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.

Neurogenin 3 is essential for the proper specification of gastric enteroendocrine cells and the maintenance of gastric epithelial cell identity

The notch signaling pathway is essential for the endocrine cell fate in various tissues including the enteroendocrine system of the gastrointestinal tract. Enteroendocrine cells are one of the four major cell types found in the gastric epithelium of the glandular stomach. To understand the molecular basis of enteroendocrine cell development, we have used gene targeting in mouse embryonic stem cells to derive an EGFP-marked null allele of the bHLH transcription factor, neurogenin 3 (ngn3). In ngn3(-/-) mice, glucagon secreting A-cells, somatostatin secreting D-cells, and gastrin secreting G-cells are absent from the epithelium of the glandular stomach, whereas the number of serotonin-expressing enterochromaffin (EC) cells is decreased dramatically. In addition, ngn3(-/-) mice display intestinal metaplasia of the gastric epithelium. Thus, ngn3 is required for the differentiation of enteroendocrine cells in the stomach and the maintenance of gastric epithelial cell identity.

Expression, processing, and secretion of gastrin in patients with colorectal carcinoma

BACKGROUND & AIMS

The relationship between gastrin and the development of colorectal carcinoma (CRC) remains controversial. Problems with previous studies include failure to measure all forms of gastrin, lack of comparison between stored and secreted gastrin, and not controlling for Helicobacter pylori infection (a known cause of hypergastrinemia). The aim of this study was to quantify progastrin and progastrin-derived peptides in the resected tumor and plasma of patients with CRC and in the antrum and plasma of normal subjects.

METHODS

Four region-specific gastrin antisera were used to measure progastrin, glycine-extended gastrin, amidated gastrin, and total gastrin.

RESULTS

Progastrin, amidated gastrin, total gastrin, and glycine-extended gastrin were detected in 100%, 69%, 56%, and 44% of tumors, respectively (n = 32). When allowing for H. pylori infection, circulating amidated gastrin levels were not significantly elevated in patients with CRC. However, compared with control H. pylori-positive and H. pylori-negative subjects, fasting plasma total gastrin levels were increased in H. pylori-positive (5.2-fold) and H. pylori-negative (2.3-fold) patients with CRC.

CONCLUSIONS

Gastrin or its processing intermediates are present in a high proportion of CRCs. Nonamidated gastrin levels are elevated in the circulation of patients with CRC regardless of H. pylori status. We conclude that gastrin should continue to be assessed as a circulating or autocrine growth factor in the development of CRC.

Reciprocal regulation of antral gastrin and somatostatin gene expression by omeprazole-induced achlorhydria

Gastric acid exerts a feedback inhibition on the secretion of gastrin from antral G cells. This study examines whether gastrin gene expression is also regulated by changes in gastric pH. Achlorhydria was induced in rats by the gastric H+/K+ ATPase inhibitor, omeprazole (100 mumol/kg). This resulted in fourfold increases in both serum gastrin (within 2 h) and gastrin mRNA levels (after 24 h). Antral somatostatin D cells probably act as chemoreceptors for gastric acid to mediate a paracrine inhibition on gastrin secretion from adjacent G cells. Omeprazole-induced achlorhydria reduced D-cell activity as shown by a threefold decrease in antral somatostatin mRNA levels that began after 24 h. Exogenous administration of the somatostatin analogue SMS 201-995 (10 micrograms/kg) prevented both the hypergastrinemia and the increase in gastrin mRNA levels caused by omeprazole-induced achlorhydria. Exogenous somatostatin, however, did not influence the decrease in antral somatostatin mRNA levels seen with achlorhydria. These data, therefore, support the hypothesis that antral D cells act as chemoreceptors for changes in gastric pH, and modulates somatostatin secretion and synthesis to mediate a paracrine inhibition on gastrin gene expression in adjacent G cells.

