Beta-cell mass dynamics in Zucker diabetic fatty rats. Rosiglitazone prevents the rise in net cell death

Effect of combining rosiglitazone with either metformin or insulin on β-cell mass and function in an animal model of Type 2 diabetes characterized by reduced β-cell mass at birth

BACKGROUND

Interventions that preserve or increase β-cell mass may also prevent Type 2 diabetes. Rosiglitazone alone, as well as in combination with metformin, prevents diabetes in people with high, yet non-diabetic glucose levels. These effects may be mediated through changes in β-cell mass. In the present study, the effect of combining rosiglitazone with metformin and/or insulin on β-cell mass and glucose levels was examined in a rat model of Type 2 diabetes.

METHODS

Diabetes-prone pups were randomized to receive rosiglitazone alone or in combination with metformin and/or insulin starting at 4 weeks of age. β-Cell mass and glucose homeostasis were examined in adulthood.

RESULTS

Rosiglitazone treatment reduced insulin resistance and partially restored β-cell mass in animals with reduced β-cell mass at birth. The addition of metformin to rosiglitazone decreased insulin resistance and reduced weight gain, but had no additional effect on β-cell mass. Conversely, the addition of insulin had no additional effect on these outcomes. Although the combination of rosiglitazone and metformin did not affect β-cell mass at 26 weeks of age, it did result in reduced body weight and insulin resistance.

CONCLUSION

The results of the present study suggest that the addition of metformin to rosiglitazone improves the metabolic profile through an effect on insulin resistance and not β-cell mass.

SGLT2 deletion improves glucose homeostasis and preserves pancreatic beta-cell function

OBJECTIVE

Inhibition of the Na(+)-glucose cotransporter type 2 (SGLT2) is currently being pursued as an insulin-independent treatment for diabetes; however, the behavioral and metabolic consequences of SGLT2 deletion are unknown. Here, we used a SGLT2 knockout mouse to investigate the effect of increased renal glucose excretion on glucose homeostasis, insulin sensitivity, and pancreatic β-cell function.

RESEARCH DESIGN AND METHODS

SGLT2 knockout mice were fed regular chow or a high-fat diet (HFD) for 4 weeks, or backcrossed onto the db/db background. The analysis used metabolic cages, glucose tolerance tests, euglycemic and hyperglycemic clamps, as well as isolated islet and perifusion studies.

RESULTS

SGLT2 deletion resulted in a threefold increase in urine output and a 500-fold increase in glucosuria, as well as compensatory increases in feeding, drinking, and activity. SGLT2 knockout mice were protected from HFD-induced hyperglycemia and glucose intolerance and had reduced plasma insulin concentrations compared with controls. On the db/db background, SGLT2 deletion prevented fasting hyperglycemia, and plasma insulin levels were also dramatically improved. Strikingly, prevention of hyperglycemia by SGLT2 knockout in db/db mice preserved pancreatic β-cell function in vivo, which was associated with a 60% increase in β-cell mass and reduced incidence of β-cell death.

CONCLUSIONS

Prevention of renal glucose reabsorption by SGLT2 deletion reduced HFD- and obesity-associated hyperglycemia, improved glucose intolerance, and increased glucose-stimulated insulin secretion in vivo. Taken together, these data support SGLT2 inhibition as a viable insulin-independent treatment of type 2 diabetes.

Rare sugar D-psicose improves insulin sensitivity and glucose tolerance in type 2 diabetes Otsuka Long-Evans Tokushima Fatty (OLETF) rats

A rare sugar, D-psicose has progressively been evaluated as a unique metabolic regulator of glucose and lipid metabolism, and thus represents a promising compound for the treatment of type 2 diabetes mellitus (T2DM). The present study was undertaken to examine the underlying effector organs of D-psicose in lowering blood glucose and abdominal fat by exploiting a T2DM rat model, Otsuka Long-Evans Tokushima Fatty (OLETF) rats. Rats were fed 5% D-psicose or 5% D-glucose supplemented in drinking water, and only water in the control for 13 weeks and the protective effects were compared. A non-diabetic Long-Evans Tokushima Otsuka (LETO), fed with water served as a counter control of OLETF. After 13 weeks feeding, D-psicose treatment significantly reduced the increase in body weight and abdominal fat mass. Oral glucose tolerance test (OGTT) showed the reduced blood glucose and insulin levels suggesting the improvement of insulin resistance in OLETF rats. Oil-red-O staining elucidated that D-psicose significantly reduced lipid accumulation in the liver. Immunohistochemical analysis showed D-psicose induced glucokinase translocation from nucleus to cytoplasm of the liver which enhances glucokinase activity and subsequent synthesis of glycogen in the liver. D-psicose also protected the pathological change of the β-cells of pancreatic islets. These data demonstrate that D-psicose controls blood glucose levels by reducing lipotoxicity in liver and by preserving pancreatic β-cell function.

Odyssey between Scylla and Charybdis through storms of carbohydrate metabolism and diabetes: a career retrospective

This research perspective allows me to summarize some of my work completed over 50 years, and it is organized in seven sections. 1) The treatment of diabetes concentrates on the liver and/or the periphery. We quantified hormonal and metabolic interactions involved in physiology and the pathogenesis of diabetes by developing tracer methods to separate the effects of diabetes on both. We collaborated in the first tracer clinical studies on insulin resistance, hypertriglyceridemia, and the Cori cycle. 2) Diabetes reflects insulin deficiency and glucagon abundance. Extrapancreatic glucagon changed the prevailing dogma and permitted precise exploration of the roles of insulin and glucagon in physiology and diabetes. 3) We established the critical role of glucagon-insulin interaction and the control of glucose metabolism during moderate exercise and of catecholamines during strenuous exercise. Deficiencies of the release and effects of these hormones were quantified in diabetes. We also revealed how acute and chronic hyperglycemia affects the expression of GLUT2 gene and protein in diabetes. 4) We outlined molecular and physiological mechanisms whereby exercise training and repetitive neurogenic stress can prevent diabetes in ZDF rats. 5) We and others established that the indirect effect of insulin plays an important role in the regulation of glucose production in dogs. We confirmed this effect in humans and demonstrated that in type 2 diabetes it is mainly the indirect effect. 6) We indicated that the muscle and the liver protected against glucose changes. 7) We described molecular mechanisms responsible for increased HPA axis in diabetes and for the diminished responses of HPA axis, catecholamines, and glucagon to hypoglycemia. We proposed a new approach to decrease the threat of hypoglycemia.

