Cyclic AMP-mediated release of peptide YY (PYY) from the isolated perfused rabbit distal colon

PACAP and VIP Neuropeptides' and Receptors' Effects on Appetite, Satiety and Metabolism

The overwhelming increase in the prevalence of obesity and related disorders in recent years is one of the greatest threats to the global healthcare system since it generates immense healthcare costs. As the prevalence of obesity approaches epidemic proportions, the importance of elucidating the mechanisms regulating appetite, satiety, body metabolism, energy balance and adiposity has garnered significant attention. Currently, gastrointestinal (GI) bariatric surgery remains the only approach capable of achieving successful weight loss. Appetite, satiety, feeding behavior, energy intake and expenditure are regulated by central and peripheral neurohormonal mechanisms that have not been fully elucidated yet. Pituitary Adenylate Cyclase-Activating Polypeptide (PACAP) and Vasoactive Intestinal Polypeptide (VIP) are members of a family of regulatory peptides that are widely distributed in parallel with their specific receptors, VPAC1R, VPAC2R and PAC1R, in the central nervous system (CNS) and in the periphery, such as in the gastrointestinal tract and its associated organs and immune cells. PACAP and VIP have been reported to play an important role in the regulation of body phenotype, metabolism and homeostatic functions. The purpose of this review is to present recent data on the effects of PACAP, VIP, VPAC1R, VPAC2R and PAC1R on the modulation of appetite, satiety, metabolism, calorie intake and fat accumulation, to evaluate their potential use as therapeutic targets for the treatment of obesity and metabolic syndrome.

The VIP/VPAC1R Pathway Regulates Energy and Glucose Homeostasis by Modulating GLP-1, Glucagon, Leptin and PYY Levels in Mice

Simple Summary

The current study is the first complete characterization of the phenotypic, metabolic, calorimetric, and homeostatic effects of VPAC1R in a null murine model. To evaluate the role of VPAC1R on body phenotype, feeding behavior, glucose/energy homeostasis, metabolic rate and plasma hormones, a long-term study was conducted in VPAC1R^−/− and WT mice. The outcome data document that VPAC1R^−/− mice have altered metabolism and insulin intolerance, with significant increase of feeding bouts, reduction of total energy expenditure and respiratory gases during both the dark and light cycle, together with elevated fasting levels of GLP-1 and PYY, and higher postprandial levels of GLP-1, glucagon leptin and PYY. These findings suggests that VPAC1R controls glucose homeostasis and energy balance by regulating plasma metabolic hormones.

Abstract

Vasoactive Intestinal Peptide binds with high affinity to VPAC1R and VPAC2R, thus regulating key physiologic functions. Previously, we documented in VIP^−/− mice a leaner body phenotype and altered metabolic hormones. Past reports described in VPAC2^−/− mice impaired circadian rhythm, reduced food intake, and altered metabolism. To better define the effects of VPAC1R on body phenotype, energy/glucose homeostasis, and metabolism, we conducted a 12-week study in a VPAC1R null model. Our results reveal that VPAC1^−/− mice experienced significant metabolic alterations during the dark cycle with greater numbers of feeding bouts (p = 0.009), lower Total Energy Expenditure (p = 0.025), VO₂ (p = 0.029), and VCO₂ (p = 0.016); as well as during the light cycle with lower Total Energy Expenditure (p = 0.04), VO₂ (p = 0.044), and VCO₂ (p = 0.029). Furthermore, VPAC1^−/− mice had significantly higher levels of GLP-1 and PYY during fasting, and higher levels of GLP-1, glucagon leptin and PYY during postprandial conditions. In addition, VPAC1^−/− mice had lower levels of glucose at 60′ and 120′, as assessed by insulin tolerance test. In conclusion, this study supports a key role for VPAC1R in the regulation of body glucose/energy homeostasis and metabolism.

Ghrelin, CCK, GLP-1, and PYY(3-36): Secretory Controls and Physiological Roles in Eating and Glycemia in Health, Obesity, and After RYGB

