Isolation and partial amino acid sequence of a 78 kDa porcine gastrin-binding protein

Cholecystokinin and gastrin receptors

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

Gastrin and cancer: a review

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

Mutation of lysine residues of the 78-kDa gastrin-binding protein reduces gastrin binding

The 78-kDa gastrin-binding protein (GBP) is a likely target for the antiproliferative effects of gastrin/cholecystokinin receptor antagonists on colorectal carcinoma cell lines. Both the N- and C-terminal halves of the GBP bind gastrin, but the affinity of the N-terminal half for gastrin is 7.2-fold higher than the affinity of the C-terminal half. In order to define the gastrin-binding sites of the GBP in greater detail, we have constructed a truncation mutant lacking residues 221-318 of the N-terminal domain and a series of point mutants in which the lysine residues in the first 220 residues of the N-terminal domain were mutated to arginine residues. The effect of these mutations on both the extent of covalent cross-linking of iodinated gastrin2,17 and on the affinity for gastrin17 was investigated. Deletion of residues 221-318 of the GBP decreased the affinity 5.5-fold and reduced, but did not abolish, the extent of covalent cross-linking. Mutation of the 17 lysines in residues 1-220 of the GBP decreased the affinity for gastrin between 1.7- and 3.5-fold and in some cases reduced, but did not abolish, the extent of covalent cross-linking. We conclude that one or more lysine residues are involved in binding of gastrin to the GBP, but that no single lysine residue is the preferred target for covalent cross-linking of iodinated gastrin2,17 to the GBP.

Inhibition of the preferential binding of actin to the N-terminal hydratase domain of the 78-kDa gastrin-binding protein by non-steroidal anti-inflammatory drugs and gastrin receptor antagonists

The 78 kDa gastrin-binding protein (GBP) is a likely target for the antiproliferative effects of gastrin receptor antagonists and non-steroidal anti-inflammatory drugs (NSAIDs) on colorectal carcinoma cells (Baldwin GS, Murphy VJ, Yang Z, and Hashimoto T. J Pharmacol Exp Ther 1998;286:1110-14). This study tested the hypotheses that the GBP bound actin, and that the interaction could be disrupted by gastrin receptor antagonists and NSAIDs. Binding of actin to the GBP was assessed by competition with (125)I-[Nle(15)]-gastrin(2,17) in a covalent cross-linking assay, and by comparison of (125)I-actin binding to the N- and C-terminal GBP domains, which had been expressed independently in E. coli as glutathione-S-transferase (GST) fusion proteins. The ability of gastrin receptor antagonists and NSAIDs to interfere with the actin-GBP interaction was measured by release of (125)I-actin from preformed complexes with the N- and C-terminal domain-GST fusion proteins. Actin purified from skeletal muscle or from gastric mucosal cytosol competed with (125)I-[Nle(15)]-gastrin(2,17) for binding to the GBP with IC(50) values of 2.6 +/- 0.7 microM, and 2.1 +/- 0.7 microM, respectively. The amount of (125)I-actin from either source bound to the N-terminal GBP domain was 8.2 times greater than the amount bound to the C-terminal domain. Binding of actin to both domains was inhibited by the gastrin receptor antagonists proglumide and benzotript, and by NSAIDs. We conclude that the GBP may associate with the cytoskeleton via an interaction between its N-terminal domain and actin, and that the association may be disrupted either by gastrin receptor antagonists or by NSAIDs.

Binding sites for progastrin-derived peptides in colonic crypts

BACKGROUND AND AIMS

Gastrin17gly acts as a growth factor for the colonic mucosa. Studies on the binding properties of the receptor involved in transducing the proliferative effects have generally been confined to colorectal carcinoma cell lines, and no investigation of gastrin17gly receptors on normal colonocytes has yet been reported. The aim of this study was to investigate the binding of 125I-[Met15]-gastrin17gly to normal colonic crypts.

METHODS

Crypts were released from normal rat and rabbit colonic mucosa by treatment with EDTA and isolated by centrifugation. The binding of 125I-[Met15]-gastrin17gly was measured in displacement experiments with increasing concentrations of either gastrin17gly, gastrin17 or gastrin receptor antagonists. The concentrations required for 50% inhibition were determined by the use of curve fitting.

RESULTS

125I-[Met15]-Gastrin17gly bound to both rat and rabbit crypts, and displacement experiments with unlabeled gastrin17gly revealed that the IC50 values were 1.0 +/- 0.6 and 0.6 +/- 0.2 micromol/L, respectively. Binding was also competed by gastrin17, with IC50 values of 2.4 +/- 1.7 and 2.4 +/- 0.7 micromol/L, respectively. Binding was inhibited by the non-selective gastrin/CCK receptor antagonists proglumide and benzotript, but not by the cholecystokinin (CCK)-A receptor antagonist L364 718, or the gastrin/CCK-B receptor antagonist L365 260.

