Impact of adenoviral transduction with SREBP1c or AMPK on pancreatic islet gene expression profile: analysis with oligonucleotide microarrays

AMPK α1 mediates the protective effect of adiponectin against insulin resistance in INS-1 pancreatic β cells

The fat-derived protein adiponectin is known to reverse the effects of insulin resistance and to lower blood glucose levels. The AMP-activated protein kinase (AMPK) signalling pathway plays a central role in metabolism and energy homeostasis. Here, to investigate the role of AMPK in the protective effect of adiponectin against insulin resistance, we established the model of high-glucose (HG)- and high-lipid (HL)-induced insulin resistance in INS-1 pancreatic β cells. We found that 25mM of glucose and 0.4mM of palmitic acid treatment significantly increased cell apoptosis and impaired insulin secretion in INS-1 cells. However, recombinant human adiponectin dramatically reduced HG- and/or HL-induced cell apoptosis and greatly improved insulin secretion. Interestingly, adiponectin treatment also activated AMPK signalling pathway by increasing the phosphorylation of Thr172 in the AMPK α subunit; 10μM of compound C, a potent AMPK inhibitor, blocked the protective effects of adiponectin against HG/HL-induced insulin resistance. Furthermore, knockout experiments by CRISPR/Cas9 technology showed that AMPK α1, but not AMPK α2, is involved in the protective effects of adiponectin. Taken together, adiponectin reversed the effects of insulin resistance via AMPK α1, which provides a novel insight into the protective mechanism of adiponectin and may be used as a new strategy for the treatment of type 2 diabetes. SIGNIFICANCE OF THE STUDY: Adiponectin can reverse the effects of insulin resistance and lower blood glucose levels. Here, adiponectin reduced HG/HL-induced cell apoptosis and greatly improved insulin secretion. These effects were blocked by AMPK inhibitor, compound C. Specifically, we found that AMPK α1, but not AMPK α2, mediates the protective effects of adiponectin, which provides a novel insight into the protective mechanism of adiponectin against insulin resistance.

Posttranslational Modifications of Lipid-Activated Nuclear Receptors: Focus on Metabolism

Posttranslational modifications (PTMs) occur to nearly all proteins, are catalyzed by specific enzymes, and are subjected to tight regulation. They have been shown to be a powerful means by which the function of proteins can be modified, resulting in diverse effects. Technological advances such as the increased sensitivity of mass spectrometry-based techniques and availability of mutant animal models have enhanced our understanding of the complexities of their regulation and the effect they have on protein function. However, the role that PTMs have in a pathological context still remains unknown for the most part. PTMs enable the modulation of nuclear receptor function in a rapid and reversible manner in response to varied stimuli, thereby dramatically altering their activity in some cases. This review focuses on acetylation, phosphorylation, SUMOylation, and O-GlcNAcylation, which are the 4 most studied PTMs affecting lipid-regulated nuclear receptor biology, as well as on the implications of such modifications on metabolic pathways under homeostatic and pathological situations. Moreover, we review recent studies on the modulation of PTMs as therapeutic targets for metabolic diseases.

The Paradoxical Effects of AMPK on Insulin Gene Expression and Glucose-Induced Insulin Secretion

The activation of AMP-activated protein kinase (AMPK) is known to repress the expression of the insulin gene and glucose-stimulated insulin secretion (GSIS). However, the mechanisms by which this occurs, as well as the effects of AMPK activation on glucolipotoxicity-induced β-cell dysfunction, have not been elucidated. To investigate the effects of 5-amino-4-imidazolecarboxamide ribonucleotide (AICAR) and peroxisome proliferator-activated receptorγ-coactivator-1α (PGC-1α) on β-cell-specific genes under glucolipotoxic conditions, we performed real-time PCR and measured insulin secretion by primary islets. To study these effects in vivo, we administered AICAR for 10 days (1 mg/g body weight) to 90% pancreatectomized hyperglycemic mice. The exposure of isolated rat and human islets to glucolipotoxic conditions and the overexpression of PGC-1α suppressed insulin and NEUROD1 mRNA expression. However, the expression of these genes was preserved by AICAR treatment and by PGC-1α inhibition. Exposure of isolated islets to glucolipotoxic conditions for 3 days decreased GSIS, which was also well maintained by AICAR treatment and by PGC-1α inhibition. The administration of AICAR to 90% pancreatectomized hyperglycemic mice improved glucose tolerance and insulin secretion. These results indicate that treatment of islets with an AMPK agonist under glucolipotoxic conditions protects against glucolipotoxicity-induced β-cell dysfunction. A better understanding of the functions of molecules such as PGC-1α and AMPK, which play key roles in intracellular fuel regulation, could herald a new era for the treatment of patients with type 2 diabetes mellitus by providing protection against glucolipotoxicity.

