Caloric restriction in obese pre-diabetic rats prevents beta-cell depletion, loss of beta-cell GLUT 2 and glucose incompetence

Multi-omics: Opportunities for research on mechanism of type 2 diabetes mellitus

Type 2 diabetes mellitus (T2DM) is a burdensome global disease. In-depth understanding of its mechanism will help to optimize diagnosis and treatment, which reduces the burden. Multi-omics research has unparalleled advantages in contributing to the overall understanding of the mechanism of this chronic metabolic disease. In the past two decades, the study of multi-omics on T2DM-related intestinal flora perturbation and plasma dyslipidemia has shown tremendous potential and is expected to achieve major breakthroughs. The regulation of intestinal flora in diabetic patients has been confirmed by multiple studies. The use of metagenomics, 16S RNA sequencing, and metabolomics has comprehensively identified the overall changes in the intestinal flora and the metabolic disturbances that could directly or indirectly participate in the intestinal flora-host interactions. Lipidomics combined with other “omics” has characterized lipid metabolism disorders in T2DM. The combined application and cross-validation of multi-omics can screen for dysregulation in T2DM, which will provide immense opportunities to understand the mechanisms behind T2DM.

Understanding Dietary Intervention-Mediated Epigenetic Modifications in Metabolic Diseases

The global prevalence of metabolic disorders, such as obesity, diabetes and fatty liver disease, is dramatically increasing. Both genetic and environmental factors are well-known contributors to the development of these diseases and therefore, the study of epigenetics can provide additional mechanistic insight. Dietary interventions, including caloric restriction, intermittent fasting or time-restricted feeding, have shown promising improvements in patients' overall metabolic profiles (i.e., reduced body weight, improved glucose homeostasis), and an increasing number of studies have associated these beneficial effects with epigenetic alterations. In this article, we review epigenetic changes involved in both metabolic diseases and dietary interventions in primary metabolic tissues (i.e., adipose, liver, and pancreas) in hopes of elucidating potential biomarkers and therapeutic targets for disease prevention and treatment.

Glucose transporters in pancreatic islets

The fine-tuning of glucose uptake mechanisms is rendered by various glucose transporters with distinct transport characteristics. In the pancreatic islet, facilitative diffusion glucose transporters (GLUTs), and sodium-glucose cotransporters (SGLTs) contribute to glucose uptake and represent important components in the glucose-stimulated hormone release from endocrine cells, therefore playing a crucial role in blood glucose homeostasis. This review summarizes the current knowledge about cell type-specific expression profiles as well as proven and putative functions of distinct GLUT and SGLT family members in the human and rodent pancreatic islet and further discusses their possible involvement in onset and progression of diabetes mellitus. In context of GLUTs, we focus on GLUT2, characterizing the main glucose transporter in insulin-secreting β-cells in rodents. In addition, we discuss recent data proposing that other GLUT family members, namely GLUT1 and GLUT3, render this task in humans. Finally, we summarize latest information about SGLT1 and SGLT2 as representatives of the SGLT family that have been reported to be expressed predominantly in the α-cell population with a suggested functional role in the regulation of glucagon release.

Metalloproteins and apolipoprotein C: candidate plasma biomarkers of T2DM screened by comparative proteomics and lipidomics in ZDF rats

Background

Early diagnosis of type 2 diabetes mellitus (T2DM) is still difficult. Screening of plasma biomarkers has great significance of optimizing diagnosis and predicting the complications of T2DM.

Methods

We used a special diet, Purina #5008, to induce diabetes in Zucker leptin receptor gene-deficient rats (fa/fa) to establish Zucker diabetic fatty (ZDF) rats, simulating the early stage of T2DM. The differentially expressed proteins (DEP) and lipids (DEL), as potential biomarkers, were screened to compare the plasma expression levels in ZDF rats and their basic diet-fed wild-type controls (fa/+) by Tandem Mass Tags (TMT) and liquid chromatography-tandem mass spectrometry.

Results

These two groups had different plasma proteins and lipids profiles consisting of 84 DEPs and, 179 DELs identified in the positive ion mode and 178 DELs in the negative ion mode, respectively. Enrichment analysis of these different indicators showed that oxidative stress, insulin resistance and metabolic disorders of glycan and lipid played an important role in generating the difference. Some markers can be used as candidate biomarkers in prediction and treatments of T2DM, such as ceruloplasmin, apolipoprotein C-I, apolipoprotein C-II and apolipoprotein C-IV.

Conclusion

These plasma differences help to optimize the diagnosis and predict the complications of T2DM, although this remains to be verified in the crowd. Trace elements related-metalloproteins, such as ceruloplasmin, and lipid metabolism and transport-related apolipoprotein C are expected to be candidate biomarkers of T2DM and should be given more attention.

Spermidine supplementation and voluntary activity differentially affect obesity-related structural changes in the mouse lung

Obesity is associated with lung function impairment and respiratory diseases; however, the underlying pathophysiological mechanisms are still elusive, and therapeutic options are limited. This study examined the effects of prolonged excess fat intake on lung mechanics and microstructure and tested spermidine supplementation and physical activity as intervention strategies. C57BL/6N mice fed control diet (10% fat) or high-fat diet (HFD; 60% fat) were left untreated or were supplemented with 3 mM spermidine, had access to running wheels for voluntary activity, or a combination of both. After 30 wk, lung mechanics was assessed, and left lungs were analyzed by design-based stereology. HFD exerted minor effects on lung mechanics and resulted in higher body weight and elevated lung, air, and septal volumes. The number of alveoli was higher in HFD-fed animals. This was accompanied by an increase in epithelial, but not endothelial, surface area. Moreover, air-blood barrier and endothelium were significantly thicker. Neither treatment affected HFD-related body weights. Spermidine lowered lung volumes as well as endothelial and air-blood barrier thicknesses toward control levels and substantially increased the endothelial surface area under HFD. Activity resulted in decreased volumes of lung, septa, and septal compartments but did not affect vascular changes in HFD-fed mice. The combination treatment showed no additive effect. In conclusion, excess fat consumption induced alveolar capillary remodeling indicative of impaired perfusion and gas diffusion. Spermidine alleviated obesity-related endothelial alterations, indicating a beneficial effect, whereas physical activity reduced lung volumes apparently by other, possibly systemic effects.

Both male and female obese ZSF1 rats develop cardiac dysfunction in obesity-induced heart failure with preserved ejection fraction

Heart failure with a preserved ejection fraction (HFpEF) is associated with multiple comorbidities, such as old age, hypertension, type 2 diabetes and obesity and is more prevalent in females. Although the male obese ZSF1 rat has been proposed as a suitable model to study the development of diastolic dysfunction and early HFpEF, studies in female animals have not been performed yet. Therefore, we aimed to characterize the cardiac phenotype in female obese ZSF1 rats and their lean counterparts. Additionally, we aimed to investigate whether differences exist in disease progression in obese male and female ZSF1 rats. Therefore, male and female ZSF1 rats, lean as well as obese (N = 6-9/subgroup), were used. Every two weeks, from 12 to 26 weeks of age, systolic blood pressure and echocardiographic measurements were performed, and venous blood was sampled. Female obese ZSF1 rats, as compared to female lean ZSF1 rats, developed diastolic dysfunction with cardiac hypertrophy and fibrosis in the presence of severe dyslipidemia, increased plasma growth differentiation factor 15 and mild hypertension, and preservation of systolic function. Although obese female ZSF1 rats did not develop hyperglycemia, their diastolic dysfunction was as severe as in the obese males. Taken together, the results from the present study suggest that the female obese ZSF1 rat is a relevant animal model for HFpEF with multiple comorbidities, suitable for investigating novel therapeutic interventions.

The benefits of physical exercise for the health of the pancreatic β-cell: a review of the evidence

NEW FINDINGS

What is the topic of this review? This review discusses the evidence of the benefits of exercise training for β-cell health through improvements in function, proliferation and survival which may have implications in the treatment of diabetes. What advances does it highlight? This review highlights how exercise may modulate β-cell health in the context of diabetes and highlights the need for further exploration of whether β-cell preserving effects of exercise translates to T1D.

ABSTRACT

Physical exercise is a core therapy for type 1 and type 2 diabetes. Whilst the benefits of exercise for different physiological systems are recognised, the effect of exercise specifically on the pancreatic β-cell is not well described. Here we review the effects of physical exercise on β-cell health. We show that exercise improves β-cell mass and function. The improved function manifests primarily through the increased insulin content of the β-cell and its increased ability to secrete insulin in response to a glucose stimulus. We review the evidence relating to glucose sensing, insulin signalling, β-cell proliferation and β-cell apoptosis in humans and animal models with acute exercise and following exercise training programmes. Some of the mechanisms through which these benefits manifest are discussed.

