Identification of progastrin in gastrinomas, antrum, and duodenum by a novel radioimmunoassay

The C-terminal flanking peptide of progastrin induces gastric cell apoptosis and stimulates colonic cell division in vivo

Progastrin (PG) is processed into a number of smaller peptides including amidated gastrin (Gamide), non-amidated glycine-extended gastrin (Ggly) and the C-terminal flanking peptide (CTFP). Several groups have reported that PG, Gamide and Ggly are biologically active in vitro and in vivo, and are involved in the development of gastrointestinal cancers. CTFP is bioactive in vitro but little is known of its effects in vivo. This study investigated the bioactivity of CTFP in vivo in normal tissues using gastrin deficient (GASKO) mice and in two mouse models of cancer (SCID mice bearing xenograft tumors expressing normal or knocked-down levels of gastrin and a mouse model of hepatic metastasis). As with Ggly, CTFP treatment stimulated colonic proliferation in GASKO mice compared to control. CTFP also significantly increased apoptosis in the gastric mucosa of male GASKO mice. CTFP did not appear to effect xenograft growth or the incidence of liver metastases. This is the first demonstration that CTFP has specific biological activity in vivo in the colon and stomach.

The C-terminal flanking peptide (CTFP) of progastrin inhibits apoptosis via a PI3-kinase-dependent pathway

Progastrin is processed to a number of peptides including glycine-extended gastrin, amidated gastrin and the C-terminal flanking peptide (CTFP). Progastrin and gastrin-gly are pro-proliferative and anti-apoptotic in gastric and colorectal cancer cell lines. The CTFP is a major form of progastrin in the stomach and colon and stimulates proliferation. However the effect of CTFP on apoptosis has not been examined. Using the human gastric carcinoma cell line AGS we show that CTFP attenuates apoptosis through a PI3-kinase pathway by stimulating the phosphorylation of Akt leading to sustained increases in the concentrations of Bcl-xL and phosphorylated Bad protein and by reducing caspase 3 activity. The anti-apoptotic effect represents an important potential mechanism for the growth promoting action of CTFP.

Peptide processing and biology in human disease

PURPOSE OF REVIEW

To describe recent advances in the processing of gastrointestinal hormones, and the consequences for human disease of mutations in the enzymes involved.

RECENT FINDINGS

Although gastrointestinal prohormones were long regarded as devoid of biological activity, recent data indicate that the prohormones for both gastrin and gastrin-releasing peptide are bioactive, through different receptors from the mature hormones. Mutations in the family of prohormone convertases responsible for the initial steps in the processing of gastrointestinal hormones are associated with several different pathophysiological conditions in humans.

SUMMARY

Human mutational studies, when taken together with the phenotypes observed in mice deficient in the prohormone convertases, emphasize the crucial importance of the processing enzymes in mammalian biology. Although the phenotypes may often be ascribed to defective production of a mature hormone or growth factor, the recognition that the precursors are independently bioactive suggests that the increased precursor concentrations may also contribute to the symptoms. The observation that the precursors often act through different receptors from the mature hormones may permit the development of precursor-selective antagonists for therapeutic use.

Production, secretion, and biological activity of the C-terminal flanking peptide of human progastrin

BACKGROUND & AIMS

Processing of progastrin, the 80-amino acid precursor of the hormone gastrin, generates a variety of peptides with distinct distributions and biological activities. However, little is known regarding the expression, secretion, and biological activity of the 6-amino acid C-terminal flanking peptide (CTFP) of progastrin. The objectives were to determine the concentration of CTFP in normal subjects and patients with gastrointestinal diseases and to investigate the biological activity of CTFP.

METHODS

CTFP, gastrin-amide (Gamide), glycine-extended gastrin (Ggly), and progastrin were measured using region-specific radioimmunoassay (RIA) in antral extracts and resected colorectal cancers (CRC) and in plasma from normal subjects (fasting and meal stimulated) and from patients with CRC, multiple endocrine neoplasia type 1 (MEN-1), or pernicious anemia. The effect of CTFP on proliferation, migration, and activation of the mitogen-activated protein kinase (MAPK) pathway in several types of gastrointestinal cell lines was determined.

RESULTS

CTFP is by far the predominant progastrin-derived peptide found in the antrum (4-fold higher than Gamide), resected CRC, and circulation (60-fold higher than Gamide) and is released after meal stimulation. The hypergastrinemic patients (MEN-1, pernicious anemia) had elevated plasma Gamide but unaltered CTFP demonstrating differential secretion of these 2 progastrin-derived peptides. Finally, CTFP stimulated proliferation and migration and activated MAPK of cells in culture.