A Simple High Efficiency Intra-Islet Transduction Protocol Using Lentiviral Vectors

Successful normalization of blood glucose in patients transplanted with pancreatic islets isolated from cadaveric donors established the proof-of-concept that Type 1 Diabetes Mellitus is a curable disease. Nonetheless, major caveats to the widespread use of this cell therapy approach have been the shortage of islets combined with the low viability and functional rates subsequent to transplantation. Gene therapy targeted to enhance survival and performance prior to transplantation could offer a feasible approach to circumvent these issues and sustain a durable functional β-cell mass in vivo. However, efficient and safe delivery of nucleic acids to intact islet remains a challenging task. Here we describe a simple and easy-to-use lentiviral transduction protocol that allows the transduction of approximately 80 % of mouse and human islet cells while preserving islet architecture, metabolic function and glucose-dependent stimulation of insulin secretion. Our protocol will facilitate to fully determine the potential of gene expression modulation of therapeutically promising targets in entire pancreatic islets for xenotransplantation purposes.

O-GlcNAcylation and the Metabolic Shift in High-Proliferating Cells: All the Evidence Suggests that Sugars Dictate the Flux of Lipid Biogenesis in Tumor Processes

Cancer cells are characterized by their high capability to proliferate. This imposes an accelerated biosynthesis of membrane compounds to respond to the need for increasing the membrane surface of dividing cells and remodeling the structure of lipid microdomains. Recently, attention has been paid to the upregulation of O-GlcNAcylation processes observed in cancer cells. Although O-GlcNAcylation of lipogenic transcriptional regulators is described in the literature (e.g., FXR, LXR, ChREBP), little is known about the regulation of the enzymes that drive lipogenesis: acetyl co-enzyme A carboxylase and fatty acid synthase (FAS). The expression and catalytic activity of both FAS and O-GlcNAc transferase (OGT) are high in cancer cells but the reciprocal regulation of the two enzymes remains unexplored. In this perspective, we collected data linking FAS and OGT and, in so doing, pave the way for the exploration of the intricate functions of these two actors that play a central role in tumor growth.

Hepatic S1P deficiency lowers plasma cholesterol levels in apoB-containing lipoproteins when LDLR function is compromised

Background

Site-1 protease (S1P) is the key enzyme required for activation of the sterol regulatory element binding proteins (SREBPs) that govern lipid synthesis. While S1P has been speculated to influence plasma apoB-containing lipoprotein (Blp) metabolism, there has been little investigative work. LDL receptor (LDLR) is the major receptor for clearing plasma LDL cholesterol (LDL-c). Proprotein convertase subtilisin kexin type 9 (PCSK9) modulates LDL-c through post-translational degradation of the LDLR.

Methods

A hepatic-specific knockdown (KD) of S1P was achieved using floxed S1P mouse models (S1P^f/f and LDLR^-/-S1P^f/f) and hepatic expression of Cre recombinase. Lipids were measured in total plasma and size fractionated plasma using colorimetric assays. Realtime polymerase chain reaction, western blotting and ELISA were used to determine hepatic expression of key genes/protein. Plasmid mediated overexpression and siRNA mediated knockdown of genes were performed in mouse primary hepatocytes to determine the mechanistic basis of PCSK9 gene regulation.

