Evidence for cholecystokinin receptor subtype in endocrine pancreas

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.

Immunohistochemical analysis of cholecystokinin A receptor distribution in the rat pancreas

Cholecystokinin (CCK) plays a major role in the regulation of pancreatic enzyme secretion based on its binding to the CCK-A receptor (CCK-AR). While CCK-AR is known to be expressed in rat islet B cells, the localization of CCK-AR in rat pancreatic A and D cells remains poorly understood. The aim of this study was to identify the localization of CCK-AR in rat pancreatic islets by means of double immunofluorescence straining with antibodies against CCK-AR, glucagon, insulin and somatostatin and with in situ hybridization to detect its transcript. CCK-AR-like immunoreactive cells were found to overlap both with glucagon-like immunoreactive cells and insulin-like immunoreactive cells but not with somatostatin-like immunoreactive cells. An in situ hybridization study using a cRNA probe for CCK-AR revealed that CCK-AR mRNA was expressed in the center and periphery of the pancreatic islets. Further to this, immunofluorecsence staining using anti-glucagon antibody was carried out after in situ hybridization using the CCK-AR cRNA probe in order to identify CCK-AR mRNA expressing cells. CCK-AR mRNA exhibited a distribution pattern almost identical to that of glucagon-like immunoreactive cells. These results show clearly that CCK-AR exists not only in B but also in A cells of the rat pancreas, suggesting that CCK regulates the secretion of insulin and glucagon at least partly via CCK-AR.

The rat pancreatic islets: a reliable tool to study islet responses to cholecystokinin receptor occupation

This study was undertaken to show that rat purified islets can be used as a reliable tool to study some aspects of human islet's physiology related to CCKR occupation. Therefore, isolated foetal, adult human and rat islets were compared for (1) CCKR subtypes mRNA and protein expression and somatostatin (SS) mRNA and (2) co-localization of these receptors with insulin, glucagon and SS. Finally, rat islets were tested for their responsiveness to stimulation. Purified human and rat islets were used for CCKR subtypes and SS mRNA estimation by RT-PCR and protein by Western blots. Receptors and hormones co-localizations were evaluated by confocal microscopy. Hormones secretion served to determine rat islets responsiveness. Islets of both species express CCKA and CCKBR mRNA and proteins and SS mRNA. The CCKAR co-localizes with insulin and glucagon and the CCKBR with SS. Insulin release was increased 5-fold in response to 16 mM glucose and SS secretion reached 1.3- and 1.7-fold increments above basal in response to forskolin and IBMX. In conclusions, human and rat islets have comparable CCKR subtypes localized on the same cells; they also express SS mRNA. The rat islets are functional as they secrete but their response to hormonal stimulation remains to be clarified. These rat islets can thus serve as tools to study islets physiology.

Biological effects of newly synthesized cholecystokinin analogs

Cholecystokinin (CCK) is a gut hormone that regulates pancreatic endocrine functions via CCK(A) receptors. CCK(4) (Trp-Met-Asp-Phe-NH(2)) has an insulinotropic effect, but is 1,000-fold less potent than CCK(8) in rodents. The in vitro potencies with respect to binding, the biological effects and the selectivity of newly synthesized CCK(4) analogs constructed by computer modelling experiments were investigated in vitro in rat pancreas and brain, INS-1 cells, and guinea pig ileum. Exchanging various amino acids, e.g. Met by either Pro or Nle, and modifying Phe by adding various substituents in different positions led to compounds which were more effective as insulin secretagogues than CCK(4) itself and even show insulinotropic effects comparable with those of CCK(8) (e. g. compounds M1 and M2 being substituted at Phe). Some compounds which possess electron withdrawing groups on the C-terminal Phe and possess a Pro instead of a Met were especially effective. The CCK(A) receptor antagonist L-364,718, but not by the CCK(B) receptor antagonist L-365,260, inhibited the insulinotropic effects. The synthetic CCK(4) compounds were not selective for the endocrine pancreas: e.g. M1 and M2 had binding activity with respect to rat brain homogenates but no activity with respect to contraction of the guinea pig ileum. The data indicate that some of the newly synthesized CCK tetrapeptides exhibit a high affinity for the CCK receptor of beta-cells and have an insulinotropic effect much higher than CCK(4).

