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

Dietary effects of Sclerocarya birrea caffra seed cake replacing soyabean meal on physiology, meat and bone quality of indigenous chickens

This study investigated marula seed cake (MSC) as alternative protein source (APS) replacing soyabean meal (SBM) in indigenous chicken diets. Four hundred, 3-week-old Boschveld chicks were randomly allocated to 5 iso-energetic-nitrogenous maize and SBM-based grower diets with 0, 10, 15, 20, and 25 % MSC, each with 5 replicate pens of 16 birds, in a completely randomised design (CRD), for 9 weeks. Results showed that dietary MSC quadratically decreased bird overall feed intake (FI) (P < 0.001) and body weight gain (BWG) (P < 0.01) as it linearly decreased the weights of hot carcass (HCW; P < 0.05), spleen (P < 0.05), jejunum (P < 0.05), ileum (P < 0.001), and caecum (P < 0.001). In contrast, MSC increased chicken serum glucose (P < 0.05), cholesterol (P = 0.001) and phosphate (P < 0.05) as it decreased its amylase activity (P < 0.01). Also, it decreased bird meat lightness at 45 min (P < 0.05) and its yellowness at 45 min (P < 0.001) and 24 h (P < 0.001) whilst it increased its redness at 45 min (P < 0.01) and 24 h (P < 0.05) post-slaughter. In addition, MSC decreased chicken bone medial diaphysis (P < 0.05) as it induced no effects (P > 0.05) on overall feed conversion efficiency (FCE) and all other parameters. In conclusion, feeding of ≤ 15 % dietary MSC is nutritionally safe for indigenous chickens whilst detrimental on bird appetite, growth and meat yield, however without significantly affecting their physiology, at higher inclusion levels.

The Regulation and Secretion of Glucagon in Response to Nutrient Composition: Unraveling Their Intricate Mechanisms

Glucagon was initially regarded as a hyperglycemic substance; however, recent research has revealed its broader role in metabolism, encompassing effects on glucose, amino acids (AAs), and lipid metabolism. Notably, the interplay of glucagon with nutrient intake, particularly of AAs, and non-nutrient components is central to its secretion. Fasting and postprandial hyperglucagonemia have long been linked to the development and progression of type 2 diabetes (T2DM). However, recent studies have brought to light the positive impact of glucagon agonists on lipid metabolism and energy homeostasis. This review explores the multifaceted actions of glucagon, focusing on its regulation, signaling pathways, and effects on glucose, AAs, and lipid metabolism. The interplay between glucagon and other hormones, including insulin and incretins, is examined to provide a mechanistic understanding of its functions. Notably, the liver-α-cell axis, which involves glucagon and amino acids, emerges as a critical aspect of metabolic regulation. The dysregulation of glucagon secretion and its impact on conditions such as T2DM are discussed. The review highlights the potential therapeutic applications of targeting the glucagon pathway in the treatment of metabolic disorders.

Amino acids and the changing face of the α-cell

Glucagon has long been defined by its glucogenic action and as a result α-cells have been characterised based largely on their interaction with glucose. Recent findings have challenged this preconception, bringing to the fore the significant role glucagon plays in amino acid breakdown and underlining the importance of amino acids in glucagon secretion. The challenge that remains is defining the mechanism that underlie these effects - understanding which amino acids are most important, how they act on the α-cell and how their actions integrate with other fuels such as glucose and fatty acids. This review will describe the current relationship between amino acids and glucagon and how we can use this knowledge to redefine the α-cell.

Mesenchymal stem cell-conditioned medium alleviates high fat-induced hyperglucagonemia via miR-181a-5p and its target PTEN/AKT signaling

BACKGROUND

α-cell dysregulation gives rise to fasting and postprandial hyperglycemia in type 2 diabetes mellitus(T2DM). Administration of Mesenchymal stem cells (MSCs) or their conditioned medium can improve islet function and enhance insulin secretion. However, studies showing the direct effect of MSCs on islet α-cell dysfunction are limited.

