Chronic effects of palmitate overload on nutrient-induced insulin secretion and autocrine signalling in pancreatic MIN6 beta cells

The reversible effects of free fatty acids on sulfonylurea-stimulated insulin secretion are related to the expression and dynamin-mediated endocytosis of KATP channels in pancreatic β cells

Objective

Lipotoxicity-induced pancreatic β cell-dysfunction results in decreased insulin secretion in response to multiple stimulus. In this study, we investigated the reversible effects of palmitate (PA) or oleate (OA) on insulin secretion and the relationship with pancreatic β-cell ATP-sensitive potassium (KATP) channels.

Methods

MIN6 cells were treated with PA and OA for 48 h and then washed out for 24 h to determine the changes in expression and endocytosis of the KATP channels and glucose-stimulated insulin secretion (GSIS) and sulfonylurea-stimulated insulin secretion (SU-SIS).

Results

MIN6 cells exposed to PA or OA showed both impaired GSIS and SU-SIS; the former was not restorable, while the latter was reversible with washout of PA or OA. Decreased expressions of both total and surface Kir6.2 and SUR1 and endocytosis of KATP channels were observed, which were also recoverable after washout. When MIN6 cells exposed to free fatty acids (FFAs) were cotreated with 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR) or dynasore, we found that endocytosis of KATP channels did not change significantly by AICAR but was almost completely blocked by dynasore. Meanwhile, the inhibition of endocytosis of KATP channels after washout could be activated by PIP2. The recovery of SU-SIS after washout was significantly weakened by PIP2, but the decrease of SU-SIS induced by FFAs was not alleviated by dynasore.

Conclusions

FFAs can cause reversible impairment of SU-SIS on pancreatic β cells. The reversibility of the effects is partial because of the changes of expression and endocytosis of Kir6.2 and SUR1 which was mediated by dynamin.

Glucose Regulates m6A Methylation of RNA in Pancreatic Islets

Type 2 diabetes is characterized by chronic hyperglycemia associated with impaired insulin action and secretion. Although the heritability of type 2 diabetes is high, the environment, including blood components, could play a major role in the development of the disease. Amongst environmental effects, epitranscriptomic modifications have been recently shown to affect gene expression and glucose homeostasis. The epitranscriptome is characterized by reversible chemical changes in RNA, with one of the most prevalent being the m⁶A methylation of RNA. Since pancreatic β cells fine tune glucose levels and play a major role in type 2 diabetes physiopathology, we hypothesized that the environment, through variations in blood glucose or blood free fatty acid concentrations, could induce changes in m⁶A methylation of RNAs in pancreatic β cells. Here we observe a significant decrease in m⁶A methylation upon high glucose concentration, both in mice and human islets, associated with altered expression levels of m⁶A demethylases. In addition, the use of siRNA and/or specific inhibitors against selected m⁶A enzymes demonstrate that these enzymes modulate the expression of genes involved in pancreatic β-cell identity and glucose-stimulated insulin secretion. Our data suggest that environmental variations, such as glucose, control m⁶A methylation in pancreatic β cells, playing a key role in the control of gene expression and pancreatic β-cell functions. Our results highlight novel causes and new mechanisms potentially involved in type 2 diabetes physiopathology and may contribute to a better understanding of the etiology of this disease.

Molecular Mechanisms of Apoptosis Induction and Its Regulation by Fatty Acids in Pancreatic β-Cells

