Glucagon-induced Glucose Production Research Articles

The Liver-α-Cell Axis in Health and in Disease.

Glucagon and insulin are the main regulators of blood glucose. While the actions of insulin are extensively mapped, less is known about glucagon. Besides glucagon's role in glucose homeostasis, there are additional links between the pancreatic α-cells and the hepatocytes, often collectively referred to as the liver-α-cell axis, that may be of importance for health and disease. Thus, glucagon receptor antagonism (pharmacological or genetic), which disrupts the liver-α-cell axis, results not only in lower fasting glucose but also in reduced amino acid turnover and dyslipidemia. Here, we review the actions of glucagon on glucose homeostasis, amino acid catabolism, and lipid metabolism in the context of the liver-α-cell axis. The concept of glucagon resistance is also discussed, and we argue that the various elements of the liver-α-cell axis may be differentially affected in metabolic diseases such as diabetes, obesity, and nonalcoholic fatty liver disease (NAFLD). This conceptual rethinking of glucagon biology may explain why patients with type 2 diabetes have hyperglucagonemia and how NAFLD disrupts the liver-α-cell axis, compromising the normal glucagon-mediated enhancement of substrate-induced amino acid turnover and possibly fatty acid β-oxidation. In contrast to amino acid catabolism, glucagon-induced glucose production may not be affected by NAFLD, explaining the diabetogenic effect of NAFLD-associated hyperglucagonemia. Consideration of the liver-α-cell axis is essential to understanding the complex pathophysiology underlying diabetes and other metabolic diseases.

Structure-Activity Relationship and Biological Investigation of SR18292 (16), a Suppressor of Glucagon-Induced Glucose Production.

Despite a myriad of available pharmacotherapies for the treatment of type 2 diabetes (T2D), challenges still exist in achieving glycemic control. Several novel glucose-lowering strategies are currently under clinical investigation, highlighting the need for more robust treatments. Previously, we have shown that suppressing peroxisome proliferator-activated receptor gamma coactivator 1-alpha activity with a small molecule (SR18292, 16) can reduce glucose release from hepatocytes and ameliorate hyperglycemia in diabetic mouse models. Despite structural similarities in 16 to known β-blockers, detailed structure-activity relationship studies described herein have led to the identification of analogues lacking β-adrenergic activity that still maintain the ability to suppress glucagon-induced glucose release from hepatocytes and ameliorate hyperglycemia in diabetic mouse models. Hence, these compounds exert their biological effects in a mechanism that does not include adrenergic signaling. These probe molecules may lead to a new therapeutic approach to treat T2D either as a single agent or in combination therapy.

1918-P: YWHAB Inhibits Hepatic Gluconeogenesis through Suppression of Glucagon Signaling Pathway

Aims/Introduction: Hyperglycemia serves as a central factor in the development and progression of metabolic diseases. Since the glucagon receptor and its antagonism have been reported to be associated with the development and progression of hyperglycemias, the factors and proteins involved in the regulation of glucagon activity and signaling are still unclear. Materials and Methods: We examined the role of YWHAB in vivo (in hepatocyte-selective Ywhab knockout and overexpressing mice) and in vitro (in mouse primary hepatocytes and human HepG2 cells). Results: In this study, we found that YWHAB was down-regulated in ob/ob mice and db/db mice. To establish function of YWHAB in glucose metabolism in vivo, we generated hepatocyte-selective Ywhab knockout mice. The knockout mice displayed impaired blood glucose homeostasis in isolated mouse primary hepatocytes, as well as the increase in glucagon-mediated hepatocytic glucose production. Overexpressing YWHAB markedly decreases the expression of key enzymes in gluconeogenesis and inactivates the glucagon signaling. Similarly, the magnifying effect on glucagon signaling of Ywhab knockout in mouse primary hepatocytes were significantly changed by the treatment of PKA inhibitor H89. Conclusions: The regulatory role of YWHAB in hepatocytes can protect against the glucagon-induced glucose production by attenuating the activation of cAMP/PKA and the expression of key enzymes in gluconeogenesis. Thus, YWHAB is a new target site identified in the control of glucagon signaling and associated metabolic diseases. Disclosure L. Wei: None. L. Ji: None. C. Zhu: None. Funding National Natural Science Foundation of China (81870603)

Berberine Attenuates Hyperglycemia by Inhibiting the Hepatic Glucagon Pathway in Diabetic Mice.

