Cholecystokinin A and B receptors are differentially expressed in normal pancreas and pancreatic adenocarcinoma

Clinical endocrinology and metabolism. Gastrin

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

The functional significance of the cholecystokinin-C (CCK-C) receptor in human pancreatic cancer

OBJECTIVES

The aim of this study was to evaluate whether the CCK-C receptor, a splice variant of the CCK-B receptor, in human pancreatic cancer cells was associated with accelerated cancer cell growth.

METHODS

In vitro, BxPC-3 cells were transfected with the antisense cDNA for the CCK-C receptor and growth of transfected cells was compared with that of wild-type (WT) and empty vector (EV)-transfected cells; expression was confirmed by RT-PCR and immunocytochemistry. In vivo, athymic nude mice bearing human BxPC-3 pancreatic cancers were treated for 28 days with either an antisense oligonucleotide specific to the CCK-C receptor, the same nucleotide sequence arranged in a scrambled fashion (nucleotide control), or vehicle (control).

RESULTS

In culture, BxPC-3 cells transfected with the antisense cDNA for the CCK-C receptor were reduced in cell number 65% compared with WT and EV-transfected cell cultures at 6 days; this difference was statistically significant (P = 0.002). Transfected cells did not respond to exogenous gastrin with growth as did WT cells. Tumors of mice treated with the antisense oligonucleotide for CCK-C were 75% smaller in volume and 83% reduced in weight (P = 0.03) compared with the control tumors.

CONCLUSION

These studies indicate that the CCK-C receptor is functional and plays a crucial role in growth of human pancreatic cancer.

Mist1 is necessary for the establishment of granule organization in serous exocrine cells of the gastrointestinal tract

Establishing a pool of granules at the luminal border is a key step during exocrine cell development in the pancreas and is necessary for efficient release of digestive enzymes through regulated exocytosis. Several proteins have been linked to maintaining granule organization, but it is unclear which regulatory mechanisms are necessary to establish organization. Based on temporal and spatial expression, the transcription factor Mist1 is an excellent candidate, and analysis of mice that do not express Mist1 (Mist1KO) reveal disrupted cell morphology in adult pancreatic acini. To address Mist1's role in establishing granule location, we have characterized the organization of pancreatic acini throughout development in Mist1KO mice. Using various histological approaches, we have determined that correct granule organization is never established in pancreatic acini of Mist1KO mice. Further examination indicates that this disruption in granule targeting may be the primary defect in Mist1KO mice as granule organization is affected in other serous exocrine cells that normally express Mist1. To identify a mechanistic link between granule targeting and the loss of Mist1 function, intercellular junctions and the expression of Rab3D were assessed. While both of these factors are affected in Mist1KO mice, these changes alone do not account for the disorganization observed in Mist1KO tissues. Therefore, we conclude that Mist1 is necessary for complete differentiation and maturation of serous exocrine cells through the combined regulation of several exocrine specific genes.

Role of small GTP binding proteins in the growth-promoting and antiapoptotic actions of gastrin

G17 has growth promoting and antiapoptotic effects on the AR4-2J pancreatic acinar cell line. We previously reported that whereas MAPK regulates G17-stimulation of AR4-2J cell proliferation, Akt mediates the antiapoptotic action of G17. We examined the signal-transduction pathways mediating G17 stimulation of AR4-2J cell growth and survival. G17 activated the small GTP binding proteins Ras, Rac, Rho, and Cdc42. Transduction of the cells with adenoviral vectors expressing dominant negative Akt, Ras, Rho, and Cdc42 but not dominant negative Rac inhibited AR4-2J cell proliferation and survival. Both exoenzyme C3 from Clostridium botulinum (C3), a toxin known to inactivate Rho, and PD98059, a MAPK inhibitor, reversed G17 inhibition of AR4-2J cell apoptosis. G17 induction of Akt activation was reduced by >60% by both dominant negative Ras and Rho and by 30% by dominant negative Cdc42. In contrast, G17-stimulated MAPK activation was blocked by >80% by dominant negative Ras but not by dominant negative Rho and Cdc42. Similar results were observed in the presence of C3. Dominant negative Rac failed to affect G17 induction of both Akt and MAPK, whereas it inhibited sorbitol by almost 50% but not G17-stimulated activation of p38 kinase. Thus G17 promotes AR4-2J cell growth and survival through the activation of multiple GTP binding proteins, which, in turn, regulate different protein kinase cascades. Whereas Ras activates Akt and MAPK, Rho and Cdc42 appear to regulate Akt and possibly other as yet unidentified kinases mediating the growth-stimulatory actions of G17.

