Gastric enterochromaffin-like cell hyperplasia and neoplasia in the rat: an indirect effect of the histamine H2-receptor antagonist, BL-6341

Histopathological and Immunohistochemical Characterization of Methyl Eugenol-induced Nonneoplastic and Neoplastic Neuroendocrine Cell Lesions in Glandular Stomach of Rats

Methyl eugenol induces neuroendocrine (NE) cell hyperplasia and tumors in F344/N rat stomach. Detailed histopathological and immunohistochemical (IHC) characterization of these tumors has not been previously reported. The objective of this study was to fill that data gap. Archived slides and paraffin blocks were retrieved from the National Toxicology Program Archives. NE hyperplasias and tumors were stained with chromogranin A, synaptophysin, amylase, gastrin, H(+)/K(+) adenosine triphosphatase (ATPase), pepsinogen, somatostatin, and cytokeratin 18 (CK18) antibodies. Many of the rats had gastric mucosal atrophy, due to loss of chief and parietal cells. The hyperplasias and tumors were confined to fundic stomach, and females were more affected than the males. Hyperplasia of NE cells was not observed in the pyloric region. Approximately one-third of the females with malignant NE tumors had areas of pancreatic acinar differentiation. The rate of metastasis was 21%, with liver being the most common site of metastasis. Immunohistochemically, the hyperplasias and tumors stained consistently with chromogranin A and synaptophysin. Neoplastic cells were also positive for amylase and CK18 and negative for gastrin, somatostatin, H(+)/K(+) ATPase, and pepsinogen. Metastatic neoplasms histologically similar to the primary neoplasm stained positively for chromogranin A and synaptophysin. Based on the histopathological and IHC features, the neoplasms appear to arise from enterochromaffin-like cells.

Consensus diagnoses and mode of action for the formation of gastric tumors in rats treated with the chloroacetanilide herbicides alachlor and butachlor

A panel of pathologists (Panel) was formed to evaluate the pathogenesis and human relevance of tumors that developed in the fundic region of rat stomachs in carcinogenicity and mechanistic studies with alachlor and butachlor. The Panel evaluated stomach sections stained with hematoxylin and eosin, neuron-specific enolase, and chromogranin A to determine the presence and relative proportion of enterochromaffin-like (ECL) cells in the tumors and concluded all tumors were derived from ECL cells. Biochemical and pathological data demonstrated the tumor formation involved a nongenotoxic threshold mode of action (MOA) initially characterized by profound atrophy of the glandular fundic mucosa that affected gastric glands, but not surface epithelium. This resulted in a substantial loss of parietal cells and a compensatory mucosal cell proliferation. The loss of parietal cells caused a marked increase in gastric pH (hypochlorhydria), leading to sustained and profound hypergastrinemia. The mucosal atrophy, together with the increased gastrin, stimulated cell growth in one or more ECL cell populations, resulting in neoplasia. ECL cell autocrine and paracrine effects led to dedifferentiation of ECL cell tumors. The Panel concluded the tumors develop via a threshold-dependent nongenotoxic MOA, under conditions not relevant to humans.

Successful drug development despite adverse preclinical findings part 1: processes to address issues and most important findings

Unexpected adverse preclinical findings (APFs) are not infrequently encountered during drug development. Such APFs can be functional disturbances such as QT prolongation, morphological toxicity or carcinogenicity. The latter is of particular concern in conjunction with equivocal genotoxicity results. The toxicologic pathologist plays an important role in recognizing these effects, in helping to characterize them, to evaluate their risk for man, and in proposing measures to mitigate the risk particularly in early clinical trials. A careful scientific evaluation is crucial while termination of the development of a potentially useful drug must be avoided. This first part of the review discusses processes to address unexpected APFs and provides an overview over typical APFs in particular classes of drugs. If the mode of action (MoA) by which a drug candidate produces an APF is known, this supports evaluation of its relevance for humans. Tailor-made mechanistic studies, when needed, must be planned carefully to test one or several hypotheses regarding the potential MoA and to provide further data for risk evaluation. Safety considerations are based on exposure at no-observed-adverse-effect levels (NOAEL) of the most sensitive and relevant animal species and guide dose escalation in clinical trials. The availability of early markers of toxicity for monitoring of humans adds further safety to clinical studies. Risk evaluation is concluded by a weight of evidence analysis (WoE) with an array of parameters including drug use, medical need and alternatives on the market. In the second part of this review relevant examples of APFs will be discussed in more detail.

Gastric achlorhydria in H/K-ATPase-deficient (Atp4a(-/-)) mice causes severe hyperplasia, mucocystic metaplasia and upregulation of growth factors

BACKGROUND

Gastric neoplasia is common in humans, yet controversy remains over contributions of chronic achlorhydria, gastrinemia and hyperplasia, to cancer risk. To study this, mice lacking the gastric H/K-ATPase (Atp4a(-/-) mice) were used to determine whether chronic loss of acid secretion, with attendant hypergastrinemia, predisposes to cancer phenotype.

METHODS

Atp4a(-/-) and Atp4a(+/+) mice, paired for age and gender, were examined at 3, 8, 12 and 20 months for histopathology, and for expression of the trefoil factor family (TFF)1-3, Reg IIIbeta, gamma and delta, osteopontin, CD44, chromogranin A, Crp-ductin, and galectin, all of which are important in cell growth.