Pancreatic β-cell failure and death contribute significantly to the pathogenesis of type 2 diabetes. One of the main factors responsible for β-cell dysfunction and subsequent cell death is chronic exposure to increased concentrations of FAs (fatty acids). The effect of FAs seems to depend particularly on the degree of their saturation. Saturated FAs induce apoptosis in pancreatic β-cells, whereas unsaturated FAs are well tolerated and are even capable of inhibiting the pro-apoptotic effect of saturated FAs. Molecular mechanisms of apoptosis induction by saturated FAs in β-cells are not completely elucidated. Saturated FAs induce ER stress, which in turn leads to activation of all ER stress pathways. When ER stress is severe or prolonged, apoptosis is induced. The main mediator seems to be the CHOP transcription factor. Via regulation of expression/activity of pro- and anti-apoptotic Bcl-2 family members, and potentially also through the increase in ROS production, CHOP switches on the mitochondrial pathway of apoptosis induction. ER stress signalling also possibly leads to autophagy signalling, which may activate caspase-8. Saturated FAs activate or inhibit various signalling pathways, i.e., p38 MAPK signalling, ERK signalling, ceramide signalling, Akt signalling and PKCδ signalling. This may lead to the activation of the mitochondrial pathway of apoptosis, as well. Particularly, the inhibition of the pro-survival Akt signalling seems to play an important role. This inhibition may be mediated by multiple pathways (e.g., ER stress signalling, PKCδ and ceramide) and could also consequence in autophagy signalling. Experimental evidence indicates the involvement of certain miRNAs in mechanisms of FA-induced β-cell apoptosis, as well. In the rather rare situations when unsaturated FAs are also shown to be pro-apoptotic, the mechanisms mediating this effect in β-cells seem to be the same as for saturated FAs. To conclude, FA-induced apoptosis rather appears to be preceded by complex cross talks of multiple signalling pathways. Some of these pathways may be regulated by decreased membrane fluidity due to saturated FA incorporation. Few data are available concerning molecular mechanisms mediating the protective effect of unsaturated FAs on the effect of saturated FAs. It seems that the main possible mechanism represents a rather inhibitory intervention into saturated FA-induced pro-apoptotic signalling than activation of some pro-survival signalling pathway(s) or metabolic interference in β-cells. This inhibitory intervention may be due to an increase of membrane fluidity.

Peroxisomes and pancreatic beta-cell lipo-dysfunction

AIMS

Pancreatic beta-cell lipo-dysfunction decreases insulin secretion and predisposes to the development of type 2 diabetes. Through targeted Pex11β knockdown and peroxisome depletion, our aim was to investigate the specific contribution of peroxisomes to palmitate mediated pancreatic beta-cell dysfunction.

METHODS

MIN6 cells were transfected with probes targeted against Pex11β, a regulator of peroxisome abundance, or with scrambled control probes. Peroxisome abundance was measured by PMP-70 protein expression. 48 h post transfection, cells were incubated with 250 μM palmitate or BSA control for a further 48 h before measurement of glucose stimulated insulin secretion and of reactive oxygen species.

RESULTS

Pex11β knockdown decreased target gene expression by >80% compared with the scrambled control (P<0.001). This led to decreased PMP-70 expression (p<0.01) and a 22% decrease in peroxisome number (p<0.05). At 25 mM glucose, palmitate treatment decreased insulin secretion by 64% in the scrambled control cells (2.54±0.25 vs 7.07±0.83 [mean±SEM] ng/h/μg protein; Palmitate vs BSA P<0.001), but by just 37% in the Pex11β knockdown cells. Comparing responses in the presence of palmitate, insulin secretion at 25 mM glucose was significantly greater in the Pex11β knockdown cells compared with the scrambled controls (4.04±0.46 vs 2.54±0.25 ng/h/μg protein; p<0.05). Reactive oxygen species generation with palmitate was lower in the Pex11β knockdown cells compared with the scrambled controls (P<0.001).

CONCLUSION

Pex11β knockdown decreased peroxisome abundance, decreased palmitate mediated reactive oxygen species generation, and reversed the inhibitory effect of palmitate on insulin secretion. These findings reveal a distinct role of peroxisomes in palmitate mediated beta-cell dysfunction.

Protein Kinases Signaling in Pancreatic Beta-cells Death and Type 2 Diabetes

Type 2 diabetes (T2D) is a worldwide serious public health problem. Insulin resistance and β-cell failure are the two major components of T2D pathology. In addition to defective endoplasmic reticulum (ER) stress signaling due to glucolipotoxicity, β-cell dysfunction or β-cell death initiates the deleterious vicious cycle observed in T2D. Although the primary cause is still unknown, overnutrition that contributes to the induction of the state of low-grade inflammation, and the activation of various protein kinases-related metabolic pathways are main factors leading to T2D. In this chapter following subjects, which have critical checkpoints regarding β-cell fate and protein kinases pathways are discussed; hyperglycemia-induced β-cell failure, chronic accumulation of unfolded protein in β-cells, the effect of intracellular reactive oxygen species (ROS) signaling to insulin secretion, excessive saturated free fatty acid-induced β-cell apoptosis, mitophagy dysfunction, proinflammatory responses and insulin resistance, and the reprogramming of β-cell for differentiation or dedifferentiation in T2D. There is much debate about selecting proposed therapeutic strategies to maintain or enhance optimal β-cell viability for adequate insulin secretion in T2D. However, in order to achieve an effective solution in the treatment of T2D, more intensive clinical trials are required on newer therapeutic options based on protein kinases signaling pathways.