CONCLUSIONS

The high and regulated expression of CTFP in healthy and diseased subjects combined with the evidence for biological activity of CTFP demonstrates that CTFP is not an inactive metabolite of progastrin processing but is a bioactive peptide with potential roles in the normal and diseased gastrointestinal tract.

Serum gastrin in Zollinger-Ellison syndrome: I. Prospective study of fasting serum gastrin in 309 patients from the National Institutes of Health and comparison with 2229 cases from the literature

The assessment of fasting serum gastrin (FSG) is essential for the diagnosis and management of patients with the Zollinger-Ellison syndrome (ZES). Although many studies have analyzed FSG levels in patients with gastrinoma, limited information has resulted from these studies because of their small size, different methodologies, and lack of correlations of FSG levels with clinical, laboratory, or tumor features in ZES patients. To address this issue, we report the results of a prospective National Institutes of Health (NIH) study of 309 patients with ZES and compare our results with those of 2229 ZES patients in 513 small series and case reports in the literature. In the NIH and literature ZES patients, normal FSG values were uncommon (0.3%-3%), as were very high FSG levels >100-fold normal (4.9%-9%). Two-thirds of gastrinoma patients had FSG values <10-fold normal that overlap with gastrin levels seen in more common conditions, like Helicobacter pylori infection or antral G-cell hyperplasia/hyperfunction. In these patients, FSG levels are not diagnostic of ZES, and gastrin provocative tests are needed to establish the diagnosis. Most clinical variables (multiple endocrine neoplasia type 1 status, presence or absence of the most common symptoms, prior medical treatment) are not correlated with FSG levels, while a good correlation of FSG values was found with other clinical features (prior gastric surgery, diarrhea, duration from onset to diagnosis). Increasing basal acid output, but not maximal acid output correlated closely with increasing FSG. Numerous tumoral features correlated with the magnitude of FSG in our study, including tumor location (pancreatic > duodenal), primary size (larger > smaller) and extent (liver metastases > local disease). In conclusion, this detailed analysis of FSG in a large number of patients with ZES allowed us to identify important clinical guidelines that should contribute to improved diagnosis and management of patients with ZES.

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.

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

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

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.

Biological Activity and Ferric Ion Binding of Fragments of Glycine-Extended Gastrin

Nonamidated gastrins such as progastrin and glycine-extended gastrin17 (Ggly) induce cell proliferation and migration in vitro and colonic mucosal proliferation in vivo. Our earlier NMR study defined the structure of Ggly and showed that ferric ions are essential to its biological activity, with the first binding to Glu7 and the second to Glu8 and Glu9 (Pannequin, J. et al. (2002) J. Biol. Chem. 277, 48602-48609). The aims of this study were to define the minimum biologically active fragment of Ggly and to determine whether ferric ions were also required for its activity. Cell-proliferation studies with Ggly fragments containing the five glutamate residues showed that the nonapeptide LE(5)AYG, the octapeptide LE(5)AY, and the heptapeptides E(5)AY and LE(5)A were fully active and that their activity was dependent on the presence of ferric ions. The activity of the hexapeptides LE(5) and E(5)A and the pentapeptide E(5) was reduced and independent of the presence of iron. The stoichiometry of ferric ion binding to LE(5)AYG, LE(5)AY, and E(5)AY, determined by absorption spectroscopy, was 2 mol/mol. NMR spectroscopy showed that the nonapeptide LE(5)AYG and shorter fragments had no defined structure and that the iron-binding sites differed from those in Ggly. We conclude that, in contrast to amidated gastrins where the C-terminal tetrapeptide is the minimum bioactive fragment, the shortest fully active fragments of Ggly are the heptapeptides LE(5)A and E(5)AY. These observations indicate that extensive proteolytic processing may not completely inactivate Ggly and that bioactive forms that are not detected by current radioimmunoassays may be present in tissues and/or plasma.

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, CCK, signaling, and cancer

