Rosiglitazone prevents the impairment of human islet function induced by fatty acids: evidence for a role of PPARgamma2 in the modulation of insulin secretion

Suppression effects of AICAR on insulin secretion involved in peroxisome proliferator-activated receptor gamma changes in INS-1 cells

BACKGROUND

AMP-activated protein kinase (AMPK) activation is known to attenuate glucose-stimulated insulin secretion (GSIS) in pancreatic beta cells. However, the underlying mechanisms are poorly understood. The purpose of this study was to examine the effects of AMPK activation on insulin secretion and to determine whether peroxisome proliferator-activated receptors (PPAR) are involved in the effects on INS-1 cells.

METHODS

INS-1 cells, insulinoma cell lines, were treated with an activator (AICAR) or inhibitor (Compound C) of AMPK as well as inhibitors of PPAR [MK886 and biphenol A diglycidyl ether (BADGE)] for different treatment times.

RESULTS

AICAR-induced AMPK activation significantly attenuated GSIS as well as insulin content. Meanwhile, AMPK activation increased the mRNA levels of both PPARalpha and PPARgamma. However, with regard to DNA binding, AMPK activation upregulated PPARgamma only, and it was possible to reduce the increment with the AMPK inhibitor. Moreover, the AICAR-induced suppression of insulin secretion can be counteracted by the PPARgamma inhibitor, BADGE but not the PPARalpha inhibitor.

CONCLUSIONS

AICAR-induced glucose-stimulated insulin secretion reduction correlates mainly with PPARgamma changes.

High glucose suppresses human islet insulin biosynthesis by inducing miR-133a leading to decreased polypyrimidine tract binding protein-expression

Background

Prolonged periods of high glucose exposure results in human islet dysfunction in vitro. The underlying mechanisms behind this effect of high glucose are, however, unknown. The polypyrimidine tract binding protein (PTB) is required for stabilization of insulin mRNA and the PTB mRNA 3′-UTR contains binding sites for the microRNA molecules miR-133a, miR-124a and miR-146. The aim of this study was therefore to investigate whether high glucose increased the levels of these three miRNAs in association with lower PTB levels and lower insulin biosynthesis rates.

Methodology/Principal Findings

Human islets were cultured for 24 hours in the presence of low (5.6 mM) or high glucose (20 mM). Islets were also exposed to sodium palmitate or the proinflammatory cytokines IL-1β and IFN-γ, since saturated free fatty acids and cytokines also cause islet dysfunction. RNA was then isolated for real-time RT-PCR analysis of miR-133a, miR-124a, miR-146, insulin mRNA and PTB mRNA contents. Insulin biosynthesis rates were determined by radioactive labeling and immunoprecipitation. Synthetic miR-133a precursor and inhibitor were delivered to dispersed islet cells by lipofection, and PTB was analyzed by immunoblotting following culture at low or high glucose. Culture in high glucose resulted in increased islet contents of miR-133a and reduced contents of miR-146. Cytokines increased the contents of miR-146. The insulin and PTB mRNA contents were unaffected by high glucose. However, both PTB protein levels and insulin biosynthesis rates were decreased in response to high glucose. The miR-133a inhibitor prevented the high glucose-induced decrease in PTB and insulin biosynthesis, and the miR-133a precursor decreased PTB levels and insulin biosynthesis similarly to high glucose.

Conclusion

Prolonged high-glucose exposure down-regulates PTB levels and insulin biosynthesis rates in human islets by increasing miR-133a levels. We propose that this mechanism contributes to hyperglycemia-induced beta-cell dysfunction.

The global diabetes epidemic as a consequence of lifestyle-induced low-grade inflammation

The recent major increase in the global incidence of type 2 diabetes suggests that most cases of this disease are caused by changes in environment and lifestyle. All major risk factors for type 2 diabetes (overnutrition, low dietary fibre, sedentary lifestyle, sleep deprivation and depression) have been found to induce local or systemic low-grade inflammation that is usually transient or milder in individuals not at risk for type 2 diabetes. By contrast, inflammatory responses to lifestyle factors are more pronounced and prolonged in individuals at risk of type 2 diabetes and appear to occur also in the pancreatic islets. Chronic low-grade inflammation will eventually lead to overt diabetes if counter-regulatory circuits to inflammation and metabolic stress are compromised because of a genetic and/or epigenetic predisposition. Hence, it is not the lifestyle change per se but a deficient counter-regulatory response in predisposed individuals which is crucial to disease pathogenesis. Novel approaches of intervention may target these deficient defence mechanisms.

Plasma C-peptide level is inversely associated with family history of type 2 diabetes in Korean type 2 diabetic patients

Type 2 diabetes is characterized by progressive β-cell dysfunction. Family history of type 2 diabetes has been known to be associated with an increased risk for the development of the disease. However, few studies have evaluated the effects of family history of diabetes on residual β-cell function in type 2 diabetic patients. We investigated associations among family histories, clinical characteristics and plasma C-peptide levels in type 2 diabetic patients. A total of 1,350 patients with type 2 diabetes were recruited. The patients with a family history of type 2 diabetes had younger age at onset of diabetes, longer diabetes duration, higher LDL-cholesterol, and lower fasting C-peptide levels than the patients without family history. When divided according to the tertiles of diabetes duration, patients with a family history of type 2 diabetes had more decreased concentrations of fasting C-peptide as duration of diabetes increased, but patients without a family history did not. Multiple regression models were used to determine the association between fasting plasma C-peptide levels and a family history of type 2 diabetes mellitus. With adjustments for age and sex, glycated hemoglobin (HbA(1C)), fasting plasma glucose, free fatty acids, body mass index and diabetes mellitus (DM) duration, there was a significant association (P < 0.01). Our results showed that a family history of diabetes was significantly associated with the progressive decline of fasting plasma C-peptide levels in Korean type 2 diabetes mellitus.

Activation of PPARdelta up-regulates fatty acid oxidation and energy uncoupling genes of mitochondria and reduces palmitate-induced apoptosis in pancreatic beta-cells

Recent evidence indicates that decreased oxidative capacity, lipotoxicity, and mitochondrial aberrations contribute to the development of insulin resistance and type 2 diabetes. The goal of this study was to investigate the effects of peroxisome proliferator-activated receptor delta (PPARdelta) activation on lipid oxidation, mitochondrial function, and insulin secretion in pancreatic beta-cells. After HIT-T15 cells (a beta-cell line) were exposed to high concentrations of palmitate and GW501516 (GW; a selective agonist of PPARdelta), we found that administration of GW increased the expression of PPARdelta mRNA. GW-induced activation of PPARdelta up-regulated carnitine palmitoyltransferase 1 (CPT1), long-chain acyl-CoA dehydrogenase (LCAD), pyruvate dehydrogenase kinase 4 (PDK4), and uncoupling protein 2 (UCP2); alleviated mitochondrial swelling; attenuated apoptosis; and reduced basal insulin secretion induced by increased palmitate in HIT cells. These results suggest that activation of PPARdelta plays an important role in protecting pancreatic beta-cells against aberrations caused by lipotoxicity in metabolic syndrome and diabetes.

Role of glucotoxicity and lipotoxicity in the pathophysiology of Type 2 diabetes mellitus and emerging treatment strategies

Type 2 diabetes mellitus is a disease characterized by persistent and progressive deterioration of glucose tolerance. Both insulin resistance and impaired insulin secretion contribute to development of Type 2 diabetes. However, whilst insulin resistance is fully apparent in the pre-diabetic condition, impairment of insulin secretion worsens over the time, being paralleled by a progressive decline in both pancreatic B-cell function and B-cell mass. Intense research has identified a number of genetic variants that may predispose to impaired B-cell function, but such predisposition can be precipitated and worsened by toxic effects of hyperglycaemia (glucotoxicity) and elevated levels of free fatty acids (lipotoxicity). All these aspects of the pathogenesis of Type 2 diabetes are discussed in this review. Moreover, treatments that target reduction in glucotoxicity or lipotoxicity are outlined, including emerging strategies that target the role of glucagon-like peptide 1 and sodium glucose co-transporter 2.

Serine-385 phosphorylation of inwardly rectifying K+ channel subunit (Kir6.2) by AMP-dependent protein kinase plays a key role in rosiglitazone-induced closure of the K(ATP) channel and insulin secretion in rats

AIMS/HYPOTHESIS

Rosiglitazone, an insulin sensitiser, not only improves insulin sensitivity but also enhances insulin secretory capacity by ameliorating gluco- and lipotoxicity in beta cells. Rosiglitazone can stimulate insulin secretion at basal and high glucose levels via a phosphatidylinositol 3-kinase (PI3K)-dependent pathway. We hypothesised that regulation of phosphorylation of the ATP-sensitive potassium (K(ATP)) channel might serve as a key step in the regulation of insulin secretion.

