Incretin hormone mimetics and analogues in diabetes therapeutics

Whey proteins as multifunctional food materials: Recent advancements in hydrolysis, separation, and peptidomimetic approaches

Whey protein derived bioactives, including α-lactalbumin, ß-lactoglobulin, bovine serum albumin, lactoferrin, transferrin, and proteose-peptones, have exhibited wide ranges of functional, biological and therapeutic properties varying from anticancer, antihypertensive, and antimicrobial effects. In addition, their functional properties involve gelling, emulsifying, and foaming abilities. For these reasons, this review article is framed to understand the relationship existed in between those compound levels and structures with their main functional, biological, and therapeutic properties exhibited either in vitro or in vivo. The impacts of hydrolysis mechanism and separation techniques in enhancing those properties are likewise discussed. Furthermore, special emphasize is given to multifunctional effects of whey derived bioactives and their future trends in ameliorating further food, pharmaceutical, and nutraceutical products. The underlying mechanism effects of those properties are still remained unclear in terms of activity levels, efficacy, and targeted effectiveness. For these reasons, some important models linking to functional properties, thermal properties and cell circumstances are established. Moreover, the coexistence of radical trapping groups, chelating groups, sulfhydryl groups, inhibitory groups, and peptide bonds seemed to be the key elements in triggering those functions and properties. Practical Application: Whey proteins are the byproducts of cheese processing and usually the exploitation of these food waste products has increasingly getting acceptance in many countries, especially European countries. Whey proteins share comparable nutritive values to milk products, particularly on their richness on important proteins that can serve immune protection, structural, and energetic roles. The nutritive profile of whey proteins shows diverse type of bioactive molecules like α-lactalbumin, ß-lactoglobulin, lactoferrin, transferrin, immunoglobulin, and proteose peptones with wide biological importance to the living system, such as in maintaining immunological, neuronal, and signaling roles. The diversification of proteins of whey products prompted scientists to exploit the real mechanisms behind of their biological and therapeutic effects, especially in declining the risk of cancer, tumor, and further complications like diabetes type 2 and hypertension risk effects. For these reasons, profiling these types of proteins using different proteomic and peptidomic approaches helps in determining their biological and therapeutic targets along with their release into gastrointestinal tract conditions and their bioavailabilities into portal circulation, tissue, and organs. The wide applicability of those protein fractions and their derivative bioactive products showed significant impacts in the field of emulsion and double emulsion stabilization by playing roles as emulsifying, surfactant, stabilizing, and foaming agents. Their amphoteric properties helped them to act as excellent encapsulating agents, particularly as vehicle for delivering important vitamins and bioactive compounds. The presence of ferric elements increased their transportation to several metal-ions in the same time increased their scavenging effects to metal-transition and peroxidation of lipids. Their richness with almost essential and nonessential amino acids makes them as selective microbial starters, in addition their richness in sulfhydryl amino acids allowed them to act a cross-linker in conjugating further biomolecules. For instance, conjugating gold-nanoparticles and fluorescent materials in targeting diseases like cancer and tumors in vivo is considered the cutting-edges strategies for these versatile molecules due to their active diffusion across-cell membrane and the presence of specific transporters to these therapeutic molecules.

Raman Microscopy Investigation of GLP-1 Peptide Association with Supported Phospholipid Bilayers

A wide range of important biological processes occur at phospholipid membranes including cell signaling, where a peptide or small molecule targets a membrane-localized receptor protein. In this work, we report the adaptation of confocal Raman microscopy to quantify populations of unlabeled glucagon-like peptide-1 (GLP-1), a membrane-active 30-residue incretin peptide, in supported phospholipid bilayers deposited on the interior surfaces of wide-pore porous silica particles. Quantification of lipid bilayer-associated peptide is achieved by measuring the Raman scattering intensity of the peptide relative to that of the supported lipid bilayer, which serves as an internal standard. The dependence of the bilayer-associated GLP-1 population on the solution concentration of GLP-1 produces an isotherm used to determine the equilibrium constant for peptide-bilayer association and the maximum peptide surface coverage. The maximum coverage of GLP-1 in the lipid bilayer was found to be only 1/5th of a full monolayer based on its hydrodynamic radius. The saturation coverage, therefore, is not limited by the size of GLP-1 but by the ability of the bilayer to accommodate the peptide at high concentrations within the bilayer. Raman spectra show that GLP-1 association with the supported bilayer is accompanied by structural changes consistent with the intercalation of the peptide into the bilayer, where the observed increase in acyl-chain order would increase the lipid density and provide free volume needed to accommodate the peptide. These results were compared with previous measurements of the association of fluorescently labeled GLP-1 with a planar-supported bilayer; the unlabeled peptide exhibits a 3-fold greater affinity for the lipid bilayer on the porous silica support, suggesting that the fluorescent label alters the GLP-1 lipid bilayer association.

DBPR108, a novel dipeptidyl peptidase-4 inhibitor with antihyperglycemic activity

AIMS

Dipeptidyl peptidase 4 (DPP-4) is a valid molecular drug target from which its inhibitors have been developed as medicines for treating diabetes. The present study evaluated a new synthetic DPP-4-specific inhibitor of small molecule DBPR108 for pharmacology and pharmacokinetic profiles.

MAIN METHODS

DBPR108 of various doses was orally administered to rats, diabetic mice, and dogs and the systemic circulating DPP-4 activities in the animals were measured to demonstrate the pharmacological mechanisms of action via DPP-4 inhibition. Upon an oral administration of DBPR108, the serum active GLP-1 and insulin levels of the rats challenged with an oral glucose ingestion were measured. Oral glucose tolerance test in diet-induced obese mice was performed to examine if DBPR108 increases the glucose tolerability in animals.

KEY FINDINGS

Orally administered DBPR108 inhibited the systemic plasma DPP-4 activities in rats, dogs and diabetic mice in a dose-dependent manner. DBPR108 caused elevated serum levels of active GLP-1 and insulin in the rats. DBPR108 dose-dependently increased the glucose tolerability in diet-induced obese (DIO) mice and, furthermore, DIO mice treated with DBPR108 (0.1 mg/kg) in combination with metformin (50 or 100 mg/kg) showed a prominently strong increase in the glucose tolerability.

SIGNIFICANCE

DBPR108 is a novel DPP-4-selective inhibitor of small molecule that demonstrated potent in vivo pharmacological effects and good safety profiles in animals. DBPR108 is now a drug candidate being further developed in the clinical studies as therapeutics for treating diabetes.

Oral self-nanoemulsifying formulation of GLP-1 agonist peptide exendin-4: development, characterization and permeability assesment on Caco-2 cell monolayer

The objective of this study was to prepare a stable self-nanoemulsifying formulation of exendin-4, which is an antidiabetic peptide. As exendin-4 is commercially available only in subcutaneous form, several attempts have been made to discover an effective oral formulation. Self-nanoemulsifying drug delivery systems are known to be suitable carriers for the oral administration of peptide drugs. Various ratios of oil, surfactant, and co-surfactant mixtures were used to determine the area in the pseudoternary phase diagram for clear nanoemulsion. The Design of Experiment approach was used for the optimization of the formulation. Blank self-nanoemulsifying formulations containing ethyl oleate as oil phase, Cremophor EL^®, and Labrasol^® as surfactant, absolute ethanol, and propylene glycol as co-solvent in various proportions were approximately 18-50 nm, 0.08-0.204 and - 3 to - 23 mV in droplet size, polydispersity index, and zeta potential, respectively. When all formulations were compared by statistical analysis, five of them with smaller droplet sizes were selected for further studies. The physical stability test was performed for 1 month at 5 °C ± 3 °C and 25 °C ± 2 °C/60% RH ± 5% RH storage conditions. As a result of the characterization and physical stability test results, ethyl oleate: Cremophor EL^®:absolute ethanol (30:52.5:17.5) formulation and four formulations containing ethyl oleate: Cremophor EL^®:Labrasol^®:propylene glycol:absolute ethanol at varying concentrations were considered for peptide encapsulation efficiency. Formulation having the highest encapsulation efficiency of exendin-4 containing ethyl oleate: Cremophor EL^®:Labrasol^®:propylene glycole:absolute ethanol (15:42.5:21.25:15.94:5.31) was selected for in vitro Caco-2 intestinal permeability study. The permeabiliy coefficient was increased by 1.5-folds by exendin-4-loaded self-nanoemulsifying formulation as compared to the exendin-4 solution. It can be concluded that intestinal permeability has been improved by self-nanoemulsifying formulation.