The efficacy of Roux-en-Y gastric-bypass (RYGB) and other bariatric surgeries in the management of obesity and type 2 diabetes mellitus and novel developments in gastrointestinal (GI) endocrinology have renewed interest in the roles of GI hormones in the control of eating, meal-related glycemia, and obesity. Here we review the nutrient-sensing mechanisms that control the secretion of four of these hormones, ghrelin, cholecystokinin (CCK), glucagon-like peptide-1 (GLP-1), and peptide tyrosine tyrosine [PYY(3-36)], and their contributions to the controls of GI motor function, food intake, and meal-related increases in glycemia in healthy-weight and obese persons, as well as in RYGB patients. Their physiological roles as classical endocrine and as locally acting signals are discussed. Gastric emptying, the detection of specific digestive products by small intestinal enteroendocrine cells, and synergistic interactions among different GI loci all contribute to the secretion of ghrelin, CCK, GLP-1, and PYY(3-36). While CCK has been fully established as an endogenous endocrine control of eating in healthy-weight persons, the roles of all four hormones in eating in obese persons and following RYGB are uncertain. Similarly, only GLP-1 clearly contributes to the endocrine control of meal-related glycemia. It is likely that local signaling is involved in these hormones' actions, but methods to determine the physiological status of local signaling effects are lacking. Further research and fresh approaches are required to better understand ghrelin, CCK, GLP-1, and PYY(3-36) physiology; their roles in obesity and bariatric surgery; and their therapeutic potentials.

Partial ileal bypass affords protection from onset of type 2 diabetes

BACKGROUND

Partial ileal bypass (PIB) in the National Institutes of Health-sponsored Program on the Surgical Control of the Hyperlipidemias (POSCH) randomized controlled trial was found to reduce plasma cholesterol, in particular low density lipoprotein cholesterol, with concomitant retardation of atherosclerotic cardiovascular disease and increased life expectancy. Glucagon-like peptide-1, related to amelioration of type 2 diabetes, is increased over 5-fold after PIB. We hypothesized that PIB, in addition to its action on cholesterol metabolism, may also prevent type 2 diabetes.

METHODS

We surveyed by telephone inquiry of former POSCH patients the 30+year posttrial incidence of type 2 diabetes or prediabetes, the presence of which was a trial exclusion criteria. We were able to contact 17.4% (n = 838) of the original POSCH population.

RESULTS

Of 66 control responders, 17 contracted type 2 diabetes (25.8%); of 80 PIB responders, 8 contracted type 2 diabetes (10%). The difference between groups was significant (P = .015 by Fisher exact test) with an odds ratio of .320 for the PIB group and an over 2-fold (2.6) increase in the incidence of type 2 diabetes in the controls. Including borderline type 2 diabetes (prediabetic) patients, these values were 22 of 66 controls (33.3%) and 10 of 80 PIB patients (12.5%), with an odds ratio of .286 and a P<.004, and again an over 2-fold (2.7) increase in the incidence of type 2 diabetes in the control patients.

CONCLUSION

PIB appears to afford partial protection from the onset of type 2 diabetes for over 30 years.

Regulation of Appetite, Body Composition, and Metabolic Hormones by Vasoactive Intestinal Polypeptide (VIP)

Vasoactive intestinal peptide (VIP) is a 28-amino acid neuropeptide that belongs to the secretin-glucagon superfamily of peptides and has 68 % homology with PACAP. VIP is abundantly expressed in the central and peripheral nervous system and in the gastrointestinal tract, where it exercises several physiological functions. Previously, it has been reported that VIP regulates feeding behavior centrally in different species of vertebrates such as goldfishes, chicken and rodents. Additional studies are necessary to analyze the role of endogenous VIP on the regulation of appetite/satiety, feeding behavior, metabolic hormones, body mass composition and energy balance. The aim of the study was to elucidate the physiological pathways by which VIP regulates appetite/satiety, feeding behavior, metabolic hormones, and body mass composition. VIP deficient (VIP -/-) and age-matched wild-type (WT) littermates were weekly monitored from 5 to 22 weeks of age using a whole body composition EchoMRI analyzer. Food intake and feeding behavior were analyzed using the BioDAQ automated monitoring system. Plasma levels of metabolic hormones including active-ghrelin, GLP-1, leptin, PYY, pancreatic polypeptide (PP), adiponectin, and insulin were measured in fasting as well as in postprandial conditions. The genetic lack of VIP led to a significant reduction of body weight and fat mass and to an increase of lean mass as the mice aged. Additionally, VIP-/- mice had a disrupted pattern of circadian feeding behavior resulting in an abolished regular nocturnal/diurnal feeding. These changes were associated with an altered secretion of adiponectin, GLP-1, leptin, PYY and insulin in VIP-/- mice. Our data demonstrates that endogenous VIP is involved in the control of appetite/satiety, feeding behavior, body mass composition and in the secretion of six different key regulatory metabolic hormones. VIP plays a key role in the regulation of body phenotype by significantly enhancing body weight and fat mass accumulation. Therefore, VIP signaling is critical for the modulation of appetite/satiety and body mass phenotype and is a potential target for future treatment of obesity.

