Nesfatin-1 exerts a direct, glucose-dependent insulinotropic action on mouse islet β- and MIN6 cells

Nesfatin-1 is a recently discovered multifunctional metabolic hormone abundantly expressed in the pancreatic islets. The main objective of this study is to characterize the direct effects of nesfatin-1 on insulin secretion in vitro using MIN6 cells and islets isolated from C57BL/6 mice. We also examined the expression of the nesfatin-1 precursor protein, nucleobindin 2 (NUCB2) mRNA, and nesfatin-1 immunoreactivity (ir) in the islets of normal mice and in the islets from mice with streptozotocin-induced type 1 diabetes and diet-induced obese (DIO) mice with type 2 diabetes. Nesfatin-1 stimulated glucose-induced insulin release in vitro from mouse islets and MIN6 cells in a dose-dependent manner. No such stimulation in insulin secretion was found when MIN6 cells/islets were incubated with nesfatin-1 in low glucose. In addition, a fourfold increase in nesfatin-1 release from MIN6 cells was observed following incubation in high glucose (16.7 mM) compared to low glucose (2 mM). Furthermore, we observed a significant reduction in both NUCB2 mRNA expression and nesfatin-1-ir in the pancreatic islets of mice with type 1 diabetes, while a significant increase was observed in the islets of DIO mice. Together, our findings indicate that nesfatin-1 is a novel insulinotropic peptide and that the endogenous pancreatic islet NUCB2/nesfatin is altered in diabetes and diet-induced obesity.     

Nesfatin-1 enhances glucose-induced insulin secretion by promoting Ca(2+) influx through L-type channels in mouse islet β-cells

Nucleobindin-2 (NUCB2)-derived nesfatin-1 located in the brain has been implicated in the satiety and control of energy metabolism. Nesfatin-1 is also produced in the periphery and present in the plasma. It has recently been reported that NUCB2/nesfatin-1 is localized in pancreatic islet β-cells in mice and rats and released from islets. However, its function in islets remains largely unknown. This study examined direct effects of nesfatin-1 on insulin release from pancreatic islets and on cytosolic Ca(2+) concentration ([Ca(2+)](i)) in single β-cells from ICR mice. In the presence of 8.3 mmol/L glucose, nesfatin-1 at 10(-10)-10(-9) mol/L tended to increase and at 10(-8) mol/L increased insulin release from isolated islets, while at 2.8 mmol/L glucose nesfatin-1 had no effect. Furthermore, nesfatin-1 at 10(-10)-10(-8) mol/L increased [Ca(2+)](i) in single β-cells in the presence of 8.3 but not 2.8 mmol/L glucose. The nesfatin-1-induced [Ca(2+)](i) increase and insulin release were inhibited by removal of extracellular Ca(2+) and by addition of nitrendipine, a blocker of voltage-dependent L-type Ca(2+) channels. Unexpectedly, the [Ca(2+)](i) responses to nesfatin-1 were unaltered by inhibitors of protein kinase A (PKA) and phospholipase A(2) (PLA(2)). These results indicate that nesfain-1 potentiates glucose-induced insulin secretion by promoting Ca(2+) influx through L-type Ca(2+) channels independently of PKA and PLA(2) in mouse islet β-cells.     

Bombesin receptor subtype-3 (BRS-3) regulates glucose-stimulated insulin secretion in pancreatic islets across multiple species