Gastrin, produced by G cells in the gastric antrum, has been identified as the circulating hormone responsible for stimulation of acid secretion from the parietal cell. Gastrin also acts as a potent cell-growth factor that has been implicated in a variety of normal and abnormal biological processes including maintenance of the gastric mucosa, proliferation of enterochromaffin-like cells, and neoplastic transformation. Here, we review the models used to study the effects of gastrin on cell proliferation in vivo and in vitro with respect to mechanisms by which this hormone might influence normal and cancerous cell growth. Specifically, human and animal models of hypergastrinemia and hypogastrinemia have been described in vivo, and several cells that express cholecystokinin (CCK)B/gastrin receptors have been used for analysis of intracellular signaling pathways initiated by biologically active amidated gastrins. The binding of gastrin or CCK to their common cognate receptor triggers the activation of multiple signal transduction pathways that relay the mitogenic signal to the nucleus and promote cell proliferation. A rapid increase in the synthesis of lipid-derived second messengers with subsequent activation of protein phosphorylation cascades, including mitogen-activated protein kinase, is an important early response to these signaling peptides. Gastrin and CCK also induce rapid Rho-dependent actin remodeling and coordinate tyrosine phosphorylation of cellular proteins including the non-receptor tyrosine kinases p125fak and Src and the adaptor proteins p130cas and paxillin. This article reviews recent advances in defining the role of gastrin and CCK in the control of cell proliferation in normal and cancer cells and in dissecting the signal transduction pathways that mediate the proliferative responses induced by these hormonal GI peptides in a variety of normal and cancer cell model systems.

Review article: gastrin and colorectal cancer

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

Glycine-extended gastrin 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.

Biological activity of carboxy-terminal gastrin analogs

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

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.

Processing and proliferative effects of human progastrin in transgenic mice

Incompletely processed gastrins have been postulated to play a role in growth of the gastrointestinal tract, but few studies have examined the effects of progastrin on mucosal proliferation in vivo. Human gastrin gene expression and progastrin processing were therefore studied in transgenic mice containing a human gastrin (hGAS) minigene, and compared to processing in mice bearing an insulin gastrin (INS-GAS) transgene that overexpresses amidated gastrin. Progastrin processing was studied using region-specific antisera and radioimmunoassays, biosynthetic labeling, immunoprecipitation, and HPLC. Proliferative effects due to overexpression of processed and unprocessed gastrin in INS-GAS and hGAS mice, respectively, were determined using routine histology and BrdU incorporation. The pancreatic islets of INS-GAS mice were able to produce carboxyamidated G-17, resulting in a twofold elevation of serum amidated gastrin, marked thickening of the oxyntic mucosa, and an increased BrdU labeling index (LI) of the gastric body. In contrast, livers of adult hGAS mice expressed abundant human gastrin mRNA and human progastrin but were unable to process this peptide to the mature amidated form, resulting in markedly elevated serum progastrin levels and normal amidated gastrin levels. Nevertheless, there was a marked increase in the BrdU labeling index of the colon in hGAS mice (LI 7.46+/-1.90%), as well as in INS-GAS mice (LI 6.16+/-1.17%), compared to age-matched, wild type control mice (LI 4.01+/-0.98%, P < 0.05). These studies suggest that incompletely processed gastrin precursors may contribute to colonic mucosal proliferation in vivo.

Growth-promoting effects of glycine-extended progastrin

Peptide alpha amidation is required to produce some hormones, such as gastrin, from their glycine-extended precursors. This terminal posttranslational processing reaction is thought to be essential for the biological activation of many peptide hormones; only amidated gastrin exerts a physiological effect that results in gastric acid secretion. However, both amidated gastrin and glycine-extended gastrin stimulate proliferation of exocrine pancreatic cell line AR4-2J through selective receptors for the substrate and the product, respectively, of peptide alpha amidation. Thus, the amidation reaction may function as a determinant of the specific biological actions of products derived from prohormones.

Current approach to the management of gastrinoma and insulinoma in adults with multiple endocrine neoplasia type I

The difficult and controversial diagnostic and therapeutic management of patients having gastrinoma or insulinoma with multiple endocrine neoplasia type I (MEN-I) has been discussed by reference to the literature and a personal series of 45 gastrinoma/MEN-I patients followed consecutively at Bichat Hospital. In both gastrinoma/ and insulinoma/MEN-I patients, anatomic distribution and morphology of tumoral process(es) are usually multiple, diffuse, of small size, and associated with endocrine cell hyperplasia and even nesidioblastosis. These features enhance the difficulty of tumor localization and eradication. Despite the dramatic development of modern medical imagery and surgical experience, the real possibility, on a long-term basis, of curing the patients from their disease remains limited, especially in the gastrinoma/MEN-I patients. In the latter group, according to our experience, persistence or recurrence of the disease after surgery is usual, and metachronous hepatic metastasis development is frequently observed when the follow-up is long enough. Patients with liver metastases, however, seem to undergo a more indolent course than sporadic gastrinoma cases. In insulinoma/MEN-I patients, removal of the functionally dominant islet cell area(s) is essential. Various preoperative and intraoperative localization techniques allow efficacious selective pancreatic surgery in many cases. The latter refinements and the promises of long-acting somatostatin analogs, if confirmed, might restrict to exceptional circumstances the indication of near-total or total pancreatectomy.

