The relationship between GPR40 and lipotoxicity of the pancreatic β-cells as well as the effect of pioglitazone

Free fatty acids (FFAs) acutely stimulate insulin secretion from pancreatic β-cells, whereas impair β-cell function following long term exposure. GPR40, a FFAs receptor, has been demonstrated to be activated by both medium and long chain FFAs and played an important role in insulin release. This study was performed to determine the contribution of GPR40 to short- and/or long-term effects of FFAs on glucose-stimulated insulin secretion (GSIS) and the expression of PDX-1 and GLUT2 in pancreatic β-cells, as well as the intervenient effects of pioglitazone on lipotoxicity of β-cells. βTC6 cell line stably expressing GPR40shRNA were established and the intervention of FFAs and pioglitazone on GSIS and expression of PDX-1 and GLUT2 in βTC6 cells was investigated. Results showed that 1-h exposure to FFAs significantly enhanced GSIS and increased expression of PDX-1 and GLUT2 in pSilencer-control transfected cells, but not in cells transfected with GPR40shRNA. While 48-h exposure to FFAs significantly impaired GSIS in pSilencer-control transfected cells as well as cells transfected with GPR40shRNA. Furthermore, pioglitazone enhanced insulin secretion in pSilencer-control transfected cells exposed to FFAs for 48 h, but not in cells transfected with GPR40shRNA. These results indicate that GPR40 mediates the short-term effects of FFAs on GSIS, but does not mediate the chronic lipotoxicity on β-cells. The reverse role of pioglitazone on lipotoxicity of β-cells may be related to GPR40.     

AMP-activated protein kinase and pancreatic/duodenal homeobox-1 involved in insulin secretion under high leucine exposure in rat insulinoma β-cells

The effect of leucine on glucose-stimulated insulin secretion (GSIS) in pancreatic β-cells is quite controversial, and mechanism involved in the effect has not been elucidated yet. Consequently, we aimed to investigate effect of leucine on GSIS and its mechanism focusing on contribution of AMP-activated protein kinase (AMPK) and pancreatic/duodenal homeobox-1 (PDX-1). Rat insulinoma β-cells (INS-1, RIN m5F, DN-PDX-1#28 and PDX-1#6) were cultured with or without leucine, AICAR (AMPK agonist) or compound C (AMPK antagonist) for 48 hrs. In contrast to control, AICAR treatment decreased GSIS at high glucose and insulin content, also impaired protein and mRNA expression of PDX-1 and its downstream targets, glucokinase (GCK) and glucose transporter 2 (GLUT2). Compound C treatment had the opposite effects. We observed that neither AICAR nor compound C could affect expression of GCK and GLUT2 when PDX-1 expression was absent. Chronic leucine exposure inhibited GSIS at high glucose and insulin content in a dose-dependent manner, concomitant with an increase in AMPK and a decrease in PDX-1, GCK and GLUT2. The inhibitory effects of leucine was potentiated by AICAR treatment and rescued by compound C treatment. Finally, the inhibition of PDX-1 could potentiate the impaired effects induced by leucine whereas overexpression of PDX-1 could protect the cell from impairment induced by leucine. The study indicated that chronic leucine might result in an increase in AMPK and then a decrease in PDX-l, in turn to depress GCK and GLUT2 resulting in decreased GSIS at high glucose and insulin content.     

The CB1 Antagonist Rimonabant Decreases Insulin Hypersecretion in Rat Pancreatic Islets

