Histamine secretion from rat enterochromaffinlike cells

Exploring the Differential Expression of a Set of Key Genes Involved in the Regulation and Functioning of the Stomach in the Post-Weaned Pig

Despite playing a key role in digestion, there is only a broad characterization of the spatiotemporal development of the three glandular regions of the stomach (cardiac, fundic and pyloric) in the weaned pig. Hence, the objective of this experiment was to explore the differential expression (DE) of a panel of key genes within the three glandular regions of the stomach. Eight pigs were sacrificed at d 8 post-weaning, and three mucosal samples were collected from each stomach's glandular regions. The expression of a panel of genes were measured using QPCR. The true cardiac gland region was characterized by increased expression of PIGR, OLFM4, CXCL8 and MUC2 relative to the two other regions (p < 0.05). The fundic gland region was characterized by increased expression of ATP4A, CLIC6, KCNQ1, HRH2, AQP4, HDC, CCKBR, CHIA, PGA5, GHRL and MBOAT4 compared to the two other regions (p < 0.05). The pyloric gland region was characterized by exclusive expression of GAST (p < 0.05). A transition region between the cardiac and fundic region (cardiac-to-oxyntic transition) was observed with a gene expression signature that resembles a cross of the signatures found in the two regions. In conclusion, unique gene expression signatures were identifiable in each of the glandular regions, with a cardiac-to-oxyntic transition region clearly identifiable in the post-weaned pigs' stomachs.

Alterations in Small Intestine and Liver Morphology, Immunolocalization of Leptin, Ghrelin and Nesfatin-1 as Well as Immunoexpression of Tight Junction Proteins in Intestinal Mucosa after Gastrectomy in Rat Model

The stomach is responsible for the processing of nutrients as well as for the secretion of various hormones which are involved in many activities throughout the gastrointestinal tract. Experimental adult male Wistar rats (n = 6) underwent a modified gastrectomy, while control rats (n = 6) were sham-operated. After six weeks, changes in small intestine (including histomorphometrical parameters of the enteric nervous plexuses) and liver morphology, immunolocalization of leptin, ghrelin and nesfatin-1 as well as proteins forming adherens and tight junctions (E-cadherin, zonula occludens-1, occludin, marvelD3) in intestinal mucosa were evaluated. A number of effects on small intestine morphology, enteric nervous system ganglia, hormones and proteins expression were found, showing intestinal enteroplasticity and neuroplasticity associated with changes in gastrointestinal tract condition. The functional changes in intestinal mucosa and the enteric nervous system could be responsible for the altered intestinal barrier and hormonal responses following gastrectomy. The results suggest that more complicated regulatory mechanisms than that of compensatory mucosal hypertrophy alone are involved.

Towards Understanding of Gastric Cancer Based upon Physiological Role of Gastrin and ECL Cells

Simple Summary

Generally, we know that cancers represent genetic changes in tumour cells, but we most often do not know the causes of cancers or how they develop. Our knowledge of the regulation of gastric acid secretion is well known, with the gastric hormone gastrin maintaining gastric acidity by stimulation of the enterochromaffin-like (ECL) cell to release histamine, which subsequently augments acid secretion. Furthermore, it seems to be a general principle that stimulation of function (which, for the ECL cell, is release of histamine) in a parallel way stimulates the proliferation of the same cell. Long-term hyperstimulation of cell division predisposes to genetic changes and, thus, development of tumours. All conditions with reduced gastric acidity result in an increased risk of gastric tumours due to elevated gastrin in order to restore gastric acidity. It is probable that Helicobacter pylori infection (the most important cause of gastric cancer), as well as drugs inhibiting gastric acid secretion induce gastric cancer in the long-term, due to an elevation of gastrin caused by reduced gastric acidity. Gastric carcinomas have been shown to express ECL cell markers, further strengthening this relationship.

Abstract

The stomach is an ideal organ to study because the gastric juice kills most of the swallowed microbes and, thus, creates rather similar milieu among individuals. Combined with a rather easy access to gastric juice, gastric physiology was among the first areas to be studied. During the last century, a rather complete understanding of the regulation of gastric acidity was obtained, establishing the central role of gastrin and the histamine producing enterochromaffin-like (ECL) cell. Similarly, the close connection between regulation of function and proliferation became evident, and, furthermore, that chronic overstimulation of a cell with the ability to proliferate, results in tumour formation. The ECL cell has long been acknowledged to give rise to neuroendocrine tumours (NETs), but not to play any role in carcinogenesis of gastric adenocarcinomas. However, when examining human gastric adenocarcinomas with the best methods presently available (immunohistochemistry with increased sensitivity and in-situ hybridization), it became clear that many of these cancers expressed neuroendocrine markers, suggesting that some of these tumours were of neuroendocrine, and more specifically, ECL cell origin. Thus, the ECL cell and its main regulator, gastrin, are central in human gastric carcinogenesis, which make new possibilities in prevention, prophylaxis, and treatment of this cancer.

The Physiology of the Gastric Parietal Cell

Parietal cells are responsible for gastric acid secretion, which aids in the digestion of food, absorption of minerals, and control of harmful bacteria. However, a fine balance of activators and inhibitors of parietal cell-mediated acid secretion is required to ensure proper digestion of food, while preventing damage to the gastric and duodenal mucosa. As a result, parietal cell secretion is highly regulated through numerous mechanisms including the vagus nerve, gastrin, histamine, ghrelin, somatostatin, glucagon-like peptide 1, and other agonists and antagonists. The tight regulation of parietal cells ensures the proper secretion of HCl. The H+-K+-ATPase enzyme expressed in parietal cells regulates the exchange of cytoplasmic H+ for extracellular K+. The H+ secreted into the gastric lumen by the H+-K+-ATPase combines with luminal Cl- to form gastric acid, HCl. Inhibition of the H+-K+-ATPase is the most efficacious method of preventing harmful gastric acid secretion. Proton pump inhibitors and potassium competitive acid blockers are widely used therapeutically to inhibit acid secretion. Stimulated delivery of the H+-K+-ATPase to the parietal cell apical surface requires the fusion of intracellular tubulovesicles with the overlying secretory canaliculus, a process that represents the most prominent example of apical membrane recycling. In addition to their unique ability to secrete gastric acid, parietal cells also play an important role in gastric mucosal homeostasis through the secretion of multiple growth factor molecules. The gastric parietal cell therefore plays multiple roles in gastric secretion and protection as well as coordination of physiological repair.

Lipopolysaccharide-induced expansion of histidine decarboxylase-expressing Ly6G+ myeloid cells identified by exploiting histidine decarboxylase BAC-GFP transgenic mice

Histamine is a biogenic amine that is chiefly produced in mast cells and basophils and elicits an allergic response upon stimulation. Histidine decarboxylase (HDC) is a unique enzyme that catalyzes the synthesis of histamine. Therefore, the spatiotemporally specific Hdc gene expression profile could represent the localization of histamine-producing cells under various pathophysiological conditions. Although the bioactivity of histamine is well defined, the regulatory mechanism of Hdc gene expression and the distribution of histamine-producing cell populations in various disease contexts remains unexplored. To address these issues, we generated a histidine decarboxylase BAC (bacterial artificial chromosome) DNA-directed GFP reporter transgenic mouse employing a 293-kb BAC clone containing the entire Hdc gene locus and extended flanking sequences (Hdc-GFP). We found that the GFP expression pattern in the Hdc-GFP mice faithfully recapitulated that of conventional histamine-producing cells and that the GFP expression level mirrored the increased Hdc expression in lipopolysaccharide (LPS)-induced septic lungs. Notably, a CD11b⁺Ly6G⁺Ly6C^low myeloid cell population accumulated in the lung during sepsis, and most of these cells expressed high levels of GFP and indeed contain histamine. This study reveals the accumulation of a histamine-producing myeloid cell population during sepsis, which likely participates in the immune process of sepsis.

The Enterochromaffin-like [ECL] Cell-Central in Gastric Physiology and Pathology

Background: Studies on the regulation of gastric and pancreatic secretion began more than 100 years ago. Secretin was the first hormone postulated to exist, initiating the field of endocrinology. Gastrin produced in the antral mucosa was the second postulated hormone, and together with histamine and acetylcholine, represent the three major gastric acid secretagogues known since 1920. For a long time, the mast cell was the only recognized histamine-producing cell in the oxyntic mucosa and, in the mid-1980s, the ECL cell was recognized as the cell producing histamine, taking part in the regulation of gastric acid secretion. Methods: This review is based upon literature research and personal knowledge. Results: The ECL cell carries the gastrin receptor, and gastrin regulates its function (histamine release) as well as proliferation. Long-term hypergastrinemia results in gastric neoplasia of variable malignancies, implying that gastric hypoacidity resulting in increased gastrin release will induce gastric neoplasia, including gastric cancer. Conclusions: The trophic effect of gastrin on the ECL cell has implications to the treatment with inhibitors of acid secretion.