Generation and characterization of dipeptidyl peptidase-IV inhibitory peptides from trypsin-hydrolyzed α-lactalbumin-rich whey proteins

Dipeptidyl peptidase-IV (DPP-IV) is an enzyme that break down the antidiabetic hormone glucagon-like peptide-1. Therefore, inhibition of DPP-IV could be an effective strategy to treat Type 2 diabetes (T2D). The α-lactalbumin-rich whey protein concentrate was hydrolyzed by trypsin, and the hydrolysates were then fractionated at a semi-preparative scale using a Superdex Gel filtration Chromatography. The peptides were analyzed by using HPLC coupled with tandem mass spectrometry (RP-HPLC-MS/MS), and their Dipeptidyl peptidase-IV inhibitory activity was determined by the enzymatic assay. Among tested fragments, a potent fragment (LDQWLCEKL), with the half-maximal inhibitory concentration (IC₅₀) of 131 μM was obtained. Further analysis shows that the LDQWLCEKL peptide corresponds to the amino acid sequence of f(115-123) in α-lactalbumin. Furthermore, LDQWLCEKL exhibited a typical non-competitive mode of inhibition. The results indicate that α-lactalbumin contains active peptides with DPP-IV inhibitory activity that may be used to prevent and treat T2D.

Hydration shell differentiates folded and disordered states of a Trp-cage miniprotein, allowing characterization of structural heterogeneity by wide-line NMR measurements

Hydration properties of folded and unfolded/disordered miniproteins were monitored in frozen solutions by wide-line 1H-NMR. The amount of mobile water as function of T (-80 °C < T < 0 °C) was found characteristically different for folded (TC5b), semi-folded (pH < 3, TCb5(H+)) and disordered (TC5b_N1R) variants. Comparing results of wide-line 1H-NMR and molecular dynamics simulations we found that both the amount of mobile water surrounding proteins in ice, as well as their thaw profiles differs significantly as function of the compactness and conformational heterogeneity of their structure. We found that (i) at around -50 °C ~50 H2Os/protein melt (ii) if the protein is well-folded then this amount of mobile water remains quasi-constant up to -20 °C, (iii) if disordered then the quantity of the lubricating mobile water increases with T in a constant manner up to ~200 H2Os/protein by reaching -20 °C. Especially in the -55 °C ↔ -15 °C temperature range, wide-line 1H-NMR detects the heterogeneity of protein fold, providing the size of the hydration shell surrounding the accessible conformers at a given temperature. Results indicate that freezing of protein solutions proceeds by the gradual selection of the enthalpically most favored states that also minimize the number of bridging waters.

Expression kinetics reveal the self-adaptive role of β cells during the progression of diabetes

OBJECTIVE

To determine the histopathological and molecular changes in β-cells at different time intervals following streptozotocin (STZ)-induced diabetes.

METHODS

STZ (65 mg/kg body weight) was given to overnight fasted rats that were sacrificed after 1, 3, and 10 days of injection. Changes in islet morphology and in the expression of various factors involved in β-cell proliferation, inflammation and apoptosis were analyzed.

RESULTS

Superoxide dismutase (Sod) expression was completely reduced and that of NF-kB and iNOS were significantly increased, along with lymphocytic infiltration in the islets within 24 h of STZ injection. In addition, the β-cell protective markers Bcl-2, IL-6, Ki67, Hif-1α, VEGF and insulin were also enhanced, indicating a compensatory response of the β-cells to the initial damaging effects. Lymphocytic infiltration decreased after 3 days of injection, accompanied by enhanced expression of both GLP-1R and GIP R. The unresponsiveness of the incretin ligands after STZ administration further suggested a compensatory approach by the incretin receptors independent of glucose regulation. After 10 days, lymphocytic infiltration and inflammatory markers again increased, along with a concomitant reduction in the expression of incretin receptors, and upregulation of the protective markers. Furthermore, the saturation peak of blood glucose indicated progressive diabetes.

CONCLUSIONS

The β-cells follow a biphasic pattern of expression of certain factors in order to achieve a balance between apoptosis, autophagy, neo-genesis, hypoxia and proliferation, and achieve homeostatic protection before the onset of diabetes. The drug interventions at an early stage, which are specific to these pathways, could be beneficial in preventing the progression of diabetes pathogenesis.

Glucagon-Like Peptide-1 (GLP-1)-Based Therapeutics: Current Status and Future Opportunities beyond Type 2 Diabetes

Glucagon-like peptide-1 (GLP-1) is secreted by intestinal L-cells following food intake, and plays an important role in glucose homeostasis due to its stimulation of glucose-dependent insulin secretion. Further, GLP-1 is also associated with protective effects on pancreatic β-cells and the cardiovascular system, decreased appetite, and weight loss, making GLP-1 derivatives an exciting treatment for type 2 diabetes and obesity. Despite these benefits, wild-type GLP-1 exhibits a short circulation time due to its poor metabolic stability and rapid renal clearance, and must be administered by injection, making it a poor therapeutic agent. Many strategies have been used to improve the circulation time of GLP-1 (e.g., mutations, unnatural amino acids, depot formulations, use of exendin-4 sequences, and fusions with high-molecular-weight proteins or polymers), with its therapeutic utility further improved by adding agonist activity for gastric inhibitory peptide and glucagon receptors. This minireview focuses on strategies that have been used to improve the pharmacokinetics of GLP-1 and provides an overview of GLP-1-based therapeutics in the pipeline.

Detailed characterisation of STC-1 cells and the pGIP/Neo sub-clone suggests the incretin hormones are translationally regulated

STC-1 is a heterogeneous plurihormonal cell line producing several prominent gut peptide hormones. pGIP/Neo is a genetically selected sub-clone of STC-1 with augmented levels of glucose-dependent insulinotropic peptide (GIP). Morphometric parameters, hormone concentrations, mRNA transcripts, hormone immunocytochemistry and nutrient utilisation/production of these two cell lines were compared. Proglucagon-derived peptides (Glucagon-like peptide-1 (GLP-1) and - 2(GLP-2)) were lower in sub-clone cells than progenitor cells. High Content Analysis found altered intracellular GLP-1, GIP, cholecystokinin (CCK) and peptide YY (PYY) levels and differing hormone co-localisation. The proportion pGIP/Neo cells containing GIP immunoreactivity (82%) was greater than STC-1 (65%), as were the proportion with 'GIP only', 'GLP-1+GIP' or 'GIP+PYY' immunoreactivity. Most surprisingly mRNA transcripts of the proglucagon and GIP genes were inversely correlated to the levels of their translated peptides. This strongly suggests that proglucagon and GIP are encoded on 'translationally regulated genes' - a characteristic possessed by other endocrine hormones. Metabolomic profiling revealed differences in cellular nutrient utilisation/production and that under normal culture conditions both cell lines exhibit signs of overflow metabolism. These studies provide an insight into the metabolism and properties of these valuable cells, suggesting for the first time that incretin hormone genes are translationally regulated.

Improving drug-like properties of insulin and GLP-1 via molecule design and formulation and improving diabetes management with device & drug delivery

There is an increased incidence of diabetes worldwide. The discovery of insulin revolutionized the management of diabetes, the revelation of glucagon-like peptide-1 (GLP-1) and introduction of GLP-1 receptor agonists to clinical practice was another breakthrough. Continued translational research resulted in better understanding of diabetes, which, in combination with cutting-edge biology, chemistry, and pharmaceutical tools, have allowed for the development of safer, more effective and convenient insulins and GLP-1. Advances in self-administration of insulin and GLP-1 receptor agonist therapies with use of drug-device combination products have further improved the outcomes of diabetes management and quality of life for diabetic patients. The synergies of insulin and GLP-1 receptor agonist actions have led to development of devices that can deliver both molecules simultaneously. New chimeric GLP-1-incretins and insulin-GLP-1-incretin molecules are also being developed. The objective of this review is to summarize molecular designs to improve the drug-like properties of insulin and GLP-1 and to highlight the continued advancement of drug-device combination products to improve diabetes management.

