Identification and characterization of islet amyloid polypeptide in mammalian gastrointestinal tract

Okinawa-Based Nordic Diet Decreases Plasma Levels of IAPP and IgA against IAPP Oligomers in Type 2 Diabetes Patients

Pancreas-derived islet amyloid polypeptide (IAPP) aggregates and deposits in the pancreas and periphery of Type 2 Diabetes (T2D) patients, contributing to diabetic complications. The excess IAPP can be removed by autoantibodies, and increased levels of immunoglobulin (Ig) G against IAPP have been reported in T2D patients. However, whether other Ig classes are also affected and if the levels can be managed is less known. This pre-post study examines IgA levels against IAPP oligomers (IAPPO-IgA) in T2D patients and assesses the impact of the Okinawa-based Nordic (O-BN) diet-a low-carbohydrate, high-fiber diet-on these levels after following the diet for 3 months. IAPP, IAPPO-IgA, and total IgA levels were measured in plasma and fecal samples from n = 30 T2D patients collected at baseline, after 3 months of diet, and after additional 4 months of unrestricted diets (a clinical follow-up). The IAPP and IAPPO-IgA levels were significantly lower after 3 months, with the latter also being significantly reduced at the clinical follow-up. The reduction in plasma IAPP and IAPPO-IgA levels correlated with reductions in plasma levels of metabolic and inflammatory markers. Hence, following the O-BN diet for at least 3 months is sufficient to reduce circulating IAPP and IAPPO-IgA levels, which may be principal in managing T2D.

Mediators of Amylin Action in Metabolic Control

Amylin (also called islet amyloid polypeptide (IAPP)) is a pancreatic beta-cell hormone that is co-secreted with insulin in response to nutrient stimuli. The last 35 years of intensive research have shown that amylin exerts important physiological effects on metabolic control. Most importantly, amylin is a physiological control of meal-ending satiation, and it limits the rate of gastric emptying and reduces the secretion of pancreatic glucagon, in particular in postprandial states. The physiological effects of amylin and its analogs are mediated by direct brain activation, with the caudal hindbrain playing the most prominent role. The clarification of the structure of amylin receptors, consisting of the calcitonin core receptor plus receptor-activity modifying proteins, aided in the development of amylin analogs with a broad pharmacological profile. The general interest in amylin physiology and pharmacology was boosted by the finding that amylin is a sensitizer to the catabolic actions of leptin. Today, amylin derived analogs are considered to be among the most promising approaches for the pharmacotherapy against obesity. At least in conjunction with insulin, amylin analogs are also considered important treatment options in diabetic patients, so that new drugs may soon be added to the only currently approved compound pramlintide (Symlin^®). This review provides a brief summary of the physiology of amylin’s mode of actions and its role in the control of the metabolism, in particular energy intake and glucose metabolism.

Mono and dual agonists of the amylin, calcitonin, and CGRP receptors and their potential in metabolic diseases

BACKGROUND

Therapies for metabolic diseases are numerous, yet improving insulin sensitivity beyond that induced by weight loss remains challenging. Therefore, search continues for novel treatment candidates that can stimulate insulin sensitivity and increase weight loss efficacy in combination with current treatment options. Calcitonin gene-related peptide (CGRP) and amylin belong to the same peptide family and have been explored as treatments for metabolic diseases. However, their full potential remains controversial.

SCOPE OF REVIEW

In this article, we introduce this rather complex peptide family and its corresponding receptors. We discuss the physiology of the peptides with a focus on metabolism and insulin sensitivity. We also thoroughly review the pharmacological potential of amylin, calcitonin, CGRP, and peptide derivatives as treatments for metabolic diseases, emphasizing their ability to increase insulin sensitivity based on preclinical and clinical studies.

MAJOR CONCLUSIONS

Amylin receptor agonists and dual amylin and calcitonin receptor agonists are relevant treatment candidates, especially because they increase insulin sensitivity while also assisting weight loss, and their unique mode of action complements incretin-based therapies. However, CGRP and its derivatives seem to have only modest if any metabolic effects and are no longer of interest as therapies for metabolic diseases.

