Noncooperative receptor interactions of glucagon and eleven analogues: inhibition of adenylate cyclase

Glucagon, cyclic AMP, and hepatic glucose mobilization: A half‐century of uncertainty

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.

Promotion of differentiation in human colon carcinoma cells by vasoactive intestinal polypeptide

The effects of vasoactive intestinal polypeptide (VIP) and dibutyryl cyclic adenosine 3':5'monophosphate (dbcAMP) on two human colon carcinoma cell lines, HCT 116 and GEO, were investigated. VIP and dbcAMP inhibited the growth of both cell lines in monolayer culture in a dose-dependent manner. Within 6 h of treatment with 1 mM dbcAMP or 0.3 microM VIP, numerous mucin-like droplets were secreted by GEO cells. VIP and dbcAMP also increased carcinoembryonic antigen (CEA) secretion. In both cell lines, a 9-fold increase in conditioned medium CEA levels was observed at 1 mM dbcAMP and a 2.6-fold increase at 1.5 microM VIP. Time- and concentration-dependent evaluation in cAMP levels were elicited by VIP in the two cell lines. Immunocytochemical studies for cell-surface glycoprotein detection in GEO cells showed that VIP induced a morphological and functional organization of mucin-secreting cells. These results indicate that VIP and dbcAMP have antiproliferative and strong differentiation-promoting effects in colon cancer cells. This is the first report of VIP-induced mucin secretion in colon tumor cells.

Quantitative analysis of internalization of glucagon by isolated hepatocytes

Biochemical methods have been used to quantitate total, acid-stable and acid-labile association of (mono[125I]iodoTyr10) glucagon with rat hepatocytes in suspension to evaluate internalization of glucagon and its receptors. Internalization is inhibited by low temperature, phenylarsine oxide, and by blocking receptor binding, consistent with receptor-mediated endocytosis. Approximately 30% of the total cell-associated hormone is internalized at 30 min of incubation. The rate declines until 90 min when the internalization of glucagon ceases, although the cells remain competent to internalize asialofetuin. From 90 min to 4 h, 27% of the maximum label internalized at 30 min remains within cells. The number of cell surface receptors decreases but the affinity of those remaining is unchanged. However, 1.7-2.7 surface receptors are lost to binding for each molecule of radiolabeled glucagon internalized. Uptake occurs according to a rate constant of 0.183 min-1 (t1/2 = 3.8 min). We conclude that (i) hepatocytes internalize a finite quantity of glucagon, implying the existence of undefined regulatory mechanisms; (ii) hormone is retained for greater than 2 h within cells and may play a physiological role within cells; and (iii) both occupied and unoccupied receptors become inaccessible to extracellular hormone as internalization proceeds; rapid recycling of receptors does not occur.

Guanine nucleotide regulation of the interconversion of the two-state hepatic glucagon receptor system of rat

To investigate whether guanine nucleotides regulate interconversion of the two-state hepatic glucagon receptor we have utilized kinetic assays of glucagon binding to partially purified rat liver plasma membranes. Dissociation of glucagon at 30 degrees C exhibited biexponential character in either the absence or presence of GTP, indicating that the system previously seen in intact hepatocytes is independent of intracellular modulators. In each case the receptors underwent a time-dependent conversion from a low affinity to a high affinity state. However, GTP decreased the fraction of receptors in the high affinity state. The rank order for stabilizing the low affinity state was Gpp(NH)p greater than GTP greater than GDP much greater than GMP = no nucleotides. Data from competition binding assays with increasing concentrations of GTP allow calculation of equilibrium constants which are 3.32 nM for glucagon and receptor in the absence of GTP, 18.6 nM for glucagon and receptor in the presence of GTP, 1.55 microM for the association of receptor and GTP presumably linked to an N protein, and 8.86 microM for the association of the glucagon-receptor complex and GTP again presumably linked to an N protein, Glucagon binding to receptor is noncooperative in both the absence and presence of GTP, distinguishing this system from the beta-adrenergic system. With GTP, binding to the low affinity state is favored because of the relative affinities reported. Therefore, GTP regulates the activation by slowing the conversion of the receptor from a low affinity to high affinity form.

