Amidated and non-amidated glucagon-like peptide-1 (GLP-1): non-pancreatic effects (cephalic phase acid secretion) and stability in plasma in humans

The interplay between leptin, glucocorticoids, and GLP1 regulates food intake and feeding behaviour

Nutritional, endocrine, and neurological signals converge in multiple brain centres to control feeding behaviour and food intake as part of the allostatic regulation of energy balance. Among the several neuroendocrine systems involved, the leptin, glucocorticoid, and glucagon-like peptide 1 (GLP1) systems have been extensively researched. Leptin is at the top hierarchical level since its complete absence is sufficient to trigger severe hyperphagia. Glucocorticoids are key regulators of the energy balance adaptation to stress and their sustained excess leads to excessive adiposity and metabolic perturbations. GLP1 participates in metabolic adaptation to food intake, regulating insulin secretion and satiety by parallel central and peripheral signalling systems. Herein, we review the brain and peripheral targets of these three hormone systems that integrate to regulate food intake, feeding behaviour, and metabolic homeostasis. We examine the functional relationships between leptin, glucocorticoids, and GLP1 at the central and peripheral levels, including the cross-regulation of their circulating levels and their cooperative or antagonistic actions at different brain centres. The pathophysiological roles of these neuroendocrine systems in dysregulated intake are explored in the two extremes of body adiposity - obesity and lipodystrophy - and eating behaviour disorders.

In vivo degradation forms, anti-degradation strategies, and clinical applications of therapeutic peptides in non-infectious chronic diseases

Current medicinal treatments for diseases comprise largely of two categories: small molecular (chemical) (e.g., aspirin) and larger molecular (peptides/proteins, e.g., insulin) drugs. Whilst both types of therapeutics can effectively treat different diseases, ranging from well-understood (in view of pathogenesis and treatment) examples (e.g., flu), to less-understood chronic diseases (e.g., diabetes), classical small molecule drugs often possess significant side-effects (a major cause of drug withdrawal from market) due to their low- or non-specific targeting. By contrast, therapeutic peptides, which comprise short sequences from naturally occurring peptides/proteins, commonly demonstrate high target specificity, well-characterized modes-of-action, and low or non-toxicity in vivo. Unfortunately, due to their small size, linear permutation, and lack of tertiary structure, peptidic drugs are easily subject to rapid degradation or loss in vivo through chemical and physical routines, thus resulting in a short half-life and reduced therapeutic efficacy, a major drawback that can reduce therapeutic efficiency. However, recent studies demonstrate that the short half-life of peptidic drugs can be significantly extended by various means, including use of enantiomeric or non-natural amino acids (AAs) (e.g., L-AAs replacement with D-AAs), chemical conjugation [e.g., with polyethylene glycol], and encapsulation (e.g., in exosomes). In this context, we provide an overview of the major in vivo degradation forms of small therapeutic peptides in the plasma and anti-degradation strategies. We also update on the progress of small peptide therapeutics that are either currently in clinical trials or are being successfully used in clinical therapies for patients with non-infectious diseases, such as diabetes, multiple sclerosis, and cancer.

In-silico study of the interactions between acylated glucagon like-peptide-1 analogues and the native receptor

We have performed a series of multiple molecular dynamics (MD) simulations of glucagon-like peptide-1 (GLP-1) and acylated GLP-1 analogues in complex with the endogenous receptor (GLP-1R) to obtain a molecular understanding of how fatty acid (FA) chain structure, acylation position on the peptide, and presence of a linker affect the binding. MD simulations were analysed to extract heatmaps of receptor-peptide interaction patterns and to determine the free energy of binding using the molecular mechanics Poisson-Boltzmann surface area (MM-PBSA) approach. The extracted free energies from MM-PBSA calculations are in qualitative agreement with experimentally determined potencies. Furthermore, the interaction patterns seen in the receptor-GLP-1 complex simulations resemble previously reported binding interactions validating the simulations. Analysing the receptor-GLP-1 analogue complex simulations, we found that the major differences between the systems stem from FA interactions and positioning of acylation in the peptide. Hydrophobic interactions between the FA chain and a hydrophobic patch on the extracellular domain contribute significantly to the binding affinity. Acylation on Lys26 resulted in noticeably more interactions between the FA chain and the extracellular domain hydrophobic patch than found for acylation on Lys34 and Lys38, respectively. The presence of a charged linker between the peptide and FA chain can potentially stabilise the complex by forming hydrogen bonds to arginine residues in the linker region between the extracellular domain and the transmembrane domain. A molecular understanding of the fatty acid structure and its effect on binding provides important insights into designing acylated agonists for GLP-1R.Communicated by Ramaswamy H. Sarma.

Glucagon-like peptide-1: Are its roles as endogenous hormone and therapeutic wizard congruent?

Glucagon-like peptide-1 (GLP-1) is a peptide derived from differential processing of the precursor for the hormone glucagon. It is secreted predominantly by endocrine cells in the gut epithelium in response to nutrient stimulation. Studies from the last 35 years have given us an idea about its physiological functions. On the basis of some of its many actions, it has also been developed into a pharmaceutical agent for the treatment of obesity and type 2 diabetes (T2DM). It is currently positioned as the most effective anti-obesity agent available and is recommended in both national and international guidelines as an effective second-in line treatment for T2DM, in particular in patients with increased cardiovascular risk. In this review, I first discuss whether the processing of proglucagon may also result in GLP-1 formation in the pancreas and in glucagon in the gut. Next, I discuss the relationship between the physiological actions of GLP-1 and the therapeutic effects of the GLP-1 receptor agonists, which are far from being congruent and generally poorly understood. These relationships illustrate both the difficulties and the benefits of bridging results obtained in the laboratory with those emerging from the clinic.

