Differentiation of gastric ECL cells is altered in CCK(2) receptor-deficient mice

The cckOMA syndrome and its relation to the Zollinger-Ellison syndrome: a diagnostic challenge

OBJECTIVE

Among patients with enteropancreatic neuroendocrine tumor syndromes only one case with a cholecystokinin (CCK) secreting tumor has been reported. She had significant hyperCCKemia leading to a specific syndrome of severe diarrheas, weight loss, repeated duodenal ulcers and a permanently contracted gallbladder with gallstones. There are, however, reasons to believe that further CCKomas exist, for instance among Zollinger-Ellison patients with normal plasma gastrin concentrations. The present review is a call to gastroenterologists for awareness of such CCKoma patients.

METHOD

After a short case report, the normal endocrine and oncological biology of CCK is described. Subsequently, the CCKoma symptoms are discussed with particular reference to the partly overlapping symptoms of the Zollinger-Ellison syndrome. In this context, the diagnostic use of truly specific CCK and gastrin assays are emphasized. The discussion also entails the problem of access to accurate CCK measurements.

CONCLUSION

Obviously, the clinical awareness about the CCKoma syndrome is limited. Moreover, it is also likely that the knowledge about the necessary specificity demands of diagnostic gastrin and CCK assays have obscured proper diagnosis of the CCKoma syndromes in man.

History of key regulatory peptide systems and perspectives for future research

Throughout the 20th Century, regulatory peptide discovery advanced from the identification of gut hormones to the extraction and characterization of hypothalamic hypophysiotropic factors, and to the isolation and cloning of multiple brain neuropeptides. These discoveries were followed by the discovery of G-protein-coupled and other membrane receptors for these peptides. Subsequently, the systems physiology associated with some of these multiple regulatory peptides and receptors has been comprehensively elucidated and has led to improved therapeutics and diagnostics and their approval by the US Food and Drug Administration. In light of this wealth of information and further potential, it is truly a time of renaissance for regulatory peptides. In this perspective, we review what we have learned from the pioneers in exemplified fields of gut peptides, such as cholecystokinin, enterochromaffin-like-cell peptides, and glucagon, from the trailblazing studies on the key stress hormone, corticotropin-releasing factor, as well as from more recently characterized relaxin-family peptides and receptors. The historical viewpoints are based on our understanding of these topics in light of the earliest phases of research and on subsequent studies and the evolution of knowledge, aiming to sharpen our vision of the current state-of-the-art and those studies that should be prioritized in the future.

Neuroendocrine mechanism of gastric acid secretion: Historical perspectives and recent developments in physiology and pharmacology

The physiology of gastric acid secretion is one of the earliest subjects in medical literature and has been continuously studied since 1833. Starting with the notion that neural stimulation alone drives acid secretion, progress in understanding the physiology and pathophysiology of this process has led to the development of therapeutic strategies for patients with acid-related diseases. For instance, understanding the physiology of parietal cells led to the developments of histamine 2 receptor blockers, proton pump inhibitors (PPIs), and recently, potassium-competitive acid blockers. Furthermore, understanding the physiology and pathophysiology of gastrin has led to the development of gastrin/CCK2 receptor (CCK2 R) antagonists. The need for refinement of existing drugs in patients have led to second and third generation drugs with better efficacy at blocking acid secretion. Further understanding of the mechanism of acid secretion by gene targeting in mice has enabled us to dissect the unique role for each regulator to leverage and justify the development of new targeted therapeutics for acid-related disorders. Further research on the mechanism of stimulation of gastric acid secretion and the physiological significances of gastric acidity in gut microbiome is needed in the future.

Hypergastrinemia Expands Gastric ECL Cells Through CCK2R+ Progenitor Cells via ERK Activation

BACKGROUND & AIMS

Enterochromaffin-like (ECL) cells in the stomach express gastrin/cholecystokinin 2 receptor CCK2R and are known to expand under hypergastrinemia, but whether this results from expansion of existing ECL cells or increased production from progenitors has not been clarified.

METHODS

We used mice with green fluorescent protein fluorescent reporter expression in ECL cells (histidine decarboxylase [Hdc]-green fluorescent protein), as well as Cck2r- and Hdc-driven Tamoxifen inducible recombinase Cre (Cck2r-CreERT2, Hdc-CreERT2) mice combined with Rosa26Sor-tdTomato (R26-tdTomato) mice, and studied their expression and cell fate in the gastric corpus by using models of hypergastrinemia (gastrin infusion, omeprazole treatment).

RESULTS

Hdc-GFP marked the majority of ECL cells, located in the lower third of the gastric glands. Hypergastrinemia led to expansion of ECL cells that was not restricted to the gland base, and promoted cellular proliferation (Ki67) in the gastric isthmus but not in basal ECL cells. Cck2r-CreERT2 mice marked most ECL cells, as well as scattered cell types located higher up in the glands, whose number was increased during hypergastrinemia. Cck2r-CreERT2⁺ isthmus progenitors, but not Hdc⁺ mature ECL cells, were the source of ECL cell hyperplasia during hypergastrinemia and could grow as 3-dimensional spheroids in vitro. Moreover, gastrin treatment in vitro promoted sphere formation from sorted Cck2r⁺Hdc^- cells, and increased chromogranin A and phosphorylated- extracellular signal-regulated kinase expression in CCK2R-derived organoids. Gastrin activates extracellular signal-regulated kinase pathways in vivo and in vitro, and treatment with the Mitogen-activated protein kinase kinase 1 inhibitor U0126 blocked hypergastrinemia-mediated changes, including CCK2R-derived ECL cell hyperplasia in vivo as well as sphere formation and chromogranin A expression in vitro.

CONCLUSIONS

We show here that hypergastrinemia induces ECL cell hyperplasia that is derived primarily from CCK2R⁺ progenitors in the corpus. Gastrin-dependent function of CCK2R⁺ progenitors is regulated by the extracellular signal-regulated kinase pathway.

