Major contributions of comparative endocrinology to the development and exploitation of the incretin concept

Variation in the Evolution and Sequences of Proglucagon and the Receptors for Proglucagon-Derived Peptides in Mammals

The mammalian proglucagon gene (Gcg) encodes three glucagon like sequences, glucagon, glucagon-like peptide-1 (GLP-1), and glucagon-like peptide-2 that are of similar length and share sequence similarity, with these hormones having cell surface receptors, glucagon receptor (Gcgr), GLP-1 receptor (Glp1r), and GLP-2 receptor (Glp2r), respectively. Gcgr, Glp1r, and Glp2r are all class B1 G protein-coupled receptors (GPCRs). Despite their sequence and structural similarity, analyses of sequences from rodents have found differences in patterns of sequence conservation and evolution. To determine whether these were rodent-specific traits or general features of these genes in mammals I analyzed coding and protein sequences for proglucagon and the receptors for proglucagon-derived peptides from the genomes of 168 mammalian species. Single copy genes for each gene were found in almost all genomes. In addition to glucagon sequences within Hystricognath rodents (e.g., guinea pig), glucagon sequences from a few other groups (e.g., pangolins and some bats) as well as changes in the proteolytic processing of GLP-1 in some bats are suggested to have functional effects. GLP-2 sequences display increased variability but accepted few substitutions that are predicted to have functional consequences. In parallel, Glp2r sequences display the most rapid protein sequence evolution, and show greater variability in amino acids at sites involved in ligand interaction, however most were not predicted to have a functional consequence. These observations suggest that a greater diversity in biological functions for proglucagon-derived peptides might exist in mammals.

Rebirth of the Incretin Concept: Its conception and early development

This paper describes the resurrection of the Incretin Concept in the early 1960s. It began with the more or less simultaneous discovery by three groups working independently in London. Dupre demonstrated that secretin given intravenously with glucose increased its rate of disappearance from the blood, McIntyre and co-workers established that hyperglycaemia evoked by oral glucose stimulated more insulin secretion than comparable hyperglycaemia produced by intravenous glucose and Marks and Samols established the insulinotropic properties of glucagon. The concept evolved with the discovery by Samols and co-workers that oral glucose stimulated the release of immunoreactive glucagon-like substances from the gut mucosa and the subsequent isolation of glucagon immunoreactive compounds, most notably oxyntomodulin and glicentin, and of gastic inhibitory polypetide (GIP). It concluded with the isolation and characterisation of glucagon-like peptide 1 (7-36) amide.

Anti-obesogenic and hypolipidemic effects of a glucagon-like peptide-1 receptor agonist derived from the saliva of the Gila monster

INTRODUCTION

Glucagon-like peptide-1 (GLP-1) receptor (R) agonists are a class of incretin mimetic drugs that have been used for the treatment of type 2 diabetes mellitus and also considered strong candidates for the treatment of obesity. The original prototypical drug in this class is the exenatide, a synthetic peptide with the same structure as the native molecule, exendin-4, found in the saliva of the Gila monster (Heloderma suspectum suspectum lizard).

OBJECTIVES

To identify and compare the anti-obesogenic, antidyslipidemic and antidiabetogenic effects of agonism in GLP-1R by exenatide on two distinct models of obesity: induced by hypothalamic injury (MSG) or high-calorie diet (DIO).

METHODS

To obtain MSG, neonatal rats were daily subcutaneously injected with 4 g monosodium glutamate/kg, for 10 consecutive days. To obtain DIO, 72-75 days old rats received hyperlipid food and 30% sucrose for drinking up to 142-145 days old. Untreated healthy rats with the same age were used as control. General biometric and metabolic parameters were measured.

RESULTS

MSG was characterized by decreased naso-anal length, food and fluid intake, plasma protein and glucose decay rate per minute after insulin administration (K_ITT), as well as increased Lee index (body mass^0.33/naso-anal length), mass of retroperitoneal and periepididymal fat pads, glycemia, triglycerides (TG), LDL and VLDL. Exenatide ameliorated K_ITT and food and fluid intake, and it also restored glycemia in MSG. DIO was characterized by glucose intolerance, increased body mass, Lee index, fluid intake, mass of retroperitoneal and periepididymal fat pads, glycemia, glycated hemoglobin (HbA1c), TG, VLDL and total cholesterol, as well as decreased food intake and K_ITT. Exenatide restored glycemia, HbA1c, TG, VLDL, total cholesterol and body mass, and it also ameliorated food and fluid intake, K_ITT and mass of retroperitoneal fat pad in DIO.

CONCLUSIONS

The hypothalamic injury and the high-calorie diet induce dyslipidemia and glycemic dysregulation in addition to obesity in rats. The usual therapeutic dose of exenatide in humans is antidiabetogenic in both these obesity models, but is anti-obesogenic and hypolipidemic only in diet-induced obesity. Agonists of GLP-1R are promising anti-obesogenic and antidyslipidemic drugs in the early stages of the obesity, in which the integrity of the nervous system was unaffected.

