Sterol regulatory element-binding protein 1 mediates liver X receptor-beta-induced increases in insulin secretion and insulin messenger ribonucleic acid levels

Adapting Physiology in Functional Human Islet Organogenesis

Generation of three-dimensional (3D)-structured functional human islets is expected to be an alternative cell source for cadaveric human islet transplantation for the treatment of insulin-dependent diabetes. Human pluripotent stem cells (hPSCs), such as human embryonic stem cells (hESCs) and human induced pluripotent stem cells (hiPSCs), offer infinite resources for newly synthesized human islets. Recent advancements in hPSCs technology have enabled direct differentiation to human islet-like clusters, which can sense glucose and secrete insulin, and those islet clusters can ameliorate diabetes when transplanted into rodents or non-human primates (NHPs). However, the generated hPSC-derived human islet-like clusters are functionally immature compared with primary human islets. There remains a challenge to establish a technology to create fully functional human islets in vitro, which are functionally and transcriptionally indistinguishable from cadaveric human islets. Understanding the complex differentiation and maturation pathway is necessary to generate fully functional human islets for a tremendous supply of high-quality human islets with less batch-to-batch difference for millions of patients. In this review, I summarized the current progress in the generation of 3D-structured human islets from pluripotent stem cells and discussed the importance of adapting physiology for in vitro functional human islet organogenesis and possible improvements with environmental cues.

Approved LXR agonists exert unspecific effects on pancreatic β-cell function

Novel agonists of the nuclear liver-X-receptor (LXR) are designed to treat metabolic disorders or cancer. The rationale to develop these new drugs is based on promising results with established LXR agonist like T0901317 and GW3965. LXRα and LXRβ are expressed in β-cells, and expression is increased by T0901317. The aim of the present study was to evaluate whether effects of these drugs on β-cell function are specific and reliably linked to LXR activation. T0901317 and GW3965, widely used as specific LXR agonists, show rapid, non-genomic effects on stimulus-secretion coupling of mouse pancreatic β-cells at low µM concentrations. T0901317 lowered the cytosolic Ca²⁺ concentration, reduced or completely inhibited action potentials, and decreased insulin secretion. GW3965 exerted similar effects on insulin secretion. T0901317 affected the production of reactive oxygen species and ATP. The involvement of the classical nuclear LXRs in T0901317- and GW3965-mediated effects in β-cells could be ruled out using LXRα, LXRβ and double knockout mice. Our results strongly suggest that LXR agonists, that are considered to be specific for this receptor, interfere with mitochondrial metabolism and metabolism-independent processes in β-cells. Thus, it is indispensable to test novel LXR agonists accompanying to ongoing clinical trials for acute and chronic effects on cell function in cellular systems and/or animal models lacking classical LXRs.

Resveratrol downregulates PCSK9 expression and attenuates steatosis through estrogen receptor α-mediated pathway in L02 cells

Proprotein convertase subtilisin kexin type 9 (PCSK9) is a promising target for treating dyslipidemia and atherosclerosis. Circulating PCSK9 levels are closely related to hepatic steatosis severity and endogenous estrogen levels. Resveratrol (RSV) is a phytoestrogens that protects against atherosclerosis and hepatic steatosis. Thus, we sought to determine whether RSV had the activities to inhibit PCSK9 expression and to attenuate lipid accumulation in free fatty acid (FFA)-induced L02 cells via ERα pathway. In this study, RSV (10, 20 μM) were cultured with L02 cells in the presence of FFA (oleate:palmitate = 2:1). RSV significantly reduced the number of lipid droplets and intracellular TG in steatotic L02 cells, and Oil red O staining and Nile red staining had the same results. Western blot analysis showed that RSV significantly reduced apoB secretion and intracellular microsomal triglyceride transporter (MTP) expression under lipid-rich conditions. Treatment with RSV reduced expression of PCSK9 while maintaining LDL receptor (LDLR) expression and LDL uptake. RSV decreased SREBP-1c expression at both mRNA and protein levels. In addition, RSV significantly reduced the expression of liver X receptor α (LXRα) mRNA in L02 cells, but did not affect the expression of liver X receptor β (LXRβ) mRNA. The luciferase reporter assays suggested that RSV inhibited SREBP-mediated transcription of PCSK9. Finally, these results could be partly reversed by Estrogen receptor α (ERα) gene silencing. These results suggest that RSV attenuates steatosis and PCSK9 expression through down-regulation of SREBP-1c expression, at least in part through ERα-mediated pathway.

Regulation of Nampt expression by transcriptional coactivator NCOA6 in pancreatic β-cells

Nuclear receptor coactivator 6 (NCOA6) is a transcriptional coactivator and crucial for insulin secretion and glucose metabolism in pancreatic β-cells. However, the regulatory mechanism of β-cell function by NCOA6 is largely unknown. In this study, we found that the transcript levels of nicotinamide phosphoribosyltransferase (Nampt) were decreased in islets of NCOA6^+/- mice compared with NCOA6^+/+ mice. Moreover, NCOA6 overexpression increased the levels of Nampt transcripts in the mouse pancreatic β-cell line NIT-1. Promoter analyses showed that transcriptional activity of the Nampt promoter was stimulated by cooperation of sterol regulatory element binding protein-1c (SREBP-1c) and NCOA6. Additional studies using mutant promoters demonstrated that SREBP-1c activates Nampt promoter through the sterol regulatory element (SRE), but not through the E-box. Using chromatin immunoprecipitation assay, NCOA6 was also shown to be directly recruited to the SRE region of the Nampt promoter. Furthermore, treatment with nicotinamide mononucleotide (NMN), a product of the Nampt reaction and a key NAD⁺ intermediate, ameliorates glucose-stimulated insulin secretion from NCOA6^+/- islets. These results suggest that NCOA6 stimulates insulin secretion, at least partially, by modulating Nampt expression in pancreatic β-cells.

Integrative Genomics Outlines a Biphasic Glucose Response and a ChREBP-RORγ Axis Regulating Proliferation in β Cells

Glucose is an important inducer of insulin secretion, but it also stimulates long-term adaptive changes in gene expression that can either promote or antagonize the proliferative potential and function of β cells. Here, we have generated time-resolved profiles of enhancer and transcriptional activity in response to glucose in the INS-1E pancreatic β cell line. Our data outline a biphasic response with a first transcriptional wave during which metabolic genes are activated, and a second wave where cell-cycle genes are activated and β cell identity genes are repressed. The glucose-sensing transcription factor ChREBP directly activates first wave enhancers, whereas repression and activation of second wave enhancers are indirect. By integrating motif enrichment within late-regulated enhancers with expression profiles of the associated transcription factors, we have identified multiple putative regulators of the second wave. These include RORγ, the activity of which is important for glucose-induced proliferation of both INS-1E and primary rat β cells.