Results

A hepatic-specific KD of S1P resulted in a 45 % and 38 % reduction in plasma total cholesterol and triglyceride levels, respectively. Hepatic S1P KD had a minimal effect on plasma Blp cholesterol (Blp-c) in S1P^f/f mice, despite significantly reducing VLDL secretion. Notably, hepatic S1P KD decreased the LDL receptor (LDLR) mRNA expression by 50 %. However, the reduction in LDLR protein levels was less than that of mRNA expression, especially under fed conditions. Further assessment of hepatic S1P deficiency revealed that it increased LDLR protein stability in vivo. Mechanistically, hepatic S1P KD was shown to decrease the liver and plasma levels of the protein proprotein convertase subtilisin/kexin type 9 (PCSK9), which degrades LDLR protein. This effect was more prominent in the fed condition and sufficient to account for the discordance in LDLR mRNA and protein levels. Furthermore, hepatic S1P was shown to regulate PCSK9 expression through activation of the SREBPs. In the LDLR^-/- background, hepatic S1P KD significantly reduced Blp-c levels.

Conclusion

Hepatic S1P is a physiological modulator of plasma Blp metabolism through its regulation of LDLR and PCSK9. Hepatic S1P is a valid target for lowering plasma Blp-c levels in the situation where LDLR function is compromised.

Live-cell imaging of vesicle trafficking and divalent metal ions by total internal reflection fluorescence (TIRF) microscopy

Total internal reflection fluorescence (TIRF) microscopy is an especially powerful tool for visualizing live cellular events. Fluorescent molecules alone provide broad information about the expression and localization of proteins and other molecules; however, the temporal and spatial resolution is confounded by signal from outside the area of interest and the intensity of the illumination required. TIRF overcomes this limitation by using the reflective properties of a laser beam to illuminate a narrow (<100 nm) strip at the surface of a cell with a relatively low powered evanescent wave, thus making it possible to measure events occurring specifically at the plasma membrane such as exocytosis, single molecule interactions, and ionic changes during signal transduction. Here we describe some of the methods for using TIRF microscopy to study the processes involved in exocytosis from excitable cells (i.e., neurons, endocrine, neuroendocrine, and exocrine cells) and the release of physiologically active substances (i.e., neurotransmitters, hormones, and mucus).The failure of regulated exocytosis is associated with various diseases such as allergy, brain dysfunction, and endocrine illness. Diabetes mellitus, which is due to an absolute (type I) or relative (type II) deficiency of insulin secretion from pancreatic β-cells, is a major area of therapeutic interest. Insulin is stored in dense core vesicles with Zn(2+) ions in pancreatic β-cells. Insulin secretion is regulated by plasma glucose concentration which acts through intracellular metabolism to influence intracellular [Ca(2+)]. However, the precise molecular mechanisms controlling insulin granule movement towards, and fusion at, the plasma membrane remain only partially understood. To tackle this problem, we have used live cell imaging techniques to image regulated exocytosis in single living β-cells alongside intracellular Ca(2+) and Zn(2+) concentrations.

Differential gene expression in pancreatic tissues of streptozocin-induced diabetic rats and genetically-diabetic mice in response to hypoglycemic dipeptide cyclo (His-Pro) treatment

