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

Cholecystokinin receptors regulate sperm protein tyrosine phosphorylation via uptake of HCO3-

Cholecystokinin (CCK), a peptide hormone and a neurotransmitter, was detected in mature sperm two decades ago. However, the exact role of CCK and the types of CCK receptors (now termed CCK1 and CCK2) in sperm have not been identified. Here, we find that CCK1 and CCK2 receptors are immunolocalized to the acrosomal region of mature sperm. The antagonist of CCK1 or CCK2 receptor strongly activated the soluble adenylyl cyclase/cAMP/protein kinase A signaling pathway that drives sperm capacitation-associated protein tyrosine phosphorylation in dose- and time-dependent manners. But these actions of stimulation were abolished when sperm were incubated in the medium in the absence of HCO3-. Further investigation demonstrated that the inhibitor of CCK1 or CCK2 receptor could accelerate the uptake of HCO3- and significantly elevate the intracellular pH of sperm. Interestingly, the synthetic octapeptide of CCK (CCK8) showed the same action and mechanism as antagonists of CCK receptors. Moreover, CCK8 and the antagonist of CCK1 or CCK2 receptor were also able to accelerate human sperm capacitation-associated protein tyrosine phosphorylation by stimulating the influx of HCO3-. Thus, the present results suggest that CCK and its receptors may regulate sperm capacitation-associated protein tyrosine phosphorylation by modulating the uptake of HCO3-.

A fluorescent ligand-binding alternative using Tag-lite® technology

G-protein-coupled receptors (GPCRs) are crucial cell surface receptors that transmit signals from a wide range of extracellular ligands. Indeed, 40% to 50% of all marketed drugs are thought to modulate GPCR activity, making them the major class of targets in the drug discovery process. Binding assays are widely used to identify high-affinity, selective, and potent GPCR drugs. In this field, the use of radiolabeled ligands has remained so far the gold-standard method. Here the authors report a less hazardous alternative for high-throughput screening (HTS) applications by the setup of a nonradioactive fluorescence-based technology named Tag-lite(®). Selective binding of various fluorescent ligands, either peptidic or not, covering a large panel of GPCRs from different classes is illustrated, particularly for chemokine (CXCR4), opioid (δ, µ, and κ), and cholecystokinin (CCK1 and CCK2) receptors. Affinity constants of well-known pharmacological agents of numerous GPCRs are in line with values published in the literature. The authors clearly demonstrate that the Tag-lite binding assay format can be successfully and reproducibly applied by using different cellular materials such as transient or stable recombinant cells lines expressing SNAP-tagged GPCR. Such fluorescent-based binding assays can be performed with adherent cells or cells in suspension, in 96- or 384-well plates. Altogether, this new technology offers great advantages in terms of flexibility, rapidity, and user-friendliness; allows easy miniaturization; and makes it completely suitable for HTS applications.

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.

The cholecystokinin-B receptor antagonist L-740,093 produces an insurmountable antagonism of CCK-4 stimulated functional response in cells expressing the human CCK-B receptor

A stable cell line expressing the human cholecystokinin-B receptor gene (hCCK-B.CHO) has been employed in an evaluation of the recently developed CCK-B receptor antagonist L-740,093. L-740,093 exhibited high affinity (IC50 0.49 nM) and selectivity (<50% displacement at CCK-A sites at 1 microM) for the human CCK-B receptor subtype as estimated from [125I]-CCK-8S displacement studies with membranes prepared from hCCK-B.CHO cells. The elevation of intracellular free Ca2+ in hCCK-B.CHO cells in response to stimulation with CCK-4 was used to evaluate the antagonist activity of L-740,093 in vitro. L-740,093 potently (IC50 5.4 nM) antagonized the 30 nM CCK-4-stimulated Ca2+ mobilization in hCCK-B.CHO cells. Further studies were performed to investigate the nature of the antagonist activity of L-740,093. When tested at 10 nM L-740,093 produced a modest rightward shift in the CCK-4 dose response curve, an effect which was accompanied by a small reduction (13%) in the maximum response to CCK-4. In the presence of 30 nM L-740,093 the maximum functional response to CCK-4 was further reduced by 45% indicating that L-740,093 behaves as an insurmountable antagonist of the human CCK-B receptor subtype.

