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

Exenatide is a medication that belongs to the class of drugs known as GLP-1RAs. It is a synthetic version of the hormone exendin-4, which is found in the saliva of the Gila monster, a venomous lizard native to the southwestern United States.⁷⁶ Exenatide is composed of 39 amino acids and shares 53% sequence identity to human GLP-1. The second position of its N-terminus is glycine (alanine in GLP-1), which is not easily degraded by the DPP-4 enzyme.⁷⁷ Compared with GLP-1, the C-terminus of its amino acid sequence contains 9 additional amino acid residues (PSSGAPPPS), which are not easily degraded by peptide chain endonucleases; thus, it has a long half-life and strong biological activity.⁷⁸ The average half-life of exenatide is 2.4 h, 2 to 3 times per day.⁷⁸

Liraglutide is a medication used to treat T2DM and obesity.^79,80 In the treatment of T2DM, liraglutide helps lower blood sugar levels by increasing insulin production and reducing glucose production by the liver.⁸¹ It also slows stomach emptying, which helps control appetite and reduce food intake.^82,83 In addition to lowering blood sugar levels, liraglutide helps to reduce body weight by suppressing appetite and reducing energy intake,^84,85 but its associated costs and need for daily injections may limit its use in individuals with obesity.^86–88 Liraglutide was first approved by the FDA in 2010 under the brand name Victoza for the treatment of T2DM.^83,89–92 Since then, it has also been approved for other indications, such as chronic weight management, under the brand name Saxenda.⁶² Liraglutide has an arginine at position 34 (GLP-1(7-37)) that is lysine, and its lysine at position 26 connects to a 16-carbon palmitic acid side chain linked by glutamic acid. Liraglutide shares 97% sequence homology with human GLP-1 and can bind to and activate GLP-1R.⁷⁸ The elimination half-life of liraglutide is 13 h, and only one injection per day is required.⁶²

Lixisenatide was developed by Sanofi to treat T2DM.^64,93 Structurally, lixisenatide is based on the exenatide structure but lacks proline at position 38, and six lysines are linked after serine at position 39.⁹⁴ The six lysine residues increase the rigidity of the molecule's structure, thus allowing its drug properties to be improved.⁷⁸ These changes stabilize its structure, prevent protein degradation of the molecule in the circulation, and increase the circulation time enough to ensure once-daily injection (compared with exenatide, which is injected two or three times daily). The average half-life of lixisenatide is approximately 3–4 h.⁹⁵ Lixisenatide is a short-acting GLP-1RA agent,^96,97 and once-daily lixisenatide can improve patient compliance to a certain extent and reduce the occurrence of hypoglycemia.^98–100 The injection is usually given within one hour before the first meal of the day, preferably at the same time each day. It is important to avoid injecting lixisenatide after a meal or in case of a missed meal. The initial recommended dosage of lixisenatide is 10 mcg once daily for at least 14 days. After the initial period, its dosage may be increased to 20 mcg once daily if additional glycemic control is needed. The maximum recommended dosage is 20 mcg once daily.¹⁰¹

Albiglutide was approved by the FDA in 2014. As a GLP-1RA,¹⁰² albiglutide works by stimulating insulin release and reducing glucagon production, leading to improved blood sugar control in patients with T2DM.^102,103 Albiglutide is a long-acting GLP-1RA injected subcutaneously once a week. The half-life of albiglutide is approximately 4 to 7 days.¹⁰² Compared with the structure of human GLP-1(7-36), the amino acid sequence of albiglutide contains an arginine at position 8 (in GLP-1, this residue is lysine), and two modified GLP-1 peptide chains are fused to human serum albumin (HSA), thereby greatly extending its half-life.^104,105 Albiglutide is typically prescribed in combination with diet and exercise. The dosage of albiglutide is usually 30 mg once a week. It is administered as a subcutaneous injection (just under the skin). The injection site can be the thigh, abdomen, or upper arm.¹⁰²

The FDA set several usage restrictions upon the initial approval of Tanzeum (albiglutide), reflecting considerations of suitability and safety for specific patient groups. Firstly, Tanzeum is not recommended as a first-line treatment for patients inadequately controlled with diet and exercise.⁴⁹ Secondly, its safety and efficacy remain unclear in patients with a history of pancreatitis as it has not been studied in this group.¹⁰⁶ Additionally, Tanzeum is not suitable for treating type 1 diabetes or diabetic ketoacidosis, related to its pharmacological action and target disease. It is also not recommended for patients with existing gastrointestinal disease to avoid potential side effects or exacerbating the condition.¹⁰⁷ Further, serious risks associated with Tanzeum use include pancreatitis, acute kidney injury, renal impairment, and pneumonia, further limiting its use in specific conditions or susceptible patients. These restrictions and warnings demonstrate the FDA's stringent consideration of patient safety in the drug approval process.¹⁰⁸ In March 2015, the FDA required a black box warning on Tanzeum due to the observed risk of thyroid C-cell tumors in animals, although it is unclear if this effect also occurs in humans.¹⁰⁸ Later, the FDA added a warning about the risk of anaphylactic shock to the medication's label. This reaction is severe and potentially life-threatening, including symptoms like unease, tingling, dizziness, itching, hives, swelling, difficulty breathing, and fainting.^81,107,109

