Pancreatic beta-cell lipotoxicity induced by overexpression of hormone-sensitive lipase

An Overview of Hormone-Sensitive Lipase (HSL)

Hormone-sensitive lipase (HSL) is a pivotal enzyme that mediates triglyceride hydrolysis to provide free fatty acids and glycerol in adipocytes in a hormonally controlled lipolysis process. Elevated plasma-free fatty acids were accompanied by insulin resistance, type-2 diabetes, and obesity. Inhibition of lipolysis through HSL inhibition may provide a mechanism to prevent the accumulation of free fatty acids and to improve the affectability of insulin and blood glucose handling in type II diabetes. The published studies that examine the structure, regulation, and function of HSL and major inhibitors were reviewed in this paper.

Recurrent Hypertriglyceridemia-Induced Pancreatitis due to Multifactorial Chylomicronemia Syndrome in a Patient With Ketosis-Prone Diabetes Mellitus

The triad of diabetic ketoacidosis (DKA), severe hypertriglyceridemia, and acute pancreatitis have been occasionally described in severely obese patients with type 2 diabetes mellitus (T2DM). Herein, we present a long-term clinical course of a Thai man with ketosis-prone diabetes mellitus (KPDM) complicated with recurrent pancreatitis due to multifactorial chylomicronemia syndrome. Genetic testing showed no mutation in lipoprotein lipase (LPL) and its co-factors. The patient was referred to multidisciplinary team for lifelong weight loss consultation, limiting intake of fat and simple carbohydrates, and adherence to lipid-lowering medications. Subsequent follow-up 1 year later showed no recurrent pancreatitis. In patients with multifactorial chylomicronemia syndrome, long-term management with dietary modifications together with pharmacotherapy remains the cornerstone of successful treatment.

Targeting mitochondrial biogenesis: a potential approach for preventing and controlling diabetes

Background

Diabetes mellitus is a lingering hyperglycemic ailment resulting in several life-threatening difficulties. Enduring hyperglycemia often persuades the buildup of reactive oxygen species that are the significant pathological makers of diabetic complications. The mitochondrial dysfunction, with mitochondrial damage and too much production of reactive oxygen species, have been proposed to be convoluted in the progress of insulin resistance. Numerous studies advocate that agents that enhance the mitochondrial number and/or decrease their dysfunction, could be greatly helpful in management of diabetes and its complications.

Main body

Mitochondrial biogenesis is an extremely delimited procedure arbitrated by numerous transcription influences, in which mitochondrial fusion and fission happen in synchronization in a standard vigorous cell. But this synchronization is greatly disturbed in diabetic condition designated by modification in the working of several important transcription factors regulating the expressions of different genes. Numerous preclinical and clinical investigations have suggested that, the compromised functions of mitochondria play a significant protagonist in development of pancreatic β-cell dysfunction, skeletal muscle insulin resistance and several diabetic complications. However, there are several phytoconstituents performing through numerous alleyways, either unswervingly by motivating biogenesis or indirectly by constraining or averting dysfunction and producing a beneficial effect on overall function of the mitochondria.

Conclusion

This review describes standard mitochondrial physiology and anomalous modifications that transpire in answer to persistent hyperglycemia in diabetes condition. It also discusses about the different phytoconstituents that can affect the biogenesis pathways of mitochondria and thus can be used in the treatment and prevention of diabetes.

β-Cell Dysfunction, Hepatic Lipid Metabolism, and Cardiovascular Health in Type 2 Diabetes: New Directions of Research and Novel Therapeutic Strategies

Cardiovascular disease (CVD) remains a major problem for people with type 2 diabetes mellitus (T2DM), and dyslipidemia is one of the main drivers for both metabolic diseases. In this review, the major pathophysiological and molecular mechanisms of β-cell dysfunction and recovery in T2DM are discussed in the context of abnormal hepatic lipid metabolism and cardiovascular health. (i) In normal health, continuous exposure of the pancreas to nutrient stimulus increases the demand on β-cells. In the long term, this will not only stress β-cells and decrease their insulin secretory capacity, but also will blunt the cellular response to insulin. (ii) At the pre-diabetes stage, β-cells compensate for insulin resistance through hypersecretion of insulin. This increases the metabolic burden on the stressed β-cells and changes hepatic lipoprotein metabolism and adipose tissue function. (iii) If this lipotoxic hyperinsulinemic environment is not removed, β-cells start to lose function, and CVD risk rises due to lower lipoprotein clearance. (iv) Once developed, T2DM can be reversed by weight loss, a process described recently as remission. However, the precise mechanism(s) by which calorie restriction causes normalization of lipoprotein metabolism and restores β-cell function are not fully established. Understanding the pathophysiological and molecular basis of β-cell failure and recovery during remission is critical to reduce β-cell burden and loss of function. The aim of this review is to highlight the link between lipoprotein export and lipid-driven β-cell dysfunction in T2DM and how this is related to cardiovascular health. A second aim is to understand the mechanisms of β-cell recovery after weight loss, and to explore new areas of research for developing more targeted future therapies to prevent T2DM and the associated CVD events.

Lipotoxic Impairment of Mitochondrial Function in β-Cells: A Review

Lipotoxicity is a major contributor to type 2 diabetes mainly promoting mitochondrial dysfunction. Lipotoxic stress is mediated by elevated levels of free fatty acids through various mechanisms and pathways. Impaired peroxisome proliferator-activated receptor (PPAR) signaling, enhanced oxidative stress levels, and uncoupling of the respiratory chain result in ATP deficiency, while β-cell viability can be severely impaired by lipotoxic modulation of PI3K/Akt and mitogen-activated protein kinase (MAPK)/extracellular-signal-regulated kinase (ERK) pathways. However, fatty acids are physiologically required for an unimpaired β-cell function. Thus, preparation, concentration, and treatment duration determine whether the outcome is beneficial or detrimental when fatty acids are employed in experimental setups. Further, ageing is a crucial contributor to β-cell decay. Cellular senescence is connected to loss of function in β-cells and can further be promoted by lipotoxicity. The potential benefit of nutrients has been broadly investigated, and particularly polyphenols were shown to be protective against both lipotoxicity and cellular senescence, maintaining the physiology of β-cells. Positive effects on blood glucose regulation, mitigation of oxidative stress by radical scavenging properties or regulation of antioxidative enzymes, and modulation of apoptotic factors were reported. This review summarizes the significance of lipotoxicity and cellular senescence for mitochondrial dysfunction in the pancreatic β-cell and outlines potential beneficial effects of plant-based nutrients by the example of polyphenols.