RESULTS

By 8 months, the glandular stomach of the Atp4a(-/-) mice doubled in weight and thickness, and several modulators of growth were increased. Female Atp4a(-/-) mice were more hyperplastic than Atp4a(-/-) males at 12 and 20 months. By 1 year, severe mucocystic hyperplasia, incomplete intestinal metaplasia, ciliated metaplasia, a shift in mucins from neutral to acidic, and inflammation were widespread. Cells in the mucus pit zone developed a pyloric-type appearance, containing large hyaline-like, periodic acid-Schiff (PAS)-negative/alcian blue-negative inclusions. But critical characteristics of gastric neoplasia, such as nuclear atypia, invasion into the muscularis mucosa, and metastases were absent. In Atp4a(-/-) mice, chromogranin A and histidine decarboxylase, RegIIIgamma and delta, TFF3, osteopontin and CD44 were upregulated while Reg IIIbeta, and TFF1 were reduced.

CONCLUSIONS

Chronic achlorhydria and hypergastrinemia in aged Atp4a(-/-) mice produced progressive hyperplasia, mucocystic and incomplete intestinal metaplasia, and the upregulation of growth factors without histological evidence of neoplasia.

Unexpected tumour findings in lifetime rodent bioassay studies--what to do?

Currently, the majority of substances tested in lifetime bioassays in rodents are not mutagenic and, therefore, at the most weakly carcinogenic, generally by epigenetic mechanisms. It thus appears obvious that only marginal increases of tumour incidences can be expected in lifetime bioassays and that, therefore, every aspect of a potential carcinogenic effect must be thoroughly evaluated. This paper describes a series of key factors, which should be looked at in order to exclude that the lifetime bioassay in question is flawed for design, technical or qualification reasons. It also provides some hints whether there is indeed a real effect and not just a variation of the spontaneous tumour incidences. Tumour findings must be seen in the context of the animal model, the pharmcokinetics and pharmcodynamics of the test substance, as well as any other observation in the present or other studies with the test substance, including non-tumour findings and--in particular--potential precursor lesions and effects on feed intake and survival. The possibility that the observed carcinogenic effects may be species-specific and not relevant for man is discussed. It is also important to check what findings are reported with similar substances or substances with the same pharmacological effect. Data from additional investigations on material of the same study and/or mechanistic studies are often needed to support the final risk assessment.

Digestive System 1

This chapter deals with the digestive system. The major and minor salivary glands and their secretions also represent and integral part of the protective mechanism of the oral cavity, and derangement of saliva production may lead to loss of integrity of the oral mucosa. Drug-induced abnormalities of taste sensation are also well-described phenomena occurring in man although human studies are necessary for the detection of these effects. Inflammation of the oral cavity may involve the buccal mucosa, the gingiva (gingivitis), the tongue (glossitis), and the peridontal tissues (peridontitis). Therapeutic agents can induce inflammatory lesions in the tongue. Moreover, a protective layer of mucus, a visco-elastic material containing high molecular weight glycoproteins produced by the major and minor salivary glands, covers the stratified squamous mucosa of the oral cavity. Salivary secretions also possess digestive enzyme activity although in herbivores and carnivores, it is usually low in contrast to high digestive enzyme activity in omnivorous species.

Amphicrine tumor

The term amphicrine refers to cells, and tumors, which show both exocrine and endocrine features. Author s aim was to analyse the characteristics of these neoplasms. 40 suspicious cases were reviewed. Mucin-stains (PAS, diastase-PAS, Stains-all, Alcian-blue), immunohistochemistry (antibodies against Neuron-Specific Enolase (NSE), and Chromogranin A (CGA), and electronmicroscopic studies were performed to demonstrate exocrine and/or endocrine features of the tumor cells. By means of these methods, 16 cases turned out to be amphicrine tumors. Among them, there were 4 sinonasal, 1 bronchial, 1 mediastinal, 8 gastrointestinal and 2 suprarenal gland neoplasms. In connection to the subject, a brief review is given of amphicrine tumor, regarding its etiological and pathological aspects. These tumors form a distinct clinicopathological entity and should be separated from both neuroendocrine tumors and adenocarcinomas.

Lack of carcinogenicity of cyanoguanidine in F344 rats

The carcinogenicity of cyanoguanidine, a compound used in the production of melamine, guanidine salts and guanamine derivatives, was examined in male and female Fischer 344 rats fed CRF-1 pulverized diets containing 0, 2.5 and 5% cyanoguanidine for up to 2 yr. The rats were randomly allocated to three groups, each consisting of 50 males and 50 females. The mean body weight gains in both sexes of the 5% group and in females of the 2.5% group were significantly lower than the control values after wk 1 of treatment. No other signs of toxicity were seen in any of the rats throughout the treatment period. Histopathologically, various tumours developed in all groups, including the control group, but these were all similar to those known to occur spontaneously in this strain of rats, and no toxicologically significant increase was found for any lesion type in the treated groups. On the basis of these results, it is concluded that cyanoguanidine exerts no carcinogenic potential in F344 rats when administered for up to 2 yr under the conditions of the present study.