Type 2 diabetes and obesity are characterized by elevated nocturnal circulating free fatty acids, elevated basal insulin secretion, and blunted glucose‐stimulated insulin secretion (GSIS). The CB1 receptor antagonist, Rimonabant, has been shown to improve glucose tolerance and insulin sensitivity in vivo but its direct effect on islets has been unclear. Islets from lean littermates and obese Zucker (ZF) and Zucker Diabetic Fatty (ZDF) rats were incubated for 24 h in vitro and exposed to 11 mmol/l glucose and 0.3 mmol/l palmitate (GL) with or without Rimonabant. Insulin secretion was determined at basal (3 mmol/l) or stimulatory (15 mmol/l) glucose concentrations. As expected, basal secretion was significantly elevated in islets from obese or GL‐treated lean rats whereas the fold increase in GSIS was diminished. Rimonabant decreased basal hypersecretion in islets from obese rats and GL‐treated lean rats without decreasing the fold increase in GSIS. However, it decreased GSIS in islets from lean rats without affecting basal secretion. These findings indicate that Rimonabant has direct effects on islets to reduce insulin secretion when secretion is elevated above normal levels by diet or in obesity. In contrast, it appears to decrease stimulated secretion in islets from lean animals but not in obese or GL‐exposed islets.     

The FFA receptor GPR40 links hyperinsulinemia, hepatic steatosis, and impaired glucose homeostasis in mouse

Obesity is typically associated with elevated levels of free fatty acids (FFAs) and is linked to glucose intolerance and type 2 diabetes. FFAs exert divergent effects on insulin secretion from beta cells: acute exposure to FFAs stimulates insulin secretion, whereas chronic exposure impairs insulin secretion. The G protein-coupled receptor GPR40 is selectively expressed in beta cells and is activated by FFAs. We show here that GPR40 mediates both acute and chronic effects of FFAs on insulin secretion and that GPR40 signaling is linked to impaired glucose homeostasis. GPR40-deficient beta cells secrete less insulin in response to FFAs, and loss of GPR40 protects mice from obesity-induced hyperinsulinemia, hepatic steatosis, hypertriglyceridemia, increased hepatic glucose output, hyperglycemia, and glucose intolerance. Conversely, overexpression of GPR40 in beta cells of mice leads to impaired beta cell function, hypoinsulinemia, and diabetes. These results suggest that GPR40 plays an important role in the chain of events linking obesity and type 2 diabetes.     

Inhibition of fatty acid translocase cluster determinant 36 (CD36), stimulated by hyperglycemia, prevents glucotoxicity in INS-1 cells

The purpose of the present study was to determine whether exposure of pancreatic islets to glucotoxic conditions changes fatty acid translocase cluster determinant 36 (CD36) and examine the role of CD36 on the induction of glucotoxicity. We measured the changes of CD36 and insulin secretion in high glucose (30 mM) exposed INS-1 cells and CD36 suppressed INS-1 cells by transfection of CD36 siRNA. The intracellular peroxide level of INS-1 cells increased in the high glucose media compared to normal glucose (5.6mM) media. The mRNA levels of insulin and PDX-1, as well as glucose stimulated insulin secretion (GSIS) were decreased in INS-1 cells exposed to high glucose media compared to normal glucose media, while CD36 and palmitate uptake were significantly elevated with exposure to high glucose media for 12h. The inhibition of CD36 reversed the decreased GSIS and intracellular peroxide level in INS-1 cells. These results suggest that high glucose may exacerbate glucotoxicity via increasing fatty acid influx by elevation of CD36 expression, and that CD36 may be a possible target molecule for preventing glucotoxicity in pancreatic beta-cells.     

Kynurenine-3-monooxygenase expression is activated in the pancreatic endocrine cells by diabetes and its blockade improves glucose-stimulated insulin secretion

Type 2 diabetes is associated with an inflammatory phenotype in the pancreatic islets. We previously demonstrated that proinflammatory cytokines potently activate the tryptophan/kynurenine pathway (TKP) in INS-1 cells and in normal rat islets. Here we examined: (1) the TKP enzymes expression in the diabetic GK islets; (2) the TKP enzymes expression profiles in the GK islets before and after the onset of diabetes; (3) The glucose-stimulated insulin secretion (GSIS) in vitro in GK islets after KMO knockdown using specific morpholino-oligonucleotides against KMO or KMO blockade using the specific inhibitor Ro618048; (4) The glucose tolerance and GSIS after acute in vivo exposure to Ro618048 in GK rats. We report a remarkable induction of the kmo gene in GK islets and in human islets exposed to proinflammatory conditions. It occurred prominently in beta cells. The increased expression and activity of KMO reflected an acquired adaptation. Both KMO knockdown and specific inhibitor Ro618048 enhanced GSIS in vitro in GK islets. Moreover, acute administration of Ro618048 in vivo improved glucose tolerance, GSIS and basal blood glucose levels in GK rats. These results demonstrate that targeting islet TKP is able to correct defective GSIS. KMO inhibition could represent a potential therapeutic strategy for type 2 diabetes.     

