Gastrin receptor genes are expressed in gastric parietal and enterochromaffin-like cells of Mastomys natalensis

Pathophysiology of Gastric NETs: Role of Gastrin and Menin

PURPOSE OF REVIEW

Neuroendocrine tumors (NETs) were initially identified as a separate entity in the early 1900s as a unique malignancy that secretes bioactive amines. GI-NETs are the most frequent type and represent a unique subset of NETs, because at least 75% of these tumors represent gastrin stimulation of the enterochromaffin-like cell located in the body of the stomach. The purpose of this review is to understand the specific role of gastrin in the generation of Gastric NETs (G-NETs).

RECENT FINDINGS

We review here the origin of enterochromaffin cells gut and the role of hypergastrinemia in gastric enteroendocrine tumorigenesis. We describe generation of the first genetically engineered mouse model of gastrin-driven G-NETs that mimics the human phenotype. The common mechanism observed in both the hypergastrinemic mouse model and human carcinoids is translocation of the cyclin-dependent inhibitor p27^kip to the cytoplasm and its subsequent degradation by the proteasome. Therapies that block degradation of p27^kip, the CCKBR2 gastrin receptor, or gastrin peptide are likely to facilitate treatment.

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.

Gastrin and Gastric Cancer

Gastric cancer is the third leading cause of cancer-related mortality worldwide. Despite progress in understanding its development, challenges with treatment remain. Gastrin, a peptide hormone, is trophic for normal gastrointestinal epithelium. Gastrin also has been shown to play an important role in the stimulation of growth of several gastrointestinal cancers including gastric cancer. We sought to review the role of gastrin and its pathway in gastric cancer and its potential as a therapeutic target in the management of gastric cancer. In the normal adult stomach, gastrin is synthesized in the G cells of the antrum; however, gastrin expression also is found in many gastric adenocarcinomas of the stomach corpus. Gastrin's actions are mediated through the G-protein-coupled receptor cholecystokinin-B (CCK-B) on parietal and enterochromaffin cells of the gastric body. Gastrin blood levels are increased in subjects with type A atrophic gastritis and in those taking high doses of daily proton pump inhibitors for acid reflux disease. In experimental models, proton pump inhibitor-induced hypergastrinemia and infection with Helicobacter pylori increase the risk of gastric cancer. Understanding the gastrin:CCK-B signaling pathway has led to therapeutic strategies to treat gastric cancer by either targeting the CCK-B receptor with small-molecule antagonists or targeting the peptide with immune-based therapies. In this review, we discuss the role of gastrin in gastric adenocarcinoma, and strategies to block its effects to treat those with unresectable gastric cancer.

Helicobacter pylori can induce heparin-binding epidermal growth factor expression via gastrin and its receptor

Both gastrin and Helicobacter pylori have been shown capable of up-regulating gene expression and protein shedding of heparin-binding epidermal growth factor (HB-EGF). Furthermore, the bacteria have previously been shown to induce serum hypergastrinemia in infected individuals. The aim of this work was to assess the extent to which the ability of H. pylori to up-regulate expression of HB-EGF can be attributed to its effect on gastrin. Gastric cells, transfected with either gastrin small interfering RNA or antisense plasmid or the gastrin/cholecystokinin-2 receptor (CCK-2R), were cultured for 24 hours with H. pylori(+/-), a CCK-2R antagonist. Gene expression levels were measured using reverse transcription-PCR, whereas protein changes were measured using ELISA, Western blotting, and immunofluorescence. H. pylori induced significantly higher levels of HB-EGF gene expression and ectodomain shedding in the CCK-2R-transfected cells than the vector control (P < 0.01). Addition of the CCK-2R inhibitor significantly decreased gene and shedding up-regulation. Gastrin down-regulation reduced the effect of the bacteria on HB-EGF gene and protein expression levels. Endogenous gastrin and CCK-2R expression were also found to be significantly up-regulated in all cell lines as a result of exposure to H. pylori (P < 0.02). Gastric mucosal tissue from H. pylori-infected individuals had significantly higher CCK-2R expression levels than noninfected (P < 0.003), and in hypergastrinemic mice, there was an increase in HB-EGF-expressing cells in the gastric mucosa and colocalization of HB-EGF with CCK-2R-positive enterochromaffin-like cells. In conclusion, gastrin and the CCK-2R play significant roles in the induction of HB-EGF gene and protein expression and ectodomain shedding by H. pylori.

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.

Effect of a gastrin/cholecystokinin B receptor antagonist, S-0509, on the omeprazole-induced proliferation of gastric mucosa in rats

Hypergastrinemia is known to cause hyperplasia of the gastric mucosa, especially in gastric enterochromaffinlike (ECL) cells. In some clinical conditions causing hypergastrinemia, such as long-term gastric-acid inhibition and gastric-mucosa atrophy, hyperplastic ECL cells may develop into gastric carcinoid tumors. A newly developed gastrin-receptor antagonist, S-0509, has been reported to block gastrin-induced stimulation of gastric-acid secretion. We therefore investigated whether S-0509 inhibits the omeprazole- and gastrin-stimulated hyperproliferation of gastric mucosa, especially of ECL cells. Daily administration of omeprazole and gastrin in male Sprague-Dawley rats induced marked hypergastrinemia and increased proliferation of gastric-mucosa cells. The numbers of ECL cells and of ECL cells producing messenger RNA for regenerating gene, a potent growth factor for gastric-mucosa cells, were also augmented by long-term administration of omeprazole and gastrin. Coadministration of S-0509 with omeprazole or gastrin almost completely inhibited the omeprazole- and gastrin-induced changes in gastric mucosa, including mucosal thickening and ECL hyperplasia. S-0509 did not induce gastric-mucosa atrophy, even when administered for as long as 4 weeks. In summary, we have found that a newly developed gastrin receptor antagonist, S-0509, inhibits omeprazole- and gastrin-induced mucosal hyperplasia, especially ECL-cell hyperplasia, in rats.

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.

Localization of calcitonin gene-related peptide receptors in rat gastric mucosa

The location of calcitonin gene-related peptide (CGRP) receptors in the rat stomach has not been elucidated. It was recently reported that the CGRP receptor is formed when a calcitonin-receptor-like receptor (CRLR) and receptor activity modifying protein (RAMP) 1 are co-expressed on the cell membrane. The aim of this study was to determine the location and the role of CGRP receptors in the rat gastric mucosa. Gene expressions of CRLR and RAMP1 were investigated by Northern blot analysis, reverse transcription-polymerase chain reaction (RT-PCR), and in situ hybridization. Immunohistochemical stainings for CGRP, somatostatin, gastrin, and chromogranin A were performed. Gastric endocrine cells were collected by counterflow-elutriation and their responses to CGRP were studied. CRLR and RAMP1 mRNA was expressed mainly in small gastric epithelial cells in the pyloric glands. The mRNA expression had a similar distribution to that of D cells. In cultured gastric endocrine cells, CGRP enhanced somatostatin production, while it inhibited the secretion of histamine and gastrin. Our results suggest that CGRP receptors are expressed in D cells in the rat gastric mucosa and control production and secretion of somatostatin.