New insights into neurohormonal regulation of pancreatic secretion

The existence of high- and low-affinity cholecystokinin (CCK)-A receptors on rodent pancreatic acini is well established. Until recently, CCK was believed to act directly on pancreatic acini to stimulate pancreatic secretion in both rodents and humans. However, conclusive evidence that human pancreatic acini lack functional CCK-A receptors has been presented. Despite substantial differences in rodent and human pancreatic physiology, CCK appears to act via vagal cholinergic pathways to mediate pancreatic secretion in both species. Structural and functional evidence suggests that CCK acts on vagal afferent fibers, which may explain how CCK doses that produce physiologic plasma CCK levels act via vagal cholinergic pathways to stimulate pancreatic secretion. Although most knowledge of vagal CCK-A receptors comes from research on rodents, physiologic studies suggest that this information is applicable to humans. In contrast to its effect on satiety, which is mediated by low-affinity vagal CCK-A receptors, CCK acts through high-affinity CCK-A receptors to evoke pancreatic secretion, suggesting that different affinity states of the vagal CCK receptors mediate different digestive functions. Vagal afferent pathways also transmit sensory information about the mechanical and physiochemical state of the digestive tract, mediated in part by serotonin, which, in turn, influences pancreatic secretion. A synergistic interaction between CCK and serotonin at the level of the nodose ganglia may explain the robust postprandial pancreatic secretion despite a modest postprandial increase in plasma CCK. Important physiologically, these findings not only explain discrepancies in previous in vivo vs. in vitro studies, but they revolutionize our current concept of the mechanism of CCK on pancreatic exocrine secretion.

Gastrins, cholecystokinins and gastrointestinal cancer

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

Heterodimerization of Type A and B Cholecystokinin Receptors Enhance Signaling and Promote Cell Growth

Dimerization of several G protein-coupled receptors has recently been described, but little is known about its clinical and functional relevance. Cholecystokinin (CCK) and gastrin are structurally related gastrointestinal and neuronal peptides whose functions are mediated by two structurally related receptors in this superfamily, the type A and B CCK receptors. We previously demonstrated spontaneous homodimerization of type A CCK receptors and the dissociation of those complexes by agonist occupation (Cheng, Z. J., and Miller, L. J. (2001) J. Biol. Chem. 276, 48040-48047). Here, for the first time, we also demonstrate spontaneous homodimerization of type B CCK receptors, as well as heterodimerization of that receptor with the type A CCK receptor. Unlike type A CCK receptor dimers, the homodimerization of type B CCK receptors was not affected by ligand occupation. However, although heterodimers of type A and B CCK receptors bound natural agonists normally, they exhibited unusual functional and regulatory characteristics. Such complexes demonstrated enhanced agonist-stimulated cellular signaling and delayed agonist-induced receptor internalization. As a likely consequence, agonist-stimulated cell growth was markedly enhanced in cells simultaneously expressing both of these receptors. Our results provide the first evidence that heterodimerization of G protein-coupled receptors can form a more "powerful" signaling unit, which has potential clinical significance in promoting cell growth.

Transient agonist-induced regulation of the cholecystokinin-A and cholecystokinin-B receptor mRNA levels in rat pancreatic acinar AR42J cells

BACKGROUND

CCK-8 and gastrin exert multiple effects in the gastrointestinal tract and the nervous system. Their actions are mediated via the G-protein coupled CCK-A and CCK-B receptors.

METHODS

Rat pancreatic acinar tumor AR42J cells express both CCK receptor subtypes. This cell line was used to characterize the agonist-dependent regulation of CCK-A and CCK-B receptor gene expression.

RESULTS

CCK-8 (10 nM) or gastrin (10 nM) reduced CCK-A receptor mRNA expression to 56% and 53%, respectively 2 h after hormonal exposure. In contrast, the level of CCK-B receptor gene expression was upregulated to 157% and 153%, respectively. These effects are most probably linked to the CCK-B receptor in AR42J cells. The phorbolester PMA (100 nM), a protein kinase C activator, downregulated CCK-A receptor expression but did not affect CCK-B receptor gene transcription. Activation of protein kinase A by forskolin (10 microM) or Bt(2)cAMP (100 microM) is not involved in the transient regulation of CCK receptor mRNA expression. Both elevated CCK-B and decreased CCK-A receptor mRNA expression returned to basal levels 6 h after continuous stimulation.