Altered adipose tissue and adipocyte function in the pathogenesis of metabolic syndrome

Over the past decade, great progress has been made in understanding the complexity of adipose tissue biology and its role in metabolism. This includes new insights into the multiple layers of adipose tissue heterogeneity, not only differences between white and brown adipocytes, but also differences in white adipose tissue at the depot level and even heterogeneity of white adipocytes within a single depot. These inter- and intra-depot differences in adipocytes are developmentally programmed and contribute to the wide range of effects observed in disorders with fat excess (overweight/obesity) or fat loss (lipodystrophy). Recent studies also highlight the underappreciated dynamic nature of adipose tissue, including potential to undergo rapid turnover and dedifferentiation and as a source of stem cells. Finally, we explore the rapidly expanding field of adipose tissue as an endocrine organ, and how adipose tissue communicates with other tissues to regulate systemic metabolism both centrally and peripherally through secretion of adipocyte-derived peptide hormones, inflammatory mediators, signaling lipids, and miRNAs packaged in exosomes. Together these attributes and complexities create a robust, multidimensional signaling network that is central to metabolic homeostasis.

High frequency of type 2 diabetes and impaired glucose tolerance in Japanese subjects with the angiopoietin-like protein 8 R59W variant

BACKGROUND

Angiopoietin-like protein 8 (ANGPTL8) is considered to be metabolically multifunctional. One notable function still to be elucidated definitively is a betatrophic role in protecting and preserving pancreatic beta-cell function. There is, however, a paucity of data regarding the role of ANGPTL8 in the etiology of type 2 diabetes (T2D), but some findings of human research have suggested the potential for significant involvement.

OBJECTIVE

To examine the frequency of T2D and impaired glucose tolerance (IGT) in Japanese subjects with the ANGPTL8 R59W variant.

METHODS

ANGPTL8 R59W (Rs2278426, c.194C > T) was determined by polymerase chain reaction-restriction fragment length polymorphism using the restriction enzyme FokI in 797 consecutive Japanese individuals. Subjects with triglyceride levels greater than or equal to 150 mg/dL were considered to be hypertriglyceridemic.

RESULTS

Genotype frequencies of ANGPTL8 R59W were as follows: wild-type RR (C/C) 53.5%, RW (C/T) 36.6%, and WW (T/T) 9.9%. T2D and IGT were significantly prevalent in WW and RW subjects relative to RR among all 797 subjects (P = .0138) and also in hypertriglyceridemic subjects (P = .0015). In multiple logistic regression models for the existence of T2D and IGT in hypertriglyceridemic subjects, the odds ratio for heterozygote RW and homozygote WW genotypes to wild-type RR was 2.406 (P = .0017) after controlling the risk factors of age, gender, and body mass index as covariates.

CONCLUSIONS

The frequency of ANGPTL8 R59W is significantly higher in Japanese subjects than in other ethnic groups. The rates of T2D and IGT were greater in subjects with the R59W variant. These findings indicate that ANGPTL8 is a participant in diabetes and a potential therapeutic target for T2D prevention, especially in East Asians.

Uncoupling protein 2 regulates daily rhythms of insulin secretion capacity in MIN6 cells and isolated islets from male mice

Objective

Upregulation of uncoupling protein 2 (UCP2) is associated with impaired glucose-stimulated insulin secretion (GSIS), which is thought to be an important contributor to pathological β cell failure in obesity and type 2 diabetes (T2D); however, the physiological function of UCP2 in the β cell remains undefined. It has been suggested, but not yet tested, that UCP2 plays a physiological role in β cells by coordinating insulin secretion capacity with anticipated fluctuating nutrient supply, such that upregulation of UCP2 in the inactive/fasted state inhibits GSIS as a mechanism to prevent hypoglycemia. Therefore, we hypothesized that daily cycles of GSIS capacity are dependent on rhythmic and predictable patterns of Ucp2 gene expression such that low Ucp2 in the active/fed phase promotes maximal GSIS capacity, whereas elevated Ucp2 expression in the inactive/fasted phase supresses GSIS capacity. We further hypothesized that rhythmic Ucp2 expression is required for the maintenance of glucose tolerance over the 24 h cycle.

Methods

We used synchronized MIN6 clonal β cells and isolated mouse islets from wild type (C57BL6) and mice with β cell knockout of Ucp2 (Ucp2-βKO; and respective Ins2-cre controls) to determine the endogenous expression pattern of Ucp2 over 24 h and its impact on GSIS capacity and glucose tolerance over 24 h.

Results

A dynamic pattern of Ucp2 mRNA expression was observed in synchronized MIN6 cells, which showed a reciprocal relationship with GSIS capacity in a time-of-day-specific manner. GSIS capacity was suppressed in islets isolated from wild type and control mice during the light/inactive phase of the daily cycle; a suppression that was dependent on Ucp2 in the β cell and was lost in islets isolated from Ucp2-βKO mice or wild type islets treated with a UCP2 inhibitor. Finally, suppression of GSIS capacity by UCP2 in the light phase was required for the maintenance of normal patterns of glucose tolerance.

Conclusions

Our study suggests that Ucp2/UCP2 in the β cell is part of an important, endogenous, metabolic regulator that controls the temporal capacity of GSIS over the course of the day/night cycle, which, in turn, regulates time-of-day glucose tolerance. Targeting Ucp2/UCP2 as a therapeutic in type 2 diabetes or any other metabolic condition must take into account the rhythmic nature of its expression and its impact on glucose tolerance over 24 h, specifically during the inactive/fasted phase.

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Ucp2 mRNA expression in MIN6 β cells and isolated islets is dynamic and rhythmic over 24 h.

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Daily cycles of glucose-stimulated insulin secretion capacity are dependent on rhythmic Ucp2 expression and UCP2 activity.

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Loss of rhythmic Ucp2 mRNA expression triggers glucose intolerance only in the light/inactive phase of the daily cycle.

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UCP2 is part of an endogenous diurnal metabolic regulator that coordinates islet function with the daily cycle of fasting and feeding.