Non-sulfated cholecystokinin-8 reduces meal size and prolongs the intermeal interval in male Sprague Dawley rats

The current study measured seven feeding responses by non-sulfated cholecystokinin-8 (NS CCK-8) in freely fed adult male Sprague Dawley rats. The peptide (0, 0.5, 1, 3, 5 and 10 nmol/kg) was given intraperitoneally (ip) prior to the onset of the dark cycle, and first meal size (MS), second meal size, intermeal interval (IMI) length, satiety ratio (SR = IMI/MS), latency to first meal, duration of first meal, number of meals and 24-hour food intake were measured. We found that NS CCK-8 (0.5 and 1.0 nmol/kg) reduced MS, prolonged IMI length and increased SR during the dark cycle. Furthermore, the specific CCK-B receptor antagonist L365, 260 (1 mg/kg, ip) attenuated these responses. These results support a possible role for NS CCK-8 in regulating food intake.

Expression of the Cholecystokinin-B Receptor in Neoplastic Gastric Cells

Gastric cancer is an important disease due to its high mortality. Despite the decline in frequency, most cases are discovered late in its course, and most of the cancer patients die within a few years of diagnosis. In addition to Helicobacter pylori gastritis, gastrin is considered an important factor in the development of this disease, and thus, cholecystokinin-B receptor (CCKBR) becomes of interest. The aim of our study was to explore whether CCKBR is expressed in stomach cancers. Thirty-seven tumors from 19 men and 18 women diagnosed with either adenocarcinoma or neuroendocrine neoplasm (NENs) were included in this study. The tumors were classified into 29 adenocarcinomas and eight NENs. Immunohistochemistry with antibodies against chromogranin A (CgA), synaptophysin and CCKBR, and in situ hybridization with probes against CgA, CCKBR and histidine decarboxylase were used to further explore these tumors. Thirty-three (89%) of the tumors expressed CCKBR protein, whereas only 20 (54%) of all tumors expressed CCKBR mRNA. Of the 20 tumors expressing CCKBR mRNA, eight were NENs and 12 were adenocarcinoma. The highest amount of CCKBR was expressed in NEN. Interestingly, a high degree of co-expression of CCKBR and CgA was observed when the two markers were examined together with in situ hybridization. In conclusion, we found that all eight NENs expressed CCKBR and neuroendocrine markers in a majority of tumor cells. The same markers were also expressed in a proportion of adenocarcinomas supporting the view that gastrin is important in the development of gastric cancer.

Deletion of Men1 and somatostatin induces hypergastrinemia and gastric carcinoids

BACKGROUND

Gastric carcinoids are slow growing neuroendocrine tumours arising from enterochromaffin-like (ECL) cells in the corpus of stomach. Although most of these tumours arise in the setting of gastric atrophy and hypergastrinemia, it is not understood what genetic background predisposes development of these ECL derived tumours. Moreover, diffuse microcarcinoids in the mucosa can lead to a field effect and limit successful endoscopic removal.

OBJECTIVE

To define the genetic background that creates a permissive environment for gastric carcinoids using transgenic mouse lines.

DESIGN

The multiple endocrine neoplasia 1 gene locus (Men1) was deleted using Cre recombinase expressed from the Villin promoter (Villin-Cre) and was placed on a somatostatin null genetic background. These transgenic mice received omeprazole-laced chow for 6 months. The direct effect of gastrin and the gastrin receptor antagonist YM022 on expression and phosphorylation of the cyclin inhibitor p27^Kip1 was tested on the human human gastric adenocarcinoma cell line stably expressing CCKBR (AGSE) and mouse small intestinal neuroendocrine carcinoma (STC)-1 cell lines.

RESULTS

The combination of conditional Men1 deletion in the absence of somatostatin led to the development of gastric carcinoids within 2 years. Suppression of acid secretion by omeprazole accelerated the timeline of carcinoid development to 6 months in the absence of significant parietal cell atrophy. Carcinoids were associated with hypergastrinemia, and correlated with increased Cckbr expression and nuclear export of p27^Kip1 both in vivo and in gastrin-treated cell lines. Loss of p27^Kip1 was also observed in human gastric carcinoids arising in the setting of atrophic gastritis.

CONCLUSIONS

Gastric carcinoids require threshold levels of hypergastrinemia, which modulates p27^Kip1 cellular location and stability.

Expression of Adenosine A2B Receptor and Adenosine Deaminase in Rabbit Gastric Mucosa ECL Cells

Adenosine is readily available to the glandular epithelium of the stomach. Formed continuously in intracellular and extracellular locations, it is notably produced from ATP released in enteric cotransmission. Adenosine analogs modulate chloride secretion in gastric glands and activate acid secretion in isolated parietal cells through A_2B adenosine receptor (A2BR) binding. A functional link between surface A2BR and adenosine deaminase (ADA) was found in parietal cells, but whether this connection is a general feature of gastric mucosa cells is unknown. Here we examine whether A2BR is expressed at the membrane of histamine-producing enterochromaffin-like (ECL) cells, the major endocrine cell type in the oxyntic mucosa, and if so, whether it has a vicinity relationship with ADA. We used a highly homogeneous population of rabbit ECL cells (size 7.5–10 µm) after purification by elutriation centrifugation. The surface expression of A2BR and ADA proteins was assessed by flow cytometry and confocal microscopy. Our findings demonstrate that A2BR and ADA are partially coexpressed at the gastric ECL cell surface and that A2BR is functional, with regard to binding of adenosine analogs and adenylate cyclase activation. The physiological relevance of A2BR and ADA association in regulating histamine release is yet to be explained.

Transcriptional and Functional Characterization of the G Protein-Coupled Receptor Repertoire of Gastric Somatostatin Cells

In the stomach, somatostatin (SST) acts as a general paracrine negative regulator of exocrine secretion of gastric acid and pepsinogen and endocrine secretion of gastrin, ghrelin, and histamine. Using reporter mice expressing red fluorescent protein (RFP) under control of the SST promotor, we have characterized the G protein-coupled receptors expressed in gastric Sst-RFP-positive cells and probed their effects on SST secretion in primary cell cultures. Surprisingly, besides SST, amylin and PYY were also highly enriched in the SST cells. Several receptors found to regulate SST secretion were highly expressed and/or enriched. 1) The metabolite receptors calcium-sensing receptor and free fatty acid receptor 4 (GPR120) functioned as positive and negative regulators, respectively. 2) Among the neurotransmitter receptors, adrenergic receptors α1a, α2a, α2b, and β1 were all highly expressed, with norepinephrine and isoproterenol acting as positive regulators. The muscarinic receptor M3 acted as a positive regulator, whereas M4 was conceivably a negative regulator. 3) Of the hormone receptors, the GLP-1 and GIP receptors, CCKb (stimulated by both CCK and gastrin) and surprisingly the melanocortin MC1 receptor were all positive regulators. 4) The neuropeptide receptors for calcitonin gene-related peptide, adrenomedullin, and vasoactive intestinal peptide acted as positive regulators, no effect was observed using galanin and nociceptin although transcripts for the corresponding receptors appeared highly expressed. 5) The SST receptors 1 and 2 functioned in an autocrine negative feedback loop. Thus, the article provides a comprehensive map of receptors through which SST secretion is regulated by hormones, neurotransmitters, neuropeptides and metabolites that act directly on the SST cells in the gastric mucosa.

Upper gastrointestinal physiology and diseases

Nordic research on physiology and pathophysiology of the upper gastrointestinal tract has flourished during the last 50 years. Swedish surgeons and physiologists were in the frontline of research on the regulation of gastric acid secretion. This research finally led to the development of omeprazole, the first proton pump inhibitor. When Swedish physiologists developed methods allowing the assessment of acid secretion in isolated oxyntic glands and isolated parietal cells, the understanding of mechanisms by which gastric acid secretion is regulated took a great step forward. Similarly, in Trondheim, Norway, the acid producing isolated rat stomach model combined with a sensitive and specific method for determination of histamine made it possible to evaluate this regulation qualitatively as well as quantitatively. In Lund, Sweden, the identification of the enterochromaffin-like cell as the cell taking part in the regulation of acid secretion by producing and releasing histamine was of fundamental importance both physiologically and clinically. Jorpes and Mutt established a center at Karolinska Institutet in Stockholm for the purification of gastrointestinal hormones in the 1960s, and Danes followed up this work by excelling in the field of determination and assessment of biological role of gastrointestinal hormones. A Finnish group was for a long period in the forefront of research on gastritis, and the authors' own studies on the classification of gastric cancer and the role of gastrin in the development of gastric neoplasia are of importance. It can, accordingly, be concluded that Nordic researchers have been central in the research on area of the upper gastrointestinal physiology and diseases.

