Metabolic signatures suggest o-phosphocholine to UDP-N-acetylglucosamine ratio as a potential biomarker for high-glucose and/or palmitate exposure in pancreatic β-cells

INTRODUCTION

Chronic exposure to high-glucose and free fatty acids (FFA) alone/or in combination; and the resulting gluco-, lipo- and glucolipo-toxic conditions, respectively, have been known to induce dysfunction and apoptosis of β-cells in Diabetes. The molecular mechanisms and the development of biomarkers that can be used to predict similarities and differences behind these conditions would help in easier and earlier diagnosis of Diabetes.

OBJECTIVES

This study aims to use metabolomics to gain insight into the mechanisms by which β-cells respond to excess-nutrient stress and identify associated biomarkers.

METHODS

INS-1E cells were cultured in high-glucose, palmitate alone/or in combination for 24 h to mimic gluco-, lipo- and glucolipo-toxic conditions, respectively. Biochemical and cellular experiments were performed to confirm the establishment of these conditions. To gain molecular insights, abundant metabolites were identified and quantified using ¹H-NMR.

RESULTS

No loss of cellular viability was observed in high-glucose while exposure to FFA alone/in combination with high-glucose was associated with increased ROS levels, membrane damage, lipid accumulation, and DNA double-strand breaks. Forty-nine abundant metabolites were identified and quantified using ¹H-NMR. Chemometric pair-wise analysis in glucotoxic and lipotoxic conditions, when compared with glucolipotoxic conditions, revealed partial overlap in the dysregulated metabolites; however, the dysregulation was more significant under glucolipotoxic conditions.

CONCLUSION

The current study compared gluco-, lipo- and glucolipotoxic conditions in parallel and elucidated differences in metabolic pathways that play major roles in Diabetes. o-phosphocholine and UDP-N-acetylglucosamine were identified as common dysregulated metabolites and their ratio was proposed as a potential biomarker for these conditions.

Chronic high glucose induced INS-1β cell mitochondrial dysfunction: a comparative mitochondrial proteome with SILAC

As glucose-stimulated insulin secretion of pancreatic β cell is triggered and promoted by the metabolic messengers derived from mitochondria, mitochondria take a central stage in the normal function of β cells. β cells in diabetics were chronically exposed to hyperglycemia stimulation, which have been reported to exert deleterious effects on β-cell mitochondria. However, the mechanism of the toxic effects of hyperglycemia on β-cell mitochondria was not clear. In this study, we characterized the biological functional changes of rat INS-1β cells and their mitochondria with chronic exposure to hyperglycemia and created a research model of chronic hyperglycemia-induced dysfunctional β cells with damaged mitochondria. Then, SILAC-based quantitative proteomic approach was used to compare the mitochondrial protein expression from high glucose treated INS-1β cells and control cells. The expression of some mitochondrial proteins was found with significant changes. Functional classification revealed most of these proteins were related with oxidative phosphorylation, mitochondrial protein biosynthesis, substances metabolism, transport, and cell death. These results presented some useful information about the effect of glucotoxicity on the β-cell mitochondria.

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.

Cellular responses of novel human pancreatic β-cell line, 1.1B4 to hyperglycemia

The novel human-derived pancreatic β-cell line, 1.1B4 exhibits insulin secretion and β-cell enriched gene expression. Recent investigations of the cellular responses of this novel cell line to lipotoxicity and cytokine toxicity revealed similarities to primary human β cells. The current study has investigated the responses of 1.1B4 cells to chronic 48 and 72 h exposure to hyperglycemia to probe mechanisms of human β-cell dysfunction and cell death. Exposure to 25 mM glucose significantly reduced insulin content (p<0.05) and glucokinase activity (p<0.01) after 72 h. Basal insulin release was unaffected but acute secretory response to 16.7 mM glucose was impaired (p<0.05). Insulin release stimulated by alanine, GLP-1, KCl, elevated Ca (2+) and forskolin was also markedly reduced after exposure to hyperglycemia (p<0.001). In addition, PDX1 protein expression was reduced by 58% by high glucose (p<0.05). Effects of hyperglycemia on secretory function were accompanied by decreased mRNA expression of INS, GCK, PCSK1, PCSK2, PPP3CB, GJA1, ABCC8, and KCNJ11. In contrast, exposure to hyperglycemia upregulated the transcription of GPX1, an antioxidant enzyme involved in detoxification of hydrogen peroxide and HSPA4, a molecular chaperone involved in ER stress response. Hyperglycemia-induced DNA damage was demonstrated by increased % tail DNA and olive tail moment, assessed by comet assay. Hyperglycemia-induced apoptosis was evident from increased activity of caspase 3/7 and decreased BCL2 protein. These observations reveal significant changes in cellular responses and gene expression in novel human pancreatic 1.1B4 β cells exposed to hyperglycemia, illustrating the usefulness of this novel human-derived cell line for studying human β-cell biology and diabetes.

