Cholinergic signaling mediates the effects of xenin-25 on secretion of pancreatic polypeptide but not insulin or glucagon in humans with impaired glucose tolerance

Glucose-dependent insulinotropic polypeptide (GIP)

BACKGROUND

Glucose-dependent insulinotropic polypeptide (GIP) was the first incretin identified and plays an essential role in the maintenance of glucose tolerance in healthy humans. Until recently GIP had not been developed as a therapeutic and thus has been overshadowed by the other incretin, glucagon-like peptide 1 (GLP-1), which is the basis for several successful drugs to treat diabetes and obesity. However, there has been a rekindling of interest in GIP biology in recent years, in great part due to pharmacology demonstrating that both GIPR agonism and antagonism may be beneficial in treating obesity and diabetes. This apparent paradox has reinvigorated the field, led to new lines of investigation, and deeper understanding of GIP.

SCOPE OF REVIEW

In this review, we provide a detailed overview on the multifaceted nature of GIP biology and discuss the therapeutic implications of GIPR signal modification on various diseases.

MAJOR CONCLUSIONS

Following its classification as an incretin hormone, GIP has emerged as a pleiotropic hormone with a variety of metabolic effects outside the endocrine pancreas. The numerous beneficial effects of GIPR signal modification render the peptide an interesting candidate for the development of pharmacotherapies to treat obesity, diabetes, drug-induced nausea and both bone and neurodegenerative disorders.

D-Ala2-GIP (1-30) promotes angiogenesis by facilitating endothelial cell migration via the Epac/Rap1/Cdc42 signaling pathway

Angiogenesis, a meticulously regulated process essential for both normal development and pathological conditions, necessitates a comprehensive understanding of the endothelial mechanisms governing its progression. Leveraging the zebrafish model and NgAgo knockdown system to identify target genes influencing angiogenesis, our study highlights the significant role of gastric inhibitory polypeptide (GIP) and its receptor (GIPR) in this process. While GIP has been extensively studied for its insulinotropic and glucagonotropic effects, its role in angiogenesis remains unexplored. This study demonstrated that GIPR knockdown induced developmental delays, morphological abnormalities, and pronounced angiogenic impairments in zebrafish embryos. Conversely, exogenous D-Ala2-GIP administration enhanced blood vessel formation in the yolk sac membrane of chick embryos. Consistent with these findings, D-Ala2-GIP treatment promoted microvessel formation in the tube formation assays and rat aortic ring models. Further investigation revealed that D-Ala2-GIP facilitated human umbilical vein endothelial cell (HUVEC) migration, a key step in angiogenesis, through the cyclic adenosine monophosphate (cAMP)-mediated activation of the Epac/Rap1/Cdc42 signaling pathway. This study provides novel insights into the angiogenic functions of GIP and its potential implications for cardiovascular biology.

The evolving story of incretins (GIP and GLP-1) in metabolic and cardiovascular disease: A pathophysiological update

The incretin hormones glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) have their main physiological role in augmenting insulin secretion after their nutrient-induced secretion from the gut. A functioning entero-insular (gut-endocrine pancreas) axis is essential for the maintenance of a normal glucose tolerance. This is exemplified by the incretin effect (greater insulin secretory response to oral as compared to "isoglycaemic" intravenous glucose administration due to the secretion and action of incretin hormones). GIP and GLP-1 have additive effects on insulin secretion. Local production of GIP and/or GLP-1 in islet α-cells (instead of enteroendocrine K and L cells) has been observed, and its significance is still unclear. GLP-1 suppresses, and GIP increases glucagon secretion, both in a glucose-dependent manner. GIP plays a greater physiological role as an incretin. In type 2-diabetic patients, the incretin effect is reduced despite more or less normal secretion of GIP and GLP-1. While insulinotropic effects of GLP-1 are only slightly impaired in type 2 diabetes, GIP has lost much of its acute insulinotropic activity in type 2 diabetes, for largely unknown reasons. Besides their role in glucose homoeostasis, the incretin hormones GIP and GLP-1 have additional biological functions: GLP-1 at pharmacological concentrations reduces appetite, food intake, and-in the long run-body weight, and a similar role is evolving for GIP, at least in animal studies. Human studies, however, do not confirm these findings. GIP, but not GLP-1 increases triglyceride storage in white adipose tissue not only through stimulating insulin secretion, but also by interacting with regional blood vessels and GIP receptors. GIP, and to a lesser degree GLP-1, play a role in bone remodelling. GLP-1, but not GIP slows gastric emptying, which reduces post-meal glycaemic increments. For both GIP and GLP-1, beneficial effects on cardiovascular complications and neurodegenerative central nervous system (CNS) disorders have been observed, pointing to therapeutic potential over and above improving diabetes complications. The recent finding that GIP/GLP-1 receptor co-agonists like tirzepatide have superior efficacy compared to selective GLP-1 receptor agonists with respect to glycaemic control as well as body weight has renewed interest in GIP, which previously was thought to be without any therapeutic potential. One focus of this research is into the long-term interaction of GIP and GLP-1 receptor signalling. A GLP-1 receptor antagonist (exendin [9-39]) and, more recently, a GIP receptor agonist (GIP [3-30] NH₂ ) and, hopefully, longer-acting GIP receptor agonists for human use will be helpful tools to shed light on the open questions. A detailed knowledge of incretin physiology and pathophysiology will be a prerequisite for designing more effective incretin-based diabetes drugs.