Protein aggregates and proteostasis in aging: Amylin and β-cell function

The ubiquitin-proteasomal-system (UPS) and the autophagy-lysosomal-system (ALS) are both highly susceptible for disturbances leading to the accumulation of cellular damage. A decline of protein degradation during aging results in the formation of oxidatively damaged and aggregated proteins finally resulting in failure of cellular functionality. Besides protein aggregation in response to oxidative damage, amyloids are a different type of protein aggregates able to distract proteostasis and interfere with cellular functionality. Amyloids are clearly linked to the pathogenesis of age-related degenerative diseases such as Alzheimer's disease. Human amylin is one of the peptides forming fibrils in β-sheet conformation finally leading to amyloid formation. In contrast to rodent amylin, human amylin is prone to form amyloidogenic aggregates, proposed to play a role in the pathogenesis of Type 2 Diabetes by impairing β-cell functionality. Since aggregates such as lipofuscin and β-amyloid are known to impair proteostasis, it is likely to assume similar effects for human amylin. In this review, we focus on the effects of IAPP on UPS and ALS and their role in amylin degradation, since both systems play a crucial role in maintaining proteome balance thereby influencing, at least in part, cellular fate and aging.

Amylin-mediated control of glycemia, energy balance, and cognition

Amylin, a peptide hormone produced in the pancreas and in the brain, has well-established physiological roles in glycemic regulation and energy balance control. It improves postprandial blood glucose levels by suppressing gastric emptying and glucagon secretion; these beneficial effects have led to the FDA-approved use of the amylin analog pramlintide in the treatment of diabetes mellitus. Amylin also acts centrally as a satiation signal, reducing food intake and body weight. The ability of amylin to promote negative energy balance, along with its unique capacity to cooperatively facilitate or enhance the intake- and body weight-suppressive effects of other neuroendocrine signals like leptin, have made amylin a leading target for the development of novel pharmacotherapies for the treatment of obesity. In addition to these more widely studied effects, a growing body of literature suggests that amylin may play a role in processes related to cognition, including the neurodegeneration and cognitive deficits associated with Alzheimer's disease (AD). Although the function of amylin in AD is still unclear, intriguing recent reports indicate that amylin may improve cognitive ability and reduce hallmarks of neurodegeneration in the brain. The frequent comorbidity of diabetes mellitus and obesity, as well as the increased risk for and occurrence of AD associated with these metabolic diseases, suggests that amylin-based pharmaceutical strategies may provide multiple therapeutic benefits. This review will discuss the known effects of amylin on glycemic regulation, energy balance control, and cognitive/motivational processes. Particular focus will be devoted to the current and/or potential future clinical use of amylin pharmacotherapies for the treatment of diseases in each of these realms.

Role of Cholesterol and Phospholipids in Amylin Misfolding, Aggregation and Etiology of Islet Amyloidosis

Amyloidosis is a biological event in which proteins undergo structural transitions from soluble monomers and oligomers to insoluble fibrillar aggregates that are often toxic to cells. Exactly how amyloid proteins, such as the pancreatic hormone amylin, aggregate and kill cells is still unclear. Islet amyloid polypeptide, or amylin, is a recently discovered hormone that is stored and co-released with insulin from pancreatic islet β-cells. The pathology of type 2 diabetes mellitus (T2DM) is characterized by an excessive extracellular and intracellular accumulation of toxic amylin species, soluble oligomers and insoluble fibrils, in islets, eventually leading to β-cell loss. Obesity and elevated serum cholesterol levels are additional risk factors implicated in the development of T2DM. Because the homeostatic balance between cholesterol synthesis and uptake is lost in diabetics, and amylin aggregation is a hallmark of T2DM, this chapter focuses on the biophysical and cell biology studies exploring molecular mechanisms by which cholesterol and phospholipids modulate secondary structure, folding and aggregation of human amylin and other amyloid proteins on membranes and in cells. Amylin turnover and toxicity in pancreatic cells and the regulatory role of cholesterol in these processes are also discussed.

Gut microbiota in health and disease

Gut microbiota is an assortment of microorganisms inhabiting the length and width of the mammalian gastrointestinal tract. The composition of this microbial community is host specific, evolving throughout an individual's lifetime and susceptible to both exogenous and endogenous modifications. Recent renewed interest in the structure and function of this "organ" has illuminated its central position in health and disease. The microbiota is intimately involved in numerous aspects of normal host physiology, from nutritional status to behavior and stress response. Additionally, they can be a central or a contributing cause of many diseases, affecting both near and far organ systems. The overall balance in the composition of the gut microbial community, as well as the presence or absence of key species capable of effecting specific responses, is important in ensuring homeostasis or lack thereof at the intestinal mucosa and beyond. The mechanisms through which microbiota exerts its beneficial or detrimental influences remain largely undefined, but include elaboration of signaling molecules and recognition of bacterial epitopes by both intestinal epithelial and mucosal immune cells. The advances in modeling and analysis of gut microbiota will further our knowledge of their role in health and disease, allowing customization of existing and future therapeutic and prophylactic modalities.