Diacylglycerol-evoked activation of PKC and PKD isoforms in regulation of glucose and lipid metabolism: a review

Protein kinase C (PKC) and Protein kinase D (PKD) isoforms can sense diacylglycerol (DAG) generated in the different cellular compartments in various physiological processes. DAG accumulates in multiple organs of the obese subjects, which leads to the disruption of metabolic homeostasis and the development of diabetes as well as associated diseases. Multiple studies proved that aberrant activation of PKCs and PKDs contributes to the development of metabolic diseases. DAG-sensing PKC and PKD isoforms play a crucial role in the regulation of metabolic homeostasis and therefore might serve as targets for the treatment of metabolic disorders such as obesity and diabetes.

What role do fat cells play in pancreatic tissue?

Background

It is now generally accepted that obesity is a major risk factor for type 2 diabetes mellitus (T2DM). Hepatic steatosis in particular, as well as visceral and ectopic fat accumulation within tissues, is associated with the development of the disease. We recently presented the first study on isolated human pancreatic adipocytes and their interaction with islets [Gerst, F., Wagner, R., Kaiser, G., Panse, M., Heni, M., Machann, J., et al., 2017. Metabolic crosstalk between fatty pancreas and fatty liver: effects on local inflammation and insulin secretion. Diabetologia 60(11):2240–2251.]. The results indicate that the function of adipocytes depends on the overall metabolic status in humans which, in turn, differentially affects islet hormone release.

Scope of Review

This review summarizes former and recent studies on factors derived from adipocytes and their effects on insulin-secreting β-cells, with particular emphasis on the human pancreas. The adipocyte secretome is discussed with a special focus on its influence on insulin secretion, β-cell survival and apoptotic β-cell death.

Major Conclusions

Human pancreatic adipocytes store lipids and release adipokines, metabolites, and pro-inflammatory molecules in response to the overall metabolic, humoral, and neuronal status. The differentially regulated adipocyte secretome impacts on endocrine function, i.e., insulin secretion, β-cell survival and death which interferes with glycemic control. This review attempts to explain why the extent of pancreatic steatosis is associated with reduced insulin secretion in some studies but not in others.

Palmitate induces neuroinflammation, ER stress, and Pomc mRNA expression in hypothalamic mHypoA-POMC/GFP neurons through novel mechanisms that are prevented by oleate

Dietary fats can modulate brain function. How free fatty acids (FFAs) alter hypothalamic pro-opiomelanocortin (POMC) neurons remain undefined. The saturated FFA, palmitate, increased neuroinflammatory and ER stress markers, as well as Pomc mRNA levels, but did not affect insulin signaling, in mHypoA-POMC/GFP-2 neurons. This effect was mediated through the MAP kinases JNK and ERK. Further, the increase in Pomc was dependent on palmitoyl-coA synthesis, but not de novo ceramide synthesis, as inhibition of SPT enhanced palmitate-induced Pomc expression, while methylpalmitate had no effect. While palmitate concomitantly induces neuroinflammation and ER stress, these effects were independent of changes in Pomc expression. Palmitate thus has direct acute effects on Pomc, which appears to be important for negative feedback, but not directly related to neuroinflammation. The monounsaturated FFA oleate completely blocked the palmitate-mediated increase in neuroinflammation, ER stress, and Pomc mRNAs. This study provides insight into the complex central metabolic regulation by FFAs.

Epicatechin potentiation of glucose-stimulated insulin secretion in INS-1 cells is not dependent on its antioxidant activity