Intracellular and extracellular adenosine triphosphate in regulation of insulin secretion from pancreatic β cells (β)

Adenosine triphosphate (ATP) synthesis and release in mitochondria play critical roles in regulating insulin secretion in pancreatic β cells. Mitochondrial dysfunction is mainly characterized by a decrease in ATP production, which is a central event in the progression of pancreatic β cell dysfunction and diabetes. ATP has been demonstrated to regulate insulin secretion via several pathways: (i) Intracellular ATP directly closes ATP-sensitive potassium channel to open L-type calcium channel, leading to an increase in free cytosolic calcium levels and exocytosis of insulin granules; (ii) A decrease in ATP production is always associated with an increase in production of reactive oxygen species, which exerts deleterious effects on pancreatic β cell survival and insulin secretion; and (iii) ATP can be co-secreted with insulin from pancreatic β cells, and the released ATP functions as an autocrine signal to modulate insulin secretory process via P2 receptors on the cell membrane. In this review, the recent findings regarding the role and mechanism of ATP synthesis and release in regulation of insulin secretion from pancreatic β cells will be summarized and discussed.

Making, baking, and breaking: the synthesis, storage, and hydrolysis of neutral lipids

The esterification of amphiphilic alcohols with fatty acids is a ubiquitous strategy implemented by eukaryotes and some prokaryotes to conserve energy and membrane progenitors and simultaneously detoxify fatty acids and other lipids. This key reaction is performed by at least four evolutionarily unrelated multigene families. The synthesis of this "neutral lipid" leads to the formation of a lipid droplet, which despite the clear selective advantage it confers is also a harbinger of cellular and organismal malaise. Neutral lipid deposition as a cytoplasmic lipid droplet may be thermodynamically favored but nevertheless is elaborately regulated. Optimal utilization of these resources by lipolysis is similarly multigenic in determination and regulation. We present here a perspective on these processes that originates from studies in model organisms, and we include our thoughts on interventions that target reductions in neutral lipids as therapeutics for human diseases such as obesity and diabetes.

Restoring a functional beta-cell mass in diabetes

Type 1 and type 2 diabetes have often been presented as disease forms that profoundly differ in the presence and pathogenic significance of a reduced beta-cell mass. We review evidence indicating that the beta-cell mass in type 1 diabetes is usually not decreased by at least 90% at clinical onset, and remains often detectable for years after diagnosis at age above 15 years. Clinical and experimental evidence also exists for a reduced beta-cell mass in type 2 diabetes where it can be the cause for and/or the consequence of dysregulated beta-cell functions. With beta-cell mass defined as number of beta-cells, these views face the limitation of insufficient data and methods for human organs. Because beta-cells can occur under different phenotypes that vary with age and with environmental conditions, we propose to use the term functional beta-cell mass as an assessment of a beta-cell population by the number of beta-cells and their phenotype or functional state. Assays exist to measure functional beta-cell mass in isolated preparations. We selected a glucose-clamp test to evaluate functional beta-cell mass in type 1 patients at clinical onset and in type 1 recipients following intraportal islet cell transplantation. Comparison of the data with those in non-diabetic controls helps targeting and monitoring of therapeutic interventions.

Glycerolipid metabolism and signaling in health and disease

Maintenance of body temperature is achieved partly by modulating lipolysis by a network of complex regulatory mechanisms. Lipolysis is an integral part of the glycerolipid/free fatty acid (GL/FFA) cycle, which is the focus of this review, and we discuss the significance of this pathway in the regulation of many physiological processes besides thermogenesis. GL/FFA cycle is referred to as a "futile" cycle because it involves continuous formation and hydrolysis of GL with the release of heat, at the expense of ATP. However, we present evidence underscoring the "vital" cellular signaling roles of the GL/FFA cycle for many biological processes. Probably because of its importance in many cellular functions, GL/FFA cycling is under stringent control and is organized as several composite short substrate/product cycles where forward and backward reactions are catalyzed by separate enzymes. We believe that the renaissance of the GL/FFA cycle is timely, considering the emerging view that many of the neutral lipids are in fact key signaling molecules whose production is closely linked to GL/FFA cycling processes. The evidence supporting the view that alterations in GL/FFA cycling are involved in the pathogenesis of "fatal" conditions such as obesity, type 2 diabetes, and cancer is discussed. We also review the different enzymatic and transport steps that encompass the GL/FFA cycle leading to the generation of several metabolic signals possibly implicated in the regulation of biological processes ranging from energy homeostasis, insulin secretion and appetite control to aging and longevity. Finally, we present a perspective of the possible therapeutic implications of targeting this cycling.

Cardiac overexpression of hormone-sensitive lipase inhibits myocardial steatosis and fibrosis in streptozotocin diabetic mice

Intracellular lipid accumulation (steatosis) and resultant lipotoxicity are key features of diabetic cardiomyopathy. Since cardiac hormone-sensitive lipase (HSL) is activated in diabetic mice, we sought to explore a pathophysiological function of cardiac HSL in the development of diabetic cardiomyopathy. Transgenic (Tg) mice with heart-specific HSL overexpression were generated, and cardiac histology, function, lipid profile, and gene expressions were analyzed after induction of diabetes by streptozotocin. Electron microscopy showed numerous lipid droplets in wild-type (Wt) hearts after 3 wk of diabetes, whereas Tg mice showed no lipid droplet accumulation. Cardiac content of acylglycerides was increased approximately 50% with diabetes in Wt mice, whereas this was blunted in Tg hearts. Cardiac lipid peroxide content was twofold lower in Tg hearts than in Wt hearts. The mRNA expressions for peroxisome proliferator-activated receptor-alpha, genes for triacylglycerol synthesis, and lipoprotein lipase were increased with diabetes in Wt hearts, whereas this induction was absent in Tg hearts. Expression of genes associated with lipoapoptosis was decreased, whereas antioxidant protein metallothioneins were increased in diabetic Tg hearts. Diabetic Wt hearts showed interstitial fibrosis and increased collagen content. However, Tg hearts displayed no overt fibrosis, concomitant with decreased expression of collagens, transforming growth factor-beta, and matrix metalloproteinase 2. Notably, mortality during the experimental period was approximately twofold lower in diabetic Tg mice compared with Wt mice. In conclusion, since HSL overexpression inhibits cardiac steatosis and fibrosis by apparently hydrolyzing toxic lipid metabolites, cardiac HSL could be a therapeutic target for regulating diabetic cardiomyopathy.

