Molecular mechanisms of incretin hormone secretion

In Silico Screening of Therapeutic Targets as a Tool to Optimize the Development of Drugs and Nutraceuticals in the Treatment of Diabetes mellitus: A Systematic Review

The Target-Based Virtual Screening approach is widely employed in drug development, with docking or molecular dynamics techniques commonly utilized for this purpose. This systematic review (SR) aimed to identify in silico therapeutic targets for treating Diabetes mellitus (DM) and answer the question: What therapeutic targets have been used in in silico analyses for the treatment of DM? The SR was developed following the guidelines of the Preferred Reporting Items Checklist for Systematic Review and Meta-Analysis, in accordance with the protocol registered in PROSPERO (CRD42022353808). Studies that met the PECo strategy (Problem, Exposure, Context) were included using the following databases: Medline (PubMed), Web of Science, Scopus, Embase, ScienceDirect, and Virtual Health Library. A total of 20 articles were included, which not only identified therapeutic targets in silico but also conducted in vivo analyses to validate the obtained results. The therapeutic targets most frequently indicated in in silico studies were GLUT4, DPP-IV, and PPARγ. In conclusion, a diversity of targets for the treatment of DM was verified through both in silico and in vivo reassessment. This contributes to the discovery of potential new allies for the treatment of DM.

Potassium voltage-gated channel subfamily H member 2 (KCNH2) is a promising target for incretin secretagogue therapies

Derived from enteroendocrine cells (EECs), glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic peptide (GIP) are pivotal incretin hormones crucial for blood glucose regulation. Medications of GLP-1 analogs and GLP-1 receptor activators are extensively used in the treatment of type 2 diabetes (T2D) and obesity. However, there are currently no agents to stimulate endogenous incretin secretion. Here, we find the pivotal role of KCNH2 potassium channels in the regulation of incretin secretion. Co-localization of KCNH2 with incretin-secreting EECs in the intestinal epithelium of rodents highlights its significance. Gut epithelial cell-specific KCNH2 knockout in mice improves glucose tolerance and increases oral glucose-triggered GLP-1 and GIP secretion, particularly GIP. Furthermore, KCNH2-deficient primary intestinal epithelial cells exhibit heightened incretin, especially GIP secretion upon nutrient stimulation. Mechanistically, KCNH2 knockdown in EECs leads to reduced K+ currents, prolonged action potential duration, and elevated intracellular calcium levels. Finally, we found that dofetilide, a KCNH2-specific inhibitor, could promote incretin secretion in enteroendocrine STC-1 cells in vitro and in hyperglycemic mice in vivo. These findings elucidate, for the first time, the mechanism and application of KCNH2 in regulating incretin secretion by EECs. Given the therapeutic promise of GLP-1 and GIP in diabetes and obesity management, this study advances our understanding of incretin regulation, paving the way for potential incretin secretagogue therapies in the treatment of diabetes and obesity.

Preclinical pharmacokinetics, pharmacodynamics, and toxicity of novel small-molecule GPR119 agonists to treat type-2 diabetes and obesity

The escalating global prevalence of type-2 diabetes (T2D) and obesity necessitates the development of novel oral medications. Agonism at G-protein coupled receptor-119 (GPR119) has been recognized for modulation of metabolic homeostasis in T2D, obesity, and fatty liver disease. However, off-target effects have impeded the advancement of synthetic GPR119 agonist drug candidates. Non-systemic, gut-restricted GPR119 agonism is suggested as an alternative strategy that may locally stimulate intestinal enteroendocrine cells (EEC) for incretin secretion, without the need for systemic drug availability, consequently alleviating conventional class-related side effects. Herein, we report the preclinical acute safety, efficacy, and pharmacokinetics (PK) of novel GPR119 agonist compounds ps297 and ps318 that potentially target gut EEC for incretin secretion. In a proof-of-efficacy study, both compounds demonstrated glucagon-like peptide-1 (GLP-1) secretion capability during glucose and mixed-meal tolerance tests in healthy mice. Furthermore, co-administration of sitagliptin with investigational compounds in diabetic db/db mice resulted in synergism, with GLP-1 concentrations rising by three-fold. Both ps297 and ps318 exhibited low gut permeability assessed in the in-vitro Caco-2 cell model. A single oral dose PK study conducted on healthy mice demonstrated poor systemic bioavailability of both agents. PK measures (mean ± SD) for compound ps297 (Cmax 23 ± 19 ng/mL, Tmax range 0.5 - 1 h, AUC0-24 h 19.6 ± 21 h*ng/mL) and ps318 (Cmax 75 ± 22 ng/mL, Tmax range 0.25 - 0.5 h, AUC0-24 h 35 ± 23 h*ng/mL) suggest poor oral absorption. Additionally, examinations of drug excretion patterns in mice revealed that around 25 % (ps297) and 4 % (ps318) of the drugs were excreted through faeces as an unchanged form, while negligible drug concentrations (<0.005 %) were excreted in the urine. These acute PK/PD assessments suggest the gut is a primary site of action for both agents. Toxicity assessments conducted in the zebrafish and healthy mice models confirmed the safety and tolerability of both compounds. Future chronic in-vivo studies in relevant disease models will be essential to confirm the long-term safety and efficacy of these novel compounds.

Pharmacological and Clinical Studies of Medicinal Plants That Inhibit Dipeptidyl Peptidase-IV

Dipeptidyl peptidase IV (DPP-IV) is an enzyme responsible for the degradation of the incretin hormone glucagon-like peptide-1 (GLP-1). DPP-IV plays a significant role in regulating blood glucose levels by modulating the activity of GLP-1. In the context of diabetes, DPP-IV inhibitors effectively block the activity of DPP-IV, hence mitigating the degradation of GLP-1. This, in turn, leads to an extension of GLP-1's duration of action, prolongs gastric emptying, enhances insulin sensitivity, and ultimately results in the reduction of blood glucose levels. Nonetheless, reported adverse events of DPP-IV inhibitors on T2DM patients make it essential to understand the activity and mechanism of these drugs, particularly viewed from the perspective of finding the effective and safe add-on medicinal plants, to be implemented in clinical practice. This review is intended to bring forth a thorough overview of plants that work by reducing DPP-IV activity, from computational technique, enzymatic study, animal experiments, and studies in humans. The articles were searched on PubMed using "Plants", "DPP-IV", "DPP-IV inhibitor", "GLP-1", "Type 2 diabetes", "diabetes", "in silico", "in vitro", "in vivo", "studies in human", "clinical study" as the query words, and filtered for ten years of publication period. Eighteen plants showed inhibition against DPP-IV as proven by in silico, in vitro, and in vivo studies; however, only ten plants were reported for efficacy in clinical studies. Several plant-based DPP-IV inhibitors, eg, Allium sativum, Morus Alba, Curcuma longa, Pterocarpus marsupium, and Taraxacum officinale, have established their functional role in inhibiting DPP-IV and have proven their effectiveness through studies in humans earning them a prominent place in therapeutic discovery.

The β2-adrenergic receptor in the apical membrane of intestinal enterocytes senses sugars to stimulate glucose uptake from the gut

The presence of sugar in the gut causes induction of SGLT1, the sodium/glucose cotransporter in intestinal epithelial cells (enterocytes), and this is accompanied by stimulation of sugar absorption. Sugar sensing was suggested to involve a G-protein coupled receptor and cAMP - protein kinase A signalling, but the sugar receptor has remained unknown. We show strong expression and co-localization with SGLT1 of the β2-adrenergic receptor (β ₂-AR) at the enterocyte apical membrane and reveal its role in stimulating glucose uptake from the gut by the sodium/glucose-linked transporter, SGLT1. Upon heterologous expression in different reporter systems, the β ₂-AR responds to multiple sugars in the mM range, consistent with estimated gut sugar levels after a meal. Most adrenergic receptor antagonists inhibit sugar signaling, while some differentially inhibit epinephrine and sugar responses. However, sugars did not inhibit binding of I¹²⁵-cyanopindolol, a β ₂-AR antagonist, to the ligand-binding site in cell-free membrane preparations. This suggests different but interdependent binding sites. Glucose uptake into everted sacs from rat intestine was stimulated by epinephrine and sugars in a β ₂-AR-dependent manner. STD-NMR confirmed direct physical binding of glucose to the β ₂-AR. Oral administration of glucose with a non-bioavailable β ₂-AR antagonist lowered the subsequent increase in blood glucose levels, confirming a role for enterocyte apical β ₂-ARs in stimulating gut glucose uptake, and suggesting enterocyte β ₂-AR as novel drug target in diabetic and obese patients. Future work will have to reveal how glucose sensing by enterocytes and neuroendocrine cells is connected, and whether β ₂-ARs mediate glucose sensing also in other tissues.