Autonomic involvement in the permanent metabolic programming of hyperinsulinemia in the high-carbohydrate rat model

Exposure to a high-carbohydrate (HC) milk formula during the suckling period results in permanent metabolic programming of hyperinsulinemia in HC rats. Previous studies have shown that hyperinsulinemia in HC rats involves a programmed hyperresponsiveness to glucose. However, the immediate onset and persistence of enhanced insulin secretion throughout life suggests a role for numerous factors that control insulin secretion. Present in vivo and in vitro studies have shown a role for altered autonomic activity, including increased parasympathetic and decreased sympathetic activities, in the maintenance of hyperinsulinemia in 100-day-old HC rats. HC rats were shown to be more sensitive to cholinergic-induced potentiation of glucose-stimulated insulin secretion (GSIS) in response to acetylcholine and showed increased sensitivity to blockade of cholinergic-induced insulin secretion by the muscarinic-type 3 receptor-specific antagonist 4-diphenylacetoxy-N-methylpiperidine. In addition, HC rats were less sensitive to adrenergic-induced inhibition of insulin secretion by oxymetazoline, whereas treatment with yohimbine resulted in increased GSIS. Furthermore, HC rats showed greater reductions in plasma insulin levels after vagotomy, as well as an attenuation of yohimbine-induced potentiation of GSIS, suggesting that yohimbine-mediated changes are mediated by parasympathetic activity. Changes in autonomic regulation of GSIS are supported by increased mRNA levels of the parasympathetic signaling molecules muscarinic-type 3 receptor, phospholipase Cbeta1, and protein kinase C-alpha and decreased levels of alpha(2a)-adrenergic receptors in islets from adult HC rats. In conclusion, metabolic programming of hyperinsulinemia throughout adulthood of HC rats involves changes in autonomic activity in response to the HC dietary intervention in the suckling period.     

Oleanolic acid enhances insulin secretion in pancreatic beta-cells

We investigated the effect of oleanolic acid, a plant-derived triterpenoid, on insulin secretion and content in pancreatic beta-cells and rat islets. Oleanolic acid significantly enhanced insulin secretion at basal and stimulatory glucose concentrations in INS-1 832/13 cells and enhanced acute glucose-stimulated insulin secretion in isolated rat islets. In the cell line the effects of oleanolic acid on insulin secretion were comparable to that of the sulfonylurea tolbutamide at basal glucose levels and with the incretin mimetic Exendin-4 under glucose-stimulated conditions, yet neither Ca(2+) nor cAMP rose in response to oleanolic acid. Chronic treatment with oleanolic acid increased total cellular insulin protein and mRNA levels. These effects may contribute to the anti-diabetic properties of this natural product.     

Sphingosine 1-phosphate (S1P) regulates glucose-stimulated insulin secretion in pancreatic beta cells

Recent studies suggest that sphingolipid metabolism is altered during type 2 diabetes. Increased levels of the sphingolipid ceramide are associated with insulin resistance. However, a role for sphingolipids in pancreatic beta cell function, or insulin production, and release remains to be established. Our studies in MIN6 cells and mouse pancreatic islets demonstrate that glucose stimulates an intracellular rise in the sphingolipid, sphingosine 1-phosphate (S1P), whereas the levels of ceramide and sphingomyelin remain unchanged. The increase in S1P levels by glucose is due to activation of sphingosine kinase 2 (SphK2). Interestingly, rises in S1P correlate with increased glucose-stimulated insulin secretion (GSIS). Decreasing S1P levels by treatment of MIN6 cells or primary islets with the sphingosine kinase inhibitor reduces GSIS. Moreover, knockdown of SphK2 alone results in decreased GSIS, whereas knockdown of the S1P phosphatase, Sgpp1, leads to a rise in GSIS. Treatment of mice with the sphingosine kinase inhibitor impairs glucose disposal due to decreased plasma insulin levels. Altogether, our data suggest that glucose activates SphK2 in pancreatic beta cells leading to a rise in S1P levels, which is important for GSIS.     

