The effect of chronic exposure to fatty acids on gene expression in clonal insulin-producing cells: studies using high density oligonucleotide microarray

Protein Kinase CK2 Contributes to Glucose Homeostasis by Targeting Fructose-1,6-Bisphosphatase 1

Glucose homeostasis is of critical importance for the survival of organisms. It is under hormonal control and often coordinated by the action of kinases and phosphatases. We have previously shown that CK2 regulates insulin production and secretion in pancreatic β-cells. In order to shed more light on the CK2-regulated network of glucose homeostasis, in the present study, a qRT-PCR array was carried out with 84 diabetes-associated genes. After inhibition of CK2, fructose-1,6-bisphosphatase 1 (FBP1) showed a significant lower gene expression. Moreover, FBP1 activity was down-regulated. Being a central enzyme of gluconeogenesis, the secretion of glucose was decreased as well. Thus, FBP1 is a new factor in the CK2-regulated network implicated in carbohydrate metabolism control.

Temporal Proteomic Analysis of Pancreatic β-Cells in Response to Lipotoxicity and Glucolipotoxicity

Chronic hyperlipidemia causes the dysfunction of pancreatic β-cells, such as apoptosis and impaired insulin secretion, which are aggravated in the presence of hyperglycemia. The underlying mechanisms, such as endoplasmic reticulum (ER) stress, oxidative stress and metabolic disorders, have been reported before; however, the time sequence of these molecular events is not fully understood. Here, using isobaric labeling-based mass spectrometry, we investigated the dynamic proteomes of INS-1 cells exposed to high palmitate in the absence and presence of high glucose. Using bioinformatics analysis of differentially expressed proteins, including the time-course expression pattern, protein-protein interaction, gene set enrichment and KEGG pathway analysis, we analyzed the dynamic features of previously reported and newly identified lipotoxicity- and glucolipotoxicity-related molecular events in more detail. Our temporal data highlight cholesterol metabolism occurring at 4 h, earlier than fatty acid metabolism that started at 8 h and likely acting as an early toxic event highly associated with ER stress induced by palmitate. Interestingly, we found that the proliferation of INS-1 cells was significantly increased at 48 h by combined treatment of palmitate and glucose. Moreover, benefit from the time-course quantitative data, we identified and validated two new molecular targets: Setd8 for cell replication and Rhob for apoptosis, demonstrating that our temporal dataset serves as a valuable resource to identify potential candidates for mechanistic studies of lipotoxicity and glucolipotoxicity in pancreatic β-cells.

Metabolic regulation of GLP-1 and PC1/3 in pancreatic α-cell line

Background and aims

An intra-islet incretin system has been recently suggested to operate through modulation of the expression and activity of proconvertase 1/3 and 2 (PC1/3, PC2) in pancreatic alpha-cell accounting for local release of GLP-1. Little is known, whether this alpha-cell activity can be affected by the metabolic alterations occurring in type 2 diabetes, such as hyperglycemia, hyperlipidemia or hyperglucagonemia.

Materials and methods

AlphaTC1/6 cells from a mice pancreatic cell line were incubated in the presence of two glucose (G) concentration (5.5 and 16.7 mM) for 16 h with or without free fatty acid, IL6 or glucagon. GLP-1 secretion was measured by ELISA and expression of PC1/3 and PC2 by RT-PCR and western blot; cell viability was determined by MTT method, Reactive Oxygen Species generation (ROS) by H₂DCFDA fluorescence and apoptosis by Annexin staining and Propidium Iodine (PI) fluorescence.

Results

Upon 16.7G incubation, GLP-1 secretion (total and active) was significantly increased in parallel with a significant increment in PC1/3 expression, a slight increase in cell viability and ROS generation and by a decrement in PC2 expression with no change in cell apoptosis. When cells were incubated at 5.5mM glucose with FFA, also an increment in GLP-1 secretion and PC1/3 expression was observed together an increment in ROS generation, a decrement in cell viability, and a modest increment in apoptosis. When incubated with 16.7mM glucose with FFA, the increment in GLP-1 secretion was reduced to basal, accompanied by an increment in apoptosis and ROS generation. This was also observed with IL-6, but in this case, no modification in ROS generation or apoptosis was observed when compared to 16.7mM glucose. The presence of glucagon did not modify any of the parameters studied.

Conclusion

These data suggest that under hyperglycemic, hyperlipidemia or inflammatory conditions, alpha cells can increase expression PC1/3 and activate GLP-1 secretion, which may contribute protecting both alpha and beta-cells from glucose and lipotoxicity, while this effect seems to be lost in the presence of both pathophysiological conditions.

Molecular phenotyping of multiple mouse strains under metabolic challenge uncovers a role for Elovl2 in glucose-induced insulin secretion

OBJECTIVE

In type 2 diabetes (T2D), pancreatic β cells become progressively dysfunctional, leading to a decline in insulin secretion over time. In this study, we aimed to identify key genes involved in pancreatic beta cell dysfunction by analyzing multiple mouse strains in parallel under metabolic stress.

METHODS

Male mice from six commonly used non-diabetic mouse strains were fed a high fat or regular chow diet for three months. Pancreatic islets were extracted and phenotypic measurements were recorded at 2 days, 10 days, 30 days, and 90 days to assess diabetes progression. RNA-Seq was performed on islet tissue at each time-point and integrated with the phenotypic data in a network-based analysis.

RESULTS

A module of co-expressed genes was selected for further investigation as it showed the strongest correlation to insulin secretion and oral glucose tolerance phenotypes. One of the predicted network hub genes was Elovl2, encoding Elongase of very long chain fatty acids 2. Elovl2 silencing decreased glucose-stimulated insulin secretion in mouse and human β cell lines.

CONCLUSION

Our results suggest a role for Elovl2 in ensuring normal insulin secretory responses to glucose. Moreover, the large comprehensive dataset and integrative network-based approach provides a new resource to dissect the molecular etiology of β cell failure under metabolic stress.

Bioactive Nutrients and Nutrigenomics in Age-Related Diseases

The increased life expectancy and the expansion of the elderly population are stimulating research into aging. Aging may be viewed as a multifactorial process that results from the interaction of genetic and environmental factors, which include lifestyle. Human molecular processes are influenced by physiological pathways as well as exogenous factors, which include the diet. Dietary components have substantive effects on metabolic health; for instance, bioactive molecules capable of selectively modulating specific metabolic pathways affect the development/progression of cardiovascular and neoplastic disease. As bioactive nutrients are increasingly identified, their clinical and molecular chemopreventive effects are being characterized and systematic analyses encompassing the "omics" technologies (transcriptomics, proteomics and metabolomics) are being conducted to explore their action. The evolving field of molecular pathological epidemiology has unique strength to investigate the effects of dietary and lifestyle exposure on clinical outcomes. The mounting body of knowledge regarding diet-related health status and disease risk is expected to lead in the near future to the development of improved diagnostic procedures and therapeutic strategies targeting processes relevant to nutrition. The state of the art of aging and nutrigenomics research and the molecular mechanisms underlying the beneficial effects of bioactive nutrients on the main aging-related disorders are reviewed herein.

