Regulation of PDK mRNA by high fatty acid and glucose in pancreatic islets

Arginine methylation of PPP1CA by CARM1 regulates glucose metabolism and affects osteogenic differentiation and osteoclastic differentiation

BACKGROUND

The imbalance between osteoblasts and osteoclasts may lead to osteoporosis. Osteoblasts and osteoclasts have different energy requirements, with aerobic glycolysis being the prominent metabolic feature of osteoblasts, while osteoclast differentiation and fusion are driven by oxidative phosphorylation.

METHODS

By polymerase chain reaction as well as Western blotting, we assayed coactivator-associated arginine methyltransferase 1 (CARM1) expression in bone tissue, the mouse precranial osteoblast cell line MC3T3-E1 and the mouse monocyte macrophage leukaemia cell line RAW264.7, and expression of related genes during osteogenic differentiation and osteoclast differentiation. Using gene overexpression (lentivirus) and loss-of-function approach (CRISPR/Cas9-mediated knockout) in vitro, we examined whether CARM1 regulates osteogenic differentiation and osteoblast differentiation by metabolic regulation. Transcriptomic assays and metabolomic assays were used to find the mechanism of action of CARM1. Furthermore, in vitro methylation assays were applied to clarify the arginine methylation site of PPP1CA by CARM1.

RESULTS

We discovered that CARM1 reprogrammed glucose metabolism in osteoblasts and osteoclasts from oxidative phosphorylation to aerobic glycolysis, thereby promoting osteogenic differentiation and inhibiting osteoclastic differentiation. In vivo experiments revealed that CARM1 significantly decreased bone loss in osteoporosis model mice. Mechanistically, CARM1 methylated R23 of PPP1CA, affected the dephosphorylation of AKT-T450 and AMPK-T172, and increased the activities of phosphofructokinase-1 and pructose-2,6-biphosphatase3, causing an up-regulation of glycolytic flux. At the same time, as a transcriptional coactivator, CARM1 regulated the expression of pyruvate dehydrogenase kinase 3, which resulted in the inhibition of pyruvate dehydrogenase activity and inhibition of the tricarboxylic acid cycle, leading to a subsequent decrease in the flux of oxidative phosphorylation.

CONCLUSIONS

These findings reveal for the first time the mechanism by which CARM1 affects both osteogenesis and osteoclast differentiation through metabolic regulation, which may represent a new feasible treatment strategy for osteoporosis.

Inhibitory Effects of Ginsenoside Compound K on Lipopolysaccharide-Stimulated Inflammatory Responses in Macrophages by Regulating Sirtuin 1 and Histone Deacetylase 4

Inflammation, an innate immune response mediated by macrophages, has been a hallmark leading to the pathophysiology of diseases. In this study, we examined the inhibitory effects of ginsenoside compound K (CK) on lipopolysaccharide (LPS)-induced inflammation and metabolic alteration in RAW 264.7 macrophages by regulating sirtuin 1 (SIRT1) and histone deacetylase 4 (HDAC4). LPS suppressed SIRT1 while promoting HDAC4 expression, accompanied by increases in cellular reactive oxygen species accumulation and pro-inflammatory gene expression; however, the addition of CK elicited the opposite effects. CK ameliorated the LPS-induced increase in glycolytic genes and abrogated the LPS-altered genes engaged in the NAD+ salvage pathway. LPS decreased basal, maximal, and non-mitochondrial respiration, reducing ATP production and proton leak in macrophages, which were abolished by CK. SIRT1 inhibition augmented Hdac4 expression along with increased LPS-induced inflammatory and glycolytic gene expression, while decreasing genes that regulate mitochondrial biogenesis; however, its activation resulted in the opposite effects. Inhibition of HDAC4 enhanced Sirt1 expression and attenuated the LPS-induced inflammatory gene expression. In conclusion, CK exerted anti-inflammatory and antioxidant properties with the potential to counteract the alterations of energy metabolism, including glycolysis and mitochondrial respiration, through activating SIRT1 and repressing HDAC4 in LPS-stimulated macrophages.

Bioactive Compounds as Inhibitors of Inflammation, Oxidative Stress and Metabolic Dysfunctions via Regulation of Cellular Redox Balance and Histone Acetylation State

Bioactive compounds (BCs) are known to exhibit antioxidant, anti-inflammatory, and anti-cancer properties by regulating the cellular redox balance and histone acetylation state. BCs can control chronic oxidative states caused by dietary stress, i.e., alcohol, high-fat, or high-glycemic diet, and adjust the redox balance to recover physiological conditions. Unique functions of BCs to scavenge reactive oxygen species (ROS) can resolve the redox imbalance due to the excessive generation of ROS. The ability of BCs to regulate the histone acetylation state contributes to the activation of transcription factors involved in immunity and metabolism against dietary stress. The protective properties of BCs are mainly ascribed to the roles of sirtuin 1 (SIRT1) and nuclear factor erythroid 2-related factor 2 (NRF2). As a histone deacetylase (HDAC), SIRT1 modulates the cellular redox balance and histone acetylation state by mediating ROS generation, regulating nicotinamide adenine dinucleotide (NAD+)/NADH ratio, and activating NRF2 in metabolic progression. In this study, the unique functions of BCs against diet-induced inflammation, oxidative stress, and metabolic dysfunction have been considered by focusing on the cellular redox balance and histone acetylation state. This work may provide evidence for the development of effective therapeutic agents from BCs.

Nicotinamide riboside, an NAD+ precursor, attenuates inflammation and oxidative stress by activating sirtuin 1 in alcohol-stimulated macrophages

Macrophages play an essential role in alcohol-induced inflammation and oxidative stress. We investigated the effects of nicotinamide riboside (NR), a natural nicotinamide adenine dinucleotide (NAD+) precursor, on alcohol-induced inflammation and oxidative stress in macrophages. NR significantly decreased ethanol-induced inflammatory gene expression, with a concomitant decrease in nuclear translocation of nuclear factor κB p65 in RAW 264.7 macrophages and mouse bone marrow-derived macrophages (BMDMs). In macrophages incubated with ethanol or acetaldehyde, NR abolished the accumulation of cellular reactive oxygen species. Ethanol decreased sirtuin 1 (SIRT1) expression and activity, and cellular NAD+ level while inducing pro-inflammatory gene expression. However, NR markedly attenuated the changes. SIRT1 inhibition augmented ethanol-induced inflammatory gene expression, but its activation elicited opposing effects. Also, ethanol did not alter glycolysis but increased glycolytic capacity, glycolytic reserve, and non-glycolytic acidification, with concomitant increases in hypoxia-induced factor 1α expression and activity, phosphorylation of pyruvate dehydrogenase, and extracellular lactate levels. Interestingly, ethanol increased mitochondrial respiration and ATP production but decreased maximal respiration and spare respiration capacity. The latter was linked to decreases in mitochondrial copy numbers. NR abolished the ethanol-induced metabolic changes in the glycolytic and oxidative phosphorylation pathways in RAW 264.7 macrophages. In conclusion, NR exerts anti-inflammatory and antioxidant properties by abrogating the inhibitory effects of ethanol on the SIRT1 pathway by increasing Sirt1 expression and its activator, NAD+. Also, SIRT1 activation and normalization of ethanol-induced changes in NAD+/NADH ratios by NR are likely crucial to counteract the changes in energy phenotypes of macrophages exposed to ethanol.