Cornelian cherry fruit improves glycaemia and manifestations of diabetes in obese Zucker diabetic fatty rats

Cornelian cherry (Cornus mas L.) was in the past frequently used in Slovak Republic; meanwhile fell into oblivion despite the fact that it is known as antidiabetic supplement. However, there is no research investigated its effect on animal model of Diabetes mellitus (DM) 2 type as it is Zucker diabetic fatty (ZDF) rats. Thus, the aim of this study was to determine the effect of C. mas fruit given orally on the development of DM symptoms in ZDF rats. In the experiment male ZDF rats (fa/fa) and their age-matched non-diabetic lean controls (fa/+) were used aged 12 weeks. Male ZDF rats were administered C. mas in two doses (500 and 1000 mg/kg body weight) using a gastric gavage for 10 weeks. One group of diabetic animals served as positive control and received only distilled water. We found significant decrease of glucose level after oral administration of C. mas in dose of 1000 mg/kg bw in pre-diabetic state of animals (until 7th week of the experiment) and significant restriction of water intake in both C. mas groups against the diabetic control. We presume that the higher dose of Cornelian cherry could be beneficial and helpful in prevention of diabetic symptoms when consumed regularly in young animals.

Alcohol-induced ketonemia is associated with lowering of blood glucose, downregulation of gluconeogenic genes, and depletion of hepatic glycogen in type 2 diabetic db/db mice

Alcoholic ketoacidosis and diabetic ketoacidosis are life-threatening complications that share the characteristic features of high anion gap metabolic acidosis. Ketoacidosis is attributed in part to the massive release of ketone bodies (e.g., β-hydroxybutyrate; βOHB) from the liver into the systemic circulation. To date, the impact of ethanol consumption on systemic ketone concentration, glycemic control, and hepatic gluconeogenesis and glycogenesis remains largely unknown, especially in the context of type 2 diabetes. In the present study, ethanol intake (36% ethanol- and 36% fat-derived calories) by type 2 diabetic db/db mice for 9 days resulted in significant decreases in weight gain (∼19.5% ↓) and caloric intake (∼30% ↓). This was accompanied by a transition from macrovesicular-to-microvesicular hepatic steatosis with a modest increase in hepatic TG (∼37% ↑). Importantly, ethanol increased systemic βOHB concentration (∼8-fold ↑) with significant decreases in blood glucose (∼4-fold ↓) and plasma insulin and HOMA-IR index (∼3-fold ↓). In addition, ethanol enhanced hepatic βOHB content (∼5-fold ↑) and hmgcs2 mRNA expression (∼3.7-fold ↑), downregulated key gluconeogenic mRNAs (e.g., Pcx, Pck1, and G6pc), and depleted hepatic glycogen (∼4-fold ↓). Furthermore, ethanol intake led to significant decreases in the mRNA/protein expression and allosteric activation of glycogen synthase (GS) in liver tissues regardless of changes in the phosphorylation of GS, GSK-3β, or Akt. Together, our findings suggest that ethanol-induced ketonemia may occur in concomitance with significant lowering of blood glucose concentration, which may be attributed to suppression of gluconeogenesis in the setting of glycogen depletion in type 2 diabetes.

A review of the molecular pathways mediating the improvement in diabetes mellitus following caloric restriction

Lifestyle modification is the cornerstone of diabetes prevention and treatment. Weight loss through caloric restriction (CR) is effective in improving glycemic control, though it is difficult for patients to follow in practice, and remains critical to achieve optimal glucose homeostasis. In this review, we look at what is known about the molecular pathways involved in CR-induced insulin sensitivity and improved insulin resistance.

Remission of Human Type 2 Diabetes Requires Decrease in Liver and Pancreas Fat Content but Is Dependent upon Capacity for β Cell Recovery

The Diabetes Remission Clinical Trial reported return and persistence of non-diabetic blood glucose control in 46% of people with type 2 diabetes of up to 6 years duration. Detailed metabolic studies were performed on a subgroup (intervention, n = 64; control, n = 26). In the intervention group, liver fat content decreased (16.0% ± 1.3% to 3.1% ± 0.5%, p < 0.0001) immediately after weight loss. Similarly, plasma triglyceride and pancreas fat content decreased whether or not glucose control normalized. Recovery of first-phase insulin response (0.04[-0.05-0.32] to 0.11[0.0005-0.51] nmol/min/m², p < 0.0001) defined those who returned to non-diabetic glucose control and this was durable at 12 months (0.11[0.005-0.81] nmol/min/m², p = 0.0001). Responders were similar to non-responders at baseline but had shorter diabetes duration (2.7 ± 0.3 versus 3.8 ± 0.4 years; p = 0.02). This study demonstrates that β cell ability to recover long-term function persists after diagnosis, changing the previous paradigm of irreversible loss of β cell function in type 2 diabetes.

Coordination of GPR40 and Ketogenesis Signaling by Medium Chain Fatty Acids Regulates Beta Cell Function

Diabetes prevalence increases with age, and β-cell dysfunction contributes to the incidence of the disease. Dietary lipids have been recognized as contributory factors in the development and progression of the disease. Unlike long chain triglycerides, medium chain triglycerides (MCT) increase fat burning in animal and human subjects as well as serum C-peptide in type 2 diabetes patients. We evaluated the beneficial effects of MCT on β-cells in vivo and in vitro. MCT improved glycemia in aged rats via β-cell function assessed by measuring insulin secretion and content. In β-cells, medium chain fatty acid (MCFA)-C10 activated fatty acid receptor 1 FFAR1/GPR40, while MCFA-C8 induced mitochondrial ketogenesis and the C8:C10 mixture improved β cell function. We showed that GPR40 signaling positively impacts ketone body production in β-cells, and chronic treatment with β-hydroxybutyrate (BHB) improves β-cell function. We also showed that BHB and MCFA help β-cells recover from lipotoxic stress by improving mitochondrial function and increasing the expression of genes involved in β-cell function and insulin biogenesis, such as Glut2, MafA, and NeuroD1 in primary human islets. MCFA offers a therapeutic advantage in the preservation of β-cell function as part of a preventative strategy against diabetes in at risk populations.

Impact of diet restriction in the management of diabetes: evidences from preclinical studies

The inappropriate dietary habits lead to the onset of age-related pathologies which include diabetes and cardiovascular ailments. Dietary restriction and nutritional therapy play an important role in the prevention of these chronic ailments. Preclinical research provides a basis for the therapeutic exploration of new dietary interventions for the clinical trials to potentiate the scientific management of diabetes and its related complications which further help in translating these nutritional improvements from bench to bedside. Within the same context, numerous therapeutically proved preclinical dietary interventions like high-fiber diet, caloric restriction, soy isoflavone-containing diets, etc., have shown the promising results for the management of diabetes and the associated complications. The focus of the present review is to highlight the various preclinical evidences of diet restriction for the management of diabetes and which will be helpful for enlightening the new ideas of nutritional therapy for future research exploration. In addition, some potential approaches are also discussed which are associated with various nutritional interventions to combat progressive diabetes and the associated disorders. Graphical abstract ᅟ.

β-Hydroxybutyrate improves β-cell mitochondrial function and survival

Pharmacological interventions aimed at improving outcomes in type 2 diabetes and achieving normoglycaemia, including insulin therapy, are increasingly common, despite the potential for substantial side effects. Carbohydrate-restricted diets that result in increased ketogenesis have effectively been used to improve insulin resistance, a fundamental feature of type 2 diabetes. In addition, limited evidence suggests that states of ketogenesis may also improve β-cell function in type 2 diabetics. Considering how little is known regarding the effects of ketones on β-cell function, we sought to determine the specific effects of β-Hydroxybutyrate (βHB) on pancreatic β-cell physiology and mitochondrial function. βHB treatment increased β-cell survival and proliferation, while also increasing mitochondrial mass, respiration and adenosine triphosphate (ATP) production. Despite these improvements, were unable to detect an increase in β-cell or islet insulin production and secretion. Collectively, these findings have two implications. Firstly, they indicate that β-cells have improved survival and proliferation in the midst of βHB, the circulating form of ketones. Secondly, insulin secretion does not appear to be directly related to apparent improvements in mitochondrial function and cellular proliferation.