Study of the population of enterochromaffin-like cells in mouse gastric mucosa after long-term treatment with ebrotidine

The possible hyperplastic effect on the mouse gastric mucosa following administration of 500 mg/kg of ebrotidine for 18 mo was investigated. The animals were taken from the study of carcinogenesis in mouse carried out with this product. Two different aspects were considered to assess such a possible hyperplastic effect. The height of the fundic and antral mucosa was microscopically measured in several points. Histologic sections obtained from standardized levels of the stomach were used. The density of argyrophil cells in the gastric mucosa has been also quantified, in order to identify enterochromaffin-like (ECL) cells, the most abundant among the cells that have this property. Grimelius' silver staining method was used to identify ECL cells. The cell count was performed under x400 magnification, and the length of mucosa examined was determined by computer-assisted image analysis. This study complements the mouse carcinogenesis study, in which no differences in tumor incidence were found between treated and control animals. The results show that administration of 500 mg/kg/day of ebrotidine for 18 mo to mice did not induce any hyperplastic effect on the gastric mucosa comprising its various cell types or any specific, diffuse, or focal hyperplasia of ECL cells.

Identity and pathogenesis of stomach tumors in Sprague-Dawley rats associated with the dietary administration of butachlor

Macroscopic stomach tumors induced in Sprague-Dawley rats during two chronic bioassays with the acetanilide herbicide butachlor at a dietary concentration of 3000 ppm, were evaluated histologically and immunohistochemically in order to determine their identity and pathogenesis. The tumors, which occurred primarily in female rats, were a heterogeneous series, including a few consisting wholly or partly of classic solid or anaplastic epithelium, but with the majority containing diffusely distributed primitive neoplastic cells. The latter had either the general appearance of undifferentiated epithelium or presented a more "mesenchyme-like" pattern where the cells were epithelioid, blastema-like, neuroendocrine-like or sarcoma-like with fascicular disposition. Gastric glandular profiles were also present, usually located near the periphery of the tumors, but in some cases extending into the diffuse tumor tissue. Most of the tumors displayed variable immunohistochemical reactivity for cytokeratin, vimentin and neuron-specific enolase but were negative for muscle-specific actin or desmin except in the stromal tracts. Detailed examination of all available gastric tissue revealed the presence of additional microscopic neoplasms and precursor hyperplastic lesions. All of these were typical gastric neuroendocrine cell lesions (gastric carcinoids) originating in the fundic mucosa but occasionally invading submucosally, and consisting of epithelial cells in organized clusters, rosettes or primitive tubules. The enterochromaffin-like (ECL) nature of these microscopic neoplasms and precursor lesions was substantiated by strong immunohistochemical reactivity for cytokeratin, neuron-specific enolase and chromogranin A, and a negative reaction for vimentin. One microscopic tumor showed a transition from differentiated neuroendocrine type in the fundic mucosa to a dispersed "mesenchyme-like" pattern in the submucosal extension. An additional finding in the butachlor-treated male and female rats was atrophy of the fundic mucosa involving, in particular, reduction in the numbers of parietal cells. This effect was dose-related, being most severe in the high-dose (3000 ppm) females. On the basis of their morphological characteristics, coupled with the continuity evident in the microscopic lesions, it is concluded that the macroscopic stomach tumors associated with the dietary administration of butachlor are poorly differentiated gastric carcinoids, in some cases admixed with a non-neuroendocrine epithelial element. Fundic ECL and stem cells are known to be under the trophic influence of gastrin, which is apparently responsible for the induction of the tumors associated with butachlor administration. Gastric tumor development involving gastrin is recognized as a secondary, hormonal mechanism of carcinogenesis, demonstrating a dose-threshold phenomenon.(ABSTRACT TRUNCATED AT 400 WORDS)

Endocrine tumours of the stomach

This review illustrates the current classification of gastric endocrine tumors: a) carcinoid tumors (associated with chromic atrophic gastritis; with Zollinger-Ellison syndrome and multiple endocrine neoplasia type I; sporadic) and b) neuroendocrine carcinomas. The pathogenetic characteristics and clinical implications that justify this classification are discussed.

A study of the mechanism of butachlor-associated gastric neoplasms in Sprague-Dawley rats

Long term administration of butachlor to Sprague-Dawley rats in a previous bioassay, resulted in the induction of gastric neoplasms which occurred only in the highest dose group (3000 ppm in the diet), primarily in females and specifically in the fundic region. The tumors were a composite of highly undifferentiated enterochromaffin-like (ECL) cells and mucus producing cells with morphologic characteristics unlike those previously described in the rat stomach. Mucosal atrophy of marked intensity was a consistent feature of the gastric mucosa in animals from the highest dose group. An additional long term study was conducted in female Sprague-Dawley rats at dietary levels of 0, 100, 1000 and 3000 ppm to explore the mechanism(s) involved in the formation of these neoplasms. Cell proliferation was evaluated in both fundic and pyloric regions of the stomachs of rats at multiple time periods from 14 days to 26 months. Mucosal thickness was determined in the fundic region at the same time intervals as were used for cell proliferation studies. Gastric pH and gastric acid production were measured after approximately 21 months of exposure. Serum gastrin levels were analyzed at 14, 60, and 120 days and at 6, 18 and 20 months. Cholecystokinin (CCK)/gastrin receptor binding studies were conducted on samples of four tumors and pooled fundic mucosa from five animals in the control group. Cell proliferation was increased in both the neck and base regions of the fundic mucosa at nearly all time points measured from 14 days to 26 months. The magnitude of the changes in the base region were substantially greater than those in the neck region. Fundic mucosal thickness was decreased beginning at the 30-day time point and continued at all intervals, being less than one half that of controls at 20 and 26 months. Gastric pH in rats from the highest dose was elevated to nearly twice control levels at 21 months. Gastric acid secretion was dramatically decreased in animals from the 3000 ppm group and was moderately decreased in the 1000 ppm group at 21 months. Hypergastrinemia was observed at the 3000 ppm level only, beginning at 120 days with progression to extremely high levels by 18 months. CCK/gastrin receptor binding was demonstrated in all tumors studied, at levels comparable to or higher than that of the pooled control sample. All changes involved only the fundic region, the site of tumor formation. Tumors occurred only in animals from the 3000 ppm level, the only level at which hypergastrinemia occurred.(ABSTRACT TRUNCATED AT 400 WORDS)