Chronic endogenous hypergastrinemia in humans: evidence for a mitogenic effect on the colonic mucosa

BACKGROUND

Information concerning the influence that gastrin may exert on the colon is fragmentary and somewhat controversial. This study analyzed the effect of chronic endogenous hypergastrinemia on cell proliferation and tumor occurrence in the human colonic mucosa.

METHODS

Twenty-three consecutive hypergastrinemic patients presenting with the Zollinger-Ellison syndrome and 18 normogastrinemic subjects were studied. All had fasting serum gastrin determination, colonoscopy, and cell kinetic measurement in two colonic sites using in vitro 5'-bromodeoxyuridine incorporation.

RESULTS

Macroscopic tumors, one endocrine and five adenomas, were found in 5 of 23 hypergastrinemic patients, without apparent relationship with the level of gastrin. The labeling indices were significantly higher in hypergastrinemic than in normogastrinemic individuals in the right and left colon, (P < 0.002 to P < 0.001) without resulting in colonic cell hyperplasia. There was no correlation between labeling indices and serum gastrin concentrations or duration of hypergastrinemia. The DNA labeling distribution along the crypt was normal in the two groups, without expansion of the proliferative zone towards the surface.

CONCLUSIONS

These results showed for the first time that long-lasting endogenous hypergastrinemia is accompanied by increased in vivo cell proliferation in the human colonic mucosa. However, the prevalence of adenomas (17.4%) in patients, all more than 50 years old, may not be different from that in the general population.

Low tissue gastrin content in the ovine distal duodenum is associated with increased percentage of G34

1. In adult sheep and in lambs, over 95% of gastrin in the abomasal antrum was G17 with small amounts of G34 and lesser amounts of Component I. 2. Low gastrin concentration in the proximal duodenum was associated with a reduced percentage of G17. 3. The proportion of G34 increased progressively down the duodenum from a mean of 7% proximally to 47% in the most distal segment, and correlated negatively in any segment with the gastrin content. 4. In both the antrum and proximal duodenum, 60-70% of the G17 was in the sulphated form. 5. The gastroepiploic venous serum contained less G17 and more G34 than the tissues and up to 19% G14.

Identification of progastrin derived peptides in colorectal carcinoma extracts

The possible production of gastrin by colorectal carcinomas has been studied. Extracts of 44 tumours and adjacent macroscopically normal tissue were examined in radioimmunoassay using the following antibodies: (i) L289 raised to a C-terminal fragment of progastrin which shows specificity for intact progastrin, but not the extreme C-terminal tryptic peptide; (ii) LW60 raised to a C-terminal fragment of progastrin which reacts with progastrin and its C-terminal tryptic peptide; (iii) 109-21 which was raised to, and reacts with, Gly-extended forms of heptadecapeptide gastrin--that is, biosynthetic intermediates on the pathway producing active gastrin; and (iv) L2 which reacts with amidated, biologically active gastrins. All samples contained detectable material in assays using LW60; in general, concentrations measured with this antibody were higher than with the other antibodies, and in particular there were higher concentrations in tumour compared with normal tissue extracts. Tumour extracts also contained higher concentrations of immunoreactivity compared with normal tissue, in assays using antibodies L289 and 109-21. In contrast, amidated gastrins were found in similar concentrations in tumour and normal tissue, and concentrations were the lowest of those recorded in the four assays. Separation on Sephadex G50 revealed peaks compatible with progastrin and its C-terminal flanking peptide, and two other peaks that are so far unidentified. In conclusion most colorectal carcinomas contain peptides derived from the gastrin precursor, progastrin, but for the most part these tumours do not convert progastrin into biologically active products.

Studies on co-localization of 7B2 and pancreatic hormones in normal and tumoural islet cells

The distribution of protein 7B2, a protein with structural characteristics of GTP-binding proteins, has been studied in normal pancreatic islets and in a series of 70 pancreatic endocrine tumours with emphasis on the co-localization of 7B2 and the different pancreatic hormones. Although all cell types of normal islets were found to store 7B2, variations from intense expression to absence of reaction were seen within each cell type. In particular, B cells showed intense immunostaining for 7B2 in small compact islets and weak or no staining in larger islets with lobular arrangement. Pancreatic polypeptide (PP) cells expressed 7B2 intensely in the PP-rich area of ventral embryological origin, but were mostly non-reactive in the PP-poor area. The A cells, located along intralobular blood vessels, were more frequently immunoreactive for 7B2 than those at the periphery of the islets. Immuno-electron microscopy revealed a preferential localization of 7B2 in secretory granules of islet cells, with more intense localization in the peripheral halo of alpha granules. Benign islet cell tumours more frequently expressed 7B2 than their malignant counterparts. Although often expressed in a lower number of tumour cells than the tumour-specific hormone, 7B2 was usually co-localized with the latter. In contrast, no relationship was found with the localization of proinsulin. It is concluded that 7B2 is a non-permanent component of the cell granule compartment, probably involved in events related to exocytosis and without relationship to intracellular prohormone processing.

