Measurement of precursors for alpha-amidated hormones by radioimmunoassay of glycine-extended peptides after trypsin-carboxypeptidase B cleavage

Gastric Residual to Predict Necrotizing Enterocolitis in Preterm Piglets As Models for Infants

BACKGROUND

Necrotizing enterocolitis (NEC) is a serious intestinal inflammatory disease in preterm infants. High volume of gastric residual (GR) after oral feedings is often used as a predictor of NEC, but evidence is limited. Using NEC-sensitive preterm piglets as models, we hypothesized that GR mass and related plasma biomarkers predict early onset of NEC.

METHODS

In total, 258 newborn preterm piglets were fed bovine milk-based formulas for 5 days. At euthanasia, the stomach, small intestine, and colon were evaluated for NEC lesions. Mass, acidity, gastrin, and bile acid levels were determined for GR content, together with gastrin, glucagon-like peptide 2 (GLP-2), and gastric inhibitory polypeptide (GIP) levels in plasma.

RESULTS

In total, 48% of piglets had NEC lesions in the small intestine and/or colon. These piglets had higher GR mass (+32%, P < 0.001) and lower gastric bile acid concentrations (-22%, P < 0.05) than piglets without NEC lesions. The positive and negative predictive values for these markers were 34%-61%. Gastric acidity, gastrin, GLP-2, and GIP levels were similar for piglets with and without NEC lesions.

CONCLUSION

Elevated GR mass correlates positively with NEC lesions but may be a poor predictor of NEC, even when combined with other biomarkers. More knowledge about gastric emptying and gut transit in preterm neonates is required to understand how GR volume and composition relate to morbidities, such as NEC, in preterm neonates.

Cardiomyocyte expression and cell-specific processing of procholecystokinin

Heart muscle cells produce peptide hormones such as natriuretic peptides. Developing hearts also express the gene for the classic intestinal hormone cholecystokinin (CCK) in amounts similar to those in the intestine and brain. However, cardiac expression of peptides other than natriuretic peptides has only been suggested using transcriptional measures or methods, with the post-translational phase of gene expression unaddressed. In this study, we examined the cardiac expression of the CCK gene in adult mammals and its expression at the protein level. Using quantitative PCR, a library of sequence-specific pro-CCK assays, peptide purification, and mass spectrometry, we demonstrate that the mammalian heart expresses pro-CCK in amounts comparable to natriuretic prohormones and processes it to a unique, triple-sulfated, and N-terminally truncated product distinct from intestinal and cerebral CCK peptides. Isoprenaline rapidly stimulated cardiac CCK gene expression in vitro and in vivo, which suggests that the cardiac-specific truncated pro-CCK may have pathophysiological relevance as a new marker of heart failure. The suggestion is confirmed by measurement of plasma from heart failure patients.

An evaluation of chromogranin A versus gastrin and progastrin in gastrinoma diagnosis and control

AIM

The value of chromogranin A (CgA) versus gastrin and progastrin in diagnosis and control of gastrinoma patients is not settled because the peptides circulate as variable mixtures. We have addressed this complexity using defined sequence-specific assays.

PATIENTS & METHODS

Six assays were applied to plasma from 40 gastrinoma patients to measure α-amidated gastrins, glycine-extended gastrins, the total progastrin product, and assays for CgA sequence (340-348) and the 'total' CgA product.

RESULTS

The gastrin/progastrin parameters did not add to the diagnosis beyond that of α-amidated gastrins, except in one patient. All gastrin parameters correlated otherwise closely. The CgA results differed. Thus, 11 patients had normal CgA concentrations. By contrast, all total CgA concentrations were elevated but correlated only moderately to gastrin.

CONCLUSION

Assays measuring α-amidated gastrins have high diagnostic value except for singular patients in whom only progastrin was elevated. By contrast, CgA measurements are not valid in diagnosis or control of gastrinomas.

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.

Gene expression profiling of gastric mucosa in mice lacking CCK and gastrin receptors

The stomach produces acid, which may play an important role in the regulation of bone homeostasis. The aim of this study was to reveal signaling pathways in the gastric mucosa that involve the acid secretion and possibly the bone metabolism in CCK1 and/or CCK2 receptor knockout (KO) mice. Gastric acid secretion was impaired and the ECL cell signaling pathway was inhibited in CCK2 receptor KO mice but not in CCK1 receptor KO mice. However, in CCK1+2 receptor double KO mice the acid secretion in response to pylorus ligation-induced vagal stimulation and the ECL cell pathway were partially normalized, which was associated with an up-regulated pituitary adenylate cyclase-activating polypeptide (PACAP) type 1 receptor (PAC1). The basal part of the gastric mucosa expressed parathyroid hormone-like hormone (PTHLH) in a subpopulation of likely ECL cells (and possibly other cells) and vitamin D3 1α hydroxylase probably in trefoil peptide2-immunoreactive cells. In conclusion, mice lacking CCK receptors exhibited a functional shift from the gastrin-CCK pathways to the neuronal pathway in control of the ECL cells and eventually the acid secretion. Taking the present data together with previous findings, we suggest a possible link between gastric PTHLH and vitamin D and bone metabolism.

Circulating gastrin concentrations in patients at increased risk of developing colorectal carcinoma

BACKGROUND AND AIM

An increase in circulating concentrations of gastrin or gastrin precursors such as progastrin and glycine-extended gastrin has been proposed to promote the development of colorectal carcinomas (CRC). The aim of this study was to investigate whether or not circulating gastrin concentrations were increased in patients with an increased risk of developing CRC.

METHOD

Patients were divided according to their risk into the five following groups: familial adenomatous polyposis (n = 20), hereditary non-polyposis colorectal cancer (n = 53), cluster of common colorectal cancers (n = 13), personal history and/or family history of adenomatous polyps or CRC (n = 150) and controls (n = 42). Radioimmunoassay with four region-specific gastrin antisera was used to measure progastrin, glycine-extended gastrin (gastrin-gly), amidated gastrin (gastrin-amide), and total gastrin in peripheral blood taken at the time of colonoscopy.

RESULTS

Compared with the control group, familial adenomatous polyposis patients had significantly higher median values of total gastrin (29.8 pM vs 16.9 pM, P = 0.003) and gastrin-amide (17.1 pM vs 12.0 pM, P = 0.015). Patients with a personal or family history of adenomatous polyps or CRC also had higher circulating concentrations of total gastrin (21.8 pM) compared with controls (P < 0.05), while patients from all groups who presented with an adenomatous polyp on the day of colonoscopy had higher concentrations of total gastrin, progastrin, and gastrin-amide than patients without polyps.

CONCLUSION

Concentrations of gastrin precursors are increased in particular groups with an increased risk of developing CRC.

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.

Juvenile polyposis of the stomach--a novel cause of hypergastrinemia

BACKGROUND

A 38-year-old female presented with a 3-year history of postprandial abdominal pain, refractory nausea, vomiting and hematemesis. She appeared malnourished and her symptoms were refractory to previous treatment with acid-suppressive drugs, prokinetics and antiemetics. Her medical history was significant for a diagnosis of juvenile polyposis syndrome at the age of 14 resulting in a transverse colectomy, and a diagnosis of Crohn's disease in her residual colon at the age of 35 resulting in a total colectomy.

