exist and helps limit the severity of disease. We demonstrate here that the mammalian target of rapamycin (mTOR) system might play an important role in this adaptation, because blocking mTORC1 (complex 1) by rapamycin in the nutritional diabetes model Psammomys obesus caused severe impairment of beta-cell function, increased beta-cell apoptosis and progression of diabetes. On the other hand, under exposure to high glucose and FFA (gluco-lipotoxicity), blocking mTORC1 in vitro reduced endoplasmic reticulum (ER) stress and beta-cell death. Thus, according to the conditions of stress, mTOR may have beneficial or deleterious effects on the beta-cell. beta-Cell function in man can be reduced without T2DM/impaired glucose tolerance (IGT). Prospective studies have shown subjects with reduced insulin response to present, several decades later, an increased incidence of IGT/T2DM. From these and other studies we conclude that T2DM develops on the grounds of beta-cells whose adaptation capacity to increased nutrient intake and/or insulin resistance is in the lower end of the normal variation. Inborn and acquired factors that limit beta-cell function are diabetogenic only in a nutritional/metabolic environment that requires high functional capabilities from the beta-cell.

Increased hepatic lipogenesis in insulin resistance and Type 2 diabetes is associated with AMPK signalling pathway up-regulation in Psammomys obesus

AMPK (AMP-activated protein kinase) has been suggested to be a central player regulating FA (fatty acid) metabolism through its ability to regulate ACC (acetyl-CoA carboxylase) activity. Nevertheless, its involvement in insulin resistance- and TD2 (Type 2 diabetes)-associated dyslipidaemia remains enigmatic. In the present study, we employed the Psammomys obesus gerbil, a well-established model of insulin resistance and TD2, in order to appreciate the contribution of the AMPK/ACC pathway to the abnormal hepatic lipid synthesis and increased lipid accumulation in the liver. Our investigation provided evidence that the development of insulin resistance/diabetic state in P. obesus is accompanied by (i) body weight gain and hyperlipidaemia; (ii) elevations of hepatic ACC-Ser79 phosphorylation and ACC protein levels; (iii) a rise in the gene expression of cytosolic ACC1 concomitant with invariable mitochondrial ACC2; (iv) an increase in hepatic AMPKα-Thr172 phosphorylation and protein expression without any modification in the calculated ratio of phospho-AMPKα to total AMPKα; (v) a stimulation in ACC activity despite increased AMPKα phosphorylation and protein expression; and (vi) a trend of increase in mRNA levels of key lipogenic enzymes [SCD-1 (stearoyl-CoA desaturase-1), mGPAT (mitochondrial isoform of glycerol-3-phosphate acyltransferase) and FAS (FA synthase)] and transcription factors [SREBP-1 (sterol-regulatory-element-binding protein-1) and ChREBP (carbohydrate responsive element-binding protein)]. Altogether, our findings suggest that up-regulation of the AMPK pathway seems to be a natural response in order to reduce lipid metabolism abnormalities, thus supporting the role of AMPK as a promising target for the treatment of TD2-associated dyslipidaemia.

Islet beta cell failure in the 60% pancreatectomised obese hyperlipidaemic Zucker fatty rat: severe dysfunction with altered glycerolipid metabolism without steatosis or a falling beta cell mass

AIMS/HYPOTHESIS

The Zucker fatty (ZF) rat subjected to 60% pancreatectomy (Px) develops moderate diabetes by 3 weeks. We determined whether a progressive fall in beta cell mass and/or beta cell dysfunction contribute to beta cell failure in this type 2 diabetes model.

METHODS

Partial (60%) or sham Px was performed in ZF and Zucker lean (ZL) rats. At 3 weeks post-surgery, beta cell mass and proliferation, proinsulin biosynthesis, pancreatic insulin content, insulin secretion, and islet glucose and lipid metabolism were measured.