Epicatechin (EC) is a monomeric flavan-3-ol. We have previously demonstrated that glucose-intolerant rats fed flavan-3-ols exhibit improved pancreatic islet function corresponding with an increase in circulating EC-derived metabolites. Thus, we speculate that EC may act as a cellular signaling molecule in vivo to modulate insulin secretion. In this study we further examined the effects of different concentrations of EC on H2O2 or hyperglycemia-induced ROS production, as well as on saturated fatty acid (SFA)-impaired glucose-stimulated insulin secretion (GSIS) in INS-1 cell line in vitro. We showed that EC at a high concentration (30 μmol/L), but not a low concentration (0.3 μmol/L), significantly decreased H2O2 or hyperglycemia-induced ROS production in INS-1 cells. However, EC (0.3 μmol/L) significantly enhanced SFA-impaired GSIS in INS-1 cells. Addition of KN-93, a CaMKII inhibitor, blocked the effect of EC on insulin secretion and decreased CaMKII phosphorylation. Addition of GW1100, a GPR40 antagonist, significantly attenuated EC-enhanced GSIS, but only marginally affected CaMKII phosphorylation. These results demonstrate that EC at a physiological concentration promotes GSIS in SFA-impaired β-cells via activation of the CaMKII pathway and is consistent with its function as a GPR40 ligand. The findings support a role for EC as a cellular signaling molecule in vivo and further delineate the signaling pathways of EC in β-cells.

Oxidative stress and calcium dysregulation by palmitate in type 2 diabetes

Free fatty acids (FFAs) are important substrates for mitochondrial oxidative metabolism and ATP synthesis but also cause serious stress to various tissues, contributing to the development of metabolic diseases. CD36 is a major mediator of cellular FFA uptake. Inside the cell, saturated FFAs are able to induce the production of cytosolic and mitochondrial reactive oxygen species (ROS), which can be prevented by co-exposure to unsaturated FFAs. There are close connections between oxidative stress and organellar Ca²⁺ homeostasis. Highly oxidative conditions induced by palmitate trigger aberrant endoplasmic reticulum (ER) Ca²⁺ release and thereby deplete ER Ca²⁺ stores. The resulting ER Ca²⁺ deficiency impairs chaperones of the protein folding machinery, leading to the accumulation of misfolded proteins. This ER stress may further aggravate oxidative stress by augmenting ER ROS production. Secondary to ER Ca²⁺ release, cytosolic and mitochondrial matrix Ca²⁺ concentrations can also be altered. In addition, plasmalemmal ion channels operated by ER Ca²⁺ depletion mediate persistent Ca²⁺ influx, further impairing cytosolic and mitochondrial Ca²⁺ homeostasis. Mitochondrial Ca²⁺ overload causes superoxide production and functional impairment, culminating in apoptosis. This vicious cycle of lipotoxicity occurs in multiple tissues, resulting in β-cell failure and insulin resistance in target tissues, and further aggravates diabetic complications.

Kinase Signaling in Apoptosis Induced by Saturated Fatty Acids in Pancreatic β-Cells

Pancreatic β-cell failure and death is considered to be one of the main factors responsible for type 2 diabetes. It is caused by, in addition to hyperglycemia, chronic exposure to increased concentrations of fatty acids, mainly saturated fatty acids. Molecular mechanisms of apoptosis induction by saturated fatty acids in β-cells are not completely clear. It has been proposed that kinase signaling could be involved, particularly, c-Jun N-terminal kinase (JNK), protein kinase C (PKC), p38 mitogen-activated protein kinase (p38 MAPK), extracellular signal-regulated kinase (ERK), and Akt kinases and their pathways. In this review, we discuss these kinases and their signaling pathways with respect to their possible role in apoptosis induction by saturated fatty acids in pancreatic β-cells.

Targeting ceramide metabolism in obesity

Obesity is a major health concern that increases the risk for insulin resistance, type 2 diabetes (T2D), and cardiovascular disease. Thus, an enormous research effort has been invested into understanding how obesity-associated dyslipidemia and obesity-induced alterations in lipid metabolism increase the risk for these diseases. Accordingly, it has been proposed that the accumulation of lipid metabolites in organs such as the liver, skeletal muscle, and heart is critical to these obesity-induced pathologies. Ceramide is one such lipid metabolite that accumulates in tissues in response to obesity, and both pharmacological and genetic strategies that reduce tissue ceramide levels yield salutary actions on overall metabolic health. We will review herein why ceramide accumulates in tissues during obesity and how an increase in intracellular ceramide impacts cellular signaling and function as well as potential mechanisms by which reducing intracellular ceramide levels improves insulin resistance, T2D, atherosclerosis, and heart failure. Because a reduction in skeletal muscle ceramide levels is frequently associated with improvements in insulin sensitivity in humans, the beneficial findings reported for reducing ceramides in preclinical studies may have clinical application in humans. Therefore, modulating ceramide metabolism may be a novel, exciting target for preventing and/or treating obesity-related diseases.