Stereological assessment of pancreatic beta-cell mass development in male Zucker Diabetic Fatty (ZDF) rats: correlation with pancreatic beta-cell function

The present study was initiated to improve our understanding of pancreatic beta-cell dynamics in male Zucker Diabetic Fatty (ZDF) rats and hence provide a framework for future diabetes studies in this animal model. Male ZDF rats from 6, 8, 10, 12, 14, 16, 20 and 26 weeks of age were subjected to an oral glucose tolerance test (OGTT). The animals were then euthanized and pancreases were removed for morphometric analyses of pancreatic beta-cell mass. As evident by a marked fourfold increase in insulin secretion, insulin resistance developed rapidly from 6 to 8 weeks of age. Simultaneously, the pancreatic beta-cell mass expanded from 6.17 ± 0.41 mg at 6 weeks of age, reaching a maximum of 16.5 ± 2.5 mg at 16 weeks of age, at which time pancreatic beta-cell mass gradually declined. The corresponding changes in glucose/insulin homeostasis were analysed using a standard insulin sensitivity index (ISI), an area under the curve (AUC) glucose-insulin index, or simple semi-fasted glucose levels. The study demonstrated that male ZDF rats underwent rapid changes in pancreatic beta-cell mass from the onset of insulin resistance to frank diabetes coupled directly to marked alterations in glucose/insulin homeostasis. The study underscores the need for a critical co-examination of glucose homeostatic parameters in studies investigating the effects of novel anti-diabetic compounds on pancreatic beta-cell mass in the male ZDF rat. A simple assessment of fasting glucose levels coupled with information about age can provide a correct indication of the actual pancreatic beta-cell mass and the physiological state of the animal.

Angiotensin I-converting enzyme type 2 (ACE2) gene therapy improves glycemic control in diabetic mice

OBJECTIVE

Several clinical studies have shown the benefits of renin-angiotensin system (RAS) blockade in the development of diabetes, and a local RAS has been identified in pancreatic islets. Angiotensin I-converting enzyme (ACE)2, a new component of the RAS, has been identified in the pancreas, but its role in β-cell function remains unknown. Using 8- and 16-week-old obese db/db mice, we examined the ability of ACE2 to alter pancreatic β-cell function and thereby modulate hyperglycemia.

RESEARCH DESIGN AND METHODS

Both db/db and nondiabetic lean control (db/m) mice were infected with an adenovirus expressing human ACE2 (Ad-hACE2-eGFP) or the control virus (Ad-eGFP) via injection into the pancreas. Glycemia and β-cell function were assessed 1 week later at the peak of viral expression.

RESULTS

In 8-week-old db/db mice, Ad-hACE2-eGFP significantly improved fasting glycemia, enhanced intraperitoneal glucose tolerance, increased islet insulin content and β-cell proliferation, and reduced β-cell apoptosis compared with Ad-eGFP. ACE2 overexpression had no effect on insulin sensitivity in comparison with Ad-eGFP treatment in diabetic mice. Angiotensin-(1-7) receptor blockade by D-Ala(7)-Ang-(1-7) prevented the ACE2-mediated improvements in intraperitoneal glucose tolerance, glycemia, and islet function and also impaired insulin sensitivity in both Ad-hACE2-eGFP- and Ad-eGFP-treated db/db mice. D-Ala(7)-Ang-(1-7) had no effect on db/m mice. In 16-week-old diabetic mice, Ad-hACE2-eGFP treatment improved fasting blood glucose but had no effect on any of the other parameters.

CONCLUSIONS

These findings identify ACE2 as a novel target for the prevention of β-cell dysfunction and apoptosis occurring in 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.

Targeting beta-cell function early in the course of therapy for type 2 diabetes mellitus

OBJECTIVE

This report examines current perspectives regarding likely mechanisms of beta-cell failure in type 2 diabetes and their clinical implications for protecting or sparing beta-cells early in the disease progression. In addition, it considers translation strategies to incorporate relevant scientific findings into educational initiatives targeting clinical practice behavior.

PARTICIPANTS

On January 10, 2009, a working group of basic researchers, clinical endocrinologists, and primary care physicians met to consider whether current knowledge regarding pancreatic beta-cell defects justifies retargeting and retiming treatment for clinical practice. Based on this meeting, a writing group comprised of four meeting participants subsequently prepared this consensus statement. The conference was convened by The Endocrine Society and funded by an unrestricted educational grant from Novo Nordisk.

EVIDENCE

Participants reviewed and discussed published literature, plus their own unpublished data.

CONSENSUS PROCESS

The summary and recommendations were supported unanimously by the writing group as representing the consensus opinions of the working group.

CONCLUSIONS

Workshop participants strongly advocated developing new systems to address common barriers to glycemic control and recommended several initial steps toward this goal. These recommendations included further studies to establish the clinical value of pharmacological therapies, continuing basic research to elucidate the nature and mechanisms of beta-cell failure in type 2 diabetes mellitus, and exploring new educational approaches to promote pathophysiology-based clinical practices. The Endocrine Society has launched a new website to continue the discussion between endocrinologists and primary care physicians on beta-cell pathophysiology in type 2 diabetes and its clinical implications. Join the conversation at http://www.betacellsindiabetes.org

Development of PACAP38 analogue with improved stability: physicochemical and in vitro/in vivo pharmacological characterization

Pituitary adenylate cyclase activating polypeptide 38 (PACAP38), one of the major peptide transmitters, has emerged as a promising drug candidate for the treatment of type 2 diabetes. In the present study, on the basis of previous structure-activity relationships, a new PACAP38 derivative, [R(15, 20, 21), L(17)]-PACAP38, was chemically synthesized with the aim of enhancing the therapeutic potential of PACAP38. The solution structure of the new derivative was almost identical to that of PACAP38 as evaluated by circular dichroic spectroscopy, and both PACAP38 and the new derivative stimulated adenylate cyclase in rat insulinoma RIN-m5F cells with EC(50) values of 4.6 and 5.5 nM, respectively. Stability studies revealed the gradual degradation of PACAPs in rat serum, although there appeared to be a 42% reduction in degradation kinetics for [R(15, 20, 21), L(17)]-PACAP38 compared with that of PACAP38. The novel derivative also exhibited more potent protective effects against streptozotocin (STZ)-induced apoptotic death of RIN-m5F cells, possibly due to the enhanced stability. The n0-STZ model, in which neonatal rats were injected with STZ at birth, developed a typical diabetic condition; however, chronic administration of [R(15, 20, 21), L(17)]-PACAP38 resulted in protection of pancreatic islets, followed by the improvement of glycemic control. Thus, the chemical modification of PACAP38 led to the development of a new promising derivative with enhanced stability and biological activity, and early administration of [R(15, 20, 21), L(17)]-PACAP38 might be of help for preventing the development of diabetes in type 2 diabetic model rats.

Role of heme oxygenase in inflammation, insulin-signalling, diabetes and obesity

Diabetes and obesity are chronic conditions associated with elevated oxidative/inflammatory activities with a continuum of tissue insults leading to more severe cardiometabolic and renal complications including myocardial infarction and end-stage-renal damage. A common denominator of these chronic conditions is the enhanced the levels of cytokines like tumour necrosis factor-alpha (TNF-α), interleukin (IL-6), IL-1β and resistin, which in turn activates the c-Jun-N-terminal kinase (JNK) and NF-κB pathways, creating a vicious cycle that exacerbates insulin resistance, type-2 diabetes and related complications. Emerging evidence indicates that heme oxygenase (HO) inducers are endowed with potent anti-diabetic and insulin sensitizing effects besides their ability to suppress immune/inflammatory response. Importantly, the HO system abates inflammation through several mechanisms including the suppression of macrophage-infiltration and abrogation of oxidative/inflammatory transcription factors like NF-κB, JNK and activating protein-1. This review highlights the mechanisms by which the HO system potentiates insulin signalling, with particular emphasis on HO-mediated suppression of oxidative and inflammatory insults. The HO system could be explored in the search for novel remedies against cardiometabolic diseases and their complications.