Diabetic studies are mostly interested in gene expression in the pancreas, the site of insulin secretion that regulates blood glucose levels. However, a single gene approach has been ruled out for many years in discovering new genes or the molecular networks involved in the induction process of diabetes. To understand the molecular mechanisms by which cyclo (His-Pro) (CHP) affects amelioration of diabetes mellitus, we performed gene expression profiling in the pancreatic tissues of two diabetic animal models, streptozocin (STZ)-induced diabetic rats (T1DM) and genetically-diabetic (C57BL/6J ob/ob) mice (T2DM). To understand the healing process of these diabetic rodents, we examined the effects of CHP on various gene expression in pancreatic tissues of both animal models. Our microarray analysis revealed that a total of 1,175 genes were down-regulated and 629 genes were up-regulated in response to STZ treatment, and the altered expression levels of numerous genes were restored to normal state upon CHP treatment. In particular, 476 genes showed significantly altered gene expression upon CHP treatment. In a functional classification, 7,198 genes were counted as differentially expressed in pancreatic tissues of STZ- and CHP-treated rats compared with control, whereas 1,534 genes were restored to normal states by CHP treatment. Microarray data demonstrated for the first time that overexpression of the genes encoding IL-1 receptor, lipid metabolic enzymes (e.g. Mte1, Ptdss1, and Sult2a1), myo-inositol oxygenase, glucagon, and somatostatin as well as down-regulation of olfactory receptor 984 and mitochondrial ribosomal protein, which are highly linked to T1DM etiology. In genetically-diabetic mice, 4,384 genes were altered in gene expression by more than 2-fold compared to the control mice, when counted differentially expressed. In genetically-diabetic mice, 4,384 genes altered in expression by higher than 2-fold were counted as differentially expressed genes in pancreatic tissues of CHP-treated mice. On the other hand, 2,140 genes were up-regulated and 2,244 genes were down-regulated by CHP treatment. The results of the microarray analysis revealed that up-regulation of IL-2, IL12a, and leptin receptor and down-regulation of PIK3 played important physiological roles in the onset of T2DM. In conclusion, we hypothesize that CHP accelerates alterations of gene expression in ameliorating diabetes and antagonizes those that induces the disease.

Human mutation within Per-Arnt-Sim (PAS) domain-containing protein kinase (PASK) causes basal insulin hypersecretion

PAS kinase (PASK) is a glucose-regulated protein kinase involved in the control of pancreatic islet hormone release and insulin sensitivity. We aimed here to identify mutations in the PASK gene that may be associated with young-onset diabetes in humans. We screened 18 diabetic probands with unelucidated maturity-onset diabetes of the young (MODY). We identified two rare nonsynonymous mutations in the PASK gene (p.L1051V and p.G1117E), each of which was found in a single MODY family. Wild type or mutant PASKs were expressed in HEK 293 cells. Kinase activity of the affinity-purified proteins was assayed as autophosphorylation at amino acid Thr307 or against an Ugp1p-derived peptide. Whereas the PASK p.G1117E mutant displayed a ∼25% increase with respect to wild type PASK in the extent of autophosphorylation, and a ∼2-fold increase in kinase activity toward exogenous substrates, the activity of the p.L1051V mutant was unchanged. Amino acid Gly1117 is located in an α helical region opposing the active site of PASK and may elicit either: (a) a conformational change that increases catalytic efficiency or (b) a diminished inhibitory interaction with the PAS domain. Mouse islets were therefore infected with adenoviruses expressing wild type or mutant PASK and the regulation of insulin secretion was examined. PASK p.G1117E-infected islets displayed a 4-fold decrease in glucose-stimulated (16.7 versus 3 mM) insulin secretion, chiefly reflecting a 4.5-fold increase in insulin release at low glucose. In summary, we have characterized a rare mutation (p.G1117E) in the PASK gene from a young-onset diabetes family, which modulates glucose-stimulated insulin secretion.

Transcriptome profiling in response to adiponectin in human cancer-derived cells

The adipocyte-derived hormone adiponectin exerts protective actions in several disorders, including some cancers. However, while growing data suggest that adiponectin could be an effective anticancer agent, its mechanism of action in cancer cells is still poorly known. Here, using microarrays, we identified a set of 1,301 genes commonly modulated in three cancer-derived cell lines in response to short-term stimulation with full-length recombinant human adiponectin. Most of these genes are involved in translation regulation, immune or stress responses, and cell proliferation. Furthermore, among genes linked to disease that were retrieved by functional enrichment tests using text mining based on PubMed analysis, we found that 66% are involved in malignant neoplasms, further supporting the link between adiponectin and cancer mechanisms. Bioinformatic analysis demonstrated the diversity of signaling pathways and transcription factors potentially mediating adiponectin effects on gene expression, illustrating the complexity of adiponectin mechanisms of action in cancer cells.