METHODS

In this study, we used high-fat diet (HFD)-induced mice and α-cell line exposure to palmitate (PA) to determine the effects of bone marrow-derived MSC-conditioned medium (bmMSC-CM) on glucagon secretion. Plasma and supernatant glucagon were detected by enzyme-linked immunosorbent assay(ELISA). To investigate the potential signaling pathways, phosphatase and tensin homolog deleted on chromosome 10 (PTEN), AKT and phosphorylated AKT(p-AKT) were assessed by Western blotting.

RESULTS

In vivo, bmMSC-CM infusion improved the glucose and insulin tolerance and protected against HFD-induced hyperglycemia and hyperglucagonemia. Meanwhile, bmMSC-CM infusion ameliorated HFD-induced islet hypertrophy and decreased α- and β-cell area. Consistently, in vitro, glucagon secretion from α-cells or primary islets was inhibited by bmMSC-CM, accompanied by reduction of intracellular PTEN expression and restoration of AKT signaling. Previous studies and the TargetScan database indicate that miR-181a and its target PTEN play vital roles in ameliorating α-cell dysfunction. We observed that miR-181a-5p was highly expressed in BM-MSCs but prominently lower in αTC1-6 cells. Overexpression or downregulation of miR-181a-5p respectively alleviated or aggravated glucagon secretion in αTC1-6 cells via the PTEN/AKT signaling pathway.

CONCLUSIONS

Our observations suggest that MSC-derived miR-181a-5p mitigates glucagon secretion of α-cells by regulating PTEN/AKT signaling, which provides novel evidence demonstrating the potential for MSCs in treating T2DM.

Functional Status of Pancreatic α and β Cells in Type 2 Diabetes Mellitus Patients with Different Plasma Triglyceride Levels: A Retrospective Analysis

OBJECTIVE

To investigate the functional status of pancreatic α and β cells in Type 2 diabetes mellitus (T2DM) patients with different plasma triglyceride (TG) levels. TG levels can be prognostic markers for T2DM.

METHODS

A total of 328 patients with T2DM were divided into three groups according to different TG levels: the TGL group: TG < 1.7 mmol/L; TGM group: 1.7 mmol/L ≤ TG < 2.3 mmol/L; and TGH group: TG ≥ 2.3 mmol/L. An oral glucose tolerance test (OGTT), insulin release test, and glucagon release test were performed in each patient. The changes of glucagon, glucagon/insulin ratio, early insulin secretion index (ΔI ₃₀/ΔG ₃₀), and area under the insulin curve (AUC_I) were compared among each group. Also, the correlations between glucagon and pancreatic β-cell function, glycosylated hemoglobin (HbA1c), and other indices were analyzed.

RESULTS

With the increase of TG, the fasting and postprandial glucagon levels, the glucagon/insulin ratio, and the area under the glucagon curve (AUC_G) presented an increasing trend. The homeostasis model assessment of insulin resistance (HOMA-IR) of the TGH group was significantly increased compared to the TGL and TGM groups. In addition to the increase in TG levels, the insulin sensitivity index (ISI), homeostasis model assessment for β-cell function index (HOMA-β), ΔI ₃₀/ΔG ₃₀, and AUC_I displayed a reducing trend. Glucagon was negatively correlated with ΔI ₃₀/ΔG ₃₀, high-density lipoprotein (HDL), HOMA-β, body mass index (BMI), ISI, and AUC_I (P < 0.05) and positively correlated with fasting blood glucose (FPG), AUC_G, HOMA-IR, HbA1c, duration, TG, low-density lipoprotein (LDL), and total cholesterol (TC) (P < 0.05).

CONCLUSION

Hypertriglyceridemia aggravated the dysfunction of pancreatic α and β cells. A reasonable control of the TG level makes it easier for blood glucose to reach the standard.