Chronic hyperinsulinemia contributes to insulin resistance under dietary restriction in association with altered lipid metabolism in Zucker diabetic fatty rats

Hyperinsulinemia is widely thought to be a compensatory response to insulin resistance, whereas its potentially causal role in the progression of insulin resistance remains to be established. Here, we aimed to examine whether hyperinsulinemia could affect the progression of insulin resistance in Zucker fatty diabetic (ZDF) rats. Male ZDF rats at 8 wk of age were fed a diet ad libitum (AL) or dietary restriction (DR) of either 15 or 30% from AL feeding over 6 wk. Insulin sensitivity was determined by hyperinsulinemic euglycemic clamp. ZDF rats in the AL group progressively developed hyperglycemia and hyperinsulinemia by 10 wk of age, and then plasma insulin rapidly declined to nearly normal levels by 12 wk of age. Compared with AL group, DR groups showed delayed onset of hyperglycemia and persistent hyperinsulinemia, leading to weight gain and raised plasma triglycerides and free fatty acids by 14 wk of age. Notably, insulin sensitivity was significantly reduced in the DR group rather than the AL group and inversely correlated with plasma levels of insulin and triglyceride but not glucose. Moreover, enhanced lipid deposition and upregulation of genes involved in lipogenesis were detected in liver, skeletal muscle, and adipose tissues of the DR group rather than the AL group. Alternatively, continuous hyperinsulinemia induced by insulin pellet implantation produced a decrease in insulin sensitivity in ZDF rats. These results suggest that chronic hyperinsulinemia may lead to the progression of insulin resistance under DR conditions in association with altered lipid metabolism in peripheral tissues in ZDF rats.

A Mechanistic Basis for the Beneficial Effects of Caloric Restriction On Longevity and Disease: Consequences for the Interpretation of Rodent Toxicity Studies

Caloric restriction in rodents has been repeatedly shown to increase life span while reducing the severity and retarding the onset of both spontaneous and chemically induced neoplasms. These effects of caloric restriction are associated with a spectrum of biochemical and physiological changes that characterize the organism's adaptation to reduced caloric intake and provide the mechanistic basis for caloric restriction's effect on longevity. Here, we review evidence suggesting that the primary adaptation appears to be a rhythmic hypercorticism in the absence of elevated adrenocorticotropin (ACTH) levels. This characteristic hypercorticism evokes a spectrum of responses, including reduced body temperature and increased metabolic efficiency, decreased mitogenic response coupled with increased rates of apoptosis, reduced inflammatory response, reduced oxidative damage to proteins and DNA, reduced reproductive capacity, and altered drug-metabolizing enzyme expression. The net effect of these changes is to (1) decrease growth and metabolism in peripheral tissues to spare energy for central functions, and (2) increase the organism's capacity to withstand stress and chemical toxicity. Thus, caloric restriction research has uncovered an evolutionary mechanism that provides rodents with an adaptive advantage in conditions of fluctuating food supply. During periods of abundance, body growth and fecundity are favored over endurance and longevity. Conversely, during periods of famine, reproductive performance and growth are sacrificed to ensure survival of individuals to breed in better times. This phenomena can be observed in rodent populations that are used in toxicity testing. Improvements over the last 30 years in animal husbandry and nutrition, coupled with selective breeding for growth and fecundity, have resulted in several strains now exhibiting larger animals with reduced survival and increased incidence of background lesions. The mechanistic data from caloric restriction studies suggest that these large animals will also be more susceptible to chemically induced toxicity. This creates a problem in comparing tests performed on animals of different weights and comparing data generated today with the historical database. The rational use of caloric restriction to control body weight to within preset guidelines is a possible way of alleviating this problem.

Calorie restriction and reversal of type 2 diabetes

Type 2 diabetes causes major global health problems and has been believed to be a lifelong condition with inevitable worsening. Steadily increasing numbers of drugs appeared to be required to achieve even modest control. Early type 2 diabetes has now been shown to be reversed by substantial weight loss and this has allowed temporal tracking of the underlying pathophysiological changes. Areas covered: In early type 2 diabetes, negative calorie balance decreases liver fat within days, and allows return of normal control of hepatic glucose production. Over 8 weeks, the negative calorie balance allows the raised levels of intra-pancreatic fat and simultaneously first phase insulin secretion to normalise. These findings are consistent with the 2008 Twin Cycle Hypothesis of the etiology and pathogenesis of type 2 diabetes. Individuals develop type 2 diabetes when they exceed their personal fat threshold for safe storage of fat and there is no difference in pathophysiology between those with BMI above or below 30 kg/m². Expert commentary: Type 2 diabetes can now be understood as a state of excess fat in liver and pancreas, and remains reversible for at least 10 years in most individuals.

Review: Development and therapeutic potential of incretin hormone analogues for type 2 diabetes

The rising prevalence of type 2 diabetes and increasing burden of diabetic complications requires new approaches to diabetes therapy. An encouraging new approach for development of future anti-diabetic drugs is based on analogues of incretin hormones, glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP). Both peptides reduce postprandial glucose by stimulating glucose-dependent insulin release and exert a number of other beneficial actions including trophic effects on the beta-cell. Efforts are currently focused on developing stable analogues of GLP-1 and GIP which are resistant to dipeptidylpeptidase IV mediated degradation and renal filtration. Thus, by increasing the half-life and potency of these incretins, they should become a new class of agents for the treatment of type 2 diabetes.

Dietary restriction preserves the mass and function of pancreatic β cells via cell kinetic regulation and suppression of oxidative/ER stress in diabetic mice

To assess the molecular mechanisms by which dietary restriction preserves the β-cell mass and function in diabetic db/db mice. Male db/db mice were divided into two groups with or without diet restriction. Daily food intake of db/db mice was adjusted to that of the control db/m mice, which was determined in advance. A dietary restriction was implemented for 6 weeks from 6 weeks of age. Islet morphology, β-cell function and gene expression profiles specific for pancreatic islet cells were compared. Food intake in db/m mice was 50% of that in db/db mice. Impaired glucose tolerance and insulin sensitivity were significantly ameliorated in db/db mice with dietary restriction. The pancreatic β-cell mass was greater in mice with dietary restriction than that in mice without intervention. The dietary restriction significantly increased cyclin D gene expression and down-regulated CAD gene expression at 12 weeks compared with untreated db/db mice. Antiapoptotic bcl-2 gene expression was significantly increased, whereas genes related to oxidative stress, ER stress and inflammatory processes, such as NADPH oxidase, CHOP10 and TNF, were markedly down-regulated in mice with dietary restriction. Dietary restriction preserved the pancreatic β-cell function and β-cell mass in diabetic db/db mice, suggesting that alimentary therapy prevented β-cell loss by suppressing cellular apoptosis and antioxidative stress in the pancreatic β cells.

Cross-fostering and improved lactation ameliorates deficits in endocrine pancreatic morphology in growth-restricted adult male rat offspring

Uteroplacental insufficiency and poor postnatal nutrition impair adult glucose tolerance and insulin secretion in male rat offspring, which can be partially ameliorated by improving postnatal nutrition. Uteroplacental insufficiency was induced in the WKY rat on day 18 of pregnancy (Restricted) compared to sham-operated Controls. Pups were then cross-fostered onto Control or Restricted mothers one day after birth resulting in: (Pup-on-Mother) Control-on-Control, Control-on-Restricted, Restricted-on-Control and Restricted-on-Restricted. Endocrine pancreatic morphology and markers of intrinsic β-cell function and glucose homeostasis were assessed in male offspring at 6 months. Pancreatic and hepatic gene expression was quantified at postnatal day 7 and 6 months. Restricted pups were born 10-15% lighter than Controls and remained lighter at 6 months. Relative islet and β-cell mass were 51-65% lower in Restricted-on-Restricted compared to Controls at 6 months. Non-fasting plasma C-reactive protein levels were also increased, suggestive of an inflammatory response. Overall, the average number of islets, small islets and proportion of β-cells per islet correlated positively with birth weight. Intrinsic β-cell function, estimated by insulin secretion relative to β-cell mass, was unaffected by Restriction, suggesting that the in vivo functional deficit was attributable to reduced mass, not function. Importantly, these deficits were ameliorated when lactational nutrition was normalized in Restricted-on-Control offspring, who also showed increased pancreatic Igf1r, Pdx1 and Vegf mRNA expression at 7 days compared to Control-on-Control and Restricted-on-Restricted. This highlights lactation as a critical period for intervention following prenatal restraint, whereby deficits in endocrine pancreatic mass and associated impaired in vivo insulin secretion can be ameliorated.