Tanzeum was discontinued by GlaxoSmithKline (GSK) in 2017, primarily due to economic factors.^110,111 Despite GSK's attempts to gain a competitive edge through low pricing, Tanzeum failed to achieve sufficient market acceptance.¹⁰⁶ In 2017, Tanzeum was removed from the preferred drug list of leading pharmacy benefit manager (PBM) company Express Scripts and was replaced by Eli Lilly's Trulicity, highlighting Tanzeum's weak market presence.^106,108 Moreover, this decision was part of a broad strategic reform led by GSK's new CEO Emma Walmsley.¹⁰⁶ This reform aimed to refocus the company's efforts on areas with higher revenue potential, such as respiratory and HIV treatments, as well as oncology and immunology.¹⁰⁸ Additionally, Tanzeum struggled to establish a significant market share in the competitive GLP-1RA market, which was another reason GSK decided to withdraw the drug globally.¹⁰⁸

Dulaglutide was approved by the FDA in 2014 for the treatment of T2DM.¹¹² Dulaglutide is a once-weekly long-acting GLP-1RA. Compared with the structure of human GLP-1(7-37), the amino acid sequence of dulaglutide contains glycine at position 8, glutamic acid at position 22, and glycine at position 36.¹¹³ It was then fused to the constant region (Fc) of modified human immunoglobulin G4 (IgG4) via a "-(Gly-Gly-Gly-Gly-Ser) 3-Ala-" bridge and exhibits an average biological half-life of 90 h.¹⁰⁵ Dulaglutide is the first large-molecule GLP-1RA, and the weekly dosing frequency of dulaglutide can greatly improve patient compliance.^{33,78,104,114}

Semaglutide is a long-acting GLP-1RA agent used once weekly to improve glycemic control in patients with T2DM.¹¹⁵ Compared with the structure of human GLP-1(7-37), the amino acid sequence of semaglutide contains diaminoisobutyric acid at position 8, arginine at position 34, and acylated lysine at position 26.³¹ Semaglutide has a longer aliphatic chain and increased hydrophobicity, but its hydrophilicity is greatly enhanced by PEG modification of the short chain. Modified DPP-4 can not only mask the enzymatic hydrolysis site of DPP-4 but also bind closely with albumin to reduce renal excretion and prolong the half-life.^35,104,116 On August 22, 2022, Novo Nordisk announced the primary results of the Phase II clinical trial for the dual-action compound CagriSema, which demonstrated effective blood sugar reduction and weight loss. CagriSema is composed of the GLP-1 RA semaglutide and the long-acting insulin analog cagrilintide, and can be administered subcutaneously once a week.¹¹⁷

Beinaglutide was the first original drug approved for the treatment of obesity in China and the third GLP-1 class reduction drug approved worldwide, and it represents a new treatment option for overweight and obese patients.^67,118,119 Beinaglutide, a recombinant human GLP-1 (rhGLP-1) polypeptide, exhibits a remarkable resemblance to human GLP-1(7-36), with nearly 100% homology.¹²⁰ This innovative compound demonstrates dose-dependent efficacy in regulating glycemic control, suppressing appetite, delaying gastric emptying, and facilitating weight reduction.¹²¹ Consequently, beinaglutide holds significant promise for advancing research in the areas of overweight/obesity and nonalcoholic steatohepatitis (NASH).^67,122 Beinaglutide is a natural human GLP-1(7-36)-NH2 expressed in Escherichia coli. This product was approved for the treatment of T2DM; it has a half-life of 11 minutes and requires 3 injections per day.⁷⁸

Polyethylene glycol liraglutide (PEG-loxenatide) is a long-acting GLP-1RA. It was the world's first PEGylated long-acting GLP-1RA.¹²³ PEG-loxenatide is used for blood glucose control in adult patients with T2DM. Structurally, the product was optimized on the basis of exenatide, and the glycine 2, methionine 14 and asparagine 28 positions were modified to improve enzyme stability and chemical stability based on the polypeptide backbone. Moreover, the C-terminal serine of the peptide was replaced by cysteine via site-directed mutagenesis performed with polyethylene glycol.⁷⁸ The half-life of PEG-loxenatide is approximately 1 week.¹²³

In recent years, the development of dual and triple agonists related to GLP-1 has been vigorously pursued.^124–126 The design concept of these multi-agonists is to simultaneously regulate multiple key metabolic pathways to achieve more effective control over blood sugar, body weight, and overall metabolic health.⁴³ Currently, these drugs are primarily in the development and clinical trial stages, but preliminary research results have shown their potential powerful effects in reducing blood sugar and body weight.^127,128

Currently, most GLP-1RAs are based on proteins or peptides, meaning they are large molecules typically administered via injection.³⁷ Small molecule GLP-1RAs are chemically synthesized, and compared to protein-based drugs, they generally have smaller molecular sizes.¹⁷¹ The development of small molecule GLP-1RAs aims to overcome some of the limitations of traditional protein or peptide-based GLP-1RAs, such as the need for injection. Small molecule drugs may offer the possibility of oral administration, which is more convenient and acceptable for patients.¹⁷¹ Additionally, small molecule drugs may have better tissue permeability, longer half-lives in the body, and lower production costs. As of now, research on small molecule GLP-1RAs is still mainly in the laboratory and early clinical trial stages.¹⁷² The challenges in developing these drugs include ensuring that they can effectively mimic the biological activity of large molecule GLP-1RAs while maintaining efficacy and selectivity. This review article will introduce some of the small molecule drugs that are currently receiving significant attention:

GLP-1 significantly influences the functionality of pancreatic β-cells and α-cells, contributing to its therapeutic effect on T2DM.^132,216,217 As a vital incretin hormone, GLP-1 enhances glucose-dependent insulin secretion.³ It also promotes proliferation and reduces apoptosis of pancreatic β-cells, thereby maintaining their quality and functionality.^130,218 At the molecular level, GLP-1 activates the cAMP response element-binding protein (CREB) via its receptor (GLP-1R), a transcription factor crucial for expressing the insulin gene.^3,130,219 GLP-1 also activates PKA and EPAC through a cAMP-dependent pathway.^1,220 PKA, a key enzyme, phosphorylates various target proteins, affecting their activity and function, which in turn promotes insulin synthesis and secretion.^178,180 The PI3K/Akt signaling pathway, activated by GLP-1, plays a vital role in maintaining pancreatic β-cell survival and promoting their proliferation.^221,222 Activation of Akt stimulates β-cell proliferation, reduces apoptosis, and enhances insulin secretion by regulating downstream effector molecules like Forkhead box protein O1 (FoxO1) and the glucose transporter type 2 (GLUT2).^223,224

GLP-1 also inhibits glucagon release from α-cells, which is beneficial for reducing blood glucose levels since glucagon promotes hepatic gluconeogenesis.^131,225 GLP-1's action on α-cells regulates glucagon release.¹³¹ The direct impact of GLP-1 on these cells slows the secretion of glucagon, essential for maintaining glucose stability, especially postprandially.^131,226 When GLP-1 binds to its receptor on α-cells, it activates intracellular cAMP production.²²⁷ In α-cells, increased cAMP affects glucagon synthesis and release.²²⁸ PKA, activated by cAMP, can regulate the activity of K-ATP channels in α-cells.^228,229 The opening of these channels is controlled by the intracellular ATP/ADP ratio.²²⁹ Specifically, PKA modifies the open state of K-ATP channels through phosphorylation, affecting the cell membrane's potential and intracellular calcium ion concentration.^230,231 By modulating the activity of K-ATP channels, GLP-1 indirectly controls the calcium signaling in α-cells, thereby influencing glucagon secretion.^232,233 GLP-1 inhibits the release of glucagon from α-cells through multiple mechanisms. GLP-1 can directly bind to the receptors on the surface of α-cells in the pancreas, inhibiting the secretion of glucagon from these cells.^234,235 Glucagon is a hormone secreted by pancreatic α-cells that stimulates the liver to release glucose. Therefore, inhibiting the release of glucagon is crucial when blood glucose levels need to be lowered. GLP-1 can stimulate pancreatic β-cells to release insulin.^236,237 Insulin not only directly lowers blood glucose levels but also further inhibits glucagon secretion from α-cells through a feedback mechanism.²³⁸ The high concentration of insulin in the local pancreatic environment creates a negative feedback effect, reducing the amount of glucagon secreted by α-cells.²³⁴ Additionally, GLP-1 can reduce blood glucose production by delaying gastric emptying and decreasing appetite, which also contributes to overall blood glucose control.^239,240 In summary, GLP-1 lowers blood glucose levels and inhibits glucagon secretion through direct action on α-cells, promotion of insulin secretion, and regulation of gastrointestinal activity.²⁴¹

GLP-1R is expressed not only on β-cells and α-cells but also on δ-cells in the pancreatic islets.²⁴² δ-cells primarily secrete somatostatin, which is an important regulatory hormone.²⁴² The expression of GLP-1R on δ-cells helps to inhibit the secretion of glucagon.²⁴² When GLP-1 binds to GLP-1R on δ-cells, it activates these cells and promotes the secretion of somatostatin.²⁴³ Somatostatin is a potent inhibitory hormone that affects the surrounding α-cells and β-cells. It acts directly on neighboring α-cells to inhibit the secretion of glucagon and can also indirectly influence α-cells by inhibiting the secretion of other hormones.²⁴³ In the local islet environment, a high concentration of somatostatin creates an inhibitory milieu, further reducing the activity of α-cells and the secretion of glucagon.²⁴⁴ Additionally, somatostatin can inhibit gastrointestinal activity, reducing the secretion of pancreatic enzymes and gastric acid, thereby indirectly decreasing the demand for glucagon.^245,246 Hence, GLP-1 activates δ-cells and promotes somatostatin secretion, forming a multi-layered inhibitory mechanism that effectively reduces glucagon secretion.

The relative expression levels of GLP-1R on α-cells and δ-cells exhibit certain differences. These variations significantly influence the role of GLP-1 in regulating islet function and glucose homeostasis.¹³¹ Research indicates that the expression level of GLP-1R on α-cells is relatively low.¹³¹ Although GLP-1Rs are present, they are limited in number, making the direct inhibitory effect of GLP-1 on α-cells relatively weak.²⁴⁷ The primary action often occurs through indirect mechanisms, such as insulin and somatostatin.²⁴⁴ Conversely, the expression level of GLP-1R on δ-cells is relatively high.²⁴⁸ GLP-1 can effectively bind to receptors on δ-cells, stimulating the secretion of somatostatin.²⁴⁸ As a broad-spectrum inhibitory hormone, somatostatin can effectively inhibit glucagon secretion from α-cells and insulin secretion from β-cells.^249,250 Relevant studies suggest that the expression of GLP-1R on δ-cells is crucial for GLP-1's regulation of somatostatin secretion and the overall inhibitory effect on glucagon.^248,251,252 This mechanism is particularly evident in the use of GLP-1-based drugs, such as GLP-1RAs, in the treatment of T2DM. In summary, the relatively higher expression of GLP-1R on δ-cells allows GLP-1 to indirectly inhibit glucagon secretion by promoting somatostatin secretion. In contrast, the lower expression of GLP-1R on α-cells results in a limited direct effect. Although these three types of cells each have distinct functions, it is more important to note that the somatostatin, glucagon, and insulin they secrete work together through mutual regulation and feedback mechanisms to maintain blood glucose balance and overall metabolic homeostasis.^253,254