CONCLUSION

These results demonstrate that CCK-A and CCK-B receptor mRNA levels are differentially regulated by their agonists via distinct signal transduction mechanisms in AR42J cells.

Peptide receptors as molecular targets for cancer diagnosis and therapy

During the past decade, proof of the principle that peptide receptors can be used successfully for in vivo targeting of human cancers has been provided. The molecular basis for targeting rests on the in vitro observation that peptide receptors can be expressed in large quantities in certain tumors. The clinical impact is at the diagnostic level: in vivo receptor scintigraphy uses radiolabeled peptides for the localization of tumors and their metastases. It is also at the therapeutic level: peptide receptor radiotherapy of tumors emerges as a serious treatment option. Peptides linked to cytotoxic agents are also considered for therapeutic applications. The use of nonradiolabeled, noncytotoxic peptide analogs for long-term antiproliferative treatment of tumors appears promising for only a few tumor types, whereas the symptomatic treatment of neuroendocrine tumors by somatostatin analogs is clearly successful. The present review summarizes and critically evaluates the in vitro data on peptide and peptide receptor expression in human cancers. These data are considered to be the molecular basis for peptide receptor targeting of tumors. The paradigmatic peptide somatostatin and its receptors are extensively reviewed in the light of in vivo targeting of neuroendocrine tumors. The role of the more recently described targeting peptides vasoactive intestinal peptide, gastrin-releasing peptide, and cholecystokinin/gastrin is discussed. Other emerging and promising peptides and their respective receptors, including neurotensin, substance P, and neuropeptide Y, are introduced. This information relates to established and potential clinical applications in oncology.

Distribution of CCK1 and CCK2 receptors in normal and diseased human pancreatic tissue

BACKGROUND & AIMS

The localization and functional role of cholecystokinin (CCK) receptor proteins in normal and diseased human pancreas, particularly in ductal pancreatic carcinomas, remain unclear.

METHODS

Tissue samples of normal human pancreas, chronic pancreatitis, and ductal pancreatic carcinomas were investigated under carefully controlled conditions for expression of CCK1 and CCK2 receptor messenger RNA (mRNA) and proteins using in situ hybridization and in vitro CCK receptor autoradiography by means of subtype-selective analogues. Synaptophysin immunohistochemistry was used concomitantly for optimal identification of islets, nerves, and tumor areas with neuroendocrine features.

RESULTS

CCK2 receptor mRNA and proteins were found abundantly in human pancreatic islets in normal pancreas and chronic pancreatitis. CCK1 receptor proteins were found occasionally in small-sized pancreatic nerves, whereas acini expressed a low density of CCK2 receptors in a few cases of chronic pancreatitis. Ductal pancreatic carcinomas rarely expressed CCK receptors; a few receptor-positive tumors, often characterized by neuroendocrine differentiation, expressed the CCK2 receptor at the mRNA or protein level. However, the main source of CCK receptors in the pancreatic tumor samples consisted of CCK2-expressing islets and/or CCK1-expressing nerves rather than neoplastic tissue.

CONCLUSIONS

These data indicate that the presence of CCK receptors in human ductal pancreatic tumor samples is mainly due to CCK2 expression in residual pancreatic islets and CCK1 in pancreatic nerves. Pancreatic acini and ductal pancreatic tumor cells very rarely express CCK2 receptors. These observations suggest that CCK analogues may not be of clinical use to target most of these cancers.

Cholecystokinin-B (CCK-B)/gastrin receptor targeting peptides for staging and therapy of medullary thyroid cancer and other CCK-B receptor expressing malignancies