Caloric restriction or telmisartan control dyslipidemia and nephropathy in obese diabetic Zücker rats

BACKGROUND

The obese Zücker diabetic fatty male rat (ZDF:Gmi™-fa) is an animal model of type II diabetes associated with obesity and related metabolic disturbances like dyslipidaemia and diabetic nephropathy. In addition, diabetic dyslipidaemia has been linked to vascular and glomerular damage too. Dietary fat restriction is a current strategy to tackle obesity and, telmisartan, as a renoprotective agent, may mediate cholesterol efflux by activating PPARγ. To test the hypothesis that both therapeutical alternatives may influence dyslipidaemia and nephropathy in the ZDF rat, we studied their effect on development of diabetes.

METHODS

Male Zücker Diabetic Fatty (ZDF) rats received a low-calorie diet, vehicle or telmisartan for 9 weeks. Blood samples were obtained for analyses of lipids and lipoproteins, LDL-oxidisability, HDL structural and functional properties. Urinalysis was carried out to estimate albumin loss. At the end of the experimental period, rats were sacrificed, liver extracted and APOA1 mRNA quantified.

RESULTS

Results indicated that low-calorie diet and telmisartan can slower the onset of overt hyperglycaemia and renal damage assessed as albuminuria. Both interventions decreased the oxidative susceptibility of LDL and hepatic APOA1 mRNA expression but only dietary restriction lowered hyperlipidaemia.

CONCLUSION

Either a dietary or pharmacologic interventions with telmisartan have important beneficial effects in terms of LDL oxidative susceptibility and progression of albuminuria in obesity related type II diabetes.

Banting Memorial lecture 2012: reversing the twin cycles of type 2 diabetes

It has become widely accepted that Type 2 diabetes is inevitably life-long, with irreversible and progressive beta cell damage. However, the restoration of normal glucose metabolism within days after bariatric surgery in the majority of people with Type 2 diabetes disproves this concept. There is now no doubt that this reversal of diabetes depends upon the sudden and profound decrease in food intake, and does not relate to any direct surgical effect. The Counterpoint study demonstrated that normal glucose levels and normal beta cell function could be restored by a very low calorie diet alone. Novel magnetic resonance methods were applied to measure intra-organ fat. The results showed two different time courses: a) resolution of hepatic insulin sensitivity within days along with a rapid fall in liver fat and normalisation of fasting glucose levels; and b) return of normal beta cell insulin secretion over weeks in step with a fall in pancreas fat. Now that it has been possible to observe the pathophysiological events during reversal of Type 2 diabetes, the reverse time course of events which determine the onset of the condition can be identified. The twin cycle hypothesis postulates that chronic calorie excess leads to accumulation of liver fat with eventual spill over into the pancreas. These self-reinforcing cycles between liver and pancreas eventually cause metabolic inhibition of insulin secretion after meals and onset of hyperglycaemia. It is now clear that Type 2 diabetes is a reversible condition of intra-organ fat excess to which some people are more susceptible than others.

Quantitative histopathological assessment of retardation of islets of langerhans degeneration in rosiglitazone-dosed obese ZDF rats using combined insulin and collagens (I and III) immunohistochemistry with automated image analysis and statistical modeling

Islets of Langerhans represent a heterogeneous population in insulin resistant and diabetic animals and humans as histological appearances and function vary substantially. Mathematical representation that reflects this morphological diversity will assist in assessment of degeneration and regeneration, enabling comparisons between species, strains, and experimental investigations. Our investigative approach used a model of islet degeneration in diabetic male obese Zucker Diabetic Fatty (ZDF) rats and evaluated its prevention using rosiglitazone treatment. Immunohistochemical staining (insulin and collagens I/III) with automated image analysis reliably measured numbers, area, clustering, and staining intensity of β-cells and degree of islet fibrosis. Finite mixture mathematical modeling for the joint probability distribution of seven islet parameters to represent islet numerical data variation provided an automatic procedure for islet category allocations as normal or abnormal. Allocations for obese ZDF controls and rosiglitazone-treated animals were significantly different, with no significant difference between the latter and lean ZDF controls, indicative of differences within islet populations of individual animals, between lean and obese rat strains and following drug treatment. Islet morphology showed clear association with mathematical characterization. Information on islet morphology obtained by histopathological assessment of single pancreatic tissue sections was confirmed by this method showing drug-induced retardation of islet of Langerhans degeneration.

Ibipinabant attenuates β-cell loss in male Zucker diabetic fatty rats independently of its effects on body weight

AIM

To test the antidiabetic efficacy of ibipinabant, this new cannabinoid receptor 1 (CB1) antagonist was compared with food-restriction-induced weight loss, rosiglitazone (4 mg/kg) and rimonabant (3 and 10 mg/kg), using parameters of glycaemic control in male Zucker diabetic fatty (ZDF) rats.

METHODS

Body weight, food and water intake, fasted and non-fasted glucose and insulin, glucose tolerance and glycosylated haemoglobin (HbA1c) were all assessed over the course of the 9-week study. Pancreatic insulin content and islet area were also evaluated.

RESULTS

At the end of the study, vehicle-treated ZDF rats were severely hyperglycaemic and showed signs of β-cell decline, including dramatic reductions in unfasted insulin levels. Ibipinanbant (10 mg/kg) reduced the following relative to vehicle controls: fasting glucose (-61%), glucose excursion area under the curve (AUC) in an oral glucose tolerance test (OGTT, -44%) and HbA1c (-50%). Furthermore, non-fasting insulin, islet area and islet insulin content were all increased (71, 40 and 76%, respectively) relative to vehicle controls by the end of the study. All of these effects were similar to those of rimonabant and rosiglitazone, where ibipinabant was slightly more effective than rimonabant at the lowest dose and somewhat less effective than rosiglitazone at all doses. These antidiabetic effects appear independent of weight loss because none of the parameters above were consistently improved by the comparable weight loss induced by food restriction.

CONCLUSIONS

Ibipinabant may have weight loss-independent antidiabetic effects and may have the potential to attenuate β-cell loss in a model of progressive β-cell dysfunction.

Voluntary running exercise prevents β-cell failure in susceptible islets of the Zucker diabetic fatty rat

Physical activity improves glycemic control in type 2 diabetes (T2D), but its contribution to preserving β-cell function is uncertain. We evaluated the role of physical activity on β-cell secretory function and glycerolipid/fatty acid (GL/FA) cycling in male Zucker diabetic fatty (ZDF) rats. Six-week-old ZDF rats engaged in voluntary running for 6 wk (ZDF-A). Inactive Zucker lean and ZDF (ZDF-I) rats served as controls. ZDF-I rats displayed progressive hyperglycemia with β-cell failure evidenced by falling insulinemia and reduced insulin secretion to oral glucose. Isolated ZDF-I rat islets showed reduced glucose-stimulated insulin secretion expressed per islet and per islet protein. They were also characterized by loss of the glucose regulation of fatty acid oxidation and GL/FA cycling, reduced mRNA expression of key β-cell genes, and severe reduction of insulin stores. Physical activity prevented diabetes in ZDF rats through sustaining β-cell compensation to insulin resistance shown in vivo and in vitro. Surprisingly, ZDF-A islets had persistent defects in fatty acid oxidation, GL/FA cycling, and β-cell gene expression. ZDF-A islets, however, had preserved islet insulin mRNA and insulin stores compared with ZDF-I rats. Physical activity did not prevent hyperphagia, dyslipidemia, or obesity in ZDF rats. In conclusion, islets of ZDF rats have a susceptibility to failure that is possibly due to altered β-cell fatty acid metabolism. Depletion of pancreatic islet insulin stores is a major contributor to islet failure in this T2D model, preventable by physical activity.