Endogenous incretin levels and risk of first incident cancer: a prospective cohort study

Concerns have been raised regarding a potentially increased risk of cancer associated with treatment with glucagon-like peptide-1 (GLP-1) receptor agonists. Here, we explored whether fasting and oral glucose tolerance test post-challenge glucose-dependent insulinotropic peptide (GIP) and GLP-1 levels were associated with incident first cancer. Within the cardiovascular re-examination arm of the population-based Malmö Diet Cancer study (n = 3734), 685 participants with a previous cancer diagnosis were excluded, resulting in 3049 participants (mean age 72.2 ± 5.6 years, 59.5% women), of whom 485 were diagnosed with incident first cancer (median follow-up time 9.9 years). Multivariable Cox-regression and competing risk regression (death as competing risk) were used to explore associations between incretin levels and incident first cancer. Higher levels of fasting GLP-1 (462 incident first cancer cases/2417 controls) showed lower risk of incident first cancer in competing risk regression (sub-hazard ratio 0.90; 95% confidence interval 0.82-0.99; p = 0.022). No association was seen for fasting GIP, post-challenge GIP, or post-challenge GLP-1 and incident first cancer. In this prospective study, none of the fasting and post-challenge levels of GIP and GLP-1 were associated with higher risk of incident first cancer; by contrast, higher levels of fasting GLP-1 were associated with lower risk of incident first cancer.

Association of GLP1R Polymorphisms With the Incretin Response

CONTEXT

Polymorphisms in the gene encoding the glucagon-like peptide-1 receptor (GLP1R) are associated with type 2 diabetes but their effects on incretin levels remain unclear.

OBJECTIVE

We evaluated the physiologic and hormonal effects of GLP1R genotypes before and after interventions that influence glucose physiology.

DESIGN

Pharmacogenetic study conducted at 3 academic centers in Boston, Massachusetts.

PARTICIPANTS

A total of 868 antidiabetic drug-naïve participants with type 2 diabetes or at risk for developing diabetes.

INTERVENTIONS

We analyzed 5 variants within GLP1R (rs761387, rs10305423, rs10305441, rs742762, and rs10305492) and recorded biochemical data during a 5-mg glipizide challenge and a 75-g oral glucose tolerance test (OGTT) following 4 doses of metformin 500 mg over 2 days.

MAIN OUTCOMES

We used an additive mixed-effects model to evaluate the association of these variants with glucose, insulin, and incretin levels over multiple timepoints during the OGTT.

RESULTS

During the OGTT, the G-risk allele at rs761387 was associated with higher total GLP-1 (2.61 pmol/L; 95% CI, 1.0.72-4.50), active GLP-1 (2.61 pmol/L; 95% CI, 0.04-5.18), and a trend toward higher glucose (3.63; 95% CI, -0.16 to 7.42 mg/dL) per allele but was not associated with insulin. During the glipizide challenge, the G allele was associated with higher insulin levels per allele (2.01 IU/mL; 95% CI, 0.26-3.76). The other variants were not associated with any of the outcomes tested.

CONCLUSIONS

GLP1R variation is associated with differences in GLP-1 levels following an OGTT load despite no differences in insulin levels, highlighting altered incretin signaling as a potential mechanism by which GLP1R variation affects T2D risk.

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

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

Nutritional control of thyroid morphogenesis through gastrointestinal hormones

Developing animals absorb nutrients either through the placenta or from ingested food; however, the mechanisms by which embryos use external nutrients for individual organ morphogenesis remain to be elucidated. In this study, we assessed nutrient-dependent thyroid follicle morphogenesis in Xenopus laevis and investigated the role of secreted gastrointestinal (GI) hormones post-feeding. We found that feeding triggers thyroid follicle formation, and the thyroid cells showed transient inactivation of cell proliferation after feeding. In addition, the thyroid cells with multi-lumina were frequently observed in the fed tadpoles. The expression of the particular GI hormone incretin, glucose-dependent insulinotropic polypeptide (GIP), responded to feeding in the intestines of Xenopus tadpoles. Inhibition of dipeptidyl peptidase 4 (Dpp4), a degradative enzyme of incretin, increased the size of the thyroid follicles by facilitating follicular lumina connection, whereas inhibition of the sodium-glucose cotransporter (SGLT) reversed the effects of Dpp4 inhibition. Furthermore, injection of GIP peptide in unfed tadpoles initiated thyroid follicle formation-without requiring feeding-and injection of an incretin receptor antagonist suppressed follicle enlargement in the fed tadpoles. Lastly, GIP receptor knockout in neonatal mice showed smaller follicles in the thyroid, suggesting that the GI hormone-dependent thyroid morphogenesis is conserved in mammals. In conclusion, our study links external nutrients to thyroid morphogenesis and provides new insights into the function of GI hormone as a regulator of organ morphology in developing animals.

Role of the gut-brain axis in energy and glucose metabolism

The gastrointestinal tract plays a role in the development and treatment of metabolic diseases. During a meal, the gut provides crucial information to the brain regarding incoming nutrients to allow proper maintenance of energy and glucose homeostasis. This gut-brain communication is regulated by various peptides or hormones that are secreted from the gut in response to nutrients; these signaling molecules can enter the circulation and act directly on the brain, or they can act indirectly via paracrine action on local vagal and spinal afferent neurons that innervate the gut. In addition, the enteric nervous system can act as a relay from the gut to the brain. The current review will outline the different gut-brain signaling mechanisms that contribute to metabolic homeostasis, highlighting the recent advances in understanding these complex hormonal and neural pathways. Furthermore, the impact of the gut microbiota on various components of the gut-brain axis that regulates energy and glucose homeostasis will be discussed. A better understanding of the gut-brain axis and its complex relationship with the gut microbiome is crucial for the development of successful pharmacological therapies to combat obesity and diabetes.

Glycaemia and body weight are regulated by sodium-glucose cotransporter 1 (SGLT1) expression via O-GlcNAcylation in the intestine

Objective

Methods

Results

Conclusion

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Intestine-specific OGT depletion results in weight loss and hypoglycaemia.

•

It reduces SGLT1 expression, resulting in glucose absorption from the gut.

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OGT knockdown may contribute to diminish glucose-induced incretin secretion.

•

OGT may regulate SGLT1 expression via the cAMP/CREB-dependent pathway.

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O-GlcNAcylation regulates SGLT1 expression in the intestine and the kidney.

Intestinal Gpr17 deficiency improves glucose metabolism by promoting GLP-1 secretion

G protein-coupled receptors (GPCRs) in intestinal enteroendocrine cells (EECs) respond to nutritional, neural, and microbial cues and modulate the release of gut hormones. Here we show that Gpr17, an orphan GPCR, is co-expressed in glucagon-like peptide-1 (GLP-1)-expressing EECs in human and rodent intestinal epithelium. Acute genetic ablation of Gpr17 in intestinal epithelium improves glucose tolerance and glucose-stimulated insulin secretion (GSIS). Importantly, inducible knockout (iKO) mice and Gpr17 null intestinal organoids respond to glucose or lipid ingestion with increased secretion of GLP-1, but not the other incretin glucose-dependent insulinotropic polypeptide (GIP). In an in vitro EEC model, overexpression or agonism of Gpr17 reduces voltage-gated calcium currents and decreases cyclic AMP (cAMP) production, and these are two critical factors regulating GLP-1 secretion. Together, our work shows that intestinal Gpr17 signaling functions as an inhibitory pathway for GLP-1 secretion in EECs, suggesting intestinal GPR17 is a potential target for diabetes and obesity intervention.

Yan et al. locate GPR17 expression in the enteroendocrine cells of human and rodent intestinal epithelium. They find that GPR17 signaling inhibits intracellular rise of cAMP and calcium and that loss of intestinal Gpr17 in rodents leads to better glucose tolerance via increased hormone secretion in response to nutrient ingestion.