GLP-1 reduces appetite and food intake through its actions in the CNS.^212,260,261 By interacting with receptors in the hypothalamus, GLP-1 influences satiety and reduces the consumption of food in both animals and humans.²⁶² It functions not only in peripheral tissues but also directly within the CNS, crossing the blood-brain barrier or being produced centrally to act on the brain.^262,263 GLP-1 regulates energy balance and food intake by acting on specific brain regions, especially the hypothalamus, a key area responsible for hunger and fullness sensations.^264–266 The hypothalamus contains various neurons that respond to different nutritional signals and hormones, like GLP-1, to regulate food intake.^265,266 When GLP-1 binds to its receptors in the hypothalamus, it activates specific neurons, including those that promote satiety (POMC/CART neurons) and inhibits those that induce hunger (NPY/AgRP neurons), thereby enhancing the feeling of fullness and reducing food intake.^233,267

GLP-1 can slow down gastric emptying through its effects on smooth muscles and the nervous system in the gastrointestinal tract.^264,268 Released at the gut's end, GLP-1 acts on its receptors in the gastrointestinal tract, leading to reduced gastric muscle contractions and extended food retention in the stomach.^264,269 Furthermore, GLP-1 can slow down gastric emptying by activating the vagus nerve, a part of the autonomic nervous system crucial for regulating the activities of the gastrointestinal tract.²⁶⁸ When activated, the vagus nerve can reduce the contraction of the stomach, thereby slowing down food emptying.²⁶⁹ The prolonged retention of food in the stomach enhances the feeling of fullness, naturally reducing overall food intake.^270,271 Additionally, slower gastric emptying helps stabilize postprandial blood glucose levels. Reducing food intake and extending satiety are significant for weight management and loss. By regulating gastric emptying, GLP-1 not only aids in short-term appetite control but may also contribute positively to long-term energy balance and weight management.¹⁷⁶

GLP-1RAs work via numerous mechanisms that contribute to weight loss, one of the most well-known of which is the gut-brain axis.^212,213,390 Within this axis, GLP-1 functions by acting on both the gut and the brain.³⁹¹ Furthermore, the combination of GLP-1-mediated signaling and the adipocyte hormone leptin has recently garnered increased interest. Notably, leptin may serve as a crucial biological signal for GLP-1, working in synergy, to decrease food intake and body weight. The effects of the leptin-GLP-1 interaction may be governed by intracellular signaling pathways, including those involving phosphorylated STAT3 and PTP1B.^15,392

In Wistar rats, a diet high in fat was found to lead to a decrease in the ratio of OPG/RANKL, which resulted in increased bone resorption and ultimately a reduction in bone mass. The administration of GLP-1RA or exendin-4 to rats fed a high-fat diet resulted in an increase in the OPG/RANKL ratio and a reduction in the degree of bone loss. It was observed that the treatment of rats on a high-fat diet with exendin-4 resulted in a decrease in the number of osteoclasts and the area of eroded surfaces, while there was an increase in the osteoid area, bone mass, and trabecular bone volume. These effects were compared to those of untreated controls that were also maintained on a high-fat diet.³⁹³ As a result, GLP-1RAs could mitigate the detrimental effects of hyperlipidemia-induced skeletal defects, leading to an improved prognosis in individuals with OA.

GLP-1 can modulate peripheral nerves through the ERK (extracellular signal-regulated kinase) signaling pathway, reducing the occurrence of neurological dysfunction.⁴⁰⁰ Researchers have discovered that GLP-1RAs exhibit a noticeable increase in the level of phosphorylated ERK1/2 within the sciatic nerve of diabetic rats. This observation led them to speculate that GLP-1 analogs might possess distinct neurotrophic properties and exert protective effects specifically on the nerve.^{71,395,401–403}

Astrocytes, the most abundant cell type in adult brain nerve tissue, have recently been recognized for their pivotal role in regulating glucose and energy homeostasis.^404–406 These cells not only respond to signals from leptin and insulin but also adapt to alterations in brain metabolism to accommodate behavioral changes by controlling glucose transport.^406,407 Furthermore, astrocytes express GLP-1R, which has been found to be crucial for their proper functioning.⁴⁰⁸ Studies have shown that the loss of GLP-1R in astrocytes can impair mitochondrial integrity, leading to the dysfunction and inhibition of glucose uptake and β-oxidation.⁴⁰⁷ GLP-1 inhibits glucose uptake in astrocytes and promotes beta-oxidation, which is essential for regulating energy balance in the brain and maintaining mitochondrial integrity.⁴⁰⁶ When GLP-1R is knocked out in astrocytes, it activates an integrated stress response, which affects the overall metabolic state by increasing the production of FGF21.⁴⁰⁵ This suggests that signaling through GLP-1R in astrocytes is crucial for maintaining the metabolic stability and functionality of the cells.⁴⁰⁶ FGF21 is recognized as a stress response factor that addresses mitochondrial dysfunction in cells. In astrocytes lacking GLP-1R, the increase in FGF21 is associated with improved systemic glucose homeostasis and memory formation.⁴⁰⁶ This indicates that FGF21 not only plays a role in cellular stress responses but also has a significant role in regulating brain function and systemic metabolism.⁴⁰⁶ Therefore, endogenous GLP-1R signaling in astrocytes plays a critical role in maintaining mitochondrial homeostasis and is dependent on FGF21 to effectively regulate glucose metabolism.^406,407