Stillen und Stillberatung

Muttermilch (Stillen) stellt die Norm der Säuglingsernährung dar. Stillen ist die Standardernährungsform für den ersten Lebensabschnitt eines Menschen. In der Folge wird bei der Einführung von Beikost aus ausschließlichem Stillen weiterbegleitendes Stillen – bis zum vollständigen Übergang zur Familienkost. Die Zusammensetzung der Muttermilch und die hormonelle Steuerung der Milchbildung sind optimal auf das Kind abgestimmt. Stillen ist jedoch kein instinktives, sondern ein sozial erlerntes Verhalten und bedarf von Anbeginn an begleitender Information und Beratung der Mutter und der Familien. Medizinisches Fachpersonal wird als kompetent in Ernährungsfragen erachtet und sollte daher über ausreichende wissenschaftlich fundierte Kenntnis bezüglich der physiologischen Vorgänge im Zusammenhang mit Stillen sowie der Kurz- und Langzeitauswirkungen von Stillen auf die Gesundheit von Mutter und Kind verfügen. Ebenso sollten die Risiken von Formulaernährung (Säuglingsfertignahrung auf Kuhmilchbasis) für die Entwicklung und verschiedene Erkrankungen bekannt sein.

Modelling of energy-driven switch for glucagon and insulin secretion

We present a mathematical model of the energy-driven metabolic switch for glucagon and insulin secretion from pancreatic alpha and beta cells, respectively. The energy status related to hormone secretion is studied for various glucose concentrations. Additionally, the physiological response is studied with regards to the presence of other metabolites, particularly the free-fatty acids. At low glucose, the ATP production in alpha cells is high due to free-fatty acids oxidation in mitochondria, which enables glucagon secretion. When the glucose concentration is elevated above the threshold value, the glucagon secretion is switched off due to the contribution of glycolytic ATP production, representing an "anaerobic switch". On the other hand, during hypoglycemia, the ATP production in beta cells is low, reflecting a "waiting state" for glucose as the main metabolite. When glucose is elevated above the threshold value, the oxidative fate of glucose in mitochondria is the main source of energy required for effective insulin secretion, i.e. the "aerobic switch". Our results show the importance of well-regulated and fine-tuned energetic processes in pancreatic alpha and beta cells required for efficient hormone secretion and hence effective blood glucose regulation. These energetic processes have to be appropriately switched on and off based on the sensing of different metabolites by alpha and beta cells. Our computational results indicate that disturbances in cell energetics (e.g. mitochondrial dysfunction), and dysfunctional metabolite sensing and distribution throughout the cell might be related to pathologies such as metabolic syndrome and diabetes.

Postprandial Lipemia Modulates Pancreatic Alpha-Cell Function in the Prediction of Type 2 Diabetes Development: The CORDIOPREV Study

Diabetes (T2DM) is a major global health issue, and developing new approaches to its prevention is of paramount importance. We hypothesized that abnormalities in lipid metabolism are involved in alpha-cell deregulation. We therefore studied the metabolic factors underlying alpha-cell dysfunction in T2DM progression after a dietary intervention (Mediterranean and low-fat). Additionally, we evaluated whether postprandial glucagon levels may be considered as a predictive factor of T2DM in cardiovascular patients. Non-T2DM participants from the CORDIOPREV study were categorized by tertiles of the area under the curve (AUC) for triacylglycerols and also by tertiles of AUC for glucagon. Our results showed that patients with higher triacylglycerols levels presented elevated postprandial glucagon (P = 0.009). Moreover, we observed higher risk of T2DM (hazard ratio: 2.65; 95% confidence interval: 1.56-4.53) in subjects with elevated glucagon. In conclusion, high postprandial lipemia may induce alpha-cell dysfunction in cardiovascular patients. Our results also showed that postprandial glucagon levels could be used to predict T2DM development.

Glucometabolic consequences of acute and prolonged inhibition of fatty acid oxidation

Excessive circulating FAs have been proposed to promote insulin resistance (IR) of glucose metabolism by increasing the oxidation of FAs over glucose. Therefore, inhibition of FA oxidation (FAOX) has been suggested to ameliorate IR. However, prolonged inhibition of FAOX would presumably cause lipid accumulation and thereby promote lipotoxicity. To understand the glycemic consequences of acute and prolonged FAOX inhibition, we treated mice with the carnitine palmitoyltransferase 1 (CPT-1) inhibitor, etomoxir (eto), in combination with short-term 45% high fat diet feeding to increase FA availability. Eto acutely increased glucose oxidation and peripheral glucose disposal, and lowered circulating glucose, but this was associated with increased circulating FAs and triacylglycerol accumulation in the liver and heart within hours. Several days of FAOX inhibition by daily eto administration induced hepatic steatosis and glucose intolerance, specific to CPT-1 inhibition by eto. Lower whole-body insulin sensitivity was accompanied by reduction in brown adipose tissue (BAT) uncoupling protein 1 (UCP1) protein content, diminished BAT glucose clearance, and increased hepatic glucose production. Collectively, these data suggest that pharmacological inhibition of FAOX is not a viable strategy to treat IR, and that sufficient rates of FAOX are required for maintaining liver and BAT metabolic function.

