Histidine decarboxylase in the transplantable argyrophilic gastric carcinoid of Praomys (Mastomys) natalensis

Characterization of gastrin/CCK receptors on gastric carcinoid tumor membrane of Mastomys natalensis

Recently, we identified the specific binding site for gastrin on the gastric carcinoid tumor of Mastomys (Praomys) natalensis. In this study, precise characterization of the gastrin binding site on these tumors was performed. Both 125I-human gastrin I (gastrin) and 125I-CCK-8 bound specifically to the cell membrane, and Scatchard analysis revealed a high affinity binding site for each ligand with similar Kd and Bmax values. The specific binding of both 125I-gastrin and 125I-CCK-8 was displaced in a concentration-dependent manner by various related peptides with a relative potency order of CCK-8 > or = gastrin < des(SO3)CCK-8. In addition, L364,718 as well as L365,260 displaced the binding of both ligands with similar potencies. Furthermore, not only gastrin but also CCK-8 increased [Ca2+]i in these tumor cells, the action of both being inhibited by L364,718 as well as by L365,260 (10(-7) M). These results suggest that the carcinoid tumor of Mastomys possesses a high affinity gastrin/CCK binding site coupled to the increase of [Ca2+]i.

Cloning and characterization of gastrin receptor from ECL carcinoid tumor of Mastomys natalensis

We report here the cDNA cloning of a putative gastrin receptor from enterochromaffin-like (ECL) carcinoid tumor of Mastomys natalensis. For this study, we used the polymerase chain reaction technique to amplify transmembrane domain sequences related to rat pancreatic cholecystokinin (CCK)-A receptor from the ECL tumor cDNA library. The amino acid sequence deduced from the cloned cDNA showed 85.7% and 49.0% identity to canine parietal cell gastrin receptor and rat pancreatic CCK-A receptor, respectively. Ligand binding studies using COS-7 cells transfected with the cDNA showed the same binding specificity for gastrin and CCK-8 as the gastrin receptor on the Mastomys carcinoid tumor membrane. Both gastrin and CCK-8 elevated free cytosolic calcium concentration in COS-7 cells expressing the cloned receptor. RNA blot analysis revealed the expression of the gastrin receptor in both Mastomys stomach and brain.

Receptors for gastrin on gastric carcinoid tumor membrane of Mastomys natalensis

Specific binding sites for human gastrin I (gastrin) were identified in a crude membrane preparation from the gastric carcinoid tumor of Mastomys (Praomys) natalensis. The binding of 125I-gastrin to the carcinoid tumor membrane was saturable, and Scatchard analysis of the data revealed a single class of binding site with a dissociation constant of 139.2 pM and a maximal binding capacity of 23.5 fmol/mg protein. Gastrin and CCK8 equipotently and dose-dependently displaced the binding of 125I-gastrin to the membrane. GTP but not ATP decreased 125I-gastrin binding to the membrane, and removal of Mg2+ attenuated this inhibitory action of GTP. The GTP-induced reduction of 125I-gastrin binding was found to be due to a decrease in binding affinity without a change in binding capacity. These results clearly indicate the presence of specific binding sites for gastrin, probably coupled to guanine nucleotide-binding protein, in the carcinoid tumor membrane of Mastomys, and suggest that gastrin has possible biological actions on these tumors.

The effect of omeprazole-induced hypergastrinemia on the oxyntic mucosa of mastomys

Mastomys is a rodent with a high incidence of spontaneous carcinoids in the acid-producing part of the stomach. The present study was conducted to examine whether hypergastrinemia could promote tumor formation in this species. Mastomys, 4 months of age, were treated for 5 months with omeprazole subcutaneously, 100 mumol/kg body weight daily, and compared with mastomys given the vehicle only. The plasma gastrin concentration and the number of antral gastrin cells were increased in the omeprazole-treated group. The hypergastrinemia was associated with elevated histidine decarboxylase activity and histamine content in the oxyntic mucosa and with a trophic effect on the oxyntic mucosa and the enterochromaffin-like cells. However, no carcinoid tumors were observed, possibly because the strain of mastomys studied does not produce carcinoids spontaneously.