The use of murine-derived fundic organoids in studies of gastric physiology

KEY POINTS

An in vitro approach to study gastric development is primary mouse-derived epithelium cultured as three-dimensional spheroids known as organoids. We have devised two unique gastric fundic-derived organoid cultures: model 1 for the expansion of gastric fundic stem cells, and model 2 for the maintenance of mature cell lineages. Organoids maintained in co-culture with immortalized stomach mesenchymal cells express robust numbers of surface pit, mucous neck, chief, endocrine and parietal cells. Histamine induced a significant decrease in intraluminal pH that was reversed by omeprazole in fundic organoids and indicated functional activity and regulation of parietal cells. Localized photodamage resulted in rapid cell exfoliation coincident with migration of neighbouring cells to the damaged area, sustaining epithelial continuity. We report the use of these models for studies of epithelial cell biology and cell damage and repair.

ABSTRACT

Studies of gastric function and disease have been limited by the lack of extended primary cultures of the epithelium. An in vitro approach to study gastric development is primary mouse-derived antral epithelium cultured as three-dimensional spheroids known as organoids. There have been no reports on the use of organoids for gastric function. We have devised two unique gastric fundic-derived organoid cultures: model 1 for the expansion of gastric fundic stem cells, and model 2 for the maintenance of mature cell lineages. Both models were generated from single glands dissociated from whole fundic tissue and grown in basement membrane matrix (Matrigel) and organoid growth medium. Model 1 enriches for a stem cell-like niche via simple passage of the organoids. Maintained in Matrigel and growth medium, proliferating organoids expressed high levels of stem cell markers CD44 and Lgr5. Model 2 is a system of gastric organoids co-cultured with immortalized stomach mesenchymal cells (ISMCs). Organoids maintained in co-culture with ISMCs express robust numbers of surface pit, mucous neck, chief, endocrine and parietal cells. Histamine induced a significant decrease in intraluminal pH that was reversed by omeprazole in fundic organoids and indicated functional activity and regulation of parietal cells. Localized photodamage resulted in rapid cell exfoliation coincident with migration of neighbouring cells to the damaged area, sustaining epithelial continuity. Thus, we report the use of these models for studies of epithelial cell biology and cell damage and repair.

Gastric acid secretion: changes during a century

The advances in knowledge of gastric physiology within the past century have been the most exciting and important in this area of interest for many decades. The aim of this presentation consists of a comprehensive review of the extensive recent literature on this topic in order to highlight milestones in the field of gastric physiology, in particular in gastric acid secretion, gastric pathophysiology, acid-related diseases and use of acid regulatory drugs. Moreover, in the 21st century there have been many epidemiologic changes as well as a decrease of Helicobacter pylori infection and gastric cancer together with an increase of gastroesophageal reflux disease and the related increase of pomp proton inhibitor wide use.

Cell-to-cell propagation of intracellular signals fluorescently visualized with acridine orange in the gastric glands of guinea pigs

Secretion from the gastric gland involves the activation of various types of cells in a coordinated manner. In order to elucidate the mechanisms underlying the coordination of secretion, we studied live fluorescence images of guinea pig gastric glands stained with acridine orange (AO). On 2 μM AO staining, individual cells were characterized by metachromatic colors and various intensities of fluorescence. When the gland was stimulated with 100 μM of histamine, green fluorescence was transiently increased in parietal cells and intermediate cells and propagated along the gland for a long distance over many cells. Local stimulation in a couple of cells with histamine in the presence of suramin also induced propagation. However, the fluorescence response was suppressed by the addition of H-89, a protein kinase A inhibitor. These findings suggest that a cAMP-dependent signal propagates intercellularly through a variety of cells to induce coordinated secretion in the entire gastric gland.

The regulation of gastric acid secretion - clinical perspectives

The purpose of this review, based upon 40 years of research, is to clear old controversies. The gastric juice is a strong acid with active enzymes (pepsin and lipase); ideal for killing swallowed microorganisms. Totally isolated rat stomach and histamine determination. Human gastric carcinomas were examined for ECL cell differentiation because tumours found in rodents after dosing with inhibitors of acid secretion were reclassified to be of ECL cell origin. The gastrin receptor is localized to the ECL cell only, where gastrin stimulates the function and growth. Drug-induced hypo-acidity induces hypergastrinaemia and ECL cell hyperplasia responsible for rebound acid hypersecretion. Every condition with long-term hypergastrinaemia disposes to ECL cell neoplasia. In man, both atrophic gastritis and gastrinoma lead to ECL cell carcinoids. Proton pump inhibitors induce hypergastrinaemia with ECL cell hyperplasia and ECL cell carcinoids that disappear when stopping treatment. The gastrin antagonist netazepide induces regression of ECL cell carcinoids due to atrophic gastritis. Human gastric carcinomas of diffuse type, particularly the signet-ring subtype, show ECL cell differentiation, suggesting involvement of gastrin in the carcinogenesis. Helicobacter pylori (Hp) causes gastritis and peptic ulcer, and when infecting the antrum only gives a slight hypergastrinaemia with acid hypersecretion predisposing to duodenal ulcer, but protecting from gastric cancer. When Hp infection spreads to oxyntic mucosa, it induces atrophy, reduced acid secretion and marked hypergastrinaemia and cancer.It is remarkable that the interaction between Hp and gastrin may explain the pathogenesis of most diseases in the upper gastrointestinal tract.

Gastric acid, calcium absorption, and their impact on bone health

Calcium balance is essential for a multitude of physiological processes, ranging from cell signaling to maintenance of bone health. Adequate intestinal absorption of calcium is a major factor for maintaining systemic calcium homeostasis. Recent observations indicate that a reduction of gastric acidity may impair effective calcium uptake through the intestine. This article reviews the physiology of gastric acid secretion, intestinal calcium absorption, and their respective neuroendocrine regulation and explores the physiological basis of a potential link between these individual systems.

Simultaneous detection of gastric acid and histamine release to unravel the regulation of acid secretion from the guinea pig stomach

Gastric acid secretion is regulated by three primary components that activate the parietal cell: histamine, gastrin, and acetylcholine (ACh). Although much is known about these regulatory components individually, little is known on the interplay of these multiple activators and the degree of regulation they pose on the gastric acid secretion mechanism. We utilized a novel dual-sensing approach, where an iridium oxide sensor was used to monitor pH and a boron-doped diamond electrode was used for the detection of histamine from in vitro guinea pig stomach mucosal sections. Under basal conditions, gastrin was shown to be the main regulatory component of the total acid secretion and directly activated the parietal cell rather than by mediating gastric acid secretion through the release of histamine from the enterochromaffin-like cell, although both pathways were active. Under stimulated conditions with ACh, the gastrin and histamine components of the total acid secretion were not altered compared with levels observed under basal conditions, suggestive that ACh had no direct effect on the enterochromaffin-like cell and G cell. These data identify a new unique approach to investigate the regulation pathways active during acid secretion and the degree that they are utilized to drive total gastric acid secretion. The findings of this study will enhance our understanding on how these signaling mechanisms vary under pathophysiology or therapeutic management.

The role of Sonic Hedgehog as a regulator of gastric function and differentiation

The Hedgehog (Hh) genes play a key role in the regulation of embryonic development and govern processes such as cell differentiation, cell proliferation, and tissue patterning. In vertebrate embryos, Hh gene expression regulates correct formation of limbs, skeleton, muscles, and organs including stomach. In the adult, the Hh pathway functions in tissue repair and regeneration, along with maintenance of stem cells. Sonic Hedgehog (Shh) signaling has been extensively studied for its role in developmental and cancer biology. Recent advances in the field of gastroenterology show that in the stomach, Shh is responsible for proper differentiation of the gastric glands. The aberrant activity of the Shh signaling pathway leads to an altered gastric differentiation program and loss of gastric acid secretion that is the predominant function of the stomach. In this chapter, we review the most recent findings that reveal the role of Shh as a regulator of gastric function and differentiation and how this signaling is dysregulated during the development of gastric cancer in response bacterial infection.

