GLP-1 receptor activation indirectly reduces hepatic lipid accumulation but does not attenuate development of atherosclerosis in diabetic male ApoE(-/-) mice

Animal studies on glucagon-like peptide-1 receptor agonists and related polyagonists in nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) is a prevalent metabolic liver disease closely associated with the epidemics of obesity and type 2 diabetes mellitus (T2DM), but without licensed pharmacological treatment to date. As glucagon-like peptide-1 (GLP-1) receptor agonists (GLP-1RAs) are approved anti-diabetic and anti-obesity medications, they were also considered a potential therapeutic option for NAFLD. Preclinical studies suggest that GLP-1RAs have a beneficial effect on major NAFLD histological outcomes, i.e., hepatic steatosis and inflammation, through multiple intrahepatic mechanisms, including increased fatty acid β-oxidation, activation of autophagy, suppression of inflammation, and oxidative stress. Data on hepatic fibrosis are limited or inconclusive, although some studies reported improvement in indices of fibrosis or prevention of fibrosis initiation or reduction of collagen deposition. Whether the positive impact of GLP-1RAs on hepatic histology is indirect, i.e., through their action on extrahepatic tissues, or whether their action is direct, i.e., through activating GLP-1R on the hepatocytes, is still a controversial issue. Alongside GLP-1RAs, newly emerging peptide polyagonists (i.e., synthetic molecules that combine the amino acid sequences of more than one peptide, thus having the ability to bind more than one receptor) are now being investigated in NAFLD with high expectations. This review summarizes the existing knowledge derived from animal studies on the effects of GLP-1RAs and GLP-1RA related peptide polyagonists on NAFLD in an attempt to illuminate areas of uncertainty and provide the groundwork for future animal and clinical research in the field.

Tirzepatide, a dual glucose-dependent insulinotropic polypeptide/glucagon-like peptide 1 receptor agonist, exhibits favourable effects on pancreatic β-cells and hepatic steatosis in obese type 2 diabetic db/db mice

AIM

Tirzepatide, a dual agonist of glucagon-like peptide receptor and glucose-dependent insulinotropic polypeptide receptor, is expected to exhibit high clinical efficacy in obese type 2 diabetic patients. We evaluated the effects of tirzepatide on pancreatic β-cells and the liver, an insulin-target organ, in a mouse model of obese type 2 diabetes mellitus.

MATERIALS AND METHODS

Obese type 2 diabetic db/db mice (BKS.Cg-/+ Leprdb/+ Leprdb/Jcl*) were used in this study. Starting at 7 weeks of age, mice were treated with tirzepatide (30 nmol/kg, subcutaneous injection twice a week) or semaglutide (200 nmol/kg, subcutaneous injection twice a week). The control group received phosphate-buffered saline (40-50 μL/subcutaneous injection twice a week). After 4 weeks of drug administration, pancreatic β-cells and the liver were removed and examined.

RESULTS

Compared to the control group, blood glucose and body weight were significantly reduced in the group that received either tirzepatide or semaglutide (p < 0.001 and p < 0.05, respectively). Fasting insulin was significantly higher in the semaglutide and tirzepatide groups compared to the control group (p < 0.001). β-Cell mass and quality of insulin granules in β-cells similarly increased in the semaglutide and tirzepatide groups compared to the control group (p < 0.05 and p < 0.001, respectively). The fat staining area in the liver in oil red O staining and the liver-spleen ratio in computed tomography showed improvement only in the tirzepatide group (p < 0.001 and p < 0.005, respectively). Liver macrophage M1/M2 ratio similarly improved with semaglutide and tirzepatide (p < 0.05).

CONCLUSION

Tirzepatide and semaglutide exhibited similar potent glucose-lowering effects. At concentrations used in the present experiments, tirzepatide exhibited more beneficial effects on β-cell-related gene expression, insulin granule count and glucose-stimulated insulin secretion compared to semaglutide. In addition, tirzepatide exhibited a stronger favourable effect on hepatic fat deposition and improved inflammation in the liver. This is the first report showing that tirzepatide, a novel diabetes drug, exhibits a superior effect on pancreatic β-cells and the liver of obese type 2 diabetic mice.

Hepatokines and MASLD: The GLP1-Ras-FGF21-Fetuin-A Crosstalk as a Therapeutic Target

The introduction of the term "Metabolic Steatotic Liver Disease" (MASLD) underscores the critical role of metabolic dysfunction in the development and progression of chronic liver disease and emphasizes the need for strategies that address both liver disease and its metabolic comorbidities. In recent years, a liver-focused perspective has revealed that altered endocrine function of the fatty liver is a key contributor to the metabolic dysregulation observed in MASLD. Due to its secretory capacity, the liver's increased production of proteins known as "hepatokines" has been linked to the development of insulin resistance, explaining why MASLD often precedes dysfunction in other organs and ultimately contributes to systemic metabolic disease. Among these hepatokines, fibroblast growth factor 21 (FGF21) and fetuin-A play central roles in regulating the metabolic abnormalities associated with MASLD, explaining why their dysregulated secretion in response to metabolic stress has been implicated in the metabolic abnormalities of MASLD. This review postulates why their modulation by GLP1-Ras may mediate the beneficial metabolic effects of these drugs, which have increased attention to their emerging role as pharmacotherapy for MASLD. By discussing the crosstalk between GLP1-Ras-FGF21-fetuin-A, this review hypothesizes that the possible modulation of fetuin-A by the novel GLP1-FGF21 dual agonist pharmacotherapy may contribute to the management of metabolic and liver diseases. Although research is needed to go into the details of this crosstalk, this topic may help researchers explore the mechanisms by which this type of pharmacotherapy may manage the metabolic dysfunction of MASLD.

Efficacy and Safety of GLP-1 Receptor Agonists in Patients With Metabolic Dysfunction-Associated Steatotic Liver Disease: A Systematic Review and Meta-Analysis of Randomized Controlled Trials

Metabolic dysfunction-associated steatotic liver disease (MASLD) poses a major global health challenge. glucagon-like peptide-1 receptor agonists (GLP-1RAs) have shown potential therapeutic benefits for MASLD patients, including improvements in liver function, inflammation, and fibrosis. This study aims to systematically review and meta-analyze randomized controlled trials (RCTs) to evaluate the efficacy and safety of GLP-1RAs in MASLD patients, focusing on hepatic outcomes, cardiovascular outcomes, anthropometric measurements, and mortality. Following PRISMA guidelines, a comprehensive database search was conducted to include RCTs assessing GLP-1RAs' effects on MASLD. Quality assessment was conducted using the Revised Cochrane Risk of Bias tool. Our meta-analysis used a random-effects model, calculating standardized mean differences for continuous outcomes to determine the agents' efficacy and safety. Additionally, funnel plots were generated to assess publication bias, ensuring the integrity of our meta-analytical findings. The review included 27 trials, revealing GLP-1RAs significantly improved hepatic function markers (alanine aminotransferase, aspartate aminotransferase, gamma-glutamyl transferase, and liver fat content) and cardiovascular risk factors (fasting blood sugar, HbA1c levels, lipid profiles). Additionally, GLP-1RAs were associated with significant reductions in body weight, BMI, subcutaneous fat, and waist circumference. GLP-1RAs demonstrate a promising therapeutic role in managing MASLD, offering benefits that extend to improving liver function, mitigating cardiovascular risk, and promoting weight loss. Further research is needed to confirm these findings and optimize GLP-1RAs' usage in MASLD treatment.

Hepatic function of glucagon-like peptide-1 and its based diabetes drugs

Incretins are gut-produced peptide-hormones that potentiate insulin secretion, especially after food intake. The concept of incretin was formed more than 100 years ago, even before insulin was isolated and utilized in the treatment of subjects with type 1 diabetes. The first incretin, glucose-dependent insulinotropic polypeptide (GIP), was identified during later 1960's and early 1970's; while the second one, known as glucagon-like peptide-1 (GLP-1), was recognized during 1980's. Today, GLP-1-based therapeutic agents [also known as GLP-1 receptor (GLP-1R) agonists, GLP-1RAs] are among the first line drugs for type 2 diabetes. In addition to serving as incretin, extra-pancreatic functions of GLP-1RAs have been broadly recognized, including those in the liver, despite the absence of GLP-1R in hepatic tissue. The existence of insulin-independent or gut-pancreas-liver axis-independent hepatic function of GLP-1RAs explains why those therapeutic agents are effective in subjects with insulin resistance and their profound effect on lipid homeostasis. Following a brief review on the discovery of GLP-1, we reviewed literature on the exploration of hepatic function of GLP-1 and GLP-1RAs and discussed recent studies on the role of hepatic hormone fibroblast growth factor 21 (FGF21) in mediating function of GLP-1RAs in animal models. This was followed by presenting our perspective views.