CONCLUSION

We conclude that the gastrin17gly binding site on normal colonic crypts has properties consistent with the gastrin/CCK-C receptor.

Binding of pepsinogen to the 78 kDa gastrin binding protein

An endogenous ligand of the 78 kDa gastrin-binding protein (GBP) has been purified from detergent extracts of porcine gastric mucosal membranes by ion exchange chromatography and preparative gel electrophoresis. The ligand bound to the GBP with high affinity (mean IC50 value of 0.31+/-0.09 microgram/ml, or 8 nM), as assessed by inhibition of cross-linking of iodinated gastrin2,17 to the GBP. Both the N- and C-terminal halves of the GBP, which had been expressed individually as glutathione-S-transferase fusion proteins in Escherichia coli, and purified on glutathione-agarose beads, bound the ligand. Two peptides derived from the ligand were purified by reversed-phase high-performance liquid chromatography (HPLC), and characterised by mass spectrometry and Edman sequencing. The peptides were 97% and 100% identical, respectively, to amino acids 119-157 and 199-219 of porcine pepsinogen A. Commercial samples of pepsinogen also bound to the GBP, with a mean IC50 value of 3.9+/-1. 2 micrograms/ml (100 nM). We conclude that the ligand is closely related, but not identical, to pepsinogen A.

Structural requirements for the binding of non-steroidal anti-inflammatory drugs to the 78 kDa gastrin binding protein

Non-steroidal anti-inflammatory drugs (NSAIDs) inhibit the proliferation of colorectal carcinoma cell lines in vitro and reduce the risk of colorectal carcinoma in vivo. The good correlation observed between the potency of NSAIDs as inhibitors of colorectal carcinoma cell proliferation and as antagonists of a 78 kDa gastrin binding protein (GBP) suggested that blockade of the GBP might contribute to the anti-proliferative effects of NSAIDs [G.S. Baldwin, V.J. Murphy, Z. Yang, T. Hashimoto, J. Pharmacol. Exp. Ther. 286 (1998) 1110-1114]. The most potent NSAID investigated was sulindac sulphide, which had an IC50 value of 40 microM. In order to investigate the structural requirements for binding to the GBP, 26 analogues of sulindac sulphide and sulindac sulphoxide were tested for their ability to inhibit the binding of iodinated gastrin to the GBP. Six of the analogues inhibited gastrin binding by more than 50% at a concentration of 1 mM. The IC50 values estimated by computer fitting of titration data were in the range of 280-940 microM. Comparison of the analogue structures suggests that a substituent with a carboxyl group is preferred in the R2 position. In addition the location of the NSAID binding site within the GBP structure was investigated. NSAIDs bound to both the N- and C-terminal halves of the GBP, and the affinities determined were similar to the values previously reported for the full-length GBP. The results reported herein represent the first step in the rational design of more potent GBP antagonists, some of which may be useful for the treatment of colorectal carcinoma.

Gastrin and gastrin receptor antagonists bind to both N- and C-terminal halves of the 78 kDa gastrin-binding protein

A 78 kDa gastrin-binding protein (GBP) has previously been identified as the target of the anti-proliferative effects of non-selective gastrin/cholecystokinin receptor antagonists on colorectal carcinoma cell lines. The GBP was related in sequence to a family of fatty acid oxidation enzymes possessing enoyl CoA hydratase and 3-hydroxyacyl CoA dehydrogenase activity. This study aims to define the binding site for gastrin and gastrin antagonists in greater detail. The N- and C-terminal halves of the porcine GBP were expressed independently as glutathione S-transferase fusion proteins in E. coli. Affinities of gastrin and gastrin antagonists for the fusion proteins were measured by competition for 125I-[Nle15]-gastrin binding in a covalent cross-linking assay. The N- and C-terminal fusion proteins bound gastrin with affinities of 9.9 +/- 6.1 and 71 +/- 48 microM, respectively (n = 3). These values were 40-fold and 300-fold lower than the affinity of the full-length GBP for gastrin (0.23 +/- 0.15 microM). In contrast, the affinities of the N- and C-terminal halves for the antagonists proglumide (22 +/- 13 and 10 +/- 4 mM, respectively) and benzotript (350 +/- 90 and 400 +/- 160 micro M, respectively) were similar to each other and to the affinities of proglumide and benzotript for the full-length GBP (5.1 +/- 3.6 mM and 200 +/- 120 microM, respectively). It is concluded that proglumide and benzotript bind independently to both the hydratase and dehydrogenase active sites of the GBP, while a single molecule of gastrin may bind simultaneously to both active sites. A model is proposed which is consistent with these data, and which will assist in the development of more potent and selective GBP antagonists.