Potential new therapeutic target for Alzheimer's disease: Glucagon-like peptide-1

Increasing evidence shows a close relationship between Alzheimer's disease (AD) and type 2 diabetes mellitus (T2DM). Recently, glucagon-like peptide-1 (GLP-1), a gut incretin hormone, has become a well-established treatment for T2DM and is likely to be involved in treating cognitive impairment. In this mini review, the similarities between AD and T2DM are summarised with the main focus on GLP-1-based therapeutics in AD.

Actions of glucagon-like peptide-1 receptor ligands in the gut

The incretin hormone glucagon-like peptide-1 (GLP-1) is inactivated by the enzyme dipeptidyl peptidase-4 even before it leaves the gut, but it seems to act predominantly via activation of intestinal sensory neurons expressing GLP-1 receptors. Thus, activation of vagal afferents is probably responsible for its effects on appetite and food intake, gastrointestinal secretion and motility, and pancreatic endocrine secretion. However, GLP-1 receptors are widely expressed in the gastrointestinal (GI) tract, including epithelial cells in the stomach, and the Brunner glands, in endocrine cells of the gut epithelium, and on mucosal lymphocytes. In this way, GLP-1 may have important local actions of epithelial protection and endocrine signalling and may interact with the immune system. We review the formation and release of GLP-1 from the endocrine L cells and its fate after release and describe the localization of its receptor throughout the GI tract and discuss its direct or indirect actions in the GI tract.

GLP-1 and Intestinal Diseases

Accumulating evidence implicates glucagon-like peptide-1 (GLP-1) to have, beyond glucose maintenance, a beneficial role in the gastrointestinal tract. Here, we review emerging data investigating GLP-1 as a novel treatment for intestinal diseases, including inflammatory bowel diseases, short-bowel syndrome, intestinal toxicities and coeliac disease. Possible beneficial mechanisms for these diseases include GLP-1′s influence on gastric emptying, its anti-inflammatory properties and its intestinotrophic effect. The current knowledge basis derives from the available GLP-1 agonist treatments in experimental animals and small clinical trials. However, new novel strategies including dual GLP-1/GLP-2 agonists are also in development for the treatment of intestinal diseases.

What Is an L-Cell and How Do We Study the Secretory Mechanisms of the L-Cell?

Synthetic glucagon-like peptide-1 (GLP-1) analogues are effective anti-obesity and anti-diabetes drugs. The beneficial actions of GLP-1 go far beyond insulin secretion and appetite, and include cardiovascular benefits and possibly also beneficial effects in neurodegenerative diseases. Considerable reserves of GLP-1 are stored in intestinal endocrine cells that potentially might be mobilized by pharmacological means to improve the body's metabolic state. In recognition of this, the interest in understanding basic L-cell physiology and the mechanisms controlling GLP-1 secretion, has increased considerably. With a view to home in on what an L-cell is, we here present an overview of available data on L-cell development, L-cell peptide expression profiles, peptide production and secretory patterns of L-cells from different parts of the gut. We conclude that L-cells differ markedly depending on their anatomical location, and that the traditional definition of L-cells as a homogeneous population of cells that only produce GLP-1, GLP-2, glicentin and oxyntomodulin is no longer tenable. We suggest to sub-classify L-cells based on their differential peptide contents as well as their differential expression of nutrient sensors, which ultimately determine the secretory responses to different stimuli. A second purpose of this review is to describe and discuss the most frequently used experimental models for functional L-cell studies, highlighting their benefits and limitations. We conclude that no experimental model is perfect and that a comprehensive understanding must be built on results from a combination of models.

Investigating Intestinal Glucagon After Roux-en-Y Gastric Bypass Surgery

CONTEXT

After Roux-en-Y gastric bypass (RYGB) surgery, postprandial plasma glucagon concentrations have been reported to increase. This occurs despite concomitant improved glucose tolerance and increased circulating plasma concentrations of insulin and the glucagon-inhibiting hormone glucagon-like peptide 1 (GLP-1).

OBJECTIVE

To investigate whether RYGB-induced hyperglucagonemia may be derived from the gut.

DESIGN AND SETTING

Substudy of a prospective cross-sectional study at a university hospital in Copenhagen, Denmark.

PARTICIPANTS

Morbidly obese individuals undergoing RYGB (n = 8) with or without type 2 diabetes.

INTERVENTIONS

Three months before and after RYGB, participants underwent upper enteroscopy with retrieval of gastrointestinal mucosal biopsy specimens. Mixed-meal tests were performed 1 week and 3 months before and after RYGB.

MAIN OUTCOME MEASURES

The 29-amino acid glucagon concentrations in plasma and in mucosal gastrointestinal biopsy specimens were assessed using mass spectrometry-validated immunoassays, and a new monoclonal antibody reacting with immunoreactive glucagon was used for immunohistochemistry.

RESULTS

Postprandial plasma concentrations of glucagon after RYGB were increased. Expression of the glucagon gene in the small intestine increased after surgery. Glucagon was identified in the small-intestine biopsy specimens obtained after, but not before, RYGB. Immunohistochemically, mucosal biopsy specimens from the small intestine harbored cells costained for GLP-1 and immunoreactive glucagon.

CONCLUSION

Increased concentrations of glucagon were observed in small-intestine biopsy specimens and postprandially in plasma after RYGB. The small intestine harbored cells immunohistochemically costaining for GLP-1 and glucagon-like immunoreactivity after RYGB. Glucagon derived from small-intestine enteroendocrine l cells may contribute to postprandial plasma concentrations of glucagon after RYGB.