Activation of calcitonin receptor and calcitonin receptor-like receptor by membrane-anchored ligands

G protein-coupled receptors (GPCRs) are the most important pharmaceutical targets, and more than 40% of drugs in use today modulate GPCR signaling. A major hurdle in the development of therapies targeting GPCRs is the drug candidate's nonselective actions in multiple tissues. The ability to spatially control GPCR signaling would provide a venue for developing therapies that require targeted GPCR signaling. Here, we show that the fusion of a RAMP1 co-receptor with the calcitonin gene-related peptide (CGRP), or calcitonin, transforms the RAMP1 from a co-receptor to bona fide membrane-anchored ligands (CGRP-RAMP1 and CAL-RAMP1). The CAL-RAMP1 selectively activates the calcitonin receptor (CR), whereas, the CGRP-RAMP1 activates both the calcitonin receptor-like receptor (CLR) and CR. Unlike a free peptide, which moves freely in the extracellular space and differentiates targets based on molecular affinity, the anchored CGRP-RAMP1 and CAL-RAMP1 ligands confine their activities to individual cells. In addition, our study showed that a CGRP8-37-RAMP1 chimera, but not RAMP1, functions as an antagonist for CGRP-RAMP1-mediated signaling, suggesting that the activation of CLR by CGRP-RAMP1 shares similar molecular mechanisms with the CGRP-mediated activation of CLR/RAMP1 receptor complexes. Taken together, our finding thus provides a novel class of ligands that activate CR and CLR exclusively in an autocrine manner and a proof-of-concept demonstration for future development of targeted therapies aimed at these receptors in specific cell populations.

Tissue Expression and Secretion of Amylin

Amylin and insulin are co-localized within the same secretory granules of pancreatic beta-cells. Acutely, the secreted ratio of amylin:insulin is comparatively invariant, but long-standing hyperglycemia may favor induction of amylin synthesis and secretion over that of insulin. Amylin is also found in much lesser quantities in the gut and other tissues. In humans, both type 1 diabetes mellitus and the later stages of type 2 diabetes mellitus are characterized by deficiency of both insulin and amylin secretion. The severity of amylin deficiency appears to correlate with the severity of insulin deficiency. This concordance of deficiencies in amylin and insulin secretion observed with the progression of diabetes mellitus is consistent with their co-localization in pancreatic beta-cells. Amylin is cleared mainly by proteolytic degradation at the kidney. The terminal t1/2 for rat amylin in rats is approximately 13 min, and that for pramlintide in humans is approximately 20-45 min.

Effects on digestive secretions

Rat amylin subcutaneously injected into rats dose-dependently inhibits pentagastrin-stimulated gastric acid secretion and protects the stomach from ethanol-induced gastritis. The ED50s for these actions (0.050 and 0.036 microg, respectively) are the lowest for any dose-dependent effect of amylin thus far described, and their similar potencies are consistent with a mechanistic (causal) association. At higher amylin doses, inhibition of gastric acid secretion was almost complete (93.4%). Gastric injury (measured by a subjective analog scale) was inhibited by up to 67%. The observation that effective doses of amylin result in plasma concentrations of 7-10 pM (i.e., within the reported range; Pieber et al., 1994) supports the interpretation that inhibition of gastric acid secretion and maintenance of gastric mucosal integrity are physiological actions of endogenous amylin. The pharmacology of these responses fits with one mediated via amylin-like receptors. Rat amylin inhibited CCK-stimulated secretion of pancreatic enzymes,amylase, and lipase by up to approximately 60% without having significant effect in the absence of CCK. ED50s for the effect were in the 0.1-0.2 microg range, calculated to produce plasma amylin excursions within the physiological range. Effects of informative ligands are consistent with the concept of amylin receptor mediation. Amylin was effective in ameliorating the severity of pancreatitis in a rodent model. The amylin analog pramlintide inhibited gallbladder emptying in mice as measured by total weight of acutely excised gallbladders. Amylin inhibition of gastric acid secretion, pancreatic enzyme secretion, and bile secretion likely represents part of an orchestrated control of nutrient appearance. Modulation of digestive function fits with a general role of amylin in regulating nutrient uptake. Rate of ingestion, rate of release from the stomach, and rate of digestion of various food groups appear to be under coordinate control.