Dysregulated glucagon drives hyperfunction in hepatic glucose output, which is the main cause of persistent hyperglycemia in type 2 diabetes. Berberine (Zhang et al., 2010) has been used as a hypoglycemic agent, yet the mechanism by which BBR inhibits hepatic gluconeogenesis remains incompletely understood. In this study, we treated diabetic mice with BBR, tested blood glucose levels, and then performed insulin, glucose lactate, and glucagon tolerance tests. Intracellular cAMP levels in hepatocytes were determined by ELISA, hepatic gluconeogenetic genes were assayed by RT-qPCR, and the phosphorylation of CREB, which is the transcriptional factor controlling the expression of gluconeogenetic genes, was detected by western blot. BBR reduced blood glucose levels, improved insulin and glucose tolerance, and suppressed lactate- and glucagon-induced hepatic gluconeogenesis in ob/ob and STZ-induced diabetic mice. Importantly, BBR blunted glucagon-induced glucose production and gluconeogenic gene expression in hepatocytes, presumably through reducing cAMP, which resulted in the phosphorylation of CREB. By utilizing a cAMP analogue, adenylate cyclase (AC), to activate cAMP synthetase, and an inhibitor of the cAMP degradative enzyme, phosphodiesterase (PDE), we revealed that BBR accelerates intracellular cAMP degradation. BBR reduces the intracellular cAMP level by activating PDE, thus blocking activation of downstream CREB and eventually downregulating gluconeogenic genes to restrain hepatic glucose production.

Autophagy mediates hepatic GRK2 degradation to facilitate glucagon-induced metabolic adaptation to fasting.

The liver plays a key role during fasting to maintain energy homeostasis and euglycemia via metabolic processes mainly orchestrated by the insulin/glucagon ratio. We report here that fasting or calorie restriction protocols in C57BL6 mice promote a marked decrease in the hepatic protein levels of G protein-coupled receptor kinase 2 (GRK2), an important negative modulator of both G protein-coupled receptors (GPCRs) and insulin signaling. Such downregulation of GRK2 levels is liver-specific and can be rapidly reversed by refeeding. We find that autophagy, and not the proteasome, represents the main mechanism implicated in fasting-induced GRK2 degradation in the liver in vivo. Reducing GRK2 levels in murine primary hepatocytes facilitates glucagon-induced glucose production and enhances the expression of the key gluconeogenic enzyme Pck1. Conversely, preventing full downregulation of hepatic GRK2 during fasting using adenovirus-driven overexpression of this kinase in the liver leads to glycogen accumulation, decreased glycemia, and hampered glucagon-induced gluconeogenesis, thus preventing a proper and complete adaptation to nutrient deprivation. Overall, our data indicate that physiological fasting-induced downregulation of GRK2 in the liver is key for allowing complete glucagon-mediated responses and efficient metabolic adaptation to fasting in vivo.

Differential effects of metformin glycinate and hydrochloride in glucose production, AMPK phosphorylation and insulin sensitivity in hepatocytes from non-diabetic and diabetic mice