Histamine 3 receptor activation mediates inhibition of acid secretion during Helicobacter-induced gastritis

AIM: To test the hypothesis that histamine 3 receptor (H3R) activation during Helicobacter infection inhibits gastric acid secretion in vivo and in vitro.

METHODS: Helicobacter felis (H. felis) infected and uninfected C57Bl/6 mice were infused with either PBS or the H3 receptor antagonist thioperamide (THIO) for 12 wk. After treatment, mice were analyzed for morphological changes and gastric acid content. Total RNA was prepared from the stomachs of each group and analyzed for changes in somatostatin and gastrin mRNA abundance by real time-polymerase chain reaction (RT-PCR). Location of H3 receptors in the stomach was analyzed by co-localization using antibodies specific for the H3 receptor and parietal cell marker H⁺, K⁺-ATPase β subunit.

RESULTS: Inflammation and parietal cell atrophy was observed after 12 wk of H. felis infection. Interestingly, treatment with the H3R antagonist thioperamide (THIO) prior to and during infection prevented H. felis-induced inflammation and atrophy. Compared to the uninfected controls, infected mice also had significantly decreased gastric acid. After eradication of H. felis with THIO treatment, gastric acidity was restored. Compared to the control mice, somatostatin mRNA abundance was decreased while gastrin gene expression was elevated during infection. Despite elevated gastric acid levels, after eradication of H. felis with THIO, somatostatin mRNA was elevated whereas gastrin mRNA was suppressed. Immunofluorescence revealed the presence of H3 receptors on the parietal cells, somatostatin-secreting D-cells as well as the inflammatory cells.

CONCLUSION: This study shows that during H. felis infection, gastric acidity is suppressed as a consequence of an inhibitory effect on the parietal cell by H3R activation. The stimulation of gastric mucosal H3Rs increases gastrin expression and release by inhibiting release of somatostatin.

The production and role of gastrin-17 and gastrin-17-gly in gastrointestinal cancers

The gastrointestinal peptide hormone gastrin is responsible for initiating the release of gastric acid in the stomach in response to the presence of food and/or humoral factors such as gastrin releasing peptide. However, it has a role in the growth and maintenance of the gastric epithelium, and has been implicated in the formation and growth of gastric cancers. Hypergastrinemia resulting from atrophic gastritis and pernicious anemia leads to hyperplasia and carcinoid formation in rats, and contributes to tumor formation in humans. Additionally, gastrin has been suspected to play a role in the formation and growth of cancers of the colon, but recent studies have instead implicated gastrin processing intermediates, such as gastrin-17-Gly, acting upon a putative, non-cholecystokinin receptor. This review summarizes the production and chemical structures of gastrin and of the processing intermediate gastrin-17-Gly, as well as their activities in the gastrointestinal tract, particularly the promotion of colon cancers.

Ghrelin, des-acyl ghrelin and nesfatin-1 in gastric X/A-like cells: role as regulators of food intake and body weight

Numerous peptides released from endocrine cells in the intestinal mucosa were established early on to be involved in the physiological regulation of food intake with a prominent role in termination of food ingestion when nutrients pass along the intestinal tract. Recently, peptides released from X/A-like endocrine cells of the gastric oxyntic mucosa were recognized as additional key players in the regulation of feeding and energy expenditure. Gastric X/A-like cells release the octanoylated peptide, ghrelin, the only known peripherally produced hormone stimulating food intake through interaction with growth hormone secretagogue 1a receptor (GHS-R1a). Additionally, non-octanoylated (des-acyl) ghrelin present in the circulation at higher levels than ghrelin is currently discussed as potential modulator of food intake by opposing ghrelin's action independent from GHS-R1a although the functional significance remains to be established. Obestatin, a ghrelin-associated peptide was initially reported as anorexigenic modulator of ghrelin's orexigenic action. However, subsequent reports did not support this contention. Interesting is the recent identification of nesfatin-1, a peptide derived from the nucleobindin2 gene prominently expressed in gastric X/A-like cells in different vesicles than ghrelin. Circulating nesfatin-1 levels vary with metabolic state and peripheral or central injection inhibits dark phase feeding in rodents. Overall, these data point to an important role of gastric X/A-like cells in food intake regulation through the expression of the orexigenic peptide ghrelin along with des-acyl ghrelin and nesfatin-1 capable of reducing food intake upon exogenous injection although their mechanisms of action and functional significance remain to be established.

Identification and characterization of nesfatin-1 immunoreactivity in endocrine cell types of the rat gastric oxyntic mucosa

Hypothalamic nesfatin-1, derived from the nucleobindin2 (NUCB2) precursor, inhibits nocturnal food intake and body weight gain in rats. Nesfatin-1 is able to cross the blood-brain barrier, suggesting a peripheral source of nesfatin-1. Many centrally acting food intake regulatory neuropeptides are also produced in the periphery, especially in the gastrointestinal tract. Therefore, we investigated the gene expression of NUCB2 and distribution of nesfatin-1-immunoreactive cells in the stomach. Microarray mRNA expression profiles in purified small endocrine cells of the gastric mucosa substantiated by quantitative RT-PCR showed significantly higher NUCB2 mRNA expression compared with brain and heart. Western blot confirmed the expression of NUCB2 protein and its transport into a secretory soluble fraction of gastric mucosal endocrine cell homogenates. Immunohistochemical colabeling for nesfatin-1 and ghrelin, histidine decarboxylase, or somatostatin revealed two subtypes of nesfatin-1-positive endocrine cells. Cells in the midportion of the glands coexpressed nesfatin-1 and ghrelin, whereas few cells in the glandular base coexpressed nesfatin-1 and somatostatin or histidine decarboxylase. High-resolution three-dimensional volume imaging revealed two separate populations of intracytoplasmic vesicles in these cells, one containing nesfatin-1 and the other ghrelin immunoreactivity. Microarray rat genome expression data of NUCB2 in small gastric endocrine cells confirmed by quantitative RT-PCR showed significant down-regulation of NUCB2 after 24 h fasting. In summary, NUCB2 mRNA expression as well as protein content is present in a specific subset of gastric endocrine cells, most of which coexpress ghrelin. NUCB2 gene expression is significantly regulated by nutritional status, suggesting a regulatory role of peripheral nesfatin-1 in energy homeostasis.

Control of gastric acid secretion in health and disease

Recent milestones in the understanding of gastric acid secretion and treatment of acid-peptic disorders include the (1) discovery of histamine H(2)-receptors and development of histamine H(2)-receptor antagonists, (2) identification of H(+)K(+)-ATPase as the parietal cell proton pump and development of proton pump inhibitors, and (3) identification of Helicobacter pylori as the major cause of duodenal ulcer and development of effective eradication regimens. This review emphasizes the importance and relevance of gastric acid secretion and its regulation in health and disease. We review the physiology and pathophysiology of acid secretion as well as evidence regarding its inhibition in the management of acid-related clinical conditions.

Autoregulation of McA-RH7777 hepatoma cell proliferation by histamine H3 receptors

Previous studies have suggested that histamine (HA) acts as an autocrine growth factor. We have explored the modulation of cell proliferation by HA using McA-RH7777 hepatoma cells. High L-histidine decarboxylase (HDC) expression and HA synthesis were found in McA-RH7777 cells. Whereas extracellular HA reached submicromolar concentrations, intracellular levels were very low, indicating that HA was secreted by the cells. McA-RH7777 cells also express H3-receptor (H3R) transcripts and proteins. Reverse transcriptase-polymerase chain reaction analysis detected only transcripts for the long isoform. Immunocytochemistry performed with a selective H3R antibody showed that most cells were immunoreactive. H3R binding sites (Bmax approximately 30 fmol/mg protein) were identified when [125I] iodoproxyfan binding was displaced by the agonist imetit. High-affinity binding also occurred at cytochrome P450 enzymes. This binding was not inhibited by HA, H3R agonists, or by a nonimidazole H3R antagonist but was displaced by imidazole H3R antagonists or by ketoconazole, a imidazole-containing cytochrome inhibitor. HA inhibited proliferation of McA-RH7777 hepatoma cells. The absence of uptake system, its much higher potency at H3Rs, and its low intracellular levels suggested that HA interacted with H3Rs rather than cytochromes. In agreement, both imidazole H3R antagonists, a nonimidazole H3R antagonist, and the HDC inhibitor alpha-monofluoromethyl histidine increased cell proliferation (up to approximately 60%), revealing a H3R-mediated inhibition by endogenous HA. Moreover, exogenous HA inhibited the increase induced by alpha-FMH or H3R antagonists with a nanomolar potency. In conclusion, our findings show that HA regulates proliferation of McA-RH7777 hepatoma cells by interacting with autoinhibitory H3Rs.