Cholesterol in Pancreatic β-Cell Death and Dysfunction: Underlying Mechanisms and Pathological Implications

The mechanisms or causes of pancreatic β-cell death as well as impaired insulin secretion, which are the principal events of diabetic etiopathology, are largely unknown. Diabetic complications are known to be associated with abnormal plasma lipid profile, mainly elevated level of cholesterol and free fatty acids. However, in recent years, elevated plasma cholesterol has been implicated as a primary modulator of pancreatic β-cell functions as well as death. High-cholesterol diet in animal models or excess cholesterol in pancreatic β-cell causes transporter desensitization and results in morphometric changes in insulin granules. Moreover, cholesterol is also held responsible to cause oxidative stress, mitochondrial dysfunction, and activation of proapoptotic markers leading to β-cell death. The present review focuses on the pathways and molecularevents that occur in the β-cell under the influence of excess cholesterol that hampers the basal physiology of the cell leading to the progression of diabetes.

Trans-11 vaccenic acid improves insulin secretion in models of type 2 diabetes in vivo and in vitro

SCOPE

Trans-11 vaccenic acid (VA) is a fatty acid produced by ruminants entering the human food supply through meat and dairy products, which appears not to have the health risks associated with industrially produced trans-fatty acids. In this study, we investigated the effect of VA on insulin secretion in vivo in rats and in vitro in human and rat islets after diabetogenic insult.

METHODS AND RESULTS

Hyperglycemic clamp showed that VA dietary supplementation for 8 weeks significantly increased glucose turnover in rats with type 2 diabetes (T2D), accompanied by an elevated plasma C-peptide concentration, indicating improved insulin secretion. The β-cell area and proliferation rate were higher in T2D+VA than T2D group. Isolated islets from T2D+VA rats had higher glucose-stimulated insulin secretion (GSIS) than T2D group. In vitro, VA treatment for 24 and 48 h significantly enhanced GSIS in rat and human islets after diabetogenic challenges. The mRNA expression of G-protein-coupled receptor 40 (GPR40) and regenerating islet-derived 1α (REG-1α) were consistently increased by VA in both rat and human islets.

CONCLUSION

These results indicate that VA may improve insulin secretion and growth of islets in T2D, at least partly by altering GPR40 and REG-1α mRNA expression.

Coordinated Actions of FXR and LXR in Metabolism: From Pathogenesis to Pharmacological Targets for Type 2 Diabetes

Type 2 diabetes (T2D) is the most prevalent metabolic disease, and many people are suffering from its complications driven by hyperglycaemia and dyslipidaemia. Nuclear receptors (NRs) are ligand-inducible transcription factors that mediate changes to metabolic pathways within the body. As metabolic regulators, the farnesoid X receptor (FXR) and the liver X receptor (LXR) play key roles in the pathogenesis of T2D, which remains to be clarified in detail. Here we review the recent progress concerning the physiological and pathophysiological roles of FXRs and LXRs in the regulation of bile acid, lipid and glucose metabolism and the implications in T2D, taking into account that these two nuclear receptors are potential pharmaceutical targets for the treatment of T2D and its complications.

Reciprocal regulation of hepatic and adipose lipogenesis by liver X receptors in obesity and insulin resistance

Liver X receptors (LXRs) regulate lipogenesis and inflammation, but their contribution to the metabolic syndrome is unclear. We show that LXRs modulate key aspects of the metabolic syndrome in mice. LXRαβ-deficient-ob/ob (LOKO) mice remain obese but show reduced hepatic steatosis and improved insulin sensitivity compared to ob/ob mice. Impaired hepatic lipogenesis in LOKO mice is accompanied by reciprocal increases in adipose lipid storage, reflecting tissue-selective effects on the SREBP, PPARγ, and ChREBP lipogenic pathways. LXRs are essential for obesity-driven SREBP-1c and ChREBP activity in liver, but not fat. Furthermore, loss of LXRs in obesity promotes adipose PPARγ and ChREBP-β activity, leading to improved insulin sensitivity. LOKO mice also exhibit defects in β cell mass and proliferation despite improved insulin sensitivity. Our data suggest that sterol sensing by LXRs in obesity is critically linked with lipid and glucose homeostasis and provide insight into the complex relationships between LXR and insulin signaling.

Genomic deletion of MAP3K7 at 6q12-22 is associated with early PSA recurrence in prostate cancer and absence of TMPRSS2:ERG fusions

6q12-22 is the second most commonly deleted genomic region in prostate cancer. Mapping studies have described a minimally deleted area at 6q15, containing MAP3K7/TAK1, which was recently shown to have tumor suppressive properties. To determine prevalence and clinical significance of MAP3K7 alterations in prostate cancer, a tissue microarray containing 4699 prostate cancer samples was analyzed by fluorescence in situ hybridization. Heterozygous MAP3K7 deletions were found in 18.48% of 2289 interpretable prostate cancers. MAP3K7 deletions were significantly associated with advanced tumor stage (P<0.0001), high Gleason grade (P<0.0001), lymph node metastasis (P<0.0108) and early biochemical recurrence (P<0.0001). MAP3K7 alterations were typically limited to the loss of one allele as homozygous deletions were virtually absent and sequencing analyses revealed no evidence for MAP3K7 mutations in 15 deleted and in 14 non-deleted cancers. There was a striking inverse association of MAP3K7 deletions and TMPRSS2:ERG fusion status with 26.7% 6q deletions in 1125 ERG-negative and 11.1% 6q deletions in 1198 ERG-positive cancers (P<0.0001). However, the strong prognostic role of 6q deletions was retained in both ERG-positive and ERG-negative cancers (P<0.0001 each). In summary, our study identifies MAP3K7 deletion as a prominent feature in ERG-negative prostate cancer with strong association to tumor aggressiveness. MAP3K7 alterations are typically limited to one allele of the gene. Together with the demonstrated tumor suppressive function in cell line experiments and lacking evidence for inactivation through hypermethylation, these results indicate MAP3K7 as a gene for which haploinsufficency is substantially tumorigenic.

Liver X receptors, nervous system, and lipid metabolism

Lipids in the nervous system are represented by cholesterol and phospholipids as constituents of cell membranes and, in particular, of myelin. Therefore, lipids are finely regulated to guarantee physiological functions. In the central nervous system, cholesterol is locally synthesized due to the presence of the blood brain barrier. In the peripheral nervous system cholesterol is either up-taken by lipoproteins and/or produced by de novo biosynthesis. Defects in lipid homeostasis in these tissues lead to structural and functional changes that often result in different pathological conditions depending on the affected pathways (i.e. cholesterol biosynthesis, cholesterol efflux, fatty acid biosynthesis etc.). Alterations in cholesterol metabolism in the central nervous system are linked to several disorders such as Alzheimer's disease, Huntington disease, Parkinson disease, Multiple sclerosis, Smith-Lemli-Opitz syndrome, Niemann-Pick type C disease, and glioblastoma. In the peripheral nervous system changes in lipid metabolism are associated with the development of peripheral neuropathy that may be caused by metabolic disorders, injuries, therapeutics, and autoimmune diseases. Transcription factors, such as the Liver X receptors (LXR), regulate both cholesterol and fatty acid metabolism in several tissues including the nervous system. In the last few years several studies elucidated the biology of LXR in the nervous system due to the availability of knock-out mice and the development of synthetic ligands. Here, we review a survey of the literature focused on the central and peripheral nervous system and in physiological and pathological settings with particular attention to the roles played by LXR in both districts.