RESULTS

ZL-Px rats maintained normal glycaemia and glucose-stimulated insulin secretion (GSIS) despite incomplete recovery of beta cell mass possibly due to compensatory enhanced islet glucose metabolism and lipolysis. ZF-Px rats developed moderate hyperglycaemia (14 mmol/l), hypertriacylglycerolaemia and relative hypoinsulinaemia. Despite beta cell mass recovery and normal arginine-induced insulin secretion, GSIS and pancreatic insulin content were profoundly lowered in ZF-Px rats. Proinsulin biosynthesis was not reduced. Compensatory increases in islet glucose metabolism above those observed in ZF-Sham rats were not seen in ZF-Px rats. Triacylglycerol content was not increased in ZF-Px islets, possibly due to lipodetoxification by enhanced lipolysis and fatty acid oxidation. Fatty acid accumulation into monoacylglycerol and diacylglycerol was increased in ZF-Px islets together with a 4.5-fold elevation in stearoyl-CoA desaturase mRNA expression.

CONCLUSIONS/INTERPRETATION

Falling beta cell mass, reduced proinsulin biosynthesis and islet steatosis are not implicated in early beta cell failure and glucolipotoxicity in ZF-Px rats. Rather, severe beta cell dysfunction with a specific reduction in GSIS and marked depletion of beta cell insulin stores with altered lipid partitioning underlie beta cell failure in this animal model of type 2 diabetes.

Glucose amplifies fatty acid-induced endoplasmic reticulum stress in pancreatic beta-cells via activation of mTORC1

Background

Palmitate is a potent inducer of endoplasmic reticulum (ER) stress in β-cells. In type 2 diabetes, glucose amplifies fatty-acid toxicity for pancreatic β-cells, leading to β-cell dysfunction and death. Why glucose exacerbates β-cell lipotoxicity is largely unknown. Glucose stimulates mTORC1, an important nutrient sensor involved in the regulation of cellular stress. Our study tested the hypothesis that glucose augments lipotoxicity by stimulating mTORC1 leading to increased β-cell ER stress.

Principal Findings

We found that glucose amplifies palmitate-induced ER stress by increasing IRE1α protein levels and activating the JNK pathway, leading to increased β-cell apoptosis. Moreover, glucose increased mTORC1 activity and its inhibition by rapamycin decreased β-cell apoptosis under conditions of glucolipotoxicity. Inhibition of mTORC1 by rapamycin did not affect proinsulin and total protein synthesis in β-cells incubated at high glucose with palmitate. However, it decreased IRE1α expression and signaling and inhibited JNK pathway activation. In TSC2-deficient mouse embryonic fibroblasts, in which mTORC1 is constitutively active, mTORC1 regulated the stimulation of JNK by ER stressors, but not in response to anisomycin, which activates JNK independent of ER stress. Finally, we found that JNK inhibition decreased β-cell apoptosis under conditions of glucolipotoxicity.

Conclusions/Significance

Collectively, our findings suggest that mTORC1 mediates glucose amplification of lipotoxicity, acting through activation of ER stress and JNK. Thus, mTORC1 is an important transducer of ER stress in β-cell glucolipotoxicity. Moreover, in stressed β-cells mTORC1 inhibition decreases IRE1α protein expression and JNK activity without affecting ER protein load, suggesting that mTORC1 regulates the β-cell stress response to glucose and fatty acids by modulating the synthesis and activity of specific proteins involved in the execution of the ER stress response. This novel paradigm may have important implications for understanding β-cell failure in type 2 diabetes.

Nutritional models of type 2 diabetes mellitus

In order to better understand the events which precede and precipitate the onset of type 2 diabetes (T2DM) several nutritional animal models have been developed. These models are generated by manipulating the diet of either the animal itself or its mother during her pregnancy and, in comparison to traditional genetic and knock out models, have the advantage that they more accurately reflect the aetiology of human T2DM. This chapter will discuss some of the most widely used nutritional models of T2DM: Diet-induced obesity (DIO) in adult rodents, and studies of prenatal and postnatal nutrition in offspring of mothers fed a low-protein diet or overnourished during pregnancy. Several common mechanisms have been identified through which these nutritional manipulations can lead to metabolic disease, including pancreatic beta-cell dysfunction, impaired insulin signalling in skeletal muscle and the excess accumulation of visceral adipose tissue and consequent deposition of non-esterified fatty acids in peripheral tissues resulting in peripheral insulin resistance. The following chapter will discuss each of these nutritional models, their application and relationship to human aetiology, and will highlight the important insights these models have provided into the pathogenesis of T2DM.

