Stimulation of incretin secretion by dietary lipid: is it dose dependent?

Diet: friend or foe of enteroendocrine cells--how it interacts with enteroendocrine cells

Gut hormones play a key role in the regulation of food intake, energy expenditure, glucose homeostasis, lipid metabolism, and a wide range of metabolic functions in response to food ingestion. These hormones are altered in metabolic diseases, such as obesity and type 2 diabetes, and are thus proposed to be possible targets for the prevention or treatment of these diseases. It is clear that food composition, macronutrients, and other non-nutrient components as well as the physical properties of food not only modulate the secretion of gut peptides but also modulate transcription and enteroendocrine cell differentiation, which ultimately modifies gut hormone response. The specific mechanisms or sensing machinery that respond to the different components of the diet have been studied for many years; however, over the last few years, new molecular genetic techniques have led to important advances, thereby allowing a deeper understanding of these mechanisms. This review addresses the current knowledge regarding enteroendocrine cells and how diet interacts with this machinery to stimulate and regulate the secretion of gut peptides. The potential for diet interventions as a promising strategy for modulating gut hormone responses to food ingestion and, ultimately, preventing or treating metabolic diseases is being emphasized considering that these diseases are currently a public health burden.

Using the lymphatics to study nutrient absorption and the secretion of gastrointestinal hormones

The lymph fistula rat model has traditionally been used to study the intestinal absorption of nutrients, especially lipids, but recently this model has also been used for studying the secretion of incretin hormones by the small intestine. The small intestine is not only responsible for the digestion and transport of dietary triacylglycerol, through the formation of chylomicrons, but it also secretes the incretin hormones glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) from enteroendocrine cells. Ultimately, both chylomicrons and incretins are found in lymph. Advantages of the lymph fistula rat model in studying chylomicron and incretin secretion are numerous and include: 1) the concentrations of incretin hormones are higher in lymph than in peripheral or portal plasma; 2) there is reduced degradation of incretin hormones by DPP-IV in the lymph compartment; 3) less dilution by the circulating fluid; 4) this model allows the continuous collection of lymph from conscious animals, eliminating any potential side effects on lymph flow and gastrointestinal function due to anesthesia; and finally, and perhaps most importantly, and 5) the concentration in the intestinal lymph provides a physiologically accurate representation of the hormonal milieu within the intestinal mucosa where incretins may interact with enteroendocrine and/or dendritic cells and signal through the enteric or autonomic neurons. The importance of GIP and GLP-1 in health and disease is becoming more apparent, especially as the prevalence of type 2 diabetes and other metabolic disorders increases. This review focuses on the use of the lymph fistula rat as a model to study the secretion of incretins, as well as dietary lipid.

Rapid post-oral stimulation of intake and flavor conditioning by glucose and fat in the mouse

Although widely assumed to have only satiating actions, nutrients in the gut can also condition increases in intake in some cases. Here we studied the time course of post-oral nutrient stimulation of ingestion in food-restricted C57BL/6J mice. In experiment 1, mice adapted to drink a 0.8% sucralose solution 1 h/day, rapidly increased their rate of licking (within 4-6 min) when first tested with an 8% glucose solution and even more so in tests 2 and 3. Other mice decreased their licking rate when switched from sucralose to 8% fructose, a sugar that is sweet like glucose but lacks positive post-oral effects in mice. The glucose-stimulated drinking is due to the sugar's post-oral rather than taste properties, because sucralose is highly preferred to glucose and fructose in brief choice tests. A second experiment showed that the glucose-stimulated ingestion is associated with a conditioned flavor preference in both intact and capsaicin-treated mice. This indicates that the post-oral stimulatory action of glucose is not mediated by capsaicin-sensitive visceral afferents. In experiment 3, mice consumed flavored saccharin solutions as they self-infused water or glucose via an intragastric (IG) catheter. The glucose self-infusion stimulated ingestion within 13-15 min in test 1 and produced a conditioned increase in licking that was apparent in the initial minute of tests 2 and 3. Experiment 4 revealed that IG self-infusions of a fat emulsion also resulted in post-oral stimulation of licking in test 1 and conditioned increases in tests 2 and 3. These findings indicate that glucose and fat can generate stimulatory post-oral signals early in a feeding session that increase ongoing ingestion and condition increases in flavor acceptance and preference revealed in subsequent feeding sessions. The test procedures developed here can be used to investigate the peripheral and central processes involved in stimulation of intake by post-oral nutrients.