The impact of GLP-1 signalling on the energy metabolism of pancreatic islet β-cells and extrapancreatic tissues

Glucagon-like peptide-1 signalling impacts glucose homeostasis and appetite thereby indirectly affecting substrate availability at the whole-body level. The incretin canonically produces an insulinotropic effect, thereby lowering blood glucose levels by promoting the uptake and inhibiting the production of the sugar by peripheral tissues. Likewise, GLP-1 signalling within the central nervous system reduces the appetite and food intake, whereas its gastric effect delays the absorption of nutrients, thus improving glycaemic control and reducing the risk of postprandial hyperglycaemia. We review the molecular aspects of the GLP-1 signalling, focusing on its impact on intracellular energy metabolism. Whilst the incretin exerts its effects predominantly via a Gs receptor, which decodes the incretin signal into the elevation of intracellular cAMP levels, the downstream signalling cascades within the cell, acting on fast and slow timescales, resulting in an enhancement or an attenuation of glucose catabolism, respectively.

Glucagon-like peptide (GLP)-1 regulation of lipid and lipoprotein metabolism

Metabolic health is highly dependent on intestinal and hepatic handling of dietary and endogenous lipids and lipoproteins. Disorders of lipid and lipoprotein metabolism are commonly observed in patients with insulin resistant states such as obesity, metabolic syndrome, and type 2 diabetes. Evidence from both animal models and human studies indicates that a major underlying factor in metabolic or diabetic dyslipidemia is the overproduction of hepatic and intestinal apolipoprotein (apo)B-containing lipoprotein particles. These particles are catabolized down into highly proatherogenic remnants, which can be taken up into the arterial intima and promote plaque development. Several gut-derived peptides have been identified as key regulators of energy metabolism; one such peptide is the incretin hormone glucagon-like peptide (GLP)-1. Our laboratory has previously demonstrated that GLP-1 can signal both centrally and peripherally to reduce postprandial and fasting lipoprotein secretion. Moreover, we have demonstrated that GLP-1 receptor (GLP-1R) agonists can ameliorate diet-induced dyslipidemia. Recently, we published evidence for a novel vagal neuroendocrine signalling pathway by which native GLP-1 may exert its anti-lipemic effects. Furthermore, we demonstrated a novel role for other gut-derived peptides in regulating intestinal lipoprotein production. Overall, ample evidence supports a key role for GLP-1R on the portal vein afferent neurons and nodose ganglion in modulating intestinal fat absorption and lipoprotein production and identifies other gut-derived peptides as novel regulators of postprandial lipemia. Insights from these data may support identification of potential drug targets and the development of new therapeutics targeting treatment of diabetic dyslipidemia.

Mechanisms of action and therapeutic applications of GLP-1 and dual GIP/GLP-1 receptor agonists

Glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) are two incretins that bind to their respective receptors and activate the downstream signaling in various tissues and organs. Both GIP and GLP-1 play roles in regulating food intake by stimulating neurons in the brain's satiety center. They also stimulate insulin secretion in pancreatic β-cells, but their effects on glucagon production in pancreatic α-cells differ, with GIP having a glucagonotropic effect during hypoglycemia and GLP-1 exhibiting glucagonostatic effect during hyperglycemia. Additionally, GIP directly stimulates lipogenesis, while GLP-1 indirectly promotes lipolysis, collectively maintaining healthy adipocytes, reducing ectopic fat distribution, and increasing the production and secretion of adiponectin from adipocytes. Together, these two incretins contribute to metabolic homeostasis, preventing both hyperglycemia and hypoglycemia, mitigating dyslipidemia, and reducing the risk of cardiovascular diseases in individuals with type 2 diabetes and obesity. Several GLP-1 and dual GIP/GLP-1 receptor agonists have been developed to harness these pharmacological effects in the treatment of type 2 diabetes, with some demonstrating robust effectiveness in weight management and prevention of cardiovascular diseases. Elucidating the underlying cellular and molecular mechanisms could potentially usher in the development of new generations of incretin mimetics with enhanced efficacy and fewer adverse effects. The treatment guidelines are evolving based on clinical trial outcomes, shaping the management of metabolic and cardiovascular diseases.

GLP1R and GIPR expression and signaling in pancreatic alpha cells, beta cells and delta cells

Glucagon-like peptide-1 receptor (GLP1R) and glucose-dependent insulinotropic polypeptide receptor (GIPR) are transmembrane receptors involved in insulin, glucagon and somatostatin secretion from the pancreatic islet. Therapeutic targeting of GLP1R and GIPR restores blood glucose levels in part by influencing beta cell, alpha cell and delta cell function. Despite the importance of the incretin-mimetics for diabetes therapy, our understanding of GLP1R and GIPR expression patterns and signaling within the islet remain incomplete. Here, we present the evidence for GLP1R and GIPR expression in the major islet cell types, before addressing signaling pathway(s) engaged, as well as their influence on cell survival and function. While GLP1R is largely a beta cell-specific marker within the islet, GIPR is expressed in alpha cells, beta cells, and (possibly) delta cells. GLP1R and GIPR engage Gs-coupled pathways in most settings, although the exact outcome on hormone release depends on paracrine communication and promiscuous signaling. Biased agonism away from beta-arrestin is an emerging concept for improving therapeutic efficacy, and is also relevant for GLP1R/GIPR dual agonism. Lastly, dual agonists exert multiple effects on islet function through GIPR > GLP1R imbalance, increased GLP1R surface expression and cAMP signaling, as well as beneficial alpha cell-beta cell-delta cell crosstalk.

Hepatic glucose metabolism in the steatotic liver

The liver is central in regulating glucose homeostasis, being the major contributor to endogenous glucose production and the greatest reserve of glucose as glycogen. It is both a target and regulator of the action of glucoregulatory hormones. Hepatic metabolic functions are altered in and contribute to the highly prevalent steatotic liver disease (SLD), including metabolic dysfunction-associated SLD (MASLD) and metabolic dysfunction-associated steatohepatitis (MASH). In this Review, we describe the dysregulation of hepatic glucose metabolism in MASLD and MASH and associated metabolic comorbidities, and how advances in techniques and models for the assessment of hepatic glucose fluxes in vivo have led to the identification of the mechanisms related to the alterations in glucose metabolism in MASLD and comorbidities. These fluxes can ultimately increase hepatic glucose production concomitantly with fat accumulation and alterations in the secretion and action of glucoregulatory hormones. No pharmacological treatment has yet been approved for MASLD or MASH, but some antihyperglycaemic drugs approved for treating type 2 diabetes have shown positive effects on hepatic glucose metabolism and hepatosteatosis. A deep understanding of how MASLD affects glucose metabolic fluxes and glucoregulatory hormones might assist in the early identification of at-risk individuals and the use or development of targeted therapies.

GLP-1 receptor agonists and atherosclerosis protection: the vascular endothelium takes center stage

Atherosclerotic cardiovascular disease is a chronic condition that often copresents with type 2 diabetes and obesity. Glucagon-like peptide-1 receptor agonists (GLP-1RAs) are incretin mimetics endorsed by major professional societies for improving glycemic status and reducing atherosclerotic risk in people living with type 2 diabetes. Although the cardioprotective efficacy of GLP-1RAs and their relationship with traditional risk factors are well established, there is a paucity of publications that have summarized the potentially direct mechanisms through which GLP-1RAs mitigate atherosclerosis. This review aims to narrow this gap by providing comprehensive and in-depth mechanistic insight into the antiatherosclerotic properties of GLP-1RAs demonstrated across large outcome trials. Herein, we describe the landmark cardiovascular outcome trials that triggered widespread excitement around GLP-1RAs as a modern class of cardioprotective agents, followed by a summary of the origins of GLP-1RAs and their mechanisms of action. The effects of GLP-1RAs at each major pathophysiological milestone of atherosclerosis, as observed across clinical trials, animal models, and cell culture studies, are described in detail. Specifically, this review provides recent preclinical and clinical evidence that suggest GLP-1RAs preserve vessel health in part by preventing endothelial dysfunction, achieved primarily through the promotion of angiogenesis and inhibition of oxidative stress. These protective effects are in addition to the broad range of atherosclerotic processes GLP-1RAs target downstream of endothelial dysfunction, which include systemic inflammation, monocyte recruitment, proinflammatory macrophage and foam cell formation, vascular smooth muscle cell proliferation, and plaque development.