METHODS

Insulin secretory responses were studied in an isolated pancreas perfusion system, cultured rat islets and MIN6 and RINm5F beta cells. Signal transduction pathways downstream of PI3K were explored to link rosiglitazone to K(ATP) channel conductance with patch clamp techniques and insulin secretion measured by ELISA.

RESULTS

Rosiglitazone stimulated AMP-activated protein kinase (AMPK) activity and induced inhibition of the K(ATP) channel conductance in islet beta cells; both effects were blocked by the PI3K inhibitor LY294002. Following stimulation of AMPK by 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR), a pharmacological activator, both AICAR-stimulated insulin secretion and inhibition of K(ATP) channel conductance were unaffected by LY294002, indicating that AMPK activation occurs at a site downstream of PI3K activity. The serine residue at amino acid position 385 of Kir6.2 was found to be the substrate phosphorylation site of AMPK when activated by rosiglitazone or AICAR.

CONCLUSIONS/INTERPRETATION

Our data indicate that PI3K-dependent activation of AMPK is required for rosiglitazone-stimulated insulin secretion in pancreatic beta cells. Phosphorylation of the Ser(385) residue of the Kir6.2 subunit of the K(ATP) channel by AMPK may play a role in insulin secretion.

Rosiglitazone aggravates nonalcoholic Fatty pancreatic disease in C57BL/6 mice fed high-fat and high-sucrose diet

OBJECTIVES

Evaluate the effect of fenofibrate, bezafibrate, and rosiglitazone on nonalcoholic fatty pancreatic disease and islet peroxisome proliferator-activated receptor-alpha (PPAR-alpha) and PPAR-beta immunostain in mice fed high-fat high-sucrose (HFHS) diet.

METHODS

Two-month-old male mice were fed standard chow (n = 10) or HFHS chow (n = 40) for 6 weeks. Afterward, HFHS mice were grouped by treatment: untreated HFHS and HFHS treated with rosiglitazone (HFHS-Ro), fenofibrate (HFHS-Fe), or bezafibrate (HFHS-Bz). Medications were administered for 5 weeks. After treatment, the pancreas was removed and analyzed by morphometry, stereology, and immunohistochemistry.

RESULTS

The HFHS-fed mice showed altered fasting glucose (+33%) and insulin (+138%); increased body (+20%) and pancreas (+28%) masses, pancreatic fat (+700%), islet hypertrophy (+38%); and decreased GLUT2 immunostain (-60%). Rosiglitazone reduced fasting glucose and insulin but induced weight gain. Fibrates impeded weight gain, but only bezafibrate prevented islet hypertrophy. The GLUT2 stain was improved in all treatments, and there were no alterations in PPAR-alpha. There were morphological signs of pancreatitis with fenofibrate, although there were no alterations in amylase and lipase. Rosiglitazone exacerbated pancreatic fat infiltration (+75% vs HFHS group), and bezafibrate increased PPAR-beta expression in pancreatic islets.

CONCLUSIONS

Rosiglitazone is shown for the first time to exacerbate pancreatic fat infiltration; therefore, precaution has to be taken when rosiglitazone is prescribed to obese patients.

Inhibition of human insulin gene transcription by peroxisome proliferator-activated receptor gamma and thiazolidinedione oral antidiabetic drugs

BACKGROUND AND PURPOSE

The transcription factor peroxisome proliferator-activated receptor gamma (PPARgamma) is essential for glucose homeostasis. PPARgamma ligands reducing insulin levels in vivo are used as drugs to treat type 2 diabetes mellitus. Genes regulated by PPARgamma have been found in several tissues including insulin-producing pancreatic islet beta-cells. However, the role of PPARgamma at the insulin gene was unknown. Therefore, the effect of PPARgamma and PPARgamma ligands like rosiglitazone on insulin gene transcription was investigated.

EXPERIMENTAL APPROACH

Reporter gene assays were used in the beta-cell line HIT and in primary mature pancreatic islets of transgenic mice. Mapping studies and internal mutations were carried out to locate PPARgamma-responsive promoter regions.

KEY RESULTS

Rosiglitazone caused a PPARgamma-dependent inhibition of insulin gene transcription in a beta-cell line. This inhibition was concentration-dependent and had an EC(50) similar to that for the activation of a reporter gene under the control of multimerized PPAR binding sites. Also in normal primary pancreatic islets of transgenic mice, known to express high levels of PPARgamma, rosiglitazone inhibited glucose-stimulated insulin gene transcription. Transactivation and mapping experiments suggest that, in contrast to the rat glucagon gene, the inhibition of the human insulin gene promoter by PPARgamma/rosiglitazone does not depend on promoter-bound Pax6 and is attributable to the proximal insulin gene promoter region around the transcription start site from -56 to +18.

CONCLUSIONS AND IMPLICATIONS

The human insulin gene represents a novel PPARgamma target that may contribute to the action of thiazolidinediones in type 2 diabetes mellitus.

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.

Pioglitazone attenuates fatty acid-induced oxidative stress and apoptosis in pancreatic beta-cells

AIMS

Thiazolidinediones (TZDs), ligands for peroxisome proliferator-activated receptor gamma, are antidiabetic agents that improve hyperglycemia by decreasing insulin resistance in obese diabetic animal models and patients with type 2 diabetes. We have studied whether pioglitazone, a TZD, can exert a direct effect against pancreatic beta-cell lipoapoptosis.

METHODS

MIN6 cells were cultured in medium containing either 5.6 (low glucose) or 25 mM glucose (high glucose) in the presence or absence of 0.5 mM palmitate for 48 h. We examined the effect of 10 microM pioglitazone on MIN6 cells on glucose-stimulated insulin secretion, cellular ATP, uncoupling protein-2 (UCP-2) mRNA expression, intracellular triglyceride content, reactive oxygen species production, the number of apoptotic cells and nuclear factor-kappaB (NF-kappaB) activity.

RESULTS

Pioglitazone recovered partly impaired glucose-stimulated insulin secretion and cellular ATP in MIN6 cell exposed to high glucose with 0.5 mM palmitate. Pioglitazone suppressed intracellular triglyceride accumulation in cells exposed to high glucose with 0.5 mM palmitate. Palmitate-induced upregulation of UCP-2 mRNA levels was suppressed by pioglitazone in a dose-dependent manner. Pioglitazone decreased palmitate-induced reactive oxygen species production in MIN6 cells by 24% and in mouse islet cells by 53%. Pioglitazone also decreased palmitate-induced NF-kappaB activity by 40% and protected beta-cells from palmitate-induced apoptosis by 22% in MIN6 cell.

CONCLUSIONS

Pioglitazone attenuated fatty acid-induced oxidative stress and apoptosis in pancreatic beta-cells. TZDs might be used as a mean for maintaining beta-cell survival and preserving capacity of insulin secretion in patients with diabetes mellitus.

Beta-cell failure in type 2 diabetes mellitus

Diabetes mellitus has been defined as a "group of metabolic diseases characterized by hyperglycemia resulting from defects in insulin secretion, insulin action, or both" and encompasses a wide range of heterogeneous conditions. Common type 2 diabetes mellitus (T2DM) results from a combination of genetic and acquired factors. However, lifestyle factors, particularly overeating and physical inactivity, are the major clinical determinants of T2DM. Insulin resistance is a common feature of T2DM, but it is unlikely to cause T2DM unless progressive loss of beta-cell function develops. Significant reduction in beta-cell function is already present at the time of T2DM diagnosis, and it continuously declines irrespective of treatment. As such, the progressive loss of beta-cell function dictates the rate of worsened glycemic control. Development of progressive deterioration accelerates via gluco- and lipotoxicity, loss of beta-cell function, and shrinkage of beta-cell mass. Understanding the causes for beta-cell failure is therefore of capital importance to develop new and more effective therapeutic strategies.

Palmitate-induced beta-cell dysfunction is associated with excessive NO production and is reversed by thiazolidinedione-mediated inhibition of GPR40 transduction mechanisms

Background

Type 2 diabetes often displays hyperlipidemia. We examined palmitate effects on pancreatic islet function in relation to FFA receptor GPR40, NO generation, insulin release, and the PPARγ agonistic thiazolidinedione, rosiglitazone.