GLP-1: Molecular mechanisms and outcomes of a complex signaling system

Meal ingestion provokes the release of hormones and transmitters, which in turn regulate energy homeostasis and feeding behavior. One such hormone, glucagon-like peptide-1 (GLP-1), has received significant attention in the treatment of obesity and diabetes due to its potent incretin effect. In addition to the peripheral actions of GLP-1, this hormone is able to alter behavior through the modulation of multiple neural circuits. Recent work that focused on elucidating the mechanisms and outcomes of GLP-1 neuromodulation led to the discovery of an impressive array of GLP-1 actions. Here, we summarize the many levels at which the GLP-1 signal adapts to different systems, with the goal being to provide a background against which to guide future research.

Type 2 diabetes risk alleles in PAM impact insulin release from human pancreatic β-cells

The molecular mechanisms underpinning susceptibility loci for type 2 diabetes (T2D) remain poorly understood. Coding variants in peptidylglycine α-amidating monooxygenase (PAM) are associated with both T2D risk and insulinogenic index. Here, we demonstrate that the T2D risk alleles impact negatively on overall PAM activity via defects in expression and catalytic function. PAM deficiency results in reduced insulin content and altered dynamics of insulin secretion in a human β-cell model and primary islets from cadaveric donors. Thus, our results demonstrate a role for PAM in β-cell function, and establish molecular mechanisms for T2D risk alleles at this locus.

GLP-1 Levels Predict Mortality in Patients with Critical Illness as Well as End-Stage Renal Disease

BACKGROUND

Glucagon-like peptide 1 (GLP-1) is an incretin hormone, which stimulates glucose-dependent insulin secretion from the pancreas and holds immune-regulatory properties. A marked increase of GLP-1 has been found in critically ill patients. This study was performed to elucidate the underlying mechanism and evaluate its prognostic value.

METHODS

GLP-1 plasma levels were determined in 3 different patient cohorts: 1) critically ill patients admitted to our intensive care unit (n = 215); 2) patients with chronic kidney disease on hemodialysis (n = 173); and 3) a control group (no kidney disease, no acute inflammation, n = 105). In vitro experiments were performed to evaluate GLP-1 secretion in response to human serum samples from the above-described cohorts.

RESULTS

Critically ill patients presented with 6.35-fold higher GLP-1 plasma level in comparison with the control group. There was a significant correlation of GLP-1 levels with markers for the severity of inflammation, but also kidney function. Patients with end-stage renal disease displayed 4.46-fold higher GLP-1 concentrations in comparison with the control group. In vitro experiments revealed a strong GLP-1-inducing potential of serum from critically ill patients, while serum from hemodialysis patients only modestly increased GLP-1 secretion. GLP-1 levels independently predicted mortality in critically ill patients and patients with end-stage renal disease.

CONCLUSIONS

Chronic and acute inflammatory processes like sepsis or chronic kidney disease increase circulating GLP-1 levels. This most likely reflects a sum effect of increased GLP-1 secretion and decreased GLP-1 clearance. GLP-1 plasma levels independently predict the outcome of critically ill and end-stage renal disease patients.

Gold nanoparticles as an efficient drug delivery system for GLP-1 peptides

In this work, the potential application of gold nanoparticles for GLP-1 analogues delivery was studied. For this purpose, the original sequence of the incretin GLP-1 was slightly modified in the C-terminal region by adding a cysteine residue to facilitate conjugation to the gold surface. The interaction between peptides and gold nanoparticles and also the colloid stability of the conjugates were studied by UV-vis spectrophotometry, TEM, IR and XPS spectroscopy. Moreover, the permeability of these conjugates was assayed using a Caco-2/goblet monolayer model. On the basis of the stability and permeability results, one of the conjugates was chosen to be administered intraperitoneally to normoglycemic rats. The intraperitoneal delivery of the GLP-1 analogue using gold nanoparticles led to decrease levels of blood glucose in the same way as native GLP-1, thereby demonstrating that the formulation of the analogue is stable in physiological conditions and maintains the activity of this incretin.

Glucagon-Like Peptide-1 and Its Class B G Protein-Coupled Receptors: A Long March to Therapeutic Successes

The glucagon-like peptide (GLP)-1 receptor (GLP-1R) is a class B G protein-coupled receptor (GPCR) that mediates the action of GLP-1, a peptide hormone secreted from three major tissues in humans, enteroendocrine L cells in the distal intestine, α cells in the pancreas, and the central nervous system, which exerts important actions useful in the management of type 2 diabetes mellitus and obesity, including glucose homeostasis and regulation of gastric motility and food intake. Peptidic analogs of GLP-1 have been successfully developed with enhanced bioavailability and pharmacological activity. Physiologic and biochemical studies with truncated, chimeric, and mutated peptides and GLP-1R variants, together with ligand-bound crystal structures of the extracellular domain and the first three-dimensional structures of the 7-helical transmembrane domain of class B GPCRs, have provided the basis for a two-domain-binding mechanism of GLP-1 with its cognate receptor. Although efforts in discovering therapeutically viable nonpeptidic GLP-1R agonists have been hampered, small-molecule modulators offer complementary chemical tools to peptide analogs to investigate ligand-directed biased cellular signaling of GLP-1R. The integrated pharmacological and structural information of different GLP-1 analogs and homologous receptors give new insights into the molecular determinants of GLP-1R ligand selectivity and functional activity, thereby providing novel opportunities in the design and development of more efficacious agents to treat metabolic disorders.

