Protein kinase Czeta is required for oleic acid-induced secretion of glucagon-like peptide-1 by intestinal endocrine L cells

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

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

Effects of Obesogenic Feeding and Free Fatty Acids on Circadian Secretion of Metabolic Hormones: Implications for the Development of Type 2 Diabetes

Circadian rhythms are 24-h internal biological rhythms within organisms that govern virtually all aspects of physiology. Interestingly, metabolic tissues have been found to express cell-autonomous clocks that govern their rhythmic activity throughout the day. Disruption of normal circadian rhythmicity, as induced by environmental factors such as shift work, significantly increases the risk for the development of metabolic diseases, including type 2 diabetes and obesity. More recently, obesogenic feeding and its fatty acid components have also been shown to be potent disruptors of normal circadian biology. Two key hormones that are released in response to nutrient intake are the anti-diabetic incretin hormone glucagon-like peptide-1, from intestinal L cells, and insulin secreted by pancreatic β cells, both of which are required for the maintenance of metabolic homeostasis. This review will focus on the circadian function of the L and β cells and how both obesogenic feeding and the saturated fatty acid, palmitate, affect their circadian clock and function. Following introduction of the core biological clock and the hierarchical organization of the mammalian circadian system, the circadian regulation of normal L and β cell function and the importance of GLP-1 and insulin in establishing metabolic control are discussed. The central focus of the review then considers the circadian-disrupting effects of obesogenic feeding and palmitate exposure in L and β cells, while providing insight into the potential causative role in the development of metabolic disease.

Nutrient infusion in the dorsal vagal complex controls hepatic lipid and glucose metabolism in rats

Hypothalamic regulation of lipid and glucose homeostasis is emerging, but whether the dorsal vagal complex (DVC) senses nutrients and regulates hepatic nutrient metabolism remains unclear. Here, we found in rats DVC oleic acid infusion suppressed hepatic secretion of triglyceride-rich very-low-density lipoprotein (VLDL-TG), which was disrupted by inhibiting DVC long-chain fatty acyl-CoA synthetase that in parallel disturbed lipid homeostasis during intravenous lipid infusion. DVC glucose infusion elevated local glucose levels similarly as intravenous glucose infusion and suppressed hepatic glucose production. This was independent of lactate metabolism as inhibiting lactate dehydrogenase failed to disrupt glucose sensing and neither could DVC lactate infusion recapitulate glucose effect. DVC oleic acid and glucose infusion failed to lower VLDL-TG secretion and glucose production in high-fat fed rats, while inhibiting DVC farnesoid X receptor enhanced oleic acid but not glucose sensing. Thus, an impairment of DVC nutrient sensing may lead to the disruption of lipid and glucose homeostasis in metabolic syndrome.

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DVC oleic acid infusion lowers hepatic secretion of VLDL-TG in chow but not HF rats

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Inhibition of ACSL in the DVC negates lipid sensing

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DVC glucose infusion lowers hepatic glucose production in chow but not HF rats

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Inhibition of FXR in the DVC enhances oleic acid but not glucose sensing in HF rats

Free Fatty Acid Receptors as Mediators and Therapeutic Targets in Liver Disease

Free fatty acid receptors (FFARs) are a class of G protein-coupled receptors (GPCRs) that have wide-ranging effects on human physiology. The four well-characterized FFARs are FFAR1/GPR40, FFAR2/GPR43, FFAR3/GPR41, and FFAR4/GPR120. Short-chain (<6 carbon) fatty acids target FFAR2/GPR43 and FFAR3/GPR41. Medium- and long-chain fatty acids (6-12 and 13-21 carbon, respectively) target both FFAR1/GPR40 and FFAR4/GPR120. Signaling through FFARs has been implicated in non-alcoholic fatty liver disease (NAFLD), non-alcoholic steatohepatitis (NASH), intestinal failure-associated liver disease (IFALD), and a variety of other liver disorders. FFARs are now regarded as targets for therapeutic intervention for liver disease, diabetes, obesity, hyperlipidemia, and metabolic syndrome. In this review, we provide an in-depth, focused summary of the role FFARs play in liver health and disease.

Carbonic anhydrase 8 (CAR8) negatively regulates GLP-1 secretion from enteroendocrine cells in response to long-chain fatty acids

Glucagon-like peptide-1 (GLP-1) is an incretin secreted from enteroendocrine preproglucagon (PPG)-expressing cells (traditionally known as L cells) in response to luminal nutrients that potentiates insulin secretion. Augmentation of endogenous GLP-1 secretion might well represent a novel therapeutic target for diabetes treatment in addition to the incretin-associated drugs currently in use. In this study, we found that PPG cells substantially express carbonic anhydrase 8 (CAR8), which has been reported to inhibit inositol 1,4,5-trisphosphate (IP3) binding to the IP3 receptor and subsequent Ca2+ efflux from the endoplasmic reticulum in neuronal cells. In vitro experiments using STC-1 cells demonstrated that Car8 knockdown increases long-chain fatty acid (LCFA)-stimulated GLP-1 secretion. This effect was reduced in the presence of phospholipase C (PLC) inhibitor; in addition, Car8 knockdown increased the intracellular Ca2+ elevation caused by α-linolenic acid, indicating that CAR8 exerts its effect on GLP-1 secretion via the PLC/IP3/Ca2+ pathway. Car8wdl null mutant mice showed significant increase in GLP-1 response to oral corn oil administration compared with that in wild-type littermates, with no significant change in intestinal GLP-1 content. These results demonstrate that CAR8 negatively regulates GLP-1 secretion from PPG cells in response to LCFAs, suggesting the possibility of augmentation of postprandial GLP-1 secretion by CAR8 inhibition.NEW & NOTEWORTHY This study focused on the physiological significance of carbonic anhydrase 8 (CAR8) in GLP-1 secretion from enteroendocrine preproglucagon (PPG)-expressing cells. We found an inhibitory role of CAR8 in LCFA-induced GLP-1 secretion in vitro and in vivo, suggesting a novel therapeutic approach to diabetes and obesity through augmentation of postprandial GLP-1 secretion by CAR8 inhibition.

(-)-Epicatechin and Anthocyanins Modulate GLP-1 Metabolism: Evidence from C57BL/6J Mice and GLUTag Cells

BACKGROUND

Generated in intestinal L cells through cleavage of proglucagon (Gcg), glucagon-like peptide 1 (GLP-1) is secreted and rapidly inactivated by dipeptidyl peptidase IV (DPP-IV). GLP-1 regulates insulin secretion and overall glucose homeostasis. The capacity of dietary bioactives to increase GLP-1 circulating levels, and therefore increase insulin secretion and glucose metabolism, has gained significant interest of late.

OBJECTIVES

We evaluated the effects of (-)-epicatechin (EC) and different anthocyanins (ACs) and AC metabolites on GLP-1 metabolism in mice and on GLUTag cells.

METHODS

We fed 6-week-old C57BL/6J male mice a control diet or a control diet supplemented with either 40 mg AC or 20 mg EC/kg body weight for 14 weeks (AC) or 15 weeks (EC). Intestinal mRNA levels of Gcg and Dpp-iv were measured. In vitro, GLUTag cells were incubated in the presence or absence of different ACs, the AC metabolite protocatechuic acid (PCA), and EC. GLP-1 secretion and the main pathways involved in its release were assessed.

RESULTS

Long-term supplementation with EC or AC increased mouse GLP-1 plasma concentrations (55% and 98%, respectively; P < 0.05). In mice, 1) EC and AC increased Gcg mRNA levels in the ileum (91%) and colon (41%), respectively (P < 0.05); and 2) AC lowered ileum Dpp-iv mRNA levels (35%), while EC decreased plasma DPP-IV activity (15%; P < 0.05). In GLUTag cells, 1) cyanidin, delphinidin, PCA, and EC increased GLP-1 secretion (53%, 33%, 53%, and 68%, respectively; P < 0.05); and 2) cyanidin, delphinidin, EC, and PCA increased cyclin adenosine monophosphate levels (25-50%; P < 0.05) and activated protein kinase A (PKA; 100%, 50%, 80%, and 86%, respectively; P < 0.05).