Principal Findings

Rosiglitazone suppressed acute palmitate-stimulated GPR40-transduced PI hydrolysis in HEK293 cells and insulin release from MIN6c cells and mouse islets. Culturing islets 24 h with palmitate at 5 mmol/l glucose induced β-cell iNOS expression as revealed by confocal microscopy and increased the activities of ncNOS and iNOS associated with suppression of glucose-stimulated insulin response. Rosiglitazone reversed these effects. The expression of iNOS after high-glucose culturing was unaffected by rosiglitazone. Downregulation of GPR40 by antisense treatment abrogated GPR40 expression and suppressed palmitate-induced iNOS activity and insulin release.

Conclusion

We conclude that, in addition to mediating acute FFA-stimulated insulin release, GPR40 is an important regulator of iNOS expression and dysfunctional insulin release during long-term exposure to FFA. The adverse effects of palmitate were counteracted by rosiglitazone at GPR40, suggesting that thiazolidinediones are beneficial for β-cell function in hyperlipidemic type 2 diabetes.

An overview of pancreatic beta-cell defects in human type 2 diabetes: Implications for treatment

Type 2 diabetes is the most common form of diabetes in humans. It results from a combination of factors that impair beta-cell function and tissue insulin sensitivity. However, growing evidence is showing that the beta-cell is central to the development and progression of this form of diabetes. Reduced islet and/or insulin-containing cell mass or volume in Type 2 diabetes has been reported by several authors. Furthermore, studies with isolated Type 2 diabetic islets have consistently shown both quantitative and qualitative defects of glucose-stimulated insulin secretion. The impact of genotype in affecting beta-cell function and survival is a very fast growing field or research, and several gene polymorphisms have been associated with this form of diabetes. Among acquired factors, glucotoxicity, lipotoxicity and altered IAPP processing are likely to play an important role. Interestingly, however, pharmacological intervention can improve several defects of Type 2 diabetes islet cells in vitro, suggesting that progression of the disease might not be relentless.

Protection of pancreatic beta-cells: is it feasible?

Hyperglycemia, which is the biochemical hallmark of type 2 diabetes, mainly results from insulin resistance and beta-cell dysfunction. However, the latter is crucial in the development of the disease because diabetes cannot occur without an impairment of insulin secretion. Beta-cell failure is also responsible for progressive loss of metabolic control in type 2 diabetic patients and the eventual need for insulin treatment. An impairment of beta-cell function can be detected in several ways and can be observed already in pre-diabetic individuals. Histopathology studies documented that beta-cell volume is reduced in pre-diabetes and, to a greater extent, in type 2 diabetes mainly because the apoptotic rate of beta-cells is increased whereas neogenesis is intact. All anti-diabetic agents can improve, directly or indirectly, beta-cell function. However, only PPAR-gamma agonists and incretin-mimetic agents seem to have favorable effects on beta-cell morphology and volume. Many trials showed that type 2 diabetes can be prevented but few of them directly addressed the issue of beta-cell protection by the intervention used in the study. It is reasonable to conclude that in these trials diabetes prevention, which was based on the use of lifestyle changes (diet and/or exercise) or different drugs (tolbutamide, acarbose, metformin, glitazones, bezafibrate, orlistat, angiotensin converting enzyme inhibitors, angiotensin II receptor blockers or pravastatin), depended also, or mainly, on a protection of the beta-cells but in most studies data on insulin secretion are not available or are insufficient to draw firm conclusions. The mechanisms of beta-cell protection in these trials, if any, remain unknown. They could be various and likely included reduced glucotoxicity, lipotoxicity, insulin resistance, inflammation, oxidant stress and/or apoptosis, an amelioration of islet blood flow and/or favorable changes in cation balance within the islets. Contrasting the decline and the eventual failure of beta-cells is crucial in preventing type 2 diabetes as well as in changing the natural history of the disease, when it occurs. The protection can be achieved in several ways but any strategy should include a change in lifestyle in order to generate a healthier islet milieu. Among anti-diabetic drugs, PPAR-gamma agonists and incretin-mimetic agents are the most promising in the protection. Among other drugs, inhibitors of the renin-angiotensin system might play a significant role. The increased worldwide diffusion of type 2 diabetes and the progressive loss of metabolic control in affected patients are clear demonstrations that the strategies to protect the beta-cells implemented so far, if any, were largely inadequate. Anti-diabetic agents targeting the intimate mechanisms of beta-cell damage might change the scenario in the near future.

The importance of beta-cell management in type 2 diabetes

Despite intervention with effective oral glucose-lowering agents, most patients with type 2 diabetes will experience a gradual loss of glycaemic control. Irrespective of underlying levels of insulin resistance, the progressive failure and loss of beta-cells are ultimately responsible for the onset of frank type 2 diabetes. The mechanisms responsible for loss of beta-cell function are likely to be multifactorial, but may involve toxicity because of elevated glucose and/or lipid levels, increased secretory demand because of insulin resistance, amyloid deposition and altered levels of cytokines. Preservation of beta-cell function is now gaining recognition as a critical target in the management of type 2 diabetes. For patients with frank type 2 diabetes, preservation of beta-cell function has the potential to reduce or stabilise the progression of type 2 diabetes and to decrease the need for additional oral glucose-lowering agents and/or insulin therapy. There is a growing body of animal/preclinical evidence for improved and preserved beta-cell function with current glucose-lowering agents, such as the thiazolidinediones, metformin and the glucagon-like peptide-1 analogue, exenatide. Clinical studies incorporating indirect measures of beta-cell function also support a protective effect with some agents. A number of novel therapies that are currently under investigation may also offer beta-cell structural and functional protection, including dipeptidyl peptidase IV inhibitors and cannabinoid receptor type 1 blockers. Emerging evidence from interventional trials suggests that both intensive lifestyle changes and pharmacotherapy can delay or possibly prevent the onset of type 2 diabetes in high-risk individuals. For patients newly diagnosed with type 2 diabetes, early and aggressive intervention strategies that combine maximal glucose-lowering efficacy alongside potential beta-cell preserving properties may provide an opportunity to delay or prevent progression of the disease.

beta-cell function and anti-diabetic pharmacotherapy

Type 2 diabetes is a chronic disease characterized by progressive worsening of glycaemic control as indicated by the United Kingdom Prospective Diabetes Study (UKPDS). The progressive nature of the disease is mainly due to continuous loss of beta-cell mass and function. Though much of this loss is due to intrinsic defects of the beta-cell several factors may accelerate such process. These include the metabolic environment where hyperglycaemia and increased circulating free-fatty acid exert a toxic effect on the beta-cell. Therefore, tight metabolic control may prevent not only the risk of long-term diabetic complication but also preserve beta-cell function. Several therapeutic agents are currently used for treatment of type 2 diabetic patients. However, their effect on maintenance of beta-cell function has not been yet systematically reviewed. By literature searching we have then analysed in detail the effect of sulfonylureas and non-sulfonylureic secretagogues, incretin-mimetics, insulin sensitizers, alpha-glucosidase inhibitors, and insulin on beta-cell function. Moreover, promising future approaches aiming at preserving beta-cell function and mass are discussed.

Comparison of the alteration of the concentration of C-peptide in 24-h urine according to the combination patterns of hypoglycemic agents in type 2 diabetes patients

AIMS

Urinary C-peptide (UCP) has been considered as a simple method for monitoring beta-cell function in diabetic patients clinically. The aim of the study is to compare the changes of 24-h urinary C-peptide levels according to subgroups divided by therapeutic agents for subjects with type 2 diabetes.

METHODS

In 206 participants, under treatment for type 2 diabetes, 24-h urinary C-peptide levels were assessed yearly for 3 years. All participants were subdivided into four groups according to the therapeutic agents. Changes for the measured values during the follow-up were compared between groups.

RESULTS

Mean HbA1C was 7.1% and mean 24-h UCP was 61.7 microg/24h and mean duration of diabetes was 8.7 years in all subjects at baseline. Mean 24-h UCP levels increased significantly from a baseline to at 36 months in the insulin sensitizers (IS) only group, in the IS plus sulfonylurea combination group and in IS plus insulin combination group (p<0.001), whereas in the sulfonylurea only group, we could not find statistically significant changes (p=0.152). Treatment with only IS significantly reduced fasting plasma glucose (FPG) and HbA1C level (p=0.045, p<0.001). Differences between baseline and last 24-h UCP were significantly different between-groups and this difference was more significant after adjustment in FPG, HbA1C, and the duration of diabetes (p=0.024). Especially, IS plus sulfonylurea combination group resulted in greatest increase of 24-h UCP (DeltaC-peptide=51.19 microg/24h).

CONCLUSIONS

This study suggested that IS, in mono- or in combination, significantly improved pancreatic beta-cell function, especially when combined with sulfonylurea as evidenced by the increase of 24-h UCP.