The liver is a main target tissue of the biguanide metformin which activates AMP-activated protein kinase (AMPK). We previously reported that administration of metformin glycinate showed a greater decrease of glycated hemoglobin A1c than a placebo in patients with type 2 diabetes mellitus (T2DM). In this study, we compared the effects of metformin hydrochloride, the oral antidiabetic drug of first choice, with those of metformin glycinate in hepatocytes from non-diabetic and diabetic mice and humans. Both formulations were equally potent regard to the reduction of basal and glucagon-induced glucose production and mRNA levels of gluconeogenic enzymes (Pck1 and G6pc) in hepatocytes from C57/Bl6 mice and humans. On the contrary, phosphorylation of AMPK and its substrate acetyl CoA carboxylase (ACC) was faster in hepatocytes treated with metformin glycinate. Likewise, we found stronger reduction in hepatocytes from obese/diabetic db/db mice of glucagon-induced glucose output and more sustained AMPK phosphorylation after treatment with metformin glycinate. Importantly, insulin sensitization regarding phosphorylation of AKT (Ser473) was enhanced in hepatocytes from db/db mice or humans pretreated with metformin glycinate. In conclusion, our data indicate that metformin glycinate may be an alternative therapy against insulin resistance during obesity and/or T2DM.

Design, synthesis, and effects of novel phenylpyrimidines as glucagon receptor antagonists.

The hormone glucagon increases blood glucose levels through increasing hepatic glucose output. In diabetic patients, dysregulation of glucagon secretion contributes to hyperglycemia. Thus, the inhibition of glucagon receptor is one target for the treatment of hyperglycemia in type 2 diabetes. Here we designed and synthesized a series of small molecules based on phenylpyrimidine. Of these, the compound (R)-7a most significantly decreased the glucagon-induced cAMP production and glucagon-induced glucose production during in vitro and in vivo assays. In addition, (R)-7a showed good efficacy in glucagon challenge tests and lowered blood glucose levels in diabetic db/db mice. Our results suggest that the compound (R)-7a could be a potential glucose-lowering agent for treating type 2 diabetes.

Midgut-Derived Activin Regulates Glucagon-like Action in the Fat Body and Glycemic Control

While high-caloric diet impairs insulin response to cause hyperglycemia, whether and how counter-regulatory hormones are modulated by high-caloric diet is largely unknown. We find that enhanced response of Drosophila adipokinetic hormone (AKH, the glucagon homolog) in the fat body is essential for hyperglycemia associated with a chronic high-sugar diet. We show that the activin type I receptor Baboon (Babo) autonomously increases AKH signaling without affecting insulin signaling in the fat body via, at least, increase of Akh receptor (AkhR) expression. Further, we demonstrate that Activin-β (Actβ), an activin ligand predominantly produced in the enteroendocrine cells (EEs) of the midgut, is upregulated by chronic high-sugar diet and signals through Babo to promote AKH action in the fat body, leading to hyperglycemia. Importantly, activin signaling in mouse primary hepatocytes also increases glucagon response and glucagon-induced glucose production, indicating a conserved role for activin in enhancing AKH/glucagon signaling and glycemic control.

CD38-mediated Ca2+ signaling contributes to glucagon-induced hepatic gluconeogenesis

CD38 is a multifunctional enzyme for the synthesis of Ca2+ second messengers. Glucagon promotes hepatic glucose production through Ca2+ signaling in the fasting condition. In this study, we investigated the role of CD38 in the glucagon signaling of hepatocytes. Here, we show that glucagon induces cyclic ADP-ribose (cADPR) production and sustained Ca2+ increases via CD38 in hepatocytes. 8-Br-cADPR, an antagonistic cADPR analog, completely blocked glucagon-induced Ca2+ increases and phosphorylation of cAMP response element-binding protein (CREB). Moreover, glucagon-induced sustained Ca2+ signals and translocation of CREB-regulated transcription coactivator 2 to the nucleus were absent and glucagon-induced glucose production and expression of glucose-6-phosphatase (G6Pase) and phosphoenolpyruvate carboxykinase (Pck1) are remarkably reduced in hepatocytes from CD38−/− mice. Furthermore, in the fasting condition, CD38−/− mice have decreased blood glucose and hepatic expression of G6Pase and Pck1 compared to wild type mice. Our data suggest that CD38/cADPR-mediated Ca2+ signals play a key role in glucagon-induced gluconeogenesis in hepatocytes, and that the signal pathway has significant clinical implications in metabolic diseases, including type 2 diabetes.