The KCNE2 potassium channel ancillary subunit is essential for gastric acid secretion

Genes in the KCNE family encode single transmembrane domain ancillary subunits that co-assemble with voltage-gated potassium (Kv) channel alpha subunits to alter their function. KCNE2 (also known as MiRP1) is expressed in the heart, is associated with human cardiac arrhythmia, and modulates cardiac Kv alpha subunits hERG and KCNQ1 in vitro. KCNE2 and KCNQ1 are also expressed in parietal cells, leading to speculation they form a native channel complex there. Here, we disrupted the murine kcne2 gene and found that kcne2 (-/-) mice have a severe gastric phenotype with profoundly reduced parietal cell proton secretion, abnormal parietal cell morphology, achlorhydria, hypergastrinemia, and striking gastric glandular hyperplasia arising from an increase in the number of non-acid secretory cells. KCNQ1 exhibited abnormal distribution in gastric glands from kcne2 (-/-) mice, with increased expression in non-acid secretory cells. Parietal cells from kcne2 (+/-) mice exhibited normal architecture but reduced proton secretion, and kcne2 (+/-) mice were hypochlorhydric, indicating a gene-dose effect and a primary defect in gastric acid secretion. These data demonstrate that KCNE2 is essential for gastric acid secretion, the first genetic evidence that a member of the KCNE gene family is required for normal gastrointestinal function.

Detection and partial sequence analysis of gastrin mRNA by using an oligodeoxynucleotide probe

We have used a specific deoxyoligonucleotide probe to detect gastrin mRNA in poly(A)-enriched RNA preparations from hog antrum. The nucleotide sequence of the oligonucleotide, d(C-T-C-C-T-C-C-A-T-C-C-A), was deduced from the unique amino acid sequence Trp-Met-Glu-Glu of gastrin. When used with hog antral RNA, the dodecanucleotide is an effective primer for the synthesis of gastrin-specific cDNA as judged by nucleotide sequence analysis of cDNA isolated by polyacrylamide gel electrophoresis. We have determined an 81-nucleotide sequence corresponding to the region of the gastrin mRNA that codes for the known amino acid sequence of the G34 progastrin intermediate species, and we have demonstrated the presence of two consecutive basic residues preceding the G34 sequence in the prohormone. Hybridization of gastrin cDNA or synthetic dodecanucleotide to hog antral RNA separated by gel electrophoresis on agarose gels in the presence of methylmercuric hydroxide indicates that the mRNA coding for gastrin is about 620 nucleotides long. These results suggest that the gastrin precursor peptide contains 110-140 amino acids. This method should be of general application for detection and characterization of mRNAs corresponding to proteins of known amino acid sequence.

Helicobacter pylori potentiates epithelial:mesenchymal transition in gastric cancer: links to soluble HB-EGF, gastrin and matrix metalloproteinase-7

BACKGROUND AND AIMS

Helicobacter pylori (H pylori) infection is a major risk factor in the development of distal gastric adenocarcinoma. Development of the invasive phenotype is associated with the phenomenon of epithelial:mesenchymal transition (EMT). Soluble heparin-binding epidermal growth factor (HB-EGF) has been implicated in this process. A study was undertaken to investigate the possibility that matrix metalloproteinase (MMP)-7 is upregulated in H pylori infection as a result of hypergastrinaemia, which may enhance shedding of HB-EGF and contribute towards EMT in gastric adenocarcinoma cell lines.

METHODS

Three gastric epithelial cell lines (AGS, MGLVA1 and ST16) were co-cultured with the pathogenic H pylori strain 60190 and non-pathogenic strain Tx30a in an in vitro infection model. Gene expression was quantified by real-time PCR, HB-EGF shedding by ELISA and protein expression by immunofluorescence or immunohistochemistry. The INS-GAS mouse, a transgenic mouse model of gastric carcinogenesis which overexpresses amidated gastrin, was used to investigate the in vivo relationship between HB-EGF, MMP-7, gastrin and EMT.