INVESTIGATIONS

Physical examination, blood analysis, esophagogastroduodenoscopy with biopsy, abdominal endoscopic ultrasound, abdominal CT scan, MRI, 24 h urine analysis, MIBG scintigraphy, ocreotide scintigraphy, fluorodeoxyglucose-PET scan and genetic testing for defined polyposis syndromes (SMAD4, BMPR1A).

DIAGNOSIS

Juvenile polyposis syndrome with outlet obstruction of the stomach and excessive hypergastrinemia.

MANAGEMENT

Continuous acid-suppressive therapy, prokinetic therapy and total parenteral nutrition. Repetitive endoscopic polypectomy (also known as debulking) was performed twice and was followed by gastrectomy with duodenoesophageal anastomosis.

Proton-pump inhibitor therapy induces acid-related symptoms in healthy volunteers after withdrawal of therapy

BACKGROUND & AIMS

Rebound acid hypersecretion (RAHS) has been demonstrated after 8 weeks of treatment with a proton-pump inhibitor (PPI). If RAHS induces acid-related symptoms, this might lead to PPI dependency and thus have important implications.

METHODS

A randomized, double-blind, placebo-controlled trial with 120 healthy volunteers was conducted. Participants were randomized to 12 weeks of placebo or 8 weeks of esomeprazole 40 mg/d followed by 4 weeks with placebo. The Gastrointestinal Symptom Rating Scale (GSRS) was filled out weekly. A score of >2 on 1 of the questions regarding heartburn, acid regurgitation, or dyspepsia was defined as a clinically relevant acid-related symptom.

RESULTS

There were no significant differences between groups in GSRS scores at baseline. GSRS scores for acid-related symptoms were significantly higher in the PPI group at week 10 (1.4 +/- 1.4 vs 1.2 +/- 0.9; P = .023), week 11 (1.4 +/- 1.4 vs 1.2 +/- 0.9; P = .009), and week 12 (1.3 +/- 1.2 vs 1.0 +/- 0.3; P = .001). Forty-four percent (26/59) of those randomized to PPI reported > or = 1 relevant, acid-related symptom in weeks 9-12 compared with 15% (9/59; P < .001) in the placebo group. The proportion reporting dyspepsia, heartburn, or acid regurgitation in the PPI group was 13 of 59 (22%) at week 10, 13 of 59 (22%) at week 11, and 12 of 58 (21%) at week 12. Corresponding figures in the placebo group were 7% at week 10 (P = .034), 5% at week 11 (P = .013), and 2% at week 12 (P = .001).

CONCLUSIONS

PPI therapy for 8 weeks induces acid-related symptoms in healthy volunteers after withdrawal. This study indicates unrecognized aspects of PPI withdrawal and supports the hypothesis that RAHS has clinical implications.

Prohormone convertases 1/3 and 2 together orchestrate the site-specific cleavages of progastrin to release gastrin-34 and gastrin-17

Cellular synthesis of peptide hormones requires PCs (prohormone convertases) for the endoproteolysis of prohormones. Antral G-cells synthesize the most gastrin and express PC1/3, 2 and 5/6 in the rat and human. But the cleavage sites in progastrin for each PC have not been determined. Therefore, in the present study, we measured the concentrations of progastrin, processing intermediates and alpha-amidated gastrins in antral extracts from PC1/3-null mice and compared the results with those in mice lacking PC2 and wild-type controls. The expression of PCs was examined by immunocytochemistry and in situ hybridization of mouse G-cells. Finally, the in vitro effect of recombinant PC5/6 on progastrin and progastrin fragments containing the relevant dibasic cleavage sites was also examined. The results showed that mouse G-cells express PC1/3, 2 and 5/6. The concentration of progastrin in PC1/3-null mice was elevated 3-fold. Chromatography showed that cleavage of the Arg(36)Arg(37) and Arg(73)Arg(74) sites were grossly decreased. Accordingly, the concentrations of progastrin products were markedly reduced, alpha-amidated gastrins (-34 and -17) being 25% of normal. Lack of PC1/3 was without effect on the third dibasic site (Lys(53)Lys(54)), which is the only processing site for PC2. Recombinant PC5/6 did not cleave any of the dibasic processing sites in progastrin and fragments containing the relevant dibasic processing sites. The complementary cleavages of PC1/3 and 2, however, suffice to explain most of the normal endoproteolysis of progastrin. Moreover, the results show that PCs react differently to the same dibasic sequences, suggesting that additional structural factors modulate the substrate specificity.

The cell-specific pattern of cholecystokinin peptides in endocrine cells versus neurons is governed by the expression of prohormone convertases 1/3, 2, and 5/6

Most peptide hormone genes are, in addition to endocrine cells, also expressed in neurons. The peptide hormone cholecystokinin (CCK) is expressed in different molecular forms in cerebral neurons and intestinal endocrine cells. To understand this difference, we examined the roles of the neuroendocrine prohormone convertases (PC) 1/3, PC2, and PC5/6 by measurement of proCCK, processing intermediates and bioactive, alpha-amidated, and O-sulfated CCK peptides in cerebral and jejunal extracts of null mice, controls, and in the PC5/6-expressing SK-N-MC cell-line. In PC1/3 null mice, the synthesis of bioactive CCK peptide in the gut was reduced to 3% of the translational product, all of which was in the form of alpha-amidated and tyrosine O-sulfated CCK-22, whereas the neuronal synthesis in the brain was largely unaffected. This is opposite to the PC2 null mice in which only the cerebral synthesis was affected. SK-N-MC cells, which express neither PC1/3 nor PC2, synthesized alone the processing intermediate, glycine-extended CCK-22. Immunocytochemistry confirmed that intestinal endocrine CCK cells in wild-type mice express PC1/3 but not PC2. In contrast, cerebral CCK neurons contain PC2 and only little, if any, PC1/3. Taken together, the data indicate that PC1/3 governs the endocrine and PC2 the neuronal processing of proCCK, whereas PC5/6 contributes only to a modest endocrine synthesis of CCK-22. The results suggest that the different peptide patterns in the brain and the gut are due to different expression of PCs.

Distinct linkage between post-translational processing and differential secretion of progastrin derivatives in endocrine cells

Prohormones often undergo extensive cellular processing prior to secretion. These post-translational processing events occur in organelles of the constitutive or regulated secretory pathway. The aim of this study was to examine the relationship between post-translational modifications and the secretory pathways taken by peptides derived from progastrin, the prohormone of gastrin, which in vivo is secreted by cells of the pyloric glands and stimulates the release of gastric acid. Targeting progastrin to compartments of the early secretory pathway shows that endoproteolytic processing is initiated in a pre-trans-Golgi network compartment of endocrine but not non-endocrine cells. The resulting N-terminal fragments of progastrin are secreted via the constitutive pathway, whereas endoproteolytically processed C-terminal fragments are secreted via the regulated or constitutive-like pathways. C-terminal fragments derived from progastrin differ in characteristic manners in levels and patterns of carboxyamidation and tyrosine sulfation in accordance with the secretory pathway taken. Point mutations introduced into a sorting motif disrupt these patterns, suggesting that differences in post-translational modifications are attributable to differential intracellular sorting of precursors. The results suggest a two-step sorting mechanism for progastrin leading to differential secretion of processed fragments via different secretory pathways.