Gastrin stimulates the growth of gastric pit cell precursors by inducing its own receptors

Gastrin/CCK-B receptors (CCKB-Rs) are present on parietal and enterochromaffin-like cells in the gastric mucosa but not on pit cells in the proliferative zone. Because serum gastrin levels are well correlated with the growth of the gastric pit, we examined whether pit precursor cells express CCKB-Rs using hypergastrinemic transgenic mice and a mouse pit precursor cell line, GSM06. In situ hybridization indicated that CCKB-R mRNA was limited to the lower one-third of the mucosa in control mice, whereas it was faintly distributed along the mid- to low glandular region in the hypergastrinemic transgenic mouse mucosa. CCKB-R-positive midglandular cells appear to have a pit cell lineage; therefore, GSM06 cells were used for an [(125)I]gastrin binding study. [(125)I]gastrin bound to the membrane fraction of the GSM06 cells when precultured with gastrin. Gastrin dose dependently induced CCKB-R expression in GSM06 cells and stimulated their growth. Thus these findings suggest that gastrin directly stimulates the growth of the pit cell lineage by inducing its own receptor in pit cell precursors.

Localization of Reg receptor in rat fundic mucosa

Reg protein has a trophic effect on gastric mucosal cells and pancreatic islets. Recently, the Reg receptor (Reg-R) has been cloned, and Reg-Reg-R interaction has been reported in the pancreas. The aim of this study was to investigate the localization of Reg-R in rat fundic mucosa. Gene expression of Reg-R was investigated with Northern blot analysis, laser capture microdissection coupled with reverse transcription-polymerase chain reaction, and in situ hybridization in the fundic mucosa, and the types of cells expressing this gene were determined. Reg-R mRNA expression was detected mainly in chief cells and parietal cells of the deep layers and faintly in surface epithelial cells and mucous neck cells of the proliferating zone. Our results suggest that regenerating protein may act not only as a regulator of gastric epithelial cell proliferation but also as a modifier of other multiple physiologic functions.

Mobilization of rat stomach ECL-cell histamine in response to short- or long-term treatment with omeprazole and/or YF 476 studied by gastric submucosal microdialysis in conscious rats

1. Mobilization of histamine from the ECL cells was monitored by gastric submucosal microdialysis in conscious rats. The ECL cells are known to operate under gastrin control and the purpose of the present study was to examine their in situ response to short-term (12 h) as well as long-term (28 days) hypergastrinaemia, induced by treatment with the proton pump inhibitor omeprazole. 2. Hypergastrinaemia promptly raised the histamine concentration in the microdialysate. The effect was prevented by CCK(2) receptor blockade (YF476). On day 7 of omeprazole treatment the microdialysate histamine concentration reached a peak, five times higher than before treatment. Subsequently (14 and 28 days), less histamine was mobilized. 3. Gastrin infusion (4 h) raised the microdialysate histamine concentration in a dose-dependent manner in fasted rats and freely fed rats and in rats treated with omeprazole for a week. However, while fasted and fed rats responded to low doses of gastrin, the omeprazole-treated rats required large doses of gastrin to respond. 4. When the amount of histamine mobilized was related to the serum gastrin concentration the following EC(50) values could be calculated: fasted rats 2.3 x 10(-10) M, freely fed rats 2.5 x 10(-10) M, omeprazole-treated rats 8.7 x 10(-10) M. The maximal histamine responses in the three groups were 18.4 pmol 4 h(-1)+/-0.8, 21.9 pmol 4 h(-1)+/-1.2 and 68.0 pmol 4 h(-1)+/-3.5, respectively. 5. The results suggest that ECL cells, exposed to a high gastrin concentration for a week, respond with a shift in the receptor-ligand binding affinity from high to low. Apparently, CCK(2) receptors of the ECL cells are subject to dynamic changes with respect to ligand-binding affinity.

Analysis of gastrin receptor gene expression in proliferating cells in the neck zone of gastric fundic glands using laser capture microdissection

Gastrin stimulates proliferation of progenitor cells in the neck zone of gastric fundic mucosa. However, whether it directly enhances this proliferation through its receptors remains unclear. We investigated the expression of gastrin receptors in neck zone proliferating cells in rat gastric fundic glands using a reverse transcription polymerase chain reaction (RT-PCR) coupled with laser capture microdissection and in situ RT-PCR. Gastrin receptor expression was identified in c-fos-expressing cells located in the neck zone, and results of the RT-PCR analysis argued against contamination by other cells, such as enterochromaffin-like, parietal or D cells. Supporting this finding, gastrin receptor gene expression was identified in the neck zone as well as base glands by in situ RT-PCR. Therefore, it is suggested that proliferating cells in the neck zone are stimulated directly by gastrin via their gastrin receptors.

Neutrophil chemoattractant 2 beta regulates expression of the Reg gene in injured gastric mucosa in rats

BACKGROUND & AIMS

Regenerating (Reg) protein has a trophic effect on gastric mucosal cells. We have shown that Reg gene expression is increased in enterochromaffin-like (ECL) cells during the healing of damaged gastric mucosa around mucosal erosion. This study was designed to explore the stimulants of Reg expression during the healing of gastric mucosal damage.

METHODS

Time course changes of the expression of genes for various proinflammatory cytokines and Reg were investigated after induction of gastric mucosal lesions in rats. The direct effect of proinflammatory cytokines on Reg gene expression and Reg protein production were investigated in vitro using counterflow elutriation-enriched rat ECL cells. CXC receptor 2 (CXCR-2) expression was investigated in ECL cells by reverse-transcription polymerase chain reaction. Reg gene expression was also investigated in rats treated by the neutralizing antibody of cytokine-induced neutrophil chemoattractant (CINC-2 beta).

RESULTS

During healing, the gene expression of several proinflammatory cytokines and Reg was markedly augmented. Among the proinflammatory cytokines, CINC-2 beta is the only cytokine in which augmented expression preceded the increase of Reg gene expression. In rats treated with CINC-2 beta neutralizing antibody, the augmentation of Reg gene expression was significantly inhibited. When ECL cells were incubated with these proinflammatory cytokines, CINC-2 beta dose-dependently increased Reg messenger RNA and Reg protein in ECL cells. CXCR-2 was identified in isolated ECL cells.

CONCLUSIONS

CINC-2 beta, expressed in damaged gastric mucosa, stimulates the production of Reg protein in ECL cells via CXCR-2 and may be involved in the accelerated healing of injured gastric mucosa.

Pathophysiology of abomasal parasitism: is the host or parasite responsible?

