Tetronothiodin, a novel CCKB receptor ligand, antagonizes cholecystokinin-induced Ca2+ mobilization in a pituitary cell line

Strategies for the construction of γ-spirocyclic butenolides in natural product synthesis

Over the last four decades, a number of γ-spirocyclic butenolide containing natural products, drugs, and medicinally useful synthetic compounds have been reported. In this review, we discuss diverse chemical approaches to synthesize γ-spiro butenolides and their application towards natural product synthesis. The collective perception of various methods may allow superior approaches capable of delivering efficient synthetic approaches to obtain γ-spiro butenolide comprising natural products and their hybrid analogues for further drug discovery and development.

Natural [4 + 2]-Cyclases

[4 + 2]-Cycloadditions are increasingly being recognized in the biosynthetic pathways of many structurally complex natural products. A relatively small collection of enzymes from these pathways have been demonstrated to increase rates of cyclization and impose stereochemical constraints on the reactions. While mechanistic investigation of these enzymes is just beginning, recent studies have provided new insights with implications for understanding their biosynthetic roles, mechanisms of catalysis, and evolutionary origin.

Spirotetronate polyketides as leads in drug discovery

The discovery of chlorothricin (1) defined a new family of microbial metabolites with potent antitumor antibiotic properties collectively referred to as spirotetronate polyketides. These microbial metabolites are structurally distinguished by the presence of a spirotetronate motif embedded within a macrocyclic core. Glycosylation at the periphery of this core contributes to the structural complexity and bioactivity of this motif. The spirotetronate family displays impressive chemical structures, potent bioactivities, and significant pharmacological potential. This review groups the family members based on structural and biosynthetic considerations and summarizes synthetic and biological studies that aim to elucidate their mode of action and explore their pharmacological potential.

Tetramic and tetronic acids: an update on new derivatives and biological aspects

Significant developments in the isolation of tetramic acids and tetronic acids, in the elucidation of their biosyntheses and their biological activities and in laboratory syntheses are reviewed with a focus on those derivatives with medicinal and pharmacological relevance. Important new members of the title compound families isolated since the year 2000 are covered as well as new biological aspects of some earlier congeners.

CCK receptor antagonists

1. The peptide hormone and neurotransmitter, cholecystokinin, is widely distributed throughout the gastrointestinal tract and central nervous system and mediates a diverse number of biological functions. 2. Two receptor subtypes, CCK-A and CCK-B, have been identified by both pharmacological characterization and molecular cloning. The CCK-A receptor is the predominant peripheral CCK receptor subtype and the CCK-B receptor is the predominant central CCK receptor. In addition, there are discrete populations of CCK-A receptors in the brain and CCK-B receptors are present in gastric mucosa. 3. Subtype selective antagonists have been developed which discriminate between the two receptor subtypes. One of the major chemical classes has exploited a benzodiazepine template present in asperlicin which was initially discovered in a natural product screen for CCK receptor antagonists. 4. The structurally related benzodiazepines L-365,260, L-740,093, and YM022 are selective antagonists of the CCK-B receptor subtype. Their in vitro pharmacological profiles have been characterized using the human CCK-B receptor expressed in CHO cells. 5. L-365,260 behaves in a manner consistent with that of a competitive antagonist and both L-740,093 and YM022 behave as insurmountable CCK-B receptor antagonists in vitro.

Cholecystokinin B-type receptor signaling is involved in human pancreatic cancer cell growth

Cholecystokinin (CCK) is known to stimulate pancreatic cancer cell growth, but no detailed CCK receptor subtype characterization and investigation of CCK receptor-mediated cellular responses in human pancreatic cancer cells have been reported thus far. In this study, CCK binding sites were identified in human pancreatic cancer cells (MIA-PaCa-2) using radioligand binding studies. Pharmacological characterization demonstrated a single class of high-affinity CCK sites on MIA-PaCa-2 cells (326 +/- 18 pM, receptor density 16.9 +/- 2.3 fmol/mg protein). These CCK binding sites displayed a typical CCKB binding profile as shown in competition studies by using different CCK-related compounds and non-peptide CCK antagonists discriminating between CCKA and CCKB sites. CCKB receptor-connected effector systems have been characterized in MIA-PaCA-2 cells, and their involvement in CCK-8S-induced proliferative effects on MIA-PaCa-2 cells has been demonstrated.