Integration of single-cell datasets reveals novel transcriptomic signatures of β-cells in human type 2 diabetes

Pancreatic islet β-cell failure is key to the onset and progression of type 2 diabetes (T2D). The advent of single-cell RNA sequencing (scRNA-seq) has opened the possibility to determine transcriptional signatures specifically relevant for T2D at the β-cell level. Yet, applications of this technique have been underwhelming, as three independent studies failed to show shared differentially expressed genes in T2D β-cells. We performed an integrative analysis of the available datasets from these studies to overcome confounding sources of variability and better highlight common T2D β-cell transcriptomic signatures. After removing low-quality transcriptomes, we retained 3046 single cells expressing 27 931 genes. Cells were integrated to attenuate dataset-specific biases, and clustered into cell type groups. In T2D β-cells (n = 801), we found 210 upregulated and 16 downregulated genes, identifying key pathways for T2D pathogenesis, including defective insulin secretion, SREBP signaling and oxidative stress. We also compared these results with previous data of human T2D β-cells from laser capture microdissection and diabetic rat islets, revealing shared β-cell genes. Overall, the present study encourages the pursuit of single β-cell RNA-seq analysis, preventing presently identified sources of variability, to identify transcriptomic changes associated with human T2D and underscores specific traits of dysfunctional β-cells across different models and techniques.

Changes in expression of genes related to glucose metabolism in liver and skeletal muscle of rats exposed to acute hypoxia

The aim of this study was to determine changes in gene expression associated with glucose metabolism in the liver and soleus muscles of rats exposed to hypoxia to improve work capacity under high altitude conditions. Rats were divided into normobaric normoxia (control) and normobaric hypoxia (hypoxia) groups (n = 7 each), and the hypoxia group was exposed to 10.5% oxygen for 90 min. Glucose metabolism-related gene expression was examined by real-time polymerase chain reaction. In the liver, the expression levels of the glucose utilization-related genes solute carrier family 2 member 1, glucokinase, and liver-type phosphofructokinase and the gluconeogenesis-related gene phosphoenolpyruvate carboxykinase 1 (Pck1) were significantly increased upon hypoxic exposure. In contrast, gene expression in the soleus was unchanged, with the exception of Pck1. The results suggest that under hypoxia, both glucose utilization and gluconeogenesis are accelerated in the liver, and liver glycogen is degraded to maintain blood glucose level.

Reduced somatostatin signalling leads to hypersecretion of glucagon in mice fed a high-fat diet

Objectives

Elevated plasma glucagon is an early symptom of diabetes, occurring in subjects with impaired glucose regulation. Here, we explored alpha-cell function in female mice fed a high-fat diet (HFD).

Methods

Female mice expressing the Ca²⁺ indicator GCaMP3 specifically in alpha-cells were fed a high-fat or control (CTL) diet. We then conducted in vivo phenotyping of these mice, as well as experiments on isolated (ex vivo) islets and in the in situ perfused pancreas.

Results

In HFD-fed mice, fed plasma glucagon levels were increased and glucagon secretion from isolated islets and in the perfused mouse pancreas was also elevated. In mice fed a CTL diet, increasing glucose reduced intracellular Ca²⁺ ([Ca²⁺]_i) oscillation frequency and amplitude. This effect was also observed in HFD mice; however, both the frequency and amplitude of the [Ca²⁺]_i oscillations were higher than those in CTL alpha-cells. Given that alpha-cells are under strong paracrine control from neighbouring somatostatin-secreting delta-cells, we hypothesised that this elevation of alpha-cell output was due to a lack of somatostatin (SST) secretion. Indeed, SST secretion in isolated islets from HFD-fed mice was reduced but exogenous SST also failed to suppress glucagon secretion and [Ca²⁺]_i activity from HFD alpha-cells, in contrast to observations in CTL mice.

Conclusions

These findings suggest that reduced delta-cell function, combined with intrinsic changes in alpha-cells including sensitivity to somatostatin, accounts for the hyperglucagonaemia in mice fed a HFD.

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HFD feeding causes hyperglucagonaemia in vivo.

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Glucagon is inadequately suppressed by glucose in HFD-fed mice.

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Alpha-cell [Ca²⁺]_i oscillations and glucagon output are elevated ex vivo in response to HFD feeding.

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SST secretion from HFD islets is reduced.

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Alpha-cells from HFD-fed mice become ‘resistant’ to SST.

Triphenyl phosphate causes a sexually dimorphic metabolism dysfunction associated with disordered adiponectin receptors in pubertal mice

The potential for triphenyl phosphate (TPhP) caused metabolic dysfunction has been documented. However, the relative mechanism of sexual dimorphic disruption on metabolism induced by TPhP remains unclear. Herein, we observed the insulin-sensitizing hormone (adiponectin) was inhibited in female serum while stimulated in males after oral administration of TPhP. Correspondingly, we found a high index of HOMA-IR in females. The primary receptors of adiponectin (AdipoR1 and AdipoR2) and the downstream: phosphorylation of AKT (pAKT) and PPAR⍺ signaling was attenuated in female liver. The disordered adiponectin/AdipoR signaling reduced hepatic glucose glycolysis and induced gluconeogenesis and finally led to the glucose intolerance in females. Also, the aberrant fatty acid β-oxidation and hepatic triacylglyceride (TG) deposition were found in female liver. Comparably, TPhP upregulated the AdipoR 1/2 and induced the downstream (pAMPK and PPAR⍺ signaling) in males. Thus, the serum glucose and hepatic TG level remained normal. However, modulation on AdipoR1/R2 and the genes related to glucose and lipid disposal in skeletal muscle has no gender-specific effect. Our research firstly revealed TPhP-induced hepatic nutrient metabolism was partially mediated by the adiponectin/AdipoR pathway in sexual-dependent manner during pubertal.

Conditional knockout of pyruvate dehydrogenase in mouse pancreatic β‑cells causes morphological and functional changes

Diabetes mellitus is a metabolic disorder predominantly caused by the dysfunction of pancreatic β-cells. This dysfunction is partly caused by the dysregulation of pyruvate dehydrogenase (PDH), which acts as an important mediator of pyruvate oxidation after glycolysis and fuels the tricarboxylic acid cycle. Previous studies have reported decreased PDH expression in rodent models and humans with type 2 diabetes mellitus (T2DM), suggesting that PDH may play an important role in the development of T2DM. However, the mechanism by which PDH affects insulin secretion and β-cell development is poorly understood. Using immunofluorescence staining, the present study found that the expression of pyruvate dehydrogenase E1-α subunit (PDHA1; encoded by the PDHA1 gene) in the islets of type 2 diabetic mice (db/db mice) was lower than in wild-type mice, which indicated the possible association between PDHA1and diabetes. To further understand this mechanism, an inducible, islet-specific PDHA1 knockout mouse (βKO) model was established. The phenotype was authenticated, and the blood glucose levels and islet function between the βKO and control mice were compared. Though no changes were found in food intake, development status, fasting blood glucose or weight between the groups, the level of insulin secretion at 30 min after glucose injection in the βKO group was significantly lower compared with the control group. Furthermore, the performed of the βKO mice on the intraperitoneal glucose tolerance test was visibly impaired when compared with the control mice. Pancreatic tissues were collected for hematoxylin and eosin staining, immunohistochemical and confocal laser-scanning microscopy analysis. Examination of the islets from the βKO mouse model indicated that abolishing the expression of PDH caused a compensatory islet enlargement and impaired insulin secretion.

