Increased Plasma Glucagon Levels Research Articles

Gluco-metabolic response to exogenous oxytocin in totally pancreatectomized patients and healthy individuals

Oxytocin has been proposed to possess glucose-stabilizing effects through the release of insulin and glucagon from the pancreas. Also, exogenous oxytocin has been shown to stimulate extrapancreatic glucagon secretion in depancreatized dogs. Here, we investigated the effect of exogenous oxytocin on circulating levels of pancreatic and gut-derived glucose-stabilizing hormones (insulin [measured as C-peptide], glucagon, glucagon-like peptide 1 [GLP-1], and glucose-dependent insulinotropic polypeptide). We studied nine pancreatectomized (PX) patients and nine healthy controls (CTRLs) (matched on age and body mass index) before, during, and after an intravenous infusion of 10 IU of oxytocin administered over 12 min. Oxytocin did not increase plasma glucagon levels, nor induce any changes in plasma glucose, C-peptide, or GIP in any of the groups. Oxytocin decreased plasma glucagon levels by 19 ± 10 % in CTRLs (from 2.0 ± 0.5 [mean ± SEM] to 1.3 ± 0.2 pmol/l, P = 0.0025) and increased GLP-1 by 42 ± 22 % in PX patients (from 9.0 ± 1.0–12.7 ± 1.0 pmol/l, P = 0.0003). Fasting plasma glucose levels were higher in PX patients compared with CTRLs (13.1 ± 1.1 vs. 5.1 ± 0.1 mmol/l, P < 0.0001). In conclusion, the present findings do not support pancreas-mediated glucose-stabilizing effects of acute oxytocin administration in humans and warrant further investigation of oxytocin’s gluco-metabolic effects.

Role of Glucagon in The Metabolic Response

Historically, glucagon is the counter-regulatory hormone of insulin. Glucagon secretion is induced by fasting conditions or hypoglycaemia to increase glucose levels. Glucagon is the dominant product of alpha cells in the islet and was first identified in 1923 during an attempt to purify insulin, where it was identified as a contaminant hyperglycaemia factor. Further research determined that the hyperglycaemic action of glucagon is mediated by increased hepatic glycogenolysis and gluconeogenesis to increase endogenous glucose production. Insulin and glucagon as opposing hormones work together for glycaemic control. Diabetic hyperglycaemia is caused by increased impaired insulin action and inappropriately elevated glucagon levels. This review summarizes an important function of glucagon is its role as a regulator of glucose homeostasis. Increased plasma glucagon levels lead to increased hepatic glucose production. The balance between insulin and glucagon is responsible for maintaining euglycaemia conditions. In conditions of hypoglycaemia, increased glucagon secretion leads to increased hepatic glucose production through a number of cellular mechanisms including suppression of glycogenesis and glycolysis and stimulation of glycogenolysis and gluconeogenesis

Urotensin II receptor knockout alleviates streptozotocin‐induced hyperglucagonemia and diabetic kidney disease

Plasma and urinary levels of UII, a potent vasoactive peptide, are elevated in diabetes mellitus (DM) patients. UII receptor (UT) expression levels are also increased in the kidneys of humans and animals with DM. Palosuran, an orally active UT antagonist, increased insulin levels and improved renal function in uninephrectomized streptozotocin (STZ)‐treated rats. However, human studies on the potential use of palosuran for kidney protection in diabetes were inconclusive. In addition to UT antagonism, palosuran can activate somatostatin receptors. Also, the reduced affinity of palosuran for UT in intact cells and tissues suggests that it may not be an optimal UT antagonist. Thus, using non‐selective and less potent UT antagonists constitutes a weakness in the scientific rigor of the prior attempts to dissect UT as a potential therapeutic target in DKD. Hence, studies using genetic animal models and highly selective and potent pharmacological antagonists are needed to fill the gaps of understanding on the pathophysiological significance of the UII system in DM. In the present study, we examine the development of hyperglycemia and DKD in the wild‐type (WT) and UT knockout (KO) mouse model of type 1 DM.STZ‐treated mice showed increased blood glucose levels, glucosuria, decreased plasma insulin levels, and diabetic kidney disease (DKD), which UT KO attenuated. However, UT KO failed to reverse STZ‐induced insulin deficiency. To understand the underlying mechanism for alleviating hyperglycemia and DKD in UT KO despite the uncorrected insulin deficiency, we tested the role of the primary insulin counterregulatory hormone, glucagon. We found that STZ treatment increased plasma glucagon levels in WT mice but not in UT KO mice. This finding was supported by double immunohistochemistry of pancreatic islet sections, where STZ treatment reduced insulin and increased glucagon staining. UT KO reversed only the effect of STZ on glucagon staining. These findings suggest that UT KO protects the glucagon‐producing alpha cells but not the insulin‐producing beta cells.UII increased glucagon secretion in a cultured pancreatic alpha cell line, which UT inhibition reversed. Next, to determine how UII triggers glucagon secretion, we performed membrane potential recordings in alpha cells using the perforated‐patch clamp technique. UII produced depolarization of alpha cells. This effect was abolished by UT inhibition. Calcium (Ca2+) imaging demonstrated that UII stimulates intracellular Ca2+ ([Ca2+]i) elevation in alpha cells, which T‐ and L‐type Ca2+ channel blockers attenuated.Taken together, the results of this study suggest that: (1) STZ‐treatment causes insulin deficiency, hyperglycemia, glucosuria, DKD, and hyperglucagonemia. (2) UT KO reverses STZ effects, except insulin deficiency. (3) UII increases glucagon secretion by alpha cells, mediated by UII‐induced membrane depolarization and Ca2+ influx via T‐ and L‐type Ca2+ channels. Therefore, the reversal of hyperglycemia, glucosuria, and DKD in UT KO might be linked to the reversal of hyperglucagonemia.