Metabolomic analysis of pancreatic beta cells following exposure to high glucose

BACKGROUND

Chronic exposure to hyperglycaemic conditions has been shown to have detrimental effects on beta cell function. The resulting glucotoxicity is a contributing factor to the development of type 2 diabetes. The objective of this study was to combine a metabolomics approach with functional assays to gain insight into the mechanism by which glucotoxicity exerts its effects.

METHODS

The BRIN-BD11 and INS-1E beta cell lines were cultured in 25 mM glucose for 20 h to mimic glucotoxic effects. PDK-2 protein expression, intracellular glutathione levels and the change in mitochondrial membrane potential and intracellular calcium following glucose stimulation were determined. Metabolomic analysis of beta cell metabolite extracts was performed using GC-MS, 1H NMR and 13C NMR.

RESULTS

Conditions to mimic glucotoxicity were established and resulted in no loss of cellular viability in either cell line while causing a decrease in insulin secretion. Metabolomic analysis of beta cells following exposure to high glucose revealed a change in amino acids, an increase in glucose and a decrease in phospho-choline, n-3 and n-6 PUFAs during glucose stimulated insulin secretion relative to cells cultured under control conditions. However, no changes in calcium handling or mitochondrial membrane potential were evident.

CONCLUSIONS

Results indicate that a decrease in TCA cycle metabolism in combination with an alteration in fatty acid composition and phosphocholine levels may play a role in glucotoxicity induced impairment of glucose stimulated insulin secretion.

GENERAL SIGNIFICANCE

Alterations in certain metabolic pathways play a role in glucotoxicity in the pancreatic beta cell.

Modulation of neuronal pentraxin 1 expression in rat pancreatic β-cells submitted to chronic glucotoxic stress

Insulin secretory granules are β-cell vesicles dedicated to insulin processing, storage, and release. The secretion of insulin secretory granule content in response to an acute increase of glucose concentration is a highly regulated process allowing normal glycemic homeostasis. Type 2 diabetes is a metabolic disease characterized by chronic hyperglycemia. The consequent prolonged glucose exposure is known to exert deleterious effects on the function of various organs, notably impairment of insulin secretion by pancreatic β-cells and induction of apoptosis. It has also been described as modifying gene and protein expression in β-cells. Therefore, we hypothesized that a modulation of insulin secretory granule protein expression induced by chronic hyperglycemia may partially explain β-cell dysfunction. To identify the potential early molecular mechanisms underlying β-cell dysfunction during chronic hyperglycemia, we performed SILAC and mass spectrometry experiments to monitor changes in the insulin secretory granule proteome from INS-1E rat insulinoma β-cells cultivated either with 11 or 30 mm of glucose for 24 h. Fourteen proteins were found to be differentially expressed between these two conditions, and several of these proteins were not described before to be present in β-cells. Among them, neuronal pentraxin 1 was only described in neurons so far. Here we investigated its expression and intracellular localization in INS-1E cells. Furthermore, its overexpression in glucotoxic conditions was confirmed at the mRNA and protein levels. According to its role in hypoxia-ischemia-induced apoptosis described in neurons, this suggests that neuronal pentraxin 1 might be a new β-cell mediator in the AKT/GSK3 apoptotic pathway. In conclusion, the modification of specific β-cell pathways such as apoptosis and oxidative stress may partially explain the impairment of insulin secretion and β-cell failure, observed after prolonged exposure to high glucose concentrations.

