Propionate inhibits glucose-induced insulin secretion in isolated rat pancreatic islets

Dietary fibers, probably by generating short chain fatty acids (SCFA) through enterobacterial fermentation, have a beneficial effect on the control of glycemia in patients with peripheral insulin resistance. We studied the effect of propionate on glucose-induced insulin secretion in isolated rat pancreatic islets. Evidence is presented that propionate, one of the major SCFA produced in the gut, inhibits insulin secretion induced by high glucose concentrations (11.1 and 16.7 mM) in incubated and perfused pancreatic islets. This short chain fatty acid reduces [U-(14)C]-glucose decarboxylation and raises the conversion of glucose to lactate. Propionate causes a significant decrease of both [1-(14)C]- (84%) and [2-(14)C]-pyruvate (49%) decarboxylation. These findings indicate pyruvate dehydrogenase as the major site for the propionate effect. These observations led us to postulate that the reduction in glucose oxidation and the consequent decrease in the ATP/ADP ratio may be the major mechanism for the lower insulin secretion to glucose stimulus induced by propionate.     

Blockade of IRS1 in isolated rat pancreatic islets improves glucose-induced insulin secretion

Several neural, hormonal and biochemical inputs actively participate in the balance of insulin secretion induced by blood glucose fluctuations. The exact role of insulin as an autocrine and paracrine participant in the control of its own secretion remains to be determined, mostly due to insufficient knowledge about the molecular phenomena that govern insulin signaling in pancreatic islets. In the present experiments we demonstrate that higher insulin receptor and insulin receptor substrates-1 and -2 (IRS1 and IRS2) concentrations are predominantly encountered in cells of the periphery of rat pancreatic islets, as compared to centrally located cells, and that partial blockade of IRS1 protein expression by antisense oligonucleotide treatment leads to improved insulin secretion induced by glucose overload, which is accompanied by lower steady-state glucagon secretion and blunted glucose-induced glucagon fall. These data reinforce the inhibitory role of insulin upon its own secretion in isolated, undisrupted pancreatic islets.     

Difference in mechanism between glyceraldehyde- and glucose-induced insulin secretion from isolated rat pancreatic islets

The effects of D-glyceraldehyde and glucose on islet function were compared in order to investigate the difference between them in the mechanism by which they induce insulin secretion. The stimulation of insulin secretion from isolated rat islets by 10 mM glyceraldehyde was not completely inhibited by either 150 microM diazoxide (an opener of ATP-sensitive K(+) channels) or 5 microM nitrendipine (an L-type Ca(2+)-channel blocker), whereas the stimulation of insulin secretion by 20 mM glucose was completely inhibited by either drug. The insulin secretion induced by glyceraldehyde was less augmented by 100 microM carbachol (a cholinergic agonist) than that induced by glucose. The stimulation of myo-inositol phosphate production by 100 microM carbachol was more marked in islets incubated with the hexose than with the triose. The content of glyceraldehyde 3-phosphate, a glycolytic intermediate, in islets incubated with glyceraldehyde was far higher than that in islets incubated with glucose, whereas the ATP content in islets incubated with the triose was significantly lower than that in islets incubated with the hexose. These results suggest that glyceraldehyde not only mimics the effect of glucose on insulin secretion but also has the ability to cause the secretion of insulin without the influx of Ca(2+ )through voltage-dependent Ca(2+) channels. The reason for the lower potency of the triose than the hexose in stimulating insulin secretion is also discussed.     