Quantification of the relationship between glycemia and beta-cell mass adaptation in vivo

Beta-cell mass dynamics play an important role in the adaptation to obesity, as well as in the pathogenesis of type 2 diabetes. Here we used a 24-hour modified hyperglycemic clamp protocol to investigate the effect of increasing glucose concentrations (15, 20, 25, or 35 mmol/L) on beta-cell mass and rates of beta-cell replication, death, and neogenesis in 6-week-old Sprague Dawley rats (n = 40). During the first 4 h of glucose infusion, plasma insulin levels rose to an approximate steady state in each group, but by the end of 24 h, there was no difference in insulin levels between any of the groups. There was also no difference in beta-cell mass between groups. Mean beta-cell replication rates displayed a linear relationship to mean plasma glucose levels in all hyperglycemic animals (r(2) = 0.98, p < 0.05). Relative to the uninfused basal control animals, replication rates were significantly reduced in the 15 mmol/L glucose group. The percentage of TUNEL-positive beta-cells was not different between groups. There was also no significant difference in markers of neogenesis. Thus, these data demonstrate that hyperglycemia for 24 h had no effect on beta-cell mass, death, or neogenesis in 6-week-old Sprague Dawley rats. We demonstrate a linear relationship, however, between hyperglycemia and beta-cell replication rates in vivo.

Rosiglitazone improves pancreatic mitochondrial function in an animal model of dysglycemia: role of the insulin-like growth factor axis

Thiazolidinediones (TZDs) improve insulin sensitivity and maintain beta cell mass. This study examined whether this effect is attributable to improved mitochondrial function in the pancreas and the potential involvement of the pancreatic insulin-like growth factor (IGF) axis in mediating this effect. Female Wistar rats were given either saline (vehicle) or nicotine (1 mg kg⁻¹ day⁻¹) during pregnancy and lactation. Following weaning, nicotine-exposed offspring were randomized to receive either vehicle or rosiglitazone (3 mg kg⁻¹ day⁻¹) until 26 weeks of age when serum and pancreas tissue were collected. The effect of rosiglitazone on nicotine-induced mitochondrial dysfunction was also examined in vitro. Fetal and neonatal nicotine exposure resulted in structural and functional mitochondrial deficits relative to saline controls. The nicotine-induced mitochondrial defects were attenuated by postnatal rosiglitazone administration. A similar effect was observed in vitro; nicotine (25 ng/ml) inhibited beta cell mitochondrial function and co-treatment with rosiglitazone (1 μM) restored enzyme activity to control levels. Fetal and neonatal nicotine exposure also altered key components of the adult pancreatic IGF axis, an effect that was not prevented by rosiglitazone treatment. Rosiglitazone treatment maintains mitochondrial structure and function in the pancreas of rats that are prone to diabetes, as well as mitochondrial function in beta cell culture. We propose that this may be an important part of the mechanism by which rosiglitazone improves beta cell mass and prevents diabetes in individuals with impaired glucose tolerance and/or impaired fasting glucose. The underlying mechanism through which rosiglitazone targets the mitochondria remains to be determined, but does not appear to involve the IGF axis.

Epigenetic mechanisms in the development of type 2 diabetes

Type 2 diabetes (T2D) is a disorder of complex genetics influenced by interactions between susceptible genetic loci and environmental perturbations. Intrauterine growth retardation is one such environmental perturbation linked to the development of T2D in adulthood. An abnormal metabolic intrauterine milieu affects fetal development by permanently modifying expression of key genes regulating beta-cell development (Pdx1) and glucose transport (Glut4) in muscle. Epigenetic regulation of gene expression is one mechanism by which genetic susceptibility and environmental insults can lead to T2D. Therefore, therapeutic agents targeting epigenetic gene regulation can ultimately be used to treat T2D; however, there is much to be learned about genome-wide epigenetic programming of health and disease before these therapies can be used in patient care.

A GIP receptor agonist exhibits beta-cell anti-apoptotic actions in rat models of diabetes resulting in improved beta-cell function and glycemic control

AIMS

The gastrointestinal hormone GIP promotes pancreatic islet function and exerts pro-survival actions on cultured beta-cells. However, GIP also promotes lipogenesis, thus potentially restricting its therapeutic use. The current studies evaluated the effects of a truncated GIP analog, D-Ala(2)-GIP(1-30) (D-GIP(1-30)), on glucose homeostasis and beta-cell mass in rat models of diabetes.

MATERIALS AND METHODS

The insulinotropic and pro-survival potency of D-GIP(1-30) was evaluated in perfused pancreas preparations and cultured INS-1 beta-cells, respectively, and receptor selectivity evaluated using wild type and GIP receptor knockout mice. Effects of D-GIP(1-30) on beta-cell function and glucose homeostasis, in vivo, were determined using Lean Zucker rats, obese Vancouver diabetic fatty rats, streptozotocin treated rats, and obese Zucker diabetic fatty rats, with effects on beta-cell mass determined in histological studies of pancreatic tissue. Lipogenic effects of D-GIP(1-30) were evaluated on cultured 3T3-L1 adipocytes.

RESULTS

Acutely, D-GIP(1-30) improved glucose tolerance and insulin secretion. Chronic treatment with D-GIP(1-30) reduced levels of islet pro-apoptotic proteins in Vancouver diabetic fatty rats and preserved beta-cell mass in streptozotocin treated rats and Zucker diabetic fatty rats, resulting in improved insulin responses and glycemic control in each animal model, with no change in body weight. In in vitro studies, D-GIP(1-30) exhibited equivalent potency to GIP(1-42) on beta-cell function and survival, but greatly reduced action on lipoprotein lipase activity in 3T3-L1 adipocytes.

CONCLUSIONS

These findings demonstrate that truncated forms of GIP exhibit potent anti-diabetic actions, without pro-obesity effects, and that the C-terminus contributes to the lipogenic actions of GIP.

Effect of Rosiglitazone and Ramipril on {beta}-cell function in people with impaired glucose tolerance or impaired fasting glucose: the DREAM trial

OBJECTIVE The objective of this study was to determine the degree to which ramipril and/or rosiglitazone changed beta-cell function over time among individuals with impaired fasting glucose (IFG) and/or impaired glucose tolerance (IGT) who participated in the Diabetes Reduction Assessment With Ramipril and Rosiglitazone Medication (DREAM) Trial, which evaluated whether ramipril and/or rosiglitazone could prevent or delay type 2 diabetes in high-risk individuals. RESEARCH DESIGN AND METHODS The present analysis included subjects (n = 982) from DREAM trial centers in Canada who had oral glucose tolerance tests at baseline, after 2 years, and at the end of the study. beta-Cell function was assessed using the fasting proinsulin-to-C-peptide ratio (PI/C) and the insulinogenic index (defined as 30-0 min insulin/30-0 min glucose) divided by homeostasis model assessment of insulin resistance (insulinogenic index [IGI]/insulin resistance [IR]). RESULTS Subjects receiving rosiglitazone had a significant increase in IGI/IR between baseline and end of study compared with the placebo group (25.59 vs. 1.94, P < 0.0001) and a significant decrease in PI/C (-0.010 vs. -0.006, P < 0.0001). In contrast, there were no significant changes in IGI/IR or PI/C in subjects receiving ramipril compared with placebo (11.71 vs. 18.15, P = 0.89, and -0.007 vs. -0.008, P = 0.64, respectively). The impact of rosiglitazone on IGI/IR and PI/C was similar within subgroups of isolated IGT and IFG + IGT (all P < 0.001). Effects were more modest in those with isolated IFG (IGI/IR: 8.95 vs. 2.13, P = 0.03; PI/C: -0.003 vs. -0.001, P = 0.07). CONCLUSIONS Treatment with rosiglitazone, but not ramipril, resulted in significant improvements in measures of beta-cell function over time in pre-diabetic subjects. Although the long-term sustainability of these improvements cannot be determined from the present study, these findings demonstrate that the diabetes preventive effect of rosiglitazone was in part a consequence of improved beta-cell function.