Glucagon-like Peptide-1 and the Central/Peripheral Nervous System: Crosstalk in Diabetes

Glucagon-like peptide-1 (GLP-1) is released in response to meals and exerts important roles in the maintenance of normal glucose homeostasis. GLP-1 is also important in the regulation of neurologic and cognitive functions. These actions are mediated via neurons in the nucleus of the solitary tract that project to multiple regions expressing GLP-1 receptors (GLP-1Rs). Treatment with GLP-1R agonists (GLP-1-RAs) reduces ischemia-induced hyperactivity, oxidative stress, neuronal damage and apoptosis, cerebral infarct volume, and neurologic damage, after cerebral ischemia, in experimental models. Ongoing human trials report a neuroprotective effect of GLP-1-RAs in Alzheimer's and Parkinson's disease. In this review, we discuss the role of GLP-1 and GLP-1-RAs in the nervous system with focus on GLP-1 actions on appetite regulation, glucose homeostasis, and neuroprotection.

A new GLP-1 analogue with prolonged glucose-lowering activity in vivo via backbone-based modification at the N-terminus

Glucagon-like peptide-1 (GLP-1) is an endogenous insulinotropic hormone with wonderful glucose-lowering activity. However, its clinical use in type II diabetes is limited due to its rapid degradation at the N-terminus by dipeptidyl peptidase IV (DPP-IV). Among the N-terminal modifications of GLP-1, backbone-based modification was rarely reported. Herein, we employed two backbone-based strategies to modify the N-terminus of tGLP-1. Firstly, the amide N-methylated analogues 2-6 were designed and synthesized to make a full screening of the N-terminal amide bonds, and the loss of GLP-1 receptor (GLP-1R) activation indicated the importance of amide H-bonds. Secondly, with retaining the N-terminal amide H-bonds, the β-peptide replacement strategy was used and analogues 7-13 were synthesized. By two rounds of screening, analogue 10 was identified. Analogue 10 greatly improved the DPP-IV resistance with maintaining good GLP-1R activation in vitro, and showed approximately a 4-fold prolonged blood glucose-lowering activity in vivo in comparison with tGLP-1. This modification strategy will benefit the development of GLP-1-based anti-diabetic drugs.

Degradation and Stabilization of Peptide Hormones in Human Blood Specimens

Plasma hormone peptides, including GLP-1, GIP, Glucagon, and OXM, possess multiple physiological roles and potential therapeutic and diagnostic utility as biomarkers in the research of metabolic disorders. These peptides are subject to proteolytic degradation causing preanalytical variations. Stabilization for accurate quantitation of these active peptides in ex vivo blood specimens is essential for drug and biomarker development. We investigated the protease-driven instability of these peptides in conventional serum, plasma, anticoagulated whole blood, as well as whole blood and plasma stabilized with protease inhibitors. The peptide was monitored by both time-course Matrix-Assisted Laser Desorption Ionization Time-to-Flight Mass Spectrometry (MALDI –TOF MS) and Ab-based assay (ELISA or RIA). MS enabled the identification of proteolytic fragments. In non-stabilized blood samples, the results clearly indicated that dipeptidyl peptidase-IV (DPP-IV) removed the N-terminal two amino acid residues from GLP-1, GIP and OXM(1-37) and not-yet identified peptidase(s) cleave(s) the full-length OXM(1-37) and its fragments. DPP-IV also continued to remove two additional N-terminal residues of processed OXM(3–37) to yield OXM(5–37). Importantly, both DPP-IV and other peptidase(s) activities were inhibited efficiently by the protease inhibitors included in the BD P800* tube. There was preservation of GLP-1, GIP, OXM and glucagon in the P800 plasma samples with half-lives > 96, 96, 72, and 45 hours at room temperature (RT), respectively. In the BD P700* plasma samples, the stabilization of GLP-1 was also achieved with half-life > 96 hours at RT. The stabilization of these variable peptides increased their utility in drug and/or biomarker development. While stability results of GLP-1 obtained with Ab-based assay were consistent with those obtained by MS analysis, the Ab-based results of GIP, Glucagon, and OXM did not reflect the time-dependent degradations revealed by MS analysis. Therefore, we recommended characterizing the degradation of the peptide using the MS-based method when investigating the stability of a specific peptide.

Robust, quantitative analysis of proteins using peptide immunoreagents, in vitro translation, and an ultrasensitive acoustic resonant sensor

A major benefit of proteomic and genomic data is the potential for developing thousands of novel diagnostic and analytical tests of cells, tissues, and clinical samples. Monoclonal antibody technologies, phage display and mRNA display, are methods that could be used to generate affinity ligands against each member of the proteome. Increasingly, the challenge is not ligand generation, rather the analysis and affinity rank-ordering of the many ligands generated by these methods. Here, we developed a quantitative method to analyze protein interactions using in vitro translated ligands. In this assay, in vitro translated ligands generate a signal by simultaneously binding to a target immobilized on a magnetic bead and to a sensor surface in a commercial acoustic sensing device. We then normalize the binding of each ligand with its relative translation efficiency in order to rank-order the different ligands. We demonstrate the method with peptides directed against the cancer marker Bcl-x_L. Our method has 4- to 10-fold higher sensitivity, using 100-fold less protein and 5-fold less antibody per sample, as compared directly with ELISA. Additionally, all analysis can be conducted in complex mixtures at physiological ionic strength. Lastly, we demonstrate the ability to use peptides as ultrahigh affinity reagents that function in complex matrices, as would be needed in diagnostic applications.