Semaglutide Improves Liver Steatosis and De Novo Lipogenesis Markers in Obese and Type-2-Diabetic Mice with Metabolic-Dysfunction-Associated Steatotic Liver Disease

Metabolic-dysfunction-associated steatotic liver disease (MASLD) is a prevalent clinical condition associated with elevated morbidity and mortality rates. Patients with MASLD treated with semaglutide, a glucagon-like peptide-1 receptor agonist, demonstrate improvement in terms of liver damage. However, the mechanisms underlaying this beneficial effect are not yet fully elucidated. We investigated the efficacy of semaglutide in halting MASLD progression using a genetic mouse model of diabesity. Leptin-receptor-deficient mice with obesity and diabetes (BKS db/db) were either untreated or administered with semaglutide for 11 weeks. Changes in food and water intake, body weight and glycemia were monitored throughout the study. Body fat composition was assessed by dual-energy X-ray absorptiometry. Upon sacrifice, serum biochemical parameters, liver morphology, lipidomic profile and liver-lipid-related pathways were evaluated. The semaglutide-treated mice exhibited lower levels of glycemia, body weight, serum markers of liver dysfunction and total and percentage of fat mass compared to untreated db/db mice without a significant reduction in food intake. Histologically, semaglutide reduced hepatic steatosis, hepatocellular ballooning and intrahepatic triglycerides. Furthermore, the treatment ameliorated the hepatic expression of de novo lipogenesis markers and modified lipid composition by increasing the amount of polyunsaturated fatty acids. The administration of semaglutide to leptin-receptor-deficient, hyperphagic and diabetic mice resulted in the amelioration of MASLD, likely independently of daily caloric intake, suggesting a direct effect of semaglutide on the liver through modulation of the lipid profile.

Isocaloric diets with varying protein levels affected energy metabolism in young adult Sprague-Dawley rats via modifying the gut microbes: A lipid imbalance was brought on by a diet with a particularly high protein content

Protein is the most important macro-nutrient when it comes to maximizing health, body composition, muscle growth, and recovery of body tissue. In recent years, it has been found that protein also plays an important role in metabolism and gut microbiota. This study was performed to investigate the effects of an isocaloric diet with different crude protein contents on the energy metabolism of Sprague-Dawley (SD) rats. Results revealed that compared with the 20% crude protein (CP; control) diet, the 38% CP diet improved serum parameters that are associated with dyslipidemia and glucose metabolic disorders in SD rats, whereas the 50% CP diet increased liver injury indicators and fatty acid synthesis-related genes and protein expression in the liver. Compared with the control diet, the 14% CP diet increased the abundance of colonic short-chain fatty acid-producing bacteria (Lachnospiraceae_NK4A136_group and Ruminiclostridium_9) and promoted colonic microbial cysteine and methionine metabolism, the 38% CP diet up-regulated colonic microbial lysine biosynthesis and degradation pathways, and the 50% CP diet down-regulated colonic mucosal cholesterol metabolism. Furthermore, the increase of multiple colonic enteropathogenic bacteria in the 50% CP group was associated with higher palmitic acid and stearic acid concentrations in the colonic microbes and lower cholesterol and arachidonic acid concentrations in the colonic mucosa. These findings revealed that the 14% CP and 38% CP diets improved rats' energy metabolism, while the 50% CP diet was accompanied by lipid metabolism imbalances and an increase in the abundance of multiple enteropathogenic bacteria.

Poly-Agonist Pharmacotherapies for Metabolic Diseases: Hopes and New Challenges

The use of glucagon-like peptide-1 (GLP-1) receptor-based multi-agonists in the treatment of type 2 diabetes and obesity holds great promise for improving glycaemic control and weight management. Unimolecular dual and triple agonists targeting multiple gut hormone-related pathways are currently in clinical trials, with recent evidence supporting their efficacy. However, significant knowledge gaps remain regarding the biological mechanisms and potential adverse effects associated with these multi-target agents. The mechanisms underlying the therapeutic efficacy of GLP-1 receptor-based multi-agonists remain somewhat mysterious, and hidden threats may be associated with the use of gut hormone-based polyagonists. In this review, we provide a critical analysis of the benefits and risks associated with the use of these new drugs in the management of obesity and diabetes, while also exploring new potential applications of GLP-1-based pharmacology beyond the field of metabolic disease.

Semaglutide Modulates Extracellular Matrix Production of LX-2 Cells via Exosomes and Improves Metabolic Dysfunction-Associated Steatotic Liver Disease (MASLD)

Metabolic dysfunction-associated steatotic liver disease (MASLD) is closely related to some metabolic disorders, such as central obesity and type 2 diabetes (T2D). Glucagon-like peptide 1 receptor agonists (GLP-1RAs), such as semaglutide, may have therapeutic roles in MASLD associated with T2D. This study aims to investigate the molecular mechanisms underlying the effectiveness of semaglutide on MASLD in terms of progression from liver steatosis to fibrosis. We characterized exosomes from ten patients with type 2 diabetes (T2D) before (T0) and after 12 months (T12) of treatment with once-weekly subcutaneous semaglutide. Six of ten patients were considered responders to therapy (R) based on MASLD severity downgrading by at least one class according to a validated ultrasonographic (US) score. Normal hepatocytes (HEPA-RG) and stellate (LX-2) cells were challenged with exosomes from R and NR patients, isolated before and after 12 months of therapy. Exosomes from both R and NR patients isolated at T0 significantly affected LX-2 viability. After 12 months of treatment, only those isolated from R patients restored cell viability, whereas those from NR patients did not. No effects were observed on HEPA-RG cells. Exosomes at T12 from R but not from NR patients significantly decreased the production of α-SMA, a marker of LX-2 activation, a liver stellate cell model, and ph-SMAD2 and CTGF, involved in fibrosis processes. TGF-β1 was not modulated by the exosomes of R and NR patients. As a downstream effect, Vimentin, Collagen 1A1, and Fibronectin extracellular matrix components were also downregulated, as measured by droplets digital PCR. In conclusion, these results shed light on the potential effectiveness of semaglutide in improving liver fibrosis in MASLD.

Semaglutide reduces tumor burden in the GAN diet-induced obese and biopsy-confirmed mouse model of NASH-HCC with advanced fibrosis

Non-alcoholic steatohepatitis (NASH) is emerging as a major cause of hepatocellular carcinoma (HCC), however, it is not resolved if compounds in late-stage clinical development for NASH may have additional therapeutic benefits in NASH-driven HCC (NASH-HCC). Here, we profiled monotherapy with semaglutide (glucagon-like-receptor-1 receptor agonist) and lanifibranor (pan-peroxisome proliferator-activated receptor agonist) in a diet-induced obese (DIO) mouse model of NASH-HCC. Disease progression was characterized in male C57BL/6 J mice fed the GAN (Gubra Amylin NASH) diet high in fat, fructose and cholesterol for 12-72 weeks (n = 15 per group). Other GAN DIO-NASH-HCC mice fed the GAN diet for 54 weeks and with biopsy-confirmed NASH (NAFLD Activity Score ≥ 5) and advanced fibrosis (stage F3) received vehicle (n = 16), semaglutide (30 nmol/kg, s.c., n = 15), or lanifibranor (30 mg/kg, p.o., n = 15) once daily for 14 weeks. GAN DIO-NASH-HCC mice demonstrated progressive NASH, fibrosis and HCC burden. Tumors presented with histological and molecular signatures of poor prognostic HCC. Consistent with clinical trial outcomes in NASH patients, both lanifibranor and semaglutide improved NASH while only lanifibranor reduced fibrosis in GAN DIO-NASH-HCC mice. Notably, only semaglutide reduced tumor burden in GAN DIO-NASH-HCC mice. In conclusion, the GAN DIO-NASH-HCC mouse is a clinical translational model of NASH-HCC. Semaglutide improves both NASH and tumor burden in GAN DIO-NASH-HCC mice, highlighting the suitability of this preclinical model for profiling novel drug therapies targeting NASH-HCC.

The intestine as an endocrine organ and the role of gut hormones in metabolic regulation

Gut hormones orchestrate pivotal physiological processes in multiple metabolically active tissues, including the pancreas, liver, adipose tissue, gut and central nervous system, making them attractive therapeutic targets in the treatment of obesity and type 2 diabetes mellitus. Most gut hormones are derived from enteroendocrine cells, but bioactive peptides that are derived from other intestinal epithelial cell types have also been implicated in metabolic regulation and can be considered gut hormones. A deeper understanding of the complex inter-organ crosstalk mediated by the intestinal endocrine system is a prerequisite for designing more effective drugs that are based on or target gut hormones and their receptors, and extending their therapeutic potential beyond obesity and diabetes mellitus. In this Review, we present an overview of gut hormones that are involved in the regulation of metabolism and discuss their action in the gastrointestinal system and beyond.

[Advantages of glucagon-like peptide-1 receptor agonists in the treatment of patients with type 2 diabetes mellitus: A review]

Glucagon-like peptide-1 receptor agonists (arGLP1) are an effective treatment for patients with type 2 diabetes mellitus (T2DM), mainly due to increasing insulin secretion and suppressing glucagon release by stimulating the respective receptors. Taking into account their positive effect on major cardiovascular events, drugs in this group with proven cardioprotective effects are recommended for patients with T2DM and clinical cardiovascular diseases or multiple cardiovascular risk factors. In this paper, we present an overview of current clinical studies on the clinical efficacy and safety of arGLP1 and discuss current prospects for arGLP1 as a therapy for patients with T2DM.

