Glycine-extended progastrin processing intermediates induce H+,K(+)-ATPase alpha-subunit gene expression through a novel receptor

Glycine-extended gastrin enhances somatostatin release from cultured rabbit fundic D-cells

The role of the peptide hormone gastrin in stimulating gastric acid secretion is well established. Mature amidated gastrin is processed from larger peptide precursor forms. Increasingly these processing intermediates, such as glycine-extended gastrin (G-Gly) and progastrin, have been shown to have biological activities of their own, often separate and complementary to gastrin. Although G-Gly is synthesized and secreted by gastric antral G-cells, the physiological functions of this putative mediator are unclear. Gastrin and cholecystokinin (CCK) stimulate the secretion of somatostatin from gastric D-cells as part of the feedback control of gastric acid. In this study the effect of G-Gly and gastrin on the release of somatostatin from rabbit fundic D-cells was examined. D-cells were obtained by collagenase-EDTA digestion and elutriation and cultured for 48 hours. With a 2 hour exposure to the peptides, gastrin but not G-Gly stimulated somatostatin release. Treatment of D-cells for 24 hours with gastrin or G-Gly individually, significantly enhanced subsequent basal as well as CCK- and GLP-1-stimulated somatostatin release. Twenty four hours exposure to gastrin combined with G-Gly synergistically enhanced basal and agonist-stimulated somatostatin release and cellular somatostatin content. Gastrin and G-Gly may be important in the longer term regulation of D-cell function.

Transcriptional profiling of gastrin-regulated genes in mouse stomach

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

Cholecystokinin and gastrin receptors

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

Gene expression profiling of gastrin target genes in parietal cells

Previous studies demonstrated that mice with a null mutation in the gene encoding the hormone gastrin have impaired gastric acid secretion. Hence, the aim of this study was to evaluate changes in the acid-secreting parietal cell in gastrin-deficient (GAS-KO) mice. Analysis of several transcripts encoding parietal cell proteins involved in gastric acid secretion showed reduced abundance in the GAS-KO stomach, including H+,K+-ATPase alpha- and beta-subunits, KCNQ1 potassium channel, aquaporin-4 water channel, and creatine kinase B, which were reversed by gastrin infusion for 1 wk. Although mRNA and protein levels of LIM and SH3 domain-containing protein-1 (LASP-1) were not greatly changed in the mutant, there was a marked reduction in phosphorylation, consistent with its proposed role as a cAMP signal adaptor protein associated with acid secretion. A more comprehensive analysis of parietal cell gene expression in GAS-KO mice was performed using the Affymetrix U74AV2 chip with RNA from parietal cells purified by flow cytometry to >90%. Comparison of gene expression in GAS-KO and wild-type mice identified 47 transcripts that differed by greater than or equal to twofold, suggesting that gastrin affects parietal cell gene expression in a specific manner. The differentially expressed genes included several genes in signaling pathways, with a substantial number (20%) known to be target genes for Wnt and Myc.

Gastrin and cancer: a review

In 1905, a Cambridge physiologist, John Sydney Edkins, initially identified a hormone responsible of gastric acid secretion, which he called gastric secretin, or gastrin. While gastrin's role in acid secretion is now well defined, more recent studies have implicated the various isoforms of gastrin in cancer. Important advances in the last decade have included the recognition of biological activity for processing intermediates such as progastrin and the glycine-extended gastrin. Here, we give an overview of the roles of these peptides in cancer, highlighted by molecular, cellular and integrated studies on animal models for progastrin-derived peptides and their receptors.