Ileal effect on blood glucose, HbA1c, and GLP-1 in Goto-Kakizaki rats

BACKGROUND

There have been enumerable studies on the effects of glucagon-like peptide-1 (GLP-1) on satiety and pancreatic islet function, stimulating the advocacy of surgical transposition of the ileum (rich in GLP-1-generating L-cells) higher in the gastrointestinal tract for earlier stimulation. In the Goto-Kakizaki rat with naturally occurring type 2 diabetes, we studied the influence of ileal exclusion (IE) and ileal resection (IR) on blood glucose, hemoglobin A1c (HbA1c), and GLP-1.

METHODS

In six control (Ctrl), 10 IE, and 10 IR rats, over 12 weeks of follow-up, we determined blood glucose, HbA1c, and GLP-1.

RESULTS

Two animals in the IE and IR groups did not survive to week 13. Both operated groups weighed more than the Ctrl group at baseline and at 13 weeks; thus, IE and IR did not retard weight gain (p < 0.05). All three groups were equally hyperglycemic at week 13: 255 ± 10.2 Ctrl, 262 ± 11.0 IE, 292 ± 17.8 IR (mg/dl ± SEM). The three groups had statistically identical markedly elevated HbA1c percentages at week 13: 14.7 ± 28 Ctrl, 11.7 ± 3.4 IE, 13.8 ± 3.5 IR (% ± SEM). The end-study GLP-1 values (pM ± SEM) were 5 ± 0.9 Ctrl, 33 ± 8.9 IE, and 25 ± 6.7 IR. P values for intergroup differences were IE vs. Ctrl 0.02, IR vs. Ctrl 0.02, and IE vs. IR 0.59.

CONCLUSIONS

Neither IE nor IR resulted in a decrease in the mean GLP-1 level. On the contrary, the exclusion or resection of the L-cell rich ileum raised GLP-1 levels 5- to 6-fold. This increase in the GLP-1 was not associated with the mitigation of hyperglycemia or elevated HbA1c levels.

Effects on GLP-1, PYY, and leptin by direct stimulation of terminal ileum and cecum in humans: implications for ileal transposition

BACKGROUND

We do not have a unified, scientifically tested theory of causation for obesity and its co-morbidities, nor do we have explanations for the mechanics of the metabolic/bariatric surgery procedures. Integral to proffered hypotheses are the actions of the hormones glucagon-like peptide-1 (GLP-1), peptide YY (PYY), and leptin. The objective of this study was to obtain blood levels of GLP-1, PYY, and leptin after stimulation of the terminal ileum and cecum by a static infusion of a food hydrolysate in morbidly obese patients undergoing a duodenal switch procedure.

SETTING

University Hospital.

METHODS

Plasma levels of GLP-1, PYY, and leptin were obtained at 0, 30, 60, 90, and 120 minutes after instillation of 240 mL of a food hydrolysate into the ileum or cecum.

RESULTS

The mean±SD GLP-1 values by cecal stimulation for 0, 30, 60, 90, and 120 minutes were: 41.3±23.2; 39.6±21.8; 38.9±19.1; 47.4±22.3; 51.7±27.3 pM, and by ileal stimulation: 55.0±32.8; 83.4±16.1; 78.7±23.8; 84.7±23.5; 76.4±25.6. The mean±SD PYY values by cecal stimulation were: 62.1±24.8; 91.1±32.8; 102.1±39.6; 119.6±37.5; 130.3±36.7, and by ileal stimulation: 73.8±41.6; 138.1±17.7; 149.5±23.3; 165.7±24.3; 155.5±29.1. Percent change in PYY levels increased ~150%, GLP-1 increased ~50%, and leptin decreased ~20%.

CONCLUSION

Direct stimulation of the human terminal ileum and cecum by a food hydrolysate elicits significant plasma GLP-1 and PYY elevations and leptin decreases, peaking at 90-120 minutes. The ileal GLP-1 and PYY responses exceed those of the cecum, and the PYY effect is about 3-fold that of GLP-1. The results of this study question the satiety premise for ileal transposition.

Hormone profiling in a novel enteroendocrine cell line pGIP/neo: STC-1

OBJECTIVE

GIP is a peptide hormone of therapeutic interest in type 2 diabetes and obesity. This study evaluated pGIP/neo STC-1 as a potential K-cell model for studying GIP secretion.

METHODS

We evaluated cellular storage and medium accumulation of GIP along with other gastrointestinal peptides cholecystokinin (CCK), peptide YY (PYY), obestatin and ghrelin over 72 h and probed possible intracellular signals (PKA, PKC, Ca(2+) and GPCR) involved in peptide hormone synthesis/secretion.