YM022 [(R)-1-[2,3-dihydro-1-(2'-methylphenacyl)-2-oxo-5-phenyl-1H-1,4- benzodiazepin-3-yl]-3-(3-methylphenyl)urea]: an irreversible cholecystokinin type-B receptor antagonist

A functional evaluation of the recently developed cholecystokinin type-B (CCK-B) receptor antagonist YM022 [(R)-1-[2,3-dihydro-1-(2'-methylphenacyl)-2-oxo-5-phenyl-1H-1,4-++ +benzodiazepin-3-yl]-3-(3-methylphenyl)urea] was undertaken in Chinese hamster ovary cells stably expressing the human CCK-B receptor gene (hCCK-B.CHO). YM022 exhibited high affinity and selectivity for the CCK-B receptor subtype as estimated from [125I]CCK8S displacement studies using membranes derived from hCCK-B.CHO and hCCK-A.CHO cells. Functional antagonist activity of YM022 was demonstrated employing CCK-4-stimulated Ca2+ mobilization in hCCK-B.CHO cells. In the presence of 30 nM YM022, the maximum effect of CCK-4 was suppressed to 48 +/- 11% of control, an effect that was accompanied by a modest rightward shift in the CCK-4 concentration-response curve. In contrast, the structurally similar CCK-B receptor antagonist L-365,260 [3R(+)-N-[2,3-dihydro-1-methyl-2-oxo-5-phenyl-1H-1,4-benzodiazepin -3-yl]-N'-(methylphenyl)urea; 30 nM-10 microM] produced progressive rightward shifts in the CCK-4 concentration-response curve, with no effect observed on the CCK-4 maximum response. Further characterization using the technique of microphysiometry revealed that the agonist activity of CCK-4 was not restored following washout after exposure to YM022. The antagonist activity of L-365,260, however, was found to be fully reversible in this system. Thus, YM022 behaves as an irreversible antagonist, whilst its structural analogue L-365,260 exhibits properties consistent with a competitive antagonist.

Biology of gut cholecystokinin and gastrin receptors

1. The stomach hormone gastrin and the intestinal hormone cholecystokinin (CCK) share a common C-terminal pentapeptide sequence but have different biological roles. Gastrin is the major stimulant of gastric acid secretion and has a growth stimulatory effect on the secretory part of the stomach. The physiological roles of CCK are the stimulation of pancreatic secretion and the contraction of the gall-bladder. 2. Several classes of receptors have been defined for peptides of the gastrin/CCK family. The CCKA receptor on pancreatic acini has a greater affinity for sulfated CCK than for gastrin, while the gastrin/CCKB receptor in gastric mucosa and brain has similar affinities for both gastrin and CCK. Potent and selective antagonists have been developed for both receptor classes. 3. The structures of the CCKA and gastrin/CCKB receptors have been deduced from the nucleotide sequences of cloned cDNA. The receptors, which both belong to the family with seven transmembrane segments, control secretion via similar signalling mechanisms. Occupation of either receptor leads to activation of phospholipase C, with resultant increases in intracellular levels of inositol triphosphate and Ca2+. Mitogenic signalling pathways are also being defined. 4. Recent studies have questioned the previous assumption that gastrin precursors are inactive. Glycine-extended gastrin17 has been shown to stimulate mitogenesis in some cell lines and may also have an autocrine role in the growth of colonic cancers. The receptors involved, which are clearly distinct in binding properties from the CCKA and gastrin/CCKB receptors, have not yet been cloned. Specific antagonists for the novel receptors will be required to define their function in further detail.