Evaluation of the Anti-Diabetic Activity of Some Common Herbs and Spices: Providing New Insights with Inverse Virtual Screening

Culinary herbs and spices are widely used as a traditional medicine in the treatment of diabetes and its complications, and there are several scientific studies in the literature supporting the use of these medicinal plants. However, there is often a lack of knowledge on the bioactive compounds of these herbs and spices and their mechanisms of action. The aim of this study was to use inverse virtual screening to provide insights into the bioactive compounds of common herbs and spices, and their potential molecular mechanisms of action in the treatment of diabetes. In this study, a library of over 2300 compounds derived from 30 common herbs and spices were screened in silico with the DIA-DB web server against 18 known diabetes drug targets. Over 900 compounds from the herbs and spices library were observed to have potential anti-diabetic activity and liquorice, hops, fennel, rosemary, and fenugreek were observed to be particularly enriched with potential anti-diabetic compounds. A large percentage of the compounds were observed to be potential polypharmacological agents regulating three or more anti-diabetic drug targets and included compounds such as achillin B from yarrow, asparasaponin I from fenugreek, bisdemethoxycurcumin from turmeric, carlinoside from lemongrass, cinnamtannin B1 from cinnamon, crocin from saffron and glabridin from liquorice. The major targets identified for the herbs and spices compounds were dipeptidyl peptidase-4 (DPP4), intestinal maltase-glucoamylase (MGAM), liver receptor homolog-1 (NR5A2), pancreatic alpha-amylase (AM2A), peroxisome proliferator-activated receptor alpha (PPARA), protein tyrosine phosphatase non-receptor type 9 (PTPN9), and retinol binding protein-4 (RBP4) with over 250 compounds observed to be potential inhibitors of these particular protein targets. Only bay leaves, liquorice and thyme were found to contain compounds that could potentially regulate all 18 protein targets followed by black pepper, cumin, dill, hops and marjoram with 17 protein targets. In most cases more than one compound within a given plant could potentially regulate a particular protein target. It was observed that through this multi-compound-multi target regulation of these specific protein targets that the major anti-diabetic effects of reduced hyperglycemia and hyperlipidemia of the herbs and spices could be explained. The results of this study, taken together with the known scientific literature, indicated that the anti-diabetic potential of common culinary herbs and spices was the result of the collective action of more than one bioactive compound regulating and restoring several dysregulated and interconnected diabetic biological processes.

Effects of intermittent hypoxia training on leukocyte pyruvate dehydrogenase kinase 1 (PDK-1) mRNA expression and blood insulin level in prediabetes patients

PURPOSE

Intermittent hypoxia training/treatment (IHT) is an emerging therapeutic approach to alleviate chronic diseases, such as diabetes. The present study investigated the effects of IHT on blood leucocyte pyruvate dehydrogenase kinase 1 (PDK-1) mRNA expression and its relationship with the changes in blood insulin level.

METHODS

Seven adult healthy volunteers and 11 prediabetic patients participated in this study. A 3-week course of IHT consisted of a 40-min session of 4 cycles of 5-min 12% O₂ and 5-min room air breathing per day, 3 sessions per week for 3 weeks (i.e., total 9 sessions of IHT). Plasma insulin levels and leukocyte PDK-1 mRNA expression were determined at various time points either under fasting condition or following oral glucose tolerance test (OGTT). Correlation between the IHT-induced changes in PDK-1 mRNA and insulin or glucose levels in the same serological samples was analyzed.

RESULTS

At pre-IHT baseline, PDK-1 mRNA expression was two times higher in prediabetes than control subjects. IHT resulted in significant augmentation in PDK-1 mRNA expression (> twofold) in prediabetes at the end of 3-week IHT and remained elevated 1 month after IHT, which was correlated with a significantly reduced insulin release and lower blood glucose after glucose loading with OGTT.

CONCLUSION

IHT can trigger beneficial effects in normalizing blood insulin levels in prediabetic patients under oral glucose load, which were closely correlated with an enhanced mRNA expression of PDK-1 in leukocytes. Further clinical trials are warranted to validate the utility of IHT as a non-invasive complementary therapy against diabetes-associated pathologies.

An integrated approach with the zebrafish model for biomonitoring of municipal wastewater effluent and receiving waters

Comprehensive monitoring of water pollution is challenging. With the increasing amount and types of anthropogenic compounds being released into water, there are rising concerns of undetected toxicity. This is especially true for municipal wastewater effluents that are discharged to surface waters. This study was designed to integrate zebrafish toxicogenomics, targeted gene expression, and morphological analyses, for toxicity evaluation of effluent discharged from two previously characterized wastewater treatment plants (WWTPs) in Pima County, Arizona, and their receiving surface water. Zebrafish embryos were exposed to organic extracts from the WWTP1 effluent that were reconstituted to represent 1× and 0.5× of the original concentration. Microarray analyses identified deregulated gene probes that mapped to 1666, 779, and 631 unique human homologs in the 1×, 0.5×, and the intersection of both groups, respectively. These were associated with 18 cellular and molecular functions ranging from cell cycle to metabolism and are involved in the development and function of 10 organ systems including nervous, cardiovascular, haematological, reproductive, and hepatic systems. Superpathway of cholesterol biosynthesis, retinoic acid receptor activation, glucocorticoid receptor and prolactin signaling were among the top 11 perturbed canonical pathways. Real-time quantitative PCR validated the expression changes of 12 selected genes. These genes were then tested on zebrafish embryos exposed to the reconstituted extract of water sampled downstream of WWTP1 and another nearby WWTP2. The expression of several targeted genes were significantly affected by the WWTP effluents and some of the downstream receiving waters. Morphological analyses using four transgenic zebrafish lines revealed potential toxicity associated with nervous, hepatic, endothelial-vascular and myeloid systems. This study demonstrated how information can be obtained using adverse outcome pathway framework to derive biological effect-based monitoring tools. This integrated approach using zebrafish can supplement analytical chemistry to provide more comprehensive monitoring of discharged effluents and their receiving waters.

