1
|
Rodgers RL. Glucagon, cyclic AMP, and hepatic glucose mobilization: A half‐century of uncertainty. Physiol Rep 2022; 10:e15263. [PMID: 35569125 PMCID: PMC9107925 DOI: 10.14814/phy2.15263] [Citation(s) in RCA: 6] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/27/2022] [Revised: 03/16/2022] [Accepted: 03/18/2022] [Indexed: 12/14/2022] Open
Abstract
For at least 50 years, the prevailing view has been that the adenylate cyclase (AC)/cyclic AMP (cAMP)/protein kinase A pathway is the predominant signal mediating the hepatic glucose‐mobilizing actions of glucagon. A wealth of evidence, however, supports the alternative, that the operative signal most of the time is the phospholipase C (PLC)/inositol‐phosphate (IP3)/calcium/calmodulin pathway. The evidence can be summarized as follows: (1) The consensus threshold glucagon concentration for activating AC ex vivo is 100 pM, but the statistical hepatic portal plasma glucagon concentration range, measured by RIA, is between 28 and 60 pM; (2) Within that physiological concentration range, glucagon stimulates the PLC/IP3 pathway and robustly increases glucose output without affecting the AC/cAMP pathway; (3) Activation of a latent, amplified AC/cAMP pathway at concentrations below 60 pM is very unlikely; and (4) Activation of the PLC/IP3 pathway at physiological concentrations produces intracellular effects that are similar to those produced by activation of the AC/cAMP pathway at concentrations above 100 pM, including elevated intracellular calcium and altered activities and expressions of key enzymes involved in glycogenolysis, gluconeogenesis, and glycogen synthesis. Under metabolically stressful conditions, as in the early neonate or exercising adult, plasma glucagon concentrations often exceed 100 pM, recruiting the AC/cAMP pathway and enhancing the activation of PLC/IP3 pathway to boost glucose output, adaptively meeting the elevated systemic glucose demand. Whether the AC/cAMP pathway is consistently activated in starvation or diabetes is not clear. Because the importance of glucagon in the pathogenesis of diabetes is becoming increasingly evident, it is even more urgent now to resolve lingering uncertainties and definitively establish glucagon’s true mechanism of glycemia regulation in health and disease.
Collapse
Affiliation(s)
- Robert L. Rodgers
- Department of Biomedical and Pharmaceutical Sciences College of Pharmacy University of Rhode Island Kingston Rhode Island USA
| |
Collapse
|
2
|
Ernest CS, Hooker AC, Karlsson MO. Methodological comparison of in vitro binding parameter estimation: sequential vs. simultaneous non-linear regression. Pharm Res 2010; 27:866-77. [PMID: 20221897 DOI: 10.1007/s11095-010-0082-1] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/03/2009] [Accepted: 02/09/2010] [Indexed: 10/19/2022]
Abstract
PURPOSE Analysis of simulated data was compared using sequential (NLR) and simultaneous non-linear regression (SNLR) to evaluate precision and accuracy of ligand binding parameter estimation. METHODS Commonly encountered experimental error, specifically residual error of binding measurements (RE), experiment-to-experiment variability (BEV) and non-specific binding (B(NS)), were examined for impact of parameter estimation using both methods. Data from equilibrium, dissociation, association and non-specific binding experiments were fit simultaneously (SNLR) using NONMEM VI compared to the common practice of analyzing data from each experiment separately and assigning these as exact values (NLR) for estimation of the subsequent parameters. RESULTS The greatest contributing factor to bias and variability in parameter estimation was RE of the measured concentrations of ligand bound; however, SNLR provided more accurate and less bias estimates. Subtraction of B(NS) from total ligand binding data provided poor estimation of specific ligand binding parameters using both NLR and SNLR. Additional methods examined demonstrated that the use of SNLR provided better estimation of specific binding parameters, whereas there was considerable bias using NLR. NLR cannot account for BEV, whereas SNLR can provide approximate estimates of BEV. CONCLUSION SNLR provided superior resolution of parameter estimation in both precision and accuracy compared to NLR.
Collapse
Affiliation(s)
- C Steven Ernest
- Department of Pharmaceutical Biosciences, Uppsala University, Uppsala, Sweden.