Nuclear medicine is engaged with the detection of pathological processes with the help of radionuclides. An interesting approach is to target antigens, symporters, or receptors with diagnostic and therapeutic radionuclides. Different peptide receptors like somatostatin, bombesin/GRP or VIP are (over)expressed on cancer cells, and are therefore an ideal target for the diagnosis and therapy in nuclear medicine with radiolabeled peptides. The somatostatin analogue OctreoScan [octreotide coupled with diethylene-triamine-pentaacetate (DTPA)] can be labeled with In-111 and is widely used in nuclear oncology for the staging of different tumors (e.g., carcinoids). Other peptides like neurotensin, bombesin/GRP, and VIP are under (pre)clinical investigations. The staging of metastatic medullary thyroid cancer (MTC) with the conventional radiological procedures is sometimes difficult. The high sensitivity of the pentagastrin stimulation test in detecting primary or metastatic MTC indicates the presence of tumor, but its localization is often not possible. This reaction of the tumor cells to the pentagastrin stimulation test suggests a widespread expression of the corresponding receptor type on human MTC. Indeed, autoradiographic studies demonstrated cholecystokinin (CCK)-B/gastrin receptors not only in over 90% of MTCs, but in a high percentage of small cell lung cancers, stromal ovarian, and potentially a variety of other tumors, including gastrointestinal adenocarcinomas, neuroendocrine tumors, and malignant glioma. The aim of our recent work was to develop and systematically optimize suitable radioligands for targeting CCK-B receptors in vivo and to investigate their role in the staging and therapy of MTC and other CCK-B receptor expressing malignancies. For this purpose, a variety of CCK/gastrin-related peptides, all having in common the C-terminal CCK receptor binding tetrapeptide sequence -Trp-Met-Asp-PheNH(2) or derivatives thereof, were investigated. They were members of the gastrin- or cholecystokinin families, or possessed characteristics of both, which differ by the intramolecular position of a tyrosyl moiety. Their stability and affinity were studied and optimized in vitro and in vivo; their biodistribution and therapeutic efficacy were tested in preclinical models. Best tumor uptake and tumor-to-nontumor ratios were obtained with members of the gastrin family, due to their superior selectivity and affinity for the CCK-B receptor subtype. Radiometal-labeled derivatives of minigastrin showed excellent targeting of CCK-B receptor expressing tissues in animals and healthy human volunteers. Preclinical therapy experiments in MTC-bearing animals showed significant antitumor efficacy. In a subsequent clinical study, 75 MTC patients with metastatic MTC were investigated; 43 suffered of known, 32 of occult disease. CCK-B receptor scintigraphy was performed with (111)In-DTPA-D-Glu(1)-minigastrin. The normal organ uptake was essentially confined to the stomach (and to a lower extent, to the gallbladder and, in premenopausal women, to normal breast tissue) as a result of CCK-B receptor specific binding, and to the kidneys as excretory organs. All tumor manifestations known from conventional imaging were visualized as early as 1 h p.i., with increasing tumor-to-background ratios over time; at least one lesion was detected in 29/32 patients with occult disease (patient-based sensitivity 91%). Among them were local recurrences, lymph node, pulmonary, hepatic, splenic, and bone (marrow) metastases. Eight patients with advanced metastatic disease were injected in a dose-escalation study with potentially therapeutic activities of a (90)Y-labeled minigastrin derivative at 4-6-weekly intervals (30-50 mCi/m(2) per injection for a maximum of four injections). Hematologic and renal were identified as the dose-limiting toxicities at the 40 and 50 mCi/m(2) levels. Two patients experienced partial remissions, 4 stabilization of their previously rapidly progressing disease. These data suggest that CCK-B receptor ligands may be a useful new class of receptor binding peptides for diagnosis and therapy of a variety of (CCK-B receptor expressing) tumor types. They allow for a sensitive and reliable staging of patients with metastatic MTC. Initial therapeutic results are promising, but nephrotoxicity is a major concern to be solved.

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

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

Cholecystokinin-B/Gastrin receptor-targeting peptides for staging and therapy of medullary thyroid cancer and other cholecystokinin-B receptor-expressing malignancies