Inhibiting Oxidative Phosphorylation In Vivo Restrains Th17 Effector Responses and Ameliorates Murine Colitis

Integration of signaling and metabolic pathways enables and sustains lymphocyte function. Whereas metabolic changes occurring during T cell activation are well characterized, the metabolic demands of differentiated T lymphocytes are largely unexplored. In this study, we defined the bioenergetics of Th17 effector cells generated in vivo. These cells depend on oxidative phosphorylation (OXPHOS) for energy and cytokine production. Mechanistically, the essential role of OXPHOS in Th17 cells results from their limited capacity to increase glycolysis in response to metabolic stresses. This metabolic program is observed in mouse and human Th17 cells, including those isolated from Crohn disease patients, and it is linked to disease, as inhibiting OXPHOS reduces the severity of murine colitis and psoriasis. These studies highlight the importance of analyzing metabolism in effector lymphocytes within in vivo inflammatory contexts and suggest a therapeutic role for manipulating OXPHOS in Th17-driven diseases.

A hypothetical model to solve the controversy over the involvement of UCP2 in palmitate-induced β-cell dysfunction

The aim of this article is to solve an existing controversy over the involvement of uncoupling protein-2 in the impairment of glucose-stimulated insulin secretion induced by chronic exposure of β-cells to palmitate. We analyzed and compared the results of studies that support and that deny the involvement of uncoupling protein-2 in this impairment. We observed that this impairment could occur in multiple stages. We provide a model in which palmitate-induced impairment of glucose-stimulated insulin secretion is proposed to occur in two stages, early stage and late stage, depending on the integrity of electron supply (glycolysis and Krebs cycle) and transport system through electron transport chain after palmitate treatment. Prolonged exposure of β-cells to palmitate can impair this system. Early-stage impairment occurs due to uncoupling by uncoupling protein-2 when this system is still intact. When this system becomes impaired, late-stage impairment occurs mainly due to reduced glucose-stimulated adenosine triphosphate production independent of uncoupling by uncoupling protein-2. The change in glucose-stimulated oxygen uptake after palmitate treatment reflects the integrity of this system and can be used to differentiate between the two stages. Some β-cells lines and islets appear to be more resistant to palmitate-induced impairment of electron supply and transport system than others, and therefore early stage is prominent in the more resistant cell lines and less prominent or absent in the less resistant cell lines. This may help to resolve the pathogenesis of diabetes and to monitor the progression of palmitate-induced β-cell dysfunction.

High glucose repatterns human podocyte energy metabolism during differentiation and diabetic nephropathy

Podocytes play a key role in diabetic nephropathy pathogenesis, but alteration of their metabolism remains unknown in human kidney. By using a conditionally differentiating human podocyte cell line, we addressed the functional and molecular changes in podocyte energetics during in vitro development or under high glucose conditions. In 5 mM glucose medium, we observed a stepwise activation of oxidative metabolism during cell differentiation that was characterized by peroxisome proliferator-activated receptor-γ coactivator 1α (PGC-1α)–dependent stimulation of mitochondrial biogenesis and function, with concomitant reduction of the glycolytic enzyme content. Conversely, when podocytes were cultured in high glucose (20 mM), stepwise oxidative phosphorylation biogenesis was aborted, and a glycolytic switch occurred, with consecutive lactic acidosis. Expression of the master regulators of oxidative metabolism transcription factor A mitochondrial, PGC-1α, AMPK, and serine–threonine liver kinase B1 was altered by high glucose, as well as their downstream signaling networks. Focused transcriptomics revealed that myocyte-specific enhancer factor 2C (MEF2C) and myogenic factor 5 (MYF5) expression was inhibited by high glucose levels, and endoribonuclease-prepared small interfering RNA–mediated combined inhibition of those transcription factors phenocopied the glycolytic shift that was observed in high glucose conditions. Accordingly, a reduced expression of MEF2C, MYF5, and PGC-1α was found in kidney tissue sections that were obtained from patients with diabetic nephropathy. These findings obtained in human samples demonstrate that MEF2C-MYF5–dependent bioenergetic dedifferentiation occurs in podocytes that are confronted with a high-glucose milieu.—Imasawa, T., Obre, E., Bellance, N., Lavie, J., Imasawa, T., Rigothier, C., Delmas, Y., Combe, C., Lacombe, D., Benard, G., Claverol, S., Bonneu, M., Rossignol, R. High glucose repatterns human podocyte energy metabolism during differentiation and diabetic nephropathy.

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.

Oral exposure of pubertal male mice to endocrine-disrupting chemicals alters fat metabolism in adult livers

The potential for the exposure of humans and wildlife to environmental endocrine-disrupting chemicals (EDCs) has been increasing. Risk assessment for such EDCs is primarily based on detecting the main endpoints related to the endocrine and reproductive systems, while the effects on glucose and fat metabolism have only received limited attention. In this study, pubertal male C57BL/6J mice were orally administered 10 mg/kg body weight cypermethrin (CYP), 100 mg/kg body weight atrazine (ATZ), and 0.1 mg/kg body weight 17α-ethynyestradiol (EE2) for 4 weeks and then switched to a high-energy diet (HD) for 8 weeks. The body weight gain in the EDC-treated groups was lower than that in the control group during exposure and then tended to show values similar to the HD group. The epididymal fat weight, cell size and serum triacylglycerol (TG) and total cholesterol (TCH) levels in the EDC-HD groups were lower than those in the HD group. The transcription of genes related to glycolytic and gluconeogenic processes in the liver was affected by EDC exposure. Furthermore, the expression levels of transcriptional factors including PPARα, PPARγ, and SREBP1C and their target genes related to fatty acid synthesis and oxidation in the liver were also influenced by early life EDC administration. The results showed that early-life-stage exposure to high doses of various environmental EDCs affected the homeostasis of glucose and fatty acid metabolism in the livers of adult male mice.

Metabolomics – the complementary field in systems biology: a review on obesity and type 2 diabetes

This paper highlights the metabolomic roles in systems biology towards the elucidation of metabolic mechanisms in obesity and type 2 diabetes.

Hyperoxia reverses glucotoxicity-induced inhibition of insulin secretion in rat INS-1 β cells

Chronic hyperglycemia has deleterious effects on pancreatic β-cell function, a process known as glucotoxicity. This study examined whether chronic high glucose (CHG) induces cellular hypoxia in rat INS-1 β cells, and whether hyperoxia (35% O2) can reverse glucotoxicity-induced inhibition of insulin secretion. CHG (33.3 mm, 96 h) reduced insulin secretion, and down-regulated insulin and pancreatic duodenal homeobox factor 1 gene expression. CHG also increased intracellular pimonidazole-protein adducts, a marker for hypoxia. CHG also enhanced hypoxia-inducible factor 1α (HIF-1α) protein expression and its DNA-binding activity, which was accompanied by a decrease in mRNA expression of glucose transporter 2 (GLUT2), glucokinase and uncoupling protein-2 and an increase in mRNA expression of GLUT1 and pyruvate dehydrogenase kinase 1. Hyperoxia restored the decrease in insulin secretion and the gene expression except for GLUT2, and suppressed intracellular hypoxia and HIF-1α activation. These results suggest that glucotoxicity may cause β-cell hypoxia. Hyperoxia might prevent glucotoxicity-induced β-cell dysfunction and improve insulin secretion.