Nematode larvae developing within the glands cause local loss of parietal cells and mucous cell hyperplasia whereas reduced acid secretion, increased serum gastrin and pepsinogen concentrations and generalized histological changes are associated with parasites in the abomasal lumen. Parietal cells with dilated canaliculi and/or degenerative changes typical of necrosis are present soon after the transplantation of adult worms, and abomasal secretion is also affected. Anaerobic bacteria survive in greater numbers as the pH rises, with bacterial densities becoming similar to ruminal populations at an abomasal pH of 4 and above. Failure to lyse bacteria may affect adversely the nutrition of the host. The parasites may initiate the pathophysiology through the release of excretory/secretory (ES) products which either act directly on parietal cells or indirectly through enterochromaffin-like (ECL) cells by provoking inflammation or by disrupting the protective mucosal defence system. Parietal cell dysfunction is proposed as a key event which leads to loss of mature chief cells and mucous cell hyperplasia, as well as hypergastrinaemia. Inflammation increases circulating pepsinogen concentrations and may also contribute to increased gastrin secretion. Stimulation of mucosal proliferation and differentiation of parietal cells in the isthmus by the raised serum gastrin levels will be beneficial by generating a new population of active parietal cells and adequate acid secretion.

Long-lasting cholecystokinin(2) receptor blockade after a single subcutaneous injection of YF476 or YM022

Histamine-forming ECL cells in the rat stomach operate under the control of gastrin. They represent a convenient target for studying cholecystokinin-B/gastrin (CCK(2)) receptor antagonists in vivo. We examined the effectiveness and duration of action of two CCK(2) antagonists, YM022 and YF476, with respect to their effect on ECL-cell histidine decarboxylase (HDC) activity in the rat. Oral administration of subcutaneous deposition of YF476 or YM022 reduced the HDC activity. The maximum/near-maximum dose for both drugs and for both modes of administration was 300 micromol kg(-1) (effects measured 24 h after dose). At this dose and time the serum concentration of YF476 was 20 - 40 nmol l(-1). The dose 300 micromol kg(-1) was used in all subsequent studies. A single subcutaneous injection of YF476 inhibited the HDC activity for 8 weeks. The circulating concentration of YF476 remained high for the same period of time (>/=15 nmol l(-1)). Subcutaneous YM022 suppressed the HDC activity for 4 weeks. A single oral dose of YF476 or YM022 inhibited the HDC activity for 2 - 3 days. Chronic gastric fistula rats were used to study the effect of subcutaneous YF476 on gastrin-stimulated acid secretion. A single injection of YF476 prevented gastrin from causing an acid response for at least 4 weeks (the longest time studied). We conclude that a single subcutaneous injection of 300 micromol kg(-1) YF476 causes blockade of CCK(2) receptors in the stomach of the rat for 8 weeks thus providing a convenient method for studies of the consequences of long-term CCK(2) receptor inhibition.

Mechanism of gastric mucosal proliferation induced by gastrin

Gastrin has a potent trophic effect on gastric fundic mucosa. When serum concentrations of gastrin are elevated, proliferation of both the progenitor cells in the glandular neck zone and enterochromaffin-like (ECL) cells in the bottom of the glands is stimulated. Because ECL cells have gastrin receptors, their proliferation is directly stimulated by gastrin. However, because the proliferation of progenitor cells cannot be directly stimulated (so far there has been no gastrin receptor demonstrated on these proliferating cells), some indirect mechanisms must be involved. Enterochromaffin-like and parietal cells are only two types of cells that have demonstrated a strong gene expression of the gastrin receptor. Furthermore, they secrete several growth factors, such as Reg protein, heparin-binding epidermal growth factor-like growth factor (HB-EGF) and amphiregulin (AR). Reg protein production by ECL cells, as well as HB-EGF and AR production by parietal cells, is stimulated by gastrin and these growth factors are potent trophic agents of progenitor cells in the neck zone of the gastric fundic mucosa. Accordingly, gastrin may stimulate the proliferation of gastric mucosal cells indirectly via these growth factors in addition to its direct trophic effect on ECL cells.

Altered influence of CCK-B/gastrin receptors on HDC expression in ECL cells after neoplastic transformation

Gastrin is one of the main factors controlling enterochromaffin-like (ECL) cell endocrine function and growth. Long-standing hypergastrinemia may give rise to ECL cell carcinoids in the gastric corpus in man and in experimental models. We have analysed the expression and function of CCK-B/gastrin receptors in normal ECL cells and in ECL cell tumours (gastric carcinoids) of the African rodent Mastomys natalensis. Hypergastrinemia induced by short-term (5 days) histamine2-receptor blockade (loxtidine) resulted in increased histidine decarboxylase (HDC) mRNA expression in the gastric oxyntic mucosa. This increase was significantly and dose-dependently reversed by selective CCK-B/gastrin receptor blockade (YM022). Long-term (12 months) hypergastrinemia, induced by histamine2-receptor blockade, gave rise to ECL cell carcinoids in the gastric oxyntic mucosa. CCK-B/gastrin receptor mRNA was only slightly elevated while HDC mRNA expression was eight-fold elevated in ECL cell carcinoids and was not influenced by CCK-B/gastrin receptor blockade. Thus CCK-B/gastrin receptor blockade of hypergastrinemic animals reduces the HDC mRNA expression in normal mucosa but not in ECL cell carcinoids. These results demonstrate that HDC mRNA expression in neoplastic ECL cells is not controlled by CCK-B/gastrin receptors.

Gastrin stimulates the growth of gastric pit with less-differentiated features

Gastrin stimulates the growth of gastric mucosa by increasing mostly its glandular region but is not known to induce the growth of a pit region where its major constituent cells, gastric surface mucous (GSM) cells, turn over rapidly. To investigate the effect of gastrin on GSM cells, we generated hypergastrinemic mice by expressing a human gastrin transgene. We obtained a hypergastrinemic mouse line whose average serum gastrin level is 671 +/- 252 pg/ml (normal level <150 pg/ml). Gastrin-positive cells were found in the fundic mucosa. The gastric mucosa exhibited hypertrophic growth, which was characterized by an elongated pit with an active proliferative zone, but the glandular region containing parietal cells was normal or reduced in size. The GSM cells contained fewer mucous granules than those of control littermates and lost reactivity to the GSM cell-specific cholera toxin beta-subunit lectin. GSM cells along the foveolar region and many mucous neck cells became Alcian blue positive, suggesting the appearance of sialomucin in these cells. We suggest that gastrin stimulates the growth of the proliferative zone of gastric glands, which results in the elongation of the pit region whose GSM cells exhibit less-differentiated features.

Enterochromaffin-like cells, a cellular source of uroguanylin in rat stomach

Uroguanylin is an endogenous peptide ligand for guanylyl cyclase-C, an apical membrane receptor predominantly located in the gastrointestinal epithelium. It regulates intestinal and renal fluid and electrolyte transport through the second messenger, cyclic GMP. Uroguanylin messenger RNA and the peptide are present in rat stomach, but the cellular source has not been identified. We separated gastric mucosal cells by size into seven fractions (F1-F7) and enriched endocrine cells into F1-F3 using counterflow elutriation. Uroguanylin messenger RNA and peptide were found in F1-F3 by Northern blot analysis and an RIA specific for rat uroguanylin. Uroguanylin-producing cells were identified as endocrine cells by immunocytochemical methods using antisera for uroguanylin, prouroguanylin, and chromogranin A, as well as by in situ hybridization cytochemistry. Double-staining showed that uroguanylin and histamine are colocalized in enterochromaffin-like (ECL) cells that release histamine, leading to the stimulation of gastric acid secretion from parietal cells. Uroguanylin is synthesized in ECL cells. These findings should contribute to elucidating the physiological functions of ECL cells and the cyclic GMP-mediated gastric ion transport mechanism.