Nucleolin interacts with US11 protein of herpes simplex virus 1 and is involved in its trafficking

Herpes simplex virus type 1 (HSV-1) infection induces profound nucleolar modifications at the functional and organizational levels, including nucleolar invasion by several viral proteins. One of these proteins is US11, which exhibits several different functions and displays both cytoplasmic localization and clear nucleolar localization very similar to that of the major multifunctional nucleolar protein nucleolin. To determine whether US11 interacts with nucleolin, we purified US11 protein partners by coimmunoprecipitations using a tagged protein, Flag-US11. From extracts of cells expressing Flag-US11 protein, we copurified a protein of about 100 kDa that was further identified as nucleolin. In vitro studies have demonstrated that nucleolin interacts with US11 and that the C-terminal domain of US11, which is required for US11 nucleolar accumulation, is sufficient for interaction with nucleolin. This association was confirmed in HSV-1-infected cells. We found an increase in the nucleolar accumulation of US11 in nucleolin-depleted cells, thereby revealing that nucleolin could play a role in US11 nucleocytoplasmic trafficking through one-way directional transport out of the nucleolus. Since nucleolin is required for HSV-1 nuclear egress, the interaction of US11 with nucleolin may participate in the outcome of infection.

Differentiation of mesenchymal stem cells derived from pancreatic islets and bone marrow into islet-like cell phenotype

BACKGROUND

Regarding regenerative medicine for diabetes, accessible sources of Mesenchymal Stem Cells (MSCs) for induction of insular beta cell differentiation may be as important as mastering the differentiation process itself.

METHODOLOGY/PRINCIPAL FINDINGS

In the present work, stem cells from pancreatic islets (human islet-mesenchymal stem cells, HI-MSCs) and from human bone marrow (bone marrow mesenchymal stem cells, BM-MSCs) were cultured in custom-made serum-free medium, using suitable conditions in order to induce differentiation into Islet-like Cells (ILCs). HI-MSCs and BM-MSCs were positive for the MSC markers CD105, CD73, CD90, CD29. Following this induction, HI-MSC and BM-MSC formed evident islet-like structures in the culture flasks. To investigate functional modifications after induction to ILCs, ultrastructural analysis and immunofluorescence were performed. PDX1 (pancreatic duodenal homeobox gene-1), insulin, C peptide and Glut-2 were detected in HI-ILCs whereas BM-ILCs only expressed Glut-2 and insulin. Insulin was also detected in the culture medium following glucose stimulation, confirming an initial differentiation that resulted in glucose-sensitive endocrine secretion. In order to identify proteins that were modified following differentiation from basal MSC (HI-MSCs and BM-MSCs) to their HI-ILCs and BM-ILCs counterparts, proteomic analysis was performed. Three new proteins (APOA1, ATL2 and SODM) were present in both ILC types, while other detected proteins were verified to be unique to the single individual differentiated cells lines. Hierarchical analysis underscored the limited similarities between HI-MSCs and BM-MSCs after induction of differentiation, and the persistence of relevant differences related to cells of different origin.

CONCLUSIONS/SIGNIFICANCE

Proteomic analysis highlighted differences in the MSCs according to site of origin, reflecting spontaneous differentiation and commitment. A more detailed understanding of protein assets may provide insights required to master the differentiation process of HI-MSCs to functional beta cells based only upon culture conditioning. These findings may open new strategies for the clinical use of BM-MSCs in diabetes.

Advances and challenges in biomarker development for type 1 diabetes prediction and prevention using omic technologies

IMPORTANCE OF THE FIELD: Biomarkers are essential for the identification of high risk children as well as monitoring of prevention outcomes for type 1 diabetes (T1D). AREAS COVERED IN THIS REVIEW: This review discusses progress, opportunities and challenges in biomarker discovery and validation using high throughput genomic, transcriptomic and proteomic technologies. The authors also suggest potential solutions to deal with the current challenges. WHAT THE READER WILL GAIN: Readers will gain an overview of the current status on T1D biomarkers, an integrated review of three omic technologies, their applications and limitations for biomarker discovery and validation, and a critical discussion of the major issues encountered in biomarker development. TAKE HOME MESSAGE: Better biomarkers are still urgently needed for T1D prediction and prevention. The high throughput omic technologies offer great opportunities but also face significant challenges that have to be solved before their potential for biomarker development is fully realized.