Disordered branched chain amino acid catabolism in pancreatic islets is associated with postprandial hypersecretion of glucagon in diabetic mice

Dysregulation of glucagon is associated with the pathophysiology of type 2 diabetes. We previously reported that postprandial hyperglucagonemia is more obvious than fasting hyperglucagonemia in type 2 diabetes patients. However, which nutrient stimulates glucagon secretion in the diabetic state and the underlying mechanism after nutrient intake are unclear. To answer these questions, we measured plasma glucagon levels in diabetic mice after oral administration of various nutrients. The effects of nutrients on glucagon secretion were assessed using islets isolated from diabetic mice and palmitate-treated islets. In addition, we analyzed the expression levels of branched chain amino acid (BCAA) catabolism-related enzymes and their metabolites in diabetic islets. We found that protein, but not carbohydrate or lipid, increased plasma glucagon levels in diabetic mice. Among amino acids, BCAAs, but not the other essential or nonessential amino acids, increased plasma glucagon levels. BCAAs also directly increased the intracellular calcium concentration in α cells. When BCAAs transport was suppressed by an inhibitor of system L-amino acid transporters, glucagon secretion was reduced even in the presence of BCAAs. We also found that the expression levels of BCAA catabolism-related enzymes and their metabolite contents were altered in diabetic islets and palmitate-treated islets compared to control islets, indicating disordered BCAA catabolism in diabetic islets. Furthermore, BCKDK inhibitor BT2 suppressed BCAA-induced hypersecretion of glucagon in diabetic islets and palmitate-treated islets. Taken together, postprandial hypersecretion of glucagon in the diabetic state is attributable to disordered BCAA catabolism in pancreatic islet cells.

133-OR: Somatostatin Receptor Type 2 Antagonism (SSTR2a) Augments Glucagon Counterregulation and Delays Hypoglycemia Onset in Type 2 Diabetic (T2D), Sprague-Dawley Rats

Background: SSTR2a rescues glucagon counterregulation in type 1 diabetic rats. Here, we investigated its potential for resisting hypoglycemic onset in insulin-treated rats with T2D.Methods: T2D was induced in 8-9-week-old, male, Sprague-Dawley rats (n=20) via 3-weeks of high-fat feeding and low-dose streptozotocin (35 mg/kg). After 1 week of basal insulin maintenance, N=10 healthy and N=20 T2D rats were treated with SSTR2a (3 mg/kg ZT-01, SC) or vehicle in a crossover design (1 wk apart), 60 min prior to hypoglycemic induction via insulin aspart bolus (6 U/kg).Results: SSTR2a increased plasma glucagon levels for up to 120 min after insulin challenge, with a 2.2-fold increase in glucagon AUC (32167 ± 6658 vs. 14428 ± 3673 pg*min/mL; p<0.01). Blood glucose was significantly elevated in treated, T2D rats for ∼30 min, and hypoglycemia onset was delayed by 17 ± 4 min (p<0.001). Neither effect was observed in SSTR2a-treated, healthy rats.Conclusions: SSTR2a increased glucagon secretion following insulin overdose and delayed the onset of hypoglycemia in T2D rats. SSTR2a may be a promising therapeutic tool for alleviating hypoglycemic risk in intensively-treated T2D.View largeDownload slideView largeDownload slide DisclosureE. G. Hoffman: None. N. Dsouza: None. J. Aiken: None. R. Liggins: Employee; Self; Zucara Therapeutics Inc. M. Riddell: Advisory Panel; Self; Zealand Pharma A/S, Consultant; Self; Lilly Diabetes, Research Support; Self; Dexcom, Inc., Insulet Corporation, Speaker’s Bureau; Self; Novo Nordisk Inc., Sanofi, Stock/Shareholder; Self; Zucara Therapeutics Inc. FundingGlycoNet