Inhibition of mitogen-activated protein kinase Erk1/2 promotes protein degradation of ATP binding cassette transporters A1 and G1 in CHO and HuH7 cells

Signal transduction modulates expression and activity of cholesterol transporters. We recently demonstrated that the Ras/mitogen-activated protein kinase (MAPK) signaling cascade regulates protein stability of Scavenger Receptor BI (SR-BI) through Proliferator Activator Receptor (PPARα) -dependent degradation pathways. In addition, MAPK (Mek/Erk 1/2) inhibition has been shown to influence liver X receptor (LXR) -inducible ATP Binding Cassette (ABC) transporter ABCA1 expression in macrophages. Here we investigated if Ras/MAPK signaling could alter expression and activity of ABCA1 and ABCG1 in steroidogenic and hepatic cell lines. We demonstrate that in Chinese Hamster Ovary (CHO) cells and human hepatic HuH7 cells, extracellular signal-regulated kinase 1/2 (Erk1/2) inhibition reduces PPARα-inducible ABCA1 protein levels, while ectopic expression of constitutively active H-Ras, K-Ras and MAPK/Erk kinase 1 (Mek1) increases ABCA1 protein expression, respectively. Furthermore, Mek1/2 inhibitors reduce ABCG1 protein levels in ABCG1 overexpressing CHO cells (CHO-ABCG1) and human embryonic kidney 293 (HEK293) cells treated with LXR agonist. This correlates with Mek1/2 inhibition reducing ABCG1 cell surface expression and decreasing cholesterol efflux onto High Density Lipoproteins (HDL). Real Time reverse transcriptase polymerase chain reaction (RT-PCR) and protein turnover studies reveal that Mek1/2 inhibitors do not target transcriptional regulation of ABCA1 and ABCG1, but promote ABCA1 and ABCG1 protein degradation in HuH7 and CHO cells, respectively. In line with published data from mouse macrophages, blocking Mek1/2 activity upregulates ABCA1 and ABCG1 protein levels in human THP1 macrophages, indicating opposite roles for the Ras/MAPK pathway in the regulation of ABC transporter activity in macrophages compared to steroidogenic and hepatic cell types. In summary, this study suggests that Ras/MAPK signaling modulates PPARα- and LXR-dependent protein degradation pathways in a cell-specific manner to regulate the expression levels of ABCA1 and ABCG1 transporters.

Evaluation of the INS-1 832/13 cell line as a beta-cell based screening system to assess pollutant effects on beta-cell function

Environmental pollutants have recently emerged as potential risk factors for metabolic diseases, urging systematic investigation of pollutant effects on metabolic disease processes. To enable risk assessment of these so-called metabolic disruptors the use of stable, robust and well-defined cell based screening systems has recently been encouraged. Since beta-cell (dys)functionality is central in diabetes pathophysiology, the need to develop beta-cell based pollutant screening systems is evident. In this context, the present research evaluated the strengths and weaknesses of the INS-1 832/13 pancreatic beta-cell line as diabetogenic pollutant screening system with a focus on beta-cell function. After optimization of exposure conditions, positive (exendin-4, glibenclamide) and negative (diazoxide) control compounds for acute insulin secretion responses were tested and those with the most profound effects were selected to allow potency estimations and ranking of pollutants. This was followed by a first explorative screening of acute bisphenol A and bis(2-ethylhexyl)phthalate effects. The same approach was applied for chronic exposures, focusing primarily on evaluation of acknowledged chronic stimulators (diazoxide, T0901317, exendin-4) or inhibitors (glibenclamide) of insulin secretion responses to select the most responsive ones for use as control compounds in a chronic pollutant testing framework. Our results showed that INS-1 832/13 cells responded conform previous observations regarding acute effects of control compounds on insulin secretion, while bisphenol A and bis(2-ethylhexyl)phthalate had limited acute effects. Furthermore, chronic exposure to known beta-cell reactive compounds resulted in deviating insulin secretion and insulin content profiles compared to previous reports. In conclusion, this INS-1 subclone appears to lack certain characteristics needed to respond appropriately to acute pollutant exposure or long term exposure to known beta-cell reactive compounds and thus seems to be, in our setting, inadequate as a diabetogenic pollutant screening system.

Lipid metabolism and neuroinflammation in Alzheimer's disease: a role for liver X receptors

Liver X receptors (LXR) are nuclear receptors that have emerged as key regulators of lipid metabolism. In addition to their functions as cholesterol sensors, LXR have also been found to regulate inflammatory responses in macrophages. Alzheimer's disease (AD) is a neurodegenerative disease characterized by a progressive cognitive decline associated with inflammation. Evidence indicates that the initiation and progression of AD is linked to aberrant cholesterol metabolism and inflammation. Activation of LXR can regulate neuroinflammation and decrease amyloid-β peptide accumulation. Here, we highlight the role of LXR in orchestrating lipid homeostasis and neuroinflammation in the brain. In addition, diabetes mellitus is also briefly discussed as a significant risk factor for AD because of the appearing beneficial effects of LXR on glucose homeostasis. The ability of LXR to attenuate AD pathology makes them potential therapeutic targets for this neurodegenerative disease.

Associations of ATP-binding cassette transporter A1 and G1 with insulin secretion in human insulinomas

OBJECTIVES

Adenosine triphosphate-binding cassette transporter A1 (ABCA1) and G1 (ABCG1) are 2 important cholesterol transporters in human pancreatic β-cells. The aim of this study was to investigate their alteration in insulinomas and their potential associations with abnormal insulin secretion in these patients.

METHODS

Six patients with insulinoma and 6 healthy controls were recruited. Lipid profiles and glucose metabolism were measured. Insulin content, ABCA1, and ABCG1 in insulinomas and the adjacent islets of the 6 patients with insulinoma were detected by immunohistochemistry or immunofluorescence.

RESULTS

Plasma total cholesterol, high-density lipoprotein, low-density lipoprotein, and triglyceride were comparable between the controls and the patients with insulinoma. Fasting glucose was less than 2.8 mmol/L, and insulin release index was greater than 0.3 in each patient. Serum insulin fell extremely, and blood glucose reached the reference range within an hour after the cutting of the tumors in 2 patients with insulinoma. Adenosine triphosphate-binding cassette transporter G1 increased in insulinomas compared with the adjacent islets. However, ABCA1 was detected neither in the adjacent islets nor in insulinomas. Adenosine triphosphate-binding cassette transporter G1 expression in insulinomas was significantly associated with fasting insulin level and insulin release index.

CONCLUSIONS

Increased ABCG1 may contribute to insulin hypersecretion in insulinomas. In contrast, the undetectable ABCA1 in insulinomas may reflect a negative feedback in insulin secretion in these patients.