Pharmacological analysis of CCK2 receptor antagonists using isolated rat stomach ECL cells

1. Gastrin stimulates rat stomach ECL cells to secrete histamine and pacreastatin, a chromogranin A (CGA)-derived peptide. The present report describes the effect of nine cholecystokinin2 (CCK2) receptor antagonists and one CCK1 receptor antagonist on the gastrin-evoked secretion of pancreastatin from isolated ECL cells. 2. The CCK2 receptor antagonists comprised three benzodiazepine derivatives L-740,093, YM022 and YF476, one ureidoacetamide compound RP73870, one benzimidazole compound JB 93182, one ureidoindoline compound AG041R and three tryptophan dipeptoids PD 134308 (CI988), PD135158 and PD 136450. The CCK1 receptor antagonist was devazepide. 3. A preparation of well-functioning ECL cells (approximately 80% purity) was prepared from rat oxyntic mucosa using counter-flow elutriation. The cells were cultured for 48 h in the presence of 0.1 nM gastrin; they were then washed and incubated with antagonist alone or with various concentrations of antagonist plus 10 nM gastrin (a maximally effective concentration) for 30 min. Gastrin dose-response curves were constructed in the absence or presence of increasing concentrations of antagonist. The amount of pancreastatin secreted was determined by radioimmunoassay. 4. The gastrin-evoked secretion of pancreastatin was inhibited in a dose-dependent manner. YM022, AG041R and YF476 had IC50 values of 0.5, 2.2 and 2.7 nM respectively. L-740,093, JB93182 and RP73870 had IC50 values of 7.8, 9.3 and 9.8 nM, while PD135158, PD136450 and PD134308 had IC50 values of 76, 135 and 145 nM. The CCK1 receptor antagonist devazepide was a poor CCK2 receptor antagonist with an IC50 of about 800 nM. 5. YM022, YF476 and AG041R were chosen for further analysis. YM022 and YF476 shifted the gastrin dose-response curve to the right in a manner suggesting competitive antagonism, while the effects of AG041R could not be explained by simple competitive antagonism. pK(B) values were 11.3 for YM022, 10.8 for YF476 and the apparent pK(B) for AG041R was 10.4.

Reversibility of cholecystokinin-B/gastrin receptor blockade: a study of the gastrin-ECL cell axis in the rat

Gastrin acts via cholecystokinin-B/gastrin receptors to control histamine- and chromogranin A-producing ECL cells, which constitute the quantitatively predominant endocrine cell population in the acid-producing part of the rat stomach. Cholecystokinin-B receptor blockade is known to suppress the activity of ECL cells and to prevent their ability to respond to gastrin stimulation. The present study examines the reversibility of long-standing cholecystokinin-B receptor blockade of ECL cells. YM022, a potent and selective cholecystokinin-B receptor antagonist, was administered in a maximally effective dose by continuous subcutaneous infusion for 4 weeks (via osmotic minipumps). The resulting receptor blockade was manifested in elevated serum gastrin concentration (due to the ensuing acid inhibition), while the serum pancreastatin concentration, oxyntic mucosal histidine decarboxylase activity, histidine decarboxylase- and chromogranin A- mRNA levels and histamine and pancreastatin concentrations were lowered. After withdrawal of YM022, all these parameters returned to normal after varying lengths of time. The serum gastrin concentration and the oxyntic mucosal histidine decarboxylase activity returned to normal within a week after termination of treatment. The serum pancreastatin concentration and the mucosal histidine decarboxylase- and chromogranin A-mRNA levels returned to normal within 2 weeks of drug withdrawal. The mucosal pancreastatin and histamine concentrations remained unchanged for about a week before gradually returning to control levels within the next two weeks. Hence, the various effects of cholecystokinin-B receptor blockade of the ECL cells are fully reversible within 1-3 weeks of drug withdrawal.

CCK2 receptor antagonists: pharmacological tools to study the gastrin-ECL cell-parietal cell axis

Gastrin-recognizing CCK2 receptors are expressed in parietal cells and in so-called ECL cells in the acid-producing part of the stomach. ECL cells are endocrine/paracrine cells that produce and store histamine and chromogranin A (CGA)-derived peptides, such as pancreastatin. The ECL cells are the principal cellular transducer of the gastrin-acid signal. Activation of the CCK2 receptor results in mobilization of histamine (and pancreastatin) from the ECL cells with consequent activation of the parietal cell histamine H2 receptor. Thus, release of ECL-cell histamine is a key event in the process of gastrin-stimulated acid secretion. The oxyntic mucosal histidine decarboxylase (HDC) activity and the serum pancreastatin concentration are useful markers for the activity of the gastrin-ECL cell axis. Powerful and selective CCK2 receptor antagonits have been developed from a series of benzodiazepine compounds. These agents are useful tools to study how gastrin controls the ECL cells. Conversely, the close control of ECL cells by gastrin makes the gastrin-ECL cell axis well suited for evaluating the antagonistic potential of CCK2 receptor antagonists with the ECL-cell HDC activity as a notably sensitive and reliable parameter. The CCK2 receptor antagonists YF476, YM022, RP73870, JB93182 and AG041R were found to cause prompt inhibition of ECL-cell histamine and pancreastatin secretion and synthesis. The circulating pancreastatin concentration is raised, was lowered when the action of gastrin on the ECL cells was blocked by the CCK2 receptor antagonists. These effects were associated with inhibition of gastrin-stimulated acid secretion. In addition, sustained receptor blockade was manifested in permanently decreased oxyntic mucosal HDC activity, histamine concentration and HDC mRNA and CGA mRNA concentrations. CCK2 receptor blockade also induced hypergastrinemia, which probably reflects the impaired gastric acid secretion (no acid feedback inhibition of gastrin release). Upon withdrawal of the CCK2 receptor antagonists, their effects on the ECL cells were readily reversible. In conclusion, gastrin mobilizes histamine from the ECL cells, thereby provoking the parietal cells to secrete acid. While CCK2 receptor blockade prevents gastrin from evoking acid secretion, it is without effect on basal and vagally stimulated acid secretion. We conclude that specific and potent CCK2 receptor antagonists represent powerful tools to explore the functional significance of the ECL cells.

Regenerating gene protein may mediate gastric mucosal proliferation induced by hypergastrinemia in rats

BACKGROUND & AIMS

Regenerating gene (Reg) has been isolated from rat regenerating pancreatic islets, and Reg protein is mitogenic to islet cells. We have recently shown that Reg gene and Reg protein are expressed in gastric enterochromaffin-like (ECL) cells. This study aimed to clarify whether gastrin enhances Reg protein production in ECL cells and whether Reg protein is mitogenic to gastric mucosal cells.