GLP-1 amidation efficiency along the length of the intestine in mice, rats and pigs and in GLP-1 secreting cell lines

XXX: Measurements of plasma concentrations of the incretin hormone GLP-1 are complex because of extensive molecular heterogeneity. This is partly due to a varying and incompletely known degree of C-terminal amidation. Given that virtually all GLP-1 assays rely on a C-terminal antibody, it is essential to know whether or not the molecule one wants to measure is amidated. We performed a detailed analysis of extractable GLP-1 from duodenum, proximal jejunum, distal ileum, caecum, proximal colon and distal colon of mice (n=9), rats (n=9) and pigs (n=8) and determined the degree of amidation and whether this varied with the six different locations. We also analyzed the amidation in 3 GLP-1 secreting cell lines (GLUTag, NCI-H716 and STC-1). To our surprise there were marked differences between the 3 species with respect to the concentration of GLP-1 in gut. In the mouse, concentrations increased continuously along the intestine all the way to the rectum, but were highest in the distal ileum and proximal colon of the rat. In the pig, very little or no GLP-1 was present before the distal ileum with similar levels from ileum to distal colon. In the mouse, GLP-1 was extensively amidated at all sampling sites, whereas rats and pigs on average had around 2.5 and 4 times higher levels of amidated compared to non-amidated GLP-1, although the ratio varied depending upon the location. GLUTag, NCI-H716 and STC-1 cells all exhibited partial amidation with 2-4 times higher levels of amidated compared to non-amidated GLP-1.

Incretin hormones and beta-cell mass expansion: what we know and what is missing?

Pancreatic beta-cell mass expands through beta-cell proliferation and neogenesis while it decreases mainly via apoptosis. The loss of balance between beta-cell death and regeneration leads to a reduction of beta-cell functional mass, thus contributing to the pathogenesis of type 2 diabetes mellitus (T2DM). The pathogenetic mechanisms causing T2DM are complex, and also include a significant reduction of the incretin effect. A better understanding of the role of incretin hormones in glucose homeostasis has led to the development of incretin-based therapies. Recently, incretin hormones have been shown to stimulate the beta-cell growth and differentiation from pancreas-derived stem/progenitor cells, as well as to exert cytoprotective, antiapoptotic effects on beta-cells. However, the role and the molecular mechanisms by which GLP-1 and its agonists regulate beta-cell mass have not been fully investigated. This review focuses the current findings and the missing understanding of the effects of incretin hormones on beta-cell mass expansion.

Incretin hormones and the satiation signal

Recent research has indicated that appetite-regulating hormones from the gut may have therapeutic potential. The incretin hormone, glucagon-like peptide-1 (GLP-1), appears to be involved in both peripheral and central pathways mediating satiation. Several studies have also indicated that GLP-1 levels and responses to meals may be altered in obese subjects. Clinical trial results have shown further that two GLP-1 receptor agonists (GLP-1 RAs), exenatide and liraglutide, which are approved for the treatment of hyperglycemia in patients with type 2 diabetes, also produce weight loss in overweight subjects without diabetes. Thus, GLP-1 RAs may provide a new option for pharmacological treatment of obesity.

Glucagon-like peptide-1 analogues: An overview

Abnormalities of the incretin axis have been implicated in the pathogenesis of type 2 diabetes mellitus. Glucagon-like peptide-1 (GLP-1) and gastroinhibitory intestinal peptide constitutes >90% of all the incretin function. Augmentation of GLP-1 results in improvement of beta cell health in a glucose-dependant manner (post-prandial hyperglycemia) and suppression of glucagon (fasting hyperglycemia), amongst other beneficial pleiotropic effects. Native GLP-1 has a very short plasma half-life and novel methods have been developed to augment its half life, such that its anti-hyperglycemic effects can be exploited. They can be broadly classified as exendin-based therapies (exenatide, exenatide once weekly), DPP-4-resistant analogues (lixisenatide, albiglutide), and analogues of human GLP-1 (liraglutide, taspoglutide). Currently, commercially available analogues are exenatide, exenatide once weekly, and liraglutide. This review aims to provide an overview of most GLP-1 analogues.

Cell-specific precursor processing

The singular gene for a peptide hormone is expressed not only in a specific endocrine cell type but also in other endocrine cells as well as in entirely different cells such as neurons, adipocytes, myocytes, immune cells, and cells of the sex-glands. The cellular expression pattern for each gene varies with development, time and species. Endocrine regulation is, however, based on the release of a given hormone from an endocrine cell to the general circulation from whose cappilaries the hormone reaches the specific target cell elsewhere in the body. The widespread expression of hormone genes in different cells and tissues therefore requires control of biogenesis and secretion in order to avoid interference with the function of a specific hormonal peptide from a particular endocrine cell. Several mechanisms are involved in such control, one of them being cell-specific processing of prohormones. The following pages present four examples of such cell-specific processing and the implications of the phenomenon for the use of peptide hormones as markers of diseases. Notably, sick cells - not least the neoplastic cells - often process prohormones in a manner different from that of the normal endocrine cells.

Peptide processing and biology in human disease

PURPOSE OF REVIEW

To describe recent advances in the processing of gastrointestinal hormones, and the consequences for human disease of mutations in the enzymes involved.

RECENT FINDINGS

Although gastrointestinal prohormones were long regarded as devoid of biological activity, recent data indicate that the prohormones for both gastrin and gastrin-releasing peptide are bioactive, through different receptors from the mature hormones. Mutations in the family of prohormone convertases responsible for the initial steps in the processing of gastrointestinal hormones are associated with several different pathophysiological conditions in humans.