Bombesin receptor subtype-3 (BRS-3) regulates energy homeostasis, and BRS-3 agonism is being explored as a possible therapy for obesity. Here we study the role of BRS-3 in the regulation of glucose-stimulated insulin secretion (GSIS) and glucose homeostasis. We quantified BRS-3 mRNA in pancreatic islets from multiple species and examined the acute effects of Bag-1, a selective BRS-3 agonist, on GSIS in mouse, rat, and human islets, and on oral glucose tolerance in mice. BRS-3 is highly expressed in human, mouse, rhesus, and dog (but not rat) pancreatic islets and in rodent insulinoma cell lines (INS-1 832/3 and MIN6). Silencing BRS-3 with small interfering RNA or pharmacological blockade with a BRS-3 antagonist, Bantag-1, reduced GSIS in 832/3 cells. In contrast, the BRS-3 agonist (Bag-1) increased GSIS in 832/3 and MIN6 cells. The augmentation of GSIS by Bag-1 was completely blocked by U73122, a phospholipase C inhibitor. Bag-1 also enhanced GSIS in islets isolated from wild-type, but not Brs3 knockout mice. In vivo, Bag-1 reduced glucose levels during oral glucose tolerance test in a BRS-3-dependent manner. BRS-3 agonists also increased GSIS in human islets. These results identify a potential role for BRS-3 in islet physiology, with agonism directly promoting GSIS. Thus, in addition to its potential role in the treatment of obesity, BRS-3 may also regulate blood glucose levels and have a role in the treatment of diabetes mellitus.     

Differential regulation of mouse pancreatic islet insulin secretion and Smad proteins by activin ligands

Aims/hypothesis

Glucose-stimulated insulin secretion (GSIS) from pancreatic beta cells is regulated by paracrine factors, the identity and mechanisms of action of which are incompletely understood. Activins are expressed in pancreatic islets and have been implicated in the regulation of GSIS. Activins A and B signal through a common set of intracellular components, but it is unclear whether they display similar or distinct functions in glucose homeostasis.

Methods

We examined glucose homeostatic responses in mice lacking activin B and in pancreatic islets derived from these mutants. We compared the ability of activins A and B to regulate downstream signalling, ATP production and GSIS in islets and beta cells.

Results

Mice lacking activin B displayed elevated serum insulin levels and GSIS. Injection of a soluble activin B antagonist phenocopied these changes in wild-type mice. Isolated pancreatic islets from mutant mice showed enhanced GSIS, which could be rescued by exogenous activin B. Activin B negatively regulated GSIS and ATP production in wild-type islets, while activin A displayed the opposite effects. The downstream mediator Smad3 responded preferentially to activin B in pancreatic islets and beta cells, while Smad2 showed a preference for activin A, indicating distinct signalling effects of the two activins. In line with this, overexpression of Smad3, but not Smad2, decreased GSIS in pancreatic islets.

Conclusions/interpretation

These results reveal a tug-of-war between activin ligands in the regulation of insulin secretion by beta cells, and suggest that manipulation of activin signalling could be a useful strategy for the control of glucose homeostasis in diabetes and metabolic disease.     

Glucose uptake and glycolytic flux in adipose tissue from rats adapted to a high-protein, carbohydrate-free diet

Rates of glucose uptake by epididymal and retroperitoneal adipose tissue in vivo, as well as rates of hexose uptake and glycolytic flux in isolated adipocytes, were determined in rats adapted to a high-protein, carbohydrate-free (HP) diet and in control rats fed a balanced (N) diet. Adaptation to the HP diet induced a significant reduction in rates of glucose uptake, estimated with 2-deoxy-[1-(3)H]-glucose, both by adipose tissue (epididymal and retroperitoneal) in vivo and by isolated adipocytes. Twelve hours after replacement of the HP diet with the balanced diet, rates of adipose tissue uptake in vivo in HP-adapted rats returned to levels that did not differ significantly from those in N-fed rats. The rate of flux in the glycolytic pathway, estimated with (3)H[5]-glucose, was also significantly reduced in adipocytes from HP-fed rats. In agreement with the above findings, the activities of hexokinase (HK), phosphofructo-1-kinase (PFK-1), and pyruvate kinase (PK) were markedly reduced in adipose tissue from HP-adapted rats. The activity of pyruvate kinase was partially reverted by diet replacement for 12 hours. The low-plasma insulin and high-glucagon levels in HP-fed rats may have played an important role in the reduction of adipose tissue glucose utilization in these animals.     