Dietary modification of metabolic pathways via nuclear hormone receptors

Nuclear hormone receptors (NHRs), as ligand-dependent transcription factors, have emerged as important mediators in the control of whole body metabolism. Because of the promiscuous nature of several members of this superfamily that have been found to bind ligand with lower affinity than the classical steroid NHRs, they consequently display a broader ligand selectivity. This promiscuous nature has facilitated various bioactive dietary components being able to act as agonist ligands for certain members of the NHR superfamily. By binding to these NHRs, bioactive dietary components are able to mediate changes in various metabolic pathways, including, glucose, cholesterol and triglyceride homeostasis among others. This review will provide a general overview of the nuclear hormone receptors that have been shown to be activated by dietary components. The physiological consequences of such receptor activation by these dietary components will then be discussed in more detail.

Molecular mechanisms of estrogen receptors' suppression of lipogenesis in pancreatic β-cells

The gonadal steroid, 17β-estradiol (E2), suppresses pancreatic islet fatty acid and glycerolipid synthesis and prevents β-cell failure in rodent models of type 2 diabetes. β-Cell estrogen receptors (ER) mediate these actions by suppressing the expression and enzymatic activity of fatty acid synthase (FAS). Here, we explored the mechanism of FAS suppression. We show that E2, and pharmacological agonists for ERα, ERβ, and the G protein-coupled ER, suppress mRNA and protein expression of the transcriptional regulators of FAS, namely, sterol regulatory element-binding protein 1c (SREBP1c) and carbohydrate response element binding protein (ChREBP) in insulin-secreting INS-1 cells. ER suppress SREBP1c and ChREBP mRNA and protein expression via an extranuclear localization. Using two mouse lines with pancreas-specific null deletion of either ERα or the signal transducer and activator of transcription 3 (STAT3), we show that ERα activation in vivo reduces SREBP1c and ChREBP mRNA expression via a direct islet action involving STAT3 activation. The master regulators of lipogenesis, liver X receptor (LXR) α and β, transcriptionally up-regulate SREBP1c and ChREBP. We find that activation of ERα, ERβ, and G protein-coupled ER suppresses LXR's mRNA expression in INS-1 cells. We also observe that activation of ERα in mouse islets in vivo suppresses LXR mRNA in a STAT3-dependent manner. Finally, we show that E2 also activates and uses AMP-activated protein kinase in INS-1 cells to suppress SREBP1c protein expression. This study identifies extranuclear ER pathways involving STAT3 and AMP-activated protein kinase in the genetic control of lipogenesis with therapeutic implications to protect β-cells in type 2 diabetes.

The zinc finger protein ZBTB20 regulates transcription of fructose-1,6-bisphosphatase 1 and β cell function in mice

BACKGROUND & AIMS

Fructose-1,6-bisphosphatase (FBP)-1 is a gluconeogenic enzyme that regulates glucose metabolism and insulin secretion in β cells, but little is known about how its transcription is controlled. The zinc finger protein ZBTB20 regulates glucose homeostasis, so we investigated its effects on expression of FBP-1.

METHODS

We analyzed gene expression using real-time reverse-transcription polymerase chain reaction, immunoblotting, and immunohistochemistry. We generated mice with β cell-specific disruption of Zbtb20 using Cre/LoxP technology. Expression of Zbtb20 in β cells was reduced using small interfering RNAs, and promoter occupancy and transcriptional regulation were analyzed by chromatin immunoprecipitation and reporter assays.

RESULTS

ZBTB20 was expressed at high levels by β cells and other endocrine cells in islets of normal mice; expression levels were reduced in islets from diabetic db/db mice. Mice with β cell-specific knockout of Zbtb20 had normal development of β cells but had hyperglycemia, hypoinsulinemia, glucose intolerance, and impaired glucose-stimulated insulin secretion. Islets isolated from these mice had impaired glucose metabolism, adenosine triphosphate production, and insulin secretion after glucose stimulation in vitro, although insulin secretion returned to normal levels in the presence of KCl. ZBTB20 knockdown with small interfering RNAs impaired glucose-stimulated insulin secretion in the β cell line MIN6. Expression of Fbp1 was up-regulated in β cells with ZBTB20 knockout or knockdown; impairments to glucose-stimulated insulin secretion were restored by inhibition of FBPase activity. ZBTB20 was recruited to the Fbp1 promoter and repressed its transcription in β cells.

CONCLUSIONS

The transcription factor ZBTB20 regulates β cell function and glucose homeostasis in mice. It might be a therapeutic target for type 2 diabetes mellitus.

Aberrant activation of liver X receptors impairs pancreatic beta cell function through upregulation of sterol regulatory element-binding protein 1c in mouse islets and rodent cell lines

AIMS/HYPOTHESIS

Liver X receptors (LXR) are important transcriptional regulators of lipid and glucose metabolism. Our previous report demonstrated that LXR activation inhibited pancreatic beta cell proliferation through cell cycle arrest. Here we explore the role of LXR activation in beta cell insulin secretion and the underlying mechanism that might be involved.

METHODS

Mouse pancreatic islets or insulin-secreting MIN6 cells were exposed to the LXR agonist, T0901317, and insulin secretion, glucose and fatty acid oxidation, and lipogenic gene expression were assessed. The unsaturated fatty acid eicosapentaenoic acid and the dominant negative sterol regulatory element binding protein 1c (SREBP1c) were used to inhibit endogenous SREBP1c and evaluate the involvement of SREBP1c in beta cell dysfunction induced by LXR activation.

RESULTS

Treatment with the LXR agonist decreased beta cell glucose sensitivity and impaired glucose-stimulated insulin secretion in vivo and in vitro. This was accompanied by derangements of beta cell glucose oxygen consumption, glucose oxidation, ATP production and intracellular voltage-gated calcium channel flux. LXR activation also regulated the expression of lipid metabolism-related genes such as Fas, Acc (also known as Acaca) and Cpt1a, and led to intracellular lipid accumulation. Further studies revealed that inhibition of SREBP1c abolished LXR activation-induced lipid accumulation and improved beta cell glucose metabolism, ATP production and insulin secretion.

CONCLUSIONS/INTERPRETATION

Our data reveal that aberrant activation of LXR reproduced the phenomenon of beta cell dysfunction in the development of type 2 diabetes in vitro and in vivo. Upregulation of SREBP1c production and the lipotoxicity mediated by it played a central role in this process.

Ras/mitogen-activated protein kinase (MAPK) signaling modulates protein stability and cell surface expression of scavenger receptor SR-BI

The mitogen-activated protein kinase (MAPK) Erk1/2 has been implicated to modulate the activity of nuclear receptors, including peroxisome proliferator activator receptors (PPARs) and liver X receptor, to alter the ability of cells to export cholesterol. Here, we investigated if the Ras-Raf-Mek-Erk1/2 signaling cascade could affect reverse cholesterol transport via modulation of scavenger receptor class BI (SR-BI) levels. We demonstrate that in Chinese hamster ovary (CHO) and human embryonic kidney (HEK293) cells, Mek1/2 inhibition reduces PPARα-inducible SR-BI protein expression and activity, as judged by reduced efflux onto high density lipoprotein (HDL). Ectopic expression of constitutively active H-Ras and Mek1 increases SR-BI protein levels, which correlates with elevated PPARα Ser-21 phosphorylation and increased cholesterol efflux. In contrast, SR-BI levels are insensitive to Mek1/2 inhibitors in PPARα-depleted cells. Most strikingly, Mek1/2 inhibition promotes SR-BI degradation in SR-BI-overexpressing CHO cells and human HuH7 hepatocytes, which is associated with reduced uptake of radiolabeled and 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyane-labeled HDL. Loss of Mek1/2 kinase activity reduces SR-BI expression in the presence of bafilomycin, an inhibitor of lysosomal degradation, indicating down-regulation of SR-BI via proteasomal pathways. In conclusion, Mek1/2 inhibition enhances the PPARα-dependent degradation of SR-BI in hepatocytes.