Pharmacology and therapeutic implications of current drugs for type 2 diabetes mellitus

Type 2 diabetes mellitus (T2DM) is a global epidemic that poses a major challenge to health-care systems. Improving metabolic control to approach normal glycaemia (where practical) greatly benefits long-term prognoses and justifies early, effective, sustained and safety-conscious intervention. Improvements in the understanding of the complex pathogenesis of T2DM have underpinned the development of glucose-lowering therapies with complementary mechanisms of action, which have expanded treatment options and facilitated individualized management strategies. Over the past decade, several new classes of glucose-lowering agents have been licensed, including glucagon-like peptide 1 receptor (GLP-1R) agonists, dipeptidyl peptidase 4 (DPP-4) inhibitors and sodium/glucose cotransporter 2 (SGLT2) inhibitors. These agents can be used individually or in combination with well-established treatments such as biguanides, sulfonylureas and thiazolidinediones. Although novel agents have potential advantages including low risk of hypoglycaemia and help with weight control, long-term safety has yet to be established. In this Review, we assess the pharmacokinetics, pharmacodynamics and safety profiles, including cardiovascular safety, of currently available therapies for management of hyperglycaemia in patients with T2DM within the context of disease pathogenesis and natural history. In addition, we briefly describe treatment algorithms for patients with T2DM and lessons from present therapies to inform the development of future therapies.

Naturally-occurring TGR5 agonists modulating glucagon-like peptide-1 biosynthesis and secretion

Selective GLP-1 secretagogues represent a novel potential therapy for type 2 diabetes mellitus. This study examined the GLP-1 secretory activity of the ethnomedicinal plant, Fagonia cretica, which is postulated to possess anti-diabetic activity. After extraction and fractionation extracts and purified compounds were tested for GLP-1 and GIP secretory activity in pGIP/neo STC-1 cells. Intracellular levels of incretin hormones and their gene expression were also determined. Crude F. cretica extracts stimulated both GLP-1 and GIP secretion, increased cellular hormone content, and upregulated gene expression of proglucagon, GIP and prohormone convertase. However, ethyl acetate partitioning significantly enriched GLP-1 secretory activity and this fraction underwent bioactivity-guided fractionation. Three isolated compounds were potent and selective GLP-1 secretagogues: quinovic acid (QA) and two QA derivatives, QA-3β-O-β-D-glycopyranoside and QA-3β-O-β-D-glucopyranosyl-(28→1)-β-D-glucopyranosyl ester. All QA compounds activated the TGR5 receptor and increased intracellular incretin levels and gene expression. QA derivatives were more potent GLP-1 secretagogues than QA. This is the first time that QA and its naturally-occurring derivatives have been shown to activate TGR5 and stimulate GLP-1 secretion. These data provide a plausible mechanism for the ethnomedicinal use of F. cretica and may assist in the ongoing development of selective GLP-1 agonists.

Pharmacological characterization and antidiabetic activity of a long-acting glucagon-like peptide-1 analogue conjugated to an antithrombin III-binding pentasaccharide

AIMS

To examine the biological characteristics of a novel glucagon-like peptide-1 (GLP-1) conjugate, in which an antithrombin III (ATIII)-binding pentasaccharide is conjugated to d-Ala(8) GLP-1 using a tetraethylene glycol linker.

METHODS

We assessed GLP-1 receptor binding, cAMP generation and insulin secretory activity of the GLP-1 conjugate in vitro. Circulating half-life, glucose homeostatic and subchronic therapeutic effectiveness were then examined in vivo.

RESULTS

The half-life of the GLP-1 conjugate in mice was ∼11 h. In vitro insulin secretion from clonal β cells and islets was increased (p < 0.001) by the conjugate. The conjugate had half maximum effective concentration values of 1.3 × 10(-7) and 9.9 × 10(-8)  M for displacement of (125) I-GLP-1 in competitive GLP-1 receptor binding and cAMP generation, respectively. Glucose tolerance in normal mice, immediately and 4 h after conjugate injection, resulted in significant (p < 0.001) improvements in blood glucose. These effects persisted for >48 h after administration. Daily treatment (21 days) of high-fat-fed and ob/ob mice with 25 nmol/kg conjugate resulted in significant improvement in glucose tolerance (p < 0.001) and reductions in glycated haemoglobin (HbA1c; p < 0.01) equivalent to or better than with exenatide or liraglutide. Treatment of C57BL/KsJ db/db mice for 15 days with 100 nmol/kg conjugate significantly (p < 0.001) reduced glucose and raised plasma insulin. Oral glucose tolerance was significantly (p < 0.001) improved and both 24-h glucose profile (p < 0.001) and HbA1c levels (p < 0.001) were reduced. Islet size (p < 0.001) and pancreatic insulin content were increased without change of islet cell proliferation or apoptosis.

CONCLUSION

These data show that d-Ala(8) GLP-1(Lys(37) ) pentasaccharide exerts significant antidiabetic actions and has a projected pharmacokinetic/pharmacodynamic profile that merits further evaluation in humans for a possible once-weekly dosing regimen.

Gut-brain connection: The neuroprotective effects of the anti-diabetic drug liraglutide

Long-acting glucagon-like peptide-1 (GLP-1) analogues marketed for type 2 diabetes (T2D) treatment have been showing positive and protective effects in several different tissues, including pancreas, heart or even brain. This gut secreted hormone plays a potent insulinotropic activity and an important role in maintaining glucose homeostasis. Furthermore, growing evidences suggest the occurrence of several commonalities between T2D and neurodegenerative diseases, insulin resistance being pointed as a main cause for cognitive decline and increased risk to develop dementia. In this regard, it has also been suggested that stimulation of brain insulin signaling may have a protective role against cognitive deficits. As GLP-1 receptors (GLP-1R) are expressed throughout the central nervous system and GLP-1 may cross the blood-brain-barrier, an emerging hypothesis suggests that they may be promising therapeutic targets against brain dysfunctional insulin signaling-related pathologies. Importantly, GLP-1 actions depend not only on the direct effect mediated by its receptor activation, but also on the gut-brain axis involving an exchange of signals between both tissues via the vagal nerve, thereby regulating numerous physiological functions (e.g., energy homeostasis, glucose-dependent insulin secretion, as well as appetite and weight control). Amongst the incretin/GLP-1 mimetics class of anti-T2D drugs with an increasingly described neuroprotective potential, the already marketed liraglutide emerged as a GLP-1R agonist highly resistant to dipeptidyl peptidase-4 degradation (thereby having an increased half-life) and whose systemic GLP-1R activity is comparable to that of native GLP-1. Importantly, several preclinical studies showed anti-apoptotic, anti-inflammatory, anti-oxidant and neuroprotective effects of liraglutide against T2D, stroke and Alzheimer disease (AD), whereas several clinical trials, demonstrated some surprising benefits of liraglutide on weight loss, microglia inhibition, behavior and cognition, and in AD biomarkers. Herein, we discuss the GLP-1 action through the gut-brain axis, the hormone’s regulation of some autonomic functions and liraglutide’s neuroprotective potential.

Whey proteins have beneficial effects on intestinal enteroendocrine cells stimulating cell growth and increasing the production and secretion of incretin hormones

Whey protein has been indicated to curb diet-induced obesity, glucose intolerance and delay the onset of type 2 diabetes mellitus. Here the effects of intact crude whey, intact individual whey proteins and beta-lactoglobulin hydrolysates on an enteroendocrine (EE) cell model were examined. STC-1 pGIP/neo cells were incubated with several concentrations of yogurt whey (YW), cheese whey (CW), beta-lactoglobulin (BLG), alpha-lactalbumin (ALA) and bovine serum albumin (BSA). The findings demonstrate that BLG stimulates EE cell proliferation, and also GLP-1 secretion (an effect which is lost following hydrolysis with chymotrypsin or trypsin). ALA is a highly potent GLP-1 secretagogue which also increases the intracellular levels of GLP-1. Conversely, whey proteins and hydrolysates had little impact on GIP secretion. This appears to be the first investigation of the effects of the three major proteins of YW and CW on EE cells. The anti-diabetic potential of whey proteins should be further investigated.