Gastric Peptides-Gastrin and Somatostatin

Gastric acid secretion (i) facilitates digestion of protein as well as absorption of micronutrients and certain medications, (ii) kills ingested microorganisms, including Helicobacter pylori, and (iii) prevents bacterial overgrowth and enteric infection. The principal regulators of acid secretion are the gastric peptides gastrin and somatostatin. Gastrin, the major hormonal stimulant for acid secretion, is synthesized in pyloric mucosal G cells as a 101-amino acid precursor (preprogastrin) that is processed to yield biologically active amidated gastrin-17 and gastrin-34. The C-terminal active site of gastrin (Trp-Met-Asp-Phe-NH₂ ) binds to gastrin/CCK₂ receptors on parietal and, more importantly, histamine-containing enterochromaffin-like (ECL) cells, located in oxyntic mucosa, to induce acid secretion. Histamine diffuses to the neighboring parietal cells where it binds to histamine H₂ -receptors coupled to hydrochloric acid secretion. Gastrin is also a trophic hormone that maintains the integrity of gastric mucosa, induces proliferation of parietal and ECL cells, and is thought to play a role in carcinogenesis. Somatostatin, present in D cells of the gastric pyloric and oxyntic mucosa, is the main inhibitor of acid secretion, particularly during the interdigestive period. Somatostatin exerts a tonic paracrine restraint on gastrin secretion from G cells, histamine secretion from ECL cells, and acid secretion from parietal cells. Removal of this restraint, for example by activation of cholinergic neurons during ingestion of food, initiates and maximizes acid secretion. Knowledge regarding the structure and function of gastrin, somatostatin, and their respective receptors is providing novel avenues to better diagnose and manage acid-peptic disorders and certain cancers. Published 2020. Compr Physiol 10:197-228, 2020.

The cholecystokinin receptor agonist, CCK-8, induces adiponectin production in rat white adipose tissue

BACKGROUND AND PURPOSE

A cholecystokinin (CCK) system has been identified in white adipose tissue (WAT). Nevertheless, the endocrine actions of CCK on WAT remain unknown. Our goal was to investigate the role of CCK in regulating the production of adiponectin, an adipokine expressed in WAT, which is pivotal in preserving energy homeostasis.

EXPERIMENTAL APPROACH

The effect of the bioactive CCK fragment CCK-8 on adiponectin production was studied both in vivo and in vitro. CCK-8 effects were characterized in rats treated with selective CCK₁ and CCK₂ receptor antagonists as well as in pre-adipocytes carrying the selective silencing of either CCK₁ or CCK₂ receptors. The influence of insulin on CCK-8 responses was also analysed.

KEY RESULTS

In WAT, CCK-8 increased plasma adiponectin levels and the expression of the adiponectin gene (Adipoq). In pre-adipocytes, CCK-8 up-regulated adiponectin production. CCK-8 effects were abolished by L-365,260, a selective CCK₂ receptor antagonist. CCK₂ receptor knockdown also abolished the effects of CCK-8 in pre-adipocytes. Moreover, in vitro CCK-8 effects were blocked by triciribine, a specific inhibitor of protein kinase B (Akt) and by the PPARγ antagonist T0070907. Silencing the expression of the insulin receptor inhibited CCK-8-induced Adipoq expression in pre-adipocytes. Furthermore, insulin potentiated the effect of CCK-8.

CONCLUSION AND IMPLICATIONS

CCK-8 stimulates adiponectin production in WAT by acting on CCK₂ receptors, through a mechanism involving both Akt and PPARγ. Moreover, CCK-8 actions are only observed in the presence of insulin. Our results could have translational value in the design of new insulin-sensitizing therapies.

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.

International Union of Basic and Clinical Pharmacology. XCVIII. Histamine Receptors

Histamine is a developmentally highly conserved autacoid found in most vertebrate tissues. Its physiological functions are mediated by four 7-transmembrane G protein-coupled receptors (H1R, H2R, H3R, H4R) that are all targets of pharmacological intervention. The receptors display molecular heterogeneity and constitutive activity. H1R antagonists are long known antiallergic and sedating drugs, whereas the H2R was identified in the 1970s and led to the development of H2R-antagonists that revolutionized stomach ulcer treatment. The crystal structure of ligand-bound H1R has rendered it possible to design new ligands with novel properties. The H3R is an autoreceptor and heteroreceptor providing negative feedback on histaminergic and inhibition on other neurons. A block of these actions promotes waking. The H4R occurs on immuncompetent cells and the development of anti-inflammatory drugs is anticipated.

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.

Gene expression profiling of gastric mucosa in mice lacking CCK and gastrin receptors

The stomach produces acid, which may play an important role in the regulation of bone homeostasis. The aim of this study was to reveal signaling pathways in the gastric mucosa that involve the acid secretion and possibly the bone metabolism in CCK1 and/or CCK2 receptor knockout (KO) mice. Gastric acid secretion was impaired and the ECL cell signaling pathway was inhibited in CCK2 receptor KO mice but not in CCK1 receptor KO mice. However, in CCK1+2 receptor double KO mice the acid secretion in response to pylorus ligation-induced vagal stimulation and the ECL cell pathway were partially normalized, which was associated with an up-regulated pituitary adenylate cyclase-activating polypeptide (PACAP) type 1 receptor (PAC1). The basal part of the gastric mucosa expressed parathyroid hormone-like hormone (PTHLH) in a subpopulation of likely ECL cells (and possibly other cells) and vitamin D3 1α hydroxylase probably in trefoil peptide2-immunoreactive cells. In conclusion, mice lacking CCK receptors exhibited a functional shift from the gastrin-CCK pathways to the neuronal pathway in control of the ECL cells and eventually the acid secretion. Taking the present data together with previous findings, we suggest a possible link between gastric PTHLH and vitamin D and bone metabolism.

Gastric carcinoids (neuroendocrine neoplasms)

Gastric neuroendocrine neoplasms of the stomach can be divided into the usually well-differentiated, hypergastrinemia-dependent type I and II lesions and the more aggressively behaving gastrin-independent type III lesions. Studying menin and its complex interrelationship with gastrin may provide insight into tumor biology at the clinical level and in terms of basic cell biology (eg, the role of the epigenome in neuroendocrine cell proliferation), and lead to potential consideration of other targets that are known candidates for molecular-based therapies in other adenocarcinomas.

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.