Analysis of variation in L-365,260 competition curves in radioligand binding assays

1. For several years, we have used the cholecystokinin (CCK)B/gastrin receptor selective antagonist, L-365,260, as a reference compound in a variety of studies in CCKB/gastrin receptor radioligand binding assays. Here, we have analysed the competition curve data sets obtained between L-365,260 and [125I]-BH-CCK8S in guinea-pig gastric gland and mouse and rat cerebral cortex preparations. 2. Competition curves obtained for L-365,260 in the mouse cortex assay were not different from rectangular hyperbolae (slope = 1.01 +/- 0.02) implying the presence of a single population of binding sites (pKI = 8.41 +/- 0.01; data from 47 experiments, slope constrained to unity). However, in the rat cortex and guinea-pig gastric gland assays, the mean slope of the competition curves was significantly less than one and the mean apparent pKI significantly lower than that obtained in the mouse cortex (slope = 0.85 +/- 0.03, 0.90 +/- 0.03; apparent pKI = 7.98 +/- 0.05, 8.07 +/- 0.05; 48 and 45 experiments, in rat and guinea-pig, respectively). The distribution of the individual pKI and slope estimates of the competition curves in these two assays was consistent with expectations for the variable expression (in terms of absolute number and proportion) of two binding sites. The two sites were characterized by pKI values for L-365,260 of 8.50 +/- 0.04 and 8.48 +/- 0.04 for the high affinity site and 7.32 +/- 0.04 and 7.22 +/- 0.06 for the low affinity site in guinea-pig and rat, respectively. 3. The affinity estimates for L-365,260, although obtained on different tissues, are consistent with data obtained from the analysis of L-365,260 antagonism of pentagastrin-stimulated responses in mouse and rat stomach (acid secretion) and guinea-pig gastric muscle (isotonic contraction) assays. To this extent, these data suggest the existence of two CCKB/gastrin receptor subtypes.

Pharmacological characterization of a Chinese hamster ovary cell line transfected with the human CCK-B receptor gene

A stable Chinese hamster ovary cell line expressing the human CCK-B receptor gene is described (hCCK-B.CHO). In radioligand binding experiments employing membranes derived from these cells the rank order of affinity estimated for a series of CCK receptor ligands (CCK-8S > CI988 > PD 135158 > pentagastrin > CCK-8NS > L-365,260 > CCK-4 > LY 288513 > devazepide > A71378 > lorglumide) was found to be in excellent agreement with CCK-B receptor pharmacology described in guinea-pig cortex. Functional coupling in hCCK-B.CHO cells was demonstrated using agonist stimulated mobilization of intracellular Ca2+, measured with the FURA-2 technique. The CCK-B receptor selective agonist CCK-4 stimulated the mobilization of intracellular Ca2+ with an estimated pEC50 value of 7.4. Consistent with CCK-B receptor pharmacology, the rank order of potency for antagonism of this response was observed to be PD 135158 > CI988 > L-365,260 >> devazepide > lorglumide. This cell line provides a powerful new tool for the evaluation and development of novel ligands acting at the human CCK-B receptor subtype.

Cholecystokinin-A specific antagonism of KSG-504 to cholecystokinin receptor binding and pancreatic secretion in mammals

