Post-translational processing of gastrin in neoplastic human colonic tissues

A novel method to detect hPG80 (human circulating progastrin) in the blood

hPG₈₀ (human circulating progastrin) is produced and released by cancer cells. We recently reported that hPG₈₀ is detected in the blood of patients with cancers from different origins, suggesting its potential utility for cancer detection. To accurately measure hPG₈₀ in the blood of patients, we developed the DxPG₈₀ test, a sandwich Enzyme-Linked Immunosorbent Assay (ELISA). This test quantifies hPG₈₀ in EDTA plasma samples. The analytical performances of the DxPG₈₀ test were evaluated using standard procedures and guidelines specific to ELISA technology. We showed high specificity for hPG₈₀ with no cross-reactivity with human glycine-extended gastrin (hG17-Gly), human carboxy-amidated gastrin (hG17-NH₂) or the CTFP (C-Terminus Flanking Peptide) and no interference with various endogenous or exogenous compounds. The test is linear between 0 and 50 pM hPG₈₀ (native or recombinant). We demonstrated a trueness of measurement, an accuracy and a variability of hPG₈₀ quantification with the DxPG₈₀ test below the 20% relative errors as recommended in the guidelines. The limit of detection of hPG₈₀ and the limit of quantification were calculated as 1 pM and 3.3 pM respectively. In conclusion, these results show the strong analytical performance of the DxPG₈₀ test to measure hPG₈₀ in blood samples.

Progastrin production transitions from Bmi1+/Prox1+ to Lgr5high cells during early intestinal tumorigenesis

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Secretion of progastrin is a signature event of early malignant transformation in the colon.

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In the healthy epithelium, progastrin is produced by a subset of enteroendocrine cells expressing both Bmi1 and Prox1.

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LGR5-high intestinal stem cells are a primary source of progastrin production in early mouse and human intestinal adenomas.

Progastrin is an unprocessed soluble peptide precursor with a well-described tumor-promoting role in colorectal cancer. It is expressed at small levels in the healthy intestinal mucosa, and its expression is enhanced at early stages of intestinal tumor development, with high levels of this peptide in hyperplastic intestinal polyps being associated with poor neoplasm-free survival in patients. Yet, the precise type of progastrin-producing cells in the healthy intestinal mucosa and in early adenomas remains unclear. Here, we used a combination of immunostaining, RNAscope labelling and retrospective analysis of single cell RNAseq results to demonstrate that progastrin is produced within intestinal crypts by a subset of Bmi1⁺/Prox1⁺/LGR5^low endocrine cells, previously shown to act as replacement stem cells in case of mucosal injury. In contrast, our findings indicate that intestinal stem cells, specified by expression of the Wnt signaling target LGR5, become the main source of progastrin production in early mouse and human intestinal adenomas. Collectively our results suggest that the previously identified feed-forward mechanisms between progastrin and Wnt signaling is a hallmark of early neoplastic transformation in mouse and human colonic adenomas.

Gastric Peptides-Gastrin and Somatostatin

Gastric acid secretion (i) facilitates digestion of protein as well as absorption of micronutrients and certain medications, (ii) kills ingested microorganisms, including Helicobacter pylori, and (iii) prevents bacterial overgrowth and enteric infection. The principal regulators of acid secretion are the gastric peptides gastrin and somatostatin. Gastrin, the major hormonal stimulant for acid secretion, is synthesized in pyloric mucosal G cells as a 101-amino acid precursor (preprogastrin) that is processed to yield biologically active amidated gastrin-17 and gastrin-34. The C-terminal active site of gastrin (Trp-Met-Asp-Phe-NH₂ ) binds to gastrin/CCK₂ receptors on parietal and, more importantly, histamine-containing enterochromaffin-like (ECL) cells, located in oxyntic mucosa, to induce acid secretion. Histamine diffuses to the neighboring parietal cells where it binds to histamine H₂ -receptors coupled to hydrochloric acid secretion. Gastrin is also a trophic hormone that maintains the integrity of gastric mucosa, induces proliferation of parietal and ECL cells, and is thought to play a role in carcinogenesis. Somatostatin, present in D cells of the gastric pyloric and oxyntic mucosa, is the main inhibitor of acid secretion, particularly during the interdigestive period. Somatostatin exerts a tonic paracrine restraint on gastrin secretion from G cells, histamine secretion from ECL cells, and acid secretion from parietal cells. Removal of this restraint, for example by activation of cholinergic neurons during ingestion of food, initiates and maximizes acid secretion. Knowledge regarding the structure and function of gastrin, somatostatin, and their respective receptors is providing novel avenues to better diagnose and manage acid-peptic disorders and certain cancers. Published 2020. Compr Physiol 10:197-228, 2020.

Do glycine-extended hormone precursors have clinical significance?

Half of the known peptide hormones are C-terminally amidated. Subsequent biogenesis studies have shown that the immediate precursor is a glycine-extended peptide. The clinical interest in glycine-extended hormones began in 1994, when it was suggested that glycine-extended gastrin stimulated cancer cell growth. Accompanying findings of gastrin gene expression in common cancers spurred the interest. The interest is now accompanied by skepticism, which is due to failure to demonstrate truly specific receptors for glycine-extended peptides and failure to demonstrate separate physiological and clinical effects of glycine-extended precursors for most other amidated hormones than gastrin and cholecystokinin (CCK). The idea of glycine-extended peptides as independent messengers was interesting. But clinical science has to move ahead from ideas that cannot be supported at key points after decades of research.

Characterization of gastrins and their receptor in solid human gastric adenocarcinomas

OBJECTIVE

The gastrin and the gastrin/CCK-B receptor genes are co-expressed in several carcinomas. The primary translational product, progastrin, however, is processed to several peptides of which only those that are α-amidated at their C-terminus are receptor ligands. So far, characterization of the progastrin-derived peptides in gastric cancer has not been reported. The authors therefore examined the molecular nature of gastrin and its receptor in human gastric carcinomas.