Genome-wide association study of circulating levels of glucagon during an oral glucose tolerance test

BackgroundIn order to explore the pathophysiology underlying type 2 diabetes we examined the impact of gene variants associated with type 2 diabetes on circulating levels of glucagon during an oral glucose tolerance test (OGTT). Furthermore, we performed a genome-wide association study (GWAS) aiming to identify novel genomic loci affecting plasma glucagon levels.MethodsPlasma levels of glucagon were examined in samples obtained at three time points during an OGTT; 0, 30 and 120 min, in two separate cohorts with a total of up to 1899 individuals. Cross-sectional analyses were performed separately in the two cohorts and the results were combined in a meta-analysis.ResultsA known type 2 diabetes variant in EYA2 was significantly associated with higher plasma glucagon level at 30 min during the OGTT (Beta 0.145, SE 0.038, P = 1.2 × 10–4) corresponding to a 7.4% increase in plasma glucagon level per effect allele. In the GWAS, we identified a marker in the MARCH1 locus, which was genome-wide significantly associated with reduced suppression of glucagon during the first 30 min of the OGTT (Beta − 0.210, SE 0.037, P = 1.9 × 10–8), equivalent to 8.2% less suppression per effect allele. Nine additional independent markers, not previously associated with type 2 diabetes, showed suggestive associations with reduced glucagon suppression during the first 30 min of the OGTT (P < 1.0 × 10–5).ConclusionsA type 2 diabetes risk variant in the EYA2 locus was associated with higher plasma glucagon levels at 30 min. Ten additional variants were suggestively associated with reduced glucagon suppression without conferring increased type 2 diabetes risk.

1919-P: Dysfunction of BCAA Catabolism in Pancreatic Alpha Cells Is Associated with Higher Glucagon Secretion in Diabetic Mice

Background: Circulating plasma glucagon level is known to increase in diabetic state. We found that postprandial plasma glucagon level significantly increased in diabetic mice more than that in the control group. However, it’s still unknown which nutrient stimulates glucagon secretion as well as the underlying mechanism of glucagon secretion enhancement in diabetic state. Therefore, we aimed to answer these questions by using diabetic mouse models and isolated islets. Methods: Plasma glucagon levels were measured by sandwich ELISA in several diabetic model mice after oral administration of various nutrients. The effects of nutrients on glucagon secretion were assessed using isolated islets from diabetic mice. The expression levels of nutrient catabolism-related factors were analyzed in diabetic mice compared with control mice. Results and Discussions: Rather than carbohydrate or lipid, protein increased plasma glucagon levels in all diabetic models including high-fat diet mice, STZ mice, and db/db mice. Among amino acids, branched-chain amino acids (BCAA), but not the other essential or nonessential amino acids, increased plasma glucagon. BCAA directly stimulated glucagon secretion in isolated islets from diabetic mice, which was suppressed by amino acid transporter inhibitor (BCH) or branched-chain alpha-ketoacid dehydrogenase (BCKDK) inhibitor (BT2). This suggests that BCAA’s stimulation of glucagon secretion is mediated by BCAA catabolism in alpha cells. We also found that the expression level of BCKDK was higher, while branched-chain amino acid transaminase 2 (BCAT2) was lower, in diabetic islets than control islets. Considering that dysfunction of BCAA catabolism has been reported in both adipocytes and hepatocytes of diabetic mice, the dysfunction of BCAA catabolism might be a common pathogenesis for metabolic disorders observed in diabetes. Disclosure E. Wada: None. M. Kobayashi: None. O. Kikuchi: None. D. Kohno: None. T. Kitamura: None.

