Differential activation of ER stress and apoptosis in response to chronically elevated free fatty acids in pancreatic beta-cells

Lack of TXNIP protects against mitochondria-mediated apoptosis but not against fatty acid-induced ER stress-mediated beta-cell death

OBJECTIVE

We have previously shown that lack of thioredoxin-interacting protein (TXNIP) protects against diabetes and glucotoxicity-induced beta-cell apoptosis. Because the role of TXNIP in lipotoxicity is unknown, the goal of the present study was to determine whether TXNIP expression is regulated by fatty acids and whether TXNIP deficiency also protects beta-cells against lipoapoptosis. RESARCH DESIGN AND METHODS: To determine the effects of fatty acids on beta-cell TXNIP expression, INS-1 cells and isolated islets were incubated with/without palmitate and rats underwent cyclic infusions of glucose and/or Intralipid prior to islet isolation and analysis by quantitative real-time RT-PCR and immunoblotting. Using primary wild-type and TXNIP-deficient islets, we then assessed the effects of palmitate on apoptosis (transferase-mediated dUTP nick-end labeling [TUNEL]), mitochondrial death pathway (cytochrome c release), and endoplasmic reticulum (ER) stress (binding protein [BiP], C/EBP homologous protein [CHOP]). Effects of TXNIP deficiency were also tested in the context of staurosporine (mitochondrial damage) or thapsigargin (ER stress).

RESULTS

Glucose elicited a dramatic increase in islet TXNIP expression both in vitro and in vivo, whereas fatty acids had no such effect and, when combined with glucose, even abolished the glucose effect. We also found that TXNIP deficiency does not effectively protect against palmitate or thapsigargin-induced beta-cell apoptosis, but specifically prevents staurosporine- or glucose-induced toxicity.

CONCLUSIONS

Our results demonstrate that unlike glucose, fatty acids do not induce beta-cell expression of proapoptotic TXNIP. They further reveal that TXNIP deficiency specifically inhibits the mitochondrial death pathway underlying beta-cell glucotoxicity, whereas it has very few protective effects against ER stress-mediated lipoapoptosis.

Palmitate attenuates insulin signaling and induces endoplasmic reticulum stress and apoptosis in hypothalamic neurons: rescue of resistance and apoptosis through adenosine 5' monophosphate-activated protein kinase activation

Hypothalamic insulin signaling is essential to the maintenance of glucose and energy homeostasis. During pathological states, such as obesity and type 2 diabetes mellitus, insulin signaling is impaired. One key mechanism involved in the development of insulin resistance is lipotoxicity, through increased circulating saturated fatty acids. Although many studies have begun to determine the underlying mechanisms of lipotoxicity in peripheral tissues, little is known about the effects of excess lipids in the brain. We used a hypothalamic, neuronal cell model, mHypoE-44, to understand how the highly prevalent nonesterified fatty acid, palmitate, affects neuronal insulin signaling. Through Western blot analysis, we discerned that prolonged exposure to palmitate impairs insulin activation, as assessed by phosphorylation of Akt. We investigated the role of endoplasmic reticulum (ER) stress, which is known to promote cellular insulin resistance and apoptosis in peripheral tissues. Palmitate treatment induced ER stress through a c-Jun N-terminal kinase (JNK)-dependent pathway because a selective JNK inhibitor blocked palmitate activation of the ER stress pathways eIF2 alpha and X-box binding protein-1. Interestingly, JNK inhibition did not prevent the palmitate-mediated cleaved caspase-3 increase, an apoptotic marker, or insulin signaling attenuation. However, pretreatment with the AMP kinase activator, aminoimidazole carboxamide ribonucleotide, blocked JNK phosphorylation and importantly prevented caspase-3 cleavage and restored insulin signaling during short-term exposure to palmitate. Thus, activation of AMP kinase prevents the deleterious effects of palmitate on hypothalamic neurons by inhibiting the onset of insulin resistance and apoptosis.