Ultrastructure of ECL cells in Mastomys after long-term treatment with H2 receptor antagonist loxtidine

Gastric ECL-cell hyperplasia and carcinoids (ECLoma) develop after 1 year in rats treated with omeprazole or 2 months in Mastomys treated with loxtidine. The aim of this study was to examine the ultrastructure of ECL cells in Mastomys after loxtidine treatment with an attempt to evaluate whether an impairment of autophagy was involved in the tumorigenesis. Mastomys were given loxtidine for 8 or 27 weeks. Morphological analysis of ECL cells showed that (1) cell size was not increased after 8 or 27 weeks; (2) secretory vesicles, a hallmark feature of welldifferentiated ECL cells, were unchanged after 8 weeks but reduced after 27 weeks; (3) granules were reduced after 8 or 27 weeks; (4) microvesicles were unchanged after the treatment; and (5) vacuoles and lipofuscin bodies were found occasionally after 8 weeks but not at 27 weeks. In addition, the appearance of ECL-cell ultrastructure differed between loxtidine-treated Mastomys and rats treated with omeprazole or subjected to antrectomy, but was similar between Mastomys treated with loxtidine for 27 weeks and mice deficient in CCK(2) receptor. We suggest that the ultrastructure of ECL cells in Mastomys after long-term treatment with loxtidine displayed an impaired formation of vacuoles and lipofuscin bodies, markers of the autophagic pathway.

Cholecystokinin and gastrin receptors targeting in gastrointestinal cancer

Cholecystokinin and Gastrin are amongst the first gastrointestinal hormone discovered. In addition to classical actions (contraction of gallbladder, growth and secretion in the stomach and pancreas), these also act as growth stimulants for gastrointestinal malignancies and cell lines. Growth of these tumours is inhibited by antagonists of the cholecystokinin and gastrin receptors. These receptors provides most promising approach in clinical oncology and several specific radiolabelled ligands have been synthesized for specific tumour targeting and therapy of tumours overexpressing these receptors. Therefore, definition of the molecular structure of the receptor involved in the autocrine/paracrine loop may contribute to novel therapies for gastrointestinal cancer. Hence, this review tries to focus on the role and distribution of these hormones and their receptors in gastrointestinal cancer with a brief talk about the clinical trial using available agonist and antagonist in gastrointestinal cancers.

Downregulation of the CCK-B receptor in pancreatic cancer cells blocks proliferation and promotes apoptosis

Gastrin stimulates the growth of pancreatic cancer cells through the activation of the cholecystokinin-B receptor (CCK-BR), which has been found to be overexpressed in pancreatic cancer. In this study, we proposed that the CCK-BR drives growth of pancreatic cancer; hence, interruption of CCK-BR activity could potentially be an ideal target for cancer therapeutics. The effect of CCK-BR downregulation in the human pancreatic adenocarcinoma cells was examined by utilizing specific CCK-BR-targeted RNA interference reagents. The CCK-BR receptor expression was both transiently and stably downregulated by transfection with selective CCK-BR small-interfering RNA or short-hairpin RNA, respectively, and the effects on cell growth and apoptosis were assessed. CCK-BR downregulation resulted in reduced cancer cell proliferation, decreased DNA synthesis, and cell cycle arrest as demonstrated by an inhibition of G(1) to S phase progression. Furthermore, CCK-BR downregulation increased caspase-3 activity, TUNEL-positive cells, and decreased X-linked inhibitor of apoptosis protein expression, suggesting apoptotic activity. Pancreatic cancer cell mobility was decreased when the CCK-BR was downregulated, as assessed by a migration assay. These results show the importance of the CCK-BR in regulation of growth and apoptosis in pancreatic cancer. Strategies to decrease the CCK-BR expression and activity may be beneficial for the development of new methods to improve the treatment for patients with pancreatic cancer.

Gastrin, cholecystokinin and gastrointestinal tract functions in mammals

The aim of the present review is to synthesise and summarise our recent knowledge on the involvement of cholecystokinin (CCK) and gastrin peptides and their receptors in the control of digestive functions and more generally their role in the field of nutrition in mammals. First, we examined the release of these peptides from the gut, focusing on their molecular forms, the factors regulating their release and the signalling pathways mediating their effects. Second, general physiological effects of CCK and gastrin peptides are described with regard to their specific receptors and the role of CCK on vagal mucosal afferent nerve activities. Local effects of CCK and gastrin in the gut are also reported, including gut development, gastrointestinal motility and control of pancreatic functions through vagal afferent pathways, including NO. Third, some examples of the intervention of the CCK and gastrin peptides are exposed in diseases, taking into account intervention of the classical receptor subtypes (CCK1 and CCK2 receptors) and their heterodimerisation as well as CCK-C receptor subtype. Finally, applications and future challenges are suggested in the nutritional field (performances) and in therapy with regards to the molecular forms or in relation with the type of receptor as well as new techniques to be utilised in detection or in therapy of disease. In conclusion, the present review underlines recent developments in this field: CCK and gastrin peptides and their receptors are the key factor of nutritional aspects; a better understanding of the mechanisms involved may increase the efficiency of the nutritional functions and the treatment of abnormalities under pathological conditions.

Achlorhydria is associated with gastric microbial overgrowth and development of cancer: Lessons learned from the gastrin knockout mouse

Gastrin and gastrin receptor-deficient mice have been used for genetic dissection of the role of gastrins in maintaining gastric homeostasis and control of acid secretion. The gastrin knockout mice are achlorhydric due to inactivation of the ECL and parietal cells. Moreover, this achlorhydria is associated with intestinal metaplasia and bacterial overgrowth, which ultimately leads to the development of gastric tumours. The association between progastrin, progastrin-derived processing intermediates and colorectal carcinogenesis has also been examined through genetic or chemical cancer induction in several mouse models, although the clinical relevance of these studies remains unproven. While others have focused on models of increased gastrin production, the present review describes the lessons learned from gastrin-deficient mice. Study of these mice helps our understanding of how dysregulation of gastrin secretion may be implicated in human disease.

Role of serotonin in intestinal inflammation: knockout of serotonin reuptake transporter exacerbates 2,4,6-trinitrobenzene sulfonic acid colitis in mice

Serotonin (5-HT) regulates peristaltic and secretory reflexes in the gut. The serotonin reuptake transporter (SERT; SLC6A4), which inactivates 5-HT, is expressed in the intestinal mucosa and the enteric nervous system. Stool water content is increased and colonic motility is irregular in mice with a targeted deletion of SERT. We tested the hypotheses that 5-HT plays a role in regulating intestinal inflammation and that the potentiation of serotonergic signaling that results from SERT deletion is proinflammatory. Rectal installation of 2,4,6-trinitrobenzene sulfonic acid (TNBS) was used to induce an immune-mediated colitis, which was compared in SERT knockout mice and littermate controls. Intestinal myeloperoxidase and histamine levels were significantly increased, whereas the survival rate and state of health were significantly decreased in TNBS-treated mice that lacked SERT. Deletion of SERT thus increases the severity of TNBS colitis. These data suggest that 5-HT and its SERT-mediated termination play roles in intestinal immune/inflammatory responses in mice.