Toxicity generated through inhibition of pyruvate carboxylase and carnitine palmitoyl transferase-1 is similar to high glucose/palmitate-induced glucolipotoxicity in INS-1 beta cells

This work was initiated to determine whether toxicity generated through inhibition of mitochondrial fuel metabolism is similar to high glucose/palmitate (HG/PA)-induced glucolipotoxicity. Influx of glucose and free fatty acids into the tricarboxylic acid (TCA) cycle was inhibited by treatment with the pyruvate carboxylase (PC) inhibitor phenylacetic acid (PAA) and carnitine palmitoyl transferase-1 (CPT-1) inhibitor etomoxir (Eto), or knockdown of PC and CPT-1. Treatment of PAA/Eto or knockdown of PC/CPT-1 induced apoptotic death in INS-1 beta cells. Similar to HG/PA treatment, PAA/Eto increased endoplasmic reticulum stress responses but decreased the Akt signal. JNK inhibitor or chemical chaperone was protective against both PAA/Eto- and HG/PA-induced cell death. All attempts to reduce [Ca²⁺](i), stimulate lipid metabolism, and increase the TCA cycle intermediate pool protected PAA/Eto-induced death as well as HG/PA-induced death. These data suggest that signals induced from impaired mitochondrial fuel metabolism play a critical role in HG/PA-induced glucolipotoxicity.

The pivotal role of pyruvate dehydrogenase kinases in metabolic flexibility

Metabolic flexibility is the capacity of a system to adjust fuel (primarily glucose and fatty acids) oxidation based on nutrient availability. The ability to alter substrate oxidation in response to nutritional state depends on the genetically influenced balance between oxidation and storage capacities. Competition between fatty acids and glucose for oxidation occurs at the level of the pyruvate dehydrogenase complex (PDC). The PDC is normally active in most tissues in the fed state, and suppressing PDC activity by pyruvate dehydrogenase (PDH) kinase (PDK) is crucial to maintain energy homeostasis under some extreme nutritional conditions in mammals. Conversely, inappropriate suppression of PDC activity might promote the development of metabolic diseases. This review summarizes PDKs’ pivotal role in control of metabolic flexibility under various nutrient conditions and in different tissues, with emphasis on the best characterized PDK4. Understanding the regulation of PDC and PDKs and their roles in energy homeostasis could be beneficial to alleviate metabolic inflexibility and to provide possible therapies for metabolic diseases, including type 2 diabetes (T2D).

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.

PDHK-2 deficiency is associated with attenuation of lipase-mediated fat consumption for the increased survival of Caenorhabditis elegans dauers

In Caenorhabditis elegans, slow fat consumption has been suggested to contribute to the extension of the survival rate during nutritionally adverse conditions. Here, we investigated the potential role of pyruvate dehydrogenase kinase (PDHK)-2, the C. elegans homolog of mammalian PDK, effects on fat metabolism under nutritional conditions. PDHK-2 was expressed at low levels under well-fed conditions but was highly induced during long-term starvation and in the dauer state. This increase in pdhk-2 expression was regulated by both DAF-16 and NHR-49. Dauer-specific induction of PDHK-2 was abolished upon entry into the post-dauer stage. Interestingly, in the long-term dauer state, stored fat levels were higher in daf-2(e1370);pdhk-2 double mutants than in daf-2(e1370), suggesting a positive relationship between PDHK-2 activity and fat consumption. PDHK-2 deficiency has been shown to lead to greater preservation of residual fats, which would be predicted to contribute to survival during the dauer state. A test of this prediction showed that the survival rates of daf-2(e1370);pdhk-2(tm3075) and daf-2(e1370);pdhk-2(tm3086) double mutants were higher than that of daf-2(e1370), suggesting that loss of either the ATP-binding domain (tm3075) or branched chain keto-acid dehydrogenase kinase domain (tm3086) of PDHK-2 leads to reduced fat consumption and thus favors increased dauer survival. This attenuated fat consumption in the long-term dauer state of C. elegans daf-2 (e1370);pdhk-2 mutants was associated with concomitant down-regulation of the lipases ATGL (adipose triglyceride lipase), HSL (hormone-sensitive lipase), and C07E3.9 (phospholipase). In contrast, PDHK-2 overexpression in wild-type starved worms induced lipase expression and promoted abnormal dauer formation. Thus, we propose that PDHK-2 serves as a molecular bridge, connecting fat metabolism and survival under nutritionally adverse conditions in C. elegans.

Pyruvate dehydrogenase E1α phosphorylation is induced by glucose but does not control metabolism-secretion coupling in INS-1E clonal β-cells

Glucose-induced insulin secretion from pancreatic β-cells depends on mitochondrial activation. In the organelle, glucose-derived pyruvate is metabolised along the oxidative and anaplerotic pathway to generate downstream signals leading to insulin granule exocytosis. Entry into the oxidative pathway is catalysed by pyruvate dehydrogenase (PDH) and controlled in part by phosphorylation of the PDH E1α subunit blocking enzyme activity. We find that glucose but not other nutrient secretagogues induce PDH E1α phosphorylation in INS-1E cells and rat islets. INS-1E cells and primary β-cells express pyruvate dehydrogenase kinase (PDK) 1, 2 and 3, which mediate the observed phosphorylation. In INS-1E cells, suppression of the two main isoforms, PDK1 and PDK3, almost completely prevented PDH E1α phosphorylation. Under basal glucose conditions, phosphorylation was barely detectable and therefore the enzyme almost fully active (90% of maximal). During glucose stimulation, PDH is only partially inhibited (to 78% of maximal). Preventing PDH phosphorylation in situ after suppression of PDK1, 2 and 3 neither enhanced pyruvate oxidation nor insulin secretion. In conclusion, although glucose stimulates E1α phosphorylation and therefore inhibits PDH activity, this control mechanism by itself does not alter metabolism-secretion coupling in INS-1E clonal β-cells.

Mechanisms underlying skeletal muscle insulin resistance induced by fatty acids: importance of the mitochondrial function

Insulin resistance condition is associated to the development of several syndromes, such as obesity, type 2 diabetes mellitus and metabolic syndrome. Although the factors linking insulin resistance to these syndromes are not precisely defined yet, evidence suggests that the elevated plasma free fatty acid (FFA) level plays an important role in the development of skeletal muscle insulin resistance. Accordantly, in vivo and in vitro exposure of skeletal muscle and myocytes to physiological concentrations of saturated fatty acids is associated with insulin resistance condition. Several mechanisms have been postulated to account for fatty acids-induced muscle insulin resistance, including Randle cycle, oxidative stress, inflammation and mitochondrial dysfunction. Here we reviewed experimental evidence supporting the involvement of each of these propositions in the development of skeletal muscle insulin resistance induced by saturated fatty acids and propose an integrative model placing mitochondrial dysfunction as an important and common factor to the other mechanisms.

The pyruvate carboxylase-pyruvate dehydrogenase axis in islet pyruvate metabolism: Going round in circles?