Hormones and oxidative stress: an overview

In aerobes, oxygen is essential for maintenance of life. However, incomplete reduction of oxygen leads to generation of reactive oxygen species. These oxidants oxidise biological macromolecules present in their vicinity and thereby impair cellular functions causing oxidative stress (OS). Aerobes have evolved both enzymatic and nonenzymatic antioxidant defences to protect themselves from OS. Although hormones as means of biological coordination involve in regulation of physiological activities of tissues by regulating metabolism, any change in their normal titre leads to pathophysiological states. While, hormones such as melatonin, insulin, oestrogen, progesterone display antioxidant features, thyroid hormone, corticosteroids and catecholamines elicit free radical generation and OS, and the role of testosterone in inducing OS is debateable. This review is an attempt to understand the impact of free radical generation and cross talk between the hormones modulating antioxidant defence system under various pathophysiological conditions.

Periodontal pathogenic bacteria, Aggregatibacter actinomycetemcomitans affect non-alcoholic fatty liver disease by altering gut microbiota and glucose metabolism

Increasing evidence indicates that periodontitis affects non-alcoholic fatty liver disease (NAFLD). We examined the relationship between periodontal bacterial infection and clinical/biochemical parameters in 52 NAFLD patients. Anti-Aggregatibacter actinomycetemcomitans (Aa) antibody titers correlated positively with visceral fat, fasting plasma insulin, and HOMA-IR; and negatively with the liver/spleen ratio. C57BL/6J mice (8-weeks-old) were given Aa or saline (control) for 6 weeks, and were fed either normal chow (NCAa, NCco) or high-fat diet (HFAa and HFco). NCAa and HFAa mice presented impaired glucose tolerance and insulin resistance compared to control mice. HFAa mice showed higher hepatic steatosis than HFco animals. Liver microarray analysis revealed that 266 genes were differentially expressed between NCAa and NCco mice. Upregulated genes in Aa-administrated mice were enriched for glucagon signaling pathway, adipocytokine signaling pathway and insulin resistance. Consistently, plasma glucagon concentration was higher in NCAa mice. In addition, Akt phosphorylation was lower in the liver of NCAa/HFAa than in NCco/HFco mice. Based on 16S rRNA sequencing, Aa administration changed composition of the gut microbiota. Metagenome prediction in gut microbiota showed upregulation of fatty acid biosynthesis and downregulation of fatty acid degradation in Aa-administered mice. Thus, infection with Aa affects NAFLD by altering the gut microbiota and glucose metabolism.

Adiponectin, Leptin, and Fatty Acids in the Maintenance of Metabolic Homeostasis through Adipose Tissue Crosstalk

Metabolism research has made tremendous progress over the last several decades in establishing the adipocyte as a central rheostat in the regulation of systemic nutrient and energy homeostasis. Operating at multiple levels of control, the adipocyte communicates with organ systems to adjust gene expression, glucoregulatory hormone exocytosis, enzymatic reactions, and nutrient flux to equilibrate the metabolic demands of a positive or negative energy balance. The identification of these mechanisms has great potential to identify novel targets for the treatment of diabetes and related metabolic disorders. Herein, we review the central role of the adipocyte in the maintenance of metabolic homeostasis, highlighting three critical mediators: adiponectin, leptin, and fatty acids.

Postprandial lipemia induces pancreatic α cell dysfunction characteristic of type 2 diabetes: studies in healthy subjects, mouse pancreatic islets, and cultured pancreatic α cells

BACKGROUND

Type 2 diabetes is associated with pancreatic α cell dysfunction, characterized by elevated fasting plasma glucagon concentrations and inadequate postprandial glucose- and insulin-induced suppression of glucagon secretion. The cause and the underlying mechanisms of α cell dysfunction are unknown.