The role of mTOR in the adaptation and failure of beta-cells in type 2 diabetes

Mammalian target of rapamycin (mTOR) is an important nutrient sensor that plays a critical role in cellular metabolism, growth, proliferation and apoptosis and in the cellular response to oxidative stress. In addition, mTOR-raptor complex, also called mammalian target of rapamycin complex 1 (mTORC1), generates an inhibitory feedback loop on insulin receptor substrate proteins. It was suggested that nutrient overload leads to insulin/insulin-like growth factor 1 resistance in peripheral insulin-responsive tissues and in the beta-cells through sustained activation of mTORC1. In this review, we summarize the literature on the regulation and function of mTOR, its role in the organism's response to nutrients and its potential impact on lifespan, insulin resistance and the metabolic adaptation to hyperglycaemia in type 2 diabetes. We also propose a hypothesis based on data in the literature as well as data generated in our laboratory, which assigns a central positive role to mTOR in the maintenance of beta-cell function and mass in the diabetic environment.

Anatomical versus functional beta-cell mass in experimental diabetes

The ability of pancreatic beta-cell mass to vary according to insulin requirements is an important component of optimal long-term control of glucose homeostasis. It is generally assumed that alteration of this property largely contributes to the impairment of insulin secretion in type 2 diabetes. However, data in humans are scarce and it is impossible to correlate beta-cell mass and function with the various stages of the disease. Thus, the importance of animal models is obvious. In rodents, increased beta-cell mass associated with an increase in the function of individual beta-cells contributes to the adaptation of the insulin response to insulin resistance in late pregnancy and in obesity. A reduction in beta-cell mass always corresponds to an alteration in insulin secretory capacity of islet tissue (Zucker diabetic fatty and Goto-Kakisaki rats, db/db mice). During regenerative processes following experimental reduction of beta-cell mass [partial pancreatectomy, streptozocin (STZ) injection], beta-cell mass increase is not associated with a corresponding improvement of beta-cell function, thus indicating that regenerative beta-cells did not achieve functional maturity. The main lesson from experimental diabetes is therefore that beta-cell mass cannot always predict functional capacity of the beta-cell tissue and that the functional beta-cell mass rather than the anatomical beta-cell mass must be taken into account at all times.

A review of islet of Langerhans degeneration in rodent models of type 2 diabetes

Type 2 diabetes mellitus (TTDM) is characterized by progressive loss of glucose control through multifactorial mechanisms. The search for an understanding of TTDM has relied on animal models since the realization of the importance of the pancreas in controlling plasma glucose concentration. Rodent models of TTDM are developed to express hyperglycemia and not islet degeneration per se. Degeneration of the islets of Langerhans with beta-cell loss is secondary to insulin resistance and is regarded as the more important lesion. Despite this, differences between models are seen in the development and progression of islet degeneration. Assessing the differences between the models is important to appreciate the various aspects of TTDM and understand their advantages as well as their deficiencies. Relevant animal models of TTDM provide opportunities to investigate important physiological and cell biological processes that may ultimately lead to development of targeted therapies. This article reviews the importance, advantages, and limitations of rodent models of TTDM in relation to the histopathological changes that characterize islet degeneration. Pathophysiological mechanisms that contribute to islet degeneration are also discussed and are placed into the context of changes in islet histological appearances.

mTOR inhibition by rapamycin prevents beta-cell adaptation to hyperglycemia and exacerbates the metabolic state in type 2 diabetes

OBJECTIVE

Mammalian target of rapamycin (mTOR) and its downstream target S6 kinase 1 (S6K1) mediate nutrient-induced insulin resistance by downregulating insulin receptor substrate proteins with subsequent reduced Akt phosphorylation. Therefore, mTOR/S6K1 inhibition could become a therapeutic strategy in insulin-resistant states, including type 2 diabetes. We tested this hypothesis in the Psammomys obesus (P. obesus) model of nutrition-dependent type 2 diabetes, using the mTOR inhibitor rapamycin.

RESEARCH DESIGN AND METHODS

Normoglycemic and diabetic P. obesus were treated with 0.2 mg x kg(-1) x day(-1) i.p. rapamycin or vehicle, and the effects on insulin signaling in muscle, liver and islets, and on different metabolic parameters were analyzed.