Appropriateness of the Zucker Diabetic Fatty rat as a model for diabetic microvascular late complications

Male obese Zucker Diabetic Fatty (ZDF) rats develop type 2 diabetes around eight weeks of age, and are widely used as a model for human diabetes and its complications. The objective of the study was to test whether the complications manifested in the kidney and nerves of ZDF rats really correspond to human diabetic complications in their being related to the hyperglycaemic state. Four groups of ZDF rats were used. One lean (Fa/?) and one obese (fa/fa) untreated group served as non-diabetic and diabetic controls. In two further groups of obese (fa/fa) rats, diabetes was prevented by pioglitazone or delayed by food restriction. All rats were monitored up to 35 weeks of age with respect to their blood glucose, HbA1c and insulin levels, their kidney function (urinary glucose excretion, renal glucose filtration, glomerular filtration rate, albumin/creatinine ratio), and their nerve function (tactile and thermal sensory threshold and nerve conduction velocity). Pioglitazone prevented the development of diabetes, while food restriction delayed its onset for 8–10 weeks. Accordingly, kidney function parameters were similar to lean non-diabetic rats in pioglitazone-treated rats and significantly improved in food-restricted rats compared with obese controls. Kidney histology paralleled the functional results. By contrast, nerve functional evaluations did not mirror the differing blood glucose levels. We conclude that the ZDF rat is a good model for diabetic nephropathy, while alterations in nerve functions were not diabetes-related.

Caloric restriction and chronic inflammatory diseases

A reduction in calorie intake [caloric restriction (CR)] appears to consistently decrease the biological rate of aging in a variety of organisms as well as protect against age-associated diseases including chronic inflammatory disorders such as cardiovascular disease and diabetes. Although the mechanisms behind this observation are not fully understood, identification of the main metabolic pathways affected by CR has generated interest in finding molecular targets that could be modulated by CR mimetics. This review describes the general concepts of CR and CR mimetics as well as discusses evidence related to their effects on inflammation and chronic inflammatory disorders. Additionally, emerging evidence related to the effects of CR on periodontal disease in non-human primates is presented. While the implementation of this type of dietary intervention appears to be challenging in our modern society where obesity is a major public health problem, CR mimetics could offer a promising alternative to control and perhaps prevent several chronic inflammatory disorders including periodontal disease.

JTT-130, a novel intestine-specific inhibitor of microsomal triglyceride transfer protein, ameliorates impaired glucose and lipid metabolism in Zucker diabetic fatty rats

AIM

Microsomal triglyceride transfer protein (MTP) takes part in the mobilization of triglyceride-rich lipoproteins from enterocytes and hepatocytes. We investigated the effects of JTT-130, a novel intestine-specific MTP inhibitor, on impaired glucose and lipid metabolism in Zucker diabetic fatty (ZDF) rats.

METHODS

Male ZDF rats were fed a regular powdered diet with or without JTT-130 as a food admixture (0.01-0.02%) for 6 weeks. Food intake, body weight, blood biochemical parameters, fecal lipid contents, hepatic lipid contents, tissue mRNA levels and glucose utilization in adipose tissues were assessed. An intraperitoneal glucose tolerance test (IPGTT) and histological analysis of the pancreas were performed.

RESULTS

JTT-130 treatment decreased food intake, glycated hemoglobin, plasma levels of glucose, triglycerides and total cholesterol, hepatic levels of triglycerides and cholesterol and hepatic mRNA levels of glucose-6-phosphatase, phosphoenolpyruvate carboxykinase and fructose-1,6-bisphosphatase. JTT-130 treatment increased fecal levels of free fatty acids and cholesterol, plasma levels of glucagon-like peptide-1 and peptide YY, mRNA levels of glucose transporter 4 (GLUT4) and lipoprotein lipase in adipose tissues and GLUT4 in muscle and glucose utilization in adipose tissues. Plasma insulin decreased after 2 weeks and increased after 4 weeks of JTT-130 treatment. Plasma glucose in the JTT-130-treated rats was lower with higher plasma insulin than in the control rats during the IPGTT. The islets of the JTT-130-treated rats were larger and contained more insulin than those of the control rats.

CONCLUSIONS

JTT-130 ameliorates impaired glucose and lipid metabolism in the ZDF rats thereby suggesting that JTT-130 could be useful for prevention and treatment of type 2 diabetes.

Pancreatic beta cell mass PET imaging and quantification with [11C]DTBZ and [18F]FP-(+)-DTBZ in rodent models of diabetes

PURPOSE

The aim of this study is to compare the utility of two positron emission tomography (PET) imaging ligands ((+)-[(11)C]dihydrotetrabenazine ([(11)C]DTBZ) and the fluoropropyl analog ([(18)F]FP-(+)-DTBZ)) that target islet β-cell vesicular monoamine transporter type II to measure pancreatic β-cell mass (BCM).

PROCEDURES

[(11)C]DTBZ or [(18)F]FP-(+)-DTBZ was injected, and serial PET images were acquired in rat models of diabetes (streptozotocin-treated and Zucker diabetic fatty) and β-cell compensation (Zucker fatty). Radiotracer standardized uptake values (SUV) were correlated to pancreas insulin content measured biochemically and histomorphometrically.

RESULTS

On a group level, a positive correlation of [(11)C]DTBZ pancreatic SUV with pancreas insulin content and BCM was observed. In the STZ diabetic model, both [(18)F]FP-(+)-DTBZ and [(11)C]DTBZ correlated positively with BCM, although only ∼25% of uptake could be attributed to β-cell uptake. [(18)F]FP-(+)-DTBZ displacement studies indicate that there is a substantial fraction of specific binding that is not to pancreatic islet β cells.

CONCLUSIONS

PET imaging with [(18)F]FP-(+)-DTBZ provides a noninvasive means to quantify insulin-positive BCM and may prove valuable as a diagnostic tool in assessing treatments to maintain or restore BCM.

Variation in characteristics of islets of Langerhans in insulin-resistant, diabetic and non-diabetic-rat strains

Assessment of the histopathological and plasma biochemical characteristics of diabetic and non-diabetic rat strains [Han and AP Wistar, lean and obese Zucker Fatty (ZF), and lean and obese Zucker Diabetic Fatty (ZDF) rats] was performed at 6 or 14 weeks of age. Wistar and lean ZF and ZDF rats showed no or minimal islet pathology or plasma biochemical alterations at both timepoints. Obese ZFs were euglycaemic at both timepoints and mildly and severely hyperinsulinaemic at 6 and 14 weeks respectively. Islet morphology was normal at 6 weeks but at 14 weeks, islet hyperplasia was present with a minority showing degenerative changes namely, beta-cell vacuolation, vascular congestion and haemorrhage with minimal mononuclear cell and T lymphocytic infiltration. Obese ZDFs were euglycaemic and moderately hyperinsulinaemic at 6 weeks and severely hyperglycaemic with minor hypoinsulinaemia at 14 weeks. Obese ZDFs at 6 weeks showed mainly normal islets with some displaying degeneration (ranging from beta-cell vacuolation alone to the features described above). At 14 weeks, islet degeneration was more severe and widespread: beta-cell death was present in numerous islets at low level. Islet beta-cell numbers were reduced or absent (with associated reduction in insulin immunostaining) within the islets that now consisted predominantly of fibroblasts, collagen and mononuclear cells. Fibroproliferation consisting of smooth muscle actin-alpha-positive tissue was associated with mononuclear cell infiltration. Some fibrous scars were visible indicative of lost islets. Islet degeneration in obese ZF and ZDF rats was not accompanied by a reduction in beta-cell proliferation or in compensatory proliferation of beta-cell neogenic clusters. In the light of recent reports of adaptive and inflammation-mediated degenerative changes in human non-insulin dependent diabetes mellitus (NIDDM) islets, the hypertrophy/hyperplasia of beta-cells and islet degeneration involving infiltration by monocyte/macrophages in obese ZF and obese ZDF rats respectively offers substantial potential for elucidation of the processes involved.

Fatty diabetic lung: altered alveolar structure and surfactant protein expression

Pulmonary dysfunction develops in type 2 diabetes mellitus (T2DM) in direct correlation with glycemia and is exacerbated by obesity; however, the associated structural derangement has not been quantified. We studied lungs from obese diabetic (fa/fa) male Zucker diabetic fatty (ZDF) rats at 4, 12, and 36 wk of age, before and after onset of T2DM, compared with lean nondiabetic (+/+) rats. Surfactant proteins A and C (SP-A and SP-C) immunoexpression in lung tissue was quantified at ages 14 and 18 wk, after the onset of T2DM. In fa/fa animals, lung volume was normal despite obesity. Numerous lipid droplets were visible within alveolar interstitium, lipofibroblasts, and macrophages, particularly in subpleural regions. Total triglyceride content was 136% higher. By 12 wk, septum volume was 21% higher, and alveolar duct volume was 36% lower. Capillary basement membrane was 29% thicker. Volume of lamellar bodies was 45% higher. By age 36 wk, volumes of interstitial collagen fibers, cells, and matrix were respectively 32, 25, and 80% higher, and capillary blood volume was 18% lower. ZDF rats exhibited a strain-specific increase in resistance of the air-blood diffusion barrier with age, which was exaggerated in fa/fa lungs compared with +/+ lungs. In fa/fa lungs, SP-A and SP-C expression were elevated at age 14-18 wk; the normal age-related increase in SP-A expression was accelerated, whereas SP-C expression declined with age. Thus lungs from obese T2DM animals develop many qualitatively similar changes as in type 1 diabetes mellitus but with extensive lipid deposition, altered alveolar type 2 cell ultrastructure, and surfactant protein expression patterns that suggest additive effects of hyperglycemia and lipotoxicity.