METHODS

Reg gene expression in response to acute and chronic hypergastrinemia was investigated in rats. Immunohistochemical studies, Northern blotting, and in situ hybridization were performed to investigate the expression of Reg protein and Reg gene. The direct effect of gastrin on Reg gene expression was investigated using isolated ECL cells, and the trophic effect of Reg protein on cultured gastric epithelial cells was assessed by [3H]thymidine uptake.

RESULTS

Both chronic hypergastrinemia and short-term gastrin administration stimulated Reg gene expression and Reg protein production in fundic mucosa. Reg gene expression was also augmented in isolated ECL cells after incubation with rat gastrin. Reg protein was mitogenic to cultured rat gastric epithelial cells.

CONCLUSIONS

Gastrin stimulates the production of Reg protein in gastric ECL cells, which may be involved in the gastrin-induced gastric mucosal cell growth.

Comparison of the signal transduction pathways activated by gastrin in enterochromaffin-like and parietal cells

BACKGROUND & AIMS

Gastrin stimulates acid secretion from parietal cells and histamine release from enterochromaffin-like (ECL) cells through identical gastrin receptors. However, gastrin has been shown to have a trophic effect only on ECL cells. The aim of this study was to compare gastrin-induced signal transduction pathways in the ECL and parietal cells of Mastomys natalensis, an African rodent.

METHODS

Both ECL and parietal cells were isolated from the gastric mucosa of M. natalensis, and intracellular signal transduction events in response to gastrin were investigated.

RESULTS

Gastrin elicited histamine release from ECL cells and acid secretion from parietal cells in association with enhanced inositol phospholipid turnover. Although gastrin increased [3H]thymidine incorporation into ECL cells, it had no effect on parietal cells. Moreover, tyrosine phosphorylation and activation of mitogen-activated protein (MAP) kinase as well as c-fos and c-jun gene expression were augmented only in ECL cells. In addition, gastrin increased the formation of guanosine triphosphate-Ras with a simultaneous decrease in guanosine diphosphate-Ras levels in ECL but not in parietal cells.

CONCLUSIONS

Although gastrin receptors are present in both ECL and parietal cells, they activate the Ras-MAP kinase pathway only in ECL cells.

Rat gastric mucosal cells express ICAM-1 and proinflammatory cytokines during indomethacin-induced mucosal injury

Adhesion molecules and cytokines are known to be involved in the formation of acute gastric mucosal injury. However, it is not clear whether the gastric mucosal cells express these molecules and modulate the inflammation. To clarify whether gastric mucosal cells express intercellular adhesion molecule-1 (ICAM-1) and proinflammatory cytokines (tumor necrosis factor-alpha (TNF-alpha), interleukin-1-alpha (IL-1-alpha), and cytokine-induced neutrophil chemoattractant-2-beta (CINC-2-beta)) in the formation of gastric mucosal injury, we have used rat indomethacin-induced gastric mucosal lesions as an in vivo model. The gene expression of all cytokines and ICAM-1 increases at the early stages of indomethacin-induced gastritis (TNF-alpha and IL-1-alpha gene expression began to increase earlier than that of ICAM-1 and CINC-2-beta) and can mainly be detected in the gastric epithelial layer. To further identify the source of those molecules, the epithelial cells were separated into seven fractions according to their sizes by a counterflow elution. ICAM-1 and CINC-2-beta gene expressions are particularly enhanced in the middle-sized cell fractions that are rich in gastric mucous-producing cells. The effect of TNF-alpha or IL-1-alpha on the gene expression of ICAM-1 and cytokines was examined by using RGM-1 cells as a model for gastric mucosal cells. RGM-1 cells show an augmented ICAM-1 and proinflammatory cytokine expression in response to TNF-alpha or IL-1-alpha stimulation. Moreover, immunohistochemical staining also reveals an increase in ICAM-1 and CINC protein production in RGM-1 cells in response to TNF-alpha stimulation. We conclude that gastric mucosal cells express various cytokines and an adhesion molecule during the formation of acute gastric mucosal injury and that they may modulate the inflammation.

Sustained cholecystokinin-B/gastrin receptor blockade does not impair basal or histamine-stimulated acid secretion in chronic gastric fistula rats

Gastrin is a physiologically important secretagogue. It is thought to stimulate parietal cells indirectly by mobilizing histamine from enterochromaffin-like (ECL) cells in the oxyntic mucosa. Gastrin stimulates the secretory activity and growth of the ECL cells via an action on cholecystokinin-B/gastrin receptors. Acute cholecystokinin-B/gastrin receptor blockade is known to inhibit gastrin-stimulated acid secretion but whether sustained cholecystokinin-B/gastrin receptor blockade will impair basal, gastrin- and histamine-stimulated acid secretion remains uncertain. The present study was designed to study the effect of long-term (4 weeks) cholecystokinin-B/gastrin receptor blockade on basal and stimulated acid secretion in conscious rats. The selective cholecystokinin-B/gastrin receptor antagonist YM022 (3 mumol.kg-1.hr-1) was given to gastric fistula rats by continuous subcutaneous infusion via osmotic minipumps for various times from 2 hr to 4 weeks. Basal, gastrin- and histamine-stimulated acid secretion were examined during and after cessation of treatment. Basal and histamine-stimulated acid secretion was not affected by YM022 during the 4 week period of administration, whereas gastrin-induced acid secretion was inhibited. YM022 induced hypergastrinaemia in freely fed rats but did not affect the serum gastrin level in fasted rats. The serum gastrin concentration and gastrin-induced acid secretion returned to control levels 3-7 days after termination of YM022 administration.

Effect of aging on gastrin receptor gene expression in rat stomach

Gastrin is a pivotal humoral factor which regulates gastric acid secretion through its receptors. There is no report, however, concerning the age-related changes of gastrin receptor gene expression in the stomach. Northern blot analysis and in situ hybridization were performed to clarify the changes of gastrin receptor expression during the aging. In situ hybridization clarified that gastrin receptor mRNA was expressed mainly in enterochromaffin-like (ECL) cells in adult rat gastric mucosa. With aging, gastrin receptor gene expression in the stomach increased with the concomitant increase in histidine decarboxylase mRNA. Since histidine decarboxylase is a marker of gastric ECL cells, the augmented gastrin receptor mRNA in aged rats may be caused by the increased ECL cells in gastric mucosa during the aging.