Glucose-dependent and glucose-sensitizing insulinotropic effect of nateglinide: comparison to sulfonylureas and repaglinide

Nateglinide, a novel D-phenylalanine derivative, stimulates insulin release via closure of K(ATP) channels in pancreatic beta-cell, a primary mechanism of action it shares with sulfonylureas (SUs) and repaglinide. This study investigated (1) the influence of ambient glucose levels on the insulinotropic effects of nateglinide, glyburide and repaglinide, and (2) the influence of the antidiabetic agents on glucose-stimulated insulin secretion (GSIS) in vitro from isolated rat islets. The EC50 of nateglinide to stimulate insulin secretion was 14 microM in the presence of 3 mM glucose and was reduced by 6-fold in 8 mM glucose and by 16-fold in 16 mM glucose, indicating a glucose-dependent insulinotropic effect. The actions of glyburide and repaglinide failed to demonstrate such a glucose concentration-dependent sensitization. When tested at fixed and equipotent concentrations (approximately 2x EC50 in the presence of 8 mM glucose) nateglinide and repaglinide shifted the EC50s for GSIS to the left by 1.7 mM suggesting an enhancement of islet glucose sensitivity, while glimepiride and glyburide caused, respectively, no change and a right shift of the EC50. These data demonstrate that despite a common basic mechanism of action, the insulinotropic effects of different agents can be influenced differentially by ambient glucose and can differentially influence the islet responsiveness to glucose. Further, the present findings suggest that nateglinide may exert a more physiologic effect on insulin secretion than comparator agents and thereby have less propensity to elicit hypoglycemia in vivo.     

Endogenous islet uncoupling protein-2 expression and loss of glucose homeostasis in ob/ob mice

We hypothesized that the loss of glucose homeostasis in ob/ob mice is associated with upregulation of islet uncoupling protein-2 (UCP2) expression, leading to impaired glucose-stimulated insulin secretion (GSIS). Changes in glucose homeostasis in lean and ob/ob mice from 5 to 16 weeks were assessed by fasting blood glucose, plasma insulin, oral glucose tolerance, and tissue insulin sensitivity. In vitro GSIS and ATP content were assayed in isolated islets, while UCP2 expression was determined by quantitative real-time PCR and immunoblotting. Short-term reduction of UCP2 expression was achieved through transfection of islets with specific small interfering RNA. Insulin resistance was detected in 5-week-old ob/ob mice, but GSIS and blood glucose levels remained normal. By 8 weeks of age, ob/ob mice displayed fasting hyperglycemia, hyperinsulinemia and glucose intolerance, and also had elevated non-esterified fatty acid concentration in plasma. In vitro, GSIS and ATP generation were impaired in ob/ob islets. Islet UCP2 expression was elevated at 5 and 8 weeks of age. Short-term knockdown of islet UCP2 increased GSIS in islets of lean mice, but had no effect in islets from ob/ob mice. Loss of glucose homeostasis and impairment of insulin secretion from isolated islets at 8 weeks in ob/ob mice is preceded by an increase in UCP2 expression in islets. Moreover, the glucolipotoxic conditions observed are predicted to increase UCP2 activity, contributing to lower islet ATP and GSIS.     

Peroxisome proliferator-activated receptor-alpha activation during pregnancy attenuates glucose-stimulated insulin hypersecretion in vivo by increasing insulin sensitivity,without impairing pregnancy-induced increases in beta-cell glucose sensing and responsiveness