The expanding incretin universe: from basic biology to clinical translation

Incretin hormones, principally glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1(GLP-1), potentiate meal-stimulated insulin secretion through direct (GIP + GLP-1) and indirect (GLP-1) actions on islet β-cells. GIP and GLP-1 also regulate glucagon secretion, through direct and indirect pathways. The incretin hormone receptors (GIPR and GLP-1R) are widely distributed beyond the pancreas, principally in the brain, cardiovascular and immune systems, gut and kidney, consistent with a broad array of extrapancreatic incretin actions. Notably, the glucoregulatory and anorectic activities of GIP and GLP-1 have supported development of incretin-based therapies for the treatment of type 2 diabetes and obesity. Here we review evolving concepts of incretin action, focusing predominantly on GLP-1, from discovery, to clinical proof of concept, to therapeutic outcomes. We identify established vs uncertain mechanisms of action, highlighting biology conserved across species, while illuminating areas of active investigation and uncertainty that require additional clarification.

Incretins and microvascular complications of diabetes: neuropathy, nephropathy, retinopathy and microangiopathy

Glucagon-like peptide-1 receptor agonists (GLP-1RAs, incretin mimetics) and dipeptidyl peptidase-4 inhibitors (DPP-4is, incretin enhancers) are glucose-lowering therapies with proven cardiovascular safety, but their effect on microvascular disease is not fully understood. Both therapies increase GLP-1 receptor agonism, which is associated with attenuation of numerous pathological processes that may lead to microvascular benefits, including decreased reactive oxygen species (ROS) production, decreased inflammation and improved vascular function. DPP-4is also increase stromal cell-derived factor-1 (SDF-1), which is associated with neovascularisation and tissue repair. Rodent studies demonstrate several benefits of these agents in the prevention or reversal of nephropathy, retinopathy and neuropathy, but evidence from human populations is less clear. For nephropathy risk in human clinical trials, meta-analyses demonstrate that GLP-1RAs reduce the risk of a composite renal outcome (doubling of serum creatinine, eGFR reduction of 30%, end-stage renal disease or renal death), whereas the benefits of DPP-4is appear to be limited to reductions in the risk of albuminuria. The relationship between GLP-1RAs and retinopathy is less clear. Many large trials and meta-analyses show no effect, but an observed increase in the risk of retinopathy complications with semaglutide therapy (a GLP-1RA) in the SUSTAIN-6 trial warrants caution, particularly in individuals with baseline retinopathy. Similarly, DPP-4is are associated with increased retinopathy risk in both trials and meta-analysis. The association between GLP-1RAs and peripheral neuropathy is unclear due to little trial evidence. For DPP-4is, one trial and several observational studies show a reduced risk of peripheral neuropathy, with others reporting no effect. Evidence in other less-established microvascular outcomes, such as microvascular angina, cerebral small vessel disease, skeletal muscle microvascular disease and autonomic neuropathies (e.g. cardiac autonomic neuropathy, gastroparesis, erectile dysfunction), is sparse. In conclusion, GLP-1RAs are protective against nephropathy, whereas DPP-4is are protective against albuminuria and potentially peripheral neuropathy. Caution is advised with DPP-4is and semaglutide, particularly for patients with background retinopathy, due to increased risk of retinopathy. Well-designed trials powered for microvascular outcomes are needed to clarify associations of incretin therapies and microvascular diseases.

Liraglutide attenuates type 2 diabetes mellitus-associated non-alcoholic fatty liver disease by activating AMPK/ACC signaling and inhibiting ferroptosis

BACKGROUND

Non-alcoholic fatty liver disease (NAFLD) is one of the most common complications of type 2 diabetes mellitus (T2DM). The pathogenesis of NAFLD involves multiple biological changes, including insulin resistance, oxidative stress, inflammation, as well as genetic and environmental factors. Liraglutide has been used to control blood sugar. But the impact of liraglutide on T2DM-associated NAFLD remains unclear. In this study, we investigated the impact and potential molecular mechanisms of inhibiting ferroptosis for liraglutide improves T2DM-associated NAFLD.

METHODS

Mice were fed on high-fat-diet and injected with streptozotocin to mimic T2DM-associated NAFLD and gene expression in liver was analysed by RNA-seq. The fast blood glucose was measured during the period of liraglutide and ferrostatin-1 administration. Hematoxylin and eosin staining was used to evaluate the pathological changes in the liver. The occurrence of hepatic ferroptosis was measured by lipid peroxidation in vivo. The mechanism of liraglutide inhibition ferroptosis was investigated by in vitro cell culture.

RESULTS

Liraglutide not only improved glucose metabolism, but also ameliorated tissue damage in the livers. Transcriptomic analysis indicated that liraglutide regulates lipid metabolism related signaling including AMPK and ACC. Furthermore, ferroptosis inhibitor rather than other cell death inhibitors rescued liver cell viability in the presence of high glucose. Mechanistically, liraglutide-induced activation of AMPK phosphorylated ACC, while AMPK inhibitor compound C blocked the liraglutide-mediated suppression of ferroptosis. Moreover, ferroptosis inhibitor restored liver function in T2DM mice in vivo.

CONCLUSIONS

These findings indicate that liraglutide ameliorates the T2DM-associated NAFLD, which possibly through the activation of AMPK/ACC pathway and inhibition of ferroptosis.

Whey protein hydrolysates improve high-fat-diet-induced obesity by modulating the brain-peripheral axis of GLP-1 through inhibition of DPP-4 function in mice

PURPOSE

Obesity is a growing global health concern. Recent literature indicates a prominent role of glucagon-like peptide-1 (GLP-1) in glucose metabolism and food intake. The synergistic action of GLP-1 in the gut and brain is responsible for its satiety-inducing effect, suggesting that upregulation of active GLP-1 levels could be an alternative strategy to combat obesity. Dipeptidyl peptidase-4 (DPP-4) is an exopeptidase known to inactivate GLP-1, suggesting that its inhibition could be a crucial strategy for effectively extending the half-life of endogenous GLP-1. Peptides derived from partial hydrolysis of dietary proteins are gaining traction due to their inhibitory activity on DPP-4.

METHODS

Whey protein hydrolysate from bovine milk (bmWPH) was produced using simulated in situ digestion, purified using RP-HPLC, and characterized for DPP-4 inhibition. The antiadipogenic and antiobesity activity of bmWPH was then studied in 3T3-L1 preadipocytes and high-fat diet-induced obesity (HFD) mice model, respectively.

RESULTS

The dose-dependent inhibitory effect of bmWPH on the catalytic activity of DPP-4 was observed. Additionally, bmWPH suppressed adipogenic transcription factors and DPP-4 protein levels, leading to a negative effect on preadipocyte differentiation. In an HFD mice model, co-administration of WPH for 20 weeks downregulated adipogenic transcription factors, resulting in a concomitant reduction in whole body weight and adipose tissues. Mice fed with bmWPH also showed a marked reduction in DPP-4 levels in WAT, liver, and serum. Furthermore, HFD mice fed with bmWPH exhibited increased serum and brain GLP levels, which led to a significant decrease in food intake.

CONCLUSION

In conclusion, bmWPH reduces body weight in HFD mice by suppressing appetite through GLP-1, a satiety-inducing hormone, in both the brain and peripheral circulation. This effect is achieved through modulation of both the catalytic and non-catalytic activity of DPP-4.

GPCR Promiscuity Reshapes Islet Physiology

The family of proglucagon peptides Includes glucagon and glucagon-like peptide 1 (GLP-1), two unique peptides derived from the same prohormone. Despite numerous similarities between the peptides, these have long been viewed as having opposing actions on metabolism. GLP-1 is described as a postprandial hormone that stimulates anabolic actions via insulin, while glucagon is viewed as a fasting hormone that drives catabolic actions to maintain euglycemia. Here, we revisit a classic article in Diabetes that first established that glucagon and GLP-1 have more in common than previously appreciated, including actions at the same receptor. Furthermore, we discuss how the impact of this observation has guided research decades later that has reshaped the view of how proglucagon hormones regulate metabolism.

Increased Glucagon Immunoreactivity in a Rat Model of Diet-induced Obesity following Sleeve Gastrectomy

OBJECTIVE

Bariatric surgery is currently the most effective treatment for obesity, and procedures such as Roux-en Y gastric bypass and sleeve gastrectomy (SG) also result in rapid improvements in insulin sensitivity and glucose tolerance. In addition, these procedures cause changes in the secretion of various gut-derived hormones. The role these hormones play in the mechanism of the beneficial effects of bariatric surgery is still debated, but nonetheless, their importance provides inspiration for novel obesity-targeted pharmacotherapies.

METHODS

Male Sprague Dawley rats were fed either regular chow or a cafeteria diet to induce obesity. A sub-group of the obese animals then underwent either sham surgery or SG.