Reducing endoplasmic reticulum stress through a macrophage lipid chaperone alleviates atherosclerosis

Macrophages exhibit endoplasmic reticulum (ER) stress when exposed to lipotoxic signals associated with atherosclerosis, although the pathophysiological significance and the underlying mechanisms remain unknown. Here, we demonstrate that mitigation of ER stress with a chemical chaperone results in marked protection against lipotoxic death in macrophages and prevents macrophage fatty acid binding protein-4 (aP2) expression. Utilizing genetic and chemical models, we show that aP2 is the predominant regulator of lipid-induced macrophage ER stress. Lipid chaperone effects are mediated by the production of phospholipids rich in monounsaturated fatty acids and bioactive lipids that render macrophages resistant to lipid-induced ER stress. Furthermore, aP2’s impact on macrophage lipid metabolism and ER stress response is mediated by upregulation of key lipogenic enzymes by the liver X receptor. Our results demonstrate the central role for lipid chaperones in regulating ER homeostasis in macrophages in atherosclerosis and that ER responses can be modified, genetically or chemically, to protect the organism against the deleterious effects of hyperlipidemia.

Reduced endoplasmic reticulum (ER)-to-Golgi protein trafficking contributes to ER stress in lipotoxic mouse beta cells by promoting protein overload

AIMS/HYPOTHESIS

Saturated fatty acids augment endoplasmic reticulum (ER) stress in pancreatic beta cells and this is implicated in the loss of beta cell mass that accompanies type 2 diabetes. However, the mechanisms underlying the induction of ER stress are unclear. Our aim was to establish whether saturated fatty acids cause defects in ER-to-Golgi protein trafficking, which may thereby contribute to ER stress via protein overload.

METHODS

Cells of the mouse insulinoma cell line MIN6 were transfected with temperature-sensitive vesicular stomatitis virus G protein (VSVG) tagged with green fluorescent protein to quantify the rate of ER-to-Golgi protein trafficking. I14 antibody, which detects only correctly folded VSVG, was employed to probe the folding environment of the ER. ER stress markers were monitored by western blotting.

RESULTS

Pretreatment with palmitate, but not oleate, significantly reduced the rate of ER-to-Golgi protein trafficking assessed using VSVG. This was not secondary to ER stress, since thapsigargin, which compromises chaperone function by depletion of ER calcium, markedly inhibited VSVG folding and promoted strong ER stress but only slightly reduced protein trafficking. Blockade of ER-to-Golgi protein trafficking with brefeldin A (BFA) was sufficient to trigger ER stress, but neither BFA nor palmitate compromised VSVG folding.

CONCLUSIONS/INTERPRETATION

Reductions in ER-to-Golgi protein trafficking potentially contribute to ER stress during lipoapoptosis. In this case ER stress would be triggered by protein overload, rather than a disruption of the protein-folding capacity of the ER.

Balancing needs and means: the dilemma of the beta-cell in the modern world

The insulin resistance of type 2 diabetes mellitus (T2DM), although important for its pathophysiology, is not sufficient to establish the disease unless major deficiency of beta-cell function coexists. This is demonstrated by the fact that near-physiological administration of insulin (CSII) achieved excellent blood glucose control with doses similar to those used in insulin-deficient type 1 diabetics. The normal beta-cell adapts well to the demands of insulin resistance. Also in hyperglycaemic states some degree of adaptation does exist and helps limit the severity of disease. We demonstrate here that the mammalian target of rapamycin (mTOR) system might play an important role in this adaptation, because blocking mTORC1 (complex 1) by rapamycin in the nutritional diabetes model Psammomys obesus caused severe impairment of beta-cell function, increased beta-cell apoptosis and progression of diabetes. On the other hand, under exposure to high glucose and FFA (gluco-lipotoxicity), blocking mTORC1 in vitro reduced endoplasmic reticulum (ER) stress and beta-cell death. Thus, according to the conditions of stress, mTOR may have beneficial or deleterious effects on the beta-cell. beta-Cell function in man can be reduced without T2DM/impaired glucose tolerance (IGT). Prospective studies have shown subjects with reduced insulin response to present, several decades later, an increased incidence of IGT/T2DM. From these and other studies we conclude that T2DM develops on the grounds of beta-cells whose adaptation capacity to increased nutrient intake and/or insulin resistance is in the lower end of the normal variation. Inborn and acquired factors that limit beta-cell function are diabetogenic only in a nutritional/metabolic environment that requires high functional capabilities from the beta-cell.

