GDF15 promotes weight loss by enhancing energy expenditure in muscle.

Caloric restriction that promotes weight loss is an effective strategy for treating non-alcoholic fatty liver disease and improving insulin sensitivity in people with type 2 diabetes1. Despite its effectiveness, in most individuals, weight loss is usually not maintained partly due to physiological adaptations that suppress energy expenditure, a process known as adaptive thermogenesis, the mechanistic underpinnings of which are unclear2,3. Treatment of rodents fed a high-fat diet with recombinant growth differentiating factor 15 (GDF15) reduces obesity and improves glycaemic control through glial-cell-derived neurotrophic factor family receptor α-like (GFRAL)-dependent suppression of food intake4-7. Here we find that, in addition to suppressing appetite, GDF15 counteracts compensatory reductions in energy expenditure, eliciting greater weight loss and reductions in non-alcoholic fatty liver disease (NAFLD) compared to caloric restriction alone. This effect of GDF15 to maintain energy expenditure during calorie restriction requires a GFRAL-ß-adrenergic-dependent signalling axis that increases fatty acid oxidation and calcium futile cycling in the skeletal muscle of mice. These data indicate that therapeutic targeting of the GDF15-GFRAL pathway may be useful for maintaining energy expenditure in skeletal muscle during caloric restriction.

Standard procedures for blood withdrawal in conscious male rats induce stress and profoundly affect glucose tolerance and secretion of glucoregulatory hormones.

OBJECTIVE: A fundamental difference between physiological and pharmacological studies in rats and humans is that withdrawal of blood from conscious rats necessitates restraint which inevitably inflicts a higher level of stress. We investigated the impact of handling on acute glucose regulation and secretion of glucoregulatory hormones in rats. METHODS: Fasted male Sprague Dawley rats (375-400 g, n = 11) were given an oral glucose tolerance test (OGTT) by gavage (2 g/kg). Blood was sampled frequently until 90 min after challenge by handheld sampling (HS) or by automated sampling (AS). In the HS experiment, blood was withdrawn by restraint and sublingual vein puncture; two weeks later, samples were obtained by AS through an implanted catheter in a carotid artery, allowing sampling without disturbing the animals. RESULTS: On the day of HS, post challenge glucose AUCs were ∼17% higher (P < 0.0001), despite gastric emptying (AUC) being reduced by ∼30% (P < 0.0001). Plasma insulin AUC was 3.5-fold lower (P < 0.001), and glucose-dependent insulinotropic peptide (GIP) AUC was reduced by ∼36% but glucagon-like peptide-1 concentrations were not affected. Glucagon concentrations were higher both before and after challenge (fold difference in AUCs = 3.3). Adrenocorticotropin (ACTH) and corticosterone AUCs were 2.4-fold and 3.6-fold higher (P < 0.001), respectively. DISCUSSION AND CONCLUSION: Our study highlights that sampling of blood from conscious rats by sublingual vein puncture inflicts stress which reduces glucose absorption and glucose tolerance and blunts secretion of insulin and GIP. As blood sampling in humans are less stressful, standard procedures of conducting OGTT's in rats by HS presumably introduce an interspecies difference that may have negative consequences for translatability of test results.

Housing-temperature reveals energy intake counter-balances energy expenditure in normal-weight, but not diet-induced obese, male mice.

Most metabolic studies on mice are performed at room temperature, although under these conditions mice, unlike humans, spend considerable energy to maintain core temperature. Here, we characterize the impact of housing temperature on energy expenditure (EE), energy homeostasis and plasma concentrations of appetite- and glucoregulatory hormones in normal-weight and diet-induced obese (DIO) C57BL/6J mice fed chow or 45% high-fat-diet, respectively. Mice were housed for 33 days at 22, 25, 27.5, and 30 °C in an indirect-calorimetry-system. We show that energy expenditure increases linearly from 30 °C towards 22 °C and is ~30% higher at 22 °C in both mouse models. In normal-weight mice, food intake counter-balances EE. In contrast, DIO mice do not reduce food intake when EE is lowered. By end of study, mice at 30 °C, therefore, had higher body weight, fat mass and plasma glycerol and triglycerides than mice at 22 °C. Dysregulated counterbalancing in DIO mice may result from increased pleasure-based eating.