Receptor binding of selectively labeled (Tyr-10) and (Tyr-13)-mono-125I-glucagons and competition by homologous 127I-labeled isomers

Two monoiodinated derivatives of glucagon were prepared by lactoperoxidase catalyzed iodination followed by separation on reverse-phase high-performance liquid chromatography. The purified (Tyr-10) and (Tyr-13)-mono-125I-labeled glucagon isomers were characterized and studied with respect to their binding to the receptors of isolated intact rat hepatocytes. The extent of steady-state binding to cellular receptor sites differed for the two labeled glucagon tracers at 37 degrees C as well as at 15 degrees C with (Tyr-10)-mono-125I-glucagon displaying higher receptor binding. The apparent equilibrium constants, Kd,app at 37 degrees C are 3.6 +/- 0.4 nM (mean +/- S.E. of three independent experiments) for the tyrosine-13-labeled tracer and 5.9 +/- 0.6 nM for the tyrosine-10-labeled glucagon with native glucagon as competitor. Since the observed Kd in the competition assay is a function of the true Kd values of the monoiodinated radioactive glucagon isomers and native glucagon, the dissociation constants were also measured with chemically identical tracer and competitor. Under these conditions, we obtained Kd values of 1.3 +/- 0.2 nM for the tyrosine-10-labeled analog and 2.0 +/- 0.2 nM for the tyrosine-13-labeled glucagon isomers confirming the higher receptor binding affinity of (Try-10)-mono-125I-glucagon. All competition curves fit the mathematical expression for a model of non-cooperative binding to a single class of receptors.

Glucagon-like peptide I stimulates insulin gene expression and increases cyclic AMP levels in a rat islet cell line

Insulin secretion is controlled by a complex set of factors. Although blood glucose levels serve as the major stimulus of insulin secretion in mammals, insulin release is also modulated by amino acids, catecholamines, glucagon, and other, intestinal hormones. The identification of factors that modulate insulin production has engendered much interest because of their potential importance in the altered dynamics of insulin secretion in response to glucose characteristic of maturity-onset diabetes mellitus. Decoding of the glucagon gene has uncovered two additional glucagon-like peptides encoded in proglucagon, the polypeptide precursor of glucagon. One of these peptides, glucagon-like peptide I, is processed from proglucagon in two forms, of 31 and 37 amino acids. We report that the smaller of the two glucagon-like peptides potently increases cAMP levels, insulin mRNA transcripts, and insulin release in cultured rat insulinoma cells. These results indicate that glucagon-like peptide I may be a physiologic modulator of insulin gene expression.

Biological activities of catfish glucagon and glucagon-like peptide

The ability of catfish glucagon and glucagon-like peptide to bind and activate mammalian glucagon receptors was investigated. Neither catfish peptide binds to glucagon receptors of rat liver, hypothalamus or pituitary. Neither stimulates adenylate cyclase activity in liver membranes. Catfish glucagon fails to activate adenylate cyclase in hypothalamic or pituitary membranes in contrast to mammalian glucagon. However, catfish glucagon-like peptide does stimulate hypothalamic and pituitary adenylate cyclase (EC50 approximately 1 pM) possibly through mammalian glucagon-like peptide receptors.

Identification of distinct receptor complexes that account for high-and low-affinity glucagon binding to hepatic plasma membranes

We have analyzed ligand-receptor complexes resulting from (i) the incubation of canine hepatic plasma membranes with [125I]iodoglucagon and (ii) subsequent gentle solubilization of receptor-bound ligand with digitonin. The complexes (molecular weight approximately equal to 500,000) retain the radiolabeled ligand during gel filtration and subsequent manipulation at 4 degrees C in the absence of covalent crosslinking. Affinity chromatography of the glucagon-receptor complexes on columns of wheat germ lectin linked to agarose resulted in two fractions, one of which was not bound by the column and the other of which was specifically eluted by N-acetylglucosamine. The presence of GTP during the incubation of plasma membranes with [125I]iodoglucagon caused about a 50% decrease in total ligand binding but affected only the ligand-receptor complexes that bound to wheat germ lectin. Moreover, it was found that the proportion of the two forms of ligand-receptor complexes identified by chromatography on wheat germ lectin depended on the degree of saturation of the membrane receptor. Thus, both the inhibition by glucagon of radiolabeled glucagon binding to membranes and the concomitantly decreased extent of association of the radiolabeled ligand with solubilized receptor complexes could be modeled in terms of two noninteracting receptor populations (having dissociation constants of about 0.35 and 4.94 X 10(-9) M). We conclude that (i) glucagon-receptor complexes formed on canine hepatic plasma membranes exist in two forms that differ after solubilization by digitonin in their avidities for wheat germ lectin, (ii) the high-and low-affinity binding of glucagon characteristic of hepatic plasma membranes arises from distinct receptor populations that probably differ in glycosylation, and (iii) the effect of GTP to decrease binding of glucagon to membranes arises from interactions of the nucleotide with the receptor complex that binds to wheat germ lectin.