MATERIALS AND METHODS

Twenty patients with adenocarcinoma underwent partial or total gastrectomy. In samples from each carcinoma, gastrin peptides were characterized, using a library of sequence-specific immunoassays. Expression was also demonstrated by immunohistochemistry. In addition, the gastrin and gastrin/CCK-B receptor gene expression was quantitated using real-time PCR, and the receptor protein demonstrated by western blotting.

RESULTS

α-Amidated gastrins were detectable in 16 of 20 carcinomas (median concentration 2.1 pmol/g tissue; range 0-386 pmol/g tissue). The tissue concentrations correlated closely to the gastrin mRNA contents (r = 0.75, p < 0.0001). Moreover, progastrin and non-amidated processing intermediates, including glycine-extended gastrins, were detected in 19 carcinomas. Immunohistochemistry corroborated gastrin expression in carcinoma cells. Chromatography revealed extensive progastrin processing with α-amidated gastrin-34 and -17 (tyrosyl-sulfated as well as non-sulfated) as major products. Finally, gastrin/CCK-B receptor mRNA and protein were detected in all tumors.

CONCLUSIONS

The results show that the elements for a local loop of α-amidated gastrins and their receptor are detectable in 80% of human gastric adenocarcinomas. Therefore, the results support the contention that locally expressed gastrin may be involved in the tumorigenesis of gastric adenocarcinomas.

P53 gene mutation increases progastrin dependent colonic proliferation and colon cancer formation in mice

Transgenic mice overexpressing human progastrin (hGAS) show colonic crypt hyper-proliferation and elevated susceptibility to colon carcinogenesis. We aimed to investigate effects of p53 mutation on colon carcinogenesis in hGAS mice. We show that introducing a p53 gene mutation further increases progastrin dependent BrdU labeling and results in markedly elevated number of aberrant crypt foci (ACF) and colonic tumors. We demonstrate that hGAS/Lgr5-GFP mice have higher number of Lgr5+ colonic stem cells per crypt when compared to Lgr5-GFP mice indicating that progastrin changes crypt biology through increased stem cell numbers and additional p53 mutation leads to more aggressive phenotype in this murine colon cancer model.

Safety of proton pump inhibitor exposure

Proton pump (H(+)/K(+)-adenosine triphosphatase) inhibitors (PPIs) are widely used to treat patients with acid-related disorders because they are generally perceived to be safe and effective. However, as with any pharmacologic agent, they have the potential for side effects. Many studies have examined the side effects of long-term or short-term PPI exposure. We review the mechanism of action of PPIs, focusing on recently released products that might have greater risks of adverse effects than older products because of increased potency and/or duration of action. We summarize the data available on the putative adverse effects of PPI therapy and propose guidelines for clinicians who prescribe these agents to limit the potential for adverse outcomes in users of these effective therapeutic agents.

The production and role of gastrin-17 and gastrin-17-gly in gastrointestinal cancers

The gastrointestinal peptide hormone gastrin is responsible for initiating the release of gastric acid in the stomach in response to the presence of food and/or humoral factors such as gastrin releasing peptide. However, it has a role in the growth and maintenance of the gastric epithelium, and has been implicated in the formation and growth of gastric cancers. Hypergastrinemia resulting from atrophic gastritis and pernicious anemia leads to hyperplasia and carcinoid formation in rats, and contributes to tumor formation in humans. Additionally, gastrin has been suspected to play a role in the formation and growth of cancers of the colon, but recent studies have instead implicated gastrin processing intermediates, such as gastrin-17-Gly, acting upon a putative, non-cholecystokinin receptor. This review summarizes the production and chemical structures of gastrin and of the processing intermediate gastrin-17-Gly, as well as their activities in the gastrointestinal tract, particularly the promotion of colon cancers.

The structure of bioactive analogs of the N-terminal region of gastrin-17

Gastrin-17 (G17) processing intermediates bind to non-CCK receptors which mediate growth of the colonic mucosa but also the formation and development of colonic cancers. In previous studies, we removed the C-terminal region of G17 to form G17(1-12) and considerably shorter C-terminally amidated and non-amidated analogs. Peptides as short as G17(1-4) continued to bind to a single site on DLD-1 human colonic carcinoma cells, while only the G17(1-6)-NH(2) and G17(1-12) peptides retained the ability to activate the receptor and stimulate cell proliferation in vitro. In this report, we studied the structure of these analogs, using a combination of ECD and VCD spectroscopy and replica exchange molecular dynamics (REMD) simulations in water, TFE, and membrane-mimicking environments, in order to determine preferred conformations that may have importance in promoting the biological activities. Mostly random meander structures, punctuated by a beta-turn at residues 1-4, were found in most peptides by REMD simulations. G17(1-3)-NH(2), which cannot form a beta-turn, failed to bind the non-CCK receptor, suggesting the importance of this feature for binding. Additionally, the beta-turn appeared more frequently in longer sequences, possibly explaining the higher affinity of the non-CCK receptor for these peptides seen previously. Finally, C-terminally amidated peptides generally showed greater formation of turn structure than their non-amidated counterparts as shown by ECD spectra, suggesting the importance of peptide length in stabilizing turn structure in N-terminal sequences, and perhaps explaining the ability of G17(1-6)-NH(2) to activate the non-CCK receptor where as the non-amidated G17(1-6) and shorter peptides do not.

Bioactivity of analogs of the N-terminal region of gastrin-17

Gastrin-17-Gly (G17-Gly) has been shown to bind to non-CCK nanomolar and micromolar affinity sites on DLD-1 and HT-29 human colonic carcinoma cells and to stimulate cellular proliferation. However, in previous studies, we showed that C-terminal truncation of the gastrin-17 (G17) to the G17 analog G17(1-12) and then to G17(1-6)-NH(2) did not remove the ability to bind to DLD-1 cells or to activate proliferation. This implies that residues and/or structural motifs required for bioactivity at these receptors rest in the N-terminal region of G17. In this work, radioligand binding studies conducted with further C-terminally truncated analogs revealed that sequences as short as G17(1-4) still bind to a single receptor with micromolar affinity. Additionally, cell proliferation assays showed that G17(1-12) stimulates proliferation of DLD-1 cells, as of HT-29 cells, but the sequences shorter than G17(1-6)-NH(2), including non-amidated G17(1-6), were incapable of stimulating proliferation. These observations indicate that the tetrapeptide pGlu-Gly-Pro-Trp is the minimum N-terminal sequence for binding to the probable growth-promoting site on DLD-1 cells. Since analogs shorter than G17(1-6) are able to bind the receptor, these peptides may be of use for developing selective antagonists.