Targeting the Gut in Obesity: Signals from the Inner Surface.

Obesity is caused by prolonged energy surplus. Current anti-obesity medications are mostly centralized around the energy input part of the energy balance equation by increasing satiety and reducing appetite. Our gastrointestinal tract is a key organ for regulation of food intake and supplies a tremendous number of circulating signals that modulate the activity of appetite-regulating areas of the brain by either direct interaction or through the vagus nerve. Intestinally derived messengers are manifold and include absorbed nutrients, microbial metabolites, gut hormones and other enterokines, collectively comprising a fine-tuned signalling system to the brain. After a meal, nutrients directly interact with appetite-inhibiting areas of the brain and induce satiety. However, overall feeding behaviour also depends on secretion of gut hormones produced by highly specialized and sensitive enteroendocrine cells. Moreover, circulating microbial metabolites and their interactions with enteroendocrine cells further contribute to the regulation of feeding patterns. Current therapies exploiting the appetite-regulating properties of the gut are based on chemically modified versions of the gut hormone, glucagon-like peptide-1 (GLP-1) or on inhibitors of the primary GLP-1 inactivating enzyme, dipeptidyl peptidase-4 (DPP-4). The effectiveness of these approaches shows that that the gut is a promising target for therapeutic interventions to achieve significant weigh loss. We believe that increasing understanding of the functionality of the intestinal epithelium and new delivery systems will help develop selective and safe gut-based therapeutic strategies for improved obesity treatment in the future. Here, we provide an overview of the major homeostatic appetite-regulating signals generated by the intestinal epithelial cells and how these signals may be harnessed to treat obesity by pharmacological means.

Plasma GDF15 levels are similar between subjects after bariatric surgery and matched controls and are unaffected by meals.

Growth differentiating factor 15 (GDF15) is expressed in the intestine and is one of the most recently identified satiety peptides. The mechanisms controlling its secretion are unclear. The present study investigated whether plasma GDF15 concentrations are meal-related and if potential responses depend on macronutrient type or are affected by previous bariatric surgery. The study included 1) volunteers ingesting rapidly vs. slowly digested carbohydrates (sucrose vs. isomaltose; n = 10), 2) volunteers who had undergone Roux-en-Y gastric bypass (RYGB) or sleeve gastrectomy (SG) surgery and unoperated matched controls ingesting a liquid mixed meal (n = 9-10 in each group), and 3) individuals with previous RYGB compared with unoperated controls ingesting isocaloric glucose, fat, or protein (n = 6 in each group). Plasma was collected after an overnight fast and up to 6 h after ingestion (≥12 time points). In cohort 1, fasting GDF15 concentrations were ∼480 pg/mL. Concentrations after sucrose or isomaltose intake did not differ from baseline (P = 0.26 to P > 0.99) and total area under the curves (tAUCs were similar between groups (P = 0.77). In cohort 2, fasting GDF15 concentrations were as follows (pg/mL): RYGB = 540 ± 41.4, SG = 477 ± 36.4, and controls = 590 ± 41.8, with no between-group differences (P = 0.73). Concentrations did not increase at any postprandial time point (over all time factor: P = 0.10) and tAUCs were similar between groups (P = 0.73). In cohort 3, fasting plasma GDF15 was similar among the groups (P > 0.99) and neither glucose, fat, nor protein intake consistently increased the concentrations. In conclusion, we find that plasma GDF15 was not stimulated by meal intake and that fasting concentrations did not differ between RYGB-, SG-, and body mass index (BMI)-matched controls when investigated during the weight stable phase after RYGB and SG.NEW & NOTEWORTHY Our combined data show that GDF15 does not increase in response to a liquid meal. Moreover, we show for the first time that ingestion of sucrose, isomaltose, glucose, fat, or protein also does not increase plasma GDF15 concentrations, questioning the role of GDF15 in regulation of food source preference. Finally, we find that neither fasting nor postprandial plasma GDF15 concentrations are increased in individuals with previous bariatric surgery compared with unoperated body mass index (BMI)-matched controls.