Chronic Exposure to Proline Causes Aminoacidotoxicity and Impaired Beta-Cell Function: Studies In Vitro

BACKGROUND

Pancreatic islet-cell dysfunction is a hallmark in the development of diabetes, but the reasons for the primary β-cell defect are still elusive. Elevated circulating proline levels have been found in subjects with insulin resistance, obesity, and type 2 diabetes. Therefore, we assessed β-cell function, gene expressions, and cell death after long-term exposure of pancreatic β-cells to excess proline in vitro.

METHODS

Isolated mouse islets and INS-1E cells were incubated with and without excess proline. After 72 h, we examined: (1) β-cell function, including basal insulin secretion (BIS) and glucose-stimulated insulin secretion (GSIS), (2) transcription factors related to insulin gene expression and enzymes involved in the tricarboxylic acid cycle and cholesterol biogenesis, (3) cellular triglycerides (TG) and cholesterol content, (4) the death of INS-1E cells and 3H thymidine incorporation, and (5) protein expression of INS-1E cells in response to proline by proteomics.

RESULTS

We found that high doses of proline increased BIS and decreased GSIS in both isolated mouse islets and INS-1E cells. MafA, insulin 1, and the cytochrome c oxidase subunit VIa polypeptide 2 mRNA expressions were all downregulated, indicating that proline impaired insulin gene transcription and mitochondrial oxidative phosphorylation. In contrast, mevalonate decarboxylase gene expression was upregulated, and simultaneously, cholesterol content in INS-1E cells was enhanced. Protein profiling of INS-1E cells revealed that cytosolic non-specific dipeptidase and α enolase were differentially expressed.

CONCLUSIONS

Our results indicate that proline-induced insulin transcription and mitochondrial oxidative phosphorylation impairment may contribute to the β-cell dysfunction observed in type 2 diabetes. Caution should be applied in interpreting the pathophysiological role of proline since very high proline concentrations were used in the experiments.

Disruption of protein-tyrosine phosphatase 1B expression in the pancreas affects β-cell function

Protein-tyrosine phosphatase 1B (PTP1B) is a physiological regulator of glucose homeostasis and energy balance. However, the role of PTP1B in pancreatic endocrine function remains largely unknown. To investigate the metabolic role of pancreatic PTP1B, we generated mice with pancreas PTP1B deletion (panc-PTP1B KO). Mice were fed regular chow or a high-fat diet, and metabolic parameters, insulin secretion and glucose tolerance were determined. On regular chow, panc-PTP1B KO and control mice exhibited comparable glucose tolerance whereas aged panc-PTP1B KO exhibited mild glucose intolerance. Furthermore, high-fat feeding promoted earlier impairment of glucose tolerance and attenuated glucose-stimulated insulin secretion in panc-PTP1B KO mice. The secretory defect in glucose-stimulated insulin secretion was recapitulated in primary islets ex vivo, suggesting that the effects were likely cell-autonomous. At the molecular level, PTP1B deficiency in vivo enhanced basal and glucose-stimulated tyrosyl phosphorylation of EphA5 in islets. Consistently, PTP1B overexpression in the glucose-responsive MIN6 β-cell line attenuated EphA5 tyrosyl phosphorylation, and substrate trapping identified EphA5 as a PTP1B substrate. In summary, these studies identify a novel role for PTP1B in pancreatic endocrine function.

RNA sequencing identifies dysregulation of the human pancreatic islet transcriptome by the saturated fatty acid palmitate

Pancreatic β-cell dysfunction and death are central in the pathogenesis of type 2 diabetes (T2D). Saturated fatty acids cause β-cell failure and contribute to diabetes development in genetically predisposed individuals. Here we used RNA sequencing to map transcripts expressed in five palmitate-treated human islet preparations, observing 1,325 modified genes. Palmitate induced fatty acid metabolism and endoplasmic reticulum (ER) stress. Functional studies identified novel mediators of adaptive ER stress signaling. Palmitate modified genes regulating ubiquitin and proteasome function, autophagy, and apoptosis. Inhibition of autophagic flux and lysosome function contributed to lipotoxicity. Palmitate inhibited transcription factors controlling β-cell phenotype, including PAX4 and GATA6. Fifty-nine T2D candidate genes were expressed in human islets, and 11 were modified by palmitate. Palmitate modified expression of 17 splicing factors and shifted alternative splicing of 3,525 transcripts. Ingenuity Pathway Analysis of modified transcripts and genes confirmed that top changed functions related to cell death. Database for Annotation, Visualization and Integrated Discovery (DAVID) analysis of transcription factor binding sites in palmitate-modified transcripts revealed a role for PAX4, GATA, and the ER stress response regulators XBP1 and ATF6. This human islet transcriptome study identified novel mechanisms of palmitate-induced β-cell dysfunction and death. The data point to cross talk between metabolic stress and candidate genes at the β-cell level.

Chronic exposure to GLP-1 increases GLP-1 synthesis and release in a pancreatic alpha cell line (α-TC1): evidence of a direct effect of GLP-1 on pancreatic alpha cells

Aims/Hypothesis

Incretin therapies, which are used to treat diabetic patients, cause a chronic supra-physiological increase in GLP-1 circulating levels. It is still unclear how the resulting high hormone concentrations may affect pancreatic alpha cells. The present study was designed to investigate the effects of chronic exposure to high GLP-1 levels on a cultured pancreatic alpha cell line.

Methods

α-TC1-6 cell line was cultured in the presence or absence of GLP-1 (100 nmol/l) for up to 72 h. In our model GLP-1 receptor (GLP-1R) was measured. After the cells were exposed to GLP-1 the levels of glucagon secretion were measured. Because GLP-1 acts on intracellular cAMP production, the function of GLP-1R was studied. We also investigated the effects of chronic GLP-1 exposure on the cAMP/MAPK pathway, Pax6 levels, the expression of prohormone convertases (PCs), glucagon gene (Gcg) and protein expression, glucagon and GLP-1 production.

Results

In our model, we were able to detect GLP-1R. After GLP-1 exposure we found a reduction in glucagon secretion. During further investigation of the function of GLP-1R, we found an activation of the cAMP/MAPK/Pax6 pathway and an increase of Gcg gene and protein expression. Furthermore we observed a significant increase in PC1/3 protein expression, GLP-1 intracellular content and GLP-1 secretion.

Conclusions/Interpretation

Our data indicate that the chronic exposure of pancreatic alpha cells to GLP-1 increases the ability of these cells to produce and release GLP-1. This phenomenon occurs through the stimulation of the transcription factor Pax6 and the increased expression of the protein convertase PC1/3.