OBJECTIVE

Because Western dietary habits cause postprandial lipemia for a major part of a day and, moreover, increase the risk of developing type 2 diabetes, we tested the hypothesis that postprandial lipemia with its characteristic elevation of triglyceride-rich lipoproteins (TGRLs) might cause pancreatic α cell dysfunction.

DESIGN

In a crossover study with 7 healthy volunteers, 2 experiments using 2 fat-enriched meals were performed on each volunteer; meal 1 was designed to increase plasma concentrations of both TGRLs and nonesterified fatty acids and meal 2 to increase TGRLs only. Intravenous glucose boli were injected at 0800 after an overnight fast and postprandially at 1300, 3 h after ingestion of a fat-enriched meal. Glucagon concentrations were measured throughout the days of the experiments. In addition to the study in humans, in vitro experiments were performed with mouse pancreatic islets and cultured pancreatic alpha TC 1 clone 9 (αTC1c9) cells, which were incubated with highly purified TGRLs.

RESULTS

In humans, postprandial lipemia increased plasma glucagon concentrations and led to an inadequate glucose- and insulin-induced suppression of glucagon. There was no difference between the 2 meal types. In mouse pancreatic islets and cultured pancreatic αTC1c9 cells, purified postprandial TGRLs induced abnormalities in glucagon kinetics comparable with those observed in humans. The TGRL-induced α cell dysfunction was due to reduced γ-aminobutyric acid A receptor activation in pancreatic α cells.

CONCLUSION

We concluded that postprandial lipemia induces pancreatic α cell dysfunction characteristic of type 2 diabetes and, therefore, propose that pancreatic α cell dysfunction could be viewed, at least partly, as a postprandial phenomenon.

Nutrient regulation of glucagon secretion: involvement in metabolism and diabetes

Glucose homeostasis is precisely regulated by glucagon and insulin, which are released by pancreatic α- and β-cells, respectively. While β-cells have been the focus of intense research, less is known about α-cell function and the actions of glucagon. In recent years, the study of this endocrine cell type has experienced a renewed drive. The present review contains a summary of established concepts as well as new information about the regulation of α-cells by glucose, amino acids, fatty acids and other nutrients, focusing especially on glucagon release, glucagon synthesis and α-cell survival. We have also discussed the role of glucagon in glucose homeostasis and in energy and lipid metabolism as well as its potential as a modulator of food intake and body weight. In addition to the well-established action on the liver, we discuss the effects of glucagon in other organs, where the glucagon receptor is expressed. These tissues include the heart, kidneys, adipose tissue, brain, small intestine and the gustatory epithelium. Alterations in α-cell function and abnormal glucagon concentrations are present in diabetes and are thought to aggravate the hyperglycaemic state of diabetic patients. In this respect, several experimental approaches in diabetic models have shown important beneficial results in improving hyperglycaemia after the modulation of glucagon secretion or action. Moreover, glucagon receptor agonism has also been used as a therapeutic strategy to treat obesity.

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.

Uncoupling protein 2 regulates glucagon-like peptide-1 secretion in L-cells

AIM: To investigate whether uncoupling protein 2 (UCP2) affects oleic acid-induced secretion of glucagon-like peptide-1 (GLP-1) in L-cells.

METHODS: mRNA and protein expression of UCP2 were analyzed in human NCI-H716 cells, which serve as a model for enteroendocrine L-cells, by quantitative reverse transcription-polymerase chain reaction and Western blotting before and after treatment with oleic acid. Localization of UCP2 and GLP-1 in NCI-H716 cells was assessed by immunofluorescence labeling. NCI-H716 cells were transiently transfected with a small interfering RNA (siRNA) that targets UCP2 (siUCP2) or with a non-specific siRNA using Lipofectamine 2000. The concentrations of bioactive GLP-1 in the medium were measured by enzyme linked immunosorbent assay.