Increased DNA methylation and decreased expression of PDX-1 in pancreatic islets from patients with type 2 diabetes

Mutations in pancreatic duodenal homeobox 1 (PDX-1) can cause a monogenic form of diabetes (maturity onset diabetes of the young 4) in humans, and silencing Pdx-1 in pancreatic β-cells of mice causes diabetes. However, it is not established whether epigenetic alterations of PDX-1 influence type 2 diabetes (T2D) in humans. Here we analyzed mRNA expression and DNA methylation of PDX-1 in human pancreatic islets from 55 nondiabetic donors and nine patients with T2D. We further studied epigenetic regulation of PDX-1 in clonal β-cells. PDX-1 expression was decreased in pancreatic islets from patients with T2D compared with nondiabetic donors (P = 0.0002) and correlated positively with insulin expression (rho = 0.59, P = 0.000001) and glucose-stimulated insulin secretion (rho = 0.41, P = 0.005) in the human islets. Ten CpG sites in the distal PDX-1 promoter and enhancer regions exhibited significantly increased DNA methylation in islets from patients with T2D compared with nondiabetic donors. DNA methylation of PDX-1 correlated negatively with its gene expression in the human islets (rho = -0.64, P = 0.0000029). Moreover, methylation of the human PDX-1 promoter and enhancer regions suppressed reporter gene expression in clonal β-cells (P = 0.04). Our data further indicate that hyperglycemia decreases gene expression and increases DNA methylation of PDX-1 because glycosylated hemoglobin (HbA1c) correlates negatively with mRNA expression (rho = -0.50, P = 0.0004) and positively with DNA methylation (rho = 0.54, P = 0.00024) of PDX-1 in the human islets. Furthermore, while Pdx-1 expression decreased, Pdx-1 methylation and Dnmt1 expression increased in clonal β-cells exposed to high glucose. Overall, epigenetic modifications of PDX-1 may play a role in the development of T2D, given that pancreatic islets from patients with T2D and β-cells exposed to hyperglycemia exhibited increased DNA methylation and decreased expression of PDX-1. The expression levels of PDX-1 were further associated with insulin secretion in the human islets.     

Taurine supplementation modulates glucose homeostasis and islet function

Taurine is a conditionally essential amino acid for human that is involved in the control of glucose homeostasis; however, the mechanisms by which the amino acid affects blood glucose levels are unknown. Using an animal model, we have studied these mechanisms. Mice were supplemented with taurine for 30 d. Blood glucose homeostasis was assessed by intraperitoneal glucose tolerance tests (IPGTT). Islet cell function was determined by insulin secretion, cytosolic Ca2+ measurements and glucose metabolism from isolated islets. Islet cell gene expression and translocation was examined via immunohistochemistry and quantitative real-time polymerase chain reaction. Insulin signaling was studied by Western blot. Islets from taurine-supplemented mice had: (i) significantly higher insulin content, (ii) increased insulin secretion at stimulatory glucose concentrations, (iii) significantly displaced the dose-response curve for glucose-induced insulin release to the left, (iv) increased glucose metabolism at 5.6 and 11.1-mmol/L concentrations; (v) slowed cytosolic Ca2+ concentration ([Ca2+]i) oscillations in response to stimulatory glucose concentrations; (vi) increased insulin, sulfonylurea receptor-1, glucokinase, Glut-2, proconvertase and pancreas duodenum homeobox-1 (PDX-1) gene expression and (vii) increased PDX-1 expression in the nucleus. Moreover, taurine supplementation significantly increased both basal and insulin stimulated tyrosine phosphorylation of the insulin receptor in skeletal muscle and liver tissues. Finally, taurine supplemented mice showed an improved IPGTT. These results indicate that taurine controls glucose homeostasis by regulating the expression of genes required for glucose-stimulated insulin secretion. In addition, taurine enhances peripheral insulin sensitivity.     