Changes in mitochondrial carriers exhibit stress-specific signatures in INS-1Eβ-cells exposed to glucose versus fatty acids

Chronic exposure of β-cells to metabolic stresses impairs their function and potentially induces apoptosis. Mitochondria play a central role in coupling glucose metabolism to insulin secretion. However, little is known on mitochondrial responses to specific stresses; i.e. low versus high glucose, saturated versus unsaturated fatty acids, or oxidative stress. INS-1E cells were exposed for 3 days to 5.6 mM glucose, 25 mM glucose, 0.4 mM palmitate, and 0.4 mM oleate. Culture at standard 11.1 mM glucose served as no-stress control and transient oxidative stress (200 µM H₂O₂ for 10 min at day 0) served as positive stressful condition. Mito-array analyzed transcripts of 60 mitochondrion-associated genes with special focus on members of the Slc25 family. Transcripts of interest were evaluated at the protein level by immunoblotting. Bioinformatics analyzed the expression profiles to delineate comprehensive networks. Chronic exposure to the different metabolic stresses impaired glucose-stimulated insulin secretion; revealing glucotoxicity and lipo-dysfunction. Both saturated and unsaturated fatty acids increased expression of the carnitine/acylcarnitine carrier CAC, whereas the citrate carrier CIC and energy sensor SIRT1 were specifically upregulated by palmitate and oleate, respectively. High glucose upregulated CIC, the dicarboxylate carrier DIC and glutamate carrier GC1. Conversely, it reduced expression of energy sensors (AMPK, SIRT1, SIRT4), metabolic genes, transcription factor PDX1, and anti-apoptotic Bcl2. This was associated with caspase-3 cleavage and cell death. Expression levels of GC1 and SIRT4 exhibited positive and negative glucose dose-response, respectively. Expression profiles of energy sensors and mitochondrial carriers were selectively modified by the different conditions, exhibiting stress-specific signatures.

Impact of glucagon-like peptide-1 (7-36) amide, isosteviol and 5-aminoimidazole-4-carboxamide 1-β-d-ribofuranoside on leucine-mediated α-cell dysfunction

AIM

To investigate the acute and chronic effects of l-leucine on pancreatic α-cell function in vitro. Furthermore, we wanted to explore if glucagon-like peptide-1 (GLP-1), isosteviol (ISV) and 5-aminoimidazole-4-carboxamide 1-β-d-ribofuranoside (AICAR) counteract changes in α-cell function induced by chronic exposure to leucine.

METHODS

Isolated mice islets were incubated with 10 mM leucine for 2 or 72 h. We investigated glucagon and insulin secretion at 2 mM and 16.7 mM glucose. In addition, we cultured clonal α-TC1-6 cells with 5 mM leucine, 5 mM leucine plus GLP-1 (10(-6) M), or ISV (10(-6) M) or AICAR (10(-5) M) at high glucose for 72 h. We measured the glucagon secretion, cholesterol (CHO) and triglyceride (TG) content, cell proliferation as well as gene expression.

RESULTS

Ten millimolar of leucine for 2 h significantly stimulated glucagon and insulin secretion both at 2 and 16.7 mM glucose in mice islets. After 72 h incubation with 10 mM leucine the glucagon secretion was enhanced at both 2 and 16.7 mM glucose, whereas the glucose-stimulated insulin secretion (16.7 mM glucose) was inhibited. Chronic exposure to 5 mM leucine increased glucagon secretion, CHO and TG content, cell proliferation and Pcsk2 (p < 0.001), MafB (p < 0.05), Gcg (p < 0.001), Prkaa1 (p < 0.01), Hmgcr (p < 0.001), Srebf2 (p < 0.001), Acaca (p < 0.001), Mtor (p < 0.05) mRNA expression in clonal α-TC1-6 cells. While GLP-1 was cable of reducing glucagon hypersecretion and Pcsk2 (p < 0.05) mRNA expression. ISV and AICAR had no effect on leucine-induced glucagon hypersecretion.

CONCLUSIONS

Long-term exposure to leucine induces hypersecretion of glucagon secretion, that is, aminoacidotoxicity and influences some key genes of pancreatic α-cells. Interestingly, GLP-1 counteracts the leucine-induced α-cell dysfunction.

Overexpression of peroxisome proliferator-activated receptor α in pancreatic β-cells improves glucose tolerance in diet-induced obese mice

Lipotoxicity is implicated in pancreatic β-cell dysfunction in obesity-induced type 2 diabetes. In vitro, activation of peroxisome proliferator-activated receptor α (PPARα) has been shown to protect pancreatic β-cells from the lipotoxic effects of palmitate, thereby preserving insulin secretion. Utilizing an adeno-associated virus (dsAAV8), overexpression of PPARα was induced specifically in pancreatic β-cells of adult, C57Bl/6 mice fed a high-fat diet for 20 weeks and carbohydrate metabolism and β-cell mass assessed. We show that overexpression of PPARα in pancreatic β-cells in vivo preserves β-cell function in obesity, and this improves glucose tolerance by preserving insulin secretion in comparison to control mice with diet-induced obesity. No changes in β-cell mass were observed in PPARα-overexpressing mice compared with diet-induced obese control animals. This model of β-cell-specific PPARα overexpression provides a useful in vivo model for elucidating the mechanisms underlying β-cell lipotoxicity in obesity-induced 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.

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.

Palmitate affects insulin receptor phosphorylation and intracellular insulin signal in a pancreatic alpha-cell line

This study investigated in a pancreatic alpha-cell line the effects of chronic exposure to palmitate on the insulin and IGF-I receptor (IGF-IR) and intracellular insulin pathways. alpha-TC1-6 cells were cultured in the presence or absence of palmitate (0.5 mmol/liter) up to 48 h. Glucagon secretion, insulin and IGF-IR autophosphorylation, and insulin receptor substrate (IRS)-1, IRS-2, phosphatidylinositol kinase (PI3K) (p85 alpha), and serine-threonine protein kinase (Akt) phosphorylated (active) forms were measured. Erk 44/42 and p38 phosphorylation (P) (MAPK pathway markers) were also measured. Because MAPK can regulate Pax6, a transcription factor that controls glucagon expression, paired box gene 6 (Pax6) and glucagon gene and protein expression were also measured. Basal glucagon secretion was increased and the inhibitory effect of acute insulin exposure reduced in alpha-TC1 cells cultured with palmitate. Insulin-stimulated insulin receptor phosphorylation was greatly reduced by exposure to palmitate. Similar results were observed with IRS-1-P, PI3K (p85 alpha), and Akt-P. In contrast, with IGF-IR and IRS-2-P, the basal levels (i.e. in the absence of insulin stimulation) were higher in cells cultured with palmitate. Similar data were obtained with Erk 44/42-P and p-38-P. Pax6 and glucagon gene and protein expression were higher in cells cultured with palmitate. In these cells cultured, specifics MAPKs inhibitors were able to reduce both Pax6 and glucagon gene and protein expression. These results indicate that alpha-cells exposed to palmitate show insulin resistance of the IRS-1/PI3K/Akt pathway that likely controls glucagon secretion. In contrast, the IRS-2/MAPKs pathway is stimulated, through an activation of the IGF-IR, leading to increased Pax6 and glucagon expression. Our data support the hypothesis that the chronic elevation of fatty acids contribute to alpha-cell dysregulation frequently observed in type 2 diabetes.