Antiulcer effects of amylin: a review

Amylin belongs to the calcitonin peptide family. Amylin is a peptide synthesized not only in the beta cells of pancreatic islets, but in small quantities also in other organs like in the intestinal and gastric mucosa, lungs and central nervous system. It is located in the same secretory granules as insulin. Amylin participates in the maintenance of glucose and calcium homeostasis. It also inhibits food intake and decreases body weight. Furthermore, amylin inhibits gastric acid secretion. It protects the gastric mucosa in ulcer models like stress, vagal stimulation, ethanol, acetic acid, reserpine and serotonine administration and pylorus ligation. This protective antiulcer is seen not only at pharmacological but also at near-physiological doses-0.5mkg/kg. Moreover amylin also exerts curative properties in the acetic acid and indomethacin ulcer models. Amylin decreases the aggressive factors like acid-pepsin secretion, increases mast cell stability and increases protective mechanisms like bicarbonate gastric secretion, dilates blood vessels, and it increases lymphatic mesenteric activity. Amylin seems to be a powerful protector of gastric mucosa in animals by increasing the stability of gastric mucosa. Further research remains, however, to be done.

Regulation of amylin release from cultured rabbit gastric fundic mucosal cells

Background

Amylin (islet amyloid polypeptide) is a hormone with suggested roles in the regulation of glucose homeostasis, gastric motor and secretory function and gastroprotection. In the gastric mucosa amylin is found co-localised with somatostatin in D-cells. The factors regulating gastric amylin release are unknown. In this study we have investigated the regulation of amylin release from gastric mucosal cells in primary culture. Rabbit fundic mucosal cells enriched for D-cells by counterflow elutriation were cultured for 40 hours. Amylin and somatostatin release over 2 hours in response to agonists were assessed.

Results

Amylin release was significantly enhanced by activation of protein kinase C with phorbol-12-myristate-13-acetate, adenylate cyclase with forskolin and elevation of intracellular calcium with A23187. Cholecystokinin (CCK), epinephrine and glucagon-like peptide-1 (GLP-1) each stimulated amylin release in a dose-dependent manner. Maximal CCK-stimulated release was greater than either epinephrine or GLP-1, even when the effects of the latter two were enhanced by isobutylmethylxanthine. Stimulated amylin release was significantly inhibited by carbachol (by 51–59%) and octreotide (by 33–42%). Somatostatin release paralleled that of amylin.

Conclusions

The cultured D-cell model provides a means of studying amylin release. Amylin secretion is stimulated by receptor-dependent and -independent activation of Ca²⁺/protein kinase C and adenylate cyclase pathways. Inhibition involves activation of muscarinic receptors and auto-regulation by somatostatin.

Direct inhibitory effect of adrenomedullin, calcitonin gene-related peptide, calcitonin, and amylin on cholecystokinin-induced contraction of guinea-pig isolated caecal circular smooth muscle cells

We recently reported the direct inhibitory effect of adrenomedullin on caecal circular smooth muscle cells via cAMP system. This study was designed to determine whether the structurally related peptides to adrenomedullin (i.e.; calcitonin gene-related peptide (CGRP), calcitonin, and amylin) can inhibit the cholecystokinin octapeptide (CCK-8)-induced contractile response by exerting a direct action on guinea-pig caecal circular smooth muscle cells, and to compare the inhibitory potency of these peptides. In addition, to elucidate each intracellular mechanisms, the effects of an inhibitor of cAMP-dependent protein kinase, inhibitors of particulate or soluble guanylate cyclase on the each peptide-induced relaxation were investigated. Adrenomedullin, CGRP, calcitonin, and amylin inhibited the contractile response produced by CCK-8 in a dose-dependent manner, with IC50 values of 0.14 nM, 0.37 nM, 5.4 nM, and 160 nM, respectively. An inhibitor of cAMP-dependent protein kinase significantly inhibited the relaxation produced by all of these peptides. On the contrary, inhibitors of particulate or soluble guanylate cyclase did not have any significant effect on the relaxation produced by these peptides. In this study, we demonstrated the direct inhibitory effects of the structurally related peptides to adrenomedullin (i.e.; CGRP, calcitonin, and amylin) on the isolated caecal circular smooth muscle cells via cAMP system. The order of potency was as follows; adrenomedullin falling dots CGRP > calcitonin > amylin.