RESULTS

Unexpectedly, rapamycin worsened hyperglycemia in diabetic P. obesus without affecting glycemia in normoglycemic controls. There was a 10-fold increase of serum insulin in diabetic P. obesus compared with controls; rapamycin completely abolished this increase. This was accompanied by weight loss and a robust increase of serum lipids and ketone bodies. Rapamycin decreased muscle insulin sensitivity paralleled by increased glycogen synthase kinase 3beta activity. In diabetic animals, rapamycin reduced beta-cell mass by 50% through increased apoptosis. Rapamycin increased the stress-responsive c-Jun NH(2)-terminal kinase pathway in muscle and islets, which could account for its effect on insulin resistance and beta-cell apoptosis. Moreover, glucose-stimulated insulin secretion and biosynthesis were impaired in islets treated with rapamycin.

CONCLUSIONS

Rapamycin induces fulminant diabetes by increasing insulin resistance and reducing beta-cell function and mass. These findings emphasize the essential role of mTOR/S6K1 in orchestrating beta-cell adaptation to hyperglycemia in type 2 diabetes. It is likely that treatments based on mTOR inhibition will cause exacerbation of diabetes.

Scanning electron microscopic analysis of intramyocellular lipid droplets in an animal model of type 2 diabetes

OBJECTIVE

To evaluate the accumulation pattern of intramyocellular lipids (IMCLs) in striated muscle during the development and progression of diabetes, using a novel scanning electron microscopic method.

METHODS AND PROCEDURES

Hyperglycemia was induced by feeding diabetes-prone (DP) Psammomys obesus a high-energy (HE) diet. Lipid accumulation within gastrocnemius muscle fibers was assessed in formalin-fixed muscle samples during the development of hyperglycemia using high resolution imaging in a scanning electron microscope. We evaluated the temporal relationship between changes in IMCL quantity and morphology and the altered glucose metabolism and assessed the effect of reversal of hyperglycemia on IMCL level and morphology. Diabetes-resistant (DR) P. obesus served as controls.

RESULTS

Lipid accumulation in the muscle fibers of DP animals was increased with the development of hyperglycemia. This was characterized by increased lipid density as well as by an abundance of large lipid droplets. Reversal of the phenotype resulted in the disappearance of large lipid droplets. The IMCL level and the distribution of lipid droplet size were similar in muscles of both the normoglycemic DR and DP animals, with an abundance of small lipid droplets. This profile was changed following a HE diet only in the DP animals.

DISCUSSION

Lipid accumulation in the muscle of P. obesus during the development of hyperglycemia is characterized by increased quantity and accumulation of large lipid droplets. These changes were reversible upon normalization of blood glucose. The evaluated methodology is a useful tool for the study of the dynamics of lipid accumulation in different metabolic conditions.

Impaired glucose-stimulated insulin secretion is coupled with exocrine pancreatic lesions in the Cohen diabetic rat

OBJECTIVE

The Cohen diabetes-sensitive rat develops postprandial hyperglycemia when fed a high-sucrose, copper-poor diet, whereas the Cohen diabetes-resistant rat maintains normoglycemia. The pathophysiological basis of diabetes was studied in the Cohen diabetic rat centering on the interplay between the exocrine and endocrine compartments of the pancreas.

RESEARCH DESIGN AND METHODS

Studies used male Cohen diabetes-sensitive and Cohen diabetes-resistant rats fed 1-month high-sucrose, copper-poor diet. Serum insulin and glucose levels were measured during glucose and insulin tolerance tests. The pancreas was evaluated for weight, insulin content, macrophage, and fat infiltration. Glucose-stimulated insulin secretion (GSIS) was determined in isolated perfused pancreas and in islets.

RESULTS

Hyperglycemic Cohen diabetes-sensitive rats exhibited reduced pancreatic weight with lipid deposits and interleukin-1beta-positive macrophage infiltration in the exocrine pancreas. Islet morphology was preserved, and total pancreatic insulin content did not differ from that of Cohen diabetes-resistant rats. Lipids did not accumulate in skeletal muscle, nor was insulin resistance observed in hyperglycemic Cohen diabetes-sensitive rats. Intravenous glucose-tolerance test revealed markedly elevated glucose levels associated with diminished insulin output. Insulin release was induced in vivo by the non-nutrient secretagogues arginine and tolbutamide, suggesting a selective unresponsiveness to glucose. Decreased GSIS was observed in the isolated perfused pancreas of the hyperglycemic Cohen diabetes-sensitive rat, whereas islets isolated from these rats exhibited glucose-dependent insulin secretion and proinsulin biosynthesis.