Sex- and age-dependent outcomes of 9-hour time-restricted feeding of a Western high-fat high-sucrose diet in C57BL/6J mice

Time-restricted feeding (TRF) is a nutritional intervention wherein food intake is limited to a consistent 8- to 10-h daily window without changes in nutritional quality or quantity. TRF can prevent and treat diet-induced obesity (DIO) and associated metabolic disease in young male mice fed an obesogenic diet, the gold standard preclinical model for metabolic disease research. Because age and sex are key biological variables affecting metabolic disease pathophysiology and response to therapies, we assessed their impact on TRF benefits by subjecting young 3-month-old or middle-aged 12-month-old male and female mice to ad libitum or TRF of a Western diet. We show that most of the benefits of TRF are age-independent but are sex-dependent. TRF protects both sexes against fatty liver and glucose intolerance while body weight benefits are observed only in males. We also find that TRF imparts performance benefits and increases survival to sepsis in both sexes.

Cardiovascular Outcomes Trials of Incretin-Based Therapies

The cardiovascular (CV) safety of glucagon-like peptide 1 (GLP-1) receptor agonists has been established in robust cardiovascular outcomes trials (CVOTs) in patients with type 2 diabetes at high CV risk. The GLP-1 receptor agonists liraglutide, dulaglutide, and injectable semaglutide demonstrated a significant CV benefit in these trials and now have indications to reduce the risk of major adverse CV events, including CV death, myocardial infarction, and stroke in adult patients with type 2 diabetes and established cardiovascular disease or high CV risk (dulaglutide). The dipeptidyl peptidase 4 inhibitors have also demonstrated CV safety in dedicated CVOTs but have not been associated with any CV benefit. Guidelines for the treatment of type 2 diabetes have evolved from a glucocentric focus to one that now focuses on reducing overall CV risk by personalizing therapy and using drugs such as GLP-1 receptor agonists with proven CV benefits.

Higher dietary insulin load and index are not associated with the risk of metabolic syndrome and obesity in Iranian adults

BACKGROUND

Data about the relation between dietary insulin load (DIL) and dietary insulin index (DII) with metabolic syndrome (MetS) and obesity are scarce. Therefore, the present study aimed to examine the association of the insulinemic potential of the diet with MetS and obesity amongst Iranian adults.

METHODS

This cross-sectional study was conducted amongst 850 adults aged 20-59 years. Dietary data were collected using a 168-item food frequency questionnaire. DIL was calculated using a standard formula and DII was obtained by dividing DIL by the total energy intake of each participant. The guidelines of the International Diabetes Federation were used to define MetS. General obesity was considered as body mass index ≥ 30 kg/m² , and abdominal obesity as waist circumference ≥ 94 cm for men and ≥ 80 cm for women.

RESULTS

Mean DIL and DII values were 101 684 ± 54 802 and 49.4 ± 33.4, respectively. The mean age of participants was 44.9 ± 10.7 years and 36.8%, 28.5% and 48.8% of participants were suffering from MetS, general and abdominal obesity, respectively. In contrast with DIL (P = .73), participants in the last quartile of DII (P = .62) had lower odds of MetS than the first quartile. There were non-significant inverse associations between DIL (P = .91, P = .85) and DII (P = .59, P = .53) with odds of general and abdominal obesity before and after the adjustment of confounders, respectively.

CONCLUSIONS

We did not observe any significant association of DIL and DII with the risk of MetS and obesity amongst the Iranian population. Further prospective studies are needed to confirm the findings of this study.

Effects of mixed meal tolerance test on gastric emptying, glucose and lipid homeostasis in obese nonhuman primates

Meal ingestion elicits a variety of neuronal, physiological and hormonal responses that differ in healthy, obese or diabetic individuals. The mixed meal tolerance test (MMTT) is a well-established method to evaluate pancreatic β-cell reserve and glucose homeostasis in both preclinical and clinical research in response to calorically defined meal. Nonhuman primates (NHPs) are highly valuable for diabetic research as they can naturally develop type 2 diabetes mellitus (T2DM) in a way similar to the onset and progression of human T2DM. The purpose of this study was to investigate the reproducibility and effects of a MMTT containing acetaminophen on plasma glucose, insulin, C-peptide, incretin hormones, lipids, acetaminophen appearance (a surrogate marker for gastric emptying) in 16 conscious obese cynomolgus monkeys (Macaca fascicularis). Plasma insulin, C-peptide, TG, aGLP-1, tGIP, PYY and acetaminophen significantly increased after meal/acetaminophen administration. A subsequent study in 6 animals showed that the changes of plasma glucose, insulin, C-peptide, lipids and acetaminophen were reproducible. There were no significant differences in responses to the MMTT among the obese NHPs with (n = 11) or without (n = 5) hyperglycemia. Our results demonstrate that mixed meal administration induces significant secretion of several incretins which are critical for maintaining glucose homeostasis. In addition, the responses to the MMTTs are reproducible in NHPs, which is important when the MMTT is used for evaluating post-meal glucose homeostasis in research.

Role of Bioactive Peptide Sequences in the Potential Impact of Dairy Protein Intake on Metabolic Health

For years, there has been an increasing move towards elucidating the complexities of how food can interplay with the signalling networks underlying energy homeostasis and glycaemic control. Dairy foods can be regarded as the greatest source of proteins and peptides with various health benefits and are a well-recognized source of bioactive compounds. A number of dairy protein-derived peptide sequences with the ability to modulate functions related to the control of food intake, body weight gain and glucose homeostasis have been isolated and characterized. Their being active in vivo may be questionable mainly due to expected low bioavailability after ingestion, and hence their real contribution to the metabolic impact of dairy protein intake needs to be discussed. Some reports suggest that the differential effects of dairy proteins-in particular whey proteins-on mechanisms underlying energy balance and glucose-homeostasis may be attributed to their unique amino acid composition and hence the release of free amino acid mixtures enriched in essential amino acids (i.e., branched-chain-amino acids) upon digestion. Actually, the research reports reviewed in this article suggest that, among a number of dairy protein-derived peptides isolated and characterized as bioactive compounds in vitro, some peptides can be active in vivo post-oral administration through a local action in the gut, or, alternatively, a systemic action on specific molecular targets after entering the systemic circulation. Moreover, these studies highlight the importance of the enteroendocrine system in the cross talk between food proteins and the neuroendocrine network regulating energy balance.

Dual Inhibition of DPP-4 and Cholinesterase Enzymes by the Phytoconstituents of the Ethanolic Extract of Prosopis cineraria Pods: Therapeutic Implications for the Treatment of Diabetes-associated Neurological Impairments

BACKGROUND

Insulin resistance causes decreased uptake of glucose which promotes the susceptibility of type 2 associated neurological impairments.

METHODS

The study was aimed to evaluate the inhibition potential of the ethanolic extract of Prosopis cineraria (EPC) pods against DPP-4 and cholinesterase enzymes by in-vitro, in-vivo and in-silico assessments. The present study consists of in vivo studies on a diabetes-induced rat model by HOMA (Homeostasis model assessment) and related parameters, in vitro studies through the DPP-4 enzyme assay and cholinesterase assays using Ellman's reaction. The in-silico studies were conducted by the molecular docking of Cinerin C with targeted enzymes. The phytochemical characterization of the extract was demonstrated through LCMS studies. The antioxidant studies on the extract were performed by FRAP and TEAC assays.

RESULTS

The extract showed 64.8% maximum inhibition of DPP-4, 34.91% inhibition of AChE and 74.35% inhibition of BuChE. The antioxidant capacity of the extract was observed to be 847.81±16.25μM Fe2+ equivalent in the FRAP assay and 0.40 ± 0.08 mmol/l of Trolox equivalent in the TEAC assay. The in vivo study showed competent glycaemic control against significant HOMA IR (1.5), HOMA % β (26.5) and HOMA % S (68.8) as well as pancreatic cell mass proliferation. The insilico analysis also revealed positive interactions of Cinerin C with targeted enzymes (DPP4 and cholinesterase).

CONCLUSION

It can be concluded that the phytoconstituents of Prosopis cineraria pod extract can be significantly considered in neuropharmacology to resolve insulin resistance-induced neurological complications as it showed inhibition against DPP-4, AChE and BuChE target enzymes.