Pyruvate is the major product of glycolysis in pancreatic β-cells, and its ultimate metabolic fate depends on the relative activities of two enzymes. The first, pyruvate carboxylase (PC) replenishes oxaloacetate withdrawn from the tricarboxylic acid (TCA) cycle via the carboxylation of pyruvate to form oxaloacetate. Flux via PC is also involved in the formation of NADPH, one of several important coupling factors for insulin secretion. In most tissues, PC activity is enhanced by increased acetyl-CoA. The alternative fate of pyruvate is its oxidative decarboxylation to form acetyl-CoA via the pyruvate dehydrogenase complex (PDC). The ultimate fate of acetyl-CoA carbon is oxidation to CO2 via the TCA cycle, and so the PDC reaction results of the irreversible loss of glucose-derived carbon. Thus, PDC activity is stringently regulated. The mechanisms controlling PDC activity include end-product inhibition by increased acetyl-CoA, NADH and ATP, and its phosphorylation (inactivation) by a family of pyruvate dehydrogenase kinases (PDHKs 1-4). Here we review new developments in the regulation of the activities and expression of PC, PDC and the PDHKs in the pancreatic islet in relation to islet pyruvate disposition and glucose-stimulated insulin secretion (GSIS).

Amino acid-induced gene expression profiling in clonal β-cell line INS-1E cells

BACKGROUND

There is abundant evidence that glucotoxicity and lipotoxicity contribute to impaired β-cell function in type 2 diabetes. Interestingly, amino acid (AA) derangement is also a characteristic part of the diabetic state. The acute effects of AA on pancreatic β-cell function have been widely explored; however, to our knowledge, the chronic effects of AA, e.g. proline (Pro), homocysteine (Hcy), and leucine (Leu), on pancreatic β-cell function and integrity have not yet been studied. We aimed to investigate global alterations in β-cell gene expression after long-term exposure of clonal INS-1E cells to elevated level of specific AA in vitro.

METHODS

Global gene expression profiling was performed to characterize genes differently modified by Pro, Hcy, and Leu, respectively, in INS-1E cells.

RESULTS

Gene expression profiling revealed significant changes in INS-1E cell mRNAs involved in the control of several aspects of β-cell function, e.g. epigenetic regulation of gene expression, metabolism, innate and adaptive immune responses, cellular signalling, protein synthesis, apoptosis, and cellular stress response. After 72 h, INS-1E cells were differentially regulated (≥1.5- or ≤ -1.5-fold) by Pro (295 transcripts), Hcy (301 transcripts), and Leu (701 transcripts). It appears that Hcy effects changes opposite to those induced by Leu and/or Pro.

CONCLUSIONS

AA appears to participate in and to influence many physiological processes including those involved in cholesterol metabolism, immune responses, and oxidative phosphorylation. Whether such events promote the β-cell dysfunction and the β-cell failure in diabetes remains to be elucidated. Our data strongly indicate that AA elevation may take part in the progressive development of type 2 diabetes.

Supplement of TCA cycle intermediates protects against high glucose/palmitate-induced INS-1 beta cell death

The aim of this study is to investigate the effect of mitochondrial metabolism on high glucose/palmitate (HG/PA)-induced INS-1 beta cell death. Long-term treatment of INS-1 cells with HG/PA impaired energy-producing metabolism accompanying with depletion of TCA cycle intermediates. Whereas an inhibitor of carnitine palmitoyl transferase 1 augmented HG/PA-induced INS-1 cell death, stimulators of fatty acid oxidation protected the cells against the HG/PA-induced death. Furthermore, whereas mitochondrial pyruvate carboxylase inhibitor phenylacetic acid augmented HG/PA-induced INS-1 cell death, supplementation of TCA cycle metabolites including leucine/glutamine, methyl succinate/α-ketoisocaproic acid, dimethyl malate, and valeric acid or treatment with a glutamate dehydrogenase activator, aminobicyclo-heptane-2-carboxylic acid (BCH), significantly protected the cells against the HG/PA-induced death. In particular, the mitochondrial tricarboxylate carrier inhibitor, benzene tricarboxylate (BTA), also showed a strong protective effect on the HG/PA-induced INS-1 cell death. Knockdown of glutamate dehydrogenase or tricarboxylate carrier augmented or reduced the HG/PA-induced INS-1 cell death, respectively. Both BCH and BTA restored HG/PA-induced reduction of energy metabolism as well as depletion of TCA intermediates. These data suggest that depletion of the TCA cycle intermediate pool and impaired energy-producing metabolism may play a role in HG/PA-induced cytotoxicity to beta cells and thus, HG/PA-induced beta cell glucolipotoxicity can be protected by nutritional or pharmacological maneuver enhancing anaplerosis or reducing cataplerosis.

Suppressed glucose metabolism in acinar cells might contribute to the development of exocrine pancreatic insufficiency in streptozotocin-induced diabetic mice

High prevalence of exocrine pancreatic insufficiency has been observed in diabetic patients. However, the underlying mechanisms are not well known. Reduced cytosolic Ca(2+) signals in pancreatic acinar cells may contribute to lower digestive enzyme secretion. It is well known that adenosine triphosphate (ATP) regulates cytosolic Ca(2+) signals in acinar cells; however, little is known as to whether diabetes impairs glucose metabolism that produces ATP in acinar cells. Streptozotocin (STZ)-induced diabetic C57BL/6 mouse model was used. Four weeks after being diabetic, pancreatic acinar cells were isolated; and amylase secretion and contents, glucose utilization and oxidation, the activities of several key enzymes for glucose metabolism, and ATP and nicotinamide adenine dinucleotide phosphate (reduced form) (NADPH) contents were determined. Compared with controls, diabetic mice had lower body weight. Cholecystokinin-8- and acetylcholine-stimulated amylase secretion was significantly impaired, and total amylase activity in acinar cells of STZ-diabetic mice was markedly reduced. Glucose utilization and oxidation were suppressed; measured enzyme activities for glucose metabolism and the ATP and NADPH contents were significantly reduced. These data indicate that glucose metabolism and ATP and NADPH productions are very important for maintaining acinar cell normal function. Reduction of ATP (reduces cytosolic Ca(2+) signals) and NADPH (reduces cell capability for antioxidative stress) productions may contribute to the development of exocrine pancreatic insufficiency in STZ-diabetic mice.

Regulation of islet beta-cell pyruvate metabolism: interactions of prolactin, glucose, and dexamethasone

Prolactin (PRL) induces beta-cell proliferation and glucose-stimulated insulin secretion (GSIS) and counteracts the effects of glucocorticoids on insulin production. The mechanisms by which PRL up-regulates GSIS are unknown. We used rat islets and insulinoma (INS-1) cells to explore the interactions of PRL, glucose, and dexamethasone (DEX) in the regulation of beta-cell pyruvate carboxylase (PC), pyruvate dehydrogenase (PDH), and the pyruvate dehydrogenase kinases (PDKs), which catalyze the phosphorylation and inactivation of PDH. PRL increased GSIS by 37% (P < 0.001) in rat islets. Glucose at supraphysiological concentrations (11 mm) increased PC mRNA in islets; in contrast, PRL suppressed PC mRNA levels in islets and INS-1 cells, whereas DEX was without effect. Neither PRL nor DEX altered PC protein or activity levels. In INS-1 cells, PRL increased PDH activity 1.4- to 2-fold (P < 0.05-0.001) at glucose concentrations ranging from 2.5-11 mm. DEX reduced PDH activity; this effect was reversed by PRL. PDK1, -2, -3, and -4 mRNAs were detected in both islets and insulinoma cells, but the latter expressed trivial amounts of PDK4. PRL reduced PDK2 mRNA and protein levels in rat islets and INS-1 cells and PDK4 mRNA in islets; DEX increased PDK2 mRNA in islets and INS-1 cells; this effect was reversed by PRL. Our findings suggest that PRL induction of GSIS is mediated by increases in beta-cell PDH activity; this is facilitated by suppression of PDKs. PRL counteracts the effects of DEX on PDH and PDK expression, suggesting novel roles for the lactogens in the defense against diabetes.