On the role of uncoupling protein-2 in pancreatic beta cells

Pancreatic beta cells secrete insulin when blood glucose levels are high. Dysfunction of this glucose-stimulated insulin secretion (GSIS) is partly responsible for the manifestation of type 2 diabetes, a metabolic disorder that is rapidly becoming a global pandemic. Mitochondria play a central role in GSIS by coupling glucose oxidation to production of ATP, a signal that triggers a series of events that ultimately leads to insulin release. Beta cells express a mitochondrial uncoupling protein, UCP2, which is rather surprising as activity of such a protein is anticipated to lower the efficiency of oxidative phosphorylation, and hence to impair GSIS. The mounting evidence demonstrating that insulin secretion is indeed blunted by UCP2 agrees with this prediction, and has provoked the idea that UCP2 activity contributes to beta cell pathogenesis and development of type 2 diabetes. Although this notion may be correct, the evolved function of UCP2 remains unclear. With this paper we aim to provide a brief account of the present state of affairs in this field, suggest a physiological role for UCP2, and highlight some of our own recent results.

The novel mechanism of recombinant human ciliary neurotrophic factor on the anti-diabetes activity

In a previous study, the ciliary neurotrophic factor (CNTF) were demonstrated to lead to weight-loss partly by up-regulating the energy metabolism and the expression of uncoupling protein-1, mitochondrial transcription factor A and nuclear respiratory factor-1 in adipose tissues or muscle. To investigate the up-stream regulators of the expression, recombinant human CNTF (rhCNTF) (0.1, 0.3 and 0.9 mg/kg/day subcutaneously) were administered to KK-Ay mice for 30 days, resulting in reduction of perirenal fat mass, serum free fatty acids and islet triacylglycerol; furthermore, the values of oral glucose tolerance test were found improved. In brown adipose tissues, the gene expressions of peroxisome proliferator-activated receptor alpha (PPARalpha) and peroxisome proliferator-activated receptor coactivator-1 alpha (PGC-1alpha) were found to be up-regulated by rhCNTF. To the best of our knowledge, the changes of gene expression of PPARalpha and PGC-1alpha represent new insights into the mechanisms of anti-diabetes by rhCNTF. In addition, the activity of mitochondrial complexII was found to be increased by rhCNTF. Stimulation of PPARalpha, PGC-1alpha, uncoupling protein-1 and enhanced activity of mitochondrial complex II may be associated with the effects of anti-diabetes. The present study indicates new mechanisms of the activity and mechanisms on anti-diabetes of rhCNTF, which may be a novel anti-diabetes reagent partly acting by enhancing energy metabolism.

Testosterone deficiency and the metabolic syndrome

Evidence is presented to link components of the metabolic syndrome to testosterone deficiency and obesity. Testosterone deficiency in hypogonadism or testosterone deprivation in normo-gonadotropic men increases fat mass as well as fasting insulin levels. Testosterone supplementation (TS) in a dose dependent manner, increase lean body mass (LBM), reduces fat mass, body mass index (BMI) and waist hip ratio in both young and elderly hypogonadal men. A negative association between T and insulin resistance as well as impaired glucose intolerance has been demonstrated and in type 2 diabetic men TS improves metabolic parameters. TS improves most components of the metabolic syndrome and also reduces inflammatory cytokines.

High-fat diets: modeling the metabolic disorders of human obesity in rodents

RESEARCH METHODS AND PROCEDURES

High-fat (HF) diet feeding can induce obesity and metabolic disorders in rodents that resemble the human metabolic syndrome. However, this dietary intervention is not standardized, and the HF-induced phenotype varies distinctly among different studies. The question which HF diet type is best to model the metabolic deterioration seen in human obesity remains unclear. Therefore, in this review, metabolic data obtained with different HF diet approaches are compiled. Both whole-body and organ-specific diet effects are analyzed.

RESULTS

On the basis of these results, we conclude that animal fats and omega-6/omega-9-containing plant oils can be used to generate an obese and insulin-resistant phenotype in rodents, whereas fish oil-fed animals do not develop these disorders.

DISCUSSION

Looking at the present data, it does not seem possible to define an ideal HF diet, and an exact definition of diet composition and a thorough metabolic characterization of the HF diet effects in a researcher's specific laboratory setting remains essential for metabolic studies with this model.

Obesity-related derangements in metabolic regulation

An epidemic surge in the incidence of obesity has occurred worldwide over the past two decades. This alarming trend has been triggered by lifestyle habits that encourage overconsumption of energy-rich foods while also discouraging regular physical activity. These environmental influences create a chronic energy imbalance that leads to persistent weight gain in the form of body fat and a host of other abnormalities in metabolic homeostasis. As adiposity increases, so does the risk of developing comorbidities such as diabetes, hypertension, and cardiovascular disease. The intimate association between obesity and systemic metabolic dysregulation has inspired a new area of biochemistry research in which scientists are seeking to understand the molecular mechanisms that link chronic lipid oversupply to tissue dysfunction and disease development. The purpose of this chapter is to review recent findings in this area, placing emphasis on lipid-induced functional impairments in the major peripheral organs that control energy flux: adipose tissue, the liver, skeletal muscle, and the pancreas.

Regulation of insulin secretion by uncoupling protein

UCPs (uncoupling proteins) can regulate cellular ATP production by uncoupling oxidative phosphorylation. UCP2 is expressed in islet beta-cells and its induction reduces glucose-stimulated insulin secretion. Under physiological conditions, superoxide, formed as a by-product of respiration, activates UCP2. This leads to reduced ATP production, which impairs closure of the ATP-dependent K+ channels to prevent insulin secretion. It is suggested that the physiological role of UCP2 is to prevent excessive superoxide generation through a feedback loop. UCP2 induction may also alter fatty acid metabolism by altering NAD/NADH or by facilitating cycling of fatty acid anions. Recently, UCP2 has been proposed to keep insulin secretion low during starvation, a function under the control of the transcription co-repressor, surtuin-1, which has been shown to bind to the UCP2 promoter. Pathological UCP2 expression or activation may suppress glucose-stimulated insulin secretion to the extent that diabetes onset is hastened. In ob/ob mice, induction of UCP2 at age 5 weeks precedes development of insulin secretion defects and hyperglycaemia. Activating protein kinase A-dependent pathways can normalize insulin secretion in UCP2-overexpressing islets. Conversely, lowering UCP2 expression may promote increased insulin secretion. UCP2 knockout mice were protected from the diabetogenic effects of a high-fat diet and their islets exhibited increased sensitivity to glucose and elevated ATP/ADP. These results support a role for UCP2 as a gene contributing to the pathogenesis of Type 2 diabetes.