Functional cross-talk between beta-catenin and NFkappaB signaling pathways in colonic crypts of mice in response to progastrin

We recently reported a critical role of NFkappaB in mediating hyperproliferative and anti-apoptotic effects of progastrin on proximal colonic crypts of transgenic mice overexpressing progastrin (Fabp-PG mice). We now report activation of beta-catenin in colonic crypts of mice in response to chronic (Fabp-PG mice) and acute (wild type FVB/N mice) progastrin stimulation. Significant increases were measured in relative levels of cellular and nuclear beta-catenin and pbeta-cat45 in proximal colonic crypts of Fabp-PG mice compared with that in wild type littermates. Distal colonic crypts were less responsive. Interestingly, beta-catenin activation was downstream of IKKalpha,beta/NFkappaB, because treatment of Fabp-PG mice with the NFkappaB essential modulator (NEMO) peptide (inhibitor of IKKalpha,beta/NFkappaB activation) significantly blocked increases in cellular/nuclear levels of total beta-catenin/pbeta-cat45/and pbeta-cat552 in proximal colons. Cellular levels of pbeta-cat33,37,41, however, increased in proximal colons in response to NEMO, probably because of a significant increase in pGSK-3betaTyr216, facilitating degradation of beta-catenin. NEMO peptide significantly blocked increases in cyclin D1 expression, thereby, abrogating hyperplasia of proximal crypts. Goblet cell hyperplasia in colonic crypts of Fabp-PG mice was abrogated by NEMO treatment, suggesting a cross-talk between the NFkappaB/beta-catenin and Notch pathways. Cellular proliferation and crypt lengths increased significantly in proximal but not distal crypts of FVB/N mice injected with 1 nM progastrin associated with a significant increase in cellular/nuclear levels of total beta-catenin and cyclin D1. Thus, intracellular signals, activated in response to acute and chronic stimulation with progastrin, were similar and specific to proximal colons. Our studies suggest a novel possibility that activation of beta-catenin, downstream to the IKKalpha,beta/NFkappaB pathway, may be integral to the hyperproliferative effects of progastrin on proximal colonic crypts.

Expression of gastrin precursors by CD133-positive colorectal cancer cells is crucial for tumour growth

Precursors of the hormone gastrin, progastrin and glycine-extended gastrin (G-gly), have been detected in colorectal polyps and tumours, and in the blood of patients with colorectal cancer (CRC), while their expression is lower in healthy subjects. The surface glycoproteins CD133 and CD44 have been identified as possible markers for CRC stem cells. Our aims were to investigate whether progastrin and G-gly are expressed by CD133-positive cells in human CRC tissues and in the human CRC cell line DLD-1, and to determine whether this expression is biologically relevant. The great majority of the cells expressing CD133 also expressed gastrin precursors in both DLD-1 cells, which retain a stem cell-like subpopulation, and human CRC specimens. The CD133high/CD44high/progastrinhigh cells gave rise to larger tumours in SCID mice compared to CD133low/CD44low/progastrinlow cells. The CD133high/CD44high/progastrinhigh cells displayed enhanced activation of the signalling molecules JAK2, STAT3, ERK1/2 and Akt, known to regulate the induction of proliferation and/or survival by gastrin precursors. Moreover, downregulation of the gastrin gene in DLD-1 cells reduced the expression of cancer stem cell markers and abolished tumour development in SCID mice. We conclude that gastrin precursors may provide a target for therapies directed against the cells responsible for tumour development and recurrence.

Flow cytometric detection of progastrin interaction with gastrointestinal cells

The unprocessed gastrin precursor, progastrin (PG), is often overexpressed in colon cancer and other malignancies where it appears to stimulate colonic growth. Overexpression of progastrin also stimulates proliferation of normal colonic mucosa, but the receptors mediating these effects have not been identified. Here we report the development of a non-radioactive assay for assessment of PG binding to normal and transformed cells. Progastrin was labeled using biotinylation, and binding of biotinylated PG to cells was assessed using flow cytometry. Using this approach, we show strong and specific binding of PG to some cell lines (IEC-6, IEC-18, HT-29, COLO320) and minimal binding to others (HeLa, DC2.4, Jurkat). We also found PG binding to several non-gut epithelial lines, such as CHO-K1, COS-6 and HEK293 cells. The specificity of binding was confirmed by competition with cold, unlabeled PG but not with glycine-extended gastrin or amidated gastrin-17. Binding was not influenced by the presence of the classical CCK-2 receptor, but was partially dependent on the charged glycosaminoglycans (GAG). The analysis of primary colonic tissues isolated from wild type C57BL/6 mouse, revealed a small epithelial subpopulation of non-hematopoietic (CD45-negative) cells that strongly interacted with PG. Surprisingly, this population was greatly expanded in gastrin knockout mice. This non-radioactive, FACS-based assay should prove useful for further characterization of cells expressing the progastrin receptor.

Role of CCK/gastrin receptors in gastrointestinal/metabolic diseases and results of human studies using gastrin/CCK receptor agonists/antagonists in these diseases

In this paper, the established and possible roles of CCK1 and CCK2 receptors in gastrointestinal (GI) and metabolic diseases are reviewed and available results from human agonist/antagonist studies are discussed. While there is evidence for the involvement of CCK1R in numerous diseases including pancreatic disorders, motility disorders, tumor growth, regulation of satiety and a number of CCK-deficient states, the role of CCK1R in these conditions is not clearly defined. There are encouraging data from several clinical studies of CCK1R antagonists in some of these conditions, but their role as therapeutic agents remains unclear. The role of CCK2R in physiological (atrophic gastritis, pernicious anemia) and pathological (Zollinger-Ellison syndrome) hypergastrinemic states, its effects on the gastric mucosa (ECL cell hyperplasia, carcinoids, parietal cell mass) and its role in acid-peptic disorders are clearly defined. Furthermore, recent studies point to a possible role for CCK2R in a number of GI malignancies. Current data from human studies of CCK2R antagonists are presented and their potential role in the treatment of these conditions reviewed. Furthermore, the role of CCK2 receptors as targets for medical imaging is discussed.