Role of endogenous GLP-1 and GIP in beta cell compensatory responses to insulin resistance and cellular stress

Role of GLP-1 and GIP in beta cell compensatory responses to beta cell attack and insulin resistance were examined in C57BL/6 mice lacking functional receptors for GLP-1 and GIP. Mice were treated with multiple low dose streptozotocin or hydrocortisone. Islet parameters were assessed by immunohistochemistry and hormone measurements were determined by specific enzyme linked immunoassays. Wild-type streptozotocin controls exhibited severe diabetes, irregularly shaped islets with lymphocytic infiltration, decreased Ki67/TUNEL ratio with decreased beta cell and increased alpha cell areas. GLP-1 and GIP were co-expressed with glucagon and numbers of alpha cells mainly expressing GLP-1 were increased. In contrast, hydrocortisone treatment and induction of insulin resistance increased islet numbers and area, with enhanced beta cell replication, elevated mass of beta and alpha cells, together with co-expression of GLP-1 and GIP with glucagon in islets. The metabolic responses to streptozotocin in GLP-1RKO and GIPRKO mice were broadly similar to C57BL/6 controls, although decreases in islet numbers and size were more severe. In contrast, both groups of mice lacking functional incretin receptors displayed substantially impaired islet adaptations to insulin resistance induced by hydrocortisone, including marked curtailment of expansion of islet area, beta cell mass and islet number. Our observations cannot be explained by simple changes in circulating incretin concentrations, suggesting that intra-islet GLP-1 and GIP make a significant contribution to islet adaptation, particularly expansion of beta cell mass and compensatory islet compensation to hydrocortisone and insulin resistance.

The dipeptidyl peptidase IV inhibitors vildagliptin and K-579 inhibit a phospholipase C: a case of promiscuous scaffolds in proteins

The long term side effects of any newly introduced drug is a subject of intense research, and often raging controversies. One such example is the dipeptidyl peptidase-IV (DPP4) inhibitor used for treating type 2 diabetes, which is inconclusively implicated in increased susceptibility to acute pancreatitis. Previously, based on a computational analysis of the spatial and electrostatic properties of active site residues, we have demonstrated that phosphoinositide-specific phospholipase C (PI-PLC) from Bacillus cereus is a prolyl peptidase using in vivo experiments. In the current work, we first report the inhibition of the native activity of PI-PLC by two DPP4 inhibitors - vildagliptin (LAF-237) and K-579. While vildagliptin inhibited PI-PLC at micromolar concentrations, K-579 was a potent inhibitor even at nanomolar concentrations. Subsequently, we queried a comprehensive, non-redundant set of 5000 human proteins (50% similarity cutoff) with known structures using serine protease (SPASE) motifs derived from trypsin and DPP4. A pancreatic lipase and a gastric lipase are among the proteins that are identified as proteins having promiscuous SPASE scaffolds that could interact with DPP4 inhibitors. The presence of such scaffolds in human lipases is expected since they share the same catalytic mechanism with PI-PLC. However our methodology also detects other proteins, often with a completely different enzymatic mechanism, that have significantly congruent domains with the SPASE motifs. The reported elevated levels of serum lipase, although contested, could be rationalized by inhibition of lipases reported here. In an effort to further our understanding of the spatial and electrostatic basis of DPP4 inhibitors, we have also done a comprehensive analysis of all 76 known DPP4 structures liganded to inhibitors till date. Also, the methodology presented here can be easily adopted for other drugs, and provide the first line of filtering in the identification of pathways that might be inadvertently affected due to promiscuous scaffolds in proteins.

The dipeptidyl peptidase IV inhibitors vildagliptin and K-579 inhibit a phospholipase C: a case of promiscuous scaffolds in proteins

The long term side effects of any newly introduced drug is a subject of intense research, and often raging controversies. One such example is the dipeptidyl peptidase-IV (DPP4) inhibitor used for treating type 2 diabetes, which is inconclusively implicated in increased susceptibility to acute pancreatitis. Previously, based on a computational analysis of the spatial and electrostatic properties of active site residues, we have demonstrated that phosphoinositide-specific phospholipase C (PI-PLC) from Bacillus cereus is a prolyl peptidase using in vivo experiments. In the current work, we first report the inhibition of the native activity of PI-PLC by two DPP4 inhibitors - vildagliptin (LAF-237) and K-579. While vildagliptin inhibited PI-PLC at micromolar concentrations, K-579 was a potent inhibitor even at nanomolar concentrations. Subsequently, we queried a comprehensive, non-redundant set of 5000 human proteins (50% similarity cutoff) with known structures using serine protease (SPASE) motifs derived from trypsin and DPP4. A pancreatic lipase and a gastric lipase are among the proteins that are identified as proteins having promiscuous SPASE scaffolds that could interact with DPP4 inhibitors. The presence of such scaffolds in human lipases is expected since they share the same catalytic mechanism with PI-PLC. However our methodology also detects other proteins, often with a completely different enzymatic mechanism, that have significantly congruent domains with the SPASE motifs. The reported elevated levels of serum lipase, although contested, could be rationalized by inhibition of lipases reported here. In an effort to further our understanding of the spatial and electrostatic basis of DPP4 inhibitors, we have also done a comprehensive analysis of all 76 known DPP4 structures liganded to inhibitors till date. Also, the methodology presented here can be easily adopted for other drugs, and provide the first line of filtering in the identification of pathways that might be inadvertently affected due to promiscuous scaffolds in proteins.

Characterisation of structurally modified analogues of glucagon as potential glucagon receptor antagonists

Acute in vitro and in vivo biological activities of four novel structural analogues of glucagon were tested. desHis(1)Pro(4)-glucagon, desHis(1)Pro(4)Glu(9)-glucagon, desHis(1)Pro(4)Glu(9)Lys(12)FA-glucagon and desHis(1)Pro(4)Glu(9)Lys(30)FA-glucagon were stable to DPP-4 degradation and dose-dependently inhibited glucagon-mediated cAMP production (p<0.05 to p<0.001). None stimulated insulin secretion in vitro above basal levels, but all inhibited glucagon-induced insulin secretion (p<0.01 to p<0.001). In normal mice all analogues antagonised acute glucagon-mediated elevations of blood glucose (p<0.05 to p<0.001) and blocked corresponding insulinotropic responses. In high-fat fed mice, glucagon-induced increases in plasma insulin (p<0.05 to p<0.001) and glucagon-induced hyperglycaemia were blocked (p<0.05 to p<0.01) by three analogues. In obese diabetic (ob/ob) mice only desHis(1)Pro(4)Glu(9)-glucagon effectively (p<0.05 to p<0.01) inhibited both glucagon-mediated glycaemic and insulinotropic responses. desHis(1)Pro(4)-glucagon and desHis(1)Pro(4)Glu(9)-glucagon were biologically ineffective when administered 8h prior to glucagon, whereas desHis(1)Pro(4)Glu(9)Lys(12)FA-glucagon retained efficacy (p<0.01) for up to 24h. Such peptide-derived glucagon receptor antagonists have potential for type 2 diabetes therapy.

Optimizing glycemic control: lixisenatide and basal insulin in combination therapy for the treatment of Type 2 diabetes mellitus

Despite availability of new treatments for patients with Type 2 diabetes mellitus (T2DM), optimal management of glycemic control remains challenging. Treatment with basal insulin can improve HbA1c, but may not be sufficient to control postprandial plasma glucose (PPG) levels. Both fasting plasma glucose (FPG) and PPG levels contribute to overall glycemic control. In patients with moderate hyperglycemia, PPG excursions have a greater contribution to overall hyperglycemia, with this contribution being greatest when HbA1c is approximately 7-8% [1] . Glucagon-like peptide-1 receptor agonists (GLP-1RAs) have been designed to restore and maintain GLP-1 levels and attenuate PPG excursions. GLP-1RAs that predominantly affect PPG may complement the FPG lowering provided by basal insulin, possibly improving overall glycemic control without additional weight gain and with limited incidence of hypoglycemia. Lixisenatide as an add-on to basal insulin lowers PPG levels, improves HbA1c control and has a beneficial effect on weight in T2DM patients.

The functional state of hormone-sensitive adenylyl cyclase signaling system in diabetes mellitus

Diabetes mellitus (DM) induces a large number of diseases of the nervous, cardiovascular, and some other systems of the organism. One of the main causes of the diseases is the changes in the functional activity of hormonal signaling systems which lead to the alterations and abnormalities of the cellular processes and contribute to triggering and developing many DM complications. The key role in the control of physiological and biochemical processes belongs to the adenylyl cyclase (AC) signaling system, sensitive to biogenic amines and polypeptide hormones. The review is devoted to the changes in the GPCR-G protein-AC system in the brain, heart, skeletal muscles, liver, and the adipose tissue in experimental and human DM of the types 1 and 2 and also to the role of the changes in AC signaling in the pathogenesis and etiology of DM and its complications. It is shown that the changes of the functional state of hormone-sensitive AC system are dependent to a large extent on the type and duration of DM and in experimental DM on the model of the disease. The degree of alterations and abnormalities of AC signaling pathways correlates very well with the severity of DM and its complications.