Efficacy of autologous bone marrow-derived stem cell transplantation in patients with type 2 diabetes mellitus

Progressive and inexorable beta-cell dysfunction is the hallmark of type 2 diabetes mellitus (T2DM) and beta-cell regeneration using stem cell therapy may prove to be an effective modality. A total of 10 patients (8 men) with T2DM for >5 years, failure of triple oral antidiabetic drugs, currently on insulin (> or = 0.7 U/kg/day) at least for 1 year, and glutamic acid decarboxylase antibody negative were included. Patients on stable doses of medications for past 3 months were recruited. Primary end points were reduction in insulin requirement by > or = 50% and improvement in glucagon-stimulated C-peptide levels at the end of 6 months of autologous bone marrow-derived stem cell transplantation (SCT), while secondary end points were a change in weight and HbA1c and lipid levels as compared to baseline. Seven patients were responders and showed a reduction in insulin requirement by 75% as compared to baseline. Mean duration to achieve the primary objective was 48 days. Three patients were able to discontinue insulin completely, although it was short-lived in one. Mean HbA1c reduction was 1% and 3 of the 7 responders had HbA1c value <7%. A significant weight loss of 5.5 kg was noted in the responders, whereas, nonresponders gained 2.2 kg of weight. However, weight loss did not correlate with reduction in insulin requirement (r = 0.68, P = 0.06). There was a significant improvement in both fasting and glucagon-stimulated C-peptide level in the group (P = 0.03) and responders (P = 0.03). HOMA-B increased significantly in the whole group (P = 0.02) and responders (P = 0.04) whereas, HOMA-IR did not change significantly (P = 0.74). Reduction in insulin doses correlated with stimulated C-peptide response at the baseline (r = 0.83, P = 0.047) and mononuclear cell count of infused stem cells (r = 0.57, P = 0.04). No serious adverse effects were noted. Our observations indicate that SCT is a safe and effective modality of treatment to improve beta-cell function in patients with T2DM. However, further large-scale studies are needed to substantiate these observations.

Molecular mechanism by which pioglitazone preserves pancreatic beta-cells in obese diabetic mice: evidence for acute and chronic actions as a PPARgamma agonist

Pioglitazone preserves pancreatic beta-cell morphology and function in diabetic animal models. In this study, we investigated the molecular mechanisms by which pioglitazone protects beta-cells in diabetic db/db mice. In addition to the morphological analysis of the islets, gene expression profiles of the pancreatic islet were analyzed using laser capture microdissection and were compared with real-time RT-PCR of db/db and nondiabetic m/m mice treated with or without pioglitazone for 2 wk or 2 days. Pioglitazone treatment (2 wk) ameliorated dysmetabolism, increased islet insulin content, restored glucose-stimulated insulin secretion, and preserved beta-cell mass in db/db mice but had no significant effects in m/m mice. Pioglitazone upregulated genes that promote cell differentiation/proliferation in diabetic and nondiabetic mice. In db/db mice, pioglitazone downregulated the apoptosis-promoting caspase-activated DNase gene and upregulated anti-apoptosis-related genes. The above-mentioned effects of pioglitazone treatment were also observed after 2 days of treatment. By contrast, the oxidative stress-promoting NADPH oxidase gene was downregulated, and antioxidative stress-related genes were upregulated, in db/db mice treated with pioglitazone for 2 wk, rather than 2 days. Morphometric results for proliferative cell number antigen and 4-hydroxy-2-noneal modified protein were consistent with the results of gene expression analysis. The present results strongly suggest that pioglitazone preserves beta-cell mass in diabetic mice mostly by two ways; directly, by acceleration of cell differentiation/proliferation and suppression of apoptosis (acute effect); and indirectly, by deceleration of oxidative stress because of amelioration of the underlying metabolic disorder (chronic effect).

Peroxisome proliferator-activated receptor translational research and clinical experience

Since the early 1970s, pharmaceutical biochemists have sought to exploit the scientific findings that were uncovered when they studied the basis for the function and mode of action of fibric acid derivatives. In the early 1970s, little was known of peroxisome proliferator-activated receptors (PPARs), even in concept. Since then, however, the development of bioactive small molecules in medicinal science has resulted in tools developed to be inserted into the PPAR-binding domain, which has resulted in the recognition of literally thousands of possible biological effects of binding configurations. In diabetes care, the first of the marketed agents from these discoveries and developments was introduced in 1996. It was potent and did its job well. However, the use of this early form of thiazolidinedione sometimes, although rarely, led to fulminant liver failure, and ultimately the drug was removed from the market. Subsequent thiazolidinediones have been developed, and 2 have been relatively successful. However, they are not without their problems. This article describes the history of the development of these drugs, identifies the valuable attributes that they possess, and gives a clear rationale as to why a quest for a "safer" PPAR agonist is still being sought.

Pdx1 and other factors that regulate pancreatic beta-cell survival

A progressive reduction in beta-cell mass occurs in the evolution of diabetes. Thus understanding the mechanisms responsible for this reduction in beta-cell mass is important for understanding the pathogenesis of diabetes and in developing novel approaches to prevention and treatment. Pancreatic duodenal homeobox 1 (Pdx1) is a transcription factor that plays a central role in pancreatic beta-cell function and survival. Complete deficiency of Pdx1 is associated with pancreatic agenesis, and partial deficiency leads to severe beta-cell dysfunction, and increases beta-cell death and diabetes both in rodent and human. Chronic hyperglycaemia and dyslipidaemia, which are major features of type 2 diabetes, cause beta-cell dysfunction via reduced Pdx1 expression. Inhibition of insulin/insulin-like growth factor (Igf) signalling followed by reduced Pdx1 expression is a common pathway induced by the majority of the mechanisms in apoptotic beta-cells. Although the report so far paid little attention to non-apoptotic beta-cell death (autophagy and necrosis), we expect these are also involved in the pathogenesis of diabetes. The potential role of Pdx1 in non-apoptotic beta-cell death should also be considered in future studies in diabetes, and in attempts to develop novel agents that target this process for prevention and treatment of the disorder.