SGLT1 in pancreatic α cells regulates glucagon secretion in mice, possibly explaining the distinct effects of SGLT2 inhibitors on plasma glucagon levels

ObjectivesIt is controversial whether sodium glucose transporter (SGLT) 2 inhibitors increase glucagon secretion via direct inhibition of SGLT2 in pancreatic α cells. The role of SGLT1 in α cells is also unclear. We aimed to elucidate these points that are important not only for basic research but also for clinical insight. MethodsPlasma glucagon levels were assessed in the high-fat, high-sucrose diet (HFHSD) fed C57BL/6J mice treated with dapagliflozin or canagliflozin. RT-PCR, RNA sequence, and immunohistochemistry were conducted to test the expression of SGLT1 and SGLT2 in α cells. We also used αTC1 cells and mouse islets to investigate the molecular mechanism by which SGLT1 modulates glucagon secretion. ResultsDapagliflozin, but not canagliflozin, increased plasma glucagon levels in HFHSD fed mice. SGLT1 and glucose transporter 1 (GLUT1), but not SGLT2, were expressed in αTC1 cells, mouse islets and human islets. A glucose clamp study revealed that the plasma glucagon increase associated with dapagliflozin could be explained as a response to acute declines in blood glucose. Canagliflozin suppressed glucagon secretion by inhibiting SGLT1 in α cells; consequently, plasma glucagon did not increase with canagliflozin, even though blood glucose declined. SGLT1 effect on glucagon secretion depended on glucose transport, but not glucose metabolism. Islets from HFHSD and db/db mice displayed higher SGLT1 mRNA levels and lower GLUT1 mRNA levels than the islets from control mice. These expression levels were associated with higher glucagon secretion. Furthermore, SGLT1 inhibitor and siRNA against SGLT1 suppressed glucagon secretion in isolated islets. ConclusionsThese data suggested that a novel mechanism regulated glucagon secretion through SGLT1 in α cells. This finding possibly explained the distinct effects of dapagliflozin and canagliflozin on plasma glucagon levels in mice.

SGLT1 in Pancreatic a Cells Regulates Glucagon Secretion in Mice, Possibly Explaining the Distinct Effects of SGLT2 Inhibitors on Plasma Glucagon Levels

A recent report described that sodium glucose cotransporter (SGLT) 1 and 2 were expressed in α cells, and dapagliflozin which is a specific SGLT2 inhibitor, increased plasma glucagon levels by inhibiting SGLT2 in α cells. However, it is still controversial whether SGLT2 inhibitors increase plasma glucagon levels in vivo. Here, we showed that dapagliflozin, but not canagliflozin which is a SGLT2/low-potency-SGLT1 inhibitor, increased plasma glucagon levels in mice fed a high-fat, high-sucrose diet (HFHSD) and diabetic db/db mice. A glucose clamp study revealed that the plasma glucagon increase associated with dapagliflozin could be explained as a response to acute declines in blood glucose. RT-PCR and whole RNA sequencing revealed that SGLT1 and glucose transporter 1 (GLUT1), but not SGLT2, were expressed in αTC1 cells and mouse islets. We showed that canagliflozin suppressed glucagon secretion by inhibiting SGLT1 in α cells; consequently, plasma glucagon did not increase with canagliflozin, even though blood glucose declined. We also showed that the SGLT1 effect on glucagon secretion depended on sodium/glucose cotransport, but not on metabolic effect of glucose. Islets from HFHSD and db/db mice displayed higher SGLT1 expression and lower GLUT1 expression than the islets from control mice. These expression levels were associated with glucagon secretion levels. Furthermore, SGLT1 inhibitor and siRNA against SGLT1 suppressed glucagon secretion in isolated islets. These data suggested that a novel mechanism regulated glucagon secretion, through SGLT1, in α cells. This finding explained the distinct effects of dapagliflozin and canagliflozin on plasma glucagon levels in mice. Disclosure T. Suga: None. O. Kikuchi: None. M. Kobayashi: None. S. Matsui: None. H.H. Hashimoto: None. T. Sasaki: None. S. Kakizaki: None. M. Yamada: None. T. Kitamura: None.