We investigated the effects of acute (24-h) peroxisome proliferator-activated receptor (PPAR)alpha activation by WY14,643 (pirinixic acid) treatment on glucose-stimulated insulin secretion (GSIS) during pregnancy, in the rat, in relation to insulin sensitivity. GSIS after iv glucose challenge (500 mg/kg) was increased at d 15 of pregnancy but was attenuated by WY14,643 treatment in vivo, with decreases in acute insulin response (51%; P < 0.001) and total suprabasal 30-min area under the insulin curve (deltaI) (55%; P < 0.001). GSIS was unaffected by WY14,643 treatment in unmated rats. Islet perifusions were employed to identify persistent effects of PPARalpha activation. GSIS was enhanced, and the glucose threshold was reduced in perifused islets from pregnant rats, but WY14,643 treatment failed to reverse these effects. WY14,643 treatment of 15-d-pregnant rats significantly lowered (by 63%; P < 0.01) the insulin resistance index [total suprabasal 30-min area under insulin curve x suprabasal 30-min area under glucose curve (deltaI x deltaG)]. A strong positive linear relationship (r = 0.92) between acute insulin response and deltaI x deltaG was evident between groups. Our studies show that acute PPARalpha activation reverses the augmented GSIS evoked by pregnancy in vivo, whereas the isolated islets retain pregnancy-induced enhancement of beta-cell glucose sensing and responsiveness. Normalization of maternal GSIS to that found in the nonpregnant state is observed in association with alleviation of maternal insulin resistance.     

Acute (24 h) activation of peroxisome proliferator-activated receptor-alpha (PPARalpha) reverses high-fat feeding-induced insulin hypersecretion in vivo and in perifused pancreatic islets

Abnormal depletion or accumulation of islet lipid may be important for the development of pancreatic beta cell failure. Long-term lipid sensing by beta cells may be co-ordinated via peroxisome proliferator-activated receptors (PPARs). We investigated whether PPARalpha activation in vivo for 24 h affects basal and glucose-stimulated insulin secretion in vivo after intravenous glucose administration and ex vivo in isolated perifused islets. Insulin secretion after intravenous glucose challenge was greatly increased by high-fat feeding (4 weeks) but glucose tolerance was minimally perturbed, demonstrating insulin hypersecretion compensated for insulin resistance. The effect of high-fat feeding to enhance glucose-stimulated insulin secretion was retained in perifused islets demonstrating a stable, long-term effect of high-fat feeding to potentiate islet glucose stimulus-secretion coupling. Treatment of high-fat-fed rats with WY14,643 for 24 h reversed insulin hypersecretion in vivo without impairing glucose tolerance, suggesting improved insulin action, and ex vivo in perfused islets. PPARalpha activation only affected hypersecretion of insulin since glucose-stimulated insulin secretion was unaffected by WY14,643 treatment in vivo in control rats or in perifused islets from control rats. Our data demonstrate that activation of PPARalpha for 24 h can oppose insulin hypersecretion elicited by high-fat feeding via stable long-term effects exerted on islet function. PPARalpha could, therefore, participate in ameliorating abnormal glucose homeostasis and hyperinsulinaemia in dietary insulin resistance via modulation of islet function, extending the established requirement for PPARalpha for normal islet lipid homeostasis.     

Leptin secretion and negative autocrine crosstalk with insulin in brown adipocytes

Leptin is an important adipocytokine whose main regulative effects on energy metabolism are exerted via activation of signalling pathways in the central nervous system. Another important regulator of energy homeostasis is insulin. The role of direct autocrine leptin effects on adipose tissue and crosstalk with insulin, in particular in the thermogenically active brown adipose tissue, remains unclear. In the present study, we have investigated leptin secretion and interaction with insulin in highly insulin-responsive immortalised mouse brown adipocytes. Leptin was secreted in a differentiation-dependent manner, and acute leptin treatment of mature adipocytes dose- and time-dependently stimulated phosphorylation of STAT3 and MAP kinase. Interestingly, acute pretreatment of fully differentiated brown adipocytes with leptin (100 nM) significantly diminished insulin-induced glucose uptake by approximately 25%. This inhibitory effect was time-dependent and maximal after 60 min of leptin prestimulation. Furthermore, it correlated with a 35% reduction in insulin-stimulated insulin receptor kinase activity after acute leptin pretreatment. Insulin-induced insulin receptor substrate-1 tyrosine phosphorylation and binding to the regulatory subunit p85 of phosphatidylinositol 3-kinase (PI 3-kinase) were diminished by approximately 60% and 40%, respectively. Taken together, this study has demonstrated strong differentiation-dependent leptin secretion in brown adipocytes and PI 3-kinase-mediated negative autocrine effects of this hormone on insulin action. Direct peripheral leptin-insulin crosstalk may play an important role in the regulation of energy homeostasis.     