Toxicokinetic and mechanistic considerations in the interpretation of the rodent bioassay

When chemicals that are nongenotoxic in conventional assays produce increases in tumor incidence in rodents in chronic bioassays, the determination of the significance of these data for human safety is a challenging task. An important first step in this process is consideration of available data on the mechanism of action and biological properties of the chemical as well as pharmacokinetic and metabolism data in the species showing the response. In recent years, there has been an increase in the understanding of so-called "secondary mechanisms" of carcinogenesis (e.g., thyroid tumors in rats following exposure to enzyme inducers). Application of these data may assist in determination of human risk. There are 2 important questions that will be explored and developed: Are there biological effects produced in the test species that could explain the increase in tumor incidence, and will these effects be reproduced in humans? What is the exposure to the chemical that is associated with the increase in tumors, and how does this relate to exposure in humans?

Time-dependent changes in enterochromaffin-like cell kinetics in stomach of hypergastrinemic rats

BACKGROUND

Hypergastrinemia has been claimed to cause first hyperplasia and then dysplasia/neoplasia of enterochromaffin-like (ECL) cells in rat stomach. The growth is thought to reflect an accelerated self replication rate of mature ECL cells. The cytokinetics and the histidine decarboxylase (HDC) activity of the ECL cells were investigated during sustained hypergastrinemia.

METHODS

Hypergastrinemia was evoked by omeprazole (400 mumol.kg-1 x day-1 orally) for up to 1 year. Immunocytochemistry for histamine was used to determine the ECL cell density and combined with [H3]-thymidine autoradiography to establish the labeling index (LI), i.e., the proportion of the ECL cells that has incorporated [H3]thymidine.

RESULTS

The ECL cell density increased progressively for 10-20 weeks in response to the hypergastrinemia and remained at a plateau for the remainder of the study. The hyperplasia was diffuse with additional micronodules at 52 weeks. The ECL cell Ll was maximally elevated after 1-2 weeks and declined to control values after 10-20 weeks of treatment. In contrast, the HDC activity remained elevated for the duration of the study.

CONCLUSIONS

The ECL cell hyperplasia reflects the transiently elevated ECL cell Ll during the early phase but is not associated with an accelerated rate of mitosis during the 10-52 weeks period. Even though with time gastrin seems to loose its ability to sustain a high ECL cell Ll it retains its ability to maintain a high HDC activity.

Pharmacology of gastric acid inhibition

Gastric acid secretion is precisely regulated by neural (acetylcholine), hormonal (gastrin), and paracrine (histamine; somatostatin) mechanisms. The stimulatory effect of acetylcholine and gastrin is mediated via increase in cytosolic calcium, whereas that of histamine is mediated via activation of adenylate cyclase and generation of cAMP. Potentiation between histamine and either gastrin or acetylcholine may reflect postreceptor interaction between the distinct pathways and/or the ability of gastrin and acetylcholine to release histamine from mucosal ECL cells. The prime inhibitor of acid secretion is somatostatin. Its inhibitory paracrine effect is mediated predominantly by receptors coupled via guanine nucleotide binding proteins to inhibition of adenylate cyclase activity. All the pathways converge on and modulate the activity of the luminal enzyme, H+,K(+)-ATPase, the proton pump of the parietal cell. Precise information on the mechanisms involved in gastric acid secretion and the identification of specific receptor subtypes has led to the development of potent drugs capable of inhibiting acid secretion. These include competitive antagonists that interact with stimulatory receptors (e.g. muscarinic M1-receptor antagonists and histamine H2-receptor antagonists) as well as non-competitive inhibitors of H+,K(+)-ATPase (e.g. omeprazole). The histamine H2-receptor antagonists (cimetidine, ranitidine, famotidine, nizatidine and roxatidine acetate) continue as first-line therapy for peptic ulcer disease and are effective in preventing relapse. Although they are generally well tolerated, histamine H2-receptor antagonists may cause untoward CNS, cardiac and endocrine effects, as well as interfering with the absorption, metabolism and elimination of various drugs. The dominance of the histamine H2-receptor antagonists is now being challenged by omeprazole. Omeprazole reaches the parietal cell via the bloodstream, diffuses through the cytoplasm and becomes activated and trapped as a sulfenamide in the acidic canaliculus of the parietal cell. Here, it covalently binds to H+,K(+)-ATPase, the hydrogen pump of the parietal cell, thereby irreversibly blocking acid secretion in response to all modes of stimulation. The main potential drawback to its use is its extreme potency which sometimes leads to virtual anacidity, gastrin cell hyperplasia, hypergastrinaemia and, in rats, to the development of carcinoid tumours. The cholinergic receptor on the parietal cell has recently been identified as an M3 subtype and that on postganglionic intramural neurones of the submucosal plexus as an M1 subtype.(ABSTRACT TRUNCATED AT 400 WORDS)