RESULTS

Following a 4-week recovery period, SG rats weighed significantly less than obese or sham-operated rats. Improvements in glucose tolerance and insulin sensitivity also occurred in the SG group, but these were not always statistically significant. We measured the intracellular lipid content of liver samples and found that obese rats showed signs of non-alcoholic fatty liver disease, which were significantly ameliorated by SG. There were significantly higher glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) responses to a standard mixed meal in the SG group, as well as paradoxically higher glucagon secretion.

CONCLUSION

These data highlight the need for more specific anti-glucagon antibodies to characterize the changes in proglucagon-derived peptide concentrations that occur following SG. Further studies are required to determine whether these peptides contribute to the therapeutic effects of SG.

The role of glucagon-like peptide-1 receptor agonists in nonalcoholic fatty liver disease

INTRODUCTION

Nonalcoholic fatty liver disease (NAFLD) is a highly prevalent disease, associated with obesity, type 2 diabetes mellitus and dyslipidemia, which can lead to liver cirrhosis and hepatocellular carcinoma in some patients. Apart from lifestyle modifications, which are the cornerstone for its management, several drugs are under evaluation, including glucagon-like peptide-1 receptor agonists (GLP-R1RAs). In this review, we summarized major clinical data concerning the effects of GLP-1RAs on NAFLD, trying to highlight existing knowledge and to elucidate areas of uncertainty, thus providing clues to potential clinical implications and research.

AREAS COVERED

Selected clinical studies on GLP-R1As in NAFLD are presented in this narrative review.

EXPERT OPINION

There is evidence that treatment with GLP-R1As in NAFLD has beneficial effects on NAFLD, i.e. improvement in liver function tests and histological improvement in hepatic steatosis and inflammation, but not fibrosis. Further research is required toward the early use of GLP-R1Αs, i.e. in NAFLD patients without fibrosis to evaluate whether they may prevent the progression to fibrosis, or in patients with advanced disease in combination with other medications, which may have additive or even synergistic effects on NAFLD.

Glucagon-like peptide 1 receptor agonists: cardiovascular benefits and mechanisms of action

Type 2 diabetes mellitus (T2DM) and obesity are metabolic disorders characterized by excess cardiovascular risk. Glucagon-like peptide 1 (GLP1) receptor (GLP1R) agonists reduce body weight, glycaemia, blood pressure, postprandial lipaemia and inflammation - actions that could contribute to the reduction of cardiovascular events. Cardiovascular outcome trials (CVOTs) have demonstrated that GLP1R agonists reduce the rates of major adverse cardiovascular events in patients with T2DM. Separate phase III CVOTs of GLP1R agonists are currently being conducted in people living with heart failure with preserved ejection fraction and in those with obesity. Mechanistically, GLP1R is expressed at low levels in the heart and vasculature, raising the possibility that GLP1 might have both direct and indirect actions on the cardiovascular system. In this Review, we summarize the data from CVOTs of GLP1R agonists in patients with T2DM and describe the actions of GLP1R agonists on the heart and blood vessels. We also assess the potential mechanisms that contribute to the reduction in major adverse cardiovascular events in individuals treated with GLP1R agonists and highlight the emerging cardiovascular biology of novel GLP1-based multi-agonists currently in development. Understanding how GLP1R signalling protects the heart and blood vessels will optimize the therapeutic use and development of next-generation GLP1-based therapies with improved cardiovascular safety.

Combined GIP receptor and GLP1 receptor agonism attenuates NAFLD in male APOE∗3-Leiden.CETP mice

BACKGROUND

Combined glucose-dependent insulinotropic polypeptide receptor (GIPR) and glucagon-like peptide-1 receptor (GLP1R) agonism is superior to single GLP1R agonism with respect to glycemic control and weight loss in obese patients with or without type 2 diabetes. As insulin resistance and obesity are strong risk factors for nonalcoholic fatty liver disease (NAFLD), in the current study we investigated the effects of combined GIPR/GLP1R agonism on NAFLD development.

METHODS

Male APOE∗3-Leiden.CETP mice, a humanized model for diabetic dyslipidemia and NAFLD when fed a high-fat high-cholesterol diet, received subcutaneous injections with either vehicle, a GIPR agonist, a GLP1R agonist, or both agonists combined every other day.

FINDINGS

GIPR and GLP1R agonism reduced body weight and additively lowered fasting plasma levels of glucose, triglycerides and total cholesterol. Strikingly, we report an additive reduction in hepatic steatosis as evidenced by lower hepatic lipid content and NAFLD scores. Underlying the lipid-lowering effects were a reduced food intake and intestinal lipid absorption and an increased uptake of glucose and triglyceride-derived fatty acids by energy-combusting brown adipose tissue. Combined GIPR/GLP1R agonism also attenuated hepatic inflammation as evidenced by a decreased number of monocyte-derived Kupffer cells and a reduced expression of inflammatory markers. Together, the reduced hepatic steatosis and inflammation coincided with lowered markers of liver injury.

INTERPRETATION

We interpretate that GIPR and GLP1R agonism additively attenuate hepatic steatosis, lower hepatic inflammation, ameliorate liver injury, together preventing NAFLD development in humanized APOE∗3-Leiden.CETP mice. We anticipate that combined GIPR/GLP1R agonism is a promising strategy to attenuate NAFLD progression in humans.

FUNDING

This work was supported by a grant from the Netherlands CardioVascular Research Initiative: the Dutch Heart Foundation, Dutch Federation of University Medical Centers, the Netherlands Organization for Health Research and Development, and the Royal Netherlands Academy of Sciences [CVON-GENIUS-II] to P.C.N.R., a Lilly Research Award Program [LRAP] Award to P.C.N.R. and S.K., a Dutch Heart Foundation [2017T016] grant to S.K., and an NWO-VENI grant [09150161910073] to M.R.B.; J.F.D.B. is supported by the Nutrition and Health initiative of the University of Groningen; Z.Y. is supported by a full-time PhD scholarship from the China Scholarship Council (201806850094 to Z.Y.).

Multifaceted Roles of GLP-1 and Its Analogs: A Review on Molecular Mechanisms with a Cardiotherapeutic Perspective

Diabetes is one of the chronic metabolic disorders which poses a multitude of life-debilitating challenges, including cardiac muscle impairment, which eventually results in heart failure. The incretin hormone glucagon-like peptide-1 (GLP-1) has gained distinct recognition in reinstating glucose homeostasis in diabetes, while it is now largely accepted that it has an array of biological effects in the body. Several lines of evidence have revealed that GLP-1 and its analogs possess cardioprotective effects by various mechanisms related to cardiac contractility, myocardial glucose uptake, cardiac oxidative stress and ischemia/reperfusion injury, and mitochondrial homeostasis. Upon binding to GLP-1 receptor (GLP-1R), GLP-1 and its analogs exert their effects via adenylyl cyclase-mediated cAMP elevation and subsequent activation of cAMP-dependent protein kinase(s) which stimulates the insulin release in conjunction with enhanced Ca²⁺ and ATP levels. Recent findings have suggested additional downstream molecular pathways stirred by long-term exposure of GLP-1 analogs, which pave the way for the development of potential therapeutic molecules with longer lasting beneficial effects against diabetic cardiomyopathies. This review provides a comprehensive overview of the recent advances in the understanding of the GLP-1R-dependent and -independent actions of GLP-1 and its analogs in the protection against cardiomyopathies.

Fibroblast growth factor-21 is required for weight loss induced by the glucagon-like peptide-1 receptor agonist liraglutide in male mice fed high carbohydrate diets

OBJECTIVE

Glucagon-like peptide-1 receptor (GLP-1R) agonists (GLP-1RA) and fibroblast growth factor-21 (FGF21) confer similar metabolic benefits. GLP-1RA induce FGF21, leading us to investigate mechanisms engaged by the GLP-1RA liraglutide to increase FGF21 levels and the metabolic relevance of liraglutide-induced FGF21.

METHODS

Circulating FGF21 levels were measured in fasted male C57BL/6J, neuronal GLP-1R knockout, β-cell GLP-1R knockout, and liver peroxisome proliferator-activated receptor alpha knockout mice treated acutely with liraglutide. To test the metabolic relevance of liver FGF21 in response to liraglutide, chow-fed control and liver Fgf21 knockout (LivFgf21-/-) mice were treated with vehicle or liraglutide in metabolic chambers. Body weight and composition, food intake, and energy expenditure were measured. Since FGF21 reduces carbohydrate intake, we measured body weight in mice fed matched diets with low- (LC) or high-carbohydrate (HC) content and in mice fed a high-fat, high-sugar (HFHS) diet. This was done in control and LivFgf21-/- mice and in mice lacking neuronal β-klotho (Klb) expression to disrupt brain FGF21 signaling.

RESULTS

Liraglutide increases FGF21 levels independently of decreased food intake via neuronal GLP-1R activation. Lack of liver Fgf21 expression confers resistance to liraglutide-induced weight loss due to attenuated reduction of food intake in chow-fed mice. Liraglutide-induced weight loss was impaired in LivFgf21-/- mice when fed HC and HFHS diets but not when fed a LC diet. Loss of neuronal Klb also attenuated liraglutide-induced weight loss in mice fed HC or HFHS diets.