Cell-specific localization of the cholecystokininA receptor in the porcine pancreas

Cholecystokinin (CCK) produced in the mucosa of the upper small intestine exerts several biological functions. Its secretion in physiological amounts is modulated by the interaction of extracellular regulators and by binding to intracellular receptors of the target cells. The relative affinity of CCK to its receptor has been characterized in various biological and pharmacological studies and it is now well established that CCK has a higher affinity to the CCKA than to the CCKB receptor. Furthermore CCK influences the secretion of pancreatic enzymes in several species but very little is known about the relationship between CCK and the islet hormone-producing cells in the pig pancreas. The localization of this receptor at the cellular level showed conflicting results in animal studies and has not been described in pigs. The aim of the present study was to characterize the precise cellular location of the CCKA receptor in the porcine pancreas. Polyclonal antiserum was raised against the N-terminal epitope of the CCKA receptor molecule and used for localization studies. Using immunohistochemistry on methanol/acetic acid-fixed, paraffin-embedded pancreas, the CCKA receptor could successfully be localized in islet cells. Parallel staining of serial sections with antibodies directed against insulin and glucagon revealed colocalization with glucagon in alpha cells. No immunoreaction was found in the exocrine pancreas. Our results support the concept that in the porcine species the stimulation of the exocrine pancreas is mediated by the CCKB rather than the CCKA receptor, as it is known for the rat species.

Activation of phospholipase C by cholecystokinin receptor subtypes with different G-protein-coupling specificities in hormone-secreting pancreatic cell lines

Phospholipase C (PLC) activity was investigated by stimulation of membrane preparations obtained from insulin (beta-TC3)-, somatostatin (Rin 1027-B2)-, and glucagon (INR1-G9)-producing pancreatic cell lines using the non-hydrolyzable GTP analogue GTPgammaS alone, the C-terminal octapeptide cholecystokinin (CCK-8), or gastrin. All compounds caused a significant 2- to 4.4-fold stimulation of PLC activity in the different cell lines, which was diminished by the non-hydrolyzable GDP analogue GDPbetaS. CCK receptor subtypes were characterized by radioligand binding experiments. High-affinity binding sites for tritiated CCK(A) receptor antagonist L-364,718 (K(d) = 0.24 nM) and tritiated CCK(B) receptor antagonist L-365,260 (K(d) = 0.13 nM) were only present in Rin 1027-B2 cells. High-affinity binding sites for both ligands were not found in beta-TC3 or INR1-G9 cells. Competition binding experiments with non-labeled CCK receptor antagonists CR 1505 (CCK(A) receptor-selective) and CR 2945 (CCK(B) receptor-selective), as well as microphysiometry experiments, resulted in the same receptor distribution. Reverse transcriptase-polymerase chain reaction confirmed the CCK receptor distribution pattern for Rin 1027-B2 cells, but in addition showed the existence of CCK(B) receptors in beta-TC3 cells. Immunoblocking experiments with C-terminal antibodies against different G-protein alpha-subunits demonstrated inhibition of CCK-stimulated PLC activity in beta-TC3 cells by G(q/11)alpha antiserum (70%), in Rin 1027-B2 cells by G(q/11)alpha antiserum (70%) and G(i)-3alpha antiserum (23%), and in INR1-G9 cells by G(q/11)alpha antiserum (60%) and G(o)alpha antiserum (45%). We conclude that CCK receptor subtypes with different G-protein-coupling specificities to PLC are present in the different hormone-secreting cells of the endocrine pancreas.

Pancreatic CCK(B) receptors: their potential roles in somatostatin release and delta-cell proliferation

In rodents, cholecystokinin (CCK) induces pancreatic enzyme secretion and pancreas growth through its CCK(A) receptors. It is unknown whether occupation of the CCK(B) receptors present in pig and human pancreas can cause the same effects. This study evaluates CCK(B) receptor expression in rat, mouse, pig, and fetal human pancreata using Northern blot, Western blot, and immunofluorescence techniques. The reported 2.7-kb CCK(B) receptor mRNA transcript in the rat brain and gastric fundus is absent in pancreas; the message was, however, detected by RT-PCR and by a CCK(B) receptor antibody as an 80-kDa protein present uniquely in islet delta-cells. Proteins of 50 and 80 kDa appear in mouse pancreas, and proteins of 50 and 115 kDa appear in pig and human pancreas, respectively, all localized in islet delta-cells. Gastrin mRNAs are strongly present in fetal rat pancreas, and the hormone is localized in islets; both are repressed 10 days after birth. In conclusion, the CCK(B) receptors are present in pancreas of four species with exclusive location in islet delta-cells. In such a location, they could be indirectly involved in the control of enzyme secretion.