CONCLUSIONS

The association of the in vivo insulin secretory defect with lipid accumulation and activated macrophage infiltration in the exocrine pancreas suggests that changes in the islet microenvironment are the culprit in the insulin secretory malfunction observed in vivo.

[Anatomical and functional plasticity of pancreatic beta-cells and type 2 diabetes]

The most common form of diabetes, type 2 diabetes (T2D) is a major Public Health issue which is receiving a great deal of attention both in industrial and public research, in order to develop new and more effective drugs. The hyperglycaemia of T2D is the result of two interdependent defects : decreased biological efficacy of insulin in target tissues (insulin resistance), and a decreased capacity for beta cells to secrete insulin in response to glucose. Furthermore, hyperglycaemia evolves with time and even with rigorous treatment there is a progressive deterioration of glucose homeostasis. Seventy five percent of DT2 patients are obese and show a perturbed lipid profile. beta-cell plasticity is a unique property of these cells to adapt their number and volume (beta-cell mass) and their function to the increased secretory demand linked to insulin resistance. This is well documented in physiological (pregnancy) as well in pathophysiological conditions (obesity, acromegaly). Although the lack of reliable techniques makes it very difficult to document it in humans, this property is likely altered in DT2, mainly as a consequence of the prolonged exposure of islet cells to high plasma levels of glucose and free fatty acids (gluco-lipotoxicity). The mechanisms by which hyperglycaemia and hyperlipidemia exert their deleterious effects on the beta-cell include the generation of Reactive Oxygen Species (ROS) and Reactive Nitrogen Species (RNS) and Advanced Glycosylation End Products (AGE). Altogether the prevailing clinical and experimental data urge us to consider that the pathophysiology of DT2 lies, at least in part, the inability of beta-cells to adapt their functional mass to the prevailing insulin demand. This re-evaluation of the pathophysiology of DT2 stimulates the research of new therapeutic approaches aimed at maintaining and/or restoring the functional beta-cell mass by targeting the mechanisms responsible for its decrease.

Congenital and environmental factors associated with adipocyte dysregulation as defects of insulin resistance

The metabolic syndrome refers to insulin resistance and its associated cluster of related cardiovascular metabolic risk factors including type 2 diabetes, hypertension, dyslipidemia and central obesity. Although many hypotheses and facts have been proposed to explain the interaction between genetic and environmental causes of this syndrome, the primary etiology of the metabolic syndrome is adipose tissue dysregulation. Firstly, the thrifty genotype and phenotype hypothesis may explain the endemic increase in type 2 diabetes and cardiovascular disease in developing countries and elucidates the congenital susceptibility and environmental triggering of the metabolic syndrome. Secondly, over-nutrition leads to fatty acid (FA) accumulation in adipocytes and to an overflow to ectopic fat storage organs. This causes functional changes in adipocytes shifting the intra-cellular metabolic pathway toward insulin resistance. Thirdly, obese subjects exhibit increased fat cell size and over-secretion of biologic adipocytokines. Fourthly, failure to adequately develop adipose tissue mass, as seen in lipodystrophy cases, causes severe insulin resistance and diabetes. Lastly, similar to human type 2 diabetes, Psammonys obesus, a desert rat which feeds mainly on low-calorie vegetation, develops the metabolic syndrome when given a diet of calorie rich food. The above evidence indicates adipocyte dysregulation and secretion of FA as well as certain molecules from overloaded adipocytes/adipokines contribute to the pathogenesis of impaired insulin secretion and insulin resistance, endothelial dysfunction, a pro-inflammatory state and promote progression of atherosclerosis. The metabolic syndrome is a modern disease resulting adipocyte dysmetabolism resulting from the paradox of the slow human evolution combined with rapid environmental changes.