Importance of gastrin in the pathogenesis and treatment of gastric tumors

In addition to regulating acid secretion, the gastric antral hormone gastrin regulates several important cellular processes in the gastric epithelium including proliferation, apoptosis, migration, invasion, tissue remodelling and angiogenesis. Elevated serum concentrations of this hormone are caused by many conditions, particularly hypochlorhydria (as a result of autoimmune or Helicobacter pylori (H pylori)-induced chronic atrophic gastritis or acid suppressing drugs) and gastrin producing tumors (gastrinomas). There is now accumulating evidence that altered local and plasma concentrations of gastrin may play a role during the development of various gastric tumors. In the absence of H pylori infection, marked hypergastrinemia frequently results in the development of gastric enterochromaffin cell-like neuroendocrine tumors and surgery to remove the cause of hypergastrinemia may lead to tumor resolution in this condition. In animal models such as transgenic INS-GAS mice, hypergastrinemia has also been shown to act as a cofactor with Helicobacter infection during gastric adenocarcinoma development. However, it is currently unclear as to what extent gastrin also modulates human gastric adenocarcinoma development. Therapeutic approaches targeting hypergastrinemia, such as immunization with G17DT, have been evaluated for the treatment of gastric adenocarcinoma, with some promising results. Although the mild hypergastrinemia associated with proton pump inhibitor drug use has been shown to cause ECL-cell hyperplasia and to increase H pylori-induced gastric atrophy, there is currently no convincing evidence that this class of agents contributes towards the development of gastric neuroendocrine tumors or gastric adenocarcinomas in human subjects.

Gastric acid secretion in cholecystokinin-1 receptor, -2 receptor, and -1, -2 receptor gene knockout mice

Gastrin is important for stimulating acid secretion as well as differentiating gastric mucosal cells via cholecystokinin-2 receptors (CCK-2Rs). In turn, CCK acts preferably via CCK-1R to release somatostatin, and somatostatin has been postulated to exhibit a tonic inhibition of gastrin bioactivity. The present study was designed to examine the hypothesis that CCK-1R and 2R may act in opposite directions in gastric acid secretion. Having generated CCK-1R(-/-), 2R(-/-), and 1R(-/-)2R(-/-) mice, we examined the regulation of gastric acid secretion in four genotypes including wild-type mice. Parietal cells possess histamine receptors, muscarinic receptors, and CCK-2Rs. Since histamine increases cAMP and carbachol increases calcium, the responses of gastric acid secretion to graded doses of histamine, carbachol, and a combination of histamine + carbachol were determined. The sensitivity to histamine did not differ among the four genotypes, while the maximal acid secretion was lower in CCK-2R(-/-) mice than in wild-type mice. In addition, sensitivity to carbachol was impaired in mice without CCK-2R. The interaction of histamine and carbachol was conserved in all genotypes. In conclusion, CCK-2R is necessary to respond to carbachol as well as to produce the maximal acid secretion, while the role of CCK-1R in acid secretion is less important.

Endocrine hyperplasia and dysplasia in the pathogenesis of gastrointestinal and pancreatic endocrine tumors

Non-neoplastic proliferative lesions of endocrine cells have been described throughout the gastrointestinal tract and pancreas. A multistep continuum from hyperplasia and dysplasia to neoplasia originally was identified and systematically defined for histamine-producing enterochromaffin-like (ECL) cells of the gastric corpus. More recently, a similar classification system was devised for gastrin (G)- and somatostatin (D)-producing cells in the duodenum. Preneoplastic lesions of the endocrine pancreas still lack a solid and widely accepted definition of a multistep growth process. Similarly, in spite of reports of carcinoid-associated endocrine cell hyperplasia, there is no systematic definition of nonneoplastic lesions of the endocrine cells of the ileum, appendix and colorectum.

Genetically engineered mice: a new paradigm to study gastric physiology

PURPOSE OF REVIEW

To summarize key aspects from recent research as well as review articles on the topic of genetic mouse models, particularly in knockout mice, that have considerably contributed to understanding the pathways and mechanisms underlying gastric physiology.

RECENT FINDINGS

A series of knockout mouse models has proven to be invaluable in elucidating the mechanism and validating the current model of acid secretion. The interaction between the gastrin-histamine and cholecystokinin-somatostatin pathways was identified using the genetic approach as being critical in regulating acid secretion. Curiously, neither ghrelin nor ghrelin receptor knockout mice displayed the expected lean phenotype. Importantly, the study of obestatin in GPR39 knockout mice could be misleading, as zinc rather than obestatin is the endogenous ligand for GPR39. The physiological roles of ghrelin and obestatin have yet to be confirmed using knockout mouse models.

SUMMARY

The knockout mouse continues to serve as an excellent model to dissect the complexity of the mechanism of gastric acid secretion and to study the physiological importance of gastric ghrelin.

Transcriptional profiling of gastrin-regulated genes in mouse stomach

Gastrin, a potent stimulator of gastric acid secretion, primarily targets the acid-secreting parietal cells and histamine-secreting enterochromaffin-like (ECL) cells in the stomach. Accordingly, gastrin-deficient (GAS-KO) mice have a severe impairment in acid secretion. The aim of this study was to characterize changes in gene expression in GAS-KO mice to identify gastrin-regulated genes and to gain insight into how gastric cell types are regulated by gastrin and acid secretion. Affymetrix microarray analysis of GAS-KO and wild-type mice identified numerous differentially expressed transcripts. The results were compared with GAS-KO mice treated with gastrin to identify genes that were gastrin responsive. Finally, genes that were primarily changed due to gastrin and not hypochlorhydria were identified by comparison to mice that are deficient in both gastrin and cholecystokinin (GAS/CCK-KO), since these mice have restored basal acid secretion. The data were validated by quantitative reverse transcriptase polymerase chain reaction analysis. Interestingly, a number of inflammatory response genes were induced in GAS-KO mice and normalized in GAS/CCK-KO mice, suggesting that they were increased in response to low gastric acid. Moreover, a number of parietal cell transcripts that were downregulated in GAS-KO mice were similarly restored in GAS/CCK-KO mice, suggesting that parietal cell changes were also primarily associated with hypochlorhydria. In contrast, ECL cell genes that were markedly downregulated in GAS-KO mice continued to be reduced in GAS/CCK-KO mice, demonstrating that gastrin coordinately regulates a number of ECL cell genes, including several involved in histamine synthesis and secretion.

