Glucagon-like peptide-1 receptor: mechanisms and advances in therapy

Abstract

The glucagon-like peptide-1 (GLP-1) receptor, known as GLP-1R, is a vital component of the G protein-coupled receptor (GPCR) family and is found primarily on the surfaces of various cell types within the human body. This receptor specifically interacts with GLP-1, a key hormone that plays an integral role in regulating blood glucose levels, lipid metabolism, and several other crucial biological functions. In recent years, GLP-1 medications have become a focal point in the medical community due to their innovative treatment mechanisms, significant therapeutic efficacy, and broad development prospects. This article thoroughly traces the developmental milestones of GLP-1 drugs, from their initial discovery to their clinical application, detailing the evolution of diverse GLP-1 medications along with their distinct pharmacological properties. Additionally, this paper explores the potential applications of GLP-1 receptor agonists (GLP-1RAs) in fields such as neuroprotection, anti-infection measures, the reduction of various types of inflammation, and the enhancement of cardiovascular function. It provides an in-depth assessment of the effectiveness of GLP-1RAs across multiple body systems-including the nervous, cardiovascular, musculoskeletal, and digestive systems. This includes integrating the latest clinical trial data and delving into potential signaling pathways and pharmacological mechanisms. The primary goal of this article is to emphasize the extensive benefits of using GLP-1RAs in treating a broad spectrum of diseases, such as obesity, cardiovascular diseases, non-alcoholic fatty liver disease (NAFLD), neurodegenerative diseases, musculoskeletal inflammation, and various forms of cancer. The ongoing development of new indications for GLP-1 drugs offers promising prospects for further expanding therapeutic interventions, showcasing their significant potential in the medical field.

Fig. 1

Growth and Timescale of Medical Publications on GLP-1 and GLP-1RAs. This figure illustrates the number of published articles over time, from 1978 to 2022, related to GLP-1 and GLP-1RAs, along with significant milestones in clinical trials. The data is visualized through three layered area graphs, each representing a different category of publications: research on GLP-1 (red area), GLP-1RAs (blue area), and clinical trials (green area). Key milestones are annotated on the timeline, including the identification of GLP-1 and GLP-2 through gene sequencing in 1983, the extraction of Exendin-4 from Gila monster venom in 1991, and the first FDA approval of a GLP-1 analog, Exenatide, in 2005. Liraglutide’s clinical trials began in 2000 and the drug’s FDA approval in 2010. Notable is the steep increase in the number of publications from 2000 onwards, reflecting a growing interest and development in the field. The peak in 2022 corresponds to GLP-1 drug sales hitting $22 billion, with the approval of semaglutide by the FDA in 2021 and the initiation of the first clinical trial for an oral diabetes drug based on GLP-1 in the same year. This graph underscores the expanding research and clinical importance of GLP-1-related therapies in the treatment of diabetes. The x-axis represents the year, ranging from 1978 to 2023, and the y-axis quantifies the number of published articles, with a scale ranging from 0 to 2500 articles annual. GLP-1RAs are artificial protein formulations that exhibit partial or complete amino acid sequence similarity to endogenous GLP-1 within the human body. These compounds possess enhanced stability, extended half-lives, and heightened biological potency, enabling them to mimic the actions of GLP-1. Ongoing advancements in research and development have resulted in the production of GLP-1RA with increasingly prolonged half-lives