Diabetes and apoptosis: lipotoxicity

Obesity is an established risk factor in the pathogenesis of insulin resistance, type 2 diabetes mellitus and cardiovascular disease; all components that are part of the metabolic syndrome. Traditionally, insulin resistance has been defined in a glucocentric perspective. However, elevated systemic levels of fatty acids are now considered significant contributors towards the pathophysiological aspects associated with the syndrome. An overaccumulation of unoxidized long-chain fatty acids can saturate the storage capacity of adipose tissue, resulting in a lipid 'spill over' to non-adipose tissues, such as the liver, muscle, heart, and pancreatic-islets. Under these circumstances, such ectopic lipid deposition can have deleterious effects. The excess lipids are driven into alternative non-oxidative pathways, which result in the formation of reactive lipid moieties that promote metabolically relevant cellular dysfunction (lipotoxicity) and programmed cell-death (lipoapoptosis). Here, we focus on how both of these processes affect metabolically significant cell-types and highlight how lipotoxicity and sequential lipoapoptosis are as major mediators of insulin resistance, diabetes and cardiovascular disease.

Pancreatic steatosis: harbinger of type 2 diabetes in obese rodents

OBJECTIVE

The aim of this study was to determine if the fat accumulation in the exocrine pancreas fat of obese Zucker diabetic fatty (ZDF) rodents, like that in their endocrine pancreas, precedes the onset of type 2 diabetes mellitus (T2DM). As the fat content of whole pancreas, but not islets, can now be measured in humans by magnetic resonance spectroscopy (MRS), such measurements could be used as a predictor of impending T2DM and an indication for preventive intervention.

ANIMALS

Obese ZDF (fa/fa) rats and lean (+/+) controls on a 6% fat diet were killed at time points from 6 to 16 weeks and total pancreatic fat was measured biochemically and electronmicroscopic examination of tissue for fat droplets was carried out.

RESULTS

Compared to lean ZDF controls, pancreatic fat was elevated above lean controls from 6 to 16 weeks of age, peaking at 10 weeks of age when hyperglycemia first appeared. The pancreatic profile of fat content in whole pancreas paralleled that of islets. Electronmicroscopic examination identified the acinar location of the fat droplets and ruled out a major contribution of intrapancreatic adipocytes.

CONCLUSION

The almost identical pattern of triglyceride overaccumulation in the exocrine and endocrine pancreas of obese rodents before the onset of T2DM suggests that MRS of the human pancreas might predict T2DM in obese subjects and permit timely interventions to prevent the disease.

Voluntary exercise and caloric restriction enhance hippocampal dendritic spine density and BDNF levels in diabetic mice

Diabetes may adversely affect cognitive function, but the underlying mechanisms are unknown. To investigate whether manipulations that enhance neurotrophin levels will also restore neuronal structure and function in diabetes, we examined the effects of wheel running and dietary energy restriction on hippocampal neuron morphology and brain-derived neurotrophic factor (BDNF) levels in db/db mice, a model of insulin resistant diabetes. Running wheel activity, caloric restriction, or the combination of the two treatments increased levels of BDNF in the hippocampus of db/db mice. Enhancement of hippocampal BDNF was accompanied by increases in dendritic spine density on the secondary and tertiary dendrites of dentate granule neurons. These studies suggest that diabetes exerts detrimental effects on hippocampal structure, and that this state can be attenuated by increasing energy expenditure and decreasing energy intake.

Lipid homeostasis, lipotoxicity and the metabolic syndrome

In the 20th century industrialized nations have become afflicted with an unprecedented pandemic of increased adiposity. In the United States, the epicenter of the epidemic, over 2/3 of the population, is overweight and 1 of every 6 Americans carries the diagnosis of metabolic syndrome. Although genes determine susceptibility to environmental factors, the epidemic is clearly due to increased consumption of calorie-dense, highly lipogenic foods, coupled with a marked decrease in physical exertion resulting from modern technologies. If this lifestyle continues, morbid consequences are virtually inevitable. They include type II diabetes and a cluster of disorders known as "the metabolic syndrome" usually appearing in middle age. The morbid consequences of the chronic caloric surplus are buffered before middle age by the partitioning of these calories as fat in the adipocyte compartment which is specifically designed to store triglycerides. Leptin has been proposed as the major hormonal regulator of the partitioning of surplus calories. However, multiple factors can determine the storage capacity of the fat tissue and when it is exceeded ectopic lipid deposition begins. The organs affected in metabolic syndrome include skeletal muscle, liver, heart and pancreas, which are now known to contain abnormal levels of triglycerides. While neutral fat is probably harmless, it is an index of ectopic lipid overload. Fatty acid derivatives can interfere with the function of the cell and ultimately lead to its demise through lipoapoptosis, the consequences of which are gradual organ failure.

Adaptation to intermittent stress promotes maintenance of beta-cell compensation: comparison with food restriction

Intermittent restraint stress delays hyperglycemia in ZDF rats better than pair feeding. We hypothesized that intermittent stress would preserve beta-cell mass through distinct mechanisms from food restriction. We studied temporal effects of intermittent stress on beta-cell compensation during pre-, early, and late diabetes. Six-week-old obese male ZDF rats were restraint-stressed 1 h/day, 5 days/wk for 0, 3, 6, or 13 wk and compared with age-matched obese ZDF rats that had been food restricted for 13 wk, and 19-wk-old lean ZDF rats. Thirteen weeks of stress and food restriction lowered cumulative food intake 10-15%. Obese islets were fibrotic and disorganized and not improved by stress or food restriction. Obese pancreata had islet hyperplasia and showed evidence of neogenesis, but by 19 wk old beta-cell mass was not increased, and islets had fewer beta-cells that were hypertrophic. Both stress and food restriction partially preserved beta-cell mass at 19 wk old via islet hypertrophy, whereas stress additionally lowered alpha-cell mass. Concomitant with maintenance of insulin responses to glucose, stress delayed the sixfold decline in beta-cell proliferation and reduced beta-cell hypertrophy, translating into 30% more beta-cells per islet after 13 wk. In contrast, food restriction did not improve insulin responses or beta-cell hyperplasia, exacerbated beta-cell hypertrophy, and resulted in fewer beta-cells and greater alpha-cell mass than with stress. Thus, preservation of beta-cell mass with adaptation to intermittent stress is related to beta-cell hyperplasia, maintenance of insulin responses to glucose, and reductions in alpha-cell mass that do not occur with food restriction.

The islet beta-cell: fuel responsive and vulnerable

The pancreatic beta-cell senses blood nutrient levels and is modulated by neurohormonal signals so that it secretes insulin according to the need of the organism. Nutrient sensing involves marked metabolic activation, resulting in the production of coupling signals that promote insulin biosynthesis and secretion. The beta-cell's high capacity for nutrient sensing, however, necessitates reduced protection to nutrient toxicity. This potentially explains why in susceptible individuals, chronic fuel surfeit results in beta-cell failure and type 2 diabetes. Here we discuss recent insights into first, the biochemical basis of beta-cell signaling in response to glucose, amino acids and fatty acids, and second, beta-cell nutrient detoxification. We emphasize the emerging role of glycerolipid/fatty acid cycling in these processes.