Binding of a valproate metabolite to the trifunctional protein of fatty acid oxidation

The anti-convulsant drug valproate causes hepatic failure in a small percentage of patients. We now report that the valproate metabolite 2,4-dien-valproate binds (IC50 = 42 microM) to the alpha-subunit of the trifunctional protein responsible for the second and third steps in the mitochondrial beta-oxidation of fatty acids. Binding of valproate itself, or of the metabolites 2-envalproate, 4-en-valproate or 3-hydroxy-4-en-valproate, is considerably weaker. We conclude that valproate-induced hepatotoxicity may be due in part to the reversible binding of the valproate metabolite 2,4-dien-valproate or its CoA ester to the alpha-subunit of the trifunctional protein with consequent inhibition of fatty acid oxidation.

Binding of progastrin fragments to the 78 kDa gastrin-binding protein

The non-selective gastrin/cholecystokinin receptor antagonists proglumide and benzotript inhibit colon carcinoma cell proliferation by binding to the 78 kDa gastrin-binding protein (GBP) (Baldwin, Proc. Natl. Acad. Sci. USA, 91 (1994) 7593-7597). However, although most colon carcinoma cell lines synthesize progastrin, production of mature amidated gastrin17 has not been observed. In order to define the structural requirements for the binding of gastrin to the GBP the affinities of various fragments of amidated and C-terminally extended gastrin17 for the GBP have been measured. The results indicate that the GBP recognizes both N- and C-termini of gastrin17. Moreover since C-terminal amidation is not a prerequisite for binding of gastrin to the GBP, the GBP is a potential target for the autocrine effects of progastrin.

Structures of the human cDNA and gene encoding the 78 kDa gastrin-binding protein and of a related pseudogene

The nucleotide sequence encoding the human 78 kDa gastrin binding protein (GBP) has been deduced from overlapping fragments generated by the polymerase chain reaction with oligonucleotides based on the sequence of the porcine GBP (Mantamadiotis, T. et al. (1993) Biochim. Biophys. Acta 1170, 211-215) and cDNA from the colonic carcinoma cell line LIM 1215 as template. The mature human GBP is 90% identical to the porcine GBP. Clones encoding the human GBP gene, which contains 19 exons, have been isolated from human genomic libraries. The positions of the exon/intron junctions are completely different from the junctions in the gene encoding the related peroxisomal trifunctional enzyme. Clones encoding a related pseudogene have also been isolated and sequenced.

Antiproliferative gastrin/cholecystokinin receptor antagonists target the 78-kDa gastrin-binding protein

Inhibition of colon carcinoma cell growth by the nonselective gastrin/cholecystokinin (CCK) receptor antagonists proglumide and benzotript provided evidence that gastrin functions as an autocrine growth factor. However, the molecular properties of the receptor mediating the antagonist effects have not been identified. A 78-kDa gastrin-binding protein (GBP), the sequence of which is related to the family of enzymes possessing enoyl-CoA hydratase and 3-hydroxyacyl-CoA dehydrogenase activities, has been previously purified from porcine gastric mucosal membranes. I now report that covalent cross-linking of 125I-labeled [Nle15]gastrin2,17 to the 78-kDa GBP is inhibited by crotonyl-CoA and by acetoacetyl-CoA. Gastrin, CCK, and their analogues also inhibit cross-linking, and the spectrum of analogue affinities correlates better with the values previously reported for binding to the gastrin/CCK-C receptor than with the values reported for binding to either the CCK-A or the gastrin/CCK-B receptor. Cross-linking is also inhibited by proglumide and benzotript, but no inhibition is seen with either the CCK-A receptor-selective antagonist L364,718 or the gastrin/CCK-B receptor-selective antagonist L365,260. The affinities of antagonists for the GBP correlate well with their affinities for the gastrin/CCK-C receptor and with their potencies for inhibition of colon carcinoma cell growth. I conclude that the 78-kDa gastrin-binding protein is (i) a member of the hydratase/dehydrogenase family of fatty acid oxidation enzymes, (ii) the gastrin/CCK-C receptor, and (iii) the target for the antiproliferative action of two gastrin/CCK receptor antagonists.