RESULTS

The pathogenic strain of H pylori significantly upregulated EMT-associated genes Snail, Slug and vimentin in all three gastric cell lines to a greater degree than the non-pathogenic strain. Pathogenic H pylori also upregulated HB-EGF shedding, a factor implicated in EMT, which was partially dependent on both gastrin and MMP-7 expression. Gastrin and MMP-7 siRNAs and MMP-7 neutralising antibody significantly reduced upregulation of HB-EGF shedding in H pylori infected gastric cell lines and reduced EMT gene expression. The effect of H pylori on EMT was also reversed by gastrin siRNA. Neutralisation of gastrin in the INS-GAS mouse model reduced expression of MMP-7, HB-EGF and key EMT proteins.

CONCLUSION

The upregulation of MMP-7 by pathogenic H pylori is partially dependent on gastrin and may have a role in the development of gastric cancer, potentially through EMT, by indirectly increasing levels of soluble HB-EGF.

Retrospective study of 77 pancreatic endocrine tumors using the immunoperoxidase method

Immunocytochemical stains for various pancreatic hormones were performed on 77 pancreatic endocrine tumors from 59 patients [17 with hypoglycemia, three with glucagonoma syndrome, 18 were Zollinger-Ellison syndrome, six with WDHA (watery, diarrhea, hypokalemia, and achlorhydria) syndrome and 15 without endocrine symptoms]. In all tumors that caused either hypoglycemia or glucagonoma syndrome, insulin and glucagon were respectively identified. On the other hand, only 10 tumors from 18 patients with Zollinger-Ellison syndrome were positive for gastrin, and only four of six patients with WDHA syndrome had a vasoactive intestinal peptides-positive tumor. Ten of 15 clinically silent tumors contained hormone-producing cells but without a consistent pattern. Ten neoplasms were negative for all hormones tested. Twenty-six tumors showed positively for more than one hormone and usually one cell type predominated. Four patients had multiple tumors which showed variation in the architecture and cellular composition. The tumors were classified into three major histopathologic groups: solid, gyriform, and glandular. The correlation between the pattern of growth and the hormonal production was generally poor. However, a pure gyriform pattern was often associated with insulin production, and glandular differentiation was commonly seen in tumors associated with Zollinger-Ellison syndrome. This study demonstrates the reliability of the immunocytochemical method for the specific identification of cell types in pancreatic endocrine tumors.

The role of matrix metalloproteinase-7 in redefining the gastric microenvironment in response to Helicobacter pylori

BACKGROUND & AIMS

Interactions between epithelial and stromal cells are important determinants of mucosal organization, but the signaling mechanisms are understood incompletely. Matrix metalloproteinase (MMP)-7 is produced uniquely in epithelia, may act on growth factors and matrix proteins, and in the stomach is increased with Helicobacter pylori infection. We have studied the role of MMP-7 in signaling between epithelial cells and a key stromal cell type, the myofibroblast.

METHODS

Immunohistochemistry and Western blotting were applied to gastric corpus biopsy specimens; primary cultures of human gastric glands and myofibroblasts were used to study the role of MMP-7 in regulating proliferation and migration of the latter, and MMP-7 substrates were identified by proteomic methods.

RESULTS

Increased abundance of the myofibroblast marker alpha-smooth muscle actin was identified in H. pylori-positive biopsy specimens. Media from H pylori-infected gastric epithelial cultures stimulated proliferation and migration of primary human gastric myofibroblasts and antisense oligonucleotide treatment indicated a role for MMP-7. Proteomic methods identified insulin-like growth factor binding protein (IGFBP)-5 as a substrate for MMP-7 in medium from gastric myofibroblasts. Knockdown of IGFBP-5 by small interfering RNA or immunoneutralization of IGF-II, abolished myofibroblast responses to MMP-7. Proliferation of gastric epithelial cells also was stimulated by MMP-7-treated myofibroblasts via IGF-II.