Regulation of mammalian gastrin/CCK receptor (CCK2R) expression in vitro and in vivo

The gastrin/CCK receptor (CCK2R) mediates the physiological functions of gastrin in the stomach, including stimulation of acid secretion and cellular proliferation and migration, but little is known about the factors that regulate its expression. We identified endogenous CCK2R expression in several cell lines and used luciferase promoter-reporter constructs to define the minimal promoter required for transcription in human gastric adenocarcinoma, AGS, and rat gastric mucosa, RGM1, cells. Consensus binding sites for SP1, C/EBP and GATA were essential for activity. Following serum withdrawal from RGM1 and AR42J cells, endogenous CCK2R mRNA abundance and the activity of a CCK2R promoter-reporter construct were significantly elevated. Transcription of CCK2R was also increased in AGS-G(R) and RGM1 cells by gastrin through mechanisms partly dependent upon protein kinase C (PKC) and mitogen/extracellular signal-regulated kinase (MEK). Gastrin significantly increased endogenous CCK2R expression in RGM1 cells, and CCK2R protein expression was elevated in the stomach of hypergastrinaemic animals. In mice with cryoulcers in the acid-secreting mucosa, CCK2R expression increased progressively in the regenerating mucosa adjacent to the ulcer repair margin, evident at 6 days postinjury and maximal at 13 days. De novo expression of CCK2R was observed in the submucosa beneath the repairing ulcer crater 6-9 days postinjury. Many of the cells in mucosa and submucosa that expressed CCK2R in response to cryoinjury were identified as myofibroblasts, since they coexpressed vimentin and smooth muscle alpha-actin but not desmin. The data suggest that increased CCK2R expression might influence the outcome of epithelial inflammation or injury and that the response may be mediated in part by myofibroblasts.

Thermodynamic analysis of ligands at cholecystokinin CCK2 receptors in rat cerebral cortex

BACKGROUND AND PURPOSE

Several studies using radioligand binding assays, have shown that measurement of thermodynamic parameters can allow discrimination of agonists and antagonists (Weiland et al., 1979; Borea et al., 1996a). Here we investigate whether agonists and antagonists can be thermodynamically discriminated at CCK(2) receptors in rat cerebral cortex.

EXPERIMENTAL APPROACH

The pK(L) of [(3)H]-JB93182 in rat cerebral cortex membranes was determined at 4, 12, 21 and 37 degrees C in 50 mM Tris-HCl buffer (buffer B pH 6.96; containing 0.089 mM bacitracin). pK(I) values of ligands of diverse chemical structure and with differing intrinsic activity (alpha), as defined by the lumen-perfused rat and mouse stomach bioassays, were determined in buffer B at 4, 12, 21 and 37 degrees C.

KEY RESULTS

[(3)H]-JB93182 labelled a homogeneous population of receptors in rat cerebral cortex at 4, 12, 21 and 37 degrees C and the pK(L) and B(max) were not altered by incubation temperature. [(3)H]-JB93182 binding reached equilibrium after 10, 50, 90 and 220 min at 37, 21, 12 and 4 degrees C, respectively. pK(I) values for R-L-365,260, R-L-740,093, YM220, PD134,308 and JB95008 were higher at 4 degrees C than at 37 degrees C. There was no effect of temperature on pK(I) values for pentagastrin, CCK-8S, S-L-365,260, YM022, PD140,376 and JB93242.

CONCLUSIONS AND IMPLICATIONS

CCK(2) receptor agonists and antagonists at rat CCK(2) receptors cannot be discriminated by thermodynamic analysis using [(3)H]-JB93182 as the radioligand.

A comparison of the effects of gastrin, somatostatin and dopamine receptor ligands on rat gastric enterochromaffin-like cell secretion and proliferation

Gastrin regulates ECL cell histamine release and is a critical determinant of acid secretion. ECL cell secretion and proliferation is inhibited by gastrin antagonists and somatostatin but little is known about the role of dopamine agonists in this process. Since the ECL cell exhibits all three classes of receptor we evaluated and compared the effects of the gastrin receptor antagonist, (YF476), lanreotide (SST agonist) and novel dopaminergic agents (BIM53061 and BIM27A760) on ECL cell histamine secretion and proliferation. Highly enriched (>98%) ECL cell preparations prepared from rat gastric mucosa using a FACS approach were studied. Real-time PCR confirmed presence of the CCK2, SS2 and SS5 and D1 receptors on ECL cells. YF476 inhibited histamine secretion and proliferation with IC(50)s of 1.25 nM and 1.3 x 10(-11) M respectively, values 10-1000x more potent than L365,260. Lanreotide inhibited secretion and proliferation (2.2 nM, 1.9 x 10(-10) M) and increased YF476-inhibited proliferation a further 5-fold. The dopamine agonist, BIM53061, inhibited gastrin-mediated ECL cell secretion and proliferation (17 nM, 6 x 10(-10) M) as did the novel dopamine/somatostatin chimera BIM23A760 (22 nM, 4.9 x 10(-10) M). Our studies demonstrate that the gastrin receptor antagonist, YF476, is the most potent inhibitor of ECL cell histamine secretion and proliferation. Lanreotide, a dopamine agonist and a dopamine/somatostatin chimera inhibited ECL cell function but were 10-1000x less potent than YF476. Agents that selectively target the CCK2 receptor may provide alternative therapeutic strategies for gastrin-mediated gastrointestinal cell secretion and proliferation such as evident in the hypergastrinemic gastric carcinoids associated with low acid states.

Novel approaches to the pharmacological blockade of gastric acid secretion

Research into new methods of controlling acid secretion is driven by existing medical needs in gastro-oesophageal reflux disease treatment. Histamine receptor subtype 3 agonists offer one approach for acid inhibition but no agent is yet undergoing clinical testing. Other, as yet unrealized strategies include preventing the fusion of the tubulovesicular elements that contain H+/K+-ATPase with the parietal cell membrane, or blocking channels that recycle K+ in the parietal cell. Of more promise are gastrin (cholecystokinin) receptor antagonists and potassium-competitive acid blockers; examples of both are in clinical development. It is probable that gastrin receptor antagonists would be used adjunctively with proton pump inhibitors, possibly for meal-induced reflux. The potassium-competitive acid blockers have attributes that may facilitate use as monotherapy for the treatment of gastro-oesophageal reflux disease. The early promise of gastrin receptor antagonists and potassium-competitive acid blockers remains to be defined in large-scale trials.

Isolation, functional characterization, and transcriptome of Mastomys ileal enterochromaffin cells

Although the enterochromaffin (EC) cell is one of the primary neuroendocrine regulatory cells of the small intestine, the lack of a purified cell system has precluded characterization of the cell and limited precise physiological evaluation. We developed methodology to obtain a pure population of Mastomys ileal EC cells, evaluated their functional regulation, and defined the transcriptome. Mastomys ilea were everted, end ligated, pronase-collagenase digested, and Nycodenz gradient centrifuged, and EC cells were collected by fluorescence-activated cell sorting (FACS) of acridine orange-labeled cells. Enrichment was confirmed by immunostaining of tryptophan hydroxylase and chromogranin A, specific EC cell markers, serotonin content, EC cell marker gene expression, and electron microscopy. Pituitary adenylate cyclase-activating polypeptide (PACAP), somatostatin, and gastrin receptor expression was determined by real-time RT-PCR. Live post-FACS-sorted cells were cultured, and the effects of forskolin, isoproterenol, acetylcholine, GABAA, PACAP-38, and gastrin on serotonin secretion were measured by ELISA. GeneChip Affymetrix profiling of FACS-sorted cells was undertaken to obtain the EC cell transcriptome. FACS produced a >70-fold enrichment of EC cells with a serotonin content of 240 +/- 22 ng/mg protein. Preparations were 99 +/- 0.7% pure by immunostaining for tryptophan hydroxylase. Vasoactive intestinal peptide/PACAP receptor 1 (VPAC1) and somatostatin receptor 2 were present, whereas PACAP receptor 1 (PAC1) and CCK2 receptors were undetectable. Forskolin, isoproterenol, and PACAP-38 stimulated serotonin secretion at EC50 values of 5 x 10(-10), 4.5 x 10(-10), and 1.2 x 10(-9) M, respectively. Isoproterenol stimulated cAMP levels by approximately 3.5 +/- 0.62-fold vs. unstimulated cells (EC50 of approximately 10(-9) M). Octreotide, acetylcholine, and GABAA inhibited serotonin secretion with IC50 values of 3 x 10(-11), 3 x 10(-10), and 2.9 x 10(-10) M, respectively. Gastrin had no effect on serotonin secretion. The naive EC cell transcriptome revealed highly expressed EC cell marker genes, the absence of marker genes for other small intestinal cell types, and a receptor profile that included cholinergic, adrenergic, dopaminergic, serotoninergic, GABAergic, and prostaglandin receptors. We were able to isolate homogeneous preparations (>99%) of live ileal EC cells and demonstrated regulation of serotonin secretion as well as established the normal EC cell transcriptome. Application of this methodology to normal and diseased human ileum will facilitate the elucidation of the pathophysiology of EC cells.