Mechanism of norepinephrine stimulation of glucose transport in isolated rat brown adipocytes

Cold exposure reverses the diabetogenic effects of high-fat feeding and markedly stimulates glucose uptake in rat brown adipose tissue (BAT). Considering that cold exposure increases the plasma levels of norepinephrine and lipolytic hormones, but decreases the levels of insulin, we have examined the effects of these agents on glucose transport in isolated rat brown adipocytes using D-[U-14C]glucose as a tracer. It was found that norepinephrine (0.1 microM), glucagon (0.1 nM) and ACTH (100 nM) all increased brown adipocyte respiration (2-10 times) and glucose transport (2-5 times). Studies with adrenergic agonists and antagonists revealed that norepinephrine increases glucose uptake via beta-adrenergic pathways. On the other hand, insulin also increased glucose transport (6 times) but inhibited (40-60 percent) the calorigenic effects of the lipolytic hormones. Both norepinephrine and glucagon potentiated the submaximal insulin responses for glucose transport, demonstrating the existence of metabolic interactions between norepinephrine-, glucagon-, and insulin-mediated glucose uptake. Remarkably, the stimulatory effects of these lipolytic agents were reproduced by dibutyryl cAMP (1 mM), isobutylmethylxanthine (0.1 mM) and palmitic acid (0.5 mM), suggesting that cAMP increases glucose transport via activation of lipolysis and thermogenesis. Considering that the stimulatory effects of norepinephrine (0.1 microM) on respiration and glucose transport were totally blocked by 2-tetradecylglycidic acid (50 microM), a specific inhibitor of mitochondrial carnitine acyl transferase, it is concluded that norepinephrine increases BAT glucose transport via fatty acid-activation of mitochondrial thermogenesis.     

Durable islet effects on insulin secretion and protein kinase A expression following exendin-4 treatment of high-fat diet-fed mice

Glucagon-like peptide 1 (GLP-1) augments glucose-stimulated insulin secretion (GSIS) through cAMP-induced activation of protein kinase A (PKA), and stimulates beta-cell proliferation and reduces beta-cell apoptosis in rodent islets. This study explored islet GSIS, PKA expression, and markers of apoptosis (caspase 3/7 activity) and proliferation (PKBalpha and pancreatic and duodenal homeobox gene 1, Pdx-1) after 2 weeks of treatment with the GLP-1 receptor agonist exendin-4 (2 nmol/kg once daily) in female mice with high-fat diet-induced insulin resistance (HFD; 58% fat by energy). Islets were isolated 20 h after the last exendin-4 injection, when effects of circulating exendin-4 had vanished. The glucose responsiveness in islets from HFD-fed mice at 8.3 mM glucose was reduced compared with islets from control mice fed a normal diet due to increased basal insulin secretion. However, GSIS increased in islets from HFD-fed exendin-4-treated animals (0.124+/-0.012 ng/h per islet in HFD-Ex-4 versus 0.062+/-0.010 in HFD, P=0.006). Furthermore, the insulin response to forskolin was increased (2.7+/-0.3 in HFD-Ex-4 versus 2.0+/-0.2 ng/h per islet in HFD, P=0.011) and PKAcat expression was increased, while PKAreg was reduced in islets from exendin-4-treated mice. In contrast, protein expression of PKBalpha, Pdx-1, and caspase 3/7 activity was not affected by exendin-4 treatment. We conclude that GLP-1 receptor activation in HFD-fed mice has durable effects on GSIS, in association with augmented signaling through the PKA pathway. These effects are seen beyond those induced by circulating exendin-4 already after 2 weeks of once-daily treatment in mice, whereas markers for islet proliferation and apoptosis were unaffected by this treatment.     