Regulation of hormone-sensitive lipase in islets

An unique isoform of hormone-sensitive lipase (HSL) is expressed in beta-cells. Recent findings suggest that HSL could be involved in the regulation of glucose stimulated insulin secretion (GSIS), however, these findings are controversial. To test the hypothesis that HSL is involved in control of normal GSIS via changes in its expression and/or activity in response to stimuli, we examined the effects of free fatty acid (FFA) loading and glucagon like peptide-1 (GLP-1) stimulation on the regulation of HSL expression and activity. With prolonged FFA loading, there was increased expression of beta-cell HSL and increased HSL hydrolytic activity in clonal beta-cells. Short-term treatment with GLP-1 increased HSL activity without changing the expression of the beta-cell isoform of HSL. Basal insulin secretion was increased, whereas GLP-1 potentiation of GSIS was decreased in islets isolated from HSL-/- mice, as compared to islets from wild type mice. Furthermore, using PancChip 2.2 cDNA microarrays (NIDDK consortium), the gene expression profile in the islets of HSL-/- mice was compared with wild type mice. Results showed changes in several metabolic pathways due to changes in lipid homeostasis caused by inactivation of HSL. Quantitative PCR for selected genes also revealed changes in genes that are related to insulin secretion, such as UCP-2. Therefore, these results suggest that the beta-cell isoform of HSL is involved in maintaining lipid homeostasis in islets and contributes to the proper control of GSIS.

Lipotoxicity

Excess fatty acids accompanied by triglyceride accumulation in parenchymal cells of multiple tissues including skeletal and cardiac myocytes, hepatocytes, and pancreatic beta cells results in chronic cellular dysfunction and injury. The process, now termed lipotoxicity, can account for many manifestations of the 'metabolic syndrome'. Most data suggest that the triglycerides serve primarily a storage function with toxicity deriving mainly from long-chain nonesterified fatty acids (NEFA) and their products such as ceramides and diacylglycerols. In the kidney, filtered NEFA carried on albumin can aggravate the chronic tubule damage and inflammatory phenotype that develop during proteinuric states and lipid loading of both glomerular and tubular cells is a common response to renal injury that contributes to progression of nephropathy. NEFA-induced mitochondrial dysfunction is the primary mechanism for energetic failure of proximal tubules during hypoxia/reoxygenation and persistent increases of tubule cell NEFA and triglycerides occur during acute renal failure in vivo in association with downregulation of mitochondrial and peroxisomal enzymes of beta oxidation. In acute renal failure models, peroxisome proliferator-activated receptor alpha ligand treatment can ameliorate the NEFA and triglyceride accumulation and limits tissue injury likely via both direct tubule actions and anti-inflammatory effects. Both acute and chronic kidney disease are associated with systemic manifestations of the metabolic syndrome.

Beta cell compensation for insulin resistance in Zucker fatty rats: increased lipolysis and fatty acid signalling

AIMS/HYPOTHESIS

The aim of this study was to determine the role of fatty acid signalling in islet beta cell compensation for insulin resistance in the Zucker fatty fa/fa (ZF) rat, a genetic model of severe obesity, hyperlipidaemia and insulin resistance that does not develop diabetes.

MATERIALS AND METHODS

NEFA augmentation of insulin secretion and fatty acid metabolism were studied in isolated islets from ZF and Zucker lean (ZL) control rats.

RESULTS

Exogenous palmitate markedly potentiated glucose-stimulated insulin secretion (GSIS) in ZF islets, allowing robust secretion at physiological glucose levels (5-8 mmol/l). Exogenous palmitate also synergised with glucagon-like peptide-1 and the cyclic AMP-raising agent forskolin to enhance GSIS in ZF islets only. In assessing islet fatty acid metabolism, we found increased glucose-responsive palmitate esterification and lipolysis processes in ZF islets, suggestive of enhanced triglyceride-fatty acid cycling. Interruption of glucose-stimulated lipolysis by the lipase inhibitor Orlistat (tetrahydrolipstatin) blunted palmitate-augmented GSIS in ZF islets. Fatty acid oxidation was also higher at intermediate glucose levels in ZF islets and steatotic triglyceride accumulation was absent.

CONCLUSIONS/INTERPRETATION

The results highlight the potential importance of NEFA and glucoincretin enhancement of insulin secretion in beta cell compensation for insulin resistance. We propose that coordinated glucose-responsive fatty acid esterification and lipolysis processes, suggestive of triglyceride-fatty acid cycling, play a role in the coupling mechanisms of glucose-induced insulin secretion as well as in beta cell compensation and the hypersecretion of insulin in obesity.

Genipin inhibits UCP2-mediated proton leak and acutely reverses obesity- and high glucose-induced beta cell dysfunction in isolated pancreatic islets

Uncoupling protein 2 (UCP2) negatively regulates insulin secretion. UCP2 deficiency (by means of gene knockout) improves obesity- and high glucose-induced beta cell dysfunction and consequently improves type 2 diabetes in mice. In the present study, we have discovered that the small molecule, genipin, rapidly inhibits UCP2-mediated proton leak. In isolated mitochondria, genipin inhibits UCP2-mediated proton leak. In pancreatic islet cells, genipin increases mitochondrial membrane potential, increases ATP levels, closes K(ATP) channels, and stimulates insulin secretion. These actions of genipin occur in a UCP2-dependent manner. Importantly, acute addition of genipin to isolated islets reverses high glucose- and obesity-induced beta cell dysfunction. Thus, genipin and/or chemically modified variants of genipin are useful research tools for studying biological processes thought to be controlled by UCP2. In addition, these agents represent lead compounds that comprise a starting point for the development of therapies aimed at treating beta cell dysfunction.

Specific inhibition of hormone-sensitive lipase improves lipid profile while reducing plasma glucose

Elevation of plasma free fatty acids has been linked with insulin resistance and diabetes. Inhibition of lipolysis may provide a mechanism to decrease plasma fatty acids, thereby improving insulin sensitivity. Hormone-sensitive lipase (HSL) is a critical enzyme involved in the hormonally regulated release of fatty acids and glycerol from adipocyte lipid stores, and its inhibition may thus improve insulin sensitivity and blood glucose handling in type 2 diabetes. In rat adipocytes, forskolin-activated lipolysis was blocked by in vitro addition of a potent and selective HSL inhibitor or by prior treatment of the animals themselves. Antilipolytic effects also were demonstrated in overnight-fasted mice, rats, and dogs with species-dependent effects on plasma free fatty acid levels but with similar reductions in plasma glycerol being observed in all species. Inhibition of HSL also reduced hyperglycemia in streptozotocin-induced diabetic rats. The data support a connection between adipose tissue lipolysis and plasma glucose levels.