Is gastrin partially responsible for body weight reduction after gastric bypass?

BACKGROUND

The rationale for bariatric surgery is to reduce food intake by gastric restriction and/or malabsorption by intestinal bypass. Unlike ghrelin, gastrin is released in response to food intake. Here we studied the possible role of gastrin in the reduction of body weight after gastric bypass surgery.

METHODS

Rats were divided into four experimental groups and were subjected to different treatments: sham operation, gastric bypass, sham operation + gastrin infusion, and gastric bypass + gastrin infusion. The gastric bypass was done by anastomosing the esophagus to the duodenal bulb without bypassing the intestine. Gastrin-17 was infused continuously for 2 months via subcutaneously implanted osmotic minipumps. Body weights were recorded; serum gastrin and ghrelin levels were measured, and the stomachs were analyzed morphologically.

RESULTS

Gastric bypass resulted in reducing the body weight, stomach weight, thickness of the oxyntic mucosa, serum gastrin concentration, and activity of the ECL cells. Gastrin infusion prevented mucosal atrophy and ECL cell inactivation, and attenuated the body weight reduction that occurred following gastric bypass. Circulating ghrelin and ghrelin-producing A-like cells in stomachs that had undergone gastric bypass were unchanged with or without gastrin infusion and are thus unlikely to be responsible for the reduced body weight.

CONCLUSION

We suggest that hypogastrinemia and impaired ECL cell function in the oxyntic mucosa of the stomach might be partially responsible for the reduction in body weight that occurs after gastric bypass.

Direct effects of nitric oxide on histamine release from rat enterochromaffin-like cells

The direct effects of nitric oxide (NO) on enterochromaffin-like (ECL) cells have not yet been demonstrated. In this study we investigated the direct effects of NO donors on rat ECL cells. The NO donor, NOR3 (10 and 100 microM), decreased gastrin-induced histamine release. 100 microM NOR3 increased cGMP levels and reduced gastrin-induced calcium influx. ODQ, an inhibitor of guanylate cyclase, completely blocked NOR3-induced inhibition of histamine release. These results suggest that NO inhibits gastric acid secretion via suppression of gastrin-induced histamine release through a pathway in which NO activates guanylate cyclase, in addition to increasing cGMP levels and reducing gastrin-induced calcium influx. The use of NO as a new type of gastric acid inhibitor that decreases histamine levels in the stomach would be beneficial as increased histamine levels resulting from use of a histamine H2 receptor antagonist or proton pump inhibitor have various effects on tumors and immunological functions.

Transcriptomes of purified gastric ECL and parietal cells: identification of a novel pathway regulating acid secretion

The gastric entero-chromaffin-like (ECL) cell plays a key regulatory role in peripheral regulation of acid secretion due to the release of histamine that stimulates acid secretion by the parietal cell. Studies in intact animals, gastric glands, and isolated cells after short-term culture have shown expression of stimulatory CCK2 and PAC1 and inhibitory SST2 and Gal1 receptors as well as histidine decarboxylase. However, the pattern of its gene expression as a neuroendocrine cell has not been explored. Comparison of gene expression by 95% pure ECL cells obtained by density gradient, elutriation, and fluorescence-assisted cell sorting with isolates of the intact fundic gastric epithelium (i.e., "subtractive hybridization") identified a variety of additional expressed gene families characteristic of this neuroendocrine cell. These include genes 1) involved in neuropeptide synthesis and secretory vesicle exocytosis, 2) involved in control of inflammation, 3) implicated in healing of the epithelium, 4) encoding inhibitory Gi protein-coupled receptors, 5) playing a role in neuroendocrine regulation of food intake, and 6) encoding proteins likely involved in maintenance of circadian rhythm, in addition to the ECL cell-specific genes histidine decarboxylase and monoamine transporter. Particularly, the inhibitory apelin receptor gene, APJ, was highly expressed in the ECL cell preparation. Because parietal cells express apelin, immunohistochemical and functional studies showed that there is an inhibitory feed back loop between the parietal and ECL cell during gastrin stimulation, providing evidence for a novel pathway of downregulation of acid secretion due to interaction between these two cell types.

ECL cell histamine mobilization and parietal cell stimulation in the rat stomach studied by microdialysis and electron microscopy

AIM

Gastrin stimulates acid secretion by mobilizing histamine from enterochromaffin-like (ECL) cells that occur predominantly at the base of the gastric glands. The parietal cells occur higher up in the glands nearer to the gastric lumen. The present study was performed to assess whether histamine is transported from the ECL cell via the microcirculation (endocrine route) or local diffusion (paracrine route).

METHODS

Totally isolated, vascularly perfused, rat stomachs were examined both in basal and gastrin-stimulated state. Histamine concentrations, determined by radioimmunoassay in venous effluent and microdialysate from an indwelling probe in the submucosa, were monitored over a period of 240 min. Gastrin-17 was infused through an arterial catheter for 120 min. The parietal cells were examined by electron microscopy, and the percentage of actively secreting parietal cells (displaying secretory canaliculi) in four regions along the glands (basal to surface, zones I-IV) was determined.

RESULTS

Gastrin stimulated acid secretion and histamine release as well as parietal cell activation. Upon gastrin stimulation, histamine concentration in the microdialysate was 2.5-fold higher than in the venous effluent (P = 0.008). The parietal cells in the upper part of the gland (zone III) were found to be activated the most.

CONCLUSION

As the histamine concentrations were higher in the tissue (microdialysate) than in blood, histamine seems to reach the parietal cells via the paracrine route. The fraction of active parietal cells seems to depend more on the age of the parietal cells than on the distance from the ECL cell.

Somatostatin, misoprostol and galanin inhibit gastrin- and PACAP-stimulated secretion of histamine and pancreastatin from ECL cells by blocking specific Ca2+ channels

The oxyntic mucosa is rich in ECL cells. They secrete histamine and chromogranin A-derived peptides, such as pancreastatin, in response to gastrin and pituitary adenylate cyclase-activating peptide (PACAP). Secretion is initiated by Ca2+ entry. While gastrin stimulates secretion by opening L-type and N-type Ca2+ channels, PACAP stimulates secretion by activating L-type and receptor-operated Ca2+ channels. Somatostatin, galanin and prostaglandin E2 (PGE2) inhibit gastrin- and PACAP-stimulated secretion from the ECL cells. In the present study, somatostatin and the PGE2 congener misoprostol inhibited gastrin- and PACAP-stimulated secretion 100%, while galanin inhibited at most 60-65%. Bay K 8644, a specific activator of L-type Ca2+ channels, stimulated ECL-cell secretion, an effect that was inhibited equally effectively by somatostatin, misoprostol and galanin (75-80% inhibition). Pretreatment with pertussis toxin, that inactivates inhibitory G-proteins, prevented all three agents from inhibiting stimulated secretion (regardless of the stimulus). Pretreatment with nifedipine (10 microM), an L-type Ca2+ channel blocker, reduced PACAP-evoked pancreastatin secretion by 50-60%, gastrin-evoked secretion by approximately 80% and abolished the response to Bay K 8644. The nifedipine-resistant response to PACAP was abolished by somatostatin and misoprostol but not by galanin. Gastrin and PACAP raised the intracellular Ca2+ concentration in a biphasic manner, believed to reflect mobilization of internal Ca2+ followed by Ca2+ entry. Somatostatin and misoprostol blocked Ca2+ entry (and histamine and pancreastatin secretion) but not mobilization of internal Ca2+. The present observations on isolated ECL cells suggest that Ca2+ entry rather than mobilization of internal Ca2+ triggers exocytosis, that gastrin and PACAP activate different (but over-lapping) Ca2+ channels, that somatostatin, misoprostol and galanin interact with inhibitory G-proteins to block Ca2+ entry via L-type Ca2+ channels, and that somatostatin and misoprostol (but not galanin) in addition block N-type and/or receptor-operated Ca2+ channels.