Insulin-Like Proteins in Plant Sources: A Systematic Review

Type 2 diabetes mellitus (T2DM) is characterized by hyperglycemia. Proteins in plant sources that enable the maintenance of the glycemic profile may be of interest in the context of T2DM. However, their mechanisms of action are unclear, unlike other bioactive compounds. This systematic review identified and described the mechanisms of action of isolated and purified proteins and peptides extracted from vegetables on the reduction of blood glucose in T2DM in experimental studies. The research was done in PubMed, ScienceDirect, Scopus, Web of Science, Embase and Virtual Health Library (VHL) databases in March 2019. The initial search retrieved 916 articles, and, after reading the title, abstract and keywords, 24 articles were eligible for full reading. Then, five articles were eligible to build this systematic review. The evaluation of the evidence and the strength of the recommendations of the studies was evaluated with the SYstematic Review Center for Laboratory animal Experimentation - SYRCLE. Studies with proteins or peptides extracted from soybean (Glycine max), corn (Zea mays), peas (Pisum sativum), costus (Costus igneus) and ginseng (Panax ginseng) were found, and all of them decreased glycemia but not by the same mechanisms. The mechanism of action of proteins extracted from Glycine max, Pisum sativum, Costus igneus were similar, acting in the insulin-mediated pathways. The peptide derived from Zea mays increased GLP-1 expression, and the peptide from Panax ginseng reduced NF-kB signaling, both resulting in stimulating the release of insulin. Therefore, bioactive proteins and peptides of plant sources act through biochemical pathways, in the modulation of insulin resistance and the hyperglycemic state. These compounds are promising in scientific research on T2DM, because there is a probable similarity of these proteins with insulin, which enables them to act as insulin-like molecules.

Dose-dependent reduction in body weight with LIK066 (licogliflozin) treatment in Japanese patients with obesity

AIMS

LIK066 (licogliflozin) is a dual sodium glucose co-transporter 1/2 inhibitor with potential benefits in weight loss. This study evaluated the efficacy, tolerability and safety of licogliflozin in Japanese adults with obesity.

MATERIALS AND METHODS

This study was a randomized, double-blind, placebo-controlled, dose-finding study to evaluate the effect of licogliflozin (2.5, 10, 25 and 50 mg once daily) in 126 Japanese patients with obesity. The primary objective was to examine the dose-response relationship of licogliflozin treatment in body weight reduction relative to placebo at 12 weeks. The secondary objectives included assessment of responder rates, change in parameters related to complications, visceral and subcutaneous fat area, and safety during 12 weeks of treatment.

RESULTS

The placebo-subtracted least square mean percentage change in body weight from baseline at week 12 was -1.99 (95% confidence interval -2.92, -0.21), -3.00 (-4.15, -1.70), -3.54 (-4.54, -2.26) and - 3.91% (-5.01, -2.77) in licogliflozin 2.5, 10, 25 and 50 mg once-daily dose groups, respectively. The proportion of responders with ≥3% reduction in body weight in the licogliflozin 2.5, 10, 25 and 50 mg once-daily dose groups were 15.8%, 55.6%, 50.0% and 56.7%, respectively, versus placebo [7.1%; P ≤0.002 for all except the 2.5 mg once-daily group (P = 0.39)]. Dose-dependent reductions were observed significantly in haemoglobin A1c, uric acid, fasting plasma glucose and potentially in the waist circumference, diastolic blood pressure and visceral fat area.

CONCLUSION

Dual inhibition of SGLT1/2 with licogliflozin treatment induced a dose-dependent reduction in body weight in Japanese patients with obesity. Treatment with licogliflozin was safe and well tolerated in this study. The study is registered with ClinicalTrials.gov (NCT03320941).

Glucagon-like peptide 1 (GLP-1)

BACKGROUND

The glucagon-like peptide-1 (GLP-1) is a multifaceted hormone with broad pharmacological potential. Among the numerous metabolic effects of GLP-1 are the glucose-dependent stimulation of insulin secretion, decrease of gastric emptying, inhibition of food intake, increase of natriuresis and diuresis, and modulation of rodent β-cell proliferation. GLP-1 also has cardio- and neuroprotective effects, decreases inflammation and apoptosis, and has implications for learning and memory, reward behavior, and palatability. Biochemically modified for enhanced potency and sustained action, GLP-1 receptor agonists are successfully in clinical use for the treatment of type-2 diabetes, and several GLP-1-based pharmacotherapies are in clinical evaluation for the treatment of obesity.

SCOPE OF REVIEW

In this review, we provide a detailed overview on the multifaceted nature of GLP-1 and its pharmacology and discuss its therapeutic implications on various diseases.

MAJOR CONCLUSIONS

Since its discovery, GLP-1 has emerged as a pleiotropic hormone with a myriad of metabolic functions that go well beyond its classical identification as an incretin hormone. The numerous beneficial effects of GLP-1 render this hormone an interesting candidate for the development of pharmacotherapies to treat obesity, diabetes, and neurodegenerative disorders.

Contribution of intestinal dipeptidyl peptidase-4 inhibition for incretin-dependent improved glucose tolerance in mice

Dipeptidyl peptidase-4 (DPP-4) inhibitors prevent the degradation of glucagon-like peptide-1 (GLP-1) and improve glycemic control. The GLP-1 insulinotropic effect involves a pathway through vagus nerve GLP-1 receptors in the gut, in addition to a direct effect on the pancreas. Therefore, this study verified whether DPP-4 inhibition in the gut contributed to the improvement of glycemic control. Anagliptin, a DPP-4 inhibitor, was administered orally or subcutaneously (with or without passing through the gastrointestinal tract, respectively) to mice. The association between blood glucose suppression following oral glucose challenge and DPP-4 inhibition in the small intestine and plasma was assessed. Oral administration of anagliptin (0.03-0.3 mg/kg) in normal mice significantly suppressed blood glucose, which was associated with an increase in insulin secretion at a dose of ≥0.1 mg/kg (P < 0.05). Subcutaneous administration of anagliptin (0.01-0.1 mg/kg) produced similar results. However, plasma DPP-4 inhibition following oral administration was weaker than that following subcutaneous administration; blood glucose suppression was significantly correlated with small intestinal DPP-4 inhibition (r = 0.949, P < 0.01), but not with plasma DPP-4 inhibition. Additionally, similar results were observed in a type 2 diabetes model (r = 0.975, P < 0.001). Thus, these results demonstrated that an improvement in glycemic control was dependent upon small intestinal DPP-4 inhibition. As these effects were accompanied by the elevation of intact GLP-1 in the portal, this suggests that improvement in glucose tolerance after anagliptin treatment might be related to an increase in GLP-1 receptor signaling in the small intestine and portal vein.

Arctium lappa contributes to the management of type 2 diabetes mellitus by regulating glucose homeostasis and improving oxidative stress: A critical review of in vitro and in vivo animal-based studies

Diabetes is a metabolic disease highly widespread worldwide, and the most common form is the type 2 diabetes mellitus (T2DM). A large number of synthetic drugs are currently available for the treatment of diabetes; however, they present various side effects and, for this reason, people are increasingly inclined to search natural alternative treatments. Among these, Arctium lappa (A. lappa) has interesting anti-diabetic activities, exerted by improving glucose homeostasis and reducing insulin-resistance. In addition, A. lappa exerts a marked antioxidant activity, an effect known to play a pivotal role in the treatment of T2DM. The purpose of this review is to analyse scientific evidence in order to evaluate the role of A. lappa and its bioactive compounds in management of T2DM. The literature search performed provided only in vitro and animal-based studies. No clinical studies have been conducted in order to investigate the effect of A. lappa on T2DM patients. However, available literature provides evidence for further clinical trials in order to confirm these claimed activities on humans.

Gastrointestinal Sensing of Meal-Related Signals in Humans, and Dysregulations in Eating-Related Disorders

The upper gastrointestinal (GI) tract plays a critical role in sensing the arrival of a meal, including its volume as well as nutrient and non-nutrient contents. The presence of the meal in the stomach generates a mechanical distension signal, and, as gastric emptying progresses, nutrients increasingly interact with receptors on enteroendocrine cells, triggering the release of gut hormones, with lipid and protein being particularly potent. Collectively, these signals are transmitted to the brain to regulate appetite and energy intake, or in a feedback loop relayed back to the upper GI tract to further adjust GI functions, including gastric emptying. The research in this area to date has provided important insights into how sensing of intraluminal meal-related stimuli acutely regulates appetite and energy intake in humans. However, disturbances in the detection of these stimuli have been described in a number of eating-related disorders. This paper will review the GI sensing of meal-related stimuli and the relationship with appetite and energy intake, and examine changes in GI responses to luminal stimuli in obesity, functional dyspepsia and anorexia of ageing, as examples of eating-related disorders. A much better understanding of the mechanisms underlying these dysregulations is still required to assist in the development of effective management and treatment strategies in the future.