Beta-cell preservation with thiazolidinediones

Progressive beta-cell dysfunction and beta-cell failure are fundamental pathogenic features of type 2 diabetes. Ultimately, the development and continued progression of diabetes is a consequence of the failure of the beta-cell to overcome insulin resistance. Strategies that aim to prevent diabetes must, therefore, ultimately aim to stabilize the progressive decline of the beta-cell. Clinical study evidence from several sources now suggests that thiazolidinediones (TZDs) have profound effects on the beta-cell, such as improving insulin secretory capacity, preserving beta-cell mass and islet structure and protecting beta-cells from oxidative stress, as well as improving measures of beta-cell function, such as insulinogenic index and homeostasis model assessment of beta-cell function (HOMA-%B). Furthermore, intervention studies suggest that TZDs have the potential to delay, stabilize and possibly even prevent the onset on diabetes in high-risk individuals, and these effects appear to accompany improvements in beta-cell function. Here, we review the evidence, from in vitro studies to large intervention trials, for the effects of TZDs on beta-cell function and the consequences for glucose-lowering therapy.

beta-cell failure in diabetes and preservation by clinical treatment

There is a progressive deterioration in beta-cell function and mass in type 2 diabetics. It was found that islet function was about 50% of normal at the time of diagnosis, and a reduction in beta-cell mass of about 60% was shown at necropsy. The reduction of beta-cell mass is attributable to accelerated apoptosis. The major factors for progressive loss of beta-cell function and mass are glucotoxicity, lipotoxicity, proinflammatory cytokines, leptin, and islet cell amyloid. Impaired beta-cell function and possibly beta-cell mass appear to be reversible, particularly at early stages of the disease where the limiting threshold for reversibility of decreased beta-cell mass has probably not been passed. Among the interventions to preserve or "rejuvenate" beta-cells, short-term intensive insulin therapy of newly diagnosed type 2 diabetes will improve beta-cell function, usually leading to a temporary remission time. Another intervention is the induction of beta-cell "rest" by selective activation of ATP-sensitive K+ (K(ATP)) channels, using drugs such as diazoxide. A third type of intervention is the use of antiapoptotic drugs, such as the thiazolidinediones (TZDs), and incretin mimetics and enhancers, which have demonstrated significant clinical evidence of effects on human beta-cell function. The TZDs improve insulin secretory capacity, decrease beta-cell apoptosis, and reduce islet cell amyloid with maintenance of neogenesis. The TZDs have indirect effects on beta-cells by being insulin sensitizers. The direct effects are via peroxisome proliferator-activated receptor gamma activation in pancreatic islets, with TZDs consistently improving basal beta-cell function. These beneficial effects are sustained in some individuals with time. There are several trials on prevention of diabetes with TZDs. Incretin hormones, which are released from the gastrointestinal tract in response to nutrient ingestion to enhance glucose-dependent insulin secretion from the pancreas, aid the overall maintenance of glucose homeostasis through slowing of gastric emptying, inhibition of glucagon secretion, and control of body weight. From the two major incretins, glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP), only the first one or its mimetics or enhancers can be used for treatment because the diabetic beta-cell is resistant to GIP action. Because of the rapid inactivation of GLP-1 by dipeptidyl peptidase (DPP)-IV, several incretin analogs were developed: GLP-1 receptor agonists (incretin mimetics) exenatide (synthetic exendin-4) and liraglutide, by conjugation of GLP-1 to circulating albumin. The acute effect of GLP-1 and GLP-1 receptor agonists on beta-cells is stimulation of glucose-dependent insulin release, followed by enhancement of insulin biosynthesis and stimulation of insulin gene transcription. The chronic action is stimulating beta-cell proliferation, induction of islet neogenesis, and inhibition of beta-cell apoptosis, thus promoting expansion of beta-cell mass, as observed in rodent diabetes and in cultured beta-cells. Exenatide and liraglutide enhanced postprandial beta-cell function. The inhibition of the activity of the DPP-IV enzyme enhances endogenous GLP-1 action in vivo, mediated not only by GLP-1 but also by other mediators. In preclinical studies, oral active DPP-IV inhibitors (sitagliptin and vildagliptin) also promoted beta-cell proliferation, neogenesis, and inhibition of apoptosis in rodents. Meal tolerance tests showed improvement in postprandial beta-cell function. Obviously, it is difficult to estimate the protective effects of incretin mimetics and enhancers on beta-cells in humans, and there is no clinical evidence that these drugs really have protective effects on beta-cells.

Thiazolidinediones improve beta-cell function in type 2 diabetic patients

Thiazolidinediones (TZDs) improve glycemic control and insulin sensitivity in patients with type 2 diabetes mellitus (T2DM). There is growing evidence from in vivo and in vitro studies that TZDs improve pancreatic beta-cell function. The aim of this study was to determine whether TZD-induced improvement in glycemic control is associated with improved beta-cell function. We studied 11 normal glucose-tolerant and 53 T2DM subjects [age 53+/-2 yr; BMI 29.4+/-0.8 kg/m2; fasting plasma glucose (FPG) 10.3+/-0.4 mM; Hb A1c 8.2+/-0.3%]. Diabetic patients were randomized to receive placebo or TZD for 4 mo. Subjects received 1) 2-h OGTT with determination of plasma glucose, insulin, and C-peptide concentrations and 2) two-step euglycemic insulin (40 and 160 mU.m-2.min-1) clamp with [3-(3)H]glucose. T2DM patients were then randomized to receive 4 mo of treatment with pioglitazone (45 mg/day), rosiglitazone (8 mg/day), or placebo. Pioglitazone and rosiglitazone similarly improved FPG, mean plasma glucose during OGTT, Hb A1c, and insulin-mediated total body glucose disposal (Rd) and decreased mean plasma FFA during OGTT (all P<0.01, ANOVA). The insulin secretion/insulin resistance (disposition) index [DeltaISR(AUC)/Deltaglucose(AUC)/IR] was significantly improved in all TZD-treated groups: +1.8+/-0.7 (PIO+drug-naïve diabetics), +0.7+/-0.3 (PIO+sulfonylurea-treated diabetics), and 0.7+/-0.2 (ROSI+sulfonylurea-withdrawn diabetics) vs. -0.2+/-0.3 in the two placebo groups (P<0.01, all TZDs vs. placebo, ANOVA). Improved insulin secretion correlated positively with increased body weight, fat mass, and Rd and inversely with decreased plasma glucose and FFA during the OGTT. In T2DM patients, TZD treatment leads to improved beta-cell function, which correlates strongly with improved glycemic control.

Review: Peroxisome proliferator-activated receptor gamma and adipose tissue--understanding obesity-related changes in regulation of lipid and glucose metabolism

CONTEXT

Adipose tissue is a metabolically dynamic organ, serving as a buffer to control fatty acid flux and a regulator of endocrine function. In obese subjects, and those with type 2 diabetes or the metabolic syndrome, adipose tissue function is altered (i.e. adipocytes display morphological differences alongside aberrant endocrine and metabolic function and low-grade inflammation).

EVIDENCE ACQUISITION

Articles on the role of peroxisome proliferator-activated receptor gamma (PPARgamma) in adipose tissue of healthy individuals and those with obesity, metabolic syndrome, or type 2 diabetes were sourced using MEDLINE (1990-2006).

EVIDENCE SYNTHESIS

Articles were assessed to provide a comprehensive overview of how PPARgamma-activating ligands improve adipose tissue function, and how this links to improvements in insulin resistance and the progression to type 2 diabetes and atherosclerosis.

CONCLUSIONS

PPARgamma is highly expressed in adipose tissue, where its activation with thiazolidinediones alters fat topography and adipocyte phenotype and up-regulates genes involved in fatty acid metabolism and triglyceride storage. Furthermore, PPARgamma activation is associated with potentially beneficial effects on the expression and secretion of a range of factors, including adiponectin, resistin, IL-6, TNFalpha, plasminogen activator inhibitor-1, monocyte chemoattractant protein-1, and angiotensinogen, as well as a reduction in plasma nonesterified fatty acid supply. The effects of PPARgamma also extend to macrophages, where they suppress production of inflammatory mediators. As such, PPARgamma activation appears to have a beneficial effect on the relationship between the macrophage and adipocyte that is distorted in obesity. Thus, PPARgamma-activating ligands improve adipose tissue function and may have a role in preventing progression of insulin resistance to diabetes and endothelial dysfunction to atherosclerosis.