Resistin induces insulin resistance in pancreatic islets to impair glucose-induced insulin release

An adipokine resistin, a small cysteine-rich protein, is one of the major risk factors of insulin resistance. In the present study, transiently resistin-expressing mice using adenovirus method showed an impaired glucose tolerance due to insulin resistance. We found that resistin-expressing mice exhibited impaired insulin secretory response to glucose. In addition, in vitro treatment with resistin for 1 day induced insulin resistance in pancreatic islets and impaired glucose-stimulated insulin secretion by elevating insulin release at basal glucose (2.8 mM) and suppressing insulin release at stimulatory glucose (8.3 mM). In addition, resistin inhibited insulin-induced phosphorylation of Akt in islets as well as other insulin target organs. Furthermore, resistin induced SOCS-3 expression in beta-cells. In conclusion, resistin induces insulin resistance in islet beta-cells at least partly via induction of SOCS-3 expression and reduction of Akt phosphorylation and impairs glucose-induced insulin secretion.     

Dehydroepiandrosterone increases beta-cell mass and improves the glucose-induced insulin secretion by pancreatic islets from aged rats

The effect of dehydroepiandrosterone (DHEA) on pancreatic islet function of aged rats, an animal model with impaired glucose-induced insulin secretion, was investigated. The following parameters were examined: morphological analysis of endocrine pancreata by immunohistochemistry; protein levels of insulin receptor, IRS-1, IRS-2, PI 3-kinase, Akt-1, and Akt-2; and static insulin secretion in isolated pancreatic islets. Pancreatic islets from DHEA-treated rats showed an increased beta-cell mass accompanied by increased Akt-1 protein level but reduced IR, IRS-1, and IRS-2 protein levels and enhanced glucose-stimulated insulin secretion. The present results suggest that DHEA may be a promising drug to prevent diabetes during aging.     

Biotin enhances ATP synthesis in pancreatic islets of the rat, resulting in reinforcement of glucose-induced insulin secretion

Previous studies showed that biotin enhanced glucose-induced insulin secretion. Changes in the cytosolic ATP/ADP ratio in the pancreatic islets participate in the regulation of insulin secretion by glucose. In the present study we investigated whether biotin regulates the cytosolic ATP/ADP ratio in glucose-stimulated islets. When islets were stimulated with glucose plus biotin, the ATP/ADP ratio increased to approximately 160% of the ATP/ADP ratio in islets stimulated with glucose alone. The rate of glucose oxidation, assessed by CO(2) production, was also about 2-fold higher in islets treated with biotin. These increasing effects of biotin were proportional to the effects seen in insulin secretion. There are no previous reports of vitamins, such as biotin, directly affecting ATP synthesis. Our data indicate that biotin enhances ATP synthesis in islets following the increased rate of substrate oxidation in mitochondria and that, as a consequence of these events, glucose-induced insulin release is reinforced by biotin.     

Phosphoinositides turnover and insulin secretion in pancreatic islets

The relationship between glucose-induced insulin secretion and metabolism of inositol phospholipid was investigated by means of an islet perifusion method and direct measuring of inositol phosphates after sonicating the islets. The results showed that the time course of inositol phospholipid breakdown is coincident with the first phase of glucose-induced insulin secretion. Analysis of the effluent perifusate as well as the water soluble inositol-containing substance after sonication of stimulated islets revealed that most of the metabolite of inositol phospholipid is inositol-triphosphate, the hydrolysis product of phosphatidylinositol-4,5-bisphosphate. On the other hand, perifusion of islets with exogenous inositol-triphosphate showed a monophasic and dose-dependent response of insulin secretion. Thus, the initial process of glucose stimulation is accompanied with the formation of inositol-triphosphate, which is a possible candidate for the triggering of first phase insulin secretion.     

Tranilast inhibits glucose-induced insulin secretion from pancreatic beta-cells.