CONCLUSIONS

In mice, EC and ACs regulated different steps in GLP-1 regulation, leading to increased plasma GLP-1. Cyanidin, delphinidin, PCA, and EC promoted GLP-1 secretion from GLUTag cells by activating the PKA-dependent pathway. These findings support the beneficial actions of these flavonoids in sustaining intestinal and glucose homeostasis through the modulation of the GLP-1 metabolism.

Nutrient-Induced Cellular Mechanisms of Gut Hormone Secretion

The gastrointestinal tract can assess the nutrient composition of ingested food. The nutrient-sensing mechanisms in specialised epithelial cells lining the gastrointestinal tract, the enteroendocrine cells, trigger the release of gut hormones that provide important local and central feedback signals to regulate nutrient utilisation and feeding behaviour. The evidence for nutrient-stimulated secretion of two of the most studied gut hormones, glucagon-like peptide 1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP), along with the known cellular mechanisms in enteroendocrine cells recruited by nutrients, will be the focus of this review. The mechanisms involved range from electrogenic transporters, ion channel modulation and nutrient-activated G-protein coupled receptors that converge on the release machinery controlling hormone secretion. Elucidation of these mechanisms will provide much needed insight into postprandial physiology and identify tractable dietary approaches to potentially manage nutrition and satiety by altering the secreted gut hormone profile.

The Role of the Bacterial Muramyl Dipeptide in the Regulation of GLP-1 and Glycemia

The host's intestinal microbiota contributes to endocrine and metabolic responses, but a dysbiosis in this environment can lead to obesity and insulin resistance. Recent work has demonstrated a role for microbial metabolites in the regulation of gut hormones, including the metabolic hormone, glucagon-like peptide-1 (GLP-1). Muramyl dipeptide (MDP) is a bacterial cell wall component which has been shown to improve insulin sensitivity and glucose tolerance in diet-induced obese mice by acting through the nucleotide oligomerization domain 2 (NOD2) receptor. The purpose of this study was to understand the effects of MDP on GLP-1 secretion and glucose regulation. We hypothesized that MDP enhances glucose tolerance by inducing intestinal GLP-1 secretion through NOD2 activation. First, we observed a significant increase in GLP-1 secretion when murine and human L-cells were treated with a fatty acid MDP derivative (L18-MDP). Importantly, we demonstrated the expression of the NOD2 receptor in mouse intestine and in L-cells. In mice, two intraperitoneal injections of MDP (5 mg/kg body weight) caused a significant increase in fasting total GLP-1 in chow-fed mice, however this did not lead to an improvement in oral glucose tolerance. When mice were exposed to a high-fat diet, they eventually lost this MDP-induced GLP-1 release. Finally, we demonstrated in L-cells that hyperglycemic conditions reduce the mRNA expression of NOD2 and GLP-1. Together these findings suggest MDP may play a role in enhancing GLP-1 during normal glycemic conditions but loses its ability to do so in hyperglycemia.

Insights into incretin-based therapies for treatment of diabetic dyslipidemia

Derangements in triglyceride and cholesterol metabolism (dyslipidemia) are major risk factors for the development of cardiovascular diseases in obese and type-2 diabetic (T2D) patients. An emerging class of glucagon-like peptide-1 (GLP-1) analogues and next generation peptide dual-agonists such as GLP-1/glucagon or GLP-1/GIP could provide effective therapeutic options for T2D patients. In addition to their role in glucose and energy homeostasis, GLP-1, GIP and glucagon serve as regulators of lipid metabolism. This review summarizes the current knowledge in GLP-1, glucagon and GIP effects on lipid and lipoprotein metabolism and frames the emerging therapeutic benefits of GLP-1 analogs and GLP-1-based multiagonists as add-on treatment options for diabetes associated dyslipidemia.

Essential Role of Syntaxin-Binding Protein-1 in the Regulation of Glucagon-Like Peptide-1 Secretion

Circadian secretion of the incretin, glucagon-like peptide-1 (GLP-1), correlates with expression of the core clock gene, Bmal1, in the intestinal L-cell. Several SNARE proteins known to be circadian in pancreatic α- and β-cells are also necessary for GLP-1 secretion. However, the role of the accessory SNARE, Syntaxin binding protein-1 (Stxbp1; also known as Munc18-1) in the L-cell is unknown. The aim of this study was to determine whether Stxbp1 is under circadian regulation in the L-cell and its role in the control of GLP-1 secretion. Stxbp1 was highly-enriched in L-cells, and STXBP1 was expressed in a subpopulation of L-cells in mouse and human intestinal sections. Stxbp1 transcripts and protein displayed circadian patterns in mGLUTag L-cells line, while chromatin-immunoprecipitation revealed increased interaction between BMAL1 and Stxbp1 at the peak time-point of the circadian pattern. STXBP1 recruitment to the cytosol and plasma membrane within 30 minutes of L-cell stimulation was also observed at this time-point. Loss of Stxbp1 in vitro and in vivo led to reduced stimulated GLP-1 secretion at the peak time-point of circadian release, and impaired GLP-1 secretion ex vivo. In conclusion, Stxbp1 is a circadian regulated exocytotic protein in the intestinal L-cell that is an essential regulatory component of GLP-1 secretion.

Intestinal sensing and handling of dietary lipids in gastric bypass-operated patients and matched controls

BACKGROUND

Altered meal-related gut hormone secretion seems important for weight loss and diabetes remission after Roux-en-Y gastric bypass (RYGB). Elucidating the responsible meal components and receptors could aid discovery of new treatments of obesity and diabetes. Enteroendocrine cells respond to digestion products of dietary triacylglycerol, especially long-chain fatty acids (LCFAs) and 2-oleoyl-glycerol (2-OG), but not medium-chain fatty acids (MCFAs).

OBJECTIVE

We examined the impact of olive oil (20 mL) and its derivates, LCFAs and 2-OG, on enteroendocrine secretions [glucagon-like peptide-1 (GLP-1), glucose-dependent insulinotropic polypeptide (GIP), cholecystokinin (CCK), peptide YY (PYY), and neurotensin (NT)] and on glucose, lipid, and bile acid metabolism in RYGB-operated and unoperated individuals.

METHODS

In an exploratory randomized crossover design, 10 RYGB-operated patients and 10 matched controls ingested 3 equimolar triacylglycerol formulations on separate days: olive oil (digested to 2-OG + LCFAs), C8-dietary oil (2-OG + MCFAs), and tricaprylin (MCFAs; negative control). Hormone responses were calculated as area under the curve (AUC).

RESULTS

Independent of group status, olive oil had greater effects than C8-dietary oil on AUCs of plasma GLP-1 (+32%; 95% CI: 23%, 43%; P < 0.01), CCK (+53%, P < 0.01), and NT (+71%, P < 0.01), whereas the effect on GIP differed between groups (+90% in controls, P < 0.01; +24% in RYGB, P = 0.10). Independent of group status, C8-dietary oil had greater effects than tricaprylin on AUCs of plasma CCK (+40%, P < 0.01) and NT (+32%, P < 0.01), but not GLP-1 (+5%; 95% CI: -2.9%, 13%; P = 0.22), whereas the effect on GIP again differed between groups (+78% in controls, P < 0.01; +39% in RYGB, P = 0.01). Distal (GLP-1/PYY/NT), but not proximal (CCK/GIP), enteroendocrine responses were generally greater in RYGB patients than in controls.

CONCLUSIONS

The combination of LCFAs plus 2-OG was substantially more effective than 2-OG plus MCFAs in stimulating enteroendocrine secretion in RYGB-operated and matched control individuals. Distal lipid-induced gut hormone release was greater after RYGB.This trial was registered at clinicaltrials.gov as NCT03223389.

Pharmacological potential of novel agonists for FFAR4 on islet and enteroendocrine cell function and glucose homeostasis

BACKGROUND

To investigate the metabolic effects of FFAR4-selective agonists on islet and enteroendocrine cell hormone release and the combined therapeutic effectiveness with DPP-IV inhibitors.