Effects of eugenol on hepatic glucose production and AMPK signaling pathway in hepatocytes and C57BL/6J mice

Eugenol is a phenylpropanoid with many pharmacological activities, but its anti-hyperglycemic activity is not yet fully explored. For in vitro study, HepG2 cells and primary rat hepatocytes were used, and glucose production was induced by adding 100nM of glucagon in the presence of gluconeogenic substrates. In animal study, hyperglycemia was induced by high fat diet (HFD) in male C57BL/6J mice, and eugenol was orally administered at 20 or 40mg per kg (E20, E40) for 15weeks. Eugenol significantly inhibited glucagon-induced glucose production and phosphorylated AMPK in the HepG2 and primary rat hepatocytes, and these effects were reversed in the presence of compound C (an AMPK inhibitor) or STO-609 (a CAMKK inhibitor). In addition, the protein and gene expression levels of CREB, CRTC2·CREB complex, PGC-1α, PEPCK and G6Pase were all significantly suppressed. Moreover, inhibition of AMPK by over-expression of dominant negative AMPK prevented eugenol from suppressions of gluconeogenic gene expression and hepatic glucose production. In animal study, plasma glucose and insulin levels of the E40 group were decreased by 31% and 63%, respectively, when compared to those of HFD control. In pyruvate tolerance tests, pyruvate-induced glucose excursions were decreased, indicating that the anti-hyperglycemic activity of eugenol is primarily due to the suppression of hepatic gluconeogenesis. In summary, eugenol effectively ameliorates hyperglycemia through inhibition of hepatic gluconeogenesis via modulating CAMKK-AMPK-CREB signaling pathway. Eugenol or eugenol-containing medicinal plants could represent a promising therapeutic agent to prevent type 2 diabetes.

Inhibition of Glycogen Phosphorylase in the Context of Type 2 Diabetes, with Focus on Recent Inhibitors Bound at the Active Site

Among the variety of approaches for pharmacological intervention in T2DM, the inhibition of GP with the aim of reducing hepatic glucose output is a validated and thoroughly investigated strategy. Both the academia and health companies participate in the search of potent inhibitors, that might be suitable for long-term treatment. As several inhibitory sites have been identified for GP, interest focuses mainly on structures that can bind at either the catalytic, the allosteric, or the new allosteric sites. Glucose-based motifs and azasugars that bind at the active site constitute the most populated class of GPis. During the last two years, significant progresses have been made, since newly proposed motifs have K(i) values in the low micromolar and even sub- micromolar range. Without ignoring previously reported structures, new series based on β-D-glucopyranosyl-pyrimidine, D-glucopyranosylidene-spiro-isoxazoline and D-glucopyranosylidene-spiro-oxathiazole motifs appear promising. A representative from this last series, with a 2-naphthyl residue was identified as the most potent GPi to date (K(i) = 0.16 µM). While no inhibition was found for sulfonium analogs, D-DAB remains the best inhibitor among five and six-membered iminosugars that showed inhibitory properties toward GP. A study of glucagon-induced glucose production in primary rat hepatocytes has suggested that amylo-1,6-glucosidase inhibitors in combination with GPis may lower glucose level in T2DM. Considering the limitations found for other potent GPis binding at other sites and the complexity of pharmacological development, the potential of glucose-based GPis is still not established firmly and more tests with cells, tissues, animals are required to better establish the risks and merits of these structures, as antidiabetic drugs. Further studies might also confirm other directions where glucose-based GPis could be useful.

Reactive oxygen species facilitate the insulin-dependent inhibition of glucagon-induced glucose production in the isolated perfused rat liver

Recent studies indicate that intracellular insulin signalling involves the formation of reactive oxygen species (ROS) by NADPH oxidases (NOX). ROS inhibit intracellular protein tyrosin phosphatases whereby phosphoprotein signalling is enhanced and prolonged. We used the isolated perfused rat liver and detected ROS formation by measuring the surface fluorescence at wavelengths specific for the intracellular ROS sensor carboxydihydrodichlorofluorescein. Insulin (2, 5, 20 nM) induced low level ROS formation that was abolished by the NOX inhibitor diphenyleneiodonium chloride (4 microM). Studying insulin-dependent inhibition of glucagon-activated glucose production showed that melatonin (50 microM), used as ROS scavenger, inhibited ROS formation and blunted the effect of insulin on glucose production. The data support the general notion that hormone-dependent ROS formation modifies intracellular signal transduction.