CONCLUSIONS

MMP-7 acts as an epithelial-derived signal increasing the bioavailability of IGF-II released from myofibroblasts. Because IGF-II acts on both stromal and epithelial cells, the findings suggest that increased MMP-7 expression contributes to redefining the niche occupied by dividing cells and leading to hyperproliferation in H pylori infection.

Antral gastrin-producing G-cells and somatostatin-producing D-cells in different states of gastric acid secretion

The number of G- and D-cells per area and the ratio of G/D-cells were investigated in biopsy specimens of the pyloric antrum from normochlorhydric subjects without peptic ulcer, from patients with duodenal ulcer, gastrinoma, pernicious anaemia, and after selective proximal vagotomy. Compared with normochlorhydric subjects antral G-cell density was significantly raised in pernicious anaemia, unchanged in duodenal ulcer, and diminished in gastrinoma patients. After vagotomy G-cell density was found to be raised if compared with patients with duodenal ulcer. D-cell density was significantly increased in gastrinoma patients, unchanged in duodenal ulcer, and diminished in pernicious anaemia and after vagotomy. The G/D-cell ratio was increased in pernicious anaemia and after vagotomy, unchanged in duodenal ulcer, and decreased in gastrinoma patients. It is concluded that the antral pH governs the ratio of G- and D-cells. Therefore, the G/D cell ratio increases in states of reduced acid secretion and decreases in massive hyperchlorhydria. Hypergastrinaemia as such does not affect the G/D-cell ratio.

Beta cell secretion--from the standpoint of a pathobiologist

A proposed model depicting the biologic events in the formation, storage and releaseof insulin in beta cells is presented. The proposed sequence of events is as follows: intracellular glucose metabolism signals the initiation of proinsulin synthesis in the endoplasmic reticulum; pro-insulin is transferred to the Golgi complex; the conversion of proinsulin to insulin probably occurs in the Golgi complex and intermediate beta granules may be present in this structure; mature beta granules enclosed in smooth membranous sacs are released into the cytoplasm and becomeattached to the microtubular-microfilament system; glucose metabolism initiates the entry of calcium which triggers the change in physical conformation of the microtubules with resultant displacement of the granules to the cell surface where they are released by emiocytosis.

The murine gastrin promoter is synergistically activated by transforming growth factor-beta/Smad and Wnt signaling pathways

The transforming growth factor-beta (TGF-beta) and Wnt/wingless pathways play critical roles in the specification of cell fate during development and also contribute to cancer formation and progression. Whereas Wnt signaling is clearly pro-oncogenic, TGF-beta signaling is cell- and context-dependent, manifesting both inhibitory and proliferative effects. The growth factor, gastrin, has previously been shown to be a downstream target of the Wnt pathway and a promoter of gastrointestinal cancer. In this study, we show that the mouse gastrin promoter is regulated synergistically by TGF-beta/Smads and beta-catenin/T-cell factor (TCF). Co-transfection of Smad3/Smad4 and beta-catenin expression constructs synergistically activated mouse gastrin promoter activity 30-60-fold in AGS cells with minimal effect seen with either construct alone. This activation was further potentiated by TGF-beta1 treatment. Mutating either the TCF binding site or the Smad-binding element (SBE) diminished the activation of gastrin expression by Smad3/Smad4 and beta-catenin and led to a loss of gastrin promoter responsiveness to TGF-beta1 treatment. Wnt and TGF-beta regulated endogenous gastrin mRNA levels in AGS cells in a similar fashion, as revealed by small interference RNA studies or overexpression of Smads and TCF4/beta-catenin. Electrophoretic mobility shift assays and DNA affinity precipitation assays showed that the putative SBE and T-cell factor (TCF) sites were able to bind a complex containing Smads and beta-catenin/TCF4. In addition, the synergy between Smads and beta-catenin/TCF4 was dependent on CREB-binding protein (CBP)/P300, as demonstrated by overexpression of CBP or E1A. Moreover, by using a heterogeneous promoter reporter system, we showed that this complex containing Smads/TCF4/beta-catenin complex was able to up-regulate transcription at isolated SBE or TCF sites. Thus, the Wnt signaling pathway is able to activate some target genes through its actions as a co-activator at non-TCF sites and has the potential to profoundly alter transcriptional responses to TGF-beta signaling.