METHODS

Insulinotropic activity and specificity of FFAR4 agonists were determined in clonal pancreatic BRIN-BD11 cells. Expression of FFAR4 was assessed by qPCR and western blotting following agonist treatment in BRIN-BD11 cells and by immunohistochemistry in mouse islets. Acute in-vivo effects of agonists was investigated after intraperitoneal (i.p.) or oral administration in lean and HFF-obese diabetic mice.

RESULTS

GSK137647 (10^-11-10^-4 M) and Compound-A (10^-10-10^-4 M) stimulated insulin secretion at 5.6 mM (p < 0.05-p < 0.001) and 16.7 mM (p < 0.05-p < 0.001) glucose in BRIN-BD11 cells, with no cytotoxicity effects as assessed by MTT. FFAR4 antagonist (AH-7614) abolished the insulintropic effect of GSK137647 (p < 0.05-p < 0.001), whilst FFAR1 antagonist (GW1100) had no effect. Incubation of BRIN-BD11 cells with GSK137647 and Compound-A increased FFAR4 (p < 0.01) gene expression at 16.7 mM glucose, with a corresponding increase in FFAR4 (p < 0.01) protein concentrations. FFAR4 upregulation was attenuated under normoglycaemic conditions. Immunohistochemistry demonstrated co-localisation of FFAR4 and insulin in mouse islets. Orally administered GSK137647 or Compound-A (0.1 µmol/kgBW) improved glucose tolerance (p < 0.001), increased plasma insulin (p < 0.001), GLP-1 (p < 0.05), GIP (p < 0.05) and induced satiety (p < 0.001) in HFF mice, with glucose-lowering effects enhanced in combination with DPP-IV inhibitor (Sitagliptin) (p < 0.05).

CONCLUSIONS

Specific FFAR4 agonism improves glucose tolerance through insulin and incretin secretion, with enhanced DPP-IV inhibition in combination with Sitagliptin.

GENERAL SIGNIFICANCE

These findings have for the first time demonstrated that selective FFAR4 activation regulates both islet and enteroendocrine hormone function with agonist combinational therapy, presenting a promising strategy for the treatment of type-2-diabetes.

Gut microbiota confers host resistance to obesity by metabolizing dietary polyunsaturated fatty acids

Gut microbiota mediates the effects of diet, thereby modifying host metabolism and the incidence of metabolic disorders. Increased consumption of omega-6 polyunsaturated fatty acid (PUFA) that is abundant in Western diet contributes to obesity and related diseases. Although gut-microbiota-related metabolic pathways of dietary PUFAs were recently elucidated, the effects on host physiological function remain unclear. Here, we demonstrate that gut microbiota confers host resistance to high-fat diet (HFD)-induced obesity by modulating dietary PUFAs metabolism. Supplementation of 10-hydroxy-cis-12-octadecenoic acid (HYA), an initial linoleic acid-related gut-microbial metabolite, attenuates HFD-induced obesity in mice without eliciting arachidonic acid-mediated adipose inflammation and by improving metabolic condition via free fatty acid receptors. Moreover, Lactobacillus-colonized mice show similar effects with elevated HYA levels. Our findings illustrate the interplay between gut microbiota and host energy metabolism via the metabolites of dietary omega-6-FAs thereby shedding light on the prevention and treatment of metabolic disorders by targeting gut microbial metabolites.

The gut microbiome is an important regulator of metabolic health. Here the authors show that intestinal bacteria metabolize dietary linoleic acid to 10-hydroxy-cis-12-octadecenoic acid (HYA) which confers host resistance to high fat diet-induced obesity in mice.

Intestinal Sensing by Gut Microbiota: Targeting Gut Peptides

There are more than 2 billion overweight and obese individuals worldwide, surpassing for the first time, the number of people affected by undernutrition. Obesity and its comorbidities inflict a heavy burden on the global economies and have become a serious threat to individuals' wellbeing with no immediate cure available. The causes of obesity are manifold, involving several factors including physiological, metabolic, neural, psychosocial, economic, genetics and the environment, among others. Recent advances in genome sequencing and metagenomic profiling have added another dimension to this complexity by implicating the gut microbiota as an important player in energy regulation and the development of obesity. As such, accumulating evidence demonstrate the impact of the gut microbiota on body weight, adiposity, glucose, lipid metabolism, and metabolic syndrome. This also includes the role of microbiota as a modulatory signal either directly or through its bioactive metabolites on intestinal lumen by releasing chemosensing factors known to have a major role in controlling food intake and regulating body weight. The importance of gut signaling by microbiota signaling is further highlighted by the presence of taste and nutrient receptors on the intestinal epithelium activated by the microbial degradation products as well as their role in release of peptides hormones controlling appetite and energy homeostasis. This review present evidence on how gut microbiota interacts with intestinal chemosensing and modulates the release and activity of gut peptides, particularly GLP-1 and PYY.

Suppression of circadian secretion of glucagon-like peptide-1 by the saturated fatty acid, palmitate

AIM

Glucagon-like peptide-1 is an incretin hormone secreted by the intestinal L-cell with a circadian rhythm that parallels expression of the core clock gene, Bmal1. Although feeding rats a high-fat/high-sucrose Western diet impairs rhythmic glucagon-like peptide-1 release, the mechanisms underlying this effect remain unclear. Therefore, the aim of this study was to determine the pathway(s) by which the saturated fat, palmitate, a major component of the Western diet, impairs circadian glucagon-like peptide-1 secretion.

METHODS

Murine mGLUTag L-cells were synchronized, and the effects of palmitate pre-treatment on gene expression and glucagon-like peptide-1 secretion were determined, in addition to metabolite quantification, mitochondrial function analysis and enzyme inhibition and activation assays. Glucagon-like peptide-1 secretion was also analysed in ileal crypt cultures from control and Bmal1 knockout mice.

RESULTS

Pre-treatment with palmitate dampened Bmal1 mRNA and protein expression and glucagon-like peptide-1 secretion at 8 but not 20 hours after cell synchronization (P < .05-.001). Glucagon-like peptide-1 release was also impaired in Bmal1 knockout cultures as compared to wild-type controls (P < .001). Palmitate pre-treatment reduced expression of the Bmal1 downstream target, nicotinamide phosphoribosyltransferase, the rate-limiting enzyme in the synthesis of NAD+ . This was paralleled by dampening of total NAD+ levels, as well as impaired mitochondrial function and ATP production (P < .05-.001). Whereas direct inhibition of nicotinamide phosphoribosyltransferase also decreased glucagon-like peptide-1 release, activation of this enzyme restored glucagon-like peptide-1 secretion in the presence of palmitate.

CONCLUSION

Palmitate impairs L-cell clock function at the peak of Bmal1 gene expression, thereby impairing mitochondrial function and ultimately rhythmic glucagon-like peptide-1 secretion.

Lactobacillus gasseri in the Upper Small Intestine Impacts an ACSL3-Dependent Fatty Acid-Sensing Pathway Regulating Whole-Body Glucose Homeostasis

Long-chain acyl-CoA synthetase (ACSL)-dependent upper small intestinal lipid metabolism activates pre-absorptive pathways to regulate metabolic homeostasis, but whether changes in the upper small intestinal microbiota alter specific fatty acid-dependent pathways to impact glucose homeostasis remains unknown. We here first find that upper small intestinal infusion of Intralipid, oleic acid, or linoleic acid pre-absorptively increases glucose tolerance and lowers glucose production in rodents. High-fat feeding impairs pre-absorptive fatty acid sensing and reduces upper small intestinal Lactobacillus gasseri levels and ACSL3 expression. Transplantation of healthy upper small intestinal microbiota to high-fat-fed rodents restores L. gasseri levels and fatty acid sensing via increased ACSL3 expression, while L. gasseri probiotic administration to non-transplanted high-fat-fed rodents is sufficient to restore upper small intestinal ACSL3 expression and fatty acid sensing. In summary, we unveil a glucoregulatory role of upper small intestinal L. gasseri that impacts an ACSL3-dependent glucoregulatory fatty acid-sensing pathway.