The effects of licogliflozin, a dual SGLT1/2 inhibitor, on body weight in obese patients with or without diabetes

BACKGROUND

There is an unmet need for a safer and more effective treatment for obesity. This study assessed the effects of licogliflozin, a dual inhibitor of sodium-glucose co-transporter (SGLT) 1/2, on body weight, metabolic parameters and incretin hormones in patients with type 2 diabetes mellitus (T2DM) and/or obesity.

METHODS

Patients with obesity (BMI, 35-50 kg/m² ) were enrolled into a 12-week study (N = 88; licogliflozin 150 mg q.d.). Patients with T2DM were enrolled into a second, two-part study, comprising a single-dose cross-over study (N = 12; 2.5 - 300 mg) and a 14-day dosing study (N = 30; 15 mg q.d). Primary endpoints included effects on body weight, effects on glucose, safety and tolerability. Secondary endpoints included urinary glucose excretion (UGE₂₄ ) and pharmacokinetics, while exploratory endpoints assessed the effects on incretin hormones (total GLP-1, PYY_3-36 , and GIP), insulin and glucagon.

RESULTS

Treatment with licogliflozin 150 mg q.d. for 12 weeks in patients with obesity significantly reduced body weight by 5.7% vs placebo (P < 0.001) and improved metabolic parameters such as significantly reduced postprandial glucose excursion (21%; P < 0.001), reduced insulin levels (80%; P < 0.001) and increased glucagon (59%; P < 0.001). In patients with T2DM, a single dose of licogliflozin 300 mg in the morning prior to an oral glucose tolerance test (OGTT) remarkably reduced glucose excursion by 93% (P < 0.001; incremental AUC_0-4h ) and suppressed insulin by 90% (P < 0.01; incremental AUC_0-4h ). Treatment with licogliflozin 15 mg q.d. for 14 days reduced 24-hour average glucose levels by 26% (41 mg/dL; P < 0.001) and increased UGE₂₄ to 100 g (P < 0.001) in patients with T2DM. In addition, this treatment regimen significantly increased total GLP-1 by 54% (P < 0.001) and PYY_3-36 by 67% (P < 0.05) post OGTT vs placebo, while significantly reducing GIP levels by 53% (P < 0.001). Treatment with licogliflozin was generally safe and well tolerated. Diarrhea (increased numbers of loose stool) was the most common adverse event in all studies (90% with licogliflozin vs 25% with placebo in the 12-week study), while a lower incidence of flatulence, abdominal pain and abdominal distension (25%-43% with licogliflozin vs 9%-11% with placebo in the 12-week study) were among the other gastrointestinal events reported.

CONCLUSION

Licogliflozin treatment (1-84 days) leads to significant weight loss and favourable changes in a variety of metabolic parameters and incretin hormones. Dual inhibition of SGLT1/2 with licogliflozin in the gut and kidneys is an attractive strategy for treating obesity and diabetes.

Association between dietary insulin index and load with obesity in adults

PURPOSE

Hyperinsulinemia is linked to the development of various chronic diseases, especially obesity given to the role of insulin responses in body fat accumulation; hence, the current study aimed to examine the association of insulinemic potential of the diet with general and abdominal obesity among a large population of Iranian adults.

METHODS

This cross-sectional study was carried out among 8691 adult participants aged 18-55 years. Dietary data were collected using a validated dish-based 106-item semi-quantitative food frequency questionnaire. Dietary insulin index (DII) was computed through considering food insulin index values published earlier. Dietary insulin load (DIL) was also calculated using a standard formula. Assessment of anthropometric measures was conducted through a self-administered questionnaire. General obesity was defined as body mass index ≥ 25 kg/m², and abdominal obesity as waist circumference ≥ 94 cm for men and ≥ 80 cm for women.

RESULTS

Mean age of study participants was 36.8 ± 8.1 years; 60.3% were women. Compared with the lowest quintile, women in the highest quintile of DIL were less likely to be abdominally obese (OR 0.73; 95% CI 0.57-0.92). Such significant association was not seen after controlling for potential confounders (OR 0.86; 95% CI 0.44-1.67). Neither in crude nor in adjusted models, we observed a significant association between DIL and general obesity among men and women. In terms of dietary insulin index, men in the top quintile of DII were more likely to be generally overweight or obese compared with those in the bottom quintile (OR 1.27; 95% CI 1.00-1.62). This association became non-significant after controlling for demographic characteristics (OR 1.14; 95% CI 0.84-1.56). Furthermore, in thefully adjusted model, women in the top quintile of DII were more likely to have general obesity compared with those in the bottom quintile (OR 1.40; 95% CI 1.07-1.84).

CONCLUSIONS

We found that adherence to a diet with a high DII was associated with greater odds of general obesity among women, but not in men. Although such information might help to draw conclusions on the practical relevance of the shown findings, further studies, specifically of prospective design, are warranted.

Bile acids drive colonic secretion of glucagon-like-peptide 1 and peptide-YY in rodents

A large number of glucagon-like-peptide-1 (GLP-1)- and peptide-YY (PYY)-producing L cells are located in the colon, but little is known about their contribution to whole body metabolism. Since bile acids (BAs) increase GLP-1 and PYY release, and since BAs spill over from the ileum to the colon, we decided to investigate the ability of BAs to stimulate colonic GLP-1 and PYY secretion. Using isolated perfused rat/mouse colon as well as stimulation of the rat colon in vivo, we demonstrate that BAs significantly enhance secretion of GLP-1 and PYY from the colon with average increases of 3.5- and 2.9-fold, respectively. Furthermore, we find that responses depend on BA absorption followed by basolateral activation of the BA-receptor Takeda-G protein-coupled-receptor 5. Surprisingly, the apical sodium-dependent BA transporter, which serves to absorb conjugated BAs, was not required for colonic conjugated BA absorption or conjugated BA-induced peptide secretion. In conclusion, we demonstrate that BAs represent a major physiological stimulus for colonic L-cell secretion. NEW & NOTEWORTHY By the use of isolated perfused rodent colon preparations we show that bile acids are potent and direct promoters of colonic glucagon-like-peptide 1 and peptide-YY secretion. The study provides convincing evidence that basolateral Takeda-G protein-coupled-receptor 5 activation is mediating the effects of bile acids in the colon and thus add to the existing literature described for L cells in the ileum.

An effective protocol for pharmacological defatting of primary human hepatocytes which is non-toxic to cholangiocytes or intrahepatic endothelial cells

Introduction

Pharmacological defatting of rat hepatocytes and hepatoma cell lines suggests that the same method could be used to ameliorate macrovesicular steatosis in moderate to severely fatty livers. However there is no data assessing the effects of those drugs on primary human liver cells. We aimed to determine the effectiveness of a pharmacological cocktail in reducing the in vitro lipid content of primary human hepatocytes (PHH). In addition we sought to determine the cytotoxicity of the cocktail towards non-parenchymal liver cells.

Methods

Steatosis was induced in PHH by supplementation with a combination of saturated and unsaturated free fatty acids. This was followed by addition of a defatting drug cocktail for up to 48 hours. The same experimental method was used with human intra-hepatic endothelial cells (HIEC) and human cholangiocytes. MTT assay was used to assess cell viability, triglyceride quantification and oil red O staining were used to determine intracellular lipids content whilst ketone bodies were measured in the supernatants following experimentation.

Results

Incubation of fat loaded PHH with the drugs over 48 hours reduced the intracellular lipid area by 54%, from 12.85% to 5.99% (p = 0.002) (percentage of total oil red O area), and intracellular triglyceride by 35%, from 28.24 to 18.30 nmol/million of cells (p<0.001). Total supernatant ketone bodies increased 1.4-fold over 48 hours in the defatted PHH compared with vehicle controls (p = 0.002). Moreover incubation with the drugs for 48 hours increased the viability of PHH by 11%, cholangiocytes by 25% whilst having no cytotoxic effects on HIEC.

Conclusion

These data demonstrate that pharmacological intervention can significantly decrease intracellular lipid content of PHH, increase fatty acids β-oxidation whilst being non-toxic to PHH, HIEC or cholangiocytes.