Acute ketosis inhibits appetite and decreases plasma concentrations of acyl ghrelin in healthy young men.

AIM: To investigate the acute effect of ketone ester (KE) ingestion on appetite and plasma concentrations of acyl ghrelin (AG), unacylated ghrelin (UAG) and glucagon-like peptide-1 (GLP-1) secretion, and to compare responses with those elicited by isocaloric glucose (GLU) administration. METHODS: We examined 10 healthy young men on three separate occasions using a placebo (PBO)-controlled crossover design. A KE versus taste-matched isovolumetric and isocaloric 50% GLU and taste-matched isovolumetric PBO vehicle was orally administered. Our main outcome measures were plasma concentrations of AG, UAG, glucose-dependent insulinotropic polypeptide (GIP) and GLP-1 along with appetite sensation scores assessed by visual analogue scale. RESULTS: KE ingestion resulted in an average peak beta-hydroxybutyrate concentration of 5.5 mM. AG and UAG were lowered by approximately 25% following both KE and GLU intake compared with PBO. In the case of AG, the differences were -52.1 (-79.4, -24.8) for KE and -48.4 (-75.4, -21.5) pg/mL for GLU intake (P < .01). Concentrations of AG remained lower with KE but returned to baseline and were comparable with PBO levels after GLU intake. GLP-1, GIP, gastrin and cholecystokinin were not affected by KE ingestion. CONCLUSION: Our results suggest that the suppressive effects on appetite sensation scores associated with hyperketonaemia are more probable to be mediated through reduced ghrelin concentrations than by increased activity of cholecystokinin, gastrin, GIP or GLP-1.

Amino acids differ in their capacity to stimulate GLP-1 release from the perfused rat small intestine and stimulate secretion by different sensing mechanisms.

The aim of this study was to explore individual amino acid-stimulated GLP-1 responses and the underlying stimulatory mechanisms, as well as to identify the amino acid-sensing receptors involved in amino acid-stimulated GLP-1 release. Experiments were primarily based on isolated perfused rat small intestines, which have intact epithelial polarization allowing discrimination between luminal and basolateral mechanisms as well as quantitative studies of intestinal absorption and hormone secretion. Expression analysis of amino acid sensors on isolated murine GLP-1 secreting L-cells was assessed by qPCR. We found that l-valine powerfully stimulated GLP-1 secretion but only from the luminal side (2.9-fold increase). When administered from the vascular side, l-arginine and the aromatic amino acids stimulated GLP-1 secretion equally (2.6- to 2.9-fold increases). Expression analysis revealed that Casr expression was enriched in murine GLP-1 secreting L-cells, whereas Gpr35, Gprc6a, Gpr142, Gpr93 (Lpar5), and the umami taste receptor subunits Tas1r3 and Tas1r1 were not. Consistently, activation of GPR35, GPR93, GPR142, and the umami taste receptor with specific agonists or allosteric modulators did not increase GLP-1 secretion (P > 0.05 for all experiments), whereas vascular inhibition of CaSR reduced GLP-1 secretion in response to luminal infusion of mixed amino acids. In conclusion, amino acids differ in their capacity to stimulate GLP-1 secretion. Some amino acids stimulated secretion only from the intestinal lumen, whereas other amino acids exclusively stimulated secretion from the vascular side, indicating that amino acid-stimulated GLP-1 secretion involves both apical and basolateral (postabsorptive) sensing mechanisms. Sensing of absorbed amino acids involves CaSR activation as vascular inhibition of CaSR markedly diminished amino acid stimulated GLP-1 release.NEW & NOTEWORTHY Using isolated perfused rat small intestines, we show that amino acids differ in their mechanisms and capacity of stimulating GLP-1 release. Furthermore, we demonstrate that sensing by GPR142, GPR35, GPR93, and the umami taste receptor (Tas1R1/Tas1R3) are not involved in amino acid stimulated GLP-1 release. In contrast to previous studies, this experimental model allows discrimination between the luminal and the vascular side of the intestine, which is essential when studying mechanisms of amino acid-stimulated GLP-1 secretion.