Genetic dissection of the signaling pathways that control gastric acid secretion

Gastric acid secretion is regulated by endocrine, paracrine and neurocrine signals via at least three pathways, the gastrin-histamine pathway, the CCK-somatostatin pathway and the neural pathway. Genetically-engineered mice, subjected to targeted gene disruption (i.e., knockout mice), have been used to dissect the signaling pathways that are responsible for the complexity of the regulation of acid secretion in vivo. Both gastrin knockout and gastrin/CCK2 receptor knockout mice displayed greatly impaired acid secretion, presumably because of the loss of the gastrin-histamine pathway. Gastrin/CCK double-knockout mice had a relatively high percentage of active parietal cells with a maintained ability to respond with copious acid secretion to pylorus ligation-evoked vagal stimulation and to a histamine challenge. The low acid secretion in gastrin knockout mice and gastrin/CCK2 receptor knockout mice and the restoration of acid secretion in gastrin/CCK double-knockout mice suggest that CCK plays an important role as inhibitor of the parietal cells via the CCK-somatostatin pathway by stimulating the CCK1 receptor of the D cell. In the absence of both the gastrin-histamine and the CCK-somatostatin pathway (as in gastrin/CCK2 receptor double-knockout mice), the control of acid secretion is probably taken over by neural pathways, explaining the high acid output. The observations illustrate the complexity and plasticity of the acid regulatory mechanisms. It seems that one pathway may be suppressed or allowed to dominate over the others depending on the circumstances.

Potassium-competitive acid blockade: a new therapeutic strategy in acid-related diseases

Current therapies to treat gastroesophageal reflux disease (GERD), peptic ulcer disease (PUD), and other acid-related diseases either prevent stimulation of the parietal cell (H2 receptor antagonists, H2RAs) or inhibit gastric H+,K+-ATPase (e.g., proton pump inhibitors, PPIs). Of the 2 approaches, the inhibition of the final step in acid production by PPIs provides more effective relief of symptoms and healing. Despite the documented efficacy of the PPIs, therapeutic doses have a gradual onset of effect and do not provide complete symptom relief in all patients. There is scope for further improvements in acid suppressive therapy to maximize healing and offer more complete symptom relief. It is unlikely that cholecystokinin2 (CCK2, gastrin) receptor antagonists, a class in clinical trials, will be superior to H2RAs or PPIs. However, a new class of acid suppressant, the potassium-competitive acid blockers (P-CABs), is undergoing clinical trials in GERD and other acid-related diseases. These drugs block gastric H+,K+-ATPase by reversible and K+-competitive ionic binding. After oral doses, P-CABs rapidly achieve high plasma concentrations and have linear, dose-dependent pharmacokinetics. The pharmacodynamic properties reflect the pharmacokinetics of this group (i.e., the effect on acid secretion is correlated with plasma concentrations). These agents dose dependently inhibit gastric acid secretion with a fast onset of action and have similar effects after single and repeated doses (i.e., full effect from the first dose). Animal studies comparing P-CABs with PPIs suggest some important pharmacodynamic differences (e.g., faster and better control of 24-hr intragastric acidity). Studies in humans comparing PPIs with P-CABs will help to define the place of this new class in the management of acid-related diseases.

The vagus regulates histamine mobilization from rat stomach ECL cells by controlling their sensitivity to gastrin

The ECL cells in the oxyntic mucosa secrete histamine in response to gastrin, stimulating parietal cells to produce acid. Do they also operate under nervous control? The present study examines histamine mobilization from rat stomach ECL cells in situ in response to acute vagal excitation and to food or gastrin following vagal or sympathetic denervation. Applying the technique of microdialysis, we monitored the release of histamine by radioimmunoassay. Microdialysis probes were placed in the submucosa on either side of the stomach, 3 days before experiments. The rats were awake during microdialysis except when subjected to electrical vagal stimulation. One-sided electrical vagal stimulation raised serum gastrin and mobilized gastric histamine. However, gastrin receptor blockade prevented the histamine mobilization, indicating that circulating gastrin accounts for the response. Vagal excitation by hypoglycaemia (insulin) or pylorus ligation did not mobilize either gastrin or histamine. The histamine response to food was almost abolished by gastrin receptor blockade, and it was halved on the denervated side after unilateral subdiaphragmatic vagotomy. While the histamine response to a near-maximally effective dose of gastrin was unaffected by vagotomy, the response to low gastrin doses was reduced significantly. Abdominal ganglionic sympathectomy failed to affect the histamine response to either food or gastrin. In conclusion, gastrin is responsible for most of the food-evoked mobilization of ECL-cell histamine. The histamine response to electrical vagal stimulation reflects the effect of circulating gastrin rather than a direct action of the vagus on the ECL cells. Vagal denervation was accompanied by an impaired histamine response to food intake, probably reflecting the right-ward shift of the serum gastrin concentration-histamine response curve. The results suggest that the vagus controls the sensitivity of the ECL cells to gastrin.

Modulating the cytokine response to treat Helicobacter gastritis

The conventional view of gastric acid secretion is that a negative feedback mechanism arises in response to high acidity, such that somatostatin keeps G-cells and parietal cells from producing more gastrin and acid, respectively. When the stomach becomes infected, for example with Helicobacter pylori (H. pylori), the feedback mechanism is impaired. In animal models, our laboratory has demonstrated that other types of bacteria besides H. pylori can cause gastritis. For example, under conditions of low acidity, gastritis is secondary to bacterial overgrowth, not production of excessive acid, thus suggesting a new paradigm for the regulation of gastric acid secretion under inflammatory conditions. Cytokines, released during the gastric inflammatory response, including IFN gamma, TNF alpha and IL-1 beta stimulate the G-cell to produce gastrin. Gastrin in turn triggers the release of acid, and hypergastrinemia suppresses somatostatin, the inhibitor of acid. The overall response results in maximal gastric acid output that acts as the stomach's most important anti-microbial agent. The increased acid secretion by the stomach in the presence of H. pylori seems to be part of the innate immune response, in that gastrin and somatostatin are reciprocally regulated by Th1 or Th2 cytokines, respectively. In a mouse model, we showed that octreotide, a somatostatin, analog, is an efficacious treatment for Helicobacter gastritis. In humans, octreotide might accelerate recovery from H. pylori infection, reducing the duration of antibiotic therapy.

Distribution, cloning and sequencing of GnRH, its receptor, and effects of gastric acid secretion of GnRH analogue in gastric parietal cells of rats

Our objective was to study the distribution of gonadotropin-releasing hormone (GnRH) and its receptor, cloning and sequencing of GnRH and its receptor gene in cultured gastric parietal cells of rats. The distribution of GnRH and its receptor mRNA were investigated through immunocytochemical ABC methods and in situ hybridization methods in cultured gastric parietal cells of rats. After isolation of the total RNA from the parietal cells, RT-PCR was conducted to obtain GnRH and its receptor cDNA. Then, the products of PCR was purified, digested by the restriction enzyme of Hind III and EcoR I, and DNA fragments of interests were cloned into pUC19 vector. The products of PCR were analyzed by sequencing with Sanger's method after identified by PCR and digestion of restriction enzyme. Gastric parietal cells showed GnRH and its receptor immunoreactivity; positive material was located in cytoplasm other than in nuclei. GnRH and its receptor mRNA hybridized signals were also detected in cytoplasm with negative nuclei. The specific amplified band of GnRH and its receptor sequences were detected through Agarose gel electrophoresis, and GnRH gene sequence is identical to that of GnRH which has been reported in rat hypothalamus and GnRH receptor sequence is identical to that of the pituitary of rat. GnRH analogue (Alarelin) could inhibit the gastric acid secretion both by direct actions on parietal cells and by inhibiting vagous function. Our data suggest that GnRH could be produced by gastric parietal cells of rats and may modulate physiological function of gastric parietal cells of rats through autocrinal and paracrinal way.