p38 MAPK Is Activated but Does Not Play a Key Role during Apoptosis Induction by Saturated Fatty Acid in Human Pancreatic β-Cells

Saturated stearic acid (SA) induces apoptosis in the human pancreatic β-cells NES2Y. However, the molecular mechanisms involved are unclear. We showed that apoptosis-inducing concentrations of SA activate the p38 MAPK signaling pathway in these cells. Therefore, we tested the role of p38 MAPK signaling pathway activation in apoptosis induction by SA in NES2Y cells. Crosstalk between p38 MAPK pathway activation and accompanying ERK pathway inhibition after SA application was also tested. The inhibition of p38 MAPK expression by siRNA silencing resulted in a decrease in MAPKAPK-2 activation after SA application, but it had no significant effect on cell viability or the level of phosphorylated ERK pathway members. The inhibition of p38 MAPK activity by the specific inhibitor SB202190 resulted in inhibition of MAPKAPK-2 activation and noticeable activation of ERK pathway members after SA treatment but in no significant effect on cell viability. p38 MAPK overexpression by plasmid transfection produced an increase in MAPKAPK-2 activation after SA exposure but no significant influence on cell viability or ERK pathway activation. The activation of p38 MAPK by the specific activator anisomycin resulted in significant activation of MAPKAPK-2. Concerning the effect on cell viability, application of the activator led to apoptosis induction similar to application of SA (PARP cleavage and caspase-7, -8, and -9 activation) and in inhibition of ERK pathway members. We demonstrated that apoptosis-inducing concentrations of SA activate the p38 MAPK signaling pathway and that this activation could be involved in apoptosis induction by SA in the human pancreatic β-cells NES2Y. However, this involvement does not seem to play a key role. Crosstalk between p38 MAPK pathway activation and ERK pathway inhibition in NES2Y cells seems likely. Thus, the ERK pathway inhibition by p38 MAPK activation does not also seem to be essential for SA-induced apoptosis.

Islet Brain 1 Protects Insulin Producing Cells against Lipotoxicity

Chronic intake of saturated free fatty acids is associated with diabetes and may contribute to the impairment of functional beta cell mass. Mitogen activated protein kinase 8 interacting protein 1 also called islet brain 1 (IB1) is a candidate gene for diabetes that is required for beta cell survival and glucose-induced insulin secretion (GSIS). In this study we investigated whether IB1 expression is required for preserving beta cell survival and function in response to palmitate. Chronic exposure of MIN6 and isolated rat islets cells to palmitate led to reduction of the IB1 mRNA and protein content. Diminution of IB1 mRNA and protein level relied on the inducible cAMP early repressor activity and proteasome-mediated degradation, respectively. Suppression of IB1 level mimicked the harmful effects of palmitate on the beta cell survival and GSIS. Conversely, ectopic expression of IB1 counteracted the deleterious effects of palmitate on the beta cell survival and insulin secretion. These findings highlight the importance in preserving the IB1 content for protecting beta cell against lipotoxicity in diabetes.

Palmitic acid-rich diet suppresses glucose-stimulated insulin secretion (GSIS) and induces endoplasmic reticulum (ER) stress in pancreatic islets in mice

The objective was to clarify whether dietary palmitic acid supplementation affects glucose-stimulated insulin secretion (GSIS) and the endoplasmic reticulum (ER) stress pathway in pancreatic islets in mice. Eight-week-old male C57BL/6J mice were randomly divided into three treatment diet groups: control diet, palmitic acid-supplemented diet (PAL) and oleic acid-supplemented diet (OLE). After 2 weeks of treatment, intraperitoneal glucose tolerance test and intraperitoneal insulin tolerance test were performed. GSIS was assessed by pancreatic perfusion in situ with basal (100 mg/dL) glucose followed by a high (300 mg/dL) glucose concentration. We measured mRNA levels of ER stress markers such as C/EBP homologous protein (CHOP), immunoglobulin heavy-chain binding protein (BIP) and X-box binding protein (XBP)-1 using real-time polymerase chain reaction (PCR) analyses in isolated islets. Immunohistochemical staining was also performed. Mice fed PAL showed significantly decreased glucose tolerance (p < 0.05). In the perfusion study, GSIS was significantly suppressed in the PAL group (p < 0.05). Semi-quantitative RT-PCR revealed that islet CHOP, BIP, and XBP-1 mRNA expression were significantly increased in the PAL group (p < 0.05). TUNEL-positive β-cells were not detected in all groups. Dietary palmitic acid-supplementation for 2 weeks might suppress GSIS and induce ER stress in pancreatic islets in mice, in the early stage of lipotoxicity.