GLP-1RAs can alleviate neuroinflammation by acting on the nervous system, thereby offering additional relief from pain in patients.^6,409,410 During a previous experiment, researchers induced pain and observed symptoms of cognitive impairment in rats through spinal nerve ligation. Then, the researchers administered exendin-4 via intrathecal injection to the experimental rats and observed a reduction in pain sensitivity, alleviation of neural inflammation, and suppression of inflammatory factors such as IL-1β, TNF-α, and iNOS. This finding suggests a direct correlation between the analgesic effects of GLP-1R signaling and its Anti-neuroinflammatory activity.⁴¹¹ In another study, 3D fluorescence microscopy was used to eliminate proteins within chondrocytes in the subchondral bone of cartilage, resulting in bone transparency and the acquisition of high-resolution 3D images. By utilizing this methodology, researchers have revealed that GLP-1RAs can exhibit affect the axon terminals of sensory neurons. Ultimately, it was revealed that cholinergic fibers are present within the subchondral bone of individuals with OA, and the release of acetylcholine (Ach) triggered by vagus nerve stimulation plays a prominent role in combating inflammation across various diseases while also providing pain relief.⁴¹² Notably, GLP-1RAs, including exendin-4, have been shown to attenuate microglial activation, resulting in a reduction in the expression of proinflammatory cytokines such as TNF-α and IL-1β. By modulating inflammatory responses, GLP-1RAs can help prevent the degeneration of dopamine-producing cells.⁴¹³ GLP-1RAs play a neuroprotective role in neurons by facilitating neuronal survival and promoting neuronal growth, thereby preserving the structural and functional integrity of synapses.^6,14 Furthermore, GLP-1 signaling indirectly enhances and restores the insulin signaling pathway in neurons, leading to a reduction in the phosphorylation of insulin receptor substrates and the burden caused by monomeric α-synuclein.⁴¹⁴ Together, these effects contribute to the protection of dopaminergic neurons (source). Exenatide has been shown to mitigate the phosphorylation of insulin receptor substrates and the accumulation of monomeric α-synuclein.⁴¹⁴ By activating the GLP-1 signaling pathway, exenatide exhibits neuroprotective effects and safeguards dopaminergic neurons.⁵

The most well-known mechanism through which GLP-1RAs affect weight loss is through the endocrine pathway, but its role in the nervous system should not be overlooked.^415–417 GLP-1RAs can exert their local effects by activating vagal dendritic terminals that innervate the gut.^418–420 This activation holds the ability to modulate food consumption by reducing food intake and conveying signals of satiety to the brain.³⁹¹ GLP-1 and GLP-1 analogs exert their effects on food intake and body weight through a myriad of neural substrates, encompassing several hypothalamic nuclei (including the arcuate nucleus of the hypothalamus, periventricular hypothalamus, and lateral hypothalamic area), hindbrain nuclei (such as the parabrachial nucleus and medial nucleus tractus solitarius), the ventral subregion of the hippocampus (vHP), and nuclei embedded within the mesolimbic reward circuitry (including the ventral tegmental area, VTA, and nucleus accumbens, NAc).^421,422 Remarkably, GLP-1R activation in certain nuclei (such as the VTA, NAc, and vHP) elicits reductions in food intake and body weight without concurrent nausea responses.³⁹²

In the research conducted by Daniel J. Drucker's team, the mechanism of anti-inflammatory action of GLP-1RAs was explored, revealing a key role for the CNS in regulating this anti-inflammatory effect.⁴²³ The study began by inducing inflammation in mice through the injection of various Toll-like receptor (TLR) agonists and then assessed the inflammation by measuring plasma tumor necrosis factor-alpha (TNF-α) levels.⁴²³ It was observed that the GLP-1RA, exendin-4, significantly reduced the TNF-α levels caused by various TLR agonists, indicating that exendin-4 can lower the TNF-α levels induced by multiple TLR agonists.⁴²³ The research further demonstrated that this anti-inflammatory effect was not mediated by GLP-1R in the blood or endothelial cells but required the GLP-1R in the CNS.

Additionally, to simulate sepsis caused by polymicrobial infection and assess the impact of GLP-1RAs, the study utilized a cecal content injection method. It was found that semaglutide, a long-acting GLP-1RA, could improve symptoms caused by sepsis, reduce body temperature, and decrease the bacterial load in multiple organs, along with the levels of inflammatory factors in plasma and lungs.⁴²³ These findings further confirmed the central role of the CNS in regulating the anti-inflammatory effects of GLP-1RAs, highlighting the potential application of GLP-1RAs in anti-inflammatory treatment.

The study also discovered the roles of α1-adrenergic and δ- and κ-opioid receptors in this process. Specifically, blocking the α1-adrenergic receptors with prazosin or the opioid receptors with nalbuphine interfered with the ability of GLP-1RAs to reduce plasma TNF-α levels, indicating the critical importance of these pathways in the anti-inflammatory effects mediated by GLP-1 activation. The α1-adrenergic receptors, found on the surface of cells, facilitate various physiological responses, including regulating vascular contraction and heart rate. The use of prazosin, an α1-adrenergic receptor blocker, showed that the ability of GLP-1RAs to reduce TNF-α in plasma was disrupted, signifying the vital role of α1-adrenergic receptors in the anti-inflammatory action of GLP-1RAs. Opioid receptors, which are associated with pain regulation, mood, and immune responses, include δ and κ types. The use of nalbuphine, a blocker of δ- and κ-opioid receptors, also disrupted the effect of GLP-1RAs on reducing TNF-α levels, highlighting the essential role of these opioid receptors in the anti-inflammatory effects mediated by GLP-1RAs.⁴²³