The high sensitivity of the pentagastrin stimulation test in detecting primary or metastatic medullary thyroid cancer (MTC) suggests a widespread expression of the corresponding receptor type on human MTC. Indeed, autoradiographic studies demonstrated cholecystokinin (CCK)-B/gastrin receptors not only in more than 90% of MTCs, but also in a high percentage of small-cell lung cancers, stromal ovarian tumors, and potentially a variety of other tumors, including gastrointestinal adenocarcinomas, neuroendocrine tumors, and malignant glioma. The aim of our work was to develop and systematically optimize suitable radioligands for targeting CCK-B receptors in vivo and to investigate their role in the staging and therapy of MTC and other CCK-B receptor expressing malignancies. For this purpose, a variety of CCK/gastrin-related peptides, all having in common the C-terminal CCK-receptor binding tetrapeptide sequence-Trp-Met-Asp-PheNH(2) or derivatives thereof, were investigated. They were members of the gastrin or cholecystokinin families or possessed characteristics of both, which differ by the intramolecular position of a tyrosyl moiety. Their stability and affinity were studied and optimized in vitro and in vivo; their biodistribution and therapeutic efficacy were tested in preclinical models. Best tumor uptake and tumor to nontumor ratios were obtained with members of the gastrin family, because of their superior selectivity and affinity for the CCK-B receptor subtype. Radiometal-labeled derivates of minigastrin showed excellent targeting of CCK-B receptor expressing tissues in animals and healthy human volunteers. Preclinical therapy experiments in MTC-bearing animals showed significant antitumor efficacy. In a subsequent clinical study, 45 MTC patients with metastatic MTC were investigated; 23 had known and 22 had occult disease. CCK-B receptor scintigraphy was performed with (111)In-diethylenetriamine pentaacetic acid-d-Glu(1)-minigastrin. The normal organ uptake was essentially confined to the stomach (and, to a lesser extent, to the gallbladder and, in premenopausal women, to normal breast tissue) as a result of CCK-B receptor specific binding and to the kidneys, as excretory organs. All tumor manifestations known from conventional imaging were visualized as early as 1 hour postinjection, with increasing tumor to background ratios over time; at least 1 lesion was detected in 20 of 22 patients with occult disease (patient-based sensitivity, 91%). Among them were local recurrences and lymph node, pulmonary, hepatic, splenic, and bone (marrow) metastases. Eight patients with advanced metastatic disease were injected in a dose-escalation study with potentially therapeutic activities of a (90)Y-labeled minigastrin derivative at 4 to 6-week intervals (30-50 mCi/m(2) per injection for a maximum of 4 injections). Hematologic and renal toxicities were identified as the dose-limiting toxicities at the 40 and 50 mCi/m(2) levels. Two patients experienced partial remissions, and 4 experienced stabilization of their previously rapidly progressing disease. These data suggest that CCK-B receptor ligands may be a useful new class of receptor-binding peptides for diagnosis and therapy of a variety of (CCK-B receptor expressing) tumor types. They allow for sensitive and reliable staging of patients with metastatic MTC. Initial therapeutic results are promising, but nephrotoxicity is a major concern to be solved.

The gastrin receptor promotes pancreatic growth in transgenic mice

INTRODUCTION

We demonstrated previously, in two different rodent models of pancreatic cancer, that the gastrin receptor is present on malignant pancreatic tumors in spite of the fact that the normal adult rat and mouse pancreas does not express gastrin receptors.

AIMS AND METHODOLOGY

To determine whether gastrin receptors mediate pancreatic growth or promote carcinogenesis or both, we created a transgenic mouse that constitutively expresses gastrin receptors in the exocrine pancreas. The transgene construct contained the full-length rat gastrin receptor cDNA sequence under the control of the rat elastase promoter.

RESULTS

Receptor presence and function on exocrine pancreatic tissue of transgenic but not control mice were confirmed by (125)I-gastrin-I binding studies and by gastrin stimulation of intracellular calcium release. Eighteen-month-old transgenic animals had larger pancreas-to-body weight ratios than their nontransgenic littermate controls (p < 0.001 for females; p < 0.01 for males); however, histopathologic examination revealed no neoplasms or other abnormalities.

CONCLUSION

In both female and male transgenic mice, the expression of the gastrin receptor in the exocrine pancreas is associated with a significant increase in pancreas weight, but it does not appear to promote the development of spontaneous pancreatic tumors.

Expression of CCK2 receptors in the murine pancreas: proliferation, transdifferentiation of acinar cells, and neoplasia

BACKGROUND & AIMS

To explore the pancreatic function of CCK2/gastrin receptor, we created ElasCCK2 transgenic mice expressing the human receptor in pancreatic exocrine cells. In previous studies, the transgenic CCK2/gastrin receptor was demonstrated to mediate enzyme release and protein synthesis. We now report results of phenotypic and long-term studies.

METHODS

Pancreas was characterized using morphometry and immunohistochemistry. ElasCCK2 mice were crossed with INS-GAS mice expressing gastrin in pancreatic beta cells to achieve continuous stimulation of the CCK2/gastrin receptor.

RESULTS

The pancreatic weight of ElasCCK2 mice was increased by 40% and correlated with an increase in the area of exocrine tissue. Alterations in pancreatic histology were apparent from postnatal day 50. Crossing the ElasCCK2 mice with INS-GAS mice resulted in development of morphologic changes in younger animals. Malignant transformation occurred in 3 of 20 homozygous ElasCCK2 mice. Although tumors had different phenotypes, they all developed through an acinar-ductal carcinoma sequence.