Recent advances in understanding GLP-1R (glucagon-like peptide-1 receptor) function. Biochem Soc Trans. 2013;41:172-179. [PMID: 23356279 DOI: 10.1042/bst20120236]

Type 2 diabetes is a major global health problem and there is ongoing research for new treatments to manage the disease. The GLP-1R (glucagon-like peptide-1 receptor) controls the physiological response to the incretin peptide, GLP-1, and is currently a major target for the development of therapeutics owing to the broad range of potential beneficial effects in Type 2 diabetes. These include promotion of glucose-dependent insulin secretion, increased insulin biosynthesis, preservation of β-cell mass, improved peripheral insulin sensitivity and promotion of weight loss. Despite this, our understanding of GLP-1R function is still limited, with the desired spectrum of GLP-1R-mediated signalling yet to be determined. We review the current understanding of GLP-1R function, in particular, highlighting recent contributions in the field on allosteric modulation, probe-dependence and ligand-directed signal bias and how these behaviours may influence future drug development.

desHis¹Glu⁹-glucagon-[mPEG] and desHis¹Glu⁹(Lys³⁰PAL)-glucagon: long-acting peptide-based PEGylated and acylated glucagon receptor antagonists with potential antidiabetic activity

Glucagon is hormone secreted from the pancreatic alpha-cells that is involved in blood glucose regulation. As such, antagonism of glucagon receptor signalling represents an exciting approach for treating diabetes. To harness these beneficial metabolic effects, two novel glucagon analogues, desHis¹Glu⁹-glucagon-[mPEG] and desHis¹Glu⁹(Lys³⁰PAL)-glucagon, has been evaluated for potential glucagon receptor antagonistic properties. Both novel peptides were completely resistant to enzymatic breakdown and significantly (P<0.05 to P<0.001) inhibited glucagon-mediated elevations of cAMP production in glucagon receptor transfected cells. Similarly, desHis¹Glu⁹-glucagon-[mPEG] and desHis¹Glu⁹(Lys³⁰PAL)-glucagon effectively antagonised glucagon-induced increases of insulin secretion from BRIN BD11 cells. When administered acutely to normal, high fat fed or ob/ob mice, both analogues had no significant effects on overall blood glucose or plasma insulin levels when compared to saline treated controls. However, desHis¹Glu⁹-glucagon-[mPEG] significantly (P<0.05) annulled glucagon-induced increases in blood glucose and plasma insulin levels in normal mice and had similar non-significant tendencies in high fat and ob/ob mice. In addition, desHis¹Glu⁹(Lys³⁰PAL)-glucagon effectively (P<0.05 to P<0.001) antagonised glucagon-mediated elevations of blood glucose levels in high fat fed and ob/ob mice, but was less efficacious in normal mice. Further studies confirmed the significant persistent glucagon receptor antagonistic properties of both novel enzyme-resistant analogues 4h post administration in normal mice. These studies emphasise the potential of longer-acting peptide-based glucagon receptor antagonists, and particularly acylated versions, for the treatment of diabetes.

Potential role of glucagon-like peptide-1 (GLP-1) in neuroprotection

The current understanding of neurodegenerative processes in sporadic diseases such as Alzheimer's disease (AD), Parkinson's disease (PD) or multiple sclerosis is very limited. Several risk factors have been identified that may shed light on the underlying mechanisms that initiate the neurodegeneration. Type 2 diabetes mellitus has been identified as a risk factor for AD and PD. In AD patients, desensitization of insulin receptors in the brain has been shown, even in non-diabetic patients. Insulin acts as a growth factor in the brain and supports neuronal repair, dendritic sprouting and synaptogenesis, and protection from oxidative stress. Importantly, several drugs have been developed to treat type 2 diabetes that re-sensitize insulin receptors and may be of use to prevent neurodegenerative processes. Glucagon-like peptide-1 (GLP-1) is a hormone that facilitates insulin release under high blood sugar conditions. Interestingly, GLP-1 also has very similar growth factor-like properties to insulin, and has been shown to reduce a range of degenerative processes. In pre-clinical studies, GLP-1 and longer-lasting protease-resistant analogues cross the blood-brain barrier, protect memory formation (AD) or motor activity (PD), protect synapses and synaptic functions, enhance neurogenesis, reduce apoptosis, protect neurons from oxidative stress, and reduce plaque formation and the chronic inflammation response in the brains of mouse models of AD, PD, amyotrophic lateral sclerosis, stroke and other degenerative diseases. GLP-1 signalling does not affect blood sugar levels in non-diabetic people and therapies that affect GLP-1 signalling have a good safety profile as shown by the chronic application of drugs currently on the market (liraglutide, Victoza(®); NovoNordisk, Copenhagen, Denmark, and exendin-4, Byetta(®); Amylin, San Diego, CA, USA). Based on the extensive evidence, several clinical trials are currently underway, testing liraglutide and exendin-4 in AD and PD patients. Therefore, GLP-1 analogues show great promise as a novel treatment for AD or other neurodegenerative conditions.

The neuroprotective effects of GLP-1: possible treatments for cognitive deficits in individuals with mood disorders

Incretins are a group of gastrointestinal hormones detected both peripherally and in the central nervous system (CNS). Recent studies have documented multiple effects of incretins on brain structure and function. Research into the neurological effects of incretins has primarily focused on animal models of neurodegenerative disorders (e.g., Alzheimer's Disease, Huntington's and Parkinson's diseases). Mood disorders (e.g. bipolar disorder (BD), major depressive disorder (MDD)) are associated with similar alterations in brain structure and function, as well as a range of cognitive deficits (e.g. memory, learning, executive function). Brain abnormalities and cognitive deficits are also found in populations with metabolic disorders (e.g., diabetes mellitus Type 2). In addition, individuals with mood disorders often have co-morbid metabolic conditions, thus treatment strategies which can effectively treat both cognitive deficits and metabolic abnormalities represent a possible integrated treatment avenue. In particular, glucagon-like peptide-1 (GLP-1) and its more stable, longer-lasting analogues have been demonstrated to exert neuroprotective and anti-apoptotic effects, reduce beta-amyloid (Aβ) plaque accumulation, modulate long-term potentiation and synaptic plasticity, and promote differentiation of neuronal progenitor cells. In animal models of behaviour, treatment with GLP-1 receptor agonists has been demonstrated to improve measures of cognitive function including learning and memory, as well as reduce depressive behaviour. Available GLP-1 treatments also have a favourable metabolic profile which includes weight loss and reduced risk for hypoglycemia. Systematic evaluation of the effects of GLP-1 treatment in psychiatric populations who evince cognitive deficits represents a promising treatment avenue.

Evaluation of insulin expression and secretion in genetically engineered gut K and L-cells

Background

Gene therapy could provide an effective treatment of diabetes. Previous studies have investigated the potential for several cell and tissue types to produce mature and active insulin. Gut K and L-cells could be potential candidate hosts for gene therapy because of their special features.

Results

In this study, we isolated gut K and L-cells to compare the potential of both cell types to produce insulin when exposed to similar conditions. The isolated pure K and L-cells were transfected with recombinant plasmids encoding insulin and with specific promoters for K or L-cells. Insulin expression was studied in response to glucose or meat hydrolysate. We found that glucose and meat hydrolysate efficiently induced insulin secretion from K and L-cells. However, the effects of meat hydrolysate on insulin secretion were more potent in both cells compared with glucose. Results of enzyme-linked immunosorbent assays showed that L-cells secreted more insulin compared with K-cells regardless of the stimulator, although this difference was not statistically significant.

Conclusion

The responses of K and L-cells to stimulation with glucose or meat hydrolysate were generally comparable. Therefore, both K and L-cells show similar potential to be used as surrogate cells for insulin gene expression in vitro. The potential use of these cells for diabetic gene therapy warrants further investigation.

Effects of duodenal-jejunal exclusion on beta cell function and hormonal regulation in Goto-Kakizaki rats

BACKGROUND

The aim of our work was to investigate the hormones that control glycemic status and in vitro β-cell function in diabetes mellitus after a duodenal-jejunal exclusion in Goto-Kakizaki rats (Taconic, Denmark).

METHODS

Twenty-three rats (age, 12-14 wk) were randomized as follows: group 1 (n = 14), no intervention (control); or group 2 (n = 9), duodenal-jejunal exclusion.