Intra-ocular transplantation of carcinoid tumours from mastomys and humans

Carcinoid tumours from man and Mastomys (Praomys) natalensis produce a variety of peptide hormones. The study of these peptide-secreting tumours has been difficult because of the small amount of tissue available and because of limitations with present cell culture systems. The aim of this study was to establish an experimental model where carcinoid tumours could be maintained and their hormone secretion studied. The intra-ocular transplantation technique was chosen for its simplicity and high rate of success. Gastric carcinoid tumours from mastomys (n = 4) and human carcinoids (n = 2) (one bronchial and one ileal) were transplanted to the anterior eye chamber of Sprague-Dawley rats. Pieces of fresh tumour tissue were injected into the anterior eye chamber of rats and allowed to grow for 4-8 weeks. Rats transplanted with human tissue were immunosuppressed by daily injections with cyclosporin A (20 mg/kg). Eye chambers were inspected regularly and plasma from transplanted rats was collected for assay of peptide YY (PYY) and glucagon. Vascularization of transplants occurred within 1-2 days after transplantation in 70-80 per cent of all experiments. Microscopic analysis of transplants demonstrated a rich supply of blood vessels to tumour cells which contained characteristic neurosecretory granules. Transplanted rats had significantly (P less than 0.05) elevated levels of PYY (44-165 pmol/l) and glucagon (67-162 pmol/l) in plasma as compared with sham-operated rats (PYY 28-40 pmol/l, glucagon 33-40 pmol/l), indicating that hormone secretion by tumour cells in oculo was maintained.(ABSTRACT TRUNCATED AT 250 WORDS)

Significance of gastric endocrine tumor and age-related gut peptide alterations in Mastomys

The Mastomys (Praomys natalensis) species are a unique natural model in which the bioactivity of gastric carcinoids may be studied. Several investigators have previously demonstrated that these tumors contain large amounts of histamine. In this study we investigated the presence of peptides associated with the neoplasm. The levels and location of gastrin, gastric inhibitory peptide (GIP), neurotensin, peptide YY (PYY), pancreatic polypeptide (PP), glucagon, bombesin, vasoactive intestinal peptide (VIP) and somatostatin (SRIF) were investigated by radioimmunoassay and immunocytochemistry. In addition the distribution of these peptides were evaluated in the gastrointestinal tract of young and old animals to investigate possible age-related changes. PYY and enteroglucagon (EG) were significantly (P less than 0.001) elevated in both tumor tissue (676 +/- 152, 551 +/- 164 pmol/g) and plasma (620 +/- 160, 500 +/- 147 pmol/l) of tumor-bearing animals. Immunocytochemistry revealed PYY- and EG-like immunoreactivity in 20-30% of tumor cells. A significant decrease (P less than 0.05) in bombesin was noted in older animals, but no changes in gastric tissue content of PYY or EG could be detected between young and old animals. Gastrin was not detected in tumors and there were no significant changes in tissue or plasma levels with age. Small bowel concentrations of VIP and PYY were higher in the older mastomys (P less than 0.05). In contrast, colonic levels of bombesin, VIP, somatostatin and PYY were significantly lower (P less than 0.05) in older mastomys compared with young. The age-related changes in several peptides may reflect an adaptive response to acid hypersecretion. The multi-hormonal character of these neoplasms suggests that these tumors develop from a pluripotential stem cell.

Adenocarcinoma and carcinoid developing spontaneously in the stomach of mutant strains of Mastomys natalensis

Neoplastic and nonneoplastic lesions developing spontaneously in antral and fundic mucosae of stomachs of mutant chamois-coloured Z (130 animals) and Y (67 animals) strains of Mastomys aged 18 to 24 months were examined histologically and histochemically. The Z strain developed both antral lesions (hyperplasia 29.2%; dysplasia 23.8%; adenocarcinoma 17.7%) and fundic carcinoid(s) (72.3%). The antral lesions were limited to the lesser curvature near the pyloric ring. Macroscopically, adenocarcinomas resembled human gastric carcinomas of either Borrmann's type I or II. Histochemically, adenocarcinoma cells were characterised by marked reduction of total mucins produced and predominance of mucins with both periodic acid-Schiff and Alcian blue reactivities (neutral and sialated class II mucins). An infiltrating adenocarcinoma was successfully transplanted into nude mice, reaching the 7th generation of transplantations over 4 years, and retained histological features of the primary tumour. The ultrastructural appearance of growing transplanted tumours supported the reduced production of mucins by adenocarcinoma cells with scarcity of mucin granules and intracellular cysts. However, the Y strain never developed antral lesions like the wild strain, developing fundic carcinoid(s) only. Microscopically, these carcinoids contained argyrophilic nonargentaffin granules, and biochemically produced histamine consistently but no 5-hydroxytryptamine (5-HT, serotonin) like those of the wild strain. Since we found unexpectedly that a line of F2 but not F1 hybrids between wild and Z strains developed the same antral lesions as Z strain, a preliminary experiment was performed to confirm the development of antral lesions in F2 hybrids newly produced by brother-sister mating. Among 41 surviving F2 offspring, 4 (9.8%) developed hyperplasia, 2 (4.9%) dysplasia and none adenocarcinoma. The numbers (6) of animals observed with these lesions approximated to their expected numbers (7.3).