A model of insulin resistance in mice, born to diabetic pregnancy, is associated with alterations of transcription-related genes in pancreas and epididymal adipose tissue

Objective. This study is conducted on a model of insulin-resistant (IR) mice born to dams which were rendered diabetic by the administration of streptozotocin. Methods. Adult IR and control offspring were selected and we determined the mRNA expression of transcription factors known to modulate pancreatic and adipose tissue activities and inflammation. Results. We observed that serum insulin increased, and the mRNA of insulin gene transcription factors, Pdx-1, Nkx6.1 and Maf-A, were upregulated in IR mice pancreas. Besides, their pancreatic functional capacity seemed to be exhausted as evidenced by low expression of pancreatic Glut2 and glucokinase mRNA. Though IR offspring exhibited reduced epididymal adipose tissue, their adipocytes seemed to be differentiated into macrophage-like cells, as they exhibited upregulated CD14 and CD68 antigens, generally expressed by macrophages. However, there was no peripheral macrophages infiltration into epididymal adipose tissue, as the expression of F4/80, a true macrophage marker, was undetectable. Furthermore, the expression of IL-6, TNF-α and TLR-2, key players of insulin resistance, was upregulated in the adipose tissue of IR offspring. Conclusion. Insulin resistant state in mice, born to diabetic pregnancy, alters the expression of function-related genes in pancreas and epididymal adipose tissue and these offspring are prone to develop metabolic syndrome.

Network analysis of hepatic genes responded to high-fat diet in C57BL/6J mice: nutrigenomics data mining from recent research findings

Obesity and its associated complications, including diabetes, dyslipidemia, atherosclerosis, and some cancers, have been a global health problem with a rapid increase of the obese population. In this study, we selected 31 obesity candidate genes in the liver of high-fat-induced obese C57BL/6J mice through investigation of literature search and analyzed functional protein-protein interaction of the genes using the STRING database. Most of the obesity candidate genes were closely connected through lipid metabolism, and in particular acyl-coenzyme A oxidase 1 appeared to be a core obesity gene. Overall, genes involved in fatty acid beta-oxidation, fatty acid synthesis, and gluconeogenesis were up-regulated, and genes involved in sterol biosynthesis, insulin signaling, and oxidative stress defense system were down-regulated with a high-fat diet. Future identification of core obesity genes and their functional targets is expected to provide a new way to prevent obesity by phytochemicals or functional foods on the basis of food and nutritional genomics.

Suggestive evidence of associations between liver X receptor β polymorphisms with type 2 diabetes mellitus and obesity in three cohort studies: HUNT2 (Norway), MONICA (France) and HELENA (Europe)

BACKGROUND

The liver X receptors (LXR) α and β regulate lipid and carbohydrate homeostasis and inflammation. Lxrβ⁻/⁻ mice are glucose intolerant and at the same time lean. We aimed to assess the associations between single nucleotide polymorphisms (SNPs) in LXRβ and risk of type 2 diabetes mellitus (T2DM), obesity and related traits in 3 separate cohort studies.

METHODS

Twenty LXRβ SNPs were identified by sequencing and genotyped in the HUNT2 adult nested case-control study for T2DM (n = 835 cases/1986 controls). Five tag-SNPs (rs17373080, rs2695121, rs56151148, rs2303044 and rs3219281), covering 99.3% of the entire common genetic variability of the LXRβ gene were identified and genotyped in the French MONICA adult study (n = 2318) and the European adolescent HELENA cross-sectional study (n = 1144). In silico and in vitro functionality studies were performed.

RESULTS

We identified suggestive or significant associations between rs17373080 and the risk of (i) T2DM in HUNT2 (OR = 0.82, p = 0.03), (ii) obesity in MONICA (OR = 1.26, p = 0.05) and (iii) overweight/obesity in HELENA (OR = 1.59, p = 0.002). An intron 4 SNP (rs28514894, a perfect proxy for rs17373080) could potentially create binding sites for hepatic nuclear factor 4 alpha (HNF4α) and nuclear factor 1 (NF1). The C allele of rs28514894 was associated with ~1.25-fold higher human LXRβ basal promoter activity in vitro. However, no differences between alleles in terms of DNA binding and reporter gene transactivation by HNF4α or NF1 were observed.

CONCLUSIONS

Our results suggest that rs17373080 in LXRβ is associated with T2DM and obesity, maybe via altered LXRβ expression.

Fructose-1,6-bisphosphatase regulates glucose-stimulated insulin secretion of mouse pancreatic beta-cells

Pancreatic β-cells can precisely sense glucose stimulation and accordingly adjust their insulin secretion. Fructose-1,6-bisphosphatase (FBPase) is a gluconeogenic enzyme, but its physiological significance in β-cells is not established. Here we determined its physiological role in regulating glucose sensing and insulin secretion of β-cells. Considerable FBPase mRNA was detected in normal mouse islets and β-cell lines, although their protein levels appeared to be quite low. Down-regulation of FBP1 in MIN6 cells by small interfering RNA could enhance the glucose-stimulated insulin secretion (GSIS), whereas FBP1-overexpressing MIN6 cells exhibited decreased GSIS. Inhibition of FBPase activity in islet β-cells by its specific inhibitor MB05032 led to significant increase of their glucose utilization and cellular ATP to ADP ratios and consequently enhanced GSIS in vitro. Pretreatment of mice with the MB05032 prodrug MB06322 could potentiate GSIS in vivo and improve their glucose tolerance. Therefore, FBPase plays an important role in regulating glucose sensing and insulin secretion of β-cells and serves a promising target for diabetes treatment.

Coronary heart disease: Significance of liver X receptor α genomics

Crosstalk between lipid peroxidation and inflammation is known to be a pathognomonic feature for the development of coronary heart disease (CHD). In this regard ligand activated liver X receptor (LXR)-α has emerged as a key molecular switch by its inherent ability to modulate an array of genes involved in these two fundamental cellular processes. In addition, LXR-α has also been found to play a role in hepatic lipogenesis and innate immunity. Although several lines of evidence in experimental model systems have established the atheroprotective nature of LXR-α, human subjects have been reported to possess a paradoxical situation in which increased blood cellular LXR-α gene expression is always accompanied by increased coronary occlusion. This apparent paradox was resolved recently by the finding that CHD patients possess a deregulated LXR-α transcriptome due to impaired ligand-receptor interaction. This blood cellular mutated LXR-α gene expression correlated specifically with the extent of coronary occlusion and hence need is felt to devise new synthetic ligands that could restore the function of this mutated LXR-α protein in order to modulate genes involved in reverse cholesterol transport and suppression of the inflammatory response leading to the effective treatment of CHD.