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

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

alpha-Linolenic acid protects renal cells against palmitic acid lipotoxicity via inhibition of endoplasmic reticulum stress

Unsaturated fatty acids may counteract the lipotoxicity associated with saturated fatty acids. Palmitic acid induced endoplasmic reticulum (ER) stress and caused apoptotic and necrotic cell death in the renal proximal tubular cell line, NRK-52E. We investigated whether alpha-linolenic acid, an unsaturated fatty acid, protected against ER stress and cell death induced by palmitic acid or by other non-nutrient ER stress generators. Incubation of NRK-52E cells for 24h with palmitic acid produced a significant increase in apoptosis and necrosis. Palmitic acid also increased levels of three indicators of ER stress - the phosphorylated form of the eukaryotic initiation factor 2alpha (eIF2alpha), C/EBP homologous protein (CHOP), and glucose regulated protein 78 (GRP78). alpha-Linolenic acid dramatically reduced cell death and levels of all three indicators of ER stress brought about by palmitic acid. Tunicamycin, which induces ER stress by glycosylation of proteins, produced similar effects to those obtained using palmitic acid; its effects were partially reversed by alpha-linolenic acid. Salubrinal (a phosphatase inhibitor) causes increased levels of the phosphorylated form of eIF2alpha - this effect was partially reversed by alpha-linolenic acid. Palmitoleate, a monosaturated fatty acid, had similar effects to those of alpha-linolenic acid. These results suggest that part of the mechanism of protection of the kidney by unsaturated fatty acids is through inhibition of ER stress, eIF2alpha phosphorylation and consequential reduction of CHOP protein expression and apoptotic renal cell death.

Glucolipotoxicity of the pancreatic beta cell

The concept of glucolipotoxicity refers to the combined, deleterious effects of elevated glucose and fatty acid levels on pancreatic beta-cell function and survival. Significant progress has been made in recent years towards a better understanding of the cellular and molecular basis of glucolipotoxicity in the beta cell. The permissive effect of elevated glucose on the detrimental actions of fatty acids stems from the influence of glucose on intracellular fatty acid metabolism, promoting the synthesis of cellular lipids. The combination of excessive levels of fatty acids and glucose therefore leads to decreased insulin secretion, impaired insulin gene expression, and beta-cell death by apoptosis, all of which probably have distinct underlying mechanisms. Recent studies from our laboratory have identified several pathways implicated in fatty acid inhibition of insulin gene expression, including the extracellular-regulated kinase (ERK1/2) pathway, the metabolic sensor Per-Arnt-Sim kinase (PASK), and the ATF6 branch of the unfolded protein response. We have also confirmed in vivo in rats that the decrease in insulin gene expression is an early defect which precedes any detectable abnormality in insulin secretion. While the role of glucolipotoxicity in humans is still debated, the inhibitory effects of chronically elevated fatty acid levels has been clearly demonstrated in several studies, at least in individuals genetically predisposed to developing type 2 diabetes. It is therefore likely that glucolipotoxicity contributes to beta-cell failure in type 2 diabetes as well as to the decline in beta-cell function observed after the onset of the disease.

PDX1 deficiency causes mitochondrial dysfunction and defective insulin secretion through TFAM suppression

Mutations in the transcription factor Pdx1 cause maturity-onset diabetes of the young 4 (MODY4). Islet transduction with dominant-negative Pdx1 (RIPDN79PDX1) impairs mitochondrial metabolism and glucose-stimulated insulin secretion (GSIS). Transcript profiling revealed suppression of nuclear-encoded mitochondrial factor A (TFAM). Herein, we show that Pdx1 suppression in adult mice reduces islet TFAM expression coinciding with hyperglycemia. We define TFAM as a direct target of Pdx1 both in rat INS1 cells and human islets. Adenoviral overexpression of TFAM along with RIPDN79PDX1 in isolated rat islets rescued mitochondrial DNA (mtDNA) copy number and restored respiratory chain activity as well as glucose-induced ATP synthesis and insulin secretion. CGP37157, which blocks the mitochondrial Na(+)/Ca(2+) exchanger, restored ATP generation and GSIS in RIPDN79PDX1 islets, thereby bypassing the transcriptional defect. Thus, the genetic control by the beta cell-specific factor Pdx1 of the ubiquitous gene TFAM maintains beta cell mtDNA vital for ATP production and normal GSIS.

Reconstruct modular phenotype-specific gene networks by knowledge-driven matrix factorization

MOTIVATION

Reconstructing gene networks from microarray data has provided mechanistic information on cellular processes. A popular structure learning method, Bayesian network inference, has been used to determine network topology despite its shortcomings, i.e. the high-computational cost when analyzing a large number of genes and the inefficiency in exploiting prior knowledge, such as the co-regulation information of the genes. To address these limitations, we are introducing an alternative method, knowledge-driven matrix factorization (KMF) framework, to reconstruct phenotype-specific modular gene networks.