The impact of short-chain fatty acids on GLP-1 and PYY secretion from the isolated perfused rat colon

The colonic epithelium harbors a large number of endocrine cells, but little is known about the endocrine functions of the colon. However, the high density of glucagon like peptide-1 (GLP-1)- and peptide-YY (PYY)-secreting L cells is of great interest because of the potential antidiabetic and antiobesity effects of GLP-1 and PYY. Short-chain fatty acids (SCFAs) produced by local bacterial fermentation are suggested to activate the colonic free fatty acid receptors FFAR2 (GPR43) and FFAR3 (GPR41), stimulating the colonic L cells. We used the isolated perfused rat colon as a model of colonic endocrine secretion and studied the effects of the predominant SCFAs formed: acetate, propionate, and butyrate. We show that luminal and especially vascular infusion of acetate and butyrate significantly increases colonic GLP-1 secretion, and to a minor extent also PYY secretion, but only after enhancement of intracellular cAMP. Propionate neither affected GLP-1 nor PYY secretion whether administered luminally or vascularly. A FFAR2- and FFAR3-specific agonist [( S)-2-(4-chlorophenyl)-3,3-dimethyl- N-(5-phenylthiazol-2-yl)butamide (CFMB)/ AR420626 ] had no effect on colonic GLP-1 output, and a FFAR3 antagonist ( AR399519 ) did not decrease the SCFA-induced GLP-1 response. However, the voltage-gated Ca²⁺-channel blocker nifedipine, the K_ATP-channel opener diazoxide, and the ATP synthesis inhibitor 2,4-dinitrophenol completely abolished the responses. FFAR2 receptor studies confirmed low-potent partial agonism of acetate, propionate, and butyrate, compared with CFMB, which is a full agonist with ~750-fold higher potency than the SCFAs. In conclusion, SCFAs may increase colonic GLP-1/PYY secretion, but FFAR2/FFAR3 do not seem to be involved. Rather, SCFAs are metabolized and appear to function as a colonocyte energy source. NEW & NOTEWORTHY By the use of in situ isolated perfused rat colon we show that short-chain fatty acids (SCFAs) primarily are used as a colonocyte energy source in the rat, subsequently triggering glucagon like peptide-1 (GLP-1) secretion independent of the free fatty acid receptors FFAR2 and FFAR3. Opposite many previous studies on SCFAs and FFAR2/FFAR3 and GLP-1 secretion, this experimental model allows investigation of the physiological interactions between luminal nutrients and secretion from cells whose function depend critically on their blood supply as well as nerve and paracrine interactions.

Bitter tastant quinine modulates glucagon-like peptide-1 exocytosis from clonal GLUTag enteroendocrine L cells via actin reorganization

Enteroendocrine L cells in the gastrointestinal tract secrete glucagon-like peptide-1 (GLP-1), which plays an important role in glucose homeostasis. Here we investigated the effect of bitter tastant quinine on GLP-1 secretion using clonal GLUTag mouse enteroendocrine L cells. We found that GLUTag cells expressed putative quinine receptors at mRNA levels. Although application of quinine resulted in an increase of intracellular Ca²⁺ levels, which was mediated by Ca²⁺ release from the endoplasmic reticulum and Ca²⁺ influx through voltage-sensitive Ca²⁺ channels, quinine had little effect on GLP-1 secretion. Total internal reflection fluorescence microscopy and immunocytochemistry revealed that GLP-1-containing vesicles remained unfused with the plasma membrane and facilitated actin polymerization beneath the plasma membrane after application of quinine, respectively. Interestingly, application of forskolin together with quinine induced GLP-1 exocytosis from the cells. These results suggest that quinine does not induce GLP-1 secretion because it facilitates Ca²⁺ increase and actin reorganization but not cAMP increase, and both Ca²⁺ and cAMP are essential for GLP-1 secretion.

GLP1 and GIP are involved in the action of synbiotics in broiler chickens

Background

In order to discover new strategies to replace antibiotics in the post-antibiotic era in meat-type chicken production, two new synbiotics were tested: (Lactobacillus salivarius IBB3154 plus galactooligosaccharide (Syn1) and Lactobacillus plantarum IBB3036 plus raffinose family oligosaccharides (Syn2).

Methods

The synbiotics were administered via syringe, using a special automatic system, into the egg air chamber of Cobb 500 broiler chicks on the 12^th day of egg incubation (2 mg of prebiotics + 10⁵ cfu bacteria per egg). Hatched roosters (total 2,400) were reared on an experimental farm, kept in pens (75 animals per pen), with free access to feed and water. After 42 d animals were slaughtered. Blood serum, pancreas, duodenum and duodenum content were collected.

Results

Syn2 increased trypsin activity by 2.5-fold in the pancreas and 1.5-fold in the duodenal content. In the duodenum content, Syn2 resulted in ca 30% elevation in lipase activity and 70% reduction in amylase activity. Syn1 and Syn2 strongly decreased expression of mRNA for GLP-1 and GIP in the duodenum and for GLP-1 receptors in the pancreas. Simultaneously, concentrations of the incretins significantly diminished in the blood serum (P < 0.05). The decreased expression of incretins coincides with changed activity of digestive enzymes in the pancreas and in the duodenal content. The results indicate that incretins are involved in the action of Syn1 and Syn2 or that they may even be their target. No changes were observed in key hormones regulating metabolism (insulin, glucagon, corticosterone, thyroid hormones, and leptin) or in metabolic indices (glucose, NEFA, triglycerides, cholesterol). Additionally, synbiotics did not cause significant changes in the activities of alanine and aspartate aminotransferases in broiler chickens. Simultaneously, the activity of alkaline phosphatase and gamma glutamyl transferase diminished after Syn2 and Syn1, respectively.

Conclusion

The selected synbiotics may be used as in ovo additives for broiler chickens, and Syn2 seems to improve their potential digestive proteolytic and lipolytic ability. Our results suggest that synbiotics can be directly or indirectly involved in incretin secretion and reception.

SCFAs strongly stimulate PYY production in human enteroendocrine cells

Peptide-YY (PYY) and Glucagon-Like Peptide-1 (GLP-1) play important roles in the regulation of food intake and insulin secretion, and are of translational interest in the field of obesity and diabetes. PYY production is highest in enteroendocrine cells located in the distal intestine, mirroring the sites where high concentrations of short chain fatty acids (SCFAs) are produced by gut microbiota. We show here that propionate and butyrate strongly increased expression of PYY but not GCG in human cell line and intestinal primary culture models. The effect was predominantly attributable to the histone deacetylase inhibitory activity of SCFA and minor, but significant contributions of FFA2 (GPR43). Consistent with the SCFA-dependent elevation of PYY gene expression, we also observed increased basal and stimulated PYY hormone secretion. Interestingly, the transcriptional stimulation of PYY was specific to human-derived cell models and not reproduced in murine primary cultures. This is likely due to substantial differences in PYY gene structure between mouse and human. In summary, this study revealed a strong regulation of PYY production by SCFA that was evident in humans but not mice, and suggests that high fibre diets elevate plasma concentrations of the anorexigenic hormone PYY, both by targeting gene expression and hormone secretion.

Bile Acid Signaling Pathways from the Enterohepatic Circulation to the Central Nervous System

Bile acids are best known as detergents involved in the digestion of lipids. In addition, new data in the last decade have shown that bile acids also function as gut hormones capable of influencing metabolic processes via receptors such as FXR (farnesoid X receptor) and TGR5 (Takeda G protein-coupled receptor 5). These effects of bile acids are not restricted to the gastrointestinal tract, but can affect different tissues throughout the organism. It is still unclear whether these effects also involve signaling of bile acids to the central nervous system (CNS). Bile acid signaling to the CNS encompasses both direct and indirect pathways. Bile acids can act directly in the brain via central FXR and TGR5 signaling. In addition, there are two indirect pathways that involve intermediate agents released upon interaction with bile acids receptors in the gut. Activation of intestinal FXR and TGR5 receptors can result in the release of fibroblast growth factor 19 (FGF19) and glucagon-like peptide 1 (GLP-1), both capable of signaling to the CNS. We conclude that when plasma bile acids levels are high all three pathways may contribute in signal transmission to the CNS. However, under normal physiological circumstances, the indirect pathway involving GLP-1 may evoke the most substantial effect in the brain.