Liver X receptor agonists augment human islet function through activation of anaplerotic pathways and glycerolipid/free fatty acid cycling

Recent studies in rodent models suggest that liver X receptors (LXRs) may play an important role in the maintenance of glucose homeostasis and islet function. To date, however, no studies have comprehensively examined the role of LXRs in human islet biology. Human islets were isolated from non-diabetic donors and incubated in the presence or absence of two synthetic LXR agonists, TO-901317 and GW3965, under conditions of low and high glucose. LXR agonist treatment enhanced both basal and stimulated insulin secretion, which corresponded to an increase in the expression of genes involved in anaplerosis and reverse cholesterol transport. Furthermore, enzyme activity of pyruvate carboxylase, a key regulator of pyruvate cycling and anaplerotic flux, was also increased. Whereas LXR agonist treatment up-regulated known downstream targets involved in lipogenesis, we observed no increase in the accumulation of intra-islet triglyceride at the dose of agonist used in our study. Moreover, LXR activation increased expression of the genes encoding hormone-sensitive lipase and adipose triglyceride lipase, two enzymes involved in lipolysis and glycerolipid/free fatty acid cycling. Chronically, insulin gene expression was increased after treatment with TO-901317, and this was accompanied by increased Pdx-1 nuclear protein levels and enhanced Pdx-1 binding to the insulin promoter. In conclusion, our data suggest that LXR agonists have a direct effect on the islet to augment insulin secretion and expression, actions that should be considered either as therapeutic or unintended side effects, as these agents are developed for clinical use.

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.

Glucose, regulator of survival and phenotype of pancreatic beta cells

The key role of glucose in regulating insulin release by the pancreatic beta cell population is not only dependent on acute stimulus-secretion coupling mechanisms but also on more long-term influences on beta cell survival and phenotype. Glucose serves as a major survival factor for beta cells via at least three actions: it prevents an oxidative redox state, it suppresses a mitochondrial apoptotic program that is triggered at reduced mitochondrial metabolic activity and it induces genes needed for the cellular responsiveness to glucose and to growth factors. Glucose-regulated pathways may link protein synthetic and proliferative activities, making glucose a permissive factor for beta cell proliferation, in check with metabolic needs. Conditions of inadequate glucose metabolism in beta cells are not only leading to deregulation of acute secretory responses but should also be considered as causes for increased apoptosis and reduced formation of beta cells, and loss of their normal differentiated state.

Elevated insulin secretion from liver X receptor-activated pancreatic beta-cells involves increased de novo lipid synthesis and triacylglyceride turnover

Increased basal and loss of glucose-stimulated insulin secretion (GSIS) are hallmarks of beta-cell dysfunction associated with type 2 diabetes. It has been proposed that elevated glucose promotes insulin secretory defects by activating sterol regulatory element binding protein (SREBP)-1c, lipogenic gene expression, and neutral lipid storage. Activation of liver X receptors (LXRs) also activates SREBP-1c and increases lipogenic gene expression and neutral lipid storage but increases basal and GSIS. This study was designed to characterize the changes in de novo fatty acid and triacylglyceride (TAG) synthesis in LXR-activated beta-cells and determine how these changes contribute to elevated basal and GSIS. Treatment of INS-1 beta-cells with LXR agonist T0901317 and elevated glucose led to markedly increased nuclear localization of SREBP-1, lipogenic gene expression, de novo synthesis of monounsaturated fatty acids and TAG, and basal and GSIS. LXR-activated cells had increased fatty acid oxidation and expression of genes involved in mitochondrial beta-oxidation, particularly carnitine palmitoyltransferase-1. Increased basal insulin release from LXR-activated cells coincided with rapid turnover of newly synthesized TAG and required acyl-coenzyme A synthesis and mitochondrial beta-oxidation. GSIS from LXR-activated INS-1 cells required influx of extracellular calcium and lipolysis, suggesting production of lipid-signaling molecules from TAG. Inhibition of diacylglyceride (DAG)-binding proteins, but not classic isoforms of protein kinase C, attenuated GSIS from LXR-activated INS-1 cells. In conclusion, LXR activation in beta-cells exposed to elevated glucose concentrations increases de novo TAG synthesis; subsequent lipolysis produces free fatty acids and DAG, which are oxidized to increase basal insulin release and activate DAG-binding proteins to enhance GSIS, respectively.

Activation of liver X receptors inhibits pancreatic islet beta cell proliferation through cell cycle arrest

AIMS/HYPOTHESIS

Liver X receptors (LXRs) are important transcriptional regulators of lipid homeostasis and proliferation in several cell types. However, the roles of LXRs in pancreatic beta cells have not been fully established. The aim of this study was to investigate the effects of LXRs on pancreatic beta cell proliferation.

METHODS

Gene expression was analysed using real-time RT-PCR. Transient transfection and reporter gene assays were used to determine the transcriptional activity of LXRs in pancreatic beta cells. Cell viability and proliferation were analysed using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT), DNA fluorometric, BrdU labelling and [(3)H]thymidine incorporation assays. Cell cycle distribution was investigated by flow cytometry analysis. Adenovirus-based RNA interference was used to knockdown LXRalpha, LXRbeta and p27 in MIN6 cells and mouse islets.

RESULTS

We found that both Lxralpha (also known as Nr1h3) and Lxrbeta (also known as Nr1h2) were expressed and transactivated the LXR response element in HIT-T15 and MIN6 cells. Activation of LXRs dose-dependently inhibited pancreatic beta cell viability and proliferation. This was accompanied by beta cell cycle arrest at the G1 phase. Furthermore, LXR activation increased levels of the p27 protein by inhibiting its degradation. Knockdown of p27 reversed these effects of LXR activation on growth inhibition and cell cycle arrest.

CONCLUSIONS/INTERPRETATION

Our observations indicate that LXR activation inhibits pancreatic beta cell proliferation through cell cycle arrest. A well-known regulator of pancreatic beta cell cycle progression, p27, is upregulated and mediates the effects of LXRs on growth inhibition in beta cells. These observations suggest the involvement of aberrant activation of LXR in beta cell mass inadequacy, which is an important step in the development of type 2 diabetes.