Post-translational processing of gastrin in neoplastic human colonic tissues

Gastrin has been postulated to stimulate proliferation in colorectal neoplasms. Although gastrin mRNA has been demonstrated to be present in colon cancer cell lines, the intact peptide had not been recovered from human colorectal neoplasms. We demonstrate that gastrin and its precursors are present in both colorectal neoplasia and adjacent normal-appearing colonic mucosa. In colonic tissue, the glycine-extended precursor form of the peptide is over 10-fold more abundant than the amidated gastrin, and progastrin is more than 700-fold more abundant. In contrast, amidated gastrin in the human antrum is the predominant form of gastrin by a factor of 10. Furthermore, the ratio of gastrin precursors to gastrin is significantly increased in neoplastic colonic mucosa when compared with normal colonic tissue. These data suggest that the processing of gastrin is unique in the human colon and that further differences in processing occur in neoplastic colonic tissue.

Regulation by gastric acid of the processing of progastrin-derived peptides in rat antral mucosa

1. Inhibition of gastric acid secretion by proton pump inhibitors like omeprazole increases the synthesis and secretion of the pyloric antral hormone gastrin. We report here how omeprazole influences the conversion of the gastrin precursor to its final products, and the abundance of mRNAs encoding proteins associated with progastrin processing in rat antral mucosa. 2. Progastrin processing was studied using a pulse-chase protocol in antral mucosa, incubated in vitro, from rats treated with omeprazole for up to 5 days. Labelled peptides were detected by on-line scintillation counting after immunoprecipitation and HPLC. The mRNAs encoding prohormone-processing enzymes were identified by Northern blot, polymerase chain reaction or ribonuclease protection assay, and their cellular origins identified by immunocytochemistry. 3. Cleavage of [3H]- and [35S]-labelled progastrins at Arg-94-95 or Arg-57-58, and amidation at Phe-92 were not influenced by pretreatment with omeprazole. In contrast, cleavage of G34 (the thirty-four amino acid amidated gastrin) at Lys-74-75 to give G17 (the seventeen amino acid amidated gastrin), and of G34-Gly to G17-Gly (G34 and G17 with COOH-terminal glycine), was increased 3-fold after treatment with omeprazole for either 1 or 5 days. 4. Approximately 20% of newly synthesized amidated and Gly-extended gastrins were secreted within 240 min of the labelling period in omeprazole-treated samples, but secretion of labelled gastrins from control tissue was undetectable over a comparable period. 5. The amidating enzyme, peptidyglycine alpha-amidating mono-oxygenase (PAM), the prohormone convertases PC1/3, PC2, PC5 and the PC2 chaperone 7B2 were localized to rat antral gastrin cells by immunocytochemistry. The relative abundance of mRNA species encoding 7B2, PC5 and PAM were unchanged after treatment with omeprazole for 5 days, whereas gastrin, PC1/3 and PC2 mRNAs are known to increase at this time. 6. The main consequence of increased cleavage at Lys-74-75 is the production of G17 and G17-Gly at the expense of G34 and G34-Gly, respectively. The latter have longer plasma half-lives, and so their increased cleavage may serve to limit the rise in plasma gastrin concentration after inhibition of acid secretion. Changes in the abundance of mRNAs encoding prohormone-processing enzymes cannot account for the rapidity of the changes in cleavage of progastrin at Lys residues after omeprazole.