Combination therapy with PPARgamma and PPARalpha agonists increases glucose-stimulated insulin secretion in db/db mice

Although peroxisome proliferator-activated receptor (PPAR)gamma agonists ameliorate insulin resistance, they sometimes cause body weight gain, and the effect of PPAR agonists on insulin secretion is unclear. We evaluated the effects of combination therapy with a PPARgamma agonist, pioglitazone, and a PPARalpha agonist, bezafibrate, and a dual agonist, KRP-297, for 4 wk in male C57BL/6J mice and db/db mice, and we investigated glucose-stimulated insulin secretion (GSIS) by in situ pancreatic perfusion. Body weight gain in db/db mice was less with KRP-297 treatment than with pioglitazone or pioglitazone + bezafibrate treatment. Plasma glucose, insulin, triglyceride, and nonesterified fatty acid levels were elevated in untreated db/db mice compared with untreated C57BL/6J mice, and these parameters were significantly ameliorated in the PPARgamma agonist-treated groups. Also, PPARgamma agonists ameliorated the diminished GSIS and insulin content, and they preserved insulin and GLUT2 staining in db/db mice. GSIS was further increased by PPARgamma and -alpha agonists. We conclude that combination therapy with PPARgamma and PPARalpha agonists may be more useful with respect to body weight and pancreatic GSIS in type 2 diabetes with obesity.     

Oleic acid interacts with GPR40 to induce Ca2+ signaling in rat islet beta-cells: mediation by PLC and L-type Ca2+ channel and link to insulin release

It has long been thought that long-chain free fatty acids (FFAs) stimulate insulin secretion via mechanisms involving their metabolism in pancreatic beta-cells. Recently, it was reported that FFAs function as endogenous ligands for GPR40, a G protein-coupled receptor, to amplify glucose-stimulated insulin secretion in an insulinoma cell line and rat islets. However, signal transduction mechanisms for GPR40 in beta-cells are little known. The present study was aimed at elucidating GPR40-linked Ca(2+) signaling mechanisms in rat pancreatic beta-cells. We employed oleic acid (OA), an FFA that has a high affinity for the rat GPR40, and examined its effect on cytosolic Ca(2+) concentration ([Ca(2+)](i)) in single beta-cells by fura 2 fluorescence imaging. OA at 1-10 microM concentration-dependently increased [Ca(2+)](i) in the presence of 5.6, 8.3, and 11.2 mM, but not 2.8 mM, glucose. OA-induced [Ca(2+)](i) increases at 11.2 mM glucose were inhibited in beta-cells transfected with small interfering RNA targeted to rat GPR40 mRNA. OA-induced [Ca(2+)](i) increases were also inhibited by phospholipase C (PLC) inhibitors, U73122 and neomycin, Ca(2+)-free conditions, and an L-type Ca(2+) channel blocker, nitrendipine. Furthermore, OA increased insulin release from isolated islets at 8.3 mM glucose, and it was markedly attenuated by PLC and L-type Ca(2+) channel inhibitors. These results demonstrate that OA interacts with GPR40 to increase [Ca(2+)](i) via PLC- and L-type Ca(2+) channel-mediated pathway in rat islet beta-cells, which may be link to insulin release.     

D-Glyceraldehyde causes production of intracellular peroxide in pancreatic islets, oxidative stress, and defective beta cell function via non-mitochondrial pathways