Comparing olive oil and C4-dietary oil, a prodrug for the GPR119 agonist, 2-oleoyl glycerol, less energy intake of the latter is needed to stimulate incretin hormone secretion in overweight subjects with type 2 diabetes

Background/objective

After digestion, dietary triacylglycerol stimulates incretin release in humans, mainly through generation of 2-monoacylglycerol, an agonist for the intestinal G protein-coupled receptor 119 (GPR119). Enhanced incretin release may have beneficial metabolic effects. However, dietary fat may promote weight gain and should therefore be restricted in obesity. We designed C4-dietary oil (1,3-di-butyryl-2-oleoyl glycerol) as a 2-oleoyl glycerol (2-OG)-generating fat type, which would stimulate incretin release to the same extent while providing less calories than equimolar amounts of common triglycerides, e.g., olive oil.

Subjects and methods

We studied the effect over 180 min of (a) 19 g olive oil plus 200 g carrot, (b) 10.7 g C4 dietary oil plus 200 g carrot and (c) 200 g carrot, respectively, on plasma responses of gut and pancreatic hormones in 13 overweight patients with type 2 diabetes (T2D). Theoretically, both oil meals result in formation of 7.7 g 2-OG during digestion.

Results

Both olive oil and C4-dietary oil resulted in greater postprandial (P ≤ 0.01) glucagon-like peptide 1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) responses (incremental area under curve (iAUC)): iAUC_GLP−1: 645 ± 194 and 702 ± 97 pM × min; iAUC_GIP: 4,338 ± 764 and 2,894 ± 601 pM × min) compared to the carrot meal (iAUC_GLP−1: 7 ± 103 pM × min; iAUC_GIP: 266 ± 234 pM × min). iAUC for GLP-1 and GIP were similar for C4-dietary oil and olive oil, although olive oil resulted in a higher peak value for GIP than C4-dietary oil.

Conclusion

C4-dietary oil enhanced secretion of GLP-1 and GIP to almost the same extent as olive oil, in spite of liberation of both 2-OG and oleic acid, which also may stimulate incretin secretion, from olive oil. Thus, C4-dietary oil is more effective as incretin releaser than olive oil per unit of energy and may be useful for dietary intervention.

Targeting the gastrointestinal tract to treat type 2 diabetes

The rising global rates of type 2 diabetes and obesity present a significant economic and social burden, underscoring the importance for effective and safe therapeutic options. The success of glucagon-like-peptide-1 receptor agonists in the treatment of type 2 diabetes, along with the potent glucose-lowering effects of bariatric surgery, highlight the gastrointestinal tract as a potential target for diabetes treatment. Furthermore, recent evidence suggests that the gut plays a prominent role in the ability of metformin to lower glucose levels. As such, the current review highlights some of the current and potential pathways in the gut that could be targeted to improve glucose homeostasis, such as changes in nutrient sensing, gut peptides, gut microbiota and bile acids. A better understanding of these pathways will lay the groundwork for novel gut-targeted antidiabetic therapies, some of which have already shown initial promise.

The Effect of a Dairy-Based Recovery Beverage on Post-Exercise Appetite and Energy Intake in Active Females

This study was designed to assess the effect of a dairy-based recovery beverage on post-exercise appetite and energy intake in active females. Thirteen active females completed three trials in a crossover design. Participants completed 60 min of cycling at 65% V̇O2peak, before a 120 min recovery period. On completion of cycling, participants consumed a commercially available dairy-based beverage (DBB), a commercially available carbohydrate beverage (CHO), or a water control (H₂O). Non-esterified fatty acids, glucose, and appetite-related peptides alongside measures of subjective appetite were sampled at baseline and at 30 min intervals during recovery. At 120 min, energy intake was assessed in the laboratory by ad libitum assessment, and in the free-living environment by weighed food record for the remainder of the study day. Energy intake at the ad libitum lunch was lower after DBB compared to H₂O (4.43 ± 0.20, 5.58 ± 0.41 MJ, respectively; p = 0.046; (95% CI: -2.28, -0.20 MJ)), but was not different to CHO (5.21 ± 0.46 MJ), with no difference between trials thereafter. Insulin and GLP-17-36 were higher following DBB compared to H₂O (p = 0.015 and p = 0.001, respectively) but not to CHO (p = 1.00 and p = 0.146, respectively). In addition, glucagon was higher following DBB compared to CHO (p = 0.008) but not to H₂O (p = 0.074). The results demonstrate that where DBB consumption may manifest in accelerated recovery, this may be possible without significantly affecting total energy intake and subsequent appetite-related responses relative to a CHO beverage.

G protein-coupled receptors: signalling and regulation by lipid agonists for improved glucose homoeostasis

G protein-coupled receptors (GPCRs) play a pivotal role in cell signalling, controlling many processes such as immunity, growth, cellular differentiation, neurological pathways and hormone secretions. Fatty acid agonists are increasingly recognised as having a key role in the regulation of glucose homoeostasis via stimulation of islet and gastrointestinal GPCRs. Downstream cell signalling results in modulation of the biosynthesis, secretion, proliferation and anti-apoptotic pathways of islet and enteroendocrine cells. GPR40 and GPR120 are activated by long-chain fatty acids (>C12) with both receptors coupling to the Gαq subunit that activates the Ca(2+)-dependent pathway. GPR41 and GPR43 are stimulated by short-chain fatty acids (C2-C5), and activation results in binding to Gαi that inhibits the adenylyl cyclase pathway attenuating cAMP production. In addition, GPR43 also couples to the Gαq subunit augmenting intracellular Ca(2+) and activating phospholipase C. GPR55 is specific for cannabinoid endogenous agonists (endocannabinoids) and non-cannabinoid fatty acids, which couples to Gα12/13 and Gαq proteins, leading to enhancing intracellular Ca(2+), extracellular signal-regulated kinase 1/2 (ERK) phosphorylation and Rho kinase. GPR119 is activated by fatty acid ethanolamides and binds to Gαs utilising the adenylate cyclase pathway, which is dependent upon protein kinase A. Current research indicates that GPCR therapies may be approved for clinical use in the near future. This review focuses on the recent advances in preclinical diabetes research in the signalling and regulation of GPCRs on islet and enteroendocrine cells involved in glucose homoeostasis.

Glucagon-like peptide-1: The missing link in the metabolic clock?

Circadian expression of clock genes in peripheral tissues is critical to the coordinated regulation of intestinal digestive and absorptive functions, insulin secretion, and peripheral tissue nutrient deposition during periods of nutrient ingestion, thereby preventing metabolic dysregulation. As glucagon-like peptide-1 is a key incretin hormone that regulates glucose-dependent insulin secretion, we hypothesized that this intestinal hormone is a player in the peripheral metabolic clock, linking nutrient ingestion to insulin secretion. We have now established that secretion of glucagon-like peptide-1 from the intestinal L cell shows a rhythmic pattern in rats and humans in vivo that is altered by circadian disruptors, such as constant light exposure, consumption of a Western diet and feeding at inappropriate times (i.e., during the light period in rodents). Interestingly, the alterations in the rhythm of the glucagon-like peptide-1 secretory responses were found to parallel the changes in the pattern of insulin responses in association with significant impairments in glucose tolerance. Furthermore, we have detected circadian clock gene expression, and showed circadian secretion of glucagon-like peptide-1 from both the murine and human L cell in vitro. These findings demonstrate that glucagon-like peptide-1 is a functional component of the peripheral metabolic clock, and suggest that altered release of glucagon-like peptide-1 might play a role in the metabolic perturbations that result from circadian disruption.

Oleic acid stimulates glucagon-like peptide-1 release from enteroendocrine cells by modulating cell respiration and glycolysis

OBJECTIVE

Glucagon-like peptide-1 (GLP-1) is a potent satiating and incretin hormone released by enteroendocrine L-cells in response to eating. Dietary fat, in particular monounsaturated fatty acids, such as oleic acid (OA), potently stimulates GLP-1 secretion from L-cells. It is, however, unclear whether the intracellular metabolic handling of OA is involved in this effect.