Effect of Combination Therapy with Liraglutide plus Canagliflozin on HGP and HbA1c vs. Each Therapy Alone in T2DM

Background: We previously have shown that SGLT2 inhibitors cause an increase in HGP accompanied with an increase in plasma glucagon concentration. We hypothesized that the increase in plasma glucagon concentration is, at least in part, responsible for the increase in HGP. The aim of the present study was to examine whether inhibition of glucagon secretion by liraglutide can prevent the increase in HGP. Research Design and Methods: 51 T2DM patients (age=51±1 yr; 40% female; BMI=34.6±0.7; diabetes duration=6.8±0.8 yr; FPG=175±7; HbA1c=8.3±0.1%) were randomized to receive for 16 weeks: (i) canagliflozin 300 mg; (ii) liraglutide 1.8 mg; or (iii) canagliflozin 300 mg plus liraglutide 1.8 mg. HGP (measured with 3-3H-glucose infusion) and plasma glucagon concentration were measured before and after 16 weeks of treatment. Results: Canagliflozin monotherapy caused a significant reduction in HbA1c (-1.1±0.2%, p&amp;lt;0.01) accompanied with an increase in plasma glucagon concentration (by 28%, p&amp;lt;0.05) and HGP (by 15%, p&amp;lt;0.05) which lasted for 16 weeks. Conversely, liraglutide monotherapy caused a 1.6±0.5% (p&amp;lt;0.01) reduction in HbA1c accompanied by a small (6%) reduction in HGP (P=NS) without significant change in fasting plasma glucagon concentration. The combination of canagliflozin plus liraglutide caused a greater reduction in HbA1c (1.9±0.5%, p&amp;lt;0.vs. canagliflozin and p=NS vs. liraglutide) and attenuated the increase in fasting plasma glucagon and basal HGP at 16 weeks. Conclusion: These results: (1) support a possible role for increased plasma glucagon levels in the long term maintenance of increase in HGP caused by SGLT2i, and (2) suggest that factors other than/in addition to glucagon contribute to the initiation of the increase in HGP. Disclosure M. Abdul-Ghani: None. A.M. Ali: None. R.A. Martinez: None. J.M. Adams: None. E. Cersosimo: Research Support; Self; AstraZeneca, VeroScience, LLC.. Speaker's Bureau; Self; AstraZeneca, Sanofi, Janssen Pharmaceuticals, Inc., Eli Lilly and Company. C.L. Triplitt: Speaker's Bureau; Self; AstraZeneca, Boehringer Ingelheim Pharmaceuticals, Inc., Janssen Pharmaceuticals, Inc., Eli Lilly and Company. Consultant; Self; Sanofi, Novo Nordisk Inc. R.A. DeFronzo: Speaker's Bureau; Self; AstraZeneca, Novo Nordisk Inc.. Advisory Panel; Self; AstraZeneca, Novo Nordisk Inc., Janssen Pharmaceuticals, Inc., Boehringer Ingelheim Pharmaceuticals, Inc., Elcelyx Therapeutics, Inc., Intarcia Therapeutics, Inc.. Research Support; Self; AstraZeneca, Boehringer Ingelheim Pharmaceuticals, Inc., Janssen Pharmaceuticals, Inc., Takeda Pharmaceuticals U.S.A., Inc..

Hepatic insulin resistance and increased hepatic glucose production in mice lacking Fgf21.

Fibroblast growth factor 21 (FGF21) is an important regulator of hepatic glucose and lipid metabolism and represents a potential pharmacological agent for the treatment of type 2 diabetes and obesity. Mice fed a ketogenic diet (KD) develop hepatic insulin resistance in association with high levels of FGF21, suggesting a state of FGF21 resistance. To address the role of FGF21 in hepatic insulin resistance, we assessed insulin action in FGF21 whole-body knock-out (FGF21 KO) male mice and their littermate WT controls fed a KD. Here, we report that FGF21 KO mice have hepatic insulin resistance and increased hepatic glucose production associated with an increase in plasma glucagon levels. FGF21 KO mice are also hypometabolic and display increased fat mass compared with their WT littermates. Taken together, these findings support a major role of FGF21 in regulating energy expenditure and hepatic glucose and lipid metabolism, and its potential role as a candidate in the treatment of diseases associated with insulin resistance.

Inhibition of the glucose transporter SGLT2 with dapagliflozin in pancreatic alpha cells triggers glucagon secretion.