Potent antidiabetic effects of rivoglitazone, a novel peroxisome proliferator-activated receptor-gamma agonist, in obese diabetic rodent models

The pharmacological effects of rivoglitazone, a novel thiazolidinedione-derivative peroxisome proliferator-activated receptor (PPAR)-gamma agonist, were characterized in vitro and in vivo. Rivoglitazone activated human PPARgamma more potently compared with rosiglitazone and pioglitazone and had little effect on PPARalpha and PPARdelta activity in luciferase reporter assays. In Zucker diabetic fatty (ZDF) rats, 14-day administration of rivoglitazone decreased the plasma glucose and triglyceride (TG) levels in a dose-dependent manner. The glucose-lowering effect of rivoglitazone was much more potent than those of pioglitazone (ED(50): 0.19 vs. 34 mg/kg) and rosiglitazone (ED(50): 0.20 vs. 28 mg/kg). In addition, rivoglitazone showed potent antidiabetic effects in diabetic db/db mice. In Zucker fatty rats, rivoglitazone at a dose of 0.1 mg/kg clearly ameliorated insulin resistance and lowered plasma TG levels by accelerating the clearance of plasma TG. Gene expression analysis in the liver and heart of ZDF rats treated with rivoglitazone for 14 days suggested that rivoglitazone may reduce hepatic glucose production and modulate the balance of the cardiac glucose/fatty acid metabolism in diabetic animals. In summary, we showed that rivoglitazone is a potent and selective PPARgamma agonist and has a potent glucose-lowering effect via improvement of the insulin resistance in diabetic animal models.

Susceptibility of pancreatic beta cells to fatty acids is regulated by LXR/PPARalpha-dependent stearoyl-coenzyme A desaturase

Chronically elevated levels of fatty acids-FA can cause beta cell death in vitro. Beta cells vary in their individual susceptibility to FA-toxicity. Rat beta cells were previously shown to better resist FA-toxicity in conditions that increased triglyceride formation or mitochondrial and peroxisomal FA-oxidation, possibly reducing cytoplasmic levels of toxic FA-moieties. We now show that stearoyl-CoA desaturase-SCD is involved in this cytoprotective mechanism through its ability to transfer saturated FA into monounsaturated FA that are incorporated in lipids. In purified beta cells, SCD expression was induced by LXR- and PPARalpha-agonists, which were found to protect rat, mouse and human beta cells against palmitate toxicity. When their SCD was inhibited or silenced, the agonist-induced protection was also suppressed. A correlation between beta cell-SCD expression and susceptibility to palmitate was also found in beta cell preparations isolated from different rodent models. In mice with LXR-deletion (LXRbeta(-/-) and LXRalphabeta(-/-)), beta cells presented a reduced SCD-expression as well as an increased susceptibility to palmitate-toxicity, which could not be counteracted by LXR or PPARalpha agonists. In Zucker fatty rats and in rats treated with the LXR-agonist TO1317, beta cells show an increased SCD-expression and lower palmitate-toxicity. In the normal rat beta cell population, the subpopulation with lower metabolic responsiveness to glucose exhibits a lower SCD1 expression and a higher susceptibility to palmitate toxicity. These data demonstrate that the beta cell susceptibility to saturated fatty acids can be reduced by stearoyl-coA desaturase, which upon stimulation by LXR and PPARalpha agonists favors their desaturation and subsequent incorporation in neutral lipids.

Dynamics of beta-cell turnover: evidence for beta-cell turnover and regeneration from sources of beta-cells other than beta-cell replication in the HIP rat

Type 2 diabetes is characterized by hyperglycemia, a deficit in beta-cells, increased beta-cell apoptosis, and islet amyloid derived from islet amyloid polypeptide (IAPP). These characteristics are recapitulated in the human IAPP transgenic (HIP) rat. We developed a mathematical model to quantify beta-cell turnover and applied it to nondiabetic wild type (WT) vs. HIP rats from age 2 days to 10 mo to establish 1) whether beta-cell formation is derived exclusively from beta-cell replication, or whether other sources of beta-cells (OSB) are present, and 2) to what extent, if any, there is attempted beta-cell regeneration in the HIP rat and if this is through beta-cell replication or OSB. We conclude that formation and maintenance of adult beta-cells depends largely ( approximately 80%) on formation of beta-cells independent from beta-cell duplication. Moreover, this source adaptively increases in the HIP rat, implying attempted beta-cell regeneration that substantially slows loss of beta-cell mass.

Long-term treatment with sergliflozin etabonate improves disturbed glucose metabolism in KK-A(y) mice

Sergliflozin etabonate, a novel oral selective low-affinity sodium glucose cotransporter (SGLT2) inhibitor, improves hyperglycemia by suppressing renal glucose reabsorption, in which SGLT2 participates as a dominant transporter. In the present study, we examined the antidiabetic profile of sergliflozin etabonate in a diabetic model, KK-A(y) mice, with symptoms of obesity and hyperinsulinemia. The blood glucose level was monitored in non-fasted female KK-A(y) mice after a single oral administration of sergliflozin etabonate. The non-fasting blood glucose level was reduced in a dose-dependent manner after a single oral administration of sergliflozin etabonate (39% reduction at 2 h after a dose of 30 mg/kg). The effects of long-term administration of sergliflozin etabonate on the blood glucose level were assessed in female KK-A(y) mice in several studies (4-day, 8-week, and 9-week administration study), in which sergliflozin etabonate was administered in the diet. The non-fasting blood glucose and plasma insulin were both lowered dose-dependently in the 4-day administration study. Long-term treatment with sergliflozin etabonate dose-dependently improved the hyperglycemia and prevented body weight gain in the 8-week study. In addition to the improvement in glycemic control, fatty liver and pancreatic beta-cell abnormalities were ameliorated in mice fed sergliflozin etabonate in the 9-week study. These data indicate that SGLT2 inhibitors could be useful to improve hyperglycemia resulting from insulin resistance without pancreatic beta-cell abuse or body weight gain. SGLT2 inhibitors may simultaneously realize both a systemic negative energy balance and correction of hyperglycemia.

The interplay of catalysis and toxicity by amyloid intermediates on lipid bilayers: insights from type II diabetes

The dynamics, energies, and structures governing protein folding are critical to biological function. Amyloidoses are a class of disease defined, in part, by the misfolding and aggregation of functional protein precursors into fibrillar states. Amyloid fibers contribute to the pathology of many diseases, including type II diabetes, Alzheimer's, and Parkinson's. In these disorders, amyloid fibers are present in affected tissues. However, it has become clear that intermediate states, rather than mature fibers, represent the cytotoxic species. In this review, we focus particularly on lipid bilayer-bound intermediates. Remarkably, the precursors of these fibers are intrinsically disordered, and yet catalysis of beta-sheet formation appears to be mediated by the stabilization of alpha-helical states. On the lipid bilayer, these intermediate species have been implicated as cytotoxic through elimination of ionic homeostasis. Recent advances are enabling insights at a molecular level that promise to provide meaningful targets for the development of therapeutics.

The role of GLP-1 in the regulation of islet cell mass

Glucagon-like peptide-1 (GLP-1) is an incretin hormone capable of restoring euglycemia in glucose-intolerant subjects and improving glucose control in individuals with type 2 diabetes. Whether the antidiabetic properties of GLP-1 are exclusively the result of its acute postprandial action is being investigated. A GLP-1-dependent differentiation of pancreatic precursor cells into mature beta-cells has been proposed. In addition, GLP-1 has been shown to have antiapoptotic activity in cultured insulin-secreting cells and in an animal model in which diabetes occurs as a consequence of an excessive rate of beta-cell apoptosis. Studies from our laboratory, and others, lead us to propose that GLP-1 is a growth factor for pancreatic cells and it is a regulator of islet cell mass. The aim of this article is to review those reports that have emphasized the role of GLP-1 as a regulator of islet cell mass as well as its insulin secretory action.