Importance of N- and C-terminal regions of gastrin-Gly for preferential binding to high and low affinity gastrin-Gly receptors

G17-Gly has been shown to stimulate the growth of DLD-1 human colon cancer cells in a biphasic manner via high and low affinity receptors. In the current study, the existence of heterogeneous receptor populations for G17-Gly on the HT-29 human colon cancer cell line was investigated. The effect of either N- or C-terminal peptide truncation on receptor binding and cell growth stimulation was also explored. [Leu15]G17-Gly bound to both high (nM) and low (microM) affinity sites on HT-29 cells. The peptide stimulated cell growth in a dose-dependent and biphasic manner with maximal stimulation at 10(-9) M peptide concentration, suggesting that, as in the case of DLD-1 cells, it is the high affinity receptor which is responsible for the growth-promoting effects. In contrast, G17(1-12) stimulated the growth of HT-29 cells in a sigmoidal fashion with an EC50 of 4.6x10(-9) M. Sequential N-terminal truncation of [Leu15]G17-Gly results in decreased binding to the high affinity G17-Gly receptor on DLD-1 cells. [Leu15]G17(11-17)Gly bound to the low affinity G17-Gly receptor with an affinity similar to that of the full sequence peptide but was unable to displace the radioligand from high affinity sites. G17(1-6)-NH2 was unable to displace [3H]G17-Gly from either site. These results suggest that the important residues for binding to the low affinity receptor are in the C-terminal region of the peptide while those required for interaction with the high affinity receptor lie further towards the N-terminus.

Expression of some tumor associated factors in human carcinogenesis and development of gastric carcinoma

AIM: To study the effect of IGF-1/IGF-1R and gastrin/CCK-BR on carcinogenesis and development of human gastric carcinoma and to explore its mechanism and provide a credible theoretical foundation for early diagnosis and molecular therapy of gastric carcinoma.

METHODS: mRNA expression levels of IGF-1/IGF-1R and gastrin/CCK-BR were assessed by RT-PCR method in gastric cancer tissues, adjacent mucosa, and tumor-free tissues from 56 patients with gastric carcinoma and normal gastric mucosae from 56 healthy controls. Tissue specimens were obtained by biopsy and confirmed by histological evaluation.

RESULTS: The mRNA levels of IGF-1/IGF-1R were increased in gastric cancer tissues compared with normal tissues from healthy controls and successively increased in tumor-free tissues, adjacent mucosa, and gastric cancer tissues. The mRNA levels of gastrin/CCK-BR were increased in gastric cancer tissues compared with normal tissues from healthy controls. There was a significant difference between gastric cancer tissues and adjacent mucosa and tumor-free tissues, but the mRNA levels of gastrin were not significantly increased in adjacent mucosa and gastric cancer tissues compared with tumor-free tissues. The mRNA levels of CCK-BR were increased in gastric cancer tissues and adjacent mucosa compared with tumor-free tissues, but not significantly increased in adjacent mucosa and gastric cancer tissues compared with gastric cancer tissues.

CONCLUSION: Overexpression of IGF-1/IGF-1R and gastrin/CCK-BR promotes the disorderly proliferation of gastric mucosa epithelia and it is of great significance in the carcinogenesis and development of gastric carcinoma.

Identity and regulation of stored and secreted progastrin-derived peptides in sheep

Amidated and nonamidated progastrin-derived peptides have distinct biological activities that are mediated by a range of receptor subtypes. The objective was to determine the nature of the stored and secreted progastrin-derived peptides and to investigate whether progastrin release is regulated by gastric acidity. Using an antiserum directed to the C terminus of progastrin for identification and to monitor purification, C-terminal flanking peptides (CTFP) of progastrin (prog(76-83), prog(77-83), and prog(78-83) in approximately equivalent amounts) were isolated and identified from extracts of sheep antrum using ion exchange, HPLC, and mass spectrometry. Only trace amounts of full-length progastrin were present. Progastrin CTFP was the predominant progastrin-derived peptide in the antrum [progastrin CTFP/gastrin amide (Gamide) = 3]. Similarly, progastrin CTFP was the major circulating form in the antral (CTFP, 710 +/- 62 pmol/liter; Gamide, 211 +/- 35 pmol/liter) and jugular (CTFP, 308 +/- 16 pmol/liter; gastrin amide, 32 +/- 3 pmol/liter) veins. Alteration of gastric acidity in sheep by iv infusion of a H/K-adenosine triphosphatase inhibitor or somatostatin or by intragastric infusion of HCl demonstrated that the CTFP concentrations changed, although to a lesser extent than the changes in circulating gastrin amide. We conclude that the CTFP of progastrin is the major stored and circulating species of the gastrin gene, and that it is secreted in a regulated fashion rather than constitutively. Because full-length progastrin is bioactive, but is only a minor antral and secreted form, determination of the biological activity of the C-terminal flanking peptides will be important for a complete understanding of gastrin endocrinology.