Furthermore, the study emphasized that neurons in specific brain regions, such as the hindbrain and hypothalamus, co-express GLP-1R along with α1-adrenergic and δ-opioid receptors, suggesting a localized mechanism within the CNS that could coordinate peripheral anti-inflammatory responses. This co-expression indicates a mechanism within the CNS allowing these neurons to sense and respond to the peripheral inflammatory state. Through the interaction of these receptors, the brain can receive signals of peripheral inflammation and respond through neural signaling, thereby modulating or alleviating the inflammatory response. This receptor co-expression on neurons in specific brain regions may enable the brain to finely regulate the body's response to inflammation. For instance, when peripheral tissues become inflamed, the related signals might be transmitted to the brain through these receptors on the neurons, and the brain could then respond to these signals via neural pathways, thus regulating the level of peripheral inflammation.⁴²³

AD is a progressive and irreversible neurodegenerative disorder characterized by an unclear etiology and pathogenesis.⁴²⁴ In AD, GLP-1(7-36) amide inhibits IL-1β transcription and prevents cognitive dysfunction, amyloid precursor protein synthesis, and cell death. It also enhances learning and memory by promoting long-term potentiation (LTP).^425,426 In a murine model of AD, the administration of GLP-1RAs effectively reduced the levels of pathological markers associated with AD. These markers include oligomeric antibodies and amyloid plaque load. Furthermore, GLP-1RAs have been shown to attenuate microglial activation and improve memory-related behaviors.⁷¹ Moreover, GLP-1RAs have demonstrated considerable therapeutic promise in animal models of both PD and AD.^427–429 The compound NLY01 is particularly effective at attenuating the activity of proinflammatory microglia and preventing the transformation of astrocytes to a reactive phenotype. This activity is instrumental in safeguarding hippocampal neurons from the deleterious impacts of glutamate-induced excitotoxicity and hypoxic conditions. In the context of AD, where neuroinflammation and neurotoxicity are prominent pathological hallmarks, the modus operandi of NLY01 offers a potential therapeutic avenue to mitigate analogous pathological mechanisms inherent to this neurodegenerative condition.⁴³⁰

PD is a chronic neurodegenerative disorder that affects the CNS and is the second most prevalent neurodegenerative disease worldwide.¹⁴ GLP-1RAs are promising pharmacological agents for treating PD due to their potential to preserve the integrity and function of dopaminergic neurons. In addition to its effects on metabolic regulation, GLP-1, when synthesized within the brain, exhibits neuroprotective properties.^397,427,431 A clinical trial titled "Liraglutide in Early Parkinson's Disease" was published in the New England Journal of Medicine, exploring the effects of liraglutide on patients with early-stage PD diagnosed within the last three years.⁴³² The research was a 14-month Phase II double-blind randomized controlled trial. The results showed that liraglutide had a modest positive effect on improving motor function and performed well in terms of safety and tolerability, although there were some manageable gastrointestinal side effects.⁴³² The study emphasizes the need for further research into the potential benefits and risks of liraglutide in patients at different stages of PD.⁴³²

Researchers have discovered that semaglutide has the ability to reduce both recurrent alcohol consumption and overall alcohol intake in rats by more than 50%.⁴³³ Specifically, alcohol-dependent rats were administered semaglutide, which resulted in a significant reduction in their alcohol consumption.⁴³³ Further investigation into the mechanism underlying the alcohol-reducing effects of semaglutide suggested that this effect may involve the modulation of alcohol-induced rewards and punishments within the brain. Researchers have also shown that semaglutide impacts the reward and punishment systems in the brains of mice, particularly in the nucleus accumbens region, which is part of the limbic system.⁴³³ It is believed that alcohol activates the brain's reward and punishment system, triggering the release of dopamine, a neurotransmitter associated with pleasure and reward, both in humans and animals. However, the administration of semaglutide appeared to block this process, potentially leading to a diminished alcohol-induced reward and punishment response within the body.⁴³³ Remarkably, compared with untreated rats, treated rats exhibited a significant reduction in alcohol intake, which was reduced by half.⁴³³ These findings highlight the potential therapeutic efficacy of semaglutide in mitigating alcohol consumption.⁴³⁴

AS is characterized by the formation of fibrofatty plaques within arterial walls and is one of the foremost global causes of mortality.⁵ GLP-1RAs, as exemplified by liraglutide and semaglutide, exhibit pronounced cardiovascular protective effects.⁴³ Liraglutide and semaglutide have been demonstrated to be effective at reducing lipid and blood pressure levels through numerous scientific investigations, thereby contributing to the mitigation of AS and cardiovascular ailments.⁴⁴² Preclinical studies have documented the inhibitory effects of GLP-1RAs on the development of AS in animal models.⁴⁴³ These agents exert their anti-atherosclerotic effects through various mechanisms, including by improving blood lipid profiles,⁴⁴⁴ preserving endothelial integrity and regulating endothelial function,⁴⁴⁵ as well as modulating inflammatory processes.⁷⁰

GLP-1RAs confer substantial benefits for individuals with AS owing to their multifaceted impact on various aspects of cardiovascular health.^39,40,446 Notably, GLP-1RAs exhibit pronounced efficacy in optimizing blood lipid profiles by effectively suppressing chylomicron secretion in the intestine, thereby mitigating the occurrence of postprandial hyperlipidemia.⁴⁴⁷ Moreover, these agents promote hemodynamic equilibrium,¹⁸ diminish thrombotic propensity,⁴⁴⁸ alleviate endothelial oxidative stress,⁴⁴⁹ attenuate inflammatory processes,⁴⁵⁰ and foster a favorable balance of the gut microbiota,⁴⁵¹ all of which collectively contribute to the salutary effects of these agents in patients with AS.