RESULTS

In group 2, levels of glucagon and leptin were lower than in group 1 at 1 week and at 8 weeks. Glucagon-like peptide 1 levels had a significant increase at 8 weeks from basal value in group 2 and this value was higher than in group 1. The insulin secretion at 60 minutes in group 2 was higher than in group 1 (group 1, 12.9 ± 12.0 μg/L vs group 2, 41.9 ± 36.3 μg/L; P < .05). Messenger RNA (mRNA) expression of insulin at 2 months was higher in the rat pancreas of the experimental group than in the control group (group 1, .99 ± .48 mRNA amount vs group 2, 1.66 ± .33 mRNA amount; P < .05).

CONCLUSIONS

Gastrojejunal bypass in this model improves glucose ratios, with a significant increase of glucagon-like peptide 1 and decrease of homeostasis model assessment, glucagon, and leptin levels after surgery. This type of surgery improves mRNA insulin expression in pancreatic islets and insulin secretion as well.

Liraglutide: clinical pharmacology and considerations for therapy

Liraglutide is a United States Food and Drug Administration (FDA)-approved glucagon-like peptide-1 (GLP-1) analog that is 97% homologous to native human GLP-1. The additional 16-carbon fatty acid chain causes noncovalent binding to albumin, which slows absorption from the injection site and protects the molecule from degradation by the enzyme dipeptidyl peptidase-4, allowing for protraction of action. Albumin binding and an elimination half-life of 13 hours combine to allow for once-daily dosing. Liraglutide 1.2 and 1.8 mg/day given as monotherapy for up to 52 weeks produced mean reductions in hemoglobin A1c (A1C) of 0.6-1.6%; combination therapy of liraglutide with oral antidiabetic agents demonstrated mean A1C reductions up to 1.5%. The satiety effect of GLP-1 receptor agonists and documented weight loss as great as 3.38 kg in clinical trials may make liraglutide ideal for obese patients with type 2 diabetes mellitus. Like other incretin-based agents, preliminary studies suggest liraglutide may also increase β-cell mass and function. Hypoglycemia is rare with liraglutide and tends to occur when used in combination with sulfonylureas; liraglutide in combination with insulin is not yet FDA approved. The pharmacokinetic parameters of liraglutide are unaffected by age, sex, race, or ethnicity, and no special recommendations for altered dosing of liraglutide need apply to populations with hepatic or renal impairment. Results from clinical trials have not shown an increased risk of medullary thyroid cancer, pancreatitis, or poor cardiovascular outcomes with liraglutide treatment. Ongoing, long-term monitoring studies continue to evaluate the safety of liraglutide treatment in these outcomes.

Tigerinin-1R: a potent, non-toxic insulin-releasing peptide isolated from the skin of the Asian frog, Hoplobatrachus rugulosus

AIM

Characterization of peptides in the skin of the Vietnamese common lowland frog Hoplobatrachus rugulosus with the ability to stimulate insulin release in vitro and improve glucose tolerance in vivo.

METHODS

Peptides in an extract of skin were purified by reversed-phase HPLC, and their abilities to stimulate the release of insulin and the cytosolic enzyme lactate dehydrogenase were determined using BRIN-BD11 clonal β cells. Insulin-releasing potencies of synthetic peptides and their effects on membrane potential and intracellular Ca²⁺ concentration were also measured using BRIN-BD11 cells. Effects on glucose tolerance and insulin release in vivo were determined in mice fed a high-fat diet to induce obesity and insulin resistance.

RESULTS

A cyclic dodecapeptide (RVCSAIPLPICH.NH₂), termed tigerinin-1R, was isolated from the skin extract that lacked short-term cytotoxic and haemolytic activity but significantly (p < 0.01) stimulated the rate of release of insulin from BRIN-BD11 cells at concentrations ≥ 0.1 nM. The maximum response was 405% of the basal rate at 5.6 mM ambient glucose concentration and 290% of basal rate at 16.7 mM glucose. C-terminal α-amidation was necessary for high potency and a possible mechanism of action of the peptide-involved membrane depolarization and an increase in intracellular Ca²⁺ concentration. Administration of tigerinin-1R (75 nmol/kg body weight) to high fat-fed mice significantly (p < 0.05) enhanced insulin release and improved glucose tolerance during the 60-min period following an intraperitoneal glucose load.

CONCLUSION

Tigerinin-1R is a potent, non-toxic insulin-releasing peptide that shows potential for development into an agent for the treatment of type 2 diabetes.

Medicinal Chemistry and Applications of Incretins and DPP-4 Inhibitors in the Treatment of Type 2 Diabetes Mellitus

Diabetes mellitus (DM) is a major metabolic disorder currently affecting over 200 million people worldwide. Approximately 90% of all diabetic patients suffer from Type 2 diabetes mellitus (T2DM). The world's economy coughs out billions of dollars annually to diagnose, treat and manage patients with diabetes. It has been shown that the naturally occurring gut hormones incretins, glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) can preserve the morphology and function of pancreatic beta cell. In addition, GIP and GLP-1 act on insulin receptors to facilitate insulin-receptor binding, resulting in optimal glucose metabolism. This review examines the medicinal chemistry and roles of incretins, specifically, GLP-1 and drugs which can mimic its actions and prevent its enzymatic degradation. The review discussed GLP-1 agonists such as exenatide, liraglutide, taspoglutide and albiglutide. The paper also identified and reviewed a number of inhibitors, which can block dipeptidyl peptidase 4 (DPP-4), the enzyme responsible for the rapid degradation of GLP-1. These DPP-4 inhibitors include sitagliptin, saxagliptin, vildagliptin and many others which are still in the experimental phase.

Beyond glucose lowering: glucagon-like peptide-1 receptor agonists, body weight and the cardiovascular system

AIM

Glucagon-like peptide-1 (GLP-1) belongs to the incretin hormone family: in the presence of elevated blood glucose, it stimulates insulin secretion and inhibits glucagon production. In addition, GLP-1 slows gastric emptying. GLP-1 secretion has also been reported to potentially affect patients with type 2 diabetes (T2DM) compared with non-diabetics and, as enzymatic inactivation by dipeptidyl peptidase-4 (DPP-4) shortens the GLP-1 half-life to a few minutes, GLP-1 receptor agonists such as exenatide twice daily (BID) and liraglutide have been developed, and have become part of the management of patients with T2DM. This review focuses on the potential beneficial effects of these compounds beyond those associated with improvements in blood glucose control and weight loss, including changes in the cardiovascular and central nervous systems.

METHODS

This was a state-of-the-art review of the literature to evaluate the relationships between GLP-1, GLP-1 receptor agonists, weight and the cardiovascular system.

RESULTS

GLP-1 receptor agonists improve glucose control and do not significantly increase the risk of hypoglycaemia. Also, this new class of antidiabetic drugs was shown to favour weight loss. Mechanisms may involve central action, direct action by reduction of food intake and probably indirect action through slowing of gastric emptying. The relative importance of each activity remains unclear. Weight loss may improve cardiovascular outcomes in patients with T2DM, although GLP-1 receptor agonists may have other direct and indirect effects on the cardiovascular system. Reductions in myocardial infarct size and improvements in cardiac function have been seen in animal models. Beneficial changes in cardiac function were also demonstrated in patients with myocardial infarcts or heart failure. Indirect effects could involve a reduction in blood pressure and potential effects on oxidation. However, the mechanisms involved in the pleiotropic effects of GLP-1 receptor agonists have yet to be completely elucidated and require further study.

CONCLUSION

These compounds may play an important role in the treatment of patients with T2DM as their potential effects go beyond glucose-lowering (weight loss, potential improvement of cardiovascular risk factors). However, to better understand their place in the management of T2DM, further experimental and clinical prospective studies are required.

Management of type 2 diabetes: new and future developments in treatment

The increasing prevalence, variable pathogenesis, progressive natural history, and complications of type 2 diabetes emphasise the urgent need for new treatment strategies. Longacting (eg, once weekly) agonists of the glucagon-like-peptide-1 receptor are advanced in development, and they improve prandial insulin secretion, reduce excess glucagon production, and promote satiety. Trials of inhibitors of dipeptidyl peptidase 4, which enhance the effect of endogenous incretin hormones, are also nearing completion. Novel approaches to glycaemic regulation include use of inhibitors of the sodium-glucose cotransporter 2, which increase renal glucose elimination, and inhibitors of 11β-hydroxysteroid dehydrogenase 1, which reduce the glucocorticoid effects in liver and fat. Insulin-releasing glucokinase activators and pancreatic-G-protein-coupled fatty-acid-receptor agonists, glucagon-receptor antagonists, and metabolic inhibitors of hepatic glucose output are being assessed. Early proof of principle has been shown for compounds that enhance and partly mimic insulin action and replicate some effects of bariatric surgery.