Evaluation of three novel cholecystokinin-B/gastrin receptor antagonists: a study of their effects on rat stomach enterochromaffin-like cell activity

Gastrin stimulates rat stomach enterochromaffin-like (ECL) cells via activation of cholecystokinin-B/gastrin receptors. The stimulation is manifested in the activation of the histamine-forming enzyme histidine decarboxylase and in the secretion of histamine and pancreastatin, a chromogranin A-derived peptide. We have examined the short-term effects of three novel cholecystokinin-B/gastrin receptor antagonists (YF476, JB93182 and AG041R) on the ECL cells in intact fasted rats. The drugs and/or gastrin were infused intravenously for 3 hr and the oxyntic mucosal histidine decarboxylase activity and the serum pancreastatin concentration were measured. We also studied the effects of the three drugs on gastric emptying in mice, a cholecystokinin-A receptor-mediated response. YF476, JB93182 and AG041R antagonized the gastrin-evoked histidine decarboxylase activation in a dose-dependent manner. YF476, JB93182 and AG041R induced maximal inhibition at 0.03, 0.1 and 0.1 mumol kg-1 hr-1, respectively; the corresponding ID50 values were 0.002, 0.008, and 0.01 mumol kg-1 hr-1. YF476 was selected for further analysis. It produced a rightward shift of the gastrin dose-response curve, consistent with competitive inhibition. Moreover, it antagonized the omeprazole-evoked histidine decarboxylase activation and the gastrin- and omeprazole-induced rise in the circulating pancreastatin concentration. None of the three drugs tested inhibited gastric emptying or prevented the cholecystokinin-8s-induced inhibition of gastric emptying at the doses tested. The results show that YF476, JB93182 and AG041R are potent and selective cholecystokinin-B/ gastrin receptor antagonists, and that YF476 is 4-5 times more potent than JB93182 and AG041R.

Functional impairment of the individual rat stomach ECL cell in response to sustained hypergastrinemia

ECL cells in the oxyntic mucosa secrete histamine and pancreastatin in response to gastrin. The present study examined gastrin-evoked ECL-cell responses over a 10-week time span in terms of individual ECL cells and unit ECL cell volume. Rats were treated with omeprazole (400 micromol/kg per day orally). The concentrations of gastrin and pancreastatin in serum and of histamine and pancreastatin in the oxyntic mucosa were measured as was the activity of the oxyntic mucosal histidine decarboxylase (HDC). The ECL cells were visualized by immunostaining of histamine and examined by electron microscopy. The total ECL cell number and volume, and the mean ECL cell diameter and volume were determined. The HDC, chromogranin A (CGA) and cholecystokinin-B (CCK-B) receptor mRNA concentrations were determined. In terms of individual ECL cells and unit ECL cell volume, the serum pancreastatin concentration, the oxyntic mucosal histamine content, HDC activity, and HDC, CGA and CCK-B receptor mRNA contents increased slowly at first and then leveled off or started to decline after 2 weeks. After 10 weeks all ECL-cell parameters (expressed per unit ECL cell volume) were back to or approaching the starting value. In conclusion, sustained hypergastrinemia first activates each individual ECL cell (with a peak after 1-2 weeks) and then causes gradual functional impairment, the activity returning towards the pre-stimulation level.

Gastrin receptor expression and function during rapid transformation of the enterochromaffin-like cells in an African rodent

The enterochromaffin-like cell (ECL) cells of the stomach are principally regulated by gastrin via a gastrin/CCK(B) receptor (G[R]) which modulates both histamine secretion and cell proliferation. In the African rodent (mastomys) hypergastrinemia generated by the histamine-2 receptor antagonist (loxtidine) results in ECL cell hyperplasia and neoplasia at 8 and 16 weeks respectively. The expression, structure and function of the G(R) during transformation is however unknown. We utilized a pure (approximately 90%) preparation of ECL cells to evaluate alterations in the G(R) utilizing immunocytochemistry, Western blot analysis, reverse transcription polymerase chain reaction (RT-PCR), 5-bromo-2-deoxyuridine uptake and phosphorylation site analysis. Although the expression of ECL cell G(R) was upregulated at both mRNA (PT-PCR) and protein (Western analysis) level, its affinity to gastrin was decreased in the hyperplastic phase and lost during transformation. The coding sequence of the G(R) of mastomys tumor ECL cells was identical to that of normal ECL cells, parietal cells and the brain. However, the mRNA sequence of the third introcytoplasmic loop of the G(R) was significantly different to other species. In addition, the G(R) exhibited phosphorylation site on serine residue(s). We have thus noted a direct correlation between hypergastrinemia and G(R) alteration and function during ECL cell transformation. It is possible that the unique mastomys gastrin receptor mediated ECL cell transformation involves the novel phosphorylation sites and a divergence in the introcytoplasmic domain.

Time-course of deactivation of rat stomach ECL cells following cholecystokinin B/gastrin receptor blockade

1 The so-called enterochromaffin-like (ECL) cells constitute 65-75% of the endocrine cells in the acid-producing part of the rat stomach. They produce and secrete histamine and pancreastatin, a chromogranin A (CGA)-derived peptide, in response to gastrin, Cholecystokinin (CCK)B/gastrin receptor blockade is known to suppress their activity. 2 We have examined the time course of the deactivation of the ECL cells following treatment with the selective CCKB receptor antagonists RP73870 and YM022. The drugs were given by continuous subcutaneous infusion for a time span of 1 h to 3 weeks and the serum gastrin concentration and various ECL cell parameters were measured (oxyntic mucosal histidine decarboxylase (HDC) activity, histamine and pancreastatin concentrations, HDC mRNA and CGA mRNA levels, and circulating pancreastatin concentration). 3 The two antagonists caused a prompt and dramatic decline in the oxyntic mucosal HDC activity and HDC mRNA level. The HDC activity started to decline after 1-2 h, was reduced by 60-70% after 6 h and was maximally suppressed (80-90%) after 24-48 h. The HDC mRNA level was reduced after 12 h and was at about 20% of the pretreatment level after 2-4 days of infusion. The ECL cell histamine concentration was lowered by about 50% after 7-10 days. 4 RP73870 and YM022 lowered the serum pancreastatin concentration and the oxyntic mucosal CGA mRNA level. The serum pancreastatin concentration was reduced by 40% after 6 h and the reduction was maximal after 2-3 days. A decline in the oxyntic mucosal CGA mRNA level was noted after 12 h with a maximal reduction after 2-4 days of infusion. The ECL cell pancreastatin concentration was reduced by 30-40% after 3 weeks. 5 The infusion of RP73870 and YM022 induced hypergastrinaemia. The serum gastrin concentration started to rise after 2-4 h, there was a 2 fold increase after 6 h and maximal increase (3-4 fold) after 2-3 days of treatment. 6 In conclusion, CCKB/gastrin receptor blockade promptly deactivates the ECL cells. Deactivation, manifested in a greatly reduced HDC activity, was apparent after 1-2 h of the infusion. The serum pancreastatin concentration and the oxyntic mucosal HDC mRNA and CGA mRNA levels were greatly reduced after 1-2 days. The ECL cell concentrations of histamine and pancreastatin declined quite slowly by comparison.