Dual PPAR α/γ Agonists: Promises and Pitfalls in Type 2 Diabetes

Type 2 diabetes mellitus is a disease of complex pathogenesis and pleiotropic clinical manifestations. The greatest clinical challenge in this disease is the prevention of the long-term complications, many of which involve cardiovascular outcomes. The peroxisome proliferator-activated receptor (PPAR) alpha and gamma isoforms of the family of nuclear transcription factors are pharmaceutical targets for therapeutic intervention because they can potentially ameliorate not only the hyperglycemia of diabetes, but also the dyslipidemia that is characteristic of this disorder (low high-density lipoprotein cholesterol, high triglycerides, small, dense low-density lipoprotein particles). Novel drugs with dual PPAR alpha and gamma activity have been under clinical development for type 2 diabetes, and they have shown promise in early studies with regard to glucose lowering and improved lipid profile when compared with the PPAR-gamma-specific thiazolidinediones. Unfortunately, the dual PPARs available to date have some of the PPAR-gamma-associated side effect profile, including fluid retention and weight gain, which have limited the further clinical development of higher doses that show improved efficacy. This review will briefly summarize our understanding of the pathogenesis of type 2 diabetes, the role of the PPAR family of receptors, and the potential for clinical use of this novel emerging class of agents that serve as dual activators of both PPAR-alpha and PPAR-gamma.

Pioglitazone acutely influences glucose-sensitive insulin secretion in normal and diabetic human islets

We have studied acute effects of the PPARgamma agonist pioglitazone in vitro on human islets from both non-diabetic and type 2 diabetic subjects. In 5 mM glucose, pioglitazone caused a transient increase in insulin secretion in non-diabetic, but not diabetic, islets. Continuous presence of the drug suppressed insulin release in both non-diabetic and diabetic islets. In islets from non-diabetic subjects, both high glucose and tolbutamide-stimulated insulin secretion was inhibited by pioglitazone. When islets were continuously perifused with 5 mM glucose, short-term pretreatment with pioglitazone caused approximately 2-fold increase in insulin secretion after drug withdrawal. Pioglitazone pretreatment of diabetic islets restored their glucose sensitivity. Examination of cytosolic free Ca(2+) concentration ([Ca(2+)](i)) in non-diabetic islets revealed slight Ca(2+) transient by pioglitazone at 3 mM glucose with no significant changes at high glucose. Our data suggest that short-term pretreatment with pioglitazone primes both healthy and diabetic human islets for enhanced glucose-sensitive insulin secretion.

Nutrient control of insulin secretion in isolated normal human islets

Pancreatic islets were isolated from 16 nondiabetic organ donors and, after culture for approximately 2 days in 5 mmol/l glucose, were perifused to characterize nutrient-induced insulin secretion in human islets. Stepwise increases from 0 to 30 mmol/l glucose (eight 30-min steps) evoked concentration-dependent insulin secretion with a threshold at 3-4 mmol/l glucose, K(m) at 6.5 mmol/l glucose, and V(max) at 15 mmol/l glucose. An increase from 1 to 15 mmol/l glucose induced biphasic insulin secretion with a prominent first phase (peak increase of approximately 18-fold) and a sustained, flat second phase ( approximately 10-fold increase), which were both potentiated by forskolin. The central role of ATP-sensitive K(+) channels in the response to glucose was established by abrogation of insulin secretion by diazoxide and reversible restoration by tolbutamide. Depolarization with tolbutamide or KCl (plus diazoxide) triggered rapid insulin secretion in 1 mmol/l glucose. Subsequent application of 15 mmol/l glucose further increased insulin secretion, showing that the amplifying pathway is operative. In control medium, glutamine alone was ineffective, but its combination with leucine or nonmetabolized 2-amino-bicyclo [2,2,1]-heptane-2-carboxylic acid (BCH) evoked rapid insulin secretion. The effect of BCH was larger in low glucose than in high glucose. In contrast, the insulin secretion response to arginine or a mixture of four amino acids was potentiated by glucose or tolbutamide. Palmitate slightly augmented insulin secretion only at the supraphysiological palmitate-to-albumin ratio of 5. Inosine and membrane-permeant analogs of pyruvate, glutamate, or succinate increased insulin secretion in 3 and 10 mmol/l glucose, whereas lactate and pyruvate had no effect. In conclusion, nutrient-induced insulin secretion in normal human islets is larger than often reported. Its characteristics are globally similar to those of insulin secretion by rodent islets, with both triggering and amplifying pathways. The pattern of the biphasic response to glucose is superimposable on that in mouse islets, but the concentration-response curve is shifted to the left, and various nutrients, in particular amino acids, influence insulin secretion within the physiological range of glucose concentrations.

Effect of rosiglitazone on the frequency of diabetes in patients with impaired glucose tolerance or impaired fasting glucose: a randomised controlled trial

BACKGROUND

Rosiglitazone is a thiazolidinedione that reduces insulin resistance and might preserve insulin secretion. The aim of this study was to assess prospectively the drug's ability to prevent type 2 diabetes in individuals at high risk of developing the condition.

METHODS

5269 adults aged 30 years or more with impaired fasting glucose or impaired glucose tolerance, or both, and no previous cardiovascular disease were recruited from 191 sites in 21 countries and randomly assigned to receive rosiglitazone (8 mg daily; n=2365) or placebo (2634) and followed for a median of 3 years. The primary outcome was a composite of incident diabetes or death. Analyses were done by intention to treat. This trial is registered at ClinicalTrials.gov, number NCT00095654.

FINDINGS

At the end of study, 59 individuals had dropped out from the rosiglitazone group and 46 from the placebo group. 306 (11.6%) individuals given rosiglitazone and 686 (26.0%) given placebo developed the composite primary outcome (hazard ratio 0.40, 95% CI 0.35-0.46; p<0.0001); 1330 (50.5%) individuals in the rosiglitazone group and 798 (30.3%) in the placebo group became normoglycaemic (1.71, 1.57-1.87; p<0.0001). Cardiovascular event rates were much the same in both groups, although 14 (0.5%) participants in the rosiglitazone group and two (0.1%) in the placebo group developed heart failure (p=0.01).

INTERPRETATION

Rosiglitazone at 8 mg daily for 3 years substantially reduces incident type 2 diabetes and increases the likelihood of regression to normoglycaemia in adults with impaired fasting glucose or impaired glucose tolerance, or both.

The metabolic syndrome: evolving evidence that thiazolidinediones provide rational therapy

The metabolic syndrome, also known as the dysmetabolic syndrome, syndrome X or the insulin resistance syndrome, refers to the clustering of cardiovascular disease risk factors that are present in many individuals who are at increased risk for both cardiovascular events and type 2 diabetes. Prediabetic subjects typically exhibit an atherogenic pattern of cardiovascular risks that is associated with hyperinsulinaemia. Thus, identification of components of the metabolic syndrome is important if patients are to be treated early enough to prevent cardiovascular events and other complications related to diabetes. Therapies targeted to specific components of the metabolic syndrome such as improving glycaemic control, managing dyslipidaemia and reducing the prothrombotic state should help to minimize cardiovascular risk, particularly if initiated early. Traditional pharmacologic agents used to manage the individual components of the metabolic syndrome do not typically impact the other components. The thiazolidinediones, a new class of agents that improve insulin resistance, have the ability, in addition to their glucose-lowering effects, to exert several powerful anti-atherogenic properties, including anti-inflammatory effects in the vascular endothelium, redistribution of visceral fat and reduction of insulin resistance, hyperinsulinaemia and hyperproinsulinaemia. This makes the thiazolidinediones ideal candidates for the early treatment of many components associated with the metabolic syndrome.

The PPARalpha/gamma dual agonist chiglitazar improves insulin resistance and dyslipidemia in MSG obese rats

1. The aim of this study was to investigate the capacity of chiglitazar to improve insulin resistance and dyslipidemia in monosodium L-glutamate (MSG) obese rats and to determine whether its lipid-lowering effect is mediated through its activation of PPARalpha. 2. Chiglitazar is a PPARalpha/gamma dual agonist. 3. The compound improved impaired insulin and glucose tolerance; decreased plasma insulin level and increased the insulin sensitivity index and decreased HOMA index. Euglycemic hyperinsulinemic clamp studies showed chiglitazar increased the glucose infusion rate in MSG obese rats. 4. Chiglitazar inhibited alanine gluconeogenesis, lowered the hepatic glycogen level in MSG obese rats. Like rosiglitazone, chiglitazar promoted the differentiation of adipocytes and decreased the maximal diameter of adipocytes. In addition, chiglitazar decreased the fibrosis and lipid accumulation in the islets and increased the size of islets. 5. Chiglitazar reduced plasma triglyceride, total cholesterol (TCHO), nonesterified fatty acids (NEFA) and low density lipoprotein-cholesterol levels; lowered hepatic triglyceride and TCHO contents; decreased muscular NEFA level. Unlike rosiglitazone, chiglitazar showed significant increase of mRNA expression of PPARalpha, CPT1, BIFEZ, ACO and CYP4A10 in the liver of MSG obese rats. 6. These data suggest that PPARalpha/gamma coagonist, such as chiglitazar, affect lipid homeostasis with different mechanisms from rosiglitazone, chiglitazar may have better effects on lipid homeostasis in diabetic patients than selective PPARgamma agonists.

