The biological significance of "enteroglucagon." Present status

Striking the Balance: GLP-1/Glucagon Co-Agonism as a Treatment Strategy for Obesity

Obesity and Type 2 diabetes represent global health challenges, and there is an unmet need for long-lasting and effective pharmacotherapies. Although long-acting glucagon-like peptide-1 (GLP-1) analogues are now in routine use for diabetes and are now being utilised for obesity per se, the need for ever better treatments has driven the development of co-agonists, with the theoretical advantages of improved efficacy by targeting multiple pathways and reduced adverse effects. In this review, we highlight the past and present progress in our understanding and development of treatments based on GLP-1/glucagon co-agonism. We also reflect on the divergent effects of varying the GLP-1:glucagon activity and ratio in the context of pre-clinical and human clinical trial findings. In particular, the multiple metabolic actions of glucagon highlight the importance of understanding the contributions of individual hormone action to inform the safe, effective and tailored use of GLP-1/glucagon co-agonists to target weight loss and metabolic disease in the future.

Glucagon-like Peptide 1 (GLP-1): An Intestinal Hormone, Signalling Nutritional Abundance, with an Unusual Therapeutic Potential

The incretin hormone, glucagon-like peptide 1 (GLP-1) has many actions; namely: (1) it enhances all steps of insulin biosynthesis and potentiates glucose-induced secretion; (2) it seems to have trophic effects on pancreatic cells; (3) it inhibits glucagon secretion; (4) it inhibits hepatic glucose production and lowers blood glucose, but does not produce severe hypoglycaemia; (5) it inhibits postprandial gastrointestinal motility and secretion; and (6) it reduces appetite and food intake. Because of this, current research is focusing upon development of a clinically practicable therapy for type 2 diabetes mellitus based on GLP-1.

Glucagon and glucagon-like peptides in fishes

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.

Pancreatic glucagon signals postprandial satiety

The hypothesis that prandial increases in circulating pancreatic glucagon initiates an important peripheral satiety signal is reviewed. Glucagon administration at the beginning of meals reduces the size of test meals in animals and humans and reduces the size of spontaneous meals in rats. Exogenous glucagon may also interact synergistically with cholecystokinin to inhibit feeding. These appear to be satiety effects because they are behaviorally specific in rats and subjectively specific in humans. Glucagon's pharmacological satiety effect is complemented by compelling evidence for a necessary contribution of endogenous glucagon to the control of meal size: administration of glucagon antibodies increases both test and spontaneous meal size in rats. Under many, but not all, conditions exogenous glucagon's satiety effect appears to originate in the liver and to be relayed to the brain via hepatic vagal afferents. Analysis of the central processing of this signal, however, has barely begun. How glucagon changes are transduced into neural afferent signals also remains an open question. The only hypothesis that has been extensively tested is that stimulation of hepatic glucose production initiates the satiety signal, but this is neither convincingly supported nor clearly rejected by currently available data. It is also not yet clear whether glucagon contributes to some forms of obesity or has potential use as a therapeutic tool in the control of eating disorders. Of the several proposed controls of hunger and satiety, glucagon appears to be one of the most likely to be physiologically relevant. This encourages further analysis of its behavioral characteristics, its neural mechanisms, and its clinical potential.

The effect of oxyntomodulin (glucagon-37) and glucagon on exocrine pancreatic secretion in the conscious rat

The inhibitory effect of glucagon on exocrine pancreas has been the subject of controversial reports. On the other hand, oxyntomodulin (bioactive enteroglucagon or glucagon-37), a 37 amino acid peptide isolated from porcine lower intestine, has been shown to be 10-20 times more potent than glucagon in inhibiting gastric acid secretion in the rat. In view of this, the effect of glucagon and oxyntomodulin on basal and caerulein-stimulated pancreatic secretion has been studied, during re-introduction of pancreatic juice into duodenum, in the conscious rat provided with pancreatic and duodenal fistulas. A depression of pancreatic function was observed with both peptides on the three parameters studied: (volume of juice secreted, bicarbonate and protein output), either under basal conditions or during stimulation by caerulein. In all the experimental conditions used, oxyntomodulin was ca. ten times more potent than glucagon in its inhibitory effect. The fact that oxyntomodulin, as what is observed in the stomach, is one order of magnitude more potent than glucagon in inhibiting pancreatic secretion suggests that the biological mechanisms by which the peptides of the glucagon-family act on exocrine pancreas are similar, or related to that present at the gastric level.

The receptors of the VIP family peptides (VIP, secretin, GRF, PHI, PHM, GIP, glucagon and oxyntomodulin). Specificities and identity

A model is proposed for the receptors of the VIP family peptides including a ligand and a cellular domain. Specificities of the receptors are due to different ligand binding sites. Three subgroups of the family can be distinguished accordingly: glucagon and oxyntomodulin; GIP; VIP, secretin r and hGRF, PHI and PHM. In the same species, the expression of these different sites is cell-specific resulting in a stoichiometry of the ligand-receptor interaction which is compatible with physiological regulation of cell function. Specificities of the interaction as studied by native and synthetic analogs is supported both by restricted sequences of amino acids (such as that including the N-terminal histidine residue), and membrane-induced configuration of the ligand. Identity of the receptors is related to their interactions with subunits of the adenylate cyclase system. Arguments are put forward indicating that the alpha subunit of the guanyl regulatory protein is a reasonable candidate for directly transducing to the adenylyl cyclase the information contained in the activated ligand-binding site subunits. Evidence of functional and molecular heterogeneity of the recognizing site and of the alpha subunits leads to the supposition that some types of specific complementarity is retained at this level of interaction, further enhancing the possibility of species and cell differences. On the other hand, the identities found in other sequences of the alpha and ras oncogene products extend to the receptor of the VIP family peptides a pattern of organization which is similar to that recently described for the insulin family of receptors. The role of ligand specific receptor mediated regulation in homologous or heterologous desensitization is reviewed in brief for the peptides of the VIP family as well as the appearance of the specific receptor during the ontogenesis or the cell differentiation. The co-distribution of plasma membrane receptors from other families further adds to the cell specificity resulting for each differentiated cell in unique patterns of recognizing site. Some examples of receptor-receptor interaction are given, indicating that the integration of the different signals by cells might occur at an early step through the transmembranair domain of the receptor.

Oxyntomodulin and its C-terminal octapeptide inhibit liquid meal-stimulated acid secretion

Oxyntomodulin (OXM), a 37-amino acid glucagon-containing peptide produced mainly in intestine and endocrine pancreas, is present in rat plasma and inhibits pentagastrin-stimulated gastric acid secretion in both the anesthetized and the conscious rat. In order to investigate the modifications in acid and water secretions in a physiological model, we set up a protocol which allowed us to study acid secretion in the conscious rat both under basal conditions (during an 18-hour fast) or after a physiological stimulus (a liquid meal). OXM (110 pmol X kg-1) did not modify the basal acid or water output in an 18-hour fasting state. When injected before the test meal, OXM (225 pmol X kg-1) sharply decreased the acid output stimulated by a liquid meal (milk), which represented 10 times the basal value. Results are compared to those obtained when pentagastrin was the stimulant. Synthetic C-terminal octapeptide of OXM was able to inhibit both basal and meal-stimulated secretions. We can conclude that OXM, or a closely related peptide containing the C-terminal octapeptide, may be a physiological regulator of gastric functions.