Differentiation of the gastric mucosa. II. Role of gastrin in gastric epithelial cell proliferation and maturation

Gastrin is the principal hormonal inducer of gastric acid secretion. The cellular targets for gastrin in the stomach are the acid-secreting parietal cell and histamine-producing enterochromaffin-like (ECL) cell. Gastrin is also a growth factor, with hypergastrinemia resulting in increased proliferation of gastric progenitor cells and a thickened mucosa. This review presents insights into gastrin function revealed by genetically engineered mouse models, demonstrating a new role for gastrin in the maturation of parietal and ECL cells. Thus, gastrin regulates many aspects of gastric physiology, with tight regulation of gastrin levels required to maintain balanced growth and function of gastric epithelial cells.

Differentiation of the gastric mucosa. I. Role of histamine in control of function and integrity of oxyntic mucosa: understanding gastric physiology through disruption of targeted genes

Many physiological functions of the stomach depend on an intact mucosal integrity; function reflects structure and vice versa. Histamine in the stomach is synthesized by histidine decarboxylase (HDC), stored in enterochromaffin-like (ECL) cells, and released in response to gastrin, acting on CCK(2) receptors on the ECL cells. Mobilized ECL cell histamine stimulates histamine H(2) receptors on the parietal cells, resulting in acid secretion. The parietal cells express H(2), M(3), and CCK(2) receptors and somatostatin sst(2) receptors. This review discusses the consequences of disrupting genes that are important for ECL cell histamine release and synthesis (HDC, gastrin, and CCK(2) receptor genes) and genes that are important for "cross-talk" between H(2) receptors and other receptors on the parietal cell (CCK(2), M(3), and sst(2) receptors). Such analysis may provide insight into the functional significance of gastric histamine.

Cholecystokinin and gastrin receptors

Cholecystokinin and gastrin receptors (CCK1R and CCK2R) are G protein-coupled receptors that have been the subject of intensive research in the last 10 years with corresponding advances in the understanding of their functioning and physiology. In this review, we first describe general properties of the receptors, such as the different signaling pathways used to exert short- and long-term effects and the structural data that explain their binding properties, activation, and regulation. We then focus on peripheral cholecystokinin receptors by describing their tissue distribution and physiological actions. Finally, pathophysiological peripheral actions of cholecystokinin receptors and their relevance in clinical disorders are reviewed.

Secretory organelles in ECL cells: effects of pharmacological blockade of the gastrin/CCK2 receptor versus its elimination by gene targeting

Histamine-producing ECL cells are numerous in the stomach. They express gastrin/CCK2 receptors and respond to gastrin by releasing histamine. Ultrastructurally, they display numerous and very characteristic secretory organelles: granules, secretory vesicles and microvesicles. This paper focuses on the impact of the gastrin/CCK2 receptor on the ultrastructure of the ECL cells. The effects of pharmacological blockade of the receptor are compared with the effects of receptor elimination following selective gene targeting. Long-term administration of powerful gastrin/CCK2 receptor antagonists was found to induce hypotrophy of rat stomach ECL cells with reduced number of granules, secretory vesicles and microvesicles. In gastrin/CCK2 receptor knockout mice ECL cells, i.e., histamine-storing cells with the characteristic ultrastructure of ECL cells, had disappeared from the oxyntic mucosa and been replaced by a novel population of endocrine-like cells. These cells harbored granules and microvesicles, but were devoid of histamine and secretory vesicles. We suggest that the gastrin/CCK2 receptor is important for the proper differentiation of the ECL cells and for maintaining their characteristic ultrastructure.

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.

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.

Synergistic inhibitory effects of gastrin and histamine receptor antagonists on Helicobacter-induced gastric cancer

BACKGROUND & AIMS

Apart from its importance as an acid secretogogue, the role of histamine as a downstream target of gastrin has not been fully explored. Previous studies have shown that the combination of hypergastrinemia and Helicobacter infection resulted in accelerated gastric cancer in mice. We used this model to examine the role of cholecystokinin 2 (CCK2)/gastrin receptor and histamine H2-receptor signaling in the development of gastric atrophy and cancer.

METHODS

Male hypergastrinemic mice (INS-GAS mice) were infected with Helicobacter felis and given the CCK2/gastrin receptor antagonist YF476 and/or the histamine H2-receptor antagonist loxtidine for 3 or 6 months. In addition, mice were treated with omeprazole alone or in combination with either YF476 or loxtidine for 3 months.

RESULTS

Mice treated with YF476 or loxtidine alone showed partial suppression of both gastric acid secretion and progression to neoplasia. The combination of YF476 plus loxtidine treatment resulted in nearly complete inhibition of both parameters. YF476 and/or loxtidine treatment did not alter the overall level of H. felis colonization but did result in significant down-regulation of the growth factors regenerating gene I and amphiregulin. Loxtidine treatment, with or without YF476, induced a mild shift in T-helper cell polarization. In contrast, omeprazole treatment resulted in mild progression of gastric hyperplasia/dysplasia, which was ameliorated by the addition of YF476 or loxtidine.

CONCLUSIONS

The combination of CCK2/gastrin- and histamine H2-receptor antagonists has synergistic inhibitory effects on development of gastric atrophy and cancer in H. felis/INS-GAS mice, while the proton pump inhibitor showed no such effects. These results support an important role for the gastrin-histamine axis in Helicobacter-induced gastric carcinogenesis.

Neuroendocrinology of gastric H+ and duodenal HCO3- secretion: the role of brain-gut axis

Gastric H+ and duodenal HCO3- secretions are precisely regulated by neuro-hormonal mechanisms at central and peripheral levels to match the rate of these secretions with the type of stimulation of sensory receptors in the head area (sight, smell, taste, etc.) and in the gastro-intestinal system. Two-way communication pathways operate between the brain and the gut, each comprising afferent fibers signaling sensory information from the gut to the brain and efferent fibers transmitting signals in opposite direction. Short intramural and long extramural reflexes are triggered as well as various gut hormones are released by feeding that "cooperate" with the "brain-gut axis" in the alteration of exocrine and endocrine gastro-duodenal secretion, motility and blood circulation. The malfunction of gastric or duodenal secretory mechanisms may lead to disturbances of gastric H+-pepsin or duodenal mucus-HCO3- secretion and to gastro-duodenal disorders and diseases. This review presents recent advances in pathophysiological mechanisms underlying gastro-duodenal secretory disorders.