Enhanced peptide secretion by gene disruption of CYM1, a novel protease in Saccharomyces cerevisiae

Saccharomyces cerevisiae is a widely used host in the production of therapeutic peptides and proteins. Here we report the identification of a novel endoprotease in S. cerevisiae. It is encoded by the CYM1 gene and is specific for the C-terminus of basic residues of heterologously expressed peptides. Gene disruption of CYM1 not only reduced the intracellular proteolysis, but also enhanced the secretion of heterologously expressed peptides such as growth hormone, pro-B-type natriuretic peptide and pro-cholecystokinin. Cym1p resembles metalloendoproteases of the pitrilysin family with the HXXEH(X)E(71-77) catalytic domain as seen in insulysin, nardilysin and human metalloprotease 1. It is a nuclear encoded protease that localizes to mitochondria without a hydrophobic N-terminal signal sequence or a C-terminal tail-anchor. The protease does not require post-translational processing prior to activation and it contains cytosolic activity that processes peptides designated for the secretory pathway prior to translocation into the endoplasmic reticulum.

Increased synthesis but decreased processing of neuronal proCCK in prohormone convertase 2 and 7B2 knockout animals

In addition to its role as a gut hormone, cholecystokinin (CCK) is a widespread and potent neurotransmitter. Its biosynthesis requires endoproteolytic cleavage of proCCK at several mono- and dibasic sites by subtilisin-like prohormone convertases (PCs). Of these, PC1 and PC2 are specific for neuroendocrine cells. We have now examined the role of PC2 and its binding protein, 7B2, in the neuronal processing of proCCK by measurement of precursor, processing-intermediates and bioactive end-products in brain extracts from PC2- and 7B2-null mice and from corresponding controls. PC2-null mice displayed a nine-fold increase of cerebral proCCK concentrations, and a two-fold increase in the concentrations of the processing-intermediate, glycine-extended CCK, whereas the concentrations of transmitter-active (i.e. alpha-amidated and O-sulfated) CCK peptides were reduced (61%). Chromatography showed that O-sulfated CCK-8 still is the predominant transmitter-active CCK in PC2-null brains, but that the fraction of intermediate-sized CCK-peptides (CCK-58, -33 and -22) was eight-fold increased. 7B2-null brains displayed a similar pattern but with less pronounced precursor accumulation. In contrast with the cerebral changes, PC2 deficiency was without effect on proCCK synthesis and processing in intestinal endocrine cells, whereas 7B2 deficiency halved the concentration of bioactive CCK in the intestine. The results show that PC2 plays a major neuron-specific role in the processing of proCCK.

Differentiation of gastric ECL cells is altered in CCK(2) receptor-deficient mice

BACKGROUND & AIMS

Gastrin stimulation of the type 2 cholecystokinin (CCK(2)) receptor results in ECL cell proliferation and histamine secretion. This report describes the effects of targeted disruption of the CCK(2) receptor gene on ECL cell morphology and function.

METHODS

The ECL cells in the oxyntic mucosa of CCK(2) receptor-deficient (knockout [KO]) vs. wild-type (WT) mice were investigated by immunocytochemical and biochemical approaches, as well as by electron microscopy.

RESULTS

Immunocytochemistry demonstrates similar numbers (cells per millimeter of horizontal length of mucosa) of pancreastatin- or vesicle monoamine transporter-2 (VMAT-2)-immunoreactive cells in WT mice and KO mice. However, only WT mice harbor histamine-immunoreactive ECL cells. The mucosal histamine content in KO mice (likely originating from mast cells) is only a minute fraction of that present in WT animals. The activity of the histamine forming enzyme, histidine decarboxylase (a marker of ECL cells), was undetectable in the oxyntic mucosa of KO mice yet was readily apparent in the mucosa from WT animals. Electron microscopy revealed numerous ECL cells in WT mice. In KO animals, these cells were replaced by an "ECL-like" cell type, characterized by a lack of secretory vesicles (a hallmark feature of normal ECL cells) and the presence of dense-core granules and microvesicles in numbers comparable to those found in WT ECL cells. Based on ultrastructural features, the ECL-like cells in KO mice can be readily distinguished from other gastric endocrine cells, including A-like cells and D cells.

CONCLUSIONS

Absence of a single gene product, the CCK(2) receptor, alters the differentiation and function of gastric ECL cells.

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.

Neuronal cell lines expressing PC5, but not PC1 or PC2, process Pro-CCK into glycine-extended CCK 12 and 22

Endocrine tumor cells in culture and in vitro cleavage assays have shown that PC1 and PC2 are capable of processing pro-CCK into smaller, intermediate and final, bioactive forms. Similar studies have shown that PC5 has the ability to process a number of propeptides. Here, we use GT1-7 (mouse hypothalamic) and SK-N-MC and SK-N-SH (human neuroblastoma) tumor cell lines to study the ability of PC5 to process pro-CCK. RT-PCR and Western blot analysis showed that the cells express PC5 mRNA and protein, but not PC1 or PC2. They were engineered to stably overexpress CCK and cell media was analyzed for pro-CCK expression and cleavage of the prohormone. Radioimmunoassays showed that pro-CCK was expressed, but no amidated CCK was detected. Lack of production of amidated CCK may be due to the lack of the appropriate carboxypeptidase and amidating enzymes. Production of glycine-extended CCK processing products was evaluated by treatment of media with carboxypeptidase B followed by analysis with a CCK Gly RIA. Glycine-extended forms of the peptide were found in the media. The predominant forms co-eluted with CCK 12 Gly and CCK 22 Gly on gel filtration chromatography. The results demonstrate that these cell lines which express PC5 and not PC1 or PC2 have the ability to process pro-CCK into intermediate, glycine-extended forms more closely resembling pro-CCK products in intestine than in brain.

A synthetic glycine-extended bombesin analogue interacts with the GRP/bombesin receptor

alpha-amidation of a peptide (which takes place from a glycine-extended precursor) is required to produce biologically active amidated hormones, such as gastrin-releasing peptide (GRP)/Pyr-Gln-Arg-Leu-Gly-Asn-Gln-Trp-Ala-Val-Gly-His-Leu-Met-NH(2) (bombesin). It was shown that glycine-extended gastrin mediates mitogenic effects on various cell lines by interacting with a specific receptor, different from the classical CCK(1) or CCK(2) receptors. On the basis of this observation, we have extended the concept of obtaining active glycine-extended forms of others amidated peptides to produce new active analogues. In this study, we have tested the biological behaviour of a synthetic analogue of the glycine-extended bombesin (para-hydroxy-phenyl-propionyl-Gln-Trp-Ala-Val-Gly-His-Leu-Met-Gly-OH or JMV-1458) on various in vitro models. We showed that compound JMV-1458 was able to inhibit specific (3-[125I]iodotyrosyl(15)) GRP ([125I]GRP) binding in rat pancreatic acini and in Swiss 3T3 cells with K(i) values of approximately 10(-8) M. In isolated rat pancreatic acini, we found that JMV-1458 induced inositol phosphates production and amylase secretion in a dose-dependent manner. In Swiss 3T3 cells, the glycine-extended bombesin analogue dose-dependently produced [3H]thymidine incorporation. By using potent GRP/bombesin receptor antagonists, we showed that this synthetic glycine-extended bombesin analogue induces its biological activities via the classical GRP/bombesin receptor.