Proteins altered by elevated levels of palmitate or glucose implicated in impaired glucose-stimulated insulin secretion

Background

Development of type 2 diabetes mellitus (T2DM) is characterized by aberrant insulin secretory patterns, where elevated insulin levels at non-stimulatory basal conditions and reduced hormonal levels at stimulatory conditions are major components. To delineate mechanisms responsible for these alterations we cultured INS-1E cells for 48 hours at 20 mM glucose in absence or presence of 0.5 mM palmitate, when stimulatory secretion of insulin was reduced or basal secretion was elevated, respectively.

Results

After culture, cells were protein profiled by SELDI-TOF-MS and 2D-PAGE. Differentially expressed proteins were discovered and identified by peptide mass fingerprinting. Complimentary protein profiles were obtained by the two approaches with SELDI-TOF-MS being more efficient in separating proteins in the low molecular range and 2D-PAGE in the high molecular range. Identified proteins included alpha glucosidase, calmodulin, gars, glucose-6-phosphate dehydrogenase, heterogenous nuclear ribonucleoprotein A3, lon peptidase, nicotineamide adenine dinucleotide hydrogen (NADH) dehydrogenase, phosphoglycerate kinase, proteasome p45, rab2, pyruvate kinase and t-complex protein. The observed glucose-induced differential protein expression pattern indicates enhanced glucose metabolism, defense against reactive oxygen species, enhanced protein translation, folding and degradation and decreased insulin granular formation and trafficking. Palmitate-induced changes could be related to altered exocytosis.

Conclusion

The identified altered proteins indicate mechanism important for altered β-cell function in T2DM.

Short-term modulation of extracellular signal-regulated kinase 1/2 and stress-activated protein kinase/c-Jun NH2-terminal kinase in pancreatic islets by glucose and palmitate: possible involvement of ceramide

OBJECTIVES

The effect of glucose and palmitate on the phosphorylation of proteins associated with cell growth and survival (extracellular signal-regulated kinase 1/2 [ERK1/2] and stress-activated protein kinase/c-Jun NH2-terminal kinase [SAPK/JNK]) and on the expression of immediate early genes was investigated.

METHODS

Groups of freshly isolated rat pancreatic islets were incubated in 10-mmol/L glucose with palmitate, LY294002, or fumonisin B1 for the measurement of the phosphorylation and the content of ERK1/2, JNK/SAPK, and v-akt murine thymoma viral oncongene (AKT) (serine 473) by immunoblotting. The expressions of the immediate early genes, c-fos and c-jun, were evaluated by reverse transcription-polymerase chain reaction.

RESULTS

Glucose at 10 mmol/L induced ERK1/2 and AKT phosphorylations and decreased SAPK/JNK phosphorylation. Palmitate (0.1 mmol/L) abolished the glucose effect on ERK1/2, AKT, and SAPK/JNK phosphorylations. LY294002 caused a similar effect. The inhibitory effect of palmitate on glucose-induced ERK1/2 and AKT phosphorylation changes was not observed in the presence of fumonisin B1. Glucose increased c-fos and decreased c-jun expressions. Palmitate and LY294002 abolished these latter glucose effects. The presence of fumonisin B1 abolished the effect induced by palmitate on c-jun expression.

CONCLUSIONS

Our results suggest that short-term changes of mitogen-activated protein kinase and AKT signaling pathways and c-fos and c-jun expressions caused by glucose are abolished by palmitate through phosphatidylinositol 3-kinase inhibition via ceramide synthesis.

Effects of elevated fatty acid and glucose concentrations on pancreatic islet function in vitro

AIMS/HYPOTHESIS

The aims of this study were to elucidate long-term effects of increased fatty acids and glucose concentrations on islet hormone secretion, triglyceride (TG) accumulation and fuel metabolism, and to determine the role of insulin on glucagon secretion.

METHODS

Isolated normal mouse islets were exposed to palmitate (0.6 mM) in the presence of high glucose (16.7 mM). After 48 h culture, glucagon secretion and content, insulin secretion and content, TG content and glucose oxidation were measured. The impact of etomoxir, an inhibitor of carnitine palmitoyl transferase-1, as well as of insulin, and alterations in gene expression were also investigated.

RESULTS

In the presence of palmitate, (i) high glucose caused no statistically significant suppression of glucagon while this was seen in the absence of palmitate; (ii) the insulin response to high glucose was impaired and (iii) an accumulation of TG and a decline in glucose oxidation were detected, whereas the glucagon content remained unchanged. However, etomoxir was capable of reducing glucagon secretion. Addition of exogenous insulin (10(-10)-10(-6) M) failed to restore alpha cell response to normal. Furthermore, 0.6 mM palmitate reduced the mRNA levels of acetyl-CoA carboxylase-1 and sterol regulatory element-binding protein-1c.

CONCLUSIONS/INTERPRETATION

In summary, high concentrations of palmitate and glucose cause a relative increase in glucagon secretion, a decline in insulin secretion, a loss of alpha cell sensitivity to glucose and an accumulation of TG. The inability of insulin to suppress glucagon may be because of insulin resistance of alpha cells.

Fructose-1,6-bisphosphatase overexpression in pancreatic beta-cells results in reduced insulin secretion: a new mechanism for fat-induced impairment of beta-cell function

OBJECTIVE

Fructose-1,6-bisphosphatase (FBPase) is a gluconeogenic enzyme that is upregulated in islets or pancreatic beta-cell lines exposed to high fat. However, whether specific beta-cell upregulation of FBPase can impair insulin secretory function is not known. The objective of this study therefore is to determine whether a specific increase in islet beta-cell FBPase can result in reduced glucose-mediated insulin secretion.

RESEARCH DESIGN AND METHODS

To test this hypothesis, we have generated three transgenic mouse lines overexpressing the human FBPase (huFBPase) gene specifically in pancreatic islet beta-cells. In addition, to investigate the biochemical mechanism by which elevated FBPase affects insulin secretion, we made two pancreatic beta-cell lines (MIN6) stably overexpressing huFBPase.

RESULTS

FBPase transgenic mice showed reduced insulin secretion in response to an intravenous glucose bolus. Compared with the untransfected parental MIN6, FBPase-overexpressing cells showed a decreased cell proliferation rate and significantly depressed glucose-induced insulin secretion. These defects were associated with a decrease in the rate of glucose utilization, resulting in reduced cellular ATP levels.

CONCLUSIONS

Taken together, these results suggest that upregulation of FBPase in pancreatic islet beta-cells, as occurs in states of lipid oversupply and type 2 diabetes, contributes to insulin secretory dysfunction.