Increased tissue concentrations of the gastrin precursor in patients treated with omeprazole

The main form of gastrin in antral mucosa, the amidated heptadecapeptide G17, is generated from an inactive precursor, progastrin, by steps involving endopeptidase cleavage and amidation. Gastrin cells are normally inhibited by gastric acid and in this study we have examined how suppression of acid by treatment with omeprazole for 6-8 weeks influences gastrin production in patients with oesophagitis. Plasma concentrations of total amidated gastrins in the fasting state increased from 18 to 43 pmol l-1; assays specific for G17-immunoreactivity indicated that the plasma concentrations of this form increased from 6 to 12 pmol l-1. In endoscopic biopsies of antral mucosa there was no change with omeprazole treatment in the concentrations of total amidated gastrins, or their immediate precursors, the Gly-extended gastrins. However, assays using an antibody that reacts with progastrin, together with size exclusion chromatography, indicated that tissue progastrin concentration increased 6-fold. The data suggest a modest net increase in gastrin production with omeprazole-treatment; because the ratio of tissue concentrations of total amidated gastrins to Gly-extended gastrins did not change, it would seem that the amidating capacity of the gastrin cell was maintained. However, the increase in progastrin concentrations suggests a relative failure of the initial steps of post-translational processing, and consequently that in certain circumstances endopeptidase cleavage of progastrin may be rate limiting.

Studies of regulation of gastrin synthesis and post-translational processing by molecular biology approaches

We have examined the regulation of gene expression and post-translational processing of progastrin by starvation and feeding in rats. An oligonucleotide complementary to rat preprogastrin cDNA was used in RNA blot and hybridization analysis to measure gastrin mRNA levels. A region-specific antibody raised against the predicted amino acid sequence of the carboxyl terminal extension of progastrin was used for quantitation of progastrin peptides. The effects of starvation and of refeeding on rat antral gastrin mRNA and pro-hormone peptide levels were examined in rats starved for 48 h and after refeeding with a solid meal. Antral gastrin mRNA concentrations decreased to a plateau level (30% of the nonstarved control) after 48 h of starvation. Immunoreactive gastrin concentration decreased threefold, but progastrin processing intermediates did not decrease significantly during fasting. Following refeeding, significant increases in antral mRNA level were detected in 1 h, and peak levels were reached by 2 h (more than two times higher than starved control). There was a rapid and significant decrease in progastrin immunoreactivity within 30 min, followed by a significant increase in gastrin immunoreactivity 2 h after refeeding. These data suggest that rapid increases of blood and tissue gastrin levels in response to food may be associated with increases in both gastrin gene expression and post-translational processing of progastrin.

Progastrin in serum from Zollinger-Ellison patients. An indicator of malignancy?

Progastrin and all of its processing products were measured in serum from 48 patients with Zollinger-Ellison syndrome, 42 patients with duodenal ulcers, and 34 normal subjects. A processing-independent gastrin analysis and a conventional radioimmunoassay for the biologically active alpha-amidated gastrins were used. In serum from normal subjects, 87% (median; range, 27%-160%) of all progastrin products were alpha-amidated gastrins, whereas they constituted only 39% (15%-130%) in serum from patients with duodenal ulcers (p less than 0.01) and 46% (16%-100%) in serum from gastrinoma patients (p less than 0.01). A significantly lower percentage of alpha-amidated gastrin was found in patients with hepatic metastases (23%) than in patients with apparently benign tumors (54%). Chromatography of serum showed that large progastrin molecules occurred mainly in patients with malignant tumors, whereas smaller glycine-extended precursors dominated in patients with benign tumors. The results indicate that the total progastrin product reflects tumor synthesis of gastrin better than conventional measurements of alpha-amidated gastrin. Moreover, the results suggest that a low degree of processing of progastrin could serve as a predictor of a malignant clinical course at an early stage of the disease.