D-Glyceraldehyde (D-GLYC) is usually considered to be a stimulator of insulin secretion but theoretically can also form reactive oxygen species (ROS), which can inhibit beta cell function. We examined the time- and concentration-dependent effects of D-GLYC on insulin secretion, insulin content, and formation of ROS. We observed that a 2-h exposure to 0.05-2 mM D-GLYC potentiated glucose-stimulated insulin secretion (GSIS) in isolated Wistar rat islets but that higher concentrations inhibited GSIS. A 24-h exposure to 2 mm D-GLYC inhibited GSIS, decreased insulin content, and increased intracellular peroxide levels (2.14 +/- 0.31-fold increase, n = 4, p < 0.05). N-Acetylcysteine (10 mM) prevented the increase in intracellular peroxides and the adverse effects of d-GLYC on GSIS. In the presence of 11.1 but not 3.0 mm glucose, koningic acid (10 microM), a specific glyceraldehyde-3-phosphate dehydrogenase inhibitor, increased intracellular peroxide levels (1.88 +/- 0.30-fold increase, n = 9, p < 0.01) and inhibited GSIS (control GSIS = p < 0.001; koningic acid GSIS, not significant). To determine whether oxidative phosphorylation was the source of ROS formation, we cultured rat islets with mitochondrial inhibitors. Neither rotenone or myxothiazol prevented D-GLYC-induced increases in islet ROS. Adenoviral overexpression of manganese superoxide dismutase also failed to prevent the effect of D-GLYC to increase ROS levels. These observations indicate that exposure to excess D-GLYC increases reactive oxygen species in the islet via non-mitochondrial pathways and suggest the hypothesis that the oxidative stress associated with elevated D-GLYC levels could be a mechanism for glucose toxicity in beta cells exposed chronically to high glucose concentrations.     

Uric acid may inhibit glucose-induced insulin secretion via binding to an essential arginine residue in rat pancreatic beta-cells

Uric acid (1a) suppresses basal insulin release in isolated rat pancreatic islets and inhibition of glucose-stimulated insulin secretion (GSIS) occurs right at hyperuricaemic levels (0.4 mM). Conversely, 1 mM guanidinium urate (2a) was completely ineffective, strongly suggesting that binding to an essential arginine residue triggers the inhibitory effect. A specific recognition of 1a molecule at the crucial beta-cell receptor is probably involved in the blocking glucose signal transduction.     

Arachidonic acid induces an increase in the cytosolic calcium concentration in single pancreatic islet beta cells.

The insulin secretagogue D-glucose induces both accumulation of nonesterified arachidonic acid (35 microM) in pancreatic islets and a rise in beta cell cytosolic [Ca++]i. Arachidonate amplifies both voltage-dependent Ca++ entry in secretory cells and depolarization-induced insulin secretion. Here, arachidonate induced a biphasic rise in [Ca++]i of Fura-2AM loaded beta cells which increased with arachidonate concentration (5-30 microM), was reversed upon washout, and was unaffected by the arachidonate oxygenase inhibitor BW755C. The sustained phase of the rise was abolished by removal of extracellular Ca++ and amplified by depolarization with KCl. The accumulation of nonesterified arachidonate in islets stimulated by D-glucose may therefore promote the D-glucose-induced rise in beta cell [Ca++]i.     

Free fatty acids increase PGC-1alpha expression in isolated rat islets

PGC-1alpha mRNA and protein are elevated in islets from multiple animal models of diabetes. Overexpression of PGC-1alpha impairs glucose-stimulated insulin secretion (GSIS). However, it is not well known which metabolic events lead to upregulation of PGC-1alpha in the beta-cells under pathophysiological condition. In present study, we have investigated effects of chronic hyperlipidemia and hyperglycemia on PGC-1alpha mRNA expression in isolated rat islets. Isolated rat islets are chronically incubated with 0, 0.2 and 0.4 mM oleic acid/palmitic acid (free fatty acids, FFA) or 5.5 and 25 mM glucose for 72 h. FFA dose-dependently increases PGC-1alpha mRNA expression level in isolated islets. FFA also increases PGC-1alpha expression in mouse beta-cell-derived beta TC3 cell line. In contrast, 25 mM glucose decreases expression level of PGC-1alpha. Inhibition of PGC-1alpha by siRNA improves FFA-induced impairment of GSIS in islets. These data suggest that hyperlipidemia and hyperglycemia regulate PGC-1alpha expression in islets differently, and elevated PGC-1alpha by FFA plays an important role in chronic hyperlipidemia-induced beta-cell dysfunction.