The Local Paracrine Actions of the Pancreatic α-Cell

Secretion of glucagon from the pancreatic α-cells is conventionally seen as the first and most important defense against hypoglycemia. Recent findings, however, show that α-cell signals stimulate insulin secretion from the neighboring β-cell. This article focuses on these seemingly counterintuitive local actions of α-cells and describes how they impact islet biology and glucose metabolism. It is mostly based on studies published in the last decade on the physiology of α-cells in human islets and incorporates results from rodents where appropriate. As this and the accompanying articles show, the emerging picture of α-cell function is one of increased complexity that needs to be considered when developing new therapies aimed at promoting islet function in the context of diabetes.

Molecular interactions of full-length and truncated GIP peptides with the GIP receptor - A comprehensive review

Enzymatic cleavage of endogenous peptides is a commonly used principle to initiate, modulate and terminate action for instance among cytokines and peptide hormones. The incretin hormones, glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1), and the related hormone glucagon-like peptide-2 (GLP-2) are all rapidly N-terminally truncated with severe loss of intrinsic activity. The most abundant circulating form of full length GIP(1-42) is GIP(3-42) (a dipeptidyl peptidase-4 (DPP-4) product). GIP(1-30)NH₂ is another active form resulting from prohormone convertase 2 (PC2) cleavage of proGIP. Like GIP(1-42), GIP(1-30)NH₂ is a substrate for DPP-4 generating GIP(3-30)NH₂ which, compared to GIP(3-42), binds with higher affinity and very efficiently inhibits GIP receptor (GIPR) activity with no intrinsic activity. Here, we review the action of these four and multiple other N- and C-terminally truncated forms of GIP with an emphasis on molecular pharmacology, i.e. ligand binding, subsequent receptor activation and desensitization. Our overall conclusion is that the N-terminus is essential for receptor activation as GIP N-terminal truncation leads to decreased/lost intrinsic activity and antagonism (similar to GLP-1 and GLP-2), whereas the C-terminal extension of GIP(1-42), as compared to GLP-1, GLP-2 and glucagon (29-33 amino acids), has no apparent impact on the GIPR in vitro, but may play a role for other properties such as stability and tissue distribution. A deeper understanding of the molecular interaction of naturally occurring and designed GIP-based peptides, and their impact in vivo, may contribute to a future therapeutic targeting of the GIP system - either with agonists or with antagonists, or both.

Nonclassical Islet Peptides: Pancreatic and Extrapancreatic Actions

The pancreas has physiologically important endocrine and exocrine functions; secreting enzymes into the small intestine to aid digestion and releasing multiple peptide hormones via the islets of Langerhans to regulate glucose metabolism, respectively. Insulin and glucagon, in combination with ghrelin, pancreatic polypeptide and somatostatin, are the main classical islet peptides critical for the maintenance of blood glucose. However, pancreatic islets also synthesis numerous ‘nonclassical’ peptides that have recently been demonstrated to exert fundamental effects on overall islet function and metabolism. As such, insights into the physiological relevance of these nonclassical peptides have shown impact on glucose metabolism, insulin action, cell survival, weight loss, and energy expenditure. This review will focus on the role of individual nonclassical islet peptides to stimulate pancreatic islet secretions as well as regulate metabolism. In addition, the more recognised actions of these peptides on satiety and energy regulation will also be considered. Furthermore, recent advances in the field of peptide therapeutics and obesity-diabetes have focused on the benefits of simultaneously targeting several hormone receptor signalling cascades. The potential for nonclassical islet hormones within such combinational approaches will also be discussed.

Dissecting the Physiology and Pathophysiology of Glucagon-Like Peptide-1

An aging world population exposed to a sedentary life style is currently plagued by chronic metabolic diseases, such as type-2 diabetes, that are spreading worldwide at an unprecedented rate. One of the most promising pharmacological approaches for the management of type 2 diabetes takes advantage of the peptide hormone glucagon-like peptide-1 (GLP-1) under the form of protease resistant mimetics, and DPP-IV inhibitors. Despite the improved quality of life, long-term treatments with these new classes of drugs are riddled with serious and life-threatening side-effects, with no overall cure of the disease. New evidence is shedding more light over the complex physiology of GLP-1 in health and metabolic diseases. Herein, we discuss the most recent advancements in the biology of gut receptors known to induce the secretion of GLP-1, to bridge the multiple gaps into our understanding of its physiology and pathology.