Pyruvate dehydrogenase kinase 1 controls mitochondrial metabolism and insulin secretion in INS-1 832/13 clonal beta-cells

Tight coupling between cytosolic and mitochondrial metabolism is key for GSIS (glucose-stimulated insulin secretion). In the present study we examined the regulatory contribution of PDH (pyruvate dehydrogenase) kinase 1, a negative regulator of PDH, to metabolic coupling in 832/13 clonal beta-cells. Knockdown of PDH kinase 1 with siRNA (small interfering RNA) reduced its mRNA (>80%) and protein level (>40%) after 72 h. PDH activity, glucose-stimulated cellular oxygen consumption and pyruvate-stimulated mitochondrial oxygen consumption increased 1.7- (P<0.05), 1.6- (P<0.05) and 1.6-fold (P<0.05) respectively. Gas chromatography/MS revealed an altered metabolite profile upon silencing of PDH kinase 1, determined by increased levels of the tricarboxylic acid cycle intermediates malate, fumarate and alpha-ketoglutarate. These metabolic alterations were associated with exaggerated GSIS (5-fold compared with 3.1-fold in control cells; P<0.01). Insulin secretion, provoked by leucine and dimethylsuccinate, which feed into the tricarboxylic acid cycle bypassing PDH, was unaffected. The oxygen consumption and metabolic data strongly suggest that knockdown of PDH kinase 1 in beta-cells permits increased metabolic flux of glucose-derived carbons into the tricarboxylic acid cycle via PDH. Enhanced insulin secretion is probably caused by increased generation of tricarboxylic acid cycle-derived reducing equivalents for mitochondrial electron transport to generate ATP and/or stimulatory metabolic intermediates. On the basis of these findings, we suggest that PDH kinase 1 is an important regulator of PDH in clonal beta-cells and that PDH kinase 1 and PDH are important for efficient metabolic coupling. Maintaining low PDH kinase 1 expression/activity, keeping PDH in a dephosphorylated and active state, may be important for beta-cells to achieve the metabolic flux rates necessary for maximal GSIS.

Reduction of islet pyruvate carboxylase activity might be related to the development of type 2 diabetes mellitus in Agouti-K mice

Pyruvate carboxylase (PC) activity is enhanced in the islets of obese rats, but it is reduced in the islets of type 2 diabetic rats, suggesting the importance of PC in beta-cell adaptation to insulin resistance as well as the possibility that PC reduction might lead to hyperglycemia. However, the causality is currently unknown. We used obese Agouti mice (AyL) as a model to show enhanced beta-cell adaptation, and type 2 diabetic db/db mice as a model to show severe beta-cell failure. After comparison of the two models, a less severe type 2 diabetic Agouti-K (AyK) mouse model was used to show the changes in islet PC activity during the development of type 2 diabetes mellitus (T2DM). AyK mice were separated into two groups: mildly (AyK-M, blood glucose <250 mg/dl) and severely (AyK-S, blood glucose >250 mg/dl) hyperglycemic. Islet PC activity, but not protein level, was increased 1.7-fold in AyK-M mice; in AyK-S mice, islet PC activity and protein level were reduced. All other changes including insulin secretion and islet morphology in AyK-M mice were similar to those observed in AyL mice, but they were worse in AyK-S mice where these parameters closely matched those in db/db mice. In 2-day treated islets, PC activity was inhibited by high glucose but not by palmitate. Our findings suggest that islet PC might play a role in the development of T2DM where reduction of PC activity might be a consequence of mild hyperglycemia and a cause for severe hyperglycemia.

Beta-cell death and proliferation after intermittent hypoxia: role of oxidative stress

Intermittent hypoxia (IH), such as occurs in sleep apnea, induces increased oxidative stress and is associated with altered glucose homeostasis. Because pancreatic beta cells are very sensitive to oxidative stress we tested whether they could be affected by IH. The effects of IH exposure (24 h/day, 5.7 and 21% O(2) alternation) in mice on beta-cell proliferation and beta-cell death were tested using Ki67 staining and TUNEL staining, respectively. To assess the role of oxidative stress in these processes, transgenic mice with beta-cell-specific overexpression of the antioxidant protein MnSOD were exposed to IH. After 4 days of IH exposure, beta-cell proliferation was increased almost fourfold. Coinciding with the increase in proliferation, the subcellular localization of the cell cycle regulator cyclin D2 was increased in the nucleus. In addition, beta-cell death was increased approximately fourfold. MnSOD transgene did not alter the effects of IH on beta-cell proliferation, but completely abrogated the IH effects on cell death. Thus, IH exposure that mimics sleep apnea can lead to increased beta-cell proliferation and cell death. Furthermore, the cell death response seems to be due to oxidative stress.

Fatty acids and beta-cell toxicity

PURPOSE OF REVIEW

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

RECENT FINDINGS

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

SUMMARY

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

Gene expression regulated by pioglitazone and exenatide in normal and diabetic rat islets exposed to lipotoxicity

BACKGROUND

Hyperlipidaemia has been suggested to contribute by pro-apoptotic actions to the loss of beta-cell mass, its secretory defects, and thereby impaired beta-cell function in type 2 diabetes. Treatment of genetically diabetic rats and also type 2 diabetic patients with pioglitazone, a PPAR-gamma agonist, lowers fasting levels of plasma glucose and triglycerides, and has been suggested to protect beta-cells against diabetic lipotoxicity in vitro and in vivo. Another recently launched anti-diabetic drug, exenatide, an incretin mimetic, has been shown to stimulate insulin secretion, growth, and proliferation of pancreatic beta-cells and to protect them against apoptosis. We aimed to investigate global alterations in beta-cell gene expression under lipotoxic conditions and the influence of in vitro treatment with pioglitazone and exenatide.

METHODS

Global gene expression profiling was thus performed to characterize genes differently regulated by palmitate, pioglitazone, and exenatide in isolated islets from non-diabetic Wistar rats and type 2 diabetic Goto-Kakizaki (GK) rats.