Characteristics of the spontaneous gastric endocrine tumor of mastomys

Mastomys is a rodent which has been reported to develop spontaneous antral endocrine tumors with acid hypersecretion and duodenal ulceration. This study documents the establishment of a breeding colony and the characterization of the tumors and their possible secretagogues. Parietal cell secretory characteristics were studied using isolated gastric glands (IGG) of both normal (n = 5) and tumor-bearing animals. Tumors (n = 6) and control gastric tissue samples were examined by light transmission microscopy and immunohistochemistry. Gastrin was measured by radioimmunoassay in both plasma and tissue. IGG were prepared by collagenase dispersion and acid sequestration assessed by [14C]AP accumulation. Secretory mechanisms of this species were identified by establishment of a histamine dose-response curve and use of 8-bromo-cAMP. Receptor and proton pump inhibitions were assessed using cimetidine (10(-5)M) and the H/K ATPase inhibitor omeprazole (10(-5]. Both reduced [14C]AP accumulation significantly (P less than 0.05). 8-Bromo-cAMP and histamine significantly stimulated [14C]AP accumulation (P less than 0.05). Although parietal cells were substantially increased in tumor animals as compared to controls, the physiological parameters of acid secretion appeared normal in both and were comparable to other species which have been studied. Tumors were Grimelius positive and contained diffuse electron-dense granules. Immunohistochemistry was negative for gastrin, bombesin, serotonin, neuron-specific enolase, calcitonin, and pancreatic polypeptide. Tumor histamine-like immunoreactivity was, however, positive. Normal stomach contained 1001 +/- 185 compared to less than 0.5 pmole/g gastrin in tumors. Plasma gastrin was normal in both groups (29 +/- 5) as compared to 26 +/- 8 pmole/liter. This study characterizes a spontaneous gastric endocrine tumor which is associated with apparent parietal cell hyperplasia and reports of increased acid secretion and duodenal ulceration. The observations are consistent with the elaboration by the tumor of a nongastrin acid-trophic secretagogue.

Gastrin and the trophic control of gastric mucosa

Gastrin is a trophic stimulant of the acid producing gastric mucosa. Experiments have been carried out in rats, in which chronic states of either low or high serum gastrin levels were induced by surgical manipulation or drug treatment. A relationship between circulating gastrin and a trophic effect could be demonstrated in the oxyntic mucosa, but not in the pancreas and small intestine. Endocrine cells in the oxyntic mucosa (the ECL cells and A-like cells) are among the target cells for the trophic action of gastrin. The functional significance of these two cell populations is unknown. There is much experimental evidence indicating that they are under functional as well as tropic control of gastrin. The vagus nerve also exerts trophic control on the oxyntic mucosa, including the endocrine cells within it. This could be demonstrated by one-sided truncal vagotomy which caused atrophy of the mucosa and hypoplasia of endocrine cells (notably the ECL cells) on the denervated side of the stomach. Conversely, portacaval shunt greatly increased the number of ECL cells. There was no hypergastrinaemia after portacaval shunt, and no trophic effect on other cell types in the oxyntic mucosa. The factors responsible for the ECL cell proliferation after portacaval shunting remain unknown. Tumours may arise spontaneously from the ECL cells. Such neoplasias have been described in Mastomys (Praomys natalensis) and in man. ECL cell hyperplasia and neoplasia in man, but not in Mastomys, are usually associated with hypergastrinaemia either as a result of a gastrin producing tumour or as a result of achylia (sometimes associated with pernicious anaemia). It is unlikely that gastrin alone is responsible for the neoplasia, though it is quite likely that long-standing hypergastrinaemia triggers or facilitates a sequence of events that ultimately leads to tumour formation, via diffuse ECL cell hyperplasia.