Advances in the treatment of type 2 diabetes mellitus

There is a rising worldwide prevalence of diabetes, especially type 2 diabetes mellitus (T2DM), which is one of the most challenging health problems in the 21st century. The associated complications of diabetes, such as cardiovascular disease, peripheral vascular disease, stroke, diabetic neuropathy, amputations, renal failure, and blindness result in increasing disability, reduced life expectancy, and enormous health costs. T2DM is a polygenic disease characterized by multiple defects in insulin action in tissues and defects in pancreatic insulin secretion, which eventually leads to loss of pancreatic insulin-secreting cells. The treatment goals for T2DM patients are effective control of blood glucose, blood pressure, and lipids (if elevated) and, ultimately, to avert the serious complications associated with sustained tissue exposure to excessively high glucose concentrations. Prevention and control of diabetes with diet, weight control, and physical activity has been difficult. Treatment of T2DM has centered on increasing insulin levels, either by direct insulin administration or oral agents that promote insulin secretion, improving sensitivity to insulin in tissues, or reducing the rate of carbohydrate absorption from the gastrointestinal tract. This review presents comprehensive and up-to-date information on the mechanism(s) of action, efficacy, pharmacokinetics, pleiotropic effects, drug interactions, and adverse effects of the newer antidiabetic drugs, including (1) peroxisome proliferator-activated-receptor-γ agonists (thiazolidinediones, pioglitazone, and rosiglitazone); (2) the incretin, glucagon-like peptide-) receptor agonists (incretin-mimetics, exenatide. and liraglutide), (3) inhibitors of dipeptidyl-peptidase-4 (incretin enhancers, sitagliptin, and vildagliptin), (4) short-acting, nonsulfonylurea secretagogue, meglitinides (repaglinide and nateglinide), (5) amylin anlog-pramlintide, (6) α-glucosidase inhibitors (miglitol and voglibose), and (7) colesevelam (a bile acid sequestrant). In addition, information is presented on drug candidates in clinical trials, experimental compounds, and some plants used in the traditional treatment of diabetes based on experimental evidence. In the opinion of this reviewer, therapy based on orally active incretins and incretin mimetics with long duration of action that will be efficacious, preserve the β-cell number/function, and block the progression of diabetes will be highly desirable. However, major changes in lifestyle factors such as diet and, especially, exercise will also be needed if the growing burden of diabetes is to be contained.

Investigating the effects of physiological bile acids on GLP-1 secretion and glucose tolerance in normal and GLP-1R-/- mice

Physiological secretion of bile acids has previously been linked to the regulation of blood glucose. GLP-1 is an intestinal peptide hormone with important glucose-lowering actions, such as stimulation of insulin secretion and inhibition of glucagon secretion. In this investigation, we assessed the ability of several bile acid compounds to secrete GLP-1 in vitro in STC-1 cells. Bile acids stimulated GLP-1 secretion from 3.3- to 6.2-fold but some were associated with cytolytic effects. Glycocholic and taurocholic acids were selected for in vivo studies in normal and GLP-1R-/- mice. Oral glucose tolerance tests revealed that glycocholic acid did not affect glucose excursions. However, taurocholic acid reduced glucose excursions by 40% in normal mice and by 27% in GLP-1R-/- mice, and plasma GLP-1 concentrations were significantly elevated 30 min post-gavage. Additional studies used incretin receptor antagonists to probe involvement of GLP-1 and GIP in taurocholic acid-induced glucose lowering. The findings suggest that bile acids partially aid glucose regulation by physiologically enhancing nutrient-induced GLP-1 secretion. However, GLP-1 secretion appears to be only part of the glucose-lowering mechanism and our studies indicate that the other major incretin GIP is not involved.

Peptidomic analysis of skin secretions from the bullfrog Lithobates catesbeianus (Ranidae) identifies multiple peptides with potent insulin-releasing activity

Using a combination of reversed-phase HPLC and electrospray mass spectrometry, peptidomic analysis of norepinephrine-stimulated skin secretions of the American bullfrog Lithobates catesbeianus Shaw, 1802 led to the identification and characterization of five newly described peptides (ranatuerin-1CBb, ranatuerin-2CBc, and -CBd, palustrin-2CBa, and temporin-CBf) together with seven peptides previously isolated on the basis of their antimicrobial activity (ranatuerin-1CBa, ranatuerin-2CBa, brevinin-1CBa, and -1CBb, temporin-CBa, -CBb, and -CBd). The abilities of the most abundant of the purified peptides to stimulate the release of insulin from the rat BRIN-BD11 clonal β cell line were evaluated. Ranatuerin-2CBd (GFLDIIKNLGKTFAGHMLDKIRCTIGTCPPSP) was the most potent peptide producing a significant stimulation of insulin release (119% of basal rate, P<0.01) from BRIN-BD11 cells at a concentration of 30nM, with a maximum response (236% of basal rate, P<0.001) at a concentration of 3μM. Ranatuerin-2CBd did not stimulate release of the cytosolic enzyme, lactate dehydrogenase at concentrations up to 3μM, indicating that the integrity of the plasma membrane had been preserved. Brevinin-1CBb (FLPFIARLAAKVFPSIICSVTKKC) produced the maximum stimulation of insulin release (285% of basal rate, P<0.001 at 3μM) but the peptide was cytotoxic at this concentration.

Acute and chronic effects of dietary fatty acids on cholecystokinin expression, storage and secretion in enteroendocrine STC-1 cells

Cholecystokinin (CCK) is a peptide hormone secreted from the I-cells of the intestine and it has important physiological actions related to appetite regulation and satiety. In this study we used STC-1 cells to investigate the effects of common dietary-derived fatty acids (FAs) on I-cell secretory function and metabolism. We extend earlier studies by measuring the acute and chronic effects of 11 FAs on CCK secretion, cellular CCK content, CCK mRNA levels, cellular DNA synthesis, cellular viability and cytotoxicity. FAs were selected in order to assess the importance of chain length, degree of saturation, and double bond position and conformation. The results demonstrate that secretory responses elicited by dietary FAs are highly selective. For example, altering the conformation of a double bond from cis to trans (i.e. oleic acid versus elaidic acid) completely abolishes CCK secretion. Lauric acid appears to adversely affect I-cell metabolism and arachidonic acid suppresses DNA synthesis. Our studies reveal for the first time that conjugated linoleic acid isoforms are particularly potent CCK secretagogues, which also boost intracellular stores of CCK. These actions of conjugated linoleic acid may explain satiating actions observed in dietary intervention studies.

Metabolic and structural properties of human obestatin {1-23} and two fragment peptides

Obestatin is a peptide produced in the oxyntic mucosa of the stomach and co-localizes with ghrelin on the periphery of pancreatic islets. Several studies demonstrate that obestatin reduces food and water intake, decreases body weight gain, inhibits gastrointestinal motility, and modulates glucose-induced insulin secretion. In this study we evaluated the acute metabolic effects of human obestatin {1-23} and fragment peptides {1-10} or {11-23} in high-fat fed mice, and then investigated their solution structure by NMR spectroscopy and molecular modelling. Obestatins {1-23} and {11-23} significantly reduced food intake (86% and 90% respectively) and lowered glucose responses to feeding, whilst leaving insulin responses unchanged. No metabolic changes could be detected following the administration of obestatin {1-10}. In aqueous solution none of the obestatin peptides possessed secondary structural features. However, in a 2,2,2-trifluoroethanol (TFE-d(3))-H(2)O solvent mixture, the structure of obestatin {1-23} was characterized by an alpha-helix followed by a single turn helix conformation between residues Pro(4) and Gln(15) and His(19) and Ala(22) respectively. Obestatin {1-10} showed no structural components whereas {11-23} contained an alpha-helix between residues Val(14) and Ser(20) in a mixed solvent. These studies are the first to elucidate the structure of human obestatin and provide clear evidence that the observed alpha-helical structures are critical for in vivo activity. Future structure/function studies may facilitate the design of novel therapeutic agents based on the obestatin peptide structure.