Progastrin expression in mammalian pancreas

Expression and processing of progastrin were examined in fetal, neonatal, and adult pancreatic tissue from five mammalian species (cat, dog, man, pig, and rat). A library of sensitive, sequence-specific immunoassays for progastrin and its products was used to monitor extractions and chromatography before and after cleavage with processing-like enzymes. The results showed that progastrin and its products are expressed in the pancreas of all species in total concentrations varying from 0.3 to 58.9 pmol/g of tissue (medians). The degree of processing was age- and species-dependent. In comparison with adult pancreatic tissue the fetal or neonatal pancreas processed a higher fraction to bioactive, C-terminally amidated gastrin. Nevertheless, the pancreatic processing was always less complete than that of the adult antral mucosa. The moderate level of expression and the attenuated processing in the adult pancreas contribute to explain previous failures to detect gastrin in normal pancreatic tissue. Our results indicate that gastrin-producing tumors in the pancreas are not ectopic, but arise from cells that normally express the gastrin gene.

Electron-cytochemical localization of alkaline phosphatase to G cells of Necturus maculosus antrum

Electron-cytochemical localization of alkaline phosphatase activity was performed on G cells of Necturus maculosus antral mucosa. Alkaline phosphatase activity was localized to the nuclear membrane, the Golgi/endoplasmic reticulum, and the limiting membranes of G cell peptide-secretion vesicles. There was no specific localization of alkaline phosphatase activity to the plasma membrane. Treatment of the tissues with levamisole (an alkaline phosphatase inhibitor) did not markedly reduce the specific alkaline phosphatase activity. Specific lead deposition was reduced by removal of the substrate from the reaction mixture. The results from this study on N. maculosus G cells demonstrate that alkaline phosphatase activity can be found in a non-mammalian gastric endocrine cell and that specific activity was localized primarily to those intracellular structures involved with protein biosynthesis.

Current diagnosis and management of Zollinger-Ellison syndrome

The treatment of patients with Zollinger-Ellison syndrome (ZES) has undergone dramatic evolution during the past decade. Although initially regarded as an incurable tumor, resection of gastrinoma for potential cure has been reported in 30% to 40% of selected patients in recent series. Conversely, although definitive control of acid hypersecretion is achieved by total gastrectomy, histamine (H2)-receptor antagonists and the newly introduced agents omeprazole and somatostatin analogues allow effective medical therapy of gastric acid overproduction. Confirmation of the diagnosis is best achieved with the I.V. secretin stimulation test, and tumor localization techniques are mandatory to identify candidates for operative tumor resection. Intraoperative sonography and careful exploration are required for tumor removal; successful tumor resection is associated with prolonged survival. The majority of patients (60%) are still found to have malignant disease at the time of diagnosis, but 10-year overall survival commonly exceeds 40%. The presence of multiple endocrine neoplasia type I (MEN-I) is seen in 10% to 25% of patients; correction of hypercalcemia alone may have therapeutic benefit in some ZES patients, and while gastrinoma resection is rarely possible, MEN-I patients demonstrate prolonged survival. The choice of medical rather than surgical therapy for acid hypersecretion depends on the suitability of each patient for careful and repeated endoscopic and chemical studies, versus the likelihood of a successful postoperative outcome. Socioeconomic, geographic, and related medical factors in each case may dictate the form of long-term antisecretory therapy. Exploration for possible tumor resection is indicated for virtually all patients who have no documented metastatic disease.

Characterization of the carboxyl terminal flanking peptide of rat progastrin

A peptide identical in structure to the carboxyl-terminal flanking nonapeptide of rat progastrin, predicted by cDNA sequence, was synthesized. The synthetic peptide was used for production of a rabbit antiserum. This antiserum was used to develop a radioimmunoassay specific for rat carboxyl terminal flanking peptide. This assay was used to monitor the purification of immunoreactivity from rat antral extracts. Gel permeation, anion exchange and reverse phase chromatography steps resulted in a single absorbance peak associated with the carboxyl terminal flanking peptide immunoreactivity. The purified peptide eluted in the same position as the synthetic peptide during all three types of chromatography. This material was shown to be identical in mass to Ser-Ala-Glu-Glu-Glu-Asp-Gln-Tyr-Asn, the predicted sequence of the carboxyl terminal nonapeptide of rat progastrin.

The constitution and properties of phosphorylated and unphosphorylated C-terminal fragments of progastrin from dog and ferret antrum

Antibodies to the extreme C-terminal tryptic (nona-) peptide fragment of porcine progastrin have been used in radioimmunoassay to identify progastrin fragments in dog, ferret and pig antral mucosa extracts and to monitor their purification. In addition to previously characterised phosphorylated and unphosphorylated C-terminal tryptic peptides of porcine progastrin a minor form corresponding to the C-terminal octapeptide (i.e. des-Ser C-terminal nonapeptide) was isolated and characterised. The latter form together with phosphorylated and unphosphorylated forms of the nonapeptides were also isolated and chemically characterised from dog antrum, and the unphosphorylated nonapeptide was characterised from ferret antrum. The primary amino acid sequences of the dog, ferret and pig nonapeptides were identical. In ferret the unphosphorylated nonapeptide predominated, and in dog the phosphorylated form predominated; in pig both forms of the nonapeptide were well represented. Intact progastrin was identified in gel filtration eluates of extracts of all 3 species, but occurred only in relatively low concentrations. The nonapeptides did not stimulate acid secretion in the conscious gastric fistula rat and they did not modify the acid response to G17. Phosphorylation of progastrin-derived peptides is evidently well conserved across a range of species even though there appear to be differences in the relative proportions of phosphorylated and unphosphorylated forms.