Contribution of adipocyte-derived factors to beta-cell dysfunction in diabetes

In addition to serving as an energy reservoir, the adipocyte has been characterized as an endocrine cell, secreting many bioactive factors which influence energy homeostasis. Being overweight, with excessive adipose tissue, is considered to be part of the pathogenesis of type 2 diabetes. Insulin resistance and beta-cell dysfunction are two major pathophysiological changes seen in type 2 diabetes. In addition to inducing insulin resistance in insulin-responsive tissues, adipocyte-derived factors play an important role in the pathogenesis of beta-cell dysfunction. Leptin, free fatty acids, adiponectin, tumor necrosis factor-alpha and interleukin-6 are all produced and secreted by adipocytes, and may directly influence aspects of beta-cell function, including insulin synthesis and secretion, insulin cell survival and apoptosis. During the progression from normal weight to obesity and on to overt diabetes, the adipocyte-derived factors contribute to the occurrence and development of beta-cell dysfunction and type 2 diabetes.

Regulation of PDK mRNA by high fatty acid and glucose in pancreatic islets

Pyruvate dehydrogenase (PDH) converts pyruvate to acetyl-CoA, links glycolysis to the Krebs cycle, and plays an important role in glucose metabolism and insulin secretion in pancreatic beta cells. In beta cells from obese and Type 2 diabetic animals, PDH activity is significantly reduced. PDH is negatively regulated by multiple pyruvate dehydrogenase kinase (PDK) isotypes (PDK subtypes 1-4). However, we do not know whether fatty acids or high glucose modulate PDKs in islets. To test this we determined PDH and PDK activities and PDK gene and protein expression in C57BL/6 mouse islets. Both high palmitate and high glucose reduced active PDH activity and increased PDK activity. The gene and protein for PDK3 were not expressed in islets. Palmitate up-regulated mRNA expression of PDK1 (2.9-fold), PDK2 (1.9-fold), and PDK4 (3.1-fold). High glucose increased PDK1 (1.8-fold) and PDK2 (2.7-fold) mRNA expression but reduced PDK4 mRNA expression by 40 percent in cultured islets. Changed PDK expression was confirmed by Western blotting. These results demonstrate that in islet cells both fat and glucose regulate PDK gene and protein expression and indicate that hyperglycemia and hyperlipidemia contribute to the decline in diabetic islet PDH activity by increasing mRNA and protein expression of PDK.

Effects of rosiglitazone and metformin on pancreatic beta cell gene expression

AIMS/HYPOTHESIS

Rosiglitazone and metformin are two oral antihyperglycaemic drugs used to treat type 2 diabetes. While both drugs have been shown to improve insulin-sensitive glucose uptake, the direct effects of these drugs on pancreatic beta cells is only now beginning to be clarified. The aim of the present study was to determine the direct effects of these agents on beta cell gene expression.

METHODS

We used reporter gene analysis to examine the effects of rosiglitazone and metformin on the activity of the proinsulin and insulin promoter factor 1 (IPF1) gene promoters in the glucose-responsive mouse beta cell line Min6. Western blot and gel retardation analyses were used to examine the effects of both drugs on the regulation of IPF1 protein production, nuclear accumulation and DNA binding activity in both Min6 cells and isolated rat islets of Langerhans.

RESULTS

Over 24 h, rosiglitazone promoted the nuclear accumulation of IPF1 and forkhead homeobox A2 (FOXA2), independently of glucose concentration, and stimulated a two-fold increase in the activity of the Ipf1 gene promoter (p<0.01). Stimulation of the Ipf1 promoter by rosiglitazone was unaffected by the presence of the peroxisome proliferator activated receptor gamma antagonist GW9662. No effect of either rosiglitazone or metformin was observed on proinsulin promoter activity. Metformin stimulated IPF1 nuclear accumulation and DNA binding activity in a time-dependent manner, with maximal effects observed after 2 h.

CONCLUSIONS/INTERPRETATION

Metformin and rosiglitazone have direct effects on beta cell gene expression, suggesting that these agents may play a previously unrecognised role in the direct regulation of pancreatic beta cell function.

Physiological and pathophysiological regulation of regional adipose tissue in the development of insulin resistance and type 2 diabetes

AIM

To survey the latest state of knowledge concerning the regulation of regional adipocytes and their role in the development of insulin resistance and type 2 diabetes.

METHODS

Data from the English-language literature on regional adipocytes, including abdominal, intramyocellular, intrahepatic and intra-islet fat as well as the adipokines and their relations to insulin resistance and type 2 diabetes, were reviewed.

RESULTS

It is not the total amount of fat but the fat that resides within skeletal muscle cell (intramyocellular fat), hepatocytes and intra-abdominally (visceral fat), via systemic and local secretion of several adipokines, that influences insulin resistance. Among the adipokines that relate to insulin resistance, adiponectin and leptin appear to have clinical relevance to human insulin resistance and others may also contribute, but their role is still inconclusive. The intra-islet fat also adversely affects beta-cell function and number (beta-cell apoptosis), eventually leading to deterioration of glucose tolerance. The abnormal location of fat observed in patients with type 2 diabetes and their relatives is conceivably partly the results of the genetically determined, impaired mitochondrial fatty acid oxidative capacity. Restriction or elimination of the fat load by weight control, regular exercise and thiazolidinediones has been shown to improve insulin resistance and beta-cell function and to delay the development of type 2 diabetes.

CONCLUSION

These data support the plausibility of an essential role of regional adipose tissue in the development of insulin resistance and type 2 diabetes.

Metformin and pioglitazone: Effectively treating insulin resistance

BACKGROUND

Insulin resistance has a complex etiology, with multiple manifestations across the organ systems involved in glucose homeostasis. Glucose-lowering drug therapies that target insulin resistance can therefore utilize different mechanistic approaches. Two key classes of insulin-sensitizing agents--the biguanides (principally metformin) and thiazolidinediones (pioglitazone and rosiglitazone)--have distinct molecular mechanisms of action and differing effects on metabolic dysfunction. This provides an opportunity for complementary beneficial effects in the treatment of type 2 diabetes and on the potential consequences of insulin resistance, such as dyslipidemia and atherosclerosis.

SCOPE

This review (based upon EMBASE and MEDLINE searches from January 1990 to April 2006) highlights the mechanistic distinctions and clinical data that support the rationale for thiazolidinedione/metformin combination therapy in patients with type 2 diabetes.

FINDINGS

The different insulin-sensitizing mechanisms of metformin and the thiazolidinediones are manifest in partially distinct effects on hepatic and peripheral glucose homeostasis, and clinical studies show improved glucose control with combination therapy. Both metformin and thiazolidinediones may also have pancreatic beta-cell preserving properties. Furthermore, they have different beneficial effects on several other metabolic risk markers and risk factors for cardiovascular disease. Whereas the thiazolidinediones (particularly pioglitazone) have greater effects on multiple aspects of dyslipidemia, metformin has anorexigenic properties. They also have distinct effects on levels of mediators involved in inflammation and endothelial dysfunction, and outcome studies suggest that either pioglitazone or metformin may reduce the risk of macrovascular events.

CONCLUSION

The distinct, but complementary, mechanisms of action of the thiazolidinediones and metformin provide the opportunity for effective combination therapy with two insulin-sensitizing agents. Such an approach has consequences, not only for improved glucose control, but also for reducing metabolic risk and potentially improving major cardiovascular disease outcomes.

Is there a role for locally produced interleukin-1 in the deleterious effects of high glucose or the type 2 diabetes milieu to human pancreatic islets?