Tranilast, N-(3,4-demethoxycinnamoyl)-anthranilic acid, is an anti-allergic agent identified as an inhibitor of mast cell degranulation. Recently, tranilast was shown to decrease albuminuria in a rat model of diabetic nephropathy and to ameliorate vascular hypertrophy in diabetic rats, suggesting that it may be clinically useful in the treatment of diabetic complications. However, the effects of tranilast on glucose tolerance have not been elucidated. Thus, the aim of this study is to investigate the effect of tranilast on insulin secretion in pancreatic beta-cells. Treatment with tranilast significantly suppressed insulin secretion in INS-1E cells and rat islets induced by 16.7 mmol/l glucose. Furthermore, tranilast inhibited tolbutamide-induced insulin secretion. Treatment with tranilast increased (86)Rb (+) efflux from COS-1 cells in which pancreatic beta-cell-type ATP-sensitive K (+) (K (ATP)) channels were reconstructed and suppressed the cytosolic ATP/ADP ratio in INS-1E cells. Interestingly, treatment with tranilast enhanced glucose uptake in INS-1E cells. In the present study, we demonstrated that tranilast inhibited glucose- and tolbutamide-induced insulin secretion through the activation of K (ATP) channels in pancreatic beta-cells.     

Opiate-prostaglandin interactions in the regulation of insulin secretion from rat islets of Langerhans in vitro

The inadequate insulin secretory response to glucose stimulation in non-insulin dependent diabetes has been attributed to many factors including high PGE2 levels blunting the secretory response, and to the existence of inhibitory opiate activity in vivo. The purpose of the present work was to see if there was a connection between these two independent theories. Radioimmunoassayable PGE2 in islets of Langerhans was found to be proportional to islet number and protein content and was typically 4 to 5pg/micrograms islet protein. Indomethacin (2.8 X 10(-5) M), sodium salicylate (1.25 X 10(-3) M) and chlorpropamide (7.2 X 10(-5) M) all lowered islet PGE2 levels and stimulated insulin release in vitro. Dynorphin (1-13), stimulated insulin release at a concentration of 6 X 10(-9) M, while lowering islet PGE2. Conversely, at a higher concentration, (6 X 10(-7) M), dynorphin had no stimulatory effect on insulin secretion and did not lower PGE2 levels in islets or in the incubation media. The stimulatory effects of dynorphin and sodium salicylate on insulin secretion were blocked by exogenous PGE2 (10(-5) M). PGE2 at a lower concentration (10(-9) M) did not exert any inhibitory effect on dynorphin- or sodium salicylate-induced insulin release. This concentration of exogenous PGE2 stimulated insulin release in the presence of 6mM glucose. Results from these experiments suggest that since an opioid peptide can lower endogenous PGE2 production in islets and since the stimulatory effects of the opioid peptide are reversed by exogenous PGE2 there may be interactions between these two modulators of insulin secretion.     

Characteristics of the biotin enhancement of glucose-induced insulin release in pancreatic islets of the rat

Perifused isolated rat islets were used to show that biotin plus 16.5 mM glucose evoked more insulin secretion than 16.5 mM glucose alone. Whether or not this reinforcement of glucose-induced insulin secretion by biotin is unique was studied by using perifused islets stimulated with 16.5 mM glucose plus 100 microM of one of various components of the vitamin B group. No effect of any of these vitamins was found on glucose-induced insulin secretion. These results indicate that biotin is unique among the members of the vitamin B group in enhancing glucose-induced insulin secretion. Static incubation experiments showed that biotin did not potentiate insulin release when the islets were incubated with an experimental solution containing either no or 2.8 mM glucose. The addition of biotin to 27.7 mM glucose, which is the maximal concentration for stimulating insulin release, did not significantly enhance the effect of the glucose on insulin release (although it did at 16.5 mM glucose). These findings indicate that biotin, by itself, does not stimulate insulin secretion, and does not enhance glucose-induced insulin secretion beyond the ability of glucose itself to stimulate insulin secretion.     

