Distribution and function of glucagon-like peptide 1 (GLP-1) in the digestive tract of mammals and the clinical use of its analogues

Recently, interest in glucagon-like peptide-1 (GLP-1) and other peptides derived from preproglucagon has increased significantly. GLP-1 is a 30-amino acid peptide hormone produced in L-type enteroendocrine cells as a response to food intake. GLP-1 is rapidly metabolized and inactivated by the dipeptidyl peptidase IV enzyme before the hormone leaves the intestine, which increases the likelihood that GLP-1 action is transmitted through sensory neurons in the intestine and liver through the GLP-1 receptor. The main actions of GLP-1 are to stimulate insulin secretion (i.e. act as incretin hormone) and inhibit glucagon secretion, thus contributing to the reduction of postprandial glucose spikes. GLP-1 also inhibits motility and gastrointestinal secretion, and therefore acts as part of the „small bowel brake” mechanism. GLP-1 also appears to be a physiological regulator of appetite and food intake. Because of these effects, GLP-1 or GLP-1 receptor agonists are now increasingly used to treat type 2 diabetes. Reduced GLP-1 secretion may contribute to the development of obesity, and excessive secretion may be responsible for postprandial reactive hypoglycemia. The use of GLP-1 agonists opens up new possibilities for the treatment of type 2 diabetes and other metabolic diseases. In the last two decades, many interesting studies covering both the physiological and pathophysiological role of GLP-1 have been published, and our understanding of GLP-1 has broadened significantly. In this review article, we have tried to describe our current understanding of how GLP-1 works as both a peripheral hormone and as a central neurotransmitter in health and disease. We focused on its biological effects on the body and the potential clinical application in relation to current research.

Adherence and persistence rates of major antidiabetic medications: a review

The objective of this paper was to review the adherence and persistence rates of major antidiabetic medication classes (i.e., metformin, sulfonylureas, sodium glucose cotransporter-2 inhibitors, dipeptidyl peptidase-4 inhibitors, insulin, glucagon-like peptide-1 receptor agonists, and thiazolidinediones) by summarizing the major findings of the studies published since 2017. In addition, we reported the potential causes for low adherence and persistence of antidiabetic medications. Based on the literature, the highest rate of adherence and persistence was consistently observed in metformin users. Second to metformin were sodium glucose cotransporter-2 inhibitors. Injectable therapies such as insulin and glucagon-like peptide-1 receptor agonists trailed low on the adherence and persistence rates. To the best of our knowledge, no studies published since the year 2017 analyzed the adherence and persistence of thiazolidinediones independently. The most frequently cited cause for low adherence and persistence was the severity of adverse events. Baseline characteristics (e.g., baseline HbA1c level), demographic information (e.g., age, gender, or ethnicity), and comorbidity profiles also had significant impacts on adherence and persistence in patients with type 2 diabetes mellitus.

Dynamics of GLP-1R peptide agonist engagement are correlated with kinetics of G protein activation

The glucagon-like peptide-1 receptor (GLP-1R) has broad physiological roles and is a validated target for treatment of metabolic disorders. Despite recent advances in GLP-1R structure elucidation, detailed mechanistic understanding of how different peptides generate profound differences in G protein-mediated signalling is still lacking. Here we combine cryo-electron microscopy, molecular dynamics simulations, receptor mutagenesis and pharmacological assays, to interrogate the mechanism and consequences of GLP-1R binding to four peptide agonists; glucagon-like peptide-1, oxyntomodulin, exendin-4 and exendin-P5. These data reveal that distinctions in peptide N-terminal interactions and dynamics with the GLP-1R transmembrane domain are reciprocally associated with differences in the allosteric coupling to G proteins. In particular, transient interactions with residues at the base of the binding cavity correlate with enhanced kinetics for G protein activation, providing a rationale for differences in G protein-mediated signalling efficacy from distinct agonists.