Expression of cholecystokinin-2/gastrin receptor in the murine pancreas modulates cell adhesion and cell differentiation in vivo

The presence of gastrin and cholecystokinin-2 (CCK2) receptors in human preneoplastic and neoplastic gastrointestinal lesions suggests a role in cancer development. In addition to the growth-promoting action of gastrin, recently a role of the cholecystokinin-2/gastrin receptor (CCK2-R) modulating cellular morphology in cultured epithelial cells has been shown. Here, we have investigated in transgenic (ElasCCK2) mice whether ectopic expression of human CCK2-R in the exocrine pancreas affected epithelial differentiation. Cellular localization of cell adhesion molecules, differentiation markers, and transcription factors was determined using immunofluorescence techniques. Before tumor formation, expression and subcellular localization of proteins of the adherens junction complex, differentiation markers, and transcription factors were altered in ElasCCK2 exocrine pancreas, indicating an evolution from an acinar to a ductal phenotype. Loss of cell polarity, defective secretion, and loss of intercellular adhesion in acini of ElasCCK2 mice was confirmed by ultrastructural analysis. Finally, expression of the transgene in mice treated with the carcinogen azaserine resulted in enhanced size of preneoplastic lesions as well as an increased degree of acinar-ductal transdifferentiation. Thus, these data represent the first evidence for the CCK2-R modulating intercellular adhesion and cell fate in vivo and show that these alterations may contribute to enhanced sensitivity of ElasCCK2 pancreas to chemical carcinogens.

Lack of histamine alters gastric mucosal morphology: comparison of histidine decarboxylase-deficient and mast cell-deficient mice

Histamine plays an important role in the regulation of gastric acid secretion; however, its role in maintenance of gastric morphology remains unclear. To clarify the necessity of histamine for gastric mucosal development and maintenance, we evaluated two different kinds of mice that lacked either mast cells (one of the gastric histamine-producing cell types) or histidine decarboxylase (HDC; a histamine-synthesizing enzyme). Measurements of stomach weight, intragastric pH, mucosal histamine levels, as well as serum gastrin and albumin levels were performed in mice. Gastric mucosal appearance was examined by immunohistochemical techniques. Although gastric mucosal histamine levels in mast cell-deficient mice were half of those observed in the wild-type mice, intragastric pH, serum gastrin levels, and gastric morphology at 12 mo were unchanged compared with the wild-type mice. In contrast, HDC-deficient mice possessed no detectable gastric histamine, but did exhibit hypergastrinemia, as well as marked increases in intragastric pH and stomach weight compared with the wild-type mice. Histological analysis revealed that 9-mo-old HDC-deficient mice demonstrated hyperplasia in the oxyntic glandular base region, as well as increased numbers of parietal and enterochromaffin-like cells. These results indicate that enterochromaffin-like cell-derived histamine is potentially involved in gastric mucosal morphology regulation.

Chronic Helicobacter pylori infection results in gastric hypoacidity and hypergastrinemia in wild-type mice but vagally induced hypersecretion in gastrin-deficient mice

Helicobacter pylori infection is a causal factor of gastric cancer (which is associated with low gastric acid secretion) or duodenal ulcer (high acid secretion). Parietal cells and ECL cells in the stomach are controlled by gastrin, which plays a crucial role in the regulation of acid secretion. The present study was undertaken to identify a possible role of gastrin in determining the different responses of the parietal cells and ECL cells to chronic H. pylori infection. Wild-type (C57BL/6J) gastrin(+/+) mice and gastrin(-/-) knockout mice, generated through targeted gene disruption and backcrossed eight times to C57BL/6J, were infected with H. pylori for 9 months. The acid output was measured 4 h after pylorus ligation (known to cause vagal excitation). The gastric mucosa was examined by immunocytochemistry with antisera to alpha-subunit of H+/K(+)-ATPase for the parietal cells, and to histamine and vesicle monoamine transporter-2 for the ECL cells, and by quantitative electron microscopy. In infected gastrin(+/+) mice, the acid output and the percentage of secreting parietal cells (freely fed state) were 20-30% of the values in uninfected controls, while the density and ultrastructure of parietal cells were normal. The infected mice had hypergastrinemia and displayed hypertrophy and hyperplasia of ECL cells. Although uninfected gastrin(-/-) mice had lower the acid output than uninfected gastrin(+/+) mice, there was a higher acid output (approximately 3 times) in infected gastrin(-/-) mice than their uninfected homologues. The numbers of parietal cells and ECL cells remained unchanged in infected gastrin(-/-) mice. In conclusion, chronic H. pylori infection results to impaired parietal-cell function (acid hyposecretion), hypergastrinemia and hyperplasia of ECL cells in wild-type mice but leads to vagally induced hypersecretion in gastrin-deficient mice.

The "normal" endocrine cell of the gut: changing concepts and new evidences

The endocrine cells of the gut are a highly specialized mucosal cell subpopulation. Within the gastrointestinal tract at least 14 different cell types produce a wide range of hormones with a specific regional distribution. The gut endocrine cells belong to the diffuse endocrine system. These cells present two regulated pathways of secretion characterized by large dense core vesicles (LDCV) and synaptic-like microvesicles (SLMV). Gut endocrine cells are recognized by the expression of several "general" markers, including the LDCV marker chromogranin A and the SLMV marker synaptophysin, in addition to the cytosolic markers neuron-specific enolase and protein gene product 9.5. The expression of different hormones identifies specific cell types. The gut endocrine cells are reputed to be terminally differentiated and incapable of proliferation. However, some data suggest that the number of gut endocrine cells may adapt in response to tissue-specific physiological stimuli. Gut endocrine cell differentiation appears to follow a "constitutive" tissue-specific pathway, which may be disrupted and investigated by genetic manipulation in mice. It is suggested that endocrine cell homeostasis is maintained by the entry of new endocrine-committed cells along the differentiation pathway and that such intermediate cells may be sensitive to physiological stimuli as well as transforming agents.

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.

Alterations of gastrin, somatostatin, G and D cells in rat gastric ulcer

AIM: To investigate the alterations of gastrin secretion of G cells, somatostatin secretion of D cells and the change of G (D) cells in rat gastric ulcer.