CONCLUSIONS

Our findings support a novel role for a GLP-1R-FGF21 axis in regulating body weight in a dietary carbohydrate-dependent manner.

The Role of GLP1-RAs in Direct Modulation of Lipid Metabolism in Hepatic Tissue as Determined Using In Vitro Models of NAFLD

Glucagon-like peptide 1 receptor agonists (GLP-1RAs) have been shown to improve glucose and lipid homeostasis, promote weight loss, and reduce cardiovascular risk factors. They are a promising therapeutic option for non-alcoholic fatty liver disease (NAFLD), the most common liver disease, associated with T2DM, obesity, and metabolic syndrome. GLP-1RAs have been approved for the treatment of T2DM and obesity, but not for NAFLD. Most recent clinical trials have suggested the importance of early pharmacologic intervention with GLP-1RAs in alleviating and limiting NAFLD, as well as highlighting the relative scarcity of in vitro studies on semaglutide, indicating the need for further research. However, extra-hepatic factors contribute to the GLP-1RA results of in vivo studies. Cell culture models of NAFLD can be helpful in eliminating extrahepatic effects on the alleviation of hepatic steatosis, modulation of lipid metabolism pathways, reduction of inflammation, and prevention of the progression of NAFLD to severe hepatic conditions. In this review article, we discuss the role of GLP-1 and GLP-1RA in the treatment of NAFLD using human hepatocyte models.

Semaglutide Has Beneficial Effects on Non-Alcoholic Steatohepatitis in Ldlr-/-.Leiden Mice

Semaglutide, a glucagon-like peptide-1 receptor agonist, is an antidiabetic medication that has recently been approved for the treatment of obesity as well. Semaglutide is postulated to be a promising candidate for the treatment of non-alcoholic steatohepatitis (NASH). Here, Ldlr-/-.Leiden mice received a fast-food diet (FFD) for 25 weeks, followed by another 12 weeks on FFD with daily subcutaneous injections of semaglutide or vehicle (control). Plasma parameters were evaluated, livers and hearts were examined, and hepatic transcriptome analysis was performed. In the liver, semaglutide significantly reduced macrovesicular steatosis (-74%, p < 0.001) and inflammation (-73%, p < 0.001) and completely abolished microvesicular steatosis (-100%, p < 0.001). Histological and biochemical assessment of hepatic fibrosis showed no significant effects of semaglutide. However, digital pathology revealed significant improvements in the degree of collagen fiber reticulation (-12%, p < 0.001). Semaglutide did not affect atherosclerosis relative to controls. Additionally, we compared the transcriptome profile of FFD-fed Ldlr-/-.Leiden mice with a human gene set that differentiates human NASH patients with severe fibrosis from those with mild fibrosis. In FFD-fed Ldlr-/-.Leiden control mice, this gene set was upregulated as well, while semaglutide predominantly reversed this gene expression. Using a translational model with advanced NASH, we demonstrated that semaglutide is a promising candidate with particular potential for the treatment of hepatic steatosis and inflammation, while for the reversal of advanced fibrosis, combinations with other NASH agents may be necessary.

Beyond the pancreas: contrasting cardiometabolic actions of GIP and GLP1

Glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide 1 (GLP1) exhibit incretin activity, meaning that they potentiate glucose-dependent insulin secretion. The emergence of GIP receptor (GIPR)-GLP1 receptor (GLP1R) co-agonists has fostered growing interest in the actions of GIP and GLP1 in metabolically relevant tissues. Here, we update concepts of how these hormones act beyond the pancreas. The actions of GIP and GLP1 on liver, muscle and adipose tissue, in the control of glucose and lipid homeostasis, are discussed in the context of plausible mechanisms of action. Both the GIPR and GLP1R are expressed in the central nervous system, wherein receptor activation produces anorectic effects enabling weight loss. In preclinical studies, GIP and GLP1 reduce atherosclerosis. Furthermore, GIPR and GLP1R are expressed within the heart and immune system, and GLP1R within the kidney, revealing putative mechanisms linking GIP and GLP1R agonism to cardiorenal protection. We interpret the clinical and mechanistic data obtained for different agents that enable weight loss and glucose control for the treatment of obesity and type 2 diabetes mellitus, respectively, by activating or blocking GIPR signalling, including the GIPR-GLP1R co-agonist tirzepatide, as well as the GIPR antagonist-GLP1R agonist AMG-133. Collectively, we update translational concepts of GIP and GLP1 action, while highlighting gaps, areas of uncertainty and controversies meriting ongoing investigation.

Short-term semaglutide treatment improves FGF21 responsiveness in primary hepatocytes isolated from high fat diet challenged mice

Metabolic functions of GLP-1 and its analogues have been extensively investigated. In addition to acting as an incretin and reducing body weight, we and others have suggested the existence of GLP-1/fibroblast growth factor 21 (FGF21) axis in which liver mediates certain functions of GLP-1 receptor agonists. In a more recent study, we found with surprise that four-week treatment with liraglutide but not semaglutide stimulated hepatic FGF21 expression in HFD-challenged mice. We wondered whether semaglutide can also improve FGF21 sensitivity or responsiveness and hence triggers the feedback loop in attenuating its stimulation on hepatic FGF21 expression after a long-term treatment. Here, we assessed effect of daily semaglutide treatment in HFD-fed mice for 7 days. HFD challenge attenuated effect of FGF21 treatment on its downstream events in mouse primary hepatocytes, which can be restored by 7-day semaglutide treatment. In mouse liver, 7-day semaglutide treatment stimulated FGF21 as well as genes that encode its receptor (FGFR1) and the obligatory co-receptor (KLB), and a battery of genes that are involved in lipid homeostasis. In epididymal fat tissue, expressions of a battery genes including Klb affected by HFD challenge were reversed by 7-day semaglutide treatment. We suggest that semaglutide treatment improves FGF21 sensitivity which is attenuated by HFD challenge.

The postprandial actions of GLP-1 receptor agonists: The missing link for cardiovascular and kidney protection in type 2 diabetes

Recent clinical trials in people with type 2 diabetes have demonstrated beneficial actions on heart and kidney outcomes following treatment with GLP-1RAs. In part, these actions are consistent with improved glucose control and significant weight loss. But GLP-1RAs may also have additive benefits by improving postprandial dysmetabolism. In diabetes, dysregulated postprandial nutrient excursions trigger inflammation, oxidative stress, endothelial dysfunction, thrombogenicity, and endotoxemia; alter hormone levels; and modulate cardiac output and regional blood and lymphatic flow. In this perspective, we explore the actions of GLP-1RAs on the postprandial state and their potential role in end-organ benefits observed in recent trials.

Glucagon-like Peptide-1 Receptor-based Therapeutics for Metabolic Liver Disease

Glucagon-like peptide-1 (GLP-1) controls islet hormone secretion, gut motility, and body weight, supporting development of GLP-1 receptor agonists (GLP-1RA) for the treatment of type 2 diabetes (T2D) and obesity. GLP-1RA exhibit a favorable safety profile and reduce the incidence of major adverse cardiovascular events in people with T2D. Considerable preclinical data, supported by the results of clinical trials, link therapy with GLP-RA to reduction of hepatic inflammation, steatosis, and fibrosis. Mechanistically, the actions of GLP-1 on the liver are primarily indirect, as hepatocytes, Kupffer cells, and stellate cells do not express the canonical GLP-1R. GLP-1RA reduce appetite and body weight, decrease postprandial lipoprotein secretion, and attenuate systemic and tissue inflammation, actions that may contribute to attenuation of metabolic-associated fatty liver disease (MAFLD). Here we discuss evolving concepts of GLP-1 action that improve liver health and highlight evidence that links sustained GLP-1R activation in distinct cell types to control of hepatic glucose and lipid metabolism, and reduction of experimental and clinical nonalcoholic steatohepatitis (NASH). The therapeutic potential of GLP-1RA alone, or in combination with peptide agonists, or new small molecule therapeutics is discussed in the context of potential efficacy and safety. Ongoing trials in people with obesity will further clarify the safety of GLP-1RA, and pivotal studies underway in people with NASH will define whether GLP-1-based medicines represent effective and safe therapies for people with MAFLD.