Oxidative Modification of Proteins in Cell Signaling

Antioxidants & Redox SignalingVol. 7, No. 5-6 Forum EditorialOxidative Modification of Proteins in Cell SignalingKenneth Hensley and Robert A. FloydKenneth HensleySearch for more papers by this author and Robert A. FloydSearch for more papers by this authorPublished Online:12 May 2005https://doi.org/10.1089/ars.2005.7.523AboutSectionsPDF/EPUB ToolsPermissionsDownload CitationsTrack CitationsAdd to favorites Back To Publication ShareShare onFacebookTwitterLinked InRedditEmail FiguresReferencesRelatedDetailsCited byLate-Onset Dietary Restriction Modulates Protein Carbonylation and Catalase in Cerebral Hemispheres of Aged Mice12 December 2013 | Cellular and Molecular Neurobiology, Vol. 34, No. 2Redox regulation of macrophage migration inhibitory factor expression in rat neuronsBiochemical and Biophysical Research Communications, Vol. 390, No. 1Reactive oxygen species facilitate the insulin-dependent inhibition of glucagon-induced glucose production in the isolated perfused rat liverWiener Medizinische Wochenschrift, Vol. 158, No. 19-20A chemical approach for detecting sulfenic acid-modified proteins in living cellsMolecular BioSystems, Vol. 4, No. 6Redox Regulation of cAMP-dependent Protein Kinase SignalingJournal of Biological Chemistry, Vol. 282, No. 30 Volume 7Issue 5-6May 2005 InformationCopyright 2005, Mary Ann Liebert, Inc.To cite this article:Kenneth Hensley and Robert A. Floyd.Oxidative Modification of Proteins in Cell Signaling.Antioxidants & Redox Signaling.May 2005.523-525.http://doi.org/10.1089/ars.2005.7.523Published in Volume: 7 Issue 5-6: May 12, 2005PDF download

Modulation of glucagon-induced glucose production by dexfenfluramine in rat hepatocytes

The mechanism of the antihyperglycaemic action of dexfenfluramine (DEXF) was investigated in isolated rat hepatocytes exposed to glucagon. Preincubation of hepatocytes with DEXF caused a dose-dependent inhibition of cyclic AMP formation by 100 nM glucagon (Ki = 0.29 mM) that was almost complete at 1 mM DEXF. Surprisingly, glucagon-induced phosphorylase activation was not affected by DEXF despite the significant drop in cyclic AMP levels. Glucose production stimulated by glucagon was inhibited by up to 48% by 1 mM DEXF, and the rate of glucose production correlated positively with the steady-state concentration of glucose 6-phosphate. DEXF also partially restored lactate + pyruvate production which was abolished by an optimal concentration of glucagon. Although DEXF was not able to prevent the inactivation of pyruvate kinase by glucagon, the lack of further accumulation of phosphoenolpyruvate in DEXF-treated cells supports the conclusion that the flux through pyruvate kinase is stimulated, probably via the increase in fructose 2,6-bisphosphate, thereby increasing glycolysis. Our results thus indicate that DEXF counteracts the inhibition of glycolysis by glucagon and that this property might contribute to the antihyperglycaemic effect of this drug. Furthermore, this study shows that, in the presence of the drug, glucagon caused phosphorylase activation and pyruvate kinase inactivation without a significant increase in cyclic AMP levels.