Oncogenic ras induces gastrin/CCKB receptor gene expression in human colon cancer cell lines LoVo and Colo320HSR

Gastrin has the ability to stimulate cell growth in some colorectal cancer cells and some of these cells also express gastrin/CCKB receptors, suggesting that gastrin and its autocrine loop are involved in their proliferation. We previously reported that oncogenic ras induced gastrin gene expression in colon cancer cells. The aim of this study was to investigate whether oncogenic ras also induces gastrin/CCKB receptor gene expression. A transiently transfected activated ras vector stimulated gastrin/CCKB receptor transcriptional activities in both Colo320HSR and LoVo cells, but these ras-increased activities were inhibited by a specific MEK inhibitor, PD98059. An RPA demonstrated that activated ras increased endogenous gastrin/CCKB receptor mRNA levels and PD98059 decreased them in LoVo cells. These findings suggest that oncogenic ras induces gastrin/CCKB receptor gene expression through some intracellular signaling pathways, including MEK, in colon cancer cell lines.

Ferric ions are essential for the biological activity of the hormone glycine-extended gastrin

Amidated and nonamidated gastrins elicit different biological effects via distinct receptors in different tissues. Amidated gastrin 17 stimulates gastric acid secretion and the development of gastric carcinoids, whereas glycine-extended gastrin 17 stimulates proliferation of the colonic mucosa and the development of colorectal cancers. Because glycine-extended gastrin 17 binds two ferric ions with high affinity (Baldwin, G. S., Curtain, C. C., and Sawyer, W. H. (2001) Biochemistry 40, 10741-10746), we have investigated the identity of the iron ligands and the role of ferric ions in biological activity. Here we report the solution structure of glycine-extended gastrin 17, determined by NMR spectroscopy. The spectral changes observed upon the addition of ferric ions revealed that Glu(7) acted as a ligand at the first ferric binding site, and that Glu(8) and Glu(9) acted as ligands at the second ferric ion binding site. Fluorescence quenching experiments confirmed that a GglyE7A mutant bound only one ferric ion. The inability of this mutant to stimulate proliferation or migration in the IMGE-5 cell line and the observation that the iron chelator desferrioxamine selectively blocked the effects of glycine-extended gastrin 17 indicated that binding of a ferric ion to Glu(7) was essential for biological activity. This is the first report of an essential role for a metal ion in the action of a hormone.

Glycine-extended gastrin-17 stimulates acid secretion only via CCK-2 receptor-induced histamine release in the totally isolated vascularly perfused rat stomach

The effects of gastrin precursors have been discussed during recent years. However, the mechanism for their action, whether through a novel receptor on the parietal cell or a cholecystokinin-2 (CCK-2) receptor on the enterochromaffin like (ECL) cells, is still not settled. This study examines the effect of glycine-extended gastrin-17 (Gly-G-17), the main non-amidated gastrin precursor, on gastric acid secretion and histamine release in the totally isolated vascularly perfused rat stomach. Glycine-extended gastrin-17 at the concentrations from 0.52 to 520 nmol L(-1) was administered to the totally isolated vascularly perfused rat stomach. Glycine-extended gastrin-17 at 52 or 520 nmol L(-1), and gastrin-17 at 0.52 nmol L(-1)were co-administered to examine whether glycine-extended gastrin augmented maximal gastrin stimulated acid secretion and histamine release. Both Gly-G-17 at 52 nmol L(-1) and gastrin-17 (G-17) at 0.52 nmol L(-1) were administered together with the histamine-2 receptor antagonist ranitidine at 10 micromol L(-1). Gastric acid and venous histamine output were measured. Glycine-extended gastrin-17 at lower concentrations from 0.52 to 5.2 nmol L(-1) did not stimulate gastric acid output or histamine release, whereas higher concentrations from 52 to 520 nmol L(-1) elicited a concentration-dependent increase in acid secretion and histamine release. The outputs of acid and histamine at 520 nmol L(-1) Gly-G-17 were at the same level as those found for G-17 at its maximally effective concentration of 0.52 nmol L(-1). Glycine-extended gastrin-17 at maximally effective concentration of 520 nmol L(-1) did not augment maximal gastrin stimulated acid secretion or histamine release. Ranitidine inhibited G-17 and Gly-G-17 stimulated acid secretion to a similar degree. This study confirms that the stimulatory effect of Gly-G-17 on gastric acid secretion is via a CCK-2 receptor on the ECL cell.