Alteration with dietary state of the activity and zonal distribution of adenylate cyclase stimulated by glucagon, fluoride and forskolin in microdissected rat liver tissue

Adenylate cyclase activated by glucagon, fluoride and forskolin was measured in liver homogenates and microdissected periportal and perivenous tissue of fed and fasted rats. A radiochemical microtest, more sensitive by 2-3 orders of magnitude as compared with the usual assay, was established for the determination of the activity in liver samples corresponding to 200-600 ng dry weight. In liver homogenates from fasted as compared to fed animals the glucagon-stimulated and fluoride-stimulated activity was increased by 1.65-fold, while the basal and the forskolin-stimulated activity remained the same. In microdissected tissue of both fed and fasted animals the activity was stimulated in about 60% of the samples by glucagon, fluoride and forskolin (responsive samples). However, in about 40% of the microdissected tissue samples the activity could not be stimulated by any of the above activators (non-responsive samples). In responsive microdissected tissue of fasted as compared to fed animals, the glucagon-stimulated and fluoride stimulated activity but not the basal and the forskolin-activated activity was increased by 2-3-fold. In responsive microdissected samples of fed animals neither the basal nor the stimulated activities showed a significant periportal to perivenous gradient. In samples of fasted animals, however, a zonal gradient was observed for the glucagon-stimulated activity exhibiting a 1.5-fold higher rate in the perivenous zone.

Hormone receptor-coupling factor-adenylate cyclase interaction: theoretical considerations

The steady-state response properties of two current models for receptor/nucleotide coupling protein/adenylate cyclase systems are examined by computer modeling techniques. In the model of Levitzki (Trends Pharmac. Sci., May, 1982, pp. 203-208), a ligand may give rise to full or partial agonist behavior only. In the model of Stadel, DeLean, and Lefkowitz (1982) configurations of the rate constants can be found which lead not only to full or partial agonist behavior, but also to varying degrees of inhibition at sufficiently high concentrations of ligand, as observed experimentally in a variety of adenylate cyclase systems. In the latter model, it is also possible to find configurations of the rate constants for which addition of a ligand will lead to inhibition of adenylate cyclase activity. The nature of partial agonism and reasons as to why it may be expected to occur for a wide variety of ligands are discussed.

Human glucagon-like peptides 1 and 2 activate rat brain adenylate cyclase

Two human glucagon-like peptides, GLP-1 and GLP-2, which are coencoded with pancreatic glucagon in the preproglucagon gene, do not significantly inhibit [125I]monoiodoglucagon binding to rat liver and brain membranes and do not activate adenylate cyclase in liver plasma membranes. Nevertheless, GLP-1 and GLP-2 were each found to be potent stimulators of both rat hypothalamic and pituitary adenylate cyclase. Only 30-50 pM concentrations of each peptide elicited half-maximal adenylate cyclase stimulation. Our data suggest that GLP-1 and GLP-2 may be neurotransmitters and/or neuroendocrine effectors, which would account for their high degree of sequence conservation through vertebrate evolution.

N alpha-Malto-glucagon and N alpha-malto, S-methyl methionine27-glucagon: preparation and characterization of two partial agonists

N alpha-Maltoglucagon was prepared by demethylation of N alpha-malto, S-methyl methionine27 glucagon, and the two derivatives were purified to greater than 99% and 99.7%, respectively. S-Methylation of glucagon lowers the reactivity of Lys-12 and provides an alternative strategy to epsilon-amino protection for directing glycosylation of glucagon to the alpha-amino group. Both derivatives are partial agonists, with their adenylate cyclase activation and binding reduced in parallel. N alpha-Maltoglucagon produces 70% and N alpha-malto, S-methyl methionine27 glucagon 40% of the maximum activity of native hormone. N alpha-Maltoglucagon binds equivalently to N alpha-biotinyl, N epsilon-acetimidoglucagon whose maximum activity is near 35%, but a pK shift of the imidazole moiety cannot account for the difference in their abilities to produce transduction. Both glycosylated derivatives bind noncooperatively and both inhibit adenylate cyclase at high concentrations. The presence of a maltose residue on the amino terminal of glucagon may be required but, alone provides insufficient structural complementarity for concanavalin A binding to occur. The glycosylated derivatives are resistant to aminopeptidase degradation, are more soluble, and the maltose residue is unlikely to cause toxicity with in vivo use. Such attributes may be advantageous in the development of other analogs.

Heterogeneity of glucagon receptors of rat hepatocytes: a synthetic peptide probe for the high affinity site

A glucagon analog with the following sequence has been synthesized: His- Ser-Gln-Gly-Thr-Phe-Thr-Ser-Asp-Tyr-Ser-Lys-Tyr-Leu-Asp-Ser-Arg-Arg -Leu-Gln-Glu-Phe-Leu-Gln-Trp-Ala-Leu-Gln-Thr. When interacting with rat hepatocytes, the analog mimics, in part, the activities of glucagon in receptor binding and inhibition of carbohydrate incorporation into glycogen. Comparison of the binding of the analog with that of glucagon demonstrates the existence of two distinct homogeneous populations of glucagon receptors. The synthetic analog acts as a specific probe for those receptors that have a high affinity for glucagon.