CONCLUSIONS

Our data show that transgenic expression of a G protein-coupled receptor can lead to cancer. This study also supports a key role of the CCK2/gastrin receptor in the development of pre- and neoplastic lesions of the pancreas. ElasCCK2 mice provide a model for carcinogenesis by transformation and dedifferentiation of acinar cells.

Human pancreatic acinar cells lack functional responses to cholecystokinin and gastrin

BACKGROUND & AIMS

Pancreatic acinar cells from various species express cholecystokinin (CCK) A, CCK-B, or a combination of these CCK receptor subtypes. The presence and functional roles of CCK receptors on human acinar cells remain unclear.

METHODS

Acini isolated from human pancreas were treated with CCK receptor agonists, CCK-8 and gastrin, and an agonist for m3 muscarinic acetylcholine receptors (m3 AchR), carbachol. Functional parameters measured included intracellular [Ca(2+)], amylase secretion, and ERK phosphorylation. Binding studies were performed using (125)I-CCK-8. Expression of messenger RNAs (mRNAs) was determined using real-time quantitative reverse-transcription polymerase chain reaction (RT-PCR) and localized by in situ hybridization.

RESULTS

Human acini did not respond to CCK agonists. In contrast, they responded to carbachol with robust increases in each of the functional parameters. Moreover, the cells responded to CCK agonists after adenoviral-mediated gene transfer of CCK-A or CCK-B receptors. A low level of specific and a high level of nonspecific binding of (125)I-CCK-8 were observed. Quantitative RT-PCR indicated that the message levels for CCK-A receptors were approximately 30-fold lower than those of CCK-B receptors, which were approximately 10-fold lower than those of m3 Ach receptors. In situ hybridization indicated the presence of m3 Ach receptor and insulin mRNA but not CCK-A or CCK-B receptor mRNAs in adult human pancreas.

CONCLUSIONS

These data indicate that human pancreatic acinar cells do not respond to CCK receptor agonists in terms of expected functional parameters and show that this is due to an insufficient level of receptor expression.

Molecular characterization and organ distribution of type A and B cholecystokinin receptors in cynomolgus monkey

Differences in the molecular structure or organ distribution of receptors can limit the usefulness of a given species for drug studies. In this work, we have studied cholecystokinin (CCK) receptors in cynomolgus monkey, an animal model useful for preclinical testing. The type A CCK receptor cDNA was cloned and predicted to encode a 428 amino acid peptide that was 98% identical to the human receptor. Only 2 of the 10 residues that were distinct from the human receptor were not present in other cloned CCK receptor species. A Chinese hamster ovary cell line that stably expressed this receptor was developed. The cynomolgus receptor expressed in this environment was functionally indistinguishable from the human receptor, binding CCK with high affinity [inhibition constant (K(I)) = 1.8 +/- 0.5 nM] and exhibiting a potent intracellular calcium signaling response to this hormone (EC(50) = 6.6 +/- 2.1 pM). Like the human type A CCK receptor, this receptor was expressed prominently in monkey gallbladder and stomach and was expressed in low levels in brain and pancreas. The type B CCK receptor cDNA was cloned from stomach and brain (450 residue receptor that is 96% identical to the human receptor), where it was highly expressed yet was undetectable in gallbladder or pancreas. This work confirms the relevance of the cynomolgus species for preclinical testing of drugs acting on the type A CCK receptor.

Selected features of nonendocrine pancreatic cancer

We survey some interesting features of gene expression in nonendocrine pancreatic cancer, the response to some less widely known agents as they impact on pancreatic cell proliferation and programmed death, and several developing approaches to therapy. The proliferative and cellular suicide responses of Panc-1 cells to the free radical spin trap, NTBN, and to the 5-lipoxygenease inhibitor, MK 886, the latter assessed with CLONTECH Atlas Human cDNA Array 1, are reviewed. Difficulties in identifying those factors whose suppression or augmentation could result in inhibition of malignantly transformed cell properties are considered.