RESULTS: Both GLP-1 and UCP2 granules were expressed mainly in the cytoplasm of NCI-H716 cells. NCI-H716 cells that secreted GLP-1 also expressed UCP2. Time-course experiments revealed that release of GLP-1 from NCI-H716 cells into the medium reached a maximum at 120 min and remained stable until at least 180 min after treatment with oleic acid (the level of GLP-1 increased about 2.3-fold as compared with the level of GLP-1 in the control cells, P < 0.05). In an experiment to determine dose dependence, stimulation of NCI-H716 cells with ≤ 8 mmol oleic acid led to a concentration-dependent release of GLP-1 into the medium; 10 mmol oleic acid diminished the release of GLP-1. Furthermore, GLP-1 secretion induced by oleic acid from NCI-H716 cells that were transfected with siUCP2 decreased to 41.8%, as compared with NCI-H716 cells that were transfected with a non-specific siRNA (P < 0.01).

CONCLUSION: UCP2 affected GLP-1 secretion induced by oleic acid. UCP2 plays an important role in L-cell secretion that is induced by free fatty acids.

Isosteviol has beneficial effects on palmitate-induced α-cell dysfunction and gene expression

BACKGROUND

Long-term exposure to high levels of fatty acids impairs insulin secretion and exaggerates glucagon secretion. The aim of this study was to explore if the antihyperglycemic agent, Isosteviol (ISV), is able to counteract palmitate-induced α-cell dysfunction and to influence α-cell gene expression.

METHODOLOGY/PRINCIPAL FINDINGS

Long-term incubation studies with clonal α-TC1-6 cells were performed in the presence of 0.5 mM palmitate with or without ISV. We investigated effects on glucagon secretion, glucagon content, cellular triglyceride (TG) content, cell proliferation, and expression of genes involved in controlling glucagon synthesis, fatty acid metabolism, and insulin signal transduction. Furthermore, we studied effects of ISV on palmitate-induced glucagon secretion from isolated mouse islets. Culturing α-cells for 72-h with 0.5 mM palmitate in the presence of 18 mM glucose resulted in a 56% (p<0.01) increase in glucagon secretion. Concomitantly, the TG content of α-cells increased by 78% (p<0.01) and cell proliferation decreased by 19% (p<0.05). At 18 mM glucose, ISV (10(-8) and 10(-6) M) reduced palmitate-stimulated glucagon release by 27% (p<0.05) and 27% (p<0.05), respectively. ISV (10(-6) M) also counteracted the palmitate-induced hypersecretion of glucagon in mouse islets. ISV (10(-6) M) reduced α-TC1-6 cell proliferation rate by 25% (p<0.05), but ISV (10(-8) and 10(-6) M) had no effect on TG content in the presence of palmitate. Palmitate (0.5 mM) increased Pcsk2 (p<0.001), Irs2 (p<0.001), Fasn (p<0.001), Srebf2 (p<0.001), Acaca (p<0.01), Pax6 (p<0.05) and Gcg mRNA expression (p<0.05). ISV significantly (p<0.05) up-regulated Insr, Irs1, Irs2, Pik3r1 and Akt1 gene expression in the presence of palmitate.

CONCLUSIONS/SIGNIFICANCE

ISV counteracts α-cell hypersecretion and apparently contributes to changes in expression of key genes resulting from long-term exposure to palmitate. ISV apparently acts as a glucagonostatic drug with potential as a new anti-diabetic drug for the treatment of type 2 diabetes.

Glucotoxicity and α cell dysfunction: involvement of the PI3K/Akt pathway in glucose-induced insulin resistance in rat islets and clonal αTC1-6 cells

AIM/HYPOTHESIS

The objective of this study was to assess how long-term exposure to high glucose affects the α cell function and whether the increased glucagon secretion is mediated via insulin resistance.

MATERIALS AND METHODS

We established a β cell-depleted rat model to obtain pure primary α cells. Furthermore, isolated rat islets and TC1-6 cells (a clonal α cell line) were exposed to high glucose (25 or 30 mmol/L) and low glucose (5.5 mmol/L) for up to 5 days to evaluate the influence of chronic glucose toxicity on glucagon secretion and glucagon gene expression. Moreover, we added insulin and/or Wortmannin to examine if the inhibitory effect of insulin on glucagon secretion was impaired by high glucose via the phosphatidylinositol 3 kinase/PKB protein kinase B pathway.