| | | | | |
Collapse
|
3
|
Unson CG, Wu CR, Sakmar TP, Merrifield RB. Selective stabilization of the high affinity binding conformation of glucagon receptor by the long splice variant of Galpha(s). J Biol Chem 2000; 275:21631-8. [PMID: 10791965 DOI: 10.1074/jbc.m002093200] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
To analyze functional differences in the interactions of the glucagon receptor (GR) with the two predominant splice variants of Galpha(s), GR was covalently linked to the short and the long forms Galpha(s)-S and Galpha(s)-L to produce the fusion proteins GR-Galpha(s)-S and GR-Galpha(s)-L. GR-Galpha(s)-S bound glucagon with an affinity similar to that of GR, while GR-Galpha(s)-L showed a 10-fold higher affinity for glucagon. In the presence of GTPgammaS, GR-Galpha(s)-L reverted to the low affinity glucagon binding conformation. Both GR-Galpha(s)-L and GR-Galpha(s)-S were constitutively active, causing elevated basal levels of cAMP even in the absence of glucagon. A mutant GR that failed to activate G(s) (G23D1R) was fused to Galpha(s)-L. G23D1R-Galpha(s)-L bound glucagon with high affinity, but failed to elevate cAMP levels, suggesting that the mechanisms of GR-mediated Galpha(s)-L activation and Galpha(s)-L-induced high affinity glucagon binding are independent. Both GR-Galpha(s)-S and GR-Galpha(s)-L bound the antagonist desHis(1)[Nle(9),Ala(11),Ala(16)]glucagon amide with affinities similar to GR. The antagonist displayed partial agonist activity with GR-Galpha(s)-L, but not with GR-Galpha(s)-S. Therefore, the partial agonist activity of the antagonist observed in intact cells appears to be due to GRs coupled to Galpha(s)-L. We conclude that Galpha(s)-S and Galpha(s)-L interact differently with GR and that specific coupling of GR to Galpha(s)-L may account for GTP-sensitive high affinity glucagon binding.
Collapse
Affiliation(s)
- C G Unson
- Rockefeller University and the Howard Hughes Medical Institute, Laboratory of Molecular Biology and Biochemistry, Rockefeller University, New York, New York 10021, USA.
| | | | | | | |
Collapse
|
4
|
Navarro I, Leibush B, Moon TW, Plisetskaya EM, Baños N, Méndez E, Planas JV, Gutiérrez J. Insulin, insulin-like growth factor-I (IGF-I) and glucagon: the evolution of their receptors. Comp Biochem Physiol B Biochem Mol Biol 1999; 122:137-53. [PMID: 10327604 DOI: 10.1016/s0305-0491(98)10163-3] [Citation(s) in RCA: 85] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/18/2022]
Abstract
Insulin and glucagon, two of the most studied pancreatic hormones bind to specific membrane receptors to exert their biological actions. Insulin-like growth factors IGF-I and IGF-II are structurally related to insulin, although they are expressed ubiquitously. The biological functions of the IGFs are mediated by different transmembrane receptors, which includes the insulin, IGF-I and IGF-II receptors. The interaction of insulin, insulin related peptides and glucagon with the corresponding receptors has been studied extensively in mammals and continues to be so. At the same time, research on ectothermic animals has made enormous progress in the recent years. This paper summarizes current knowledge on insulin, IGF-I and glucagon receptors, from a comparative point of view with special attention to non-mammalian vertebrates. The review covers adult and mostly typical target tissues, and with very few exceptions, developmental aspects are not considered. Binding characteristics, tissue distribution and structure of insulin and IGF-I receptors will be considered first, because both ligands and receptors are structurally related and have overlapping functions. These sections will be followed by similar distribution of information on glucagon receptors. Readers interested in either structure or functions of insulin, IGFs and glucagon in nonmammalian vertebrates are referred to other reviews (Mommsen TP, Plisetskaya EM. Insulin in fishes and agnathans: history, structure and metabolic regulation. Rev Aquat Sci 1991;4:225-259; Mommsen TP, Plisetskaya EM. Metabolic and endocrine functions of glucagon-like peptides: evolutionary and biochemical perspectives. Fish Physiol Biochem 1993;11:429-438; Duguay SJ, Mommsen TP. Molecular aspects of pancreatic peptides. In: Sherwood NM, Hew CL, editors, Fish Physiology. vol 13. 1994:225-271; Plisetskaya EM, Mommsen TP. Glucagon and glucagon-like peptides in fishes. Int Rev Citol 1996;168:187-257.).
Collapse
Affiliation(s)
- I Navarro
- Departament de Fisiologia, Facultat de Biologia, Universitat de Barcelona, Spain
| | | | | | | | | | | | | | | |
Collapse
|
5
|
Abstract
The classic concept of gastrointestinal endocrinology is that of a few peptides released to the circulation from endocrine cells, which are interspersed among other mucosal cells in the upper gastrointestinal tract. Today more than 30 peptide hormone genes are known to be expressed throughout the digestive tract, which makes the gut the largest endocrine organ in the body. Moreover, development in cell and molecular biology now makes it feasible to describe a new biology for gastrointestinal hormones based on five characteristics. 1) The structural homology groups the hormones into families, each of which is assumed to originate from a common ancestral gene. 2) The individual hormone gene is often expressed in multiple bioactive peptides due to tandem genes encoding different hormonal peptides, alternative splicing of the primary transcript, or differentiated processing of the primary translation product. By these mechanisms, more than 100 different hormonally active peptides are produced in the gastrointestinal tract. 3) In addition, gut hormone genes are widely expressed, also outside the gut. Some are expressed only in neuroendocrine cells, whereas others are expressed in a multitude of different cells, including cancer cells. 4) The different cell types often express different products of the same gene, "cell-specific expression." 5) Finally, gastrointestinal hormone-producing cells release the peptides in different ways, so the same peptide may act as an acute blood-borne hormone, as a local growth factor, as a neurotransmitter, and as a fertility factor. The new biology suggests that gastrointestinal hormones should be conceived as intercellular messengers of general physiological impact rather than as local regulators of the upper digestive tract.