Autocrine and paracrine signaling through neuropeptide receptors in human cancer

Autocrine and paracrine signaling leading to stimulation of tumor cell growth is a common theme in human cancers. In addition to polypeptide growth factors such as EGF family members which signal through receptor tyrosine kinases, accumulating evidence supports the autocrine and paracrine involvement of specific neuropeptides with defined physiologic actions as neurotransmitters and gut hormones in lung, gastric, colorectal, pancreatic and prostatic cancers. These neuropeptides, including gastrin-releasing peptide, neuromedin B, neurotensin, gastrin, cholecystokinin and arginine vasopressin bind seven transmembrane-spanning receptors that couple to heterotrimeric G proteins. Studies with human small cell lung cancer (SCLC) cells support a requirement for balanced signaling through G(q) and G(12/13) proteins leading to intracellular Ca2+ mobilization, PKC activation and regulation of the ERK and JNK MAP kinase pathways. While specific neuropeptide antagonists offer promise for interrupting the single neuropeptide autocrine systems operating in pancreatic and prostatic cancers, SCLC is exemplified by multiple, redundant neuropeptide autocrine systems such that tumor growth cannot be inhibited with a single specific antagonist. However, a novel class of neuropeptide derivatives based on the substance P sequence have been defined that exhibit broad specificity for neuropeptide receptors and induce apoptosis in SCLC by functioning as biased agonists that stimulate discordant signal transduction. Thus, interruption of autocrine and paracrine neuropeptide signaling with specific antagonists or broad-spectrum biased agonists offer promising new therapeutic approaches to the treatment of human cancers.

Genetic analysis of pancreatic duct hyperplasia in Otsuka Long-Evans Tokushima Fatty rats: possible association with a region on rat chromosome 14 that includes the disrupted cholecystokinin-A receptor gene

An Otsuka Long-Evans Tokushima Fatty (OLETF) strain of rat spontaneously developed hyperglycemia, hyperinsulinemia, insulin resistance and mild obesity, which had been studied as animal model for type II diabetes mellitus (T2DM). Recently, we observed that this strain coincidentally developed atypical hyperplasia of the choledocho-pancreatic ductal epithelium with a complete incidence. In an effort to locate genes responsible for this hyperplasia, we prepared 288 backcross progeny from a mating between OLETF rats and BN rats (which do not develop hyperplasia), and performed a genome-wide scan using 207 polymorphic genetic markers. We observed a prominent association of hyperplasia with a region involving a marker locus D14Mit4 (P = 0.00020, Fisher's exact test) and Cckar (the cholecystokinin-A receptor gene; P = 0.00025, Fisher's exact test) which is known to be disrupted in an OLETF strain. Our findings indicated that epithelial hyperplasia of the choledocho-pancreatic duct is associated with a region on rat chromosome 14 around the Cckar gene in an additive fashion with another two susceptible loci, each on chromosome 9 and 7. This implied the possibility that Cckar deficiency could result in a predisposition towards pancreatic duct hyperplasia.

G protein-coupled receptor signaling in human ductal pancreatic cancer cells: neurotensin responsiveness and mitogenic stimulation

Neuropeptides and their corresponding G protein-coupled receptors (GPCRs) are increasingly implicated in the autocrine/paracrine stimulation of growth of human cancers. We report that neurotensin induced rapid Ca2+ mobilization from intracellular stores followed by Ca2+ influx in five human ductal pancreatic cancer cell lines: HPAF-II, Capan-1, Capan-2, PANC-1, and MIA PaCa-2. In addition, most cell lines exhibited Ca2+ responses to multiple neuropeptides including bombesin, bradykinin, cholecystokinin, and vasopressin and to bioactive lipids, including lysophosphatidic acid (LPA), that also act via GPCRs. The well-differentiated line HPAF-II responded to at least seven independent GPCR agonists. The concentrations of neurotensin required to induce half-maximal effects (EC50) in HPAF-II and PANC-1 cells were 5 and 8nM, respectively. Digital fluorescence image analysis to measure Ca2+ responses in single cells revealed that 90% or more of HPAF-II and PANC-1 cells responded to 10nM neurotensin. Addition of neurotensin to PANC-1 cells also induced rapid and dose-dependent extracellular-regulated protein kinase (ERK-1 and ERK-2) activation and subsequently, stimulated DNA synthesis. The signaling complexity of GPCRs uncovered by these studies reveals a new aspect in the biology of human pancreatic cancer and could offer the basis for new approaches to the treatment of this disease.

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.