RESULTS

Considering the reconstruction of gene network as a matrix factorization problem, we first use the gene expression data to estimate a correlation matrix, and then factorize the correlation matrix to recover the gene modules and the interactions between them. Prior knowledge from Gene Ontology is integrated into the matrix factorization. We applied this KMF algorithm to hepatocellular carcinoma (HepG2) cells treated with free fatty acids (FFAs). By comparing the module networks for the different conditions, we identified the specific modules that are involved in conferring the cytotoxic phenotype induced by palmitate. Further analysis of the gene modules of the different conditions suggested individual genes that play important roles in palmitate-induced cytotoxicity. In summary, KMF can efficiently integrate gene expression data with prior knowledge, thereby providing a powerful method of reconstructing phenotype-specific gene networks and valuable insights into the mechanisms that govern the phenotype.

Reduced endoplasmic reticulum luminal calcium links saturated fatty acid-mediated endoplasmic reticulum stress and cell death in liver cells

Chronic exposure to elevated free fatty acids, in particular long chain saturated fatty acids, provokes endoplasmic reticulum (ER) stress and cell death in a number of cell types. The perturbations to the ER that instigate ER stress and activation of the unfolded protein in response to fatty acids in hepatocytes have not been identified. The present study employed H4IIE liver cells and primary rat hepatocytes to examine the hypothesis that saturated fatty acids induce ER stress via effects on ER luminal calcium stores. Exposure of H4IIE liver cells and primary hepatocytes to palmitate and stearate reduced thapsigargin-sensitive calcium stores and increased biochemical markers of ER stress over similar time courses (6 h). These changes preceded cell death, which was only observed at later time points (16 h). Co-incubation with oleate prevented the reduction in calcium stores, induction of ER stress markers and cell death observed in response to palmitate. Inclusion of calcium chelators, BAPTA-AM or EGTA, reduced palmitate- and stearate-mediated enrichment of cytochrome c in post-mitochondrial supernatant fractions and cell death. These data suggest that redistribution of ER luminal calcium contributes to long chain saturated fatty acid-mediated ER stress and cell death.

Effects of palmitate on ER and cytosolic Ca2+ homeostasis in beta-cells

There are strong links between obesity, elevated free fatty acids, and type 2 diabetes. Specifically, the saturated fatty acid palmitate has pleiotropic effects on beta-cell function and survival. In the present study, we sought to determine the mechanism by which palmitate affects intracellular Ca2+, and in particular the role of the endoplasmic reticulum (ER). In human beta-cells and MIN6 cells, palmitate rapidly increased cytosolic Ca2+ through a combination of Ca2+ store release and extracellular Ca2+ influx. Palmitate caused a reversible lowering of ER Ca2+, measured directly with the fluorescent protein-based ER Ca2+ sensor D1ER. Using another genetically encoded indicator, we observed long-lasting oscillations of cytosolic Ca2+ in palmitate-treated cells. In keeping with this observed ER Ca2+ depletion, palmitate induced rapid phosphorylation of the ER Ca2+ sensor protein kinase R-like ER kinase (PERK) and subsequently ER stress and beta-cell death. We detected little palmitate-induced insulin secretion, suggesting that these Ca2+ signals are poorly coupled to exocytosis. In summary, we have characterized Ca2+-dependent mechanisms involved in altered beta-cell function and survival induced by the free fatty acid palmitate. We present the first direct evidence that free fatty acids reduce ER Ca2+ and shed light on pathways involved in lipotoxicity and the pathogenesis of type 2 diabetes.

Glucose amplifies fatty acid-induced endoplasmic reticulum stress in pancreatic beta-cells via activation of mTORC1

Background

Palmitate is a potent inducer of endoplasmic reticulum (ER) stress in β-cells. In type 2 diabetes, glucose amplifies fatty-acid toxicity for pancreatic β-cells, leading to β-cell dysfunction and death. Why glucose exacerbates β-cell lipotoxicity is largely unknown. Glucose stimulates mTORC1, an important nutrient sensor involved in the regulation of cellular stress. Our study tested the hypothesis that glucose augments lipotoxicity by stimulating mTORC1 leading to increased β-cell ER stress.