Expression of α2-Receptor-Mediated Responses by Insulin in Primary Culture of Rat Hepatocytes

The effects of the α2-adrenergic agonist, clonidine, on the glucagon-stimulated glucose output from serum-free cultures of adult rat hepatocytes were examined in vitro. When hepatocytes were cultured with 10 nM dexamethasone under the serum-free condition, 1 or 10 μM clonidine did not inhibit the glucagon-induced glucose production. In contrast, clonidine dose-dependently inhibited the activity concomitantly with suppression of hepatocyte cAMP production by glucagon when they were cultured with 10 nM dexamethasone and 10 nM insulin. The inhibitory effects of clonidine were completely blocked by prior treatment of hepatocytes with pertussis toxin (100 ng/ml). In addition, forskolin-stimulated cAMP production was also inhibited by α2-adrenergic agonists (clonidine and oxymetazoline) in a dose-dependent manner when hepatocytes were cultured with 10 nM dexamethasone and 10 nM insulin. The inhibitory effects of α2-adrenergic agonists on forskolin-stimulated cAMP production were specifically blocked when they were combined with the α2-adrenergic antagonist yohimbine. Hepatocytes cultured with dexamethasone alone showed no response to the α2-adrenergic agonists. The α2-response was abolished when cycloheximide (0.5 μM) was added to the cultures. These results suggest that insulin develops α2-adrenergic responsiveness through new protein synthesis during the primary culture of adult rat hepatocytes.

15] cAMP analog antagonists of cAMP action

The first direct evidence that (Rp)-cAMPS was a cAMP antagonist and could oppose the action of cAMP was a study done in hepatocytes in 1983. This study demonstrated that (Rp)-cAMPS is an intracellular inhibitor of cAMP action. Subsequently, this analog has been used to study several cAMPdependent systems: glucagon-induced glucose production in hepatocytes; isoproterenol-induced lipolysis in adipocytes; cAMP activation of the cellular slime mold, Dictyostelium discoideum; cAMP-induced phosphorylation of microtubule-associated protein (MAP2) in brain tissue; and hormone-induced steroidogenesis in cultured granulosa and Leydig cells. (Rp)-cAMPS has not yet been fully exploited as an intracellular antagonist of cAMP action for several reasons: lack of availability due to difficulties involved in synthesis and purification, lack of potency, and lack of cell membrane permeability in some cell culture systems. Nevertheless, this compound, the only known intracellular antagonist of cAMP, provides a means of distinguishing cAMP-dependent from cAMP-independent cellular events, which was not previously available.-

Beta-adrenergic contribution to glucagon-induced glucose production and insulin secretion in uremia.

Spontaneous or propranolol-induced hypoglycemia can occur in uremic humans. We studied glucose kinetics (using [3-3H]glucose) in five uremic humans 24 h after hemodialysis and in seven normal controls. The effect of glucagon infusion at rates of 3, 6, 12, and 18 ng X kg-1 X min-1 at 60-min intervals was compared with either saline or beta-adrenergic blockade (propranolol infusion). In uremics, plasma glucose increased by 20-25% and by 40-50% at the 3 and 6 ng X kg-1 X min-1 glucagon doses, respectively, with no further increases at higher infusion rates. Glucose production increased transiently and in tandem with glucose uptake at each glucagon increment (P less than 0.0001). During beta-adrenergic blockade, the effect of glucagon in stimulating glucose production was blunted by 14-24% at the 6-18 ng X kg-1 X min-1 doses (P less than 0.05). During saline infusion, plasma insulin concentrations increased progressively to peak levels fourfold above basal at the 18 ng X kg-1 X min-1 dose. This increase in plasma insulin was virtually abolished by concomitant beta-adrenergic blockade (P = 0.0002). In contrast to uremic subjects, normal controls exhibited lesser degrees of hyperglycemia and hyperinsulinemia at all glucagon infusion rates. Propranolol infusion had no effect on the increments in glucose production and uptake nor on the plasma insulin response. These results suggest that in uremic humans propranolol independently reduces the hepatic response to glucagon and the insulin secretory response to hyperglycemia and/or hyperglucagonemia. These observations provide a possible mechanism for the adrenergic regulation of glucose homeostasis in uremia.