SUMMARY

Human mutational studies, when taken together with the phenotypes observed in mice deficient in the prohormone convertases, emphasize the crucial importance of the processing enzymes in mammalian biology. Although the phenotypes may often be ascribed to defective production of a mature hormone or growth factor, the recognition that the precursors are independently bioactive suggests that the increased precursor concentrations may also contribute to the symptoms. The observation that the precursors often act through different receptors from the mature hormones may permit the development of precursor-selective antagonists for therapeutic use.

Glucagon and glucagon-like peptides 1 and 2

The glucagon gene is expressed not only in the alpha cells of the pancreatic islets but also in the endocrine cells of the intestinal epithelium (so-called L-cells), and in certain neurons of the brain stem. Whereas in the pancreas, glucagon, the hyperglycaemic hormone, is cleaved out of the 160 amino acid precursor, proglucagon, leaving behind proglucagon fragments (PG 1-30 and PG 72-158, the so-called major proglucagon fragment (MPGF)) that are probably inactive, the intestinal processing leads to the formation of glicentin (PG 1-69; action uncertain) and glucagon-like peptides 1 (PG 78-107amide, a potent incretin homone, regulating insulin secretion, glucagon secretion, gastrointestinal motility and appetite) and 2 (PG 126-158, a regulator of gut mucosal growth and integrity). The two prohormone convertases PC2 and PC1/3, respectively, are responsible for the differential processing. After their release, the hormones are eliminated mainly in the kidneys, but both GLP-2 and in particular GLP-1, but not glucagon, are metabolized both locally and in the circulation and liver by dipeptidyl peptidase 4 (DPP-4) which inactivates the peptides, suggesting that GLP-1 acts locally rather than in an endocrine manner. A number of transcription factors have been identified that can at least partly explain the differential cellular expression of the glucagon gene as well as the differential tissue-specific processing of the precursor.

The physiology of glucagon-like peptide 1

Glucagon-like peptide 1 (GLP-1) is a 30-amino acid peptide hormone produced in the intestinal epithelial endocrine L-cells by differential processing of proglucagon, the gene which is expressed in these cells. The current knowledge regarding regulation of proglucagon gene expression in the gut and in the brain and mechanisms responsible for the posttranslational processing are reviewed. GLP-1 is released in response to meal intake, and the stimuli and molecular mechanisms involved are discussed. GLP-1 is extremely rapidly metabolized and inactivated by the enzyme dipeptidyl peptidase IV even before the hormone has left the gut, raising the possibility that the actions of GLP-1 are transmitted via sensory neurons in the intestine and the liver expressing the GLP-1 receptor. Because of this, it is important to distinguish between measurements of the intact hormone (responsible for endocrine actions) or the sum of the intact hormone and its metabolites, reflecting the total L-cell secretion and therefore also the possible neural actions. The main actions of GLP-1 are to stimulate insulin secretion (i.e., to act as an incretin hormone) and to inhibit glucagon secretion, thereby contributing to limit postprandial glucose excursions. It also inhibits gastrointestinal motility and secretion and thus acts as an enterogastrone and part of the "ileal brake" mechanism. GLP-1 also appears to be a physiological regulator of appetite and food intake. Because of these actions, GLP-1 or GLP-1 receptor agonists are currently being evaluated for the therapy of type 2 diabetes. Decreased secretion of GLP-1 may contribute to the development of obesity, and exaggerated secretion may be responsible for postprandial reactive hypoglycemia.

Is Glucagon-like peptide-1, an agent treating diabetes, a new hope for Alzheimer's disease?

Glucagon-like peptide-1 (GLP-1) has been endorsed as a promising and attractive agent in the treatment of type 2 diabetes mellitus (T2DM). Both Alzheimer's disease (AD) and T2DM share some common pathophysiologic hallmarks, such as amyloid beta (Abeta), phosphoralation of tau protein, and glycogen synthase kinase-3. GLP-1 possesses neurotropic properties and can reduce amyloid protein levels in the brain. Based on extensive studies during the past decades, the understanding on AD leads us to believe that the primary targets in AD are the Abeta and tau protein. Combine these findings, GLP-1 is probably a promising agent in the therapy of AD. This review was focused on the biochemistry and physiology of GLP-1, communities between T2DM and AD, new progresses of GLP-1 in treating T2MD and improving some pathologic hallmarks of AD.

Biology of incretins: GLP-1 and GIP

This review focuses on the mechanisms regulating the synthesis, secretion, biological actions, and therapeutic relevance of the incretin peptides glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1). The published literature was reviewed, with emphasis on recent advances in our understanding of the biology of GIP and GLP-1. GIP and GLP-1 are both secreted within minutes of nutrient ingestion and facilitate the rapid disposal of ingested nutrients. Both peptides share common actions on islet beta-cells acting through structurally distinct yet related receptors. Incretin-receptor activation leads to glucose-dependent insulin secretion, induction of beta-cell proliferation, and enhanced resistance to apoptosis. GIP also promotes energy storage via direct actions on adipose tissue, and enhances bone formation via stimulation of osteoblast proliferation and inhibition of apoptosis. In contrast, GLP-1 exerts glucoregulatory actions via slowing of gastric emptying and glucose-dependent inhibition of glucagon secretion. GLP-1 also promotes satiety and sustained GLP-1-receptor activation is associated with weight loss in both preclinical and clinical studies. The rapid degradation of both GIP and GLP-1 by the enzyme dipeptidyl peptidase-4 has led to the development of degradation-resistant GLP-1-receptor agonists and dipeptidyl peptidase-4 inhibitors for the treatment of type 2 diabetes. These agents decrease hemoglobin A1c (HbA1c) safely without weight gain in subjects with type 2 diabetes. GLP-1 and GIP integrate nutrient-derived signals to control food intake, energy absorption, and assimilation. Recently approved therapeutic agents based on potentiation of incretin action provide new physiologically based approaches for the treatment of type 2 diabetes.