Clinical studies of liraglutide, a novel, once-daily human glucagon-like peptide-1 analog for improved management of type 2 diabetes mellitus

Liraglutide, a new glucagon-like peptide-1 (GLP-1)-receptor agonist with 97% homology to human GLP-1, can be administered once/day independent of meals in patients with type 2 diabetes mellitus. Clinical trials have demonstrated its efficacy in controlling hyperglycemia, helping patients achieve hemoglobin A(1c) level goals; in facilitating weight loss, and in improving indexes of beta-cell function when used alone or in combination with metformin, glimepiride, or rosiglitazone. These studies also suggest that liraglutide may be associated with modest improvements in systolic blood pressure. Data from a comparative trial of liraglutide and insulin glargine have suggested that liraglutide provides greater glycemic control with less weight gain, and another study demonstrated that liraglutide provides greater improvements in glycemic control with less hypoglycemia than exenatide and with comparable weight loss. Although liraglutide is well tolerated and is associated with low rates of hypoglycemia, transient and mild nausea can occur when therapy is initiated. However, rates of hypoglycemia appear to be lower and nausea appears to be less persistent with liraglutide than with exenatide. Even though data on the long-term use of liraglutide are still needed, this drug may provide a useful treatment option in patients poorly controlled with dietary modification and exercise and in those whose diabetes is inadequately controlled by oral antidiabetic agents.

Differentiating incretin therapies based on structure, activity, and metabolism: focus on liraglutide

The incretin effect, mediated by glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP), plays an important role in the regulation of insulin secretion in response to oral glucose. The discovery of deficiencies in incretin pathways associated with development of type 2 diabetes mellitus has propelled the growth of incretin-based therapies in patients with this disease. The basic rationale for incretin-based therapies, including both GLP-1-receptor agonists and dipeptidyl peptidase-4 (DPP-4) inhibitors is reviewed, focusing on their roles in glucose regulation and potential therapeutic benefits. Increased awareness of the differences among incretin mimetics, GLP-1 analogs, and DPP-4 inhibitors, including their structures, half-lives, dosages, hemoglobin A(1c)-lowering capacities, effects on weight, and adverse events will help shape the future of these therapeutic agents. Improved understanding of the mechanism of action and clinical effects of incretin-based therapies will help advance their appropriate use within clinical practice.

Roux-en-Y gastric bypass promotes expression of PDX-1 and regeneration of β-cells in Goto-Kakizaki rats

AIM: To study the effects of Roux-en-Y gastric bypass (RYGB) on the expression of pancreatic duodenal homeobox-1 (PDX-1) and pancreatic β-cell regeneration/ neogenesis, and their possible mechanisms in diabetics.

METHODS: Three groups of randomly selected non-obese diabetic Goto-Kakizaki (GK) rats were subjected to RYGB, sham-RYGB and sham-operation (sham-op) surgery, respectively. The rats were euthanized at post-operative 1, 2, 4 and 12 wk. Their pancreases were resected and analyzed using reverse transcription polymerase chain reaction to detect the mRNA of PDX-1. Anti-PDX-1 immunohistochemical (IHC) staining and Western blotting were used to detect the protein of PDX-1. Double IHC staining of anti-Brdu and -insulin was performed to detect regenerated β-cells. The index of double Brdu and insulin positive cells was calculated.

RESULTS: In comparison with sham-RYGB and sham-op groups, a significant increase in the expressions of PDX-1 mRNA in RYGB group was observed at all experimental time points (1 wk: 0.378 ± 0.013 vs 0.120 ± 0.010, 0.100 ± 0.010, F = 727.717, P < 0.001; 2 wk: 0.318 ± 0.013 vs 0.110 ± 0.010, 0.143 ± 0.015, F = 301.509, P < 0.001; 4 wk: 0.172 ± 0.011 vs 0.107 ± 0.012, 0.090 ± 0.010, F = 64.297, P < 0.001; 12 wk: 0.140 ± 0.007 vs 0.120 ± 0.010, 0.097 ± 0.015, F = 16.392, P < 0.001); PDX-1 protein in RYGB group was also increased significantly (1 wk: 0.61 ± 0.01 vs 0.21 ± 0.01, 0.15 ± 0.01, F = 3031.127, P < 0.001; 2 wk: 0.55 ± 0.00 vs 0.15 ± 0.01, 0.17 ± 0.01, F = 3426.455, P < 0.001; 4 wk: 0.39 ± 0.01 vs 0.18 ± 0.01, 0.22 ± 0.01, F = 882.909, P < 0.001; 12 wk: 0.41 ± 0.01 vs 0.20 ± 0.01, 0.18 ± 0.01, F = 515.833, P < 0.001). PDX-1 mRNA and PDX-1 protein production showed no statistical significance between the two sham groups. Many PDX-1 positive cells could be found in the pancreatic islets of the rats in RYGB group at all time points. In addition, the percentage of Brdu-insulin double staining positive cells was higher in RYGB group than in the other two groups (1 wk: 0.22 ± 0.13 vs 0.03 ± 0.06, 0.03 ± 0.06, P < 0.05; 2 wk: 0.28 ± 0.08 vs 0.00 ± 0.00, 0.03 ± 0.06, P < 0.05; 4 wk: 0.24 ± 0.11 vs 0.07 ± 0.06, 0.00 ± 0.00, P < 0.001; 12 wk: 0.20 ± 0.07 vs 0.03 ± 0.06, 0.00 ± 0.00, P < 0.05).

CONCLUSION: RYGB can increase the expression of pancreatic PDX-1 and induce the regeneration of β-cells in GK rats. The associated regeneration of islet cells may be a possible mechanism that how RYGB could improve type 2 diabetes mellitus.

Val(8)GLP-1 rescues synaptic plasticity and reduces dense core plaques in APP/PS1 mice

Diabetes is a risk factor for Alzheimer's disease. We tested the effects of Val(8)GLP-1, an enzyme-resistant analogue of the incretin hormone glucagon-like peptide 1 originally developed to treat diabetes in a mouse model of Alzheimer's disease that expresses mutated amyloid precursor protein (APP) and presenilin-1. We tested long term potentiation (LTP) of synaptic plasticity, inflammation response, and plaque formation. Val(8)GLP-1 crosses the blood-brain barrier when administered via intraperitoneal injection. Val(8)GLP-1 protected LTP in 9- and 18-month-old Alzheimer's disease mice when given for 3 weeks at 25 nmol/kg intraperitoneally. LTP was also enhanced in 18-month-old wild type mice, indicating that Val(8)GLP-1 also ameliorates age-related synaptic degenerative processes. Paired-pulse facilitation was also enhanced. The number of beta-amyloid plaques and microglia activation in the cortex increased with age but was not reduced by Val(8)GLP-1. In 18-month-old mice, however, the number of Congo red positive dense-core amyloid plaques was reduced. Treatment with Val(8)GLP-1 might prevent or delay neurodegenerative processes.

A GIP receptor agonist exhibits beta-cell anti-apoptotic actions in rat models of diabetes resulting in improved beta-cell function and glycemic control

AIMS

The gastrointestinal hormone GIP promotes pancreatic islet function and exerts pro-survival actions on cultured beta-cells. However, GIP also promotes lipogenesis, thus potentially restricting its therapeutic use. The current studies evaluated the effects of a truncated GIP analog, D-Ala(2)-GIP(1-30) (D-GIP(1-30)), on glucose homeostasis and beta-cell mass in rat models of diabetes.

MATERIALS AND METHODS

The insulinotropic and pro-survival potency of D-GIP(1-30) was evaluated in perfused pancreas preparations and cultured INS-1 beta-cells, respectively, and receptor selectivity evaluated using wild type and GIP receptor knockout mice. Effects of D-GIP(1-30) on beta-cell function and glucose homeostasis, in vivo, were determined using Lean Zucker rats, obese Vancouver diabetic fatty rats, streptozotocin treated rats, and obese Zucker diabetic fatty rats, with effects on beta-cell mass determined in histological studies of pancreatic tissue. Lipogenic effects of D-GIP(1-30) were evaluated on cultured 3T3-L1 adipocytes.

RESULTS

Acutely, D-GIP(1-30) improved glucose tolerance and insulin secretion. Chronic treatment with D-GIP(1-30) reduced levels of islet pro-apoptotic proteins in Vancouver diabetic fatty rats and preserved beta-cell mass in streptozotocin treated rats and Zucker diabetic fatty rats, resulting in improved insulin responses and glycemic control in each animal model, with no change in body weight. In in vitro studies, D-GIP(1-30) exhibited equivalent potency to GIP(1-42) on beta-cell function and survival, but greatly reduced action on lipoprotein lipase activity in 3T3-L1 adipocytes.

CONCLUSIONS

These findings demonstrate that truncated forms of GIP exhibit potent anti-diabetic actions, without pro-obesity effects, and that the C-terminus contributes to the lipogenic actions of GIP.