Expression of gastrin and transforming growth factor-alpha during duct to islet cell differentiation in the pancreas of duct-ligated adult rats

In adult rats islet cell neogenesis can be stimulated by partial duct ligation. Duct to islet cell differentiation is thought to be regulated by growth factors such as gastrin and transforming growth factor-alpha (TGF alpha). To test this hypothesis, we examined the expression of gastrin and TGF alpha at the mRNA and protein level in pancreatic tissue following partial duct ligation. Pancreatic specimens were investigated on days 3, 5, 7 and 14 after duct ligation by means of non-isotopic in situ hybridization, immunocytochemistry and Western blotting. Gastrin mRNA was strongly expressed in newly developed duct-like cell structures in the ligated tail portion of the pancreas before the period of pronounced islet cell neogenesis (days 5 and 7), and immunostaining for gastrin peptides was positive at days 5-7. In the non-ligated head portion and in control pancreases, gastrin was not expressed. Expression of TGF alpha was found to be increased in the ligated tail portion of the pancreas on day 3 and particularly on day 5, while there was no enhanced signal in the non-ligated part. Western blotting revealed two different TGF alpha isoforms (18 kDa and 42 kDa) in the ligated tail part and three isoforms (18 kDa, 24 kDa and 42 kDa) in the non-ligated head part and in untreated pancreases. The induction of gastrin and TGF alpha expression preceded the peak in the bromodeoxyuridine pulse labelling index of beta cells, known from a previous study to occur on day 7. We conclude that pancreas duct ligation induces the overexpression of gastrin and TGF alpha in the first days following ligation. Since ductal cells are known to give rise to endocrine cells after duct ligation, gastrin and TGF alpha may play a role as growth factors in islet neogenesis.

Hormonally defined pancreatic and duodenal neuroendocrine tumors differ in their transcription factor signatures: expression of ISL1, PDX1, NGN3, and CDX2

We recently identified the transcription factor (TF) islet 1 gene product (ISL1) as a marker for well-differentiated pancreatic neuroendocrine tumors (P-NETs). In order to better understand the expression of the four TFs, ISL1, pancreatico-duodenal homeobox 1 gene product (PDX1), neurogenin 3 gene product (NGN3), and CDX-2 homeobox gene product (CDX2), that mainly govern the development and differentiation of the pancreas and duodenum, we studied their expression in hormonally defined P-NETs and duodenal (D-) NETs. Thirty-six P-NETs and 14 D-NETs were immunostained with antibodies against the four pancreatic hormones, gastrin, serotonin, calcitonin, ISL1, PDX1, NGN3, and CDX2. The TF expression pattern of each case was correlated with the tumor's hormonal profile. Insulin-positive NETs expressed only ISL1 (10/10) and PDX1 (9/10). Glucagon-positive tumors expressed ISL1 (7/7) and were almost negative for the other TFs. Gastrin-positive NETs, whether of duodenal or pancreatic origin, frequently expressed PDX1 (17/18), ISL1 (14/18), and NGN3 (14/18). CDX2 was mainly found in the gastrin-positive P-NETs (5/8) and rarely in the D-NETs (1/10). Somatostatin-positive NETs, whether duodenal or pancreatic in origin, expressed ISL1 (9/9), PDX1 (3/9), and NGN3 (3/9). The remaining tumors showed labeling for ISL1 in addition to NGN3. There was no association between a particular TF pattern and NET features such as grade, size, location, presence of metastases, and functional activity. We conclude from our data that there is a correlation between TF expression patterns and certain hormonally defined P-NET and D-NET types, suggesting that most of the tumor types originate from embryologically determined precursor cells. The observed TF signatures do not allow us to distinguish P-NETs from D-NETs.