Dietary-fat-induced obesity in mice results in beta cell hyperplasia but not increased insulin release: evidence for specificity of impaired beta cell adaptation

AIMS/HYPOTHESIS

Increased dietary fat intake is associated with obesity and insulin resistance, but studies have shown that the subsequent increase in insulin release is not appropriate for this obesity-induced insulin resistance. We therefore sought to determine whether the impaired beta cell adaptation is due to inadequate expansion of the beta cell population or to a lack of an adaptive increase in insulin release.

METHODS

Male mice were fed diets containing increasing amounts of fat (15, 30 or 45% of energy intake) for 1 year, after which islet morphology and secretory function were assessed.

RESULTS

Increased dietary fat intake was associated with a progressive increase in body weight (p<0.001). Fractional beta cell area (total beta cell area/section area) was increased with increasing dietary fat (1.36+/-0.39, 2.46+/-0.40 and 4.93+/-1.05%, p<0.001), due to beta cell hyperplasia, and was positively and highly correlated with body weight (r2=0.68, p<0.005). In contrast, insulin release following i.p. glucose did not increase with increasing dietary fat (118+/-32, 108+/-47 and 488+/-200 pmol/l per mmol/l, p=0.07) and did not correlate with body weight (r2=0.11). When this response was examined relative to fractional beta cell area (insulin release/fractional beta cell area), it did not increase but rather tended to decrease with increasing dietary fat (157+/-55, 43+/-13 and 97+/-53 [pmol/l per mmol/l]/%, p=0.06) and did not correlate with body weight (r2=0.02).

CONCLUSIONS/INTERPRETATION

Long-term fat feeding is associated with an increase in the beta cell population but an inadequate functional adaptation. Thus, a functional rather than a morphological abnormality appears to underlie dietary-fat-induced beta cell dysfunction.

RESISTANCE TO EXCESSIVE BODYWEIGHT GAIN IN RISPERIDONE-INJECTED RATS

1. The present study was carried out to explain the resistance of rats injected subcutaneously with risperidone, the atypical antipsychotic drug, for 21 consecutive days at 0.1 mg/kg per day (a dose equivalent to the one used for patients) to result in an excessive bodyweight despite the increase in diet-uptake in rats against risperidone-induced decrease in body temperature. 2. Rectal temperature measurements were made in 8-week-old male Sprague-Dawley rats maintained under standard laboratory conditions using a 12 h daylight cycle. A s.c. injection of risperidone (0.05 mg/kg) produced hypothermia in rats, which was observed during the daily injection for 21 consecutive days. 3. Sera, white and brown adipose tissues, skeletal muscle and liver were extracted from 8-week-old male Sprague-Dawley rats injected subcutaneously with risperidone (0.01 or 0.1 mg/kg per day) or a vehicle for 21 consecutive days. Serum levels of lipids, ketones and thyroid hormone were measured. The mRNA expression levels in these tissues and organs of the genes encoding the substances involved in heat production and/or lipid metabolism were investigated by using quantitative real-time polymerase chain reaction amplification. 4. Serum nonesterified fatty acid levels in risperidone 0.1 mg/kg per day s.c. injected rats were significantly lower than those in vehicle-injected ones. Serum beta-hydroxybutyrate levels in risperidone-injected rats tended to decrease compared with those in vehicle-injected ones. The serum level of neither triiodothyronine nor thyroxine was affected by risperidone s.c. injection at the doses examined, although their values were within normal limits. 5. Risperidone injection (0.1 mg/kg per day) for 21 consecutive days upregulated mRNA expressions in white adipose tissue of uncoupling protein 3 which dissipates energy as heat; peroxisome proliferator-activated receptor (PPAR) gamma coactivator 1alpha which activates mitochondrial biogenesis to expand the oxidative machinery; and PPARalpha which is necessary for the fat-depletion of adipocytes for thermogenesis. The mRNA of lipogenic enzymes (acetyl-CoA carboxylase alpha, fatty-acid synthase and glycerol-3-phosphate acyltransferase), hormone sensitive lipase and beta1-adrenoceptor were also enhanced in white adipose tissue by the injection of 0.1 mg/kg per day risperidone. 6. These findings suggest that the materials for heat generation in white adipose tissue would be readily supplied, which in turn would reduce a storage of lipids in white adipose tissue resulting in the lower rate of bodyweight gain of rats.

The mitochondrial uncoupling-protein homologues

Uncoupling protein(UCP)1 is an integral membrane protein that is located in the mitochondrial inner membrane of brown adipocytes. Its physiological role is to mediate a regulated, thermogenic proton leak. UCP2 and UCP3 are recently identified UCP1 homologues. They also mediate regulated proton leak, and might function to control the production of superoxide and other downstream reactive oxygen species. However, their role in normal physiology remains unknown. Recent studies have shown that UCP2 has an important part in the pathogenesis of type-2 diabetes. The obscure roles of the UCP homologues in normal physiology, together with their emerging role in pathophysiology, provide exciting potential for further investigation.

Pancreatic β-Cell Lipoprotein Lipase Independently Regulates Islet Glucose Metabolism and Normal Insulin Secretion

Lipid and glucose metabolism are adversely affected by diabetes, a disease characterized by pancreatic beta-cell dysfunction. To clarify the role of lipids in insulin secretion, we generated mice with beta-cell-specific overexpression (betaLPL-TG) or inactivation (betaLPL-KO) of lipoprotein lipase (LPL), a physiologic provider of fatty acids. LPL enzyme activity and triglyceride content were increased in betaLPL-TG islets; decreased LPL enzyme activity in betaLPL-KO islets did not affect islet triglyceride content. Surprisingly, both betaLPL-TG and betaLPL-KO mice were strikingly hyperglycemic during glucose tolerance testing. Impaired glucose tolerance in betaLPL-KO mice was present at one month of age, whereas betaLPL-TG mice did not develop defective glucose homeostasis until approximately five months of age. Glucose-simulated insulin secretion was impaired in islets isolated from both mouse models. Glucose oxidation, critical for ATP production and triggering of insulin secretion mediated by the ATP-sensitive potassium (KATP) channel, was decreased in betaLPL-TG islets but increased in betaLPL-KO islets. Islet ATP content was not decreased in either model. Insulin secretion was defective in both betaLPL-TG and betaLPL-KO islets under conditions causing calcium-dependent insulin secretion independent of the KATP channel. These results show that beta-cell-derived LPL has two physiologically relevant effects in islets, the inverse regulation of glucose metabolism and the independent mediation of insulin secretion through effects distal to membrane depolarization.