Discovery of triazole-based uracil derivatives bearing amide moieties as novel dipeptidyl peptidase-IV inhibitors

Dipeptidyl peptidase-IV (DPP-4) is a validated target for T2DM treatment. We previously reported a novel series of triazole-based uracil derivatives bearing aliphatic carboxylic acids with potent DPP-4 inhibitory activities in vitro, but these compounds showed poor hypoglycemic effects in vivo. Herein we further optimized the triazole moiety by amidation of the carboxylic acid to improve in vivo activities. Two series of compounds 3a-f and 4a-g were designed and synthesized. By screening in DPP-4, compound 4c was identified as a potent DPP-4 inhibitor with the IC₅₀ value of 28.62 nM. Docking study revealed compound 4c has a favorable binding mode and interpreted the SAR of these analogs. DPP-8 and DPP-9 tests indicated compound 4c had excellent selectivity over DPP-8 and DPP-9. Further in vivo evaluations revealed that compound 4c showed more potent hypoglycemic activity than its corresponding carboxylic acid in ICR mice and dose-dependently reduced glucose levels in type 2 diabetic C57BL/6 mice. The overall results have shown that compound 4c could be a promising lead for further development of novel DPP-4 agents treating T2DM.

Could hop-derived bitter compounds improve glucose homeostasis by stimulating the secretion of GLP-1?

Hops (Humulus lupulus L.) is by far the greatest contributors to the bitter property of beer. Over the past years, a large body of evidence demonstrated the presence of taste receptors in different locations of the oral cavity. In addition to the taste buds of the tongue, cells expressing these receptors have been identified in olfactory bulbs, respiratory and gastrointestinal tract. In the gut, the attention was mainly directed to sweet Taste Receptor (T1R) and bitter Taste Receptor (T2R) receptors. In particular, T2R has shown to modulate secretion of different gut hormones, mainly Glucagon-like Peptide 1 (GLP-1), which are involved in the regulation of glucose homeostasis and the control of gut motility, thereby increasing the sense of satiety. Scientific interest in the activity of bitter taste receptors emerges because of their wide distribution in the human species and the large range of natural substances that interact with them. Beer, whose alcohol content is lower than in other common alcoholic beverages, contains a considerable amount of bitter compounds and current scientific evidence shows a direct effect of beer compounds on glucose homeostasis. The purpose of this paper is to review the available literature data in order to substantiate the novel hypothesis of a possible direct effect of hop-derived bitter compounds on secretion of GLP-1, through the activation of T2R, with consequent improvement of glucose homeostasis.

RABIF/MSS4 is a Rab-stabilizing holdase chaperone required for GLUT4 exocytosis

Rab GTPases are switched from their GDP-bound inactive conformation to a GTP-bound active state by guanine nucleotide exchange factors (GEFs). The first putative GEFs isolated for Rabs are RABIF (Rab-interacting factor)/MSS4 (mammalian suppressor of Sec4) and its yeast homolog DSS4 (dominant suppressor of Sec4). However, the biological function and molecular mechanism of these molecules remained unclear. In a genome-wide CRISPR genetic screen, we isolated RABIF as a positive regulator of exocytosis. Knockout of RABIF severely impaired insulin-stimulated GLUT4 exocytosis in adipocytes. Unexpectedly, we discovered that RABIF does not function as a GEF, as previously assumed. Instead, RABIF promotes the stability of Rab10, a key Rab in GLUT4 exocytosis. In the absence of RABIF, Rab10 can be efficiently synthesized but is rapidly degraded by the proteasome, leading to exocytosis defects. Strikingly, restoration of Rab10 expression rescues exocytosis defects, bypassing the requirement for RABIF. These findings reveal a crucial role of RABIF in vesicle transport and establish RABIF as a Rab-stabilizing holdase chaperone, a previously unrecognized mode of Rab regulation independent of its GDP-releasing activity. Besides Rab10, RABIF also regulates the stability of two other Rab GTPases, Rab8 and Rab13, suggesting that the requirement of holdase chaperones is likely a general feature of Rab GTPases.

Abscisic Acid: A Novel Nutraceutical for Glycemic Control

Abscisic acid is naturally present in fruits and vegetables, and it plays an important role in managing glucose homeostasis in humans. According to the latest U.S. dietary survey, about 92% of the population might have a deficient intake of ABA due to their deficient intake of fruits and vegetables. This review summarizes the in vitro, preclinical, mechanistic, and human translational findings obtained over the past 15 years in the study of the role of ABA in glycemic control. In 2007, dietary ABA was first reported to ameliorate glucose tolerance and obesity-related inflammation in mice. The most recent findings regarding the topic of ABA and its proposed receptor lanthionine synthetase C-like 2 in glycemic control and their interplay with insulin and glucagon-like peptide-1 suggest a major role for ABA in the physiological response to a glucose load in humans. Moreover, emerging evidence suggests that the ABA response might be dysfunctional in diabetic subjects. Follow on intervention studies in healthy individuals show that low-dose dietary ABA administration exerts a beneficial effect on the glycemia and insulinemia profiles after oral glucose load. These recent findings showing benefits in humans, together with extensive efficacy data in mouse models of diabetes and inflammatory disease, suggest the need for reference ABA values and its possible exploitation of the glycemia-lowering effects of ABA for preventative purposes. Larger clinical studies on healthy, prediabetic, and diabetic subjects are needed to determine whether addressing the widespread dietary ABA deficiency improves glucose control in humans.

Lysophosphatidylinositol-induced activation of the cation channel TRPV2 triggers glucagon-like peptide-1 secretion in enteroendocrine L cells

The lysophosphatidylinositol (LPI) has crucial roles in multiple physiological processes, including insulin exocytosis from pancreatic islets. However, the role of LPI in secretion of glucagon-like peptide-1 (GLP-1), a hormone that enhances glucose-induced insulin secretion, is unclear. Here, we used the murine enteroendocrine L cell line GLUTag and primary murine small intestinal cells to elucidate the mechanism of LPI-induced GLP-1 secretion. Exogenous LPI addition increased intracellular Ca²⁺ concentrations ([Ca²⁺] _i ) in GLUTag cells and induced GLP-1 secretion from both GLUTag and acutely prepared primary intestinal cells. The [Ca²⁺] _i increase was suppressed by an antagonist for G protein-coupled receptor 55 (GPR55) and by silencing of GPR55 expression, indicating involvement of G_q and G_12/13 signaling pathways in the LPI-induced increased [Ca²⁺] _i levels and GLP-1 secretion. However, GPR55 agonists did not mimic many of the effects of LPI. We also found that phospholipase C inhibitor and Rho-associated kinase inhibitor suppressed the [Ca²⁺] _i increase and that LPI increased the number of focal adhesions, indicating actin reorganization. Of note, blockage or silencing of transient receptor potential cation channel subfamily V member 2 (TRPV2) channels suppressed both the LPI-induced [Ca²⁺] _i increase and GLP-1 secretion. Furthermore, LPI accelerated TRPV2 translocation to the plasma membrane, which was significantly suppressed by a GPR55 antagonist. These findings suggest that TRPV2 activation via actin reorganization induced by G_q and G_12/13 signaling is involved in LPI-stimulated GLP-1 secretion in enteroendocrine L cells. Because GPR55 agonists largely failed to mimic the effects of LPI, its actions on L cells are at least partially independent of GPR55 activation.

Fatty acid and mineral receptors as drug targets for gastrointestinal disorders

Nutrient-sensing receptors, including fatty acid receptors (FFA1–FFA4), Ca2+-sensing receptors and Zn2+-sensing receptors, are involved in several biological processes. These receptors are abundantly expressed in the GI tract, where they have been shown to play crucial roles in regulating GI function. This review provides an overview of the GI functions of fatty acid and mineral receptors, including the regulation of gastric and enteroendocrine functions, GI motility, ion transport and cell growth. Recently, several lines of evidence have implicated these receptors as promising therapeutic targets for the treatment of GI disorders, for example, inflammatory bowel disease, colorectal cancer, metabolic syndrome and diarrheal diseases. A future perspective on drug discovery research targeting these receptors is discussed.