Progastrin processing during antral G-cell hypersecretion in humans

Using radioimmunoassays specific for essential processing sites of human progastrin in combination with chromatography before and after cleavage with trypsin and carboxypeptidase B, we have examined antral biopsy specimens and serum from 10 hypergastrinemic patients with fundic atrophic gastritis and 7 normal control subjects. Four types of processing were studied: N-terminal proteolysis (at the N-terminus of component I, gastrin 34, and gastrin 17); C-terminal proteolysis (at the C-terminus of the amide donor, glycine93 in preprogastrin); alpha-carboxyamidation (of phenylalanine92); and O-sulfation (of tyrosine87). The results show that progastrin during permanent G-cell hypersecretion is less completely processed with respect to C-terminal proteolysis, alpha-amidation, and tyrosine-sulfation. In contrast, the degree of N-terminal proteolysis is normal. Thus, the processing of progastrin adjacent to the active site of gastrin is more restrictively controlled than N-terminal processing during G-cell hypersecretion associated with pernicious anemia.

The human gastrin precursor. Characterization of phosphorylated forms and fragments

There is a potential phosphorylation site in the C-terminal region of the precursor for the acid-stimulating hormone gastrin, which is immediately adjacent to an important cleavage point. In the present study we have sought to identify, separate, quantify and characterize phosphorylated and unphosphorylated forms of human progastrin and its fragments. Identification was made by two radioimmunoassays: (a) a novel assay employing an antibody raised to intact human progastrin; and (b) an assay using antibody reacting with the C-terminal tryptic fragment of human progastrin, as well as progastrin itself. Two forms of human progastrin isolated from a gastrinoma were separated by ion-exchange h.p.l.c., and had similar elution positions on reverse-phase h.p.l.c. and on gel filtration. The more acidic peptide contained close to equimolar amounts of phosphate. On trypsinization, peptides were released that co-eluted on ion-exchange h.p.l.c. with, and had the immunochemical properties of, naturally occurring C-terminal fragments of progastrin. One of the latter was isolated and shown by Edman degradation after derivatization with ethanethiol to have the sequence Ser (P)-Ala-Glu-Asp-Glu-Asn. Similar peptides occur in antral mucosa resected from ulcer patients. The unphosphorylated forms of progastrin predominated, whereas the phosphorylated forms of the C-terminal fragments were predominant. This distribution could be explained by preferential cleavage of phosphorylated progastrin. We conclude that in human progastrin, Ser-96 can occur in the phosphorylated form; this residue immediately follows a pair of basic residues (Arg-Arg) that are cleaved during synthesis of the biologically active product.

Zollinger-Ellison syndrome. Current concepts in diagnosis and management

The Zollinger-Ellison syndrome, although uncommon, is not rare, and most patients with the disorder present with clinical manifestations similar to those of patients with common peptic ulcer. Early studies emphasized death due to complications of massive gastric acid hypersecretion. However, with the availability of potent antisecretory agents to control acid secretion, death is now more frequently associated with the metastatic potential of slowly growing but malignant gastrinomas. Therefore, physicians should maintain a high degree of suspicion of the Zollinger-Ellison syndrome in assessing patients with either chronic peptic ulcer or unexplained secretory diarrhea. An evaluation aimed at early diagnosis of the Zollinger-Ellison syndrome should be instituted in such patients and should begin with a determination of the fasting serum gastrin level. At least 50 percent of patients with gastrinoma will have nondiagnostic serum gastrin concentrations and will therefore require provocative testing to establish the correct diagnosis. After the presence of the syndrome is established, patients should be treated with a potent antisecretory agent in doses sufficient to reduce basal acid output to less than 10 mmol in the hour preceding administration of the next dose. Although some patients may be maintained satisfactorily in this manner for extended periods, an approach aimed at tumor localization and extirpation is recommended in most patients. Preoperative evaluation should begin with CT scanning with intravenous contrast material. Selective angiography, and occasionally, portal venous sampling for gastrin, should be performed if the location and extent of tumor remain in question. If metastatic disease is demonstrated, or if MEN-I is present, surgery aimed at tumor resection, although it is occasionally effective, will probably be unsuccessful. Because of the considerable morbidity and mortality associated with pancreatoduodenectomy, it should not be performed for unresectable tumor in the head of the pancreas. In other patients with the Zollinger-Ellison syndrome, exploratory surgery should be performed; this should include a careful search for, and resection of, all pancreatic and extrapancreatic gastrinomas. With this approach, it is likely that at least 20 percent of all patients with the Zollinger-Ellison syndrome can be cured.