Protein kinase C-mediated inhibition of transmembrane signalling through CCK(A) and CCK(B) receptors

1. The rat CCK(A) and CCK(B) receptors were stably expressed in Chinese hamster ovary (CHO-09) cells in order to compare modes of signal transduction and effects of protein kinase C (PKC) thereupon. 2. Spectrofluorophotometry of Fura-2-loaded cells revealed that both receptors retained their pharmacological characteristics following expression in CHO cells. Sulphated cholecystokinin-(26-33)-peptide amide (CCK-8-S) increased the cytosolic Ca2+ concentration ([Ca2+]i) in CCK(A) cells, measured as an increase in Fura-2 fluorescence emission ratio, 1000 fold more potently than its non-sulphated form (CCK-8-NS) (EC50 values of 0.19 nM and 0.18 microM, respectively). By contrast, CCK-8-S and CCK-8-NS were equally potent in CCK(B) cells (EC50 values of 0.86 nM and 1.18 nM, respectively). The CCK(A) receptor agonist JMV-180 increased [Ca2+]i only in CCK(A) cells. Likewise, pentagastrin increased [Ca2+]i only in CCK(B) cells. Finally, CCK-8-S-induced Ca2+ signalling through the CCK(A) receptor was most potently inhibited by the CCK(A) receptor antagonist L364,718, whereas the CCK(B) receptor antagonist L365,260 was more potent in CCK(B) cells. 3. Receptor-mediated activation of adenylyl cyclase was measured in the presence of the inhibitor of cyclic nucleotide phosphodiesterase activity, 3-isobutyl-1-methylxanthine. CCK-8-S and, to a lesser extent, CCK-8-NS, but not JMV-180 or pentagastrin, stimulated the accumulation of cyclicAMP in CCK(A) cells. By contrast, none of these agonists increased cyclicAMP in CCK(B) cells. 4. Short-term (3 min) pretreatment with the PKC activator 12-O-tetradecanoylphorbol 13-acetate (TPA) evoked a rightward shift of the dose-response curve for the Ca2+ mobilizing effect of CCK-8-S in both cell lines. In addition, short-term TPA pretreatment markedly reduced CCK-8-S-induced cyclicAMP accumulation in CCK(A) cells. In both cases, the inhibitory effect of TPA was abolished by the PKC inhibitors, GF-109203X and staurosporine, whereas no inhibition was observed with the inactive phorbol ester, 4-alpha-phorbol 12-myristate 13-acetate. 5. During prolonged TPA treatment, the cells gradually recovered from phorbol ester inhibition and in the case of CCK-8-S-induced Ca2+ mobilization complete recovery was achieved after 24 h of TPA treatment. Western blot analysis revealed that this recovery was paralleled by down-regulation of PKC-alpha, suggesting the involvement of this PKC isotype in the inhibitory action of TPA. 6. This study demonstrates that following expression in CHO cells (i) both CCK(A) and CCK(B) receptors are coupled to Ca2+ mobilization, (ii) only CCK(A) receptors are coupled to cyclicAMP formation and (iii) with both receptors signalling is inhibited by PKC.

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.

Full and partial agonist activity of C-terminal cholecystokinin peptides at the cloned human CCK-A receptor expressed in Chinese hamster ovary cells

The agonist activities of the C-terminal cholecystokinin peptides sulfated cholecystokinin octapeptide (CCK-8S), non-sulfated cholecystokinin octapeptide (CCK-8NS), pentagastrin and CCK-4 at the cloned human CCK-A receptor expressed in Chinese hamster ovary cells were evaluated in two functional assays of receptor activation. [125I]-CCK-8S displacement studies employing membranes derived from these cells revealed the expected rank order of affinity for a number of CCK receptor ligands. CCK-8S was a potent agonist in (i) stimulating the mobilization of intracellular free Ca2+, measured with the Ca2+ sensitive fluorescent indicator FURA-2, and (ii) stimulating increases in extracellular acidification rates, measured by microphysiometry. Consistent with their lower affinities for CCK-A receptors, CCK-8NS, pentagastrin and CCK-4 were weaker agonists in both functional assays. In addition, these peptides exhibited partial agonist activity relative to the maximum response observed with CCK-8S in both assays. These results demonstrate that CCK-8S represents the minimum ligand requirement for both high affinity and full agonist activity at the human CCK-A receptor subtype.