Cellular distribution, regulated expression, and functional role of the anorexigenic peptide, NUCB2/nesfatin-1, in the testis

Nesfatin-1, product of the precursor NEFA/nucleobindin2 (NUCB2), was initially identified as anorectic hypothalamic neuropeptide, acting in a leptin-independent manner. In addition to its central role in the control of energy homeostasis, evidence has mounted recently that nesfatin-1 is also produced in peripheral metabolic tissues, such as pancreas, adipose, and gut. Moreover, nesfatin-1 has been shown to participate in the control of body functions gated by whole-body energy homeostasis, including puberty onset. Yet, whether, as is the case for other metabolic neuropeptides, NUCB2/nesfatin-1 participates in the direct control of gonadal function remains unexplored. We document here for the first time the expression of NUCB2 mRNA in rat, mouse, and human testes, where NUCB2/nesfatin-1 protein was identified in interstitial mature Leydig cells. Yet in rats, NUCB2/nesfatin-1 became expressed in Sertoli cells upon Leydig cell elimination and was also detected in Leydig cell progenitors. Although NUCB2 mRNA levels did not overtly change in rat testis during pubertal maturation and after short-term fasting, NUCB2/nesfatin-1 content significantly increased along the puberty-to-adult transition and was markedly suppressed after fasting. In addition, testicular NUCB2/nesfatin-1 expression was up-regulated by pituitary LH, because hypophysectomy decreased, whereas human choriogonadotropin (super-agonist of LH receptors) replacement enhanced, NUCB2/nesfatin-1 mRNA and peptide levels. Finally, nesfatin-1 increased human choriogonadotropin-stimulated testosterone secretion by rat testicular explants ex vivo. Our data are the first to disclose the presence and functional role of NUCB2/nesfatin-1 in the testis, where its expression is regulated by developmental, metabolic, and hormonal cues as well as by Leydig cell-derived factors. Our observations expand the reproductive dimension of nesfatin-1, which may operate directly at the testicular level to link energy homeostasis, puberty onset, and gonadal function.     

Modulation of β-cell function: a translational journey from the bench to the bedside

Both decreased insulin secretion and action contribute to the pathogenesis of type 2 diabetes (T2D) in humans. The insulin receptor and insulin signalling proteins are present in the rodent and human β-cell and modulate cell growth and function. Insulin receptors and insulin signalling proteins in β-cells are critical for compensatory islet growth in response to insulin resistance. Rodents with tissue-specific knockout of the insulin receptor in the β-cell (βIRKO) show reduced first-phase glucose-stimulated insulin secretion (GSIS) and with aging develop glucose intolerance and diabetes, phenotypically similar to the process seen in human T2D. Expression of multiple insulin signalling proteins is reduced in islets of patients with T2D. Insulin potentiates GSIS in isolated human β-cells. Recent studies in humans in vivo show that pre-exposure to insulin increases GSIS, and this effect is diminished in persons with insulin resistance or T2D. β-Cell function correlates to whole-body insulin sensitivity. Together, these findings suggest that pancreatic β-cell dysfunction could be caused by a defect in insulin signalling within β-cell, and β-cell insulin resistance may lead to a loss of β-cell function and/or mass, contributing to the pathophysiology of T2D.     

Ethanol influence on calcium uptake and insulin release by rat islets

Effect of acute ethanol treatment on simultaneous 45Ca++ uptake and insulin response to glucose was measured in isolated rat pancreatic islets. Ethanol, given ip 1 gm/Kg 1 h prior to sacrifice of the animal, decreased significantly 45Ca++ uptake and insulin response to 8.3 and 16.7 mM glucose. Addition of ethanol to the incubation media inhibited 45Ca++ uptake and insulin release in a dose-related matter. Ionophore A23187, which is known to enhance 45Ca++ efflux, decreased 45Ca++ uptake without affecting insulin release. Inhibitory effects of ethanol and ionophore A23187 were not additive when islets were exposed to both test substances simultaneously. Forskolin, an activator of the adenylate cyclase system potentiated the glucose mediated insulin response in rat islets. However, ethanol decreased the insulin response of islets exposed to glucose and forskolin. The data show that ethanol inhibits 45Ca++ uptake response to glucose and that ethanol influence on insulin release may involve a site beyond the formation of cyclic AMP in the process of excitation-secretion coupling.     