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.

The gastrin gene promoter is regulated by p73 isoforms in tumor cells

p73, a new p53 family member, is a transcription factor that is increasingly recognized in cancer research as an important player in tumorigenesis as well as in chemotherapeutic drug sensitivity. Despite the substantial structural and functional similarities to p53, accumulating evidence suggests that p53 and p73 may differently regulate their transcriptional targets. In this study, we have investigated the role of p73 in regulation of the gastrin gene promoter. Gastrin is a peptide hormone and an important factor in determining the progression of a number of human malignancies. Our results show that p73 can bind to the gastrin promoter. This leads to transcriptional upregulation of gastrin mRNA. We also found that the levels of gastrin and p73 transcripts correlate in primary gastric tumors. Taken together, our results demonstrate a novel mechanism for regulation of gastrin gene transcription and support a concept that p53 and p73 may have different biological roles in tumors.

Glycine-extended gastrin activates two independent tyrosine-kinases in upstream of p85/p110 phosphatidylinositol 3-kinase in human colonic tumour cells

AIM: To investigate whether Src, JAK2 and phosphatidylinositol 3-kinase (PI3K) pathways are involved in the proliferation of human colonic tumour cells induced by glycine-extended gastrin (G-gly), the precursor of the mature amidated gastrin and to elucidate the molecular interaction between these three kinases in response to this peptide.

METHODS: Using the human colonic tumour cell line HCT116 as a model, we first measured the activation of PI3K, p60-Src and JAK2 in response to G-gly by in vitro kinase assays. Then we investigated the involvement of these kinases in G-gly-induced cell proliferation by MTT test.

RESULTS: G-gly stimulation induced p60-Src, JAK2 and PI3K activation in HCT116. The different pathways were involved in proliferation of human colon cancer cells induced by G-gly. Furthermore, we found that both Src and JAK2 were necessary to PI3K regulation by this peptide. However, we did not find any cross-talk between the two tyrosine kinases.

CONCLUSION: Our results suggest that the p60-Src/PI3K and JAK2/PI3K pathways act independently to mediate G-gly proliferative effect on human colonic tumour cells.

Glycine-extended gastrin inhibits apoptosis in colon cancer cells via separate activation of Akt and JNK pathways

Glycine-extended gastrin (G-Gly) is produced by colon cancers and has growth promoting and anti-apoptotic effects in the colonic epithelium. We have examined the anti-apoptotic effects of G-Gly and the signal transduction pathways involved. G-Gly stimulated HT-29 cell proliferation in a concentration dependent manner and inhibited serum-starvation and celecoxib-induced apoptosis. Inhibition of signalling via c-Jun NH2-terminal kinase (JNK) with SP600125 or PI3-kinase/Akt with LY294002 abolished the effects of G-Gly. G-Gly significantly increased phosphorylation of both JNK and Akt. The JAK2 inhibitor AG490 abolished the anti-apoptotic effect of G-Gly and inhibited phosphorylation of Akt but not of JNK. G-Gly stimulated tyrosine phosphorylation of JAK2. G-Gly-increased activation of AP-1 was JNK-dependant and activation of STAT3 was JAK2-dependant. We conclude that G-Gly promotes growth and inhibits apoptosis in colon cancer cells. These effects are mediated via the JAK2, PI3-kinase/Akt and JNK pathways. Activation of JAK2 is upstream of Akt but not of JNK.

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.

Signaling pathways associated with colonic mucosa hyperproliferation in mice overexpressing gastrin precursors

MTI/G-Gly mice and hGAS mice, overexpressing glycine-extended gastrin (G-Gly) and progastrin, respectively, display colonic mucosa hyperplasia, hyperproliferation, and an increased susceptibility to intestinal neoplasia. Here, we have used these transgenic mice to analyze in vivo the modulation of intracellular signaling pathways that may be responsible for the proliferative effects of gastrin precursors. The expression, activation, and localization of signaling and cell-to-cell adhesion molecules were studied using immunofluorescence and Western blot techniques on colonic tissues derived from MTI/G-Gly, hGAS, or wild-type FVB/N mice. These analyses revealed an up-regulation of Src tyrosine kinase and related signaling pathways [phosphatidyl inositol 3'-kinase (PI3K)/Akt, Janus-activated kinase (JAK) 2, signal transducer and activator of transcription (STAT) 3, and extracellular-signal regulated kinases (ERK)] in both MTI/G-Gly and hGAS mice compared with the wild-type control animals as well as an overexpression of transforming growth factor-alpha (TGF-alpha). In contrast, overexpression of the gastrin precursors did not affect the activation status of STAT1 nor the expression and the distribution of adhesion proteins (focal adhesion kinase, cadherins, and catenins). We report for the first time that the transition from a normal colonic epithelium to a hyperproliferative epithelium in MTI/G-Gly and hGAS mice may be a consequence of the up-regulation of Src, PI3K/Akt, JAK2, STAT3, ERKs, and TGF-alpha. Deregulation of cell adhesion, a late event in tumor progression, does not occur in these transgenic models.