Fatty acid-induced effect on glucagon secretion is mediated via fatty acid oxidation

BACKGROUND

While the effect of fatty acids and ectopic triglyceride storage in pancreatic beta cells has been well-defined, only limited information is available on alpha cells. This study evaluates the long-term impact of fatty acids on alpha cell function and proliferation as well as fatty acid oxidation.

METHODS

Clonal alpha cells were cultured with fatty acids in the presence of high glucose for up to 3 days. The influence of fatty acids on glucagon secretion, glucagon content and triglyceride accumulation from 24 to 72 h was investigated. After a - 72 h culture, cell proliferation, carnitine palmitoyl transferase-1 mRNA level and the effect of etomoxir were also elucidated.

RESULTS

Fatty acids stimulated glucagon secretion and increased triglyceride accumulation in a time- and dose-dependent manner, but inhibited alpha cell proliferation. Lower concentrations (0.125-0.25 mM) of fatty acids significantly increased glucagon secretion at 48 and 72 h, but did not affect triglyceride content. However, a marked increment in triglyceride accumulation occurred in the presence of 0.5 mM fatty acids. Fatty acids caused an up-regulation of the expression of carnitine palmitoyl transferase-1 gene. Etomoxir (1 microM) reversed fatty acid-induced glucagon hypersecretion, but did not inhibit carnitine palmitoyl transferase-1 mRNA level.

CONCLUSIONS

Our data indicates that compared with triglyceride accumulation, glucagon secretion is more sensitive to changes in fatty acid concentration. The effect of fatty acids on the glucagon response is mediated through their oxidation. The high carnitine palmitoyl transferase-1 gene expression and the accumulation of triglyceride may initially be a compensatory oxidation reaction to elevated fatty acids.

Stevioside Counteracts Beta-Cell Lipotoxicity without Affecting Acetyl CoA Carboxylase

Chronic exposure to high levels of free fatty acids impairs beta-cell function (lipotoxicity). Then basal insulin secretion (BIS) is increased and glucose-stimulated insulin secretion (GSIS) is inhibited. Acetyl CoA carboxylase (ACC) acts as the sensor for insulin secretion in pancreatic beta-cells in response to glucose and other nutrients. Stevioside (SVS), a diterpene glycoside, has recently been shown to prevent glucotoxic effect by regulating ACC activity. The aim of this study was to investigate whether SVS can alleviate impaired beta-cell function by regulating ACC activity. We exposed isolated rat islets and the clonal beta-cell line, INS-1E, to palmitate concentrations of 1.0 or 0.6 mM, respectively, for a period of 24 h to 120 h. The results showed that lipotoxicity occurred in rat islets after 72 h exposure to 1.0 mM palmitate. The lipotoxicity was counteracted by 10(-6) M SVS (n = 8, p < 0.001). Similar results were obtained in INS-1E cells. Neither SVS nor palmitate had any effect on the gene expression of ACC, insulin 2, and glucose transporter 2 in INS-1E cells. In contrast, palmitate significantly increased the gene expression of carnitine palmitoyl transporter 1 (n = 6, p = 0.003). However, the addition of SVS to palmitate did not counteract this effect (n = 6, p = 1.0). During lipotoxicity, SVS did not alter levels of ACC protein, phosphorylated-ACC, ACC activity or glucose uptake. Our results showed that SVS counteracts the impaired insulin secretion during lipotoxicity in rat islets as well as in INS-1E cells without affecting ACC activity.

Gene expression in human small intestinal mucosa in vivo is mediated by iron-induced oxidative stress

Iron-induced oxidative stress in the small intestine may alter gene expression in the intestinal mucosa. The present study aimed to determine which genes are mediated by an iron-induced oxidative challenge in the human small intestine. Eight healthy volunteers [22 yr(SD2)] were tested on two separate occasions in a randomized crossover design. After duodenal tissue sampling by gastroduodenoscopy, a perfusion catheter was inserted orogastrically to perfuse a 40-cm segment of the proximal small intestine with saline and, subsequently, with either 80 or 400 mg of iron as ferrous gluconate. After the intestinal perfusion, a second duodenal tissue sample was obtained. Thiobarbituric acid-reactive substances, an indicator of lipid peroxidation, in intestinal fluid samples increased significantly and dose dependently at 30 min after the start of perfusion with 80 or 400 mg of iron, respectively ( P < 0.001). During the perfusion with 400 mg of iron, the increase in thiobarbituric acid-reactive substances was accompanied by a significant, momentary rise in trolox equivalent antioxidant capacity, an indicator of total antioxidant capacity ( P < 0.05). The expression of 89 gene reporters was significantly altered by both iron interventions. Functional mapping showed that both iron dosages mediated six distinct processes. Three of those processes involved G-protein receptor coupled pathways. The other processes were associated with cell cycle, complement activation, and calcium channels. Iron administration in the small intestine induced dose-dependent lipid peroxidation and a momentary antioxidant response in the lumen, mediated the expression of at least 89 individual gene reporters, and affected at least six biological processes.

Stevioside counteracts the alpha-cell hypersecretion caused by long-term palmitate exposure

Long-term exposure to fatty acids impairs beta-cell function in type 2 diabetes, but little is known about the chronic effects of fatty acids on alpha-cells. We therefore studied the prolonged impact of palmitate on alpha-cell function and on the expression of genes related to fuel metabolism. We also investigated whether the antihyperglycemic agent stevioside was able to counteract these effects of palmitate. Clonal alpha-TC1-6 cells were cultured with palmitate in the presence or absence of stevioside. After 72 h, we evaluated glucagon secretion, glucagon content, triglyceride (TG) content, and changes in gene expression. Glucagon secretion was dose-dependently increased after 72-h culture, with palmitate at concentrations >or=0.25 mM (P< 0.05). Palmitate (0.5 mM) enhanced TG content of alpha-cells by 73% (P< 0.01). Interestingly, stevioside (10(-8) and 10(-6) M) reduced palmitate-stimulated glucagon release by 22 and 45%, respectively (P< 0.01). There was no significant change in glucagon content after 72-h culture with palmitate and/or stevioside. Palmitate increased carnitine palmitoyltransferase I (CPT I) mRNA level, whereas stevioside enhanced CPT I, peroxisome proliferator-activated receptor-gamma, and stearoyl-CoA desaturase gene expressions in the presence of palmitate (P<0.05). In conclusion, long-term exposure to elevated fatty acids leads to a hypersecretion of glucagon and an accumulation of TG content in clonal alpha-TC1-6 cells. Stevioside was able to counteract the alpha-cell hypersecretion caused by palmitate and enhanced the expression of genes involved in fatty acid metabolism. This indicates that stevioside may be a promising antidiabetic agent in treatment of type 2 diabetes.