Gastrimmune-induced antigastrin-17 antibodies inhibit acid secretion in a rat fistula model

BACKGROUND

Gastrimmune is an immunogenic form of gastrin. It raises in situ antibodies against two proliferative forms of gastrin: amidated and glycine-extended gastrin-17. It has been shown to have a therapeutic action in several in vivo tumour models. Following immunization, due to the complex equilibrium that exists between the antibodies and gastrin, it is not technically feasible to assay for free gastrin.

AIM

To determine the effect of Gastrimmune-induced antigastrin antibodies on acid secretion.

METHOD

A rat gastric fistula model was used. Animals (six per group) were immunized with a control immunogen or ascending doses of Gastrimmune. Acid output was measured following infusion of increasing doses of gastrin-17 and pentagastrin.

RESULTS

Gastrimmune-induced antibodies significantly reduced gastrin-17-stimulated acid output compared to control animals (Gastrimmune at 200 microg/rat vs. control; acid output following 30 ng gastrin-17, 0.01 vs. 0.16, P < 0.001; following 120 ng gastrin-17, 0.022 vs. 0.29, P < 0.001).

CONCLUSIONS

Gastrimmune significantly inhibits gastrin-17-stimulated acid output. This biological assay suggests that the antigastrin antibodies effectively bind gastrin-17. In addition to its use as an antineoplastic agent, Gastrimmune may have a role as an acid-decreasing agent in oesophagogastric pathology.

Involvement of phosphatidylinositol 3-kinase and mitogen-activated protein kinases in glycine-extended gastrin-induced dissociation and migration of gastric epithelial cells

The various molecular forms of gastrin can act as promoters of proliferation and differentiation in different regions of the gastrointestinal tract. We report a novel stimulatory effect of glycine-extended gastrin(17) only on cell/cell dissociation and cell migration in a non-tumorigenic mouse gastric epithelial cell line (IMGE-5). In contrast, both amidated and glycine-extended gastrin(17) stimulated proliferation of IMGE-5 cells via distinct receptors. Glycine-extended gastrin(17)-induced dissociation preceded migration and was blocked by selective inhibitors of phosphatidylinositol 3-kinase (PI3-kinase) but did not require mitogen-activated protein (MAP) kinase activation. Furthermore, glycine-extended gastrin(17) induced a PI3-kinase-mediated tyrosine phosphorylation of the adherens junction protein beta-catenin, partial dissociation of the complex between beta-catenin and the transmembrane protein E-cadherin, and delocalization of beta-catenin into the cytoplasm. Long lasting activation of MAP kinases by glycine-extended gastrin(17) was specifically required for the migratory response, in contrast to the involvement of a rapid and transient MAP kinase activation in the proliferative response to both amidated and glycine-extended gastrin(17). Therefore, the time course of MAP kinase activation appears to be a critical determinant of the biological effects mediated by this pathway. Together with the involvement of PI3-kinase in the dissociation of adherens junctions, long term activation of MAP kinases seems responsible for the selectivity of this novel effect of G(17)-Gly on the adhesion and migration of gastric epithelial cells.