METHODS

First we determined the optimal medium for the bioenergetics measurements. Then we examined the effect of OA on the metabolism of the immortalized enteroendocrine GLUTag cell model and assessed GLP-1 release in parallel. We measured oxygen consumption rate and extracellular acidification rate in response to OA and to different metabolic inhibitors with the Seahorse extracellular flux analyzer.

RESULTS

OA increased cellular respiration and potently stimulated GLP-1 release. The fatty acid oxidation inhibitor etomoxir did neither reduce OA-induced respiration nor affect the OA-induced GLP-1 release. In contrast, inhibition of the respiratory chain or of downstream steps of aerobic glycolysis reduced the OA-induced GLP-1 release, and an inhibition of the first step of glycolysis by addition of 2-deoxy-d-glucose even abolished it.

CONCLUSION

These findings indicate that an indirect stimulation of glycolysis is crucial for the OA-induced release of GLP-1.

Stimulation of incretin secreting cells

The incretin hormones glucose-dependent insulinotropic polypeptide (GIP) and glucagon like peptide-1 (GLP-1) are secreted from enteroendocrine cells in the gut and regulate physiological and homeostatic functions related to glucose control, metabolism and food intake. This review provides a systematic summary of the molecular mechanisms underlying secretion from incretin cells, and an understanding of how they sense and interact with lumen and vascular factors and the enteric nervous system through transporters and G-protein coupled receptors (GPCRs) present on their surface to ultimately culminate in hormone release. Some of the molecules described below such as sodium coupled glucose transporter 1 (SGLT1), G-protein coupled receptor (GPR) 119 and GPR40 are targets of novel therapeutics designed to enhance endogenous gut hormone release. Synthetic ligands at these receptors aimed at treating obesity and type 2 diabetes are currently under investigation.

Free-fatty acid receptor-4 (GPR120): Cellular and molecular function and its role in metabolic disorders

Over the last decade, a subfamily of G protein-coupled receptors that are agonized by endogenous and dietary free-fatty acids (FFA) has been discovered. These free-fatty acid receptors include FFA2 and FFA3, which are agonized by short-chained FFA, as well as FFA1 and FFA4, which are agonized by medium-to-long chained FFA. Ligands for FFA1 and FFA4 comprise the family of long chain polyunsaturated omega-3 fatty acids including α-linolenic acid (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA), suggesting that many of the long-known beneficial effects of these fats may be receptor mediated. In this regard, FFA4 has gathered considerable interest due to its role in ameliorating inflammation, promoting insulin sensitization, and regulating energy metabolism in response to FFA ligands. The goal of this review is to summarize the body of evidence in regard to FFA4 signal transduction, its mechanisms of regulation, and its functional role in a variety of tissues. In addition, recent endeavors toward discovery of small molecules that modulate FFA4 activity are also presented.

Potential mechanism of enhanced postprandial glucagon-like peptide-1 release following treatment with a diacylglycerol acyltransferase 1 inhibitor

Studies have demonstrated that blockade of diacylglycerol acyltransferase 1 (DGAT1) leads to prolonged release of glucagon‐like peptide 1 (GLP‐1) after meal challenge. The current study was undertaken to investigate the mechanism of action underlying the elevated levels of GLP‐1 release following pharmacological inhibition of DGAT1. We utilized a potent, specific DGAT1 inhibitor, compound A, to investigate the changes in intestinal lipid profile in a mouse model after oral administration of the compound and challenge with tracer containing fatty meal. [¹³C₁₈]‐oleic acid and LC‐MS were employed to trace the fate of dietary fatty acids provided as part of a meal challenge in lean mice. Lipid profiles in plasma, proximal to distal segments of intestine, and feces were evaluated at various times following the meal challenge to study the kinetics of fatty acid absorption, synthesis into complex lipids, and excretion. Pharmacological inhibition of DGAT1 led to reduction of postprandial total and newly synthesized triglyceride (TG) excursion and significant increases in TG and FFA levels in the distal portion of intestine enriched with enteroendocrine L cells. Enhanced levels of FFA and cholesteryl ester were observed via fecal fat profiling. DGAT1 inhibition leads to enhancement of carbon flow to the synthesis of phosphatidylcholine within the intestine. DGAT1 inhibition markedly increases levels of TG and FFA in the distal intestine, which could be the predominant contributor to the prolonged and enhanced postprandial GLP‐1 release. Inactivation of DGAT1 could provide potential benefit in the treatment of dysmetabolic diseases.

Increased Glucose-induced Secretion of Glucagon-like Peptide-1 in Mice Lacking the Carcinoembryonic Antigen-related Cell Adhesion Molecule 2 (CEACAM2)

Carcinoembryonic antigen-related cell adhesion molecule 2 (CEACAM2) regulates food intake as demonstrated by hyperphagia in mice with the Ceacam2 null mutation (Cc2(-/-)). This study investigated whether CEACAM2 also regulates insulin secretion. Ceacam2 deletion caused an increase in β-cell secretory function, as assessed by hyperglycemic clamp analysis, without affecting insulin response. Although CEACAM2 is expressed in pancreatic islets predominantly in non-β-cells, basal plasma levels of insulin, glucagon and somatostatin, islet areas, and glucose-induced insulin secretion in pooled Cc2(-/-) islets were all normal. Consistent with immunofluorescence analysis showing CEACAM2 expression in distal intestinal villi, Cc2(-/-) mice exhibited a higher release of oral glucose-mediated GLP-1, an incretin that potentiates insulin secretion in response to glucose. Compared with wild type, Cc2(-/-) mice also showed a higher insulin excursion during the oral glucose tolerance test. Pretreating with exendin(9-39), a GLP-1 receptor antagonist, suppressed the effect of Ceacam2 deletion on glucose-induced insulin secretion. Moreover, GLP-1 release into the medium of GLUTag enteroendocrine cells was increased with siRNA-mediated Ceacam2 down-regulation in parallel to an increase in Ca(2+) entry through L-type voltage-dependent Ca(2+) channels. Thus, CEACAM2 regulates insulin secretion, at least in part, by a GLP-1-mediated mechanism, independent of confounding metabolic factors.

Pharmacology and physiology of gastrointestinal enteroendocrine cells

Gastrointestinal (GI) polypeptides are secreted from enteroendocrine cells (EECs). Recent technical advances and the identification of endogenous and synthetic ligands have enabled exploration of the pharmacology and physiology of EECs. Enteroendocrine signaling pathways stimulating hormone secretion involve multiple nutrient transporters and G protein-coupled receptors (GPCRs), which are activated simultaneously under prevailing nutrient conditions in the intestine following a meal. The majority of studies investigate hormone secretion from EECs in response to single ligands and although the mechanisms behind how individual signaling pathways generate a hormonal output have been well characterized, our understanding of how these signaling pathways converge to generate a single hormone secretory response is still in its infancy. However, a picture is beginning to emerge of how nutrients and full, partial, or allosteric GPCR ligands differentially regulate the enteroendocrine system and its interaction with the enteric and central nervous system. So far, activation of multiple pathways underlies drug discovery efforts to harness the therapeutic potential of the enteroendocrine system to mimic the phenotypic changes observed in patients who have undergone Roux-en-Y gastric surgery. Typically obese patients exhibit ∼30% weight loss and greater than 80% of obese diabetics show remission of diabetes. Targeting combinations of enteroendocrine signaling pathways that work synergistically may manifest with significant, differentiated EEC secretory efficacy. Furthermore, allosteric modulators with their increased selectivity, self-limiting activity, and structural novelty may translate into more promising enteroendocrine drugs. Together with the potential to bias enteroendocrine GPCR signaling and/or to activate multiple divergent signaling pathways highlights the considerable range of therapeutic possibilities available. Here, we review the pharmacology and physiology of the EEC system.