Esculentin-2CHa(1-30) and its analogues: stability and mechanisms of insulinotropic action

The insulin-releasing effects, cellular mechanisms of action and anti-hyperglycaemic activity of 10 analogues of esculentin-2CHa lacking the cyclic C-terminal domain (CKISKQC) were evaluated. Analogues of the truncated peptide, esculentin-2CHa(1-30), were designed for plasma enzyme resistance and increased biological activity. Effects of those analogues on insulin release, cell membrane integrity, membrane potential, intracellular Ca²⁺ and cAMP levels were determined using clonal BRIN-BD11 cells. Their acute effects on glucose tolerance were investigated using NIH Swiss mice. d-Amino acid substitutions at positions 7(Arg), 15(Lys) and 23(Lys) and fatty acid (l-octanoate) attachment to Lys at position 15 of esculentin-2CHa(1-30) conveyed resistance to plasma enzyme degradation whilst preserving insulin-releasing activity. Analogues, [d-Arg⁷,d-Lys¹⁵,d-Lys²³]-esculentin-2CHa(1-30) and Lys¹⁵-octanoate-esculentin-2CHa(1-30), exhibiting most promising profiles and with confirmed effects on both human insulin-secreting cells and primary mouse islets were selected for further analysis. Using chemical inhibition of adenylate cyclase, protein kinase C or phospholipase C pathways, involvement of PLC/PKC-mediated insulin secretion was confirmed similar to that of CCK-8. Diazoxide, verapamil and Ca²⁺ omission inhibited insulin secretion induced by the esculentin-2CHa(1-30) analogues suggesting an action on K_ATP and Ca²⁺ channels also. Consistent with this, the analogues depolarised the plasma membrane and increased intracellular Ca²⁺ Evaluation with fluorescent-labelled esculentin-2CHa(1-30) indicated membrane action, with internalisation; however, patch-clamp experiments suggested that depolarisation was not due to the direct inhibition of K_ATP channels. Acute administration of either analogue to NIH Swiss mice improved glucose tolerance and enhanced insulin release similar to that observed with GLP-1. These data suggest that multi-acting analogues of esculentin-2CHa(1-30) may prove useful for glycaemic control in obesity-diabetes.

Evolution of genes for incretin hormones and their receptors

The incretin hormones glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are essential components in the regulation of blood glucose levels in mammals. These two incretins are produced by evolutionarily related genes and these hormones show similarity in sequence as both are glucagon-like sequences. Genes for these hormones have been identified in a number of diverse vertebrate species indicating that they originated prior to the earliest divergences of vertebrate species. However, analysis of functional and sequence data suggest that each of these hormones acquired incretin activity independently, and only since the divergence of tetrapods from fish. Not only are the hormones related, but so are their receptors. Like the hormones, the incretin action of the receptors is not a product of a shared common ancestral history, as the receptors for GLP-1 and GIP are not most closely related. Further study of the physiological functions of GLP-1 and GIP in additional vertebrates is required to better understand the origin of incretin action.

Insulin-releasing properties of the frog skin peptide pseudin-2 and its [Lys18]-substituted analogue

Pseudin-2 is a cationic α-helical peptide that was first isolated from the skin of the paradoxical frog Pseudis paradoxa on the basis of its antimicrobial activity. We have investigated the insulin-releasing properties and cytotoxicity of the peptide, together with selected analogues with increased cationicity and hydrophobicity. At concentrations in the range 10-9–10-6 m, pseudin-2, and its [Lys18], [Phe8], and [d-Lys3,d-Lys10,d-Lys14] derivatives, stimulated insulin release from the BRIN-BD11 clonal β-cell line without increasing release of lactate dehydrogenase. The [Lys18] analogue was the most potent (46% increase in insulin release at 10-9 m) and the most effective (215% increase in insulin release at 10-6 m). The more cationic [Lys3,Lys10,Lys14] and [Lys3,Lys10,Lys14,Lys21] analogues lacked insulinotropic action and the more hydrophobic [Phe16] analogue was cytotoxic at concentrations ≥10-7 m. Pseudin-2 and [Lys18]-pseudin-2 had no effect on intracellular calcium concentrations and stimulated insulin release in the absence of external calcium. [Lys18]-pseudin-2 (10-8 m) stimulated insulin release in the presence of diazoxide and verapamil. Our results demonstrate that pseudin-2 stimulates insulin secretion from BRIN-BD11 cells by a mechanism involving Ca2+-independent pathways and identify [Lys18]-pseudin-2 as a peptide that may have potential for development as a therapeutically valuable insulinotropic agent for the treatment of type 2 diabetes.