Evidence that the etiology of the syndrome containing type 2 diabetes mellitus results from abnormal magnesium metabolism

Evidence is reviewed supporting the presence of an inherited structural defect in the plasma membranes of somatic cells of humans who have type 2 diabetes mellitus and sodium-sensitive essential hypertension. This magnesium-binding defect (MgBD) consists of a decreased content of tightly bound Mg2+ ion in the cell membrane and limits the amount of Mg2+ that enters the cell, some of which combines with ATP4-, produced by the cell, to form MgATP2-, the currency of metabolic energy. Consequently, in both prediabetes and overt diabetes, the intracellular concentration of the interdependent Mg2+ and MgATP2- ions is significantly less than normal. These 2 ions are required as cofactors and (or) substrates for some 300 enzyme systems in human metabolism, many of which are involved with insulin. Thus the decreased activities of particular ones of these enzyme systems due to the decreased intracellular [Mg2+] and its dependent [MgATP2-] are responsible for (i) insulin resistance and (ii) decreased insulin secretion and (or) production, the 2 pathophysiological processes required for the occurrence of type 2 diabetes mellitus. These 2 processes can account for all of the morbid symptoms associated with this disease. Thus, the decreased intracellular concentration of the interdependent Mg2+ and MgATP2- ions constitutes the etiology of genetic predisposition to type 2 diabetes mellitus and can be corrected by 2 identified peptide Mg2+-binding promoters that are derived from the carboxyl terminal of the tachykinin substance P and occur in normal blood plasma. Decreased intracellular [Mg2+] and [MgATP2-] can also result from a dietary deficiency of magnesium or from an abnormal accumulation of saturated fatty acids in cell membranes, which inhibits the entrance of Mg2+ into the cell; thus it is also the etiology not only of diabetes caused by magnesium deficiency, but also of the "lipotoxic" type 2 diabetes mellitus. Although these pathologies cannot be corrected by the Mg2+-binding promoters, they can be corrected, respectively, by dietary magnesium supplementation or by exercise plus dietary caloric and lipid restriction. Theoretically, the disease syndrome containing type 2 diabetes mellitus may involve approximately 30% of the population.

Setting the stage: possible mechanisms by which acute contraction restores insulin sensitivity in muscle

It has long been known that acute exercise can dramatically improve insulin sensitivity in previously insulin-resistant muscle; however, the precise mechanisms underlying this clinically significant interaction remain unknown. Using hindlimb perfusions in obese Zucker rats, our group found that acute muscle contraction synergistically improved insulin-stimulated glucose transport in skeletal muscle, but contrary to our hypothesis, these findings were not associated with either improved insulin signaling or decreased intramuscular lipid metabolites. A further analysis revealed that the improved insulin sensitivity was associated with a robust increase in mitochondrial energy flux. These findings and reports from other labs suggest that mitochondrial energy flux and mitochondrial oxidative capacity may govern insulin sensitivity and override insulin signaling defects associated with obesity. This review will discuss the effects of acute exercise to enhance insulin sensitivity in previously insulin-resistant muscle and present possible novel mechanisms by which alterations in mitochondrial energy metabolism may play a regulatory role.

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.

Skeletal muscle metabolic dysfunction in obesity and metabolic syndrome

Obesity and the related metabolic syndrome have become a worldwide epidemic. Inactivity appears to be a primary causative factor in the pathogenesis of this obesity and metabolic syndrome. There are two possible, perhaps not mutually exclusive, events that may lead to intramyocellular lipid accumulation and mitochondrial dysfunction in patients with obesity. First, obesity, with high intake-associated lipid accumulation in muscle may interfere with cellular mitochondrial function through generation of reactive oxygen species leading to lipid membrane peroxidative injury and disruption of mitochondrial membrane-dependent enzymes. This in turn leads to impaired oxidative metabolism. Secondly, a primary defect in mitochondrial oxidative metabolism may be responsible for a reduction in fatty acid oxidation leading to intramyocellular lipid accumulation as a secondary event. Non-invasive techniques such as proton (1H) and phosphorus (31P) magnetic resonance spectroscopy, coupled with specific magnetic resonance imaging techniques, may facilitate the investigation of the effects of various ergometric interventions on the pathophysiology of obesity and the metabolic syndrome. Exercise has positive effects on glucose metabolism, aerobic metabolism, mitochondrial density, and respiratory chain proteins in patients with metabolic syndrome, and we propose that this may be due to the exercise effects on AMP kinase, and a prospective physiological mechanism for this benefit is presented. A physiological model of the effect of intramyocellular lipid accumulation on oxidative metabolism and insulin mediated glucose uptake is proposed.

Adipogenic capacity and the susceptibility to type 2 diabetes and metabolic syndrome

To determine whether adipocyte storage capacity influences the onset and severity of type 2 diabetes and other components of the metabolic syndrome, we made normal and db/db mice resistant to obesity by overexpressing leptin receptor-b on the aP2-Lepr-b promoter. On a 4% diet, these mice have no phenotype, but on a 60% fat diet, they resist diet-induced obesity because constitutive adipocyte-specific overexpression of Lepr-b prevents obesity via the antilipogenic autocrine/paracrine action of leptin on adipocytes. After 8 months on the same 60% fat diet, body fat of transgenic mice was 70% below WT controls. Cardiac and liver fat was elevated in the transgenics, and their hyperinsulinemia was more marked, suggesting greater insulin resistance. The aP2-Lepr-b transgene also prevented obesity in db/db mice; at 10 weeks of age their body fat was half that of the db/db mice. This lack of obesity was attributable to reduced expression of sterol regulatory element binding protein-1c and its target lipogenic enzymes in adipose tissue and a 6-fold increase in Pref-1 mRNA. Severe diabetes was present in transgenics at 4 weeks of age, 10 weeks before db/db controls. Echocardiographic evidence of cardiomyopathy appeared at 10 weeks, weeks before the db/db mice. Histologically, loss of beta cells and myocardial fibrosis was present in the transgenic group at least 6 weeks before the db/db mice. These results suggest that the expression level of genes that regulate the adipogenic response to overnutrition profoundly influences the age of onset and severity of diet-induced type 2 diabetes and co-morbidities.

Swim training prevents hyperglycemia in ZDF rats: mechanisms involved in the partial maintenance of beta-cell function

Exercise improves glucose tolerance in obese rodent models and humans; however, effects with respect to mechanisms of beta-cell compensation remain unexplained. We examined exercise's effects during the progression of hyperglycemia in male Zucker diabetic fatty (ZDF) rats until 19 wk of age. At 6 wk old, rats were assigned to 1) basal--euthanized for baseline values; 2) exercise--swam individually for 1 h/day, 5 days/wk; and 3) controls (n = 8-10/group). Exercise (13 wk) resulted in maintenance of fasted hyperinsulinemia and prevented increases in fed and fasted glucose (P < 0.05) compared with sham-exercised and sedentary controls (P < 0.05). Beta-cell function calculations indicate prolonged beta-cell adaptation in exercised animals alone. During an intraperitoneal glucose tolerance test (IPGTT), exercised rats had lower 2-h glucose (P < 0.05) vs. controls. Area-under-the-curve analyses from baseline for IPGTT glucose and insulin indicate improved glucose tolerance with exercise was associated with increased insulin production and/or secretion. Beta-cell mass increased in exercised vs. basal animals; however, mass expansion was absent at 19 wk in controls (P < 0.05). Hypertrophy and replication contributed to expansion of beta-cell mass; exercised animals had increased beta-cell size and bromodeoxyuridine incorporation rates vs. controls (P < 0.05). The relative area of GLUT2 and protein kinase B was significantly elevated in exercised vs. sedentary controls (P < 0.05). Last, we show formation of ubiquitinated protein aggregates, a response to cellular/oxidative stress, occurred in nonexercised 19 wk-old ZDF rats but not in lean, 6 wk-old basal, or exercised rats. In conclusion, improved beta-cell compensation through increased beta-cell function and mass occurs in exercised but not sedentary ZDF rats and may be in part responsible for improved glucoregulation.

Forgotten but not gone: the rediscovery of fatty heart, the most common unrecognized disease in America

Until 60 years ago, fatty heart was an accepted clinical entity. Since then, its very existence has been questioned, despite the fact that 2 of 3 Americans are now obese or overweight and obesity has been shown to be correlated with cardiac functional abnormalities. In 2000, a syndrome of "lipotoxic cardiomyopathy" resembling earlier pathologic descriptions of fatty human hearts was described in rodents, and fatty infiltration of cardiomyocytes was subsequently reported in patients with congestive failure. Now, magnetic resonance spectroscopy has been adapted to permit routine noninvasive screening for fatty heart. The use of this technique in human volunteers indicates that cardiomyocyte fat correlates well with body mass index and is elevated in uncomplicated obesity. It is more severe when glucose tolerance becomes abnormal or diabetes is present. It is associated with impaired diastolic filling, even in seemingly asymptomatic obese volunteers. Because fatty heart can be readily prevented by lifestyle modification and pharmacologic interventions that reduce caloric intake and increase fatty acid oxidation, it seems important to recognize its existence so as to intervene as early as possible.