The effects of KSG-504 ((S)-arginium (R)-4-[-N-(3-methoxypropyl)-N-pentylcarbamoyl]-5-(2- naphthylsulfonyl) pentanoate monohydrate), a new cholecystokinin (CCK)-receptor antagonist, on 125I-CCK-8 binding to rat pancreatic, canine gallbladder and guinea pig cerebrocortical membranes and the pancreatic amylase release from isolated rat acini stimulated by several kinds of secretagogues, including CCK, were investigated. The 125I-CCK-8 saturation experiment showed that pancreatic, gallbladder and cerebrocortical CCK receptors had a single high affinity binding component with dissociation constants (Kd) of 0.18, 0.31 and 0.88 nM, respectively. The maximum numbers of specific binding sites (Bmax) in these membranes were 1012, 52 and 20 fmol/mg protein, respectively. KSG-504 and CCK-8 displaced specific 125I-CCK-8 binding to CCK receptors in all membrane preparations in a competitive manner. The affinity of KSG-504 for pancreatic (Ki = 173 nM) and gallbladder (Ki = 283 nM) CCK receptors were > 3 orders of magnitude higher than its affinity for cerebrocortical CCK receptors. KSG-504 also inhibited 125I-gastrin-I binding to guinea pig gastric glands, but the IC50 value (18.2 microM) was apparently much higher. CCK-8-stimulated amylase release from isolated pancreatic acini of rats was antagonized by KSG-504 in a concentration-dependent manner. KSG-504 did not affect amylase release stimulated by secretagogues such as gastrin-releasing peptide, carbachol, vasoactive intestinal peptide and A23187. These results indicate that KSG-504 acts as a CCK-A-receptor-specific antagonist in the pancreas and gallbladder.

Cholecystokinin induced signaling in rat glioma C6 cells

We investigated the effect of sulphated cholecystokinin octapeptide (CCK-8S) on free intracellular calcium concentration, phosphatidylinositol metabolism, and protein phosphorylation in C6 cells, a rat glioma cell line which was shown to express CCKB type receptors. Increase in [Ca2+]i by both influx across the cell membrane and release from internal stores was demonstrated by utilizing a laser confocal imaging system. Because CCK-8S produced a transient elevation of inositol triphosphate level participation of InsP3 in calcium signaling in C6 cells is very likely. Protein kinase C seems to be involved in CCK-8S induced signaling in rat glioma C6 cells as demonstrated by using in vivo phosphorylation experiments.

Characterization of YM022: its CCKB/gastrin receptor binding profile and antagonism to CCK-8-induced Ca2+ mobilization

We investigated the antagonistic activity of (R)-1-[2,3-dihydro-1-(2'- methylphenacyl)-2-oxo-5-phenyl-1H-1,4-benzodiazepin-3-yl]-3- (3-methylphenyl) urea (YM022), a benzodiazepine derivative, at CCKB/gastrin receptors. This compound potently inhibited [125I]CCK-8 binding to rat brain CCKB/gastrin receptors with a Ki value of 0.26 nM, but it showed weak affinity for rat pancreas CCKA receptors (Ki = 270 nM). Selectivity for CCKB/gastrin receptors was 1000-fold greater than that for CCKA receptors. Changes in intracellular free Ca2+ concentration ([Ca2+]i) in response to CCK-8 were measured in a rat anterior pituitary cell line GH3 by fura-2 fluorometry. CCK-8 (1-100 nM) dose-dependently increased [Ca2+]i in these cells, whereas YM022 had no effect on baseline [Ca2+]i even at the highest concentration of 100 nM. YM022 inhibited the mobilization of [Ca2+]i elicited by 10 nM CCK-8 in a concentration-dependent manner with an IC50 value of 4 nM. In conclusion, YM022 is an extremely potent and highly selective antagonist of CCKB/gastrin receptors. This compound is therefore useful for studying the physiological and pharmacological roles of CCKB/gastrin receptors.

CCKB receptor signaling in rat pituitary GH3 cells. CCK-8S-induced intracellular calcium mobilization by Ca2+ release and Ca2+ influx

We describe the effect of sulphated cholecystokinin octapeptide (CCK-8S) on [Ca2+]i in rat pituitary GH3 cells. Investigations were performed on fluo-3 loaded cells by using a confocal imaging system MRC-600 (Bio-Rad). Because CCK-8S mobilized intracellular calcium in cells bathed in Ca(2+)-free buffer it must be able to release calcium from internal stores. Furthermore, influx of Ca2+ from outside the cells seems to contribute to CCK-8S induced increases in [Ca2+]i as demonstrated by calcium mobilization in GH3 cells preincubated with thapsigargin in Ca2+ containing buffer.