Responses of gut and pancreatic hormones, bile acids, and fibroblast growth factor-21 differ to glucose, protein, and fat ingestion after gastric bypass surgery.

Postprandial gut hormone responses change after Roux-en-Y gastric bypass (RYGB), and we investigated the impact of glucose, protein, and fat (with and without pancreas lipase inhibition) on plasma responses of gut and pancreas hormones, bile acids, and fibroblast growth factor 21 (FGF-21) after RYGB and in nonoperated control subjects. In a randomized, crossover study 10 RYGB operated and 8 healthy weight-matched control subjects were administered 4 different 4-h isocaloric (200 kcal) liquid meal tests containing >90 energy (E)% of either glucose, protein (whey protein), or fat (butter with and without orlistat). The primary outcome was glucagon-like peptide-1 (GLP-1) secretion (area under the curve above baseline). Secondary outcomes included responses of peptide YY (PYY), glucose-dependent insulinotropic polypeptide (GIP), cholecystokinin (CCK), glicentin, neurotensin, ghrelin, insulin, glucagon, bile acids, and FGF-21. In the RYGB group the responses of GLP-1, GIP, glicentin, FGF-21, and C-peptide were increased after glucose compared with the other meals. The neurotensin and bile acids responses were greater after fat, while the glucagon and CCK responses were greater after protein ingestion. Furthermore, compared with control subjects, RYGB subjects had greater responses of total PYY after glucose, glucagon after glucose and fat, glicentin after glucose and protein, and GLP-1 and neurotensin after all meals, while GIP and CCK responses were lower after fat. Ghrelin responses did not differ between meals or between groups. Orlistat reduced all hormone responses to fat ingestion, except for ghrelin in the RYGB group. In conclusion, after RYGB glucose is a more potent stimulator of most gut hormones, especially for the marked increased secretion of GLP-1 compared with fat and protein.NEW & NOTEWORTHY We investigated the impact of glucose, protein, and fat meals on intestinal and pancreatic hormones, bile acid, and fibroblast growth factor 21 (FGF-21) secretion in gastric bypass-operated patients compared with matched nonoperated individuals. The fat meal was administered with and without a pancreas lipase inhibitor. We found that the impact of the different meals on gut hormones, bile, and FGF 21 secretion differ and was different from the responses observed in nonoperated control subjects.

Ghrelin Does Not Directly Stimulate Secretion of Glucagon-like Peptide-1.

CONTEXT: The gastrointestinal hormone ghrelin stimulates growth hormone secretion and appetite, but recent studies indicate that ghrelin also stimulates the secretion of the appetite-inhibiting and insulinotropic hormone glucagon-like peptide-1 (GLP-1). OBJECTIVE: To investigate the putative effect of ghrelin on GLP-1 secretion in vivo and in vitro. SUBJECTS AND METHODS: A randomized placebo-controlled crossover study was performed in eight hypopituitary subjects. Ghrelin or saline was infused intravenously (1 pmol/min × kg) after collection of baseline sample (0 min), and blood was subsequently collected at time 30, 60, 90, and 120 minutes. Mouse small intestine was perfused (n = 6) and GLP-1 output from perfused mouse small intestine was investigated in response to vascular ghrelin administration in the presence and absence of a simultaneous luminal glucose stimulus. Ghrelin receptor expression was quantified in human (n = 11) and mouse L-cells (n = 3) by RNA sequencing and RT-qPCR, respectively. RESULTS: Ghrelin did not affect GLP-1 secretion in humans (area under the curve [AUC; 0-120 min]: ghrelin infusion = 1.37 ± 0.05 min × nmol vs. saline infusion = 1.40 ± 0.06 min × nmol [P = 0.63]), but induced peripheral insulin resistance. Likewise, ghrelin did not stimulate GLP-1 secretion from the perfused mouse small intestine model (mean outputs during baseline/ghrelin infusion = 19.3 ± 1.6/25.5 ± 2.0 fmol/min, n = 6, P = 0.16), whereas glucose-dependent insulinotropic polypeptide administration, used as a positive control, doubled GLP-1 secretion (P < 0.001). Intraluminal glucose increased GLP-1 secretion by 4-fold (P < 0.001), which was not potentiated by ghrelin. Finally, gene expression of the ghrelin receptor was undetectable in mouse L-cells and marginal in human L-cells. CONCLUSIONS: Ghrelin does not interact directly with the L-cell and does not directly affect GLP-1 secretion.