Peroxisome proliferator-activated receptor gamma activation restores islet function in diabetic mice through reduction of endoplasmic reticulum stress and maintenance of euchromatin structure

The nuclear receptor peroxisome proliferator-activated receptor gamma (PPAR-gamma) is an important target in diabetes therapy, but its direct role, if any, in the restoration of islet function has remained controversial. To identify potential molecular mechanisms of PPAR-gamma in the islet, we treated diabetic or glucose-intolerant mice with the PPAR-gamma agonist pioglitazone or with a control. Treated mice exhibited significantly improved glycemic control, corresponding to increased serum insulin and enhanced glucose-stimulated insulin release and Ca(2+) responses from isolated islets in vitro. This improved islet function was at least partially attributed to significant upregulation of the islet genes Irs1, SERCA, Ins1/2, and Glut2 in treated animals. The restoration of the Ins1/2 and Glut2 genes corresponded to a two- to threefold increase in the euchromatin marker histone H3 dimethyl-Lys4 at their respective promoters and was coincident with increased nuclear occupancy of the islet methyltransferase Set7/9. Analysis of diabetic islets in vitro suggested that these effects resulting from the presence of the PPAR-gamma agonist may be secondary to improvements in endoplasmic reticulum stress. Consistent with this possibility, incubation of thapsigargin-treated INS-1 beta cells with the PPAR-gamma agonist resulted in the reduction of endoplasmic reticulum stress and restoration of Pdx1 protein levels and Set7/9 nuclear occupancy. We conclude that PPAR-gamma agonists exert a direct effect in diabetic islets to reduce endoplasmic reticulum stress and enhance Pdx1 levels, leading to favorable alterations of the islet gene chromatin architecture.

Impact of incretin therapy on islet dysfunction: an underlying defect in the pathophysiology of type 2 diabetes

Glucose homeostasis is governed by a complex interplay of hormonal signaling and modulation. Insulin, glucagon, amylin, the incretin hormones glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1), and other hormones and enzymes interact to maintain glucose homeostasis and normal cellular metabolism. Derangements in these hormonal interactions, particularly insulin deficits and impaired insulin action, result in the development of type 2 diabetes-but only in individuals who have experienced significant dysfunction or loss of beta-cells, located in the pancreatic islets of Langerhans. Much less is known about the impact of alpha-cell dysregulation on glucose homeostasis, although it has been demonstrated that glucagon-secreting alpha-cells, also located in the pancreatic islets, play an important role in glucose metabolism. Because beta-cell dysfunction occurs early in the course of type 2 diabetes and is progressive, early intervention with therapies that improve beta-cell function is desirable. In addition to reducing HbA1c and fasting plasma glucose, the recently developed diabetes therapies GLP-1 receptor agonists (eg, exenatide, liraglutide) and dipeptidyl peptidase-4 (DPP-4) inhibitors (eg, sitagliptin, vildagliptin) appear to have beneficial effects on beta-cell dysfunction and, possibly, on alpha-cell dysregulation.

Gene expression regulated by pioglitazone and exenatide in normal and diabetic rat islets exposed to lipotoxicity

BACKGROUND

Hyperlipidaemia has been suggested to contribute by pro-apoptotic actions to the loss of beta-cell mass, its secretory defects, and thereby impaired beta-cell function in type 2 diabetes. Treatment of genetically diabetic rats and also type 2 diabetic patients with pioglitazone, a PPAR-gamma agonist, lowers fasting levels of plasma glucose and triglycerides, and has been suggested to protect beta-cells against diabetic lipotoxicity in vitro and in vivo. Another recently launched anti-diabetic drug, exenatide, an incretin mimetic, has been shown to stimulate insulin secretion, growth, and proliferation of pancreatic beta-cells and to protect them against apoptosis. We aimed to investigate global alterations in beta-cell gene expression under lipotoxic conditions and the influence of in vitro treatment with pioglitazone and exenatide.

METHODS

Global gene expression profiling was thus performed to characterize genes differently regulated by palmitate, pioglitazone, and exenatide in isolated islets from non-diabetic Wistar rats and type 2 diabetic Goto-Kakizaki (GK) rats.

RESULTS

Gene expression profiling revealed significant changes in islet mRNAs involved in control of several aspects of beta-cell function, e.g. epigenetic regulation of gene expression, cell differentiation and morphogenesis, also metabolism, response to stimulus, transport, and signal transduction. Pioglitazone and exenatide appear to significantly impact epigenetic processes, e.g. stable alterations in gene expression potential, which arise during development and cell proliferation. Bcl2-like 1 (Bcl2l1), an anti-apoptotic protein, and Bcl2 modifying factor (Bmf), a pro-apoptotic protein, were both down-regulated by pioglitazone and exenatide in the presence of palmitate in diabetic GK islets. In contrast, Bmf was downregulated by pioglitazone in the presence of palmitate in non-diabetic Wistar islets. Exposure of non-diabetic Wistar islets to palmitate led to a reduction in the expression of PPAR beta/delta. This suggests that palmitate may increase the accumulation of triglycerides by reducing PPAR signalling. Moreover, treatment with either pioglitazone or exenatide restored and increased the expression of PPAR beta/delta in non-diabetic Wistar islets.

CONCLUSIONS

Taking into account that these drugs target different components of the epigenetic machinery, our findings suggest that they might participate in restoring normal gene activity in dysfunctional islets and that additive benefits may occur. Whether such events contribute to the beta-cell sparing, proliferative, and anti-apoptotic effects of these drugs in diabetes remains to be elucidated.

Thiazolidinediones: effects on the development and progression of type 2 diabetes and associated vascular complications

In addition to reducing hyperglycaemia, the metabolic actions of TZDs (pioglitazone and rosiglitazone) in theory might improve the prognosis of patients with type 2 diabetes. However, it appears from recent data that pioglitazone and rosiglitazone have different cardiovascular risk profiles. The scope of this paper is to examine the benefits and risks of pioglitazone and rosiglitazone. Three large clinical studies (DREAM, and ADOPT with rosiglitazone; PROactive with pioglitazone) have recently been reported. A lower annual rate of decline of ss-cell function observed with rosiglitazone in the ADOPT study, compared with metformin and glyburide (glibenclamide), along with a reduced progression to insulin use seen with pioglitazone in the PROactive study, provides evidence that TZDs are effective in treating progressive hyperglycaemia. In PROactive, although the primary endpoint was not met, pioglitazone was associated with a reduction in a secondary composite endpoint of clinical cardiovascular events in high-risk patients with existing macrovascular disease who were already receiving other glycaemic and cardiovascular medications. Further evidence supporting an anti-atherogenic effect of pioglitazone was gained from the PERISCOPE study of carotid intima-media thickness. Recent controversy concerning a possible increased risk of myocardial infarction associated with rosiglitazone has fuelled uncertainty about the risk-benefit profile of this agent. In 2008, an update of an American Diabetes Association-European Association for the Study of Diabetes consensus statement on initiation and adjustment of therapy in patients with type 2 diabetes advised clinicians against using rosiglitazone. Skeletal fractures have recently emerged as a side effect of both TZDs. Available data suggest that cardiovascular benefits observed with pioglitazone might not be a class effect of TZDs.