The gastrins: their production and biological activities

Gastric epithelial organization and function are controlled and maintained by a variety of endocrine and paracrine mediators. Peptides encoded by the gastrin gene are an important part of this system because targeted deletion of the gene, or of the gastrin-CCKB receptor gene, leads to decreased numbers of parietal cells and decreased gastric acid secretion. Recent studies indicate that the gastrin precursor, preprogastrin, gives rise to a variety of products, each with a distinctive spectrum of biological activity. The conversion of progastrin to smaller peptides is regulated by multiple mechanisms including prohormone phosphorylation and secretory vesicle pH. Progastrin itself stimulates colonic epithelial proliferation; biosynthetic intermediates (Gly-gastrins) stimulate colonic epithelial proliferation and gastric epithelial differentiation; and C-terminally amidated gastrins stimulate colonic proliferation, gastric epithelial proliferation and differentiation, and acid secretion. The effects of progastrin-derived peptides on gastric epithelial function are mediated in part by release of paracrine factors that include histamine, epidermal growth factor (EGF)-receptor ligands, and Reg. The importance of the appropriate regulation of this system is shown by the observation that prolonged moderate hypergastrinemia in transgenic mice leads to remodelling of the gastric epithelium, and in the presence of Helicobacter, to gastric cancer.

Review article: gastrin and colorectal cancer

The polypeptide hormone gastrin was identified nearly a hundred years ago and its role in the regulation of acid secretion is well established. Gastrin also acts as a growth factor and is trophic for the normal gastric oxyntic mucosa. This growth promoting action has led to the extensive investigation of its role in carcinogenesis, in particular colorectal neoplasia. The relationship between gastrin and colorectal adenocarcinoma has been subject to controversy, however the findings from several recent studies have resulted in a clearer understanding of the mechanism of action of gastrin in this is common cancer. The majority of colorectal cancers produce their own gastrin, which may act in an autocrine manner. The tumour cells also express gastrin/CCKB receptors (and/or a combination of isoforms) which mediate the proliferative action. This locally produced gastrin gives rise to a small increase in systemic gastrin levels. Autocrine gastrin may also have a role in tumour development, as expression occurs early in the adenoma-carcinoma sequence. In addition, several studies using animal models have shown that systemic hypergastrinaemia promotes the proliferation of both normal and neoplastic colonic epithelium. Hyperproliferative colonic epithelium in the presence of hypergastrinaemia has been recorded in humans and a well-designed epidemiological study has demonstrated an increased incidence of colorectal cancer. Gastrin is a potential therapeutic target in the treatment of colorectal cancer and several approaches have been assessed. Receptor antagonists and antisecretory agents have been demonstrated to be ineffectual. Novel methods of inhibition, including the use of anti-gastrin antibodies, are currently being evaluated.

Glycine-extended gastrin synergizes with gastrin 17 to stimulate acid secretion in gastrin-deficient mice

BACKGROUND & AIMS

Studies in gastrin-deficient mice have demonstrated critical roles for gastrin peptides in the regulation of gastric acid secretion, but the relative contributions of amidated (G-17) and glycine-extended (G17-Gly) gastrin remain unclear. We examined the effects of these 2 forms of gastrin on acid secretion in gastrin-deficient mice.

METHODS

Sixty gastrin-deficient mice received infusions of saline, or 1, 6, or 14 days of amidated gastrin 17 (G-17), G17-Gly, or both G-17 and G17-Gly at 10 nmol. kg(-1). h(-1). Twenty-four gastrin-deficient mice were then infused for 14 days with 1, 2, or 5 nmol. kg(-1). h(-1) of G-17 or G-17 and G17-Gly. Acid secretion was determined 4 hours after pyloric ligation, and gastric tissue was processed for histology, immunohistochemistry, and electron microscopy.