Berberine acutely inhibits insulin secretion from beta-cells through 3',5'-cyclic adenosine 5'-monophosphate signaling pathway

Berberine, a hypoglycemic agent, has recently been shown to activate AMP-activated protein kinase (AMPK) contributing to its beneficial metabolic effects in peripheral tissues. However, whether berberine exerts a regulatory effect on beta-cells via AMPK or other signaling pathways and counteracts glucolipotoxicity remains uncertain. In the present study, the impact of berberine on beta-cell function was investigated in vivo and in vitro. In high-fat-fed rats, berberine treatment for 6 wk significantly decreased plasma glucose and insulin levels before and after an oral glucose challenge along with the reduction of body weight and improvement of blood lipid profile. In accordance with the in vivo results, berberine acutely decreased glucose-stimulated insulin secretion (GSIS) and palmitate-potentiated insulin secretion in MIN6 cells and rat islets. However, pretreated with berberine for 24 h augmented the response of MIN6 cells and rat islets to glucose and attenuated the glucolipotoxicity. Berberine acutely increased AMPK activity in MIN6 cells. However, compound C, an AMPK inhibitor, completely reversed troglitazone-suppressed GSIS, not berberine-suppressed GSIS. Otherwise, berberine decreased cAMP-raising agent-potentiated insulin secretion in MIN6 cells and rat islets. These results suggest that the activation of AMPK is required for troglitazone-suppressed GSIS, whereas cAMP signaling pathway contributes, at least in part, to the regulatory effect of berberine on insulin secretion.

Microarray analyses of SREBP-1a and SREBP-1c target genes identify new regulatory pathways in muscle

In this study we have identified the target genes of sterol regulatory element binding protein (SREBP)-1a and SREBP-1c in primary cultures of human skeletal muscle cells, using adenoviral vectors expressing the mature nuclear form of human SREBP-1a or SREBP-1c combined with oligonucleotide microarrays. Overexpression of SREBP-1a led to significant changes in the expression of 1,315 genes (655 upregulated and 660 downregulated), whereas overexpression of SREBP-1c modified the mRNA level of 514 genes (310 upregulated and 204 downregulated). Gene ontology analysis indicated that in human muscle cells SREBP-1a and -1c are involved in the regulation of a large number of genes that are at the crossroads of different functional pathways, several of which are not directly connected with cholesterol and lipid metabolism. Six hundred fifty-two of all genes identified to be differentially regulated on SREBP overexpression had a sterol regulatory element (SRE) motif in their promoter sequences. Among these, 429 were specifically regulated by SREBP-1a, 69 by SREBP-1c, and 154 by both 1a and 1c. Because both isoforms recognize the same binding motif, we determined whether some of these functional differences could depend on the environment of the SRE motifs in the promoters. Results from promoter analysis showed that different combinations of transcription factor binding sites around the SRE binding motifs may determine regulatory networks of transcription that could explain the superposition of lipid and cholesterol metabolism with various other pathways involved in adaptive responses to stress like hypoxia and heat shock, or involvement in the immune response.

Glucose-induced de novo synthesis of fatty acyls causes proportional increases in INS-1E cellular lipids