Mitochondrial dysfunction and type 2 diabetes

Maintenance of normal blood glucose levels depends on a complex interplay between the insulin responsiveness of skeletal muscle and liver and glucose-stimulated insulin secretion by pancreatic beta cells. Defects in the former are responsible for insulin resistance, and defects in the latter are responsible for progression to hyperglycemia. Emerging evidence supports the potentially unifying hypothesis that both of these prominent features of type 2 diabetes are caused by mitochondrial dysfunction.

Transgenic mice overexpressing nuclear SREBP-1c in pancreatic beta-cells

Influx of excess fatty acids and the resultant accumulation of intracellular triglycerides are linked to impaired insulin secretion and action in the pathogenesis of type 2 diabetes. Sterol regulatory element-binding protein (SREBP)-1c is a transcription factor that controls cellular synthesis of fatty acids and triglycerides. SREBP-1c is highly expressed in high-energy and insulin-resistant states. To investigate effects of this synthetic lipid regulator on insulin secretion, we generated transgenic mice overexpressing nuclear SREBP-1c under the insulin promoter. beta-Cell-specific expression of SREBP-1c caused reduction in islet mass and impaired glucose-stimulated insulin secretion and was associated with accumulation of triglycerides, suppression of pancreas duodenal homeobox-1, and upregulation of uncoupling protein 2 gene expression. The mice presented with impaired glucose tolerance that was exacerbated by a high-energy diet. Taken together with enhanced insulin secretion from SREBP-1-null islets, these data suggest that SREBP-1c and endogenous lipogenesis could be involved in beta-cell dysfunction and diabetes.

AMPK activation as a strategy for reversing the endothelial lipotoxicity underlying the increased vascular risk associated with insulin resistance syndrome

The endotheliopathy associated with insulin resistance syndrome appears to result largely from excessive free fatty acid (FFA) exposure that boosts endothelial production of diacylglycerol, thereby activating protein kinase C. This endothelial "lipotoxicity" can be alleviated by very-low-fat diets and by appropriate weight loss. In addition, pharmacological activation of endothelial AMP-activated kinase (AMPK), as with the drug metformin, has the potential to decrease the FFA content of endothelial cells by stimulating fat oxidation; AMPK may also suppress endothelial de novo synthesis of diacylglycerol by inhibiting glycerol-3-phosphate acyltransferase. These considerations may rationalize the superior impact of metformin therapy on the macrovascular health of diabetics. More generally, metformin - or, preferably, better tolerated activators of AMPK - may have considerable potential for promoting vascular health in the large proportion of the adult population afflicted with insulin resistance syndrome.

Analysis of the contribution to type 2 diabetes susceptibility of sequence variation in the gene encoding stearoyl-CoA desaturase, a key regulator of lipid and carbohydrate metabolism

AIMS/HYPOTHESIS

Stearoyl-CoA desaturase (SCD) is emerging as a key regulator of lipid and carbohydrate metabolism. Scd-null mice display a beneficial metabolic phenotype characterised by resistance to obesity, diabetes and hyperlipidaemia. The human homologue, SCD, maps to a region of chromosome 10 linked to type 2 diabetes, and SCD activity correlates with insulin sensitivity. Given this strong positional and biological candidacy, the present study sought to establish whether sequence variation in SCD influences susceptibility to type 2 diabetes and related traits.

METHODS

The SCD gene was resequenced in 23 diabetic subjects. Six variants within coding and adjacent sequence, including a non-synonymous SNP in exon 5 (M224L), were selected for genotyping in a primary set of 608 diabetic subjects and 600 control subjects.

RESULTS

There was no association (at the allele, genotype or haplotype level) with type 2 diabetes, although genotype frequencies at the +14301 A>C SNP in the 3' untranslated region showed borderline association (p~0.06) when evidence for linkage was taken into account. However, replication studies (350 young-onset diabetic patients; 747 controls) failed to confirm any relationship with diabetes for this variant. No significant associations were seen for diabetes-related traits including BMI and waist-to-hip ratio.

CONCLUSIONS/INTERPRETATION

The present study, the first reported analysis of this gene, indicates that the SCD variants typed do not explain chromosome-10-encoded susceptibility to type 2 diabetes. Although this study provided no evidence that SCD sequence variation influences diabetes susceptibility or related traits, SCD remains a major target for pharmaceutical and/or environmental manipulation.

Effect of rosiglitazone versus insulin on the pancreatic beta-cell function of subjects with type 2 diabetes

OBJECTIVE

In a previous study, we found observational evidence of improvement in beta-cell function when rosiglitazone was added to a failing oral antihyperglycemic regimen consisting of sulfonylureas and metformin. To confirm our previous observations, we designed and performed a prospective, randomized, and controlled study.

RESEARCH DESIGN AND METHODS

A total of 17 subjects with type 2 diabetes, inadequately controlled on a maximized oral antihyperglycemic double regimen of glimepiride and metformin, were randomized to the addition of rosiglitazone or insulin to their treatment regimens for a period of 6 months. At baseline and at 6 months, the following were performed: measurement of fasting plasma glucose, fasting proinsulin, and insulin levels; frequently sampled intravenous glucose tolerance test; and glucagon stimulation test for C-peptide.

RESULTS

Nine subjects were randomized to the addition of 8 mg rosiglitazone, and eight subjects were randomized to the addition of one injection of insulin (premixed 70/30) before their evening meal. The treatment groups were well matched for age, duration of diabetes, and BMI. Most important, the HbA(1c) was well matched between groups before treatment (8.7 +/- 0.3 and 9.0 +/- 0.3%; NS) and at the end of the 6 months (7.8 +/- 0.5 and 7.8 +/- 0.3%; NS). After 6 months, at the end of the study, there was a significant improvement in acute insulin response to glucose in the rosiglitazone group (+15.3 microIU x ml(-1) x 10 min(-1); P < 0.001) that led to an increase in the disposition index from 0.18 at baseline to 4.18 at 6 months (P = 0.02). Furthermore, at the end of the study, the proinsulin-to-insulin ratio had decreased in the rosiglitazone group by 36% (P = 0.03) but did not change significantly in the insulin treatment group.

CONCLUSIONS

Rosiglitazone, but not insulin, induced a recovery of pancreatic beta-cell function, as evidenced by the restoration of the first-phase insulin response to glucose, improvement in the disposition index, and a decrease in the proinsulin-to-insulin ratio in subjects with type 2 diabetes in whom oral antihyperglycemic therapy failed. This improvement was independent of the correction of glucotoxicity.