Closing the gastrin loop in pancreatic carcinoma: coexpression of gastrin and its receptor in solid human pancreatic adenocarcinoma

BACKGROUND

Alpha-amidated gastrin promotes the growth of nontransfected pancreatic cell lines expressing the gastrin/cholecystokinin (CCK)-B receptor. Gastrin/CCK-B and CCK-A receptors recently were demonstrated in human pancreatic adenocarcinomas, but to the authors' knowledge expression of their ligands to date have not been adequately investigated. As a prerequisite for making suggestions regarding local growth stimulation, the authors examined whether gastrin and the homologous CCK peptides as well as their specific receptors were expressed in consecutively collected solid human pancreatic adenocarcinomas.

METHODS

Using a library of radioimmunoassays specific for different epitopes on proCCK, progastrin, their processing intermediates, and bioactive end products, CCK and gastrin gene expression was measured in extracts of solid human pancreatic adenocarcinomas (n = 19), resection margins (n = 15), and normal pancreatic tissue (n = 8). Moreover, CCK, CCK-A receptor, and gastrin/CCK-B receptor mRNA were measured by reverse transcriptase-polymerase chain reaction.

RESULTS

Amidated gastrins were synthetized in 14 of 19 carcinomas (median, 0.4 pmol/g; range, < 0.1-84.0 pmol/g) and in 12 of 15 resection margin samples (median, 0.3 pmol/g; range, < 0.1-6.1 pmol/g). In contrast, normal human pancreatic tissue expressed only traces of poorly processed progastrin. Gastrin/CCK-B receptor mRNA was present in all carcinomas, resection margins, and normal pancreatic tissue. CCK-A receptor mRNA was detected in most tumors, but neither the mature ligands (alpha-amidated and O-sulfated CCK peptides) nor their precursors were expressed in carcinoma and normal pancreatic tissue.

CONCLUSIONS

The results of the current study demonstrate that alpha-amidated gastrin peptides and their receptor invariably are coexpressed in pancreatic adenocarcinoma. Therefore these findings support the contention of a role for local gastrin regulatory mechanisms, but no CCK mechanisms, in pancreatic carcinoma.

Impaired feedback of gastric functions in carboxypeptidase E-deficient mice

Carboxypeptidase E deficiency as seen in the fat/fat mice is associated with reduced antral somatostatin content but tripling of the progastrin product. Thus, fat/fat mice are able to maintain normal tissue concentrations of bioactive alpha-amidated gastrin in spite of grossly attenuated progastrin processing. After induction of achlorhydria, however, neither the amount of alpha-amidated gastrin nor the total progastrin product increased in the fat/fat mice. This is contrary to what is seen in wild-type mice. Furthermore, the synthesis of antral somatostatin and fundic chromogranin A is also abnormal. Hence the results suggest a breakdown in the feedback loop that regulates gastric acid secretion.

Excretion of progastrin products in human urine

The renal handling of carboxyamidated gastrins, NH2-terminal progastrin fragments, and glycine-extended gastrins was examined in healthy volunteers. The respective urinary clearances after a meal amounted to 0.09 +/- 0.02%, 0.17 +/- 0.04% (P < 0.05), and 0.04 +/- 0.01% (P < 0.01) of the glomerular filtration rate. During intravenous infusion of carboxyamidated gastrin-17, progastrin fragment-(1-35), and glycine-extended gastrin-17, the respective urinary clearances amounted to 0.08 +/- 0.02, 0.46 +/- 0.08, and 0. 02 +/- 0.01%, respectively, of the glomerular filtration rate. The metabolic clearance rate of the three peptides was 24.4 +/- 1.3, 6.0 +/- 0.4, and 8.6 +/- 0.7 ml. kg-1. min-1. A maximum rate for tubular transport or degradation of the peptides could not be determined, nor was a renal plasma threshold recorded. Plasma concentrations and urinary excretion rates correlated for gastrin-17 and progastrin fragment-(1-35) (r = 0.94 and 0.97, P < 0.001), whereas the excretion of glycine-extended gastrin diminished with increasing plasma concentrations. We conclude that renal excretion of progastrin products is negligible compared with renal metabolism and that renal handling of the peptides depends on their molecular structure. Hence, the kidneys exhibited a higher excretion of NH2-terminal progastrin fragments than of carboxyamidated and especially glycine-extended gastrins.

Adult carboxypeptidase E-deficient fat/fat mice have a near-total depletion of brain CCK 8 accompanied by a massive accumulation of glycine and arginine extended CCK: identification of CCK 8 Gly as the immediate precursor of CCK 8 in rodent brain

Cholecystokinin (CCK) amide concentrations were reduced over 85% in all the major brain regions of carboxypeptidase E (Cpe)(fat)/Cpe(fat) mice in comparison to control mice. Using an radioimmunoassay (RIA) specific for glycine-extended CCK (CCK Gly), low levels of CCK Gly were detected in control (0.65 ng/g tissue) and were even lower in Cpe(fat)/Cpe(fat) (0.246 ng/g) mice brain extracts. After treatment with carboxypeptidase B, the level of CCK Gly in Cpe(fat)/Cpe(fat) in these brain extracts was elevated to 33.5 ng/g, about 51-fold higher than in control. On gel-filtration chromatography and high-performance liquid chromatography (HPLC), this material coeluted with CCK 8 Gly. These results demonstrate that CPE is required for the correct processing of arginine- and glycine-extended CCK in all major regions of the mouse brain. These results support the hypothesis that CCK 8 Gly is the immediate precursor of CCK 8 amide in mouse brain, not larger amidated forms like CCK 22 or CCK 33.

Altered processing of procholecystokinin in carboxypeptidase E-deficient fat mice: differential synthesis in neurons and endocrine cells

The fat mouse strain exhibits a late-onset obesity syndrome associated with a mutation in the gene encoding carboxypeptidase E (CPE). CPE plays a central role in the biosynthesis of many regulatory peptides. Therefore, we examined the processing of procholecystokinin (proCCK) in the brain (neurons) and small intestine (endocrine cells) of fat/fat mice. In the brain, bioactive CCK was markedly reduced (7.9+/-1.0 pmol/g in fat/fat mice vs. 82.5+/-11.2 pmol/g in controls), but the concentration of the CPE substrate, glycylarginine-extended CCK, was elevated 105-fold. In contrast, the concentration of bioactive CCK in intestinal endocrine cells was unaffected. Endocrine cell processing was, nevertheless, altered with a 33-fold increase in glycyl-arginine-extended CCK. Interestingly, although total proCCK products were normal in the brain they were elevated 3-fold in the intestine, indicating that biosynthesis is upregulated in endocrine cells but not neurons to compensate for the processing defect. These results demonstrate that the CPE mutation differentially affects CCK processing in these two cell types. Intestinal CCK synthesis more closely resembles progastrin processing, suggesting the presence of an endocrine-specific biosynthetic regulatory mechanism not present in neurons.