Liver Fibroblast Growth Factor 21 (FGF21) is Required for the Full Anorectic Effect of the Glucagon-Like Peptide-1 Receptor Agonist Liraglutide in Male Mice fed High Carbohydrate Diets

Glucagon-like peptide-1 receptor (GLP-1R) agonists and fibroblast growth factor 21 (FGF21) confer similar metabolic benefits. Studies report that GLP-1RA induce FGF21. Here, we investigated the mechanisms engaged by the GLP-1R agonist liraglutide to increase FGF21 levels and the metabolic relevance of liraglutide-induced FGF21. We show that liraglutide increases FGF21 levels via neuronal GLP-1R activation. We also demonstrate that lack of liver Fgf21 expression confers partial resistance to liraglutide-induced weight loss. Since FGF21 reduces carbohydrate intake, we tested whether the contribution of FGF21 to liraglutide-induced weight loss is dependent on dietary carbohydrate content. In control and liver Fgf21 knockout (Liv ^{Fgf21 -/-} ) mice fed calorically matched diets with low- (LC) or high-carbohydrate (HC) content, we found that only HC-fed Liv ^{Fgf21 -/-} mice were resistant to liraglutide-induced weight loss. Similarly, liraglutide-induced weight loss was partially impaired in Liv ^{Fgf21 -/-} mice fed a high-fat, high-sugar (HFHS) diet. Lastly, we show that loss of neuronal β-klotho expression also diminishes liraglutide-induced weight loss in mice fed a HC or HFHS diet, indicating that FGF21 mediates liraglutide-induced weight loss via neuronal FGF21 action. Our findings support a novel role for a GLP-1R-FGF21 axis in regulating body weight in the presence of high dietary carbohydrate content.

The interplay between nonalcoholic fatty liver disease and atherosclerotic cardiovascular disease

Therapeutic approaches that lower circulating low-density lipoprotein (LDL)-cholesterol significantly reduced the burden of cardiovascular disease over the last decades. However, the persistent rise in the obesity epidemic is beginning to reverse this decline. Alongside obesity, the incidence of nonalcoholic fatty liver disease (NAFLD) has substantially increased in the last three decades. Currently, approximately one third of world population is affected by NAFLD. Notably, the presence of NAFLD and particularly its more severe form, nonalcoholic steatohepatitis (NASH), serves as an independent risk factor for atherosclerotic cardiovascular disease (ASCVD), thus, raising interest in the relationship between these two diseases. Importantly, ASCVD is the major cause of death in patients with NASH independent of traditional risk factors. Nevertheless, the pathophysiology linking NAFLD/NASH with ASCVD remains poorly understood. While dyslipidemia is a common risk factor underlying both diseases, therapies that lower circulating LDL-cholesterol are largely ineffective against NASH. While there are no approved pharmacological therapies for NASH, some of the most advanced drug candidates exacerbate atherogenic dyslipidemia, raising concerns regarding their adverse cardiovascular consequences. In this review, we address current gaps in our understanding of the mechanisms linking NAFLD/NASH and ASCVD, explore strategies to simultaneously model these diseases, evaluate emerging biomarkers that may be useful to diagnose the presence of both diseases, and discuss investigational approaches and ongoing clinical trials that potentially target both diseases.

Acute Cholesterol-Lowering Effect of Exendin-4 in Ldlr-/- and C57BL/6J Mice

AIMS

We previously reported that glucagon-like peptide-1 receptor agonists (GLP-1RAs) reduced serum low-density lipoprotein cholesterol (LDL-C) levels in patients with type 2 diabetes mellitus receiving statins, which increased LDL receptor (LDLR) expression. Nevertheless, it remains unclear how much LDLR expression contributes to the LDL-C-lowering effect of GLP-1RAs. We examined the effect of a GLP-1RA, namely, exendin-4, on serum LDL-C levels and its mechanism in Ldlr^-/- and C57BL/6J mice.

METHODS

Ten-week-old Ldlr^-/- and C57BL/6J mice received exendin-4 or saline for 5 days, and serum lipid profiles and hepatic lipid levels were examined. Cholesterol metabolism-related gene expression and protein levels in the liver and ileum and the fecal bile acid (BA) composition were also examined.

RESULTS

Exendin-4 treatment significantly decreased serum very-low-density lipoprotein cholesterol (VLDL-C) and LDL-C levels and mature hepatic SREBP2 levels and increased hepatic Insig1/2 mRNA expression in both mouse strains. In Ldlr^-/- mice, exendin-4 treatment also significantly decreased hepatic cholesterol levels and fecal BA excretion, decreased hepatic Cyp7a1 mRNA expression, and increased small intestinal Fgf15 mRNA expression. In C57BL/6J mice, exendin-4 treatment significantly decreased small intestinal NPC1L1 levels.

CONCLUSIONS

Our findings demonstrate that exendin-4 treatment decreased serum VLDL-C and LDL-C levels in a manner that was independent of LDLR. Exendin-4 treatment might decrease serum cholesterol levels by lowering hepatic SREBP2 levels and cholesterol absorption in Ldlr^-/- and C57BL/6J mice. Exendin-4 treatment might decrease cholesterol absorption by different mechanisms in Ldlr^-/- and C57BL/6J mice.

Semaglutide May Alleviate Hepatic Steatosis in T2DM Combined with NFALD Mice via miR-5120/ABHD6

Objective

Although the pathogenesis of non-alcoholic fatty liver disease (NAFLD) has been extensively studied, the role of its underlying pathogenesis remains unclear, and there is currently no approved therapeutic strategy for NAFLD. The purpose of this study was to observe the beneficial effects of Semaglutide on NAFLD in vivo and in vitro, as well as its potential molecular mechanisms.

Methods

Semaglutide was used to treat type 2 diabetes mellitus (T2DM) combined with NAFLD mice for 12 weeks. Hepatic function and structure were evaluated by liver function, blood lipids, liver lipids, H&E staining, oil red staining and Sirius staining. The expression of α/β hydrolase domain-6 (ABHD6) was measured by qPCR and Western blotting in vivo and in vitro. Then, dual-luciferase reporter assay was performed to verify the regulation of the upstream miR-5120 on ABHD6.

Results

Our data revealed that Semaglutide administration significantly improved liver function and hepatic steatosis in T2DM combined with NAFLD mice. Furthermore, compared with controls, up-regulation of ABHD6 and down-regulation of miR-5120 were found in the liver of T2DM+NAFLD mice and HG+FFA-stimulated Hepa 1-6 hepatocytes. Interestingly, after Semaglutide intervention, ABHD6 expression was significantly decreased in the liver of T2DM+NAFLD mice and in HG+FFA-stimulated Hepa 1-6 hepatocytes, while miR-5120 expression was increased. We also found that miR-5120 could regulate the expression of ABHD6 in hepatocytes, while Semaglutide could modulate the expression of ABHD6 through miR-5120. In addition, GLP-1R was widely expressed in mouse liver tissues and Hepa 1-6 cells. Semaglutide could regulate miR-5120/ABHD6 expression through GLP-1R.

Conclusion

Our data revealed the underlying mechanism by which Semaglutide improves hepatic steatosis in T2DM+NAFLD, and might shed new light on the pathological role of miR-5120/ABHD6 in the pathogenesis of T2DM+NAFLD.

The Molecular Determinants of Glucagon-like Peptide Secretion by the Intestinal L cell

The intestinal L cell secretes a diversity of biologically active hormones, most notably the glucagon-like peptides, GLP-1 and GLP-2. The highly successful introduction of GLP-1-based drugs into the clinic for the treatment of patients with type 2 diabetes and obesity, and of a GLP-2 analog for patients with short bowel syndrome, has led to the suggestion that stimulation of the endogenous secretion of these peptides may serve as a novel therapeutic approach in these conditions. Situated in the intestinal epithelium, the L cell demonstrates complex relationships with not only circulating, paracrine, and neural regulators, but also ingested nutrients and other factors in the lumen, most notably the microbiota. The integrated input from these numerous secretagogues results in a variety of temporal patterns in L cell secretion, ranging from minutes to 24 hours. This review combines the findings of traditional, physiological studies with those using newer molecular approaches to describe what is known and what remains to be elucidated after 5 decades of research on the intestinal L cell and its secreted peptides, GLP-1 and GLP-2.

Old and new classes of glucose-lowering agents as treatments for non-alcoholic fatty liver disease: A narrative review

Non-alcoholic fatty liver disease (NAFLD) has become the most common chronic liver disease with a global prevalence of about 55% in people with type 2 diabetes mellitus (T2DM). T2DM, obesity and NAFLD are three closely inter-related pathological conditions. In addition, T2DM is one of the strongest clinical risk factors for the faster progression of NAFLD to non-alcoholic steatohepatitis (NASH), cirrhosis and hepatocellular carcinoma. Increasing evidence suggests that newer classes of glucose-lowering drugs, such as peroxisome proliferator-activated receptor agonists, glucagon-like peptide-1 receptor agonists, dipeptidyl peptidase-4 inhibitors or sodium-glucose cotransporter-2 inhibitors, could reduce the rates of NAFLD progression. This narrative review aims to briefly summarize the recent results from randomized controlled trials testing the efficacy and safety of old and new glucose-lowering drugs for the treatment of NAFLD or NASH in adults both with and without coexisting T2DM.