COOH-terminal 26-amino acid residues of progastrin are sufficient for stimulation of mitosis in murine colonic epithelium in vivo

Transgenic mice (hGAS) that overexpress human progastrin are more susceptible than wild-type mice (FVB/N) to the induction of colonic aberrant crypt foci (ACF) and adenomas by the chemical carcinogen azoxymethane. We have previously shown significantly increased levels of colonic mitosis in hGAS compared with FVB/N mice after gamma-radiation. To investigate whether the effects of progastrin observed in hGAS colon require the presence of other forms of circulating gastrin, we have crossed hGAS (hg(+/+)) with gastrin knockout (G(-/-)) mice to generate mice that express progastrin and no murine gastrin (G(-/-)hg(+/+)). After azoxymethane, G(-/-)hg(+/+) mice developed significantly more ACF than control G(-/-)hg(-/-) mice (which do not express any forms of gastrin). G(-/-)hg(+/+) mice also exhibited significantly increased colonic mitosis both before and after exposure to 8 Gray Gy gamma-radiation or 50 mg/kg azoxymethane compared with G(-/-)hg(-/-). Treatment of G(-/-)hg(-/-) mice with synthetic progastrin (residues 21-101 of human preprogastrin) or G17 extended at its COOH terminus corresponding to the COOH-terminal 26-amino-acid residues of human preprogastrin (residues 76-101, G17-CFP) resulted in continued colonic epithelial mitosis after gamma-radiation, whereas glycine-extended gastrin-17 and the COOH-terminal tryptic fragment of progastrin [human preprogastrin-(96-101)] had no effect. Immunoneutralization with an antibody against G17-CFP before gamma-radiation significantly decreased colonic mitosis in G(-/-)hg(+/+) mice to levels similar to G(-/-)hg(-/-). We conclude that progastrin does not require the presence of other forms of gastrin to exert proliferative effects on colonic epithelia and that the portion of the peptide responsible for these effects is contained within amino acid residues 76-101 of human preprogastrin.

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.

Gastrin: old hormone, new functions

It is exactly a century since the gastric hormone gastrin was first described as a blood-borne regulator of gastric acid secretion. The identities of the main active forms of the hormone (the "classical gastrins") and their cellular and molecular sites of action in regulating acid secretion have all attracted sustained attention. However, recent work on peptides derived from the gastrin precursor that do not stimulate acid secretion ("non-classical gastrins"), together with studies on mice over-expressing the gene, or in which the gastrin gene has been deleted, suggest hitherto unsuspected roles in regulating cell proliferation, migration, and differentiation. Moreover, microarray and proteomic studies have identified previously unsuspected target genes of the classical gastrins. Some of the newer actions have implications for our understanding of the progression to cancer in oesophagus, stomach, pancreas and colon, all of which have recently been linked in one way or another to dysfunctional signalling involving products of the gastrin gene. The present review focuses on recent progress in understanding the biology of both classical and non-classical gastrins.

Gastrins, cholecystokinins and gastrointestinal cancer

The gastrointestinal peptide hormones gastrin and cholecystokinin (CCK) are well known for their ability to stimulate gastric acid secretion and pancreatic enzyme secretion, respectively. The suggestion that gastrin and CCK might also promote the development of cancers of the gastrointestinal tract has been controversial, but an increasing body of evidence now supports the view that the amidated and non-amidated forms of gastrin act as growth factors via different receptors in different regions of the gut. For example, animal experiments indicate that amidated gastrins are involved in cellular differentiation and repair in the gastric mucosa, and synergize with Helicobacter pylori infection in the development of gastric carcinoma. In contrast, non-amidated gastrins stimulate colonic mucosal growth, accelerate the early steps in colorectal carcinoma formation, and are elevated in the tumour and circulation of patients with colorectal cancer. Although human pancreatic carcinomas express CCK-1 and CCK-2 receptors, the role of gastrins and CCK in pancreatic carcinogenesis is yet to be established. Further investigation of the possible role of the CCK-2 receptor in gastric and pancreatic neoplasia, and of the hypothesis that gastrin precursors act as autocrine growth factors in colorectal carcinoma, is warranted. However, therapies aimed at the gastrins must be targeted to the relevant gastrin/gastrin receptor combination.

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.

Progastrin stimulates murine colonic epithelial mitosis after DNA damage

BACKGROUND & AIMS

Transgenic mice that overexpress progastrin are more susceptible than either wild-type mice or mice that overexpress amidated gastrin to chemical carcinogen-induced colonic adenomas. We have investigated whether alterations in the regulation of apoptosis or mitosis after DNA damage contribute to the effects of progastrin on murine colonic epithelium.

METHODS

Apoptosis and mitosis were assessed on a cell positional basis in murine intestinal epithelium after gamma-irradiation. Mice analyzed were progastrin overexpressing, gastrin overexpressing, gastrin knockout, and their wild-type counterparts. The expression of cell cycle regulators was analyzed by gene array and Western blotting.

RESULTS

Apoptosis was induced to similar levels in the small intestinal and colonic crypts of all mice 4.5 hours after 8 Gy gamma-radiation. Colonic mitosis was inhibited to almost undetectable levels by 8Gy gamma-radiation in wild-type, gastrin-knockout, and gastrin-overexpressing mice. However, significant colonic mitosis persisted in progastrin-overexpressing mice up to 24 hours after 8Gy gamma-radiation. Increased levels of cdk4 and cyclin D1 proteins were found in the colonic epithelium of progastrin-overexpressing mice relative to wild-type animals after gamma-radiation.

CONCLUSIONS

After DNA damage by gamma-radiation, mice with elevated progastrin exhibit significantly higher levels of colonic mitosis than wild-type or gastrin-overexpressing mice. Persistently elevated cdk4 and cyclin D1 in progastrin overexpressing mice accounts for the capacity of colon cells to continue with the cell cycle after DNA damage.

Involvement of cholecystokinin/gastrin-related peptides and their receptors in clinical gastrointestinal disorders

In this paper the possible roles of cholecystokinin (CCK), gastrin, or gastrin-related peptides and their receptors in human gastrointestinal diseases are reviewed. For CCK/CCK(A) receptors (CCK(A)-R), the evidence for their proposed involvement in diseases caused by impaired CCK release or CCK(A)-R mutations, pancreatic disorders (acute/chronic pancreatitis), gastrointestinal motility disorders (gallbladder disease, irritable bowel syndrome), pancreatic tumor growth and satiety disorders, is briefly reviewed. The evidence that has established the involvement of gastrin/CCK(B)-R in mediating the action of hypergastrinaemic disorders, mediating hypergastrinaemic effects on the gastric mucosa (ECL hyperplasia, carcinoids, parietal cell mass), and acid-peptic diseases, is reviewed. The evidence for their possible involvement in mediating growth of gastric and pancreatic tumours and possible involvement of gastrin-related peptides in colon cancers, is reviewed briefly.