Renal tubular transport and metabolism of carboxyamidated and glycine-extended gastrins in pigs

Renal handling of postprandial and intravenously administered gastrin was investigated in anaesthetised pigs. The fractional extraction of postprandial carboxyamidated and glycine-extended gastrin in the kidneys was 0.21 +/- 0.01 and 0.16 +/- 0.02, but the respective urinary clearance comprised only 0.57 +/- 0.03 and 0.44 +/- 0.05% of the GFR (P < 0.02). The respective total body clearance of carboxyamidated and glycine-extended gastrin-17 (gastrin-17 and gastrin-17Gly) during continuous infusion was 22.9 +/- 1.5 and 19.6 +/- 1.4 mL kg-1 min-1 (NS), and the renal fractional extraction of the peptides was 0.31 +/- 0.03 and 0.29 +/- 0.05, respectively. The kidneys accounted for 8% of total body clearance of gastrin-17. Renal filtration rate of gastrin-17 exceeded renal extraction rate (9.739 +/- 0.487 vs. 6.407 +/- 0.321 pmol min-1). Urinary clearance of gastrin-17 and gastrin-17Gly amounted only 0.91 +/- 0.16 and 0.13 +/- 0.03%, respectively, of the GFR (P < 0.01), but urinary excretion rate correlated with the filtered amount of the peptides (r = 0.93, P < 0.01). Neither was a renal plasma threshold recorded nor was a Tm value for tubular uptake or degradation of gastrin achieved in spite of supraphysiological plasma levels of the peptides. The results indicate that filtered gastrin is almost completely removed in the renal tubules, primary by metabolism although part of the absorbed peptides may be returned to the circulation in intact form. The process for uptake or metabolism has a high capacity but varies with the molecular form of gastrin.

Accurate measurement of cholecystokinin in plasma

Shortage of reliable plasma assays has hampered studies of cholecystokinin (CCK). The assay problems are low plasma concentrations, extensive molecular heterogeneity, and close homology of CCK to gastrin, which circulates in higher concentrations. To develop an accurate CCK RIA, antibodies were raised in rabbits, guinea pigs, and mice in titers from 200 to 4 000 000. The specificity of the antisera was tested with homologous peptides, and tissue and plasma extracts. Rabbit 92128 produced antibodies in high titer (≥500 000) with sufficient avidity (K \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(_{eff}^{{^\circ}}\) \end{document} ≥ 1012 mol−1) and the desired specificity. The antiserum binds the bioactive forms of CCK with equimolar potency and displays no reactivity with gastrin. CCK concentrations in plasma from healthy humans rose from 1.13 ± 0.10 pmol/L (mean ± SE, n = 26) to 4.92 ± 0.34 pmol/L after a mixed meal. Chromatography of human plasma revealed traces of CCK-58, a predominance of CCK-33 and CCK-22, and moderate amounts of CCK-8. The results show that it is possible to produce specific CCK-antisera using a sulfated CCK-12 analog.

CCK biosynthesis and processing: recent progress and future challenges

The peptide cholecystokinin (CCK), like other peptides which pass through the regulated secretory pathway, undergoes a number of post-translational modifications during its biosynthesis including tyrosine sulfation, endoproteolytic cleavage, and trimming by carboxypeptidases. This minireview summarizes what is known about this process in endocrine cells and in the Cpe(fat)/Cpe(fat) mouse and points out what challenges remain for future research.

Disturbed progastrin processing in carboxypeptidase E-deficient fat mice

The fat mouse strain exhibits a late-onset obesity syndrome associated with a mutation in the gene encoding carboxypeptidase E (CPE). Since CPE plays a central role in the biosynthesis of a number of regulatory peptides, including gastrin, we examined the biogenesis and processing of progastrin in fat/fat mice by measuring gastrin mRNA, carboxyamidated gastrin and its processing intermediates in the stomach. The tissue concentration of carboxyamidated (i.e. bioactive) gastrin was only slightly reduced (601 +/- 28 pmol/g in fat/fat mice vs. 715 +/- 43 pmol/g in wild-type controls). However, progastrin processing intermediates accumulated excessively with an 86-fold increase in the concentration of the CPE substrate, glycyl-arginine extended gastrin, and a seven-fold increase in the concentration of glycine-extended gastrin. Accordingly, the total progastrin product was doubled, as was the concentration of gastrin mRNA. Plasma concentrations of carboxyamidated gastrin were, however slightly reduced both in fasted fat/fat mice and postprandially. The results show that the CPE mutation diminishes the efficiency of progastrin processing, but gastrin synthesis is nevertheless increased to maintain an almost normal production of bioactive gastrins. By comparison with other neuroendocrine prohormones, progastrin processing in CPE-deficient mice is unique. Hence, the increase of glycine-extended gastrin in combination with normal levels of carboxyamidated gastrin suggests that G-cells may have another biosynthetic pathway for gastrin.

New consensus features for tyrosine O-sulfation determined by mutational analysis

Tyrosine sulfation is an ubiquitous modification of proteins synthesized along the secretory pathway. It enhances protein-protein interactions and may be necessary for the bioactivity of secreted proteins and peptides. To predict tyrosine sulfation, a consensus has been proposed based on sequence comparisons of known substrates and on in vitro studies using synthetic peptides. This consensus predicts the presence of acidic residues on the amino-terminal side of the target tyrosine as the key feature. Using site-directed mutagenesis, we have examined the role of residues neighboring the sulfation site of an intact protein, human progastrin, in vivo. The results show that the charge of the residue in the amino-terminal position (-1) of the tyrosine is critical and can be neutral or acidic, whereas a basic residue abolishes sulfation. In addition, the degree of sulfation is influenced by the residues in positions -2 and -3. Hence, surprisingly a basic residue in position -2 enhances sulfation. Our data suggest a considerably broader range of substrates for the tyrosylprotein sulfotransferase than hitherto assumed and that the tyrosylprotein sulfotransferase is cell-specifically expressed.

Heterologous expression of human cholecystokinin in Saccharomyces cerevisiae. Evidence for a lysine-specific endopeptidase in the yeast secretory pathway

Precursors of the human regulatory peptide cholecystokinin (CCK) have been expressed in Saccharomyces cerevisiae, and the post-translational processing of secreted CCK-related products analyzed. Recombinant plasmids expressing native human prepro-CCK and a hybrid molecule encompassing the prepro leader of the yeast alpha-mating pheromone fused to pro-CCK were examined. The latter construct resulted in considerably higher levels of pro-CCK secretion and was therefore analyzed in more detail. Two of the protein modifications essential for CCK bioactivity, C-terminal alpha-amidation and tyrosyl sulfation, were not detected in S. cerevisiae. Proteolytic cleavage of pro-CCK occurred C-terminally of three basic sites; (i) Arg105-Arg106 which, upon exposure to carboxypeptidase activity, leads to the production of glycine-extended CCK; (ii) Arg95 to produce CCK-8 related processing intermediates; and (iii) Lys81 resulting in CCK-22 related products. To elucidate which protease(s) are involved in these endoproteolytic cleavage events, pro-CCK was expressed in yeast mutants lacking various combinations of the Mkc7, Yap3, and Kex2 proteases. Only in S. cerevisiae strains deficient in Kex2 function was any of the above mentioned pro-CCK cleavages abolished, namely processing at the Arg105-Arg106 and Arg95 sites. This suggests that mammalian Kex2-like serine proteases may process pro-CCK at single arginine residues. Our data suggests that an as yet uncharacterized endopeptidase(s) in the S. cerevisiae secretory pathway is responsible for the lysine-specific cleavage of pro-CCK.