RESULTS

Infusion of G-17 increased acid secretion in a dose-dependent manner with a peak at 5 nmol. kg(-1). h(-1) and a subsequent decrease in acid secretion at higher doses. Infusion of G17-Gly alone had no effect on acid secretion, but coinfusion with G-17 resulted in significantly higher levels of acid secretion at all doses examined than infusion with G-17 alone. The potentiating effect of G17-Gly on G-17-induced acid secretion was associated with increased parietal cell activation but was independent of changes in parietal and enterochromaffin-like cell number, fundic proliferation rates, and H(+),K(+)-adenine triphosphatase expression. G17-Gly also prevented the formation of vacuolar canaliculi and lipofuscin bodies in the parietal cells induced by G-17.

CONCLUSIONS

G17-Gly appears to synergize with G-17 to up-regulate acid secretion and prevent parietal cell degradation. These results suggest that G17-Gly plays an important role in parietal cell function.

Hydrolysis by somatic angiotensin-I converting enzyme of basic dipeptides from a cholecystokinin/gastrin and a LH-RH peptide extended at the C-terminus with gly-Arg/Lys-arg, but not from diarginyl insulin

Endoproteolytic cleavage of protein prohormones often generates intermediates extended at the C-terminus by Arg-Arg or Lys-Arg, the removal of which by a carboxypeptidase (CPE) is normally an important step in the maturation of many peptide hormones. Recent studies in mice that lack CP activity indicate the existence of alternative tissue or plasma enzymes capable of removing C-terminal basic residues from prohormone intermediates. Using inhibitors of angiotensin I-converting enzyme (ACE) and CP, we show that both these enzymes in mouse serum can remove the basic amino acids from the C-terminus of CCK5-GRR and LH-RH-GKR, but only CP is responsible for converting diarginyl insulin to insulin. ACE activity removes C-terminal dipeptides to generate the Gly-extended peptides, whereas CP hydrolysis gives rise to CCK5-GR and LH-RH-GK, both of which are susceptible to the dipeptidyl carboxypeptidase activity of ACE. Somatic ACE has two similar protein domains (the N-domain and the C-domain), each with an active site that can display different substrate specificities. CCK5-GRR is a high-affinity substrate for both the N-domain and C-domain active sites of human sACE (Km of 9.4 microm and 9.0 microm, respectively) with the N-domain showing greater efficiency (kcat : Km ratio of 2.6 in favour of the N-domain). We conclude that somatic forms of ACE should be considered as alternatives to CPs for the removal of basic residues from some Arg/Lys-extended peptides.

Biological activity of carboxy-terminal gastrin analogs

Amidated forms of gastrin are derived by post-translational processing of a large precursor peptide and stimulate gastric acid secretion via the gastrin/CCK(B) receptor. Non-amidated biosynthetic intermediates may exert biological effects through other mechanisms, but their effect on gastric acid secretion is unclear. Amidated gastrins stimulate acid secretion mainly by releasing histamine from mucosal enterochromaffin-like cells. This study examines the effects on histamine release from the vascularly perfused rat stomach of amidated gastrin-17, COOH-terminal glycine-extended gastrin-17, gastrin-17 extended at the COOH-terminal including the remaining progastrin sequence, and carboxy-terminal progastrin fragments (SAEDEN and GRRSAEDEN). Carboxy-terminal extended gastrins induced histamine release which was inhibited by the gastrin/CCK(B) antagonist L-740,093, but had to be given in concentrations 100-fold higher than amidated gastrin-17 to produce comparable effects. These progastrin-derived peptides are found in high concentrations in some patients with the Zollinger-Ellison syndrome and may contribute to acid hypersecretion and other gastrin/CCK(B) receptor mediated responses.