Inhibitory effect of a long-acting somatostatin analogue on EGF-stimulated cell proliferation in Capan-2 cells

Numerous studies have reported diverse effects of gut-derived regulatory peptides on growth of the normal pancreas, pancreatic neoplasms induced experimentally in animals, and pancreatic cancer cell lines, but the results of these investigations are rather controversial. The stimulatory effect of epidermal growth factor (EGF) on cell proliferation of pancreatic cell lines is well established. Whether this action can be modulated by somatostatin is not clear. Furthermore, it is not certain whether another regulatory peptide, cholecystokinin (CCK), affects the proliferation of these cells. In the present study we investigated the presence of CCK-A and CCK-B, as well as somatostatin-2 (SSTR2) receptors by RT-PCR, and studied the actions of EGF, CCK and octreotide on DNA synthesis in the human pancreatic adenocarcinoma cell line Capan-2. Octreotide, a long-acting somatostatin analogue was used as somatostatin agonist. Cells were cultured in RPMI-1640 medium. They were incubated in serum free medium containing 0.2% BSA in the absence (control) or the presence of the peptides. [3H]-thymidine incorporation into DNA was measured after 48 h of incubation. By means of RT-PCR analysis we were able to demonstrate SSTR2 expression, but not CCK-A or CCK-B receptor mRNA in Capan-2 cells. DNA synthesis evaluated by [3H]-thymidine incorporation was found to be increased by 45.2 +/- 5.6% in response to EGF (10(-8) M) and decreased by 11.7 +/- 2.6% to octreotide (10(-8) M) compared to controls (P < 0.01). The increase in [3H]-thymidine incorporation was significantly lower when EGF treatment was combined with octreotide administration (10.1 +/- 2.5% over control). In the concentration range of 10(-11)-10(-8) M, CCK did not alter significantly the incorporation of [3H]-thymidine into DNA in Capan-2 cells. In conclusion, these data support a role for EGF as a growth factor for the human pancreatic cancer cell Capan-2. Somatostatin may play an important role in regulating cell proliferation in Capan-2 cells both under basal, and growth factor-stimulated conditions. Our results suggest, however, that CCK receptors are not expressed, and CCK does not affect cell proliferation in this transformed pancreatic cell line.

Arginine 197 of the cholecystokinin-A receptor binding site interacts with the sulfate of the peptide agonist cholecystokinin

The knowledge of the binding sites of G protein-coupled cholecystokinin receptors represents important insights that may serve to understand their activation processes and to design or optimize ligands. Our aim was to identify the amino acid of the cholecystokinin-A receptor (CCK-AR) binding site in an interaction with the sulfate of CCK, which is crucial for CCK binding and activity. A three-dimensional model of the [CCK-AR-CCK] complex was built. In this model, Arg197 was the best candidate residue for a ionic interaction with the sulfate of CCK. Arg197 was exchanged for a methionine by site-directed mutagenesis. Wild-type and mutated CCK-AR were transiently expressed in COS-7 cells for pharmacological and functional analysis. The mutated receptor on Arg197 did not bind the agonist radioligand 125I-BH-[Thr, Nle]-CCK-9; however, it bound the nonpeptide antagonist [3H]-SR27,897 as the wild-type receptor. The mutant was approximately 1,470- and 3,200-fold less potent than the wild-type CCK-AR to activate G proteins and to induce inositol phosphate production, respectively. This is consistent with the 500-fold lower potency and 800-fold lower affinity of nonsulfated CCK relative to sulfated CCK on the wild-type receptor. These data, together with those showing that the mutated receptor failed to discriminate nonsulfated and sulfated CCK while it retained other pharmacological features of the CCK-AR, strongly support an interaction between Arg197 of the CCK-AR binding site and the sulfate of CCK. In addition, the mutated CCK-AR resembled the low affinity state of the wild-type CCK-AR, suggesting that Arg197-sulfate interaction regulates conformational changes of the CCK-AR that are required for its physiological activation.

Cholecystokinin-A receptor messenger RNA expression in human pancreatic cancer

Cholecystokinin (CCK) is a gut peptide hormone known to stimulate postprandial gallbladder contraction and pancreatic enzyme secretion. It has also been shown to induce the growth of normal pancreas and of malignant and premalignant lesions in rodents. Although CCK has been shown to promote the growth of human adenocarcinoma cell lines, its role in the growth of human pancreatic adenocarcinomas in vivo is less clear. Localization of CCK receptors to neoplastic cells within resected human tissue specimens would be suggestive of its potential action as an in vivo promoter of human pancreatic cancer. Resected tissue specimens of pancreatic adenocarcinomas were therefore studied by both reverse transcriptase-polymerase chain reaction (RT-PCR) and in situ hybridization for the presence of CCK-A receptors. Ninety percent of studied tumors demonstrated CCK-A expression by RT-PCR, and this expression was localized to neoplastic cells by in situ hybridization. An increase in the expression of CCK receptors is a mechanism by which pancreatic malignancies may gain a significant growth stimulus.