METHODS: An acetic-acid-induced rat gastric ulcer model was established. The histological structure of rat antral mucosa and the ultrastructure of mucosal cells were observed generally, through microscope and through electron microscope. The content of gastrin or somatostatin in serum or in antral tissue was measured via radioimmunoassay. The shape, number, size of G (D) cells, and the ratio of number and size of G/D cells were viewed and analyzed with immunohistochemical technique and image analysis system. G (D) cells and the secretive gastrin (somatostatin) granules in G (D) cells were observed through immunoel-ectron microscope and analyzed in image analysis system.

RESULTS: G (D) cells and the secretive gastrin (somatostatin) granules in G (D) cells were observed through immunoelectron microscope successfully. In gastric ulcer rat the secretive gastrin in G cells increased, the secretive somatostatin in D cells declined, the number of G cells increases and the size of G cells declined; both the number and the size of D cells declined, both the ratio of the number and size of G/D cells increased, both the content of gastrin in serum and in antral tissue increased, and both the content of somatostatin in serum and in antral tissue declined.

CONCLUSION: The rat gastric ulcer induces the changes of G cells and D cells, secretive gastrin in G cells and secretive somatostatin in D cells, as well as the contents of gastrin and somatostatin.

Altered control of gastric acid secretion in gastrin-cholecystokinin double mutant mice

BACKGROUND & AIMS

Three pathways control gastric acid secretion: the gastrin-enterochromaffin-like (ECL) cell axis, the vagus-parietal cell axis, and the cholecystokinin (CCK)-D cell axis. Mice lacking gastrin or both gastrin and CCK were examined to determine the role of the hormones.

METHODS

Acid was measured after pylorus ligation, and biopsies from gastrin knockout (KO), gastrin-CCK double-KO, and wild-type (WT) mice were collected for biochemical, immunocytochemical, and electron-microscopic examination.

RESULTS

The ECL cells were inactive in both groups of mutant mice but the cell number was unaffected. Both parietal cell number and level of H(+)/K(+)-ATPase messenger RNA (mRNA) were reduced in the mutant strains, but gastrin-CCK double-KO mice displayed more active parietal cells and larger acid output than the gastrin KO mice. The acid response to histamine in double-KO mice was unchanged whereas that to gastrin was diminished, but it could be restored by infusion of gastrin. Oxyntic D-cell density was the same in both mutant strains, but the D cells were more active in the gastrin KO than in the double-KO mice. CCK infusion in gastrin-CCK double-KO mice raised the somatostatin mRNA level and inhibited acid secretion to the level seen in gastrin KO mice. Vagotomy and atropine abolished acid secretion in all 3 groups of mice.

CONCLUSIONS

Lack of gastrin impairs the gastrin-ECL axis, whereas lack of gastrin and CCK impairs both hormonal pathways. In the gastrin-CCK double-KO mice, acid secretion is only controlled by cholinergic vagal stimulation, which normalizes the acid output.

Impaired gastric secretion and lack of trophic responses to hypergastrinemia in M3 muscarinic receptor knockout mice

BACKGROUND & AIMS

The physiologic significance of the M(3) muscarinic receptor is unclear due to an absence of specific ligand. In the present study, M(3) receptor knockout (KO) mice were used to elucidate the role of M(3) receptors in gastric acid secretion and gastric mucosal integrity.

METHODS

M(3) KO versus wild-type mice aged 1 month to 2 years were included. Gastric acid secretion was assessed by both direct intragastric pH measurement and pylorus ligation. Serum gastrin and gastric mucosal histamine levels were determined by radioimmunoassay and enzyme-linked immunosorbent assay, respectively. Morphologic analysis was performed by both immunohistochemistry and transmission electron microscopy.

RESULTS

Fasted M(3) KO mice exhibited higher intragastric pH, lower acid output after pylorus ligation, a lower proportion of active parietal cells, and higher serum gastrin levels than fasted wild-type mice. Acid secretion in response to carbachol, histamine, gastrin 17, and 2-deoxy-D-glucose was impaired in the mutant mice. Although carbachol was still able to cause approximately 30% acid output in M(3) KO mice, the acid secretion was inhibited by pirenzepine or famotidine. Despite remarkable hypergastrinemia in M(3) KO mice, there were no trophic responses in the oxyntic mucosa with respect to the mucosal thickness, proliferation rate, and numbers of parietal and enterochromaffin-like cells. Cholecystokinin type 2 receptor antagonist YM022 was without the effect in M(3) KO mice.

CONCLUSIONS

The present study shows that M(3) receptors are essential for basal acid secretion, a fully acid secretory response to histamine and gastrin, and the trophic responses of oxyntic mucosa to gastrin.

Gastric secretion

PURPOSE OF REVIEW

Gastric acid facilitates the digestion of protein and the absorption of iron, calcium, and vitamin B12. It also protects against bacterial overgrowth and enteric infection, including prion disease. When homeostatic mechanisms malfunction, the volume and concentration of acid may overwhelm mucosal defense mechanisms, leading to duodenal ulcer, gastric ulcer, and gastroesophageal reflux disease. This article reviews recent knowledge contributing to understanding of the regulation of gastric acid secretion at the central, peripheral, and intracellular levels.

RECENT FINDINGS

The vagus nerve contains afferent fibers that transmit sensory information from the stomach to the nucleus of the solitary tract. Input from the nucleus of the solitary tract is relayed to vagal efferent neurons that originate from two brain stem nuclei: the nucleus ambiguus and the dorsal motor nucleus of the vagus. The latter is also influenced by thyrotropin-releasing hormone neurons that act centrally to stimulate acid secretion. The main peripheral stimulants of acid secretion are the hormone gastrin and the paracrine amine histamine. Gastrin stimulates acid secretion directly and, more importantly, indirectly by releasing histamine from fundic enterochromaffin-like cells. Gastrin also exerts trophic effects on various tissues, including the gastric and intestinal mucosa. The main inhibitor of acid secretion is somatostatin. Somatostatin, acting via ssTR2 receptors, exerts a tonic paracrine inhibitory influence on the secretion of gastrin, histamine, and acid secretion. Calcitonin gene-related peptide, adrenomedullin, amylin, atrial natriuretic peptide, and pituitary adenylate cyclase-activating polypeptide all stimulate somatostatin secretion and thus inhibit acid secretion. HK-ATPase, the proton pump of the parietal cell, is stored within cytoplasmic tubulovesicles during the resting state, but during stimulation, it is shuttled to the canalicular membrane by a poorly understood mechanism that probably involves soluble N-ethylmaleimide-sensitive factor attachment protein receptor proteins. The proton pump inhibitor, pantoprazole, is unique in that it binds cysteine 822, located deep within the membrane domain of the alpha-subunit. The difficulty that reducing agents, such as glutathione, have in reaching cysteine 822 may be responsible for the longer half-time for acid recovery observed with pantoprazole. Hypergastrinemia, induced by proton pump inhibitors, enhances expression of cyclooxygenase-2 and hence prostaglandins within parietal cells, a feedback pathway that may protect the stomach against acid-induced damage.