Energy restriction prevents the development of type 2 diabetes in Zucker diabetic fatty rats: coordinated patterns of gene expression for energy metabolism in insulin-sensitive tissues and pancreatic islets determined by oligonucleotide microarray analysis

Energy restriction (ER) causes metabolic improvement in the prediabetic and diabetic state. Little information exists on the mechanism of action of ER, for example, on the changes at the transcriptional gene level in insulin-sensitive tissues. To gain further insight, we have investigated changes in gene expressions in skeletal muscle, liver, fat, and pancreatic islets after ER in male Zucker diabetic fatty rats. Eighteen Zucker diabetic fatty rats were divided at the age of 7 weeks into a control group (ad libitum diet) and an ER group (30% ER compared with the control group). Blood glucose, weight, and food intake were measured weekly. After 5 weeks, blood samples, and skeletal muscle, liver, visceral fat (epididymal fat pads), and islets tissue were collected. Gene expression was quantified with high-density oligonucleotide, microarray GeneChip technology. ER ameliorated the development of hyperglycemia, increased the levels of plasma insulin, and reduced plasma total cholesterol and the glucagon-insulin ratio (P < .05). In skeletal muscle, the expression of 55 genes increased and 245 decreased involving genes related to glucose metabolism (eg, phosphorylase kinase, pyruvate dehydrogenase kinase 4), lipid metabolism (eg, carnitine palmitoyltransferase 1, fatty acid transporter), and signaling pathways (eg, mitogen-activated protein kinases, protein kinase C). In the liver, the expression of 123 genes increased and 103 decreased involving genes related primarily to lipid metabolism. In pancreatic islets, the expression of 110 genes increased and that of 127 decreased, whereas in visceral fat, the expression of 279 genes increased and that of 528 decreased. ER counteracts the development of diabetes and causes changes in the expression of multiple genes involved in glucose and lipid metabolism in skeletal muscle, liver, and pancreatic islets, which may play an important role for the prevention of diabetes.

Prevention of hyperglycemia in Zucker diabetic fatty rats by exercise training: effects on gene expression in insulin-sensitive tissues determined by high-density oligonucleotide microarray analysis

Exercise training (ET) causes metabolic improvement in the prediabetic and diabetic states. However, only little information exists on the changes to ET at the transcriptional level in insulin-sensitive tissues. We have investigated the gene expression changes in skeletal muscle, liver, fat, and pancreatic islets after ET in male Zucker diabetic fatty (ZDF) rats. Eighteen ZDF rats (7 weeks old) were divided in a control and ET group. Exercise was performed using a motorized treadmill (20 m/min 1 hour daily for 6 days a week). Blood glucose, weight, and food intake were measured weekly. After 5 weeks, blood samples, soleus muscle, liver, visceral fat (epididymal fat pads), and islet tissue were collected. Gene expression was quantified with Affymetrix RG-U34A array (16 chips). Exercise training ameliorates the development of hyperglycemia and reduces plasma free fatty acid and the level of glucagon-insulin ratio (P < .05). In skeletal muscle, the expression of 302 genes increased, whereas that of 119 genes decreased. These changes involved genes related to skeletal muscle plasticity, Ca(2+) signals, energy metabolism (eg, glucose transporter 1, phosphorylase kinase), and other signaling pathways as well as genes with unknown functions (expressed sequence tags). In the liver, expression of 148 genes increased, whereas that of 199 genes decreased. These were primarily genes involved in lipogenesis and detoxification. Genes coding for transcription factors were changed in parallel in skeletal muscle and liver tissue. Training did not markedly influence the gene expression in islets. In conclusion, ET changes the expression of multiple genes in the soleus muscle and liver tissue and counteracts the development of diabetes, indicating that ET-induced changes in gene transcription may play an important role en the prevention of diabetes.

High-fat feeding during gestation and nursing period have differential effects on the insulin secretory capacity in offspring from normal Wistar rats

Restriction of protein or energy intake during gestation or early life is linked to developmental defects in the endocrine pancreas and insulin resistance.

AIMS

To study whether a saturated fatty acid-rich diet during gestation and/or after the weaning period may be detrimental to the insulin secretory capacity later in life.

STUDY DESIGN

Female Wistar rats were fed diets rich in carbohydrate (CHO) or saturated fat (SAFA) during pregnancy. The male offspring were split into five subgroups: after birth group 1 (control) continued on CHO and group 3 on SAFA. Group 2 continued on the CHO diet during the nursing period but changed to SAFA post weaning. Group 4 continued on SAFA, but changed to CHO post weaning. For group 5 the offspring of mothers given a SAFA diet were changed to nursing mothers on a CHO diet immediately after birth, and continued on the same diet post weaning. After 14 wk, the islets of Langerhans were isolated for determination of insulin secretory capacity in static incubation and dynamic perifusion experiments.

RESULTS

We found a negative correlation (Coef: -3.1, 95% CI: -6.1 to -0.0, p < 0.05) between a diet rich in saturated fat fed to mothers during gestation and a positive correlation (Coef: 4.4, 95% CI: 0.9 to 7.8, p = 0.01) between nursing mothers' diet and the capability to secrete insulin in the offspring.

CONCLUSION

Our results indicate the importance of applying a nutrient-balanced diet during pregnancy and the nursing period on the later insulin secretory capacity in the offspring.

The short-term effect of fatty acids on glucagon secretion is influenced by their chain length, spatial configuration, and degree of unsaturation: studies in vitro

The influence of fatty acids on beta cell function has been well established whereas little is known about the role of fatty acids on alpha cell function. The aim of our study was to investigate the short-term effects of chain length, spatial configuration, and degree of unsaturation of fatty acids on glucagon secretion from isolated mouse islets and alpha tumor cell 1 clone 6 cells (alpha TC1-6 cells). Glucagon release was measured with different saturated and unsaturated fatty acids as well as cis and trans isomers of fatty acids at low and high glucose. Palmitate (0.1-0.5 mmol/L) immediately stimulated glucagon release in a dose-dependent manner from both isolated islets and alpha TC 1-6 cells. The longer chain length of saturated fatty acids, the higher glucagon responses were obtained. The average fold increase in glucagon to saturated fatty acids (0.3 mmol/L) compared to control was octanoate 1.5, laurate 2.0, myristate 2.9, palmitate 5.4, and stearate 6.2, respectively. Saturated fatty acids were more effective than unsaturated fatty acids in stimulating glucagon secretion. At an equimolar concentration, trans-fatty acids were more potent than their cis isomers. Fatty acids immediately stimulate glucagon secretion from isolated mouse islets pancreatic alpha cells. The chain length, spatial configuration, and degree of unsaturation of fatty acids influence the glucagonotropic effect.