Autocrine stimulation of AR4-2J rat pancreatic tumor cell growth by glycine-extended gastrin

Glycine-extended gastrin (gastrin-Gly) stimulates proliferation of AR4-2J pancreatic tumor cell line through a specific receptor, different from the gastrin-cholecystokinin B receptor. Our purpose was to determine whether AR4-2J cells produced gastrin-Gly and then whether the peptide was involved in an autocrine loop. First, proliferation of AR4-2J cells in serum-free medium was inhibited by a gastrin anti-sense oligodeoxynucleotide phosphorothioate and by antibodies specific for gastrin-Gly. In contrast, antibodies specific for alpha-amidated gastrin were without effect. By using RT-PCR, we have shown that AR4-2J cells expressed gastrin mRNA. The presence of gastrin-Gly, but not alpha-amidated gastrin, in serum-free media was detected by radioimmunoanalysis. Gel chromatography revealed that the predominant molecular forms secreted were glycine-extended gastrin-34 and gastrin- 17. Furthermore, epidermal growth factor (EGF), a stimulator of gastrin gene transcription, modulates gastrin-Gly secretion by AR4-2J. These data together suggest that gastrin-Gly is an autocrine growth factor for AR4-2J cells and that it participates with EGF in the regulation of AR4-2J-cell proliferation.

Autocrine stimulation of AR4-2J rat pancreatic tumor cell growth by glycine-extended gastrin

Glycine-extended gastrin (gastrin-Gly) stimulates proliferation of AR4-2J pancreatic tumor cell line through a specific receptor, different from the gastrin-cholecystokinin B receptor. Our purpose was to determine whether AR4-2J cells produced gastrin-Gly and then whether the peptide was involved in an autocrine loop. First, proliferation of AR4-2J cells in serum-free medium was inhibited by a gastrin anti-sense oligodeoxynucleotide phosphorothioate and by antibodies specific for gastrin-Gly. In contrast, antibodies specific for alpha-amidated gastrin were without effect. By using RT-PCR, we have shown that AR4-2J cells expressed gastrin mRNA. The presence of gastrin-Gly, but not alpha-amidated gastrin, in serum-free media was detected by radioimmunoanalysis. Gel chromatography revealed that the predominant molecular forms secreted were glycine-extended gastrin-34 and gastrin- 17. Furthermore, epidermal growth factor (EGF), a stimulator of gastrin gene transcription, modulates gastrin-Gly secretion by AR4-2J. These data together suggest that gastrin-Gly is an autocrine growth factor for AR4-2J cells and that it participates with EGF in the regulation of AR4-2J-cell proliferation.

An efficient cellular system for mutational analysis of prohormone processing

A novel system for heterologous expression of prohormones based on transient transfection of the HIT beta-cell line was established using human progastrin as a model. Progastrin was expressed at high levels compared to other gene transfer systems in endocrine cells, and the processing pattern was similar to that of normal antral gastrin cells. Thus, gastrin was partially tyrosine O-sulfated and carboxyamidated. Cell extracts contained mainly gastrin-17 and gastrin-34 and the corresponding glycine-extended forms. In contrast, the media contained more incompletely processed gastrin forms. This suggests that gastrin was directed to the regulated secretory pathway but that some progastrin products were constitutively secreted. Glucose increased both the level of gastrin gene expression and maturation to carboxyamidated peptides, indicating that glucose influences the activity of the amidation enzyme complex, peptidylglycine alpha-amidating mono-oxygenase (PAM), in insulin cells. Mutational analysis of tyrosine sulfation of gastrin demonstrated that substitution of the uncharged residue carboxy-terminal to the tyrosine with an acidic residue does not increase sulfation in contrast to previous results, where the amino-terminal residue was replaced with an acidic residue. The mutant peptides displayed sulfation-dependent processing, supporting our recent suggestion that tyrosine sulfation increases the proteolytic processing of prohormones.

Vitamin D3 effects on basal and cAMP modulated expression of cholecystokinin and somatostatin genes in a rat medullary thyroid carcinoma cell line [CA-77]

The regulation of cholecystokinin and somatostatin expression by vitamin D and cyclic AMP in the rat medullary thyroid carcinoma cell line CA-77 was investigated. Treatment with 100 nmol/l vitamin D did not affect cholecystokinin mRNA and peptide concentrations significantly; somatostatin mRNA level increased 6 times and the somatostatin peptide concentration increased 2-fold after 5 days of drug treatment. Under the same experimental conditions cyclic AMP increased cholecystokinin mRNA level 4.5 times and the cellular cholecystokinin-peptide concentration 2-fold; somatostatin mRNA and peptide concentrations were not significantly changed. Cyclic AMP stimulated peptide secretion from the cells were not affected by vitamin D, but cyclic AMP mediated increase in CCK peptide concentration was significantly inhibited by vitamin D (p < 0.05).

Quantitation of procholecystokinin and its products in plasma by processing-independent analysis

A procedure for processing-independent quantitation of procholecystokinin (proCCK) and its products has been applied to plasma. The procedure is based on tryptic cleavage after Lys61 and Arg71 with subsequent monospecific radioimmuno-analysis of fragment 62-71 of human proCCK, which again corresponds to fragment 1-10 of CCK-22. The detection limit of the analysis was 0.2 pmol/l. Plasma was extracted with ethanol. In plasma from 13 healthy volunteers the basal concentration with the above-mentioned radioimmunoassay was 1.1 +/- 0.1 pmol/l (mean +/- S.E.M.) before, and 13.7 +/- 0.6 pmol/l after, incubation with trypsin. Two hours after ingestion of a mixed meal, the plasma concentration was 2.0 +/- 0.1 pmol/l before, and 21.7 +/- 1.2 pmol/l after tryptic cleavage. With a conventional CCK radioimmunoassay specific for the C-terminally amidated and O-sulfated bioactive epitope, the concentration was 1.0 +/- 0.1 pmol/l in the basal state and 4.2 +/- 0.4 pmol/l 2 h after a meal. Tryptic cleavage did not increase the concentrations of amidated, bioactive CCK peptides. In plasma from 37 patients with the carcinoid syndrome, the basal concentration of proCCK and its products was 14.1 (2.8-150.4) pmol/l (median (range)), compared with 0.3 (0-18.8) pmol/l for carboxyamidated CCK. Only two patients had significantly elevated CCK concentrations. We conclude that processing-independent analysis is useful for quantitation of proCCK and its products in plasma, since it quantitates CCK cell secretion more accurately than conventional CCK assays.