Epidermal growth factor stimulation of the human gastrin promoter requires Sp1

Growth factors coordinately regulate a variety of different genes to stimulate cellular proliferation. In the stomach, gastrin, epidermal growth factor (EGF), and transforming growth factor-alpha all mediate gastric mucosal homeostasis by promoting cell renewal. We have previously shown that EGF and phorbol esters stimulate the human gastrin promoter through a novel GC-rich DNA element 5'-(68)GGGGCGGGGTGGGGGG-53 called gERE (gastrin EGF response element). In this report, we show that three factors bind to this element, the transcription factor Sp1 and two fast migrating complexes designated gastrin EGF response proteins (gERP 1 and 2). To understand how these factors bind and confer EGF responsiveness, mutations of gERE were tested in vitro for protein binding and in vivo for promoter activation. Both gel shift assays and UV cross-linking studies revealed that the factors bind to overlapping domains, Sp1 to the 5' half-site and gERP 1 and 2 to the 3' half-site. Placing either the 5' or 3' mutations upstream of a minimal gastrin promoter abolished EGF induction. Therefore both the 5' and 3' domains were required to confer EGF induction. Collectively, these results demonstrate that complex interactions between Sp1 and other factors binding to overlapping gERE half-sites confer EGF responsiveness to the gastrin promoter.

Adherens junctions and tight junctions are regulated via different pathways by progastrin in epithelial cells

Adhesion between neighbouring epithelial cells is a crucial and tightly controlled process. In the gastrointestinal tract, the integrity of cell-cell contacts is essential for the regulation of electrolyte absorption and for the prevention of tumour metastasis. We recently showed that migration of the gastric epithelial cell line IMGE-5 is stimulated by the nonamidated form of the hormone gastrin(17). Here, we examine the effect on cell-cell adhesion of the prohormone progastrin, the concentration of which is increased in the plasma of patients with colorectal carcinoma. Progastrin induced the dissociation of both tight junction (TJ) and adherens junction (AJ) complexes in IMGE-5 cells. In progastrin-secreting DLD-1 human colorectal carcinoma cells, expression of an antisense gastrin construct restored membrane localisation of zonula occludens-1 (ZO-1), occludin, beta-catenin and E-cadherin. This restoration was reversed by treatment with exogenous progastrin. Endogenous or exogenous progastrin also increased the paracellular flux of mannitol, and induced cell migration of several gastrointestinal cell lines. In addition, progastrin enhanced Src tyrosine kinase activity and induced a spatial delocalisation of protein kinase C alpha. Using dominant-negative mutants and pharmacological inhibitors, we showed that the stimulation of Src kinase activity was essential for the regulation of TJs. By contrast, the dissociation of AJs involved phosphatidylinositol 3-kinase, partly through the formation of a complex with protein kinase C alpha. We conclude that separate pathways mediate the disruption of AJs and TJs by progastrin. Either pathway may contribute to the co-carcinogenic role of this prohormone in colorectal carcinoma.

Gastric acid secretion and gastrin production in the short bowel syndrome

Excess gastric acid secretion and gastrin production may occur in patients with the short bowel syndrome but the two measurements have never been made simultaneously in man in response to a food stimulus. Using the technique of intragastric titration, this was carried out in eight patients after extensive small bowel resection resulting mainly from vascular occlusion and in eight matched normal control subjects. Basal acid output and peak acid output in response to pentagastrin was also measured separately. Although peak and integrated serum gastrin concentrations were significantly greater in patients (450 +/- SE 109 pg/ml; 113 +/- 2.9X10(-3) pg/ml/min) compared with control subjects (174 +/- 98 pg/ml; 6.1 +/- 2.0X10(-3) pg/ml/min p less than 0.05), no concomitant increase in acid secretion was shown either during intragastric titration or in response to pentagastrin. These findings indicate that there is no rationale for treating these patients with long term anti-ulcer therapy.