Type 2 diabetes (T2D) is characterized by chronic hyperglycemia resulting from a deficiency in insulin signaling, because of insulin resistance and/or defects in insulin secretion; it is also associated with increases in glucagon and endogenous glucose production (EGP). Gliflozins, including dapagliflozin, are a new class of approved oral antidiabetic agents that specifically inhibit sodium-glucose co-transporter 2 (SGLT2) function in the kidney, thus preventing renal glucose reabsorption and increasing glycosuria in diabetic individuals while reducing hyperglycemia. However, gliflozin treatment in subjects with T2D increases both plasma glucagon and EGP by unknown mechanisms. In spite of the rise in EGP, T2D patients treated with gliflozin have lower blood glucose levels than those receiving placebo, possibly because of increased glycosuria; however, the resulting increase in plasma glucagon levels represents a possible concerning side effect, especially in a patient population already affected by hyperglucagonemia. Here we demonstrate that SGLT2 is expressed in glucagon-secreting alpha cells of the pancreatic islets. We further found that expression of SLC5A2 (which encodes SGLT2) was lower and glucagon (GCG) gene expression was higher in islets from T2D individuals and in normal islets exposed to chronic hyperglycemia than in islets from non-diabetics. Moreover, hepatocyte nuclear factor 4-α (HNF4A) is specifically expressed in human alpha cells, in which it controls SLC5A2 expression, and its expression is downregulated by hyperglycemia. In addition, inhibition of either SLC5A2 via siRNA-induced gene silencing or SGLT2 via dapagliflozin treatment in human islets triggered glucagon secretion through KATP channel activation. Finally, we found that dapagliflozin treatment further promotes glucagon secretion and hepatic gluconeogenesis in healthy mice, thereby limiting the decrease of plasma glucose induced by fasting. Collectively, these results identify a heretofore unknown role of SGLT2 and designate dapagliflozin an alpha cell secretagogue.

Glucagon signaling in the heart: Activation or inhibition?

Glucagon signaling in the heart: Activation or inhibition?

Vildagliptin Reduces Glucagon during Hyperglycemia and Sustains Glucagon Counterregulation during Hypoglycemia in Type 1 Diabetes

The dipeptidyl peptidase-4 inhibitor, vildagliptin, inhibits glucagon secretion at hyperglycemia but appears to enhance glucagon counterregulation during hypoglycemia in type 2 diabetes. The objective of the investigation was to study whether vildagliptin also improves α-cell function in type 1 diabetes (T1D). The study was a single-center, double-blind, randomized, placebo-controlled crossover study involving 28 patients with C-peptide negative and antibody positive T1D [21 males, seven females, glycosylated hemoglobin 57.9 mmol/mol (7.5%)]. Patients received vildagliptin (50 mg twice a day) or placebo as an add-on to their insulin therapy for 4 wk each. On d 28 of the respective treatment period, patients were served a standard meal (500 kcal) to raise the circulating incretin hormone levels followed by a hyperinsulinemic hypoglycemic clamp at 2.5 mmol/liter. The increase in plasma glucagon levels during the 30-min hypoglycemic clamp (min 165-195 of the test) was measured. During the meal, glucagon levels were lower with vildagliptin than with placebo (120 min area under the curve(glucagon) 2.4±0.2 vs. 2.6±0.2 nmol/liter×minutes, P=0.022 for between group difference). In contrast, during hypoglycemia, the glucagon counterregulation was not reduced by vildagliptin (increase in glucagon 1.5±1.0 pmol/liter with vildagliptin vs. 1.7±0.8 pmol/liter with placebo, P=NS). In addition, the counterregulatory responses in epinephrine, norepinephrine, cortisol, and pancreatic polypeptide were not different between the treatments. During the 4-wk treatment period, vildagliptin reduced the mean glycosylated hemoglobin, whereas there was no change with placebo [between group difference was -3.4±1.0 mmol/mol (-0.32±0.09%; P=0.002)] from baseline of 57.9 mmol/mol (7.5%). Vildagliptin, although inhibiting glucagon secretion during hyperglycemia, does not compromise the glucagon counterregulatory response during hypoglycemia in T1D.

Characterization of major elements of insulin signaling cascade in chicken adipose tissue: Apparent insulin refractoriness