Extragastric effects of gastrin gene knock-out mice

Gastrin is a peptide hormone that regulates both acid secretion and growth of the gastric oxyntic mucosa. Recent studies suggest that gastrin, in both its amidated, and less processed forms (glycine-extended gastrin and progastrin) may also exert biological activity in other organs in the gastrointestinal tract. This article will review the studies performed to date addressing the physiological role of gastrin outside of the gastric mucosa, with particular emphasis on the information gleaned from gastrin-deficient mice. Most of these studies address the potential role for the less processed forms of gastrin in regulating the proliferation of the colonic mucosa and colon cancers. There is also some data to support a potential role for gastrin in the regulation of the pancreas and the kidney, although the effects of gastrin deficiency on the function of these organs in mice have not yet been rigorously studied.

Metabolism of recombinant progastrin in sheep

Precursor forms of peptide hormones may be biologically active with effects distinct from the mature end product. Nonamidated progastrin-derived peptides stimulate growth of colonic epithelium and are elevated in the circulation of patients with colorectal carcinomas, whereas the amidated end product is the major regulator of gastric acidity. Using region-specific radioimmunoassays, we here compared the in vitro and in vivo metabolism of recombinant human progastrin-(6-80) and two other nonamidated gastrins, gastrin-17-Gly and Tyr(70)-progastrin-(71-80). Although progastrin-(6-80) was very stable in vitro, both progastrin-(6-80) and gastrin-17-Gly were degraded in vivo. The in vivo data were best fitted by a double-exponential decay curve, and the half-lives for progastrin-(6-80) (t1/2alpha = 5.1 +/- 1.1, t(1/2)beta = 42 +/- 11 min) were significantly (P < 0.05) longer than for gastrin-17-Gly (t(1/2)alpha = 2.2 +/- 0.6, t(1/2)beta = 13 +/- 1 min). Tyr(70)-progastrin-(71-80) was degraded more rapidly. Comparison with amidated gastrins suggests that peptide length, rather than sequence, is the critical determinant of clearance. Progastrin has the clearance characteristics to be considered a circulating hormone.

Gastrin biosynthesis in canine G cells

Gastrin requires extensive posttranslational processing for full biological activity. It is presumed that progastrin is cleaved at pairs of basic amino acids by a prohormone convertase to form a glycine-extended intermediate (G-Gly) that serves as a substrate for peptidyl-glycine alpha-amidating monooxygenase (PAM), resulting in COOH-terminally amidated gastrin. To confirm the nature of progastrin processing in a primary cell line, we performed [(35)S]methionine-labeled pulse-chase biosynthetic experiments in canine antral G cells. Radiolabeled progastrin reached a peak earlier than observed for G-Gly or amidated gastrin. G-Gly radioactivity accumulated in G cells and preceded the appearance of radioactivity in amidated gastrin. The conversion of G-Gly to amidated gastrin was enhanced by the PAM cofactor ascorbic acid. To determine whether one member of the prohormone convertase family (PC2) was responsible for progastrin cleavage, G cells were incubated with PC2 antisense oligonucleotide probes. Cells treated with antisense probes had reduced PC2 expression, an accumulation of radiolabeled progastrin, and a delay in the formation of amidated gastrin. Progastrin in antral G cells is cleaved via PC2 to form G-Gly that is converted to amidated gastrin via the actions of PAM.

Progastrin processing differs in 7B2 and PC2 knockout animals: a role for 7B2 independent of action on PC2

Cellular synthesis of neuroendocrine peptides requires prohormone convertases (PCs). In order to determine the role of PC2 for gastrin synthesis, we examined antral extracts from mice lacking PC2 or its chaperone, 7B2. The overall concentrations of precursors and alpha-amidated gastrins were similar in all mice. Chromatography, however, revealed that while the K(53)-K(54) site was almost fully cleaved in controls and half cleaved in PC2 null mice, only 23% was cleaved in 7B2 null mice. The results show that PC2 and 7B2 both are required for synthesis of the main form of gastrin (gastrin-17), and that 7B2 exhibits effects beyond PC2-mediated cleavages.

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.

Autocrine gastrins in colon cancer cells Up-regulate cytochrome c oxidase Vb and down-regulate efflux of cytochrome c and activation of caspase-3

Suppression of the gastrin gene in human colon cancer cells by stably expressing antisense (AS) gastrin RNA results in significant growth suppression of AS cells. To understand mechanisms mediating the growth effects of autocrine gastrins, differential expression of transcripts by AS and control (C) clones of a representative cell line (HCT-116) was analyzed to identify target genes of autocrine gastrins. Six differentially expressed transcripts were confirmed and sequenced. Of these, the RNA and protein levels of cytochrome c oxidase (COX) Vb were significantly higher in C versus AS cells. The expression of COX Vb by colon cancer cells was proportional to the expression of gastrin. Higher levels of COX Vb coprecipitated with cytochrome c in the mitochondria of C versus AS cells. Treatment of mitochondria with digitonin resulted in a 2-fold higher release of cytochrome c from AS versus C mitochondria. As a corollary, the cytosolic levels of cytochrome c were significantly higher in AS versus C cells, which correlated with approximately 2- and approximately 3-fold higher activation of caspase-9 and -3, respectively, in AS versus C cells in response to camptothecin. Thus, autocrine gastrins may support growth/survival of cells by up-regulating COX Vb, which may decrease the sensitivity of the cancer cells to apoptotic stimuli by increasing retention of cytochrome c in mitochondria.

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.