RESULTS

Results demonstrate for the first time that pGIP/Neo STC-1 cells produce and secrete 3 to 6 times more GIP than STC-1. The cells clearly retain the ability to synthesize and secrete CCK and PYY but reduced levels indicate a shift towards a predominantly K-cell phenotype. Furthermore, gastric peptides such as obestatin and ghrelin are not produced in either STC-1 or pGIP/Neo STC-1 cells.

DISCUSSION

This study demonstrates the potential usefulness of pGIP/Neo cells for studying GIP secretion and further investigations will establish its suitability for investigating hormone release in vitro.

Examining acute and chronic effects of short- and long-chain fatty acids on peptide YY (PYY) gene expression, cellular storage and secretion in STC-1 cells

PURPOSE

Peptide YY (PYY) is a gastrointestinal hormone with physiological actions regulating appetite and energy homoeostasis. The cellular mechanisms by which nutrients stimulate PYY secretion from intestinal enteroendocrine cells are still being elucidated.

METHODS

This study comprehensively evaluated the suitability of intestinal STC-1 cells as an in vitro model of PYY secretion. PYY concentrations (both intracellular and in culture media) with other intestinal peptides (CCK, GLP-1 and GIP) demonstrated that PYY is a prominent product of STC-1 cells. Furthermore, acute and chronic PYY responses to 15 short (SCFAs)- and long-chain (LCFAs) dietary fatty acids were measured alongside parameters for DNA synthesis, cell viability and cytotoxicity.

RESULTS

We found STC-1 cells to be reliable secretors of PYY constitutively releasing PYY into cell culture media (but not into non-stimulatory buffer). We demonstrate for the first time that STC-1 cells produce PYY mRNA transcripts; that STC-1 cells produce specific time- and concentration-dependent PYY secretory responses to valeric acid; that linoleic acid and conjugated linoleic acid 9,11 (CLA 9,11) are potent PYY secretagogues; and that chronic exposure of SCFAs and LCFAs can be detrimental to STC-1 cells.

CONCLUSIONS

Our studies demonstrate the potential usefulness of STC-1 cells as an in vitro model for investigating nutrient-stimulated PYY secretion in an acute setting. Furthermore, our discovery that CLA directly stimulates L-cells to secrete PYY indicates another possible mechanism contributing to the observed effects of dietary CLA on weight loss.

CCK, PYY and PP: the control of energy balance

The control of food intake consists of neural and hormonal signals between the gut and central nervous system (CNS). Gut hormones such as CCK, PYY and PP signal to important areas in the CNS involved in appetite regulation to terminate a meal. These hormones can act directly via the circulation and activate their respective receptors in the hypothalamus and brainstem. In addition, gut vagal afferents also exist, providing an alternative pathway through which gut hormones can communicate with higher centres through the brainstem. Animal and human studies have demonstrated that peripheral administration of certain gut hormones reduces food intake and leads to weight loss. Gut hormones are therefore potential targets in the development of novel treatments for obesity and analogue therapies are currently under investigation.

Inhibition of apical sodium-dependent bile acid transporter as a novel treatment for diabetes

Bile acids are recognized as metabolic modulators. The present study was aimed at evaluating the effects of a potent Asbt inhibitor (264W94), which blocks intestinal absorption of bile acids, on glucose homeostasis in Zucker Diabetic Fatty (ZDF) rats. Oral administration of 264W94 for two wk increased fecal bile acid concentrations and elevated non-fasting plasma total Glp-1. Treatment of 264W94 significantly decreased HbA1c and glucose, and prevented the drop of insulin levels typical of ZDF rats in a dose-dependent manner. An oral glucose tolerance test revealed up to two-fold increase in plasma total Glp-1 and three-fold increase in insulin in 264W94 treated ZDF rats at doses sufficient to achieve glycemic control. Tissue mRNA analysis indicated a decrease in farnesoid X receptor (Fxr) activation in small intestines and the liver but co-administration of a Fxr agonist (GW4064) did not attenuate 264W94 induced glucose lowering effects. In summary, our results demonstrate that inhibition of Asbt increases bile acids in the distal intestine, promotes Glp-1 release and may offer a new therapeutic strategy for type 2 diabetes mellitus.