The new biology of gastrointestinal hormones

The classic concept of gastrointestinal endocrinology is that of a few peptides released to the circulation from endocrine cells, which are interspersed among other mucosal cells in the upper gastrointestinal tract. Today more than 30 peptide hormone genes are known to be expressed throughout the digestive tract, which makes the gut the largest endocrine organ in the body. Moreover, development in cell and molecular biology now makes it feasible to describe a new biology for gastrointestinal hormones based on five characteristics. 1) The structural homology groups the hormones into families, each of which is assumed to originate from a common ancestral gene. 2) The individual hormone gene is often expressed in multiple bioactive peptides due to tandem genes encoding different hormonal peptides, alternative splicing of the primary transcript, or differentiated processing of the primary translation product. By these mechanisms, more than 100 different hormonally active peptides are produced in the gastrointestinal tract. 3) In addition, gut hormone genes are widely expressed, also outside the gut. Some are expressed only in neuroendocrine cells, whereas others are expressed in a multitude of different cells, including cancer cells. 4) The different cell types often express different products of the same gene, "cell-specific expression." 5) Finally, gastrointestinal hormone-producing cells release the peptides in different ways, so the same peptide may act as an acute blood-borne hormone, as a local growth factor, as a neurotransmitter, and as a fertility factor. The new biology suggests that gastrointestinal hormones should be conceived as intercellular messengers of general physiological impact rather than as local regulators of the upper digestive tract.

Gastric effects of the CCK-B/gastrin receptor ligand CI-988 in cynomolgus monkeys

We have previously demonstrated that the CCK-B/gastrin receptor ligand CI-988 induces gastric gland degeneration and atrophy in cynomolgus monkeys, an effect consistent with gastrin receptor antagonism and inhibition of gastrin's trophic effects on oxyntic mucosa. However, gastrin receptor ligands of the dipeptoid chemical series to which CI-988 belongs have been reported to act as agonists or antagonists towards gastrin-related events, depending on the animal model and the functional endpoint examined. To investigate further these apparently conflicting data, basal gastric acid secretion was monitored acutely in conscious monkeys given CI-988 orally at 10 mg/kg or intravenously at 0.01 mumol/kg/hr and histological changes in gastric mucosa were evaluated in monkeys given CI-988 orally at 5, 25 or 75 mg/kg/day for 4 weeks. Degeneration and atrophy of gastric glands occurred at 25 and 75 mg/kg with statistically significant decrements in gastric mucosal height at 75 mg/kg. In addition, CI-988 stimulated gastric acid secretion when given either orally or intravenously. Co-administration of the structurally unrelated CCK-B/gastrin antagonist L-365,260 completely blocked CI-988-stimulated acid secretion, confirming that CI-988's agonist effect on acid secretion is mediated by the gastrin receptor. Assuming that gastric mucosal degeneration is the result of inhibition of gastrin's trophic activity, CI-988 appears to induce paradoxical agonist and antagonist gastrin-receptor mediated effects.

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.

The G cell

The study of gastrin continues to serve as an excellent model for gastrointestinal regulatory processes. This review highlights some recent advances in the field by outlining gastrin biosynthesis, summarizing current understanding of gastrin receptors, describing the regulation of gastrin release, and discussing the clinical implications of gastrin in the pathogenesis of peptic ulcer disease. Emphasis is on three emerging areas of gastrin research: the novel finding that one of gastrin's posttranslational processing intermediates has biological activity distinct from that of the mature peptide; elucidation of gastrin's signal transduction mechanisms that mediate the trophic effects of the peptide; and the role of gastrin in peptic ulcer disease pathogenesis secondary to Helicobacter pylori infection.