Fig. 2

Mechanisms of Blood Glucose Reduction by GLP-1 in Pancreatic α, β, and δ Cells. This illustration demonstrates how GLP-1 reduces blood glucose levels by acting on different pancreatic cell types. In β cells, GLP-1(9–36) activates the GLP-1R, which increases cAMP levels, subsequently activating PKA and CREB, leading to the promotion of insulin secretion. The PI3K/Akt pathway enhances glucose sensitivity in β cells, promoting insulin secretion. This pathway also supports β cell survival and proliferation, ensuring an adequate β cell mass to maintain normal insulin secretion. This signaling cascade results in increased insulin gene expression, enhanced protein synthesis, improved cell survival, and reduced apoptosis. Additional metabolic effects include fatty acid oxidation, gluconeogenesis, and energy expenditure. In α cells, GLP-1(7-36) mainly regulates function through indirect mechanisms. GLP-1R expression is lower in α cells compared to β and δ cells, resulting in relatively less direct action of GLP-1 on α cells. Through paracrine effects via β cells, GLP-1 enhances insulin secretion, which in turn inhibits glucagon secretion from α cells. Additionally, GLP-1 promotes somatostatin secretion from δ cells, which inhibits glucagon secretion from α cells. GLP-1(7-36) can suppress glucagon secretion in α cells by increasing cAMP levels, activating PKA and EPAC, leading to a decrease in intracellular calcium concentration and reduced glucagon release. GLP-1(9–36) inhibits glucagon secretion by activating inhibitory G proteins (Gi/o) and suppressing PKA activity through a GCGR-dependent mechanism. By promoting the undocking of secretory granules (SG), GLP-1(9–36) reduces the number of granules available for exocytosis, thereby decreasing the release of glucagon. δ cells primarily secrete somatostatin. GLP-1(7-36) regulates somatostatin secretion by modulating calcium channels and affecting membrane potential changes. The action of GLP-1(7-36) on δ cells may be more indirect, such as through the influence on hormones secreted by β and α cells (insulin and glucagon), which indirectly affects δ cell somatostatin secretion. Additionally, somatostatin can inhibit gastrointestinal activities, reducing the secretion of pancreatic enzymes and gastric acid, thereby indirectly lowering the demand for glucagon

Fig. 3

The Effects of GLP-1RAs on osteoclasts and osteoblasts. GLP-1RAs aid in weight loss by regulating the gut-brain axis and interacting with leptin, while weight loss can alleviate the harmful effects of obesity on the body, particularly in knee OA, by reducing joint loading and inflammation. Obesity disrupts bone metabolism and leads to increased bone resorption, but GLP-1RAs can inhibit this damage and improve bone health by increasing the OPG/RANKL ratio, reducing osteoclast activity, and promoting bone formation

Fig. 4

The Effects of GLP-1RAs on Musculoskeletal System. GLP-1RAs inhibit chondrocyte apoptosis, reduce inflammation, and protect articular chondrocytes in OA and RA through various mechanisms, including suppressing cytokine release, inhibiting the NF-κB pathway, and reducing inflammation-related gene expression. They have shown to reduce the inflammatory response by decreasing the expression of proinflammatory cytokines and inhibiting matrix metalloproteinase activity and cell signaling pathways. GLP-1R expression is found in macrophages and fibroblast-like synoviocytes, which are important for maintaining synovial fluid homeostasis. GLP-1R signaling stimulates calcitonin secretion, which inhibits bone resorption, and disruption of GLP-1R leads to increased bone resorption and decreased calcitonin expression. GLP-1RAs have pleiotropic effects on skeletal muscle, including inhibiting muscle atrophy, preserving muscle strength, and enhancing exercise endurance, through GLP-1R-mediated signaling pathways. GLP-1RAs can influence the composition of the phospholipid layer on cartilage, leading to beneficial effects on joint health and potentially facilitating repair of existing damage in individuals with OA

Fig. 5

The effects of GLP-1RAs on nervous system. GLP-1RAs have diverse effects, including alleviating neuroinflammation and pain in OA, reducing food intake and improving body weight, protecting peripheral nerves, maintaining astrocyte function and metabolic homeostasis, and showing potential therapeutic benefits in AD and PD. Semaglutide, a diabetes drug, can also reduce alcohol consumption by modulating the brain’s reward and punishment systems

Fig. 6

The Effects of GLP-1RAs on Carotid Body Activation. GLP-1 by inhibiting the chemoreception in carotid body cell activity to adjust the new mechanism of sympathetic nerve excitability, and points out that GLP-1 agonists can inhibit the origin of the carotid body around chemical reflection to lighten the sympathetic nerve excitability, which is expected to improve the sympathetic activity of the patients with high blood pressure, reduce blood pressure levels