Palatinose and oleic acid act together to prevent pancreatic islet disruption in nondiabetic obese Zucker rats

We showed previously that 8-wk consumption of a diet containing palatinose (P, a slowly-absorbed sucrose analogue) and oleic acid (O) ameliorates but a diet containing sucrose (S) and linoleic acid (L) aggravates metabolic abnormalities in Zucker fatty (fa/fa) rats. In this study, we aimed to identify early changes in metabolism in rats induced by certain combinations of carbohydrates and fatty acids. Specifically, male Zucker fatty rats were fed an isocaloric diet containing various combinations of carbohydrates (P; S) and fatty acids (O; L). After 4 wk, no significant differences in body weight, visceral fat mass, plasma parameters (glucose, insulin, lipids, and adipokines), hepatic adiposity and gene expression, and adipose inflammation were observed between dietary groups. In contrast, pancreatic islets of palatinose-fed (PO and PL) rats were smaller and less fibrotic than sucrose-fed (SO and SL) rats. The abnormal alpha-cell distribution and sporadic staining of active caspase-3 common to islets of linoleic-acid-fed rats were not observed in oleic-acid-fed (PO and SO) rats. Accordingly, progressive beta-cell loss was seen in SL rats, but not in PO rats. These findings suggest that pancreatic islets may be initial sites that translate the effects of different combinations of dietary carbohydrates and fats into metabolic changes.

Morphology of pancreatic islets: a time course of pre-diabetes in Zucker fatty rats

The Zucker fatty rat (fa/fa; ZR) is considered as a model for pre-diabetes, as characterised by a genetic defect in the leptin receptor, which results in hyperphagia, insulin resistance, hyperinsulinaemia, hyperlipoproteinaemia, and obesity. These animals become glucose intolerant but do not develop type 2 diabetes. As a consequence of increased adiposity and insulin resistance, the endocrine pancreas of ZR undergoes adaptive and compensatory changes. Measurements of the time course of the pathological changes by the histological analysis of the pancreatic islet in combination with metabolic parameters are an effective way to reveal disease progression. A loss in glucose tolerance occurs in ZR by 10 weeks of age and progressively worsens by 19 weeks of age. This process is accompanied by impaired islet histology, changes of beta-cell mass, and impaired islet function. The early expression of insulin resistance and glucose intolerance in ZR results in morphological and functional changes of pancreatic islets despite their capability to maintain normoglycaemia.

Pituitary adenylate cyclase-activating polypeptide attenuates streptozotocin-induced apoptotic death of RIN-m5F cells through regulation of Bcl-2 family protein mRNA expression

Oxidative stress, followed by the apoptotic death of pancreatic beta cells, is considered to be one of causative agents in the evolution of the type 2 diabetic state; therefore, the protection of beta cells can comprise an efficacious strategy for preventing type 2 diabetes. In the present study, RIN-m5F cells (i.e. the rat insulinoma beta cell line) were stimulated with streptozotocin, resulting in a time- and concentration-dependent release of lactate dehydrogenase. There appeared to be significant apoptotic cell death after 2 h of treatment with streptozotocin at 10 mM, as demonstrated by terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling staining and 2.6-fold activation of cellular caspase-3, an apoptotic enzyme. By contrast, some neuropeptides of the glucagon-secretin family and coenzyme Q(10), an endogenous mitochondrial antioxidant, could attenuate streptozotocin cytotoxicity, and especially pituitary adenylate cyclase-activating polypeptide (PACAP), at a concentration of 10(-7) M, exhibited 34% attenuation of lactate dehydrogenase release from streptozotocin-treated RIN-m5F cells. Quantitative RT-PCR experiments indicated the inhibitory effect of PACAP on streptozotocin-evoked up-regulation of pro-apoptotic factor (Noxa and Bax) and a 2.3-fold enhancement of Bcl-2 mRNA expression, a pro-survival protein, was also observed after addition of PACAP. The data obtained suggest the anti-apoptotic role of PACAP in streptozotocin-treated RIN-m5F cells through the regulation of pro-apoptotic and pro-survival factors.

Exercise training decreases proinflammatory profile in Zucker diabetic (type 2) fatty rats

OBJECTIVE

In the present study we evaluated the effect of exercise on the plasma levels of proinflammatory cytokines, interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNF-alpha), and the anti-inflammatory molecule uric acid in the Zucker diabetic fatty (ZDF) rats that are more prone to develop type 2 diabetes mellitus.

METHODS

Sixteen obese ZDF (Gmi fa/fa) rats (8 wk old, 228.40 +/- 4.05 g) were randomly assigned to one of two groups (n = 8 each): an exercise-trained group and a sedentary one. In addition, 16 lean ZDF (Gmi +/+) rats (8 wk old, 199.00 +/- 3.50 g) were subjected to identical sedentary and exercise conditioning (n = 8 each). Initially, rats swam 15 min/d (5 d/wk) in a 36 degrees C bath. The exercise protocol was gradually increased by 15 min/d until a swimming period of 1 h/d (1 wk) was attained. Thereafter, rats swam 1 h/d, 3 d/wk, for an additional period of 11 wk. Rats were sacrificed 48 h after the last training period and the blood and pancreas were collected. Circulating levels of glucose, glycosylated hemoglobin, total cholesterol, triglycerides, insulin, uric acid, IL-6, and TNF-alpha were assessed. The concentrations of proinflammatory cytokines in the pancreas were also evaluated.

RESULTS

In the diabetic ZDF (fa/fa) rats, exercise decreased hyperuricemia (-37.3%) and IL-6 and TNF-alpha levels (-16.9% and -12.7% respectively) and maintained the weight of the pancreas at near normal. Immunohistochemistry revealed a marked decrease in the expression of TNF-alpha and IL-6 in the pancreatic islet cells of ZDF (fa/fa) rats.

CONCLUSION

These results indicate that aerobic exercise is anti-inflammatory in nature.

Beta-cell failure as a complication of diabetes

Type 2 diabetes mellitus is a complex disease characterized by beta-cell failure in the setting of insulin resistance. In early stages of the disease, pancreatic beta-cells adapt to insulin resistance by increasing mass and function. As nutrient excess persists, hyperglycemia and elevated free fatty acids negatively impact beta-cell function. This happens by numerous mechanisms, including the generation of reactive oxygen species, alterations in metabolic pathways, increases in intracellular calcium and the activation of endoplasmic reticulum stress. These processes adversely affect beta-cells by impairing insulin secretion, decreasing insulin gene expression and ultimately causing apoptosis. In this review, we will first discuss the regulation of beta-cell mass during normal conditions. Then, we will discuss the mechanisms of beta-cell failure, including glucotoxicity, lipotoxicity and endoplasmic reticulum stress. Further research into mechanisms will reveal the key modulators of beta-cell failure and thus identify possible novel therapeutic targets. Type 2 diabetes mellitus is a multifactorial disease that has greatly risen in prevalence in part due to the obesity and inactivity that characterize the modern Western lifestyle. Pancreatic beta-cells possess the potential to greatly expand their function and mass in both physiologic and pathologic states of nutrient excess and increased insulin demand. beta-cell response to nutrient excess occurs by several mechanisms, including hypertrophy and proliferation of existing beta-cells, increased insulin production and secretion, and formation of new beta-cells from progenitor cells [1, 2]. Failure of pancreatic beta-cells to adequately expand in settings of increased insulin demand results in hyperglycemia and diabetes. In this review, we will first discuss the factors involved in beta-cell growth and then discuss the mechanisms by which beta-cell expansion fails and leads to beta-cell failure and diabetes (Fig. 1).