A processing-independent assay for human procholecystokinin and its products

In order to develop a processing-independent analysis for procholecystokinin (proCCK) and its products, antibodies were raised against the synthetic fragment 62-71 of human proCCK. All rabbits (n = 8) responded to the immunization. One (No. 89,009) produced antibodies of particularly high titer (1:350,000), homogeneity (Sips' index approximately 1.0) and binding affinity (K0 eff approximately 0.88 x 10(12) l/mol). A radioimmunoassay using this antiserum and [125I]tyrosine-extended fragment 62-71 measured the total CCK mRNA product after tryptic cleavage at Lys61 in normal and neoplastic tissue independent of the degree of precursor processing. In addition to previously known CCK producing tumors, CCK was found also in a thoracic round-cell tumor (Askin tumor) and in brain tumors (gliomas and astrocytomas). These tumors processed proCCK poorly. Thus, they contained 11 and 23 (mean n = 5) pmol/g of proCCK and its products before, versus 71 and 99 (mean) pmol/g after tryptic cleavage, respectively. Accordingly, gel chromatography revealed significant amounts of unprocessed proCCK, large molecular forms of glycine-extended CCKs and the well-known carboxyamidated and tyrosine O-sulfated bioactive CCK-83, -58, -33, -22 and -8. We conclude that monospecific antibodies directed against the N-terminus of sequence 62-71 of human proCCK are suitable for processing-independent analysis (PIA) for proCCK and its products. Moreover, we suggest that such PIA should be used for quantitation of CCK gene expression at peptide level in normal tissue and tumors.

Identification of gastrin component I as gastrin-71. The largest possible bioactive progastrin product

Gastrin component I is the largest hormonally active form of gastrin. In order to determine its structure, we isolated progastrin-derived peptides from normal human antral tissue. A radioimmunoassay specific for sequence 20-25 of human progastrin was developed to monitor the purifications. After four or five steps of reverse-phase chromatography, the peptides were pure and could be identified by a combination of microsequence, amino acid and mass spectral analysis as well as by a library of sequence-specific immunoassays. In addition to intact progastrin 1-80, fragments 1-71, 1-35, 6-35, 20-35, and 20-36 of progastrin were identified. Only the 71-amino-acid peptide contained at its C-terminus the alpha-amidated bioactive site (Trp-Met-Asp-Phe-NH2). This unoheptacontapeptide amide (gastrin-71) corresponds to component I and is the largest possible bioactive product of progastrin. Its structure shows that progastrin is used in its entirety for biosynthesis of active peptides. The occurrence of fragments 6-35, 20-35, and 20-36 demonstrate that antral progastrin is partially cleaved at two monobasic sites (Arg5 and Arg19) in addition to processing at the three C-terminal dibasic sites. The results show that both the N- and C-terminal parts of antral progastrin undergo extensive processing. The results also suggest that progastrin may follow two different processing pathways of which the less trafficked releases gastrin-71.

Ileal expression of gastrin and cholecystokinin. In search of a related hormone

Antibodies against the common active site of cholecystokinin (CCK) and gastrin stain three endocrine cell-types in the gut: G-cells (that synthesize gastrin) I-cells (that synthesize CCK), and TG-cells (whose product is unknown). In order to examine whether TG-cells either process progastrin or proCCK in a specific manner, or express a novel gastrin-CCK related hormone, we studied the distal porcine ileum, which have far more TG- than G- and I-cells. Ileal CCK and gastrin mRNA corresponded to those of the antroduodenal mucosa. Gel, ion-exchange and reversed-phase chromatography monitored with sequence-specific immunoassays showed that the ileal mucosa on average contain 0.3 and 7.6 pmol/g progastrin and proCCK, respectively; 1.1 and 13.5 pmol/g glycine-extended intermediates; and 1.1 and 24.8 pmol/g of bioactive carboxyamidated gastrin and CCK, respectively. Gastrin was present only as non-sulfated gastrin-34, whereas CCK occurred in forms similar to those of the proximal intestine. Although ileal progastrin processing is tissue specific, the amounts of gastrin and CCK are too small to explain the TG-cells. Moreover, since the ileal extracts were without traces of other peptides having the C-terminus common to gastrin and CCK, the results suggest that TG-cells produce a peptide, which is only weakly related to gastrin and CCK.

Glycine-extended post-translational processing intermediates of gastrin and cholecystokinin in the gut

Cholecystokinin (CCK) and gastrin are two polypeptide hormones of the gut that share complete structural homology in their carboxyl-terminal pentapeptide. Both peptides are biologically activated from their glycine-extended precursor forms by a carboxyl-terminal alpha-amidation reaction. In the present studies we used region specific antisera to characterize the carboxyl-terminally amidated and glycine-extended forms of gastrin and CCK in mammalian intestine. Multiple amidated molecular forms of gastrin and CCK and their corresponding glycine-extended forms were detected throughout the most of the small bowel. Although, we detected substantial amounts of glycine-extended CCK in the proximal rat duodenum, we detected none of the corresponding amidated molecular forms. In contrast, the proximal duodenum of dog and hog contained both glycine-extended and amidated CCK. These findings suggest that there may be peptide, tissue and species specific differences in expression and activity of the peptide alpha-amidating enzyme.

The molecular nature of cholecystokinin in plasma. An in vivo immunosorption study in rabbits

The nature of cholecystokinin (CCK) in rabbit plasma was examined by means of a novel in vivo immunosorption procedure. Cholecystokinin (CCK) antibodies in 11 rabbit antisera were denatured, and the released peptides characterized by size and reversed-phase chromatography. Five of six antisera specific for the COOH terminus of CCK contained substantial amounts of CCK-22- and CCK-8-like peptides and small amounts of CCK-33-like peptides (range, 120 to 1140 nmol/l antiserum). In contrast, neither antisera for the NH2-terminus and mid-sequence of porcine CCK-33 nor antisera against the glycine-extended COOH terminus released CCK peptides. Postprandial acidified plasma from non-immunized rabbits concentrated in vitro also contained mainly CCK-22- and -8-like peptides, whereas extracts of rabbit duodenum and jejunum in addition contained forms resembling CCK-58, -39, and/or -33. The results show that mainly small molecular forms of CCK circulate in rabbits, and that NH2-terminal and mid-sequences of porcine and human CCK-33 differ from those of rabbit CCK-33. The results support the contention that plasma in most mammals contains small molecular forms of CCK.