The Anthocyanin Delphinidin 3-Rutinoside Stimulates Glucagon-Like Peptide-1 Secretion in Murine GLUTag Cell Line via the Ca2+/Calmodulin-Dependent Kinase II Pathway

Glucagon-like peptide-1 (GLP-1) is an incretin hormone secreted from enteroendocrine L-cells. Although several nutrients induce GLP-1 secretion, there is little evidence to suggest that non-nutritive compounds directly increase GLP-1 secretion. Here, we hypothesized that anthocyanins induce GLP-1 secretion and thereby significantly contribute to the prevention and treatment of diabetes. Delphinidin 3-rutinoside (D3R) was shown to increase GLP-1 secretion in GLUTag L cells. The results suggested that three hydroxyl or two methoxyl moieties on the aromatic ring are essential for the stimulation of GLP-1 secretion. Notably, the rutinose moiety was shown to be a potent enhancer of GLP-1 secretion, but only in conjunction with three hydroxyl moieties on the aromatic ring (D3R). Receptor antagonist studies revealed that D3R-stimulates GLP-1 secretion involving inositol 1,4,5-trisphosphate receptor-mediated intracellular Ca²⁺ mobilization. Treatment of GLUTag cells with a Ca²⁺/calmodulin-dependent kinaseII (CaMKII) inhibitor (KN-93) abolished D3R-stimulated GLP-1 secretion. In addition, treatment of GLUTag cells with D3R resulted in activation of CaMKII. Pre-treatment of cells with a G protein-coupled receptor (GPR) 40/120 antagonist (GW1100) also significantly decreased D3R-stimulated GLP-1 secretion. These observations suggest that D3R stimulates GLP-1 secretion in GLUTag cells, and that stimulation of GLP-1 secretion by D3R is mediated via Ca²⁺-CaMKII pathway, which may possibly be mediated by GPR40/120. These findings provide a possible molecular mechanism of GLP-1 secretion in intestinal L-cells mediated by foods or drugs and demonstrate a novel biological function of anthocyanins in regards to GLP-1 secretion.

Possible abilities of dietary factors to prevent and treat diabetes via the stimulation of glucagon-like peptide-1 secretion

There is a pressing need for countermeasures against diabetes, which has increased in incidence, becoming a global issue. Glucagon-like peptide-1 (GLP-1), a molecule secreted in enteroendocrine L cells in the lower small and large intestines, is thought to be one of the most important molecular targets for the prevention and treatment of diabetes. There has been increasing interest in the possible ability of dietary factors to treat diabetes via modulating GLP-1 secretion. There is thought to be a close relationship between incretin and diet, and the purported best approach for using dietary factors to increase GLP-1 activity is promotion of secretion of endogenous GLP-1. It have been reported that nutrients as well as various non-nutrient dietary factors can function as GLP-1 secretogogues. Here, we present our findings on the GLP-1 secretion-stimulating functions of two dietary factors, curcumin and extract of edible sweet potato leaves, which contain caffeoylquinic acid derivatives. However, it is necessary to reveal in greater detail the stimulation of GLP-1 secretion by dietary factors for preventing and treating diabetes. It is desirable to clarify the exact GLP-1 secretory pathway, the effect of metabolites derived from dietary factors in gut lumen, and the relationship between incretin and meal.

Fatty acid sensing in the gut and the hypothalamus: in vivo and in vitro perspectives

The ability to properly sense both ingested and circulating nutrients is crucial for the maintenance of metabolic homeostasis. As such, both the gastrointestinal tract and the hypothalamus have demonstrated the capacity to sense and effectively respond to nutrients, such as fatty acids, to control food intake and glucose production to regulate energy and glucose homeostasis. In modern, Westernized societies, obesity and diabetes rates continue to rise unabated, due in part to an increase in highly palatable high-fat diet consumption. Thus, our understanding in the ability of the body to successfully monitor lipids is more vital than ever. This review details the current understanding of both the gut and the brain, specifically the hypothalamus, in sensing fatty acids. Highlighting both in vivo and in vitro studies, we explore some of the mechanisms upon which different fatty acids activate enteroendocrine and neural lipid-sensing signaling mechanisms to subsequently lower food intake and glucose production to ultimately regulate metabolic homeostasis. A better understanding of these lipid-sensing pathways could lay the groundwork for successful pharmacological targets for the treatment of obesity and diabetes.

Evaluation of the insulin-releasing and glucose-lowering effects of GPR120 activation in pancreatic β-cells

AIMS

To assess the potency and selectivity of various GPR120 agonists and to determine the cellular localization of GPR120 in clonal β-cells and pancreatic islets.

METHODS

Insulin secretion and alterations in intracellular Ca(2+) and cAMP response to glucose and GPR120 agonists, including endogenous agonists α-linolenic acid (ALA), docosahexaenoic acid (DHA), eicosapentaenoic acid (EPA) and a synthetic analogue (GW-9508), were examined using clonal pancreatic BRIN-BD11 cells, mouse pancreatic islets and in vivo studies using NIH Swiss mice. Cytotoxicity was assessed by lactate dehydrogenase release. Cellular localization of GPR120 was explored by double-staining immunohistochemistry.

RESULTS

The most potent and selective GPR120 agonist tested was ALA (half maximum effective concentration 1.2 × 10(-8) mol/l) with a maximum stimulation of insulin secretion of 53% at 10(-4) mol/l (p < 0.001) in BRIN-BD11 cells. Stimulation of insulin secretion was also observed with GW-9508 (6.4 × 10(-8) mol/l; 47%), EPA (7.9 × 10(-8) mol/l; 36%) and DHA (1.0 × 10(-7) mol/l; 50%). Results were corroborated by islet studies, with no evidence of cytotoxic effects. Dose-dependent insulin secretion by GPR120 agonists was glucose-sensitive and accompanied by significant elevations of intracellular Ca(2+) and cAMP. Immunocytochemistry showed GPR120 expression on BRIN-BD11 cells and was confined to islet β-cells with no distribution on α-cells. Administration of GPR120 agonists (0.1 µmol/kg body weight) in glucose tolerance studies significantly reduced plasma glucose and augmented insulin release in mice.

CONCLUSIONS

These results indicate that GPR120 is expressed on pancreatic β-cells and that agonists at this receptor are potent insulin secretagogues with therapeutic potential for type 2 diabetes.

Glucagon-Like Peptide 2 Improves Cholestasis in Parenteral Nutrition--Associated Liver Disease

BACKGROUND

Parenteral nutrition-associated liver disease (PNALD) remains a significant cause of morbidity and mortality in neonates with intestinal failure. Although glucagon-like peptide-2 (GLP-2) is being advanced as therapy, the effect of GLP-2 treatment on PNALD is unknown. We aim to investigate the effect of exogenous GLP-2 administration on hepatic function in a neonatal piglet model of PNALD.

METHODS

Neonatal piglets (aged 2-6 days) underwent jugular venous catheterization to receive isonitrogenous, isocaloric parenteral nutrition (PN). Piglets were allocated to 2 groups: group 1 (n = 8) received saline while group 2 (n = 7) received GLP-2 (at 11 nmol/kg/d). After 17 days, piglets underwent terminal laparotomy, and bile flow was measured. Liver specimens were analyzed histologically and with immunoperoxidase staining. Age-matched sow-reared control piglets (group 3, n = 8) were used for comparison.

RESULTS

Both groups 1 and 2 receiving PN developed cholestasis relative to sow-reared controls, as evidenced by a decrease in bile flow and increase in serum total bilirubin. However, group 2 had improved bile flow (1.35 vs 0.73 µL/g; P = .02) and diminished bilirubin (38.0 vs 78.5 µmol/L; P = .008) compared with group 1. Group 2 also had lower serum alanine aminotransferase levels, a marker of liver injury. Histologically, the liver specimens in group 1 had marked hepatocyte pigmentation, which was decreased in group 2 specimens.

CONCLUSIONS

The exogenous administration of GLP-2 is associated with the improvement of cholestasis and liver injury. This study introduces a novel role for GLP-2 in improving PNALD in the setting of prolonged PN duration.