Efficacy of L-Arabinose in Lowering Glycemic and Insulinemic Responses: The Modifying Effect of Starch and Fat

L-arabinose is a bio-active compound derived from the side-streams of plant food processing. L-arabinose lowers glycemic and insulinemic responses when added to simple water-based sugary liquids. However, the effect in more complex foods, including fat and starch, is inconsistent. This study assessed the effect of fat or starch in a sugary drink on the efficacy of L-arabinose. Twenty-three healthy volunteers (12 female/11 male; aged 24 ± 3 years; BMI 23 ± 3 kg/m2) participated in a randomised cross-over trial with six drinks: control: 50 g sucrose in water; fat: control + 22 g oil; starch: control + 50 g starch; and all three with and without the addition of 5 g L-arabinose. The addition of L-arabinose to the control drink lowered glucose and insulin peaks by 15% and 52%; for the fat drink by 8% and 45%; and for the starch drink by 7% and 29%. For all three drinks, adding L-arabinose increased glucagon-like peptide 1 (GLP-1) responses and lowered Glucose-dependent insulinotropic polypeptide (GIP) responses. Despite adding large quantities of starch and fat to sugary drinks, L-arabinose significantly lowered postprandial glycemic and insulinemic responses in healthy subjects. These findings suggest that L-arabinose can be functional in more complex foods; however, the factors affecting its efficacy in solid food matrices need to be studied in more detail.

Effects of omega fatty acids on the short-term postprandial satiety related peptides in rats

We aimed to assess the effects of omega fatty acids on time depending on responses of satiety hormones. Sixty adult rats were randomly divided into 4 groups; linoleic acid (LA), α-linolenic acid (ALA), eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) groups. For each fatty acid, the dose of 400 mg/kg was applied by oral gavage. Blood samples were taken after the 15, 30, 60 and 120 minutes. Ghrelin, cholecystokinin (CCK), glucagon-like peptide-1 (GLP-1), peptide YY (PYY), leptin and insulin hormones were analyzed by ELISA. We observed the significant increases (p<0.05) of the levels of CCK between n-3 (ALA, at 60th min; EPA, at 30th and 60th min and DHA, at 60 min) and n-6 (LA) supplemented rats. The highest GLP-1 levels were in ALA (0.70 ng/mL) and DHA (0.67 ng/mL) supplemented groups at 60th and 120th min indicating n-3 fatty acids efficiency on satiety compared to LA. It seems that ALA at 60th min and EPA at 120th min could provide the highest satiety effect with the highest insulin response, while the efficiency of LA supplementation on insulin-induced satiety diminished. The only significant change in AUC values among all hormones was in the CCK of the ALA group (p=0.004). The level of leptin increased in DHA and EPA supplemented rats (p=0.140). Our results showed that dietary omega fatty acids influenced the releasing of hormones in different ways possibly depending on chain length or saturation degree. Comprehensive studies need to be addressed for each fatty acid on satiety-related peptide hormones.

Effects of Glucagon-Like Peptide-1 Receptor Agonist Exendin-4 on the Reinstatement of Cocaine-Mediated Conditioned Place Preference in Mice

A high percentage of relapse to compulsive cocaine-taking and cocaine-seeking behaviors following abstinence constitutes a major obstacle to the clinical treatment of cocaine addiction. Thus, there is a substantial need to develop effective pharmacotherapies for the prevention of cocaine relapse. The reinstatement paradigm is known as the most commonly used animal model to study relapse in abstinent human addicts. The primary aim of this study is to investigate the potential effects of systemic administration of glucagon-like peptide-1 receptor agonist (GLP-1RA) exendin-4 (Ex4) on the cocaine- and stress-triggered reinstatement of cocaine-induced conditioned place preference (CPP) in male C57BL/6J mice. The biased CPP paradigm was induced by alternating administration of saline and cocaine (20 mg/kg), followed by extinction training and then reinstatement by either a cocaine prime (10 mg/kg) or exposure to swimming on the reinstatement test day. To examine the effects of Ex4 on the reinstatement, Ex4 was systemically administered 1 h after the daily extinction session. Additionally, we also explored the associated molecular basis of the behavioral effects of Ex4. The expression of nuclear factor κβ (NF-κβ) in the nucleus accumbens (NAc) was detected using Western blotting. As a result, all animals that were treated with cocaine during the conditioning period successfully acquired CPP, and their CPP response was extinguished after 8 extinction sessions. Furthermore, the animals that were exposed to cocaine or swimming on the reinstatement day showed a significant reinstatement of CPP. Interestingly, systemic pretreatment with Ex4 was sufficient to attenuate cocaine- and stress-primed reinstatement of cocaine-induced CPP. Additionally, the expression of NF-κβ, which was upregulated by cocaine, was normalized by Ex4 in the cocaine-experienced mice. Altogether, our study reveals the novel effect of Ex4 on the reinstatement of cocaine-induced CPP and suggests that GLP-1R agonists appear to be highly promising drugs in the treatment of cocaine use disorder.