Qing-Hua Granule induces GLP-1 secretion via bitter taste receptor in db/db mice

Qing-Hua Granule (QHG), the modified formulation of a classical Chinese prescription named Gegen Qinlian Decoction, was clinically employed to treat type 2 diabetes mellitus (T2DM) through regulation of glucagon-like peptide-1 (GLP-1). However, the potential mechanism is unknown. We investigate whether QHG induces GLP-1 secretion via activation of bitter taste receptor (TAS2R) pathway in the gastrointestinal tract of db/db mice. The db/db mice were intragastrically (i.g.) administered QHG (low/medium/high dose) once daily for 8 weeks. GLP-1 secretion was evaluated. The bitter receptor signaling pathway, which regulates GLP-1 secretion, including TAS2R5 (a subtype of TAS2R), α-gustducin (Gαgust), 1-phosphatidylinositol-4, 5-bisphosphate phosphodiesterase beta-2 (PLCβ2), transient receptor potential cation channel subfamily M member 5 (TRPM5), was assessed by quantitative real-time polymerase chain reaction (qRT-PCR), Western blot and immunohistochemistry (IHC). The biochemical observations of ileum and pancreas tissue were detected histopathologically. Acquity Ultra Performance LCTM - Micromass ZQ 2000 (UPLC-MS) was used for the phytochemical analysis. QHG exhibited significant and dose-dependent effect on GLP-1 secretion in db/db mice, along with significant up-regulation of TAS2R5 mRNA level and activation of TAS2R pathway (p<0.05). In addition, QHG improved the histopathological structure of ileum and pancreatic tissue. Seventeen compounds were identified in QHG. In conclusion, QHG induces GLP-1 secretion in db/db mice, most likely through the bitter taste receptor pathway.

Dose-dependent effect of glucose on GLP-1 secretion involves sweet taste receptor in isolated perfused rat ileum [Rapid Communication]

Luminal glucose is an important stimulus for glucagon-like peptide 1 (GLP-1) secretion from intestinal endocrine cells. However, the effects of luminal glucose concentration on GLP-1 secretion remain unknown. In this study, we investigated the effect of luminal glucose concentrations (3.5, 5, 10, 15, and 20 mmol/L) on GLP-1 secretion from isolated perfused rat ileum. Results showed that the perfusate glucose concentration dose-dependently stimulated GLP-1 secretion from isolated perfused rat ileum, which was eliminated by the sweet taste receptor inhibitor gurmarin (30 μg/mL), but not inhibited by phloridzin (1 mmol/L), a Na⁺-coupled glucose transporters inhibitor. We conclude that luminal glucose induced GLP-1 secretion from perfused rat ileum in a concentration-dependent manner. This secretion was mediated by sweet taste receptor transducing signal for GLP-1 release on the gut of rat.

Gut hormones such as amylin and GLP-1 in the control of eating and energy expenditure

The control of meal size is the best studied aspect of the control of energy balance, and manipulation of this system constitutes a promising target to treat obesity. A major part of this control system is based on gastrointestinal hormones such as glucagon-like peptide-1 (GLP-1) or amylin, which are released in response to a meal and which limit the size of an ongoing meal. Both amylin and GLP-1 have also been shown to increase energy expenditure in experimental rodents, but mechanistically we know much less how this effect may be mediated, which brain sites may be involved, and what the physiological relevance of these findings may be. Most studies indicate that the effect of peripheral amylin is centrally mediated via the area postrema, but other brain areas, such as the ventral tegmental area, may also be involved. GLP-1's effect on eating seems to be mainly mediated by vagal afferents projecting to the caudal hindbrain. Chronic exposure to amylin, GLP-1 or their analogs decrease food intake and body weight gain. Next to the induction of satiation, amylin may also constitute an adiposity signal and in fact interact with the adiposity signal leptin. Amylin analogs are under clinical consideration for their effect to reduce food intake and body weight in humans, and similar to rodents, amylin analogs seem to be particularly active when combined with leptin analogs.

Glucagon-like peptide-1, glucagon-like peptide-2, and lipid metabolism

PURPOSE OF REVIEW

Glucagon-like peptide-1 (GLP-1) is the best known incretin hormone able to potentiate glucose-induced insulin secretion. Moreover, GLP-1 is currently under intensive investigation as a potential crucial mediator of beneficial metabolic effects after bariatric surgery, because of its eating inhibitory, antiobesity, and antidiabetes effects. This review briefly summarizes recent findings on the specific effects of GLP-1 on lipoprotein metabolism. The related hormone GLP-2 is derived from the same precursor gene; its effects on lipoprotein metabolism will also be discussed briefly.

RECENT FINDINGS

Pharmacological activation of the GLP-1 system has beneficial effects on obesity-induced alterations of lipoprotein metabolism. These benefits can be observed with direct GLP-1 receptor agonists like liraglutide or exendin-4, but also with inhibitors of dipeptidyl peptidase IV (DPP-IV), which reduce the breakdown of endogenous GLP-1. The role of GLP-2-related pathways on lipid levels and metabolism are less clear, but some effects (e.g. increased intestinal chylomicron output) are opposite to GLP-1.

SUMMARY

Activation of the GLP-1-dependent pathways may perhaps translate into a lower cardiovascular risk. Understanding how GLP-1 and GLP-2 regulate and interact in the control of lipoprotein metabolism will set the stage for the development of new strategies to treat dyslipidaemia in obesity, diabetes, and other cardiometabolic diseases.

The regulation of function, growth and survival of GLP-1-producing L-cells

Glucagon-like peptide-1 (GLP-1) is a peptide hormone, released from intestinal L-cells in response to hormonal, neural and nutrient stimuli. In addition to potentiation of meal-stimulated insulin secretion, GLP-1 signalling exerts numerous pleiotropic effects on various tissues, regulating energy absorption and disposal, as well as cell proliferation and survival. In Type 2 Diabetes (T2D) reduced plasma levels of GLP-1 have been observed, and plasma levels of GLP-1, as well as reduced numbers of GLP-1 producing cells, have been correlated to obesity and insulin resistance. Increasing endogenous secretion of GLP-1 by selective targeting of the molecular mechanisms regulating secretion from the L-cell has been the focus of much recent research. An additional and promising strategy for enhancing endogenous secretion may be to increase the L-cell mass in the intestinal epithelium, but the mechanisms that regulate the growth, survival and function of these cells are largely unknown. We recently showed that prolonged exposure to high concentrations of the fatty acid palmitate induced lipotoxic effects, similar to those operative in insulin-producing cells, in an in vitro model of GLP-1-producing cells. The mechanisms inducing this lipototoxicity involved increased production of reactive oxygen species (ROS). In this review, regulation of GLP-1-secreting cells is discussed, with a focus on the mechanisms underlying GLP-1 secretion, long-term regulation of growth, differentiation and survival under normal as well as diabetic conditions of hypernutrition.

GLP-1 based therapies: clinical implications for gastroenterologists

The gut-derived incretin hormone, glucagon-like peptide 1 (GLP-1) lowers postprandial blood glucose levels by stimulating insulin and inhibiting glucagon secretion. Two novel antihyperglycaemic drug classes augment these effects; GLP-1 receptor agonists and inhibitors of the GLP-1 degrading enzyme dipeptidyl peptidase 4. These so called GLP-1 based or incretin based drugs are increasingly used to treat type 2 diabetes, because of a low risk of hypoglycaemia and favourable effect on body weight, blood pressure and lipid profiles. Besides glucose control, GLP-1 functions as an enterogastrone, causing a wide range of GI responses. Studies have shown that endogenous GLP-1 and its derived therapies slow down digestion by affecting the stomach, intestines, exocrine pancreas, gallbladder and liver. Understanding the GI actions of GLP-1 based therapies is clinically relevant; because GI side effects are common and need to be recognised, and because these drugs may be used to treat GI disease.

Convertible visceral fat as a therapeutic target to curb obesity

New therapeutic and preventative strategies are needed to address the growing obesity epidemic. In animal models, brown adipose tissue activation and the associated heat produced contribute to countering obesity and the accompanying metabolic abnormalities. Adult humans also have functional brown fat. Here, we present and discuss the concepts of murine and human white adipose tissue plasticity and the transdifferentiation of white adipocytes into brown adipocytes. Human visceral adipocytes - which are crucial contributors to the burden of obesity and its complications - are particularly susceptible to such transdifferentiation. Therefore, we propose that this process should be a focus of anti-obesity research. Approved drugs that have browning properties as well as future drugs that target molecular pathways involved in white-to-brown visceral adipocyte transdifferentiation may provide new avenues for obesity therapy.