Alterations in gastric mucosal lineages induced by acute oxyntic atrophy in wild-type and gastrin-deficient mice

In addition to their role in gastric acid secretion, parietal cells secrete a number of growth factors that may influence the differentiation of other gastric lineages. Indeed, oxyntic atrophy is considered the most significant correlate with increased risk for gastric adenocarcinoma. We studied the alterations in gastric mucosal lineages elicited by acute oxyntic atrophy induced by treatment of C57BL/6 and gastrin-deficient mice with the parietal cell protonophore [S-(R*,S*)]-N-[1-(1,3-benzodioxol-5-yl)butyl]-3,3-diethyl-2-[4-[(4-methyl-1-piperazinyl)carbonyl]phenoxy]-4-oxo-1-azetidinecarboxamide (DMP-777). In both wild-type and gastrin knockout mice, DMP-777 elicited the rapid loss of parietal cells within 2 days of treatment. In wild-type mice, oxyntic atrophy was accompanied by a rapid increase in 5-bromo-2'-deoxyuridine-labeled proliferative cells and attendant increase in surface cell numbers. However, gastrin knockout mice did not demonstrate significant foveolar hyperplasia and showed a blunted proliferative response. After 7 days of treatment in wild-type mice, a second proliferative population emerged at the base of fundic glands along with the development of a mucous cell metaplasia expressing TFF2/spasmolytic polypeptide (SPEM). However, in gastrin knockout mice, SPEM expressing both TFF2 mRNA and protein developed after only 1 day of DMP-777 treatment. In wild-type mice, all changes induced by DMP-777 were reversed 14 days after cessation of treatment. In gastrin-deficient mice, significant SPEM was still present 14 days after the cessation of treatment. The results indicate that foveolar hyperplasia requires the influence of gastrin, whereas SPEM develops in response to oxyntic atrophy independent of gastrin, likely through transdifferentiation of chief cells.

Expression of the endocytic proteins dynamin and amphiphysin in rat gastric enterochromaffin-like cells

Dynamin and amphiphysin play crucial roles in a variety of endocytic processes. Previous investigations of expression and functions of these proteins were performed mostly on neurons. The aim of this study was to investigate the presence and interaction of dyn and amph in gastric enterochromaffin-like cells. These endocrine cells of the gastric mucosa play a pivotal role in the regulation of acid secretion. Exocytosis of histamine-containing secretory vesicles has been described in detail. However, the mechanisms of endocytosis are unknown in this neuroendocrine cell type. Using RT-PCR and western blotting, we detected dynamin-1, -2 and -3 in highly enriched isolated enterochromaffin-like cells. Dynamin-1 and -2 were expressed at similar high levels, whereas dynamin-3 was of low abundance. Immunofluorescence microscopy located dynamin-1 and -2 to the cytoplasm and cell surface, whereas dynamin-3 was distributed differently in the perinuclear area. The presence of amphiphysin-1 and -2 RNAs was revealed by RT-PCR and a new splice variant of amphiphysin-2 was detected. Amphiphysin-1 and -2 were also detected in enterochromaffin-like cells by immunohistochemistry in the same locations as dynamin-1 and -2. Amphiphysin-1 and dynamin-1 co-immunoprecipitated with amphiphysin-2. In addition, dynamin-1 and amphiphysin-2 partially colocalized at the plasma membrane. Our results confirm the interaction of dynamin and amphiphysin and imply a role in endocytosis in enterochromaffin-like cells. To our knowledge, this is the first demonstration of the co-expression of all three dynamin isoforms in a non-tumor cell.

Isolation and receptor profiling of ileal enterochromaffin cells

AIM AND METHODS

Enterochromaffin (EC) cells, interspersed throughout the gastrointestinal mucosa, provide most of the serotonin of the body and control intestinal motility, secretion and absorption. We purified EC cells from the rat ileum by a combination of elutriation and density gradient centrifugation in order to characterize the function of this important cell type.

RESULTS

Immunostaining showed that there were 84% serotonin-positive cells in the highly enriched EC fraction as compared with 12% in unfractionated cells, yielding a approximately sevenfold enrichment. Serotonin measurements in the cell suspensions indicated a seven to 14-fold enrichment. Presence of alpha- and beta-adrenoreceptor isoforms, muscarinic M3 and gama-aminobutyric acid (GABA)-A receptors was confirmed by RT-PCR and cytochemistry. Increased expression of VMAT-1 and GABA-A mRNA was also shown by quantitative TaqMan PCR using EC cell RNA. Serotonin release in isolated EC cells was stimulated by noradrenaline, and to a smaller extent, by carbachol, while GABA addition was without effect.

CONCLUSION

Our data provide a basis for a new approach to characterize receptors on this unique cell type.

Rebound hypersecretion after inhibition of gastric acid secretion

Drugs inhibiting gastric acid secretion are widely used because of the high prevalence of acid-related disorders. However, from clinical experience it seems that symptom relapse is common after withdrawal of these drugs. Experimental as well as clinical studies have demonstrated an increased acid secretion after a period of treatment with either histamine 2 receptor antagonists or proton pump inhibitors. Rebound hypersecretion is likely to reflect the following sequence of events: Long-term inhibition of acid output is accompanied by elevated serum gastrin levels, leading to enterochromaffin-like cell activation and proliferation, resulting in increased amounts of histamine being mobilized from these cells to stimulate the parietal cells. The clinical consequences of rebound hypersecretion have not been settled.

Regression of type II gastric carcinoids in multiple endocrine neoplasia type 1 patients with Zollinger-Ellison syndrome after surgical excision of all gastrinomas

Enterochromaffin-like (ECL) tumors are documented in patients with hypergastrinemia secondary to chronic atrophic gastritis or with Zollinger-Ellison syndrome and multiple endocrine neoplasia type 1 (ZES-MEN-1). In patients with ECL tumors and atrophic gastritis, normogastrinemia after antrectomy has resulted in resolution, regression, or stabilization of ECL tumors. The natural history of ECL tumors associated with ZES-MEN-1 following normalization of gastrin levels after gastrinoma resection has not been previously reported. The purpose of this study was to determine the course of ECL tumors in patients with ZES-MEN-1 following normalization of serum gastrin levels after gastrinoma resection. Two patients with ZES-MEN-1 had biopsy-proven ECL tumors on endoscopic evaluation. They then underwent surgical exploration that included distal pancreatectomy, enucleation of pancreatic head tumors, duodenotomy with excision of submucosal tumors, and peripancreatic lymphadenectomy. Gastric ECL tumors larger than 1.0 cm were locally excised. Patients underwent long-term follow-up with biochemical and endoscopic surveillance. Normogastrinemia was achieved and sustained following gastrinoma resection in two patients with ZES-MEN-1. Periodic endoscopic surveillance over a 6-year period showed complete resolution of the ECL tumors. The development of ECL tumors associated with ZES-MEN-1 is multifactorial. Studies identified a genetic influence on tumor growth with loss of heterozygosity at the MEN-1 gene locus in ECL tumors. The resolution of ECL tumors in ZES-MEN-1 patients who are normogastrinemic indicates that an elevated gastrin level is a primary initiator for development of these tumors. Therefore both genetic defects and hypergastrinemia are causative agents. Normalization of serum gastrin levels is critical for the prevention of aggressive forms of ECL tumors.

The cholecystokinin2-receptor mediates calcitonin secretion, gene expression, and proliferation in the human medullary thyroid carcinoma cell line, TT

Gastrin-induced release of calcitonin from medullary thyroid carcinomas (MTC) is based on the expression of the cholecystokinin(2)-receptor (CCK(2)R) in these tumors. Recently, we have shown that the CCK(2)R is expressed not only in MTC but also in C-cells within the normal thyroid gland. The functions of the CCK(2)R in MTC and C-cells are largely unknown. We therefore explored the effects of gastrin-induced CCK(2)R stimulation in the highly differentiated MTC cell line, TT. CCK(2)R expression in TT-cells is detectable by RT-PCR as well as immunocytochemistry. Stimulation of the CCK(2)R by gastrin induces immediate release of calcitonin from TT-cells. Moreover, quantitative (LightCycler) RT-PCR demonstrates that gastrin stimulates transcription of the calcitonin and chromogranin A genes in TT-cells. TT-cell proliferation, assessed by counting of viable cells and (3)H-thymidine uptake, is markedly increased by gastrin. This effect is inhibited by the CCK(2)R-specific antagonist L-365,260. Our findings suggest physiological functions for the CCK(2)R in calcitonin-secretion and gene expression as well as a pathophysiological role in MTC proliferation. CCK(2)R antagonists might have therapeutic potential in these tumors.