Synthesis and purification of amyloidogenic peptides

The polypeptide hormone amylin forms amyloid deposits in patients with type 2 diabetes mellitus. Amyloid-forming peptides are often very difficult to synthesize and purify. Amylin and fragments of amylin are no exception. In this paper we describe the efficient synthesis and purification of two amyloidogenic fragments of human amylin. One fragment corresponds to residues 17 to 37 of the full-length hormone and the other corresponds to residues 24 to 37. These fragments have previously been identified in vivo and have been shown to form amyloid in vitro. The strategy used to elucidate appropriate conditions for the synthesis and purification of these peptides is generally applicable to other peptides that are difficult to synthesize. These peptides were prepared using solid-phase peptide synthesis with Fmoc alpha-amino protection. The effects of varying the solvent, side-chain-protecting group and choice of cleavage conditions were examined. The use of NMP as the main solvent and cleavage with trifluoroacetic acid, phenol, ethanedithiol, thioanisole, and water proved to be optimal. 1,1,1,3, 3,3-Hexafluoro-2-propanol (HFIP) was found to be the best solvent for solubilizing the crude peptides. A wide range of HPLC conditions for the purification of the peptides were examined and an acetonitrile-based solvent system with HCl as the ion pairing agent provided efficient purification.

Analysis of amylin cleavage products provides new insights into the amyloidogenic region of human amylin

Human amylin is the primary component of amyloid deposits found in the pancreatic beta-cells of patients with type 2 diabetes mellitus. Recently, two fragments of amylin have been identified in vivo. One fragment contains residues 17 to 37 of human amylin (AMYLIN17-37) and the other contains residues 24 to 37 (AMYLIN24-37). The secondary structure and amyloid forming ability of each peptide was determined at pH 5.5(+/-0.3) and pH 7.4(+/-0.3). Results at these two values of pH were very similar. Both peptides are predominantly unstructured in solution (CD) but adopt a significant amount of beta-sheet secondary structure upon aggregation (FTIR). Transmission electron microscopy (TEM) confirmed the presence of amyloid fibrils. AMYLIN24-37 was further dissected by studying peptides corresponding to residues 24 to 29 and 30 to 37. The AMYLIN30-37 peptide forms amyloid deposits. Samples of the 24 to 29 fragment which had TFA as the associated counterion formed ordered deposits but samples associated with HCl did not. Residues 20 to 29 are traditionally thought to be the amyloidogenic region of amylin, but this study demonstrates that peptides derived from other regions of amylin are capable of forming amyloid, and hence indicates that these regions of amylin can play a role in amyloid formation.

Gastric amylin expression. Cellular identity and lack of requirement for the homeobox protein PDX-1. A study in normal and PDX-1-deficient animals with a cautionary note on antiserum evaluation

The gene encoding amylin is implicated in the generation of amyloid in the islets of Langerhans of diabetics and is believed to be regulated by the homeodomain transcription factor PDX-1. Although gastric mucosa also produces amylin, studies on its cellular site of production have yielded highly divergent results, localizing this peptide to either gastrin, serotonin, or somatostatin cells or to combinations thereof. Using region-specific amylin antisera in combination with reverse transcriptase-polymerase chain reaction, we now document that the majority of cells expressing amylin correspond to somatostatin cells. Only a small subpopulation of gastrin cells contained immunoreactive amylin. Studies of PDX-1-deficient mice, which fail to develop gastrin cells while possessing normal numbers of somatostatin cells, revealed no detectable change in gastric amylin expression. These data show that neither normal gastrin cell development nor PDX-1 expression is needed for gastric amylin expression.

Examination of somatostatin involvement in the inhibitory action of GIP, GLP-1, amylin and adrenomedullin on gastric acid release using a new SRIF antagonist analogue

1. The effect of a new type 2 selective somatostatin (SRIF) receptor antagonist (DC-41-33) on somatostatin-induced inhibition of pentagastrin-stimulated gastric acid secretion in conscious, chronic gastric fistula equipped rats was studied. 2. Infused intravenously, DC-41-33 dose-dependently inhibits SRIF-induced inhibition of pentagastrin-stimulated gastric acid secretion with an IC50 of 31.6+/-1.2 nmol kg(-1) versus 10 nmol kg(-1) SRIF and blocks the inhibitory effects of SRIF when simultaneously co-infused. Its effectiveness provides additional evidence that SRIF-inhibition of gastric acid release is a SRIF type 2 receptor-mediated process. 3. DC-41-33 is able to completely reverse the inhibitory effect of glucose-dependent insulinotropic polypeptides, GIP and GIP-(1-30)NH2, and glucagon-like polypeptide, GLP-1(7-36)NH2, on pentagastrin-stimulated gastric acid secretion thus confirming that they exert these effects through stimulation of endogenous SRIF release. 4. DC-41-33 only partially blocks potent amylin and adrenomedullin-induced inhibition of gastric acid secretion, therefore suggesting that somatostatin may not function as a primary mediator in the action of these peptides. 5. Our results indicate that DC-41-33, is a potent in vivo inhibitor of exogenous and endogenous SRIF in rats. It represents a new class of SRIF analogues which should eventually provide excellent tools for further evaluating the many physiological roles of SRIF and its five receptor subtypes.