Different degrees of beta-cell failure and apoptosis are present in type 1 and type 2 diabetes. It has been recently suggested that high glucose-induced beta-cell apoptosis in type 2 diabetes shares a final common pathway with type 1 diabetes, involving interleukin-1beta (IL-1beta) production by beta-cells, nuclear factor-kappaB (NF-kappaB) activation, and death via Fas-FasL. The aim of this study was to test whether human islet exposure to high glucose in vitro, or to the type 2 diabetes environment in vivo, induces IL-1beta expression and consequent activation of NF-kappaB-dependent genes. Human islets were isolated from five normoglycemic organ donors. The islets were cultured for 48 h to 7 days at 5.6, 11, or 28 mmol/l glucose. For comparative purposes, islets were also exposed to IL-1beta. Gene mRNA expression levels were assessed by real-time RT-PCR in a blinded fashion. Culture of the human islets at 11 and 28 mmol/l glucose induced a four- to fivefold increase in medium insulin as compared with 5.6 mmol/l glucose, but neither IL-1beta nor IL-1 receptor antagonist (IL-1ra) expression changed. IL-1beta and IL-1ra protein release to the medium was also unchanged. Stimulated human monocytes, studied in parallel, released >50-fold more IL-1beta than the islets. There was also no glucose-induced islet Fas expression. Expression of the NF-kappaB-dependent genes IkappaB-alpha and monocyte chemoattractant protein (MCP)-1 was induced in human islets by IL-1beta but not by high glucose. In a second set of experiments, human islets were isolated from seven type 2 diabetic patients and eight control subjects. The findings on mRNA levels were essentially the same as in the in vitro experiments, namely the in vivo diabetic state did not induce IL-1beta, Fas, or MCP-1 expression in human islets, and also did not modify IL-1ra expression. The present findings suggest that high glucose in vitro, or the diabetic milieu in vivo, does not induce IL-1beta production or NF-kappaB activation in human islets. This makes it unlikely that locally produced IL-1beta is an important mediator of glucotoxicity to human islets and argues against the IL-1beta-NF-kappaB-Fas pathway as a common mediator for beta-cell death in type 1 and type 2 diabetes.

Rosiglitazone/Metformin

The thiazolidinedione rosiglitazone and the biguanide metformin are effective antihyperglycaemic agents with different modes of action; rosiglitazone primarily increases insulin sensitivity, whereas metformin primarily reduces hepatic glucose output. Antihyperglycaemic combination therapy is often required to achieve effective glycaemic control. A fixed-dose formulation of rosiglitazone/metformin was recently approved in the EU and the US for the treatment of type 2 diabetes mellitus in patients inadequately controlled on metformin monotherapy. Bioequivalence between the fixed-dose combination tablet and coadministration of rosiglitazone with metformin at the same dosage has been established in a pharmacokinetic study. Fixed-dose rosiglitazone/metformin 8 mg/2g per day reduced glycosylated haemoglobin (HbA1c) and fasting plasma glucose (FPG) levels to a significantly greater extent than metformin 3 g/day in patients with type 2 diabetes in a 24-week, randomised, double-blind study. Rosiglitazone plus metformin was significantly more effective than metformin alone at reducing HbA1c and FPG levels in patients with type 2 diabetes in three 26-week, randomised, double-blind, placebo-controlled studies. Rosiglitazone plus metformin was generally well tolerated in all studies and had a tolerability profile similar to that of metformin monotherapy. Mild or moderate symptomatic hypoglycaemia was reported in <or=4.4% of rosiglitazone plus metformin recipients.

Beta-cell preservation: a potential role for thiazolidinediones to improve clinical care in Type 2 diabetes

Type 2 diabetes is caused by progressively increasing insulin resistance coupled with deteriorating beta-cell function, and there is a growing body of evidence to suggest that both of these defects precede hyperglycaemia by many years. Several studies have demonstrated the importance of maintaining beta-cell function in patients with Type 2 diabetes. This review explores parameters used to indicate beta-cell dysfunction, in Type 2 diabetes and in individuals with a predisposition to the disease. A genetic element undoubtedly underlies beta-cell dysfunction; however, a number of modifiable components are also associated with beta-cell deterioration, such as chronic hyperglycaemia and elevated free fatty acids. There is also evidence for a link between pro-inflammatory cytokines and impairment of insulin-signalling pathways in the beta-cell, and the potential role of islet amyloid deposition in beta-cell deterioration continues to be a subject for debate. The thiazolidinediones are a class of agents that have demonstrated clinical improvements in indices of beta-cell dysfunction and have the potential to improve beta-cell function. Data are accumulating to show that this therapeutic group offers a number of advantages over traditionally employed oral agents, and these data demonstrate the growing importance of thiazolidinediones in Type 2 diabetes management.

Long-term treatment with rosiglitazone and metformin reduces the extent of, but does not prevent, islet amyloid deposition in mice expressing the gene for human islet amyloid polypeptide

Islet amyloid deposition in type 2 diabetes is associated with reduced beta-cell mass. Therefore, interventions aimed at reducing islet amyloid formation may help preserve beta-cell mass in type 2 diabetes. Rosiglitazone and metformin act by different mechanisms to improve insulin sensitivity and thereby reduce beta-cell secretory demand, resulting in decreased release of insulin and islet amyloid polypeptide (IAPP), the unique constituent of islet amyloid deposits. We hypothesized that this reduced beta-cell secretory demand would lead to reduced islet amyloid formation. Human IAPP (hIAPP) transgenic mice, a model of islet amyloid, were treated for 12 months with rosiglitazone (1.5 mg.kg(-1).day(-1), n = 19), metformin (1 g.kg(-1).day(-1), n = 18), or control (n = 17). At the end of the study, islet amyloid prevalence (percent islets containing amyloid) and severity (percent islet area occupied by amyloid), islet mass, beta-cell mass, and insulin release were determined. Islet amyloid prevalence (44 +/- 8, 13 +/- 4, and 11 +/- 3% for control, metformin-, and rosiglitazone-treated mice, respectively) and severity (9.2 +/- 3.0, 0.22 +/- 0.11, and 0.10 +/- 0.05% for control, metformin-, and rosiglitazone-treated mice, respectively) were markedly reduced with both rosiglitazone (P < 0.001 for both measures) and metformin treatment (P < 0.001 for both measures). Both treatments were associated with reduced insulin release assessed as the acute insulin response to intravenous glucose (2,189 +/- 857, 621 +/- 256, and 14 +/- 158 pmol/l for control, metformin-, and rosiglitazone-treated mice, respectively; P < 0.05 for metformin vs. control and P < 0.005 for rosiglitazone vs. control), consistent with reduced secretory demand. Similarly, islet mass (33.4 +/- 7.0, 16.6 +/- 3.6, and 12.2 +/- 2.1 mg for control, metformin-, and rosiglitazone-treated mice, respectively) was not different with metformin treatment (P = 0.06 vs. control) but was significantly lower with rosiglitazone treatment (P < 0.05 vs. control). When the decreased islet mass was accounted for, the islet amyloid-related decrease in beta-cell mass (percent beta-cell mass/islet mass) was ameliorated in both rosiglitazone- and metformin-treated animals (57.9 +/- 3.1, 64.7 +/- 1.4, and 66.1 +/- 1.6% for control, metformin-, and rosiglitazone-treated mice, respectively; P < 0.05 for metformin or rosiglitazone vs. control). In summary, rosiglitazone and metformin protect beta-cells from the deleterious effects of islet amyloid, and this effect may contribute to the ability of these treatments to alleviate the progressive loss of beta-cell mass and function in type 2 diabetes.

The FFA receptor GPR40 links hyperinsulinemia, hepatic steatosis, and impaired glucose homeostasis in mouse

Obesity is typically associated with elevated levels of free fatty acids (FFAs) and is linked to glucose intolerance and type 2 diabetes. FFAs exert divergent effects on insulin secretion from beta cells: acute exposure to FFAs stimulates insulin secretion, whereas chronic exposure impairs insulin secretion. The G protein-coupled receptor GPR40 is selectively expressed in beta cells and is activated by FFAs. We show here that GPR40 mediates both acute and chronic effects of FFAs on insulin secretion and that GPR40 signaling is linked to impaired glucose homeostasis. GPR40-deficient beta cells secrete less insulin in response to FFAs, and loss of GPR40 protects mice from obesity-induced hyperinsulinemia, hepatic steatosis, hypertriglyceridemia, increased hepatic glucose output, hyperglycemia, and glucose intolerance. Conversely, overexpression of GPR40 in beta cells of mice leads to impaired beta cell function, hypoinsulinemia, and diabetes. These results suggest that GPR40 plays an important role in the chain of events linking obesity and type 2 diabetes.