Nateglinide stimulates glucagon-like peptide-1 release by human intestinal L cells via a K(ATP) channel-independent mechanism

A reduced incretin effect is one of the well-known characteristics of patients with type 2 diabetes, and impaired release of glucagon-like peptide-1 (GLP-1) has been reported to be at least partly involved. In this study, we investigated the effect of nateglinide on GLP-1 release in vivo and in vitro. The GLP-1 level in the portal blood at 20 min after oral administration of nateglinide to Wistar rats was about twice that in vehicle-treated rats. To clarify whether this effect of nateglinide was related to direct stimulation of intestinal cells, in vitro studies were performed using human intestinal L cells (NCI-H716). Nateglinide stimulated GLP-1 release in a concentration-dependent manner from 500 µM, along with transient elevation of the intracellular calcium level. However, diazoxide, nitrendipine, and dantrolene did not block this effect of nateglinide. In addition, the major metabolite of nateglinide, tolbutamide, and mitiglinide, all of which augment insulin secretion by the pancreatic islets, had no effect on GLP-1 release by this cell line. On the other hand, capsazepine significantly inhibited the promotion of GLP-1 release by nateglinide in a concentration-dependent manner. These findings indicate that nateglinide directly stimulates GLP-1 release by intestinal L cells in a K(ATP) channel-independent manner. A novel target of nateglinide may be involved in increasing intracellular calcium to stimulate GLP-1 release, e.g., the transient receptor potential channels. Taken together, the present findings indicate that promotion of GLP-1 release from intestinal L cells may be another important mechanism by which nateglinide restores early-phase insulin secretion and regulates postprandial glucose metabolism.

Dietary effects on incretin hormone secretion

The delivery of nutrients from the stomach into the duodenum and their subsequent interaction with the small intestine to stimulate incretin hormone release are central determinants of the glycemic response. The incretin effect has hitherto been attributed to the secretion of glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) from enteroendocrine cells in the intestinal epithelium. A number of recent studies have yielded fundamental insights into the influence of individual nutrients on incretin release and the mechanisms involved in the detection of carbohydrates, fats, and proteins by enteroendocrine cells, including the K(ATP) channel, sodium-glucose cotransporter 1 (SGLT1), sweet taste receptors, G-protein-coupled receptors (GPRs), and oligopeptide transporter 1 (PepT1). Dietary modification, including modifying macronutrient composition or the consumption of "preloads" in advance of a meal, represents a novel approach to manipulate the incretin response and thereby regulate glucose homeostasis in patients with type 2 diabetes. This review focuses on the effects of individual nutrients on incretin hormone secretion, our current understanding of the signaling mechanisms that trigger secretion by enteroendocrine cells, and the therapeutic implications of these observations.

Changes in gastrointestinal hormones and leptin after Roux-en-Y gastric bypass surgery

BACKGROUND

Roux-en-Y gastric bypass (RYGB) imparts long-term weight loss, the mechanisms for which are not well understood. Changes in leptin and gastrointestinal (GI) hormones, including glucagon-like peptide 1 (GLP-1), peptide YY (PYY), and ghrelin, may contribute to the relative success of RYGB compared with conventional weight loss methods. This study evaluated changes in GI hormones and leptin post-RYGB. The study also evaluated whether GI hormones differed after a short-term dose of protein or fat.

METHODS

GLP-1, PYY, ghrelin, and leptin were assessed in 16 women before RYGB and up to 1 year after RYGB. Plasma was collected before and at several times after a short-term equicaloric dose of protein or fat.

RESULTS

GLP-1 area under the curve (AUC) increased at week 6 and 1 year in the fat beverage (FAT-BEV) group compared with baseline. PYY AUC remained elevated at 1 year in the FAT-BEV group. Ghrelin AUC decreased at week 2, week 6, and 1 year in the protein beverage (PRO-BEV) group compared with baseline. Ghrelin AUC was lower in the PRO-BEV group compared with the FAT-BEV group at week 6. Fasted leptin decreased at all visits in both groups and was lower in the FAT-BEV group compared with the PRO-BEV group at 1 year.

CONCLUSIONS

Changes from baseline were evident for all GI hormones and leptin; some differences were evident soon after surgery (ghrelin, leptin), whereas others were maintained long term (GLP-1, PYY, ghrelin, leptin). In response to a short-term stimulus, protein suppressed ghrelin and fat potently stimulated GLP-1 and PYY. Future work in this area is warranted.

Nutrition and L and K-enteroendocrine cells

PURPOSE OF REVIEW

The review highlights the influence of nutrients over the secretion of several hormones produced by enteroendocrine cells in the gastrointestinal tract that secrete incretin hormones. These hormones influence glucose homeostasis; food intake; gastric, pancreatic and hepatic secretions; and gastric and intestinal motility, and these aspects are summarized in this review.

RECENT FINDINGS

This study provides an overview of recent advances in our understanding of the physiology of the incretins, glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP), as well as of oxyntomodulin. A better understanding of the secretion and action of these hormones at their receptors was made possible by new techniques that allow investigation of individual enteroendocrine cells.

SUMMARY

The better understanding of the function of the gastrointestinal incretin hormones and their implications for improving glucose homeostasis and perhaps influencing food intake and appetite as well, new research in this area will help combat metabolic diseases such as type 2 diabetes and obesity.