N-terminal glycation of cholecystokinin-8 abolishes its insulinotropic action on clonal pancreatic B-cells

Monoglycated cholecystokinin octapeptide (Asp(1)-glucitol CCK-8) was prepared under hyperglycaemic reducing conditions and purified by reverse phase-high performance liquid chromatography. Electrospray ionisation mass spectrometry and automated Edman degradation demonstrated that CCK-8 was glycated specifically at the amino-terminal Asp(1) residue. Effects of Asp(1)-glucitol CCK-8 and CCK-8 on insulin secretion were examined using glucose-responsive clonal BRIN-BD11 cells. In acute (20 min) incubations, 10(-10) mol/l CCK-8 enhanced insulin release by 1.2-1.5-fold at 5.6-11.1 mmol/l glucose. The stimulatory effect induced by 10(-10) mol/l CCK-8 was abolished following glycation. At 5.6 mmol/l glucose, CCK-8 at concentrations ranging from 10(-11) to 10(-7) mol/l induced a significant 1.6-1.9-fold increase in insulin secretion. Insulin output in the presence of Asp(1)-glucitol CCK-8 over the concentration range 10(-11)-10(-7) mol/l was decreased by 21-35% compared with CCK-8, and its insulinotropic action was effectively abolished. Asp(1)-glucitol CCK-8 at 10(-8) mol/l also completely blocked the stimulatory effects of 10(-11)-10(-8) mol/l CCK-8. These data indicate that structural modification by glycation at the amino-terminal Asp(1) residue effectively abolishes and/or antagonises the insulinotropic activity of CCK-8.

Ontogeny and species differences in the pancreatic expression and localization of the CCK(A) receptors

We have evaluated the presence and localization of the CCK(A) receptor in rat, mouse, pig and human fetal pancreas by Northern, Western blots and immunofluorescence techniques. In the rat, parallelism exists between development of the CCK(A) receptor mRNA and protein with maximal peaks of expression during the suckling period. In the course of pancreatitis induction, CCK(A) receptor mRNA were maximally expressed and sustained during the gland's regeneration. In the rat and mouse pancreas, the CCK(A) receptor protein is localized around the acinar cells and beta cells of the islets of Langerhans. In the adult pig and fetal human pancreas, the CCK(A) receptor proteins were detected by Western blot. By immunofluorescence, its detection was possible only in the islet of Langerhans of the pig pancreas. These new findings support the views that CCK plays important and various roles in specific physiological systems of the pancreas of different species.

CCK receptor subtype in insulin-producing cells: a combined functional and in situ hybridization study in rat islets and a rat insulinoma cell line

Cholecystokinin (CCK) stimulates insulin secretion. It is, however, not established whether CCK receptors are expressed in insulin-producing cells. We therefore investigated, by in situ hybridization, whether CCK-A or CCK-B receptor mRNA could be detected in normal rat pancreatic islets and in the rat insulinoma cell line, RINm5F. The CCK-A, but not the CCK-B, receptor transcript was detected in both islets and RINm5F cells. Islet CCK-A receptors were mostly confined to the center of the islets corresponding to the distribution of the B cells. In RINm5F cells, insulin release was not significantly affected by cholecystokinin (CCK)-8-S (10(-13) to 10(-7) M), which is in contrast to the insulinotropic effect of CCK-8-S in normal rat islets. Similarly, in FURA-2AM-loaded cells, CCK-8-S (10(-11) to 10(-7) M) was without effect on the intracellular Ca2+ concentration ([Ca2+]ic) in RINm5F cells, whereas CCK-8-S (10(-7) M) markedly increased [Ca2+]ic (by 366+/-2 nM (P < 0.001) in normal rat islet cells. We conclude that the CCK-A, but not the CCK-B, receptor subtype is expressed in both normal rat islets and in the rat insulinoma-derived cell line RINmS5F. There is, however, a functional difference between normal islets and the RINm5F cells with respect to effects of CCK-8-S on insulin release and [Ca2+]ic.

Effects of C-terminal fragments of cholecystokinin on plasma insulin and glucagon concentrations in sheep

The effects of three C-terminal fragments of cholecystokinin (CCK) (CCK-8-sulphated form [SF], CCK-8-non-sulphated form [NSF] and CCK-4) on insulin and glucagon secretion were examined in sheep in vivo. Each CCK fragment was injected intravenously at a wide range of doses (1 pmol to 3 x 10(5) pmol kg-1). CCK-8(SF) had the lowest threshold dose (10 pmol kg-1) and a maximal response dose of 10(3) pmol kg-1 for increasing plasma insulin concentration; the respective threshold doses of CCK-8(NSF) and CCK-8 for increasing plasma insulin were 30 and 100 times greater than that of CCK-8(SF). A maximal insulin response was not obtained at the highest doses of CCK-8(NSF) or CCK-4 tested (3 x 10(3) and 3 x 10(5) pmol kg-1, respectively). These results indicate that CCK-A type receptors rather than CCK-B receptors may be involved in CCK-induced insulin secretion in sheep. None of the CCK fragments affected plasma glucagon concentration. The lack of a glucagon response to exogenous CCK-fragments may be one of the characteristics of the endocrine pancreatic responses of ruminant species.