SUMMARY

In the past year, significant advances have been made in understanding of the regulation of gastric acid secretion. Ultimately, these advances should lead to improved therapies to prevent and treat acid-related disorders. Gastric acid secretion must be precisely controlled at a variety of levels to prevent disease caused by hyperchlorhydria and hypochlorhydria. The mechanisms include neural (central and peripheral), hormonal, paracrine, and intracellular pathways that operate in concert to switch acid secretion on during ingestion of a meal and off during the interdigestive period. A better understanding of the physiology of acid secretion in health and disease should eventually lead to improved therapies to prevent and treat acid-related disorders.

Pharmacological control of gastric acid secretion for the treatment of acid-related peptic disease: past, present, and future

Pharmacological agents, such as histamine H(2) receptor antagonists and acid pump inhibitors, are now the most frequently used treatment for such acid-related diseases as gastroduodenal ulcers and reflux esophagitis. Based on increased understanding of the precise mechanisms of gastric acid secretion at the level of receptors, enzymes, and cytoplasmic signal transduction systems, further possibilities exist for the development of effective antisecretory pharmacotherapy. Gastrin CCK(2) receptor antagonists and locally active agents appear to represent promising therapies for the future. Development of gene targeting techniques has allowed production of genetically engineered transgenic and knockout mice. Such genetic technology has increased the investigative power for pharmacotherapy for not only antisecretory agents, but also treatment of mucosal diseases, such as atrophy, hyperplasia, and cancer. Elucidation of the origin of gastric parietal cells also represents an interesting investigative target that should allow a better understanding of not only acid-related diseases, but also the evolution of the stomach as an acid-secreting organ.

Gastric phenotypic abnormality in cholecystokinin 2 receptor null mice

Gastrin, released from antral G-cells, plays an important role in the regulation of gastric acid secretion and is trophic for the stomach. The cholecystokinin type 2 (CCK)2 receptor (previously referred to as CCK-B/gastrin receptors) is expressed in both parietal cells and ECL cells in the oxyntic mucosa of stomach. Gastric phenotypic abnormality has been observed in CCK2 receptor null (gene knock-out) mice. Such mice displayed markedly impaired gastric acid secretion, atrophy of the oxyntic mucosa and hypergastrinaemia. The impaired acid secretion may be the result of a reduced parietal cell mass, a reduced proportion of actively secreting parietal cells (with secretory canaliculi), and a replacement of ECL cells by histamine-free ECL-like cells. The ECL-like cells, observed in the CCK2 receptor null mice, lacked the hallmark features of wild-type ECL cells, i.e. histamine and cytoplasmic secretory vesicles. However, they had the features of endocrine cells, such as the content of pancreastatin (a fragment of chromogranin A), with cytoplasmic small dense-core granules and microvesicles. We propose that the replacement of ECL cells by ECL-like cells in the mutant mice reflects an altered differentiation of the same precursors that develop into ECL cells in wild-type mice. Thus, studies of CCK2 receptor null mice demonstrate the importance of the receptor in the regulation of gastric acid secretion and in the differentiation of ECL cells in the oxyntic mucosa of stomach.

Role of gastrin in the development of gastric mucosa, ECL cells and A-like cells in newborn and young rats

Histamine-producing ECL cells and ghrelin-producing A-like cells are endocrine/paracrine cell populations in the acid-producing part of the rat stomach. While the A-like cells operate independently of gastrin, the ECL cells respond to gastrin with mobilization of histamine and chromogranin A (CGA)-derived peptides, such as pancreastatin. Gastrin is often assumed to be the driving force behind the postnatal development of the gastric mucosa in general and the ECL cells in particular. We tested this assumption by examining the oxyntic mucosa (with ECL cells and A-like cells) in developing rats under the influence of YF476, a cholecystokinin-2 (CCK(2)) receptor antagonist. The drug was administered by weekly subcutaneous injections starting at birth. The body weight gain was not affected. Weaning occurred at days 15-22 in both YF476-treated and age-matched control rats. Circulating gastrin was low at birth and reached adult levels 2 weeks after birth. During and after weaning (but not before), YF476 greatly raised the serum gastrin concentration (because of abolished acid feedback inhibition of gastrin release). The weight of the stomach was unaffected by YF476 during the first 2-3 weeks after birth. From 4 to 5 weeks of age, the weight and thickness of the gastric mucosa were lower in YF476-treated rats than in controls. Pancreastatin-immunoreactive cells (i.e. all endocrine cells in the stomach) and ghrelin-immunoreactive cells (A-like cells) were few at birth and increased gradually in number until 6-8 weeks of age (control rats). At first, YF476 did not affect the development of the pancreastatin-immunoreactive cells, but a few weeks after weaning, the cells were fewer in the YF476 rats. The ECL-cell parameters (oxyntic mucosal histamine and pancreastatin concentrations, the histidine decarboxylase (HDC) activity, the HDC mRNA levels and serum pancreastatin concentration) increased slowly until weaning in both YF476-treated and control rats. From then on, there was a further increase in the ECL-cell parameters in control rats but not in YF476 rats. The postnatal development of the ghrelin cells (i.e. the A-like cells) and of the A-like cell parameters (the oxyntic mucosal ghrelin concentration and the serum ghrelin concentrations) was not affected by YF476 at any point. We conclude that gastrin affects neither the oxyntic mucosa nor the endocrine cells before weaning. After weaning, CCK(2) receptor blockade is associated with a somewhat impaired development of the oxyntic mucosa and the ECL cells. While gastrin stimulation is of crucial importance for the onset of acid secretion during weaning and for the activation of ECL-cell histamine formation and secretion, the mucosal and ECL-cell growth at this stage is only partly gastrin-dependent. In contrast, the development of the A-like cells is independent of gastrin at all stages.