Endocrine disruptive effects of polychlorinated aromatic hydrocarbons on intestinal cholecystokinin in rats

The ubiquitous and persistent nature of polychlorinated aromatic hydrocarbons (PCAHs) in our environment and the risk of exposure to PCAHs have provoked concern over their potential toxicity. In humans, exposure to PCAHs is aimed chiefly at epithelial cells residing in the intestinal mucosa, because oral intake of contaminated food is a major source of PCAHs. The purpose of this study, therefore, was to examine the effects of chronic exposure to various PCAHs [i.e. 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 2,3,4,7,8-pentachlorodibenzofuran (PeCDF), 3,3',4,4',5-pentachlorobiphenyl (PCB-126), and 2,2'4,4'5,5'-hexachlorobiphenyl (PCB-153)], given alone or as mixtures, on intestinal cholecystokinin (CCK) peptide and messenger RNA levels. We show that chronic PCAH treatment significantly lowers intestinal levels of stored CCK peptide. Intestinal CCK messenger RNA levels are not affected. In addition, 3,3',4,4',5-pentachlorobiphenyl treatment increased intestinal insulin-like growth factor-binding protein-3 levels in a dose-related manner. Acute 2,3,7,8-tetrachlorodibenzo-p-dioxin treatment of intestinal CCK cells lowered levels of CCK-processing enzymes (i.e. prohormone convertase-1 and -2). Together, these data indicate that PCAHs may decrease intestinal levels of stored CCK peptide by affecting the intestinal insulin-like growth factor system and CCK processing.

Progastrin expression predisposes mice to colon carcinomas and adenomas in response to a chemical carcinogen

BACKGROUND & AIMS

Processing intermediates of preprogastrin (gly-gastrin and progastrin), termed nonamidated gastrins, are mitogenic for several cell types including colonic epithelial cells. However, presently it is not known if nonamidated gastrins play a role in colon carcinogenesis and if the effects are similar to those of amidated gastrins.

METHODS

Colon carcinogenesis in response to azoxymethane (AOM) was examined in transgenic mice overexpressing either progastrin (hGAS) or amidated gastrin (INS-GAS), compared with that in wild-type (WT) mice.

RESULTS

In AOM-treated groups, the total number of tumors per colon was significantly higher in hGAS (4.8+/-0.34) than INS-GAS (3.0+/-0.16) and WT (2.7+/-0.35) mice. Total numbers of adenocarcinomas and adenomas per animal colon were also significantly higher in hGAS than INS-GAS and WT mice. The size of the tumors was greater in hGAS mice, resulting in a significantly higher tumor burden per mouse in the hGAS mice than INS-GAS and WT mice. Although >90% of the tumors were located in the distal half of the colon in INS-GAS and WT mice, a significant number (42%) were present at the proximal end of the colon in hGAS mice.

CONCLUSIONS

The results suggest that the risk for developing colon carcinomas and adenomas in response to AOM is significantly increased in mice expressing high levels of progastrin, but not amidated gastrins.

Role of autocrine and endocrine gastrin-like peptides in colonic carcinogenesis

Colon carcinogenesis is a multistep process that involves deletions, mutations, and changes in expression of genes that regulate growth, differentiation, and apoptosis. Hyperproliferation can initiate dysplastic growth, resulting in accumulation of genetic defects and progression of colon cancer. Although genetic instability, because of inheritance of specific genetic defects, plays a dominant role in familial cancers, in the majority of sporadic cancers hyperproliferation is likely to play a permissive role in initiation and progression of the disease. Thus factors that regulate growth, differentiation, and apoptosis are likely to play an important role in colon carcinogenesis. Autocrine gastrins, insulin-like growth factor-II, transforming growth factor-alpha, and endocrine gastrins have been implicated in the tumorigenic potential of colon cancer cells. In this article we focus on the role of endocrine and autocrine gastrins in colon cancer and review recent advances that suggest a role of processing intermediates of gastrin in colon carcinogenesis.

Lessons from genetically engineered animal models. III. Lessons learned from gastrin gene deletion in mice

Gastrin is the principal hormonal inducer of gastric acid secretion. Chronic hypergastrinemia, leading to hypersecretion of gastric acid and increased proliferation of parietal and enterochromaffin-like (ECL) cells, has been well described. In contrast, the physiological consequences of chronic gastrin deficiency had been poorly understood until the recent genetic engineering of mouse mutants containing a gastrin gene deletion by homologous recombination in embryonic stem cells. This themes article describes the consequences of constitutive gastrin deficiency on the development and physiology of the stomach. A lack of gastrin disrupts basal gastric acid secretion and renders the acid secretory system unresponsive to acute histaminergic, cholinergic, and gastrinergic stimulation. The defect in acid secretion is greater than would have been predicted from previous studies in which gastrin action was acutely blocked. Cellular changes include thinning of the gastric mucosa in the gastrin-deficient mice, with a reduction in parietal cells and reduced expression of markers of parietal and ECL cell-differentiated functions. The results suggest that gastrin is required for the functional maturation of the acid-secretory system.

Glycine-extended gastrin regulates HEK cell growth

Posttranslational processing of progastrin to a carboxy terminally amidated form (G-NH(2)) is essential for its effect on gastric acid secretion and other biological effects mediated by gastrin/CCK-B receptors. The immediate biosynthetic precursor of G-NH(2), glycine-extended gastrin (G-Gly), does not stimulate gastric acid secretion at physiological concentrations but is found in high concentrations during development. G-NH(2) and G-Gly have potent growth stimulatory effects on gastrointestinal tissues, and G-NH(2) can stimulate proliferation of human kidney cells. Thus we sought to explore the actions of G-NH(2) and G-Gly on the human embryonic kidney cell line HEK 293. HEK 293 cells showed specific binding sites for (125)I-labeled Leu(15)-G17-NH(2) and (125)I-Leu(15)-G(2-17)-Gly. Both G-NH(2) and G-Gly induced a dose-dependent increase in [(3)H]thymidine incorporation, and both peptides together significantly increased [(3)H]thymidine incorporation above the level of either peptide alone. G-NH(2) and G-Gly were detected by radioimmunoassay in serum-free conditioned media. Antibodies directed against G-NH(2) and G-Gly lead to a significant reduction in [(3)H]thymidine incorporation. G-NH(2) but not G-Gly increased intracellular Ca(2+) concentration. We conclude that G-NH(2) and G-Gly act cooperatively via distinct receptors to stimulate the growth of a nongastrointestinal cell line (HEK 293) in an autocrine fashion.