Ganglioside GM3 as a gatekeeper of obesity-associated insulin resistance: Evidence and mechanisms

Gangliosides constitute a large family of sialic acid-containing glycosphingolipids which play a key regulatory role in a diverse array of cellular processes, including receptor-associated signalling. Accordingly, the aberrant production of the ganglioside GM3 has been linked to pathophysiological changes associated with obesity, which in turn can lead to metabolic disorders such as insulin resistance and type 2 diabetes mellitus. This review examines the role of GM3 in mediating obesity-induced perturbations in metabolic function, including impaired insulin action. By doing so, we highlight the potential use of therapies targeting GM3 biosynthesis in order to counteract obesity-related metabolic disorders.

Metabolic memory of ß-cells controls insulin secretion and is mediated by CaMKII

Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) functions both in regulation of insulin secretion and neurotransmitter release through common downstream mediators. Therefore, we hypothesized that pancreatic ß-cells acquire and store the information contained in calcium pulses as a form of "metabolic memory", just as neurons store cognitive information. To test this hypothesis, we developed a novel paradigm of pulsed exposure of ß-cells to intervals of high glucose, followed by a 24-h consolidation period to eliminate any acute metabolic effects. Strikingly, ß-cells exposed to this high-glucose pulse paradigm exhibited significantly stronger insulin secretion. This metabolic memory was entirely dependent on CaMKII. Metabolic memory was reflected on the protein level by increased expression of proteins involved in glucose sensing and Ca(2+)-dependent vesicle secretion, and by elevated levels of the key ß-cell transcription factor MAFA. In summary, like neurons, human and mouse ß-cells are able to acquire and retrieve information.

Inhibition of pancreatic β-cell Ca2+/calmodulin-dependent protein kinase II reduces glucose-stimulated calcium influx and insulin secretion, impairing glucose tolerance

Glucose-stimulated insulin secretion (GSIS) from pancreatic β-cells is caused by Ca(2+) entry via voltage-dependent Ca(2+) channels. CaMKII is a key mediator and feedback regulator of Ca(2+) signaling in many tissues, but its role in β-cells is poorly understood, especially in vivo. Here, we report that mice with conditional inhibition of CaMKII in β-cells show significantly impaired glucose tolerance due to decreased GSIS. Moreover, β-cell CaMKII inhibition dramatically exacerbates glucose intolerance following exposure to a high fat diet. The impairment of islet GSIS by β-cell CaMKII inhibition is not accompanied by changes in either glucose metabolism or the activities of KATP and voltage-gated potassium channels. However, glucose-stimulated Ca(2+) entry via voltage-dependent Ca(2+) channels is reduced in islet β-cells with CaMKII inhibition, as well as in primary wild-type β-cells treated with a peptide inhibitor of CaMKII. The levels of basal β-cell cytoplasmic Ca(2+) and of endoplasmic reticulum Ca(2+) stores are also decreased by CaMKII inhibition. In addition, CaMKII inhibition suppresses glucose-stimulated action potential firing frequency. These results reveal that CaMKII is a Ca(2+) sensor with a key role as a feed-forward stimulator of β-cell Ca(2+) signals that enhance GSIS under physiological and pathological conditions.

The class I histone deacetylase inhibitor MS-275 prevents pancreatic beta cell death induced by palmitate

Elevation of the dietary saturated fatty acid palmitate contributes to the reduction of functional beta cell mass in the pathogenesis of type 2 diabetes. The diabetogenic effect of palmitate is achieved by increasing beta cell death through induction of the endoplasmic reticulum (ER) stress markers including activating transcription factor 3 (Atf3) and CAAT/enhancer-binding protein homologous protein-10 (Chop). In this study, we investigated whether treatment of beta cells with the MS-275, a HDAC1 and HDAC3 activity inhibitor which prevents beta cell death elicited by cytokines, is beneficial for combating beta cell dysfunction caused by palmitate. We show that culture of isolated human islets and MIN6 cells with MS-275 reduced apoptosis evoked by palmitate. The protective effect of MS-275 was associated with the attenuation of the expression of Atf3 and Chop. Silencing of HDAC3, but not of HDAC1, mimicked the effects of MS-275 on the expression of the two ER stress markers and apoptosis. These data point to HDAC3 as a potential drug target for preserving beta cells against lipotoxicity in diabetes.