Potential roles of GPR120 and its agonists in the management of diabetes

Free fatty acids (FFAs) serve not only as nutrients that provide energy but also as extracellular signaling molecules that manipulate intracellular physiological events through FFA receptors (FFARs) such as FFAR4. FFAR4 is also known as G-protein coupled receptor 120 (GPR120). The main role of GPR120 is to elicit FFA regulation on metabolism homeostasis. GPR120 agonism correlates with prevention of the occurrence and development of metabolic disorders such as obesity and diabetes. GPR120 activation directly or indirectly inhibits inflammation, modulates hormone secretion from the gastrointestinal tract and pancreas, and regulates lipid and/or glucose metabolism in adipose, liver, and muscle tissues, which may help prevent obesity and diabetes. This review summarizes recent advances in physiological roles of GPR120 in preventing insulin resistance and protecting pancreatic islet function, and examines how resident GPR120 in the pancreas may be involved in modulating pancreatic islet function.

Role of vesicle-associated membrane protein 2 in exocytosis of glucagon-like peptide-1 from the murine intestinal L cell

AIMS/HYPOTHESIS

Glucagon-like peptide-1 (GLP-1), secreted by the enteroendocrine L cell, is an incretin hormone that potently stimulates insulin secretion. Although signalling pathways promoting GLP-1 release are well characterised, the mechanisms by which GLP-1-containing granules fuse to the L cell membrane are unknown. As soluble NSF attachment proteins (SNAREs) are known to mediate granule-membrane fusion, the role of vesicle-associated membrane proteins (VAMPs) in GLP-1 exocytosis was examined.

METHODS

SNARE expression was determined in murine GLUTag L cells by RT-PCR and immunoblot and in primary murine L cells by immunofluorescence. Co-immunoprecipitation was used to examine SNARE interactions, while tetanus toxin (TetX)-mediated cleavage of VAMP was used with a GLP-1 secretion assay and total internal reflection fluorescence microscopy to determine the role of VAMP2 in exocytosis.

RESULTS

VAMP2 was expressed in murine L cells and localised to secretory granules in GLUTag cells. VAMP1/3 and the core membrane proteins syntaxin1a and synaptosomal-associated protein 25 kDa (SNAP25) were also detected. TetX cleaved VAMPs in GLUTag cells. However, only VAMP2 interacted with syntaxin1a, as did SNAP25 and Munc18-1. TetX treatment of GLUTag cells prevented glucose-dependent insulinotrophic peptide- and oleic-acid-stimulated GLP-1 secretion (p < 0.05-0.01), as well as K(+)-stimulated single-cell exocytosis (p < 0.05-0.001), while TetX-resistant VAMP2 expression rescued GLP-1 secretion (p < 0.01-0.001).

CONCLUSIONS/INTERPRETATION

Together, these findings indicate an essential role for VAMP2 in GLP-1 exocytosis from the GLUTag L cell in response to a variety of established secretagogues. An improved understanding of the mechanisms governing the release of GLP-1 may lead to new therapeutic approaches to enhance the levels of this incretin hormone in patients with type 2 diabetes.

Glucagon-like peptide-1: glucose homeostasis and beyond

Glucagon-like peptide-1 (GLP-1), an incretin hormone secreted primarily from the intestinal L-cells in response to meals, modulates nutrient homeostasis via actions exerted in multiple tissues and cell types. GLP-1 and its analogs, as well as compounds that inhibit endogenous GLP-1 breakdown, have become an effective therapeutic strategy for many subjects with type 2 diabetes. Here we review the discovery of GLP-1; its synthesis, secretion, and elimination from the circulation; and its multiple pancreatic and extrapancreatic effects. Finally, we review current options for GLP-1-based diabetes therapy, including GLP-1 receptor agonism and inhibition of GLP-1 breakdown, as well as the benefits and drawbacks of different modes of therapy and the potential for new therapeutic avenues.

Acyl chain length, saturation, and hydrophobicity modulate the efficiency of dietary fatty acid absorption in adult humans

Intestinal fat absorption is known to be, overall, a highly efficient process, but much less is known about the efficiency with which individual dietary fatty acids (FA) are absorbed by the adult small intestine. We therefore measured the absorption efficiency of the major dietary FA using sucrose polybehenate (SPB) as a nonabsorbable marker and analyzed how it is modulated by acyl chain physicochemical properties and polymorphisms of proteins involved in chylomicron assembly. Dietary FA absorption efficiency was measured in 44 healthy subjects fed a standard diet containing 35% fat and 5% SPB. FA and behenic acid (BA) were measured in homogenized diets and stool samples by gas chromatography-mass spectroscopy, and coefficients of absorption for each FA were calculated as 1 - [(FA/BA)feces/(FA/BA)diet]. Absorption coefficients for saturated FA decreased with increasing chain length and hydrophobicity (mean ± SE) and ranged from 0.95 ± 0.02 for myristate (14:0), 0.80 ± 0.03 for stearate (18:0), to 0.26 ± 0.02 for arachidate (20:0). Absorption coefficients for unsaturated FA increased with increasing desaturation from 0.79 ± 0.03 for elaidic acid (18:1t), 0.96 ± 0.01 for linoleate (18:2), to near complete absorption for eicosapentaenoic (20:5) and docosahexaenoic (22:6) acids. Of several common genetic polymorphisms in key proteins involved in the chylomicron assembly pathway, only the intestinal fatty acid-binding protein-2 A54T allele (rs1799883) had any impact on FA absorption. We conclude that acyl chain length, saturation, and hydrophobicity are the major determinants of the efficiency with which dietary FA are absorbed by the adult small intestine.

Dietary lipids and sweeteners regulate glucagon-like peptide-2 secretion

Glucagon-like peptide-2 (GLP-2) is a potent intestinal growth factor derived from enteroendocrine L cells. Although food intake is known to increase GLP-2 secretion, its regulatory mechanisms are largely unknown as a result of its very short half-life in venules. The aims of this study were to compare the effects of luminal nutrients on the stimulation of GLP-2 secretion in vivo using lymph samples and to clarify the involvement of the sweet taste receptor in this process in vitro. Lymph samples were collected from the thoracic duct after bolus administration of dietary lipids or sweetening agents into the duodenum of rats. Human enteroendocrine NCI-H716 cells were also used to compare the effects of various nutrients on GLP-2 secretion. GLP-2 concentrations were measured by ELISA in vivo and in vitro. GLP-2 secretion was enhanced by polyunsaturated fatty acid- and monounsaturated fatty acid-rich dietary oils, dietary carbohydrates, and some kinds of sweeteners in rats; this effect was reproduced in NCI-H716 cells using α-linolenic acid (αLA), glucose, and sweeteners. GLP-2 secretion induced by sweetening agents was inhibited by lactisole, a sweetness-antagonizing inhibitor of T1R3. In contrast, lactisole was unable to inhibit GLP-2 secretion induced by αLA alone. Our results suggested that fatty acid- and sweetener-induced GLP-2 secretion may be mediated by two different pathways, with the sweet taste receptor involved in the regulation of the latter.

Gustducin couples fatty acid receptors to GLP-1 release in colon

Sweet taste receptor subunits and α-gustducin found in enteroendocrine cells of the small intestine have been implicated in release of the incretin hormones glucagon-like peptide 1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) in response to glucose and noncaloric sweeteners. α-Gustducin has also been found in colon, although its function there is unclear. We examined expression of α-gustducin, GLP-1, and GIP throughout the intestine. The number of α-gustducin-expressing cells and those coexpressing α-gustducin together with GLP-1 and/or GIP increased from small intestine to colon. α-Gustducin also was coexpressed with fatty acid G protein-coupled receptor (GPR) 40, GPR41, GPR43, GPR119, GPR120, and bile acid G protein-coupled receptor TGR5 in enteroendocrine cells of the colon. In colon, GPR43 was coexpressed with GPR119 and GPR120, but not with TGR5. Treatment of colonic mucosa isolated from wild-type mice with acetate, butyrate, oleic acid, oleoylethanolamide, or lithocholic acid stimulated GLP-1 secretion. However, GLP-1 release in response to these fatty acids was impaired in colonic tissue from α-gustducin knockout mice.