Functional and molecular defects of pancreatic islets in human type 2 diabetes

To shed further light on the primary alterations of insulin secretion in type 2 diabetes and the possible mechanisms involved, we studied several functional and molecular properties of islets isolated from the pancreata of 13 type 2 diabetic and 13 matched nondiabetic cadaveric organ donors. Glucose-stimulated insulin secretion from type 2 diabetic islets was significantly lower than from control islets, whereas arginine- and glibenclamide-stimulated insulin release was less markedly affected. The defects were accompanied by reduced mRNA expression of GLUT1 and -2 and glucokinase and by diminished glucose oxidation. In addition, AMP-activated protein kinase activation was reduced. Furthermore, the expression of insulin was decreased, and that of pancreatic duodenal homeobox-1 (PDX-1) and forkhead box O1 (Foxo-1) was increased. Nitrotyrosine and 8-hydroxy-2'-deoxyguanosine concentrations, markers of oxidative stress, were significantly higher in type 2 diabetic than control islets, and they were correlated with the degree of glucose-stimulated insulin release impairment. Accordingly, 24-h exposure to glutathione significantly improved glucose-stimulated insulin release and decreased nitrotyrosine concentration, with partial recovery of insulin mRNA expression. These results provide direct evidence that the defects of insulin secretion in type 2 diabetic islets are associated with multiple islet cell alterations. Most importantly, the current study shows that the functional impairment of type 2 diabetic islets can be, at least in part, reversible. In this regard, it is suggested that reducing islet cell oxidative stress is a potential target of human type 2 diabetes therapy.

Transcriptional regulation of lipid metabolism by fatty acids: a key determinant of pancreatic beta-cell function

BACKGROUND: Optimal pancreatic beta-cell function is essential for the regulation of glucose homeostasis in both humans and animals and its impairment leads to the development of diabetes. Type 2 diabetes is a polygenic disease aggravated by environmental factors such as low physical activity or a hypercaloric high-fat diet. RESULTS: Free fatty acids represent an important factor linking excess fat mass to type 2 diabetes. Several studies have shown that chronically elevated free fatty acids have a negative effect on beta-cell function leading to elevated insulin secretion basally but with an impaired response to glucose. The transcription factors PPARalpha, PPARgamma and SREBP-1c respond to changing fat concentrations in tissues, thereby coordinating the genomic response to altered metabolic conditions to promote either fat storage or catabolism. These transcription factors have been identified in beta-cells and it appears that each may exert influence on beta-cell function in health and disease. CONCLUSION: The role of the PPARs and SREBP-1c as potential mediators of lipotoxicity is an emerging area of interest.

Effects of prolonged in vitro exposure to sulphonylureas on the function and survival of human islets

The direct effects of prolonged exposure to sulphonylureas on the function and survival of human islets are unknown. This study assessed the insulin content, glucose-stimulated insulin release, islet cell apoptosis, and mRNA expression of insulin and GLUT-1 in isolated human islets cultured in the presence of therapeutical concentrations of glimepiride (10 microM), glibenclamide (10 microM), or chlorpropamide (600 microM). Islets were prepared by collagenase digestion and density gradient purification from 18 multiorgan donors and were then exposed for 24 h to the different sulphonylureas. Insulin content decreased significantly following culture with any sulphonylurea compound. In response to an acute challenge with 3.3 and 16.7 mM glucose, insulin release from the control islets accounted for 1.9 +/- 0.5% and 4.9 +/- 1.7% of total insulin content (P<.01), respectively. Glucose responsiveness was preserved in islets precultured in the presence of glimepiride, whereas high glucose level did not elicit any significant increase of insulin secretion from islets preincubated with glibenclamide or chlorpropamide. These alterations were reverted by an additional 48-h incubation in drug-free conditions. The amount of apoptotic cells did not differ significantly among the experimental groups. Quantitative RT-PCR studies showed that, compared with the control islets, cells preincubated with glibenclamide or chlorpropamide had an increased expression of insulin mRNA, with no change in the expression of GLUT-1. In conclusion, prolonged exposure of human islets to different sulphonylureas causes different disturbances of islet cell function, with glimepiride showing milder effects, as compared with chlorpropamide and glibenclamide.

Thiazolidinediones and Insulin

The range of therapeutic modalities to treat type 2 diabetes mellitus has broadened in recent years. Biguanides and thiazolidinediones are the two currently available classes of anti-hyperglycemic agents with insulin-sensitizing properties. Thiazolidinediones, in particular, have received much attention, not only for the well documented hepatotoxicity of troglitazone that led to its removal from the market in 2000, but also for the emerging data that support the beneficial effects of the thiazolidinedione class of drugs on beta-cell rejuvenation and cardiovascular risk reduction. In the US, thiazolidinediones are indicated either as monotherapy or in combination with a sulfonylurea, metformin, or insulin in cases where diet, exercise, and a single drug fail. In contrast, the UK National Institute for Clinical Excellence included in its re-appraisal of 'glitazones' in August 2003 the continued exclusion from licensed use in the UK of combination therapy with thiazolidinediones and insulin. When added to insulin therapy, thiazolidinediones appear to effectively lower glucose levels and reduce insulin dosage in clinical trials involving individuals with poorly controlled type 2 diabetes. However, weight gain, hypoglycemia, and fluid retention pose problems in certain patients. The fluid retention may exacerbate or even precipitate congestive heart failure, which usually necessitates discontinuation of the drug. Risk stratification and careful management of patients at risk for heart failure, including those taking insulin concomitantly, allow healthcare providers to safely administer combination therapy with thiazolidinediones in patients with type 2 diabetes. Hepatic toxicity with currently available thiazolidinediones has been found to be minimal overall.

Effect of PPAR-gamma activation and inhibition on glucose-stimulated insulin release in INS-1e cells

Peroxisome proliferator-activated receptor (PPAR)-gamma is expressed in human beta-cells and in the rat beta-cell line INS-1. Previous studies have suggested that PPAR-gamma agonism (e.g., thiazolidinediones) enhances glucose-stimulated insulin secretion (GSIS) from islets or INS-1 cells. We tested the direct effect on insulin release by INS-1e of a PPAR-gamma agonist (Ro4389679-000-001 at 0.2 and 0.4 micromol/l) and a PPAR-gamma antagonist (SR202 at 0.2 and 0.4 mmol/l). Cells were incubated in 11 mmol/l glucose for 96 h and then challenged with 3.3, 7.5, 11.0, and 20.0 mmol/l glucose for 1 h. Under these control conditions, insulin concentrations in the medium rose from 19 +/- 4 ng/ml (mean +/- SE) to 82 +/- 5, 107 +/- 11, and 103 +/- 10 ng/ml (P <0.0001 by ANOVA). Preincubation for 48 h with the PPAR-gamma agonist potentiated GSIS (to 154 +/- 14 and 156 +/- 12 ng/ml at 20 mmol/l glucose, P <0.01). Cell insulin content was not altered by either acute glucose challenge or PPAR-gamma agonist coincubation. Preincubation for 48 h with SR202 at the higher dose caused a 30% inhibition of GSIS, with no change in cell insulin contents. When cells were preincubated with 11 mmol/l glucose plus 1 mmol/l oleate, GSIS was significantly potentiated (by 30%, P <0.0001); adding Ro4389679-000-001 or SR202 to these preincubations reduced GSIS to the respective levels seen in the absence of oleate (P <0.0001 for both effects). In conclusion, INS-1e cells display a PPAR-gamma tone that is symmetrically modulated and competitively stimulated by oleate.

Rosiglitazone, but not glyburide, reduces circulating proinsulin and the proinsulin:insulin ratio in type 2 diabetes

An elevation in the ratio of proinsulin (PI) to immunoreactive insulin (IRI) is inversely related to beta-cell function in type 2 diabetes, and increased PI is an independent risk factor for coronary heart disease. An objective of the present studies was to assess the effects of the thiazolidinedione insulin sensitizer, rosiglitazone, on indirect markers of beta-cell function and cardiovascular risk in people with type 2 diabetes by measuring plasma PI and the PI:IRI ratio. Parameters of insulin processing, including plasma PI and PI:IRI ratios, were determined in type 2 diabetes patients enrolled in two randomized double-blind studies comparing the effects of rosiglitazone (4 or 8 mg/d) with placebo (study 1, 26-wk treatment) or the sulfonylurea glyburide (study 2, 52-wk treatment). Treatment with rosiglitazone for 26 wk (study 1) produced significant dose-dependent decreases in both plasma PI concentrations (18-29%) and the PI:IRI ratio compared with baseline (7-14%) and placebo (19-29%) (P < 0.001). A significant increase in the PI:IRI ratio in placebo-treated patients occurred (P < 0.001). In study 2, rosiglitazone also significantly reduced both plasma PI and the PI:IRI ratio compared with baseline (P < 0.001). In contrast, glyburide significantly increased both plasma PI (45%; P < 0.001) and the PI:IRI ratio (10%) (P < 0.05 vs. baseline). These results show that rosiglitazone and glyburide have differential effects on absolute PI levels and the PI:IRI ratio in people with type 2 diabetes.