Metabolic mechanisms of failure of intraportally transplanted pancreatic beta-cells in rats: role of lipotoxicity and prevention by leptin

The objective of this study was to determine whether the late failure of beta-cells in islets transplanted via the portal vein is caused by excess insulin-stimulated lipogenesis and lipotoxicity and, if so, whether the damage can be prevented by reducing lipogenesis surrounding the islets. Based on the premise that high portal vein levels of nutrients and incretins would stimulate hyperinsulinemia, thereby inducing intense lipogenesis in nearby hepatocytes, normal islets were transplanted into livers of syngeneic streptozotocin-induced diabetic recipients. Hydrolysis of the surrounding fat would flood the islet grafts with fatty acids that could damage and destroy the beta-cells. Reducing lipogenesis by leptin or caloric restriction should prevent or reduce the destruction. After a rise after transplantation, insulin levels gradually declined and hyperglycemia increased. Four weeks after transplantation mRNA of the lipogenic transcription factor, sterol regulatory element-binding protein-1c (SREBP-1c) and its lipogenic target enzymes were elevated in livers of these recipients, as was triacylglycerol content. Positive oil red O staining for lipids and immunostaining for SREBP-1 were observed in hepatocytes surrounding islets with damaged beta-cells. Leptin-induced lipopenia prevented and caloric restriction reduced steatosis, hyperglycemia, and apoptotic beta-cell destruction. Excessive SREBP-1c-mediated lipogenesis, induced in hepatocytes by insulin hypersecretion, is followed by beta-cell destruction in the grafts and reappearance of diabetes. Graft failure is prevented by blocking lipogenesis. The results suggest that strict antilipogenic intervention might improve outcomes after human islet transplantation.

Attenuation of type 2 diabetes mellitus in the male Zucker diabetic fatty rat: the effects of stress and non-volitional exercise

To date, a limited number of studies have investigated the effects of exercise on the maintenance of endocrine pancreatic adaptations to worsening insulin resistance. In particular, the roles of stress hormones that are associated with commonly used forced-exercise paradigms are not fully explained. To examine the effects of exercise per se in ameliorating pancreatic decompensation over time, we investigated the role of forced swimming and sham exercise stress on the development of type 2 diabetes mellitus in the Zucker diabetic fatty (ZDF) rat. Thirty-two male ZDF rats were obtained at 5 weeks of age and all went through a 1-week acclimatization period. They were then divided into 4 groups: basal (euthanized at 6 weeks of age), exercise (1 h/d; 5 d/wk), sham exercise (sham), and non-treated controls (n = 8 per group). After 6 weeks of treatment, an intraperitoneal glucose tolerance test was performed and animals were euthanized for tissue analysis. By 5 weeks of treatment, controls had elevated fed and fasted glycemia (>11.1 and 7.1 mmol/L, respectively; both P < .05), whereas exercise and sham rats remained euglycemic. At euthanasia, there were elevations in fed insulin levels in exercise and sham rats compared with basal animals (both P < .05). Despite improvements in fed and fasting glucose levels in sham rats, glucose tolerance in sham-treated rats (intraperitoneal glucose tolerance test) was similar to controls, whereas glucose levels were similar in exercised trained and basal rats. After 6 weeks, gastrocnemius glycogen content was higher in exercised rats and sham rats when compared with age-matched controls, whereas muscle glucose transporter 4 levels were similar between groups. Compared with controls, the exercise group had increased beta cell proliferation, beta cell mass, and partial maintenance of normal islet morphology. Sham rats also displayed beta cell compensation, as evidenced by increased fasting insulin levels and partial preservation of normal islet morphology. Finally, at the time of euthanasia, plasma corticosterone was increased in sham and control rats but was at basal levels in the exercise group. In summary, both exercise and sham treatment delay the progression of type 2 diabetes mellitus in the male ZDF rat by distinct mechanisms related to pancreatic function and improvements in peripheral glucose disposal.

Diabetes: insulin resistance and derangements in lipid metabolism. Cure through intervention in fat transport and storage

We present multiple findings on derangements in lipid metabolism in type 2 diabetes. The increase in the intracellular deposition of triglycerides (TG) in muscles, liver and pancreas in subjects prone to diabetes is well documented and demonstrated to attenuate glucose metabolism by interfering with insulin signaling and insulin secretion. The obesity often associated with type 2 diabetes is mainly central, resulting in the overload of abdominal adipocytes with TG and reducing fat depot capacity to protect other tissues from utilizing a large proportion of dietary fat. In contrast to subcutaneous adipocytes, the central adipocytes exhibit a high rate of basal lipolysis and are highly sensitive to fat mobilizing hormones, but respond poorly to lipolysis restraining insulin. The enlarged visceral adipocytes are flooding the portal circulation with free fatty acids (FFA) at metabolically inappropriate time, when FFA should be oxidized, thus exposing nonadipose tissues to fat excess. This leads to ectopic TG accumulation in muscles, liver and pancreatic beta-cells, resulting in insulin resistance and beta-cell dysfunction. This situation, based on a large number of observations in humans and experimental animals, confirms that peripheral adipose tissue is closely regulated, performing a vital role of buffering fluxes of FFA in the circulation. The central adipose tissues tend to upset this balance by releasing large amounts of FFA. To reduce the excessive fat outflow from the abdominal depots and prevent the ectopic fat deposition it is important to decrease the volume of central fat stores or increase the peripheral fat stores. One possibility is to downregulate the activity of lipoprotein lipase, which is overexpressed in abdominal relatively to subcutaneous fat stores. This can be achieved by gastrointestinal bypass or gastroplasty, which decrease dietary fat absorption, or by direct means that include surgical removal of mesenteric fat. Indirect treatment consists of the compliant application of drastic lifestyle change comprising both diet and exercise and pharmacotherapy that reduces mesenteric fat mass and activity. The first step should be an attempt to effectively induce a lifestyle change. Next comes pharmacotherapy including acarbose, metformin, PPARgamma, or PPARgammaalpha agonists, statins and orlistat, estrogens in postmenopausal women or testosterone in men. Among surgical procedures, gastric bypass has been proven to produce beneficial results in advance of other surgical techniques, the evidence basis of which still needs strengthening.

Leptinomimetic effects of the AMP kinase activator AICAR in leptin-resistant rats: prevention of diabetes and ectopic lipid deposition

AIMS/HYPOTHESIS

Leptin has been shown to activate AMP-activated protein kinase (AMPK), an enzyme that regulates the activities of key enzymes of lipid synthesis and metabolism. We assess here (i) whether AMPK activity is diminished in rodents deficient in leptin or the leptin receptor, and (ii) the effects of treating the diabetes-prone, leptin-receptor-deficient Zucker Diabetic Fatty (ZDF) rat with an AMPK activator.

METHODS

AMPK activity and related parameters were measured in muscle and or liver of fa/fa and ZDF rats and ob/ob mice. We also explored the effect of treatment with the AMPK activator 5-aminoimidazole 4-carboxamide 1-beta-D ribofuranoside (AICAR) (7.4 mmol/l, on Monday, Wednesday and Friday for 15 weeks, beginning at 7 weeks of age) on the phenotype of the ZDF rat.

RESULTS

AMPK activity was diminished in muscle and/or liver of fa/fa (leptin-receptor-deficient, non-diabetic) and ZDF (leptin-receptor-deficient, diabetes-prone) rats and ob/ob mice (leptin-deficient). ZDF rats that had free access to food became hyperglycaemic (22.2 mmol/l) and hyperphagic after 2 to 5 weeks and remained so during the remainder of the study. Treatment of ZDF rats with AICAR prevented the development of diabetes, as well as increases of triglyceride content in liver, muscle and the pancreatic islets. It also attenuated the morphological abnormalities observed in the islets of untreated rats. Rats diet-matched with the AICAR-treated animals developed diabetes of intermediate severity and showed decreases in triglyceride content in the islets, but not in liver or muscle.

CONCLUSIONS/INTERPRETATION

The results indicate that a deficiency of leptin or the leptin receptor is associated with a decrease in AMPK activity in muscle and/or liver. They also suggest that treatment with an AMPK activator prevents the development of diabetes and ectopic lipid accumulation in the ZDF rat.