Gastrin and cholecystokinin in the Eastern Grey kangaroo, Macropus giganteus giganteus

Gastrin and CCK are believed to have a common ancestor. The gastrin structure has probably evolved from CCK-like peptides at a stage later than the amphibians. To trace the evolution of gastrin and CCK we have determined their structures in an Australian marsupial, the Eastern Grey kangaroo. The brain CCK was identical to CCK-8 of most mammals. The larger form of kangaroo gastrin was a 33mer with the sequence pELHPQDLPHLMTDLSKKKGPWQEEDAAY(SO3)GWMDF-NH2. The 11 italic residues indicate differences from human gastrin. Gastrin-15 and gastrin-16 comprised about 70% of the total immunoreactivity and resulted from cleavage after the second and third residue, respectively, of the unusual tribasic cleavage site. The smaller forms of kangaroo gastrin differ from most other mammalian gastrins in that the N-termini are not blocked with a pyroGlu moiety. Unlike other gastrins, kangaroo gastrin is more than 95% sulfated. The present study indicates that the gastrin structure, as defined by having a Tyr at position 6 from the C-terminus, evolved before the marsupials diverged from the other mammals 130 million years ago.

cDNA deduced procionin. Structure and expression in protochordates resemble that of procholecystokinin in mammals

Using an improved 3' RACE (PCR) amplification system containing oligonucleotide primer with an inosine at ambiguous codon positions and inverse PCR to amplify the 5' ends, we have isolated and characterized cDNA clones which encode cionin, a protochordean homologue of the mammalian hormones, cholecystokinin (CCK) and gastrin. The full-length cloned cDNA of 510 bp encoded a 128 amino acid preprocionin. Reverse transcription-PCR and subsequent cDNA cloning revealed that cionin mRNA is expressed in both the neuronal ganglion and the gut of the protochordate Ciona intestinalis. The primary structure of procionin resembles that of proCCK more than that of progastrin. Sequence-specific immunochemical analysis showed that the cionin gene is expressed also at peptide level in both the gut and the neural ganglion. The neuronal processing of procionin is, however, more complete both with respect to carboxyamidation and tyrosine O-sulfation. Hence, the tissue-specific expression of the cionin gene in Ciona intestinalis resembles that of the CCK gene in mammals.

N-terminal fragments of intestinal cholecystokinin: evidence for release of CCK-8 by cleavage on the carboxyl side of Arg74 of proCCK

From porcine duodenal mucosa we have identified three major procholecystokinin (proCCK) fragments: desoctaCCK-33, desnonaCCK-33 and desnonaCCK-39. (DesoctaCCK-33 means CCK-33 devoid of the 8 C-terminal amino acids, etc.). The fragments were purified by immunoaffinity chromatography and three steps of reverse phase HPLC monitored by a radioimmunoassay specific for the N-terminal part of CCK-33. The structures could be deduced from the proCCK sequence by N-terminal sequence determination and mass spectrometry. Whereas desnona-fragments of CCK have been described before, this is the first finding of a desoctaCCK, and it indicates that CCK-8 is released from the longer forms by endogenous cleavage of the Arg-Asp-bond. A carboxypeptidase B-like exopeptidase subsequently must produce the desnona-fragments by removing the arginine residue.

Peptide alpha-amidation activity in human plasma: relationship to gastrin processing

OBJECTIVE AND DESIGN

C-terminal amidation is an essential processing step towards bioactivation of many peptides including gastrin. This reaction is catalysed by peptidylglycine alpha-amidating mono-oxygenase (PAM, EC 1.14.17.3) which converts the glycine extended precursors on their carboxyl termini to the des-glycine amidated peptide products. In the case of gastrin, most of the amidation is thought to occur in the antrum. However substantial quantities of glycine extended gastrin and PAM are present in plasma. It is unclear whether circulating PAM reflects the secretory activity of the gastrin secreting cell or whether PAM is involved in the postsecretory processing of gastrin. The aim of the present study was to relate the circulating amidation activity to the plasma concentrations of glycine extended and amidated gastrins.

PATIENTS AND MEASUREMENTS

Plasma PAM, gastrin-amide and gastrin-gly were measured in subjects with different gastrin secretory status: healthy subjects basally and following a meal, members of families with multiple endocrine neoplasia type 1 (MEN-1) with normal and high plasma gastrin, and patients with hypergastrinaemic atrophic gastritis.

RESULTS

Patients with MEN-1 and hypergastrinaemia tended to have a higher plasma PAM activity than MEN-1 subjects with normal circulating G-NH2 indicating a cosecretion of hormone and PAM. However in contradistinction to patients with medullary thyroid carcinoma, PAM activity does not appear to be a useful tumour marker of gastrinoma. Hypergastrinaemia from a non-tumour source (hypergastrinaemic non-atrophic gastritis) was associated with a lower plasma PAM activity than in normal subjects and may reflect the secretion of a greater proportion of already amidated gastrin. In general, there was no relationship between plasma PAM activity and the ratio of amidated to non-amidated gastrin suggesting that circulating PAM was not involved in the amidation of gastrin. Feeding increased circulating gastrin but had no effect on plasma PAM activity.

CONCLUSION

The results support the view that gastrin is amidated at the site of its synthesis and that hypergastrinaemia is associated with elevated plasma amidating enzyme activity only when the gastrin originates from tumour sources.

Developmental expression of the gastrin and cholecystokinin genes in rat colon

BACKGROUND

To elucidate the hypothesis that gastrin and cholecystokinin (CCK) are local growth factors for colorectal mucosa, we have examined the peptide gene expression in rat colon during development.

METHODS

Northern analysis, reverse transcription PCR, and sequence-specific radioimmunoassays were the essential methods.

RESULTS

High concentrations of gastrin and CCK messenger RNA were found in the fetal colon. At birth, gastrin and CCK mRNA's were both undetectable but increased subsequently towards adult life. The fetal colon contained 5.5 and 4.2 pmol/g tissue gastrin and CCK, respectively. After birth, carboxyamidated gastrin disappeared from the colon, whereas the concentration of CCK remained at 1 pmol/g. Glycine-extended gastrin and CCK were also present in the fetal colon, but towards adult life they decreased below 0.2 pmol/g. In contrast, progastrin and proCCK were detectable at all ages.

CONCLUSIONS

Rat colon expresses the gastrin and CCK genes throughout life. The posttranslational maturation of progastrin, however, ceases shortly after birth, indicating that gastrin may play a role in the developing colon. Whether CCK influences the development remains to be shown.

Expression but incomplete maturation of progastrin in colorectal carcinomas

BACKGROUND

To evaluate the hypothesis that gastrin is a local growth factor in colonic carcinomas, the expression of gastrin messenger RNA (mRNA) and peptides were examined in five human colon carcinoma cell lines, 12 solid colon carcinomas, and normal colonic tissue.

METHODS

Northern analysis, reverse-transcription PCR, and a library of sequence-specific radioimmunoassays were the principal methods.

RESULTS

Cell lines, tumors, and normal tissue all expressed a gastrin mRNA of 0.7 kilobases, and all cell lines contained incompletely processed progastrin (range, 17-54 fmol/10(6) cells). Two cell lines secreted progastrin into the media (LoVo, 25 +/- 3 pmol/L; HCT116; 12 +/- 2 pmol/L). Normal colonic tissue and all the solid tumors also contained progastrin, the concentration being higher in tumors (range, 0.4-2 pmol/g) than in normal tissue (range, 0.1-0.2 pmol/g). Only one tumor contained carboxyamidated gastrins.

CONCLUSIONS

Normal and neoplastic colonic mucosa both express the gastrin gene, but the posttranslational phase of expression is attenuated. The incomplete processing and low level of expression suggest that autocrine gastrin secretion has only minor significance for normal adult and most neoplastic colonic tissue.

Post-translational processing of gastrin in neoplastic human colonic tissues

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