The role of insulin in chicken adipose tissue appears weak or questionable. In a first study, proximal and distal components of the insulin signaling cascade were characterized in abdominal adipose tissue of fasted or fed chickens for the first time. Similar measurements were performed on epididymal adipose tissue from fasted or fed rats for comparison. Tyrosine phosphorylation of IR beta subunit, IRS-1 and Shc and phosphorylation of downstream components (Akt and MAPK ERK1/2) were significantly reduced as expected by fasting in rat, but not in chicken. Phosphorylation of MAPK P38 was increased by fasting in chicken but not in rat. Phosphorylation of AMPK was not affected in the conditions investigated in either species. Whatever the nutritional state, the protein levels of IR and IRS-1 were lower in chicken than in rat, whereas those of Shc, Akt, AMPK, MAPK ERK2 and MAPK P38 were similar in both species. In fed state, PI3K activity was higher in chicken than in rat. Insulin sensitivity of insulin cascade was further investigated in chicken adipose tissue following in vivo insulin neutralization for 1 or 5h in fed chickens. Insulin privation did not alter early insulin signaling steps (IRβ, IRS-1 and Shc) or downstream elements (Akt, P70S6K, S6 ribosomal protein, AMPK, MAPK ERK2 and MAPK P38). Finally, phosphorylation of the transcription factor Creb was increased by 2-fold by 5h fasting or 5h insulin privation, most likely in response to an increase in plasma glucagon levels. Thus, insulin signaling is markedly different in chicken abdominal adipose tissue from that operating in mammals making chicken an interesting model of insulin resistance or refractoriness.

Effects of biotin deficiency on pancreatic islet morphology, insulin sensitivity and glucose homeostasis

Several studies have revealed that physiological concentrations of biotin are required for the normal expression of critical carbohydrate metabolism genes and for glucose homeostasis. However, the different experimental models used in these studies make it difficult to integrate the effects of biotin deficiency on glucose metabolism. To further investigate the effects of biotin deficiency on glucose metabolism, we presently analyzed the effect of biotin deprivation on glucose homeostasis and on pancreatic islet morphology. Three-week-old male BALB/cAnN Hsd mice were fed a biotin-deficient or a biotin-control diet (0 or 7.2 μmol of free biotin/kg diet, respectively) over a period of 8 weeks. We found that biotin deprivation caused reduced concentrations of blood glucose and serum insulin concentrations, but increased plasma glucagon levels. Biotin-deficient mice also presented impaired glucose and insulin tolerance tests, indicating defects in insulin sensitivity. Altered insulin signaling was linked to a decrease in phosphorylated Akt/PKB but induced no change in insulin receptor abundance. Islet morphology studies revealed disruption of islet architecture due to biotin deficiency, and an increase in the number of α-cells in the islet core. Morphometric analyses found increased islet size, number of islets and glucagon-positive area, but a decreased insulin-positive area, in the biotin-deficient group. Glucagon secretion and gene expression increased in islets isolated from biotin-deficient mice. Our results suggest that biotin deficiency promotes hyperglycemic mechanisms such as increased glucagon concentration and decreased insulin secretion and sensitivity to compensate for reduced blood glucose concentrations. Variations in glucose homeostasis may participate in the changes observed in pancreatic islets.

A Switch From Prohormone Convertase (PC)-2 to PC1/3 Expression in Transplanted α-Cells Is Accompanied by Differential Processing of Proglucagon and Improved Glucose Homeostasis in Mice

OBJECTIVE—Glucagon, which raises blood glucose levels by stimulating hepatic glucose production, is produced in α-cells via cleavage of proglucagon by prohormone convertase (PC)-2. In the enteroendocrine L-cell, proglucagon is differentially processed by the alternate enzyme PC1/3 to yield glucagon-like peptide (GLP)-1, GLP-2, and oxyntomodulin, which have blood glucose–lowering effects. We hypothesized that alteration of PC expression in α-cells might convert the α-cell from a hyperglycemia-promoting cell to one that would improve glucose homeostasis. RESEARCH DESIGN AND METHODS—We compared the effect of transplanting encapsulated PC2-expressing αTC-1 cells with PC1/3-expressing αTCΔPC2 cells in normal mice and low-dose streptozotocin (STZ)-treated mice. RESULTS—Transplantation of PC2-expressing α-cells increased plasma glucagon levels and caused mild fasting hyperglycemia, impaired glucose tolerance, and α-cell hypoplasia. In contrast, PC1/3-expressing α-cells increased plasma GLP-1/GLP-2 levels, improved glucose tolerance, and promoted β-cell proliferation. In GLP-1R−/− mice, the ability of PC1/3-expressing α-cells to improve glucose tolerance was attenuated. Transplantation of PC1/3-expressing α-cells prevented STZ-induced hyperglycemia by preserving β-cell area and islet morphology, possibly via stimulating β-cell replication. However, PC2-expressing α-cells neither prevented STZ-induced hyperglycemia nor increased β-cell proliferation. Transplantation of αTCΔPC2, but not αTC-1 cells, also increased intestinal epithelial proliferation. CONCLUSIONS—Expression of PC1/3 rather than PC2 in α-cells induces GLP-1 and GLP-2 production and converts the α-cell from a hyperglycemia-promoting cell to one that lowers blood glucose levels and promotes islet survival. This suggests that alteration of proglucagon processing in the α-cell may be therapeutically useful in the context of diabetes.