GLP-1 and GLP-2 as yin and yang of intestinal lipoprotein production: evidence for predominance of GLP-2-stimulated postprandial lipemia in normal and insulin-resistant states

The glucagon-like peptides (GLP-1 and GLP-2) are processed from the proglucagon polypeptide and secreted in equimolar amounts but have opposite effects on chylomicron (CM) production, with GLP-1 significantly reducing and GLP-2 increasing postprandial chylomicronemia. In the current study, we evaluated the apparent paradoxical roles of GLP-1 and GLP-2 under physiological conditions in the Syrian golden hamster, a model with close similarity to humans in terms of lipoprotein metabolism. A short (30-min) intravenous infusion of GLP-2 resulted in a marked increase in postprandial apolipoprotein B48 (apoB48) and triglyceride (TG) levels in the TG-rich lipoprotein (TRL) fraction, whereas GLP-1 infusion decreased lipid absorption and levels of TRL-TG and apoB48. GLP-1 and GLP-2 coinfusion resulted in net increased lipid absorption and an increase in TRL-TG and apoB48. However, prolonged (120-min) coinfusion of GLP-1 and GLP-2 decreased postprandial lipemia. Blocking dipeptidyl peptidase-4 activity resulted in decreased postprandial lipemia. Interestingly, fructose-fed, insulin-resistant hamsters showed a more pronounced response, including possible hypersensitivity to GLP-2 or reduced sensitivity to GLP-1. In conclusion, under normal physiological conditions, the actions of GLP-2 predominate; however, when GLP-1 activity is sustained, the hypolipidemic action of GLP-1 predominates. Pharmacological inhibition of GLP-1 degradation tips the balance toward an inhibitory effect on intestinal production of atherogenic CM particles.

The gut endocrine system as a coordinator of postprandial nutrient homoeostasis

Hormones from the gastrointestinal (GI) tract are released following food ingestion and trigger a range of physiological responses including the coordination of appetite and glucose homoeostasis. The aim of this review is to discuss the pathways by which food ingestion triggers secretion of cholecystokinin (CCK), glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) and the altered patterns of gut hormone release observed following gastric bypass surgery. Our understanding of how ingested nutrients trigger secretion of these gut hormones has increased dramatically, as a result of physiological studies in human subjects and animal models and in vitro studies on cell lines and primary intestinal cultures. Specialised enteroendocrine cells located within the gut epithelium are capable of directly detecting a range of nutrient stimuli through a range of receptors and transporters. It is concluded that the arrival of nutrients at the apical surface of enteroendocrine cells is a major stimulus for gut hormone release, thereby coupling these endocrine signals to the arrival of absorbed nutrients in the bloodstream.

A potent class of GPR40 full agonists engages the enteroinsular axis to promote glucose control in rodents

Type 2 diabetes is characterized by impaired glucose homeostasis due to defects in insulin secretion, insulin resistance and the incretin response. GPR40 (FFAR1 or FFA1) is a G-protein-coupled receptor (GPCR), primarily expressed in insulin-producing pancreatic β-cells and incretin-producing enteroendocrine cells of the small intestine. Several GPR40 agonists, including AMG 837 and TAK-875, have been disclosed, but no GPR40 synthetic agonists have been reported that engage both the insulinogenic and incretinogenic axes. In this report we provide a molecular explanation and describe the discovery of a unique and potent class of GPR40 full agonists that engages the enteroinsular axis to promote dramatic improvement in glucose control in rodents. GPR40 full agonists AM-1638 and AM-6226 stimulate GLP-1 and GIP secretion from intestinal enteroendocrine cells and increase GSIS from pancreatic islets, leading to enhanced glucose control in the high fat fed, streptozotocin treated and NONcNZO10/LtJ mouse models of type 2 diabetes. The improvement in hyperglycemia by AM-1638 was reduced in the presence of the GLP-1 receptor antagonist Ex(9–39)NH₂.

Role of fatty acid transport protein 4 in oleic acid-induced glucagon-like peptide-1 secretion from murine intestinal L cells

The antidiabetic intestinal L cell hormone glucagon-like peptide-1 (GLP-1) enhances glucose-dependent insulin secretion and inhibits gastric emptying. GLP-1 secretion is stimulated by luminal oleic acid (OA), which crosses the cell membrane by an unknown mechanism. We hypothesized that L cell fatty acid transport proteins (FATPs) are essential for OA-induced GLP-1 release. Therefore, the murine GLUTag L cell model was used for immunoblotting, [(3)H]OA uptake assay, and GLP-1 secretion assay as determined by radioimmunoassay following treatment with OA ± phloretin, sulfo-N-succinimidyl oleate, or siRNA against FATP4. FATP4(-/-) and cluster-of-differentiation 36 (CD36)(-/-) mice received intraileal OA, and plasma GLP-1 was measured by sandwich immunoassay. GLUTag cells were found to express CD36, FATP1, FATP3, and FATP4. The cells demonstrated specific (3)H[OA] uptake that was dose-dependently inhibited by 500 and 1,000 μM unlabeled OA (P < 0.001). Cell viability was not altered by treatment with OA. Phloretin and sulfo-N-succinimidyl oleate, inhibitors of protein-mediated transport and CD36, respectively, also decreased [(3)H]OA uptake, as did knockdown of FATP4 by siRNA transfection (P < 0.05-0.001). OA dose-dependently increased GLP-1 secretion at 500 and 1,000 μM (P < 0.001), whereas phloretin, sulfo-N-succinimidyl oleate, and FATP4 knockdown decreased this response (P < 0.05-0.01). FATP4(-/-) mice displayed lower plasma GLP-1 at 60 min in response to intraileal OA (P < 0.05), whereas, unexpectedly, CD36(-/-) mice displayed higher basal GLP-1 levels (P < 0.01) but a normal response to intraileal OA. Together, these findings demonstrate a key role for FATP4 in OA-induced GLP-1 secretion from the murine L cell in vitro and in vivo, whereas the precise role of CD36 remains unclear.

Uncoupling protein 2 regulates glucagon-like peptide-1 secretion in L-cells

AIM: To investigate whether uncoupling protein 2 (UCP2) affects oleic acid-induced secretion of glucagon-like peptide-1 (GLP-1) in L-cells.

METHODS: mRNA and protein expression of UCP2 were analyzed in human NCI-H716 cells, which serve as a model for enteroendocrine L-cells, by quantitative reverse transcription-polymerase chain reaction and Western blotting before and after treatment with oleic acid. Localization of UCP2 and GLP-1 in NCI-H716 cells was assessed by immunofluorescence labeling. NCI-H716 cells were transiently transfected with a small interfering RNA (siRNA) that targets UCP2 (siUCP2) or with a non-specific siRNA using Lipofectamine 2000. The concentrations of bioactive GLP-1 in the medium were measured by enzyme linked immunosorbent assay.

RESULTS: Both GLP-1 and UCP2 granules were expressed mainly in the cytoplasm of NCI-H716 cells. NCI-H716 cells that secreted GLP-1 also expressed UCP2. Time-course experiments revealed that release of GLP-1 from NCI-H716 cells into the medium reached a maximum at 120 min and remained stable until at least 180 min after treatment with oleic acid (the level of GLP-1 increased about 2.3-fold as compared with the level of GLP-1 in the control cells, P < 0.05). In an experiment to determine dose dependence, stimulation of NCI-H716 cells with ≤ 8 mmol oleic acid led to a concentration-dependent release of GLP-1 into the medium; 10 mmol oleic acid diminished the release of GLP-1. Furthermore, GLP-1 secretion induced by oleic acid from NCI-H716 cells that were transfected with siUCP2 decreased to 41.8%, as compared with NCI-H716 cells that were transfected with a non-specific siRNA (P < 0.01).

CONCLUSION: UCP2 affected GLP-1 secretion induced by oleic acid. UCP2 plays an important role in L-cell secretion that is induced by free fatty acids.