Composite gastric carcinoma and precursor lesions with amphicrine features in chronic atrophic gastritis

A composite carcinoma of the gastric body consisting of endocrine and mucous epithelial cells with interspersed amphicrine cells is reported together with ultrastructural and immunocytochemical documentation of endocrine and nonendocrine differentiation. The tumor was associated with hypergastrinemia related to chronic atrophic gastritis (achlorhydria) and with multiple proliferative lesions, such as intramucosal microcarcinoid (IMC) and endocrine cell proliferations of the micronodular and linear type, which are currently regarded as carcinoid precursor changes. Ultrastructurally, a composite architecture with amphicrine features was demonstrated in the primary tumor, IMC, and liver metastases. On the other hand, the endocrine cell proliferations exclusively contained gastrin and enterochromaffinlike cells. Immunostaining with antibodies to calcitonin documented a number of positive cells both in the primary and in the metastatic sites. This is the first report of mixed exocrine-endocrine-amphicrine components both in a metastasizing carcinoma and in its precursor lesions in a chronic hypergastrinemic state. Unlike previously reported lesions, the endocrine component was unexpectedly composed of calcitonin cells, which are not usually present in the gastric mucosa.

Focal oxyntic gland atrophy with endocrine cell hyperplasia in Zollinger-Ellison syndrome during omeprazole treatment

Development of focal gland atrophy of the oxyntic mucosa was found in two patients with the Zollinger-Ellison syndrome undergoing long-term treatment with omeprazole. The atrophic areas revealed florid proliferation of endocrine cells in the form of both intraglandular crescents and micronodular hyperplasia. This proliferation was significantly more pronounced than in the remaining non-atrophic mucosa. The possible relationship of these changes to long-standing pharmacological therapy for gastric acid suppression is discussed.

Ultrastructural investigations of the enterochromaffin-like (ECL) cells in three different rat strains (Sprague-Dawley, Fischer 344, Wistar) after treatment with the H+,K(+)-ATPase inhibitor pantoprazole

The purpose of the present study was the evaluation of ultrastructural characteristics of the enterochromaffin-like (ECL) cells in the fundic mucosa of three different rat strains without treatment and after treatment with the H+, K(+)-ATPase inhibitor pantoprazole. In the study, 20 one year old female Sprague Dawley (SD), Fischer 344 (F) and Wistar (W) rats each were treated orally for three months with 4 mg pantoprazole/kg/d or with the vehicle only. The control animals showed close conformity of ECL cell density and morphology in all three strains. Treatment with pantoprazole led to a significant increase in serum gastrin concentration and GPC density in all strains. However, the electron microscopically determined ECL cell density was markedly increased in the SD strain only. Ultrastructurally all treated rats showed activation of the ECL cells, and enhanced histamine release. The SD and F strains had an enhanced proportion of large ECL cell granules, with the F rats also showing an increased granule density. In contrast, the treated W rats were found to have a lower granule density and a higher proportion of small and medium sized granules compared to their controls.

Effects of omeprazole and ranitidine on plasma gastrin concentration and stomach gastrin content in rats

Prolonged fasting and longer time between dosing and sampling reduced the plasma gastrin concentrations after omeprazole (80 mumol/kg x 2 for 14 days) treatment in male rats whereas the amounts of tissue gastrin were essentially unchanged during these initial experiments. After 28 days omeprazole (80 mumol/kg x 2) or ranitidine (375 mumol/kg x 4) that produced corresponding inhibition of acid secretion, increased the tissue gastrin content by 114 and 59%. A low dose of omeprazole (20 mumol/kg x 2) also raised the gastric gastrin content (41%), whereas no change was noted on treatment with a low dose of ranitidine (125 mumol x 4). Following recovery for 28 days no significant increases in gastrin were observed. 1, 3, 7, 14 or 28 days of treatment with omeprazole (80 mumol/kg x 2) gradually increased the gastric gastrin content being significantly raised already after 3 days. We conclude that a) measuring the tissue gastrin content may be the preferable method when changes in gastrin following long-term treatment with acid inhibiting drugs are to be determined, b) the amount of gastrin in the stomach increases rapidly following treatment with omeprazole and is approximately doubled following 28 days of treatment and c) after treatment for 28 days omeprazole was found to cause greater elevations in the tissue gastrin content than ranitidine despite similar degrees of basal acid inhibition.