Loss of Bmal1 leads to uncoupling and impaired glucose-stimulated insulin secretion in β-cells

The circadian clock has been shown to regulate metabolic homeostasis. Mice with a deletion of Bmal1, a key component of the core molecular clock, develop hyperglycemia and hypoinsulinemia, suggesting β-cell dysfunction. However, the underlying mechanisms are not fully known. In this study, we investigated the mechanisms underlying the regulation of β-cell function by Bmal1. We studied β-cell function in global Bmal1-/- mice, in vivo and in isolated islets ex vivo, as well as in rat insulinoma cell lines with shRNA-mediated Bmal1 knockdown. Global Bmal1-/- mice develop diabetes secondary to a significant impairment in glucose-stimulated insulin secretion (GSIS). There is a blunting of GSIS in both isolated Bmal1-/- islets and in Bmal1 knockdown cells, as compared to controls, suggesting that this is secondary to a loss of cell-autonomous effect of Bmal1. In contrast to previous studies, in these Bmal1-/- mice on a C57Bl/6 background, the loss of stimulated insulin secretion, interestingly, is with glucose but not to other depolarizing secretagogues, suggesting that events downstream of membrane depolarization are largely normal in Bmal1-/- islets. This defect in GSIS occurs as a result increased mitochondrial uncoupling with consequent impairment of glucose-induced mitochondrial potential generation and ATP synthesis, due to an upregulation of Ucp2. Inhibition of Ucp2, in isolated islets, leads to a rescue of the glucose-induced ATP production and insulin secretion in Bmal1-/- islets. Thus, Bmal1 regulates mitochondrial energy metabolism to maintain normal GSIS and its disruption leads to diabetes due to a loss of GSIS.     

Leptin suppression of insulin secretion and gene expression in human pancreatic islets: implications for the development of adipogenic diabetes mellitus

Previously we demonstrated the expression of the long form of the leptin receptor in rodent pancreatic beta-cells and an inhibition of insulin secretion by leptin via activation of ATP-sensitive potassium channels. Here we examine pancreatic islets isolated from pancreata of human donors for their responses to leptin. The presence of leptin receptors on islet beta-cells was demonstrated by double fluorescence confocal microscopy after binding of a fluorescent derivative of human leptin (Cy3-leptin). Leptin (6.25 nM) suppressed insulin secretion of normal islets by 20% at 5.6 mM glucose. Intracellular calcium responses to 16.7 mM glucose were rapidly reduced by leptin. Proinsulin messenger ribonucleic acid expression in islets was inhibited by leptin at 11.1 mM, but not at 5.6 mM glucose. Leptin also reduced proinsulin messenger ribonucleic acid levels that were increased in islets by treatment with 10 nM glucagon-like peptide-1 in the presence of either 5.6 or 11.1 mM glucose. These findings demonstrate direct suppressive effects of leptin on insulin-producing beta-cells in human islets at the levels of both stimulus-secretion coupling and gene expression. The findings also further indicate the existence of an adipoinsular axis in humans in which insulin stimulates leptin production in adipocytes and leptin inhibits the production of insulin in beta-cells. We suggest that dysregulation of the adipoinsular axis in obese individuals due to defective leptin reception by beta-cells may result in chronic hyperinsulinemia and may contribute to the pathogenesis of adipogenic diabetes.     