Neurohormonal regulation of histamine and pancreastatin secretion from isolated rat stomach ECL cells

ECL cells are numerous in the acid-producing part of the rat stomach. They are rich in histamine and pancreastatin, a chromogranin A-derived peptide, and they secrete these products in response to gastrin. We have examined how isolated ECL cells respond to a variety of neuromessengers and peptide hormones. Highly purified (85%) ECL cells were collected from rat stomach using repeated counter-flow elutriation and cultured for 48 h before experiments were conducted. The ECL cells responded to gastrin, sulphated cholecystokinin-8 and to high K+ and Ca2+ with the parallel secretion of histamine and pancreastatin. Glycine-extended gastrin was without effect. Forskolin, an activator of adenylate cyclase, induced secretion, whereas isobutylmethylxanthine, a phosphodiesterase inhibitor, raised the basal release without enhancing the gastrin-evoked stimulation. Maximum stimulation with gastrin resulted in the release of 30% of the secretory products. Numerous neuromessengers and peptide hormones were screened for their ability to stimulate secretion and to inhibit gastrin-stimulated secretion. Pituitary adenylate cyclase activating peptide (PACAP)-27 and -38 stimulated secretion of both histamine and pancreastatin with a potency greater than that of gastrin and with the same efficacy. Related peptides, such as vasoactive intestinal peptide, helodermin and helospectin, stimulated secretion with lower potency. The combination of EC100 gastrin and EC50 PACAP produced a greater response than gastrin alone. None of the other neuropeptides or peptide hormones tested stimulated secretion. Serotonin, adrenaline, noradrenaline and isoprenaline induced moderate secretion at high concentrations. Muscarinic receptor agonists did not stimulate secretion, and histamine and selective histamine receptor agonists and antagonists were without effect. This was the case also with GABA, aspartate and glutamate. Somatostatin and galanin, but none of the other agents tested, inhibited gastrin-stimulated secretion. Our results reveal that not only gastrin but also PACAP is a powerful excitant of the ECL cells, that not only somatostatin, but also galanin can suppress secretion, that muscarinic receptor agonists fail to evoke secretion, and that histamine (and pancreastatin) does not evoke autofeedback inhibition.

Cholecystokinin-B/gastrin receptor blockade suppresses the activity of rat stomach ECL cells

Gastrin controls the histamine- and chromogranin A-producing enterochromaffin-like (ECL) cells, the predominant endocrine cell population in the acid-producing part of the rat stomach. They are responsible for most of the circulating pancreastatin, a chromogranin A-derived peptide. The present study examines the ability of two potent and highly selective cholecystokinin-B/gastrin receptor antagonists, RP73870 and YM022, to incapacitate the ECL cells. The two antagonists were given by continuous subcutaneous infusion to otherwise untreated rats and to hypergastrinaemic rats treated with gastrin-17 (continuous subcutaneous infusion) or omeprazole (orally) for 7 days. Several parameters reflecting ECL cell activity were measured: The oxyntic mucosal histidine decarboxylase activity, the histamine concentration, the histidine decarboxylase mRNA and chromogranin A mRNA concentrations, and the serum pancreastatin concentration. In addition, the serum gastrin concentration was measured. RP73870 and YM022 greatly lowered the oxyntic mucosal histidine decarboxylase activity and the histidine decarboxylase mRNA and chromogranin A mRNA concentrations, and also reduced the oxyntic mucosal histamine concentration and the serum pancreastatin concentration. Moreover, they raised the serum gastrin concentration. With respect to blockade of histidine decarboxylase activity, 1.0 mumol.kg-1.hr-1 was an almost maximally effective dose for both RP73870 and YM022. The corresponding ID50 values were 0.04 and 0.05 mumol.kg-1.hr-1. RP73870 and YM022 inhibited the hypergastrinaemia-evoked rise in all ECL-cell parameters. The results suggest that sustained cholecystokinin-B/gastrin receptor blockade causes lasting deactivation of the ECL cells.

Induction of heparin binding epidermal growth factor-like growth factor and amphiregulin mRNAs by gastrin in the rat stomach

The present study was designed to investigate whether heparin-binding epidermal growth factor-like growth factor and its related peptides are expressed in response to gastrin in rat stomach. Rat gastrin-17I (2.5 nmol/kg/hour) or gastrin-17I plus gastrin receptor antagonist, L-740,093 (2.0 mg/kg/hour), was injected intravenously into male Sprague-Dawley rats. RNA was extracted from oxyntic mucosa, and heparin-binding epidermal growth factor-like growth factor and related peptide gene expression was estimated using a ribonuclease protection assay. The level of transforming growth factor-alpha mRNA did not change at any time point during the experiment. In contrast, the levels of heparin-binding epidermal growth factor-like growth factor and amphiregulin mRNA were significantly increased within 3 hours following gastrin infusion and reached maximum levels 6 and 12 hours later, respectively. Continuous infusion of gastrin significantly increased oxyntic mucosal proliferation. Gastrin receptor antagonist significantly inhibited gastrin-induced heparin-binding epidermal growth factor-like growth factor and amphiregulin gene expression and gastrin-induced oxyntic mucosal proliferation. These findings indicate that heparin-binding epidermal growth factor-like growth factor and amphiregulin genes are induced by gastrin and that they play a role in the trophic action of gastrin on oxyntic mucosa.

Developmental gene expression of gastrin receptor in rat stomach

Gastrin, which is present in fetal plasma, may have important roles in the development of gastric mucosa, since it is not only a potent stimulator of gastric acid secretion but also a growth promoting factor. Gastrin regulates various cellular functions via its receptors on cell membrane. Therefore, in order to elucidate a role for gastrin in the development of gastrointestinal system during gestation, Northern blot analysis was performed. The results of the study suggested that gastrin receptor is mainly present on parietal cells. Furthermore, proton pump and gastrin receptor gene expressions in parietal cells were strongly stimulated by the administration of exogenous gastrin. In conclusion, gastrin may be involved in the developmental change of parietal cells through its receptors.

Distribution of prostaglandin E receptors in the rat gastrointestinal tract

AIMS

In order to study the role of prostaglandin in the regulation of the gastrointestinal functions, gene expression of prostaglandin receptors along the rat gastrointestinal tracts were investigated.

METHODS

Rats were used for the study. The combination of counterflow elutriation separation of mucosal cells and Northern blot analysis was used to detect the gene expression of prostaglandin receptors in gastrointestinal tracts.

RESULTS

In small intestine and colon, prostaglandin E2 EP1 and EP3 receptor mRNAs were mainly localized in the deeper intestinal wall containing muscle layers. EP4 receptor gene expression, on the other hand, was detected in the intestinal mucosal layer. In the stomach, EP1 mRNA was detected in gastric muscle layers, whereas EP3 and EP4 receptor gene expression was mainly present in the gastric mucosal layer containing epithelial cells. In gastric epithelial cells, parietal cells were found to have both EP3 and EP4 receptors. At lower concentrations, prostaglandin E2 inhibited gastric acid secretion by parietal cells probably through EP4 receptors. At higher concentrations, however, it stimulated it. On the other hand, mucous cells possessed only EP4 receptor mRNA.