Effect of ezetimibe on incretin secretion in response to the intestinal absorption of a mixed meal

Ezetimibe is a potent inhibitor of cholesterol absorption by enterocytes. Although ezetimibe minimally affects the absorption of triglyceride, it is unknown whether ezetimibe affects the secretion of the incretin hormones glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1). It has been shown that ezetimibe-treated mice are protected from diet-induced insulin resistance. Since GIP and GLP-1 promote the actions of insulin, we hypothesized that ezetimibe may affect the secretion of GIP and GLP-1 by enteroendocrine cells into lymph in response to the intestinal absorption of a mixed meal (Ensure). To test this hypothesis, we used the lymph fistula rat model to determine GIP and GLP-1 concentrations in lymph during the 2 h after the infusion of Ensure. Ezetimibe significantly reduced lymphatic cholesterol output during fasting, without coincident decreases in glucose, protein, and triglyceride outputs. However, ezetimibe did not influence cholesterol output after infusion of Ensure. Interestingly, ezetimibe significantly reduced the secretion of both GIP and GLP-1 into lymph after the infusion of Ensure. Therefore, the inhibitory effect of ezetimibe on GIP and GLP-1 secretion by enteroendocrine cells occurs outside of the effects of glucose, protein, or triglyceride secretion by the intestine.

Differential responses of the incretin hormones GIP and GLP-1 to increasing doses of dietary carbohydrate but not dietary protein in lean rats

Previous studies have shown that oral ingestion of nutrients stimulates secretion of the incretin hormones glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1); however, it is unclear whether there is a dose-dependent response between the amount of nutrient ingested and the secretion of the hormones in vivo. Using our lymph fistula rat model, we previously demonstrated that both GIP and GLP-1 responded dose dependently to increasing amounts of infused dietary lipid and that the GLP-1-secreting cells were more sensitive to changes in intestinal lipid content. In the present study, we investigated the dose-dependent relationships between incretin secretion and the two remaining macronutrients, carbohydrate and protein. To accomplish this objective, the major mesenteric lymphatic duct of male Sprague-Dawley rats was cannulated. Each animal received a single bolus (3 ml) of saline, dextrin, whey protein, or casein hydrolysate (0.275, 0.55, 1.1, 2.2, 4.4 kcal) via a surgically inserted duodenal or ileal feeding tube. Lymph was continuously collected for 3 h and analyzed for GIP and GLP-1 content. Both GIP and GLP-1 outputs responded dose dependently to increasing amounts of dietary carbohydrate but not protein. Additionally, we found that the GIP-secreting cells were more sensitive than the GLP-1-secreting cells to changes in intestinal carbohydrate content.

Colesevelam improves insulin resistance in a diet-induced obesity (F-DIO) rat model by increasing the release of GLP-1

Bile acid sequestrants have been shown to lower glucose levels in patients with type 2 diabetes. To investigate how colesevelam (CL) HCl improves hyperglycemia, studies were conducted in diet-induced obesity (F-DIO) rats, which develop insulin resistance when fed a high-energy (high fat/high sucrose) diet (HE). The rats were fed HE; HE + 2% CL; HE + 0.02% SC-435 (SC), an apical sodium-dependent bile acid transporter inhibitor; and regular chow (controls). After 4 wk of treatment, both in the HE group and the SC + HE group, plasma glucose and insulin levels remained elevated compared with baseline values throughout an oral glucose tolerance test (OGTT). In contrast, in the CL + HE group, plasma glucose levels returned to baseline by the end of the test, and insulin peaked in 15-30 min and then returned to baseline. CL induced release of glucagon-like peptide-1 (GLP-1) because the area under the curve of plasma total GLP-1 in the CL + HE group was significantly greater than in the HE group during the OGTT. Bile acid concentrations in the portal blood did not decrease in the HE group but declined significantly both in the CL + HE and SC + HE groups with reduced farnesoid X receptor activation compared with controls. We concluded that CL reduces plasma glucose levels by improving insulin resistance in this rat model. It is unlikely that the improvement is attributable to decreased bile acid flux to the liver but is likely secondary to induced GLP-1 secretion, which improves insulin release.

Using the lymph fistula rat model to study incretin secretion

The past several decades have witnessed a flourish of interest in the field of incretin biology. The importance of the two incretin hormones, glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1), in health and disease is becoming more apparent as the prevalence of type 2 diabetes and other metabolic disorders escalates. Rodent models have become indispensable in the study of the physiological function of GIP and GLP-1; however, investigators have run into several roadblocks when untangling the regulation of incretin secretion in these systems. The low circulating levels of the incretin hormones combined with sensitivity of the currently available assays require substantial amounts of blood to be removed from an animal if the hormones are to be analyzed over a period of time. Because of these limitations, continuous monitoring of GIP and GLP-1 secretion becomes difficult. A more effective means of studying incretin secretion in small animal models is therefore desirable. This chapter evaluates the use of the lymph fistula rat as a model to study the secretion of incretins. Lymph fistula models, in a variety of animals, have been used for decades to study the absorption and transport of lipid and lipophilic compounds; however, only recently has the value of this model been appreciated as a tool to explore incretin secretion.