Effect of annexin I on insulin secretion through surface binding sites in rat pancreatic islets

This study investigates the effect of extracellular annexin I (Anx I) on regulating insulin secretion in isolated rat pancreatic islets. Results show that Anx I stimulates insulin release in pancreatic islets regardless of the presence or absence of extracellular Ca2+. In particular, confocal microscopy shows that Anx I binds to the surface of islet cells in the absence of extracellular Ca2+. However, insulin secretion through Anx I significantly decreases in trypsin-treated islets. Likewise, there is minimal binding of Anx I to the surface of trypsin-treated islets. Anti-Anx I polyclonal antibody also inhibits the stimulating effect of Anx I on insulin secretion. These results indicate that Anx I is capable of binding to the cell surface receptor, in order to regulate the stimulation of insulin release in rat pancreatic islets.     

Tacrolimus suppresses glucose-induced insulin release from pancreatic islets by reducing glucokinase activity

Tacrolimus is widely used for immunosuppressant therapy, including various organ transplantations. One of its main side effects is hyperglycemia due to reduced insulin secretion, but the mechanism remains unknown. We have investigated the metabolic effects of tacrolimus on insulin secretion at a concentration that does not influence insulin content. Twenty-four-hour exposure to 3 nM tacrolimus reduced high glucose (16.7 mM)-induced insulin secretion (control 2.14 +/- 0.08 vs. tacrolimus 1.75 +/- 0.02 ng.islet(-1).30 min(-1), P < 0.01) without affecting insulin content. In dynamic experiments, insulin secretion and NAD(P)H fluorescence during a 20-min period after 10 min of high-glucose exposure were reduced in tacrolimus-treated islets. ATP content and glucose utilization of tacrolimus-treated islets in the presence of 16.7 mM glucose were less than in control (ATP content: control 9.69 +/- 0.99 vs. tacrolimus 6.52 +/- 0.40 pmol/islet, P < 0.01; glucose utilization: control 103.8 +/- 6.9 vs. tacrolimus 74.4 +/- 5.1 pmol.islet(-1).90 min(-1), P < 0.01). However, insulin release from tacrolimus-treated islets was similar to that from control islets in the presence of 16.7 mM alpha-ketoisocaproate, a mitochondrial fuel. Glucokinase activity, which determines glycolytic velocity, was reduced by tacrolimus treatment (control 65.3 +/- 3.4 vs. tacrolimus 49.9 +/- 2.8 pmol.islet(-1).60 min(-1), P < 0.01), whereas hexokinase activity was not affected. These results indicate that glucose-stimulated insulin release is decreased by chronic exposure to tacrolimus due to reduced ATP production and glycolysis derived from reduced glucokinase activity.     

Regulation of insulin and glucagon secretion from rat pancreatic islets in vitro by somatostatin analogues

Somatostatin is an inhibitor of hormone secretion through specific receptors (sst1-5). The aim of this study was to investigate the putative regulatory role of somatostatin analogues on the secretion of insulin and glucagon by rat pancreatic islets. After 48 h exposure only the non-selective agonists (somatostatin, octreotide and SOM-230) inhibited insulin accumulation. The inhibition of insulin secretion was accompanied by increased islet insulin contents. None of the analogues showed a consistent effect on the glucagon accumulation in the medium after 48 h. Since we observed a difference in the regulatory effect between the non-selective and selective analogues, combinations of selective analogues were studied. Combination of sst2+sst5 agonists inhibited the medium insulin accumulation, while combination of sst1+sst2 analogues caused a decrease in glucagon accumulation. After removal of somatostatin a rebound effect with increased insulin secretion were observed. This effect was reversed after 6 h. For SOM-230 insulin secretion continued to be suppressed even after the analogue was removed and returned to control values after 3 h. As for glucagon secretion there was an initial decline after culture with octreotide, while the other substances failed to induce any changes. In summary, non-selective somatostatin analogues or combinations of receptor selective analogues may cause inhibition of hormone secretion from rat pancreatic islets. For insulin and glucagon, combinations of sst2+sst5 and sst1+sst2, respectively may exert this effects. Thus, our data suggest that more than one sst must be involved to down-regulate islet glucagon and insulin secretion.     