RESULTS

Both glucagon secretion and glucagon gene expression were increased in response to 5 days exposure to high glucose. While a moderate insulin concentration slightly inhibits glucagon secretion from rat islets and α TC1-6 cells at high glucose, a pronounced increase in glucagon secretion was observed at low glucose. We found that the insulin-mediated activity of the phosphatidylinositol 3 kinase/PKB protein kinase B pathway in the α cell was markedly impaired by chronic exposure to high glucose.

CONCLUSION

The hypersecretion of glucagon induced by glucotoxicity may be secondary to insulin resistance of the α cell induced by impaired activity of the insulin signaling pathway.

A high-fat diet reduces ceramide synthesis by decreasing adiponectin levels and decreases lipid content by modulating HMG-CoA reductase and CPT-1 mRNA expression in the skin

SCOPE

Molecules involved in skin function are greatly affected by nutritional conditions. However, the mechanism linking high-fat (HF) diets with these alterations is not well understood. This study aimed to investigate the molecular changes in skin function that result from HF diets.

METHODS AND RESULTS

Sprague-Dawley rats were fed HF diets for 28 days. The skin levels of ceramide, lipids and mRNAs involved in lipid metabolism were evaluated using TLC, oil red O staining and quantitative PCR, respectively. The serum adiponectin concentration was determined by ELISA. HF diets led to reduced ceramide levels and lowered skin lipid content. They also decreased mRNA levels of serine palmitoyltransferase (SPT) and 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase in the skin and those of peroxisome proliferator-activated receptor-α -PPAR-α), which upregulates SPT and HMG-CoA reductase expression. The HF diets reduced the serum concentration of adiponectin, which acts upstream of PPAR-α. Finally, these diets led to increased mRNA levels of carnitine palmitoyltransferase-1, the rate-limiting enzyme that acts in β-oxidation.

CONCLUSION

Our study suggests that HF diets reduce ceramide and lipid synthesis in the skin by reducing levels of SPT and HMG-CoA reductase through lowered adiponectin and PPAR-α activity. Additionally, they decrease lipid content by enhancing β-oxidation.

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.

Dose- and Glucose-Dependent Effects of Amino Acids on Insulin Secretion from Isolated Mouse Islets and Clonal INS-1E Beta-Cells

BACKGROUND

The influence of glucose and fatty acids on beta-cell function is well established whereas little is known about the role of amino acids (AAs).

METHODS

Islets isolated from NMRI mice were incubated overnight. After preincubation, isolated islets as well as clonal INS-1E beta-cells were incubated for 60 min in a modified Krebs Ringer buffer containing glucose and AAs.

RESULTS

At 16.7 mmol/l (mM) glucose, L-arginine, L-lysine, L-alanine, L-proline, L-leucine, and L-glutamine potentiated glucose-stimulated insulin secretion dose-dependently, while DL-homocysteine inhibited insulin secretion. Maximal insulin stimulation was obtained at 20 mM L-proline, L-lysine, L-alanine, L-arginine (islets: 2.5 to 6.7 fold increase; INS-1E cells: 1.6 to 2.2 fold increase). L-glutamine and L-leucine only increased glucose-stimulated (16.7 mM) insulin secretion (INS-1E cells: 1.5 and 1.3 fold, respectively) at an AA concentration of 20 mM. Homocysteine inhibited insulin secretion both at 5.6 mM and 16.7 mM glucose. At glucose levels ranging from 1.1 to 25 mM, the equimolar concentration of 10 mM, L-proline, L-lysine, L-arginine increased insulin secretion from mouse islets and INS-1E cells at all glucose levels applied, with a maximal effect obtained at 25 mM glucose. At a concentration of 10 mM, L-arginine and L-lysine had the highest insulinotropic potency among the AAs investigated.

CONCLUSION

L-arginine, L-lysine, L-alanine, L-proline, L-leucine and L-glutamine acutely stimulate insulin secretion from mouse islets and INS-1E cells in a dose- and glucose-dependent manner, whereas DL-homocysteine inhibits insulin release.