Leptin inhibition of insulin secretion from isolated human islets

Leptin is a hormone produced and secreted from the adipose tissue. Its physiological actions include the regulation of satiety, food intake and energy balance. The production of leptin is increased by high insulin levels. Here, we demonstrate that leptin acts as an inhibitor of glucose-induced (20 mM) insulin secretion from isolated human islets. No effect was observed in the presence of lower glucose levels (2.8 and 10 mM glucose). The pancreatic β-cell might represent a target of a direct physiological action of leptin. We suggest the presence of an “adipo-insular axis” in which leptin mediates negative feedback from the adipose tissue to the endocrine pancreas. Received: 21 July 1997 / Accepted in revised form: 1 October 1997     

Adaptive response in beta-cell function in pancreatic islets isolated from partially pancreatectomized rats.

Before clinical onset of insulin-dependent diabetes mellitus a decreasing pancreatic beta-cell mass maintains glucose homeostasis. We currently aimed to study the function of pancreatic islets isolated 2 weeks after a 60% partial pancreatectomy (P) or after a sham operation (S) on adult rats. Experiments on the islets were subsequently performed acutely (day 0) and after 1 week (day 7) of tissue culture in medium RPMI 1640 (11.1 mM glucose) + 10% calf serum. There was no difference in the body weight 2 weeks after surgery. The pancreatic remnant weight of the P rats was 35% less than the pancreatic weight in the S rats. The islet DNA content was 25% higher in the islets of the P rats on day 0, indicating a stimulated islet growth. However, this difference did not remain after culture for 7 days. Islet proinsulin mRNA content and (pro)insulin biosynthesis rates were slightly increased in the islets of P rats on day 0, which could be due to the increased islet mass. The islet insulin content was not different on day 0, but was higher after culture in the islets of the P rats. The islet rates of glucose oxidation and insulin release were markedly higher in the P rats on day 0, suggesting a selective effect on these processes. A higher glucose oxidation rate was, however, not evident on day 7. The relative fraction of insulin-positive cells was slightly lowered in the islets of the P rats on day 0.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of the amino-terminal portion of human growth hormone on glucose clearance and metabolism in normal, diabetic, hypophysectomized, and diabetic-hypophysectomized rats

A naturally occurring pituitary peptide, human (h) GH-(1-43) potentiates insulin action. The present study has compared the effects of acute (30-60 min) and chronic (3-6 days) injections of synthetic hGH-(1-43), hGH, and insulin in normal, diabetic, hypophysectomized, and diabetic-hypophysectomized rats. Male rats (150-250 g) received injections of saline, insulin (50-200 mU), hGH (200 micrograms), or hGH-(1-43) (200-400 micrograms) with or without insulin. Hormone and glucose were injected simultaneously for glucose tolerance tests. Basal and insulin-stimulated [U-14C]glucose oxidation to 14CO2 in adipose tissue were measured in vitro after in vivo treatments; insulin release by isolated pancreatic islets was determined in vitro. Acute injections of hGH-(1-43) with insulin dramatically increased glucose clearance in diabetic (P less than 0.05) and hypophysectomized (P less than 0.01) rats. In diabetic-hypophysectomized rats acute injections of hGH-(1-43) significantly lowered the elevated basal blood glucose level (P less than 0.025) and stimulated [U-14C]glucose oxidation to 14CO2 in adipose tissue (P less than 0.05); it did not increase the glucose clearance rate during glucose administration. Chronic treatment of diabetic rats with hGH-(1-43) did not lower the elevated blood glucose level significantly, but it stimulated [U-14C]glucose oxidation to 14CO2 in adipose tissue; the oxidation was further stimulated by treatment with insulin. Chronic injections of hGH-(1-43) slightly lowered blood glucose levels in hypophysectomized rats (P less than 0.025) despite a diminished release in vitro of insulin from pancreatic islets (P less than 0.05). Therefore, these experiments show hGH-(1-43) to be an insulin potentiator that increases insulin-stimulated glucose clearance and glucose oxidation without an increase in insulin secretion, and they suggest that the peptide may have a physiological role in regulating carbohydrate metabolism.