Leptin suppresses basal insulin secretion from rat pancreatic islets

The effects of leptin on insulin secretion from pancreatic islets of Sprague–Dawley rats were examined in vitro. In a basal glucose medium (5.5 mM), insulin secretion from isolated islets was significantly decreased after addition of a recombinant leptin (80 nM) (3.20±0.14 nmol/10 islets/h) compared with that before the addition (4.41±0.30 nmol/10 islets/h). Although significant leptin suppression of insulin secretion was not observed under a glucose-stimulated (11.1 mM) condition, these results suggest that a negative feedback system may exist between leptin and insulin, which increases the production of leptin from adipose tissues.     

EphA-Ephrin-A-mediated beta cell communication regulates insulin secretion from pancreatic islets

In vertebrates, beta cells are aggregated in the form of pancreatic islets. Within these islets, communication between beta cells inhibits basal insulin secretion and enhances glucose-stimulated insulin secretion, thus contributing to glucose homeostasis during fasting and feeding. In the search for the underlying molecular mechanism, we have discovered that beta cells communicate via ephrin-As and EphAs. We provide evidence that ephrin-A5 is required for glucose-stimulated insulin secretion. We further show that EphA-ephrin-A-mediated beta cell communication is bidirectional: EphA forward signaling inhibits insulin secretion, whereas ephrin-A reverse signaling stimulates insulin secretion. EphA forward signaling is downregulated in response to glucose, which indicates that, under basal conditions, beta cells use EphA forward signaling to suppress insulin secretion and that, under stimulatory conditions, they shift to ephrin-A reverse signaling to enhance insulin secretion. Thus, we explain how beta cell communication in pancreatic islets conversely affects basal and glucose-stimulated insulin secretion to improve glucose homeostasis.     

Phorbol-ester-induced down-regulation of protein kinase C in mouse pancreatic islets. Potentiation of phase 1 and inhibition of phase 2 of glucose-induced insulin secretion.

The influence of down-regulation of protein kinase C on glucose-induced insulin secretion was studied. A 22-24 h exposure of mouse pancreatic islets to the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA; 0.16 microM) in RPMI 1640 culture medium (8.3 mM-glucose, 0.43 mM-Ca2+) abolished TPA (0.16 microM)-induced insulin secretion and led to a potentiation of phase 1 and a decrease in phase 2 of glucose-induced insulin secretion. Thus, although the total insulin release during 40 min of perfusion with glucose (16.7 mM) (45-85 min) was unaffected, the percentage released during phase 1 (45-55 min) was increased from 12.9 +/- 1.5 (4)% in controls to 35.8 +/- 3.9 (4)% in TPA-treated islets (P less than 0.01), and the percentage released during phase 2 (65-85 min) was decreased from 63.2 +/- 3.9 (4)% to 35.3 +/- 1.4 (4)% (P less than 0.005). In contrast, TPA exposure in TCM 199 medium (5.5 mM-glucose, 1.26 mM-Ca2+) caused a total abolition of both phases 1 and 2 of glucose-induced secretion. However, inclusion of the alpha 2-adrenergic agonists adrenaline (10 microM) or clonidine (10 microM), or lowering of the Ca2+ concentration in TCM 199 during down-regulation, preserved and potentiated phase 1 of glucose-induced secretion. Furthermore, perifusion of islets in the presence of staurosporine (1 microM), an inhibitor of protein kinase C, potentiated phase 1 and inhibited phase 2 of glucose-induced secretion. In addition, down-regulation of protein kinase C potentiated phase 1 and inhibited phase 2 of carbamoylcholine (100 microM)-induced insulin secretion at 3.3 mM-glucose, and abolished the potentiating effect of carbamoylcholine (100 microM) at 16.7 mM-glucose. These results substantiate a role for protein kinase C in insulin secretion, and suggest that protein kinase C inhibits phase 1 and stimulates phase 2 of both glucose-induced and carbamoylcholine-induced insulin secretion.