Liver fatty acid binding protein gene ablation enhances age-dependent weight gain in male mice

Although studies performed in vitro and with transfected cells in culture suggest a role for liver fatty acid binding protein (L-FABP) in regulating fatty acid oxidation and fat deposition, the physiological significance of this possibility is not completely clear. To begin to address this question, the effect of L-FABP gene ablation on phenotype of standard rodent chow-fed male mice was examined with increasing age up to 18 months. While young (2-3 months old) L-FABP null mice displayed no visually obvious phenotype, with increasing age >9 months the L-FABP null mice were visibly larger, exhibiting increased body weight due to increased fat and lean tissue mass. Liver lipid concentrations were unaffected by L-FABP gene ablation with the exception of triacylglycerol, which was decreased by 74% in the livers of 3-month-old mice. Likewise, serum lipid levels were not altered in L-FABP null mice with the exception of triacylglycerol, which was increased in the serum of 18-month-old mice. Increased body weight, fat tissue mass, and lean tissue mass in 18-month-old L-FABP null mice were accompanied by increased hepatic levels of low-density lipoprotein (LDL) receptor, peroxisome proliferator-activated receptor (PPAR) alpha, and PPARalpha-regulated proteins such as fatty acid transport protein (FATP), fatty acid translocase (FAT/CD36), carnitine palmitoyl transferase I (CPT I), and lipoprotein lipase (LPL). A key enzyme in cholesterol biosynthesis, 3-hydroxy-3-methylglutaryl Coenzyme A (HMG-CoA) reductase, was down-regulated in L-FABP null mice. These findings were consistent with a proposed role for L-FABP as an important physiological regulator of PPARalpha.

The effect of T0901317 on ATP-binding cassette transporter A1 and Niemann-Pick type C1 in apoE-/- mice

Although a range of studies indicated Liver X receptor (LXR) activation inhibited the development of atherosclerosis in animal models, the mechanism of this effect for LXR agonists has not been fully understood. A recent study has suggested LXR activators increased the amount of free cholesterol in the plasma membrane of human macrophages by inducing Niemann-Pick type C1 (NPC1) gene expression. Therefore, we hypothesize that LXRs may also promote NPC1 expression in vivo. Here we investigated the effect of a synthetic LXR agonist T0901317 on ATP-binding cassette transporter A1 (ABCA1) and NPC1 in apolipoprotein E knockout (apoE-/-) mice. Male apoE-/- mice were randomized into four groups: baseline group (n = 10), vehicle group (n = 14), prevention group (n = 14), and treatment group (n = 14). En face analysis and Oil red O staining were used to examine the aortic atherosclerotic lesions. Macrophage content of aortic root atherosclerotic lesions and cholesterol efflux form peritoneal macrophages were measured. Gene and protein expression was analyzed by real-time quantitative polymerase chain reaction and Western blotting, respectively. T0901317 treatment reduced aortic atherosclerotic lesion area by 64.2% in prevention group (P < 0.001) and 58.3% in treatment group (P < 0.001) and resulted in a reduction in macrophage content. Plasma triglyceride, total cholesterol, high-density lipoprotein cholesterol, and apoA-I concentrations were markedly increased in T0901317-treated groups. T0901317 also promoted ABCA1 and NPC1 gene and protein levels in the aorta, liver, and small intestine of apoE-/- mice and significantly increased cholesterol efflux from peritoneal macrophages. T0901317 upregulates ABCA1 and NPC1. This study gives us a new insight into the mechanism for antiatherogenic effect of LXR synthetic agonists.

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.

Acute-phase serum amyloid A as a marker of insulin resistance in mice

Acute-phase serum amyloid A (A-SAA) was shown recently to correlate with obesity and insulin resistance in humans. However, the mechanisms linking obesity-associated inflammation and elevated plasma A-SAA to insulin resistance are poorly understood. Using high-fat diet- (HFD-) fed mice, we found that plasma A-SAA was increased early upon HFD feeding and was tightly associated with systemic insulin resistance. Plasma A-SAA elevation was due to induction of Saa1 and Saa2 expression in liver but not in adipose tissue. In adipose tissue Saa3 was the predominant isoform and the earliest inflammatory marker induced, suggesting it is important for initiation of adipose tissue inflammation. To assess the potential impact of A-SAA on adipose tissue insulin resistance, we treated 3T3-L1 adipocytes with recombinant A-SAA. Intriguingly, physiological levels of A-SAA caused alterations in gene expression closely resembling those observed in HFD-fed mice. Proinflammatory genes (Ccl2, Saa3) were induced while genes critical for insulin sensitivity (Irs1, Adipoq, Glut4) were down-regulated. Our data identify HFD-fed mice as a suitable model to study A-SAA as a biomarker and a novel possible mediator of insulin resistance.

Liver TAG transiently decreases while PL n-3 and n-6 fatty acids are persistently elevated in insulin resistant mice

Changes in fatty acid metabolism associated with insulin resistance have been described in rats and humans but have not been well characterized in the frequently used mouse model of diet-induced obesity. To analyse the early phase as well as established insulin resistance, C57BL/6 mice were placed for 1 or 16 weeks on a high fat diet (1w-HFD, 16w-HFD). Endocrine and metabolic parameters indicated that 1w-HFD mice showed a moderate but significant induction of insulin resistance while 16w-HFD mice exhibited profound obesity-associated insulin resistance and dyslipidemias. Significant alterations in fatty acid composition were observed in plasma and liver in both groups. Liver phospholipid-associated arachidonate and docosahexaenoate were increased in both 1w-HFD and 16w-HFD mice, possibly due to increased expression of the desaturases Fads1 and Fads2. Unexpectedly, SCD1 activity and gene expression in liver were decreased in the 1w-HFD group accompanied by diminished total hepatic lipid levels, while they were increased in chronically fed mice. Our data indicate that the early phase of HFD-induced insulin resistance is not associated with elevated liver lipid concentration. Furthermore, the early and persistent rise of arachidonate and docosahexaenoate indicates that insulin resistance is not due to insufficient availability (or concentrations) of polyunsaturated fatty acids as postulated previously.

Nutrigenomics, beta-cell function and type 2 diabetes

INTRODUCTION

The present investigation was designed to investigate the accuracy and precision of lactate measurement obtained with contemporary biosensors (Chiron Diagnostics, Nova Biomedical) and standard enzymatic photometric procedures (Sigma Diagnostics, Abbott Laboratories, Analyticon).

MATERIALS AND METHODS

Measurements were performed in vitro before and after the stepwise addition of 1 molar sodium lactate solution to samples of fresh frozen plasma to systematically achieve lactate concentrations of up to 20 mmol/l.

RESULTS

Precision of the methods investigated varied between 1% and 7%, accuracy ranged between 2% and -33% with the variability being lowest in the Sigma photometric procedure (6%) and more than 13% in both biosensor methods.

CONCLUSION

Biosensors for lactate measurement provide adequate accuracy in mean with the limitation of highly variable results. A true lactate value of 6 mmol/l was found to be presented between 4.4 and 7.6 mmol/l or even with higher difference. Biosensors and standard enzymatic photometric procedures are only limited comparable because the differences between paired determinations presented to be several mmol. The advantage of biosensors is the complete lack of preanalytical sample preparation which appeared to be the major limitation of standard photometry methods.