Expression analysis of a human hepatic cell line in response to palmitate

Saturated fat plays a role in common debilitating diseases such as obesity, type 2 diabetes, and coronary heart disease. It is also clear that certain fatty acids act as regulators of metabolism via both direct and indirect signalling of target tissues. As the molecular mechanisms of saturated fatty acid signalling in the liver are poorly defined, hepatic gene expression analysis was undertaken in a human hepatocyte cell line after incubation with palmitate. Profiling of mRNA expression using cDNA microarray analysis revealed that 162 of approximately 18,000 genes tested were differentially expressed after incubation with palmitate for 48 h. Altered transcription profiles were observed in a wide variety of genes, including genes involved in lipid and cholesterol transport, cholesterol catabolism, cell growth and proliferation, cell signalling, beta-oxidation, and oxidative stress response. While palmitate signalling has been examined in pancreatic beta-cells, this is the first report showing that palmitate regulates expression of numerous genes via direct molecular signalling mechanisms in liver cells.

Free fatty acids and cytokines induce pancreatic beta-cell apoptosis by different mechanisms: role of nuclear factor-kappaB and endoplasmic reticulum stress

Apoptosis is probably the main form of beta-cell death in both type 1 diabetes mellitus (T1DM) and T2DM. In T1DM, cytokines contribute to beta-cell destruction through nuclear factor-kappaB (NF-kappaB) activation. Previous studies suggested that in T2DM high glucose and free fatty acids (FFAs) are beta-cell toxic also via NF-kappaB activation. The aims of this study were to clarify whether common mechanisms are involved in FFA- and cytokine-induced beta-cell apoptosis and determine whether TNFalpha, an adipocyte-derived cytokine, potentiates FFA toxicity through enhanced NF-kappaB activation. Apoptosis was induced in insulinoma (INS)-1E cells, rat islets, and fluorescence-activated cell sorting-purified beta-cells by oleate, palmitate, and/or cytokines (IL-1beta, interferon-gamma, TNFalpha). Palmitate and IL-1beta induced a similar percentage of apoptosis in INS-1E cells, whereas oleate was less toxic. TNFalpha did not potentiate FFA toxicity in primary beta-cells. The NF-kappaB-dependent genes inducible nitric oxide synthase and monocyte chemoattractant protein-1 were induced by IL-1beta but not by FFAs. Cytokines activated NF-kappaB in INS-1E and beta-cells, but FFAs did not. Moreover, FFAs did not enhance NF-kappaB activation by TNFalpha. Palmitate and oleate induced C/EBP homologous protein, activating transcription factor-4, and immunoglobulin heavy chain binding protein mRNAs, X-box binding protein-1 alternative splicing, and activation of the activating transcription factor-6 promoter in INS-1E cells, suggesting that FFAs trigger an endoplasmic reticulum (ER) stress response. We conclude that apoptosis is the main mode of FFA- and cytokine-induced beta-cell death but the mechanisms involved are different. Whereas cytokines induce NF-kappaB activation and ER stress (secondary to nitric oxide formation), FFAs activate an ER stress response via an NF-kappaB- and nitric oxide-independent mechanism. Our results argue against a unifying hypothesis for the mechanisms of beta-cell death in T1DM and T2DM.

Long-Term exposure of INS-1 cells to cis and trans fatty acids influences insulin release and fatty acid oxidation differentially

The importance of elevated levels of fatty acids in the pathogenesis of the deteriorated beta-cell function present in type 2 diabetes has been established. Long-term exposure of the beta-cell to high levels of fatty acids causes enhanced insulin secretion at low glucose (basal insulin release), while glucose-stimulated insulin secretion (GSIS) is decreased or unchanged. We have previously demonstrated that the spatial configuration of fatty acids (cis and trans isomers) is of importance for the acute impact on the beta-cell function. In this study we aimed to elucidate whether the spatial configuration also influenced beta-cell function after long-term exposure. Thus, we compared the effect of 3 days culture of INS-1 cells with cis (cis C 18:1-11) and trans vaccenic acid (trans C 18:1-11), as well as oleic (cis C 18:1-9) and elaidic acid (trans C 18:1-9), on basal and glucose-stimulated insulin release. All fatty acids tested increased basal insulin release; however, a significantly lower basal insulin release was demonstrated for cells cultured with 0.3 to 0.4 mmol/L trans vaccenic acid compared to equimolar levels of the cis isomer. GSIS was not changed by cis or trans vaccenic acid or by oleic acid, whereas it was stimulated by 0.3 to 0.4 mmol/L elaidic acid. The mechanisms behind the fatty acid-induced changes in the beta cells have been linked to changes in glucose and fatty acid oxidation. We demonstrated an increased fatty acid oxidation in beta cells after long-term exposure to all of the tested fatty acids. Interestingly, both trans isomers (trans vaccenic and elaidic acid) induced higher fatty acid oxidation than the cis isomers (cis vaccenic and oleic acid, respectively). No changes in glucose oxidation were found when INS-1 cells were cultured with either of the fatty acids. The increased fatty acid oxidation was associated with an increased content of carnitine palmitoyltransferase I (CPT-I) mRNA, but no difference in the content of CPT-I mRNA to the different fatty acids was found. Insulin mRNA expression in beta cells was not affected by the fatty acids. In conclusion, we have demonstrated that the pathological changes in insulin secretion from INS-1 cells to long-term culture with elevated levels of fatty acids are more pronounced for the cis (cis vaccenic acid and oleic acid) rather than the trans isomers (trans vaccenic acid and elaidic acid). We suggest that this, at least in part, may be explained by a lower fatty acid oxidation in cells cultured with the cis compared to the trans fatty acid isomers. Apparently, the difference in fatty acid oxidation was not caused by an increased induction of CPT-I mRNA, nor by changes in glucose oxidation or insulin mRNA in beta cells chronically exposed to the fatty acids.

Uncoupling protein 2 and islet function

Stressors such as chronic hyperglycemia or hyperlipidemia may lead to insufficient insulin secretion in susceptible individuals, contributing to type 2 diabetes. The molecules mediating this effect are just beginning to be identified. Uncoupling protein (UCP)-2 may be one such negative modulator of insulin secretion. Accumulating evidence shows that beta-cell UCP2 expression is upregulated by glucolipotoxic conditions and that increased activity of UCP2 decreases insulin secretion. Mitochondrial superoxide has been identified as a posttranslational regulator of UCP2 activity in islets; thus, UCP2 may provide protection to beta-cells at one level while simultaneously having detrimental effects on insulin secretion. Interestingly, the latter appears to be the dominant outcome, because UCP2 knockout mice display an increased beta-cell mass and retained insulin secretion capacity in the face of glucolipotoxicity.

Chronic effects of fatty acids on pancreatic beta-cell function: new insights from functional genomics

Type 2 diabetes can be viewed as a failure of the pancreatic beta-cell to compensate for peripheral insulin resistance with enhanced insulin secretion. This failure is explained by both a relative loss of beta-cell mass as well as secretory defects that include enhanced basal secretion and a selective loss of sensitivity to glucose. These features are reproduced by chronic exposure of beta-cells to fatty acids (FAs), suggesting that hyperlipidemia might contribute to decompensation. Using MIN6 cells pretreated for 48 h with oleate or palmitate, we have previously defined alterations in global gene expression by transcript profiling and described additional secretory changes to those already established (Busch A-K, Cordery D, Denyer G, Biden TJ: Diabetes 51:977-987, 2002). In contrast to a modest decoupling of glucose-stimulated insulin secretion, FA pretreatment markedly enhanced the secretory response to an acute subsequent challenge with FAs. We propose that this apparent switch in sensitivity from glucose to FAs would be an appropriate response to hyperlipidemia in vivo and thus plays a positive role in beta-cell compensation for insulin resistance. Altered expression of dozens of genes could contribute to this switch, and allelic variations in any of these genes could (to varying degrees) impair beta-cell compensation and thus contribute to conditions ranging from impaired glucose tolerance to frank diabetes.