Adrenergic activation of electrogenic K+ secretion in guinea pig distal colonic epithelium: desensitization via the Y2-neuropeptide receptor

Adrenergic activation of electrogenic K+ secretion in isolated mucosa from guinea pig distal colon was desensitized by peptide-YY (PYY). Addition of PYY or neuropeptide-Y (NPY) to the bathing solution of mucosae in Ussing chambers suppressed the short-circuit current (Isc) corresponding to electrogenic Cl- secretion, whether stimulated by epinephrine (epi), prostaglandin-E2 (PGE2), or carbachol (CCh). Neither peptide markedly inhibited the large transient component of synergistic secretion (PGE2 + CCh). Sustained Cl- secretory Isc was inhibited approximately 65% by PYY or NPY, with IC50s of 4.1 +/- 0.9 nM and 9.4 +/- 3.8 nM, respectively. This inhibition was eliminated by BIIE0246, an antagonist of the Y2-neuropeptide receptor (Y2-NpR), but not by Y1-NpR antagonist BVD10. Adrenergic sensitivity for activation of K+ secretion in the presence of Y2-NpR blockade by BIIE0246 was (EC50s) 2.9 +/- 1.2 nM for epi and 13.3 +/- 1.0 nM for norepinephrine, approximately fourfold greater than in the presence of PYY. Expression of mRNA for both Y1-NpR and Y2-NpR was indicated by RT-PCR of RNA from colonic mucosa, and protein expression was indicated by immunoblot. Immunoreactivity (ir) for Y1-NpR and Y2-NpR was distinct in basolateral membranes of columnar epithelial cells in the crypts of Lieberkühn as well as intercrypt surface epithelium. Adrenergic nerves in proximity with crypts were detected by ir for dopamine-beta-hydroxylase, and a portion of these nerves also contained NPY(ir). BIIE0246 addition increased secretagog-activated Isc, consistent with in vitro release of either PYY or NPY. Thus PYY and NPY were able to suppress Cl- secretory capacity and desensitize the adrenergic K+ secretory response, providing a direct inhibitory counterbalance against secretory activation.

Neuropeptide Y inhibits the protein kinase C-stimulated Cl(-) secretion in the human colonic cell line HT29cl.19A cell line via multiple sites

Neuropeptide Y is known to exert inhibitory effects on ion secretion in the intestine by reducing the activity of adenylyl cyclase. In the human intestinal epithelial cell line HT29cl.19A, it has been previously shown that neuropeptide Y inhibits the electrophysiological phenomena related to Cl(-) secretion, when induced by elevation of cAMP via forskolin. Moreover, the secretion induced via elevation of intracellular calcium levels via muscarinic activation can be inhibited by neuropeptide Y. Part of these inhibitions appeared to be due to lowered calcium activity in the epithelial cells, thereby reducing the basolateral K(+) conductance. The phorbol ester 4-phorbol-12,13-dibutyrate (PDB) can induce secretion in this cell line via activation of protein kinase C as well; however, the effect of neuropeptide Y on this pathway has not yet been studied. In the present experiments, it is shown that neuropeptide Y inhibits the PDB-induced secretion at two sides: one located in the apical membrane and another in the basolateral membrane. It is shown that the latter effect can, at least partially, be explained via a direct effect of neuropeptide Y on the K(+) conductance. This was concluded from the observation that neuropeptide Y could also reduce basolateral K(+) conductance when intracellular calcium was dramatically increased by ionomycin. The observed inhibitory effects suggest that neuropeptide Y is a very powerful antisecretory peptide in human intestinal epithelial cells.

Role of the neuroendocrine system in pathogenesis of gastroenteritis

The concept of neuroendocrine modulation of infectious gastroenteritis adds another dimension to the pathophysiology of diarrhoeal diseases. Furthermore it opens up new avenues for therapeutic intervention. Until now, most interest has been directed at enterotoxin-producing bacteria, notably Vibrio cholerae and the enterotoxigenic Escherichia coli. However, more recently neuroendocrine recruitment has been implicated by other pathogens. The roles of vasoactive intestinal peptide, 5-hydroxytryptamine, tachykinins, nitric oxide and opioids are explored in this review. In addition new insights in the contradictory galanin story are discussed.

Effect of carbachol on the release of peptide YY from isolated vascularly and luminally perfused rat ileum

Possible cholinergic control on the release of PYY from intestine into the lumen or blood vessel was studied by radioimmunoassay in the isolated perfused rat ileum. The basal release of PYY into the lumen was 43.1 +/- 8.9 pg/min, which was comparable with that into the vasculature (35.2 +/- 2.6 pg/min). The administration of 1 microM carbachol into the vascular perfusate resulted in a more than 40-fold increase of the luminal release but only a twofold increase of the vascular release. Carbachol-induced release of PYY into both lumen and vasculature was completely blocked by atropine, but not by hexamethonium. Tetrodotoxin abolished carbachol-induced release of PYY into lumen and vasculature. These data suggest that the ileal PYY release, into either lumen or vasculature, is under the control of postganglionic cholinergic neurons via muscarinic receptors.