Beta cell death in hyperglycaemic Psammomys obesus is not cytokine-mediated

AIMS/HYPOTHESIS

It has recently been proposed that IL-1beta may be responsible for beta cell death in type 2 diabetes mellitus. Major support for this assumption was derived from experiments in the gerbil Psammomys obesus (sand rat), a model for nutritionally induced non-insulin-dependent type 2 diabetes. Using gerbil-specific primers for the analysis of gene expression, we investigated the validity of this hypothesis.

METHODS

Gene expression was analysed by real-time RT-PCR of isolated and laser-microdissected islets and by in situ RT-PCR, both in beta cells and in immune cells, as well as in lymph nodes and spleen.

RESULTS

We were unable to detect Il-1beta and the IL-1beta-inducible enzyme inducible nitric oxide synthase (iNos) by in situ RT-PCR, either in the pancreatic beta cells, or in the small number of non-activated immune cells of healthy and diabetic Psammomys obesus after 1 and 3 weeks on a high-energy diet. Very low levels of Il-1beta and iNos mRNA were detectable in collagenase-isolated and laser-microdissected islets of normoglycaemic gerbils by real-time RT-PCR without any increase of these mRNAs in islets from diabetic animals. These results were confirmed by electron microscopy with immunogold staining for IL-1beta and insulin.

CONCLUSIONS/INTERPRETATION

The diabetic syndrome induced in Psammomys obesus by high-energy diet is a classical type 2 diabetes model, which does not show any evidence of an involvement of the proinflammatory cytokine IL-1beta or of activated immune cells in its pathogenesis. This is clearly at variance with the situation in type 1 diabetes.

Regulation of insulin secretion and proinsulin biosynthesis by succinate

Succinate stimulates insulin secretion and proinsulin biosynthesis. We studied the effects of reduced nicotinamide adenine dinucleotide phosphate (NADPH)-modulating pathways on glucose- and succinate-stimulated insulin secretion and proinsulin biosynthesis in the rat and the insulin-resistant Psammomys obesus. Disruption of the anaplerotic pyruvate/malate shuttle by phenylacetic acid inhibited glucose- and succinate-stimulated insulin secretion and succinate-stimulated proinsulin biosynthesis in both species. In contrast, phenylacetic acid failed to inhibit glucose-stimulated proinsulin biosynthesis in P. obesus islets. Inhibition of the NADPH-consuming enzyme neuronal nitric oxide synthase (nNOS) with l-N(G)-nitro-l-arginine methyl ester or with N(G)-monomethyl-l-arginine(G) doubled succinate-stimulated insulin secretion in rat islets, suggesting that succinate- and nNOS-derived signals interact to regulate insulin secretion. In contrast, nNOS inhibition had no effect on succinate-stimulated proinsulin biosynthesis in both species. In P. obesus islets, insulin secretion was not stimulated by succinate in the absence of glucose, whereas proinsulin biosynthesis was increased 5-fold. Conversely, under stimulating glucose levels, succinate doubled insulin secretion, indicating glucose-dependence. Pyruvate ester and inhibition of nNOS partially mimicked the permissive effect of glucose on succinate-stimulated insulin secretion, suggesting that anaplerosis-derived signals render the beta-cells responsive to succinate. We conclude that beta-cell anaplerosis via pyruvate carboxylase is important for glucose- and succinate-stimulated insulin secretion and for succinate-stimulated proinsulin biosynthesis. In P. obesus, pyruvate/malate shuttle dependent and independent pathways that regulate proinsulin biosynthesis coexist; the latter can maintain fuel stimulated biosynthetic activity when the succinate-dependent pathway is inhibited. nNOS signaling is a negative regulator of insulin secretion, but not of proinsulin biosynthesis.

Islet beta cell failure in type 2 diabetes

The major focus of this Review is on the mechanisms of islet beta cell failure in the pathogenesis of obesity-associated type 2 diabetes (T2D). As this demise occurs within the context of beta cell compensation for insulin resistance, consideration is also given to the mechanisms involved in the compensation process, including mechanisms for expansion of beta cell mass and for enhanced beta cell performance. The importance of genetic, intrauterine, and environmental factors in the determination of "susceptible" islets and overall risk for T2D is reviewed. The likely mechanisms of beta cell failure are discussed within the two broad categories: those with initiation and those with progression roles.