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.

Abscisic acid stimulates the release of insulin and of GLP-1 in the rat perfused pancreas and intestine.

AIMS: Previous results indicate that nanomolar concentrations of abscisic acid (ABA) stimulate insulin release from ß-pancreatic cells in vitro and that oral ABA at 50 mg/kg increases plasma GLP-1 in the fasted rat. The aim of this study was to test the effect of ABA on the perfused rat pancreas and intestine, to verify the insulin- and incretin-releasing actions of ABA in controlled physiological models. MATERIALS AND METHODS: Rat pancreas and small intestine were perfused with solutions containing ABA at high-micromolar concentrations, or control secretagogues. Insulin and GLP-1 concentrations in the venous effluent were analysed by radioimmunoassay, and ABA levels were determined by ELISA. RESULTS: High micromolar concentrations of ABA induced GLP-1 secretion from the proximal half of the small intestine and insulin secretion from pancreas. GLP-1 stimulated ABA secretion from pancreas in a biphasic manner. Notably, a positive correlation was found between the ABA area under the curve (AUC) and the insulin AUC upon GLP-1 administration. CONCLUSION: Our results indicate the existence of a cross talk between GLP-1 and ABA, whereby ABA stimulates GLP-1 secretion, and vice versa. Release of ABA could be considered as a new promising molecule in the strategy of type 2 diabetes treatment and as a new endogenous hormone in the regulation of glycaemia.

Neuromedin U Does Not Act as a Decretin in Rats.

Studies on isolated pancreatic islets suggest that neuromedin U (NMU), a brain and gastrointestinal peptide, acts as a decretin hormone, inhibiting glucose-stimulated insulin secretion. We investigated whether this effect could be reproduced in vivo and in isolated perfused rat pancreas. Unlike the incretin hormone, glucagon-like peptide 1 (GLP-1), intravenous NMU administration had no effects on blood glucose and plasma insulin and glucagon in vivo. Moreover, NMU neither changed insulin, glucagon, or somatostatin secretion from isolated perfused rat pancreas, nor affected GLP-1-stimulated insulin and somatostatin secretion. For NMU to act as a decretin hormone, its secretion should increase following glucose ingestion; however, glucose did not affect NMU secretion from isolated perfused rat small intestine, which contained extractable NMU. Furthermore, the two NMU receptors were not detected in endocrine rat or human pancreas. We conclude that NMU does not act as a decretin hormone in rats.

Peptide production and secretion in GLUTag, NCI-H716, and STC-1 cells: a comparison to native L-cells.

GLUTag, NCI-H716, and STC-1 are cell lines that are widely used to study mechanisms underlying secretion of glucagon-like peptide-1 (GLP-1), but the extent to which they resemble native L-cells is unknown. We used validated immunoassays for 14 different hormones to analyze peptide content (lysis samples; n = 9 from different passage numbers) or peptide secretion in response to buffer (baseline), and after stimulation with 50 mM KCl or 10 mM glucose + 10 µM forskolin/3-isobutyl-1-methylxanthine (n = 6 also different passage numbers). All cell lines produced and processed proglucagon into GLP-1, GLP-2, glicentin, and oxyntomodulin in a pattern (prohormone convertase (PC)1/3 dependent) similar to that described for human gut. All three cell lines showed basal secretion of GLP-1 and GLP-2, which increased after stimulation. In contrast to freshly isolated murine L-cells, all cell lines also expressed PC2 and secreted large amounts of pancreatic glucagon. Neurotensin and somatostatin storage was low and secretion was not consistently increased by stimulation. STC-1 cells released more glucose-dependent insulinotropic polypeptide than GLP-1 at baseline (P < 0.01) and KCl elevated its secretion (P < 0.05). Peptide YY, which normally co-localizes with GLP-1 in distal L-cells, was not detected in any of the cell lines. GLUTag and STC-1 cells also expressed vasoactive intestinal peptide, but none expressed pancreatic polypeptide or insulin. GLUTag contained and secreted large amounts of CCK, while NCI-H716 did not store this peptide and STC-1 contained low amounts. Our results show that hormone production in cell line models of the L-cell has limited similarity to the natural L-cells.