GLP-1RAs have been shown to exert beneficial effects on the maintenance of endothelial integrity. One study suggested that GLP-1RAs exert direct endothelial protective effects by activating the GLP-1R-dependent AMPK/Akt/eNOS pathway. This pathway facilitates the generation of NO, thereby contributing to the preservation of vascular health and endothelial function. Additionally, GLP-1RAs facilitate the maintenance of endothelial barrier integrity, thus playing a crucial role in preventing vascular leakage.⁴⁵² Endothelial cells exhibit enhanced NO production and concurrent suppression of endothelin formation, resulting in vascular smooth muscle relaxation and vasodilation mediated by the endothelium (e.g., GLP-1, exenatide, and liraglutide).⁴⁵³ Indeed, GLP-1RAs, including exenatide, liraglutide, and semaglutide, have been linked to the diminished expression of matrix metalloproteinases (MMPs).^454–456 MMPs are a class of enzymes that play a crucial role in the breakdown of extracellular matrix components. Excessive MMP activity can lead to the weakening of fibrous caps and increase the risk of plaque rupture in atherosclerotic lesions.⁴⁵⁴ By reducing MMP expression, GLP-1R stimulation may help preserve the integrity of fibrous caps and mitigate the risk of plaque rupture.

GLP-1RAs have been shown to reduce the levels of systemic inflammatory markers.^457,458 Importantly, the effective management of inflammation is widely acknowledged to play a crucial role in the prevention of cardiovascular diseases.^459,460 The anti-inflammatory effects of GLP-1RAs contribute to the attenuation of atherosclerotic plaque lesion development through various mechanisms. First, GLP-1RAs have been shown to inhibit the expression and release of proinflammatory cytokines, such as IL-6 and TNF-α, thereby inhibiting the overall inflammatory response.⁴⁶¹ GLP-1RAs can suppress the activation of nuclear factor-kappa B (NF-κB), a key transcription factor involved in the regulation of inflammatory processes.⁴⁶² Inhibition of the NF-κB signaling pathway leads to the decreased production of inflammatory mediators, including adhesion molecules and chemokines, which are crucial for the recruitment of immune cells to sites of inflammation.⁴⁶³ In addition, liraglutide can delay the formation of AS by inducing cell cycle arrest in vascular smooth muscle cells through the AMPK pathway.⁵

The prevalence of left ventricular ejection fraction (LVEF)-preserved heart failure continues to increase. The symptoms in patients are severe and often accompanied by functional impairments, especially in the obese population. Heart failure with preserved ejection fraction (HFpEF) accounts for approximately half of all heart failure cases, with patients bearing a high symptomatic burden and physical limitations that impact their daily lives. These limitations include fatigue, shortness of breath, reduced exercise capacity, and limb swelling. Currently, there are no approved targeted therapies for LVEF-preserved heart failure associated with obesity. Moreover, GLP-1R has not yet been identified on human myocardial cells, negating any direct effects of GLP-1RAs on myocardial cells.⁴⁷⁰

Recently, studies have shown that the GLP-1RA Meg peptide also enters the ejection fraction reserve following heart failure therapy. The results of a trial of semaglutide in obese patients with HFpEF showed a significant amelioration of symptoms and improvements in motor function and weight loss compared with patients treated with a placebo.⁴⁷⁰ In this study, researchers randomized 529 patients with heart failure with a preserved ejection fraction and body mass index (weight in kilograms in the square of the height in meters) ≥30 to receive once-weekly injections of semaglutide (2.4 mg) or placebo for 52 weeks. The two primary endpoints were the Kansas City Cardiomyopathy Questionnaire clinical summary score (KCCQ-CSS, which ranges from 0 to 100) and the Kansas City Cardiomyopathy Questionnaire Clinical Summary Score (KCCQ-CSS). Higher scores indicate fewer symptoms and physical limitations and weight change from baseline. The confirmatory secondary endpoints included the change in the 6-min walking distance, a hierarchical composite of death and heart failure events; the difference between the change in the KCCQ-CSS and the change in the 6-min walking distance; and the change in the C-reactive protein (CRP) level.⁴⁷⁰ In this trial, semaglutide benefited patients with heart failure and preserved ejection fraction by intervening with upstream metabolic drivers. This agent differs from earlier therapies that were designed to reduce myocardial loading or induce neurohumoral blockade. These positive results indicate that the change in myocardial cells may not be the primary driver of HFpEF; instead, the multisystem pathologic processes that are associated with this condition have long been well established as drivers of its clinical presentation and outcomes.⁴⁷⁰

One could argue that the success of sodium-glucose cotransporter 2 (SGLT2) inhibitors, and now of GLP-1RAs, suggests that metabolic abnormalities play an important role in driving HFpEF.⁴⁷⁰ SGLT2 inhibition is also known to benefit patients with heart failure and a reduced ejection fraction.⁴⁷¹ The mechanisms by which SGLT2 inhibitors treat heart failure include lowering blood glucose levels by inhibiting the SGLT2 protein in the renal tubules and reducing glucose reabsorption in the tubules.⁴⁷² Promoting the urinary excretion of sodium and water reduces fluid retention and hypervolaemia, thereby reducing the load on the heart.⁴⁷³ It also improves myocardial energy metabolism, increases fatty acid oxidation in the myocardium, and improves myocardial energy supply.⁴⁷⁴ Moreover, these agents reduce the risks of myocardial inflammation and fibrosis and improve cardiac structure and function.⁴⁷⁵ By the way, a clinical study published in "Nature Metabolism" detailed the effects of semaglutide and dapagliflozin (an SGLT2 inhibitor) on blood sugar control in patients with T2DM of different pathophysiological types.⁴⁷⁶ It was found that semaglutide performed better than dapagliflozin in reducing HbA1c levels.⁴⁷⁶ The study results suggest that semaglutide may be a more suitable choice for patients with severe insulin deficiency, while dapagliflozin might be effective for a broader range of patients with metabolic abnormalities.⁴⁷⁶