Raised concentrations of glucose for extended periods of time have detrimental effects on the insulin-producing beta-cell. As de novo synthesis of lipids has been observed under such conditions, it was hypothesized that newly formed lipids may preferentially contain saturated fatty acids, which in particular have been associated with impaired beta-cell function. Glucose-induced de novo synthesis of fatty acids in INS-1E cells cultured in 5.5, 11, 20 or 27 mM glucose for 5 days was assessed by high-resolution magic-angle-spinning (HR-MAS) NMR spectroscopy and gas chromatography-mass spectrometry (GC-MS). The glucose origin of the increase in fatty acyls was verified by replacing glucose with [1-13C]glucose during culture followed by analysis with two-dimensional 1H-13C NMR spectroscopy. The composition of the fatty acyls was determined by GC-MS. Fatty acyls determined by HR-MAS (1)H NMR spectroscopy were increased fivefold in INS-1E cells cultured in 20 or 27 mM glucose compared with cells cultured in 5.5 mM glucose. The five most abundant fatty acids with their relative percentages in INS-1E cells cultured in 5.5 mM glucose were oleate (33%), palmitate (25%), stearate (19%), octadecenoate (13%) and palmitoleate (4.4%). These proportions were not affected by glucose- induced de novo synthesis in INS-1E cells cultured in 11, 20 or 27 mM glucose. It is concluded that glucose-induced de novo lipid synthesis results in accumulation of both saturated and unsaturated fatty acids in specific proportions that are identical with those present under control conditions.

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.

Transcriptional mechanism of suppression of insulin gene expression by AMP-activated protein kinase activator 5-amino-4-imidazolecarboxamide riboside (AICAR) in beta-cells

It is well known that the activation of AMP-activated protein kinase (AMPK) represses insulin gene expression and glucose-stimulated insulin secretion. However, how this effect is achieved and the effects of AMPK activation on glucolipotoxicity-induced beta-cell dysfunction have not been elucidated. We investigate whether BETA2 gene expression are involved in the AMPK-mediated regulation of insulin gene expression in normal and dysfunctional beta-cells. BETA2 gene expression and protein levels were significantly decreased by AICAR treatment and those were associated with the suppression of BETA2 promoter activity and DNA binding activity. These results demonstrate that the expressions of BETA2 and insulin gene are positively regulated by glucose and negatively by AMPK. Therefore, AMPK may function as a key molecule, which conveys extracellular metabolic signals into the cells and finely tunes expression of beta-cell specific transcription factors in response to glucose level.

Troglitazone acutely activates AMP-activated protein kinase and inhibits insulin secretion from beta cells

Changes in AMP-activated protein kinase (AMPK) activity contribute to the regulation of insulin secretion. Troglitazone has been shown to lower serum insulin levels and protect beta cell function. The aim of the present study was to examine the effects of troglitazone on AMPK activity and insulin secretion in beta cells. Isolated rat islets and MIN6 cells were treated for a short (1 h) or a long time (20 h) with troglitazone. One-hour troglitazone treatment activated AMPK and inhibited both glucose-stimulated insulin secretion (GSIS) and the response of insulin secretion to combined stimuli of glucose and palmitate. Long (20 h) treatment with troglitazone caused a sustained phosphorylation of AMPK and acetyl-CoA carboxylase, and increased GSIS after withdrawal of the drug. This study provided evidence that troglitazone activated AMPK in beta cells. In addition to the insulin-sensitizing effects in peripheral tissues, troglitazone also directly inhibits insulin hypersecretion by the elevated glucose and fatty acids, and thus protects beta cells from glucolipotoxicity.

AMP-activated protein kinase--the fat controller of the energy railroad

AMP-activated protein kinase plays an important role in the regulation of lipid metabolism in response to metabolic stress and energy demand. It is also under endocrine control. AMPK acts at multiple steps and has a central role controlling fatty acid, triglyceride, and cholesterol synthesis, as well as the oxidation of fatty acids through direct phosphorylation effects and the control of gene transcription. As such, it can be considered to be the fat controller of the energy railroad. It is thought that AMPK may be a major mediator of the health benefits of exercise in mitigating the development of obesity and age-onset diseases.

Palmitate activates AMP-activated protein kinase and regulates insulin secretion from beta cells

AMP-activated protein kinase (AMPK) is an energy sensor that regulates cellular metabolism. Changes in AMPK activity contribute to the regulation of insulin secretion. Epidemiological evidence links the ingestion of saturated fatty acid with hyperinsulinemia. The aim of the present study was to examine the effects of palmitate on beta cell AMPK activity and insulin secretion. Isolated rat islets and MIN6 beta cells were treated acutely (5-60 min) or chronically (24 h) with palmitate. Insulin secretion, AMPK and acetyl CoA carboxylase phosphorylation were assessed. The acute effects of palmitate included AMPK activation and augmentation in insulin secretion. Activation of AMPK by 24h pretreatment with palmitate suppressed glucose-stimulated insulin secretion, but not the response of insulin secretion to combined stimuli of glucose and palmitate. This study demonstrated that palmitate availability affected beta cell AMPK activity. In beta cells, an increase in AMPK activity may be required for fatty acid-induced fatty acid oxidation and prevention of lipotoxicity.