Novel insulin sensitizer modulates nutrient sensing pathways and maintains β-cell phenotype in human islets

Major bottlenecks in the expansion of human β-cell mass are limited proliferation, loss of β-cell phenotype, and increased apoptosis. In our previous studies, activation of Wnt and mTOR signaling significantly enhanced human β-cell proliferation. However, isolated human islets displayed insulin signaling pathway resistance, due in part to chronic activation of mTOR/S6K1 signaling that results in negative feedback of the insulin signaling pathway and a loss of Akt phosphorylation and insulin content. We evaluated the effects of a new generation insulin sensitizer, MSDC-0160, on restoring insulin/IGF-1 sensitivity and insulin content in human β-cells. This novel TZD has low affinity for binding and activation of PPARγ and has insulin-sensitizing effects in mouse models of diabetes and ability to lower glucose in Phase 2 clinical trials. MSDC-0160 treatment of human islets increased AMPK activity and reduced mTOR activity. This was associated with the restoration of IGF-1-induced phosphorylation of Akt, GSK-3, and increased protein expression of Pdx1. Furthermore, MSDC-0160 in combination with IGF-1 and 8 mM glucose increased β-cell specific gene expression of insulin, pdx1, nkx6.1, and nkx2.2, and maintained insulin content without altering glucose-stimulated insulin secretion. Human islets were unable to simultaneously promote DNA synthesis and maintain the β-cell phenotype. Lithium-induced GSK-3 inhibition that promotes DNA synthesis blocked the ability of MSDC-0160 to maintain the β-cell phenotype. Conversely, MSDC-0160 prevented an increase in DNA synthesis by blocking β-catenin nuclear translocation. Due to the counteracting pathways involved in these processes, we employed a sequential ex vivo strategy to first induce human islet DNA synthesis, followed by MSDC-0160 to promote the β-cell phenotype and insulin content. This new generation PPARγ sparing insulin sensitizer may provide an initial tool for relieving inherent human islet insulin signaling pathway resistance that is necessary to preserve the β-cell phenotype during β-cell expansion for the treatment of diabetes.

Nutrition-/diet-induced changes in gene expression in pancreatic β-cells

β-Cell dysfunction is a critical component in the development of type 2 diabetes. Whilst both genetic and environmental factors contribute to the development of the disease, relatively little is known about the molecular network that is responsible for diet-induced functional changes in pancreatic β-cells. Recent genome-wide association studies for diabetes-related traits have generated a large number of candidate genes that constitute possible links between dietary factors and the genetic susceptibility for β-cell failure. Here, we summarize recent approaches for identifying nutritionally regulated transcripts in islets on a genome-wide scale. Polygenic mouse models for type 2 diabetes have been instrumental for investigating the mechanism of diet-induced β-cell dysfunction. Enhanced oxidative metabolism, triggered by a combination of dietary carbohydrates and fat, appears to play a critical role in the pathophysiology of diet-induced impairment of islets. More systematic studies of gene-diet interactions in β-cells of rodent models in combination with genetic profiling might reveal the regulatory circuits fundamental for the understanding of diet-induced impairments of β-cell function in humans.

Roles of ceramide and sphingolipids in pancreatic β-cell function and dysfunction

Recent technical advances have re-invigorated the study of sphingolipid metabolism in general, and helped to highlight the varied and important roles that sphingolipids play in pancreatic β-cells. Sphingolipid metabolites such as ceramide, glycosphingolipids, sphingosine 1-phosphate and gangliosides modulate many β-cell signaling pathways and processes implicated in β-cell diabetic disease such as apoptosis, β-cell cytokine secretion, ER-to-golgi vesicular trafficking, islet autoimmunity and insulin gene expression. They are particularly relevant to lipotoxicity. Moreover, the de novo synthesis of sphingolipids occurs on many subcellular membranes, in parallel to secretory vesicle formation, traffic and granule maturation events. Indeed, the composition of the plasma membrane, determined by the activity of neutral sphingomyelinases, affects β-cell excitability and potentially insulin exocytosis while another glycosphingolipid, sulfatide, determines the stability of insulin crystals in granules. Most importantly, sphingolipid metabolism on internal membranes is also strongly implicated in regulating β-cell apoptosis.