Characterisation of CCKB receptors on GH3 pituitary cells: receptor activation is linked to Ca2+ mobilisation

Cholecystokinin receptors on GH3 rat anterior pituitary cells have been characterised using radioligand binding and Ca2+ mobilisation. [125I]Bolton Hunter CCK-8s (BHCCK) bound dose dependently to GH3 cells (Kd 85 pM, Bmax 23 fmol/mg protein). Competition curves with CCK-8s (IC50 2.4 nM), pentagastrin (IC50 25 nM) and devazepide (IC50 820 nM), were consistent with a population predominantly of CCKB receptors. Binding of [125I]BHCCK to lysed cells was inhibited by 10 microM GTP-gamma-S and 10 microM GppNHp, suggesting the receptor was linked to a guanine nucleotide binding protein. Intracellular Ca2+ mobilisation was a functional consequence of CCKB receptor activation in GH3 cells using the fluorescent dye fura-2. CCK-8s (0.1 nM-1 microM) and the selective CCKB receptor agonist, pentagastrin, (0.1 nM-100 microM) dose dependently increased intracellular Ca2+ with a similar maximal increase of 2.85-fold and 2.77-fold respectively. Response to a submaximal dose of the CCKB receptor agonist pentagastrin (100 nM) was dose dependently blocked by the CCKB receptor antagonist L-365,260. GH3 cells may therefore provide a useful model to study CCKB receptor coupling in a pituitary cell line.

Desensitization of cholecystokininB receptors in GH3 cells

Desensitization of the cholecystokinin (CCK) octapeptide (CCK-8)-induced rise in intracellular free calcium concentration ([Ca2+]i) was characterized in GH3 cells, a pituitary tumor cell line, which are known to possess CCKB receptor subtype. The CCK-8-induced [Ca2+]i transient was reduced following the initial application of CCK-8. A similar desensitization of the CCK-8-induced response was observed following the first application of thyrotropin-releasing hormone (TRH). By contrast, the TRH-induced response was not desensitized by the preceding application of CCK-8. Desensitization of the CCK-8-induced [Ca2+]i transient was associated with diminished inositol 1,4,5-trisphosphate formation. The recovery of desensitization of the CCK-8-induced response was delayed by a phosphoserine/phosphothreonine phosphatase inhibitor, calyculin A (100 nM). The responsiveness to CCK-8 was also reduced by phorbol 12,13-dibutyrate (PDBu), and this effect of PDBu was completely abolished by preincubation with staurosporine. Staurosporine significantly attenuated the desensitization caused by preincubation with CCK-8, but this effect was too small to attribute the desensitization to the protein kinase C transduction pathway alone. It is likely that desensitization of CCK receptors involves multiple transduction pathways.

Species specificity of pharmacological characteristics of CCK-B receptors

Novel CCK-B receptor antagonists, tetronothiodin and L-156,586, showed different affinities for CCK-B receptors in brain membranes from human, rat, guinea pig and mouse. [125I]CCK-8 bound to these membranes with a similar affinity. However, tetronothiodin was most potent in rat (IC50 = 3.6 nM), followed by guinea pig (96 nM), human (210 nM) and mouse (280 nM). L-156,586 bound with highest affinity to membranes from guinea pig (11 nM), and with lowest affinity to membranes from mouse (220 nM). These results suggest the existence of species specificity of CCK-B receptors, and that these two compounds are useful tools for discrimination between these receptors.

GH3 cells, an anterior pituitary cell line, express CCK-B receptors

We found that GH3 cells, a rat anterior pituitary tumor cell line, expressed a single class of high-affinity binding sites for radiolabeled cholecystokinin octapeptide (CCK-8) with a Kd of 48 pM. The binding sites had high affinity for CCK-8, CCK-4, gastrin I, and L-365,260 (CCK-B antagonist), and had low affinity for devazepide (CCK-A antagonist), indicating that the binding sites are CCK-B receptors. GTP and its stable analogues inhibited radiolabeled CCK-8 binding to GH3 cell membranes, suggesting a coupling of CCK-B receptors to a G-protein.