Overexpression of glycine-extended gastrin in transgenic mice results in increased colonic proliferation

Gastrin is a peptide hormone involved in the growth of both normal and malignant gastrointestinal tissue. Recent studies suggest that the glycine-extended biosynthetic intermediates mediate many of these trophic effects, but the in vivo relevance of glycine-extended gastrin (G-Gly) has not been tested. We have generated mice (MTI/G-GLY) that overexpress progastrin truncated at glycine-72 to evaluate the trophic effects of G-Gly in an in vivo model. MTI/G-GLY mice have elevated serum and colonic mucosal levels of G-Gly compared with wild-type mice. MTI/G-GLY mice had a 43% increase in colonic mucosal thickness and a 41% increase in the percentage of goblet cells per crypt. MTI/G-GLY mice exhibited increased colonic proliferation compared with wild-type controls, with an expansion of the proliferative zone into the upper third of the colonic crypts. Continuous infusion of G-Gly into gastrin-deficient mice for two weeks also resulted in elevated G-Gly levels, a 10% increase in colonic mucosal thickness, and an 81% increase in colonic proliferation when compared with gastrin-deficient mice that received saline alone. To our knowledge, these studies demonstrate for the first time that G-Gly's contribute to colonic mucosal proliferation in vivo.

Expression of CCKB/gastrin receptor isoforms in gastro-intestinal tumour cells

Anti-serum raised against the human cholecystokinin B (CCKB)/gastrin receptor was used in Western blotting to differentiate the cellular locations of receptor isoforms expressed by human gastro-intestinal (GI) tumour cell lines. Using anti-serum directed against the amino-terminal extracellular tail of the CCKB/gastrin receptor, 8/9 cell lines were shown to express immunoreactive proteins of either m.w. 70 or 40 kDa, or both. Both isoforms were found to be associated with intracellular, non-nuclear membranes, whereas only the 70 kDa protein was expressed in the plasma membrane. Receptor expression was related to gastrin production and secretion. Both progastrin and glycine-extended gastrin-17 were produced and secreted by the tumour cell lines; however, carboxy amidated gastrin was not detected by radioimmunoassay. A CCKB/gastrin receptor transfectant NIH3T3 cell line did not produce detectable gastrin and showed exclusive expression of the 70 kDa receptor on the plasma membrane. One cell line had <50 pg/ml cell-associated progastrin and no detectable receptor form. Cell lines expressing 50-150 pg/ml had both 40 and 70 kDa receptor forms. Those expressing >150 pg/ml progastrin had only the 40 kDa isoform, which was shown to be exclusively expressed on intracellular, non-nuclear membranes, in one of the cell lines. Of the 4 cell lines exclusively expressing the lower m.w. receptor, 3 had gastrin present within the cell, which was not secreted. Thus, if cell-associated gastrin induces a proliferative effect, it may be by an intracrine pathway. Our study has identified the presence of CCKB/gastrin receptor isoforms in different cellular locations and may help toward understanding the complex autocrine and intracrine pathways mediated by gastrin peptides.

Gastrin and colorectal cancer: a prospective study

BACKGROUND & AIMS

Gastrin is a putative promoter of colorectal carcinomas. The aim of this study was to evaluate the temporal relationship between gastrinemia and development of colorectal malignancy.

METHODS

We conducted a nested case-control study among 128,992 subscribers to a health maintenance program who had participated in a multiphasic health checkup between 1964 and 1969. Serum had been frozen since the checkup and the cohort followed up for cancer. Of 1881 incident colorectal carcinoma cases, 250 were randomly selected; 1 control without cancer was matched to each case by age, sex, education, and date of serum collection. Stored sera were tested for Helicobacter pylori immunoglobulin G and for gastrin and glycine-extended gastrin.

RESULTS

Verified cases included 166 colon cancers, 58 rectal cancers, and 9 with cancer in both locations. A mean of 15.3 years had elapsed between serum collection and diagnosis of cancer. Median gastrin levels were similar in cases and controls (41.7 vs. 40.7 pg/mL). However, a gastrin level above normal was associated with increased risk for colorectal malignancy (odds ratio, 3.9; 95% confidence interval, 1.5-9.8). If this association is causal, 8.6% of colorectal cancers could be attributed to high serum gastrin level.

CONCLUSIONS

Hypergastrinemia is associated with an increased risk of colorectal carcinoma.

Processing-independent analysis in the diagnosis of gastrinomas

BACKGROUND

This study evaluates whether a new analytic principle, processing-independent analysis (PIA), offers better specificity and sensitivity than the conventional gastrin radioimmunoassay in the diagnosis of gastrinomas.

METHODS

Plasma concentrations of alpha-amidated gastrins and the total progastrin product were measured with radioimmunoassay and with PIA, respectively, in 512 samples taken for gastrin measurement and in a selected group of gastrinoma patients (n=10).

RESULTS

Among the 512 patients were 9 with gastrinomas. In plasma from these patients the median degree of amidation (ratio of alpha-amidated gastrins to total progastrin product) was 75% (range, 25-98%), whereas in the other groups the medians varied from 41% to 86%. In the second group of gastrinoma patients all had a degree of amidation of less than 50%.

CONCLUSIONS

In screening for gastrinomas PIA offered no diagnostic advantages in comparison with conventional gastrin radioimmunoassay. However, in selected patients who in spite of normal or slightly increased concentrations of amidated gastrins were still suspected of having gastrinoma, additional measurement of the total progastrin product showed incomplete processing of progastrin and thus proved helpful in establishing the diagnosis.

Tyrosine phosphorylation of insulin receptor substrate-1 and activation of the PI-3-kinase pathway by glycine-extended gastrin precursors

Glycine-extended gastrin precursors (G-Gly) were considered as processing intermediates devoid of biological activity. However, we have recently identified selective receptors for G-Gly which mediate the proliferative effects of this precursor. Little is known about the signaling pathways activated by G-Gly. In the present study, we demonstrate that PI-3-kinase is rapidly and transiently activated by G-Gly. We also observed a rapid increase in the tyrosine phosphorylation of IRS-1 and an activation of the PI-3-kinase in anti-IRS-1 immunoprecipitates, suggesting that PI-3-kinase may be activated by association with tyrosine phosphorylated IRS-1. We also demonstrated that gastrin precursors activate the serine/threonine kinase, p70 kDa S6 kinase (p70S6K), through a wortmannin sensitive pathway.