Collapse
Affiliation(s)
- J F Rehfeld
- Department of Clinical Biochemistry, Rigshospitalet, University of Copenhagen, Denmark
| |
Collapse
|
6
|
Bass J, Kurose T, Pashmforoush M, Steiner DF. Fusion of insulin receptor ectodomains to immunoglobulin constant domains reproduces high-affinity insulin binding in vitro. J Biol Chem 1996; 271:19367-75. [PMID: 8702623 DOI: 10.1074/jbc.271.32.19367] [Citation(s) in RCA: 36] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/01/2023] Open
Abstract
A unique feature of the insulin receptor is that it is dimeric in the absence of ligand. Dimerization of two adjacent transmembrane domain (TMD) alpha helices has been shown to be critical in receptor kinase activation. Moreover, previous work has suggested that the TMD is involved in stabilizing the high-affinity binding site; soluble receptors expressed after simple truncation at the ectodomain-TMD junction have reduced affinity for insulin. To further examine this issue, we have replaced the TMD and intracellular domain of the soluble human insulin receptor (HIRs) with constant domains from immunoglobulin Fc and lambda subunits (HIRs-Fc and HIRs-lambda). Studies of receptor biosynthesis and binding characteristics were performed following transient transfection of receptor cDNAs into human embryonal kidney 293 cells. Each hybrid receptor was initially synthesized as a single chain proreceptor, followed by cleavage into alpha- and beta-Fc or beta-lambda subunits. The majority of secreted protein appeared in the cell medium as fully processed heterotetramer. Fc fragments released from HIRs-Fc by papain digestion and analyzed by nonreducing SDS-polyacrylamide gel electrophoresis were dimeric. Furthermore, dissociation constants for both chimeras were similar to those for the full-length holoreceptor (wild-type receptor, Kd1 = 200 pM and Kd2 = 2 nM; HIRs-Fc, Kd1 = 200 pM and Kd2 = 40 nM; and HIRs-lambda, Kd1 = 200 pM and Kd2 = 5 nM). These results extend previous observations that dimerization of the membrane-proximal ectodomain is necessary to maintain an intact high-affinity insulin-binding site.
Collapse
Affiliation(s)
- J Bass
- Department of Medicine, Section of Endocrinology, University of Chicago, Chicago, Illinois 60637, USA
| | | | | | | |
Collapse
|
7
|
Abstract
The objective of the present studies was to determine whether the existence of functional glucagon receptors could be established on lympoid cells. The glucagon receptor, which positively regulates adenylate cyclase, is a member of the superfamily of seven transmembrane domain G-protein coupled receptors. Previously reported specific binding with [125I]-glucagon to a variety of lymphoid and myeloid cell preparations suggests that glucagon receptors are expressed within the immune system. In the present study, Northern analysis of polyA RNA isolated from primary mouse and rat derived lymphoid tissues and lymphoid cell lines EL-4.IL-2, Jurkat E6-1, CH12LX, and BCL1-3B3 cells were probed with a 32P-labeled human hepatic glucagon receptor. Mouse spleen and thymus, rat spleen, and the B cell line, CH12LX, all possessed a single 1.5 kb fragment (BCL1-3B3, 1.4 kb) which hybridized to the glucagon receptor cDNA probe, as compared to mouse liver which exhibited a 2.8 kb fragment. EL-4.IL-2 and Jurkat E6-1 cells possessed a 3.7 kb fragment with an additional 2.75 kb band present in Jurkat E6-1 cells. Treatment of mouse splenocytes and T- and B-lymphoma cells with glucagon (0 - 100 nM) produced a dose-dependent enhancement in intracellular cAMP which was maximal at 5 min post treatment followed by a gradual decline. Direct addition of glucagon to spleen cell cultures over a broad concentration range produced no effect on either lymphoproliferation following stimulation with anti-CD3 mAb, or LPS nor on the antibody forming cell (AFC) response to sRBC. Conversely, glucagon effectively reversed the suppression of the sRBC AFC response produced by delta9-tetrahydocannabinol (delta9-THC), and partially reversed the suppression produced by 2',3'-dideoxyadenosine, both of which are potent inhibitors of adenylate cyclase. These studies confirm the expression of functional glucagon receptors on lymphoid cells.