Intermittent hepatic dearterialization induces glucose intolerance: An experimental study in the rat

Intermittent hepatic dearterialization is used in the palliative treatment of liver malignancy. However, its metabolic consequences are not established. Therefore the influences of the procedure on the plasma insulin, glucagon and glucose responses were studied in healthy rats and in rats with a tumour inoculated subcapsularly into the liver. To study the influence on stimulated islet hormone secretion we infused arginine intravenously (7 mg/min) for 30 min, because arginine is known to stimulate the secretion of both insulin and glucagon. During hepatic dearterialization, hyperglycaemia developed; mean(s.e.m.) blood glucose levels after 60 min of dearterialization were 20.2(1.3) mM versus 14.7(1.5)mM in controls (P less than 0.001). Concomitantly, compensatory hyperinsulinaemia and hypoglucagonaemia occurred. Furthermore, during both dearterialization and in the immediate reperfusion phase, the arginine-induced increase in plasma insulin levels was impaired (P less than 0.001), whereas the arginine-induced increase in plasma glucagon levels was not significantly affected. These changes were qualitatively the same in tumour-free and tumour-bearing rats. We conclude that glucose intolerance develops during selective hepatic dearterialization, which is evident both from basal hyperglycaemia and impaired insulin secretion.

Regulation of glucagon secretion by glucose transporter type 2 (glut2) and astrocyte-dependent glucose sensors

Ripglut1;glut2-/- mice have no endogenous glucose transporter type 2 (glut2) gene expression but rescue glucose-regulated insulin secretion. Control of glucagon plasma levels is, however, abnormal, with fed hyperglucagonemia and insensitivity to physiological hypo- or hyperglycemia, indicating that GLUT2-dependent sensors control glucagon secretion. Here, we evaluated whether these sensors were located centrally and whether GLUT2 was expressed in glial cells or in neurons. We showed that ripglut1;glut2-/- mice failed to increase plasma glucagon levels following glucoprivation induced either by i.p. or intracerebroventricular 2-deoxy-D-glucose injections. This was accompanied by failure of 2-deoxy-D-glucose injections to activate c-Fos-like immunoreactivity in the nucleus of the tractus solitarius and the dorsal motor nucleus of the vagus. When glut2 was expressed by transgenesis in glial cells but not in neurons of ripglut1;glut2-/- mice, stimulated glucagon secretion was restored as was c-Fos-like immunoreactive labeling in the brainstem. When ripglut1;glut2-/- mice were backcrossed into the C57BL/6 genetic background, fed plasma glucagon levels were also elevated due to abnormal autonomic input to the alpha cells; glucagon secretion was, however, stimulated by hypoglycemic stimuli to levels similar to those in control mice. These studies identify the existence of central glucose sensors requiring glut2 expression in glial cells and therefore functional coupling between glial cells and neurons. These sensors may be activated at different glycemic levels depending on the genetic background.

Gender difference in the glucagon response to glucopenic stress in mice.

A gender difference in the glucagon response to insulin-induced hypoglycemia was previously demonstrated in humans. Whether this reflects a gender difference in autonomic activation or in pancreatic alpha-cell regulation is not known. We investigated the glucagon, epinephrine, and norepinephrine responses to neuroglycopenic stress induced by 2-deoxy-D-glucose (2-DG) or insulin in female and male mice. 2-DG increased plasma glucagon levels by 559 +/- 68% in females versus 281 +/- 46% in males (P < 0.01). Plasma levels of epinephrine or norepinephrine after 2-DG administration did not differ between genders. During insulin-induced hypoglycemia, the glucagon response was similarly higher in females (P < 0.001), whereas the plasma catecholamine response was higher in males (P < 0.05). In vivo, the glucagon response to carbachol or clonidine was higher in females (P < 0.05). In isolated islets, the glucagon response to carbachol (100 microM; P = 0.003) but not to clonidine (1 microM) was larger in females. We conclude that in addition to a larger alpha-cell mass (previously described in female mice), an increased sensitivity of the glucagon-producing alpha-cell to cholinergic activation contributes to the larger glucagon response to glucopenic stress in female mice.