Ontogeny of gastrin and cholecystokinin in the colon and duodenum of sheep

The different roles of gastrin and cholecystokinin in the fetus compared to the adult may be reflected in different distribution patterns. Re-expression of these fetal patterns is often seen in tumours of the adult. Using region-specific antisera and chromatography, we have determined the ontogeny of amidated gastrin (G-amide), glycine extended gastrin (G-gly), and cholecystokinin (CCK) in various segments of the colon and compared it to the developmental profile in the duodenum. Fetal sheep aged 80-90, 115-125 and 135-144 days (term is 145 days), 7-14 day lamb, and adult sheep were examined. In the colon, higher concentrations of G-amide (2.8 +/- 0.2 pmol/g) and CCK (11.7 +/- 1.6 pmol/g) were measured in the fetus while G-gly (0.7 +/- 0.1 pmol/g) was higher in the adult compared to other age groups. The calculated G-gly/G-amide ratio was 0.4 in the fetus and 1.4 in the adult while the CCK/G-amide ratios were 5 in the fetus and 13 in the adult. The duodenum of the lamb rather than the fetus contained the highest concentrations of G-amide, G-gly and CCK (40.3 +/- 9.7, 2.0 +/- 0.4, 109.0 +/- 14.3 pmol/g, respectively) and at concentrations exceeding that in the colon. The results demonstrate two major developmentally regulated features. Firstly as the colon matures, there is a gradual switch between the expression of the gastrin and CCK genes and secondly, the processing to G-amide is attenuated. These findings suggest that non-amidated gastrin should be examined for a potential role as a growth factor in colorectal carcinogenesis.

Gastrin and glycine-extended progastrin processing intermediates induce different programs of early gene activation

We recently reported that gastrin and glycine-extended progastrin processing intermediates (G-Gly) exert growth-promoting effects on AR4-2J cells (derived from rat pancreas) via interaction with distinct receptors. In this study we sought to investigate the mechanisms by which gastrin and G-Gly stimulate cell proliferation. While gastrin increased [Ca2+]i in AR4-2J cells, G-Gly had no effect. Similarly, G-Gly had no effect either on basal and 10(-7) M vasoactive intestinal polypeptide-stimulated cAMP generation, although gastrin is known to inhibit cAMP generation. Gastrin dose dependently stimulated AR4-2J cell mRNA content of both c-fos and c-jun, two genes known to function in regulating cell proliferation, but G-Gly had no effect. Gastrin also induced the expression of luciferase in AR4-2J cells transfected with a construct consisting of a luciferase reporter gene coupled to the serum response element of the c-fos gene promoter. In similar fashion, gastrin stimulated the activity of mitogen-activated protein kinase, an enzyme known to mediate the induction of the c-fos serum response element in response to growth factor stimulation. Although G-Gly had none of these effects of gastrin in AR4-2J cells, it stimulated activity of c-Jun amino-terminal kinase, an enzyme known to phosphorylate and transcriptionally activate c-Jun. These data support the notion that gastrin stimulates cell proliferation by inducing c-fos and c-jun gene expression, while G-Gly acts by post-translationally regulating early gene transcriptional activation. Our studies represent a novel model in which both the precursor and the product of a key processing reaction, peptide alpha-amidation, act cooperatively to stimulate cell proliferation via distinct receptors linked to different signal transduction pathways.

Epidermal growth factor induces H+,K+-ATPase alpha-subunit gene expression through an element homologous to the 3' half-site of the c-fos serum response element

Epidermal growth factor (EGF) acutely inhibits acid secretion; however, prolonged administration of EGF has been reported to increase acid production. We undertook these studies to examine whether the physiological effects of EGF on acid secretion are mediated by regulation of gastric H+,K+-ATPase, the principle enzyme responsible for acid secretion. EGF in concentrations equivalent to those in plasma increased H+,K(+)-ATPase alpha-subunit mRNA levels. Using H+,K(+)-ATPase-luciferase constructs transfected into primary cultured parietal cells, a significant step up in EGF inducibility was observed between bases -162 and -156 (5'-GACATGG-3') relative to the cap site. This EGF response element (ERE) conferred EGF inducibility when linked to homologous and heterologous promoters. The ERE is homologous to the 3' half-site of the c-fos serum response element to which rNFIL-6, rE12, and SRE-ZBP bind. Electrophoretic mobility shift assays using an ERE probe and parietal cell nuclear extracts revealed a specific DNA-protein complex, the formation of which was changed by neither E12 and NFIL-6 consensus oligonucleotides nor antibodies for NFIL-6, SRE-ZBP, and E12. Our studies indicate that EGF induces gastric H+,K(+)-ATPase alpha-subunit gene expression via an interaction between a specific ERE and a novel transcriptional factor and that this may be a physiologic mechanism by which EGF regulates acid secretion.