Treatment of diabetes mellitus has borne much fruit in the prevention of cardiovascular disease

Cardiovascular (CV) disease is the most alarming complication of diabetes mellitus (DM), and a strategy aiming at cardiovascular event prevention in diabetes mellitus has long been debated. Large landmark clinical trials have shown cardiovascular benefits of intensive glycemic control as a 'legacy effect' in newly diagnosed type 2 diabetes mellitus. In contrast, we have learned that excessive intervention aimed at strong glycemic control could cause unexpected cardiovascular death in patients who are resistant to treatments against hyperglycemia. It has also been shown that the comprehensive multifactorial intervention for cardiovascular risk factors that was advocated in the current guideline provided substantial cardiovascular event reduction. The impact of classical antidiabetic agents launched before 1990s on cardiovascular events is controversial. Although there are many clinical or observational studies assessing the impact of those agents on cardiovascular events, the conclusions are inconsistent owing to variable patient backgrounds and concomitant antidiabetic agents among the studies. Moreover, most of them were not large-scale, randomized, cardiovascular outcome trials. In contrast, GLP-1RA (glucagon-like peptide-1 receptor agonist) and SGLT2 (sodium-glucose cotransporter 2) inhibitors have demonstrated undeniable cardiovascular benefits in large-scale, randomized, controlled trials. Whereas GLP-1RAs decrease atherosclerotic disease, especially stroke, SGLT2 inhibitors mainly prevent heart failure. SGLT2 inhibitors are superior to GLP-1RAs with respect to hard renal outcomes. Therefore, it can be said that drugs such as GLP-1RAs and SGLT2 inhibitors that prevent cardiovascular events, in addition to their glucose-lowering effect, are incredible novel tools that we have gained for use in diabetic treatment.

Application of glucagon-like peptide-1 receptor antagonists in fibrotic diseases

Fibrosis can occur in various organs, leading to structural destruction, dysfunction, and even organ failure. Hence, organ fibrosis is being actively researched worldwide. Glucagon-like peptide-1 (GLP-1), a naturally occurring hormone, binds to a G-protein-coupled receptor widely distributed in the pancreas, kidney, lung, heart, gastrointestinal tract, and other organs. Synthetic GLP-1 analogs can be used as GLP-1 receptor agonists (GLP-1RAs) for treating diabetes mellitus. In recent years, GLP-1RAs have also been found to exert anti-inflammatory, antioxidant, and cardiovascular protective effects. GLP-1RAs have also been shown to inhibit fibrosis of solid organs, such as the lung, heart, liver, and kidney. In this review, we discuss the advancements in research on the role of GLP-1RAs in the fibrosis of the heart, lung, liver, kidney, and other organs to obtain new clues for treating organ fibrosis.

Orphan GPR146: an alternative therapeutic pathway to achieve cholesterol homeostasis?

Atherosclerosis predisposes to myriad cardiovascular complications, including myocardial infarction and stroke. Statins have revolutionised cholesterol management but they do not work for all patients, particularly those with familial hypercholesterolaemia (FH). Genome-wide association studies have linked SNPs at orphan G protein-coupled receptor 146 (GPR146) to human atherosclerosis but how GPR146 influences serum cholesterol homeostasis was only recently described. Gpr146 deletion in mice reduces serum cholesterol and atherosclerotic plaque burden, confirming GPR146 as a potential therapeutic target for managing circulating cholesterol. Critically, this effect was independent of the low-density lipoprotein receptor. While still an orphan, the activation of GPR146 by serum suggests identification of its endogenous ligand is tantalisingly close. Herein, we discuss the evidence for GPR146 inhibition as a treatment for atherosclerosis.

Liraglutide stimulates the β-catenin signaling cascade in mouse epididymal fat tissue

Although canonical Wnt signaling pathway activation was shown to negatively regulate adipogenesis, recent investigations suggest that Wnt pathway effectors TCF7L2 and β-catenin (β-cat) in adipose tissues are also involved in energy homeostasis during adulthood. In assessing the metabolic beneficial effect of GLP-1-based diabetes drugs in high-fat diet (HFD)-challenged mice, we observed that liraglutide treatment affected the expression of a battery of adipose tissue-specific genes, including those that encode adiponectin and leptin, mainly in epididymal white adipose tissue (eWAT). Fourteen-week HFD challenge repressed TCF7L2 and β-cat S675 phosphorylation in eWAT, while such repression was reversed by liraglutide treatment (150 µg/kg body weight daily) during weeks 10-14. In Glp1r-/-mice, liraglutide failed in stimulating TCF7L2 or β-cat in eWAT. We detected Glp1r expression in mouse eWAT and its level is enriched in its stromal vascular fraction (SVF). Mouse eWAT-SVF showed reduced expression of Tcf7l2 and its Tcf7l2 level could not be stimulated by liraglutide treatment; while following adipogenic differentiation, rat eWAT-SVF showed elevated Tcf7l2 expression. Direct in vitro liraglutide treatment in eWAT-SVF stimulated CREB S133, β-cat S675 phosphorylation, and cellular cAMP level. Thus, cAMP/β-cat signaling cascade can be stimulated by liraglutide in eWAT via GLP-1R expressed in eWAT-SVF.

The anti-inflammatory feature of glucagon-like peptide-1 and its based diabetes drugs—Therapeutic potential exploration in lung injury

Since 2005, GLP-1 receptor (GLP-1R) agonists (GLP-1RAs) have been developed as therapeutic agents for type 2 diabetes (T2D). GLP-1R is not only expressed in pancreatic islets but also other organs, especially the lung. However, controversy on extra-pancreatic GLP-1R expression still needs to be further resolved, utilizing different tools including the use of more reliable GLP-1R antibodies in immune-staining and co-immune-staining. Extra-pancreatic expression of GLP-1R has triggered extensive investigations on extra-pancreatic functions of GLP-1RAs, aiming to repurpose them into therapeutic agents for other disorders. Extensive studies have demonstrated promising anti-inflammatory features of GLP-1RAs. Whether those features are directly mediated by GLP-1R expressed in immune cells also remains controversial. Following a brief review on GLP-1 as an incretin hormone and the development of GLP-1RAs as therapeutic agents for T2D, we have summarized our current understanding of the anti-inflammatory features of GLP-1RAs and commented on the controversy on extra-pancreatic GLP-1R expression. The main part of this review is a literature discussion on GLP-1RA utilization in animal models with chronic airway diseases and acute lung injuries, including studies on the combined use of mesenchymal stem cell (MSC) based therapy. This is followed by a brief summary.

Therapeutic Potential of Semaglutide, a Newer GLP-1 Receptor Agonist, in Abating Obesity, Non-Alcoholic Steatohepatitis and Neurodegenerative diseases: A Narrative Review

INTRODUCTION

Semaglutide, a peptidic GLP-1 receptor agonist, has been clinically approved for treatment of type 2 diabetes mellitus and is available in subcutaneous and oral dosage form. Diabetes, insulin resistance, and obesity are responsible for the pathological manifestations of non-alcoholic steatohepatitis (NASH). Similarly, insulin resistance in brain is also responsible for neurodegeneration and impaired cognitive functions.

BACKGROUND

Observations from phase-3 clinical trials like SUSTAIN and PIONEER indicated anti-obesity potential of semaglutide, which was established in STEP trials. Various pre-clinical and phase-2 studies have indicated the therapeutic potential of semaglutide in non-alcoholic steatohepatitis and neurodegenerative disorders like Parkinson's and Alzheimer's disease.

DISCUSSION

Significant weight reduction ability of semaglutide has been demonstrated in various phase-3 clinical trials, for which recently semaglutide became the first long-acting GLP-1 receptor agonist to be approved by the United States Food and Drug Administration for management of obesity. Various pre-clinical and clinical studies have revealed the hepatoprotective effect of semaglutide in NASH and neuroprotective effect in Parkinson's and Alzheimer's disease.

CONCLUSION

Many GLP-1 receptor agonists have shown hepatoprotective and neuroprotective activity in animal and human trials. As semaglutide is an already clinically approved drug, successful human trials would hasten its inclusion into therapeutic treatment of NASH and neurodegenerative diseases. Semaglutide improves insulin resistance, insulin signalling pathway, and reduce body weight which are responsible for prevention or progression of NASH and neurodegenerative diseases.

GLP-1 mimetics as a potential therapy for nonalcoholic steatohepatitis

Nonalcoholic steatohepatitis (NASH), as a severe form of nonalcoholic fatty liver disease (NAFLD), is characterized by liver steatosis, inflammation, hepatocellular injury and different degrees of fibrosis. The pathogenesis of NASH is complex and multifactorial, obesity and type 2 diabetes mellitus (T2DM) have been implicated as major risk factors. Glucagon-like peptide-1 receptor (GLP-1R) is one of the most successful drug targets of T2DM and obesity, and its peptidic ligands have been proposed as potential therapeutic agents for NASH. In this article we provide an overview of the pathophysiology and management of NASH, with a special focus on the pharmacological effects and possible mechanisms of GLP-1 mimetics in treating NAFLD/NASH, including dual and triple agonists at GLP-1R, glucose-dependent insulinotropic polypeptide receptor or glucagon receptor.