The role of gut hormones in glucose homeostasis

The gastrointestinal tract has a crucial role in the control of energy homeostasis through its role in the digestion, absorption, and assimilation of ingested nutrients. Furthermore, signals from the gastrointestinal tract are important regulators of gut motility and satiety, both of which have implications for the long-term control of body weight. Among the specialized cell types in the gastrointestinal mucosa, enteroendocrine cells have important roles in regulating energy intake and glucose homeostasis through their actions on peripheral target organs, including the endocrine pancreas. This article reviews the biological actions of gut hormones regulating glucose homeostasis, with an emphasis on mechanisms of action and the emerging therapeutic roles of gut hormones for the treatment of type 2 diabetes mellitus.

C-terminal analogues of parathyroid hormone: effect of C-terminus function on helical structure, stability, and bioactivity

We have studied the effects of C-terminal group modifications (amide, methylamide, dimethylamide, aldehyde, and alcohol) on the conformation, adenylyl cyclase stimulation (AC), or binding of parathyroid hormone (hPTH) analogues, hPTH(1-28)NH(2) and hPTH(1-31)NH(2). hPTH(1-31)NH(2) has a C-terminal alpha-helix bounded by residues 17-29 [Chen, Z., et al. (2000) Biochemistry 39, 12766]. In both cases, relative to the natural analogue with a carboxyl C-terminus, the amide and methylamide had increased helix content whereas the dimethylamide forms had CD spectra more similar to the carboxyl one. Conformational effects were more pronounced with hPTH(1-28) than with hPTH(1-31), with increases in helix content of approximately 30% in contrast to 10%. Stabilization of the C-terminal helix of residues 1-28 seemed to correlate with an ability of the C-terminal function to H-bond appropriately. None of the analogues affected the AC stimulating activity significantly, but there was an up to 15-fold decrease in the level of apparent binding of the carboxyl hPTH(1-28) analogue compared to that of the methylamide and a 4-fold decrease in the level of binding of the aldehyde or dimethylamide. There was no significant change in binding activities for the 1-31 analogues. These observations are consistent with previous studies that imply the importance of a region of the hormone's C-terminal alpha-helix for tight binding to the receptor. They also show that modulation of helix stability does have an effect on the binding of the hormone, but only when the C-terminus is at the putative end of the helix. The similarity of AC stimulation even when binding changed 10-fold can be explained by assuming greater efficacy of the weaker binding PTH-receptor complexes in stimulating AC.

Glucagon-like peptide 1 (GLP-1) and metabolic diseases

Glucagon-like peptide-1 (GLP-1) is a gastrointestinal hormone, mainly secreted after meals, which enhances glucose-induced insulin secretion and induces satiety. It has been reported that GLP-1 levels after a mixed meal and after an oral glucose load are reduced in patients with Type 2 diabetes. The reduction of oral glucose-stimulated active GLP-1 levels in patients with Type 2 diabetes has also been observed during euglycemic iperinsulinemic clamp. The reduction of post-prandial circulating active GLP-1 in Type 2 diabetic subjects, as a consequence of chronic hyperglycemia, could contribute to the reduction of early post-prandial insulin secretion; in fact, the administration of GLP-1 receptor antagonists to healthy volunteers elicits both an impairment of meal-induced insulin secretion and an increase of post-prandial glycemia similar to that observed in Type 2 diabetes. GLP-1 is rapidly inactivated by dipeptidyl peptidase IV (DPP-IV), an enzyme produced by endothelial cells in different districts and that circulates in plasma. It is still not clear whether the reduction of mealor oral-glucose stimulated GLP-1 levels in Type 2 diabetic patients is due to impairment of secretion, increase of degradation, or both. The major limitation of using GLP-1 to treat diabetic patients is the short half-life of the native compound. There are now several compounds in various stages of pre-clinical or clinical development for the treatment of Type 2 diabetes that utilize the GLP-1 signaling pathway; these include GLP-1 receptor agonists with extended half-lives, and inhibitors of DPP-IV that increase circulating levels of endogenous, intact and bioactive GLP-1.

Structure and function studies of glucagon-like peptide-1 (GLP-1): the designing of a novel pharmacological agent for the treatment of diabetes

Glucagon-like peptide-1 (GLP-1) is a proglucagon-derived peptide secreted from gut endocrine cells in response to nutrient ingestion. The multifaceted actions of GLP-1 include the following: (1) the stimulation of insulin secretion and of its gene expression, (2) the inhibition of glucagon secretion, (3) the inhibition of food intake, (4) the proliferation and differentiation of beta cells, and (5) the protection of beta-cells from apoptosis. The therapeutic utility of the native GLP-1 molecule is limited by its rapid enzymatic degradation by a serine protease termed dipeptidyl peptidase-IV (DPP-IV). The present article reviews the research studies aimed at elucidating the biosynthesis, metabolism, and molecular characteristics of GLP-1 since it is from these studies that the development of a GLP-1-like pharmacological agent may be derived.