If SGLT2 inhibitors and GLP-1RAs are effective in patients with heart failure (regardless of whether the ejection fraction is reduced or preserved), the commonalities between the two types of heart failure may be greater than is often thought. If so, the two types of heart failure may be very similar, except that the cause of heart failure with a preserved ejection fraction may not be the same as that with a reduced ejection fraction.^477–479 If this is indeed the case, then both types of heart failure are syndromes caused by multiple metabolic and inflammatory changes; however, heart failure with reduced ejection fraction also has a regional cause. Therefore, intrinsic cardiac load and capacity do not improve in patients with heart failure with reduced ejection fraction, and patients HFpEF benefit only from the treatment of abnormal metabolism and inflammation.^475,480,481

The encouraging results obtained with semaglutide in patients with heart failure and a preserved ejection fraction may provide a new option for this patient population, in which additional therapy, as well as another upstream therapy for patients with a higher BMI who have indications for this condition, is urgently needed. The clinical translation of these trial results, which will be important for the comparison of GLP-1RAs with SGLT2 inhibitors in the treatment of patients with heart failure and a preserved ejection fraction, remains to be determined.^{470,478,482–484}

When dietary nutrients are ingested, endogenous incretins (GIP and GLP-1) activate K and L cells in the gut, which then secrete GIP and GLP-1.^488–490 In the pancreas, this stimulates the secretion of insulin and inhibits the secretion of glucagon.⁴⁹¹ In the brain, it reduces appetite and improves satiety.^130,133 In the gastrointestinal tract, it lowers the synthesis and secretion of triglycerides.⁴⁹¹ By regulating appetite, insulin secretion, and lipid metabolism, GLP-1RAs have potential benefits in the treatment of NAFLD, NASH, and T2DM.^130,492,493

NASH is a liver disease primarily caused by fat accumulation, which can progress to liver fibrosis, cirrhosis, or even liver cancer.^494–498 The role of GLP-1 in NASH has garnered attention due to its potential in regulating metabolism, improving insulin sensitivity, and exerting anti-inflammatory effects.^17,499–503 Insulin resistance, a common occurrence in NASH patients, is a key driver of the disease's progression.^504–506 GLP-1 enhances the insulin signaling pathway in the liver by activating the GLP-1R, especially through the phosphorylation of insulin receptor substrates and the activation of the PI3K/Akt signaling pathway, thereby increasing hepatic insulin sensitivity, facilitating glucose uptake and utilization, and reducing hepatic gluconeogenesis.^3,507

GLP-1RAs reduce liver fat accumulation by activating AMPK, which inhibits fatty acid synthesis enzymes and promotes fatty acid β-oxidation, thus diminishing lipid droplet accumulation in hepatocytes.^508–511 Additionally, GLP-1 mitigates liver inflammation and fibrosis by inhibiting the NF-κB pathway, reducing the release of pro-inflammatory cytokines such as TNF-α and IL-6, and thus suppressing inflammatory pathways.^307,413

In terms of apoptosis inhibition, GLP-1 activates the PI3K/Akt signaling pathway, enhances the expression of the anti-apoptotic protein Bcl-2, and inhibits the activation of caspase family proteins, reducing cell apoptosis.²⁰⁶ This helps prevent the progression of NASH to liver fibrosis and cirrhosis. The weight-loss effect of GLP-1RAs also benefits NASH improvement, by reducing appetite and increasing energy expenditure to promote weight loss, thereby indirectly ameliorating NASH.^206,512 Thus, GLP-1 and its agonists slow down the progression of NASH and may positively impact the reversal of liver fibrosis.^513,514 These signaling pathways suggest that GLP-1 and its receptor agonists may influence the progression of NASH through multiple mechanisms, providing several potential therapeutic targets including: FOXO1, a transcription factor that plays a critical role in regulating glucose and lipid metabolism. GLP-1 can improve abnormalities in glucose metabolism and excessive lipid accumulation by modulating the activity of FOXO1.^5,514 Sirt1, a protein deacetylase, plays a key role in delaying cellular aging and regulating metabolism.^515–517 GLP-1 may improve the liver's antioxidant capacity and metabolic function by activating Sirt1, thereby helping to alleviate the pathological changes associated with NASH.

A 2023 study published in the "Journal of Hepatology" reported on a Phase IIa trial aimed at evaluating the efficacy and safety of efinopegdutide in patients with NAFLD, comparing it to semaglutide.¹⁴⁴ Efinopegdutide is a dual agonist for GLP-1R and GIPR.¹⁴⁴ The study used a randomized, open-label design and employed magnetic resonance imaging technology (MRI-PDFF) to measure liver fat content (LFC) after 24 weeks of treatment. Results showed that efinopegdutide was more effective in reducing LFC compared to semaglutide. Additionally, the study assessed weight and metabolic responses, finding that efinopegdutide's tolerability was similar to that of semaglutide, although certain gastrointestinal side effects were more common. Overall, efinopegdutide emerged as a promising option for the treatment of NAFLD.¹⁴⁴

GLP-1 and its receptor agonists show some potential in the treatment of gastrointestinal cancers, although this remains an emerging area of research.^518–521