Endocrine cells producing regulatory peptides

Recent data on the immunolocalization of regulatory peptides and related propeptide sequences in endocrine cells and tumors of the gastrointestinal tract, pancreas, lung, thyroid, pituitary (ACTH and opioids), adrenals and paraganglia have been revised and discussed. Gastrin, xenopsin, cholecystokinin (CCK), somatostatin, motilin, secretin, GIP (gastric inhibitory polypeptide), neurotensin, glicentin/glucagon-37 and PYY (peptide tyrosine tyrosine) are the main products of gastrointestinal endocrine cells; glucagon, CRF (corticotropin releasing factor), somatostatin, PP (pancreatic polypeptide) and GRF (growth hormone releasing factor), in addition to insulin, are produced in pancreatic islet cells; bombesin-related peptides are the main markers of pulmonary endocrine cells; calcitonin and CGRP (calcitonin gene-related peptide) occur in thyroid and extrathyroid C cells; ACTH and endorphins in anterior and intermediate lobe pituitary cells, alpha-MSH and CLIP (corticotropin-like intermediate lobe peptide) in intermediate lobe cells; met- and leu-enkephalins and related peptides in adrenal medullary and paraganglionic cells as well as in some gut (enterochromaffin) cells; NPY (neuropeptide Y) in adrenaline-type adrenal medullary cells, etc.. Both tissue-appropriate and tissue-inappropriate regulatory peptides are produced by endocrine tumours, with inappropriate peptides mostly produced by malignant tumours.

Progastrin and its glycine-extended posttranslational processing intermediates in human gastrointestinal tissues

We examined the posttranslational modification of gastrin in human gastrointestinal and Zollinger-Ellison tumor tissues using antibodies specific for progastrin and its glycine-extended processing intermediates. Gel filtration of antral and duodenal extracts on Sephadex G-50 revealed multiple molecular forms of immunoreactive glycine-extended processing intermediates corresponding to known molecular forms of gastrin and cholecystokinin. Immunoaffinity chromatography studies of antral mucosal and gastrinoma extracts identified a molecular form of glycine-extended processing intermediates that was characterized by an amino terminus identical to that of gastrin heptadecapeptide. Large differences in the relative contents of precursors and products of gastrin synthesis were found in gastrinoma tissue as compared to gastrointestinal mucosal extracts. These studies suggest the potential importance of glycine-extended peptides as intermediates in the posttranslational processing of gastrin and cholecystokinin in humans and indicate that gastrin processing mechanisms may be altered in Zollinger-Ellison tumors.

Expression of the C-terminal flanking peptide of human progastrin in human gastroduodenal mucosa, G-cell hyperplasia and islet cell tumours producing gastrin

Three antisera to the C-terminally extended form of gastrin or the C-terminal flanking peptide of progastrin were used in an attempt to investigate the post-translational processing of progastrin at the cellular level by light and electron microscopical immunocytochemistry. In the normal human gastric antrum, the G-cell secretory granules were found to contain both gastrin and the C-terminal progastrin determinants (progastrin 87-93, 87-95 and 93-101). Immunostaining of serial sections at the light microscopical level revealed that duodenal gastrin-containing cells also express the C-terminal progastrin determinants, as well as gastrin-34. In foetal tissue, cells containing C-terminal gastrin and the C-flanking peptide of progastrin were first seen at 8 weeks of gestation, in the duodenum. They were not found in the stomach until the 11th week. In hyperplastic G-cells and in gastrin-producing tumour cells, the level of C-terminal peptide immunoreactivity was variable and often lower than that seen in normal antrum and only minimal immunoreactivity could be detected using electron immunocytochemistry. This was interpreted as representing altered post-translational processing of progastrin in modified G-cells.

Isolation and characterization of the intact gastrin precursor from a gastrinoma

Antibodies to the extreme C-terminal region of human progastrin have been used to monitor the isolation of high-Mr immunoreactive material in a gastrinoma extract. Microsequence analysis of the product revealed amino acid residues in the first 18 positions corresponding to those predicted from the cDNA sequence for preprogastrin starting at position 22; the sequence and immunochemical data together allow the identification of this material as intact progastrin. Implications for gastrin biosynthesis are discussed.