Collapse
Affiliation(s)
- W S Koh
- Department of Pharmacology and Toxicology, Michigan State University, East Lansing, MI, USA
| | | | | | | |
Collapse
|
8
|
Abstract
Glucagon and glucagon-like peptides (GLPs) are coencoded in the vertebrate proglucagon gene. Large differences exist between fishes and other vertebrates in gene structure, peptide expression, peptide chemistry, and function of the hormones produced. Here we review selected aspects of glucagon and glucagon-like peptides in vertebrates with special focus on the contributions made by analysis of piscine systems. Our topics range from the history of discovery to gene structure and expression, through primary structures and regulation of plasma concentrations to physiological effects and message transduction. In fishes, the pancreas synthesizes glucagon and GLP-1, while the intestine may contribute oxyntomodulin, glucagon, GLP-1, and GLP-2. The pancreatic gene is short and lacks the sequence for GLP-2. GLP-1, which is produced exclusively in its biologically active form, is a potent metabolic hormone involved in regulation of liver glycogenolysis and gluconeogenesis. The responsiveness of isolated hepatocytes to glucagon is limited to high concentrations, while physiological concentrations of GLP-1 effectively regulate hepatic metabolism. Plasma concentrations of GLP-1 are higher than those of glucagon, and liver is identified as the major site of removal of both hormones from fish plasma. Ultimately, GLP-1 and glucagon exert effects on glucose metabolism that directly and indirectly oppose several key actions of insulin. Both glucagon and GLP-1 show very weak insulinotropic activity, if any, when tested on fish pancreas. Intracellular message transduction for glucagon, especially at slightly supraphysiological concentrations, involves cAMP and protein kinase A, while pathways for GLP are largely unknown and may involve a multitude of messengers, including cAMP. In spite of fundamental differences in GLP-1 function between fishes and mammals, fish GLP-1 is as powerful an insulinotropin for mammalian B-cells as mammalian GLP-1 is a metabolic hormone if tested on piscine liver.
Collapse
Affiliation(s)
- E M Plisetskaya
- School of Fisheries, University of Washington, Seattle 98195, USA
| | | |
Collapse
|
9
|
Affiliation(s)
- J Christophe
- Department of Experimental Surgery, Medical School, Université Libre de Bruxelles, Belgium
| |
Collapse
|
10
|
|
11
|
Carruthers CJ, Unson CG, Kim HN, Sakmar TP. Synthesis and expression of a gene for the rat glucagon receptor. Replacement of an aspartic acid in the extracellular domain prevents glucagon binding. J Biol Chem 1994. [DOI: 10.1016/s0021-9258(19)62047-8] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/22/2022] Open
|
12
|
Abstract
The discovery of aspartic acid at position 9 in glucagon to be a critical residue for transduction has spurred renewed efforts to identify other strategic residues in the peptide sequence that dictate either receptor binding or biological activity. It also became apparent from further studies that Asp9 operates in conjunction with His1 in the activation mechanism that follows binding to the glucagon receptor. Indeed, it was later demonstrated that the protonatable histidine imidazole is important for transduction. It is likely that the interaction of a positively charged histidine 1 with a negatively charged aspartic acid 9 might be part of the triggering step at the molecular level. Two other aspartic acid residues in glucagon are capable of assuming a similar role, namely that of contributing to an electrostatic attraction with histidine via a negative carboxylate. These studies were conducted to investigate the role of aspartic acid 15 and 21 in glucagon action. Evidence reported here, gathered from 31 replacement analogs, supports the idea that in the absence of the requisite carboxyl group at position 9, histidine utilizes Asp21 or Asp15 as a compensatory site. Asp15 was also found to be indispensable for binding and may serve to tether the hormone to the receptor protein at the binding site. It is also demonstrated that these new findings promote the design of better glucagon antagonists.
Collapse
Affiliation(s)
- C G Unson
- Rockefeller University, New York, New York 10021
| | | | | |
Collapse
|
13
|
Post S, Miyazaki H, Tager H. Identification of a Mg(2+)- and guanyl nucleotide-dependent glucagon receptor cycle by use of permeabilized canine hepatocytes. J Biol Chem 1992; 267:25776-85. [DOI: 10.1016/s0021-9258(18)35677-1] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/19/2022] Open
|
14
|
Unson CG, Macdonald D, Ray K, Durrah TL, Merrifield RB. Position 9 replacement analogs of glucagon uncouple biological activity and receptor binding. J Biol Chem 1991. [DOI: 10.1016/s0021-9258(18)49911-5] [Citation(s) in RCA: 74] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/27/2022] Open
|