Limited role for SREBP-1c in defective glucose-induced insulin secretion from Zucker diabetic fatty rat islets: a functional and gene profiling analysis

Accumulation of intracellular lipid may contribute to defective insulin secretion in type 2 diabetes. Although Zucker diabetic fatty (ZDF; fa/fa) rat islets are fat-laden and overexpress the lipogenic master gene, sterol regulatory element binding protein 1c (SREBP-1c), the contribution of SREBP-1c to the secretory defects observed in this model remains unclear. Here we compare the gene expression profile of lean control (fa/+) and ZDF rat islets in the absence or presence of dominant-negative SREBP-1c (SREBP-1c DN). ZDF islets displayed elevated basal insulin secretion at 3 mmol/l glucose but a severely depressed response to 17 mmol/l glucose. While SREBP-1c DN reduced basal insulin secretion from ZDF islets, glucose-stimulated insulin secretion was not improved. Of 57 genes differentially regulated in ZDF islets and implicated in glucose metabolism, vesicle trafficking, ion fluxes, and/or exocytosis, 21 were upregulated and 5 were suppressed by SREBP-1c DN. Genes underrepresented in ZDF islets were either unaffected (Glut-2, Kir6.2, Rab3), stimulated (voltage-dependent Ca(2+) channel subunit alpha1D, CPT2, SUR2, rab9, syt13), or inhibited (syntaxin 7, secretogranin-2) by SREBP-1c inhibition. Correspondingly, SREBP-1c DN largely corrected decreases in the expression of the transcription factors Pdx-1 and MafA but did not affect the abnormalities in Pax6, Arx, hepatic nuclear factor-1alpha (HNF1alpha), HNF3beta/Forkhead box-a2 (Foxa2), inducible cyclic AMP early repressor (ICER), or transcription factor 7-like 2 (TCF7L2) expression observed in ZDF islets. We conclude that upregulation of SREBP-1c and mild increases in triglyceride content do not explain defective glucose-stimulated insulin secretion from ZDF rats. However, overexpression of SREBP-1c may contribute to enhanced basal insulin secretion in this model.

AMP-activated protein kinase: a new beta-cell glucose sensor?: Regulation by amino acids and calcium ions

Stimulation of AMP-activated protein kinase (AMPK) in skeletal muscle and liver is seen as an exciting prospect for the treatment of type 2 diabetes. However, we have recently demonstrated that changes in AMPK activity accompany the exposure of pancreatic islet beta-cells to elevated glucose concentrations and may be involved in the activation of insulin secretion. Here, we discuss this hypothesis and explore the potential role of changes in AMPK activity in the actions of other secretagogues. Amino acids decreased AMPK activity in MIN6 beta-cells with an order of potency for inhibition: arg=leu < gln= leu + glu < glucose, which was closely correlated with the stimulation of insulin release (r2=0.76). By contrast, increases in intracellular Ca2+ concentration provoked by cell depolarization with KCl activated AMPK in the face of increased free intracellular ATP concentrations. Elevation of intracellular cAMP levels with isobutylmethylxanthine or forskolin had no effect on AMPK activity. We conclude that metabolizable amino acids regulate AMPK in the beta-cell via increases in the cytosolic ATP/AMP ratio and via phosphorylation by the upstream kinase LKB1. Intracellular Ca2+ ions may activate AMPK by calmodulin kinase 1 kinase-mediated phosphorylation. The latter may act as a novel feedback mechanism to inhibit excessive insulin secretion under some circumstances.