Principal Findings

We found that glucose amplifies palmitate-induced ER stress by increasing IRE1α protein levels and activating the JNK pathway, leading to increased β-cell apoptosis. Moreover, glucose increased mTORC1 activity and its inhibition by rapamycin decreased β-cell apoptosis under conditions of glucolipotoxicity. Inhibition of mTORC1 by rapamycin did not affect proinsulin and total protein synthesis in β-cells incubated at high glucose with palmitate. However, it decreased IRE1α expression and signaling and inhibited JNK pathway activation. In TSC2-deficient mouse embryonic fibroblasts, in which mTORC1 is constitutively active, mTORC1 regulated the stimulation of JNK by ER stressors, but not in response to anisomycin, which activates JNK independent of ER stress. Finally, we found that JNK inhibition decreased β-cell apoptosis under conditions of glucolipotoxicity.

Conclusions/Significance

Collectively, our findings suggest that mTORC1 mediates glucose amplification of lipotoxicity, acting through activation of ER stress and JNK. Thus, mTORC1 is an important transducer of ER stress in β-cell glucolipotoxicity. Moreover, in stressed β-cells mTORC1 inhibition decreases IRE1α protein expression and JNK activity without affecting ER protein load, suggesting that mTORC1 regulates the β-cell stress response to glucose and fatty acids by modulating the synthesis and activity of specific proteins involved in the execution of the ER stress response. This novel paradigm may have important implications for understanding β-cell failure in type 2 diabetes.

Fatty acids and beta-cell toxicity

PURPOSE OF REVIEW

The rising incidence of type 2 diabetes is due, in part, to the detrimental effects of certain fatty acids on pancreatic beta-cell function and viability. The present review examines recent advances in the understanding of the molecular mechanisms by which fatty acids influence the life and death of beta cells.

RECENT FINDINGS

There are important differences in the cytotoxic potential of fatty acids, with long-chain saturated molecules being the most potent. By contrast, monounsaturates and polyunsaturates are relatively well tolerated and, in some cases, are actively cytoprotective. The mechanisms underlying the toxicity of the saturates may reflect a decrease in protein processing, which drives the accumulation of unfolded proteins in the endoplasmic reticulum. This triggers an apoptotic response by virtue of enhanced endoplasmic reticulum stress and induction of CHOP-10 synthesis. Alterations in the regulatory control of other proapoptotic genes via changes in microRNA synthesis may also contribute. The cytoprotection deriving from incubation with long-chain mono-unsaturates is probably receptor mediated and involves antagonistic actions on the effector arm of the endoplasmic reticulum stress pathway.

SUMMARY

The findings have implications for the development of new therapeutic agents designed to minimize beta-cell dysfunction and the loss of beta-cell viability in type 2 diabetes.

Drak2 overexpression results in increased beta-cell apoptosis after free fatty acid stimulation

Drak2 is a serine threonine kinase in the death-associated protein family. In this study, we investigated its role in free fatty acid (FFA)-induced islet apoptosis. Drak2 mRNA and protein were rapidly induced in islet beta-cells after FFA stimulation. Such Drak2 upregulation was accompanied by increased beta-cell apoptosis, which was inhibited by Drak2 knockdown using siRNA. Conversely, transgenic (Tg) Drak2 overexpression led to aggravated beta-cell apoptosis triggered by FFA. Drak2 overexpression in islets compromised the increase of anti-apoptotic factors, such as Bcl-2, Bcl-xL and Flip, upon FFA assault. Further in vivo experiments demonstrated that Drak2 Tg mice presented compromised glucose tolerance in a diet-induced obesity model. Our data show that Drak2 is detrimental to islet survival in the presence of excessive lipid.

25-Hydroxycholesterol-3-sulfate regulates macrophage lipid metabolism via the LXR/SREBP-1 signaling pathway

The oxysterol receptor LXR is a key transcriptional regulator of lipid metabolism. LXR increases expression of SREBP-1, which in turn regulates at least 32 genes involved in lipid synthesis and transport. We recently identified 25-hydroxycholesterol-3-sulfate (25HC3S) as an important regulatory molecule in the liver. We have now studied the effects of 25HC3S and its precursor, 25-hydroxycholesterol (25HC), on lipid metabolism as mediated by the LXR/SREBP-1 signaling in macrophages. Addition of 25HC3S to human THP-1-derived macrophages markedly decreased nuclear LXR protein levels. 25HC3S administration was followed by dose- and time-dependent decreases in SREBP-1 mature protein and mRNA levels. 25HC3S decreased the expression of SREBP-1-responsive genes, acetyl-CoA carboxylase-1, and fatty acid synthase (FAS) as well as HMGR and LDLR, which are key proteins involved in lipid metabolism. Subsequently, 25HC3S decreased intracellular lipids and increased cell proliferation. In contrast to 25HC3S, 25HC acted as an LXR ligand, increasing ABCA1, ABCG1, SREBP-1, and FAS mRNA levels. In the presence of 25HC3S, 25HC, and LXR agonist T0901317, stimulation of LXR targeting gene expression was repressed. We conclude that 25HC3S acts in macrophages as a cholesterol satiety signal, downregulating cholesterol and fatty acid synthetic pathways via inhibition of LXR/SREBP signaling. A possible role of oxysterol sulfation is proposed.