Neuroendocrine cell hyperplasia and neuroendocrine carcinoma of the rodent fundic stomach

Certain substances when given orally to rats have effects on the neuroendocrine cells of the fundic stomach. Such compounds also have effects on acid or its secretion, which is to a greater or lesser extent suppressed, with a consequent rise in serum gastrin, followed by an increase in the number of histamine-secreting ECL cells. These changes are seen with the histamine H2 receptor antagonists loxtidine, SKF 93479, ICI 162,846 and ranitidine; with the hypolipidaemic agents clofibrate, ciprofibrate and benzofibrate; with sodium bicarbonate and pentagastrin; and with omeprazole, a potent inhibitor of the parietal cell proton pump mechanism. Changes in the pH of the rat stomach stimulate the neuroendocrine G cells of the pylorus to secrete gastrin, which acts on the ECL cells of the fundus causing the production of histamine, which in turn stimulates the parietal cell. This sequence leads to an excess of circulating gastrin, which is detectable within 5 days. Subsequently increases in the number of ECL cells occur, the hyperplasia being related to hypergastrinaemia and the degree of acid suppression. The hyperplastic response is rapid, being so obvious with loxtidine at 39 days that there is good reason to suppose it could well be detected earlier. Using omeprazole, hyperplasia was found at 28 days after oral doses of 140 mg/kg/day. In order to get an equivalent degree of acid suppression with ranitidine it was necessary to deliver 420 mg/kg/day by subcutaneous infusion using an osmotic minipump, when hyperplasia occurred. Interestingly, only omeprazole produced a hyperplastic response of G cells. Such results reflect the covalent binding of omeprazole to the proton pump as opposed to the competitive binding of ranitidine to the histamine H2 receptor site. In addition to ECL cell hyperplasia there is ample evidence from lifetime studies in rats and mice that neoplasia may result. Neuroendocrine carcinomas (carcinoids) of the rat fundic stomach have been observed with loxtidine, omeprazole, SKF 93479 and ICI 162,846. They are seen late in the 2-year rat studies and are most unlikely to have arisen purely as an extension of the hyperplastic response. It is possible that the prolonged disturbance of gastric homoestasis resulting from achlorhydria result in the production of a carcinogen or carcinogens, in which event it is not too surprising, in view of the neuroendocrine hyperplasia, that the tumours seen are neuroendocrine carcinomas.(ABSTRACT TRUNCATED AT 400 WORDS)

Gastric carcinoids and their precursor lesions. A histologic and immunohistochemical study of 23 cases

A histologic and immunohistochemical study was carried out in 23 unselected nonantral gastric carcinoids and their precursor lesions classified according to Solcia et al. None of the patients showed Zollinger-Ellison syndrome. Two variants of carcinoids showing distinctive pathologic and pathogenetic characteristics were identified on the basis of presence or absence of associated chronic atrophic gastritis type A (A-CAG). Chronic atrophic gastritis type A was found in 19 cases showing either single or multiple neoplasms, tumor extension limited to the mucosa or submucosa, consistent endocrine cell precursor changes in extratumoral mucosa, and consistent hypergastrinemia and/or G cell hyperplasia. Associated precursor lesions were only hyperplastic in all but two cases with single carcinoids whereas they were also dysplastic in all but one case with multiple carcinoids. The four tumors arising in nonatrophic mucosa were all single, more aggressive, and not associated with extratumoral endocrine cell proliferations or with signs of gastrin hypersecretion. Tumor cells were diffusely immunoreactive for chromogranin A and synaptophysin but usually negative for chromogranin B or HISL-19. Scattered serotonin cells were found in ten carcinoids. They were more frequent in infiltrating than in intramucosal tumors as were the less represented pancreatic polypeptide cells whereas the reverse was found for alpha-subunit-containing cells. These results are of relevance for tumor pathogenesis and may provide the rationale for a less aggressive therapeutic approach in the patients.

Partial gastric corpectomy results in hypergastrinemia and development of gastric enterochromaffinlike-cell carcinoids in the rat

Studies in the rat have shown that partial gastric corpectomy, in which about 75% of the acid-producing oxyntic mucosa was removed, leads to markedly reduced acid secretion and a feedback increase in the plasma gastrin levels. Ten weeks after operation, the gastric enterochromaffin (ECL)-like cell density in the remaining part of the oxyntic mucosa had increased significantly. In the present study, the effects on the gastric ECL cells of lifelong persistent hypergastrinemia induced by partial (75%) corpectomy have been investigated. Seventy-five partially corpectomized rats and 40 control rats were investigated for plasma gastrin and oxyntic mucosal changes in a 124-week study. The partially corpectomized rats showed increased plasma gastrin levels after the operation; the mean increase compared with the controls was almost 10-fold during the entire study. The remaining oxyntic mucosa of the partially corpectomized rats differed from that of control rats in two respects, showing first general hypertrophy and second a marked hyperplasia of argyrophil ECL cells. The degree and incidence of these changes increased towards the end of the study, i.e., in the aging rats. An age-related increase in ECL-cell density occurred spontaneously also in the control rats but to a lesser extent than in the partially corpectomized group. ECL-cell carcinoids were found in the oxyntic mucosa of 26 of the 75 partially corpectomized rats. The first carcinoid was found 78 weeks after the beginning of the study. Six rats with carcinoids (23%) were found before week 104 (2 years) and the remainder, 20 (77%), were discovered later. No carcinoid tumor was found in the control rats. It is concluded that lifelong hypergastrinemia induced by partial corpectomy leads to the development of ECL-cell carcinoids in the oxyntic mucosa of some rats towards the end of their life span. This observation strongly supports the hypothesis that the gastric ECL-cell carcinoids found in rats treated with antisecretory drugs are caused by long-standing hypergastrinemia developing secondary to inhibition of gastric acid secretion.