On high-frequency insulin oscillations

Insulin is released in a pulsatile manner, which results in oscillatory concentrations in blood. The oscillatory secretion improves release control and enhances the hormonal action. Insulin oscillates with a slow ultradian periodicity (approximately 140 min) and a high-frequency periodicity (approximately 6-10 min). Only the latter is reviewed in this article. At least 75% of the insulin secretion is released in a pulsatile manner. Individuals prone to developing diabetes or with overt type 2 diabetes are characterized by irregular oscillations of plasma insulin. Many factors have impact on insulin pulsatility such as age, insulin resistance and glycemic level. In addition, tiny glucose oscillations are capable of entraining insulin oscillations in healthy people in contrast to type 2 diabetic individuals emphasizing a profound disruption of the beta-cells in type 2 diabetes to sense or respond to physiological glucose excursions. A crucial question is how approximately 1,000,000 islets, each containing from a few to several thousand beta-cells, can be coordinated to secrete insulin in a pulsatile manner. This is blatantly a very complex operation to control involving an intra-pancreatic neural network, an intra-islet communication and metabolic oscillations in the beta-cell itself. Overnight beta-cell rest, e.g. during somatostatin administration, improves the disordered pulsatile insulin secretion in type 2 diabetes. Acute as well as long-term administration of sulphonylureas (SU) leads to substantial amplification (approximately 50%) of the pulsatile insulin secretion in type 2 diabetes. This is probably cardinal in terms of governing the hepatic glucose release in type 2 diabetes. Whether sulfonylureas also improve the ability of the beta-cells to sense glucose fluctuations remains to be explored. Thiazolidinediones reduce the pulsatile insulin secretion without affecting regularity, but appear to improve the ability of the beta-cell to be entrained by small glucose excursions. Finally, similar to SUs, the incretin hormone GLP-1 also results in an augmented pulsatile burst mass in both healthy and diabetic individuals, in the latter group, however, without influencing the disorderliness of pulses. This review will briefly describe the high-frequency insulin pulsatility during physiologic and pathophysiologic conditions as well as the influence of some hypoglycemic compounds on the insulin oscillations.

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.

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.

Effect of chronic rosiglitazone, metformin and glyburide treatment on beta-cell mass, function and insulin sensitivity in mZDF rats

Here we investigate the effect of rosiglitazone (RSG), metformin (MET) and glyburide (GLIB) on plasma glucose levels, beta-cell mass, function and insulin sensitivity in 10-week-old diabetic male Zucker diabetic fatty (mZDF) rats using quantitative morphometry and a mathematical model beta-cell mass, insulin and glucose kinetics (betaIG). At treatment start, 10-week-old diabetic mZDF rats were severely hyperglycaemic and had very low beta-cell function (insulin secretory capacity). RSG treatment significantly lowered plasma glucose levels in 67% of the mZDF rats. MET was effective at lowering plasma glucose levels in 33% of the mZDF rats, while GLIB was completely ineffective at lowering blood glucose levels in 10-week-old mZDF rats. RSG treatment prevented the fall in beta-cell mass after 6-8 weeks of treatment accompanied by a significant decrease in beta-cell death while MET treatment had no effect on beta-cell mass. RSG treatment increased insulin sensitivity 10-fold, increased beta-cell function fivefold and modestly increased beta-cell mass 1.4-fold. MET treatment increased insulin sensitivity fourfold, with no significant effect on beta-cell function or mass. Although RSG treatment was highly successful in lowering plasma glucose levels, the 33% of mZDF rats that did not respond to the treatment had significantly lower beta-cell function prior to treatment start compared with the responder group. Thus, the low level of beta-cell function at treatment start may explain why none of these agents were completely effective at lowering blood glucose levels in 10-week-old diabetic mZDF rats. Nevertheless, these data suggest that the preservation of beta-cell mass and improvement in beta-cell function play a role in the overall beneficial effect of RSG in 10-week-old diabetic mZDF rats.

Rosiglitazone prevents diabetes by increasing beta-cell mass in an animal model of type 2 diabetes characterized by reduced beta-cell mass at birth

AIM

Interventions that preserve or increase beta-cell mass may also prevent type 2 diabetes. Rosiglitazone prevents diabetes in people with high glucose levels who have impaired glucose tolerance and/or impaired fasting glucose. The effect of this drug on both glucose levels and beta-cell mass was studied in a rat model of diabetes, characterized by reduced beta-cell mass at birth with normoglycaemia, and progression to dysglycaemia with age.

METHODS

Female Wistar rats were given either saline (vehicle) or nicotine during pregnancy and lactation. Offspring of saline-exposed dams were given vehicle and offspring of nicotine-exposed dams were randomized to receive either vehicle or rosiglitazone starting at weaning. Beta-cell mass, proliferation and apoptosis were determined at birth and at 4 and 26 weeks of age. Glucose homeostasis was examined following sequential oral glucose tolerance tests (OGTT).

RESULTS

Rosiglitazone treatment prevented the development of dysglycaemia in nicotine-exposed animals. The ability of rosiglitazone to preserve normoglycaemia appeared to be because of its ability to increase beta-cell mass through a combination of enhanced beta-cell proliferation and decreased beta-cell apoptosis.

CONCLUSIONS

These results suggest that if rosiglitazone administration is started prior to the onset of glucometabolic abnormalities, it prevents the onset of dysglycaemia by partially restoring beta-cell mass in animals with reduced beta-cell mass at birth.

Thiazolidinediones and the preservation of beta-cell function, cellular proliferation and apoptosis

The thiazolidinediones (TZDs) or glitazones are pharmaceutical agents that have profound effects on energy expenditure and conservation. They also exert significant anti-inflammatory effects and influence cell proliferation and cell death. The drugs are primarily used in clinical practice in the treatment of patients with type 2 diabetes mellitus, a disorder of insulin resistance that occurs when the pancreatic beta-cells are unable to produce adequate amounts of insulin to maintain euglycaemia. Loss of pancreatic beta-cell function in type 2 diabetes is progressive and often precedes overt diabetes by 10 years or more, as was shown by the United Kingdom Prospective Diabetes Study. Any therapeutic or preventive approach that would limit or reverse loss of beta-cell function in diabetes would have profound effects on the morbidity associated with this widespread disease. Evidence suggesting a potential role of TZDs in preserving beta-cell function in type 2 diabetes as well as the ability of these agents to exert anti-inflammatory and proapoptotic anticancer effects, and their ability to promote cellular proliferation in various organs is reviewed.