Pharmacological and biochemical evidence for the simultaneous expression of CCKB/gastrin and CCKA receptors in the pig pancreas

1. In the pig, the secretory response of the pancreas is not inhibited by the antagonist MK329 suggesting that cholecystokininA (CCKA) receptors are not involved. 2. Membranes were isolated from the pancreas of 6 Large White pigs to characterize their CCK receptors. 3. The binding of [125I]-BH-[Thr, Nle]CCK-9 was dependent on pH, maximal after a 90 min incubation period, saturable and reversible. Saturation analysis of the binding demonstrated a single class of high affinity sites (Kd = 0.22 +/- 0.02 nM) and a binding capacity, Bmax = 110.64 +/- 12.50 fmol mg-1 protein. 4. Competition binding by agonists and antagonists of CCKA and CCKB/gastrin receptors demonstrated the presence of two distinct binding components, sites presenting a high affinity for [Thr, Nle]CCK-9, gastrin, PD 135158, L-365, 260 and a low affinity for MK329, SR 27897, and sites presenting a high affinity for [Thr, Nle]CCK-9, MK329, SR 27897 and a low affinity for gastrin, PD 135158, L-365,260. 5. These pharmacological data demonstrate the presence of both CCKA and CCKB/gastrin receptors in the pig pancreas, the latter being predominant. 6. Two distinct membrane proteins (50 and 85-100 kDa, respectively) display pharmacological features of CCKB/gastrin and CCKA receptors. 7. In pigs, as in calves and humans, CCKB/gastrin receptors are predominant in the pancreas.

Cholecystokinin-A and cholecystokinin-B/gastrin receptor mRNA expression in the gastrointestinal tract and pancreas of the rat and man. A polymerase chain reaction study

BACKGROUND

Gastrin and cholecystokinin (CCK) are thought to exert trophic effects on the gastrointestinal tract and pancreas. Two types of receptors have been cloned, CCK-A and CCK-B/ gastrin. We have examined the occurrence of CCK-A and CCK-B receptor mRNA in the brain, digestive tract, pancreas, and kidney of the rat and man by Northern blot and reverse transcribed polymerase chain reaction (RT-PCR).

METHODS

Total RNA was isolated from rat tissues and reverse transcribed into cDNA. cDNA from brain, kidney, and pancreas of the rat and man and from human whole stomach were commercially available. Northern blot and a PCR technique based on Taq polymerase-antibody interaction and using CCK-A and CCK-B receptor-specific primers, followed by Southern blot analysis, were the methods used.

RESULTS

By means of Northern blots, CCK-A receptor mRNA was detected in rat fundus mucosa and pancreas but not in the remaining GI tract or brain. By means of RT-PCR, CCK-A receptor mRNA was demonstrated in the brain and the mucosa of the fundus, antrum, duodenum, and colon, kidney, pancreas and pancreatic islets. CCK-B receptor mRNA was detected by Northern blot analysis in the brain and the fundus mucosa but not in the rest of the digestive tract and not in the pancreas, pancreatic islets, or kidney. By RT-PCR, expression of CCK-B receptor mRNA could also be detected in antrum mucosa. In man, CCK-A receptor mRNA was detected in the brain, stomach, pancreas, and kidney, whereas CCK-B receptor mRNA was found in the brain, stomach, and pancreas but not in the kidney. Cloning and DNA-sequence analysis of the PCR-amplified rat and human CCK-A and CCK-B receptor DNA fragments, which cover the protein-encoding regions of the intracellular loop C3, showed complete sequence homology as compared with published rat and human sequences.

CONCLUSIONS

It appears unlikely that CCK will have effects in the ileum, at least not effects mediated by CCK-A receptors. It also appears unlikely that physiologic concentrations of gastrin in the circulation will promote growth (or exert other effects) in the pancreas, duodenum, ileum, and colon, since CCK-B receptor mRNA is not expressed or is poorly expressed in these tissues.