RESULTS

Gene expression profiling revealed significant changes in islet mRNAs involved in control of several aspects of beta-cell function, e.g. epigenetic regulation of gene expression, cell differentiation and morphogenesis, also metabolism, response to stimulus, transport, and signal transduction. Pioglitazone and exenatide appear to significantly impact epigenetic processes, e.g. stable alterations in gene expression potential, which arise during development and cell proliferation. Bcl2-like 1 (Bcl2l1), an anti-apoptotic protein, and Bcl2 modifying factor (Bmf), a pro-apoptotic protein, were both down-regulated by pioglitazone and exenatide in the presence of palmitate in diabetic GK islets. In contrast, Bmf was downregulated by pioglitazone in the presence of palmitate in non-diabetic Wistar islets. Exposure of non-diabetic Wistar islets to palmitate led to a reduction in the expression of PPAR beta/delta. This suggests that palmitate may increase the accumulation of triglycerides by reducing PPAR signalling. Moreover, treatment with either pioglitazone or exenatide restored and increased the expression of PPAR beta/delta in non-diabetic Wistar islets.

CONCLUSIONS

Taking into account that these drugs target different components of the epigenetic machinery, our findings suggest that they might participate in restoring normal gene activity in dysfunctional islets and that additive benefits may occur. Whether such events contribute to the beta-cell sparing, proliferative, and anti-apoptotic effects of these drugs in diabetes remains to be elucidated.

Prolonged L-alanine exposure induces changes in metabolism, Ca2+ handling and desensitization of insulin secretion in clonal pancreatic β-cells

Acute insulin-releasing actions of amino acids have been studied in detail, but comparatively little is known about the β-cell effects of long-term exposure to amino acids. The present study examined the effects of prolonged exposure of β-cells to the metabolizable amino acid L-alanine. Basal insulin release or cellular insulin content were not significantly altered by alanine culture, but acute alanine-induced insulin secretion was suppressed by 74% (P<0.001). Acute stimulation of insulin secretion with glucose, KCl or KIC (2-oxoisocaproic acid) following alanine culture was not affected. Acute alanine exposure evoked strong cellular depolarization after control culture, whereas AUC (area under the curve) analysis revealed significant (P<0.01) suppression of this action after culture with alanine. Compared with control cells, prior exposure to alanine also markedly decreased (P<0.01) the acute elevation of [Ca2+]i (intracellular [Ca2+]) induced by acute alanine exposure. These diminished stimulatory responses were partially restored after 18 h of culture in the absence of alanine, indicating reversible amino-acid-induced desensitization. 13C NMR spectra revealed that alanine culture increased glutamate labelling at position C4 (by 60%; P<0.01), as a result of an increase in the singlet peak, indicating increased flux through pyruvate dehydrogenase. Consistent with this, protein expression of the pyruvate dehydrogenase kinases PDK2 and PDK4 was significantly reduced. This was accompanied by a decrease in cellular ATP (P<0.05), consistent with diminished insulin-releasing actions of this amino acid. Collectively, these results illustrate the phenomenon of β-cell desensitization by amino acids, indicating that prolonged exposure to alanine can induce reversible alterations to metabolic flux, Ca2+ handling and insulin secretion.

Malic enzyme is present in mouse islets and modulates insulin secretion

AIMS/HYPOTHESIS

The pyruvate-malate shuttle is a metabolic cycle in pancreatic beta cells and is important for beta cell function. Cytosolic malic enzyme (ME) carries out an essential step in the shuttle by converting malate to pyruvate and generating NADPH. In rat islets the pyruvate-malate shuttle may regulate insulin secretion and it has been shown to play a critical role in adaptation to obesity and insulin resistance. However, ME has not been demonstrated in mouse islets and three reports indicate that mouse islets contain no ME activity. If mouse islets lack ME, rat and mouse islets must regulate insulin secretion by different mechanisms.

METHODS

We measured ME activity by a fluorometric enzymatic assay and Me mRNA by real-time PCR. ME activity was also measured in streptozotocin-treated mouse islets. FACS-purified beta cells were obtained from MIP-GFP mouse islets, agouti-L obese mouse islets and mouse beta cell line MIN-6. Insulin secretion and NADPH/NADP(+) ratios were measured in Me siRNA-treated beta cells.

RESULTS

ME activity and Me mRNA were present in C57BL/6 mouse islets. ME activity was reduced in streptozotocin-treated mouse islets. ME activity was also measurable in FACS-purified mouse beta cells. In addition, ME activity was significantly increased in obese agouti-L mouse islets and the mouse MIN-6 cell line. Me siRNA inhibited ME activity and reduced glucose-stimulated insulin secretion and also inhibited NADPH products.

CONCLUSIONS/INTERPRETATION

Mouse islets contain ME, which plays a significant role in regulating insulin secretion.

The role of pyruvate carboxylase in insulin secretion and proliferation in rat pancreatic beta cells

AIMS/HYPOTHESIS

Pyruvate carboxylase (PC) or pyruvate dehydrogenase (PDH) is required to transfer carbons from pyruvate into the Krebs cycle. PC activity is preserved in the islets of obese animals, but it is reduced in the islets of animal models of type 2 diabetes, suggesting that PC is important in beta cell adaptation to insulin resistance and that PC reduction may lead to beta cell failure.

METHODS

To confirm the significance of PC, we first lowered activity using Pc (now known as Pcx) small interfering RNA (siRNA) in INS-1 cells and in dispersed rat islet cells. Second, we overexpressed PC in INS-1 cells, and third, we inhibited PDH by overexpressing the gene encoding pyruvate dehydrogenase kinase 4 (Pdk4) in INS-1 cells.

RESULTS

Treatment of INS-1 cells or dispersed rat islet cells with Pc siRNA resulted in a significant reduction in insulin secretion in both cell types and reduced proliferation in INS-1 cells. This treatment also reduced the content of oxaloacetate, malate and ATP, as well as the NADPH:NADP(+) ratio and activity of the pyruvate-malate shuttle. Overexpression of PC in INS-1 cells led to an elevation of insulin secretion and cell proliferation, whereas inhibition of PDH activity by overexpressing Pdk4 in INS-1 cells did not reduce insulin secretion.

CONCLUSIONS/INTERPRETATION

Our findings indicate that the PC pathway in beta cells might play a key role in pyruvate metabolism, insulin secretion and cell proliferation.

Life and death decisions of the pancreatic β-cell: the role of fatty acids

Both stimulatory and detrimental effects of NEFAs (non-esterified fatty acids) on pancreatic β-cells have been recognized. Acute exposure of the pancreatic β-cell to high glucose concentrations and/or saturated NEFAs results in a substantial increase in insulin release, whereas chronic exposure results in desensitization and suppression of secretion, followed by induction of apoptosis. Some unsaturated NEFAs also promote insulin release acutely, but they are less toxic to β-cells during chronic exposure and can even exert positive protective effects. Therefore changes in the levels of NEFAs are likely to be important for the regulation of β-cell function and viability under physiological conditions. In addition, the switching between endogenous fatty acid synthesis or oxidation in the β-cell, together with alterations in neutral lipid accumulation, may have critical implications for β-cell function and integrity. Long-chain acyl-CoA (formed from either endogenously synthesized or exogenous fatty acids) controls several aspects of β-cell function, including activation of specific isoenzymes of PKC (protein kinase C), modulation of ion channels, protein acylation, ceramide formation and/or NO-mediated apoptosis, and transcription factor activity. In this review, we describe the effects of exogenous and endogenous fatty acids on β-cell metabolism and gene and protein expression, and have explored the outcomes with respect to insulin secretion and β-cell integrity.