Differential regulation of the ER stress response by long-chain fatty acids in the pancreatic beta-cell

Recent evidence indicates that treatment of pancreatic beta-cells with long chain fatty acids can lead to the development of an ER (endoplasmic reticulum) stress response. This is manifest as the activation of some components of the PERK [RNA-dependent protein kinase-like ER eIF2alpha (eukaryotic initiation factor 2alpha) kinase]-dependent arm of ER stress and is seen most dramatically when cells are treated with long-chain saturated fatty acids (e.g. palmitate). By contrast, the equivalent mono-unsaturates (e.g. palmitoleate) are much less effective and they can even attenuate the ER stress response to palmitate. This may be due to the regulation of eIF2alpha phosphorylation in cells exposed to mono-unsaturates. The present review discusses the differential effects of saturated and mono-unsaturated fatty acids on ER stress in beta-cells and considers the extent to which regulation of this pathway may be involved in mediating their effects on viability.

Role of nuclear receptors in the modulation of insulin secretion in lipid-induced insulin resistance

In healthy individuals, a hyperbolic relationship exists between whole-body insulin-sensitivity and insulin secretion. Thus, for any difference in insulin-sensitivity, a reciprocal proportionate change occurs in insulin secretion. Such a feedback loop is evident in healthy individuals ingesting diets high in saturated fat and in late pregnancy where, despite lipid-induced insulin resistance, glucose tolerance is maintained through augmented GSIS (glucose-stimulated insulin secretion). NRs (nuclear receptors) are members of a superfamily of ligand-regulated and orphan transcription factors. On activation by a cognate ligand, many ligand-activated NRs recruit the RXR (retinoid X receptor) for heterodimer formation. Such NRs include the PPARs (peroxisome-proliferator-activated receptors), which are involved in lipid sensing and liporegulation. PPARs exert important lipid-lowering effects in vivo, thereby opposing the development of lipid-induced insulin resistance by relieving the inhibition of insulin-stimulated glucose disposal by muscle and lowering the necessity for augmented GSIS to counter lipid-induced insulin resistance. Long-chain fatty acids are proposed as natural PPAR ligands and some specific endogenous pathways of lipid metabolism are believed to generate PPAR agonists. Other NRs, e.g. the LXR (liver X receptor), which senses expansion of the metabolically active pool of cholesterol, and the FXR (farnesoid X receptor; NR1H4), which, like the LXR, is involved in sterol metabolism, also modulate systemic lipid levels and insulin-sensitivity. In this review, we discuss how these NRs impact insulin secretion via effects on the insulin-sensitivity-insulin secretion feedback loop and, in some cases, via direct effects on the islet itself. In addition, we discuss interactions between these nutrient/metabolite-responsive NRs and NRs that are central to the action of metabolically important hormones, including (i) the glucocorticoid receptor, critical for maintaining glucose homoeostasis in stress, inflammation and during fasting, and (ii) the thyroid hormone receptors, vital for maintenance of oxidative functions. We present data indicating that the RXR occupies a key role in directly modulating islet function and that its heterodimerization with at least two of its partners modulates GSIS.

It's not how fat you are, it's what you do with it that counts

Mechanisms underlying obesity-related metabolic disorders are poorly understood. Samuel Virtue and Antonio Vidal-Puig explore the evidence for an emerging hypothesis that attributes metabolic complications not to obesity per se, but to an individual's capacity for adipose tissue expandability.

An update on lipotoxic endoplasmic reticulum stress in pancreatic β-cells

The UPR (unfolded protein response) or ER (endoplasmic reticulum) stress response was first described 20 years ago. The field of ER stress has expanded tremendously since, moving from basic biology in yeast to human neurodegenerative, inflammatory, cardiovascular and neoplastic diseases. The ER stress response has also been implicated in diabetes development, affecting both insulin production by pancreatic β-cells and insulin sensitivity in peripheral tissues. In the present mini-review, we focus on recent progress in the field of ER stress in pancreatic β-cells. Recent advances in the understanding of lipotoxic ER stress and β-cell recovery from ER stress are discussed.