Hormone-sensitive lipase deficiency in mouse islets abolishes neutral cholesterol ester hydrolase activity but leaves lipolysis, acylglycerides, fat oxidation, and insulin secretion intact

Lipids are thought to serve as coupling factors in insulin secretion. Hormone-sensitive lipase (HSL) is expressed in pancreatic beta-cells and could potentially regulate insulin secretion via mobilization of stored triglycerides. Here, we examined the impact of HSL deficiency on fuel metabolism and insulin secretion in mouse islets. Lack of HSL resulted in abrogation of neutral cholesterol ester hydrolase activity, whereas diglyceride lipase activity remained intact. Although glucose stimulates lipolysis in rat islets, elevation of glucose with or without addition of cAMP failed to increase lipolysis in mouse islets regardless of genotype, as indicated by release of glycerol from islets. Storage of lipids, assayed as total acylglycerides, was unaltered in HSL null islets, and oxidation of fatty acids or glucose was not different. The intracellular rise in Ca(2+) triggered by glucose and its subsequent oscillations was unaffected in HSL null islets. Accordingly, insulin secretion in static incubations of islets, in response to fuel- and nonfuel secretagogues, was in no instance significantly different between wild-type and HSL null mice. The lacking impact of HSL deficiency on insulin secretion may be attributed to the failure of insulin secretagogues to stimulate lipolysis. Consequently, a regulatory function of lipid mobilization in insulin secretion in the mouse appears unlikely.

Hormone-sensitive lipase--new roles for an old enzyme

Although described initially as an intracellular adipocyte-specific triacylglycerol lipase, it is now clear that HSL (hormone-sensitive lipase) is expressed in multiple tissues and plays a number of roles in lipid metabolism, including that of a neutral cholesteryl ester hydrolase. The major isoform is a single polypeptide with a molecular mass of approx. 84 kDa and which comprises three major domains: a catalytic domain, a regulatory domain encoding several phosphorylation sites and an N-terminal domain involved in protein-protein and protein-lipid interactions. The activity of HSL is regulated acutely by several mechanisms, including reversible phosphorylation by a number of different protein kinases, translocation to different sites within the cell and interaction with a number of proteins, some of which may serve to direct the inhibitory products of HSL away from the protein. It is also apparent from work with HSL null mice that more than one enzyme species may be classified as a hormone-sensitive lipase. The possible presence of HSL in macrophages remains controversial, and the role of the protein in pancreatic beta-cells has yet to be fully elucidated. Altered expression of HSL in different cell types may be associated with a number of pathological states, including obesity, atherosclerosis and Type II diabetes.

Hormone-sensitive lipase has a role in lipid signaling for insulin secretion but is nonessential for the incretin action of glucagon-like peptide 1

We previously reported decreased glucose-stimulated insulin secretion (GSIS) in hormone-sensitive lipase-null mice (HSL(-/-)), both in vivo and in vitro. The focus of the current study was to gain further insight into the signaling role and regulation of lipolysis in islet tissue. The effect of glucagon-like peptide 1 (GLP-1) on GSIS was also studied, as GLP-1 could augment GSIS via protein kinase A activation of HSL and lipolysis. Freshly isolated islets from fasted and fed male HSL(-/-) and wild-type (HSL(+/+)) mice were studied at ages 4 and 7 months. Neutral cholesteryl ester hydrolase activity was markedly reduced in islets from both 4- and 7-month-old male HSL(-/-) mice, whereas a marked deficiency in triglyceride lipase activity became evident only in the older mice. The deficiencies in lipase activities were associated with higher islet triglyceride content and reduced lipolysis at basal glucose levels. Lipolysis was stimulated by high glucose in islets of both wild-type and HSL-null mice. Severe deficiencies in GSIS were found, but only in islets from 7-month-old, fasted, male HSL(-/-) mice. GSIS was less affected in 4-month-old fasted male HSL(-/-) mice and not reduced in female mice. Exogenous delivery of free fatty acids (FFAs) rescued GSIS, supporting the view that the lack of endogenous FFA supply for lipid-signaling processes in HSL(-/-) mice was responsible for the loss of GSIS. GLP-1 also rescued GSIS in HSL(-/-) mice, indicating that signaling via HSL is not a major pathway for its incretin effect. Thus, the secretory phenotype of HSL-null mice is gender dependent, increases with age, and is influenced by the nutritional state. Under most circumstances, the major determinant of lipolytic flux in the beta-cell involves an enzyme(s) other than HSL that is acutely activated by glucose. Our results support the view that the availability of endogenous FFA through HSL and an additional enzyme(s) is involved in providing lipid moieties for beta-cell signaling for secretion in response to glucose.

Non-esterified fatty acids are deleterious for human pancreatic islet function at physiological glucose concentration

AIMS/HYPOTHESIS

Whether excess glucose (glucotoxicity) and excess non-esterified fatty acids (lipotoxicity) act synergistically or separately to alter beta-cell function on Type 2 diabetes remains controversial. We examined the influence of non-esterified fatty acids, with or without concomitant increased glucose concentrations, on human islet function and on the expression of genes involved in lipid metabolism.

METHODS

Human islets isolated from non-diabetic and non-obese donors were cultured with 5.5, 16 or 30 mmol/l glucose, and when appropriate with 1 or 2 mmol/l non-esterified fatty acids. After 48 h, glucose-stimulated insulin secretion, insulin content, triglyceride content and expression of different genes were evaluated.

RESULTS

Non-esterified fatty acids decreased glucose-stimulated insulin secretion, insulin content and increased triglyceride content of human isolated islets, independently from the deleterious effect of glucose. Increased glucose concentrations also decreased glucose-stimulated insulin secretion and insulin content, but had no influence on triglyceride content. Glucose-stimulated insulin secretion of islets appeared to be significantly correlated with their triglyceride content. Glucose and non-esterified fatty acids modified the gene expression of carnitine palmitoyltransferase-I, acetyl-CoA carboxylase, acyl-CoA oxidase and uncoupling protein 2.

CONCLUSION/INTERPRETATION

In our model of isolated human islets, increased glucose and non-esterified fatty acids separately reproduced the two major beta-cell alterations observed in vivo, i.e. loss of glucose-stimulated insulin secretion and reduction in islet insulin content. Our results also suggest that this deleterious effect was, at least in part, mediated by modifications in lipid metabolism gene expression.