Clinical endocrinology and metabolism. Gastrin

The gastric hormone gastrin stimulates gastric acid secretion and epithelial cell proliferation. Multiple active products are generated from the precursor, preprogastrin, including the well-characterized amidated gastrins acting at the cholecystokinin-2 (CCK-2, or gastrin-CCK(B)) receptor, and others that may be growth factors in a range of cancers. Plasma concentrations of the amidated gastrins are elevated as a consequence of gastrin-secreting tumours (gastrinomas) and in conditions in which the normal inhibition of the antral G-cell by acid is depressed, for example chronic atrophic gastritis and prolonged treatment with proton pump inhibitors. There may also be increased gastrin release in Helicobacter pylori infection. Provocative tests for the diagnosis of gastrinoma include the secretin and calcium infusion tests. Hypergastrinaemia is associated with enterochromaffin-like (ECL) cell proliferation; the factors that determine progression to ECL cell dysplasia and gastric ECL cell carcinoid tumours are discussed. Several strategies for inhibiting the effects of gastrin are under evaluation, and their potential application is discussed.

Regulation of rat antral gastrin and somatostatin gene expression during starvation and after refeeding

Antral gastrin and somatostatin gene expression during starvation and after refeeding with liquid meals of varying composition were studied. Northern and slot-blot hybridization analyses showed that starvation caused a marked decrease in antral gastrin messenger RNA (mRNA) level by 12 hours associated with an increase in somatostatin mRNA. After 48 hours of fasting, antral gastrin mRNA was 26% and somatostatin mRNA was 136% of their prefasting levels. Refeeding caused increased 2-hour integrated gastrin mRNA levels after liquid peptone (+45%), phenylalanine (+31%), and olive oil (+13%), but no changes were observed with glucose or saline solutions. Integrated 2-hour immunoreactive antral gastrin content was increased after peptone (+106%), phenylalanine (+68%), and olive oil (+32%) meals but was not increased after glucose (-11%) or saline (-10%). In some cases, both gastrin mRNA and peptide responses could be measured as early as 15 minutes. The same nutrients that increased gastrin mRNA levels caused decreased 2-hour integrated somatostatin mRNA levels; peptone (-30%), phenylalanine (-28%), and olive oil (-21%), but neither glucose nor saline, altered somatostatin mRNA levels. These results suggest that antral gastrin and somatostatin genes were regulated in opposite directions, in a coordinate manner, by specific gastric nutrients that stimulate gastrin release.

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.

Gastrins in cat and dog: evidence for a biosynthetic relationship between the large molecular forms of gastrin and heptadecapeptide gastrin

1. Extracts of antral, duodenal and jejunal mucosa contained the same concentrations and molecular forms of gastrin in cat and dog. In both species component III constituted 93%, component II 4% and components I and IV each 1% of the total immunoreactivity. 2. During electrical vagal stimulation or feeding in (a) anaesthetized cats with ligated kidney vessels and resected jejunum and ileum, (b) anaesthetized normal cats or (c) conscious normal cats, release of the large molecular forms of gastrin, components I and II, was not detectable. 3. Luminal perfusates of cat antrum contained only component III and occasionally less than 2% of component IV. 4. Intravenous injections into conscious cats of components I, II and III isolated from cat antrum revealed a significantly slower elimination of the large components (t1/2 approximately 4.2 and 4.0 min respectively) than of component III (t1/2 approximately 1.1 min). Thus the absence of components I and II in cat blood cannot be due to rapid degradation. 5. During feeding, components I and II constituted 20% of the total gastrin immunoreactivity in antral venous blood of dogs. This amount is sufficient to account for the predominance of large components in peripheral venous blood in dogs considering their slow metabolic clearance rates. 6. Luminal perfusates of dog antrum contained components I, II and III in proportions corresponding to those found in antrum. 7. The results indicate that components I and II are synthesized in the same proportions in gastrin cells of cat and dog. In the cat, components I and II are, however, never released from the cells. Since these components contain heptadecapeptide gastrin within their sequeaces, they probably represent biosynthetic precursors of the principal gastrin, the heptadecapeptide (component III). The predominance of component III in gastric juice suggests that the feline gastrin cell secretes only component III, which degrades to component IV in blood.