Investigation on the mechanisms involved in the central protective effect of amylin on gastric ulcers in rats

1. The mechanisms involved in the protective effect of amylin (administered into the brain ventricle, i.c.v.) on gastric ulcers induced by the oral administration of ethanol 50% (EtOH, 2 ml/rat) or indomethacin (indomethacin, 20 mg kg(-1), at a dosing volume of 5 ml) were investigated in rats. 2. The possible involvement of endogenous nitric oxide (NO) in the beneficial effect of amylin against EtOH-induced ulcers was examined. The inhibitor of NO-synthesis, NG-nitro-L-arginine methyl ester (L-NAME, 70 mg kg(-1), s.c.) was injected 30 min before amylin (2.2 microg/rat, i.c.v.) followed by EtOH after a further 30 min. Rats were sacrificed 1 h after EtOH. L-NAME completely removed the protective effect of amylin. 3. The interaction between amylin and gastric nonprotein sulfhydryl groups was studied. The rats were treated with N-ethyl-maleimide (NEM, 25 mg kg(-1), s.c.) 30 min before amylin (2.2 microg/rat, i.c.v.) followed by EtOH 30 min after or by indomethacin 5 min after amylin. Rats were sacrificed 1 h or 6 h respectively after EtOH or indomethacin. NEM counteracted the protective effect of amylin against EtOH-induced ulcers but not against those provoked by indomethacin. 4. To determine whether amylin was able to promote ulcer healing, the peptide was injected 5 min after EtOH or 1 h after indomethacin. In the case of EtOH, the beneficial effect of amylin was lost whereas it was still effective on indomethacin-induced ulcers. 5. The results indicate that: the mechanisms involved in the antiulcer effects of amylin are different in these two types of gastric lesions probably because of the different etiopathology of various types of ulcers. Endogenous NO and nonprotein sulfhydryl groups are involved in the mucosal protective effects of amylin on EtOH and not on indomethacin-induced ulcers. Furthermore the effectiveness of amylin against indomethacin-induced lesions when administered after the ulcerogenic process has started suggests that amylin is involved not only in the protection but also in the healing mechanisms in this type of ulcer.

Amylin and gastrointestinal activity

1. Amylin is a new pancreatic islet peptide with a role in the maintenance of glucose homeostasis. 2. Amylin is predominantly present in the beta cells of the pancreas and to a lesser extent in the gastrointestinal tract and in the nervous system, where amylin mRNA is also present along with specific binding sites. 3. Amylin given peripherally or centrally inhibits acid gastric secretion in a dose-dependent manner and has a protective effect against indomethacin- or ethanol-induced ulcers only when injected centrally. 4. Subcutaneous or central injection of amylin produces a dose-dependent inhibition of gastric emptying, which may contribute to the activity of amylin in the regulation of carbohydrate absorption. In addition amylin inhibits food intake both when injected peripherally or centrally. 5. Amylin may thus be considered a novel brain-gut peptide taking part in the rapid endocrine response during digestion to maintain euglycemia.

Adrenomedullin, amylin, calcitonin gene-related peptide and their fragments are potent inhibitors of gastric acid secretion in rats

Adrenomedullin, amylin and calcitonin gene-related peptides (CGRP) share close sequence homology and have overlapping spectra of biological activities, particularly with respect to cardiovascular and gastrointestinal functions. Comparisons of the effects of these three peptides on gastric acid release have been made by i.v. infusions in conscious rats equipped with gastric fistulae. All peptides were extremely potent inhibitors of basal, pentagastrin- and 2-deoxy-D-glucose-stimulated gastric acid secretion with IC50 values in the subnanomolar to nanomolar range. These effects were not inhibited by C-terminal extra-cyclic fragments of the peptides which often act as competitive receptor antagonists in other biological systems. At high concentrations C-terminal fragments of human adrenomedullin and rat alpha-calcitonin gene-related peptide displayed some receptor agonist activity. Furthermore, the N-terminally situated disulfide-bridged ring fragments, human adrenomedullin-(15-22), rat amylin-(1-8) and rat alpha-calcitonin gene-related peptide-(1-8), retained significant gastric acid inhibitory potencies thus suggesting involvement of receptor(s) with significantly differing ligand binding profiles than those characterized previously.