CONCLUSIONS

Thus, it is suggested that prostaglandin E2 modulates gastrointestinal functions through at least three different prostaglandin receptors (EP1, EP3, and EP4), each of which has a distinct contribution in the gastrointestinal tract.

Regenerating gene expression in normal gastric mucosa and indomethacin-induced mucosal lesions of the rat

Regenerating (reg) gene expression was tested in rat gastrointestinal mucosa to investigate the role played by this gene in the healing of mucosal lesions. Expression of reg mRNA was higher in the stomach than in any other region of the gastrointestinal tract. The gastric cells that expressed reg mRNA were located in the deepest mucosal layer and were small in diameter. In an injured state following indomethacin treatment, reg gene expression was markedly augmented, accompanied by an increase of c-fos expression and healing of the mucosal lesions. These results suggest a role of the reg gene in the healing of gastrointestinal mucosal lesions.

Evaluation of the specificity and potency of a series of cholecystokinin-B/gastrin receptor antagonists in vivo

The potency and specificity of five proposed cholecystokinin-B receptor antagonists, YM022, RP73870, L-740,093, L-365,260 and LY288513, were studied in rats and mice. Gastrin activates rat stomach histidine decarboxylase via cholecystokinin-B/gastrin receptors. To examine cholecystokinin-B receptor-mediated effects of the five drugs, they were infused intravenously to fasted rats and the histidine decarboxylase activity in the oxyntic mucosa was determined. While YM022, RP73870, L-740,093 and L-365,260 failed to activate histidine decarboxylase, they dose-dependently antagonized the gastrin-induced histidine decarboxylase activation. LY288513 had no effect in the doses tested. The maximal inhibitory effect of L-365,260, L-740,093, RP73870 and YM022 on histidine decarboxylase, activated by the intravenous infusion of an ED50 does of gastrin (0.4 nmoles/kg/hr), was seen at doses of 3, 0.3, 0.1 and 0.1 mumoles/kg/hr, respectively; the corresponding ID50 values were 0.4, 0.02, 0.007 and 0.004 mumoles/kg/h. In a follow-up study, YM022 and RP73870 were found to produce a rightward shift of the gastrin dose-response curve, which is consistent with competitive inhibition. The effect of the five drugs on a cholecystokinin-A receptor-mediated response was examined by studying gastric emptying in mice. Cholecystokinin-8s, given by a subcutaneous bolus injection, dose-dependently inhibits gastric emptying. The specific cholecystokinin-A receptor antagonist devazepide (given intravenously as a bolus injection) antagonized the effect of cholecystokinin-8s in a dose-dependent manner, with an ID50 value of 28 nmoles/kg. None of the drugs inhibited the gastric emptying or prevented the cholecystokinin-8s-induced effect at the doses tested. The results indicate that YM022, RP73870, L-740,093 and L-365,260 act as cholecystokinin-B receptor antagonists in vivo, being without measurable agonistic activity. Furthermore, they do not interact with cholecystokinin-A receptors at te doses tested. Among the cholecystokinin-B receptor antagonists studied YM022 and RP73870 are superior, the rank order of potency being YM022 > or = RP73870 > L-740,093 > L-365,260.

Cholecystokinin-A and cholecystokinin-B/gastrin receptor mRNA expression in the gastrointestinal tract and pancreas of the rat and man. A polymerase chain reaction study

BACKGROUND

Gastrin and cholecystokinin (CCK) are thought to exert trophic effects on the gastrointestinal tract and pancreas. Two types of receptors have been cloned, CCK-A and CCK-B/ gastrin. We have examined the occurrence of CCK-A and CCK-B receptor mRNA in the brain, digestive tract, pancreas, and kidney of the rat and man by Northern blot and reverse transcribed polymerase chain reaction (RT-PCR).

METHODS

Total RNA was isolated from rat tissues and reverse transcribed into cDNA. cDNA from brain, kidney, and pancreas of the rat and man and from human whole stomach were commercially available. Northern blot and a PCR technique based on Taq polymerase-antibody interaction and using CCK-A and CCK-B receptor-specific primers, followed by Southern blot analysis, were the methods used.

RESULTS

By means of Northern blots, CCK-A receptor mRNA was detected in rat fundus mucosa and pancreas but not in the remaining GI tract or brain. By means of RT-PCR, CCK-A receptor mRNA was demonstrated in the brain and the mucosa of the fundus, antrum, duodenum, and colon, kidney, pancreas and pancreatic islets. CCK-B receptor mRNA was detected by Northern blot analysis in the brain and the fundus mucosa but not in the rest of the digestive tract and not in the pancreas, pancreatic islets, or kidney. By RT-PCR, expression of CCK-B receptor mRNA could also be detected in antrum mucosa. In man, CCK-A receptor mRNA was detected in the brain, stomach, pancreas, and kidney, whereas CCK-B receptor mRNA was found in the brain, stomach, and pancreas but not in the kidney. Cloning and DNA-sequence analysis of the PCR-amplified rat and human CCK-A and CCK-B receptor DNA fragments, which cover the protein-encoding regions of the intracellular loop C3, showed complete sequence homology as compared with published rat and human sequences.

CONCLUSIONS

It appears unlikely that CCK will have effects in the ileum, at least not effects mediated by CCK-A receptors. It also appears unlikely that physiologic concentrations of gastrin in the circulation will promote growth (or exert other effects) in the pancreas, duodenum, ileum, and colon, since CCK-B receptor mRNA is not expressed or is poorly expressed in these tissues.

Cholecystokinin-B receptor ligands of the dipeptoid series act as agonists on rat stomach histidine decarboxylase

BACKGROUND & AIMS

The effect of gastrin on the enterochromaffin-like cells in the rat stomach is mediated by cholecystokinin (CCK)-B receptors and manifested as activation of histidine decarboxylase (HDC). The dipeptoids PD 136450, PD 135158, and PD 134308 are thought to be selective CCK-B receptor antagonists. The effects of the dipeptoids and of gastrin-17 and sulfated CCK-8 on rat stomach HDC activity were examined.

METHODS

Drugs were infused intravenously or subcutaneously to fasted rats, and the HDC activity was determined.

RESULTS

The dipeptoids activated HDC with maximal responses (50%-60% of the maximal response to gastrin) at 1 mumol.kg-1.h-1. Rat gastrin-17 activated HDC maximally at 3 nmol.kg-1.h-1, and sulfated CCK-8 produced maximal response at 20 nmol.kg-1.h-1. The CCK-B receptor antagonist L-365,260 inhibited the HDC activation induced by gastrin, sulfated CCK-8, or the dipeptoids. The dipeptoids did not inhibit the gastrin-induced HDC activation.

CONCLUSIONS

Gastrin, sulfated CCK-8, and the dipeptoids activated rat stomach HDC. L-365,260 but not devazepide inhibited the HDC activation. Thus, the dipeptoids, which failed to inhibit the gastrin-induced HDC activation, act as CCK-B receptor agonists and not as antagonists. It is important to recognize this to ensure appropriate interpretation of data obtained with these drugs.