Gene and protein kinase expression profiling of reactive oxygen species-associated lipotoxicity in the pancreatic beta-cell line MIN6

Oligonucleotide microarrays were used to define oleic acid (OA)-regulated gene expression and proteomic technology to screen protein kinases in MIN6 insulinoma cells. The effects of oxidative stress caused by OA and potential protective effects of N-acetyl-L-cysteine (NAC), a scavenger of reactive oxygen species (ROS), on global gene expression and beta-cell function were investigated. Long-term exposure of MIN6 cells to OA led to a threefold increase in basal insulin secretion, a 50% decrease in insulin content, an inhibition of glucose-stimulated insulin secretion (GSIS), and a twofold increase in the level of ROS. The addition of NAC normalized both the OA-induced insulin content and ROS elevation, but it failed to restore GSIS. Microarray studies and subsequent quantitative PCR analysis showed that OA consistently regulated the expression of 45 genes involved in metabolism, cell growth, signal transduction, transcription, and protein processing. The addition of NAC largely normalized the expression of the OA-regulated genes involved in cell growth and differentiation but not other functions. A protein kinase screen showed that OA regulated the expression and/or phosphorylation levels of kinases involved in stress-response mitogen-activated protein kinase, phosphatidylinositol 3-kinase, and cell cycle control pathways. Importantly, these findings indicate that chronic OA exposure can impair beta-cell function through ROS-dependent and -independent mechanisms.

Palmitate inhibition of insulin gene expression is mediated at the transcriptional level via ceramide synthesis

Chronic exposure to elevated levels of fatty acids impairs pancreatic beta cell function, a phenomenon thought to contribute to the progressive deterioration of insulin secretion in type 2 diabetes. We have previously demonstrated that prolonged exposure of isolated islets to elevated levels of palmitate inhibits preproinsulin mRNA levels in the presence of high glucose concentrations. However, whether this occurs via transcriptional or post-transcriptional mechanisms has not been determined. In addition, the nature of the lipid metabolites involved in palmitate inhibition of insulin gene expression is unknown. In this study, we show that palmitate decreases glucose-stimulated preproinsulin mRNA levels in isolated rat islets, an effect that is not mediated by changes in preproinsulin mRNA stability, but is associated with inhibition of glucose-stimulated insulin promoter activity. Prolonged culture of isolated islets with palmitate is associated with increased levels of intracellular ceramide. Palmitate-induced ceramide generation is prevented by inhibitors of de novo ceramide synthesis. Further, exogenous ceramide inhibits insulin mRNA levels, whereas blockade of de novo ceramide synthesis prevents palmitate inhibition of insulin gene expression. We conclude that prolonged exposure to elevated levels of palmitate affects glucose-stimulated insulin gene expression via transcriptional mechanisms and ceramide synthesis.

Nutrigenomics: goals and strategies

Nutrigenomics is the application of high-throughput genomics tools in nutrition research. Applied wisely, it will promote an increased understanding of how nutrition influences metabolic pathways and homeostatic control, how this regulation is disturbed in the early phase of a diet-related disease and to what extent individual sensitizing genotypes contribute to such diseases. Ultimately, nutrigenomics will allow effective dietary-intervention strategies to recover normal homeostasis and to prevent diet-related diseases.

Antihyperglycemic and blood pressure-reducing effects of stevioside in the diabetic Goto-Kakizaki rat

Stevioside, a glycoside present in the leaves of the plant, Stevia rebaudiana Bertoni (SrB), has acute insulinotropic effects in vitro. Its potential antihyperglycemic and blood pressure-lowering effects were examined in a long-term study in the type 2 diabetic Goto-Kakizaki (GK) rat. Rats were fed 0.025 g x kg(-1) x d(-1) of stevioside (purity > 99.6%) for 6 weeks. An intra-arterial catheter was inserted into the rats after 5 weeks, and conscious rats were subjected to arterial glucose tolerance test (2.0 g x kg(-1)) during week 6. Stevioside had an antihyperglycemic effect (incremental area under the glucose response curve [IAUC]): 985 +/- 20 (stevioside) versus 1,575 +/- 21 (control) mmol/L x 180 minutes, (P <.05), it enhanced the first-phase insulin response (IAUC: 343 +/- 33 [stevioside] v 136 +/- 24 [control] microU/mL insulin x 30 minutes, P <.05) and concomitantly suppressed the glucagon levels (total AUC: 2,026 +/- 234 [stevioside] v 3,535 +/- 282 [control] pg/mL x 180 minutes, P <.05). In addition, stevioside caused a pronounced suppression of both the systolic (135 +/- 2 v 153 +/- 5 mm Hg; P <.001) and the diastolic blood pressure (74 +/- 1 v 83 +/- 1 mm Hg; P <.001). Bolus injections of stevioside (0.025 g x kg(-1)) did not induce hypoglycemia. Stevioside augmented the insulin content in the beta-cell line, INS-1. Stevioside may increase the insulin secretion, in part, by induction of genes involved in glycolysis. It may also improve the nutrient-sensing mechanisms, increase cytosolic long-chain fatty acyl-coenzyme A (CoA), and downregulate phosphodiesterase 1 (PDE1) estimated by the microarray gene chip technology. In conclusion, stevioside enjoys a dual positive effect by acting as an antihyperglycemic and a blood pressure-lowering substance; effects that may have therapeutic potential in the treatment of type 2 diabetes and the metabolic syndrome.

Reproducibility of targeted gene expression measurements in human islets of Langerhans

The expression of 47 genes involved in the biosynthesis and secretion of insulin, apoptosis, and cellular stress was evaluated in isolated human islets using cDNA probes arrayed on nitrocellulose membranes. Isolated human islets were cultured for four days, or one month, with glucose present at a concentration of either 5.5 or 16.7 mmol/L. Extracted islet total RNA was used to generate [32P]dATP-labelled complex cDNA targets and hybridised with immobilised cDNA arrays. The positive expression of 45 mRNA transcripts in isolated human islets was documented. The coefficient of variance for relative levels of expression of transcripts was <25% for 9, 25-50% for 22, and 50-100% for 10, indicating good reproducibility between islet preparations from five different human pancreas donors. This study demonstrates the utility of nitrocellulose-based cDNA arrays for a focused reproducible analysis of gene expression changes in human islets of Langerhans.