Glucose stimulates neurotensin secretion from the rat small intestine by mechanisms involving SGLT1 and GLUT2, leading to cell depolarization and calcium influx.

Neurotensin (NT) is a neurohormone produced in the central nervous system and in the gut epithelium by the enteroendocrine N cell. NT may play a role in appetite regulation and may have potential in obesity treatment. Glucose ingestion stimulates NT secretion in healthy young humans, but the mechanisms involved are not well understood. Here, we show that rats express NT in the gut and that glucose gavage stimulates secretion similarly to oral glucose in humans. Therefore, we conducted experiments on isolated perfused rat small intestine with a view to characterize the cellular pathways of secretion. Luminal glucose (20% wt/vol) stimulated secretion but vascular glucose (5, 10, or 15 mmol/l) was without effect. The underlying mechanisms depend on membrane depolarization and calcium influx, since the voltage-gated calcium channel inhibitor nifedipine and the KATP channel opener diazoxide, which causes hyperpolarization, eliminated the response. Luminal inhibition of the sodium-glucose cotransporter 1 (SGLT1) (by phloridzin) eliminated glucose-stimulated release as well as secretion stimulated by luminal methyl-α-D-glucopyranoside (20% wt/vol), a metabolically inactive SGLT1 substrate, suggesting that glucose stimulates secretion by initial uptake by this transporter. However, secretion was also sensitive to GLUT2 inhibition (by phloretin) and blockage of oxidative phosphorylation (2-4-dinitrophenol). Direct KATP channel closure by sulfonylureas stimulated secretion. Therefore, glucose stimulates NT secretion by uptake through SGLT1 and GLUT2, both causing depolarization either as a consequence of sodium-coupled uptake (SGLT1) or by closure of KATP channels (GLUT2 and SGLT1) secondary to the ATP-generating metabolism of glucose.

Measurement of the incretin hormones: glucagon-like peptide-1 and glucose-dependent insulinotropic peptide.

The two incretin hormones, glucagon-like peptide 1 (GLP-1) and glucose-dependent insulinotropic peptide (GIP), are secreted from the gastrointestinal tract in response to meals and contribute to the regulation of glucose homeostasis by increasing insulin secretion. Assessment of plasma concentrations of GLP-1 and GIP is often an important endpoint in both clinical and preclinical studies and, therefore, accurate measurement of these hormones is important. Here, we provide an overview of current approaches for the measurement of the incretin hormones, with particular focus on immunological methods.

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

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

Targeting the intestinal L-cell for obesity and type 2 diabetes treatment.

Degradation-resistant glucagon-like peptide-1 (GLP-1) mimetics and GLP-1 enhancers (inhibitors of dipeptidyl peptidase-4, the enzyme which degrades and inactivates GLP-1) have been used for treatment of type 2 diabetes mellitus since 2005-2006. Cutting-edge research is now focusing on uncovering the secretory mechanisms of the GLP-1-producing cells (L-cells) with the purpose of developing agonists that enhance endogenous hormone secretion. Since GLP-1 co-localizes with other anorectic peptides, cholecystokinin, oxyntomodulin/glicentin and peptide YY, L-cell targeting might cause release of several hormones at the same time, providing additive effects on appetite and glucose regulation. In this review, we explore the role of proglucagon-derived peptides and other L-cell co-localizing hormones, in appetite regulation and the mechanism regulating their secretion.