Treatment of Type 2 diabetes mellitus based on glucagon-like peptide-1

Glucagon-like peptide-1 (GLP-1) is a peptide hormone released from the gut mucosa in response to meal ingestion. Its actions include stimulation of all steps of insulin gene expression, as well as beta-cell growth, inhibition of glucagon secretion, inhibition of hepatic glucose production, inhibition of gastrointestinal secretion and motility, and inhibition of appetite and food intake. Physiologically, therefore, GLP-1 is thought to act as an incretin hormone (intestinal hormones that enhance meal-related insulin secretion) and as one of the hormones of the ileal brake mechanism (endocrine inhibition of gastrointestinal motility and secretion in the presence of nutrients in the lower small intestine). However, because of these same actions, the hormone can normalise the blood glucose of patients with Type 2 diabetes mellitus, and, in contradistinction to insulin and sulphonylurea, it does not cause hypoglycaemia. Therefore, treatment of Type 2 diabetes based on GLP-1 is currently being investigated. As a peptide, it must be administered parenterally, and, in addition, it is metabolised extremely rapidly. However, several methods to circumvent these problems have already been developed. A GLP-1- based therapy of diabetes mellitus and perhaps also obesity is therefore likely to become a realistic alternative to current therapies of these disorders.

Sequence-based Design of Kinase Inhibitors Applicable for Therapeutics and Target Identification

A platform for specifically modulating kinase-dependent signaling using peptides derived from the catalytic domain of the kinase is presented. This technology, termed KinAce, utilizes the canonical structure of protein kinases. The targeted regions (subdomain V and subdomains IX and X) are analyzed and their sequence, three-dimensional structure, and involvement in protein-protein interaction are highlighted. Short myristoylated peptides were derived from the target regions of the tyrosine kinases c-Kit and Lyn and the serine/threonine kinases 3-phosphoinositide-dependent kinase-1 (PDK1) and Akt/protein kinase B (PKB). For each kinase an active designer peptide is shown to selectively inhibit the signaling of the kinase from which it is derived, and to inhibit cancer cell proliferation in the micromolar range. This technology emerges as an applicable tool for deriving sequence-based selective inhibitors for a broad range of protein kinases as hits that may be further developed into drugs. Moreover, it enables identification of novel kinase targets for selected therapeutic indications as demonstrated in the KinScreen application.

Glycine-extended adrenomedullin exerts vasodilator effect through amidation in the rat aorta

Human adrenomedullin (hAM) is an endogenous peptide that has potent vasodilator activity. Mature AM is biosynthesized from its intermediate form, glycine-extended AM (AM-gly), by carboxy-terminal amidation. AM-gly is generally considered to be biologically inactive but is a major molecular form in human and rat plasma. The present study demonstrated that recombinant human AM-gly (hAM-gly) elicits potent vasodilator effect on isolated rat aorta. In aortic rings, hAM-gly produced dose-dependent (0.1-100 nM) relaxation in phenylephrine-precontracted strips (pD(2) 8.4+/-0.5). The vasorelaxant potency of hAM-gly was comparable to that of hAM (pD(2) 8.6+/-0.2) but hAM-gly took a significantly (P<0.01) longer time to reach the maximal relaxation compared with hAM (T(max) 23+/-4 vs. 5+/-2 min). Vasorelaxant responses to hAM-gly were abolished by endothelial removal. N(omega)-nitro-L-arginine (L-NNA) and AM(22-52) significantly (P<0.01) reduced the vasodilator effect of hAM-gly. Furthermore, 4-phenyl-3-butenoic acid (PBA), an alpha-amidation enzyme inhibitor, significantly (P<0.05) inhibited the vasorelaxant responses to hAM-gly without any effect on the hAM-induced relaxation, suggesting the possible process of amidation in the rat aorta. We further clarified that the aorta has the ability to convert exogenous hAM-gly to mature hAM and the conversion is inhibited by PBA. These results suggest that the circulating AM-gly may play a role in regulating vascular tone and increased plasma AM-gly may be involved in the pathophysiology of cardiovascular diseases.

Biological actions and therapeutic potential of the glucagon-like peptides

The glucagon-like peptides (GLP-1 and GLP-2) are proglucagon-derived peptides cosecreted from gut endocrine cells in response to nutrient ingestion. GLP-1 acts as an incretin to lower blood glucose via stimulation of insulin secretion from islet beta cells. GLP-1 also exerts actions independent of insulin secretion, including inhibition of gastric emptying and acid secretion, reduction in food ingestion and glucagon secretion, and stimulation of beta-cell proliferation. Administration of GLP-1 lowers blood glucose and reduces food intake in human subjects with type 2 diabetes. GLP-2 promotes nutrient absorption via expansion of the mucosal epithelium by stimulation of crypt cell proliferation and inhibition of apoptosis in the small intestine. GLP-2 also reduces epithelial permeability, and decreases meal-stimulated gastric acid secretion and gastrointestinal motility. Administration of GLP-2 in the setting of experimental intestinal injury is associated with reduced epithelial damage, decreased bacterial infection, and decreased mortality or gut injury in rodents with chemically induced enteritis, vascular-ischemia reperfusion injury, and dextran sulfate-induced colitis. GLP-2 also attenuates chemotherapy-induced mucositis via inhibition of drug-induced apoptosis in the small and large bowel. GLP-2 improves intestinal adaptation and nutrient absorption in rats after major small bowel resection, and in humans with short bowel syndrome. The actions of GLP-2 are mediated by a distinct GLP-2 receptor expressed on subsets of enteric nerves and enteroendocrine cells in the stomach and small and large intestine. The beneficial actions of GLP-1 and GLP-2 in preclinical and clinical studies of diabetes and intestinal disease, respectively, has fostered interest in the potential therapeutic use of these gut peptides. Nevertheless, the actions of the glucagon-like peptides are limited in duration by enzymatic inactivation via cleavage at the N-terminal penultimate alanine by dipeptidyl peptidase IV (DP IV). Hence, inhibitors of DP IV activity, or DP IV-resistant glucagon-like peptide analogues, may be alternative therapeutic approaches for treatment of human diseases.