Protection by amylin of gastric erosions induced by indomethacin or ethanol in rats

1. The effect of amylin on gastric ulcers induced by oral administration of indomethacin (Indo, 20 mg kg-1 at a dosing volume of 5 ml) or ethanol 50% (EtOH, 1 ml/rat) was investigated in conscious rats. 2. Amylin given intracerebroventricularly (0.22, 0.66 and 2.2 micrograms/rat, i.c.v.) demonstrated a dose-dependent cytoprotective effect against both Indo and EtOH-induced ulcers. In contrast, amylin, given subcutaneously at doses effective in inhibiting acid gastric secretion (2.5, 10 and 40 micrograms kg-1, s.c.), did not show any cytoprotective effect. 3. The interaction between amylin and endogenous nitric oxide (NO) in the maintenance of gastric mucosal integrity was investigated by pretreating the rats with a selective inhibitor of NO-synthesis, NG-nitro-L-arginine methyl ester (L-NAME, 25 and 70 mg kg-1, s.c.). Administration of L-NAME to rats did not significantly increase the degree of the Indo-induced ulcer index and was not able to remove the protective effect of amylin on Indo-induced ulcers, thus excluding a role for endogenous NO in mediating the protective effect of this peptide. 4. To determine whether the cytoprotective effect of amylin was mediated by endogenous prostaglandins, we studied the effect of amylin (2.2 micrograms/rat, i.c.v.) on EtOH- induced ulcers in rats pretreated with Indo (10 mg kg-1, s.c.) to inhibit prostanoid biosynthesis; Indo was injected 30 min before amylin and EtOH after a further 30 min. Pretreatment with Indo did not significantly increase the ulcer index induced by EtOH but counteracted the ability of amylin to prevent the ulcer formation. 5. These findings suggest that amylin exerts a gastroprotective activity that is not strictly related to inhibition of acid gastric secretion and can be partly explained through a prostaglandin-dependent mechanism mediated by receptors for the peptide in the brain. Amylin might be considered as a new brain-gut peptide.

Islet amyloid polypeptide in the gut and pancreas: localization, ontogeny and gut motility effects

The occurrence of islet amyloid polypeptide (IAPP) in the gut and pancreas of several species and during ontogeny of the rat, was studied using immunocytochemistry. Effects of IAPP on rat ileal smooth muscle were assessed in vitro. Islets of most, but not all, species examined, displayed IAPP in insulin cells and, in some species, also in somatostatin- and peptide YY (PYY)-containing cells. In the gut, expression of IAPP varied among species; when present, IAPP was most abundant in the proximal part and co-localized with somatostatin, PYY, gastrin/cholecystokinin, enteroglucagon or serotonin. IAPP was first demonstrated at embryonic day 12 and 16 in islet and gastrointestinal endocrine cells, respectively. IAPP relaxed gut muscle and reduced electrical field stimulation-evoked contractions, presumably by inhibiting acetylcholine release. Thus, IAPP expression in islets is consistent with an important role for IAPP in fuel metabolism; the gastrointestinal expression and motor effects of IAPP suggest that IAPP may modulate gastrointestinal function.

Different influence of CGRP (8-37), an amylin and CGRP antagonist, on the anorectic effects of cholecystokinin and bombesin in diabetic and normal rats

Because previous studies had suggested that the anorectic effects of cholecystokinin (CCK) and bombesin (BBS) depend partly on the release of amylin or calcitonin gene-related peptide (CGRP), we investigated the influence of the amylin and CGRP receptor antagonist CGRP (8-37) on the anorectic effects of CCK and BBS in streptozotocin (STZ)-diabetic and nondiabetic rats. STZ-diabetic rats had significantly lower plasma amylin and insulin concentrations than nondiabetic control rats. Amylin (5 micrograms/kg or 2.5 micrograms/rat) injected IP at dark onset after 24-h food deprivation elicited an anorectic effect of similar extent in STZ-diabetic and control rats. Under similar conditions, CCK (0.25 and 2 micrograms/kg) and BBS (5 micrograms/kg) reduced food intake in both STZ-diabetic and nondiabetic rats. These effects were markedly attenuated by CGRP (8-37) (10 micrograms/kg) in non-diabetics but not in STZ-diabetic rats. It is concluded that part of the anorectic effects of CCK and BBS depend on the release of amylin from pancreatic B-cells.