Liver X Receptor: Crosstalk Node for the Signaling of Lipid Metabolism, Carbohydrate Metabolism, and Innate Immunity

Liver X Receptor-α (LXRα, also known as NR1H3) and LXRβ (NR1H2) are members of the nuclear receptor superfamily of ligand-activated transcription factors, a superfamily which includes the more widely known glucocorticoid receptor, estrogen receptor, thyroid receptor, and peroxisome proliferator-activated receptors. The LXRs are activated by physiologic sterol ligands (e.g., oxysterols) and by synthetic agonists. In recent years, our understanding of the importance of LXRs has expanded across several fields of (patho-)physiology. Perhaps best known from a sizeable literature as homeostatic 'cholesterol sensors' that drive transcriptional programs promoting cellular cholesterol efflux, 'reverse cholesterol transport,' and bile acid synthesis, more recent roles for LXRs in glucose homeostasis, atherosclerosis, and innate immunity have also been identified. These discoveries complement an emerging literature that continues to draw surprisingly intimate connections between host metabolism and host defense. The present review will discuss the roles of LXR in the signaling of metabolism and innate immunity, and the potential for synthetic LXR agonists as novel therapeutics in dyslipidemia, atherosclerosis, disordered glucose metabolism, and inflammation.

SREBP1 is required for the induction by glucose of pancreatic beta-cell genes involved in glucose sensing

Previous studies have reported both positive and negative effects of culture of islets at high glucose concentrations on regulated insulin secretion. Here, we have reexamined this question in mouse islets and determined the role of changes in lipid synthesis in the effects of glucose. Glucose-stimulated insulin secretion (GSIS) and gene expression were examined in islets from C57BL/6 mice or littermates deleted for sterol-regulatory element binding protein-1 (SREBP1) after 4 days of culture at high glucose concentrations. Culture of control islets at 30 versus 8 mmol/l glucose led to enhanced secretion at both basal (3 mmol/l) and stimulatory (17 mmol/l) glucose concentrations and to enhanced triacylglycerol accumulation. These changes were associated with increases in the expression of genes involved in glucose sensing (glucose transporter 2, glucokinase, sulfonylurea receptor 1, inwardly rectifying K(+) channel 6.2), differentiation (pancreatic duodenal homeobox 1), and lipogenesis (Srebp1, fatty acid synthase, acetyl-coenzyme A carboxylase 1, stearoyl-coenzyme A desaturase 1). When cultured at either 8 or 30 mmol/l glucose, SREBP1-deficient (SREBP1(-/-)) islets displayed reduced GSIS and triacylglycerol content compared with normal islets. Correspondingly, glucose induction of the above genes in control islets was no longer observed in SREBP1(-/-) mouse islets. We conclude that enhanced lipid synthesis mediated by SREBP1c-dependent genes is required for the adaptive changes in islet gene expression and insulin secretion at high glucose concentrations.

Stevioside improves pancreatic beta-cell function during glucotoxicity via regulation of acetyl-CoA carboxylase

Chronic hyperglycemia is detrimental to pancreatic beta-cells, causing impaired insulin secretion and beta-cell turnover. The characteristic secretory defects are increased basal insulin secretion (BIS) and a selective loss of glucose-stimulated insulin secretion (GSIS). Several recent studies support the view that the acetyl-CoA carboxylase (ACC) plays a pivotal role for GSIS. We have shown that stevioside (SVS) enhances insulin secretion and ACC gene expression. Whether glucotoxicity influences ACC and whether this action can be counteracted by SVS are not known. To investigate this, we exposed isolated mouse islets as well as clonal INS-1E beta-cells for 48 h to 27 or 16.7 mM glucose, respectively. We found that 48-h exposure to high glucose impairs GSIS from mouse islets and INS-1E cells, an effect that is partly counteracted by SVS. The ACC dephosphorylation inhibitor okadaic acid (OKA, 10(-8) M), and 5-aminoimidazole-4-carboxamide-1-beta-d-ribofuranoside (AICAR, 10(-4) M), an activator of 5'-AMP protein kinase that phosphorylates ACC, eliminated the beneficial effect of SVS. 5-Tetrade-cyloxy-2-furancarboxylic acid (TOFA), the specific ACC inhibitor, blocked the effect of SVS as well. During glucotoxity, ACC gene expression, ACC protein, and phosphorylated ACC protein were increased in INS-1E beta-cells. SVS pretreatment further increased ACC gene expression with strikingly elevated ACC activity and increased glucose uptake accompanied by enhanced GSIS. Our studies show that glucose is a potent stimulator of ACC and that SVS to some extent counteracts glucotoxicity via increased ACC activity. SVS possesses the potential to alleviate negative effects of glucotoxicity in beta-cells via a unique mechanism of action.

Chronic activation of liver X receptor induces beta-cell apoptosis through hyperactivation of lipogenesis: liver X receptor-mediated lipotoxicity in pancreatic beta-cells

Liver X receptor (LXR)alpha and LXRbeta play important roles in fatty acid metabolism and cholesterol homeostasis. Although the functional roles of LXR in the liver, intestine, fat, and macrophages are well established, its role in pancreatic beta-cells has not been clearly defined. In this study, we revealed that chronic activation of LXR contributes to lipotoxicity-induced beta-cell dysfunction. We observed significantly elevated expression of LXR in the islets of diabetic rodent models, including fa/fa ZDF rats, OLETF rats, and db/db mice. In primary pancreatic islets and INS-1 insulinoma cells, activation of LXR with a synthetic ligand, T0901317, stimulated expression of the lipogenic genes ADD1/SREBP1c, FAS, and ACC and resulted in increased intracellular lipid accumulation. Moreover, chronic LXR activation induced apoptosis in pancreatic islets and INS-1 cells, which was synergistically promoted by high glucose conditions. Taken together, we suggest lipid accumulation caused by chronic activation of LXR in beta-cells as a possible cause of beta-cell lipotoxicity, a key step in the development of type 2 diabetes.

Effect of T0901317 on Hepatic Proinflammatory Gene Expression in ApoE−/− Mice Fed a High-fat/high-cholesterol Diet

OBJECTIVE

In present study, we employed cDNA-based microarray technique to investigate the effect of a synthetic LXR ligand T0901317 on hepatic gene expression of proinflammatory cytokines in apolipoprotein E knockout mice fed an atherogenic diet.

METHODS AND RESULTS

Male 8-week-old apoE-/- mice were randomly divided into four groups, baseline group, vehicle group, prevention group and treatment group. All of the mice were fed a high-fat/high-cholesterol diet with or without LXR agonist T0901317 for 8 or 14 weeks. Gene array analysis found 17 atherosclerosis-related genes with a 2- to 8-fold difference in expression level between vehicle-treated group and T0901317-treated group. It induced mRNA expression of proinflammatory cytokine tumor necrosis factor (TNF), but inhibited gene expression of several other proinflammatory cytokines including interleukin (IL)-1alpha, IL-6, and IL-7 in the liver. C-reactive protein, TNF, matrix metalloproteinase-9, IL-1alpha, IL-6, and IL-7 were verified by real-time quantitative PCR. Next, enzyme-linked immunosorbent assay analyses showed up-regulation of TNFalpha levels and down-regulation of IL-alpha, IL-6, IL-7 levels in plasma sample.

CONCLUSION

The synthetic LXR agonist T0901317 has paradoxical roles in hepatic gene expression of proinflammatory cytokines in apoE-/- mice.