Proposed mechanism of induction of gastric carcinoids: the gastrin hypothesis

Novel, powerful and long-acting inhibitors of gastric acid secretion include second generation H2-blockers and so-called proton pump inhibitors, such as omeprazole. Gastric carcinoids were found to develop in experimental animals as a consequence of continuous long-term administration of several of these highly effective anti secretory drugs. This unwanted side effect is now thought to reflect the fact (1) that pharmacological blockade of acid secretion results in hypergastrinaemia, and (2) that long-standing hypergastrinaemia gives rise to hyperplasia of certain endocrine cells, the so-called ECL cells, in the gastric mucosa. The carcinoids that develop in the rat stomach after lifelong treatment with antisecretagogues arise from the ECL cells. The proposed sequence of events is acid blockade--hypergastrinaemia--ECL cell hyperplasia--carcinoid. This concept, referred to as the gastrin hypothesis, maintains that the ECL cell hyperplasia (and possibly the carcinoids) is a consequence of long-term continuous hypergastrinaemia.

Gastrin stimulates the self-replication rate of enterochromaffinlike cells in the rat stomach. Effects of omeprazole, ranitidine, and gastrin-17 in intact and antrectomized rats

The enterochromaffinlike cells in the rat stomach are rich in histamine and are thought to be under the influence of gastrin. The effect of sustained endogenous and exogenous hypergastrinemia on the activity and proliferation rate of the enterochromaffinlike cells was studied by determining the histidine decarboxylase activity and histamine concentration and by combining histamine immunocytochemistry and autoradiography after in vivo labeling with [3H]thymidine. The proliferation rate of the stem cells in the oxyntic mucosal progenitor zone was also studied. Exogenous hypergastrinemia was induced by infusion of rat gastrin-17 (60 nmol.kg-1.day-1). Endogenous hypergastrinemia was induced by inhibition of gastric acid secretion with omeprazole (80 mumol.kg-1.day-1) or ranitidine (1200 mumol.kg-1.day-1). The effect of omeprazole was also studied in antrectomized rats. In intact rats, all treatments resulted in elevated plasma gastrin levels and were accompanied by an increase in the histidine decarboxylase activity and the histamine content of the oxyntic mucosa. This resulted in an increase in the enterochromaffinlike cell proliferation rate, leading to enterochromaffinlike cell hyperplasia. The number of labeled stem cells was increased, but this effect was not as pronounced as in the enterochromaffinlike cells. In antrectomized rats, the inhibition of acid secretion by omeprazole did not result in elevated plasma gastrin or in an increase in the activity or number of enterochromaffinlike cells, indicating that omeprazole per se had no effect on these cells. These data support the view that gastrin stimulates the proliferation rate of both enterochromaffinlike cells and stem cells. Gastrin also stimulates the activity of the enterochromaffinlike cells.

Quantitative ultrastructure of endocrine cells of oxyntic mucosa in Zollinger-Ellison syndrome. Correspondence with light microscopic findings

The endocrine cells of the oxyntic mucosa of five patients with longstanding Zollinger-Ellison syndrome were quantitatively investigated with electron microscopy and two light microscopic methods (Grimelius and immunostaining for chromogranin A). Ultrastructurally, the volume density of endocrine cells was 3.2% +/- 1.1% of the mucosal epithelial component, a 168% increase (P less than 0.001) over the value found in normal subjects. Of the six endocrine cell types of human oxyntic mucosa, only enterochromaffinlike cells increased in cell density (65% +/- 15% of the total endocrine cell mass), size, and number of cell profiles per unit area. The enterochromaffinlike cells also underwent morphological changes of secretory granules with a decrease in vacuolated forms, increase in elongated profiles, and appearance of granules with a punctate structure of the core. The latter variety of granules was previously observed only in carcinoid tumors of the oxyntic mucosa and is possibly related to the enterochromaffinlike cell hyperplasia-neoplasia sequence seen in hypergastrinemic patients. A positive relationship was found between endocrine cell densities evaluated ultrastructurally and with chromogranin A immunostaining. It is concluded that in Zollinger-Ellison syndrome, the trophic effects induced by longstanding hypergastrinemia are strictly selective for enterochromaffinlike cells and are associated with ultrastructural features typical for enterochromaffinlike cell tumors.

Potential hazards of long-term acid suppression

Neuroendocrine cell (carcinoid) tumours have been reported in the acid-secreting part of the stomach of rodents after long-term administration of a range of potent chemically diverse antisecretory agents. Although evidence shows a link between the sequence of acid suppression, hypergastrinaemia, and neuroendocrine cell hyperplasia, other factors are also thought to be involved in neoplastic transformation. Prolonged hypochlorhydria or achlorhydria resulting in bacterial colonization of the stomach may allow the generation of carcinogenic substances. Other as yet unidentified trophic factors may be involved in tumour formation. In view of the potential risks associated with these agents, there must be concern about the possible consequences in man of marked suppression of acid. It seems wise to limit the use of these more potent agents to situations in which conventional therapy has failed and to short-term treatment.