Amylin given by central or peripheral routes decreases gastric emptying and intestinal transit in the rat

The effect of rat amylin on gastric emptying and intestinal transit in the rat was examined. Amylin administered intracerebroventricularly (1, 2, 2.5 or 4 micrograms/rat) produced the maximal decrease in gastric emptying and intestinal transit at the dose of 2.5 micrograms/rat. Higher doses produced a lower effect. Peripheral administration (25, 50 or 100 micrograms/kg) produced dose-dependent effects. Pre-treatment with neostigmine blocked the effect of amylin when it was centrally injected, while the effect of amylin given peripherally was partially reduced. Pre-treatment with domperidone decreased the inhibitory effect of peripherally injected amylin, but no effect was observed when the peptide was centrally injected.

Islet amyloid polypeptide is expressed in endocrine cells of the gastric mucosa in the rat and mouse

BACKGROUND/AIMS

Islet amyloid polypeptide (IAPP) or amylin is a novel islet hormone candidate with a suggested role in the regulation of glucose homeostasis and the pathogenesis of non-insulin dependent diabetes mellitus. Occurrence of IAPP in the gastrointestinal tract of rats and humans has also been shown. The expression of IAPP in the stomach of the rat and mouse and the possible colocalization of IAPP and known gastric hormones were investigated in this study.

METHODS

In situ hybridization, immunofluorescence, and combined in situ hybridization and immunocytochemistry were used.

RESULTS

IAPP messenger RNA and IAPP-like immunoreactivity were shown in the same endocrine cells in the antrum and fundus of the rat and in the antrum of the mouse. IAPP was expressed in a major population of somatostatin-immunoreactive cells as well as in small populations of gastrin- and peptide YY-immunoreactive cells.

CONCLUSIONS

Our results establish the synthesis and storage of IAPP in gastric endocrine cells in the rat and mouse. The extensive colocalization of IAPP with somatostatin and to a lesser extent with gastrin and peptide YY suggests that IAPP may modulate endocrine activity in the gastric mucosa in a paracrine and/or autocrine mode.

Amylin given by central and peripheral routes inhibits acid gastric secretion

The effect of rat amylin on acid gastric secretion in the pylorus-ligated, unanesthetized rat system (Shay test) was examined. Amylin administered peripherally (2.5, 5, 10, 40, 100, or 160 micrograms/kg, SC) or intracerebroventricularly (1.5, 2.7, or 5 micrograms/rat, ICV) decreased acid gastric secretion in a dose-dependent manner. Central administration of amylin gave a stronger suppression of gastric secretion than peripheral injection. In addition, ICV-injected amylin inhibited insulin-stimulated acid gastric secretion and was effective in suppressing acid gastric secretion in rats depleted of somatostatin by pretreatment with cysteamine. This study suggests that amylin may participate in the central regulation of acid gastric secretion and indicates a possible biological function of amylin as a gastrointestinal peptide.

Islet amyloid polypeptide gene expression in the endocrine pancreas of the rat: a combined in situ hybridization and immunocytochemical study

The expression of the islet amyloid polypeptide (IAPP) gene within the endocrine pancreas and its correlation with insular neuroendocrine peptide localization were investigated in the rat. In situ hybridization with a 35S-labelled IAPP-mRNA specific oligonucleotide probe was combined with immunocytochemistry. In situ hybridization alone showed strong autoradiographic labelling of the pancreatic islets. In situ hybridization combined with immunocytochemistry for IAPP, revealed labelling of the IAPP-immunoreactive cells. However, when in situ hybridization was combined with immunocytochemistry for proinsulin, we noted a lack of proinsulin immunoreactivity in some peripherally located autoradiographically labelled islet cells. Furthermore, combination of in situ hybridization and immunocytochemistry for somatostatin showed autoradiographic labelling of somatostatin cells to a varying degree. This was further confirmed by showing cellular co-localization of IAPP and somatostatin by immunocytochemical double staining. We conclude that IAPP is mainly synthesized in insulin cells. Additionally, a subpopulation of the somatostatin cells is capable of IAPP synthesis. This may account for the relatively small reduction in the content of IAPP-mRNA in islets compared to the marked reduction of insulin mRNA after streptozotocin-induced diabetes in rats as previously reported.