Omeprazole and ranitidine, antisecretagogues with different modes of action, are equally effective in causing hyperplasia of enterochromaffin-like cells in rat stomach

Female rats were treated for 28 days with high doses of the gastric acid secretion inhibitors omeprazole and ranitidine. Omeprazole, which is long-acting, was given orally once daily. Ranitidine, which is short-acting, was given by continuous infusion (via osmotic minipumps, implanted subcutaneously). The aim was to produce a similar degree of acid inhibition with the two drugs. The inhibition of acid secretion over the day and night was more pronounced in the omeprazole-treated rats (maximal inhibition 100%, minimum 85%) than in those receiving ranitidine (mean 70%). In both groups, there was a great increase in plasma gastrin, somewhat greater after omeprazole than after ranitidine. The gastrin concentration in the antrum was almost doubled by both treatments and there was a moderate increase in the number of antral gastrin cells in the omeprazole-treated rats. The number of enterochromaffin-like (ECL) cells (per visual field) increased in the oxyntic mucosa to the same extent (greater than 100%) in the ranitidine- and omeprazole-treated rats. Apart from the gastrin cells in the antrum and the ECL cells in the corpus no other gastric endocrine cell type seemed to respond to treatments with antisecretagogues. We conclude that, regardless of the type of antisecretagogue used, effective and long-term suppression of gastric acid secretion results in sustained hypergastrinemia and increased number of ECL cells. Conceivably therefore, the ECL cell hyperplasia reflects the trophic effect of gastrin.

A review of the effects of long-term acid inhibition in animals

Studies with H2-receptor antagonists have revealed a trophic effect on the gastric mucosa - an effect which has been ascribed to hypergastrinaemia secondary to acid inhibition. Such hyperplasia of oxyntic mucosal cells has also been demonstrated in chronic toxicity studies following profound, long-standing inhibition of gastric acid secretion with omeprazole. The central role of gastrin in this effect was clearly demonstrated in the omeprazole studies, as antrectomy prevented this effect in both rats and dogs. The hyperplasia was fully reversible in both species. The close correlation between serum gastrin and hyperplasia of enterochromaffin-like (ECL) cells in the rat oxyntic mucosa has been demonstrated in a large number of experiments using different means to induce hypergastrinaemia, including administration of exogenous gastrin, treatment with antisecretory drugs and partial fundectomy. The hyperplasia of ECL cells was fully reversible even after 1 year of sustained gastric acid inhibition following treatment with a high dose of omeprazole. Marked long-standing hypergastrinaemia explains the findings of gastric ECL cell carcinoids in the life-long rat toxicity studies with both omeprazole and other inhibitors of gastric acid secretion.

Gastric morphological changes including carcinoid tumors in animals treated with a potent hypolipidemic agent, ciprofibrate

Oral administration of ciprofibrate, a potent hypolipidemic compound, to rats for 2 or more weeks at doses of 20 mg/kg/day or more resulted in hypertrophy and increased eosinophilia of the oxyntic cells in the gastric mucosa. Ultrastructural evaluation revealed small secretory canaliculi with small microvilli in these cells, changes consistent with the inhibition of acid secretion. After longer administration (e.g., greater than 2 months at 20 mg/kg/day), hyperplasia of the neuroendocrine cells (in particular, the enterochromaffin-like cells) was present in the fundic mucosa of the stomach. After life-time (2-year) administration at 10 mg/kg/day, neuroendocrine cell hyperplasia was accompanied by formation of malignant carcinoid tumors in the fundus of 5 of 59 male and 1 of 60 female rats. In contrast, administration of ciprofibrate to mice at 20 mg/kg/day for 2 months was not associated with oxyntic or neuroendocrine cell changes, a finding consistent with the lack of gastric carcinoid tumors in a 2-year mouse study. Similarly, no significant changes were induced in the marmoset stomach by doses as high as 100 mg/kg/day for 6 months. These findings are consistent with the hypothesis that the formation of gastric carcinoid tumors following ciprofibrate administration is a phenomenon that occurs specifically in those species such as the rat where this compound has significant gastric antisecretory activity.

Gastric ECL-cell hyperplasia and carcinoids in rodents following chronic administration of H2-antagonists SK&F 93479 and oxmetidine and omeprazole

The histamine H2-receptor antagonist SK&F 93479 induced gastric neuroendocrine (carcinoid) ECL-cell tumor formation in 6/34 male and 8/37 female rats treated for 22-24 months at 1,000 mg/kg/day po. Focal ECL-cell hyperplasia was present in 21/34 males and 15/37 females, with local infiltration through the muscularis mucosae in half these cases. No focal hyperplasias or carcinoids were present after 200 mg/kg/day po treatment. Investigative studies showed evidence for marked and sustained hypergastrinemia increasing on chronic dosing which was capable of restoring gastric acid secretion and pH to near control values. Using morphometric analysis of immunoperoxidase anti-chromogranin A stained sections, a dose-related and time-dependent neuroendocrine ECL-cell hyperplasia was correlated with the sustained elevated hypergastrinemia. A 21-month mouse oncogenicity study showed no focal neuroendocrine cell hyperplasia or carcinoid tumor induction, but a diffuse neuroendocrine cell hyperplasia and an increase in multifocal glandular hyperplasia of the oxyntic mucosa was observed in mice treated with 1,000 mg/kg SK&F 93479 po. The morphological changes observed in both rat and mouse were considered to be secondary to the hypergastrinemia resulting from the pharmacological suppression of gastric acid secretion by SK&F 93479. These changes were also observed to a more marked degree following omeprazole treatment and were only slight following oxmetidine treatment in the rat.